Physiol Rev
84: 835– 867, 2004; 10.1152/physrev.00036.2003.
Molecular Structure and Physiological Functions
of GABAB Receptors
BERNHARD BETTLER, KLEMENS KAUPMANN, JOHANNES MOSBACHER, AND MARTIN GASSMANN
Pharmazentrum, Department of Clinical-Biological Sciences, Institute of Physiology, University of Basel, and
Novartis Institutes for Biomedical Research, Novartis Pharma AG, Basel, Switzerland
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
I. Introduction
II. Native GABAB Receptors
A. Coupling to G proteins
B. Coupling to Ca2⫹ channels
C. Coupling to K⫹ channels
D. Coupling to adenylyl cyclase
E. Pharmacology of native GABAB receptors
III. Cloned GABAB Receptors
A. Molecular structure
B. Molecular studies on native GABAB receptors
IV. Implication in Disease
A. Addiction
B. Epilepsy
C. Gastrointestinal disease
D. Nociception
E. Genetic linkage studies
V. Novel GABAB Compounds
A. Allosteric modulators
B. Subtype-selective ligands
VI. Summary and Outlook
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Bettler, Bernhard, Klemens Kaupmann, Johannes Mosbacher, and Martin Gassmann. Molecular Structure
and Physiological Functions of GABAB Receptors. Physiol Rev 84: 835– 867, 2004; 10.1152/physrev.00036.2003.—
GABAB receptors are broadly expressed in the nervous system and have been implicated in a wide variety of
neurological and psychiatric disorders. The cloning of the first GABAB receptor cDNAs in 1997 revived interest in
these receptors and their potential as therapeutic targets. With the availability of molecular tools, rapid progress was
made in our understanding of the GABAB system. This led to the surprising discovery that GABAB receptors need
to assemble from distinct subunits to function and provided exciting new insights into the structure of G proteincoupled receptors (GPCRs) in general. As a consequence of this discovery, it is now widely accepted that GPCRs can
exist as heterodimers. The cloning of GABAB receptors allowed some important questions in the field to be
answered. It is now clear that molecular studies do not support the existence of pharmacologically distinct GABAB
receptors, as predicted by work on native receptors. Advances were also made in clarifying the relationship between
GABAB receptors and the receptors for ␥-hydroxybutyrate, an emerging drug of abuse. There are now the first
indications linking GABAB receptor polymorphisms to epilepsy. Significantly, the cloning of GABAB receptors
enabled identification of the first allosteric GABAB receptor compounds, which is expected to broaden the spectrum
of therapeutic applications. Here we review current concepts on the molecular composition and function of GABAB
receptors and discuss ongoing drug-discovery efforts.
I. INTRODUCTION
GABA is the major inhibitory neurotransmitter in the
central nervous system (CNS) and as such plays a key role
in modulating neuronal activity. GABA mediates its action
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via distinct receptor systems, the ionotropic GABAA and
metabotropic GABAB receptors. Unlike GABAA receptors
that form ion channels, GABAB receptors address second
messenger systems through the binding and activation of
guanine nucleotide-binding proteins (G proteins; for other
0031-9333/04 $15.00 Copyright © 2004 the American Physiological Society
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BETTLER, KAUPMANN, MOSBACHER, AND GASSMANN
Physiol Rev • VOL
drug targets and at the same time provide a means to
determine the specificity of GABAB compounds in vivo.
Throughout this review emphasis is given to developments in the field that followed the initial cloning of
GABAB receptors. However, where molecular findings
extend or challenge earlier work, special attention is
given to older literature.
As a consequence of intense research efforts that
followed isolation of the first GABAB cDNAs, several
laboratories reported the cloning of a second GABAB
receptor cDNA, termed GABAB(2) (see sect. IIIA2). Several
groups published a most important discovery related to
GABAB(2). As shown for the first time for a G proteincoupled receptor (GPCR), the GABAB receptor was not a
single protein but instead consisted of two distinct subunits, neither of which was functional on its own. This
finding was of interest to a large scientific community,
and very quick progress was made in dissecting the roles
of the individual subunits in receptor activation, assembly, and signaling (see sect. IIIA). Much of this review is
devoted to this topic, as it fundamentally changed our
view on the structure and functioning of GPCRs. The
search for GABAB receptor subtypes did not lead to the
expected identification of additional GABAB cDNAs, although a number of GABAB-related cDNAs were identified in the process. The apparent lack of molecular heterogeneity was a surprise to many in the field who expected a variety of pharmacologically distinct GABAB
subunits, as predicted from work on native receptors.
Efforts therefore turned toward identifying GABAB isoforms (see sect. IIIA4), receptor-associated proteins (see
sect. IIIA9), receptor modifications (see sect. IIIA10), and
endogenous factors (see sect. IIIA6) that possibly were
responsible for generating pharmacological differences.
Once more, some unexpected findings were made. Several groups reported that transcription factors, such as
CREB2/ATF-4, are able to directly interact with GABAB
receptors. At the same time, with the puzzling lack of
pharmacologically distinct receptor subtypes, it became
important to understand to which known GABAB functions the cloned receptor subunits contribute in vivo. To
address this question, many laboratories studied the expression of cloned GABAB subunits in vivo (see sect.
IIIB3) or disabled GABAB genes in mice (see sect. IIIB1),
which greatly clarified the role of individual subunits in
GABAB receptor physiology. The overt phenotypes of
GABAB knockout mice also pointed at the neuronal systems that crucially depend on GABAB-receptor activity. A
long-standing question in the field concerns the relationship between GABAB receptors and the receptors for
␥-hydroxybutyrate (GHB), a metabolite of GABA and
emerging drug of abuse. It is a matter of much debate
whether specific GHB receptors exist and whether they
are related to GABAB receptors. This question was addressed using molecular tools (see sect. IIIB2). Following
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recent reviews on GABAB receptors, see Refs. 45, 55).
Dysfunction of GABA-mediated synaptic transmission in
the CNS is believed to underlie various nervous system
disorders. For example, hypoactivity of the GABA system
was linked to epilepsy, spasticity, anxiety, stress, sleep
disorders, depression, addiction, and pain. On the contrary, hyperactivity of the GABAergic system was associated with schizophrenia (20). GABA research, because of
its medical relevance, has always attracted a great deal of
attention in academia and industry. Over the years pharmaceutical companies successfully exploited the GABA
system and introduced a number of drugs to the market.
However, despite considerable drug-discovery efforts, baclofen (␤-chlorophenyl-GABA, Lioresal) currently remains the only available GABAB medication. Baclofen, a
lipophilic derivative of GABA, was synthesized in 1962 in
an attempt to enhance the blood-brain barrier penetrability of the endogenous neurotransmitter. Baclofen was
introduced to the market in 1972 and is used to treat
spasticity and skeletal muscle rigidity in patients with
spinal cord injury, multiple sclerosis, amyotrophic lateral
sclerosis, and cerebral palsy (44). GABAB agonists
showed promising therapeutic effects in a whole range of
other indications, but their side effects, including sedation, tolerance, and muscle relaxation, prevented further
development (see sect. IV). Many researchers in the field
assumed that a dissociation of the therapeutic effects
from the side effects was achievable with more selective
GABAB drugs. This assumption was based on a large body
of literature suggesting the existence of pharmacologically distinct GABAB receptor subtypes in the brain (see
sect. IIE). A more selective interference with the GABAB
system appeared feasible but crucially depended on the
cloning of the predicted receptor subtypes. Therefore, it
was mostly commercial interests that were driving efforts
to isolate a GABAB receptor cDNA. GABAB receptors
were not cloned until 1997 and thus remained the last of
the major neurotransmitter receptors to be characterized
at the molecular level (169). At first glance this is quite
surprising, considering that GABAB receptors were identified as early as in 1980 (46). In retrospect, it is clear that
many cloning attempts failed because of the unexpected
properties of GABAB receptors. We therefore review
some of the strategies that were applied when trying to
isolate GABAB receptors and discuss why these approaches failed (see sect. IIIA1). Identification of the first
GABAB cDNAs renewed commercial interests in these
receptors for a number of reasons. Most importantly, the
cloning generated the necessary tools to isolate the expected additional members of the GABAB receptor family.
Moreover, high-throughput compound screening, using
molecularly defined receptors and functional assay systems, suddenly became practicable. GABAB receptors
were now also amenable to gene-targeting technology.
This was supposed to help validate the most promising
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
receptor cloning, several studies tried to establish a link
between polymorphisms in GABAB genes and congenital
human diseases (see sect. IVE). Taking recent developments in the field into account, we also touch on the most
promising indications for GABAB drugs (see sect. IV). Last
but not least, the cloned receptors were used to establish
high-throughput compound screens based on functional
assays, which yielded the first allosteric compounds acting at GABAB receptors (see sect. VA).
II. NATIVE GABAB RECEPTORS
Bowery et al. (46) were the first to discover that
GABA reduces norepinephrine release by activating a
bicuculline- and isoguvacine-insensitive receptor, which
they named the GABAB receptor. Evidence for a coupling
of GABAB receptors to G proteins came from the sensitivity of agonist affinity to GTP analogs (10, 144). Studies
using N-ethylmaleimide (NEM), islet activated protein
(IAP), pertussis toxin, or antisense knock-down provided
evidence that GABAB receptors predominantly couple to
Gi␣- and Go␣-type G proteins (9, 59, 122, 223, 228). It is
now well established that presynaptic GABAB receptors
repress Ca2⫹ influx by inhibiting Ca2⫹ channels in a membrane-delimited manner via the G␤␥ subunits (see sect.
IIB). Postsynaptic GABAB receptors trigger the opening of
K⫹ channels, again through the G␤␥ subunits (see sect.
IIC). This results in a hyperpolarization of the postsynaptic neuron that underlies the late phase of inhibitory
postsynaptic potentials (IPSPs) (201). Besides modulating
ion channels through G␤␥, GABAB receptors activate and
inhibit adenylyl cyclase via the Gi␣/Go␣ and G␤␥ subunits
(see sect. IID). Recent work by Hirono et al. (145) shows
that GABAB activity enhances mGlu1 responses at excitatory synapses in the cerebellum. This facilitation is
suggested to require Ca2⫹ release from internal stores
through a mechanism that involves Gi␣/Go␣-linked phospholipase C (PLC) activation and a cooperative upregulation of GPCR signaling by the G␤␥ subunits. This is reminiscent of synergistic interactions seen with GABAB and
␤-adrenergic receptors (42). Importantly, G protein-independent GABAB effects on neurotransmitter release were
also suggested (132).
B. Coupling to Ca2ⴙ Channels
Presynaptic GABAB receptors are subdivided into
those that control GABA release (autoreceptors) and
those that inhibit all other neurotransmitter release (heteroreceptors). In most preparations, GABAB receptors
mediate their presynaptic effects through a voltage-dePhysiol Rev • VOL
pendent inhibition of high-voltage activated Ca2⫹ channels of the N type (Cav2.2) or P/Q type (Cav2.1) (4, 61, 223,
225, 236, 261, 265, 299, 320). Both types of Ca2⫹ channels
are expressed in presynaptic terminals and were shown to
trigger neurotransmitter release (349). A postsynaptic inhibition of Ca2⫹ channels by GABAB receptors was also
postulated (130). It was shown that GABAB receptors
couple to different types of Ca2⫹ channels depending on
the input site (266). The inhibition of Ca2⫹ inward currents is voltage dependent and varies between 10 and 42%
among studies (73, 78, 294). Since Ca2⫹ influx and transmitter release are correlated with a third to fourth power
law (349), GABAB agonists frequently inhibit more than
90% of neurotransmitter release with a less than 50%
inhibition of Ca2⫹-channel activity. This inhibition is modulated by the action potential frequency, where strong
depolarization relieves Ca2⫹ channels from their G␤␥mediated inhibition (140, 152, 354). This particular property of presynaptic Ca2⫹ channels may differentially modulate action potential trains, depending on their frequency
(53). GABAB receptors are also described to either inhibit
(4, 202, 208) or facilitate (309) L-type Ca2⫹ channels. The
latter effect was shown to be indirect and to depend on
protein kinase C (PKC) activity. Similarly, GABAB receptors also inhibit or disinhibit T-type Ca2⫹ channels (83,
107, 219, 303, 304).
C. Coupling to Kⴙ Channels
GABAB receptors induce a slow inhibitory postsynaptic current (late IPSC) through activation of inwardly
rectifying K⫹ channels (GIRK or Kir3) (201, 300). Accordingly, GABAB-induced late IPSCs can be inhibited by the
Kir3 channel blocker Ba2⫹ (159, 264, 325), and they usually exhibit a reversal potential similar to the K⫹ equilibrium potential (188, 220). The physiological effect of Kir3channel activation is normally a K⫹ efflux, resulting in a
hyperpolarization. The time course of the late IPSC, with
a time to peak of 50 –250 ms and decay times of 100 and
500 ms, clearly differs from that of the fast IPSC, which is
GABAA receptor mediated (188, 249). Baclofen-induced
outward currents in hippocampal neurons are absent in
Kir3.2 and GABAB(1) knockout mice, corroborating the
prominent role of Kir3 channels in mediating the effects
of GABAB receptors (201, 300). The rectification properties of synaptically evoked late IPSCs differed between
studies. On the one hand, the stimulus-evoked and spontaneous late IPSCs in dopaminergic neurons are inwardly
rectifying and similar to those activated by baclofen (134).
On the other hand, baclofen also induces linear or even
outwardly rectifying conductances, suggesting that channels other than Kir3 can contribute to the late IPSC. These
other channels may include fast inactivating, voltagegated K⫹ channels (289) and small-conductance Ca2⫹-
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A. Coupling to G Proteins
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D. Coupling to Adenylyl Cyclase
All of the known nine adenylyl cyclase isoforms are
expressed in neuronal tissue. Gi␣ and Go␣ proteins, the
predominant transducers of GABAB receptors, inhibit
most of them (311). Many studies have reported that
GABAB receptors inhibit forskolin-stimulated cAMP formation, but others also observed a stimulation of cAMP
production (45, 55). Gi␣ and Go␣ proteins inhibit adenylyl
cyclase types I, III, V, and VI, while G␤␥ stimulates adenylyl cyclase types II, IV, and VII. This stimulation depends on the presence of Gs␣, which results from the
activation of GPCRs by, e.g., norepinephrine, isoprenaline, histamine, or vasoactive intestinal polypeptide (311,
321). Therefore, the stimulatory action of GABAB receptors on cAMP levels is a consequence of G protein crosstalk and depends on the expression of adenylyl cyclase
isoforms together with GABAB and Gs␣-coupled GPCRs.
Both the inhibition and enhancement of cAMP levels by
GABAB receptor activation were confirmed in vivo using
microdialysis (133). Many ion channels are targets of the
cAMP-dependent kinase (protein kinase A or PKA). Accordingly, a GABAB receptor-mediated modulation of K⫹
channels via cAMP was reported (116). Significantly, the
activity of GABAB receptors on adenylyl cyclase is expected to modulate neuronal function on a longer time
scale (see sect. IIIA9).
GABAB receptors were repeatedly implicated in synaptic plasticity (87, 229, 246, 247, 334). Until recently, it
was unclear whether GABAB receptors can influence plasticity processes through the cAMP pathway. Recent experiments now demonstrate that G protein-mediated signaling through GABAB receptors retards the recruitment
of synaptic vesicles during sustained activity and after
short-term depression (290). This retardation occurs
through a lowering of cAMP, which blocks the stimulatory effect of the increased Ca2⫹ concentration on vesicle
recruitment. In this signaling pathway, cAMP and Ca2⫹/
calmodulin cooperate to enhance vesicle priming.
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E. Pharmacology of Native GABAB Receptors
Bowery et al. (45) recently published a comprehensive and detailed review on currently available GABAB
ligands (45). Here, we only focus on one particular aspect
of GABAB receptor pharmacology: the discrepancies in
the potency of agonists and antagonists in different biochemical and physiological paradigms. Numerous biochemical studies indicate pharmacological differences between auto- and heteroreceptors and even within autoand heteroreceptors (15, 39 – 41, 248, 323). However, the
proposal of presynaptic receptor subtypes based on neurotransmitter release experiments has been open to dispute (336). Electrophysiological and release experiments
suggest distinctions between pre- and postsynaptic
GABAB receptors as well (65, 76, 85, 88, 96, 115, 262, 268,
325, 352). Accordingly, published half-maximal effective
concentrations for baclofen in pharmacological studies
vary considerably and range between 100 nM and 100 ␮M
(Table 1).
The rank order of agonist and antagonist binding
affinities at GABAB(1) and native GABAB receptors is identical (169). This, together with the reasons outlined in
sections IIIA4 and vB, makes it unlikely that molecularly
distinct GABAB receptor subtypes or isoforms underlie
these pharmacological differences. Obviously, the ratio of
receptors and effectors can determine the apparent potency of receptor agonists (174). Agonist potency may
also depend on the concentration of divalent cations in
the extracellular buffer (see sect. IIIA6), the association
with lipid rafts (18), the phosphorylation state of subunits
(see sect. IIIA10), or the type of G protein that is present
in the cell. These factors may also explain why the agonist
affinity at GABAB receptors increases 10-fold during postnatal development (206). This said, all experiments with
native GABAB receptors reporting changes in the rank
order of ligand efficacies remain unexplained (40, 85).
These experiments would normally clearly argue in favor
of pharmacologically distinct receptor subtypes.
III. CLONED GABAB RECEPTORS
A. Molecular Structure
1. Expression cloning of GABAB receptors
GABAB receptors were cloned in 1997 (169), close to
20 years after their discovery by Bowery et al. (46). In
retrospect, we understand the reasons that prevented an
earlier isolation of GABAB cDNAs. To begin with, a purification of GABAB receptor proteins proved difficult.
There were no radioligands that bound irreversibly or
with high affinity to the receptor. Moreover, receptor
function was inevitably lost in the presence of solubilizing
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activated K⫹ channels (SK channels) (116). Accordingly,
the fast inactivating A-type K⫹-channel blocker 4-aminopyridine inhibits a baclofen-induced current in guinea pig
hippocampal neurons (153, 243). Moreover, GABA activates Ca2⫹-sensitive K⫹ channels (36) and small-conductance K⫹ channels (36, 89) in rat hippocampal neurons.
Possibly, GABAB receptors enhance the activity of SK
channels by inhibiting the production of cAMP after an
action potential-induced Ca2⫹ influx (116). In addition to
the well-documented coupling of GABAB receptors to
postsynaptic K⫹ channels, GABAB receptors also appear
to activate Ba2⫹-sensitive K⫹ channels at presynaptic
sites (325). Likely these presynaptic K⫹ channels are of
the Kir3 type, devoid of the Kir3.2 subunit and assembled
from Kir3.1 and Kir3.4 subunits (201).
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GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
TABLE
1. Potencies of GABAB compounds
Region
Drug
Ca channel inhibition
Ca2⫹ channel inhibition
IPSCs
IPSCs
Glu release
Glu release
Glu release
Glu release
Glu release
GABA release
GABA release
GABA release
GABA release
GABA release
[K⫹]out
evoked IPSCs
Firing
Contraction
SST release
Evoked IPSCs
Evoked IPSCs
Binding [3H]GABA
Binding [3H]GABA
Baclofen inhibition
Baclofen inhibition
mEPSCs
mIPSCs
Baclofen
Baclofen
Baclofen
CGP55845
Baclofen
3-APPA
CGP44532
CGP35348
CGP52432
Baclofen
3-APPA
CGP44532
CGP35348
CGP52432
Baclofen
Baclofen
Baclofen
Baclofen
CGP36742
Baclofen
CGP35348
SCH50911
CGP35348
SCH50911
CGP35348
Baclofen
Baclofen
IC50/EC50, ␮M
3
1.3
7
0.5
0.1
0.05
0.5
⬎100
10
⬎100
0.09
0.84
1
10
39
0.7
0.5
31
0.14
0.48
5 (Kd)
1
62
1.8
25
4.4
4.3
Reference Nos.
74
78
285
285
39
39
39
39
39
39
39
39
39
39
242
230
124
203
38
308
308
37
37
37
37
158
158
PAG, periaqueductal gray; IPSCs, inhibitory postsynaptic currents; EPSCs, excitatory postsynaptic currents; Kd, dissociation constant.
concentrations of detergents, most likely because of the
dissociation of GABAB(1) and GABAB(2) subunits. This
made it impossible to trace the protein during the purification steps using functional assay systems, such as, e.g.,
GTP␥[35S] binding. Nevertheless, the purification of a putative 80-kDa GABAB-receptor protein was reported
(233). The molecular mass of the 80-kDa GABAB protein
does not match the molecular mass of cloned GABAB
subunits, and a relationship with the latter is unlikely. No
amino acid sequence of the 80-kDa protein was disclosed,
and hence, its molecular structure remains enigmatic. As
a consequence of the problems associated with purifying
a GABAB-receptor protein, no antibodies for cloned
GABAB subunits were available prior to cloning.
In pregenome times, expression cloning was the
most successful approach for isolation of a neurotransmitter receptor. In essence, expression cloning circumvents the requirement for protein purification and instead
uses a specific biological activity as the basis of cDNA
identification. An inherent advantage of such a screening
approach is that the isolated cDNA clones are usually
full-length and immediately available for functional studies. Xenopus oocytes are the expression system of choice
when using electrophysiological screening techniques.
The method relies on the ability of oocytes to translate
protein from injected cRNA (RNA transcribed from
cDNA). Expressed receptors in the oocyte membrane can
then be detected using two-electrode voltage-clamp or
Physiol Rev • VOL
similar recording techniques. This procedure was used to
isolate a number of G protein-coupled neurotransmitter
receptors, including mGlu receptors (218). Several laboratories, including ours, explored strategies using Xenopus oocytes when attempting to clone a GABAB cDNA.
Because GABAB receptors do not couple to PLC, it was
impossible to apply expression-cloning strategies based
on increases in intracellular [Ca2⫹] and subsequent activation of Ca2⫹-activated Cl⫺ channels. Others and we
therefore supplied Xenopus oocytes with effector Kir3
channels (201) or promiscuous G proteins that permitted
the detection of weak functional GABAB responses (306).
We eventually abandoned expression cloning in Xenopus
oocytes because the electrophysiological GABAB responses were unreliable and were lost in the process of
splitting the brain cDNA pool into smaller pools. This loss
of functional responses is readily explained by the fact
that both GABAB(1) and GABAB(2) subunits are required to
assemble a functional receptor. The concomitant serial
isolation of the two distinct cDNA clones is virtually
impossible because the functional response is missing
whenever the two cDNAs segregate into different pools in
the course of narrowing down the active cDNAs.
We reasoned that for the isolation of a GABAB cDNA,
a screening approach using a radioligand binding assay
rather than an electrophysiological read-out would be
more promising. Such a screening does not depend on a
functional coupling of the receptor and solely relies on
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mIL cells
PAG neurons
Midbrain cultures
Midbrain cultures
Spinal cord
Spinal cord
Spinal cord
Spinal cord
Spinal cord
Spinal cord
Spinal cord
Spinal cord
Spinal cord
Spinal cord
Hippocampus
Supraoptic nucleus
SN dopaminergic
Colonic circular muscle
Neocortical neurons
SN reticulara
SN reticulara
Rat brain
Rat brain
Trachea
Trachea
Spinal cord
Spinal cord
Read-Out
2⫹
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BETTLER, KAUPMANN, MOSBACHER, AND GASSMANN
2. GABAB receptor heteromerization
While GABAB(1) subunits showed many of the expected features of native GABAB receptors in terms of
structure and distribution, they surprisingly did not efficiently couple to their effector systems (214). Moss and
colleagues (80) were the first to demonstrate that
GABAB(1) proteins are retained in the endoplasmatic reticulum (ER) when expressed in heterologous cells. This
is taken to explain the 100- to 150-fold lower affinity for
agonists that is observed with recombinant GABAB(1) subunits compared with native GABAB receptors (169). Presumably, the failure of GABAB(1) to traffic to the cell
surface in the absence of GABAB(2) prevents the interaction with the G protein in the plasma membrane, which is
necessary to stabilize the high-affinity conformation of
the binding site (see sect. IIIA8). The search for a missing
factor, which traffics GABAB(1) subunits to the cell surface and renders them functional, therefore became an
important objective. The search ended with the remarkable publication of three consecutive papers in Nature by
groups from GlaxoWellcome, Novartis, and Synaptic
Pharmaceuticals, as well as three subsequent papers from
BASF-LYNX Bioscience, Merck, and the Laboratory of
Molecular Neurobiology at Boston University (163, 170,
185, 216, 239, 343). All six papers describe the identification of the GABAB(2) subunit, which must be coexpressed
with GABAB(1a) or GABAB(1b) subunits to form a functional receptor. This finding represented the first compelling evidence for heteromerization among the GPCRs.
Recombinant heteromeric GABAB(1,2) receptors couple to
all prominent effector systems of native GABAB receptors, that is, adenylyl cyclase, Kir3-type K⫹ channels, and
P/Q- and N-type Ca2⫹ channels (98, 100, 214). When the
GABAB(2) subunit is coexpressed with GABAB(1), agonist
potency more closely approximates that of native receptors (214). Kaupmann et al. (170) still observed a 10-fold
lower affinity of recombinant GABAB(1,2) receptors as
opposed to brain receptors, which may be explained
by limiting amounts of the G protein in the heterologous
cells (170).
The reason for the intracellular retention of
GABAB(1) subunits is the presence of an ER-retention
signal, the four-amino acid motif RSRR, in its cytoplasmic
tail (210, 250). ER-retention signals of the RXR type were
FIG. 1. Phylogenetic tree of human Family 3
GPCRs. The predicted seven transmembrane domains
for receptors were aligned using Clustal W (version
1.82) (324), using default parameters (gap open penalty, 10; gap extension penalty, 1.0; and protein weight
matrix blosum). The receptors are as follows:
GABAB(1) (GenBank accession number NW001470);
GABA B(2) (AF056085); GABA B -related receptor,
GABABL (XM093467); Ca2⫹-sensing receptor, CaSR
(S83176); metabotropic glutamate receptors, mGluR1
(NM_000838), mGluR2 (NM_000839), mGluR3
(NM_000840), mGluR4 (NM_000841), mGluR5
(NM_000842), mGluR6 (NM_000843), mGluR7
(NM_000844), mGluR8 (NM_00845); taste receptors,
T1R1 (NM_138697), TIR2 (AH011576), TIR3
(BK000152); and a family of orphan receptors, RAIG1
(AF095448), GPRC5B (AF202640), GPRC5C
(AF207989), GPRC5D (NM_018654). Not indicated are
the members of a large family of putative pheromone
receptors that were identified in rodents.
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the exposure of a high-affinity binding site. With the development of high-affinity GABAB radioligand antagonists,
such as 125I-CGP64213 (Kd ⫽ 1.2 ⫾ 0.2 nM), expression
cloning using a binding assay became feasible and allowed the isolation of GABAB(1a) and GABAB(1b) cDNAs
(169). The two encoded proteins derive from the same
gene and differ in their extracellular NH2-terminal domains (see sect. IIIA4A). The molecular structure of the
cloned GABAB proteins revealed all the hallmarks of a
GPCR. Specifically, the sequence of GABAB(1) subunits
exhibited seven transmembrane domains and similarity
with Family 3 (also named Family C) GPCRs. It is now
known that Family 3 GPCRs comprise metabotropic glutamate (mGlu), the Ca2⫹-sensing (CaS), vomeronasal, and
taste receptors (Fig. 1).
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
Physiol Rev • VOL
biochemical responses (169, 171). This suggests that
GABAB(1) could be functional either alone or in combination with an unknown protein. It is difficult to reconcile
functional GABAB(1) responses with the efficient ER retention observed with this subunit. Even when wild-type
GABAB(1) is artificially targeted to the cell surface by
masking the ER-retention signal with the COOH-terminal
domain of GABAB(2), functional responses are not observed (250). It may be speculated that the weak and
infrequent coupling of homomeric GABAB(1) or GABAB(2)
to cAMP and Kir3 channels depends on presently unknown endogenous factors. On the other hand, it remains
unclear whether occasional endogenous expression of
the partner subunit in heterologous cells could be responsible for the rare responses that were seen when
GABAB(2) or GABAB(1) was transfected alone. In this context it is interesting to compare GABAB receptors with
NMDA receptors that are similarly assembled with subunits that contain the RXR-type ER-retention signal (302).
For unknown reasons, and similar to GABAB subunits, the
NMDA receptor subunit NR1 occasionally generates functional responses in HEK293 cells or Xenopus oocytes in
the absence of the masking subunit (222).
Despite several observations suggesting that GPCRs
could form dimers or higher-order oligomers (7), the conventional perception was that GPCRs exist at the cell
surface as monomers that couple to G proteins. For the
most part it is the characterization of GABAB receptors
that changed our view. It is now generally agreed that
GPCRs can form homo- and heterodimers. In the last
couple of years, it became more and more evident that
heteromerization between GPCRs is not that rare a phenomenon. There are now several examples where even
distantly related GPCRs form heteromeric complexes (1,
117, 164, 284). This raises fascinating combinatorial possibilities and may generate a level of regulatory and pharmacological diversity that we did not anticipate. Because
GPCRs are major pharmacological targets, this recognition will have important implications for the development
and screening of new drugs.
3. Invertebrate GABAB receptors
Development and organization of the nervous system
are substantially different between vertebrates and invertebrates. Neurotransmitters evolved the ability to activate
a range of ion channels and second messenger systems,
but their relative importance as sensory, inter- or motorneuron signaling molecules varies widely between phyla.
Although the evidence is still sketchy, it currently appears
that most mammalian neurotransmitter receptors have
counterparts in invertebrates (337). Invertebrate GABAB
receptors were described in echinoderms (90, 91), mollusks (11, 287), and arthropods (101, 226, 254, 276, 335).
Invertebrate GABAB functions were mostly studied at the
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also observed in other multisubunit proteins, such as, for
example, the KATP channels (355) or N-methyl-D-aspartate
(NMDA) receptors (302). It was recently proposed that
the sequence context of the RSRR motif in GABAB(1) is
crucial for ER retention, and the motif was extended to
include the sequence QLQXRQQLRSRR (125). The ERretention signal in GABAB(1) is masked from its ER-anchoring mechanism through the interaction with the
COOH terminus of GABAB(2), thus allowing for delivery of
the GABAB(1,2) complex to the cell surface. This ERretention mechanism is suggested to prevent incorrectly
folded GABAB receptors from reaching the cell surface
and to represent a quality control mechanism. Some laboratories identified GABAB(2) in the yeast two-hybrid system when using GABAB(1) COOH-terminal sequences as
bait, directly demonstrating that the COOH-terminal domains of GABAB(1) and GABAB(2) interact with each other
(185, 343). Subsequent deletion analysis mapped the interaction to ␣-helical coiled-coil domains of 32–35 amino
acids in length. Coiled-coil domains are dimerization motifs that are found in numerous structural, trafficking, and
regulatory proteins, such as, e.g., in leucine-zipper transcription factors. Mixing equimolar amounts of recombinant GABAB(1) and GABAB(2) peptides corresponding to
the predicted coiled-coil domains indeed produced parallel coiled-coil heteromers under physiological conditions
(166). In contrast, individual GABAB(1) or GABAB(2) peptides folded into relatively unstable homodimers or remained largely unstructured, suggesting that homodimeric receptors are not efficiently formed through
coiled-coil interaction. Mutational analysis confirmed that
the COOH-terminal coiled-coil interaction of GABAB receptors is essential for surface trafficking of the heteromer (57, 210, 250). Surprisingly, however, this interaction
is not an absolute requirement for assembly of the receptor complex. It was demonstrated that COOH-terminally
truncated GABAB(1) and GABAB(2) subunits can form fully
functional receptors when expressed in heterologous
mammalian cells (57, 250). This indicates that the transmembrane domains and/or extracellular domains (ECDs)
encode surfaces that are sufficient for heteromerization,
which is in agreement with findings with the COOH-terminally truncated GABAB(1e) splice variant (301). Furthermore, this agrees with data that suggest that other Family
3 GPCRs, e.g., the mGlu1, mGlu5, and CaS receptors,
homodimerize in their ECDs (278, 286, 328).
Challenging the general assumption that GABAB receptors necessarily need to heteromerize for function are
infrequent responses seen with receptor subunits expressed in isolation. For example, GABAB(2) was found to
couple to adenylyl cyclase in the absence of GABAB(1)
(185, 216), which is at odds with the proposal that only
GABAB(1) subunits are able to bind ligands (179). Similarly, when expressed alone in heterologous cells,
GABAB(1) yields infrequent electrophysiological and small
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ogy that was described for insect GABAB receptors (12,
150, 295) and therefore do not suggest the existence of
additional GABAB subunits.
Physiological or pharmacological approaches have
not found any evidence for GABAB receptors in helminths
(337). GABA-induced muscle relaxation in the nematode
Caenorhabditis only depends on the unc-49 gene products, which form GABAA-like ionotropic receptors (14).
However, the Caenorhabditis genome database reveals
the presence of orthologs for both GABAB(1) and
GABAB(2) (179). Most residues of the GABA bindingpocket in GABAB(1) are conserved from Caenorhabditis
to human (179). This is in contrast to the complete lack of
evolutionary constraint placed on the binding pocket of
GABAB(2), which suggests that this subunit does not constitute a binding site for an endogenous ligand (see sect.
IIIA5).
4. GABAB-receptor isoforms
When it became apparent that probably all GABAB
receptors in the vertebrate brain are the sole products of
the GABAB(1) and GABAB(2) genes, much attention focused on subunit isoforms. Many in the field wondered
whether isoforms encoded pharmacological differences
and accounted for the heterogeneity observed with native
GABAB receptors. Rapidly numerous GABAB isoforms
were identified (recently reviewed in Ref. 29). A close
inspection of GABAB gene structures indicates that not all
of these splice variants are real and that some do not
occur across different species. A summary of confirmed
GABAB isoforms is shown in Figure 2. While in most
laboratories mixing and matching of isoforms did not
produce GABAB receptors with distinct functional and
FIG. 2. Protein structure of vertebrate
GABAB(1) isoforms. The sushi domains (SD),
seven transmembrane domains (7TM), coiled-coil
domains (CC), and the ER-retention signals
(RSRR) are indicated. Unfortunately, unrelated
variants identified in humans and rats both were
named GABAB(1c). Thus the variants described in
the rat are better referred to as GABAB(1c-a) or
GABAB(1c-b), depending on whether their aminoterminal domain is related to GABAB(1a) or
GABAB(1b), respectively. The 31-amino acid insertion between the second extracellular loop (ECL2)
and the fifth transmembrane domain in GABAB(1c-a),
GABAB(1c-b), and GABAB(1f) corresponds to a 93-bp
exon located between exons 19 and 20, which is not
conserved in human and mouse. Dark gray segments
represent the unique carboxy-terminal tails of
GABAB(1d), GABAB(1e), and GABAB(1g). In GABAB(1f),
skipping of exon 5 results in an in-frame deletion of
7 amino acids (⌬7aa). References for all published
variants are shown in parentheses.
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neuromuscular junction of echinoderms, where GABA
elicits inhibitory and excitatory responses (90). GABAB
responses were also observed in invertebrate CNS neurons (150, 287). Like their mammalian orthologs, invertebrate GABAB receptors activate G proteins and regulate
K⫹ and Ca2⫹ currents (226, 287).
The only cloned invertebrate GABAB receptors are
those of Drosophila melanogaster (224). Two of the three
subunits that were described, d-GABAB(1) and d-GABAB(2),
show clear sequence identity to the mammalian subunits.
A third subunit, d-GABAB(3), seems to be specific for
insects and is of unknown function. Surprisingly, Drosophila GABAB receptors do not contain the sushi domains that are found in the mammalian GABAB(1a) subunit
(see sect. IIIA4A), although sushi domains exist in the
Drosophila genome. For example, sushi repeats are found
in hikaru genki, a Drosophila protein that is secreted from
presynaptic terminals during the period of synapse development (148). Analysis of the expression pattern in the
embryonic nervous system revealed that d-GABAB(1) and
d-GABAB(2) are expressed in overlapping regions. Upon
coexpression in Xenopus oocytes or mammalian cell
lines, d-GABAB(1) and d-GABAB(2) form a functional heteromeric d-GABAB(1,2) receptor that couples to Gi␣/Go␣type G proteins. Therefore, heteromerization is not only a
prerequisite for mammalian but also for invertebrate
GABAB function. d-GABAB(1,2) receptors exhibit a unique
pharmacology. GABA and 3-aminopropylphosphonous
acid (3-APPA) are, as expected, d-GABAB(1,2) agonists.
However, baclofen has no longer agonistic properties and
instead acts as an antagonist. Furthermore, neither
saclofen nor CGP35348 antagonizes d-GABAB(1,2) receptors, in contrast to their activity at mammalian receptors.
d-GABAB(1,2) receptors closely reproduce the pharmacol-
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
FIG. 3. Evolutionary conservation of the GABAB receptor antagonist binding site. Photoaffinity labeling of GABAB receptors from different species. Brain membranes from the species indicated were labeled
with 125I-CGP71872. In the case of Drosophila melanogaster and Haemonchus concortus, whole animals were analyzed. cDNA cloning revealed that Drosophila GABAB receptors exhibit a unique pharmacology
and do not interact with a number of known agonists and antagonists.
Physiol Rev • VOL
sushi repeats with proteins in the extracellular matrix or
on the surface of neighboring cells. A recent report suggests that the GABAB(1a) sushi-repeats interact with the
extracellular matrix protein fibulin (118). This proposal is
of significant interest, also from a drug discovery point of
view (see sect. VI).
Numerous studies indicate that GABAB(1a) and
GABAB(1b) show differences in their spatial and temporal
expression patterns (Fig. 4, B–D) (21, 32, 33, 104, 169 –171,
194, 206, 267, 270). A pre- versus postsynaptic localization
was suggested for both isoforms, but was never directly
demonstrated (21, 32, 171). A striking example of a differential expression of GABAB(1a) and GABAB(1b) is found
in the cerebellum (Fig. 4B) (33, 171). GABAB(1a) transcripts are confined to the granule cell layer that comprises the cell bodies of the parallel fibers, which are
excitatory to the Purkinje cell dendrites in the molecular
layer. By comparison GABAB(1b) transcripts are mostly
expressed in Purkinje cells, the dendrites of which possess GABAB receptors that are postsynaptic to GABAergic
basket and stellate cells or glutamatergic parallel fibers.
In dorsal root ganglia the density of GABAB(1a) transcripts
is high as opposed to GABAB(1b) transcripts (32).
GABAB(1b) protein is generally expressed at higher levels
in the adult brain compared with fetal brain, whereas the
opposite is seen during development (Fig. 4D) (56, 104,
206, 217). These spatial and temporal differences in the
expression of the GABAB(1a) and GABAB(1b) subunits highlight the separate transcriptional regulation and suggest
distinct functional roles.
A number of laboratories compared the pharmacology of GABAB(1a) and GABAB(1b) and did not detect significant differences (49, 121, 170, 206). However, there are
isolated reports that claim that GABAB(1a) and GABAB(1b)
can be separated by pharmacological or functional means
(26, 192, 237). Especially, the proposal that the anticonvulsant gabapentin is an agonist at GABAB(1a,2), but not at
GABAB(1b,2), attracted a great deal of attention (26, 27,
237). Not only was gabapentin suggested to be subunit
specific, but it was also shown to selectively activate
postsynaptic GABAB receptors. This remains highly controversial, as a number of other laboratories were unable
to reproduce these findings, using similar and additional
experimental approaches (161, 190, 255). In these latter
studies, no clear effect of gabapentin on GABAB receptors
was seen in recombinant systems, in brain slice preparations, or in vivo, even when using high concentrations of
the drug.
B) MINOR SUBUNIT VARIANTS. Several additional GABAB(1)
isoforms were reported. GABAB(1c) and GABAB(1d) were
identified in rat cDNA libraries (156, 260). GABAB(1c) is
characterized by an in-frame insertion of 31 amino acids
between the second extracellular loop and the fifth transmembrane domain. This extra sequence is not conserved
in humans, casting doubt on its significance. Nonetheless,
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pharmacological properties, others reported differences
that are, however, highly controversial (see sect. IIIA4A).
GABAB isoforms may afford the means for a differential
subcellular targeting and/or coupling to distinct intracellular signaling pathways. To some extent a coupling to
different effector systems could mimic a differential pharmacology and explain some of the differences that were
observed with native GABAB receptors.
A) PREDOMINANT SUBUNIT VARIANTS: GABAB(1a) AND GABAB(1b). The
most abundant GABAB-receptor isoforms are GABAB(1a)
and GABAB(1b), which exhibit dissimilarity in the ECD
(169). The GABAB(1a) and GABAB(1b) isoforms are the
only variants that are highly conserved among different
species (Fig. 3). The first 147 amino acids of the mature
GABAB(1a) isoform are replaced in GABAB(1b) with a sequence of 18 amino acids. Contrary to the general assumption, GABAB(1a) and GABAB(1b) are not generated by
NH2-terminal alternative splicing. The distinct ECD in
GABAB(1b) results from the presence of an alternative
transcription initiation site within the GABAB(1a) intron
upstream of exon 6, thereby extending exon 6 at its 5⬘-end
(Fig. 4A) (217, 260). Presumably, GABAB(1a) and
GABAB(1b) use different promoters, with the GABAB(1b)
promoter being buried within GABAB(1a) intron sequences. Alternative NH2 termini are rather exceptional
for GPCRs and are not observed in any of the closely
related Family 3 GPCRs. GABAB(1a) and GABAB(1b) primarily differ by the presence of a pair of sushi repeats in
the GABAB(1a)-specific domain (28, 135). Sushi repeats,
also known as short consensus repeats (SCRs), were
originally identified in complement proteins as a module
that is involved in protein-protein interactions. They are
mostly found in proteins that are involved in cell-cell
adhesion and were never before observed in a neurotransmitter receptor. Sushi repeats have yet to exhibit a function in the context of the GABAB receptor. It is tempting
to speculate that GABAB(1a) is targeted to or retained at
specific subcellular regions by means of interaction of its
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BETTLER, KAUPMANN, MOSBACHER, AND GASSMANN
the rat GABAB(1c) protein forms a functional receptor
when expressed with GABAB(2) in heterologous cells
(260). An isoform that differs at the NH2 terminus from
the GABAB(1a) isoform was identified in humans (56, 217).
Unfortunately, this variant was also named GABAB(1c).
Human GABAB(1c) is similar to GABAB(1a) yet lacks one
sushi repeat because the splice machinery skips exon 4.
The human GABAB(1c) mRNA expression pattern parallels
that of GABAB(1a). GABAB(1d) results from the failure to
splice out the last intron of the GABAB(1) gene and has a
divergent COOH terminus that deletes half the coiledcoiled domain, including the ER-retention motif. It is impossible to generate GABAB(1d) in human and mouse due
to poor sequence conservation, again rising doubts about
the physiological relevance of this splice event. GABAB(1e)
encodes the extracellular ligand-binding domain of the
GABAB(1a) subunit (301). GABAB(1e) is generated by skipping of exon 15 and is detected both in rats and humans.
Physiol Rev • VOL
While the GABAB(1e) transcript is a minor component in
the CNS, it is very prominent in peripheral tissues.
GABAB(1e) is secreted into the culture medium when expressed in transfected mammalian cells. Additionally, it
also forms stable heteromeric complexes with GABAB(2)
at the plasma membrane, providing additional evidence
that the coiled-coil interaction is not the only dimerization
interface between GABAB(1) and GABAB(2) (57, 125, 250).
Should the GABAB(1e) protein occur in vivo, then it could
affect GABAB receptor function in a dominant-negative
manner. GABAB(1f) was identified as a rat transcript. It
contains the in-frame deletion of exon 5 resulting in a
21-bp deletion in the ECD, as well as a COOH-terminal
insertion corresponding to intron 22 (156, 339).
GABAB(1g), an additional truncated GABAB(1a) isoform,
was identified in rat tissue (340). GABAB(1g) is characterized by an insertion of 124 bp between exon 4 and 5,
which generates a frameshift. GABAB(1g) encodes a
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FIG. 4. Differential expression of GABAB(1a) and GABAB(1b) transcripts. A: schematic representation of the 5⬘-end
of the GABAB(1) gene. The GABAB(1b) isoform uses an alternative transcription initiation site within the GABAB(1a) intron
upstream of exon 6. Thus the first 147 amino acids of the mature GABAB(1a) isoform are replaced in GABAB(1b) with a
sequence of 18 amino acids. The amino-terminal extracellular domains of GABAB(1a) and GABAB(1b) primarily differ by
the presence of a pair of sushi repeats encoded by GABAB(1a) exons 3 and 4. B: spatial distribution of GABAB(1) mRNA
in the cerebellum using variant-specific riboprobes. GABAB(1a) transcripts are predominantly observed in the granule cell
layer (GL), whereas GABAB(1b) mRNA is present in Purkinje cells (P). C: photoaffinity cross-linking of GABAB receptor
proteins. GABAB(1a) and GABAB(1b) are differentially expressed throughout the central nervous system. D: developmental
expression of GABAB receptors. GABAB(1b) protein is generally expressed at higher levels in the adult brain compared
with fetal brain, whereas GABAB(1a) protein is more abundant during development. Numbers refer to postnatal days.
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
5. The GABA binding site
All GABAB agonists and competitive antagonists bind
to the ECD of the GABAB(1) subunit, as shown for other
Family 3 GPCRs as well (207). The ECD of GABAB(1) can
be expressed as a soluble protein. The truncated protein
mostly retains the binding properties of wild-type receptors, indicating that it folds independently from the transmembrane domains. Structural analysis of the GABAB(1)
ECD reveals a weak sequence homology with bacterial
periplasmic binding proteins, such as the leucine-binding
protein (LBP) (169). GABAB(1a) and GABAB(1b), which
differ in their ECD (see sect. IIIA4A), share the entire
bacterial homology domain (110). In all GABAB subunits
the LBP-like domain is linked to the first transmembrane
domain via a short sequence that lacks the cysteine-rich
region conserved between the other members of Family 3
GPCRs. In the mGlu receptors, this cysteine-rich region
appears to be necessary for the LBP-like domain to bind
glutamate (244).
The X-ray structure of periplasmic-binding proteins
reveals a binding pocket that is made up by two globular
lobes (lobes I and II) separated by a hinge region. The two
lobes close upon ligand binding, similar to a Venus flytrap
when touched by an insect (275). Homology models of the
GABAB(1) ligand-binding domain, based on the X-ray
structures of the bacterial proteins, have guided mutational analysis of the GABA binding site (110, 111). Key
for both agonist and antagonist binding are S246, S269,
D471, and E465 in lobe I, as well as Y366 in lobe II (Fig. 5).
GABA and baclofen are thought to bind via their carboxylic group to the hydroxyl groups of S246 and Y366. E465
is then believed to bind to the NH2-terminal end of GABA.
D471, which was originally proposed to undergo an ionic
interaction with GABA, now appears more important for
correct folding of lobe I. Mutation of S247 and Q312
increases the affinity of agonists while decreasing the
affinity of antagonists. This supports a model where the
LBP-like domain exists in two conformational states, an
open and a closed state, where binding of ligands favors
the closed state (110). According to the three-dimensional
FIG. 5. Three-dimensional model of the Venus flytrap module (VFTM) of GABAB(1). A model of GABA
docked into the ligand-binding site of GABAB(1). The
putative interactions between GABA and the amino
acid residues of GABAB(1) are shown with dotted lines
(see sect. IIIA5 for details). [Adapted from Kniazeff et
al. (179); courtesy of Dr. J.-P. Pin.]
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COOH-terminally truncated polypeptide of 239 amino
acid residues of unknown function. The distribution of
GABAB(c-g) isoform mRNA was exclusively studied using
Northern blot, PCR, or in situ hybridization. For none of
the GABAB(c-g) splice variants the existence of a protein in
vivo was demonstrated yet [in contrast to the GABAB(1a)
and GABAB(1b) variants].
With regard to GABAB(2), all initial papers report a
single transcript (163, 170, 185, 216, 239, 343). Subsequently, two additional transcripts, GABAB(2b) and
GABAB(2c), were identified in human tissue (75). Analysis
of human GABAB(2) genomic sequences reveals that the
intron-exon boundaries required to generate these transcripts do not match known consensus sequences for
splice junctions (217). GABAB(2b) and GABAB(2c) transcripts are likely to represent artifacts arising during
cDNA synthesis and/or PCR amplification. Therefore, at
the present time, there are no confirmed GABAB(2) splice
variants.
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6. The Ca2⫹ binding site
GABAB receptors share sequence similarity with
mGlu and CaS receptors that are sensitive to Ca2⫹. In the
CaS receptor the primary determinant for Ca2⫹ recognition is in the ECD (129). The effect of Ca2⫹ on mGlu
receptors is heavily disputed. Some reports show that
Ca2⫹ can directly activate mGlu1, mGlu3, and mGlu5
receptors (183), whereas others claim that Ca2⫹ is an
allosteric modulator rather than an agonist (234, 296).
Because of the effects of Ca2⫹ on Family 3 GPCRs, two
groups investigated the possible regulation of GABAB
receptors by Ca2⫹ (112, 345). This led to the discovery of
a Ca2⫹ binding site in the GABAB(1) subunit as the first
allosteric site of GABAB receptors. Accordingly, it was
shown that Ca2⫹ potentiates GABA-stimulated GTP␥[35S]
binding in membranes expressing native or recombinant
GABAB receptors. The effect of Ca2⫹ depends on the
agonist, with baclofen being less sensitive than GABA or
3-APPA.
The residues that confer to GABAB receptors the
ability to sense Ca2⫹ were identified (112). The S269A
mutation (see sect. IIIA5) in the LBP-like domain of
GABAB(1) renders the otherwise functional GABAB(1,2)
receptor Ca2⫹ insensitive. S269 localizes to the GABAbinding pocket, next to S246 that interacts with agonists
(111). S269 in GABAB(1) aligns with S170 in the CaS rePhysiol Rev • VOL
ceptor, a residue that is involved in Ca2⫹ activation of the
CaS receptor (48). Allosteric regulation of GABAB(1) by
Ca2⫹ is supposed to stabilize the activated closed conformational state (112). Possibly, Ca2⫹ compensates for the
lack of the ␣-amino group in GABA, and the contact of
Ca2⫹ with S269 optimizes positioning of the carboxylic
group of GABA for contacting S246. Alternatively Ca2⫹
does not directly interact with S269 but affects the positioning of its hydroxyl group. This may allow the formation of an additional hydrogen bond with the carboxylic
group of GABA. The EC50 value for Ca2⫹ modulation of
GABA binding at GABAB receptors is with 37 ␮M rather
low. Under normal physiological conditions, with [Ca2⫹]
in the blood and cerebrospinal fluid in the millimolar
range, the Ca2⫹ site of GABAB receptors is saturated.
Allosteric regulation by Ca2⫹ may, however, become significant under pathological conditions, when extracellular
[Ca2⫹] is low. This could be the case following ischemia
(191) or epileptic seizures (138).
7. Molecular determinants of G protein coupling
A) MOLECULAR DETERMINANTS IN THE G PROTEIN. The cloning of
GABAB receptors allowed characterizing the G protein
interaction in heterologous systems. Franek et al. (103)
used chimeric G proteins to identify the specific regions
of the Gi␣/Go␣ subunits that are important for their interaction with GABAB receptors. HEK293 cells expressing
recombinant GABAB(1) or GABAB(2) alone or in combination do not activate PLC through Gq␣-type G proteins.
However, heteromeric GABAB(1,2) receptors do activate
PLC via chimeric Gqi␣ and Gqo␣ proteins, in which the five
COOH-terminal residues of Gi␣ or Go␣ replace those of
Gq␣. Therefore, like other GPCRs, GABAB receptors recognize the very COOH terminus of G␣ subunits (35). The
amino acid residue at position ⫺4 in the COOH terminus
of G␣ proteins was shown to be most important for the
coupling to GABAB receptors.
B) MOLECULAR DETERMINANTS IN GABAB RECEPTOR SUBUNITS.
Questions were raised as to what domains of the heteromeric GABAB(1,2) complex are involved in the interaction
with G proteins. Two reports showed that neither the
COOH-terminal intracellular domain of GABAB(1) nor that
of GABAB(2) is needed for coupling to chimeric G proteins
in a heterologous system (57, 250). Both groups expressed
receptor constructs together with chimeric Gqi␣ or Gqo␣
proteins in HEK293 cells and measured the increase of the
intracellular [Ca2⫹] following PLC activation. These findings were confirmed by others who found that the COOH
termini of GABAB(1) and GABAB(2) influence G protein
coupling but that they are not an absolute requirement for
function (125). In contrast, it was reported that the deletion of the GABAB(2) COOH terminus impairs the ability of
recombinant receptors to activate Kir3-type K⫹ channels
in Xenopus oocytes (211). The reason for this discrepancy
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model, a direct interaction with the second lobe is only
possible in the closed form of the LBP-like domain, as
shown for other receptors (8, 245). Some mutations differentially affect the binding of agonists. For example, the
potency of GABA is decreased 30-fold by the S269A mutation, whereas the potency of baclofen remains unaltered (112). As is discussed in section IIIA6, this correlates
with distinct effects of Ca2⫹ on GABA and baclofen binding (112). Interestingly, mutation of Y366 in lobe II not
only decreases the affinity of GABA and baclofen, but also
converts baclofen into an antagonist (111). The recently
obtained crystal structure of the dimeric ECD of mGlu1 in
the presence and absence of glutamate has essentially
validated the homology models for GABAB subunits described above (186).
Most people will agree that GABAB(2) does not bind
agonists or antagonists and does not function when expressed alone (see sect. IIIA2 for conflicting reports).
However, GABAB(2) contains the large ECD that binds
ligands in GABAB(1), the mGlu and CaS receptors. It was
speculated that an alternative, as yet unknown, ligand
binds to the ECD of GABAB(2). According to a recent
phylogenetic analysis of GABAB subunits from various
species, this appears rather unlikely (179). However, it is
conceivable that the recently developed allosteric modulators bind specifically to the GABAB(2) subunit (see
sect. VA).
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
8. Intra- and intermolecular events controlling
receptor function
As described above, a large body of work has aimed
at defining the structural requirements for ligand binding,
subunit interaction, and G protein coupling (see sect. III,
A5–A7). The interdependence of heteromerization, surface trafficking, and effector coupling makes it difficult to
assign a defined molecular function to structural elePhysiol Rev • VOL
ments. Nevertheless, a number of laboratories reached
similar conclusions regarding the sequence of intra- and
intermolecular events that take place when activating a
GABAB receptor (109, 125, 253, 280). A scheme that accommodates most of the available data predicts that the
ECD of GABAB(1) is the only determinant for GABA binding, while the ECD of GABAB(2) is necessary for receptor
activation and for increasing agonist affinity. The hepathelical region and the cytoplasmic tail of GABAB(2) is the
prime determinant of G protein coupling, but GABAB(1) is
clearly necessary to optimize the coupling efficiency. A
model was proposed where a conformational change
within the dimeric ECDs of GABAB(1) and GABAB(2) is
responsible for the stabilization of an active dimeric form
of the transmembrane domains (Fig. 6). Hence, there is an
allosteric interaction between the ligand-binding domain
and the effector transmembrane domains. It is assumed
that the GABAB heteromer differs from the homodimers
formed by other Family 3 GPCRs with respect to the
functional coupling between binding and effector domains. In the GABAB receptor the conformations of the
effector and binding domains are probably tightly coupled, that is, the two domains are either both in an active
or inactive conformation (253). This model is reminiscent
of a two-state model for receptor activation.
9. Interacting proteins
The discovery that receptor activity modifying proteins (RAMPs) can change the pharmacology of a GPCR
triggered an intense search for GABAB receptor-associated proteins (221). Many people in the field wondered
whether interacting proteins could account for the pharmacological differences that were observed with native
GABAB receptors (see sect. IIE). A number of candidate
proteins were identified in yeast two-hybrid screens, using the COOH-terminal domains of GABAB(1) or GABAB(2)
as baits (Fig. 7). There are no reports claiming pharmacological changes upon expression of these proteins
together with GABAB(1), GABAB(2), or GABAB(1,2). This
aside, interacting proteins constitute potential targets
for ligands that modulate GABAB function in a more
specific way.
Three laboratories described that the COOH-terminal
domain of GABAB(1) interacts with members of the ATF/
CREB family of transcription factors, that is, ATF4/
CREB2 and ATFx (235, 333, 342). Gadd153, also known as
CHOP, is an additional leucine-zipper transcription factor
that was described to bind to GABAB receptors (297).
These findings are both intriguing and provocative, as
they suggest for the first time a direct interaction between
a GPCR and transcription factors. The interaction between CREB2/ATF-4 and GABAB(1) takes place between
the leucine-zipper and the coiled-coil domain, respectively. Although it is formally not ruled out that this
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is unclear but may relate to the fact that PLC is activated
by G␣, whereas Kir3 channels are activated by G␤␥. There
is now good evidence that the heptahelical region of the
GABAB(2) subunit directs the coupling to G proteins. With
the use of chimeric GABAB(1) and GABAB(2) subunits with
swapped ECDs, it was shown that only the heptahelical
region of GABAB(2) is absolutely necessary for G protein
signaling (109, 212). However, the heptahelical region of
GABAB(1) significantly improves coupling efficacy. These
studies further showed that both the GABAB(1) and
GABAB(2) ECDs are required for function. It was later
found that all GABAB(2) intracellular loops are important
for receptor coupling to Kir3 channels, whereas those of
GABAB(1) could be replaced with those of GABAB(2) without affecting function (97, 211, 280). Particular attention
was set on addressing the role of the second intracellular
(i2) loop since there is evidence that this region is critical
for G protein coupling in Family 3 GPCRs (119). Exchanging the i2 loops between GABAB(1) and GABAB(2) did not
result in the formation of functional receptors. Hence, the
i2 loop of GABAB(2) needs to be correctly positioned with
respect to the other intracellular domains. Sequence comparison between the i2 and i3 loops of GABAB and the
related mGlu receptors highlighted clear differences between GABAB(1) and GABAB(2). Mutational analysis confirmed the functional importance of conserved residues
(K586, M587, and K590) in the i2 loop of GABAB(2) and
indicates that the GABAB(1) i2 loop lacks the requirements
for interaction with G proteins (97, 280). Mutation of
K686, a basic residue in the i3 loop of GABAB(2) that plays
a critical role in G protein coupling of mGlu1 and CaS
receptors (66, 102), suppresses functional coupling to G
proteins in HEK293 and cerebellar granule cells, corroborating a similar role for this residue (97).
The question arises as to with how many G proteins
a dimeric GPCR can interact. It was shown that the cytoplasmic surface of monomeric rhodopsin is too small to
anchor both the G␣ and G␤␥ subunits (195). Only a rhodopsin homodimer provides sufficient interface to anchor
both G␣ and G␤␥. Although not generally accepted yet,
the data on rhodopsin imply that all GPCRs need to homoor heterodimerize for function. We therefore expect that
the GABAB(1,2) heterodimer binds one G protein only; one
GABAB subunit probably interacts with G␣, while the
other subunit interacts with G␤␥.
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BETTLER, KAUPMANN, MOSBACHER, AND GASSMANN
interaction is artificial, increasing evidence points toward
a physiological relevance. Colocalization between GABAB
receptors and ATF4/CREB2 is observed in the soma and
dendrites of cultured hippocampal neurons as well as in
retinal amacrine cells in situ (235, 333). Translocation of
ATF4/CREB2 either into or out of the nucleus was seen
following GABAB receptor activation (333, 342). The reason for these opposing effects is unclear, but possibly
relates to differences in the neuronal cultures that were
used. Furthermore, stimulation of GABAB receptors re-
FIG. 7. GABAB receptor interacting
proteins. Through the activation of Gi␣/
Go␣-type G proteins GABAB receptors
couple to pre- and postsynaptic effector
systems. A number of proteins were found
to interact with the cytoplasmic tails of
GABAB subunits. Among the interacting
proteins are leucine-zipper transcription
factors (CREB2/ATF-4, CHOP), suggesting
that GABAB receptors mediate long-term
metabolic changes involving protein synthesis. Furthermore, a number of scaffolding and adaptor proteins such as 14 –3-3,
N-ethylmaleimide-sensitive factor (NSF),
tamalin, and MUPP-1 were found to interact with the coiled-coil domains of either
GABAB(1) and/or GABAB(2). It is proposed
that these proteins regulate receptor
dimerization, intracellular trafficking, and
synaptic localization. ␤-Filamin, an actinbinding protein, is supposed to tether the
GABAB heterodimer to the cytoskeleton
via interaction with the GABAB(2) subunit.
The extracellular sushi repeats (Su1 and
Su2) of GABAB(1a) are proposed to interact with the extracellular matrix protein
fibulin. HNK-1, a carbohydrate carried by
extracellular matrix proteins such as tenascin, binds to the extracellular domains of
both GABAB(1a) and GABAB(1b).
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FIG. 6. Putative activation mechanism of the GABAB heterodimer. Left: schematic representation of the inactive
receptor with both Venus flytrap modules (VFTMs) in the open state and in the resting orientation. Binding of GABA in
the VFTM of GABAB(1) induces its closing (step 1). The closed VFTM of GABAB(1) induces a change in the relative
orientation of the two VFTMs to reach the active orientation (step 2). The possible closure of the VFTM of GABAB(2)
without bound ligand is not indicated. The small white circles represent the axis allowing the opening and closing of each
VFTM. The large white circle represents the axis for the change in the relative orientation of the two VFTMs. (Courtesy
of Dr. J.-P. Pin.)
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
Physiol Rev • VOL
zipper region and a COOH-terminal PDZ-binding motif.
PDZ domain proteins are important for the scaffolding of
receptors at synapses and in epithelia (182). MUPP1, a
multivalent PDZ protein, interacts with a stretch of 10
amino acids proximal to the coiled-coil domain of
GABAB(2) (214). Interestingly, this sequence does not conform to the PDZ domain-binding consensus sequence.
MUPP1 was initially shown to interact with another
GPCR, the 5-HT2c receptor (17). MUPP1 may serve as an
adaptor protein, linking GABAB(2) to various signaling
molecules. Intriguingly, the GABAB(1) COOH terminus
contains a putative PDZ domain-binding consensus sequence, LYK. However, no PDZ domain protein is reported to interact with GABAB(1). Finally, the actin-binding protein ␤-filamin is proposed to tether GABAB receptors via the GABAB(2) subunit to the cytoskeleton (341).
Two reports suggest that GABAB receptor extracellular domains interact with other proteins. Fibulin, an
extracellular matrix protein, apparently binds to the sushi
repeats of the GABAB(1a) subunit (118). This finding is of
special interest, as we still do not know whether mechanisms are in place to target GABAB(1a) and GABAB(1b) to
different subcellular regions, e.g., pre- and postsynaptic
sites. The other report provides evidence that the HNK-1
carbohydrate carried by many neural extracellular matrix
proteins, among them tenascin-R and tenascin-C, binds to
GABAB receptors (288). HNK-1 was proposed to regulate
GABAA receptor-mediated perisomatic inhibition by suppression of postsynaptic GABAB receptor activity. HNK-1
does not bind to the sushi repeats of GABAB(1a).
GABAB receptors were found to be associated with
lipid rafts, specialized plasma membrane microdomains
that function as platforms for signaling complexes (18).
Lipid rafts are enriched in cholesterol/sphingolipids and
contain specific populations of membrane-associated proteins, such as G proteins and other signaling molecules.
For example, Gi␣- and Go␣-type G proteins, the main
transducers of GABAB receptors, are enriched in the lipidraft fraction extracted from cerebellar membranes. It remains to be seen whether lipid rafts can segregate GABAB
receptor populations, e.g., GABAB(1a,2) and GABAB(1b,2),
and in some ways account for the functional heterogeneity observed in vivo.
10. Phosphorylation and desensitization studies
Modulation of GPCR activity by intracellular kinases
is well-known and usually the trigger for activity-dependent desensitization (327). In general, phosphorylation of
the receptor protein is followed by interaction with cytoplasmic accessory proteins called ␤-arrestins, which interfere with the receptor to G protein coupling and promote rapid endocytosis. PKC is reported to suppress
GABAB-mediated inhibition of neurotransmitter release
(322). Similarly, PKA is described to desensitize GABAB
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sulted in transcriptional activation of ATF4/CREB2-responsive reporter genes (235, 342). Paradoxically, in one
case pertussis toxin blocked transcriptional activation,
whereas in the other case pertussis toxin was ineffective.
GABAB signaling through CREB proteins was already proposed before cloning (16, 157). In these earlier studies
baclofen silenced transcription instead of activating it.
These earlier findings are more in agreement with the
well-known inhibitory effect of baclofen on cAMP production. Nerve growth factor (NGF) and brain-derived
neurotrophic factor (BDNF) genes may well be the targets
of GABAB-mediated transcriptional regulation. Both the
production of NGF and BDNF are stimulated after treatment of rats with GABAB receptor antagonists (137).
Taken together, the data suggest a novel and unique
mechanism of signal transduction to the nucleus that
results from activation of GABAB receptors. Although still
conflicting, the data linking GABAB receptors to transcription factors suggest that these receptors mediate longterm metabolic effects requiring new protein synthesis.
Additionally, GABAB receptors may signal through G protein-independent effector pathways that modulate neurotransmitter release (132). A G protein-independent signaling was recently also proposed for the related mGlu1
receptors (142, 143) and for CASK/Lin2 (149).
The intracellular COOH terminus of GABAB(1) associates with two members of the 14 –3-3 family of signaling
proteins (81). The interacting region of GABAB(1) overlaps
with the coiled-coil domain. Consequently, 14 –3-3 proteins could interfere with GABAB(1,2) heteromerization.
14 –3-3 proteins were implicated in GPCR signaling before
(23, 269). For example, they directly influence G protein
coupling efficiency and act as scaffolds that recruit proteins modulating G protein activity, such as protein kinases and regulators of G protein signaling (RGS). In
addition, owing to their dimeric nature, 14 –3-3 proteins
might be directly implicated in the clustering of GABAB
receptors at pre- or postsynaptic sites. The functional
consequences of 14 –3-3 binding to GABAB receptors are,
however, unknown.
There are additional scaffolding proteins that interact with GABAB receptors. NEM-sensitive factor, or NSF,
an ATPase critical for intracellular trafficking, interacts
with the COOH terminus of GABAB(2) (341). NSF also
interacts with GABAA receptors through its binding of
GABARAP (177) and possibly provides a structural link
between the ionotropic and metabotropic GABA receptor
systems. Tamalin is yet another scaffolding protein that
interacts with GABAB as well as mGlu receptors (176).
Tamalin is also proposed to interact with guanine nucleotide-exchange factor cytohesins and to promote intracellular trafficking and cell-surface expression of associated
receptors. Tamalin comprises multiple protein-interacting
domains, including a 95-kDa postsynaptic density protein
(PSD-95)/discs-large/ZO-1 (PDZ) domain (131), a leucine-
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BETTLER, KAUPMANN, MOSBACHER, AND GASSMANN
B. Molecular Studies on Native GABAB Receptors
1. GABAB-deficient mice
Given that cloning efforts did not substantiate the
claim for receptor heterogeneity, it became important to
understand to which GABAB functions the cloned receptors can contribute in vivo. To address this question, three
laboratories disabled the GABAB(1) gene in mice (272,
274, 300). Strikingly, the GABAB(2) subunit is heavily
downregulated in all three GABAB(1)⫺/⫺ mouse lines. This
requirement of GABAB(1) for stable expression of
GABAB(2) supports that in vivo most GABAB(2) protein is
associated with GABAB(1), in agreement with biochemical
studies (21). Only the GABAB(1)⫺/⫺ mice generated on the
Balb/c genetic background are viable (300). The
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GABAB(1)⫺/⫺ mice generated on other genetic backgrounds die within 3– 4 wk after birth, thus precluding
behavioral analysis of adult animals (272, 274). Strain
differences in viability of knockout mice are not uncommon (256). The overt phenotype of all GABAB(1)⫺/⫺
mouse strains includes spontaneous epileptic seizures,
and these seizures may be suppressed to some extent in
the Balb/c genetic background. A reduced seizure activity,
in turn, may rescue mice from lethality. Upon GABAB
agonist application, adult Balb/c GABAB(1)⫺/⫺ mice show
neither the typical muscle relaxation, hypothermia, nor
delta electroencephalogram (EEG) waves. These behavioral findings are paralleled by a loss of all detectable
biochemical and electrophysiological GABAB responses
in GABAB(1)⫺/⫺ mice. This demonstrates that GABAB(1) is
an essential component of pre- and postsynaptic GABAB
receptors in the CNS. This finding rules out the presence
of functional homomeric GABAB(2) receptors and is consistent with the proposal that the GABAB(1) subunit is
solely responsible for binding GABA (179). Knockout
studies further indicate that the GABAB(1) subunit is an
essential requirement for GABAB receptor function in the
peripheral nervous system (PNS) and the enteric nervous
system (293). Therefore, gene-targeting experiments do
not substantiate the existence of GABAB subtypes in the
periphery, claimed by studies in which different rank
orders of GABAB agonist affinities were reported (see
sect. IIE). GABAB(2)-deficient mice should further clarify
whether molecular subtypes of GABAB receptors exist in
vivo, and whether GABAB(1) can participate in GABAB
receptors in the absence of GABAB(2).
Theoretically, it remains possible that additional
GABAB subunits exist, which could explain the discrepancy between the historically diverse receptor pharmacology in vivo on the one hand, and the lack of pharmacologically distinct cloned receptors on the other hand.
However, with most of the human genome sequence available, and despite extensive data-mining efforts, no
GABAB genes other than GABAB(1) and GABAB(2) were
identified. All GABAB-related proteins that were isolated
do not form functional GABAB receptors when expressed
together with GABAB(1) and/or GABAB(2) (47, 50, 54, 69,
72, 281). Altogether, this indicates that the likelihood of
identifying additional GABAB receptor genes in the future
is small.
2. GHB activity at GABAB receptors
GHB is a metabolite of GABA that is present at
micromolar concentration in the brain (24). An abundance of high-affinity [3H]GHB binding sites is present in
the cortex and hippocampus, but the physiological role of
endogenous GHB is unclear. Behavioral effects are usually only observed when endogenous GHB levels increase
above physiological levels, either due to a genetic disease
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receptors expressed in Xenopus oocytes (353). Subsequently, it was observed that PKC- and PKA-dependent
signaling pathways mediate the modulation of GABAB
activity in response to estrogen (173, 189). More recent
molecular studies contrast these earlier findings. Moss
and colleagues (82) reported that PKA phosphorylation of
S892 in the cytoplasmic tail of GABAB(2) reduces rather
than increases receptor desensitization, probably as a
result of stabilizing the receptor complex at the cell surface. This challenges the conventional view that phosphorylation is a negative modulator of GPCR signaling.
S892 phosphorylation in GABAB(2) was also observed in
another experimental paradigm (350). It was shown that
withdrawal from repeated cocaine treatment produces an
increase in the basal level of extracellular GABA in the rat
accumbens (see sect. IVA). The increase in extracellular
GABA is paralleled by diminished Ser-892 phosphorylation, suggesting a functional desensitization of GABAB
autoreceptors. At first glance contradictory, Bouvier and
colleagues (258) identified the G protein-coupled receptor
kinase 4 (GRK4) as the kinase that promotes desensitization of GABAB receptors. Surprisingly, however, this desensitization occurred in the absence of ligand-induced
receptor phosphorylation and could be promoted by
GRK4 mutants deleted of their kinase domain. Again,
these results are at odds with the generally accepted
model linking the kinase activity of GRKs to their role in
receptor desensitization. GABAB receptors can also
change the phosphorylation state of other proteins by
influencing intracellular signaling pathways. For example,
GABAB receptors increase the activity of P2X-type ATP
receptors or the calcium-dependent K⫹ current (IAHP)
through a downregulation of cAMP levels, resulting in
decreased PKA activity and subsequent dephosphorylation of the ion channel (116, 120). Possibly the interplay
between the GABA and ATP neurotransmitter systems
could be exploited for the treatment of neuropathic pain
(see sect. IVD).
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
Physiol Rev • VOL
typical GPCR topology are visible in the COOH-terminal
part of the protein, which exhibits no significant homology to known proteins. All the more puzzling, the mRNA
of the putative GHB receptor is particularly abundant in
the cerebellum, where GHB binding sites are low or absent (24). Furthermore, the GHB receptor antagonist
NCS-382 has no activity at the cloned GHB protein. In
summary, these findings make it unlikely that the cloned
protein corresponds to the high-affinity [3H]GHB binding
sites in the brain.
3. Cellular and subcellular distribution
A) mRNA DISTRIBUTION. GABAB(1) and GABAB(2) transcripts are broadly expressed in the CNS of various vertebrate species (2, 22, 25, 32, 33, 75, 95, 163, 171, 194, 200,
231, 232, 307, 326, 356). In the rat brain, GABAB(1) mRNA
is detectable in almost all neuronal cell populations, with
highest levels of expression in hippocampus, thalamic
nuclei, and cerebellum (Fig. 8). High levels of GABAB(2)
mRNA are seen in the piriform cortex, hippocampus, and
medial habenula. Moreover, GABAB(2) mRNA is abundant
in all layers of the cortex, in the thalamus, and in cerebellar Purkinje cells (25, 95, 163, 170). The in situ hybridization patterns of the GABAB(1) and GABAB(2) subunits
are mostly overlapping (Fig. 8) and qualitatively parallel
those of GABAB agonist and antagonist binding sites (33,
171, 326). From the subunit expression patterns it is obvious that heteromeric GABAB(1,2) receptors constitute
the majority of native GABAB binding sites. This does not
exclude that the subunits also form receptors independent of each other. Several studies tried to find evidence
for this by looking for differences in the expression pattern of GABAB(1) and GABAB(2) transcripts. The most
striking difference is seen in the striatum/caudate putamen region, where high GABAB(1) mRNA levels contrast
low to nondetectable GABAB(2) mRNA levels (Fig. 8) (25,
75, 95, 170). However, immunohistochemical analysis
shows that the GABAB(2) protein is significantly expressed in this structure (see sect. IIIB3B). PCR approaches detect GABAB(1) mRNA, but not GABAB(2)
mRNA, in most peripheral human tissues analyzed (56).
The reason for this is unclear.
B) RECEPTOR IMMUNOHISTOCHEMISTRY AND AUTORADIOGRAPHY. Antibodies directed against COOH- and NH2-terminal sequences of GABAB(1) and GABAB(2) were produced by
several laboratories and are available from a number of
commercial sources. Additionally, GABAB(1) subunits can
be traced using the photoaffinity cross-linker 125ICGP71872 (169) or the radioligands [3H]CGP54626 (33)
and [3H]CGP64239 (172, 326). The only radioligand that is
currently commercially available is [3H]CGP54626.
In general, the immunohistochemical localization of
GABAB(1) and GABAB(2) subunits (56, 71, 213, 313) correlates well with physiological and autoradiographic data
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or to exogenous administration of GHB. Patients suffering
from GHB aciduria, a congenital enzyme defect causing
such a GHB accumulation, exhibit psychomotor retardation, delayed or absent speech, hypotonia, ataxia, hyporeflexia, seizures, and EEG abnormalities. When administered exogenously, acute increases in GHB induce a large
spectrum of behavioral responses. Since 1990, GHB has
been more and more abused for its euphoric, sedative,
and anabolic (body-building) effects (241). In the past
GHB was clinically used as an anesthetic, while in recent
times it is shown to normalize sleep patterns in narcoleptic patients (241, 329). Preliminary preclinical and clinical
data suggest that GHB is also useful in the therapy of
alcoholism, nicotine, and opiate dependency (108), similar to baclofen (see sect. IVA).
Some actions of GHB clearly involve GABAB receptors (162, 198, 279). GHB binds to native and recombinant
GABAB receptors, however with significantly lower affinity than to its cognate high-affinity [3H]GHB binding sites
in the brain (198). Therefore, GHB is only expected to
activate GABAB receptors under abuse conditions, where
high concentrations of GHB are reached. If endogenous
GHB is involved in active signaling, high-affinity GHB
receptors, likely related to brain [3H]GHB binding sites,
are expected to mediate these effects. The availability of
GABAB-deficient mice provided the opportunity to study
GHB effects in the absence of coincident GABAB effects
(168, 274). After GHB application, GABAB(1)⫺/⫺ mice
showed neither the typical hypolocomotion, hypothermia,
increase in striatal dopamine synthesis, nor EEG deltawave induction seen in wild-type mice. This indicates that
these GHB effects are all mediated by GABAB receptors.
Autoradiography reveals a similar spatial distribution of
[3H]GHB binding sites in brains of GABAB(1)⫺/⫺ and wildtype mice, demonstrating that GABAB subunits are not
part of high-affinity [3H]GHB binding sites. Millimolar
concentrations of GHB induce small GTP␥[35S] responses
in brain membrane preparations from wild-type, but not
from GABAB(1)⫺/⫺ mice. The GTP␥[35S] responses in wildtype mice are blocked by the GABAB antagonist
CGP54626, but not by the GHB antagonist NCS-382. This,
together with additional data (63), suggests that the GHBinduced GTP␥[35S] responses are exclusively mediated by
GABAB receptors and not by the high-affinity [3H]GHB
binding sites, as proposed in earlier studies (277, 315).
Recently, the cloning of a putative GHB receptor
from a rat hippocampal cDNA library was reported (6).
However, several findings related to this putative GHB
receptor require clarification. For example, the NH2-terminal part of the cloned GHB receptor identifies it as a
member of the tetraspannin gene family, with four transmembrane regions and no significant similarity to GPCRs.
Nevertheless, the cloned protein is reported to exhibit
seven transmembrane regions and to activate G proteins.
No additional transmembrane domains to achieve the
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BETTLER, KAUPMANN, MOSBACHER, AND GASSMANN
on the distribution of native GABAB receptors (170). The
level of GABAB protein expression in the molecular layer
of the cerebellum is not uniform (104, 213, 267). The
pattern of intensely stained bands of Purkinje cells coincides with anatomical and functional compartments previously defined by zebrin, a brain-specific aldolase with no
obvious link to GABAB receptors (104). It is thus conceivable that the regulatory elements governing the gene expression of zebrins may also entrain the expression of
GABAB receptors. Where analyzed, the pattern of
GABAB(1) protein expression matches well with that of
GABAB(2). Significantly, electron microscopy confirmed a
colocalization of GABAB(1) and GABAB(2) protein at synaptic and extrasynaptic sites (104, 170, 184). In general,
GABAB(1) immunoreactivity appears to be abundant in the
cytoplasm of neurons (313). It was therefore speculated
that the GABAB(2) protein is the limiting factor that determines the level of expression of the functional heteromer
at the cell surface. Several studies suggested a mismatch
of GABAB(1) and GABAB(2) mRNA expression in the basal
ganglia (25, 75, 95, 170), but it appears that significant
levels of GABAB(2) protein are nevertheless present in this
area (56, 71). A differential expression of GABAB(1) and
GABAB(2) protein was observed in subpopulations of striatal neurons (240). However, a recent study comparing
GABAB(1) and GABAB(2) protein expression in different
brain regions found no evidence for a GABAB(1) subunit
expression in the absence of GABAB(2) (71). In conclusion, it appears that most, possibly all, GABAB receptors
are of the heteromeric GABAB(1,2) type. This is in good
agreement with biochemical data (21) and GABAB(1)
knockout studies (272, 274, 293, 300). Nonetheless, we
will have to await the analysis of GABAB(2) knockout
mice to know whether GABAB(1), like GABAB(2), is functionally silent in the absence of its partner subunit.
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Colocalization studies with glutamic acid decarboxylase 67 (GAD67) addressed whether GABAB(1,2) is expressed in GABAergic cells (213, 240). Surprisingly, only
few cell populations, e.g., Purkinje cells, were found to
coexpress GAD67 and GABAB(1). More often GAD67-positive terminals were surrounded by GABAB(1)-positive cell
bodies. In the rat neostriatum, GABAB(2) and GAD67 do
not often colocalize (240). Instead, GABAB(1) protein was
mostly detected on glutamatergic and dopaminergic terminals. GABAB(1) is also expressed on glutamatergic and
dopaminergic terminals in monkey brain (68). This localization implies that GABAB receptors are activated by
spillover GABA from adjacent synapses. Immunohistochemical data therefore question whether GABAB(1) can
fulfill autoreceptor functions. However, GABAB(1) knockout studies clearly demonstrate that this subunit is necessary for regulating GABA release (272, 300). Most likely,
GABAB protein expression levels at GABAergic terminals
are below the detection limit of immunohistochemical
methods. In this context it is interesting to note that
GABAB receptors are mostly located extra- or perisynaptically (170, 184), a finding that is consistent with the
observation that inhibitors of GABA uptake, which promote diffusion of GABA out of the synaptic cleft, greatly
enhance IPSPs (155).
GABAB(1a) and GABAB(1b) proteins are differentially
regulated during postnatal maturation (Fig. 4D) (104,
206). GABAB(1a) expression is highest within the first
postnatal days (P0 –P5) and then decreases to adult levels.
In contrast, immunoreactivity for GABAB(1b) increases
from P5 onwards and reaches a maximum at P10, followed by a gradual decline to adult levels. A similar
expression profile was observed using photoaffinity labeling of GABAB(1a) and GABAB(1b), followed by gel electrophoresis and quantitative autoradiography (206). In the
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FIG. 8. Comparison of the distribution of GABAB(1) and GABAB(2) subunit mRNA in the rat brain. Transcripts of
GABAB(1) and GABAB(2) are coexpressed at similar levels throughout the brain, with a few exceptions. For example,
GABAB(2) transcripts are less abundant than GABAB(1) transcripts in the caudate putamen (CP) and the olfactory bulb
(OB).
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
Physiol Rev • VOL
stomach and testis. In the same study GABAB(1b) was
selectively detected in kidney and liver, while GABAB(1a)
was expressed in the adrenal gland, pituitary, spleen, and
prostate. Immunoblot analysis demonstrated the presence of the GABAB(1) protein, but not the GABAB(2) protein, in uterus and spleen (56). Surprisingly, an earlier
Northern blot analysis revealed no evidence for significant GABAB(1) mRNA expression in the spleen (169).
Studies with GABAB(1)⫺/⫺ mice confirmed that, as in the
CNS, the GABAB(1) subunit is an essential requirement for
GABAB function in the enteric nervous system and the
PNS (293). Specifically, it was shown that the effects of
baclofen in both ileum and urinary bladder were absent in
GABAB(1)⫺/⫺ mice.
Peripheral expression of GABAB subunits is not restricted to cells of neuronal origin. As in neurons, both
GHB (see sect. IIIB2) and baclofen activate an inwardly
rectifying K⫹ channel in rat cardiomyocytes (199).
GABAB(1) and GABAB(2) expression in cardiomyocytes
was confirmed by immunoblotting and in coimmunoprecipitation studies. In rat ventricular myocytes, the two
subunits are expressed in the sarcolemma and along the
transverse tubular system. GABAB(1a), GABAB(1c), and
GABAB(2) mRNA were detected in rat testis and sperm
(136). GABAB(1) protein localizes to the head of sperm
cells, as shown in immunofluorescence experiments. It
was speculated that GABAB receptors play a role in fertilization, e.g., in the induction of the acrosome reaction
by GABA. However, male Balb/c GABAB(1)⫺/⫺ mice are
fertile (B. Bettler and M. Gassmann, unpublished observation), casting doubt on this proposal.
IV. IMPLICATION IN DISEASE
Baclofen, the only GABAB drug on the market, is
prescribed as an antispastic agent and muscle relaxant for
the treatment of patients suffering from multiple sclerosis
as well as hemi- or tetraplegia. For severe cases of spasticity, baclofen is administered intrathecally. This is also
the route of administration for stiff-man syndrome patients that suffer from rigidity of skeletal muscles. Baclofen is further used in the treatment of spasticity due to
traumatic/hypoxic brain injury, dystonia, and in cerebral
palsy movement disorders. Therapy with baclofen efficiently alleviates the rigidity associated with Lewy-body
dementia and the spasms following tetanus infections. It
is generally assumed that baclofen exerts its therapeutic
effects by inhibiting the release of excitatory neurotransmitters and neuropeptides in the spinal cord.
Over the years, a number of clinical observations
suggested that GABAB compounds might offer the means
to ease the symptoms of drug withdrawal (see sect. IVA)
and to treat neuropathic pain (see sect. IVD). Furthermore, GABAB compounds are of therapeutic interest for
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adult rat brain GABAB(1b) protein levels exceed GABAB(1a)
levels in most structures. Exceptions are the olfactory
bulb and the striatum where higher levels of GABAB(1a)
than GABAB(1b) protein are detected.
Two laboratories succeeded in obtaining antibodies
that discriminate between the GABAB(1a) and GABAB(1b)
isoforms (104, 184). Although these antibodies work well
on immunoblots, the GABAB(1a) antibodies are not suitable for use in fixed tissue. Nevertheless, a distinct and
highly selective immunohistochemical distribution for
each of the two isoforms could be inferred by subtractive
analysis using GABAB(1b) and pan GABAB(1) antibodies
(104). GABAB(1b) is predominantly expressed in Purkinje
cells and GABAB(1a) mostly confined to granule cells (30,
104, 151, 267). This was taken to suggest a pre- versus
postsynaptic localization of GABAB(1a) versus GABAB(1b),
respectively (32, 33). Conversely, GABAB(1a) appears to be
located postsynaptically on the cell bodies in thalamocortical circuits (271). In dorsal root ganglia the density of
GABAB(1a) immunoreactivity is much higher than that of
GABAB(1b) (267). This supports the association of
GABAB(1a) with presynaptic receptors in the primary afferent terminals. In biochemical experiments, however,
GABAB(1a) protein accumulated in the postsynaptic density fraction (21). While some additional studies suggested a differential pre- and postsynaptic function of the
GABAB(1a) and GABAB(1b) isoforms (104, 194, 267), it
emerges that they probably both contribute to pre- and
postsynaptic functions (71). GABAB receptors are also
expressed on nonneuronal cells of the brain (70). Specifically, GABAB receptor subunits were found on astrocytic
processes surrounding both symmetric and asymmetric
synapses in the CA1 hippocampal region and on activated
cultured microglia. In this context it is interesting to note
that an activity-dependent potentiation of inhibitory synaptic transmission between hippocampal interneurons
and CA1 pyramidal neurons was described (167). It was
suggested that interneuronal firing elicits a GABAB receptor-mediated elevation of Ca2⫹ in surrounding astrocytes,
which in turn potentiates inhibitory transmission. Activation of GABAB receptors on astrocytes may therefore be
necessary for activity-dependent modulation of inhibitory
synapses in the hippocampus.
C) GABAB RECEPTORS IN PERIPHERAL TISSUES. GABAB receptors
are also expressed in many peripheral tissues. Using reverse transcriptase-PCR analysis, GABAB(1) mRNA expression was detectable in all peripheral organs examined, including heart, spleen, lung, liver, small intestine,
large intestine, kidney, stomach, adrenal, testis, ovary,
and urinary bladder (64, 156, 301, 339). This supports a
physiological role for GABAB receptors in the control of a
wide variety of peripheral organs. GABAB(1) protein expression in peripheral tissues was confirmed using 125ICGP71872 photoaffinity labeling (19). These experiments
showed that GABAB(1a) and GABAB(1b) are coexpressed in
853
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BETTLER, KAUPMANN, MOSBACHER, AND GASSMANN
A. Addiction
There is now good evidence, for the most part both in
humans and animals, that GABAB agonists can reduce the
craving for drugs such as cocaine, heroin, alcohol, and
nicotine (79). Preliminary clinical studies with cocaineabusing patients reported a reduced craving for cocaine
after baclofen administration (51, 197). GABAB agonists
are also effective in clinical studies of alcohol abuse (3).
Regardless of these highly promising findings, the pharmaceutical industry is reluctant to enter the drug-abuse
market. One reason is that there are no GABAB agonists
with lasting patent protection available. Another reason is
that clinical trials are considered highly problematic and
risky because of multidrug abusers, dropouts, and possible lawsuits.
Therapeutic effects of baclofen in humans are supported by experiments with animals. A number of studies
showed that baclofen suppresses intravenous cocaine
self-administration in rodents at low doses of injected
cocaine, but not at high doses (⬎1.5 mg/kg) (52, 58, 282,
310). Baclofen is well-known to cause motor side-effects
and to reduce locomotion (252). It is therefore important
to rule out that the therapeutic actions of baclofen on
drug self-administration are mediated by these side effects. To address this question the effects of GABAB
agonists on cocaine- and food-reinforced responses were
compared. Baclofen and CGP44532, a high-affinity GABAB
Physiol Rev • VOL
agonist, are much more effective on cocaine-reinforced
than on food-reinforced responding, thus validating the
therapeutic concept (52, 282, 310). In rats, withdrawal
from repeated cocaine treatment produces an increase in
the basal levels of extracellular GABA in the accumbens
(350). The increase may be mediated by GABAB autoreceptor desensitization, likely the result of diminished
S892 phosphorylation of the GABAB(2) subunit (see sect.
IIIA10). Baclofen is reported to reduce self-administration
of heroin (351). Rats that received a low dose of baclofen
(0.5 mg/kg) resumed self-administration of heroin when a
5-day treatment with baclofen was followed by 2 baclofen-free days. This is in contrast to rats that received a
high dose of baclofen (1 mg/kg), which did not resume
drug administration. This suggests that high and low
doses of baclofen activate distinct cellular mechanisms.
Low doses of baclofen (3 mg/kg) reduce alcohol intake in
alcohol-preferring Long-Evans rats (86). A reduction of
alcohol intake after baclofen treatment was also seen in
Wistar rats (77). However, Smith et al. (314), using LongEvans rats and 5 days of baclofen treatment before the
start of the dark cycle, reported that baclofen reduced
alcohol intake only during the first hour and increased
intake over the next 23 h (314). Possible motor impairing
and sedative effects of baclofen on alcohol and heroin
self-administration were addressed. As shown in the experiments with cocaine, baclofen did not impair food
intake, and it reduced alcohol intake if an ethanol versus
water choice was given to the animals (77, 86, 314).
Other GABAergic compounds reproduce the therapeutic effects of baclofen on drugs of abuse. ␥-VinylGABA inhibits the GABA transaminase and thereby elevates GABA levels. Like baclofen, ␥-vinyl-GABA reduces
the reinforcing properties of cocaine, heroin, and ethanol
in animals, at doses that do not decrease locomotion (178,
338, 351). ␥-Vinyl-GABA also reduces the acquisition and
expression of condition place preference by cocaine and
nicotine (92, 93). It further alleviates the effects of cocaine in an electrical brain stimulation paradigm (187).
GHB, an endogenous metabolite of GABA (24), shows
promising effects in situations involving opiate withdrawal, alcohol, and nicotine dependence (108). GHB is
also shown to decrease cocaine self-administration in rats
(215). The effects of high doses of GHB are almost certainly mediated by GABAB receptors (see sect. IIIB2).
More recently, GHB has become a drug of abuse itself
(241). Studies are therefore needed to address whether
GABAB agonists reduce craving and withdrawal symptoms, or whether they just mimic the effects of the abused
drug.
There is evidence that GABAB agonists attenuate the
reinforcing effects of abused drugs by influencing the
mesolimbic dopamine system (93, 282). Drugs of abuse
increase extracellular dopamine levels in the accumbens,
a brain region that is believed to be involved in the reward
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the treatment of intestinal (see sect. IVC) and pulmonary
disorders (67, 94), bladder dysfunctions (257), epilepsy
(see sect. IVB), and memory loss (227). Indeed, mice with
a disruption in the GABAB(1) gene support that GABAB
drugs could be used to manage pain, strengthen memory,
and treat epilepsy (272, 300). Moreover, GABAB(1)⫺/⫺
mice pointed at indications that were not necessarily
linked to the GABAB system. For example, GABAB(1)⫺/⫺
mice exhibit a hyperactive phenotype reminiscent of dopamine-transporter DAT knockout mice that are similarly
aroused by novelty and respond with hyperlocomotion to
a new environment (300, 316). It is commonly assumed
that hyperactive behaviors are related to a hyperdopaminergic state. It is therefore conceivable that the loss of
GABAB control over dopamine release triggers the behavioral abnormalities in GABAB(1)⫺/⫺ mice. A GABAB brake
on dopamine release could possibly be exploited to attenuate motor problems of Parkinson’s disease or attentiondeficit hyperactivity-disorder (ADHD) patients. Furthermore, GABAB(1)⫹/⫺ mice show enhanced prepulse inhibition compared with littermates, suggesting that GABAB(1)
knockout mice exhibit sensorimotor gating abnormalities
(272). Compounds interfering with GABAB receptors may
therefore be beneficial in schizophrenia. Below we review
some of the most promising indications for GABAB drugs.
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
855
(106, 127). In contrast, after induction of generalized seizures in rats with electroshock, an upregulation of the
GABAB(1b) mRNA, but not of the GABAB(1a) mRNA, was
observed (31). This suggests a selective involvement of
GABAB splice variants in the regulation of electroshockinduced seizure activity.
To date, the data indicate that GABAB compounds
could be most useful in the pharmacotherapy of absence
seizures in children. The finding, however, that high doses
of certain GABAB antagonists induce convulsions in rats
renders clinical trials problematic (332). Clearly a wide
therapeutic window will be a critical requirement when
entering clinical trials with GABAB antagonists.
B. Epilepsy
C. Gastrointestinal Disease
GABAB receptors have repeatedly been implicated
into the etiology of epilepsies. However, the first genetic
link between GABAB receptors and human epilepsy was
only provided very recently (113). It appears that a
GABAB(1) polymorphism not only confers a highly increased susceptibility to temporal lobe epilepsy but also
influences the severity of the disease.
GABAB antagonists suppress the absence seizures
seen in the lh/lh lethargic mice (147) and the genetic
absence-epilepsy rats from Strasbourg (GAERS) (209),
while agonists exacerbate the seizures. Apparently, blocking of thalamic GABAB receptors reverses an excess of
inhibition that is the cause of absence seizures (83, 305).
It was proposed that GABAB-mediated IPSPs have a
“priming” function towards the generation of low-threshold Ca2⫹ potentials, thereby facilitating burst-firing of the
type observed in absence epilepsy (83). The primary gene
defect in lh/lh mice is a mutation in the Ca2⫹ channel
␤-subunit, which affects the interaction with the G␤␥
subunits of the activated G protein. The phenotype of
lh/lh mice involves an upregulation of GABAB binding
sites, which is expected to facilitate absence seizure development. Paradoxically, the overt phenotype of GABAB
receptor-deficient mice includes spontaneous seizures, including sporadic absence-type seizures (272, 300). However, the absence-type seizures seen in the GABAB(1)⫺/⫺
mice are not directly comparable to the “typical” absence
seizures observed in the GAERS. The seizures in the
GAERS are characterized by frequent and short EEG
bursts, while the ones seen in the GABAB(1)⫺/⫺ mice are
rare, of much longer duration, and indicative of “atypical”
absence seizures. A lack of GABAB signaling may also
underlie the clonic-type seizures in the weaver mouse
(126). The weaver mouse exhibits a mutation in the
Kir3.2 K⫹ channel gene, which leads to a postsynaptic
loss of GABAB inhibition and to seizure development
(201). Kainic acid-induced seizures in rats are characterized by a downregulation of GABAB receptor subunits
Physiol Rev • VOL
From a medicinal chemistry point of view it is often
easier to develop peripheral drugs than central drugs.
Peripheral GABAB drugs are expected to be largely devoid of the known side effects that are mostly of central
origin and may well be superior to current therapies. The
cloned GABAB subunits are expressed in the periphery
(see sect. IIIB3C) and involved in the regulation of organs
such as, e.g., bladder and ileum (293). GABAB receptors
were also described on vagal-afferent terminals in the
medullary brain stem region, which is important in the
integration of sensory information from the stomach and
the pharynx. Baclofen reduces the rate of transient lower
esophageal sphincter relaxations and the number of reflux episodes, in humans and animals, which attracted a
great deal of attention (34, 60, 146, 181, 193, 196, 318).
GABAB compounds may therefore open new possibilities
in the treatment of disorders such as gastroesophageal
reflux disease (60, 196).
D. Nociception
Baclofen exerted antinociceptive effects in clinical
trials involving trigeminal, glossopharyngeal, vagoglossopharyngeal and ophtalmic-postherpetic neuralgias,
diabetic neuropathy, and migraine (44, 105, 139, 312).
Although baclofen is used clinically to treat neuropathic pain and, when administered intrathetically, to
attenuate pain associated with spinal injury (141) or
stroke (319), its use as a general analgesic is limited
because of its sedative properties and the rapid development of tolerance to its pain-relieving activity. Baclofen also exhibited antinociceptive properties in rodent models of acute pain, such as the tail-flick, acetic
acid writhing, Formalin, and hot-plate tests (13, 273).
Moreover, baclofen showed antinociceptive and antiallodynic actions in chronic pain models in rats (84, 344).
Baclofen likely exerts its effects both in the brain (154,
160) and the spinal cord (204). One of the few cortical
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and reinforcement circuitry (346, 347). Activation of
GABAB receptors in the accumbens reduces firing of dopaminergic cells and inhibits the release of dopamine
(99). Similarly, ␥-vinyl-GABA reduces the release of dopamine in the neostriatum and accumbens after cocaine
and nicotine administration (92, 93). The GABAB antagonist SCH5091 blocks this effect, suggesting that ␥-vinylGABA acts through GABAB receptors. Altogether, it appears that GABAB activity blocks the increase in dopamine release that is otherwise induced by drugs of abuse
(351). Accordingly, rats reduce self-administration of cocaine after they receive an injection of baclofen into the
accumbens and the ventral tegmental area (165, 310).
856
BETTLER, KAUPMANN, MOSBACHER, AND GASSMANN
E. Genetic Linkage Studies
The GABAB(1) and GABAB(2) genes map to human
chromosomes 6p21.3 and 9q22, respectively (123, 238).
Given the potential implication of GABAB receptors in the
etiology of epilepsies (see sect. IVB), a number of studies
addressed a direct involvement of the GABAB(1) gene. The
result of a recent study indicates that a GABAB(1) polymorphism, G1465A, indeed confers a highly increased
susceptibility to temporal lobe epilepsy (113). This polymorphism also influences the severity of this common
epileptic disease. This agrees with earlier studies that
proposed a dysfunctional GABAB system as one of the
causes of temporal lobe epilepsy (127, 298, 348). A possible involvement of GABAB(1) receptors in idiopathic generalized epilepsies was investigated in linkage and association studies (259, 292). Yet another study explored a
link to childhood absence epilepsy (283). However, these
latter studies provided no evidence for an involvement of
GABAB(1) polymorphisms in these two other forms of
epilepsy. Similarly, Anaya et al. (5) found no linkage
between the GABAB(1) gene and epilepsy in Caucasian
patients. Other linkage studies addressed a potential involvement of the GABAB(1) gene in conditions of alcohol
dependence (292) and in panic disorders (291). However,
GABAB(1) polymorphisms do not account for the genetic
variance of alcohol dependence, nor is there an indication
for increased vulnerability to panic disorders. The
GABAB(2) gene maps in the vicinity to the locus for hereditary sensory neuropathy type-1 (HSN-1) (238). A possible involvement of GABAB(2) in the etiology of HSN-1
has not yet been investigated.
Physiol Rev • VOL
V. NOVEL GABAB COMPOUNDS
A. Allosteric Modulators
The side effects of baclofen, principally sedation,
tolerance, and motor impairment, limit its utility for the
treatment of many diseases. Novel GABAB drugs that
largely lack these components are therefore much sought
after. In the absence of pharmacological subtypes, alternative strategies for achieving selectivity must be considered. For instance, positive allosteric modulators may
provide a means to dissociate the unwanted side effects
seen with baclofen. Allosteric modulators discriminate
between activated and nonactivated receptor states. They
will enhance the endogenous activity of GABA, in contrast to agonists that will activate every GABAB receptor
they reach, independently of synaptic activity. To be effective, allosteric GABAB drugs therefore rely on receptor
activity stimulated by endogenous GABA. The demonstration of an intrinsic brake on memory impairment, locomotion, bladder activity, and nociception in GABAB(1)⫺/⫺
mice (293, 300) is therefore of great importance (see sect.
IIIB1). This suggests that under physiological conditions
disease-relevant neuronal systems are under temporary
or tonic control of GABAB receptors and that a treatment
with allosteric modulators is possible.
Using the cloned GABAB receptors, researchers in
several laboratories developed functional assay systems.
For instance, the activation of G proteins by GABAB(1,2)
can be measured using the GTP␥[35S] binding assay (330).
Alternatively mobilization of intracellular Ca2⫹ can be
detected in cell lines expressing GABAB(1,2) in combination with chimeric G proteins (250). Functional assay
systems permitted the identification of the first synthetic
allosteric GABAB compounds (330, 331). CGP7930,
CGP13501, GS39783, and related allosteric compounds
are structurally distinct from GABAB agonists and markedly enhance agonist-stimulated responses at GABAB receptors (Fig. 9). Notably, these allosteric compounds
have little or no intrinsic activity and do not directly
activate GABAB receptors. Where analyzed, GABA concentration-response curves in the presence of fixed concentrations of the allosteric modulator indicate an increase of both the potency and the maximum efficacy of
GABA at GABAB(1,2) receptors. The published allosteric
compounds do not discriminate between GABAB(1a,2) and
GABAB(1b,2), the two predominant GABAB receptor populations in the nervous system (see sect. IIIA4A). The
binding sites for allosteric modulators are unknown, but
all compounds require the presence of GABAB(2) to exert
their allosteric effect. Possibly, the compounds bind to
the GABAB(2) subunit itself or to the GABAB(1)/GABAB(2)
interface. All compounds are hydrophobic, and it is conceivable that they interact with the transmembrane do-
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areas consistently activated by painful stimuli is the
rostral agranular insular cortex (RAIC) where GABA
robustly inhibits neuronal activity (160). Selective activation of GABAB receptor-bearing RAIC neurons produces hyperalgesia through projections to the amygdala, an area involved in pain and fear. In the dorsal
horn of the spinal cord, baclofen inhibits the release of
glutamate and substance P from both large and small
fibers (255). This explains why intrathecal application
of baclofen relieves central pain in patients with spinal
lesions (141) or after cerebral strokes (319). Acute pain
tests with GABAB receptor-deficient mice support that
GABAB receptors participate in nociceptive pathways
(300). GABAB(1)⫺/⫺ mice exhibit pronounced hyperalgesia to noxious heat in the hot-plate and tail-flick tests
as well as reduced paw-withdrawal thresholds to mechanical pressure. Changes in the noxious thermal and
mechanical threshold suggest that there is a loss of
intrinsic GABAB tone in the nociceptive system of
GABAB(1)⫺/⫺ mice. The lack of GABAB receptors most
probably results in an increased central hyperexcitability of the spinal nociceptive pathways.
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
857
FIG. 9. Positive allosteric modulators acting at GABAB receptors. Chemical structures described by Urwyler et al.
(330, 331). CGP7930 [2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol], its aldehyde analog CGP13501, and
GS39783 (N,N⬘-dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4,6-diamine) are shown.
Physiol Rev • VOL
mains (62, 205, 251). It is to be hoped that in the near
future noncompetitive GABAB antagonists, based on
novel chemical structures, will be identified as well.
B. Subtype-Selective Ligands
To date the only prominent molecular distinction in
the GABAB system is based on the two splice variants
GABAB(1a) and GABAB(1b) (see sect. IIIA4A). Essentially,
these two variants represent the only means for directing
the search for novel GABAB drugs toward molecularly
distinct receptor populations. The differential expression
patterns of GABAB(1a) and GABAB(1b) point to a functional
heterogeneity, but it is unknown which physiological effects relate to which isoform. Most importantly, no conclusion can be drawn regarding their specific contributions to muscle-relaxant effects and tonic brakes on locomotion, hyperalgesia, memory impairment, and bladder
activity. Likewise, it remains unclear which splice variants are involved in pre- and postsynaptic functions (21,
32, 68, 71, 104, 267, 271).
Appreciating the unique roles played by GABAB(1a)
and GABAB(1b) is essential to fully exploit GABAB receptors for therapeutic uses. It was argued that the anticonvulsant, antihyperalgesic, and anxiolytic drug gabapentin
is able to distinguish the two variants (26, 237). In particular, gabapentin was claimed to be active at GABAB(1a,2)
but not at GABAB(1b,2) receptors. However, in most people’s hands gabapentin has no activity at GABAB receptors (161, 190, 317). The reason for this discrepancy is
unclear. Given the lack of truly selective ligands, it will
probably be necessary to take a genetic approach to
dissociate in vivo functions of GABAB(1a,2) and
GABAB(1b,2) receptors. Mice with selective ablations of
the GABAB(1a) or GABAB(1b) subunits should allow exposing the influence of splice variants on the manifestation of
physiological and behavioral traits. Such mice will also
help to evaluate whether compounds specifically targeted
to GABAB(1a,2) or GABAB(1b,2) are likely to have distinct
enough effects to warrant drug discovery efforts.
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mains, similar to allosteric modulators at mGlu receptors
(see below).
Recently, Kerr et al. (175) described the arylalkylamines as a different class of positive allosteric GABAB
modulators. Fendiline, prenylamine, and F551 potentiated
hyperpolarizing responses to baclofen in rat cortical
slices. Surprisingly, fendiline and F551 were inactive at
GABAB receptors located on inhibitory terminals. Given
that potentiation of ligand responses in acute slices can
be mediated by a variety of mechanisms, including G
protein cross-talk, second messenger modulation, or intercellular signaling, the interpretation of the experiments
with arylalkylamines is difficult. Because there is currently no known molecular basis for a distinction of
GABAB receptors on different terminals, these findings
clearly need to be confirmed and extended to include
studies with recombinant GABAB receptors. Phenylalkylamines, such as NPS467 and NPS568, are structurally
related to arylalkylamines and allosterically modulate the
CaS receptors by binding to the hydrophobic transmembrane domains (128). It is therefore assumed that the
arylalkylamines bind to the transmembrane domains of
GABAB receptors as well. Positive allosteric compounds
were recently also obtained for an additional member of
the Family 3 GPCRs, the mGlu1 receptors (180). The
residues that are critical for allosteric modulation of
mGlu1 are again located in the transmembrane domains.
It is likely that positive allosteric modulators act by stabilizing the active state of the transmembrane domains.
Positive allosteric modulation is not the sole possibility to fine-tune GABAB receptors. High-throughput
screening using functional read-outs allows identification
of noncompetitive GABAB antagonists as well, as, e.g.,
shown for the mGlu receptors (114). Noncompetitive
mGlu antagonists, such as CPCCOEt, MPEP, and BAY36 –
7620, decrease the maximal effect of glutamate without
changing its affinity, clearly indicating that they interact at
a site other than the glutamate binding site. Similar to
positive allosteric modulators, noncompetitive mGlu receptor antagonists interact with the transmembrane do-
858
BETTLER, KAUPMANN, MOSBACHER, AND GASSMANN
VI. SUMMARY AND OUTLOOK
Physiol Rev • VOL
We thank Dr. J.-P. Pin for the contribution of figures and
the anonymous reviewers for their suggestions.
B. Bettler is supported by the Swiss Science Foundation
Grant 3100 – 067100.01.
Address for reprint requests and other correspondence: B.
Bettler, Pharmazentrum, Dept. of Clinical-Biological Sciences,
Institute of Physiology, Univ. of Basel, Klingelbergstr. 50, CH4056 Basel, Switzerland (E-mail: [email protected]).
REFERENCES
1. Abdalla S, Lother H, and Quitterer U. AT1-receptor heterodimers show enhanced G-protein activation and altered receptor sequestration. Nature 407: 94 –98, 2000.
2. Abellan MT, Adell A, Honrubia MA, Mengod G, and Artigas F.
GABAB-R1 receptors in serotonergic neurons: effects of baclofen
on 5-HT output in rat brain. Neuroreport 11: 941–945, 2000.
3. Addolorato G, Caputo F, Capristo E, Domenicali M, Bernardi
M, Janiri L, Agabio R, Colombo G, Gessa GL, and Gasbarrini
G. Baclofen efficacy in reducing alcohol craving and intake: a
preliminary double-blind randomized controlled study. Alcohol 37:
504 –508, 2002.
4. Amico C, Marchetti C, Nobile M, and Usai C. Pharmacological
types of calcium channels and their modulation by baclofen in
cerebellar granules. J Neurosci 15: 2839 –2848, 1995.
5. Anaya M, Romero T, Sofia RD, and Yunis EJ. Linkage disequilibrium of HLA-A11 and A1 with one of the polymorphisms of the
gamma-aminobutyric acid receptor type B. Tissue Antigens 58:
324 –328, 2001.
6. Andriamampandry C, Taleb O, Viry S, Muller C, Humbert JP,
Gobaille S, Aunis D, and Maitre M. Cloning and characterization
of a rat brain receptor that binds the endogenous neuromodulator
gamma-hydroxybutyrate (GHB). FASEB J 17: 1691–1693, 2003.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
Recent years have seen rapid advances in our understanding of GABAB receptors. This has not only significantly advanced the GABAB field but also fundamentally
changed our view of the structure and signaling of GPCRs
in general. Research on GABAB receptors very much contributed to the now widely accepted idea that GPCRs
form homo- or heterodimers. Although no consensus exists on the role of GPCR dimerization, structural studies
on rhodopsin suggest that GPCRs necessarily need to
dimerize to bind the G protein (195). In addition to
GABAB receptors, there are now several examples of
GPCRs where heteromerization changes pharmacological
and signaling properties (7, 43). Because GPCRs represent major drug targets and are the single largest family of
cell surface receptors in the human genome, the concept
of heterodimerization is likely to have significant impact
on drug development. It is, for example, conceivable that
some of the several hundred orphan GPCRs will only
function in combination with a distinct subunit.
Searches in the human genome databases failed to
identify the expected variety of GABAB subunits that
would readily explain the heterogeneity described for
native receptors. While the existence of as yet unidentified splice variants and alternative quaternary assemblies
is never totally ruled out as a source of receptor heterogeneity, it is now generally established that the pharmacological diversity proposed before receptor cloning can
no longer be maintained. To date, no GABAB compound
unequivocally differentiates molecular variants of GABAB
receptors. Reported differences in the potency of agonists
probably relate to differences in receptor reserve or in
differences in the various downstream effectors that were
analyzed. Additionally, the lipid environment or receptor
modifications may wrongly suggest subtypes. As a consequence of the lack of heterogeneity, the possibilities for
selective interference with the GABAB system are now
limited. Essentially there are only two GABAB receptor
populations, GABAB(1a,2) and GABAB(1b,2), which are
abundant in vivo. However, it is still unknown whether
these two receptors exhibit functional differences, for
example, by localizing to pre- versus postsynaptic sites, or
to inhibitory versus excitatory terminals. The search for
GABAB(1a) selective compounds will almost certainly necessitate the identification of protein(s) that interact selectively with one of the isoforms.
Despite the obvious lack of pharmacological subtypes, the availability of functional assays based on recombinant receptors has led to the discovery of novel
GABAB compounds. Notably the repertoire of GABAB
compounds was expanded to include positive allosteric
modulators (175, 263, 330, 331). Chances are that positive
allosteric compounds will be therapeutically effective,
since a basal GABAB activity is present in many disease-
relevant neuronal pathways (272, 300). Functional highthroughput screens may allow the identification of noncompetitive antagonists as well. Here the hope is that new
antagonists with distinct chemical structures will have
altered pharmacokinetic/distribution profiles that help to
separate clinical efficacy from side effects. A big help for
drug discovery efforts is the availability of GABAB knockout mice, which allow optimizing receptor selectivity of
novel GABAB compounds.
Partial agonists that activate sensitized receptors at
lower doses than inactive receptors mediating the side
effects would represent another possibility for more selective interference with the GABAB system. However, no
laboratory reported the identification of new partial or full
agonists (or competitive antagonists) in recombinant
high-throughput screens. This is probably a consequence
of the fact that only GABAB(1) contains a GABA binding
site. Compounds that are selective for the GABA binding
site of GABAB(1) were identified a long time ago, using
radioligand antagonist displacement in rat brain membranes. A functional screening assay offers no particular
advantages as opposed to previous binding assays for
identifying competitive ligands. For these reasons we do
not expect that many novel structures for competitive
ligands will be reported as a consequence of functional
high-throughput screens.
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
Physiol Rev • VOL
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
antihyperalgesic agent gabapentin is an agonist at brain gammaaminobutyric acid type B receptors negatively coupled to voltagedependent calcium channels. J Pharmacol Exp Ther 298: 15–24,
2001.
Bertrand S, Nouel D, Morin F, Nagy F, and Lacaille JC. Gabapentin actions on Kir3 currents and N-type Ca2⫹ channels via
GABA-B receptors in hippocampal pyramidal cells. Synapse 50:
95–109, 2003.
Bettler B, Kaupmann K, and Bowery N. GABA-B receptors:
drugs meet clones. Curr Opin Neurobiol 8: 345–350, 1998.
Billinton A, Ige AO, Bolam JP, White JH, Marshall FH, and
Emson PC. Advances in the molecular understanding of GABA-B
receptors. Trends Neurosci 24: 277–282, 2001.
Billinton A, Ige AO, Wise A, White JH, Disney GH, Marshall
FH, Waldvogel HJ, Faull RL, and Emson PC. Gaba-B receptor
heterodimer-component localisation in human brain. Brain Res 77:
111–124, 2000.
Billinton A, Stean TO, Bowery NG, and Upton N. Gaba-B1
splice variant mRNAs are differentially affected by electroshock
induced seizure in rats. Neuroreport 11: 3817–3822, 2000.
Billinton A, Upton N, and Bowery NG. Gaba-B receptor isoforms GBR1a and GBR1b, appear to be associated with pre- and
postsynaptic elements respectively in rat and human cerebellum.
Br J Pharmacol 126: 1387–1392, 1999.
Bischoff S, Leonhard S, Reymann N, Schuler V, Shigemoto R,
Kaupmann K, and Bettler B. Spatial distribution of GABA-BR1
receptor mRNA and binding sites in the rat brain. J Comp Neurol
412: 1–16, 1999.
Blackshaw LA, Smid SD, O’Donnell TA, and Dent J. Gaba-B
receptor-mediated effects on vagal pathways to the lower oesophageal sphincter and heart. Br J Pharmacol 130: 279 –288, 2000.
Blahos J II, Mary S, Perroy J, De Colle C, Brabet I, Bockaert
J, and Pin JP. Extreme C terminus of G protein alpha-subunits
contains a site that discriminates between Gi-coupled metabotropic glutamate receptors. J Biol Chem 273: 25765–25769, 1998.
Blaxter TJ, Carlen PL, Davies MF, and Kujtan PW. Gammaaminobutyric acid hyperpolarizes rat hippocampal pyramidal cells
through a calcium-dependent potassium conductance. J Physiol
373: 181–194, 1986.
Bolser DC, Blythin DJ, Chapman RW, Egan RW, Hey JA, Rizzo
C, Kuo SC, and Kreutner W. The pharmacology of SCH 50911: a
novel, orally-active GABA-B receptor antagonist. J Pharmacol Exp
Ther 274: 1393–1398, 1995.
Bonanno G, Carita F, Cavazzani P, Munari C, and Raiteri M.
Selective block of rat and human neocortex GABA-B receptors
regulating somatostatin release by a GABA-B antagonist endowed
with cognition enhancing activity. Neuropharmacology 38: 1789 –
1795, 1999.
Bonanno G, Fassio A, Sala R, Schmid G, and Raiteri M. Gaba-B
receptors as potential targets for drugs able to prevent excessive
excitatory amino acid transmission in the spinal cord. Eur J Pharmacol 362: 143–148, 1998.
Bonanno G, Fassio A, Schmid G, Severi P, Sala R, and Raiteri
M. Pharmacologically distinct GABA-B receptors that mediate inhibition of GABA and glutamate release in human neocortex. Br J
Pharmacol 120: 60 – 64, 1997.
Bonanno G and Raiteri M. Multiple GABA-B receptors. Trends
Pharmacol Sci 14: 259 –261, 1993.
Bourne HR and Nicoll R. Molecular machines integrate coincident synaptic signals. Cell 72: 65–75, 1993.
Bouvier M. Oligomerization of G-protein-coupled transmitter receptors. Nat Rev Neurosci 2: 274 –286, 2001.
Bowery NG. Gaba-B receptor pharmacology. Annu Rev Pharmacol Toxicol 33: 109 –147, 1993.
Bowery NG Bettler B, Froestl W, Gallagher JP, Marshall F,
Raiteri M, Bonner TI, and Enna SJ. International Union of
Pharmacology XXXIII. Mammalian gamma-aminobutyric acid B receptors: structure and function. Pharmacol Rev 54: 247–264, 2002.
Bowery NG, Hill DR, Hudson AL, Doble A, Middlemiss DN,
Shaw J, and Turnbull M. (⫺)Baclofen decreases neurotransmitter release in the mammalian CNS by an action at a novel GABA
receptor. Nature 283: 92–94, 1980.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
7. Angers S, Salahpour A, and Bouvier M. Dimerization: an emerging concept for G protein-coupled receptor ontogeny and function.
Annu Rev Pharmacol Toxicol 42: 409 – 435, 2002.
8. Armstrong N, Sun Y, Chen GQ, and Gouaux E. Structure of a
glutamate-receptor ligand-binding core in complex with kainate.
Nature 395: 913–917, 1998.
9. Asano T and Ogasawara N. Uncoupling of gamma-aminobutyric
acid B receptors from GTP-binding proteins by N-ethylmaleimide:
effect of N-ethylmaleimide on purified GTP-binding proteins. Mol
Pharmacol 29: 244 –249, 1986.
10. Asano T, Ui M, and Ogasawara N. Prevention of the agonist
binding to gamma-aminobutyric acid B receptors by guanine nucleotides and islet-activating protein, pertussis toxin, in bovine
cerebral cortex. Possible coupling of the toxin-sensitive GTP-binding proteins to receptors. J Biol Chem 260: 12653–12658, 1985.
11. Azatian KV, Ayrapetyan SN, and Carpenter DO. Metabotropic
GABA receptors regulate acetylcholine responses on snail neurons.
Gen Pharmacol 29: 67–72, 1997.
12. Bai D and Sattelle D. A GABA-B receptor on an identified insect
motor neurone. J Exp Biol 198: 889 – 894, 1995.
13. Balerio GN and Rubio MC. Baclofen analgesia: involvement of
the GABAergic system. Pharmacol Res 46: 281–286, 2002.
14. Bamber BA, Beg AA, Twyman RE, and Jorgensen EM. The
Caenorhabditis elegans unc-49 locus encodes multiple subunits of
a heteromultimeric GABA receptor. J Neurosci 19: 5348 –5359,
1999.
15. Banerjee PK and Snead OC III. Presynaptic gamma-hydroxybutyric acid (GHB) and gamma-aminobutyric acid-B (GABA-B) receptor-mediated release of GABA and glutamate (GLU) in rat thalamic
ventrobasal nucleus (VB): a possible mechanism for the generation
of absence-like seizures induced by GHB. J Pharmacol Exp Ther
273: 1534 –1543, 1995.
16. Barthel F, Kienlen Campard P, Demeneix BA, Feltz P, and
Loeffler JP. GABA-B receptors negatively regulate transcription in
cerebellar granular neurons through cyclic AMP responsive element binding protein-dependent mechanisms. Neuroscience 70:
417– 427, 1996.
17. Becamel C, Figge A, Poliak S, Dumuis A, Peles E, Bockaert J,
Lubbert H, and Ullmer C. Interaction of serotonin 5-hydroxytryptamine type 2C receptors with PDZ10 of the multi-PDZ domain
protein MUPP1. J Biol Chem 276: 12974 –12982, 2001.
18. Becher A, White JH, and McIlhinney RA. The gamma-aminobutyric acid receptor B, but not the metabotropic glutamate receptor
type-1, associates with lipid rafts in the rat cerebellum. J Neurochem 79: 787–795, 2001.
19. Belley M, Sullivan R, Reeves A, Evans J, O’Neill G, and Ng GY.
Synthesis of the nanomolar photoaffinity GABA-B receptor ligand
CGP71872 reveals diversity in the tissue distribution of GABA-B
receptor forms. Bioorg Med Chem 7: 2697–2704, 1999.
20. Benes FM and Berretta S. GABAergic interneurons: implications
for understanding schizophrenia and bipolar disorder. Neuropsychopharmacology 25: 1–27, 2001.
21. Benke D, Honer M, Michel C, Bettler B, and Mohler H.
Gamma-aminobutyric acid type B receptor splice variant proteins
GBR1a and GBR1b are both associated with GBR2 in situ and
display differential regional and subcellular distribution. J Biol
Chem 274: 27323–27330, 1999.
22. Benke D, Michel C, and Mohler H. Structure of GABA-B receptors in rat retina. J Recept Signal Transduct Res 22: 253–266, 2002.
23. Benzing T, Yaffe MB, Arnould T, Sellin L, Schermer B, Schilling B, Schreiber R, Kunzelmann K, Leparc GG, Kim E, and
Walz G. 14 –3-3 interacts with regulator of G protein signaling
proteins and modulates their activity. J Biol Chem 275: 28167–
28172, 2000.
24. Bernasconi R, Mathivet P, Bischoff S, and Marescaux C.
Gamma-hydroxybutyric acid: an endogenous neuromodulator with
abuse potential? Trends Pharmacol Sci 20: 135–141, 1999.
25. Berthele A, Platzer S, Weis S, Conrad B, and Tolle TR. Expression of GABA-B1 and GABA-B2 mRNA in the human brain.
Neuroreport 12: 3269 –3275, 2001.
26. Bertrand S, Ng GY, Purisai MG, Wolfe SE, Severidt MW,
Nouel D, Robitaille R, Low MJ, O’Neill GP, Metters K, Lacaille JC, Chronwall BM, and Morris SJ. The anticonvulsant,
859
860
BETTLER, KAUPMANN, MOSBACHER, AND GASSMANN
Physiol Rev • VOL
66. Chang W, Chen TH, Pratt S, and Shoback D. Amino acids in the
second and third intracellular loops of the parathyroid Ca2⫹-sensing receptor mediate efficient coupling to phospholipase C. J Biol
Chem 275: 19955–19963, 2000.
67. Chapman RW, Hey JA, Rizzo CA, and Bolser DC. Gaba-B
receptors in the lung. Trends Pharmacol Sci 14: 26 –29, 1993.
68. Charara A, Heilman TC, Levey AI, and Smith Y. Pre- and
postsynaptic localization of GABA-B receptors in the basal ganglia
in monkeys. Neuroscience 95: 127–140, 2000.
69. Charles KJ, Calver A, Jourdain S, and Pangalos M. Distribution of a GABA-B-like receptor protein in the rat central nervous
system. Brain Res 989: 135–146, 2003.
70. Charles KJ, Deuchars J, Davies CH, and Pangalos MN. Gaba-B
receptor subunit expression in glia. Mol Cell Neurosci 24: 214 –223,
2003.
71. Charles KJ, Evans ML, Robbins MjJ, Calver AR, Leslie RA,
and Pangalos MN. Comparative immunohistochemical localisation of GABA-B1a, GABA-B1b and GABA-B2 subunits in rat brain,
spinal cord and dorsal root ganglion. Neuroscience 106: 447– 467,
2001.
72. Cheng Y and Lotan R. Molecular cloning and characterization of
a novel retinoic acid-inducible gene that encodes a putative G
protein-coupled receptor. J Biol Chem 273: 35008 –35015, 1998.
73. Chieng B and Bekkers JM. Gaba-B, opioid and alpha2 receptor
inhibition of calcium channels in acutely-dissociated locus coeruleus neurones. Br J Pharmacol 127: 1533–1538, 1999.
74. Chronwall BM, Davis TD, Severidt MW, Wolfe SE, McCarson
KE, Beatty DM, Low MJ, Morris SJ, and Enna SJ. Constitutive
expression of functional GABA-B receptors in mIL-tsA58 cells requires both GABA-B1 and GABA-B2 genes. J Neurochem 77: 1237–
1247, 2001.
75. Clark JA, Mezey E, Lam AS, and Bonner TI. Distribution of the
GABA-B receptor subunit gb2 in rat CNS. Brain Res 860: 41–52,
2000.
76. Colmers WF and Williams JT. Pertussis toxin pretreatment discriminates between pre- and postsynaptic actions of baclofen in rat
dorsal raphe nucleus in vitro. Neurosci Lett 93: 300 –306, 1988.
77. Colombo G, Agabio R, Carai MA, Lobina C, Pani M, Reali R,
Addolorato G, and Gessa GL. Ability of baclofen in reducing
alcohol intake and withdrawal severity: preclinical evidence. Alcohol Clin Exp Res 24: 58 – 66, 2000.
78. Connor M, Schuller A, Pintar JE, and Christie MJ. Mu-opioid
receptor modulation of calcium channel current in periaqueductal
grey neurons from C57B16/J mice and mutant mice lacking MOR-1.
Br J Pharmacol 126: 1553–1558, 1999.
79. Cousins MS, Roberts DC, and De Wit H. Gaba-B receptor agonists for the treatment of drug addiction: a review of recent findings. Drug Alcohol Depend 65: 209 –220, 2002.
80. Couve A, Filippov AK, Connolly CN, Bettler B, Brown DA,
and Moss SJ. Intracellular retention of recombinant GABA-B receptors. J Biol Chem 273: 26361–26367, 1998.
81. Couve A, Kittler JT, Uren JM, Calver AR, Pangalos MN,
Walsh FS, and Moss SJ. Association of GABA-B receptors and
members of the 14 –3-3 family of signaling proteins. Mol Cell Neurosci 17: 317–328, 2001.
82. Couve A, Thomas P, Calver AR, Hirst WD, Pangalos MN,
Walsh FS, Smart TG, and Moss SJ. Cyclic AMP-dependent protein kinase phosphorylation facilitates GABA-B receptor-effector
coupling. Nat Neurosci 5: 415– 424, 2002.
83. Crunelli V and Leresche N. A role for GABA-B receptors in
excitation and inhibition of thalamocortical cells. Trends Neurosci
14: 16 –21, 1991.
84. Cui JG, Meyerson BA, Sollevi A, and Linderoth B. Effect of
spinal cord stimulation on tactile hypersensitivity in mononeuropathic rats is potentiated by simultaneous GABA-B and adenosine
receptor activation. Neurosci Lett 247: 183–186, 1998.
85. Cunningham MD and Enna SJ. Evidence for pharmacologically
distinct GABA-B receptors associated with cAMP production in rat
brain. Brain Res 720: 220 –224, 1996.
86. Daoust M, Saligaut C, Lhuintre JP, Moore N, Flipo JL, and
Boismare F. Gaba transmission, but not benzodiazepine receptor
stimulation, modulates ethanol intake by rats. Alcohol 4: 469 – 472,
1987.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
47. Brauner-Osborne H, Jensen AA, Sheppard PO, Brodin B,
Krogsgaard-Larsen P, and O’Hara P. Cloning and characterization of a human orphan family C G-protein coupled receptor
GPRC5D. Biochim Biophys Acta 1518: 237–248, 2001.
48. Brauner-Osborne H, Jensen AA, Sheppard PO, O’Hara P, and
Krogsgaard-Larsen P. The agonist-binding domain of the calcium-sensing receptor is located at the amino-terminal domain.
J Biol Chem 274: 18382–18386, 1999.
49. Brauner-Osborne H and Krogsgaard-Larsen P. Functional
pharmacology of cloned heterodimeric GABA-B receptors expressed in mammalian cells. Br J Pharmacol 128: 1370 –1374, 1999.
50. Brauner-Osborne H and Krogsgaard-Larsen P. Sequence and
expression pattern of a novel human orphan G-protein-coupled
receptor, GPRC5B, a family C receptor with a short amino-terminal
domain. Genomics 65: 121–128, 2000.
51. Brebner K, Froestl W, and Roberts DC. The GABA-B antagonist
CGP56433A attenuates the effect of baclofen on cocaine but not
heroin self-administration in the rat. Psychopharmacology 160:
49 –55, 2002.
52. Brebner K, Phelan R, and Roberts DC. Intra-VTA baclofen attenuates cocaine self-administration on a progressive ratio schedule of reinforcement. Pharmacol Biochem Behav 66: 857– 862,
2000.
53. Brody DL and Yue DT. Relief of G-protein inhibition of calcium
channels and short-term synaptic facilitation in cultured hippocampal neurons. J Neurosci 20: 889 – 898, 2000.
54. Calver A, Michalovich D, Testa T, Robbins JM, Jaillard C, Hill
J, Szekeres PG, Charles KJ, Jourdain S, Holbrook JD, Boyfield I, Patel N, Medhurst AD, and Pangalos M. Molecular
cloning and characterisation of a novel GABA-B related G-protein
coupled receptor. Mol Brain Res 110: 305–317, 2003.
55. Calver AR, Davies CH, and Pangalos M. Gaba-B receptors: from
monogamy to promiscuity. Neurosignals 11: 299 –314, 2002.
56. Calver AR, Medhurst AD, Robbins MJ, Charles KJ, Evans ML,
Harrison DC, Stammers M, Hughes SA, Hervieu G, Couve A,
Moss SJ, Middlemiss DN, and Pangalos MN. The expression of
GABA-B1 and GABA-B2 receptor subunits in the CNS differs from
that in peripheral tissues. Neuroscience 100: 155–170, 2000.
57. Calver AR, Robbins MJ, Cosio C, Rice SQ, Babbs AJ, Hirst
WD, Boyfield I, Wood MD, Russell RB, Price GW, Couve A,
Moss SJ, and Pangalos MN. The C-terminal domains of the
GABA-B receptor subunits mediate intracellular trafficking but are
not required for receptor signaling. J Neurosci 21: 1203–1210, 2001.
58. Campbell UC, Lac ST, and Carroll ME. Effects of baclofen on
maintenance and reinstatement of intravenous cocaine self-administration in rats. Psychopharmacology 143: 209 –214, 1999.
59. Campbell V, Berrow N, and Dolphin AC. Gaba-B receptor modulation of Ca2⫹ currents in rat sensory neurones by the G protein
G(o): antisense oligonucleotide studies. J Physiol 470: 1–11, 1993.
60. Cange L, Johnsson E, Rydholm H, Lehmann A, Finizia C,
Lundell L, and Ruth M. Baclofen-mediated gastro-oesophageal
acid reflux control in patients with established reflux disease.
Aliment Pharmacol Ther 16: 869 – 873, 2002.
61. Cardozo DL and Bean BP. Voltage-dependent calcium channels
in rat midbrain dopamine neurons: modulation by dopamine and
GABA-B receptors. J Neurophysiol 74: 1137–1148, 1995.
62. Carroll FY, Stolle A, Beart PM, Voerste A, Brabet I, Mauler F,
Joly C, Antonicek H, Bockaert J, Muller T, Pin JP, and
Prezeau L. Bay36 –7620: a potent non-competitive mGlu1 receptor
antagonist with inverse agonist activity. Mol Pharmacol 59: 965–
973, 2001.
63. Castelli MP, Ferraro L, Mocci I, Carta F, Carai MAM, Antonelli T, Tanganelli S, Cignarella G, and Gessa GL. Selective
␥-hydroxybutyric acid receptor ligands increase extracellular glutamate in the hippocampus, but fail to activate G protein and to
produce the sedative/hypnotic effect of ␥-hydroxybutyric acid.
J Neurochem 203: 722–732, 2003
64. Castelli MP, Ingianni A, Stefanini E, and Gessa GL. Distribution of GABA-B receptor mRNAs in the rat brain and peripheral
organs. Life Sci 64: 1321–1328, 1999.
65. Chan PK, Leung CK, and Yung WH. Differential expression of
pre- and postsynaptic GABA-B receptors in rat substantia nigra
pars reticulata neurones. Eur J Pharmacol 349: 187–197, 1998.
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
Physiol Rev • VOL
108. Gallimberti L, Spella MR, Soncini CA, and Gessa GL. Gammahydroxybutyric acid in the treatment of alcohol and heroin dependence. Alcohol 20: 257–262, 2000.
109. Galvez T, Duthey B, Kniazeff J, Blahos J, Rovelli G, Bettler B,
Prezeau L, and Pin JP. Allosteric interactions between GB1 and
GB2 subunits are required for optimal GABA-B receptor function.
EMBO J 20: 2152–2159, 2001.
110. Galvez T, Parmentier ML, Joly C, Malitschek B, Kaupmann K,
Kuhn R, Bittiger H, Froestl W, Bettler B, and Pin JP. Mutagenesis and modeling of the GABA-B receptor extracellular domain
support a venus flytrap mechanism for ligand binding. J Biol Chem
274: 13362–13369, 1999.
111. Galvez T, Prezeau L, Milioti G, Franek M, Joly C, Froestl W,
Bettler B, Bertrand HO, Blahos J, and Pin JP. Mapping the
agonist-binding site of GABA-B type 1 subunit sheds light on the
activation process of GABA-B receptors. J Biol Chem 275: 41166 –
41174, 2000.
112. Galvez T, Urwyler S, Prezeau L, Mosbacher J, Joly C, Malitschek B, Heid J, Brabet I, Froestl W, Bettler B, Kaupmann
K, and Pin JP. Ca(2⫹) requirement for high-affinity gamma-aminobutyric acid (GABA) binding at GABA-B receptors: involvement
of serine 269 of the GABA-BR1 subunit. Mol Pharmacol 57: 419 –
426, 2000.
113. Gambardella A, Manna I, Labate A, Chifari R, La Russa A,
Serra P, Cittadella R, Bonavita S, Andreoli V, Lepiane E,
Sasanelli F, Di Costanzo A, Zappia M, Tedeschi G, Aguglia U,
and Quattrone A. Gaba-B receptor 1 polymorphism (G1465A) is
associated with temporal lobe epilepsy. Neurology 60: 560 –563,
2003.
114. Gasparini F, Kuhn R, and Pin JP. Allosteric modulators of group
I metabotropic glutamate receptors: novel subtype-selective ligands and therapeutic perspectives. Curr Opin Pharmacol 2: 43–
49, 2002.
115. Gemignani A, Paudice P, Bonanno G, and Raiteri M. Pharmacological discrimination between gamma-aminobutyric acid type B
receptors regulating cholecystokinin and somatostatin release
from rat neocortex synaptosomes. Mol Pharmacol 46: 558 –562,
1994.
116. Gerber U and Gahwiler BH. Gaba-B and adenosine receptors
mediate enhancement of the K⫹ current, IAHP, by reducing adenylyl cyclase activity in rat CA3 hippocampal neurons. J Neurophysiol 72: 2360 –2367, 1994.
117. Gines S, Hillion J, Torvinen M, Le Crom S, Casado V, Canela
EI, Rondin S, Lew JY, Watson S, Zoli M, Agnati LF, Verniera
P, Lluis C, Ferre S, Fuxe K, and Franco R. Dopamine D1 and
adenosine A1 receptors form functionally interacting heteromeric
complexes. Proc Natl Acad Sci USA 97: 8606 – 8611, 2000.
118. Ginham R, Blein S, Barlow P, White J, and McIlhinney RA.
Interaction of “Sushi” domain of GABA-BR1a subunit with the
extracellular matrix protein fibulin. FENS Abstr 1: 144.146, 2002.
119. Gomeza J, Joly C, Kuhn R, Knopfel T, Bockaert J, and Pin JP.
The second intracellular loop of metabotropic glutamate receptor 1
cooperates with the other intracellular domains to control coupling
to G-proteins. J Biol Chem 271: 2199 –2205, 1996.
120. Gomez-Villafuertes R, Pintor J, Gualix J, and Miras-Portugal
MT. Gaba-B receptor-mediated presynaptic potentiation of ATP
ionotropic receptors in rat midbrain synaptosomes. Neuropharmacology 44: 311–323, 2003.
121. Green A, Walls S, Wise A, Green RH, Martin AK, and Marshall
FH. Characterization of [(3)H]-CGP54626A binding to heterodimeric GABA-B receptors stably expressed in mammalian
cells. Br J Pharmacol 131: 1766 –1774, 2000.
122. Greif GJ, Sodickson DL, Bean BP, Neer EJ, and Mende U.
Altered regulation of potassium and calcium channels by GABA-B
and adenosine receptors in hippocampal neurons from mice lacking Galpha(o). J Neurophysiol 83: 1010 –1018, 2000.
123. Grifa A, Totaro A, Rommens JM, Carella M, Roetto A, Borgato L, Zelante L, and Gasparini P. Gaba (gamma-amino-butyricacid) neurotransmission: identification and fine mapping of the
human GABA-B receptor gene. Biochem Biophys Res Commun
250: 240 –245, 1998.
124. Grobaski KC, Ping H, Dasilva HM, Bowery NG, Connelly ST,
and Shepard PD. Responses of rat substantia nigra dopamine-
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
87. Davies CH, Starkey SJ, Pozza MF, and Collingridge GL. Gaba
autoreceptors regulate the induction of LTP. Nature 349: 609 – 611,
1991.
88. Deisz RA, Billard JM, and Zieglgansberger W. Presynaptic and
postsynaptic GABA-B receptors of neocortical neurons of the rat in
vitro: differences in pharmacology and ionic mechanisms. Synapse
25: 62–72, 1997.
89. De Koninck Y and Mody I. Endogenous GABA activates smallconductance K⫹ channels underlying slow IPSCs in rat hippocampal neurons. J Neurophysiol 77: 2202–2208, 1997.
90. Devlin CL. The pharmacology of gamma-aminobutyric acid and
acetylcholine receptors at the echinoderm neuromuscular junction.
J Exp Biol 204: 887– 896, 2001.
91. Devlin CL and Schlosser W. Gamma-aminobutyric acid modulation of acetylcholine-induced contractions of a smooth muscle
from an echinoderm (Sclerodactyla briareus). Invert Neurosci 4:
1– 8, 1999.
92. Dewey SL, Brodie JD, Gerasimov M, Horan B, Gardner EL,
and Ashby CR Jr. A pharmacologic strategy for the treatment of
nicotine addiction. Synapse 31: 76 – 86, 1999.
93. Dewey SL, Morgan AE, Ashby CR, Horan B Jr, Kushner SA,
Logan J, Volkow ND, Fowler JS, Gardner EL, and Brodie JD.
A novel strategy for the treatment of cocaine addiction. Synapse
30: 119 –129, 1998.
94. Dicpinigaitis PV, Dobkin JB, Rauf K, and Aldrich TK. Inhibition of capsaicin-induced cough by the gamma-aminobutyric acid
agonist baclofen. J Clin Pharmacol 38: 364 –367, 1998.
95. Durkin MM, Gunwaldsen CA, Borowsky B, Jones KA, and
Branchek TA. An in situ hybridization study of the distribution of
the GABA-B2 protein mRNA in the rat CNS. Brain Res 71: 185–200,
1999.
96. Dutar P and Nicoll RA. Pre- and postsynaptic GABA-B receptors
in the hippocampus have different pharmacological properties.
Neuron 1: 585–591, 1988.
97. Duthey B, Caudron S, Perroy J, Bettler B, Fagni L, Pin JP, and
Prezeau L. A single subunit (GB2) is required for G-protein activation by the heterodimeric GABA-B receptor. J Biol Chem 277:
3236 –3241, 2002.
98. Easter A and Spruce AE. Recombinant GABA-B receptors
formed from GABA-B1 and GABA-B2 subunits selectively inhibit
N-type Ca(2⫹) channels in NG108 –15 cells. Eur J Pharmacol 440:
17–25, 2002.
99. Erhardt S, Mathe JM, Chergui K, Engberg G, and Svensson
TH. Gaba-B receptor-mediated modulation of the firing pattern of
ventral tegmental area dopamine neurons in vivo. NaunynSchmiedebergs Arch Pharmacol 365: 173–180, 2002.
100. Filippov AK, Couve A, Pangalos MN, Walsh FS, Brown DA,
and Moss SJ. Heteromeric assembly of GABA-BR1 and GABA-BR2
receptor subunits inhibits Ca(2⫹) current in sympathetic neurons.
J Neurosci 20: 2867–2874, 2000.
101. Fischer Y and Parnas I. Activation of GABA-B receptors at individual release boutons of the crayfish opener neuromuscular junction produces presynaptic inhibition. J Neurophysiol 75: 1377–
1385, 1996.
102. Francesconi A and Duvoisin RM. Role of the second and third
intracellular loops of metabotropic glutamate receptors in mediating dual signal transduction activation. J Biol Chem 273: 5615–
5624, 1998.
103. Franek M, Pagano A, Kaupmann K, Bettler B, Pin JP, and
Blahos J. The heteromeric GABA-B receptor recognizes G-protein
alpha subunit C-termini. Neuropharmacology 38: 1657–1666, 1999.
104. Fritschy JM, Meskenaite V, Weinmann O, Honer M, Benke D,
and Möhler H. Gaba-B receptor splice variants GB1a and GB1b in
rat brain: developmental regulation, cellular distribution and extrasynaptic localization. Eur J Neurosci 11: 761–768, 1999.
105. Fromm GH. The pharmacology of trigeminal neuralgia. Clin Neuropharmacol 12: 185–194, 1989.
106. Furtinger S, Bettler B, and Sperk G. Altered expression of
GABA-B receptors in the hippocampus after kainic-acid-induced
seizures in rats. Brain Res 113: 107–115, 2003.
107. Futatsugi Y and Riviello JJ. Mechanisms of generalized absence
epilepsy. Brain Dev 20: 75–79, 1998.
861
862
125.
126.
127.
128.
130.
131.
132.
133.
134.
135.
136.
137.
138.
139.
140.
141.
142.
143.
144.
145.
146.
containing neurones to (⫺)-HA-966 in vitro. Br J Pharmacol 120:
575–580, 1997.
Grunewald S, Schupp BJ, Ikeda SR, Kuner R, Steigerwald F,
Kornau HC, and Kohr G. Importance of the gamma-aminobutyric
acid B receptor C-termini for G-protein coupling. Mol Pharmacol
61: 1070 –1080, 2002.
Guatteo E, Fusco FR, Giacomini P, Bernardi G, and Mercuri
NB. The weaver mutation reverses the function of dopamine and
GABA in mouse dopaminergic neurons. J Neurosci 20: 6013– 6020,
2000.
Haas KZ, Sperber EF, Moshe SL, and Stanton PK. Kainic
acid-induced seizures enhance dentate gyrus inhibition by downregulation of GABA-B receptors. J Neurosci 16: 4250 – 4260, 1996.
Hammerland LG, Garrett JE, Hung BC, Levinthal C, and Nemeth EF. Allosteric activation of the Ca2⫹ receptor expressed in
Xenopus laevis oocytes by NPS 467 or NPS 568. Mol Pharmacol 53:
1083–1088, 1998.
Hammerland LG, Krapcho KJ, Garrett JE, Alasti N, Hung BC,
Simin RT, Levinthal C, Nemeth EF, and Fuller FH. Domains
determining ligand specificity for Ca2⫹ receptors. Mol Pharmacol
55: 642– 648, 1999.
Harayama N, Shibuya I, Tanaka K, Kabashima N, Ueta Y, and
Yamashita H. Inhibition of N- and P/Q-type calcium channels by
postsynaptic GABA-B receptor activation in rat supraoptic neurones. J Physiol 509: 371–383, 1998.
Harris BZ and Lim WA. Mechanism and role of PDZ domains in
signaling complex assembly. J Cell Sci 114: 3219 –3231, 2001.
Harrison NL. On the presynaptic action of baclofen at inhibitory
synapses between cultured rat hippocampal neurones. J Physiol
422: 433– 446, 1990.
Hashimoto T and Kuriyama K. In vivo evidence that GABA-B
receptors are negatively coupled to adenylate cyclase in rat striatum. J Neurochem 69: 365–370, 1997.
Hausser MA and Yung WH. Inhibitory synaptic potentials in
guinea-pig substantia nigra dopamine neurones in vitro. J Physiol
479: 401– 422, 1994.
Hawrot E, Xiao Y, Shi QL, Norman D, Kirkitadze M, and
Barlow PN. Demonstration of a tandem pair of complement protein modules in GABA-B receptor 1a. FEBS Lett 432: 103–108, 1998.
He XB, Hu JH, Wu Q, Yan YC, and Koide SS. Identification of
GABA-B receptor in rat testis and sperm. Biochem Biophys Res
Commun 283: 243–247, 2001.
Heese K, Otten U, Mathivet P, Raiteri M, Marescaux C, and
Bernasconi R. Gaba-B receptor antagonists elevate both mRNA
and protein levels of the neurotrophins nerve growth factor (NGF)
and brain-derived neurotrophic factor (BDNF) but not neurotrophin-3 (NT-3) in brain and spinal cord of rats. Neuropharmacology
39: 449 – 462, 2000.
Heinemann U, Konnerth A, Pumain R, and Wadman WJ. Extracellular calcium and potassium concentration changes in
chronic epileptic brain tissue. Adv Neurol 44: 641– 661, 1986.
Hering-Hanit R. Baclofen for prevention of migraine. Cephalalgia
19: 589 –591, 1999.
Herlitze S, Garcia DE, Mackie K, Hille B, Scheuer T, and
Catterall WA. Modulation of Ca2⫹ channels by G-protein beta
gamma subunits. Nature 380: 258 –262, 1996.
Herman RM, D’Luzansky SC, and Ippolito R. Intrathecal baclofen suppresses central pain in patients with spinal lesions. A
pilot study. Clin J Pain 8: 338 –345, 1992.
Heuss C and Gerber U. G-protein-independent signaling by Gprotein-coupled receptors. Trends Neurosci 23: 469 – 475, 2000.
Heuss C, Scanziani M, Gahwiler BH, and Gerber U. G-proteinindependent signaling mediated by metabotropic glutamate receptors. Nat Neurosci 2: 1070 –1077, 1999.
Hill DR, Bowery NG, and Hudson AL. Inhibition of GABA-B
receptor binding by guanyl nucleotides. J Neurochem 42: 652– 657,
1984.
Hirono M, Yoshioka T, and Konishi S. Gaba-B receptor activation enhances mGluR-mediated responses at cerebellar excitatory
synapses. Nat Neurosci 4: 1207–1216, 2001.
Hirsch DP, Tytgat GN, and Boeckxstaens GE. Transient lower
oesophageal sphincter relaxations: a pharmacological target for
Physiol Rev • VOL
147.
148.
149.
150.
151.
152.
153.
154.
155.
156.
157.
158.
159.
160.
161.
162.
163.
164.
165.
166.
gastro-oesophageal reflux disease? Aliment Pharmacol Ther 16:
17–26, 2002.
Hosford DA, Clark S, Cao Z, Wilson WA, Lin FH, Morrisett RA
Jr, and Huin A. The role of GABA-B receptor activation in absence
seizures of lethargic (lh/lh) mice. Science 257: 398 – 401, 1992.
Hoshino M, Suzuki E, Nabeshima Y, and Hama C. Hikaru genki
protein is secreted into synaptic clefts from an early stage of
synapse formation in Drosophila. Development 122: 589 –597, 1996.
Hsueh YP, Wang TF, Yang FC, and Sheng M. Nuclear translocation and transcription regulation by the membrane-associated
guanylate kinase CASK/LIN-2. Nature 404: 298 –302, 2000.
Hue B. Functional assay for GABA receptor subtypes of a cockroach giant interneuron. Arch Insect Biochem Physiol 18: 147–157,
1991.
Ige AO, Bolam JP, Billinton A, White JH, Marshall FH, and
Emson PC. Cellular and sub-cellular localisation of GABA-B1 and
GABA-B2 receptor proteins in the rat cerebellum. Brain Res 83:
72– 80, 2000.
Ikeda SR. Voltage-dependent modulation of N-type calcium channels by G-protein beta gamma subunits. Nature 380: 255–258, 1996.
Inoue M, Matsuo T, and Ogata N. Characterization of pre- and
postsynaptic actions of (⫺)-baclofen in the guinea-pig hippocampus in vitro. Br J Pharmacol 84: 843– 851, 1985.
Ipponi A, Lamberti C, Medica A, Bartolini A, and MalmbergAiello P. Tiagabine antinociception in rodents depends on
GABA-B receptor activation: parallel antinociception testing and
medial thalamus GABA microdialysis. Eur J Pharmacol 368: 205–
211, 1999.
Isaacson JS, Solis JM, and Nicoll RA. Local and diffuse synaptic
actions of GABA in the hippocampus. Neuron 10: 165–175, 1993.
Isomoto S, Kaibara M, Sakurai-Yamashita Y, Nagayama Y,
Uezono Y, Yano K, and Taniyama K. Cloning and tissue distribution of novel splice variants of the rat GABA-B receptor. Biochem Biophys Res Commun 253: 10 –15, 1998.
Ito Y, Ishige K, Zaitsu E, Anzai K, and Fukuda H. Gammahydroxybutyric acid increases intracellular Ca2⫹ concentration and
nuclear cyclic AMP-responsive element- and activator protein 1
DNA-binding activities through GABAB receptor in cultured cerebellar granule cells. J Neurochem 65: 75– 83, 1995.
Iyadomi M, Iyadomi I, Kumamoto E, Tomokuni K, and Yoshimura M. Presynaptic inhibition by baclofen of miniature EPSCs
and IPSCs in substantia gelatinosa neurons of the adult rat spinal
dorsal horn. Pain 85: 385–393, 2000.
Jarolimek W, Bijak M, and Misgeld U. Differences in the Cs
block of baclofen and 4-aminopyridine induced potassium currents
of guinea pig CA3 neurons in vitro. Synapse 18: 169 –177, 1994.
Jasmin L, Rabkin SD, Granato A, Boudah A, and Ohara PT.
Analgesia and hyperalgesia from GABA-mediated modulation of
the cerebral cortex. Nature 424: 316 –320, 2003.
Jensen AA, Mosbacher J, Elg S, Lingenhoehl K, Lohmann T,
Johansen TN, Abrahamsen B, Mattsson JP, Lehmann A, Bettler B, and Brauner-Osborne H. The anticonvulsant gabapentin
(neurontin) does not act through gamma-aminobutyric acid-B receptors. Mol Pharmacol 61: 1377–1384, 2002.
Jensen K and Mody I. Ghb depresses fast excitatory and inhibitory synaptic transmission via GABA-B receptors in mouse neocortical neurons. Cereb Cortex 11: 424 – 429, 2001.
Jones KA, Borowsky B, Tamm JA, Craig DA, Durkin MM, Dai
M, Yao WJ, Johnson M, Gunwaldsen C, Huang LY, Tang C,
Shen Q, Salon JA, Morse K, Laz T, Smith KE, Nagarathnam D,
Noble SA, Branchek TA, and Gerald C. Gaba-B receptors function as a heteromeric assembly of the subunits GABA-BR1 and
GABA-BR2. Nature 396: 674 – 679, 1998.
Jordan BA, Trapaidze N, Gomes I, Nivarthi R, and Devi LA.
Oligomerization of opioid receptors with beta 2-adrenergic receptors: a role in trafficking and mitogen-activated protein kinase
activation. Proc Natl Acad Sci USA 98: 343–348, 2001.
Kalivas PW and Duffy P. D1 receptors modulate glutamate transmission in the ventral tegmental area. J Neurosci 15: 5379 –5388,
1995.
Kammerer RA, Frank S, Schulthess T, Landwehr R, Lustig A,
and Engel J. Heterodimerization of a functional GABA-B receptor
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
129.
BETTLER, KAUPMANN, MOSBACHER, AND GASSMANN
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
167.
168.
169.
170.
172.
173.
174.
175.
176.
177.
178.
179.
180.
181.
182.
183.
184.
Physiol Rev • VOL
185.
186.
187.
188.
189.
190.
191.
192.
193.
194.
195.
196.
197.
198.
199.
200.
201.
202.
203.
tors relative to synaptic sites in the rat cerebellum and ventrobasal
thalamus. Eur J Neurosci 15: 291–307, 2002.
Kuner R, Kohr G, Grunewald S, Eisenhardt G, Bach A, and
Kornau HC. Role of heteromer formation in GABA-B receptor
function. Science 283: 74 –77, 1999.
Kunishima N, Shimada Y, Tsuji Y, Sato T, Yamamoto M, Kumasaka T, Nakanishi S, Jingami H, and Morikawa K. Structural
basis of glutamate recognition by a dimeric metabotropic glutamate receptor. Nature 407: 971–977, 2000.
Kushner SA, Dewey SL, and Kornetsky C. The irreversible
gamma-aminobutyric acid (GABA) transaminase inhibitor gammavinyl-GABA blocks cocaine self-administration in rats. J Pharmacol
Exp Ther 290: 797– 802, 1999.
Lacaille JC. Postsynaptic potentials mediated by excitatory and
inhibitory amino acids in interneurons of stratum pyramidale of the
CA1 region of rat hippocampal slices in vitro. J Neurophysiol 66:
1441–1454, 1991.
Lagrange AH, Wagner EJ, Ronnekleiv OK, and Kelly MJ.
Estrogen rapidly attenuates a GABA-B response in hypothalamic
neurons. Neuroendocrinology 64: 114 –123, 1996.
Lanneau C, Green A, Hirst WD, Wise A, Brown JT, Donnier E,
Charles KJ, Wood M, Davies CH, and Pangalos MN. Gabapentin is not a GABA-B receptor agonist. Neuropharmacology 41:
965–975, 2001.
Lazarewicz JW. Calcium transients in brain ischemia: role in
neuronal injury. Acta Neurobiol Exp 56: 299 –311, 1996.
Leaney JL and Tinker A. The role of members of the pertussis
toxin-sensitive family of G proteins in coupling receptors to the
activation of the G protein-gated inwardly rectifying potassium
channel. Proc Natl Acad Sci USA 97: 5651–5656, 2000.
Lehmann A, Bremner-Danielsen M, Branden L, and Karrberg
L. Inhibitory effects of GABA-B receptor agonists on swallowing in
the dog. Eur J Pharmacol 448: 67–70, 2002.
Liang F, Hatanaka Y, Saito H, Yamamori T, and Hashikawa T.
Differential expression of gamma-aminobutyric acid type B receptor-1a and -1b mRNA variants in GABA and non-GABAergic neurons of the rat brain. J Comp Neurol 416: 475– 495, 2000.
Liang Y, Fotiadis D, Filipek S, Saperstein DA, Palczewski K,
and Engel A. Organization of the G protein-coupled receptors
rhodopsin and opsin in native membranes. J Biol Chem 278: 21655–
21662, 2003.
Lidums I, Lehmann A, Checklin H, Dent J, and Holloway RH.
Control of transient lower esophageal sphincter relaxations and
reflux by the GABA-B agonist baclofen in normal subjects. Gastroenterology 118: 7–13, 2000.
Ling W, Shoptaw S, and Majewska D. Baclofen as a cocaine
anti-craving medication: a preliminary clinical study. Neuropsychopharmacology 18: 403– 404, 1998.
Lingenhoehl K, Brom R, Heid J, Beck P, Froestl W, Kaupmann
K, Bettler B, and Mosbacher J. Gamma-hydroxybutyrate is a
weak agonist at recombinant GABA-B receptors. Neuropharmacology 38: 1667–1673, 1999.
Lorente P, Lacampagne A, Pouzeratte Y, Richards S, Malitschek B, Kuhn R, Bettler B, and Vassort G. Gamma-aminobutyric acid type B receptors are expressed and functional in mammalian cardiomyocytes. Proc Natl Acad Sci USA 97: 8664 – 8669,
2000.
Lu XY, Ghasemzadeh MB, and Kalivas PW. Regional distribution and cellular localization of gamma-aminobutyric acid subtype
1 receptor mRNA in the rat brain. J Comp Neurol 407: 166 –182,
1999.
Lüscher C, Jan LY, Stoffel M, Malenka RC, and Nicoll RA. G
protein-coupled inwardly rectifying K⫹ channels (GIRKs) mediate
postsynaptic but not presynaptic transmitter actions in hippocampal neurons. Neuron 19: 687– 695, 1997.
Maguire G, Maple B, Lukasiewicz P, and Werblin F. Gammaaminobutyrate type B receptor modulation of L-type calcium channel current at bipolar cell terminals in the retina of the tiger
salamander. Proc Natl Acad Sci USA 86: 10144 –10147, 1989.
Mako E, Ronai AZ, Adam G, Juhasz G, Ritter L, Lestar B, and
Crunelli V. Modulation by GABA-B and delta opioid receptors of
neurally induced responses in isolated guinea-pig taenia coli and
human colonic circular muscle. J Physiol 94: 135–138, 2000.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
171.
is mediated by parallel coiled-coil alpha-helices. Biochemistry 38:
13263–13269, 1999.
Kang J, Jiang L, Goldman SA, and Nedergaard M. Astrocytemediated potentiation of inhibitory synaptic transmission. Nat
Neurosci 1: 683– 692, 1998.
Kaupmann K, Cryan JF, Wellendorph P, Mombereau C, Sansig G, Klebs K, Schmutz M, Froestl W, Van Der Putten H,
Mosbacher J, Bräuner-Osborne H, Waldmeier P, and Bettler
B. Specific ␥-hydroxybutyrate-binding sites but loss of pharmacological effects of ␥-hydroxybutyrate in GABAB(1)-deficient mice.
Eur J Neurosci. 18: 2722–2730, 2003.
Kaupmann K, Huggel K, Heid J, Flor PJ, Bischoff S, Mickel
SJ, Mcmaster G, Angst C, Bittiger H, Froestl W, and Bettler
B. Expression cloning of GABA-B receptors uncovers similarity to
metabotropic glutamate receptors. Nature 386: 239 –246, 1997.
Kaupmann K, Malitschek B, Schuler V, Heid J, Froestl W,
Beck P, Mosbacher J, Bischoff S, Kulik A, Shigemoto R, Karschin A, and Bettler B. Gaba-B receptor subtypes assemble into
functional heteromeric complexes. Nature 396: 683– 687, 1998.
Kaupmann K, Schuler V, Mosbacher J, Bischoff S, Bittiger H,
Heid J, Froestl W, Leonhard S, Pfaff T, Karschin A, and
Bettler B. Human gamma-aminobutyric acid type B receptors are
differentially expressed and regulate inwardly rectifying K⫹ channels. Proc Natl Acad Sci USA 95: 14991–14996, 1998.
Keir MJ, Barakat MJ, Dev KK, Bittiger H, Bettler B, and
Henley JM. Characterisation and partial purification of the
GABA-B receptor from the rat cerebellum using the novel antagonist [3H]CGP 62349. Brain Res 71: 279 –289, 1999.
Kelly MJ, Lagrange AH, Wagner EJ, and Ronnekleiv OK.
Rapid effects of estrogen to modulate G protein-coupled receptors
via activation of protein kinase A and protein kinase C pathways.
Steroids 64: 64 –75, 1999.
Kenakin T. Differences between natural and recombinant G-protein coupled receptor systems with varying receptor/G-protein stoichiometry. Trends Pharmacol Sci 18: 456 – 464, 1997.
Kerr DI, Ong J, Puspawati NM, and Prager RH. Arylalkylamines
are a novel class of positive allosteric modulators at GABA-B
receptors in rat neocortex. Eur J Pharmacol 451: 69 –77, 2002.
Kitano J, Kimura K, Yamazaki Y, Soda T, Shigemoto R, Nakajima Y, and Nakanishi S. Tamalin, a PDZ domain-containing
protein, links a protein complex formation of group 1 metabotropic
glutamate receptors and the guanine nucleotide exchange factor
cytohesins. J Neurosci 22: 1280 –1289, 2002.
Kittler JT, Rostaing P, Schiavo G, Fritschy JM, Olsen R,
Triller A, and Moss SJ. The subcellular distribution of GABARAP
and its ability to interact with NSF suggest a role for this protein in
the intracellular transport of GABA-A receptors. Mol Cell Neurosci
18: 13–25, 2001.
Knapp CM, Foye MM, Ciraulo DA, and Kornetsky C. The type
IV phosphodiesterase inhibitors, Ro 20 –1724 and rolipram, block
the initiation of cocaine self-administration. Pharmacol Biochem
Behav 62: 151–158, 1999.
Kniazeff J, Galvez T, Labesse G, and Pin JP. No ligand binding
in the GB2 subunit of the GABA-B receptor is required for activation and allosteric interaction between the subunits. J Neurosci 22:
7352–7361, 2002.
Knoflach F, Mutel V, Jolidon S, Kew JN, Malherbe P, Vieira E,
Wichmann J, and Kemp JA. Positive allosteric modulators of
metabotropic glutamate 1 receptor: characterization, mechanism
of action, and binding site. Proc Natl Acad Sci USA 98: 13402–
13407, 2001.
Koek GH, Sifrim D, Lerut T, Janssens J, and Tack J. Effect of
the GABA-B agonist baclofen in patients with symptoms and duodeno-gastro-oesophageal reflux refractory to proton pump inhibitors. Gut 52: 1397–1402, 2003.
Kornau HC, Schenker LT, Kennedy MB, and Seeburg PH.
Domain interaction between NMDA receptor subunits and the
postsynaptic density protein PSD-95. Science 269: 1737–1740, 1995.
Kubo Y, Miyashita T, and Murata Y. Structural basis for a
Ca2⫹-sensing function of the metabotropic glutamate receptors.
Science 279: 1722–1725, 1998.
Kulik A, Nakadate K, Nyiri G, Notomi T, Malitschek B, Bettler B, and Shigemoto R. Distinct localization of GABA-B recep-
863
864
BETTLER, KAUPMANN, MOSBACHER, AND GASSMANN
Physiol Rev • VOL
224. Mezler M, Muller T, and Raming K. Cloning and functional
expression of GABA-B receptors from Drosophila. Eur J Neurosci
13: 477– 486, 2001.
225. Mintz IM and Bean BP. Gaba-B receptor inhibition of P-type Ca2⫹
channels in central neurons. Neuron 10: 889 – 898, 1993.
226. Miwa A, Ui M, and Kawai N. G protein is coupled to presynaptic
glutamate and GABA receptors in lobster neuromuscular synapse.
J Neurophysiol 63: 173–180, 1990.
227. Mondadori C, Moebius HJ, and Zingg M. Cgp36742, an orally
active GABA-B receptor antagonist, facilitates memory in a social
recognition test in rats. Behav Brain Res 77: 227–229, 1996.
228. Morishita R, Kato K, and Asano T. Gaba-B receptors couple to
G proteins Go, Go* and Gi1 but not to Gi2. FEBS Lett 271: 231–235,
1990.
229. Mott DD and Lewis DV. Facilitation of the induction of long-term
potentiation by GABA-B receptors. Science 252: 1718 –1720, 1991.
230. Mouginot D, Kombian SB, and Pittman QJ. Activation of presynaptic GABA-B receptors inhibits evoked IPSCs in rat magnocellular neurons in vitro. J Neurophysiol 79: 1508 –1517, 1998.
231. Munoz A, Defelipe J, and Jones EG. Patterns of GABA-BR1a,b
receptor gene expression in monkey and human visual cortex.
Cereb Cortex 11: 104 –113, 2001.
232. Munoz A, Huntsman MM, and Jones EG. Gaba-B receptor gene
expression in monkey thalamus. J Comp Neurol 394: 118 –126,
1998.
233. Nakayasu H, Nishikawa M, Mizutani H, Kimura H, and
Kuriyama K. Immunoaffinity purification and characterization of
gamma-aminobutyric acid (GABA) B receptor from bovine cerebral
cortex. J Biol Chem 268: 8658 – 8664, 1993.
234. Nash MS, Saunders R, Young KW, Challiss RA, and Nahorski
SR. Reassessment of the Ca2⫹ sensing property of a type I metabotropic glutamate receptor by simultaneous measurement of inositol
1,4,5-trisphosphate and Ca2⫹ in single cells. J Biol Chem 276:
19286 –19293, 2001.
235. Nehring RB, Horikawa HP, El Far O, Kneussel M, Brandstatter JH, Stamm S, Wischmeyer E, Betz H, and Karschin A. The
metabotropic GABA-B receptor directly interacts with the activating transcription factor 4. J Biol Chem 275: 35185–35191, 2000.
236. Newberry NR and Nicoll RA. Comparison of the action of baclofen with gamma-aminobutyric acid on rat hippocampal pyramidal cells in vitro. J Physiol 360: 161–185, 1985.
237. Ng GY, Bertrand S, Sullivan R, Ethier N, Wang J, Yergey J,
Belley M, Trimble L, Bateman K, Alder L, Smith A, McKernan
R, Metters K, O’Neill GP, Lacaille JC, and Hebert TE. Gammaaminobutyric acid type B receptors with specific heterodimer composition and postsynaptic actions in hippocampal neurons are
targets of anticonvulsant gabapentin action. Mol Pharmacol 59:
144 –152, 2001.
238. Ng GY, McDonald T, Bonnert T, Rigby M, Heavens R, Whiting
P, Chateauneuf A, Coulombe N, Kargman S, Caskey T, Evans
J, O’Neill GP, and Liu Q. Cloning of a novel G-protein-coupled
receptor GPR 51 resembling GABA-B receptors expressed predominantly in nervous tissues and mapped proximal to the hereditary
sensory neuropathy type 1 locus on chromosome 9. Genomics 56:
288 –295, 1999.
239. Ng GYK, Clark J, Coulombe N, Ethier N, Hebert TE, Sullivan
R, Kargman S, Chateauneuf A, Tsukamoto N, McDonald T,
Whiting P, Mezey E, Johnson Mp Liu Q, Kolakowski LF,
Evans JF, Bonner TI, and O’Neill GP. Identification of a
GABA-B receptor subunit, gb2, required for functional GABA-B
receptor activity. J Biol Chem 274: 7607–7610, 1999.
240. Ng TK and Yung KK. Subpopulations of neurons in rat substantia
nigra display GABA-BR2 receptor immunoreactivity. Brain Res
920: 210 –216, 2001.
241. Nicholson KL and Balster RL. Ghb: a new and novel drug of
abuse. Drug Alcohol Depend 63: 1–22, 2001.
242. Obrocea GV and Morris ME. Changes in [K⫹]o evoked by baclofen in guinea pig hippocampus. Can J Physiol Pharmacol 76:
148 –154, 1998.
243. Ogata N, Inoue M, and Matsuo T. Contrasting properties of K⫹
conductances induced by baclofen and gamma-aminobutyric acid
in slices of the guinea pig hippocampus. Synapse 1: 62– 69, 1987.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
204. Malan TP, Mata HP, and Porreca F. Spinal GABA-A and GABA-B
receptor pharmacology in a rat model of neuropathic pain. Anesthesiology 96: 1161–1167, 2002.
205. Malherbe P, Kratochwil N, Knoflach F, Zenner MT, Kew JN,
Kratzeisen C, Maerki HP, Adam G, and Mutel V. Mutational
analysis and molecular modeling of the allosteric binding site of a
novel, selective, non-competitive antagonist of the metabotropic
glutamate 1 receptor. J Biol Chem 278: 8340 – 8347, 2003.
206. Malitschek B, Rüegg D, Heid J, Kaupmann K, Bittiger H,
Fröstl W, Bettler B, and Kuhn R. Developmental changes in
agonist affinity at GABA-BR1 receptor variants in rat brain. Mol Cell
Neurosci 12: 56 – 64, 1998.
207. Malitschek B, Schweizer C, Keir M, Heid J, Froestl W, Mosbacher J, Kuhn R, Henley J, Joly C, Pin JP, Kaupmann K, and
Bettler B. The N-terminal domain of gamma-aminobutyric acid-B
receptors is sufficient to specify agonist and antagonist binding.
Mol Pharmacol 56: 448 – 454, 1999.
208. Marchetti C, Carignani C, and Robello M. Voltage-dependent
calcium currents in dissociated granule cells from rat cerebellum.
Neuroscience 43: 121–133, 1991.
209. Marescaux C, Vergnes M, Liu Z, Depaulis A, and Bernasconi
R. Gaba-B receptor involvement in the control of genetic absence
seizures in rats. Epilepsy Res Suppl 9: 131–138, 1992.
210. Margeta-Mitrovic M, Jan YN, and Jan LY. A trafficking checkpoint controls GABA-B receptor heterodimerization. Neuron 27:
97–106, 2000.
211. Margeta-Mitrovic M, Jan YN, and Jan LY. Function of GB1 and
GB2 subunits in G protein coupling of GABA-B receptors. Proc Natl
Acad Sci USA 98: 14649 –14654, 2001.
212. Margeta-Mitrovic M, Jan YN, and Jan LY. Ligand-induced signal
transduction within heterodimeric GABA-B receptor. Proc Natl
Acad Sci USA 98: 14643–14648, 2001.
213. Margeta-Mitrovic M, Mitrovic I, Riley RC, Jan LY, and Basbaum AI. Immunohistochemical localization of GABA-B receptors
in the rat central nervous system. J Comp Neurol 405: 299 –321,
1999.
214. Marshall FH, Jones KA, Kaupmann K, and Bettler B. Gaba-B
receptors: the first 7TM heterodimers. Trends Pharmacol Sci 20:
396 –399, 1999.
215. Martellotta MC, Balducci C, Fattore L, Cossu G, Gessa GL,
Pulvirenti L, and Fratta W. Gamma-hydroxybutyric acid decreases intravenous cocaine self-administration in rats. Pharmacol
Biochem Behav 59: 697–702, 1998.
216. Martin SC, Russek SJ, and Farb DH. Molecular identification of
the human GABA-BR2: cell surface expression and coupling to
adenylyl cyclase in the absence of GABA-BR1. Mol Cell Neurosci
13: 180 –191, 1999.
217. Martin SC, Russek SJ, and Farb DH. Human GABA-BR genomic
structure: evidence for splice variants in GABA-BR1 but not GABABR2. Gene 278: 63–79, 2001.
218. Masu M, Tanabe Y, Tsuchida K, Shigemoto R, and Nakanishi
S. Sequence and expression of a metabotropic glutamate receptor.
Nature 349: 760 –765, 1991.
219. Matsushima T, Tegner J, Hill RH, and Grillner S. Gaba-B
receptor activation causes a depression of low- and high-voltageactivated Ca2⫹ currents, postinhibitory rebound, and postspike
afterhyperpolarization in lamprey neurons. J Neurophysiol 70:
2606 –2619, 1993.
220. McCormick DA. GABA as an inhibitory neurotransmitter in human cerebral cortex. J Neurophysiol 62: 1018 –1027, 1989.
221. McLatchie LM, Fraser NJ, Main MJ, Wise A, Brown J, Thompson N, Solari R, Lee MG, and Foord SM. Ramps regulate the
transport and ligand specificity of the calcitonin-receptor-like receptor. Nature 393: 333–339, 1998.
222. Meguro H, Mori H, Araki K, Kushiya E, Kutsuwada T,
Yamazaki M, Kumanishi T, Arakawa M, Sakimura K, and
Mishina M. Functional characterization of a heteromeric NMDA
receptor channel expressed from cloned cDNAs. Nature 357: 70 –
74, 1992.
223. Menon-Johansson AS, Berrow N, and Dolphin AC. G(o) transduces GABA-B receptor modulation of N-type calcium channels in
cultured dorsal root ganglion neurons. Pflügers Arch 425: 335–343,
1993.
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
Physiol Rev • VOL
263. Pin JP, Parmentier ML, and Prezeau L. Positive allosteric modulators for gamma-aminobutyric acid-B receptors open new routes
for the development of drugs targeting family 3 G-protein-coupled
receptors. Mol Pharmacol 60: 881– 884, 2001.
264. Pitler TA and Alger BE. Differences between presynaptic and
postsynaptic GABA-B mechanisms in rat hippocampal pyramidal
cells. J Neurophysiol 72: 2317–2327, 1994.
265. Poncer JC, McKinney RA, Gahwiler BH, and Thompson SM.
Either N- or P-type calcium channels mediate GABA release at
distinct hippocampal inhibitory synapses. Neuron 18: 463– 472,
1997.
266. Poncer JC, McKinney RA, Gahwiler BH, and Thompson SM.
Differential control of GABA release at synapses from distinct
interneurons in rat hippocampus. J Physiol 528: 123–130, 2000.
267. Poorkhalkali N, Juneblad K, Jonsson AC, Lindberg M, Karlsson O, Wallbrandt P, Ekstrand J, and Lehmann A. Immunocytochemical distribution of the GABA-B receptor splice variants
GABA-B R1a and R1b in the rat CNS and dorsal root ganglia. Anat
Embryol 201: 1–13, 2000.
268. Pozza MF, Manuel NA, Steinmann M, Froestl W, and Davies
CH. Comparison of antagonist potencies at pre- and post-synaptic
GABA-B receptors at inhibitory synapses in the CA1 region of the
rat hippocampus. Br J Pharmacol 127: 211–219, 1999.
269. Prezeau L, Richman JG, Edwards SW, and Limbird LE. The
zeta isoform of 14 –3-3 proteins interacts with the third intracellular
loop of different alpha2-adrenergic receptor subtypes. J Biol Chem
274: 13462–13469, 1999.
270. Princivalle A, Spreafico R, Bowery N, and De Curtis M. Layerspecific immunocytochemical localization of GABA-BR1a and
GABA-BR1b receptors in the rat piriform cortex. Eur J Neurosci
12: 1516 –1520, 2000.
271. Princivalle AP, Pangalos MN, Bowery NG, and Spreafico R.
Distribution of GABA-B1a, GABA-B1b and GABA-B2 receptor protein in cerebral cortex and thalamus of adult rats. Neuroreport 12:
591–595, 2001.
272. Prosser HM, Gill CH, Hirst WD, Grau E, Robbins M, Calver A,
Soffin EM, Farmer CE, Lanneau C, Gray J, Schenck E, Warmerdam BS, Clapham C, Reavill C, Rogers DC, Stean T, Upton
N, Humphreys K, Randall A, Geppert M, Davies CH, and
Pangalos MN. Epileptogenesis and enhanced prepulse inhibition
in GABA-B1 deficient mice. Mol Cell Neurosci 17: 1059 –1070, 2001.
273. Przesmycki K, Dzieciuch JA, Czuczwar SJ, and Kleinrok Z. An
isobolographic analysis of drug interaction between intrathecal
clonidine and baclofen in the formalin test in rats. Neuropharmacology 37: 207–214, 1998.
274. Quéva C, Bremner-Danielsen M, Edlund A, Ekstrand AJ, Elg
S, Erickson S, Johannson T, Lehmann A, and Mattsson JP.
Effects of GABA agonists on body temperature regulation in
GABA-B1⫺/⫺ mice. Br J Pharmacol: 1– 8, 2003.
275. Quiocho FA and Ledvina PS. Atomic structure and specificity of
bacterial periplasmic receptors for active transport and chemotaxis: variation of common themes. Mol Microbiol 20: 17–25, 1996.
276. Rathmayer W and Djokaj S. Presynaptic inhibition and the participation of GABA-B receptors at neuromuscular junctions of the
crab Eriphia spinifrons. J Comp Physiol 186: 287–298, 2000.
277. Ratomponirina C, Hode Y, Hechler V, and Maitre M. Gammahydroxybutyrate receptor binding in rat brain is inhibited by guanyl
nucleotides and pertussis toxin. Neurosci Lett 189: 51–53, 1995.
278. Ray K and Hauschild BC. Cys-140 is critical for metabotropic
glutamate receptor-1 dimerization. J Biol Chem 275: 34245–34251,
2000.
279. Ren X and Mody I. Gamma-hydroxybutyrate (GHB) reduces MAP
kinase phosphorylation via GABA-B receptor activation in mouse
frontal cortex and hippocampus. J Biol Chem 15: 15, 2003.
280. Robbins MJ, Calver AR, Filippov AK, Hirst WD, Russell RB,
Wood MD, Nasir S, Couve A, Brown DA, Moss SJ, and Pangalos MN. Gaba-B2 is essential for G-protein coupling of the GABA-B
receptor heterodimer. J Neurosci 21: 8043– 8052, 2001.
281. Robbins MJ, Michalovich D, Hill J, Calver AR, Medhurst AD,
Gloger I, Sims M, Middlemiss DN, and Pangalos MN. Molecular
cloning and characterization of two novel retinoic acid-inducible
orphan G-protein coupled receptors (GPRC5B and GPRC5C).
Genomics 67: 8 –18, 2000.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
244. Okamoto T, Sekiyama N, Otsu M, Shimada Y, Sato A, Nakanishi S, and Jingami H. Expression and purification of the extracellular ligand binding region of metabotropic glutamate receptor
subtype 1. J Biol Chem 273: 13089 –13096, 1998.
245. Olah GA, Trakhanov S, Trewhella J, and Quiocho FA. Leucine/
isoleucine/valine-binding protein contracts upon binding of ligand.
J Biol Chem 268: 16241–16247, 1993.
246. Olpe HR, Karlsson G, Pozza MF, Brugger F, Steinmann M, Van
Riezen H, Fagg G, Hall RG, Froestl W, and Bittiger H. Cgp
35348: a centrally active blocker of GABA-B receptors. Eur J Pharmacol 187: 27–38, 1990.
247. Olpe HR, Woerner W, and Ferrat T. Stimulation parameters
determine role of GABA-B receptors in long-term potentiation.
Experientia 49: 542–546, 1993.
248. Ong J, Marino V, Parker DA, and Kerr DI. Differential effects of
phosphonic analogues of GABA on GABA-B autoreceptors in rat
neocortical slices. Naunyn-Schmiedebergs Arch Pharmacol 357:
408 – 412, 1998.
249. Otis TS, De Koninck Y, and Mody I. Characterization of synaptically elicited GABA-B responses using patch-clamp recordings in
rat hippocampal slices. J Physiol 463: 391– 407, 1993.
250. Pagano A, Rovelli G, Mosbacher J, Lohmann T, Duthey B,
Stauffer D, Ristig D, Schuler V, Meigel I, Lampert C, Stein T,
Prezeau L, Blahos J, Pin J, Froestl W, Kuhn R, Heid J, Kaupmann K, and Bettler B. C-terminal interaction is essential for
surface trafficking but not for heteromeric assembly of GABA-B
receptors. J Neurosci 21: 1189 –1202, 2001.
251. Pagano A, Ruegg D, Litschig S, Stoehr N, Stierlin C, Heinrich
M, Floersheim P, Prezeau L, Carroll F, Pin JP, Cambria A,
Vranesic I, Flor PJ, Gasparini F, and Kuhn R. The non-competitive antagonists 2-methyl-6-(phenylethynyl)pyridine and 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester
interact with overlapping binding pockets in the transmembrane
region of group I metabotropic glutamate receptors. J Biol Chem
275: 33750 –33758, 2000.
252. Paredes RG and Agmo A. The GABA-B antagonist CGP 35348
inhibits the effects of baclofen on sexual behavior and motor
coordination. Brain Res Bull 36: 495– 497, 1995.
253. Parmentier ML, Prezeau L, Bockaert J, and Pin JP. A model
for the functioning of family 3 GPCRs. Trends Pharmacol Sci 23:
268 –274, 2002.
254. Parnas I, Rashkovan G, Ong J, and Kerr DI. Tonic activation of
presynaptic GABA-B receptors in the opener neuromuscular junction of crayfish. J Neurophysiol 81: 1184 –1191, 1999.
255. Patel S, Naeem S, Kesingland A, Froestl W, Capogna M, Urban L, and Fox A. The effects of GABA-B agonists and gabapentin
on mechanical hyperalgesia in models of neuropathic and inflammatory pain in the rat. Pain 90: 217–226, 2001.
256. Pearson H. Surviving a knockout blow. Nature 415: 8 –9, 2002.
257. Pehrson R, Lehmann A, and Andersson KE. Effects of gammaaminobutyrate B receptor modulation on normal micturition and
oxyhemoglobin induced detrusor overactivity in female rats. J Urol
168: 2700 –2705, 2002.
258. Perroy J, Adam L, Qanbar R, Chenier S, and Bouvier M.
Phosphorylation-independent desensitization of GABA-B receptor
by GRK4. EMBO J 22: 3816 –3824, 2003.
259. Peters HC, Kämmer G, Volz A, Kaupmann K, Ziegler A, Bettler B, Epplen JT, Sander T, and Riess O. Mapping, genomic
structure, and polymorphisms of the human GABAB-R1 receptor
gene: evaluation of its involvement in idiopathic generalized epilepsy. Neurogenetics 2: 47–54, 1998.
260. Pfaff T, Malitschek B, Kaupmann K, Prezeau L, Pin JP, Bettler B, and Karschin A. Alternative splicing generates a novel
isoform of the rat metabotropic GABAB-R1 receptor. Eur J Neurosci 11: 2874 –2882, 1999.
261. Pfrieger FW, Gottmann K, and Lux HD. Kinetics of GABA-B
receptor-mediated inhibition of calcium currents and excitatory
synaptic transmission in hippocampal neurons in vitro. Neuron 12:
97–107, 1994.
262. Phelan KD. N-ethylmaleimide selectively blocks presynaptic
GABA-B autoreceptor but not heteroreceptor-mediated inhibition
in adult rat striatal slices. Brain Res 847: 308 –313, 1999.
865
866
BETTLER, KAUPMANN, MOSBACHER, AND GASSMANN
Physiol Rev • VOL
301.
302.
303.
304.
305.
306.
307.
308.
309.
310.
311.
312.
313.
314.
315.
316.
317.
318.
319.
320.
Der Putten H, and Bettler B. Epilepsy, hyperalgesia, impaired
memory, and loss of pre- and postsynaptic GABA-B responses in
mice lacking GABA-B1. Neuron 31: 47–58, 2001.
Schwarz DA, Barry G, Eliasof SD, Petroski RE, Conlon PJ,
and Maki RA. Characterization of gamma-aminobutyric acid receptor GABA-B1e, a GABA-B1 splice variant encoding a truncated
receptor. J Biol Chem 275: 32174 –32181, 2000.
Scott DB, Blanpied TA, Swanson GT, Zhang C, and Ehlers
MD. An NMDA receptor ER retention signal regulated by phosphorylation and alternative splicing. J Neurosci 21: 3063–3072,
2001.
Scott RH and Dolphin AC. Regulation of calcium currents by a
GTP analogue: potentiation of (⫺)-baclofen-mediated inhibition.
Neurosci Lett 69: 59 – 64, 1986.
Scott RH, Wootton JF, and Dolphin AC. Modulation of neuronal
T-type calcium channel currents by photoactivation of intracellular
guanosine 5⬘-O-(3-thio)triphosphate. Neuroscience 38: 285–294,
1990.
Seidenbecher T and Pape HC. Contribution of intralaminar thalamic nuclei to spike-and-wave-discharges during spontaneous seizures in a genetic rat model of absence epilepsy. Eur J Neurosci 13:
1537–1546, 2001.
Sekiguchi M, Sakuta H, Okamoto K, and Sakai Y. Gaba-B
receptors expressed in Xenopus oocytes by guinea pig cerebral
mRNA are functionally coupled with Ca2⫹-dependent Cl⫺ channels
and with K⫹ channels, through GTP-binding proteins. Brain Res 8:
301–309, 1990.
Serrats J, Artigas F, Mengod G, and Cortes R. Gaba-B receptor
mRNA in the raphe nuclei: co-expression with serotonin transporter and glutamic acid decarboxylase. J Neurochem 84: 743–752,
2003.
Shen KZ and Johnson SW. Presynaptic GABA-B and adenosine
A1 receptors regulate synaptic transmission to rat substantia nigra
reticulata neurones. J Physiol 505: 153–163, 1997.
Shen W and Slaughter MM. Metabotropic GABA receptors facilitate L-type and inhibit N-type calcium channels in single
salamander retinal neurons. J Physiol 516: 711–718, 1999.
Shoaib M, Swanner LS, Beyer CE, Goldberg SR, and Schindler
CW. The GABA-B agonist baclofen modifies cocaine self-administration in rats. Behav Pharmacol 9: 195–206, 1998.
Simonds WF. G protein regulation of adenylate cyclase. Trends
Pharmacol Sci 20: 66 –73, 1999.
Sindrup SH and Jensen TS. Pharmacotherapy of trigeminal neuralgia. Clin J Pain 18: 22–27, 2002.
Sloviter RS, Ali-Akbarian L, Elliott RC, Bowery BJ, and Bowery NG. Localization of GABA-BR1 receptors in the rat hippocampus by immunocytochemistry and high resolution autoradiography,
with specific reference to its localization in identified hippocampal
interneuron subpopulations. Neuropharmacology 38: 1707–1721,
1999.
Smith BR, Boyle AE, and Amit Z. The effects of the GABA-B
agonist baclofen on the temporal and structural characteristics of
ethanol intake. Alcohol 17: 231–240, 1999.
Snead OC III. Evidence for a G protein-coupled gamma-hydroxybutyric acid receptor. J Neurochem 75: 1986 –1996, 2000.
Spielewoy C, Roubert C, Hamon M, Nosten-Bertrand M, Betancur C, and Giros B. Behavioural disturbances associated with
hyperdopaminergia in dopamine-transporter knockout mice. Behav
Pharmacol 11: 279 –290, 2000.
Stringer JL and Lorenzo N. The reduction in paired-pulse inhibition in the rat hippocampus by gabapentin is independent of
GABA-B receptor activation. Epilepsy Res 33: 169 –176, 1999.
Symonds E, Butler R, and Omari T. The effect of the GABA-B
receptor agonist baclofen on liquid and solid gastric emptying in
mice. Eur J Pharmacol 470: 95–97, 2003.
Taira T, Kawamura H, Tanikawa T, Iseki H, Kawabatake H,
and Takakura K. A new approach to control central deafferentation pain: spinal intrathecal baclofen. Stereotact Funct Neurosurg
65: 101–105, 1995.
Takahashi T, Kajikawa Y, and Tsujimoto T. G-protein-coupled
modulation of presynaptic calcium currents and transmitter release by a GABA-B receptor. J Neurosci 18: 3138 –3146, 1998.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
282. Roberts DC, Andrews MM, and Vickers GJ. Baclofen attenuates
the reinforcing effects of cocaine in rats. Neuropsychopharmacology 15: 417– 423, 1996.
283. Robinson R, Taske N, Sander T, Heils A, Whitehouse W,
Goutieres F, Aicardi J, Lehesjoki AE, Siren A, Laue Friis M,
Kjeldsen MJ, Panayiotopoulos C, Kennedy C, Ferrie C, Rees
M, and Gardiner RM. Linkage analysis between childhood absence epilepsy and genes encoding GABA-A and GABA-B receptors, voltage-dependent calcium channels, and the ECA1 region on
chromosome 8q. Epilepsy Res 48: 169 –179, 2002.
284. Rocheville M, Lange DC, Kumar U, Patel SC, Patel RC, and
Patel YC. Receptors for dopamine and somatostatin: formation of
hetero-oligomers with enhanced functional activity. Science 288:
154 –157, 2000.
285. Rohrbacher J, Jarolimek W, Lewen A, and Misgeld U. Gaba-B
receptor-mediated inhibition of spontaneous inhibitory synaptic
currents in rat midbrain culture. J Physiol 500: 739 –749, 1997.
286. Romano C, Yang WL, and O’Malley L. Metabotropic glutamate
receptor 5 is a disulfide-linked dimer. J Biol Chem 271: 28612–
28616, 1996.
287. Rubakhin SS, Szucs A, Gurin VN, and Rozsa KS. Inhibition of
calcium spikes by gamma-amino-butyric acid in the neurons of
Lymnaea stagnalis L. Acta Biol Hung 46: 375–380, 1995.
288. Saghatelyan AK, Snapyan M, Gorissen S, Meigel I, Mosbacher
J, Kaupmann K, Bettler B, Kornilav AV, Nifantiev NE, Sakanyan V, Schachner M, and Dityatev A. Recognition molecule
associated carbohydrate inhibits postsynaptic GABA-B receptors: a
mechanism for homeostatic regulation of GABA release in perisomatic synapses. Mol Cell Neurosci 24: 271–282, 2003.
289. Saint DA, Thomas T, and Gage PW. Gaba-B agonists modulate a
transient potassium current in cultured mammalian hippocampal
neurons. Neurosci Lett 118: 9 –13, 1990.
290. Sakaba T and Neher E. Direct modulation of synaptic vesicle
priming by GABA-B receptor activation at a glutamatergic synapse.
Nature 424: 775–778, 2003.
291. Sand PG, Godau C, Riederer P, Peters C, Franke P, Nothen
MM, Stober G, Fritze J, Maier W, Propping P, Lesch KP, Riess
O, Sander T, Beckmann H, and Deckert J. Exonic variants of
the GABA-B receptor gene and panic disorder. Psychiatr Genet 10:
191–194, 2000.
292. Sander T, Peters C, Kammer G, Samochowiec J, Zirra M,
Mischke D, Ziegler A, Kaupmann K, Bettler B, Epplen JT, and
Riess O. Association analysis of exonic variants of the gene encoding the GABA-B receptor and idiopathic generalized epilepsy.
Am J Med Genet 88: 305–310, 1999.
293. Sanger GJ, Munonyara ML, Dass N, Prosser H, Pangalos MN,
and Parsons ME. Gaba-B receptor function in the ileum and
urinary bladder of wildtype and GABA-B1 subunit null mice. Auton
Autacoid Pharmacol 22: 147–154, 2002.
294. Santos AE, Carvalho CM, Macedo TA, and Carvalho AP. Regulation of intracellular [Ca2⫹] and GABA release by presynaptic
GABA-B receptors in rat cerebrocortical synaptosomes. Neurochem Int 27: 397– 406, 1995.
295. Sattelle DB, Pinnock RD, Wafford KA, and David JA. GABA
receptors on the cell-body membrane of an identified insect motor
neuron. Proc R Soc Lond B Biol Sci 232: 443– 456, 1988.
296. Saunders R, Nahorski SR, and Challiss RA. A modulatory effect
of extracellular Ca2⫹ on type 1alpha metabotropic glutamate receptor-mediated signalling. Neuropharmacology 37: 273–276, 1998.
297. Sauter K, Kaupmann K, Bettler B, Mohler H, and Benke D.
Direct interaction of GABA-B receptors with the transcription factor CHOP. FENS Abstr 1: 144.112, 2001.
298. Scanziani M, Debanne D, Muller M, Gahwiler BH, and Thompson SM. Role of excitatory amino acid and GABA-B receptors in
the generation of epileptiform activity in disinhibited hippocampal
slice cultures. Neuroscience 61: 823– 832, 1994.
299. Scholz KP and Miller RJ. Gaba-B receptor-mediated inhibition of
Ca2⫹ currents and synaptic transmission in cultured rat hippocampal neurones. J Physiol 444: 669 – 686, 1991.
300. Schuler V, Luscher C, Blanchet C, Klix N, Sansig G, Klebs K,
Schmutz M, Heid J, Gentry C, Urban L, Fox A, Spooren W,
Jaton AL, Vigouret J, Pozza M, Kelly PH, Mosbacher J,
Froestl W, Kaslin E, Korn R, Bischoff S, Kaupmann K, Van
GABAB RECEPTOR STRUCTURE AND PHYSIOLOGY
Physiol Rev • VOL
338. Wegelius K, Halonen T, and Korpi ER. Gamma-vinyl GABA
decreases voluntary alcohol consumption in alcohol-preferring AA
rats. Pharmacol Toxicol 73: 150 –152, 1993.
339. Wei K, Eubanks JH, Francis J, Jia Z, and Snead OC III.
Cloning and tissue distribution of a novel isoform of the rat GABABR1 receptor subunit. Neuroreport 12: 833– 837, 2001.
340. Wei K, Jia Z, Wang YT, Yang J, Liu CC, and Snead OC III.
Cloning and characterization of a novel variant of rat GABA-BR1
with a truncated C-terminus. Brain Res 89: 103–110, 2001.
341. White JH, Ginham R, Pontier S, Wise A, Blein S, Barlow P,
Bouvier M, and McIlhinney RA. The heterodimeric GABA-B
receptor and associated proteins. FENS Abstr 1: 062.061, 2002.
342. White JH, McIllhinney RA, Wise A, Ciruela F, Chan WY, Emson PC, Billinton A, and Marshall FH. The GABA-B receptor
interacts directly with the related transcription factors CREB2 and
ATFx. Proc Natl Acad Sci USA 97: 13967–13972, 2000.
343. White JH, Wise A, Main MJ, Green A, Fraser NJ, Disney GH,
Barnes AA, Emson P, Foord SM, and Marshall FH. Heterodimerization is required for the formation of a functional
GABA-B receptor. Nature 396: 679 – 682, 1998.
344. Wiesenfeld-Hallin Z, Aldskogius H, Grant G, Hao JX, Hokfelt
T, and Xu XJ. Central inhibitory dysfunctions: mechanisms and
clinical implications. Behav Brain Sci 20: 420 – 425, 1997.
345. Wise A, Green A, Main MJ, Wilson R, Fraser N, and Marshall
FH. Calcium sensing properties of the GABA-B receptor. Neuropharmacology 38: 1647–1656, 1999.
346. Wise RA, Leone P, Rivest R, and Leeb K. Elevations of nucleus
accumbens dopamine and DOPAC levels during intravenous heroin
self-administration. Synapse 21: 140 –148, 1995.
347. Wise RA, Newton P, Leeb K, Burnette B, Pocock D, and
Justice JB Jr. Fluctuations in nucleus accumbens dopamine concentration during intravenous cocaine self-administration in rats.
Psychopharmacology 120: 10 –20, 1995.
348. Wu C and Leung LS. Partial hippocampal kindling decreases
efficacy of presynaptic GABA-B autoreceptors in CA1. J Neurosci
17: 9261–9269, 1997.
349. Wu LG and Saggau P. Presynaptic inhibition of elicited neurotransmitter release. Trends Neurosci 20: 204 –212, 1997.
350. Xi ZX, Ramamoorthy S, Shen H, Lake R, Samuvel DJ, and
Kalivas PW. GABA transmission in the nucleus accumbens is
altered after withdrawal from repeated cocaine. J Neurosci 23:
3498 –3505, 2003.
351. Xi ZX and Stein EA. Baclofen inhibits heroin self-administration
behavior and mesolimbic dopamine release. J Pharmacol Exp Ther
290: 1369 –1374, 1999.
352. Yamada K, Yu B, and Gallagher JP. Different subtypes of
GABA-B receptors are present at pre- and postsynaptic sites within
the rat dorsolateral septal nucleus. J Neurophysiol 81: 2875–2883,
1999.
353. Yoshimura M, Yoshida S, and Taniyama K. Desensitization by
cyclic AMP-dependent protein kinase of GABA-B receptor expressed in Xenopus oocytes. Life Sci 57: 2397–2401, 1995.
354. Zamponi GW, Bourinet E, Nelson D, Nargeot J, and Snutch
TP. Crosstalk between G proteins and protein kinase C mediated
by the calcium channel alpha1 subunit. Nature 385: 442– 446, 1997.
355. Zerangue N, Schwappach B, Jan YN, and Jan LY. A new ER
trafficking signal regulates the subunit stoichiometry of plasma
membrane K(ATP) channels. Neuron 22: 537–548, 1999.
356. Zhang C, Bettler B, and Duvoisin RM. Differential localization of
GABA-B receptors in the mouse retina. Neuroreport 9: 3493–3497,
1998.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
321. Tang WJ and Gilman AG. Type-specific regulation of adenylyl
cyclase by G protein beta gamma subunits. Science 254: 1500 –1503,
1991.
322. Taniyama K, Niwa M, Kataoka Y, and Yamashita K. Activation
of protein kinase C suppresses the gamma-aminobutyric acid-B
receptor-mediated inhibition of the vesicular release of noradrenaline and acetylcholine. J Neurochem 58: 1239 –1245, 1992.
323. Teoh H, Malcangio M, and Bowery NG. Gaba, glutamate and
substance P-like immunoreactivity release: effects of novel
GABA-B antagonists. Br J Pharmacol 118: 1153–1160, 1996.
324. Thompson JD, Higgins DG, and Gibson TJ. Clustal W: improving the sensitivity of progressive multiple sequence alignment
through sequence weighting, position-specific gap penalties and
weight matrix choice. Nucleic Acids Res 22: 4673– 4680, 1994.
325. Thompson SM and Gahwiler BH. Comparison of the actions of
baclofen at pre- and postsynaptic receptors in the rat hippocampus
in vitro. J Physiol 451: 329 –345, 1992.
326. Towers S, Princivalle A, Billinton A, Edmunds M, Bettler B,
Urban L, Castro-Lopes J, and Bowery NG. Gaba-B receptor
protein and mRNA distribution in rat spinal cord and dorsal root
ganglia. Eur J Neurosci 12: 3201–3210, 2000.
327. Tsao P and Von Zastrow M. Downregulation of G protein-coupled
receptors. Curr Opin Neurobiol 10: 365–369, 2000.
328. Tsuji Y, Shimada Y, Takeshita T, Kajimura N, Nomura S,
Sekiyama N, Otomo J, Usukura J, Nakanishi S, and Jingami
H. Cryptic dimer interface and domain organization of the extracellular region of metabotropic glutamate receptor subtype 1.
J Biol Chem 275: 28144 –28151, 2000.
329. Tunnicliff G and Raess BU. Gamma-hydroxybutyrate (orphan
medical). Curr Opin Invest Drugs 3: 278 –283, 2002.
330. Urwyler S, Mosbacher J, Lingenhoehl K, Heid J, Hofstetter K,
Froestl W, Bettler B, and Kaupmann K. Positive allosteric modulation of native and recombinant gamma-aminobutyric acid-B receptors by 2,6-Di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol (CGP7930) and its aldehyde analog CGP13501. Mol Pharmacol
60: 963–971, 2001.
331. Urwyler S, Pozza MF, Lingenhoehl K, Mosbacher J, Lampert
C, Froestl W, Koller M, and Kaupmann K. GS39783 (N,N⬘Dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4,6-diamine) and
structurally related compounds: novel allosteric enhancers of
gamma-aminobutyric acid-B receptor function. J Pharmacol Exp
Ther 3: 3, 2003.
332. Vergnes M, Boehrer A, Simler S, Bernasconi R, and Marescaux C. Opposite effects of GABA-B receptor antagonists on absences and convulsive seizures. Eur J Pharmacol 332: 245–255,
1997.
333. Vernon E, Meyer G, Pickard L, Dev K, Molnar E, Collingridge
GL, and Henley JM. Gaba-B receptors couple directly to the
transcription factor ATF4. Mol Cell Neurosci 17: 637– 645, 2001.
334. Vogt KE and Nicoll RA. Glutamate and gamma-aminobutyric acid
mediate a heterosynaptic depression at mossy fiber synapses in the
hippocampus. Proc Natl Acad Sci USA 96: 1118 –1122, 1999.
335. Wachowiak M and Cohen LB. Presynaptic inhibition of primary
olfactory afferents mediated by different mechanisms in lobster
and turtle. J Neurosci 19: 8808 – 8817, 1999.
336. Waldmeier PC, Wicki P, Feldtrauer JJ, Mickel SJ, Bittiger H,
and Baumann PA. GABA and glutamate release affected by
GABA-B receptor antagonists with similar potency: no evidence for
pharmacologically different presynaptic receptors. Br J Pharmacol
113: 1515–1521, 1994.
337. Walker RJ, Brooks HL, and Holden-Dye L. Evolution and overview of classical transmitter molecules and their receptors. Parasitology 113: S3–S33, 1996.
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