British Journal of Anaesthesia 114 (3): 491–8 (2015)
Advance Access publication 23 August 2014 . doi:10.1093/bja/aeu269
TRANSLATIONAL RESEARCH
Propofol modulates phasic and tonic GABAergic currents
in spinal ventral horn interneurones
V. S. Eckle 1*, U. Rudolph 2, B. Antkowiak1 and C. Grasshoff 1
1
2
Experimental Anaesthesiology Section, Department of Anaesthesiology and Intensive Care, Eberhard-Karls-University, Tübingen, Germany
Laboratory of Genetic Neuropharmacology, McLean Hospital and Department of Psychiatry, Harvard Medical School, Belmont, MA, USA
* Corresponding author. E-mail: [email protected]
Editor’s key points
† The effect of propofol on
GABA(A) receptors in the
spinal ventral horn was
studied.
† Whole-cell recordings from
organotypic spinal cord slices
from mice were used.
† Propofol depressed ventral
horn interneurones by
inhibiting phasic GABA(A)
receptor responses.
† At high concentrations, there
was a shift from phasic to
tonic inhibition.
† This may reflect the
depression of nociceptive
reflexes at high propofol
concentrations.
Background. Surgical interventions like skin incisions trigger withdrawal reflexes which
require motor neurones and local circuit interneurones in the spinal ventral horn. This
region plays a key role in mediating immobilizing properties of the GABAergic
anaesthetic propofol. However, it is unclear how propofol modulates GABA(A) receptors
in the spinal ventral horn and whether tonic or phasic inhibition is involved.
Methods. Organotypic spinal cord tissue slices were prepared from mice. Whole-cell
recordings were performed for quantifying effects of propofol on GABA(A) receptormediated phasic transmission and tonic conductance.
Results. Propofol increased GABAergic phasic transmission by a prolongation of the decay
time constant in a concentration-dependent manner. The amount of the charge
transferred per inhibitory post-synaptic current, described by the area under the curve,
was significantly augmented by 1 mM propofol (P,0.01). A GABA(A) receptor-mediated
tonic current was not induced by 1 mM propofol but at a concentration of 5 mM (P,0.05).
Conclusions. Propofol depresses ventral horn interneurones predominantly by phasic
rather than by tonic GABA(A) receptor-mediated inhibition. However, the present results
suggest that the involvement of a tonic inhibition might contribute to the efficacy of
propofol to depress nociceptive reflexes at high concentrations of the anaesthetic.
Keywords: anaesthetics i.v., propofol; brain, anaesthesia, molecular effects; ions, ion
channels, pharmacology; pharmacology, propofol; spinal cord, GABA
Accepted for publication: 21 May 2014
Painful stimuli induced by intraoperative surgical interventions
like skin incisions activate dorsal horn neurones of the spinal
cord which synapse onto motoneurones directly or indirectly
via interneurones.1 These interneurones are involved in the
generation of locomotor patterns which coordinate movements and nociceptive reflexes.2 The i.v. anaesthetic propofol
has been found to depress nociceptive reflexes via actions on
spinal interneurones in the ventral horn, but in contrast did
not depress spinal dorsal horn neurones.3 In the same study,
Kungys and colleagues could demonstrate that the depression
of spinal ventral horn interneurones was mediated by GABA(A)
receptors. The results of a previous in vitro study using organotypic cultured slices corroborate the hypothesis that propofol
effects in the spinal ventral horn are predominantly mediated
via a modulation of GABA(A) receptors.4
In the central nervous system, there are two distinct modes
of GABAergic transmission, namely phasic and tonic activation of GABA(A) receptors.5 Phasic transmission is triggered
by GABA release from presynaptic terminals into the synaptic cleft. This fast activation of post-synaptic GABA(A) receptors takes place at synaptic sites and produces inhibitory
post-synaptic currents (IPSCs), whereas an activation of
extrasynaptic-located GABA(A) receptors induces tonic inhibitory currents.5 6
Although the modulation of tonic GABA(A) receptormediated inhibition by general anaesthetics plays a significant
role in anaesthetic-induced hypnosis and amnesia,7 8 it is
unknown whether tonic GABAergic currents are involved in
the depression of nociceptive reflexes. In order to address
this question, the effects of propofol on spinal ventral horn
interneurones were analysed in the present study. In this
context, changes of both spontaneous IPSCs and tonic currents
were measured at clinically relevant concentrations of the anaesthetic. Furthermore, changes in spontaneous action potential activity were quantified at an interneuronal network level
in cultured organotypic slices of the spinal cord.
& The Author 2014. Published by Oxford University Press on behalf of the British Journal of Anaesthesia. All rights reserved.
For Permissions, please email: [email protected]
BJA
Eckle et al.
Methods
Results
Organotypic spinal cultures
Propofol modulates phasic GABAergic currents
in spinal ventral horn interneurones at clinically
relevant concentrations
All procedures were performed in accordance with institutional and federal guidelines, including the German law on
animal experimentation, and were approved by the Animal
Care Committee (Eberhard-Karls-University, Tübingen, Germany)
and the Regierungspräsidium Tübingen and followed relevant
aspects of the ARRIVE guidelines. Organotypic slice cultures
for extracellular recordings were obtained from 129/Sv×129/
SvJ wild-type and homozygous b3 (N265M) knock-in mice on
the same genetic background. These knock-in mice contained
a point mutation at position 265, where asparagine was substituted by methionine.9 Spinal slice cultures for whole-cell
patch-clamp recordings were prepared from C57BL6J mice
(Charles River, Sulzfeld, Germany). For additional information, see Supplementary material.
Electrophysiology
Extracellular recordings and whole-cell patch-clamp experiments were performed as described previously.10 For additional information, see Supplementary material.
Data analysis
The data were analysed with an in-house software written in
OriginPro version 7 (OriginLab Corp., Northampton, MA, USA)
and MATLAB version 7.7 (The MathWorks Inc., Natick, MA,
USA). IPSC decays were fitted with a mono-exponential function. The area under the curve (AUC) was derived from the respective decay times and IPSC amplitude values. The data
analysis of extracellular recordings was performed as
described previously.4 After close inspection of the raw data,
action potentials were detected by setting a threshold well
above baseline noise. The mean firing rate was obtained
from single or multiunit activity; it was defined as the
number of detected action potentials divided by the recording
time of 180 s. All data sets were tested for normal distribution
by the Kolmogorov–Smirnov test. Parametric data were analysed by a two-tailed Student’s t-test, or analysis of variance
followed by a Bonferroni post hoc test. For non-parametric
data, a Mann –Whitney test was performed (*P,0.05,
**P,0.01, ***P,0.001). A P-value of ≥0.05 was defined as nonsignificant (n.s.). Parametric data are presented as mean (SD).
For non-parametric data, the median and inter-quartile
range is given. The inhibitory effect of propofol on network
firing activity was fitted best with Hill’s equation: y¼Bottom+
(Top2Bottom/1+10(logEC50 – x)×Hill Slope), where the bottom
was defined as the Y value at the bottom plateau and the top
was set as the Y value at the top plateau. The concentration
that produces 50% of maximal inhibition (logEC50) was
reported as the IC50. The goodness of the fit was quantified
by R 2 (GraphPad Prism software).
492
Phasic GABAergic transmission takes place at synaptic sites.
The neurotransmitter GABA activates post-synaptic GABA(A)
receptors. The subsequent opening of chloride channels
allows chloride influx and hyperpolarizes the respective postsynaptic neurones.5 This conductance can be quantified as
IPSCs by a whole-cell patch-clamp technique as displayed in
Figure 1. For this purpose, commissural interneurones were
visually identified in lamina VIII of the ventral horn and
voltage clamped at 270 mV as previously reported.11 The
mean capacitance of these cells was 32.7 (4.3) pF (n¼11).
The effects of propofol on spontaneous IPSCs were studied at
two different anaesthetic concentrations. At 500 nM, propofol provides, on a behavioural level, hypnosis, whereas at 1 mM,
nociceptive reflexes are blocked.12 The two concentrations are
within the clinically relevant range of the anaesthetic. Cumulative data revealed that propofol prolonged GABAergic IPSC
decay times in a concentration-dependent manner (Fig. 2A).
Under control conditions, the decay time was 25.2 (5.6) ms
(n¼13), while at 500 nM propofol, the decay time increased
about 23.5 (7.5%) [absolute value 29.2 (6.2) ms, n¼5, P,0.01,
t-test], and prolonged at 1 mM propofol about 62.8 (16.2)% compared with control [absolute value 43.2 (13.8) ms, n¼7, P,0.01].
The average of the analysed experiments showed that the amplitude of GABAergic IPSCs was not significantly changed by application of propofol (Fig. 2B). Under control conditions, the
IPSC amplitudes were 57 (6.4) pA (n¼12).
In a following step, we investigated how propofol alters the
efficacy of GABAergic synaptic transfer. An increase in synaptic
efficacy is indicated by an increase in the amount of total
charge transferred during the course of an averaged synaptic
event (AUC). Synaptic efficacy can be enhanced by an increase
in the amplitude, by prolongation of the decay time of inhibitory events, or both. Since IPSC amplitudes were basically not
altered and IPSC decay times were well fitted with monoexponentials, the prolongation of decay times was translated
into an increase in the total amount of charge transferred
per IPSC (P,0.01, Fig. 2C). Absolute values were 1.41 (0.6)
pC under control conditions (n¼12), 1.66 (0.6) pC (n¼5) for
500 nM propofol, and 2.1 (0.6) pC (n¼7) for 1 mM propofol. Additionally, propofol reduced the IPSC event rate at both tested
concentrations (Fig. 2D – F). This effect reached significance
level at 1 mM [frequency reduction about 53.1 (21.5)%, n¼7,
P,0.01, t-test]. Absolute values were 4.2 (3.8) Hz for 500 nM
propofol [5.53 (3.7) Hz for the respective control] and 3.3 (2.8)
Hz for 1 mM propofol [6.7 (4.4) Hz for the respective control].
Tonic GABAergic inhibition by propofol in spinal
ventral horn interneurons
Ambient GABA could activate extrasynaptic GABA(A) receptors
and thereby induce tonic GABAergic currents.5 Tonic inhibition
BJA
Propofol and GABAergic inhibition in the ventral horn
A
B
Control
Control
1 µM propofol
10 pA
50 ms
C
Cumulative probability (%)
1 µM propofol
100
50
Control
1 µM Propofol
0
0
20
40
60
80
100
Decay time (ms)
D
+Bicuculline
1s
Cumulative probability (%)
200 pA
100
50
Control
1 µM Propofol
0
0
50
100
150
200
250
Amplitude (pA)
Fig 1 (A) Original whole-cell patch-clamp recording from one cell in organotypic spinal culture showing spontaneous GABAergic IPSCs. After establishing the control condition (upper traces), 1 mM propofol was applied (middle traces). Bicuculline abolished IPSC events completely (lower traces).
(B) Averaged IPSC events from traces in (A). (C) Cumulative probability plot of IPSC decay values resulting from shown representative experiment.
(D) Cumulative distribution of IPSC amplitudes from the same recording. Blue, control; green, propofol.
has been shown to regulate the inhibitory tone in several
regions of the central nervous system.13 – 15 Thus, propofol
might induce a tonic current by activating extrasynaptic
GABA(A) receptors, thereby attenuating action potential activity of ventral horn interneurones.4 The modulation of a
tonic conductance can be quantified as a shift in the
holding current of the corresponding neurone.10 In recordings from whole-cell patch-clamped spinal interneurones,
holding currents remained unchanged by application of 1
mM propofol as demonstrated exemplarily in Figure 3A and
B. Subsequent application of the selective GABA(A) receptor
antagonist bicuculline (100 mM) did not basically alter the
holding current of the tested neurones (Fig. 3E). In order to estimate the impact of propofol on extrasynaptic GABAergic
currents, we investigated in a further step a higher propofol
concentration. Remarkably, at a high concentration (5 mM),
propofol significantly induced GABAergic tonic currents
(Fig. 3C – E; n¼5, P,0.05).
493
BJA
Eckle et al.
40
**
20
0
5
7
500 nM
1 µM
D
20
5
7
0
–20
–40
500 nM
1 µM
E
Cumulative probability (%)
Control
1 µM Propofol
Change in AUC (%)
60
40
100
**
80
60
40
20
0
5
7
500 nM
1 µM
F
100
100 pA
50
Control
1 µM Propofol
0
0
100 200 300 400 500
Inter-event interval (ms)
Change in frequency (%)
**
80
C
Change in amplitude (%)
B
Change in decay (%)
A
0
5
7
500 nM
1 µM
–20
–40
–60
–80
–100
**
1s
Fig 2 (A) Summary of the change in decay time with respect to control conditions. (B) Pooled data of amplitude of GABAergic IPSCs. (C) AUC derived
from the decay time and amplitude values. (D) Representative traces from the same recording in Figure 1 and the counted IPSC events for each
condition (marked as pink bars above the respective trace). (E) Cumulative distribution of IPSC frequency from one representative experiment
(same cell as in D). (F) IPSC frequency. Number of experiments shown as insets.
Inhibitory effects of propofol on ventral horn
interneurones are predominantly mediated by
b3-containing GABA(A) receptors
In an in vivo study, Kungys and colleagues3 demonstrated
that the depression of spinal ventral horn interneurones is predominantly mediated by GABA(A) receptors. Other authors
reported earlier that propofol-mediated suppression of movements in response to noxious stimuli was completely abolished
in mice harbouring a point mutation (N265M) in the second
transmembrane region of the b3 subunit of GABA(A) receptors.9
This leads to the conclusion that propofol effects are mediated by GABA(A) receptors containing a b3 subunit. In order
to test this hypothesis, we compared the effects of propofol
on spinal network activity in cultured organotypic slices
from wild-type and b3 (N265M) knock-in mice. Spontaneous
action potential activity was measured by extracellular recordings in the ventral horn area (Fig. 4). Concentration –response
curves were fitted by a Hill’s equation (R 2 0.77 for wild type
and 0.42 for b3 mutant). As previously demonstrated in rats,4
full depression of spontaneous activity could not be achieved
at the tested concentrations. The maximum depression
of spike firing rate (Vmax) reached 77.6 (16.8)% (n¼10) in wildtype mice, whereas the Vmax was 36.8 (26.5%) (n¼7) at 5 mM
propofol in the mutant mice. Hill slope values were 0.68
(0.12) and 0.59 (0.4) for wild type and mutant, respectively.
In wild-type mice, the IC50 (SD) was close to the clinically
494
relevant range [1.66 (0.17) mM, n¼41], while it was clearly
beyond at 3.1 (0.9) mM (n¼38) in b3 (N265M) knock-in mice.
Statistical analysis revealed that propofol altered the firing
activity in wild-type mice at the tested concentrations compared with normalized control condition, while in b3 (N265M)
knock-in mice, only the highest propofol concentration (5 mM)
had a significant effect. This observation supports the notion
that the depression of spinal neurones is predominantly
mediated by GABA(A) receptors containing a b3 subunit. At
higher concentrations, propofol slightly diminished network activity in mutant mice (Fig. 4C), an effect that might be mediated
by tonic GABAergic currents.
Discussion
I.V. anaesthetics like propofol exert their anaesthetic actions
by modulating inhibitory neurotransmission via GABA(A)
receptors in different regions of the central nervous system.16
In the present study, the specific modulation of GABAergic
transmission by propofol was investigated in the spinal
ventral horn.3 17 The major findings of the present work indicate that propofol modulates both phasic GABA(A) receptormediated currents and tonic GABAergic currents, but the
latter at higher concentrations of the anaesthetic. The
plasma concentration of propofol causing surgical immobility
in 50% of the patients has been estimated to be between 10
and 15.2 mg ml21,12 18 which corresponds to 1–1.5 mM.19 20
BJA
Propofol and GABAergic inhibition in the ventral horn
A
1 µM Propofol + TTX
125 pA
20 s
+ Bicuculline
B
+ Bicuculline
C
All points count
20 000
15 000
5 µM PRO
10 000
5000
50 pA
0
–300
–250
–200
–150
Baseline (pA)
–50
20 s
Bicuculline
1 µM PRO
D
–100
E
*
80 000
Δ Holding current (pA)
All points count
50
60 000
40 000
20 000
0
–250
–200
–150
–100
–50
–50
–100
–150
0
1 µM (n=6)
Baseline (pA)
5 µM PRO
0
5 µM (n=5)
Bicuculline
Fig 3 (A) The effect of propofol on tonic GABAergic current (representative trace). Propofol (PRO, 1 mM) was applied with the selective glycine receptor antagonist strychnine plus tetrodotoxin (TTX). After 12 min, bicuculline was washed in. (B) All points count histogram of the same experiment
in (A). (C) The effect of 5 mM propofol (representative trace). (D) All points count histogram of the same experiment as in (C). (E) Pooled data showing
effects of propofol on induction of tonic GABA(A) current.
In contrast to the moderate propofol concentrations applied in
our experiments, Bieda and MacIver8 performed experiments
in acute adult rat brain slices using concentrations of 5, 10,
and 30 mM. They justified their choice for higher concentrations
by a high diffusional/binding barrier for entry of compounds in
acute brain slices which they assumed to be particularly
495
BJA
Eckle et al.
A
B
Control (wild type)
Control (b3 mutant)
1 µM Propofol (b3 mutant)
1 µM Propofol (wild type)
125 µV
100 µV
2s
2s
C
100
Depression of spike rate (%)
***
***
75
***
***
50
***
***
25
wild type
b3 mutant
0
1
2
3
4
5
Propofol (µM)
Fig 4 (A) Original traces from one representative extracellular recording showing spontaneous action potential firing under control conditions
(upper trace) and after application of 1 mM propofol (lower trace) in the spinal ventral horn from wild-type mice. (B) Representative experiment
from b3 (N265M) knock-in mice (b3 mutant), showing spontaneous action potential firing activity in control condition (upper trace) and propofol
(lower trace). (C) Extracellular recordings of spontaneous action potential firing in the spinal ventral horn [blue line, filled circles , wild type; green line,
open circles, b3 (N265M) knock-in mutant ]. The clinically relevant range is displayed as a pink bar. Each data point (mean and SD) is from seven to 10
experiments.
important for propofol with its strong protein binding and high
lipid solubility.21 In conclusion, the authors assumed the actual
free effective site concentrations of propofol in their experiments to be much lower than the applied concentrations.8 In
contrast to acute brain slices, organotypic cultures have the advantage of reduced drug diffusion times up to minutes,22 since
they flatten in vitro to a quasi-monolayer, thereby reducing the
thickness from 300 mm at the time of preparation to 50 mm
496
after cultivation.23 Thus, we assume that the concentrationrange of propofol in the current study covers clinically relevant
concentrations of the anaesthetic.
Experiments in brainstem neurones of the nucleus of the
solitary tract showed that GABA(A) receptor-mediated tonic
currents were less sensitive to propofol than phasic GABA(A)
receptor-mediated currents.24 25 In the present work, similar
results were found in spinal ventral horn neurones. In contrast
BJA
Propofol and GABAergic inhibition in the ventral horn
to our findings and to the results obtained from the brain stem,
tonic currents were found to be more sensitive to propofol
than phasic IPSCs in hippocampal and thalamocortical neurones.7 26 However, in hippocampal pyramidal and interneurones, propofol produced a 20-fold increase in GABA phasic
currents compared with only a three-fold increase in tonic currents.8 Our results suggest that propofol modulates synaptic
and extrasynaptic GABA(A) receptors on spinal ventral horn
interneurones but with a distinctly different sensitivity, that
is, synaptic receptors at moderate concentrations and extrasynaptic receptors at high concentrations. This observation most
likely reflects different subunit compositions compared with
thalamocortical or hippocampal neurones. Multiple subunits
exist, which form GABA(A) receptors with different biophysical
characteristics and functions. In particular, receptor subtypes
containing a1–3bg2 subunits mediate largely phasic inhibition,
whereas those assembled from a4–6bd subunits are predominantly responsible for a tonic inhibition.27 GABA(A) receptormediated tonic currents in the hippocampus are modulated
by anaesthetics like midazolam and propofol or thiopental and
isoflurane.7 28 Results from hippocampal pyramidal neurones
demonstrated that tonic inhibition is mediated by a5 subunitcontaining GABA(A) receptors and plays a key role in cognitive
processes thereby linking amnesia, an important quality of
general anaesthesia, to a subgroup of GABA(A) receptors.29
Kretschmannova and colleagues reported an accumulation of
the a4 subunit-containing GABA(A) receptors in the thalamus
and dentate gyrus of female Y365/7Fmice. These mice exhibited
a gender-specific enhancement of tonic inhibition associated
with a dramatic increase in etomidate- and propofol-mediated
hypnosis.30
Focusing on GABAergic IPSCs, propofol caused a decline in
the frequency of events. This observation is consistent with previous findings in spinal ventral horn interneurones from experiments with sevoflurane and etomidate.11 31 Experiments
comparing the effects of etomidate on spontaneous and miniature IPSCs demonstrated that etomidate reduced the spontaneous IPSC frequency without altering the frequency of
miniature IPSCs.31 As etomidate acts via a modulation of
GABA(A) receptors, the results suggest that presynaptic
GABA(A) receptors are involved in the control of GABA release
in the spinal ventral horn.9 31 A similar, concentrationdependent decline in IPSC frequency was observed with propofol in the current study supporting the idea that propofol
hyperpolarizes presynaptic GABAergic interneurones and
thereby reduces the release of GABA. However, such a reduction in spontaneous IPSCs depends on the respective brain
region under review since propofol does not alter the frequencies of GABAergic spontaneous IPSCs in second-order neurones
of the solitary tract nucleus.25 But does the concentrationdependent reduction in spontaneous GABAergic IPSCs by
propofol in spinal ventral horn interneurones impair the
ability of the anaesthetic to depress interneuronal network
activity? Anaesthetic-induced depression of GABA release,
caused by a presynaptic mechanism of action, is expected to
increase the excitability of these cells. In contrast, anaesthetic
actions mediated via post-synaptic GABA(A) receptors
decrease the excitability of ventral horn neurones. Thus, preand post-synaptic actions of propofol affect neurones in the
ventral horn in opposing ways, which means that the reduction
in GABAergic IPSC frequency counteracts the hyperpolarization
by a post-synaptic GABA(A) receptor-mediated chloride influx.
On a network level, these opposing effects translate into a
limited efficacy of propofol to depress action potential activity
in the spinal ventral horn as demonstrated for organotypic
cultures obtained from mice in the current study and for rats
in previous experiments.4 Similar findings were also observed
in decerebrated rats, where propofol depressed ventral horn
neurones to a limited percentage of control values.3 Considering that anaesthetic actions in the spinal ventral horn determine the efficacy of an anaesthetic in depressing nociceptive
reflexes, it can be assumed that propofol anaesthesia is less
capable of preventing spontaneous movements. Although
experimental conditions in vitro are different from the depression of nociceptive reflexes in vivo, our results reflect closely
those obtained from studies in humans, where i.v. GABAergic
anaesthetics were far less effective in depressing involuntary
movements caused by painful stimuli compared with volatile
anaesthetics.32 – 34 However, in clinical study settings, immobility as a measurement for depression of pain-induced
movements can also be achieved using propofol at concentrations approximately five-fold higher than those required for
hypnosis.12 The current results suggest that a shift from
phasic to tonic inhibition occurs at increasing concentrations
of propofol and that a tonic inhibition might contribute to an
enhanced efficacy of propofol to depress nociceptive reflexes.
Supplementary material
Supplementary material is available at British Journal of
Anaesthesia online.
Authors’ contributions
V.S.E. and C.G. conceived and performed the study, analysed
the data, and approved the final manuscript. U.R. and B.A.
helped to perform the study and approved the final
manuscript.
Acknowledgements
The authors thank Claudia Holt and Helga Garcı́a for excellent
technical assistance, and Harald Hentschke for providing analysis software tools.
Declaration of interest
None declared.
Funding
This study was supported by institutional funding.
References
1 Antognini JF, Carstens E. In vivo characterization of clinical anaesthesia and its components. Br J Anaesth 2002; 89: 156–66
497
BJA
2 Kiehn O. Locomotor circuits in the mammalian spinal cord. Annu
Rev Neurosci 2006; 29: 279–306
3 Kungys G, Kim J, Jinks SL, Atherley RJ, Antognini JF. Propofol produces immobility via action in the ventral horn of the spinal cord
by a GABAergic mechanism. Anesth Analg 2009; 108: 1531–7
4 Grasshoff C, Antkowiak B. Propofol and sevoflurane depress spinal
neurons in vitro via different molecular targets. Anesthesiology
2004; 101: 1167– 76
5 Farrant M, Nusser Z. Variations on an inhibitory theme: phasic and
tonic activation of GABA(A) receptors. Nat Rev Neurosci 2005; 6:
215– 29
6 Glykys J, Mody I. The main source of ambient GABA responsible for
tonic inhibition in the mouse hippocampus. J Physiol 2007; 582:
1163– 78
7 Bai D, Zhu G, Pennefather P, Jackson MF, MacDonald JF, Orser BA.
Distinct functional and pharmacological properties of tonic and
quantal inhibitory postsynaptic currents mediated by gammaaminobutyric acid(A) receptors in hippocampal neurons. Mol
Pharmacol 2001; 59: 814–24
8 Bieda MC, MacIver MB. Major role for tonic GABAA conductances
in anesthetic suppression of intrinsic neuronal excitability. J Neurophysiol 2004; 92: 1658–67
9 Jurd R, Arras M, Lambert S, et al. General anesthetic actions in vivo
strongly attenuated by a point mutation in the GABA(A) receptor
beta3 subunit. FASEB J 2003; 17: 250– 2
10 Eckle V-S, Antkowiak B. ALX 1393 inhibits spontaneous network
activity by inducing glycinergic tonic currents in the spinal ventral
horn. Neuroscience 2013; 253: 165–71
11 Eckle V-S, Hauser S, Drexler B, Antkowiak B, Grasshoff C. Opposing
actions of sevoflurane on GABAergic and glycinergic synaptic
inhibition in the spinal ventral horn. PLoS One 2013; 8: e60286
12 Smith C, McEwan AI, Jhaveri R, et al. The interaction of fentanyl on
the Cp50 of propofol for loss of consciousness and skin incision.
Anesthesiology 1994; 81: 820– 8
13 Mody I, Pearce RA. Diversity of inhibitory neurotransmission
through GABAA receptors. Trends Neurosci 2004; 27: 569–75
14 Muller E, Le-Corronc H, Legendre P. Extrasynaptic and postsynaptic
receptors in glycinergic and GABAergic neurotransmission: a division of labor? Front Mol Neurosci 2008; 1: 3
15 Takazawa T, MacDermott AB. Glycinergic and GABAergic tonic inhibition fine tune inhibitory control in regionally distinct subpopulations of dorsal horn neurons. J Physiol 2010; 588: 2571–87
16 Rudolph U, Antkowiak B. Molecular and neuronal substrates for
general anaesthetics. Nat Rev Neurosci 2004; 5: 709–20
17 Jinks SL, Bravo M, Hayes SG. Volatile anesthetic effects on midbrainelicited locomotion suggest that the locomotor network in the
ventral spinal cord is the primary site for immobility. Anesthesiology
2008; 108: 1016– 24
18 Kazama T, Ikeda K, Morita K. Reduction by fentanyl of the Cp50
values of propofol and hemodynamic responses to various noxious
stimuli. Anesthesiology 1997; 87: 213–27
Eckle et al.
19 Franks NP, Lieb WR. Which molecular targets are most relevant to
general anaesthesia? Toxicol Lett 1998; 100 –101: 1 –8
20 Rehberg B, Duch DS. Suppression of central nervous system sodium
channels by propofol. Anesthesiology 1999; 91: 512–20
21 Gredell JA, Turnquist PA, MacIver MB, Pearce RA. Determination of
diffusion and partition coefficients of propofol in rat brain tissue:
implications for studies of drug action in vitro. Br J Anaesth 2004;
93: 810– 7
22 Antkowiak B. Different actions of general anesthetics on the firing
patterns of neocortical neurons mediated by the GABA(A) receptor.
Anesthesiology 1999; 91: 500– 11
23 Drexler B, Hentschke H, Antkowiak B, Grasshoff C. Organotypic cultures as tools for testing neuroactive drugs—link between in-vitro
and in-vivo experiments. Curr Med Chem 2010; 17: 4538– 50
24 Jin Y-H, Zhang Z, Mendelowitz D, Andresen MC. Presynaptic actions
of propofol enhance inhibitory synaptic transmission in isolated
solitary tract nucleus neurons. Brain Res 2009; 1286: 75– 83
25 McDougall SJ, Bailey TW, Mendelowitz D, Andresen MC. Propofol
enhances both tonic and phasic inhibitory currents in second-order
neurons of the solitary tract nucleus (NTS). Neuropharmacology
2008; 54: 552–63
26 Belelli D, Peden DR, Rosahl TW, Wafford KA, Lambert JJ. Extrasynaptic GABAA receptors of thalamocortical neurons: a molecular target
for hypnotics. J Neurosci 2005; 25: 11513–20
27 Olsen RW, Sieghart W. International union of pharmacology.
LXX. subtypes of g-Aminobutyric AcidA receptors: classification
on the basis of subunit composition, pharmacology, and function.
Update. Pharmacol Rev 2008; 60: 243–60
28 Bieda MC, Su H, MacIver MB. Anesthetics discriminate between
tonic and phasic gamma-aminobutyric acid receptors on hippocampal CA1 neurons. Anesth Analg 2009; 108: 484– 90
29 Caraiscos VB, Elliott EM, You-Ten KE, et al. Tonic inhibition in mouse
hippocampal CA1 pyramidal neurons is mediated by a5 subunitcontaining g-aminobutyric acid type A receptors. Proc Natl Acad
Sci USA 2004; 101: 3662– 7
30 Kretschmannova K, Hines R, Revilla-Sanchez R, et al. Enhanced
tonic inhibition influences the hypnotic and amnestic actions of
the intravenous anesthetics etomidate and propofol. J Neurosci
2013; 33: 7264– 73
31 Grasshoff C, Jurd R, Rudolph U, Antkowiak B. Modulation of presynaptic {beta}3-containing GABAA receptors limits the immobilizing
actions of GABAergic anesthetics. Mol Pharmacol 2007; 72: 780–7
32 Ashworth J, Smith I. Comparison of desflurane with isoflurane or
propofol in spontaneously breathing ambulatory patients. Anesth
Analg 1998; 87: 312– 8
33 Smith I, Thwaites AJ. Target-controlled propofol vs. sevoflurane: a
double-blind, randomised comparison in day-case anaesthesia.
Anaesthesia 1999; 54: 745– 52
34 Watson KR, Shah MV. Clinical comparison of ‘single agent’ anaesthesia with sevoflurane versus target controlled infusion of propofol. Br J Anaesth 2000; 85: 541– 6
Handling editor: H. F. Galley
498