Division of Pharmacology and Toxicology
Faculty of Pharmacy
University of Helsinki
Finland
New insights to the brain functions of prolyl oligopeptidase
IIDA PELTONEN
ACADEMIC DISSERTATION
To be presented, with the permission of the Faculty of Pharmacy at the University of
Helsinki, for public examination at Viikki Biocentre, Auditorium 1041, on November 2nd 2012
at 12 noon.
Supervisors:
Professor Pekka T. Männistö, MD, PhD
Division of Pharmacology and Toxicology
Faculty of Pharmacy
University of Helsinki
Finland
Docent J. Arturo García-Horsman, PhD
Division of Pharmacology and Toxicology
Faculty of Pharmacy
University of Helsinki
Finland
Reviewers:
Docent Jouni Sirviö, PhD
Oy Sauloner Ltd
Kuopio
Finland
Professor Eero Vasar, MD, PhD
Department of Physiology
University of Tartu
Estonia
Opponent:
© Iida Peltonen 2012
ISBN 978-952-10-8288-7 (paperback)
ISBN 978-952-10-8289-4 (PDF)
ISSN 1799-7372
Unigrafia
Helsinki, Finland 2012
Associate Professor Markus Forsberg, PhD
School of Pharmacy
University of Eastern Finland
Finland
ABSTRACT
ABBREVIATIONS
LIST OF ORIGINAL PUBLICATIONS
1. INTRODUCTION .......................................................................................... 1
2. REVIEW OF THE LITERATURE ...................................................................... 3
2.1 Basic characteristics of neuropeptides ..................................................................... 3
2.1.1 Neuropeptide families and their distribution and expression in the CNS .................. 4
2.1.2 Coexistence of neuropeptides with other messengers .............................................. 4
2.2 Neuropeptides - from synthesis to signalling ........................................................... 6
2.2.1 Biosynthesis, storage and release ............................................................................... 6
2.2.2 Neuropeptide receptors.............................................................................................. 9
2.2.3 Neuromodulatory functions in the CNS ...................................................................... 9
2.2.4 Neuropeptide-processing enzymes ............................................................................ 9
2.3 Common brain neuropeptides that are also putative PREP substrates .................... 10
2.3.1 AVP ............................................................................................................................ 11
2.3.2 Oxytocin .................................................................................................................... 11
2.3.3 Neurotensin............................................................................................................... 12
2.3.4 Somatostatin ............................................................................................................. 14
2.3.5 SP ............................................................................................................................... 15
2.4 The effects of neuropeptides: focus on behaviour .................................................. 16
2.4.1 Motor behaviour ....................................................................................................... 17
2.4.2 Learning and memory ............................................................................................... 18
2.4.3 Affective behaviours ................................................................................................. 20
2.4.4 Other behavioural effects ......................................................................................... 22
2.5 The importance of PREP in neuropeptide metabolism ............................................ 24
2.6 Concluding remarks of the literature review .......................................................... 25
3. AIMS OF THE STUDY ................................................................................. 26
4. MATERIALS AND METHODS ..................................................................... 27
4.1 Animals ................................................................................................................. 27
4.2 Drugs .................................................................................................................... 27
4.3 Stereotaxic surgery (II, III) ...................................................................................... 28
4.3.1 Administration of neurotoxins (II) ............................................................................. 28
4.3.2 Intracerebral injections of NTS1 agonist and antagonist (III) ................................... 29
4.4 Intraperitoneal injections of NTS1 agonist and antagonist (unpublished)................ 29
4.5 The ex vivo study with a single dose of thioridazine (I) ........................................... 30
4.6 Behavioural testing methods (IV)........................................................................... 30
4.6.1 Eight-arm radial maze (IV)......................................................................................... 30
4.6.2 Locomotor activity (IV) .............................................................................................. 31
4.6.3 Rotational behaviour (unpublished) ......................................................................... 31
4.7 Analytical procedures ............................................................................................ 32
4.7.1 Tissue preparation (I, II) ............................................................................................ 32
4.7.2 PREP activity assay (I, II) ............................................................................................ 32
4.7.3 Validation of PREP activity measurements (unpublished) ........................................ 32
4.7.4 Kinetic measurements (I) .......................................................................................... 33
4.7.5 HPLC analysis of dopamine, DOPAC and HVA in tissue samples (III, rotational
behaviour) .......................................................................................................................... 33
4.7.6 Immunofluorescence (II, III) ...................................................................................... 33
4.7.7 Laser scanning microscopy (II, III) ............................................................................. 34
4.8 Statistical analysis ................................................................................................. 34
4.8.1 Radial-arm maze (IV) ................................................................................................. 34
4.8.2 Locomotor activity (IV) .............................................................................................. 34
4.8.3 Enzyme activity assays (I) .......................................................................................... 34
4.8.4 Intracerebral injections in neurotensin study (III) .................................................... 34
5. RESULTS ................................................................................................... 36
5.1 PREP activity measurements (I, II).......................................................................... 36
5.1.1 Validation of PREP activity measurements (unpublished) ........................................ 36
5.1.2 The effects of psychoactive compounds on the activity of PREP (I) ......................... 37
5.1.3 Kinetic analysis of ketanserin, thioridazine and VPA (I) ............................................ 38
5.1.4 PREP activity after a single dose of thioridazine in the mouse brain (I) ................... 38
5.1.5 PREP activity in projection areas after the lesions of selected neurotransmitter
nuclei (II) ............................................................................................................................. 38
5.2 The expression of immunoreactive PREP (II, III) ..................................................... 40
5.2.1 The expression of immunoreactive PREP after neurotransmitter lesions (II) .......... 40
5.2.2 Colocalization of PREP and neurotensin or NTS1 in the striatum, the NAcc, the SN
and the VTA (III).................................................................................................................. 40
5.3 Effects of PREP inhibitor, NTS1 agonist and antagonist on the levels of dopamine and
its metabolites in the striatum and NAcc (III) ............................................................... 41
5.3.1 Injection of NTS1 agonist and antagonist to the striatum (III) ................................. 41
5.3.2 Injection of NTS1 agonist and antagonist to the nucleus accumbens (III)................ 42
5.3.3 Intraperitoneal injections of NTS1 agonist and antagonist (unpublished) ............... 43
5.4 The effects of PREP inhibition in behavioural models of CNS diseases (IV,
unpublished)............................................................................................................... 44
5.4.1 The effect of PREP inhibitor KYP-2047 on the memory and learning of young and
old rats in eight-arm radial maze (IV)................................................................................. 44
5.4.2 Locomotor activity after a single administration of PREP inhibitor KYP-2047 (IV) ... 44
5.4.3 The effect of PREP inhibitor KYP-2047 on the rotational behaviour after an
unilateral 6-OHDA-lesion of MFB (unpublished) ............................................................... 46
6. DISCUSSION ............................................................................................. 47
6.1 PREP as a potential physiological target of psychopharmacological compounds ..... 47
6.2 The effects of selective neurotransmitter lesions on the activity and expression of
immunoreactive PREP protein ..................................................................................... 48
6.3 The colocalization and functional interaction of PREP with neurotensin and its
receptor in the nigrostriatal and mesolimbic dopaminergic pathways ......................... 49
6.4 The effect of PREP inhibition on motor functions, memory and learning, and
rotational behaviour in rats......................................................................................... 52
6.5 General discussion ................................................................................................ 54
7. SUMMARY AND CONCLUSIONS ................................................................ 56
8. ACKNOWLEDGEMENTS ............................................................................ 58
9. REFERENCES ............................................................................................. 60
APPENDIX: Original publications I-IV
ABSTRACT
Prolyl oligopeptidase (PREP, also known as POP; EC 3.4.21.26) is an 80-kDa serine protease
hydrolyzing peptides smaller than 30-mer at the carboxyl side of an internal proline-residue.
PREP is widely distributed in the body and the cytosolic form is abundantly expressed also in
the brain. Several neuropeptides have been suggested to be hydrolyzed by PREP, such as
arginine-vasopressin (AVP), oxytocin and substance P (SP), through which PREP has been
connected to central nervous system (CNS) functions including learning, memory and mood.
However, the break-down of these peptides by PREP has not been conclusively
demonstrated in vivo. One of the major criticisms against the neuropeptide theory has been
the different locations of intracellular PREP and the mainly extracellular neuropeptides.
Although PREP has mostly been studied as a neuropeptide-cleaving enzyme, there are also
other connections between PREP and brain diseases, such as the altered levels of PREP in
the plasma of patients with CNS disorders and the location of PREP in neurons connected to
important brain functions. Recent studies show that PREP may have effects on intracellular
protein aggregation via protein-protein interactions.
The purpose of this study was to find novel actions and elucidate the relevance
of proposed physiological roles of PREP in the rodent brain using different neurochemical
and behavioural methods. Brain penetrating PREP inhibitors were used as tools in many of
the studies. First, we studied the PREP inhibition capabilities of common psychoactive drugs.
Since the pathophysiology of CNS diseases is not fully understood, we tested whether PREP
could be a physiological target for these compounds. Second, we examined the
colocalization and interaction of PREP with several neurotransmitter systems by lesioning
the major nuclei of these transmitters with neurotoxins and then studying the effects of the
lesions on PREP activity and immunoreactivity in the respective projection areas. Third, the
colocalization and functional interaction of PREP with neurotensin and its receptor NTS1
were studied in the midbrain dopaminergic pathways. Finally, the effect of PREP inhibition
on behaviour was tested in animal models of memory, locomotor activity and Parkinson’s
disease.
We found that psychoactive compounds did not considerably inhibit PREP at
concentrations achieved in the human therapy. Lesioning of the major neurotransmitter
nuclei did not affect PREP activity or immunoreactivity in the projection areas indicating that
PREP is not present in the long projection neurons or not connected to their functions. PREP
was highly colocalized with neurotensin and NTS1 in the ventral tegmental area (VTA) and
with NTS1 in the substantia nigra (SN). In addition, PREP inhibitor was shown to increase the
dopamine levels in the SN and VTA in a NTS1-dependent manner. We found also that PREP
inhibition increases locomotor activity in rats, but does not improve memory or affect
rotational behaviour in a rat model of Parkinson’s disease.
Conclusively, although the precise role of PREP in the brain remains unclear,
our studies substantiated the view that the major physiological role for PREP in the brain is
not the improvement of memory. Interestingly, our results suggested that PREP may be
involved in motor functions and in the neurotensin-mediated dopaminergic signalling in the
SN and VTA through intracellular mechanisms.
ABBREVIATIONS
ACh
ACTH
ANOVA
AVP
BNST
CCK
CD
CRH
CNS
CV
5,7-DHT
DOPAC
DSP-4
GABA
GPCR
HPLC
5-HT
HVA
i.c.
i.c.v.
i.p.
IP3
LD
LDCV
MFB
MPOA-AH
MPTP
mRNA
NAcc
NKA
NKB
NTS1
6-OHDA
OTR
PFC
PNS
POMC
PPI
PREP
RER
s.c.
SEM
SN
SP
SSV
Tβ4
TRH
VPA
VTA
Acetylcholine
Corticotropin
Analysis of variance
Arginine-vasopressin
Bed nucleus of the stria terminalis
Cholecystokinin
Carbidopa
Corticotropin-releasing hormone
Central nervous system
Coefficient of variation
5,7-Dihydroxytryptamine
3,4-Dihydroxyphenylacetic acid
N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride
Gamma-aminobutyric acid
G-protein-coupled receptor
High-performance liquid chromatography
5-Hydroxytryptamine (serotonin)
Homovanillic acid
Intracerebral
Intracerebroventricular
Intraperitoneal
Inositol-1,4,5-triphosphate
L-Dopa
Large dense core vesicle
Medial forebrain bundle
Medial preoptic-anterior hypothalamic area
1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine
Messenger ribonucleic acid
Nucleus accumbens
Neurokinin A
Neurokinin B
Neurotensin receptor 1
6-Hydroxydopamine
Oxytocin receptor
Prefrontal cortex
Peripheral nervous system
Pro-opiomelanocortin
Pre-pulse inhibition of the startle response
Prolyl oligopeptidase
Rough endoplasmic reticulum
Subcutaneous
Standard error of the mean
Substantia nigra
Substance P
Small synaptic vesicle
Thymosin beta-4
Thyrotropin-releasing hormone
Valproate
Ventral tegmental area
LIST OF ORIGINAL PUBLICATIONS
This dissertation is based on the following publications, herein referred by their Roman
numerals (I-IV):
I
Peltonen I, Männistö PT: Effects of diverse psychopharmacological substances
on the activity of brain prolyl oligopeptidase. Basic Clin Pharmacol Toxicol
108(1):46-54, 2011
II
Peltonen I, Myöhänen TT, Männistö PT: Association of prolyl oligopeptidase
with conventional neurotransmitters in the brain. CNS Neurol Disord Drug
Targets, 10(3):311-318, 2011
III
Peltonen I, Myöhänen TT, Männistö PT: Different interactions of prolyl
oligopeptidase and neurotensin in dopaminergic function of the rat
nigrostriatal and mesolimbic pathways. Neurochem Res, 37(9):2033-2041,
2012
IV
Peltonen I, Jalkanen AJ, Sinervä V, Puttonen KA, Männistö PT: Different
effects of scopolamine and inhibition of prolyl oligopeptidase on mnemonic
and motility functions of young and 8- to 9-month-old rats in the radial-arm
maze. Basic Clin Pharmacol Toxicol, 106(4):280-287, 2010
Reprints were made with the permission of the copyright holders.
1. INTRODUCTION
The term neuropeptide was first used by David de Wied in the early 1970’s to describe
peptides in the brain without any endocrine functions (Bohus and De Wied 1966; De Wied
1971). He noticed that the brain was sensitive to peptide hormones and their derivatives.
Eventually, his pioneering work led to the discovery of corticotropin (ACTH). Thereafter,
research on neuropeptides has been carried out for over 40 years. Hundreds of
neuropeptides have been found, and their functional significance has been intensively
studied (Hökfelt et al. 2000).
Currently, there is no effective treatment for the majority of central nervous
system (CNS) diseases, such as Alzheimer’s disease, Parkinson’s disease, schizophrenia,
depression and bipolar disorder (Malavolta and Cabral 2011). One goal of the neuropeptide
research has been the development of drugs for CNS diseases. However, there are several
problems related to peptidergic pharmacotherapy including in vivo stability, route of
administration, poor blood-brain barrier penetration and complexity of manufacture
(Malavolta and Cabral 2011). Therefore, several receptor-specific non-peptide ligands with
nanomolar affinity have been developed, which have helped to define functional roles for
neuropeptides (Hökfelt et al. 2003). Another approach to affect neuropeptidergic systems
has been the modulation of neuropeptide degradation by inhibiting neuropeptidehydrolyzing enzymes. With enzyme inhibitors, the stability and penetration problems of
neuropeptide receptor ligands may be avoided (Fricker 2005).
Prolyl oligopeptidase (PREP; EC 3.4.21.26; also known as prolyl endopeptidase,
PEP, POP or PO) was identified by Walter et al. 1971 from bovine uterus as an oxytocin
cleaving enzyme. Thereafter PREP has been connected to the hydrolysis of neuropeptides.
PREP is an 80 kDa serine protease belonging to the family S9 of the serine carboxypeptidase
clan (Rawlings and Barrett 1994). Its special feature is the hydrolysis of small (shorter than
30 amino acids) peptides containing proline, such as the neuropeptides arginine-vasopressin
(AVP), neurotensin, oxytocin and substance P (SP). PREP cleaves the -Pro-Xaa- bond at the
carboxyl side of a proline residue, where Xaa is any amino acid other than proline
(Cunningham and O'Connor 1997; Polgar 2002). PREP family is widely distributed in
organisms ranging from bacterial and archaeal species to mammals (Venäläinen et al. 2004).
PREP is found in most rat and human tissues, but the highest activities have been measured
in the brain (Kato et al. 1980; Irazusta et al. 2002; Myöhänen et al. 2009). PREP is considered
to be mainly an intracellular and cytosolic enzyme constitutively expressed in all cells.
However, a membrane-bound form has been described, too (O'Leary and O'Connor 1995;
García-Horsman et al. 2007; Tenorio-Laranga et al. 2008). PREP activity has also been
measured in plasma and cerebrospinal fluid suggesting the existence of an extracellular
form or secretion of the enzyme, which has not been proved (Brandt et al. 2007).
In the rat brain, high PREP immunoreactivities have been found in the cerebral
cortex, especially primary motor cortex and somatosensory cortex, the striatum,
hippocampus, lateral septum and cerebellum (Myöhänen et al. 2008b). High amounts of
PREP messenger ribonucleic acid (mRNA) have been found in the cerebellum and
hypothalamus (Bellemere et al. 2004). Highest PREP activity levels in the rat brain have been
1
measured from the cortex, striatum and cerebellum (Irazusta et al. 2002; Agirregoitia et al.
2005). Generally, PREP has been present in neurons using gamma-aminobutyric (GABA) and
acetylcholine (ACh) as neurotransmitters, but to a lesser extent in dopaminergic neurons
(Myöhänen et al. 2008b). Due to its location in the brain and suggested importance in
neuropeptide break-down, PREP has been connected to CNS diseases related to
neuropeptidergic malfunction (for review, see García-Horsman et al. 2007 and Männistö et
al. 2007).
PREP has mainly been studied as a memory-modulating enzyme, since many of
the putative substrate peptides, including AVP, thyrotropin-releasing hormone (TRH) and
SP, have been shown to possess memory-enhancing effects (Brandt et al. 2007). Several
PREP inhibitors have been developed with the purpose to prevent and treat memory
disorders by inhibiting the degradation of promnesic neuropeptides and thus elevating their
levels in the brain (Männistö et al. 2007). Despite the intensive research, the final
conclusions about the role of PREP in memory and learning and neuropeptide break-down
(reviewed by García-Horsman et al. 2007 and Männistö et al. 2007 are yet to be drawn.
A large amount of evidence about the role of PREP in CNS diseases comes from
studies about the levels of PREP activity in the plasma or serum of patients with psychiatric
or neurological disorders (Maes et al. 1994; Maes et al. 1995; Mantle et al. 1996; Maes et al.
1999; Maes et al. 2001), which correlated with the severity of the disease or with the drug
treatment. For example, Maes et al. 1994 and 1995 reported that PREP activity in plasma
was significantly lower in patients with major depression than in healthy volunteers.
Antidepressant fluoxetine and the mood stabilizer valproate (VPA) restored the altered
PREP activity levels to normal. Interestingly, VPA was also shown to inhibit PREP activity
(Cheng et al. 2005). In Parkinson patients, the activity of PREP (Mantle et al. 1996) and the
levels of a putative PREP substrate neurotensin (Fernandez et al. 1996) have been altered.
Active PREP has also been found to increase the aggregation of α-synuclein, the main
component in Lewy bodies, the hallmarks of Parkinson’s disease (Brandt et al. 2008;
Myöhänen et al. 2012).
Interestingly, novel roles for PREP independent of its hydrolytic activity have
been proposed, including the involvement in the inositol-1,4,5-trisphosphate (IP3) signalling
(Williams and Harwood 2000), in intracellular trafficking and protein secretion (Schulz et al.
2005), and neuronal growth via protein–protein interactions (Di Daniel et al. 2009).
The present study aimed to gain new insights about the functions of PREP in
the brain by studying PREP as a target of psychopharmaceutical compounds, the localization
of PREP in the projection neurons of major classical neurotransmitters, the interaction of
PREP with the neuropeptide neurotensin in the midbrain dopaminergic pathways and the
effects of PREP inhibition in animal models of behaviour. The literature part of the thesis will
focus on the characteristics and effects of putative PREP substrate peptides AVP,
neurotensin, oxytocin, somatostatin and SP on behaviour. As neuropeptides act as
neuromodulators in the CNS, their behavioural effects are complex and difficult to interpret.
However, since the physiological function of PREP and particularly its role in behaviour are
not known, it is important to review the conceivable behavioural effects of putative PREP
subtrates.
2
2. REVIEW OF THE LITERATURE
2.1 Basic characteristics of neuropeptides
Neuropeptides represent the largest and most diverse class of signalling molecules in the
CNS. Currently, hundreds of different neuropeptides are known and the discovery period
does not seem to be over yet (Hökfelt et al. 2000). With the growing number of known
peptides and small proteins affecting neurons, the exact definition to pinpoint the essence
of neuropeptides has lacked behind. Although there are common hallmarks for
neuropeptides, such as the biosynthesis by neurons, regulated release and the ability to
modulate or mediate neural functioning by acting on receptors, there is no exact definition
of the term neuropeptide (Burbach 2010). For example, neuropeptides can be defined as
proteineous substances produced by neural cells and acting on any neural or non-neural
substrate or target. That definition covers all neural secretory peptides and proteins of the
brain regardless of their size, such as growth factors, chemokines and peptide hormones of
the endocrine system (Burbach 2010). However, a more conservative definition of
neuropeptides takes into account also the size of the peptides.
Currently, neuropeptides are mostly considered to be typically composed of 340 amino-acid residues and they act as neurotransmitters or neuromodulators in the CNS
(Salio et al. 2006). Every neuropeptide has a unique amino acid sequence which determines
its biological functions (Hook et al. 2008). Although the effects of neuropeptides on target
neurons are diverse, they most commonly modify the membrane excitability (Salio et al.
2006). They can also regulate gene transcription, local blood flow, synaptogenesis and glial
cell architecture (Theodosis et al. 1986; Cauli et al. 2004; Landgraf and Neumann 2004).
After the 1970s, it has been understood that neurons are able to manufacture
and utilize more than a single molecule for signalling (Hökfelt et al. 2000). Neuropeptides
often coexist in neurons with classical small-molecule neurotransmitters such as glutamate,
catecholamines, ACh and 5-hydroxytryptamine (serotonin; 5-HT) (Hökfelt et al. 2000).
However, neuropeptides are in many ways different from them (Hökfelt et al. 2000). They
are about 50 times larger than the classical neurotransmitters giving them many more
recognition sites for receptors (Salio et al. 2006). Therefore neuropeptides bind to their
receptors with a higher affinity than the classical neurotransmitters and they can produce
biological effects even when released in low quantities. Neuropeptides are also more stable
in the extracellular space of the brain compared to classical neurotransmitters (e.g. the halflives for oxytocin and AVP are about 20 min) (Mens et al. 1983). Furthermore, neuropeptide
actions are not targeted or associated with particular synapses, since they can diffuse far
away from the releasing site. Pharmacokinetically, neuropeptides are between the classical
neurotransmitters and hormones in the neuron-to-neuron communication (Salio et al.
2006). The co-excistence of neuropeptides and other messengeres allows the neurons to
communicate fast and slowly, with neurons close by and far away.
3
2.1.1 Neuropeptide families and their distribution and expression in the CNS
Bioactive peptides are found in the whole animal kingdom, but also in plants (Hökfelt et al.
2000). Peptide families have ancient origins. Examples of neuropeptides and their families in
the CNS are listed in Table 2-1. The best studied neuropeptide family includes two
hypothalamic magnocellular neurosecretory peptides oxytocin and AVP (Hökfelt et al.
2000).
Intensive research has provided information about the distribution of
neuropeptides in the CNS. They can be found in all parts of the CNS and peripheral nervous
system (PNS), but all peptides have their own distribution patterns. Some peptides are
present at high levels under normal circumstances, whereas others are normally expressed
at very little or undetectable levels and require a specific stimulus, e.g. a nerve injury, for
their expression to increase (Hökfelt et al. 2000). There are also peptides expressed only in
early stages of development. In general, peptides can be expressed in one or several of the
above-mentioned manners, which can also be neuron type specific.
2.1.2 Coexistence of neuropeptides with other messengers
When neuropeptides were discovered, it was believed that peptidergic systems were
separate from the neurons containing classical neurotransmitters. Thereafter, it has been
understood that neurons are able to manufacture and utilize more than a single molecule
for signalling (Hökfelt 1991). Releasing a mixture of messenger molecules provides a neuron
the possibility to signal with variable velocities and functions (Kupfermann 1991). This
coexistence is now considered a common trait of all neurotransmission (Merighi 2002).
Neuropeptides can coexist with several other types of neurotransmitters, such
as other neuropeptides, classical neurotransmitters, the gaseous neurotransmitter nitric
oxide and certain growth factors (Salio et al. 2006). It is generally believed that when a
neuropeptide coexists with a classical neurotransmitter, the principal messenger molecule is
the latter one. Therefore neuropeptides are considered to be the modulators of the actions
of the classical neurotransmitters (Hökfelt et al. 2000). The coexistence is so common, that
it is more of an exception if there are no classical neurotransmitters present in the
neuropeptidergic neurons. This seems to be the case in the hypothalamic magnocellular
neurons, where oxytocin and AVP are the principal messengers. However, there is no rule
about the colocalization combinations of neurotransmitters and neuropeptides. Examples of
the coexistence of neuropeptides with classical neurotransmitters are given in Table 2-2.
4
Table 2-1. Examples of neuropeptides in the CNS, their families and amino acid sequences. The
sequences of the neuropeptides susceptible to hydrolysis by PREP in vitro are highlighted in bold.
Modified from Hökfelt et al. 2000 and Salio et al. 2006.
Neuropeptide
Amino acid sequence
Hypothalamic peptides
Arginine-vasopressin Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2
Oxytocin
Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2
Hypothalamic releasing and inhibiting peptides
CRH
Ser-Glu-Glu-Pro-Pro-Ile-Ser-Leu-Asp-Leu-Thr-Phe-His-Leu-Leu-Arg-GluVal-Val-Glu-Met-Ala-Arg-Ala-Glu-Gln-Leu-Ala-Gln-Gln-Ala-His-Ser-AsnArg-Lys-Leu-Met-Glu-Ile-Ile-NH2
LHRH
pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2
Somatostatin
Ser-Ala-Asn-Ser-Asn-Pro-Ala-Met-Ala-Pro-Arg-Glu-Arg-Lys-Ala-GlyCys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-Cys-NH2
TRH
pGlu-His-Pro-NH2
Neuropeptide Y and related peptides
Neuropeptide Y
Tyr-Pro-Ser-Lys-Pro-Asp-Asn-Pro-GIy-Glu-Asp-Ala-Pro-Ala-Glu-Asp-LeuAIa-Arg-Tyr-Tyr-Ser-Ala-Leu-Arg-His-Tyr-Ile-Asn-Leu-Ile-Thr-Arg-GInArg-Tyr-NH2
Opioid peptides
Dynorphin A
Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln
Met-enkephalin
Tyr-Gly-Gly-Phe-Met
Leu-enkephalin
Tyr-Gly-Gly-Phe-Leu
Tachykinins
Neurokinin A
His-Lys-Thr-Asp-Ser-Phe-Val-Gly-Leu-Met-NH2
Neurokinin B
Asp-Met-His-Asp-Phe-Phe-Val-Gly-Leu-Met-NH2
Substance P
Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2
VIP-glucagon family
VIP
His-Ser-Asp-Ala-Val-Phe-Thr-Asp-Asn-Tyr-Thr-Arg-Leu-Arg-Lys-Gln-MetAla-Val-Lys-Lys-Tyr-Leu-Asn-Ser-Ile-Leu-Asn-NH2
Other peptides
ACTH
Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-Gly-Lys-Lys-ArgArg-Pro-Val-Lys-Val-Tyr-Pro-Asn-Gly-Ala-Glu-Asp-Glu-Ser-Ala-Glu-AlaPhe-Pro-Leu-Glu-Phe
CCK
Asp-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-Phe-NH2
Corticostatin
Gly-Ile-Cys-Ala-Cys-Arg-Arg-Arg-Phe-Cys-Pro-Asn-Ser-Glu-Arg-Phe-SerGly-Tyr-Cys-Arg-Val-Asn-Gly-Ala-Arg-Tyr-Val-Arg-Cys-Cys-Ser-Arg-Arg
Endomorphin-1
Tyr-Pro-Trp-Phe-NH2
Endomorphin-2
Tyr-Pro-Phe-Phe-NH2
Galanin
Gly-Trp-Thr-Leu-Ann-Ser-Ala-Gly-Tyr-Leu-Leu-Gly-Pro-His-Ala-Ile-AspAsn-His-Arg-Ser-Phe-Ser-Asp-Lys-His-Gly-Leu-Thr-NH2
α-MSH
Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2
Orexin
Arg-Ser-Gly-Pro-Gly-Leu-Gln-Gly-Arg-Leu-Gln-Arg-Leu-Leu-Gln-AlaSer-Gly-Asn-His-Ala-Ala-Gly-Ile-Leu-Thr-Met-NH2
Neurotensin
pGlu-Leu-Tyr-Glu-Asn-Lys-Pro-Arg-Arg-Pro-Tyr-Ile-Leu
Secretin
H2N-His-Ser-Asp-Gly-Thr-Phe-Thr-Ser-Glu-Leu-Ser-Arg-Leu-Arg-Asp-SerAla-Arg-Leu-Gln-Arg-Leu-Leu-Gln-Gly-Leu-Val-CONH2
ACTH, corticotropin; CCK, cholecystokinin; CRH, corticotropin-releasing hormone; LHRH, luteinizinghormone-releasing hormone; α-MSH, α-melanocyte-stimulating hormone; TRH, thyrotropin-releasing
hormone; VIP, vasoactive intestinal polypeptide.
5
Table 2-2. Examples of the coexistence of neuropeptides with classical neurotransmitters in the CNS.
Modified from Binder et al. 2001 and Salio et al. 2006.
Neuropeptide
Classical neurotransmitter
CNS area
Substance P
5-HT
Medulla oblongata
Dorsal raphe nuclei
Medulla oblongata
Spinal cord
Hypothalamus
Retinal ganglion cells
Spinal cord
Noradrenaline
Aspartate
GABA
Glutamate
Somatostatin
GABA
Visual cortex
Entorhinal cortex
Suprachiasmatic nucleus
Amygdala
Hippocampus
Acetylcholine
Neurotensin
Dopamine
VTA
Substantia nigra
GABA
NAcc shell
NAcc core
Striatum
CNS, central nervous system; GABA, gamma-aminobutyric acid; 5-HT, 5-hydroxytryptamine; NAcc,
nucleus accumbens; VTA, ventral tegmental area.
Co-existence and co-release of classical neurotransmitters and neuropeptides is often called
co-transmission regardless of the existence of a true interaction. Indeed, based on the
differences in the synthesis, storage, release and targets of classical neurotransmitters and
neuropeptides, it is possible that interaction does not always occur (Yang et al. 1996). On
the other hand, multiple transmitters released to the extracellular space can result in
interactive actions even though they would not be released from the same neuron.
2.2 Neuropeptides - from synthesis to signalling
2.2.1 Biosynthesis, storage and release
There are several common features in the biosynthesis of most neuropeptides, including the
synthesis as a part of larger inactive protein precursors known as proneuropeptides or
prohormones (Fig. 2-1) (Kitabgi 2006a). The synthesis of neuropeptides begins with the
translation of the mRNAs and the generation of preproneuropeptides (Hook et al. 2008).
Thereafter, the proteolytic processing starts in the rough endoplasmic reticulum (RER),
where the proneuropeptide is released form the preproneuropeptide. The
proneuropeptides are routed through the Golgi apparatus and packaged into large dense
core vesicles (LDCV, also known as large granlular vesicles) together with the processing
proteases (Seidah 2011). Processing enzymes recognize the specific cleavage sites, which
are often flanked by clusters of basic residues (Kitabgi 2006a). The proneuropeptides may
6
contain copies of only one or several different peptides in their sequence. As an example,
TRH is cleaved form a proneuropeptide containing five copies of TRH, whereas
proopiomelanocortin (POMC) precursor is cleaved into ACTH, α-melanocyte stimulating
hormone (α-MSH) and β-endorphin, which is an opioid peptide (Fig. 2-1) (Hökfelt et al.
2000). Also other enzymatic modifications can happen in the LDCVs, such as glycosylation,
C-terminal amidation, acetylation, phosphorylation and sulfation (Hökfelt et al. 2000).
Finally, LDCVs can contain one ore more cleavage products from the same precursor
polypeptide.
LDCVs are usually targeted to axon terminals, but also some LDCVs are
transported to dendrites (Merighi et al. 2011). In the terminals, there are also small synaptic
vesicles (SSVs) containing classical neurotransmitters. LDCVs differ from them in the way
how they are stored and released from the presynaptic neuron (Fig. 2-2). SSVs are usually
located in large amounts near the synaptic axon terminal. Their release requires a high
elevation in the intracellular Ca2+ concentration close to the Ca2+ channels of the synapse.
Figure 2-1. Example of the structure of proneuropeptides (neuropeptide precursor proteins).
Neuropeptides are synthezised as proneuropeptides and processed proteolytically from mono- (R), di(KR, RK, RR, KK) and multibasic (KKRR) sites to release the active neuropeptide. Some
proneuropeptides, like the proneuropeptides of neuropeptide Y (NPY), galanin, corticotropinreleasing hormone (CRF) and arginine-vasopressin (AVP), contain only one copy of the active
neuropeptide. Whereas the proneuropeptide proenkephalin contains several copies of
(Met)enkephalin (M), one copy of (Leu)enkephalin (L), and one copy of related opioid peptides (MEArg-Phe; H) and ME-Arg-Gly-Cleu (O). Additionally some proneuropeptides carry copies of discrete
peptides, such as the proneuropeptide pro-opiomelanocortin (POMC), which produces corticotropin
(ACTH), α-melanocyte -stimulating hormone (α-MSH) and β-endorphin (Hook et al. 2008).
7
LDCVs are stored away from the synaptic membrane in variable amounts, and
their release is triggered by a diffuse increase in the Ca2+ concentration inside the terminal
(Verhage et al. 1991). Interestingly, LDCVs are released anywhere at the terminal
membrane. It has been suggested that approximately 50 % of the release events occur at
extra-synaptic locations, also in dendrites (de Wit et al. 2009). Co-stored neuropeptides are
released at the same time, but the neuron can regulate the relative proportions of individual
neuropeptides when they are synthezised. Apparently also the different dissolving rates of
the co-stored neuropeptides from the LDCVs may regulate the speed of peptide secretion.
Vesicles empty their contents to the extracellular space by either classical exocytosis, where
the vesicle membrane is completely fused with the plasma membrane, or with a so called
“kiss and run” principle, where a transient pore is formed to release part of the transmitters.
LDCVs typically release their content using exocytosis (Harata et al. 2001).
Figure 2-2. A simplified figure about the synthesis and transport of neuropeptides. First, the inactive
precursor proneuropeptide is synthesized in the endoplasmic reticulum (ER). Then the precursor is
transferred to the Golgi apparatus for packaging. The proneuropeptides are packaged in large dense
core vesicles (LDCV) together with processing enzymes which then cut out the bioactive peptides on
the way to the synaptic terminal. Neuropeptide-containing LDCVs reside in variable amounts in the
terminal area, whereas the small synaptic vesicles (SSV) containing classical neurotransmitters
cluster close to the synaptic axon terminal. TGN, trans-Golgi network. Modified from Bean et al.
1994.
8
2.2.2 Neuropeptide receptors
Neuropeptides signal through specific G-protein-coupled receptors (GPCRs) that are coupled
to various intracellular signalling cascades. Their localization follows distinct patterns
generally away from the synaptic cleft and usually also away from the ligand peptides,
resulting in the so-called peptide/receptor mismatch (Merighi et al. 2011). The expression of
neuropeptide receptors can be functionally regulated. For example, activity-dependent
translocation of neuropeptide receptors from LDCVs’ to terminals’ membranes occurs at
least in the kappa opioid receptors of the hypothalamus (Shuster et al. 1999). The
differences in the expression of neuropeptide receptors can in turn change the overall
effects of released neuropeptides. After agonist binding, neuropeptide GPCRs are
internalized and recycled back to the cell membrane.
2.2.3 Neuromodulatory functions in the CNS
As mentioned in chapter 2.1.2, neuropeptides are generally considered to modulate the
actions of classical neurotransmitters. The modulation of fast neurotransmission can occur
pre- and postsynaptically affecting both excitatory and inhibitory neurotransmission
(Merighi et al. 2011). There are different ways for the neuropeptides to modulate
neurotransmission, although the intracellular mechanisms are not entirely understood. The
simplest kind of interaction occurs in the postsynaptic membrane when a neuropeptide
GPCR alteres the signal transduction properties of the ion channel receptor opened by a
classical neurotransmitter (Merighi et al. 2011). This can change the affinity of the receptor
for the neurotransmitter. Also, the number of receptors on the plasma membrane can be
altered, resulting in an increase or decrease in the neurotransmission. Presynaptic
modulation exists usually through presynaptic autoreceptors of one or more
neurotransmitters which control their own release. As neuropeptides can diffuse far away
from the site that they have been released, there is a possibility for them to control
neurotransmission also at different synaptic sites.
2.2.4 Neuropeptide-processing enzymes
Peptidases play several pivotal roles in the life of neuropeptides: on one hand they are
responsible for neuropeptide synthesis, but on the other hand they metabolize and
inactivate neuropeptides determining the range and time span of neuropeptidergic
signalling. Generally, the biosynthetic processing occurs intracellularly and the metabolism
extracellularly. The processing enzymes are called exopeptidases, if they cleave amino acids
from the end of the peptide chain, whereas endopeptidases break peptide bonds of
nonterminal amino acids. The peptide precursor molecules contain basic amino acids
separating the bioactive and spacer regions (Fig. 2-1). Two endopeptidases (proprotein
convertase 1, EC3.4.21.93; and proprotein convertase 2, EC3.4.21.94) appear to be involved
in the cleavage of the precursor molecules generating intermediate peptides with Cterminal basic residues (Fricker 2005). Thereafter, the basic residues are removed by
carboxypeptidase E (EC 3.4.17.10). Sometimes the peptide needs further processing, such as
C-terminal amidation by peptidylglycine-α-amidating monooxygenase (EC 1.14.17.3). Also
9
N-terminal acetylation or other modifications like glycosylation and phosphorylation can
occur. However, the enzymes responsible for these modifications are not well characterized.
Neuropeptide signalling can be terminated by overlapping mechanisms,
including peptide degradation by peptidases on the surface of the cell, receptor uncoupling
from heterotrimeric G-proteins and receptor endocytosis (Defea et al. 2000). The peptidases
and the mechanisms how neuropeptides are cleaved are highly conserved in both at the
level of protein sequence and catalytic properties (Isaac et al. 2009). Neuropeptidemetabolizing enzymes are usually localized extracellularly; some of them are attached to the
extracellular surface with transmembrane segments or they are secreted to the extracellular
space without any anchors. Commonly, these enzymes belong to a group of thermolysin-like
metalloendopeptidases including neprilysin (neutral endopeptidase/enkephalinase
EC3.4.24.11; EP 24.11), angiotensin-converting enzyme (EC3.4.15.1; ACE), neurolysin
(EC3.4.24.16; EP 24.16), endothelin-converting enzyme (EC3.4.24.71; ECE) and EP 24.15
(EC3.4.24.15; thimet oligopeptidase, TOP) (Kim et al. 2003).
A common feature for the metabolism of neuropeptides is that the specificity
is wide; several different peptidases can participate on the degradation of a certain
neuropeptide and a certain peptidase can degrade several neuropeptides (Fricker 2005). For
example, neurotensin is cleaved in the synaptic membranes of the rat brain by EP 24.15, EP
24.11 and EP 24.16 (Kitabgi 2006b). Unfortunately, the wide specificity of the processing
enzymes has hindered the development of enzyme inhibitors as drug candidates. Since a
relatively small number of peptidases are responsible for the processing of majority of
neuropeptides, the inhibition of one enzyme affects several different neuropeptides and
fails to produce specific effects. Neuropeptide degradation can also produce shorter but still
active peptides. That seems to be the case with SP, which C- and N-terminal fragments are
known to possess different effects (Nikolaus et al. 2000). Interestingly, neuropeptides are
released in relatively high (micromolar) concentrations. Given the affinity of neuropeptides
to their receptors, even a 100-fold decrease in these concentrations in the extracellular
space by a peptidase would still leave a sufficient number of intact neuropeptide molecules
to reach the receptors (Landgraf and Neumann 2004).
2.3 Common brain neuropeptides that are also putative PREP substrates
Neuropeptides play an important role in the normal function of CNS. Although there are
several neuropeptides present in the CNS, this chapter will focus on those known as
putative PREP substrates, at least in vitro (for review, see García-Horsman et al. 2007 and
Männistö et al. 2007). In theory, PREP can cleave short peptides with an internal proline
residue (Cunningham and O'Connor 1997). However, as mentioned in the Introduction, the
relevance of PREP in neuropeptide metabolism in vivo has been questioned. Still, it is
important to understand the effects of the putative PREP substrates in the brain. The basic
characteristics of the behaviourally most interesting putative PREP substrate peptides are
reviewed here, and their effects on behaviour are discussed in chapter 2.4.
10
2.3.1 AVP
AVP was first detected 1895 by Oliver and Schäfer as a neurohypophysial hormone that
altered blood pressure (Caldwell et al. 2008). This 9 amino acid peptide is biologically active
in the periphery and in the CNS. It is mainly synthesized in the magnocellular cells of the
supraoptic nucleus and paraventricular nucleus which project to the posterior pituitary
(Brownstein et al. 1980). These neurons secrete AVP to the blood stream where it acts on
the vascular system. However, there are also small AVP containing neuron populations,
mainly in the paraventricular nucleus, bed nucleus of the stria terminalis (BNST), medial
amygdala and suprachiasmatic nucleus, whose projections remain in the brain (Fig. 2-3).
Also the distribution of AVP receptors suggests that the peptide has a role as a
neuropeptide transmitter in the brain. Of the three known receptor types for AVP, V1A is
located in the periphery (liver, kidney and vasculature) and widely in the brain. V1B is
prominent in the anterior pituitary and in many other tissues and throughout the brain. V2
is mainly located in the kidney (Caldwell et al. 2008). AVP is known to regulate various
behaviours, especially social behaviours related to affiliation, social and non-social memory,
as well as mood and aggression (Caldwell et al. 2008). It is also known to control the
circadian hormone secretion (Kalsbeek et al. 2010).
Figure 2-3. Major arginine-vasopressin (AVP) nuclei and their projections in a sagittal section of the
rat brain (modified from McEwen 2004). BNST, bed nucleus of the stria terminalis; LC, locus
coeruleus; LS, lateral septum; VTA, ventral tegmental area.
2.3.2 Oxytocin
The nonapeptide oxytocin is synthesized in neurons of the supraoptic and paraventricular
nuclei of the hypothalamus, which project to the posterior pituitary and release oxytocin
into the bloodstream (Landgraf and Neumann 2004). This peripherally acting oxytocin is
critical for the processes of lactation and parturition. Oxytocin neurons of the
paraventricular nucleus also send projections to many regions within the brain, including
11
the hippocampus, amygdala, and hypothalamus (Fig. 2-4). Oxytocin excerts its effects via
oxytocin receptors (OTR), GPCRs which are located in discrete brain regions, such as the
central nucleus of the amygdala and the ventromedial nucleus of the hypothalamus
(Tribollet et al. 1992; Bale et al. 2001). Oxytocin is involved in various behaviours such as
maternal care, aggression, pair bonding, sexual behaviour, social memory and anxietyrelated behaviour. Although the behavioural effects of oxytocin have been extensively
studied, they seem to differ depending on the species, strain, gender, behavioural test
conditions, mode of drug administration and other experimental conditions (Neumann
2008).
Figure 2-4. Sagittal illustration of major oxytocin nuclei and their projections in the rat brain
(modified from McEwen 2004). MFB, medial forebrain bundle; MS, medial septum; SN, substantia
nigra.
2.3.3 Neurotensin
Neurotensin is a tridecapeptide found in the CNS as well as in the gastrointestinal tract. The
gene encoding preproneurotensin also contains the sequence of a related 6 amino acid
peptide neuromedin N (Kitabgi et al. 1992). Neurotensin-producing neurons as well as their
projections are abundantly expressed in the brain (Fig. 2-5), giving the peptide a wide range
of effects. Most abundantly neurotensin is found in the amygdala, lateral septum, BNST, SN
and ventral tegmental area (VTA) (Uhl 1982). Neurotensin exerts its effects through two
GPCRs, the high-affinity NTS1 and the low-affinity NTS2 (Mustain et al. 2011). NTS1 is found
throughout the CNS, especially in the dopaminergic neurons of the midbrain, in the small
and large intestine, liver and in various malignancies. NTS2 is located more scatteredly in the
CNS than NTS1, but not in the gastrointestinal tract. Also two single-transmembrane domain
receptors bind neurotensin (NTS3 and sorLA/LR1), but their role is not fully understood.
12
Figure 2-5. Neurotensinergic pathways in a sagittal section of the rat brain (adapted from St-Gelais
et al. 2006). BNST, bed nucleus of the stria terminalis; DBB, diagonal band of Broca; DR, dorsal raphe
nucleus; GP, globus pallidus; HT, hypothalamus; LS, lateral septum; NAcc, nucleus accumbens; PAG,
periacqueductal grey; PB, parabrachial nucleus; PFC, prefrontal cortex; SN, substantia nigra; TD,
dorsal thalamic nucleus; VP, ventral pallidum; VTA, ventral tegmental area.
Since its discovery in the 1970s, the effects of neurotensin have been studied
extensively. Neurotensin acts as a neurotransmitter and a modulator of other signals. The
close association between neurotensin and the mesocorticolimbic and nigrostriatal
dopamine systems has served as a rationale to study interaction between these systems.
Indeed, neurotensin has been well established as a modulator of dopaminergic
neurotransmission (for a review, see Binder et al. 2001 and St-Gelais et al. 2006). Therefore,
neurotensin has been implicated in CNS conditions such as schizophrenia, Parkinson’s
disease and addiction, which are characterized by abnormalities in dopaminergic functions.
Neurotensin can affect dopaminergic transmission via two mechanisms: it modulates the
activity of dopaminergic neurons and controls the release of dopamine (St-Gelais et al.
2006). The activating effect occurs via a Ca2+ -dependent excitatory effect (St-Gelais et al.
2004) and the release of dopamine is controlled with a separate indirect antagonistic effect
on dopamine D2 receptors (Binder et al. 2001; Jomphe et al. 2006; Fawaz et al. 2009). The
synaptic localization of NTS1 and D2 receptors determines whether the overall dopaminergic
signalling is enhanced or inhibited (reviewed by Binder et al. 2001, also summarised in Fig.
6-1).
Although neurotensin is best known for its effects on the dopaminergic
transmission, there is also evidence about the connections of neurotensin with cholinergic,
5-HT, GABAergic and glutamatergic neurotransmission (St-Gelais et al. 2006). Neurotensin
receptors are expressed in these neurons in variable amounts and neurotensin can have
either stimulatory or inhibitory effects (Mustain et al. 2011). Furthermore, neurotensin has
been reported to affect the activity of glial cells, such as astrocytes and microglia, suggesting
a role for neurotensin in the inflammation or trauma processes.
13
2.3.4 Somatostatin
Somatostatin (somatotropin release-inhibiting factor, SRIF) is a cyclic peptide first known as
an inhibitor of growth hormone release from the anterior pituitary (Siehler et al. 2008).
Somatostatin is found in the CNS, but also in the endocrine tissues and in the gastrointestinal tract. It has a wide range of biological actions, including inhibition of the secretion
of growth hormone, insulin, glucagon and gastrin, as well as other hormones secreted by
the pituitary and gastrointestinal tract. There are two biologically active forms of
somatostatin in mammals, the 14 amino acid peptide SRIF14 that is more predominant in
the CNS, and a longer N-terminally extended form SRIF28. Both are produced by a same
gene and cleaved from a common proneuropeptide (Vezzani and Hoyer 1999). In mammals,
six somatostatin GPCRs (sst1, sst2A, sst2B, sst3, sst4 and sst5) have been cloned (Viollet et
al. 2008). The distribution of somatostatin receptors is given in Table 2-3. According to the
similarities in the structural and pharmacological features, somatostatin receptors have
been divided in two groups (for a review, see Olias et al. 2004). In the CNS, somatostatin and
its receptors are located in regionally varying amounts (Epelbaum 1986; Schindler et al.
1996).
High somatostatin immunoreactivity has been found in the hypothalamus,
amygdala, preoptic area, hippocampus, striatum, cerebral cortex, olfactory regions, and
brainstem. Somatostatinergic neurons can be classified in two subcategories, the longprojecting neurons and short GABAergic interneurons acting within microcircuits (Viollet et
al. 2008). In addition to the role in the neuroendocrine processes, somatostatin is known to
control neuronal activity in the CNS (Viollet et al. 2008). Via its receptors, somatostatin
exerts several different functions (see Viollet et al. 2008), including the presynaptic
inhibition of glutamate and GABA release. Therefore it has been connected to the
modulation of sleep, eating and drinking, locomotor activity, cognition and nociception
(Vezzani and Hoyer 1999).
Table 2-3. Somatostatin receptors in the murine brain, adapted from Viollet et al. 2008.
Brain area
Main somatostatin receptors
Olfactory bulb
sst2, sst3, ss4
Anterior olfactory nucleus
sst2
Neocortex
sst2, sst4
Hippocampus:
CA1
sst2, sst4
CA3
sst2
Dentate gyrus
sst2
Amygdala
sst2
Lateral hypothalamus
sst1, sst2
Locus coeruleus
sst2
14
2.3.5 SP
The undecapeptide SP is one of the best-known and most-studied neuropeptides. It was first
discovered as early as in the 1930s (Mantyh 2002) from brain and intestine extracts, and
later identified in the 1970s (Chang et al. 1971). SP is found in the CNS, with especially high
levels in the hypothalamus and SN (Fig. 2-6), as well as in the spinal cord and PNS. SP
belongs to the neuropeptide family called tachykinins together with neurokinin A (NKA) and
B (NKB), which share the similar C-terminal amino acid sequence (Phe-X-Gly-Leu-Met x NH2)
(Saria 1999; Hökfelt et al. 2001). They are encoded by two genes, preprotachykinin I and
preprotachykinin II (Hökfelt et al. 2001). NKB is only produced by the latter gene. Three
different tachykinin receptors are known: NK1, NK2 and NK3 receptors. Tachykinin peptides
bind to these receptors in the following order of affinity; SP binds with highest affinity to the
NK1 receptor, whereas NKA prefers NK2 receptors and NKB preferably binds to the NK3
receptors (Saria 1999). NK1 receptors are highly expressed in the striatum, nucleus
accumbens (NAcc), hippocampus, lateral nucleus of the hypothalamus, habenula,
interpeduncular nucleus, nucleus of the tractus solitarius, raphe nuclei and medulla
oblongata (Otsuka and Yoshioka 1993).
Based on the anatomical distribution in the CNS and PNS, SP and its receptors
have been connected to several pathophysiological conditions, including pain, movement
disorders, memory and learning, anxiety, emesis and depression (Quartara and Maggi
1998). In the CNS, SP frequently resides the same neuron as other tachykinins and classical
transmitters such as dopamine, ACh, 5-HT, GABA and glutamate (Hasenöhrl et al. 2000).
Indeed, SP has been shown to regulate the release of these neurotransmitters or their
effects on the target neurons (for a review, refer to Otsuka and Yoshioka 1993). Many
antagonists have been developed for the SP-preferring NK1 receptor, some of which
penetrate to the brain after oral administration (Quartara and Maggi 1998). However, the
species variation in the primary sequence of the NK1 receptive site has hindered the
research regarding the physiological functions of SP (Saria 1999). Furthermore, it has been
suggested that SP may act as a precursor for two N- and C-terminal fragments having
separate biological functions.
The different effects of SP and its fragments have been connected to selective
and site-specific changes in the activities of classical neurotransmitters. For example, SP and
its C-terminal fragment, but not the N-terminal sequence, increase extracellular dopamine
in NAcc (Boix et al. 1992). Therefore, the behavioural effects of SP may be determined by
the site of action within the brain and the way of the hydrolysis of the peptide. It has been
also suggested, that the actions of SP or its fragments are terminated by an active uptake
mechanism (Nakata et al. 1981).
15
Figure 2-6. Substance P containing neurons and their projections in a sagittal section of the rat brain
(modified from Otsuka and Yoshioka 1993). BNST, bed nucleus of the stria terminalis; DR, dorsal
raphe nucleus; GP, globus pallidus; LC, locus coeruleus; LS, lateral septum; NAcc, nucleus accumbens;
NX, vagal nerve; PFC, prefrontal cortex; PGi, paragigantocellular reticular nucleus; RMg, nucleus
raphe magnus; ROb, nucleus raphe obsculus; RPa, nucleus raphe pallidus; SN, substantia nigra; Sol,
solitary nucleus; SpV, spinal trigeminal nucleus; VP, ventral pallidum; VTA, ventral tegmental area.
2.4 The effects of neuropeptides: focus on behaviour
As neuropeptides can act as neurotransmitters or modulators, regulators of hormonal
secretion and cerebral blood flow, or as mediators of immune system and blood-brain
barrier permeability (Malavolta and Cabral 2011), they must have an impact on behaviour.
Furthermore, the malfunction of neuropeptidergic systems has been connected to several
CNS disorders. However, due to the abundance of neuropeptides and the wide spectrum of
their functions in the brain also make the interpretation of the effects of a single
neuropeptide on a specific type of behaviour complicated. This chapter will focus on the
data obtained about the behavioural effects of some of the most common neuropeptides
that are also putative substrates of PREP in vitro (AVP, neurotensin, oxytocin, somatostatin
and SP) on experimental animals. The chapter will concentrate on some effects of these
neuropeptides on motor behaviour, memory and learning and affective behaviours, and
some other behavioural effects relevant to this thesis.
The research about the behavioural effects of neuropeptides has been
hindered by the lack of selective non-peptide receptor agonists/antagonists, the very
limited permeability of these peptides across the blood-brain barrier, and their fast
degradation. The studies have been mostly done by injecting the neuropeptides into the
brain, since they commonly do not reach the CNS when administered by other routes.
Neuropeptide receptor agonists or antagonists have also been developed, some of them
able to reach the brain. In addition, some studies in knockout mice are reviewed. Some of
the neuropeptides have structural similarities, such as oxytocin and AVP, and unspecificity
16
between them may occur. Therefore, recently more specific techniques such as the
overexpression of neuropeptides or their receptors with virally mediated gene transfer have
been used. It is important to keep in mind that the manipulation of a single component is
likely to affect several other systems. For example using knockout mice or chronic
treatments can have long-term consequences making the data interpretation difficult. All
techniques have their own advantages and pitfalls, but together they can contribute to
understanding the behavioural effects of neuropeptides.
Table 2-4 summarizes the behavioural effects discussed in this chapter.
2.4.1 Motor behaviour
Several neuropeptides affect motor functions by direct or indirect mechanisms. Increased or
decreased locomotor activity in an open field study can indicate multiple effects of a
neuropeptide. For instance, locomotor activity can be decreased because of a sedative or
anxiogenic effect.
Neuropeptides that have been reported to increase locomotor activity, include
AVP and somatostatin. Intracerebroventricular (i.c.v.) and subcutaneous (s.c.)
administration of AVP has been associated with an increase in locomotor activity in young
adult rats, which could be related to an increase in dopaminergic activity (DiMichele et al.
1996). AVP can also modulate locomotion related to circadian clock (Li et al. 2009) or
dehydrated state, via V1A receptors (Tsunematsu et al. 2008). Somatostatin receptors in the
striatum and other basal ganglia nuclei are involved in regulation of motor behavior
(Semenova et al. 2010). Somatostatin can modulate locomotor activity by activating sst1,
sst2 and sst4 receptors in the ventral pallidum and sst2 and sst4 receptors in the striatum.
However, the reports about the effects of somatostatin on motor functions have not always
been uniform. I.c.v and local brain administrations of somatostatin increased the locomotor
activity in several studies (Vecsei et al. 1984; Viollet et al. 2000). However, there are also
studies reporting no effect of somatostatin on the locomotor activity (Vecsei et al. 1984).
The reports about the effects of SP and oxytocin on locomotor activity are
more complex. I.c.v. injections of SP enhance stereotypical grooming and scratching
behaviour in mice (Hall et al. 1987). NK1 receptor agonist GR 73632 has also increased
locomotor activity in guinea pigs (Severini et al. 2002). Although the effects of
intraperitoneal (i.p.) and s.c. injections of SP in mice have been variable, SP has decreased
spontaneous locomotor activity and counteracted amphetamine-induced hyperactivity.
Oxytocin can affect locomotor activity differently with different doses and in male and
female rats (Uvnas-Moberg 1994). In an open field test, low dose of oxytocin increased the
exploratory activity of rats in the centre of the field. In contrast, about 100-fold higher doses
suppressed the locomotor activity (Uvnas-Moberg et al. 1994). In male rats, the s.c. injected
oxytocin has sedative effects (Uvnas-Moberg et al. 1994), whereas in females, it increases or
decreases locomotor activity depending on the oestrous cycle and sex hormones (Petersson
et al. 1998). Oxytocin has also been reported to enhance locomotor activity induced by
clonidine, an α2-receptor agonist. This finding connected the motor activity-increasing
effects to the activation of noradrenergic neurons in the locus coeruleus (Petersson et al.
1999).
17
As a proposed endogeneous antipsychotic, neurotensin has been mostly
studied in animal models related to schizophrenia, such as stimulant-induced
hyperlocomotion or antipsychotic-induced hypolocomotion. However, also spontaneous
locomotor activity has been studied. I.c.v. and systemic neurotensin or its analogue have
reduced both spontaneous locomotor activity and amphetamine-induced hyperlocomotion
(Casti et al. 2004; Caceda et al. 2006). However, only the stimulation of neurotensin system
seems to affect locomotion, since neurotensin antagonists have not affected mouse
spontaneous locomotor activity when administered i.p. (Casti et al. 2004). In addition, it has
been shown that although single i.c.v. administration of neurotensin blocks the
amphetamine-induced hyperlocomotion, chronic administration can potentiate it (Norman
et al. 2008). However, pretreatment with a NTS1 antagonist has not modified the locomotor
activity induced by stimulants, suggesting that endogenous neurotensin is not involved in
stimulant-induced hyperlocomotion (Caceda et al. 2012). On the other hand, haloperidolinduced hypolocomotion was alleviated by neurotensin antagonists SR48692 and SR142948
(i.p.) (Casti et al. 2004).
2.4.2 Learning and memory
The ability to remember is a prerequisite for functioning in life. With great simplification,
memory processes involve acquisition, consolidation, reinforcement and retrieval of
information. The role of cholinergic system in learning and memory is well established
(Gulpinar and Yegen 2004). However, these and other memory-related functions are
modulated by a number of neuropeptides. Indeed, several neuropeptides, such as AVP,
oxytocin, ACTH, CRF, somatostatin, neurotensin and SP have been suggested to modulate
learning and memory (Kovacs and De Wied 1994). To be able to exert these effects,
neuropeptides or their active fragments need to act at brain areas responsible for memory
functions. Although the whole picture is not fully unveiled, there is basic understanding
about the brain regions involved in cognitive processes, including the hippocampus,
amygdala, medial septum and neocortex (Gulpinar and Yegen 2004). The best documented
neuropeptides involved in memory and learning processes are AVP and oxytocin (Gulpinar
and Yegen 2004). Next paragraphs summarize the data obtained about the effects of AVP,
neurotensin, oxytocin, somatostatin and SP on learning and memory behaviours in
experimental animals.
Promnesic neuropeptides
The neuropeptides with reported promnesic properties include AVP, somatostatin and SP
(Kovacs and De Wied 1994). AVP in the ventral hippocampus is connected to information
processing, memory consolidation, storage and retrieval (Gulpinar and Yegen 2004).
Although i.c.v. injected AVP has been shown to improve learning and memory in avoidance
paradigms, it has not been as clear if AVP enhances memory in other type of tests
(Klimkiewicz 2001). The effects on memory have been proposed to be facilitated mainly via
the AVP-mediated activation of hippocampal interneurons (Dietrich and Allen 1997). Some
fragments of AVP (4-9 and 5-8) have been shown to be more potent than the native peptide
(Dietrich and Allen 1997; Fujiwara et al. 1997). Indeed, it has been suggested, that the AVP418
fragment is the most important in memory processes (Klimkiewicz 2001). For example, s.c.
injection of AVP4-9 fragment has enhanced working and reference memory in the radial-arm
maze (Dietrich and Allen 1997). Furthermore, AVP projections from the medial amygdala
and BNST to the lateral septum are associated with social memory (Bluthe et al. 1990;
Bluthe et al. 1993). Social memory and social recognition are important for an individual to
distinguish a friend from foe according to the visual, olfactory or auditory cues, and to
choose the appropriate behaviour for the situation. To support this, it has been shown in
Brattleboro rats lacking AVP, that the injection of AVP into the lateral septum facilitates
social memory (Engelmann and Landgraf 1994). Also in normal rats infusions of V1A
antagonists or antisense V1A oligonucleotides into the lateral septum have this kind of
memory impaired (Engelmann and Landgraf 1994; Landgraf et al. 1995). Knockout mice
have been used to show that the deficiency of V1A impairs social memory which can be
reversed with the overexpression of V1A in the lateral septum (Bielsky et al. 2004; Bielsky et
al. 2005).
Although best known as a potential endogenous antipsychotic, neurotensin
has also been connected to memory functions (Caceda et al. 2006). Neurotensin may affect
learning and reinforcement especially through the NTS1 receptors in the central nucleus of
amygdala (Laszlo et al. 2010). When neurotensin was administered bilaterally to this
nucleus, it enhanced the passive avoidance learning in the rat possibly due to the
modulation of mesolimbic dopaminergic system (Laszlo et al. 2012).
The role of somatostatin is well acknowledged in the memory consolidation
and retention processes (Viollet et al. 2000). Early studies showed that i.c.v injected
somatostatin and its analogue are able to reverse the avoidance response impairing effects
of cysteamine, a somatostatin-depleting agent, in a conditioned active avoidance test
(Schettini et al. 1988). Similar results have been obtained with passive avoidance learning
test (Vecsei et al. 1984; Matsuoka et al. 1995). Matsuoka et al. 1994 showed that the
memory-improving effects of somatostatin would be mediated via enhanced cholinergic
transmission in the hippocampus. In particular, intrahippocampal injections of somatostatin
enhance the rate of acquisition of a spatial discrimination task in a radial maze (Lamirault et
al. 2001). Later, somatostatin has been shown to regulate the homeostasis in the
hippocampus through modulation of sst2 and sst4 receptors, which seem to cooperate in
the control of hippocampus- and striatum-mediated memory functions (Gastambide et al.
2010). Especially hippocampal sst4 is involved in the selection of memory strategies
(Gastambide et al. 2009).
SP has been connected to memory processes in several studies (Hasenöhrl et
al. 2000). The most important finding has been that the N- and C-terminal fragments of SP
have distinct roles in learning and reinforcement (Hasenöhrl et al. 2000). The N-terminal
fragment appears to be more important in memory promoting, whereas the C-terminal
sequence mediates the reinforcement. These effects have been obtained both after the
peripheral and central injections to the nucleus basalis of the ventral pallidum (Huston and
Hasenohrl 1995). Also the injections of these fragments to the NAcc caused similar effects,
where the N-terminal fragment facilitated learning (Gaffori et al. 1984).
9
19
Amnestic neuropeptides
Oxytocin exerts opposite effects on fear-motivated avoidance behaviour as AVP (Kovacs and
De Wied 1994). Therefore it has been proposed that oxytocin is an amnestic neuropeptide.
Oxytocin impairs memory consolidation when injected to the hippocampal dentate gyrus or
dorsal raphe nucleus (Kovacs and De Wied 1994). Also, oxytocin injected to the Meynert
nucleus has increased the latency of Morris water maze task in rats, indicating impairment
on spatial learning (Wu and Yu 2004).
Taken together, there are several neuropeptides present at the brain areas related to
memory and learning, such as the hippocampus, where they regulate neurotransmission.
Therefore they can have various effects on memory processes via several different
mechanisms. However, there are no particular neuropeptidergic mechanisms that could be
responsible for the modulation of learning or memory processes (Kovacs and De Wied
1994). Instead, several different neuropeptides with different localization and origin are cooperating with each other and also with classical transmitters. In some instances a particular
neuropeptide becomes more effective in a particular behavioural situation. Interestingly,
some of these neuropeptides may contribute to plastic changes in the connectivity of
neurons which are reconstructed during learning and memory formation.
2.4.3 Affective behaviours
Neuropeptides are considered to tune the direct communication of neurons, and this tuning
may be important in mood. Many neuropeptides are located in the important brain areas
for the generation and perception of emotion and stress, such as the hypothalamus, limbic
structures, amygdalar nuclei, BNST and frontal cortex structures (Papathanassoglou et al.
2010). Indeed, several neuropeptides have been connected to affective behaviours or even
to the pathophysiology of psychiatric disorders, such as schizophrenia, anxiety and
depression (De Wied and Sigling 2002; Landgraf 2006). Especially neuropeptides considered
to possess opiate-like, neuroleptic-like and amphetamine-like activity may be involved in
these processes (De Wied and Sigling 2002). The following paragraphs focus on the effects
of AVP, neurotensin, oxytocin, somatostatin and SP on affective behaviours, such as
psychotic, anxious and depressive behaviours.
Psychotic behaviour
Of all neuropeptides, neurotensin is best characterized in the regulation of psychotic
behaviour. After Charles Nemeroff published his paper about the role of neurotensin as an
endogenous antipsychotic in 1980, the theory got a lot of attention (Nemeroff 1980).
Nemeroff proposed that neurotensin inhibits dopaminergic neurotransmission like
antipsychotics, and that schizophrenia is partially caused by a lack of neurotensin in the
brain.
There are several facts supporting the role of neurotensin as an endogenous
antipsychotic, mostly obtained from studies with experimental animals. First, neurotensin
and its receptors are abundantly expressed in the dopaminergic neurons of the midbrain
(Binder et al. 2001), where neurotensin is known to modulate dopaminergic transmission.
20
Second, studies where neurotensin has been injected in the VTA or NAcc have shown that
the effects of neurotensin are similar as those of antipsychotics. For example, both
neurotensin and antipsychotics increase the firing rate of dopaminergic neurons when
injected to VTA (Hollerman et al. 1992), whereas injections to the NAcc cause the inhibition
of D2 receptors (Li et al. 1995). The antipsychotic-like effect of neurotensin is also seen in
the behaviour of the animals; it causes sedation, muscle relaxation, hypotermia and
stereotypical chewing. Third, there are several studies showing a possible link between
neurotensin and dopamine in the pre-pulse inhibition of the startle response (PPI) test
which is a commonly used animal model of schizophrenia. It refers to the ability of a weak
stimulus to inhibit the response to a following strong stimulus. PPI is altered in
schizophrenic patients and the disruption of PPI can be produced in rodents by dopamine
agonists or glutamatergic NMDA receptor antagonists. The injection of neurotensin to the
NAcc reduces amphetamine induced PPI in rats (Feifel et al. 1999). However, the same
effect is not seen with injections to the VTA (Feifel and Reza 1999). Similarly as atypical
antipsychotic drugs, the systemically administered neurotensin agonists, NT69L and
PD149163, have shown to be effective in pharmacologically induced PPI model. Feifel et al.
2011 showed that neurotensin agonist PD149163 elevated PPI also in brown Norway rats (a
rat strain that has genetically deficits in PPI) similarly to clozapine. The behavioural effects of
the systemically administered neurotensin agonists are similar as with the direct injections
to the NAcc (Caceda et al. 2006). This suggests that postsynaptic neurotensin receptors in
the NAcc are stimulated also after the peripheral administration. Furthermore, neurotensin
knockout mice have a disrupted PPI (Kinkead et al. 2005).
There is less evidence about the effects of other neuropeptides on psychotic
behaviour. The brain AVP system may have a role in both positive and negative symptoms of
schizophrenia, probably due to interactions with the glutamatergic and dopaminergic
pathways (Frank and Landgraf 2008). However, it has been suggested that AVP analogues
would have potentially alleviating effects on schizophrenic behaviour (Matsuoka et al.
2005). Also compelling preclinical evidence indicates that oxytocin has a role in
schizophrenia (Feifel 2012). Somatostatin fragments have been linked to schizophrenia and
especially the positive symptoms of schizophrenia, since cysteamine, the somatostatindepleting agent has reversed the amphetamine-induced deficits in PPI model (Feifel and
Minor 1997). I.c.v administered somatostatin-28 has caused deficits in PPI, which was then
partially reversed with a selective sst1 antagonist SRA-880 (Semenova et al. 2010). Although
SP modulates dopamine release in the dopaminergic neurons of the midbrain, it has not
been studied in the animal models of schizophrenia. However, it has been suggested that
the NK1/NK3 antagonists may be effective in the treatment of schizophrenia (Catalani et al.
2011).
Anxiety and depression
Anxiety is a protective, if at times uncomfortable, emotion that is fundamental to
adaptation and survival. However, excessive anxiety can be disabling and interfere with
normal life. Several neuropeptides or their analogues have been reported to possess
anxiolytic or anxiogenic effects.
21
A range of studies have shown that AVP has anxiogenic properties (Mak et al.
2012). Antagonists of V1A (Bleickardt et al. 2009) and V1B (Griebel et al. 2002) have reduced
the anxiety-related behavior in the elevated plus-maze following i.p. administration. Similar
results have also been found in AVP knockout mice (Bielsky et al. 2004). In i.c.
administration studies, the exact anxiolytic-like effects of V1A antagonists have been
connected to the ventral hippocampus, whereas the anxiolytic-like effects of V1B
antagonism may be specific to the dorsal hippocampus (Engin and Treit 2008).
Brain oxytocin is an important regulator of fear and stress responses. Oxytocin
is anxiolytic in female and male rats, and in mice, when administered centrally (Windle et al.
1997; Ring et al. 2006; Blume et al. 2008). However, the OTR antagonists have only
produced anxiety in pregnant or lactating female rats (Neumann et al. 2000). Therefore it
has been suggested that the activation of the oxytocin system during the peripartum period
is required for the anxiolytic effect. The central amygdala (Bale et al. 2001) and
hypothalamic paraventricular nucleus (Blume et al. 2008) have been connected the
anxiolytic effects of oxytocin.
Recently, also somatostatin has been connected to affective behaviours, such
as anxiety and depression (Yeung and Treit 2012). Somatostatin receptors are extensively
expressed in limbic areas such as the amygdala, septum and hippocampus that are related
to the affective behaviours. It has been shown that i.c.v. administered somatostatin
produces anxiolytic effects in the elevated plus-maze and antidepressant effects in forced
swim test in rats (Engin et al. 2008).
SP has been linked to anxiety and depression in several studies. First, SP may
be involved in the action of antidepressants (Shirayama et al. 1996) and anxiolytic drugs
(Brodin et al. 1994), because these conditions have low levels of SP in the brain. More
importantly, the intracerebral (i.c.) injection of SP agonists have produced anxiogenic (De
Araujo et al. 1999) and SP antagonists anxiolytic effects (File 1997) when studied in rats with
different animal models. Therefore it seems the severity of anxiety and SP activity go hand
in hand (Hasenöhrl et al. 2000). However, the effects are dependent on the specific brain
area and the dose of SP (Nikolaus et al. 2000). SP and its C-terminal fragment have had
anxiogenic effects when injected to the dorsal periaqueductal grey (De Araujo et al. 1999),
but when injected to the ventral pallidum, both N- and C-terminal fragments of SP have
produced anxiolytic effects (Nikolaus et al. 2000). With low doses, SP has been reported to
be anxiolytic and with higher doses anxiogenic (Hasenohrl et al. 1998).
2.4.4 Other behavioural effects
Affiliative behaviours
Affiliation comprises social bonding between individuals, such as relationships between
sexual partners or parents and infants. These behaviours have been studied using species
that express monogamy and biparental care of the offspring, such as the monogamous
prairie and pine voles. Interestingly, their behaviour has been compared to the nonmonogamous species of the same genus Microtus (montane and meadow voles).
Young et al. 1999 showed that a species-specific distribution of V1A is
important for the development of affiliative behaviours. It has also been shown that AVP
22
antagonists injected i.c.v. block the social bonding in voles measured by a partner
preference test, whereas AVP infusions promote partner preference (Cho et al. 1999).
Interestingly, intra-septal AVP increases and V1A antagonist decreases paternal behaviour in
sexually naïve male prairie voles supporting the role of AVP in the affiliative behaviours
(Wang et al. 1994). The mechanisms behind the effects are not fully understood. However,
it has been shown that also AVP and dopamine interact, especially in the ventral pallidum,
to mediate pair bonding (Young et al. 2008). These findings suggest that the interaction
between AVP and dopamine systems is important in the regulation affiliative behaviours.
Aggressive behaviour
The AVP projections originating from the BNST and medial amygdala to the caudal lateral
septum have been associated with aggressive behaviour in males, depending on the species
examined (de Vries and Miller 1998). When V1A antagonists are given orally or injected to
the medial preoptic-anterior hypothalamic area (MPOA-AH), they block the aggressionrelated flank-marking behaviour in the extensively studied Syrian hamsters indicating that
MPOA-AH and V1A receptors are important in controlling aggressive behaviour (Ferris and
Potegal 1988; Ferris et al. 2006). Interestingly, gonadal steroids control the biosynthesis of
V1A (Young et al. 2000). However, there are some species differences in the AVP-mediated
regulation of aggressive behaviours.
I.c.v. injection of SP has reduced fighting in mice that were made aggressive
with isolation (Hall et al. 1987), and different peptide fragments of SP can have opposite
effects on aggressive behaviour.
Table 2-4. A summary table about the effects of neuropeptides and putative in vitro PREP substrates,
including arginine-vasopressin (AVP), neurotensin, oxytocin, somatostatin and substance P (SP), on
behaviour, including locomotor activity, memory and learning, psychotic behaviour and anxiety and
depression. See the text for further details. ↑, mostly increasing effect; ↓, mostly decreasing effect;
↕, both increasing and decreasing effects reported; – effects not known.
Type of behaviour
AVP
Neurotensin
Oxytocin
Somatostatin
SP
Locomotor activity
↑
↓
↕
↕
↕
Memory and learning
↑
↑
↓
↑
↕
Psychotic behaviour
↓
↓
↓
↑
–
Anxiety and depression
↑
↓
↓
↓
↕
23
2.5 The importance of PREP in neuropeptide metabolism
The discovery of PREP as an oxytocin-cleaving enzyme in the bovine uterus set the course of
the PREP research closely around neuropeptides for about 40 years. Throughout these
years, PREP was strongly believed to participate in the neuropeptide degradation. This issue
has been comprehensively reviewed by Männistö et al. 2007 and Tenorio-Laranga et al.
2011 and it will be discussed later also in the experimental part of this thesis. Therefore this
chapter will cover only the main findings and problems related to PREP and neuropeptides.
There are a couple of important reasons why PREP has been proposed to be
involved in the neuropeptide metabolism. First, the catalytic properties of PREP. PREP
cleaves proline-containing neuropeptides shorter than 30 amino acids from the internal side
of a C-terminal proline (Cunningham and O'Connor 1997). PREP cleaves the Pro-Xaa bond at
the carboxyl side, where Xaa is any other amino acid other than proline (Cunningham and
O'Connor 1997). Since proline is a unique amino acid with a peptide stabilizing cyclic
structure, it reduces the susceptibility of a peptide for the proteolytic hydrolysis. As several
broad selectivity peptidases can not hydrolyze proline-containing peptides, specific
peptidases, like PREP, are needed. Many of the bioactive peptides, such as SP, TRH, AVP and
neurotensin contain proline and are short enough to be degraded by PREP (García-Horsman
et al. 2007). Second, PREP is expressed in several tissues, but most abundantly in the brain.
Therefore it has been suggested that PREP plays an important role in the regulation of brain
neuropeptide levels.
The development of PREP inhibitors has enabled the close investigation of the
importance of PREP in neuropeptide metabolism and physiological processes. In the 1990s,
PREP was mostly studied as a memory-modulating enzyme, since many of its proposed
substrates have been connected to memory and learning (Cunningham and O'Connor 1997).
In several studies, PREP inhibitors increased neuropeptide levels (Toide et al. 1995b; Toide
et al. 1996; Bellemere et al. 2003) and improved memory (Toide et al. 1995a; Morain et al.
2002; Jalkanen et al. 2007). However, the increasing effects of PREP inhibitors on
neuropeptide levels were not uniform. Usually these effects were observed after a single
but not chronic administration, and they varied with the dose and brain area studied. Some
studies failed to show any increase in the neuropeptide levels (Toide et al. 1995b; Jalkanen
et al. 2007; Jalkanen et al. 2011) and in memory and learning. The conclusion was that PREP
does not have a significant effect on the neuropeptide levels (Jalkanen et al. 2007; Männistö
et al. 2007).
The major dilemma in the role of PREP in neuropeptide metabolism is the
intracellular location of PREP. As mentioned above, the enzymes cleaving extracellular
active neuropeptides are either anchored to the cell membrane or secreted to the
extracellular space. Inside the cell, the enzymes participating in the biosynthesis of the
neuropeptides are packaged inside the LDCVs with the precursor molecules. Cytosolic
enzymes are not known to participate in neuropeptide processing and there is no evidence
indicating that PREP would be inside the LDCVs. Even though PREP has been detected in the
synaptosomal membranes and in plasma and cerebrospinal fluid, the quantities of PREP
have been very small compared to the tissue levels (Tenorio-Laranga et al. 2008; Tenorio24
Laranga et al. 2010). Therefore, the direct role of PREP in the degradation of extracellular
neuropeptides is currently unlikely.
However, Tenorio-Laranga et al. 2011 suggested a novel role for PREP in
peptide processing. As a cytosolic enzyme, PREP could generate or degrade active peptides
derived from cytosolic proteins. As an example, the tetrapeptide acetyl-N-Ser-Asp-Lys-Pro
(Ac-SDKP) derived from the intracellular thymosin beta-4 (Tβ4), is an active molecule
important in angiogenesis (Cavasin 2006). PREP can not cleave Ac-SDKP from the full length
Tβ4, but following another peptidase PREP can detach Ac-SDKP which then promotes
angiogenesis in vivo (Myöhänen et al. 2011).
2.6 Concluding remarks of the literature review
Neuropeptides can have various functions in the CNS, but they are best known as
modulators of neurotransmission. They can affect diverse behavioural aspects in several
parts of the brain. The complex involvement of neuropeptides in multiple processes makes
it difficult to distinguish the role of a single peptide in a particular behaviour. Furthermore, it
is difficult to affect the levels or functions of a single neuropeptide in a single area of the
brain. This has to be taken into consideration when assessing the effects of PREP on the
functions of neuropeptides. More interestingly, even the cleavage products of
neuropeptides can act as individual signalling fragments, as it is the case with SP. Therefore,
the inhibition or enhancement of neuropeptidergic signalling can have various and
unpredictable effects. The phenomen has also raised concerns about the possiblility to use
neuropeptide agonists or antagonists as drug candidates.
25
3. AIMS OF THE STUDY
Despite the intensive research, the physiological role of PREP is still unknown. This study
aimed to discover novel and clarify the supposed functions of PREP in the brain with
different biochemical and behavioural testing methods. The specific aims of this study were:
I
To study the role of PREP as a target for various psychopharmacological compounds.
II To investigate the colocalization and
neurotransmitter systems in the brain.
interaction
of
PREP
with
major
III To explore the colocalization and functional connection of PREP with neurotensin
system in the nigrostriatal and mesolimbic dopaminergic pathways.
IV To find out whether PREP inhibition affects behaviour in animal models of locomotor
activity, memory and learning and Parkinson’s disease in rats.
Figure 3-1. The aims of the study in a simplified scheme. Solid lines represent proved interactions and
dash lines putative interactions.
26
4. MATERIALS AND METHODS
4.1 Animals
Animals were purchased from Harlan Laboratories, the Netherlands (II, III, IV) and Biocenter
2 or 3, Helsinki, Finland (I). Invariably, the animals were housed under controlled conditions
(temperature 20 ± 2°C, 12-h light/dark cycle, water available ad libitum). Animals had free
access to food (Harlan Teklad Global Diets, The Netherlands) in all experiments except in the
radial-arm maze study (IV). The detailed description of the restricted diet used in the radialarm maze study is in paper IV. All animal experiments were performed according to the
Council of Europe (directive 86/609) and Finnish guidelines and approved by the State
Provincial Office of Southern Finland.
9-11 weeks old male Wistar rats (n = 8) were used for testing the effect of
various CNS drugs on the activity of PREP (I). 15 male NMRI mice weighing 35-45 g were
used for the ex vivo study with a single dose of thioridazine (I). In radial-arm maze
experiments (IV), young (3-month-old; n = 31) and old (8- to 9-month-old, n = 51) male
Wistar rats were used. Locomotor activity (IV) was studied with 11 young (3-month-old)
male Wistar rats. The effects of selective neurotransmitter lesions on the activity and
expression of PREP were studied with male Wistar rats (n = 65) weighing 250-350 g on the
operation day (II). The effects of neurotensin receptor modulation and PREP inhibition on
the levels of dopamine and the co-localization of PREP with neurotensin and NTS1 in the rat
brain were studied with 128 and 6 male Wistar rats, respectively (III and unpublished data).
The rotational behaviour was tested with 36 male Wistar rats weighing 260-350 g on the day
of operation (unpublished).
4.2 Drugs
Suc-Gly-Pro-AMC (N-succinyl-glycyl-prolyl-7-amino-4-methylcoumarin), the substrate for
PREP activity assay (studies I, II) was purchased from Bachem (AG, Bubendorf, Switzerland)
and dissolved in assay buffer (0.1 M Na-K-phosphate buffer, pH 7.0). AMC (7-amino-4methylcoumarin; Sigma-Aldrich Chemie GmbH, Steinheim, Germany) was used as a
fluorescence standard in the PREP activity assay (I, II) and dissolved in DMSO. For details of
the psychoactive compounds used in the PREP activity assays of study I, see Table 1 in paper
I.
PREP inhibitors JTP-4819 (2(S)-[[2(S)-(Hydroxyacetyl)-1-pyrrolidinyl]carbonyl]N-phenylmethyl)-1-pyrrolidinecarboxamide) and KYP-2047 (4-phenylbutanoyl-L-prolyl-2(S)cyanopyrrolidine) were synthesised by Dr. Erik Wallén (University of Eastern Finland,
Department of Pharmaceutical Chemistry, Kuopio, Finland). JTP-4819 was dissolved in assay
buffer or saline and used in PREP activity assays and in the ex vivo study (I). KYP-2047 was
given to the rats in the radial-arm maze studies (IV), in the rotational experiments
(unpublished) and in the study of the interaction of PREP and neurotensin (III). It was
dissolved in 5% Tween 80 because of its low solubility in water. In rat brain, 3 mg/kg KYP2047 causes 85% inhibition of PREP activity ten minutes after its i.p. administration
27
(compound 2B in Venäläinen et al. 2006) and the inhibition lasts for several hours. NTS1
agonists JMV-449 (Tocris Bioscience, Bristol, United Kingdom) and PD149163 (NIMH
Chemical Synthesis and Drug Supply Program) were dissolved in saline. Saline was also used
to dissolve NTS1 antagonist SR142948 (Tocris Bioscience, Bristol, United Kingdom) (III).
For the neurotransmitter lesion study (II), 6-hydroxydopamine (6-OHDA) was
obtained from Sigma-Aldrich and dissolved in 0.2 mg/ml ascorbic acid solution. Ibotenic acid
(Sigma-Aldrich), 5,7-dihydroxytryptamine (5,7-DHT; Sigma-Aldrich) and DSP-4 (N-(2chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride; Sigma-Aldrich) were dissolved in
saline. In the radial-arm maze study (IV), scopolamine hydrobromide (Sigma-Aldrich)
dissolved in saline was used. For the ex vivo study (I), thioridazine hydrochloride (SigmaAldrich) was dissolved in saline.
In the rotational experiments (unpublished), L-3,4-dihydroxyphenylalanine
methyl ester hydrochloride (L-dopa, LD; Sigma-Aldrich) was dissolved in 0,25 %
carboxymethylcellulose gel (Sigma-Aldrich). Carbidopa (CD; Orion Pharma Oyj, Espoo,
Finland) was added in the same gel to give a suspension. Saline was used to dissolve
apomorphine hydrochloride hemihydrate (Sigma-Aldrich).
All treatments were given i.p. in all other studies exept in studies II and III
where some of the neurotoxins (6-OHDA, 5,7-DHT and ibotenic acid) and NTS1 ligands (JMV449 and SR142948) were injected i.c. In the rotation experiments, apomorphine was given
s.c. Injection volumes (i.p. and s.c. injections) were 0.1 ml/100 g for rats, and 0.1 ml/10 g for
mice. Injection volumes for i.c. injections are described in detail in II and III. Drug doses were
calculated as free base.
4.3 Stereotaxic surgery (II, III)
Stereotaxic surgery was used to administer neurotoxins (6-OHDA, 5,7-DHT and ibotenic
acid) in study II and NTS1 ligands (agonist JMV-449 and antagonist SR142948) in study III to
specific areas of the rat brain. The rats were operated under general isoflurane anesthesia
(4.5 % for induction, 2-3 % for maintenance; Baxter Oy, Helsinki, Finland) using a stereotaxic
frame (Stoelting, Wood Dale, IL, USA). Lidocaine (Orion Pharma) was used to give a local
anesthetic effect. After drilling a small hole to the skull, the injections of were given with a
10 μl microinjection syringe (Hamilton, Bonaduz, Switzerland). In the end of the operation,
the rats received a single dose of tramadol (1 mg/kg, s.c.; Orion Pharma) for postoperative
pain.
4.3.1 Administration of neurotoxins (II)
In study II, two cholinergic nuclei (the medial septum and Meynert nucleus), one 5-HT
nucleus (the dorsal raphe) and a dopaminergic pathway (the medial forebrain bundle; MFB)
were lesioned with neurotoxins. Only one nucleus was lesioned from each rat. Unilateral
lesion of the MFB was also used to study the effects of PREP inhibitor on the rotational
behaviour (unpublished). The details of the i.c. neurotoxin injections are given in Table 4-1.
The coordinates of the injections were calculated relative to bregma according to the atlas
of Paxinos and Watson 1997.
28
Table 4-1. Details of intracerebral injections (studies II and III).
Coordinates from
Injected
Injection
bregma
Study
Dose
chemical
site
A/P
L/M D/V
Injection
volume
Infusion
rate
Control
Dopaminergic
neurotoxin
(6-OHDA)
II,
rotation
MFB
-4.4
+1.2
-8.3
10 µg
4 µl
0.5
μl/min
Intact
side
5-HT
neurotoxin
(5,7-DHT)
II
Dorsal
raphe
-7.08
0.4
-6.0
0.08 μg
0.2 µl
0.2
μl/min
Saline
II
Meynert
nucleus
-1.32
3.4
-7.4
3.0 µg
3 µl
1 μl/min
Intact
side
II
Medial
septum
0.72
0.0
-7.0
3.0 µg
3 µl
1 μl/min
Saline
III
Striatum
+1.0
+3.0
-5.0
5.0 µg
5 µl
1 µl/min
Saline
Cholinergic
neurotoxin
(IBA)
NTS1 agonist
(JMV-449) or
antagonist
(SR142948)
NTS1 agonist
(JMV-449) or
III
NAcc
+1.7 +1.1 -7.0 5.0 µg
5 µl
1 µl/min Saline
antagonist
(SR142948)
A/P, anterior/posterior; 5,7-DHT, 5,7-dihydroxytryptamine; D/V, dorsal/ventral; 6-OHDA, 6hydroxydopamine; 5-HT, 5-hydroxytryptamine; IBA, ibotenic acid; L/M, lateral/medial; MFB, medial
forebrain bundle; NAcc, nucleus accumbens; NTS1, neurotensin receptor 1.
4.3.2 Intracerebral injections of NTS1 agonist and antagonist (III)
NTS1 agonist JMV-449 or antagonist SR142948 was injected i.c. either to the striatum or
NAcc (Table 4-1). 1 h after the injection the rats were decapitated, the brains were removed
and dissected on ice. Tissue samples were collected from the striatum, NAcc, SN and VTA,
frozen on dry ice and stored -80 °C before the high-performance liquid chromatography
(HPLC) analysis of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid
(HVA). A group of rats received PREP inhibitor 5 mg/kg KYP-2047 30 min before the i.c.
injection.
4.4 Intraperitoneal injections of NTS1 agonist and antagonist (unpublished)
NTS1 agonist PD149163 and antagonist SR142948 were injected i.p. (1 mg/kg). Control rats
received an injection of saline. 15, 30 or 60 min after the administration the rats were
decapitated and the brains were removed and dissected on ice (n = 6 in each treatment
group/time point). The dissected tissues were frozen on dry ice and stored -80 °C until the
HPLC analysis of dopamine, DOPAC and HVA.
29
4.5 The ex vivo study with a single dose of thioridazine (I)
Saline, thioridazine (10 mg/kg; an antipsychotic compound that also inhibits PREP with its
therapeutical plasma concentrations) or JTP-4819 (10 mg/kg) was injected i.p. to mice. 1 h
after the injection, the mice were decapitated, their brains were removed and the cortex,
the cerebellum and the prefrontal cortex (PFC) were dissected on ice and stored in -80°C
until the measurement of PREP activity.
4.6 Behavioural testing methods (IV)
4.6.1 Eight-arm radial maze (IV)
The protocol for the radial-arm maze experiment is described in detail in IV. Briefly, the rats’
task was to find pieces of chocolate breakfast cerial (SunMuro, TopFood Oy, Finland) at the
end of four randomly chosen arms. The testing was done in a dark room, and the rats had to
navigate in the maze just by looking at the visual cues, such as the furniture. To keep the
rats motivated in the task, they were kept on a restricted diet. The rats were first habituated
(Fig. 4-1, days 1 and 2) to the maze, trained the task (Fig. 4-1, days 3-11) and the
experiments were done on days 12-15. The experiment started when the rat was placed on
the central platform and ended when all rewards were eaten or if 10 min had passed. The
training and the experiments were conducted twice a day with a 1 h gap in between. The
same four arms were always baited for the same rat.
Figure 4-1. Experimental design for the eight-arm radial maze experiments (study IV).
30
On the testing days, each rat was injected i.p. twice 30 min before the first trial of the day.
The treatment groups were saline + Tween (SAL-TWE; young, n = 8; old, n = 12), saline + 5
mg/kg KYP-2047 (SAL-KYP; young, n = 8; old, n = 13), 0.4 mg/kg scopolamine + Tween (SCOTWE; young, n = 8; old, n = 13) or 0.4 mg/kg scopolamine + 5 mg/kg KYP-2047 (SCO-KYP;
young, n = 7; old, n = 13). The experimenter was blind for the treatments and kept track of
the time the rats spent in the maze, their visits in each arm and the number of eaten
rewards. On testing day 3 (day 14), rats did the test only once and the gap was prolonged to
24 h so that the second test was on testing day 4 (day 15) without drug injections.
4.6.2 Locomotor activity (IV)
The effect of 5 % Tween 80 or KYP-2047 (3 or 5 mg/kg) on the locomotor activity of 3-month
old rats was measured for 60 min in the MED Associates open field activity monitor (43.2 x
43.2 x 30.5 cm; six boxes; St. Albans, GA, USA) directly after the 30 min habituation period
and the i.p. injections. The distance travelled and rearings were registered via infrared
photo beam interruptions. The locomotor activity experiments were conducted as a crossover study meaning that all treatments were given to all rats. The washout period was one
week.
4.6.3 Rotational behaviour (unpublished)
The rotational behaviour was studied two weeks after the 6-OHDA lesion to the MFB with
an apomorphine (0.1 mg/kg, s.c.) control rotation (n = 36; experimental design in Fig. 4-2.) in
the rotometer bowls (MED Associates Inc., GA, USA). Thereafter, two priming rotations with
LD (10 mg/kg, i.p.) and CD (30 mg/kg, i.p.) were done three and four weeks post-lesion to
rats that had more than 30 contralateral rotations in the apomorphine control rotation.
Priming was done to sensitize the rats to LD (Brotchie 2005). Eight rats that rotated the
most were selected to the final experiments. LD/CD-induced turning behaviour was
monitored weekly for 120 min. The rats received KYP-2047 (3, 10 or 30 mg/kg), vehicle (5 %
Tween 80) or 10mg/kg KYP-2047 without LD/CD in a cross-over manner 30 min prior to the
experiment. After the rotation experiments, the success of the lesion was confirmed by
analyzing the dopamine levels with HPLC in both striata.
Figure 4-2. Experimental design for the rotational experiments. APO, apomorphine; CD, carbidopa;
LD, L-dopa; MFB, medial forebrain bundle.
31
4.7 Analytical procedures
4.7.1 Tissue preparation (I, II)
For the enzyme activity measurements in study I, procedure described in Venäläinen et al.
2002 was used. The animals were decapitated, the brains were removed and homogenized
with assay buffer (0.1 M Na-K-phosphate buffer, pH 7.0) with a mechanical (rats; Kontes
Microtube Pellet Pestle Rod with Motor, Vineland, NJ, USA) or ultrasound (mice; Rinco
Ultrasonics, Arbon, Switzerland) homogenizer. The protocol described in Myöhänen et al.
2008c was used to prepare the samples for enzyme activity measurements in study II. Two
weeks post lesion the rats were deeply anesthetized with pentobarbital and then perfused
intracardially with phosphate-buffered saline. The brains were removed, dissected on ice
and the samples were cooled on dry ice. The homogenization was done to the assay buffer
with an ultrasound homogenizer (the striatum, hippocampus and PFC samples) or a
mechanical homogenizer (the cortex and cerebellum). Thereafter the homogenate was
centrifuged at 10 000 g, 4°C, for 20 min. The supernatants were collected, diluted to the
final concentration with assay buffer if needed and stored at -80°C until the enzyme activity
assay.
For tissue preparation in paper II and III, a similar protocol as in Myöhänen et
al. 2008c was used. Two weeks post lesion, the rats were deeply anesthetized with
pentobarbital and perfused first with phosphate-buffered saline and then with 4%
paraformaldehyde in 0.1 M phosphate buffer, pH 7.4. Next, the rats were decapitated and
the brains were removed, post-fixed with 4% paraformaldehyde and sucrose and stored at
-80°C until sectioning into 40μm cryosections with a microtome (Leica SM2010, Leica
Microsystems Inc., Bannockburn, IL, USA).
4.7.2 PREP activity assay (I, II)
The PREP activity assays in studies I and II were performed as described earlier in
Venäläinen et al. 2002. In brief, sample was preincubated with 0.1 M Na-K-phosphate buffer
(pH 7.0) for 30 min at 30°C. The reaction was initiated by adding substrate (4 mM Suc-GlyPro-AMC) and the plate was incubated for 60 min at 30 °C. The reaction was terminated
with 1 M Na-acetate buffer (pH 4.2). The formation of fluorescent AMC was measured with
a Wallac 1420 VICTOR2 fluorescence plate reader (PerkinElmer, Waltham, MA, USA) with the
excitation and emission wavelengths of 355 and 460 nm, respectively. The reaction
velocities are given as nmol AMC/min/mg protein.
4.7.3 Validation of PREP activity measurements (unpublished)
Earlier, the effects of reaction conditions, such as the solvents, pH and temperature have
been studied by Venäläinen et al. 2002. The method with rat brain homogenate was
validated by studying the effects of pipetting order and keeping reagents on ice during the
experiment. Also the intra-assay variation, day-to-day variation and variation in the
standards were studied. Intra-assay and day-to-day variation were calculated with a high
(control) and low (1000 µM VPA) PREP activity level. In the intra-assay test, every second
32
well on the plate was a control well and every second a 1000 µM VPA well. Day-to-day
variation was calculated by adding the same high and low activity wells into other
measurement’s plates on ten consecutive experiment days. Mean, standard deviation and
coefficient of variation (CV) were calculated from both PREP activity levels. Variation in the
standard wells was calculated by measuring the same well plate three times with the
fluorescence plate reader, and from ten different well plates. Mean, standard deviation and
CV were calculated from the fluorescence values.
4.7.4 Kinetic measurements (I)
The PREP inhibition type and kinetic parameters (IC50 and Ki) were measured from
ketanserin, thioridazine and VPA with the PREP activity assay described in 4.7.2 using
different substrate concentrations (50, 100, 200, 300 and 400 µM). IC50 values were
calculated with the sigmoidal dose-response equation using GraphPad Prism version 5.02
(GraphPad Software, San Diego, CA, USA). The intercept of the Y-axis in the same equation
gave the Ki values. The reversibility of PREP inhibition was tested by first incubating
ketanserin, VPA and thioridazine with the brain homogenate in room temperature for 2 hr
in a Pierce Slide-A-Lyzer Dialysis Unit (3,500 MWCO, Rockford, IL, USA) floating in the
incubation buffer and then measuring PREP activity as described in chapter 4.7.2.
4.7.5 HPLC analysis of dopamine, DOPAC and HVA in tissue samples (III, rotational
behaviour)
HPLC electrochemical detection method was used in study III and after the rotational
experiments for the analysis of the levels of dopamine, DOPAC and HVA. The method is
described earlier by Airavaara et al. 2006 and just minor changes were made to the method.
The column was heated to 40°C. The mobile phase consisted of 0.1 M NaH2PO4 buffer (pH =
3), 220 mg/l of octane sulfonic acid, MeOH 6%, and 0.2 mM EDTA.
4.7.6 Immunofluorescence (II, III)
Immunofluorescence of free-floating sections was used in studies II and III. The protocol is
described earlier by Myöhänen et al. 2008a and Myöhänen et al. 2008b. Information of used
primary antibodies is presented in Table 4-2. Briefly, sections were first incubated in 10%
normal goat serum (Vector laboratories, Burlingame, CA, USA) in tris-buffered saline
followed by an overnight incubation with respective antibodies at room temperature. After
washes with tris-buffered saline, the sections were incubated with secondary antibody (for
choline acetyltransferase, ChAT, 5-HT and dopamine-β-hydroxylase, DβH; dilution 1:500;
FITC-conjugated goat anti-rabbit IgG, Pierce Biotechnology, Rockford, IL, USA; for
neurotensin; dilution 1:500; Texas-red conjugated goat anti-rabbit IgG polyclonal antibody,
Thermo Scientific, Rockford, IL, USA; for NTS1; dilution 1:500; Texas-red conjugated goat
polyclonal secondary antibody to guinea pig IgG, Abcam, Cambridge, UK; for PREP; dilution
1:500, FITC conjugated rabbit anti-chicken IgY, Pierce Biotechnology) for 2 h. The sections
were placed to objective glasses and a mounting medium (Vectashield with 4’,6-diamino-2phenylindole, DAPI; Vector laboratories) was used to demonstrate the nuclei of the cells.
33
Colocalization in paper III was calculated by counting the percentage of all neurotensin or
NTS1 cells containing colocalization with PREP. The colocalization was classified as low (0-19
% of all neurotensin- or NTS1-containing neurons), moderate (20-49 %) or high (50 % or
more).
4.7.7 Laser scanning microscopy (II, III)
Immunofluorescence stained sections were analyzed and photographed using Leica TCS SP2
AOBS (Leica Microsystems Inc.) equipped with an argon-He/Ne laser mounted on an
inverted Leica DM IRE2 microscope (Leica Microsystems Inc.). Only brightness and contrast
were adjusted in the pictures with Adobe Photoshop CS2 software (version 9.0, Adobe
Systems Incorporated, San Jose, CA, USA).
4.8 Statistical analysis
4.8.1 Radial-arm maze (IV)
Results from the radial-arm maze experiments were analyzed with GraphPad Prism using
two-way analysis of variance (ANOVA) and Bonferroni multiple group comparison test as a
post hoc test.
4.8.2 Locomotor activity (IV)
Predictive Analysis Software (PASW) Statistics version 18 (SPSS Inc., Chicago, IL, USA) was
used to analyze the data from the locomotor activity measurements (II) with one-way
ANOVA for repeated measures and Tukey’s test as a post hoc test. The data in Fig. 5-7 was
analyzed with one-way ANOVA and Tukey’s test as a post hoc test with GraphPad Prism.
4.8.3 Enzyme activity assays (I)
The results from the ex vivo study with a single dose of thioridazine were analyzed with oneway ANOVA and Tukey’s test as a post hoc test using PASW Statistics version 18.
4.8.4 Intracerebral injections in neurotensin study (III)
Two-way ANOVA was used to analyze the overall effects of NTS1 agonist and KYP-2047 on
the levels of dopamine and dopamine metabolites (combined DOPAC+HVA) with PASW
Statistics version 18. The pairwise comparisons (saline vs. NTS1 agonist groups; saline vs.
saline KYP-2047 groups) were calculated with Student’s unpaired t-test.
34
Table 4-2. List of primary antibodies used in studies II and III.
Antigen
PREP
ChAT
5-HT
DβH
Neurotensin
NTS1
Marker for
Species
PREP protein
Chicken IgY
Cholinergic cells
Rabbit IgG
5-HT cells
Rabbit IgG
Noradrenergic cells
Rabbit IgG
Neurotensin protein
Rabbit IgG
NTS1
Guinea pig IgG
Purified pig
PREP
Synthetic peptide
from porcine ChAT.
Sequence: HGLFSSYRLPGHT
QDTLVAQKSSNH2
Whole-length 5HT
Cloned from human
DβH, synthetic
CPTSQGRSPAGTVVSIamide
Neurotensin
conjugated to BSA
Synthetic peptide:
TVGTHNGLEHSTFNMTIEP
GRVQAYC, corresponding
to amino acids 279-302 of
rat neurotensin receptor
Millipore/Chemicon,
Temecula, CA, USA
AB5042
1:2000
Sigma
Millipore/Chemicon
Millipore/Chemicon
Abcam
Product #
Dilution used
Venäläinen
et al. 2006
1:500
S5545
1:4000
AB1536
1:250
AB5496
1:500
AB66216
1:500
Specificity and
reference
Myöhänen
et al. 2007
Pre-incubation
control,
manufacturer’s
datasheet
Western blot,
manufacturer’s
datasheet
Immunogen
35
Manufacturer
Djebaili et al. 2005
Pre-incubation
control; Williams and
Manufacturer's datasheet
Beitz 1989; Dufourny
et al. 1999
ChAT, choline acetyl transferase; DβH, dopamine-β-hydroxylase; 5-HT, 5-hydroxytryptamine (serotonin); NTS1, neurotensin receptor 1; PREP, prolyl
oligopeptidase
5. RESULTS
5.1 PREP activity measurements (I, II)
5.1.1 Validation of PREP activity measurements (unpublished)
The pipetting order in the PREP activity assay affects the results possibly due to the
difference in the incubation times in the wells; it is simply impossible to pipette all the
reagents manually to the reaction with an exact speed. This was taken into account by
dividing the wells containing different concentrations of compounds evenly on the 48-well
plate. It was also noticed in the validation experiments, that keeping the rat brain
homogenate on ice when pipetting reduces the variation, but keeping the whole plate on
ice does not affect the results. Overall, there was very little variation in the intra-assay and
day-to-day variation tests. The CVs in the intra-assay variation experiment were 3.76 % for
control wells and 3.99 % for 1000 μM VPA wells (Fig. 5-1A). In the day-to-day variation test,
the CV for control wells was 6.69 % and for 1000 µM VPA wells 6.14 % (Fig. 5-1B).
Figure 5-1. Intra-assay (A) and day-to-day (B) variation in the PREP activity measurements studied
with high (control wells) and low (1000 µM valproate) PREP activity levels using rat brain
homogenate. In the intra-assay test (A), every second well on the plate was a control well and every
second a valproate well. In the test of day-to-day variation the control and valproate wells were
added into other measurement’s plates on ten consecutive experiment days. In intra-assay test, the
fluorescence values decrease slightly towards the last wells of the plate in the pipetting order of
substrate and rat brain homogenate. The means of PREP activities in control and valproate wells are
represented as black lines in both figures.
The fluorescence values of the standards varied day-to-day, but also in the
intra-assay study where the same plate was measured in the fluorescence plate reader
three times in a row. The CVs were around 18-27 %. The problem was later found to be
related to the varying temperature in the standard wells and it was fixed by making sure
that the temperature of all added liquids was equal before the reading of the plate.
36
5.1.2 The effects of psychoactive compounds on the activity of PREP (I)
Because changes in serum PREP activity have been linked to several psychiatric disorders
and the exact mechanisms of actions of the commonly used psychopharmaceuticals are not
entirely known, we tested their effects on the activity of PREP. Altogether 38 compounds
were tested. The results of the PREP activity measurements are listed comprehensively in
Table 1 in paper I and summarized in Table 5-1. In total, 18 compounds inhibited PREP
activity over 20 % from the control activity at 10 µM concentration (Table 5-1). Thioridazine
and VPA inhibited PREP at their therapeutic plasma concentrations (Fig. 2 in paper I).
Table 5-1. Psychoactive compounds that inhibited PREP activity over 20 % of the control activity at 10
µM concentration, their therapeutic plasma concentrations (if available) and brain/plasma ratios as
reported in the literature. Valproate inhibited PREP only 3 % at 10 µM concentration, but it is listed
here because it inhibits PREP with its therapeutic plasma concentration.
PREP inhibition
Therapeutic plasma
Reported brain/plasma ratio
(%) at 10 µM
concentration (µM)
Buspirone
25
0.0026-0.010
Chlorpromazine #
55
0.095-0.47
11.5
Citalopram
27
0.03-0.61
10
Clozapine #
40
0.3-0.9
24
Desipramine #
20
0.038-1.9
20
Duloxetine
21
0.016-0.46
Escitalopram
39
0.063-0.2
Flupentixol
56
0.0023-0.035
Imipramine #
29
0.18-0.54
3-4, 25
Ketanserin
49
0.13-1.3
1
Lamotrigine
22
12-60
1
Levomepromazine
32
0.015-0.04
10
Prazosin
91
0.0026-0.08
<1
Prochlorperazine #
77
0.02-0.09
420
Promazine #
64
0.035-0.18
62.5
Risperidone
56
0.0073-0.05
<1
Ritanserin
33
Thioridazine *#
91
0.27-5.4
2.2, 1.4
Valproate *#
3
280-700
15.1
*, inhibits PREP with therapeutic plasma concentration; #, inhibits PREP 30-50 % at presumably
achievable brain concentration
Compound
Due to the lipophilicity of many psychopharmacological compounds, their
concentrations in the brain may be higher than in plasma. With literature-based possible
brain
concentrations,
chlorpromazine,
clozapine,
desipramine,
imipramine,
prochlorperazine and promazine inhibited PREP 30-50% from the control activity. The
inhibition curves and IC50 values of six most potent PREP inhibitors are shown in Fig. 1 in
paper I.
37
5.1.3 Kinetic analysis of ketanserin, thioridazine and VPA (I)
The inhibition kinetics of ketanserin, thioridazine and VPA, some of the most potent
putative PREP inhibitors, were calculated to determine their PREP inhibition type. The nonlinear IC50 values of ketanserin are characteristic of mixed inhibition (Fig. 5-2). For
thioridazine, the IC50 values remained constant as a function of substrate concentration,
which is typical for a non-competitive inhibitor (Fig. 5-2). VPA was a competitive inhibitor,
since the IC50 values increased linearly as a function of substrate concentration (Fig. 5-2). In
the test for the reversibility of PREP inhibition, ketanserin, thioridazine and VPA turned out
to be reversible inhibitors, since the PREP activity was restored back to control levels after
the 2 h incubation (data not shown).
Figure 5-2. The IC50 values of ketanserin, thioridazine and valproate plotted against the substrate
concentrations. The substrate was Suc-Gly-Pro-AMC, temperature 30°C and pH 7.0. Valproate is a
competitive inhibitor, since its IC50 values increased linearly with the increasing substrate
concentration, typically for competitive inhibition. The IC50 values of thioridazine remained nearly
constant, which is typical for non-competitive inhibition. Ketanserin is a mixed type inhibitor. IC50
values were calculated by a sigmoidal dose-response equation with GraphPad Prism version 5.02.
The values are represented as mean (± SEM) of 3–4 experiments.
5.1.4 PREP activity after a single dose of thioridazine in the mouse brain (I)
Since thioridazine inhibited PREP in vitro, we conducted an ex vivo study in mice to test
whether it could be potent enough to inhibit PREP in vivo. The single i.p. injection of
thioridazine (10 mg/kg) did not inhibit PREP in the cerebellum, cortex or PFC 1 h after the
administration (Fig. 4 in paper I). However, JTP-4819 (10 mg/kg, i.p.) inhibited PREP
statistically significantly in all brain areas (JTP-4819 versus saline; p < 0.0001 in the PFC and
the cerebellum; p < 0.001 in cortex).
5.1.5 PREP activity in projection areas after the lesions of selected neurotransmitter nuclei
(II)
The purpose of this study was to find out whether PREP would be present in long
cholinergic, 5-HT, noradrenergic or dopaminergic projection neurons. Chemical lesions were
made in the rat medial septum, Meynert nucleus, dorsal raphe and locus coeruleus or the
MFB, the terminals which contain mRNA, protein or activity of PREP. The activity of PREP in
the respective terminal projection areas was measured. The projection areas were identified
using Paxinos 2004. Table 5-2. summarises the effect of the lesions of selected
38
Table 5-2. The activity of PREP in the respective projection areas two weeks after a lesion of the dorsal raphe, locus coeruleus, medial forebrain bundle,
medial septum or Meynert nucleus of the rat brain.
Projection areas
Cortex
Hippocampus
Striatum
PFC
Cerebellum
Lesion sites
Treatment
Dorsal
raphe (5-HT)
SAL
1.4 ± 0.2
0.8 ± 0.04
1.0 ± 0.09
0.8 ± 0.06
0.9 ± 0.2
5,7-DHT
1.1 ± 0.2
0.8 ± 0.05
1.0 ± 0.05
0.9 ± 0.03
1.1 ± 0.1
SAL
1.5 ± 0.08
0.7 ± 0.02
0.8 ± 0.04
0.7 ± 0.04
1.7 ± 0.2
DSP-4
1.5 ± 0.06
0.8 ± 0.06
0.8 ± 0.04
0.7 ± 0.03
1.7 ± 0.08
Control
-
-
1.2 ± 0.06
-
-
6-OHDA
-
-
1.1 ± 0.08
-
-
SAL
0.8 ± 0.04
0.9 ± 0.05
-
-
-
IBA
0.8 ± 0.02
1.0 ± 0.06
-
-
-
Control
1.0 ± 0.04
-
-
-
-
IBA
1.0 ± 0.06
-
-
-
-
Locus coeruleus
(noradrenergic)
39
Medial forebrain bundle
(dopaminergic)
Medial septum (cholinergic)
Meynert nucleus (cholinergic)
PREP-activity in rat brain (nmol AMC/min/mg prot)
PFC = prefrontal cortex, SAL = saline, 5,7-DHT = 5,7-dihydroxytryptamine, DSP-4 = a noradrenergic neurotoxin, Control = intact side used as a control, 6OHDA = 6-hydroxydopamine, 5-HT, 5-hydroxytryptamine (serotonin); IBA = ibotenic acid. All values are represented as mean ± SEM of 3-5 values.
neurotransmitter nuclei on the activity of PREP. Only the PREP activity values of lesioned
and control samples of the same lesion’s projection areas are comparable. No statistically
significant changes in the activity of PREP were observed in the projection areas (the cortex
and hippocampus) after the cholinergic lesion of the medial septum.
The cholinergic lesion of the Meynert nucleus did not cause any changes to the
activity of PREP in its projection area (the cortex). Also when the dopaminergic innervation
of the MFB was lesioned, the activity of PREP in the striatum did not change. The lesion of
the 5-HT nucleus (the dorsal raphe) did not affect the activities of PREP in the respective
projection areas (the cortex, hippocampus, striatum, cerebellum and PFC), either. Finally, no
effect on the PREP activity was seen in any of the projection areas after the lesion of the
noradrenergic nucleus (the locus coeruleus) by DSP-4. The effects of the lesions on
immunoreactive PREP are described in 5.2.1.
5.2 The expression of immunoreactive PREP (II, III)
5.2.1 The expression of immunoreactive PREP after neurotransmitter lesions (II)
The expression level of immunoreactive PREP protein was determined from the
corresponding projection areas of the lesioned nuclei using immunofluorescence (Paxinos
2004). PREP immunofluorescence was studied from the hippocampus and cortex; two
memory-related areas rich in PREP (Myöhänen et al. 2008b). The hippocampus was stained
from the medial septum, dorsal raphe and locus coeruleus -lesioned rats, and the primary
somatosensory cortex was stained after the lesion of Meynert nucleus. The
neurotransmitter lesions did not affect the expression level of immunoreactive PREP in the
corresponding projecting areas of the lesioned nuclei (Fig. 2 in paper II). The expression of
PREP in the striatum after the MFB lesion was not changed either (data not shown) as
previously described (Myöhänen et al. 2008b).
5.2.2 Colocalization of PREP and neurotensin or NTS1 in the striatum, the NAcc, the SN
and the VTA (III)
PREP is located in the same brain areas as neurotensin and NTS1 in the nigrostriatal and
mesolimbic dopaminergic pathways. Therefore we tested if there is colocalization between
them. The colocalization of PREP with neurotensin and NTS1 in the rat striatum, NAcc,
substantia nigra (SN) and VTA was studied with immunofluorescence. Neurotensin
immunofluorescence was found in vesicle-like structures in the striatum, SN, NAcc and VTA
(Fig. 1 in paper III). PREP and neurotensin immunoreactivities were highly colocalized in the
VTA (Fig. 1P in paper III), but the colocalization was moderate in the striatum and SN (Fig. 1D
and H in paper III). The colocalization was low in the NAcc (Fig. 1L in paper III). NTS1
immunoreactivity was detected in all brain areas (the striatum, SN, NAcc and VTA; Fig. 2 in
paper III) also in vesicle-like formations. Colocalization of PREP and NTS1 was high in the SN
and VTA (Fig. 2H and P in paper III), but low in the striatum and NAcc (Fig. 2D and L in paper
III).
40
5.3 Effects of PREP inhibitor, NTS1 agonist and antagonist on the levels of
dopamine and its metabolites in the striatum and NAcc (III)
In this study, our aim was to measure if there is a functional interaction of PREP and NTS1 in
the nigrostriatal and mesolimbic dopaminergic pathways. We injected NTS1 agonist (JMV449; 5 µg) and antagonist (SR142948; 5 µg) to the striatum or NAcc and measured the levels
of dopamine and its metabolites in the striatum and SN, or NAcc and VTA. A group of rats
received PREP inhibitor KYP-2047 (5 mg/kg; i.p.) 30 min before the i.c. injections.
5.3.1 Injection of NTS1 agonist and antagonist to the striatum (III)
The striatal injection of NTS1 agonist JMV-449 (5 µg) tended to increase the levels of
dopamine in the striatum (saline vs. NTS1 agonist, p = 0.052; Fig. 5-3A) and in the SN (saline
vs. NTS1 agonist, p = 0.149; Fig. 5-3B). The levels of dopamine metabolites were also slightly
increased by the NTS1 agonist in the striatum and SN (Fig. 5-3C and D). NTS1 antagonist
(SR142948, 5 µg) had a significantly different effect on the dopamine and dopamine
metabolite levels in the SN (Fig. 5-3B and D) compared to the NTS1 agonist (NTS1 agonist vs.
antagonist, p < 0.05).
Figure 5-3. The effects of intrastriatally administered saline, neurotensin receptor 1 agonist (5 µg
JMV-449; NTago) and antagonist (5 µg SR142948; NTant) on the levels of dopamine (DA) in the rat
striatum (STR; A) and substantia nigra (SN; B) 1 h after their injection. The levels of dopamine
metabolites (DOPAC and HVA) were analyzed from the STR (C) and SN (D). PREP inhibitor KYP-2047 (5
mg/kg) was given intraperitoneally 30 min before the intracerebral injections. Overall KYP-2047 or
neurotensin receptor 1 (NTS1) ligand effects were calculated with two-way ANOVA. Values are
represented as mean ± SEM, n = 4-6. * p < 0.05 NTago vs NTant groups.
41
PREP inhibitor KYP-2047 treatment (5 mg/kg, i.p.) decreased the dopamine levels in the
striatum significantly (overall KYP-2047 effect in the two-way ANOVA, p < 0.001; Fig. 5-3A),
but not in the SN (Fig. 5-3B). The effects of NST1 agonist or antagonist on dopamine or
dopamine metabolite levels were not modified by KYP-2047 in the striatum and SN (Fig. 53).
5.3.2 Injection of NTS1 agonist and antagonist to the nucleus accumbens (III)
When injected to the NAcc, NTS1 agonist JMV-449 increased the levels of dopamine in the
VTA (saline vs. NTS1 agonist groups, p = 0.026; Fig. 5-4B) and dopamine metabolites in the
NAcc (saline vs. NTS1 agonist groups, p = 0.048; Fig. 5-4C) and VTA (saline vs. NTS1 agonist
groups, p = 0.003; Fig. 5-4D). NTS1 antagonist (SR142948) injection in the NAcc had a
significantly different effect on the dopamine (NTS1 agonist vs. antagonist groups, p = 0.015;
Fig. 5-4B) and metabolite levels (NTS1 agonist vs. antagonist groups, p = 0.011; Fig. 5-4D) in
the VTA compared to the NTS1 agonist. KYP-2047 treatment had a significant increasing
effect on dopamine metabolites in the VTA (overall KYP-2047 effect in two-way ANOVA, p =
0.001; Fig. 5-4D).
Figure 5-4. The effects of saline, neurotensin receptor 1 agonist (5 µg JMV-449; NTago) and antagonist
(5 µg SR142948; NTant) on the levels of dopamine (DA) in the rat nucleus accumbens (NAcc; A) and
ventral tegmental area (VTA; B) 1 h after their injection to the NAcc. The levels of dopamine
metabolites (DOPAC and HVA) were analyzed from the NAcc (C) and VTA (D). PREP inhibitor KYP2047 (5 mg/kg) was given intraperitoneally 30 min before the intracerebral injections. Overall KYP2047 or neurotensin receptor 1 (NTS1) ligand effects were calculated with two-way ANOVA. Values
are represented as mean ± SEM, n = 4-6, except in NAcc injection saline and NTago control groups n =
8-10. * p < 0.05, ** p < 0.01 Saline vs. NTago groups. # p < 0.05, ## p < 0.01 NTago vs. NTant groups.
42
5.3.3 Intraperitoneal injections of NTS1 agonist and antagonist (unpublished)
To avoid the stereotaxic operation, we tested if we can see changes in dopamine and
dopamine metabolite levels in the striatum, SN, NAcc and VTA after the i.p. injections of
saline and blood-brain barrier crossing NTS1 ligands (agonist PD149163 and antagonist
SR142948; 1 mg/kg). The levels of dopamine or dopamine metabolites DOPAC and HVA did
not change significantly in the striatum, NAcc, SN and VTA 60 min after the i.p. injections of
either saline, NTS1 agonist (1 mg/kg PD149163) or antagonist (1 mg/kg SR142948) (Fig. 55A-C). We also studied the levels of dopamine, DOPAC and HVA 15 and 30 min after the
injections, but found no changes (data not shown). Since no differences were observed, the
impact of PREP inhibitor KYP-2047 on the NTS1 ligands’ effects was not tested.
Figure 5-5. The effects of NTS1 agonist (PD149163 NTago) and antagonist (SR142948, NTant) 1 mg/kg,
i.p. on the levels of dopamine (A), DOPAC (B) and HVA (C) in the rat striatum (STR), nucleus
accumbens (NAcc), substantia nigra (SN) and ventral tegmental area (VTA) 60 min after the injection.
Mean ± SEM, n = 6/treatment.
43
5.4 The effects of PREP inhibition in behavioural models of CNS diseases (IV,
unpublished)
The behavioural effects of PREP inhibitor KYP-2047 were tested in three animal models of
CNS disorders; the eight-arm radial maze (study IV), the locomotor activity test (study IV)
and in Ungerstedt’s turning model, which is a model for Parkinson’s disease (unpublished).
Radial-arm maze study was performed to clarify the controversial role of PREP inhibitors in
memory and learning. Locomotor activity was tested after the radial-arm maze experiments
showed that PREP inhibition may increase the locomotor activity. Finally, the experiments
with the turning model were performed to see the effects of PREP inhibition in
dopaminergic signalling after the 6-OHDA lesion of the MFB.
5.4.1 The effect of PREP inhibitor KYP-2047 on the memory and learning of young and old
rats in eight-arm radial maze (IV)
The purpose of this study was to measure the effects of PREP inhibitor KYP-2047 on the
scopolamine-impaired memory in young and old rats in the eight-arm radial maze. All
results of the experiment are presented in the Fig. 1, 2 and 3 in paper IV. PREP inhibitor KYP2047 (5 mg/kg) did not have an effect on the memory and learning of scopolamine-treated
young and old rats in the radial arm maze. KYP-2047 treatment did not significantly improve
the error rates (Fig. 1 in paper IV) or the time spent in the maze (Fig. 2 in paper IV) in young
and old rats. However, the young and old rats responded differently to scopolamine
treatment and to the task in the maze.
The young rats were faster (Fig. 2 in paper IV) and they conducted statistically
significantly (SAL-TWE groups young versus old; p < 0.05 on days 2-4) fewer errors in the
maze than the old rats (Fig. 1 in paper IV). Scopolamine treatment increased the time spent
in the maze (Fig. 2 paper IV) significantly in both young (SAL-TWE versus SCO-TWE, p<0.05
on day 1 trial 1 and day 3 trial 1) and old rats (SAL-TWE versus SCO-TWE, in all trials:
p<0.0001) and the error rates of old rats (Fig. 1 in paper IV; SAL-TWE versus SCO-TWE; p <
0.05 on day 1 trials 1 and 2, and on the first trials of days 2 and 3). KYP-2047 increased the
frequencies (arms visited/minute) of young rats statistically significantly (Fig. 5-6; SAL-TWE
versus SAL-KYP, p < 0.05 on day 2 trials 1-2 and on day 3 trial 1).
5.4.2 Locomotor activity after a single administration of PREP inhibitor KYP-2047 (IV)
To test whether PREP inhbition affects the locomotor activity, vehicle or KYP-2047 (3 or 5
mg/kg) was given to young rats and the locomotor activity was measured for 60 min. KYP2047 (3 mg/kg) increased the locomotor activity of young rats in 0-50 min significantly (p <
0.05) as seen in the activity curve of Fig. 4 in paper IV. The higher dose (5 mg/kg) did not
quite cause a significant increase (p = 0.14) in the locomotor activity. The same data is
presented in a cumulative form in Fig. 5-7.
44
Figure 5-6. Effects of PREP inhibitor KYP-2047 on frequencies (arms visited/minute) of young rats in
the radial-arm maze. During testing days 1 and 2, the first trial was done 30 min after the i.p.
injection of 5 mg/kg KYP-2047 (KYP), 0.4 mg/kg scopolamine (SCO), 5 % Tween 80 (TWE) or saline
(SAL) combinations (SAL-TWE, SAL-KYP, SCO-TWE or SCO-KYP). The second trial was performed 1 h
after the first trial. On testing day 3, the gap of trials 1 and 2 was prolonged to 24 h so that trial 2
was on the testing day 4. Values are represented as mean (± SEM). *p<0.05, **p<0.01 SAL-KYP group
versus SAL-TWE group (Bonferroni multiple group comparison test). n = 8 in all groups except in SCOKYP group n = 7.
Figure 5-7. The locomotor activity of young rats in 60 min measured immediately after the i.p.
injection of vehicle (VEH; 5 % Tween 80) or PREP inhibitor KYP-2047 (3 or 5 mg/kg, i.p.). The study
was done in a cross-over manner with one week washout period. In each group, n = 11, *p<0.05 as
analyzed using one-way ANOVA and Tukey’s test as a post hoc test.
45
5.4.3 The effect of PREP inhibitor KYP-2047 on the rotational behaviour after an unilateral
6-OHDA-lesion of MFB (unpublished)
Since alterations in plasma PREP activity have been connected to Parkinson’s disease
(Mantle et al. 1996), the effects of PREP inhibition on the LD/CD-induced rotational
behaviour was measured once a week starting 5 weeks after the unilateral 6-OHDA lesion
(10 µg) of the MFB. The LD/CD-induced turning behaviour rats was not affected by PREP
inhibitor KYP-2047 treatment (3, 10 or 30 mg/kg; i.p. 30 min prior to the experiment). 3
mg/kg KYP-2047 had a tendency to decrease the contralateral rotations, but not significantly
(Fig. 5-8A). We also tested whether KYP-2047 (10 mg/kg) alone would have an effect on the
rotational behaviour. However, when given KYP-2047 without LD/CD, the rats did not
practically rotate at all (data not shown). The HPLC analysis of striatal dopamine levels
showed that the MFB lesions were successful since there was only approximately 3 %
dopamine left in the lesion side striata compared to the striata on the intact side (Fig. 5-8B.).
We also wanted to test whether PREP activity in the striatum is changed after the lesion, but
there was no change (Fig. 5-8C).
Figure 5-8. Rotational experiments with KYP-2047 on unilaterally 6-OHDA A) L-dopa/carbidopa
(10/30 mg/kg) –induced contralateral rotations in 120 min. Vehicle (5 % Tween 80; VEH) or KYP-2047
(3, 10 or 30 mg/kg) were injected to MFB-lesioned rats i.p. 30 min prior to the experiment. Rotational
behavior was measured once a week in a cross-over manner. B) Dopamine levels in intact and
lesioned (6-OHDA) striatum. Only approximately 3 % of dopamine was left in the lesioned striata
confirmig the success of the lesions. C) PREP activity in intact side and lesioned (10 µg 6-OHDA)
striatum. 6-OHDA-lesion did not affect the activity of PREP in striatum. Results are represented as
mean ± SEM, n = 8.
46
6. DISCUSSION
6.1 PREP as a potential physiological target of psychopharmacological
compounds
Changes in plasma or serum PREP-like activity have been linked to several psychiatric
diseases, such as depression, mania and schizophrenia (Brandt et al. 2007). PREP activity has
been significantly lower in patients with major depression than in healthy volunteers (Maes
et al. 1995). On the other hand, plasma PREP activity has been significantly higher in
patients with mania and schizophrenia compared to healthy control subjects (Maes et al.
1995). Interestingly, the altered plasma PREP activity levels are restored to normal levels
with the antidepressant fluoxetine and the mood stabilizer VPA. Even though the relevance
of the plasma PREP measurements has been questioned, PREP has also been connected to
mood disorders via other mechanisms. Interestingly, one possibility for PREP to affect mood
is the interference of the phosphoinositol signalling cascade (Williams et al. 1999).
Furthermore, the mood stabilizer VPA has been shown to inhibit PREP (Cheng et al. 2005).
These above mentioned findings proposed that the connection of commonly used
psychopharmacocigal substances and PREP activity should be more closely studied.
Therefore, in study I we were intrested to see whether PREP activity is inhibited by
psychopharmaceuticals, such as the antipsychotics, anxiolytics, antidepressants,
anticonvulsants or mood stabilizing drugs.
We studied extensively the effects of various psychopharmaceuticals on PREP
activity and found several weak inhibitors, some of which could inhibit PREP also at their
achievable brain concentrations. The most inhibitors were found in the group of
antipsychotics, since all antipsychotics that we tested, except haloperidol and sulpiride,
inhibited PREP. As a group, antipsychotics are often considered to be very ‘dirty’ drugs
because of their actions on various neurotransmitter receptors, such as dopaminergic, αadrenergic, muscarinic, 5-HT and histaminergic receptors (Table 1 in paper I). It seems that
there is something in the chemical structure of antipsychotics that will also affect PREP
activity. The antipsychotic thioridazine and anticonvulsant/mood stabilizer VPA were the
only compounds that inhibited PREP with their therapeutic plasma concentrations.
Interestingly, a common mechanism of action to all compounds that inhibited PREP in our
experiments is the antagonism of adrenergic α-receptors, as listed in Table 1 in paper I.
Despite these in vitro results, it seems unlikely that CNS drugs would inhibit
PREP in vivo, since their potency to inhibit PREP is considerably low (Ki values in µ-molar
range) in relation to the therapeutic levels. Second, the CNS drugs work mainly on receptors
or other target proteins on the membrane of the neurons. Although PREP activity has also
been measured from body fluids such as plasma and cerebrospinal fluid, PREP is currently
believed to be mostly intracellularly located. Therefore the compounds need to penetrate
cell membrane before they are able to encounter PREP. In our studies, we used brain
homogenate, where the cell membranes have been mechanically broken. Very little is
known about the intracellular concentrations and effects of the CNS drugs even though they
47
are known to be very lipophilic and penetrate membranes, such as the blood-brain barrier.
Lastly, even the most potent PREP inhibitor in our studies, thioridazine, did not inhibit PREP
in the ex vivo study with mice. However, the brain homogenate was diluted before
measuring PREP activity and that can result in a false negative outcome. Still, our studies
showed that these compounds may not be potent enough to inhibit PREP at their
therapeutical concentrations.
6.2 The effects of selective neurotransmitter lesions on the activity and
expression of immunoreactive PREP protein
In study II, we lesioned the major nuclei of the nerve tracts of common small-molecule
neurotransmitters, and studied their effects on PREP activity and immunofluorescence
expression in the respective projection areas. The purpose of this study was to investigate
whether PREP is located in the terminal areas of ACh, 5-HT, noradrenaline and dopamine
projection neurons. The lesioned nuclei were chosen based on their projections to areas
known to contain PREP protein, PREP mRNA or high PREP activity, e.g. the cortex, striatum,
cerebellum, hippocampus and hypothalamus (Fig. 1B-C in paper II) (Bellemere et al. 2004;
Myöhänen et al. 2007; Myöhänen et al. 2008b). Our results showed that PREP activity and
immunoreactivity did not change in the terminal areas after the lesions, suggesting that
there is no PREP, or no direct connection between the ACh, 5-HT, noradrenaline and
dopamine systems and PREP, at least in the long projection neurons. It has to be pointed
out though that we did not study the immunoreactivity of PREP from all of the projection
areas of the lesioned nuclei.
PREP has been linked to number CNS disorders, such as the cognitive,
neurodegenerative and psychiatric conditions (Maes et al. 1995; Maes et al. 1996; Toide et
al. 1998; Myöhänen et al. 2012), which are in turn often connected to abnormalities in the
functions of conventional small-molecule neurotransmitters. For quite some time, PREP has
been thought to affect the functions of neurotransmitters by cleaving the neuropeptides
(Männistö et al. 2007), which usually coexist in neurons with one or more classic
neurotransmitters as co-transmitters or modulators (Hökfelt et al. 2003). For instance,
GABAergic neurons can use SP as a neurotransmitter (McGinty 2007). Therefore, by cleaving
neuropeptides, PREP could change the functions of neurotransmitters. However, it has been
shown that for example PREP and SP are only poorly colocalized in the brain (Myöhänen et
al. 2008a). Also, as the role of PREP as neuropeptide-cleaving enzyme is questionable, PREP
may not be connected to neurotransmission via neuropeptide hydrolysis (Männistö et al.
2007; Jalkanen et al. 2011).
Despite the intesive research in the animal models of cognition, there is a lack
of studies about the direct effects of PREP on neurotransmitter functions, and the most data
comes from the immunohistochemical studies. Myöhänen et al. 2007 have studied the
localization of PREP in the rat and human brain and also its spatial association with
neurotransmitters and neuropeptides (Myöhänen et al. 2008b). They found PREP in
GABAergic, cholinergic and glutamatergic neurons in the brain, pointing to the fact that
48
PREP may be involved in excitatory and inhibitory neurotransmission (Myöhänen et al.
2008b). Also, a few studies have reported functional associations of PREP with
neurotransmitters. For example, PREP inhibitor JTP-4819 has elevated ACh activity in the
cortex of aged rats, suggesting that PREP is connected to cholinergic neurotransmission
(Toide et al. 1995a). In another study, an antagonist of the glutamatergic NMDA receptor
has increased PREP activity in the posterior cingulate/retrosplenial cortices (Arif et al. 2007).
Quite recently Jalkanen et al. 2012 studied the effects of two PREP inhibitors KYP-2047 and
JTP-4819 with microdialysis and found that, if anything, PREP inhibitors decrease the levels
of ACh and dopamine in the rat striatum.
Our results show that PREP is either not present in these long projection
neurons or that there is no such signal coming from the presynaptic neuron that could affect
PREP activity or expression in the postsynaptic neuron. Earlier immunohistochemical studies
support the view that PREP is absent from these projection neurons of the ACh, 5-HT,
noradrenaline and dopamine systems (Myöhänen et al. 2008b). It seems more probable that
PREP is located in the cholinergic interneurons and in glutamatergic and GABAergic neurons,
especially in the cortex, thalamus and nigrostriatal pathway. The other possibility is that
PREP activity and expression are not dependent on the signals coming from the soma to the
terminal area.
One explanation is that the activation of microglia in the lesioned areas is
masking the changes in the PREP activity or expression in the terminal areas caused by the
lesions. It has been shown that PREP is significantly upregulated upon microglial activation
(Klegeris et al. 2008). After the 6-OHDA lesion of the MFB such activation lasts at least 15
days or even longer (Marinova-Mutafchieva et al. 2009). In our study, the activity and
expression of PREP was studied two weeks post-lesion. Therefore it is possible that the
upregulation of PREP due to microglial activation affects our results.
As a conclusion, in study II we showed that there was no change in the PREP
activity or protein expression after devastating the long projection neurons that use ACh, 5HT, noradrenaline or dopamine as transmitters. The results support the earlier findings
showing that PREP is not present in the projection areas of major long aminergic or
cholinergic projection neurons but rather in short GABAergic, glutamatergic and cholinergic
interneurons.
6.3 The colocalization and functional interaction of PREP with neurotensin
and its receptor in the nigrostriatal and mesolimbic dopaminergic pathways
In study III, we were interested in the possible colocalization and interaction of PREP with
neurotensin and NTS1 in the nigrostriatal and mesolimbic dopaminergic pathways. As
previously mentioned in chapter 2.5, PREP has been thought to partcipate physiological
processes through the cleavage of neuropeptides but not without criticism mostly due to
the anatomical dissociation of intracellular PREP and neuropeptides, which are secreted to
the extracellular space. Therefore PREP may not be able to cleave the neuropeptides in vivo
if not secreted, a situation that has not been demonstrated. However, the location of PREP
49
in the perinuclear space, and the changes in secretion at different PREP levels, have
suggested the role of intracellular PREP in protein maturation and secretion (Schulz et al.
2005). Indeed, PREP has been shown to be associated with intracellular membranes in
neurons, such as the RER and the Golgi apparatus (Myöhänen et al. 2008b; Tenorio-Laranga
et al. 2008). Previous studies have shown that PREP, neurotensin and NTS1 should be
located in the same neurons in the nigrostriatal and mesolimbic dopaminergic pathways,
where neurotensin is known to regulate dopaminergic signalling (Binder et al. 2001). The
anatomical location and known and proposed functional relationships of PREP, NTS1 and D2
in nigrostriatal and mesolimbic pathways are summarized in Figure 6-1.
We found that PREP was highly colocalized with neurotensin and NTS1 in the
VTA and moderately in the striatum and SN. However, in the NAcc, the colocalization was
low. Therefore, at least in the VTA there is a possibility for PREP, neurotensin and NTS1 to
interact intracellularly. Interestingly, the colocalization data was supported by the results of
the i.c. injections of NTS1 agonist (JMV-449). Striatally injected JMV-449 tended to increase
the levels of dopamine and its metabolites in the striatum and SN. The injection of JMV-449
to NAcc increased dopamine levels in the VTA and dopamine metabolite levels in the NAcc
and VTA. NTS1 antagonist (SR142948) reversed the dopamine- and metabolite-increasing
effects of JMV-449 in the SN and VTA.
Interestingly, KYP-2047 treatment 30 min before the i.c. injections decreased
the dopamine levels in the striatum. In the mesolimbic pathway, KYP-2047 increased the
levels of dopamine metabolites in the VTA. In the SN and VTA, NTS1 antagonist tended to
reverse the mild increasing effects of PREP inhibitor (KYP-2047) on the dopamine and
metabolite levels. These findings support the view about the putative role of PREP in
intracellular signalling, e.g. in the vesicle transport. Since PREP has been earlier shown to be
located close to microtubules, RER and the Golgi network, it seems possible that PREP may
control the transportation of dopamine or neurotensin vesicles or the vesicles formed after
the internalization of NTS1. The decrease of the dopamine levels in the terminal areas, like
the striatum, could be caused by a malfunction in the vesicle transportation. A similar
decrease of striatal dopamine levels was also found in the microdialysis study of Jalkanen et
al. 2012 with two PREP inhibitors KYP-2047 and JTP-4819. Furthermore, the effects of KYP2047 on dopamine and its metabolite levels in the SN and VTA may be mediated via NTS1.
These results indicate that there is an interaction between PREP and
neurotensin system in the nigrostriatal and mesolimbic dopaminergic pathways resulting in
changes in the dopaminergic signalling. Furthermore, the interaction seems to be different
in these two dopaminergic pathways.
50
Figure 6-1. The location, known and proposed interactions of dopaminergic D2 receptor (D2),
neurotensin (NT), neurotensin receptor 1 (N1; in the text NTS1) and prolyl oligopeptidase (PREP) in
nigrostriatal and mesolimbic dopaminergic pathways. The contents of the bordered and numbered
grey boxes are represented in detail in the grey coloured boxes on the right. Summarized from Binder
et al. 2001 and Myöhänen et al. 2009.
Known interactions:
1. In the nigrostriatal and mesolimbic dopaminergic pathway, NTS1 and D2 receptors are located
presynaptically. NTS1 inhibits the D2 autoreceptor leading to an increase in the release of presynaptic
dopamine.
2. In the mesolimbic dopaminergic pathway, NTS1 and D2 receptors are located also postsynaptically.
Postsynaptic NTS1 inhibits postsynaptic D2 receptor and the net effect is decrease in the
dopaminergic signalling.
Proposed interactions:
3. PREP is located in the same neurons with neurotensin and/or NTS1. PREP might regulate
dopaminergic neurotransmission via two pathways:
a) PREP could have an effect on NTS1 after its internalization for example through an interaction with
the vesicles. Interaction with NTS1 could lead to a change in dopaminergic signalling.
b) PREP could influence the processing of neurotensin or its precursor peptide or their vesicle
trafficking.
DA, dopamine; GABA, gamma-aminobutyric acid; VTA, ventral tegmental area.
51
6.4 The effect of PREP inhibition on motor functions, memory and learning,
and rotational behaviour in rats
The data about the role of PREP and PREP inhibitors in behaviour is not conclusive. In fact,
the only kind of behavioural effects that have been extensively studied are the effects of
PREP inhibitors on cognitive processes (reviewed in Männistö et al. 2007), and even those
results are far from consistent. We studied the effects of a PREP inhibitor KYP-2047 on
behaviour in three animal models: locomotor activity (IV), eight-arm radial maze (IV) and
rotational behaviour (unpublished). The data about the behavioural effects (except the data
from cognitive studies, which has been reviewed by Männistö et al. 2007 of PREP inhibitors
from previous and our studies is listed in Table 6-1.
In the radial-arm maze study, KYP-2047 (5 mg/kg) did not improve the memory
and learning of young or old rats in the radial-arm maze. There were clear differences in the
performances of the young and old rats. Generally, old rats needed more time to complete
the task than young rats, and they made more errors. Also the responses to scopolamine
and KYP-2047 treatments were different in young and old rats. Old rats were more sensitive
to the impairing effects of scopolamine than the young rats. Although KYP-2047 did not
improve rats’ memory in the radial-arm maze, it increased the motility of young rats in the
experiments by increasing the frequencies (arms visited per minute). Therefore, we tested
the effects of KYP-2047 (3 and 5 mg/kg) on locomotor activity in a separate test and saw an
increase in the locomotor activity. In the rotation experiments (unpublished), 3 mg/kg KYP2047 had only a tendency to decrease the LD/CD-induced contralateral rotations in 120 min.
The results of the radial-arm maze study highlight the fact that the memoryimproving effects of PREP inhibitors have not been attested. Although there are several
studies supporting the role of PREP in improvement of memory and learning, the data is
inconsistent and the underlying mechanisms have not been detected (Männistö et al. 2007).
One of the best studied PREP inhibitor in the animal models of cognitive functions is JTP4819 (Männistö et al. 2007). It has been shown to improve memory in many animal models,
such as the Morris water maze (Shinoda et al. 1996; Toide et al. 1997), radial-arm maze
(Miyazaki et al. 1998) and passive avoidance test (Toide et al. 1995a; Shinoda et al. 1996),
but the effects have rarely been robust or dose-dependent. Another intesively studied PREP
inhibitor S-17092 has even been in clinical studies (Morain et al. 2002). But obviously even
S-17092 has not been promising enough, since the latest reports from the human studies
date back to year 2007 (Morain et al. 2007).
A common theory to explain the role of PREP in memory and learning has been
the increase in the levels of cognition-enhancing neuropeptides, such as SP, TRH and AVP,
by PREP inhibition. However, the theory has been more like an assumption than a proven
fact. The effects of PREP inhibitors on neuropeptide levels have been tested, but the results
have varied with the PREP inhibitor used, the length of the treatment and the brain area
studied (Männistö et al. 2007). Jalkanen et al. 2007 studied the levels of neurotensin and SP
after acute and 10 days JTP-4819 and KYP-2047 treatment, and found no changes in the SP
levels in the cortex, hippocampus and hypothalamus and in neurotensin levels in the
hypothalamus. They concluded that there are many other enzymes participating in the
52
neuropeptide metabolism and blocking one enzyme may not be sufficient enough to
increase the levels of neuropeptides. Recently, the same group used microdialysis, and saw
that PREP inhibitors JTP-4819 and KYP-2047 do not increase the levels of neurotensin and SP
in the rat striatum (Jalkanen et al. 2011). Therefore, it seems unlikely that PREP would
participate in cognitive or other behavioural processes through neuropeptide metabolism.
All of our behavioural experiments were related to motor functions. We saw
an increase in the frequencies in the radial-arm maze and in the locomotor activity in young
rats. Interestingly, PREP is known to be present in the brain areas participating in motor
functions, such as the nigrostriatal system, primary motor cortex and cerebellum
(Myöhänen et al. 2007). However, there is no evidence about the functions of PREP in these
brain areas. One suggestion is that PREP may regulate the activity of striatal GABAergic
neurons, although that has never been measured. According to the results obtained in study
III, the inhibition of PREP does not affect robustly the dopaminergic neurotransmission in
the nigrostriatal system, at least not in a way that would change behaviour. Also further
studies about the motility-increasing effects of PREP inhibition are still needed. The test of
locomotor activity is quite rough, and does not reveal any mechanisms behind the increase
in the motility. Also, small changes in the dopaminergic signalling are difficult to see with
the rotational experiments.
In addition to our findings, also the other studies listed in Table 6-1 show that
the effects of PREP inhibitors on behaviour are not clear. PREP inhibitor Z-321 did not have
effects on locomotor activity, when tested with three different doses p.o. (Oosuka et al.
2000). In the same study, also the effect of Z-321 on sexual receptivity was tested, but it was
only impaired with the highest dose (300 mg/kg). A couple of PREP inhibitors have been
tested in the animal models of depression (Khlebnikova et al. 2009a; Khlebnikova et al.
2009b; Khlebnikova et al. 2012). In these tests, PREP inhibitors reversed some of the
depressive symptoms caused by the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6tetrahydropyridine (MPTP) or dipeptidyl peptidase IV inhibitor. However, even though the
rats were characterized as depressed after the 14 days MPTP treatment, MPTP has also
other kind of behavioural effects, such as the impairment of motor functions. Therefore it is
difficult to separate the effects of PREP inhibitors on depressive symptoms and on MPTPtreated rats in general and whether that is valid in the human depression. However, these
findings point to the fact that PREP inhibitors may exerts some kind of effects on
neurotransmission via classical or other neurotransmitter molecules that in turn cause mild
effects on behaviour.
The effects of PREP on behaviour have not been studied thoroughly enough to
make any conclusions. In future, PREP knockout mice will provide more data about the role
of PREP in behaviour. PREP knockout (Di Daniel et al. 2009) and knockdown mice
(D'Agostino et al. 2012) have already been created but their behavioural phenotype has not
been completely reported. They are more aggressive and hyperactive (Di Daniel et al. 2009)
supporting our behavioural data. The behavioural changes caused by PREP inhibitors may be
modest, or they are undetectable in the animal models previously used. They may also be
varying with age, sex and species, which should be also taken into account when planning
future studies.
53
Table 6-1. The behavioural effects of PREP inhibitors in animal models other than cognition.
PREP
INHIBITOR
TREATMENT
ANIMAL
BEHAVIOURAL
TEST
FURTHER
DETAILS
RESULT
REFERENCE
ANIMAL MODELS OF PARKINSON’S DISEASE
KYP-2047
3, 10, 30
mg/kg i.p.
single dose
Male rat
3-4
months
LD/CD-induced
rotational
behaviour
Unilateral 6OHDA lesion (10
µg) of the MFB
↔
Peltonen
et al.
unpublished
↑ 3 mg/kg
↔ 5 mg/kg
Study IV
↔
Oosuka et al.
2000
↓ Depressive
symptoms
Khlebnikova
et al. 2009b
ANIMAL MODELS OF MOTOR FUNCTIONS
KYP-2047
Z-321
3, 5 mg/kg
i.p.
single dose
100, 200 or
300 mg/kg
p.o.
single dose
Male rat
3 months
Locomotor
activity 60 min
Female
rats
Locomotor
activity 180 min
Estrogen and
progesteronetreated
ovariectomized
ANIMAL MODELS OF DEPRESSION
Z-Ala-ProOH
3 mg/kg
i.p.
7 days
Benzyloxycarbonylmethionyl2(S)cyanopyrrolidine
1 mg/kg
i.p.
14 days
Benzyloxycarbonylmethionyl2(S)cyanopyrrolidine
2 mg/kg
i.p.
10 days
Male rat
Several
different tests
for depressive
symptoms
MPTP 20 mg/kg
i.p. 14 days
Male rat
Several
different tests
for depressive
symptoms
MPTP 20 mg/kg
i.p. 14 days
Male rat
Forced
swimming test
DPPIV-inhibitor
during postnatal
period (days 518)
↓ Behavioural
despair,
swimming
behaviour
↔ Other
depressive
symptoms
↓ Depressionlike behaviour
↑ Normalizing
effect on PREP
activity in the
brain
Khlebnikova
et al. 2009a
Khlebnikova
et al. 2012
OTHER ANIMAL MODELS
Z-321
100, 200 or
300 mg/kg
p.o.
single dose
Female
rats
Estrogen and
progesteronetreated
ovariectomized
Sexual
receptivity
↔ 100 and
200 mg/kg
↓ 300 mg/kg
Oosuka et al.
2000
CD, carbidopa; DPPIV, dipeptidyl peptidase IV; LD, L-dopa; MFB, medial forebrain bundle; MPTP, 1Methyl-4-phenyl-1,2,3,6-tetrahydropyridine; 6-OHDA, 6-hydroxydopamine
6.5 General discussion
The physiological role of PREP has been studied for several decades with the main focus on
the cleavage of neuropeptides and the cognitive effects related to the putative physiological
substrates. Several PREP inhibitors have been developed, but they have not provided
enough evidence about the functions of PREP. Although these inhibitors have been
extensively studied as memory-modulating molecules, only two have progressed into the
clinical trials. There is still an obvious lack of studies concerning the overall effects of PREP
54
inhibitors on in vitro as well as in vivo functions. For example, there is not much data about
the pharmacodynamic, pharmacokinetic and toxicity profiles of the PREP inhibitors. In
addition, PREP has been mostly studied in animal models related to cognitive functions, but
the data about other behavioural studies is scarce. Therefore, the potential of PREP as a
therapeutic target for the treatment of CNS diseases has remained undefined. Also, the
newly discovered effects of PREP alternative to its hydrolytic activity, such as the proteinprotein interactions (Di Daniel et al. 2009; Myöhänen et al. 2012), may offer new insights in
studying the enzyme.
Our studies affirmed the view that PREP may not be as important in the
hydrolysis of neuropeptides or in the regulation of memory and learning as it has been
previously thought. We conducted several experiments related to PREP and
neurotransmission, such as the lesions of the neurotransmitter nuclei in paper II and study
about the colocalization and interaction of PREP and neurotensin in paper III. All of the
behavioural experiments can be considered to also measure neurotransmission. Our results
are not easily interpreted. Especially the interaction of PREP with dopaminergic systems
needs to be considered carefully. First of all, the lesion study (paper II) showed that
lesioning the MFB does not affect the activity or expression of PREP in the striatum. Also,
after the similar lesion, PREP inhibitor KYP-2047 did not affect the behaviour of the rats in
the rotational experiments. Both of these findings suggest that PREP is not connected to
dopaminergic signalling in the nigrostriatal dopaminergic pathway. However, in study III, the
single i.p. treatment with PREP inhibitor decreased the levels of dopamine in the striatum
and increased the levels of dopamine metabolites in the VTA. Furthermore, KYP-2047
treatment also increased locomotor activity of young rats after acute treatment suggesting
an involvement in the dopaminergic signalling, although locomotor activity can be also
regulated via other mechanisms.
In conclusion, PREP may control dopaminergic signalling as well as other
signalling cascades, and eventually behaviour, possibly through intracellular mechanisms
that should be further confirmed. Neuropeptidergic processes can be involved, but the
cleavage of neuropeptides extracellularly may not be the sole or most important mechanism
of action for PREP. Furthermore, as discussed in the literature part of this thesis, the effects
of neuropeptides on behaviour are complex and may vary depending on the brain area,
species, sex and peptide fragment. Interpreting the effects of PREP may be even more
difficult, since it may also possess specific effects in certain brain areas.
55
7. SUMMARY AND CONCLUSIONS
The present studies were done to discover novel and clarify reputed functions of PREP in the
rodent brain. Our results support the view that PREP inhibitors may not be as important in
the cognitive functions as previously thought. Instead, our findings suggest that PREP may
be involved in dopaminergic signalling and motor functions. The following specific
conclusions were drawn from the results:
I
PREP activity was inhibited by several psychoactive compounds. However, even
though these compounds may inhibit PREP in the brain, the action usually occurred
at concentrations that cannot be achieved in the therapy. Therefore PREP is
probably not the molecular target for known psychopharmacological compounds.
II Lesioning the nuclei of the tracts using classical modulatory neurotransmitters did
not affect the activity and expression of immunoreactive PREP in respective
projection areas suggesting that there is no PREP in the terminal areas of cholinergic
or aminergic long projection neurons. Instead, the present and previous findings
suggest that PREP is located in short GABAergic, cholinergic and glutamatergic
interneurons.
III There are different interactions between PREP and neurotensin or its receptor NTS1
in the nigrostriatal and mesolimbic dopaminergic pathways. Based on these findings,
we hypothesize that PREP may control the transport of dopamine or neurotensin
vesicles thereby affecting site-dependently dopaminergic signalling.
IV PREP inhibition increased locomotor activity in young rats but had no effect on the
memory and learning in radial-arm maze or rotational behaviour after a unilateral 6OHDA lesion. The increase in the locomotor activity suggests a role for PREP in
motoric functions.
56
Figure 7-1. The schematic conclusions of the study. Proved interactions are marked with a solid line
and putative interactions with a dash line.
57
8. ACKNOWLEDGEMENTS
This study was carried out at the Division of Pharmacology and Toxicology, Faculty of
Pharmacy, University of Helsinki during the years 2008-2012.
I wish to express my sincere gratitude to my supervisors Professor Pekka T. Männistö and
Docent J. Arturo García-Horsman for their guidance throughout my studies.
I am grateful for Professor Raimo Tuominen, Head of the Division of Pharmacology and
Toxicology, for creating a friendly working atmosphere and supporting me as a teaching
assistant.
I want to express my great appreciation to the reviewers of my thesis, Docent Jouni Sirviö
and Professor Eero Vasar, for their thorough investigation of my thesis and their valuable
comments. I would like to thank Associate Professor Markus Forsberg for agreeing to be the
opponent of this dissertation. The evaluators of my research plan – Associate Professor
Markus Forsberg, PhD Jarkko Venäläinen and PhD Jofre Tenorio-Laranga – are sincerely
acknowledged for their valuable remarks.
I am grateful for all the co-authors of these studies. This project could not have been
successful without the valuable contribution of Docent Timo Myöhänen. Furthermore, I
want to thank PhD Aaro Jalkanen and M. Sc. (Pharm) Katja Puttonen for their contribution
to the radial-arm maze study. I wish to thank Anna Niemi, Kati Rautio and Marjo Vaha for
skillful technical assistance.
I would like to express my gratitude to PhD Jarkko Venäläinen for his advice in enzyme
kinetics and for his support throughout my studies, especially during the preparation of my
master’s thesis and second manuscript.
I am forever thankful for my current and former colleagues at the Division of Pharmacology
and Toxicology. I have truly enjoyed working with all of you, both in the lab and planning
teaching-related things. Because of you, it has always been fun to come to work! In
particular, I want to thank my dearest girlfriends Susanne Bäck, Marjo Piltonen, Nadia
Schendzielorz and Reeta Talka, for everything we have experienced together during these
years. I would have been lost without your friendship and support in the scientific and notso-scientific things of life!
I wish to thank the personnel of Arabianranta pharmacy for the opportunity to maintain my
hands-on pharmacy skills in a warm and supporting atmosphere. I have shared so many fun
moments with you!
58
I wish to acknowledge the Graduate School in Pharmaceutical Research, the Finnish
Pharmacological Society, the Finnish Pharmaceutical Society, The Finnish Pharmacists'
Society, the Chancellor’s Travel Grant, the Finnish Concordia Fund, the Finnish Drug
Research Foundation, the Association of Pharmacy Teachers and Researchers, the Emil
Aaltonen Foundation and the 7th Framework Programme of Health of the European
Commission (NEUROPRO) for the financial support of these studies.
I want to thank my lovely friends for enriching my life in so many ways. I would like to
express special thanks to Marja Halonen, Miia Järvinen, Terhi Majava, Eeva Pörsti, Kaisa
Pakarinen and Satu Pihlaja for their friendship during and after our undergraduate studies.
My warmest and dearest thanks belong to Oskari, for all the love and happiness you have
brought into my life.
Finally, I owe my deepest gratitude to my family. I want to thank my beloved parents Päivi
and Kari and my dear brother Eemeli for their endless love, support and confidence in me. I
am particularly grateful for my dear mother Päivi for always being there for me.
Helsinki, October 2012
Iida
59
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