Immunocytochemical localization of neurons expressing 5-HT-moduline in the mouse brain

Role of the serotonergic system in alcohol dependence: from animal models to clinics

Alcohol dependence remains among the most common substance abuse problems worldwide, and compulsive alcohol consumption is a significant public health concern. Alcohol is an addictive drug that alters brain function through interactions with multiple neurotransmitter systems. These neurotransmitter systems mediate the reinforcing effects of alcohol. Specifically, the serotonergic system is important in mediating alcohol reward, preference, dependence, and craving. In this review chapter, we first discuss the serotonin system as it relates to alcoholism, and then outline interactions between this system and other neurotransmitter systems. We emphasize the serotonin transporter and its possible role in alcoholism, then present several serotonergic receptors and discuss their contribution to alcoholism, and finally assess the serotonin system as a target for pharmacotherapy, with an emphasis on current and potential treatments.

The relationship between serotonin receptor 1B polymorphisms A-161T and alcohol dependence

BACKGROUND

Several studies have suggested that the serotonin receptor 1B gene (5HT1B) may be important in the pathogenesis of alcohol dependence (alcoholism; ALC; AD). We examined whether 5HT1B gene A-161T polymorphisms (rs130058) are a susceptibility factor for total AD and subgroups of AD. We further explored correlation of this 5HT1B gene variant between anxiety-depression alcoholism (ANX/DEP ALC) and antisocial alcoholism (antisocial ALC) subgroups because of the high comorbidity of anxiety-depression, antisocial personality disorder, and AD.

METHODS

We recruited 522 Han Chinese in Taiwan for this study: 322 AD patients and 200 controls. The patient group was recruited primarily from medical teaching hospitals; patients with antisocial alcoholism were recruited from Taiwanese prisons. Individuals with AD were classified into 3 homogeneous clinical subgroups -- pure alcoholism (pure ALC), ANX/DEP ALC, and antisocial ALC -- using DSM-IV diagnosis. The 5HT1B gene A-161T polymorphism was determined using PCR-RFLP.

RESULTS

No significant differences in genotypic and allelic frequencies were found between controls and the total AD group or between controls and the 3 AD subgroups. However, there were significant differences in the 5HT1B gene A-161T polymorphism at both the genotype and allelic levels between the ANX/DEP ALC and antisocial ALC subgroups.

CONCLUSIONS

This study suggests that the 5HT1B gene A-161T polymorphism alone is not a risk factor for increasing susceptibility to either AD or its subtypes. However, 5HT1B gene A-161T polymorphisms might be one of the common genetic factors between the ANX/DEP ALC and antisocial ALC subgroups.

Anxiolytic profile of HG1, a 5-HT-moduline antagonist, in three mouse models of anxiety

HG1 is a new 5-HT-moduline antagonist which is itself an endogenous tetrapeptide specifically acting as an antagonist of 5-HT(1B) auto- and heteroreceptors. Blockade of endogenous 5-HT-moduline might provoke anxiolysis, so it could be a new therapeutic target in anxiety disorders. The aim of our study was to examine the effects of HG1 in three mouse models of anxiety: the four plates test (FPT), the black and white (B&W) model and the elevated plus maze (EPM). Male Swiss mice were intraperitoneally and acutely administered HG1 at the doses of 8, 16, 32 and 64 mg/kg. In these three tests, HG1 exhibited an anxiolytic profile similar to that of diazepam, the referential benzodiazepine compound, without affecting locomotor activity. In the three models used, HG1 was as efficient as benzodiazepine and may consequently exert its anxiolytic effects via the GABA-ergic system. We cannot exclude that it might also act through 5-HT receptors and rather have the profile of a selective serotonin reuptake inhibitor.

GABA, serotonin and serotonin receptors in the rat inferior colliculus

GABA and serotonin in the mammalian inferior colliculus both have a restrictive effect on fearful and aversive behavior. Immunohistochemical techniques were used to study the relationship between GABA and serotonin receptors in the central nucleus of the rat inferior colliculus. Neurons positive for 5HT1B are more numerous than those displaying 5HT1A receptors. Approximately two-thirds of GABA-positive neurons are associated with serotonin receptors.

5-HT moduline: an endogenous inhibitor of 5-HT(1B/1D)-mediated contraction in pulmonary arteries

(1) 5-HT moduline (5-HTm) is tetrapeptide (Leu-Ser-Ala-Leu) previously shown to act as a specific endogenous antagonist to central 5-HT(1B/1D) receptors. Its effects were investigated in rat and rabbit pulmonary arteries (PAs). (2) In rabbit PAs, contractile responses to the 5-HT(1B/1D) receptor agonist 5-carboxamidotryptamine (5-CT) were inhibited by 1 and 10 micro M 5-HTm in a non-competitive fashion with the maximum contractile response (E(max), per cent of response to 50 mM KCl) being reduced from 65.6+/-7% (n=6) to 39.7+/-6.5% (n=6) and 25.2+/-7.9 (n=4), respectively. The ability of 5-HTm to inhibit responses to 5-CT was increased by the aminopeptidase inhibitor bestatin (10 micro M). (3) In the rabbit PAs, the nitric oxide synthase inhibitor, N(omega)-nitro-L-arginine methylester (L-NAME) potentiated responses to 5-CT (E(max): 106+/-22.5 (n=4)) and this response was also inhibited by 10 micro M 5-HTm (E(max): 38+/-13% (n=8)). (4) 5-HTm (10 micro M) inhibited responses to 5-CT in rat PAs, the E(max) being reduced from 24.8+/-4.1% (n=7) to 15.5+/-3.7% (n=9). 5-HTm induced relaxation of 5-CT-pre-constricted rat PAs with a pIC(50) of 9.0+/-0.6 (n=9). (5) In PAs from chronic hypoxic, pulmonary hypertensive rats, the maximum response to 5-CT was increased to 80+/-8.5% (n=11). 5-HTm reduced this response to 34.4+/-6.3% (n=12). L-NAME markedly inhibited the ability of 5-HTm to inhibit responses to 5-CT (E(max) before 5-HTm: 100.5+/-16% (n=5), E(max) after 5-HTm: 107+/-11.3% (n=4)). (6) In conclusion we show here for the first time that 5-HTm is a non-competitive inhibitor of 5-HT(1B/1D) receptor-mediated constriction in PAs. In rat PAs, L-NAME can inhibit this effect of 5-HTm.

Activation of 5HT1B receptors inhibits glycinergic synaptic inputs to mammalian motoneurons during postnatal development

We investigated whether 5-HT(1B) receptor-mediated inhibition of evoked glycinergic inhibitory postsynaptic currents (eIPSCs) in hypoglossal motoneurons (HMs) changed postnatally. In HMs from postnatal days 2-3 (P2-3, neonate) and P10-11 (juvenile) rats bath application of 5-HT (10 microM) caused a not significantly different large reduction in eIPSC amplitude to 35.0+/-22.5% (mean+/-S.D.) and 35.4+/-10.6% of control; respectively. The dose-response relationship for the 5-HT(1B) receptor agonist, CP-93,129, revealed that the mean agonist concentration at half-maximal inhibition (IC(50)) was similar, 1.6 and 2.0 nM, respectively. Additionally, strong antibody labeling of 5-HT(1B) receptors in the hypoglossal motor nucleus was observed in neonates, juveniles and adults. These results demonstrate that over the postnatal period studied, 5-HT(1B) receptor-mediated inhibition of glycinergic eIPSCs is not age dependent.

Molecular, pharmacological and functional diversity of 5-HT receptors

Serotonin (5-hydroxytryptamine, 5-HT) is probably unique among the monoamines in that its effects are subserved by as many as 13 distinct heptahelical, G-protein-coupled receptors (GPCRs) and one (presumably a family of) ligand-gated ion channel(s). These receptors are divided into seven distinct classes (5-HT(1) to 5-HT(7)) largely on the basis of their structural and operational characteristics. Whilst this degree of physical diversity clearly underscores the physiological importance of serotonin, evidence for an even greater degree of operational diversity continues to emerge. The challenge for modern 5-HT research has therefore been to define more precisely the properties of the systems that make this incredible diversity possible. Much progress in this regard has been made during the last decade with the realisation that serotonin is possibly the least conservative monoamine transmitter and the cloning of its many receptors. Coupled with the actions of an extremely avid and efficient reuptake system, this array of receptor subtypes provides almost limitless signalling capabilities to the extent that one might even question the need for other transmitter systems. However, the complexity of the system appears endless, since posttranslational modifications, such as alternate splicing and RNA editing, increase the number of proteins, oligomerisation and heteromerisation increase the number of complexes, and multiple G-protein suggest receptor trafficking, allowing phenotypic switching and crosstalk within and possibly between receptor families. Whether all these possibilities are used in vivo under physiological or pathological conditions remains to be firmly established, but in essence, such variety will keep the 5-HT community busy for quite some time. Those who may have predicted that molecular biology would largely simplify the life of pharmacologists have missed the point for 5-HT research in particular and, most probably, for many other transmitters. This chapter is an attempt to summarise very briefly 5-HT receptor diversity. The reward for unravelling this complex array of serotonin receptor--effector systems may be substantial, the ultimate prize being the development of important new drugs in a range of disease areas.

The proliferation of human T lymphoblastic cells induced by 5-HT1B receptors activation is regulated by 5-HT-moduline

Serotonin (5-hydroxytryptamine, 5-HT) is a well-known neurotransmitter and immunomodulator, which has been reported to affect the function of cells in the immune system. The purpose of the herein reported experiments was to investigate whether serotonin could regulate the proliferation of a human T lymphoblastic leukemia cell line (CCRF-CEM cells) and to characterize the 5-HT receptor(s) involved in this phenomenon using a pharmacological approach. The herein presented results show that serotonin alone stimulated the proliferation of CCRF-CEM cells and that this effect could be mimicked by two 5-HT1B/1D receptor agonists (L-694,247 and GR 46611). Serotonin- or L-694,247-induced increase in cell proliferation was inhibited by a selective 5-HT1B receptor antagonist, SB-224289. A recently identified endogenous tetrapeptide, 5-HT-moduline (Leu-Ser-Ala-Leu, LSAL), which specifically antagonizes 5-HT1B/1D receptor activity, was also shown to reverse the stimulating action of L-694,247 on T cell proliferation. Taken together, these results establish the existence of a direct serotonergic control of the T cell proliferation mediated through h5-HT1B receptors. In addition, these results are in favour of an immunomodulatory role of 5-HT-moduline.

Circadian variation in the activity of the 5-HT(1B) autoreceptor in the region of the suprachiasmatic nucleus, measured by microdialysis in the conscious freely-moving rat

Intracerebral microdialysis was used to examine the function of the terminal 5-hydroxytryptamine(1B) (5-HT(1B)) autoreceptor in the region of the suprachiasmatic nuclei (SCN) of freely moving conscious rats at six time points or zeitgeber times (ZTs) across the light:dark cycle. Infusion of the 5-HT(1A/1B) agonist 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridyl)-1H-indole (RU24969) (1 microM) via the microdialysis probe produced a decrease in 5-HT output when applied at ZTs 3, 6, 15 and 21 (69.8+/-11.9, 59+/-11.7, 43.9+/-17.2 and 45.7+/-17.0% respectively). At ZTs 9 and 18 RU24969 (1 microm) failed to affect the 5-HT output significantly (28.0+/-11 and 32.8+/-24.6% decrease respectively). The profile of inhibition of 5-HT output following infusion of RU24969 (1 microM) at ZT 6 was unaffected by concurrent infusion of the specific 5-HT(1A) antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xanecarboxamide trihydrochloride (WAY100635) (1 microM) (52.48+/-17.5% decrease). The data demonstrate a circadian rhythm in the activity of the 5-HT(1B) autoreceptor in the region of the SCN.

5-HT-moduline controls serotonergic activity: implication in neuroimmune reciprocal regulation mechanisms

The serotonergic neurotransmission is known as a neuromodulatory system exerting its activity in the central nervous system (CNS) as well as at the periphery. The anatomical and morphological organization of the system based on a marked centralization of the cellular bodies and the large, almost ubiquitary, presence of axonal projections of the neurons is in good agreement with this modulatory role. Furthermore, a very high number of varicosities located along the axonal branches are capable of releasing serotonin (5-HT). The amine stimulates a number of different specific receptor types which allows 5-HT to exert different activities on its various cellular targets. Among these receptors, the 5-HT1B subtypes play a particular role as they are autoreceptors located on 5-HT neurons terminals and heteroreceptors located on non-serotonergic terminals where they control the release of the neurotransmitter. 5-HT-moduline, an endogenous tetrapeptide, regulates the efficacy of these 5-HT1B receptors, hence, is able to control the serotonergic activity in a synchronous manner for the various varicosities from a single neuron and thus may favour the differential effect of that neuron on distinct cerebral functions. Accordingly, the peptide allows the 'fine tuning' of the cerebral activity by the serotonergic system to elaborate the response given by the brain to a particular stimulus, that is, stress situations. At the periphery, the serotonergic system also appears to possess a regulatory activity via 5-HT1B receptors. In particular, the receptors located on immunocompetent cells control their activity and are themselves regulated by 5-HT-moduline likely originating from adrenal medulla and released after acute stress. The serotonergic system appears to play a major role in the reciprocal signalling existing between the neuronal and the immune system. The participation of 5-HT-moduline is likely in physiological functions as well as in pathological disorders affecting central and peripheral activities.

5-Hydroxytryptamine-moduline: a novel endogenous peptide involved in the control of anxiety

The serotonergic system is considered as a neuromodulatory system interacting with other neurotransmissions in the brain and participating in the elaboration of an adapted response of the central nervous system to external stimuli. Indeed, serotonin is involved in a large number of physiological events, such as temperature regulation, sleep, learning and memory, behaviour, sexual function, hormonal secretions and immune activity, and in parallel, it is also implicated in pathological disorders particularly in stress, anxiety, aggressivity and depression. At least 14 different types of serotonin receptors mediate serotonergic activity and among them, serotonin-1B receptors play an important role in the control of the serotonergic function. Serotonin-1B receptors are autoreceptors localized on serotonergic neuron terminals (varicosities) where they inhibit the evoked release of serotonin and its biosynthesis; they are also heteroreceptors located on non-serotonergic terminals, where they inhibit the release of the corresponding neurotransmitters (acetylcholine, GABA, noradrenaline, etc.). 5-Hydroxytryptamine-moduline, an endogenous tetrapeptide (Leu-Ser-Ala-Leu) recently isolated and characterized from rat and bovine brain extracts, was shown to specifically interact with serotonin1B receptors as an allosteric modulator having antagonistic properties in vitro and in vivo. Immuncytochemical studies using specific polyclonal anti-peptide antibodies have shown that this peptide is distributed heterogeneously in mouse brain and located in areas which also contain serotonin-1B receptors. Moreover, the content of these cerebral tissues in 5-hydroxytryptamine-moduline is affected by stress. In the present work, polyclonal anti-5-hydroxytryptamine-moduline antibodies were administered to mice via intracerebroventricular injections to study the in vivo effects of a lowering (or suppression) of this neuropeptide in the central nervous system. The inactivation of the peptide by the specific antibodies significantly modified the behaviour of the animals in two behavioural tests, the open-field and elevated plus-maze, known to be animal models related to anxiety behaviour. Treated mice displayed behaviour consistent with an anxiolytic effect of the antibody, suggesting a potential role of 5-hydroxytryptamine-moduline in the control of anxiety.

Endoproteolytic activity in mammalian brain membranes cleaves 5-hydroxytryptamine-moduline into dipeptides

This work was intended to determine which enzymatic activities from crude synaptosomal mammalian brain membranes could qualify for the status of 5-hydroxytryptamine-moduline (5-HT-moduline, LSAL, Leu-Ser-Ala-Leu) inactivating enzymes. An enzymatic assay for 5-HT-moduline metabolism was developed using [3H]5-HT-moduline measurement and high performance liquid chromatography (HPLC) technique to identify and quantify 5-HT-moduline metabolites. 5-HT-moduline metabolism displayed all characteristics of metalloprotease activity: sensitivity to divalent ion chelators, reactivation by Zn2+ ions and a pH optimum in the 7-8 range. Bestatin, an aminopeptidase inhibitor, allowed the identification of two enzymatic activities responsible for this metabolism: a bestatin-sensitive aminopeptidase and an endoprotease cleaving 5-HT-moduline into LS (Leu-Ser) and AL (Ala-Leu) dipeptides. This latter enzyme was shown to have a Km of 37.1 +/- 3.6 microM and a Vmax of 5.5 micromol min(-1) l(-1) per mg of protein. Moreover, this enzyme was insensitive to peptidyl dipeptidase A (angiotensin converting enzyme, EC 3.4.15.1), endothelin converting enzyme and neutral endopeptidase (neprylisin, EC 3.4.24.11) inhibitors and displayed some specificity among 5-HT-moduline-analogues and in particular recognized only tetrapeptides. These results, together with the isolation of the LS and AL metabolites [Rousselle, J.C., Massot, O., Delepierre, M., Zifa, E., Rousseau, B., Fillion, G., 1996. Isolation and characterization of an endogenous peptide from rat brain interacting specifically with the serotonergic 1B receptor subtypes. J. Biol. Chem. 271, 726-735] during the purification process of 5-HT-moduline are strong arguments for the physiological implication of this endoprotease in 5-HT-moduline metabolism.

Acute stress induces a differential increase of 5-HT-moduline (LSAL) tissue content in various rat brain areas

5-HT-moduline is an endogenous cerebral tetrapeptide (LSAL) which specifically interacts as an allosteric modulator with 5-HT1B receptors controlling serotonergic activity [O. Massot, J.C. Rousselle, M.P. Fillion, B. Grimaldi, I. Cloez-Tayarani, A. Fugelli, N. Prudhomme, L. Seguin, B. Rousseau, M. Plantefol, R. Hen, G. Fillion, 5-Hydroxytryptamine-moduline, a new endogenous cerebral peptide, controls the serotonergic activity via its specific interaction with 5-hydroxytryptamine1B/1D receptors, Mol. Pharmacol. 50 (1996) 752-762; J.C. Rousselle, O. Massot, M. Delepierre, E. Zifa, G. Fillion, Isolation and characterization of an endogenous peptide from rat brain interacting specifically with the serotonergic1B receptor subtypes, J. Biol. Chem. 271 (1996) 726-735; J.C. Rousselle, M. Plantefol, M.P. Fillion, O. Massot, P.J. Pauwels, G. Fillion, Specific interaction of 5-HT-moduline with human 5-HT1b as well as 5-HT1d receptors expressed in transfected cultured cells, Naunyn-Schmiedeberg's Arch. Pharmacol. 358 (1998) 279-286]. Cerebral tissue contents of 5-HT-moduline were determined in various rat brain areas after an acute restraint stress, and after repetition of this stress, to examine whether or not mechanisms involving this peptide could be affected by stress situations. The measurement of the peptide was carried out using specific polyclonal antibodies [B. Grimaldi, M.P. Fillion, A. Bonnin, J.C. Rousselle, O. Massot, G. Fillion, Immunocytochemical localization of neurons expressing 5-HT-moduline in the mouse brain, Neuropharmacology 36 (1997) 1079-1087] in a dot-ELISA (enzyme-linked-immunosorbent assay) assay in cortex, hippocampus, hypothalamus, substantia nigra, striatum and in adrenal glands. Tissue contents of 5-HT-moduline progressively and transiently increased in most studied brain regions and reached a maximal value 20 min after the beginning of the restraint stress. The increase in 5-HT-moduline tissue contents represented 323% of the value observed in unstressed control animals in the cortex, 207% in the hippocampus, 149% in the hypothalamus and 156% in the substantia nigra. Thereafter, the peptide content of the latter tissues diminished during the last 20 min of restraint and returned to control values within 1 h after the end of the stress period. The striatum did not show any significant variation of 5-HT-moduline content during restraint stress. In adrenal glands, the 5-HT-moduline content rapidly decreased (60% of controls) after the beginning of the restraint stress, the effect of this stress being progressively less pronounced, still representing 80% of controls after 40 min. Repetition of the restraint stress daily for 3 weeks totally abolished the effect of the stress on variations of 5-HT-moduline tissue content in all the studied brain regions. These results show that an acute restraint stress induces a rapid and significant increase in the amount of 5-HT-moduline contained in various brain areas. This phenomenon is likely to be related to the stress-induced 5-HT1B receptor desensitization which was previously demonstrated.

Molecular, cellular and physiological characteristics of 5-HT-moduline, a novel endogenous modulator of 5-HT1B receptor subtype

The serotonergic transmission is considered as a neuromodulatory system in the Central Nervous System. 5-HT1B receptors play an important role in this modulatory activity. We have purified from mammalian brain an endogenous peptide, LSAL, we called 5-HT-moduline, interacting specifically with 5-HT1B receptors. This interaction is characterized by a high affinity (Ki = 10(-10) M) and a non-competitive mechanism. Direct [3H]5-HT-moduline binding revealed a single population of sites having an apparent affinity constant close to 10(-10) M. Autoradiographic studies showed a brain distribution of [3H]5-HT-moduline binding sites closely related to the 5-HT1B receptors. In functional studies, the peptide is able to reverse the activity of a 5-HT1B agonist in the nanomolar range. Furthermore, this antagonist effect is also observed in vivo on mice behavior. Immunocytochemistry revealed an heterogeneous distribution of 5-HT-moduline in mouse brain. The labeled structures correspond to cellular profiles with axon-like prolongations. Moreover, in vitro, LSAL is released in a Ca++, K(+)-dependent manner. Therefore, 5-HT-moduline behaves as a neurotransmitter. The fact that 5-HT-moduline induces the desensitization of 5-HT1B receptors reflects the existence of a novel and efficient mechanism able to rapidly modulate the serotonergic activity.

5-HT-moduline, a 5-HT(1B/1D) receptor endogenous modulator, interacts with dopamine release measured in vivo by microdialysis

5-Hydroxytryptamine-moduline (5-HT-moduline) is an endogenous tetrapeptide (Leu-Ser-Ala-Leu) recently isolated and characterized from mammalian brain. This compound interacts with 5-HT1B receptors as a non-competitive, high-affinity antagonist and has the properties of an allosteric modulator. 5-HT-moduline could play an important role in the regulation of serotonergic transmission and also, through heteroreceptors, dopaminergic transmission. The aim of this work was to examine the potential ability of 5-HT-moduline to modify the basal extracellular concentration of dopamine and its metabolites (3-methoxytyramine, dihydroxyphenylacetic acid and homovanillic acid), in the rat striatum and to determine its potential interaction with the stimulating activity of a specific 5-HT1B receptor agonist, 3-(1,2,5,6-tetrahydropyrid-4-yl) pyrrolo [3,2-b] pyrid-5-one (CP-93,129), on the release of dopamine. The technique is based on in vivo microdialysis using probes implanted in the striatum of the conscious rat. Results showed that the perfusion of 5-HT-moduline directly into this structure (1.25 mM) increased the striatal level of dopamine by two-fold (104% of the absolute basal release values, P = 0.0015) and that of 3-methoxytyramine by 3-fold (293%, P = 0.0001) without any change in the terminal metabolite concentrations. The intrastriatal administration of CP-93,129 induced a statistically significant, dose-dependent increase of dopamine levels (P < 0.0001). Coperfusion of 5-HT-moduline did not significantly alter the effect of CP-93,129 at 0.1 and 0.5 mM, but appeared to have an additive effect on the lowest dose (P = 0.0406). The results obtained show that 5-HT-moduline directly administered into the striatum increases the release of dopamine in this area. Presumably, this effect results from the desensitization of 5-HT1B receptors located on dopamine terminals. However, the fact that a 5-HT1B receptor agonist (CP-93,129) also increased the release of dopamine in the striatum and that 5-HT-moduline exhibited a slight additive effect with that of a low concentration of CP-93,129 suggests that the two substances interact with different mechanisms.