Contractile effects of 5-hydroxytryptamine and 5-carboxamidotryptamine in the equine jejunum

The use of human prokinetic drugs in colic horses leads to inconsistent results. This might be related to differences in gastrointestinal receptor populations. The motor effects of 5-hydroxytryptamine (5-HT; serotonin) on the equine mid-jejunum were therefore studied. Longitudinal muscle preparations were set up for isotonic measurement. 5-HT induced tonic contractions with superimposed phasic activity; these responses were not influenced by tetrodotoxin and atropine, suggesting a non-neurogenic, non-cholinergic pathway. The 5-HT receptor antagonists GR 127935 (5-HT(1B,D)), ketanserin (5-HT(2A)), SB 204741 (5-HT(2B)), RS 102221 (5-HT(2C)), granisetron (5-HT(3)), GR 113808 (5-HT(4)) and SB 269970 (5-HT(7)) had no influence on the 5-HT-induced response; the 5-HT(1A) receptor antagonists NAN 190 (pK(b)=8.13+/-0.06) and WAY 100635 (pK(b)=8.69+/-0.07), and the 5-HT(1,2,5,6,7) receptor antagonist methysergide concentration-dependently inhibited the 5-HT-induced contractile response. The 5-HT(1,7) receptor agonist 5-carboxamidotryptamine (5-CT) induced a contractile response similar to that of 5-HT; its effect was not influenced by tetrodotoxin and atropine, and SB 269970, but antagonised by WAY 100635. 8-OHDPAT, buspiron and flesinoxan, which are active at rat and human 5-HT(1A) receptors, had no contractile influence. These results suggest that the contractile effect of 5-HT in equine jejunal longitudinal muscle is due to interaction with muscular 5-HT receptors, which cannot be characterised between the actually known classes of 5-HT receptors.     

On the mechanism of anti-hyperthermic effects of LY379268 and LY487379, group II mGlu receptors activators, in the stress-induced hyperthermia in singly housed mice

Earlier studies have demonstrated that the agonists of the mGlu(2/3) receptors produced anxiolytic actions after peripheral administration. However, the mechanism of their action is still not clear. Therefore the aim of the present study was to specify the role of the GABAergic and serotonergic system in the mechanism of the anxiolytic activity of group II mGlu receptor activators by using the stress induced hyperthermia test (SIH) in singly housed mice. We used an orthosteric mGlu(2/3) receptor agonist, LY379268, which induced anti-hyperthermic efficacy in the doses of 1-5mg/kg (73% of inhibition after a highest dose). The effect of the second ligand used, a mGlu(2) receptor positive modulator (PAM), LY487379, was observed in a dose range of 0.5-5mg/kg and reached 53% of the inhibition. The blockade of GABAergic system by GABA(A) receptor antagonist flumazenil (10mg/kg) or GABA(B) receptor antagonist CGP55845 (10mg/kg), and the blockade of serotonergic system by 5-HT(1A) receptor antagonist WAY100635 (0.1 and 1mg/kg) or 5-HT(2A/2C) receptor antagonist ritanserin (0.5mg/kg) had no influence on the anti-hyperthermic effect induced by effective dose of LY379268. However, the action of the effective dose of LY487379 was enhanced when co-administered with flumazenil, WAY100635 (0.1mg/kg) and ritanserin. Similar results were observed for the subeffective dose of LY379268 (0.5mg/kg). WAY100635 in a dose of 1mg/kg did not induce any enhancing effect on the activity of compounds. Therefore, it seems that the antagonism towards GABA(A) receptors, presynaptic 5-HT(1A) and postsynaptic 5-HT(2A/2C) receptors is responsible for the phenomenon. This article is part of a Special Issue entitled 'Anxiety and Depression'.     

In vivo evidence that 5-HT(2C) receptors inhibit 5-HT neuronal activity via a GABAergic mechanism

BACKGROUND AND PURPOSE: Recent evidence suggests that 5-HT(2C) receptor activation may inhibit midbrain 5-HT neurones by activating neighbouring GABA neurones. This hypothesis was tested using the putative selective 5-HT(2C) receptor agonist, WAY 161503. EXPERIMENTAL APPROACH: The effect of WAY 161503 on 5-HT cell firing in the dorsal raphe nucleus (DRN) was investigated in anaesthetised rats using single unit extracellular recordings. The effect of WAY 161503 on DRN GABA neurones was investigated using double label immunohistochemical measurements of Fos, glutamate decarboxylase (GAD) and 5-HT(2C) receptors. Finally, drug occupancy at 5-HT(2A) receptors was investigated using rat positron emission tomography and ex vivo binding studies with the 5-HT(2A) receptor radioligand [(11)C]MDL 100907. KEY RESULTS: WAY 161503 caused a dose-related inhibition of 5-HT cell firing which was reversed by the 5-HT(2) receptor antagonist ritanserin and the 5-HT(2C) receptor antagonist SB 242084 but not by the 5-HT(1A) receptor antagonist WAY 100635. SB 242084 pretreatment also prevented the response to WAY 161503. The blocking effects of SB 242084 likely involved 5-HT(2C) receptors because the drug did not demonstrate 5-HT(2A) receptor occupancy in vivo or ex vivo. The inhibition of 5-HT cell firing induced by WAY 161503 was partially reversed by the GABA(A) receptor antagonist picrotoxin. Also, WAY 161503 increased Fos expression in GAD positive DRN neurones and DRN GAD positive neurones expressed 5-HT(2C) receptor immunoreactivity. CONCLUSIONS AND IMPLICATIONS: These findings indicate that WAY 161503 inhibits 5-HT cell firing in the DRN in vivo, and support a mechanism involving 5-HT(2C) receptor-mediated activation of DRN GABA neurones.     

Effects of the serotonin 5-HT(2) antagonist, ritanserin, and the serotonin 5-HT(1A) antagonist, WAY 100635, on cocaine-seeking in rats

Manipulations of serotonergic systems have been shown to modify many of the behavioral effects of cocaine. It was recently demonstrated that serotonin (5-HT) depletions produced by inhibition of tryptophan hydroxylase reduced cocaine-seeking in an animal model. The present study was designed to determine whether pretreatment with specific 5-HT antagonists might also decrease cocaine-seeking. The effect of pretreatment with the 5-HT(2) antagonist, ritanserin (0.0, 1.0, or 10.0 mg/kg), or the 5-HT(1A) antagonist, WAY 100635 (0. 0, 0.1, 0.3, or 1.0 mg/kg), on cocaine (5.0, 10.0, or 20.0 mg/kg)-produced reinstatement of extinguished drug-taking behavior was measured. Although ritanserin was ineffective, WAY 100635 attenuated cocaine-produced reinstatement in a dose-dependent manner. These effects of WAY 100635 appeared to be specific since responding maintained by saccharin self-administration remained high following pretreatment with 0.3 or 1.0 mg/kg WAY 100635. These data suggest a role of 5-HT(1A), but not 5-HT(2), receptors in cocaine-seeking.     

Potentiation of excitatory serotonergic responses by MK-801 in the medial prefrontal cortex

New atypical antipsychotics show a greater affinity to serotonergic rather than to dopamine receptors, suggesting that serotonin (5-HT) has a major role in the pathophysiology and treatment of schizophrenia. The goal of this study was to characterise the response of pyramidal neurons in the medial prefrontal cortex (mPFC) to 5-HT and NMDA before and after administration of the NMDA receptor antagonist, MK-801 (dizocilpine), a well-validated pharmacological model of psychosis. mPFC pyramidal (glutamatergic) neurons were recorded in urethane-anaesthetised rats. The responses to NMDA and 5-HT were assessed using in vivo electrophysiology and microiontophoresis. The 5-HT_2A/2C antagonist ritanserin and the 5-HT_1A antagonist WAY100635 were used to block 5-HT responses. MK-801 decreased the NMDA-induced excitatory responses and increased NMDA-evoked burst activity among mPFC pyramidal neurons. Three subpopulations of pyramidal cells were identified according to their responses to 5-HT: excitation (33%), inhibition (40%) and non-response (27%). The inhibitory responses were blocked by WAY100635 in 100% of cases, but not by ritanserin; the excitatory responses were blocked by ritanserin in 75% of cases, but not by WAY100635. The administration of MK-801 potentiated the firing rate of excitatory responses but did not modify the inhibitory responses induced by microiontophoretic application of 5-HT. These results suggest that MK-801 modifies 5-HT synapses in the mPFC by potentiating the excitatory 5-HT_2A/2C responses and attenuating NMDA excitations. These data indicate that 5-HT excitatory transmission is selectively impaired at the mPFC level in this pharmacological model of schizophrenia.     

Characterization of the 5-hydroxytryptamine receptors mediating contraction in the intestine of Suncus murinus.

1. The effects of 5-HT and 5-HT agonists to induce contraction and the 5-HT receptors mediating these effects were investigated in the proximal, central and terminal intestinal segments of Suncus murinus. 2. The contraction curves to 5-HT (3 nM - 30 microM) were shifted to the right by methysergide (1 microM) and ritanserin (0.1 microM), without affecting the maximum response. 3. In the central and terminal segments (but not the proximal segments) ondansetron (1 microM) and atropine (1 microM) significantly attenuated the contractions to higher concentrations of 5-HT. The selective 5-HT4 receptor antagonist SB204070 (1 nM), failed to modify 5-HT induced contractions in any segment examined. 4. 5-carboxamidotryptamine, alpha-methyl-5-HT and 5-methoxytryptamine (0.003 - 3.0 microM) induced contractions but unlike 5-HT, higher concentrations of these three agents failed to increase the response or were associated with a decrease in response. 2-methyl-5-HT (0.03 - 1.0 microM) was ten times less potent than 5-HT to induce contraction but achieved the same maximum response. 5. The contractions induced by the lower concentrations of 2-methyl-5-HT (0.03 - 1.0 microM) in all segments were markedly reduced or abolished by methysergide (1.0 microM); the response to the higher concentrations of 2-methyl-5-HT (3 - 30.0 microM) were markedly reduced by atropine (1.0 microM) and ondansetron (1.0 microM). 6. In all segments examined, tetrodotoxin (1 microM) significantly reduced the 5-HT-induced contraction. 7. It is concluded that the 5-HT-induced contraction was mediated via 5-HT2 (ritanserin sensitive) receptors in all regions of the intestine, with 5-HT3 (ondansetron sensitive) receptors mediating an additional major component in the central and terminal regions.     

Further pharmacological characterization of 5-HT2C receptor agonist-induced inhibition of 5-HT neuronal activity in the dorsal raphe nucleus in vivo

Background and purpose:

Recent experiments using non-selective 5-hydroxytryptamine (5-HT)_2C receptor agonists including WAY 161503 suggested that midbrain 5-HT neurones are under the inhibitory control of 5-HT_2C receptors, acting via neighbouring gamma-aminobutyric acid (GABA) neurones. The present study extended this pharmacological characterization by comparing the actions of WAY 161503 with the 5-HT_2C receptor agonists, Ro 60-0275 and 1-(3-chlorophenyl) piperazine (mCPP), as well as the non-selective 5-HT agonist lysergic acid diethylamide (LSD) and the 5-HT releasing agent 3,4-methylenedioxymethamphetamine (MDMA).

Experimental approach:

5-HT neuronal activity was measured in the dorsal raphe nucleus (DRN) using extracellular recordings in anaesthetized rats. The activity of DRN GABA neurones was assessed using double-label immunohistochemical measurements of Fos and glutamate decarboxylase (GAD).

Key results:

Ro 60-0175, like WAY 161503, inhibited 5-HT neurone firing, and the 5-HT_2C antagonist SB 242084 reversed this effect. mCPP also inhibited 5-HT neurone firing (∼60% neurones) in a SB 242084-reversible manner. LSD inhibited 5-HT neurone firing; however, this effect was not altered by either SB 242084 or the 5-HT_2A/C receptor antagonist ritanserin but was reversed by the 5-HT_1A receptor antagonist WAY 100635. Similarly, MDMA inhibited 5-HT neurone firing in a manner reversible by WAY 100635, but not SB 242084 or ritanserin. Finally, both Ro 60-0275 and mCPP, like WAY 161503, increased Fos expression in GAD-positive DRN neurones.

Conclusions and implications:

These data strengthen the hypothesis that midbrain 5-HT neurones are under the inhibitory control of 5-HT_2C receptors, and suggest that the 5-HT_2C agonists Ro 60-0175, mCPP and WAY 161503, but not LSD or MDMA, are useful probes of the mechanism(s) involved.     

Pharmacological characterization of 5-Hydroxytryptamine-receptor subtypes in circular muscle from the rat stomach

5-Hydroxytryptamine (5-HT) modulates gastric motility and gastric emptying via a variety of 5-HT receptor subtypes. However, regional and functional differences among 5-HT receptor subtypes in the rat stomach are not fully investigated. Thus, we aimed to characterize 5-HT receptor subtypes involved in the 5-HT-induced contractions in the isolated antral, corporal and fundic circular muscles of the rat stomach by measuring the contractile force. 5-HT induced concentration-dependent contractions in the antrum, corpus and fundus. 5-HT-induced antral contractions were partly blocked by atropine and enhanced by tetrodotoxin (TTX). Neither atropine nor TTX affected the corporal or the fundic contractions to 5-HT. In the antrum, 5-HT-induced contractions were inhibited by methysergide, tended to be inhibited by ketanserin, enhanced by SB-203186, but were not affected by WAY-100635, GR127935, RS-127445, ondansetron, or SB-269970. In the corpus, 5-HT-induced contractions were inhibited by ketanserin or methysergide. In the fundus, 5-HT-induced contractions were blocked by methysergide or RS-127445, but were enhanced by cinanserin or SB-203186. It is thus concluded that contractile responses to 5-HT in the antrum are mediated by 5-HT receptors on both smooth muscle and neurons whilst in the corpus and fundus responses are mainly mediated by 5-HT receptors on smooth muscle. Moreover, the antrum presents the contractile 5-HT2A and 5-HT2B receptors and the relaxant 5-HT4 receptors. The corpus presents the contractile 5-HT2A receptors, and the fundus presents the contractile 5-HT2B receptors and the relaxant 5-HT2A and 5-HT4 receptors.     

Anxiolytic-like action of MTEP expressed in the conflict drinking Vogel test in rats is serotonin dependent

The purpose of the present study was to investigate whether the anxiolytic-like action of a selective and brain penetrable group I metabotropic glutamate (mGlu5) receptor antagonist 3-[(2-methyl-1,3-tiazol-4-yl)ethynyl]-pyridine (MTEP) is dependent upon the serotonergic system. Experiments were performed on male Wistar rats. The Vogel conflict drinking test was used to detect anxiolytic-like activity. MTEP administered intraperitoneally at doses of 1, 3 and 6 mg/kg induced anxiolytic-like effect. The potential anxiolytic effect of MTEP (1 mg/kg) was inhibited by a nonselective 5-HT receptor antagonist metergoline (2 mg/kg i.p.) and 5-HT2A/2C receptor antagonist ritanserin (0.5 mg/kg i.p.), but not by a 5-HT1A receptor antagonist N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridynyl)cyclohexane-carboxamide (WAY 100635) (0.1 mg/kg i.p). The anxiolytic effect of MTEP (6 mg/kg) was attenuated by ritanserin (1 mg/kg i.p.). Moreover, MTEP-induced a dose-dependent release of serotonin in the frontal cortex. The obtained results suggest that the potential anxiolytic effect of the mGlu5 receptor antagonist MTEP is due to the increased serotonin release with subsequent activation of 5-HT2A/2C receptors, most probably located postsynaptically, but not by the 5-HT1A receptors.     

Differential influence of selective 5-HT5A vs 5-HT1A, 5-HT1B,or 5-HT2C receptor blockade upon light-induced phase shifts in circadian activity rhythms: Interaction studies with citalopram

Though serotonergic mechanisms modulate circadian rhythms, roles of individual serotonin (5-HT) receptors remain uncertain since data are lacking for antagonists. Herein, both the 5-HT_5A receptor antagonist, A843277 (10 mg/kg), and the 5-HT_1B antagonist, SB224289 (1 mg/kg), inhibited light-induced phase advances in hamster circadian wheel-running rhythms. Conversely, though 5-HT_1A and 5-HT₇ receptors are likewise implicated in circadian scheduling, their blockade by WAY100635 (0.5 mg/kg) and SB269970 (1 mg/kg), respectively, was ineffective. Since actions of 5-HT reuptake inhibitors are modified by antagonists, we evaluated their influence on suppression of phase advances by citalopram (10 mg/kg). Its action was potentiated by WAY100635 and the 5-HT_2C antagonist, SB242084 (1 mg/kg), but not by A842377, SB224289, SB269970, and antagonists at 5-HT_2A (MDL100907) and 5-HT₆ (SB399885) receptors. In conclusion, this is the first in vivo evidence for an influence of 5-HT_5A receptors upon circadian rhythms, but no single class of 5-HT receptor mediates their control by citalopram.     

Pharmacological profile of the 5-HT-induced inhibition of cardioaccelerator sympathetic outflow in pithed rats: correlation with 5-HT1 and putative 5-ht5A/5B receptors

Continuous infusions of 5-hydroxytryptamine (5-HT) inhibit the tachycardiac responses to preganglionic (C7-T1) sympathetic stimulation in pithed rats pretreated with desipramine. The present study identified the pharmacological profile of this inhibitory action of 5-HT. The inhibition induced by intravenous (i.v.) continuous infusions of 5-HT (5.6 microg x kg-1x min-1) on sympathetically induced tachycardiac responses remained unaltered after i.v. treatment with saline or the antagonists GR 127935 (5-HT1B/1D), the combination of WAY 100635 (5-HT1A) plus GR 127935, ritanserin (5-HT2), tropisetron (5-HT3/4), LY215840 (5-HT7) or a cocktail of antagonists/inhibitors consisting of yohimbine (alpha2), prazosin (alpha1), ritanserin, GR 127935, WAY 100635 and indomethacin (cyclooxygenase), but was abolished by methiothepin (5-HT1/2/6/7 and recombinant 5-ht5A/5B). These drugs, used in doses high enough to block their respective receptors/mechanisms, did not modify the sympathetically induced tachycardiac responses per se. I.v. continuous infusions of the agonists 5-carboxamidotryptamine (5-CT; 5-HT1/7 and recombinant 5-ht5A/5B), CP 93129 (r5-HT1B), sumatriptan (5-HT1B/1D), PNU-142633 (5-HT1D) and ergotamine (5-HT1B/1D and recombinant 5-ht5A/5B) mimicked the above sympatho-inhibition to 5-HT. In contrast, the agonists indorenate (5-HT1A) and LY344864 (5-ht1F) were inactive. Interestingly, 5-CT-induced cardiac sympatho-inhibition was abolished by methiothepin, the cocktail of antagonists/inhibitors, GR 127935 or the combination of SB224289 (5-HT1B) plus BRL15572 (5-HT1D), but remained unchanged when SB224289 or BRL15572 were given separately. Therefore, 5-HT-induced cardiac sympatho-inhibition, being unrelated to 5-HT2, 5-HT3, 5-HT4, 5-ht6, 5-HT7 receptors, alpha1/2-adrenoceptor or prostaglandin synthesis, seems to be primarily mediated by (i). 5-HT1 (probably 5-HT1B/1D) receptors and (ii). a novel mechanism antagonized by methiothepin that, most likely, involves putative 5-ht5A/5B receptors.     

Characteristics of 5-hydroxytryptamine receptors involved in contraction of feline ileal longitudinal smooth muscle

A number of studies have demonstrated that 5-hydroxytryptamine (5-HT) can induce muscle contraction or relaxation response and enhance secretion in the gastrointestinal tract via a multiplicity of 5-HT receptor subtypes. In the present study, we investigated the pharmacological characterization of the 5-HT-induced contractile response in longitudinal smooth muscle isolated from the feline ileum. Addition of 5-HT into muscle chambers enhanced the basal tone and spontaneous activity in a concentration-dependent manner. The neurotoxin tetrodotoxin did not alter the 5-HT-induced contraction of the longitudinal muscles. Neither atropine nor guanethidine affected the contraction. The 5-HT agonists, 5-methylserotonin hydrochloride and mosapride, also evoked concentration-dependent contractions. The 5-HT-induced contraction was enhanced by the 5HT(2) receptor antagonist ketanserin and the 5-HT(3) receptor antagonist ondansetron but was inhibited by the 5-HT(1) receptor antagonist methysergide and 5-HT(4) receptor antagonist GR113808. These results indicate that 5-HT(1) and 5-HT(4) receptors may mediate the contraction of the 5-HT-induced response and 5-HT(2) and 5-HT(3) receptors may mediate 5-HT-induced relaxation in feline ileal longitudinal smooth muscles.     

5-Hydroxytryptamine receptors,especially the 5-HT4 receptor,in guinea pig urinary bladder

The function of 5-hydroxytryptamine (5-HT) receptors, especially the 5-HT4 receptor, in the urinary bladder were examined in preparations isolated from the guinea pig by in vitro receptor autoradiography and determinations of mechanical activity and acetylcholine (ACh) release. Specific [125I]SB207710 binding sites were detected evenly throughout the urinary bladder. 5-HT (3 x 10(-8)-10(-4) M) caused contractions of strips of the urinary bladder, in a concentration dependent manner. Ketanserin antagonized the 5-HT-induced contractions, while granisetron and SB204070 antagonized the contractions induced by high concentrations of 5-HT. Atropine inhibited the contractions induced by high concentrations of 5-HT. Ketanserin prevented the 5-HT-induced contractions in the presence of atropine, but granisetron and SB204070 did not affect the contractions under such a condition. 5-HT enhanced the electrically-stimulated (5 Hz, 0.5 ms) outflow of [3H]acetylcholine from strips preloaded with [3H]choline, and the enhancement was antagonized by granisetron and SB204070. Thus, the contractile response to 5-HT was mediated by activations of 5-HT2, 5-HT3 and 5-HT4 receptors. The 5-HT2 receptor may be a property of high affinity to 5-HT and located on the smooth muscle cells. The 5-HT4 as well as 5-HT3 receptor may be a property of low affinity to 5-HT and located on the cholinergic neurons.     

Increased REM sleep after intra-dorsal raphe nucleus injection of flesinoxan or 8-OHDPAT: prevention with WAY 100635.

The effects of 8-OHDPAT and flesinoxan, two selective 5-HT(1A) receptor agonists, and of WAY 100635, a selective 5-HT(1A) receptor antagonist, on spontaneous sleep were studied in adult rats implanted for chronic sleep recordings. The serotonergic ligands were microinjected directly into the dorsal raphe nucleus (DRN). Direct administration of flesinoxan (25.0-50.0 ng) into the DRN induced a significant increment of REM sleep (REMS) during the second and third 2 h of recording. Microinjection of 8-OHDPAT (50.0 ng) induced similar effects on REMS during the second 2 h of recording. On the other hand, intra-DRN injection of WAY 100635 (12.5-50.0 ng) significantly reduced REMS during the second 2 h recording period. REM sleep values had also decreased significantly during the first 2 h of recording after the 50 ng dose. Pretreatment with WAY 100635 (25.0 or 50.0 ng) prevented the increase of REMS induced by flesinoxan (25.0 ng) during the second two recording hours. Our findings support the proposal that activation of somatodendritic 5-HT(1A) receptors in the DRN increases REMS, whereas their blockade induces the opposite effect.     

Differential effects of 5-HT2C receptor ligands on place conditioning and locomotor activity in rats

Effects of the 5-hydroxytryptamine (5-HT)(1A/1B/2C) receptor agonist N-[3-(trifluoromethyl)phenyl] piperazine (TFMPP, 0-3.0 mg/kg s.c.) and the 5-HT2C receptor agonist 8,9-dichloro-2,3,4,4a-tetrahydro-1H-pyrazino[1,2-a]quinoxalin-5(6H)-one (WAY 161503, 0-3.0 mg/kg s.c.) in place conditioning were measured in male Sprague-Dawley rats. Effects of TFMPP, alone and with the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-2-pyridinyl-cyclohexanecarboxamine (WAY 100635), the 5-HT(1B) receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-1,1'-biphenyl-4-carboxamide (GR 127935) or the 5-HT2C receptor antagonist 6-chloro-5-methyl-1-[[2-(2-methylpyrid-3-yloxy)pyrid-5-yl]carbamoyl]indoline (SB 242084) and of WAY 161503 alone and with SB 242084 on locomotor activity were also assessed. Neither TFMPP nor WAY 161503 induced place conditioning. WAY 161503 (1.0 and 3.0 mg/kg s.c.) decreased locomotor activity; SB 242084 (1.0 mg/kg i.p.) blocked this effect. Reduced locomotor activity following TFMPP was blocked by SB 242084 but not WAY 100635 (0.1 mg/kg s.c.) or GR 127935 (3.0 mg/kg s.c.). Behaviourally relevant levels of 5-HT2C receptor stimulation may not exert reinforcing effects, although other studies indicate that such manipulations alter reinforcing effects of drugs of abuse.     

Effect of combined administration of 5-HT1A or 5-HT1B/1D receptor antagonists and antidepressants in the forced swimming test

In the present study, we examined effects of the selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitor citalopram, the 5-HT/noradrenaline reuptake inhibitor imipramine, the selective noradrenaline reuptake inhibitor desipramine or the monoamine oxidase-A inhibitor moclobemide, administered in combination with the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridynyl)cyclohexanecarboxamide (WAY 100635) or the 5-HT(1B/1D) receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-[1,2,4]oxadiazol-3-yl)1,1'-biphenyl-4-carboxamide (GR 127935) and the 5-HT(1B) receptor antagonist N-[3-(2-dimethylamino) ethoxy-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-carboxamide (SB 216641) in the forced swimming test in rats. When given alone, citalopram (20 and 30 mg/kg), imipramine (20 mg/kg), desipramine (20 mg/kg), moclobemide (20 mg/kg), WAY 100635 (0.1 and 1 mg/kg), GR 127935 (10 and 20 mg/kg) or SB 216641 (2 mg/kg) did not shorten the immobility time of rats. Co-administration of WAY 100635 (0.1 and 1 mg/kg) and citalopram (20 mg/kg), or imipramine (20 mg/kg), or moclobemide (20 mg/kg) did not affect the immobility time of rats, whereas WAY 100635 given jointly with desipramine (20 mg/kg) induced a weak anti-immobility effect. GR 127935 (10 and 20 mg/kg) or SB 216641 (2 mg/kg) co-administered with imipramine, desipramine or moclobemide, but not citalopram, produced a significant anti-immobility action in the forced swimming test in rats. These results indicate that the blockade of 5-HT(1B) rather than 5-HT(1A) receptors may facilitate the anti-immobility effect of imipramine, desipramine or moclobemide in the forced swimming test. No interaction was observed between 5-HT(1A) or 5-HT(1B/1D) receptor antagonists and citalopram.     

Involvement of 5-HT1A receptors in behavioural effects of the cannabinoid receptor agonist CP 55,940 in male rats

We have studied the possible interaction between the cannabinoid receptor agonist CP 55,940 (1 and 50 microg/kg) and the 5-HT1A receptor antagonist WAY 100635 (1 mg/kg) in the modulation of plus-maze and holeboard activity in Wistar adult male rats. In the plus-maze, the higher dose of CP 55,940 induced an anxiogenic-like effect, whereas the lower dose induced anxiolytic-like responses. The 5-HT1A antagonist, which was silent in this test, attenuated the anxiogenic, but not the anxiolytic, effect of CP 55,940. In the holeboard, the higher dose of CP 55,940 significantly decreased head-dipping duration, and WAY 100635, which did not affect exploratory head-dipping when administered alone, antagonized this effect. The administration of WAY 100635 significantly increased grooming behaviour, and this effect was inhibited by the two doses of CP 55,940, which did not exert any effect, per se, on this parameter. We provide the first evidence implicating 5-HT1A receptors in anxiety-related behavioural responses to a cannabinoid agonist.     

Antipsychotic-like effect by combined treatment with citalopram and WAY 100635: involvement of the 5-HT2C receptor

Catalepsy occurs following high dopamine (DA) D2 blockade by typical antipsychotic drugs (APDs). We showed that a combination of a high dose of citalopram, a selective serotonin reuptake inhibitor (SSRI) and the selective 5-HT1A receptor antagonist WAY 100635 produces significant catalepsy in rats, similar to APDs. Here, we investigated the potential antipsychotic activity of lower doses of citalopram+WAY 100635, using the conditioned avoidance response (CAR) test. Cataleptogenic liability of the combination was evaluated with the catalepsy test. Citalopram and WAY 100635 in combination, but not when given alone, produced a significant antipsychotic action in CAR without significant catalepsy, similar to the effect of a low dose of the typical APD haloperidol. Pretreatment with a selective 5-HT2C receptor antagonist, SB 242084, completely prevented the citalopram/WAY 100635-induced suppression of CAR indicating an involvement of the 5-HT2C receptor. In summary, treatment with an SSRI/5-HT1A antagonist combination might prove beneficial in psychiatric disorders with psychotic/depressive symptoms.     

Vasodilator and vasoconstrictor responses induced by 5-hydroxytryptamine in the in situ blood autoperfused hindquarters of the anaesthetized rat

In the present study we attempted to characterise the responses and receptors involved in the effects of 5-hydroxytryptamine (5-HT, serotonin) in in situ autoperfused rat hindquarters. Intra-arterial administration of the lowest doses of 5-HT used (0.12-12.5 ng/kg) induced vasodilator responses, whereas the highest doses (25-1000 ng/kg) produced vasoconstriction. The vasodilator effect was inhibited by methiothepin (a non-specific 5-HT(1,2,5,6,7) receptor antagonist) and by a 5-HT(1D/1B) receptor antagonist, i.e., 3-[4-(4-chlorophenyl)piperazin-1-yl]-1,1-diphenyl-2-propanolol (BRL 15572), but not by ritanserin (a selective 5-HT(2) receptor antagonist), 5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3-f] indole (SB 206553, a selective 5-HT(2B/2C) receptor antagonist) or mesulergine (a non-specific serotonergic antagonist that shows affinity to the 5-HT(7) receptor). This vasodilator effect was mimicked by administration of a selective 5-HT(1) receptor agonist - 5-carboxamidotryptamine (5-CT) - and by 2-[5-[3-(4-methylsulphonylamino)benzyl-1,2,4-1 H-indol-3-yl]ethanamine (L-694,247, a selective 5-HT(1D/1B) receptor agonist). Methiothepin, but not mesulergine, inhibited 5-CT-induced vasodilatation and the selective 5-HT(1D/1B) receptor antagonist (BRL 15572) inhibited the vasodilator action induced by L-694,247.The vasoconstrictor effect of 5-HT was significantly decreased by methiothepin, ritanserin and SB 206553, and was mimicked by alpha-methyl-5-HT (a selective 5-HT(2) receptor agonist) but not by administration of BW 723C86, a selective 5HT(2B) receptor agonist. Ritanserin, SB 206553 and spiperone (a non-specific 5-HT(1/2A) receptor antagonist) inhibited the alpha-methyl-5HT-induced vasoconstriction.Our data suggest that the vasodilator response induced by 5-HT in autoperfused rat hindquarters is mainly mediated by 5-HT(1D/1B) receptors, whereas the vasoconstrictor effect is mainly due to the activation of 5-HT(2A) receptors.     

Characterization of serotonergic mechanisms involved in the behavioural inhibition induced by 5-hydroxytryptophan in a modified light-dark test in mice

In a modified light-dark exploration test in mice, 5-hydroxytryptophan, at doses (25-50 mg/kg) that approximately doubled the 5-HT content in the cerebral cortex, reduced the time spent by mice in the white compartment, suggesting an anxiogenic effect. Depletion of brain 5-HT content with p-chlorophenylalanine (300 mg/kg/day for three consecutive days) resulted in an anxiolytic-like effect. Conversely, the 5-HT reuptake blocker fluoxetine reduced the time spent by mice in the white compartment. No significant interaction of either p-chlorophenylalanine or fluoxetine with 5-hydroxytryptophan was found. Several 5-HT agents, some of them with an intrinsic anxiolytic-like effect in this test, were studied in combination with 5-hydroxytryptophan. All of the drugs with a selective affinity at 5-HT1A receptors interacted significantly with 5-hydroxytryptophan. The suppressant effect of 5-hydroxytryptophan was antagonized or reversed by buspirone, a partial agonist at postsynaptic 5-HT1A receptors, and also by the "silent" 5-HT1A receptor antagonist WAY 100635, but not by the full agonist 8-OH-DPAT. The 5-HT2 receptor antagonist ritanserin partly counteracted the 5-hydroxytryptophan effect at the lower dose used. The 5-HT3 receptor antagonist ondansetron was able to prevent, at a low dose, the anxiogenic effect of 5-hydroxytryptophan; however, the 5-HT3 antagonists VA21B7 and granisetron as well as the 5-HT3/5-HT4 antagonist tropisetron and the selective 5-HT4 receptor antagonist RS 23597-190 were ineffective. The results appear to be consistent with the hypothesis that relates increased activity of the 5-HT systems to increased anxiety. Even though different 5-HT receptor subtypes may be involved in the anxiogenic effect of a high dose of 5-hydroxytryptophan, postsynaptic 5-HT1A receptors appear to play a prominent role. Administration of 5-hydroxytryptophan may consequently represent a valid approach to analyse further the role of 5-HT agents, in particular those acting at 5-HT1A receptors, in animal models of anxiety.