Evidence for the preferential involvement of 5-HT2A serotonin receptors in stress- and drug-induced dopamine release in the rat medial prefrontal cortex.

The mechanism(s) by which serotonin modulates dopamine release in the medial prefrontal cortex is not known, although studies suggest an involvement of 5-HT2 family receptors. We employed in vivo microdialysis and putatively selective 5-HT2A antagonists (M100907, MDL 11,939, SR46349B) to determine if 5-HT2A receptors are responsible for both drug- and stress-induced DA release in the medial prefrontal cortex. MDL 11,939 and SR46349B receptor-binding studies indicated, for the first time, that only MDL 11,939 had greater selectivity for the 5-HT2A vs the 5-HT2C receptor subtypes similar to M100907, and that both showed low or no affinity for non-5-HT2 receptors. Reverse dialysis with 5-HT2A antagonists had little or no effect on basal dopamine efflux. However, intracortical administration of MDL 11,939 or M100907 attenuated dopamine release induced by systemic administration of the 5-HT2 agonist DOI. Dual-probe microdialysis demonstrated that systemic DOI also increased glutamate concentrations in the ventral tegmental area (VTA). This was blocked by intracortical M100907. Cortical perfusion with M100907, or the atypical antipsychotic drug risperidone, but not the 5-HT2B/C ligand SB 206553, also decreased dopamine release induced physiologically by stress. These results indicate that stimulation of cortical 5-HT2A receptors increases the release of dopamine from the mesocortical system. They suggest that this effect may be mediated by increases in glutamate release from corticotegmental projections to the VTA. Additionally, they indicate that cortical 5-HT2A receptors modulate evoked dopamine release, such as that observed physiologically following mild stress. These findings may have implications for the pharmacological treatment of disorders resulting from or exacerbated by stress.     

Serotonin 5-HT3 antagonists do not alter the discriminative stimulus properties of cocaine

The central nervous system (CNS) of the rat is known to contain serotonin (5-HT) type -3 receptors (5-HT3). Behavioral evidence suggests that 5-HT3 receptors interact with mesolimbic dopamine (DA) systems and that 5-HT3 antagonists can interfere with the hyperlocomotive effects of amphetamine and cocaine and the rewarding and stimulus effects of morphine, nicotine and ethanol. Cocaine, which blocks the reuptake of DA, norepinephrine (NE), and 5-HT in the CNS, also may be an antagonist at 5-HT3 receptors. The purpose of the present study was to determine whether systemic administration of the 5-HT3 antagonists ICS 205930 or MDL 72222 could mimic or block the discriminative stimulus properties of cocaine. Once rats (N = 16) were trained to discriminate cocaine (10 mg/kg) from saline, substitution tests with various doses of cocaine (0.313-10 mg/kg), ICS 205930 (2-24 mg/kg), and MDL 72222 (2-16 mg/kg) were conducted. Cocaine produced a dose-related increase in cocaine-appropriate responding while the 5-HT3 antagonists engendered primarily saline-lever responding. Neither ICS 205930 nor MDL 72222 were able to antagonize the stimulus effects of cocaine (5 mg/kg). Response rates were not significantly reduced when the 5-HT3 antagonists were given in combination with cocaine. The results indicate that although 5-HT3 antagonists can inhibit some of the unconditioned behavioral effects of psychomotor stimulants, the discriminative stimulus effects of cocaine remain intact.     

5-HT4 receptor-mediated modulation of 5-HT release in the rat hippocampus in vivo.

1. In the present study, the ability of the 5-hydroxytryptamine, receptor (5-HT4 receptor) to modulate the release of 5-HT in the hippocampus of freely-moving rats was investigated by the in vivo microdialysis technique. 2. The 5-HT4 receptor agonist, renzapride (1.0-100 microM, administered via the microdialysis probe) increased extracellular hippocampal levels of 5-HT in concentration-dependent manner (approximately 200% maximal increase). The ability of renzapride (100 microM, administered via the microdialysis probe) to elevate extracellular levels of 5-HT remained in the presence of the selective 5-HT reuptake blocker, paroxetine (1.0 microM, administered via the microdialysis probe). Furthermore, another 5-HT4 receptor agonist 5-methoxytryptamine (5-MeOT; 10 microM, administered via the microdialysis probe, in the presence of the non-5-HT4 5-HT receptor antagonists pindolol (10 microM) and methysergide (10 microM)) maximally elevated extracellular levels of 5-HT by approximately 450% in the rat hippocampus. The elevation of extracellular 5-HT levels induced by either renzapride (100 microM) or 5-MeOT (10 microM) was completely prevented by combined administration of the selective 5-HT4 receptor antagonist, GR113808 (100 nM, administered via the microdialysis probe). GR113808 (100 nM, administered via the microdialysis probe) administered alone, however, reduced extracellular hippocampal 5-HT levels by some 60%. 3. Systemic administration of the 5-HT1A receptor agonist, 8-OH-DPAT (0.1 mg kg-1, s.c.) reduced extracellular levels of 5-HT in the rat hippocampus by approximately 40%. Prior administration of 8-OH-DPAT (0.1 mg kg-1, s.c.), with an associated reduction of extracellular hippocampal 5-HT levels by approximately 40-50%, however, failed to prevent a subsequent elevation of extracellular levels of 5-HT induced by renzapride (100 microM, administered via the microdialysis probe). 4. Systemic administration of the 5-HT4 receptor agonist, renzapride (0.25 and 1.0 mg kg-1, i.p.) increased extracellular levels of 5-HT in the hippocampus in a dose-dependent manner. The higher dose of renzapride increasing extracellular 5-HT levels by some 200%. The selective 5-HT4 receptor antagonist, GR125487D (1.0-100 micrograms kg-1, i.p.) caused a dose-dependent reduction in extracellular levels of 5-HT in the hippocampus (maximally approximately 80% reduction). Prior administration of GR125487D (10 micrograms kg-1, i.p.) prevented the elevation of extracellular levels of 5-HT induced by renzapride (1.0 mg kg-1, i.p.). 5. In conclusion, the present study provides evidence that activation of the 5-HT4 receptor facilitates 5-HT release in the rat hippocampus in vivo.     

Peripherally administered serotonin 5-HT3 receptor antagonists reduce inflammatory pain in rats

Intraplantar administration of serotonin 5-HT3 receptor antagonists ICS 205-930 and MDL 72222 (1-100 micrograms; 50 microliters) produced dose-related analgesia against formalin-induced acute- and Freunds adjuvant-induced chronic-inflammatory pain in rats. 5-HT3 receptor antagonists had greater effect in the chronic pain test than in the acute paradigm. In both tests, ICS 205-930 was more potent than MDL 72222. These data further support the involvement of peripheral 5-HT3 sites in inflammatory pain, and suggest the utility of selective 5-HT3 receptor antagonists as peripheral analgesics.     

Opposing roles for 5-HT1B and 5-HT3 receptors in the control of 5-HT release in rat hippocampus in vivo.

1. Intracerebral microdialysis was used to determine whether 5-hydroxytryptamine (5-HT) release in the ventral hippocampus of rats anaesthetized with chloral hydrate was modulated by 5-HT3 receptors. 2. It was confirmed that 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU 24969), a selective 5-HT1B receptor agonist, decreased 5-HT release in a dose- and concentration-related manner when administered i.p. (1 and 5 mg kg-1) or via the dialysis probe (0.1 and 1 microM) respectively. The effect of RU 24969 infusion (1 microM) was attenuated by concurrent infusion of metitepine (10 microM) into the hippocampus. 3. When infused into the hippocampus for 15 min, the selective 5-HT3 receptor agonist, 2-methyl-5-hydroxytryptamine (2-methyl-5-HT; 0.1- 10 microM) increased dialysate 5-HT levels in a concentration-related manner; an effect which was abolished by concurrent infusion of 3-tropanyl-3,5-dichlorobenzoate (1 microM, MDL 72222), a selective 5-HT3 antagonist. 4. MDL 72222 had no effects on hippocampal 5-HT release when administered via the dialysis probe (1 or 10 microM). 5. The data show that 5-HT3 and 5-HT1B receptors have opposing roles in the control of 5-HT release in the hippocampus, with 5-HT3 receptors facilitating and 5-HT1B receptors inhibiting 5-HT efflux, respectively. They also indicate that the facilitatory 5-HT3 receptors are not tonically activated.     

Enhancement of 5-HT1B and 5-HT1D receptor antagonist effects on extracellular 5-HT levels in the guinea-pig brain following concurrent 5-HT1A or 5-HT re-uptake site blockade.

The effects of selective serotonin re-uptake inhibitor (SSRI), paroxetine, and 5-HT1A, 5-HT1B and 5-HT1B/1D receptor antagonists on in vivo extracellular 5-HT levels in the guinea-pig frontal cortex and dorsal hippocampus were investigated using the technique of microdialysis. The aim of the study was to further investigate the autoreceptor roles of the 5-HT1A, 5-HT1B and 5-HT1D receptors in the median vs dorsal raphe nuclei. In the frontal cortex, 5-HT1A (WAY 100635, 1 mg/kg i.p.) or 5-HT1B (SB-224289, 4 mg/kg i.p.) receptor antagonists had no effect on extracellular levels of 5-HT, whilst the mixed 5-HT1B/1D receptor antagonist (GR 127935, 0.3 mg/kg i.p) produced a significant decrease in extracellular 5-HT levels. Paroxetine (10 microM) significantly increased extracellular 5-HT levels when perfused locally into the cortex. Administration of SB-224289, followed 120 min later by WAY 100635, had no effect on extracellular 5-HT levels. In contrast, sequential administration of either WAY 100635 and GR 127935, or SB-224289 and paroxetine significantly increased extracellular 5-HT levels. In the dorsal hippocampus, whilst 5-HT1A receptor antagonism elicited by administration of WAY 100635 had no effect, both 5-HT1B and mixed 5-HT1B/1D receptor blockade significantly increased extracellular 5-HT levels. Administration of SB-224289 followed 120 min later with WAY 100635, or WAY 100635 followed 30 min later with GR 127935, potentiated the effect of the three compounds alone, significantly increasing extracellular 5-HT levels. These data demonstrate that to simultaneously increase extracellular 5-HT in both frontal cortex and dorsal hippocampus of the guinea-pig brain concurrent 5-HTA1A, 5-HT1B and 5-HT1D receptor blockade is required. Whereas in the dorsal hippocampus, 5-HT1B receptor blockade is sufficient to elicit an increase in extracellular 5-HT levels.     

Effects of 5-HT3 receptor antagonists on 5-HT and nicotinic depolarizations in guinea-pig submucosal neurones.

1. Intracellular recordings were made from neurones of the guinea-pig submucosal plexus. The effects of several 5-hydroxytryptamine3 (5-HT3) receptor antagonists on depolarizations produced by ionophoretic application of 5-HT and acetylcholine, as well as on fast excitatory postsynaptic potentials (fast e.p.s.ps) produced by nerve stimulation were examined. 2. ICS 205-930, GR 38032F, MDL 72222, cocaine and curare all inhibited the fast e.p.s.p. as well as the depolarizations in response to 5-HT and acetylcholine (ACh) ionophoresis in a dose-dependent fashion. 3. IC50 values for ICS 205-930, GR 38032F, MDL 72222, cocaine and curare in inhibiting the 5-HT mediated depolarizations were 12 nM, 100 nM, 3 microM, 3 microM and 20 microM, respectively. 4. IC50 values for ICS 205-930, GR 38032F, MDL 72222, cocaine and curare in inhibiting the nicotinic depolarizations were 4 microM, 12 microM, 11 microM, 6 microM and 17 microM, respectively. Similar IC50 values were obtained for inhibition of the fast e.p.s.ps by these antagonists. 5. The nicotinic receptor blocker, hexamethonium, inhibited the nicotinic depolarization and the fast e.p.s.p. with IC50 values of 10 microM. Hexamethonium (10 microM-5 mM) did not alter the depolarization induced by 5-HT. 6. These results demonstrate that the pharmacological profile of 5-HT3 receptors present on submucosal neurones is identical to that of 5-HT3 receptors on myenteric neurones and, thus, provide evidence that the enteric neuronal 5-HT3 receptor forms a receptor subtype distinct from that characterized in other parts of the autonomic nervous system.     

Pretreatment with serotonin 5-HT<Subscript>3</Subscript> receptor antagonists produces no observable blockade of long-term motor sensitization to cocaine in rats

Abstract Rationale. Serotonin 5-HT₃ receptor antagonists are proposed to serve as potential anti-addictive and anti-psychotic therapies. Objective. The present study re-examined the hypothesis that the activation of 5-HT₃ receptors is required for the development of long-term motor sensitization to repeated cocaine (COC). Methods. Rats were pretreated with vehicle (VEH) or one of three 5-HT₃ receptor antagonists, MDL 72222, Y-25130 or ondansetron, 30 min prior to seven daily COC or saline (SAL) injections. Three weeks later, animals were challenged with COC in a test for sensitization. For comparison, the effects of pretreatment with the dopamine (DA) receptor antagonist fluphenazine and a combination of fluphenazine and Y-25130 were assessed. Results. Pretreatment with ondansetron, Y-25130, fluphenazine and their combination significantly attenuated COC-induced behaviors during repeated treatment but not on the test for sensitization. MDL 72222 pretreatment enhanced motor sensitization on the test day. In repeated SAL rats, pretreatment with ondansetron, Y-25130 and fluphenazine+Y-25130 enhanced COC-induced rearing on the test for sensitization. Conclusions. These data indicate the effects of 5-HT₃ receptor antagonists on both acute COC-induced motor behavior and COC-induced motor sensitization are compound-selective. As none of the 5-HT₃ receptor antagonists attenuated the magnitude of the sensitized motor response to COC in the long term, these data also indicate that like DA receptor activation, 5-HT₃ receptor activation is necessary for the full expression of acute COC-induced motor hyperactivity, but it is not required for the development of long-term motor sensitization. Electronic Publication     

Ethanol and negative feedback regulation of mesolimbic dopamine release in rats

 The objectives of this study were to examine the relationship between somatodendritic and terminal field dopamine (DA) release following manipulation of DA D₂ receptors in the ventral tegmental area (VTA), systemic administration of ethanol, and inhibition of DA uptake in the nucleus accumbens (ACB). Perfusion of 5, 25 and 100 μM quinpirole (a D₂ agonist), or sulpiride (a D₂ antagonist) through the microdialysis probe in the VTA produced dose-related decreases or increases, respectively, in the extracellular levels of DA in both the VTA and ACB of adult Wistar rats. The IP administration of 2–3 g/kg ethanol produced a sustained increase in the extracellular levels of DA (150–200% of baseline) in the ACB for at least 2 h after injection, whereas only a transient increase was observed in the VTA. Local perfusion of the ACB with 100 μM GBR12909, a DA uptake inhibitor, elevated the extracellular levels of DA in the ACB to approximately 400% of baseline, but decreased the extracellular levels of DA in the VTA to approximately 50% of baseline. Overall, the results suggest that (a) there is an association between somatodendritic and terminal field DA release when D₂ cell body autoreceptors in the VTA are manipulated, (b) elevating synaptic levels of DA in the terminal field activates a long-loop negative feedback system to the VTA, and (c) different mechanisms may be mediating the actions of ethanol on DA neuronal activity and terminal DA release. Received: 18 November 1997 / Final version: 29 January 1998     

Inactivation of 5-HT(2C) receptors potentiates consequences of serotonin reuptake blockade.

The enhancement of central serotonin system function underlies the therapeutic effects of selective serotonin reuptake inhibitors (SSRIs), which have become the most commonly used class of antidepressant agents. However, many individuals experience depressive episodes that are resistant to SSRI treatment. Homeostatic mechanisms that limit the extent to which SSRIs enhance serotonergic neurotransmission have been implicated in this phenomenon. Here, we report a novel strategy for enhancing the efficacy of SSRIs. Using in vivo microdialysis methods in rats, the nonselective 5-HT2 receptor antagonist ketanserin was observed to produce a robust augmentation of citalopram-, fluoxetine-, and sertraline-induced elevations of hippocampal extracellular serotonin levels. Similar effects were also observed in cortex. The potentiation of SSRI-induced increases in hippocampal serotonin levels was reproduced by the 5-HT(2C) receptor-selective antagonists SB 242084 and RS 102221, but not by the 5-HT(2A) receptor-selective antagonist MDL 100 907. Although 5-HT(2C) receptor antagonists augmented the actions of SSRIs, they had no effect on extracellular serotonin levels or tail suspension responses when administered alone. These results were in strong accord with independent findings using a line of 5-HT(2C) receptor-null mutant mice. Although this mutation did not affect baseline extracellular serotonin levels or tail suspension test (TST) behavior, it enhanced fluoxetine effects on serotonin levels and immobility in the TST. These findings reveal an unanticipated pharmacological action of 5-HT(2C) receptors that warrants consideration in the development of novel strategies for the treatment of depression.     

Long-term blockade of the expression of cocaine sensitization by ondansetron, a 5-HT(3) receptor antagonist

Intermittent cocaine administration induces sensitization (reverse tolerance) to its behavioral effects. The mechanism(s) mediating sensitization is not clear, however, previous research has implicated 5-HT(3) receptors in the expression of sensitization. The present experiment evaluated the ability of the 5-HT(3) receptor antagonist, ondansetron, administered during withdrawal from chronic intermittent cocaine administration, to block the expression of sensitization. Rats were pretreated for 14 days by daily subcutaneous injections of either 40 mg/kg cocaine or 0.9% saline. During the first 5 days of withdrawal from this pretreatment regimen, all rats received a daily subcutaneous injection of 0-1.0 mg/kg ondansetron. On days 7, 14 or 28 of withdrawal from the cocaine pretreatment, the rats received a 15.0-mg/kg cocaine challenge. Ambulatory behavior was automatically recorded for 60 min. Ondansetron had no significant effect on the subsequent behavioral response to cocaine in the saline control subjects. In contrast, daily injections of ondansetron blocked the expression of sensitization at all withdrawal times. We thus report that it is possible to permanently block the expression of sensitization once it has developed by administering a 5-HT(3) receptor antagonist.     

Flibanserin,a potential antidepressant drug,lowers 5-HT and raises dopamine and noradrenaline in the rat prefrontal cortex dialysate: role of 5-HT(1A) receptors

(1) Using in vivo intracerebral microdialysis in conscious, freely moving rats, we examined the effect of flibanserin, a potential antidepressant drug with high affinity for human 5-HT(1A) receptors and four-50-fold lower affinity for 5-HT(2A) and D(4) receptors, on basal extracellular concentrations of serotonin (5-hydroxytryptamine, 5-HT), dopamine (DA) and noradrenaline (NA) in selected regions of the rat brain. (2) Flibanserin at 3 and 10 mg kg(-1) significantly reduced extracellular 5-HT in the prefrontal cortex (by 30 and 45%) and dorsal raphe (35 and 44%), but had no effect on extracellular 5-HT in the ventral hippocampus. The 3 and 10 mg kg(-1) doses raised extracellular NA to a similar extent in the prefrontal cortex (47 and 50%). In all, 10 mg kg(-1) raised extracellular DA in the prefrontal cortex (63%) whereas 3 mg kg(-1) had no significant effect. (3) Pretreatment with the selective 5-HT(1A) receptor antagonist WAY100,635 (0.3 mg kg(-1)) 30 min before 10 mg kg(-1) flibanserin completely antagonized the latter's effects on extracellular 5-HT, DA and NA in the prefrontal cortex. WAY100,635 by itself had no effect on cortical extracellular monoamines. (4) The results show that the stimulation of 5-HT(1A) receptors plays a major role in the effect of flibanserin on brain extracellular 5-HT, DA and NA.     

Opposite effects of antidepressants and corticosterone on the sensitivity of hippocampal CA1 neurons to 5-HT1A and 5-HT4 receptor activation

Using extracellular and intracellular ex vivo recording techniques we studied changes in the reactivity of hippocampal pyramidal CA1 neurons to serotonin (5-HT) and to the 5-HT1A- and 5-HT4 receptor agonists (+/-)-2-dipropylamino-8-hydroxy- 1,2,3 ,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT) and zacopride, respectively, evoked by repeated electroconvulsive shock (ECS), imipramine and corticosterone treatments. Rats were subjected to ECS for 1 or 10 days, treated with imipramine for 1, 7, 14 or 21 days (10 mg/kg p.o., twice daily) and with corticosterone for 7 days (10 mg/kg s.c., twice daily). Hippocampal slices were prepared 2 days after the last treatment. Activation of 5-HT1A receptors decreased the amplitude of population spikes evoked by stimulation of the Schaffer/collateral-commissural pathway and hyperpolarized CA1 cells. Activation of 5-HT4 receptors increased the population spike amplitude and decreased the amplitude of slow afterhyperpolarization. Both repeated ECS and imipramine enhanced the effects related to 5-HT1A receptor activation and attenuated the effects of 5-HT4 receptor activation. The action of imipramine was significant after a 7-day treatment and reached a maximum after 14 daily applications, remaining at the same level in a group of animals treated for 21 days. Repeated corticosterone attenuated the inhibitory effect of 5-HT and 8-OH-DPAT on the population spike amplitude and enhanced the increase in population spike amplitude induced by zacopride. These findings indicate that antidepressant treatments and repeated corticosterone have opposite effects on hippocampal responsiveness to 5-HT1A and 5-HT4 receptor activation. In consequence, antidepressants enhance, whereas corticosterone reduces the 5-HT-mediated inhibition of hippocampal CA1 cells, which may be relevant to the antidepressant and pro-depressant effects of either treatment, respectively.     

5-HT2 receptors exert a state-dependent regulation of dopaminergic function: studies with MDL 100,907 and the amphetamine analogue, 3,4-methylenedioxymethamphetamine.

The highly selective 5-HT2 receptor antagonist, MDL 100,907, was used to explore the role of serotonin in the stimulation of dopaminergic function produced by the amphetamine analogue 3,4-methylenedioxymethamphetamine (MDMA). MDL 100,907 blocked MDMA-stimulated dopamine synthesis in vivo without affecting basal synthesis. The long-term deficits in 5-HT concentrations believed to be a consequence of MDMA-induced dopamine release were also blocked by MDL 100,907 over the same dose range. In vivo microdialysis confirmed that 5-HT2 receptor blockade with MDL 100,907 attenuated MDMA-induced increases in extracellular concentrations of striatal dopamine. In contrast to its effect on MDMA-induced synthesis, MDL 100,907 did not alter dopamine synthesis stimulated by haloperidol or reserpine. In vivo dopamine release produced by haloperidol was also unaffected by MDL 100,907. The results suggest a permissive role for 5-HT2 receptors in the activation of the dopamine system which occurs during states of high serotonergic activity or during conditions of elevated dopamine efflux with high D2 receptor occupancy.     

The effects of chronic treatment with amitriptyline and MDL 72394 on the control of 5-HT release in vivo

The purpose of the experiments reported were to determine whether chronic treatment with either a monoamine oxidase (MAO) inhibitor or an uptake inhibitor would increase extracellular levels of 5-HT in vivo and whether such treatment resulted in a down-regulation of the 5-HT1B-mediated decrease in extracellular levels of 5-HT. Rats were given either saline, (E)-beta-fluoromethyline-m-tyrosine (MDL 72394 0.25 mg/kg p.o.) or amitriptyline (10 mg/kg p.o.) once a day for 21 days. Twenty-four hr after the final injection, dialysis loops were implanted into the frontal cortices of these rats and basal extracellular levels of 5-HT were measured. The effect of the 5-HT1 receptor agonist 5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)1H indole succinate (RU 24969 10 mg/kg i.p.) on the extracellular level of 5-HT was then studied. Basal levels of 5-HT in saline-treated rats was found to be 27.9 +/- 3.9 fmol/20 microliters perfusate. Chronic treatment with amitriptyline had no effect on extracellular levels of 5-HT but chronic treatment with MDL 72394 significantly increased extracellular levels of 5-HT. Chronic treatment with either MDL 72394 or amitriptyline had no significant effect on the ability of RU 24969 to reduce extracellular levels of 5-HT. These results suggest that 5-HT1B receptors are not down-regulated in response to chronically raised extracellular levels of 5-HT.     

Attenuation by the 5-HT1A receptor agonist osemozotan of the behavioral effects of single and repeated methamphetamine in mice

This study examined the effects of the selective 5-HT1A receptor agonist osemozotan on repeated methamphetamine (METH)-induced behavioral sensitization and single METH-induced locomotor stimulant effect in mice, and then the neurochemical mechanisms using in vivo microdialysis. Repeated administration of METH for 7 days enhanced METH challenge-induced locomotor activity, and this sensitization was observed even after its withdrawal for 7-14 days. Administration of osemozotan to METH-sensitized mice inhibited the maintenance of behavioral sensitization. This effect was blocked by a low dose of WAY100635, a selective 5-HT1A receptor antagonist. A METH challenge increased the extracellular levels of dopamine (DA), 5-HT, and noradrenaline in the prefrontal cortex, but only the increase in 5-HT release was enhanced by repeated METH administration. This augmented response of 5-HT release was attenuated by osemozotan in a WAY100635-sensitive way. A single administration of osemozotan to drug na?ve mice inhibited METH-induced locomotor stimulant effect and reduced METH-induced increase in prefrontal 5-HT, but not DA, release. These results suggest that prefrontal 5-HT release is involved at least partly in the effects of osemozotan on single and repeated METH-induced behavioral effects in mice, and imply that the 5-HT1A receptors may have a potential therapeutic value in the remission of schizophrenia.     

Effects of the 5-HT2 receptor antagonist, ritanserin, on biogenic amines in the rat nucleus accumbens.

The effects of acute ritanserin treatment on dopamine (DA) and serotonin (5-HT) release and metabolism were studied in tissue and microdialysis samples of the nucleus accumbens in rats. Administration of a moderate dose of ritanserin elicited decreases in DA and 5-HT in tissue with concomitant increases in extracellular fluid. These data show that selective 5-HT2 receptor antagonism modulates DA as well as 5-HT neurotransmission and adds support to the suggestion that 5-HT/DA interactions may be important in the treatment of psychoses.     

5-HT1B receptors modulate release of [3H]dopamine from rat striatal synaptosomes: further evidence using 5-HT moduline, polyclonal 5-HT1B receptor antibodies and 5-HT1B receptor knock-out mice

In previous paper based on classical pharmacological tools, we identified a Gi protein-coupled presynaptic 5-hydroxytryptamine (5-HT) 1B receptor causing inhibition of dopamine (DA) release in rat striatal synaptosomes. It was the aim of the present study to further explore this receptor, using 5-HT moduline, a polyclonal antibody directed against 5-HT1B receptors and 5-HT1B receptor knock-out mice. Preincubation of rat striatal synaptosomes with 5-HT moduline (0.1, 1, or 10 microM) significantly reduced the inhibitory effect of CP93,129, a selective rat 5-HT1B receptor agonist, on K+-evoked overflow of [3H]DA in a non-competitive manner: 5-HT moduline did not modify the IC50 of CP93,129, but concentration-dependently reduced the maximal inhibitory effect. Preincubation of rat striatal synaptosomes with a specific polyclonal 5-HT1B receptor antibody also resulted in a significant attenuation of the inhibitory effect of CP93,129 on K+-evoked overflow of [3H]DA. In female 129/Sv wild-type mice, CP93,129 and 5-carboxyamidotryptamine maleate (5-CT), a non-selective 5-HT1B receptor agonist, inhibited the K+-evoked [3H]DA overflow in a concentration-dependent manner. Sumatriptan, a selective rat 5-HT1D receptor agonist, did not modify the overflow of [3H]DA. SB224289, a selective 5-HT1B receptor antagonist, abolished the inhibitory effects of CP93,129 and 5-CT. The inhibitory effects of CP93,129 and 5-CT were absent in synaptosomes from 5-HT1B receptor knockout mice. No compensatory inhibition effect in mutant mice was observed using sumatriptan. In conclusion, the results show that a non-competitive antagonist of the 5-HT1B receptor concentration-dependently decreases the maximal inhibitory effect of a 5-HT1B receptor agonist on the synaptosomal K+-evoked release of [3H]DA in striatum. Moreover, a specific antibody raised against the receptor and particularly directed against a region of the receptor protein involved in signal transduction, namely the coupling with the G-protein, also antagonizes the inhibitory effect of the stimulation of 5-HT1B receptor on the release of [3H]DA. Ultimately the disruption of 5-HT1B receptor gene in 5-HT1B knock-out mice leads to a total suppression of the effect of 5-HT1B receptor agonists on [3H]DA release. These observations further support our previous observations using selective agonists/antagonists, indicating that 5-HT1B receptors control the release of neuronal DA as presynaptic heteroreceptors.     

Antagonism at 5-HT(2A) receptors potentiates the effect of haloperidol in a conditioned avoidance response task in rats

High affinity for serotonin-2A (5-HT(2A)) over dopamine (DA) D(2) receptors is a leading hypothesis for clozapine's favorable therapeutic profile. Recent preclinical studies also indicate that a sufficient antipsychotic effect might be obtained by a combined high 5-HT(2A)/low D(2) receptor blockade. Thus, addition of a 5-HT(2A) receptor antagonist to an ineffective dose of a D(2) receptor antagonist produces a robust antipsychotic-like effect in the conditioned avoidance response (CAR) test. Electrophysiological and biochemical studies also show that 5-HT(2A) receptor antagonists can confer an atypical (clozapine-like) profile on a D(2) receptor antagonist. Improved therapeutic efficacy by adjunctive 5-HT(2A) receptor antagonist treatment to a traditional D(2) receptor blocking regimen has been suggested. However, the ability of 5-HT(2A) receptor blockade to protect against, or ameliorate, parkinsonian symptoms still remains unclear. Using the CAR and the catalepsy (CAT) tests as indices for antipsychotic activity and extrapyramidal side effect (EPS) liability, respectively, the effects of the selective 5-HT(2A) receptor antagonist MDL 100,907 in combination with the DA D(2) receptor antagonists haloperidol or raclopride were studied in rats. Haloperidol (0.025 or 0.1 mg/kg sc, -30 min) produced a dose-dependent suppression of CAR. Pretreatment with MDL 100,907 (0.5, 1.0, or 1.5 mg/kg sc; -60 min) enhanced and prolonged the haloperidol-induced suppression of CAR without escape failures. MDL 100,907 (1 mg/kg sc, -60 min) had no effect on CAT when coadministered with ineffective doses of raclopride. Raclopride (1 mg/kg sc, -30 min) alone produced a submaximal cataleptic response that was significantly enhanced by pretreatment with MDL 100,907. The present results confirm and extend previous results by showing that 5-HT(2A) receptor blockade can enhance the antipsychotic-like effects of a very low dose of a commonly used traditional antipsychotic. 5-HT(2A) receptor blockade does not, however, prevent EPS (CAT). The therapeutic advantage of this combination might, therefore, operate within a fairly narrow window.     

Bradycardic responses to microinjection of N-methyl-D-aspartate into the nucleus tractus solitarius are inhibited by local activation of 5-HT(3) receptors

Previous reports have described that glutamate ionotropic receptors in the nucleus tractus solitarius (NTS) are involved in the reflex control of heart rate, and that such a control can be inhibited by NTS-5-HT(3) receptor stimulation. In the present study, we examined in urethane anaesthetized rats the effects of intra-NTS microinjection of 1-(m-chlorophenyl)-biguanide (CPBG), a potent and selective 5-HT(3) receptor agonist, on the cardiovascular responses to local administration of glutamate ionotropic receptor agonists. Intra-NTS microinjection of CPBG reduced the atropine-sensitive bradycardia elicited by local microinjection of NMDA without affecting the cardiovascular responses to intra-NTS microinjections of AMPA or kainic acid. The reduction by CPBG of the NMDA-evoked cardiac response was blocked by prior intra-NTS microinjection of granisetron, a 5-HT(3) receptor antagonist, as well as bicuculline, a GABA(A) receptor antagonist. These results suggest that the stimulation of NTS 5-HT(3) receptors specifically reduces, via a GABA-dependent mechanism, the cardiac response to local NMDA administration.