A comparison of the behavioural effects of 5-HT2A and 5-HT2C receptor agonists in the pigeon

Activity at the 5-HT2A receptor versus that of the 5-HT2C receptor was studied in three behavioural paradigms. In pigeons trained to discriminate 0.32 mg/kg of 1-(2,5-diemethoxy-4-iodophenyl)-2-aminopropane (DOI) (a mixed 5-HT2A/C receptor agonist) from vehicle, quipazine (0.1-1 mg/kg) and m-chlorophenylpiperazine (mCPP) (1-3 mg/kg) substituted for DOI in a dose-related manner, and this generalization was blocked by MDL100907 (0.0001-0.01 mg/kg), a selective 5-HT2A receptor antagonist. RO60-0175 (a relatively selective 5-HT2C agonist) induced partial substitution at 3 mg/kg that was antagonized by both MDL100907 and by 3 mg/kg of SB242084, a relatively selective 5-HT2C antagonist. MK212 (a mixed 5-HT2C/A agonist) induced partial substitution that was antagonized by SB242084, but not by MDL100907. On a progressive ratio 5 operant schedule (PR5) for food reinforcement, DOI, quipazine, mCPP, MK212 and R060-0175 decreased the break point; mCPP, DOI, MK212 and quipazine also induced vomiting. Although MDL100907 antagonized both the reductions of break point and vomiting, SB242084 only partially attenuated the decrease in break point observed with MK212 and DOI, and was unable to eliminate vomiting. Thus pharmacological activity at the 5-HT2A receptor can be behaviourally distinguished from pharmacological activity at the 5-HT2C receptor in the pigeon. Furthermore, the decrease in the break point of a PR5 schedule induced by 5-HT2C receptor agonists may be related to decreased appetite, whereas that induced by 5-HT2A receptor agonists may be due to unrelated factors, such as emesis.     

Thermogenic effect of YM348, a novel 5-HT2C-receptor agonist, in rats

We have investigated the effect of S-2-(7-ethyl-1H-furo[2,3-g]indazol-1-yl)-1-methylethylamine (YM348), a novel 5-HT(2C)-receptor agonist, on body temperature and energy expenditure in Wistar rats. m-Chlorophenylpiperazine (mCPP) and S-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine (RO 60-0175) were used as reference 5-HT(2C)-receptor agonists. Administration of YM348, mCPP and RO 60-0175 dose-dependently and significantly increased body temperature in rats. YM348- or RO 60-0175-induced hyperthermia was significantly attenuated by the non-selective 5-HT(2)-receptor antagonist methysergide and the selective 5-HT(2C)-receptor antagonist SB242084, but not by the selective 5-HT(2A)-receptor antagonist MDL100907. mCPP-induced hyperthermia was significantly attenuated by methysergide, SB242084 and MDL100907. In addition to the increase in body temperature, YM348, mCPP and RO 60-0175 produced dose-related and significant increases in energy expenditure. YM348-, mCPP- and RO 60-0175-induced increases in energy expenditure were significantly attenuated by methysergide and SB242084 but not by MDL100907. These results suggested that 5-HT(2C)-receptor stimulation increased body temperature and energy expenditure and that the 5-HT(2C)-receptor was the target receptor in the thermogenic effect of YM348 in Wistar rats.     

Evidence that central 5-HT2A and 5-HT2B/C receptors regulate 5-HT cell firing in the dorsal raphe nucleus of the anaesthetised rat

1. Systemic administration of phenethylamine-derived, 5-hydroxytryptamine(2) (5-HT(2)) receptor agonists inhibits the firing of midbrain 5-HT neurones, but the 5-HT receptors involved are poorly defined, and the contribution of peripheral mechanisms is uncertain. This study addresses these issues using extracellular recordings of 5-HT neurones in the dorsal raphe nucleus of anaesthetised rats. 2. The 5-HT(2) receptor agonists DOI ((+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride) and DOB ((+/-)-2,5-dimethoxy-4-bromoamphetamine hydrobromide), caused a dose-related (10-100 micro g kg(-1) i.v.) inhibition of 5-HT neuronal activity, with the highest dose reducing firing rates by >80%. 3. Pretreatment with the 5-HT(2) receptor antagonist ritanserin (1 mg kg(-1) i.v.) completely blocked the action of DOI. The 5-HT(2A) receptor antagonist MDL 100,907 (0.2 mg kg(-1) i.v.) blocked the action of both DOI and DOB. In comparison, the 5-HT(2B/C) receptor antagonist SB 206553 (0.5 mg kg(-1) i.v.) caused a small, but statistically significant, shift to the right in the dose response to DOI and DOB. 4. Pretreatment with the peripherally acting 5-HT(2) receptor antagonist BW 501C67 (0.1 mg kg(-1) i.v.) had no effect on the DOI-induced inhibition of 5-HT cell firing, but completely blocked the DOI-induced rise in mean arterial blood pressure. 5. These data indicate that the inhibition of 5-HT cell firing induced by systemic administration of DOI and DOB is mediated predominantly by the 5-HT(2A) receptor-subtype, but that 5-HT(2B/C) receptors also play a minor role. Moreover, central and not peripheral mechanisms are involved. Given evidence that 5-HT(2) receptors are not located on 5-HT neurones, postsynaptic 5-HT feedback mechanisms are implicated.     

5-HT(2A) and 5-HT(2C) receptor stimulation are differentially involved in the cortical dopamine efflux-Studied in 5-HT(2A) and 5-HT(2C) genetic mutant mice

Both 5-HT(2A) and 5-HT(2C) receptors modulate cortical dopamine efflux, but in opposite directions. We have now compared the ability of the three 5-HT(2A/2C) receptor agonists, DOI (R(-)-2,5-dimethoxy-4-iodoamphetamine), mCPP (meta-chlorophenylpiperazine) and MK-212 (6-Chloro-2-(piperazinyl) pyrazine), to modulate cortical dopamine efflux in 5-HT(2A) and 5-HT(2C) genetic mutant mice. In the 5-HT(2A) mice, the preferential 5-HT(2A) receptor agonist DOI (2.5mg/kg, s.c.) induced a slight but significant increase in cortical dopamine efflux only in the wild type (WT) mice; MK-212 (2.5mg/kg) reduced dopamine efflux in both WT and receptor knockout (KO) mice; moreover, MCPP, 2.5mg/kg, had no effect in either types. In 5-HT(2C) mice, DOI increased dopamine efflux in both types; while MK-212 decreased dopamine efflux in the WT, but not the receptor KO mice. These results provide new evidence that 5-HT(2A) receptor stimulation enhances and 5-HT(2C) receptor stimulation inhibits cortical dopamine efflux, and suggest the effects of DOI, MK-212 and mCPP on the cortical dopamine efflux are due to their different abilities on 5-HT(2A) and 5-HT(2C) receptors stimulation. Of these three agents, only DOI, the more selective 5-HT(2A) receptor agonist, is hallucinogenic. The absence of hallucinations with mCPP may be due to its relatively more potent 5-HT(2C) receptor agonist effect, inhibiting the ability of mCPP to enhance dopamine efflux in cortical and perhaps limbic regions as well. The present data provide additional evidence that hallucinations are due, in part, to 5-HT(2A) rather than 5-HT(2C) receptor stimulation. These findings suggest that 5-HT(2C) receptor agonists may be useful as antipsychotics, consistent with previous suggestions.     

Behavioral effects of dipropyltryptamine in rats: evidence for 5-HT1A and 5-HT2A agonist activity

These studies investigated the role of serotonin-1A (5-HT1A) and 5-HT2A receptors in the behavioral effects of dipropyltryptamine (DPT). Eight rats discriminated 0.56 mg/kg 2,5-dimethoxy-4-methylamphetamine (DOM) from saline and responded under a fixed ratio 5 schedule of food presentation; 12 other rats were used for observational studies. DOM and DPT increased responding on the DOM lever with 3.2 mg/kg DPT producing greater than 95% responding on the DOM lever; this effect of DPT was antagonized by the 5-HT2A receptor antagonist MDL100907. In another study, the 5-HT1A and 5-HT7 receptor agonist 8-OH-DPAT produced lower-lip retraction and, at larger doses, flat body posture; DPT alone produced flat body posture and not lower-lip retraction; MDL100907 alone did not produce either effect. Pretreatment with DPT blocked 8-OH-DPAT-elicited lower-lip retraction, suggesting antagonist activity of DPT at 5-HT1A receptors; however, in the presence of MDL100907 DPT produced not only flat body posture but also lower-lip retraction, suggesting that agonist activity of DPT at 5-HT2A receptors masked agonist activity at 5-HT1A receptors. Lower-lip retraction and flat body posture by DPT in the presence of MDL100907 were attenuated by the 5-HT1A receptor antagonist WAY100635. These findings suggest that DPT has agonist activity at 5-HT1A and 5-HT2A receptors and that effects at 5-HT2A receptors mask effects at 5-HT1A receptors.     

Effects of the 5-HT1C/5-5-HT2 receptor agonists DOI and alpha-methyl-5-HT on plasma glucose and insulin levels in the rat

Administration of the 5-HT1C/5-HT2 receptor agonist 1-(2,5-dimethoxy-4- iodophenyl)-2-aminopropane (DOI, 0.125-2.0 mg/kg i.v.) triggered dose-dependent increases in plasma glucose; plasma insulin levels remained unchanged. Pretreatment with the 5-HT1C/5-HT2 receptor antagonists LY 53857, ritanserin, or the mixed 5-HT2/alpha 1-adrenoceptor antagonist ketanserin either diminished or prevented the hyperglycemic effect of DOI (0.5 mg/kg). Administration of the mixed 5-HT1C receptor agonists/5-HT2 receptor antagonists 1-(3-chlorophenyl)-piperazine (mCPP) or 1-(3-trifluoromethyl)phenyl)piperazine level (TFMPP) did not affect plasma glucose levels. However, pretreatment with mCPP or TFMPP decreased DOI-induced hyperglycemia in a dose-dependent manner. The alpha 2-adrenoceptor antagonist idazoxan and the ganglionic blocker hexamethonium both decreased DOI-induced hyperglycemia, Whilst the alpha 1-adrenoceptor antagonist prazosin amplified the rise in plasma glucose elicited by DOI. The peripherally acting 5-HT1C/5-HT2 receptor agonist alpha-methyl-5-HT (0.5-1.0 mg/kg i.v.) triggered a rise in plasma glucose levels that was associated with an increase in plasma insulin levels. Pretreatment with LY 53857 diminished alpha-methyl-5-HT-induced hyperglycemia. These data indicate that 5-HT2 receptors, but not 5-HT1C receptors, and catecholaminergic systems, mediate DOI-induced hyperglycemia. Moreover, it is suggested that the inhibition of insulin release by DOI is centrally mediated, and that activation of peripheral 5-HT2 receptors may affect glycemia.     

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.     

Evidence that genetic variation in 5-HT transporter expression is linked to changes in 5-HT2A receptor function

Variability in expression of the 5-HT transporter (5-HTT) gene in the human population has been associated with a range of behavioural phenotypes. The underlying mechanisms are unclear but may involve changes in 5-HT receptor levels and/or signalling. The present study used a novel 5-HTT overexpressing transgenic mouse to test the hypothesis that variability in 5-HTT expression may alter 5-HT(2A) receptor function. In wildtype mice, the 5-HT(2) receptor agonist DOI increased regional brain mRNA expression of two immediate early genes (c-fos and Arc), and induced head twitches, and both effects were abolished by pre-treatment with the 5-HT(2A) receptor antagonist MDL 100907. In 5-HTT overexpressing mice, DOI induced a greater increase in both c-fos and Arc mRNA expression in cortical brain regions, and more head twitches, compared to wildtype mice. Autoradiographic and in situ hybridisation experiments showed that 5-HT(2A) receptor binding sites and 5-HT(2A) receptor mRNA did not differ between transgenic and wildtype mice. Finally, the transgenic mice had lower regional brain 5-HT levels compared to wildtype mice. This depletion of 5-HT may underpin the increase in 5-HT(2A) receptor function because in wildtype mice 5-HT depletion using the 5-HT synthesis inhibitor, p-chlorophenylalanine, enhanced the head twitch response to DOI. These data demonstrate that elevated 5-HTT expression is accompanied by increased 5-HT(2A) receptor function, an effect possibly mediated by decreased availability of synaptic 5-HT. Variation in levels of 5-HTT expression may therefore be a source of variability in 5-HT(2A) receptor function, which may be an important modifier of 5-HTT-linked phenotypes.     

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.     

5-HT2 receptors differentially modulate dopamine-mediated auto-inhibition in A9 and A10 midbrain areas of the rat

5-HT (20 microM) enhanced dopamine (DA) D2-like receptor mediated reduction of the firing rate of DA neurons in the substantia nigra pars compacta (A9) and ventral tegmental area (A10) in a rat midbrain slice preparation. Quinpirole (30 nM) induced a mean reduction of the firing rate in A9 and A10 DA neurons to 64 +/- 4%, respectively, 71 +/- 5% of the baseline value. Bath application of 5-HT in the presence of quinpirole further reduced the firing rate to 37 +/- 7% in A9 and 33 +/- 13% in A10. The 5-HT2 receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI, 500 nM) enhanced quinpirole-induced reduction of firing rate of A10 DA neurons, but not of A9 DA neurons, suggesting that different 5-HT receptor subtypes are involved in modulation of dopamine D2-like receptor mediated inhibition in the two regions. The selective 5-HT2A receptor antagonist MDL100907 and the selective 5-HT2C receptor antagonist SB242084 (50 and 500 nM) both abolished the enhancement of quinpirole-induced reduction by either 5-HT or DOI, suggesting the involvement of direct and indirect (possibly via interneurons) modulation pathways in A10. The involvement of 5-HT and specific 5-HT2 receptors in augmentation of auto-inhibition in A10 could have important implications for our understanding of the mechanism of atypical antipsychotic drug action.     

mCPP-induced hyperactivity in 5-HT2C receptor mutant mice is mediated by activation of multiple 5-HT receptor subtypes

The serotonin receptor agonist mCPP induces hyperlocomotion in 5-HT2C receptor knockout (KO) mice or in the presence of a 5-HT2C receptor antagonist. In the present group of experiments, we evaluate the role of 5-HT1A, 5-HT1B and 5-HT2A receptors in mCPP-induced hyperactivity in 5-HT2C KO mice. We also assess the ability of agonists at these receptors to induce hyperactivity in wildtype (WT) mice pre-treated with a selective 5-HT2C receptor antagonist. As previously reported, mCPP (3 mg/kg) induced hyperactivity in 5-HT2C KO mice. A combination of the 5-HT1B receptor agonist CP-94,253 (20 mg/kg) and the 5-HT1A receptor agonist 8-OH-DPAT (0.5 mg/kg) induced marked hyperactivity in WT but not in 5-HT2C KO mice, nor in mice treated with the selective 5-HT2C receptor antagonist, SB 242084 (1.5 mg/kg). Neither CP-94,253 nor 8-OH-DPAT had any intrinsic effect on locomotion in WTs. mCPP-induced hyperactivity was attenuated in 5-HT2C KO mice by the 5-HT1B receptor antagonist SB 224289 (2.5 mg/kg), and the 5-HT2A receptor antagonists ketanserin (0.3 mg/kg) and M100907 (0.01 mg/kg) but not by the 5-HT1A receptor antagonist WAY 100635 (1 mg/kg). The 5-HT(2A/2B/2C) receptor agonist, Ro 60-0175 (3 mg/kg), induced a modest increase in locomotor activity in WT mice pre-treated with SB 242084. However, the combination of Ro 60-0175 with CP-94,253 induced a substantial increase in activity in 5-HT2C KO mice, an effect comparable to mCPP-induced hyperactivity. Thus, joint activation of 5-HT1A and 5-HT1B receptors stimulates locomotion in WT mice but this response is dependent on a functional 5-HT2C receptor population and hence is absent in 5-HT2C KO mice. By contrast, mCPP-induced hyperactivity depends on the inactivation of a separate 5-HT2C receptor population and is mediated by 5-HT2A and 5-HT1B receptor activation.     

Agonist action at 5-HT1C receptors facilitates 5-HT1A receptor-mediated spontaneous tail-flicks in the rat

In rats lightly restrained in plastic cylinders, subcutaneous administration of the selective, high efficacy 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), induced spontaneous tail-flicks, that is, tail-flicks in the absence of extraneous stimulation. The putative 5-HT1B receptor agonist, CGS 12066B, the mixed 5-HT1B/1C receptor agonists, 1-((3-(trifluoromethyl)phenyl]piperazine (TFMPP) and 1-(3-chlorophenyl)piperazine (mCPP), the 5-HT1C/2 receptor agonist, [+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the 5-HT1B/1C/2 receptor agonist, quipazine, did not, in contrast, elicit tail-flicks when applied alone. However, TFMPP, mCPP, DOI and quipazine, but not CGS 12066B, each potentiated the action of 8-OH-DPAT. Further, in the presence of TFMPP, mCPP and DOI, the dose-response curve for the induction of tail-flicks by 8-OH-DPAT was both steeper and shifted to the left. Tail-flicks induced by another high efficacy 5-HT1A receptor agonist, lisuride, were also enhanced by TFMPP, mCPP and DOI. The 5-HT1A receptor partial agonists, buspirone and (+/-)-flesinoxan, evoked tail-flicks only in the presence of TFMPP, mCPP or DOI. The mixed 5-HT1C/2 receptor antagonists, ritanserin and ICI 169,369, did not modify the action of 8-OH-DPAT alone but abolished the potentiation of 8-OH-DPAT-induced tail-flicks by DOI and TFMPP. Further, the selective 5-HT1A receptor antagonist, BMY 7378, blocked tail-flicks induced by both 8-OH-DPAT alone and 8-OH-DPAT plus DOI or TFMPP. A common property of those drugs potentiating 8-OH-DPAT-induced tail-flicks is an agonist action at 5-HT1C receptors and the data indicate that it is this mechanism which underlies the facilitation of tail-flicks.     

Modulation of 5-HT(2A) receptor-mediated head-twitch behaviour in the rat by 5-HT(2C) receptor agonists

The pharmacology of several commonly described 5-hydroxytryptamine (5-HT)(2C) receptor agonists was investigated in vivo and in vitro at rat 5-HT(2A), 5-HT(2B), and 5-HT(2C) receptors. The 5-HT(2C) receptor agonist, (S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine fumarate (Ro 60-0175), did not induce a significant head-twitch response when given alone, yet when administered to rats subsequent to an acute challenge with the selective 5-HT(2C) receptor antagonist, 6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy) pyridin-3-yl carbomyl] indoline (SB-242084), a robust head-twitch response was observed which was blocked by the selective 5-HT(2A) receptor antagonists R(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl-ethyl)]-4-piperidine-methanol (MDL 100907) or ketanserin. The preferential 5-HT(2C) receptor agonists Ro 60-0175, 6-chloro-2-[1-piperazinyl]-pyrazine HCl (MK-212), 1-(3-chlorophenyl)piperazine hydrochloride (mCPP), 1-(3-trifluoromethylphenyl)piperazine hydrochloride (TFMPP), and (S)-3-[(2,3-dihydro-5-methoxy-1H-inden-4-yl)oxy]-pyrollidine HCl (ORG-37684), the 5-HT(2A/2C) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI), the 5-HT(2B) receptor agonist 1-[5-thienylmethoxy-1-1H-3-indoyl] propan-2-amine hydrochloride (BW-723C86), and nor-D-fenfluramine were administered to rats subsequent to an acute challenge of SB-242084. Under such conditions, each agonist, with the exception of BW-723C86, induced a dose-dependent increase in the incidence of head twitches. The pharmacology of the same agonists was determined at cloned rat 5-HT(2) receptors using a fluorometric imaging plate reader (FLIPR). Both the in vivo and in vitro data suggest that for some ligands, previous reports have overestimated their in vivo selectivity for the 5-HT(2C) receptor.     

Effects of 5-HT2A receptor stimulation on the discrimination of durations by rats

We recently found that rats' ability to discriminate durations of exteroceptive stimuli is disrupted by the non-selective 5-HT receptor agonist quipazine. Ketanserin reversed this effect, suggesting that the effect may be mediated by 5-HT2A receptors. Here, we report that the 5-HT2A/2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) also disrupts temporal discrimination, and that this effect can be reversed by ketanserin and the highly selective 5-HT2A receptor antagonist (+/-)2,3-dimethoxyphenyl-1-[2-(4-piperidine)-methanol] (MDL-100907). Twenty rats were trained to discriminate durations in a discrete-trials psychophysical procedure. In each 50-s trial, a light was presented for t seconds, following which two levers (A and B) were presented. A response on A was reinforced if t < 25 s, and a response on B if t > 25 s. Logistic psychometric curves were fitted to the proportional choice of B (%B) for derivation of timing indices [T50: time corresponding to %B = 50; Weber fraction: (T75-T25)/2T50, where T75 and T25 are times corresponding to %B = 75 and 25, respectively]. DOI 0.25 mg kg (subcutaneous) significantly increased the Weber fraction and tended to increase T50. Ketanserin 2 mg kg (subcutaneous) did not alter either parameter, but completely antagonized the effects of DOI. Similarly, MDL-100907 0.5 and 1 mg kg (intraperitoneal) did not affect performance, but completely antagonized the effects of DOI. The results indicate that the mixed 5-HT2A/2C receptor agonist DOI disrupts temporal discrimination via stimulation of 5-HT2A receptors.     

Role of 5-HT1B, 5-HT2A and 5-HT2C receptors in the generalization of 5-HT receptor agonists to the ethanol cue in the rat

Although accumulating evidence suggests that serotonergic drugs are able to substitute for the ethanol (EtOH) cue in rats, it is still unclear which 5-HT receptor subtypes are responsible for this phenomenon, and whether these receptors are critically involved in the EtOH cue. In the present study, rats were trained to discriminate EtOH (1000 mg/kg, i.p., t-15 min) from saline in a two-lever food-reinforced procedure, and it was investigated to which extent serotonergic compounds with a certain level of specificity for either 5-HT1B, 5-HT2A or 5-HT2C receptors generalized to the EtOH cue. Subsequently, the involvement of these receptor subtypes was ascertained by the use of selected 5-HT receptor antagonists. The 5-HT1B receptor agonist CP 94,253 (0.3-5 mg/kg, i.p.) and the mixed 5-HT(2C/1B) receptor agonist mCPP (0.1-1 mg/kg, i.p.), but not the preferential 5-HT2A receptor agonist DOI (0.3-1 mg/kg, i.p.), completely generalized to the EtOH cue. Complete generalization of the former two compounds coincided with a decrease in response rate. In antagonism studies, it was shown that the 5-HT1B receptor antagonist GR 127935 (10 mg/kg, i.p.) completely blocked generalization of CP 94,253 to the EtOH cue, suggesting that stimulation of 5-HT1B receptors produces discriminative stimulus effects which are similar to those of EtOH. GR 127935 (10 mg/kg, i.p.), as well as the mixed 5-HT(1B/2C) receptor antagonist metergoline (1 mg/kg, i.p.), and the 5-HT2C receptor antagonist SB 206,553 (1 mg/kg, i.p.) completely blocked generalization of mCPP to the EtOH cue. This suggests that 5-HT1B and 5-HT2C receptors are required for the generalization of mCPP to the EtOH cue. The present findings indicate that activation of 5-HT1B and 5-HT2C, but not of 5-HT2A receptors, mimics the EtOH cue. However, the finding that neither metergoline, nor the 5-HT2A receptor antagonist MDL 100,907 blocked the EtOH cue, suggests that these receptors play only a minor role in the discriminative stimulus effects of a moderately low dose of EtOH.     

In-vivo assessment of 5-HT2A and 5-HT2C antagonistic properties of newer antipsychotics

The effects of serotonin (5-HT) receptor ligands on the MK 212 (6-chloro-2[1-piperazinyl]pyrazine) discriminative stimulus and quipazine-induced head twitches were studied in rats. 5-HT1A (8-OH-DPAT) and preferential 5-HT2A (DOI) receptor agonists did not generalize to the discriminative stimulus. The 5-HT2B/2C-receptor antagonist, SB 206553 (5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2 ,3-f]indole), and the 5-HT2A/2C-receptor antagonist, ritanserin, acted as potent antagonists, whereas the 5-HT2A-receptor antagonist, MDL 100.151 ([(+/-)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4- piperidine-methanol), produced minor and inconsistent inhibition. SB 206553 was a weak antagonist against quipazine-induced head twitches, whereas MDL 100.151 and ritanserin were potent antagonists. This suggests that the MK 212 discriminative stimulus is mediated by 5-HT2C receptors, while quipazine-induced head twitches are mediated primarily by 5-HT2A receptors. The effects on quipazine-induced head twitches were comparable to previously published effects on the DOI discriminative stimulus. 5-HT2A- and 5-HT2C-receptor antagonistic potencies of clozapine, olanzapine, risperidone, sertindole and ziprasidone were compared in the same models. Clozapine showed similar potencies in both models, while sertindole, olanzapine and risperidone inhibited quipazine-induced effects more potently than the MK 212 discriminative stimulus. Ziprasidone exerted a minor preference for 5-HT2A- compared to 5-HT2C-receptor-mediated effects. The ratio between in vivo inhibitory potencies at 5-HT2A and 5-HT2C receptors did not correlate with corresponding ratios from in-vitro affinity and ex-vivo occupancy studies in the literature.     

Fluvoxamine exerts anorexic effect in 5-HT2C receptor mutant mice with heterozygous mutation of beta-endorphin gene

Serotonin (5-hydroxytryptamine; 5-HT) 2C receptors and the downstream melanocortin pathway are suggested to mediate the anorexic effects of m-chlorophenylpiperazine (mCPP) and fenfluramine. We previously reported that fluvoxamine, a selective serotonin reuptake inhibitor, together with pharmacological inactivation of 5-HT2C receptors exert feeding suppression through activation of 5-HT1B receptors in mice. Here, we report that fluvoxamine exerted anorexic effects in 5-HT2C receptor mutant mice with heterozygous mutation of beta-endorphin gene (2CREnd mice), whereas fluvoxamine had no effect on food intake in age-matched wild-type mice and 5-HT2C receptor mutant mice, which are associated with decreases in hypothalamic proopiomelanocortin (POMC) expression. mCPP suppressed food intake in 5-HT2C receptor mutant mice, 2CREnd mice and age-matched wild-type mice. These results suggest that fluvoxamine-induced feeding suppression requires a perturbation of 5-HT2C receptor and beta-endorphin signalling plus functional hypothalamic POMC activity, whereas mCPP-induced feeding suppression does not always require functional 5-HT2C receptor, beta-endorphin, and POMC activity in mice.     

5-HT2 receptor antagonism reduces hyperactivity induced by amphetamine, cocaine, and MK-801 but not D1 agonist C-APB.

The hyperlocomotion induced by the noncompetitive NMDA antagonist MK-801 (0.3 mg/kg SC) in mice was attenuated by the nonselective 5-HT2 antagonist ritanserin (0.12 and 0.25 mg/kg SC) and by the 5-HT2A selective antagonist MDL100907 (0.05 and 0.1 mg/kg SC). SB242084 (0.25-1.0 mg/kg), a selective 5-HT(2C) antagonist, had no effect on MK-801-induced hyperactivity. These same doses of ritanserin and MDL100907 reduced the hyperactivity induced by cocaine (10 mg/ kg). Amphetamine (2.5 mg/kg SC) induced hyperlocomotion that was also attenuated by ritanserin (0.064).25 mg/kg SC). The hyperlocomotion induced by the D1 agonist C-APB (1.0 mg/kg) is not altered by pretreatment with ritanserin or MDL100907. This suggests that compounds that increase locomotor activity via indirectly increasing dopaminergic activity (either by increased release or blockade of reuptake) require the activation of a 5-HT2A receptor. Activity of compounds that act directly at the postsynaptic dopamine receptors such as C-APB is not dependent on such a mechanism. This suggests a selective involvement of 5-HT2A receptors but not 5-HT2c receptors in the mediation of the behavioral effects of compounds that increase synaptic concentration of dopamine but not directly acting agonists. This implies that the 5-HT2A receptors modulate elevation of extracellular dopamine, not the postsynaptic sensitivity of dopamine neurons.     

Effect of atypical antipsychotic drugs on 5-HT2 receptors in the rat orbito-frontal cortex: an in vivo electrophysiological study

Low doses of the atypical antipsychotic drug risperidone are effective in patients with obsessive compulsive disorder (OCD) not responding to serotonin (5-HT) reuptake inhibitors, although higher doses have been reported to induce OCD symptoms in psychotic patients. Since such atypical antipsychotics exert, in addition to dopamine, 5-HT₂ receptor antagonistic properties, it was deemed essential to investigate the electrophysiological effect of these agents on 5-HT₂ receptors in the rat orbito-frontal cortex (OFc), a brain region implicated in OCD. Microiontophoretic application of the GABA_A receptor antagonist bicuculline had no effect on the suppressant effect of neuronal activity in the OFc induced by microiontophoretic application of the preferential 5-HT_2A and 5-HT_2C receptor agonists (+)-1-(4-iodo-2, 5-dimethoxyphenyl)-2-aminopropane (DOI) and m-chlorophenyl-piperazine (mCPP), respectively, but it antagonized the effect of GABA on the same neurons. These results indicate a lack of involvement of GABA interneurons in the suppressant effect of DOI and mCPP. While the 5-HT₂ receptor antagonist ritanserin (2 mg/kg, IV) attenuated the inhibitory effect of DOI and mCPP in the medial prefrontal cortex (mPFc), the inhibition was unaffected in the OFc. In the mPFc, the effect of DOI and mCPP was blocked by both clozapine (1.0 and 10 mg/kg, IV) and risperidone (0.1 and 1.0 mg/kg, IV). In the OFc, only the suppressant effect of mCPP was attenuated by both doses of clozapine but only by the high dose of risperidone. These results suggest that the 5-HT₂ response in the OFc is more akin to the 5-HT_2C subtype and that the deleterious effect sometimes observed with high doses of risperidone and clozapine may be due to a decrease in 5-HT neurotransmission. In contrast, the beneficial effect of low doses of risperidone may be due, in part, to the antagonism of dopamine receptors. Received: 22 June 1998/Final version: 8 October 1998