Characterizing the effects of 5-HT2C receptor ligands on motor activity and feeding behaviour in 5-HT2C receptor knockout mice

5-HT_2C receptor agonists have considerable therapeutic potential, however there is little in vivo data to compare the potency and selectivity of 5-HT_2C receptor agonists. Since 5-HT_2C receptor agonists reduce locomotor activity and food intake, changes in these drug-induced behaviours in 5-HT_2C receptor knockout mice could provide a means to examine receptor selectivity in-vivo. Initially this study compared older 5-HT_2C agonists mCPP and MK212, to newer, apparently more selective compounds: Ro 60-0175, WAY161503, CP809,101 and lorcaserin (APD356) on motor activity in wild-type, and 5-HT_2C receptor knockout mice. Two 5-HT_2C receptor antagonists SB242084 and SDZ SER 082 were also examined. mCPP did not significantly alter activity in wild-type mice, but enhanced activity in knockout animals. MK212 (3 and 10 mg/kg) and Ro 60-0175 (1 and 3 mg/kg) reduced activity in wild-type but not knockout animals. At 10 mg/kg, Ro 60-0175 reduced activity in knockout animals, suggesting loss of 5-HT_2C receptor selectivity. CP809,101 and lorcaserin reduced activity in wild-type but not knockout mice. In subsequent feeding studies, Ro 60-0175 and lorcaserin reduced food intake in wild-type animals only. Selectivity of effect for mCPP was marginal. The antagonist SB242084 increased activity in wild-type animals but not in knockout mice; SB242084 did not alter feeding in either genotype. SDZ SER 082 reduced activity in both genotypes implying poor selectivity for 5-HT_2C receptors. The data demonstrate that studying food intake, and particularly motor behaviour, in the 5-HT_2C receptor knockout mouse is a useful and relatively simple approach for screening 5-HT_2C receptor ligands in vivo.     

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.     

Implication of 5-HT<Subscript>2</Subscript> receptor subtypes in the mechanism of action of antidepressants in the four plates test

Rationale The selective serotonin reuptake inhibitors (SSRIs) and the serotonin and noradrenaline reuptake inhibitors (SNRIs) increase synaptic levels of serotonin, leading to an increased activation of a multitude of specific postsynaptic 5-HT receptors. However, it is not yet known which 5-HT receptor subtypes mediate the therapeutic effects of antidepressants.Methods The effects of the SSRI, paroxetine and the SNRI, venlafaxine were evaluated in the mouse four plates test (FPT).Results Paroxetine administered intraperitoneally (IP) (0.5, 2–8 mg/kg) potently augmented the number of punished passages accepted by mice in this paradigm. The effects of paroxetine (8 mg/kg) were not reversed by the selective 5-HT_2C receptor antagonist, RS 10-2221 (0.1 mg/kg and 1 mg/kg) or the selective 5-HT_2B/2C receptor antagonist SB 206553 (0.1 mg/kg and 1 mg/kg), at doses which lack an effect when administered alone. In contrast, the selective 5-HT_2A receptor antagonist, SR 46349B (0.1 mg/kg and 1 mg/kg) completely abolished the paroxetine-induced increase in punished passages. The acute administration of venlafaxine induced an anxiolytic-like effect in the FPT at the doses of 2–16 mg/kg. This effect was reversed by the 5-HT_2B/2C receptor antagonist as did SR 46349B, for both doses administered. Our results strongly suggest that activation of 5-HT_2A receptors is critically involved in the anxiolytic activity of paroxetine, whereas the 5-HT_2A and 5-HT_2B receptors are involved in the anti-punishment action of venlafaxine in the FPT. The co-administration of selective 5-HT_{2A, 2B, 2C} receptor agonists (DOI, 0.06 mg/kg and 0.25 mg/kg; BW 723C86, 0.5 mg/kg and 2 mg/kg and RO 60-0175, 0.06 mg/kg and 0.25 mg/kg), respectively, was subsequently investigated. The effects of sub-active doses of paroxetine (0.25 mg/kg and 1 mg/kg) were augmented by BW 723C86 and RO 60-0175 receptor agonist challenge. The anti-punishment effects of venlafaxine (0.25 mg/kg and 1 mg/kg) were potentialised only by DOI co-administration.Conclusion These results indicate that the co-administration of 5-HT₂ receptor agonists with paroxetine and venlafaxine may provide a powerful tool for enhancing the clinical efficacy of these antidepressants.     

Impulsive action induced by amphetamine, cocaine and MK801 is reduced by 5-HT(2C) receptor stimulation and 5-HT(2A) receptor blockade

Previous work has shown that 5-HT_2C receptor agonists and 5-HT_2A receptor antagonists reduce impulsive action, as well as the locomotor stimulant effect of psychomotor stimulants. Since psychomotor stimulants also increase impulsive action we examined the effects of the 5-HT_2C receptor agonist Ro60-0175, and the 5-HT_2A receptor antagonist M100907 on impulsive action induced by amphetamine, cocaine and the NMDA receptor antagonist MK801 (dizocilpine). Impulsive action was measured in adult male Long-Evans rats as premature responding in the 5-choice serial reaction time (5-CSRT) test. Initially, we determined that amphetamine (0.3 mg/kg), cocaine (15 mg/kg) and MK801 (0.03 mg/kg) induced comparable premature response rates of approximately 50-70 per session, compared to 10-15 responses under baseline conditions. Each drug and its vehicle were then tested in combination with Ro60-0175 (0.1 and 0.6 mg/kg) or its vehicle, or M100907 (0.5 mg/kg) or its vehicle. At 0.1 mg/kg Ro60-0175 did not modify the effects of amphetamine, cocaine or MK801. In contrast, the 0.6 mg/kg dose reduced premature responses induced by amphetamine, cocaine and MK801. M100907 also reduced premature responding induced by all three of these drugs. In general, treatment with Ro60-0175 or M100907 by itself did not consistently alter any of the other aspects of task performance in the 5-CSRT test including number of trials completed, and accuracy of responding. These data show that activation of 5-HT_2C receptors and blockade of 5-HT_2A receptors have seemingly similar functional effects on a measure of impulsive action.     

Anxiolytic properties of agomelatine,an antidepressant with melatoninergic and serotonergic properties: role of 5-HT<Subscript>2C</Subscript> receptor blockade

Rationale The novel antidepressant agent, agomelatine, behaves as an agonist at melatonin receptors and as an antagonist at serotonin (5-HT)_2C receptors.Objectives To determine whether, by virtue of its antagonist properties at 5-HT_2C receptors, agomelatine elicits anxiolytic properties in rats.Methods Employing a combined neurochemical and behavioural approach, actions of agomelatine were compared to those of melatonin, the selective 5-HT_2C receptor antagonist, SB243,213, and the benzodiazepine, clorazepate.Results In unfamiliar pairs of rats exposed to a novel environment, agomelatine enhanced the time devoted to active social interaction, an action mimicked by clorazepate and by SB243,213. In a Vogel conflict procedure, agomelatine likewise displayed dose-dependent anxiolytic activity with a maximal effect comparable to clorazepate, and SB243,213 was similarly active in this procedure. In a plus-maze procedure in which clorazepate significantly enhanced percentage entries into open arms, agomelatine revealed only modest activity and SB243,213 was inactive. Further, like SB243,213, and in contrast to clorazepate, agomelatine did not suppress ultrasonic vocalizations emitted by rats re-exposed to an environment associated with an aversive stimulus. Whereas clorazepate reduced dialysate levels of 5-HT and noradrenaline in hippocampus and frontal cortex of freely moving rats, agomelatine did not affect extracellular levels of 5-HT and elevated those of noradrenaline. SB243,213 acted similarly to agomelatine. Melatonin, which did not modify extracellular levels of 5-HT or noradrenaline, was ineffective in all models of anxiolytic activity. Furthermore, the selective melatonin antagonist, S22153, did not modify anxiolytic properties of agomelatine in either the social interaction or the Vogel Conflict tests.Conclusions In contrast to melatonin, and reflecting blockade of 5-HT_2C receptors, agomelatine is active in several models of anxiolytic properties in rodents. The anxiolytic profile of agomelatine differs from that of benzodiazepines from which it may also be distinguished by its contrasting influence on corticolimbic monoaminergic pathways.     

Orthostatic hypotensive effect of antipsychotic drugs in Wistar rats by in vivo and in vitro studies of α1-adrenoceptor function

Rationale It has been suggested that the increase in serotonin transmission induced by indirect agonists such as fenfluramine and fluoxetine attenuates cue-elicited reinstatement of cocaine-seeking in rats through a 5-HT_2C receptor-dependent mechanism. Objective We investigated whether Ro 60-0175, a nonselective 5-HT_2B–2C agonist, influences cue-elicited reinstatement of cocaine-seeking behavior. We evaluated the 5-HT_2C receptor’s role in Ro 60-0175 by studying its interaction with SB-242,084, a selective 5-HT_2C antagonist. The study also explored whether Ro 60-0175 influences cue-elicited seeking behavior associated with sucrose, a highly palatable nutritive reinforcer. Materials and methods Different groups of free-feeding rats were trained to associate discriminative stimuli (S^Ds) with the availability of cocaine or a sucrose pellet or no-reward in two-lever operant cages. Cocaine and sucrose pellets were available under an FR1 schedule of reinforcement, and each reinforcer was followed by a 20-s timeout signaled by a cue light coming above the active lever. After extinction of reinforced responding in the absence of cue, the reinforcer-associated stimuli were reintroduced in reinstatement sessions in which reinforcers were withheld. Results Ro 60-0175, at IP doses from 0.1 to 1 mg/kg, dose-dependently reduced cocaine-seeking behavior, while 1 mg/kg had no such effect for the sucrose pellet. Pretreatment with 1 mg/kg SC SB-242,084 completely prevented the effect on cocaine-seeking behavior. Conclusions These findings, provided they can be extrapolated to abstinent human addicts, suggest therapeutic potential for the selective 5-HT_2C agonist in preventing cue-controlled cocaine-seeking and relapse. An erratum to this article can be found at     

Post-synaptic 5-HT1A receptor involvement in yawning and penile erections induced by apomorphine,physostigmine and mCPP in rats

Apomorphine and mCPP induced yawning associated with penile erections in rats, whereas physostigmine induced only yawns. Apomorphineinduced yawning and penile erections were antagonized by low doses of raclopride, whereas physostigmineinduced yawning and mCPP-induced effects were only partly inhibited at high doses of raclopride. Scopolamine as well as clozapine antagonized yawning and penile erections induced by apomorphine, mCPP and physostigmine. Similarly, the 5-HT_1A agonists 8-OH-DPAT and S 14506 inhibited yawning and penile erections induced by apomorphine, mCPP and physostigmine, and at similar doses induced lower lip retraction and hyperreactivity to handling. The /5-HT_1A antagonist tertatolol reversed the inhibitory effects of 8-OH-DPAT and S 14506 on drug-induced yawning and penile erections and increased apomorphine-and physostigmine-induced yawn frequency but not penile erection frequency. Like tertatolol, propranolol increased apomorphine- and physostigmine-induced yawn frequency, whereas ICI 118551 increased only physostigmine-induced yawning. 8-OH-DPAT-and S 14506-induced lower lip retraction and hyperre-activity to handling were also significantly antagonized by tertatolol. Finally,p-chlorophenylalanine pretreatment produced about 95% depletion in 5-HT in hypothalamus, hippocampus, striatum and frontal cortex and modified neither the responses of the inducing drugs nor the inhibitory effects of 8-OH-DPAT and S 14506 on drug-induced yawning and penile erections. These data suggest (i) that a final cholinergic mechanism blocked by scopolamine and clozapine is involved in drug-induced yawning and penile erections, (ii) that the effects of S 14506 depend upon stimulation of 5-HT_1A receptors, (iii) that the inhibitory effects of 5-HT_1A agonists on yawning and penile erections probably occurred at final steps on the pathways involved in the expression of yawning and penile erections and that these effects could be related to other effects of the 5-HT_1A agonists, (iv) that a tonic inhibitory influence could interfere with the expression of yawning but not with that of penile erections and (v) that presynaptic 5-HT_1A receptors do not seem to play an important role in the inhibitory effects of 5-HT_1A agonists on drug-induced yawning and penile erections in rats.     

Genetic and pharmacological evidence that 5-HT2C receptor activation, but not inhibition, affects motivation to feed under a progressive ratio schedule of reinforcement

Previous work showed that 5-HT_2C receptor agonists reduce cocaine self-administration on a progressive ratio (PR) schedule of reinforcement, whereas a 5-HT_2C receptor antagonist enhances responding for cocaine. The present experiments examined the effects of Ro60-0175 (5-HT_2C agonist) and SB242084 (5-HT_2C receptor antagonist) in rats on responding for food on a PR schedule; responding was also determined in mice lacking functional 5-HT_2C receptors. In food-restricted rats, lever pressing reinforced by regular food pellets or sucrose pellets was reduced by Ro60-0175. This effect was blocked by SB242084, and was absent in mice lacking functional 5-HT_2C receptors. A number of studies examined the effects of SB242084 on responding for food under a variety of conditions. These included manipulation of food type (regular pellets versus sucrose pellets), nutritional status of the animals (food restriction versus no restriction), and rate of progression of the increase in ratio requirements on the PR schedule. In all cases there was no evidence of enhanced responding for food by SB242084. Mice lacking functional 5-HT_2C receptors did not differ from wildtype mice in responding for food in either food-restricted or non-restricted states. The effects of Ro60-0175 are consistent with its effects on food consumption and motivation to self-administer cocaine. Unlike their effects on cocaine self-administration, pharmacological blockade of 5-HT_2C receptors, and genetic disruption of 5-HT_2C receptor function do not alter the motivation to respond for food. Because the 5-HT_2C receptor exerts a modulatory effect on dopamine function, the differential effects of reduced 5-HT_2C receptor mediated transmission on responding for food versus cocaine may relate to a differential role of this neurotransmitter in mediating these two behaviours.     

Investigation of the role of 5-HT2 receptor subtypes in the control of the bladder and the urethra in the anaesthetized female rat

Background and purpose:

Micturition is controlled by central 5-HT-containing pathways. 5-HT₂ receptors have been implicated in this system especially in control of the urethra, which is a drug target for treating urinary incontinence. This study investigates the role of each of the three subtypes of this receptor with emphasis on sphincter regulation.

Experimental approach:

Recordings of urethral and bladder pressure, external urethral sphincter (EUS) EMG, as well as the micturition reflex induced by bladder distension along with blood pressure and heart rate were made in anaesthetized rats. The effects of agonists and antagonists for 5-HT₂ receptor subtypes were studied on these variables.

Key results:

The 5-HT_2C agonists Ro 60-0175, WAY 161503 and mCPP, i.v., activated the EUS, increased urethral pressure and inhibited the micturition reflex. The effects of Ro 60-0175 on the EUS were blocked by the 5-HT_2C antagonist SB 242084 and the 5-HT_2A antagonists, ketanserin and MDL 100907. SB 242084 also blocked the inhibitory action on the reflex, while the 5-HT_2B antagonist RS 127445 only blocked the increase in urethral pressure. The 5-HT_2A receptor agonist DOI given i.v. or i.t. but not i.c.v. activated the EUS.

Conclusions and implications:

5-HT_2A/2C receptors located in the sacral spinal cord activate the EUS, while central 5-HT_2C receptors inhibit the micturition reflex and 5-HT_2B receptors, probably at the level of the urethra, increase urethral smooth muscle tone. Furthermore, 5-HT_2B and 5-HT_2C receptors do not seem to play an important role in the physiological regulation of micturition.     

Agomelatine in the tree shrew model of depression: Effects on stress-induced nocturnal hyperthermia and hormonal status

The antidepressive drug agomelatine combines the properties of an agonist of melatonergic receptors 1 and 2 with an antagonist of the 5-HT_2C receptor. We analyzed the effects of agomelatine in psychosocially stressed male tree shrews, an established preclinical model of depression. Tree shrews experienced daily social stress for a period of 5 weeks and were concomitantly treated with different drugs daily for 4 weeks. The effects of agomelatine (40 mg/kg/day) were compared with those of the agonist melatonin (40 mg/kg/day), the inverse 5-HT_2C antagonist S32006 (10 mg/kg/day), and the SSRI fluoxetine (15 mg/kg/day). Nocturnal core body temperature (CBT) was recorded by telemetry, and urinary norepinephrine and cortisol concentrations were measured.Chronic social stress induced nocturnal hyperthermia. Agomelatine normalized the CBT in the fourth week of the treatment (T4), whereas the other drugs did not significantly counteract the stress-induced hyperthermia. Agomelatine also reversed the stress-induced reduction in locomotor activity. Norepinephrine concentration was elevated by the stress indicating sympathetic hyperactivity, and was normalized in the stressed animals treated with agomelatine or fluoxetine but not in those treated with melatonin or S32006. Cortisol concentration was elevated by stress but returned to basal levels by T4 in all animals, irrespective of the treatment.These observations show that agomelatine has positive effects to counteract stress-induced physiological processes and to restore the normal rhythm of nocturnal CBT. The data underpin the antidepressant properties of agomelatine and are consistent with a distinctive profile compared to its constituent pharmacological components and other conventional agents.     

Agonist diversity in 5-HT2C receptor-mediated weight control in rats

Rationale Food intake and energy expenditure are the two main determinants of body weight. Given that 5-HT_2C receptor agonists are reported to have effects on both energy expenditure and food intake, this strongly suggests that 5-HT_2C receptor agonists have excellent potential for development as antiobesitiy drugs. One important issue in antiobesity drug development is whether the effects of the compound are maintained during chronic drug treatment.Objectives The purpose of the present study was to investigate the effect of repeated oral administration of three 5-HT_2C receptor agonists, m-chlorophenylpiperazine (mCPP), d(S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine (RO60-0175) and (S)-2-(7-ethyl-1H-furo[2,3-g]indazol-1-yl)-1-methylethylamine (YM348), on food intake and energy expenditure in rats.Results In the food intake study, mCPP, RO60-0175 and YM348 decreased food intake in a dose-dependent manner on day 1 of administration. On day 14 of repeated administration, the hypophagic effect of YM348 was lost and that of mCPP was reduced. In contrast, the hypophagic effect of RO60-0175 was maintained even after repeated administration. The hypophagic effects of all agonists were significantly inhibited by a 5-HT_2C receptor antagonist, SB242084. In contrast to the hypophagic effects, no drug tolerance developed with respect to the hyperthermic effects of mCPP, RO60-0175, and YM348. The hyperthermic effects of these drugs were also inhibited by SB242084.Conclusions Together, the difference between compounds in their hypophagic effects and the similarity in their hyperthermic effects suggest a diversity in agonists in 5-HT_2C receptor-mediated weight control in rats.     

Pharmacological profile of YM348, a novel, potent and orally active 5-HT2C receptor agonist

YM348, (S)-2-(7-ethyl-1H-furo[2,3-g]indazol-1-yl)-1-methylethylamine, showed a high affinity for cloned human 5-HT(2C) receptors (K(i): 0.89 nM). The functional selectivity for 5-HT(2C) receptors in the 5-HT(2) receptor family was the highest among 5-HT(2C) receptor agonists, including m-chlorophenylpiperazine (mCPP) and Ro60-0175 ((S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine). Oral administration of YM348 induced penile erections and hypolocomotion in rats, being completely inhibited by a selective 5-HT(2C) receptor antagonist, SB242084 (6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy) pyridin-3-yl carbamoyl] indoline). The dose-response curve for penile erections, unlike that for hypolocomotion, was an inverted U-shape in the dose range of 0.0677-2.03 mg/kg. A selective 5-HT(2A) receptor antagonist, MDL100907 (R(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol), and a selective 5-HT(2B) receptor antagonist, RS-127445 (2-amino-4-(4-fluoronaphth-1-yl)-6-isopropylpyrimidine), had no effect on the decline in penile erection frequency at 2.03 mg/kg of YM348. YM348 did not affect blood pressure at 2.03 mg/kg. In conclusion, YM348 is a novel, potent and orally active 5-HT(2C) receptor agonist, and neither the activation of 5-HT(2A) or 5-HT(2B) receptors nor a cardiovascular effect is likely to contribute to the inverted U-shape dose-response curve for penile erections.     

The serotonin<Subscript>2C</Subscript> receptor agonist Ro-60-0175 attenuates effects of nicotine in the five-choice serial reaction time task and in drug discrimination

Rationale There is evidence that serotonin_2C (5-HT_2C) receptors can modulate some behavioural effects of nicotine, but the generality of this action is not known. Objective To analyse the influence of the 5-HT_2C agonist Ro-60-0175 on responses to nicotine in the five-choice serial reaction time task (5-CSRTT) and on its discriminative stimulus effect; these procedures constitute models for attention-enhancing and subjective effects of nicotine, respectively. Materials and methods In the 5-CSRTT, rats were trained to obtain food reinforcers by detecting light stimuli and then challenged with Ro-60-0175 (0.3–0.8 mg/kg) and nicotine (0.2 mg/kg). For drug discrimination studies, rats were trained to discriminate nicotine (0.2 mg/kg) from saline in a two-lever procedure using a tandem schedule of food reinforcement. Results In the 5-CSRTT, nicotine positively influenced most response indices, confirming previous results. Ro-60-0175 increased response latencies and omission errors and reduced anticipatory responding but had little effect on response accuracy; importantly, it counteracted the effects of nicotine on response speed and omission errors. Pentobarbitone (10–14 mg/kg) also slowed performance of the 5-CSRTT but did not weaken the nicotine-induced enhancement of performance. In the drug discrimination procedure, Ro-60-0175 was not generalised with nicotine but shifted the nicotine dose–response curve to the right in a dose-related manner. Conclusions The data suggest that selective occupancy of 5-HT_2C receptors can attenuate some effects of nicotine in the 5-CSRTT and weaken the nicotine discriminative stimulus; these effects cannot be explained by a sedative action of Ro-60-0175. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

5-HT1A receptor agonists prevent in rats the yawning and penile erections induced by direct dopamine agonists

The new compound (+) S-20499, an amino chromane derivative (8[-4[N-(5-methoxychromane-3yl)N-propyl]aminobutyl] azaspiro[4–5] décane-7,9 dione), is a high affinity full 5-HT_1A agonist. We have investigated its effects on dopaminergic transmission. (+) S-20499 displayed a 10^–8 M affinity for D₂ dopamine (DA) receptors, 100 fold lower than for 5-HT_1A receptors. The hypothermic effect of the drug was reversed by haloperidol in mice, suggesting that it behaves as a direct dopamine agonist. However, increasing doses of (+) S-20499 induced neither yawning nor penile erections, which constitute characteristic responses of direct DA agonists administered at low doses. In addition, (+) S-20499 prevented the apomorphine (100 µg/kg SC) induced yawning and penile erections. This inhibition appears to result from the stimulation of 5-HT_1A receptors since it is an effect shared by both buspirone (from 5 mg/kg) and 8-OH-DPAT (from 0.10 mg/kg). In addition, when rats are treated with the 5-HT_1A receptor antagonist tertatolol (2–5 mg/kg; SC), increasing doses of (+) S-20499 elicit the expected yawns and penile erections. It is concluded that the 5-HT_1A agonist property opposes to that of D₂ dopamine receptor stimulation with regard to yawning and penile erections.     

Discriminative stimulus properties of the novel serotonin (5-HT)2C receptor agonist, RO 60-0175: a pharmacological analysis

Employing a Fixed-Ratio 10, food-reinforced protocol, rats were trained to recognize the discriminative stimulus (DS) properties of the novel, potent, 5-HT2C agonist, Ro 60-0175 (2.5 mg/kg, i.p.). This schedule generated appropriate drug versus vehicle responding after 50 + 5 training sessions and Ro 60-0175 elicited full (100%) drug selection with an effective dose50 (ED50) of 0.6 mg/kg, i.p.. The 5-HT2C receptor agonists, mCPP and MK 212, fully generalized to Ro 60-0175 with ED50s of 0.8 and 0.4 mg/kg, s.c., respectively, whereas the preferential 5-HT2B agonist, BW 723C86 ( > 10.0 mg/kg, s.c.) and the 5-HT2A agonist, DOI ( > 2.5 mg/kg, s.c.), were ineffective. The 5-HT2A/2B/2C receptor antagonist, mianserin, dose-dependently blocked the DS properties of Ro 60-0175 with an ED50 of 0.7 mg/kg, s.c. This action was mimicked by the novel, 5-HT2B/2C antagonist, SB 206,553 (ED50 = 0.3 mg/kg, s.c.), whereas the selective 5-HT2A antagonist, MDL 100,907 ( > 0.63 mg/kg, s.c.), was ineffective. Further, the selective 5-HT2C antagonist, SB 242,084, dose-dependently and fully blocked drug selection (ED50 = 0.2 mg/kg, i.p.), whereas the selective 5-HT2B antagonist, SB 204,741, was not active ( > 0.63 mg/kg, i.p.). In conclusion, these data demonstrate that Ro 60-0175 generates a robust DS and suggest that activation of 5-HT2C receptors is the principal mechanism underlying its DS properties.     

Agomelatine,a melatonin agonist with antidepressant properties

Agomelatine (β-methyl-6-chloromelatonin), which is structurally homologous to melatonin, is a potent agonist of melatonin MT1 and MT2 receptors as well as an antagonist of serotonin 5-HT_2C receptors. Agomelatine appears to improve sleep without causing daytime sedation. It has not been found to be associated with sexual side effects and discontinuation symptoms. Three placebo-controlled trials, one of them a dose finding study and two of them pivotal trials, suggest that agomelatine is an antidepressant at doses of 25 – 50 mg/day. Agomelatine appears to be well tolerated, without sexual or cardiac adverse effects, weight gain or discontinuation syndromes. Animal studies suggest a possible neuroprotective action of agomelatine, although there are more data in favor of an anxiolytic effect. Substantially more research is needed to establish its role in the treatment of mood and circadian rhythm disorders.     

Effects of the antidepressant trazodone,a 5-HT<Subscript>2A/2C</Subscript> receptor antagonist,on dopamine-dependent behaviors in rats

Rationale 5-Hydroxytryptamine, via stimulation of 5-HT_2C receptors, exerts a tonic inhibitory influence on dopaminergic neurotransmission, whereas activation of 5-HT_2A receptors enhances stimulated DAergic neurotransmission. The antidepressant trazodone is a 5-HT_2A/2C receptor antagonist.Objectives To evaluate the effect of trazodone treatment on behaviors dependent on the functional status of the nigrostriatal DAergic system.Methods The effect of pretreatment with trazodone on dexamphetamine- and apomorphine-induced oral stereotypies, on catalepsy induced by haloperidol and apomorphine (0.05 mg/kg, i.p.), on ergometrine-induced wet dog shake (WDS) behavior and fluoxetine-induced penile erections was studied in rats. We also investigated whether trazodone induces catalepsy in rats.Results Trazodone at 2.5–20 mg/kg i.p. did not induce catalepsy, and did not antagonize apomorphine (1.5 and 3 mg/kg) stereotypy and apomorphine (0.05 mg/kg)-induced catalepsy. However, pretreatment with 5, 10 and 20 mg/kg i.p. trazodone enhanced dexamphetamine stereotypy, and antagonized haloperidol catalepsy, ergometrine-induced WDS behavior and fluoxetine-induced penile erections. Trazodone at 30, 40 and 50 mg/kg i.p. induced catalepsy and antagonized apomorphine and dexamphetamine stereotypies.Conclusions Our results indicate that trazodone at 2.5–20 mg/kg does not block pre- and postsynaptic striatal D2 DA receptors, while at 30, 40 and 50 mg/kg it blocks postsynaptic striatal D2 DA receptors. Furthermore, at 5, 10 and 20 mg/kg, trazodone blocks 5-HT_2A and 5-HT_2C receptors. We suggest that trazodone (5, 10 and 20 mg/kg), by blocking the 5-HT_2C receptors, releases the nigrostriatal DAergic neurons from tonic inhibition caused by 5-HT, and thereby potentiates dexamphetamine stereotypy and antagonizes haloperidol catalepsy.     

Influence of the novel antidepressant and melatonin agonist/serotonin2C receptor antagonist,agomelatine, on the rat sleep–wake cycle architecture

Rationale  The novel antidepressant, agomelatine, behaves as an agonist at melatonin MT₁ and MT₂ receptors and as an antagonist at serotonin (5-HT)_2C receptors. In animal models and clinical trials, agomelatine displays antidepressant properties and re-synchronizes disrupted circadian rhythms. Objectives  The objectives of this study were to compare the influence of agomelatine upon sleep–wake states to the selective melatonin agonists, melatonin and ramelteon, and to the selective 5-HT_2C receptor antagonist, S32006. Methods  Rats were administered with vehicle, agomelatine, ramelteon, melatonin, or S32006, at the onset of either dark or light periods. Polygraphic recordings were performed and changes determined over 24 h, i.e., number and duration of sleep–wake episodes, latencies to rapid eye movement (REM) and slow-wave (SWS) sleep, power band spectra of the electroencephalogram (EEG), and circadian changes. Results  Administered at light phase onset, no changes were induced by agomelatine. In contrast, administered shortly before dark phase, agomelatine (10 and 40 mg/kg, per os) enhanced duration of REM and SWS sleep and decreased wake state for 3 h. Melatonin (10 mg/kg, per os) induced a transient enhancement in REM sleep followed by a reduction in REM and SWS sleep and an increase in waking. Ramelteon (10 mg/kg, per os) provoked a transient increase in REM sleep. Finally, S32006 (10 mg/kg, intraperitoneally), administered at dark phase onset, mimicked the increased SWS provoked by agomelatine, yet diminished REM sleep. Conclusions  Agomelatine possesses a distinctive EEG profile compared with melatonin, ramelteon, and S32006, possibly reflecting co-joint agonist and antagonist properties at MT₁/MT₂ and 5-HT_2C receptors, respectively. An erratum to this article can be found at     

Involvement of 5-HT1C-receptors in drug-induced penile erections in rats

Drug-induced penile erections (PE) were initially suggested to be 5-HT_1B receptor mediated. However, since the discovery of the 5-HT_1C receptor a number of compounds, considered to be 5-HT_1B selective, appear to bind more strongly to the 5-HT_1C receptor and this prompted a re-evaluation of the receptor subtype involved in PE induction. PE could be induced by the 5-HT agonists mCPP (0.22–2.2 mg/kg), TFMPP (0.46–1.0 mg/kg) and MK 212 (0.1–1.0 mg/kg). The 5-HT agonist DOI (0.022–0.22 mg/kg) did not induce PE in placebo-pretreated rats but in rats pretreated with various 5-HT₂ antagonists it did. These compounds have in common a strong affinity for the 5-HT_1C receptor. mCPP (0.46 mg/kg)-induced PE could be antagonized by the 5-HT antagonists metergoline, cyproheptadine, mesulergine, mianserin, ritanserin and ketanserin. Their ED₅₀s were 0.04, 0.4, 0.03, 0.06, 0.4 and 2 mg/kg, respectively. The potency of both the agonists to induce, and the antagonists to inhibit, PE was found to be dependent on their selectivity for the 5-HT_1C receptor versus the 5-HT₂ receptor. Spiperone (0.1–1.0 mg/kg) and GR 38032F (1–10 mg/kg) did not antagonise mCPP-induced PE. 8-OH-DPAT and 5MeODMT counteracted mCPP (0.46 mg/kg)-induced PE. Their ED₅₀s were 0.03 and 0.4 mg/kg, respectively. DOI counteracted mCPP induced PE only at doses above 1 mg/kg, whereas CGS 12066B (1.0–10 mg/kg) was inactive. The results suggest that PE are induced by activation of the 5-HT_1C receptor and are functionally inhibited by activation of 5-HT_1A or 5-HT₂ receptors.     

Effects of serotonin (5-HT)2 receptor ligands on depression-like behavior during nicotine withdrawal

A pronounced withdrawal syndrome including depressed mood prevents cigarette smoking cessation. We tested if blockade or activation of serotonin (5-HT)₂ receptors affected the time of immobility (as an indirect measure of depression-like behavior) in naïve animals and in those withdrawn from chronic nicotine in the forced swim test (FST). The antidepressant imipramine was used as a control. In the FST, the selective 5-HT_2A receptor antagonist M100,907 (1-2 mg/kg, but not 0.5 mg/kg), the selective 5-HT_2C receptor antagonist SB 242,084 (0.3-1 mg/kg, but not 0.1 mg/kg), the 5-HT_2C receptor agonists Ro 60-0175 (10 mg/kg, but not 3 mg/kg) and WAY 163,909 (1.5-10 mg/kg, but not 0.75 mg/kg) as well as imipramine (30 mg/kg, but not 15 mg/kg) decreased the immobility time while the non-selective 5-HT₂ receptor agonist DOI (0.1-1 mg/kg) was inactive in naïve rats. We found an increase in immobility time in rats that were withdrawn from nicotine exposure after 5 days of chronic nicotine treatment. This effect increased from day 1 until day 10 following withdrawal of nicotine, with maximal withdrawal effects on day 3. M100,907 (1 mg/kg), SB 242,084 (0.3 mg/kg), Ro 60-0175 (3 mg/kg), WAY 163,909 (0.75-1.5 mg/kg) and imipramine (15-30 mg/kg) shortened the immobility time in rats that had been removed from nicotine exposure for 3 days. Locomotor activity studies indicated that the effects of SB 242,084 might have been non-specific, as we noticed enhanced basal locomotion in naïve rats. This data set demonstrates that 5-HT_2A receptor antagonist and 5-HT_2C receptor agonists exhibited effects similar to antidepressant drugs and abolished the depression-like effects in nicotine-withdrawn rats. These drugs should be considered as adjuncts to smoking cessation therapy, to ameliorate abstinence-induced depressive symptoms.