5-HT2C receptor involvement in female rat lordosis behavior

Adult, hormone-primed, ovariectomized rats (CDF-344) with bilateral implants within the ventromedial nucleus of the hypothalamus (VMN), were injected with 0.5 microgram estradiol benzoate followed 48 h later with 500 microgram progesterone. This priming produced rats with 2 different levels of sexual receptivity. Rats with a lordosis to mount ratio (L/M)>/=0.5 were used to examine the potential lordosis-inhibiting effects of the 5-HT2A receptor antagonist, R(+)-a-(2, 3-dimethoxyphenyl)-1-[2(4-fluoro-phenylethyl)]-4-piperidine-methanol (MDL 100,907), and the 5-HT2C receptor antagonist, 5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2, 3-f]indole (SB 206553). Rats with low sexual receptivity (L/M<0.5) were bilaterally infused with the 5-HT2A/2C receptor agonist, (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI), or DOI plus either MDL 100,907 or SB 206553 to determine if either drug would attenuate the lordosis-facilitating effects of DOI. The 5-HT2C receptor antagonist, but not the 5-HT2A receptor antagonist, effectively inhibited lordosis behavior. Similarly, SB 206553 was more effective than MDL 100,907 in reducing the DOI-induced increase in lordosis responding. However, both drugs limited the duration of lordosis responding initiated by DOI. These results are consistent with prior suggestions that 5-HT2A/2C receptors within the VMN are involved in the modulation of lordosis behavior and lead to the suggestion that 5-HT2C, rather than 5-HT2A, receptors are primarily responsible for the effects of 5-HT2 receptor-active drugs on lordosis behavior.     

WAY100635 and female rat lordosis behavior

Sexually receptive proestrous rats with bilateral cannulae in the ventromedial nucleus of the hypothalamus (VMN) were infused with 200 ng of (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) or with 8-OH-DPAT plus varying concentrations (200 to 2000 ng) of the 5-HT1A receptor antagonist, N-[2[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride (WAY100635). 8-OH-DPAT inhibited lordosis behavior within 15 min of the infusion and every dose of WAY100635 prevented the inhibition. When non-sexually receptive, ovariectomized rats, hormonally primed with 0.5 microg estradiol benzoate and 500 microg progesterone, were infused with WAY100635 (400 to 2000 ng), the 5-HT1A receptor antagonist did not facilitate lordosis responding. These findings support earlier findings that activation of 5-HT1A receptors in the mediobasal hypothalamus inhibits lordosis behavior. However, they further demonstrate that tonic activation of 5-HT1A receptors is not responsible for the absence of sexual receptivity in suboptimally hormonally primed ovariectomized rats.     

Progesterone attenuates the effect of the 5-HT1A receptor agonist, 8-OH-DPAT,and of mild restraint on lordosis behavior

Ovariectomized, hormone-primed rats were used to test the hypothesis that progesterone treatment attenuated the effects of the 5-HT(1A) receptor agonist, (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), on female rat lordosis behavior. Based upon prior evidence that prepriming with estradiol benzoate (EB) reduced the ability of 8-OH-DPAT to inhibit lordosis behavior, rats were preprimed with 10 microg EB 7 days before a second priming with 10 microg EB followed 48 h later with 500 microg progesterone or vehicle. Independent of the presence of progesterone, prepriming with EB attenuated the lordosis-inhibiting effects of systemic treatment with 8-OH-DPAT. However, progesterone also reduced the effects of 8-OH-DPAT and this effect was also seen in females primed only once with EB. In contrast, progesterone was relatively ineffective in attenuating the effects of bilateral infusion with 8-OH-DPAT into the ventromedial nucleus of the hypothalamus (VMN). The failure of progesterone to substantially reduce the effects of VMN infusion with 8-OH-DPAT contrasts with prior studies in which estrogen's protective action against the drug did include the VMN. Thus, while both estrogen and progesterone reduce the lordosis-inhibiting effect of 8-OH-DPAT, the mechanisms responsible for the effects of the two gonadal hormones may be different. Priming with progesterone also prevented the effects of 5 min of restraint. When rats were hormonally primed with EB and oil, rats showed a transient, but significant, decline in lordosis behavior 5 and 10 min after restraint. Rats primed with EB and progesterone were unaffected by the restraint. These results are discussed in terms of their implications for the role of progesterone in altering the 5-HT(1A) receptor modulation of lordosis behavior.     

Role of 5-HT(2) receptor subtypes in depletion of 5-HT induced by p-chloroamphetamine in the mouse frontal cortex

A serotonin (5-hydroxytryptamine, 5-HT)-releasing drug, p-chloroamphetamine elicited decreases in 5-HT levels in the mouse frontal cortex. 5-HT reduction elicited by p-chloroamphetamine was inhibited by the 5-HT(2A/2B/2C) receptor antagonist, LY 53857 and the 5-HT(2A) receptor antagonist, ketanserin. However, the 5-HT(2B/2C) receptor antagonist, SB 206553, enhanced it. LY 53857 and ketanserin can inhibit hyperthermia elicited by p-chloroamphetamine, although SB 206553 enhances it. The effects of the 5-HT(2) receptor antagonists on neurotoxicity are very similar to those on hyperthermia. Since hyperthermia facilitates neurotoxicity induced by amphetamine analogue, these 5-HT(2) receptor antagonists may modify 5-HT depletion induced by p-chloroamphetamine through responses to body temperature.     

Mild restraint reduces the time hormonally primed rats spend with sexually active males

The effect of 5 min of restraint on the time sexually-receptive females spend in the compartment of a sexually active male was examined. Ovariectomized females, hormonally primed with 10 microg estradiol benzoate and 500 microg progesterone (EP) or primed only with estradiol benzoate (EO) were used. After the restraint or home-cage experience, females were tested for 30 min in a chamber that allowed the female to escape to a small "burrow". Females, subjected to restraint, left the male's compartment faster and spent significantly less time in the male's compartment than did non-restrained females. This was true for both EP and EO females. When females were injected with the 5-HT(2B/2C)-receptor antagonist, SB 206553, 15 min before restraint, time spent in the male's compartment was even further reduced. However, additional studies indicated that it was the stress of the injection rather than the action of the drug that was responsible for the female's behavior. These findings are discussed in terms of their significance to the understanding of the female's reproductive response to stress and are compared to prior findings, where lordosis behavior was significantly reduced by restraint.     

Differential effects of 5-HT(2A) and 5-HT(2C) receptor blockade on strategy-switching

Recent experiments indicate that blockade of serotonin (5-HT) 2A and 2C receptors have differential effects on reversal learning. The present experiments investigated the effects of the 5-HT(2A) receptor antagonist, ketanserin and 5-HT(2C) receptor antagonist, SB242084 on acquisition and strategy-switching in a visual cue-response paradigm. Long-Evans rats were trained in a cross-maze to enter an arm based on color (visual cue version) or a specific turn response (response version). Systemic treatment with ketanserin did not affect initial learning of a visual cue or response discrimination, but ketanserin at 0.5 mg/kg significantly enhanced a switch between visual cue and response strategies. Ketanserin facilitated strategy-switching by inhibiting responses to a previously relevant strategy without affecting choices to never-reinforced strategies. Treatment with SB242084 (0.5, 1.0 or 2.0 mg/kg) did not affect acquisition of a visual cue or response discrimination. SB242084 treatment also did not affect strategy-switching. The present findings suggest that blockade of 5-HT(2A), but not 5-HT(2C), receptors enhance strategy switching.     

Tropisetron increases the inhibitory effect of mild restraint on lordosis behavior of hormonally primed, ovariectomized rats

Ovariectomized rats, hormonally primed with 10 μg estradiol benzoate and 500 μg progesterone are resistant to the lordosis-inhibiting effects of a 5 min restraint experience. However, modulation of the serotonergic (5-HT) system alters this resistance to stress. In the following experiment, ovariectomized Fischer inbred rats were hormonally primed with 10 μg estradiol benzoate and 500 μg progesterone. The effect of 5 min restraint on sexual behavior was examined after bilateral hypothalamic infusion or intraperitoneal (ip) treatment with the 5-HT(3) receptor antagonist, 3-tropanylindole-3-carboxylate hydrochloride (tropisetron). Infusion with 50 or 100 ng tropisetron inhibited lordosis behavior. When rats were infused with 10 or 25 ng tropisetron, rats showed normal lordosis behavior. However, when infusion with 10 or 25 ng tropisetron was combined with 5 min restraint, lordosis behavior was inhibited. These findings are consistent with prior work that has implicated hypothalamic serotonin in control of lordosis behavior and in the effect of mild restraint on the behavior. In contrast to the effects of the intracranial infusion, intraperitoneal injection with 1.0 or 2.0 mg/kg tropisetron did not amplify the effects of restraint.     

Serotonin (1A) receptor involvement in acute 3,4-methylenedioxymethamphetamine (MDMA) facilitation of social interaction in the rat

The current study assessed whether various co-administered serotonin (5-HT) receptor antagonists could prevent some of the acute behavioral effects of 3,4-methylenedioxymethamphetamine (MDMA, "Ecstasy") in rats. In the social interaction test, MDMA (5 mg/kg) significantly increased the duration of total social interaction between two conspecifics meeting for the first time. Microanalysis showed that MDMA increased adjacent lying and approach behaviours while reducing anogenital sniffing. MDMA (5 mg/kg) also caused elements of the serotonin syndrome including low body posture and piloerection. In the emergence test, MDMA significantly increased hide time and emergence latency indicating increased anxiety-like behavior. Pretreatment with the 5HT 1A receptor antagonist, WAY 100635 (1 mg/kg), prevented MDMA-induced increases in social interaction and markers of the serotonin syndrome while the 5-HT 1B receptor antagonist GR 55562 (1 mg/kg) and 5-HT 2A receptor antagonist ketanserin (1 mg/kg) were ineffective. The 5-HT 2B/2C receptor antagonist, SB 206553 (2 mg/kg), prevented MDMA-induced prosocial effects but caused pronounced thigmotaxis (hyperactivity at the periphery of the testing chamber). The anxiogenic effect of MDMA on the emergence test was not prevented by pretreatment with any of the 5-HT receptor antagonists tested. These results indicate that prosocial effect of MDMA may involve 5-HT 1A and possibly 5-HT 2B/2C receptors. In contrast, MDMA-induced generalised anxiety, as measured by the emergence test, seems unlikely to involve the 5-HT 1A, 5-HT 1B or 5-HT 2A, 5-HT 2B or 5-HT 2C receptors.     

Effects of 5-HT(2A) and 5-HT(2C) receptor antagonists on acute and chronic dyskinetic effects induced by haloperidol in rats

An important limitation of classical antipsychotic drugs such as haloperidol (HAL) is their liability to induce extrapyramidal motor symptoms acutely and tardive dyskinetic syndromes when given chronically. These effects are less likely to occur with newer antipsychotic drugs, an attribute that is often thought to result from their serotonin-2 (5-HT(2)) receptor antagonistic properties. In the present study, we used selected doses of the 5-HT(2A) antagonist M100,907, the 5-HT(2C) antagonist SB242,084 and the mixed 5-HT(2A/C) antagonist ketanserin to re-examine the respective roles of 2A vs. 2C 5-HT(2) receptor subtypes in both acute and chronic motor effects induced by HAL. Acutely, SB242,084 (0.5 mg/kg) reduced HAL-induced catalepsy, while M100,907 (0.5 mg/kg) and ketanserin (1 mg/kg) were without effect. None of the drugs reduced HAL-induced Fos expression in the striatum or frontal cortex, and M100,907 actually potentiated HAL-induced Fos expression in the n. accumbens. In rats chronically treated with HAL, both ketanserin and SB242,084 attenuated vacuous chewing movements, while M100,907 had no effect. In addition, 5-HT(2C) but not 5-HT(2A) mRNA levels were altered in several brain regions after chronic HAL. These results highlight the importance of 5-HT2(2C) receptors in both acute and chronic motoric side effects of HAL, and suggest that 5-HT(2C) antagonism could be targeted as a key property in the development of new antipsychotic medications.     

The role of delta-opioid receptors in estrogen facilitation of lordosis behavior

The present study investigated the role of delta-opioid receptors (ORs) in estrogen facilitation of female rat reproductive behavior (lordosis). Infusion of 2 microg of the selective delta-OR agonist [D-Pen(2),D-Pen(5)]-enkephalin (DPDPE), into the third ventricle facilitated lordosis behavior in ovariectomized (OVX) rats injected with estrogen (E) 48 and 24 h before behavioral testing. Pretreatment with the selective delta-OR antagonist naltrindole (NTDL) blocked DPDPE effects on lordosis behavior. Ventricular infusion of NTDL (40 microg) also suppressed lordosis behavior in fully receptive OVX rats primed with both E and progesterone (P). In addition, NTDL blocked lordosis behavior when infused into the ventromedial nucleus of the hypothalamus (VMH) but not into the medial preoptic area (mPOA). Site-specific infusion of DPDPE into the VMH had dose-dependent, dual effects on lordosis behavior. While a very low dose of DPDPE (0.01 microg) facilitated lordosis behavior, a higher dose (1.0 microg) inhibited receptivity in OVX rats primed with E and a low dose (50 microg) of P. We used 3H-DPDPE to measure the density of delta-ORs in OVX rats treated with vehicle or with E by receptor autoradiography. E treatment did not have any effect on the density of DPDPE binding sites in the VMH, mPOA, medial amygdala, or caudate putamen. The behavioral effects of the ligands used in this study suggest that activation of delta-OR in the VMH by endogenous opioids facilitates estrogen-dependent lordosis behavior.     

Activation of protein kinase C in the hypothalamic ventromedial nucleus or the midbrain central gray facilitates lordosis

Many neurotransmitters and neuropeptides can act through the hypothalamic ventromedial nucleus (VMN) or midbrain central gray (MCG) to facilitate lordosis. Since these lordosis-facilitating agents can also stimulate the phosphoinositide (PI) second-messenger pathway, it was hypothesized that direct activation of this pathway can also potentiate the behavior. To evaluate this possibility, a phorbol ester, TPA (12-O-tetradecanoyl phorbol 13-acetate), was used to activate a key enzyme, protein kinase C (PKC), of the PI pathway in ovariectomized (OVX) rats either primed or not primed with estrogen. These female rats were paired with males for mating tests before and after an intracerebral infusion of TPA, and both the lordosis quotient (LQ) and the lordosis strength (LS) were measured. Bilateral infusion of TPA (5 μg/0.5 μl or 0.2 μg/0.2 μl, but not 0.1 μg/0.2 μl/side) into the VMN or MCG of estrogen-primed subjects facilitated both LQ and LS in 30 min, peaked at 60–90 min, and the facilitation lasted for more than 180 min. This facilitatory effect of TPA was: (1) not observed in OVX rats not primed with estrogen; (2) not observed if the infused TPA did not reach both sides of the VMN or MCG; (3) not mimicked by 4-phorbol 12,13-didecanoate, which does not activate PKC; (4) blocked by PKC inhibitors (H7 10 mM or staurosporine 1 μM, 0.2 μl/ side), which by themeselves did not facilitate lordosis; and (5) was not affected by pretreatment of the progestin antagonist RU486. These observations indicate that TPA facilitates lordosis in a dose-dependent fashion by activating, and not by depleting, PKC in the VMN or MCG, and that the TPA effect requires estrogen priming but not the activation of progestin receptors. Thus, the PI pathway or the activation of PKC may be a common mediator for lordosis facilitation in these two brain regions; and the requirement of estrogen priming further raises the possibility that this second-messenger system or its substrates in the VMN and MCG are modulated by estrogen.     

Lordosis-inhibiting effect of progesterone in male and female rats with septal lesions

The inhibitory role of progesterone (P) in regulating lordosis was investigated in male and female rats with septal lesions (SL). Male rats with SL showed lordosis quotients (LQ) as high as female rats with SL and female control rats without brain surgery after injection of 50 microg/kg estradiol benzoate (EB) followed by 0.5 mg P 44 h later. Even when primed with 5 mg P 1 h prior to the 50 microg EB-injection, the mean LQs were still high in all groups. When the dose of EB was decreased to 5 microg/kg, all rats showed high-score LQs. In contrast, all animals in both male and female in which 5 mg P was injected 1 h before 5 microg EB, showed low LQs. These results suggest that P is effective in suppressing lordosis enhanced by estrogen in either male rats or females. Furthermore, the high dose of estrogen overcomes the inhibitory action of P on lordosis in both sexes.     

Evidence for a 5-HT2A receptor mode of action in the anxiolytic-like properties of DOI in mice

DOI [(+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane] displays a high affinity for the rat 5-HT2A, 5-HT2B and 5-HT2C receptors (pKi 7.3, 7.4 and 7.8, respectively) and acts as an agonist. DOI (0.5-4 mg/kg, i.p. 30 min pre-test) increased the number of punished passages in the mouse four plates test (FPT). The anti-punishment action of DOI (1 mg/kg, i.p. 30 min pre-test) was abolished by prior treatment with the selective 5-HT2A receptor antagonist SR 46949B (0.1 and 1 mg/kg, i.p. 45 min pre-test) but not by the selective 5-HT2C receptor antagonist RS 10-2221 (0.1 and 1 mg/kg, i.p. 45 min pre-test) nor the selective 5-HT2C/2B receptor antagonist SB 206553 (0.1 and 1 mg/kg, i.p. 45 min pre-test). An anxiolytic-like action was also observed for DOI (1 mg/kg) in the elevated plus maze (EPM). The anxiolytic-like action of DOI (1 mg/kg, i.p. 30 min pre-test) was antagonised by pre-treatment with SR 46949B (0.125 and 0.5 mg/kg, i.p. 45 min pre-test) but not by RS 10-2221 (0.1 and 1 mg/kg, i.p. 45 min pre-test) nor SB 206553 (0.1 and 1 mg/kg, i.p. 45 min pre-test). In conclusion, DOI produced an anxiolytic-like profile in the mouse FPT and EPM. These effects are likely to be 5-HT2A receptor mediated.     

The PKC inhibitor, bisindolymaleimide, blocks DOI's attenuation of the effects of 8-OH-DPAT on female rat lordosis behavior

Ovariectomized rats with bilateral cannulae near the ventromedial nucleus of the hypothalamus were hormonally primed with 10 microg estradiol benzoate and 500 microg progesterone. Sexually receptive females were infused bilaterally with 200 ng of the 5-HT(1A) receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), or with a combination of 200 ng 8-OH-DPAT and 2000 ng of the 5-HT(2) receptor agonist, (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane HCl (DOI). 8-OH-DPAT inhibited lordosis behavior and DOI reduced this inhibition. However, if females were preinfused with the PKC inhibitor, bisindolymaleimide I hydrochloride (BIM), DOI's effect was eliminated. BIM's attenuation of the effects of DOI was time-dependent. When BIM was infused 90 min, but not 30 min, before the 5-HT receptor agonists, BIM eliminated DOI's protection against the lordosis-inhibiting effects of 8-OH-DPAT. A concentration of BIM as low as 10(-5) nmol in a 0.5 microl infusion volume was effective and there was little evidence of dose responsivity between 10(-5) and 10(-1) nmol of BIM. In contrast, prior infusion with vehicle or with 10(-7) nmol BIM had no impact on the female's response to the 5-HT receptor agonists. These findings allow the suggestion that DOI's ability to increase PKC may be responsible for attenuation of the effects of 8-OH-DPAT on lordosis behavior.     

Attenuation of the lordosis-inhibiting effects of 8-OH-DPAT by TFMPP and quipazine

Regularly cycling, proestrous female rats received infusions of 200 ng of the serotonin (5-HT) 1A receptor agonist, (±) 8-hydroxy 2-(di-n-propylamino) tetralin-HBr (8-OH-DPAT), or 200 ng 8-OH-DPAT and 1000 or 2000 ng of N-(3-trifluoro-methylphenyl) piperazine hydrochloride (TFMPP) or 2-(1-piperazinyl) quinoline dimaleate (quipazine). Infusions were made bilaterally into the ventromedial nucleus of the hypothalamus (VMN). Animals receiving 200 ng 8-OH-DPAT exhibited a decline in lordosis behavior following infusion. Rats receiving 8-OH-DPAT and 1000 or 2000 ng quipazine or TFMPP were protected from the lordosis-inhibiting effects of 8-OH-DPAT, alone. Although both quipazine and TFMPP act on multiple 5-HT receptors, they overlap in their agonist action at 5-HT₂ receptors. Consequently, these results provide further evidence supporting the contention that within the VMN, both 5-HT_1A and 5-HT₂ receptor subtypes contribute to the modulation of lordosis behavior in the female rat. The data are discussed in terms of the relative potency of 5-HT at 5-HT receptors mediating inhibition and facilitation of lordosis behavior.     

In vitro autoradiography of serotonin 5-HT(2A/2C) receptor-activated G protein: guanosine-5'-(gamma-[(35)S]thio)triphosphate binding in rat brain

Agonist activation of G protein-coupled receptors induces an increase in the binding of guanosine 5'-(gamma-[(35)S]thio)triphosphate ([(35)S]GTPgammaS); this increase in binding has been used as a tool to investigate receptor interaction with the heterotrimer guanine nucleotide-binding regulatory protein (G protein). The present study uses agonist-stimulated [(35)S]GTPgammaS binding to characterize serotonin 5-HT(2A/2C) receptors in rat brain membrane fractions and demonstrate the anatomical localization of the receptors by in vitro autoradiography on slide-mounted sections. The stimulatory effect of the agonist [1-(2,5-dimethoxy-4-iodophenyl)]-2 aminopropane (DOI) is compared to that of serotonin (5-HT). Autoradiography revealed a similar localization of DOI- and 5-HT-stimulated binding of [(35)S]GTPgammaS in distinct areas of prefrontal and parietal cortex, consistent with previously reported 5-HT(2A) receptor distribution. Specific binding was demonstrated in the frontal and parietal cortex, medial prefrontal, and cingular and orbital-insular areas as well as in the hippocampal formation, septal areas, the nucleus accumbens, and the choroid plexus. MDL 100105, a specific 5-HT(2A) antagonist, and ketanserin, an antagonist of 5-HT(2A/2C) receptors, blocked DOI stimulation in all labeled areas, whereas 5-HT stimulation was only partially blocked (70-80%). A small but significant inhibition was observed with the specific antagonist of 5-HT(2C/2B), SB 206553. This autoradiographic technique provides a useful tool for measuring in situ changes in specific receptor-Gq protein coupling in anatomically discrete brain regions, under physiological and pathological conditions.     

5-HT2B receptors do not modulate sensitivity to colonic distension in rats with acute colorectal hypersensitivity

The 5-HT4 receptor agonist tegaserod is an effective prokinetic agent that increases gastrointestinal secretion and reduces visceral sensitivity. Tegaserod has both 5-HT4 receptor agonist and 5-HT2B receptor antagonist activity, the latter being a less potent effect of the drug. In a rat model of colonic hypersensitivity, selective 5-HT4 receptor antagonists only partially reversed the antihyperalgesic effect of tegaserod suggesting that non-5-HT4 receptor-mediated mechanisms may also be involved in its overall antihyperalgesic action. The objective of this study was to determine whether 5-HT2B receptors play a role in colonic hypersensitivity. A visceromotor response (VMR) in acutely sensitized animals (intracolonic acetic acid, 0.6%, 1.5 mL) quantified colonic hypersensitivity. Acetic acid produced an increase in the VMR at all distension pressures. However, neither the 5-HT2B receptor agonist BW 723C86, the 5-HT2B antagonist SB204741 or the 5-HT2B/2C antagonist SB 206553 caused any significant inhibition of the VMR. In summary, in the same rodent model in which tegaserod has previously been shown to produce a potent antihyperalgesic effect, 5-HT2B receptors do not appear to mediate colonic hypersensitivity. We conclude that 5-HT2B receptor-mediated mechanisms are unlikely to play a role in the antihyperalgesic action of tegaserod in man.     

Clozapine and haloperidol differentially alter the constitutive activity of central serotonin2C receptors in vivo.

BACKGROUND: Central serotonin2C (5-HT2C) receptors are known to play a role in the mechanism of action of the antipsychotic drugs (APDs) clozapine and haloperidol. However, evidence for the involvement of the constitutive activity of 5-HT2C receptors in the dopamine (DA)ergic effects of APDs is lacking in vivo. METHODS: Using in vivo microdialysis in halothane-anesthetized rats, we assessed the ability of selective 5-HT2C compounds to modulate the release of DA induced by haloperidol and clozapine in the nucleus accumbens and striatum. RESULTS: Both APDs induced a dose-dependent increase in accumbal and striatal DA extracellular levels. The effect of .01 mg/kg haloperidol was potentiated by the 5-HT2C inverse agonist SB 206553 (5 mg/kg) but unaltered by the 5-HT2C antagonists SB 243213 and SB 242084 (1 mg/kg). Conversely, the effect of 1 mg/kg clozapine, a dose able to reverse the decrease in DA outflow induced by the 5-HT2C agonist Ro 60-0175 (3 mg/kg), was unaffected by SB 206553 but blocked by SB 243213 (1 mg/kg) and SB 242084 (.3 and 1 mg/kg). CONCLUSIONS: These results show that clozapine and haloperidol differentially alter the constitutive activity of 5-HT2C receptors and suggest that clozapine behaves as a 5-HT2C inverse agonist in vivo.     

Noradrenergic transmission and female sexual behavior of guinea pigs

Treatment with the dopamine beta-hydroxylase (DBH) inhibitor U-14,624 (50, 100, or 150 mg/kg) blocked the induction of lordosis behavior be estradiol benzoate (EB) and progesterone (P) in ovariectomized guinea pigs. After treatment with U-14,624 (100 mg/kg), norepinephrine (NE) content of medial basal hypothalamus, preoptic area and cortex was reduced (by 55%) and dopamine (DA) content of medial basal hypothalamus was increased (by 155%) during the period when females treated with EB and P normally display lordosis. Treatment with the NE receptor stimulator clonidine (1.0 mg/kg) restored lordosis behavior in females treated with EB, P, and U-14,624 (100 mg/kg), but the putative DA and serotonin (5-HT) receptor blockers pimozide (1.0 mg/kg) and methysergide (20.0 mg/kg) were ineffective in this respect. Thus, inhibition of lordosis after treatment with U-14,624 appeared to be attributable primarily to a reduction in NE neurotransmission, rather than to increase in DA or 5-HT activity. Because clonidine induced lordosis in females treated with EB, P, and U-14,624, it seemed unlikely that the facilitatory effects of clonidine on lordosis were mediated by activation of presynaptic alpha-adrenergic receptors (i.e. inhibitory NE autoreceptors) rather than by postsynaptic alpha-receptors. In addition, pretreatment with the postsynaptic alpha-adrenergic antagonist phenoxybenzamine (20.0 mg/kg) blocked the facilitation of lordosis by clonidine (1.0 mg/kg) in females primed with EB alone and with EB plus P. Thus, the facilitatory effects of clonidine on lordosis appear to be mediated by activation of postsynaptic alpha-adrenergic (i.e. NE) receptors. The results of this study provide further evidence that NE neurotransmission facilitates the expression of female sexual behavior in guinea pigs.     

Modulation of dopamine release by striatal 5-HT2C receptors

Previous work has demonstrated that dopamine (DA) transmission is regulated by serotonin-2C (5-HT2C) receptors but the site(s) in the brain where these receptors are localized is not known. The present work utilized in vivo microdialysis to investigate the modulation of DA release by 5-HT2C receptors localized in the nerve terminal regions of the mesocortical and nigrostriatal DA pathways. Microdialysis probes implanted in the striatum or the prefrontal cortex (PFC) measured dialysate DA concentrations, while the selective 5-HT2B/2C inverse agonist SB 206553 was given locally by reverse dialysis into these terminal regions. Additionally, the effects of the 5-HT2C agonist mCPP on striatal DA were measured. Local administration of SB 206553 (0.1-100 microM) into the striatum increased DA efflux in a concentration-dependent manner. Systemic administration of mCPP (1.0 mg/kg i.p.) decreased striatal DA and attenuated the SB 206553-induced increase. In contrast, infusion of SB 206553 (0.1-500 microM) by reverse dialysis into the PFC had no significant effect on basal DA efflux in this region. Additionally, high concentrations of SB 206553 had no effect on high potassium (K(+))-stimulated DA release in the PFC. These data contribute to a body of evidence indicating that 5-HT2C receptors inhibit nigrostriatal dopaminergic transmission. In addition, the results suggest that the nigrostriatal system is regulated by 5-HT2C receptors localized in the dorsal striatum. Elucidating the mechanisms by which serotonin (5-HT) modulates striatal and prefrontocortical DA concentrations may lead to improvements in the treatment of diverse syndromes such as schizophrenia, Parkinson's disease, anxiety, drug abuse, and/or depression.