Involvement of the serotonergic neuronal system in phencyclidine-induced place aversion in rats

The possible involvement of the serotonergic neuronal system in aversive motivation produced by phencyclidine [1-(1-phenylcyclohexyl)piperidine; PCP] was investigated using a place-conditioning paradigm in rats. PCP (4 mg/kg, i.p.) produced place aversion in this task as reported previously (Kitaichi K, Noda Y, Hasegawa T, Furukawa H, Nabeshima T. Acute phencyclidine induces aversion, but repeated phencyclidine induces preference in the place conditioning test in rats. Eur J Pharmacol 1996;318:7-9). The blockade of serotonin2A (5-HT2A) receptors using the antagonist ritanserin (3 and 10 mg/kg, p.o.) significantly attenuated this aversive property of PCP whereas lesions of serotonergic neurons using 5,7-dihydroxytryptamine (5,7-DHT, 100 microg/animal, i.c.v.) failed to affect it. Repeated PCP treatment (10 mg/kg, i.p. for 14 days), which is enough to diminish the stereotyped 5-HT2A receptor-mediated head-twitch behavior, also decreased the place aversion. These results suggest that the serotonergic neuronal system, specifically the 5-HT2A receptor, may play a critical role in producing PCP-induced place aversion.

Effect of 5-hydroxytryptamine-2 and alpha-adrenergic receptor antagonists on the 5-hydroxytryptamine-induced decrease in rabbit masseter muscle blood flow

A previous study showed a significant decrease in blood flow in the rabbit masseter during infusion of 5-hydroxytryptamine (5-HT) (10(-9) mol/l). The aim of the present study was to test the hypothesis that blockade of the 5-HT2 or the alpha-adrenergic receptor would inhibit the 5-HT-induced decrease of microcirculatory blood flow in the masseter. In 12 rabbits, the masseters were infused with 5-HT (10(-9) mol/l) in combination with the alpha-adrenoceptor antagonist phentolamine (10(-6) mol/l) or the 5-HT2-receptor antagonist ritanserin (10(-6) mol/l). The effect on microcirculatory blood flow was measured by laser-Doppler flowmetry. Infusion of 5-HT induced a significant decrease in blood flow. Inclusion of ritanserin in the 5-HT infusion solution significantly inhibited this decrease, while inclusion of phentolamine did not. This study therefore showed that the 5-HT2 receptor antagonist ritanserin inhibited the 5-HT-induced decrease in microcirculatory blood flow in the rabbit masseter. This decrease in blood flow is thus mediated by the 5-HT2 receptor.

Serotonin depolarizes hippocampal interneurones in the rat stratum oriens by interaction with 5HT2 receptors

Patch-clamp recording techniques were used to examine the effect of serotonin (5HT) upon interneurones contained in the stratum oriens layer of hippocampal slices. Bath application of 1-20 microM 5HT depolarized neurones by the induction of an inward current at -60 mV. This inward current was Na+-dependent in nature, was mimicked by the 5HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and was inhibited by pre-incubation with the 5HT2 receptor antagonist ritanserin.

Blockade of 5-hydroxytryptamine (serotonin)-2 receptors alters interleukin-1-induced changes in rat sleep

Recent data suggest that interleukin-1-induced enhancement of non-rapid eye movement sleep is mediated, in part, by the serotonergic system. To determine if sleep changes induced by interleukin-1 are mediated by a specific serotonergic receptor subtype, we evaluated interleukin-1 effects on sleep in rats pretreated with the 5-hydroxytryptamine (serotonin)-2 receptor antagonist ritanserin. Ritanserin (0.63 mg/kg, intraperitoneally) by itself did not alter sleep-wake behavior, although it did reduce cortical brain temperature. Interleukin-1 (5 ng, intracerebroventricularly) enhanced non-rapid eye movement sleep, suppressed rapid eye movement sleep, and induced a moderate febrile response. Pretreatment with ritanserin completely blocked the febrile response to interleukin-1 and abolished the interleukin-1-induced enhancement in non-rapid eye movement sleep that occurred during postinjection hours 3-4, without altering interleukin-1 effects on rapid eye movement sleep. The present data suggest that serotonin may partially mediate interleukin-1 effects on sleep by interacting with 5-hydroxytryptamine (serotonin)-2 receptors. These results also suggest that interactions between the serotonergic system and interleukin-1 may be important in regulating sleep-wake behavior.

Involvement of 5-HT1B receptors in collar-induced hypersensitivity to 5-hydroxytryptamine of the rabbit carotid artery

In humans intimal thickening is aprerequisite of atherosclerosis. Application of a silicone collar around the rabbit carotid artery induces an intimal thickening but in addition it increases the sensitivity to the vasoconstrictor action of serotonin (5-hydroxytryptamine, 5-HT). The 5-HT receptors involved in collar-induced hypersensitivity to 5-HT were investigated using several agonists and antagonists. One week after placement of collars around both carotid arteries of anaesthetized rabbits, rings (2 mm width) from inside (=collar) and outside (=sham) the collars were mounted in organ baths (10 ml) for isometric force measurements at 6 g loading tension. Collared rings were more sensitive to the contractile effect of 5-HT (7.6 fold) and 5-carboxamidotryptamine (31 fold, 5-CT, 5-HT1 agonist) in cumulative concentration response curves. Sumatriptan (5-HT1B/1D agonist) caused concentration-dependent constrictions in collared rings only. Collar placement did not significantly alter pA2 values (Schild regression) or apparent pKb values (non-linear regression) of spiperone and methysergide (mixed 5-HT2A/5-HT1 antagonists) or ketanserin and ritanserin (5-HT2A antagonists), indicating unchanged binding characteristics of the 5-HT2A receptor. However, the reduced slope of the Schild regression pointed to a heterogeneous receptor population in collared rings. In contrast, the apparent pKb value of methiothepin (5-HT1B antagonist) was significantly reduced by collar placement, and its antagonism shifted from non-surmountable in sham rings to surmountable in collared segments. Taken together, this study demonstrates that the serotonergic receptor involved in the hypersensitivity to 5-HT of rabbit collared carotid artery is a 5-HT1B receptor subtype.

Clozapine and other 5-hydroxytryptamine-2A receptor antagonists alter the subcellular distribution of 5-hydroxytryptamine-2A receptors in vitro and in vivo

In this study, we demonstrate that clozapine and other atypical antipsychotic drugs induce a paradoxical internalization of 5-hydroxytryptamine-2A receptors in vitro and a redistribution of 5-hydroxytryptamine-2A receptors in vivo. We discovered that clozapine, olanzapine, risperidone and the putative atypical antipsychotic drug MDL 100,907 all induced 5-hydroxytryptamine-2A receptor internalization in fibroblasts stably expressing the 5-hydroxytryptamine-2A receptor in vitro. Two 5-hydroxytryptamine-2A antagonists (mianserin and ritanserin), which have been demonstrated to reduce negative symptoms in schizophrenia, also caused 5-hydroxytryptamine-2A receptor internalization. Four different drugs, each devoid of 5-hydroxytryptamine-2A antagonist activity, had no effect on the subcellular distribution of 5-hydroxytryptamine-2A receptors in vitro. Treatment of rats for seven days with clozapine induced an increase in intracellular 5-hydroxytryptamine-2A receptor-like immunoreactivity in pyramidal neurons, while causing a decrease in labeling of apical dendrites in the medial prefrontal cortex. This redistribution of 5-hydroxytryptamine-2A receptors in pyramidal neurons was also seen when rats were chronically treated with another atypical antipsychotic drug, olanzapine. The typical antipsychotic drug haloperidol, however, did not induce a redistribution of 5-hydroxytryptamine-2A receptors in pyramidal neurons in the medial prefrontal cortex. Taken together, these results demonstrate that several atypical antipsychotic drugs with high 5-hydroxytryptamine-2A receptor affinities induce a redistribution of 5-hydroxytryptamine-2A receptors both in vivo and in vitro. It is conceivable that the loss of 5-hydroxytryptamine-2A receptors from the apical dendrites of pyramidal neurons is important for the beneficial effects of atypical antipsychotic drugs and other 5-hydroxytryptamine-2A antagonists in schizophrenia.

Selective blockade of serotonin-2C/2B receptors enhances mesolimbic and mesostriatal dopaminergic function: a combined in vivo electrophysiological and microdialysis study

Electrophysiological techniques and in vivo microdialysis were used to investigate the relative contribution of central serotonin-2C/2B and serotonin-2A receptor subtypes in the control of mesolimbic and nigrostriatal dopaminergic function. Thus, extracellular single-unit recordings were performed from neurochemically identified dopamine neurons in the ventral tegmental area and the substantia nigra pars compacta, as well as simultaneous monitoring of accumbal and striatal basal dopamine release in anesthetized rats following the administration of serotonin-2C/2B (SB 206553), serotonin-2A (SR 46349B) or serotonin-2A/2B/2C (ritanserin) antagonists. Administration of SB 206553 (40-160 microg/kg, i.v.) caused a dose-dependent increase in the basal firing rate of ventral tegmental area and nigral dopamine neurons, reaching its maximum (45.2 and 28.5%, respectively) following 160 microg/kg. Moreover, burst activity was significantly enhanced by SB 206553 in the ventral tegmental area only. In contrast, injection of SR 46349B (40-160 microg/kg, i.v.), and ritanserin (40-160 microg/kg, i.v.) did not cause any significant change in the basal activity of these neurons. Basal dopamine release was significantly enhanced in both the nucleus accumbens (42%) and the striatum (33%) following the intraperitoneal administration of 5 mg/kg SB 206553. In contrast, SR 46349B (0.5 mg/kg, s.c.) and ritanserin (0.63 mg/kg, i.p.) failed to affect basal dopamine output in both regions. Taken together, these data indicate that the central serotonergic system exerts a tonic inhibitory control of mesolimbic and nigrostriatal dopaminergic pathway activity and that the serotonin-2C/2B receptor subtypes are involved in this effect. Moreover, these findings might open new possibilities for the employment of serotonin-2C/2B receptor antagonists in the treatment of neuropsychiatric disorders related to a hypofunction of central dopaminergic neurons.

[Effect of 5HT2 receptor blockade on the startle reflex and its prepulse inhibition in mice and rats of various strains]

Effects of 5-HT2 receptor blockade on the amplitude of startle reflex, induced by an unexpected sound, and on its prepulse inhibition (PPI) were studied on mice of CBA strain and rats of Wistar and the genetically predisposed to catalepsy (GC) strains. The effect was dependent on type and dose of 5-HT2 antagonist used: 5-HT2A antagonist ketanserin increased startle amplitude at the dose of 0.5 mg/kg and decreased it at the dose of 2 mg/kg. Mixed 5-HT2A/2C antagonist ritanserin (0.1 and 0.2 mg/kg) markedly increased startle in mice. Ketanserin and cyproheptadine produced opposite effects on startle reflex in rats with inherited neuropathology and in rats with normal genotype: marked decrease in GC rats and increase in Wistar rats was shown. Ketanserin and cyproheptadine produced a pronounced potentiation of PPI in mice and rats of both strains, ritanserin was ineffective. Results suggest 5-HT2 receptors implication in both startle and PPI regulation with 5-HT2C receptors in startle response and 5-HT2A in PPI predominant involvement.

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.

Acute p-chloroamphetamine increases striatal preprotachykinin mRNA: role of the serotonin 2A/2C receptor

Acute administration of p-chloroamphetamine (pCA) significantly increased (+90%) preprotachykinin (PPT) mRNA levels in the rat striatum. Administration of the serotonin2A/2C receptor antagonist, ritanserin, blocked the pCA-induced increase in PPT mRNA levels. alpha-Methyl-p-tyrosine pretreatment (alpha-MT, to reduce dopamine transmission) inhibited the pCA-induced increase in PPT mRNA levels. These results indicate that the pCA-induced increase in striatal PPT mRNA expression is mediated by serotonin2A/2C receptors but also requires dopamine tone.

The motivation for beer in rats: effects of ritanserin, naloxone and SR 141716

Rats were given two weeks of home cage access to either "near-beer" (a beverage that tastes like beer but contains <0.5% ethanol v/v) or near-beer with added ethanol (4.5% v/v), which is simply referred to as "beer". The two groups of rats (near-beer and beer) were then trained on a "lick-based progressive ratio paradigm" in operant chambers in which an ever increasing number of licks had to be emitted for each successive fixed unit of near-beer or beer delivered. Break points (the ratio at which responding ceased) for near-beer and beer were approximately equal under baseline conditions. Rats were then tested for the effects of the 5HT2A/2C receptor antagonist ritanserin (0.625, 2.5 or 10 mg/kg), the opioid receptor antagonist naloxone (0.625, 2.5 or 10 mg/kg) or the cannabinoid CB1 receptor antagonist SR 141716 (0.3, 1 or 3 mg/kg). All three drugs caused a dose-dependent reduction of break-points and locomotor activity in both the beer and near-beer groups. However, the effects of SR 141716 and naloxone, but not ritanserin, on breakpoints were significantly more pronounced on rats drinking beer compared to those drinking near-beer. There were no such differential effects of any of the drugs on locomotor activity across the two groups. These results suggest that both SR 141716 and naloxone differentially affect the motivation to consume alcoholic beverages and may thus have potential as drugs for the treatment of alcohol craving.

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 GABAA receptor antagonist bicuculline had no effect on the suppressant effect of neuronal activity in the OFc induced by microiontophoretic application of the preferential 5-HT2A and 5-HT2C 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, i.v.) 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, i.v.) and risperidone (0.1 and 1.0 mg/kg, i.v.). 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-HT2C 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.

Differential EEG effects of the anxiolytic drugs, deramciclane (EGIS-3886), ritanserin and chlordiazepoxide in rats

The influence of serotonergic and benzodiazepine type anxiolytic drugs on the cortical activation and sleep-wakefulness cycle were compared by evaluating the effects of ritanserin and deramciclane (EGIS-3886), two 5-HT2 receptor antagonists, and chlordiazepoxide on the electroencephalogram (EEG) in freely moving rats. Following drug administration (1, 3, and 10 mg/kg, PO for all drugs), EEG was continuously sampled for 6 h and power spectra were calculated for every 5 s to assess changes in slow wave activity and sleep phases. In a separate test, anticonvulsant effects of the drugs were examined in mice. Both deramciclane and ritanserin slightly increased total time spent in deep sleep (DS) and lengthened sleep episodes. In contrast, chlordiazepoxide had a strong inhibitory action on DS, sleep time being shifted to more superficial light sleep (LS). The incidence and length of the high voltage spindle (HVS) episodes characteristic for the motionless, awake rat were increased at the highest dose of both deramciclane and ritanserin, while it was decreased by chlordiazepoxide. In mice, chlordiazepoxide had a marked anticonvulsant effect, while deramciclane was moderately effective and ritanserin ineffective. In conclusion, the 5-HT2 receptor antagonist anxiolytic drugs seem to be superior compared to the benzodiazepine type anxiolytic drug, chlordiazepoxide, as ritanserin and deramciclane improved sleep quality by increasing sleep episode length and time spent in DS, while chlordiazepoxide enhanced sleep fragmentation and decreased DS.

Beneficial effect of serotonin 5-HT2-receptor antagonism on renal blood flow autoregulation in cyclosporin-treated rats

Renal blood flow (RBF) autoregulation reappears in postischemic rat kidneys during serotonin (5-HT2) antagonism. The aim of the present study was to analyze whether 5-HT2 antagonism can ameliorate impaired RBF autoregulation in rats treated with 20 mg/kg per d cyclosporin A during 10 d. Autoregulation of RBF was assessed during stepwise lowering of renal perfusion pressure from 110 to 70 mmHg by gradual compression of the aorta. Autoregulation was lost in the cyclosporin A-treated rats. During administration of the 5-HT2 antagonist ritanserin (0.6 mg/kg intravenous bolus, followed by 1.2 mg/kg per h intravenous infusion during 1 h), autoregulation acutely reappeared. Intrarenal bolus injections of a selective 5-HT2-agonist, 2,5 dimethoxy-4-iodoamphetamine hydrochloride, elicited a significantly stronger renal vasocontraction in cyclosporin A-treated rats than in control rats. This finding was also observed with serotonin after nitric oxide-synthase blockade. These results (1) show the importance of 5-HT2-receptor-mediated vasoconstriction in the suppression of vasodilatory autoregulation of RBF in experimental cyclosporin A-induced renal dysfunction and (2) demonstrate that the complete loss of RBF autoregulation is not due to damage of the vascular smooth muscle cells.

Serotonergic 5-HT2A receptors important for the oestradiol-induced surge of luteinising hormone-releasing hormone in the rat

Serotonin (5-HT) plays a role in mediating the oestradiol-induced surge of luteinising hormone (LH), but so far the 5-HT receptor subtype involved has not been identified. Our previous in-situ hybridization and pharmacological studies suggest that the action of 5-HT involves the 5-HT2A receptor. The aim of the present study was to investigate this possibility by the direct approach of determining whether 5-HT2A receptor antagonists block the oestradiol-induced surge of luteinising hormone releasing hormone (LHRH). Adult female Wistar rats, which had shown at least two consecutive 4-day oestrous cycles, were ovariectomised under halothane anaesthesia in the morning of dioestrus and injected with vehicle (arachis oil) alone or oestradiol benzoate (OB). At 12.00 h of the next day, presumptive pro-oestrus, the animals were injected intraperitoneally with one of three 5-HT2A antagonists, a selective 5-HT reuptake inhibitor (fluoxetine), or the appropriate vehicles; hypophysial portal blood was then collected under alphaxalone anaesthesia between 15.00 and 19.00 h. The amount of LHRH released into hypophysial portal blood during consecutive 30-min periods was determined by radioimmunoassay. As expected, oestradiol, but not oil, triggered a surge of LHRH in hypophysial portal blood with a peak at about 16.00 h of presumptive pro-oestrus. This oestradiol-induced surge of LHRH was blocked by ketanserin, ritanserin and the highly selective 5-HT2A receptor antagonist, RP62203, but not by fluoxetine. These results provide the first direct evidence that the 5-HT2A receptor plays an important role in the oestradiol-induced surge of LHRH.

Effect of 5-HT2 receptor antagonists on a cranial nerve reflex in the rabbit: evidence for inverse agonism

This study examined the role of the serotonin 5-HT2 receptor in motor function by examining the effect of antagonists on the motor performance of a cranial nerve reflex, the nictitating membrane (NM) reflex of the rabbit. The NM reflex was elicited by varying intensities of a tactile stimulus and the magnitudes of the elicited responses were measured at each intensity. Dose-response curves were obtained for the effects of several 5-HT2 receptor antagonists on response magnitude. d-Bro-molysergic acid diethylamide (BOL), LY-53,857 and ketanserin had no significant effect on the magnitude of the NM reflex, indicating that they are neutral antagonists. However, the 5-HT2 receptor antagonists ritanserin, MDL-11,939 and mianserin produced a significant reduction in response magnitude with no significant effects on response frequency, suggesting that they were acting as inverse agonists at the 5-HT2 receptor. The reduction in reflex magnitude produced by mianserin (10 micromol/kg) was fully blocked by BOL (5.8 micromol/kg), supporting the conclusion that mianserin was producing a reduction in reflex magnitude through an effect at the 5-HT2 receptor. The occurrence of inverse agonism suggests the possible existence of constitutive activity in vivo. We conclude that the 5-HT2 receptor (either 2A or 2C) plays an important role in motor function, perhaps by providing a tonic influence on motor systems.

The pharmacology of impulsive behaviour in rats IV: the effects of selective serotonergic agents on a paced fixed consecutive number schedule

An impulsive cognitive style may affect behaviour in several different ways, including rapid decision making, intolerance of the delay of reward and a tendency to terminate chains of responses prematurely. It has been proposed to measure the last of these in rats using fixed consecutive number (FCN) schedules. The present study uses a modified version of the FCN procedure in which responding was paced by retracting the response lever for short periods between presses. In this way, the experimenter can control the maximum rate of responding. The procedure was made up of two components. In both, the schedule requirement was FCN 8, but in the Fast component lever presses were spaced by a minimum of 2.5 s and in the Slow component by a minimum of 5 s. Alterations in impulsivity were inferred from changes in the mean chain length and the distribution of chain lengths. The 5-HT1A agonist, 8-OH-DPAT (0.03-0.3 mg/kg), increased chain lengths within a narrow dose range, whereas the 5-HT1A antagonist, WAY 100 635 (0.03-0.3 mg/kg), reduced chain lengths. The 5-HT2 agonist, DOI (0.1-1.0 mg/kg), markedly reduced chain lengths, whereas the 5-HT2 antagonist, ritanserin (0.03-0.3 mg/kg), had no effect. The 5-HT1A/1b agonist, RU 24969 (0.03-0.3 mg/kg), reduced chain lengths. The 5-HT releaser, p-chloramphetamine (0.1-1.0 mg/kg), had a weak, biphasic effect, slightly reducing the number of short chains at the lowest dose tested and slightly increasing this number at the highest dose. Other drugs tested, citalopram (1.0-10.0 mg/kg), metergoline (0.3-3.0 mg/kg) and MDL-72222 (0.1-3.0 mg/kg), had no significant effects. These results suggest that stimulation of 5-HT1A receptors reduces impulsivity, whereas stimulation of 5-HT2 receptors increases it. These data are in agreement with previous results using the DRL-72 schedule, and indicate that there is no simple role for serotonin in the control of impulsivity.

Effects of 5-hydroxytryptamine2 receptor antagonism on the behavioral activation and immediate early gene expression induced by dizocilpine

The noncompetitive N-methyl-D-aspartate (NMDA) antagonists dizocilpine and phencyclidine cause behavioral changes in animals that can be blocked by antipsychotic agents, implicating NMDA receptors in the expression of schizophrenic symptoms. In the present study, we examined the effects of dizocilpine (0.1-3.0 mg/kg s.c.) on locomotor activity and on the expression of c-fos and hsp-70 immediate-early genes (IEGs) in mice. Results indicate that dizocilpine increases locomotor activity and selectively increases the expression of c-fos and hsp-70 in the posterior cingulate cortex. Haloperidol (0.01-0.1 mg/kg) and clozapine (0.6-1.25 mg/kg) block both the locomotor response and the increased IEG immunoreactivity induced by dizocilpine (0.6 mg/kg). The 5-HT2 antagonists ritanserin (0.06-0.25 mg/kg), ketanserin (0.03-0.12 mg/kg) and amesergide (0. 3-1.25 mg/kg) also significantly attenuated the locomotor response to dizocilpine. Haloperidol and clozapine suppressed the head weaving induced by dizocilpine, but ritanserin, as previously reported did not. Although some attenuation of the c-fos and hsp-70 immunoreactivity was seen with the 5-HT2 antagonists it was less pronounced than that induced by haloperidol or clozapine. In conclusion, 5-HT2 antagonists as well as antipsychotic compounds attenuate the locomotor response to dizocilpine in mice. Haloperidol and clozapine appear to be more effective, however, in attenuating the expression of c-fos and hsp-70 in the posterior cingulate gyrus than 5-HT2 antagonists ritanserin, ketanserin or amesergide. We thus have seen a dissociation in the capacity of compounds to alter the effects on behavior and IEG expression after dizocilpine administration.

Serotonin antagonists do not attenuate activity-induced phase shifts of circadian rhythms in the Syrian hamster

A variety of observations from several rodent species suggest that a serotonin (5-HT) input to the suprachiasmatic nucleus (SCN) circadian pacemaker may play a role in resetting or entrainment of circadian rhythms by non-photic stimuli such as scheduled wheel running. If 5-HT activity within the SCN is necessary for activity-induced phase shifting, then it should be possible to block or attenuate these phase shifts by reducing 5-HT release or by blocking post-synaptic 5-HT receptors. Animals received one of four serotonergic drugs and were then locked in a novel wheel for 3 h during the mid-rest phase, when novelty-induced activity produces maximal phase advance shifts. Drugs tested at several doses were metergoline (5-HT1/2 antagonist; i.p.), (+)-WAY100135 (5-HT1A postsynaptic antagonist, which may also reduce 5-HT release by an agonist effect at 5-HT1A raphe autoreceptors; i.p.), NAN-190 (5-HT1A postsynaptic antagonist, which also reduces 5-HT release via an agonist effect at 5-HT1A raphe autoreceptors; i.p.) and ritanserin (5-HT2/7 antagonist; i.p. and i.c.v.). Mean and maximal phase shifts to running in novel wheels were not significantly affected by any drug at any dose. These results do not support a hypothesis that 5-HT release or activity at 5HT1, 2 and 7 receptors in the SCN is necessary for the production of activity-induced phase shifts in hamsters.

Brain mechanisms of social anxiety disorder

The neurobiology of social anxiety disorder is poorly understood, although preliminary research has suggested several possible biological abnormalities. Challenge studies have demonstrated that subjects with social anxiety disorder have a sensitivity to carbon dioxide, cholecystokinin, and caffeine somewhere between that of panic disorder patients and normal controls. Serotonergic pathways may play a role in social anxiety disorder, as shown by the clinical effectiveness of selective serotonin reuptake inhibitors, plus fenfluramine and m-chlorophenylpiperazine challenge studies. Dopaminergic function and striatal dopamine uptake appear to be reduced in social anxiety disorder. There is also evidence for cardiovascular and adrenergic abnormalities. Recently, positron emission tomography has begun to identify brain regions that appear to be uniquely activated in this condition. These results offer the promise of an understanding of the brain mechanisms of social anxiety disorder, but much further research is needed to fully elucidate the neurobiological cause(s) that exist.

p-Chloroamphetamine, a serotonin-releasing drug, elicited in rats a hyperglycemia mediated by the 5-HT1A and 5-HT2B/2C receptors

The effects of a serotonin (5-HT) releasing drug, p-chloroamphetamine, on plasma glucose levels were investigated in rats. p-Chloroamphetamine elicited a significant hyperglycemia. The hyperglycemic effects of p-chloroamphetamine were completely prevented by the 5-HT synthesis inhibitor, p-chlorophenylalanine. Prior adrenodemedullation abolished the hyperglycemia elicited by p-chloroamphetamine. p-Chloroamphetamine-induced hyperglycemia was prevented by methysergide, which blocks the 5-HT1 and 5-HT2 receptor, the 5-HT1A/1B/2C receptor antagonist, (-)-propranolol, the selective 5-HT1A receptor antagonist, 4-(2'-methoxyphenyl-1-[2'-n-2"pyridinyl)-p-iodobenzamido]-ethyl-pi perazine (p-MPPI), the 5-HT2A/2B/2C receptor antagonists, ritanserin and 4-isopropyl-7-methyl-9-(2-hydroxy-1-methyl-propoxycarbonyl)-4,6A,7 ,8,9,10,10A-octahydro-indolo[4,3-FG]quinolone maleate(LY 53857). However, the 5-HT3 and 5-HT4 receptor antagonist, tropisetron, the 5-HT4 receptor antagonist, 2-methoxy-4-amino-5-chloro-benzoic acid 2-(diethylamino) ethyl ester (SDZ 205-557), and the 5-HT2A receptor antagonist, ketanserin, did not affect the p-chloroamphetamine-induced hyperglycemia. These results suggest that p-chloroamphetamine-induced hyperglycemia is elicited by an enhanced 5-HT release and facilitated adrenaline release. Moreover, our results indicate that p-chloroamphetamine-induced hyperglycemia is mediated by 5-HT1A and 5-HT2B/2C receptors.

5-HT2A receptor antagonists block MK-801-induced stereotypy and hyperlocomotion

The present study was undertaken to examine the effect of 5-HT2A receptor antagonists on MK-801 (5-methyl-10,11-dihydro-5H-dibenzo (a,d) cyclohepten-5,10-imine)-induced stereotypy and hyperlocomotion. MK-801 (0.1, 0.25 and 0.5 mg/kg) dose-dependently increased stereotypy and locomotion in mice. The 5-HT2A receptor antagonists, ketanserin (2.5, 5 and 10 mg/kg) and ritanserin (0.5, 1 and 2 mg/kg), dose-dependently blocked MK-801 (0.5 mg/kg)-induced hyperlocomotion. Only the higher dose (2 mg/kg) of seganserin could block locomotor activity. Similarly, ketanserin (2.5, 5 and 10 mg/kg), ritanserin (1 and 2 mg/kg) and seganserin (0.5, 1 and 2 mg/kg) dose-dependently blocked MK-801 (0.5 mg/kg)-induced stereotypy. The results suggest the involvement of 5-HT2A receptors in MK-801-induced stereotypy and hyperlocomotion. The lack of effect on spontaneous locomotion further suggests that 5-HT2A receptor antagonists will be less prone to induce psychomotor side-effects.

An investigation into the effects of 5-HT agonists and receptor antagonists on ethanol self-administration in the rat

Pharmacological manipulation leading to altered 5-HT function has been widely demonstrated to reduce ethanol intake in free choice tests. The aim of the present study was to examine the effects of a range of compounds known to influence 5-HT neurotransmission, including selective 5-HT receptor agonists and antagonists, on ethanol ingestion and maintained behaviour in an operant self-administration paradigm. Female Sprague-Dawley rats were trained to respond for 8% ethanol (v/v) in a 60-min test by a previously described technique. The number of responses and ethanol reinforcers (dipper deliveries), ethanol consumption (g/kg of body weight), and locomotor activity (LMA) were measured following administration of 5-HT agonists (5-HT, d-fenfluramine, fluoxetine, buspirone, TFMPP, and DOI) and antagonists (metergoline, ritanserin, and ondansetron) 30 min prior to testing. d-Fenfluramine, fluoxetine, buspirone, TFMPP, and DOI all produced a reduction in ethanol ingestion and maintained behaviour at doses that failed to reduce LMA. Conversely, metergoline and ritanserin only reduced ethanol self-administration at doses that concomitantly reduced LMA. 5-HT and ondansetron were without effect on any measure. These results demonstrate that, under the present experimental conditions, activation of central 5-HT1A, 5-HT1B, and 5-HT2 receptors reduced ethanol intake and reinforced behaviour in an operant paradigm.

Serotonin 5-HT2c agonists mimic the effect of light pulses on circadian rhythms

The serotonin agonist quipazine has been shown to cause phase shifts in melatonin and activity rhythms and to induce c-fos in the suprachiasmatic nucleus of rats. In this study, in vivo pharmacological characterisation of the phase shifting properties of serotonin agonists has been performed, with a view to determining the receptor sub-types involved. Agonists for the 5-HT2a/2c receptors, (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI, 0.1 mg/k), 1-(3-chlorophenyl)-piperazine HCl (mCPP, 2 mg/kg) and N-(3-trifluoromethylphenyl)-piperazine HCl (TFMPP, 2 mg/kg) injected at CT18 resulted in acute transient inhibition of melatonin production and delays in the onset of production on the following nights of 1.2+/-0.2, 1.7+/-0.3 and 1. 4+/-0.8 h respectively. Drugs specific for 5-HT1a/7 and 5-HT3 receptors failed to affect melatonin production. At a dose of 0.07 micromole/kg, the serotonin antagonist, ritanserin inhibited the DOI induced phase delay whereas ketanserin was ineffective at this dose, providing strong evidence that DOI was acting through 5-HT2c receptors. DOI (0.5 mg/kg) at CT18 provoked a phase delay in the core body temperature rhythm of similar magnitude to that following a light pulse. Administration of DOI but not agonists active at other receptor sites resulted in the appearance of c-Fos in the ventrolateral division of the suprachiasmatic nucleus (SCN) at CT18 but not at CT6. Ritanserin was more potent than ketanserin at inhibiting the DOI induced increase in c-Fos labelled cells in the SCN. When rats were pre-treated with metergoline (15 mg/kg), ritanserin (3 mg/kg) or LY 53,857 (3 mg/kg) prior to a 2 lx/ 1 min light pulse, none of the drugs significantly inhibited the responses to light. The results of these experiments indicate that serotonergic agonists active at the 5-HT2c receptor mimic the effects of light on 2 independent rhythms and activate SCN neurones in the rat.

Open-field and LPS-induced sickness behavior in young chickens: effects of embryonic cocaine and/or ritanserin

Exposure to drugs of abuse during embryogenesis may adversely affect nervous, immune, and endocrine systems development. We compared exposure on embryonic day 18 (E18) by single or multiple cocaine (COC) injections (56.25 mg/kg total dose for both) or saline on hatching and activity measures. In saline-exposed controls, repeated testing, age, and gender affected activity levels. A single or multiple COC injections increased the median latency to explore and multiple COC injections decreased the median number of lines crossed by female chicks in the open field. We also determined if pretreatment with the serotonin2 (5-HT2) receptor antagonist ritanserin could attenuate COC's effects on open-field behavior as well as behaviors sensitive to immune system stimulation (lipopolysaccharide (LPS)-induced sickness behavior). Eggs containing embryos were pretreated on E17 with 0.4 mg ritanserin/kg or its vehicle followed by multiple COC injections or saline on E18. E18 COC treatment decreased the median number of lines crossed and distress vocalizations in females. Ritanserin pretreatment mitigated the COC induced effects. E18 COC exposure also suppressed LPS-induced sickness behaviors in both males and females, increasing food consumption and the time spent awake and active, as well as decreasing the time spent sleeping. Ritanserin alone had no effect on the food consumed or time spent active, nor did this dose affect COC-induced alterations in sickness behavior. Ritanserin alone decreased time spent sleeping and also failed to affect the COC-induced suppression. Thus, embryonic COC exposure can suppress open field and LPS-induced sickness behavior in the young chick, and ritanserin pretreatment can block the former, but not the latter effects at the dose chosen for these experiments.