Phencyclidine-induced dopamine-dependent behaviors in chronic haloperidol-treated rats

Risperidone prevents the development of supersensitivity, but not tolerance, to phencyclidine in rats treated with subacute phencyclidine

We investigated whether risperidone, a 5-HT2/dopamine-D2 receptor antagonist, inhibits the development of tolerance and supersensitivity to PCP and whether subacute administration of PCP with risperidone affects the [3H]MK-801 binding in rat brain, in comparison with dopamine-D2 receptor antagonist haloperidol and 5-HT2 receptor antagonist ritanserin. In rats treated with PCP (10 mg/kg, i.p.) for 14 days, PCP (10 mg/kg, i.p.)-induced hyperlocomotion, rearing and sniffing were potentiated (supersensitivity), and head-weaving, head-twitch, backpedalling and turning were diminished (tolerance). The development of supersensitivity to PCP was blocked by oral co-administration of risperidone (2.4 mg/kg, p.o.) and haloperidol (1.0 mg/kg, p.o.) for 14 days, but not ritanserin (10 mg/kg, p.o.) and risperidone (0.8 mg/kg, p.o.), while no drugs prevented the development of tolerance to PCP. Both risperidone (2.4 mg/kg, p.o.) and haloperidol (1.0 mg/kg, p.o.) also inhibited the cross-supersensitivity to methamphetamine (MAP; 2.5 mg/kg, i.p.)-induced rearing in rats treated with PCP for 14 days. The profiles of [3H]MK-801 binding in discrete brain areas did not change after subacute administration of PCP alone or in combination with risperidone, haloperidol or ritanserin for 14 days. Therefore, it is suggested that subacute administration of PCP may cause functional changes in the dopaminergic neuronal transmission under conditions where the binding of [3H]MK-801 in discrete brain areas is unchanged, and that co-administration of risperidone may block these PCP-induced changes in neuronal function.

Effects of risperidone on phencyclidine-induced behaviors: comparison with haloperidol and ritanserin

In this study, we investigated whether risperidone, a serotonin-S2A (5-HT2A)/dopamine-D2 (D2)-receptor antagonist, inhibits phencyclidine (PCP)-induced stereotyped behaviors in comparison with haloperidol and ritanserin. Moreover, we also attempted to investigate the effects of these antipsychotics on the contents of dopamine, serotonin (5-HT) and their metabolites in rat striatum and frontal cortex. In rats, PCP (5 mg/kg, i.p.) caused hyperlocomotion and stereotyped behaviors, including sniffing, head-weaving, backpedalling and turning. Both resperidone (0.8-2.4 mg/kg, p.o.) and haloperidol (0.3-1.0 mg/kg, p.o.) inhibited these behaviors, except for backpedalling, in a dose-dependent manner. PCP (10 mg/kg, i.p.) produced hyperlocomotion and stereotyped behaviors, including rearing, sniffing head-twitch, backpedalling and turning. Risperidone (0.8-2.4 mg/kg, p.o.) inhibited both hyperlocomotion and PCP-induced behaviors, except for backpedalling, while ritanserin (3-10 mg/kg, p.o.) inhibited only the head-twitch. These results suggest that risperidone may have an antipsychotic effect on schizophrenia as well as PCP psychosis in humans by exerting a mixed 5-HT2A/D2 antagonism. Neurochemically, the increasing effects of risperidone on the content of DOPAC and the ratio of DOPAC to dopamine in the striatum were lower than those of haloperidol. These findings may support the view that the extrapyramidal side effects of risperidone are lower than those of haloperidol in clinical situations.

Phencyclidine (PCP) injected in the nucleus accumbens increases extracellular dopamine and serotonin as measured by microdialysis

Phencyclidine (PCP; 20 micrograms in 0.5 microliter) was tested by local brain injection for neurochemical effects in the nucleus accumbens and striatum of rats. Changes in dopamine turnover could not be detected in postmortem tissue assays. In contrast, extracellular levels of dopamine significantly increased as measured by microdialysis in freely moving animals. PCP also increased extracellular levels of serotonin and decreased 3,4-dihydroxyphenylacetic acid (DOPAC), but did not change homovanillic acid (HVA) or 5-hydroxyindoleacetic acid (5HIAA). Microdialysis suggests that PCP acts in some dopamine terminal regions to increase extracellular dopamine and serotonin.

Development of tolerance and supersensitivity to phencyclidine in rats after repeated administration of phencyclidine

In rats treated with phencyclidine (PCP) repeatedly (PCP 10 mg/kg per day for 14 days), the back-pedalling, head-weaving and turning induced by PCP were attenuated (tolerance), while PCP-induced sniffing, rearing and ambulation were potentiated (supersensitivity). The behavior induced by the direct and indirect serotonin (5-HT) agonists, 5-methoxy-N,N-dimethyltryptamine and p-chloroamphetamine, was attenuated, while the sniffing, rearing or licking induced by the direct and indirect dopamine (DA) agonists, apomorphine and methamphetamine, were potentiated in the chronic PCP-treated rats. The DA and 5-HT contents in the nucleus accumbens and the ratio of HVA to DA in the striatum increased following the repeated PCP administration. Pentobarbital-induced sleep time did not change in the chronic PCP-treated rats as compared with the control rats. In addition, there was no significant difference between the disappearance rate of PCP in the brain of the rats treated with PCP repeatedly and the rate in the control rats. These results suggest that functional changes in the dopaminergic and serotonergic neuronal systems develop on repeated administration of PCP but that such changes do not develop in the hepatic drug-metabolizing system. In addition, tolerance develops in the serotonergic neuronal system while supersensitivity develops in the dopaminergic neuronal system. Biochemical findings suggest that increased mesolimbic dopaminergic neuronal function plays an important role in the development of the supersensitivity.

Potentiation of phencyclidine-induced stereotyped behaviors in rats by thiorphan and bestatin

We investigated the effects of thiorphan, a specific inhibitor of enkephalinase A and bestatin, a specific inhibitor of aminopeptidase, on the stereotyped behaviors induced by phencyclidine (PCP) in cannulated rats. The PCP-induced turning was significantly potentiated when thiorphan (50 micrograms) and bestatin (50 micrograms) were injected simultaneously into the rat lateral ventricle. The increase of PCP-induced turning was completely antagonized by the pretreatment with naloxone. Thiorphan (50 micrograms) or bestatin (50 micrograms) alone failed to potentiate PCP-induced turning. Thiorphan and/or bestatin did not affect significantly the PCP-induced head weaving and back-pedalling except that thiorphan (50 micrograms) potentiated the PCP-induced head weaving 15-18 min after the PCP injection. The combination of thiorphan and bestatin alone did not induce any behavioral change. These results suggest that thiorphan and bestatin produce an increase of endogenous enkephalins and that as a result, PCP-induced turning may be enhanced.