Kazumasa Yamaguchi

NE-100, a novel sigma receptor ligand: In vivo tests

It has been suggested that sigma receptor antagonists may be useful as antipsychotic drugs. N, N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100) is a novel compound with high affinity for the sigma receptor (IC50 = 4.16 nM), but low affinity (IC50 > 10,000 nM) for D1, D2, 5-HT1A, 5-HT2 and phencyclidine (PCP) receptors. The head-weaving behavior induced by either (+)SKF10047 or PCP was dose-dependently antagonized by NE-100 with oral ED50 at 0.27 and 0.12 mg/kg, respectively. NE-100 did not affect dopamine agonists-induced stereotyped behavior and/or hyperactivity. NE-100 failed to induce catalepsy in rats. These findings indicate that NE-100 may have antipsychotic activity without the liability of motor side effects typical of neuroleptics.

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.

Serotonergic involvement in phencyclidine-induced behaviors

Administration of 5-10 mg/kg of phencyclidine (PCP) caused stereotyped behaviors including sniffing, backpedalling, head weaving and turning in rats. The PCP-induced stereotyped behaviors (backpedalling, head weaving and turning) were attenuated by serotonin (5-HT) depleters [reserpine, p-chlorophenylalanine, p-chloroamphetamine (PCA)] and 5-HT receptor antagonist (cyproheptadine). PCP-induced head weaving and turning were potentiated by 5-HT precursor (tryptophan) and 5-HT releaser (PCA). PCP-induced head weaving were potentiated also by monoamine oxidase inhibitor (pargyline) and 5-HT reuptake inhibitor (imipramine). PCP 5-10 mg/kg significantly increased the content of 5-HT in the thalamus/hypothalamus at 30 and 60 min after the injection, except PCP 5 mg/kg at 60 min. PCP 7.5 and 10 mg/kg increased the rate of increment of 5-HT by pargyline in the thalamus/hypothalamus at 30 and 60 min after the injection, respectively. PCP 10 mg/kg significantly increased the contents of 5-HIAA in the striatum and thalamus/hypothalamus at 30 min, but decreased that of 5-HIAA in all discrete brain areas except the stratium at 60 min after the injection. PCP also significantly prevented the depletion of 5-HT by PCA in all discrete brain areas except the stratium at 60 min after the injection. From these results, PCP-induced stereotyped behaviors are related to an increased serotonergic neuronal activity due to 5-HT releasing action and/or inhibitory action of 5-HT uptake-by this drug.

Effect of lesions in the striatum, nucleus accumbens and medial raphe on phencyclidine-induced stereotyped behaviors and hyperactivity in rats.

The effect of lesioning the striatum, nucleus accumbens and medial raphe on phencyclidine(PCP)-induced stereotyped behaviors and hyperactivity was investigated to determine the site or sites of actions of PCP in rats. Bilateral lesions of the striatum diminished or abolished all the parameters of PCP-induced stereotyped behaviors, including sniffing, back pedalling, turning and head weaving 7 days after the operation. The medial raphe lesion significantly reduced PCP-induced back pedalling and head weaving. Bilateral lesions of the ventral portion of the nucleus accumbens did not affect the PCP-induced stereotyped behaviors. On the contrary, none of the lesions altered the sensitivity to PCP-induced hyperactivity 7 days after the operation. These results suggest that PCP-induced stereotyped behaviors may be mediated in the striatum and the medial raphe but not the nucleus accumbens. Furthermore, PCP-induced hyperactivity may not result from PCP effects on these discrete brain areas.

Sex-dependent differences in the pharmacological actions and pharmacokinetics of phencyclidine in rats

This study was designed to assess the sex differences in phencyclidine(PCP)-induced ambulatory activity in an open-field, stereotyped behaviors, motor incoordination, tremor, salivation, the regional and subcellular distributions of PCP in the brain and the half-life of PCP in the brain and plasma. Female rats appeared to be more sensitive to PCP as evidenced by hyperactivity, stereotyped behaviors, motor incoordination, tremor, salivation and ataxia. The concentrations of PCP in female rat brain were higher than in the male rats in some discrete brain areas and subcellular fractions. The half-life of PCP in the brain and plasma was longer in female rats than in male rats. The inverse relationship of pharmacological responses to PCP and biotransformation of PCP in both sexes of rats suggests that sex differences in pharmacological actions of PCP depend largely on differences in ability to biotransform the drug.

Phencyclidine-induced stereotyped behaviors in rats following specific neurotoxin lesions of the striatum.

All the components of phencyclidine(PCP)-induced stereotyped behaviors, including sniffing, backpedalling, turning and head weaving were significantly decreased in rats following kainic acid lesion of the striatum. In the 6-hydroxydopamine lesioned rats, the behavioral score of PCP-induced stereotyped sniffing was similar to that in the sham-operated rats, while other components were significantly decreased. In addition, only the PCP-induced backpedalling and head weaving were significantly attenuated in the 5,6-dihydroxytryptamine lesioned rats. These results suggest that not only dopaminergic but also serotonergic and other systems in the striatum may play important roles in PCP-induced stereotyped behaviors.

The atypical antipsychotic profile of NRA0045, a novel dopamine D4 and 5-hydroxytryptamine2A receptor antagonist, in rats

The atypical antipsychotic profile of (R)‐(+)‐2‐amino‐4‐(4‐fluorophenyl)‐5‐[1‐[4‐(4‐fluorophenyl)‐4‐oxobutyl] pyrrolidin‐3‐yl] thiazole (NRA0045), a potent dopamine D4 and 5‐hydroxytryptamine (5‐HT)2A receptor antagonist, was examined in rats. Spontaneous locomotor activity was decreased dose‐dependently with i.p. administration of clozapine (ED50 3.7 mg kg−1), haloperidol (ED50 0.1 mg kg−1) and chlorpromazine (ED50 0.9 mg kg−1), whereas inhibition of this type of behaviour induced by i.p. administration of NRA0045, at doses up to 10 mg kg−1, did not exceed 50%. Locomotor hyperactivity induced by methamphetamine (MAP, 2 mg kg−1, i.p.) in rats (a model of antipsychotic activity) was dose‐dependently antagonized by NRA0045 (ED50 0.4 mg kg−1, i.p., and 0.3 mg kg−1, p.o., respectively), clozapine (ED50 0.3 mg kg−1, i.p. and 0.8 mg kg−1, p.o., respectively), haloperidol (ED50 0.02 mg kg−1, i.p. and 0.1 mg kg−1, p.o., respectively), chlorpromazine (ED50 0.3 mg kg−1, i.p. and 3.3 mg kg−1, p.o., respectively). In contrast, the MAP (3 mg kg−1, i.v.)‐induced stereotyped behaviour in rats (a model of extrapyramidal symptoms) was not affected by NRA0045 or clozapine, at the highest dose given (30 mg kg−1, i.p.). Haloperidol (ED50 0.3 mg kg−1, i.p.) and chlorpromazine (ED50 4.8 mg kg−1, i.p.) strongly blocked the MAP‐induced stereotyped behaviour. NRA0045 and clozapine selectively blocked behaviour associated with activation of the mesolimbic/mesocortical dopamine neurones rather than nigrostriatal dopamine neurones. Extracellular single‐unit recording studies demonstrated that MAP (1 mg kg−1, i.v.) decreased the firing rate in the substantia nigra (A9) and ventral tegmental area (A10) dopamine neurones in anaesthetized rats. NRA0045 completely reversed the inhibitory effects of MAP on A10 dopamine neurones (ED50 0.1 mg kg−1, i.v.), whereas the inhibitory effects of MAP on A9 dopamine neurones were not affected by NRA0045, in doses up to 1 mg kg−1 (i.v.). Clozapine completely reversed the inhibitory effects of MAP on A10 dopamine neurones (ED50 1.9 mg kg−1, i.v.) and on A9 dopamine neurones (ED50 2.5 mg kg−1, i.v.). Haloperidol completely reversed the inhibitory effects of MAP on A10 (ED50 0.03 mg kg−1, i.v.) and on A9 dopamine neurones (0.02 mg kg−1, i.v.). NRA0045, like clozapine, was more potent in reversing the effects of MAP on A10 than A9 dopamine neurones. Prepulse inhibition (PPI) is impaired markedly in humans with schizophrenia. The disruption of PPI in rats by apomorphine (0.5 mg kg−1, s.c.) was reversed significantly by NRA0045 (3 mg kg−1, i.p.), clozapine (3 mg kg−1, i.p.) and haloperidol (0.3 mg kg−1, i.p.). Phencyclidine (PCP) elicits predominantly psychotic symptoms in normal humans and in schizophrenics. NRA0045 (0.03–0.3 mg kg−1, i.p.) and clozapine (0.1–1 mg kg−1, i.p.) significantly and dose‐dependently shortened the PCP(1.25 mg kg−1, i.p.)‐induced prolonged swimming latency in rats in a water maze task, whereas haloperidol (0.01–0.1 mg kg−1, i.p.) did not significantly alter swimming latency. These findings suggest that NRA0045 may have unique antipsychotic activities without the liability of motor side effects typical of classical antipsychotics.

NE-100, a novel sigma receptor ligand : effect on phencyclidine-induced behaviors in rats, dogs and monkeys

Phencyclidine (PCP)-induced psychosis is a useful animal model for studies on schizophrenia. N, N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)-phenyl]- ethylamine monohydrochloride (NE-100) had no effect on conditioned avoidance responses (CAR) in rats, whereas, the PCP-induced impairment of avoidance inhibition was attenuated by NE-100. The PCP-induced ataxia or decreased attention in rhesus monkeys was to some extent overcome by NE-100. In dogs, PCP-induced either head-weaving behavior or ataxia, effects which were blocked by NE-100. Administration of PCP led to an increase in beta-2 and a decrease in delta relative power (RP) activity in cortical background spectral electroencephalographics (ECoG) in dogs. While NE-100 in itself showed no significant change in beta-2 and delta RP, NE-100 did block the PCP-induced beta-2 increase and delta decrease. These findings indicate that NE-100 attenuates the effect of PCP in experimental animals. This drug is being considered as a therapeutic for the treatment of patients in the schizophrenia.

Methysergide-induced precipitated withdrawal syndrome in phencyclidine-dependent rats

Chronic phencyclidine (PCP) administration has been shown to produce tolerance to a number of its pharmacological actions. We have suggested that PCP interacts with the 5-HT2 receptors since it inhibits [3H]spiperone binding to 5-HT2 receptors in vitro. In the present study, we investigated whether methysergide (a 5-HT2 receptor blocker) induces the precipitated withdrawal syndrome in PCP-tolerant rats. The body weight of the rats in the abrupt and precipitated withdrawal groups was significantly lower 5 days and 1-5 days after withdrawal, respectively, than that in the control group. Furthermore, other typical precipitated abstinence syndrome characteristics such as jumping, wet-dog shake and ptosis were also observed in the precipitated withdrawal group. These results suggest that PCP produces its behavioral effects via an agonistic interaction with 5-HT2 receptor sites and that our method may be very useful for the development of a rat model for studying physical dependence on PCP.

Phencyclidine-induced head-weaving and head-twitch through interaction with 5-HT1 and 5-HT2 receptors in reserpinized rats

Phencyclidine mainly produced head-weaving and head-twitches at doses of 5-7.5 mg/kg and of 7.5-12.5 mg/kg, respectively. Phencyclidine-induced head-twitches and head-weaving were blocked by pretreatment with ritanserin (1 mg/kg), a selective serotonin (5-HT)2 receptor antagonist and with pindolol (20 mg/kg, s.c.), a 5-HT1 receptor antagonist, respectively. In reserpine-pretreated rats, the degree of utilization of 5-HT and the number of 5-HT1 ([3H]5-HT) and 5-HT2 ([3H]ketanserin) binding sites were significantly increased compared with the figures for the vehicle-pretreated rats. The intensity of phencyclidine-induced head-weaving (at the dose of 2.5 mg/kg) and head-twitch (at the doses of 2.5 and 5 mg/kg) was significantly increased in reserpine-pretreated rats compared with that of vehicle-pretreated rats. Furthermore, in the reserpine-pretreated rats, the intensity of phencyclidine (1.25 mg/kg)-induced head-weaving and head-twitches was increased in combination with imipramine, while the intensity of phencyclidine (2.5 mg/kg)-induced head-weaving and head-twitch was decreased by pretreatment with mianserin, a non-selective 5-HT receptor antagonist. These results indicate that phencyclidine induced head-weaving by interacting with 5-HT1 receptors, indirectly after the release of 5-HT and/or with some other mechanisms and induced head-twitch by interacting with 5-HT2 receptors directly and/or indirectly.