Effects of bilateral lesions in the striatum or nucleus accumbens on the cataleptogenic activity of neuroleptics in rats

Paradoxical kinesia may no longer be a paradox waiting for 100 years to be unraveled

Parkinson's disease (PD) is a progressive neurodegenerative disorder mainly characterized by bradykinesia and akinesia. Interestingly, these motor disabilities can depend on the patient emotional state. Disabled PD patients remain able to produce normal motor responses in the context of urgent or externally driven situations or even when exposed to appetitive cues such as music. To describe this phenomenon Souques coined the term "paradoxical kinesia" a century ago. Since then, the mechanisms underlying paradoxical kinesia are still unknown due to a paucity of valid animal models that replicate this phenomenon. To overcome this limitation, we established two animal models of paradoxical kinesia. Using these models, we investigated the neural mechanisms of paradoxical kinesia, with the results pointing to the inferior colliculus (IC) as a key structure. Intracollicular electrical deep brain stimulation, glutamatergic and GABAergic mechanisms may be involved in the elaboration of paradoxical kinesia. Since paradoxical kinesia might work by activation of some alternative pathway bypassing basal ganglia, we suggest the IC as a candidate to be part of this pathway.

Clozapine prolongs hypotonic immobility in rats with bilateral 6-hydroxydopamine lesions of the striatum

Clozapine does not induce classical catalepsy. One explanation is that endogenous dopamine (DA) readily displaces clozapine from postsynaptic striatal DA receptors. If the latter were true, then depletion of striatal DA should permit clozapine to induce classical catalepsy. Rats received bilateral sham or 6-hydroxydopamine (6-OHDA) lesions of the striatum. Catalepsy using the bar test was assessed on days 21, 28 and 35 after i.p. vehicle, haloperidol (0.75 mg/kg) or clozapine (20 mg/kg). Brains were harvested on day 36 and striatal DA was assayed. A 97% depletion of striatal DA was associated with a significant increase in the duration of immobility after haloperidol or clozapine administration. Both in sham as well as in 6-OHDA lesioned animals, however, the duration of immobility was greater after haloperidol than after clozapine. Furthermore, clozapine-treated animals were hypotonic and did not show the classic rigidity of haloperidol catalepsy. While clozapine's low propensity to induce immobility in animals and extrapyramidal symptoms in man may depend in part on endogenous striatal DA, other mechanisms must also be involved.

Dopamine D1- and D2-dependent catalepsy in the rat requires functional NMDA receptors in the corpus striatum, nucleus accumbens and substantia nigra pars reticulata

This study investigated the anticataleptic activity of MK-801 versus the D1 antagonist SCH 23390 and the D2 antagonist raclopride, using the horizontal bar test in the rat. MK-801, 0.2 mg/kg i.p., strongly opposed the cataleptogenic actions of SCH 23390 and raclopride administered systemically (1 and 3 mg/kg i.p., respectively), or directly into the corpus striatum (CS) or nucleus accumbens (NAc; 1 and 10 microg, respectively). Conversely, intraCS and intraNAc pretreatment with MK-801 (10 microg) markedly attenuated the cataleptic response to a systemic injection of SCH 23390 or raclopride. In the latter experiments the anticataleptic effect of MK-801 was pronounced and sustained (> 2 h), except with intraCS MK-801 versus raclopride, where it was initially profound but only short-lived (15 min). Stereotaxic injection of MK-801 (1 microg) into the substantia nigra pars reticulata (SNr) prevented catalepsy developing to either dopamine D1 or D2 receptor antagonism. These results indicate there must be unimpeded glutamate neurotransmission in the CS and NAc before catalepsy can develop fully to D1 and D2 dopamine receptor blockade in these structures. The weaker glutamate-D2 interaction in the CS than in the NAc may be related to differences in the N-methyl-D-aspartate receptor subpopulations in these nuclei. Finally, the ability of intranigral MK-801 to diminish both D1- and D2-dependent catalepsy suggests the SNr acts as a common output pathway for the expression of both forms of catalepsy in the rat.

Behavioral effects of clozapine and dopamine receptor subtypes

The atypical neuroleptic clozapine (CLZ) is an extremely effective antipsychotic that produces relatively few motoric side effects. However, CLZ displays limited antagonism at the dopamine (DA) D2 receptor, the receptor commonly thought to mediate the antipsychotic activity of neuroleptics. The mechanism of action behind the efficacy of CLZ remains to be determined. Miller, Wickens and Beninger [Progr. Neurobiol., 34, 143-184 (1990)] propose a "D1 hypothesis of antipsychotic action" that may explain the antipsychotic effects of CLZ. This hypothesis is built on the interactions between D2, cholinergic and D1 mechanisms in the striatum. These authors assert that although typical neuroleptics block D2 receptors, it is through an indirect action on D1 receptors that their antipsychotic action is manifest. The extra-pyramidal side effects produced by typical neuroleptics are hypothesized to be due to an indirect action on cholinergic receptors. It is argued that the anticholinergic properties of CLZ negate the D2 (motor side effects) action of CLZ, allowing CLZ to diminish psychotic symptoms through a direct action on D1 receptors. Thus, CLZ may function as a D1 receptor antagonist in behavioral paradigms. The current paper reviews and compares the behavioral profile of CLZ to those produced by D2- and D1-selective antagonists with specific reference to unconditioned and conditioned behaviors in order to more fully evaluate the "D1 hypothesis of CLZ action". Although the actions of CLZ remain unique, they do share some striking similarities with D1 receptor antagonists especially in tests of unconditioned behavior, possibly implicating the D1 receptor in the action of this antipsychotic drug.

KF17837: a novel selective adenosine A2A receptor antagonist with anticataleptic activity

KF17837 is a novel selective adenosine A2A receptor antagonist. Oral administration of KF17837 (2.5, 10.0 and 30.0 mg/kg) significantly ameliorated the cataleptic responses induced by intracerebroventricular administration of an adenosine A2A receptor agonist, CGS 21680 (10 micrograms), in a dose-dependent manner. KF17837 also reduced the catalepsy induced by haloperidol (1 mg/kg i.p.) and by reserpine (5 mg/kg i.p.). These anticataleptic effects were exhibited dose dependently at doses from 0.625 and 2.5 mg/kg p.o., respectively. Moreover, KF17837 (0.625 mg/kg p.o.) potentiated the anticataleptic effects of a subthreshold dose of L-3,4-dihydroxyphenylalanine (L-DOPA; 25 mg/kg i.p.) plus benserazide (6.25 mg/kg i.p.). These results suggested that KF17837 is a centrally active adenosine A2A receptor antagonist and that the dopaminergic function of the nigrostriatal pathway is potentiated by adenosine A2A receptor antagonists. Furthermore, KF17837 may be a useful drug in the treatment of parkinsonism.

Possible noradrenergic dysfunction in schizophrenia

In spite of extensive studies over the last 2 decades to find direct evidence in support of the dopamine hypothesis of schizophrenia, no undisputed experimental data has been obtained. In contrast, estimation of noradrenalin (another major catecholamine) and its metabolites in postmortem brain and in the cerebrospinal fluid appears to be producing consistent results. To understand the meaning of this change for the pathogenesis of the illness, we have carried out animal experiments in which reproducibility of schizophrenic signs and symptoms by noradrenergic dysfunction, and treatability of the disorder by modulation of noradrenergic activity were studied. First, psychophysiological signs in skin conductance responsiveness (nonhabituating or nonresponding change) and smooth pursuit eye movement (spiky or stepwise pursuit) could be reproduced by enhancing or suppressing central noradrenergic activity. Behavioral abnormalities resembling schizophrenic symptoms are known to be reproducible by over- or underactivity of the system (overarousal or underarousal syndrome). Secondly, the action of various drugs capable of modulating schizophrenic symptoms was analyzed in relation to noradrenergic activity. Haloperidol, in particular, had a potent suppressing effect on skin conductance activity (spontaneous fluctuation rate and habituation rate) when administered chronically, suggesting its inhibitory action on noradrenergic activity.

Nicotine potentiation of haloperidol-induced catalepsy: striatal mechanisms

Nicotine potentiated the catalepsy produced by haloperidol. The excitotoxin quinolinic acid (QA) selectively destroys striatal neurons when injected directly into the striatum. Bilateral QA lesions of the rat striatum (150 nmol) significantly reduced the catalepsy produced by haloperidol as well as the ability of nicotine to potentiate haloperidol-induced catalepsy. A second experiment examined whether the ability of nicotine to potentiate haloperidol-induced catalepsy was associated with a potentiation of dopamine turnover following haloperidol. Nicotine alone produced a mild increase in dopamine turnover relative to saline treated controls while haloperidol produced a marked increase in dopamine turnover relative to saline- and nicotine-treated controls. However, the combined administration of haloperidol and nicotine did not further elevate dopamine turnover over that observed following haloperidol alone. The results indicated that: 1) nicotine could not potentiate haloperidol-induced catalepsy without an intact striatum and 2) the behavioral effect of nicotine and haloperidol cotreatment was not due to any change in dopamine turnover.

Multiple neurochemical action of clozapine: a quantitative autoradiographic study of DA2, opiate and benzodiazepine receptors in the rat brain after long-term treatment

The atypical neuroleptic clozapine has clinical and behavioral properties that differ not only from the typical compounds, but also from atypical ones. It interacts with the dopaminergic systems, but also produces effects on the serotoninergic, GABA-ergic, cholinergic systems. In spite of the amount of papers devoted to its study, the profile of the neurochemical action of this drug is still confuse. In this paper we investigated the DA2-, opiate- and benzodiazepine-receptor modifications induced by the long term (21 days) treatment with clozapine 20 mg/kg/day in the rat brain. We found a decrease of DA2 receptor density in the target areas of the mesolimbocortical system (ventral n. caudate-putamen, cerebral cortex except for the anterior cingulate at the most anterior level and the n. accumbens) and a decrease of opiate and benzodiazepine receptors in the cerebral cortex and in the olfactory tubercle. Opiate receptors increase in the patches of the striatum. We also compared these effects with those produced by long-term (21 days), low-dosage (0.5 mg/kg day) haloperidol.

Striatal and nucleus accumbens D1/D2 dopamine receptors in neuroleptic catalepsy

Haloperidol (2.5-10 micrograms) injected bilaterally into the ventro-rostral striatum or into the nucleus accumbens induced dose-dependent catalepsy whereas its injection into the dorso-rostral striatum (2.5 micrograms) was ineffective. Similarly, the specific antagonist of D1 receptors, SCH 23390 (1-5 micrograms), injected into the ventro-rostral striatum or nucleus accumbens, as well as the specific antagonist of D2 receptors, sulpiride, injected into the ventro-rostral striatum (0.02-15 micrograms) or nucleus accumbens (1-15 micrograms), induced a dose-dependent catalepsy. Both drugs (SCH 23390 2 micrograms, sulpiride 0.5 micrograms) were ineffective when injected into the dorso-rostral striatum. Doses of sulpiride about 100 times lower than those injected into the nucleus accumbens were sufficient to evoke an equipotent catalepsy when injected into the ventro-rostral striatum. However, similar doses of haloperidol and SCH 23390, injected into the ventro-rostral striatum and nucleus accumbens, evoked a similar catalepsy. It is concluded that (1) the catalepsy induced by systemic administration of haloperidol seems to result from the action of this drug on both the ventro-rostral striatum and the nucleus accumbens, (2) both D1 and D2 dopamine receptors in the ventro-rostral striatum are involved in the cataleptogenic action of neuroleptics, and (3) in the nucleus accumbens, only D1 dopamine receptors seem to play an important role in this phenomenon.

Lateral hypothalamus-zona incerta region as an output station for the catalepsy induced by the blockade of striatal D1 and D2 dopamine receptors

Our previous study reported that the blockade of GABAA receptors of the lateral hypothalamus-zona incerta region (LH-ZI) by local injections of bicuculline methiodide inhibited the haloperidol-induced catalepsy. The aim of the present study was to determine (1) whether the blockade of GABAA receptors of the LH-ZI may counteract the catalepsy evoked by SCH 23390 and by sulpiride, and (2) whether the GABAA receptors of the LH-ZI affect the function of the striatal dopaminergic system. Bicuculline methiodide (2.5 and 5 ng/side) injected bilaterally into the LH-ZI inhibited in a dose-dependent manner the catalepsy induced by SCH 23390 administered peripherally (0.2 mg/kg s.c.). SCH 23390 (2 micrograms/side) and sulpiride (1 microgram/side) injected bilaterally into the rostroventral part of the striatum induced potent catalepsy. The catalepsy induced by injection of SCH 23390 (2 micrograms) and sulpiride (1 microgram) into the striatum was inhibited by bicuculline methiodide (2.5 ng and 5 ng) injected into the LH-ZI. Neither bicuculline (5 ng/side) nor muscimol (50 ng/side) injected bilaterally into the LH-ZI changed the levels of dopamine and its intraneuronal metabolite, 3,4-dihydroxyphenyl-acetic acid, or the concentration of noradrenaline and 5-hydroxyindole-acetic acid measured in the striatum and nucleus accumbens by HPLC with an electrochemical detection. It is concluded that GABAA receptors of the LH-ZI are an output station for the catalepsy induced by the blockade of the striatal D2 and D1 dopamine receptors.

The ontogeny of SCH 23390-induced catalepsy in male and female rats exposed to ethanol in utero

The maturation of sensitivity to the cataleptic actions of selective D1 dopamine receptor blockade was assessed in rats exposed prenatally to ethanol. Pregnant rats received an average of 12.68 g/kg body weight of ethanol per day through a liquid diet. Control dams were either pair-fed an isocaloric liquid diet without ethanol or given ad lib lab chow. Separate groups of male and female offspring of these dams were administered either 0.33 or 1.00 mg/kg of the selective dopamine D1 receptor antagonist SCH 23390 at postnatal days 13, 17 or 21, and catalepsy was measured 0, 15 and 30 min later. The results are consistent with a hypothesis that prenatal ethanol exposure alters behaviors mediated by nigrostriatal dopamine systems. Overall, prenatal ethanol-exposed animals showed less catalepsy than control rats. Further, there is a gender effect in that dopamine antagonist-induced catalepsy matures earlier in normal male than in normal female controls. Prenatal alcohol exposure appears to delay the emergence of mature behavioral responses to dopamine blockade in male but not female rats.

Chronic low-dose haloperidol effects on self-stimulation rate-intensity functions

Animals responding for biphasic square wave stimulation to the VTA were treated for 26 days with a low dose (0.07 mg/kg) of the neuroleptic haloperidol and tested at 1 h post-injection. Initially the drug induced a pronounced lateral displacement of the baseline rate-intensity function, concomitant with a depression in slope. Over the course of chronic treatment, partial tolerance was observed to the drug-induced increases in threshold concomitant with the onset of a significant suppression in peak response rate. Biochemical tolerance to stimulated dopamine metabolism (as per cent non-drug control) was significant only for mesolimbic (versus neostriatal) regions, in animals receiving haloperidol according to pre- and post-test administration schedules. The observation of sensitization to peak rate reductions parallels previous reports for spontaneous locomotor activity measures and is compatible with depolarization inactivation mechanisms proposed to account for delayed-onset clinical effects. Further, selective biochemical tolerance in mesolimbic regions supports suggestions that mesolimbic dopamine is important as a substrate for subtle low dose neuroleptic effects which may be relevant for studying pharmacotherapeutic treatment issues.

Evidence for the independent role of GABA synapses of the zona incerta-lateral hypothalamic region in haloperidol-induced catalepsy

Bicuculline (0.5-50 ng), injected bilaterally into the zona incerta-lateral hypothalamus (ZI-LH) of the rat, inhibited catalepsy evoked by haloperidol (1 mg/kg s.c.) in a dose-dependent manner. The same effect was obtained by injections of bicuculline directed towards the ventromedial thalamic nucleus (Vm), but then higher doses of the drug were necessary (10-50 ng). Muscimol (10-50 ng), injected into the ZI-LH, evoked a state of catalepsy almost identical to that of haloperidol. Bicuculline (50 ng) abolished the catalepsy evoked by muscimol (25 ng). Bicuculline injected into ZI-LH in doses of 0.5-2.5 ng did not change locomotor activity of rats as measured in photoresistor actometers, whereas it had a slightly stimulating effect at a dose of 5 ng. A comparison between the doses of bicuculline injected into the ZI-LH and Vm suggests that, irrespective of Vm synapses, GABA synapses of this region are involved in the mediation of haloperidol-induced catalepsy. A similar conclusion regarding the catalepsy and rigidity induced by morphine was drawn previously. It seems, therefore, that the catalepsy antagonism of bicuculline is independent of the action of the drug which promotes the locomotor stimulation.

Antipsychotic properties of new N-(4-substituted-1-piperazinylethyl)- and N-(4-substituted-1-piperidinylethyl)-phthalimides

A series of N-(4-phenyl- and 4-pyridyl-1-piperazinylethyl)- and N-(4-phenyl-1-piperidinylethyl)-phthalmides were synthesized and tested for antipsychotic activity. All compounds suppressed the spontaneous motor activity and the apomorphine-induced climbing in mice and pergolide-induced locomotor activity in rats, demonstrating psychotropic properties equal to the corresponding properties of sulpiride. Although the compounds, like sulpiride, were less potent than haloperidol in blocking the locomotor activities, they caused no catalepsy, a major side effect following treatment with conventional antipsychotic agents. It is likely that the new compounds produce their neuroleptic activities through inhibition of limbic dopamine receptors.

Effects of serotonergic activity in nucleus accumbens septi on drug-induced circling

The effects of injections of 5-hydroxytryptamine (5-HT) into the nucleus accumbens and lesions of the nucleus accumbens induced by 5,7-dihydroxytryptamine (5,7-DHT) on drug-induced circling were investigated in rats with unilateral nigrostriatal lesions induced by 6-hydroxydopamine (6-OHDA). Injections of 5-HT (60-120 micrograms in 1 microliter; 1 microliter/min) into the nucleus accumbens caused a significant decrease in the circling response to 5.0 mg/kg of d-amphetamine (s.c.). The distribution of radioactivity after intracerebral injections of [3H]5-HT using these parameters showed that although much of the injected material was retained in the nucleus accumbens there was also considerable spread to the frontal cortex. However, in further behavioural experiments, using an injection procedure (0.5 microliter; 0.11 microliter/min) which caused much greater retention of injected material in the nucleus accumbens, with minimal spread to the frontal cortex, the ability of 5-HT injected into the accumbens to block amphetamine-induced circling was not diminished. Moreover, injections of 5-HT into the frontal cortex did not have any effect on amphetamine-induced circling. Lesions of the nucleus accumbens induced by 5,7-DHT caused a significant enhancement of the contralateral circling response to 1.0 mg/kg of apomorphine and a similar but non-significant tendency to increase the circling responses to several other doses of apomorphine and amphetamine. The results provide evidence that serotonergic mechanisms in the nucleus accumbens inhibit circling behaviour generated by unilateral activation of nigrostriatal dopaminergic mechanisms.

Role of mesencephalic reticular formation in cholinergic-induced catalepsy and anticholinergic reversal of neuroleptic-induced catalepsy

The present experiments investigate the brain sites involved in the elicitation of catalepsy by cholinergic agonists and neuroleptics. Microinjection of acetylcholine chloride (50 micrograms) in combination with eserine (2.5 micrograms) into the ventral mesencephalic reticular formation (MRF) elicited catalepsy. Microinjection of atropine sulfate (5 micrograms) into the same sites reversed the catalepsy of rats treated with haloperidol (1.5 mg/kg) 2 h earlier, but did not reverse morphine-induced (30 mg/kg, 1 h) catalepsy. Haloperidol (25 micrograms) injected into the nucleus accumbens septi (NAS) resulted in catalepsy as severe as that caused by an identical injection into the caudate nucleus. Catalepsy caused by intraNAS haloperidol occurred with a shorter latency than that resulting from intracaudate haloperidol, and was reversed by systemic scopolamine (0.4 mg/kg). On the basis of these results it is suggested that the ventral MRF is a site for the elicitation of catalepsy by cholinergic agonists and for the reversal of neuroleptic-induced catalepsy by anticholinergics, and that neuroleptic-induced catalepsy involves blockade of dopamine receptors in both the NAS and caudate nucleus.

Cataleptogenic potency of the antipsychotic drugs is inversely correlated with neuronal activity in the amygdaloid complex of the rat

At doses known to elicit catalepsy in rats, haloperidol (1.0 and 2.0 mg/kg) and pimozide (4.0 mg/kg), injected intraperitoneally, failed to alter the spontaneous activity of neurons in the amygdaloid complex of locally anesthetized, immobilized rats. In contrast, clozapine and thioridazine, which are devoid of cataleptic effects even at high doses (10.0 and 20.0 mg/kg), caused a dramatic and prolonged increase in firing rate, whereas chlorpromazine, which produces relatively mild catalepsy at doses of 5.0 and 10.0 mg/kg, produced an intermediate response. These results, which were obtained throughout the amygdaloid complex, indicate that the cataleptogenic potency of the antipsychotic drugs is inversely correlated with their ability to accelerate neuronal activity. This finding is discussed in relation to the known mechanisms of action of these drugs on various neurotransmitter systems in the amygdaloid complex.

Kainic acid injections in the striatum alter the cataleptic and locomotor effects of drugs influencing dopaminergic and cholinergic systems

Rats with bilateral injections of kainic acid into the striatum were tested for their motor responsiveness to drugs influencing dopaminergic and cholinergic systems. The kainic acid-induced lesions potentiated the locomotor response to both the dopaminergic agonist, d-amphetamine, and the cholinergic antagonist, scopolamine, attenuated the cataleptic response to the dopaminergic antagonist, haloperidol, and potentiated the cataleptic acid and convulsive responses to the cholinergic agonist, pilocarpine. The analogy of these pharmacological effects with those induced by similar drugs in patients with Huntington's disease supports the view that this animal preparation is a useful model of Huntington's disease. The opposite effects of haloperidol and pilocarpine on catalepsy in kainic acid-lesioned rats suggest that more work should be done to elucidate the mechanism behind this conflict before full support can be given to its use as a model system for evaluating possible pharmacotherapy in Huntington's disease.

Differential actions of classical and atypical antipsychotic drugs on spontaneous neuronal activity in the amygdaloid complex

Classical antipsychotic drugs such as haloperidol produce akinesia and catalepsy, whereas clozapine and related atypical antipsychotics fail to elicit these behaviors even at relatively high doses. Despite these behavioral differences, a cataleptic dose of haloperidol (2.0 mg/kg) produces changes in neuronal activity in the neostriatum and nucleus accumbens comparable to those produced by a non-cataleptic dose of clozapine (20.0 mg/kg). To further elucidate the brain mechanisms underlying the differential behavioral response to these drugs, an electrophysiological analysis was extended to neurons in the rat amygdaloid complex. Whereas an intraperitoneal injection of 2.0 mg/kg haloperidol generally failed to alter the firing rate of amygdaloid neurons, 20.0 mg/kg clozapine typically produced a prolonged increase in activity. Similarly, clozapine, but not haloperidol, reversed the depression of firing rate produced by 1.0 mg/kg d-amphetamine. The results suggest that neurons in the amygdaloid complex are more responsive to antipsychotic drugs devoid of extrapyramidal side effects than to antipsychotics which elicit parkinsonian-like motor dysfunctions.

Haloperidol-induced catalepsy is mediated by postsynaptic dopamine receptors

Antipsychotic or neuroleptic drugs, which block central dopamine receptors, produce a behavioural state in animals in which they fail to correct externally imposed postures. This is referred to as catalepsy. Previous lesion studies have shown that the dopamine receptors in the striatum are involved in this neuroleptic-induced catalepsy. Dopamine receptors, identified by the specific, high-affinity binding of the potent neuroleptic haloperiodol, have been shown to be equally distributed postsynaptically on striatal neurones and presynaptically on cortico-striatal terminals. Because the electrolytic lesioning studies have unavoidably damaged both pre- and postsynaptic striatal dopamine receptors, it is not known whether these two receptors are separately involved in neuroleptic-induced catalepsy. Using kainic acid and cortical ablation to destroy postsynaptic and presynaptic dopamine receptors, respectively, the present study demonstrates that the cataleptic effects of haloperidol are apparently mediated by dopamine receptors localised postsynaptically on striatal neurones.

"Classical" and "atypical" antipsychotic drugs: differential antagonism of amphetamine- and apomorphine-induced alterations of spontaneous neuronal activity in the neostriatum and nucleus accumbens

The ability of clozapine and haloperidol to antagonize the depression of firing rate produced by d-amphetamine and apomorphine in the neostriatum and nucleus accumbens was tested in immobilized, locally anesthetized rats. In the neostriatum, an intraperitoneal injection of 2.5 mg/kg d-amphetamine or 1.0 mg/kg apomorphine produced a prolonged inhibition of neuronal activity that was reversed by a subjsequent injection of either 20 mg/kg clozapine or 2.0 mg/kg haloperidol. An analysis of the onset and magnitude of the blockade revealed that clozapine was more effective than haloperidol in reversing the amphetamine response but that both antipsychotic drugs produced a comparable blockade of the apomorphine-induced depression. Similar results were obtained in the nucleus accumbens. The data indicate that although clozapine acts equieffectively in the neostriatum and nucleus accumbens, this atypical antipsychotic drug, aside from blocking postsynaptic dopamine receptors, may exert at least some of its effects by preventing dopamine release.

The involvement of serotonergic neurons in the central nervous system as the possible mechanism for slow head-shaking behavior induced by methamphetamine in rats

Following the IV administration of d-methamphetamine (MA), rats showed slow head shaking (SHS) and stereotyped gnawing (SG) behaviors in a dose-dependent manner. Methysergide, cyrpoheptadine, and p-chlorophenylalanine given intracerebroventricularly (ICV) or systemically significantly blocked SHS behavior induced by 10 mg/kg MA. Combined administration of L-5-hydroxytryptophan and peripheral decarboxylase inhibitor (Ro 4-4602) enhanced SHS behavior. Tyrosine hydroxylase inhibitor (H44/68) blocked SG behaviors, but dopamine-beta-hydroxylase inhibitors (FLA 63 and U-14, 624) and combined administration of L-3,4-dihydroxyphenylalanine and Ro-4-4602 enhanced it. These drugs did not affect SHS behavior. Phentolamine, phenoxybenzamine, clonidine, isoproterenol, and propranolol given ICV or systemically showed no effect on either SHS or SG behaviors. These results suggest that SHS behavior is produced by the activation of seronergic neurons in the central nervous system and are consistent with the view that SG behaviors are mediated through the release of dopamine. Some neuroleptics inhibited SHS as well as SG behaviors, but the older of inhibitory activity of neuroleptics onSHS behavior was quite different from their effects on SG behaviors induced by MA or apomorphine.