Alteration of the neuronal response to amphetamine in the neostriatum by pretreatment with a centrally acting anticholinergic

Amphetamine-induced changes in behavior and caudate extracellular acetylcholine

In vivo microdialysis was used to study the effects of amphetamine on caudate extracellular acetylcholine and to compare these effects to the drug-induced behavioral response profile. Consistent with an inhibitory dopamine/acetylcholine interaction, the dopamine receptor agonist, apomorphine, decreased acetylcholine concentrations, while the dopamine receptor antagonist, haloperidol, increased acetylcholine. In contrast, an intermediate dose of amphetamine (1.75 mg/kg), did not significantly alter acetylcholine levels. Furthermore, a higher dose of amphetamine (5.0 mg/kg) promoted a two-fold increase in acetylcholine levels, and the increase paralleled the appearance of oral stereotypies in these animals. These results suggest that the effects of amphetamine on caudate acetylcholine, which may be implicated in the appearance of stereotyped behaviors, are not strictly dependent on caudate dopamine receptor activation.

Cholinergic blockade with scopolamine in adult cats. Effects on the behaviors evoked by apomorphine and amphetamine

1. The aim of this work is to analyse the role that the cholinergic system could play in the production of the behaviors evoked by apomorphine and amphetamine in adult cats. These two drugs were injected s.c. in separate sessions, before and after a s.c. administration of scopolamine which blocked the muscarinic receptors. The pre and post-scopolamine results of the behaviors produced by the two catecholaminergic drugs were compared using the non-parametric Wilcoxon signed rank test. 2. In a previous step a dose-response study of the behavioral effects of scopolamine, in doses of 0.05, 0.1, 0.4 and 0.8 mg/kg was carried out in ten cats. The Kruskal-Wallis and the non-parametric multiple comparison tests were employed. A dose-dependent decrease in motility (locomotion) and a dose-dependent increase in inappetence and pupillary dilation were found. 3. In thirteen cats which were injected with 2 mg/kg of apomorphine and 2.5 mg/kg of amphetamine the findings were: 1--apomorphine after scopolamine produced a decrease in the hypermotility, compared with the results observed with the former drug previous to scopolamine; 2--with amphetamine an increase in immobility and a decrease in indifference were observed. 4. The authors conclude that the decrease in motility recorded with apomorphine and amphetamine after scopolamine, could be attributed to the proper effect of scopolamine. No explanation could be found for the decrease in indifference found by injecting amphetamine after scopolamine. 5. Considering the antagonistic effect between the dopaminergic and the cholinergic systems and that the latter one has an arousal effect, we postulate that the behavioral indifference produced by apomorphine and amphetamine could be the result of a kind of blockade of the cholinergic system when the catecholaminergic system is activated through the administration of the two cited drugs.

Cholinergic modulation of stimulant-induced behavior

Stimulant-induced stereotypy, presumably mediated by nigrostriatal dopaminergic neurons, can be altered by the administration of cholinergic agonists or antagonists. Cholinergic, striatal interneurons have been postulated as the site of these effects, although the specific site of interaction between cholinergic and dopaminergic systems is unknown. The study reported here examined the effects of the cholinesterase inhibitor physostigmine, and several other cholinergic and anticholinergic drugs, on stimulant-induced behavior. Stereotypy was inhibited by physostigmine to the same degree whether induced by direct (apomorphine) or by indirect acting (amphetamine and methylphenidate) stimulants. The results are interpreted as indicating that the site of cholinergic modulation of stimulant-induced stereotypy is postsynaptic to the dopaminergic neurons which mediate stereotypy.

"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.