Effects of acute and chronic clozapine and haloperidol on in vitro release of acetylcholine and dopamine from striatum and nucleus accumbens

DR Compton and KM Johnson

Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston.

The purpose of this investigation was to determine if striatal or nucleus accumbens dopamine (DA) release, ACh release or DA receptor function were altered by acute and chronic haloperidol or clozapine treatment in a manner consistent with the reported pharmacological effects of each drug on A9 and A10 DA cell bodies and projection areas, when experiments were performed without a drug-free, or washout, period after drug treatment. The release of neurotransmitters reported here was evaluated using a slice-superfusion assay system. Transmitter release was induced either by an electrical field (for DA and ACh) or by application of either amphetamine or amfonelic acid (DA only). Dopaminergic receptor function was assessed by inhibiting electrically stimulated ACh release with in vitro TL-99 (a dopaminergic agonist) and by reversing that inhibition with in vitro neuroleptics or with ex vivo experimental paradigms (the in vitro analysis of transmitter release subsequent to in vivo drug administration). These data suggest that although there are differences between haloperidol and clozapine, there is no difference between the degree of postsynaptic DA receptor blockade produced that can be attributed to the duration of neuroleptic treatment. Chronic clozapine (20 mg/kg x 21 days) reversed TL-99- induced inhibition of ACh release in the nucleus accumbens only, whereas chronic haloperidol (0.5 mg/kg x 21 days) produced a similar reversal in both brain areas. One possible explanation for the lack of effect of chronic clozapine treatment in the striatum is that carrier- mediated (amphetamine-stimulated) DA release is enhanced in the striatum but not in the nucleus accumbens, suggesting that the potential DA receptor block in the striatum may be compromised by enhanced striatal DA levels. Acute haloperidol (0.5 mg/kg) was found to increase electrically stimulated ACh release in the striatum and DA release in the nucleus accumbens. Tolerance developed in the striatum, but not the nucleus accumbens, with repeated administration. However, acute clozapine had no effect on ACh release in either area, but it was found to enhance DA release in the striatum, an effect to which tolerance developed with chronic administration. Further, comparison of these data with data obtained using haloperidol and clozapine in vitro suggests that it is unlikely that these effects are due to residual drug still present in these tissues at the time of experimentation. These data are discussed with regard to electrophysiological and pharmacological differences observed between clozapine and haloperidol on the activity of A9 and A10 DA cells after chronic neuroleptic treatment.

Volume 248, Issue 2, pp. 521-530, 02/01/1989
Copyright © 1989 by American Society for Pharmacology and Experimental Therapeutics

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