Anticholinergic properties of antipsychotic drugs and their relation to extrapyramidal side-effects

The effects of haloperidol, alone and in combination with atropine, were compared with the effects of clozapine, alone and in combination with physostigmine, in a variety of tests commonly used to characterize neuroleptic compounds. It was found that clozapine in combination with physostigmine did not present the profile of activity of a classical neuroleptic agent; neither did haloperidol in combination with atropine present that of clozapine. In fact, some effects of haloperidol (catalepsy) were antagonized by atropine, while others (induction of striatal DA-receptor hypersensitivity) were enhanced. It is concluded that the interaction between dopaminergic and cholinergic systems in the striatum is highly complex, and that a neuroleptic possessing both potent DA-receptor blocking and muscarinic anticholinergic activity, while being less likely to cause parkinsonism in patients, would be more likely to induce tardive dyskinesias.