The effects of some atypical neuroleptics on apomorphine-induced behaviors as a measure of their relative potencies in blocking presynaptic versus postsynaptic dopamine receptors.

The effects of the atypical neuroleptics clozapine, thioridazine and sulpiride on behaviors induced by apomorphine were recorded, using a time-sampling observational paradigm. A low dose of apomorphine (0.1 mg/kg, SC) produced hypomotility. Of the neuroleptics tested, only sulpiride antagonized this hypomotility. Apomorphine in higher doses (0.2-1.0 mg/kg, SC) produced stereotyped behaviors (sniffing down and licking or gnawing). All three atypical neuroleptics antagonized stereotypy. The effects of sulpiride on apomorphine-induced hypomotility and stereotypy are consistent with the notion that this drug has strong presynaptic and weak postsynaptic blocking effects at dopamine receptors. The mechanisms of action of clozapine and thioridazine may be different from that of sulpiride. Perhaps the anticholinergic activities of these drugs mediate some of their behavioral effects. The effects of these atypical neuroleptics on apomorphine-induced stereotypy are opposite in direction to their effects on amphetamine-induced stereotypy, suggesting that these two behavioral patterns are not measures of the same neural process.