Potentiation of haloperidol-induced catalepsy by dopamine agonists: possible involvement of central 5-hydroxytryptamine

Apomorphine (0.12--2 mg/kg, SC) and d-amphetamine (1--8 mg/kg, IP) were each able, at certain doses, to potentiate the cataleptic state produced by the neuroleptic agent, haloperidol (1 mg/kg, IP). In subsequent biochemical experiments, in which the effects of combinations of apomorphine or d-amphetamine and haloperidol on brain monoamine levels were studied, this behavioural observation was seen to be related to an enhanced utilisation of 5-hydroxytryptamine (5-HT) in certain brain regions. The results suggest not only the possible involvement of 5-HT in the production of catalepsy, but also that the effects of these 'classical' dopamine agonists on other central transmitter systems should be considered when interpreting their various behavioural responses.