Single and repeated administration of neuroleptic drugs to rats: effects on striatal dopamine-sensitive adenylate cyclase and locomotor activity produced by tranylcypromine and L-tryptophan or L-Dopa.

Injection of tranylcypromine and L-tryptophan results in rats displaying behavioural changes including hyperactivity, probably due to stimulation of post-synaptic 5-hydroxytryptamine (5-HT) receptors. Increased locomotor activity of a different type is elicited by injection of tranylcypromine and L-dopa, a procedure which increased dopaminergic function in the brain. It has now been demonstrated that the neuroleptic drugs, chlorpromazine, alpha-flupenthixol, haloperidol and spiroperidol block both syndromes. The inhibition produced by these drugs on 5-HT-induced hyperactivity is probably because a dopaminergic system is involved in the behavioural expression of the 5-HT induced hyperactivity. The structurally related drugs with no neuroleptic activity (ethopropazine, promethazine and beta-flupenthixol)are without effect on thses hyperactivity syndromes. Also ineffective were the neuroleptics pimozide and clozapine. Striatal dopamine sensitive adenylate cyclase activity in vitro was inhibited by the administration of chlorpromazine (100 mg/kg) in vivo. Rats treated for 4 or more days with chlorpromazine, alpha-flupenthixol, spiroperidol and haloperidol subsequently showed enhanced locomotor activity in response to tranylcypromine and L-Dopa. Administration of those drugs which did not block hyperactivity acutely did not result in enhancement. Only chlorpromazine, when given for 4 days, enhanced the hyperactivity response following tranylcypromine and L-tryptophan, probably because the drug also blocks 5-HT receptors. In rats displaying enhanced behavioural responses no evidence was found for enhanced sensitivity of striatal adenylate cyclase to dopamine.