Non-opiate beta-endorphin fragments and dopamine--III. gamma-Type endorphins and various neuroleptics counteract the hypoactivity elicited by injection of apomorphine into the nucleus accumbens.

The hypoactivity in rats induced by small doses of apomorphine, injected bilaterally into the nucleus accumbens area of the brain, could be antagonized by pretreatment with the neuroleptic-like neuropeptide des-enkephalin-gamma-endorphin (DE gamma E, beta-endorphin 6-17) as well as with the neuroleptic drugs haloperidol, sulpiride and clozapine injected into the accumbens. Dose-response studies revealed that a dose of 100 pg DE gamma E completely inhibited the apomorphine-induced hypomotility. The influence of DE gamma E appeared to be specific for gamma-type endorphins, since alpha-type endorphins were inactive in this respect. Treatment with DE gamma E injected into the accumbens for 4 days resulted in an enhancement of apomorphine-induced hypoactivity. It is concluded that gamma-type endorphins may control the activity of dopaminergic transmission in the nucleus accumbens, a suggestion which may be of significance for the purported neuroleptic-like and antipsychotic action of gamma-type endorphins.