beta-Endorphin involvement in the antidopaminergic effect of caerulein.

Caerulein has been shown to possess a long-lasting antagonistic effect on amphetamine hyperactivity in rats when given in combination with haloperidol. We found that this effect of caerulein involved beta-endorphin. Naloxone pretreatment and hypophysectomy abolished the caerulein effect, while intracerebroventricular or intra-nucleus accumbens injection of beta-endorphin together with haloperidol administration produced an effect similar to that of caerulein. The results suggest that the long-term antagonism of the amphetamine effect of caerulein is mediated by the endogenous opioid beta-endorphin.