Behavioral evidence for the involvement of endogenous opioids in the interaction between cholecystokinin and brain dopamine systems.

The hyperactivity induced by injection of the dopamine (DA) agonist apomorphine into the nucleus accumbens of rats was dose dependently inhibited by intra-accumbal pretreatment with cholecystokinin (CCK-8) (ED50, 0.34 ng). Local treatment with the opioid antagonist naloxone antagonized this inhibitory action of CCK-8 (ED50, 18 ng), but did not change the blocking effect of haloperidol on the apomorphine-induced hyperactivity. These data show that endogenous opioids are concerned in certain interactions between CCK-8 and DA systems.