Abstract
Subtype-selective antagonists of the peripheral-type (CCK-A) and the central-type (CCK-B) cholecystokinin (CCK) receptors were employed to determine the receptor subtype(s) mediating the modulatory actions of CCK on dopamine-induced changes in exploratory activity at three sites in the mesolimbic pathway of the rat. The CCK-A antagonist L-364,718 (10 ng) blocked CCK potentiation of dopamine-induced hyperlocomotion in the medial posterior nucleus accumbens. The CCK-B antagonist CI-988 (20 ng) blocked CCK inhibition of dopamine-induced hyperlocomotion in the anterior nucleus accumbens. The CCK-B antagonists CI-988 (20 ng) and L-365,260 (10 ng) blocked CCK potentiation of dopamine-induced hypolocomotion in the ventral tegmental area. These data indicate a CCK-B pharmacology in the cell body and anterior terminal field, and a CCK-A pharmacology in the posterior terminal field, of the mesolimbic dopamine pathway. Behavioral analyses using the selective CCK antagonists did not detect a contribution of endogenous CCK to exploratory locomotion. L-364,718 (10 ng), L-365,260 (10 ng), and CI-988 (20 ng or 2 micrograms), microinjected into the medial posterior nucleus accumbens, anterior nucleus accumbens, or ventral tegmental area, had no effect on baseline exploratory locomotion or on dopamine-induced changes in exploratory locomotion. Using a dark-induced hyperlocomotion paradigm, the CCK antagonists at these doses at these sites and intraperitoneally had no effect on the high levels of exploratory locomotor activity exhibited by the rats in the dark testing environment, or the lower levels of exploratory activity in the lighted environment. Endogenous CCK may not be released during dopamine-induced hyperlocomotion or dark-induced hyperlocomotion, or endogenous CCK may not contribute significantly to exploratory behaviors mediated through the mesolimbic dopamine pathway. Utilization of these potent, selective, nonpeptide CCK antagonists, with the doses, vehicles, and routes of administration developed in the present studies, will guide further investigations into the role of endogenous CCK in other facets of mesolimbic function.