Receptor selectivity of cholecystokinin effects on mesoaccumbens dopamine neurons

Extracellular recording techniques were combined with antidromic stimulation to examine the effects of C-terminal cholecystokinin (CCK) fragments and CCK antagonists on the activity of identified mesoaccumbens dopamine (MADA) neurons in chloral hydrate-anesthetized rats. These experiments were designed to determine the receptor selectivity of sulfated CCK octapeptide (CCK-8S) effects on MADA cells. Neither CCK tetrapeptide (CCK-4) nor unsulfated CCK octapeptide (CCK-8U) significantly altered MADA cell basal firing rate or responsiveness to the inhibitory effects of the D2 DA agonist quinpirole. As reported previously for ventral tegmental area DA cells, CCK-8S produced increases or decreases in the firing rate of most MADA cells sampled. CCK-8S also enhanced the sensitivity of MADA neurons to quinpirole-induced inhibition. This increase in sensitivity to quinpirole was blocked by pretreatment with the nonselective CCK receptor antagonist proglumide and the preferential CCK-A receptor antagonist CR 1409 but not by the preferential CCK-B receptor antagonist L-365,260. The inactivity of CCK-4 and CCK-8U in these tests and the results with the antagonists suggest that the effects of CCK-8S on MADA neuronal activity are mediated by CCK-A receptors.