Comparison of the effects of the cholecystokinin-B receptor antagonist, PD 134308, and the cholecystokinin-A receptor antagonist, L-364,718, on dopamine neuronal activity in the substantia nigra and ventral tegmental area.

Extracellular single-unit recording techniques were used to study the effects of the cholecystokinin-A (CCK-A) antagonist, L-364,718, and the CCK-B antagonist, PD 134308, on DA neuronal activity in chloral hydrate anesthetized rats. Neither L-364,718 (0.1-1.6 mg/kg i.v.) nor PD 134308 (0.1-6.4 mg/kg) altered the basal firing rate of substantia nigra or ventral tegmental area DA neurons. The ability of PD 134308 and L-364,718 to alter the apomorphine-induced inhibition of substantia nigra DA neurons was assessed. Pretreatment with L-364,718 (0.6 or 4.16 mg/kg i.v.) did not shift the apomorphine dose-response curve (0.5-32 micrograms/kg i.v.). In contrast, PD 134308 (0.6 or 6.4 mg/kg i.v.) produced dose-related, significant shifts to the right of the apomorphine dose-response curves. However, these effects were small in comparison to the haloperidol (0.1 mg/kg i.p.)-induced shift of the apomorphine curve. These data suggest that in the substantia nigra there may be a tonic level of CCK release that, through actions on CCK-B receptors, may modulate DA agonist-induced inhibition of DA neuronal activity.