Potentiation of licking in rats by stimulation of both D-1 and D-2 dopamine receptors.

Apomorphine-induced licking in rats was assessed by recording the total number of licks by direct observation. Apomorphine induced licking dose dependently. The maximum response was obtained by 0.5 mg/kg of the drug and 30 min after drug administration. Pre-treatment with dopamine antagonists, sulpiride and SCH 23390 decreased the apomorphine effect. Pre-treatment of animals with reserpine+α-methyl-p-tyrosine (AMPT) increased apomorphine-induced licking. In normal rats the D-2 agonist quinpirole and the D-1 agonist SKF 38393 also induced significant licking. The effect of quinpirole or SKF 38393 was decreased by reserpine+AMPT pre-treatment. Combined treatment with SKF 38393 and quinpirole induced more intense licking in both reserpinized and non-reserpinized animals. It is therefore concluded that the apomorphine-induced licking is mediated through both D-1 and D-2 receptors, and that pre-treatment with reserpine hypersensitizes these receptors to the drug effect. However, for either SKF 38393- or quinpirole-induced licking, the presence of endogenous dopamine seems essential.