Dopamine D2 receptor mediation of the discriminative stimulus properties of LY 171555 (quinpirole).

The mechanisms subserving the behavioral effects of the putatively selective dopamine D2 agonist LY 171555 (0.025 mg/kg) were analyzed in rats (N = 23) using a two-lever, water-reinforced drug discrimination task. Substitution (generalization) tests showed that the DA agonist apomorphine mimicked LY 171555 while the selective D1 agonist SKF 38393 did not. When given in combination with the training drug, haloperidol but not the D1 antagonist Sch 23390 dose dependently blocked the LY 171555 cue; given alone, Sch 23390 mimicked LY 171555 and haloperidol elicited responding on the saline-appropriate lever. These results suggest that stimulus properties of LY 171555 are mediated primarily by D2-containing neuronal systems but that the effects of stimulating either D1 or D2 receptor subtypes in vivo are complex and interdependent.