SKF 38393 and apomorphine modify locomotion and exploration in rats placed on a holeboard by separate actions at dopamine D-1 and D-2 receptors.

Horizontal motor activity, rearing and head dipping were recorded automatically in rats placed on a holeboard and taken as indices of locomotion and exploration. Other behaviours were assessed visually using a video camera. SKF 38393, 15-30 mg/kg (D-1 agonist), suppressed all three behavioural measures more effectively in habituated than in naive rats. These actions were blocked by SCH 23390 (0.05 mg/kg, D-1 antagonist) but not by haloperidol (0.05 mg/kg, D-2 antagonist). Apomorphine modified behaviour biphasically, causing haloperidol-resistant sedation at a low, presynaptic dose (0.05 mg/kg) and haloperidol-sensitive hyperactivity and stereotyped sniffing at a larger, postsynaptic dose (0.5 mg/kg, especially after habituation). Neither action was antagonised by SCH 23390. The results support the conclusion that D-1 and D-2 dopamine recognition sites can separately influence the processes controlling locomotion and exploratory activity in normal rats.