Ontogeny of biphasic locomotor effects of quinpirole

The effects of the dopamine D2/D3 receptor agonist quinpirole (LY171555) on locomotor activity were tested on rats of 10, 15, 20, 30, and 60 days of age. In two separate experiments, doses of 0 (vehicle), 0.02, 0.2, or 2.0 mg/kg quinpirole were injected SC into rats at each age, and their effects measured either for 2 h at 15-min intervals, or 30 min at 5-min intervals. At 10, 15, and 20 days of age, quinpirole significantly increased distance travelled in a dose-dependent manner. At 30 and 60 days of age, quinpirole significantly decreased distance travelled early in the session and increased it later. These results suggest that a dopamine autoreceptor begins to function between 20 and 30 days of age. Concomitant with the appearance of quinpirole-induced locomotor suppression early in the session, the amount of quinpirole-induced activation late in the session declined.