ONTOGENY OF BIPHASIC LOCOMOTOR EFFECTS OF QUINPIROLE

The effects of the dopamine D-2/D-3 receptor agonist quinpirole (LY171 555) on locomotor activity were tested on rats of 10, 15, 20, 30, and 60 days of age. In two separate experiments, doses of O (vehicle), 0.0 2, 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 sig nificantly increased distance travelled in a dose-dependent manner. At 30 and 60 days of age, quinpirole significantly decreased distance tr avelled early in the session and increased it later. These results sug gest that a dopamine autoreceptor begins to function between 20 and 30 days of age. Concomitant with the appearance of quinpirole-induced lo comotor suppression early in the session, the amount of quinpirole-ind uced activation late in the session declined.