Biphasic effect of D-2 agonist quinpirole on locomotion and movements.

The effects of the D-2 agonist quinpirole on forward progression, and on vertical and lateral movements, were measured for 2 h in rats injected with either saline, 0.03, 0.125, 0.5 or 8 mg/kg of the drug. Results showed that the drug had a biphasic effect: the lowest dose decreased and the high doses increased the amount of locomotion and of movement. The decrease in activity produced by the low dose had its onset within minutes after administration of the drug; persistent and marked hyperactivity was observed from about 60-80 min after injection of 0.5-8 mg/kg of quinpirole. Moreover, in animals injected with intermediate doses, the excitation was preceded by a brief period of reduced locomotion. It is suggested that the biphasic effect may reflect two independent actions of the drug, possibly on activity in the nucleus accumbens.