Biphasic effects of morphine on locomotor activity in hamsters

Two experiments investigated the dose and time related effects of morphine sulfate on the running wheel activity of golden Syrian hamsters. Compared with saline controls, a low dose (5 mg/kg) increased locomotor activity, whereas high doses (10, 20 and 40 mg/kg) produced an initial dose-related depression in activity, a gradual dose-related recovery and finally a period of hyperactivity. The results are discussed in terms of similar biphasic effects of morphine observed in rats.