Modifications of ambulation-increasing effect of morphine and buprenorphine following repeated administration in mice

Interaction of behavioral effects of morphine (10 and 20 mg/kg s. c.) and buprenorphine (0.3 and 1 mg/kg s. c.) were investigated by means of ambulatory activity following repeated administration of either drug in mice. The repeated administration of buprenorphine (1 mg/kg, but not 0.3 mg/kg) total of 5 times at intervals of 3-4 days induced a gradual decrease in the sensitivity to its ambulation-increasing effect. The buprenorphine-experienced mice showed an increase in the sensitivity to the ambulation-increasing effect of morphine (20 mg/kg) in parallel with the doses of buprenorphine. On the other hand, the ambulation-increasing effect of morphine (10 and 20 mg/kg) progressively enhanced during the repeated administration schedule. The morphine-experienced mice demonstrated a decrease in the sensitivity to buprenorphine (1 mg/kg). The present results may reflect differential characteristics between morphine and buprenorphine in their ambulation-increasing effects in mice.