Morphine antagonizes pentobarbital-induced anesthesia.

Morphine was administered by the intracerebroventricular (i.c.v.) route to pentobarbital-anesthetized rabbits. Small doses of morphine (less than 150 micrograms) potentiated, but larger doses (greater than 250 micrograms) shortened, the duration of anesthesia. In naltrexone-pretreated animals, all doses of morphine employed acted only as an analeptic. Atropine, but not atropine methylbromide, blocked the analeptic effect of morphine, indicating that a central cholinergic mechanism was involved in this response. Tolerance to the analeptic effect was not evident. These results suggest that morphine exerts an arousal action which is usually masked by the dominant narcotic properties, but which becomes evident when administered intracerebroventricularly or in the presence of naltrexone.