Characterization of withdrawal syndrome of morphine-dependent rats prepared by intermittent infusion technique.

The morphine withdrawal syndrome was studied in rats which had been made dependent on morphine administered by the intermittent infusion technique. Rats made rapidly dependent on morphine by an hourly infusion of 0.12-4 mg/kg/h showed a withdrawal syndrome when they were abruptly withdrawn, after infusion for 7 days, or when they were challenged by naloxone after infusion for 4 days. Abruptly withdrawn rats showed a marked weight loss and other mild symptoms. The weight loss seems to mainly due to anorexia, partly because it was attenuated by IV feeding throughout the withdrawal and partly because the fasted rats showed a weight loss comparable to the withdrawn rats. The naloxone-precipitated withdrawal syndrome showed characteristics, which, from their time course of incidence and their three groups; motor excitation, cholinergic signs, and others. These groups and their interrelationships were discussed. All characteristics were suppressed by deep anesthesia with ether or pentobarbital. A sudden fall in blood pressure was indicated in the anesthetized morphine-dependent rats immediately after the naloxone challenge. This suggests that the intrinsic withdrawal syndrome was progressing even under anesthesia.