Weight loss and shock-elicited aggression as indices of morphine abstinence in rats

Weight loss and shock-elicited aggression have been compared as quantitative indices of morphine abstinence in rats. A range of doses of morphine was administered to rats by i.p. injection twice daily for 12-15 days. After injections were stopped, morphine-abstinent rats lost weight precipitously, and showed an increased frequency of fighting in response to aversive stimulation (foot-shock). Recovery of weight appeared complete after 15-20 days but a significant increase in aggression was found at 18 days post-withdrawal; this virtually disappeared after 52 days. Both the amount of weight lost and the frequency of fighting increased as a function of the previous maintenance dose of morphine; the effective dose range appeared similar for these two indices. Weight loss was much less variable than fighting, had the advantage of rapid, objective measurement, and appeared to be the more reliable index of abstinence.