Schedule-controlled behavior in the morphine-dependent rat.

The behavioral effects of acute and chronic administration of morphine and its withdrawal were studied using schedule-controlled responsing in the rat under a differential reinforcement of low rates of responding (DRL) schedule of food presentation. Acute morphine administration had a biphasic effect on response rate. Low dses (1.8-5.6 mg/kg) tended to produce a small increase and higher doses (10-30 mg/kg) decreased responding. Physical dependence was produced by twice daily injections, with an initial dose of 40 mg/kg/day which was increased by 80 mg/kg/day until reaching 600 mg/kg/day which was continued for 14 days. Throughout chronic administration the pattern of responding remained disrupted resulting in a 27-47 percent decrease in presentations of the reinforcer, while response rate was more variable and generally decreased. The effects of morphine withdrawal lasted 5 days and produced an initial maked decrease in reinforecements per hour and a biphasic change in response rate. A marked decrease in responding early in withdrawal (22.5 hr) was followed by a marked and more prolonged (70.5-118.5 hr) response rate increase.