Contrasting effects of morphine on schedule-controlled behavior in the chimpanzee and baboon

Schedule-controlled key pressing was maintained in two chimpanzees and three baboons under a multiple 10-minute fixed-interval (FI 10-min) 30-response fixed-ratio (FR 30) schedule of food delivery. Characteristic rates and patterns of responding were maintained under the FI and FR schedules, and the performance of the two species differed in no systematic way. The acute i.m. administration of morphine (0.1-3.0 mg/kg) prior to selected 2-hour sessions increased mean rates of responding under the FI schedule in the chimpanzee, but decreased responding in the baboon. At a dose of 3.0 mg/kg of morphine, responding under the FI schedule in the chimpanzee increased 4-fold and responding in the baboon decreased to less than 25% of control levels. Mean response rates under the FR schedule were also increased by morphine in the chimpanzee, but responding under the FR schedule was little affected in the baboon except at the higher doses which decreased response rates below control levels. Respiratory rate in the chimpanzee was markedly depressed at 5.6 mg/kg of morphine and one chimpanzee died. A similar depression of respiration was not observed in the increase responding in a nonhuman primate, the chimpanzee, and that the behavioral effects of morphine in the chimpanzee are qualitatively different from the effects in monkeys.