Changes in response rates and reinforcement thresholds for intracranial self-stimulation during morphine withdrawal

Rats were implanted with stimulating electrodes in the medial forebrain bundle-lateral hypothalamus and were trained in an auto-titration brain self-stimulation paradigm. When response rates and reinforcement thresholds were stable, the animals were implanted with subcutaneous osmotic minipumps (Alzet, 2ML1) which continually delivered morphine (1.2 mg/kg/hr as the base, n = 16) or saline (10.0 microliter/hr, n = 11). After one week the pumps were removed, and the animals were again tested in the auto-titration paradigm following the daily administration of either saline (spontaneous withdrawal) or 1.0 mg/kg naloxone (precipitated withdrawal). During the eight-day withdrawal phase there was a decrease in the rate of lever-pressing for the morphine dependent animals and this was greatest on the first day. The magnitude of the decrease was greater in the precipitated withdrawal group than in the spontaneous withdrawal group and an increase in the reinforcement threshold occurred only with precipitated withdrawal. Animals in both groups lost weight when measured each morning, but the precipitated group showed greater weight loss during the day. In addition, animals in the precipitated withdrawal group had diarrhea and showed a higher incidence of withdrawal signs than both the non-dependent (control) and spontaneous withdrawal groups. These experiments provide a detailed account of opiate withdrawal following the continuous subcutaneous infusion of a small dose of morphine for one week.