Effects of morphine, naltrexone, and dextrorphan in untreated and morphine-treated pigeons

Six pigeons, trained to peck a response key on a fixed-ratio 20 schedule of food reinforcement, were used to examine the effects of morphine, naltrexone, and dextrorphan, before, during, and after chronic treatment with increasing doses of morphine (10.0-100.0 mg/kg/day). Tolerance developed to the rate-decreasing effect of the daily maintenance doses of morphine within 2 days of each dose increase. A small amount of tolerance to morphine and supersensitivity to naltrexone was evident within the 1st week of morphine treatment (10.0 mg/kg/day). Continued administration of morphine (32.0-100.0 mg/kg/day) produced further tolerance to morphine and supersensitivity to naltrexone, as evidenced by a 5-fold increase in the dose of morphine, and 1,000-fold decrease in the dose of naltrexone, necessary to suppress responding. By the 4th week of treatment (100.0 mg/kg/day), a modest tolerance had also developed to the rate-decreasing effects of dextrorphan. Suppression of responding by naltrexone, but not morphine or dextrorphan, was accompanied by a loss in body weight over the 1-to 2-h session in morphine-maintained pigeons; both weight loss and reduced response rates also occurred on termination of morphine treatment. Sensitivity to the rate-decreasing effects of morphine and naltrexone was near normal within 10 days following termination of morphine treatment. The dramatic changes in sensitivity to naltrexone and morphine produced by daily morphine injections, as well as the ability to generate complete dose-effect curves within a single session, indicate that this behavioral preparation may provide sensitive concurrent measures of narcotic tolerance and supersensitivity to antagonists in the pigeon.