Narcotic discrimination in pigeons: antagonism by naltrexone

In pigeons trained to discriminate between morphine (10 mg/kg) and saline, both morphine and ethylketazocine produced dose-related morphine-appropriate responding. The maximum effect produced by meperidine, however, was only 60% of that produced by morphine or ethylketazocine. Naltrexone (0.1-1.0 mg/kg) produced dose-related shifts to the right in the dose-response curves for the discriminative stimulus and rate-decreasing effects of morphine and ethylketazocine without affecting the response produced by meperidine. Thus, in contrast to the effects observed in other species, morphine and ethylketazocine produce similar discriminative effects in the pigeon. In addition, the morphine-like discriminative effects and the rate-decreasing effects of meperidine in the pigeon are not mediated by the naltrexone-sensitive mechanisms which mediate these effects of morphine or ethylketazocine.