Quantitative analysis of naloxone antagonism of the discriminative stimulus properties of morphine in the pigeon.

Pigeons were trained to discriminate morphine (5.0 mg/kg) from saline under a second-order fixed ratio 10 (fixed-ratio 5) color-tracking schedule for food reinforcement. After reliable stimulus control was established, cumulative graded doses of morphine (0.3-30.0 mg/kg) were tested and resulted in dose-dependent increases in morphine-appropriate key pecking and decreases in response rate. Cumulative doses of naloxone (0.1-10.0 mg/kg) or consecutive injections of saline did not elicit morphine-appropriate responding or affect response rate. Pre-treatment with naloxone (0.1-1.0 mg/kg) before determination of cumulative dose-effect curves for morphine caused the morphine generalization curves to be shifted, in a parallel manner, rightward. Dose-ratio analysis of naloxone antagonism of morphine generalization, using a Schild plot with the slope constrained to -1, gave an apparent pA2 value (95% confidence limits) of 6.53 (6.18-6.89).