Fig. 3.

Discriminative stimulus effects of morphine in control (black) and NVP (gray) rats trained to discriminate 1.5 mg/kg morphine from saline. Circles represent substitution tests with discrete morphine doses, while diamonds represent assessment of interoceptive effects of morphine following a pre-session injection of 0.1 mg/kg naltrexone. Abscissa, injection condition, where SAL and TD represent training sessions where saline or the morphine training dose, respectively, were administered, or where various doses of morphine were substituted. Ordinate, mean (±SEM) percent of total responses emitted on the morphine-appropriate lever. No between-group statistical comparisons were significant