Fig. 2.

Reinforcing effects of morphine in control (black) and NVP (gray) rats trained to respond for food pellets under an FR20 schedule, then maintained on intravenous morphine under continuous reinforcement. Abscissa, injection condition, where Saline represents substitution sessions where intravenous saline was delivered contingently, or where various doses of morphine were available for self-administration. Ordinate, mean (±SEM) number of reinforcers earned. Hash marks indicate significant differences from the appropriate saline control condition, while asterisks indicate significant differences from control at that dose, as determined by one-way repeated measures ANOVA and Tukey's HSD test (p<0.05)