Figure 3. Social reinforcement is more sensitive than food reinforcement to treatment with IP acid alone or morphine alone.

(A) IP acid is more potent to decrease social- vs. food self-administration. Abscissa: concentration of lactic acid administered IP in a volume of 1.0 ml/kg. Ordinate: number of reinforcers per 30-min session expressed as a percent of the vehicle (water) baseline. (B) Morphine was more potent to decrease social- vs. food self-administration, whereas ketoprofen did not affect responding for either reinforcer. Abscissa: morphine or ketoprofen (Keto) dose in mg/kg. Ordinate: number of reinforcers per session expressed as a percent of the vehicle (saline) baseline. Raw vehicle baseline data are reported in text, and Table 1 shows EC₅₀/ED₅₀ values determined from each concentration- or dose-effect curve. Filled points indicate values significantly different from vehicle baseline as determined by analysis of the raw data using one-way ANOVA followed by Dunnett’s post hoc test, p<0.05. All points show mean ± SEM from 6 rats.