Fig. 2.

Morphine dose response curve following cumulative administration (A). EC₅₀ values were generated from nonlinear regression analysis (B). Adult males exhibit lower morphine EC₅₀ values than females and aged males (C). Data presented in milligrams of morphine per kilogram of body weight. *Significant differences between adult male and adult female, adult male and aged male, and adult male and aged female. p<0.05 calculated by Tukey’s post hoc test. Graphs indicate mean ± SEM.