After the discrimination training, dose-response tests were performed. Figure 4A shows the morphine dose-response function with morphine-appropriate responding during the 20-min session. There was a significant dose effect (F3,21 = 14.59, p<0.001) with higher morphine doses producing higher morphine-appropriate responding. However, no genotype (F1,7 = 0.72, p>0.05) or sex (F1,7 = 0.07, p>0.05) effects and no interactions (p's>0.05) were found. Figure 4B shows that response rates declined with increasing morphine dose (dose effect, F3,21 = 6.51, p<0.01). There were no significant effects of sex (F1,7 = 0.01, p>0.05) or genotype (F1,7 = 1.88, p>0.05) and no significant interactions (p's>0.05). Statistical analysis for both the morphine-appropriate responding and the response rate data were also performed without the data for the highest morphine testing dose (10 mg/kg) which tended to produce non-responding behavior in both genotypes. These analyses, however, revealed no significant genotype differences (data not shown).
