Self-administration, extinction and reinstatement were analyzed using two-way mixed ANOVAs. In case of significant interactions or significant main effects, multiple comparisons were performed using Newman–Keuls post hoc method to identify significant differences between groups. Group differences in mRNA levels of each gene in each brain region and in plasma hormonal levels were analyzed using one-way ANOVA (high reinstatement, low reinstatement and heroin self-administration non-responder groups). To explore possible relationships between gene expressions/stress hormones and individual vulnerability to stress-induced reinstatement, total responses made on the active lever on reinstatement tests and neurochemical measures were also examined by linear regression. The accepted level of significance was p<0.05.
