In the CPu, one-way ANOVA just failed to show a significant effect of the Group [F(2,21) = 3.39, p=0.08] (Table 1). However, the presence of individual differences in groups created by median split was confirmed by a significant negative correlation found between the CPu D2 receptor mRNA levels and foot shock-induced responding on the active lever during the stress induced reinstatement in all heroin self-administration rats (r = −0.67, n = 18, p<0.005) (Fig. 5). Since the H-RI rats responded significantly more after foot shock stress than the L-RI rats, a Student t-test was carried out: there was marginally significant difference between the H-RI and L-RI groups [t (16) = 4.40, p = 0.05]. When the D2 mRNA levels were analyzed with heroin-induced lever responding in the H-RI and L-RI rats, no significant correlation was found (data not shown). In the VTA or AP, no significant group differences in the D2 mRNA expression were found (Table 1).
