As shown in Fig. 2C, animals displayed reinstatement of operant responding induced by acute foot-shock stress, with large individual differences. Therefore, using the median-split approach, the animals were divided into the high (H-RI) and low reinstatement (L-RI) groups, and there was approximate 3-fold difference in mean foot shock-induced reinstatement of drug-paired lever responding between the H-RI and L-RI rats (Fig. 2E). ANOVA revealed significant main effects of Group [F(2,51) = 29.9, p<0.001], Lever [F(1,51) = 55.5, p<0.001] and Group x Lever interaction [F(2,51) = 26.0, p<0.001]. Newman-Keuls post hoc tests showed that the H-RI rats responded significantly more on the active lever than the inactive lever during the 3-h session following exposure to 15-min intermittent foot shock stress (p<0.05). Neither the L-RI nor the NR rats showed differences between active and inactive lever responding. Following the acute foot shock stress, both the H-RI and L-RI rats responded significantly more on the active lever than the NR rats (p<0.001 and p<0.05, respectively), and the H-RI rats responded significantly more on the active lever than the L-RI rats (p<0.001) (Fig. 2E).
