As mentioned in the self-administration section below, we used an acquisition criterion to identify sub-groups of drug self-administration responders (n=20) and non-responders (n=6) that differed in amount of heroin exposure during self-administration. Within the group of drug self-administration responders, we also observed large individual differences in levels of lever-pressing during the acute foot shock-induced reinstatement session. Thus, we employed the median-split approach [e.g., 27] to further subdivide this group into “high” (H-RI) and “low” (L-RI) stress-reinstatement rats.
