specificity of the stress-defined H-RI and L-RI groups, all rats were also tested for reinstatement of lever pressing induced by a heroin prime. Nine days after heroin self-administration, specific brain regions were collected to quantify AVP, POMC and orexin gene expressions, and the alterations were analyzed in the H-RI and L-RI groups. Because availability of dopamine D2 receptor (D2) binding and D2 gene expression in the striatum have been associated with cocaine seeking [25, 26], we also determined if the changes in striatal D2 expression, as well as in the stress hormones (ACTH, corticosterone and prolactin), were correlated with vulnerability to stress-induced heroin-seeking.
