Although there was no group difference in total heroin intake, or active/inactive lever responding during self-administration, or after heroin-induced reinstatement, the H-RI group displayed more responding on the active lever (and on the inactive lever) during the initial extinction sessions, suggesting an enhanced tendency to respond to drug withdrawal-associated stress, as reflected by elevated stress hormones [e.g., 16, 32]. Following foot shock stress, the responding on the active lever in the H-RI rats was significantly higher (3 fold) than that of the L-RI rats, and the H-RI rats also displayed an elevated AVP mRNA level in the Me/BLA, a brain region known to play an important role in drug-seeking behaviors [33]. This effect seemed to be gene-specific (no effect on POMC mRNA levels in the same region) and region-specific (no effect on AVP mRNA levels in the medial hypothalamus). In a separate pilot experiment, levels of AVP mRNA in the Me/BLA were also compared between the rats that self-administered saline and self-administered heroin without the reinstatement tests by foot shock stress. We found no AVP mRNA alteration in the rats
