and possibly species-dependent. For example, early studies showed that peripherally administered OXT slightly increased heroin self-administration in rats (Ree and Wied, 1977). In support of this finding, central OXT administration did not prevent the acquisition of morphine-conditioned place preference (CPP), and instead, increased the expression of CPP in rats (Moaddab et al., 2015). In contrast, systemically administered OXT decreased both the acquisition and maintenance of heroin self-administration (Kovacs et al., 1985), and inhibited heroin self-administration in heroin-tolerant, but not heroin-naïve rats and mice (Kovacs et al., 1985, Kovács and Van Ree, 1985). Further, site-specific micro infusion of OXT into the NAc or ventral hippocampus attenuated heroin self-administration in heroin-dependent rats, and this effect was prevented by OXT receptor blockade (Ibragimov et al., 1987). However, it is important to note that only a single, low dose of heroin was tested in these early studies making it difficult to determine OXT effect on opioids is inhibiting or potentiating. Future studies are necessary to determine dose-response information.
