HeroinReinstatement2007 12

Increased drug availability can precipitate a rapid transition to compulsive drug use in both vulnerable humans and laboratory animals.
Recent studies have shown that despite equivalent levels of psychomotor sensitization, only rats with prolonged, but not limited, access
to cocaine self-administration respond to the priming effects of cocaine on drug seeking, as measured in a within-session reinstatement
model of drug craving. In this model, drug seeking is first extinguished and then reinstated by non-contingent presentations of the drug
alone in the absence of response-contingent stimuli. Here, we assessed the generality of this observation in rats with daily short (1 h, ShA)
vs long access (6 h, LgA) to i.v. heroin self-administration. As expected, heroin intake by LgA rats (n¼24) increased over time to become
excessive compared to heroin intake by ShA rats (n¼24). After escalation, LgA rats tended to be less sensitive to heroin-induced
locomotion (7.5–30 mg, i.v.) than ShA rats. In contrast, only LgA rats, not ShA rats, responded to the priming effects of heroin, as
measured by the ability of heroin alone (7.5–30 mg, i.v.) to reinstate extinguished drug-seeking behavior. Finally, during the course of
heroin intake escalation, a large proportion of LgA rats developed self-injury (mostly targeting the nails and digit tips of the forepaws), a
negative consequence not seen in ShA rats. This study reproduces and extends previous research on compulsive cocaine use by showing
that heroin-induced reinstatement is also specific to compulsive drug use and dissociable from heroin-induced reward and psychomotor
sensitization