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 (n=24) increased over time to become excessive compared to heroin intake by ShA rats (n=24). After escalation, LgA rats tended to be less sensitive to heroin-induced locomotion (7.5-30 microg, 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 microg, 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
19-06-2008, 08:28
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