The neural circuitry underlying reinstatement of heroin-seeking behavior in an animal model of relapse.

Reinstatement of extinguished drug-seeking has been utilized in the study of the neural substrates of relapse to drugs of abuse, particularly cocaine. However, limited studies have examined the circuitry that drives the reinstatement of heroin-seeking behavior in the presence of conditioned cues, or by heroin itself. In order to test the hypothesis that the circuitry underlying reinstatement in heroin-experienced animals would show overlapping, yet distinct differences from cocaine-experienced animals, we used transient inhibition of several cortical, striatal, and limbic brain regions during reinstatement of heroin-seeking produced by heroin-paired cues, or by a single priming dose of heroin. Rats lever pressed for i.v. heroin discretely paired with a conditioned stimulus (CS) during daily 3-h sessions for a period of 2 weeks, followed by daily extinction of lever responding. Subsequent reinstatement of heroin-seeking was measured as lever responding in the absence of heroin reinforcement. The first set of reinstatement tests involved response-contingent CS presentations following bilateral intracranial infusion of either a combination of GABA receptor agonists (baclofen-muscimol, B/M) or vehicle (saline) into one of 13 different brain regions. The second set of reinstatement tests involved a single heroin injection (0.25 mg/kg, s.c.) following either B/M or vehicle infusions. Our results showed that vehicle-infused animals reinstated to both CS presentations and a priming injection of heroin, while B/M inactivation of several areas known to be important for the reinstatement of cocaine-seeking also attenuated heroin-seeking in response to CS presentations and/or a priming dose of heroin. However, as predicted, inactivation of areas previously shown to not affect cocaine-seeking significantly attenuated heroin-seeking, supporting the hypothesis that the circuitry underlying the reinstatement of heroin-seeking is more diffusely distributed than that for cocaine.