Environmental contexts previously associated with drug use provoke relapse to drug use in humans and reinstatement of drug seeking in animal models of drug relapse. We examined whether context-induced reinstatement of cocaine seeking is mediated by activation of context-selected nucleus accumbens neurons. We trained rats to self-administer cocaine in Context A and extinguished their lever-pressing in a distinct Context B. On test day, reexposure to the cocaine-associated Context A reinstated cocaine seeking and increased expression of the neural activity marker Fos in 3.3% of accumbens shell and 1.6% of accumbens core neurons. To assess a causal role for these activated neurons, we used the Daun02 inactivation procedure to selectively inactivate these neurons. We trained c-fos-lacZ transgenic rats to self-administer cocaine in Context A and extinguished their lever-pressing in Context B. On induction day, we exposed rats to either Context A or a novel Context C for 30 min and injected Daun02 or vehicle into accumbens shell or core 60 min later. On test day, 3 d after induction day, the ability of Context A to reinstate cocaine seeking and increase neuronal activity in accumbens shell was attenuated when Daun02 was previously injected after exposure to Context A. Daun02 injections after exposure to the novel Context C had no effect on context-induced reinstatement of cocaine seeking despite much greater numbers of Fos-expressing neurons induced by Context C. Daun02 injections in accumbens core had no effect. Our data suggest that context-induced reinstatement of cocaine seeking is mediated by activation of context-selected accumbens shell but not core neuronal ensembles.

Context-induced reinstatement of drug seeking is a well established animal model for assessing the neural mechanisms underlying context-induced drug relapse, a major factor in human drug addiction. Neural activity in striatum has previously been shown to contribute to context-induced reinstatement of heroin, cocaine, and alcohol seeking, but not yet for methamphetamine seeking. In this study, we found that context-induced reinstatement of methamphetamine seeking increased expression of the neural activity marker Fos in dorsal but not ventral striatum. Reversible inactivation of neural activity in dorsolateral but not dorsomedial striatum using the GABA agonists muscimol and baclofen decreased context-induced reinstatement. Based on our previous findings that Fos-expressing neurons play a critical role in conditioned drug effects, we assessed whether context-induced reinstatement was associated with molecular alterations selectively induced within context-activated Fos-expressing neurons. We used fluorescence-activated cell sorting to isolate reinstatement-activated Fos-positive neurons from Fos-negative neurons in dorsal striatum and used quantitative PCR to assess gene expression within these two populations of neurons. Context-induced reinstatement was associated with increased expression of the immediate early genes Fos and FosB and the NMDA receptor subunit gene Grin2a in only Fos-positive neurons. RNAscope in situ hybridization confirmed that Grin2a, as well as Grin2b, expression were increased in only Fos-positive neurons from dorsolateral, but not dorsomedial, striatum. Our results demonstrate an important role of dorsolateral striatum in context-induced reinstatement of methamphetamine seeking and that this reinstatement is associated with unique gene alterations in Fos-expressing neurons.