Extinction Training after Cocaine Self-Administration Induces Glutamatergic Plasticity to Inhibit Cocaine Seeking

Learning to inhibit drug seeking can be an important strategy for inhibiting relapse, and this can be modeled by extinguishing drug seeking in response to a drug-paired context. Rats were either extinguished or withdrawn without extinction training (abstinence) from cocaine self-administration, and measurements of postsynaptic density proteins in the core and shell subcompartments of the nucleus accumbens were compared with yoked-saline controls. Only extinguished rats had elevations of PSD-95, Homer1b/c, and Narp in the postsynaptic density of the core, whereas no proteins measured were altered in the postsynaptic density of the shell in either extinguished or abstinent rats. Using a biotinylation strategy, it was found that surface expression of mGluR5 was reduced only in the core of extinguished animals. Although both extinguished and abstinent animals showed a reduction in long-term potentiation elicited in the core by stimulating prefrontal cortex, blunted long-term depression was observed only in extinguished rats. These data indicate that the elevation in Homer1b/c in the core may have sequestered mGluR5 away from the membrane surface and that the loss of surface mGluR5 inhibits long-term depression. Accordingly, when Homer1c was overexpressed in the core of cocaine-naive rats with an adenoassociated virus, long-term depression was inhibited. This mechanism may contribute to the inhibition of cocaine seeking by extinction training because overexpression of Homer1c in the core also inhibited cue-induced reinstatement of cocaine seeking. These data identify a cellular mechanism that may contribute to extinction-induced inhibition of cocaine seeking.