GluN3-containing NMDA receptors in the rat nucleus accumbens core contribute to incubation of cocaine craving

Cue-induced cocaine craving progressively intensifies (incubates) after withdrawal from cocaine self-administration in rats and humans. In rats, the expression of incubation ultimately depends on Ca2+-permeable AMPARs that accumulate in synapses onto medium spiny neurons (MSNs) in the NAc core. However, the delay in their accumulation (∼1month after drug self-administration ceases) suggests earlier waves of plasticity. This prompted us to conduct the first study of NMDAR transmission in NAc core during incubation, focusing on the GluN3 subunit, which confers atypical properties when incorporated into NMDARs, including insensitivity to Mg2+ block and Ca2+ impermeability. Whole-cell patch-clamp recordings were conducted in MSNs of adult male rats 1-68 d after discontinuing extended-access saline or cocaine self-administration. NMDAR transmission was enhanced after 5 d of cocaine withdrawal, and this persisted for at least 68 d of withdrawal. The earliest functional alterations were mediated through increased contributions of GluN2B-containing NMDARs, followed by increased contributions of GluN3-containing NMDARs. As predicted by GluN3- NMDAR incorporation, fewer MSN spines exhibited NMDAR-mediated Ca2+ entry. GluN3A knockdown in NAc core was sufficient to prevent incubation of craving, consistent with biotinylation studies showing increased GluN3A surface expression, although array tomography studies suggested that adaptations involving GluN3B also occur. Collectively, our data show that a complex cascade of NMDAR and AMPAR plasticity occurs in NAc core, potentially through a homeostatic mechanism, leading to persistent increases in cocaine cue reactivity and relapse vulnerability. This is a remarkable example of experience-dependent glutamatergic plasticity evolving over a protracted window in the adult brain.