Functional and Structural Plasticity in Cocaine-Seeking Ensembles of the Nucleus Accumbens Core
Relapse vulnerability in substance use disorder (SUD) is primarily driven by cue-induced activation of neurons within the nucleus accumbens core (NAcore), among other contributing factors. Neuronal ensembles within the NAcore, defined here as selectively co-activated neurons during specific behavioral experiences, are essential during cocaine sensitization and recall. While transient synaptic plasticity (t-SP) has been widely observed in general neuronal populations within the NAcore during reinstatement, its ensemble-specific dynamics remain unclear. Here, we used c-Fos-TRAP2-based tagging to identify cocaine-seeking ensembles in mice following cocaine intravenous self-administration, extinction, and cue-induced reinstatement. Structural spine plasticity was assessed via confocal microscopy, and functional changes were measured using whole-cell electrophysiology across multiple reinstatement time points. Ensemble neurons exhibited enhanced dendritic spine head diameter (dh) and AMPA/NMDA (A/N) ratios following cue exposure, consistent with t-SP. Notably, spine classification revealed a reduction in mature spines during reinstatement, suggesting morphological remodeling rather than new spine formation in both ensemble and non-ensemble cells. Non-ensemble neurons exhibited classical functional transient synaptic plasticity, characterized by increased A/N ratios but no significant changes in dh. To begin assessing if presynaptic vesicle release impacts t-SP, paired-pulse ratio analysis indicated no differences in population or time point. Importantly, ensemble neurons displayed elevated A/N ratio following cocaine exposure, suggesting prior silent synapse maturation. These findings demonstrate that t-SP is not uniformly distributed across NAcore neurons but differs significantly between ensemble and non-ensemble neurons. By linking ensemble identity to both structural and functional plasticity, this study refines our understanding of cue-induced relapse mechanisms. Significance StatementRelapse in substance use disorder is strongly driven by cue-induced reactivation of neuronal ensembles in the nucleus accumbens core. While transient synaptic potentiation has been widely described in bulk neuronal populations within the nucleus accumbens core, its ensemble-specific expression has remained unclear. Here, we combined c-Fos-TRAP2 tagging, confocal imaging, and slice electrophysiology to show that transient synaptic potentiation is selectively expressed in behaviorally relevant ensembles. By linking ensemble identity with structural and functional plasticity during cue-induced cocaine seeking, these findings refine current models of relapse and identify ensemble-specific plasticity as a potential target for therapeutic intervention.