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 (d_h) 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 d_h. 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.