Synaptic plasticity in cocaine-seeking ensembles of the nucleus accumbens core

Cue-induced seeking engages neuronal ensembles within the nucleus accumbens core (NAcore), with neuronal ensembles defined here as neurons coactivated during specific behavioral experiences that have been implicated in cued-reinstatement. Although transient synaptic plasticity has been widely observed in unidentified ensemble and non-ensemble neuronal populations in the NAcore during reinstatement, its expression within behaviorally relevant ensembles remains unclear. Here, we used c-Fos-TRAP2-based tagging to characterize structural and functional synaptic plasticity within ensembles during cocaine-seeking in mice following cocaine intravenous self-administration, extinction, and cue-induced reinstatement. Structural plasticity was measured via spine confocal imaging, and functional changes were evaluated by AMPA/NMDA ratios using whole-cell electrophysiology across reinstatement time points. Ensemble neurons exhibited increased dendritic spine head diameter during cue-induced reinstatement and were functionally potentiated relative to non-ensemble neurons. Spine classification showed reduced mature spines during reinstatement in both ensemble and non-ensemble cells, suggesting morphological remodeling rather than new spine formation. Non-ensemble neurons showed no change in spine head diameter during reinstatement but did exhibit an increased AMPA/NMDA ratio during cued-reinstatement. Paired-pulse ratio analysis suggested that yoked-cocaine exposure decreased presynaptic vesicle release probability, while operant cocaine exposure had no effect. Ensemble neurons showed an elevated AMPA/NMDA ratio following cocaine exposure, regardless of whether intake was yoked or contingent. Together, these findings suggest that ensemble and non-ensemble neurons undergo distinct forms of synaptic plasticity during cue-induced reinstatement. By distinguishing ensemble-specific structural plasticity from non-ensemble functional plasticity, this study refines the current understanding of mechanisms underlying cue-induced relapse. SIGNIFICANCE STATEMENT: In preclinical models of substance use disorder drug seeking is associated with cue-induced reactivation of neuronal ensembles in the nucleus accumbens core. While transient synaptic plasticity has been extensively described in non-selective neuronal populations pooling recordings of both ensemble and non-ensemble neurons of the nucleus accumbens core, ensemble-specific plasticity remains unclear. Here, we combined c-Fos-TRAP2 tagging, confocal imaging, and slice electrophysiology to show that structural synaptic plasticity 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 plasticity within the ensemble as a potential target for therapeutic intervention.