Refinement of Nucleus Accumbens Neuronal Dynamics during Cocaine Self-Administration Training.

Drug addiction is an acquired motivational-behavioral state that begins with drug taking, which is composed of a series of phases, including initial acquisition, stabilization, habituation, and maintenance. In rodent models of cocaine self-administration, the forebrain region nucleus accumbens (NAc) has been critically implicated in the acquisition-maintenance process of drug-taking and drug-seeking behaviors. However, it remains unknown how NAc neurons shift their activity patterns in response to these phasic transitions during cocaine taking. To examine this, we used GCaMP6m-based in vivo Ca2+ imaging in male mice to monitor activities of principal medium spiny neurons (MSNs) in the NAc across 11 d of cocaine self-administration. Behaviorally, mice exhibited progressive stabilization of operant responding and locomotion across 11 d of cocaine self-administration. During the early training days, we detected a portion of NAc neurons-a potential neuronal ensemble-that exhibited increased activities temporally contingent to the lever-press for cocaine. The number of NAc neurons exhibiting contingent activity increased progressively over the first three training days and then decreased gradually during the later training days, exhibiting expansion-refinement dynamics that may correspond to the acquisition and subsequent stabilization/maintenance of cocaine self-administration. Using a neuron-tracking technique, we found that the lever-press-contingent NAc ensemble exhibited substantial compositional dynamics, with neurons dropping into and out across training days. These activity features of lever-press-contingent neurons may represent key circuit dynamics of the NAc that transition the acquisition toward the maintenance of cocaine-taking behavior.