Jin, L.; Qi, X.; Liu, J.; Wright, W. J.; Schall, T. A.; Li, K.-L.; Zeng, B.; Wang, C.; Wang, L.; Dong, Y.

Drug addiction is an acquired motivational-behavioral state that begins with drug taking, which is comprised 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 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 to monitor activities of principal medium spiny neurons (MSNs) in the NAc across eleven days of cocaine self-administration. Behaviorally, mice exhibited progressive stabilization of operant responding and locomotion across 11 days 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.