Relapse to cocaine-seeking is regulated by medial habenula Nr4a2

After exposure to drugs of abuse, the reward circuit can experience persistent changes that are thought to underlie relapse behavior. These changes can be epigenetic in nature, and here we identify an epigenetic regulator of memory processes, nuclear orphan receptor subfamily4 groupA member2 (NR4A2 aka NURR1), as necessary for relapse to cocaine-seeking behavior. Nr4a2 is an epigenetically regulated immediate early gene and transcription factor that is highly expressed in the medial habenula (MHb). Despite the well-studied contributions of the MHb to nicotine-associated behaviors, the MHb is not classically included in reward circuits. Therefore, the role of MHb NR4A2 in cocaine-seeking and relapse behavior remains largely unknown. This is a key open question as NR4A2 is a powerful regulator of memory processes dependent on epigenetic mechanisms. In this study, we examined the role of MHb NR4A2 in cued reinstatement of cocaine self-administration in mice, and found that over-expressing a dominant negative form of Nr4a2 (Nurr2c) in the MHb results in a near complete block of   relapse-like behavior. We used single-nucleus transcriptomics to characterize the molecular cascade following the manipulation of Nr4a2, revealing changes in transcriptional networks related to addiction, neuroplasticity, and glutamatergic signaling. Together, these results place the MHb in the reward circuit as a pivotal regulator of relapse behavior and demonstrate the importance of NR4A2 as a key mechanism driving relapse behavior in the MHb.