Habenular μ-opioid receptor knockout and chronic systemic receptor blockade promote negative affect and heighten nociceptive sensitivity

The μ-opioid receptor (MOR), a subtype of opioid G protein-coupled receptor, is expressed in multiple brain circuits and is particularly enriched in the habenula, a small epithalamic structure implicated in aversive states. MOR dysfunction has been linked to several psychiatric and nociceptive disorders. Identifying the key brain regions mediating the behavioral consequences of disrupted MOR signaling can shed light on the role of the opioid system in mood and pain regulation. In this study, we administered methocinnamox (MCAM), a long-acting, pseudo-irreversible MOR antagonist, acutely or chronically to adult C57BL/6J mice. A comprehensive behavioral battery was used to assess affective, social, and pain behavior. A single MCAM administration (10 mg/kg, s.c.) did not alter baseline behavior, but blocked opioid-induced analgesia, suggesting that basal µ-opioid tone does not contribute to these behaviors. In contrast, chronic MCAM administration (10 mg/kg, s.c., 3x/week for 4 weeks) led to increased anxiety-like behavior and decreased sociability, as well as enhanced mechanical allodynia and thermal hyperalgesia. Remarkably, selective knockout of habenular MORs in adult Oprm1 ^fl/fl mice reproduced key features of the chronic MCAM phenotype, including anxiety-like behavior and mechanical hyperalgesia. Together, these findings reveal that sustained inhibition of MOR signaling disrupts affective and nociceptive processing and highlight the habenula as a node mediating key behavioral deficits of disrupted opioid signaling.