Mu opioid receptor mRNA and protein localization across the rat and mouse habenula

The habenula (Hb), which contains medial and lateral subdivisions (MHb and LHb, respectively), has high intensity mu opioid binding and receptor (MOR) expression, yet the details of MOR localization across these regions remains debated. MHb and LHb participate in largely non-overlapping neural circuits, therefore accurately resolving MOR expression across MHb and LHb is critical for understanding how MOR ligands impact behaviors. Here we utilized <i>in situ</i> hybridization (ISH) and immunocytochemistry (ICC) to systematically map <i>Oprm1</i> mRNA and MOR protein throughout the habenular complex. We studied both rat and mouse tissue to evaluate conserved expression across two common research species. Importantly, we found mRNA and protein in both the MHb and LHb in both. In rat, 39 ± 3% (MHb) and 21 ± 4% (LHb) of cells expressed <i>Oprm1</i>. These proportions were greater in mouse: 57 ± 1% (MHb) and 32 ± 4% (LHb). Within each species, <i>Oprm1</i> labeling density per positive cell was greater in MHb compared to LHb (<i>p</i> < 0.0001 for rat and mouse). The highest intensity labeling was localized along the lateral edge of the MHb for both methods. ICC showed MOR localized to fibers and somata in MHb and LHb. In LHb, MOR labeling was most dense in intermediate sections along the anterior-posterior (AP) axis. In rats we also observed higher density labeling in dorsal LHb at intermediate AP levels and medial LHb more posteriorly. These results indicate that both MHb and LHb can contribute to MOR mediated actions through their respective circuits.