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Mu Opioid Receptor mRNA and Protein Localization Across the Rat and Mouse Habenula.
Aashka K Popat1, Rhiana C Simon1,2, Beatriz B Aoyama1
1UCSF Weill Institute for Neurosciences, Department of Neurology, University of California, San Francisco, California, USA.
The Journal of Comparative Neurology
|June 6, 2026
Summary
Mu opioid receptor (MOR) mRNA and protein are present in both the medial habenula (MHb) and lateral habenula (LHb) in rats and mice. This finding is crucial for understanding how MOR ligands influence behavior via these distinct neural circuits.
Area of Science:
- Neuroscience
- Molecular Biology
- Pharmacology
Background:
- The habenula (Hb) complex, comprising medial (MHb) and lateral (LHb) subdivisions, exhibits significant mu opioid receptor (MOR) binding and expression.
- Understanding MOR localization within the MHb and LHb is critical, as these regions engage in distinct neural circuits, influencing behavior differently.
- Accurate mapping of MOR expression is essential for elucidating the behavioral impacts of MOR ligands.
Purpose of the Study:
- To systematically map the distribution of Oprm1 mRNA and MOR protein within the rat and mouse habenular complex.
- To compare MOR expression patterns between the MHb and LHb subdivisions.
- To investigate species-specific differences in MOR localization between rats and mice.
Main Methods:
- Utilized in situ hybridization (ISH) to detect Oprm1 mRNA.
- Employed immunocytochemistry (ICC) to visualize MOR protein.
- Performed parallel mapping in Oprm1-Venus/Venus reporter mice for validation.
- Analyzed tissue from both Sprague Dawley rats and C57BL/6 mice.
Main Results:
- Oprm1 mRNA and MOR protein were detected in both MHb and LHb in both rat and mouse species.
- Oprm1 expression was higher in mice (MHb: 57±1%, LHb: 32±4%) compared to rats (MHb: 39±3%, LHb: 21±4%).
- Higher MOR puncta density per positive cell was observed in MHb versus LHb in both species (p < 0.0001).
- Intense MOR labeling was concentrated along the lateral edge of the MHb.
- ICC revealed MOR in both fibers and cell bodies within MHb and LHb.
- LHb exhibited densest MOR labeling in intermediate anterior-posterior sections, with regional variations in rats (dorsal and medial).
Conclusions:
- Both MHb and LHb express MOR, indicating they can mediate MOR ligand actions through their respective neural circuits.
- The differential expression patterns suggest distinct roles for MOR signaling in the MHb and LHb.
- These findings provide a detailed map of MOR distribution, aiding future research on opioid-related behaviors.
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