A multiscale atlas of Oprm1-expressing neurons in the central nervous system: Brain-wide distribution, circuit functions, and translational therapeutic implications

The mu-opioid receptor (MOR), encoded by the Oprm1 gene, critically modulates diverse physiological processes including pain perception, reward behaviors, emotional regulation, and autonomic control. The genetic complexity and region-specific distribution of Oprm1-expressing neurons underpin both the therapeutic actions and adverse effects of opioid drugs. In this comprehensive review, we systematically construct a multiscale atlas of Oprm1-expressing neurons throughout the central nervous system (CNS) by integrating genetic labeling, neuroanatomical mapping, functional circuit analyses, and translational perspectives. We highlight three interconnected aspects: (1) brain-wide regional distribution patterns, revealing enriched Oprm1 expression across sensory, limbic, and autonomic circuits; (2) functional heterogeneity of Oprm1-expressing neurons, elucidating their distinct roles in nociception, reward processing, emotional and neuroendocrine regulation, with a particular emphasis on sex differences and adaptive plasticity under stress; (3) translational opportunities for therapeutic interventions, focusing on innovative strategies such as circuit-specific opioid modulation and biased agonism, designed to optimize analgesic benefits while minimizing addiction liability and respiratory depression. Furthermore, we critically examine existing challenges and knowledge gaps, including receptor trafficking mechanisms, dynamic changes in Oprm1 expression under conditions such as chronic opioid exposure or stress, interspecies differences, and network-level opioid signaling dynamics. This integrative framework provides essential insights into MOR neurobiology, facilitating the development of next-generation opioid therapeutics that leverage precise modulation of neural circuits and molecular pharmacological advancements. The translational implications of understanding these aspects are emphasized throughout this review, aiming to bridge basic neurobiological findings with clinical applications.