Opioid receptor, mu 1 (OPRM1, MOR) is a receptor for natural and synthetic opioids including morphine, heroin, DAMGO, fentanyl, etorphine, buprenorphin and methadone. It has inhibitory action and functions to lower cAMP levels. In the brain, it regulates the excitability of neurons, and activation by various agonists such as morphine results in various emotional and physiological effects such as euphoria, nausea, lowered respiration and blood pressure, sedation, analgesia and constipation. In immunohistochemistry, OPRM1 has highest positivity in the cerebellum, basal ganglia, olfactory region, cerebral cortex and amygdala in the brain and is also highly expressed in seminiferous tubules in the testes. It has low levels of positivity in endocrine tissues. References: The UniProt Consortium. Nucleic Acids Res. 47: D506-515 (2019); Nucleic Acids Res. 2016 Jan 4;44(D1):D733-45, PMID:26553804