In the context of neuroscience and cell biology, acronym for "Mu Opioid Receptor". This receptor is widely distributed, with high expression in places like the neocortex, the basal ganglia, amygdala, periaqueductal gray, the dorsal horn of the spinal cord, and the enteric nervous system. This receptor has been implicated in things like nociception, mood, respiratory and cardiovascular functions and gut motility. Its endogenous ligands are B endorphin and the enkephalin peptides.