Mu opioid receptor (MOR) is a receptor for beta-endorphin, inhibits neurotransmitter release by reducing calcium ion currents and increasing potassium ion conductance. MOR signaling and regulation are strongly agonist-dependent. MOR mediates positive reinforcement following direct (morphine) or indirect (alcohol, cannabinoids, nicotine) activation. MOR plays a genetic role in the control of gut inflammation. MOR-deficient mice are highly susceptible to colon inflammation, with a 50% mortality rate occurring 3 days after administration of TNBS that induces inflammation. MOR agonists regulate cytokine production and T cell proliferation and might be new therapeutic molecules in inflammatory bowel disease.