Mu, delta, and kappa opioid receptors (MOR (mu opioid receptor), DOR (delta opioid receptor), KOR (kappa opioid receptor))

The mu, delta, and kappa opioid receptors are three closely related but genetically distinct members of the opioid receptor family, all belonging to the GPCR superfamily. They are widely distributed in the central and peripheral nervous systems, mediating physiological responses to endogenous peptides (beta-endorphin for mu, enkephalins for delta, dynorphin for kappa) and exogenous opioids. These receptors modulate neurotransmission and neuronal excitability, especially in pain pathways, mood regulation, and autonomic functions. The mu opioid receptor is associated with strong analgesic effects but carries high risks of addiction and respiratory depression. Delta opioid receptor agonists can mediate analgesia but are less clinically exploited and may require inflammatory conditions for efficacy. Kappa opioid receptors are involved in analgesia, diuresis, and mood, with agonists often producing dysphoria. Recent advances include development of biased agonists to optimize therapeutic benefits and limit adverse effects.

Other names

mu receptorμ receptordelta receptorδ receptorkappa receptorκ receptor

Agonism involves activation inducing signaling via Gi/o proteins, inhibition of adenylyl cyclase, modulation of ion channels, and production of analgesia. Antagonism blocks the receptor and reverses opioid effects (e.g., naloxone, naltrexone). Biased agonism refers to selective activation of G-protein or β-arrestin pathways to optimize therapeutic profiles, such as achieving safer analgesia with lower side effect burden.