Opioid Receptors

Endogenous opiates such as endorphins, endomorphins, and enkephalins, as well as opiate drugs (including morphine) exert their effects by binding to opioid receptors which belong to the superfamily of G-protein coupled receptors. Three “classic” types of opioid receptors have been identified: mu (µ)-opioid (MOP) receptor, delta (δ)-opioid (DOP) receptor, and kappa (κ)-opioid (KOP) receptor. Recently, the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor was also described. Despite its significant sequence homology, its pharmacological profile differs greatly from those of the classic µ, δ, and κ receptors.