opioid receptor
a receptor that binds opioids (including endogenous opioids) and mediates their effects via G proteins. It is generally agreed that there are at least three classes: mu (μ), kappa (κ), and delta (δ) opioid receptors. Opioid receptors are widely distributed in the brain, spinal cord, and periphery, and each type of receptor is differentially distributed. Mu opioid receptors are largely responsible for the analgesic and euphoric effects associated with opioid use. Most exogenously administered opioids bind to mu opioid receptors, which also mediate the respiratory depression, sedation, and reduced gastrointestinal motility associated with opioids. Kappa opioid receptors are localized primarily in the dorsal root ganglia of the spinal cord. Stimulation of these receptors produces more modest analgesia and dysphoric responses and may also be responsible for some of the perceptual and cognitive effects
of opioids. Delta opioid receptors may potentiate activity of opioids at the mu opioid receptor site and have a less direct involvement in the production of analgesia. The more recently discovered nociceptin/orphanin FQ (N/OFQ) receptor, included in the opioid receptor family, has not been completely characterized.