Opioid Receptors are a group of inhibitory G protein-coupled receptors (GPCRs) with opioids as ligands. The endogenous opioids are dynorphins, enkephalins, endorphins, endomorphins and nociceptin. Opioid receptors are currently classified into three groups, κ, δ and μ receptors. An opioid-like receptor with a high degree of homology to μ, δ and κ-opioid receptors has been cloned and termed the orphan opioid receptor (NOP, ORL1). This receptor has a distinct pharmacological profile from the classical opioid receptors and an endogenous ligand, nociceptin (orphanin FQ) has been identified.
δ opioid receptors play a role in analgesia, motor integration, gastrointestinal motility, olfaction, respiration, cognitive function and mood driven behavior. The enkephalins are generally considered to be the preferred endogenous ligands.
μ Opioid receptors are distributed throughout the neuraxis (neocortex, thalamus, nucleus accumbens, hippocampus, amygdala) and in the peripheral nervous system (myenteric neurons and vas deferens). μ opioid receptors have been implicated in respiration, cardiovascular functions, feeding, learning and memory, intestinal transit, locomotor activity, thermoregulation, hormone secretion and immune functions.
κ Opioid receptors are located predominantly in the cerebral cortex, nucleus accumbens, claustrum and hypothalamus and have been implicated in the regulation of nociception, diuresis, feeding and neuroendocrine secretions. Dynorphins A and B and α-neoendorphin appear to be the endogenous ligands.