Endogenous opiates such as endorphins, endomorphins, and enkephalins, as well as opiate drugs (including morphine) exert their effects by binding to opioid 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.¹ 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.²

The opioid receptors belong to the G protein-coupled receptor (GPCR) superfamily whose members share a common structure of seven putative transmembrane domains, an extracellular amino terminus, a cytoplasmic carboxyl terminus, and a third intracellular loop important for binding G proteins.¹

All three receptors mediate opioid-induced analgesia. Supraspinal analgesia is mainly mediated by the µ-receptors, whereas µ-, δ-, and κ-receptors participate in the control of pain at the spinal level. These receptors also mediate the mood-altering properties of opioids.³

Of the opioid receptors, the µ-opioid receptor has been the most extensively studied due to its important role in mediating the actions of morphine and other analgesic agents, as well as other addictive drugs such as heroin.¹ The µ-opioid receptors are expressed in the central nervous system (CNS) and in the peripherial nervous system.⁴  The highest densities are found in the thalamus, caudate putamen, neocortex, amygdala, and other brain regions known to have well established roles in pain and analgesia.⁵