Opioid receptor
Opioid receptors are a group of G-protein coupled receptors with opioids as ligands. The endogenous opioids are dynorphins, enkephalins and endorphins. The opioid receptors are similar, ~40% homologous, to G protein binds to the third intracellular loop of the opioid receptors. Both in mice and humans the genes for the various receptor subtypes are located on different chromosomes.
The μ-receptors exist mostly presynaptically in the periaqueductal gray region, and in the superficial dorsal horn of the spinal cord. Other areas where μ-receptors have been located include the external plexiform layer of the olfactory bulb, the nucleus accumbens, in several layers of the cerebral cortex and in some of the nuclei of the amygdala. The μ-receptor has high affinity for enkephalins and beta-endorphin but low affinity for dynorphins. Opioid alkaloids such as morphine, codeine, and methadone also bind to the μ-receptor.
Activation of the μ receptor by an agonist such as morphine causes analgesia, sedation, reduced blood pressure, itching, nausea, euphoria, decreased respiration, miosis (constricted pupils) and decreased bowel motility often leading to constipation. Some of these effects, such as sedation, euphoria and decreased respiration, tend to disappear with continued use as tolerance develops. Analgesia, miosis and reduced bowel motility tend to persist; little tolerance develops to these effects. Although tolerance to respiratory depression develops relatively quickly, it is the single most adverse side effect of opioid use; it is how overdoses kill. Opioid overdoses can be rapidly reversed with any of several opioid antagonists, drugs that bind to the μ receptors more strongly than most agonists but do not activate them. This displaces the agonist drug, countering its effects.
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