Opioid receptors are a group of G protein-coupled receptors with opioids as ligands. The endogenous opioids are dynorphins, enkephalins, endorphins, endomorphins and nociceptin. Opioid receptors are distributed widely in the brain, and are found in the spinal cord and digestive tract. Opioid receptors are molecules, or sites, within the body that are activated by opioid substances. Opioid receptors inhibit the transmission of impulse in excitatory pathways within the human body system. These pathways include the serotonin, catecholamine, and substance P pathways, which are all implicated in pain perception and feelings of well-being. Opioid receptors are further subclassified into mu, delta, and kappa receptors. All the classes, while exhibiting differing modes of action, share some basic similarities. They all are driven by the potassium pump mechanism, which is found on the plasma membrane of the majority of cells.
