opioid receptor

An opioid receptor is the molecular lock that opioid drugs fit into. The body did not build these locks for morphine; it built them for its own painkilling molecules. Opioid drugs simply happen to have the right shape to turn the same lock — which is why a poppy extract can switch off pain in a human nervous system.

There are three main types: mu (μ), delta (δ), and kappa (κ). All are G-protein-coupled receptors that signal through Gi/Go proteins, so when activated they lower cyclic AMP, close certain calcium channels, and open potassium channels. The net effect is to make pain-transmitting neurons less excitable and less likely to release their signaling chemicals. The mu receptor is the most clinically important: it mediates most of the analgesia, but also the euphoria, the respiratory depression, the constipation, and the dependence.

Because one receptor type drives both the benefit and the worst harms, separating them has proved very hard. Researchers have pursued kappa-selective drugs, biased agonists that favour pain-relieving signaling over harmful pathways, and peripherally restricted opioids, but no design has fully escaped the trade-off. Understanding which receptor a drug hits, and how strongly, explains much of its clinical profile.