Opioid Agonists

The opioid receptors - mu, delta and kappa - are G protein-coupled receptors for endogenous endorphins, enkephalins and dynorphins. Agonist binding couples them to inhibitory G proteins, which suppresses adenylate cyclase, reduces calcium currents and increases potassium conductance, so neurotransmitter release falls and the transmission of pain is blunted. Drugs in this class activate one or more of those receptors and so produce analgesia, together with the sedation, respiratory depression and gut stasis that follow from the same mechanism. A drug belongs here because it switches an opioid receptor on; the mu receptor is the one that binds morphine, fentanyl, methadone and buprenorphine.