The three main opioid receptor subtypes
Opioid effects are mediated chiefly through three receptor subtypes, conventionally called mu, kappa, and delta. All are G-protein-coupled receptors that reduce neuronal excitability and dampen pain signalling. The mu receptor is most associated with strong analgesia and also with the effects that make opioids dangerous, including respiratory depression, sedation, and the development of dependence. Kappa and delta receptors contribute to analgesia and to mood and other effects to varying degrees.
Because these receptors sit on pain pathways in both the spinal cord and the brain, opioids act at multiple levels of the nervous system.
Agonists, partial agonists, and antagonists
Full agonists such as morphine activate the receptor maximally. Partial agonists such as buprenorphine bind tightly but produce a submaximal effect, which gives them a ceiling on some responses. Antagonists such as naloxone occupy the receptor without activating it and can rapidly displace other opioids, which is the basis for reversing an overdose.
Opioids carry serious risks of respiratory depression, dependence, and overdose. This is general educational information about pharmacology, not medical advice; opioid use must be directed and monitored by a qualified clinician.