Opioid Antagonists

The opioid receptors are G protein-coupled receptors for endogenous opioid peptides; the mu receptor binds beta-endorphin, endomorphin and enkephalin peptides as well as natural and synthetic opioids such as morphine, fentanyl, methadone and buprenorphine. Agonist binding couples the receptor predominantly to pertussis-toxin-sensitive Gi and Go proteins, and the downstream responses include inhibition of adenylate cyclase, inhibition of N-type and L-type calcium channels and activation of inward rectifying potassium channels. Drugs in this class occupy the receptor without activating it: the naltrexone label describes it as a synthetic congener of oxymorphone with no opioid agonist properties that competitively binds opioid receptors and may block the effects of endogenous opioids. Reversal is the property of the shorter-acting members, and it belongs to them specifically - the naloxone label states that naloxone prevents or reverses the effects of opioids including respiratory depression, sedation and hypotension, and is indicated for complete or partial reversal of opioid depression. Naltrexone's label makes no such claim.