A receptor antagonist is a ligand that inhibits the function of an agonist and inverse agonist for a specific receptor.
Antagonists may be naturally occurring as is the case with physiological antagonists, or they may be synthetic and made to mimic the body's physiological antagonists. There are multiple antagonists for most receptors. A synthetic antagonist may compete with a physiological antagonist for binding sites upon receptors. Similarly, endogenous antagonists may also compete with one another. The antagonist which is bound is usually the one with the higher affinity for the receptor.
Experiments in mice show nalmefene is a universal opiate antagonist and binds more effectively to central opiate receptors than naloxone or naltrexone. Nalmefene occupies the central µ-opiod receptors in the brain. There is also evidence that it occupies the kappa-opioid receptors. When people are given nalmefene in tests, the prolactin level in their blood rises. Prolactin is the pituitary hormone that, among other things, stimulates breasts to produce milk.