Opioid receptor

Opioid receptors are "cell membrane proteins that bind opioids and trigger intracellular changes which influence the behavior of cells. The endogenous ligands for opioid receptors in mammals include three families of peptides, the enkephalins, endorphins, and dynorphins. The receptor classes include mu, delta, and kappa receptors. Sigma receptors bind several psychoactive substances, including certain opioids, but their endogenous ligands are not known."

Opioid analgesics stimulate opioid receptors.

Classification

Opioid receptors

Receptor	Functions
Delta	Analgesia,
Kappa	Analgesia, inhibition of gastrointestinal motility, psychotropic effect
Mu	Analgesia, inhibition of gastrointestinal motility, inhibition of respiration, sedation and physical dependency

There a several cell surface receptors for opioids. All are G-protein-coupled receptors.

kappa receptor

Kappa receptors are a "class of opioid receptors recognized by its pharmacological profile. Kappa opioid receptors bind dynorphins with a higher affinity than endorphins which are themselves preferred to enkephalins."

mu receptor

Mu receptors are a "class of opioid receptors recognized by its pharmacological profile. Mu opioid receptors bind, in decreasing order of affinity, endorphins, dynorphins, met-enkephalin, and leu-enkephalin. They have also been shown to be molecular receptors for morphine."

Alvimopan is a selective mu receptor antagonist used for treatment of postoperative ileus after bowel resection. Methylnaltrexone, is approved by the United States Food and Drug Administration for the treatment of constipation due to opioid analgesics, is another example of a mu receptor antagonist.

delta receptor

Delta receptors are a "class of opioid receptors recognized by its pharmacological profile. Delta opioid receptors bind endorphins and enkephalins with approximately equal affinity and have less affinity for dynorphins."