Opioid receptor

Opiod 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."

Classification

There a several cell surface receptors for opioids. All are 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."

Methylnaltrexone, an investigational drug for treating constipation due to opioid analgesics, is an 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."