Molecular regulation of opioid receptors.

Opioid actions are initiated at membrane receptors which couple to cellular effectors through G protein-mediated pathways. In the central nervous system opioids reduce neuronal activity through the inhibition of voltage-dependent Ca2+ channels, the activation of K+ channels and the inhibition of adenylyl cyclase. A significant clinical limitation to opioid therapy is the development of tolerance, a biological event that has been linked to agonist effects at the receptor level. Molecular studies on the consequences of opioid receptor regulation will provide a better understanding of the cellular mechanisms involved in the agonist-mediated events in tolerance development.