Endogenous opioids and their receptors.

Since the endogenous opioid peptides bind to more than one of the types of binding sites, it is important to have synthetic compounds that bind almost exclusively at one site. There are now such agonists available but antagonists are still required that interact exclusively with one opioid site. The results obtained with opioid peptides or non-peptides having such qualities will be the physiological basis for a correlation of the binding at mu-, delta- and kappa-receptors with their pharmacological effects. It is important to realize that almost all endogenous opioid ligands are degraded by peptidases and that it is necessary to have synthetic non-toxic inhibitors of those peptidases that play a role in opioid transmission. Related to this problem is the need to develop methods for the study of the release of various endogenous opioid peptides under physiological conditions.