A multiplicity of opiate receptors and enkephalin neuronal systems.

Properties of the opiate receptor, such as its discrimination of agonists and antagonists, and the relationship between receptor localization and central sites of pain perception suggested that this receptor might interact with a normally occurring opiate-like substance. Such opioid peptides have now been described in at least two systems of distinct neurons which occur in areas particularly associated with pain and emotion. Understanding the relationship between the multiple opiate receptors described pharmacologically and those distinguished by binding studies in methionine- or leucine-enkephalin-containing neurons might be useful for the development of new drugs that suppress pain or alter emotion without producing tolerance or withdrawal effects.