The opiate receptor and morphine-like peptides in the brain

Opiate receptors, neuronal membrane proteins that have been identified by the direct binding to membranes of radioactive opiates, have been shown to mediate the pharmacological effects of opiate drugs. Examination of brain extracts for substances that mimic effects of opiates on the opiate receptor permitted identification of the enkephalins, 2 peptides, each containing 5 amino acids, which are the normal substrates for the opiate receptor. Enkephalins are contained in specific neurons localized to areas of the brain enriched in opiate receptors. Enkephalin-containing neurons and opiate receptors are concentrated in portions of the brain that mediate pain perception, emotional behavior, and other functions altered by opiates. β-Endorphin, an opiate-like peptide containing 31 amino acids, is localized to the pituitary gland from which it can be released into the circulation to act presumably at peripheral target organs. In the brain β-endorphin is concentrated in the hypothalamus.