Opioids, Endogenous

Abstract

Endogenous opioid peptides represent a significant class of peptide neurotransmitters in brain and peripheral tissues. They share the common property of high affinity binding to one or more of three principal types of opioid receptor, µ, δ, and κ. The endogenous opioid peptides include three classes: the enkephalins, β‐endorphin, and dynorphin. These peptides share the pentapeptide sequence of either Met‐ or Leu‐enkephalin. The peptides in each class are synthesized from high molecular weight precursors; trypsin‐like cleavage between pairs of basic amino acid residues yields intermediate and final peptide products. Each class of peptides possesses a unique regional distribution in the brain, and mediates a variety of functions, including analgesia, respiration, thermoregulation, neuroendocrine regulation, and modulation of immune cell function. Although synthesis of numerous synthetic analogues of opioid peptides have not produced analgesic drugs of practical importance, they have produced analogues with extraordinary specificity and stability which provide valuable information about the structure of opioid binding sites.