Endogenous opioid peptides: historical aspects

Abstract

This short chapter serves as an introduction to the following contributions on endogenous opioid peptides and gives some of the more important historical developments in the search for an endogenous ligand for the opiate receptors. One general point has attracted our interest for some time, namely the fact that certain alkaloids found in plants have functional correlates in animal tissues. Although there are certain structural similarities between the two classes of compounds, the plant alkaloids are much more resistant to enzymic inactivation in animal tissues than the corresponding compounds of animal origin, and for this reason are often potent neurotoxic agents. Examples of such pairs of compounds are muscarine and muscimol, derived from the toadstool Amanita muscaria, which correspond to the neurotransmitters acetylcholine and γ-aminobutyric acid (GABA), respectively. Other pairs are nicotine and acetylcholine and also ephedrine and noradrenaline. To these may now be added morphine from Papaver somniferum and the class of peptides known as enkephalins and endorphins.