[The pharmacology of various classes of cerebral opioid receptors]

The research on endogenous opioid is only a decade old but the considerable number and the variety of studies devoted to this subject suggest that these neuropeptides might play a pivotal role in various biological functions. The most abundant opioid peptides enkephalins are synthesized as large precursors. They bind to several classes of receptors as mu and delta types and are degraded by specific enzymes (aminopeptidase M, enkephalinase, dipeptidylaminopeptidase) belonging to the group of metallopeptidases. The analysis of the functions of the enkephalinergic system can now be investigated by using recently designed selective mu (DAGO, TRIMU 5), delta (DTLET, DEPDPE), kappa (U 50, 488) agonists or antagonists (ICI 174, 864 for the delta type) and kelatorphan a complete inhibitor of enkephalin metabolism. The former probes were obtained by a rational approach based on the conformational adaptability of the endogenous peptides while inhibitors of enkephalin degrading enzymes were designed by taking into account crystallographic data on metallopeptidases. mu and delta receptors present distinct distributions in the brain. Enkephalinase visualized by autoradiography seems to be closely associated with opioid receptors. Pain control could be insured in brain structures by mu receptor-stimulation whereas both mu and delta types might be involved at the level of the spinal cord. In both cases, a "physiological" analgesia is produced by kelatorphan.