Radioligands for probing opioid receptors.

The three endogenous opioid precursors of almost 30000 Da are pro-opiocortin, proenkephalin and prodynorphin. Pro-opiocortin contains beta-endorphin, melanotropins and ACTH. Proenkephalin yields one [Leu5]enkephalin, three [Met5]enkephalins, one [Met5] enkephalyl-Arg-Arg-Val-NH2 (metorphamide or adrenorphin), one [Met5]enkephalyl-Arg-Gly-Leu and one [Met5]enkephalyl-Arg-Phe. [Leu5]enkephalin is common to all fragments of prodynorphin; its carboxyl extension by Arg-Lys leads to alpha- and beta-neo-endorphin and its carboxyl extension by Arg-Arg gives two dynorphins A and B of 17 and 13 amino acids, respectively. Another endogenous peptide is dynorphin A (1-8). The three main opioid binding sites are mu, delta and kappa. Their analysis has been facilitated by the synthesis of analogues of peptides and non-peptide compounds, which have selective agonist or antagonist action at only one site. The various physiological roles of the three types of the opiate receptor have so far not been sufficiently investigated.