Opioid peptides and opiates differ in receptor selectivity.

(1) The binding properties of opioid receptors in whole rat cerebrum have been studied with tritium-labelled dihydromorphine, naltrexone and Leu-enkephalin as radioindicators and various opioid peptides and opiates as competitors. (2) Dihydromorphine shows high affinity binding to sites which are competitively blocked by various opiate agonists and antagonists (DHM sites). (3) Naltrexone binds to additional sites which also strongly bind other narcotic antagonists (NAL sites). (4) The morphinomimetic enkephalins show high selectivity for DHM sites while ACTH fragments and somatostatin show less selectivity. (5) Leu-enkephalin appears to bind to separate sites (EKN sites) with similar affinity to that for DHM sites. (6) Both dihydromorphine and naltrexone show higher affinity for DHM sites than for EKN sites. (7) In conclusion, at least three kinds of opioid binding sites are observable. These differences in receptor populations may relate to functional differences.