Endogenous opioid peptides: comparative evaluation of their receptor affinities in the mouse brain.

The affinities of certain endogenous opioid peptides and related sequences for mu, delta and k opiate receptors have been determined in membrane preparations from mouse brain. It was found that the KIs for the delta receptor changed very little when the sequence of the enkephalins was enlarged; on the contrary, the mu and especially the k activity were highly dependent not only on the specificity of the sequence but also on the length of the peptide. Most of the peptides have similar affinities for more than one receptor type. The enkephalins are the most selective for the delta receptor. beta-Endorphin, BAM-12P, BAM-22P, peptide E and dynorphin A display the best potency at the mu site. Dynorphin A (1-8), dynorphin A and dynorphin B are the most selective for the k site.