Selectivities of opioid peptide analogues as agonists and antagonists at the δ‐receptor

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Abstract
1 The endogenous opioid ligands interact with more than one of the μ-, δ- and κ-binding sites. By the use of binding assays and bioassays, enkephalin analogues have been assessed for their selectivity for binding at the (5-binding site and for their agonist and antagonist activities at the δ-receptor. The electrically-induced contractions of myenteric plexus-longitudinal muscle preparations of the guinea-pig ileum were inhibited by μ- and κ-receptor ligands. Inhibitions were seen with μ-, δ- and κ-receptor ligands in the mouse vas deferens, mainly with μ-receptor ligands in the rat vas deferens and only with κ-receptor ligands in the rabbit vas deferens. 2 From observations on a considerable number of [Leu5] enkephalin analogues, it has been concluded that [d-Pen2, d-Pen5] enkephalin and [d-Pen2, l-Pen5] enkephalin are the most selective δ-agonists and that N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH is the most selective antagonist (Aib = α-aminoisobutyric acid). The binding of these peptides at the δ-site is 99% of the total binding. As to potency, the agonists are superior to the antagonists.