Activity profiles of dalargin and its analogues in μ‐, δ‐ and κ‐opioid receptor selective bioassays

To elucidate the structural features ensuring action of [D‐Ala2, Leu5]‐enkephalyl‐Arg (dalargin), a series of dalargin analogues were tested for their effectiveness in depressing electrically‐evoked contractions of the guinea‐pig myenteric plexus‐longitudinal muscle preparations (μ‐ and κ‐opioid receptors) and the vasa deferentia of the hamster (δ‐opioid receptors), mouse (μ‐, δ‐ and κ‐opioid receptors), rat (similar to μ‐opioid receptors) and rabbit (κ‐opioid receptors). The naloxone KB values in the myenteric plexus were also obtained. [L‐Ala2]‐dalargin was 19 times less potent than dalargin, and its pharmacological activity was peptidase‐sensitive. The ratio of δ‐activity to μ‐activity for [L‐Ala2]‐dalargin was 6.78, and KB was 7.9 nM. This emphasizes the role that D‐configuration of Ala2 plays in determining the active folding of dalargin molecule as well as in conferring resistance to peptidases. [Met5]‐dalargin was equipotent to dalargin in the myenteric plexus, but was more potent in the vasa deferentia of hamster and mouse (KB=5.5 nM). Leu5 and the interdependence of Leu5 and D‐Ala2 are of importance for the selectivity of dalargin for μ‐opioid receptors. Dalarginamide was more potent and selective for μ‐opioid receptors than dalargin, whilst dalarginethylamide, though equipotent to dalarginamide in the myenteric plexus, was more potent at δ‐opioid receptors (KB=5.0 nM). [D‐Phe4]‐dalarginamide and N‐Me‐[D‐Phe4]‐dalarginamide were inactive indicating the contribution of L‐configuration of Phe4 to the pharmacological potency of dalargin. N‐Me‐[L‐Phe4]‐dalarginamide possessed the highest potency and selectivity for μ‐opioid receptors (the ratio of δ‐activity to μ‐activity was 0.00053; KB=2.6 nM). The CONH2 terminus combined with the N‐methylation of L‐Phe4 increased the potency and selectivity of dalargin for μ‐opioid receptors. British Journal of Pharmacology (1999) 128, 569–576; doi:10.1038/sj.bjp.0702825