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

Abstract

1. To elucidate the structural features ensuring action of [D-Ala2, Leu5]-enkephalyl-Arg (dalargin), a series of dalargin analogues were tested for their effectivenesss 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 K(B) values in the myenteric plexus were also obtained. 2. [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 K(B) 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. 3. [Met5]-dalargin was equipotent to dalargin in the myenteric plexus, but was more potent in the vasa deferentia of hamster and mouse (K(B) = 5.5 nM). Leu5 and the interdependence of Leu5 and D-Ala2 are of importance for the selectivity of dalargin for μ-opioid receptors. 4. 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 (K(B) = 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. 5. N-Me-[L-Phe4]-dalarginamide possessed the highest potency and selectivity for δ-opioid receptors (the ratio of δ-activity to μ-activity was 0.00053; K(B) = 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.