Agonist and antagonist profiles of [D-Ala2,Glu4]deltorphin and its [Cys4]- and [Ser4]-substituted derivatives: further evidence of opioid delta receptor multiplicity.

Pharmacological evidence has suggested the presence of two supraspinal opioid delta receptor subtypes in the mouse, termed delta-1 and delta-2. [D-Pen2,D-Pen5]enkephalin (DPDPE) is thought to be primarily an agonist at the opioid delta-1 subtype, whereas H2N-Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2 ([D-Ala2,Glu4]deltorphin) is a selective agonist at the delta-2 subtype. Based on previous reports suggesting that a receptor sulfhydryl group may be critical for ligand binding to the opioid delta receptor, the present investigation has attempted to discover whether this concept extends to the opioid delta-2 receptor. For this purpose, a cysteine-substituted deltorphin was synthesized and the potential agonist and antagonist properties of this compound, H2N-Tyr-D-Ala-Phe-Cys-Val-Val-Gly-NH2 ([D-Ala2,Cys4]deltorphin), were evaluated in an antinociceptive assay after i.c.v. administration to mice and stability in mouse brain was determined. As a control a serine-substituted deltorphin was also prepared and the potential agonist and antagonist properties of this compound, H2N-Tyr-D-Ala-Phe-Ser-Val-Val-Gly-NH2 ([D-Ala2,Ser4]deltorphin), as well as those of the parent deltorphin, [D-Ala2,Glu4]deltorphin, were evaluated. Acutely, [D-Ala2,Cys4]deltorphin, [D-Ala2,Ser4]deltorphin and [D-Ala2,Glu4]deltorphin each produced dose-related antinociceptive effects.(ABSTRACT TRUNCATED AT 250 WORDS)