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. Pretreatment with a single i.c.v. injection of [D-Ala², Cys⁴]deltorphin, [D-Ala², Ser⁴]deltorphin or [D-Ala², Glu⁴]deltorphin blocked the antinociceptive effects of [D-Ala², Glu⁴]deltorphin (delta-2 agonist) for up to 24 hr, but not the antinociceptive effects of DPDPE (delta-1 agonist). Pretreatment with a single dose [D-Ser², Leu⁵, Thr⁶]enkephalin (DSLET) (delta-2 agonist) failed to affect [D-Ala², Glu⁴]deltorphin-mediated antinociception, suggesting that the antagonistic actions of the deltorphin derivatives were not due to the production of acute tolerance. From these data, we postulate that an intact thiol group is apparently not critical for the binding of these ligands at the opioid delta-2 receptor.