Pharmacological characterization of [D-Ala²,Leu⁵,Ser⁶]enkephalin (DALES): antinociceptive actions at the δ_{non-complexed}-opioid receptor

Substantial evidence has been accumulated which suggests that opioid δ receptors may be distinguished on the basis of their involvement in the modulation (i.e., increase or decrease in potency) of μ-mediated antinociception. On this basis, it has been hypothesized that some opioid δ receptors exist within a functional complex with μ receptors (δ_complexed (δ_cx) receptors) while other δ sites do not (δ_{non-complexed} (δ_ncx) receptors). Recent work with [D-Ala²,Leu⁵,Cys⁶]enkephalin (DALCE) has demonstrated that this compound produces initial antinociceptive actions, does not modulate morphine antinociception and appears to bind irreversibly to the δ_ncx site, presumably by means of thiol-disulfide exchange between the receptor and the cysteine sulfhydryl group. To determine if a structural basis exists for actions at the hypothesized δ_ncx receptor, in the present study we report the synthesis and pharmacological characterization of [D-Ala²,Leu⁵,Ser⁶]enkephalin (DALES), a close structural analogue of DALCE. If a structural basis for action at the δ_ncx site exists, then DALES would be predicted to produce antinociception, fail to modulate morphine antinociception and, since it lacks the free sulfhydryl group present in DALCE, fail to exhibit irreversible antagonistic actions; these predictions were supported. Additionally, pretreatment with DALCE at - 24 h, but not with DALES, blocked DALES-induced antinociception. These observations in vivo support the concept of a structural basis for activity at the hypothesized δ_ncx site and suggest that DALES, like DALCE, may be a useful probe for pharmacological characterization of putative δ receptor subtypes.