The delta-opioid receptor is sufficient, but not necessary, for spinal opioid-adrenergic analgesic synergy

Spinal administration of opioid and α₂-adrenergic receptor (α₂AR) agonists produces analgesia, and agonists interact synergistically when coadministered. The molecular mechanism underlying this synergy is largely unknown. Pharmacological studies have identified both the delta and the mu-opioid receptors (DOR and MOR) as candidate receptors capable of interacting synergistically with α₂AR agonists. However, recent studies attribute the antinociceptive effect of DOR agonists to actions at the MOR, calling the role of DOR in opioid-adrenergic synergy into question. Other studies suggesting that DOR is implicated in morphine antinociception raise the possibility that DOR is nonetheless required for morphine synergy with α₂AR agonists. This study aimed to determine whether DOR activation is sufficient and necessary to mediate opioid-adrenergic synergistic interactions in the spinal cord. The antinociceptive effects of clonidine, [D-Ala²]-deltorphin II (DeltII), morphine, and [D-Ala², N-Me-Phe⁴, Gly-ol⁵]-enkephalin (DAMGO) were evaluated using the substance P (SP) behavioral assay in wild type (WT) and DOR-knockout (KO) mice. Opioid-adrenergic drug interactions were evaluated after spinal coadministration of clonidine with DeltII, morphine, or DAMGO. Isobolo-graphic analyses of dose-response curves determined whether interactions were synergistic or additive. The absence of DeltII antinociceptive efficacy in DOR-KO confirmed its selectivity in the SP assay. Although DeltII 1 clonidine interacted syner-gistically in WT mice, no interaction with clonidine was observed in DOR-KO mice. Clonidine was synergistic with morphine in both mouse strains. DAMGO did not synergize with clonidine in either strain. These findings confirm that although other opioid receptors can interact synergistically with α₂AR agonists, DOR is sufficient for spinal opioid-adrenergic interactions.