TY - JOUR T1 - Induction of δ Opioid Receptor Function by Up-Regulation of Membrane Receptors in Mouse Primary Afferent Neurons JF - Molecular Pharmacology JO - Mol Pharmacol SP - 1688 LP - 1698 DO - 10.1124/mol.105.014829 VL - 68 IS - 6 AU - Wendy Walwyn AU - Nigel T. Maidment AU - Matthew Sanders AU - Christopher J. Evans AU - Brigitte L. Kieffer AU - Tim G. Hales Y1 - 2005/12/01 UR - http://molpharm.aspetjournals.org/content/68/6/1688.abstract N2 - It is not clear whether primary afferent neurons express functional cell-surface δ opioid receptors. We examined δ receptor coupling to Ca2+ channels in mouse dorsal root ganglion neurons under basal conditions and after δ receptor up-regulation. [d-Ala2,Phe4,Gly5-ol]-enkephalin (DAMGO), [d-Ala2,d-Leu5]-enkephalin (DADLE), trans-(±)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl) benzene-acetamide methanesulfonate (U-50,488H; 1 μM), and baclofen (50 μM) inhibited Ca2+ currents, whereas the δ-selective ligands [d-Pen2,Pen5]-enkephalin (DPDPE) and deltorphin II (1 μM) did not. The effect of DADLE (1 μM) was blocked by the μ-antagonist d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 300 nM) but not by the δ-antagonist Tyr-1,2,3,4-tetrahydroisoquinoline-Phe-Phe-OH (300 nM), implicating μ receptors. Despite a lack of functional δ receptors, flow cytometry revealed cell-surface δ receptors. We used this approach to identify conditions that up-regulate δ receptors, including μ receptor gene deletion in dorsal root ganglion neurons of μ-/- mice and 18-h incubation of μ+/+ neurons with CTAP followed by brief (10-min) DPDPE exposure. Under these conditions, the expression of cell-surface δ receptors was up-regulated to 149 ± 9 and 139 ± 5%, respectively; furthermore, DPDPE and deltorphin II (1 μM) inhibited Ca2+ currents in both cases. Viral replacement of μ receptors in μ-/- neurons reduced δ receptor expression to μ+/+ levels, restored the inhibition of Ca2+ currents by DAMGO, and abolished δ receptor coupling. Our observations suggest that δ receptor-Ca2+ channel coupling in primary afferent fibers may have little functional significance under basal conditions in which μ receptors predominate. However, up-regulation of cell-surface δ receptors induces their coupling to Ca2+ channels. Pharmacological approaches that increase functional δ receptor expression may reveal a novel target for analgesic therapy. ER -