Induction of delta opioid receptor function by up-regulation of membrane receptors in mouse primary afferent neurons.
It is not clear whether primary afferent neurons express functional cell-surface opioid receptors. We examined delta receptor coupling to Ca2+ channels in mouse dorsal root ganglion neurons under basal conditions and after 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 microM), and baclofen (50 microM) inhibited Ca2+ currents, whereas the -selective ligands [D-Pen2,Pen5]-enkephalin (DPDPE) and deltorphin II (1 microM) did not. The effect of DADLE (1 microM) was blocked by the mu-antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 300 nM) but not by the -antagonist Tyr-1,2,3,4-tetrahydroisoquinoline-Phe-Phe-OH (300 nM), implicating mu receptors. Despite a lack of functional delta receptors, flow cytometry revealed cell-surface receptors. We used this approach to identify conditions that up-regulate receptors, including mu receptor gene deletion in dorsal root ganglion neurons of mu-/- mice and 18-h incubation of mu+/+ neurons with CTAP followed by brief (10-min) DPDPE exposure. Under these conditions, the expression of cell-surface delta receptors was up-regulated to 149 +/- 9 and 139 +/- 5%, respectively; furthermore, DPDPE and deltorphin II (1 microM) inhibited Ca2+ currents in both cases. Viral replacement of mu receptors in mu-/- neurons reduced delta receptor expression to mu+/+ levels, restored the inhibition of Ca2+ currents by DAMGO, and abolished receptor coupling. Our observations suggest that receptor-Ca2+ channel coupling in primary afferent fibers may have little functional significance under basal conditions in which mu receptors predominate. However, up-regulation of cell-surface delta receptors induces their coupling to Ca2+ channels. Pharmacological approaches that increase functional delta receptor expression may reveal a novel target for analgesic therapy.
Walwyn
Wendy
W
Department of Psychiatry and Biobehavioral Sciences, Center for Health Sciences, University of California, Los Angeles, USA.
Maidment
Nigel T
NT
Sanders
Matthew
M
Evans
Christopher J
CJ
Kieffer
Brigitte L
BL
Hales
Tim G
TG
D000818:Animals; D015220:Calcium Channels / Q000378:metabolism; D005727:Ganglia, Spinal / Q000166:cytology; D008024:Ligands; D051379:Mice; D018345:Mice, Knockout; D009475:Neurons, Afferent / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000032:analysis / Q000235:genetics; D011957:Receptors, Opioid / Q000032:analysis / Q000235:genetics*; D017465:Receptors, Opioid, delta / Q000032:analysis / Q000235:genetics / Q000502:physiology*; D017450:Receptors, Opioid, mu / Q000032:analysis / Q000378:metabolism; D015854:Up-Regulation
D015220:Calcium Channels; D008024:Ligands; D011956:Receptors, Cell Surface; D011957:Receptors, Opioid; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu