Coincident signalling between the Gi/Go-coupled delta-opioid receptor and the Gq-coupled m3 muscarinic receptor at the level of intracellular free calcium in SH-SY5Y cells

In SH-SY5Y cells, activation of delta -opioid receptors with [D-Pen(2,5)]-e nkephalin (DPDPE; 1 muM) did not alter the intracellular free Ca2+ concentr ation [Ca2+](i). However, when DPDPE was applied during concomitant Gq-coup led m3 muscarinic receptor stimulation by carbachol or oxotremorine-M, it p roduced an elevation of [Ca2+](i). The DPDPE-evoked increase in [Ca2+](i) w as abolished when the carbachol-sensitive intracellular Ca2+ store was empt ied. There was a marked difference between the concentration-response relat ionship for the elevation of [Ca2+](i) by carbachol (EC50 13 muM, Hill slop e 1) and the concentration-response relationship for carbachol's permissive action in revealing the delta -opioid receptor-mediated elevation of [Ca2] (EC50 0.7 mM; Hill slope 1.8). Sequestration of free G protein beta gamma dimers by transient transfection of cells with a beta gamma binding protei n (residues 495-689 of the C terminal tail of G protein-coupled receptor ki nase 2) reduced the ability of delta opioid receptor activation to elevate [Ca2+](i). However, DPDPE did not elevate either basal or oxotremorine-M-ev oked inositol phosphate production indicating that delta -opioid receptor a ctivation did not stimulate phospholipase C. Furthermore, delta -opioid rec eptor activation did not result in the reversal of muscarinic receptor dese nsitization, membrane hyperpolarization or stimulation of sphingosine kinas e. There was no coincident signalling between the delta -opioid receptor an d the lysophosphatidic acid receptor which couples to elevation of [Ca2+](i ) in SH-SY5Y cells by a PLC-independent mechanism. In SH-SY5Y cells the coi ncident signalling between the endogenously expressed delta -opioid and m3 muscarinic receptors appears to occur in the receptor activation-Ca2+ relea se signalling pathway at a step after the activation of phospholipase C.