Co-incident signalling between μ-opioid and M₃ muscarinic receptors at the level of Ca²⁺ release from intracellular stores: lack of evidence for Ins(1,4,5)P₃ receptor sensitization

Samways, Damien S. K.; Li, Wen-hong; Conway, Stuart J.; Holmes, Andrew B.; Bootman, Martin D. and Henderson, Graeme (2003). Co-incident signalling between μ-opioid and M₃ muscarinic receptors at the level of Ca²⁺ release from intracellular stores: lack of evidence for Ins(1,4,5)P₃ receptor sensitization. Biochemical Journal, 375(3) pp. 713–720.

DOI: https://doi.org/10.1042/BJ20030508

Abstract

Activation of G_i/G_o-coupled opioid receptors increases [Ca²⁺]_i (intracellular free-Ca²⁺ concentration), but only if there is concomitant G_q-coupled receptor activation. This G_i/G_o-coupled receptor-mediated [Ca²⁺]_i increase does not appear to result from further production of InsP₃ [Ins(1,4,5)P₃] in SH-SY5Y cells. In the present study, fast-scanning confocal microscopy revealed that activation of μ-opioid receptors alone by 1 μM DAMGO ([D-Ala, NMe-Phe, Gly-ol]-enkephalin) did not stimulate the InsP₃-dependent elementary Ca²⁺-signalling events (Ca²⁺ puffs), whereas DAMGO did evoke Ca²⁺ puffs when applied during concomitant activation of M3 muscarinic receptors with 1 μM carbachol. We next determined whether μ-opioid receptor activation might increase [Ca²⁺]_i by sensitizing the InsP₃ receptor to InsP₃. DAMGO did not potentiate the amplitude of the [Ca²⁺]_i increase evoked by flash photolysis of the caged InsP₃ receptor agonist, caged 2,3-isopropylidene-InsP₃, whereas the InsP₃ receptor sensitizing agent, thimerosal (10 μM), did potentiate this response. DAMGO also did not prolong the rate of decay of the increase in [Ca²⁺]_i evoked by flash photolysis of caged 2,3-isopropylidene-InsP₃. Furthermore, DAMGO did not increase [Ca²⁺]_i in the presence of the cell-membrane-permeable InsP₃ receptor agonist, InsP₃ hexakis(butyryloxymethyl) ester. Therefore it appears that μ-opioid receptors do not increase [Ca²⁺]_i through either InsP₃ receptor sensitization, enhancing the releasable pool of Ca²⁺ or inhibition of Ca²⁺ removal from the cytoplasm.

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