Co-incident signalling between mu-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.

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 Ins P3 [Ins(1,4,5) P3] in SH-SY5Y cells. In the present study, fast-scanning confocal microscopy revealed that activation of mu-opioid receptors alone by 1 muM DAMGO ([L-Ala, NMe-Phe, Gly-ol]-enkephalin) did not stimulate the Ins P3-dependent elementary Ca2+-signalling events (Ca2+ puffs), whereas DAMGO did evoke Ca2+ puffs when applied during concomitant activation of M3 muscarinic receptors with 1 muM carbachol. We next determined whether mu-opioid receptor activation might increase [Ca2+]i by sensitizing the Ins P3 receptor to Ins P3. DAMGO did not potentiate the amplitude of the [Ca2+]i increase evoked by flash photolysis of the caged Ins P3 receptor agonist, caged 2,3-isopropylidene-Ins P3, whereas the Ins P3 receptor sensitizing agent, thimerosal (10 muM), 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-Ins P3. Furthermore, DAMGO did not increase [Ca2+]i in the presence of the cell-membrane-permeable Ins P3 receptor agonist, Ins P3 hexakis(butyryloxymethyl) ester. Therefore it appears that mu-opioid receptors do not increase [Ca2+]i through either Ins P3 receptor sensitization, enhancing the releasable pool of Ca2+ or inhibition of Ca2+ removal from the cytoplasm.