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.
Samways
Damien S K
DS
Department of Pharmacology, University of Bristol, University Walk, Bristol BS8 1TD, UK.
Li
Wen-hong
WH
Conway
Stuart J
SJ
Holmes
Andrew B
AB
Bootman
Martin D
MD
Henderson
Graeme
G
D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology; D002217:Carbachol / Q000494:pharmacology; D045744:Cell Line, Tumor; D003593:Cytoplasm / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D006801:Humans; D053496:Inositol 1,4,5-Trisphosphate Receptors; D043587:Receptor, Muscarinic M3 / Q000502:physiology*; D018160:Receptors, Cytoplasmic and Nuclear / Q000819:agonists / Q000502:physiology; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000502:physiology*; D015398:Signal Transduction; D013849:Thimerosal / Q000494:pharmacology; D014466:Ultraviolet Rays
D015220:Calcium Channels; C506926:ITPR1 protein, human; D053496:Inositol 1,4,5-Trisphosphate Receptors; D043587:Receptor, Muscarinic M3; D018160:Receptors, Cytoplasmic and Nuclear; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D013849:Thimerosal; D002110:Caffeine; D002217:Carbachol; D002118:Calcium