title: Possible role of phosphatidylinositol 4,5 bisphosphate in luteinizing hormone releasing hormone-mediated M-current inhibition in bullfrog sympathetic neurons.
abstract: Luteinizing hormone releasing hormone (LHRH) is a physiological modulator of neuronal excitability in bullfrog sympathetic ganglia (BFSG). Actions of LHRH involve suppression of the noninactivating, voltage-dependent M-type K+ channel conductance (gM). We found, using whole-cell recordings from these neurons, that LHRH-induced suppression of gM was attenuated by the phospholipase C (PLC) inhibitor U73122 (10 microM) but not by the inactive isomer U73343 (10 microM). Buffering internal Ca2+ to 117 nM with intracellular 20 mM BAPTA + 8 mM Ca2+ or to < 10 nM with intracellular 20 mM BAPTA + 0.4 mM Ca2+ did not attenuate LHRH-induced gM suppression. Suppression of gM by LHRH was not antagonized by the inositol 1,4,5 trisphosphate (InsP3) receptor antagonist heparin (approximately 300 microM). Preventing phosphatidylinositol-4,5-bisphosphate (PIP2) synthesis by blocking phosphatidylinositol-4-kinase with wortmannin (10 microM) or with the nonhydrolysable ATP analogue AMP-PNP (3 mM) prolonged recovery of LHRH-induced gM suppression. This effect was not produced by blocking phosphatidyl inositol-3-kinase with LY294002 (10 microM). Rundown of gM was attenuated when cells were dialysed with 240 microM di-octanoyl PIP2 or 240 microM di-octanoyl phosphatidylinositol-3,4,5-trisphosphate (PIP3) but not with 240 microM di-octanoyl phosphatidylcholine. LHRH-induced gM suppression was competitively antagonized by dialysis with 240 microM di-octanoyl PIP2, but not with di-octanoyl phosphatidylcholine. These results would be expected if LHRH-induced gM suppression reflects a PLC-mediated decrease in plasma membrane PIP2 levels.
authors.affiliation: Centre for Neuroscience and Department of Pharmacology, 9.75 Medical Sciences Building, University of Alberta, Edmonton, Alberta, T6G 2H7, Canada.
authors.forename: Christopher P
authors.initials: CP
authors.lastname: Ford
authors.forename: Patrick L
authors.initials: PL
authors.lastname: Stemkowski
authors.forename: Peter A
authors.initials: PA
authors.lastname: Smith
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002478:Cells, Cultured; D002614:Chelating Agents / Q000494:pharmacology; D003956:Dialysis / Q000379:methods; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004533:Egtazic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D004553:Electric Conductivity; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D005728:Ganglia, Sympathetic / Q000166:cytology*; D007987:Gonadotropin-Releasing Hormone / Q000502:physiology*; D006493:Heparin / Q000494:pharmacology; D042541:Intracellular Space / Q000187:drug effects; D015640:Ion Channel Gating / Q000187:drug effects; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D009474:Neurons / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D018408:Patch-Clamp Techniques / Q000379:methods; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000502:physiology*; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D011892:Rana catesbeiana / Q000502:physiology; D013997:Time Factors
chemical_list: D002614:Chelating Agents; D004791:Enzyme Inhibitors; D019269:Phosphatidylinositol 4,5-Diphosphate; D015221:Potassium Channels; D007987:Gonadotropin-Releasing Hormone; D004533:Egtazic Acid; D006493:Heparin; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium