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.

Activation of Epidermal Growth Factor Receptor Inhibits KCNQ2/3 Current through Two Distinct Pathways: Membrane PtdIns(4,5)P2Hydrolysis and Channel Phosphorylation

“…It was therefore proposed that channel inhibition by G q -protein-coupled receptors might result from the depletion of membrane PtdIns(4,5)P 2 as a result of its hydrolysis (Suh and Hille, 2002;Ford et al, 2003Ford et al, , 2004Winks et al, 2005).…”

L‐ and N‐current but not M‐current inhibition by M1 muscarinic receptors requires DAG lipase activity

“…The present model proposes that depletion of phosphatidylinositol-4,5-bisphosphate (PIP 2 ) by phospholipase C (PLC) during M 1 R stimulation establishes a concentration gradient of PIP 2 in the membrane. As a pool of PIP 2 associated with KCNQ channels disengages and diffuses down the gradient, M-current decreases Hille, 2002, 2005;Zhang et al, 2003;Ford et al, 2004; reviewed in Delmas and Brown, 2005).…”

Signalling Events Employed in the Hypertonic Activation of Cation Channels in HeLa Cells

“…In contrast, with U73343 [22][23][24] as an inactive analogue of U73122, currents were virtually identical to control values (i.e. -3.47 ± 0.61 pA/pF, n = 5; Figure 3a).…”

“…In the presence of genistein [9,[17][18][19], pertussis toxin [9,20,21] and U73122 [9,[22][23][24], HICC currents were significantly reduced to -1.08 ± 0.16 pA/pF (n = 6, p < 0.001), -1.09 ± 0.55 pA/pF (n = 10, p < 0.01) and -0.48 ± 0.02 pA/pF (n = 5, p < 0.001), respectively. In contrast, with U73343 [22][23][24] as an inactive analogue of U73122, currents were virtually identical to control values (i.e.…”