Nitric oxide modulation of calcium‐activated potassium channels in postganglionic neurones of avian cultured ciliary ganglia

Abstract

1 A study has been made of the modulation of calcium-activated potassium channels in cultured neurones of avian ciliary ganglia by sodium nitroprusside and L-arginine.2 Sodium nitroprusside (100 gM) reduced the net outward current by 22± 1% at 4.8 ms (mean ± s.e.mean) and 25 ± 1% at 350 ms during a test depolarization to + 40 mV from a holding potential of -40 mV. The outward current remained reduced for the duration of the recording following a single application of sodium nitroprusside. These effects did not occur if the influx of calcium ions was first blocked with Cd2+ (500 ytM). Application of ferrocyanide (100 j.M) reduced the net outward current by only 6 ± 3% at 350 ms during a test depolarization to + 40 mV.3 L-Arginine (270 IM) reduced the net outward current on average by 19 ± 2% at 4.8 ms and 22 ± 2% at 350 ms during a test depolarization to + 40 mV. The current remained in this reduced state for the duration of the recording following a single application of L-arginine. These effects were reduced to 11 ± 1% at 4.8 ms and 11 ± 2% at 350 ms in the presence of Nw-nitro-L-arginine methyl ester (L-NAME, 100 gM). 4 In order to alleviate the dependence of calcium-activated potassium channels (Ik(Ca)) on the inward flux of calcium ions, the patch-clamp pipettes were filled with a solution containing 100 gM CaC12, and the Ca2+ in the bathing solution was replaced with EGTA. Under these conditions sodium nitroprusside reduced the total outward current during a depolarizing pulse of + 40 mV by 9 ± 1% at 4.8 ms and by 36 ± 3% at 350 ms. L-Arginine (270 gM) reduced this current under the same conditions by 9 ± 1% at 4.8ms and by 35 ± 2% at 350ms. 5 Calcium-activated potassium currents were sensitive to apamin (50 nM), as this reduced the outward current by 23 ± 3% at 350 ms when a high calcium-containing pipette was used during a depolarizing command to +40 mV. L-Arginine still decreased the outward current in the presence of apamin (50 nM), by 5 ± 1% at 4.8 ms and by 19 ± 2% at 350 ms, indicating that L-arginine could reduce an apamin-insensitive Ik(Ca) 6 Calcium-activated potassium currents were also sensitive to charybdotoxin (10 nM), as this reduced the outward current by 34 ± 4% at 350 ms when a high calcium-containing pipette was used during a depolarizing command to + 40 mV. L-Arginine still decreased the outward current in the presence of charybdotoxin, by 6 ± 1% at 4.8 ms and 12 ± 4% at 350 ms, showing that L-arginine could reduce a charybdotoxin-insensitive Ik(Ca). 7 The present results indicate that NO-synthase in ciliary ganglia can modulate Ik(Ca) by a method which is independent of the action of NO on the calcium channels. The Ik(ca) is decreased significantly at 4.8 ms into a depolarizing pulse, at a time that would decrease the rate of repolarization of the action potential. Ik(Ca) is also reduced at longer times (350 ms), indicating an affect on the inactivating process.

Expression of gLTP in Sympathetic Ganglia from Stress-hypertensive Rats: Molecular Evidence

“…Stimulation of guanylyl cyclase by NO triggers increased production of cyclic guanine monophosphate (cGMP), which, in turn, phosphorylates the enzyme, phosphodiesterase, so that it can no longer break down cyclic adenosine monophosphate (cAMP; Maurice and Haslam 1990). The resulting elevated level of cAMP increases the phosphorylation of calciumactivated potassium channel in the terminal membrane, which leads to a reduction in the opening of the channel and a slight depolarization of the terminal (Cetiner and Bennett 1993). As a result, the incoming action potential duration is enhanced, leading to an increase in calcium influx, which enhances transmitter release (Scott and Bennett 1993a).…”

Nitric oxide modulation of quantal secretion in chick ciliary ganglia.

“…One possible way in which second messengers activated by NO may enhance quantal secretion is to increase the duration of the terminal action potential (Cetiner & Bennett, 1993). In order to test this possibility, the duration of the action potential in the nerve terminal calyx was determined in the presence of SNP (100 ,UM).…”

“…7). It has been suggested that cAMP might be elevated during the initiation of LTP by cGMP inhibiting cAMP-phosphodiesterase in the calyx (Cetiner & Bennett, 1993 quantal secretion by NO, ganglia were exposed to 100 /SM 8-Br-cAMP and a quantal analysis was carried out. The EPSPs were significantly increased by 44 + 9% (P < 0 05; n = 4).…”

NO decreases evoked quantal ACh release at a synapse of Aplysia by a mechanism independent of Ca2+ influx and protein kinase G.

“…tIC and D). This is not true of all neurones as NO has been shown to modify K+ conductance in cultured neurones of avian ciliary ganglia (Cetiner & Bennett, 1993).…”