Effects of arachidonic acid on sodium currents in rat dorsal root ganglion neurons

Brain Research 950 (2002) 95–102 www.elsevier.com / locate / bres

Research report

Effects of arachidonic acid on sodium currents in rat dorsal root ganglion neurons Geon Young Lee, Yong Kyoo Shin, Chung Soo Lee, Jin-Ho Song* Department of Pharmacology, Chung-Ang University, College of Medicine, 221 Heuk-Suk Dong, Dong-Jak Ku, Seoul 156 -756, South Korea Accepted 7 May 2002

Abstract The effects of arachidonic acid on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium currents in rat dorsal root ganglion neurons were assessed using the whole-cell patch-clamp method. Both sodium currents were modulated in a similar way by extracellular application of arachidonic acid. Arachidonic acid increased the currents at lower depolarizing potentials, while it suppressed the currents at higher depolarizing potentials and at less negative holding potentials. These effects were due to the shifts of both the conductance–voltage curve and the steady-state inactivation curve in the hyperpolarizing direction. Indomethacin, a cyclooxygenase inhibitor, suppressed the arachidonic acid-induced shift of the conductance–voltage curve but not that of the steady-state inactivation curve. 5,8,11,14-Eicosatetraynoic acid, a non-metabolizable arachidonic acid analog, failed to shift the conductance–voltage curve but still produced the shift of the steady-state inactivation curve. Thus it is assumed that the effect of arachidonic acid on the sodium channel activation is caused by the metabolite(s) of arachidonic acid. However, the effect on the steady-state sodium channel inactivation is exerted by arachidonic acid itself. It is suggested that arachidonic acid, by modulating sodium currents, may alter the excitability of sensory neurons depending on the resting membrane potential.  2002 Elsevier Science B.V. All rights reserved. Theme: Excitable membranes and synaptic transmission Topic: Sodium channels Keywords: Arachidonic acid; Sodium current; Sensory neuron; Tetrodotoxin

1. Introduction Arachidonic acid is a component of membrane lipids and is liberated by the activation of phospholipase A 2 , or by the sequential activation of phospholipase C and diacylglycerol lipase. The liberated arachidonic acid serves as a precursor of many substances such as prostaglandins, leukotriens and thromboxanes, which mediate various biological activities [15]. However, arachidonic acid itself also functions as an intra- and intercellular messenger. Various ion channels are modulated by arachidonic acid directly or indirectly. For instance arachidonic acid directly modulates NMDA receptor channels in cerebellar granule cells [19], large conductance calcium-activated potassium channels in pulmonary artery smooth muscle cells [17], *Corresponding author. Tel.: 182-2-820-5686; fax: 182-2-817-7115. E-mail address: [email protected] (J.-H. Song).

calcium channels in smooth muscle cells of ileum [22], transient A-type potassium channels expressed in Xenopus oocytes [26], and outward-rectifying potassium channels in microglia [27]. However, arachidonic acid modulates calcium-activated potassium channels in adrenal chromaffin cells via lipoxygenase metabolites [25], and the voltage-dependent outward potassium channels in neocortical neurons via cyclooxygenase metabolites [30]. Extracellular arachidonic acid suppressed the currents through sodium channels of skeletal and cardiac muscle types [3,29]. The suppression was mostly due to the shift of the steady-state inactivation curve to the hyperpolarizing potential. Arachidonic acid decreased the probability of channel opening without change in the single-channel conductance. The effects of arachidonic acid were not prevented by inhibitors of lipoxygenase, cyclooxygenase, or cytochrome P450 epoxygenase, which suggested that arachidonic acid itself might be responsible for the effects.

0006-8993 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0006-8993( 02 )03008-1

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Dorsal root ganglion (DRG) neurons are primary sensory neurons and express two distinct sodium currents based on their sensitivity to tetrodotoxin (TTX). A fastinactivating TTX-sensitive (TTX-S) sodium current is expressed in almost all DRG neurons, while a slowinactivating TTX-resistant (TTX-R) sodium current is preferentially expressed in small neurons [7,18,21]. TTX-R sodium currents are strongly implicated in the nociceptive processes. TTX-R sodium channels are co-expressed with capsaicin-receptors in nociceptive neurons [2], they are translocated from somata to the site of nerve injury and accumulated there [20], administration of oligodeoxynucleotide antisense to them decreases the prostaglandin E 2 -induced hyperalgesia [16], and null-mutant mice for them are less sensitive to noxious stimuli [1]. Arachidonic acid enhances formalin-induced nociceptive responses, while dexamethasone, a phospholipase A 2 inhibitor, reduces them [5]. Arachidonic acid is metabolized by cyclooxygenase to prostaglandins, of which prostaglandin E 2 and I 2 are the main hyperalgesic metabolites [24]. It has been reported that prostaglandin E 2 increases the amplitude of TTX-R sodium current, lowers the activation threshold of the current, and increases its rate of activation and inactivation [4,8,11,12]. Thus the modulation of TTX-R sodium current is regarded as one of the mechanisms for sensitization of nociceptors by prostaglandin E 2 . Since arachidonic acid, a precursor of prostaglandins, has been shown to modulate sodium currents in other tissues, it may also modulate sodium currents in sensory neurons and play a certain role in nociception before it is metabolized. The present study was undertaken to elucidate how arachidonic acid modulates sodium currents in rat DRG neurons.

2. Materials and methods

2.1. Cell preparation Newborn rats (2–6 days postnatal) were rapidly decapitated under ethyl ether anesthesia. The vertebral column was removed and cut longitudinally. The dorsal root ganglia were plucked from between the vertebrae of the spinal column. The ganglia were incubated at 36 8C first in a solution of collagenase (1.25 mg / ml, type II-S, Sigma Chemical Co., St. Louis, MO) for 15 min, and then in a solution of trypsin (2.5 mg / ml, Type IX, Sigma) for 10 min in Ca 21 - and Mg 21 -free phosphate-buffered saline (Sigma). After enzyme treatment, ganglia were rinsed with Dulbecco’s Modified Eagle Medium (DMEM, GibcoBRL, Grand Island, NY) supplemented with fetal bovine serum (10%, v / v, GibcoBRL). Single cells were mechanically dissociated by repeated triturations using a fire-polished Pasteur pipette and plated on poly-L-lysine (Sigma)-coated glass coverslips (12 mm, Warner Instruments Co., Hamden, CT). Cells were maintained in DMEM containing

fetal bovine serum at 36 8C in a humidified atmosphere (95% air–5% CO 2 ) for 2–7 h before patch clamp experiments.

2.2. Electrophysiological recording Cells attached to coverslip were transferred into a recording chamber on the stage of an inverted microscope. Ionic currents were recorded under voltage-clamp conditions by the whole-cell patch clamp technique [14]. Suction pipettes (0.9–1.0 MV) were fabricated from borosilicate glass capillary tubes (G150TF-4, Warner Instrument Co.) using a two-step vertical puller (PP-83, Narishige, Tokyo, Japan) and heat-polished with a microforge (MF-83, Narishige). The pipette solution contained (in mM): NaCl 10, CsCl 65, CsF 70, HEPES 10, at pH 7.2 with CsOH. The external solution contained (in mM): NaCl 30, choline chloride 120, tetraethylammonium chloride 20, D-glucose 5, HEPES 5, MgCl 2 1, CaCl 2 1, at pH 7.4 with tetraethylammonium hydroxide. Lanthanum (LaCl 3 , 10 mM) was added to block calcium currents. An Ag–AgCl pellet / 3 M KCl-agar bridge was used for the reference electrode. Whole cell currents were recorded with an Axopatch-1D patch-clamp amplifier (Axon Instruments, Foster City, CA), and digitized by an analog-to-digital interface (Digidata 1200A, Axon Instruments). Currents were filtered with a low-pass Bessel filter at 5 kHz and sampled at 50 kHz using pCLAMP6 software (Axon Instruments) on an IBM-compatible PC. Series resistance was compensated 60–70%. Capacitative and leakage currents were subtracted by using a P1P/ 4 procedure. The liquid junction potential between internal and external solution was an averaged 24 mV. The data shown in this paper were corrected for the liquid junction potential. All experiments were performed at 22–24 8C. Arachidonic acid, 5,8,11,14-eicosatetraynoic acid (ETYA), indomethacin (all purchased from Sigma) were dissolved in dimethylsulfoxide (DMSO) as 10 or 30 mM stock solutions, which were stored frozen at 220 8C in small aliquots for up to 3 weeks. They were diluted in the external solution to the desired concentrations just before experiment. The concentration of DMSO in the external solution after dilution of chemicals was fixed to 0.1% (v / v). A period of longer than 10 min was allowed after the establishment of the whole-cell recording configuration to ensure adequate equilibration between the internal pipette solution and the cell interior and to obtain a stable membrane current.

2.3. Data analysis Data were analyzed by a combination of pCLAMP6 and SigmaPlot (Jandel Scientific, San Rafael, CA) software programs. All results are presented as mean6S.E.M. and n represents the number of the cells examined. Statistical

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significance was determined by P,0.05, using an unpaired Student’s t-test.

3. Results

3.1. Effect of arachidonic acid on the sodium current amplitude TTX-S and TTX-R sodium currents were isolated from each other on the basis of TTX-sensitivity and the differences in their current kinetics. A fast current recorded in large cells was completely blocked by TTX 100 nM and was identified as TTX-S sodium current (Fig. 1). In small cells both fast and slow currents were recorded and upon application of TTX 100 nM the fast current disappeared. The remaining slow current was identified as TTX-R sodium current (Fig. 2). Arachidonic acid had both stimulatory and inhibitory

Fig. 2. Effects of arachidonic acid (AA) on TTX-R sodium currents. Currents were evoked by 40 ms step depolarizations to 220 mV (A, B) or 0 mV (C, D) from a holding potential of 2100 mV (A, C) or 280 mV (B, D) every 15 s. Thin line, control current traces. Thick line, current traces recorded after 5 min application of arachidonic acid (10 mM).

Fig. 1. Effects of arachidonic acid (AA) on TTX-S sodium currents. Currents were evoked by 10 ms step depolarizations to 240 mV (A, B) or 0 mV (C, D) from a holding potential of 2100 mV (A, C) or 280 mV (B, D) every 15 s. Thin line, control current traces. Thick line, current traces recorded after 5 min application of arachidonic acid (10 mM).

effects on sodium currents depending on the holding potential and the depolarizing potential. After 5 min application of arachidonic acid 10 mM, TTX-S sodium current evoked by a depolarization to 240 mV from a holding potential of 2100 mV was increased by 420696% (n57), and one evoked from a holding potential of 280 mV was increased by 194619% (n57). However, arachidonic acid 10 mM decreased TTX-S sodium current evoked by a depolarization to 0 mV from a holding potential of 2100 mV by 563% (n57), and one evoked from a holding potential of 280 mV by 1865% (n57) (Fig. 1). TTX-R sodium current was modulated by arachidonic acid similarly to TTX-S sodium current (Fig. 2). Arachidonic acid 10 mM increased TTX-R sodium current in response to a depolarizing pulse to 220 mV from 2100 mV and one from 280 mV by 184617% (n57) and 149610% (n57), respectively, but decreased the current in response to a depolarizing pulse to 0 mV by 763% (n57) and 663% (n57), respectively.

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3.2. Effects of arachidonic acid on the activation of sodium channels The observation that arachidonic acid increased the current amplitude and accelerated the rise and decay of the current at lower voltage suggested that arachidonic acid might modulate the activation voltage of sodium channels. To verify this, the effects of arachidonic acid on the current–voltage relationship and the conductance–voltage relationship of sodium currents were tested. Sodium currents were evoked by step depolarizations to various test potentials in 5-mV increments delivered every 5 s from a holding potential of 2110 mV. The current– voltage relationship curve was constructed by plotting the current amplitude as a function of the test potential. TTX-S and TTX-R sodium currents started to appear at around 250 mV and 230 mV, respectively, and reached the peak amplitude at around 220 mV and 25 mV, respectively (Fig. 3A and C). In the presence of arachidonic acid 10 mM these voltages were shifted to the hyperpolarizing direction. Arachidonic acid increased the amplitude of both

types of sodium currents at the negative slope region of the current–voltage relationship curve but hardly affected the currents at the positive slope region. In order to quantify the effects of arachidonic acid on the channel activation, the conductance–voltage relationship curves for the two types of sodium currents were constructed (Fig. 3B and D). The curves were drawn according to the Boltzmann equation, g /gmax 5 1 / h1 1 exp [(Vg0.5 2Vg) /k]j, where g is conductance, gmax is maximum conductance, Vg is test potential, Vg0.5 is the potential at which g is 0.5gmax , and k is the slope factor. Conductance was calculated by using the equation, g 5 I /(Vg 2 Vrev ), where I is current amplitude and Vrev is the reversal potential. Arachidonic acid shifted the curves for both types of sodium currents in the hyperpolarizing direction. The conductance–voltage data were best fitted when Vg0.5 was 228.962.0 mV and k was 6.0560.32 mV (n57) in TTX-S sodium currents, and 213.961.6 mV and 5.3060.29 mV (n57), respectively, in TTX-R sodium currents. In control experiment where DMSO (0.1%, v / v) was superfused for 5 min these values were spontaneously changed by 22.760.4 mV and 0.0060.40 mV (n57), respectively, in TTX-S sodium currents, and 21.360.5 mV and 0.3060.14 mV (n57), respectively, in TTX-R sodium currents. In the presence of arachidonic acid 10 and 30 mM for 5 min Vg0.5 values were shifted by 27.760.6 mV (n57, P,0.001) and 29.760.7 mV (n57, P,0.001), respectively, in TTX-S sodium currents (Fig. 3B and 5A), and –4.060.8 mV (n57, P,0.05) and 26.060.9 mV (n57, P,0.001), respectively, in TTX-R sodium currents (Fig. 3D and 5B), all of which were significantly different from the control values. For both types of sodium currents k values were not significantly changed by arachidonic acid.

3.3. Effects of arachidonic acid on the steady-state inactivation of sodium channels

Fig. 3. Effects of arachidonic acid (AA) on the activation of TTX-S (A, B) and TTX-R (C, D) sodium currents. (A, C) Representative current– voltage (I–V ) relationship curves. Currents were evoked by 10 ms (A) or 40 ms (C) step depolarizations to various test potentials from a holding potential of 2110 mV. Current amplitude was plotted as a function of the test potential. (B, D) The conductance–voltage (C–V ) curves. The curves were drawn according to the Boltzmann equation (see text). s, Control; d, arachidonic acid 10 mM for 5 min (B, n57; D, n57); g, conductance; gmax , maximum conductance.

The effects of arachidonic acid on the steady-state inactivation of sodium channels were investigated. The holding potential was changed to various levels for 10 s, and was immediately followed by a step depolarization to 0 mV. The current amplitude normalized to the maximum current amplitude was plotted as a function of the holding potential (Fig. 4). The steady-state inactivation curves were drawn according to the Boltzmann equation, I /Imax 5 1 / h1 1 exp[(Vh 2Vh 0.5 ) /k]j, where I is current amplitude, Imax is maximum current amplitude, Vh is holding potential, Vh 0.5 is the potential at which I is 0.5Imax , and k is the slope factor. Arachidonic acid shifted the curves for both types of sodium currents in the hyperpolarizing direction. The curves were best fitted when Vh 0.5 was 274.060.9 mV and k was 6.5960.18 mV (n58) in TTX-S sodium currents, and 260.861.4 mV and 4.9560.07 mV (n57), respectively, in TTX-R sodium currents. The

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3.4. Suppression of the arachidonic acid-induced shifts of the activation voltages by indomethacin Indomethacin inhibits cyclooxygenase and hence the production of prostaglandins. It was investigated whether the effects of arachidonic acid on sodium currents were mediated through its metabolite prostaglandins by using indomethacin. Indomethacin suppressed the shifts of the activation voltages caused by arachidonic acid. Indomethacin 10 mM alone for 5 min produced shifts of Vg0.5 values by 23.360.5 mV (n57) in TTX-S sodium currents, and 23.160.7 mV (n57) in TTX-R sodium currents, which were not significantly different from the spontaneous shifts (Fig. 5). A co-application of arachidonic acid 10 mM and indomethacin 10 mM for 5 min produced shifts of Vg0.5 values by 22.660.8 mV (n57) in TTX-S sodium currents, and 22.560.4 mV (n58) in TTX-R sodium currents (Fig. 5). The shifts were far less than those produced by arachidonic acid 10 mM alone and were not significantly different from the spontaneous shifts. Indomethacin, however, could not suppress the shifts of the steady-state inactivation voltages caused by arachidonic acid. Indomethacin 10 mM alone for 5 min produced shifts of the Vh 0.5 values by 22.260.4 mV (n57) in TTX-S sodium currents, and 22.660.5 mV (n57) in TTX-R sodium currents, which were not significantly different from the spontaneous shifts (Fig. 6). When arachidonic acid 10 mM and indomethacin 10 mM were added together the values were shifted by 211.060.8 mV (n57, P,0.001) in TTX-S sodium currents, and Fig. 4. Effects of arachidonic acid (AA) on the steady-state inactivation curves for TTX-S (A) and TTX-R (B) sodium currents. The membrane potential was held at various levels for 10 s, and then the current was evoked by a step depolarization to 0 mV. The current amplitude was normalized to the maximum current amplitude. The curves were drawn according to the Boltzmann equation (see text). s, Control; d, arachidonic acid 10 mM for 5 min (A, n58; B, n57).

spontaneous shifts of Vh 0.5 and k in the presence of DMSO (0.1%, v / v) for 5 min were 21.460.4 mV and 0.0360.04 mV (n57), respectively, in TTX-S sodium currents, and 21.560.5 mV and 0.0960.21 mV (n57), respectively, in TTX-R sodium currents. Arachidonic acid 10 and 30 mM for 5 min produced shifts of Vh 0.5 values by 26.560.5 mV (n58, P,0.001) and 29.961.0 mV (n57, P,0.001), respectively, in TTX-S sodium currents (Fig. 4A and 6A), and 210.561.1 mV (n57, P,0.001) and 210.960.7 mV (n57, P,0.001), respectively, in TTX-R sodium currents (Fig. 4B and 6B), all of which were significantly different from the spontaneous shifts. Arachidonic acid, however, did not cause a significant change of the slope factor k in either type of sodium current.

Fig. 5. Comparison of the shift in the conductance–voltage relationship curves for TTX-S (A) and TTX-R (B) sodium currents. Vg0.5 , voltage at which the conductance is half-maximum; AA 10, arachidonic acid 10 mM; AA 30, arachidonic acid 30 mM; IM 10, indomethacin 10 mM; ETYA 10, eicosatetraynoic acid 10 mM. *P,0.05, ***P,0.001, compared with control.

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Fig. 6. Comparison of the shift in the steady-state inactivation curves for TTX-S (A) and TTX-R (B) sodium currents. Vh 0.5 , the half-maximum inactivation voltage; AA 10, arachidonic acid 10 mM; AA 30, arachidonic acid 30 mM; IM 10, indomethacin 10 mM; ETYA 10, eicosatetraynoic acid 10 mM. ***P,0.001, compared with control.

213.461.0 mV (n57, P,0.001) in TTX-R sodium currents (Fig. 6). The shifts were greater than those produced by arachidonic acid 10 mM alone.

3.5. Effects of eicosatetraynoic acid on sodium currents 5,8,11,14-Eicosatetraynoic acid (ETYA) is an analog of arachidonic acid but is not metabolized. ETYA hardly affected the activation process of sodium currents. ETYA 10 mM for 5 min produced shifts of Vg0.5 values for TTX-S and TTX-R sodium currents by 20.460.9 mV (n57, P,0.05) and 11.160.6 mV (n57, P,0.05), respectively, which were less than the spontaneous shifts or even opposite direction (Fig. 5). ETYA, however, shifted the steady-state inactivation voltages of sodium currents. In the presence of ETYA 10 mM for 5 min the Vh 0.5 values for TTX-S and TTX-R sodium currents were shifted by 24.960.7 mV (n57, P,0.001) and 27.860.4 mV (n57, P,0.001), respectively (Fig. 6). The shifts were significantly different from the spontaneous shifts but less than those caused by the same concentration of arachidonic acid.

4. Discussion Arachidonic acid modulated two types of sodium currents in rat DRG neurons in a voltage-dependent manner. The amplitude of TTX-S sodium current evoked by a depolarizing potential of 240 mV was increased by

arachidonic acid, and the effect was greater when the holding potential was 2100 mV than when it was 280 mV. However, arachidonic acid decreased the current amplitude at higher depolarizing potential of 0 mV, the decrease being greater when the holding potential was 280 mV than when it was 2100 mV (Fig. 1). Similarly, arachidonic acid increased TTX-R sodium current evoked by a lower depolarizing potential of 220 mV, but slightly decreased the current evoked by a higher depolarizing potential of 0 mV (Fig. 2). The difference of the effects between holding potential of 2100 mV and 280 mV was not as large as it was in TTX-S sodium currents. The dependence of the effects of arachidonic acid on sodium currents on both the depolarizing potential and the holding potential suggested that arachidonic acid might change both the activation and the inactivation voltages of sodium currents. Indeed, arachidonic acid shifted the conductance–voltage relationship curves of both types of sodium currents in the hyperpolarizing direction (Fig. 3). This would lower the threshold for the action potential and increase the excitability of sensory neurons. The effects were greater at 30 mM than at 10 mM, implying that the effects were dose-dependent. Arachidonic acid increased both sodium currents at the negative slope regions of the current–voltage curves. However, arachidonic acid did not change the currents at the positive slope regions. Thus arachidonic acid does not seem to change the maximum conductance. The steady-state inactivation curves of both types of sodium currents were also shifted in the hyperpolarizing direction (Fig. 4). Since a long holding potential duration of 10 s was used, it is not clear whether fast or slow inactivation was altered by arachidonic acid. The inhibitory effect of arachidonic acid on the sodium current amplitude at the depolarizing potential of 0 mV (Figs. 1 and 2) appears to be mostly due to the shift of the steady-state inactivation curve, since the maximum conductance in the current–voltage curve, which was measured at 2110 mV, was not changed (Fig. 3). The steadystate inactivation curve for TTX-R sodium current lies at more depolarized potential than that for TTX-S sodium current, and this explains the observation that the current inhibition by arachidonic acid was dependent on holding potential in TTX-S sodium current, while it was not so much in TTX-R sodium current. By shifting the steadystate inactivation voltage of sodium channels arachidonic acid is expected to suppress the excitation of sensory neurons. The suppression, however, might be greater in cells expressing TTX-S sodium channels than those expressing TTX-R sodium channels, since at the resting membrane potential of 280 mV [23] |30% of TTX-S sodium channels are inactivated while TTX-R sodium channels are largely relieved from the inactivation. Indomethacin by itself did not affect the voltage dependence of sodium channels. However, it suppressed the arachidonic acid-induced shift of the conductance–voltage

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curves of both sodium channels with no effect on the arachidonic acid-induced shift of the steady-state inactivation curves. ETYA produced shifts of the steady-state inactivation curves of both sodium currents, but it did not shift the conductance–voltage curves. Therefore it is assumed that the effect of arachidonic acid on the sodium channel activation is not caused by arachidonic acid molecule but by its metabolite(s). This is well supported by other investigations that prostaglandin E 2 increased the amplitude of and produced a shift of the conductance– voltage curve of TTX-R sodium current in DRG neurons [4,8,12,11]. On the contrary, the shift of the steady-state inactivation curve is supposed to be caused directly by arachidonic acid. This view is in agreement with previously reported observations that the shifts of the steady-state inactivation curves of skeletal and cardiac sodium channels produced by arachidonic acid were not prevented by inhibitors of its metabolism [3], and prostaglandin E 2 did not affect the steady-state inactivation of TTX-R sodium channels in DRG neurons [11]. Extracellular arachidonic acid has been shown to suppress sodium currents in many tissues, such as cardiac muscles, skeletal muscles and striatal neurons, by causing a hyperpolarizing shift of the steady-state inactivation curve, without affecting the activation voltage [3,29,6,10,28]. In DRG neurons, however, arachidonic acid had both excitatory and inhibitory effects on sodium currents regardless of current types. The effects were rendered by the hyperpolarizing shift of both the conductance–voltage and the steady-state inactivation curves, respectively. The former effect was proven to be caused by the metabolite(s) of arachidonic acid, probably prostaglandin(s), and the latter by arachidonic acid itself. Thus the molecular ratio of arachidonic acid and its metabolite(s) would be an important factor to determine whether arachidonic acid increases or suppresses the excitability of sensory neurons. Nevertheless, at the resting potential of 280 mV arachidonic acid is expected to increase the excitability of neurons with TTX-R sodium currents but to decrease those with TTX-S sodium currents due to their different steadystate inactivation voltages. TTX-S sodium currents are expressed in all populations of DRG neurons and thus arachidonic acid would depress all senses generally. TTXR sodium currents take part in nociception, and the metabolites of arachidonic acid by shifting the conductance–voltage curve would sensitize the nociceptive neurons. Multiple sodium channel isoforms are expressed in DRG neurons. Na v 1.1, Na v 1.6 and Na v 1.7 are TTX-S sodium channels, and Na v 1.8 and Na v 1.9 are TTX-R sodium channels [13]. Their distribution is heterogeneous among different cell types and their kinetic properties are somewhat different [9]. Thus it is possible that different sodium channel isoforms might be differentially modulated by arachidonic acid and its metabolites in DRG neurons.

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Acknowledgements This work was supported by Korean Research Foundation Grant (KRF-2000-003-F00025).

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