Tetrodotoxin-sensitive and -resistant Na+ channel currents in subsets of small sensory neurons of rats

Voltage-activated Na+ channels in the primary sensory neurons are important for generation of action potentials and regulation of neurotransmitter release. The Na+ channels expressed in different types of dorsal root ganglion (DRG) neurons are not fully known. In this study, we determined the possible difference in tetrodotoxin-sensitive (TTX-S) and -resistant (TTX-R) Na+ channel currents between isolectin B4 (IB4)-positive and IB4-negative small DRG neurons. Whole-cell voltage- and current-clamp recordings were performed in acutely isolated DRG neurons labeled with and without IB4 conjugated to Alexa Fluor 594. The peak Na+ current density was significantly higher in IB4-negative than IB4-positive DRG neurons. While all the IB4-negative neurons had a prominent TTX-S Na+ current, the TTX-R Na+ current was present in most IB4-positive cells. Additionally, the evoked action potential had a higher activation threshold and a longer duration in IB4-positive than IB4-negative neurons. TTX had no effect on the evoked action potential in IB4-positive neurons, but it inhibited the action potential generation in about 50% IB4-negative neurons. This study provides complementary new information that there is a distinct difference in the expression level of TTX-S and TTX-R Na+ channels between IB4-negative than IB4-positive small-diameter DRG neurons. This difference in the density of TTX-R Na+ channels is responsible for the distinct membrane properties of these two types of nociceptive neurons.     

TNF-α enhances the currents of voltage gated sodium channels in uninjured dorsal root ganglion neurons following motor nerve injury

The ectopic discharges observed in uninjured dorsal root ganglion (DRG) neurons following various lesions of spinal nerves have been attributed to functional alterations of voltage-gated sodium channels (VGSCs). Such mechanisms may be important for the development of neuropathic pain. However, the pathophysiology underlying the functional modulation of VGSCs following nerve injury is largely unknown. Here, we studied this issue with use of a selective lumbar 5 ventral root transection (L5-VRT) model, in which dorsal root ganglion (DRG) neurons remain intact. We found that the L5-VRT increased the current densities of TTX-sensitive Na channels as well as currents in Nav1.8, but not Nav1.9 channels in uninjured DRG neurons. The thresholds of action potentials decreased and firing rates increased in DRG neurons following L5-VRT. As we found that levels of tumor necrosis factor-alpha (TNF-α) increased in cerebrospinal fluid (CSF) and in DRG tissue after L5-VRT, we tested whether the increased TNF-α might result in the changes in sodium channels. Indeed, recombinant rat TNF (rrTNF) enhanced the current densities of TTX-S and Nav1.8 in cultured DRG neurons dose-dependently. Furthermore, genetic deletion of TNF receptor 1 (TNFR-1) in mice attenuated the mechanical allodynia and prevented the increase in sodium currents in DRG neurons induced by L5-VRT. These data suggest that the increase in sodium currents in uninjured DRG neurons following nerve injury might be mediated by over-production of TNF-α.     

Comparative study of the distribution of the alpha-subunits of voltage-gated sodium channels in normal and axotomized rat dorsal root ganglion neurons

We compared the distribution of the α‐subunit mRNAs of voltage‐gated sodium channels Nav1.1–1.3 and Nav1.6–1.9 and a related channel, Nax, in histochemically identified neuronal subpopulations of the rat dorsal root ganglia (DRG). In the naïve DRG, the expression of Nav1.1 and Nav1.6 was restricted to A‐fiber neurons, and they were preferentially expressed by TrkC neurons, suggesting that proprioceptive neurons possess these channels. Nav1.7, ‐1.8, and ‐1.9 mRNAs were more abundant in C‐fiber neurons compared with A‐fiber ones. Nax was evenly expressed in both populations. Although Nav1.8 and ‐1.9 were preferentially expressed by TrkA neurons, other α‐subunits were expressed independently of TrkA expression. Actually, all IB4⁺ neurons expressed both Nav1.8 and ‐1.9, and relatively limited subpopulations of IB4⁺ neurons (3% and 12%, respectively) expressed Nav1.1 and/or Nav1.6. These findings provide useful information in interpreting the electrophysiological characteristics of some neuronal subpopulations of naïve DRG. After L5 spinal nerve ligation, Nav1.3 mRNA was up‐regulated mainly in A‐fiber neurons in the ipsilateral L5 DRG. Although previous studies demonstrated that nerve growth factor (NGF) and glial cell‐derived neurotrophic factor (GDNF) reversed this up‐regulation, the Nav1.3 induction was independent of either TrkA or GFRα1 expression, suggesting that the induction of Nav1.3 may be one of the common responses of axotomized DRG neurons without a direct relationship to NGF/GDNF supply. J. Comp. Neurol. 510:188–206, 2008. © 2008 Wiley‐Liss, Inc.     

N-Ethylmaleimide modulation of tetrodotoxin-sensitive and tetrodotoxin-resistant sodium channels in rat dorsal root ganglion neurons

The effects of N-ethylmaleimide (NEM), an alkylating reagent to protein sulfhydryl groups, on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium channels in rat dorsal root ganglion (DRG) neurons were studied using the whole cell configuration of patch-clamp technique. When currents were evoked by step depolarizations to 0 mV from a holding potential of −80 mV NEM decreased the amplitude of TTX-S sodium current, but exerted little or no effect on that of TTX-R sodium current. The inhibitory effect of NEM on TTX-S sodium channel was mainly due to the shift of the steady-state inactivation curve in the hyperpolarizing direction. NEM did not affect the voltage-dependence of the activation of TTX-S sodium channel. The steady-state inactivation curve for TTX-R sodium channel was shifted by NEM in the hyperpolarizing direction as that for TTX-S sodium channel. NEM caused a change in the voltage-dependence of the activation of TTX-R sodium channel unlike TTX-S sodium channel. After NEM treatment, the amplitudes of TTX-R sodium currents at test voltages below −10 mV were increased, but those at more positive voltages were not affected. This was explained by the shift in the conductance–voltage curve for TTX-R sodium channels in the hyperpolarizing direction after NEM treatment.     

N-Ethylmaleimide modulation of tetrodotoxin-sensitive and tetrodotoxin-resistant sodium channels in rat dorsal root ganglion neurons

The effects of N-ethylmaleimide (NEM), an alkylating reagent to protein sulfhydryl groups, on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium channels in rat dorsal root ganglion (DRG) neurons were studied using the whole cell configuration of patch-clamp technique. When currents were evoked by step depolarizations to 0 mV from a holding potential of -80 mV NEM decreased the amplitude of TTX-S sodium current, but exerted little or no effect on that of TTX-R sodium current. The inhibitory effect of NEM on TTX-S sodium channel was mainly due to the shift of the steady-state inactivation curve in the hyperpolarizing direction. NEM did not affect the voltage-dependence of the activation of TTX-S sodium channel. The steady-state inactivation curve for TTX-R sodium channel was shifted by NEM in the hyperpolarizing direction as that for TTX-S sodium channel. NEM caused a change in the voltage-dependence of the activation of TTX-R sodium channel unlike TTX-S sodium channel. After NEM treatment, the amplitudes of TTX-R sodium currents at test voltages below -10 mV were increased, but those at more positive voltages were not affected. This was explained by the shift in the conductance-voltage curve for TTX-R sodium channels in the hyperpolarizing direction after NEM treatment.     

Differential properties of tetrodotoxin-sensitive and tetrodotoxin-resistant sodium channels in rat dorsal root ganglion neurons.

TTX-sensitive (TTX-S) and TTX-resistant (TTX-R) sodium channel currents were analyzed in acutely dissociated dorsal root ganglion (DRG) neurons isolated from 3-12-d-old and adult rats. Currents were recorded using the whole-cell patch-clamp technique. TTX-R current was more likely to be present in younger animals (3-7 d), whereas TTX-S current was more common in older animals (7-10 d), although TTX-R current was recorded from adult rat DRG neurons. The TTX-R and TTX-S currents differed in their steady-state inactivation, with 50% inactivation voltage at -40 +/- 5 mV (n = 10) for TTX-R currents and -70 +/- 4 mV (n = 10) for TTX-S currents. These current types also differed in their activation kinetics, with 50% activation values of -15 +/- 5 mV (n = 5) for TTX-R currents and -26 +/- 6 mV (n = 5) for TTX-S currents. The interactions of TTX-R and TTX-S channels with various pharmacological agents and divalent cations were studied. The Kd values for TTX-S and TTX-R currents were estimated to be 0.3 nM and 100 microM for TTX, 0.5 nM and 10 microM for saxitoxin, and 50 microM and 200 microM for lidocaine, respectively. TTX-S channels did not exhibit a marked use-dependent block by lidocaine, whereas lidocaine significantly decreased TTX-R current in a use-dependent manner at frequencies ranging from 1 to 33.3 Hz. Several external divalent cations exerted different effects on these current types.(ABSTRACT TRUNCATED AT 250 WORDS)     

Modulation of sodium currents in rat dorsal root ganglion neurons by sulfur dioxide derivatives

The effect of sulfur dioxide (SO2) derivatives, a common air pollutant and exists in vivo as an equilibrium between bisulfate and sulfite, on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium channels in cultured post-natal dorsal root ganglion (DRG) neurons were studied using the whole cell configuration of patch-clamp technique. SO2 derivatives on two types of sodium currents were either inhibitory or stimulatory depending on the kinetic parameters tested. At a holding potential of -80 mV, SO2 derivatives suppressed TTX-S sodium currents when depolarizing potential was negative to -30 mV and TTX-R sodium currents when negative to -10 mV but they increased them when the depolarizing potential was positive to -30 or -10 mV. SO2 derivatives shifted the conductance-voltage curve for TTX-R sodium currents in the depolarizing direction but had little effect on that for TTX-S sodium currents. The steady-state inactivation curve for TTX-R sodium channel was shifted by SO2 derivatives in the depolarizing direction as that for TTX-S sodium channel. SO2 derivatives changed the reversal potential and increased the maximum conductance of two types of sodium channels. SO2 derivatives postponed the activating time and delayed the inactivation of sodium currents. The results suggest that SO2 derivatives would increase the excitability of neurons and alter the ion selectivity for two types of sodium currents.     

Block of sodium currents in rat dorsal root ganglion neurons by diphenhydramine

To elucidate the local anesthetic mechanism of diphenhydramine, its effects on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium currents in rat dorsal root ganglion (DRG) neurons were examined by the whole-cell voltage clamp method. Diphenhydramine blocked TTX-S and TTX-R sodium currents with K(d) values of 48 and 86 microM, respectively, at a holding potential of -80 mV. It shifted the conductance-voltage curve for TTX-S sodium currents in the depolarizing direction but had little effect on that for TTX-R sodium currents. Diphenhydramine caused a shift of the steady-state inactivation curve for both types of sodium currents in the hyperpolarizing direction. The time-dependent inactivation became faster and the recovery from the inactivation was slowed by diphenhydramine in both types of sodium currents. Diphenhydramine produced a profound use-dependent block when the cells were repeatedly stimulated with high-frequency depolarizing pulses. The use-dependent block was more pronounced in TTX-R sodium currents. The results show that diphenhydramine blocks sodium channels of sensory neurons similarly to local anesthetics.     

Taurine-induced modulation of voltage-sensitive Na+ channels in rat dorsal root ganglion neurons

The physiological role of taurine, an abundant free amino acid in the neural system, is still poorly understood. The aim of this study was to investigate its effect on TTX-sensitive (TTX-S) and TTX-resistant (TTX-R) Na+ currents in enzymatically dissociated neurons from rat dorsal root ganglion (DRG) with conventional whole-cell recording manner under voltage-clamp conditions. A TTX-S Na+ current was recorded preferentially from large DRG neurons and a TTX-R Na+ current preferentially from small ones. For TTX-S Na+ channel, taurine of the concentration > or = 10 mM shifted the activation curve in the depolarizing direction and the inactivation curve in the hyperpolarizing direction. There was no change in the activation curve for TTX-R Na+ channel and the inactivation curve was shifted in the hyperpolarizing direction slightly in the presence of taurine > or = 20 mM. When the recovery kinetics was examined, the presence of taurine resulted in a slower recovery from inactivation of TTX-S currents and no change of TTX-R ones. All the effects of taurine were weakly concentration-dependent and partly recovered quite slowly after washout. Our data indicate that taurine alters the properties of Na+ currents in intact DRG neurons. These may contribute to the understanding of taurine as a natural neuroprotectant and the potential of taurine as a useful medicine for the treatment of sensory neuropathies.     

Na(v)1.5 underlies the 'third TTX-R sodium current' in rat small DRG neurons

In addition to slow-inactivating and persistent TTX-R Na(+) currents produced by Na(v)1.8 and Na(v)1.9 Na(+) channels, respectively, a third TTX-R Na(+) current with fast activation and inactivation can be recorded in 80% of small neurons of dorsal root ganglia (DRG) from E15 rats, but in only 3% of adult small DRG neurons. The half-time for activation, the time constant for inactivation, and the midpoints of activation and inactivation of the third TTX-R Na(+) currents are significantly different from those of Na(v)1.8 and Na(v)1.9 Na(+) currents. The estimated TTX K(i) (2.11+/-0.34 microM) of the third TTX-R Na(+) current is significantly lower than those of Na(v)1.8 and Na(v)1.9 Na(+) currents. The Cd(2+) sensitivity of third TTX-R Na(+) current is closer to cardiac Na(+) currents. A concentration of 1 mM Cd(2+) is required to completely block this current, which is significantly lower than the 5 mM required to block Na(v)1.8 and Na(v)1.9 currents. The third TTX-R Na(+) channel is not co-expressed with Na(v)1.8 and Na(v)1.9 Na(+) channels in DRG neurons of E18 rats, at a time when all three currents show comparable densities. The physiological and pharmacological profiles of the third TTX-R Na(+) current are similar to those of the cardiac Na(+) channel Na(v)1.5 and RT-PCR and restriction enzyme polymorphism analysis, show a parallel pattern of expression of Na(v)1.5 in DRG during development. Taken together, these results demonstrate that Na(v)1.5 is expressed in a developmentally regulated manner in DRG neurons and suggest that Na(v)1.5 Na(+) channel produces the third TTX-R current.     

Tetrodotoxin-resistant sodium channels Na(v)1.8/SNS and Na(v)1.9/NaN in afferent neurons innervating urinary bladder in control and spinal cord injured rats

Tetrodotoxin-resistant (TTX-R) sodium channels Na(v)1.8/SNS and Na(v)1.9/NaN are preferentially expressed in small diameter dorsal root ganglia (DRG) neurons. The urinary bladder is innervated by small afferent neurons from L6/S1 DRG, of which approximately 75% exhibit high-threshold action potentials that are mediated by TTX-R sodium channels. Following transection of the spinal cord at T8, the bladder becomes areflexic and then gradually hyper-reflexic, and there is an attenuation of the TTX-R sodium currents in bladder afferent neurons. In the present study, we demonstrate that Na(v)1.8 is expressed in both bladder and non-bladder afferent neurons, while Na(v)1.9 is expressed in non-bladder afferent neurons but is rarely observed in bladder afferent neurons. In spinal cord transected rats 28-32 days following transection, there is a decreased expression of Na(v)1.8 sodium channels in bladder afferents, but no change in the expression of Na(v)1.8 in non-bladder afferent neurons. Both bladder and non-bladder afferent neurons exhibit limited increases in Na(v)1.9 expression following spinal cord transection. These results demonstrate that the expression of TTX-R channels in bladder afferent neurons changes after spinal cord transection, and these changes may contribute to the increased excitability of these neurons following spinal cord injury.     

Expression and kinetic properties of Na(+) currents in rat cardiac dorsal root ganglion neurons

The expression and properties of voltage-gated Na(+) currents in cardiac dorsal root ganglion (DRG) neurons were assessed in this study. Cardiac DRG neurons were labelled by injecting the Fast Blue fluorescent tracer into the pericardium. Recordings were performed from 138 cells. Voltage-dependent Na(+) currents were found in 115 neurons. There were 109 neurons in which both tetrodotoxin-sensitive (TTX-S, blocked by 1 microM of TTX) and tetrodotoxin-resistant (TTX-R, insensitive to 1 microM of TTX) Na(+) currents were present. Five cells expressed TTX-R current only and one cell only the TTX-S current. The kinetic properties of Na(+) currents and action potential waveform parameters were measured in neurons with cell membrane capacitance ranging from 15 to 75 pF. The densities of TTX-R (110.0 pA/pF) and TTX-S (126.1 pA/pF) currents were not significantly different. Current threshold was significantly higher for TTX-R (-34 mV) than for TTX-S (-40.4 mV) currents. V(1/2) of activation for TTX-S current (-19.6 mV) was significantly more negative than for TTX-R current (-9.2 mV), but k factors did not differ significantly. V(1/2) and the k constant for inactivation for TTX-S currents were -35.6 and -5.7 mV, respectively. These values were significantly lower than those recorded for TTX-R current for which V(1/2) and k were -62.3 and -7.7 mV, respectively. The action potential threshold was lower, the 10-90% rise time and potential width were shorter before than after the application of TTX. Based on this we drew the conclusion that action potential recorded before adding tetrodotoxin was mainly TTX-S current dependent, while the action potential recorded after the application of toxin was TTX-R current dependent. We also found 23 cells with mean membrane capacitance ranging from 12 to 35 pF (the smallest labelled DRG cells found in this study) that did not express the Na(+) current. The function of these cells is unclear. We conclude that the overwhelming majority of cardiac dorsal root ganglion neurons in which voltage-dependent Na(+) currents were present, exhibited both TTX-S and TTX-R Na(+) currents with remarkably similar expression and kinetic properties.     

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

Lysophosphatidic acid (LPA), a simple phospholipid, induces pain. To elucidate an involvement of ion channel mechanism in the LPA-induced pain, its effects on sodium currents in rat dorsal root ganglion (DRG) neurons were investigated. LPA suppressed tetrodotoxin-sensitive (TTX-S) sodium current, but increased tetrodotoxin-resistant (TTX-R) sodium current, when currents were evoked by step depolarizations to 0 mV from a holding potential of -80 mV. In both types of currents, LPA produced a hyperpolarizing shift of both activation and inactivation voltages. LPA had a negligible effect on the maximal conductance of TTX-S current, but increased that of TTX-R current. The results suggest that the enhancement of TTX-R current may contribute to the LPA-induced pain.     

ATP evokes different currents in TTX-sensitive and TTX-resistant cells of dorsal root ganglia

The relationship between the level of expression of tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium currents and the occurrence of two kinetically different ATP-induced currents in rat dorsal root ganglion neurons was studied. ATP evoked two distinct types of currents, one with fast activation and desensitization (I-fast) and the other with slow and persistent development (I-slow). In all TTX-S cells which expressed solely TTX-S sodium currents ATP evoked I-fast. The other cells expressed a variable proportion of TTX-S and TTX-R sodium currents. Only 15% of these TTX-R+S cells responded to ATP with I-fast. I-slow was evoked in both cell types but the magnitude of response was much greater in TTX-R+S cells. This result suggests that a different array of ion channels is equipped in different types of sensory neurons.     

Voltage-dependent block of sodium channels in mammalian neurons by the oxadiazine insecticide indoxacarb and its metabolite DCJW

Indoxacarb is a newly developed insecticide with high insecticidal activity and low toxicity to non-target organisms. Its metabolite, DCJW, is known to block compound action potentials in insect nerves and to inhibit sodium currents in cultured insect neurons. However, little is known about the effects of these compounds on the sodium channels of mammalian neurons. We compared the effects of indoxacarb and DCJW on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium channels in rat dorsal root ganglion neurons by using the whole-cell patch clamp technique. Indoxacarb and DCJW at 1-10 microM slowly and irreversibly blocked both TTX-S and TTX-R sodium channels in a voltage-dependent manner. The sodium channel activation kinetics were not significantly modified by 1 microM indoxacarb or 1 microM DCJW. The steady-state fast and slow inactivation curves were shifted in the hyperpolarization direction by 1 microM indoxacarb or 1 microM DCJW indicating a higher affinity of the inactivated sodium channels for these insecticides. These shifts resulted in an enhanced block at more depolarized potentials, thus explaining voltage-dependent block, and an apparent difference in the sensitivity of TTX-R and TTX-S channels to indoxacarb and DCJW near the resting potential. Indoxacarb and its metabolite DCJW cause toxicity through their action on the sodium channels.