Contactin regulates the current density and axonal expression of tetrodotoxin-resistant but not tetrodotoxin-sensitive sodium channels in DRG neurons

Contactin, a glycosyl-phosphatidylinositol (GPI)-anchored predominantly neuronal cell surface glycoprotein, associates with sodium channels Nav1.2, Nav1.3 and Nav1.9, and enhances the density of these channels on the plasma membrane in mammalian expression systems. However, a detailed functional analysis of these interactions and of untested putative interactions with other sodium channel isoforms in mammalian neuronal cells has not been carried out. We examined the expression and function of sodium channels in small-diameter dorsal root ganglion (DRG) neurons from contactin-deficient (CNTN-/-) mice, compared to CNTN+/+ litter mates. Nav1.9 is preferentially expressed in isolectin B4 (IB4)-positive neurons and thus we used this marker to subdivide small-diameter DRG neurons. Using whole-cell patch-clamp recording, we observed a greater than two-fold reduction of tetrodotoxin-resistant (TTX-R) Nav1.8 and Nav1.9 current densities in IB4+ DRG neurons cultured from CNTN-/- vs. CNTN+/+ mice. Current densities for TTX-sensitive (TTX-S) sodium channels were unaffected. Contactin's effect was selective for IB4+ neurons as current densities for both TTX-R and TTX-S channels were not significantly different in IB4- DRG neurons from the two genotypes. Consistent with these results, we have demonstrated a reduction in Nav1.8 and Nav1.9 immunostaining on peripherin-positive unmyelinated axons in sciatic nerves from CNTN-/- mice but detected no changes in the expression for the two major TTX-S channels Nav1.6 and Nav1.7. These data provide evidence of a role for contactin in selectively regulating the cell surface expression and current densities of TTX-R but not TTX-S Na+ channel isoforms in nociceptive DRG neurons; this regulation could modulate the membrane properties and excitability of these neurons.     

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.     

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.     

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.     

Differential action potentials and firing patterns in injured and uninjured small dorsal root ganglion neurons after nerve injury

The profile of tetrodotoxin sensitive (TTX-S) and resistant (TTX-R) Na(+) channels and their contribution to action potentials and firing patterns were studied in isolated small dorsal root ganglion (DRG) neurons after L5/L6 spinal nerve ligation (SNL). Total TTX-R Na(+) currents and Na(v) 1.8 mRNA were reduced in injured L5 DRG neurons 14 days after SNL. In contrast, TTX-R Na(+)currents and Na(v) 1.8 mRNA were upregulated in uninjured L4 DRG neurons after SNL. Voltage-dependent inactivation of TTX-R Na(+) channels in these neurons was shifted to hyperpolarized potentials by 4 mV. Two types of neurons were identified in injured L5 DRG neurons after SNL. Type I neurons (57%) had significantly lower threshold but exhibited normal resting membrane potential (RMP) and action potential amplitude. Type II neurons (43%) had significantly smaller action potential amplitude but retained similar RMP and threshold to those from sham rats. None of the injured neurons could generate repetitive firing. In the presence of TTX, only 26% of injured neurons could generate action potentials that had smaller amplitude, higher threshold, and higher rheobase compared with sham rats. In contrast, action potentials and firing patterns in uninjured L4 DRG neurons after SNL, in the presence or absence of TTX, were not affected. These results suggest that TTX-R Na(+) channels play important roles in regulating action potentials and firing patterns in small DRG neurons and that downregulation in injured neurons and upregulation in uninjured neurons confer differential roles in shaping electrogenesis, and perhaps pain transmission, in these neurons.     

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.     

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.     

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.     

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.     

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)     

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.     

Activation of Tetrodotoxin-Resistant Sodium Channel NaV1.9 in Rat Primary Sensory Neurons Contributes to Melittin-Induced Pain Behavior

Tetrodotoxin-resistant (TTX-R) sodium channels Na_V1.8 and Na_V1.9 in dorsal root ganglion (DRG) neurons play important roles in pathological pain. We recently reported that melittin, the major toxin of whole bee venom, induced action potential firings in DRG neurons even in the presence of a high concentration (500 nM) of TTX, indicating the contribution of TTX-R sodium channels. This hypothesis is fully investigated in the present study. After subcutaneous injection of melittin, Na_V1.8 and Na_V1.9 significantly upregulate mRNA and protein expressions, and related sodium currents also increase. Double immunohistochemical results show that Na_V1.8-positive neurons are mainly medium- and small-sized, whereas Na_V1.9-positive ones are only small-sized. Antisense oligodeoxynucleotides (AS ODNs) targeting Na_V1.8 and Na_V1.9 are used to evaluate functional significance of the increased expressions of TTX-R sodium channels. Behavioral tests demonstrate that AS ODN targeting Na_V1.9, but not Na_V1.8, reverses melittin-induced heat hypersensitivity. Neither Na_V1.8 AS ODN nor Na_V1.9 AS ODN affects melittin-induced mechanical hypersensitivity. These results provide previously unknown evidence that upregulation of Na_V1.9, but not Na_V1.8, in small-sized DRG neurons contributes to melittin-induced heat hypersensitivity. Furthermore, melittin-induced biological effect indicates a potential strategy to study properties of TTX-R sodium channels.     

Gating and modulation of presumptive NaV1.9 channels in enteric and spinal sensory neurons

The NaV1.9 subunit is expressed in nociceptive dorsal root ganglion (DRG) neurons and sensory myenteric neurons in which it generates 'persistent' tetrodotoxin-resistant (TTX-R) Na+ currents of yet unknown physiological functions. Here, we have analyzed these currents in details by combining single-channel and whole-cell recordings from cultured rat DRG and myenteric neurons. Comparison of single-channel with whole-cell data indicates that recording using internal CsCl best reflects the basic electrical features of NaV1.9 currents. Inclusion of fluoride in the pipette solution caused a negative shift in the activation and inactivation gates of NaV1.9 but not NaV1.8. Fluoride acts by promoting entry of NaV1.9 channels into a preopen closed state, which causes a strong bias towards opening and enhances the ability of sensory neurons to sustain spiking. Thus, the modulation of the resting-closed states of NaV1.9 channels strongly influences nociceptor excitability and may provide a mechanism by which inflammatory mediators alter pain threshold.     

Resveratrol inhibits Na+ currents in rat dorsal root ganglion neurons

Resveratrol, a phytoalexin found in grapevines, exerts neuroprotective, cancer chemopreventive, antiinflammatory and cardioprotective activities. Studies have also shown that resveratrol exhibits analgesic effects. Cyclooxygenase inhibition and K+ channel opening have been suggested as underlying mechanisms for the resveratrol-induced analgesia. Here, we investigated the effects of resveratrol on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) Na+ currents in rat dorsal root ganglion (DRG) neurons. Resveratrol suppressed both Na+ currents evoked at 0 mV from -80 mV. TTX-S Na+ current (K(d), 72 microM) was more susceptible to resveratrol than TTX-R Na+ current (K(d), 211 microM). Although the activation voltage of TTX-S Na+ current was shifted in the depolarizing direction by resveratrol, that of TTX-R Na+ current was not. Resveratrol caused a hyperpolarizing shift of the steady-state inactivation voltage and slowed the recovery from inactivation of both Na+ currents. However, no frequency-dependent inhibition of resveratrol on either type of Na+ current was observed. The suppression and the unfavorable effects on the kinetics of Na+ currents in terms of the excitability of DRG neurons may make a great contribution to the analgesia by resveratrol.     

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.     

Expression and Role of Voltage-Gated Sodium Channels in Human Dorsal Root Ganglion Neurons with Special Focus on Nav1.7, Species Differences,and Regulation by Paclitaxel

Voltage-gated sodium channels(Navs) play an important role in human pain sensation. However, the expression and role of Nav subtypes in native human sensory neurons are unclear. To address this issue, we obtained human dorsal root ganglion(hDRG) tissues from healthy donors. PCR analysis of seven DRG-expressed Nav subtypes revealed that the hDRG has higher expression of Nav1.7(~50% of total Nav expression) and lower expression of Nav1.8(~12%), whereas the mouse DRG has higher expression of Nav1.8(~45%) and lower expression of Nav1.7(~18%). To mimic Nav regulation in chronic pain, we treated hDRG neurons in primary cultures with paclitaxel(0.1-1 μmol/L) for 24 h. Paclitaxel increased the Nav 1.7 but not Nav1.8 expression and also increased the transient Na~+ currents and action potential firing frequency in small-diameter(50 μm) hDRG neurons. Thus, the hDRG provides a translational model in which to study"human pain in a dish" and test new pain therapeutics.     

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.