The voltage-gated sodium channel Nav1.8 is only expressed in subsets of neurons in dorsal root ganglia (DRG) and trigeminal and nodose ganglia. We have isolated mouse partial length Nav1.8 cDNA clones spanning the exon 17 sequence, which have 17 nucleotide substitutions and 12 predicted amino acid differences from the published sequence. The absence of a mutually exclusive alternative exon 17 was confirmed by sequencing 4.1 kilobases of genomic DNA spanning exons 16–18 of Scn10a. A novel cDNA isoform was identified, designated Nav1.8c, which results from alternative 3′-splice site selection at a CAG/CAG motif to exclude the codon for glutamine 1031 within the interdomain cytoplasmic loop IDII/III. The ratio of Nav1.8c (CAG-skipped) to Nav1.8 (CAG-inclusive) mRNA in mouse is ~2:1 in adult DRG, trigeminal ganglion, and neonatal DRG. A Nav1.8c isoform also occurs in rat DRG, but is less common. Of the two other tetrodotoxin-resistant channels, no analogous alternative splicing of mouse Nav1.9 was detected, whereas rare alternative splicing of Nav1.5 at a CAG/CAG motif resulted in the introduction of a CAG trinucleotide. This isoform, designated Nav1.5c, is conserved in rat and encodes an additional glutamine residue that disrupts a putative CK2 phosphorylation site. In summary, novel isoforms of Nav1.8 and Nav1.5 are each generated by alternative splicing at CAG/CAG motifs, which result in the absence or presence of predicted glutamine residues within the interdomain cytoplasmic loop IDII/III. Mutations of sodium channels within this cytoplasmic loop have previously been demonstrated to alter electrophysiological properties and cause cardiac arrhythmias and epilepsy.
The expression of voltage-gated sodium channels is regulated at multiple levels, and in this study we addressed the potential for alternative splicing of the Nav1.2, Nav1.3, Nav1.6 and Nav1.7 mRNAs. We isolated novel mRNA isoforms of Nav1.2 and Nav1.3 from adult mouse and rat dorsal root ganglia (DRG), Nav1.3 and Nav1.7 from adult mouse brain, and Nav1.7 from neonatal rat brain. These alternatively spliced isoforms introduce an additional exon (Nav1.2 exon 17A and topologically equivalent Nav1.7 exon 16A) or exon pair (Nav1.3 exons 17A and 17B) that contain an in-frame stop codon and result in predicted two-domain, truncated proteins. The mouse and rat orthologous exon sequences are highly conserved (94-100% identities), as are the paralogous Nav1.2 and Nav1.3 exons (93% identity in mouse) to which the Nav1.7 exon has only 60% identity. Previously, Nav1.3 mRNA has been shown to be upregulated in rat DRG following peripheral nerve injury, unlike the downregulation of all other sodium channel transcripts. Here we show that the expression of Nav1.3 mRNA containing exons 17A and 17B is unchanged in mouse following peripheral nerve injury (axotomy), whereas total Nav1.3 mRNA expression is upregulated by 33% (P=0.003), suggesting differential regulation of the alternatively spliced transcripts. The alternatively spliced rodent exon sequences are highly conserved in both the human and chicken genomes, with 77-89% and 72-76% identities to mouse, respectively. The widespread conservation of these sequences strongly suggests an additional level of regulation in the expression of these channels, that is also tissue-specific.
DRG; brain; alternative splicing; Scn2a; Scn3a; Scn9a
Sensory neurons of the dorsal root ganglia (DRG) express multiple voltage-gated sodium (Na) channels that substantially differ in gating kinetics and pharmacology. Small-diameter (<25 µm) neurons isolated from the rat DRG express a combination of fast tetrodotoxin-sensitive (TTX-S) and slow TTX-resistant (TTX-R) Na currents while large-diameter neurons (>30 µm) predominately express fast TTX-S Na current. Na channel expression was further investigated using single-cell RT-PCR to measure the transcripts present in individually harvested DRG neurons. Consistent with cellular electrophysiology, the small neurons expressed transcripts encoding for both TTX-S (Nav1.1, Nav1.2, Nav1.6, Nav1.7) and TTX-R (Nav1.8, Nav1.9) Na channels. Nav1.7, Nav1.8 and Nav1.9 were the predominant Na channels expressed in the small neurons. The large neurons highly expressed TTX-S isoforms (Nav1.1, Nav1.6, Nav1.7) while TTX-R channels were present at comparatively low levels. A unique subpopulation of the large neurons was identified that expressed TTX-R Na current and high levels of Nav1.8 transcript. DRG neurons also displayed substantial differences in the expression of neurofilaments (NF200, peripherin) and Necl-1, a neuronal adhesion molecule involved in myelination. The preferential expression of NF200 and Necl-1 suggests that large-diameter neurons give rise to thick myelinated axons. Small-diameter neurons expressed peripherin, but reduced levels of NF200 and Necl-1, a pattern more consistent with thin unmyelinated axons. Single-cell analysis of Na channel transcripts indicates that TTX-S and TTX-R Na channels are differentially expressed in large myelinated (Nav1.1, Nav1.6, Nav1.7) and small unmyelinated (Nav1.7, Nav1.8, Nav1.9) sensory neurons.
Sodium channel; dorsal root ganglia; single-cell RT-PCR; Necl-1; NF200; peripherin
Dysregulation of voltage-gated sodium channels (Navs) is believed to play a major role in nerve fiber hyperexcitability associated with neuropathic pain. A complete transcriptional characterization of the different isoforms of Navs under normal and pathological conditions had never been performed on mice, despite their widespread use in pain research. Navs mRNA levels in mouse dorsal root ganglia (DRG) were studied in the spared nerve injury (SNI) and spinal nerve ligation (SNL) models of neuropathic pain. In the SNI model, injured and non-injured neurons were intermingled in lumbar DRG, which were pooled to increase the tissue available for experiments.
A strong downregulation was observed for every Navs isoform expressed except for Nav1.2; even Nav1.3, known to be upregulated in rat neuropathic pain models, was lower in the SNI mouse model. This suggests differences between these two species. In the SNL model, where the cell bodies of injured and non-injured fibers are anatomically separated between different DRG, most Navs were observed to be downregulated in the L5 DRG receiving axotomized fibers. Transcription was then investigated independently in the L3, L4 and L5 DRG in the SNI model, and an important downregulation of many Navs isoforms was observed in the L3 DRG, suggesting the presence of numerous injured neurons there after SNI. Consequently, the proportion of axotomized neurons in the L3, L4 and L5 DRG after SNI was characterized by studying the expression of activating transcription factor 3 (ATF3). Using this marker of nerve injury confirmed that most injured fibers find their cell bodies in the L3 and L4 DRG after SNI in C57BL/6 J mice; this contrasts with their L4 and L5 DRG localization in rats. The spared sural nerve, through which pain hypersensitivity is measured in behavioral studies, mostly projects into the L4 and L5 DRG.
The complex regulation of Navs, together with the anatomical rostral shift of the DRG harboring injured fibers in C57BL/6 J mice, emphasize that caution is necessary and preliminary anatomical experiments should be carried out for gene and protein expression studies after SNI in mouse strains.
Activating transcription factor 3 (ATF3); Dorsal root ganglia (DRG); Nerve injury; Neuropathic pain; Quantitative real time polymerase chain reaction (qRT-PCR); Sciatic nerve; Spared nerve injury (SNI); Spinal nerve ligation (SNL); Voltage-gated sodium channels (Navs)
Rapid and effective treatment of cancer-induced bone pain remains a clinical challenge and patients with bone metastasis are more likely to experience severe pain. The voltage-gated sodium channel Nav1.8 plays a critical role in many aspects of nociceptor function. Therefore, we characterized a rat model of cancer pain and investigated the potential role of Nav1.8.
Adult female Wistar rats were used for the study. Cancer pain was induced by inoculation of Walker 256 breast carcinosarcoma cells into the tibia. After surgery, mechanical and thermal hyperalgesia and ambulation scores were evaluated to identify pain-related behavior. We used real-time RT-PCR to determine Nav1.8 mRNA expression in bilateral L4/L5 dorsal root ganglia (DRG) at 16-19 days after surgery. Western blotting and immunofluorescence were used to compare the expression and distribution of Nav1.8 in L4/L5 DRG between tumor-bearing and sham rats. Antisense oligodeoxynucleotides (ODNs) against Nav1.8 were administered intrathecally at 14-16 days after surgery to knock down Nav1.8 protein expression and changes in pain-related behavior were observed.
Tumor-bearing rats exhibited mechanical hyperalgesia and ambulatory-evoked pain from day 7 after inoculation of Walker 256 cells. In the advanced stage of cancer pain (days 16-19 after surgery), normalized Nav1.8 mRNA levels assessed by real-time RT-PCR were significantly lower in ipsilateral L4/L5 DRG of tumor-bearing rats compared with the sham group. Western-blot showed that the total expression of Nav1.8 protein significantly decreased bilaterally in DRG of tumor-bearing rats. Furthermore, as revealed by immunofluorescence, only the expression of Nav1.8 protein in small neurons down regulated significantly in bilateral DRG of cancer pain rats. After administration of antisense ODNs against Nav1.8, Nav1.8 protein expression decreased significantly and tumor-bearing rats showed alleviated mechanical hyperalgesia and ambulatory-evoked pain.
These findings suggest that Nav1.8 plays a role in the development and maintenance of bone cancer pain.
CCL2 [chemokine (C–C motif) ligand 2] contributes to the inflammation-induced neuropathic pain through activating VGSC (voltage-gated sodium channel)-mediated nerve impulse conduction, but the underlying mechanism is currently unknown. Our study aimed to investigate whether PKC (protein kinase C)–NF-κB (nuclear factor κB) is involved in CCL2-induced regulation of voltage-gated sodium Nav1.8 currents and expression. DRG (dorsal root ganglion) neurons were prepared from adult male Sprague–Dawley rats and incubated with various concentration of CCL2 for 24 h. Whole-cell patch-clamps were performed to record the Nav1.8 currents in response to the induction by CCL2. After being pretreated with 5 and10 nM CCL2 for 16 h, CCR2 [chemokine (C–C motif) receptor 2] and Nav1.8 expression significantly increased and the peak currents of Nav1.8 elevated from the baseline 46.53±4.53 pA/pF to 64.28±3.12 pA/pF following 10 nM CCL2 (P<0.05). Compared with the control, significant change in Nav1.8 current density was observed when the CCR2 inhibitor INCB3344 (10 nM) was applied. Furthermore, inhibition of PKC by AEB071 significantly eliminated CCL2-induced elevated Nav1.8 currents. In vitro PKC kinase assays and autoradiograms suggested that Nav1.8 within DRG neurons was a substrate of PKC and direct phosphorylation of the Nav1.8 channel by PKC regulates its function in these neurons. Moreover, p65 expression was significantly higher in CCL2-induced neurons (P<0.05), and was reversed by treatment with INCB3344 and AEB071. PKC–NF-κB are involved in CCL2-induced elevation of Nav1.8 current density by promoting the phosphorylation of Nav1.8 and its expression.
Cytokine CCL2 is responsible for promoting voltage-gated sodium Nav1.8 current density and expression, which mediates nerve impulse conduction and induces inflammatory nociception. PKC phosphorylates Nav1.8 to increase its current density and PKC–NF-κB are involved in inducing the up-regulation of Nav1.8.
CCL2; CCR2; dorsal root ganglion (DRG); Nav1.8; nociception; PKC; CCL2, chemokine (C–C motif) ligand 2; CCR2, chemokine (C–C motif) receptor 2; DRG, dorsal root ganglion; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; NF-κB, nuclear factor κB; PKC, protein kinase C; TEA-Cl, tetraethylammonium-Cl; TRPV1, transient receptor potential vanilloid 1; TTX-R, tetrodotoxin-resistant; VGSC, voltage-gated sodium channel
Voltage-gated sodium channel Nav1.7 is preferentially expressed in dorsal root ganglion (DRG) and sympathetic neurons within the peripheral nervous system. Homozygous or compound heterozygous loss-of-function mutations in SCN9A, the gene which encodes Nav1.7, cause congenital insensitivity to pain (CIP) accompanied by anosmia. Global knock-out of Nav1.7 in mice is neonatal lethal reportedly from starvation, suggesting anosmia. These findings led us to hypothesize that Nav1.7 is the main sodium channel in the peripheral olfactory sensory neurons (OSN, also known as olfactory receptor neurons).
We used multiplex PCR-restriction enzyme polymorphism, in situ hybridization and immunohistochemistry to determine the identity of sodium channels in rodent OSNs.
We show here that Nav1.7 is the predominant sodium channel transcript, with low abundance of other sodium channel transcripts, in olfactory epithelium from rat and mouse. Our in situ hybridization data show that Nav1.7 transcripts are present in rat OSNs. Immunostaining of Nav1.7 and Nav1.6 channels in rat shows a complementary accumulation pattern with Nav1.7 in peripheral presynaptic OSN axons, and Nav1.6 primarily in postsynaptic cells and their dendrites in the glomeruli of the olfactory bulb within the central nervous system.
Our data show that Nav1.7 is the dominant sodium channel in rat and mouse OSN, and may explain anosmia in Nav1.7 null mouse and patients with Nav1.7-related CIP.
Small neurons of the dorsal root ganglion (DRG) express five of the nine known voltage-gated sodium channels. Each channel has unique biophysical characteristics which determine how it contributes to the generation of action potentials (AP). To better understand how AP amplitude is maintained in nociceptive DRG neurons and their centrally projecting axons, which are subjected to depolarization within the dorsal horn, we investigated the dependence of AP amplitude on membrane potential, and how that dependence is altered by the presence or absence of sodium channel Nav1.8.
In small neurons cultured from wild type (WT) adult mouse DRG, AP amplitude decreases as the membrane potential is depolarized from -90 mV to -30 mV. The decrease in amplitude is best fit by two Boltzmann equations, having V1/2 values of -73 and -37 mV. These values are similar to the V1/2 values for steady-state fast inactivation of tetrodotoxin-sensitive (TTX-s) sodium channels, and the tetrodotoxin-resistant (TTX-r) Nav1.8 sodium channel, respectively. Addition of TTX eliminates the more hyperpolarized V1/2 component and leads to increasing AP amplitude for holding potentials of -90 to -60 mV. This increase is substantially reduced by the addition of potassium channel blockers. In neurons from Nav1.8(-/-) mice, the voltage-dependent decrease in AP amplitude is characterized by a single Boltzmann equation with a V1/2 value of -55 mV, suggesting a shift in the steady-state fast inactivation properties of TTX-s sodium channels. Transfection of Nav1.8(-/-) DRG neurons with DNA encoding Nav1.8 results in a membrane potential-dependent decrease in AP amplitude that recapitulates WT properties.
We conclude that the presence of Nav1.8 allows AP amplitude to be maintained in DRG neurons and their centrally projecting axons even when depolarized within the dorsal horn.
In rats expression of the Nav1.7 voltage-gated sodium channel isoform is restricted to the peripheral nervous system and is abundant in the sensory neurons of the dorsal root ganglion. We expressed the rat Nav1.7 sodium channel α subunit together with the rat auxiliary β1 and β2 subunits in Xenopus laevis oocytes and assessed the effects of the pyrethroid insecticide tefluthrin on the expressed currents using the two-electrode voltage clamp method. Tefluthrin at 100 µM modified of Nav1.7 channels to prolong inactivation of the peak current during a depolarizing pulse, resulting in a marked "late current" at the end of a 40-ms depolarization, and induced a sodium tail current following repolarization. Tefluthrin modification was enhanced up to two-fold by the application of a train of up to 100 5-ms depolarizing prepulses. These effects of tefluthrin on Nav1.7 channels were qualitatively similar to its effects on rat Nav1.2, Nav1.3 and Nav1.6 channels assayed previously under identical conditions. However, Nav1.7 sodium channels were distinguished by their low sensitivity to modification by tefluthrin, especially compared to Nav1.3 and Nav1.6 channels. It is likely that Nav1.7 channels contribute significantly to the tetrodotoxin-sensitive, pyrethroid-resistant current found in cultured dorsal root ganglion neurons. We aligned the complete amino acid sequences of four pyrethroid-sensitive isoforms (house fly Vssc1; rat Nav1.3, Nav1.6 and Nav1.8) and two pyrethroid-resistant isoforms (rat Nav1.2 and Nav1.7) and found only a single site, located in transmembrane segment 6 of homology domain I, at which the amino acid sequence was conserved among all four sensitive isoform sequences but differed in the two resistant isoform sequences. This position, corresponding to Val410 of the house fly Vssc1 sequence, also aligns with sites of multiple amino acid substitutions identified in the sodium channel sequences of pyrethroid-resistant insect populations. These results implicate this single amino acid polymorphism in transmembrane segment 6 of sodium channel homology domain I as a determinant of the differential pyrethroid sensitivity of rat sodium channel isoforms.
voltage-gated sodium channel; Nav1.7 isoform; pyrethroid; tefluthrin; peripheral nervous system; dorsal root ganglion
Voltage-gated sodium channels (Nav) mediate neuronal action potentials. Tetrodotoxin inhibits all Nav isoforms, but Nav1.8 and Nav1.9 are relatively tetrodotoxin-resistant (TTX-r) compared to other isoforms. Nav1.8 is highly expressed in dorsal root ganglion neurons and is functionally linked to nociception, but the sensitivity of TTX-r isoforms to inhaled anesthetics is unclear.
The sensitivities of heterologously expressed rat TTX-r Nav1.8 and endogenous tetrodotoxin-sensitive (TTX-s) Nav to the prototypic inhaled anesthetic isoflurane were tested in mammalian ND7/23 cells using patch-clamp electrophysiology.
From a holding potential of −70 mV, isoflurane (0.53±0.06 mM, ~1.8 MAC at 24°C) reduced normalized peak Na+ current (INa) of Nav1.8 to 0.55±0.03 and of endogenous TTX-s Nav to 0.56±0.06. Isoflurane minimally inhibited INa from a holding potential of −140 mV. Isoflurane did not affect voltage-dependence of activation, but significantly shifted voltage-dependence of steady-state inactivation by −6 mV for Nav1.8 and by −7 mV for TTX-s Nav. IC50 values for inhibition of peak INa were 0.67±0.06 mM for Nav1.8 and 0.66±0.09 mM for TTX-s Nav; significant inhibition occurred at clinically relevant concentrations as low as 0.58 MAC. Isoflurane produced use-dependent block of Nav1.8; at a stimulation frequency of 10 Hz, 0.56±0.08 mM isoflurane reduced INa to 0.64±0.01 vs. 0.78±0.01 for control.
Isoflurane inhibited the tetrodotoxin-resistant isoform Nav1.8 with potency comparable to that for endogenous tetrodotoxin-sensitive Nav isoforms, indicating that sensitivity to inhaled anesthetics is conserved across diverse Nav family members. Block of Nav1.8 in dorsal root ganglion neurons could contribute to the effects of inhaled anesthetics on peripheral nociceptive mechanisms.
The NaV1.7 tetrodotoxin-sensitive voltage-gated sodium channel isoform plays a critical role in nociception. In rodent models of diabetic neuropathy, increased NaV1.7 in dorsal root ganglion (DRG) neurons correlates with the emergence of pain-related behaviors characteristic of painful diabetic neuropathy (PDN). We examined the effect of transgene-mediated expression of enkephalin on pain-related behaviors and their biochemical correlates in DRG neurons. Transfection of DRG neurons by subcutaneous inoculation of a herpes simplex virus (HSV)-based vector expressing proenkephalin (PE) reversed nocisponsive behavioral responses to heat, cold, and mechanical pressure characteristic of PDN. Vector-mediated enkephalin production in vivo prevented the increase in DRG NaV1.7 observed in PDN, an effect that correlated with inhibition of phosphorylation of p38 MAP kinase and protein kinase C (PKC). Primary DRG neurons in vitro exposed to 45 mM glucose for 18 hrs also demonstrated an increase in NaV1.7 and increased phosphorylation of p38 and PKC; these changes were prevented by transfection in vitro with the enkephalin-expressing vector. The effect of hyperglycemia on NaV1.7 production in vitro was mimicked by exposure to PMA, and blocked by the myristolated PKC inhibitor 20–28 or the p38 inhibitor SB202190; the effect of vector-mediated enkephalin on NaV1.7 levels was prevented by naltrindole. The results of these studies suggest that activation of the presynaptic delta opioid receptor by enkephalin prevents the increase in neuronal NaV1.7 in DRG through inhibition of PKC and p38. These results establish a novel interaction between the delta opioid receptor and voltage gated sodium channels.
pain; diabetic neuropathy; sodium channel; gene therapy; herpes simplex; enkephalins
Voltage-gated sodium channels (Nav) are essential for the generation and conduction of action potentials. Peripheral inflammation increases the expression of Nav1.7 and Nav1.8 in dorsal root ganglion (DRG) neurons, suggesting that they participate in the induction and maintenance of chronic inflammatory pain. However, how Nav1.7 and Nav1.8 are regulated in the DRG under inflammatory pain conditions remains unclear. Using a complete Freund’s adjuvant (CFA)-induced chronic inflammatory pain model and Western blot analysis, we found that phosphorylated Akt (p-Akt) was significantly increased in the ipsilateral L4/5 DRGs of rats on days 3 and 7 after intraplantar CFA injection. Immunohistochemistry showed that the percentage of p-Akt-positive neurons in the DRG was also significantly increased in the ipsilateral L4/5 DRGs at these times. Moreover, CFA injection increased the colocalization of p-Akt with Nav1.7 and Nav1.8 in L4/5 DRG neurons. Pretreatment of rats with an intrathecal injection of Akt inhibitor IV blocked CFA-induced thermal hyperalgesia and CFA-induced increases in Nav1.7 and Nav1.8 in the L4/5 DRGs on day 7 after CFA injection. Our findings suggest that the Akt pathway participates in inflammation-induced upregulation of Nav1.7 and Nav1.8 expression in DRG neurons. This participation might contribute to the maintenance of chronic inflammatory pain.
This article presents that inhibition of Akt blocks CFA-induced thermal hyperalgesia and CFA-induced increases in dorsal root ganglion Nav1.7 and Nav1.8. These findings have potential implications for use of Akt inhibitors to prevent and/or treat persistent inflammatory pain.
Nav1.7; Nav1.8; Akt; Dorsal root ganglion; Inflammatory pain
Functional alterations in the properties of Aβ afferent fibers may account for the increased pain sensitivity observed under peripheral chronic inflammation. Among the voltage-gated sodium channels involved in the pathophysiology of pain, Nav1.8 has been shown to participate in the peripheral sensitization of nociceptors. However, to date, there is no evidence for a role of Nav1.8 in controlling Aβ-fiber excitability following persistent inflammation.
Distribution and expression of Nav1.8 in dorsal root ganglia and sciatic nerves were qualitatively or quantitatively assessed by immunohistochemical staining and by real time-polymerase chain reaction at different time points following complete Freund’s adjuvant (CFA) administration. Using a whole-cell patch-clamp configuration, we further determined both total INa and TTX-R Nav1.8 currents in large-soma dorsal root ganglia (DRG) neurons isolated from sham or CFA-treated rats. Finally, we analyzed the effects of ambroxol, a Nav1.8-preferring blocker on the electrophysiological properties of Nav1.8 currents and on the mechanical sensitivity and inflammation of the hind paw in CFA-treated rats.
Our findings revealed that Nav1.8 is up-regulated in NF200-positive large sensory neurons and is subsequently anterogradely transported from the DRG cell bodies along the axons toward the periphery after CFA-induced inflammation. We also demonstrated that both total INa and Nav1.8 peak current densities are enhanced in inflamed large myelinated Aβ-fiber neurons. Persistent inflammation leading to nociception also induced time-dependent changes in Aβ-fiber neuron excitability by shifting the voltage-dependent activation of Nav1.8 in the hyperpolarizing direction, thus decreasing the current threshold for triggering action potentials. Finally, we found that ambroxol significantly reduces the potentiation of Nav1.8 currents in Aβ-fiber neurons observed following intraplantar CFA injection and concomitantly blocks CFA-induced mechanical allodynia, suggesting that Nav1.8 regulation in Aβ-fibers contributes to inflammatory pain.
Collectively, these findings support a key role for Nav1.8 in controlling the excitability of Aβ-fibers and its potential contribution to the development of mechanical allodynia under persistent inflammation.
Aβ-fibers; Allodynia; Complete Freund’s adjuvant; Electrophysiology; Sodium channel blocker
Tetrodotoxin-resistant (TTX-R) sodium channels NaV1.8 and NaV1.9 in sensory neurons were known as key pain modulators. Comparing with the widely reported NaV1.8, roles of NaV1.9 on inflammatory pain are poorly studied by antisense-induced specific gene knockdown. Here, we used molecular, electrophysiological and behavioral methods to examine the effects of antisense oligodeoxynucleotide (AS ODN) targeting NaV1.8 and NaV1.9 on inflammatory pain. Following complete Freund's adjuvant (CFA) inflammation treatment, NaV1.8 and NaV1.9 in rat dorsal root ganglion (DRG) up-regulated mRNA and protein expressions and increased sodium current densities. Immunohistochemical data demonstrated that NaV1.8 mainly localized in medium and small-sized DRG neurons, whereas NaV1.9 only expressed in small-sized DRG neurons. Intrathecal (i.t.) delivery of AS ODN was used to down-regulate NaV1.8 or NaV1.9 expressions confirmed by immunohistochemistry and western blot. Unexpectedly, behavioral tests showed that only NaV1.8 AS ODN, but not NaV1.9 AS ODN could reverse CFA-induced heat and mechanical hypersensitivity. Our data indicated that TTX-R sodium channels NaV1.8 and NaV1.9 in primary sensory neurons played distinct roles in CFA-induced inflammatory pain and suggested that antisense oligodeoxynucleotide-mediated blocking of key pain modulator might point toward a potential treatment strategy against certain types of inflammatory pain.
Sodium channel NaV1.7 is preferentially expressed within dorsal root ganglia (DRG), trigeminal ganglia and sympathetic ganglion neurons and their fine-diamter axons, where it acts as a threshold channel, amplifying stimuli such as generator potentials in nociceptors. Gain-of-function mutations and variants (single amino acid substitutions) of NaV1.7 have been linked to three pain syndromes: Inherited Erythromelalgia (IEM), Paroxysmal Extreme Pain Disorder (PEPD), and Small Fiber Neuropathy (SFN). IEM is characterized clinically by burning pain and redness that is usually focused on the distal extremities, precipitated by mild warmth and relieved by cooling, and is caused by mutations that hyperpolarize activation, slow deactivation, and enhance the channel ramp response. PEPD is characterized by perirectal, periocular or perimandibular pain, often triggered by defecation or lower body stimulation, and is caused by mutations that severely impair fast-inactivation. SFN presents a clinical picture dominated by neuropathic pain and autonomic symptoms; gain-of-function variants have been reported to be present in approximately 30% of patients with biopsy-confirmed idiopathic SFN, and functional testing has shown altered fast-inactivation, slow-inactivation or resurgent current. In this paper we describe three patients who house the NaV1.7/I228M variant.
We have used clinical assessment of patients, quantitative sensory testing and skin biopsy to study these patients, including two siblings in one family, in whom genomic screening demonstrated the I228M NaV1.7 variant. Electrophysiology (voltage-clamp and current-clamp) was used to test functional effects of the variant channel.
We report three different clinical presentations of the I228M NaV1.7 variant: presentation with severe facial pain, presentation with distal (feet, hands) pain, and presentation with scalp discomfort in three patients housing this NaV1.7 variant, two of which are from a single family. We also demonstrate that the NaV1.7/I228M variant impairs slow-inactivation, and produces hyperexcitability in both trigeminal ganglion and DRG neurons.
Our results demonstrate intra- and interfamily phenotypic diversity in pain syndromes produced by a gain-of-function variant of NaV1.7.
Sensory neurons in the dorsal root ganglion express two kinds of tetrodotoxin resistant (TTX-R) isoforms of voltage-gated sodium channels, NaV1.8 and NaV1.9. These isoforms play key roles in the pathophysiology of chronic pain. Of special interest is NaV1.9: our previous studies revealed a unique property of the NaV1.9 current, i.e., the NaV1.9 current shows a gradual and notable up-regulation of the peak amplitude during recording (“spontaneous augmentation of NaV1.9”). However, the mechanism underlying the spontaneous augmentation of NaV1.9 is still unclear. In this study, we examined the effects of protein kinases A and C (PKA and PKC), on the spontaneous augmentation of NaV1.9. The spontaneous augmentation of the NaV1.9 current was significantly suppressed by activation of PKA, whereas activation of PKA did not affect the voltage dependence of inactivation for the NaV1.9 current. On the contrary, the finding that activation of PKC can affect the voltage dependence of inactivation for NaV1.9 in the perforated patch recordings, where the augmentation does not occur, suggests that the effects of PMA are independent of the augmentation process. These results indicate that the spontaneous augmentation of NaV1.9 was regulated directly by PKA, and indirectly by PKC.
Na+ channel; tetrodotoxin; dorsal root ganglion; patch clamp; PKA; PKC
Sensory neurons in the dorsal root ganglia (DRG) express a subset of voltage dependent sodium channels (NaV) including NaV1.1, 1.6, 1.7, 1.8 and 1.9. Previous work supported preferential localization of NaV1.8 channels to small-medium diameter, nociceptive afferent neurons. However, we recently published evidence that NaV1.8 was the dominant NaV channel expressed in the somas of small, medium and large diameter muscle afferent neurons, which is consistent with other reports. Here, we extend those results to show that NaV1.8 expression is not correlated with afferent neuron diameter. Using immunocytochemistry, we found NaV1.8 expression in ~50% of sensory afferent neurons with diameters ranging from 20 to 70 µm. In addition, electrophysiological analysis shows that the kinetic and inactivation properties of NaV1.8 current are invariant with neuron size. These data add further support to the idea that NaV1.8 contributes to the electrical excitability of both nociceptive and non-nociceptive sensory neurons.
cutaneous afferents; muscle afferents; dorsal root ganglia neurons; Tetrodotoxin-resistant (TTX-R)
Members of the degenerin/epithelial (DEG/ENaC) sodium channel family are mechanosensors in C elegans, and Nav1.7 and Nav1.8 voltage-gated sodium channel knockout mice have major deficits in mechanosensation. β and γENaC sodium channel subunits are present with acid sensing ion channels (ASICs) in mammalian sensory neurons of the dorsal root ganglia (DRG). The extent to which epithelial or voltage-gated sodium channels are involved in transduction of mechanical stimuli is unclear.
Here we show that deleting β and γENaC sodium channels in sensory neurons does not result in mechanosensory behavioural deficits. We had shown previously that Nav1.7/Nav1.8 double knockout mice have major deficits in behavioural responses to noxious mechanical pressure. However, all classes of mechanically activated currents in DRG neurons are unaffected by deletion of the two sodium channels. In contrast, the ability of Nav1.7/Nav1.8 knockout DRG neurons to generate action potentials is compromised with 50% of the small diameter sensory neurons unable to respond to electrical stimulation in vitro.
Behavioural deficits in Nav1.7/Nav1.8 knockout mice reflects a failure of action potential propagation in a mechanosensitive set of sensory neurons rather than a loss of primary transduction currents. DEG/ENaC sodium channels are not mechanosensors in mouse sensory neurons.
Mechanotransduction; Sodium channels; Pain; Nav1.7; Nav1.8; ENaCs
Voltage-gated sodium channels are important sites for the neurotoxic actions of pyrethroid insecticides in mammals. The pore-forming α subunits of mammalian sodium channels are encoded by a family of 9 genes, designated Nav1.1 - Nav1.9. Native sodium channels in the adult central nervous system (CNS) are heterotrimeric complexes of one of these 9 α subunits and two auxiliary (β) subunits. Here we compare the functional properties and pyrethroid sensitivity of the rat and human Nav1.3 isoforms, which are abundantly expressed in the developing CNS. Coexpression of the rat Nav1.3 and human Nav1.3 α subunits in combination with their conspecific β1 and β2 subunits in Xenopus laevis oocytes gave channels with markedly different inactivation properties and sensitivities to the pyrethroid insecticide tefluthrin. Rat Nav1.3 channels inactivated more slowly than human Nav1.3 channels during a depolarizing pulse. The rat and human channels also differed in their voltage dependence of steady-state inactivation. Exposure of rat and human Nav1.3 channels to 100 μM tefluthrin in the resting state produced populations of channels that activated, inactivated and deactivated more slowly than unmodified channels. For both rat and human channels, application of trains of depolarizing prepulses enhanced the extent of tefluthrin modification approximately twofold; this result implies that tefluthrin may bind to both the resting and open states of the channel. Modification of rat Nav1.3 channels by 100 μM tefluthrin was four-fold greater than that measured in parallel assays with human Nav1.3 channels. Human Nav1.3 channels were also less sensitive to tefluthrin than rat Nav1.2 channels, which are considered to be relatively insensitive to pyrethroids. These data provide the first direct comparison of the functional and pharmacological properties of orthologous rat and human sodium channels and demonstrate that orthologous channels with a high degree of amino acid sequence conservation differ in both their functional properties and their sensitivities to pyrethroid insecticides.
Nav1.3; oocyte; sodium channel; pyrethroid; tefluthrin; rat; human
Peripheral neuropathic pain is a disabling condition resulting from nerve injury. It is characterized by the dysregulation of voltage-gated sodium channels (Navs) expressed in dorsal root ganglion (DRG) sensory neurons. The mechanisms underlying the altered expression of Navs remain unknown. This study investigated the role of the E3 ubiquitin ligase NEDD4-2, which is known to ubiquitylate Navs, in the pathogenesis of neuropathic pain in mice. The spared nerve injury (SNI) model of traumatic nerve injury–induced neuropathic pain was used, and an Nav1.7-specific inhibitor, ProTxII, allowed the isolation of Nav1.7-mediated currents. SNI decreased NEDD4-2 expression in DRG cells and increased the amplitude of Nav1.7 and Nav1.8 currents. The redistribution of Nav1.7 channels toward peripheral axons was also observed. Similar changes were observed in the nociceptive DRG neurons of Nedd4L knockout mice (SNS-Nedd4L–/–). SNS-Nedd4L–/– mice exhibited thermal hypersensitivity and an enhanced second pain phase after formalin injection. Restoration of NEDD4-2 expression in DRG neurons using recombinant adenoassociated virus (rAAV2/6) not only reduced Nav1.7 and Nav1.8 current amplitudes, but also alleviated SNI-induced mechanical allodynia. These findings demonstrate that NEDD4-2 is a potent posttranslational regulator of Navs and that downregulation of NEDD4-2 leads to the hyperexcitability of DRG neurons and contributes to the genesis of pathological pain.
Painful neuropathy is a common complication of diabetes. Previous studies have identified significant increases in the amount of voltage gated sodium channel isoforms NaV1.7 and NaV1.3 protein in the dorsal root ganglia (DRG) of rats with streptozotocin (STZ)-induced diabetes. We found that gene transfer-mediated release of the inhibitory neurotransmitters enkephalin or gamma amino butyric acid (GABA) from DRG neurons in diabetic animals reduced pain-related behaviors coincident with a reduction in NaV1.7 protein levels in DRG in vivo. To further evaluate the role of NaVα subunit levels in DRG in the pathogenesis of pain in diabetic neuropathy, we constructed a non-replicating herpes simplex virus (HSV)-based vector expressing a microRNA (miRNA) against NaVα subunits.
Subcutaneous inoculation of the miRNA-expressing HSV vector into the feet of diabetic rats to transduce DRG resulted in a reduction in NaVα subunit levels in DRG neurons, coincident with a reduction in cold allodynia, thermal hyperalgesia and mechanical hyperalgesia.
These data support the role of increased NaVα protein in DRG in the pathogenesis of pain in diabetic neuropathy, and provide a proof-of-principle demonstration for the development of a novel therapy that could be used to treat intractable pain in patients with diabetic neuropathy.
Diabetes; Pain; Neuropathy; Gene therapy; Sodium channel
Diabetic neuropathy is a common form of peripheral neuropathy, yet the mechanisms responsible for pain in this disease are poorly understood. Alterations in the expression and function of voltage-gated tetrodotoxin-resistant (TTX-R) sodium channels have been implicated in animal models of neuropathic pain, including models of diabetic neuropathy. We investigated the expression and function of TTX-sensitive (TTX-S) and TTX-R sodium channels in dorsal root ganglion (DRG) neurons and the responses to thermal hyperalgesia and mechanical allodynia in streptozotocin-treated rats between 4–8 weeks after onset of diabetes. Diabetic rats demonstrated a significant reduction in the threshold for escape from innocuous mechanical pressure (allodynia) and a reduction in the latency to withdrawal from a noxious thermal stimulus (hyperalgesia). Both TTX-S and TTX-R sodium currents increased significantly in small DRG neurons isolated from diabetic rats. The voltage-dependent activation and steady-state inactivation curves for these currents were shifted negatively. TTX-S currents induced by fast or slow voltage ramps increased markedly in neurons from diabetic rats. Immunoblots and immunofluorescence staining demonstrated significant increases in the expression of Nav1.3 (TTX-S) and Nav1.7 (TTX-S) and decreases in the expression of Nav1.6 (TTX-S) and Nav1.8 (TTX-R) in diabetic rats. The level of serine/threonine phosphorylation of Nav1.6 and Nav1.8 increased in response to diabetes. In addition, increased tyrosine phosphorylation of Nav1.6 and Nav1.7 was observed in DRGs from diabetic rats. These results suggest that both TTX-S and TTX-R sodium channels play important roles and that differential phosphorylation of sodium channels involving both serine/threonine and tyrosine sites contributes to painful diabetic neuropathy.
Pain is a common and debilitating accompaniment of neuropathy that occurs as a complication of diabetes. In the current study, we examined the effect of continuous release of gamma amino butyric acid (GABA), achieved by gene transfer of glutamic acid decarboxylase (GAD67) to dorsal root ganglia (DRG) in vivo using a nonreplicating herpes simplex virus (HSV)-based vector (vG) in a rat model of painful diabetic neuropathy (PDN). Subcutaneous inoculation of vG reduced mechanical hyperalgesia, thermal hyperalgesia and cold allodynia in rats with PDN. Continuous release of GABA from vector transduced cells in vivo prevented the increase in the voltage gated sodium channel isoform 1.7 (NaV1.7) protein that is characteristic of PDN. In vitro, infection of primary DRG neurons with vG prevented the increase in NaV1.7 resulting from exposure to hyperglycemia. The effect of vector-mediated GABA on NaV1.7 levels in vitro was blocked by phaclofen but not by bicuculline, a GABAB receptor effect that was blocked by pertussis toxin-(PTX) interference with Gα(i/o) function. Taken in conjunction with our previous observation that continuous activation of delta opioid receptors by vector-mediated release of enkephalin also prevents the increase in NaV1.7 in DRG exposed to hyperglycemia in vitro or in vivo, the observations in this report suggest a novel common mechanism through which activation of G protein coupled receptors (GPCR) in DRG neurons regulate the phenotype of the primary afferent.
painful diabetic neuropathy; voltage gated sodium channels; gene transfer; gene therapy; pain; diabetes
Several voltage-gated sodium channels (Navs) from nociceptive nerve fibers have been identified as important effectors in pain signaling. The objective of this study is to investigate the electroacupuncture (EA) analgesia mechanism by changing the expression of Navs in mice dorsal root ganglia (DRG). We injected carrageenan and complete Freund's adjuvant (CFA) into the mice plantar surface of the hind paw to induce inflammation and examined the antinociception effect of EA at the Zusanli (ST36) acupoint at 2 Hz low frequency. Mechanical hyperalgesia was evaluated by using electronic von Frey filaments, and thermal hyperalgesia was assessed using Hargreaves' test. Furthermore, we observed the expression and quality of Navs in DRG neurons. Our results showed that EA reduced mechanical and thermal pain in inflammatory animal model. The expression of Nav1.7 and Nav1.8 was increased after 4 days of carrageenan- and CFA-elicited inflammatory pain and further attenuated by 2 Hz EA stimulation. The attenuation cannot be observed in Nav1.9 sodium channels. We demonstrated that EA at Zusanli (ST36) acupoint at 2 Hz low-frequency stimulation attenuated inflammatory pain accompanied by decreasing the expression of Nav1.7 and 1.8, rather than Nav1.9, sodium channels in peripheral DRG neurons.
The mechanisms underlying neuropathic pain induction are very complex but might involve abnormal spontaneous activity in the sensory dorsal root ganglion (DRG). Voltage-gated sodium channels in the DRG are essential for the genesis of abnormal spontaneous neuronal activity. In the present study, we examined the changes in expression of the voltage-gated sodium channel Nav1.1 in the DRG after peripheral nerve injury. Western blot analysis showed that the level of Nav1.1 protein in the ipsilateral L5 DRG was significantly increased on days 3 and 7 after fifth lumbar spinal nerve ligation. Immunohistochemical study further confirmed a marked increase in the percentage of Nav1.1-positive cells in the ipsilateral DRG on day 3 after fifth lumbar spinal nerve ligation. Similarly, on day 7 after sciatic nerve axotomy, the amount of Nav1.1 protein and the percentage of Nav1.1-positive cells in the ipsilateral L5 DRG were also significantly increased. Our results suggest that an early increase in DRG Nav1.1 expression after peripheral nerve injury might be involved in the induction of neuropathic pain.
Nav1.1; sodium channel; neuropathic pain; dorsal root ganglion; rat