Inactivation properties of sodium channel Nav1.8 maintain action potential amplitude in small DRG neurons in the context of depolarization

<p>Abstract</p> <p>Background</p> <p>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 Na_v1.8.</p> <p>Results</p> <p>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 V_1/2values of -73 and -37 mV. These values are similar to the V_1/2values for steady-state fast inactivation of tetrodotoxin-sensitive (TTX-s) sodium channels, and the tetrodotoxin-resistant (TTX-r) Na_v1.8 sodium channel, respectively. Addition of TTX eliminates the more hyperpolarized V_1/2component 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 Na_v1.8(-/-) mice, the voltage-dependent decrease in AP amplitude is characterized by a single Boltzmann equation with a V_1/2value of -55 mV, suggesting a shift in the steady-state fast inactivation properties of TTX-s sodium channels. Transfection of Na_v1.8(-/-) DRG neurons with DNA encoding Na_v1.8 results in a membrane potential-dependent decrease in AP amplitude that recapitulates WT properties.</p> <p>Conclusion</p> <p>We conclude that the presence of Na_v1.8 allows AP amplitude to be maintained in DRG neurons and their centrally projecting axons even when depolarized within the dorsal horn.</p

Rat brain Na+ channel mRNAs in non-excitable Schwann cells

AbstractThe expression of rat brain voltage-sensitive Na+ channel mRNAs in Schwann cells was examined using in situ hybridization cytochemistry and RT-PCR. The mRNAs of rat brain Na+ channel subtype II and III, but not subtype I, were detected in cultured Schwann cells from sciatic nerve and in intact sciatic nerve, which contains Schwann cells but not neuronal cell bodies. These results indicate that rat brain Na+ channel mRNAs, which have been considered as mainly neuronal-type messages, are also expressed in glial cells in vitro and in vivo

Sodium channel expression in the ventral posterolateral nucleus of the thalamus after peripheral nerve injury

Peripheral nerve injury is known to up-regulate the expression of rapidly-repriming Nav1.3 sodium channel within first-order dorsal root ganglion neurons and second-order dorsal horn nociceptive neurons, but it is not known if pain-processing neurons higher along the neuraxis also undergo changes in sodium channel expression. In this study, we hypothesized that after peripheral nerve injury, third-order neurons in the ventral posterolateral (VPL) nucleus of the thalamus undergo changes in expression of sodium channels. To test this hypothesis, adult male Sprague-Dawley rats underwent chronic constriction injury (CCI) of the sciatic nerve. Ten days after CCI, when allodynia and hyperalgesia were evident, in situ hybridization and immunocytochemical analysis revealed up-regulation of Nav1.3 mRNA, but no changes in expression of Nav1.1, Nav1.2, or Nav1.6 in VPL neurons, and unit recordings demonstrated increased background firing, which persisted after spinal cord transection, and evoked hyperresponsiveness to peripheral stimuli. These results demonstrate that injury to the peripheral nervous system induces alterations in sodium channel expression within higher-order VPL neurons, and suggest that misexpression of the Nav1.3 sodium channel increases the excitability of VPL neurons injury, contributing to neuropathic pain

Genes encoding the β1 subunit of voltage-dependent Na+ channel in rat, mouse and human contain conserved introns

AbstractWe provide evidence in this study that the 86-bp insert in the β1.2 mRNA isoform of the voltage gated sodium channel is an intron. Transcripts still retaining this intron were detected in all tissues where the β1 gene expression was investigated. We also show that the exon/intron boundaries of the last two introns are conserved among rat, mouse and human β1 gene. Unlike the highly conserved cDNAs, introns in only the rat and mouse genes are highly related. The last intron is very short (86–90 bp) and is located in the 3′ untranslated sequence, both uncommon properties of mammalian pre-mRNA introns

Voltage-gated sodium channels (NaV) in GtoPdb v.2021.3

Sodium channels are voltage-gated sodium-selective ion channels present in the membrane of most excitable cells. Sodium channels comprise of one pore-forming &#945; subunit, which may be associated with either one or two &#946; subunits [177]. &#945;-Subunits consist of four homologous domains (I-IV), each containing six transmembrane segments (S1-S6) and a pore-forming loop. The positively charged fourth transmembrane segment (S4) acts as a voltage sensor and is involved in channel gating. The crystal structure of the bacterial NavAb channel has revealed a number of novel structural features compared to earlier potassium channel structures including a short selectivity filter with ion selectivity determined by interactions with glutamate side chains [274]. Interestingly, the pore region is penetrated by fatty acyl chains that extend into the central cavity which may allow the entry of small, hydrophobic pore-blocking drugs [274]. Auxiliary &#946;1, &#946;2, &#946;3 and &#946;4 subunits consist of a large extracellular N-terminal domain, a single transmembrane segment and a shorter cytoplasmic domain.The nomenclature for sodium channels was proposed by Goldin et al., (2000) [144] and approved by the NC-IUPHAR Subcommittee on sodium channels (Catterall et al., 2005, [52])

Voltage-gated sodium channels (NaV) in GtoPdb v.2023.1

Sodium channels are voltage-gated sodium-selective ion channels present in the membrane of most excitable cells. Sodium channels comprise of one pore-forming &#945; subunit, which may be associated with either one or two &#946; subunits [179]. &#945;-Subunits consist of four homologous domains (I-IV), each containing six transmembrane segments (S1-S6) and a pore-forming loop. The positively charged fourth transmembrane segment (S4) acts as a voltage sensor and is involved in channel gating. The crystal structure of the bacterial NavAb channel has revealed a number of novel structural features compared to earlier potassium channel structures including a short selectivity filter with ion selectivity determined by interactions with glutamate side chains [278]. Interestingly, the pore region is penetrated by fatty acyl chains that extend into the central cavity which may allow the entry of small, hydrophobic pore-blocking drugs [278]. Auxiliary &#946;1, &#946;2, &#946;3 and &#946;4 subunits consist of a large extracellular N-terminal domain, a single transmembrane segment and a shorter cytoplasmic domain.The nomenclature for sodium channels was proposed by Goldin et al., (2000) [146] and approved by the NC-IUPHAR Subcommittee on sodium channels (Catterall et al., 2005, [53])

Voltage-gated sodium channels (version 2019.4) in the IUPHAR/BPS Guide to Pharmacology Database

Sodium channels are voltage-gated sodium-selective ion channels present in the membrane of most excitable cells. Sodium channels comprise of one pore-forming &#945; subunit, which may be associated with either one or two &#946; subunits [176]. &#945;-Subunits consist of four homologous domains (I&#8211;IV), each containing six transmembrane segments (S1&#8211;S6) and a pore-forming loop. The positively charged fourth transmembrane segment (S4) acts as a voltage sensor and is involved in channel gating. The crystal structure of the bacterial NavAb channel has revealed a number of novel structural features compared to earlier potassium channel structures including a short selectivity filter with ion selectivity determined by interactions with glutamate side chains [268]. Interestingly, the pore region is penetrated by fatty acyl chains that extend into the central cavity which may allow the entry of small, hydrophobic pore-blocking drugs [268]. Auxiliary &#946;1, &#946;2, &#946;3 and &#946;4 subunits consist of a large extracellular N-terminal domain, a single transmembrane segment and a shorter cytoplasmic domain.The nomenclature for sodium channels was proposed by Goldin et al., (2000) [143] and approved by the NC-IUPHAR Subcommittee on sodium channels (Catterall et al., 2005, [51])

Intense isolectin-B4 binding in rat dorsal root ganglion neurons distinguishes c-fiber nociceptors with broad action potentials and high nav1.9 expression

Binding to isolectin-B4 (IB4) and expression of tyrosine kinase A (trkA) (the high-affinity NGF receptor) have been used to define two different subgroups of nociceptive small dorsal root ganglion (DRG) neurons. We previously showed that only nociceptors have high trkA levels. However, information about sensory and electrophysiological properties in vivo of single identified IB4-binding neurons, and about their trkA expression levels, is lacking. IB4-positive (IB4+) and small dark neurons had similar size distributions. We examined IB4-binding levels in >120 dye-injected DRG neurons with sensory and electrophysiological properties recorded in vivo. Relative immunointensities for trkA and two TTX-resistant sodium channels (Nav1.8 and Nav1.9) were also measured in these neurons. IB4+ neurons were classified as strongly or weakly IB4+. All strongly IB4+ neurons were C-nociceptor type (C-fiber nociceptive or unresponsive). Of 32 C-nociceptor-type neurons examined, ~50% were strongly IB4+, ~20% were weakly IB4+ and ~30% were IB4–. A{delta} low-threshold mechanoreceptive (LTM) neurons were weakly IB4+ or IB4–. All 33 A-fiber nociceptors and all 44 A{alpha}/beta-LTM neurons examined were IB4–. IB4+ compared with IB4– C-nociceptor-type neurons had longer somatic action potential durations and rise times, slower conduction velocities, more negative membrane potentials, and greater immunointensities for Nav1.9 but not Nav1.8. Immunointensities of IB4 binding in C-neurons were positively correlated with those of Nav1.9 but not Nav1.8. Of 23 C-neurons tested for both trkA and IB4, ~35% were trkA+/IB4+ but with negatively correlated immunointensities; 26% were IB4+/trkA–, and 35% were IB4–/trkA+. We conclude that strongly IB4+ DRG neurons are exclusively C-nociceptor type and that high Nav1.9 expression may contribute to their distinct membrane properties

Neurophysiology

Contains reports on seven research projects.National Institutes of Health (Training Grant 5 TO1 EY00090)Bell Laboratories (Grant