Search

Functional bowel disorder patients can suffer from chronic abdominal pain, likely due to visceral hypersensitivity to mechanical stimuli. As there is only a limited understanding of the basis of chronic visceral hypersensitivity (CVH), drug-based management strategies are ill defined, vary considerably, and include NSAIDs, opioids, and even anticonvulsants. We previously reported that the 1.1 subtype of the voltage-gated sodium (NaV; NaV1.1) channel family regulates the excitability of sensory nerve fibers that transmit a mechanical pain message to the spinal cord. Herein, we investigated whether this channel subtype also underlies the abdominal pain that occurs with CVH. We demonstrate that NaV1.1 is functionally upregulated under CVH conditions and that inhibiting channel function reduces mechanical pain in 3 mechanistically distinct mouse models of chronic pain. In particular, we use a small molecule to show that selective NaV1.1 inhibition (a) decreases sodium currents in colon-innervating dorsal root ganglion neurons, (b) reduces colonic nociceptor mechanical responses, and (c) normalizes the enhanced visceromotor response to distension observed in 2 mouse models of irritable bowel syndrome. These results provide support for a relationship between NaV1.1 and chronic abdominal pain associated with functional bowel disorders.

The Nav1.1 voltage-gated sodium channel is a critical contributor to excitability in the brain, where pathological loss of function leads to such disorders as epilepsy, Alzheimer's disease, and autism. This voltage-gated sodium (Nav) channel subtype also plays an important role in mechanical pain signaling by primary afferent somatosensory neurons. Therefore, pharmacologic modulation of Nav1.1 represents a potential strategy for treating excitability disorders of the brain and periphery. Inactivation is a complex aspect of Nav channel gating and consists of fast and slow components, each of which may involve a contribution from one or more voltage-sensing domains. Here, we exploit the Hm1a spider toxin, a Nav1.1-selective modulator, to better understand the relationship between these temporally distinct modes of inactivation and ask whether they can be distinguished pharmacologically. We show that Hm1a inhibits the gating movement of the domain IV voltage sensor (VSDIV), hindering both fast and slow inactivation and leading to an increase in Nav1.1 availability during high-frequency stimulation. In contrast, ICA-121431, a small-molecule Nav1.1 inhibitor, accelerates a subsequent VSDIV gating transition to accelerate entry into the slow inactivated state, resulting in use-dependent block. Further evidence for functional coupling between fast and slow inactivation is provided by a Nav1.1 mutant in which fast inactivation removal has complex effects on slow inactivation. Taken together, our data substantiate the key role of VSDIV in Nav channel fast and slow inactivation and demonstrate that these gating processes are sequential and coupled through VSDIV. These findings provide insight into a pharmacophore on VSDIV through which modulation of inactivation gating can inhibit or facilitate Nav1.1 function.

Itch (pruritis) and pain represent two distinct sensory modalities; yet both have evolved to alert us to potentially harmful external stimuli. Compared with pain, our understanding of itch is still nascent. Here, we report a new clinical case of debilitating itch and altered pain perception resulting from the heterozygous de novo p.L811P gain-of-function mutation in [Na.sub.V]1.9, a voltage-gated sodium ([Na.sub.V]) channel subtype that relays sensory information from the periphery to the spine.To investigate the role of [Na.sub.V]1.9 in itch, we developed a mouse line in which the channel is N-terminally tagged with a fluorescent protein, thereby enabling the reliable identification and biophysical characterization of [Na.sub.V]1.9- expressing neurons. We also assessed [Na.sub.V]1.9 involvement in itch by using a newly created [Na.sub.V][1.9.sup.-/-] and [Na.sub.V][1.9.sup.L799P/WT] mouse model. We found that [Na.sub.V]1.9 is expressed in a subset of nonmyelinated, nonpeptidergic small-diameter dorsal root ganglia (DRGs). In WT DRGs, but not those of [Na.sub.V][1.9.sup.-/-] mice, pruritogens altered action potential parameters and [Na.sub.V] channel gating properties. Additionally, [Na.sub.V][1.9.sup.-/-] mice exhibited a strong reduction in acute scratching behavior in response to pruritogens, whereas [Na.sub.V][1.9.sup.L799P/WT] mice displayed increased spontaneous scratching. Altogether, our data suggest an important contribution of [Na.sub.V]1.9 to itch signaling., IntroductionThe somatosensory nervous system detects sensory modalities such as pain, itch, and temperature sensitivity and transfers them from the periphery to the spinal cord and eventually to the sensory cortex [...]

Purpose: Guanfacine is an α2A-adrenergic receptor agonist, FDA-approved to treat attention-deficit hyperactivity disorder and high blood pressure, typically as an extended-release formulation up to 7 mg/day. In our dysautonomia clinic, we observed that off-label use of short-acting guanfacine at 1 mg/day facilitated symptom relief in two families with multiple members presenting with severe generalized anxiety. We also noted anecdotal improvements in associated dysautonomia symptoms such as hyperhidrosis, cognitive impairment, and palpitations. We postulated that a genetic deficit existed in these patients that might augment guanfacine susceptibility. Methods: We used whole-exome sequencing to identify mutations in patients with shared generalized anxiety and dysautonomia symptoms. Guanfacine-induced changes in the function of voltage-gated Na+ channels were investigated using voltage-clamp electrophysiology. Results: Whole-exome sequencing uncovered the p.I739V mutation in SCN9A in the proband of two nonrelated families. Moreover, guanfacine inhibited ionic currents evoked by wild-type and mutant NaV1.7 encoded by SCN9A, as well as other NaV channel subtypes to a varying degree. Conclusion: Our study provides further evidence for a possible pathophysiological role of NaV1.7 in anxiety and dysautonomia. Combined with off-target effects on NaV channel function, daily administration of 1 mg short-acting guanfacine may be sufficient to normalize NaV channel mutation-induced changes in sympathetic activity, perhaps aided by partial inhibition of NaV1.7 or other channel subtypes. In a broader context, expanding genetic and functional data about ion channel aberrations may enable the prospect of stratifying patients in which mutation-induced increased sympathetic tone normalization by guanfacine can support treatment strategies for anxiety and dysautonomia symptoms. [ABSTRACT FROM AUTHOR]

Early in the pandemic, clinicians recognized an overlap between Long COVID symptoms and dysautonomia, suggesting autonomic nervous system (ANS) dysfunction. Our clinical experience at Johns Hopkins with primary dysautonomia suggested heritability of sympathetic dysfunction, manifesting primarily as hyperhidrosis and as other dysautonomia symptoms. Whole exome sequencing revealed mutations in genes regulating electrical signaling in the nervous system, thus providing a genetic basis for the sympathetic overdrive observed. We hypothesize that dysautonomia in Long COVID requires two molecular hits: a genetic vulnerability to prime the ANS and a SARS-CoV-2 infection, as an immune trigger, to further disrupt ANS function resulting in increased sympathetic activity. Indeed, Long COVID patients show signs of chronic inflammation and autoimmunity. We have translated this two-hit concept to the clinic using ion channel inhibitors to target genetic susceptibility and immunomodulators to treat inflammation. This multi-hit hypothesis shows promise for managing Long COVID and merits further study. [ABSTRACT FROM AUTHOR]

Voltage-gated sodium (Na[sub.v]) channels initiate action potentials in most neurons, including primary afferent nerve fibres of the pain pathway. Local anaesthetics block pain through non-specific actions at all Na[sub.v] channels, but the discovery of selective modulators would facilitate the analysis of individual subtypes of these channels and their contributions to chemical, mechanical, or thermal pain. Here we identify and characterize spider (Heteroscodra maculata) toxins that selectively activate the Na[sub.v]1.1 subtype, the role of which in nociception and pain has not been elucidated. We use these probes to show that Na[sub.v]1.1-expressing fibres are modality-specific nociceptors: their activation elicits robust pain behaviours without neurogenic inflammation and produces profound hypersensitivity to mechanical, but not thermal, stimuli. In the gut, high-threshold mechanosensitive fibres also express Na[sub.v]1.1 and show enhanced toxin sensitivity in a mouse model of irritable bowel syndrome. Together, these findings establish an unexpected role for Na[sub.v]1.1 channels in regulating the excitability of sensory nerve fibres that mediate mechanical pain., Author(s): Jeremiah D. Osteen [1]; Volker Herzig [2]; John Gilchrist [3]; Joshua J. Emrick [1]; Chuchu Zhang [1]; Xidao Wang [4]; Joel Castro [5, 6]; Sonia Garcia-Caraballo [5, 6]; Luke [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.tips.2009.12.007 Byline: Frank Bosmans (1)(2), Kenton J. Swartz (1) Abstract: Voltage-activated sodium (Nav) channels are essential in generating and propagating nerve impulses, placing them amongst the most widely targeted ion channels by toxins from venomous organisms. An increasing number of spider toxins have been shown to interfere with the voltage-driven activation process of mammalian Nav channels, possibly by interacting with one or more of their voltage sensors. This review focuses on our existing knowledge of the mechanism by which spider toxins affect Nav channel gating and the possible applications of these toxins in the drug discovery process. Author Affiliation: (1) Molecular Physiology and Biophysics Section, Porter Neuroscience Research Center National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892 USA (2) Laboratory of Toxicology, University of Leuven, 3000 Leuven, Belgium

Voltage-activated sodium ([Na.sub.v]) channels are crucial for the generation and propagation of nerve impulses, and as such are widely targeted by toxins and drugs. The four voltage sensors in [Na.sub.v] [...]

Background and Purpose: Voltage-gated sodium (NaV ) channels are expressed de novo in carcinomas where their activity promotes invasiveness. Breast and colon cancer cells express the neonatal splice variant of NaV 1.5 (nNaV 1.5), which has several amino acid substitutions in the domain I voltage-sensor compared with its adult counterpart (aNaV 1.5). This study aimed to determine whether nNaV 1.5 channels could be distinguished pharmacologically from aNaV 1.5 channels.Experimental Approach: Cells expressing either nNaV 1.5 or aNaV 1.5 channels were exposed to low MW inhibitors, an antibody or natural toxins, and changes in electrophysiological parameters were measured. Stable expression in EBNA cells and transient expression in Xenopus laevis oocytes were used. Currents were recorded by whole-cell patch clamp and two-electrode voltage-clamp, respectively.Key Results: Several clinically used blockers of NaV channels (lidocaine, procaine, phenytoin, mexiletine, ranolazine, and riluzole) could not distinguish between nNaV 1.5 or aNaV 1.5 channels. However, two tarantula toxins (HaTx and ProTx-II) and a polyclonal antibody (NESOpAb) preferentially inhibited currents elicited by either nNaV 1.5 or aNaV 1.5 channels by binding to the spliced region of the channel. Furthermore, the amino acid residue at position 211 (aspartate in aNaV 1.5/lysine in nNaV 1.5), that is, the charge reversal in the spliced region of the channel, played a key role in the selectivity, especially in antibody binding.Conclusion and Implications: We conclude that the cancer-related nNaV 1.5 channel can be distinguished pharmacologically from its nearest neighbour, aNaV 1.5 channels. Thus, it may be possible to design low MW compounds as antimetastatic drugs for non-toxic therapy of nNaV 1.5-expressing carcinomas. [ABSTRACT FROM AUTHOR]

ω-Grammotoxin-SIA (GrTX-SIA) was originally isolated from the venom of the Chilean rose tarantula and demonstrated to function as a gating modifier of voltage-gated Ca2+ (CaV) channels. Later experiments revealed that GrTX-SIA could also inhibit voltage-gated K+ (KV) channel currents via a similar mechanism of action that involved binding to a conserved S3-S4 region in the voltage-sensing domains (VSDs). Since voltage-gated Na+ (NaV) channels contain homologous structural motifs, we hypothesized that GrTX-SIA could inhibit members of this ion channel family as well. Here, we show that GrTX-SIA can indeed impede the gating process of multiple NaV channel subtypes with NaV1.6 being the most susceptible target. Moreover, molecular docking of GrTX-SIA onto NaV1.6, supported by a p.E1607K mutation, revealed the voltage sensor in domain IV (VSDIV) as being a primary site of action. The biphasic manner in which current inhibition appeared to occur suggested a second, possibly lower-sensitivity binding locus, which was identified as VSDII by using KV2.1/NaV1.6 chimeric voltage-sensor constructs. Subsequently, the NaV1.6p.E782K/p.E838K (VSDII), NaV1.6p.E1607K (VSDIV), and particularly the combined VSDII/VSDIV mutant lost virtually all susceptibility to GrTX-SIA. Together with existing literature, our data suggest that GrTX-SIA recognizes modules in NaV channel VSDs that are conserved among ion channel families, thereby allowing it to act as a comprehensive ion channel gating modifier peptide. [ABSTRACT FROM AUTHOR]