The distribution of low-threshold TTX-resistant Na⁺ currents in rat trigeminal ganglion cells

Understanding the physiological role of NaV1.9: Challenges and opportunities for pain modulation

Voltage-activated Na⁺ (Na_V) channels are crucial contributors to rapid electrical signaling in the human body. As such, they are among the most targeted membrane proteins by clinical therapeutics and natural toxins. Several of the nine mammalian Na_V channel subtypes play a documented role in pain or other sensory processes such as itch, touch, and smell. While causal relationships between these subtypes and biological function have been extensively described, the physiological role of Na_V1.9 is less understood. Yet, mutations in Na_V1.9 can cause striking disease phenotypes related to sensory perception such as loss or gain of pain and chronic itch. Here, we explore our current knowledge of the mechanisms by which Na_V1.9 may contribute to pain and elaborate on the challenges associated with establishing links between experimental conditions and human disease. This review also discusses the lack of comprehensive insights into Na_V1.9-specific pharmacology, an unfortunate situation since modulatory compounds may have tremendous potential in the clinic to treat pain or as precision tools to examine the extent of Na_V1.9 participation in sensory perception processes.

Attenuation of Sensory Transmission Through the Rat Trigeminal Ganglion by GABA Receptor Activation

While the trigeminal ganglion is often considered a passive conduit of sensory transmission, neurons and satellite glial cells (SGCs) within it can release neurotransmitters and express neuroreceptors. Some trigeminal ganglion neurons contain the neurotransmitter γ-aminobutyric acid (GABA) and express GABA receptors. There is behavioral evidence that increased GABA levels in the trigeminal ganglion decreases nociception, while a loss of GABA receptors results in hyperalgesia, although the neural mechanisms for this remain to be investigated. In this study, the expression of GABA receptors by trigeminal ganglion neurons that innervate rat labial skin and masseter muscle was compared using immunohistochemistry. The effect of intraganglionic administration of GABA receptor agonists was investigated by single unit recording of trigeminal brainstem and ganglion neuron responses to stimulation of the labial skin and/or masseter muscle in anesthetized rats. The mean frequency of expression of GABA_A and GABA_B receptors by masseter and labial skin ganglion neurons was 62.5% and 92.7%, and 55.4% and 20.3%, respectively. The expression of both GABA receptors was significantly greater in skin ganglion neurons. Masticatory muscle evoked brainstem trigeminal neuron responses were significantly attenuated by intraganglionic injection of muscimol (GABA_A) but not baclofen (GABA_B). The mechanical sensitivity of slow and fast conducting masticatory muscle afferent fibers was decreased and increased, respectively, by intraganglionic injection of both muscimol and baclofen. Activation of GABA_A receptors may exert a gating effect on sensory transmission through the trigeminal ganglion by decreasing putative nociceptive input and enhancing innocuous sensory input.

Maladaptive activation of Nav1.9 channels by nitric oxide causes triptan-induced medication overuse headache

Medication-overuse headaches (MOH) occur with both over-the-counter and pain-relief medicines, including paracetamol, opioids and combination analgesics. The mechanisms that lead to MOH are still uncertain. Here, we show that abnormal activation of Nav1.9 channels by Nitric Oxide (NO) is responsible for MOH induced by triptan migraine medicine. Deletion of the Scn11a gene in MOH mice abrogates NO-mediated symptoms, including cephalic and extracephalic allodynia, photophobia and phonophobia. NO strongly activates Nav1.9 in dural afferent neurons from MOH but not normal mice. Abnormal activation of Nav1.9 triggers CGRP secretion, causing artery dilatation and degranulation of mast cells. In turn, released mast cell mediators potentiates Nav1.9 in meningeal nociceptors, exacerbating inflammation and pain signal. Analysis of signaling networks indicates that PKA is downregulated in trigeminal neurons from MOH mice, relieving its inhibitory action on NO-Nav1.9 coupling. Thus, anomalous activation of Nav1.9 channels by NO, as a result of chronic medication, promotes MOH.

The Role of Voltage-Gated Sodium Channels in Pain Signaling

Acute pain signaling has a key protective role and is highly evolutionarily conserved. Chronic pain, however, is maladaptive, occurring as a consequence of injury and disease, and is associated with sensitization of the somatosensory nervous system. Primary sensory neurons are involved in both of these processes, and the recent advances in understanding sensory transduction and human genetics are the focus of this review. Voltage-gated sodium channels (VGSCs) are important determinants of sensory neuron excitability: they are essential for the initial transduction of sensory stimuli, the electrogenesis of the action potential, and neurotransmitter release from sensory neuron terminals. Nav1.1, Nav1.6, Nav1.7, Nav1.8, and Nav1.9 are all expressed by adult sensory neurons. The biophysical characteristics of these channels, as well as their unique expression patterns within subtypes of sensory neurons, define their functional role in pain signaling. Changes in the expression of VGSCs, as well as posttranslational modifications, contribute to the sensitization of sensory neurons in chronic pain states. Furthermore, gene variants in Nav1.7, Nav1.8, and Nav1.9 have now been linked to human Mendelian pain disorders and more recently to common pain disorders such as small-fiber neuropathy. Chronic pain affects one in five of the general population. Given the poor efficacy of current analgesics, the selective expression of particular VGSCs in sensory neurons makes these attractive targets for drug discovery. The increasing availability of gene sequencing, combined with structural modeling and electrophysiological analysis of gene variants, also provides the opportunity to better target existing therapies in a personalized manner.

Prostaglandin E2-mediated upregulation of neuroexcitation and persistent tetrodotoxin-resistant Na(+) currents in Ah-type trigeminal ganglion neurons isolated from adult female rats

Prostaglandin-E2 (PGE2) is a very important inflammatory mediator and PGE2-mediated neuroexcitation in sex-specific distribution of Ah-type trigeminal ganglion neurons (TGNs) isolated from adult female rats is not fully addressed. The whole-cell patch-clamp experiment was performed to verify the effects of PGE2, forskolin, and GPR30-selective agonist (G-1) on action potential (AP) and tetrodotoxin-resistant (TTX-R) Na(+) currents in identified Ah-type TGNs. The results showed that the firing frequency was increased in Ah- and C-types by PGE2, which was simulated by forskolin and inhibited by Rp-cyclic adenosine monophosphate (cAMP), while G-1 mimicked this effect only in Ah-types, which was abolished by GPR30-selective antagonist (G-15). Although the amplitude of AP was increased in Ah- and C-types, increased maximal upstroke velocity was confirmed only in Ah-types, suggesting distinct alternations in current density and/or voltage-dependent property of Na(+) channels. With 1.0 μM PGE2, TTX-R Na(+) currents were upregulated without changing the current-voltage relationship and voltage-dependent activation in C-types, however, the TTX-R Na(+) current was augmented in Ah-types, peaked voltage and the voltage-dependent activation were both shifted toward hyperpolarized direction with faster slope. Intriguingly, the low-threshold persistent TTX-R component was activated from -60 mV and increased almost double at -30 mV compared with ∼30-40% increment of TTX-R component being activated at ∼-10 mV. Additionally, the change in TTX-R component of Ah-types was equivalent well with that in C-type TGNs. Taken these data together, we conclude that PGE2 modulates the neuroexcitation via cAMP-mediated upregulation of TTX-R Na(+) currents in both cell-types with hormone-dependent feature, especially persistent TTX-R Na(+) currents in sex-specific distribution of myelinated Ah-type TGNs.

The role of Nav1.9 channel in the development of neuropathic orofacial pain associated with trigeminal neuralgia

BACKGROUND

Trigeminal neuralgia is accompanied by severe mechanical, thermal and chemical hypersensitivity of the orofacial area innervated by neurons of trigeminal ganglion (TG). We examined the role of the voltage-gated sodium channel subtype Nav1.9 in the development of trigeminal neuralgia.

RESULTS

We found that Nav1.9 is required for the development of both thermal and mechanical hypersensitivity induced by constriction of the infraorbital nerve (CION). The CION model does not induce change on Nav1.9 mRNA expression in the ipsilateral TG neurons when evaluated 9 days after surgery.

CONCLUSIONS

These results demonstrate that Nav1.9 channels play a critical role in the development of orofacial neuropathic pain. New routes for the treatment of orofacial neuropathic pain focussing on regulation of the voltage-gated Nav1.9 sodium channel activity should be investigated.

Effects of curcumin on TTX-R sodium currents of dorsal root ganglion neurons in type 2 diabetic rats with diabetic neuropathic pain

Type 2 diabetic mellitus (T2DM) has reached pandemic status and shows no signs of abatement. Diabetic neuropathic pain (DNP) is generally considered to be one of the most common complications of T2DM, which is also recognized as one of the most difficult types of pain to treat. As one kind of peripheral neuropathic pain, DNP manifests typical chronic neuralgia symptoms, including hyperalgesia, allodynia, autotomy, and so on. The injured dorsal root ganglion (DRG) is considered as the first stage of the sensory pathway impairment, whose neurons display increased frequency of action potential generation and increased spontaneous activities. These are mainly due to the changed properties of voltage-gated sodium channels (VGSCs) and the increased sodium currents, especially TTX-R sodium currents. Curcumin, one of the most important phytochemicals from turmeric, has been demonstrated to effectively prevent and/or ameliorate diabetic mellitus and its complications including DNP. The present study demonstrates that the TTX-R sodium currents of small-sized DRG neurons isolated from DNP rats are significantly increased. Such abnormality can be efficaciously ameliorated by curcumin.

Gain-of-function mutations in SCN11A cause familial episodic pain

Many ion channel genes have been associated with human genetic pain disorders. Here we report two large Chinese families with autosomal-dominant episodic pain. We performed a genome-wide linkage scan with microsatellite markers after excluding mutations in three known genes (SCN9A, SCN10A, and TRPA1) that cause similar pain syndrome to our findings, and we mapped the genetic locus to a 7.81 Mb region on chromosome 3p22.3-p21.32. By using whole-exome sequencing followed by conventional Sanger sequencing, we identified two missense mutations in the gene encoding voltage-gated sodium channel Nav1.9 (SCN11A): c.673C>T (p.Arg225Cys) and c.2423C>G (p.Ala808Gly) (one in each family). Each mutation showed a perfect cosegregation with the pain phenotype in the corresponding family, and neither of them was detected in 1,021 normal individuals. Both missense mutations were predicted to change a highly conserved amino acid residue of the human Nav1.9 channel. We expressed the two SCN11A mutants in mouse dorsal root ganglion (DRG) neurons and showed that both mutations enhanced the channel's electrical activities and induced hyperexcitablity of DRG neurons. Taken together, our results suggest that gain-of-function mutations in SCN11A can be causative of an autosomal-dominant episodic pain disorder.