alpha-SNS produces the slow TTX-resistant sodium current in large cutaneous afferent DRG neurons

Protocol to isolate dorsal root ganglion neurons from embryonic rats by immunopanning and characterize them using RNAscope and immunofluorescence

Dorsal root ganglion (DRG) neurons innervate the periphery of the body to transmit somatosensory information, including touch, pain, and temperature. Here, we present a protocol for isolating rat DRG neurons using an immunopanning technique. We describe steps for dissecting DRGs from embryonic day (E)15 rat embryos, triturating ganglionic cells, and purifying DRG neurons. We also detail procedures for mRNA or protein quantification using RNAscope and immunofluorescence. This protocol has potential applications for studying DRG neurons during development and disease. For complete details on the use and execution of this protocol, please refer to Kantarci et al.1.

Nociceptor sodium channels shape subthreshold phase, upstroke, and shoulder of action potentials

Voltage-gated sodium channels (VGSCs) in the peripheral nervous system shape action potentials (APs) and thereby support the detection of sensory stimuli. Most of the nine mammalian VGSC subtypes are expressed in nociceptors, but predominantly, three are linked to several human pain syndromes: while Nav1.7 is suggested to be a (sub-)threshold channel, Nav1.8 is thought to support the fast AP upstroke. Nav1.9, as it produces large persistent currents, is attributed a role in determining the resting membrane potential. We characterized the gating of Nav1.1-Nav1.3 and Nav1.5-Nav1.9 in manual patch clamp with a focus on the AP subthreshold depolarization phase. Nav1.9 exhibited the most hyperpolarized activation, while its fast inactivation resembled the depolarized inactivation of Nav1.8. For some VGSCs (e.g., Nav1.1 and Nav1.2), a positive correlation between ramp current and window current was detected. Using a modified Hodgkin-Huxley model that accounts for the time needed for inactivation to occur, we used the acquired data to simulate two nociceptive nerve fiber types (an Aδ- and a mechano-insensitive C-nociceptor) containing VGSC conductances according to published human RNAseq data. Our simulations suggest that Nav1.9 is supporting both the AP upstroke and its shoulder. A reduced threshold for AP generation was induced by enhancing Nav1.7 conductivity or shifting its activation to more hyperpolarized potentials, as observed in Nav1.7-related pain disorders. Here, we provide a comprehensive, comparative functional characterization of VGSCs relevant in nociception and describe their gating with Hodgkin-Huxley-like models, which can serve as a tool to study their specific contributions to AP shape and sodium channel-related diseases.

Spike desensitisation as a mechanism for high-contrast selectivity in retinal ganglion cells

In the vertebrate retina, several dozens of parallel channels relay information about the visual world to the brain. These channels are represented by the different types of retinal ganglion cells (RGCs), whose responses are rendered selective for distinct sets of visual features by various mechanisms. These mechanisms can be roughly grouped into synaptic interactions and cell-intrinsic mechanisms, with the latter including dendritic morphology as well as ion channel complement and distribution. Here, we investigate how strongly ion channel complement can shape RGC output by comparing two mouse RGC types, the well-described ON alpha cell and a little-studied ON cell that is EGFP-labelled in the Igfbp5 mouse line and displays an unusual selectivity for stimuli with high contrast. Using patch-clamp recordings and computational modelling, we show that a higher activation threshold and a pronounced slow inactivation of the voltage-gated Na+ channels contribute to the distinct contrast tuning and transient responses in ON Igfbp5 RGCs, respectively. In contrast, such a mechanism could not be observed in ON alpha cells. This study provides an example for the powerful role that the last stage of retinal processing can play in shaping RGC responses.

There is no F in APC: Using physiological fluoride-free solutions for high throughput automated patch clamp experiments

Fluoride has been used in the internal recording solution for manual and automated patch clamp experiments for decades because it helps to improve the seal resistance and promotes longer lasting recordings. In manual patch clamp, fluoride has been used to record voltage-gated Na (NaV) channels where seal resistance and access resistance are critical for good voltage control. In automated patch clamp, suction is applied from underneath the patch clamp chip to attract a cell to the hole and obtain a good seal. Since the patch clamp aperture cannot be moved to improve the seal like the patch clamp pipette in manual patch clamp, automated patch clamp manufacturers use internal fluoride to improve the success rate for obtaining GΩ seals. However, internal fluoride can affect voltage-dependence of activation and inactivation, as well as affecting internal second messenger systems and therefore, it is desirable to have the option to perform experiments using physiological, fluoride-free internal solution. We have developed an approach for high throughput fluoride-free recordings on a 384-well based automated patch clamp system with success rates >40% for GΩ seals. We demonstrate this method using hERG expressed in HEK cells, as well as NaV1.5, NaV1.7, and KCa3.1 expressed in CHO cells. We describe the advantages and disadvantages of using fluoride and provide examples of where fluoride can be used, where caution should be exerted and where fluoride-free solutions provide an advantage over fluoride-containing solutions.

Pre-Synaptic GABAA in NaV1.8+ Primary Afferents Is Required for the Development of Punctate but Not Dynamic Mechanical Allodynia following CFA Inflammation

Hypersensitivity to mechanical stimuli is a cardinal symptom of neuropathic and inflammatory pain. A reduction in spinal inhibition is generally considered a causal factor in the development of mechanical hypersensitivity after injury. However, the extent to which presynaptic inhibition contributes to altered spinal inhibition is less well established. Here, we used conditional deletion of GABA_A in NaV1.8-positive sensory neurons (Scn10a^Cre;Gabrb3^fl/fl) to manipulate selectively presynaptic GABAergic inhibition. Behavioral testing showed that the development of inflammatory punctate allodynia was mitigated in mice lacking pre-synaptic GABA_A. Dorsal horn cellular circuits were visualized in single slices using stimulus-tractable dual-labelling of c-fos mRNA for punctate and the cognate c-Fos protein for dynamic mechanical stimulation. This revealed a substantial reduction in the number of cells activated by punctate stimulation in mice lacking presynaptic GABA_A and an approximate 50% overlap of the punctate with the dynamic circuit, the relative percentage of which did not change following inflammation. The reduction in dorsal horn cells activated by punctate stimuli was equally prevalent in parvalbumin- and calretinin-positive cells and across all laminae I–V, indicating a generalized reduction in spinal input. In peripheral DRG neurons, inflammation following complete Freund’s adjuvant (CFA) led to an increase in axonal excitability responses to GABA, suggesting that presynaptic GABA effects in NaV1.8⁺ afferents switch from inhibition to excitation after CFA. In the days after inflammation, presynaptic GABA_A in NaV1.8⁺ nociceptors constitutes an “open gate” pathway allowing mechanoreceptors responding to punctate mechanical stimulation access to nociceptive dorsal horn circuits.

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na+ and Ca2+ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K+ channels. Because they display limited central side effects, peripherally restricted Na+ and Ca2+ channel blockers and K+ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Nav1.3, Nav1.7, Nav1.8, Cav3.2, and HCN2 and activators of Kv7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K+ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na+ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca2+ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing "pain" as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Gene replacement ameliorates deficits in mouse and human models of cyclin-dependent kinase-like 5 disorder

Cyclin-dependent kinase-like 5 disorder is a severe neurodevelopmental disorder caused by mutations in the X-linked cyclin-dependent kinase-like 5 (CDKL5) gene. It predominantly affects females who typically present with severe early epileptic encephalopathy, global developmental delay, motor dysfunction, autistic features and sleep disturbances. To develop a gene replacement therapy, we initially characterized the human CDKL5 transcript isoforms expressed in the brain, neuroblastoma cell lines, primary astrocytes and embryonic stem cell-derived cortical interneurons. We found that the isoform 1 and to a lesser extent the isoform 2 were expressed in human brain, and both neuronal and glial cell types. These isoforms were subsequently cloned into recombinant adeno-associated viral (AAV) vector genome and high-titre viral vectors were produced. Intrajugular delivery of green fluorescence protein via AAV vector serotype PHP.B in adult wild-type male mice transduced neurons and astrocytes throughout the brain more efficiently than serotype 9. Cdkl5 knockout male mice treated with isoform 1 via intrajugular injection at age 28–30 days exhibited significant behavioural improvements compared to green fluorescence protein-treated controls (1012 vg per animal, n = 10 per group) with PHP.B vectors. Brain expression of the isoform 1 transgene was more abundant in hindbrain than forebrain and midbrain. Transgene brain expression was sporadic at the cellular level and most prominent in hippocampal neurons and cerebellar Purkinje cells. Correction of postsynaptic density protein 95 cerebellar misexpression, a major fine cerebellar structural abnormality in Cdkl5 knockout mice, was found in regions of high transgene expression within the cerebellum. AAV vector serotype DJ efficiently transduced CDKL5-mutant human induced pluripotent stem cell-derived neural progenitors, which were subsequently differentiated into mature neurons. When treating CDKL5-mutant neurons, isoform 1 expression led to an increased density of synaptic puncta, while isoform 2 ameliorated the calcium signalling defect compared to green fluorescence protein control, implying distinct functions of these isoforms in neurons. This study provides the first evidence that gene therapy mediated by AAV vectors can be used for treating CDKL5 disorder.

Voltage- and calcium-gated ion channels of neurons in the vertebrate retina

In this review, we summarize studies investigating the types and distribution of voltage- and calcium-gated ion channels in the different classes of retinal neurons: rods, cones, horizontal cells, bipolar cells, amacrine cells, interplexiform cells, and ganglion cells. We discuss differences among cell subtypes within these major cell classes, as well as differences among species, and consider how different ion channels shape the responses of different neurons. For example, even though second-order bipolar and horizontal cells do not typically generate fast sodium-dependent action potentials, many of these cells nevertheless possess fast sodium currents that can enhance their kinetic response capabilities. Ca2+ channel activity can also shape response kinetics as well as regulating synaptic release. The L-type Ca2+ channel subtype, CaV1.4, expressed in photoreceptor cells exhibits specific properties matching the particular needs of these cells such as limited inactivation which allows sustained channel activity and maintained synaptic release in darkness. The particular properties of K+ and Cl- channels in different retinal neurons shape resting membrane potentials, response kinetics and spiking behavior. A remaining challenge is to characterize the specific distributions of ion channels in the more than 100 individual cell types that have been identified in the retina and to describe how these particular ion channels sculpt neuronal responses to assist in the processing of visual information by the retina.

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.

Characteristics of sensory neuronal groups in CGRP-cre-ER reporter mice: Comparison to Nav1.8-cre, TRPV1-cre and TRPV1-GFP mouse lines

Peptidergic sensory neurons play a critical role in nociceptive pathways. To precisely define the function and plasticity of sensory neurons in detail, new tools such as transgenic mouse models are needed. We employed electrophysiology and immunohistochemistry to characterize in detail dorsal root ganglion (DRG) neurons expressing an inducible CGRP^cre-ER (CGRP-cre⁺); and compared them to DRG neurons expressing Nav1.8^cre (Nav1.8-cre⁺), TRPV1^cre (TRPV1-cre⁺) and TRPV1-GFP (V1-GFP⁺). Tamoxifen effectively induced CGRP^cre-ER production in DRG. ≈87% of CGRP^cre-ER-expressing neurons were co-labeled CGRP antibody. Three small and two medium-large-sized (5HT3a⁺/NPY2R^- and NPY2R⁺) neuronal groups with unique electrophysiological profiles were CGRP-cre⁺. Nav1.8-cre⁺ neurons were detected in all CGRP-cre⁺ groups, as well as in 5 additional neuronal groups: MrgprD⁺/TRPA1^-, MrgprD⁺/TRPA1⁺, TRPV1⁺/CGRP^-, vGLUT3⁺ and ≈30% of trkC⁺ neurons. Differences between TRPV1^cre and Nav1.8^cre reporters were that unlike TRPV1-cre⁺, Nav1.8-cre⁺ expression was detected in non-nociceptive vGLUT3⁺ and trkC⁺ populations. Many TRPV1-cre⁺ neurons did not respond to capsaicin. In contrast, V1-GFP⁺ neurons were in 4 groups, each of which was capsaicin-sensitive. Finally, none of the analyzed reporter lines showed cre-recombination in trkB⁺, calbindin⁺, 70% of trkC⁺ or parvalbumin⁺ neurons, which together encompassed ≈20% of Nav1.8-cre^- DRG neurons. The data presented here increases our knowledge of peptidergic sensory neuron characteristics, while showing the efficiency and specificity manipulation of peptidergic neurons by the CGRP^cre-ER reporter. We also demonstrate that manipulation of all C- and A-nociceptors is better achieved with TRPV1-cre reporter. Finally, the described approach for detailed characterization of sensory neuronal groups can be applied to a variety of reporter mice.

Ambroxol for the treatment of fibromyalgia: science or fiction?

Fibromyalgia appears to present in subgroups with regard to biological pain induction, with primarily inflammatory, neuropathic/neurodegenerative, sympathetic, oxidative, nitrosative, or muscular factors and/or central sensitization. Recent research has also discussed glial activation or interrupted dopaminergic neurotransmission, as well as increased skin mast cells and mitochondrial dysfunction. Therapy is difficult, and the treatment options used so far mostly just have the potential to address only one of these aspects. As ambroxol addresses all of them in a single substance and furthermore also reduces visceral hypersensitivity, in fibromyalgia existing as irritable bowel syndrome or chronic bladder pain, it should be systematically investigated for this purpose. Encouraged by first clinical observations of two working groups using topical or oral ambroxol for fibromyalgia treatments, the present paper outlines the scientific argument for this approach by looking at each of the aforementioned aspects of this complex disease and summarizes putative modes of action of ambroxol. Nevertheless, at this point the evidence basis for ambroxol is not strong enough for clinical recommendation.

Topical ambroxol for the treatment of neuropathic pain. An initial clinical observation

BACKGROUND

Neuropathic pain is difficult to treat, and the available options are often inadequate. The expectorant ambroxol also acts as a strong local anaesthetic and blocks sodium channels about 40 times more potently than lidocaine. It preferentially inhibits the channel subtype Nav 1.8, which is expressed especially in nociceptive C-fibres. In view of the low toxicity of ambroxol, it seemed reasonable to try using it for the treatment of neuropathic pain that failed to respond to other standard options.

MATERIAL AND METHODS

The medical records of seven patients with severe neuropathic pain and pain reduction following topical ambroxol treatment are reported retrospectively. As standard therapies had not proved sufficient, a topical ambroxol 20% cream was repeatedly applied by the patients in the area of neuropathic pain.

RESULTS

The reasons for neuropathic pain were postherpetic neuralgia (2 ×), mononeuropathy multiplex, phantom pain, deafferentation pain, postoperative neuralgia and foot neuropathy of unknown origin. The individual mean pain intensity reported was between 4 and 6/10 (NRS), maximum pain at 6-10/10 (NRS). The pain reduction achieved individually following ambroxol cream was 2-8 points (NRS) within 5-30 min and lasted for 3-8 h. Pain attacks were reduced in all five patients presenting with this problem. Four patients with no improvement after lidocaine 5% and one patient with no response to capsaicin 8% nevertheless experienced a pain reduction with topical ambroxol. No patient reported any side effects or skin changes during a treatment that has since been continued for up to 4 years.

CONCLUSION

Ambroxol acts as a strong local anaesthetic and preferentially inhibits the nociceptively relevant sodium channel subtype Nav 1.8. For the first time, we report below on a relevant pain relief following topical ambroxol 20% cream in patients with neuropathic pain. In view of the positive side effect profile, the clinical benefit in patients with pain should be investigated further.

[Topical ambroxol for the treatment of neuropathic pain: A first clinical observation. German version]

BACKGROUND

Neuropathic pain is difficult to treat and available options are frequently not sufficient. The expectorant ambroxol also works as a strong local anesthetic and blocks sodium channels about 40 times more potently than lidocaine. Ambroxol preferentially inhibits the channel subtype Nav 1.8, which is expressed particularly in nociceptive C fibers. Due to the low toxicity, topical ambroxol seemed to represent a reasonable therapeutic attempt for treatment of neuropathic pain resistant to other standard options.

MATERIALS AND METHODS

Medical records of 7 patients with severe neuropathic pain, in whom many attempts at treatment with approved substances were not sufficient or possible, are reported retrospectively. Patients were then treated with topical ambroxol 20% cream applied in the area of neuropathic pain.

RESULTS

Causes of neuropathic pain were postherpetic neuralgia (2-×), mononeuropathy multiplex, phantom pain, deafferentation pain, postoperative neuralgia and an unclear allodynia of the foot. Mean pain intensity was reported as 4-6/10 on a numeric rating scale (NRS) and maximum pain intensity as 6-10/10. Pain reduction following ambroxol cream was 2-8 points (NRS) within 15-30 min and lasted 3-8 h. Pain attacks were reduced in all 5 patients presenting this problem. Topical ambroxol achieved pain reduction in 4 patients with no improvement after lidocaine 5% and 1 patient with no response to capsaicin 8%. No adverse events or skin changes have been observed, and the longest treatment duration is currently 4 years.

CONCLUSION

Ambroxol acts as a strong local anesthetic and preferentially inhibits the nociceptive-relevant sodium channel subtype Nav 1.8. For the first time, we report relevant pain reduction following topical Ambroxol 20% cream in patients with neuropathic pain. Regarding the advantageous profile with rare side effects, the clinical benefit for pain patients should be further investigated.

Acid modulation of tetrodotoxin-sensitive Na+ channels in large-sized trigeminal ganglion neurons

Voltage-gated Na⁺ channels in primary afferent neurons can be divided into tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) Na⁺ channels. Although previous studies have shown the acid modulation of TTX-R Na⁺ channels, the effect of acidic pH on tetrodotoxin-sensitive (TTX-S) Na⁺ channels is still unknown. Here we report the effect of acidic pH on TTX-S Na⁺ channels expressed in large-sized trigeminal ganglion (TG) neurons using a whole-cell patch clamp technique. The application of acidic extracellular solution decreased the peak amplitude of TTX-S currents (I_Na) in a pH-dependent manner, but weak acid (≥pH 6.0) had no inhibitory effect on TTX-S I_Na. Acidic pH (pH 6.0) shifted both the activation and steady-state fast inactivation relationships of TTX-S Na⁺ channels toward depolarized potentials. However, acidic pH (pH 6.0) had no effect on use-dependent inhibition in response to high-frequency stimuli, development of inactivation, and accelerated the recovery from inactivation of TTX-S Na⁺ channels, suggesting that TTX-S Na⁺ channels in large-sized TG neurons are less sensitive to acidic pH. Given that voltage-gated Na⁺ channels play a pivotal role in the generation and conduction of action potentials in neural tissues, the insensitivity of TTX-S Na⁺ channels expressed in large-sized TG neurons to acidic pH would ensure transmission of innocuous tactile sensation from orofacial regions at acidic pH conditions.

Sodium channel diversity in the vestibular ganglion: NaV1.5, NaV1.8, and tetrodotoxin-sensitive currents

Firing patterns differ between subpopulations of vestibular primary afferent neurons. The role of sodium (NaV) channels in this diversity has not been investigated because NaV currents in rodent vestibular ganglion neurons (VGNs) were reported to be homogeneous, with the voltage dependence and tetrodotoxin (TTX) sensitivity of most neuronal NaV channels. RT-PCR experiments, however, indicated expression of diverse NaV channel subunits in the vestibular ganglion, motivating a closer look. Whole cell recordings from acutely dissociated postnatal VGNs confirmed that nearly all neurons expressed NaV currents that are TTX-sensitive and have activation midpoints between -30 and -40 mV. In addition, however, many VGNs expressed one of two other NaV currents. Some VGNs had a small current with properties consistent with NaV1.5 channels: low TTX sensitivity, sensitivity to divalent cation block, and a relatively negative voltage range, and some VGNs showed NaV1.5-like immunoreactivity. Other VGNs had a current with the properties of NaV1.8 channels: high TTX resistance, slow time course, and a relatively depolarized voltage range. In two NaV1.8 reporter lines, subsets of VGNs were labeled. VGNs with NaV1.8-like TTX-resistant current also differed from other VGNs in the voltage dependence of their TTX-sensitive currents and in the voltage threshold for spiking and action potential shape. Regulated expression of NaV channels in primary afferent neurons is likely to selectively affect firing properties that contribute to the encoding of vestibular stimuli.

Exon 11 skipping of SCN10A coding for voltage-gated sodium channels in dorsal root ganglia

The voltage-gated sodium channel Na(V)1.8 (encoded by SCN10A) is predominantly expressed in dorsal root ganglia(DRG) and plays a critical role in pain perception. We analyzed SCN10A transcripts isolated from human DRGs using deep sequencing and found a novel splice variant lacking exon 11, which codes for 98 amino acids of the domain I/II linker. Quantitative PCR analysis revealed an abundance of this variant of up to 5–10% in human, while no such variants were detected in mouse or rat. Since no obvious functional differences between channels with and without the exon-11 sequence were detected, it is suggested that SCN10A exon 11 skipping in humans is a tolerated event.

Functional up-regulation of Nav1.8 sodium channel in Aβ afferent fibers subjected to chronic peripheral inflammation

Background

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, Na_v1.8 has been shown to participate in the peripheral sensitization of nociceptors. However, to date, there is no evidence for a role of Na_v1.8 in controlling Aβ-fiber excitability following persistent inflammation.

Methods

Distribution and expression of Na_v1.8 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 I_Na and TTX-R Na_v1.8 currents in large-soma dorsal root ganglia (DRG) neurons isolated from sham or CFA-treated rats. Finally, we analyzed the effects of ambroxol, a Na_v1.8-preferring blocker on the electrophysiological properties of Na_v1.8 currents and on the mechanical sensitivity and inflammation of the hind paw in CFA-treated rats.

Results

Our findings revealed that Na_v1.8 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 I_Na and Na_v1.8 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 Na_v1.8 in the hyperpolarizing direction, thus decreasing the current threshold for triggering action potentials. Finally, we found that ambroxol significantly reduces the potentiation of Na_v1.8 currents in Aβ-fiber neurons observed following intraplantar CFA injection and concomitantly blocks CFA-induced mechanical allodynia, suggesting that Na_v1.8 regulation in Aβ-fibers contributes to inflammatory pain.

Conclusions

Collectively, these findings support a key role for Na_v1.8 in controlling the excitability of Aβ-fibers and its potential contribution to the development of mechanical allodynia under persistent inflammation.

Regulation/modulation of sensory neuron sodium channels

The pseudounipolar sensory neurons of the dorsal root ganglia (DRG) give rise to peripheral branches that convert thermal, mechanical, and chemical stimuli into electrical signals that are transmitted via central branches to the spinal cord. These neurons express unique combinations of tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) Na(+) channels that contribute to the resting membrane potential, action potential threshold, and regulate neuronal firing frequency. The small-diameter neurons (<25 μm) isolated from the DRG represent the cell bodies of C-fiber nociceptors that express both TTX-S and TTX-R Na(+) currents. The large-diameter neurons (>35 μm) are typically low-threshold A-fibers that predominately express TTX-S Na(+) currents. Peripheral nerve damage, inflammation, and metabolic diseases alter the expression and function of these Na(+) channels leading to increases in neuronal excitability and pain. The Na(+) channels expressed in these neurons are the target of intracellular signaling cascades that regulate the trafficking, cell surface expression, and gating properties of these channels. Post-translational regulation of Na(+) channels by protein kinases (PKA, PKC, MAPK) alter the expression and function of the channels. Injury-induced changes in these signaling pathways have been linked to sensory neuron hyperexcitability and pain. This review examines the signaling pathways and regulatory mechanisms that modulate the voltage-gated Na(+) channels of sensory neurons.

NaV1.8 channels are expressed in large, as well as small, diameter sensory afferent neurons

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.

Tetrodotoxin-resistant voltage-dependent sodium channels in identified muscle afferent neurons

Muscle afferents are critical regulators of motor function (Group I and II) and cardiovascular responses to exercise (Group III and IV). However, little is known regarding the expressed voltage-dependent ion channels. We identified muscle afferent neurons in dorsal root ganglia (DRGs), using retrograde labeling to examine voltage-dependent sodium (Na(V)) channels. In patch-clamp recordings, we found that the dominant Na(V) current in the majority of identified neurons was insensitive to tetrodotoxin (TTX-R), with Na(V) current in only a few (14%) neurons showing substantial (>50%) TTX sensitivity (TTX-S). The TTX-R current was sensitive to a Na(V)1.8 channel blocker, A803467. Immunocytochemistry demonstrated labeling of muscle afferent neurons by a Na(V)1.8 antibody, which further supported expression of these channels. A portion of the TTX-R Na(V) current appeared to be noninactivating during our 25-ms voltage steps, which suggested activity of Na(V)1.9 channels. The majority of the noninactivating current was insensitive to A803467 but sensitive to extracellular sodium. Immunocytochemistry showed labeling of muscle afferent neurons by a Na(V)1.9 channel antibody, which supports expression of these channels. Further examination of the muscle afferent neurons showed that functional TTX-S channels were expressed, but were largely inactivated at physiological membrane potentials. Immunocytochemistry showed expression of the TTX-S channels Na(V)1.6 and Na(V)1.7 but not Na(V)1.1. Na(V)1.8 and Na(V)1.9 appear to be the dominant functional sodium channels in small- to medium-diameter muscle afferent neurons. The expression of these channels is consistent with the identification of these neurons as Group III and IV, which mediate the exercise pressor reflex.

Nav1.8 expression is not restricted to nociceptors in mouse peripheral nervous system

A vast diversity of salient cues is sensed by numerous classes of primary sensory neurons, defined by specific neuropeptides, ion channels, or cytoskeletal proteins. Recent evidence has demonstrated a correlation between the expression of some of these molecular markers and transmission of signals related to distinct sensory modalities (eg, heat, cold, pressure). Voltage-gated sodium channel Na(v)1.8 has been reported to be preferentially expressed in small-diameter unmyelinated sensory afferents specialized for the detection of noxious stimuli (nociceptors), and Na(v)1.8-Cre mice have been widely used to investigate gene function in nociceptors. However, the identity of neurons in which Cre-mediated recombination occurs in these animals has not been resolved, and whether expression of Na(v)1.8 in these neurons is dynamic during development is not known, rendering interpretation of conditional knockout mouse phenotypes problematic. Here, we used genetics, immunohistochemistry, electrophysiology, and calcium imaging to precisely characterize the expression of Na(v)1.8 in the peripheral nervous system. We demonstrate that 75% of dorsal root ganglion (DRG) neurons express Na(v)1.8-Cre, including >90% of neurons expressing markers of nociceptors and, unexpectedly, a large population (∼40%) of neurons with myelinated A fibers. Furthermore, analysis of DRG neurons' central and peripheral projections revealed that Na(v)1.8-Cre is not restricted to nociceptors but is also expressed by at least 2 types of low-threshold mechanoreceptors essential for touch sensation, including those with C and Aβ fibers. Our results indicate that Na(v)1.8 underlies electrical activity of sensory neurons subserving multiple functional modalities, and call for cautious interpretation of the phenotypes of Na(v)1.8-Cre-driven conditional knockout mice.

Single-cell analysis of sodium channel expression in dorsal root ganglion neurons

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, and Nav1.7) and TTX-R (Nav1.8 and 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, and 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, and Nav1.7) and small unmyelinated (Nav1.7, Nav1.8, and Nav1.9) sensory neurons.

Simulation in Sensory Neurons Reveals a Key Role for Delayed Na+ Current in Subthreshold Oscillations and Ectopic Discharge: Implications for Neuropathic Pain

Somata of primary sensory neurons are thought to contribute to the ectopic neural discharge that is implicated as a cause of some forms of neuropathic pain. Spiking is triggered by subthreshold membrane potential oscillations that reach threshold. Oscillations, in turn, appear to result from reciprocation of a fast active tetrodotoxin-sensitive Na+ current ( INa+) and a passive outward IK+ current. We previously simulated oscillatory behavior using a transient Hodgkin–Huxley-type voltage-dependent INa+ and ohmic leak. This model, however, diverged from oscillatory parameters seen in live cells and failed to produce characteristic ectopic discharge patterns. Here we show that use of a more complete set of Na+ conductances—which includes several delayed components—enables simulation of the entire repertoire of oscillation-triggered electrogenic phenomena seen in live dorsal root ganglion (DRG) neurons. This includes a physiological window of induction and natural patterns of spike discharge. An INa+ component at 2–20 ms was particularly important, even though it represented only a tiny fraction of overall INa+ amplitude. With the addition of a delayed rectifier IK+ the singlet firing seen in some DRG neurons can also be simulated. The model reveals the key conductances that underlie afferent ectopia, conductances that are potentially attractive targets in the search for more effective treatments of neuropathic pain.

Neurotrophin-3 significantly reduces sodium channel expression linked to neuropathic pain states

Neuropathic pain resulting from chronic constriction injury (CCI) is critically linked to sensitization of peripheral nociceptors. Voltage gated sodium channels are major contributors to this state and their expression can be upregulated by nerve growth factor (NGF). We have previously demonstrated that neurotrophin-3 (NT-3) acts antagonistically to NGF in modulation of aspects of CCI-induced changes in trkA-associated nociceptor phenotype and thermal hyperalgesia. Thus, we hypothesized that exposure of neurons to increased levels of NT-3 would reduce expression of Na(v)1.8 and Na(v)1.9 in DRG neurons subject to CCI. In adult male rats, Na(v)1.8 and Na(v)1.9 mRNAs are expressed at high levels in predominantly small to medium size neurons. One week following CCI, there is reduced incidence of neurons expressing detectable Na(v)1.8 and Na(v)1.9 mRNA, but without a significant decline in mean level of neuronal expression, and similar findings observed immunohistochemically. There is also increased accumulation/redistribution of channel protein in the nerve most apparent proximal to the first constriction site. Intrathecal infusion of NT-3 significantly attenuates neuronal expression of Na(v)1.8 and Na(v)1.9 mRNA contralateral and most notably, ipsilateral to CCI, with a similar impact on relative protein expression at the level of the neuron and constricted nerve. We also observe reduced expression of the common neurotrophin receptor p75 in response to CCI that is not reversed by NT-3 in small to medium sized neurons and may confer an enhanced ability of NT-3 to signal via trkA, as has been previously shown in other cell types. These findings are consistent with an analgesic role for NT-3.

Tetrodotoxin-resistant voltage-gated sodium channels Na(v)1.8 and Na(v)1.9 are expressed in the retina

Voltage-gated sodium channels (VGSCs) are one of the fundamental building blocks of electrically excitable cells in the nervous system. These channels are responsible for the generation of action potentials that are required for the communication of neuronal signals over long distances within a cell. VGSCs are encoded by a family of nine genes whose products have widely varying biophysical properties. In this study, we have detected the expression of two atypical VGSCs (Na(v)1.8 and Na(v)1.9) in the retina. Compared with more common VGSCs, Na(v)1.8 and Na(v)1.9 have unusual biophysical and pharmacological properties, including persistent sodium currents and resistance to the canonical sodium channel blocker tetrodotoxin (TTX). Our molecular biological and immunohistochemical data derived from mouse (Mus musculus) retina demonstrate expression of Na(v)1.8 by retinal amacrine and ganglion cells, whereas Na(v)1.9 is expressed by photoreceptors and Müller glia. The fact that these channels exist in the central nervous system (CNS) and exhibit robust TTX resistance requires a re-evaluation of prior physiological, pharmacological, and developmental data in the visual system, in which the diversity of VGSCs has been previously underestimated.

Channel, neuronal and clinical function in sodium channelopathies: from genotype to phenotype

What is the relationship between sodium channel function, neuronal function and clinical status in channelopathies of the nervous system? Given the central role of sodium channels in the generation of neuronal activity, channelopathies involving sodium channels might be expected to cause either enhanced sodium channel function and neuronal hyperexcitability associated with positive clinical manifestations such as seizures, or attenuated channel function and neuronal hypoexcitability associated with negative clinical manifestations such as paralysis. In this article, I review observations showing that changes in neuronal function and clinical status associated with channelopathies are not necessarily predictable solely from the altered physiological properties of the mutated channel itself. I discuss evidence showing that cell background acts as a filter that can strongly influence the effects of ion channel mutations.

The role of sodium channels in neuropathic pain

Our knowledge of the ion channels, receptors and signalling mechanisms involved in pain pathophysiology, and which specific channels play a role in subtypes of pain such as neuropathic and inflammatory pain, has expanded considerably in recent years. It is now clear that in the neuropathic state the expression of certain channels is modified, and that these changes underlie the plasticity of responses that occur to generate inappropriate pain signals from normally trivial inputs. Pain is modulated by a subset of the voltage-gated sodium channels, including Nav1.3, Nav1.7, Nav1.8 and Nav1.9. These isoforms display unique expression patterns within specific tissues, and are either up- or down-regulated upon injury to the nervous system. Here we describe our current understanding of the roles of sodium channels in pain and nociceptive information processing, with a particular emphasis on neuropathic pain and drugs useful for the treatment of neuropathic pain that act through mechanisms involving block of sodium channels. One of the future challenges in the development of novel sodium channel blockers is to design and synthesise isoform-selective channel inhibitors. This should provide substantial benefits over existing pain treatments.

Dissecting the role of sodium currents in visceral sensory neurons in a model of chronic hyperexcitability using Nav1.8 and Nav1.9 null mice

Tetrodotoxin-resistant (TTX-R) sodium currents have been proposed to underlie sensory neuronal hyperexcitability in acute inflammatory models, but their role in chronic models is unknown. Since no pharmacological tools to separate TTX-R currents are available, this study employs Na(v)1.8 and Na(v)1.9 null mice to evaluate these currents roles in a chronic hyperexcitability model after the resolution of an inflammatory insult. Transient jejunitis was induced by infection with Nippostrongylus brasiliensis (Nb) in Na(v)1.9 and Na(v)1.8 null, wild-type and naïve mice. Retrogradely labelled dorsal root ganglia (DRG) neurons were harvested on day 20-24 post-infection for patch clamp recording. Rheobase and action potential (AP) parameters were recorded as measures of excitability, and Na(v)1.9 and Na(v)1.8 currents were recorded. DRG neuronal excitability was significantly increased in post-infected mice compared to sham animals, despite the absence of ongoing inflammation (sham = 1.9 +/- 0.3, infected = 3.6 +/- 0.7 APs at 2x rheobase, P = 0.02). Hyperexcitability was associated with a significantly increased amplitude of TTX-R currents. Hyperexcitability was maintained in Na(v)1.9(-/-) mice, but hyperexcitability was absent and APs were blunted in Na(v)1.8(-/-) mice. This study identifies a critical role for Na(v)1.8 in chronic post-infectious visceral hyperexcitability, with no contribution from Na(v)1.9. Nb infection-induced hyperexcitability is not observed in Na(v)1.8(-/-) mice, but is still present in Na(v)1.9(-/-) mice. It is not clear whether hyperexcitability is due to a change in the function of Na(v)1.8 channels or a change in the number of Na(v)1.8 channels.

The Role of Sodium Channels in Chronic Inflammatory and Neuropathic Pain

Clinical and experimental data indicate that changes in the expression of voltage-gated sodium channels play a key role in the pathogenesis of neuropathic pain and that drugs that block these channels are potentially therapeutic. Clinical and experimental data also suggest that changes in voltage-gated sodium channels may play a role in inflammatory pain, and here too sodium-channel blockers may have therapeutic potential. The sodium-channel blockers of interest include local anesthetics, used at doses far below those that block nerve impulse propagation, and tricyclic antidepressants, whose analgesic effects may at least partly be due to blockade of sodium channels. Recent data show that local anesthetics may have pain-relieving actions via targets other than sodium channels, including neuronal G protein-coupled receptors and binding sites on immune cells. Some of these actions occur with nanomolar drug concentrations, and some are detected only with relatively long-term drug exposure. There are 9 isoforms of the voltage-gated sodium channel alpha-subunit, and several of the isoforms that are implicated in neuropathic and inflammatory pain states are expressed by somatosensory primary afferent neurons but not by skeletal or cardiovascular muscle. This restricted expression raises the possibility that isoform-specific drugs might be analgesic and lacking the cardiotoxicity and neurotoxicity that limit the use of current sodium-channel blockers.

PERSPECTIVE

Changes in the expression of neuronal voltage-gated sodium channels may play a key role in the pathogenesis of both chronic neuropathic and chronic inflammatory pain conditions. Drugs that block these channels may have therapeutic efficacy with doses that are far below those that impair nerve impulse propagation or cardiovascular function.

Transient outward K+ currents in rat dissociated subfornical organ neurones and angiotensin II effects

Although angiotensin II inhibits transient outward K+ currents (I(A)s) in subfornical organ neurones, there is no evidence concerning which Kv channels are involved. We investigated I(A)-generating Kv channels in dissociated rat subfornical organ neurones, using molecular, electrophysiological and pharmacological techniques, and studied the effects of angiotensin II. Conventional RT-PCR showed the presence of mRNAs for channels of the Kv3.4, Kv1.4 and Kv4 families, which are capable of generating I(A)s. Tetraethylammonium at 1 mm, which blocks Kv3 channel-derived currents, and blood-depressing substance-I, a Kv3.4-specific blocker, at 2 microm suppressed the I(A)-like component of whole-cell outward currents in some neurones. 4-Aminopyridine at 5 mm inhibited I(A)s in the presence of tetraethylammonium at 1 mm. Cd2+ at 300 microm shifted the activation and inactivation curves of the 4-aminopyridine-sensitive and tetraethylammonium-resistant I(A)s positively. The tetraethylammonium-resistant I(A)s showed fast and slow components during the process of recovery from inactivation, but the slow component was not seen in all neurones. The time constant of the fast recovery component was less than 200 ms, while that of the slow recovery component was around 1 s. Using single-cell RT-PCR, mRNAs for Kv4.2 and Kv4.3L were detected frequently, but those for Kv1.4 and Kv3.4 were seen only rarely. Angiotensin II at 30 nm inhibited the fast recovery component of tetraethylammonium-resistant I(A)s in many neurones. These results suggest that the fast recovery component of the tetraethylammonium-resistant I(A) in subfornical organ neurones depends upon Kv4, and that it can be modulated by angiotensin II.

Sodium-mediated axonal degeneration in inflammatory demyelinating disease

Axonal degeneration is a major cause of permanent neurological deficit in multiple sclerosis (MS). The mechanisms responsible for the degeneration remain unclear, but evidence suggests that a failure to maintain axonal sodium ion homeostasis may be a key step that underlies at least some of the degeneration. Sodium ions can accumulate within axons due to a series of events, including impulse activity and exposure to inflammatory factors such as nitric oxide. Recent findings have demonstrated that partial blockade of sodium channels can protect axons from nitric oxide-mediated degeneration in vitro, and from the effects of neuroinflammatory disease in vivo. This review describes some of the reasons why sodium ions might be expected to accumulate within axons in MS, and recent observations suggesting that it is possible to protect axons from degeneration in neuroinflammatory disease by partial sodium channel blockade.

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.

Diclofenac inhibition of sodium currents in rat dorsal root ganglion neurons

The effects of diclofenac, a nonsteroidal anti-inflammatory drug (NSAID), on the fast tetrodotoxin-sensitive (TTX-S) and the slow tetrodotoxin-resistant (TTX-R) sodium currents in rat dorsal root ganglion neurons were investigated using the whole-cell patch-clamp method. Diclofenac suppressed both sodium currents in a dose-dependent manner. The apparent dissociation constants for the diclofenac suppression of TTX-S and TTX-R sodium currents were estimated to be 14 and 97 microM, respectively, at a holding potential of -80 mV. Diclofenac had no effect on the kinetic parameters of the activation process in either type of sodium current. However, diclofenac produced shifts of the steady-state inactivation curves in the hyperpolarizing direction in both types of sodium currents in a dose-dependent manner. At sufficiently negative holding potentials, the inhibitory effects of diclofenac on both types of sodium currents were minimal. The results suggested that diclofenac might bind to sodium channels with a greater affinity when they are in the inactivated state than when they are in the resting state. Effects of other NSAIDs (acetylsalicylic acid, antipyrin, indomethacin and flufenamic acid) on sodium currents were tested. Among these, only flufenamic acid suppressed the sodium currents to a considerable extent. Thus, the chemical structure of each NSAID, not the inhibition of cyclooxygenase, seems to be an important determinant in the sodium current inhibition. The suppression of sodium currents in sensory neurons by diclofenac and flufenamic acid would contribute to their analgesic activity in addition to their inhibition of cyclooxygenase.

The TTX-resistant sodium channel Nav1.8 (SNS/PN3): expression and correlation with membrane properties in rat nociceptive primary afferent neurons

We have examined the distribution of the sensory neuron-specific Na+ channel Nav1.8 (SNS/PN3) in nociceptive and non-nociceptive dorsal root ganglion (DRG) neurons and whether its distribution is related to neuronal membrane properties. Nav1.8-like immunoreactivity (Nav1.8-LI) was examined with an affinity purified polyclonal antiserum (SNS11) in rat DRG neurons that were classified according to sensory receptive properties and by conduction velocity (CV) as C-, Adelta- or Aalpha/beta. A significantly higher proportion of nociceptive than low threshold mechanoreceptive (LTM) neurons showed Nav1.8-LI, and nociceptive neurons had significantly more intense immunoreactivity in their somata than LTM neurons. Results showed that 89, 93 and 60% of C-, Adelta- and Aalpha/beta-fibre nociceptive units respectively and 88% of C-unresponsive units were positive. C-unresponsive units had electrical membrane properties similar to C-nociceptors and were considered to be nociceptive-type neurons. Weak positive Nav1.8-LI was also present in some LTM units including a C LTM, all Adelta LTM units (D hair), about 10% of cutaneous LTM Aalpha/beta-units, but no muscle spindle afferent units. Nav1.8-LI intensity was negatively correlated with soma size (all neurons) and with dorsal root CVs in A- but not C-fibre neurons. Nav1.8-LI intensity was positively correlated with action potential (AP) duration (both rise and fall time) in A-fibre neurons and with AP rise time only in positive C-fibre neurons. It was also positively correlated with AP overshoot in positive neurons. Thus high levels of Nav1.8 protein may contribute to the longer AP durations (especially in A-fibre neurons) and larger AP overshoots that are typical of nociceptors.

Selective expression of a persistent tetrodotoxin-resistant Na+ current and NaV1.9 subunit in myenteric sensory neurons

Voltage-gated Na(+) currents play critical roles in shaping electrogenesis in neurons. Here, we have identified a TTX-resistant Na(+) current (TTX-R I(Na)) in duodenum myenteric neurons of guinea pig and rat and have sought evidence regarding the molecular identity of the channel producing this current from the expression of Na(+) channel alpha subunits and the biophysical and pharmacological properties of TTX-R I(Na). Whole-cell patch-clamp recording from in situ neurons revealed the presence of a voltage-gated Na(+) current that was highly resistant to TTX (IC(50), approximately 200 microm) and selectively distributed in myenteric sensory neurons but not in interneurons and motor neurons. TTX-R I(Na) activated slowly in response to depolarization and exhibited a threshold for activation at -50 mV. V(1/2) values of activation and steady-state inactivation were -32 and -31 mV in the absence of fluoride, respectively, which, as predicted from the window current, generated persistent currents. TTX-R I(Na) also had prominent ultraslow inactivation, which turns off 50% of the conductance at rest (-60 mV). Substituting CsF for CsCl in the intracellular solution shifted the voltage-dependent parameters of TTX-R I(Na) leftward by approximately 20 mV. Under these conditions, TTX-R I(Na) had voltage-dependent properties similar to those reported previously for NaN/Na(V)1.9 in dorsal root ganglion neurons. Consistent with this, reverse transcription-PCR, single-cell profiling, and immunostaining experiments indicated that Na(V)1.9 transcripts and subunits, but not Na(V)1.8, were expressed in the enteric nervous system and restricted to myenteric sensory neurons. TTX-R I(Na) may play an important role in regulating subthreshold electrogenesis and boosting synaptic stimuli, thereby conferring distinct integrative properties to myenteric sensory neurons.

The cerebellum: organization, functions and its role in nociception

Our vision of the cerebellum has been gradually transformed throughout the last century from a 'little brain' to a 'neuronal machine' capable of multitasks, all arguably based on a principle computational model. We review here the main functions of the cerebellum in light of its organization and connectivity. In addition to providing a clear and extensive review of the cerebellar literature, we emphasize the role of the cerebellum in nociception, which is novel to the neurophysiology of pain. However, it is premature to conclude that the cerebellum influences sensory experience in the absence of clinical data.

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.

Molecular determinant of Na(v)1.8 sodium channel resistance to the venom from the scorpion Leiurus quinquestriatus hebraeus

The scorpion venom from Leiurus quinquestriatus (LQTX) alters the kinetics of tetrodotoxin (TTX)-sensitive channels such as the skeletal muscle sodium channel Na(v)1.4. In this study, we tested the effects of LQTX on the TTX-resistant sodium current generated by Na(v)1.8 channels in sensory neurons. Na(v)1.8 current was found to be resistant to LQTX, whereas LQTX slowed inactivation of the current generated by Na(v)1.4 and induced a persistent current. LQTX has been shown to bind the S3-S4 linker of domain four (D4S3-S4) of rat brain Na(v)1.2 sodium channels. Sequence analysis shows that the D4S3-S4 linker is longer in Na(v)1.8 than in Na(v)1.4 by four amino acids: Serine; Leucine; Glutamic acid; and Aspargine (SLEN). Na(v)1.4-SLEN, a chimera construct carrying SLEN at the analogous position in the D4S3-S4 linker, was also found to be resistant to LQTX. Therefore, we conclude that the tetrapeptide SLEN at the D4S3-S4 linker region is sufficient to make Na(v)1.8 resistant to LQTX.

The presence and role of the tetrodotoxin-resistant sodium channel Na(v)1.9 (NaN) in nociceptive primary afferent neurons

This is the first examination of sensory receptive properties and associated electrophysiological properties in vivo of dorsal root ganglion (DRG) neurons that express the TTX-resistant sodium channel Na(v)1.9 (NaN). Intracellular recordings in lumbar DRGs in Wistar rats enabled units with dorsal root C-, Adelta-, or Aalpha/beta-fibers to be classified as nociceptive, low-threshold mechanoreceptive (LTM), or unresponsive. Intracellular dye injection enabled subsequent immunocytochemistry for Na(v)1.9-like immunoreactivity (Na(v)1.9-LI). Na(v)1.9-LI was expressed selectively in nociceptive-type (C- and A-fiber nociceptive and C-unresponsive) units. Of the nociceptive units, 64, 54, and 31% of C-, Adelta-, and Aalpha/beta-fiber units, respectively, were positive for Na(v)1.9-LI. C-unresponsive units were included in the nociceptive-type group on the basis of their nociceptor-like membrane properties; 91% were positive. Na(v)1.9-LI was undetectable in Adelta- or Aalpha/beta-fiber LTM units and in one C-LTM unit. Na(v)1.9-LI intensity was correlated negatively with soma size and conduction velocity in nociceptive units and with conduction velocity in C-fiber units. There was a positive correlation with action potential rise time in nociceptive-type units with membrane potentials equal to or more negative than -50 mV. The data provide direct evidence that Na(v)1.9 is expressed selectively in (but not in all) C- and A-fiber nociceptive-type units and suggest that Na(v)1.9 contributes to membrane properties that are typical of nociceptive neurons.

GDNF and NGF reverse changes in repriming of TTX-sensitive Na(+) currents following axotomy of dorsal root ganglion neurons

Uninjured C-type rat dorsal root ganglion (DRG) neurons predominantly express slowly inactivating TTX-resistant (TTX-R) and slowly repriming TTX-sensitive (TTX-S) Na(+) currents. After peripheral axotomy, TTX-R current density is reduced and rapidly repriming TTX-S currents emerge and predominate. The change in TTX-S repriming kinetics is paralleled by an increase in the level of transcripts and protein for the Na(v)1.3 sodium channel alpha-subunit, which is known to exhibit rapid repriming. Changes in Na(+) current profile and kinetics in DRG neurons may substantially alter neuronal excitability and could contribute to some states of chronic pain associated with injury of sensory neurons. In the present study, we asked whether glial-derived neurotrophic factor (GDNF) and nerve growth factor (NGF), which have been shown to prevent some axotomy-induced changes such as the loss of TTX-R Na(+) current expression in DRG neurons, can ameliorate the axotomy-induced change in TTX-S Na(+) current repriming kinetics. We show that intrathecally administered GDNF and NGF, delivered individually, can partially reverse the effect of axotomy on the repriming kinetics of TTX-S Na(+) currents. When GDNF and NGF were co-administered, the repriming kinetics were fully rescued. We observed parallel effects of GDNF and NGF on the Na(v)1.3 sodium channel transcript levels in axotomized DRG. Both GDNF and NGF were able to partially reverse the axotomy-induced increase in Na(v)1.3 mRNA, with GDNF plus NGF producing the largest effect. Our data indicate that both GDNF and NGF can partially reverse an important effect of axotomy on the electrogenic properties of sensory neurons and that their effect is additive.

NaN/Nav1.9: a sodium channel with unique properties

The Na(v)1.9 Na(+) channel (also known as NaN) is preferentially expressed in nociceptive neurons of the dorsal root ganglia (DRG) and trigeminal ganglia. Na(v)1.9 produces a persistent, tetrodotoxin-resistant current with wide overlap between activation and steady-state inactivation, and appears to modulate resting potential and to amplify small depolarizations. These unique properties indicate that Na(v)1.9 has significant effects on the electroresponsive properties of primary nociceptive neurons. Downregulation of Na(v)1.9, which results from a lack of peripheral glial cell-derived neurotrophic factor following peripheral axotomy, might retune DRG neurons and contribute to their hyperexcitability after nerve injury. Thus, Na(v)1.9 appears to play a key role in nociception and is an attractive target in the search for more effective treatments for pain.

[Nociceptors and mediators in acute inflammatory pain]

OBJECTIVE

To bring together the most recent data concerning the physiology of nociceptors at a time when there has been significant progress in the understanding of these.

DATA SOURCES

References were obtained from computerised bibliographic data banks (Medline and others) and the authors' personal documents.

DATA SYNTHESIS

Nociceptive impulses are generated at the periphery in unmyelinated fibres called nociceptors, the responses of which depend on the tissue environment. Numerous mediators can activate, sensitise or "wake up" nociceptor: kinins (bradykinin, kallidin and their metabolites), pro-inflammatory cytokines (TNF alpha, IL-6, IL-1 beta, IL-8), anti-inflammatory cytokines (IL-4, IL-6, IL-10, IL-12, IL-13), prostanoids (PGE2, PGI2), lipo-oxygenases (leucotrienes such LTB4 or 15-HETE), the "central mediators of the immune response" (NF-kappa B), growth factors such as neurotrophins (NGF, BDNF, NT-3 and NT-4/5), peptides (substance P, CGRP, Neurokinin A), nitric oxide, histamine, serotonin, proteases, excitatory amino acids, adrenergic amines and opioids. The release of neuromediators by primary afferent fibres in the spinal cord may be summarised by successively considering calcium channels, presynaptic receptors, excitatory amino acids and peptides.

CONCLUSION

Sensitisation phenomena are not exclusively peripheral but also central in origin and these are interlinked.

Differences in the size of the somatic action potential overshoot between nociceptive and non-nociceptive dorsal root ganglion neurones in the guinea-pig

Intracellular action potentials evoked by dorsal root stimulation were intracellularly recorded from L6 and S1 dorsal root ganglion neurones in deeply anaesthetised guinea-pigs in vivo. Units were classed as C, Adelta or Aalpha/beta units and as nociceptive, low-threshold mechanoreceptive or unresponsive. Units with membrane potentials of at least -40 mV and action potentials with an amplitude of >20 mV were included. Nociceptive neurones had significantly larger somatic action potential overshoots than low-threshold mechanoreceptors in C, Adelta and Aalpha/beta units. A higher proportion of low-threshold mechanoreceptors than of nociceptors had action potentials that failed to overshoot in all conduction velocity groups. 60% of muscle spindle afferents failed to overshoot. The size of the overshoot was correlated positively with log(10) action potential duration, log(10) action potential rise time, log(10) afterhyperpolarisation duration, action potential amplitude and membrane potential and negatively (weakly) with log(10) conduction velocity.We conclude that nociceptive neurones are more likely to have somatic action potential overshoots than low-threshold mechanoreceptors in any conduction velocity group. This effect was not due to electrode properties or conduction failure at site(s) of failure of action potential regeneration. Differences in overshoot may affect the influence of neuronal firing on cellular processes. If an overshooting action potential is used as a selection criterion, a bias towards nociceptive neurones is likely to occur. An overshooting action potential coupled with a long afterhyperpolarisation or broad action potential may help in identifying sensory neurones as nociceptive.

Oscillatory mechanism in primary sensory neurones

Ectopic spike activity, generated at low levels in intact sensory dorsal root ganglia and intensified following axotomy, is an important cause of neuropathic pain. The spikes are triggered by subthreshold membrane potential oscillations. The depolarizing phase of oscillation sinusoids is due to a phasic voltage-sensitive Na(+) conductance (gNa(+)). Here we examine the repolarizing phase for which K(+) conductance (gK(+)) is implicated. In vivo, gK(+) blockers have excitatory effects inconsistent with the elimination of oscillations. Indeed, using excised dorsal root ganglia in vitro, we found that gK(+) block does not eliminate oscillations; on the contrary, it has a variety of facilitatory effects. However, oscillations were eliminated by shifting the K(+) reversal potential so as to neutralize voltage-insensitive K(+) leak channels. Based on these data, we propose a novel oscillatory model: oscillation sinusoids are due to reciprocation between a phasically activating voltage-dependent, tetrodotoxin-sensitive Na(+) conductance and passive, voltage-independent K(+) leak. In drug-free media, voltage-sensitive K(+) channels act to suppress oscillations and increase their frequency. Numerical simulations support this model and account for the effects of gK(+) block. Oscillations in dorsal root ganglia neurones appear to be based on the simplest possible configuration of ionic conductances compatible with sustained high frequency oscillatory behaviour. The oscillatory mechanism might be exploited in the search for novel analgesic drugs.

Glycosylation alters steady-state inactivation of sodium channel Nav1.9/NaN in dorsal root ganglion neurons and is developmentally regulated

Na channel NaN (Na(v)1.9) produces a persistent TTX-resistant (TTX-R) current in small-diameter neurons of dorsal root ganglia (DRG) and trigeminal ganglia. Na(v)1.9-specific antibodies react in immunoblot assays with a 210 kDa protein from the membrane fractions of adult DRG and trigeminal ganglia. The size of the immunoreactive protein is in close agreement with the predicted Na(v)1.9 theoretical molecular weight of 201 kDa, suggesting limited glycosylation of this channel in adult tissues. Neonatal rat DRG membrane fractions, however, contain an additional higher molecular weight immunoreactive protein. Reverse transcription-PCR analysis did not show additional longer transcripts that could encode the larger protein. Enzymatic deglycosylation of the membrane preparations converted both immunoreactive proteins into a single faster migrating band, consistent with two states of glycosylation of Na(v)1.9. The developmental change in the glycosylation state of Na(v)1.9 is paralleled by a developmental change in the gating of the persistent TTX-R Na(+) current attributable to Na(v)1.9 in native DRG neurons. Whole-cell patch-clamp analysis demonstrates that the midpoint of steady-state inactivation is shifted 7 mV in a hyperpolarized direction in neonatal (postnatal days 0-3) compared with adult DRG neurons, although there is no significant difference in activation. Pretreatment of neonatal DRG neurons with neuraminidase causes an 8 mV depolarizing shift in the midpoint of steady-state inactivation of Na(v)1.9, making it indistinguishable from that of adult DRG neurons. Our data show that extensive glycosylation of rat Na(v)1.9 is developmentally regulated and changes a critical property of this channel in native neurons.