Analysis of the variation in use-dependent inactivation of high-threshold tetrodotoxin-resistant sodium currents recorded from rat sensory neurons

Use-Dependent Relief of Inhibition of Nav1.8 Channels by A-887826

Sodium channel inhibitors used as local anesthetics, antiarrhythmics, or antiepileptics typically have the property of use-dependent inhibition, whereby inhibition is enhanced by repetitive channel activation. For targeting pain, Nav1.8 channels are an attractive target because they are prominent in primary pain-sensing neurons, with little or no expression in most other kinds of neurons, and a number of Nav1.8-targeted compounds have been developed. We examined the characteristics of Nav1.8 inhibition by one of the most potent Nav1.8 inhibitors so far described, A-887826, and found that when studied with physiologic resting potentials and physiologic temperatures, inhibition had strong "reverse use dependence", whereby inhibition was relieved by repetitive short depolarizations. This effect was much stronger with A-887826 than with A-803467, another Nav1.8 inhibitor. The use-dependent relief from inhibition was seen in both human Nav1.8 channels studied in a cell line and in native Nav1.8 channels in mouse dorsal root ganglion (DRG) neurons. In native Nav1.8 channels, substantial relief of inhibition occurred during repetitive stimulation by action potential waveforms at 5 Hz, suggesting that the phenomenon is likely important under physiologic conditions. SIGNIFICANCE STATEMENT: Nav1.8 sodium channels are expressed in primary pain-sensing neurons and are a prime current target for new drugs for pain. This work shows that one of the most potent Nav1.8 inhibitors, A-887826, has the unusual property that inhibition is relieved by repeated short depolarizations. This "reverse use dependence" may reduce inhibition during physiological firing and should be selected against in drug development.

The anticonvulsant phytocannabinoids CBGVA and CBDVA inhibit recombinant T-type channels

Introduction: Cannabidiol (CBD) has been clinically approved for intractable epilepsies, offering hope that novel anticonvulsants in the phytocannabinoid class might be developed. Looking beyond CBD, we have recently reported that a series of biosynthetic precursor molecules found in cannabis display anticonvulsant properties. However, information on the pharmacological activities of these compounds on CNS drug targets is limited. The current study aimed to fill this knowledge gap by investigating whether anticonvulsant phytocannabinoids affect T-type calcium channels, which are known to modulate neuronal excitability, and may be relevant to the anti-seizure effects of this class of compounds. Materials and methods: A fluorescence-based assay was used to screen the ability of the phytocannabinoids to inhibit human T-type calcium channels overexpressed in HEK-293 cells. A subset of compounds was further examined using patch-clamp electrophysiology. Alphascreen technology was used to characterise selected compounds against G-protein coupled-receptor 55 (GPR55) overexpressed in HEK-293 cells, as GPR55 is another target of the phytocannabinoids. Results: A single 10 µM concentration screen in the fluorescence-based assay showed that phytocannabinoids inhibited T-type channels with substantial effects on Ca_v3.1 and Ca_v3.2 channels compared to the Ca_v3.3 channel. The anticonvulsant phytocannabinoids cannabigerovarinic acid (CBGVA) and cannabidivarinic acid (CBDVA) had the greatest magnitudes of effect (≥80% inhibition against Ca_v3.1 and Ca_v3.2), so were fully characterized in concentration-response studies. CBGVA and CBDVA had IC₅₀ values of 6 μM and 2 µM on Ca_v3.1 channels; 2 μM and 11 µM on Ca_v3.2 channels, respectively. Biophysical studies at Ca_v3.1 showed that CBGVA caused a hyperpolarisation shift of steady-state inhibition. Both CBGVA and CBDVA had a use-dependent effect and preferentially inhibited Ca_v3.1 current in a slow inactivated state. CBGVA and CBDVA were also shown to antagonise GPR55. Conclusion and implications: These findings show that CBGVA and CBDVA inhibit T-type calcium channels and GPR55. These compounds should be further investigated to develop novel therapeutics for treating diseases associated with dysfunctional T-type channel activity.

Functional subgroups of rat and human sensory neurons: a systematic review of electrophysiological properties

Sensory neurons are responsible for the generation and transmission of nociceptive signals from the periphery to the central nervous system. They encompass a broadly heterogeneous population of highly specialized neurons. The understanding of the molecular choreography of individual subpopulations is essential to understand physiological and pathological pain states. Recently, it became evident that species differences limit transferability of research findings between human and rodents in pain research. Thus, it is necessary to systematically compare and categorize the electrophysiological data gained from human and rodent dorsal root ganglia neurons (DRGs). In this systematic review, we condense the available electrophysiological data defining subidentities in human and rat DRGs. A systematic search on PUBMED yielded 30 studies on rat and 3 studies on human sensory neurons. Defined outcome parameters included current clamp, voltage clamp, cell morphology, pharmacological readouts, and immune reactivity parameters. We compare evidence gathered for outcome markers to define subgroups, offer electrophysiological parameters for the definition of neuronal subtypes, and give a framework for the transferability of electrophysiological findings between species. A semiquantitative analysis revealed that for rat DRGs, there is an overarching consensus between studies that C-fiber linked sensory neurons display a lower action potential threshold, higher input resistance, a larger action potential overshoot, and a longer afterhyperpolarization duration compared to other sensory neurons. They are also more likely to display an infliction point in the falling phase of the action potential. This systematic review points out the need of more electrophysiological studies on human sensory neurons.

Cannabidiol Inhibition of Murine Primary Nociceptors: Tight Binding to Slow Inactivated States of Nav1.8 Channels

The nonpsychoactive phytocannabinoid cannabidiol (CBD) has been shown to have analgesic effects in animal studies but little is known about its mechanism of action. We examined the effects of CBD on intrinsic excitability of primary pain-sensing neurons. Studying acutely dissociated capsaicin-sensitive mouse DRG neurons at 37°C, we found that CBD effectively inhibited repetitive action potential firing, from 15-20 action potentials evoked by 1 s current injections in control to 1-3 action potentials with 2 μm CBD. Reduction of repetitive firing was accompanied by a reduction of action potential height, widening of action potentials, reduction of the afterhyperpolarization, and increased propensity to enter depolarization block. Voltage-clamp experiments showed that CBD inhibited both TTX-sensitive and TTX-resistant (TTX-R) sodium currents in a use-dependent manner. CBD showed strong state-dependent inhibition of TTX-R channels, with fast binding to inactivated channels during depolarizations and slow unbinding on repolarization. CBD alteration of channel availability at various voltages suggested that CBD binds especially tightly [K _d (dissociation constant), ∼150 nm] to the slow inactivated state of TTX-R channels, which can be substantially occupied at voltages as negative as -40 mV. Remarkably, CBD was more potent in inhibiting TTX-R channels and inhibiting action potential firing than the local anesthetic bupivacaine. We conclude that CBD might produce some of its analgesic effects by direct effects on neuronal excitability, with tight binding to the slow inactivated state of Na_v1.8 channels contributing to effective inhibition of repetitive firing by modest depolarizations.SIGNIFICANCE STATEMENT Cannabidiol (CBD) has been shown to inhibit pain in various rodent models, but the mechanism of this effect is unknown. We describe the ability of CBD to inhibit repetitive action potential firing in primary nociceptive neurons from mouse dorsal root ganglia and analyze the effects on voltage-dependent sodium channels. We find that CBD interacts with TTX-resistant sodium channels in a state-dependent manner suggesting particularly tight binding to slow inactivated states of Na_v1.8 channels, which dominate the overall inactivation of Na_v1.8 channels for small maintained depolarizations from the resting potential. The results suggest that CBD can exert analgesic effects in part by directly inhibiting repetitive firing of primary nociceptors and suggest a strategy of identifying compounds that bind selectively to slow inactivated states of Na_v1.8 channels for developing effective analgesics.

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.

Upregulation of T-type Ca2+ channels in long-term diabetes determines increased excitability of a specific type of capsaicin-insensitive DRG neurons

Background

Previous studies have shown that increased excitability of capsaicin-sensitive DRG neurons and thermal hyperalgesia in rats with short-term (2–4 weeks) streptozotocin-induced diabetes is mediated by upregulation of T-type Ca²⁺ current. In longer–term diabetes (after the 8th week) thermal hyperalgesia is changed to hypoalgesia that is accompanied by downregulation of T-type current in capsaicin-sensitive small-sized nociceptors. At the same time pain symptoms of diabetic neuropathy other than thermal persist in STZ-diabetic animals and patients during progression of diabetes into later stages suggesting that other types of DRG neurons may be sensitized and contribute to pain. In this study, we examined functional expression of T-type Ca²⁺ channels in capsaicin-insensitive DRG neurons and excitability of these neurons in longer-term diabetic rats and in thermally hypoalgesic diabetic rats.

Results

Here we have demonstrated that in STZ-diabetes T-type current was upregulated in capsaicin-insensitive low-pH-sensitive small-sized nociceptive DRG neurons of longer-term diabetic rats and thermally hypoalgesic diabetic rats. This upregulation was not accompanied by significant changes in biophysical properties of T-type channels suggesting that a density of functionally active channels was increased. Sensitivity of T-type current to amiloride (1 mM) and low concentration of Ni²⁺ (50 μM) implicates prevalence of Ca_v3.2 subtype of T-type channels in the capsaicin-insensitive low-pH-sensitive neurons of both naïve and diabetic rats. The upregulation of T-type channels resulted in the increased neuronal excitability of these nociceptive neurons revealed by a lower threshold for action potential initiation, prominent afterdepolarizing potentials and burst firing. Sodium current was not significantly changed in these neurons during long-term diabetes and could not contribute to the diabetes-induced increase of neuronal excitability.

Conclusions

Capsaicin-insensitive low-pH-sensitive type of DRG neurons shows diabetes-induced upregulation of Ca_v3.2 subtype of T-type channels. This upregulation results in the increased excitability of these neurons and may contribute to nonthermal nociception at a later-stage diabetes.

The role of slow and persistent TTX-resistant sodium currents in acute tumor necrosis factor-α-mediated increase in nociceptors excitability

Tetrodotoxin-resistant (TTX-r) sodium channels are key players in determining the input-output properties of peripheral nociceptive neurons. Changes in gating kinetics or in expression levels of these channels by proinflammatory mediators are likely to cause the hyperexcitability of nociceptive neurons and pain hypersensitivity observed during inflammation. Proinflammatory mediator, tumor necrosis factor-α (TNF-α), is secreted during inflammation and is associated with the early onset, as well as long-lasting, inflammation-mediated increase in excitability of peripheral nociceptive neurons. Here we studied the underlying mechanisms of the rapid component of TNF-α-mediated nociceptive hyperexcitability and acute pain hypersensitivity. We showed that TNF-α leads to rapid onset, cyclooxygenase-independent pain hypersensitivity in adult rats. Furthermore, TNF-α rapidly and substantially increases nociceptive excitability in vitro, by decreasing action potential threshold, increasing neuronal gain and decreasing accommodation. We extended on previous studies entailing p38 MAPK-dependent increase in TTX-r sodium currents by showing that TNF-α via p38 MAPK leads to increased availability of TTX-r sodium channels by partial relief of voltage dependence of their slow inactivation, thereby contributing to increase in neuronal gain. Moreover, we showed that TNF-α also in a p38 MAPK-dependent manner increases persistent TTX-r current by shifting the voltage dependence of activation to a hyperpolarized direction, thus producing an increase in inward current at functionally critical subthreshold voltages. Our results suggest that rapid modulation of the gating of TTX-r sodium channels plays a major role in the mediated nociceptive hyperexcitability of TNF-α during acute inflammation and may lead to development of effective treatments for inflammatory pain, without modulating the inflammation-induced healing processes.

Expression of 5-HT3 receptors and TTX resistant sodium channels (Na(V)1.8) on muscle nerve fibers in pain-free humans and patients with chronic myofascial temporomandibular disorders

BACKGROUND

Previous studies have shown that 5-HT3-antagonists reduce muscle pain, but there are no studies that have investigated the expression of 5-HT3-receptors in human muscles. Also, tetrodotoxin resistant voltage gated sodium-channels (NaV) are involved in peripheral sensitization and found in trigeminal ganglion neurons innervating the rat masseter muscle. This study aimed to investigate the frequency of nerve fibers that express 5-HT3A-receptors alone and in combination with NaV1.8 sodium-channels in human muscles and to compare it between healthy pain-free men and women, the pain-free masseter and tibialis anterior muscles, and patients with myofascial temporomandibular disorders (TMD) and pain-free controls.

METHODS

Three microbiopsies were obtained from the most bulky part of the tibialis and masseter muscles of seven and six healthy men and seven and six age-matched healthy women, respectively, while traditional open biopsies were obtained from the most painful spot of the masseter of five female patients and from a similar region of the masseter muscle of five healthy, age-matched women. The biopsies were processed by routine immunohistochemical methods. The biopsy sections were incubated with monoclonal antibodies against the specific axonal marker PGP 9.5, and polyclonal antibodies against the 5-HT3A-receptors and NaV1.8 sodium-channels.

RESULTS

A similar percentage of nerve fibers in the healthy masseter (85.2%) and tibialis (88.7%) muscles expressed 5-HT3A-receptors. The expression of NaV1.8 by 5-HT3A positive nerve fibers associated with connective tissue was significantly higher than nerve fibers associated with myocytes (P < .001). In the patients, significantly more fibers per section were found with an average of 3.8 ± 3 fibers per section in the masseter muscle compared to 2.7 ± 0.2 in the healthy controls (P = .024). Further, the frequency of nerve fibers that co-expressed NaV1.8 and 5-HT3A receptors was significantly higher in patients (42.6%) compared to healthy controls (12.0%) (P < .001).

CONCLUSIONS

This study showed that the 5-HT3A-receptor is highly expressed in human masseter and tibialis muscles and that there are more nerve fibers that express 5-HT3A-receptors in the masseter of women with myofascial TMD compared to healthy women. These findings indicate that 5-HT3-receptors might be up-regulated in myofascial TMD and could serve as potential biomarkers of chronic muscle pain.

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.

A transparent organic transistor structure for bidirectional stimulation and recording of primary neurons

Real-time stimulation and recording of neural cell bioelectrical activity could provide an unprecedented insight in understanding the functions of the nervous system, and it is crucial for developing advanced in vitro drug screening approaches. Among organic materials, suitable candidates for cell interfacing can be found that combine long-term biocompatibility and mechanical flexibility. Here, we report on transparent organic cell stimulating and sensing transistors (O-CSTs), which provide bidirectional stimulation and recording of primary neurons. We demonstrate that the device enables depolarization and hyperpolarization of the primary neuron membrane potential. The transparency of the device also allows the optical imaging of the modulation of the neuron bioelectrical activity. The maximal amplitude-to-noise ratio of the extracellular recording achieved by the O-CST device exceeds that of a microelectrode array system on the same neuronal preparation by a factor of 16. Our organic cell stimulating and sensing device paves the way to a new generation of devices for stimulation, manipulation and recording of cell bioelectrical activity in vitro and in vivo.

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

The distribution of low-threshold tetrodotoxin-resistant (TTX-r) Na(+) current and its co-expression with high-threshold TTX-r Na(+) current were studied in randomly selected acutely dissociated rat trigeminal ganglion (non-identified TG cells) and TG cells serving the temporomandibular joint (TMJ-TG cells). Conditions previously shown to enhance Na(V)1.9 channel-mediated currents (holding potential (HP) -80 mV, 130-mM fluoride internally) were employed to amplify the low-threshold Na(+) current. Under these conditions, detectable low-threshold Na(+) current was exhibited by 16 out of 21 non-identified TG cells (average, 1810 ± 358 pA), and by nine of 14 TMJ-TG cells (average, 959 ± 525 pA). The low-threshold Na(+) current began to activate around -55 mV and was inactivated by holding TG cells at -60 mV and delivering 40-ms test potentials (TPs) to 0 mV. The inactivation was long lasting, recovering only 8 ± 3% over a 5-min period after the HP was returned to -80 mV. Following low-threshold Na(+) current inactivation, high-threshold TTX-r Na(+) current, evoked from HP -60 mV, was observed. High-threshold Na(+) current amplitude averaged 16,592 ± 3913 pA for TPs to 0 mV, was first detectable at an average TP of -34 ± 1.3 mV, and was ½ activated at -7.1 ± 2.3 mV. In TG cells expressing prominent low-threshold Na(+) currents, changing the external solution to one containing 0 mM Na(+) reduced the amount of current required to hold the cells at -80 mV through -50 mV, the peak effect being observed at HP -60 mV. TG cells recorded from with a more physiological pipette solution containing chloride instead of fluoride exhibited small low-threshold Na(+) currents, which were greatly increased upon superfusion of the TG cells with the adenylyl cyclase (AC) activator forskolin. These data suggest two hypotheses: (1) low- and high-threshold Na(V)1.9 and Na(V)1.8 channels, respectively, are frequently co-expressed in TG neurons serving the TMJ and other structures, and (2), Na(V)1.9 channel-mediated currents are small under physiological conditions, but may be enhanced by inflammatory mediators that increase AC activity, and may mediate an inward leak that depolarizes TG neurons, increasing their excitability.

Repetitive activity slows axonal conduction velocity and concomitantly increases mechanical activation threshold in single axons of the rat cranial dura

The passage of an action potential along a peripheral axon modulates the conduction velocity of subsequent action potentials. In C-neurones with unmyelinated axons repetitive activity progressively slows axonal conduction velocity and in microneurographic recordings from healthy human subjects the magnitude of this slowing can be used to predict the receptive properties of individual axons. Recently, a reduction in the number of available voltage-gated sodium channels (Na(V)) through inactivation has been implicated as the predominant factor responsible for the slowing of axonal conduction. Since Na(V)s are also responsible for the initiation of action potentials in sensory nerve terminals, changes in their availability may be expected to affect activation threshold for sensory stimuli. To examine this proposal, dynamic mechanical stimuli were used to make precise estimates of activation threshold in single unmyelinated axons innervating the rat cranial dura mater. Decreases in axonal conduction velocity induced by repetitive electrical stimulation were paralleled by an increase in mechanical activation threshold. Application of TTX (10-20 nM) also slowed axonal conduction velocity in all 11 fibres examined and in 9 of these this resulted in a parallel increase in mechanical activation threshold. We interpret this as indicating that a reduction in available Na(V) number contributes to both axonal conduction velocity slowing and the observed parallel increase in mechanical activation threshold. The slowing of axonal conduction velocity observed during repetitive activity thus represents a form of accommodation, i.e. self inhibition, which is likely to be decisive in limiting peripheral input to the spinal dorsal horn and thereby regulating processes that could otherwise lead to central sensitization.

Up-regulation of low-threshold tetrodotoxin-resistant Na+ current via activation of a cyclic AMP/protein kinase A pathway in nociceptor-like rat dorsal root ganglion cells

The effects of forskolin on low-threshold tetrodotoxin-resistant (TTX-r) Na(+) currents was studied in small diameter (average ≈ 25 μm) dorsal root ganglion (DRG) cells. All DRG cells included in the study were categorized as type-2 or non-type-2 based on the expression of a low-threshold A-current. In all type-2 and some non-type-2 DRG cells held at -80 mV, the adenylyl cyclase (AC) activator forskolin (10 μM) up-regulated TTX-r Na(+) currents evoked with steps to -55 mV through -35 mV (low-threshold current). Up-regulation of low-threshold current by forskolin was mimicked by the protein kinase A (PKA) agonist Sp-cAMPs and the inflammatory mediator serotonin, and blocked by the PKA antagonist Rp-cAMPs. Forskolin-induced up-regulation of low-threshold current evoked from a holding potential of -60 mV was blocked by 40 ms steps to 0 mV, which presumably induced a long lasting inactivation of the low-threshold channels. Reducing to 3 ms the duration of steps to 0 mV, significantly increased the number of DRG cells where low-threshold current was up-regulated by forskolin, presumably by reducing the long-lasting inactivation of the low-threshold channels. In the same cells, high-threshold current, evoked by 40 ms or 3 ms steps to 0 mV, was consistently up-regulated by forskolin. The selective Na(V)1.8 channel blocker A-803467 markedly blocked high-threshold current but not low-threshold current. The different voltage protocols observed to activate and inactivate the low- and high-threshold currents, and the observation that A-803467 blocked high- but not low-threshold current suggests that the two currents were mediated by different channels, possibly Na(V)1.8 and Na(V)1.9, respectively. Inflammatory mediators may simultaneously up-regulate Na(V)1.8 and Na(V)1.9 channels in the same nociceptor via a AC/PKA signaling pathway, increasing nociceptor signaling strength, and lowering nociceptor threshold, respectively.

A novel slow-inactivation-specific ion channel modulator attenuates neuropathic pain

Voltage-gated ion channels are implicated in pain sensation and transmission signaling mechanisms within both peripheral nociceptors and the spinal cord. Genetic knockdown and knockout experiments have shown that specific channel isoforms, including Na(V)1.7 and Na(V)1.8 sodium channels and Ca(V)3.2 T-type calcium channels, play distinct pronociceptive roles. We have rationally designed and synthesized a novel small organic compound (Z123212) that modulates both recombinant and native sodium and calcium channel currents by selectively stabilizing channels in their slow-inactivated state. Slow inactivation of voltage-gated channels can function as a brake during periods of neuronal hyperexcitability, and Z123212 was found to reduce the excitability of both peripheral nociceptors and lamina I/II spinal cord neurons in a state-dependent manner. In vivo experiments demonstrate that oral administration of Z123212 is efficacious in reversing thermal hyperalgesia and tactile allodynia in the rat spinal nerve ligation model of neuropathic pain and also produces acute antinociception in the hot-plate test. At therapeutically relevant concentrations, Z123212 did not cause significant motor or cardiovascular adverse effects. Taken together, the state-dependent inhibition of sodium and calcium channels in both the peripheral and central pain signaling pathways may provide a synergistic mechanism toward the development of a novel class of pain therapeutics.

Sodium channels in normal and pathological pain

Nociception is essential for survival whereas pathological pain is maladaptive and often unresponsive to pharmacotherapy. Voltage-gated sodium channels, Na(v)1.1-Na(v)1.9, are essential for generation and conduction of electrical impulses in excitable cells. Human and animal studies have identified several channels as pivotal for signal transmission along the pain axis, including Na(v)1.3, Na(v)1.7, Na(v)1.8, and Na(v)1.9, with the latter three preferentially expressed in peripheral sensory neurons and Na(v)1.3 being upregulated along pain-signaling pathways after nervous system injuries. Na(v)1.7 is of special interest because it has been linked to a spectrum of inherited human pain disorders. Here we review the contribution of these sodium channel isoforms to pain.

Serotonin upregulates low- and high-threshold tetrodotoxin-resistant sodium channels in the same subpopulation of rat nociceptors

The modulation by serotonin (5-HT) of low- and high-threshold tetrodotoxin- (TTX) resistant Na(+) currents was studied in small-diameter (approximately 25 microm) acutely-isolated rat dorsal root ganglion (DRG) cells. Each DRG cell included in the study was classified as type 2 or non-type 2, based on expression of a low-threshold A-type K(+) current. When cells of either type were recorded from using a CsF based internal solution and a holding potential (HP) of -80 mV, the apparent threshold for activation of TTX-resistant Na(+) currents ranged from -75 to -60 mV. A 500 ms prepulse to -60 mV greatly suppressed currents evoked by test potentials (TPs) to -75 through -35 mV. A similar scenario was observed when the CsF based internal solution was changed for one that contained CsCl, except that the apparent threshold of activation was shifted by about +25 mV, and currents evoked by TPs to -55 to -35 mV in the absence of a prepulse were much smaller than their counterparts observed with the CsF internal. These data suggest two types of TTX-resistant Na(+) currents; one with a low-threshold for activation that is enhanced by the presence of fluoride inside the cell and is inactivated by holding at -60 mV, and one with a higher threshold for activation that is not inactivated by holding at -60 mV. In type 2 DRG cells, 10 microM 5-HT upregulated low-threshold currents evoked by TPs to -55 to -35 mV from HP -80 mV, as well as high-threshold currents evoked by more depolarized TPs from HP -60 mV. However, when cells were held at -60 mV, 5-HT did not upregulate currents evoked by TPs to -35 or -30 mV, suggesting that the low-threshold current was nearly completely inactivated. Previous studies have suggested that type 2 DRG cells are nociceptor cell bodies. If 5-HT produces similar effects in type 2 DRG cell peripheral receptors, the upregulation of the low-threshold currents may serve to lower the threshold for nociception, while the upregulation of the high-threshold current may strengthen nociceptive signals.

Conduction velocity is regulated by sodium channel inactivation in unmyelinated axons innervating the rat cranial meninges

Axonal conduction velocity varies according to the level of preceding impulse activity. In unmyelinated axons this typically results in a slowing of conduction velocity and a parallel increase in threshold. It is currently held that Na(+)-K(+)-ATPase-dependent axonal hyperpolarization is responsible for this slowing but this has long been equivocal. We therefore examined conduction velocity changes during repetitive activation of single unmyelinated axons innervating the rat cranial meninges. In direct contradiction to the currently accepted postulate, Na(+)-K(+)-ATPase blockade actually enhanced activity-induced conduction velocity slowing, while the degree of velocity slowing was curtailed in the presence of lidocaine (10-300 microm) and carbamazepine (30-500 microm) but not tetrodotoxin (TTX, 10-80 nm). This suggests that a change in the number of available sodium channels is the most prominent factor responsible for activity-induced changes in conduction velocity in unmyelinated axons. At moderate stimulus frequencies, axonal conduction velocity is determined by an interaction between residual sodium channel inactivation following each impulse and the retrieval of channels from inactivation by a concomitant Na(+)-K(+)-ATPase-mediated hyperpolarization. Since the process is primarily dependent upon sodium channel availability, tracking conduction velocity provides a means of accessing relative changes in the excitability of nociceptive neurons.

Multiple sodium channels and their roles in electrogenesis within dorsal root ganglion neurons

Dorsal root ganglion neurons express an array of sodium channel isoforms allowing precise control of excitability. An increasing body of literature indicates that regulation of firing behaviour in these cells is linked to their patterns of expression of specific sodium channel isoforms, which have been discovered to possess distinct biophysical characteristics. The pattern of expression of sodium channels differs in different subclasses of DRG neurons and is not fixed but, on the contrary, changes in response to a variety of disease insults. Moreover, modulation of channels by their environment has been found to play an important role in the response of these neurons to stimuli. In this review we illustrate how excitability can be finely tuned to provide contrasting firing templates in different subclasses of DRG neurons by selective deployment of various sodium channel isoforms, by plasticity of expression of these proteins, and by interactions of these sodium channel isoforms with each other and with other modulatory molecules.