Electrophysiological properties of sodium current subtypes in small cells from adult rat dorsal root ganglia

Differential state-dependent Nav1.8 inhibition by suzetrigine, LTGO-33, and A-887826

Nav1.8 sodium channels are expressed in pain-sensing neurons, and some Nav1.8 inhibitors significantly reduce pain in clinical trials. Several Nav1.8 inhibitors have an unusual state dependence whereby inhibition is relieved by depolarization. We compared the state-dependent action of several Nav1.8 channel inhibitors to test whether inhibition is relieved during action potential (AP) firing under physiological conditions to produce "reverse use dependence." A-887826 inhibition was substantially relieved by AP waveforms applied at 20 Hz at 37°C. In contrast, there was no relief during AP trains with suzetrigine (VX-548) or LTGO-33, even though inhibition could be effectively removed by long, strong depolarizations. These differences were explained by differences in the voltage dependence and kinetics with which the compounds dissociate from depolarized channels and rebind to resting state channels. Suzetrigine required the strongest depolarizations for relief (midpoint +33 mV) and relief was slow (tau >300 ms at +20 mV), so almost no relief occurred during an AP waveform. Relief from A-887826 required weaker depolarizations (midpoint +13 mV) and was much faster, so some relief occurred during each AP waveform and accumulated during 20-Hz trains. LTGO-33 required the weakest depolarizations for relief (midpoint -11 mV) and relief was even faster than for A-887826, but reinhibition between AP waveforms was far faster than for A-887826, so that relief did not accumulate during AP trains at 20 Hz. The results show that, unlike A-887826, there is no use-dependent relief of inhibition by suzetrigine or LTGO-33 with physiological voltage waveforms at physiological temperatures, but each for different reasons.

Modulation of human dorsal root ganglion neuron firing by the Nav1.8 inhibitor suzetrigine

Nav1.8 voltage-gated sodium channels are strongly expressed in human primary pain-sensing neurons (nociceptors) and a selective Nav1.8 inhibitor VX-548 (suzetrigine) has shown efficacy for treating acute pain in clinical trials. Nociceptors also express other sodium channels, notably Nav1.7, raising the question of how effectively excitability of the neurons is reduced by inhibition of Nav1.8 channels alone. We used VX-548 to explore this question, recording from dissociated human dorsal root ganglion neurons at 37 °C. Applying VX-548 at 10 nM (about 25 times the IC50 determined using cloned human Nav1.8 channels at 37 °C) had only small effects on action potential threshold and upstroke velocity but substantially reduced the peak and shoulder. Counterintuitively, VX-548 shortened the refractory period-likely reflecting reduced potassium channel activation by the smaller, narrower action potential-sometimes resulting in faster firing. Generally, repetitive firing during depolarizations was diminished but not eliminated by VX-548. Voltage clamp analysis suggested two reasons that repetitive firing often remains in 10 to 100 nM VX-548. First, many neurons had such large Nav1.8 currents that even 99% inhibition leaves nA-level Nav1.8 current that could help drive repetitive firing. Second, Nav1.7 current dominated during initial spikes and could also contribute to repetitive firing. The ability of human neurons to fire repetitively even with >99% inhibition of Nav1.8 channels may help explain the incomplete analgesia produced by even the largest concentrations of VX-548 in clinical studies.

Na V 1.8/Na V 1.9 double deletion mildly affects acute pain responses in mice

ABSTRACT

The 2 tetrodotoxin-resistant (TTXr) voltage-gated sodium channel subtypes Na V 1.8 and Na V 1.9 are important for peripheral pain signaling. As determinants of sensory neuron excitability, they are essential for the initial transduction of sensory stimuli, the electrogenesis of the action potential, and the release of neurotransmitters from sensory neuron terminals. Na V 1.8 and Na V 1.9, which are encoded by SCN10A and SCN11A , respectively, are predominantly expressed in pain-sensitive (nociceptive) neurons localized in the dorsal root ganglia (DRG) along the spinal cord and in the trigeminal ganglia. Mutations in these genes cause various pain disorders in humans. Gain-of-function missense variants in SCN10A result in small fiber neuropathy, while distinct SCN11A mutations cause, i. a., congenital insensitivity to pain, episodic pain, painful neuropathy, and cold-induced pain. To determine the impact of loss-of-function of both channels, we generated Na V 1.8/Na V 1.9 double knockout (DKO) mice using clustered regularly interspaced short palindromic repeats/Cas-mediated gene editing to achieve simultaneous gene disruption. Successful knockout of both channels was verified by whole-cell recordings demonstrating the absence of Na V 1.8- and Na V 1.9-mediated Na + currents in Na V 1.8/Na V 1.9 DKO DRG neurons. Global RNA sequencing identified significant deregulation of C-LTMR marker genes as well as of pain-modulating neuropeptides in Na V 1.8/Na V 1.9 DKO DRG neurons, which fits to the overall only moderately impaired acute pain behavior observed in DKO mice. Besides addressing the function of both sodium channels in pain perception, we further demonstrate that the null-background is a very valuable tool for investigations on the functional properties of individual human disease-causing variants in Na V 1.8 or Na V 1.9 in their native physiological environment.

TrkB Agonist (7,8-DHF)-Induced Responses in Dorsal Root Ganglia Neurons Are Decreased after Spinal Cord Injury: Implication for Peripheral Pain Mechanisms

Brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) are known to contribute to both protective and pronociceptive processes. However, their contribution to neuropathic pain after spinal cord injury (SCI) needs further investigation. In a recent study utilizing TrkBF616A mice, it was shown that systemic pharmacogenetic inhibition of TrkB signaling with 1NM-PP1 (1NMP) immediately after SCI delayed the onset of pain hypersensitivity, implicating maladaptive TrkB signaling in pain after SCI. To examine potential neural mechanisms underlying the behavioral outcome, patch-clamp recording was performed in small-diameter dissociated thoracic (T) dorsal root ganglia (DRG) neurons to evaluate TrkB signaling in uninjured mice and after T10 contusion SCI. Bath-applied 7,8-dihydroxyflavone (7,8-DHF), a selective TrkB agonist, induced a robust inward current in neurons from uninjured mice, which was attenuated by 1NMP treatment. SCI also decreased 7,8-DHF-induced current while increasing the latency to its peak amplitude. Western blot revealed a concomitant decrease in TrkB expression in DRGs adjacent to the spinal lesion. Analyses of cellular and membrane properties showed that SCI increased neuronal excitability, evident by an increase in resting membrane potential and the number of spiking neurons. However, SCI did not increase spontaneous firing in DRG neurons. These results suggest that SCI induced changes in TrkB activation in DRG neurons even though these alterations are likely not contributing to pain hypersensitivity by nociceptor hyperexcitability. Overall, this reveals complex interactions involving TrkB signaling and provides an opportunity to investigate other, presumably peripheral, mechanisms by which TrkB contributes to pain hypersensitivity after SCI.

A review of dorsal root ganglia and primary sensory neuron plasticity mediating inflammatory and chronic neuropathic pain

Pain is a sensory state resulting from complex integration of peripheral nociceptive inputs and central processing. Pain consists of adaptive pain that is acute and beneficial for healing and maladaptive pain that is often persistent and pathological. Pain is indeed heterogeneous, and can be expressed as nociceptive, inflammatory, or neuropathic in nature. Neuropathic pain is an example of maladaptive pain that occurs after spinal cord injury (SCI), which triggers a wide range of neural plasticity. The nociceptive processing that underlies pain hypersensitivity is well-studied in the spinal cord. However, recent investigations show maladaptive plasticity that leads to pain, including neuropathic pain after SCI, also exists at peripheral sites, such as the dorsal root ganglia (DRG), which contains the cell bodies of sensory neurons. This review discusses the important role DRGs play in nociceptive processing that underlies inflammatory and neuropathic pain. Specifically, it highlights nociceptor hyperexcitability as critical to increased pain states. Furthermore, it reviews prior literature on glutamate and glutamate receptors, voltage-gated sodium channels (VGSC), and brain-derived neurotrophic factor (BDNF) signaling in the DRG as important contributors to inflammatory and neuropathic pain. We previously reviewed BDNF's role as a bidirectional neuromodulator of spinal plasticity. Here, we shift focus to the periphery and discuss BDNF-TrkB expression on nociceptors, non-nociceptor sensory neurons, and non-neuronal cells in the periphery as a potential contributor to induction and persistence of pain after SCI. Overall, this review presents a comprehensive evaluation of large bodies of work that individually focus on pain, DRG, BDNF, and SCI, to understand their interaction in nociceptive processing.

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.

Changes of the biophysical properties of voltage-gated Na+ currents during maturation of the sodium-taste cells in rat fungiform papillae

Taste cells are a heterogeneous population of sensory receptors that undergo continuous turnover. Different chemo-sensitive cell lines rely on action potentials to release the neurotransmitter onto nerve endings. The electrical excitability is due to the presence of a tetrodotoxin-sensitive, voltage-gated sodium current (I_Na ) similar to that found in neurons. Since the biophysical properties of neuronal I_Na change during development, we wondered whether the same also occurred in taste cells. Here, we used the patch-clamp recording technique to study I_Na in salt-sensing cells (sodium cells) of rat fungiform papillae. We identified these cells by exploiting the known blocking effect of amiloride on ENaC, the sodium (salt) receptor. Based on the amplitude of I_Na , which is known to increase during development, we subdivided sodium cells into two groups: cells with small sodium current (SSC cells; I_Na  < 1 nA) and cells with large sodium current (LSC cells; I_Na  > 1 nA). We found that: the voltage dependence of activation and inactivation significantly differed between these subsets; a slowly inactivating sodium current was more prominent in LSC cells; membrane capacitance in SSC cells was larger than in LSC cells. mRNA expression analysis of the α-subunits of voltage-gated sodium channels in fungiform taste buds supported the functional data. Lucifer Yellow labelling of recorded cells revealed that our electrophysiological criterion for distinguishing two broad groups of taste cells was in good agreement with morphological observations for cell maturity. Thus, all these findings are consistent with developmental changes in the voltage-dependent properties of sodium-taste cells. KEY POINTS: Taste cells are sensory receptors that undergo continuous turnover while they detect food chemicals and communicate with afferent nerve fibres. The voltage-gated sodium current (I_Na ) is a key ion current for generating action potentials in fully differentiated and chemo-sensitive taste cells, which use electrical signalling to release neurotransmitters. Here we show that, during the maturation of rat taste cells involved in salt detection (sodium cells), the biophysical properties of I_Na , such as voltage dependence of activation and inactivation, change significantly. Our results help reveal how taste cells gain electrical excitability during turnover, a property critical to their operation as chemical detectors that relay sensory information to nerve fibres.

Propranolol modulation of tetrodotoxin-resistant Na+ channels in dural afferent neurons

Propranolol, a representative adrenergic β-receptor antagonist, is widely used to prevent migraine attacks. Although propranolol is well known to inhibit tetrodotoxin-resistant (TTX-R) Na+ channels in cardiac myocytes, it is unclear whether the drug modulates these channels expressed in dural afferent neurons. In this study, we examined the effects of propranolol on TTX-R Na+ channels in medium-sized dural afferent neurons identified by the fluorescent dye DiI. The TTX-R Na+ currents (INa) were recorded from acutely isolated DiI-positive neurons using a whole-cell patch clamp technique under voltage-clamp conditions. Propranolol inhibited the noninactivating steady-state component more potently than the peak component of transient TTX-R INa. Propranolol also potently inhibited the slow voltage ramp-induced TTX-R INa in a concentration-dependent manner, suggesting that it preferentially inhibited the noninactivating or persistent INa in DiI-positive neurons. Propranolol had little effect on voltage dependence, but it increased the extent of the use-dependent inhibition of TTX-R Na+ channels. Propranolol also accelerated the onset of inactivation and retarded recovery from inactivation in these channels. Under current-clamp conditions, propranolol decreased the number of action potentials elicited by depolarizing current stimuli. In conclusion, the propranolol-mediated preferential inhibition of persistent INa and modulation of the inactivation kinetics of TTX-R Na+ channels might represent additional mechanisms for migraine prophylaxis.

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.

Magnolol inhibits sodium currents in freshly isolated mouse dorsal root ganglion neurons

The voltage-gated sodium channel (VGSC) currents in dorsal root ganglion (DRG) neurons contain mainly TTX-sensitive (TTX-S) and TTX-resistant (TTX-R) Na+ currents. Magnolol (Mag), a hydroxylated biphenyl compound isolated from the bark of Magnolia officinalis, has been well documented to exhibit analgesic effects, but its mechanism is not yet fully understood. The aim of the present study was to investigate whether the antinociceptive effects of Mag is through inhibition of Na+ currents. Na+ currents in freshly isolated mouse DRG neurons were recorded with the whole cell patch clamp technique. Results showed that Mag inhibited TTX-S and TTX-R Na+ currents in a concentration-dependent manner. The IC50 values for block of TTX-S and TTX-R Na+ currents were 9.4 and 7.0 μmol/L, respectively. Therefore, TTX-R Na+ current was more susceptible to Mag than TTX-S Na+ current. For TTX-S Na+ channel, 10 μmol/L Mag shifted the steady state inactivation curve toward more negative by 9.8 mV, without affecting the activation curve. For TTX-R Na+ channel, 7 μmol/L Mag shifted the steady state activation and inactivation curves toward more positive and negative potentials by 6.5 and 11.7 mV, respectively. In addition, Mag significantly postponed recovery of TTX-S and TTX-R Na+ currents from inactivation, and produced frequency dependent blocks of both subtypes of Na+ currents. These results suggest that the inhibitory effects of Mag on Na+ channels may contribute to its analgesic effect.

An Index Combining Lost and Remaining Nerve Fibers Correlates with Pain Hypersensitivity in Mice

Multiple peripheral nerves are known to degenerate after nerve compression injury but the correlation between the extent of nerve alteration and pain severity remains unclear. Here, we used intravital two-photon fluorescence microscopy to longitudinally observe changes in cutaneous fibers in the hind paw of Nav1.8-Cre-tdTomato mice after chronic constriction injury (CCI). Results showed that the CCI led to variable loss of the skin nerve plexus and intraepidermal nerve fibers. The timing of Nav1.8 nerve fiber loss correlated with the development of mechanical hypersensitivity. We compared a scoring approach that assessed whole-paw nerve degeneration with an index that quantified changes in the nerve plexus and terminals in multiple small regions of interest (ROI) from intravital images of the third and fifth toe tips. We found that the number of surviving nerve fibers was not linearly correlated with mechanical hypersensitivity. On the contrary, at 14 days after CCI, the moderately injured mice showed greater mechanical hypersensitivity than the mildly or severely injured mice. This indicates that both surviving and injured nerves are required for evoked neuropathic pain. In addition, these two methods may have the estimative effect as diagnostic and prognostic biomarkers for the assessment of neuropathic pain.

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.

Puerarin Relieves Paclitaxel-Induced Neuropathic Pain: The Role of Nav1.8 β1 Subunit of Sensory Neurons

Currently there is no effective treatment available for clinical patients suffering from neuropathic pain induced by chemotherapy paclitaxel. Puerarin is a major isoflavonoid extracted from the Chinese medical herb kudzu root, which has been used for treatment of cardiovascular disorders and brain injury. Here, we found that puerarin dose-dependently alleviated paclitaxel-induced neuropathic pain. At the same time, puerarin preferentially reduced the excitability and blocked the voltage-gated sodium (Na_v) channels of dorsal root ganglion (DRG) neurons from paclitaxel-induced neuropathic pain rats. Furthermore, puerarin was a more potent blocker of tetrodotoxin-resistant (TTX-R) Na_v channels than of tetrodotoxin-sensitive (TTX-S) Na_v channels in chronic pain rats’ DRG neurons. In addition, puerarin had a stronger blocking effect on Na_v1.8 channels in DRG neurons of neuropathic pain rats and β1 subunit siRNA can abolish this selective blocking effect on Na_v1.8. Together, these results suggested that puerarin may preferentially block β1 subunit of Na_v1.8 in sensory neurons contributed to its anti-paclitaxel induced neuropathic pain effect.

Familial episodic limb pain in kindreds with novel Nav1.9 mutations

We previously performed genetic analysis in six unrelated families with infantile limb pain episodes, characterized by cold-induced deterioration and mitigation in adolescence, and reported two new mutations p.R222H/S in SCN11A responsible for these episodes. As no term described this syndrome (familial episodic pain: FEP) in Japanese, we named it as”小児四肢疼痛発作症”. In the current study, we recruited an additional 42 new unrelated Japanese FEP families, between March 2016 and March 2018, and identified a total of 11 mutations in SCN11A: p.R222H in seven families, and p.R225C, p.F814C, p.F1146S, or p.V1184A, in independent families. A founder mutation, SCN11A p.R222H was confirmed to be frequently observed in patients with FEP in the Tohoku region of Japan. We also identified two novel missense variants of SCN11A, p.F814C and p.F1146S. To evaluate the effects of these latter two mutations, we generated knock-in mouse models harboring p.F802C (F802C) and p.F1125S (F1125S), orthologues of the human p.F814C and p.F1146S, respectively. We then performed electrophysiological investigations using dorsal root ganglion neurons dissected from the 6–8 week-old mice. Dissected neurons of F802C and F1125S mice showed increased resting membrane potentials and firing frequency of the action potentials (APs) by high input–current stimulus compared with WT mice. Furthermore, the firing probability of evoked APs increased in low stimulus input in F1125S mice, whereas several AP parameters and current threshold did not differ significantly between either of the mutations and WT mice. These results suggest a higher level of excitability in the F802C or F1125S mice than in WT, and indicate that these novel mutations are gain of function mutations. It can be expected that a considerable number of potential patients with FEP may be the result of gain of function SCN11A mutations.

Modulation of tetrodotoxin-resistant Na+ channels by amitriptyline in dural afferent neurons

Migraine is characterized by recurrent and disabling headaches; therefore, several drugs have been widely prescribed to prevent acute migraine attacks. Amitriptyline, a tricyclic antidepressant, is among the most commonly administered. It is poorly known, however, whether amitriptyline modulates the excitability of dural afferent neurons that transmit pain signals from the dura mater. In this study, the effects of amitriptyline on tetrodotoxin-resistant (TTX-R) Na⁺ channels were examined in acutely isolated rat dural afferent neurons, which were identified by the fluorescent dye DiI. The TTX-R Na⁺ currents (I_Na) were recorded from medium-sized DiI-positive neurons using a whole-cell patch clamp technique. Amitriptyline (3 μM) slightly reduced the peak component of transient I_Na and induced a marked decrease in the steady-state component of transient TTX-R I_Na, as well as in the slow ramp-induced TTX-R I_Na. Our findings suggest that amitriptyline specifically inhibits persistent Na⁺ currents mediated by TTX-R Na⁺ channels. While amitriptyline had minor effects on voltage-activation/inactivation, it increased the extent of the use-dependent inhibition of TTX-R Na⁺ channels. Amitriptyline also affected the inactivation kinetics of TTX-R Na⁺ channels by significantly accelerating the inactivation of TTX-R Na⁺ channels and slowing the subsequent recovery. Amitriptyline decreased the number of action potentials by increasing the threshold for their generation. In conclusion, the amitriptyline-mediated diverse modulation of TTX-R Na⁺ channels would be, at least in part, responsible for its prophylactic efficacy for migraine attacks.

Current Status of the New Antiepileptic Drugs in Chronic Pain

Antiepileptic drugs (AEDs) are extensively used worldwide to treat a wide range of disorders other than epilepsy, such as neuropathic pain, migraine, and bipolar disorder. Due to this situation more than 20 new third-generation AEDs have been introduced in the market recently. The future design of new AEDs must also have potential to help in the non-epileptic disorders. The wide acceptance of second generation AEDs for the management of various non-epileptic disorders has caused the emergence of generics in the market. The wide use of approved AEDs outside epilepsy is based on both economic and scientific reasons. Bipolar disorders, migraine prophylaxis, fibromyalgia, and neuropathic pain represent the most attractive indication expansion opportunities for anticonvulsant developers, providing blockbuster revenues. Strong growth in non-epilepsy conditions will see Pfizer's Lyrica become the market leading brand by 2018. In this review, we mainly focus on the current status of new AEDs in the treatment of chronic pain and migraine prophylaxis. AEDs have a strong analgesic potential and this is demonstrated by the wide use of carbamazepine in trigeminal neuralgia and sodium valproate in migraine prophylaxis. At present, data on the new AEDs for non-epileptic conditions are inconclusive. Not all AEDs are effective in the management of neuropathic pain and migraine. Only those AEDs whose mechanisms of action are match with pathophysiology of the disease, have potential to show efficacy in non-epileptic disorder. For this better understanding of the pathophysiology of the disease and mechanisms of action of new AEDs are essential requirement before initiating pre-clinical and clinical trials. Many new AEDs show good results in the animal model and open-label studies but fail to provide strong evidence at randomized, placebo-controlled trials. The final decision regarding the clinical efficacy of the particular AEDs in a specific non-epileptic disorder should be withdrawal from randomized placebo trials rather than open-label studies; otherwise this may lead to off-label uses of drug. The purpose of the present review is to relate the various mechanisms of action of new AEDs to pathophysiological mechanisms and clinical efficacy in neuropathic pain and migraine.

Use dependence of peripheral nociceptive conduction in the absence of tetrodotoxin-resistant sodium channel subtypes

KEY POINTS

This study examines conduction in peripheral nerves and its use dependence in tetrodotoxin-resistant (TTXr) sodium channel (Nav 1.8, Nav 1.9) knockout and wildtype animals. We observed use-dependent decreases of single fibre and compound action potential amplitude in peripheral mouse C-fibres (wildtype). This matches the previously published hypothesis that increased Na/K-pump activity is not the underlying mechanism for use-dependent changes of neural conduction. Knocking out TTXr sodium channels influences use-dependent changes of conductive properties (action potential amplitude, latency, conduction safety) in the order Nav 1.8 KO > Nav 1.9KO > wildtype. This is most likely explained by different subsets of presumably (relatively) Nav 1.7-rich conducting fibres in knockout animals as compared to wildtypes, in combination with reduced per-pulse sodium influx.

ABSTRACT

Use dependency of peripheral nerves, especially of nociceptors, correlates with receptive properties. Slow inactivation of voltage-gated sodium channels has been discussed to be the underlying mechanism - pointing to a receptive class-related difference of sodium channel equipment. Using electrophysiological recordings of single unmyelinated cutaneous fibres and their compound action potential (AP), we evaluated use-dependent changes in mouse peripheral nerves, and the contribution of the tetrodotoxin-resistant (TTXr) sodium channels Nav 1.8 and Nav 1.9 to these changes. Nerve fibres were electrically stimulated using single or double pulses at 2 Hz. Use-dependent changes of latency, AP amplitude, and duration as well as the fibres' ability to follow the stimulus were evaluated. AP amplitudes substantially diminished in used fibres from C57BL/6 but increased in Nav 1.8 knockout (KO) mice, with Nav 1.9 KO in between. Activity-induced latency slowing was in contrast the most pronounced in Nav 1.8 KOs and the least in wildtype mice. The genotype was also predictive of how long fibres could follow the double pulsed stimulus with wildtype fibres blocking first and Nav 1.8 KO fibres enduring the longest. In contrast, changes in spike duration were less pronounced and displayed no significant tendency. Thus, all major measures of peripheral nerve accommodation (amplitude, latency and durability) depended on genotype. All use-dependent changes appeared in the order NaV 1.8 KO > NaV 1.9 KO > wildtype, which is most likely explained by the relative contribution of Nav 1.7 varying in the same order and the amounts of per-pulse sodium influx expected in the opposite order.

Infantile Pain Episodes Associated with Novel Nav1.9 Mutations in Familial Episodic Pain Syndrome in Japanese Families

Painful peripheral neuropathy has been correlated with various voltage-gated sodium channel mutations in sensory neurons. Recently Nav1.9, a voltage-gated sodium channel subtype, has been established as a genetic influence for certain peripheral pain syndromes. In this study, we performed a genetic study in six unrelated multigenerational Japanese families with episodic pain syndrome. Affected participants (n = 23) were characterized by infantile recurrent pain episodes with spontaneous mitigation around adolescence. This unique phenotype was inherited in an autosomal-dominant mode. Linkage analysis was performed for two families with 12 affected and nine unaffected members, and a single locus was identified on 3p22 (LOD score 4.32). Exome analysis (n = 14) was performed for affected and unaffected members in these two families and an additional family. Two missense variants were identified: R222H and R222S in SCN11A. Next, we generated a knock-in mouse model harboring one of the mutations (R222S). Behavioral tests (Hargreaves test and cold plate test) using R222S and wild-type C57BL/6 (WT) mice, young (8-9 weeks old; n = 10-12 for each group) and mature (36-38 weeks old; n = 5-6 for each group), showed that R222S mice were significantly (p < 0.05) more hypersensitive to hot and cold stimuli than WT mice. Electrophysiological studies using dorsal root ganglion neurons from 8-9-week-old mice showed no significant difference in resting membrane potential, but input impedance and firing frequency of evoked action potentials were significantly increased in R222S mice compared with WT mice. However, there was no significant difference among Nav1.9 (WT, R222S, and R222H)-overexpressing ND7/23 cell lines. These results suggest that our novel mutation is a gain-of-function mutation that causes infantile familial episodic pain. The mouse model developed here will be useful for drug screening for familial episodic pain syndrome associated with SCN11A mutations.

A novel substituted aminoquinoline selectively targets voltage-sensitive sodium channel isoforms and NMDA receptor subtypes and alleviates chronic inflammatory and neuropathic pain

Recent understanding of the systems that mediate complex disease states, has generated a search for molecules that simultaneously modulate more than one component of a pathologic pathway. Chronic pain syndromes are etiologically connected to functional changes (sensitization) in both peripheral sensory neurons and in the central nervous system (CNS). These functional changes involve modifications of a significant number of components of signal generating, signal transducing and signal propagating pathways. Our analysis of disease-related changes which take place in sensory neurons during sensitization led to the design of a molecule that would simultaneously inhibit peripheral NMDA receptors and voltage sensitive sodium channels. In the current report, we detail the selectivity of N,N-(diphenyl)-4-ureido-5,7-dichloro-2-carboxy-quinoline (DCUKA) for action at NMDA receptors composed of different subunit combinations and voltage sensitive sodium channels having different α subunits. We show that DCUKA is restricted to the periphery after oral administration, and that circulating blood levels are compatible with its necessary concentrations for effects at the peripheral cognate receptors/channels that were assayed in vitro. Our results demonstrate that DCUKA, at concentrations circulating in the blood after oral administration, can modulate systems which are upregulated during peripheral sensitization, and are important for generating and conducting pain information to the CNS. Furthermore, we demonstrate that DCUKA ameliorates the hyperalgesia of chronic pain without affecting normal pain responses in neuropathic and inflammation-induced chronic pain models.

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.

Indomethacin inhibits tetrodotoxin-resistant Na(+) channels at acidic pH in rat nociceptive neurons

Non-steroidal anti-inflammatory drugs (NSAIDs) are well-known inhibitors of cyclooxygenases (COXs) and are widely used for the treatment of inflammatory pain; however several NSAIDs display COX-independent analgesic action including the inhibition of voltage-gated Na(+) channels expressed in primary afferent neurons. In the present study, we examined whether NSAIDs modulate tetrodotoxin-resistant (TTX-R) Na(+) channels and if this modulation depends on the extracellular pH. The TTX-R Na(+) currents were recorded from small-sized trigeminal ganglion neurons by using a whole-cell patch clamp technique. Among eight NSAIDs tested in this study, several drugs, including aspirin and ibuprofen, did not affect TTX-R Na(+) channels either at pH 7.4 or at pH 6.0. However, we found that indomethacin, and, to a lesser extent, ibuprofen and naproxen potently inhibited the peak amplitude of TTX-R Na(+) currents at pH 6.0. The indomethacin-induced inhibition of TTX-R Na(+) channels was more potent at depolarized membrane potentials. Indomethacin significantly shifted both the voltage-activation and voltage-inactivation relationships to depolarizing potentials at pH 6.0. Indomethacin accelerated the development of inactivation and retarded the recovery from inactivation of TTX-R Na(+) channels at pH 6.0. Given that indomethacin and several other NSAIDs could further suppress local nociceptive signals by inhibiting TTX-R Na(+) channels at an acidic pH in addition to the classical COX inhibition, these drugs could be particularly useful for the treatment of inflammatory pain.

pH-dependent inhibition of tetrodotoxin-resistant Na(+) channels by diclofenac in rat nociceptive neurons

Non-steroidal anti-inflammatory drugs (NSAIDs) are widely used for the treatment of inflammatory pain. It is well established that NSAIDs exert their analgesic effects by inhibiting cyclooxygenase to prevent the production of prostaglandins; however, several NSAIDs including diclofenac also modulate other ion channels expressed in nociceptive neurons. In this study, we investigated the pH-dependent effects of diclofenac on tetrodotoxin-resistant (TTX-R) Na(+) channels in rat trigeminal sensory neurons by using the whole-cell patch clamp technique. Diclofenac decreased the peak amplitude of TTX-R Na(+) currents (INa) in a concentration dependent manner. While diclofenac had little effect on the voltage-activation relationship, it significantly shifted the steady-state fast inactivation relationship toward hyperpolarized potentials. Diclofenac increased the extent of use-dependent inhibition of TTX-R Na(+) currents. Diclofenac also significantly accelerated the development of inactivation and retarded the recovery from inactivation of TTX-R Na(+) channels. The effects of diclofenac on TTX-R Na(+) channels were stronger at pH 6.0 than at pH7.4 for most of the parameters tested. Considering that the extracellular pH falls in inflamed tissues, and that TTX-R Na(+) channels expressed on nociceptive neurons are implicated in the prostaglandin-mediated development and maintenance of inflammatory hyperalgesia, our findings could provide an additional analgesic effect of diclofenac under acidic pH conditions.

Role played by NaV 1.7 channels on thin-fiber muscle afferents in transmitting the exercise pressor reflex

Voltage-gated sodium channels (NaV) 1.7 are highly expressed on the axons of somatic afferent neurons and are thought to play an important role in the signaling of inflammatory pain. NaV 1.7 channels are classified as tetrodotoxin (TTX)-sensitive, meaning that they are blocked by TTX concentrations of less than 300 nM. These findings prompted us to determine in decerebrated, unanesthetized rats, the role played by NaV 1.7 channels in the transmission of muscle afferent input evoking the exercise pressor reflex. We first showed that the exercise pressor reflex, which was evoked by static contraction of the triceps surae muscles, was reversibly attenuated by application of 50 nM TTX, but not 5 nM TTX, to the L4-L5 dorsal roots (control: 21 ± 1 mmHg, TTX: 8 ± 2 mmHg, recovery: 21 ± 3 mmHg; n = 6; P < 0.01). We next found that the peak pressor responses to contraction were significantly attenuated by dorsal root application of 100 nM Ssm6a, a compound that is a selective NaV 1.7 channel inhibitor. Removal of Ssm6a restored the reflex to its control level (control: 19 ± 3 mmHg, Ssm6a: 10 ± 1 mmHg, recovery: 19 ± 4 mmHg; n = 6; P < 0.05). Compound action potentials recorded from the L4 and L5 dorsal roots and evoked by single-pulse stimulation of the sciatic nerve showed that both TTX and Ssm6a attenuated input from group III, as well as group IV afferents. We conclude that NaV 1.7 channels play a role in the thin-fiber muscle afferent pathway evoking the exercise pressor reflex.

Voltage-gated sodium channels

Voltage-gated sodium channels play a key role in the transmission of sensory information about the status of organs in the periphery. Sensory fibers contain a heterogeneous yet specific distribution of voltage-gated sodium channel isoforms. Major efforts by industry and academic groups are underway to develop medicines that interrupt inappropriate signaling for a number of clinical indications by taking advantage of this specific distribution of channel isoforms. This review highlights recent advances in the study of human channelopathies, animal toxins and channel structure that may facilitate the development of selective voltage-gated sodium channel blockers.

Acid modulation of tetrodotoxin-resistant Na⁺ channels in rat nociceptive neurons

Under pathological conditions including inflammation, ischemia and incision, extracellular pH falls down as low as 5.4. Although some mediators play pivotal roles in the development and maintenance of inflammatory hyperalgesia by affecting the functional properties of tetrodotoxin-resistant (TTX-R) Na(+) channels, the roles of tissue acidosis in nociceptive transmission mediated by TTX-R Na(+) channels are largely unknown. In the present study, we have investigated the effect of acidic pH on TTX-R Na(+) currents (I(Na)) in small-sized sensory neurons isolated from rat trigeminal ganglia using a whole-cell patch clamp technique. Acidic pH decreased the peak amplitude of TTX-R I(Na) in a pH-dependent manner, but weak acid (≥pH 6.0) had a minor inhibitory effect on the TTX-R I(Na). Acidic pH also significantly shifted both the activation and steady-state fast inactivation relationships toward depolarized potentials. In addition, acidic pH had little effect on the use-dependent inhibition, and significantly retarded the development of inactivation and accelerated the recovery from inactivation of TTX-R Na(+) channels. The results suggest that weak acid (≥pH 6.0) makes TTX-R Na(+) channels to be suitable for the repetitive activation at depolarized membrane potentials. Given that both tissue acidosis and inflammatory mediators in inflamed or injured tissues act synergistically to promote nociceptive transmission by affecting the functional properties of TTX-R Na(+) channels, these channels would be, at least in part, a good target to treat inflammatory 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.

Peripheral drive in Aα/β-fiber neurons is altered in a rat model of osteoarthritis: changes in following frequency and recovery from inactivation

Purpose

To determine conduction fidelity of Aα/β-fiber low threshold mechanoreceptors in a model of osteoarthritis (OA).

Methods

Four weeks after cutting the anterior cruciate ligament and removing the medial meniscus to induce the model, in vivo intracellular recordings were made in ipsilateral L4 dorsal root ganglion neurons. L4 dorsal roots were stimulated to determine the refractory interval and the maximum following frequency of the evoked action potential (AP). Neurons exhibited two types of response to paired pulse stimulation.

Results

One type of response was characterized by fractionation of the evoked AP into an initial nonmyelinated-spike and a later larger-amplitude somatic-spike at shorter interstimulus intervals. The other type of response was characterized by an all-or-none AP, where the second evoked AP failed altogether at shorter interstimulus intervals. In OA versus control animals, the refractory interval measured in paired pulse testing was less in all low threshold mechanoreceptors. With train stimulation, the maximum rising rate of the nonmyelinated-spike was greater in OA nonmuscle spindle low threshold mechanoreceptors, possibly due to changes in fast kinetics of currents. Maximum following frequency in Pacinian and muscle spindle neurons was greater in model animals compared to controls. Train stimulation also induced an inactivation and fractionation of the AP in neurons that showed fractionation of the AP in paired pulse testing. However, with train stimulation this fractionation followed a different time course, suggesting more than one type of inactivation.

Conclusion

The data suggest that joint damage can lead to changes in the fidelity of AP conduction of large diameter sensory neurons, muscle spindle neurons in particular, arising from articular and nonarticular tissues in OA animals compared to controls. These changes might influence peripheral drive of spinal excitability and plasticity, thus contributing to OA sensory abnormalities, including OA pain.

Upregulation of cystathionine β-synthetase expression contributes to visceral hyperalgesia induced by heterotypic intermittent stress in rats

Background

Hydrogen sulfide (H₂S) functions as a neuromodulator, but whether it modulates visceral pain is not well known. This study was designed to determine the role for the endogenous H₂S producing enzyme cystathionine β-synthetase (CBS) and cystathionine γ-lyase (CSE) in a validated rat model of visceral hyperalgesia (VH).

Methods

VH was induced by nine-day heterotypic intermittent stress (HIS). Abdominal withdrawal reflex (AWR) scores were determined by measuring the visceromoter responses to colorectal distension (CRD). Dorsal root ganglia (DRG) neurons innervating the colon were labeled by injection of DiI (1,1'-dioleyl-3,3,3',3-tetramethylindocarbocyanine methanesulfonate) into the colon wall. Patch clamp recording techniques were employed to examine excitability and sodium channel currents of colon specific DRG neurons. Tissues from colon related thoracolumbar DRGs were analyzed for CBS, CSE and sodium channel expression.

Results

HIS significantly increased the visceromotor responses to CRD in association with an upregulated expression of CBS not CSE proteins in colon related DRGs. Administration of O-(Carboxymethyl)hydroxylamine hemihydrochloride (AOAA), an inhibitor of CBS, attenuated the AWR scores in HIS-treated rats, in a dose dependent fashion. In contrast, AOAA did not produce any effect on AWR scores in healthy control rats. AOAA reversed the potentiation of sodium channel current densities of colon specific DRG neurons of HIS rats. To further confirm the role for CBS-H₂S signaling, NaHS was used to mimic the production of H₂S by CBS. Application of NaHS significantly enhanced neuronal excitability and potentiated sodium channel current densities of colon DRG neurons from healthy control rats. Furthermore, AOAA reversed the upregulation of Na_V1.7 and Na_V1.8 in colon related DRGs of HIS rats.

Conclusion

Our results suggest that upregulation of CBS expression might play an important role in developing VH via sensitization of sodium channels in peripheral nociceptors, thus identifying a specific neurobiological target for the treatment of VH in functional bowel syndromes.

In vitro hyperglycemia enhances sodium currents in dorsal root ganglion neurons: an effect attenuated by carbamazepine

Neuropathy is often seen in uncontrolled diabetes and the mechanisms involved for neuropathic pain are poorly understood. Hyperglycemia is a consequence of chronic uncontrolled diabetes and it is postulated to produce neuropathic pain. Therefore, in this study, we have investigated the effects of hyperglycemia on Na(+) channel kinetics in cultured dorsal root ganglion (DRG) neurons from neonatal rats using whole-cell patch-clamp technique. Hyperglycemia-induced increase in density of tetrodotoxin resistant (TTXr) Na(+) currents was increased in time- and concentration-dependent manner. The increase was maximal with 60 mM and 24 h. There was no change Na(+) current density in time-matched control neurons. The conductance curve of TTXr Na(+) current shifted leftward after 24 h exposure to 45 mM glucose. Carbamazepine (CBZ, 100 μM) depressed TTXr Na(+) current in neurons incubated with control (17.26), 45 and 60 mM of glucose. The depression observed with CBZ in the presence of high glucose, i.e., 45 mM (86.5±4.9%) was significantly greater than control (61.6±1.8%). Hyperglycemia also increased reactive oxygen species (ROS) activity and was attenuated by CBZ. These results suggest that short-term exposure of DRG neurons to high glucose concentrations enhance the Na(+) channel activity, and were attenuated by CBZ via ROS-dependent mechanisms.

Neuroplastic alteration of TTX-resistant sodium channel with visceral pain and morphine-induced hyperalgesia

The discovery of the tetrodotoxin-resistant (TTX-R) Na⁺ channel in nociceptive neurons has provided a special target for analgesic intervention. In a previous study we found that both morphine tolerance and persistent visceral inflammation resulted in visceral hyperalgesia. It has also been suggested that hyperexcitability of sensory neurons due to altered TTX-R Na⁺ channel properties and expression contributes to hyperalgesia; however, we do not know if some TTX-R Na⁺ channel property changes can be triggered by visceral hyperalgesia and morphine tolerance, or whether there are similar molecular or channel mechanisms in both situations. To evaluate the effects of morphine tolerance and visceral inflammation on the channel, we investigated the dorsal root ganglia (DRG) neuronal change following these chronic treatments. Using whole-cell patch clamp recording, we recorded TTX-R Na⁺ currents in isolated adult rat lumbar and sacral (L6−S2) DRG neurons from normal and pathologic rats with colon inflammatory pain or chronic morphine treatment. We found that the amplitudes of TTX-R Na⁺ currents were significantly increased in small-diameter DRG neurons with either morphine tolerance or visceral inflammatory pain. Meanwhile, the result also showed that those treatments altered the kinetics properties of the electrical current (ie, the activating and inactivating speed of the channel was accelerated). Our current results suggested that in both models, visceral chronic inflammatory pain and morphine tolerance causes electrophysiological changes in voltage-gated Na channels due to the chronic administration of these medications. For the first time, the present investigation explored the adaptations of this channel, which may contribute to the hyperexcitability of primary afferent nerves and hyperalgesia during these pathologic conditions. The results also suggest that neurophysiologic mechanisms of morphine tolerance and visceral hyperalgesia are related at the TTX-R Na⁺ channel.

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.

Characteristics of sodium currents in rat geniculate ganglion neurons

Geniculate ganglion (GG) cell bodies of chorda tympani (CT), greater superficial petrosal (GSP), and posterior auricular (PA) nerves transmit orofacial sensory information to the rostral nucleus of the solitary tract. We have used whole cell recording to investigate the characteristics of the Na(+) channels in isolated Fluorogold-labeled GG neurons that innervate different peripheral receptive fields. GG neurons expressed two classes of Na(+) channels, TTX sensitive (TTX-S) and TTX resistant (TTX-R). The majority of GG neurons expressed TTX-R currents of different amplitudes. TTX-R currents were relatively small in 60% of the neurons but were large in 12% of the sampled population. In a further 28% of the neurons, TTX completely abolished all Na(+) currents. Application of TTX completely inhibited action potential generation in all CT and PA neurons but had little effect on the generation of action potentials in 40% of GSP neurons. Most CT, GSP, and PA neurons stained positively with IB(4), and 27% of the GSP neurons were capsaicin sensitive. The majority of IB(4)-positive GSP neurons with large TTX-R Na(+) currents responded to capsaicin, whereas IB(4)-positive GSP neurons with small TTX-R Na(+) currents were capsaicin insensitive. These data demonstrate the heterogeneity of GG neurons and indicate the existence of a subset of GSP neurons sensitive to capsaicin, usually associated with nociceptors. Since there are no reports of nociceptors in the GSP receptive field, the role of these capsaicin-sensitive neurons is not clear.

Na+ channel Scn1b gene regulates dorsal root ganglion nociceptor excitability in vivo

Nociceptive dorsal root ganglion (DRG) neurons express tetrodotoxin-sensitive (TTX-S) and -resistant (TTX-R) Na(+) current (I(Na)) mediated by voltage-gated Na(+) channels (VGSCs). In nociceptive DRG neurons, VGSC β2 subunits, encoded by Scn2b, selectively regulate TTX-S α subunit mRNA and protein expression, ultimately resulting in changes in pain sensitivity. We hypothesized that VGSCs in nociceptive DRG neurons may also be regulated by β1 subunits, encoded by Scn1b. Scn1b null mice are models of Dravet Syndrome, a severe pediatric encephalopathy. Many physiological effects of Scn1b deletion on CNS neurons have been described. In contrast, little is known about the role of Scn1b in peripheral neurons in vivo. Here we demonstrate that Scn1b null DRG neurons exhibit a depolarizing shift in the voltage dependence of TTX-S I(Na) inactivation, reduced persistent TTX-R I(Na), a prolonged rate of recovery of TTX-R I(Na) from inactivation, and reduced cell surface expression of Na(v)1.9 compared with their WT littermates. Investigation of action potential firing shows that Scn1b null DRG neurons are hyperexcitable compared with WT. Consistent with this, transient outward K(+) current (I(to)) is significantly reduced in null DRG neurons. We conclude that Scn1b regulates the electrical excitability of nociceptive DRG neurons in vivo by modulating both I(Na) and I(K).

The action of prostaglandins on ion channels

Prostaglandins, in particular PGE(2) and prostacyclin PGI(2) have diverse biological effects. Most importantly, they are involved in inflammation and pain. Prostaglandins in nano- and micromolar concentrations sensitize nerve cells, i.e. make them more sensitive to electrical or chemical stimuli. Sensitization arises from the effect of prostaglandins on ion channels and occurs both at the peripheral terminal of nociceptors at the site of tissue injury (peripheral sensitization) and at the synapses in the spinal cord (central sensitization). The first step is the binding of prostaglandins to receptors in the cell membrane, mainly EP and IP receptors. The receptors couple via G proteins to enzymes such as adenylate cyclase and phospholipase C (PLC). Activation of adenylate cyclase leads to increase of cAMP and subsequent activation of protein kinase A (PKA) or PKA-independent effects of cAMP, e.g. mediated by Epac (=exchange protein activated by cAMP). Activation of PLC causes increase of inositol phosphates and increase of cytosolic calcium. This article summarizes the effects of PGE(2), PGE(1), PGI2 and its stable analogues on non-selective cation channels and sodium, potassium, calcium and chloride channels. It describes the mechanism responsible for the facilitatory or inhibitory prostaglandin effects on ion channels. Understanding these mechanisms is essential for the development of useful new analgesics.

Aquaporin-1 tunes pain perception by interaction with Na(v)1.8 Na+ channels in dorsal root ganglion neurons

Aquaporin-1 (AQP1) water channels are expressed in the plasma membrane of dorsal root ganglion (DRG) neurons. We found reduced osmotic water permeability in freshly isolated DRG neurons from AQP1(-/-) versus AQP1(+/+) mice. Behavioral studies showed greatly reduced thermal inflammatory pain perception in AQP1(-/-) mice evoked by bradykinin, prostaglandin E(2), and capsaicin as well as reduced cold pain perception. Patch clamp of freshly isolated DRG neurons showed reduced action potential firing in response to current injections. Single action potentials after pulse current injections showed reduced maximum inward current, suggesting impaired Na(v)1.8 Na(+) function. Whole-cell Na(v)1.8 Na(+) currents in Na(v)1.8-expressing ND7-23 cells showed slowed frequency-dependent inactivation after AQP1 transfection. Immunoprecipitation studies showed AQP1- Na(v)1.8 Na(+) interaction, which was verified in live cells by single-particle tracking of quantum dot-labeled AQP1. Our results implicate the involvement of AQP1 in DRG neurons for the perception of inflammatory thermal pain and cold pain, whose molecular basis is accounted for, in part, by reduced Na(v)1.8-dependent membrane Na(+) current. AQP1 is, thus, a novel target for pain management.

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.

Developmental switch of the expression of ion channels in human dendritic cells

Modulation of the expression and activity of plasma membrane ion channels is one of the mechanisms by which immune cells can regulate their intracellular Ca(2+) signaling pathways required for proliferation and/or differentiation. Voltage-gated K+ channels, inwardly rectifying K+ channels, and Ca(2+)-activated K+ channels have been described to play a major role in controlling the membrane potential in lymphocytes and professional APCs, such as monocytes, macrophages, and dendritic cells (DCs). Our study aimed at the characterization and identification of ion channels expressed in the course of human DC differentiation from monocytes. We report in this study for the first time that immature monocyte-derived DCs express voltage-gated Na+ channels in their plasma membrane. The analysis of the biophysical and pharmacological properties of the current and PCR-based cloning revealed the presence of Nav1.7 channels in immature DCs. Transition from the immature to a mature differentiation state, however, was accompanied by the down-regulation of Nav1.7 expression concomitant with the up-regulation of voltage-gated Kv1.3 K+ channel expression. The presence of Kv1.3 channels seems to be common for immune cells; hence, selective Kv1.3 blockers may emerge as candidates for inhibiting various functions of mature DCs that involve their migratory, cytokine-secreting, and T cell-activating potential.

Isoflurane inhibits the tetrodotoxin-resistant voltage-gated sodium channel Nav1.8

BACKGROUND

Voltage-gated sodium channels (Nav) mediate neuronal action potentials. Tetrodotoxin inhibits all Nav isoforms, but Nav1.8 and Nav1.9 are relatively tetrodotoxin-resistant (TTX-r) compared to other isoforms. Nav1.8 is highly expressed in dorsal root ganglion neurons and is functionally linked to nociception, but the sensitivity of TTX-r isoforms to inhaled anesthetics is unclear.

METHODS

The sensitivities of heterologously expressed rat TTX-r Nav1.8 and endogenous tetrodotoxin-sensitive (TTX-s) Nav to the prototypic inhaled anesthetic isoflurane were tested in mammalian ND7/23 cells using patch-clamp electrophysiology.

RESULTS

From a holding potential of -70 mV, isoflurane (0.53 +/- 0.06 mM, 1.8 minimum alveolar concentration at 24 degrees C) reduced normalized peak Na current (INa) of Nav1.8 to 0.55 +/- 0.03 and of endogenous TTX-s Nav to 0.56 +/- 0.06. Isoflurane minimally inhibited INa from a holding potential of -140 mV. Isoflurane did not affect voltage-dependence of activation, but it significantly shifted voltage-dependence of steady-state inactivation by -6 mV for Nav1.8 and by -7 mV for TTX-s Nav. IC50 values for inhibition of peak INa were 0.67 +/- 0.06 mM for Nav1.8 and 0.66 +/- 0.09 mM for TTX-s Nav; significant inhibition occurred at clinically relevant concentrations as low as 0.58 minimum alveolar concentration. Isoflurane produced use-dependent block of Nav1.8; at a stimulation frequency of 10 Hz, 0.56 +/- 0.08 mM isoflurane reduced INa to 0.64 +/- 0.01 versus 0.78 +/- 0.01 for control.

CONCLUSION

Isoflurane inhibited the tetrodotoxin-resistant isoform Nav1.8 with potency comparable to that for endogenous tetrodotoxin-sensitive Nav isoforms, indicating that sensitivity to inhaled anesthetics is conserved across diverse Nav family members. Block of Nav1.8 in dorsal root ganglion neurons could contribute to the effects of inhaled anesthetics on peripheral nociceptive mechanisms.

Axonal ion channels from bench to bedside: a translational neuroscience perspective

Over recent decades, the development of specialised techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel dysfunction to the pathophysiology of common neurological conditions including epilepsy, multiple sclerosis, spinal cord injury, peripheral neuropathy, episodic ataxia, amyotrophic lateral sclerosis and neuropathic pain. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, novel clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps and ion exchange processes activated during impulse conduction in peripheral axons. These non-invasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and disease. This review will provide a translational perspective, focusing on an overview of the investigational method, the clinical utility in assessing the biophysical basis of ectopic symptom generation in peripheral nerve disease and a review of the major findings of excitability studies in acquired and inherited neurological disease states.

Trinitrobenzenesulphonic acid colitis alters Na 1.8 channel expression in mouse dorsal root ganglia neurons

Visceral inflammation evokes hyperexcitability in nociceptive dorsal root ganglia (DRG) neurons and these changes are associated with increased voltage-gated sodium channel (Na(v)) 1.8 current density, but the molecular determinants of these changes are unclear. This study used Western blotting to measure changes in Na(v) 1.7, 1.8 and 1.9 protein expression during trinitrobenzenesulphonic acid (TNBS) colitis and quantitative polymerase chain reaction (PCR) to examine corresponding changes in mRNA. Colonic neurons were labelled with the retrograde tracer Fast Blue injected into the wall of the distal colon and quantitative PCR performed on laser-captured labelled colonic neurons from ganglia at T9-13 or unlabelled DRG neurons from the upper spinal cord. Immunohistochemistry and western blots were performed on whole DRG from the same sites. Fast Blue-labelled neurons demonstrated Na(v) 1.7, 1.8 and 1.9 immunoreactivity. On day 7 of colitis, which correlated with electrophysiological studies, there was a threefold increase in Na(v) 1.8 protein in ganglia from T9 to 13, but Na(v) 1.7 and 1.9 levels were unchanged. There was no corresponding change in the Na(v) 1.8 alpha-subunit mRNA levels. However, on days 2 and 4, Na(v) 1.8 mRNA was decreased 10-fold. Na(v) 1.8 protein and mRNA levels were unchanged in neurons isolated from ganglia in the upper spinal cord, where colonic neurons are not found. These findings suggest that the TNBS evoked increase in Na(v) 1.8 currents is associated with increased numbers of channels. The absence of corresponding changes in transcript suggests a translational or post-translational mechanism, but the 10-fold recovery of transcript preceding this time point also demonstrates a complex transcriptional regulation.

Voltage-clamp and current-clamp recordings from mammalian DRG neurons

We provide here detailed electrophysiological protocols to study voltage-gated sodium channels and to investigate how wild-type and mutant channels influence firing properties of transfected mammalian dorsal root ganglion (DRG) neurons. Whole-cell voltage-clamp recordings permit us to analyze kinetic and voltage-dependence properties of ion channels and to determine the effect and mode of action of pharmaceuticals on specific channel isoforms. They also permit us to analyze the role of individual sodium channels and their mutant derivatives in regulating firing of DRG neurons. Five to ten cells can be recorded daily, depending on the extent of analysis that is required. Because of different internal solutions that are used in voltage-clamp and current-clamp recordings, only limited information can be obtained from recording the same neuron in both modes. These electrophysiological studies help to elucidate the role of specific channels in setting threshold and suprathreshold responses of neurons, under normal and pathological conditions.

Pregabalin in the treatment of chronic pain: an overview

Chronic 'pathological' pain is sustained by mechanisms of peripheral and central sensitization, which are being increasingly investigated at the molecular and cellular levels. The molecular determinants of nociceptive sensitization are natural targets for potential analgesic drugs used in the treatment of different forms of pain. Most of these determinants are common to all forms of chronic pain, and it is therefore not surprising that drugs specifically targeted for the treatment of neuropathic pain are effective in relieving nociceptive inflammatory pain and vice versa. The molecular mechanisms of sensitization that occur in peripheral nociceptors and the dorsal horns of the spinal cord are putative targets for context-dependent drugs, i.e. drugs that are able to discriminate between 'normal' and 'pathological' pain transmission. Among these, pregabalin and gabapentin bind to the alpha(2)delta subunit of voltage-sensitive Ca2+ channels, which sustain the enhanced release of pain transmitters at the synapses between primary afferent fibres and second-order sensory neurons under conditions of chronic pain. Pregabalin in particular represents a remarkable example of a context-dependent analgesic drug that acts at a critical step of nociceptive sensitization. Preclinical and clinical data suggest that pregabalin is more than a structural and functional analogue of gabapentin and may be effective in the treatment of nociceptive inflammatory pain that is resistant to gabapentin.

Citrobacter rodentium colitis evokes post-infectious hyperexcitability of mouse nociceptive colonic dorsal root ganglion neurons

To investigate the possible contribution of peripheral sensory mechanisms to abdominal pain following infectious colitis, we examined whether the Citrobacter rodentium mouse model of human E. coli infection caused hyperexcitability of nociceptive colonic dorsal root ganglion (DRG) neurons and whether these changes persisted following recovery from infection. Mice were gavaged with C. rodentium or distilled water. Perforated patch clamp recordings were obtained from acutely dissociated Fast Blue labelled colonic DRG neurons and afferent nerve recordings were obtained from colonic afferents during ramp colonic distensions. Recordings were obtained on day 10 (acute infection) and day 30 (infection resolved). Following gavage, colonic weights, myeloperoxidase (MPO) activity, stool cultures, and histological scoring established that infection caused colitis at day 10 which resolved by day 30 in most tissues. Electrophysiological recordings at day 10 demonstrated hyperexcitability of colonic DRG neurons (40% mean decrease in rheobase, P = 0.02; 50% mean increase in action potential discharge at twice rheobase, P = 0.02). At day 30, the increase in action potential discharge persisted (approximately 150% increase versus control; P = 0.04). In voltage clamp studies, transient outward (I(A)) and delayed rectifier (I(K)) currents were suppressed at day 10 and I(A) currents remained suppressed at day 30. Colonic afferent nerve recordings during colonic distension demonstrated enhanced firing at day 30 in infected animals. These studies demonstrate that acute infectious colitis evokes hyperexcitability of colonic DRG neurons which persists following resolution of the infection and that suppression of I(A) currents may play a role. Together, these findings suggest that peripheral pain mechanisms could contribute to post-infectious symptoms in conditions such as post-infectious irritable bowel syndrome.