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

Unusual Voltage-Gated Sodium Currents as Targets for Pain

Pain is a serious health problem that impacts the lives of many individuals. Hyperexcitability of peripheral sensory neurons contributes to both acute and chronic pain syndromes. Because voltage-gated sodium currents are crucial to the transmission of electrical signals in peripheral sensory neurons, the channels that underlie these currents are attractive targets for pain therapeutics. Sodium currents and channels in peripheral sensory neurons are complex. Multiple-channel isoforms contribute to the macroscopic currents in nociceptive sensory neurons. These different isoforms exhibit substantial variations in their kinetics and pharmacology. Furthermore, sodium current complexity is enhanced by an array of interacting proteins that can substantially modify the properties of voltage-gated sodium channels. Resurgent sodium currents, atypical currents that can enhance recovery from inactivation and neuronal firing, are increasingly being recognized as playing potentially important roles in sensory neuron hyperexcitability and pain sensations. Here we discuss unusual sodium channels and currents that have been identified in nociceptive sensory neurons, describe what is known about the molecular determinants of the complex sodium currents in these neurons. Finally, we provide an overview of therapeutic strategies to target voltage-gated sodium currents in nociceptive neurons.

Conotoxins That Could Provide Analgesia through Voltage Gated Sodium Channel Inhibition

Chronic pain creates a large socio-economic burden around the world. It is physically and mentally debilitating, and many sufferers are unresponsive to current therapeutics. Many drugs that provide pain relief have adverse side effects and addiction liabilities. Therefore, a great need has risen for alternative treatment strategies. One rich source of potential analgesic compounds that has emerged over the past few decades are conotoxins. These toxins are extremely diverse and display selective activity at ion channels. Voltage gated sodium (Na_V) channels are one such group of ion channels that play a significant role in multiple pain pathways. This review will explore the literature around conotoxins that bind Na_V channels and determine their analgesic potential.

Post-translational modifications of voltage-gated sodium channels in chronic pain syndromes

In the peripheral sensory nervous system the neuronal expression of voltage-gated sodium channels (Navs) is very important for the transmission of nociceptive information since they give rise to the upstroke of the action potential (AP). Navs are composed of nine different isoforms with distinct biophysical properties. Studying the mutations associated with the increase or absence of pain sensitivity in humans, as well as other expression studies, have highlighted Nav1.7, Nav1.8, and Nav1.9 as being the most important contributors to the control of nociceptive neuronal electrogenesis. Modulating their expression and/or function can impact the shape of the AP and consequently modify nociceptive transmission, a process that is observed in persistent pain conditions. Post-translational modification (PTM) of Navs is a well-known process that modifies their expression and function. In chronic pain syndromes, the release of inflammatory molecules into the direct environment of dorsal root ganglia (DRG) sensory neurons leads to an abnormal activation of enzymes that induce Navs PTM. The addition of small molecules, i.e., peptides, phosphoryl groups, ubiquitin moieties and/or carbohydrates, can modify the function of Navs in two different ways: via direct physical interference with Nav gating, or via the control of Nav trafficking. Both mechanisms have a profound impact on neuronal excitability. In this review we will discuss the role of Protein Kinase A, B, and C, Mitogen Activated Protein Kinases and Ca++/Calmodulin-dependent Kinase II in peripheral chronic pain syndromes. We will also discuss more recent findings that the ubiquitination of Nav1.7 by Nedd4-2 and the effect of methylglyoxal on Nav1.8 are also implicated in the development of experimental neuropathic pain. We will address the potential roles of other PTMs in chronic pain and highlight the need for further investigation of PTMs of Navs in order to develop new pharmacological tools to alleviate pain.

Pharmacogenetics of analgesic drugs

• Individual variability in pain perception and differences in the efficacy of analgesic drugs are complex phenomena and are partly genetically predetermined.

• Analgesics act in various ways on the peripheral and central pain pathways and are regarded as one of the most valuable but equally dangerous groups of medications.

• While pharmacokinetic properties of drugs, metabolism in particular, have been scrutinised by genotype–phenotype correlation studies, the clinical significance of inherited variants in genes governing pharmacodynamics of analgesics remains largely unexplored (apart from the µ-opioid receptor).

• Lack of replication of the findings from one study to another makes meaningful personalised analgesic regime still a distant future.

• This narrative review will focus on findings related to pharmacogenetics of commonly used analgesic medications and highlight authors’ views on future clinical implications of pharmacogenetics in the context of pharmacological treatment of chronic pain.

Activation of κ Opioid Receptors in Cutaneous Nerve Endings by Conorphin-1, a Novel Subtype-Selective Conopeptide, Does Not Mediate Peripheral Analgesia

Selective activation of peripheral κ opioid receptors (KORs) may overcome the dose-limiting adverse effects of conventional opioid analgesics. We recently developed a vicinal disulfide-stabilized class of peptides with subnanomolar potency at the KOR. The aim of this study was to assess the analgesic effects of one of these peptides, named conorphin-1, in comparison with the prototypical KOR-selective small molecule agonist U-50488, in several rodent pain models. Surprisingly, neither conorphin-1 nor U-50488 were analgesic when delivered peripherally by intraplantar injection at local concentrations expected to fully activate the KOR at cutaneous nerve endings. While U-50488 was analgesic when delivered at high local concentrations, this effect could not be reversed by coadministration with the selective KOR antagonist ML190 or the nonselective opioid antagonist naloxone. Instead, U-50488 likely mediated its peripheral analgesic effect through nonselective inhibition of voltage-gated sodium channels, including peripheral sensory neuron isoforms NaV1.8 and NaV1.7. Our study suggests that targeting the KOR in peripheral sensory nerve endings innervating the skin is not an alternative analgesic approach.

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.

Ca2+ toxicity due to reverse Na+/Ca2+ exchange contributes to degeneration of neurites of DRG neurons induced by a neuropathy-associated Nav1.7 mutation

Gain-of-function missense mutations in voltage-gated sodium channel Nav1.7 have been linked to small-fiber neuropathy, which is characterized by burning pain, dysautonomia and a loss of intraepidermal nerve fibers. However, the mechanistic cascades linking Nav1.7 mutations to axonal degeneration are incompletely understood. The G856D mutation in Nav1.7 produces robust changes in channel biophysical properties, including hyperpolarized activation, depolarized inactivation, and enhanced ramp and persistent currents, which contribute to the hyperexcitability exhibited by neurons containing Nav1.8. We report here that cell bodies and neurites of dorsal root ganglion (DRG) neurons transfected with G856D display increased levels of intracellular Na(+) concentration ([Na(+)]) and intracellular [Ca(2+)] following stimulation with high [K(+)] compared with wild-type (WT) Nav1.7-expressing neurons. Blockade of reverse mode of the sodium/calcium exchanger (NCX) or of sodium channels attenuates [Ca(2+)] transients evoked by high [K(+)] in G856D-expressing DRG cell bodies and neurites. We also show that treatment of WT or G856D-expressing neurites with high [K(+)] or 2-deoxyglucose (2-DG) does not elicit degeneration of these neurites, but that high [K(+)] and 2-DG in combination evokes degeneration of G856D neurites but not WT neurites. Our results also demonstrate that 0 Ca(2+) or blockade of reverse mode of NCX protects G856D-expressing neurites from degeneration when exposed to high [K(+)] and 2-DG. These results point to [Na(+)] overload in DRG neurons expressing mutant G856D Nav1.7, which triggers reverse mode of NCX and contributes to Ca(2+) toxicity, and suggest subtype-specific blockade of Nav1.7 or inhibition of reverse NCX as strategies that might slow or prevent axon degeneration in small-fiber neuropathy.

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.

Contribution of persistent sodium currents to the excitability of tonic firing substantia gelatinosa neurons of the rat

The roles of persistent Na(+) currents (INaP) in intrinsic membrane properties were examined in rat substantia gelatinosa (SG) neurons of the trigeminal subnucleus caudalis using a conventional whole-cell patch clamp technique. In a voltage-clamp mode, riluzole inhibited the slow voltage ramp-induced INaP but had little effect on the peak amplitude of transient Na(+) currents in SG neurons. In a current-clamp mode, most SG neurons exhibited spontaneous action potentials and tonic firing pattern. Riluzole reduced both spontaneous and elicited action potentials in a concentration-dependent manner. The present results suggest that the riluzole-sensitive INaP plays an important role in the excitability of SG neurons and are thus, likely to contribute to the modulation of nociceptive transmission from the orofacial tissues.

Cellular hyper-excitability caused by mutations that alter the activation process of voltage-gated sodium channels

Voltage-gated sodium channels (Na_v) are widely expressed as macro-molecular complexes in both excitable and non-excitable tissues. In excitable tissues, the upstroke of the action potential is the result of the passage of a large and rapid influx of sodium ions through these channels. Na_V dysfunction has been associated with an increasingly wide range of neurological, muscular and cardiac disorders. The purpose of this review is to summarize the recently identified sodium channel mutations that are linked to hyper-excitability phenotypes and associated with the alteration of the activation process of voltage gated sodium channels. Indeed, several clinical manifestations that demonstrate an alteration of tissue excitability were recently shown to be strongly associated with the presence of mutations that affect the activation process of the Na_v. These emerging genotype-phenotype correlations have expanded the clinical spectrum of sodium channelopathies to include disorders which feature a hyper-excitability phenotype that may or may not be associated with a cardiomyopathy. The p.I141V mutation in SCN4A and SCN5A, as well as its homologous p.I136V mutation in SCN9A, are interesting examples of mutations that have been linked to inherited hyperexcitability myotonia, exercise-induced polymorphic ventricular arrhythmias and erythromelalgia, respectively. Regardless of which sodium channel isoform is investigated, the substitution of the isoleucine to valine in the locus 141 induces similar modifications in the biophysical properties of the Na_v by shifting the voltage-dependence of steady state activation toward more negative potentials.

Contribution of mechanosensitive ion channels to somatosensation

Mechanotransduction, the conversion of a mechanical stimulus into an electrical signal, is a central mechanism to several physiological functions in mammals. It relies on the function of mechanosensitive ion channels (MSCs). Although the first single-channel recording from MSCs dates back to 30 years ago, the identity of the genes encoding MSCs has remained largely elusive. Because these channels have an important role in the development of mechanical hypersensitivity, a better understanding of their function may lead to the identification of selective inhibitors and generate novel therapeutic pathways in the treatment of chronic pain. Here, I will describe our current understanding of the role MSCs may play in somatosensation and the potential candidate genes proposed to encode them.

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.

Sensory Neurons, Ion Channels, Inflammation and the Onset of Neuropathic Pain

Neuropathic pain often fails to respond to conventional pain management procedures. here we review the aetiology of neuropathic pain as would result from peripheral neuropathy or injury. We show that inflammatory mediators released from damaged nerves and tissue are responsible for triggering ectopic activity in primary afferents and that this, in turn, provokes increased spinal cord activity and the development of ‘central sensitization’. Although evidence is mounting to support the role of interleukin-1β, prostaglandins and other cytokines in the onset of neuropathic pain, the clinical efficacy of drugs which antagonize or prevent the actions of these mediators is yet to be determined. basic science findings do, however, support the use of pre-emptive analgesia during procedures which involve nerve manipulation and the use of anti-inflammatory steroids as soon as possible following traumatic nerve injury.

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.

Correlation of the electrophysiological profiles and sodium channel transcripts of individual rat dorsal root ganglia neurons

Voltage gated sodium channels (Na_v channels) play an important role in nociceptive transmission. They are intimately tied to the genesis and transmission of neuronal firing. Five different isoforms (Na_v1.3, Na_v1.6, Na_v1.7, Na_v1.8, and Na_v1.9) have been linked to nociceptive responses. A change in the biophysical properties of these channels or in their expression levels occurs in different pathological pain states. However, the precise involvement of the isoforms in the genesis and transmission of nociceptive responses is unknown. The aim of the present study was to investigate the synergy between the different populations of Na_v channels that give individual neurons a unique electrophysical profile. We used the patch-clamp technique in the whole-cell configuration to record Na_v currents and action potentials from acutely dissociated small diameter DRG neurons (<30 μm) from adult rats. We also performed single cell qPCR on the same neurons. Our results revealed that there is a strong correlation between Na_v currents and mRNA transcripts in individual neurons. A cluster analysis showed that subgroups formed by Na_v channel transcripts by mRNA quantification have different biophysical properties. In addition, the firing frequency of the neurons was not affected by the relative populations of Na_v channel. The synergy between populations of Na_v channel in individual small diameter DRG neurons gives each neuron a unique electrophysiological profile. The Na_v channel remodeling that occurs in different pathological pain states may be responsible for the sensitization of the neurons.

Alteration of the mu opioid receptor: Ca2+ channel signaling pathway in a subset of rat sensory neurons following chronic femoral artery occlusion

The exercise pressor reflex, a crucial component of the cardiovascular response under physiological and pathophysiological states, is activated via metabolic and mechanical mediators that originate from contracting muscles and stimulate group III and IV afferents. We reported previously that stimulation of mu opioid receptors (MOR), expressed in both afferents, led to a significant attenuation of the reflex in rats whose femoral arteries had been occluded for 72 h. The present study examined the effect of arterial occlusion on the signaling components involved in the opioid-mediated modulation of Ca(2+) channels in rat dorsal root ganglion neurons innervating the triceps surae muscles. We focused on neurons that were transfected with cDNA coding for enhanced green fluorescent protein whose expression is driven by the voltage-gated Na(+) channel 1.8 (Na(V)1.8) promoter region, a channel expressed primarily in nociceptive neurons. With the use of a small interference RNA approach, our results show that the pertussis toxin-sensitive Gα(i3) subunit couples MOR with Ca(2+) channels. We observed a significant leftward shift of the MOR agonist [D-Ala2-N-Me-Phe4-Glycol5]-enkephalin concentration-response relationship in neurons isolated from rats with occluded arteries compared with those that were perfused freely. Femoral occlusion did not affect Ca(2+) channel density or the fraction of the main Ca(2+) channel subtype. Furthermore, Western blotting analysis indicated that the leftward shift did not result from either increased Gα(i3) or MOR expression. Finally, all neurons from both groups exhibited an inward current following exposure of the transient potential receptor vanilloid 1 (TRPV1) agonist, 8-methyl-N-vanillyl-6-nonenamide. These findings suggest that sensory neurons mediating the exercise pressor reflex express Na(V)1.8 and TRPV1 channels, and femoral occlusion alters the MOR pharmacological profile.

The discovery of benzenesulfonamide-based potent and selective inhibitors of voltage-gated sodium channel Na(v)1.7

The voltage gated sodium channel Nav1.7 represents an interesting target for the treatment of pain. Human genetic studies have identified the crucial role of Nav1.7 in pain signaling. Herein, we report the design and synthesis of a novel series of benzenesulfonamide-based Nav1.7 inhibitors. Structural-activity relationship (SAR) studies were undertaken towards improving Nav1.7 activity and minimizing CYP inhibition. These efforts resulted in the identification of compound 12k, a highly potent Nav1.7 inhibitor with a thousand-fold selectivity over Nav1.5 and negligible CYP inhibition.

Pharmacological fractionation of tetrodotoxin-sensitive sodium currents in rat dorsal root ganglion neurons by μ-conotoxins

BACKGROUND AND PURPOSE

Adult rat dorsal root ganglion (DRG) neurons normally express transcripts for five isoforms of the α-subunit of voltage-gated sodium channels: NaV 1.1, 1.6, 1.7, 1.8 and 1.9. Tetrodotoxin (TTX) readily blocks all but NaV 1.8 and 1.9, and pharmacological agents that discriminate among the TTX-sensitive NaV 1-isoforms are scarce. Recently, we used the activity profile of a panel of μ-conotoxins in blocking cloned rodent NaV 1-isoforms expressed in Xenopus laevis oocytes to conclude that action potentials of A- and C-fibres in rat sciatic nerve were, respectively, mediated primarily by NaV 1.6 and NaV 1.7.

EXPERIMENTAL APPROACH

We used three μ-conotoxins, μ-TIIIA, μ-PIIIA and μ-SmIIIA, applied individually and in combinations, to pharmacologically differentiate the TTX-sensitive INa of voltage-clamped neurons acutely dissociated from adult rat DRG. We examined only small and large neurons whose respective INa were >50% and >80% TTX-sensitive.

KEY RESULTS

In both small and large neurons, the ability of the toxins to block TTX-sensitive INa was μ-TIIIA < μ-PIIIA < μ-SmIIIA, with the latter blocking ≳90%. Comparison of the toxin-susceptibility profiles of the neuronal INa with recently acquired profiles of rat NaV 1-isoforms, co-expressed with various NaV β-subunits in X. laevis oocytes, were consistent: NaV 1.1, 1.6 and 1.7 could account for all of the TTX-sensitive INa , with NaV 1.1 < NaV 1.6 < NaV 1.7 for small neurons and NaV 1.7 < NaV 1.1 < NaV 1.6 for large neurons.

CONCLUSIONS AND IMPLICATIONS

Combinations of μ-conotoxins can be used to determine the probable NaV 1-isoforms underlying the INa in DRG neurons. Preliminary experiments with sympathetic neurons suggest that this approach is extendable to other neurons.

Channelopathies, painful neuropathy, and diabetes: which way does the causal arrow point?

Diabetes mellitus, a major global health problem, is commonly associated with painful peripheral neuropathy, which can substantially erode quality of life. Despite its clinical importance, the pathophysiology of painful diabetic neuropathy is incompletely understood. It has traditionally been thought that diabetes may cause neuropathy in patients with appropriate genetic makeup. Here, we propose a hypothesis whereby painful neuropathy is not a complication of diabetes, but rather occurs as a result of mutations that, in parallel, confer vulnerability to injury in pancreatic β cells and pain-signaling dorsal root ganglion (DRG) neurons. We suggest that mutations of sodium channel NaV1.7, which is present in both cell types, may increase susceptibility for development of diabetes via β cell injury and produce painful neuropathy via a distinct effect on DRG neurons.

Painful neuropathies: the emerging role of sodium channelopathies

Pain is a frequent debilitating feature reported in peripheral neuropathies with involvement of small nerve (Aδ and C) fibers. Voltage-gated sodium channels are responsible for the generation and conduction of action potentials in the peripheral nociceptive neuronal pathway where NaV 1.7, NaV 1.8, and NaV 1.9 sodium channels (encoded by SCN9A, SCN10A, and SCN11A) are preferentially expressed. The human genetic pain conditions inherited erythromelalgia and paroxysmal extreme pain disorder were the first to be linked to gain-of-function SCN9A mutations. Recent studies have expanded this spectrum with gain-of-function SCN9A mutations in patients with small fiber neuropathy and in a new syndrome of pain, dysautonomia, and small hands and small feet (acromesomelia). In addition, painful neuropathies have been recently linked to SCN10A mutations. Patch-clamp studies have shown that the effect of SCN9A mutations is dependent upon the cell-type background. The functional effects of a mutation in dorsal root ganglion (DRG) neurons and sympathetic neuron cells may differ per mutation, reflecting the pattern of expression of autonomic symptoms in patients with painful neuropathies who carry the mutation in question. Peripheral neuropathies may not always be length-dependent, as demonstrated in patients with initial facial and scalp pain symptoms with SCN9A mutations showing hyperexcitability in both trigeminal ganglion and DRG neurons. There is some evidence suggesting that gain-of-function SCN9A mutations can lead to degeneration of peripheral axons. This review will focus on the emerging role of sodium channelopathies in painful peripheral neuropathies, which could serve as a basis for novel therapeutic strategies.

NaV1.5 sodium channel window currents contribute to spontaneous firing in olfactory sensory neurons

Olfactory sensory neurons (OSNs) fire spontaneously as well as in response to odor; both forms of firing are physiologically important. We studied voltage-gated Na(+) channels in OSNs to assess their role in spontaneous activity. Whole cell patch-clamp recordings from OSNs demonstrated both tetrodotoxin-sensitive and tetrodotoxin-resistant components of Na(+) current. RT-PCR showed mRNAs for five of the nine different Na(+) channel α-subunits in olfactory tissue; only one was tetrodotoxin resistant, the so-called cardiac subtype NaV1.5. Immunohistochemical analysis indicated that NaV1.5 is present in the apical knob of OSN dendrites but not in the axon. The NaV1.5 channels in OSNs exhibited two important features: 1) a half-inactivation potential near -100 mV, well below the resting potential, and 2) a window current centered near the resting potential. The negative half-inactivation potential renders most NaV1.5 channels in OSNs inactivated at the resting potential, while the window current indicates that the minor fraction of noninactivated NaV1.5 channels have a small probability of opening spontaneously at the resting potential. When the tetrodotoxin-sensitive Na(+) channels were blocked by nanomolar tetrodotoxin at the resting potential, spontaneous firing was suppressed as expected. Furthermore, selectively blocking NaV1.5 channels with Zn(2+) in the absence of tetrodotoxin also suppressed spontaneous firing, indicating that NaV1.5 channels are required for spontaneous activity despite resting inactivation. We propose that window currents produced by noninactivated NaV1.5 channels are one source of the generator potentials that trigger spontaneous firing, while the upstroke and propagation of action potentials in OSNs are borne by the tetrodotoxin-sensitive Na(+) channel subtypes.

[Update on the pharmacology and effects of local anesthetics]

Local anesthetics (LA) are broadly used in all disciplines and it could be considered that relatively little is reflected on the mechanisms of action of this old substance group. However, several molecular mechanisms of LAs mediating wanted and unwanted effects remain to be explored. Furthermore, the number of indications for application of LAs seems to be expanding. The local anesthetic effect of LAs is primarily mediated by a potent inhibition of voltage-gated sodium channels. However, this effect is due to much more than the interaction of LAs with one single molecule. Most recent studies indicated that the development of selective local anesthetics might be possible and LAs also interact with several other membrane molecules. Although the relevance of these effects is still unclear, they might play a role in systemic analgesia, tissue protection and anti-inflammatory effects of LA. The therapeutic index of systemically applied LA is very narrow. Systemic application is formally not permitted because the impending systemic toxicity is still a life-threatening complication. Although the cardiac and central nervous toxicity at least partly result from an unselective block of neuronal and cardiac sodium channels, preclinical studies suggest the involvement of several mechanisms. A local LA toxicity is less clinically impressive; however, all LAs induce a significant tissue toxicity for which the underlying mechanisms have been partly identified. This review reports on recent findings on mechanisms and on the clinical relevance of some LA-induced effects which are of relevance for anesthesiological activities.

Dynamic-clamp analysis of wild-type human Nav1.7 and erythromelalgia mutant channel L858H

The link between sodium channel Nav1.7 and pain has been strengthened by identification of gain-of-function mutations in patients with inherited erythromelalgia (IEM), a genetic model of neuropathic pain in humans. A firm mechanistic link to nociceptor dysfunction has been precluded because assessments of the effect of the mutations on nociceptor function have thus far depended on electrophysiological recordings from dorsal root ganglia (DRG) neurons transfected with wild-type (WT) or mutant Nav1.7 channels, which do not permit accurate calibration of the level of Nav1.7 channel expression. Here, we report an analysis of the function of WT Nav1.7 and IEM L858H mutation within small DRG neurons using dynamic-clamp. We describe the functional relationship between current threshold for action potential generation and the level of WT Nav1.7 conductance in primary nociceptive neurons and demonstrate the basis for hyperexcitability at physiologically relevant levels of L858H channel conductance. We demonstrate that the L858H mutation, when modeled using dynamic-clamp at physiological levels within DRG neurons, produces a dramatically enhanced persistent current, resulting in 27-fold amplification of net sodium influx during subthreshold depolarizations and even greater amplification during interspike intervals, which provide a mechanistic basis for reduced current threshold and enhanced action potential firing probability. These results show, for the first time, a linear correlation between the level of Nav1.7 conductance and current threshold in DRG neurons. Our observations demonstrate changes in sodium influx that provide a mechanistic link between the altered biophysical properties of a mutant Nav1.7 channel and nociceptor hyperexcitability underlying the pain phenotype in IEM.

Up-regulation of NaV1.7 sodium channels expression by tumor necrosis factor-α in cultured bovine adrenal chromaffin cells and rat dorsal root ganglion neurons

BACKGROUND

Tumor necrosis factor-α (TNF-α) is not only a key regulator of inflammatory response but also an important pain modulator. TNF-α enhances both tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant Na channel currents in dorsal root ganglion (DRG) neurons. However, it remains unknown whether TNF-α affects the function and expression of the TTX-S NaV1.7 Na channel, which plays crucial roles in pain generation.

METHODS

We used cultured bovine adrenal chromaffin cells expressing the NaV1.7 Na channel isoform and compared them with cultured rat DRG neurons. The expression of TNF receptor 1 and 2 (TNFR1 and TNFR2) in adrenal chromaffin cells was studied by Semiquantitative reverse transcription-polymerase chain reaction. The effects of TNF-α on the expression of NaV1.7 were examined with reverse transcription-polymerase chain reaction and Western blot analysis. Results were expressed as mean ± SEM.

RESULTS

TNFR1 and TNFR2 were expressed in adrenal chromaffin cells, as well as reported in DRG neurons. TNF-α up-regulated NaV1.7 mRNA by 132% ± 9% (N = 5, P = 0.004) in adrenal chromaffin cells, as well as 117% ± 2% (N = 5, P < 0.0001) in DRG neurons. Western blot analysis showed that TNF-α increased NaV1.7 protein up to 166% ± 24% (N = 5, corrected P < 0.0001) in adrenal chromaffin cells, concentration- and time-dependently.

CONCLUSIONS

TNF-α up-regulated NaV1.7 mRNA in both adrenal chromaffin cells and DRG neurons. In addition, TNF-α up-regulated the protein expression of the TTX-S NaV1.7 channel in adrenal chromaffin cells. Our findings may contribute to understanding the peripheral nociceptive mechanism of TNF-α.

Voltage-gated sodium channel expression in mouse DRG after SNI leads to re-evaluation of projections of injured fibers

Background

Dysregulation of voltage-gated sodium channels (Na_vs) is believed to play a major role in nerve fiber hyperexcitability associated with neuropathic pain. A complete transcriptional characterization of the different isoforms of Na_vs under normal and pathological conditions had never been performed on mice, despite their widespread use in pain research. Na_vs mRNA levels in mouse dorsal root ganglia (DRG) were studied in the spared nerve injury (SNI) and spinal nerve ligation (SNL) models of neuropathic pain. In the SNI model, injured and non-injured neurons were intermingled in lumbar DRG, which were pooled to increase the tissue available for experiments.

Results

A strong downregulation was observed for every Na_vs isoform expressed except for Na_v1.2; even Na_v1.3, known to be upregulated in rat neuropathic pain models, was lower in the SNI mouse model. This suggests differences between these two species. In the SNL model, where the cell bodies of injured and non-injured fibers are anatomically separated between different DRG, most Na_vs were observed to be downregulated in the L5 DRG receiving axotomized fibers. Transcription was then investigated independently in the L3, L4 and L5 DRG in the SNI model, and an important downregulation of many Na_vs isoforms was observed in the L3 DRG, suggesting the presence of numerous injured neurons there after SNI. Consequently, the proportion of axotomized neurons in the L3, L4 and L5 DRG after SNI was characterized by studying the expression of activating transcription factor 3 (ATF3). Using this marker of nerve injury confirmed that most injured fibers find their cell bodies in the L3 and L4 DRG after SNI in C57BL/6 J mice; this contrasts with their L4 and L5 DRG localization in rats. The spared sural nerve, through which pain hypersensitivity is measured in behavioral studies, mostly projects into the L4 and L5 DRG.

Conclusions

The complex regulation of Na_vs, together with the anatomical rostral shift of the DRG harboring injured fibers in C57BL/6 J mice, emphasize that caution is necessary and preliminary anatomical experiments should be carried out for gene and protein expression studies after SNI in mouse strains.

Chronic stress mediators act synergistically on colonic nociceptive mouse dorsal root ganglia neurons to increase excitability

BACKGROUND

Stress hormones can signal to colonic dorsal root ganglia (DRG) neurons and may play a role in sustained hyperexcitability of nociceptors.

METHODS

Mouse DRG neurons were exposed overnight to epinephrine (Epi) 5 nM and/or corticosterone (Cort) 1 μM or prior water-avoidance stress. Patch clamp recordings, visceromotor reflexes (VMRs) and molecular studies were conducted.

KEY RESULTS

Water-avoidance stress induced neuronal hyperexcitability. Incubation of DRG neurons in both Cort and Epi (but neither alone) induced hyperexcitability (rheobase decreased 51%, p < 0.05; action potential discharge increased 95%, p < 0.01); this was blocked by antagonists of the β2 adrenoreceptor (butoxamine, But) and Cort receptor (mifepristone) in combination or alone. Stress hormones enhanced voltage-gated Nav 1.7 currents (p < 0.05) and suppressed IA (p < 0.0001) and IK+ (p < 0.05) currents. Furthermore, stress hormones increased DRG β2 adrenoreceptor mRNA (59%, p = 0.007) and protein (125%, p < 0.05), also Nav 1.7 transcript (45%, p = 0.004) and protein (114%, p = 0.002). In whole-animal studies, the WAS hyperexcitability of DRG neurons was blocked by antagonists of the β2 and glucocorticoid receptors (GCR) but together they paradoxically increased VMRs to colorectal balloon distension.

CONCLUSIONS & INFERENCES

Stress mediators Epi and Cort activate β2 and GCR on DRG neurons which synergistically induce hyperexcitability of nociceptive DRG neurons and cause corresponding changes in voltage-gated Na(+) and K(+) currents. Furthermore, they increase the expression of β2 adrenoreceptors and Nav1.7 channels, suggesting transcriptional changes could contribute to sustained signaling following stress. The paradoxical effects of But and mifepristone in electrophysiological compared to VMR testing may reflect different peripheral and central actions on sensory signaling.

Repeated functional convergent effects of NaV1.7 on acid insensitivity in hibernating mammals

Hibernating mammals need to be insensitive to acid in order to cope with conditions of high CO2; however, the molecular basis of acid tolerance remains largely unknown. The African naked mole-rat (Heterocephalus glaber) and hibernating mammals share similar environments and physiological features. In the naked mole-rat, acid insensitivity has been shown to be conferred by the functional motif of the sodium ion channel NaV1.7. There is now an opportunity to evaluate acid insensitivity in other taxa. In this study, we tested for functional convergence of NaV1.7 in 71 species of mammals, including 22 species that hibernate. Our analyses revealed a functional convergence of amino acid sequences, which occurred at least six times independently in mammals that hibernate. Evolutionary analyses determined that the convergence results from both parallel and divergent evolution of residues in the functional motif. Our findings not only identify the functional molecules responsible for acid insensitivity in hibernating mammals, but also open new avenues to elucidate the molecular underpinnings of acid insensitivity in mammals.

Voltage-Gated Sodium Channels as Insecticide Targets

Voltage-gated sodium channels are critical for the generation and propagation of action potentials. They are the primary target of several classes of insecticides, including DDT, pyrethroids and sodium channel blocker insecticides (SCBIs). DDT and pyrethroids preferably bind to open sodium channels and stabilize the open state, causing prolonged currents. In contrast, SCBIs block sodium channels by binding to the inactivated state. Many sodium channel mutations are associated with knockdown resistance (kdr) to DDT and pyrethroids in diverse arthropod pests. Functional characterization of kdr mutations together with computational modelling predicts dual pyrethroid receptor sites on sodium channels. In contrast, the molecular determinants of the SCBI receptor site remain largely unknown. In this review, we summarize current knowledge about the molecular mechanisms of action of pyrethroids and SCBIs, and highlight the differences in the molecular interaction of these insecticides with insect versus mammalian sodium channels.

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.

Inhibition of tetrodotoxin-resistant sodium current in dorsal root ganglia neurons mediated by D1/D5 dopamine receptors

Background

Dopaminergic fibers originating from area A11 of the hypothalamus project to different levels of the spinal cord and represent the major source of dopamine. In addition, tyrosine hydroxylase, the rate-limiting enzyme for the synthesis of catecholamines, is expressed in 8-10% of dorsal root ganglia (DRG) neurons, suggesting that dopamine may be released in the dorsal root ganglia. Dopamine has been shown to modulate calcium current in DRG neurons, but the effects of dopamine on sodium current and on the firing properties of small DRG neurons are poorly understood.

Results

The effects of dopamine and dopamine receptor agonists were tested on the tetrodotoxin-resistant (TTX-R) sodium current recorded from acutely dissociated small (diameter ≤ 25 μm) DRG neurons. Dopamine (20 μM) and SKF 81297 (10 μM) caused inhibition of TTX-R sodium current in small DRG neurons by 23% and 37%, respectively. In contrast, quinpirole (20 μM) had no effects on the TTX-R sodium current. Inhibition by SKF 81297 of the TTX-R sodium current was not affected when the protein kinase A (PKA) activity was blocked with the PKA inhibitory peptide (6–22), but was greatly reduced when the protein kinase C (PKC) activity was blocked with the PKC inhibitory peptide (19–36), suggesting that activation of D1/D5 dopamine receptors is linked to PKC activity. Expression of D1and D5 dopamine receptors in small DRG neurons, but not D2 dopamine receptors, was confirmed by Western blotting and immunofluorescence analysis. In current clamp experiments, the number of action potentials elicited in small DRG neurons by current injection was reduced by ~ 30% by SKF 81297.

Conclusions

We conclude that activation of D1/D5 dopamine receptors inhibits TTX-R sodium current in unmyelinated nociceptive neurons and dampens their intrinsic excitability by reducing the number of action potentials in response to stimulus. Increasing or decreasing levels of dopamine in the dorsal root ganglia may serve to adjust the sensitivity of nociceptors to noxious stimuli.

Co-expression of Na(V)β subunits alters the kinetics of inhibition of voltage-gated sodium channels by pore-blocking μ-conotoxins

BACKGROUND AND PURPOSE

Voltage-gated sodium channels (VGSCs) are assembled from two classes of subunits, a pore-bearing α-subunit (NaV 1) and one or two accessory β-subunits (NaV βs). Neurons in mammals can express one or more of seven isoforms of NaV 1 and one or more of four isoforms of NaV β. The peptide μ-conotoxins, like the guanidinium alkaloids tetrodotoxin (TTX) and saxitoxin (STX), inhibit VGSCs by blocking the pore in NaV 1. Hitherto, the effects of NaV β-subunit co-expression on the activity of these toxins have not been comprehensively assessed.

EXPERIMENTAL APPROACH

Four μ-conotoxins (μ-TIIIA, μ-PIIIA, μ-SmIIIA and μ-KIIIA), TTX and STX were tested against NaV 1.1, 1.2, 1.6 or 1.7, each co-expressed in Xenopus laevis oocytes with one of NaV β1, β2, β3 or β4 and, for NaV 1.7, binary combinations of thereof.

KEY RESULTS

Co-expression of NaV β-subunits modifies the block by μ-conotoxins: in general, NaV β1 or β3 co-expression tended to increase kon (in the most extreme instance by ninefold), whereas NaV β2 or β4 co-expression decreased kon (in the most extreme instance by 240-fold). In contrast, the block by TTX and STX was only minimally, if at all, affected by NaV β-subunit co-expression. Tests of NaV β1 : β2 chimeras co-expressed with NaV 1.7 suggest that the extracellular portion of the NaV β subunit is largely responsible for altering μ-conotoxin kinetics.

CONCLUSIONS AND IMPLICATIONS

These results are the first indication that NaV β subunit co-expression can markedly influence μ-conotoxin binding and, by extension, the outer vestibule of the pore of VGSCs. μ-Conotoxins could, in principle, be used to pharmacologically probe the NaV β subunit composition of endogenously expressed VGSCs.

Voltage gated sodium and calcium channel blockers for the treatment of chronic inflammatory pain

The inflammatory response is a natural response of the body that occurs immediately following tissue damage, which may be due to injury, infection or disease. The acute inflammatory response is an essential mechanism that promotes healing and a key aspect is the ensuing pain, which warns the subject to protect the site of injury. Thus, it is common to see a zone of primary sensitization as well as consequential central sensitization that generally, is maintained by a peripheral drive from the zone of tissue injury. Inflammation associated with chronic pain states, such as rheumatoid and osteoarthritis, cancer and migraine etc. is deleterious to health and often debilitating for the patient. Thus there is a large unmet clinical need. The mechanisms underlying both acute and chronic inflammatory pain are extensive and complex, involving a diversity of cell types, receptors and proteins. Among these the contribution of voltage gated sodium and calcium channels on peripheral nociceptors is critical for nociceptive transmission beyond the peripheral transducers and changes in their distribution, accumulation, clustering and functional activities have been linked to both inflammatory and neuropathic pain. The latter has been the main area for trials and use of drugs that modulate ion channels such as carbamazepine and gabapentin, but given the large peripheral drive that follows tissue damage, there is a clear rationale for blocking voltage gated sodium and calcium channels in these pain states. It has been hypothesized that pain of inflammatory origin may evolve into a condition that resembles neuropathic pain, but mixed pains such as low back pain and cancer pain often include elements of both pain states. This review considers the therapeutic potential for sodium and calcium channel blockers for the treatment of chronic inflammatory pain states.

The κ-opioid receptor agonist U-50488 blocks Ca2+ channels in a voltage- and G protein-independent manner in sensory neurons

BACKGROUND AND OBJECTIVES

κ-Opioid receptor (κ-OR) activation is known to play a role in analgesia and central sedation. The purpose of the present study was to examine the effect of the κ-OR agonist, U-50488 (an arylacetamide), on Ca channel currents and the signaling proteins involved in acutely isolated rat dorsal root ganglion (DRG) neurons expressing the putative promoter region of the tetrodotoxin-resistant Na channel (NaV 1.8) that is known to be involved in pain transmission.

METHODS

Acutely isolated rat DRG neurons were transfected with cDNA coding for enhanced green fluorescent protein (EGFP), whose expression is driven by the NaV 1.8 promoter region. Thereafter, the whole-cell variant of the patch-clamp technique was used to record Ca channel currents in neurons expressing EGFP.

RESULTS

Exposure of EGFP-expressing DRG neurons to U-50488 (0.3-40 μM) led to voltage-independent inhibition of the Ca channel currents. The modulation of the Ca currents did not appear to be mediated by the Gα protein subfamilies: Gαi/o, Gαs, Gαq/11, Gα14, and Gαz. Furthermore, dialysis of the hydrolysis-resistant GDP analog, GDP-β-S (1 mM), did not affect the U-50488-mediated blocking effect, ruling out involvement of other G protein subunits. Finally, U-50488 (20 μM) blocked Ca channels heterologously expressed in HeLa cells that do not express κ-OR.

CONCLUSION

These results suggest that the antinociceptive actions mediated by U-50488 are likely due to both a direct block of Ca channels in sensory neurons as well as G protein modulation of Ca currents via κ-OR-expressing neurons.

TMEM16C facilitates Na(+)-activated K+ currents in rat sensory neurons and regulates pain processing

TMEM16C belongs to the TMEM16 family, which includes the Ca²⁺-activated Cl^– channels (CaCCs) TMEM16A and TMEM16B and a small conductance Ca²⁺-activated, non-selective cation channel (SCAN), TMEM16F. Here we report that in rat dorsal root ganglia (DRG) TMEM16C is expressed mainly in the IB4 positive, non-peptidergic nociceptors that also express the sodium-activated potassium (KNa) channel Slack. Together these channel proteins promote KNa channel activity and dampen neuronal excitability. DRG from TMEM16C knock out rats have reduced Slack expression, broadened action potential and increased excitability. Moreover, the TMEM16C knock out rats as well as rats with Slack knockdown via intrathecal injection of siRNA exhibit increased thermal and mechanical sensitivity. Experiments involving heterologous expression in HEK293 cells further show that TMEM16C modulates the single channel activity of Slack channels and increases its sodium sensitivity. Our study thus reveals that TMEM16C enhances KNa channel activity in DRG neurons and regulate the processing of pain messages.

Persistent mechanical allodynia positively correlates with an increase in activated microglia and increased P-p38 mitogen-activated protein kinase activation in streptozotocin-induced diabetic rats

BACKGROUND

In experimental early painful diabetic neuropathy, persistent hyperglycaemia induces dys-regulated sodium channel (Navs) expression in the dorsal root ganglion (DRG) and activates microglia in the spinal dorsal horn (SDH). However, information on diabetes-induced chronic neuropathic pain is limited. Therefore, we investigated abnormal Navs in the DRG and activated glial cells in the SDH of diabetic rats with chronic neuropathic pain.

METHODS

Sixty-six rats were divided into diabetic and control groups: control rats (n = 18; 1 mL of normal saline via the right femoral vein) and diabetic rats [n = 48; 60 mg/kg streptozotocin (STZ) via the right femoral vein]. Hindpaw behavioural tests, Navs expression in the DRG, activation of glial cells in the SDH and the number of neurons in the SDH were measured at 1 and 2 weeks, and 1, 2, 3 and 6 months following saline and STZ administration.

RESULTS

All diabetic rats exhibited hyperglycaemia from day 7 to 6 months. The diabetic rats decreased withdrawal threshold to mechanical stimuli but had blunted responses to thermal stimuli. Consistent up-regulation of Nav1.3 and down-regulation of Nav1.8 was observed. Microglial cells were activated early in the SDH and lasted for 6 months. A positive correlation between mechanical allodynia, Nav1.3 and microglial activation was observed. In addition, microglia activation in the SDH of STZ-induced diabetes was mediated, in part, by phosphorylation of p-38 mitogen-activated protein kinase.

CONCLUSIONS

Diabetic rats showed hindpaw mechanical allodynia for 6 months. Persistent mechanical allodynia was positively associated with sustained increased activation of Nav1.3 and increased p38 phosphorylation in activated microglia.

Indoxacarb, Metaflumizone, and Other Sodium Channel Inhibitor Insecticides: Mechanism and Site of Action on Mammalian Voltage-Gated Sodium Channels

Sodium channel inhibitor (SCI) insecticides were discovered almost four decades ago but have only recently yielded important commercial products (eg., indoxacarb and metaflumizone). SCI insecticides inhibit sodium channel function by binding selectively to slow-inactivated (non-conducting) sodium channel states. Characterization of the action of SCI insecticides on mammalian sodium channels using both biochemical and electrophysiological approaches demonstrates that they bind at or near a drug receptor site, the "local anesthetic (LA) receptor." This mechanism and site of action on sodium channels differentiates SCI insecticides from other insecticidal agents that act on sodium channels. However, SCI insecticides share a common mode of action with drugs currently under investigation as anticonvulsants and treatments for neuropathic pain. In this paper we summarize the development of the SCI insecticide class and the evidence that this structurally diverse group of compounds have a common mode of action on sodium channels. We then review research that has used site-directed mutagenesis and heterologous expression of cloned mammalian sodium channels in Xenopus laevis oocytes to further elucidate the site and mechanism of action of SCI insecticides. The results of these studies provide new insight into the mechanism of action of SCI insecticides on voltage-gated sodium channels, the location of the SCI insecticide receptor, and its relationship to the LA receptor that binds therapeutic SCI agents.

Effect of amitriptyline on tetrodotoxin-resistant Nav1.9 currents in nociceptive trigeminal neurons

Background

Amitriptyline (AMI) is tricyclic antidepressant that has been widely used to manage various chronic pains such as migraines. Its efficacy is attributed to its blockade of voltage-gated sodium channels (VGSCs). However, the effects of AMI on the tetrodotoxin-resistant (TTX-r) sodium channel Na_v1.9 currents have been unclear to present.

Results

Using a whole-cell patch clamp technique, this study showed that AMI efficiently inhibited Na_v1.9 currents in a concentration-dependent manner and had an IC₅₀ of 15.16 μM in acute isolated trigeminal ganglion (TG) neurons of the rats. 10 μM AMI significantly shifted the steady-state inactivation of Na_v1.9 channels in the hyperpolarizing direction without affecting voltage-dependent activation. Surprisingly, neither 10 nor 50 μM AMI caused a use-dependent blockade of Na_v1.9 currents elicited by 60 pulses at 1 Hz.

Conclusion

These data suggest that AMI is a state-selective blocker of Na_v1.9 channels in rat nociceptive trigeminal neurons, which likely contributes to the efficacy of AMI in treating various pains, including migraines.

Novel pathogenic pathways in diabetic neuropathy

Diabetic peripheral neuropathy (DPN) is a common complication affecting more than one third of diabetes mellitus (DM) patients. Although all cellular components participating in peripheral nerve function are exposed to and affected by the metabolic consequences of DM, nodal regions, areas of intense interactions between Schwann cells and axons, may be particularly sensitive to DM-induced alterations. Nodes are enriched in insulin receptors, glucose transporters, Na(+) and K(+) channels, and mitochondria, all implicated in the development and progression of DPN. Latest results particularly reinforce the idea that changes in ion-channel function and energy metabolism, both of which depend on axon-glia crosstalk, are among the important contributors to DPN. These insights provide a basis for new therapeutic approaches aimed at delaying or reversing DPN.

An animal model of oxaliplatin-induced cold allodynia reveals a crucial role for Nav1.6 in peripheral pain pathways

Cold allodynia, pain in response to cooling, occurs during or within hours of oxaliplatin infusion and is thought to arise from a direct effect of oxaliplatin on peripheral sensory neurons. To characterize the pathophysiological mechanisms underlying acute oxaliplatin-induced cold allodynia, we established a new intraplantar oxaliplatin mouse model that rapidly developed long-lasting cold allodynia mediated entirely through tetrodotoxin-sensitive Nav pathways. Using selective inhibitors and knockout animals, we found that Nav1.6 was the key isoform involved, while thermosensitive transient receptor potential channels were not involved. Consistent with a crucial role for delayed-rectifier potassium channels in excitability in response to cold, intraplantar administration of the K(+)-channel blocker 4-aminopyridine mimicked oxaliplatin-induced cold allodynia and was also inhibited by Nav1.6 blockers. Intraplantar injection of the Nav1.6 activator Cn2 elicited spontaneous pain, mechanical allodynia, and enhanced 4-aminopyridine-induced cold allodynia. These findings provide behavioural evidence for a crucial role of Nav1.6 in multiple peripheral pain pathways including cold allodynia.

Multiple actions of phi-LITX-Lw1a on ryanodine receptors reveal a functional link between scorpion DDH and ICK toxins

We recently reported the isolation of a scorpion toxin named U1-liotoxin-Lw1a (U1-LITX-Lw1a) that adopts an unusual 3D fold termed the disulfide-directed hairpin (DDH) motif, which is the proposed evolutionary structural precursor of the three-disulfide-containing inhibitor cystine knot (ICK) motif found widely in animals and plants. Here we reveal that U1-LITX-Lw1a targets and activates the mammalian ryanodine receptor intracellular calcium release channel (RyR) with high (fM) potency and provides a functional link between DDH and ICK scorpion toxins. Moreover, U1-LITX-Lw1a, now described as ϕ-liotoxin-Lw1a (ϕ-LITX-Lw1a), has a similar mode of action on RyRs as scorpion calcines, although with significantly greater potency, inducing full channel openings at lower (fM) toxin concentrations whereas at higher pM concentrations increasing the frequency and duration of channel openings to a submaximal state. In addition, we show that the C-terminal residue of ϕ-LITX-Lw1a is crucial for the increase in full receptor openings but not for the increase in receptor subconductance opening, thereby supporting the two-binding-site hypothesis of scorpion toxins on RyRs. ϕ-LITX-Lw1a has potential both as a pharmacological tool and as a lead molecule for the treatment of human diseases that involve RyRs, such as malignant hyperthermia and polymorphic ventricular tachycardia.

A 'toothache tree' alkylamide inhibits Aδ mechanonociceptors to alleviate mechanical pain

In traditional medicine, the 'toothache tree' and other plants of the Zanthoxylum genus have been used to treat inflammatory pain conditions, such as toothache and rheumatoid arthritis. Here we examined the cellular and molecular mechanisms underlying the analgesic properties of hydroxy-α-sanshool, the active alkylamide produced by Zanthoxylum plants. Consistent with its analgesic effects in humans, sanshool treatment in mice caused a selective attenuation of mechanical sensitivity under naïve and inflammatory conditions, with no effect on thermal sensitivity. To elucidate the molecular mechanisms by which sanshool attenuates mechanical pain, we performed single fibre recordings, calcium imaging and whole-cell electrophysiology of cultured sensory neurons. We found that: (1) sanshool potently inhibits Aδ mechanonociceptors that mediate both sharp acute pain and inflammatory pain; (2) sanshool inhibits action potential firing by blocking voltage-gated sodium currents in a subset of somatosensory neurons, which express a unique combination of voltage-gated sodium channels; and (3) heterologously expressed Nav1.7 is most strongly inhibited by sanshool as compared to other sodium channels expressed in sensory neurons. These results suggest that sanshool targets voltage-gated sodium channels on Aδ mechanosensory nociceptors to dampen excitability and thus induce 'fast pain' analgesia.