Discovery of a novel series of pyridone amides as NaV1.8 inhibitors

The NaV1.8 channel, mainly found in the peripheral nervous system, is recognized as one of the key factors in chronic pain. The molecule VX-150 was initially promising in targeting this channel, but the phase II trials of VX-150 did not show expected pain relief results. By analyzing the interaction mode of VX-150 and NaV1.8, we developed two series with a total of 19 molecules and examined their binding affinity to NaV1.8 in vitro and analgesic effect in vivo. One compound, named 2j, stood out with notable activity against the NaV1.8 channel and showed effective pain relief in models of chronic inflammatory pain and neuropathic pain. Our research points to 2j as a strong contender for developing safer pain-relief treatments.

Discovery of selective NaV1.8 inhibitors based on 5-chloro-2-(4,4-difluoroazepan-1-yl)-6-methyl nicotinamide scaffold for the treatment of pain

The Na_V1.8 channel is a genetically validated target for pain and it is mostly expressed in the peripheral nervous system. Based on the disclosed structures of Na_V1.8-selective inhibitors, we designed and synthesized a series of compounds by introducing bicyclic aromatic fragments based on the nicotinamide scaffold. In this research, a systematic structure-activity relationship study was carried out. While compound 2c possessed moderate inhibitory activity (IC₅₀ = 50.18 ± 0.04 nM) in HEK293 cells stably expressing human Na_V1.8 channels, it showed potent inhibitory activity in DRG neurons and isoform selectivity (>200-fold against human Na_V1.1, Na_V1.5 and Na_V1.7 channels). Moreover, the analgesic potency of compound 2c was identified in a post-surgical mouse model. These data demonstrate that compound 2c can be further evaluated as a non-addictive analgesic agent with reduced cardiac liabilities.

µ-Theraphotoxin-Pn3a inhibition of CaV3.3 channels reveals a novel isoform-selective drug binding site

Low voltage-activated calcium currents are mediated by T-type calcium channels Ca_V3.1, Ca_V3.2, and Ca_V3.3, which modulate a variety of physiological processes including sleep, cardiac pace-making, pain, and epilepsy. Ca_V3 isoforms’ biophysical properties, overlapping expression, and lack of subtype-selective pharmacology hinder the determination of their specific physiological roles in health and disease. We have identified μ-theraphotoxin Pn3a as the first subtype-selective spider venom peptide inhibitor of Ca_V3.3, with >100-fold lower potency against the other T-type isoforms. Pn3a modifies Ca_V3.3 gating through a depolarizing shift in the voltage dependence of activation thus decreasing Ca_V3.3-mediated currents in the normal range of activation potentials. Paddle chimeras of K_V1.7 channels bearing voltage sensor sequences from all four Ca_V3.3 domains revealed preferential binding of Pn3a to the S3-S4 region of domain II (Ca_V3.3^DII). This novel T-type channel pharmacological site was explored through computational docking simulations of Pn3a, site-directed mutagenesis, and full domain II swaps between Ca_V3 channels highlighting it as a subtype-specific pharmacophore. This research expands our understanding of T-type calcium channel pharmacology and supports the suitability of Pn3a as a molecular tool in the study of the physiological roles of Ca_V3.3 channels.

Towards Structure-Guided Development of Pain Therapeutics Targeting Voltage-Gated Sodium Channels

Voltage-gated sodium (Na_V) channels are critical molecular determinants of action potential generation and propagation in excitable cells. Normal Na_V channel function disruption can affect physiological neuronal signaling and lead to increased sensitivity to pain, congenital indifference to pain, uncoordinated movement, seizures, or paralysis. Human genetic studies have identified human Na_V1.7 (hNa_V1.7), hNa_V1.8, and hNa_V1.9 channel subtypes as crucial players in pain signaling. The premise that subtype selective Na_V inhibitors can reduce pain has been reinforced through intensive target validation and therapeutic development efforts. However, an ideal therapeutic has yet to emerge. This review is focused on recent progress, current challenges, and future opportunities to develop Na_V channel targeting small molecules and peptides as non-addictive therapeutics to treat pain.

Structural optimization, synthesis and in vitro synergistic anticancer activities of combinations of new N3-substituted dihydropyrimidine calcium channel blockers with cisplatin and etoposide

T-type calcium channels are considered potential drug targets to combat cancer. Combining T-type calcium channel blockers with conventional chemotherapy drugs represents a promising strategy towards successful cancer treatment. From this perspective, we report in this study the design and synthesis of a novel series of N3-sustituted dihydropyrimidines (DHPMs) as anticancer adjuvants to cisplatin (Cis) and etoposide (Eto). Full spectral characterization of the new compounds was done using FT-IR, ¹H NMR, ¹³C NMR, and HRMS. Structure elucidation was confirmed by 2D NMR ¹H-H COSY, HSQC and NOESY experiments. Novel derivatives were tested for their Ca²⁺ channel blocking activity by employing the whole cell patch-clamp technique. Results demonstrated that most compounds were potential T-type calcium channel blockers with the triazole-based C12 and C13 being the most selective agents against Ca_V3.2 channel. Further electrophysiological studies demonstrated that C12 and C13 inhibited Ca_V3.2 currents with respective affinity of 2.26 and 1.27 µM, and induced 5 mV hyperpolarizing shifts in the half-inactivation potential. Subsequently, C12 and C13 were evaluated for their anticancer activities alone and in combination with Cis and Eto against A549 and MDA-MB 231 cancer cells. Interestingly, both compounds exhibited potential anticancer effects with IC₅₀ values < 5 µM. Combination studies revealed that both compounds had synergistic effects (combination index CI < 1) on Cis and Eto through induction of apoptosis (p53 activation and up-regulation of BAX and p21 gene expression). Importantly, in silico physicochemical and ADMET assessment of both compounds revealed their potential drug-like properties with decreased risk of cardiac toxicity. Hence, C12 and C13 are promising anticancer adjuvants through inhibition of Ca_V3.2 T-type calcium channels, thereby serving as eminent leads for further modification.

Modulation of human T-type calcium channels by synthetic cannabinoid receptor agonists in vitro

BACKGROUND AND PURPOSE

Consumption of Synthetic Cannabinoid Receptor agonists (SCRAs) is associated with severe adverse reactions including seizures, arrhythmias and death, but the molecular mechanisms surrounding SCRA toxicity are not yet established. These disease-like symptoms are also synonymous with altered T-type calcium channel activity which controls rhythmicity in the heart and brain. This study examined whether SCRAs alter T-type activity and whether this represents a possible mechanism of toxicity.

EXPERIMENTAL APPROACH

Fluorescence-based and electrophysiology assays were used to screen 16 structurally related synthetic cannabinoids for their ability to inhibit human T-type calcium channels expressed in HEK293 cells. The most potent compounds were then further examined using patch clamp electrophysiology.

KEY RESULTS

MDMB-CHMICA and AMB-CHMINACA potently blocked Cav3.2 with IC50 values of 1.5 and 0.74 μM respectively. Current inhibition increased from 47 to 80% and 45-87% respectively when the channel was in slow-inactivated state. Both SCRAs had little effect on steady state inactivation, however MDMB-CHMICA significantly shifted the half activation potential by -7mV. Neither drug produced frequency dependent block, in contrast to the phytocannabinoid Δ9-THC.

CONCLUSIONS AND IMPLICATIONS

SCRAs are potent agonists of CB1 receptors and can be extremely toxic, but observed toxicity also resembles symptoms associated with altered Cav3.2 activity. Many SCRAs tested were potent modulators of Cav3.2, raising the possibility that SC toxicity may be due in part to Cav3.2 modulation. This potent T-type channel modulation suggests the possibility of SCRAs as a new drug class with potential to treat diseases associated with altered T-type channel activity. This article is part of the special issue on 'Cannabinoids'.

Status of peripheral sodium channel blockers for non-addictive pain treatment

The effective and safe treatment of pain is an unmet health-care need. Current medications used for pain management are often only partially effective, carry dose-limiting adverse effects and are potentially addictive, highlighting the need for improved therapeutic agents. Most common pain conditions originate in the periphery, where dorsal root ganglion and trigeminal ganglion neurons feed pain information into the CNS. Voltage-gated sodium (Na_V) channels drive neuronal excitability and three subtypes - Na_V1.7, Na_V1.8 and Na_V1.9 - are preferentially expressed in the peripheral nervous system, suggesting that their inhibition might treat pain while avoiding central and cardiac adverse effects. Genetic and functional studies of human pain disorders have identified Na_V1.7, Na_V1.8 and Na_V1.9 as mediators of pain and validated them as targets for pain treatment. Consequently, multiple Na_V1.7-specific and Na_V1.8-specific blockers have undergone clinical trials, with others in preclinical development, and the targeting of Na_V1.9, although hampered by technical constraints, might also be moving ahead. In this Review, we summarize the clinical and preclinical literature describing compounds that target peripheral Na_V channels and discuss the challenges and future prospects for the field. Although the potential of peripheral Na_V channel inhibition for the treatment of pain has yet to be realized, this remains a promising strategy to achieve non-addictive analgesia for multiple pain conditions.

Carisbamate blockade of T-type voltage-gated calcium channels

OBJECTIVES

Carisbamate (CRS) is a novel monocarbamate compound that possesses antiseizure and neuroprotective properties. However, the mechanisms underlying these actions remain unclear. Here, we tested both direct and indirect effects of CRS on several cellular systems that regulate intracellular calcium concentration [Ca²⁺ ]_i .

METHODS

We used a combination of cellular electrophysiologic techniques, as well as cell viability, Store Overload-Induced Calcium Release (SOICR), and mitochondrial functional assays to determine whether CRS might affect [Ca²⁺ ]_i levels through actions on the endoplasmic reticulum (ER), mitochondria, and/or T-type voltage-gated Ca²⁺ channels.

RESULTS

In CA3 pyramidal neurons, kainic acid induced significant elevations in [Ca²⁺ ]_i and long-lasting neuronal hyperexcitability, both of which were reversed in a dose-dependent manner by CRS. Similarly, CRS suppressed spontaneous rhythmic epileptiform activity in hippocampal slices exposed to zero-Mg²⁺ or 4-aminopyridine. Treatment with CRS also protected murine hippocampal HT-22 cells against excitotoxic injury with glutamate, and this was accompanied by a reduction in [Ca²⁺ ]_i . Neither kainic acid nor CRS alone altered the mitochondrial membrane potential (ΔΨ) in intact, acutely isolated mitochondria. In addition, CRS did not affect mitochondrial respiratory chain activity, Ca²⁺ -induced mitochondrial permeability transition, and Ca²⁺ release from the ER. However, CRS significantly decreased Ca²⁺ flux in human embryonic kidney tsA-201 cells transfected with Ca_v 3.1 (voltage-dependent T-type Ca²⁺ ) channels.

SIGNIFICANCE

Our data indicate that the neuroprotective and antiseizure activity of CRS likely results in part from decreased [Ca²⁺ ]_i accumulation through blockade of T-type Ca²⁺ channels.

An oral NaV1.8 blocker improves motor function in mice completely deficient of myelin protein P0

Mice deficient of myelin protein P0 are established models of demyelinating Charcot-Marie-Tooth (CMT) disease. Dysmyelination in these mice is associated with an ectopic expression of the sensory neuron specific sodium channel isoform NaV1.8 on motor axons. We reported that in P0+/-, a model of CMT1B, the membrane dysfunction could be acutely improved by a novel oral NaV1.8 blocker referred to as Compound 31 (C31, Bioorg. Med. Chem. Lett. 2010, 20, 6812; AbbVie Inc.). The aim of this study was to investigate the extent to which C31 treatment could also improve the motor axon function in P0-/-, a CMT model with a much more severe neuropathy. We found that the progressive impairment of motor performance from 1 to 4 months of age in P0-/- could be acutely reversed by C31 treatment. The effect was associated with an improvement of the amplitude of the plantar CMAP evoked by tibial nerve stimulation. The corresponding motor nerve excitability studies by "threshold tracking" showed changes after C31 consistent with attenuation of a resting membrane depolarization. Our data suggest that the depolarizing motor conduction failure in P0-/- could be acutely improved by C31. This provides proof-of-concept that treatment with oral subtype-selective NaV1.8 blockers could be used to improve the motor function in severe forms of demyelinating CMT.

Progression of motor axon dysfunction and ectopic Nav1.8 expression in a mouse model of Charcot-Marie-Tooth disease 1B

Mice heterozygously deficient for the myelin protein P0 gene (P0+/-) develop a slowly progressing neuropathy modeling demyelinating Charcot-Marie-Tooth disease (CMT1B). The aim of the study was to investigate the long-term progression of motor dysfunction in P0+/- mice at 3, 7, 12 and 20months. By comparison with WT littermates, P0+/- showed a decreasing motor performance with age. This was associated with a progressive reduction in amplitude and increase in latency of the plantar compound muscle action potential (CMAP) evoked by stimulation of the tibial nerve at ankle. This progressive functional impairment was in contrast to the mild demyelinating neuropathy of the tibial nerve revealed by histology. "Threshold-tracking" studies showed impaired motor axon excitability in P0+/- from 3months. With time, there was a progressive reduction in threshold deviations during both depolarizing and hyperpolarizing threshold electrotonus associated with increasing resting I/V slope and increasing strength-duration time constant. These depolarizing features in excitability in P0+/- as well as the reduced CMAP amplitude were absent in P0+/- NaV1.8 knockouts, and could be acutely reversed by selective pharmacologic block of NaV1.8 in P0+/-. Mathematical modeling indicated an association of altered passive cable properties with a depolarizing shift in resting membrane potential and increase in the persistent Na(+) current in P0+/-. Our data suggest that ectopic NaV1.8 expression precipitates depolarizing conduction failure in CMT1B, and that motor axon dysfunction in demyelinating neuropathy is pharmacologically reversible.

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.

A 3.7 kb fragment of the mouse Scn10a gene promoter directs neural crest but not placodal lineage EGFP expression in a transgenic animal

Under physiological conditions, the voltage-gated sodium channel Nav1.8 is expressed almost exclusively in primary sensory neurons. The mechanism restricting Nav1.8 expression is not entirely clear, but we have previously described a 3.7 kb fragment of the Scn10a promoter capable of recapitulating the tissue-specific expression of Nav1.8 in transfected neurons and cell lines (Puhl and Ikeda, 2008). To validate these studies in vivo, a transgenic mouse encoding EGFP under the control of this putative sensory neuron specific promoter was generated and characterized in this study. Approximately 45% of dorsal root ganglion neurons of transgenic mice were EGFP-positive (mean diameter = 26.5 μm). The majority of EGFP-positive neurons bound isolectin B4, although a small percentage (∼10%) colabeled with markers of A-fiber neurons. EGFP expression correlated well with the presence of Nav1.8 transcript (95%), Nav1.8-immunoreactivity (70%), and TTX-R INa (100%), although not all Nav1.8-expressing neurons expressed EGFP. Several cranial sensory ganglia originating from neurogenic placodes, such as the nodose ganglion, failed to express EGFP, suggesting that additional regulatory elements dictate Scn10a expression in placodal-derived sensory neurons. EGFP was also detected in discrete brain regions of transgenic mice. Quantitative PCR and Nav1.8-immunoreactivity confirmed Nav1.8 expression in the amygdala, brainstem, globus pallidus, lateral and paraventricular hypothalamus, and olfactory tubercle. TTX-R INa recorded from EGFP-positive hypothalamic neurons demonstrate the usefulness of this transgenic line to study novel roles of Nav1.8 beyond sensory neurons. Overall, Scn10a-EGFP transgenic mice recapitulate the majority of the Nav1.8 expression pattern in neural crest-derived sensory neurons.

Endostatin inhibits T-type Ca2+ channel current in guinea pig ventricular myocyte

Endostatin, a fragment of collagen XVIII, is known as an endogenous angiogenesis inhibitor, and its serum concentration increases in various cardiovascular diseases. T-type Ca²⁺ channel, low voltage-activated Ca²⁺ channel, is not expressed in adult ventricular myocytes. Re-expression of T-type Ca²⁺ channels in cardiac myocytes is thought to be involved in the development of cardiac hypertrophy. We examined the effects of endostatin on T-type Ca²⁺ channel current by whole-cell patch clamp technique in freshly isolated adult guinea pig ventricular myocytes, which exceptionally express T-type Ca²⁺ channels. Although endostatin 300 ng/ml had no effect on L-type Ca²⁺ current, it significantly inhibited T-type Ca²⁺ current. These data indicate that endostatin can be an endogenous inhibitor of T-type Ca²⁺ channels in the cardiac myocytes.

T-type calcium channels in chronic pain: mouse models and specific blockers

Pain is a quite frequent complaint accompanying numerous pathologies. Among these pathological cases, neuropathies are retrieved with identified etiologies (chemotherapies, diabetes, surgeries…) and also more diffuse syndromes such as fibromyalgia. More broadly, pain is one of the first consequences of the majority of inherited diseases. Despite its importance for the quality of life, current pain management is limited to drugs that are either old or with a limited efficacy or that possess a bad benefit/risk ratio. As no new pharmacological concept has led to new analgesics in the last decades, the discovery of medications is needed, and to this aim the identification of new druggable targets in pain transmission is a first step. Therefore, studies of ion channels in pain pathways are extremely active. This is particularly true with ion channels in peripheral sensory neurons in dorsal root ganglia (DRG) known now to express unique sets of these channels. Moreover, both spinal and supraspinal levels are clearly important in pain modulation. Among these ion channels, we and others revealed the important role of low voltage-gated calcium channels in cellular excitability in different steps of the pain pathways. These channels, by being activated nearby resting membrane potential have biophysical characteristics suited to facilitate action potential generation and rhythmicity. In this review, we will review the current knowledge on the role of these channels in the perception and modulation of pain.

Calcium-permeable ion channels in pain signaling

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

T-type calcium channel blockers as neuroprotective agents

T-type calcium channels are expressed in many diverse tissues, including neuronal, cardiovascular, and endocrine. T-type calcium channels are known to play roles in the development, maintenance, and repair of these tissues but have also been implicated in disease when not properly regulated. Calcium channel blockers have been developed to treat various diseases and their use clinically is widespread due to both their efficacy as well as their safety. Aside from their established clinical applications, recent studies have suggested neuroprotective effects of T-type calcium channel blockers. Many of the current T-type calcium channel blockers could act on other molecular targets besides T-type calcium channels making it uncertain whether their neuroprotective effects are solely due to blocking of T-type calcium channels. In this review, we discuss these drugs as well as newly developed chemical compounds that are designed to be more selective for T-type calcium channels. We review in vitro and in vivo evidence of neuroprotective effects by these T-type calcium channel blockers. We conclude by discussing possible molecular mechanisms underlying the neuroprotective effects by T-type calcium channel blockers.

A gain-of-function voltage-gated sodium channel 1.8 mutation drives intense hyperexcitability of A- and C-fiber neurons

Therapeutic use of general sodium channel blockers, such as lidocaine, can substantially reduce the enhanced activity in sensory neurons that accompanies chronic pain after nerve or tissue injury. However, because these general blockers have significant side effects, there is great interest in developing inhibitors that specifically target subtypes of sodium channels. Moreover, some idiopathic small-fiber neuropathies are driven by gain-of-function mutations in specific sodium channel subtypes. In the current study, we focus on one subtype, the voltage-gated sodium channel 1.8 (Nav1.8). Nav1.8 is preferentially expressed in nociceptors, and gain-of-function mutations in Nav1.8 result in painful mechanical hypersensitivity in humans. Here, we used the recently developed gain-of-function Nav1.8 transgenic mouse strain, Possum, to investigate Nav1.8-mediated peripheral afferent hyperexcitability. This gain-of-function mutation resulted in markedly increased mechanically evoked action potential firing in subclasses of Aβ, Aδ, and C fibers. Moreover, mechanical stimuli initiated bursts of action potential firing in specific subpopulations that continued for minutes after removal of the force and were not susceptible to conduction failure. Surprisingly, despite the intense afferent firing, the behavioral effects of the Nav1.8 mutation were quite modest, as only frankly noxious stimuli elicited enhanced pain behavior. These data demonstrate that a Nav1.8 gain-of-function point mutation contributes to intense hyperexcitability along the afferent axon within distinct sensory neuron subtypes.

Pharmacological Inhibition of Voltage-gated Ca(2+) Channels for Chronic Pain Relief

Chronic pain is a major therapeutic problem as the current treatment options are unsatisfactory with low efficacy and deleterious side effects. Voltage-gated Ca2+ channels (VGCCs), which are multi-complex proteins consisting of α1, β, γ, and α2δ subunits, play an important role in pain signaling. These channels are involved in neurogenic inflammation, excitability, and neurotransmitter release in nociceptors. It has been previously shown that N-type VGCCs (Cav2.2) are a major pain target. U.S. FDA approval of three Cav2.2 antagonists, gabapentin, pregabalin, and ziconotide, for chronic pain underlies the importance of this channel subtype. Also, there has been increasing evidence that L-type (Cav1.2) or T-type (Cav3.2) VGCCs may be involved in pain signaling and chronic pain. In order to develop novel pain therapeutics and to understand the role of VGCC subtypes, discovering subtype selective VGCC inhibitors or methods that selectively target the inhibitor into nociceptors would be essential. This review describes the various VGCC subtype inhibitors and the potential of utilizing VGCC subtypes as targets of chronic pain. Development of VGCC subtype inhibitors and targeting them into nociceptors will contribute to a better understanding of the roles of VGCC subtypes in pain at a spinal level as well as development of a novel class of analgesics for chronic pain.

Analgesic effect of a mixed T-type channel inhibitor/CB2 receptor agonist

BACKGROUND

Cannabinoid receptors and T-type calcium channels are potential targets for treating pain. Here we report on the design, synthesis and analgesic properties of a new mixed cannabinoid/T-type channel ligand, NMP-181.

RESULTS

NMP-181 action on CB1 and CB2 receptors was characterized in radioligand binding and in vitro GTPγ[35S] functional assays, and block of transiently expressed human Cav3.2 T-type channels by NMP-181 was analyzed by patch clamp. The analgesic effects and in vivo mechanism of action of NMP-181 delivered spinally or systemically were analyzed in formalin and CFA mouse models of pain. NMP-181 inhibited peak CaV3.2 currents with IC50 values in the low micromolar range and acted as a CB2 agonist. Inactivated state dependence further augmented the inhibitory action of NMP-181. NMP-181 produced a dose-dependent antinociceptive effect when administered either spinally or systemically in both phases of the formalin test. Both i.t. and i.p. treatment of mice with NMP-181 reversed the mechanical hyperalgesia induced by CFA injection. NMP-181 showed no antinocieptive effect in CaV3.2 null mice. The antinociceptive effect of intrathecally delivered NMP-181 in the formalin test was reversed by i.t. treatment of mice with AM-630 (CB2 antagonist). In contrast, the NMP-181-induced antinociception was not affected by treatment of mice with AM-281 (CB1 antagonist).

CONCLUSIONS

Our work shows that both T-type channels as well as CB2 receptors play a role in the antinociceptive action of NMP-181, and also provides a novel avenue for suppressing chronic pain through novel mixed T-type/cannabinoid receptor ligands.

Attenuation of autonomic reflexes by A803467 may not be solely caused by blockade of NaV 1.8 channels

In decerebrated rats, we determined the dose of A803467, a NaV 1.8 antagonist, needed to attenuate the reflex pressor responses to femoral arterial injections of lactic acid (24 mM; ~0.1 ml) and capsaicin (0.1 μg), agents which stimulate thin fiber afferents having NaV 1.8 channels. We also determined whether the dose of A803467 needed to attenuate these reflex responses affected the responses of muscle spindle afferents to tendon stretch and succinylcholine (200 μg). Spindle afferents are not supplied with NaV 1.8 channels, and consequently their responses to these stimuli should not be influenced by A803467. Pressor responses to lactic acid and capsaicin were not altered by 500 μg of A803467 (n=6). A803467 in a dose of 1mg, however, significantly reduced (p<0.05; n=12) the pressor responses to lactic acid (23 ± 5 to 7 ± 3 Δmm Hg) and capsaicin (47 ± 5 to 31 ± 5 ΔmmHg). Surprisingly, we also found that 1mg of A803467 reduced the responses of 10 spindle afferents to succinylcholine (34 ± 11 to 4 ± 3 Δimp/s; p<0.05) and stretch (83 ± 17 to 0.4 ± 1 Δimp/s; p<0.05). We conclude that A803467 reduces the reflex response to lactic acid and capsaicin; however, it may be working on multiple channels, including NaV 1.8, other NaVs as well as voltage-gated calcium channels.

Molecular and functional determinants of local anesthetic inhibition of NaChBac

In our recent publication, we describe the local anesthetic (LA) inhibition of the prokaryotic voltage gated sodium channel NaChBac. Despite the numerous functional and putative structural differences with the mammalian sodium channels, the data show that LA compounds effectively and reversibly inhibit NaChBac channels in a concentration range similar to resting blockade on eukaryotic Navs. In addition to current reduction, LA application accelerated channel inactivation kinetics of NaChBac which could be accounted for in a simple state-model whereby local anesthetics increase the probability of entering the inactivated state. We have further explored what state (or states) local anesthetic blockade of NaChBac could pertain to eukaryotic sodium channels, and what molecular similarities exist between these disparate channel families. Here we show that the rate of recovery from inactivation remains unaffected in the presence of local anesthetics. Further, we show that two sites that support use-dependent inhibition in eukaryotic channels, do not affect block to the same extent when mutated in NaChBac channels. The data indicate that the molecular determinants and the inherent mechanisms for LA block are likely to be divergent between bacterial and eukaryotic Navs, but future experiments will help define possible similarities.