Presynaptic Cav3.2 channels regulate excitatory neurotransmission in nociceptive dorsal horn neurons

SUMOylation and DeSUMOylation: Tug of War of Pain Signaling

SUMOylation is a post-translational modification that attaches a small ubiquitin-like modifier (SUMO) group to a target protein via SUMO ligases, while deSUMOylation refers to the removal of this SUMO group by sentrin-specific proteases (SENPs). Although the functions of these processes have been well described in the nucleus, the role of SUMOylation and deSUMOylation in regulating ion channels is emerging as a novel area of study. Despite this, their contributions to pain signaling remain less clear. Therefore, this review consolidates the current evidence on the link(s) between SUMOylation, deSUMOylation, and pain, with a specific focus on ion channels expressed in the sensory system. Additionally, we explore the role of SUMOylation in the expression and function of kinases, vesicle proteins, and transcription factors, which result in the modulation of certain ion channels contributing to pain. Altogether, this review aims to highlight the relationship between SUMOylation and deSUMOylation in the modulation of ion channels, ultimately exploring the potential therapeutic role of these processes in chronic pain.

Facilitation of Ca V 3.2 channel gating in pain pathways reveals a novel mechanism of serum-induced hyperalgesia

The Ca V 3.2 isoform of T-type voltage-gated calcium channels plays a crucial role in regulating the excitability of nociceptive neurons; the endogenous molecules that modulate its activity, however, remain poorly understood. Here, we used serum proteomics and patch-clamp physiology to discover a novel peptide albumin (1-26) that facilitates channel gating by chelating trace metals that tonically inhibit Ca V 3.2 via H191 residue. Importantly, serum also potently modulated T-currents in human and rodent dorsal root ganglion (DRG) neurons. In vivo pain studies revealed that injections of serum and albumin (1-26) peptide resulted in robust mechanical and heat hypersensitivity. This hypersensitivity was abolished with a T-channel inhibitor, in Ca V 3.2 null mice and in Ca V 3.2 H191Q knock-in mice. The discovery of endogenous chelators of trace metals in the serum deepens our understanding of the role of Ca V 3.2 channels in neuronal hyperexcitability and may facilitate the design of novel analgesics with unique mechanisms of action.

Contribution of T-type calcium channel isoforms to cold and mechanical sensitivity in naïve and oxaliplatin-treated mice of both sexes

BACKGROUND AND PURPOSE

The chemotherapy agent oxaliplatin can give rise to oxaliplatin-induced peripheral neuropathy (OIPN). Here, we investigated whether T-type calcium channels (Cav3) contribute to OIPN.

EXPERIMENTAL APPROACH

We chronically treated mice with oxaliplatin and assessed pain responses and changes in expression of Cav3.2 calcium channels. We also tested the effects of T-type channel blockers on cold sensitivity in wild-type and Cav3.2 null mice.

KEY RESULTS

Oxaliplatin treatment led to mechanical and cold hypersensitivity in male and female mice. Mechanical hypersensitivity persisted in Cav3.2 null mice of both sexes. Intraperitoneal or intrathecal delivery of pan T-type channel inhibitors attenuated mechanical hypersensitivity in wild-type but not Cav3.2 null mice. Remarkably cold hypersensitivity occurred in female but not male Cav3.2 null mice even without oxaliplatin treatment. Unexpectedly, intrathecal, intraplantar or intraperitoneal delivery of T-type channel inhibitors Z944 or TTA-P2 transiently induced cold hypersensitivity in both male and female wild-type mice. Acute knockdown of specific Cav3 isoforms revealed that the depletion of Cav3.1 in males and depletion of either Cav3.1 or Cav3.2 in females triggered cold hypersensitivity. Finally, reducing Cav3.2 expression by disrupting the interactions between Cav3.2 and the deubiquitinase USP5 with the small organic molecule II-2 reversed oxaliplatin-induced mechanical and cold hypersensitivity and importantly did not trigger cold allodynia.

CONCLUSION AND IMPLICATIONS

Altogether, our data indicate that T-type channels differentially contribute to the regulation of cold and mechanical hypersensitivity, and raise the possibility that T-type channel blockers could promote cold allodynia.

ML218 modulates calcium binding protein, oxidative stress, and inflammation during ischemia-reperfusion brain injury in mice

Cerebral ischemia disrupts calcium homeostasis in the brain causing excitotoxicity, oxidative stress, inflammation, and neuronal cell apoptosis. During ischemic conditions, T-type calcium channel channels contribute to increase in intracellular calcium ions in both neurons and glial cells therefore, the current study hypothesizes the antagonism of these channels using ML218, a novel specific T-Type inhibitor in experimental model of cerebral ischemia-reperfusion (CI/R) brain injury. CI/R injury was induced in Swiss Albino mice by occlusion of common carotid arteries followed by reperfusion. Animals were assessed for learning and memory (MWM), motor coordination (Rota rod), neurological function (neurological deficit score), cerebral infarction, edema, and histopathological alterations. Biochemical assessments were made for calcium binding proteins (Calmodulin- CaM, calcium/calmodulin-dependent protein kinase II-CaMKII, S100B), oxidative stress (4-hydroxy 2-nonenal-4-HNE, glutathione-GSH, inflammation (nuclear factor kappa-light-chain-enhancer of activated B-p65-NF-kB, tumor necrosis factor-TNF-α, interleukin-IL-10) inducible nitric oxide synthase (iNOS) levels, and acetylcholinesterase activity (AChE) in brain supernatants. Furthermore, serum levels of NF-kB, iNOS, and S100B were also assessed. CI/R animals showed impairment in learning, memory, motor coordination, and neurological function along with increase in cerebral infarction, edema, and histopathological alterations. Furthermore, increase in brain calcium binding proteins, oxidative stress, inflammation, and AChE activity along with serum NF-kB, iNOS, and S100B levels were recorded in CI/R animals. Administration of ML218 (5 mg/kg and 10 mg/kg; i.p.) was observed to recuperate CI/R induced impairments in behavioral, biochemical, and histopathological analysis. Hence, it may be concluded that ML218 mediates neuroprotection during CI/R via decreasing brain and serum calcium binding proteins, inflammation, iNOS, and oxidative stress markers.

Development of Tetrahydroquinoline-Based Inhibitors for Chronic Pain

Chronic pain affects a substantial portion of the population, posing a significant health challenge. Current treatments often come with limitations and side effects, necessitating novel therapeutic approaches. Our study focuses on disrupting the Cav3.2-USP5 interaction as a strategy for chronic pain management. Through structure-activity relationship studies of a tetrahydroquinoline (THQ) scaffold, we identified a family of lead molecules that demonstrated potent inhibition of the Cav3.2-USP5 interaction. In vitro pharmacokinetic assessments and in vivo studies support the efficacy and drug-like properties of the lead compounds in mouse models of acute and chronic pain. Dependence on the Cav3.2 channels was validated in Cav3.2 null mice, consistent with the proposed mode of action of these small molecules. These findings provide a novel chronic pain treatment strategy, highlighting the potential of these small molecules for further development.

Cannabinol (CBN) Influences the Ion Channels and Synaptic-Related Genes in NSC-34 Cell Line: A Transcriptomic Study

Neurological disorders such as Alzheimer's, Parkinson's, amyotrophic lateral sclerosis, and schizophrenia are associated with altered neuronal excitability, resulting from dysfunctions in the molecular architecture and physiological regulation of ion channels and synaptic transmission. Ion channels and synapses are regarded as suitable therapeutic targets in modern pharmacology. Cannabinoids have received great attention as an original therapeutic approach for their effects on human health due to their ability to modulate the neurotransmitter release through interaction with the endocannabinoid system. In our study, we explored the effect of cannabinol (CBN) through next-generation sequencing analysis of NSC-34 cell physiology. Our findings revealed that CBN strongly influences the ontologies related to ion channels and synapse activity at all doses tested. Specifically, the genes coding for calcium and potassium voltage-gated channel subunits, and the glutamatergic and GABAergic receptors (Cacna1b, Cacna1h, Cacng8, Kcnc3, Kcnd1, Kcnd2, Kcnj4, Grik5, Grik1, Slc17a7, Gabra5), were up-regulated. Conversely, the genes involved into serotoninergic and cholinergic pathways (Htr3a, Htr3b, Htr1b, Chrna3, Chrnb2, Chrnb4), were down-regulated. These findings highlight the influence of CBN in the expression of genes involved into ion influx and synaptic transmission.

Functional remodeling of presynaptic voltage-gated calcium channels in superficial layers of the dorsal horn during neuropathic pain

N- and P/Q-type voltage-gated Ca2+ channels are critical for synaptic transmission. While their expression is increased in the dorsal root ganglion (DRG) neuron cell bodies during neuropathic pain conditions, less is known about their synaptic remodeling. Here, we combined genetic tools with 2-photon Ca2+ imaging to explore the functional remodeling that occurs in central presynaptic terminals of DRG neurons during neuropathic pain. We imaged GCaMP6s fluorescence responses in an ex vivo spinal cord preparation from mice expressing GCaMP6s in Trpv1-Cre lineage nociceptors. We show that Ca2+ transient amplitude is increased in central terminals of these neurons after spared nerve injury, and that this increase is mediated by both N- and P/Q-type channels. We found that GABA-B receptor-dependent inhibition of Ca2+ transients was potentiated in the superficial layer of the dorsal horn. Our results provide direct evidence toward nerve injury-induced functional remodeling of presynaptic Ca2+ channels in Trpv1-lineage nociceptor terminals.

Ion Channel and Transporter Involvement in Chemotherapy-Induced Peripheral Neurotoxicity

The peripheral nervous system can encounter alterations due to exposure to some of the most commonly used anticancer drugs (platinum drugs, taxanes, vinca alkaloids, proteasome inhibitors, thalidomide), the so-called chemotherapy-induced peripheral neurotoxicity (CIPN). CIPN can be long-lasting or even permanent, and it is detrimental for the quality of life of cancer survivors, being associated with persistent disturbances such as sensory loss and neuropathic pain at limb extremities due to a mostly sensory axonal polyneuropathy/neuronopathy. In the state of the art, there is no efficacious preventive/curative treatment for this condition. Among the reasons for this unmet clinical and scientific need, there is an uncomplete knowledge of the pathogenetic mechanisms. Ion channels and transporters are pivotal elements in both the central and peripheral nervous system, and there is a growing body of literature suggesting that they might play a role in CIPN development. In this review, we first describe the biophysical properties of these targets and then report existing data for the involvement of ion channels and transporters in CIPN, thus paving the way for new approaches/druggable targets to cure and/or prevent CIPN.

Effect of ABT-639 on Cav3.2 channel activity and its analgesic actions in mouse models of inflammatory and neuropathic pain

Cav3.2 T-type calcium channels are important targets for pain relief in rodent models of inflammatory and neuropathic pain. Even though many T-type channel blockers have been tested in mice, only one molecule, ABT-639, has been tested in phase II clinical studies and did not produce analgesic effects over placebo. Here we examined the effects of ABT-639 on Cav3.2 channel activity in tsA-201 cells and dorsal root ganglion (DRG) neurons, in comparison with another established Cav3.2 inhibitor Z944. These experiments revealed that Z944 mediated ∼100-fold more potent inhibition of Cav3.2 currents than ABT-639, with the latter blocking channel activity by less than 15 percent when applied at a concentration of 30 μM. A slight increase in ABT-639 potency was observed at more depolarized holding potentials, suggesting that this compound may act preferentially on inactivated channels. We tested the effects of both compounds in the Complete Freund's Adjuvant (CFA) model of chronic inflammatory pain, and in partial sciatic nerve injury model of neuropathic pain in mice. In the neuropathic pain model, both Z944 and ABT-639 reversed mechanical hypersensitivity to similar degrees when delivered systemically, but remarkably, when delivered intrathecally, only Z944 was effective. In the CFA model, both compounds reversed thermal hyperalgesia upon systemic delivery, but only Z944 mediated pain relief upon intrathecal delivery, indicating that ABT-639 acts primarily at peripheral sites. ABT-639 lost its analgesic effects in CFA treated Cav3.2 null mice, indicating that these channels are essential for ABT-639-mediated pain relief despite its poor inhibition of Cav3.2 currents.

Local Administration of the Phytochemical, Quercetin, Attenuates the Hyperexcitability of Rat Nociceptive Primary Sensory Neurons Following Inflammation Comparable to lidocaine

Although in vivo local injection of quercetin into the peripheral receptive field suppresses the excitability of rat nociceptive trigeminal ganglion (TG) neurons, under inflammatory conditions, the acute effects of quercetin in vivo, particularly on nociceptive TG neurons, remain to be determined. The aim of this study was to examine whether acute local administration of quercetin into inflamed tissue attenuates the excitability of nociceptive TG neurons in response to mechanical stimulation. The mechanical escape threshold was significantly lower in complete Freund's adjuvant (CFA)-inflamed rats compared to before CFA injection. Extracellular single-unit recordings were made from TG neurons of CFA-induced inflammation in anesthetized rats in response to orofacial mechanical stimulation. The mean firing frequency of TG neurons in response to both non-noxious and noxious mechanical stimuli was reversibly inhibited by quercetin in a dose-dependent manner (1-10 mM). The mean firing frequency of inflamed TG neurons in response to mechanical stimuli was reversibly inhibited by the local anesthetic, 1% lidocaine (37 mM). The mean magnitude of inhibition on TG neuronal discharge frequency with 1 mM quercetin was significantly greater than that of 1% lidocaine. These results suggest that local injection of quercetin into inflamed tissue suppresses the excitability of nociceptive primary sensory TG neurons. PERSPECTIVE: Local administration of the phytochemical, quercetin, into inflamed tissues is a more potent local analgesic than voltage-gated sodium channel blockers as it inhibits the generation of both generator potentials and action potentials in nociceptive primary nerve terminals. As such, it contributes to the area of complementary and alternative medicines.

Central Channelopathies in Obesity

As obesity has raised heightening awareness, researchers have attempted to identify potential targets that can be treated for therapeutic intervention. Focusing on the central nervous system (CNS), the key organ in maintaining energy balance, a plethora of ion channels that are expressed in the CNS have been inspected and determined through manipulation in different hypothalamic neural subpopulations for their roles in fine-tuning neuronal activity on energy state alterations, possibly acting as metabolic sensors. However, a remaining gap persists between human clinical investigations and mouse studies. Despite having delineated the pathways and mechanisms of how the mouse study-identified ion channels modulate energy homeostasis, only a few targets overlap with the obesity-related risk genes extracted from human genome-wide association studies. Here, we present the most recently discovered CNS-specific metabolism-correlated ion channels using reverse and forward genetics approaches in mice and humans, respectively, in the hope of illuminating the prospects for future therapeutic development.

The Role of Neuroactive Steroids in Analgesia and Anesthesia: An Interesting Comeback?

Published evidence over the past few decades suggests that general anesthetics could be neurotoxins especially when administered at the extremes of age. The reported pathology is not only at the morphological level when examined in very young and aged brains, given that, importantly, newly developing evidence suggests a variety of behavioral impairments. Since anesthesia is unavoidable in certain clinical settings, we should consider the development of new anesthetics. A promising and safe solution could be a new family of anesthetics referred to as neuroactive steroids. In this review, we summarize the currently available evidence regarding their anesthetic and analgesic properties.

Neuropathic pain; what we know and what we should do about it

Neuropathic pain can result from injury to, or disease of the nervous system. It is notoriously difficult to treat. Peripheral nerve injury promotes Schwann cell activation and invasion of immunocompetent cells into the site of injury, spinal cord and higher sensory structures such as thalamus and cingulate and sensory cortices. Various cytokines, chemokines, growth factors, monoamines and neuropeptides effect two-way signalling between neurons, glia and immune cells. This promotes sustained hyperexcitability and spontaneous activity in primary afferents that is crucial for onset and persistence of pain as well as misprocessing of sensory information in the spinal cord and supraspinal structures. Much of the current understanding of pain aetiology and identification of drug targets derives from studies of the consequences of peripheral nerve injury in rodent models. Although a vast amount of information has been forthcoming, the translation of this information into the clinical arena has been minimal. Few, if any, major therapeutic approaches have appeared since the mid 1990's. This may reflect failure to recognise differences in pain processing in males vs. females, differences in cellular responses to different types of injury and differences in pain processing in humans vs. animals. Basic science and clinical approaches which seek to bridge this knowledge gap include better assessment of pain in animal models, use of pain models which better emulate human disease, and stratification of human pain phenotypes according to quantitative assessment of signs and symptoms of disease. This can lead to more personalized and effective treatments for individual patients. Significance statement: There is an urgent need to find new treatments for neuropathic pain. Although classical animal models have revealed essential features of pain aetiology such as peripheral and central sensitization and some of the molecular and cellular mechanisms involved, they do not adequately model the multiplicity of disease states or injuries that may bring forth neuropathic pain in the clinic. This review seeks to integrate information from the multiplicity of disciplines that seek to understand neuropathic pain; including immunology, cell biology, electrophysiology and biophysics, anatomy, cell biology, neurology, molecular biology, pharmacology and behavioral science. Beyond this, it underlines ongoing refinements in basic science and clinical practice that will engender improved approaches to pain management.

Diabetes-Induced Amplification of Nociceptive DRG Neuron Output by Upregulation of Somatic T-Type Ca2+ Channels

The development of pain symptoms in peripheral diabetic neuropathy (PDN) is associated with the upregulation of T-type Ca2+ channels (T-channels) in the soma of nociceptive DRG neurons. Moreover, a block of these channels in DRG neurons effectively reversed mechanical and thermal hyperalgesia in animal diabetic models, indicating that T-channel functioning in these neurons is causally linked to PDN. However, no particular mechanisms relating the upregulation of T-channels in the soma of nociceptive DRG neurons to the pathological pain processing in PDN have been suggested. Here we have electrophysiologically identified voltage-gated currents expressed in nociceptive DRG neurons and developed a computation model of the neurons, including peripheral and central axons. Simulations showed substantially stronger sensitivity of neuronal excitability to diabetes-induced T-channel upregulation at the normal body temperature compared to the ambient one. We also found that upregulation of somatic T-channels, observed in these neurons under diabetic conditions, amplifies a single action potential invading the soma from the periphery into a burst of multiple action potentials further propagated to the end of the central axon. We have concluded that the somatic T-channel-dependent amplification of the peripheral nociceptive input to the spinal cord demonstrated in this work may underlie abnormal nociception at different stages of diabetes development.

Adiponectin receptor 1-mediated stimulation of Cav3.2 channels in trigeminal ganglion neurons induces nociceptive behaviors in mice

BACKGROUND

Adipokines, including adiponectin, are implicated in nociceptive pain; however, the underlying cellular and molecular mechanisms remain unknown.

METHODS

Using electrophysiological recording, immunostaining, molecular biological approaches and animal behaviour tests, we elucidated a pivotal role of adiponectin in regulating membrane excitability and pain sensitivity by manipulating Cav3.2 channels in trigeminal ganglion (TG) neurons.

RESULTS

Adiponectin enhanced T-type Ca2+ channel currents (IT) in TG neurons through the activation of adiponectin receptor 1 (adipoR1) but independently of heterotrimeric G protein-mediated signaling. Coimmunoprecipitation revealed a physical association between AdipoR1 and casein kinase II alpha-subunits (CK2α) in the TG, and inhibiting CK2 activity by chemical inhibitor or siRNA targeting CK2α prevented the adiponectin-induced IT response. Adiponectin significantly activated protein kinase C (PKC), and this effect was abrogated by CK2α knockdown. Adiponectin increased the membrane abundance of PKC beta1 (PKCβ1). Blocking PKCβ1 pharmacologically or genetically abrogated the adiponectin-induced IT increase. In heterologous expression systems, activation of adipoR1 induced a selective enhancement of Cav3.2 channel currents, dependent on PKCβ1 signaling. Functionally, adiponectin increased TG neuronal excitability and induced mechanical pain hypersensitivity, both attenuated by T-type channel blockade. In a trigeminal neuralgia model induced by chronic constriction injury of infraorbital nerve, blockade of adipoR1 signaling suppressed mechanical allodynia, which was prevented by silencing Cav3.2.

CONCLUSION

Our study elucidates a novel signaling cascade wherein adiponectin stimulates TG Cav3.2 channels via adipoR1 coupled to a novel CK2α-dependent PKCβ1. This process induces neuronal hyperexcitability and pain hypersensitivity. Insight into adipoR-Cav3.2 signaling in sensory neurons provides attractive targets for pain treatment.

Current Drug Development Overview: Targeting Voltage-Gated Calcium Channels for the Treatment of Pain

Voltage-gated calcium channels (VGCCs) are targeted to treat pain conditions. Since the discovery of their relation to pain processing control, they are investigated to find new strategies for better pain control. This review provides an overview of naturally based and synthetic VGCC blockers, highlighting new evidence on the development of drugs focusing on the VGCC subtypes as well as mixed targets with pre-clinical and clinical analgesic effects.

Role of T CD4+ cells, macrophages, C-low threshold mechanoreceptors and spinal Cav 3.2 channels in inflammation and related pain-like symptoms in murine inflammatory models

BACKGROUND AND PURPOSE

T-type calcium channels, mainly the Ca_v 3.2 subtype, are important contributors to the nociceptive signalling pathway. We investigated their involvement in inflammation and related pain-like symptoms.

EXPERIMENTAL APPROACH

The involvement of Ca_v 3.2 and T-type channels was investigated using genetic and pharmacological inhibition to assess mechanical allodynia/hyperalgesia and oedema development in two murine inflammatory pain models. The location of Ca_v 3.2 channels involved in pain-like symptoms was studied in mice with Ca_v 3.2 knocked out in C-low threshold mechanoreceptors (C-LTMR) and the use of ABT-639, a peripherally restricted T-type channel inhibitor. The anti-oedema effect of Ca_v 3.2 channel inhibition was investigated in chimeric mice with immune cells deleted for Ca_v 3.2. Lymphocytes and macrophages from either green fluorescent protein-targeted Ca_v 3.2 or KO mice were used to determine the expression of Ca_v 3.2 protein and the functional status of the cells.

KEY RESULTS

Ca_v 3.2 channels contributed to the development of pain-like symptoms and oedema in the two murine inflammatory pain models. Our results provided evidence of the involvement of Ca_v 3.2 channels located on C-LTMRs and spinal cord in inflammatory pain. Ca_v 3.2 channels located in T cells and macrophages contribute to the inflammatory process.

CONCLUSION AND IMPLICATIONS

Ca_v 3.2 channels play crucial roles in inflammation and related pain, implying that targeting of Ca_v 3.2 channels with pharmacological agents could be an attractive and readily evaluable strategy in clinical trials, to relieve chronic inflammatory pain in patients.

Regulation of Presynaptic Calcium Channels

Voltage-gated calcium channels (VGCCs), especially Cav2.1 and Cav2.2, are the major mediators of Ca2+ influx at the presynaptic membrane in response to neuron excitation, thereby exerting a predominant control on synaptic transmission. To guarantee the timely and precise release of neurotransmitters at synapses, the activity of presynaptic VGCCs is tightly regulated by a variety of factors, including auxiliary subunits, membrane potential, G protein-coupled receptors (GPCRs), calmodulin (CaM), Ca2+-binding proteins (CaBP), protein kinases, various interacting proteins, alternative splicing events, and genetic variations.

The Role of Voltage-Gated Calcium Channels in Basal Ganglia Neurodegenerative Disorders

Calcium (Ca2+) plays a central role in regulating many cellular processes and influences cell survival. Several mechanisms can disrupt Ca2+ homeostasis to trigger cell death, including oxidative stress, mitochondrial damage, excitotoxicity, neuroinflammation, autophagy, and apoptosis. Voltage-gated Ca2+ channels (VGCCs) act as the main source of Ca2+ entry into electrically excitable cells, such as neurons, and they are also expressed in glial cells such as astrocytes and oligodendrocytes. The dysregulation of VGCC activity has been reported in both Parkinson's disease (PD) and Huntington's (HD). PD and HD are progressive neurodegenerative disorders (NDs) of the basal ganglia characterized by motor impairment as well as cognitive and psychiatric dysfunctions. This review will examine the putative role of neuronal VGCCs in the pathogenesis and treatment of central movement disorders, focusing on PD and HD. The link between basal ganglia disorders and VGCC physiology will provide a framework for understanding the neurodegenerative processes that occur in PD and HD, as well as a possible path towards identifying new therapeutic targets for the treatment of these debilitating disorders.

Betulinic acid analogs inhibit N- and T-type voltage-gated calcium channels to attenuate nerve-injury associated neuropathic and formalin models of pain

Over the past three decades, there has been a significant growth in the use of natural products, with approximately 80% of individuals using them for some aspect of primary healthcare. Our laboratories have identified and studied natural compounds with analgesic effects from dry land plants or their associated fungus during the past ten years. Here, we isolated and characterized thirteen betulin analogs and fifteen betulinic acid analogs for their capacity to prevent calcium influx brought on by depolarization in sensory neurons. The in vitro inhibition of voltage-gated calcium channels by the top drugs was then assessed using whole cell patch clamp electrophysiology. In vivo experiments, conducted at two sites, evaluated the best compound in acute and tonic, neuropathic, inflammatory, post-operative and visceral models of pain. We found that the betulinic acid analog 8 inhibited calcium influx in rat dorsal root ganglion neurons by inhibiting N- (CaV2.2) and T- (CaV3) type voltage-gated calcium channels. Moreover, intrathecal delivery of analog 8 had analgesic activity in both spared nerve injury model of neuropathic pain and acute and tonic pain induced by formalin. The results presented herein highlight the potential antinociceptive properties of betulinic acid analog 8 and set the stage for the development of novel non-opioid pain therapeutics based on the triterpenoid scaffold of betulinic acid.

Opioid-induced hyperalgesia: Are thalamic T-type calcium channels treatment targets?

Opioid-induced hyperalgesia (OIH) is a state of paradoxically enhanced pain transmission, termed nociceptive sensitization, described to occur in both humans and animals after repeated administration of opioid drugs, including rapidly acting remifentanil. However, molecular mechanisms of OIH remain understudied. In this issue of the JCI, Yan Jin and colleagues provided strong evidence that hyperexcitable thalamocortical networks drive remifentanil-induced hyperalgesia in a rodent model of postsurgical pain. Furthermore, the authors specifically identified an important role of the CaV3.1 isoform of low-voltage-activated or T-type calcium channels (T-channels) in this process. Further experiments are needed to determine whether thalamic T channels could serve as targets for the treatment of OIH.

Targeting intrinsically disordered regions facilitates discovery of calcium channels 3.2 inhibitory peptides for adeno-associated virus-mediated peripheral analgesia

ABSTRACT

Ample data support a prominent role of peripheral T-type calcium channels 3.2 (Ca V 3.2) in generating pain states. Development of primary sensory neuron-specific inhibitors of Ca V 3.2 channels is an opportunity for achieving effective analgesic therapeutics, but success has been elusive. Small peptides, especially those derived from natural proteins as inhibitory peptide aptamers (iPAs), can produce highly effective and selective blockade of specific nociceptive molecular pathways to reduce pain with minimal off-target effects. In this study, we report the engineering of the potent and selective iPAs of Ca V 3.2 from the intrinsically disordered regions (IDRs) of Ca V 3.2 intracellular segments. Using established prediction algorithms, we localized the IDRs in Ca V 3.2 protein and identified several Ca V 3.2iPA candidates that significantly reduced Ca V 3.2 current in HEK293 cells stably expressing human wide-type Ca V 3.2. Two prototype Ca V 3.2iPAs (iPA1 and iPA2) derived from the IDRs of Ca V 3.2 intracellular loops 2 and 3, respectively, were expressed selectively in the primary sensory neurons of dorsal root ganglia in vivo using recombinant adeno-associated virus (AAV), which produced sustained inhibition of calcium current conducted by Ca V 3.2/T-type channels and significantly attenuated both evoked and spontaneous pain behavior in rats with neuropathic pain after tibial nerve injury. Recordings from dissociated sensory neurons showed that AAV-mediated Ca V 3.2iPA expression suppressed neuronal excitability, suggesting that Ca V 3.2iPA treatment attenuated pain by reversal of injury-induced neuronal hypersensitivity. Collectively, our results indicate that Ca V 3.2iPAs are promising analgesic leads that, combined with AAV-mediated delivery in anatomically targeted sensory ganglia, have the potential to be a selective peripheral Ca V 3.2-targeting strategy for clinical treatment of pain.

The calcium channel terminator: hasta la vista pain

Chronic pain remains a major burden and is difficult to treat. N-type calcium channels may be a suitable therapeutic target for analgesics, and a new study from Colecraft and colleagues utilizes a clever new way to modulate their expression to achieve therapeutic benefits in preclinical models of neuropathic pain.

Central and peripheral contributions of T-type calcium channels in pain

Chronic pain is a severely debilitating condition that reflects a long-term sensitization of signal transduction in the afferent pain pathway. Among the key players in this pathway are T-type calcium channels, in particular the Cav3.2 isoform. Because of their biophysical characteristics, these channels are ideally suited towards regulating neuronal excitability. Recent evidence suggests that T-type channels contribute to excitability of neurons all along the ascending and descending pain pathways, within primary afferent neurons, spinal dorsal horn neurons, and within pain-processing neurons in the midbrain and cortex. Here we review the contribution of T-type channels to neuronal excitability and function in each of these neuronal populations and how they are dysregulated in chronic pain conditions. Finally, we discuss their molecular pharmacology and the potential role of these channels as therapeutic targets for chronic pain.

CaV3.2 calcium channels contribute to trigeminal neuralgia

ABSTRACT

Trigeminal neuralgia (TN) is a rare but debilitating disorder characterized by excruciating facial pain, with a higher incidence in women. Recent studies demonstrated that TN patients present mutations in the gene encoding the CaV3.2 T-type calcium channel, an important player in peripheral pain pathways. Here we characterize the role of CaV3.2 channels in TN at two levels. First, we examined the biophysical properties of CACNA1H variants found in TN patients. Second, we investigated the role of CaV3.2 in an animal model of trigeminal neuropathic pain. Whole cell patch clamp recordings from four different mutants expressed in tsA-201 cells (E286K in the pore loop of domain I, H526Y, G563R and P566T in the domain I-II linker) identified a loss-of-function in activation in the E286K mutation and gain-of-function in the G563R and P566T mutations. Moreover, a loss-of-function in inactivation was observed with the E286K and H526Y mutations. Cell surface biotinylation revealed no difference in channel trafficking among the variants. The G563R mutant also caused a gain-of-function in the firing properties of transfected trigeminal ganglion neurons. In female and male mice, constriction of the infraorbital nerve (CION) induced facial thermal heat hyperalgesia. Block of T-type channels with Z944 resulted in antihyperalgesia. The effect of Z944 was absent in CaV3.2-/- mice, indicating that CaV3.2 is the molecular target of the antihyperalgesic Z944 effect. Finally, ELISA analysis revealed increased CaV3.2 channel expression in the spinal trigeminal subnucleus caudalis. Altogether, the present study demonstrates an important role of CaV3.2 channels in trigeminal pain.

The Mechanisms of Plasticity of Nociceptive Ion Channels in Painful Diabetic Neuropathy

Treating pain in patients suffering from small fiber neuropathies still represents a therapeutic challenge for health care providers and drug developers worldwide. Unfortunately, none of the currently available treatments can completely reverse symptoms of either gain or loss of peripheral nerve sensation. Therefore, there is a clear need for novel mechanism-based therapies for peripheral diabetic neuropathy (PDN) that would improve treatment of this serious condition. In this review, we summarize the current knowledge on the mechanisms and causes of peripheral sensory neurons damage in diabetes. In particular, we focused on the subsets of voltage-gated sodium channels, TRP family of ion channels and a CaV3.2 isoform of T-type voltage-gated calcium channels. However, even though their potential is well-validated in multiple rodent models of painful PDN, clinical trials with specific pharmacological blockers of these channels have failed to exhibit therapeutic efficacy. We argue that understanding the development of diabetes and causal relationship between hyperglycemia, glycosylation, and other post-translational modifications may lead to the development of novel therapeutics that would efficiently alleviate painful PDN by targeting disease-specific mechanisms rather than individual nociceptive ion channels.

A Synthetically Accessible Small-Molecule Inhibitor of USP5-Cav3.2 Calcium Channel Interactions with Analgesic Properties

Cav3.2 calcium channels are important mediators of nociceptive signaling in the primary afferent pain pathway, and their expression is increased in various rodent models of chronic pain. Previous work from our laboratory has shown that this is in part mediated by an aberrant expression of deubiquitinase USP5, which associates with these channels and increases their stability. Here, we report on a novel bioactive rhodanine compound (II-1), which was identified in compound library screens. II-1 inhibits biochemical interactions between USP5 and the Cav3.2 domain III-IV linker in a dose-dependent manner, without affecting the enzymatic activity of USP5. Molecular docking analysis reveals two potential binding pockets at the USP5-Cav3.2 interface that are distinct from the binding site of the deubiquitinase inhibitor WP1130 (a.k.a. degrasyn). With an understanding of the ability of some rhodanines to produce false positives in high-throughput screening, we have conducted several orthogonal assays to confirm the validity of this hit, including in vivo experiments. Intrathecal delivery of II-1 inhibited both phases of formalin-induced nocifensive behaviors in mice, as well as abolished thermal hyperalgesia induced by the delivery of complete Freund's adjuvant (CFA) to the hind paw. The latter effects were abolished in Cav3.2 null mice, thus confirming that Cav3.2 is required for the action of II-1. II-1 also mediated a robust inhibition of mechanical allodynia induced by injury to the sciatic nerve. Altogether, our data uncover a novel class of analgesics─well suited to rapid structure-activity relationship studies─that target the Cav3.2/USP5 interface.

IQM-PC332, a Novel DREAM Ligand with Antinociceptive Effect on Peripheral Nerve Injury-Induced Pain

Neuropathic pain is a form of chronic pain arising from damage of the neural cells that sense, transmit or process sensory information. Given its growing prevalence and common refractoriness to conventional analgesics, the development of new drugs with pain relief effects constitutes a prominent clinical need. In this respect, drugs that reduce activity of sensory neurons by modulating ion channels hold the promise to become effective analgesics. Here, we evaluated the mechanical antinociceptive effect of IQM-PC332, a novel ligand of the multifunctional protein downstream regulatory element antagonist modulator (DREAM) in rats subjected to chronic constriction injury of the sciatic nerve as a model of neuropathic pain. IQM-PC332 administered by intraplantar (0.01–10 µg) or intraperitoneal (0.02–1 µg/kg) injection reduced mechanical sensitivity by ≈100% of the maximum possible effect, with ED₅₀ of 0.27 ± 0.05 µg and 0.09 ± 0.01 µg/kg, respectively. Perforated-patch whole-cell recordings in isolated dorsal root ganglion (DRG) neurons showed that IQM-PC332 (1 and 10 µM) reduced ionic currents through voltage-gated K⁺ channels responsible for A-type potassium currents, low, T-type, and high voltage-activated Ca²⁺ channels, and transient receptor potential vanilloid-1 (TRPV1) channels. Furthermore, IQM-PC332 (1 µM) reduced electrically evoked action potentials in DRG neurons from neuropathic animals. It is suggested that by modulating multiple DREAM–ion channel signaling complexes, IQM-PC332 may serve a lead compound of novel multimodal analgesics.

Voltage-dependent CaV3.2 and CaV2.2 channels in nociceptive pathways

Noxious stimuli like cold, heat, pH change, tissue damage, and inflammation depolarize a membrane of peripheral endings of specialized nociceptive neurons which eventually results in the generation of an action potential. The electrical signal is carried along a long axon of nociceptive neurons from peripheral organs to soma located in dorsal root ganglions and further to the dorsal horn of the spinal cord where it is transmitted through a chemical synapse and is carried through the spinal thalamic tract into the brain. Two subtypes of voltage-activated calcium play a major role in signal transmission: a low voltage-activated Ca_V3.2 channel and a high voltage-activated Ca_V2.2 channel. The Ca_V3.2 channel contributes mainly to the signal conductance along nociceptive neurons while the principal role of the Ca_V2.2 channel is in the synaptic transmission at the dorsal horn. Both channels contribute to the signal initiation at peripheral nerve endings. This review summarizes current knowledge about the expression and distribution of these channels in a nociceptive pathway, the regulation of their expression and gating during pain pathology, and their suitability as targets for pharmacological therapy.

Animal Venom Peptides Cause Antinociceptive Effects by Voltage-gated Calcium Channels Activity Blockage

Pain is a complex phenomenon that is usually unpleasant and aversive. It can range widely in intensity, quality, and duration and has diverse pathophysiologic mechanisms and meanings. Voltage-gated sodium and calcium channels are essential to transmitting painful stimuli from the periphery until the dorsal horn of the spinal cord. Thus, blocking voltage-gated calcium channels (VGCCs) can effectively control pain refractory to treatments currently used in the clinic, such as cancer and neuropathic pain. VGCCs blockers isolated of cobra Naja naja kaouthia (α-cobratoxin), spider Agelenopsis aperta (ω-Agatoxin IVA), spider Phoneutria nigriventer (PhTx3.3, PhTx3.4, PhTx3.5, PhTx3.6), spider Hysterocrates gigas (SNX-482), cone snails Conus geographus (GVIA), Conus magus (MVIIA or ziconotide), Conus catus (CVID, CVIE and CVIF), Conus striatus (SO- 3), Conus fulmen (FVIA), Conus moncuri (MoVIA and MoVIB), Conus regularis (RsXXIVA), Conus eburneus (Eu1.6), Conus victoriae (Vc1.1.), Conus regius (RgIA), and spider Ornithoctonus huwena (huwentoxin-I and huwentoxin-XVI) venoms caused antinociceptive effects in different acute and chronic pain models. Currently, ziconotide is the only clinical used N-type VGCCs blocker peptide for chronic intractable pain. However, ziconotide causes different adverse effects, and the intrathecal route of administration also impairs its use in a more significant number of patients. In this sense, peptides isolated from animal venoms or their synthetic forms that act by modulating or blocking VGCCs channels seem to be a relevant prototype for developing new analgesics efficacious and well tolerated by patients.

Change in Cav3.2 T-Type Calcium Channel Induced by Varicella-Zoster Virus Participates in the Maintenance of Herpetic Neuralgia

Pain, as the most prevalent neurological complication of herpes zoster (HZ), may occur before or during the rash onset or even after the rash has recovered. Particularly, postherpetic neuralgia (PHN) is a refractory chronic condition, usually defined as pain persisting for 3 months or longer from the onset of HZ. Pain evoked by HZ impairs the normal physical and emotional functions of the patients, severely reducing their quality of life. However, how zoster-associated pain occurs and develops into PHN are elusive, making PHN difficult to predict. Uncovering the pathogenesis of zoster-associated pain (or HN) helps us to better understand the onset of PHN and supports developing more effective treatments. In this study, we successfully constructed a model for zoster-associated pain through varicella-zoster virus (VZV) infections of mouse footpads and pain behavior assessments. Next, we used the Kyoto Encyclopedia of Genes and Genomes (KEGG) and the Gene Ontology (GO) to analyze PHN rodent dorsal root ganglion (DRG) gene microarray data and found that calcium signal disorder might be involved in the onset of PHN. By using reverse transcription real-time fluorescent quantitative PCR (RT-qPCR) and Western blotting, we confirmed that VZV infection could significantly upregulate the expression of T-type calcium channel Cav3.2 in DRG and spinal dorsal horn (SDH). Intrathecal administration of Cav3.2 blocker (2R/S)-6-prenylnaringenin (6-PNG) relieved mechanical and thermal hyperalgesia induced by VZV. Taken together, our data indicated that VZV might participate in the occurrence and development of HN by upregulating the expression of Cav3.2 in DRG and SDH. These findings will help to reveal the underlying mechanisms on long-lasting pain and PHN formation, providing a new insight that Cav3.2 can be the promising drug target for remitting PHN.

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

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

Targeting the T-type calcium channel Cav3.2 in GABAergic arcuate nucleus neurons to treat obesity

OBJECTIVE

Cav3.2, a T-type low voltage-activated calcium channel widely expressed throughout the central nervous system, plays a vital role in neuronal excitability and various physiological functions. However, the effects of Cav3.2 on energy homeostasis remain unclear. Here, we examined the role of Cav3.2 expressed by hypothalamic GABAergic neurons in the regulation of food intake and body weight in mice and explored the underlying mechanisms.

METHODS

Male congenital Cana1h (the gene coding for Cav3.2) global knockout (Cav3.2KO) mice and their wild type (WT) littermates were first used for metabolic phenotyping studies. By using the CRISPR-Cas9 technique, Cav3.2 was selectively deleted from GABAergic neurons in the arcuate nucleus of the hypothalamus (ARH) by specifically overexpressing Cas9 protein and Cav3.2-targeting sgRNAs in ARH Vgat (VgatARH) neurons. These male mutants (Cav3.2KO-VgatARH) were used to determine whether Cav3.2 expressed by VgatARH neurons is required for the proper regulation of energy balance. Subsequently, we used an electrophysiological patch-clamp recording in ex vivo brain slices to explore the impact of Cav3.2KO on the cellular excitability of VgatARH neurons.

RESULTS

Male Cav3.2KO mice had significantly lower food intake than their WT littermate controls when fed with either a normal chow diet (NCD) or a high-fat diet (HFD). This hypophagia phenotype was associated with increased energy expenditure and decreased fat mass, lean mass, and total body weight. Selective deletion of Cav3.2 in VgatARH neurons resulted in similar feeding inhibition and lean phenotype without changing energy expenditure. These data provides an intrinsic mechanism to support the previous finding on ARH non-AgRP GABA neurons in regulating diet-induced obesity. Lastly, we found that naringenin extract, a predominant flavanone found in various fruits and herbs and known to act on Cav3.2, decreased the firing activity of VgatARH neurons and reduced food intake and body weight. These naringenin-induced inhibitions were fully blocked in Cav3.2KO-VgatARH mice.

CONCLUSION

Our results identified Cav3.2 expressed by VgatARH neurons as an essential intrinsic modulator for food intake and energy homeostasis, which is a potential therapeutic target in the treatment of obesity.

Painful diabetic neuropathy leads to functional CaV3.2 expression and spontaneous activity in skin nociceptors of mice

Painful diabetic neuropathy occurs in approximately 20% of diabetic patients with underlying pathomechanisms not fully understood. We evaluated the contribution of the CaV3.2 isoform of T-type calcium channel to hyperglycemia-induced changes in cutaneous sensory C-fiber functions and neuropeptide release employing the streptozotocin (STZ) diabetes model in congenic mouse strains including global knockouts (KOs). Hyperglycemia established for 3-5 weeks in male C57BL/6J mice led to major reorganizations in peripheral C-fiber functions. Unbiased electrophysiological screening of mechanosensitive single-fibers in isolated hairy hindpaw skin revealed a relative loss of (polymodal) heat sensing in favor of cold sensing. In healthy CaV3.2 KO mice both heat and cold sensitivity among the C-fibers seemed underrepresented in favor of exclusive mechanosensitivity, low-threshold in particular, which deficit became significant in the diabetic KOs. Diabetes also led to a marked increase in the incidence of spontaneous discharge activity among the C-fibers of wildtype mice, which was reduced by the specific CaV3.2 blocker TTA-P2 and largely absent in the KOs. Evaluation restricted to the peptidergic class of nerve fibers - measuring KCl-stimulated CGRP release - revealed a marked reduction in the sciatic nerve by TTA-P2 in healthy but not diabetic wildtypes, the latter showing CGRP release that was as much reduced as in healthy and, to the same extent, in diabetic CaV3.2 KOs. These data suggest that diabetes abrogates all CaV3.2 functionality in the peripheral nerve axons. In striking contrast, diabetes markedly increased the KCl-stimulated CGRP release from isolated hairy skin of wildtypes but not KO mice, and TTA-P2 reversed this increase, strongly suggesting a de novo expression of CaV3.2 in peptidergic cutaneous nerve endings which may contribute to the enhanced spontaneous activity. De-glycosylation by neuraminidase showed clear desensitizing effects, both in regard to spontaneous activity and stimulated CGRP release, but included actions independent of CaV3.2. However, as diabetes-enhanced glycosylation is decisive for intra-axonal trafficking, it may account for the substantial reorganizations of the CaV3.2 distribution. The results may strengthen the validation of CaV3.2 channel as a therapeutic target of treating painful diabetic neuropathy.

Involvement of T-type calcium channels in the mechanism of low dose morphine-induced hyperalgesia in adult male rats

It has been shown that systemic and local administration of ultra-low dose morphine induced a hyperalgesic response via mu-opioid receptors. However, its exact mechanism(s) has not fully been clarified. It is documented that mu-opioid receptors functionally couple to T-type voltage dependent Ca⁺² channels. Here, we investigated the role of T-type calcium channels, amiloride and mibefradil, on the induction of low-dose morphine hyperalgesia in male Wistar rats. The data showed that morphine (0.01 μg i.t. and 1 μg/kg i.p.) could elicit hyperalgesia as assessed by the tail-flick test. Administration of amiloride (5 and 10 μg i.t.) and mibefradil (2.5 and 5 μg i.t.) completely blocked low-dose morphine-induced hyperalgesia in spinal dorsal horn. Amiloride at doses of 1 and 5 mg/kg (i.p.) and mibefradil (9 mg/kg ip) 10 min before morphine (1 μg/kg i.p.) inhibited morphine-induced hyperalgesia. Our results indicate a role for T-type calcium channels in low dose morphine-induced hyperalgesia in rats.

Alpha lipoic acid attenuates evoked and spontaneous pain following surgical skin incision in rats

Our previous studies have implicated CaV3.2 isoform of T-type Ca2+ channels (T-channels) in the development of postsurgical pain. We have also previously established that different T-channel antagonists can alleviate in vivo postsurgical pain. Here we investigated the analgesic potential of another T-channel blocker and endogenous antioxidant molecule, α-lipoic acid (ALA), in a postsurgical pain model in rats. Our in vivo results suggest that single and repetitive intraperitoneal injections of ALA after surgery or preemptively, significantly reduced evoked mechanical hyperalgesia following surgical paw incision. Furthermore, repeated preemptive systemic injections of ALA effectively alleviated spontaneous postsurgical pain as determined by dynamic weight-bearing testing. We expect that our preclinical study may lead to further investigation of analgesic properties and mechanisms of analgesic action of ALA in patients undergoing surgery.

The terpenes camphene and alpha-bisabolol inhibit inflammatory and neuropathic pain via Cav3.2 T-type calcium channels

T-type calcium channels are known molecular targets of certain phytocannabinoids and endocannabinoids. Here we explored the modulation of Cav3.2 T-type calcium channels by terpenes derived from cannabis plants. A screen of eight commercially available terpenes revealed that camphene and alpha-bisabolol mediated partial, but significant inhibition of Cav3.2 channels expressed in tsA-201 cells, as well as native T-type channels in mouse dorsal root ganglion neurons. Both compounds inhibited peak current amplitude with IC50s in the low micromolar range, and mediated an additional small hyperpolarizing shift in half-inactivation voltage. When delivered intrathecally, both terpenes inhibited nocifensive responses in mice that had received an intraplantar injection of formalin, with alpha-bisabolol showing greater efficacy. Both terpenes reduced thermal hyperalgesia in mice injected with Complete Freund's adjuvant. This effect was independent of sex, and absent in Cav3.2 null mice, indicating that these compounds mediate their analgesic properties by acting on Cav3.2 channels. Both compounds also inhibited mechanical hypersensitivity in a mouse model of neuropathic pain. Hence, camphene and alpha-bisabolol have a wide spectrum of analgesic action by virtue of inhibiting Cav3.2 T-type calcium channels.

Neuromedin B receptor stimulation of Cav3.2 T-type Ca2+ channels in primary sensory neurons mediates peripheral pain hypersensitivity

Background: Neuromedin B (Nmb) is implicated in the regulation of nociception of sensory neurons. However, the underlying cellular and molecular mechanisms remain unknown.

Methods: Using patch clamp recording, western blot analysis, immunofluorescent labelling, enzyme-linked immunosorbent assays, adenovirus-mediated shRNA knockdown and animal behaviour tests, we studied the effects of Nmb on the sensory neuronal excitability and peripheral pain sensitivity mediated by Cav3.2 T-type channels.

Results: Nmb reversibly and concentration-dependently increased T-type channel currents (I_T) in small-sized trigeminal ganglion (TG) neurons through the activation of neuromedin B receptor (NmbR). This NmbR-mediated I_T response was G_q protein-coupled, but independent of protein kinase C activity. Either intracellular application of the QEHA peptide or shRNA-mediated knockdown of G_β abolished the NmbR-induced I_T response. Inhibition of protein kinase A (PKA) or AMP-activated protein kinase (AMPK) completely abolished the Nmb-induced I_T response. Analysis of phospho-AMPK (p-AMPK) revealed that Nmb significantly activated AMPK, while AMPK inhibition prevented the Nmb-induced increase in PKA activity. In a heterologous expression system, activation of NmbR significantly enhanced the Cav3.2 channel currents, while the Cav3.1 and Cav3.3 channel currents remained unaffected. Nmb induced TG neuronal hyperexcitability and concomitantly induced mechanical and thermal hypersensitivity, both of which were attenuated by T-type channel blockade. Moreover, blockade of NmbR signalling prevented mechanical hypersensitivity in a mouse model of complete Freund's adjuvant-induced inflammatory pain, and this effect was attenuated by siRNA knockdown of Cav3.2.

Conclusions: Our study reveals a novel mechanism by which NmbR stimulates Cav3.2 channels through a G_βγ-dependent AMPK/PKA pathway. In mouse models, this mechanism appears to drive the hyperexcitability of TG neurons and induce pain hypersensitivity.

Loganin Ameliorates Painful Diabetic Neuropathy by Modulating Oxidative Stress, Inflammation and Insulin Sensitivity in Streptozotocin-Nicotinamide-Induced Diabetic Rats

Loganin is an iridoid glycoside with antioxidant, anti-inflammatory, glucose-lowering activities which may address the pathological mechanisms of painful diabetic neuropathy (PDN) related to inflammation, oxidative stress, and hyperglycemia. This study investigated the underlying mechanisms of action of loganin on PDN. The in vivo model of PDN was established by streptozotocin-nicotinamide (STZ-NA) induction in Sprague Dawley (SD) rats. Subsequently, loganin (5 mg/kg) was administered by daily intraperitoneal injection. High-glucose stimulated human SH-SY5Y cells co-incubated with loganin were used to mimic the in vitro model of PDN. Loganin improved PDN rats' associated pain behaviors (allodynia and hyperalgesia), insulin resistance index (HOMA-IR), and serum levels of superoxide dismutase (SOD), catalase and glutathione. Loganin also reduced pain-associated channel protein CaV3.2 and calcitonin gene-related peptide (CGRP) in the surficial spinal dorsal horn of PDN rats. Loganin inhibited oxidative stress and NF-κB activation and decreased the levels of mRNA and protein of proinflammatory factors IL-1β and TNF-α. Moreover, loganin attenuated insulin resistance by modulating the JNK-IRS-1 (insulin receptor substrate-1)-Akt-GSK3β signaling pathway in PDN rats. These results suggested that loganin improved PDN-mediated pain behaviors by inhibiting oxidative stress-provoked inflammation in the spinal cord, resulting in improved neuropathic pain.

Reversal of Bortezomib-Induced Neurotoxicity by Suvecaltamide, a Selective T-Type Ca-Channel Modulator, in Preclinical Models

Simple Summary

Chemotherapy-induced peripheral neurotoxicity (CIPN) is a side-effect of anti-cancer medications, which can lead to pain, disruptions to movement, and eventually results in the need to interrupt or stop chemotherapy. This study sought to test whether the drug suvecaltamide could help to reduce the impact of the chemotherapy agent bortezomib (BTZ) on symptoms of CIPN using animal models and human cells. Suvecaltamide did reverse negative changes in nerve conduction velocity and intraepidermal nerve fiber density indicative of CIPN in rats, and did not interfere with the anti-cancer effect of BTZ. These results indicate that suvecaltamide could potentially be useful for patients experiencing CIPN, although further mechanistic and molecular studies in vitro and in vivo are required before clinical trials.

Abstract

This study evaluated suvecaltamide, a selective T-type calcium channel modulator, on chemotherapy-induced peripheral neurotoxicity (CIPN) and anti-cancer activity associated with bortezomib (BTZ). Rats received BTZ (0.2 mg/kg thrice weekly) for 4 weeks, then BTZ alone (n = 8) or BTZ+suvecaltamide (3, 10, or 30 mg/kg once daily; each n = 12) for 4 weeks. Nerve conduction velocity (NCV), mechanical threshold, β-tubulin polymerization, and intraepidermal nerve fiber (IENF) density were assessed. Proteasome inhibition was evaluated in peripheral blood mononuclear cells. Cytotoxicity was assessed in human multiple myeloma cell lines (MCLs) exposed to BTZ alone (IC₅₀ concentration), BTZ+suvecaltamide (10, 30, 100, 300, or 1000 nM), suvecaltamide alone, or vehicle. Tumor volume was estimated in athymic nude mice bearing MCL xenografts receiving vehicle, BTZ alone (1 mg/kg twice weekly), or BTZ+suvecaltamide (30 mg/kg once daily) for 28 days, or no treatment (each n = 8). After 4 weeks, suvecaltamide 10 or 30 mg/kg reversed BTZ-induced reduction in NCV, and suvecaltamide 30 mg/kg reversed BTZ-induced reduction in IENF density. Proteasome inhibition and cytotoxicity were similar between BTZ alone and BTZ+suvecaltamide. BTZ alone and BTZ+suvecaltamide reduced tumor volume versus the control (day 18), and BTZ+suvecaltamide reduced tumor volume versus BTZ alone (day 28). Suvecaltamide reversed CIPN without affecting BTZ anti-cancer activity in preclinical models.

Targeting T-type/CaV3.2 channels for chronic pain

T-type calcium channels regulate neuronal excitability and are important contributors of pain processing. CaV3.2 channels are the major isoform expressed in nonpeptidergic and peptidergic nociceptive neurons and are emerging as promising targets for pain treatment. Numerous studies have shown that CaV3.2 expression and/or activity are significantly increased in spinal dorsal horn and in dorsal root ganglia neurons in different inflammatory and neuropathic pain models. Pharmacological campaigns to inhibit the functional expression of CaV3.2 for treatment of pain have focused on the development of direct channel blockers, but none have produced lead candidates. Targeting the proteins that regulate the trafficking or transcription, and the ones that modify the channels via post-translational modifications are alternative means to regulate expression and function of CaV3.2 channels and hence to develop new drugs to control pain. Here we synthesize data supporting a role for CaV3.2 in numerous pain modalities and then discuss emerging opportunities for the indirect targeting of CaV3.2 channels.

Breeding of Cav2.3 deficient mice reveals Mendelian inheritance in contrast to complex inheritance in Cav3.2 null mutant breeding

High voltage-activated Ca_v2.3 R-type Ca²⁺ channels and low voltage-activated Ca_v3.2 T-type Ca²⁺ channels were reported to be involved in numerous physiological and pathophysiological processes. Many of these findings are based on studies in Ca_v2.3 and Ca_v3.2 deficient mice. Recently, it has been proposed that inbreeding of Ca_v2.3 and Ca_v3.2 deficient mice exhibits significant deviation from Mendelian inheritance and might be an indication for potential prenatal lethality in these lines. In our study, we analyzed 926 offspring from Ca_v3.2 breedings and 1142 offspring from Ca_v2.3 breedings. Our results demonstrate that breeding of Ca_v2.3 deficient mice shows typical Mendelian inheritance and that there is no indication of prenatal lethality. In contrast, Ca_v3.2 breeding exhibits a complex inheritance pattern. It might be speculated that the differences in inheritance, particularly for Ca_v2.3 breeding, are related to other factors, such as genetic specificities of the mutant lines, compensatory mechanisms and altered sperm activity.

The T-type calcium channel antagonist, Z944, reduces spinal excitability and pain hypersensitivity

Background and Purpose

T‐type voltage‐gated calcium channels are an emerging therapeutic target for neurological disorders including epilepsy and pain. Inhibition of T‐type channels reduces the excitability of peripheral nociceptive sensory neurons and reverses pain hypersensitivity in male rodent pain models. However, administration of peripherally restricted T‐type antagonists failed to show efficacy in multiple clinical and preclinical pain trials, suggesting that inhibition of peripheral T‐type channels alone may be insufficient for pain relief.

Experimental Approach

We utilized the selective and CNS‐penetrant T‐type channel antagonist, Z944, in electrophysiological, calcium imaging and behavioural paradigms to determine its effect on lamina I neuron excitability and inflammatory pain behaviours.

Key Results

Voltage‐clamp recordings from lamina I spinal neurons of adult rats revealed that approximately 80% of neurons possess a low threshold T‐type current, which was blocked by Z944. Due to this highly prevalent T‐type current, Z944 potently blocked action‐potential evoked somatic and dendritic calcium transients in lamina I neurons. Moreover, application of Z944 to spinal cord slices attenuated action potential firing rates in over half of laminae I/II neurons. Finally, we found that intraperitoneal injection of Z944 (1–10 mg·kg⁻¹) dose‐dependently reversed mechanical allodynia in the complete Freund's adjuvant model of persistent inflammatory pain, with a similar magnitude and time course of analgesic effects between male and female rats.

Conclusion and Implications

T‐type calcium channels critically shape the excitability of lamina I pain processing neurons and inhibition of these channels by the clinical stage antagonist Z944 potently reverses pain hypersensitivity across sexes.

A modulator of the low-voltage-activated T-type calcium channel that reverses HIV glycoprotein 120-, paclitaxel-, and spinal nerve ligation-induced peripheral neuropathies

The voltage-gated calcium channels CaV3.1-3.3 constitute the T-type subfamily, whose dysfunctions are associated with epilepsy, psychiatric disorders, and chronic pain. The unique properties of low-voltage-activation, faster inactivation, and slower deactivation of these channels support their role in modulation of cellular excitability and low-threshold firing. Thus, selective T-type calcium channel antagonists are highly sought after. Here, we explored Ugi-azide multicomponent reaction products to identify compounds targeting T-type calcium channel. Of the 46 compounds tested, an analog of benzimidazolonepiperidine-5bk (1-{1-[(R)-{1-[(1S)-1-phenylethyl]-1H-1,2,3,4-tetrazol-5-yl}(thiophen-3-yl)methyl]piperidin-4-yl}-2,3-dihydro-1H-1,3-benzodiazol-2-one) modulated depolarization-induced calcium influx in rat sensory neurons. Modulation of T-type calcium channels by 5bk was further confirmed in whole-cell patch clamp assays in dorsal root ganglion (DRG) neurons, where pharmacological isolation of T-type currents led to a time- and concentration-dependent regulation with a low micromolar IC50. Lack of an acute effect of 5bk argues against a direct action on T-type channels. Genetic knockdown revealed CaV3.2 to be the isoform preferentially modulated by 5bk. High voltage-gated calcium, as well as tetrodotoxin-sensitive and -resistant sodium, channels were unaffected by 5bk. 5bk inhibited spontaneous excitatory postsynaptic currents and depolarization-evoked release of calcitonin gene-related peptide from lumbar spinal cord slices. Notably, 5bk did not bind human mu, delta, or kappa opioid receptors. 5bk reversed mechanical allodynia in rat models of HIV-associated neuropathy, chemotherapy-induced peripheral neuropathy, and spinal nerve ligation-induced neuropathy, without effects on locomotion or anxiety. Thus, 5bk represents a novel T-type modulator that could be used to develop nonaddictive pain therapeutics.

Glycosylation of CaV3.2 Channels Contributes to the Hyperalgesia in Peripheral Neuropathy of Type 1 Diabetes

Our previous studies implicated glycosylation of the Ca_V3.2 isoform of T-type Ca²⁺ channels (T-channels) in the development of Type 2 painful peripheral diabetic neuropathy (PDN). Here we investigated biophysical mechanisms underlying the modulation of recombinant Ca_V3.2 channel by de-glycosylation enzymes such as neuraminidase (NEU) and PNGase-F (PNG), as well as their behavioral and biochemical effects in painful PDN Type 1. In our in vitro study we used whole-cell recordings of current-voltage relationships to confirm that Ca_V3.2 current densities were decreased ~2-fold after de-glycosylation. Furthermore, de-glycosylation induced a significant depolarizing shift in the steady-state relationships for activation and inactivation while producing little effects on the kinetics of current deactivation and recovery from inactivation. PDN was induced in vivo by injections of streptozotocin (STZ) in adult female C57Bl/6j wild type (WT) mice, adult female Sprague Dawley rats and Ca_V3.2 knock-out (KO mice). Either NEU or vehicle (saline) were locally injected into the right hind paws or intrathecally. We found that injections of NEU, but not vehicle, completely reversed thermal and mechanical hyperalgesia in diabetic WT rats and mice. In contrast, NEU did not alter baseline thermal and mechanical sensitivity in the Ca_V3.2 KO mice which also failed to develop painful PDN. Finally, we used biochemical methods with gel-shift analysis to directly demonstrate that N-terminal fragments of native Ca_V3.2 channels in the dorsal root ganglia (DRG) are glycosylated in both healthy and diabetic animals. Our results demonstrate that in sensory neurons glycosylation-induced alterations in Ca_V3.2 channels in vivo directly enhance diabetic hyperalgesia, and that glycosylation inhibitors can be used to ameliorate painful symptoms in Type 1 diabetes. We expect that our studies may lead to a better understanding of the molecular mechanisms underlying painful PDN in an effort to facilitate the discovery of novel treatments for this intractable disease.

Preemptive Analgesic Effect of Intrathecal Applications of Neuroactive Steroids in a Rodent Model of Post-Surgical Pain: Evidence for the Role of T-Type Calcium Channels

Preemptive management of post-incisional pain remains challenging. Here, we examined the role of preemptive use of neuroactive steroids with activity on low-voltage activated T-type Ca²⁺ channels (T-channels) and γ-aminobutyric acid A (GABA_A) receptors in the development and maintenance of post-incisional pain. We use neuroactive steroids with distinct effects on GABA_A receptors and/or T-channels: Alphaxalone (combined GABAergic agent and T-channel inhibitor), ECN (T-channel inhibitor), CDNC24 (GABAergic agent), and compared them with an established analgesic, morphine (an opioid agonist without known effect on either T-channels or GABA_A receptors). Adult female rats sustained the skin and muscle incision on the plantar surface of the right paw. We injected the agents of choice intrathecally either before or after the development of post-incisional pain. The pain development was monitored by studying mechanical hypersensitivity. Alphaxalone and ECN, but not morphine, are effective in alleviating mechanical hyperalgesia when administered preemptively whereas morphine provides dose-dependent pain relief only when administered once the pain had developed. CDNC24 on the other hand did not offer any analgesic benefit. Neuroactive steroids that inhibit T-currents—Alphaxalone and ECN—unlike morphine, are effective preemptive analgesics that may offer a promising therapeutic approach to the treatment of post-incisional pain, especially mechanical hypersensitivity.

Contribution of T-Type Calcium Channels to Spinal Cord Injury-Induced Hyperexcitability of Nociceptors

A hyperexcitable state and spontaneous activity of nociceptors have been suggested to play a critical role in the development of chronic neuropathic pain following spinal cord injury (SCI). In male rats, we employed the action potential-clamp technique to determine the underlying ionic mechanisms responsible for driving SCI-nociceptors to a hyperexcitable state and for triggering their spontaneous activity. We found that the increased activity of low voltage activated T-type calcium channels induced by the injury sustains the bulk (∼60-70%) of the inward current active at subthreshold voltages during the interspike interval in SCI-nociceptors, with a modest contribution (∼10-15%) from tetrodotoxin (TTX)-sensitive and TTX-resistant sodium channels and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. In current-clamp recordings, inhibition of T-type calcium channels with 1 μm TTA-P2 reduced both the spontaneous and the evoked firing in response to current injections in SCI-nociceptors to a level similar to sham-nociceptors. Electrophysiology in vitro was then combined with the conditioned place preference (CPP) paradigm to determine the relationship between the increased activity of T-type channels in SCI-nociceptors and chronic neuropathic pain following SCI. The size of the interspike T-type calcium current recorded from nociceptors isolated from SCI rats showing TTA-P2-induced CPP (responders) was ∼6 fold greater than the interspike T-type calcium current recorded from nociceptors isolated from SCI rats without TTA-P2-induced CPP (non-responders). Taken together, our data suggest that the increased activity of T-type calcium channels induced by the injury plays a primary role in driving SCI-nociceptors to a hyperexcitable state and contributes to chronic neuropathic pain following SCI.SIGNIFICANCE STATEMENT Chronic neuropathic pain is a major comorbidity of spinal cord injury (SCI), affecting up to 70-80% of patients. Anticonvulsant and tricyclic antidepressant drugs are first line analgesics used to treat SCI-induced neuropathic pain, but their efficacy is very limited. A hyperexcitable state and spontaneous activity of SCI-nociceptors have been proposed as a possible underlying cause for the development of chronic neuropathic pain following SCI. Here, we show that the increased activity of T-type calcium channels induced by the injury plays a major role in driving SCI-nociceptors to a hyperexcitable state and for promoting their spontaneous activity, suggesting that T-type calcium channels may represent a pharmacological target to treat SCI-induced neuropathic pain.

Deficiency of glucagon gene-derived peptides induces peripheral polyneuropathy in mice

Although diabetic polyneuropathy (DPN) is the commonest diabetic complication, its pathology remains to be clarified. As previous papers have suggested the neuroprotective effects of glucagon-like peptide-1 in DPN, the current study investigated the physiological indispensability of glucagon gene-derived peptides (GCGDPs) including glucagon-like peptide-1 in the peripheral nervous system (PNS). Neurological functions and neuropathological changes of GCGDP deficient (gcg-/-) mice were examined. The gcg-/- mice showed tactile allodynia and thermal hyperalgesia at 12-18 weeks old, followed by tactile and thermal hypoalgesia at 36 weeks old. Nerve conduction studies revealed a decrease in sensory nerve conduction velocity at 36 weeks old. Pathological findings showed a decrease in intraepidermal nerve fiber densities. Electron microscopy revealed a decrease in circularity and an increase in g-ratio of myelinated fibers and a decrease of unmyelinated fibers in the sural nerves of the gcg-/- mice. Effects of glucagon on neurite outgrowth were examined using an ex vivo culture of dorsal root ganglia. A supraphysiological concentration of glucagon promoted neurite outgrowth. In conclusion, the mice with deficiency of GCGDPs developed peripheral neuropathy with age. Furthermore, glucagon might have neuroprotective effects on the PNS of mice. GCGDPs might be involved in the pathology of DPN.

Novel neuroactive steroid with hypnotic and T-type calcium channel blocking properties exerts effective analgesia in a rodent model of post-surgical pain

BACKGROUND AND PURPOSE

Neuroactive steroid (3β,5β,17β)-3-hydroxyandrostane-17-carbonitrile (3β-OH) is a novel hypnotic and voltage-dependent blocker of T-type calcium channels. Here, we examine its potential analgesic effects and adjuvant anaesthetic properties using a post-surgical pain model in rodents.

EXPERIMENTAL APPROACH

Analgesic properties of 3β-OH were investigated in thermal and mechanical nociceptive tests in sham or surgically incised rats and mice, with drug injected either systemically (intraperitoneal) or locally via intrathecal or intraplantar routes. Hypnotic properties of 3β-OH and its use as an adjuvant anaesthetic in combination with isoflurane were investigated using behavioural experiments and in vivo EEG recordings in adolescent rats.

KEY RESULTS

A combination of 1% isoflurane with 3β-OH (60 mg·kg^-1 , i.p.) induced suppression of cortical EEG and stronger thermal and mechanical anti-hyperalgesia during 3 days post-surgery, when compared to isoflurane alone and isoflurane with morphine. 3β-OH exerted prominent enantioselective thermal and mechanical antinociception in healthy rats and reduced T-channel-dependent excitability of primary sensory neurons. Intrathecal injection of 3β-OH alleviated mechanical hyperalgesia, while repeated intraplantar application alleviated both thermal and mechanical hyperalgesia in the rats after incision. Using mouse genetics, we found that Ca_V 3.2 T-calcium channels are important for anti-hyperalgesic effect of 3β-OH and are contributing to its hypnotic effect.

CONCLUSION AND IMPLICATIONS

Our study identifies 3β-OH as a novel analgesic for surgical procedures. 3β-OH can be used to reduce T-channel-dependent excitability of peripheral sensory neurons as an adjuvant for induction and maintenance of general anaesthesia while improving analgesia and lowering the amount of volatile anaesthetic needed for surgery.