Upregulation of T-type Ca2+ channels in primary sensory neurons in spinal nerve injury

The analgesic effects of botulinum neurotoxin by modulating pain-related receptors; A literature review

Botulinum neurotoxins (BoNTs), produced by Clostridium botulinum, have been used for the treatment of various central and peripheral neurological conditions. Recent studies have suggested that BoNTs may also have a beneficial effect on pain conditions. It has been hypothesized that one of the mechanisms underlying BoNTs' analgesic effects is the inhibition of pain-related receptors' transmission to the neuronal cell membrane. BoNT application disrupts the integration of synaptic vesicles with the cellular membrane, which is responsible for transporting various receptors, including pain receptors such as TRP channels, calcium channels, sodium channels, purinergic receptors, neurokinin-1 receptors, and glutamate receptors. BoNT also modulates the opioidergic system and the GABAergic system, both of which are involved in the pain process. Understanding the cellular and molecular mechanisms underlying these effects can provide valuable insights for the development of novel therapeutic approaches for pain management. This review aims to summarize the experimental evidence of the analgesic functions of BoNTs and discuss the cellular and molecular mechanisms by which they can act on pain conditions by inhibiting the transmission of pain-related receptors.

Involvement of interaction of Cav3.2 and nociceptive TRPA1 in pathological pain transmission

T-type Ca2+ channels and TRPA1 expressed in sensory neurons are involved in pain. We previously demonstrated a functional interaction of these channels under physiological conditions. Here we investigated the possible involvement of these channels in inflammatory pain condition. We also evaluated the relationship of these channels endogenously expressed in RIN-14B, a rat pancreatic islet tumor cell line. In dorsal root ganglion (DRG) neurons innervated inflammatory side, [Ca2+]i increases induced by 15 mM KCl (15K) were enhanced in neurons responded to AITC. This enhancement was not observed in genetically TRPA1-deficient neurons. The T-type and AITC-induced currents were larger in neurons of the inflammatory side than in those of the control one. In DRGs of the inflammatory side, the protein expression of Cav3.2, but not TRPA1, was increased. In RIN-14B, 15K-induced [Ca2+]i increases were decreased by blockers of T-type Ca2+ channel and TRPA1, and by TRPA1-silencing. Immunoprecipitation suggested the coexistent of these channels in sensory neurons and RIN-14B. In mice with inflammation, mechanical hypersensitivity was suppressed by blockers of both channels. These data suggest that the interaction of Cav3.2 with TRPA1 in sensory neurons is enhanced via the augmentation of the activities of both channels under inflammatory conditions, indicating that both channels are therapeutic targets for inflammatory pain.

Potential role of Schwann cells in neuropathic pain

Neuropathic pain (NPP) is a common syndrome associated with most forms of disease, which poses a serious threat to human health. NPP may persist even after the nociceptive stimulation is eliminated, and treatment is extremely challenging in such cases. Schwann cells (SCs) form the myelin sheaths around neuronal axons and play a crucial role in neural information transmission. SCs can secrete trophic factors to nourish and protect axons, and can further secrete pain-related factors to induce pain. SCs may be activated by peripheral nerve injury, triggering the transformation of myelinated and non-myelinated SCs into cell phenotypes that specifically promote repair. These differentiated SCs provide necessary signals and spatial clues for survival, axonal regeneration, and nerve regeneration of damaged neurons. They can further change the microenvironment around the regions of nerve injury, and relieve the pain by repairing the injured nerve. Herein, we provide a comprehensive overview of the biological characteristics of SCs, discuss the relationship between SCs and nerve injury, and explore the potential mechanism of SCs and the occurrence of NPP. Moreover, we summarize the feasible strategies of SCs in the treatment of NPP, and attempt to elucidate the deficiencies and defects of SCs in the treatment of NPP.

Inhibition of T-Type Calcium Channels With TTA-P2 Reduces Chronic Neuropathic Pain Following Spinal Cord Injury in Rats

Spinal cord injury (SCI)-induced neuropathic pain (SCI-NP) develops in up to 60 to 70% of people affected by traumatic SCI, leading to a major decline in quality of life and increased risk for depression, anxiety, and addiction. Gabapentin and pregabalin, together with antidepressant drugs, are commonly prescribed to treat SCI-NP, but their efficacy is unsatisfactory. The limited efficacy of current pharmacological treatments for SCI-NP likely reflects our limited knowledge of the underlying mechanism(s) responsible for driving the maintenance of SCI-NP. The leading hypothesis in the field supports a major role for spontaneously active injured nociceptors in driving the maintenance of SCI-NP. Recent data from our laboratory provided additional support for this hypothesis and identified the T-type calcium channels as key players in driving the spontaneous activity of SCI-nociceptors, thus providing a rational pharmacological target to treat SCI-NP. To test whether T-type calcium channels contribute to the maintenance of SCI-NP, male and female SCI and sham rats were treated with TTA-P2 (a blocker of T-type calcium channels) to determine its effects on mechanical hypersensitivity (as measured with the von Frey filaments) and spontaneous ongoing pain (as measured with the conditioned place preference paradigm), and compared them to the effects of gabapentin, a blocker of high voltage-activated calcium channels. We found that both TTA-P2 and gabapentin reduced mechanical hypersensitivity in male and females SCI rats, but surprisingly only TTA-P2 reduced spontaneous ongoing pain in male SCI rats. PERSPECTIVES: SCI-induced neuropathic pain, and in particular the spontaneous ongoing pain component, is notoriously very difficult to treat. Our data provide evidence that inhibition of T-type calcium channels reduces spontaneous ongoing pain in SCI rats, supporting a clinically relevant role for T-type channels in the maintenance of SCI-induced neuropathic pain.

MEPIRAPIM-derived synthetic cannabinoids inhibit T-type calcium channels with divergent effects on seizures in rodent models of epilepsy

Background: T-type Ca²⁺ channels (Ca_v3) represent emerging therapeutic targets for a range of neurological disorders, including epilepsy and pain. To aid the development and optimisation of new therapeutics, there is a need to identify novel chemical entities which act at these ion channels. A number of synthetic cannabinoid receptor agonists (SCRAs) have been found to exhibit activity at T-type channels, suggesting that cannabinoids may provide convenient chemical scaffolds on which to design novel Ca_v3 inhibitors. However, activity at cannabinoid type 1 (CB₁) receptors can be problematic because of central and peripheral toxicities associated with potent SCRAs. The putative SCRA MEPIRAPIM and its analogues were recently identified as Ca_v3 inhibitors with only minimal activity at CB₁ receptors, opening the possibility that this scaffold may be exploited to develop novel, selective Ca_v3 inhibitors. Here we present the pharmacological characterisation of SB2193 and SB2193F, two novel Ca_v3 inhibitors derived from MEPIRAPIM. Methods: The potency of SB2193 and SB2193F was evaluated in vitro using a fluorometric Ca²⁺ flux assay and confirmed using whole-cell patch-clamp electrophysiology. In silico docking to the cryo-EM structure of Ca_v3.1 was also performed to elucidate structural insights into T-type channel inhibition. Next, in vivo pharmacokinetic parameters in mouse brain and plasma were determined using liquid chromatography-mass spectroscopy. Finally, anticonvulsant activity was assayed in established genetic and electrically-induced rodent seizure models. Results: Both MEPIRAPIM derivatives produced potent inhibition of Ca_v3 channels and were brain penetrant, with SB2193 exhibiting a brain/plasma ratio of 2.7. SB2193 was further examined in mouse seizure models where it acutely protected against 6 Hz-induced seizures. However, SB2193 did not reduce spontaneous seizures in the Scn1a ^+/- mouse model of Dravet syndrome, nor absence seizures in the Genetic Absence Epilepsy Rat from Strasbourg (GAERS). Surprisingly, SB2193 appeared to increase the incidence and duration of spike-and-wave discharges in GAERS animals over a 4 h recording period. Conclusion: These results show that MEPIRAPIM analogues provide novel chemical scaffolds to advance Ca_v3 inhibitors against certain seizure types.

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.

Histone methylation-mediated microRNA-32-5p down-regulation in sensory neurons regulates pain behaviors via targeting Cav3.2 channels

In this study, we identify microRNA-32-5p (miR-32-5p) as a key functional noncoding RNA in trigeminal-mediated neuropathic pain. We report that injury-induced histone methylation attenuates the binding of glucocorticoid receptor to the promoter region of the miR-32-5p gene and decreases the expression of miR-32-5p, in turn promoting the development of neuropathic pain through regulation of Cav3.2 channels. miRNA-mediated gene regulation has been proposed as a therapeutic approach in neuropathic pain. Our findings identify miR-32-5p replenishment as a therapeutic strategy for treating chronic neuropathic pain.

microRNA (miRNA)–mediated gene regulation has been studied as a therapeutic approach, but its functional regulatory mechanism in neuropathic pain is not well understood. Here, we identify that miRNA-32-5p (miR-32-5p) is a functional RNA in regulating trigeminal-mediated neuropathic pain. High-throughput sequencing and qPCR analysis showed that miR-32-5p was the most down-regulated miRNA in the injured trigeminal ganglion (TG) of rats. Intra-TG injection of miR-32-5p agomir or overexpression of miR-32-5p by lentiviral delivery in neurons of the injured TG attenuated established trigeminal neuropathic pain. miR-32-5p overexpression did not affect acute physiological pain, while miR-32-5p down-regulation in intact rats was sufficient to cause pain-related behaviors. Nerve injury increased the methylated histone occupancy of binding sites for the transcription factor glucocorticoid receptor in the miR-32-5p promoter region. Inhibition of the enzymes that catalyze H3K9me2 and H3K27me3 restored the expression of miR-32-5p and markedly attenuated pain behaviors. Further, miR-32-5p–targeted Cav3.2 T-type Ca²⁺ channels and decreased miR-32-5p associated with neuropathic pain caused an increase in Cav3.2 protein expression and T-type channel currents. Conversely, miR-32-5p overexpression in injured TG suppressed the increased expression of Cav3.2 and reversed mechanical allodynia. Together, we conclude that histone methylation-mediated miR-32-5p down-regulation in TG neurons regulates trigeminal neuropathic pain by targeting Cav3.2 channels.

T-type Ca2+ channels play a dual role in modulating the excitability of dorsal root ganglia neurons

A subgroup of low-threshold dorsal root ganglia (DRG) neurons discharge action potentials (APs) with an afterdepolarizing potential (ADP). The ADP is formed by T-type Ca²⁺ currents. It is known that T-type Ca²⁺ currents contribute to neuropathic pain. However, the change in ADP-firing of injured DRG neurons has not been widely studied yet. Here we applied patch clamp to record ADP-firing and T-type Ca²⁺ currents in intact and chronically compressed DRG (CCD) neurons and examined T-type Ca²⁺ channel proteins expression with western blotting. After CCD injury, the incidences of both ADP firing and non-ADP burst firing increased, and T-type Ca²⁺ channels contributed to both of these firing patterns. The neurons discharging large-amplitude-ADP firing were TTX-insensitive, implying that high-density T-type Ca²⁺ channels might cooperate with TTX-insensitive Na⁺ channels to reduce the AP threshold. By contrast, the neurons displaying non-ADP burst firing were TTX-sensitive, implying that low density T-type Ca²⁺ channels may cooperate with TTX-sensitive Na⁺ channels to increase AP number. In DRG neurons, T-type Ca²⁺ currents density varied widely, ranging between 100 pA/pF and 5 pA/pF. After injury, the proportion of DRG neurons with large T-type Ca²⁺ currents increased in parallel with the increase in the incidence of large-amplitude-ADP firing. And in addition to Cav3.2, Cav3.3 channels are also likely to contribute to low-threshold firing. The data revealed that T-type Ca²⁺ channels may play a dual role in modulating the injured neurons' high excitability through a cooperative process with Na⁺ channels, thereby contributing to neuropathic pain.

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.

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.

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.

Neuropathic pain modeling: Focus on synaptic and ion channel mechanisms

Animal models of pain consist of modeling a pain-like state and measuring the consequent behavior. The first animal models of neuropathic pain (NP) were developed in rodents with a total lesion of the sciatic nerve. Later, other models targeting central or peripheral branches of nerves were developed to identify novel mechanisms that contribute to persistent pain conditions in NP. Objective assessment of pain in these different animal models represents a significant challenge for pre-clinical research. Multiple behavioral approaches are used to investigate and to validate pain phenotypes including withdrawal reflex to evoked stimuli, vocalizations, spontaneous pain, but also emotional and affective behaviors. Furthermore, animal models were very useful in investigating the mechanisms of NP. This review will focus on a detailed description of rodent models of NP and provide an overview of the assessment of the sensory and emotional components of pain. A detailed inventory will be made to examine spinal mechanisms involved in NP-induced hyperexcitability and underlying the current pharmacological approaches used in clinics with the possibility to present new avenues for future treatment. The success of pre-clinical studies in this area of research depends on the choice of the relevant model and the appropriate test based on the objectives of the study.

The role of cyclin-dependent kinase 5 in neuropathic pain

The chronification of pain can be attributed to changes in membrane receptors and channels underlying neuronal plasticity and signal transduction largely within nociceptive neurons that initiate and maintain pathological pain states. These proteins are subject to dynamic modification by posttranslational modifications, creating a code that controls protein function in time and space. Phosphorylation is an important posttranslational modification that affects ∼30% of proteins in vivo. Increased phosphorylation of various nociceptive ion channels and of their modulators underlies sensitization of different pain states. Cyclin-dependent kinases are proline-directed serine/threonine kinases that impact various biological and cellular systems. Cyclin-dependent kinase 5 (Cdk5), one member of this kinase family, and its activators p35 and p39 are expressed in spinal nerves, dorsal root ganglia, and the dorsal horn of the spinal cord. In neuropathic pain conditions, expression and/or activity of Cdk5 is increased, implicating Cdk5 in nociception. Experimental evidence suggests that Cdk5 is regulated through its own phosphorylation, through increasing p35's interaction with Cdk5, and through cleavage of p35 into p25. This narrative review discusses the molecular mechanisms of Cdk5-mediated regulation of target proteins involved in neuropathic pain. We focus on Cdk5 substrates that have been linked to nociceptive pathways, including channels (eg, transient receptor potential cation channel and voltage-gated calcium channel), proteins involved in neurotransmitter release (eg, synaptophysin and collapsin response mediator protein 2), and receptors (eg, glutamate, purinergic, and opioid). By altering the phosphoregulatory "set point" of proteins involved in pain signaling, Cdk5 thus appears to be an attractive target for treating neuropathic pain conditions.

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.

Peripheral Pain Modulation of Chrysaora pacifica Jellyfish Venom Requires Both Ca2+ Influx and TRPA1 Channel Activation in Rats

The venom of jellyfish triggers severe dermal pain along with inflammation and tissue necrosis, and occasionally, induces internal organ dysfunction. However, the basic mechanisms underlying its cytotoxic effects are still unknown. Here, we report one of the mechanisms involved in peripheral pain modulation associated with inflammatory and neurotoxic oxidative signaling in rats using the venom of jellyfish, Chrysaora pacifica (CpV). This jellyfish is identified by brown tentacles carrying nematocysts filled with cytotoxic venom that induces severe pain, pruritus, tentacle marks, and blisters. The subcutaneous injection of CpV into rat forepaws in behavioral tests triggered nociceptive response with a decreased threshold for mechanical pain perception. These responses lasted up to 48 h and were completely blocked by verapamil and TTA-P2, T-type Ca²⁺ channel blockers, or HC030031, a transient receptor potential cation ankyrin 1 (TRPA1) channel blocker, while another Ca²⁺ channel blocker, nimodipine, was ineffective. Also, treatment with Ca²⁺ chelators (EGTA and BaptaAM) significantly alleviated the CpV-induced pain response. These results indicate that CpV-induced pain modulation may require both Ca²⁺ influx through the T-type Ca²⁺ channels and activation of TRPA1 channels. Furthermore, CpV induced Ca²⁺-mediated oxidative neurotoxicity in the dorsal root ganglion (DRG) and cortical neurons dissociated from rats, resulting in decreased neuronal viability and increased intracellular levels of ROS. Taken together, CpV may activate Ca²⁺-mediated oxidative signaling to produce excessive ROS acting as an endogenous agonist of TRPA1 channels in the peripheral terminals of the primary afferent neurons, resulting in persistent inflammatory pain. These findings provide strong evidence supporting the therapeutic effectiveness of blocking oxidative signaling against pain and cytotoxicity induced by jellyfish venom.

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.

Pathophysiological roles and therapeutic potential of voltage-gated ion channels (VGICs) in pain associated with herpesvirus infection

Herpesvirus is ranked as one of the grand old members of all pathogens. Of all the viruses in the superfamily, Herpes simplex virus type 1 (HSV-1) is considered as a model virus for a variety of reasons. In a permissive non-neuronal cell culture, HSV-1 concludes the entire life cycle in approximately 18–20 h, encoding approximately 90 unique transcriptional units. In latency, the robust viral gene expression is suppressed in neurons by a group of noncoding RNA. Historically the lesions caused by the virus can date back to centuries ago. As a neurotropic pathogen, HSV-1 is associated with painful oral lesions, severe keratitis and lethal encephalitis. Transmission of pain signals is dependent on the generation and propagation of action potential in sensory neurons. T-type Ca²⁺ channels serve as a preamplifier of action potential generation. Voltage-gated Na⁺ channels are the main components for action potential production. This review summarizes not only the voltage-gated ion channels in neuropathic disorders but also provides the new insights into HSV-1 induced pain.

Transcriptomic analysis of glycan-processing genes in the dorsal root ganglia of diabetic mice and functional characterization on Cav3.2 channels

Ca_v3.2 T-type calcium channels play an essential role in the transmission of peripheral nociception in the dorsal root ganglia (DRG) and alteration of Ca_v3.2 expression is associated with the development of peripheral painful diabetic neuropathy (PDN). Several studies have previously documented the role of glycosylation in the expression and functioning of Ca_v3.2 and suggested that altered glycosylation of the channel may contribute to the aberrant expression of the channel in diabetic conditions. In this study, we aimed to analyze the expression of glycan-processing genes in DRG neurons from a leptin-deficient genetic mouse model of diabetes (db/db). Transcriptomic analysis revealed that several glycan-processing genes encoding for glycosyltransferases and sialic acid-modifying enzymes were upregulated in diabetic conditions. Functional analysis of these enzymes on recombinant Ca_v3.2 revealed an unexpected loss-of-function of the channel. Collectively, our data indicate that diabetes is associated with an alteration of the glycosylation machinery in DRG neurons. However, individual action of these enzymes when tested on recombinant Ca_v3.2 cannot explain the observed upregulation of T-type channels under diabetic conditions.

Abbreviations: Galnt16: Polypeptide N-acetylgalactosaminyltransferase 16; B3gnt8: UDP-GlcNAc:betaGal beta-1,3-N-acetylglucosaminyltransferase 8; B4galt1: Beta-1,4-galactosyltransferase 1; St6gal1: Beta-galactoside alpha-2,6-sialyltransferase 1; Neu3: Sialidase-3

Betulinic acid, derived from the desert lavender Hyptis emoryi, attenuates paclitaxel-, HIV-, and nerve injury-associated peripheral sensory neuropathy via block of N- and T-type calcium channels

The Federal Pain Research Strategy recommended development of nonopioid analgesics as a top priority in its strategic plan to address the significant public health crisis and individual burden of chronic pain faced by >100 million Americans. Motivated by this challenge, a natural product extracts library was screened and identified a plant extract that targets activity of voltage-gated calcium channels. This profile is of interest as a potential treatment for neuropathic pain. The active extract derived from the desert lavender plant native to southwestern United States, when subjected to bioassay-guided fractionation, afforded 3 compounds identified as pentacyclic triterpenoids, betulinic acid (BA), oleanolic acid, and ursolic acid. Betulinic acid inhibited depolarization-evoked calcium influx in dorsal root ganglion (DRG) neurons predominantly through targeting low-voltage-gated (Cav3 or T-type) and CaV2.2 (N-type) calcium channels. Voltage-clamp electrophysiology experiments revealed a reduction of Ca, but not Na, currents in sensory neurons after BA exposure. Betulinic acid inhibited spontaneous excitatory postsynaptic currents and depolarization-evoked release of calcitonin gene-related peptide from lumbar spinal cord slices. Notably, BA did not engage human mu, delta, or kappa opioid receptors. Intrathecal administration of BA reversed mechanical allodynia in rat models of chemotherapy-induced peripheral neuropathy and HIV-associated peripheral sensory neuropathy as well as a mouse model of partial sciatic nerve ligation without effects on locomotion. The broad-spectrum biological and medicinal properties reported, including anti-HIV and anticancer activities of BA and its derivatives, position this plant-derived small molecule natural product as a potential nonopioid therapy for management of chronic pain.

Analgesic transient receptor potential vanilloid-1-active compounds inhibit native and recombinant T-type calcium channels

BACKGROUND AND PURPOSE

T-type calcium (Ca_v 3) and transient receptor potential vanilloid-1 (TRPV1) channels play central roles in the control of excitability in the peripheral nervous system and are regarded as potential therapeutic pain targets. Modulators that either activate or inhibit TRPV1-mediated currents display analgesic properties in various pain models despite opposing effects on their connate target, TRPV1. We explored the effects of TRPV1-active compounds on Ca_v 3-mediated currents.

EXPERIMENTAL APPROACH

Whole-cell patch clamp recordings were used to examine the effects of TRPV1-active compounds on rat dorsal root ganglion low voltage-activated calcium currents and recombinant Ca_v 3 isoforms in expression systems.

KEY RESULTS

The classical TRPV1 agonist capsaicin as well as TRPV1 antagonists A-889425, BCTC, and capsazepine directly inhibited Ca_v 3 channels. These compounds altered the voltage-dependence of activation and inactivation of Ca_v 3 channels and delayed their recovery from inactivation, leading to a concomitant decrease in T-type current availability. The TRPV1 antagonist capsazepine potently inhibited Ca_v 3.1 and 3.2 channels (K_D  < 120 nM), as demonstrated by its slow off rate. In contrast, neither the TRPV1 agonists, Palvanil and resiniferatoxin, nor the TRPV1 antagonist AMG9810 modulated Ca_v 3-mediated currents.

CONCLUSIONS AND IMPLICATIONS

Analgesic TRPV1-active compounds inhibit Ca_v 3 currents in native and heterologous systems. Hence, their analgesic effects may not be exclusively attributed to their actions on TRPV1, which has important implications in the current understanding of nociceptive pathways. Importantly, our results highlight the need for attention in the experimental design used to address the analgesic properties of Ca_v 3 channel inhibitors.

Partial sciatic nerve ligation leads to an upregulation of Ni2+-resistant T-type Ca2+ currents in capsaicin-responsive nociceptive dorsal root ganglion neurons

Background

Neuropathic pain resulting from peripheral nerve lesions is a common medical condition, but current analgesics are often insufficient. The identification of key molecules involved in pathological pain processing is a prerequisite for the development of new analgesic drugs. Hyperexcitability of nociceptive DRG-neurons due to regulation of voltage-gated ion-channels is generally assumed to contribute strongly to neuropathic pain. There is increasing evidence, that T-type Ca²⁺-currents and in particular the Ca_v3.2 T-type-channel isoform play an important role in neuropathic pain, but experimental results are contradicting.

Purpose

To clarify the role of T-type Ca²⁺-channels and in particular the Ca_v3.2 T-type-channel isoform in neuropathic pain.

Methods

The effect of partial sciatic nerve ligation (PNL) on pain behavior and the properties of T-type-currents in nociceptive DRG-neurons was tested in wild-type and Ca_v3.2-deficient mice.

Results

In wild-type mice, PNL of the sciatic nerve caused neuropathic pain and an increase of T-type Ca²⁺-currents in capsaicin-responsive neurons, while capsaicin-unresponsive neurons were unaffected. Pharmacological experiments revealed that this upregulation was due to an increase of a Ni²⁺-resistant Ca²⁺-current component, inconsistent with Ca_v3.2 up-regulation. Moreover, following PNL Ca_v3.2-deficient mice showed neuropathic pain behavior and an increase of T-Type Ca²⁺-currents indistinguishable to that of PNL treated wild-type mice.

Conclusion

These data suggest that PNL induces an upregulation of T-Type Ca²⁺-currents in capsaicin-responsive DRG-neurons mediated by an increase of a Ni²⁺-insensitive current component (possibly Ca_v3.1 or Ca_v3.3). These findings provide relevance for the development of target specific analgesic drugs.

Antigen retrieval pre-treatment causes a different expression pattern of Cav3.2 in rat and mouse spinal dorsal horn

Ca_v3 channels consist of three isoforms, Ca_v3.1 (α1G), Ca_v3.2 (α1H), and Ca_v3.3 (α1I), which produce low-threshold spikes that trigger burst firings in nociceptive neurons of the spinal dorsal horn (SDH) and dorsal root ganglion (DRG). Although Ca_v3.2 plays a crucial role in pathological pain, it is distribution in SDH still remains controversial. One study showed that Ca_v3.2 is ubiquitously expressed in neurons, but another study implied that Ca_v3.2 is expressed restricted to astrocytes. To unravel these discrepancies, we used methods of immunohistochemistry either with or without antigen retrieval (AR) pre-treatment to detect Ca_v3 in SDH and DRG from both rats and mice. Moreover, Ca_v3.2 mRNA was detected in mice SDH using in situ hybridization. We found that the expression pattern of Ca_v3.2 but not Ca_v3.1 and Ca_v3.3 in SDH were largely different with or without AR pre-treatment, which showed a neuron- like and an astrocyte-like appearance, respectively. Double staining further demonstrated that Ca_v3.2 was mainly costained with the neuronal marker NeuN in the presence of AR but was with glial fibrillary acidic protein (GFAP, marker for astrocytes) in the absence of AR pre-treatment. Importantly, Ca_v3.2 mRNA was mainly colocalized with Ca_v3.2 but not GFAP. Together, our findings indicate that AR pretreatment or not impacts the expression pattern of Ca_v3.2, which may make a significant contribution to the future study of Ca_v3.2 in SDH.

Nerve injury elevates functional Cav3.2 channels in superficial spinal dorsal horn

Cav3 channels play an important role in modulating chronic pain. However, less is known about the functional changes of Cav3 channels in superficial spinal dorsal horn in neuropathic pain states. Here, we examined the effect of partial sciatic nerve ligation (PSNL) on either expression or electrophysiological properties of Cav3 channels in superficial spinal dorsal horn. Our in vivo studies showed that the blockers of Cav3 channels robustly alleviated PSNL-induced mechanical allodynia and thermal hyperalgesia, which lasted at least 14 days following PSNL. Meanwhile, PSNL triggered an increase in both mRNA and protein levels of Cav3.2 but not Cav3.1 or Cav3.3 in rats. However, in Cav3.2 knockout mice, PSNL predominantly attenuated mechanical allodynia but not thermal hyperalgesia. In addition, the results of whole-cell patch-clamp recordings showed that both the overall proportion of Cav3 current-expressing neurons and the Cav3 current density in individual neurons were elevated in spinal lamina II neurons from PSNL rats, which could not be recapitulated in Cav3.2 knockout mice. Altogether, our findings reveal that the elevated functional Cav3.2 channels in superficial spinal dorsal horn may contribute to the mechanical allodynia in PSNL-induced neuropathic pain model.

5-Oxo-hexahydroquinoline: an attractive scaffold with diverse biological activities

5-Oxo-hexahydroquinoline (5-oxo-HHQ) represents a biologically attractive fused heterocyclic core. Various synthetic analogs of 5-oxo-HHQ have been synthesized and assessed for different biological activities. Some derivatives have exhibited myorelaxant, analgesic, anticancer, antibacterial, antifungal, antitubercular, antimalarial, antioxidant, anti-inflammatory, multidrug resistance reversal, anti-Alzheimer, neuroprotective, antidiabetic, antidyslipidemic and antiosteoporotic activities. This review provides a comprehensive report regarding the preparation and pharmacological characterization of 5-oxo-HHQ derivatives that have been reported so far. This information will be beneficial for medicinal chemists in the field of drug discovery to design and develop new and potent therapeutical agents bearing the 5-oxo-HHQ nucleus.

Gene therapy for neuropathic pain using dorsal root ganglion-targeted helper-dependent adenoviral vectors with GAD67 expression

INTRODUCTION

Currently available medications for neuropathic pain are of limited efficacy. Moreover, they are administered systemically and are associated with significant side effects. Ideally, one can circumvent systemic side effects if such treatment can be administered by delivery of the therapeutic agent directly to the diseased neurons. Towards this end, we previously reported the production of a recombinant helper-dependent adenovirus (HDAd) armed with a tissue-specific homing peptide to deliver transgenes targeting sensory neurons with high efficacy.

OBJECTIVES

To develop an effective gene therapy for neuropathic pain by producing a dorsal root ganglion (DRG)-targeted HDAd vector that specifically expresses glutamic acid decarboxylase (GAD) 67 (HDAd-DRG-GAD67).

METHODS

We produced spinal nerve transection (SNT) mice as a neuropathic pain model and delivered HDAd-DRG-GAD67 by injection into spinal nerve or intrathecally to these animals. We evaluated the therapeutic efficacy by measuring ion channel gene expression and quantifying mechanical allodynia, a representative symptom of neuropathic pain, in treated animals.

RESULTS

Glutamic acid decarboxylase expression by HDAd-DRG-GAD67 reduced allodynia significantly in SNT mice. In addition, HDAd-DRG-GAD67 had a much greater transduction efficacy and expressed the therapeutic gene for a much longer time and at a lower dose of viral particles than wild-type HDAd. We found that SNT induced the upregulation of Cav3.2 mRNA in the DRG and GAD67 overexpression suppressed the elevation. Furthermore, the HDAd-DRG-GAD67-induced allodynia amelioration occurred even when we delayed intrathecal delivery of the therapeutic vector to day 7 after SNT.

CONCLUSION

HDAd-mediated DRG-targeted gene therapy delivering GAD67 is an efficacious treatment for neuropathic pain in SNT mice.

Increased expression of CaV3.2 T-type calcium channels in damaged DRG neurons contributes to neuropathic pain in rats with spared nerve injury

Ion channels are very important in the peripheral sensitization in neuropathic pain. Our present study aims to investigate the possible contribution of Ca_V3.2 T-type calcium channels in damaged dorsal root ganglion neurons in neuropathic pain. We established a neuropathic pain model of rats with spared nerve injury. In these model rats, it was easy to distinguish damaged dorsal root ganglion neurons (of tibial nerve and common peroneal nerve) from intact dorsal root ganglion neurons (of sural nerves). Our results showed that Ca_V3.2 protein expression increased in medium-sized neurons from the damaged dorsal root ganglions but not in the intact ones. With whole cell patch clamp recording technique, it was found that after-depolarizing amplitudes of the damaged medium-sized dorsal root ganglion neurons increased significantly at membrane potentials of −85 mV and −95 mV. These results indicate a functional up-regulation of Ca_V3.2 T-type calcium channels in the damaged medium-sized neurons after spared nerve injury. Behaviorally, blockade of Ca_V3.2 with antisense oligodeoxynucleotides could significantly reverse mechanical allodynia. These results suggest that Ca_V3.2 T-type calcium channels in damaged medium-sized dorsal root ganglion neurons might contribute to neuropathic pain after peripheral nerve injury.

Recent advances in the development of T-type calcium channel blockers for pain intervention

Cav 3.2 T-type calcium channels are important regulators of pain signals in the afferent pain pathway, and their activities are dysregulated during various chronic pain states. Therefore, it is reasonable to predict that inhibiting T-type calcium channels in dorsal root ganglion neurons and in the spinal dorsal horn can be targeted for pain relief. This is supported by early pharmacological studies with T-type channel blockers, such as ethosuximide, and by analgesic effects of siRNA depletion of Cav 3.2 channels. In the past 5 years, considerable effort has been applied towards identifying novel classes of T-type calcium channel blockers. Here, we review recent developments in the discovery of novel classes of T-type calcium channel blockers, and their analgesic effects in animal models of pain and in clinical trials.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Upregulation of Cav3.2 T-type calcium channels in adjacent intact L4 dorsal root ganglion neurons in neuropathic pain rats with L5 spinal nerve ligation

Besides the injured peripheral dorsal root ganglion (DRG) neurons, the adjacent intact DRG neurons also have important roles in neuropathic pain. Ion channels including Ca_v3.2 T-type calcium channel in the DRG neurons are important in the development of neuropathic pain. In the present study, we aimed to examine the expression of Ca_v3.2 T-type calcium channels in the intact DRG neurons in neuropathic pain. A neuropathic pain model of rat with lumbar 5 (L5) spinal nerve ligation (SNL) was established, in which the L4 DRG was separated from the axotomized L5 DRG, and the molecular, morphological and electrophysiological changes of Ca_v3.2 T-type calcium channels in L4 DRG neurons were investigated. Western blotting showed that total and membrane protein levels of Ca_v3.2 in L4 DRG neurons increased, and voltage-dependent patch clamp recordings revealed an increased T-type current density with a curve shift to the left in steady-state activation in the acutely isolated L4 DRG neurons in neuropathic pain rats. Immunofluorescent staining further showed that the membrane expression of Ca_v3.2 increased in CGRP-, IB4-positive small neurons and NF200-positive large ones. In conclusion, the membrane expression and the function of Ca_v3.2 T-type calcium channels are increased in the intact L4 DRG neurons in neuropathic pain rats with peripheral nerve injury like SNL.

Contribution of S4 segments and S4-S5 linkers to the low-voltage activation properties of T-type CaV3.3 channels

Voltage-gated calcium channels contain four highly conserved transmembrane helices known as S4 segments that exhibit a positively charged residue every third position, and play the role of voltage sensing. Nonetheless, the activation range between high-voltage (HVA) and low-voltage (LVA) activated calcium channels is around 30–40 mV apart, despite the high level of amino acid similarity within their S4 segments. To investigate the contribution of S4 voltage sensors for the low-voltage activation characteristics of Ca_V3.3 channels we constructed chimeras by swapping S4 segments between this LVA channel and the HVA Ca_V1.2 channel. The substitution of S4 segment of Domain II in Ca_V3.3 by that of Ca_V1.2 (chimera IIS4C) induced a ~35 mV shift in the voltage-dependence of activation towards positive potentials, showing an I-V curve that almost overlaps with that of Ca_V1.2 channel. This HVA behavior induced by IIS4C chimera was accompanied by a 2-fold decrease in the voltage-dependence of channel gating. The IVS4 segment had also a strong effect in the voltage sensing of activation, while substitution of segments IS4 and IIIS4 moved the activation curve of Ca_V3.3 to more negative potentials. Swapping of IIS4 voltage sensor influenced additional properties of this channel such as steady-state inactivation, current decay, and deactivation. Notably, Domain I voltage sensor played a major role in preventing Ca_V3.3 channels to inactivate from closed states at extreme hyperpolarized potentials. Finally, site-directed mutagenesis in the Ca_V3.3 channel revealed a partial contribution of the S4-S5 linker of Domain II to LVA behavior, with synergic effects observed in double and triple mutations. These findings indicate that IIS4 and, to a lesser degree IVS4, voltage sensors are crucial in determining the LVA properties of Ca_V3.3 channels, although the accomplishment of this function involves the participation of other structural elements like S4-S5 linkers.

Accumulation of Cav3.2 T-type Calcium Channels in the Uninjured Sural Nerve Contributes to Neuropathic Pain in Rats with Spared Nerve Injury

Injuries to peripheral nerve fibers induce neuropathic pain. But the involvement of adjacent uninjured fibers to pain is not fully understood. The present study aims to investigate the possible contribution of Ca_v3.2 T-type calcium channels in uninjured afferent nerve fibers to neuropathic pain in rats with spared nerve injury (SNI). Aβ-, Aδ- and C-fibers of the uninjured sural nerve were sensitized revealed by in vivo single-unit recording, which were accompanied by accumulation of Ca_v3.2 T-type calcium channel proteins shown by Western blotting. Application of mibefradil, a T-type calcium channel blocker, to sural nerve receptive fields increased mechanical thresholds of Aβ-, Aδ- and C-fibers, confirming the functional involvement of accumulated channels in the sural nerve in SNI rats. Finally, perineural application of mibefradil or TTA-P2 to the uninjured sural nerve alleviated mechanical allodynia in SNI rats. These results suggest that axonal accumulation of Ca_v3.2 T-type calcium channels plays an important role in the uninjured sural nerve sensitization and contributes to neuropathic pain.

Genetic risk between the CACNA1I gene and schizophrenia in Chinese Uygur population

Background

Schizophrenia (SCZ) is a common mental disorder with high heritability, and genetic factors play a major role in the pathogenesis. Recent researches indicated that the CACNA1I involved in calcium channels probably affect the potential pathogenesis of SCZ.

Results

In this study, we attempted to investigate whether the CACNA1I gene contributes the risk to SCZ in the Uighur Chinese population, and performed a case-control study involving 985 patient samples and 1218 normal controls to analyze nine SNPs within the CACNA1I gene. Among these sites, six SNPs were significantly associated with SCZ in the allele distribution: rs132575 (adjusted P_allele = 0.039, OR = 1.159), rs713860 (adjusted P_allele = 0.039, OR = 0.792), rs738168 (adjusted P_allele = 0.039, OR = 0.785), rs136805 (adjusted P_allele = 0.014, OR = 1.212), rs5757760 (adjusted P_allele = 0.042, OR = 0.873) and rs5750871 (adjusted P_allele = 0.039, OR = 0.859). In addition, two SNPs turned to be risk factors for SCZ not only in the allele distribution, but also in the genotype distribution: rs132575 (adjusted P_genotype = 0.037) and rs136805 (adjusted P_genotype = 0.037).

Conclusions

Overall, the present study provided evidence that significant association exists between the CACNA1I gene and SCZ in the Uighur Chinese population, subsequent validation of functional analysis and genetic association studies are needed to further extend this study.

Glycosylation of voltage-gated calcium channels in health and disease

Voltage-gated calcium channels (VGCCs) are transmembrane proteins that translate electrical activities into intracellular calcium elevations and downstream signaling pathways. They serve essential physiological functions including communication between nerve cells, muscle contraction, cardiac activity, and release of hormones and neurotransmitters. Asparagine-linked glycosylation has emerged as an essential post-translational modification to control the number of channels embedded in the plasma membrane but also their functional gating properties. This review provides a comprehensive overview about the current state of knowledge on the role of glycosylation in the expression and functioning of VGCCs, and discusses how variations in the glycosylation of the channel proteins can contribute to pathological conditions.

Chronic intrathecal infusion of T-type calcium channel blockers attenuates CaV3.2 upregulation in nerve-ligated rats

OBJECTIVE

T-type channel (TCC) Ca_V3.2 plays a pivotal role in pain transmission. In this study, we examined the effects of intrathecal TCC blockers on Ca_V3.2 expression in a L5/6 spinal nerve ligation (SNL) pain model. The neurotoxicity of TCC blockers were also evaluated.

METHODS

Male Sprague-Dawley rats (200-250 g) were used for right L5/6 SNL to induce neuropathic pain. Intrathecal infusion of saline or TCC blockers [mibefradil (0.7 μg/h) or ethosuximide (60 μg/h)] was started after surgery for 7 days. Fluorescent immunohistochemistry and Western blotting were used to determine the expression pattern and protein level of Ca_V3.2. Hematoxylin-eosin and toluidine blue staining were used to evaluate the neurotoxicity of tested agents.

RESULTS

Seven days after SNL, Ca_V3.2 protein levels were upregulated in ipsi-lateral L5/6 spinal cord and dorsal root ganglia (DRG) in immunofluorescence and Western blotting studies. Compared with the saline-treated group, rats receiving mibefradil or ethosuximide showed significant lower Ca_V3.2 expression in the spinal cord and DRG. No obvious histopathologic change in hematoxylin-eosin and toluidine blue staining were observed in all tested groups.

CONCLUSION

In this study, we demonstrate that SNL-induced Ca_V3.2 upregulation in the spinal cord and DRG was attenuated by intrathecal infusion of mibefradil or ethosuximide. No obvious neurotoxicity effects were observed in all the tested groups. Our data suggest that continuous intrathecal infusion of TCC blockers may be considered as a promising alternative for the treatment of nerve injury-induced pain.

A Cav3.2/Stac1 molecular complex controls T-type channel expression at the plasma membrane

Low-voltage-activated T-type calcium channels are essential contributors to neuronal physiology where they play complex yet fundamentally important roles in shaping intrinsic excitability of nerve cells and neurotransmission. Aberrant neuronal excitability caused by alteration of T-type channel expression has been linked to a number of neuronal disorders including epilepsy, sleep disturbance, autism, and painful chronic neuropathy. Hence, there is increased interest in identifying the cellular mechanisms and actors that underlie the trafficking of T-type channels in normal and pathological conditions. In the present study, we assessed the ability of Stac adaptor proteins to associate with and modulate surface expression of T-type channels. We report the existence of a Ca_v3.2/Stac1 molecular complex that relies on the binding of Stac1 to the amino-terminal region of the channel. This interaction potently modulates expression of the channel protein at the cell surface resulting in an increased T-type conductance. Altogether, our data establish Stac1 as an important modulator of T-type channel expression and provide new insights into the molecular mechanisms underlying the trafficking of T-type channels to the plasma membrane.

[Rebound depolarization of substantia gelatinosa neurons and its modulatory mechanisms in rat spinal dorsal horn]

OBJECTIVE

To investigate the rebound depolarization of substantia gelatinosa (SG) neurons in rat spinal dorsal horn and explore its modulatory mechanisms to provide better insights into rebound depolarization-related diseases.

METHODS

Parasagittal slices of the spinal cord were prepared from 3- to 5-week-old Sprague-Dawley rats. The electrophysiologic characteristics and responses to hyperpolarization stimulation were recorded using whole-cell patch-clamp technique. The effects of hyperpolarization-activated cyclic nucleotide gated cation (HCN) channel blockers and T-type calcium channel blockers on rebound depolarization of the neurons were studied.

RESULTS

A total of 63 SG neurons were recorded. Among them, 23 neurons showed no rebound depolarization, 19 neurons showed rebound depolarization without spikes, and 21 neurons showed rebound depolarization with spikes. The action potential thresholds of the neurons without rebound depolarization were significantly higher than those of the neurons with rebound depolarization and spikes (-28.7∓1.6 mV vs -36.0∓2.0 mV, P<0.05). The two HCN channel blockers CsCl and ZD7288 significantly delayed the latency of rebound depolarization with spike from 45.9∓11.6 ms to 121.6∓51.3 ms (P<0.05) and from 36.2∓10.3 ms to 73.6∓13.6 ms (P<0.05), respectively. ZD7288 also significantly prolonged the latency of rebound depolarization without spike from 71.9∓35.1 ms to 267.0∓68.8 ms (P<0.05). The T-type calcium channel blockers NiCl2 and mibefradil strongly decreased the amplitude of rebound depolarization with spike from 19.9∓6.3 mV to 9.5∓4.5 mV (P<0.05) and from 26.1∓9.4 mV to 15.5∓5.0 mV (P<0.05), respectively. Mibefradil also significantly decreased the amplitude of rebound depolarization without spike from 14.3∓3.0 mV to 7.9∓2.0 mV (P<0.05).

CONCLUSION

Nearly two-thirds of the SG neurons have rebound depolarizations modulated by HCN channel and T-type calcium channel.

Effect of the T-type channel blocker KYS-05090S in mouse models of acute and neuropathic pain

T-type channels are important contributors to the initiation and the maintenance of chronic pain states. Blocking T-type channels is therefore a possible therapeutic strategy for relieving pain. Here, we report the Cav3.2 T-type channel blocking action of a previously reported small organic molecule, KYS-05090S. This compound was able to reduce transiently expressed Cav3.2 currents with low micromolar affinity and mediated a hyperpolarizing shift in half-inactivation potential. KYS-05090S was then tested in models of acute and neuropathic pain. KYS-05090S (10 μg/10 μl delivered intrathecally) significantly reduced acute pain induced by formalin in both the tonic and inflammatory phases. Its antinociceptive effect was not observed when delivered to Cav3.2 null-mice revealing a Cav3.2-dependent mechanism. KYS-05090S also reduced neuropathic pain in a model of partial sciatic nerve injury. Those results indicate that KYS-05090S mediates a potent analgesic effect in inflammatory and neuropathic pain through T-type channel modulation, suggesting that its scaffold could be explored as a new class of analgesic compounds.

Analgesic effect of a broad-spectrum dihydropyridine inhibitor of voltage-gated calcium channels

Voltage-activated calcium channels are important facilitators of nociceptive transmission in the primary afferent pathway. Consequently, molecules that block these channels are of potential use as pain therapeutics. Our group has recently reported on the identification of a novel class of dihydropyridines (DHPs) that included compounds with preferential selectivity for T-type over L-type channels. Among those compounds, M4 was found to be an equipotent inhibitor of both Cav1.2 L- and Cav3.2 T-type calcium channels. Here, we have further characterized the effects of this compound on other types of calcium channels and examined its analgesic effect when delivered either spinally (i.t.) or systemically (i.p.) to mice. Both delivery routes resulted in antinociception in a model of acute pain. Furthermore, M4 was able to reverse mechanical hyperalgesia produced by nerve injury when delivered intrathecally. M4 retained partial activity when delivered to Cav3.2 null mice, indicating that this compound acts on multiple targets. Additional whole-cell patch clamp experiments in transfected tsA-201 cells revealed that M4 also effectively blocks Cav3.3 (T-type) and Cav2.2 (N-type) currents. Altogether, our data indicate that broad-spectrum inhibition of multiple calcium channel subtypes can lead to potent analgesia in rodents.

Expression and Regulation of Cav3.2 T-Type Calcium Channels during Inflammatory Hyperalgesia in Mouse Dorsal Root Ganglion Neurons

The Cav3.2 isoform of the T-type calcium channel is expressed in primary sensory neurons of the dorsal root ganglion (DRG), and these channels contribute to nociceptive and neuropathic pain in rats. However, there are conflicting reports on the roles of these channels in pain processing in rats and mice. In addition, the function of T-type channels in persistent inflammatory hyperalgesia is poorly understood. We performed behavioral and comprehensive histochemical analyses to characterize Cav3.2-expressing DRG neurons and examined the regulation of T-type channels in DRGs from C57BL/6 mice with carrageenan-induced inflammatory hyperalgesia. We show that approximately 20% of mouse DRG neurons express Cav3.2 mRNA and protein. The size of the majority of Cav3.2-positive DRG neurons (69 ± 8%) ranged from 300 to 700 μm2 in cross-sectional area and 20 to 30 μm in estimated diameter. These channels co-localized with either neurofilament-H (NF-H) or peripherin. The peripherin-positive cells also overlapped with neurons that were positive for isolectin B4 (IB4) and calcitonin gene-related peptide (CGRP) but were distinct from transient receptor potential vanilloid 1 (TRPV1)-positive neurons during normal mouse states. In mice with carrageenan-induced inflammatory hyperalgesia, Cav3.2 channels, but not Cav3.1 or Cav3.3 channels, were upregulated in ipsilateral DRG neurons during the sub-acute phase. The increased Cav3.2 expression partially resulted from an increased number of Cav3.2-immunoreactive neurons; this increase in number was particularly significant for TRPV1-positive neurons. Finally, preceding and periodic intraplantar treatment with the T-type calcium channel blockers mibefradil and NNC 55-0396 markedly reduced and reversed mechanical hyperalgesia during the acute and sub-acute phases, respectively, in mice. These data suggest that Cav3.2 T-type channels participate in the development of inflammatory hyperalgesia, and this channel might play an even greater role in the sub-acute phase of inflammatory pain due to increased co-localization with TRPV1 receptors compared with that in the normal state.

NMP-7 inhibits chronic inflammatory and neuropathic pain via block of Cav3.2 T-type calcium channels and activation of CB2 receptors

Background

T-type calcium channels and cannabinoid receptors are known to play important roles in chronic pain, making them attractive therapeutic targets. We recently reported on the design, synthesis and analgesic properties of a novel T-type channel inhibitor (NMP-7), which also shows mixed agonist activity on CB₁ and CB₂ receptors in vitro. Here, we analyzed the analgesic effect of systemically delivered NMP-7 (intraperitoneal (i.p.) or intragstric (i.g.) routes) on mechanical hypersensitivity in inflammatory pain induced by Complete Freund’s Adjuvant (CFA) and neuropathic pain induced by sciatic nerve injury.

Results

NMP-7 delivered by either i.p. or i.g. routes produced dose-dependent inhibition of mechanical hyperalgesia in mouse models of inflammatory and neuropathic pain, without altering spontaneous locomotor activity in the open-field test at the highest active dose. Neither i.p. nor i.g. treatment reduced peripheral inflammation per se, as evaluated by examining paw edema and myeloperoxidase activity. The antinociception produced by NMP-7 in the CFA test was completely abolished in Ca_V3.2-null mice, confirming Ca_V3.2 as a key target. The analgesic action of intraperitoneally delivered NMP-7 was not affected by pretreatment of mice with the CB₁ antagonist AM281, but was significantly attenuated by pretreatment with the CB₂ antagonist AM630, suggesting that CB₂ receptors, but not CB₁ receptors are involved in the action of NMP-7 in vivo.

Conclusions

Overall, our work shows that NMP-7 mediates a significant analgesic effect in a model of persistent inflammatory and chronic neuropathic pain by way of T-type channel modulation and CB₂ receptor activation. Thus, this study provides a novel therapeutic avenue for managing chronic pain conditions via mixed CB ligands/T-type channel blockers.

The deubiquitinating enzyme USP5 modulates neuropathic and inflammatory pain by enhancing Cav3.2 channel activity

T-type calcium channels are essential contributors to the transmission of nociceptive signals in the primary afferent pain pathway. Here, we show that T-type calcium channels are ubiquitinated by WWP1, a plasma-membrane-associated ubiquitin ligase that binds to the intracellular domain III-IV linker region of the Cav3.2 T-type channel and modifies specific lysine residues in this region. A proteomic screen identified the deubiquitinating enzyme USP5 as a Cav3.2 III-IV linker interacting partner. Knockdown of USP5 via shRNA increases Cav3.2 ubiquitination, decreases Cav3.2 protein levels, and reduces Cav3.2 whole-cell currents. In vivo knockdown of USP5 or uncoupling USP5 from native Cav3.2 channels via intrathecal delivery of Tat peptides mediates analgesia in both inflammatory and neuropathic mouse models of mechanical hypersensitivity. Altogether, our experiments reveal a cell signaling pathway that regulates T-type channel activity and their role in nociceptive signaling.