Differential expression of slow and fast-repriming tetrodotoxin-sensitive sodium currents in dorsal root ganglion neurons.

Sodium channel Nav1.7 triggers the generation of nociceptive action potentials and is important in sending pain signals under physiological and pathological conditions. However, studying endogenous Nav1.7 currents has been confounded by co-expression of multiple sodium channel isoforms in dorsal root ganglion (DRG) neurons. In the current study, slow-repriming (SR) and fast-repriming (FR) tetrodotoxin-sensitive (TTX-S) currents were dissected electrophysiologically in small DRG neurons of both rats and mice. Three subgroups of small DRG neurons were identified based on the expression pattern of SR and FR TTX-S currents. A majority of rat neurons only expressed SR TTX-S currents, while a majority of mouse neurons expressed additional FR TTX-S currents. ProTx-II inhibited SR TTX-S currents with variable efficacy among DRG neurons. The expression of both types of TTX-S currents was higher in Isolectin B4-negative (IB4-) compared to Isolectin B4-positive (IB4+) neurons. Paclitaxel selectively increased SR TTX-S currents in IB4- neurons. In simulation experiments, the Nav1.7-expressing small DRG neuron displayed lower rheobase and higher frequency of action potentials upon threshold current injections compared to Nav1.6. The results suggested a successful dissection of endogenous Nav1.7 currents through electrophysiological manipulation that may provide a useful way to study the functional expression and pharmacology of endogenous Nav1.7 channels in DRG neurons.

Anti-epileptic/pro-epileptic effects of sodium channel modulators from Buthus martensii Karsch.

The East Asian scorpion Buthus martensii Karsch (BmK) is one of the classical traditional Chinese medicines for treating epilepsy for over a thousand years. Neurotoxins purified from BmK venom are considered as the main active ingredients, acting on membrane ion channels. Voltage-gated sodium channels (VGSCs) play a crucial role in the occurrence of epilepsy, which make them become important drug targets for epilepsy. Long chain toxins of BmK, composed of 60-70 amino acid residues, could specifically recognize VGSCs. Among them, α-like neurotoxins, binding to the receptor site-3 of VGSC, induce epilepsy in rodents and can be used to establish seizure models. The ß or ß-like neurotoxins, binding to the receptor site-4 of VGSC, have significant anticonvulsant effects in epileptic models. This review aims to illuminate the anticonvulsant/convulsant effects of BmK polypeptides by acting on VGSCs, and provide potential frameworks for the anti-epileptic drug-design.

Oxaliplatin Depolarizes the IB4- Dorsal Root Ganglion Neurons to Drive the Development of Neuropathic Pain Through TRPM8 in Mice.

Use of chemotherapy drug oxaliplatin is associated with painful peripheral neuropathy that is exacerbated by cold. Remodeling of ion channels including TRP channels in dorsal root ganglion (DRG) neurons contribute to the sensory hypersensitivity following oxaliplatin treatment in animal models. However, it has not been studied if TRP channels and membrane depolarization of DRG neurons serve as the initial ionic/membrane drives (such as within an hour) that contribute to the development of oxaliplatin-induced neuropathic pain. In the current study, we studied in mice (1) in vitro acute effects of oxaliplatin on the membrane excitability of IB4+ and IB4- subpopulations of DRG neurons using a perforated patch clamping, (2) the preventative effects of a membrane-hyperpolarizing drug retigabine on oxaliplatin-induced sensory hypersensitivity, and (3) the preventative effects of TRP channel antagonists on the oxaliplatin-induced membrane hyperexcitability and sensory hypersensitivity. We found (1) IB4+ and IB4- subpopulations of small DRG neurons displayed previously undiscovered, substantially different membrane excitability, (2) oxaliplatin selectively depolarized IB4- DRG neurons, (3) pretreatment of retigabine largely prevented oxaliplatin-induced sensory hypersensitivity, (4) antagonists of TRPA1 and TRPM8 channels prevented oxaliplatin-induced membrane depolarization, and (5) the antagonist of TRPM8 largely prevented oxaliplatin-induced sensory hypersensitivity. These results suggest that oxaliplatin depolarizes IB4- neurons through TRPM8 channels to drive the development of neuropathic pain and targeting the initial drives of TRPM8 and/or membrane depolarization may prevent oxaliplatin-induce neuropathic pain.

[The new target of Rapamycin: lysosomal calcium channel TRPML1].

Rapamycin (Rap) is an immunosuppressant, which is mainly used in the anti-rejection of organ transplantation. Meanwhile, it also shows great potential in the fields of anticancer, neuroprotection and anti-aging. Rap can inhibit the activity of mammalian target of Rap (mTOR). It activates the transcription factor EB (TFEB) to up-regulate lysosomal function and eliminates the inhibitory effect of mTOR on ULK1 (unc-51 like autophagy activating kinase 1) to promote autophagy. Recent research showed that Rap can directly activate the lysosomal cation channel TRPML1 in an mTOR-independent manner. TRPML1 activation releases lysosomal calcium. Calcineurin functions as the sensor of the lysosomal calcium signal and activates TFEB, thus promoting lysosome function and autophagy. This finding has greatly broadened and deepened our understanding of the pharmacological roles of Rap. In this review, we briefly introduce the canonical Rap-mTOR-ULK1/TFEB signaling pathway, and then discuss the discovery of TRPML1 as a new target of Rap and the pharmacological potential of this novel Rap-TRPML1-Calcineurin-TFEB pathway.

[Cancer pain, a serious threat to patientsmemory].

A large number of cancer patients suffer from pain. Growing evidence suggested that pain might be a serious risk factor for cancer patients. The shared modulators and modulation pathways between neural system and tumor cells, such as various neurotransmitters and neurogenic cytokines, provide essential basis for the effect of pain on tumor. In this article, we reviewed some possible mechanism of this process from two aspects: the systematic regulation of central nervous system on endocrine and immunity, and the regional regulation of peripheral nerves on tumor cells. The aim of this review is to provide more innovative knowledge about pain and cancer and to emphasize the importance of anti-pain in the therapy of cancer.

Understanding Genotypes and Phenotypes of the Mutations in Voltage- Gated Sodium Channel α Subunits in Epilepsy.

BACKGROUND & OBJECTIVE: Voltage-gated sodium channels (VGSCs) are responsible for the generation and propagation of action potentials in most excitable cells. In general, a VGSC consists of one pore-forming α subunit and two auxiliary ß subunits. Genetic alterations in VGSCs genes, including both α and ß subunits, are considered to be associated with epileptogenesis as well as seizures. This review aims to summarize the mutations in VGSC α subunits in epilepsy, particularly the pathophysiological and pharmacological properties of relevant VGSC mutants. CONCLUSION: The review of epilepsy-associated VGSC α subunits mutants may not only contribute to the understanding of disease mechanism and genetic modifiers, but also provide potential theoretical targets for the precision and individualized medicine for epilepsy.

Inhibitory Effects of Columbianadin on Nociceptive Behaviors in a Neuropathic Pain Model, and on Voltage-Gated Calcium Currents in Dorsal Root Ganglion Neurons in Mice.

Radix angelicae pubescentis (RAP) has been used in Chinese traditional medicine to treat painful diseases such as rheumatism and headache. A previous study has reported that columbianadin (CBN), a major coumarin in RAP inhibits acute and inflammatory pain behaviors. However, the effects of CBN on neuropathic pain behaviors, and the potential underlying mechanism have not been reported. In the present study, the effects of CBN, compared to another major coumarin of RAP osthole (OST), on oxaliplatin-induced neuropathic pain behaviors and on the voltage-gated calcium currents in small dorsal root ganglion (DRG) neurons were studied in mice. It was found that CBN and OST inhibited both mechanical and cold hypersensitivity induced by oxaliplatin. Moreover, CBN and OST might preferentially inhibit T- and L-type calcium currents (Ica). The inhibitory effects of CBN and OST on the oxaliplatin-induced mechanical allodynia were prevented by gabapentin. These results suggest that CBN, as well as OST might inhibit neuropathic pain behaviors through an inhibition of T- and L-type calcium currents in nociceptive DRG neurons.

Voltage-gated Sodium Channels in Sensory Information Processing.

Objective & Background: Voltage-gated sodium channels (VGSCs) and potassium channels are critical in the generation of action potentials in the nervous system. VGSCs and potassium channels play important roles in the five fundamental senses of vision, audition, olfaction, taste and touch. Dysfunctional VGSCs are associated with clinical sensory symptoms, such as hyperpselaphesia, parosphresia, and so on. Conclusion: This short review highlights the recent advances in the study of VGSCs in sensory information processing and discusses the potential role of VGSCs to serve as pharmacological targets for the treatment of sensory system diseases.

Visual deprivation induce cross-modal enhancement of olfactory perception.

The underlying mechanisms responsible for enhanced olfactory perception of congenital blind humans remain elusive so far. Here, animal behavioral test showed that congenital visual deprivation (from postnatal day 0-28) or one-week visual deprivation during juvenile stage (from postnatal day 21-28) could reduce the latency time of food-seeking but increase the odor discrimination performance of rodents. The enhanced olfactory perception induced by one-week visual deprivation could be returned to base level when visual input was recovered. Accordingly, local field potential (LFP) oscillation recording in vivo showed that the power of high-frequency ß and γ oscillations were increased in olfactory bulb (OB) and anterior piriform cortex (aPC) of vision deprived animals. This research discovered the enhancement of olfactory perception and adaptive plasticity of oscillations in olfactory system of rodents induced by visual deprivation, which may facilitate better understanding of mechanisms underlying cross-modal plasticity.

Region specific contribution of ASIC2 to acidosis-and ischemia-induced neuronal injury.

Acidosis in the brain plays a critical role in neuronal injury in neurological diseases, including brain ischemia. One key mediator of acidosis-induced neuronal injury is the acid-sensing ion channels (ASICs). Current literature has focused on ASIC1a when studying acid signaling. The importance of ASIC2, which is also widely expressed in the brain, has not been appreciated. We found here a region-specific effect of ASIC2 on acid-mediated responses. Deleting ASIC2 reduced acid-activated current in cortical and striatal neurons, but had no significant effect in cerebellar granule neurons. In addition, we demonstrated that ASIC2 was important for ASIC1a expression, and that ASIC2a but not 2b facilitated ASIC1a surface trafficking in the brain. Further, we showed that ASIC2 deletion attenuated acidosis/ischemia-induced neuronal injury in organotypic hippocampal slices but had no effect in organotypic cerebellar slices. Consistent with an injurious role of ASIC2, we showed that ASIC2 deletion significantly protected the mouse brain from ischemic damage in vivo. These data suggest a critical region-specific contribution of ASIC2 to neuronal injury and reveal an important functional difference between ASIC2a and 2b in the brain.

Brain natriuretic peptide suppresses pain induced by BmK I, a sodium channel-specific modulator, in rats.

BACKGROUND: A previous study found that brain natriuretic peptide (BNP) inhibited inflammatory pain via activating its receptor natriuretic peptide receptor A (NPRA) in nociceptive sensory neurons. A recent study found that functional NPRA is expressed in almost all the trigeminal ganglion (TG) neurons at membrane level suggesting a potentially important role for BNP in migraine pathophysiology. METHODS: An inflammatory pain model was produced by subcutaneous injection of BmK I, a sodium channel-specific modulator from venom of Chinese scorpion Buthus martensi Karsch. Quantitative PCR, Western Blot, and immunohistochemistry were used to detect mRNA and protein expression of BNP and NPRA in dorsal root ganglion (DRG) and dorsal horn of spinal cord. Whole-cell patch clamping experiments were conducted to record large-conductance Ca2+-activated K+ (BKCa) currents of membrane excitability of DRG neurons. Spontaneous and evoked pain behaviors were examined. RESULTS: The mRNA and protein expression of BNP and NPRA was up-regulated in DRG and dorsal horn of spinal cord after BmK I injection. The BNP and NPRA was preferentially expressed in small-sized DRG neurons among which BNP was expressed in both CGRP-positive and IB4-positive neurons while NPRA was preferentially expressed in CGRP-positive neurons. BNP increased the open probability of BKCa channels and suppressed the membrane excitability of small-sized DRG neurons. Intrathecal injection of BNP significantly inhibited BmK-induced pain behaviors including both spontaneous and evoked pain behaviors. CONCLUSIONS: These results suggested that BNP might play an important role as an endogenous pain reliever in BmK I-induced inflammatory pain condition. It is also suggested that BNP might play a similar role in other pathophysiological pain conditions including migraine.

Allosteric interactions between receptor site 3 and 4 of voltage-gated sodium channels: a novel perspective for the underlying mechanism of scorpion sting-induced pain.

BACKGROUND: BmK I, a site-3-specific modulator of voltage-gated sodium channels (VGSCs), causes pain and hyperalgesia in rats, while BmK IT2, a site-4-specific modulator of VGSCs, suppresses pain-related responses. A stronger pain-related effect has been previously attributed to Buthus martensi Karsch (BmK) venom, which points out the joint pharmacological effect in the crude venom. METHODS: In order to detect the joint effect of BmK I and BmK IT2 on ND7-23 cells, the membrane current was measured by whole cell recording. BmK I and BmK IT2 were applied successively and jointly, and the synergistic modulations of VGSCs on ND7-23 cells were detected. RESULTS: Larger peak INa and more negative half-activation voltage were elicited by joint application of BmK I and BmK IT2 than by application of BmK I or BmK IT2 alone. Compared to the control, co-applied BmK I and BmK IT2 also significantly prolonged the time constant of inactivation. CONCLUSIONS: Our results indicated that site-4 toxin (BmK IT2) could enhance the pharmacological effect induced by site-3 toxin (BmK I), suggesting a stronger effect elicited by both toxins that alone usually exhibit opposite pharmacological effects, which is related to the allosteric interaction between receptor site 3 and site 4. Meanwhile, these results may bring a novel perspective for exploring the underlying mechanisms of scorpion sting-induced pain.

Lipid bilayer modification alters the gating properties and pharmacological sensitivity of voltage-gated sodium channel.

Voltage-gated sodium channels (VGSCs) are widely distributed in most cells and tissues, performing many physiological functions. As one kind of membrane proteins in the lipid bilayer, whether lipid composition plays a role in the gating and pharmacological sensitivity of VGSCs still remains unknown. Through the application of sphingomyelinase D (SMaseD), the gating and pharmacological sensitivity of the endogenous VGSCs in neuroblastoma ND7-23 cell line to BmK I and BmK AS, two sodium channel-specific modulators from the venom of Buthus martensi Karsch (BmK), were assessed before and after lipid modification. The results showed that, in ND7-23 cells, SMaseD did not change the gating properties of VGSCs. However, SMaseD application altered the slope factor of activation with the treatment of 30 nmol/L BmK I, but caused no significant effects at 100 and 500 nmol/L BmK I. With low concentration of BmK I (30 and 100 nmol/L) treatment, the application of SMaseD exerted hyperpolarizing effects on both slow-inactivation and steady-state inactivation, and increased the recovery time constant, whereas total inactivation and recovery remained unaltered at 500 nmol/L BmK I. Meanwhile, SMaseD modulation hyperpolarized the voltage dependence of slow-inactivation at 0.1 nmol/L BmK AS and altered the slope factor of slow-inactivation at 10 nmol/L BmK AS, whereas other parameters remained unchanged. These results indicated a possibility that the lipid bilayer would disturb the pharmacological sensitivity of VGSCs for the first time, which might open a new way of developing new drugs for treating sodium channelopathies.

Spinal 5-HT3AR contributes to BmK I-induced inflammatory pain in rats.

Subcutaneous injection of BmK I could be adopted to well establish a novel pain model. Moreover, 5-hydroxytryptamine (serotonin, 5-HT) receptor is involved in regulating animal pain-related behaviors. However, the underlying mechanism of 5-HT3R on BmK I-induced pain remains unclear. Animal behavioral testing, RT-PCR and Western blotting were used to yield the following results: first, intraplantar (i.pl.) injection of BmK I (10 µg) induced elevated mRNA and protein levels of 5-HT3AR in bilateral L4-L5 spinal cord; Second, intrathecal (i.t.) injection of ondansetron (a specific antagonist of 5-HT3AR) reduced spontaneous pain responses, attenuated unilateral thermal and bilateral mechanical hypersensitivity elicited by BmK I; Microglia could be activated by BmK I (i.pl.) in both sides of L4-L5 spinal cord, and this effect was reversed by intrathecal pre-treatment with 5-HT3AR antagonist. Meanwhile, the 5-HT3AR in L4-L5 spinal cord was almost co-localized with NeuN (a marker of nerve cell), but not co-expressed with Iba-1 (a marker of microglia). Finally, the expression level of CX3CL1 and CX3CR1 was reduced by intrathecal pre-treatment with ondansetron. Our results indicate that both 5-HT3AR signaling pathway and microglia are activated in the process of induction and maintenance of BmK I-induced pain nociception. Meanwhile, our results suggest that the neuronal 5-HT3AR may communicate with microglia indirectly via CX3CL1 which is involved in regulating the BmK I-induced hyperalgesia and sensitization.

Two aspects of ASIC function: Synaptic plasticity and neuronal injury.

Extracellular brain pH fluctuates in both physiological and disease conditions. The main postsynaptic proton receptor is the acid-sensing ion channels (ASICs). During the past decade, much progress has been made on protons, ASICs, and neurological disease. This review summarizes the recent progress on synaptic role of protons and our current understanding of how ASICs contribute to various types of neuronal injury in the brain. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Allosteric interactions between receptor site 3 and 4 of voltage-gated sodium channels: a novel perspective for the underlying mechanism of scorpion sting-induced pain

Background BmK I, a site-3-specific modulator of voltage-gated sodium channels (VGSCs), causes pain and hyperalgesia in rats, while BmK IT2, a site-4-specific modulator of VGSCs, suppresses pain-related responses. A stronger pain-related effect has been previously attributed to Buthus martensi Karsch (BmK) venom, which points out the joint pharmacological effect in the crude venom.Methods In order to detect the joint effect of BmK I and BmK IT2 on ND7-23 cells, the membrane current was measured by whole cell recording. BmK I and BmK IT2 were applied successively and jointly, and the synergistic modulations of VGSCs on ND7-23 cells were detected.Results Larger peak I Na and more negative half-activation voltage were elicited by joint application of BmK I and BmK IT2 than by application of BmK I or BmK IT2 alone. Compared to the control, co-applied BmK I and BmK IT2 also significantly prolonged the time constant of inactivation.Conclusions Our results indicated that site-4 toxin (BmK IT2) could enhance the pharmacological effect induced by site-3 toxin (BmK I), suggesting a stronger effect elicited by both toxins that alone usually exhibit opposite pharmacological effects, which is related to the allosteric interaction between receptor site 3 and site 4. Meanwhile, these results may bring a novel perspective for exploring the underlying mechanisms of scorpion sting-induced pain.(AU)

Allosteric interactions between receptor site 3 and 4 of voltage-gated sodium channels: a novel perspective for the underlying mechanism of scorpion sting-induced pain

Background BmK I, a site-3-specific modulator of voltage-gated sodium channels (VGSCs), causes pain and hyperalgesia in rats, while BmK IT2, a site-4-specific modulator of VGSCs, suppresses pain-related responses. A stronger pain-related effect has been previously attributed to Buthus martensi Karsch (BmK) venom, which points out the joint pharmacological effect in the crude venom.Methods In order to detect the joint effect of BmK I and BmK IT2 on ND7-23 cells, the membrane current was measured by whole cell recording. BmK I and BmK IT2 were applied successively and jointly, and the synergistic modulations of VGSCs on ND7-23 cells were detected.Results Larger peak I Na and more negative half-activation voltage were elicited by joint application of BmK I and BmK IT2 than by application of BmK I or BmK IT2 alone. Compared to the control, co-applied BmK I and BmK IT2 also significantly prolonged the time constant of inactivation.Conclusions Our results indicated that site-4 toxin (BmK IT2) could enhance the pharmacological effect induced by site-3 toxin (BmK I), suggesting a stronger effect elicited by both toxins that alone usually exhibit opposite pharmacological effects, which is related to the allosteric interaction between receptor site 3 and site 4. Meanwhile, these results may bring a novel perspective for exploring the underlying mechanisms of scorpion sting-induced pain.