Role of voltage-gated Na+ channels in hypoxia-induced neuronal injuries

Vitamin D Attenuates Hypoxia-Induced Injury in Rat Primary Neuron Cells through Downregulation of the Dual Oxidase 1 (DUOX1) Gene

BACKGROUND This study aimed to investigate the mechanisms underlying the neuroprotective effects of vitamin D. MATERIAL AND METHODS Rat primary neuron cells were incubated under a hypoxia condition [a hypoxic chamber mixed with anaerobic gas (90% N₂, 5% CO₂) and 5% O₂] to induce cell injury. Cell transfection was performed to overexpress or suppress the expression of dual oxidase 1 (DUOX1). The malondialdehyde (MDA) and superoxide dismutase (SOD) levels were detected using a MDA (A003-2) or SOD (A001-1) kit. DUOX1 mRNA levels were detected using RT-PCR. Hypoxia-inducible factor-1alpha (HIF-1alpha), DUOX1, vitamin D receptor (VDR), NF-kappaB protein expressions were determined by western blotting. Cell apoptosis and reactive oxygen species (ROS) were evaluated by flow cytometry. RESULTS ROS increased significantly after hypoxic treatment. The expressions of HIF-1alpha and DUOX1 were significantly increased after hypoxic treatment. Vitamin D could decrease ROS level, apoptotic neuron cells and DUOX1 expression, and increase VDR expression. Downregulation of DUOX1 significantly decreased MDA level and apoptotic percentages of neuron cells, increased SOD level, and counteracted the hypoxia-induced increase of NF-kappaB signal. Further study showed that overexpression of DUOX1 significantly increased MDA level, ROS level, apoptotic percentages of neuron cells, and NF-kappaB nuclear signaling, while decreased SOD level. Vitamin D significantly counteracted the effects of DUOX1 overexpression induced injury in rat primary neuron cells. CONCLUSIONS Our study indicated that vitamin D may protect neuron cells from hypoxia-induced injury by regulating DUOX1 via the NF-kappaB signaling pathway.

A Novel Model for Studying Voltage-Gated Ion Channel Gene Expression during Reversible Ischemic Stroke

The dysfunction of voltage-gated ion channels contributes to the pathology of ischemic stroke. In this study, we developed rat models of transient ischemic attack (TIA) and reversible ischemic neurological deficit (RIND) that was induced via the injection of artificial embolic particles during full consciousness, that allow us to monitor the neurologic deficit and positron emission tomography (PET) scans in real-time. We then evaluated the infarction volume of brain tissue was confirmed by 2,3,5-triphenyl tetrazolium chloride (TTC) staining, and gene expressions were evaluated by quantitative real-time PCR (qPCR). We found that rats with TIA or RIND exhibited neurological deficits as determined by negative TTC and PET findings. However, the expression of voltage-gated sodium channels in the hippocampus was significantly up-regulated in the qPCR array study. Furthermore, an altered expression of sodium channel β-subunits and potassium channels, were observed in RIND compared to TIA groups. In conclusion, to our knowledge, this is the first report of the successful evaluation of voltage-gated ion channel gene expression in TIA and RIND animal models. This model will aid future studies in investigating pathophysiological mechanisms, and in developing new therapeutic compounds for the treatment of TIA and RIND.

Can the combination of hyperthermia, seizures and ion channel dysfunction cause fatal post-ictal cerebral edema in patients with SCN1A mutations?

A 21-year-old male with an SCN1A mutation died of cerebral herniation 3 h after a seizure occurring during physical activity. Cases of fatal cerebral edema in patients with SCN1A mutations after fever and status epilepticus have been recently reported raising the question whether sodium channel dysfunction may contribute to cerebral edema and thereby contribute to the increased premature mortality in Dravet Syndrome. We report on our patient and discuss whether the combination of hyperthermia and ion channel dysfunction may not only trigger seizures but also a fatal pathophysiological cascade of cerebral edema and herniation leading to cardiorespiratory collapse.

Acute Hypoxia Activates an ENaC-like Channel in Rat Pheochromocytoma (PC12) Cells

Cells can resist and even recover from stress induced by acute hypoxia, whereas chronic hypoxia often leads to irreversible damage and eventually death. Although little is known about the response(s) to acute hypoxia in neuronal cells, alterations in ion channel activity could be preferential. This study aimed to elucidate which channel type is involved in the response to acute hypoxia in rat pheochromocytomal (PC12) cells as a neuronal cell model. Using perfusing solution saturated with 95% N(2) and 5% CO(2), induction of cell hypoxia was confirmed based on increased intracellular Ca(2+) with diminished oxygen content in the perfusate. During acute hypoxia, one channel type with a conductance of about 30 pS (2.5 pA at -80 mV) was activated within the first 2~3 min following onset of hypoxia and was long-lived for more than 300 ms with high open probability (P(o), up to 0.8). This channel was permeable to Na(+) ions, but not to K(+), Ca(+), and Cl(-) ions, and was sensitively blocked by amiloride (200 nM). These characteristics and behaviors were quite similar to those of epithelial sodium channel (ENaC). RT-PCR and Western blot analyses confirmed that ENaC channel was endogenously expressed in PC12 cells. Taken together, a 30-pS ENaC-like channel was activated in response to acute hypoxia in PC12 cells. This is the first evidence of an acute hypoxia-activated Na(+) channel that can contribute to depolarization of the cell.

Phenytoin inhibits the persistent sodium current in neocortical neurons by modifying its inactivation properties

The persistent Na⁺ current (I_NaP) is important for neuronal functions and can play a role in several pathologies, although it is small compared to the transient Na⁺ current (I_NaT). Notably, I_NaP is not a real persistent current because it undergoes inactivation with kinetics in the order of tens of seconds, but this property has often been overlooked. Na⁺ channel blockers, drugs used for treating epilepsy and other diseases, can inhibit I_NaP, but the mechanism of this action and the conditions in which I_NaP can be actually inhibited have not been completely clarified yet. We evaluated the action of phenytoin (PHT), a prototype anti-epileptic Na⁺ channel blocker, on I_NaP inactivation in pyramidal neurons of rat sensorimotor cortical slices at different concentrations, from 5 to 100 µM. PHT did not modify I_NaP evoked with depolarizing voltage ramps of 50 or 100 mVs⁻¹, but decreased I_NaP evoked by slower voltage ramps (10 mVs⁻¹). However, at all of the tested concentrations, PHT decreased I_NaP evoked by faster ramps when they were preceded by inactivating pre-pulses. Moreover, PHT shifted towards negative potentials the voltage-dependence of I_NaP inactivation and accelerated its kinetics of development also at depolarized potentials (+40 mV), not consistently with a simple inactivated state stabilizer. Therefore, our study shows a prominent PHT effect on I_NaP inactivation rather than an open channel block, which is instead often implied. I_NaP is inhibited by PHT only in conditions that induce major I_NaP inactivation. These results highlight the importance of I_NaP inactivation not only for physiological functions but also as drug target, which could be shared by other therapeutic drugs. Through this action PHT can reduce I_NaP-induced long-lasting pathological depolarisations and intracellular sodium overload, whereas shorter I_NaP actions should not be modified. These properties set the conditions of efficacy and the limits of PHT as I_NaP inhibitor.

Protective action of tetramethylpyrazine on the medulla oblongata in rats with chronic hypoxia

Tetramethylpyrazine (TMP), one of the active ingredients of the Chinese herb Lingusticum Wallichii Frantchat (Chuan Xiong), plays an important role in neuroprotection. However, the protective effect of TMP on the medulla oblongata, the most important region of the brain for cardiovascular and respiratory control, during chronic hypoxia remains unclear. In this study, we examined the neuroprotective effect of TMP on the medulla oblongata after chronic hypoxic injury in rats. Male Sprague-Dawley rats were randomly divided into four groups: control group, TMP group, chronic hypoxia group, and chronic hypoxia+TMP group. Rats were exposed to hypoxia (10% (v/v) O₂) or normoxia for 6 h daily for 14 days. TMP (80 mg/kg) or vehicle (saline) was injected intraperitoneally 30 min before experimentation. Loss of neurons in the pre-Bötzinger complex, the nucleus ambiguus, the nucleus tractus solitarius, the hypoglossal nucleus and the facial nucleus were evaluated by Nissl staining. Superoxide dismutase (SOD) activity and malondialdehyde (MDA) content were measured, and apoptosis was monitored using the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) method. The level of Bcl-2 mRNA and Bax mRNA was quantitatively measured by RT-PCR analysis. TMP protected Nissl bodies of neurons from injury in all nuclei observed, and reduced the loss of neurons in the nucleus ambiguus, the nucleus tractus solitarius, and the hypoglossal nucleus in rats subjected to chronic hypoxia. TMP upregulated SOD activity and inhibited the increase in MDA content in the medulla oblongata of hypoxic rats. In addition, TMP decreased the rate of apoptosis index (the percentage of apoptotic cells against the total number of cells) in all medullary structures examined, excepting the nucleus ambiguus and inhibited the decrease in Bcl-2 mRNA levels in the medulla oblongata following hypoxia. Our findings indicate that TMP may protect the medullary structures that are involved in cardiovascular and respiratory control from injury induced by chronic hypoxia in rats via its anti-oxidant and anti-apoptotic effects.

Na+ channelopathies and epilepsy: recent advances and new perspectives

Mutations of ion channel genes have a major role in the pathogenesis of several epilepsies, confirming that some epilepsies are disorders due to the impairment of ion channel function (channelopathies). Voltage-gated Na(+) channels (VGSCs) play an essential role in neuronal excitability; it is, therefore, not surprising that most mutations associated with epilepsy have been identified in genes coding for VGSCs subunits. Epilepsies linked to VGSCs mutations range in severity from mild disorders, such as benign neonatal-infantile familial seizures and febrile seizures, to severe and drug-resistant epileptic encephalopathies. SCN1A is the most clinically relevant of all of the known epilepsy genes, several hundred mutations have been identified in this gene. This review will summarize recent advances and new perspectives on Na(+) channels and epilepsy. A better understanding of the genetic basis and of how gene defects cause seizures is mandatory to direct future research for newer selective and more efficacious treatments.

Hypoxia-induced cellular and vascular changes in the nucleus tractus solitarius and ventrolateral medulla

Major changes in arterial pressure, autonomic, and respiratory activity occur in response to hypoxia. We analyzed structural damage and increased vascular permeability in the ventrolateral medulla and nucleus tractus solitarius, which control autonomic, respiratory, and cardiovascular functions in adult Wistar rats subjected to 2 hours of hypoxia (7% oxygen + 93% nitrogen) for up to 14 days after hypoxicexposure. Brainstem tissue levels of vascular endothelial growth factor (VEGF), nitric oxide (NO), and glutamate were significantly increased over control levels after hypoxic injury. By electron microscopy, swollen neurons and dendrites, degenerating axons, disrupted myelin sheaths, and swollen astrocyte processes were observed in the nucleus tractus solitarius and ventrolateral medulla. Leakage of intravenously administered horseradish peroxidase was observed through vascular walls in hypoxic rats. These results suggest that increased VEGF and NO production in hypoxia resulted in increased vascular permeability, which, along with increased levels of glutamate, may have induced structural alterations of the neurons, dendrites, and axons. Administration of the antioxidant neurohormone melatonin (10mg/kg) before and after the hypoxia reduced VEGF, NO, and glutamate levels and improved ultrastructural abnormalities induced by hypoxia exposure, suggesting that it may have a therapeutic potential in reducing hypoxia-associated brainstem damage.

Nitric oxide modulation of voltage-gated calcium current by S-nitrosylation and cGMP pathway in cultured rat hippocampal neurons

Nitric oxide (NO) plays an important role in many physiological and pathophysiological processes in the brain. In this study, we examined the mechanistic effects of an NO donor, diethylenetriamine/nitric oxide adduct (DETA/NO) on the voltage-gated calcium currents in cultured rat hippocampal neurons. DETA/NO stimulated the calcium currents and slightly increased the channel sensitivity to depolarizing voltages. The effect of DETA/NO on the calcium current was blocked by either depleting the NO in DETA/NO or by pretreating the neurons with NEM, a thiol-specific alkylating agent, suggesting an involvement of S-nitrosylation in the current response to NO. In addition, activation of the cGMP pathway by 8-Br-cGMP inhibited the calcium current in the neurons. Also, inhibition of guanylyl cyclase by 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (ODQ) increased the current response to DETA/NO. Taken together, our results demonstrate that both S-nitrosylation and cGMP pathway are involved in the NO modulation of the hippocampal calcium current.

Neuro-bioenergetic concepts in cancer prevention and treatment

Cancer remains one of the most difficult and elusive disorders to prevent and treat, despite great efforts in research and treatment over the last 30 years. Researchers have tried to understand the pathogenesis of cancer by discovering the single cellular mechanism or pathway derived from a genetic mutation. There are limited efforts made toward discovering a unified concept of cancer. We propose a neuro-bioenergetic concept of cancer pathogenesis based on the central mechanism of cellular hyperexcitability via inducible overexpression of voltage-gated ion channels, ligand-gated channels and neurotransmitters. Exploration of this concept could lead to a better understanding of the cause of cancer as well as developing more effective and specific strategies toward cancer prevention and treatment.

Dopamine transporters are involved in the onset of hypoxia-induced dopamine efflux in striatum as revealed by in vivo microdialysis

Although many studies have revealed alterations in neurotransmission during ischaemia, few works have been devoted to the neurochemical effects of mild hypoxia, a situation encountered during life in altitude or in several pathologies. In that context, the present work was undertaken to determine the in vivo mechanisms underlying the striatal dopamine efflux induced by mild hypoxaemic hypoxia. For that purpose, the extracellular concentrations of dopamine and its metabolite 3,4-dihydroxyphenyl acetic acid were simultaneously measured using brain microdialysis during acute hypoxic exposure (10% O(2), 1h) in awake rats. Hypoxia induced a +80% increase in dopamine. Application of the dopamine transporters inhibitor, nomifensine (10 microM), just before the hypoxia prevented the rise in dopamine during the early part of hypoxia; in contrast the application of nomifensine after the beginning of hypoxia, failed to alter the increase in dopamine. Application of the voltage-dependent Na(+) channel blocker tetrodotoxin abolished the increase in dopamine, whether administered just before or after the beginning of hypoxia. These data show that the neurochemical mechanisms of the dopamine efflux may change over the course of the hypoxic exposure, dopamine transporters being involved only at the beginning of hypoxia.

Angiotensin type 2 receptor neuroprotection against chemical hypoxia is dependent on the delayed rectifier K+ channel, Na+/Ca2+ exchanger and Na+/K+ ATPase in primary cortical cultures

We have previously reported that angiotensin II (Ang II) protects cortical neurons from chemical-induced hypoxia through activation of the angiotensin type 2 (AT(2)) receptor. Here, we show in mouse primary neuronal cultures that the AT(2) receptor neuroprotection results from the activation of the delayed rectifier K(+) channel as well as the involvement of the Na(+)/Ca(2+) exchanger (NCX) and Na(+)/K(+) ATPase (ATPase). Roles of the K(+) channel, NCX and ATPase were determined using the specific blockers alpha-dendrotoxin, KB-R7943 and ouabain, respectively. Sodium azide (10mM) induced apoptosis in 40% of neurons. Inhibition of the AT(1) receptor with losartan (1 microM) facilitated angiotensin II mediated neuroprotection by reducing sodium azide-induced apoptosis 61.8 +/- 5.6%, while inhibition of the AT(2) receptor with PD123319 (1 microM) showed no neuroprotection. These results suggest that angiotensin II neuroprotection is mediated through the AT(2) receptor and requires inhibition of the AT(1) receptor in order to facilitate its effect. To determine the roles of delayed rectifier K(+) channel, NCX and ATPase cultures were pretreated with alpha-dendrotoxin (10nM), KB-R7943 (100 nM) and ouabain (100 nM), which significantly attenuated AT(2) receptor mediated neuroprotection. These findings further suggest that the mechanism of AT(2) receptor mediated neuroprotection is coupled to activation of the delayed rectifier K(+) channel, NCX and ATPase.

Inhibitory effects of angiotensin on NMDA-induced cytotoxicity in primary neuronal cultures

Primary cultures from the hypothalamus/thalamus/septum/midbrain (HTSM) region of 1-day-old mice were used to investigate the effects of angiotensin on NMDA excitotoxicity. Cell viability was determined following exposure to 1-10 mM glutamate or 0.01-10 mM NMDA. Cells exposed to 1 mM glutamate or 1 mM NMDA for 24 h showed a significant increase in cell death as determined by propidium iodide staining. HTSM cultures treated with 0.1 mM NMDA revealed both DNA laddering and positive staining for TUNEL, suggesting apoptosis as the primary mechanism for the cell death. We also determined whether angiotensin II (Ang II) modulated NMDA-induced cell death in HTSM-cultured neurons. Cells pre-treated with 10 nM Ang II showed a decrease in NMDA-induced cytotoxicity, TUNEL staining and DNA laddering. NMDA-induced cell death was also reduced when cells were pre-treated with the AT1 receptor antagonist losartan. In this study, we have shown that NMDA and glutamate induce cell death through the NMDA receptor, and that Ang II, acting primarily through the AT2 receptor, can attenuate this response.

Maturation of neuronal excitability in hippocampal neurons of mice chronically exposed to cyclic hypoxia

To examine the effects of chronic cyclic hypoxia on neuronal excitability and function in mice, we exposed mice to cyclic hypoxia for 8 h daily (9 cycles/h) for approximately 2 wk (starting at 2-3 days of age) and examined the properties of freshly dissociated hippocampal neurons obtained from slices. Compared with control (Con) hippocampal CA1 neurons, exposed neurons (CYC) had similar resting membrane potentials (V(m)) and action potentials (AP). CYC neurons, however, had a lower rheobase than Con neurons. There was also an upregulation of the Na(+) current density (333 +/- 84 pA/pF, n = 18) in CYC compared with that of Con neurons (193 +/- 20 pA/pF, n = 27, P < 0.03). Na(+) channel characteristics were significantly altered by hypoxia. For example, the steady-state inactivation curve was significantly more positive in CYC than in Con (-60 +/- 6 mV, n = 8, for CYC and -71 +/- 3 mV, n = 14, for Con, P < 0.04). The time constant for deactivation (tau(d)) was much shorter in CYC than in Con (at -100 mV, tau(d)=0.83 +/- 0.23 ms in CYC neurons and 2.29 +/- 0.38 ms in Con neurons, P = 0.004). We conclude that the increased neuronal excitability in mice neurons treated with cyclic hypoxia is due to alterations in Na(+) channel characteristics and/or Na(+) channel expression. We hypothesize from these and previous data from our laboratory (Gu XQ and Haddad GG. J Appl Physiol 91: 1245-1250, 2001) that this increased excitability is a reflection of an enhanced central nervous system maturation when exposed to low O(2) conditions in early postnatal life.

Radix paeoniae rubra suppression of sodium current in acutely dissociated rat hippocampal CA1 neurons

The effect of Radix paeoniae rubra (RPR) on voltage-gated sodium channel (VGSC) currents (I(Na)) was examined in freshly isolated rat hippocampal CA1 neurons using whole-cell patch-clamp technique under voltage-clamp conditions. RPR suppressed I(Na) without affecting the current activation, inactivation and deactivation. The amplitude of I(Na) decreased by approximately 18.4% within a few seconds of 0.8 mg/ml RPR exposure. RPR (0.8 mg/ml) shifted the steady-state inactivation curves of I(Na) to negative potentials, with hyperpolarizing direction shift of V(1/2) of 10.0 mV. The time course of I(Na) recovery from inactivation was prolonged significantly by 0.8 mg/ml RPR. RPR (0.8 mg/ml) also enhanced the activity-dependent attenuation of I(Na) and decreased the fraction of activated channels. These results suggested that RPR suppressed hippocampal CA1 I(Na) by shifting the inactivation curve in hyperpolarizing direction, slowing the recovery time course from inactivation, enhancing the activity-dependent attenuation and decreasing the number of activatable channels. RPR suppression on I(Na) might predict the protective effect during brain ischemia in hippocampal CA1 neurons.