Effect of prolonged O2 deprivation on Na+ channels: differential regulation in adult versus fetal rat brain

Down-regulation of Inwardly Rectifying K+ Currents in Astrocytes Derived from Patients with Monge's Disease

Chronic mountain sickness (CMS) or Monge's disease is a disease in highlanders. These patients have a variety of neurologic symptoms such as migraine, mental fatigue, confusion, dizziness, loss of appetite, memory loss and neuronal degeneration. The cellular and molecular mechanisms underlying CMS neuropathology is not understood. In the previous study, we demonstrated that neurons derived from CMS patients' fibroblasts have a decreased expression and altered gating properties of voltage-gated sodium channel. In this study, we further characterize the electrophysiological properties of iPSC-derived astrocytes from CMS patients. We found that the current densities of the inwardly rectifying potassium (Kir) channels in CMS astrocytes (-5.7 ± 2.2 pA/pF at -140 mV) were significantly decreased as compared to non-CMS (-28.4 ± 3.4 pA/pF at -140 mV) and sea level subjects (-28.3 ± 5.3 pA/pF at -140 mV). We further demonstrated that the reduced Kir current densities in CMS astrocytes were caused by their decreased protein expression of Kir4.1 and Kir2.3 channels, while single channel properties (i.e., P_o, conductance) of Kir channel in CMS astrocytes were not altered. In addition, we found no significant differences of outward potassium currents between CMS and non-CMS astrocytes. As compared to non-CMS and sea level subjects, the K⁺ uptake ability in CMS astrocytes was significantly decreased. Taken together, our results suggest that down-regulation of Kir channels and the resulting decreased K⁺ uptake ability in astrocytes could be one of the major molecular mechanisms underlying the neurologic manifestations in CMS patients.

Altered iPSC-derived neurons' sodium channel properties in subjects with Monge's disease

Monge's disease, also known as chronic mountain sickness (CMS), is a disease that potentially threatens more than 140 million highlanders during extended time living at high altitudes (over 2500m). The prevalence of CMS in Andeans is about 15-20%, suggesting that the majority of highlanders (non-CMS) are rather healthy at high altitudes; however, CMS subjects experience severe hypoxemia, erythrocytosis and many neurologic manifestations including migraine, headache, mental fatigue, confusion, and memory loss. The underlying mechanisms of CMS neuropathology are not well understood and no ideal treatment is available to prevent or cure CMS, except for phlebotomy. In the current study, we reprogrammed fibroblast cells from both CMS and non-CMS subjects' skin biopsies into the induced pluripotent stem cells (iPSCs), then differentiated into neurons and compared their neuronal properties. We discovered that CMS neurons were much less excitable (higher rheobase) than non-CMS neurons. This decreased excitability was not caused by differences in passive neuronal properties, but instead by a significantly lowered Na(+) channel current density and by a shift of the voltage-conductance curve in the depolarization direction. Our findings provide, for the first time, evidence of a neuronal abnormality in CMS subjects as compared to non-CMS subjects, hoping that such studies can pave the way to a better understanding of the neuropathology in CMS.

δ-Opioid receptor activation modified microRNA expression in the rat kidney under prolonged hypoxia

Hypoxic/ischemic injury to kidney is a frequently encountered clinical problem with limited therapeutic options. Since microRNAs are differentially involved in hypoxic/ischemic events and δ-opioid receptor (DOR) activation is known to protect against hypoxic/ischemic injury, we speculated on the involvement of DOR activation in altering the microRNA (miRNA) expression in kidney under hypoxic condition. We selected 31 miRNAs based on microarray data for quantitative PCR analysis. Among them, 14 miRNAs were significantly altered after prolonged hypoxia, DOR activation or a combination of both. We found that 1) DOR activation alters miRNA expression profiles in normoxic conditions; 2) hypoxia differentially alters miRNA expression depending on the duration of hypoxia; and 3) DOR activation can modify hypoxia-induced changes in miRNA expression. For example, 10-day hypoxia reduced the level of miR-212 by over 70%, while DOR activation could mimic such reduction even in normoxic kidney. In contrast, the same stress increased miR-29a by >100%, which was reversed following DOR activation. These first data suggest that hypoxia comprehensively modifies the miRNA profile within the kidney, which can be mimicked or modified by DOR activation. Ascertaining the targeted pathways that regulate the diverse cellular and molecular functions of miRNA may provide new insights into potential therapies for hypoxic/ischemic injury of the kidney.

δ-opioid receptor activation and microRNA expression of the rat cortex in hypoxia

Prolonged hypoxic/ischemic stress may cause cortical injury and clinically manifest as a neurological disability. Activation of the δ-opioid receptor (DOR) may induce cortical protection against hypoxic/ischemic insults. However, the mechanisms underlying DOR protection are not clearly understood. We have recently found that DOR activation modulates the expression of microRNAs (miRNAs) in the kidney exposed to hypoxia, suggesting that DOR protection may involve a miRNA mechanism. To determine if the miRNAs expressed in the cortex mediated DOR neuroprotection, we examined 19 miRNAs that were previously identified as hypoxia- and DOR-regulated miRNAs in the kidney, in the rat cortex treated with UFP-512, a potent and specific DOR agonist under hypoxic condition. Of the 19 miRNAs tested, 17 were significantly altered by hypoxia and/or DOR activation with the direction and amplitude varying depending on hypoxic duration and times of DOR treatment. Expression of several miRNAs such as miR-29b, -101b, -298, 324-3p, -347 and 466b was significantly depressed after 24 hours of hypoxia. Similar changes were seen in normoxic condition 24 hours after DOR activation with one-time treatment of UFP-512. In contrast, some miRNAs were more tolerant to hypoxic stress and showed significant reduction only with 5-day (e.g., miR-31 and -186) or 10-day (e.g., miR-29a, let-7f and -511) exposures. In addition, these miRNAs had differential responses to DOR activation. Other miRNAs like miRs-363* and -370 responded only to the combined exposure to hypoxia and DOR treatment, with a notable reduction of >70% in the 5-day group. These data suggest that cortical miRNAs are highly yet differentially sensitive to hypoxia. DOR activation can modify, enhance or resolve the changes in miRNAs that target HIF, ion transport, axonal guidance, free radical signaling, apoptosis and many other functions.

Spontaneous electrical activity in the human fetal cortex in vitro

Our knowledge about the developing human cerebral cortex is based on the analysis of fixed postmortem material. Here we use electrical recordings from unfixed human postmortem tissue to characterize the synaptic physiology and spontaneous network activity of pioneer cortical neurons ("subplate neurons"). Our electrophysiological experiments show that functional glutamate or GABA ionotropic receptors are expressed on human subplate (SP) neurons as early as 20 gestational weeks. Extracellular (synaptic) stimulations evoked postsynaptic potentials in a very small fraction of SP neurons, suggesting that functional synaptic contacts are rare at midgestation. Although synaptic inputs were scarce, we regularly observed spontaneous (unprovoked) electrical activity among human SP neurons, comprised of sustained plateau depolarizations and bursts of action potential firing, which resembled cortical UP and DOWN states in the adult neocortex. Plateau depolarizations and bursts of action potential firing are thought to depend on the mature morphology and physiology of adult cortical network. However, our current data reveal that similar cortical rhythm is generated by a very immature ensemble of human fetal neurons. In the relative absence of sensory inputs, as in development in utero, or in slow-wave sleep (i.e., throughout the entire lifespan), the spontaneous slow oscillatory pattern (UP and DOWN states) is a fundamental aspect of human cortical physiology.

Two types of independent bursting mechanisms in inspiratory neurons: an integrative model

The network of coupled neurons in the pre-Bötzinger complex (pBC) of the medulla generates a bursting rhythm, which underlies the inspiratory phase of respiration. In some of these neurons, bursting persists even when synaptic coupling in the network is blocked and respiratory rhythmic discharge stops. Bursting in inspiratory neurons has been extensively studied, and two classes of bursting neurons have been identified, with bursting mechanism depends on either persistent sodium current or changes in intracellular Ca(2+), respectively. Motivated by experimental evidence from these intrinsically bursting neurons, we present a two-compartment mathematical model of an isolated pBC neuron with two independent bursting mechanisms. Bursting in the somatic compartment is modeled via inactivation of a persistent sodium current, whereas bursting in the dendritic compartment relies on Ca(2+) oscillations, which are determined by the neuromodulatory tone. The model explains a number of conflicting experimental results and is able to generate a robust bursting rhythm, over a large range of parameters, with a frequency adjusted by neuromodulators.

Glibenclamide improves neurological function in neonatal hypoxia-ischemia in rats

Recent studies demonstrated that sulfonylurea receptor 1 (SUR 1) regulated nonselective cation channel, the NC(Ca-ATP) channel, is involved in brain injury in rodent models of stroke. Block of SUR 1 with sulfonylurea such as glibenclamide has been shown to be highly effective in reducing cerebral edema, infarct volume and mortality in adult rat models of ischemic stroke. In this study, we tested glibenclamide in both severe and moderate models of neonatal hypoxia-ischemia (HI) in postnatal day 10 Sprague-Dawley rat pups. A total of 150 pups were used in the present study. Pups were subjected to unilateral carotid artery ligation followed by 2.5 or 2 h of hypoxia in the severe and moderate HI models, respectively. In the severe HI model, glibenclamide, administered immediately after HI and on postoperative Day 1, was not effective in attenuating short-term effects (brain edema and infarct volume) or long-term effects (brain weight and neurological function) of neonatal HI. In the moderate HI model, when injected immediately after HI and on postoperative Day 1, glibenclamide at 0.01 mg/kg improved several neurological parameters at 3 weeks after HI. We conclude that glibenclamide provided some long-term neuroprotective effect after neonatal HI.

Physiological and molecular evidence of heat acclimation memory: a lesson from thermal responses and ischemic cross-tolerance in the heart

Sporadic findings in humans suggest that reinduction of heat acclimation (AC) after its loss occurs markedly faster than that during the initial AC session. Animal studies substantiated that the underlying acclimatory processes are molecular. Here we test the hypothesis that faster reinduction of AC (ReAC) implicates "molecular memory." In vivo measurements of colonic temperature profiles during heat stress and ex vivo assessment of cross-tolerance to ischemia-reperfusion or anoxia insults in the heart demonstrated that ReAC only needs 2 days vs. the 30 days required for the initial development of AC. Stress gene profiling in the experimental groups highlighted clusters of transcriptionally activated genes (37%), which included heat shock protein (HSP) genes, antiapoptotic genes, and chromatin remodeling genes. Despite a return of the physiological phenotype to its preacclimation state, after a 1 mo deacclimation (DeAC) period, the gene transcripts did not resume their preacclimation levels, suggesting a dichotomy between genotype and phenotype in this system. Individual detection of hsp70 and hsf1 transcripts agreed with these findings. HSP72, HSF1/P-HSF1, and Bcl-xL protein profiles followed the observed dichotomized genomic response. In contrast, HSP90, an essential cytoprotective component mismatched transcriptional activation upon DeAC. The uniform activation of the similarly responding gene clusters upon De-/ReAC implies that reacclimatory phenotypic plasticity is associated with upstream denominators. During AC, DeAC, and ReAC, the maintenance of elevated/phosphorylated HSF1 protein levels and transcriptionally active chromatin remodeling genes implies that chromatin remodeling plays a pivotal role in the transcriptome profile and in preconditioning to rapid cytoprotective acclimatory memory.

Molecular physiology of preconditioning-induced brain tolerance to ischemia

Ischemic tolerance describes the adaptive biological response of cells and organs that is initiated by preconditioning (i.e., exposure to stressor of mild severity) and the associated period during which their resistance to ischemia is markedly increased. This topic is attracting much attention because preconditioning-induced ischemic tolerance is an effective experimental probe to understand how the brain protects itself. This review is focused on the molecular and related functional changes that are associated with, and may contribute to, brain ischemic tolerance. When the tolerant brain is subjected to ischemia, the resulting insult severity (i.e., residual blood flow, disruption of cellular transmembrane gradients) appears to be the same as in the naive brain, but the ensuing lesion is substantially reduced. This suggests that the adaptive changes in the tolerant brain may be primarily directed against postischemic and delayed processes that contribute to ischemic damage, but adaptive changes that are beneficial during the subsequent test insult cannot be ruled out. It has become clear that multiple effectors contribute to ischemic tolerance, including: 1) activation of fundamental cellular defense mechanisms such as antioxidant systems, heat shock proteins, and cell death/survival determinants; 2) responses at tissue level, especially reduced inflammatory responsiveness; and 3) a shift of the neuronal excitatory/inhibitory balance toward inhibition. Accordingly, an improved knowledge of preconditioning/ischemic tolerance should help us to identify neuroprotective strategies that are similar in nature to combination therapy, hence potentially capable of suppressing the multiple, parallel pathophysiological events that cause ischemic brain damage.

Heat acclimation and cross-tolerance against novel stressors: genomic–physiological linkage

Heat acclimation (AC) is a "within lifetime" reversible phenotypic adaptation, enhancing thermotolerance and heat endurance via a transition to "efficient" cellular performance when acclimatory homeostasis is reached. An inseparable outcome of AC is the development of cross-tolerance (C-T) against novel stressors. This chapter focuses on central plasticity and the molecular-physiological linkage of acclimatory and C-T responses. A drop in temperature thresholds (T-Tsh) for activation of heat-dissipation mechanisms and an elevated T-Tsh for thermal injury development imply autonomic nervous system (ANS) and cytoprotective network involvement in these processes. During acclimation, the changes in T-Tsh for heat dissipation are biphasic. Initially T-Tsh drops, signifying the early autonomic response, and is associated with perturbed peripheral effector cellular performance. Pre-acclimation values return when acclimatory homeostasis is achieved. The changes in the ANS suggest that acclimatory plasticity involves molecular and cellular changes. These changes are manifested by the activation of central peripheral molecular networks and post-translational modifications. Sympathetic induction of elevated HSP 72 reservoirs, with faster heat shock response, is only one example of this. The global genomic response, detected using gene-chips and cluster analyses imply upregulation of genes encoding ion channels, pumps, and transporters (markers for neuronal excitability) in the hypothalamus at the onset of AC and down regulation of metabotrophic genes upon long term AC. Peripherally, the transcriptional program indicates a two-tier defense strategy. The immediate transient response is associated with the maintenance of DNA and cellular integrity. The sustained response correlates with long-lasting cytoprotective-signaling networks. C-T is recorded against cerebral hypoxia, hyperoxia, and traumatic brain injury. Using the highly developed ischemic/reperfused heart model as a baseline, it is evident that C-T stems via protective shared pathways developed with AC. These comprise constitutive elevation of HIF 1alpha and associated target pathways, HSPs, anti-apoptosis, and antioxidative pathways. Collectively the master regulators of AC and C-T are still enigmatic; however, cutting-edge investigative techniques, using a broad molecular approach, challenge current ideas, and the data accumulated will pinpoint novel pathways and provide new perspectives.

Chronic hypobaric hypoxia induces tolerance to acute hypoxia and up-regulation in alpha-2 adrenoceptor in rat locus coeruleus

Hypoxia preconditioning has been shown to produce tolerance against brain injuries. The hypothesis of this study is that chronic hypobaric hypoxia may also induce acute hypoxia tolerance. We used intracellular recording in slices from rats exposed to chronic hypobaric hypoxia (exposed) and control to investigate the effects of chronic hypobaric hypoxia on the physiology of locus coeruleus (LC) including neuronal excitability. The results showed 35.7% reduced spontaneous firing rate and no change for membrane potential and input resistance in exposed neurons. In response to the alpha-2 adrenoceptor (A2R) agonist clonidine, both the hyperpolarizing potency and efficacy were increased indicated by a decreased EC(50) (control: 30.9 nM and exposed: 19.7 nM) and a 50.5% increase in maximum hyperpolarized potential, respectively. A2R binding sites were also increased 21% in exposed neurons measured by radioligand [(3)H]rauwolscine binding assay. When treated with acute N(2)-hypoxia, the cell survival time (ST) was longer in exposed neurons, suggesting that a tolerance was induced. In addition, the ST for both groups of LC neurons was decreased by the A2R antagonist yohimbine and increased by the glutamate receptor antagonist kynurenic acid but not by MK-801; the decreased percentage of ST by yohimbine was larger and the increased percentage by kynurenic acid was smaller in exposed neurons. The results suggested that up-regulation of A2R and altered non-NMDA glutamate receptor function induced by chronic hypobaric hypoxia may underlie, in part, the decreased LC neuronal excitability and acute hypoxia tolerance.

Acclimatory-phase specificity of gene expression during the course of heat acclimation and superimposed hypohydration in the rat hypothalamus

The induction of the heat-acclimated phenotype involves reprogramming the expression of genes encoding both constitutive and inducible proteins. In this investigation, we studied the global genomic response in the hypothalamus during heat acclimation, with and without combined hypohydration stress. Rats were acclimated for 2 days (STHA) or for 30 days (LTHA) at 34 degrees C. Hypohydration (10% decrease in body weight) was attained by water deprivation. 32P-labeled RNA samples from the hypothalamus were hybridized onto cDNA Atlas array (Clontech no. 1.2) membranes. Clustering and functional analyses of the expression profile of a battery of genes representing various central regulatory functions of body homeostasis demonstrated a biphasic acclimation profile with a transient upregulation of genes encoding ion channels, transporters, and transmitter signaling upon STHA. After LTHA, most genes returned to their preacclimation expression levels. In both STHA and LTHA, genes encoding hormones and neuropeptides, linked with metabolic rate and food intake, were downregulated. This genomic profile, demonstrating an enhanced transcription of genes linked with neuronal excitability during STHA and enhanced metabolic efficiency upon LTHA, is consistent with our previously established integrative acclimation model. The response to hypohydration was characterized by an upregulation of a large number of genes primarily associated with the regulation of ion channels, cell volume, and neuronal excitability. During STHA, the response was transiently desensitized, recovering upon LTHA. We conclude that hypohydration overrides the heat acclimatory status. It is notable that STHA and hypohydration gene profiles are analogous with the physiological profile described in the response to various types of brain injury.

Inhibition of Qi site of mitochondrial complex III with antimycin A decreases persistent and transient sodium currents via reactive oxygen species and protein kinase C in rat hippocampal CA1 cells

Hypoxia-induced inhibition of Qi site of mitochondrial complex III under hypoxia has received attention, but its downstream pathways remain unclear. In this paper, we used Qi site inhibitor antimycin A to mimic the inhibition of the Qi site of mitochondrial complex III and studied the effects of the inhibition of this site on persistent sodium currents, transient sodium currents, and neuronal excitability in rat hippocampal CA1 cells with whole cell patch-clamp methods. The results showed that antimycin A decreased the amplitude of both persistent and transient sodium currents; antioxidant 2-mercaptopropionylglycine or 1,10 phenanthroline abolished the effect of antimycin A; the complex III Qo site inhibitor stigmatellin, the protein kinase C inhibitor chelerythrine, but not the protein kinase A inhibitor H89, canceled the effect of antimycin A; antimycin A decreased the amplitude of both persistent and transient sodium currents only at more depolarized membrane potentials and the decrease percentage of both persistent and transient sodium currents after antimycin A at potentials above -50 mV increased with the change in potentials toward more depolarized direction; exogenous application of H2O2 inhibited the amplitude of both persistent and transient sodium currents; the amount of current required to trigger spikes was increased and the number of spikes produced by varying levels of currents was decreased by antimycin A. These results suggest that the inhibition of Qi site of mitochondrial complex III decreases both persistent and transient sodium currents via reactive oxygen species and protein kinase C in rat hippocampal CA1 cells.

Intermittent hypoxia modulates Na+ channel expression in developing mouse brain

Because our previous work showed that intermittent hypoxia alters neuronal excitability and Na+ current density, we examined in this work the effect of intermittent hypoxia on Na+ channel subtypes using 3H-saxitoxin (3H-STX) autoradiography and immunoblotting. Mice were exposed to intermittent hypoxia for 2 or 4 weeks from postnatal day 2 or 3. A 2-week intermittent hypoxia reduced cerebral STX binding density with significant decrease in Na(v)1.2 in the rostral and Na(v)1.1 in the caudal regions. In contrast, a 4-week intermittent hypoxia tended to increase STX binding density in most brain regions. Our data suggest that intermittent hypoxia differentially regulates plasma membrane Na+ channels in the developing brain, depending on duration of intermittent hypoxia.

Fetal brain hypometabolism during prolonged hypoxaemia in the llama

In this study we looked for additional evidence to support the hypothesis that fetal llama reacts to hypoxaemia with adaptive brain hypometabolism. We determined fetal llama brain temperature, Na(+) and K(+) channel density and Na(+)-K(+)-ATPase activity. Additionally, we looked to see whether there were signs of cell death in the brain cortex of llama fetuses submitted to prolonged hypoxaemia. Ten fetal llamas were instrumented under general anaesthesia to measure pH, arterial blood gases, mean arterial pressure, heart rate, and brain and core temperatures. Measurements were made 1 h before and every hour during 24 h of hypoxaemia (n = 5), which was imposed by reducing maternal inspired oxygen fraction to reach a fetal arterial partial pressure of oxygen (P(a,O(2))) of about 12 mmHg. A normoxaemic group was the control (n = 5). After 24 h of hypoxaemia, we determined brain cortex Na(+)-K(+)-ATPase activity, ouabain binding, and the expression of NaV1.1, NaV1.2, NaV1.3, NaV1.6, TREK1, TRAAK and K(ATP) channels. The lack of brain cortex damage was assessed as poly ADP-ribose polymerase (PARP) proteolysis. We found a mean decrease of 0.56 degrees C in brain cortex temperature during prolonged hypoxaemia, which was accompanied by a 51% decrease in brain cortex Na(+)-K(+)-ATPase activity, and by a 44% decrease in protein content of NaV1.1, a voltage-gated Na(+) channel. These changes occurred in absence of changes in PARP protein degradation, suggesting that the cell death of the brain was not enhanced in the fetal llama during hypoxaemia. Taken together, these results provide further evidence to support the hypothesis that the fetal llama responds to prolonged hypoxaemia with adaptive brain hypometabolism, partly mediated by decreases in Na(+)-K(+)-ATPase activity and expression of NaV channels.

Oxygen-sensing neurons in the central nervous system

This mini-review summarizes the present knowledge regarding central oxygen-chemosensitive sites with special emphasis on their function in regulating changes in cardiovascular and respiratory responses. These oxygen-chemosensitive sites are distributed throughout the brain stem from the thalamus to the medulla and may form an oxygen-chemosensitive network. The ultimate effect on respiratory or sympathetic activity presumably depends on the specific neural projections from each of these brain stem oxygen-sensitive regions as well as on the developmental age of the animal. Little is known regarding the cellular mechanisms involved in the chemotransduction process of the central oxygen sensors. The limited information available suggests some conservation of mechanisms used by other oxygen-sensing systems, e.g., carotid body glomus cells and pulmonary vascular smooth muscle cells. However, major gaps exist in our understanding of the specific ion channels and oxygen sensors required for transducing central hypoxia by these central oxygen-sensitive neurons. Adaptation of these central oxygen-sensitive neurons during chronic or intermittent hypoxia likely contributes to responses in both physiological conditions (ascent to high altitude, hypoxic conditioning) and clinical conditions (heart failure, chronic obstructive pulmonary disease, obstructive sleep apnea syndrome, hypoventilation syndromes). This review underscores the lack of knowledge about central oxygen chemosensors and highlights real opportunities for future research.

Molecular adaptations for survival during anoxia: lessons from lower vertebrates

Anoxia-tolerant neurons from several species of animals may offer unparalleled opportunities to identify strategies that might be employed to enhance the hypoxia or ischemia tolerance of vulnerable neurons. In this review, the authors describe how the response of hypoxia-tolerant neurons to limited oxygen supply involves a suite of mechanisms that reduce energy expenditure in concert with decreased energy availability. This response avoids energy depletion, excitotoxic neuronal death, and apoptosis. Suppression of ion channel functions, particularly those of the ionotropic glutamate receptors, is a response common in hypoxia-tolerant neurons. The depression of excitability thereby achieved is essential given that the fundamental response to oxygen lack in anoxia-tolerant cells is a throttling down of metabolism to "pilot-light" levels. Many different types of processes have been found to down-regulate ion channel function. These include phosphorylation control, interactions with intracellular and extracellular ions, removal of active receptors from the neurolemma, and the direct sensing of oxygen by Na+ and K+ channels. Changes in [Ca2+]i may initiate a protective down-regulation of many different pumps or channels. Transcriptional events leading to differential and/or decreased expression of receptors, proteins, and their subunits are probably very important but little studied.

Alterations of single potassium channel activity in CA1 pyramidal neurons after transient forebrain ischemia

Selective neuronal injury in the CA1 zone of hippocampus following transient cerebral ischemia has been well documented. Extracellular potassium concentration markedly increases during ischemia/hypoxia. Accumulating evidence has indicated that the outward potassium currents, including delayed rectifier potassium current, not only influence membrane excitability but also mediate apoptosis. It has been shown that the amplitude of delayed rectifier potassium current in CA1 neurons significantly increased after cerebral ischemia. To elucidate the mechanisms underlying the changes of potassium currents following ischemia, single potassium channel activities of rat CA1 neurons were compared before and after transient forebrain ischemia. Using cell-attached configuration, depolarizing voltage steps activated outward single channel events. The channel properties, the kinetics and pharmacology of these events resemble the delayed rectifier potassium current. After ischemia, the unitary amplitude of single channels significantly increased, the open probability, mean open time and open time constant also significantly increased while the conductance remained unchanged. These data indicate that the increase of single channel activity is responsible, at least in part, for the increase of delayed rectifier potassium current in CA1 neurons after cerebral ischemia.

Effect of prolonged hypoxia on Na+ channel mRNA subtypes in the developing rat cortex

Voltage-gated Na+ channels are regulated in response to oxygen deprivation in the mammalian cortex. Past investigations have demonstrated that Na+ channel protein expression is up-regulated in the immature brain exposed to prolonged hypoxia. Since it is unknown as to which Na+ channel subtype(s) is involved in this regulation, we used RT-PCR to assess the effect of hypoxia on Na+ channel I, II and III alpha-subunit mRNA expression in the developing rat cortex. Na+ channel II mRNA tended to increase during early development, whereas Na+ channel I and III did not change or slightly decreased with age. Hypoxic exposure for 1-day had no effect on Na+ channel expression, while 5-day hypoxia significantly increased Na+ channel III density, with a slight increase in Na+ channel I and no appreciable change in Na+ channel II. These results suggest that Na+ channel subtype expression in the developing cortex is differentially regulated in response to prolonged hypoxic exposure.

Chronic hypoxia modulates diaphragm function in the developing rat

We studied the effect of chronic hypoxia on contractile properties and neuromuscular transmission in the developing rat diaphragm. We hypothesized that chronic hypoxia delays maturation of neuromuscular transmission. Phrenic nerve hemidiaphragm preparations were harvested from 3- to 26-day-old rats and littermates raised in 9.5% oxygen. Specific force, contraction time, and one-half relaxation time were measured. Each diaphragm was stimulated directly or via its nerve with 1-s trains at 10–100 Hz. Contraction time and one-half relaxation time decreased with advancing age in both groups, with a greater rate of decrease in hypoxic diaphragms. Specific force was lower for hypoxic diaphragms compared with controls. Diaphragms from the 3- to 10-day-old control and hypoxic groups generated less force in response to stimulation at frequencies >40 Hz but did so to a greater degree with nerve stimulation. Nerve stimulation of diaphragms from 11- to 18-day-old hypoxic rats showed a greater decrease in force with increasing frequency compared with age-matched controls. Diaphragms from 19- to 26-day-old rats showed no difference between the hypoxic and control groups. We conclude that chronic hypoxia leads to diaphragms that generate lower specific force as well as to a delayed maturation of mechanisms involved in neuromuscular transmission.

Heterogeneous increases of cytoplasmic calcium: distinct effects on down-regulation of cell surface sodium channels and sodium channel subunit mRNA levels

1. Long-term (> or = 12 h) treatment of cultured bovine adrenal chromaffin cells with A23187 (a Ca(2+) ionophore) or thapsigargin (TG) [an inhibitor of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)] caused a time- and concentration-dependent reduction of cell surface [(3)H]-saxitoxin (STX) binding capacity, but did not change the K:(D:) value. In A23187- or TG-treated cells, veratridine-induced (22)Na(+) influx was reduced (with no change in veratridine EC(50) value) while it was enhanced by alpha-scorpion venom, beta-scorpion venom, or Ptychodiscus brevis toxin-3, like in nontreated cells. 2. The A23187- or TG-induced decrease of [(3)H]-STX binding was diminished by BAPTA-AM. EGTA also inhibited the decreasing effect of A23187. A23187 caused a rapid, monophasic and persistent increase in intracellular concentration of Ca(2+) ([Ca(2+)](i)) to a greater extent than that observed with TG. 2,5-Di-(t-butyl)-1,4-benzohydroquinone (DBHQ) (an inhibitor of SERCA) produced only a rapid monophasic increase in [Ca(2+)](i), without any effect on [(3)H]-STX binding. 3. Reduction in [(3)H]-STX binding capacity induced by A23187 or TG was attenuated by Gö6976 (an inhibitor of conventional protein kinase C) or calpastatin peptide (an inhibitor of calpain). When the internalization rate of cell surface Na(+) channels was measured in the presence of brefeldin A (an inhibitor of vesicular exit from the trans-Golgi network), A23187 or TG accelerated the reduction of [(3)H]-STX binding capacity. 4. Six hours treatment with A23187 lowered Na(+) channel alpha- and beta(1)-subunit mRNA levels, whereas TG had no effect. 5. These results suggest that elevation of [Ca(2+)](i) caused by A23187, TG or DBHQ exerted differential effects on down-regulation of cell surface functional Na(+) channels and Na(+) channel subunit mRNA levels.

delta-, but not mu- and kappa-, opioid receptor activation protects neocortical neurons from glutamate-induced excitotoxic injury

Recent observations from our laboratory have led us to hypothesize that delta-opioid receptors may play a role in neuronal protection against hypoxic/ischemic or glutamate excitotocity. To test our hypothesis in this work, we used two independent methods, i.e., "same field quantification" of morphologic criteria and a biochemical assay of lactate dehydrogenase (LDH) release (an index of cellular injury). We used neuronal cultures from rat neocortex and studied whether (1) glutamate induces neuronal injury as a function of age and (2) activation of opioid receptors (delta, mu and kappa subtypes) protects neurons from glutamate-induced injury. Our results show that glutamate induced neuronal injury and cell death and this was dependent on glutamate concentration, exposure period and days in culture. At 4 days, glutamate (up to 10 mM, 4 h-exposure) did not cause apparent injury. After 8-10 days in culture, neurons exposed to a much lower dose of glutamate (100 microM, 4 h) showed substantial neuronal injury as assessed by morphologic criteria (>65%, n=23, P<0.01) and LDH release (n=16, P<0. 001). Activation of delta-opioid receptors with 10 microM DADLE reduced glutamate-induced injury by almost half as assessed by the same criteria (morphologic criteria, n=21, P<0.01; LDH release, n=16, P<0.01). Naltrindole (10 microM), a delta-opioid receptor antagonist, completely blocked the DADLE protective effect. Administration of mu- and kappa-opioid receptor agonists (DAMGO and U50488H respectively, 5-10 microM) did not induce appreciable neuroprotection. Also, mu- or kappa-opioid receptor antagonists had no appreciable effect on the glutamate-induced injury. This study demonstrates that activation of neuronal delta-opioid receptors, but not mu- and kappa-opioid receptors, protect neocortical neurons from glutamate excitotoxicity.

Differential changes of potassium currents in CA1 pyramidal neurons after transient forebrain ischemia

CA1 pyramidal neurons are highly vulnerable to transient cerebral ischemia. In vivo studies have shown that the excitability of CA1 neurons progressively decreased following reperfusion. To reveal the mechanisms underlying the postischemic excitability change, total potassium current, transient potassium current, and delayed rectifier potassium current in CA1 neurons were studied in hippocampal slices prepared before ischemia and at different time points following reperfusion. Consistent with previous in vivo studies, the excitability of CA1 neurons decreased in brain slices prepared at 14 h following transient forebrain ischemia. The amplitude of total potassium current in CA1 neurons increased approximately 30% following reperfusion. The steady-state activation curve of total potassium current progressively shifted in the hyperpolarizing direction with a transient recovery at 18 h after ischemia. For transient potassium current, the amplitude was transiently increased approximately 30% at approximately 12 h after reperfusion and returned to control levels at later time points. The steady-state activation curve also shifted approximately 20 mV in the hyperpolarizing direction, and the time constant of removal of inactivation markedly increased at 12 h after reperfusion. For delayed rectifier potassium current, the amplitude significantly increased and the steady-state activation curve shifted in the hyperpolarizing direction at 36 h after reperfusion. No significant change in inactivation kinetics was observed in the above potassium currents following reperfusion. The present study demonstrates the differential changes of potassium currents in CA1 neurons after reperfusion. The increase of transient potassium current in the early phase of reperfusion may be responsible for the decrease of excitability, while the increase of delayed rectifier potassium current in the late phase of reperfusion may be associated with the postischemic cell death.

Immunohistochemical study on the distribution of the type I and type II voltage-gated sodium channels in the gerbil cerebellum

In this study, we investigated the distribution of the type I and type II Na+ channels in the gerbil cerebellum by immunohistochemistry. Strong uniform staining for type I was observed in the granular layer, whereas there was little evidence of concentrated labeling in the cell bodies and processes of Purkinje cells. The most intense staining for type II was observed in the cell bodies and dendrites of Purkinje cells, with a strong signal in the molecular layer. This localization study has shown clearly that the type I and type II Na+ channel subunits have differential distribution in the gerbil cerebellum, for the first time. The present study may provide useful data for the future investigations to understand the roles of voltage-gated sodium channels in neurological pathways.

Short- and long-term differential effects of neuroprotective drug NS-7 on voltage-dependent sodium channels in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells, NS-7 [4-(4-fluorophenyl)-2-methyl-6-(5-piperidinopentyloxy) pyrimidine hydrochloride], a newly-synthesized neuroprotective drug, inhibited veratridine-induced (22)Na(+) influx via voltage-dependent Na(+) channels (IC(50)=11.4 microM). The inhibition by NS-7 occurred in the presence of ouabain, an inhibitor of Na(+),K(+) ATPase, but disappeared at higher concentration of veratridine, and upon the washout of NS-7. NS-7 attenuated veratridine-induced (45)Ca(2+) influx via voltage-dependent Ca(2+) channels (IC(50)=20.0 microM) and catecholamine secretion (IC(50)=25.8 microM). Chronic (>/=12 h) treatment of cells with NS-7 increased cell surface [(3)H]-STX binding by 86% (EC(50)=10.5 microM; t(1/2)=27 h), but did not alter the K(D) value; it was prevented by cycloheximide, an inhibitor of protein synthesis, or brefeldin A, an inhibitor of vesicular transport from the trans-Golgi network, but was not associated with increased levels of Na(+) channel alpha- and beta(1)-subunit mRNAs. In cells subjected to chronic NS-7 treatment, (22)Na(+) influx caused by veratridine (site 2 toxin), alpha-scorpion venom (site 3 toxin) or beta-scorpion venom (site 4 toxin) was suppressed even after the extensive washout of NS-7, and veratridine-induced (22)Na(+) influx remained depressed even at higher concentration of veratridine; however, either alpha- or beta-scorpion venom, or Ptychodiscus brevis toxin-3 (site 5 toxin) enhanced veratridine-induced (22)Na(+) influx as in nontreated cells. These results suggest that in the acute treatment, NS-7 binds to the site 2 and reversibly inhibits Na(+) channels, thereby reducing Ca(2+) channel gating and catecholamine secretion. Chronic treatment with NS-7 up-regulates cell surface Na(+) channels via translational and externalization events, but persistently inhibits Na(+) channel gating without impairing the cooperative interaction between the functional domains of Na(+) channels.

Increased neuronal excitability after long-term O(2) deprivation is mediated mainly by sodium channels

We have previously observed that prolonged O(2) deprivation alters membrane protein expression and membrane properties in the central nervous system. In this work, we studied the effect of prolonged O(2) deprivation on the electrical activity of rat cortical and hippocampal neurons during postnatal development and its relationship to Na(+) channels. Rats were raised in low O(2) environment (inspired O(2) concentration = 9.5+/-0.5%) for 3-4 weeks, starting at an early age (2-3 days old). Using electrophysiologic recordings in brain slices, RNA analysis (northern and slot blots) and saxitoxin (a specific ligand for Na(+) channels) binding autoradiography, we addressed two questions: (1) does long-term O(2) deprivation alter neuronal excitability in the neocortical and hippocampal neurons during postnatal development? and (2) if so, what are the main mechanisms responsible for the change in excitability in the exposed brain? Our results show that (i) baseline membrane properties of cortical and hippocampal CA1 neurons from rats chronically exposed to hypoxia were not substantially different from those of naive neurons; (ii) acute stress (e.g., hypoxia) elicited a markedly exaggerated response in the exposed neurons as compared to naive ones; (iii) chronic hypoxia tended to increase Na(+) channel mRNA and saxitoxin binding density in the cortex and hippocampus as compared to control ones; and (iv) the enhanced neuronal response to acute hypoxia in the exposed cortical and CA1 neurons was considerably attenuated by applying tetrodotoxin, a voltage-sensitive Na(+) channel blocker, in a dose-dependent manner. We conclude that prolonged O(2) deprivation can lead to major electrophysiological disturbances, especially when exposed neurons are stressed acutely, which renders the chronically exposed neurons more vulnerable to subsequent micro-environmental stress. We suggest that this Na(+) channel-related over-excitability is likely to constitute a molecular mechanism for some neurological sequelae, such as epilepsy, resulting from perinatal hypoxic encephalopathy.