Effect of anoxia on intracellular and extracellular potassium activity in hypoglossal neurons in vitro

Electrically Controlled Vasodilator Delivery from PEDOT/Silica Nanoparticle Modulates Vessel Diameter in Mouse Brain

Vascular damage and reduced tissue perfusion are expected to majorly contribute to the loss of neurons or neural signals around implanted electrodes. However, there are limited methods of controlling the vascular dynamics in tissues surrounding these implants. This work utilizes conducting polymer poly(ethylenedioxythiophene) and sulfonated silica nanoparticle composite (PEDOT/SNP) to load and release a vasodilator, sodium nitroprusside, to controllably dilate the vasculature around carbon fiber electrodes (CFEs) implanted in the mouse cortex. The vasodilator release is triggered via electrical stimulation and the amount of release increases with increasing electrical pulses. The vascular dynamics are monitored in real-time using two-photon microscopy, with changes in vessel diameters quantified before, during, and after the release of the vasodilator into the tissues. This work observes significant increases in vessel diameters when the vasodilator is electrically triggered to release, and differential effects of the drug release on vessels of different sizes. In conclusion, the use of nanoparticle reservoirs in conducting polymer-based drug delivery platforms enables the controlled delivery of vasodilator into the implant environment, effectively altering the local vascular dynamics on demand. With further optimization, this technology could be a powerful tool to improve the neural electrode-tissue interface and study neurovascular coupling.

Biophysical models reveal the relative importance of transporter proteins and impermeant anions in chloride homeostasis

Fast synaptic inhibition in the nervous system depends on the transmembrane flux of Cl^- ions based on the neuronal Cl^- driving force. Established theories regarding the determinants of Cl^- driving force have recently been questioned. Here, we present biophysical models of Cl^- homeostasis using the pump-leak model. Using numerical and novel analytic solutions, we demonstrate that the Na⁺/K⁺-ATPase, ion conductances, impermeant anions, electrodiffusion, water fluxes and cation-chloride cotransporters (CCCs) play roles in setting the Cl^- driving force. Our models, together with experimental validation, show that while impermeant anions can contribute to setting [Cl^-]_i in neurons, they have a negligible effect on the driving force for Cl^- locally and cell-wide. In contrast, we demonstrate that CCCs are well-suited for modulating Cl^- driving force and hence inhibitory signaling in neurons. Our findings reconcile recent experimental findings and provide a framework for understanding the interplay of different chloride regulatory processes in neurons.

Cells called neurons in the brain communicate by triggering or inhibiting electrical activity in other neurons. To inhibit electrical activity, a signal from one neuron usually triggers specific receptors on the second neuron to open, which allows particles called chloride ions to flow into or out of the neuron.

The force that moves chloride ions (the so-called ‘chloride driving force’) depends on two main factors. Firstly, chloride ions, like other particles, tend to move from an area where they are plentiful to areas where they are less abundant. Secondly, chloride ions are negatively charged and are therefore attracted to areas where the net charge (determined by the mix of positively and negatively charged particles) is more positive than their current position.

It was previously believed that a group of proteins known as CCCs, which transport chloride ions and positive ions together across the membranes surrounding cells, sets the chloride driving force. However, it has recently been suggested that negatively charged ions that are unable to cross the membrane (or ‘impermeant anions’ for short) may set the driving force instead by contributing to the net charge across the membrane. Düsterwald et al. used a computational model of the neuron to explore these two possibilities.

In the simulations, altering the activity of the CCCs led to big changes in the chloride driving force. Changing the levels of impermeant anions altered the volume of cells, but did not drive changes in the chloride driving force. This was because the flow of chloride ions across the membrane led to a compensatory change in the net charge across the membrane.

Düsterwald et al. then used an experimental technique called patch-clamping in mice and rats to confirm the model’s predictions. Defects in controlling the chloride driving force in brain cells have been linked with epilepsy, stroke and other neurological diseases. Therefore, a better knowledge of these mechanisms may in future help to identify the best targets for drugs to treat such conditions.

Ion dynamics during seizures

Changes in membrane voltage brought about by ion fluxes through voltage and transmitter-gated channels represent the basis of neural activity. As such, electrochemical gradients across the membrane determine the direction and driving force for the flow of ions and are therefore crucial in setting the properties of synaptic transmission and signal propagation. Ion concentration gradients are established by a variety of mechanisms, including specialized transporter proteins. However, transmembrane gradients can be affected by ionic fluxes through channels during periods of elevated neural activity, which in turn are predicted to influence the properties of on-going synaptic transmission. Such activity-induced changes to ion concentration gradients are a feature of both physiological and pathological neural processes. An epileptic seizure is an example of severely perturbed neural activity, which is accompanied by pronounced changes in intracellular and extracellular ion concentrations. Appreciating the factors that contribute to these ion dynamics is critical if we are to understand how a seizure event evolves and is sustained and terminated by neural tissue. Indeed, this issue is of significant clinical importance as status epilepticus—a type of seizure that does not stop of its own accord—is a life-threatening medical emergency. In this review we explore how the transmembrane concentration gradient of the six major ions (K⁺, Na⁺, Cl⁻, Ca²⁺, H⁺and HCO3−) is altered during an epileptic seizure. We will first examine each ion individually, before describing how multiple interacting mechanisms between ions might contribute to concentration changes and whether these act to prolong or terminate epileptic activity. In doing so, we will consider how the availability of experimental techniques has both advanced and restricted our ability to study these phenomena.

AMP kinase regulates K-ATP currents evoked by NMDA receptor stimulation in rat subthalamic nucleus neurons

Our lab recently showed that N-methyl-D-aspartate (NMDA) evokes ATP-sensitive K(+) (K-ATP) currents in subthalamic nucleus (STN) neurons in slices of the rat brain. Both K-ATP channels and 5'-adenosine monophosphate-activated protein kinase (AMPK) are considered cellular energy sensors because their activities are influenced by the phosphorylation state of adenosine nucleotides. Moreover, AMPK has been shown to regulate K-ATP function in a variety of tissues including pancreas, cardiac myocytes, and hypothalamus. We used whole-cell patch clamp recordings to study the effect of AMPK activation on K-ATP channel function in STN neurons in slices of the rat brain. We found that bath or intracellular application of the AMPK activators A769662 and PT1 augmented tolbutamide-sensitive K-ATP currents evoked by NMDA receptor stimulation. The effect of AMPK activators was blocked by the AMPK inhibitor dorsomorphin (compound C), and by STO609, an inhibitor of the upstream AMPK activator CaMKKβ. AMPK augmentation of NMDA-induced K-ATP current was also blocked by intracellular BAPTA and by inhibitors of nitric oxide synthase and guanylyl cyclase. However, A769662 did not augment currents evoked by the K-ATP channel opener diazoxide. In the presence of NMDA, A769662 inhibited depolarizing plateau potentials and burst firing, both of which could be antagonized by tolbutamide or dorsomorphin. These studies show that AMPK augments NMDA-induced K-ATP currents by a Ca(2+)-dependent process that involves nitric oxide and cGMP. By augmenting K-ATP currents, AMPK activation would be expected to dampen the excitatory effect of glutamate-mediated transmission in the STN.

Voltage-gated potassium channels at the crossroads of neuronal function, ischemic tolerance, and neurodegeneration

Voltage-gated potassium (Kv) channels are widely expressed in the central and peripheral nervous system and are crucial mediators of neuronal excitability. Importantly, these channels also actively participate in cellular and molecular signaling pathways that regulate the life and death of neurons. Injury-mediated increased K(+) efflux through Kv2.1 channels promotes neuronal apoptosis, contributing to widespread neuronal loss in neurodegenerative disorders such as Alzheimer's disease and stroke. In contrast, some forms of neuronal activity can dramatically alter Kv2.1 channel phosphorylation levels and influence their localization. These changes are normally accompanied by modifications in channel voltage dependence, which may be neuroprotective within the context of ischemic injury. Kv1 and Kv7 channel dysfunction leads to neuronal hyperexcitability that critically contributes to the pathophysiology of human clinical disorders such as episodic ataxia and epilepsy. This review summarizes the neurotoxic, neuroprotective, and neuroregulatory roles of Kv channels and highlights the consequences of Kv channel dysfunction on neuronal physiology. The studies described in this review thus underscore the importance of normal Kv channel function in neurons and emphasize the therapeutic potential of targeting Kv channels in the treatment of a wide range of neurological diseases.

Ionic storm in hypoxic/ischemic stress: can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K(+) efflux and Na(+)-, Ca(2+)- and Cl(-)-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly delta-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na(+) influx through the membrane and reduces the increase in intracellular Ca(2+), thus decreasing the excessive leakage of intracellular K(+). Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na(+) channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.

Na+ mechanism of delta-opioid receptor induced protection from anoxic K+ leakage in the cortex

Activation of delta-opioid receptors (DOR) attenuates anoxic K(+) leakage and protects cortical neurons from anoxic insults by inhibiting Na(+) influx. It is unknown, however, which pathway(s) that mediates the Na(+) influx is the target of DOR signal. In the present work, we found that, in the cortex, (1) DOR protection was largely dependent on the inhibition of anoxic Na(+) influxes mediated by voltage-gated Na(+) channels; (2) DOR activation inhibited Na(+) influx mediated by ionotropic glutamate N-methyl-D-aspartate (NMDA) receptors, but not that by non-NMDA receptors, although both played a role in anoxic K(+) derangement; and (3) DOR activation had little effect on Na(+)/Ca(2+) exchanger-based response to anoxia. We conclude that DOR activation attenuates anoxic K(+) derangement by restricting Na(+) influx mediated by Na(+) channels and NMDA receptors, and that non-NMDA receptors and Na(+)/Ca(2+) exchangers, although involved in anoxic K(+) derangement in certain degrees, are less likely the targets of DOR signal.

Activation of DOR attenuates anoxic K+ derangement via inhibition of Na+ entry in mouse cortex

We have recently found that in the mouse cortex, activation of delta-opioid receptor (DOR) attenuates the disruption of K(+) homeostasis induced by hypoxia or oxygen-glucose deprivation. This novel observation suggests that DOR may protect neurons from hypoxic/ischemic insults via the regulation of K(+) homeostasis because the disruption of K(+) homeostasis plays a critical role in neuronal injury under hypoxic/ischemic stress. The present study was performed to explore the ionic mechanism underlying the DOR-induced neuroprotection. Because anoxia causes Na(+) influx and thus stimulates K(+) leakage, we investigated whether DOR protects the cortex from anoxic K(+) derangement by targeting the Na(+)-based K(+) leakage. By using K(+)-sensitive microelectrodes in mouse cortical slices, we showed that 1) lowering Na(+) concentration and substituting with impermeable N-methyl-D-glucamine caused a concentration-dependent attenuation of anoxic K(+) derangement; 2) lowering Na(+) concentration by substituting with permeable Li(+) tended to potentiate the anoxic K(+) derangement; and 3) the DOR-induced protection against the anoxic K(+) responses was largely abolished by low-Na(+) perfusion irrespective of the substituted cation. We conclude that external Na(+) concentration greatly influences anoxic K(+) derangement and that DOR activation likely attenuates anoxic K(+) derangement induced by the Na(+)-activated mechanisms in the cortex.

delta-, but not mu-, opioid receptor stabilizes K(+) homeostasis by reducing Ca(2+) influx in the cortex during acute hypoxia

Past work has shown that delta-opioid receptor (DOR) activation by [D-Ala(2),D-Leu(5)]-enkephalin (DADLE) attenuated the disruption of K(+) homeostasis induced by hypoxia or oxygen-glucose deprivation (OGD) in the cortex, while naltrindole, a DOR antagonist blocked this effect, suggesting that DOR activity stabilizes K(+) homeostasis in the cortex during hypoxic/ischemic stress. However, several important issues remain unclear regarding this new observation, especially the difference between DOR and other opioid receptors in the stabilization of K(+) homeostasis and the underlying mechanism. In this study, we asked whether DOR is different from micro-opioid receptors (MOR) in stabilizing K(+) homeostasis and which membrane channel(s) is critically involved in the DOR effect. The main findings are that (1) similar to DADLE (10 microM), H-Dmt-Tic-NH-CH (CH(2)--COOH)-Bid (1-10 microM), a more specific and potent DOR agonist significantly attenuated anoxic K(+) derangement in cortical slice; (2) [D-Ala(2), N-Me-Phe(4), glycinol(5)]-enkephalin (DAGO; 10 microM), a MOR agonist, did not produce any appreciable change in anoxic disruption of K(+) homeostasis; (3) absence of Ca(2+) greatly attenuated anoxic K(+) derangement; (4) inhibition of Ca(2+)-activated K(+) (BK) channels with paxilline (10 microM) reduced anoxic K(+) derangement; (5) DADLE (10 microM) could not further reduce anoxic K(+) derangement in the Ca(2+)-free perfused slices or in the presence of paxilline; and (6) glybenclamide (20 microM), a K(ATP) channel blocker, decreased anoxia-induced K(+) derangement, but DADLE (10 microM) could further attenuate anoxic K(+) derangement in the glybenclamide-perfused slices. These data suggest that DOR, but not MOR, activation is protective against anoxic K(+) derangement in the cortex, at least partially via an inhibition of hypoxia-induced increase in Ca(2+) entry-BK channel activity.

Cortical delta-opioid receptors potentiate K+ homeostasis during anoxia and oxygen-glucose deprivation

Central neurons are extremely vulnerable to hypoxic/ischemic insult, which is a major cause of neurologic morbidity and mortality as a consequence of neuronal dysfunction and death. Our recent work has shown that delta-opioid receptor (DOR) is neuroprotective against hypoxic and excitotoxic stress, although the underlying mechanisms remain unclear. Because hypoxia/ischemia disrupts ionic homeostasis with an increase in extracellular K(+), which plays a role in neuronal death, we asked whether DOR activation preserves K(+) homeostasis during hypoxic/ischemic stress. To test this hypothesis, extracellular recordings with K(+)-sensitive microelectrodes were performed in mouse cortical slices under anoxia or oxygen-glucose deprivation (OGD). The main findings in this study are that (1) DOR activation with [D-Ala(2), D-Leu(5)]-enkephalinamide attenuated the anoxia- and OGD-induced increase in extracellular K(+) and decrease in DC potential in cortical slices; (2) DOR inhibition with naltrindole, a DOR antagonist, completely abolished the DOR-mediated prevention of increase in extracellular K(+) and decrease in DC potential; (3) inhibition of protein kinase A (PKA) with N-(2-[p-bromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide dihydrochloride had no effect on the DOR protection; and (4) inhibition of protein kinase C (PKC) with chelerythrine chloride reduced the DOR protection, whereas the PKC activator (phorbol 12-myristate 13-acetate) mimicked the effect of DOR activation on K(+) homeostasis. These data suggest that activation of DOR protects the cortex against anoxia- or ODG-induced derangement of potassium homeostasis, and this protection occurs via a PKC-dependent and PKA-independent pathway. We conclude that an important aspect of DOR-mediated neuroprotection is its early action against derangement of K(+) homeostasis during anoxia or ischemia.

Alterations of potassium currents in ischemia-vulnerable and ischemia-resistant neurons in the hippocampus after ischemia

CA1 pyramidal neurons in the hippocampus die 2-3 days following transient forebrain ischemia, whereas CA3 pyramidal neurons and granule cells in the dentate gyrus remain viable. Excitotoxicity is the major cause of ischemic cell death, and potassium currents play important roles in regulating the neuronal excitability. The present study compared the changes of potassium currents in acutely dissociated hippocampal neurons at different intervals after ischemia. In CA1 neurons, the amplitude of rapid inactivating potassium currents (I(A)) was significantly increased at 14 h and returned to control levels at 38 h after ischemia; the rising slope and decay time constant of I(A) were accordingly increased after ischemia. The activation curve of I(A) in CA1 neurons shifted to the depolarizing direction at 38 h after ischemia. In granule cells, the amplitude and rising slope of I(A) were significantly increased at 38 h after ischemia; the inactivation curves of I(A) shifted toward the depolarizing direction accordingly at 38 h after ischemia. The I(A) remained unchanged in CA3 neurons after ischemia. The amplitudes of delayed rectifier potassium currents (I(Kd)) in CA1 neurons were progressively increased after ischemia. No significant difference in I(Kd) was detected in CA3 and granule cells at any time points after reperfusion. These results indicated that the voltage dependent potassium currents in hippocampal neurons were differentially altered after cerebral ischemia. The up-regulation of I(A) in dentate granule cells might have protective effects. The increase of I(Kd) in CA1 neurons might be associated with the neuronal damage after ischemia.

Activation of voltage-sensitive sodium channels during oxygen deprivation leads to apoptotic neuronal death

Sodium (Na(+)) entry into neurons during hypoxia is known to be associated with cell death. However, it is not clear whether Na(+) entry causes cell death and by what mechanisms this increased Na(+) entry induces death. In this study we used cultures of rat neocortical neurons to show that an increase in intracellular sodium (Na(i)(+)) through voltage-sensitive sodium channels (VSSCs), during hypoxia contributes to apoptosis. Hypoxia increased Na(i)(+) and induced neuronal apoptosis, as assessed by electron microscopy, annexin V staining, and terminal UDP nick end labeling staining. Reducing Na(+) entry with the VSSC blocker, tetrodotoxin (TTX), attenuated apoptotic neuronal death via a reduction in caspase-3 activation. Since the attenuation of apoptosis by TTX during hypoxia suggested that the activation of VSSCs and Na(+) entry are crucial events in hypoxia-induced cell death, we also determined whether the activation of VSSCs per se could lead to apoptosis under resting conditions. Increasing Na(+) entry with the VSSC activator veratridine also induced neuronal apoptosis and caspase-3 activation. These data indicate that a) Na(+) entry via VSSCs during hypoxia leads to apoptotic cell death which is mediated, in part, by caspase-3 and b) activation of VSSCs during oxygen deprivation is a major event by which hypoxia induces cell death.

Impact of mitochondrial inhibition on excitability and cytosolic Ca2+ levels in brainstem motoneurones from mouse

Motoneurones (MNs) are particularly affected by the inhibition of mitochondrial metabolism, which has been linked to their selective vulnerability during pathophysiological states like hypoxia and amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disorder. To elucidate underlying events, we used sodium cyanide (CN) as a pharmacological inhibitor of complex IV of the mitochondrial respiratory chain ('chemical hypoxia') and investigated the cellular response in vulnerable and resistant neurone types. Bath application of 2 mm CN activated TTX-insensitive Na+ conductances in vulnerable hypoglossal MNs, which depolarized these MNs by 10.2 +/- 1.1 mV and increased their action potential activity. This response was mimicked by sodium azide (2 mm) and largely prevented by preincubation with the antioxidants ascorbic acid (1 mm) and Trolox (750 microm), indicating an involvement of reactive oxygen species (ROS) in the activation mechanism. CN also elevated cytosolic [Ca2+] levels through (i) Ca2+ release from mitochondria-controlled stores, (ii) significant retardation of cytosolic Ca2+ clearance rates, even when cytosolic ATP levels were held constant during whole-cell recording, and (iii) secondary Ca2+ influx during elevated firing rates. Blocking mitochondrial ATP production additionally raised cytosolic Ca2+ levels and prolonged recovery of Ca2+ transients with a delay of 5-6 min. Comparative studies on hypoglossal MNs, facial MNs and dorsal vagal neurones suggested that CN responses were dominated by the activation of K+ conductances in resistant neurones, thus reducing excitability during mitochondrial inhibition. In summary, our observations therefore support a model where selective MN vulnerability results from a synergistic accumulation of risk factors, including low cytosolic Ca2+ buffering, strong mitochondrial impact on [Ca2+]i, and a mitochondria-controlled increase in electrical excitability during metabolic disturbances.

Membrane potential of CA3 hippocampal pyramidal cells during postnatal development

A depolarized resting membrane potential has long been considered to be a universal feature of immature neurons. Despite the physiological importance, the underlying mechanisms of this developmental phenomenon are poorly understood. Using perforated-patch, whole cell, and cell-attached recordings, we measured the membrane potential in CA3 pyramidal cells in hippocampal slices from postnatal rats. With gramicidin perforated-patch recordings, membrane potential was -44 +/- 4 (SE) mV at postnatal days P0-P2, and it progressively shifted to -67 +/- 2 mV at P13-15. A similar developmental change of the membrane potential has been also observed with conventional whole cell recordings. However, the value of the membrane potential deduced from the reversal potential of N-methyl-d-aspartate channels in cell-attached recordings did not change with age and was -77 +/- 2 mV at P2 and -77 +/- 2 mV at P13-14. The membrane potential measured using whole cell recordings correlated with seal and input resistance, being most depolarized in neurons with high, several gigaohms, input resistance and low seal resistance. Simulations revealed that depolarized values of the membrane potential in whole cell and perforated-patch recordings could be explained by a shunt through the seal contact between the pipette and membrane. Thus the membrane potential of CA3 pyramidal cells appears to be strongly negative at birth and does not change during postnatal development.

Calmodulin and calmodulin-dependent kinase II mediate neuronal cell death induced by depolarization

Depolarization has been known to play an important role in the neuronal damage that occurs following cerebral ischemia. In the present study, we investigated the roles of calmodulin (CaM) and CaM-dependent enzymes in depolarization-induced neuronal cell death. Treatment of primary cortical neurons with 10 microM veratridine, a voltage sensitive Na(+) channel activator, induced cell death as indicated by lactate dehydrogenase leakage from neurons. CaM antagonists (calmidazolium, trifluoperazine, W-7, and W-5) inhibited cell death induced by veratridine in a concentration-dependent manner. CaM kinase II (CaMKII) inhibitors (KN-62, KN-93, and myristoylated autocamtide-2 related inhibitory peptide), but not inhibitors of nitric oxide synthase or calcineurin, prevented veratridine-induced neuronal cell death. Veratridine rapidly activated CaMKII in neurons, and CaM antagonists and a CaMKII inhibitor suppressed the CaMKII activation. These results suggest that the CaM-CaMKII pathway contributes to depolarization-evoked cell death in neurons.

Slow ATP loss and the defense of ion homeostasis in the anoxic frog brain

For most vertebrates, cutting off the oxygen supply to the brain results in a rapid (within minutes) loss of ATP, the failure of ATP-dependent ion-transport process, subsequent anoxic depolarization of neuronal membrane potential and consequential neuronal death. The few species that survive brain anoxia for days or months, such as the freshwater turtle Trachemys scripta, avoid anoxic depolarization and maintain brain ATP levels through a coordinated downregulation of brain energy demand processes. The frog Rana pipiens represents an intermediate in anoxia-tolerance, being able to survive brain anoxia for hours. However, the anoxic frog brain does not defend its energy stores. Instead, anoxia-tolerance appears to be related to a retarded rate of ATP depletion. To investigate the relationship between this slow ATP depletion and the loss of ionic homeostasis, cerebral extracellular K+ concentrations were monitored and ATP levels measured during anoxia, during the initial phase of anoxic depolarization and during complete anoxic depolarization. Extracellular K+ levels were maintained at normoxic levels for at least 3 h of anoxia, while ATP content decreased by 35 %. When ATP levels reached 0.33±0.06 mmol l–1 (mean ± s.e.m., N=5), extracellular K+ levels slowly started to increase. This value is thought to represent a critical ATP concentration for the maintenance of ion homeostasis. When extracellular [K+] reached an inflection value of 4.77±0.84 mmol l–1 (mean ± s.e.m., N=5), approximately 1 h later, the brain quickly depolarized. Part of the reduction in ATP demand was attributable to an approximately 50 % decrease in the rate of K+ efflux from the anoxic frog brain, which would also contribute to the retarded rate of increase in extracellular [K+] during the initial phase of anoxic depolarization. However, unlike the anoxia-tolerant turtle brain, adenosine did not appear to be involved in the downregulation of K+ leakage in the frog brain. The increased anoxia-tolerance of the frog brain is thought to be a matter more of slow death than of enhanced protective mechanisms.

Effects of ion channel blockade on the distribution of Na, K, Ca and other elements in oxygen-glucose deprived CA1 hippocampal neurons

The pathophysiology of brain ischemia and reperfusion injury involves perturbation of intraneuronal ion homeostasis. To identify relevant routes of ion flux, rat hippocampal slices were perfused with selective voltage- or ligand-gated ion channel blockers during experimental oxygen-glucose deprivation and subsequent reperfusion. Electron probe X-ray microanalysis was used to quantitate water content and concentrations of Na, K, Ca and other elements in morphological compartments (cytoplasm, mitochondria and nuclei) of individual CA1 pyramidal cell bodies. Blockade of voltage-gated channel-mediated Na+ entry with tetrodotoxin (1 microM) or lidocaine (200 microM) significantly reduced excess intraneuronal Na and Ca accumulation in all compartments and decreased respective K loss. Voltage-gated Ca2+ channel blockade with the L-type antagonist nitrendipine (10 microM) decreased Ca entry and modestly preserved CA1 cell elemental composition and water content. However, a lower concentration of nitrendipine (1 microM) and the N-, P-subtype Ca2+ channel blocker omega-conotoxin MVIIC (3 microM) were ineffective. Glutamate receptor blockade with the N-methyl-D-aspartate (NMDA) receptor-subtype antagonist 3-(2-carboxypiperazin-4-yl) propyl-1-phosphonic acid (CPP; 100 microM) or the alpha-amino-3-hydroxy-5-methyl-4-isoazole propionic acid (AMPA) receptor subtype blocker 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM/100 microM glycine) completely prevented Na and Ca accumulation and partially preserved intraneuronal K concentrations. Finally, the increase in neuronal water content normally associated with oxygen-glucose deprivation/reperfusion was prevented by Na+ channel or glutamate receptor blockade. Results of the present study demonstrate that antagonism of either postsynaptic NMDA or AMPA glutaminergic receptor subtypes provided nearly complete protection against ion and water deregulation in nerve cells subjected to experimental ischemia followed by reperfusion. This suggests activation of ionophoric glutaminergic receptors is involved in loss of neuronal osmoregulation and ion homeostasis. Na+ channel blockade also effectively diminished neuronal ion and water derangement during oxygen-glucose deprivation and reperfusion. Prevention of elevated Nai+ levels is likely to provide neuroprotection by decreasing presynaptic glutamate release and by improving cellular osmoregulation, adenosine triphosphate utilization and Ca2+ clearance. Thus, we suggest that voltage-gated tetrodotoxin-sensitive Na+ channels and glutamate-gated ionotropic NMDA or AMPA receptors are important routes of ion flux during nerve cell injury induced by oxygen-glucose deprivation/reperfusion.

Blockade of U50488H on potassium currents of acutely isolated mouse hippocampal CA3 pyramidal neurons

The actions of the opioid agonist U50488H on IA and IK were examined in acutely isolated mouse hippocampal CA3 pyramidal neurons using the whole-cell patch clamp technique. U50488H caused a concentration dependent, rapidly developing and reversible inhibition of voltage-activated IA and IK. The inhibitory actions were still observed in the presence of 30 microM naloxone or 5 microM nor-binaltorphimine dihydrochloride. The IC50 values for the blockade of IA and IK were calculated as 20.1.9 and 3.7 microM, respectively. In the presence of 3.3 microM U50488H, repetitive stimulation induced use-dependent inhibition of IA and IK. A 10 microM concentration of U50488H positively shifted the half-activation membrane potential of IA by +11 mV, but negatively shifted IK by -14 mV. These results demonstrate that U50488H can directly inhibit neuronal IA and IK without involvement of the activation of kappa-opioid receptors.

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.

Differential fall in ATP accounts for effects of temperature on hypoxic damage in rat hippocampal slices

Intracellular recordings, ATP and cytosolic calcium measurements from CA1 pyramidal cells in rat hippocampal slices were used to examine the mechanisms by which temperature alters hypoxic damage. Hypothermia (34 degrees C) preserved ATP (1.7 vs. 0.8 nM/mg) and improved electrophysiologic recovery of the CA1 neurons after hypoxia; 58% of the neurons subjected to 10 min of hypoxia (34 degrees C) recovered their resting and action potentials, while none of the neurons at 37 degrees C recovered. Increasing the glucose concentration from 4 to 6 mM during normothermic hypoxia improved ATP (1.3 vs. 0.8 nM/mg) and mimicked the effects of hypothermia; 67% of the neurons recovered their resting and action potentials. Hypothermia attenuated the membrane potential changes and the increase in intracellular Ca(2+) (212 vs. 384 nM) induced by hypoxia. Changing the glucose concentration in the artificial cerebrospinal fluid primarily affects ATP levels during hypoxia. Decreasing the glucose concentration from 4 to 2 mM during hypothermic hypoxia worsened ATP, cytosolic Ca(2+), and electrophysiologic recovery. Ten percent of the neurons subjected to 4 min of hypoxia at 40 degrees C recovered their resting and action potentials; this compared with 60% of the neurons subjected to 4 min of normothermic hypoxia. None of the neurons subjected to 10 min of hypoxia at 40 degrees C recovered their resting and action potentials. Hyperthermia (40 degrees C) worsens the electrophysiologic changes and induced a greater increase in intracellular Ca(2+) (538 vs. 384 nM) during hypoxia. Increasing the glucose concentration from 4 to 8 mM during 10 min of hyperthermic hypoxia improved ATP (1.4 vs. 0.6 nM/mg), Ca(2+) (267 vs. 538 nM), and electrophysiologic recovery (90 vs. 0%). Our results indicate that the changes in electrophysiologic recovery with temperature are primarily due to changes in ATP and that the changes in depolarization and Ca(2+) are secondary to these ATP changes. Both primary and secondary changes are important for explaining the improved electrophysiologic recovery with hypothermia.

Comparison of changes evoked by GABA (γ-aminobutyric acid) and anoxia in [K+]o, [Cl-]o, and [Na+]o in stratum pyramidale and stratum radiatum of the guinea pig hippocampus

Ion-selective microelectrode recordings were made to assess a possible contribution of extracellular γ-aminobutyric acid (GABA) accumulation to early responses evoked in the brain by anoxia and ischemia. Changes evoked by GABA or N2 in [K+]o, [Cl-]o, [Na+]o, and [TMA+]o were recorded in the cell body and dendritic regions of the stratum pyramidale (SP) and stratum radiatum (SR), respectively, of pyramidal neurons in CA1 of guinea pig hippocampal slices. Bath application of GABA (1-10 mM) for approximately 5 min evoked changes in [K+]o and [Cl-]o with respective EC50 levels of 3.8 and 4.1 mM in SP, and 4.7 and 5.6 mM in SR. In SP 5 mM GABA reversibly increased [K+]o and [Cl-]o and decreased [Na+]o; replacement of 95% O2 -5% CO2 by 95% N2 -5% CO2 for a similar period of time evoked changes which were for each ion in the same direction as those with GABA. In SR both GABA and N2 caused increases in [K+]o and decreases in [Cl-]o and [Na+]o. The reduction of extracellular space, estimated from levels of [TMA+]o during exposures to GABA and N2, was 5-6% and insufficient to cause the observed changes in ion concentration. Ion changes induced by GABA and N2 were reversibly attenuated by the GABAA receptor antagonist bicuculline methiodide (BMI, 100 µM). GABA-evoked changes in [K+]o in SP and SR and [Cl-]o in SP were depressed by >=90%, and of [Cl-]o in SR by 50%; N2-evoked changes in [K+]o in SP and SR were decreased by 70% and those of [Cl-]o by 50%. BMI blocked Δ [Na+]o with both GABA and N2 by 20-30%. It is concluded that during early anoxia: (i) accumulation of GABA and activation of GABAA receptors may contribute to the ion changes and play a significant role, and (ii) responses in the dendritic (SR) regions are greater than and (or) differ from those in the somal (SP) layers. A large component of the [K+]o increase may involve a GABA-evoked Ca2+-activated gk, secondary to [Ca2+]i increase. A major part of [Cl-]o changes may arise from GABA-induced gCl and glial efflux, with strong stimulation of active outward transport and anion exchange at SP, and inward Na+/K+/2Cl- co-transport at SR. Na+ influx is attributable mainly to Na+-dependent transmitter uptake, with only a small amount related to GABAA receptor activation. Although the release and (or) accumulation of GABA during anoxia might be viewed as potentially protectant, the ultimate role may more likely be an important contribution to toxicity and delayed neuronal death.

Key words: brain slices, ion-selective microelectrodes, stratum pyramidale, stratum radiatum, bicuculline methiodide, extracellular space shrinkage.

Enhancing our understanding of the molecular responses to hypoxia in mammals using Drosophila melanogaster

Drosophila melanogaster has been used as a genetic model, especially in the past decade, to examine normative biological processes and disease conditions very effectively. These span a wide range of major issues such as aging, cancer, embryogenesis, neural development, apoptosis, and alcohol intoxication. Here, we detail how the Drosophila melanogaster can be used as a genetic model to study the molecular and genetic underpinnings of the response to hypoxia. In our study of the basis of anoxia tolerance, one of the potent approaches that we use is a mutagenesis screen to identify loss-of-function mutants that are anoxia sensitive. The major advantage of this approach is that it is not biased for any particular gene or gene product. Although our screen is in progress, we already have evidence that this approach is useful.

Neuronal mechanisms mediating the integration of respiratory responses to hypoxia

The activation of peripheral chemoreceptors by hypoxia or electrical stimulation of the carotid sinus nerve elicited a hypoxic respiratory response consisting of both stimulatory and subsequent or simultaneous inhibitory components (hypoxic respiratory stimulation and depression). Both components have different time domains of responses (time-dependent response), providing an integrated respiratory response to hypoxia. This review has focused on the neuroanatomical and neurophysiological correlations responsible for these responses and their neuropharmacological mechanisms. Hypoxic respiratory depression is characterized by the initial activation of respiration followed by a progressive and gradual decline in ventilation during prolonged and/or severe hypoxic exposure (biphasic response). The responsible mechanisms for the depression are located within the central nervous system and may be dependent upon activity from peripheral chemoreceptor. Two underlying mechanisms contributing to the depression have been advocated. (1) Change in synaptic transmission: Within the neuronal network controlling the hypoxic respiratory response, hypoxia might induce the enhancement of inhibitory neurotransmission (modulation), disfacilitation of excitatory neruotransmission or both. (2) Change in the membrane property of respiratory neurons: Hypoxia might suppress the membrane excitability of respiratory neurons composing the hypoxic respiratory response via modulating ion channels, leading to hyperpolarization or depolarization blocking of the neurons. However, the quantitative aspects of Pao(2) (degree and duration of hypoxic exposure) to induce these changes and the susceptibility of both mechanisms to the Pao(2) level have not yet been clearly elucidated.

Differential responses of respiratory nuclei to anoxia in rhythmic brain stem slices of mice

The response of the neonatal respiratory system to hypoxia is characterized by an initial increase in ventilation, which is followed within a few minutes by a depression of ventilation below baseline levels. We used the transverse medullary slice of newborn mice as a model system for central respiratory control to investigate the effects of short-lasting periods of anoxia. Extracellular population activity was simultaneously recorded from the ventral respiratory group (VRG) and the hypoglossus (XII) nucleus (a respiration-related motor output nucleus). During anoxia, respiratory frequency was modulated in a biphasic manner and phase-locked in both the VRG and the XII. The amplitude of phasic respiratory bursts was increased only in the XII and not in the VRG. This increase in XII burst amplitude commenced approximately 1 min after the anoxic onset concomitant with a transient increase in tonic activity. The burst amplitude remained elevated throughout the entire 5 min of anoxia. Inspiratory burst amplitude in the VRG, in contrary, remained constant or even decreased during anoxia. These findings represent the first simultaneous extracellular cell population recordings of two respiratory nuclei. They provide important data indicating that rhythm generation is altered in the VRG without a concomitant alteration in the VRG burst amplitude, whereas the burst amplitude is modulated only in the XII nucleus. This has important implications because it suggests that rhythm generation and motor pattern generation are regulated separately within the respiratory network.

O(2) deprivation inhibits Ca(2+)-activated K(+) channels via cytosolic factors in mice neocortical neurons

O(2) deprivation induces membrane depolarization in mammalian central neurons. It is possible that this anoxia-induced depolarization is partly mediated by an inhibition of K(+) channels. We therefore performed experiments using patch-clamp techniques and dissociated neurons from mice neocortex. Three types of K(+) channels were observed in both cell-attached and inside-out configurations, but only one of them was sensitive to lack of O(2). This O(2)-sensitive K(+) channel was identified as a large-conductance Ca(2+)-activated K(+) channel (BK(Ca)), as it exhibited a large conductance of 210 pS under symmetrical K(+) (140 mM) conditions, a strong voltage-dependence of activation, and a marked sensitivity to Ca(2+). A low-O(2) medium (PO(2) = 10-20 mmHg) markedly inhibited this BK(Ca) channel open probability in a voltage-dependent manner in cell-attached patches, but not in inside-out patches, indicating that the effect of O(2) deprivation on BK(Ca) channels of mice neocortical neurons was mediated via cytosol-dependent processes. Lowering intracellular pH (pH(i)), or cytosolic addition of the catalytic subunit of a cAMP-dependent protein kinase A in the presence of Mg-ATP, caused a decrease in BK(Ca) channel activity by reducing the sensitivity of this channel to Ca(2+). In contrast, the reducing agents glutathione and DTT increased single BK(Ca) channel open probability without affecting unitary conductance. We suggest that in neocortical neurons, (a) BK(Ca) is modulated by O(2) deprivation via cytosolic factors and cytosol-dependent processes, and (b) the reduction in channel activity during hypoxia is likely due to reduced Ca(2+) sensitivity resulting from cytosolic alternations such as in pH(i) and phosphorylation. Because of their large conductance and prevalence in the neocortex, BK(Ca) channels may be considered as a target for pharmacological intervention in conditions of acute anoxia or ischemia.

Rapid and slow swelling during hypoxia in the CA1 region of rat hippocampal slices

The role of swelling in hypoxic/ischemic neuronal injury is incompletely understood. We investigated the extent and time course of cell swelling during hypoxia, and recovery of cell volume during reoxygenation, in the CA1 region of rat hippocampal slices in vitro. Cell swelling was measured optically and compared with simultaneous measurements of the extracellular DC potential, extracellular [K+], and synaptic transmission in the presence and absence of hypoxic depolarization. Hypoxia-induced swelling consisted of rapid and/or slow components. Rapid swelling was observed frequently and always occurred simultaneously with hypoxic depolarization. Additionally, rapid swelling was followed by a prolonged phase of swelling that was approximately 15 times slower. Less frequently, slow swelling occurred independently, without either hypoxic depolarization or a preceding rapid swelling. For slices initially swelling rapidly, recovery of both cell volume and the slope of field excitatory postsynaptic potentials were best correlated with the duration of hypoxia (r = 0.77 and 0.87, respectively). This was also the case for slices initially swelling slowly (r = 0.70 and 0.58, respectively). In contrast, the degree of recovery of cell volume was the same at 30 or 60 min of reoxygenation, indicating that prolonging the duration of reoxygenation within these limits was ineffective in improving recovery. Spectral measurements indicated that the hypoxia-induced changes in light transmittance were related to changes in cell volume and not changes in the oxidation state of mitochondrial cytochromes. The persistent impairment of synaptic transmission in slices swelling slowly (i.e., without hypoxic depolarization) indicates that swelling may play a role in this injury and that hypoxic depolarization is not required. Additionally, the correlation between the degree of recovery of cell volume and the degree of recovery of synaptic transmission during reoxygenation supports a role for swelling in hypoxic neuronal injury.

Cell-attached measurements of the firing threshold of rat hippocampal neurones

1. The cell-attached configuration of the patch-clamp technique was used to assess resting membrane potential and firing threshold of CA1 pyramidal cells and interneurones of rat hippocampal slices. 2. Resting potential was inferred from the reversal potential of voltage-gated K+ currents with symmetrical intracellular and pipette K+ concentrations. Its mean value was -74 +/- 9 mV for silent interneurones (mean +/- s.d.; n = 17) and -84 +/- 7 mV for silent pyramidal cells (n = 8). Spontaneous action currents occurred in thirteen out of thirty-two interneurones and two out of ten pyramidal cells. In active cells, membrane potential values fluctuated by up to 20 mV, due in part to the large hyperpolarizations that followed an action current. 3. Membrane potential values determined from K+ current reversal were 13 +/- 6 mV more hyperpolarized than those measured in whole-cell recordings from the same neurones (n = 8), probably due to a Donnan equilibrium potential between pipette and cytoplasm. 4. Firing threshold of silent cells was determined by elevating external K+ until action currents were generated, while membrane potential was monitored from the cell-attached K+ current reversal. Spike threshold was attained at -49 +/- 8 mV for interneurones (n = 17) and at -60 +/- 8 mV for pyramidal cells (n = 8). Increasing external Ca2+ from 2 to 4 mM shifted the neuronal voltage threshold by +5 mV, without affecting resting potential. 5. For comparison with these values, we examined how the rate of membrane polarization influenced firing threshold in whole-cell records. Ramp current injections, of duration 15-1500 ms, revealed that current threshold followed a classical strength-duration relationship. In contrast voltage threshold, determined from current injection or by elevating extracellular K+, varied little with the rate of membrane polarization. 6. The state of activation and inactivation of Na+ and K+ currents might contribute to the stability of the voltage threshold. Cell-attached records showed that 79 +/- 10 % of Na+ channels and 64 +/- 10 % of K+ channels were available for activation at resting potential in silent cells (n = 8). As cells were depolarized to threshold, Na+ current availability was reduced to 23 +/- 10 %, and K+ current availability to 31 +/- 12 %. 7. The speed of transition into the inactivated states also appears to contribute to the invariance of threshold for all but the fastest depolarizations. At potentials close to threshold, the rate of inactivation of Na+ and K+ followed a double exponential time course, such that Na+ currents were 62 % inactivated and K+ currents were 63 % inactivated within 15 ms.

Hypoxia and neuronal function under in vitro conditions

Neurons in the mammalian CNS are highly sensitive to the availability of oxygen. Hypoxia can alter neuronal function and can lead to neuronal injury or death. The underlying changes in the membrane properties of single neurons have been studied in vitro in slice preparations obtained from various brain areas. Hypoxic changes of membrane potential and input resistance correspond to a decrease in ATP concentration and an increase in internal Ca2+ concentration. Functional modifications consisting of substantial membrane depolarization and failure of synaptic transmission can be observed within a few minutes following onset of hypoxia. The hypoxic depolarization accompanied by a hyperexcitability is a trigger signal for induction of neuronal cell death and is mediated mainly by activation of glutamate receptors. The mechanisms of the hypoxic hyperpolarization are more complex. Two types of potassium channels contribute to the hyperpolarization, the Ca(2+)- and the ATP-activated potassium channel. A number of neurotransmitters and neuromodulators is involved in the preservation of normal cell function during hypoxia. Therefore, hypoxia-induced cellular changes are unlikely to have a single, discrete pathway. The complexity of cellular changes implies that several strategies may be useful for neuroprotection and a successful intervention may be dependent upon drug action at more than one target site.

Oxygen/glucose deprivation in hippocampal slices: altered intraneuronal elemental composition predicts structural and functional damage

Effects of oxygen/glucose deprivation (OGD) on subcellular elemental composition and water content were determined in nerve cell bodies from CA1 areas of rat hippocampal slices. Electron probe x-ray microanalysis was used to measure percentage water and concentrations of Na, P, K, Cl, Mg, and Ca in cytoplasm, nucleus, and mitochondria of cells exposed to normal and oxygen/glucose deficient medium. As an early (2 min) consequence of OGD, evoked synaptic potentials were lost, and K, Cl, P, and Mg concentrations decreased significantly in all morphological compartments. As exposure to in vitro OGD continued, a negative DC shift in interstitial voltage occurred ( approximately 5 min), whereas general elemental disruption worsened in cytoplasm and nucleus (5-42 min). Similar elemental changes were noted in mitochondria, except that Ca levels increased during the first 5 min of OGD and then decreased over the remaining experimental period (12-42 min). Compartmental water content decreased early (2 min), returned to control after 12 min of OGD, and then exceeded control levels at 42 min. After OGD (12 min), perfusion of hippocampal slices with control oxygenated solutions (reoxygenation) for 30 min did not restore synaptic function or improve disrupted elemental composition. Notably, reoxygenated CA1 cell compartments exhibited significantly elevated Ca levels relative to those associated with 42 min of OGD. When slices were incubated at 31 degreesC (hypothermia) during OGD/reoxygenation, neuronal dysfunction and elemental deregulation were minimal. Results show that in vitro OGD causes loss of transmembrane Na, K, and Ca gradients in CA1 neurons of hippocampal slices and that hypothermia can obtund this damaging process and preserve neuronal function.

Oxygen and ion concentrations in normoxic and hypoxic brain cells

The goal of the present contribution is to discuss the relationships among brain oxygen tension, energy (ATP) level, and ion gradients and movements. The function of the CNS, the generation and transmission of impulses, is determined to a large extent by the movements of ions. Hence elucidation of these relationships is necessary to the understanding of how brain works. Moreover, such knowledge is indispensable for the design of rational therapies for treatment of a large group of pathological states caused by lack of oxygen. This paper is partly a review and partly an original contribution although the former involves to a considerable extent, results obtained in our laboratories. It is divided into 3 parts: a) a very brief general introduction which reminds the reader some well-known facts; b) presentation and discussion of data; and c) conclusions and/or predictions.

Neurotransmitters and neuromodulators controlling the hypoxic respiratory response in anaesthetized cats

1. The contributions of neurotransmitters and neuromodulators to the responses of the respiratory network to acute hypoxia were analysed in anaesthetized cats. 2. Samples of extracellular fluid were collected at 1-1.5 min time intervals by microdialysis in the medullary region of ventral respiratory group neurones and analysed for their content of glutamate, gamma-aminobutyric acid (GABA), serotonin and adenosine by high performance liquid chromatography. Phrenic nerve activity was correlated with these measurements. 3. Levels of glutamate and GABA increased transiently during early periods of hypoxia, coinciding with augmented phrenic nerve activity and then fell below control during central apnoea. Serotonin and adenosine increased slowly and steadily with onset of hypoxic depression of phrenic nerve activity. 4. The possibility that serotonin contributes to hypoxic respiratory depression was tested by microinjecting the 5-HT-1A receptor agonist 8-OH-DPAT into the medullary region that is important for rhythmogenesis. Hypoxic activation of respiratory neurones and phrenic nerve activity were suppressed. Microinjections of NAN-190, a 5-HT-1A receptor blocker, enhanced hypoxic augmentation resulting in apneustic prolongation of inspiratory bursts. 5. The results reveal a temporal sequence in the release of neurotransmitters and neuromodulators and suggest a specific role for each of them in the sequential development of hypoxic respiratory disturbances.

Ionic mechanisms underlying depolarizing responses of an identified insect motor neuron to short periods of hypoxia

Hypoxia can dramatically disrupt neural processing because energy-dependent homeostatic mechanisms are necessary to support normal neuronal function. In a human context, the long-term effects of such disruption may become all too apparent after a "stroke," in which blood-flow to part of the brain is compromised. We used an insect preparation to investigate the effects of hypoxia on neuron membrane properties. The preparation is particularly suitable for such studies because insects respond rapidly to hypoxia, but can recover when they are restored to normoxic conditions, whereas many of their neurons are large, identifiable, and robust. Experiments were performed on the "fast" coxal depressor motoneuron (Df) of cockroach (Periplaneta americana). Five-minute periods of hypoxia caused reversible multiphasic depolarizations (10-25 mV; n = 88), consisting of an initial transient depolarization followed by a partial repolarization and then a slower phase of further depolarization. During the initial depolarizing phase, spontaneous plateau potentials normally occurred, and inhibitory postsynaptic potential frequency increased considerably; 2-3 min after the onset of hypoxia all electrical activity ceased and membrane resistance was depressed. On reoxygenation, the membrane potential began to repolarize almost immediately, becoming briefly more negative than the normal resting potential. All phases of the hypoxia response declined with repeated periods of hypoxia. Blockade of ATP-dependent Na/K pump by 30 microM ouabain suppressed only the initial transient depolarization and the reoxygenation-induced hyperpolarization. Reduction of aerobic metabolism between hypoxic periods (produced by bubbling air through the chamber instead of oxygen) had a similar effect to that of ouabain. Although the depolarization seen during hypoxia was not reduced by tetrodotoxin (TTX; 2 microM), lowering extracellular Na+ concentration or addition of 500 microM Cd2+ greatly reduced all phases of the hypoxia-induced response, suggesting that Na influx occurs through a TTX-insensitive Cd2+-sensitive channel. Exposure to 20 mM tetraethylammonium and 1 mM 3,4-diaminopyridine increased the amplitude of the hypoxia-induced depolarization, suggesting that activation of K channels may normally limit the amplitude of the hypoxia response. In conclusion we suggest that the slow hypoxia-induced depolarization on motoneuron Df is mainly carried by a TTX-resistant, Cd2+-sensitive sodium influx. Ca2+ entry may also make a direct or indirect contribution to the hypoxia response. The fast transient depolarization appears to result from block of the Na/K pump, whereas the reoxygenation-induced hyperpolarization is largely caused by its subsequent reactivation.

Sodium influx plays a major role in the membrane depolarization induced by oxygen and glucose deprivation in rat striatal spiny neurons

BACKGROUND AND PURPOSE

Striatal spiny neurons are selectively vulnerable to ischemia, but the ionic mechanisms underlying this selective vulnerability are unclear. Although a possible involvement of sodium and calcium ions has been postulated in the ischemia-induced damage of rat striatal neurons, the ischemia-induced ionic changes have never been analyzed in this neuronal subtype.

METHODS

We studied the effects of in vitro ischemia (oxygen and glucose deprivation) at the cellular level using intracellular recordings and microfluorometric measurements in a slice preparation. We also used various channel blockers and pharmacological compounds to characterize the ischemia-induced ionic conductances.

RESULTS

Spiny neurons responded to ischemia with a membrane depolarization/inward current that reversed at approximately -40 mV. This event was coupled with an increased membrane conductance. The simultaneous analysis of membrane potential changes and of variations in [Na+]i and [Ca2+]i levels showed that the ischemia-induced membrane depolarization was associated with an increase of [Na+]i and [Ca2+]i. The ischemia-induced membrane depolarization was not affected by tetrodotoxin or by glutamate receptor antagonists. Neither intracellular BAPTA, a Ca2+ chelator, nor incubation of the slices in low-Ca2+-containing solutions affected the ischemia-induced depolarization, whereas it was reduced by lowering the external Na+ concentration. High doses of blockers of ATP-dependent K+ channels increased the membrane depolarization observed in spiny neurons during ischemia.

CONCLUSIONS

Our findings show that, although the ischemia-induced membrane depolarization is coupled with a rise of [Na+]i and [Ca2+]i, only the Na+ influx plays a prominent role in this early electrophysiological event, whereas the increase of [Ca2+]i might be relevant for the delayed neuronal death. We also suggest that the activation of ATP-dependent K+ channels might counteract the ischemia-induced membrane depolarization.

ATP-sensitive K+ channel activation provides transient protection to the anoxic turtle brain

There is wide speculation that ATP-sensitive K+ (KATP) channels serve a protective function in the mammalian brain, being activated during periods of energy failure. The aim of the present study was to determine if KATP channels also have a protective role in the anoxia-tolerant turtle brain. After ouabain administration, rates of change in extracellular K+ were measured in the telencephalon of normoxic and anoxic turtles (Trachemys scripta). The rate of K+ efflux was reduced by 50% within 1 h of anoxia and by 70% at 2 h of anoxia, and no further decrease was seen at 4 h of anoxia. The addition of the KATP channel blocker glibenclamide or 2,3-butanedione monoxime prevented the anoxia-induced decrease in K+ efflux during the first hour of anoxia, but the effect of these blockers was diminished at 2 h of anoxia and was not seen after 4 h of anoxia. This pattern of change in KATP channel blocker sensitivity can be related to a previously established temporary fall and subsequent recovery of tissue ATP during early anoxia. We suggest that activated KATP channels are involved in the downregulation of membrane ion permeability (channel arrest) during the initial energy crisis period but are switched off when the full anoxic state is established and tissue ATP levels have been restored. We also found that, in contrast to those in mammals, KATP channels are not a major route for K+ efflux in the energy-depleted turtle brain.

Blockade of bepridil on IA and IK in acutely isolated hippocampal CA1 neurons

The effects of bepridil, an antianginal agent with antiarrhythmic action, on voltage-dependent K+ currents in the CA1 pyramidal neurons acutely isolated from rat hippocampus were studied by means of whole-cell patch clamp techniques. Current recordings were made in the presence of TTX to block Na+ current. Depolarizing test pulses activated two components of outward K+ currents: a rapidly activating and inactivating component, IA; and a delayed component, IK. Results showed that bepridil reduced the amplitude of IA and IK, and exerted its inhibitory action in time- and dose-dependent manner. Half-blocking concentrations (IC50) of bepridil on IA and IK were 17.8 microM and 1.7 microM, respectively. 10 microM bepridil suppressed IA and IK by 46.7% and 77.1% at +30 mV of depolarization, respectively. When IK was activated nearly uncontaminated with IA by holding at -50 mV, 10 microM bepridil inhibited IK by 71.6% at +30 mV of depolarization; 10 microM bepridil positively shifted the voltage-dependent of activation curves of IA and IK 12.1 mV and 28.7 mV, respectively. These results suggested that blockade on K+ currents by bepridil is preferential for IK, and contributes to the protection brain against ischemic damage.

Comparison of brain slice chamber designs for investigations of oxygen deprivation in vitro

We have systematically compared anoxia-induced effects in the CA1 stratum pyramidale of rat hippocampal slices maintained in Haas-type interface, and fully-submerged recording chambers to determine whether heterogeneous responses occur in response to oxygen deprivation, between these brain slice chambers. Extracellular K+ concentration ([K+]e), extracellular d.c. potential, and synaptically-evoked field potentials were measured using a K+-selective microelectrode. In slices maintained in the interface chamber, anoxia resulted in a dramatic disturbance of K homeostasis; during the first approximately 3 min of anoxia, [K+]e increased to ca. 5 mM and synaptic transmission was abolished. Thereafter, [K+]e increased explosively to ca. 35 mM, and a negative shift in the extracellular d.c. potential occurred. In slices maintained fully-submerged, even prolonged anoxia (60 min) caused only a very modest increase in [K+]e to ca. 5 mM and a negative shift in the extracellular potential never occurred. However, when glucose deprivation was combined with anoxia, both a dramatic increase in [K+]e and a negative shift in the d.c. potential were observed. We conclude that major differences exist in the degree of disruption of ionic homeostasis by anoxia in rat hippocampal slices when they are maintained at a gas-liquid interface, compared to when they are completely immersed. Possible factors underlying this difference include heterogeneous rates of glucose diffusion into the tissue slices, and washout from the extracellular space of accumulating substances such as K+ by the perfusing artificial cerebrospinal fluid (ACSF). The changes observed in the interface chamber may more closely replicate the dramatic disturbance of K+ homeostasis that is commonly observed in rodent brain in response to anoxia in vivo.

Creatine protects the central respiratory network of mammals under anoxic conditions

The effect of creatine (Cr) on the response of the respiratory center to anoxia was analyzed at different postnatal stages in a brainstem slice preparation of mice. Spontaneous rhythmic activity was recorded from hypoglossal rootlets (XII) and from identified neurons within the preBötzinger complex using the whole cell patch clamp technique. The hypoxic response was evaluated in slices from animals (n = 46), which received normal nutrition (controls, n = 16), from litters of animals fed with Cr (2 g/kg/day; nutrition group, n = 8), or after incubating slices for 3 h in Cr (200 microM) (incubation group, n = 22). ATP was measured in slices from controls and Cr-incubated slices which underwent 30-min anoxia. In neonatal animals (P0-5), amplitudes of hypoglossal bursts increased initially during anoxia by 14% in controls and by 41% in Cr-supplemented animals when compared with preanoxic values. Hypoglossal burst duration increased by 3% in controls, but by 18% in the Cr-nutrition group. In brainstem slices, the initial increase of amplitudes changed from 14% (controls) to 59% (Cr incubation) and prolongation of bursts from 3% (controls) to 37% (Cr incubation) compared with preanoxic values. In juvenile controls (P6-13), burst amplitude and duration increased by 12 and 14% during early anoxia when referred to preanoxic values. In slices from Cr-pretreated animals, increases of 48% (amplitude) and 21% (burst duration) occurred. The ATP levels remained constant during a 30-min anoxic period in the Cr-pretreated group compared with a decrease of 44% in slices from controls. Our data suggest that Cr can ameliorate hypoxic energy failure. Further studies will examine the neuroprotective potential in humans.

Ion transport and membrane potential in CNS myelinated axons I. Normoxic conditions

Compound resting membrane potential was recorded by the grease gap technique during normoxic conditions (37 degrees C) in rat optic nerve, a representative CNS myelinated tract. Mean potential was -47 +/- 3 (SD) mV and remained stable for 2-3 h. Input impedance of a single optic nerve axon was calculated to be approximately 5 Gomega. Contribution of the Na+ pump to resting axonal potential is estimated at -7 mV. Ouabain (10 microM to 10 mM) evoked a dose-dependent depolarization that was maximal at >/=1 mM, depolarizing the nerves to approximately 35-40% of control after 60 min. Inhibiting energy metabolism (CN- and iodoacetate) during high-dose ouabain (1-10 mM) exposure caused an additional depolarization, suggesting additional ATP-dependent, ouabain-insensitive ion transport systems. Perfusion with zero-Na+ (choline substituted) caused a transient hyperpolarization, that was greater than with tetrodotoxin (TTX; 1 microM) alone, indicating both TTX-sensitive and -insensitive Na+ influx pathways in resting rat optic nerve axons. Resting probability (P)K:PNa is calculated at 20:1. In contrast to choline-substituted solution, Li+-substituted zero-Na+ perfusate caused a rapid depolarization due to Na+ pump inhibition and the ability of Li+ to permeate the Na+ channel. TTX reduced, but did not prevent, ouabain- or zero-Na+/Li+-induced depolarization. We conclude that the primary Na+ influx path in resting rat optic nerve axons is the TTX-sensitive Na+ channel, with evidence for additional TTX-insensitive routes permeable to Na+ and Li+. In addition, maintenance of membrane potential is critically dependent on continuous Na+ pump activity due to the relatively high exchange of Na+ (via the above mentioned routes) and K+ across the membrane of resting optic axons.

The electrogenic effects of Na(+)-K(+)-ATPase in rat auditory thalamus

1. The electrogenic effects of the Na(+)-K(+)-ATPase in thalamic neurones were investigated by means of intracellular and whole-cell patch-clamp recording techniques in rat medial geniculate body (MGB) maintained in vitro. 2. In twenty-six out of thirty-one neurones recorded intracellularly, application of the Na(+)-K+ pump inhibitor strophanthidin induced two different types of membrane depolarization: a small, reversible depolarization with a peak amplitude of 4 +/- 2.6 mV or a prolonged depolarization of large amplitude (48.6 +/- 9.0 mV) with or without a decrease in apparent membrane resistance. Blockade of glutamate receptors with kynurenic acid or 6-cyano-7-nitroquinoxaline-2,3-dione and (+/-)-2-amino-5-phosphonopentanoic acid did not prevent either type of pump response, but the large depolarization was not seen when the medium contained the sodium channel blocker TTX. 3. Whole-cell patch-clamp recording showed that the small membrane depolarization is mediated by an inward membrane current (39.00 +/- 5.70 pA) that exhibited a weak voltage dependence. An inward current of similar amplitude was also induced in MGB cells when the pipette solution contained nominally zero Na+ or when K+ was temporarily omitted from the extracellular medium. The large membrane depolarization or the corresponding membrane current was not observed in whole-cell conditions. 6. Transient inhibition of the electrogenic Na(+)-K(+)-ATPase consistently led to a change in the mode of synaptic transmission in MGB cells, during which the synaptically evoked burst response was either blocked or converted into a single spike discharge. 7. Taken together, these data suggest that blockade of the electrogenic pump produces a dual membrane effect in mammalian thalamic neurones: a small electrogenic membrane depolarization and a large depolarization response that can be prevented by artificially maintaining the transmembrane ionic gradients. The electrogenic activity of the Na(+)-K(+)-ATPase may play an important role in setting the mode of synaptic transmission in sensory thalamus.

Multiple effects of serotonin on membrane properties of trigeminal motoneurons in vitro

Intracellular recordings from guinea pig trigeminal motoneurons (TMNs) in brain stem slices were used to determine the underlying ionic mechanisms responsible for our previously demonstrated enhancement of TMN excitability during jaw movements by serotonin (5-HT). 5-HT (0.5-100 microM) depolarized motoneurons and increased input resistance in the majority of neurons tested. Additionally, 5-HT reduced the amplitude of the postspike medium-duration afterhyperpolarization, decreased the current threshold for maintained spike discharge, and increased the maximum slope of the steady-state spike frequency-current relationship. Under voltage clamp, from holding potentials close to resting potential, 5-HT produced an inward current and a decrease in instantaneous slope conductance, suggesting a reduction in a resting K+ leak conductance (I(leak)). The instantaneous current-voltage (I-V) relationship for the inward 5-HT current (I(5-HT)) was linear throughout most of the voltage range tested. However, the steady-state I-V relationship showed some degree of inward rectification at potentials starting around -70 mV. The mean reversal potential for the instantaneous I(5-HT) was -86.2 +/- 4.5 (SE) mV (n = 9), a value slightly negative to the predicted potassium equilibrium potential of -82 mV in these neurons. In the presence of 2 mM Ba2+, 5-HT application did not produce a further reduction in input conductance, but did expose a Ba2+-insensitive residual inward current that was resistant to Cs+ application. The instantaneous I-V relationship during 5-HT application in the presence of Ba2+ was shifted downward and parallel to control, suggesting that Ba2+ and 5-HT block the same resting I(leak). The residual Ba2+- and Cs+-insensitive component of the total inward I(5-HT) was voltage independent and was blocked when the extracellular Na+ was replaced by choline, suggesting that the predominant charge carrier for this residual current is Na+. 5-HT enhanced a hyperpolarization-activated cationic current, I(h). In the presence of Ba2+, the time course of I(5-HT) resembled that of I(h) and showed a similar voltage dependence that was blocked by extracellular Cs+ (1-3 mM). The effects of 5-HT on membrane potential, input resistance, and I(h) were partially mimicked by 5-HT2 agonists and suppressed by 5-HT2 antagonists. It is concluded that 5-HT enhances TMN membrane excitability through modulation of multiple intrinsic membrane conductances. This provides for a mechanism(s) to fine tune the input-output discharge properties of these neurons, thus providing them with greater flexibility in output in response to time-varying synaptic inputs during various movements of the jaw.

Opposite membrane potential changes induced by glucose deprivation in striatal spiny neurons and in large aspiny interneurons

We have studied the electrophysiological effects of glucose deprivation on morphologically identified striatal neurons recorded from a corticostriatal slice preparation. The large majority of the recorded cells were spiny neurons and responded to aglycemia with a slow membrane depolarization coupled with a reduction of the input resistance. In voltage-clamp experiments aglycemia caused an inward current. This current was associated with a conductance increase and reversed at -40 mV. The aglycemia-induced membrane depolarization was not affected by tetrodotoxin (TTX) or 6-cyano-7-nitroquinoxaline-2,3-dione plus aminophosphonovalerate, antagonists acting respectively on AMPA and NMDA glutamate receptors. Also, the intracellular injection of bis(2-aminophenoxy)ethane-N,N, N',N'-tetra-acetic acid, a calcium (Ca2+) chelator, and low Ca2+/high Mg2+-containing solutions failed to reduce this phenomenon. Conversely, it was reduced by lowering external sodium (Na+) concentration. A minority of the recorded cells had the morphological characteristics of large aspiny interneurons and the electrophysiological properties of "long-lasting afterhyperpolarization (LA) cells." These cells responded to aglycemia with a membrane hyperpolarization/outward current that was coupled with an increased conductance. This current was not altered by TTX, blockers of ATP-dependent potassium (K+) channels, and adenosine A1 receptor antagonists, whereas it was reduced by solutions containing low Ca2+/high Mg2+. This current reversed at -105 mV and was blocked by barium, suggesting the involvement of a K+ conductance. We suggest that the opposite membrane responses of striatal neuronal subtypes to glucose deprivation might account for their differential neuronal vulnerability to aglycemia and ischemia.

Cellular adaptive responses to low oxygen tension: apoptosis and resistance

Oxygen plays such a critical role in the central nervous system that a specialized mechanism of oxygen delivery to neurons is required. Reduced oxygen tension, or hypoxia, may have severe detrimental effects on neuronal cells. Several studies suggest that hypoxia can induce cellular adaptive responses that overcome apoptotic signals in order to minimize hypoxic injury or damage. Adaptive responses of neuronal cells to hypoxia may involve activation of various ion channels, as well as induction of specific gene expression. For example, ATP sensitive K+ channels are activated by hypoxia in selective neuronal cells, and may play a role in cell survival during hypoxia/anoxia. Additionally, hypoxia-induced c-Jun, bFGF and NGF expression appear to be associated with prevention (or delay) of neuronal cell apoptosis. In this paper, these adaptive responses to hypoxia in neuronal cells are discussed to examine the possible role of hypoxia in pathophysiology of diseases.

K+ ions and channels: mechanisms of sensing O2 deprivation in central neurons

This is an exciting area of research for at least two reasons: (1) it has high clinical visibility and potential implications that transcend age, tissue, and cell type. (2) Currently there are very powerful armamentaria to solve questions and we are starting to apply this in this area of research. Hence, the possibility for understanding how hypoxia induces injury or why do cells survive anoxia is within reach. I hope that within the foreseeable future we will be able to solve some of these questions at the molecular level and start to target some of the important pathways for pharmacologic and drug interventions.

Modulation of K+ channels by intracellular ATP in human neocortical neurons

ATP-modulated K+ channels play an important role in regulating membrane excitability during metabolic stress. To characterize such K+ channels from the human brain, single channel currents were studied in excised inside-out patches from freshly dissociated human neocortical neurons. Three currents that were sensitive to physiological concentrations of ATP and selectively permeable to K+ were identified. One of these currents had a unitary conductance of approximately 47 pS and showed a strong inward rectification with symmetric K+ concentrations across the membrane. This K+ current was inhibited by ATP in a concentration-dependent manner with an IC50 (half-inhibition of channel activity) of approximately 130 microM. Channel activity also was suppressed by ADP, non-hydrolyzable ATP analogue AMP-PNP, and sulfonylurea receptor/ channel blocker glibenclamide. The second K+ current had a unitary conductance of approximately 200 pS and showed a weak inward rectification. Similarly, this current was inhibited by ATP (IC50 = 350 microM), AMP-PNP, and glibenclamide. Unlike the small-conductance ATP-inhibitable K+ channel (S-KATP), activation of this large-conductance K+ channel (L-KATP) required the presence of micromolar concentration of Ca2+ in the internal solution, but charybdotoxin did not inhibit this channel. The third K+ current was also Ca2+ dependent and had a large conductance (approximately 280 pS). It was inhibited by external charybdotoxin, iberiotoxin, and tetraethylammonium. In contrast to the other two KATP channels, ATP enhanced channel open-state probability and unitary conductance, and glibenclamide at concentration of 10-20 microM had no inhibitory effect on this current. K+ channels that have single-channel and pharmacological properties similar to these three human ATP-modulated K+ channels also were observed in experiments on rat neocortical neurons. These results therefore indicate that KATP channels are expressed in human neocortical neurons, and two distinct KATP channels (S-KATP and L-KATP) exist in the human and rat neurons. The observation that ATP at different concentrations modulates different K+ channels suggests that metabolic rate may be continuously sensed in neurons with resulting alterations in neuronal membrane excitability.

O2-sensing mechanisms in excitable cells: role of plasma membrane K+ channels

Although carotid chemosensitive glomus cells have been the most extensively studied from the vantage point of how cells sense the lack of O2, it is clear that all tissues sense O2 deprivation. In addition, all mammalian cells can trigger a cascade of events that, depending on the severity and duration of hypoxia-induced stress, can lead to permanent injury and death or to adaptation and survival. Crucial in this cascade, we believe, how the cascade is initiated, how O2 lack is detected by cells, and how these initial steps can activate further processes. In this chapter, we focus on the initial steps of O2 sensing in tissues most commonly studied, i.e. carotid glomus cells, central neurons, smooth muscle cells, and neuro-epithelial bodies of the airways. Recently it has become clear that plasma membranes of various tissues can sense the lack of O2, not only indirectly via alterations in the intracellular milieu (such as pH, Ca, ATP, etc), but also directly through an unknown mechanism that involves plasma-membrane K channels and possibly other membrane proteins. This latter mechanism is suspected to be totally independent of cytosolic changes because excised patches from plasma membranes were used in these experiments from carotid cells and neurons. There are a number of questions in this exciting area of research that pertain to the role of this plasma-membrane O2-sensing mechanism in the overall cell response, identification of all the important steps in O2 sensing, differences between O2-tolerant and O2-susceptible cells, and differences between acute and chronic cell responses to lack of O2.

Activation by intracellular ATP of a potassium channel in neurones from rat basomedial hypothalamus

1. Cell-attached recordings from isolated glucose-sensitive hypothalamic neurones show that on removal of extracellular glucose there is an increased action current frequency concomitant with decreased single-channel activity. Conversely activation of single K+ channels was observed when extracellular glucose was increased. Isolation of membrane patches into the inside-out configuration following cell-attached recording demonstrated the presence of an ATP-activated K+ channel. 2. The ATP-activated K+ channel was characterized by a mean single-channel conductance of 132 pS in symmetrical 140 mM KCl solutions. Single-channel open-state probability (Po) was not calcium dependent, and the presence of calcium did not prevent activation of the channel by ATP. 3. Activation of the channel by ATP was concentration dependent and the Po of the ATP-activated channel was unaffected by membrane voltage, regardless of the degree of activation elicited by ATP. 4. Open and closed time histograms were constructed from inside-out and cell-attached recordings and were consistent with a single open and two closed states. Channel openings were grouped in bursts. Application of ATP, in isolated patches, and glucose, in cell-attached patches, increased the burst duration and number of bursts per second and decreased the slow closed-state time constant. In neither case was there a significant change in the fast closed-state time constant nor the open-state time constant. 5. The non-hydrolysable ATP analogue adenylylimidodiphosphate (AMP(PNP)) and 'Mg2(+)-free' ATP produced little change in the Po of the ATP-activated K+ channel when applied to the intracellular surface of excised patches. These results suggest that activation of this channel is via an enzymic mechanism. 6. ADP, GTP and GDP also activated the channel in a Mg(2+)-dependent manner. ADP and ATP activated the channel in an additive manner and neither GTP nor GDP inhibited channel activity induced by ATP. 7. It is concluded that the ATP-activated K+ channel observed in isolated inside-out patches from hypothalamic neurones is the same as the channel activated by an increase in the concentration of extracellular glucose in cell-attached recordings from glucose-sensitive neurones.

Hyperthermia depletes adenosine triphosphate and decreases glutamate uptake in rat hippocampal slices

The central nervous system is especially vulnerable to hyperthermia-induced dysfunction, yet the mechanism for this susceptibility is poorly understood. High levels of adenosine triphosphate are necessary to maintain normal re-uptake of glutamate and aspartate, the major excitatory amino acids, by excitatory amino acid co-transporters. We hypothesized that excitotoxic neurotransmitters accumulate extracellularly when hyperthermia depletes adenosine triphosphate, leading to decreased uptake or release of excitatory amino acids by these co-transporters. Incubation of hippocampal slices at 42 degrees C, a temperature that results in coma in vivo, reduced adenosine triphosphate to 70% of control values and decreased uptake of the transportable excitatory amino acid analogue, D,L threo-beta-hydroxyaspartate, to 50% of control values. The degree of adenosine triphosphate depletion induced by hyperthermia was highly correlated with decreases in excitatory amino acid uptake. Severe adenosine triphosphate depletion (< or = 20% of control) induced by hyperthermia in combination with metabolic insults was highly correlated with the release of endogenous glutamate and aspartate. Preloading slices with excitatory amino acid analogues potentiated hyperthermia-induced alterations of excitatory amino acid transport, strongly suggesting that the hyperthermia-induced changes were largely due to altered excitatory amino acid co-transporter activity. Immunocytochemical studies suggested glutamate-like immunoreactivity was lost from axonal terminals during hyperthermia in a similar manner to losses induced by metabolic toxins. Hyperthermia due to infectious diseases or heat stroke my induce disorientation and coma. These dysfunctions may be due, in part, to altered excitatory amino acid transport induced by adenosine triphosphate depletion.

Isolation and chromosomal localization of a human ATP-regulated potassium channel

KATP channels are K+ channels whose activity is inhibited by the presence of and enhanced by the absence of cytosolic ATP. This property allows KATP channels to sense cellular intermediary metabolism and directly couple this information to the modulation of membrane excitability. Indeed, recent studies from our laboratory and others have suggested that activation of KATP channels during anoxia is important in the response and adaptation of central neurons to hypoxia. In order to identify KATP channels from human brain, we performed a polymerase chain reaction (PCR) using human cerebral cortex mRNA and primers derived from the ROMK1 sequence, a cDNA clone encoding an ATP-regulated potassium channel, recently isolated from rat kidney. We thus identified a novel 308-bp PCR product, pKCNJ1, whose expression was found to be restricted to a 3.0-kb band in the kidney by probing a human multiple tissue northern blot, pKCNJ1 was then used to isolate genomic clones and, using fluorescence in situ hybridization (FISH) to human metaphase chromosomes, was mapped to chromosome 11q.