Rapid decrease of high affinity ouabain binding sites in hippocampal CA1 region following short-term global cerebral ischemia in rat

The Autophagic and Apoptotic Death of Forebrain Neurons of Rats with Global Brain Ischemia Is Diminished by the Intranasal Administration of Insulin: Possible Mechanism of Its Action

Insulin is a promising neuroprotector. To better understand the mechanism of insulin action, it was important to show its ability to diminish autophagic neuronal death in animals with brain ischemic and reperfusion injury. In forebrain ischemia and reperfusion, the number of live neurons in the hippocampal CA1 region and frontal cortex of rats decreased to a large extent. Intracerebroventricular administration of the autophagy and apoptosis inhibitors to ischemic rats significantly increased the number of live neurons and showed that the main part of neurons died from autophagy and apoptosis. Intranasal administration of 0.5 IU of insulin per rat (before ischemia and daily during reperfusion) increased the number of live neurons in the hippocampal CA1 region and frontal brain cortex. In addition, insulin significantly diminished the level of autophagic marker LC3B-II in these forebrain regions, which markedly increased during ischemia and reperfusion. Our studies demonstrated for the first time the ability of insulin to decrease autophagic neuronal death, caused by brain ischemia and reperfusion. Insulin administered intranasally activated the Akt-kinase (activating the mTORC1 complex, which inhibits autophagy) and inhibited the AMP-activated protein kinase (which activates autophagy) in the hippocampus and frontal cortex of rats with brain ischemia and reperfusion.

Role of NKCC1 and KCC2 in Epilepsy: From Expression to Function

As a main inhibitory neurotransmitter in the central nervous system, γ-aminobutyric acid (GABA) activates chloride-permeable GABAa receptors (GABAa Rs) and induces chloride ion (Cl⁻) flow, which relies on the intracellular chloride concentration ([Cl⁻]_i) of the postsynaptic neuron. The Na-K-2Cl cotransporter isoform 1 (NKCC1) and the K-Cl cotransporter isoform 2 (KCC2) are two main cation-chloride cotransporters (CCCs) that have been implicated in human epilepsy. NKCC1 and KCC2 reset [Cl⁻]_i by accumulating and extruding Cl⁻, respectively. Previous studies have shown that the profile of NKCC1 and KCC2 in neonatal neurons may reappear in mature neurons under some pathophysiological conditions, such as epilepsy. Although increasing studies focusing on the expression of NKCC1 and KCC2 have suggested that impaired chloride plasticity may be closely related to epilepsy, additional neuroelectrophysiological research aimed at studying the functions of NKCC1 and KCC2 are needed to understand the exact mechanism by which they induce epileptogenesis. In this review, we aim to briefly summarize the current researches surrounding the expression and function of NKCC1 and KCC2 in epileptogenesis and its implications on the treatment of epilepsy. We will also explore the potential for NKCC1 and KCC2 to be therapeutic targets for the development of novel antiepileptic drugs.

Na, K-ATPase α3 is a death target of Alzheimer patient amyloid-β assembly

Neurodegeneration correlates with Alzheimer's disease (AD) symptoms, but the molecular identities of pathogenic amyloid β-protein (Aβ) oligomers and their targets, leading to neurodegeneration, remain unclear. Amylospheroids (ASPD) are AD patient-derived 10- to 15-nm spherical Aβ oligomers that cause selective degeneration of mature neurons. Here, we show that the ASPD target is neuron-specific Na(+)/K(+)-ATPase α3 subunit (NAKα3). ASPD-binding to NAKα3 impaired NAKα3-specific activity, activated N-type voltage-gated calcium channels, and caused mitochondrial calcium dyshomeostasis, tau abnormalities, and neurodegeneration. NMR and molecular modeling studies suggested that spherical ASPD contain N-terminal-Aβ-derived "thorns" responsible for target binding, which are distinct from low molecular-weight oligomers and dodecamers. The fourth extracellular loop (Ex4) region of NAKα3 encompassing Asn(879) and Trp(880) is essential for ASPD-NAKα3 interaction, because tetrapeptides mimicking this Ex4 region bound to the ASPD surface and blocked ASPD neurotoxicity. Our findings open up new possibilities for knowledge-based design of peptidomimetics that inhibit neurodegeneration in AD by blocking aberrant ASPD-NAKα3 interaction.

Cation-chloride cotransporters NKCC1 and KCC2 as potential targets for novel antiepileptic and antiepileptogenic treatments

In cortical and hippocampal neurons, cation-chloride cotransporters (CCCs) control the reversal potential (EGABA) of GABAA receptor-mediated current and voltage responses and, consequently, they modulate the efficacy of GABAergic inhibition. Two members of the CCC family, KCC2 (the major neuron-specific K-Cl cotransporter; KCC isoform 2) and NKCC1 (the Na-K-2Cl cotransporter isoform 1 which is expressed in both neurons and glial cells) have attracted much interest in studies on GABAergic signaling under both normal and pathophysiological conditions, such as epilepsy. There is tentative evidence that loop diuretic compounds such as furosemide and bumetanide may have clinically relevant antiepileptic actions, especially when administered in combination with conventional GABA-mimetic drugs such as phenobarbital. Furosemide is a non-selective inhibitor of CCCs while at low concentrations bumetanide is selective for NKCCs. Search for novel antiepileptic drugs (AEDs) is highly motivated especially for the treatment of neonatal seizures which are often resistant to, or even aggravated by conventional AEDs. This review shows that the antiepileptic effects of loop diuretics described in the pertinent literature are based on widely heterogeneous mechanisms ranging from actions on both neuronal NKCC1 and KCC2 to modulation of the brain extracellular volume fraction. A promising strategy for the development of novel CCC-blocking AEDs is based on prodrugs that are activated following their passage across the blood-brain barrier. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

Low-dose cardiotonic steroids increase sodium-potassium ATPase activity that protects hippocampal slice cultures from experimental ischemia

The sodium-potassium ATPase (Na/K ATPase) is a major ionic transporter in the brain and is responsible for the maintenance of the Na(+) and K(+) gradients across the cell membrane. Cardiotonic steroids such as ouabain, digoxin and marinobufagenin are well-characterized inhibitors of the Na/K ATPase. Recently, cardiotonic steroids have been shown to have additional effects at concentrations below their IC(50) for pumping. The cardiotonic steroids ouabain, digoxin, and marinobufagenin all show an inverted U-shaped dose-response curve with inhibition of pumping at concentrations near their IC(50), while increasing Na/K ATPase activity at doses below their IC(50). This stimulatory effect of cardiotonic steroids was observed in vitro in hippocampal slice cultures as well as in the hippocampus in vivo. Increased Na/K ATPase activity has been shown to protect slice culture neurons from hypoxia-hypoglycemia. Ouabain protected slice culture neurons from experimental ischemia at concentrations that increased Na/K ATPase. This protective effect was observed when ouabain was dosed 30min before, or 2h following experimental ischemia. Ouabain no longer protected against experimental ischemia if the increase of Na/K ATPase was blocked. These data suggest that the protective effect of ouabain was due to increased Na/K ATPase activity. The demonstration of a neuroprotective effect of cardiotonic steroids could potentially assist in the treatment of stroke since digoxin, one of the cardiotonic steroids examined in this study, has approval by the Food and Drug Administration and can be safely administered at the concentrations that increase Na/K ATPase activity.

Cation-chloride cotransporters and neuronal function

Recent years have witnessed a steep increase in studies on the diverse roles of neuronal cation-chloride cotransporters (CCCs). The versatility of CCC gene transcription, posttranslational modification, and trafficking are on par with what is known about ion channels. The cell-specific and subcellular expression patterns of different CCC isoforms have a key role in modifying a neuron's electrophysiological phenotype during development, synaptic plasticity, and disease. While having a major role in controlling responses mediated by GABA(A) and glycine receptors, CCCs also show close interactions with glutamatergic signaling. A cross-talk among CCCs and trophic factors is important in short-term and long-term modification of neuronal properties. CCCs appear to be multifunctional proteins that are also involved in shaping neuronal structure at various stages of development, from stem cells to synaptogenesis. The rapidly expanding work on CCCs promotes our understanding of fundamental mechanisms that control brain development and functions under normal and pathophysiological conditions.

Susceptibility of hippocampal neurons to mechanically induced injury

Experimental models of traumatic cortical brain injury in rodents reveal that specific regions of the hippocampus (e.g., CA3 and hilar subfields) are severely injured despite their distance from the initial insult. Hippocampal neurons may be intrinsically more vulnerable to mechanical insult than cortical neurons due to increased NMDA receptor densities and lower energy capacities, as evidenced by increased susceptibility to ischemic insults. The selective vulnerability of hippocampal neurons was evaluated using an in vitro model of TBI in which either primary rat cortical or hippocampal neurons (E17) seeded onto silicone substrates were subjected to graded levels of mechanical stretch. Although cortical neurons exhibited significantly longer increases in stretch-induced membrane permeability, injury of hippocampal neurons resulted in larger increases in intracellular free calcium concentration [Ca(2+)](i) and cell death. [ATP](i) deficits due to stretch were apparent by 60 min after injury in cortical neurons but recovered by 24 h, whereas significant deficits in [ATP](i) were not observed in hippocampal neurons until 24 h after injury. MK801 pretreatment decreased the stretch-induced [Ca(2+)](i) transients in both hippocampal and cortical cultures, thereby negating the regional specificity. However, MK801 pretreatment did not improve hippocampal viability and paradoxically, significantly increased cell death among cortical neurons. As the hippocampus is the primary brain region responsible for the memory deficits and epileptic seizures associated with TBI, understanding why this region is selectively damaged could lead to the development of more accurate mechanical tolerances as well as effective pharmaceutical agents.

Inhibition of Na(+),K(+)-ATPase activity in cultured rat cerebellar granule cells prevents the onset of apoptosis induced by low potassium

In cerebellar granule cells in culture, lowering of extracellular [K(+)] results in apoptotic death (D'Mello, S.R., Galli, C., Ciotti, T. and Calissano, P., Induction of apoptosis in cerebellar granule neurons by low potassium: inhibition of death by insulin-like growth factor I and cAMP, Proc. Natl. Acad. Sci. USA, 90 (1993) 10989-10993). In this model, we studied the influence of Na(+), K(+)-ATPase inhibition on apoptosis. We demonstrate that cell death (93+/-2 vs. 46+/-1.6%) as well as fragmentation of nuclear DNA induced by low extracellular potassium were prevented by addition of ouabain (0.1 mM), a specific inhibitor of the Na(+),K(+)-ATPase. Blockade of glutamatergic N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors by 5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine hydrogen maleate (MK-801; 20 microM) and 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX; 50 microM) did not inhibit the protective effect of ouabain. 24 h treatment with ouabain also decreased cell death induced by Fe(2+)/ascorbic acid (74+/-2% to 49+/-3%). We speculate that ouabain pretreatment enhances the resistance against low [K(+)]-induced apoptosis independent of glutamate-receptor activation. Since this effect can be mimicked by a free-radical generating system, we suggest an antioxidative effect underlying ouabain-induced neuroprotection.

Relationships between ATP depletion, membrane potential, and the release of neurotransmitters in rat nerve terminals. An in vitro study under conditions that mimic anoxia, hypoglycemia, and ischemia

BACKGROUND AND PURPOSE

It is known that the extracellular accumulation of glutamate during anoxia/ischemia is responsible for initiating neuronal injury. However, little information is available on the release of monoamines and whether the mechanism of its release resembles that of glutamate, which may itself influence the release of monoamines by activating presynaptic receptors. This study was designed to characterize the release of both amino acids and monoamines under chemical conditions that mimic anoxia, hypoglycemia, and ischemia.

METHODS

The contents of synaptosomes in adenine nucleotides (ATP, ADP, and AMP), amino acids (aspartate, glutamate, taurine, and gamma-aminobutyric acid), and monoamines (dopamine, noradrenaline, and 5-hydroxytryptamine) were measured by high-performance liquid chromatography, after the synaptosomes were subjected to anoxia (KCN + oligomycin), hypoglycemia (2 mmol/L 2-deoxyglucose in glucose-free medium), and ischemia (anoxia plus hypoglycemia).

RESULTS

The anoxia- and ischemia-induced release or noradrenaline, dopamine, 5-hydroxytryptamine, and glutamate correlated well with ATP depletion. The correlation observed between glutamate levels and the release of dopamine and 5-hydroxytryptamine in ischemic conditions suggests a functional linkage between the two transmitter systems. However, the antagonists of presynaptic glutamate receptors failed to alter the amount of monoamines released. The inhibition of Na+,K+-ATPase by ouabain had an effect similar to that produced by ischemia.

CONCLUSIONS

The decrease in Na+ and K+ gradients resulting from the energy depletion of the synaptosomes under ischemic conditions or resulting from the inhibition of Na+, K+-ATPase by ouabain promotes the reversal of the neurotransmitter transporters. The decrease in uptake of neurotransmitters may also contribute to the rise in the extracellular concentration of different transmitters observed during brain ischemia.

Complete cerebral ischemia with short-term survival in rats induced by cardiac arrest. I. Extracellular accumulation of Alzheimer's beta-amyloid protein precursor in the brain

The distribution of beta-amyloid protein precursor (APP) was investigated immunocytochemically in rats subjected to global cerebral ischemia (GCI) induced by cardiac arrest. Rats underwent 10 min of GCI with 3, 6, and 12 h and 2 and 7 days of survival. APP immunostaining was found extracellular and intracellularly. Multiple extracellular APP immunoreactive deposits around and close to the vessels appeared as soon as 3 h after GCI. Extracellular accumulation of APP occurred frequently in the hippocampus, cerebral and cerebellar cortex, basal ganglia and thalamus and rarely in the brain stem. These deposits were labelled with antibodies against the N-terminal, beta-amyloid peptide, and C-terminal domains of APP. Our data suggests that either proteolytically cleaved fragments of the full-length APP or the entire APP molecule accumulates extracellularly after GCI. This findings may not only implicate the participation of APP in postischemic tissue damage but also suggest the involvement of pathomechanisms operating in ischemia in Alzheimer's disease pathology.

The effects of the purinergic system on digitalis-induced epileptiform activity

It has been suggested that endogenous chemical substances, such as adenosine, released during a seizure attack, may act as anticonvulsants in vivo. We have investigated electrophysiologically the effects of purinoceptor agonists and antagonists on the epileptiform activity induced by intracortical digitalis in anesthetized rats. Intracortical injections of 1, 2, or 4 micrograms digitalis (desacetyl lanatocid C) caused an epileptiform electrocorticogram (ECoG). The application of adenosine (25 or 100 microM) or adenosine triphosphate (ATP) (3 mM) after desacetyl lanatocid C blocked the epileptiform activity. beta, gamma-Methylene ATP (0.1-0.8 mM), a stable analog of ATP, produced inhibition and then death. The epileptogenic effect of desacetyl lanatocid C was enhanced by theophylline (1 mM); however, suramin (1 mM) changed the pattern of epilepsy. These results indicate that the purinergic system may be involved in the mechanism of action of digitalis glycosides.

Early blood-brain barrier changes in the rat following transient complete cerebral ischemia induced by cardiac arrest

This study examined regional patterns of increased vascular permeability following transient global cerebral ischemia. Rats underwent 3.5, 5 or 10 min of cardiac vessel bundle occlusion, i.e. cardiac arrest. The animals were killed at 2, 3, 5 and 15 min, or 1, 3, 6 and 24 h after global cerebral ischemia. Thirty minutes before the end of each blood recirculation period, the electron dense protein tracer--horseradish peroxidase (HRP) was intravenously injected and rats were perfusion-fixed for light and electron microscopic analysis. Control rats showed no HRP leakage. Post-ischemic rats demonstrated random blood-brain barrier (BBB) alterations. Permeability alterations were spotty and widespread in cortical, thalamic, basal ganglia, hippocampal, brain stem regions, cerebellum and white matter. Peroxidase extravasation frequently involved arterioles, veins and venules surrounded by perivascular spaces. Routes of increased HRP permeability included endothelial cell (EC) vesiculo-canalicular profiles and diffuse leakage through damaged ECs. Barrier damage determined by HRP permeability revealed a biphasic nature. The first stage appeared immediately after ischemia at the 2nd min and involved the 1st post-insult hour. There was no HRP leakage in rats sacrificed 3 h after insult. BBB opening appeared again 6 h after ischemia and remained open 24 h after cardiac arrest. The openings of BBB did not increase in frequency with longer periods of ischemia and recirculation. These results demonstrate that cardiac arrest produces a spotty BBB disturbances at vessel bifurcations and suggest that BBB changes associated with cardiac arrest may be multifactorial in time course and location.

Protein kinase C as an early and sensitive marker of ischemia-induced progressive neuronal damage in gerbil hippocampus

In the model of transient brain ischemia of 6-min duration in gerbils we have estimated: 1. The concentration of brain gangliosides: A significant decrease to about 70% of control was observed selectively in the hippocampus at 3 and 7 d after ischemia. 2. The activity of Na+,K(+)-ATPase: The enzyme activity was not affected in either hippocampus nor in cerebral cortex. 3. The malonaldehyde (MDA) concentration: The levels of MDA had increased at 30 min after ischemia up to 123 and 129% of control in hippocampus and cerebral cortex, respectively. 4. Immunoreactivity of protein kinase C detected by Western blotting: In hippocampus the early translocation toward membranes was followed by a decrease in total enzyme content at 6, 24, 72, and 96 h of postischemic recovery. Also, a sharp increase of 50 kDa isoform (PKM) was noticed immediately and at the early recovery times. The behavior of these biochemical markers of ischemic brain injury in the hippocampus after the short (6 min) insult was contrasted with their reaction in the cerebral cortex as well as after prolongation of the ischemia to 15 min. These results taken together indicate that an early increase in PKC translocation followed by a decrease is the most symptomatic for selective, delayed, postischemic hippocampal injury, resulting from short duration (6 min) ischemia of the gerbil brain.

Ion homeostasis in rat brain in vivo: intra- and extracellular [Ca2+] and [H+] in the hippocampus during recovery from short-term, transient ischemia

Changes in intra- and extracellular [Ca2+] and [H+], together with alterations in tissue PO2 and local blood flow, were measured in areas CA1 and CA3 of the hippocampus during recovery (up to 8 h) after an 8-min period of low-flow ischemia. Restoration of blood supply was followed by an immediate rise in flow and tissue PO2 above normal, with large fluctuations in both persisting for up to 4 h. In area CA1, [Ca2+]i decreased rapidly from an ischemic mean value of 30 microM to a control mean level of 73.1 nM in 20-30 min, whereas normalization of [Ca2+]e took approximately 1 h. Recovery of [Ca2+]i was accelerated by preischemic administration of a calcium antagonist, nifedipine, and a free radical scavenger, N-tert-butyl-alpha-phenylnitrone (PBN), but not by MK-801, a blocker of N-methyl-D-aspartate receptors. There was a secondary rise in [Ca2+]i in many cells beginning approximately 2 h after reperfusion. This was attenuated somewhat by PBN but not clearly influenced by either nifedipine or MK-801. Changes of [Ca2+]i in area CA3 were much smaller and slightly slower than in area CA1 and were not affected by the drugs mentioned above. In both areas CA1 and CA3, pHe and pHi fell during ischemia to an average value of 6.2, from which there was a rapid initial recovery in the first 5-10 min when blood flow was restored. Thereafter tissue pH rose slowly and did not reach control levels for approximately 1 h, and in some microareas not at all. It is concluded that (a) effective mechanisms for restoring normal [Ca2+]i remain intact after 8 min of low-flow ischemia; (b) in neurons of area CA1, some insidious change in the homeostasis of calcium triggers a secondary rise in its free cytosolic concentration, which may be causally related to activation of irreversible cell damage; and (c) the changes in [Ca2+]i and [Ca2+]e during and following 8 min of ischemia can be adequately accounted for by movements of a fixed pool of Ca between intra- and extracellular compartments, and possible mechanisms are discussed.

Abnormal Ca2+ homeostasis before cell death revealed by whole cell recording of ischemic CA1 hippocampal neurons

Slices were made from the hippocampus of gerbils following transient ischemia achieved by clamping the carotid arteries for 5 min, and changes in the electrophysiology of CA1 pyramidal neurons were studied by whole cell patch-clamp recording as well as conventional intracellular recording. The great majority of CA1 neurons in slices made 2.5-3 days after ischemia showed reduced resting potentials and were easily depolarized by prolonged low-frequency stimulation or by tetanic stimulation of the Schaffer collateral/commissural input. This stimulus-induced depolarization was accelerated by intracellular injection of D-myo-inositol 1,4,5-triphosphate, which depolarized membrane potentials towards 0 mV without synaptic input stimulation. Intracellular application of BAPTA, a Ca2+ chelator, effectively blocked the stimulus-induced depolarization. When recording from ischemic neurons with patch pipettes containing both D-myo-inositol 1,4,5-triphosphate and BAPTA, excitatory postsynaptic currents were transiently potentiated by stimulation, but the membrane potential did not show stimulus-induced depolarization and remained steady for long periods. These results lend support to the view that the intracellular Ca2+ regulation system is severely disturbed following ischemia, and that input fiber stimulation leads to abnormal Ca2+ accumulation in ischemic neurons resulted in neuronal death. The reduction of free Ca2+ inside the ischemic neuron by BAPTA apparently saves neurons which are otherwise destined to delayed neuronal death.

Glucose and oxygen deprivation induces a Ca(2+)-mediated decrease in (Na(+)+K+)-ATPase activity in rat brain slices

Exposure of rat brain cortical slices to a medium lacking in glucose, oxygen or both glucose and oxygen, resulted in a decrease of the tissue ATP content and a reduction of (Na(+)+K+)-ATPase activity in membranes prepared from the slices. These treatments also inhibited partial reactions of (Na(+)+K+)-ATPase such as Na(+)-dependent phosphorylation and K(+)-stimulated phosphatase, as well as specific binding of [3H]ouabain in membranes prepared from the slices. Glucose deprivation and hypoxia decreased (Na(+)+K+)-ATPase activity in the absence of extracellular Ca2+, but the effects were blocked by 1,2-bis(2-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid tetra-acetomethyl ester (BAPTA-AM), a chelator of intracellular Ca2+. Metabolic inhibitors mimicked the effects of glucose deprivation and hypoxia. The effect of glucose-free hypoxia was dependent on extracellular Ca2+. It was blocked by Mg2+ at high concentration, bepridil or amiloride, but not by voltage-sensitive Ca2+ channel antagonists and glutamate receptor antagonists. None of the drugs tested here, except for dithiothreitol, affected the inhibitory effect of glucose-free hypoxia on the enzyme activity. In contrast to brain (Na(+)+K+)-ATPase, the kidney enzyme was insensitive to glucose and oxygen deprivation and metabolic inhibitors which depleted the tissue ATP.

Neurochemical correlates of selective neuronal loss following cerebral ischemia: role of decreased Na+,K(+)-ATPase activity

In order to investigate the role of Na+,K(+)-ATPase in the development of neuronal necrosis following cerebral ischemia, ischemia was induced in gerbils by occluding the common carotid artery unilaterally for 10 min. A time-course analysis revealed that significant reductions of the Na+,K(+)-ATPase activity in the cerebral cortex and hippocampus were manifested at 15 min, 30 min, and 1 h, and returned to the control level one day following recirculation. No apparent alterations of the Mg(2+)-ATPase activity, on the other hand, were obtained throughout the experimental period. Furthermore, Scatchard analyses of [3H]ouabain binding to the cerebral cortex membranes disclosed that the Bmax values invariably decreased without any change of Kd values following ischemia. It has also been shown that treatment of the animals with an agent known to mitigate ischemic neuronal necrosis, i.e. BY-1949, significantly reversed such derangements. These results suggest that the recovery of decreased Na+,K(+)-ATPase activity shortly after ischemia exerts a protective effect against ischemic brain damage.

Inhibition of sodium-potassium-ATPase: a potentially ubiquitous mechanism contributing to central nervous system neuropathology

Direct and indirect evidence suggests that Na+/K(+)-ATPase activity is reduced or insufficient to maintain ionic balances during and immediately after episodes of ischemia, hypoglycemia, epilepsy, and after administration of excitotoxins (glutamate agonists). Recent results show that inhibition of this enzyme results in neuronal death, and thus a hypothesis is proposed that a reduction and/or inhibition of this enzyme contributes to producing the central neuropathy found in the above disorders, and identifies potential mechanisms involved. While the extent of inhibition of Na+/K(+)-ATPase during ischemia, hypoglycemia and epilepsy may be insufficient to cause neuronal death by itself, unless the inhibition is severe and prolonged, there are a number of interactions which can lead to a potentiation of the neurotoxic actions of glutamate, a prime candidate for causing part of the damage following trauma. Presynaptically, inhibition of the Na+/K(+)-ATPase destroys the sodium gradient which drives the uptake of acidic amino acids and a number of other neurotransmitters. This results in both a block of reuptake and a stimulation of the release not only of glutamate but also of other neurotransmitters which modulate the neurotoxicity of glutamate. An exocytotic release of glutamate can also occur as inhibition of the enzyme causes depolarization of the membrane, but exocytosis is only possible when ATP levels are sufficiently high. Postsynaptically, the depolarization could alleviate the magnesium block of NMDA receptors, a major mechanism for glutamate-induced neurotoxicity, while massive depolarization results in seizure activity. With less severe inhibition, the retention of sodium results in osmotic swelling and possible cellular lysis. A build-up of intracellular calcium also occurs via voltage-gated calcium channels following depolarization and as a consequence of a failure of the sodium-calcium exchange system, maintained by the sodium gradient.

The neurotoxicity of ouabain, a sodium-potassium ATPase inhibitor, in the rat hippocampus

Intrahippocampal injection of 1 nmol ouabain, a sodium/potassium (Na+,K(+)-)ATPase inhibitor, produced a necrotic lesion within 4 days, characterised by a massive invasion by foaming macrophages. A lower dose of ouabain (0.1 nmol) produced a more discrete lesion of all groups of neuronal perikarya in the hippocampus, with only a minimal degree of glial infiltration. The neuronal perikaryal death produced in the subicular, CA1 and CA2 regions was only partially decreased by intraperitoneal injections of the anticonvulsants diazepam and MK-801; these drugs were without effect in the CA3 or hilar interneuronal regions. At neither dose of ouabain was there any indication of neuronal loss in brain regions outside the hippocampus, typically produced by prolonged seizure activity. It is suggested that ouabain has a two-fold action, a release of toxic acidic amino acids and a prolonged depolarization of neurons leading to osmolysis or calcium necrosis.