Relationship between the neuroprotective effect of Na+/H+ exchanger inhibitor SM-20220 and the timing of its administration in a transient middle cerebral artery occlusion model of rats

Disruption of ion homeostasis in the neurogliovascular unit underlies the pathogenesis of ischemic cerebral edema

Cerebral edema is a major cause of morbidity and mortality following ischemic stroke, but its underlying molecular pathophysiology is incompletely understood. Recent data have revealed the importance of ion flux via channels and transporters expressed in the neurogliovascular unit in the development of ischemia-triggered cytotoxic edema, vasogenic edema, and hemorrhagic conversion. Disruption of homeostatic mechanisms governing cell volume regulation and epithelial/endothelial ion transport due to ischemia-associated energy failure results in the thermodynamically driven re-equilibration of solutes and water across the CSF-blood and blood-brain barriers that ultimately increases the brain's extravascular volume. Additionally, hypoxia, inflammation, and other stress-triggered increases in the functional expression of ion channels and transporters normally expressed at low levels in the neurogliovascular unit cause disruptions in ion homeostasis that contribute to ischemic cerebral edema. Here, we review the pathophysiological significance of several molecular mediators of ion transport expressed in the neurogliovascular unit, including targets of existing FDA-approved drugs, which might be potential nodes for therapeutic intervention.

Intravenous HOE-642 reduces brain edema and Na uptake in the rat permanent middle cerebral artery occlusion model of stroke: evidence for participation of the blood-brain barrier Na/H exchanger

Cerebral edema forms in the early hours of ischemic stroke by processes involving increased transport of Na and Cl from blood into brain across an intact blood-brain barrier (BBB). Our previous studies provided evidence that the BBB Na-K-Cl cotransporter is stimulated by the ischemic factors hypoxia, aglycemia, and arginine vasopressin (AVP), and that inhibition of the cotransporter by intravenous bumetanide greatly reduces edema and infarct in rats subjected to permanent middle cerebral artery occlusion (pMCAO). More recently, we showed that BBB Na/H exchanger activity is also stimulated by hypoxia, aglycemia, and AVP. The present study was conducted to further investigate the possibility that a BBB Na/H exchanger also participates in edema formation during ischemic stroke. Sprague-Dawley rats were subjected to pMCAO and then brain edema and Na content assessed by magnetic resonance imaging diffusion-weighed imaging and magnetic resonance spectroscopy Na spectroscopy, respectively, for up to 210 minutes. We found that intravenous administration of the specific Na/H exchange inhibitor HOE-642 significantly decreased brain Na uptake and reduced cerebral edema, brain swelling, and infarct volume. These findings support the hypothesis that edema formation and brain Na uptake during the early hours of cerebral ischemia involve BBB Na/H exchanger activity as well as Na-K-Cl cotransporter activity.

Sodium influx pathways during and after anoxia in rat hippocampal neurons

Mechanisms that contribute to Na+ influx during and immediately after 5 min anoxia were investigated in cultured rat hippocampal neurons loaded with the Na+-sensitive fluorophore sodium-binding benzofuran isophthalate. During anoxia, an influx of Na+ in the face of reduced Na+,K+-ATPase activity caused a rise in [Na+]i. After the return to normoxia, Na+,K+-ATPase activity mediated the recovery of [Na+]i despite continued Na+ entry. Sodium influx during and after anoxia occurred through multiple pathways and increased the longer neurons were maintained in culture. Under the experimental conditions used, Na+ entry during anoxia did not reflect the activation of ionotropic glutamate receptors, TTX- or lidocaine-sensitive Na+ channels, plasmalemmal Na+/Ca2+ exchange, Na+/H+ exchange, or HCO3--dependent mechanisms; rather, contributions were received from a Gd3+-sensitive pathway activated by reactive oxygen species and Na+/K+/2Cl- cotransport in neurons maintained for 6-10 and 11-14 d in vitro (DIV), respectively. Sodium entry immediately after anoxia was not attributable to the activation of ionotropic glutamate receptors, voltage-activated Na+ channels, or Na+/K+/2Cl- cotransport; rather, it occurred via Na+/Ca2+ exchange, Na+/H+ exchange, and a Gd3+-sensitive pathway similar to that observed during anoxia; 11-14 DIV neurons received an additional contribution from an -dependent mechanism(s). The results provide insight into the intrinsic mechanisms that contribute to disturbed internal Na+ homeostasis during and immediately after anoxia in rat hippocampal neurons and, in this way, may play a role in the pathogenesis of anoxic or ischemic cell injury.

Astrocytic contributions to bioenergetics of cerebral ischemia

Astrocytes are multifunctional cells that interact with neurons and other astrocytes in signaling and metabolic functions, and their resistance to pathophysiological conditions can help restrict loss of tissue after an ischemic event provided adequate nutrients are supplied to support their requirements. Astrocytes have substantial oxidative capacity and mechanisms to upregulate glycolytic capability when respiration is impaired. An astrocytic enzyme that synthesizes a powerful activator of glycolysis is not present in neurons, endowing astrocytes with the ability to sustain ATP production under restrictive conditions. The monocarboxylic acid transporter (MCT) isoforms predominating in astrocytes are optimized to facilitate very large increases in lactate flux as lactate concentration increases within (1-3 mM) and above (>3 mM) the normal range. In sharp contrast, the major neuronal MCT serves as a barrier to increased transmembrane transport as lactate rises above 1 mM, restricting both entry and efflux. Lactate can serve as fuel during recovery from ischemia but direct evidence that lactate is oxidized by neurons (vs. astrocytes) to maintain synaptic function is lacking. Astrocytes have critical roles in regulation of ionic homeostasis and control of extracellular glutamate levels, and spreading depression associated with ischemia places high demands on energy supplies in astrocytes and contributes to metabolic exhaustion and demise. Disruption of Ca2+ homeostasis, generation of oxygen free radicals and nitric oxide, and mitochondrial depolarization contribute to astrocyte death during and after a metabolic insult. Novel pharmaceutical agents targeted to astrocytes and hyperoxic therapy that restores penumbral oxygen level during energy failure might improve postischemic outcome.

Neuroprotective efficacy of repinotan HCl, a 5-HT1A receptor agonist, in animal models of stroke and traumatic brain injury

Repinotan is a highly potent 5-HT1A receptor agonist with strong neuroprotective efficacy in animal models of middle cerebral artery occlusion and traumatic brain injury. In this study, we characterized the time window for neuroprotective effects of repinotan in animal models. In the permanent middle cerebral artery occlusion model, repinotan showed neuroprotective efficacy when administered as a triple bolus injection (0.3-100 microg/kg) or an intravenous infusion (0.3-100 microg/kg per hour). A 73% reduction in infarct volume was observed with a 3 microg/kg intravenous bolus, and a 65% reduction was observed with a 3 and 10 microg/kg per hour intravenous infusion. When delayed until 5 hours after occlusion, repinotan (10 microg/kg per hour) reduced infarct volume by 43%. In the transient middle cerebral artery occlusion model, repinotan (10 microg/kg per hour) administered immediately after occlusion reduced infarct volume by 97%, and a delay to 5 hours reduced infarct volume by 81%. In the acute subdural hematoma model, repinotan (3 and 10 microg/kg per hour) reduced infarct volume by 65%. In this model, repinotan (3 microg/kg per hour) administered 5 hours after occlusion reduced infarct volume by 54%. The favorable neuroprotective efficacy, broad dose-response curve, and prolonged therapeutic window observed in all models strongly suggest that repinotan is a promising candidate for treating acute ischemic stroke in humans.