The brain is particularly vulnerable to ischemia. Complete interruption of blood flow to the brain for only 5 minutes triggers the death of vulnerable neurons in several brain regions, whereas 20-40 minutes of ischemia is required to kill cardiac myocytes or kidney cells. In part, the prominent vulnerability of brain tissue to ischemic damage reflects its high metabolic rate. Although the human brain represents only about 2.5% of body weight, it accounts for 25% of basal metabolism, a metabolic rate 3.5 times higher even than that of the brains of other primate species. In addition, central neurons have a nearexclusive dependence on glucose as an energy substrate, and brain stores of glucose or glycogen are limited. However, over the last 15 years, evidence has emerged indicating that energetics considerations and energy substrate limitations are not solely responsible for the brain's heightened vulnerability to ischemia. Rather, it appears that the brain's intrinsic cell-cell and intracellular signaling mechanisms, normally responsible for information processing, become harmful under ischemic conditions, hastening energy failure and enhancing the final pathways underlying ischemic cell death in all tissues, including free radical production, activation of catabolic enzymes, membrane failure, apoptosis, and inflammation. Since these common pathways are explored in other accompanying JCI Perspectives, we will emphasize the role of injury-enhancing signaling mechanisms specific to the central nervous system (CNS) and discuss potential therapeutic approaches to interrupting these mechanisms. Mechanisms of injury after ischemiaCerebral ischemia may be either transient and followed by reperfusion, or essentially permanent. A region of the brain may be affected, as occurs during an arterial or venous stroke, or the entire brain may become globally ischemic, as occurs during a cardiac arrest. In addition to such settings where ischemia is the primary insult, ischemia may also contribute secondarily to brain damage in the setting of mass lesions, hemorrhage, or trauma.Within seconds of cerebral ischemia, local cortical activity as detected by electroencephalography ceases; if the ischemia is global, unconsciousness rapidly ensues (witness the Stokes-Adams attack). This massive shutdown of neural activity is induced by K + efflux from neurons, mediated initially by the opening of voltage-dependent K + channels and later by ATPdependent K + channels, leading to transient plasma membrane hyperpolarization. A few minutes later, despite this energy sparing response, an abrupt and dramatic redistribution of ions occurs across the plasma membrane, associated with membrane depolarization (efflux of K + and influx of Na + , Cl -, and Ca 2+ ). This "anoxic depolarization" results in the excessive release of neurotransmitters, in particular, glutamate, promoting further spatial spread of cellular depolarization, depletion of energy stores, and advancement of injury cascades (see below). Neurotransmitter-induced toxicityGlutamate-i...

Murine cortical cultures containing both neurons and glia (days in vitro 13-15) were exposed to periods of oxygen-glucose deprivation (5-30 min) too brief to induce neuronal death. Cultures "preconditioned" by sublethal oxygen-glucose deprivation exhibited 30-50% less neuronal death than controls when exposed to a 45-55 min period of oxygen-glucose deprivation 24 hr later. This preconditioning-induced neuroprotection was specific in that neuronal death induced by exposure to excitotoxins or to staurosporine was not attenuated. Neuroprotection was lost if the time between the preconditioning and severe insult were decreased to 7 hr or increased to 72 hr and was blocked if the NMDA antagonist 100 microM 3-((D)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid was applied during the preconditioning insult. This was true even if the duration of preconditioning was increased as far as possible (while still remaining sublethal). A similar preconditioning effect was also produced by sublethal exposure to high K+, glutamate, or NMDA but not to kainate or trans-1-aminocyclopentane-1, 3-dicarboxylic acid.

Abstract:We studied the novel hypothesis that an upmodulation of channels for outward delayed rectifier K ϩ current (I K ) plays a key role in ceramide-induced neuronal apoptosis. Exposure for 6 -10 h to the membrane-permeable C 2 -ceramide (25 M) or to sphingomyelinase (0.2 unit/ml), but not to the inactive ceramide analogue C 2 -dihydroceramide (25 M), enhanced the whole-cell I K current without affecting the transient A-type K ϩ current and increased caspase activity, followed by neuronal apoptosis 24 h after exposure onset. Tetraethylammonium (TEA) or 4-chloro-N,N-diethyl-N-heptylbenzenebutanaminium tosylate (clofilium), at concentrations inhibiting I K , attenuated the C 2 -ceramide-induced caspase-3-like activation as well as neuronal apoptosis. Raising extracellular K ϩ to 25 mM similarly blocked the C 2 -ceramideinduced cell death; the neuroprotection by 25 mM K ϩ or TEA was not eliminated by blocking voltage-gated Ca 2ϩ channels. An inhibitor of tyrosine kinases, herbimycin A (10 nM) or lavendustin A (0.1-1 M), suppressed I K enhancement and/or apoptosis induced by C 2 -ceramide. It is suggested that ceramide-induced I K current enhancement is mediated by tyrosine phosphorylation and plays a critical role in neuronal apoptosis. Key Words: Outward delayed rectifier I K channel-Ceramide-Neuronal apoptosis-Tyrosine phosphorylation. J. Neurochem. 73, 933-941 (1999).Ceramide, a long-chain sphingolipid generated intracellularly on hydrolysis of sphingomyelin, has been implicated as a second messenger molecule involved in several forms of intracellular signaling, including those triggered by cytokines, growth factors, and stress (Obeid and

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Recent studies have suggested that rats subjected to transient global brain ischemia develop depressed expression of GluR-B in CA1 hippocampal neurons. The present study was performed to determine whether a similar change in AMPA receptor expression could be triggered in vitro by sublethal oxygen-glucose deprivation in rat hippocampal neuronal cultures. mRNA was extracted from individual hippocampal neurons via patch electrodes and amplified by RT-PCR 24-48 hr after sublethal oxygen-glucose deprivation. Compared with controls, insulted neurons expressed increased levels of GluR-D flop. As an indication that this change in receptor expression was functionally significant, insulted cultures exhibited increased AMPA- or kainate-induced 45Ca2+ accumulation sensitive to Joro spider toxin and increased vulnerability to kainate-induced death. These data support the hypothesis that exposure to ischemia may enhance subsequent hippocampal neuronal vulnerability to AMPA receptor-mediated excitotoxicity by modifying the relative expression of AMPA receptor subunits in a manner that promotes Ca2+ permeability.