Inhibition of protein synthesis during repetitive cortical spreading depression

Spreading depolarization induced by amygdala micro-injury prevents disruption of fear memory extinction in rats

Spreading depolarization (SD), a self-propagating wave of near-complete breakdown of the transmembrane ion gradients with water influx, regularly occurs in traumatized human brain. Here, we investigated long-term neurobehavioral consequences of injury-triggered SDs. Recently, we revealed that SD is reliably triggered by micro-injury of the amygdala, a key brain structure involved in fear processing. Using the classical Pavlovian fear conditioning paradigm, we investigated effects of the post-retrieval amygdala micro-injury and associated SD on fear memory in rats. We found that neither SD nor micro-injury alone affect fear response 24 h later but profoundly change it in subsequent extinction phase. If bilateral injury of the amygdala did not induce SD, fear extinction was severely impaired, while conditioned fear in rats with the identical amygdala injury triggering SD was successfully extinguished similarly to naïve rats. Our study provides first experimental evidence for involvement of injury-induced SD in extinction of traumatic fear memory.

Does shuttling of glycogen-derived lactate from astrocytes to neurons take place during neurotransmission and memory consolidation?

Glycogen levels in resting brain and its utilization rates during brain activation are high, but the functions fulfilled by glycogenolysis in living brain are poorly understood. Studies in cultured astrocytes have identified glycogen as the preferred fuel to provide ATP for Na⁺ ,K⁺ -ATPase for the uptake of extracellular K⁺ and for Ca²⁺ -ATPase to pump Ca²⁺ into the endoplasmic reticulum. Studies in astrocyte-neuron co-cultures led to the suggestion that glycogen-derived lactate is shuttled to neurons as oxidative fuel to support glutamatergic neurotransmission. Furthermore, both knockout of brain glycogen synthase and inhibition of glycogenolysis prior to a memory-evoking event impair memory consolidation, and shuttling of glycogen-derived lactate as neuronal fuel was postulated to be required for memory. However, lactate shuttling has not been measured in any of these studies, and procedures to inhibit glycogenolysis and neuronal lactate uptake are not specific. Testable alternative mechanisms to explain the observed findings are proposed: (i) disruption of K⁺ and Ca²⁺ homeostasis, (ii) release of gliotransmitters, (iii) imposition of an energy crisis on astrocytes and neurons by inhibition of mitochondrial pyruvate transport by compounds used to block neuronal monocarboxylic acid transporters, and (iv) inhibition of astrocytic filopodial movements that secondarily interfere with glutamate and K⁺ uptake from the synaptic cleft. Evidence that most pyruvate/lactate derived from glycogen is not oxidized and does not accumulate suggests predominant glycolytic metabolism of glycogen to support astrocytic energy demands. Sparing of blood-borne glucose for use by neurons is a reasonable explanation for the requirement for glycogenolysis in neurotransmission and memory processing.

Phosphoinositide 3-Kinase γ Restrains Neurotoxic Effects of Microglia After Focal Brain Ischemia

Phosphoinositide 3-kinase γ (PI3Kγ) is linked to neuroinflammation and phagocytosis. This study was conducted to elucidate conjectural differences of lipid kinase-dependent and kinase-independent functions of PI3Kγ in the evolvement of brain damage induced by focal cerebral ischemia/reperfusion. Therefore, PI3Kγ wild-type, knockout, and kinase-dead mice were subjected to middle cerebral artery occlusion followed by reperfusion. Tissue damage and cellular composition were assessed by immunohistochemical stainings. In addition, microglial cells derived from respective mouse genotypes were used for analysis of PI3Kγ effects on phagocytic activity, matrix metalloproteinase-9 release, and cAMP content under conditions of oxygen/glucose deprivation and recovery. Brain infarction was more pronounced in PI3Kγ-knockout mice compared to wild-type and kinase-dead mice 48 h after reperfusion. Immunohistochemical analyses revealed a reduced amount of galectin-3/MAC-2-positive microglial cells indicating that activated phagocytosis was reduced in ischemic brains of knockout mice. Cell culture studies disclosed enhanced metalloproteinase-9 secretion in supernatants derived from microglia of PI3Kγ-deficient mice after 2-h oxygen/glucose deprivation and 48-h recovery. Furthermore, PI3Kγ-deficient microglial cells showed a failed phagocytic activation throughout the observed recovery period. Lastly, PI3Kγ-deficient microglia exhibited strongly increased cAMP levels in comparison with wild-type microglia or cells expressing kinase-dead PI3Kγ after oxygen/glucose deprivation and recovery. Our data suggest PI3Kγ kinase activity-independent control of cAMP phosphodiesterase as a crucial mediator of microglial cAMP regulation, MMP-9 expression, and phagocytic activity following focal brain ischemia/recirculation. The suppressive effect of PI3Kγ on cAMP levels appears critical for the restriction of ischemia-induced immune cell functions and in turn tissue damage.

Continuous online microdialysis using microfluidic sensors: dynamic neurometabolic changes during spreading depolarization

Microfluidic glucose biosensors and potassium ion selective electrodes were used in an in vivo study to measure the neurochemical effects of spreading depolarizations (SD), which have been shown to be detrimental to the injured human brain. A microdialysis probe implanted in the cortex of rats was connected to a microfluidic PDMS chip containing the sensors. The dialysate was also analyzed using our gold standard, rapid sampling microdialysis (rsMD). The glucose biosensor performance was validated against rsMD with excellent results. The glucose biosensors successfully monitored concentration changes, in response to SD wave induction, in the range of 10-400 μM with a second time-resolution. The data show that during a SD wave, there is a time delay of 62 ± 24.8 s (n = 4) between the onset of the increase in potassium and the decrease in glucose. This delay can be for the first time demonstrated, thanks to the high-temporal resolution of the microfluidic sensors sampling from a single tissue site (the microdialysis probe), and it indicates that the decrease in glucose is due to the high demand of energy required for repolarization.

Induction of neurogenesis in the adult dentate gyrus by cortical spreading depression

BACKGROUND AND PURPOSE

Spreading depression (SD) is an epiphenomenon of neurological disorders, like stroke or traumatic brain injury. These diseases have been associated with an increased neurogenesis in the adult rodent dentate gyrus. Such proliferative activity can also be induced by conditions that--like SD--coincide with a disturbed neuronal excitability, eg, epilepsy. Thus we hypothesized that SD might likewise influence hippocampal neurogenesis and potentially act as mediator of injury-induced neurogenesis.

METHODS

Repetitive cortical SD were induced by epidural application of 3 mol/L KCl. At different time points thereafter dentate gyrus neurogenesis was investigated by means of intraperitoneal bromodeoxyuridine injections and immunocytochemistry. Spatial learning and memory was tested in a Morris water maze.

RESULTS

Cortical SD significantly increased proliferative activity in the ipsilateral subgranular zone on days 2 and 4. We detected about 280% more newborn cells in the dentate gyrus of rats that received bromodeoxyuridine during the first week after SD and were allowed to recover for 6 weeks. Most of these cells expressed the mature neuronal marker NeuN. The mitogenic action of SD was suppressed by systemic administration of the NMDA receptor antagonist MK-801. Behavioral performance of SD animals in the Morris water maze did not improve significantly.

CONCLUSIONS

From our data we postulate that the increased dentate gyrus neurogenesis observed after brain injury may at least partly be mediated by SD-like epiphenomena. Furthermore they indicate that even a strongly enhanced dentate gyrus neurogenesis may occur without significant improvements in hippocampus-dependent spatial learning and memory.

Temporal profile of connexin 43 expression after photothrombotic lesion in rat brain

Following focal ischemic injury, several mechanisms lead to secondary expansion of the affected area and therefore increase the initial damage. We thoroughly investigated the expression of astrocytic connexin 43 (Cx43) after photothrombosis in rat brain. The temporal profile of Cx43 mRNA as well as protein expression was studied in remote, structurally uninjured cortical and hippocampal areas. The hippocampal formation revealed an increased number of Cx43 mRNA positive astrocytes and an up-regulated protein expression exclusively in the ipsilateral stratum oriens. We assume a participation of this region in glia scar formation. While Cx43 mRNA positive cells were transiently increased, immunoreactivity was reduced in the somatosensory cortex of injured hemispheres. The observed decrease of Cx43 protein in the post-ischemic cerebral cortex implies an impairment of gap junctional intercellular communication which might be detrimental to the brain.

Microarray‐based long‐term detection of genes differentially expressed after cortical spreading depression

Spreading depression (SD) is a slowly propagating wave of neuronal depolarization altering ion homeostasis, blood flow and energy metabolism without causing irreversible damage of the tissue. As SD has been implicated in several neurological diseases including migraine and stroke, understanding these disorders requires systematic knowledge of the processes modified by SD. Thus, we induced repetitive SD in the rat cerebral cortex by topical application of 3 m KCl for approximately 2 h and evaluated the kinetics of SD-induced changes in cortical gene expression for up to 30 days using Affymetrix RAE230A arrays. The temporal profile showed a rapid expression of immediate early genes, genes associated with inflammation, metabolism, stress and DNA repair, ion transport, and genes that play a role in growth/differentiation. Stress-response genes could still be detected after 24 h. At this time, induced genes were mainly related to the cell membrane and adhesion, or to the cytoskeleton. A subset of genes was still affected even 30 days after SD. Real-time polymerase chain reactions and immunohistochemistry confirmed the microarray results for several of the transcripts. Our findings demonstrate a temporal pattern of gene expression which might promote tissue remodeling and cortical plasticity, and might probably account for the mediation of neuronal tolerance towards subsequent ischemia.

Bilateral induction of the S-100A9 gene in response to spreading depression is modulated by the cyclooxygenase-2 activity

Cyclooxygenase-2 (COX-2) was reported to be induced in the infarcted human brain. Spreading depression (SD) is thought to play a role in this induction. In this study, we correlated the expression of SD-associated genes with COX-2 production in brains after SD. Rats were divided into 3 groups: rats that did not undergo SD (group I saline controls, n=7), rats that underwent unilateral SD as a result of KCl application (group II, n=9), and rats that were pretreated with the selective COX-2 inhibitor, JTE-522 3 h before the induction of SD (group III, n=7). The expression of the SD-associated genes, S-100A9, and mitogen-activated proteinkinase phosphatase (cpg21) was analyzed 2 h later using a cDNA array. In group II, COX-2 and cpg21 mRNA expression, as determined by RT-PCR, were significantly upregulated in the hemisphere undergoing SD. While the expression of S-100A9 mRNA was bilaterally upregulated in these animals, this expression was significantly reduced in group III, and was accompanied by reduced bilateral production of PGE(2). Thus, the bilateral induction of expression of the S-100A9 gene in response to SD was associated with COX-2 activation.

Measurement of regional rates of cerebral protein synthesis with L-[1-11C]leucine and PET with correction for recycling of tissue amino acids: II. Validation in rhesus monkeys

The confounding effect of recycling of amino acids derived from tissue protein breakdown into the precursor pool for protein synthesis has been an obstacle to adapting in vivo methods for determination of regional rates of cerebral protein synthesis (rCPS) to positron emission tomography (PET). We used a kinetic modeling approach to estimate lambda, the fraction of the precursor pool for protein synthesis derived from arterial plasma, and to measure rCPS in three anesthetized adult monkeys dynamically scanned after a bolus injection of L-[1-11C]leucine. In the same animals, lambda was directly measured in a steady-state terminal experiment, and values showed excellent agreement with those estimated in the PET studies. In three additional monkeys rCPS was determined with the quantitative autoradiographic L-[1-14C]leucine method. In whole brain and cerebellum, rates of protein synthesis determined with the autoradiographic method were in excellent agreement with those determined with PET, and regional values were in good agreement when differences in spatial resolution of the two methods were taken into account. Low intrasubject variability was found on repeated PET studies. Our results in anesthetized monkey indicate that, by using a kinetic modeling approach to correct for recycling of tissue amino acids, quantitatively accurate and reproducible measurement of rCPS is possible with L-[1-11C]leucine and PET.

Spreading depression activates unfolded protein response

Preconditioning is a process where a preceding non-lethal form of stress activates a stress response that protects cells against an otherwise lethal form of stress. Preconditioning can be induced in various ways including short-term ischemia or spreading depression. Here we investigated the effect of 1 h repetitive spreading depression on the unfolded protein response (UPR), a stress response activated under conditions associated with endoplasmic reticulum (ER) dysfunction. Spreading depression induced processing of xbp1 mRNA, indicative of an activation of UPR. Processing of xbp1 was paralleled by a rise in grp78 mRNA levels resulting from an activation of a signal transduction pathway that depends on protein synthesis. Preconditioning-induced activation of UPR may preserve ER functioning under pathological conditions interfering with ER functions.

Regulation of GABA transporter mRNA and protein after photothrombotic infarct in rat brain

Animal models of focal ischemic infarcts reveal an impaired GABAergic (gamma-aminobutyric acid) neurotransmission. GABA, the main inhibitory neurotransmitter, is primarily taken up by specific sodium-dependent transporters. As these transporters play a crucial role in maintaining levels of GABA concentration, they may be functionally involved in ischemic processes. We investigated whether the mRNA and protein expression of GAT-1, the dominant neuronal GABA transporter, is altered after cortical infarct induced by photothrombosis in Wistar rats. In situ hybridization was performed to analyze GAT-1 mRNA-positive cells in cortical brain regions and the hippocampus. The lesion dramatically raised the number of GABA transporter mRNA-expressing cells in all investigated cortical regions. Double-labeling studies with a general neuronal marker and a marker for astrocytes revealed that cells expressing GAT-1 mRNA after photothrombosis are neurons. The mRNA expression pattern of all hippocampal subfields remained unchanged. In contrast, cortical GAT-1 protein density was only slightly affected and surprisingly in the opposite way. In the primary and secondary somatosensory cortex, density values were significantly reduced. Immunoreactivity was not altered in all investigated hippocampal areas. We found a marked discordance between the increased number of cells expressing GAT-1 mRNA in the cortex and the reduced tissue GAT-1 protein content. Focal brain ischemia obviously triggers mechanisms that interfere with GAT-1 transcriptional regulation and protein synthesis or turnover.

Astrocytes and stroke: networking for survival?

Astrocytes are now known to be involved in the most integrated functions of the central nervous system. These functions are not only necessary for the normally working brain but are also critically involved in many pathological conditions, including stroke. Astrocytes may contribute to damage by propagating spreading depression or by sending proapoptotic signals to otherwise healthy tissue via gap junction channels. Astrocytes may also inhibit regeneration by participating in formation of the glial scar. On the other hand, astrocytes are important in neuronal antioxidant defense and secrete growth factors, which probably provide neuroprotection in the acute phase, as well as promoting neurogenesis and regeneration in the chronic phase after injury. A detailed understanding of the astrocytic response, as well as the timing and location of the changes, is necessary to develop effective treatment strategies for stroke patients.

Depolarization induces insulin-like growth factor binding protein-2 expression in vivo via NMDA receptor stimulation

The effect of depolarization and N-methyl-D-aspartate (NMDA) receptor blockade on insulin-like growth factor-I (IGF-I), IGF binding protein-2 (IGFBP-2) and IGFBP-4 expression was analysed in vivo. Depolarization was induced in adult rat brains by applying 3 M KCl to the exposed cortex for 10 min. A subgroup of animals also received daily injections of MK-801. Four days after KCl exposure, the brains were analysed by in situ hybridization, immunohistochemistry and TUNEL. A significant upregulation of IGFBP-2 mRNA and protein was detected in astrocytes after KCl exposure This upregulation was reduced by MK-801 treatment. No alterations in IGF-I or IGFBP-4 mRNA levels were noted. We did not detect TUNEL positive cells, morphological signs of necrosis or apoptosis, or neuronal loss in the depolarized zone. Taken together, these findings indicate that upregulation of IGFBP-2 by depolarization is mediated by NMDA receptors, and, as no neuronal damage was detected, astrocytic NMDA receptors may be responsible for this upregulation.

Adenovirus-mediated GDNF and CNTF pretreatment protects against striatal injury following transient middle cerebral artery occlusion in mice

During the last few years, adenoviral gene transfer techniques have achieved increasing interest in the treatment of neurodegenerative diseases. However, gene therapy requires that delivered genes are translated into proteins. This may pose a problem in focal ischemia where protein synthesis is compromized. The present study was conducted to find out the feasibility of adenoviral GDNF and CNTF delivery in transient focal ischemia, as induced by 30 min of intraluminar middle cerebral artery (MCA) occlusion in mice. Injections of vehicle, of an adenoviral vector deleted in the E1 region (Ad-dE1) and of vectors expressing the GDNF (Ad-GDNF), CNTF (Ad-CNTF), or GFP (Ad-EGFP) gene from a CMV promoter were stereotactically placed in the dorsolateral striatum, i.e., the core of the MCA territory, and focal ischemia was induced seven days later. Thread occlusion resulted in disseminated injury of the striatum, but not the overlying cortex. The number of viable neurons was significantly increased after 1 and 3 days of reperfusion both in Ad-GDNF and Ad-CNTF as compared with vehicle or Ad-dE1-treated animals, whereas the number of injured cells was significantly reduced, as shown by cresyl violet staining, terminal transferase biotinylated-dUTP nick end-labeling (TUNEL), and immunocytochemistry for activated caspase-3. Interestingly, the protective effects of Ad-GDNF were similarly strong in areas of the striatum adjacent and remote of the adenoviral infusion site, while Ad-CNTF showed pronounced rescue effects in the surrounding, but rather little effects distant to the infusion. The present study demonstrates that adenoviral delivery of neurotrophic factors may be a useful tool for the treatment of focal ischemia.

Spreading depression induces expression of calcium-independent protein kinase C subspecies in ischaemia-sensitive cortical layers: regulation by N-methyl-D-aspartate receptors and glucocorticoids

Spreading depression is a wave of sustained depolarization challenging the energy metabolism of the cells without causing irreversible damage. In the ischaemic brain, sreading depression-like depolarization contributes to the evolution of ischaemia to infarction. The depolarization is propagated by activation of N-methyl-D-aspartate receptors, but changes in signal transduction downstream of the receptors are not known. Because protein phosphorylation is a general mechanism whereby most cellular processes are regulated, and inhibition of N-methyl-D-aspartate receptors or protein kinase C is neuroprotective, the expression of protein kinase C subspecies in spreading depression was examined. Cortical treatment with KCl induced an upregulation of protein kinase Cdelta and zeta messenger RNA at 4 and 8 h, whereas protein kinase Calpha, beta, gamma and epsilon did not show significant changes. The gene induction was the strongest in layers 2 and 3, and was followed by an increased number of protein kinase Cdelta-immunoreactive neurons. Protein kinase Cdelta and zeta inductions were inhibited by pretreatment with an N-methyl-D-aspartate receptor antagonist, dizocilpine maleate, which also blocked spreading depression propagation, and with dexamethasone, which acted without blocking the propagation. Quinacrine, a phospholipase A2 inhibitor, reduced only protein kinase C5 induction. In addition, N(G)(-nitro-L-arginine methyl ester, a nitric oxide synthase inhibitor, did not influence protein kinase Cdelta or zeta induction, whereas 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione, an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate/kainate receptor antagonist, and the cyclo-oxygenase inhibitors indomethacin and diclophenac tended to increase gene expression. The data show that cortical spreading depression induces Ca2(+)-independent protein kinase C subspecies delta and zeta, but not Ca(2+)-dependent subspecies, through activation of N-methyl-D-aspartate receptors and phospholipase A2. Even though the signal pathway is similar to the induction described previously in ischaemia for genes implicated in delayed neuronal death, the gene inductions observed here are not necessarily pathogenetic, but may represent a general reaction to metabolic stress.

Cortical spreading depression increases protein synthesis and upregulates basic fibroblast growth factor

Protective effects of cortical spreading depression (CSD) against ischemic damage have been demonstrated in cortex when elicited at either 24 h or 3 days prior to ischemia. The present study was carried out to investigate possible mechanisms of neuroprotection following CSD. In Sprague-Dawley rats, 5 M KCl, 5 M NaCl, or physiological saline was applied to the cortex for 1 h. Repetitive CSD waves were elicited only in the KCl group. Measurements of cerebral glucose utilization demonstrated a marked reduction in affected cortex and subcortical regions in both the NaCl and the KCl groups, whereas cortical and hippocampal protein synthesis was discretely increased only in the KCl group. Immunohistochemistry of GFAP demonstrated a rapid activation in reactive astrocytes at 3 days in the KCl group whereas only a discrete activation was observed in the NaCl group. Similar changes were observed for basic fibroblast growth factor. Our results suggest that CSD-induced ischemic tolerance is not due to a reduction in energy metabolism but rather is associated with an upregulation of trophic factors and glial cell activation which might provide a mechanism for a long-lasting neuroprotection.

Lesion-remote metabolic changes after neocortical cold injury in rats

Lesion-remote metabolic changes were examined 1-7 days after neocortical cold injury using tissue ATP, glucose and lactate bioluminescent imaging, pH-dependent fluoroscopy and cerebral protein synthesis (CPS) autoradiography. One day after lesioning an alkaline pH shift (0.35 +/- 0.19 units above contralateral) was noticed in the lesion-remote cortex, the underlying white matter, the striatum, hippocampus and thalamus, which slowly resolved within 7 days and probably reflected the spread of vasogenic edema. Closely associated with the pH shift, elevations in tissue glucose and lactate levels were found, which reached maximum levels after 3 days (7.4 +/- 2.4 vs 4.2 +/- 1.2 micromol/g glucose, 6.6 +/- 2.3 vs 2.1 +/- 0.6 micromol/g lactate) but, in contrast to the alkalosis, remained elevated after 1 week. Thus, neocortical trauma is associated with long-lasting metabolic changes, which are intimately linked with the distribution of post-traumatic alkalosis.

Spreading depression-induced gene expression is regulated by plasma glucose

BACKGROUND AND PURPOSE

Plasma glucose and spreading depression (SD) are both determinants of brain ischemia. The purpose of this study was to examine whether plasma glucose affects SD-induced gene expression in the cortex.

METHODS

SD was induced by topical application of KCl. Hyperglycemia and hypoglycemia were induced by intraperitoneal injection of glucose and insulin, respectively. The expression of c-fos, cyclooxygenase-2 (COX-2), protein kinase C-delta (PKCdelta), and heme oxygenase-1 (HO-1) was determined by in situ hybridization.

RESULTS

SD alone induced expression of c-fos (by 340%), COX-2 (210%), HO-1 (470%), and PKCdelta (410%). Hypoglycemia (2.4+/-0.9 mmol/L) alone did not induce gene expression, and hyperglycemia (22.1+/-3.7 mmol/L) alone induced only c-fos by 42%. When hypoglycemia was induced 30 minutes before SD, c-fos induction was enhanced by 145%, but the induction of HO-1 and PKCdelta was reduced to 43% and 64%, respectively. When hyperglycemia was induced 30 minutes before SD, c-fos induction was enhanced by 388% and COX-2 expression by 53%, whereas the induction of PKCdelta and HO-1 was reduced to 54% and 51%, respectively. The frequency, amplitude, and duration of direct current potentials were unaltered in hyperglycemic SD animals, whereas in hypoglycemic animals the duration was increased by 47%.

CONCLUSIONS

While SD induces expression of several genes, the availability of glucose regulates the extent of the gene induction. The effect of glucose is different on early-response genes (c-fos and COX-2) compared with late-response genes. Plasma glucose may contribute to neuronal damage partially by regulating gene expression.

Cortical spreading depression activates trophic factor expression in neurons and astrocytes and protects against subsequent focal brain ischemia

We recently reported that cortical spreading depression (CSD), used to precondition rat brain, reduced cortical infarction volume resulting from focal cerebral ischemia by middle cerebral artery occlusion (MCAO) 3 days later. The mechanisms underlying this protective effect by CSD remains to be explored. In this study, we confirm that CSD is neuroprotective when KCl is applied epidurally rather than intracortically. Neocortical infarct volume was 101.3+/-48.5 mm3 and 45.3+/-44.1 mm3 in the sham and CSD group, respectively (p<0.05). Using image analysis, we identified the cortical region spared from infarction by the prior CSD. We then determined the distribution of brain-derived neurotrophic factor (BDNF) and basic fibroblast growth factor (bFGF) mRNA and the time course of their expression in groups of animals treated with CSD and their controls. We also examined the response of astrocytes to CSD using glial fibrillary acidic protein (GFAP) as a marker. In situ hybridization (done at 0, 3, 12, 24, 72 or 168 h after CSD) showed significant elevation of BDNF mRNA in the cortex immediately after CSD in a distribution surrounding the spared cortex, while bFGF mRNA rose 12 h after CSD and appeared more within the core of the ischemic region. Immunohistochemistry (done at 1, 3 or 7 days after CSD) demonstrated GFAP in the neocortex, with a peak at 3 days after CSD. Heat shock protein 72 (HSP72) expression was not affected by CSD. We concluded that upregulation of trophic factors and activation of glial cells may contribute to the neuroprotection induced by CSD.

Effects of a traumatic neocortical lesion on cerebral metabolism and gene expression of rats

The effects of a traumatic neocortical lesion, induced by transcranial cold injury, on brain metabolism and gene expression were examined. The surrounding of the lesions was characterized by increased glucose and lactate levels without major disturbances of protein synthesis or energy state. A transient pH decrease by 0.4 units was noticed 1 h post-injury, which shifted towards alkaline values by 3 h. The metabolic disturbances did not differ between injured animals with spontaneous spreading depressions (SD, n = 14) and those without SD (n = 7). In SD animals, c-fos mRNA was strongly elevated in the injury-remote cortex, but hsp72 mRNA was not enhanced. Thus, in contrast to focal ischemia, the metabolic dysfunction around traumatic cortical lesions is not aggravated by SD.

Effects of a traumatic neocortical lesion on cerebral metabolism and gene expression of rats

The effects of a traumatic neocortical lesions, induced by transcranial cold injury, on brain metabolism and gene expression were examined. The surrounding of the lesions was characterized by increased glucose and lactate levels without major disturbances of protein synthesis or energy state. A transient pH decrease by 0.4 units was noticed 1 h post-injury, which shifted towards alkaline values by 3 h. The metabolic disturbances did not differ between injured animals with spontaneous spreading depressions (SD, n = 14) and those without SD (n = 7). In SD animals, c-fos mRNA was strongly elevated in the injury-remote cortex, but hsp72 mRNA was not enhanced. Thus, in contrast to focal ischemia, the metabolic dysfunction around traumatic cortical lesions is not aggravated by SD.

Quantitative measurement of cerebral protein synthesis in vivo: theory and methodological considerations

The true rate of cerebral protein synthesis can be calculated from the ratio of labeled proteins to integrated arterial plasma amino acid specific activity (SA) only when the fraction of amino acid precursor pool dilution is known. In the following, current experimental designs on the measurement of cerebral protein synthesis are discussed and compared to our own approach in which the determination of regional precursor pool dilution by recycled unlabeled leucine is combined with the quantitation of regional cerebral protein synthesis rates. For this purpose, a constant arterial plasma leucine SA level is maintained for 45 min by programmed intravenous infusion which is sufficient for complete equilibrium between tissue leucine pool SAs and plasma free leucine SA. In addition to the regional assessment of the precursor dilution factor, protein radioactivity can be determined in the same tissue sample or in parallel brain sections of the same animal by quantitative autoradiography. It is then possible to calculate the actual rate of protein synthesis using the correct fraction of precursor pool dilution. This renders our approach particularly suitable for the quantitative measurement of regional CPS under pathological conditions.

Cortical spreading depression induces long-term alterations of BDNF levels in cortex and hippocampus distinct from lesion effects: implications for ischemic tolerance

Cortical spreading depression (CSD) protects hippocampal and cortical neurons from an otherwise lethal ischemic insult delivered days later. The present study was undertaken to evaluate changes in the expression of BDNF following CSD, distinct from lesion effects and its possible involvement in delayed ischemic tolerance. CSD was elicited by KCl application and a cortical lesion was made by hyperosmolar NaCl application. BDNF mRNA was examined by in situ hybridization and Northern blot up to 7 days post-CSD. BDNF protein content was measured by ELISA. In the cortex, BDNF protein was mildly elevated despite minimal increases of mRNA in the NaCl lesion group. CSD specifically up-regulated BDNF mRNA at 4 h, followed by a delayed secondary increase at 2-3 days. BDNF protein exhibited smaller biphasic increases at 24 h and 3-7 days post-CSD which were significantly higher than the NaCl lesion group. In the hippocampus, BDNF protein levels showed a delayed decrease in both groups independent of mRNA changes, but CSD specifically delayed this decrease. Thus, CSD can alter BDNF levels independent of lesion effects. The increased BDNF following CSD in the cortex is consistent with the involvement of BDNF in cortical ischemic tolerance. BDNF could not, however, be directly related to ischemic tolerance in the hippocampus.

Relationship between extracellular glutamate concentration and voltage-sensitive calcium channel function in focal cerebral ischemia in the rat

Ischemic cell death occurs when extracellular glutamate levels increase, causing tissue depolarization and an excessive rise in intracellular calcium concentrations. The relative occurrence of the depolarization events and the changes in glutamate concentration in ischemia have not been studied. In a model of focal cerebral ischemia in the rat, three measurements were made simultaneously in vivo: cerebral blood flow (CBF) by the H2-clearance method, extracellular glutamate concentration by microdialysis, and activation of the voltage-sensitive calcium channel (VSCC) by its binding to [3H]nimodipine. Effects of probe implantation on these measurements were accounted for. The CBF to control ratio obtained during the experiments spanned the range of 1.08 to 0.07. Binding to [3H]nimodipine became significantly activated when CBF fell to approximately 0.49 of its control value while extracellular glutamate concentrations increased significantly only at a CBF ratio of < 0.33. Activation of the VSCC at this high CBF ratio may be due to ischemic depolarization, which has been shown to activate the binding to [3H]nimodipine. It may be useful to define a CBF threshold of 50% of normal in focal ischemia for opening of the VSCC. The same threshold has been linked to an overall depression of protein synthesis and to activation of a number of molecular responses.

Induction of spreading depression in the ischemic hemisphere following experimental middle cerebral artery occlusion: effect on infarct morphology

This study was undertaken to test whether transient depolarizations occurring in periinfarct regions are important in contributing to infarct spread and maturation. Following middle cerebral artery (MCA) occlusion we stimulated the ischemic penumbra with recurrent waves of spreading depression (SD) and correlated the histopathological changes with the electrophysiological recordings. Halothane-anesthetized, artificially ventilated Sprague-Dawley rats underwent repetitive stimulation of SD in intact brain (Group 1; n = 8) or photothrombotic MCA occlusion coupled with ipsilateral common carotid artery occlusion (Groups 2 and 3, n = 9 each). The electroencephalogram and direct current (DC) potential were recorded for 3 h in the parietal cortex, which represented the periinffarct border zone in ischemic rats. In Group 2, only spontaneously occurring negative DC shifts occurred; in Group 3, the (nonischemic) frontal pole of the ischemic hemisphere was electrically stimulated to increase the frequency of periinfarct DC shifts. Animals underwent perfusion-fixation 24 h later, and volumes of complete infarction and scattered neuronal injury ("incomplete infarction") were assessed on stained coronal sections by quantitative planimetry. Electrical induction of SD in Group 1 did not cause morphological injury. During the initial 3 h following MCA occlusion, the number of spontaneous periinfarct depolarization in Group 2 (7.0 +/- 1.5 DC shifts) was doubled in Group 3 by frontal current application (13.4 +/- 2.7 DC shifts; p < 0.001). The duration as well as the integrated negative amplitude of DC shifts over time were significantly greater in Group 3 than in Group 2 rats (duration, 5.7 +/- 3.8 vs. 4.1 +/- 2.5 min; p < 0.05). Histopathological examination disclosed well-defined areas of pannecrosis surrounded by a cortical rim exhibiting selectively damaged acidophilic neurons and astrocytic swelling in otherwise normal-appearing brain. Induction of SD in the ischemic hemisphere led to a significant increase in the volume of incomplete infarction (19.0 +/- 6.1 mm3 in Group 3 vs. 10.3 +/- 5.1 mm3 in Group 2; p < 0.01) and of total ischemic injury (100.7 +/- 41.0 mm3 in Group 3 vs. 66.5 +/- 24.7 mm3 in Group 2; p < 0.05). The integrated magnitude of DC negativity per experiment correlated significantly with the volume of total ischemic injury (r = 0.780, p < 0.0001). Thus, induction of SD in the ischemic hemisphere accentuated the development of scattered neuronal injury and increased the volume of total ischemic injury. This observation may be explained by the fact that with limited perfusion reserve, periinfarct depolarization are associated with episodic energy failure in the acute ischemic penumbra.

Differential effect of NMDA and AMPA receptor blockade on protein synthesis in the rat infarct borderzone

We investigated whether the known neuroprotective effects of two selective glutamate receptor antagonists, the NMDA antagonist MK-801 and the AMPA antagonist NBQX, are reflected in the regional cerebral protein synthesis rates (CPSR) in rats with middle cerebral artery occlusion (MCAO). Rats treated with either saline, MK-801 (5 mg/kg i.p.) or NBQX (30 mg/kg i.p. x 3) were subjected to permanent MCAO. Regional CPSR and volumes of gray matter structures displaying normal CPSR were measured in coronal cryosections of the brain by quantitative autoradiography following an i.v. bolus injection of 35S-labelled L-methionine 2 h after occlusion. MCAO completely inhibited protein synthesis in the lateral part of striatum and part of the adjacent frontoparietal cortex corresponding to the ischemic focus. Surrounding this, a metabolic penumbra with approximately 50% reductions in CPSR was present. Treatment with MK-801 significantly increased the volume of tissue with normal CPSR in the ischemic hemisphere compared to controls, whereas this was not seen with NBQX treatment. The results suggest that MK-801 and NBQX have different effects on peri-infarct protein synthesis after MCAO. Since both compounds reduce infarct size, it is questionable that acute inhibition of protein synthesis in focal ischemia is of significant importance to the final outcome of a stroke lesion.

Glutamate-mediated injury in focal cerebral ischemia: the excitotoxin hypothesis revised

Neuronal injury following focal cerebral ischemia is widely attributed to the excitotoxic effects of glutamate. However, critical analysis of published data on glutamate toxicity in vitro and the comparison of these data with in vivo release of glutamate and the therapeutic effect of glutamate antagonists raises doubts about a neurotoxic mechanism. An alternative explanation for glutamate-mediated injury is hypoxia due to peri-infarct spreading depression-like depolarizations. These depolarizations are triggered in the core of the ischemic infarct and spread at irregular intervals into the peri-infarct surrounding. In ischemically uncompromised tissue, the metabolic workload associated with spreading depression is coupled to an increase in blood flow and oxygen supply, assuring maintenance of oxidative respiration. In the penumbra region of focal ischemia, the hemodynamic constraints of collateral blood circulation prevail the adequate adjustment of oxygen delivery, leading to transient episodes of relative tissue hypoxia. The hypoxic episodes cause a suppression of protein synthesis, a gradual deterioration of energy metabolism and a progression of irreversibly damaged tissue into the penumbra zone. The generation of peri-infarct spreading depressions and the associated metabolic workload can be suppressed by NMDA and non-NMDA antagonists. As a result, the penumbral inhibition of protein synthesis and the progressing energy failure is also prevented, and the volume of ischemic infarct decreases. Interventions to improve ischemic resistance should therefore aim at improving the oxygen supply or reducing the metabolic workload in the penumbra region.

MK-801, a glutamate antagonist, lowers flow threshold for inhibition of protein synthesis after middle cerebral artery occlusion of rat

The effect of the glutamate antagonist MK-801 on the ischemic threshold of energy metabolism and protein synthesis (CPS) was studied in rats submitted to 3 h occlusion of the left middle cerebral artery (MCA). Local blood flow and CPS were measured by double tracer autoradiography, and local ATP content by bioluminescence imaging. In untreated animals breakdown of energy metabolism occurred at flow values below 15 +/- 1 and CPS inhibition below 51 +/- 15 ml/100 g/min (means +/- S.D.). MK-801 treatment (3 mg/kg immediately after MCA occlusion) did not change the ischemic flow threshold of energy failure (16 +/- 3 ml/100 g/min) but lead to a highly significant decline of the perfusion threshold for the inhibition of CPS to 19 +/- 4 ml/100 g/min (P < 0.01). Our data demonstrate that MK-801 dramatically reduces the threshold for the suppression of protein synthesis which could explain previously reported therapeutical effects on the reduction of brain infarct size.