2-Amino-5-phosphonovalerate attenuates the severe hypoglycemia-induced loss of perforant path-evoked field potentials in the rat hippocampus

Glutamate oxaloacetate transaminase enables anaplerotic refilling of TCA cycle intermediates in stroke-affected brain

Ischemic stroke results in excessive release of glutamate, which contributes to neuronal cell death. Here, we test the hypothesis that otherwise neurotoxic glutamate can be productively metabolized by glutamate oxaloacetate transaminase (GOT) to maintain cellular energetics and protect the brain from ischemic stroke injury. The GOT-dependent metabolism of glutamate was studied in primary neural cells and in stroke-affected C57-BL6 mice using magnetic resonance spectroscopy and GC-MS. Extracellular Glu sustained cell viability under hypoglycemic conditions and increased GOT-mediated metabolism in vitro Correction of stroke-induced hypoxia using supplemental oxygen in vivo lowered Glu levels as measured by ¹H magnetic resonance spectroscopy. GOT knockdown abrogated this effect and caused ATP loss in the stroke-affected brain. GOT overexpression increased anaplerotic refilling of tricarboxylic acid cycle intermediates in mouse brain during ischemic stroke. Furthermore, GOT overexpression not only reduced ischemic stroke lesion volume but also attenuated neurodegeneration and improved poststroke sensorimotor function. Taken together, our results support a new paradigm that GOT enables metabolism of otherwise neurotoxic extracellular Glu through a truncated tricarboxylic acid cycle under hypoglycemic conditions.-Rink, C., Gnyawali, S., Stewart, R., Teplitsky, S., Harris, H., Roy, S., Sen, C. K., Khanna, S. Glutamate oxaloacetate transaminase enables anaplerotic refilling of TCA cycle intermediates in stroke-affected brain.

Glucose deprivation induces reticulum stress by the PERK pathway and caspase-7- and calpain-mediated caspase-12 activation

Glucose is the main energy source in brain and it is critical for correct brain functioning. Type 1 diabetic patients might suffer from severe hypoglycemia if exceeding insulin administration, which can lead to acute brain injury if not opportunely corrected. The mechanisms leading to hypoglycemic brain damage are not completely understood and the role of endoplasmic reticulum (ER) stress has not been studied. ER stress resulting from the accumulation of unfolded or misfolded proteins in the ER is counteracted by the unfolded protein response (UPR). When the UPR is sustained, apoptotic death might take place. We have examined UPR activation during glucose deprivation (GD) in hippocampal cultured neurons and its role in the induction of apoptosis. Activation of the PERK pathway of the UPR was observed, as increased phosphorylation of eIF2α and elevated levels of the transcription factor ATF4, occurred 30 min after GD and the levels of the chaperone protein, GRP78 and the transcription factor CHOP, increased after 2 h of GD. In addition, we observed an early activation of caspase-7 and 12 during GD, while caspase-3 activity increased only transiently during glucose reintroduction. Inhibition of caspase-3/7 and the calcium-dependent protease, calpain, significantly decreased caspase-12 activity. The ER stress inhibitor, salubrinal prevented neuronal death and caspase-12 activity. Results suggest that the PERK pathway of the UPR is involved in GD-induced apoptotic neuronal death through the activation of caspase-12, rather than the mitochondrial-dependent caspase pathway. In addition, we show that calpain and caspase-7 are soon activated after GD and mediate caspase-12 activation and neuronal death.

Neuronal damage and cognitive impairment associated with hypoglycemia: An integrated view

The aim of the present review is to offer a current perspective about the consequences of hypoglycemia and its impact on the diabetic disorder due to the increasing incidence of diabetes around the world. The main consequence of insulin treatment in type 1 diabetic patients is the occurrence of repetitive periods of hypoglycemia and even episodes of severe hypoglycemia leading to coma. In the latter, selective neuronal death is observed in brain vulnerable regions both in humans and animal models, such as the cortex and the hippocampus. Cognitive damage subsequent to hypoglycemic coma has been associated with neuronal death in the hippocampus. The mechanisms implicated in selective damage are not completely understood but many factors have been identified including excitotoxicity, oxidative stress, zinc release, PARP-1 activation and mitochondrial dysfunction. Importantly, the diabetic condition aggravates neuronal damage and cognitive failure induced by hypoglycemia. In the absence of coma prolonged and severe hypoglycemia leads to increased oxidative stress and discrete neuronal death mainly in the cerebral cortex. The mechanisms responsible for cell damage in this condition are still unknown. Recurrent moderate hypoglycemia is far more common in diabetic patients than severe hypoglycemia and currently important efforts are being done in order to elucidate the relationship between cognitive deficits and recurrent hypoglycemia in diabetics. Human studies suggest impaired performance mainly in memory and attention tasks in healthy and diabetic individuals under the hypoglycemic condition. Only scarce neuronal death has been observed under moderate repetitive hypoglycemia but studies suggest that impaired hippocampal synaptic function might be one of the causes of cognitive failure. Recent studies have also implicated altered mitochondrial function and mitochondrial oxidative stress.

Hypoglycemia, brain energetics, and hypoglycemic neuronal death

Hypoglycemia is a common and serious problem among diabetic patients receiving treatment with insulin or other glucose-lowering drugs. Moderate hypoglycemia impairs neurological function, and severe hypoglycemia leads to death of selectively vulnerable neurons. Recent advances have shed new light on the underlying processes that cause neuronal death in hypoglycemia and the factors that may render specific neuronal populations especially vulnerable to hypoglycemia. In addition to its clinical importance, the pathophysiology of hypoglycemia is an indicator of the unique bioenergetic properties of the central nervous system, in particular the metabolic coupling of neuronal and astrocyte metabolism. This review will focus on relationships between bioenergetics and brain dysfunction in hypoglycemia, the neuronal cell death program triggered by hypoglycemia, and the role of astrocytes in these processes.

Effects of MK-801 upon local cerebral glucose utilisation in conscious rats following unilateral lesion of caudal entorhinal cortex

Local cerebral glucose utilisation was examined in 62 discrete regions of conscious rats following unilateral ibotenic acid lesion of the caudal entorhinal cortex, and subsequent pharmacological challenge with (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist. Fourteen days after unilateral lesion of the entorhinal cortex, there were no significant alterations in local cerebral glucose use except within the lesioned entorhinal cortex (reduced by 31% compared to sham-operated control animals). In sham-operated animals, systemic administration of MK-801 (0.5 mg/kg, i.v.) induced anatomically organised alterations in glucose use with increases in olfactory areas, subicular complex and some limbic areas (posterior cingulate cortex, mammillary body and anteroventral thalamic nucleus), and decreases in the inferior colliculus and neocortex (auditory, sensory-motor, somatosensory and frontal cortices). In animals with unilateral entorhinal cortex lesions, the metabolic response to MK-801 differed significantly from the response to the drug in sham-lesioned animals in a number of regions, viz. hippocampus, molecular layer (ipsilateral to lesion), entorhinal cortex (ipsilateral), dentate gyrus (ipsilateral), presubiculum (bilateral), parasubiculum (bilateral) and nucleus accumbens (bilateral). The ability of MK-801 to reduce glucose use in the neocortex was not altered by entorhinal cortex lesion. These data suggest that the functional consequences of non-competitive NMDA receptor blockade are dependent in some areas upon the integrity of the perforant pathway from the entorhinal cortex to the hippocampus.

Rapid, sensitive, and simple method for quantification of both neurotoxic and neurotrophic effects of NMDA on cultured cerebellar granule cells

A simple and sensitive method adapted from the staining of living cells with fluorescein diacetate was developed to rapidly estimate the number of living cells remaining in a culture dish 24 hr after a few min of NMDA treatment of cerebellar neurons. This method consists of the measurement, after cell lysis, of the total amount of fluorescein produced from fluorescein diacetate by the living granule cells present in each culture dish. We show that this method can also be used to quantify the survival effect of chronic exposure of granule cells to either K+ or NMDA. In both cases, the fluorescence measured was found to be proportional to the number of fluorescein-labelled cells counted under a fluorescence microscope, indicating that the present method can be used to quantify both toxic and trophic effects of NMDA on cerebellar granule cells. This study confirms that these two NMDA effects occur at the same NMDA concentration, and both are inhibited by MK 801 in the same concentration range. We showed, moreover, that granule neurons developed in the presence of NMDA are much less sensitive to NMDA toxicity than neurons developed in K(+)-enriched medium.

Unlike hypoxia, hypoglycemia does not preferentially destroy GABAergic neurons in developing rat neocortex explants in culture

We tested whether hypoglycemia, like hypoxia, would preferentially destroy GABAergic nerve cells in the neocortex. To this end, rat neocortex explants dissected from 6-day-old rat pups and cultured up to a developmental stage approximately comparable to that of the newborn human neocortex, were exposed to hypoglycemia for different periods. Quantitative light microscopic and immunocytochemical evaluation of the cultures demonstrated that hypoglycemia does not preferentially destroy GABAergic but rather non-GABAergic neurons, a finding quite opposite to what was found after hypoxia. Recent biochemical data from other laboratories which seem to support this difference in neuronal vulnerability are discussed. It is concluded that perinatal hypoglycemia may not form such a serious threat with respect to the genesis of epilepsy as does hypoxia.

Blood flow and metabolism in heterotopic cerebellar grafts during hypoglycemia

Hypoglycemia-induced disturbances of brain metabolism and neuronal injury exhibit a distinct predilection for forebrain structures, in particular the caudate-putamen, hippocampus and cerebral cortex, whereas the cerebellum is remarkably resistant. In an attempt to assess the biological basis of this differential regional vulnerability, we have used a neural transplantation technique to compare hemodynamic and metabolic changes in cerebellum during severe hypoglycemia with those in heterotopic cerebellar grafts. To this end, the cerebellar anlage of fetal rat brain (day 15 of gestation) was stereotactically transplanted into the vulnerable caudate-putamen. Following a differentiation period of 8 weeks the grafts had developed into an organotypic population of mature cells with laminar histoarchitecture. Host animals were then subjected to insulin-induced hypoglycemia. After 15 min of isoelectric EEG, blood flow was increased throughout the brain but residual glucose consumption was significantly higher in cerebellum (0.29 mumol/g per min) and cerebellar grafts (0.22 mumol/g per min) as a result of increased glucose extraction. Hypoglycemia caused a depletion of ATP in all brain structures except cerebellum where normal levels were maintained. Correlation of local ATP content and glucose utilization revealed a threshold-like decline of ATP at a glucose utilization rate of 0.27 mumol/g per min. ATP, in consequence, was normal in cerebellum but partially depleted in cerebellar grafts. It is concluded that the resistance of cerebellum to hypoglycemia is due to its capacity for higher glucose extraction at low blood glucose levels, and that this unique intrinsic property is preserved after heterotopic transplantation.