Kalpana Barhwal

Bacopa monniera leaf extract ameliorates hypobaric hypoxia induced spatial memory impairment

Hypobaric hypoxia induced memory impairment has been attributed to several factors including increased oxidative stress, depleted mitochondrial bioenergetics, altered neurotransmission and apoptosis. This multifactorial response of the brain to hypobaric hypoxia limits the use of therapeutic agents that target individual pathways for ameliorating hypobaric hypoxia induced memory impairment. The present study aimed at exploring the therapeutic potential of a bacoside rich leaf extract of Bacopa monniera in improving the memory functions in hypobaric conditions. The learning ability was evaluated in male Sprague Dawley rats along with memory retrieval following exposure to hypobaric conditions simulating an altitude of 25,000 ft for different durations. The effect of bacoside administration on apoptosis, cytochrome c oxidase activity, ATP levels, and oxidative stress markers and on plasma corticosterone levels was investigated. Expression of NR1 subunit of N-methyl-d-aspartate receptors, neuronal cell adhesion molecules and was also studied along with CREB phosphorylation to elucidate the molecular mechanisms of bacoside action. Bacoside administration was seen to enhance learning ability in rats along with augmentation in memory retrieval and prevention of dendritic atrophy following hypoxic exposure. In addition, it decreased oxidative stress, plasma corticosterone levels and neuronal degeneration. Bacoside administration also increased cytochrome c oxidase activity along with a concomitant increase in ATP levels. Hence, administration of bacosides could be a useful therapeutic strategy in ameliorating hypobaric hypoxia induced cognitive dysfunctions and other related neurological disorders.

Differential temporal response of hippocampus, cortex and cerebellum to hypobaric hypoxia: A biochemical approach

Hypobaric hypoxia is known to cause cognitive dysfunctions and memory impairment. The present study aimed at exploring the occurrence of oxidative stress in hypobaric hypoxia and the differential temporal response of the hippocampus, cerebellum following hypobaric hypoxia. Animals were divided into control, 3 days, 7 days and 14 days exposure groups and were exposed to an altitude of 25,000 ft. Our study revealed an increase in lactate dehydrogenase activity along with increase in free radical generation and lipid peroxidation. We also noted depletion in the antioxidants and decrease in glutathione reductase and superoxide dismutase activity. There was significant decrease in reduced glutathione levels in the exposure groups when compared to the control which was accompanied by a concomitant increase in oxidized glutathione levels. Increase in glutamate dehydrogenase activity was observed coinciding with the decrease in glutathione levels which was accompanied with an increase in expression of vesicular glutamate transporter. The hippocampus was found to be more vulnerable to hypobaric hypoxia-induced oxidative stress in comparison to the cortex and cerebellum. An interesting observation was the onset of acclimatization on prolonged exposure to hypobaric hypoxia for a period of 14 days. Hypobaric hypoxia was found to affect various regions of the brain differentially and the response of each region varied as a function of time.

Ceftriaxone rescues hippocampal neurons from excitotoxicity and enhances memory retrieval in chronic hypobaric hypoxia.

Exposure to high altitude is known to cause impairment in cognitive functions in sojourners. The molecular events leading to this behavioral manifestation, however, still remain an enigma. The present study aims at exploring the nature of memory impairment occurring on chronic exposure to hypobaric hypoxia and the possible role of glutamate in mediating it. Increased ionotropic receptor stimulation by glutamate under hypobaric hypoxic conditions could lead to calcium mediated excitotoxic cell death resulting in impaired cognitive functions. Since glutamate is cleared from the synapse by the Glial Glutamate Transporter, upregulation of the transporter can be a good strategy in preventing excitotoxic cell death. Considering previous reports on upregulation of the expression of Glial Glutamate Transporter on ceftriaxone administration, the therapeutic potential of ceftriaxone in ameliorating hypobaric hypoxia induced memory impairment was investigated in male Sprague Dawley rats. Exposure to hypobaric hypoxia equivalent to an altitude of 7600 m for 14 days lead to oxidative stress, chromatin condensation and neuronal degeneration in the hippocampus. This was accompanied by delayed memory retrieval as evident from increased latency and pathlength in Morris Water Maze. Administration of ceftriaxone at a dose of 200 mg/kg for 7 days and 14 days during the exposure on the other hand improved the performance of rats in the water maze along with decreased oxidative stress and enhanced neuronal survival when compared to hypoxic group without drug administration. An increased expression of Glial Glutamate Transporter was also observed following drug administration indicating faster clearance of glutamate from the synapse. The present study not only brings to light the effect of longer duration of exposure to hypobaric hypoxia on the memory functions, but also indicates the pivotal role played by glutamate in mediating excitotoxic neuronal degeneration at high altitude. The therapeutic potential of ceftriaxone in providing neuroprotection in excitotoxic conditions by increasing Glial Glutamate Transporter expression and thereby enhancing glutamate uptake from the synapse has also been explored.

ACETYL-L-CARNITINE (ALCAR) PREVENTS HYPOBARIC HYPOXIA-INDUCED SPATIAL MEMORY IMPAIRMENT THROUGH EXTRACELLULAR RELATED KINASE-MEDIATED NUCLEAR FACTOR ERYTHROID 2-RELATED FACTOR 2 PHOSPHORYLATION

Exposure to hypobaric hypoxia, a condition involving decreased availability of oxygen is known to be associated with oxidative stress, neurodegeneration and memory impairment. The multifactorial response of the brain and the complex signaling pathways involved therewith limits the therapeutic efficacy of several antioxidants in ameliorating hypobaric hypoxia-induced memory impairment. The present study was therefore aimed at investigating the potential of acetyl-l-carnitine (ALCAR), a known antioxidant that has been reported to augment neurotrophin-mediated survival mechanisms, in ameliorating hypoxia-induced neurodegeneration and memory impairment. Nuclear factor erythroid 2-related factor 2 (Nrf2) is a key transcription factor involved in the cellular defense mechanism against oxidative stress related to brain injury and neurological disorders. The study was designed to understand the mechanisms involving Nrf2 stabilization following exposure to hypobaric hypoxia. The results displayed reference memory impairment in Sprague-Dawley rats exposed to hypobaric hypoxia (7620 m) for 14 consecutive days which however improved on administration of ALCAR during hypoxic exposure. The study also revealed Nrf2 regulated augmented antioxidant response on administration of ALCAR which was through a novel tyrosine kinase A (TrkA) receptor-mediated mechanism. A decrease in free radical generation, lipid peroxidation and protein oxidation was also observed along with a concomitant increase in thioredoxin and reduced glutathione levels on administration of ALCAR during exposure to hypobaric hypoxia. The present study therefore reveals the therapeutic potential of ALCAR under conditions of hypobaric hypoxia and elucidates a novel mechanism of action of the drug.

NR1 and GluR2 expression mediates excitotoxicity in chronic hypobaric hypoxia

Hypobaric hypoxia has been reported to cause memory dysfunction. The possible molecular mechanism involved, however, remains to be explored. The role that glutamate and its receptors play in causing excitotoxicity in ischemia and neurodegenerative diseases indicates the possible occurrence of a similar phenomenon in hypobaric hypoxia. The present study aimed to elucidate the molecular events occurring at glutamatergic synapses in hypobaric hypoxia using Sprague‐Dawley rats as a model system. The animals were exposed to an altitude of 7,600 m for different durations. Hypobaric hypoxia was found to cause oxidative stress, chromatin condensation, and neurodegeneration. A temporal change in the expression of the ionotropic receptors of glutamate was also observed. Expression of the N‐methyl‐D‐aspartate (NMDA) receptor increased, and expression of glutamate receptor subunit 2 of the α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazoleproprionate receptor decreased. We also observed increased activity of glutamate dehydrogenase, indicating greater synthesis and release of glutamate after 3 and 7 days of exposure. Administration of a selective NMDA antagonist during exposure was found to ameliorate neuronal degeneration, providing evidence for the occurrence of excitotoxicity in hypobaric hypoxia. Our study indicates that excitotoxicity occurs in hypobaric hypoxia. This study also indicates the appropriate period for drug administration during exposure to hypobaric hypoxia and establishes ionotropic receptors of glutamate as potential therapeutic targets for ameliorating high‐altitude‐induced cognitive dysfunction.

Chronic hypobaric hypoxia induced apoptosis in CA1 region of hippocampus: A possible role of NMDAR mediated p75NTR upregulation

Hypobaric hypoxia has been implicated with neural degeneration and memory loss. Though there has been considerable knowledge on the role of the p75(NTR) in triggering apoptosis, the occurrence of a similar mechanism in hypoxic stress still remains to be explored. We, in the present study, have tried to explore the role of p75(NTR) in mediating apoptosis in hypobaric hypoxia. Male Sprague Dawley rats were exposed to an altitude of 7,620 m for different durations. To study the contribution of apoptosis to hypobaric hypoxia induced cell death in the hippocampus, rat brains were examined for the occurrence of apoptosis by determining the number of cells showing DNA breaks using terminal deoxynucleotidyl transferase-mediated UTP nick end labeling (TUNEL) assay and chromatin condensation using Hoechst staining along with estimation of caspase activity and expression of active Caspase 3. Expression of p75(NTR) was studied to determine its possible role in triggering apoptosis in hypobaric hypoxia. Exposure to hypobaric hypoxia was found to progressively increase the number of TUNEL positive and Hoechst positive cells along with increase in caspase activity, thus suggesting apoptotic mode of cell death. p75(NTR) was found to be upregulated on prolonged exposure to hypobaric hypoxia corresponding to the increase in the number of apoptotic cells. Further, reduced expression of p75(NTR) expression by antisense nucleotide administration significantly decreased apoptosis in the CA1 region of hippocampus. Blocking of NMDA receptors by MK801 interestingly decreased p75(NTR) expression and the number of TUNEL positive cells as compared to hypoxic animals. These findings suggest the regulation of p75(NTR) by NMDA receptors and its role in inducing apoptosis in hypoxia.

Disrupting monotony during social isolation stress prevents early development of anxiety and depression like traits in male rats

BackgroundAlthough there have been several reports on social isolation induced mood alterations, the independent contribution of monotonous environment in mediating mood alterations has been less studied. In view of the above, the present study is aimed at investigating the relative contribution of monotony towards mood alterations during isolation stress. Monotony was induced in a specially designed isolation chamber in male Sprague-Dawley rats in the presence or absence of isolation by housing animals singly (SH) or in pairs (PH). Novel objects were introduced to disrupt monotony in singly housed animals (SHNO) or paired housed animals (PHNO). Behavioural alterations were assessed using Open field test (OFT), Elevated Plus Maze (EPM) and Forced Swim Test (FST). Neuro-morphological changes in the CA3 region of hippocampus were studied by cresyl violet and golgi-cox staining. Hippocampal serotonin and 5-hydroxy indole acetic acid (5-HIAA) levels were estimated along with the expression of phospho-insulin like growth factor-1 receptor (pIGF-1R) and phospho cyclic AMP response-element binding protein (pCREB). Serotonin was depleted by administering Para-chlorophenylalanine (PCPA) to a separate PH group (PHPCPA), PHNO group (PHNOPCPA) and SHNO group (SHNOPCPA) to determine the role of serotonin in mediating monotony induced emotional mal-adaptations.ResultsThe results showed anxiety and depression like traits in both PH and SH groups during behavioural test such as OFT, EPM and FST. Pyknosis along with decrease in apical dendritic arborization was observed in the CA3 region of SH group along with decrease in serotonin and reduced expression of pIGF-1R and pCREB. Disrupting monotony through intervention of novel objects in PHNO and SHNO groups ameliorated anxiety and depression like traits and augmented pIGF-1R along with increase in serotonin level. Depletion of hippocampal serotonin level by PCPA administration in PHNOPCPA and SHNOPCPA groups on the other hand resulted in altered mood state despite disruption of monotony by novel objects intervention.ConclusionThe findings of our study suggest that monotonous environment independently contributes to impairment in mood state and disrupting monotony by intervention of novel objects during social isolation prevents mood disorders and emotional maladaptation through up regulation of hippocampal pIGF-1R and increase in serotonin.

Corticosterone synthesis inhibitor metyrapone ameliorates chronic hypobaric hypoxia induced memory impairment in rat

Chronic exposure to hypobaric hypoxia causes oxidative stress and neurodegeneration leading to memory impairment. The present study aimed at investigating the role of corticosterone in hypoxia induced neurodegeneration and effect of metyrapone, a corticosterone synthesis inhibitor that reduces the stress induced elevation of corticosterone without affecting the basal level, in ameliorating chronic hypobaric hypoxia induced cognitive decline. Rats were exposed to simulated altitude of 25,000 ft for 0, 3, 7, 14 and 21 days to determine the temporal alterations in corticosterone and its receptors following exposure to hypobaric hypoxia. Our results showed an elevation of corticosterone in plasma and hippocampal tissue following 7 days of exposure, which declined on prolonged hypoxic exposure for 21 days. A concomitant increase in ROS and lipid peroxidation was observed along with depletion of intracellular antioxidants. Glucocorticoid and mineralocorticoid receptors were upregulated on 3 and 7 days of hypoxic exposure. Though expression of Glut1 and Glut3 were upregulated on 3 days of hypoxic exposure, sharp decline in Glut1 expression following 7 days of hypoxic exposure leads to reduced neuronal glucose uptake. Administration of metyrapone from 3rd to 7th day of hypoxic exposure to suppress hypoxia induced increase in corticosterone levels resulted in reduced oxidative damage, neurodegeneration and improvement of intracellular energy status. The metyrapone treated hypoxic animals performed better in the Morris Water Maze. Further, administration of exogenous corticosterone along with metyrapone during hypoxic exposure blunted the neuroprotective effect of metyrapone indicating a role for corticosterone in mediating hypobaric hypoxia induced neurodegeneration and memory impairment.

Insulin receptor A and Sirtuin 1 synergistically improve learning and spatial memory following chronic salidroside treatment during hypoxia.

Hypoxia has been reported to cause hippocampal neurodegeneration resulting in learning and memory deficits. In the present study, we investigated the potential of salidroside, a glucoside derivative of tyrosol, in ameliorating hypoxia‐induced neurodegeneration and memory impairment. Morris water maze test showed improvement in learning and spatial memory of salidroside‐treated hypoxic rats correlating with increased dendritic intersections and arborization. Salidroside administration increased phosphorylation of insulin receptor subunit A (IRA) at Y972, Y1162/63, and Y1146 sites and subsequent activation of AMP‐activated protein kinase (AMPK) α subunit isoforms pAMPKα1 and pAMPKα2 resulting in mitochondrial biogenesis. Contrarily, silencing of IRA in salidroside‐supplemented hypoxic hippocampal cells could not improve cell viability or alter pAMPKα1 and pAMPKα2 expression. Rats administered with salidroside showed elevated expression of phosphorylated cAMP response element‐binding protein in the hippocampus. Salidroside administration also resulted in increased sirtuin 1 (SIRT1) activity through a cytochrome P4502E1 (CYP2E1)‐regulated mechanism that was independent of pIRA. Taken together, these findings suggest a synergistic role of pIRA and SIRT1 in salidroside‐mediated neuroprotection, mitochondrial biogenesis, and cognitive improvement during hypoxia.

Global hypoxia induced impairment in learning and spatial memory is associated with precocious hippocampal aging

Both chronological aging and chronic hypoxia stress have been reported to cause degeneration of hippocampal CA3 neurons and spatial memory impairment through independent pathways. However, the possible occurrence of precocious biological aging on exposure to single episode of global hypoxia resulting in impairment of learning and memory remains to be established. The present study thus aimed at bridging this gap in existing literature on hypoxia induced biological aging. Male Sprague Dawley rats were exposed to simulated hypobaric hypoxia (25,000ft) for different durations and were compared with aged rats. Behavioral studies in Morris Water Maze showed decline in learning abilities of both chronologically aged as well as hypoxic rats as evident from increased latency and pathlength to reach target platform. These behavioral changes in rats exposed to global hypoxia were associated with deposition of lipofuscin and ultrastructural changes in the mitochondria of hippocampal neurons that serve as hallmarks of aging. A single episode of chronic hypobaric hypoxia exposure also resulted in the up-regulation of pro-aging protein, S100A9 and down regulation of Tau, SNAP25, APOE and Sod2 in the hippocampus similar to that in aged rats indicating hypoxia induced accelerated aging. The present study therefore provides evidence for role of biological aging of hippocampal neurons in hypoxia induced impairment of learning and memory.