The present study attempted to clarify whether over-secretion of glucocorticoids in the serum caused by increased hypothalamus-pituitary-adrenal activity induces oxidative stress in the rat brain, and how the stress causes the emergence of cognitive deficits. When rats were subcutaneously injected with corticosterone, lipid hydroperoxides and protein carbonyls increased markedly in the hippocampus in association with a decrease in activity of antioxidative enzymes, such as superoxide dismutase, catalase and glutathione peroxidase. These results suggest that high-level corticosterone in the serum induces reactive oxygen species (ROS), leading to oxidative damage in the hippocampus. After administration of corticosterone to rats, glucose and superoxide levels in the serum increased markedly. Furthermore, pyramidal cell apoptosis was observed to accompany the loss of glucocorticoid receptors at the cornus ammonis 1 region of the hippocampus. Rats injected with corticosterone showed marked deficits in memory function. The present results imply that ROS generated from the glycation reaction of increased glucose levels caused by gluconeogenesis activation through glucocorticoid with proteins in the serum attack the hippocampus to induce neurodegeneration, resulting in cognitive deficits in rats.

To elucidate whether oxidative stress induces cognitive deficit, and whether nerve cells in the hippocampus, which modulates learning and memory functions in the brain, are damaged by oxidative stress and during aging, the influence of hyperoxia as oxidative stress on either the cognitive function of rats or the oxidative damage of nerve cells was investigated. Young rats showed better learning ability than both old rats and vitamin E-deficient young rats. Vitamin E-supplemented young rats showed similar ability to young control rats. After they learned the location of the platform in the Morris water maze test, the young rats and vitamin E-supplemented young rats were subjected to oxidative stress for 48 h, and the old rats and vitamin E-deficient young rats were kept in normal atmosphere. The memory function of the old rats and vitamin E-deficient young rats declined even when they were not subjected to oxidative stress for 48 h. In contrast, the young rats maintained their memory function for 4 days after the oxidative stress. However, their learning abilities suddenly declined toward that of the normal old rats after 5 days. At this point, nerve cell loss and apoptosis were observed in the hippocampal CA 1 region of young rats. Vitamin E-supplementation in the young rats prevented either memory deficit or the induction of delayed-type apoptosis. The old rats and vitamin E-deficient young rats kept in normal atmosphere for 48 h also showed apoptosis in the hippocampus. Also, 10 days after oxidative stress, amyloid β-like substances appeared in the CA-1 region of control young rats; these substances were also observed in the CA-1 region of the old rats and vitamin E-deficient young rats. These results suggest that reactive oxygen species (ROS) generated by oxidative stress induced amyloid β-like substances and delayed-type apoptosis in the rat hippocampus, resulting in cognitive deficit. Since amyloid β in Alzheimer's disease characterized by cognitive deficit induces neuronal cell death, it is reasonable to consider that amyloid β deposition in the brain may be associated with memory dysfunction. The results of this study imply that age-related hippocampal neuronal damage is prevented by vitamin E supplementation due to the antioxidant effect of vitamin E.

Reactive oxygen species (ROS) may attack several types of tissues and chronic exposure to ROS may attenuate various biological functions and increase the risk of several types of serious disorders. It is known that treatments with ROS attack neurons and induce cell death. However, the mechanisms of neuronal change by ROS prior to induction of cell death are not yet understood. Here, it was found that treatment of neurons with low concentrations of hydrogen peroxide induced neurite injury, but not cell death. Unusual bands located above the original collapsin response mediator protein (CRMP)-2 protein were detected by western blotting. Treatment with tocopherol or tocotrienols significantly inhibited these changes in neuro2a cells and cerebellar granule neurons (CGCs). Furthermore, prevention by tocotrienols of hydrogen peroxide-induced neurite degeneration was stronger than that by tocopherol. These findings indicate that neurite beading is one of the early events of neuronal degeneration prior to induction of death of hydrogen peroxide-treated neurons. Treatment with tocotrienols may protect neurite function through its neuroprotective function.

It is well known that reactive oxygen species (ROS) attack several living tissues and increase the risk of development and progression of serious diseases. In neuronal level, ROS induce cell death in concentration-dependent fashion. However, little is known about the mechanisms of neuronal changes by ROS prior to induction of cell death. Here we found that treatment of cerebellar granule neurons (CGCs) with 0.5 μM hydrogen peroxide induced axonal injury, but not cell death. The number of dendrites remarkably decreased in hydrogen peroxide-treated CGCs, and extensive beading was observed on survival dendrites. In addition, an abnormal band of the original collapsin response mediator protein (CRMP)-2 was detected by Western blotting in hydrogen peroxide-treated CGCs. Treatment with each tocotrienol isoform prevented axonal and dendrite degeneration and induction of the abnormal band of the original band of CRMP-2 in hydrogen peroxide-treated CGCs. These results indicate that treatment with tocotrienols may therefore be neuroprotective in the presence of hydrogen peroxide by preventing changes to the CRMP-2 that occur before neuron death.

Summary In order to verify whether vitamin E improves the cognitive impairment induced through aging, aged rats fed a vitamin E-supplemented diet had their learning and memory functions assessed in comparison with the aged rats fed a normal diet using a Morris water maze test. Although normal aged rats showed very poor learning ability concerning the place of a platform in the water maze apparatus, the aged rats fed the vitamin Esupplemented diet learned the place with a marked speed in only 5 trials. After old animals showed the maximum learning ability, they were kept in a normal atmosphere for 48 h without a trial followed by an assessment of their memory function using the same apparatus. The vitamin E-supplementation to aged rats resulted in marked retention of their maximum memory function, although normal aged rats showed a significant memory loss of about 60%. Pyrroloquinoline quinone (PQQ), which increases in the production of nerve growth factor, and protects neurons, had a similar effect on cognitive function to that of vitamin E in the aged rats. These results suggest that vitamin E may improve cognitive deficit caused through aging by not only its neuro-protecting effect but an antioxidant efficacy. Key Words vitamin E, cognitive deficit, learning, memory, aged rat Although it has been recognized that normal aging is accompanied by declines in cognitive performance, the precise mechanisms leading to this deficit during aging are not well understood. It is evident that these declines arise from neurodegeneration through several factors during aging such as stroke, cerebral infarction and oxidative stress. There is substantial notion that oxidative stress is relevant to the aging process. Oxidative stress occurs at the time of an imbalance between reactive oxygen species (ROS) generation and its detoxification by antioxidants in living tissues, so that aging is considered to be accumulation of oxidative damage of living tissues through oxidative stress experienced over a long period of time ( 1 , 2 ).Among organs in living tissues, neurons in the brain are considered to be more vulnerable to oxidative stress than other organs, leading to neuronal oxidative damage, and neurodegenerative disorders such as Alzheimer's disease (AD), Parkinsonism and senile dementia ( 3 ). It is well-characterized that there are increased regional levels of oxidative stress in the AD brain, and hence recent studies have demonstrated a decrease in polyunsaturated fatty acids, increased levels of lipid peroxidation markers, protein oxidation, and DNA and RNA oxidation in AD ( 4 ). Based on the oxidative stress theory of brain aging, our previous study revealed that the levels of thiobarbituric acid reactive substances (TBARS), lipid hydroperoxides, F 2 -isoprostane and conjugated dienes increase significantly with oxidative stress in the rat brain, and the activity of antioxidative enzymes and vitamin E content in the brain decrease markedly ( 5-7 ). Furthermore, in accordance with these phenomena, young rats subjected to ox...