The putative role of free radicals in the loss of neuronal functioning in senescence

The Prevailing Role of Topoisomerase 2 Beta and its Associated Genes in Neurons

Topoisomerase 2 beta (TOP2β) is an enzyme that alters the topological states of DNA by making a transient double-strand break during the transcription process. The direct interaction of TOP2β with DNA strand results in transcriptional regulation of certain genes and some studies have suggested that a particular set of genes are regulated by TOP2β, which have a prominent role in various stages of neuron from development to degeneration. In this review, we discuss the role of TOP2β in various phases of the neuron's life. Based on the existing reports, we have compiled the list of genes, which are directly regulated by the enzyme, from different studies and performed their functional classification. We discuss the role of these genes in neurogenesis, neuron migration, fate determination, differentiation and maturation, generation of neural circuits, and senescence.

Comparison of Effects of Different Antidotes on Tabun-Induced Cognitive Impairment in Rats Using Water Maze

In the past, scientists focused on the development of antidotes (mainly anticholinergics in combination with reactivators of inhibited acetylcholinesterase – oximes) to increase the number of surviving nerve agent-intoxicated individuals. Recently, they are interested in antidotes able not only to protect nerve agent-poisoned men from lethal toxic effects but also to improve their life quality by improvement of their central cognitive functions. In our study, the water maze was used to measure spatial working learning and memory in the case of tabun-induced cognitive impairment in albino Wistar rats. Antidotal treatment consisted of atropine alone or a combination of atropine with an oxime (obidoxime, trimedoxime or oxime HI-6). Our results suggest that atropine alone is not sufficient as a treatment for saving cognitive functions impaired by tabun. On the other hand, the addition of oxime to atropine contributes to improvement of cognitive performance in tabun-poisoned rats regardless of type of oxime.

Age-dependent effects of esculetin on mood-related behavior and cognition from stressed mice are associated with restoring brain antioxidant status

Dietary antioxidants might exert an important role in the aging process by relieving oxidative damage, a likely cause of age-associated brain dysfunctions. This study aims to investigate the influence of esculetin (6,7-dihydroxycoumarin), a naturally occurring antioxidant in the diet, on mood-related behaviors and cognitive function and its relation with age and brain oxidative damage. Behavioral tests were employed in 11-, 17- and 22-month-old male C57BL/6J mice upon an oral 35day-esculetin treatment (25mg/kg). Activity of antioxidant enzymes, GSH and GSSG levels, GSH/GSSG ratio, and mitochondrial function were analyzed in brain cortex at the end of treatment in order to assess the oxidative status related to mouse behavior. Esculetin treatment attenuated the increased immobility time and enhanced the diminished climbing time in the forced swim task elicited by acute restraint stress (ARS) in the 11- and 17-month-old mice versus their counterpart controls. Furthermore, ARS caused an impairment of contextual memory in the step-through passive avoidance both in mature adult and aged mice which was partially reversed by esculetin only in the 11-month-old mice. Esculetin was effective to prevent the ARS-induced oxidative stress mostly in mature adult mice by restoring antioxidant enzyme activities, augmenting the GSH/GSSG ratio and increasing cytochrome c oxidase (COX) activity in cortex. Modulation of the mood-related behavior and cognitive function upon esculetin treatment in a mouse model of ARS depends on age and is partly due to the enhancement of redox status and levels of COX activity in cortex.

Estradiol and neurodegenerative oxidative stress

Estradiol is a potent preventative against neurodegenerative disease, in part, by activating antioxidant defense systems scavenging reactive oxygen species, limiting mitochondrial protein damage, improving electron transport chain activity and reducing mitochondrial DNA damage. Estradiol also increases the activity of complex IV of the electron transport chain, improving mitochondrial respiration and ATP production under normal and stressful conditions. However, the high oxidative cellular environment present during neurodegeneration makes estradiol a poor agent for treatment of existing disease. Oxidative stress stimulates the production of the hydroperoxide-dependent hydroxylation of estradiol to the catecholestrogen metabolites, which can undergo reactive oxygen species producing redox cycling, setting up a self-generating toxic cascade offsetting any antioxidant/antiapoptotic effects generated by the parent estradiol. Additional disease-induced factors can further perpetuate this cycle. For example dysregulation of the catecholamine system could alter catechol-O-methyltransferase-catalyzed methylation, preventing removal of redox cycling catecholestrogens from the system enhancing pro-oxidant effects of estradiol.

The free-radical scavenger edaravone restores the differentiation of human neural precursor cells after radiation-induced oxidative stress

Recently, it has been elucidated that cognitive dysfunction following cranial radiotherapy might be linked to the oxidative stress-induced impairment of hippocampal neurogenesis that is mediated by proliferating neural stem or progenitor cells. The novel free-radical scavenger edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one) has been clinically used to reduce neuronal damage following ischemic stroke. Here, we demonstrate via in vitro studies that edaravone protects human neural stem cells (NSCs) from cell death and restores their differentiation ability after irradiation; however, the protective effect of edaravone is not observed in human brain tumor cells. Our study may shed some light on the beneficial effects of free-radical scavengers in impaired neurogenesis following cranial radiation therapy.

Oxidative stress and aberrant signaling in aging and cognitive decline

Brain aging is associated with a progressive imbalance between antioxidant defenses and intracellular concentrations of reactive oxygen species (ROS) as exemplified by increases in products of lipid peroxidation, protein oxidation, and DNA oxidation. Oxidative conditions cause not only structural damage but also changes in the set points of redox-sensitive signaling processes including the insulin receptor signaling pathway. In the absence of insulin, the otherwise low insulin receptor signaling is strongly enhanced by oxidative conditions. Autophagic proteolysis and sirtuin activity, in turn, are downregulated by the insulin signaling pathway, and impaired autophagic activity has been associated with neurodegeneration. In genetic studies, impairment of insulin receptor signaling causes spectacular lifespan extension in nematodes, fruit flies, and mice. The predicted effects of age-related oxidative stress on sirtuins and autophagic activity and the corresponding effects of antioxidants remain to be tested experimentally. However, several correlates of aging have been shown to be ameliorated by antioxidants. Oxidative damage to mitochondrial DNA and the electron transport chain, perturbations in brain iron and calcium homeostasis, and changes in plasma cysteine homeostasis may altogether represent causes and consequences of increased oxidative stress. Aging and cognitive decline thus appear to involve changes at multiple nodes within a complex regulatory network.

Fruit polyphenols and their effects on neuronal signaling and behavior in senescence

The onset of age-related neurodegenerative diseases superimposed on a declining nervous system could exacerbate the motor and cognitive behavioral deficits that normally occur in senescence. It is likely that, in cases of severe deficits in memory or motor function, hospitalization and/or custodial care would be a likely outcome. This means that unless some way is found to reduce these age-related decrements in neuronal function, healthcare costs will continue to rise exponentially. Thus, it is extremely important to explore methods to retard or reverse the age-related neuronal deficits as well as their subsequent, behavioral manifestations. Applying molecular biological approaches to slow aging in the human condition may be years away. So it is important to determine what methods can be used today to increase healthy aging, forestall the onset of these diseases, and create conditions favorable to obtaining a "longevity dividend" in both financial and human terms. In this regard, epidemiological studies indicate that consumption of diets rich in antioxidants and anti-inflammatory compounds, such as those found in fruits and vegetables, may lower the risk of developing age-related neurodegenerative diseases, such as Alzheimer's or Parkinson's diseases (AD and PD). Research suggests that the polyphenolic compounds found in fruits, such as blueberries, may exert their beneficial effects by altering stress signaling and neuronal communication, suggesting that interventions may exert protection against age-related deficits in cognitive and motor function. The purpose of this article is to discuss the benefits of these interventions in rodent models and to describe the putative molecular mechanisms involved in their benefits.

The influence of antidotal treatment of low-level tabun exposure on cognitive functions in rats using a water maze

In this study, the influence of antidotal treatment of tabun poisoning on cognitive function, in the case of low-level tabun exposure, was studied. The impairment of cognitive function was evaluated by the measurement of spatial learning and memory in rats poisoned with a sublethal dose of tabun and treated with atropine alone or in combination with newly developed oximes {K027 [1-(4-hydroxyiminomethyl- pyridinium)-3-(4-carbamoylpyridinium) propane dibromide] and K048 [1-(4-hydroxyimino- methylpyridinium)-3-(4-carbamoylpyridinium) butane dibromide]} or currently available oxime (trimedoxime), using the Morris water maze. While atropine alone caused an impairment of studied cognitive functions, the addition of an oxime to atropine contributes to the improvement of cognitive performance of treated tabun-poisoned rats regardless of the type of oxime. The differences in the ameliorative effects of oximes on atropine-induced mnemonic deficits were not significant. Therefore, each low-level nerve agent exposure should be treated by complex antidotal treatment consisting of anticholinergic drug and oxime.

Oxidative Stress and Inflammation in Brain Aging: Nutritional Considerations

Aging can be defined as the condition where stressors are not counteracted by protective functions, leading to a dysregulation in development. These changes can be translated into decrements in neuronal functioning accompanied by behavioral declines, such as decreases in motor and cognitive performance, in both humans and animals. When coupled with genetic alterations, the ultimate expression of these changes is seen in diseases such as Alzheimer disease (AD). This association will be discussed in the last section of this chapter. In this review we will describe motor and cognitive deficits in behavior due to aging, and show how these deficits are related to increased vulnerability to oxidative stress, inflammation or signaling. Importantly, using muscarinic receptors as examples, we will also try to show that the sensitivity to these insults may be differentially expressed among neurotransmitter receptor subtypes.

Reversing the deleterious effects of aging on neuronal communication and behavior: beneficial properties of fruit polyphenolic compounds

Despite elegant research involving molecular biology studies and determination of the genetic mechanisms of aging, practical information on how to forestall or reverse the deleterious effects of aging may be years away. If this is the case, then it is prudent to try to establish other methods that can be used now to alter the course of aging. Numerous epidemiologic studies have indicated that individuals who consume diets containing large amounts of fruits and vegetables may reduce their risk for developing age-related diseases such as Alzheimer disease. Research from our laboratory suggested that dietary supplementation with fruit or vegetable extracts high in antioxidants (eg, blueberry or spinach extracts) might decrease the enhanced vulnerability to oxidative stress that occurs in aging. These reductions might be expressed as improvements in motor and cognitive behavior. Additional research suggested that mechanisms in addition to antioxidant and antiinflammatory activities might be involved in the beneficial effects of these extracts; the most important of these might be their ability to increase cellular signaling and neuronal communication.

Effect of age on the radial arm water maze—a test of spatial learning and memory

Aged rats show decrements in performance on cognitive tasks that require the use of spatial learning and memory. We used the 8-arm radial water maze (RAWM) to measure spatial learning as a function of age in young (6 months) and old (21 months) male F344 rats. Rats were placed in the RAWM in different start arms with the same goal arm for 3 days (five trials/day); the goal arm was changed on day 4. Old rats demonstrated spatial impairment as evidenced by increased latencies to find the hidden platform on day 4. Old rats made significantly more errors, both reference and working memory errors, than young rats on all days. It is likely that the old rats utilized non-spatial strategies to solve the task, and therefore were impaired in learning a new platform location. The RAWM is a reliable, sensitive, and powerful additional test to assess age-related spatial learning and memory deficits, combining the advantages of the Morris water maze and the radial arm maze while minimizing the disadvantages.

Cognitive deficits induced by 56Fe radiation exposure

Exposing rats to particles of high energy and charge (e.g., 56Fe) disrupts neuronal systems and the behaviors mediated by them; these adverse behavioral and neuronal effects are similar to those seen in aged animals. Because cognition declines with age, and our previous study showed that radiation disrupted Morris water maze spatial learning and memory performance, the present study used an 8-arm radial maze (RAM) to further test the cognitive behavioral consequences of radiation exposure. Control rats or rats exposed to whole-body irradiation with 1.0 Gy of 1 GeV/n high-energy 56Fe particles (delivered at the alternating gradient synchrotron at Brookhaven National Laboratory) were tested nine months following exposure. Radiation adversely affected RAM performance, and the changes seen parallel those of aging. Irradiated animals entered baited arms during the first 4 choices significantly less than did controls, produced their first error sooner, and also tended to make more errors as measured by re-entries into non-baited arms. These results show that irradiation with high-energy particles produces age-like decrements in cognitive behavior that may impair the ability of astronauts to perform critical tasks during long-term space travel beyond the magnetosphere.

Role of oxidative stress and antioxidants in neurodegenerative diseases

Neurodegenerative diseases (NDD) are a group of illness with diverse clinical importance and etiologies. NDD include motor neuron disease such as amyotrophic lateral sclerosis (ALS), cerebellar disorders, Parkinson's disease (PD), Huntington's disease (HD), cortical destructive Alzheimer's disease (AD) and Schizophrenia. Numerous epidemiological and experimental studies provide many risk factors such as advanced age, genetic defects, abnormalities of antioxidant enzymes, excitotoxicity, cytoskeletal abnormalities, autoimmunity, mineral deficiencies, oxidative stress, metabolic toxicity, hypertension and other vascular disorders. Growing body of evidence implicates free radical toxicity, radical induced mutations and oxidative enzyme impairment and mitochondrial dysfunction due to congenital genetic defects in clinical manifestations of NDD. Accumulation of oxidative damage in neurons either primarily or secondarily may account for the increased incidence of NDD such as AD, ALS and stroke in aged populations. The molecular mechanisms of neuronal degeneration remain largely unknown and effective therapies are not currently available. Recent interest has focused on antioxidants such as carotenoids and in particular lycopene, a potent antioxidant in tomatoes and tomato products, flavonoids and vitamins as potentially useful agents in the management of human NDD. The pathobiology of neurodegenerative disorders with emphasis on genetic origin and its correlation with oxidative stress of neurodegenerative disorders will be reviewed and the reasons as to why brain constitutes a vulnerable site of oxidative damage will be discussed. The article will also discuss the potential free radical scavenger, mechanism of antioxidant action of lycopene and the need for the use of antioxidants in the prevention of NDD.

Spatial learning and memory deficits induced by exposure to iron-56-particle radiation

It has previously been shown that exposing rats to particles of high energy and charge (HZE) disrupts the functioning of the dopaminergic system and behaviors mediated by this system, such as motor performance and an amphetamine-induced conditioned taste aversion; these adverse behavioral and neuronal effects are similar to those seen in aged animals. Because cognition declines with age, spatial learning and memory were assessed in the Morris water maze 1 month after whole-body irradiation with 1.5 Gy of 1 GeV/nucleon high-energy (56)Fe particles, to test the cognitive behavioral consequences of radiation exposure. Irradiated rats demonstrated cognitive impairment compared to the control group as seen in their increased latencies to find the hidden platform, particularly on the reversal day when the platform was moved to the opposite quadrant. Also, the irradiated group used nonspatial strategies during the probe trials (swim with no platform), i.e. less time spent in the platform quadrant, fewer crossings of and less time spent in the previous platform location, and longer latencies to the previous platform location. These findings are similar to those seen in aged rats, suggesting that an increased release of reactive oxygen species may be responsible for the induction of radiation- and age-related cognitive deficits. If these decrements in behavior also occur in humans, they may impair the ability of astronauts to perform critical tasks during long-term space travel beyond the magnetosphere.

The effects of dietary antioxidants on psychomotor performance in aged mice

Male C57BL/6NIA mice were provided one of six different antioxidant diets: vitamin E, glutathione, vitamin E plus glutathione, melatonin, strawberry extract, or control, beginning at 18 months of age. A battery of motor tests--rod walk, wire hang, plank walk, and inclined screen-was administered either: 1) before dietary treatment and then 6 months later at 24 months of age: or 2) only after 6 months of dietary treatment at age 24 months. An untreated group of 4-month-old mice served as young controls. Psychomotor performance was lower in 18-month-old mice compared with 4-month-old mice in the rod walk, wire hang, and inclined screen tests; however, no further decline was seen from 18 to 24 months on any measure. Chronic dietary antioxidant treatments were not effective in reversing age-related deficits in psychomotor behavior, except for the glutathione diet on inclined screen performance. It seems that motor performance deteriorates profoundly with age, because deficits at 18 months of age were as severe as they were at 24 months, and these age-associated motor deficits may be difficult to reverse, even with antioxidant treatment.

Vitamin K-dependent proteins in the developing and aging nervous system

Although the warfarin embryopathy syndrome, with its neurologic and bone abnormalities, has been known for decades, the role of vitamin K in the brain has not been studied systematically. Recently, it was demonstrated that vitamin K-dependent carboxylase expression is temporally regulated in a tissue-specific manner with high expression in the nervous system during the early embryonic stages and with liver expression after birth and in adult animals. This finding, along with the discovery of wide distribution of the novel vitamin K-dependent growth factor, Gas6, in the central nervous system, provides compelling evidence of a biologic role of vitamin K during the development of the nervous system. In animals and bacteria, vitamin K was observed to influence the brain sulfatide concentration and the activity and synthesis of an important enzyme involved in brain sphingolipids biosynthesis. Taken together, previous research results point to a possible role of vitamin K in the nervous system, especially during its development. Hence, the knowledge of the biologic role of vitamin K in the brain may be important for unveiling the mechanisms of normal and pathologic development and aging of the nervous system. The role of the vitamin K-dependent protein Gas6 in activation of signal transduction events in the brain in light of the age-related changes in the nervous system is also discussed.

The effects of aging and oxidative stress on psychomotor and cognitive behavior

Decrements in motor and cognitive function occur in aging, possibly due to oxidative stress-induced damage to the brain. Declines in antioxidant defense mechanisms have been postulated as a causative factor in these age-related decrements, however a clear link between oxidative stress (OS) and behavioral changes in aging has yet to be established. This review shows that age-validated psychomotor and cognitive tests are sensitive to behavioral deficits under different models of OS, including: 1) decreasing OS protection by depleting glutathione and then increasing the OS with dopamine; 2) 100% oxygen exposure; and 3) radiation. Furthermore, interventions that reduce OS result in concurrent improvements in age-associated behavioral deficits. Therefore, age-related changes in behavior may result from an inability to cope with OS that occurs throughout the life-span.

Current status of antioxidant therapy for Alzheimer's Disease

Accumulating evidence from preclinical and clinical studies supports the hypothesis that oxidative stress may be associated with the onset and progression of Alzheimer's Disease (AD). Antioxidant therapies are being promoted in the lay press to enhance mental functions and delay cognitive losses with aging. An increasing number of physicians are also recommending antioxidant therapies, such as high dose vitamin E, for subjects with AD and other neurodegenerative disorders. High dose vitamin E, ginkgo biloba, and selegiline are three putative antioxidants that have been tested in randomized multicenter trial conditions in the US. This paper summarizes the oxidative stress hypothesis of AD and reviews the strengths and limitations of published antioxidant studies in AD in relation to the role of such therapies in practice.

Psychomotor and spatial memory performance in aging male Fischer 344 rats

Psychomotor and spatial memory performance were examined in male Fischer 344 rats that were 6, 12, 15, 18, and 22 months of age, to assess these parameters as a function of age and to determine at what age these behaviors begin to deteriorate. Complex motor behaviors, as measured by rod walk, wire suspension, plank walk, inclined screen, and accelerating rotarod performance, declined steadily with age, with most measures being adversely affected as early as 12 to 15 months of age. Spatial learning and memory performance, as measured by the working memory version of the Morris water maze (MWM), showed decrements at 18 and 22 months of age (higher latencies on the working memory trial), with some change noticeable as early as 12-15 months of age (no improvement on the second trial following a 10-min retention interval); these differences were not due to swim speed. Therefore, complex motor and spatial memory behaviors show noticeable declines early in the lifespan of the male Fisher 344 rat. This cross-sectional age analysis study using the latest behavioral techniques determines the minimal age at which psychomotor and spatial learning and memory behaviors deteriorate; this information is important when planning for longitudinal studies where interventions are tested for their efficacy in preventing or restoring age-related behavioral deficits.

Coping with the stress of immigration among new immigrants to Israel from Commonwealth of Independent States (CIS) who were exposed to Chernobyl: the effect of age

This study examined the differential effect of age on coping and psychological measures among immigrants from Commonwealth of Independent States (CIS) to Israel. Some of these immigrants originated in the Republics adjacent to the Chernobyl Power Plant, site of the 1986 accident. The sample consisted of 708 immigrants who were interviewed between the years 1993-1995 with an average age of 47.5 (sd 11.8). This sample was reinterviewed approximately a year and three months later (n = 520). The sample included two exposure groups--high exposed and low exposed based on the estimated levels of ground cesium contamination from the IAEA maps and a comparison group matched by age, gender, and year of immigration. Those over the age of sixty-five were disadvantaged, compared to those aged fifty to sixty-four, and younger, when it came to the tasks of immigrant absorption; learning the language, working and acquiring an income, and establishing alternative social networks which could offer support in times of illness. The psychological variables showed that over time, somatization, depression, and post-traumatic stress disorder (PTSD) symptoms related to Chernobyl improved, however at a much slower pace for older immigrants (aged 55 and over) compared to younger ones.

Spatial learning and memory deficits induced by dopamine administration with decreased glutathione

Administration of buthionine sulfoximine (BSO) selectively inhibits glutathione (GSH) biosynthesis and induces a GSH deficiency. Decreased GSH levels in the brain may result in less oxidative stress (OS) protection, because GSH contributes substantially to intracellular antioxidant defense. Under these conditions, administration of the pro-oxidant, dopamine (DA), which rapidly oxidizes to form reactive oxygen species, may increase OS. To test the cognitive behavioral consequences of decreased GSH, BSO (3.2 mg in 30 microliters, intracerebroventricularly) was administered to male Fischer 344 rats every other day for 4 days. In addition, DA (15 microliters of 500 microM) was administered every day [either 1 h after BSO (BSO + DA group) or 1 h before BSO (DA + BSO group), when given on the same day as BSO] and spatial learning and memory assessed (Morris water maze, six trials/day). BSO + DA rats, but not DA + BSO rats, demonstrated cognitive impairment compared to a vehicle group, as evidenced by increased latencies to find the hidden platform, particularly on the first trial each day. Also, the BSO + DA group utilized non-spatial strategies during the probe trials (swim with no platform): i.e., less time spent in the platform quadrant, fewer crossings and longer latencies to the previous platform location, and more time spent in the platform quadrant, fewer crossings and longer latencies to the previous platform location, and more time spent around the edge of the pool rather than in the platform zone. Therefore, the cognitive behavioral consequences of decreasing GSH brain levels with BSO in conjunction with DA administration depends on the order of administration. These findings are similar to those seen previously on rod and plank walking performance, as well as to those seen in aged rats, suggesting that the oxidation of DA coupled with a reduced capacity to respond to oxidative stress may be responsible for the induction of age-related cognitive deficits.

Receptor- and age-selective effects of dopamine oxidation on receptor-G protein interactions in the striatum

The striatum contains a high concentration of oxidizable dopamine (DA), and the aged organism shows a decreased ability to respond to oxidative stress (OS), making this area extremely vulnerable to free radical insult. To determine the receptor specificity of this putative increase in OS sensitivity, striatal slices from 6- and 24-month-old animals were incubated (30 min, 37 degrees C) in a modified Krebs medium containing 0 to 500 microM DA with or without a preincubation (15 min) in a nitrone trapping agent, 1 or 5 mM alpha-phenyl-n-tert-butyl nitrone (PBN), and changes in low Km GTPase activity (an index of receptor-G protein coupling/uncoupling) assessed in muscarinic, 5-HT1A D1, and D2 receptors stimulated with carbachol, 8 OH-DPAT-HBr, SKF 38393, or quinelorane, respectively. DA exposure induced selective decreases in the stimulated activity in all of these receptor systems, and an overall increase in conjugated dienes (56%) of the young. In the case of carbachol and 8 OH-DPAT-HBr, the DA-induced deficits in GTPase stimulation were seen primarily in the young (61 and 32%, respectively), while DA-induced deficits in quinelorane (D2) stimulation were seen in both age groups. In the case of SKF 38393-stimulation (D1) the DA-induced deficits were higher in the striatal tissue from the old. The DA-induced decreases in carbachol stimulated GTPase activity in the tissue from the young could be prevented by pretreatment with PBN or the DA uptake inhibitor, nomifensin. No effect of nomifensin was seen in the old, because their DA uptake mechanisms were already compromised. These results suggest that although age-related declines in DA uptake may provide some protection against the OS effects in muscarinic or 5-HT1A receptors, other factors may increase the vulnerability of DA neurons to OS, even with reductions in DA uptake.

Learning disorders in thymectomized mice: a new screening model for cognitive enhancer

Although the close relationship between the thymus and neuroendocrine system during the aging process has been well documented, influence of the thymus on the cognitive function of the central nervous system remains unknown. Male ddY mice were thymectomized 3-4 weeks after birth. Learning behavior, tested in a step-down test and in a spatial memory task, was significantly impaired in thymectomized mice at 10 months, but not before 5 months, after thymectomy. Reduced immune response was also not obvious before 10 months. These results suggested that thymectomy in young adult life in mice not only impaired the immune response, but also deteriorated the learning and memory ability, and that learning disorders in thymectomized mice could be utilized as a new screening model for cognitive enhancer.

Cholesterol: a two-edged sword in brain aging

Previous research from several laboratories has indicated that cholesterol (CHO) accumulates in neuronal membranes and alters their structural and signal transduction (ST) properties during aging. The possible reasons for these increases in membrane CHO have not been specified. However, present findings suggest that such accumulation may actually serve to protect neuronal tissue from oxidative damage. Striatal slices (6, 24 month rats) were preincubated in 1 mM CHO (30 min) followed by incubation with H2O2 (10 microM, 30 min). The slices were then either superfused with 30 mM KCl in the presence or absence of 500 microM oxotremorine (Ox), and K(+)-evoked dopamine release (K(+)-ERDA) examined or assessed for carbachol-stimulated low K(m) GTPase activity. The results indicated that CHO incubation prior to H2O2 in either age group was effective in preventing H2O2 reductions in both non-Ox-enhanced K(+)-ERDA and Ox conditions, as well as sodium nitroprusside (SNP 150 microM)-induced decreases in K(+)-ERDA. In addition, H2O2-induced deficits in carbachol-stimulated low K(m) GTPase activity were reduced in the striatal tissue from the old animals pretreated with CHO. However, if the slices were incubated in H2O2 prior to CHO exposure, CHO enhanced the H2O2 effects in the tissue from the old animals. Thus, depending upon the order of exposure, CHO functioned to enhance or retard the effects of oxidative stress, in an age-dependent manner.

Oxidative stress and age-related neuronal deficits

Research from our laboratory has indicated that the loss of sensitivity that occurs in several receptor systems as a function of age may be an index of an increasing inability to respond to oxidative stress (OS). This loss occurs partially as a result of altered signal transduction (ST). Assessments have involved determining the nature of age-related reductions in oxotremorine enhancement of K(+)-evoked dopamine release (K(+)-ERDA) from superfused striatal slices. Using this model, we have found that 1. Reductions can be restored with in vivo administration of the free-radical trapping agent, N-tert-butyl-alpha-phenylnitrone (PBN); 2. Decrements in DA release induced by NO or H2O2 from striatal slices from both young and old animals could be restored with alpha-tocopherol or PBN; 3. ST decrements, such as those seen in aging, could be induced with radiation exposure; and 4. Pre-incubation of the striatal slices with cholesterol decreased subsequent deleterious effects of NO or OH. on DA release. Thus, cholesterol, which increases in neuronal membranes as a function of age, may function as a potent antioxidant and protectant against neuronal damage. These results suggest that therapeutic efforts to restore cognitive deficits in aging and age-related disease might begin with antioxidant reversal of ST decrements.

Age differences in sensitivity to H2O2- or NO-induced reductions in K(+)-evoked dopamine release from superfused striatal slices: reversals by PBN or Trolox

Previous research has indicated that many age-related functional alterations may be the result of a decreased ability of the organism to respond to oxidative stress (OS). However, this hypothesis is based on indirect indices of function (e.g., increased vulnerability of hepatocytes from senescent animals to H2O2-induced DNA damage, increases in lipofuscin accumulation). More direct tests of this hypothesis, especially as it relates to brain aging, have not been extensively undertaken. Present experiments were carried out to make such tests by examining age differences in the sensitivity to OS on reductions in striatal dopamine (DA) release. Thus, K(+)-evoked DA (K(+)-ERDA) release from superfused striatal slices from young (6-8 month) and old (24-25 month) animals was examined following either: (a) application of the NO-generator sodium nitroprusside or (b) preincubation with H2O2. In order to assess the specific effects of OS on muscarinic (mAChR) sensitivity, oxotremorine-enhancement of K(+) -ERDA was examined following incubation with H2O2. Results showed that the striatal tissue from the old animals showed greater sensitivity to both H2O2 and NO than young animals, and stimulated DA decreased at lower concentrations of these agents (e.g., NO--100 microM young, 30 microM old). In addition, H2O2 was also effective in reducing oxo-enhanced K(+)-ERDA and was more effective as a function of age. If the striatal tissue was incubated in either Trolox (alpha-tocopherol) or alpha-phenyl-n-tert-butyl nitrone (PBN) prior to OS, the negative effects of NO. and H2O2 were reversed in both age groups. Results are discussed in terms of age-related membrane and endogenous antioxidant alterations that could induce increases in sensitivity to OS and the specificity of antioxidants in reducing this sensitivity in key functional systems.

Thymectomy-induced deterioration of learning and memory in mice

Male ddY mice were thymectomized 4 weeks after birth. Learning behaviors, tested in passive and active avoidance performances and in a spatial memory task, were significantly impaired in thymectomized mice at 10 months after thymectomy, in addition to the reduced immune response. Contents of hypothalamic norepinephrine and hypothalamic choline acetyltransferase activity were significantly increased in thymectomized mice. These results suggested that thymectomy at young adult life in mice not only impaired the immune response, but also deteriorated the learning and memory ability.