Neural plasticity in aging and Alzheimer's disease: some selected comments

The implication of protein malnutrition on cardiovascular control systems in rats

The malnutrition in early life is associated with metabolic changes and cardiovascular impairment in adulthood. Deficient protein intake-mediated hypertension has been observed in clinical and experimental studies. In rats, protein malnutrition also increases the blood pressure and enhances heart rate and sympathetic activity. In this review, we discuss the effects of post-weaning protein malnutrition on the resting mean arterial pressure and heart rate and their variabilities, cardiovascular reflexes sensitivity, cardiac autonomic balance, sympathetic and renin-angiotensin activities and neural plasticity during adult life. These insights reveal an interesting prospect on the autonomic modulation underlying the cardiovascular imbalance and provide relevant information on preventing cardiovascular diseases.

Upregulation of calcium binding protein, S100A6, in activated astrocytes is linked to glutamate toxicity

S100A6 (calcyclin), an EF-hand calcium binding protein, is considered to exert various functions, e.g., cell proliferation and differentiation, calcium homeostasis, and neuronal degeneration. In this study, we aimed to investigate whether S100A6 might be linked to glutamate toxicity using three animal models and pharmacological interventions. We first examined the age-related changes in S100A6 immunoreactivity in the mouse hippocampus, considering that an important negative aspect of brain aging is linked to increased extracellular glutamate. The surface area of S100A6-positive (+) astrocytes was significantly larger in aged mice than in young mice, while the numbers of S100β+ astrocytes did not change with age. In the second experiment, we examined the alterations in S100A6 immunoreactivity in the injured hypoglossal nucleus, because glutamate toxicity is considered to contribute to neuronal death after axotomy. There was no apparent S100A6 immunoreactivity in the hypoglossal nucleus of sham control animals. However, intense labeling for S100A6 in activated astrocytes was observed in the axotomized hypoglossal nucleus of mice. Administration of ceftriaxone, an astrocyte glutamate transporter enhancer, to axotomized mice significantly decreased the immunoreactivity for S100A6. In the third experiment, we tested an animal model of epilepsy using kainic acid (KA), a glutamate analog. In the mouse hippocampus after KA injection, S100A6 immunoreactivity was significantly increased in astrocytes, and pyknotic changes were observed in CA3 pyramidal neurons. Treatment of MK-801, an N-methyl-d-aspartate receptor antagonist, counteracted the KA-induced increase in S100A6 immunoreactivity, and reduced the numbers of pyknotic neurons. Our results indicate that upregulation of astrocytic S100A6 in response to extracellular glutamate may be involved in neuronal damage under pathophysiological conditions.

Glutamate uptake in cultured astrocytes depends on age: a study about the effect of guanosine and the sensitivity to oxidative stress induced by H(2)O(2)

Relatively few studies have been conducted to investigate the relationship between glutamate and development and/or aging. Rat cortical astrocyte cultures were used as a model to investigate glutamate uptake during development. The immunocontent of the markers glial fibrillary acidic protein (GFAP) and S100B increased, while basal secretion of S100B decreased, in astrocytes from 10 to 40 days in vitro (DIV). Basal glutamate uptake increased with age. Exposure to hydrogen peroxide decreased glutamate uptake more potently at 40 than 10 DIV. Moreover, 40 DIV astrocytes showed earlier loss of integrity (at 6 h) than 10 DIV astrocytes (at 24 h) after H(2)O(2) exposure. Addition of guanosine stimulated glutamate uptake only in 10 DIV astrocytes. The present work shows that mature astrocytes in culture present some neurochemical alterations also observed in astrocytes of aged animals. These results can contribute to the understanding of some consequences of the excitotoxicity and oxidative stress during brain aging.

Plasticity of GABA(a) system during ageing: focus on vestibular compensation and possible pharmacological intervention

The lesion of the vestibular end organ evokes static and dynamic symptoms, which spontaneously regress during a complex process known as 'vestibular compensation'. Vestibular compensation is age-dependent and involves several transmitter-identified pathways in the central nervous system. In this paper we studied the time course of vestibular compensation in adult (3 months) and old (24 months) rats and correlated behavioral recovery with modifications of glutamic acid decarboxylase (GAD) mRNA expression and benzodiazepine receptor density in different brain areas. Compensation in adult rats was complete 28 days after hemilabyrinthectomy, whereas old rats still showed significant behavioral impairment. A higher GABAergic tone was found in old rats, as indicated by higher benzodiazepine receptor density in lateral vestibular nucleus and higher mRNA level for glutamic acid decarboxylase in cerebral cortex and medial vestibular nucleus. In adult, compensated rats, benzodiazepine receptor density in the vestibular nuclei was normal 28 days after lesion, whereas GAD mRNA level was higher in anterior cingulate cortex, only. On the contrary, these parameters were still altered in anterior cingulate and somatosensory cortex, basal ganglia, vestibular nuclei and cerebellum in old rats 28 days after vestibular lesion. We also evaluated the effect of the ergoline derivative nicergoline on behavioral and neurochemical correlates of vestibular compensation in old rats. Nicergoline treatment attenuated the severity of oculomotor and postural symptoms after vestibular lesion and reversed most of these age- and lesion-induced alterations in GAD mRNA expression. Thus, lesion-related alterations of the GABAergic transmission and behavioral profile after vestibular lesion are age-dependent.

Plasticidade neural: relações com o comportamento e abordagens experimentais

As interações entre os estímulos ambientais e as respostas de um organismo determinam as propriedades comportamentais que lhe garantem adaptação a diferentes situações e individualidade comportamental. A interação organismo-ambiente também diferencia e molda os circuitos neurais, que caracterizam a plasticidade e a individualidade neural do organismo. Os estudos sobre plasticidade neural incluem aqueles que manipulam o ambiente e analisam mudanças em circuitos neurais e outros que enfatizam recuperação comportamental após lesão do sistema nervoso. Diferentes questões relativas à fisiologia e ao comportamento, como também à morfologia, à bioquímica e à genética, são abordadas. Este trabalho procura caracterizar diferentes abordagens no estudo da plasticidade neural, indicando as suas relações com a análise do comportamento e da aprendizagem. A investigação dos efeitos que a interação organismo-ambiente produz sobre os sistemas neurais subjacentes ao comportamento é enfatizada como interessante.

Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization

Mental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in "higher brain functions" such as learning, memory, perception, self-awareness and consciousness continuously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time provide the basis for a variety of neuropsychiatric disorders. It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly "creates" and "re-creates" the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. "morphoregulatory" molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challenge as it potentially interferes with those mechanisms that at the same time provide the basis for "higher brain function".

Effects of taurine on ozone-induced memory deficits and lipid peroxidation levels in brains of young, mature, and old rats

To determine the antioxidant effects of taurine on changes in memory and lipid peroxidation levels in brain caused by exposure to ozone, we carried out two experiments. In the first experiment, 150 rats were separated into three experimental blocks (young, mature, and old) with five groups each and received one of the following treatments: control, taurine, ozone, taurine before ozone, and taurine after ozone. Ozone exposure was 0.7-0.8 ppm for 4 h and taurine was administered ip at 43 mg/kg, after or before ozone exposure. Subsequently, rats were tested in passive avoidance conditioning. In the second experiment, samples from frontal cortex, hippocampus, striatum, and cerebellum were obtained from 60 rats (young and old), using the same treatments with 1 ppm ozone. Results show both an impairment in short-term and long-term memory with ozone and an improvement with taurine after ozone exposure, depending on age. In contrast to young rats, old rats showed peroxidation in all control groups and an improvement in memory with taurine. When taurine was applied before ozone, we found high peroxidation levels in the frontal cortex of old rats and the hippocampus of young rats; in the striatum, peroxidation caused by ozone was blocked when taurine was applied either before or after ozone exposure.

Alzheimer's disease-related gene expression in the brain of senescence accelerated mouse

The levels of Alzheimer's disease (AD)-related genes, including beta-amyloid precursor protein(APP), presenilin-1 (PS-1), PS-2, apoE, tau, c-fos, neural cell adhesion molecular 180 (NCAM-180), TGF-beta 1, IL-1 alpha/beta, IL-6, TNF-alpha/beta, alpha-2-Macroglobulin (alpha 2M), class II major histocompatibility antigen la (MHCII la), bcl-2 alpha, glucocorticoid receptor-alpha (GR alpha) and mineralocorticoid receptor (MR) mRNAs were determined by reverse transcription polymerase chain reaction (RT-PCR) in the hippocampus and cerebral cortex of senescence accelerated mouse (SAM). The levels of TGF-beta 1, IL-1 alpha, TNF-beta, c-fos, NCAM-180, PS-1 and APP mRNAs were normally expressed in SAMP8 compared with age-matched other subline that is resistant (SAMR1). The levels of apoE, GR alpha and MR mRNAs in the hippocampus of SAMP8, especially GR alpha, were evidently lower than those in the hippocampus of SAMR1. While bcl-2 alpha, PS-2 and tau mRNA levels of SAMP8 were significantly higher than those of SAMR1. Inflammatory cytokines (IL-1 beta, IL-6, TNF-alpha), alpha 2M and MHCII la antigen mRNAs were not detected in the brain of SAM. The differences of gene expression in the cerebral cortex were less evident than in the hippocampus. The results indicated that some genes abnormally expressed in the AD brain were also found in the brain of SAMP8, which may contribute to its age-related deterioration of learning and memory. Our results also suggested that functional and pathological changes which occurred in the brain of SAMP8 possessed some different aspects in comparison with the AD in consideration of the differences in gene expression.

Pregnenolone reverses the age-dependent accumulation of glial fibrillary acidic protein within astrocytes of specific regions of the rat brain

Although aged-related modifications of astrocytes have been frequently described, little is known so far about the signals responsible for these modifications. Since it is well demonstrated that astrocytes are highly responsive to a variety of steroids, we hypothesized that modifications of cerebral astrocytes may result from the age-related decrease of circulating steroids. In the present study, we investigated the effects of the chronic administration of pregnenolone (PREG), the precursor of all steroid hormones, on the age-related extension of astrocytic processes in various brain regions. In adult (2-3 month-old) and aged (22-24 month-old) rats, quantitative image analysis was used to estimate, within each region, the number of astrocyte cell bodies immunostained (IS) for S100, and the surface occupied by astrocytic cell bodies and processes IS for glial fibrillary acidic protein (GFAP). In all regions, the surface occupied by GFAP-IS structures was increased in the aged vs. the adult rats, whereas no significant modifications were observed in the number of S100-IS cell bodies. Chronic administration of PREG to aged rats induced a marked decrease in the surface occupied by GFAP-IS structures in the cortex, amygdala and thalamus, without any significant effect on the number of S100-IS cell bodies present in these regions. By contrast, PREG had no significant effect when administered to adult animals. These data suggest that decreased levels of circulating steroid hormones may be responsible for the age-dependent modifications of the astrocytes present in various brain regions, and that these modifications can be at least partly corrected by the administration of PREG.

Trk receptor alterations in Alzheimer's disease

The expression of trk receptors in postmortem normal, Huntington's disease and Alzheimer's disease human brains was investigated using immunohistochemistry, in-situ hybridisation and Western blotting. Alzheimer's disease hippocampi displayed an increase in trkA receptor levels in astrocytes in the CA1 region, some of which were associated with beta-amyloid-positive plaques. Truncated trkB receptors were found in high levels in senile plaques, while the full-length receptor was expressed in glial-like cells in the hippocampus of Alzheimer's disease brains. In-situ hybridisation studies indicated that trk receptor mRNA was also elevated in Alzheimer's. The appearance of trkA and trkB receptors in astrocytes and plaques in Alzheimer's disease might be related to beta-amyloid deposition and could be implicated in the development of Alzheimer's disease.

Regional changes in expression of NCAM, GFAP, and S100 in aging rat brain

In aging brain degenerative processes occur. However, the aging brain still have regenerative capacity although diminished compared to young rats. The neural cell adhesion molecule (NCAM) may be involved in neuroplasticity during regenerative events. In this study, the polypeptide composition and amount of NCAM was determined in regions of brain from young, mature and old rats. During adult life, the amount of NCAM decreased in several brain regions whereas in aged rats, NCAM was enhanced in all brain regions examined. The amount of the glial fibrillary acidic protein (GFAP) increased during aging in all brain regions reflecting general gliosis in the aged rat brain. The amount of the neuro- and gliotrophic protein S100 increased from young adult to mature age in all brain regions investigated followed by a decrease during old age. Aged rats were tested in a Morris water maze and a group of rats (20%) with learning impairment was defined. However, no differences in amount of NCAM, GFAP, or S100 were observed between aged rats with and without spatial learning impairment.

Structural plasticity of synapses in Alzheimer's disease

Plasticity of the synaptic contact zone was previously observed following loss of synapses in the cerebral cortex of normal aging humans. The present study was undertaken to determine if there was quantitative evidence of synapse loss and synapse plasticity in the inferior temporal, superior parietal, parieto-occipital, and superior frontal cortical regions in Alzheimer's disease (AD), and how such changes related to the neurofibrillary tangles and amyloid plaques. The results showed that age at autopsy did not correlate with the numbers of synapses, plaques, or tangles. However, the numbers of synapses strongly reflected the pathology of AD; in all four brain regions, there were fewer synapses as the numbers of plaques and tangles increased. In the inferior temporal and superior parietal cortices, the loss of synapses was accompanied by an increase in the synaptic contact length. The results suggest that, in some cerebral cortical brain regions, synapses are capable of plasticity changes, even when the pathology of AD and loss of synapses are severe.