Net dendritic stability of layer II pyramidal neurons in F344 rat entorhinal cortex from 12 to 37 months

Aging of Brain: Role of Estrogen

The brain undergoes many structural and functional changes during aging. Some of these changes are regulated by estrogens which act mainly through their intracellular receptors, estrogen receptor ERalpha and ERbeta. The expression of these receptors is regulated by several factors including their own ligand estrogen, and others such as growth hormone and thyroid hormone. The levels of these factors decrease during aging which in turn influence estrogen signaling leading to alterations in brain functions. In the present paper, we review the effects of aging on brain structure and function, and estrogen action and signaling during brain aging. The findings suggest key role of estrogen in the maintenance of brain functions during aging.

Apical vulnerability to dendritic retraction in prefrontal neurones of ageing SAMP10 mouse: a model of cerebral degeneration

The SAMP10 mouse is a model of accelerated ageing in which senescence is characterized by age-related atrophy of the cerebral cortex and limbic structures, poor learning and memory task performance with depressive behaviour and cholinergic and dopaminergic alterations. Here we studied age-related changes in the dendritic arbors and spine density of pyramidal cells in the medial prefrontal cortex of SAMP10 mice using a quantitative Golgi method. Dendrites of prefrontal neurones gradually retracted with ageing towards the soma with the relative preservation of overall complexity. Apical dendrites were much more severely affected than basal dendrites. The combined length of the apical dendrites and spine density were decreased by 45% and 55%, respectively, in mice at 12 months, compared with mice at 3 months of age. Immunohistochemical and immunoblot analyses indicated that expression of microtubule-associated protein (MAP) 2, a marker of dendrites, decreased in an age-related manner not only in the anterior cortex but also in the posterior cortex and olfactory structures in SAMP10 mice. Decreased expression of MAP2 mRNA caused the decrease in MAP2 protein expression. These results suggest that retraction of apical, but not of basal dendrites, with a loss of spines in prefrontal neurones, appears to be responsible for poor learning and memory performance in aged SAMP10 mice. It is also suggested that age-related dendritic retraction occurs in a wide area including the entire cerebral cortex and olfactory structures.

Age-related and laminar-specific dendritic changes in the medial frontal cortex of the rat

Early hypotheses that normal brain aging involves widespread loss of neurons have been revised in light of accumulating evidence that, in most regions of the brain, the number of neurons is stable throughout adulthood and senescence. It is not clear, however, that all aspects of neuronal structure are similarly maintained, and anatomical changes are likely to contribute to age-related declines in cognitive function. The extent and pattern of dendritic branches is one likely target for age-dependent regulation since dendrites remain plastic into adulthood and since dendrites, as the site of most synapses, critically regulate neuronal function. This study quantified the dendritic extent and geometry of superficial and deep pyramidal neurons in the medial frontal cortex of Brown Norway rats from young adulthood through senescence. This region of cortex is of specific interest given its involvement in a variety of cognitive functions that change with age. In the present study, age-related changes in dendritic extent were found to occur with remarkable specificity. Superficial, but not deep, pyramidal neurons exhibited ongoing dendritic growth after 2 months-of-age and then dendritic regression after 18 months-of-age. Apical and basal dendrites were similarly regulated; in each arbor adult growth and regression were limited to terminal dendritic segments. The focal specificity of age-related changes suggests several possible regulatory mechanisms, including regional changes in trophic support and in neuronal activity. Although restricted to specific neuronal populations, dendritic regression in aged animals is likely to contribute to cognitive changes associated with senescence.

The effect of age and testosterone on the expression of glial fibrillary acidic protein in the rat cerebellum

Testosterone reversed the age-related increase in glial fibrillary acidic protein (GFAP) in the male rat cerebellum, a brain region not generally associated with gonadal steroid hormone sensitivity. This supports the hypothesis that a decrease in circulating testosterone contributes to age-related increase in GFAP. These data also suggest that reductions in circulating gonadal steroids during aging could render the brain more susceptible to neurodegeneration and that hormone replacement therapy might have value in neurodegenerative disease intervention.

Layer-specific dendritic regression of pyramidal cells with ageing in the human prefrontal cortex

The dendritic field of pyramidal neurons in cortical layers IIIc and V of the prefrontal cortex in ageing humans was studied. The three-dimensional branching pattern of the basilar dendrites of Golgi-Cox impregnated neurons was analysed in the middle frontal gyrus (areas 9 and 46) in eight subjects between the ages of 49 and 90 years, all without a neurological or psychiatric disorder. The results revealed a significant regression of the layer V dendritic pattern with increasing age, but the layer IIIc neurons did not show any age-related changes. Together with our earlier data on the postnatal development of the same cell types in the prefrontal cortex, we hypothesize that the layer V neurons in the prefrontal cortex start to regress from the fifth decade onwards, in contrast to the layer IIIc neurons which remain stable from puberty on. We conclude that pyramidal cells in layer IIIc and V in a similar cortical region undergo a differential ageing effect.

Region-specific stability of dendritic extent in normal human aging and regression in Alzheimer's disease. I. CA1 of hippocampus

Pyramidal neurons in two subdivisions of CA1 (CA1c and CA1a + b) of hippocampus from human brains obtained at autopsy were studied in Golgi Cox-stained tissue. Seventeen cases were a part of a normal aging series, ranging in age from 43 to 95 years; and 5 cases had Alzheimer's disease (AD). Dendritic extent of apical and basal trees was found to be stable in normal aging. In AD there was a significant loss of total dendritic length and/or average segment length for the apical and basal trees of both subdivisions of CA1. This finding is consistent with the findings of severe pathology in CA1 reported by others. The reductions in overall dendritic extent in CA1a + b in AD could be attributed largely to alterations in the lengths of the terminal segments. Apical and basal trees of CA1c were more severely affected by AD than those of CA1a + b and showed more widespread reductions in numbers of segments as well as lengths of segments.