Quantitative investigation of the morphological plasticity of synaptic junctions in rat dentate gyrus during aging

Astroglia in Alzheimer's Disease

Alzheimer's disease is the most common cause of dementia. Cellular changes in the brains of the patients suffering from Alzheimer's disease occur well in advance of the clinical symptoms. At the cellular level, the most dramatic is a demise of neurones. As astroglial cells carry out homeostatic functions of the brain, it is certain that these cells are at least in part a cause of Alzheimer's disease. Historically, Alois Alzheimer himself has recognised this at the dawn of the disease description. However, the role of astroglia in this disease has been understudied. In this chapter, we summarise the various aspects of glial contribution to this disease and outline the potential of using these cells in prevention (exercise and environmental enrichment) and intervention of this devastating disease.

Estrogen deficiency is associated with hippocampal morphological remodeling of early postmenopausal mice

Estrogen (E₂) deficiency is reported to involve in the impairment of cognition in postmenopausal women. However, the morphological basis is still unclear. In the present study, using transmission electron microscopy (TEM), we observed the ultrastructure of hippocampus in female C57BL/6 mice at the age of 18 months (18 M) which is considered as the early stage of postmenopause (n = 8). Compared with control mice aged 6 M (n = 8), we identified that the morphological changes in the hippocampus of these menopausal mice were mitochondrial damage, lipofuscin deposition and microtubule degradation. Notably, after E₂ was subcutaneously injected into mice aged 16 M with a dosage of 3.5 μg/kg every three days for two months in the 18 M + E₂ group (n = 8), mitochondrial damage and lipofuscin deposition in the DG region of hippocampus were prevented, but the degraded microtubules in the hippocampus of postmenopausal mice were failed to restore. These data suggest that hippocampal ultrastructure remodeling in mice can be initiated at the early stage of postmenopause, E₂ supplementation could only have an effect on mitochondrial damage and lipofuscin increase.

Low intensity rTMS has sex-dependent effects on the local response of glia following a penetrating cortical stab injury

Repetitive transcranial magnetic stimulation (rTMS), a non-invasive form of brain stimulation, has shown experimental and clinical efficacy in a range of neuromodulatory models, even when delivered at low intensity (i.e. subthreshold for action potential generation). After central nervous system (CNS) injury, studies suggest that reactive astrocytes and microglia can have detrimental but also beneficial effects; thus modulating glial activity, for example through application of rTMS, could potentially be a useful therapeutic tool following neurotrauma. Immunohistochemistry was used to measure the effect of low intensity rTMS (LI-rTMS) on GFAP (astrocyte), IBA1 (microglial), and CS56 (proteoglycan) expression in a unilateral penetrating cortical stab injury model of glial scarring in young adult and aged male and female C57BL6/J mice. Mice received contralateral low frequency, ipsilateral low frequency, ipsilateral high frequency or sham LI-rTMS (4-5mT intensity), for two weeks following injury. There was no significant difference in the overall volume of tissue containing GFAP positive (⁺) astrocytes, IBA1⁺ microglia, or proteoglycan expression, between sham and LI-rTMS-treated mice of all ages and sex. Importantly however, the density of GFAP⁺ astrocytes and IBA1⁺ microglia immediately adjacent to the injury was significantly reduced following ipsilateral low and high frequency stimulation in adult and aged females (p≤0.05), but was significantly increased in adult and aged males (p≤0.05). LI-rTMS effects were generally of greater magnitude in aged mice compared to young adult mice. These results suggest that sex differences need to be factored into therapeutic rTMS protocols. In particular, more work analyzing frequency and intensity specific effects, especially in relation to age and sex, is required to determine how rTMS can best be used to modify glial reactivity and phenotype following neurotrauma.

Ultrastructural synaptic changes associated with neurofibromatosis type 1: a quantitative analysis of hippocampal region CA1 in a Nf1(+/-) mouse model

Neurofibromatosis type 1 (NF1) is one of the most frequently diagnosed autosomal dominant inherited disorders resulting in neurological dysfunction, including an assortment of learning disabilities and cognitive deficits. To elucidate the neural mechanisms underlying the disorder, we employed a mouse model (Nf1(+/-) ) to conduct a quantitative analysis of ultrastructural changes associated with the NF1 disorder. Using both serial light and electron microscopy, we examined reconstructions of the CA1 region of the hippocampus, which is known to play a central role in many of the dysfunctions associated with NF1. In general, the morphology of synapses in both the Nf1(+/-) and wild-type groups of animals were similar. No differences were observed in synapse per neuron density, pre- and postsynaptic areas, or lengths. However, concave synapses were found to show a lower degree of curvature in the Nf1(+/-) mutant than in the wild type. These results indicate that the synaptic ultrastructure of Nf1(+/-) mice appears relatively normal with the exception of the degree of synaptic curvature in concave synapses, adding further support to the importance of synaptic curvature in synaptic plasticity, learning, and memory.

Astroglia in dementia and Alzheimer's disease

Astrocytes, the most numerous cells in the brain, weave the canvas of the grey matter and act as the main element of the homoeostatic system of the brain. They shape the microarchitecture of the brain, form neuronal-glial-vascular units, regulate the blood-brain barrier, control microenvironment of the central nervous system and defend nervous system against multitude of insults. Here, we overview the pathological potential of astroglia in various forms of dementias, and hypothesise that both atrophy of astroglia and reactive hypertrophic astrogliosis may develop in parallel during neurodegenerative processes resulting in dementia. We also show that in the transgenic model of Alzheimer's disease, reactive hypertrophic astrocytes surround the neuritic plaques, whereas throughout the brain parenchyma astroglial cells undergo atrophy. Astroglial atrophy may account for early changes in synaptic plasticity and cognitive impairments, which develop before gross neurodegenerative alterations.

Hippocampal granule cells in normal aging: insights from electrophysiological and functional imaging experiments

Normal aging, in the absence of neurodegenerative disease, can provide important insights into the mechanisms by which the brain can maintain cognitive abilities across the lifespan. Hippocampal-dependent memory processes can become vulnerable as age advances. The focus of this chapter is the contribution of hippocampal granule cells to cognitive impairments that are observed during aging. A number of alterations in structure, function, and gene expression have been observed in aged granule cells, any of which may lead to adaptive, compensatory or detrimental consequences to hippocampal function. As the average life span of humans continues to increase, those who reach 100 years or beyond is more common. Individuals that have aged successfully, and exhibit high levels of cognitive ability can provide useful clues into the enormous potential possessed by the mammalian brain.

Cholinergic system, rearing environment and trajectory learning during aging in mice

Three, 12- and 20-month-old C57BL6/J mice, reared in standard conditions or in enriched environments, were administered subcutaneously either scopolamine hydrobromide, 0.6 or 1.2 mg kg(-1), or physiological saline (control mice) 15 min before testing their abilities to find an invisible platform in a modified version of the Morris water maze, the starting point being kept unchanged throughout the experiment to allow the aged animals to solve the task. The results demonstrated that: 1) All control mice, whatever their age, were able to learn the platform location, but the number of trials needed to reach the learning criterion (3 consecutive trials in less than 8 s) increased with age; 2) All the scopolamine-treated mice, whatever their age, were also able to learn the platform location. However, compared to age-matched controls, the number of trials needed to reach the learning criterion was greater; 3) Rearing the animals in an enriched environment antagonized the effect of scopolamine, but only in the youngest (3 month-old) mice. All control and scopolamine-treated mice, whatever their age and their rearing environment, remembered, 7 days later, the platform location.

Changes in synaptic ultrastructure during reactive synaptogenesis in the rat dentate gyrus

Advances in stereology, combined with continuing relevance to aging, as well as recovery from disease and injury make the reexamination of reactive synaptogenesis (RS) overdue. Moreover, recent mathematical models have suggested novel aspects of morphology, such as compartmentalization, may have profound effects on synaptic transmission. Given these novel findings, their correlation with other models of synaptic plasticity, and their potential significance for behavioral function, the precise nature of these changes need to be explored through quantitative morphometry. Towards this goal, the synaptic morphology of the dentate gyrus was assessed via serial electron microscopy at 3, 6, 10, 15, and 30 days following unilateral entorhinal cortex lesions. Foremost, the results showed that degree of curvature is a plastic feature of synapses. During RS, concave synapses showed an immediate/long-lasting increase in curvature, suggesting their importance in the compensation response. Flat synapses showed unique changes in growth, having implications for development and activation following synaptogenesis. Moreover, changes in size and curvature showed a different dynamic depending on proximity from damage. In the directly denervated MML, synapses showed an increase in curvature proportionate to increases in size. In the neighboring IML, however, these changes were independent-increases in curvature far surpassed synaptic growth.

Impact of aging on hippocampal function: plasticity, network dynamics, and cognition

Aging is associated with specific impairments of learning and memory, some of which are similar to those caused by hippocampal damage. Studies of the effects of aging on hippocampal anatomy, physiology, plasticity, and network dynamics may lead to a better understanding of age-related cognitive deficits. Anatomical and electrophysiological studies indicate that the hippocampus of the aged rat sustains a loss of synapses in the dentate gyrus, a loss of functional synapses in area CA1, a decrease in the NMDA-receptor-mediated response at perforant path synapses onto dentate gyrus granule cells, and an alteration of Ca(2+) regulation in area CA1. These changes may contribute to the observed age-related impairments of synaptic plasticity, which include deficits in the induction and maintenance of long-term potentiation (LTP) and lower thresholds for depotentiation and long-term depression (LTD). This shift in the balance of LTP and LTD could, in turn, impair the encoding of memories and enhance the erasure of memories, and therefore contribute to cognitive deficits experienced by many aged mammals. Altered synaptic plasticity may also change the dynamic interactions among cells in hippocampal networks, causing deficits in the storage and retrieval of information about the spatial organization of the environment. Further studies of the aged hippocampus will not only lead to treatments for age-related cognitive impairments, but may also clarify the mechanisms of learning in adult mammals.

FK960, a potential anti-dementia drug, increases synaptic density in the hippocampal CA3 region of aged rats

There is accumulating evidence suggesting that synapse formation in the adult brain is dynamically regulated, and that this regulation plays a role in cognitive function. A decrease in synaptic density is reportedly related to memory deficits in aged animals as well as in Alzheimer's patients. FK960 [N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate], a novel anti-dementia drug, has been shown to ameliorate experimental amnesia in rats and monkeys through activation of the somatostatinergic nervous system in the hippocampus. Furthermore, FK960 has been shown to be considerably more effective in a model of spontaneous amnesia in aged rats than cholinesterase inhibitors. In the present electron microscopy study, we demonstrated that the density of axodendritic and axosomatic synapses in the hippocampal CA3 region of aged rats was reduced compared to young rats, and that repeated treatment with FK960 for either 3 or 21 days dose-dependently reversed these deficits in aged rats. This FK960-induced increase in synaptic density was transient and density returned to basal levels at 8 days after the final dose. In contrast, FK960 did not alter synaptic density in the cingulate cortex or hippocampal CA1 region in aged rats, nor the CA3 region of young rats. Collectively, these results suggest that FK960 can selectively and reversibly increase synaptic density in the hippocampal CA3 region of aged rats, and that this activity may play a role in its cognitive-enhancing action.

Total regional and global number of synapses in the human brain neocortex

An estimator of the total number of synapses in neocortex of human autopsy brains based on unbiased stereological principles is described. Each randomly chosen cerebral hemisphere was stratified into the four major neocortical regions. Uniform sampling with a varying sampling fraction in each region of neocortex was performed. The total volume of each neocortical region was estimated using point counting according to Cavalieri's principle. The ethanolic phosphotungstic acid staining technique was modified for synapses in human autopsy brains. The numerical density of synapses in each neocortical region studied was estimated using the disector at the electron microscopical level. The total number of neocortical synapses in each region was estimated as the product of the total volume of neocortex and the numerical density of synapses. The influence of the postmortem fixation delay on the number of synapses was investigated in five large mammals (one dog, one cow, and three pigs), the brains of which were kept under conditions similar to those under which human corpses are normally kept. The apparent decrease of 3.9% in the numerical density of synapses in the large mammals following a 2-day fixation delay was not significant. The average total number of synapses in the neocortex of five young male brains was 164 x 10(12) (CV = 0.17). An analysis of the precision of the estimate of the total number of synapses in neocortex indicates that blocks represent both the major source of variation and the largest workload. Using eight blocks per brain the imprecision of the estimate is, however, only 66% of the total variance.

Age-related changes in the synapses of the rat's neostriatum

By means of transmission electron microscopy, the age-related changes in axospinous (ASS) and axodendritic (ADS) synapses in the dorsal part of the rostral neostriatum in two groups of Wistar rats: young (3-month-old), and senescent (25-month-old) were examined. The changes in different parameters, characterizing the ASS and ADS: synaptic density (SD), number of synaptic vesicles (SV), number of synaptic contact zone (SCZ), and number of dendritic spines, bearing synapses (DS) were investigated morphometrically. The SD of the ASS decreased significantly during aging, but the SD of the ADS did not changed significantly. The mean area of the synaptic boutons increased significantly during aging in two types of synapses. The mean number of vesicles per synaptic bouton increased, but the number of vesicles per microm2 of synaptic bouton, and per microm3 of the neuropil decreased. The mean SCZ length increased in both types of synapses. The total SCZ length per 1000 microm2 of the neuropil, and the total area of the SCZ per 1000 microm3 of the neuropil decreased in ASS, but the same parameters of the ADS did not changed significantly. The mean number of synaptic DS per 1000 microm2 of the neropil decreased during aging, but the mean area of the synaptic DS increased. The present results support the hypothesis that the synaptic contacts change significantly during aging, and the ASS are more vulnerable during aging than the ADS.

Dietary restriction modulates synaptic structural dynamics in the aging hippocampus

A computer-assisted morphometric study has been carried out on the synaptic ultrastructural features in the hippocampus of 14-month old (DR14) and 27-month old (DR27) dietary restricted (-50% lipids and -35% carbohydrates) rats. Age-matched controls were maintained on an ad libitum (AL) feeding schedule. Synaptic numeric density (Nv), surface density (Sv) and average area (S) were the parameters measured. In old AL vs. adult AL animals, Nv decreased to a not significant extent, while S increased and Sv decreased significantly. In DR14 rats vs. AL littermates Nv increased significantly, but S and Sv were unchanged. DR27 rats vs. age-matched AL controls showed a significant increase of Nv and Sv while S was significantly decreased. Comparing DR14 vs. DR27, no significant difference due to age was documented. Both in DR14 and in DR27 groups the percent distribution of S showed a marked increase of smaller contact zones. Despite reporting on discrete aspects of synaptic ultrastructure, Nv and S are supported to be in an inverse relationship which aims at maintaining Sv constant. Thus, these three ultrastructural parameters when taken together per experimental group, appear to provide information on synaptic morphological rearrangements. In this context, the percent increase of smaller synapses in DR animals is consistent with the idea of a marked remodelling process. Considering previous data from the same groups of rats reporting significant changes in neuronal membrane lipid composition and fluidity, we interpret our findings to account for a positive modulation of dietary restriction on the synaptic structural dynamics.

Glial and neuronal cell numbers and cytochrome oxidase activity in CA1 and CA3 during postnatal development and aging of the rat

Stereological methods (neuron and glial cell numbers) and histochemical methods (cytochrome c oxidase) were used to study postnatal development and aging of the CA1 and CA3 hippocampal areas in male rats. No changes were observed in 10 microns sections in the neuronal population of areas CA1 and CA3 in any of the groups (14 days, 21 days, adult-90 days and elderly-22 months). Statistical differences were found in the number of glial cells in both the CA1 and CA3 areas. An increase was observed in cytochrome oxidase (CO) activity in the CA1 area in the 14 day old rats compared to the other groups while in area CA3 this parameter increased in the 14 and 21 day old groups and the group of adult rats. No significant changes in CO activity were found in the elderly rats in both areas. These results are discussed in the light of those recorded in other areas of the limbic system.

Embryonic (PSA) N-CAM reveals chains of migrating neuroblasts between the lateral ventricle and the olfactory bulb of adult mice

In the brain of adult mice, cell division persists in the subventricular zone (SVZ) of the lateral ventricles. These SVZ cells migrate rostrally 3-5 mm to the olfactory bulb, where they differentiate into neurons. We have investigated the distribution of PSA-N-CAM in the adult mouse forebrain. Immunoreactivity for PSA-N-CAM precisely reveals the migratory pathway of SVZ cells. This pathway of PSA-N-CAM positive cells starts in the lateral wall of the lateral ventricle, where immunopositive cells form weblike patterns. The PSA-N-CAM positive pathway extends rostrally between the corpus callosum and the striatum into the anterior ventral telencephalon, and then into the core of the olfactory bulb. Experiments in which [3H]-thymidine was injected systemically indicated that the majority of the dividing cells on the SVZ of the lateral ventricle and along the migratory pathway are positive to PSA-N-CAM or closely associated with PSA-N-CAM. Microinjection of [3H]-thymidine into the SVZ of the lateral ventricle to label a small patch of dividing SVZ cells shows that neuroblasts that migrated away from the injection site are positive or are closely associated with other cells that are positive for PSA-N-CAM. Migrating cells are tethered together, forming long chains of immunopositive cells. The migratory pathway is formed by 30-40 of these immunopositive chains. Radially oriented individual PSA-N-CAM positive cells were observed in the olfactory bulb. These cells seem to have broken away from chains of immunopositive cells in the core of the olfactory bulb and to be migrating to more superficial layers. Little is known about the mechanisms of tangential migration during development and in adulthood. The cell-cell arrangement revealed by PSA-N-CAM staining suggests new models for this form of neuronal migration. PSA-N-CAM localization along the migratory pathway to the olfactory bulb suggests that in the adult brain this molecule plays a role in migration of neuronal precursors.

Cerebrovascular, neuronal, and behavioral effects of long-term Ca2+ channel blockade in aging normotensive and hypertensive rat strains

The pathogenesis of essential hypertension is not fully understood, but most of the cardiovascular, metabolic, neurogenic, and humoral abnormalities are explained by dysfunctions in the control of intracellular Ca2+ concentrations in the cells of the vascular wall. Most theories of disturbed calcium regulation focus on the calcium concentration within vascular smooth muscle cells. The implications of hypertension for the increased calcium content of aging arteries seem to be clear, but were only studied in the peripheral circulation; hypertension prominently augments the aging-related accumulation of calcium in the vessel wall. Although the contribution of calcium overload in hypertensive cerebrovascular damage is well documented, it is not clear yet if hypertension per se is the main cause of hypertension-associated calcium-dependent cerebral damage. Thus far, the hypotensive effects of most calcium antagonists were extensively described, and their efficacy in stroke prevention was proven. Earlier studies indicated that chronic administration of nimodipine revealed a protective effect in the occurrence of strokes in SHR-SP rats, yielding a decreased mortality rate. Because nimodipine did not lower the extremely high blood pressure of these animals, the mechanisms behind such nimodipine-induced stroke prevention may be attributed to a direct cerebrovascular and/or neuronal action of nimodipine. Hypertension is generally considered a vascular pathologic condition, and most research has been directed towards the influences of hypertension on large peripheral arteries such as the aorta and coronary artery. The influence of the CNS on the regulation of cardiovascular system and blood pressure regulation was described in detail, and the role of the CNS in hypertension also was the subject of study. The increased risk of stroke in hypertensive subjects generated numerous studies on the precise nature of compromised cerebrovascular functioning under hypertensive conditions. Few data are available on Ca2+ alterations in cerebral neurons during hypertension. Honda et al. demonstrated that voltage-dependent Ca2+ uptake was higher in cortical synaptosomes from SHR than form normotensive animals and suggested that an important alteration in Ca2+ channel characteristics may occur in SHR brain synaptosomes. Although the density of L-type calcium channels was shown to be higher in the hippocampus of SHR rats, others reported that the number of L-type calcium channels was significantly lower in the brain of SHR rats than WKY normotensive controls. The latter data suggest that hypertension may be associated with similar alterations in neuronal calcium homeostasis as demonstrated for aging in normotensive subjects.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic mitochondria and ageing: computer-assisted morphometry in rat cerebellar glomeruli

The ultrastructural features of synaptic mitochondria have been investigated by means of computer-assisted morphometry in the cerebellar glomeruli of young, adult and old rats. The volume fraction occupied by mitochondria (volume density: Vv), the number of organelles/ microm(3) (numerical density: Nv), the average volume (V) and the average length (Sk) of the mitochondria were the parameters measured in electron microscopic photos taken at an enlargement of x 8000. Vv showed a lifespan constancy. Nv was increased in the adult group vs. the young, but it was decreased in old animals as compared with the other 2 groups of age studied. V and Sk showed the same age-dependent changes: a significant decrease in the adults and a significant increase in the old rats vs. the other groups analysed, respectively. We interpret these findings to represent age-dependent morphofunctional adaptations occurring at synaptic mitochondria as a response to actual energy demands from the synaptic connectivity they subserve. Although impaired, the dynamic morphology of synaptic mitochondria appears to be capable of consistent compensative ultrastructural rearrangements in the central nervous system of old individuals.

Dynamic morphology of the synaptic junctional areas during aging: the effect of chronic acetyl-L-carnitine administration

The ultrastructural features of hippocampal synaptic contact zones have been investigated by means of computer-assisted morphometry in rats of 6, 12 and 22 months of age and in age-matched animals chronically treated with ALCAR at a daily dose of 50 mg/100 g body weight from the age of 1 month up to the day of sacrifice. The number of synapses/microns 3 (Nv), the average size of the junctional areas (S) and the total area of the synaptic contact zones/microns 3 (Sv) were measured in tissue samples stained by means of the ethanol phosphotungstic acid (E-PTA) preferential technique for synaptic membranes. In control animals Nv was constant between 6 and 12 months of age, but significantly decreased in 22-month-old rats; S did not show significant differences due to age; Sv was unchanged between 6 and 12 months, but it decreased significantly in the old animals. In ALCAR treated rats Nv increased and S decreased significantly vs. Age-matched controls. Sv showed a lifespan constancy among the groups of age analysed. In ALCAR treated rats the number of contact areas smaller than 0.08 micron 2 increased by 18, 9 and 10% at 6, 12 and 22 months of age, respectively. ALCAR administration resulted in a lifespan modulation of synaptic structural dynamics. A proper metabolism at nerve terminals is accounted to play a crucial role in synaptic remodelling potential: on the basis of current research data, it is suggested that ALCAR may improve neuronal bioenergetic mechanisms.

Decreased synaptic density in aged brains and its prevention by rearing under enriched environment as revealed by synaptophysin contents

Changes in synaptic density in various brain regions were assessed among different age groups of rats maintained in ordinary small cages, as determined by synaptophysin assay. The synaptophysin content in hippocampus decreases as early as in the adult stage. The most remarkable decrement occurs in occipital cortex. In other regions, synaptophysin contents decrease in senescence to 60-77% of the respective peak values during young and adult stages. The other rat group reared under enriched environment in a large cage until 30 months of age was examined for synaptic density, and was revealed to maintain the similar levels as in young, or even higher levels in frontal, temporal, entorhinal cortices and hippocampus. These results indicate that the synaptic density in cerebrum decreases in senescence and this decrease can be prevented by rearing under enriched environment.

Morphological plasticity of synaptic mitochondria during aging

A morphometric investigation has been carried out on the synaptic mitochondria of cerebellar glomeruli in young, adult and old rats by means of a computer-assisted image analysis technique. Mitochondrial volume density (Vv), numerical density (Nv), average volume (V) and average length (Skeleton = Sk) were investigated in tissue samples fixed, embedded and sectioned according to conventional electron microscopic methods. Vv was unchanged in the three groups of age taken into account. Nv was significantly increased in adult vs. young animals, whereas it was decreased in the old group as compared to both the other two groups investigated. V and Sk showed the same age-dependent changes: they significantly decreased in the adult vs. the young and the old groups of rats while increased significantly in the old rats vs. both the adult and young animals. A percentage distribution of Sk demonstrated that in the old group 20.6% of the population of synaptic mitochondria accounts for elongated organelles (> 5 microns) as compared to 8.6% and 5.3% in young and adult animals, respectively. The present findings match the changes previously reported by us on the ultrastructure of synaptic contact zones both in rats and human beings, and support the idea of an age-dependent dynamic adaptation in the morphology of synaptic mitochondria to cope with the metabolic needs of the pattern of synaptic connectivity they subserve.

Synaptic plasticity in the dentate gyrus of aged rats is altered after chronic nimodipine application

We examined ultrastructural correlates of synaptic plasticity in the hippocampus of young (3 months) vs aged (30 months) Wistar rats and established the effects of the calcium antagonist nimodipine in animals chronically treated from 24 to 30 months. The effects of nimodipine was studied since this compound improves hippocampal neuronal physiology and enhances cognitive function during aging. In the supragranular layer of the dentate gyrus we found a 24% decrease in synaptic density (Nv) in aged animals, while synaptic size (S) was not significantly altered. After nimodipine treatment Nv in aged rats was not significantly different from young adults, thus being significantly increased compared to age-matched controls. The size of synapses was not significantly altered after nimodipine administration. Total synaptic surface area (Sv) in nimodipine-treated animals was significantly increased compared to aged controls, however, Sv remained significantly lower than in young adults. These data indicate that chronic administration of nimodipine enables granular cells in the dentate gyrus to maintain its number of synaptic contacts during the aging process. Furthermore, the presented influence of nimodipine on synaptic plasticity processes may underlie previously reported improved cognitive functioning of aged animals treated similarly with nimodipine.

Age-related loss of axospinous synapses formed by two afferent systems in the rat dentate gyrus as revealed by the unbiased stereological dissector technique

Previous attempts to elucidate whether a loss of hippocampal synapses occurs during aging provided conflicting results, possibly due to the unavailability, at the time, of unbiased methods for synapse quantitation. This study was designed to reexamine the issue by means of modern technical procedures that provide unbiased estimates of synaptic numbers. Groups of 14 young adult (5 months old) and 14 aged (28 months old) male Fischer-344 rats were compared. Synapses were examined in the middle (MML) and inner (IML) molecular layer of the hippocampal dentate gyrus, where synaptic contacts are predominantly formed by different systems of afferents, the entorhinal and commissural-associational fibers, respectively. The number of synapses per neuron was estimated with the aid of the stereological dissector technique. The results showed that the total number of synaptic contacts per neuron was significantly diminished in the MML (by 23.6%) and IML (by 22.7%) of aged rats relative to young adults. This age-related synaptic loss involved axospinous, but not axodendritic, junctions of the MML (-24.4%) and IML (-24.0%). Both perforated and nonperforated axospinous synapses (distinguished by a discontinuous or continuous postsynaptic density, respectively) exhibited an age-dependent decrease in numbers, though this decrease did not reach statistical significance in the case of perforated junctions of the IML. The observed age-related loss of axospinous synapses may underlie the reduction in the amplitude of excitatory postsynaptic potentials and the decline in functional synaptic plasticity detected in the dentate gyrus of senescent rats.

Mapping of the distribution of polysialylated neural cell adhesion molecule throughout the central nervous system of the adult rat: an immunohistochemical study

In the nervous system, the neural cell adhesion molecule changes at the cell surface during development, from a form highly enriched in polysialic residues to several isoforms containing much less sialic acid, and is thought to participate in the structuring of neuronal groups and in the establishment of neuronal connections. Recent observations have indicated, however, that it may not be restricted to developing tissues since it is still present in certain adult neuronal centres which can undergo morphological reorganization. In this study, therefore, we examined systematically the distribution of polysialylated neural cell adhesion molecule immunoreactivity throughout the central nervous system of adult male and female rats, using light microscopic immunocytochemistry and immunoblot analysis with an antibody that specifically recognizes the polysialic residues of the molecule. Concomitantly, we compared this immunoreactivity to that due to all isoforms of the neural cell adhesion molecule, detected with a polyclonal serum raised against the NH2-terminal of the protein. Immunoreactivity due to the polysialylated isoform was consistently visualized in several discrete areas of the adult brain and spinal cord. An intercellular punctate immunolabelling characterized the staining in certain hypothalamic and thalamic nuclei, superficial laminae of the dorsal horn of the spinal cord, ventral portion of the dentate gyrus of the hippocampus, lateral geniculate, parabrachial and habenular nuclei, bed nucleus of the stria terminalis, mesencephalic central gray and olfactory bulb. In other areas, such as the piriform cortex, dorsal aspect of the dentate gyrus and fimbria and lamina X of the spinal cord, isolated neuronal-like cells were either completely filled with immunolabel or showed a surface reaction on their cell bodies and processes. Highly immunoreactive isolated glial-like cells were also noted within the ependymal layer of the central canal and lateral ventricles and at times in the peripheral white matter of the spinal cord. In contrast to this discrete localization, staining due to all isoforms of the neural cell adhesion molecule was widespread and diffuse throughout the brain and spinal cord. The expression of the polysialylated isoform in the supraoptic nucleus and hippocampus was confirmed by immunoblot analysis; it occurred together with weakly sialylated isoforms. No obvious differences were detected in the amount or distribution of immunoreactivity due to the polysialylated isoform in relation to the sex or age of the animals (between three and 12 months of age). Our study thus demonstrates that well-defined areas of the central nervous system of the adult rat continue to express the polysialylated isoform of the neural cell adhesion molecule.(ABSTRACT TRUNCATED AT 400 WORDS)

Enlargement of synaptic size as a compensative reaction in aging and dementia

A quantitative investigation has been carried out on synaptic contact zones of dentate gyrus supragranular layer and cerebellar glomeruli in autoptic samples from adult, old and demented patients. During physiological aging and senile dementia, the synaptic average area was significantly increased as compared to adult values in both the CNS areas investigated. Conversely, the number of contacts and their total surface contact area per unit volume of tissue were decreased. Current literature reports that, in animal models, enlarged synapses undergo perforations and splitting to modify synaptic connectivity. As against these assumptions, the increased synaptic size observed in our study appears to represent a compensative reaction of old and demented CNS to counteract the reduction in number and in total contact area of the synaptic junctions.

Food restriction in female Wistar rats, IV. Morphometric parameters of cerebellar synapses

The effect of food restriction on morphometric parameters of cerebellar synapses has been evaluated. A decrease in the number and surface density of synapses has been observed comparing 6 and 27-28 month old rats. Food restriction prevented the loss of the number and attenuated the reduction of the surface density of synaptic contacts occurring during aging. Present data support the idea that food restriction delays the appearance of age-related modifications of synaptic structures and may explain the improvement of motoric coordination and performance found in dietary restricted old animals.

Senile dementia: a threshold phenomenon of normal aging? A contribution to the functional reserve hypothesis of the brain

Neurochemical investigations with normal aging brains show that in the first 70 years of life no major changes of the glycolytic pathway can be observed. Only in the following decades does a significant decrease of brain metabolic turnover occur. Changes in nerve cell size, one of the most relevant parameters in evaluating a diffuse nerve cell atrophy, appear in the brain cortex not earlier than between 85 and 94 years of age; a 21% nerve cell shrinkage is the mean. The results demonstrate that a significant decrease in turnover of the glycolytic pathway is followed by a significant but moderate shrinkage of the nerve cells after a delay of 10-15 years. Similar investigations in brains from senile demented subjects demonstrate that the change in glycolytic turnover is much more a quantitative than a qualitative phenomenon. In comparison with age-matched controls a decrease in glycolytic turnover of more than 60% is observed. Morphometric investigations of the nerve cell sizes in the brain cortex of senile demented subjects showed a decrease of 45-55% when compared with age-matched controls. When normal aging is compared with senile dementia it seems that old age dementia is a threshold phenomenon which starts if the glycolytic turnover drops below 50% of its value in young healthy adults. Physiological aging, however, stays within the range of the reserve capacity of normal brain performance. In conclusion, it seems that the exhaustion of the functional reserve capacity may shift an aging brain into a dementia syndrome.

Neurobiology of the aging brain: morphological alterations at synaptic regions

Computer-assisted morphometric studies have been carried out on nerve cell terminal regions in rats of different ages. The numerical density (Nv), the average area (S) and the surface density (Sv) of ethanol phosphotungstic acid stained (E-PTA) synaptic junctions were evaluated in cerebellar glomeruli and dentate gyrus supragranular layers. The volume density (Vv), the average volume (V) and the numerical density (Nvm) of synaptic mitochondria were measured in the cerebellar glomeruli of young (3 months), adult (11 months) and old (28 months) animals. We found that during aging Nv and Sv undergo a significant decrease, whereas S is significantly increased. The mitochondrial Vv is unchanged in all the age groups analysed, whereas in old rats V is increased and Nvm decreased, respectively. We interpret our results as supporting that the old CNS retains part of its remodelling activity and is capable of adaptive morphological response at synaptic terminal regions.

Quantitation of synaptic density in the septal nuclei of young and aged Fischer 344 rats

Synaptic density in the medial and lateral septal nuclei was examined in 3 and 24-28 months of age Fischer 344 rats. The lateral nucleus had a higher synaptic density than the medial region in both age groups. There were no statistically significant differences in synapse density in either region as a function of age, but the data suggested a subpopulation of aged animals which did show an age-related decline in synaptic density in the lateral, but not medial area of the septum. These data indicate that sample size may be an important variable in assessing possible age-related differences in synaptic density, since a broad range of values, some significantly below the range of young animals, exists in the aged brain.

Electron microscopic morphometric studies on the effects of idebenone on the synaptic remodelling activity in the hippocampus and cerebellum in normal old as well as in vitamin E-deficient adult rats

Electron microscopic morphometric investigation has been carried out on the synaptic junctions of the hippocampal dentate gyri and cerebellar glomeruli of normal, old female Wistar rats (29 months of age), and vitamin E-deficient, female adult rats (11 months of age) of the same strain. The vitamin E-deficient diet was maintained from the age of 1 month for the subsequent 10-month period. Both the normal old and the vitamin E-deficient rats were treated with a daily dose of 50 mg oxidized idebenone/kg body w/day or with its solvent (5% gum arabic) through a gastric tube during the last month before killing them. The following morphometric parameters were evaluated in the hippocampal dentate gyrus and cerebellar glomerulus: the average length of the synapses (L) the surface density (S(v)) and the numerical density (N(v)) of the synaptic contact zones. Although the idebenone treatment caused a tendency to improve these parameters in both brain compartments studied, these improvements did not reach statistical significance in the cerebellum, but did so in the case of hippocampal N(v). Vitamin E deprivation caused the usual, known alterations of the synaptic parameters. Idebenone treatment during the last month of this experiment compensated the decrease of S(v) in both the hippocampus and the cerebellum; however, its protective effect was significant only in the case of hippocampus. Idebenone effect manifests itself in the increase of L, contributing mainly to the increase of S(v), since N(v) remained practically invariate. Placebo treatments did not result in any significant alterations in the vitamin E-deficient group.

Morphological adaptive response of the synaptic junctional zones in the human dentate gyrus during aging and Alzheimer's disease

A computer-assisted morphometric study has been carried out on ethanol phosphotungstic acid (E-PTA) stained synaptic junctions in the human dentate gyrus supragranular layer from adult, old and Alzheimer's disease (AD)-affected patients. The number of synapses per unit volume of tissue (Nv = numerical density), the average area of the single junction (S) and the total area of the synaptic contact zones in a unit volume of tissue (Sv = surface density) were the 3 parameters taken into account. The synapse to neurone ratio was also calculated for each patient. During physiological aging, Nv and Sv significantly decreased and S increased, respectively. In the AD hippocampi, Nv and Sv underwent a further decrease which was in the range of more than 40% with reference to the adult values. S was the same as the old control group. In comparison with the adult values, the number of synapse/neurone decreased by 15.6 and 48% in old and AD patients, respectively. Nv, S and Sv, while reporting on discrete ultrastructural features of the synaptic junctional zones, are closely related to each other and, taken together per group of patients, may represent a reliable index of the morphological adaptive changes taking place at the synapses. Thus, the significant increase of S both in old and AD hippocampi may be regarded as a CNS plastic response to aging and disease, although the marked decrease of Nv and Sv supports that in AD synaptic ultrastructural alterations proceed beyond a critical threshold for functional recovery.

Computer-assisted morphometry of synaptic plasticity during aging and dementia

A computer assisted morphometric study has been carried out on synaptic membranes in the dentate gyrus supragranular layer and cerebellar glomerulus from adult, old and demented patients. Numerical (Nv) and surface (Sv) densities as well as average area (S) of the synaptic contact zones were calculated directly on electron microscopic negatives by means of an ASBA (Wild Leitz, AG) image analyzer properly programmed. The results showed a decrease of Nv in both the CNS areas investigated during aging and, to a higher extent, in senile dementia. S was found to be significantly increased in old and demented CNS as compared with adult values. In the old hippocampus Sv was decreased by 40% whereas no significant difference was present between old and adult cerebellum; in senile dementia this parameter underwent a significant decrease in both areas investigated. We interpret the present findings in terms of morphological remodelling capability of the synaptic junctional zones during aging and disease.

Region-selective decline of in vivo lipid synthesis in the aged rat visual system

[14C]palmitate and [3H]choline were injected intravitreally and, at the same time, intraventricularly in Wistar male rats at 4, 10, and 24 mo of age. The precursor incorporation into lipids of the retina, optic nerve tract, superior colliculus, and lateral geniculate body was followed for 2 h. The specific radioactivity of precursors pool (choline, phosphorylcholine, and free fatty acids) showed a marked decrease in optic nerve tract and lateral geniculate body of aged rats, whereas in retinal tissue and superior colliculus no changes were observed as a function of age. In rats of the three age groups, whole retina and superior colliculus showed neither changes of choline incorporation into phosphatidylcholine and sphingomyelin nor alteration of palmitate incorporation into diacylglycerols, triacylglycerols, and major phospholipid classes as a function of age. In sharp contrast, the optic nerve tract and, to a lesser extent, the lateral geniculate body exhibited a significant age-related decline of either the incorporation of both precursors into all lipid classes or the specific radioactivities of endogenous precursor pools. We concluded that the visual pathway structures are metabolically affected in a different manner by aging. Particularly, the ability of the retina and superior colliculus to metabolize lipids appeared to be age invariant. The marked decline of lipid biosynthesis with age, for some visual structures, is consistent with the trend generally observed in metabolic turnover and function of other CNS regions.

Age-dependent alterations in hippocampal synaptic plasticity: relation to memory disorders

In this paper, we review the evidence indicating that the common disturbance in recent memory associated with aging is a consequence of functional and structural impairment in the hippocampal formation. In the Fischer 344 rat, an experimental model of the human age-related memory disorder was developed. The majority of aged rats of this strain show impaired performance in the 8-arm radial maze in a manner typical of young rats with bilateral hippocampal lesions. Aged animals also exhibit rapid decay of LTP and slower kindling of the perforant path-dentate synapse. Furthermore, quantitative morphometric analysis of the hippocampal synaptic architecture revealed that aged, memory-impaired rats had a specific loss of perforated axospinous synapses in the middle third of the dentate gyrus molecular layer; the extent of loss was directly related to the degree of memory dysfunction. Most important was the fact that the electrophysiological and morphological abnormalities did not appear in equally old animals with good memory.

Age-dependent deterioration of neuronal membranes and the pathogenesis of Alzheimer's disease: a hypothesis

Senile dementia of the Alzheimer's type (SDAT) is considered either a specific pathological condition unrelated to normal aging or an accumulation of brain aging alterations leading to clinical dysfunction. In the present paper a hypothesis is formulated to reconcile these two issues to a common denominator, i.e. the increased age-dependent deterioration of the neuronal membrane. If this holds true, normal aging and SDAT may be considered as two different aspects of the same problem and specific interventions to slow down the former can delay the onset of the latter to older ages.

Age-related morphological rearrangements of synaptic junctions in the rat cerebellum and hippocampus

A quantitative investigation has been carried out on the age-related morphological changes of the synaptic junctions in the cerebellar glomeruli and in the supragranular layer of the hippocampal dentate gyrus of young (3 months), adult (12 months) and old (28-30 months) rats. The numerical (Nv) and surface (Sv) density as well as the average length (L) of E-PTA stained synapses was calculated by means of morphometric methods. The results we obtained showed a similar trend in both these areas of the CNS. Nv significantly increased between young and adult rats and significantly decreased in the old group, when compared both to young and adult animals. Sv appeared to be unchanged comparing young and adult animals, whereas it was significantly reduced in the old group. L showed a decrease between 3 and 12 months of age and appeared to be markedly increased in the old animals when compared to adult values. From the present findings it can be inferred that number (Nv) and size (L) of the synapses are in a close inverse relationship which, through the organism's life span, aims to maintain the constancy of the surface contact area (Sv) among the dendritic network. By considering Nv, L and Sv altogether per age group, we were able to obtain a reliable measurement of the morphological aspect of synaptic plasticity through different periods of the life. With regard to aging we found that, despite the 'compensative synaptogenesis' brought about by the increased size (L) of the synaptic appositions, the reactive capacity of old nerve cells is seriously impaired.