Comparison of synaptic changes in the precentral and postcentral cerebral cortex of aging humans: a quantitative ultrastructural study

Age-related upregulation of perineuronal nets on inferior collicular cells that project to the cochlear nucleus

Introduction

Disruptions to the balance of excitation and inhibition in the inferior colliculus (IC) occur during aging and underlie various aspects of hearing loss. Specifically, the age-related alteration to GABAergic neurotransmission in the IC likely contributes to the poorer temporal precision characteristic of presbycusis. Perineuronal nets (PNs), a specialized form of the extracellular matrix, maintain excitatory/inhibitory synaptic environments and reduce structural plasticity. We sought to determine whether PNs increasingly surround cell populations in the aged IC that comprise excitatory descending projections to the cochlear nucleus.

Method

We combined Wisteria floribunda agglutinin (WFA) staining for PNs with retrograde tract-tracing in three age groups of Fischer Brown Norway (FBN) rats.

Results

The data demonstrate that the percentage of IC-CN cells with a PN doubles from ~10% at young age to ~20% at old age. This was true in both lemniscal and non-lemniscal IC.

Discussion

Furthermore, the increase of PNs occurred on IC cells that make both ipsilateral and contralateral descending projections to the CN. These results indicate that reduced structural plasticity in the elderly IC-CN pathway, affecting excitatory/inhibitory balance and, potentially, may lead to reduced temporal precision associated with presbycusis.

Additive effect of transcranial direct current stimulation (tDCS) in combination with multicomponent training on elderly physical function capacity: a randomized, triple blind, controlled trial

PURPOSE

To evaluate the additive effect of Transcranial Direct Current Stimulation (tDCS) associated with multi-component training (MT) on the functional capacity (FC) of older adults and to assess whether these effects remain after the end of training. The secondary objectives were to evaluate the locomotion capacity, balance, functional independence, and quality of life and correlate them with functional capacity.

METHODOLOGY

Twenty-eight older adults were randomized into two groups: experimental (MT associated with active tDCS - a-tDCS) and control (MT associated with sham tDCS - s-tDCS). The FC was measured by the Glittre-ADL test, locomotion capacity by the 6-minute walk test, balance by the BESTest, functional independence by the FIM, and quality of life by the WHQOL. The assessments were performed pre-, post-intervention, and 30-day follow-up.

RESULTS

There was a significant decrease in the time to the Glittre-ADL test when comparing the a-tDCS and s-tDCS groups after the interventions (139.77 ± 21.62, 205.10 ± 43.02, p < .001) and at the 30-day follow-up (142.74 ± 17.12, 219.55 ± 54.05, p < .001), respectively. There was a moderate correlation between FC and locomotion capacity and balance.

CONCLUSIONS

The addition of tDCS potentiated the results of MT to impact FC, maintaining the positive results longer. Locomotion and balance influenced the improvement of functional capacity.

Differential effects of group III metabotropic glutamate receptors on spontaneous inhibitory synaptic currents in spine-innervating double bouquet and parvalbumin-expressing dendrite-targeting GABAergic interneurons in human neocortex

Diverse neocortical GABAergic neurons specialize in synaptic targeting and their effects are modulated by presynaptic metabotropic glutamate receptors (mGluRs) suppressing neurotransmitter release in rodents, but their effects in human neocortex are unknown. We tested whether activation of group III mGluRs by L-AP4 changes GABAA receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) in 2 distinct dendritic spine-innervating GABAergic interneurons recorded in vitro in human neocortex. Calbindin-positive double bouquet cells (DBCs) had columnar "horsetail" axons descending through layers II-V innervating dendritic spines (48%) and shafts, but not somata of pyramidal and nonpyramidal neurons. Parvalbumin-expressing dendrite-targeting cell (PV-DTC) axons extended in all directions innervating dendritic spines (22%), shafts (65%), and somata (13%). As measured, 20% of GABAergic neuropil synapses innervate spines, hence DBCs, but not PV-DTCs, preferentially select spine targets. Group III mGluR activation paradoxically increased the frequency of sIPSCs in DBCs (to median 137% of baseline) but suppressed it in PV-DTCs (median 92%), leaving the amplitude unchanged. The facilitation of sIPSCs in DBCs may result from their unique GABAergic input being disinhibited via network effect. We conclude that dendritic spines receive specialized, diverse GABAergic inputs, and group III mGluRs differentially regulate GABAergic synaptic transmission to distinct GABAergic cell types in human cortex.

Age-related ultrastructural changes in the lateral cortex of the inferior colliculus

Temporal precision, a key component of sound and speech processing in the inferior colliculus (IC), depends on a balance of inhibition and excitation, and this balance degrades during aging. The cause of disrupted excitatory-inhibitory balance in aging is unknown, however changes at the synapse are a likely candidate. We sought to determine whether synaptic changes occur in the lateral cortex of the IC (IClc), a multimodal nucleus that processes lemniscal, intrinsic, somatosensory, and descending auditory input. Using electron microscopic techniques across young, middle age and old Fisher Brown Norway rats, our results demonstrate minimal loss of synapses in middle age, but significant (∼28%) loss during old age. However, in middle age, targeting of GABAergic dendrites by GABAergic synapses is increased and the active zones of excitatory synapses (that predominantly target GABA-negative dendrites) are lengthened. These synaptic changes likely result in a net increase of excitation in the IClc during middle age. Thus, disruption of excitatory-inhibitory balance in the aging IClc may be due to synaptic changes that begin in middle age.

Three-Dimensional Synaptic Organization of Layer III of the Human Temporal Neocortex

In the present study, we have used focused ion beam/scanning electron microscopy (FIB/SEM) to perform a study of the synaptic organization of layer III of Brodmann's area 21 in human tissue samples obtained from autopsies and biopsies. We analyzed the synaptic density, 3D spatial distribution, and type (asymmetric/symmetric), as well as the size and shape of each synaptic junction of 4945 synapses that were fully reconstructed in 3D. Significant differences in the mean synaptic density between autopsy and biopsy samples were found (0.49 and 0.66 synapses/μm3, respectively). However, in both types of samples (autopsy and biopsy), the asymmetric:symmetric ratio was similar (93:7) and most asymmetric synapses were established on dendritic spines (75%), while most symmetric synapses were established on dendritic shafts (85%). We also compared several electron microscopy methods and analysis tools to estimate the synaptic density in the same brain tissue. We have shown that FIB/SEM is much more reliable and robust than the majority of the other commonly used EM techniques. The present work constitutes a detailed description of the synaptic organization of cortical layer III. Further studies on the rest of the cortical layers are necessary to better understand the functional organization of this temporal cortical region.

Three-dimensional analysis of synaptic organization in the hippocampal CA1 field in Alzheimer's disease

Alzheimer's disease is the most common form of dementia, characterized by a persistent and progressive impairment of cognitive functions. Alzheimer's disease is typically associated with extracellular deposits of amyloid-β peptide and accumulation of abnormally phosphorylated tau protein inside neurons (amyloid-β and neurofibrillary pathologies). It has been proposed that these pathologies cause neuronal degeneration and synaptic alterations, which are thought to constitute the major neurobiological basis of cognitive dysfunction in Alzheimer's disease. The hippocampal formation is especially vulnerable in the early stages of Alzheimer's disease. However, the vast majority of electron microscopy studies have been performed in animal models. In the present study, we performed an extensive 3D study of the neuropil to investigate the synaptic organization in the stratum pyramidale and radiatum in the CA1 field of Alzheimer's disease cases with different stages of the disease, using focused ion beam/scanning electron microscopy (FIB/SEM). In cases with early stages of Alzheimer's disease, the synapse morphology looks normal and we observed no significant differences between control and Alzheimer's disease cases regarding the synaptic density, the ratio of excitatory and inhibitory synapses, or the spatial distribution of synapses. However, differences in the distribution of postsynaptic targets and synaptic shapes were found. Furthermore, a lower proportion of larger excitatory synapses in both strata were found in Alzheimer's disease cases. Individuals in late stages of the disease suffered the most severe synaptic alterations, including a decrease in synaptic density and morphological alterations of the remaining synapses. Since Alzheimer's disease cases show cortical atrophy, our data indicate a reduction in the total number (but not the density) of synapses at early stages of the disease, with this reduction being much more accentuated in subjects with late stages of Alzheimer's disease. The observed synaptic alterations may represent a structural basis for the progressive learning and memory dysfunctions seen in Alzheimer's disease cases.

Carnosine research in relation to aging brain and neurodegeneration: A blessing for geriatrics and their neuronal disorders

Carnosine, an endogenous dipeptide (β-Ala-l-His), is enriched in prefrontal cortex and olfactory bulb of the brain, blood and also in muscle. It has mainly antioxidant and antiglycating properties which makes this molecule unique. Its content reduces during aging and aging-induced neurodegenerative diseases. Aging is a progressive biological process that leads to develop the risk factors of diseases and death. During aging the morphological, biochemical, cellular and molecular changes occur in brain and blood including other tissues. The objective of this review is to combine the updated information from the existing literature about the aging-induced neurodegeneration and carnosine research to meet the lacuna of mechanism of carnosine. The grey matter and white matter loses its normal ratio in aging, and hence the brain volume and weight. Different aging related neurodegenerative disorders arise due to loss of neurons, and synapses as a result of proteinopathies in some cases. Carnosine, being an endogenous biomolecule and having antioxidant, antiglycating properties has shown its potency to counteract erroneous protein biosynthesis, stress, activated microglial and astrocyte activity, and different neurodegenerative disorders. It (carnosine) can also inhibit the metal ion-induced degeneration by acting as a metal chelator. In this review the trends in carnosine research in relation to aging brain and neurodegeneration have been discussed with a view to its (carnosine) eligibility (including its mechanism of action) to be used as a promising neurotherapeutic for the betterment of elderly populations of our society at the national and international levels in near future.

The effects of transcranial alternating current stimulation (tACS) at individual alpha peak frequency (iAPF) on motor cortex excitability in young and elderly adults

Transcranial alternating current stimulation (tACS) can modulate brain oscillations, cortical excitability and behaviour. In aging, the decrease in EEG alpha activity (8–12 Hz) in the parieto-occipital and mu rhythm in the motor cortex are correlated with the decline in cognitive and motor functions, respectively. Increasing alpha activity using tACS might therefore improve cognitive and motor function in the elderly. The present study explored the influence of tACS on cortical excitability in young and old healthy adults. We applied tACS at individual alpha peak frequency for 10 min (1.5 mA) to the left motor cortex. Transcranial magnetic stimulation was used to assess the changes in cortical excitability as measured by motor-evoked potentials at rest, before and after stimulation. TACS increased cortical excitability in both groups. However, our results also suggest that the mechanism behind the effects was different, as we observed an increase and decrease in intracortical inhibition in the old group and young group, respectively. Our results indicate that both groups profited similarly from the stimulation. There was no indication that tACS was more effective in conditions of low alpha power, that is, in the elderly.

Chewing-side Preference is Involved in Differential Cortical Activation Patterns during Tongue Movements after Bilateral Gum-chewing: a Functional Magnetic Resonance Imaging Study

Contralateral dominance in the activation of the primary sensorimotor cortex (S1/M1) during tongue movements (TMs) has been shown to be associated with a chewing-side preference (CSP). However, little is known about its interaction with chewing-related cortical activation. Functional magnetic resonance imaging was performed before and after gum-chewing in six subjects who exhibited a left CSP to determine the relationship between the CSP and activation patterns in the S1/M1 during TMs. Before the subjects chewed the gum, activation foci were found in the bilateral S1/M1. In the left hemisphere, both signal intensity and the area of activation significantly increased during TMs within 10 min after subjects chewed gum. Moreover, this augmented activation significantly decreased within 20 min during tongue protrusion and leftward movement. In the right hemisphere, there were no marked changes during TMs. These results suggest that bilateral gum-chewing enhances activation of the S1/M1 ipsilateral to the CSP during TMs.

Synaptic pathology: A shared mechanism in neurological disease

Synaptic proteomes have evolved a rich and complex diversity to allow the exquisite control of neuronal communication and information transfer. It is therefore not surprising that many neurological disorders are associated with alterations in synaptic function. As technology has advanced, our ability to study the anatomical and physiological function of synapses in greater detail has revealed a critical role for both central and peripheral synapses in neurodegenerative disease. Synapse loss has a devastating effect on cellular communication, leading to wide ranging effects such as network disruption within central neural systems and muscle wastage in the periphery. These devastating effects link synaptic pathology to a diverse range of neurological disorders, spanning Alzheimer's disease to multiple sclerosis. This review will highlight some of the current literature on synaptic integrity in animal models of disease and human post-mortem studies. Synaptic changes in normal brain ageing will also be discussed and finally the current and prospective treatments for neurodegenerative disorders will be summarised.

Differential effects of motor cortical excitability and plasticity in young and old individuals: a Transcranial Magnetic Stimulation (TMS) study

Aging is associated with changes in the motor system that, over time, can lead to functional impairments and contribute negatively to the ability to recover after brain damage. Unfortunately, there are still many questions surrounding the physiological mechanisms underlying these impairments. We examined cortico-spinal excitability and plasticity in a young cohort (age range: 19–31) and an elderly cohort (age range: 47–73) of healthy right-handed individuals using navigated transcranial magnetic stimulation (nTMS). Subjects were evaluated with a combination of physiological [motor evoked potentials (MEPs), motor threshold (MT), intracortical inhibition (ICI), intracortical facilitation (ICF), and silent period (SP)] and behavioral [reaction time (RT), pinch force, 9 hole peg task (HPT)] measures at baseline and following one session of low-frequency (1 Hz) navigated repetitive TMS (rTMS) to the right (non-dominant) hemisphere. In the young cohort, the inhibitory effect of 1 Hz rTMS was significantly in the right hemisphere and a significant facilitatory effect was noted in the unstimulated hemisphere. Conversely, in the elderly cohort, we report only a trend toward a facilitatory effect in the unstimulated hemisphere, suggesting reduced cortical plasticity and interhemispheric communication. To this effect, we show that significant differences in hemispheric cortico-spinal excitability were present in the elderly cohort at baseline, with significantly reduced cortico-spinal excitability in the right hemisphere as compared to the left hemisphere. A correlation analysis revealed no significant relationship between cortical thickness of the selected region of interest (ROI) and MEPs in either young or old subjects prior to and following rTMS. When combined with our preliminary results, further research into this topic could lead to the development of neurophysiological markers pertinent to the diagnosis, prognosis, and treatment of neurological diseases characterized by monohemispheric damage and lateralized motor deficits.

Severe stroke: patient profile and predictors of favorable outcome

BACKGROUND

Severe stroke carries high rates of mortality and morbidity. The aims of this study were to determine the characteristics of patients who initially presented with severe ischemic stroke, and to identify acute and subacute predictors of favorable clinical outcome in these patients.

METHODS

An observational cohort study, Acute Stroke Registry and Analysis of Lausanne (ASTRAL), was analyzed, and all patients presenting with severe stroke - defined as a National Institute of Health Stroke Scale score of ≥ 20 on admission - were compared with all other patients. In a multivariate analysis, associations with demographic, clinical, pathophysiologic, metabolic and neuroimaging factors were determined. Furthermore, we analyzed predictors of favorable outcome (modified Rankin scale score of ≤ 3 at 3 months) in the subgroup of severe stroke patients.

RESULTS

  Of 1915 consecutive patients, 243 (12.7%) presented with severe stroke. This was significantly associated with cardio-embolic stroke mechanism (odds ratio [OR] 1.74, 95% confidence interval [CI] 1.19-2.54), unknown stroke onset (OR 2.35, 95% CI 1.14-4.83), more neuroimaging signs of early ischemia (mostly computed tomography; OR 2.65, 95% CI 1.79-3.92), arterial occlusions on acute imaging (OR 27.01, 95% CI 11.5-62.9), fewer chronic radiologic infarcts (OR 0.43, 95% CI 0.26-0.72), lower hemoglobin concentration (OR 0.97, 95% CI 0.96-0.99), and higher white cell count (OR 1.05, 95% CI 1.00-1.11). In the 68 (28%) patients with favorable outcomes despite presenting with severe stroke, this was predicted by lower age (OR 0.94, 95% CI 0.92-0.97), preceding cerebrovascular events (OR 3.00, 95% CI 1.01-8.97), hypolipemic pretreatment (OR 3.82, 95% CI 1.34-10.90), lower acute temperature (OR 0.43, 95% CI 0.23-0.78), lower subacute glucose concentration (OR 0.74, 95% CI 0.56-0.97), and spontaneous or treatment-induced recanalization (OR 4.51, 95% CI 1.96-10.41).

CONCLUSIONS

Severe stroke presentation is predicted by multiple clinical, radiologic and metabolic variables, several of which are modifiable. Predictors in the 28% of patients with favorable outcome despite presenting with severe stroke include hypolipemic pretreatment, lower acute temperature, lower glucose levels at 24 h, and arterial recanalization.

Synaptic genes are extensively downregulated across multiple brain regions in normal human aging and Alzheimer's disease

Synapses are essential for transmitting, processing, and storing information, all of which decline in aging and Alzheimer's disease (AD). Because synapse loss only partially accounts for the cognitive declines seen in aging and AD, we hypothesized that existing synapses might undergo molecular changes that reduce their functional capacity. Microarrays were used to evaluate expression profiles of 340 synaptic genes in aging (20-99 years) and AD across 4 brain regions from 81 cases. The analysis revealed an unexpectedly large number of significant expression changes in synapse-related genes in aging, with many undergoing progressive downregulation across aging and AD. Functional classification of the genes showing altered expression revealed that multiple aspects of synaptic function are affected, notably synaptic vesicle trafficking and release, neurotransmitter receptors and receptor trafficking, postsynaptic density scaffolding, cell adhesion regulating synaptic stability, and neuromodulatory systems. The widespread declines in synaptic gene expression in normal aging suggests that function of existing synapses might be impaired, and that a common set of synaptic genes are vulnerable to change in aging and AD.

The BDNF val(66)met polymorphism is not related to motor function or short-term cortical plasticity in elderly subjects

The brain derived neurotrophic factor (BDNF) val(66)met polymorphism affects function of the motor system in young subjects, but little is known about motor system effects in the elderly. The current study assessed motor system physiology and behavior, plus a measure of short-term motor cortex plasticity using transcranial magnetic stimulation, in 38 elderly subjects, then examined whether findings varied in relation to BDNF genotype. Baseline data were also collected from 14 young subjects. At baseline, elderly subjects had poorer motor performances, larger motor cortex maps, and smaller motor evoked potentials compared to young subjects. Degree of age-related differences in neurophysiology correlated inversely with motor performance, for example, larger map area correlated with weaker pinch grip force (r=-0.42, P=0.01). In elderly subjects, baseline behavior and neurophysiology did not differ in relation to BDNF genotype. In addition, although map area increased significantly (P=0.03) across 30 min of exercise, this change did not vary according to BDNF genotype. Aging is associated with changes in neurophysiology that might represent a compensatory response. The data do not support an association between BDNF genotype and behavior, neurophysiology, or short-term cortical plasticity in the motor system of healthy elderly subjects.

Synaptic loss in the inferior temporal gyrus in mild cognitive impairment and Alzheimer's disease

Alzheimer's disease (AD) is a slowly progressing form of dementia characterized in its earliest stages as a loss of memory. Individuals with amnestic mild cognitive impairment (aMCI) may be in the earliest stages of the disease and represent an opportunity to identify pathological changes related to the progression of AD. Synaptic loss is one of the hallmarks of AD and associated with cognitive impairment. The inferior temporal gyrus plays an important role in verbal fluency, a cognitive function affected early in the onset of AD. Unbiased stereology coupled with electron microscopy was used to quantify total synaptic numbers in lamina 3 of the inferior temporal gyrus from short postmortem autopsy tissue harvested from subjects who died at different cognitive stages during the progression of AD. Individuals with aMCI had significantly fewer synapses (36%) compared to individuals with no cognitive impairment. Individuals with AD showed a loss of synapses very similar to the aMCI cohort. Synaptic numbers correlated highly with Mini Mental State Examination scores and a test of category verbal fluency. These results demonstrate that the inferior temporal gyrus is affected during the prodromal stage of the disease and may underlie some of the early AD-related clinical dysfunctions.

Corticomotor plasticity and learning of a ballistic thumb training task are diminished in older adults

This study examined changes in corticomotor excitability and plasticity after a thumb abduction training task in young and old adults. Electromyographic (EMG) recordings were obtained from right abductor pollicis brevis (APB, target muscle) and abductor digiti minimi (ADM, control muscle) in 14 young (18-24 yr) and 14 old (61-82 yr) adults. The training task consisted of 300 ballistic abductions of the right thumb to maximize peak thumb abduction acceleration (TAAcc). Transcranial magnetic stimulation (TMS) of the left primary motor cortex was used to assess changes in APB and ADM motor evoked potentials (MEPs) and short-interval intracortical inhibition (SICI) before, immediately after, and 30 min after training. No differences in corticomotor excitability (resting and active TMS thresholds, MEP input-output curves) or SICI were observed in young and old adults before training. Motor training resulted in improvements in peak TAAcc in young (177% improvement, P < 0.001) and old (124%, P = 0.005) subjects, with greater improvements in young subjects (P = 0.002). Different thumb kinematics were observed during task performance, with increases in APB EMG related to improvements in peak TAAcc in young (r(2) = 0.46, P = 0.008) but not old (r(2) = 0.09, P = 0.3) adults. After training, APB MEPs were 50% larger (P < 0.001 compared with before) in young subjects, with no change after training in old subjects (P = 0.49), suggesting reduced use-dependent corticomotor plasticity with advancing age. These changes were specific to APB, because no training-related change in MEP amplitude was observed in ADM. No significant association was observed between change in APB MEP and improvement in TAAcc with training in individual young and old subjects. SICI remained unchanged after training in both groups, suggesting that it was not responsible for the diminished use-dependent corticomotor plasticity for this task in older adults.

O envelhecimento cortical e a reorganização neural após o acidente vascular encefálico (AVE): implicações para a reabilitação

Este artigo apresenta uma síntese sobre o envelhecimento do córtex cerebral humano e uma revisão das abordagens para a reabilitação do controle motor após o acidente vascular encefálico (AVE). Na discussão sobre as implicações clínicas na compensação das perdas, é enfatizado que os profissionais de reabilitação devem incentivar os pacientes idosos a usarem os dois membros superiores para a realização das atividades da vida diária (AVDs) ao invés de reforçarem o uso do membro superior não afetado.

Synapses are lost during aging in the primate prefrontal cortex

An electron microscopic analysis has been carried out on the effects of age on the numerical density of both excitatory (asymmetric) and inhibitory (symmetric) synapses in the neuropil of layers 2/3 and of layer 5 in area 46 from the frontal cortex of behaviorally tested rhesus monkeys. There is no change in the lengths of synaptic junctions with age or in the percentage distribution of synapses relative to the postsynaptic spines and dendritic shafts. However, in layers 2/3 there is an overall loss of about 30% of synapses from 5 to 30 years of age, and both asymmetric and symmetric synapses are lost at the same rate. In layer 5 the situation is different; the overall loss of synapses is only 20% and this is almost entirely due to a loss of asymmetric synapses, since there is no significant loss of symmetric synapses from this layer with age. When the synapse data are correlated with the overall cognitive impairment shown by the monkeys, it is found that there is a strong correlation between the numerical density of asymmetric synapses in layers 2/3 and cognitive impairment, with a weaker correlation between symmetric synapse loss and cognitive impairment. In layer 5 on the other hand there is no correlation between synapse loss and cognitive impairment. However synapse loss is not the only factor causing cognitive impairment, since in previous studies of area 46 we have found that age-related alteration in myelin in this frontal area also significantly contributes to cognitive decline. The synapse loss is also considered in light of earlier studies, which show that the frequency of spontaneous excitatory synaptic responses is reduced with age in layers 2/3 neurons.

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.

Aging is associated with contrasting changes in local and distant cortical connectivity in the human motor system

Pathophysiological changes in neurological and neuropsychiatric diseases are increasingly described in terms of abnormal network connectivity. However, the anatomical integrity and efficacy of connections among multiple brain regions change with aging, even in healthy adults. We combined low-frequency transcranial magnetic stimulation and positron emission tomography to study the age-related changes in regional activation and effective connectivity, associated with voluntary action by healthy adults between 22 and 68 years old. Contrasting effects of aging on the motor network were seen using analyses of regional activation, effective connectivity mediating task-related neuronal activation and effective connectivity in response to transcranial magnetic stimulation. Low-frequency rTMS reduced cerebral blood flow during both movement and resting conditions, at the site of stimulation and neighboring frontal cortex. Aging was associated with increased movement-related activation in premotor cortex, bilaterally. Increasing age also increased the susceptibility of the cortex to the inhibitory effects of rTMS, at the site of stimulation and its contralateral homologue. Moreover, older subjects showed enhanced local effective connectivity, centered on the left premotor cortex, but reduced effective connectivity between distant motor-related cortical areas. We discuss these results in relation to the HAROLD model of aging and propose that there are differential effects of aging on local and distributed neuronal subpopulations in the motor network. This differential effect of aging has important implications for the study of neurodegenerative and cerebrovascular diseases that primarily affect older people, as well as our understanding of the normal aging process.

Functional activation imaging in aging and dementia

With life expectancy increasing continuously, the effects of neurodegeneration on brain function are a topic of ever increasing importance. Thus there is a need for tools and models that probe both the functional consequences of neurodegenerative processes and compensatory mechanisms that might occur. As neurodegenerative burden and compensatory mechanisms may change over time, these tools will ideally be applied multiple times over the lifespan. Specifically, in order to elucidate whether brain-activation patterns in Alzheimer's disease (AD) and in healthy aging follow general rules in the context of degeneration and compensation, it is necessary to compare functional brain-activation patterns during different states of neurodegeneration. This article integrates the findings of functional activation studies at different stages of neurodegeneration: in healthy aging, in subjects at high risk of developing dementia, in subjects with mild cognitive impairment (MCI), and in patients suffering from AD. We review existing theoretical models that aim to explain the underlying mechanisms of functional activation changes in aging and dementia, and we propose an integrative account, which allows for different neural response patterns depending on the amount of neuronal damage and the recruitment of compensatory pathways.

Age-related changes in head and eye coordination

The effect of ageing upon head movements during gaze shifts is unknown. We have investigated age-related changes in head and eye coordination in a group of healthy volunteers. Horizontal head and eye movements were recorded in 53 subjects, aged between 20 and 83 years, during the performance of saccades, antisaccades, smooth pursuit and a reading task. The subjects were divided into three groups, young subjects (20-40 years), middle-aged subjects (41-60 years) and older subjects (over 60 years). Logarithmic transformations of the head gain were significantly greater in the older subjects compared to the young subjects during the saccadic task (P=0.001), antisaccadic task (P=0.004), smooth pursuit at 20 degrees/s (P=0.001) and 40 degrees/s (P=0.005), but not reading. For saccadic and antisaccadic tasks, the increase in transformed head gain was non-linear with significant differences between older and middle-aged subjects but not middle-aged and young subjects. Head movement tendencies were highly consistent for related tasks. Head movement gain during gaze shifts significantly increases with age, which may contribute to dizziness and balance problems experienced by the elderly.

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.

Quantitative assessment of possible age-related change in synaptic numbers in the human frontal cortex

To investigate possible age-associated changes in human synaptic connectivity, superior-middle frontal cortex (Brodmann area 9) was evaluated with ultrastructural techniques. Short post mortem autopsy tissue was obtained from 37 cognitive normal individuals ranging in age from 20 to 89 years. A minimum of five subjects represented each decade of life. Synaptic volume density (Nv) was quantified in lamina III and V of the superior-middle frontal cortex employing the physical disector. The stereological assessment demonstrated maintenance of Nv in both lamina III and V of the frontal cortex. The lack of synaptic decline in the frontal cortex in neurologically normal individuals older than 65 years lends support to the idea that many stereotypic views of age-related changes in the CNS do not apply to all brain regions. It also suggests that synaptic loss observed in pathological conditions such as Alzheimer's disease, may be the result of the disease process and not a consequence of normal aging.

Loss of presynaptic and postsynaptic structures is accompanied by compensatory increase in action potential-dependent synaptic input to layer V neocortical pyramidal neurons in aged rats

Reduction in both presynaptic and postsynaptic structures in the aging neocortex may significantly affect functional synaptic properties in this area. To directly address this issue, we combined whole-cell patch-clamp recording of spontaneously occurring postsynaptic currents (PSCs) with morphological analysis of layer V pyramidal neurons in the parietal cortex of young adult (1- to 2-month-old) and aged (28- to 37-month-old) BN x F344 F(1) hybrid rats. Analysis of spontaneous PSCs was used to contrast functional properties of basal synaptic input with structural alterations in the dendritic tree of pyramidal neurons and density of terminals in contact with these cells. We observed significant changes in a number of morphological parameters of pyramidal neurons in aged rats. These include smaller cell body size and fewer basal dendritic branches (but not of oblique dendrites and dendritic tufts) and spines. Ultrastructural analysis also revealed a lower density of presynaptic terminals per unit length of postsynaptic membrane of labeled pyramidal neurons in the aged brain. This reduction in both presynaptic and postsynaptic elements was paralleled by a significant decrease in frequency of tetrodotoxin-insensitive miniature (action potential-independent) PSCs (mPSCs). The frequency of excitatory and inhibitory mPSCs was reduced to the same extent. In contrast, no significant change was observed in the frequency of spontaneous PSCs recorded in absence of tetrodotoxin (sPSCs), indicating an increase in action potential-dependent (frequency(sPSCs) - frequency(mPSCs)) input to pyramidal neurons in the aged group. This functional compensation may explain the lack of drastic loss of spontaneous neuronal activity in normal aging.

Age-related synaptic changes in sensorimotor cortex of the Brown Norway X fischer 344 rat

Investigations of age-related changes in synapse density have yielded contradictory conclusions. The goal of the present study was to determine whether there is a significant decline in the number of cortical synapses in old age. Therefore, brains from 10-, 15-, and 32-month-old Brown Norway X Fischer 344 rats were prepared for electron microscopy and synapses were counted in a stereotaxically-identified region of sensorimotor cortex. Within this cortical area, synapses were counted in layers 2 and 4 and the data have been presented both as number of synapses per volume of neuropil and as the ratio of synapses per neuron. Results indicated that there was a decline in synapse density between 15 and 32 months in layer 2, but not in layer 4. This decline was significant not only for total synapses, but also for subcategories of synapses when classified by presynaptic features or postsynaptic element. Specifically, there was a significant decline in presumptive inhibitory synaptic terminals, i.e., those containing nonround synaptic vesicles, as well as a significant decline in synapses that contact dendritic spines.

Histology and histochemistry of the aging cerebral cortex: an overview

This review contributes to a new vision of the most important findings in the aging cerebral cortex as elucidated by modern histology and histochemistry. It includes an overview of the macroscopic and microscopic changes involved, not only in normal aging, but also in the main age-related neurodegenerative diseases. Finally, the most accepted theories about aging as well as the implications of nitric oxide in this process are described.

Synaptic numbers across cortical laminae and cognitive performance of the rat during ageing

In this study, we have investigated the changes in the number of individual presynaptic boutons in the neocortex of rats and correlated them with cognitive performance. Brown Norway x Fischer 344 F1 hybrid rats, aged from one to 24 months, were used. Using synaptophysin as a marker for presynaptic boutons, we found that in the parietal II region of the neocortex an age-related decrease in the density of immunostained punctae representing presynaptic boutons occurred. Regression analysis showed that this decline in the number of presynaptic boutons correlates with ageing (r=0.495, P<0.05). Interestingly, we found that this age-related depletion of presynaptic boutons was more intense in the deeper cortical lamina, such as laminae V and VI (mean decrease of 18%), than in the superficial laminae (mean decrease of 8% in laminae I-IV). Using the Morris water maze test, we observed that young rats acquired the task at twice the speed of aged animals (48.9 +/- 9.0 s and 91.0 +/- 4.9 s for young and aged animals, respectively). Furthermore, at the end of the training period, the aged cohort still showed significantly higher escape latencies in the Morris water maze. The present findings support the concept that the decline in cognitive performances in ageing is related to the loss of synapses in the cerebral cortex.

Synaptophysin in spinal anterior horn in aging and ALS: an immunohistological study

Aged-related spinal cord changes such as neuronal loss have been related to the degree of clinical severity of amyotrophic lateral sclerosis (ALS); morphological data on synapses are, however, wanting. Variations in synaptophysin (Sph) expression in aging and ALS were thus studied at the level of lower motor neurons in 40 controls with non-neurological diseases and 11 cases of ALS. Control sections of formalin fixed paraffin embedded cervical (C7/8), thoracic (T10) and lumbar spinal cord (L5) and C6, C7, C8 and L5 of ALS cases were stained with haematoxylin and eosin, luxol fast blue (LFB), and immunostained with a mouse monoclonal antibody against Sph. The neuropil of the anterior horn (AH) in all control cases demonstrated Sph positivity. A dot-like pattern of positivity of presynaptic terminals on soma of motor neurons and fine immunoreactivity along neuronal processes were observed. A significant reduction of Sph immunostaining was observed in the neuropil with increasing age and 3 different somatic patterns were seen: a- well preserved Sph reactivity around the soma and the proximal dendrites of histologically normal neurons; b- few chromatolytic neurons showing large numbers of dot-like presynaptic terminals around the cell body and in a "fused" pattern; c- intense, diffuse, and homogeneous reactivity of some neurons. Attenuation of Sph reactivity in the AH neuropil, to its complete loss, was observed in all ALS cases. In addition to patterns a-c, two additional microscopic findings were noted in ALS: d- chromatolytic neurons showing complete absence of Sph reactivity; e- absence of Sph reactivity around the soma and the proximal dendrites of histologically normal surviving neurons. Our findings demonstrate that there is a decrease in Sph immunostaining with aging, thus suggesting an alteration in dendritic networks of the AH with aging. Changes in the pattern of Sph immunoreactivity in cell bodies may represent synaptic plasticity and/or degeneration. Reinnervation may also be a possible mechanism as a response to neuronal loss in oldest control cases. Sph reactivity results may thus lend support to the presence of superimposed aging components in ALS cases which may give an insight into explaining the increasing severity of the disease which is encountered with advancing age.

An analysis of contemporary morphological concepts of synaptic remodelling in the CNS: perforated synapses revisited

Perforated synapses refer to a synaptic type found in the central nervous system. They are characterized by their large size and by a discontinuity of the postsynaptic density when viewed in transverse sections, and by a doughnut or horseshoe shape when viewed in en face views. Of recent morphological studies, one approach has followed their characteristics throughout development and maturity, while others have concentrated on their probable roles in activities including kindling, long-term potentiation, spatial working memory, differential rearing, and the functioning of neuroleptics. An assessment is made of the hypotheses and models that have proved determinative in the emergence of perforated synapses as being significant in synaptic plasticity. Their distribution and frequency are summarized, with emphasis on the importance of unbiased stereological procedures in their analysis. Using three-dimensional approaches various subtypes are recognized. Of these, a complex or fragmented subtype appears of especial significance in synaptic plasticity. Ideas regarding the life-cycle of perforated synapses are examined. The view that they originate from conventional, non-perforated synapses, enlarge, and subsequently split to give rise to a new generation of non-perforated synapses, is critically assessed. According to an alternative model, perforated and non-perforated synapses constitute separate populations from early in their development, each representing complementary forms of synaptic plasticity. An attempt is also made to discover whether synaptic studies on the human brain in normal aging and in Alzheimer's disease throw light on the role of perforated synapses in synaptic plasticity. The loss of synapses in Alzheimer's disease may include a loss of perforated synapses - of particular relevance for an understanding of certain neuropathological conditions.

Synapse loss in anterior horn neurons in amyotrophic lateral sclerosis

This report deals with an ultrastructural investigation of the synapses of anterior horn neurons in the lumbar spinal cords of five patients with amyotrophic lateral sclerosis (ALS) who had mild neuronal depletion. Specimens from five age-matched, neurologically normal individuals served as controls. In each instance, the autopsy was performed within 3 h after death. A statistically significant decrease in cell body area, number of synapses and total synaptic length was found in the normal-appearing neurons of the ALS patients. The alterations were more pronounced in neurons with central chromatolysis. However, despite an approximately 20% reduction in the number of synapses, the length of the active synaptic zone of the normal-appearing neurons in the ALS patients was not diminished. This observation may be accounted for by a plasticity to the loss of synapses which maintained the active zone of the remaining synapses to increase synaptic efficiency. It is suggested that when the plasticity of the active zone reaches its limit, the continuing loss of synapses may lead to functional impairment. The capacity of the active synaptic zone to respond to progressive denervation of the anterior horn neurons may preserve motor function or slow the development of motor deficits in the early stage of degeneration of the lower motor neurons.

Synapse loss in the temporal lobe in Alzheimer's disease

The temporal lobe is a well-documented area showing neuropathological and neurochemical changes in Alzheimer's disease (AD). Autopsy tissue was obtained from the superior temporal (Brodmann area 22) and the middle temporal (Brodmann area 21) regions of the cortex from patients with AD (n = 10; postmortem time < 13 hr) and age-matched control subjects (n = 10; postmortem time < 13 hr). Ultrastructural examination of the tissue revealed a highly significant AD-related decline in synaptic numbers in lamina III and V in both the superior and the middle temporal gyrus. Both normal control and AD tissue samples demonstrated a significant negative correlation between the number of synapses and the synapse size, as indexed by the length of the postsynaptic density. This change in synaptic size appears to compensate for the loss of synaptic numbers when viewed in terms of total synaptic contact area. Although the AD tissue showed an average 30% decline in synaptic numbers, the total synaptic contact area remained largely unchanged in both regions of the cortex. There were no significant correlations with age, postmortem time, or number of plaques observed in these areas. The loss of synaptic contacts in AD does not appear to be isolated to a particular cortical region or lamina.

Regional synaptic pathology in Alzheimer's disease

Synaptic pathology in Alzheimer's disease (AD) may occur diffusely or may have regional predilections. A new antibody called EP10 which detects synaptophysin-like immunoreactivity was used to study synapses in postmortem brain tissue. Four brain regions from cases of AD and controls were studied. Controls with a wide range of ages were used to investigate the possibility of age-related changes in synaptophysin-like immunoreactivity. A significant reduction in the EP10 antigen was observed to occur with age in the control caudate but not in the hippocampus or temporal or occipital cortices. Antigen levels were significantly reduced in the hippocampus (77%) and the temporal cortex (54%) in AD. The expected abnormal pallor of the outer two-thirds of the dentate gyrus molecular layer was observed with immunocytochemistry. In the temporal cortex, the reduction in synaptophysin-like immunoreactivity was inversely correlated with the neurofibrillary tangle count. No such relationship existed in the hippocampus. These results suggest that at least certain components of the synaptic loss in AD occur regionally and are disproportionately large in the hippocampus.

Axon terminals on Betz cell somata of area 4 in rhesus monkey throughout adulthood

Previous work in our laboratory demonstrated an age-related decline in the size of Betz cell somata in cortical area 4 of the adult rhesus monkey brain. The present study was conducted to determine whether changes might also occur in the axon terminals upon these cortical cells. Tissue from area 4 was collected from seven rhesus monkeys and prepared for electron microscopy. The ages of the monkeys ranged from 5 to 35 years, covering the entire adult life span of this species. A total of 140 Betz cell profiles (20 per monkey) were examined. Measurements of these profiles confirmed our earlier finding of a decline in the perimeters of Betz cell somata with advancing age. The 1,540 axon terminals upon these cells, however, remained unchanged in size and length of membrane apposition, as well as in their number of mitochondria throughout the adult life (greater than or equal to 5 years) of the rhesus monkey. In addition, the total number of axon terminals on Betz cells did not change with age. Because the axosomatic terminals showed no age-associated changes, the material was used to calculate parametric characteristics of Betz cells and associated terminals. Betz cell somata of the rhesus monkey were estimated to have a mean membrane surface area of 5,700 microns2. Axosomatic terminals on Betz cell somata had a mean appositional area of about 3.33 microns2 and covered about 15% of the somal surface. Thus, on average, each Betz cell appeared to receive approximately 260 axosomatic terminals. There were also some conspicuous age-associated changes in the motor cortex that were not quantified. These included an accumulation of lipofuscin and the presence of a novel inclusion body in the somata of Betz cells. Age-related occurrences in the neuropil included the degeneration of axons and their myelin, membrane-bound holes, and neuritic (senile) plaques.

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.

Robust synaptic plasticity of striatal cells following partial deafferentation

Partial ablation of the cerebral cortical input to the neostriatum generates a rapid lasting effect on the size of remaining synaptic sites. The neocortex was lesioned in adult rats and the neostriatum was analyzed for effects on remaining spines of principal cells during the period from 2 to 40 days. There was an increase in the size of spine heads, boutons and synaptic contact sites. The spine heads became very complex and a corresponding bouton enlargement was accompanied by an increase in the number of synaptic vesicles. By two days, the average profile length of postsynaptic membrane densities (PSDs) had increased by 25% representing an equivalent 50% increase in synaptic contact area. The number of synaptic sites was reduced on each principal neuron of the lesioned group. Comparison of the number of sites per unit volume to their average contact area revealed a reciprocal relationship indicating a conservation in the total synaptic contact area on each neuron. This effect was consistent for all postsurgical days. The lack of a significant return of synaptic number by 40 days indicates that axonal sprouting is not a major factor in neuronal plasticity in the adult striatum. The rapid increase in the size of spines, boutons and synaptic sites at remaining connections suggests that dendrites are the first to initiate the plasticity response in adult neurons through postsynaptic attachments and their corresponding receptor structure. The underlying mechanism of this plasticity may be through a conservation of macromolecules forming postsynaptic membrane specializations on target neurons. Remaining axons appear to follow the dendritic response with a plasticity generating presynaptic appositional specializations to match the contact area of the postsynaptic site.

Quantitative assessment of cortical synaptic density in Alzheimer's disease

Significant progress has been made over the last decade in delineating the neuropathological and neurochemical changes in the brains of patients with Alzheimer's disease (AD). Less well studied are the actual synaptic connections of affected areas of the brain, such as the cerebral cortex. Because the final common pathway for neurotransmission involves synaptic integrity, we quantitatively assessed synaptic number and synapse size in lamina III and V of human frontal cortex (Brodmann area 9) in patients with AD and age-matched controls. Samples were also matched for postmortem interval, and artifacts associated with postmortem change were eliminated. We found a significant decrease in synaptic number per unit volume in both lamina, more marked in lamina III (-42%) than V (-29%). In both normal controls and AD brains, there was a negative correlation between synapse number and synapse size as indexed by the length of the postsynaptic density (PSD); cortical samples with fewer synapses had larger synapse size. This appeared to be a compensatory response, rather than a selective loss of small synapses, since the total amount of synaptic contact area per unit volume did not decline in lamina V (despite a 29% loss of synapses); in lamina III it was reduced 11% despite a 42% loss of synapses. The loss of synapses in AD is widespread and significant in frontal cortex; there is observable compensation by enlargement of synaptic size. This compensatory effort is overcome by the continuing loss of synapses in areas most affected by the degeneration.

Nuclear pore complex frequency in CA1 pyramidal cells of the aging rat

The frequency and the diameter of nuclear pore complexes, and the nuclear perimeter, were studied in CA1 pyramidal cells of the hippocampi from 3-, 9-, 24-, and 30-month-old rats (Fischer 344). No changes with age in any of these parameters were observed. This finding is discussed in terms of varied responses of different brain areas to the effects of aging.

Contralateral influences on patellar tendon reflexes in young and old adults

In an effort to more fully investigate age-related changes in spinal reflex parameters, we measured force-time characteristics of the patellar tendon reflex in aged subjects and contrasted these with data obtained from college-age individuals. We also conditioned the tendon jerk with a tap to the contralateral tendon. The results showed a marked tendon reflex enhancement in the old group, consisting of greater overall reflex force produced by the quadriceps. In both groups, the contralateral conditioning stimulus produced a short-latency inhibition (at 25 msec) followed by a longer-latency facilitation (beginning at 75 msec). Both the early inhibition and the later reflex enhancement were greater in the aged subjects. We suggest that some age-related change may occur at the spinal level to compensate for decrements in more complex motor functioning.

Effects of aging on the neurons within area 17 of rhesus monkey cerebral cortex

A light and electron microscopic examination of area 17 of the visual cortex in well-fixed young (5-6 years) and old (25-35 years) rhesus monkeys was carried out to determine the effects of age on neurons. The analyses were made in a portion of area 17 on the lateral surface of the hemisphere just caudal to the lunate sulcus. Light microscopic measurements of the mean cortical depth in vertically oriented 1-micron-thick sections reveal no obvious thinning with age, and the mean diameters of neuronal nuclei do not change with age. On the basis of counts of neuronal profiles containing nuclei in 250-microns-wide strips of 1-micron-thick sections passing through the entire depth of the cortex, no significant neuronal loss could be detected. These findings are consistent with our electron microscopic observations on this area of the cortex, for in the old monkeys the neurons show little cytological evidence of advanced age beyond the presence of a few lipofuscin granules, although the neuropil contains some profiles of degenerating small-caliber dendrites, myelinated axons, and a few axon terminals. Large vacuoles, some 10 microns or more in diameter, are present in the neuropil of the old animals. Some of these vacuoles appear to represent a late stage in the degeneration of myelinated axons, for they are bounded by a thin, laminated sheath. Other large vacuoles, of unknown origin, often contain membranous debris and have an attenuated limiting membrane. It is concluded that the cell bodies of neurons in area 17 of old rhesus monkeys do not show significant structural changes due to age, although some of the neuronal processes in the neuropil are affected.