Age-related changes in basal dendrite and dendritic spine of hippocampal pyramidal neurons (CA1) among SAMP1TA/Ngs--quantitative analysis by the rapid Golgi method

Interneuron Simplification and Loss of Structural Plasticity As Markers of Aging-Related Functional Decline

Changes in excitatory neuron and synapse structure have been recognized as a potential physical source of age-related cognitive decline. Despite the importance of inhibition to brain plasticity, little is known regarding aging-associated changes to inhibitory neurons. Here we test for age-related cellular and circuit changes to inhibitory neurons of mouse visual cortex. We find no substantial difference in inhibitory neuron number, inhibitory neuronal subtypes, or synapse numbers within the cerebral cortex of aged mice compared with younger adults. However, when comparing cortical interneuron morphological parameters, we find differences in complexity, suggesting that arbors are simplified in aged mice. In vivo two-photon microscopy has previously shown that in contrast to pyramidal neurons, inhibitory interneurons retain a capacity for dendritic remodeling in the adult. We find that this capacity diminishes with age and is accompanied by a shift in dynamics from balanced branch additions and retractions to progressive prevalence of retractions, culminating in a dendritic arbor that is both simpler and more stable. Recording of visually evoked potentials shows that aging-related interneuron dendritic arbor simplification and reduced dynamics go hand in hand with loss of induced stimulus-selective response potentiation (SRP), a paradigm for adult visual cortical plasticity. Chronic treatment with the antidepressant fluoxetine reversed deficits in interneuron structural dynamics and restored SRP in aged animals. Our results support a structural basis for age-related impairments in sensory perception, and suggest that declines in inhibitory neuron structural plasticity during aging contribute to reduced functional plasticity.SIGNIFICANCE STATEMENT Structural alterations in neuronal morphology and synaptic connections have been proposed as a potential physical basis for age-related decline in cognitive function. Little is known regarding aging-associated changes to inhibitory neurons, despite the importance of inhibitory circuitry to adult cortical plasticity and the reorganization of cortical maps. Here we show that brain aging goes hand in hand with progressive structural simplification and reduced plasticity of inhibitory neurons, and a parallel decline in sensory map plasticity. Fluoxetine treatment can attenuate the concurrent age-related declines in interneuron structural and functional plasticity, suggesting it could provide an important therapeutic approach for mitigating sensory and cognitive deficits associated with aging.

SAMP8 mice as a neuropathological model of accelerated brain aging and dementia: Toshio Takeda's legacy and future directions

Senescence accelerated mice P8 (SAMP8) show significant age-related deteriorations in memory and learning ability in accordance with early onset and rapid advancement of senescence. Brains of SAMP8 mice reveal an age-associated increase of PAS-positive granular structures in the hippocampal formation and astrogliosis in the brain stem and hippocampus. A spongy degeneration in the brain stem appears at 1 month of age and reaches a maximum at 4-8 months. In addition, clusters of activated microglia also appear around the vacuoles in the brain stem. β/A4(Aβ) protein-like immunoreactive granular structures are observed in various regions and increase in number markedly with age. Other age-associated histological changes include cortical atrophy, neuronal cell loss in locus coeruleus and lateral tegmental nuclei, intraneuronal accumulation of lipopigments in Purkinje cells and eosinophilic inclusion bodies in thalamic neurons. A blood-brain barrier dysfunction and astrogliosis are also prominent with advancing age in the hippocampus. These changes are generally similar to the pathomorphology of aging human brains and characterized by their association with some specific glioneuronal reactions. As for the hallmarks of Alzheimer brains, tau morphology has not yet been confirmed regardless of the age-related increase in phosphorylated tau in SAMP8 mice brains, but early age-related Aβ deposition in the hippocampus has recently been published. SAMP8 mice are, therefore, not only a senescence-accelerated model but also a promising model for Alzheimer's disease and other cognitive disorders.

Disease-toxicant interactions in manganese exposed Huntington disease mice: early changes in striatal neuron morphology and dopamine metabolism

YAC128 Huntington's disease (HD) transgenic mice accumulate less manganese (Mn) in the striatum relative to wild-type (WT) littermates. We hypothesized that Mn and mutant Huntingtin (HTT) would exhibit gene-environment interactions at the level of neurochemistry and neuronal morphology. Twelve-week-old WT and YAC128 mice were exposed to MnCl₂-4H₂O (50 mg/kg) on days 0, 3 and 6. Striatal medium spiny neuron (MSN) morphology, as well as levels of dopamine (DA) and its metabolites (which are known to be sensitive to Mn-exposure), were analyzed at 13 weeks (7 days from initial exposure) and 16 weeks (28 days from initial exposure). No genotype-dependent differences in MSN morphology were apparent at 13 weeks. But at 16 weeks, a genotype effect was observed in YAC128 mice, manifested by an absence of the wild-type age-dependent increase in dendritic length and branching complexity. In addition, genotype-exposure interaction effects were observed for dendritic complexity measures as a function of distance from the soma, where only YAC128 mice were sensitive to Mn exposure. Furthermore, striatal DA levels were unaltered at 13 weeks by genotype or Mn exposure, but at 16 weeks, both Mn exposure and the HD genotype were associated with quantitatively similar reductions in DA and its metabolites. Interestingly, Mn exposure of YAC128 mice did not further decrease DA or its metabolites versus YAC128 vehicle exposed or Mn exposed WT mice. Taken together, these results demonstrate Mn-HD disease-toxicant interactions at the onset of striatal dendritic neuropathology in YAC128 mice. Our results identify the earliest pathological change in striatum of YAC128 mice as being between 13 to 16 weeks. Finally, we show that mutant HTT suppresses some Mn-dependent changes, such as decreased DA levels, while it exacerbates others, such as dendritic pathology.

Involvement of BDNF in age-dependent alterations in the hippocampus

It is known since a long time that the hippocampus is sensitive to aging. Thus, there is a reduction in the hippocampal volume during aging. This age-related volume reduction is paralleled by behavioral and functional deficits in hippocampus-dependent learning and memory tasks. This age-related volume reduction of the hippocampus is not a consequence of an age-related loss of hippocampal neurons. The morphological changes associated with aging include reductions in the branching pattern of dendrites, as well as reductions in spine densities, reductions in the densities of fibers projecting into the hippocampus as well as declines in the rate of neurogenesis. It is very unlikely that a single factor or a single class of molecules is responsible for all these age-related morphological changes in the hippocampus. Nevertheless, it would be of advantage to identify possible neuromodulators or neuropeptides that may contribute to these age-related changes. In this context, growth factors may play an important role in the maintenance of the postnatal hippocampal architecture. In this review it is hypothesized that brain-derived neurotrophic factor (BDNF) is a factor critically involved in the regulation of age-related processes in the hippocampus. Moreover, evidences suggest that disturbances in the BDNF-system also affect hippocampal dysfunctions, as e.g. seen in major depression or in Alzheimer disease.

Structure and function of dendritic spines within the hippocampus

Most excitatory input in the hippocampus impinges on dendritic spines. Therefore, the dendritic spines are likely to be of major importance for neural processing. The morphology of dendritic spines is very diverse and changes in spine size as well as in their density are thought to reflect changes in the strength of synaptic transmission. Thus, alterations in dendritic spine densities or shape are suspected to be morphological manifestations of psychopathological, pathophysiological, physiological and/or behavioural changes. However, in spite of a long history of research, the specific function of dendritic spines within the hippocampal formation is still not well understood. This review will shed light on the hippocampal dendritic spines, their ultrastructure and morphology, as well as their supposed roles in neuronal plasticity and in certain mental illnesses.

The cytoarchitecture and soma-dendritic arbors of the pyramidal neurons of aged rat sensorimotor cortex: an intracellular dye injection study

We studied the cytoarchitecture and dendritic arbors of the output neurons of the sensorimotor cortex of aged rats and found that although individual cortical layer became thinner, the overall cytoarchitecture and neuron densities remained comparable to those of young adults. To find out whether aging affects cortical outputs we studied the soma-dendritic arbors of layers III and V pyramidal neurons, main output neurons of the cerebral cortex, using brain slice intracellular dye injection technique. With a fluorescence microscope, selected neurons were filled with fluorescence dye under visual guidance. Injected slices were resectioned into thinner sections for converting the injected dye into non-fading material immunohistochemically. The long apical dendritic trunk and branches could be routinely revealed. This allowed us to reconstruct and study the dendritic arbors of these neurons in isolation in 300-microm-thick dimension. Analysis shows that their cell bodies did not shrink, but the densities of spines on dendrites and the total dendritic length significantly reduced. Among spines, those with long thin stalks thought to be involved in memory acquisition appeared to be reduced. These could underlie the compromise of sensorimotor functions following aging.

The bite-raised condition in aged SAMP8 mice induces dendritic spine changes in the hippocampal region

The present study examined the effect of the bite-raised condition on the number of dendritic spines on hippocampal pyramidal cells in SAMP8 mice and related the results with learning ability in a water maze test. Aged mice in the bite-raised condition had reduced learning ability and a lower number of CA1 pyramidal cell dendritic spines. The results suggest that the bite-raised condition exacerbates the age-related spatial learning impairment, and that this may be due to the degeneration of hippocampal dendritic spines.

Age-related alterations in hippocampal spines and deficiencies in spatial memory in mice

Alterations in neuronal morphology occur in the brain during normal aging, but vary depending on neuronal cell types and brain regions. Such alterations have been related to memory and cognitive impairment. Changes in hippocampal spine densities are thought to represent a morphological correlate of altered brain functions associated with hippocampal-dependent learning and memory. We therefore have analyzed the impact of aging on different hippocampal-dependent learning tasks and on changes in dendritic spines of CA1 hippocampal and dentate gyrus neurons by analyzing adult (6-7 months) and aged (21-22 months) C57/Bl6 mice. We found a significant decrease in spine numbers of basal CA1 dendrites and decreases in spine length of apical dendrites of CA1 and dentate gyrus neurons. Furthermore, aged mice exhibited significant deficits in hippocampus-dependent learning tasks, such as the probe trial of the Morris water maze and T maze learning. Given the fact that there is no neuronal loss in the hippocampus in aged mice (von Bohlen und Halbach and Unsicker [2002] Eur. J. Neurosci. 16:2434-2440), we suggest that the memory and cognitive decline in the context of aging may be accompanied by rather subtle anatomical changes, such as numbers and morphology of dendritic spines.

Molarless-induced changes of spines in hippocampal region of SAMP8 mice

We examined the effect of the molarless condition on the dendritic spines of hippocampal pyramidal cells in SAMP8 mice in comparison to its effect on learning ability in a maze test. The molarless condition caused a decrease in the number of the spines of CA1 pyramidal cells only in the aged mice showing a reduced learning ability. The results suggest the involvement of the molarless condition in an attenuation of input activities in the hippocampal synapses.

Neurotrophic Effect of Magnolol in the Hippocampal CA1 Region of Senescence-Accelerated Mice (SAMP1)

Magnolol has neurotrophic effects in primary cultured rat cortical neurons, which are expressed as the promotion of neurite outgrowth and neuronal survival. In this study, we investigated the protective effect of magnolol against age-related neuronal loss in the hippocampus using senescence-accelerated mouse (SAMP1). Magnolol (5, 10 mg/kg) was orally administered once a day for 14 d to 2- or 4-month-old mice, and evaluation was carried out when the mice were 4 or 6 months old. The density of neurofibrils decreased with aging in the stratum radiatum of the CA1 region in the hippocampus of SAMP1, not SAMR1. Treatment with magnolol significantly prevented the decrease of neurofibrils in the CA1, when it was administered in 2-month-olds. However, administration at 4 months of age did not result in a preventive effect. These findings suggest that the administration of magnolol before the initiation of neuronal loss may result in a protective effect in the hippocampus.

Expression of presenilin 1 and synapse-related proteins during postnatal development is not different between accelerated senescence-prone and -resistant mice

SAMP1TA/Ngs is an inbred strain of senescence-accelerated mice in which there is delayed development of cognitive functions and dendritic spine formation compared with normal control SAMR1TA//Ngs mice. It is hypothesized that abnormalities might be in the postnatal expression of synapse-related proteins in SAMP1TA/Ngs mice. Quantitative western blot analyses showed that the postnatal developmental changes in the expression of synaptophysin, post-synaptic density protein 95 and presenilin 1 in the cerebrum were similar between SAMP1TA/Ngs and SAMR1TA//Ngs mice. Therefore, the expression of synapse-related proteins was not disturbed in SAMP1TA/ Ngs mice regardless of reported abnormal numbers of dendritic spines during postnatal development. Immunohistochemical studies showed that the expression of synaptophysin in the neuropil increased postnatally with development in the same way in SAMP1TA/Ngs and SAMR1TA//Ngs mice. Presenilin 1 expression was relatively high at age 5 days in the neuropil of the cerebral cortex and decreased with postnatal development in the same way in SAMP1TA/Ngs and SAMR1TA//Ngs mice. At age 5 days the distribution of presenilin 1 was similar to the distribution of synaptophysin in that there were two separate immunoreactive patterns: a subpial band and patches in the middle layers reminiscent of barrels. These findings suggest that presenilin 1 is transiently expressed in the neuropil to induce synaptogenesis, and then its expression decreases overall.

Age-related alterations in responses of the nucleus basalis magnocellularis neurons to frontal cortex stimulation in rats

The present study investigated the age-related alterations in responses of the nucleus basalis magnocellularis (nbM) neurons to frontal cortex (FCX) stimulation. Single unit extracellular recording from the nbM neurons were obtained with glass micropipettes in urethane-anesthetized rats. A total of 137 units were located within the nbM in the three age groups (young, 3 months; adult, 12 months; old, 24 months). FCX stimulation elicited responses in 91% of the 137 neurons. Most of them were excited. The frequency of occurrence of excitatory responses in the nbM neurons was decreased with aging. The thresholds and latencies of excitatory responses evoked by FCX stimulation were increased in old rats. The mean peak-firing rate of exciting phase was gradually reduced with aging. These findings indicate that there might be some functional changes in the nbM neurons with aging.

Age-related alterations in responses of nucleus basalis magnocellularis neurons to peripheral nociceptive stimuli

The present study investigated the effects of peripheral noxious stimuli on the spontaneous activity of the nucleus basalis magnocellularis (nbM) neurons in young, adult and old rats. Single unit extracellular recordings from the nbM neurons were obtained with glass micropipettes in urethane-anesthetized rats. A total of 104 units were antidromically identified as nbM-cortical neurons. Noxious but not innocuous mechanical stimulation elicited responses in 72% of the 104 neurons. Most of them were excited. The receptive fields were usually very large and bilateral. Most of the neurons also responded to noxious thermal, chemical and electrical stimuli. No marked differences were observed in the incidence of neurons having different spontaneous firing rates, firing patterns and response type among the three age groups. However, the latency of responses evoked by noxious thermal or electrical stimulation and the threshold of excitatory responses evoked by electrical stimulation were increased with aging. The duration and peak-firing rate of excitatory responses evoked by noxious thermal, chemical or electrical stimulation were decreased in old rats. These findings indicate that there might be some functional changes in the nbM neurons and its projection pathway with aging, which impair their responsive ability to peripheral nociceptive stimuli.

Ageing of the brain

The brains of individuals who are cognitively normal show age-related changes that include an overall reduction in the brain volume and weight and enlargement of the brain ventricles. These changes are partly the result of nerve cell loss but accurate estimates of neuronal loss are notoriously difficult to make. There is loss of synapses and dendritic pruning in the aged brain but in selected areas rather than globally. Neurofibrillary tangles and senile plaques are the neuropathological hallmark of Alzheimer's disease in which they are more abundant and widespread than in the brains of intellectually intact elderly people. Alzheimer's disease has, therefore, been regarded as accelerated brain ageing, however, since there is a strong genetic contribution to developing the disease it implies that it may not be the inevitable, even if frequent, consequence of old age. The interplay between genetic and environmental factors probably determines the degree of pathological brain ageing and whether or not individuals develop dementia.

A possible emerging role of phytochemicals in improving age-related neurological dysfunctions: a multiplicity of effects

It is rare to see a day pass in which we are not told through some popular medium that the population is becoming older. Along with this information comes the "new" revelation that as we enter the next millennium there will be increases in age-associated diseases (e.g., cancer, cardiovascular disease) including the most devastating of these, which involve the nervous system (e.g., Alzheimer's disease [AD] and Parkinson's disease [PD]). It is estimated that within the next 50 years approximately 30% of the population will be aged 65 years or older. Of those between 75 and 84 years of age, 6 million will exhibit some form of AD symptoms, and of those older than 85 years, over 12 million will have some form of dementia associated with AD. What appears more ominous is that many cognitive changes occur even in the absence of specific age-related neurodegenerative diseases. Common components thought to contribute to the manifestation of these disorders and normal age-related declines in brain performance are increased susceptibility to long-term effects of oxidative stress (OS) and inflammatory insults. Unless some means is found to reduce these age-related decrements in neuronal function, health care costs will continue to rise exponentially. Thus, it is extremely important to explore methods to retard or reverse age-related neuronal deficits as well as their subsequent, behavioral manifestations. Fortunately, the growth of knowledge in the biochemistry of cell viability has opened new avenues of research focused at identifying new therapeutic agents that could potentially disrupt the perpetual cycle of events involved in the decrements associated with these detrimental processes. In this regard, a new role in which certain dietary components may play important roles in alleviating certain disorders are beginning to receive increased attention, in particular those involving phytochemicals found in fruits and vegetables.

Age-related changes in the brains of senescence-accelerated mice (SAM): association with glial and endothelial reactions

Twelve substrains of inbred senescence-accelerated mice (SAM) have been developed, among which the SAMP8 and SAMP10 strains show a significant age-related deterioration in learning and memory for passive and active avoidance tasks. These strains have, respectively, a low and high incidence of systemic senile amyloidosis. Although we found no amyloid deposits in their brain parenchyma, a variety of age-related alterations were identified, involving neurons, glia, and vessels in the brain tissues. Here we review the degenerative changes in aged SAMP8 and SAMP10 brains. These changes are generally similar to the pathology of aging human brain and may be characterized by their association with some specific glial reactions.

Quantitative age-related changes in apical dendrites and dendritic spines of CA1 pyramidal neurons among senescence accelerated mice (SAMP1TA/Ngs)

SAMP1TA/Ngs, a substrain of senescence accelerated mouse, is a useful animal model for research on brain dysfunction due to senescence. In a previous study it was reported that the age-related changes in basal dendrites and spines of CA1 pyramidal neurons coincide with the behavioral characteristics found in SAMP1TA/Ngs. The goal of the present study was to investigate morphological changes in apical dendrites and dendritic spines of CA1 pyramidal neurons among 3-, 5-, 7-month-old SAMP1TA/Ngs. Pyramidal neurons of the hippocampus were stained by the rapid Golgi method, and the number of apical dendrites, the number of their spines and the density of the dendritic spines were evaluated. The number and density of the spines of apical dendrites were significantly higher at 5 months than at 3 or 7 months of age. We propose that the low number of dendritic spines in 3-month-old animals was caused by immaturity, while the changes in the density and number of dendritic spines in 7-month-old mice were due to accelerated aging. The data on the morphology of apical dendrites are a useful complement to the results reported previously. The findings of the present study also support the hypothesis that this model mouse demonstrates changes in respective developmental stages, i.e. immaturity, adulthood and senescence. This pattern of postnatal growth has special meaning because it indicates the usefulness of the strain in the study of geriatric disorders in humans.