Age-related spatial learning impairment is unrelated to spinophilin immunoreactive spine number and protein levels in rat hippocampus

Synaptic density in aging mice measured by [18F]SynVesT-1 PET

Synaptic alterations in certain brain structures are related to cognitive decline in neurodegeneration and in aging. Synaptic loss in many neurodegenerative diseases can be visualized by positron emission tomography (PET) imaging of synaptic vesicle glycoprotein 2A (SV2A). However, the use of SV2A PET for studying synaptic changes during aging is not particularly explored. Thus, in the present study, PET ligand [¹⁸F]SynVesT-1, which binds to SV2A, was used to investigate synaptic density at different ages in healthy mice. Wild type C57BL/6 mice divided into three age groups (4-5 months (n = 7), 12-14 months (n = 11), 17-19 months (n = 7)) were PET scanned with [¹⁸F]SynVesT-1. Brain retention of [¹⁸F]SynVesT-1 expressed as the volume of distribution (V_IDIF) was calculated using an image-derived input function. Estimates of V_IDIF were derived using either a one-tissue compartment model (1TCM), a two-tissue compartment model (2TCM), or the Logan plot with blood input to find the best-fit model for [¹⁸F]SynVesT-1. After the PET scans, tissue sections were immunostained for the detection of SV2A and neuronal markers. We found that [¹⁸F]SynVesT-1 data acquired 60 min post intravenously injection and analyzed with 1TCM described the brain pharmacokinetics of the radioligand in mice well. [¹⁸F]SynVesT-1 brain retention was lower in the oldest group of mice, indicating a decrease in synaptic density in this age group. However, no gradual age-dependent decrease in synaptic density at a region-specific level was observed. Immunostaining indicated that SV2A expression and neuron numbers were similar across all three age groups. In general, these data obtained in healthy aging mice are consistent with previous findings in humans where synaptic density appeared stable during aging up to a certain age, after which a small decrease is observed.

The improvement of Coreopsis tinctoria essential oil on learning and memory impairment of d-galactose-induced mice through Nrf2/NF-κB pathway

Essential oil of Coreopsis tinctoria (EOC) is a essential substance extracted from Coreopsis tinctoria with the excellent anti-oxidant effect. However, it is still unclear whether EOC can improve learning and memory impairment and its mechanism. The purpose of this study was to investigate the effect of EOC on learning and memory impairment induced by D-galactose (D-gal) in mice and reveal its mechanism. The composition of EOC was analyzed by GC-MS, and the results showed that the highest content was D-limonene. The follow-up experiments were conducted by comparing EOC with D-limonene. The aging model was established by subcutaneous injection of D-gal, and donepezil, D-limonene and EOC were given by intragastric administration. It was found that EOC and D-limonene significantly improved learning and memory impairment induced by D-gal through the Morris water maze and step-through tests. Pathological and biochemical analysis showed that the hippocampal morphologic of mice was damage and the activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) induced by D-gal were decreased, while the content of malondialdehyde (MDA) was increased, while EOC and D-limonene could reverse the morphological changes and reduce oxidative damage. In addition, EOC and D-limonene significantly increased body weight and organ coefficients, including liver, spleen and kidney. Moreover, EOC and D-limonene improved the expression of nuclear factor E2 related factor 2 (Nrf2) pathway and inhibited nuclear transcription factors-κB (NF-κB) pathway. In summary, the results showed that EOC and D-limonene could improve learning and memory impairment induced by D-gal through Nrf2/ NF-κB pathway. It was clear that as a mixture, EOC was better than D-limonene on improving learning and memory impairment.

Loss of precuneus dendritic spines immunopositive for spinophilin is related to cognitive impairment in early Alzheimer's disease

Precuneus (PreC) cortex is affected with amyloid plaques early in Alzheimer's disease (AD), and this pathology may be associated with alterations in PreC synapses and cognitive impairment. We quantified the spinophilin-immunoreactive (ir) dendritic spine density and the intensity of spinophilin immunofluorescence, the latter as a measure of relative protein levels of spinophilin, in PreC lamina III from 33 subjects with clinical diagnoses of no cognitive impairment (NCI), mild cognitive impairment (MCI), mild-moderate AD (mAD), or severe AD (sAD). Both measures of spinophilin were lower in mAD and sAD compared with NCI. The MCI group had higher protein levels of spinophilin compared with mAD and sAD, and higher spinophilin-ir dendritic spine density compared with sAD. Lower spinophilin-ir dendritic spine density and relative protein levels of spinophilin were associated with greater amyloid beta (Aβ) plaque burden, detected with a derivative of Pittsburgh compound-B (6-CN-PiB), and worse cognitive performance. Clinical onset of AD is marked by the loss of PreC spinophilin-ir dendritic spines that is related to Aβ pathology and may contribute to cognitive symptoms early in the disease.

Effect of behavioral testing on spine density of basal dendrites in the CA1 region of the hippocampus modulated by (56)Fe irradiation

A unique feature of the space radiation environment is the presence of high-energy charged particles, including (56)Fe ions, which can present a significant hazard to space flight crews during and following a mission. (56)Fe irradiation-induced cognitive changes often involve alterations in hippocampal function. These alterations might involve changes in spine morphology and density. In addition to irradiation, performing a cognitive task can also affect spine morphology. Therefore, it is often hard to determine whether changes in spine morphology and density are due to an environmental challenge or group differences in performance on cognitive tests. In this study, we tested the hypothesis that the ability of exploratory behavior to increase specific measures of hippocampal spine morphology and density is affected by (56)Fe irradiation. In sham-irradiated mice, exploratory behavior increased basal spine density in the CA1 region of the hippocampus and the enclosed blade of the dentate gyrus. These effects were not seen in irradiated mice. In addition, following exploratory behavior, there was a trend toward a decrease in the percent stubby spines on apical dendrites in the CA3 region of the hippocampus in (56)Fe-irradiated, but not sham-irradiated, mice. Other hippocampal regions and spine measures affected by (56)Fe irradiation showed comparable radiation effects in behaviorally naïve and cognitively tested mice. Thus, the ability of exploratory behavior to alter spine density and morphology in specific hippocampal regions is affected by (56)Fe irradiation.

Effects of YC-1 on learning and memory functions of aged rats

Background

The aim of this study was to investigate the effects of a potent nitric oxide-guanylate cyclase activator, 3-(5′-hydroxymethyl-2′-furyl)-1-benzylindazole (YC-1), on learning and memory functions in aged rats.

Material/Methods

Rats were divided into 2 groups as 4-month-old and 24-month-old rats. Rats received YC-1 (1 mg/kg/day) for 2 weeks long-term. Morris water maze (MWM) and passive avoidance (PA) tests were used to determine learning and memory functions.

Results

In the MWM test, there is a significant increase in the acquisition latency (1–4 days) of 24-month-old rats. There is a significant reduction in the “time spent in the escape platform’s quadrant” in 24-month-old rats compared to 4-month-old rats in the probe trial of the MWM test. YC-1 treatment reversed the reduction of the “time spent in the escape platform’s quadrant” of 24-month-old rats. In the PA test, there was no significant difference in the 1^st-day latency of rats in all groups. On the 2nd day, retention latency significantly decreased in the 24-month-old rats compared to 4-month-olds. YC-1 reversed the diminished retention latency in 24-month-old rats. YC-1 treatment and aging did not affect results of the locomotor activity test or the foot-shock sensitivity test, suggesting our results were not due to a change in motor activity or disability of the animals.

Conclusions

Our findings suggest that activation of the NO-sGC-cGMP pathway plays an important role in spatial and emotional learning and memory functions in aged rats.

Aging-induced proteostatic changes in the rat hippocampus identify ARP3, NEB2 and BRAG2 as a molecular circuitry for cognitive impairment

Disturbed proteostasis as a particular phenotype of the aging organism has been advanced in C. elegans experiments and is also conceived to underlie neurodegenerative diseases in humans. Here, we investigated whether particular changes in non-disease related proteostasis can be identified in the aged mammalian brain, and whether a particular signature of aberrant proteostasis is related to behavioral performance of learning and memory. Young (adult, n = 30) and aged (2 years, n = 50) Wistar rats were tested in the Morris Water Maze (MWM) to distinguish superior and inferior performers. For both young and old rats, the best and worst performers in the MWM were selected and the insoluble proteome, termed aggregome, was purified from the hippocampus as evidence for aberrant proteostasis. Quantitative proteomics (iTRAQ) was performed. The aged inferior performers were considered as a model for spontaneous, age-associated cognitive impairment. Whereas variability of the insoluble proteome increased with age, absolute changes in the levels of insoluble proteins were small compared to the findings in the whole C. elegans insoluble proteome. However, we identified proteins with aberrant proteostasis in aging. For the cognitively impaired rats, we identified a changed molecular circuitry of proteins selectively involved in F-actin remodeling, synapse building and long-term depression: actin related protein 3 (ARP3), neurabin II (NEB2) and IQ motif and SEC7 domain-containing protein 1 (BRAG2). We demonstrate that aberrant proteostasis is a specific phenotype of brain aging in mammals. We identify a distinct molecular circuitry where changes in proteostasis are characteristic for poor learning and memory performance in the wild type, aged rat. Our findings 1. establish the search for aberrant proteostasis as a successful strategy to identify neuronal dysfunction in deficient cognitive behavior, 2. reveal a previously unknown functional network of proteins (ARP3, NEB2, BRAG2) involved in age-associated cognitive dysfunction.

Involvement of α2-antiplasmin in dendritic growth of hippocampal neurons

The α2-Antiplasmin (α2AP) protein is known as a principal physiological inhibitor of plasmin, but we previously demonstrated that it acts as a regulatory factor for cellular functions independent of plasmin. α2AP is highly expressed in the hippocampus, suggesting a potential role for α2AP in hippocampal neuronal functions. However, the role for α2AP was unclear. This study is the first to investigate the involvement of α2AP in the dendritic growth of hippocampal neurons. The expression of microtubule-associated protein 2, which contributes to neurite initiation and neuronal growth, was lower in the neurons from α2AP⁻/⁻ mice than in the neurons from α2AP⁺/⁺ mice. Exogenous treatment with α2AP enhanced the microtubule-associated protein 2 expression, dendritic growth and filopodia formation in the neurons. This study also elucidated the mechanism underlying the α2AP-induced dendritic growth. Aprotinin, another plasmin inhibitor, had little effect on the dendritic growth of neurons, and α2AP induced its expression in the neurons from plaminogen⁻/⁻ mice. The activation of p38 MAPK was involved in the α2AP-induced dendritic growth. Therefore, our findings suggest that α2AP induces dendritic growth in hippocampal neurons through p38 MAPK activation, independent of plasmin, providing new insights into the role of α2AP in the CNS.

Increased postsynaptic density protein-95 expression in the frontal cortex of aged cognitively impaired rats

In the present work we studied synaptic protein concentrations in relation to behavioral performance. Long-Evans rats, aged 22-23 months, were classified for individual expression of place memory in the Morris water maze, in reference to young adults. Two main subgroups of aged rats were established: the Aged cognitively Unimpaired (AU) had search accuracy within the range (percent of time in training sector within mean ± 2 SEM) of young rats and the Aged cognitively Impaired (AI) rats had search accuracy below this range. Samples from the hippocampus and frontal cortex of all the AI, AU and young rats were analyzed for the expression of postsynaptic protein PSD-95 by Image J analysis of immunohistochemical data and by Western blots. PSD-95 expression was unchanged in the hippocampus, but, together with synaptophysin, was significantly increased in the frontal cortex of the AI rats. A significant correlation between individual accuracy (time spent in the training zone) and PSD-95 expression was observed in the aged group. No significant effect of age or PSD-95 expression was observed in the learning of a new position. All together, these data suggest that increased expression of PSD-95 in the frontal cortex of aged rats co-occurs with cognitive impairment that might be linked to functional alterations extending over frontal networks.

Remodeling of axo-spinous synapses in the pathophysiology and treatment of depression

Dendritic spines provide a compartment for assembly and functional organization of synaptic machinery that plays a fundamental role in neuronal communication and neuroplasticity. Studies in humans as well as in animal models have demonstrated abnormal spine architecture in several psychiatric disorders, including depression and other stress-related illnesses. The negative impact of stress on the density and organization of spines is thought to contribute to the behavioral deficits caused by stress exposure. Moreover, there is now evidence that medication-induced recovery involves changes in synaptic plasticity and dendrite morphology, including increased expression of pre- and postsynaptic plasticity-related proteins, as well as the density and function of axo-spinous synapses. Here we review the evidence from brain imaging and postmortem studies demonstrating that depression is accompanied by structural and functional alterations of cortical and limbic brain regions, including the prefrontal cortex, hippocampus and amygdala. In addition, we present more direct evidence from basic research studies that exposure to stress alters spine morphology, function and plasticity and that antidepressants, particularly new rapid acting agents, reverse these effects. Elucidation of the signaling pathways and molecular mechanisms that control spine synapse assembly and plasticity will contribute to a better understanding of the pathophysiology of depression and development of novel, more effective therapeutic agents.

Age-related memory impairment is associated with disrupted multivariate epigenetic coordination in the hippocampus

Mounting evidence linking epigenetic regulation to memory-related synaptic plasticity raises the possibility that altered chromatin modification dynamics might contribute to age-dependent cognitive decline. Here we show that the coordinated orchestration of both baseline and experience-dependent epigenetic regulation seen in the young adult hippocampus is lost in association with cognitive aging. Using a well-characterized rat model that reliably distinguishes aged individuals with significant memory impairment from others with normal memory, no single epigenetic mark or experience-dependent modification in the hippocampus uniquely predicted differences in the cognitive outcome of aging. The results instead point to a multivariate pattern in which modification-specific, bidirectional chromatin regulation is dependent on recent behavioral experience, chronological age, cognitive status, and hippocampal region. Whereas many epigenetic signatures were coupled with memory capacity among young adults and aged rats with preserved cognitive function, such associations were absent among aged rats with deficits in hippocampal memory. By comparison with the emphasis in current preclinical translational research on promoting chromatin modifications permissive for gene expression, our findings suggest that optimally successful hippocampal aging may hinge instead on enabling coordinated control across the epigenetic landscape.

The ageing cortical synapse: hallmarks and implications for cognitive decline

Normal ageing is associated with impairments in cognitive function, including memory. These impairments are linked, not to a loss of neurons in the forebrain, but to specific and relatively subtle synaptic alterations in the hippocampus and prefrontal cortex. Here, we review studies that have shed light on the cellular and synaptic changes observed in these brain structures during ageing that can be directly related to cognitive decline in young and aged animals. We also discuss the influence of the hormonal status on these age-related alterations and recent progress in the development of therapeutic strategies to limit the impact of ageing on memory and cognition in humans.

Age-related deficits in spatial memory and hippocampal spines in virgin, female Fischer 344 rats

Effects of aging on memory and brain morphology were examined in aged, 21-month-old, and young, 4-month-old, Fischer 344 female rats. Spatial memory was assessed using the object placement task, and dendritic spine density was determined on pyramidal neurons in the hippocampus following Golgi impregnation. Consistent with previous studies, aged females showed poorer object placement performance than young subjects. Young subjects significantly discriminated the location of objects with a 1.5-hour intertrial delay while aged subjects did not. Spine density of basal dendrites on CA1 pyramidal cells was 16% lower in the aged subjects as compared to the young subjects. No differences in spine density were found between young and aged subjects in basal dendrites of CA1 or in either dendritic field of CA3 pyramidal neurons. Thus, decreased hippocampal CA1 dendritic spine density in aged rats may contribute to poorer spatial memory as compared to young rats. The possibility that the neuroplastic changes observed in this study may pertain only to female subjects having had a specific set of life experiences is discussed. Different factors, such as reproductive status, diet, and handling may contribute to neuroplasticity of the brain during aging; however, this view requires further examination.

Selective vulnerability of neurons in layer II of the entorhinal cortex during aging and Alzheimer's disease

All neurons are not created equal. Certain cell populations in specific brain regions are more susceptible to age-related changes that initiate regional and system-level dysfunction. In this respect, neurons in layer II of the entorhinal cortex are selectively vulnerable in aging and Alzheimer's disease (AD). This paper will cover several hypotheses that attempt to account for age-related alterations among this cell population. We consider whether specific developmental, anatomical, or biochemical features of neurons in layer II of the entorhinal cortex contribute to their particular sensitivity to aging and AD. The entorhinal cortex is a functionally heterogeneous environment, and we will also review data suggesting that, within the entorhinal cortex, there is subregional specificity for molecular alterations that may initiate cognitive decline. Taken together, the existing data point to a regional cascade in which entorhinal cortical alterations directly contribute to downstream changes in its primary afferent region, the hippocampus.

Enriched environment increases spinophilin mRNA expression and spinophilin immunoreactive dendritic spines in hippocampus and cortex

Housing rodents in an enriched environment (EE) induces structural and functional plasticity in the adult brain, including increased dendritic sprouting and number of dendritic spines. However, the molecular mechanisms behind EE-induced brain plasticity remain largely unknown. Circadian rhythm plays an important role in memory processing but the neurobiological mechanisms of how circadian rhythm affects memory and brain plasticity remain controversial. In the current study, we studied the expression of spinophilin, a protein highly enriched in dendritic spines and involved in spine morphology and synaptic plasticity, to examine the effects of EE and circadian rhythm in rats housed in EE for different periods of time. Spinophilin mRNA expression was studied by in situ hybridization and the density of spinophilin immunoreactive puncta was quantified after immunohistochemical staining. Compared to rats living in a deprived environment (DE), we found a transient increase in the density of spinophilin immunoreactive puncta in hippocampus and cortex after 1 week of EE housing and persistent elevations of spinophilin mRNA expression during 1-4 weeks of environmental enrichment. Increased spinophilin expression was found during the light phase of the diurnal cycle, but not the dark phase. Thus, enriched housing altered the diurnal variation in spinophilin mRNA expression, suggesting that circadian modulation is likely to be important for experience dependent plasticity. The current results suggest a possible role for spinophilin in neuronal plasticity induced by environmental enrichment, but further studies are needed to establish a cause-effect relation.

Synaptic strength and postsynaptically silent synapses through advanced aging in rat hippocampal CA1 pyramidal neurons

Synaptic dysfunction is thought to contribute to age-related learning impairments. Detailed information regarding the presence of silent synapses and the strength of functional ones through advanced aging, however, is lacking. Here we used paired-pulse minimal stimulation techniques in CA1 stratum radiatum to determine whether the amplitude of spontaneous and evoked miniature excitatory postsynaptic currents (sEPSCs and eEPSCs, respectively) changes over the lifespan of rats in hippocampal CA1 pyramidal neurons, and whether silent synapses are present in adult and aged rats. The amplitudes of both sEPSCs and eEPSCs at resting membrane potential (i.e., clamped at -65 mV) initially increased between 2 weeks and 3 months, but then remained constant through 36 months of age. The potency of the eEPSCs at depolarized membrane potentials (i.e., clamped at +40 mV), however, was highest among 36-month old rats. Additionally, presynaptically silent synapses in CA1 stratum radiatum disappeared between 2 weeks and 3 months, but postsynaptically silent synapses were present through advanced aging. The similarity of silent and functional synapses in CA1 hippocampus at resting membrane potentials throughout adulthood in rats may indicate that impairments in the mechanisms of synaptic plasticity and its subsequent stabilization, rather than deficient synaptic transmission, underlie age-related cognitive decline. Such a notion is consistent with the increased amplitude of synaptic currents at depolarized potentials, perhaps suggesting an upregulation in the expression of synaptic NMDA receptors once rats reach advanced age.