Age-related cerebral atrophy in nonhuman primates predicts cognitive impairments

In humans, but not in nonhuman primates, a clear relationship has been established between age-associated cognitive decline and atrophy of specific brain regions. We evaluated age-related cerebral atrophy and cognitive alterations in mouse lemur primates. Cerebral atrophy was evaluated by in vivo magnetic resonance imaging in 34 animals aged from 1.9 to 11.8 years. The caudate and splenium were atrophied in most older animals, whereas shrinkage of the hippocampus, entorhinal cortex, and septal region was identified in a subgroup of the older animals. The temporal and cingulate cortex also exhibited a severe atrophy, whereas frontal and parietal areas were spared. Measures of cognitive ability in 16 animals studied by magnetic resonance imaging (MRI) showed that both executive functions and spatial memory declined with aging. Impairment of executive functions in older animals was associated with atrophy of the septal region while spatial memory performance was related to atrophy of the hippocampus and entorhinal cortex. Mouse lemurs are the first nonhuman primates in which a clear relationship is established between age-associated cognitive alteration and cerebral atrophy.     

PIB binding in aged primate brain: Enrichment of high-affinity sites in humans with Alzheimer's disease

Aged nonhuman primates accumulate large amounts of human-sequence amyloid β (Aβ) in the brain, yet they do not manifest the full phenotype of Alzheimer's disease (AD). To assess the biophysical properties of Aβ that might govern its pathogenic potential in humans and nonhuman primates, we incubated the benzothiazole imaging agent Pittsburgh Compound B (PIB) with cortical tissue homogenates from normal aged humans, humans with AD, and from aged squirrel monkeys, rhesus monkeys, and chimpanzees with cerebral Aβ-amyloidosis. Relative to humans with AD, high-affinity PIB binding is markedly reduced in cortical extracts from aged nonhuman primates containing levels of insoluble Aβ similar to those in AD. The high-affinity binding of PIB may be selective for a pathologic, human-specific conformation of multimeric Aβ, and thus could be a useful experimental tool for clarifying the unique predisposition of humans to Alzheimer's disease.     

Preservation of hippocampal neuron numbers in aged rhesus monkeys

To investigate whether or not aging of nonhuman primates is accompanied by a region-specific neuron loss in the hippocampal formation, we used the optical fractionator technique to obtain stereological estimates of unilateral neuron numbers of the hippocampi of eight young (0-4 years) and five aged (18-31 years) male rhesus monkeys (Macaca mulatta). Our results show a preservation of neurons (mean x 10 (3)+/-S.D.x10(3)) in the subiculum (young=588+/-124, aged=612+/-207), CA1 (young=1051+/-249, aged=1318+/-311), CA2 (young=100+/-18, aged=113+/-12), CA3 (young=478+/-125, aged=509+/-139), hilus (young=337+/-115, aged=394+/-90), and dentate gyrus (young=5550+/-1725, aged=7799+/-2087) of the hippocampal formation. These results confirm a previous stereological study in rhesus monkeys, but are in conflict with data for humans, showing age-dependent region-specific alterations in the hippocampal formation.     

Age-related cognitive deficits in rhesus monkeys mirror human deficits on an automated test battery

Aged non-human primates are a valuable model for gaining insight into mechanisms underlying neural decline with aging and during the course of neurodegenerative disorders. Behavioral studies are a valuable component of aged primate models, but are difficult to perform, time consuming, and often of uncertain relevance to human cognitive measures. We now report findings from an automated cognitive test battery in aged primates using equipment that is identical, and tasks that are similar, to those employed in human aging and Alzheimer's disease (AD) studies. Young (7.1+/-0.8 years) and aged (23.0+/-0.5 years) rhesus monkeys underwent testing on a modified version of the Cambridge Automated Neuropsychological Test Battery (CANTAB), examining cognitive performance on separate tasks that sample features of visuospatial learning, spatial working memory, discrimination learning, and skilled motor performance. We find selective cognitive impairments among aged subjects in visuospatial learning and spatial working memory, but not in delayed recall of previously learned discriminations. Aged monkeys also exhibit slower speed in skilled motor function. Thus, aged monkeys behaviorally characterized on a battery of automated tests reveal patterns of age-related cognitive impairment that mirror in quality and severity those of aged humans, and differ fundamentally from more severe patterns of deficits observed in AD.     

Senile plaques and neurofibrillary changes in the brain of an aged lemurian primate, Microcebus murinus.

In some aged Microcebus brains (8- to 11-year-old animals) dramatic atrophy is found, particularly of the cortex, the hippocampus, the basal ganglia, the brainstem and the cerebellum, associated with a conspicuous increase in the size of the cerebral ventricles. These morphological changes are accompanied by certain histological profiles indicative of pathology. In the cortex, these histological changes consist of 1) a large number of senile plaques composed of degenerated neurites sometimes surrounding an amyloid plaque, 2) amyloid deposits in the vascular walls and 3) dense bundles of argyrophilic filaments in numerous pyramidal neurons. All these lesions resemble changes associated with Alzheimer's disease in man. The degenerative changes observed in the Microcebus brain are accompanied by behavioral changes. At the moment these preliminary studies, carried out on the smallest of all primates, do not prove that the degeneration is of the Alzheimer type, but do indicate that Microcebus murinus may well be a good model for the study of cerebral aging, providing a comparison with cerebral ageing in humans. The size, life span and cost of the animal provide further advantages when compared with other nonhuman primates.     

Effect of neural precursor proliferation level on neurogenesis in rat brain during aging and after focal ischemia

The observed age-related decline in neurogenesis may result from reduced proliferation or increased death rate of neuronal precursor cells (NPCs). We found that caspase-3, but not caspase-6, -7, or -9, was activated in NPCs in neurogenic regions of young, young-adult, middle-aged and aged rat brains. The number of capase-3-immunoreactive cells was highest in young and lowest in aged rats. Surprisingly, intraventricular administration of a caspase-3 inhibitor failed to restore the number of BrdU-positive cells in the aged dentate gyrus, suggesting that the age-related decline in neurogenesis may be attributable primarily to reduced proliferation. Additionally, we also found that NPCs in the subventricular zone of young-adult and aged rat brain were increased after focal cerebral ischemia, suggesting that the increase in neurogenesis induced by ischemia may result from an increase in the rate of NPC proliferation, but not from a decrease in NPC death. Thus, our results suggest that age-related and injury-induced changes in the rate of neurogenesis are controlled at the level of NPC proliferation. Furthermore, our results may imply that the mechanisms that maintain a stable population of NPCs in the normal adult and in the ischemic brain, which account for the observed age-dependent reduction or injury-induced increases in neurogenesis, impinge on the regulation of cell division at the NPC level.     

大鼠室管膜下区和齿状回神经前体细胞增殖的衰老性变化

为了观察大鼠脑内神经前体细胞增殖的增龄性变化、探讨其在脑老化机制中的作用 ,本研究取不同年龄的大鼠经腹腔注射 Brd U标记处于增殖状态的神经前体细胞 ,用抗 Brd U抗体进行免疫组化反应 ,镜下观察脑内神经前体细胞的分布 ,并计数作定量分析。结果发现 ,各年龄组的室管膜下区及齿状回颗粒下层有 Brd U阳性细胞分布 ;上述各部位 Brd U阳性细胞数和标记率均呈明显的增龄性下降 ,幼年组和青年组之间 ,以及青年组和老年组之间的差异均有非常显著性意义 (P<0 .0 1)。结果表明 ,神经前体细胞的增殖能力随衰老而明显下降。这可能与衰老脑的神经元补偿能力及神经可塑性降低有关     

Changes in the expression of the NR2B subunit during aging in macaque monkeys

Humans, non-human primates and rodents show declines in spatial memory abilities with increased age. Some of these declines in mice are related to changes in the expression of the epsilon2 (epsilon2) (NR2B) subunit of the N-methyl-D-aspartate receptor. The purpose of this study was to determine whether primates show changes during aging in the mRNA expression of the NR2B subunit. In situ hybridization was performed on tissue sections from three different ages of Rhesus monkeys (Macaca mulatta; 6-8, 10-12, and 24-26 years). There was a significant decrease in the mRNA expression of the NR2B subunit overall in the prefrontal cortex and in the caudate nucleus between young and old monkeys. There were no significant changes in NR2B mRNA expression in the hippocampus or the parahippocampal gyrus. The results in the prefrontal cortex, caudate and hippocampus were similar to those seen previously in C57BL/6 mice during aging, which suggests that mice may be useful as a model for primates to further examine the age-related changes in the expression of the NR2B subunit of the NMDA receptor in several important regions of the brain.     

Effects of aging and hormonal status on bimanual motor coordination in the rhesus monkey

Studies of age-related changes in motor function in nonhuman primates have been based exclusively on unimanual motor tasks. In this study, we examined whether aging affects bimanual motor coordination in the monkey model. In addition, we compared performance of ovariectomized and intact females on the task, to examine whether estrogen deficiency impairs motor function. The task required 29 rhesus monkeys (6-26 years old) to extract a maximum of 15 raisins from a testing hole-board. While the task could most efficiently be performed with two hands, other motor strategies were possible. The number of raisins extracted per minute was measured in each of eight sessions, the first and last of which were videotaped for analysis of motor patterns. The number of raisins retrieved per minute declined significantly with age. All monkeys improved with practice, but aged monkeys improved more slowly than young ones. The proportion of bimanual actions tended to increase between the first and the last sessions but was not significantly different between young and aged monkeys. Hormonal status did not affect performance. Finally, performance on the bimanual task was significantly correlated with performance on a previously administered unimanual motor test emphasizing speed, suggesting that age-related motor slowing may explain deficits in both tasks.     

Growth and Development at the Sphenoethmoidal Junction in Perinatal Primates

Integration of the sphenoid and ethmoid bones during early postnatal development is poorly described in the literature. A uniquely prolonged patency of sphenoethmoidal synchondrosis or prespheno‐septal synchondrosis (PSept) has been attributed to humans. However, the sphenoethmoidal junction has not been studied using a comparative primate sample. Here, we examined development of the sphenoethmoidal interface using ontogenetic samples of Old and New World monkeys, strepsirrhine primates (lemurs and lorises), and a comparative sample of other mammals. Specimens ranging from late fetal to 1 month postnatal age were studied using histology, immunohistochemistry, and micro‐computed tomography methods. Our results demonstrate that humans are not unique in anterior cranial base growth at PSept, as it is patent in all newborn primates. We found two distinctions within our sample. First, nearly all primates exhibit an earlier breakdown of the nasal capsule cartilage that abuts the orbitosphenoid when compared to nonprimates. This may facilitate earlier postnatal integration of the basicranium and midface and may enhance morphological plasticity in the region. Second, the PSept exhibits a basic dichotomy between strepsirrhines and monkeys. In strepsirrhines, the PSept has proliferating chondrocytes that are primarily oriented in a longitudinal plane, as in other mammals. In contrast, monkeys have a convex anterior end of the presphenoid with a radial boundary of cartilaginous growth at PSept. Our findings suggest that the PSept acts as a “pacemaker” of longitudinal facial growth in mammals with relatively long snouts, but may also contribute to facial height and produce a relatively taller midface in anthropoid primates. Anat Rec, 300:2115–2137, 2017. © 2017 Wiley Periodicals, Inc.     

Normal aging results in decreased synaptic excitation and increased synaptic inhibition of layer 2/3 pyramidal cells in the monkey prefrontal cortex

Executive system function, mediated largely by the prefrontal cortex (PFC), often declines significantly with normal aging in humans and non-human primates. The neural substrates of this decline are unknown, but age-related changes in the structural properties of PFC neurons could lead to altered synaptic signaling and ultimately to PFC dysfunction. The present study addressed this issue using whole-cell patch clamp assessment of excitatory and inhibitory postsynaptic currents (PSCs) in layer 2/3 pyramidal cells in in vitro slices of the PFC from behaviorally characterized young (< or =12 years old) and aged (> or =19 years old) rhesus monkeys. Behaviorally, aged monkeys were significantly impaired in performance on memory and executive system function tasks. Physiologically, the frequency of spontaneous glutamate receptor-mediated excitatory PSCs was significantly reduced in cells from aged monkeys, while the frequency of spontaneous GABAA receptor-mediated inhibitory PSCs was significantly increased. In contrast, there was no effect of age on the frequency, amplitude, rise time or decay time of action potential-independent miniature excitatory and inhibitory PSCs. The observed change in excitatory-inhibitory synaptic balance likely leads to significantly altered signaling properties of layer 2/3 pyramidal cells in the PFC with age.     

Prominent hippocampal CA3 gene expression profile in neurocognitive aging

Research in aging laboratory animals has characterized physiological and cellular alterations in medial temporal lobe structures, particularly the hippocampus, that are central to age-related memory deficits. The current study compares molecular alterations across hippocampal subregions in a rat model that closely mirrors individual differences in neurocognitive features of aging humans, including both impaired memory and preserved function. Using mRNA profiling of the CA1, CA3 and dentate gyrus subregions, we have distinguished between genes and pathways related to chronological age and those associated with impaired or preserved cognitive outcomes in healthy aged Long-Evans rats. The CA3 profile exhibited the most prominent gene expression differences related to cognitive status and of the three subregions, best distinguished preserved from impaired function among the aged animals. Within this profile differential expression of synaptic plasticity and neurodegenerative disease-related genes suggests recruitment of adaptive mechanisms to maintain function and structural integrity in aged unimpaired rats that does not occur in aged impaired animals.     

β-Endorphin: Effect on thermoregulation in aged monkeys

In previous experiments small doses of the opiate morphine produced greater hyperthermia in aged than in younger sub-human primates. To test whether this augmented response is due to enhanced sensitivity of CNS opioid receptors with age. β-endorphin (0.625–5 μg), an endogenous opioid peptide, was injected into the lateral cerebral ventricle (ICV) of young (<9 years) and aged (>9 years) squirrel monkeys. Significantly greater hyperthermias developed in the older primates after each dose. In the aged monkeys, all but the smallest dose increased core temperature about 1.5°C within 1 hr after injection. Mean rectal temperature in the y younger animals rose 0.5–0.7° after all but the largest dose (1–1.5°C rise). Both groups maintained an elevated body temperature after central β-endorphin throughout the 5 hr recording period. 1.25 μgβ-endorphin given ICV in a hot environment (30°C) caused greater hyperthermia in older animals. This dose given in the cold (18°C) caused large changes in temperature of the aged monkeys, either hyperthermia or marked decreases, whereas the young primates developed only moderate rises in body temperature. The same dose of morphine sulfate (1.25 μg) ICV produced similar changes in core temperature in the two age groups in each ambient temperature. These results indicate that: (1) stimulation of CNS opioid receptors influences thermoregulation and (2) aging increases responsiveness to such stimulation.     

Aged vervet monkeys developing transthyretin amyloidosis with the human disease-causing Ile122 allele: a valid pathological model of the human disease

Mutant forms of transthyretin (TTR) cause the most common type of autosomal-dominant hereditary systemic amyloidosis. In addition, wild-type TTR causes senile systemic amyloidosis, a sporadic disease seen in the elderly. Although spontaneous development of TTR amyloidosis had not been reported in animals other than humans, we recently determined that two aged vervet monkeys (Chlorocebus pygerythrus) spontaneously developed systemic TTR amyloidosis. In this study here, we first determined that aged vervet monkeys developed TTR amyloidosis and showed cardiac dysfunction but other primates did not. We also found that vervet monkeys had the TTR Ile122 allele, which is well known as a frequent mutation-causing human TTR amyloidosis. Furthermore, we generated recombinant monkey TTRs and determined that the vervet monkey TTR had lower tetrameric stability and formed more amyloid fibrils than did cynomolgus monkey TTR, which had the Val122 allele. We thus propose that the Ile122 allele has an important role in TTR amyloidosis in the aged vervet monkey and that this monkey can serve as a valid pathological model of the human disease. Finally, from the viewpoint of molecular evolution of TTR in primates, we determined that human TTR mutations causing the leptomeningeal phenotype of TTR amyloidosis tended to occur in amino acid residues that showed no diversity throughout primate evolution. Those findings may be valuable for understanding the genotype-phenotype correlation in this inherited human disease.     

Hippocampal volume is preserved and fails to predict recognition memory impairment in aged rhesus monkeys (Macaca mulatta)

Aged monkeys exhibit deficits in memory mediated by the medial temporal lobe system, similar to the effects of normal aging in humans. The contribution of structural deterioration to age-associated memory loss was explored using magnetic resonance imaging techniques. We quantified hippocampal, cerebral and ventricular volumes in young (n = 6, 9-12 years) and aged (n = 6, 24-29 years) rhesus monkeys. Eleven subjects were tested on a recognition memory task, delayed non-matching-to-sample (DNMS). Compared to young animals, aged monkeys exhibited robust learning deficits and significant memory impairments when challenged with longer retention intervals. Hippocampal volume was statistically equivalent across age groups, differing by less than 6%, and there was no correlation between this measure and DNMS performance. Variability in cerebral volume was greater in the aged compared to young monkeys and this parameter was marginally correlated with DNMS performance with a 10-min delay. These findings confirm and extend the conclusion of recent post-mortem histological analyses demonstrating that normal cognitive aging occurs independently of gross structural deterioration in the primate hippocampus.     

alpha-1 Adrenergic receptors stimulation induces the proliferation of neural progenitor cells in vitro

The proliferation of neural progenitor cells (NPCs) is regulated by classical neurotransmitters such as dopamine, serotonin and acetylcholine, via its own receptors. Previous studies have reported that the depletion of L-norepinephrine decreases the proliferation of NPCs in the adult rat hippocampus and it has been suggested that L-norepinephrine regulates the proliferation of NPCs. However, it remains unknown whether or not adrenergic receptors are involved in the increased proliferation of NPCs. In the present study, an MTT cell proliferation assay was carried out in order to investigate the roles played by adrenergic receptors in the proliferation of NPCs. We demonstrated that L-epinephrine enhanced the proliferation of embryonic NPCs in vitro. In addition, the alpha-1 adrenergic receptor agonist L-phenylephrine was found to enhance the proliferation of NPCs, whereas an alpha-adrenergic antagonist and selective alpha-1 antagonists significantly inhibited cell proliferation increases induced by L-epinephrine and L-phenylephrine. These results suggest that stimulation with alpha-1 adrenergic receptors induces the proliferation of embryonic NPCs.     

The antitumorigenic response of neural precursors depends on subventricular proliferation and age

Glioblastomas, the most aggressive primary brain tumors, occur almost exclusively in adult patients. Neural precursor cells (NPCs) are antitumorigenic in mice, as they can migrate to glioblastomas and induce tumor cell death. Here, we show that the antitumor effect of NPCs is age-dependently controlled by cell proliferation in the subventricular zone (SVZ) and that NPCs accumulating at a glioblastoma are diverted from their normal migratory path to the olfactory bulb. Experimentally induced cortical glioblastomas resulted in decreased subventricular proliferation in adult (postnatal day 90) but not in young (postnatal day 30) mice. Adult mice supplied fewer NPCs to glioblastomas and had larger tumors than young mice. Apart from the difference in proliferation, there was neither a change in cell number and death rate in the SVZ nor a change in angiogenesis and immune cell density in the tumors. The ability to kill glioblastomas was similar in NPCs isolated from young and adult mice. The proliferative response of NPCs to glioblastomas depended on the expression of D-type cyclins. In young mice, NPCs express the cyclins D1 and D2, but the expression of cyclin D1 is lost during aging, and in adult NPCs only cyclin D2 remains. In young and adult cyclin D2-deficient mice we observed a reduced supply of NPCs to glioblastomas and the generation of larger tumors compared with wild-type mice. We conclude that cyclin D1 and D2 are nonredundant for the antitumor response of subventricular NPCs. Loss of a single D-type cyclin results in a smaller pool of proliferating NPCs, lower number of NPCs migrating to the tumor, and reduced antitumor activity. Disclosure of potential conflicts of interest is found at the end of this article.     

Brain aging in humans,chimpanzees (Pan troglodytes), and rhesus macaques (Macaca mulatta): magnetic resonance imaging studies of macro- and microstructural changes

Among primates, humans are uniquely vulnerable to many age-related neurodegenerative disorders. We used structural and diffusion magnetic resonance imaging (MRI) to examine the brains of chimpanzees and rhesus monkeys across each species' adult lifespan, and compared these results with published findings in humans. As in humans, gray matter volume decreased with age in chimpanzees and rhesus monkeys. Also like humans, chimpanzees showed a trend for decreased white matter volume with age, but this decrease occurred proportionally later in the chimpanzee lifespan than in humans. Diffusion MRI revealed widespread age-related decreases in fractional anisotropy and increases in radial diffusivity in chimpanzees and macaques. However, both the fractional anisotropy decline and the radial diffusivity increase started at a proportionally earlier age in humans than in chimpanzees. Thus, even though overall patterns of gray and white matter aging are similar in humans and chimpanzees, the longer lifespan of humans provides more time for white matter to deteriorate before death, with the result that some neurological effects of aging may be exacerbated in our species.     

Calorie restriction in nonhuman primates: assessing effects on brain and behavioral aging

Dietary caloric restriction (CR) is the only intervention repeatedly demonstrated to retard the onset and incidence of age-related diseases, maintain function, and extend both lifespan and health span in mammals, including brain and behavioral function. In 70 years of study, such beneficial effects have been demonstrated in rodents and lower animals. Recent results emerging from ongoing studies of CR in humans and nonhuman primates suggest that many of the same anti-disease and anti-aging benefits observed in rodent studies may be applicable to long-lived species. Results of studies in rhesus monkeys indicate that CR animals (30% less than controls) are healthier than fully-fed counterparts based on reduced incidence of various diseases, exhibit significantly better indices of predisposition to disease and may be aging at a slower rate based on analysis of selected indices of aging. The current review discusses approaches taken in studies of rhesus monkeys to analyze age-related changes in brain and behavioral function and the impact of CR on these changes. Approaches include analyses of gross and fine locomotor performance as well as brain imaging. In a related study it was observed that short-term CR (6 months) in adult rhesus monkeys can provide protection against a neurotoxic insult. Increasing interest in the CR paradigm will expand its role in demonstrating how nutrition can modulate the rate of aging and the mechanisms responsible for this modulation.     

Synaptic correlates of memory and menopause in the hippocampal dentate gyrus in rhesus monkeys

Aged rhesus monkeys exhibit deficits in hippocampus-dependent memory, similar to aging humans. Here we explored the basis of cognitive decline by first testing young adult and aged monkeys on a standard recognition memory test (delayed nonmatching-to-sample test; DNMS). Next we quantified synaptic density and morphology in the hippocampal dentate gyrus (DG) outer (OML) and inner molecular layer (IML). Consistent with previous findings, aged monkeys were slow to learn DNMS initially, and they performed significantly worse than young subjects when challenged with longer retention intervals. Although OML and IML synaptic parameters failed to differ across the young and aged groups, the density of perforated synapses in the OML was coupled with recognition memory accuracy. Independent of chronological age, monkeys classified on the basis of menses data as peri- or post-menopausal scored worse on DNMS, and displayed lower OML perforated synapse density, than premenopausal monkeys. These results suggest that naturally occurring reproductive senescence potently influences synaptic connectivity in the DG OML, contributing to individual differences in the course of normal cognitive aging.