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.     

Recognition memory deficits in a subpopulation of aged monkeys resemble the effects of medial temporal lobe damage.

The present study examined individual differences in recognition memory function in a group of Old World monkeys (Macaca mulatta). Four young (9-11 years) and 10 aged (22-33 years) monkeys were tested in the same delayed-nonmatching-to-sample (DNMS) recognition memory procedure that has been widely used to study the effects of experimental hippocampal lesions in young subjects. Animals were first trained to a 90% correct learning criterion in the DNMS task using a 10-second delay between the sample and recognition phase of each trial. The memory demands of the task were then increased by gradually extending the retention interval from 15 seconds to 10 minutes. Three of the aged monkeys performed as accurately as young subjects at all delays. The remaining aged monkeys performed well at the shortest delays (15 and 30 seconds), but progressively greater impairments emerged across delays of 60 seconds, 2 minutes, and 10 minutes. These results suggest that recognition memory is only compromised in a subpopulation of aged monkeys. Moreover, aged monkeys that are impaired in the DNMS task exhibit the same delay-dependent pattern of deficits that is the hallmark of memory dysfunction resulting from medial temporal lobe damage.     

Effects of aging on visual recognition memory in the rhesus monkey

As part of an effort to develop a primate model of human age-related memory dysfunction, performance by six rhesus monkeys 26 to 27 years of age was compared to that of six young adult monkeys (four to five years of age) on a trial unique delayed nonmatching to sample (DNMS) task. This task assesses the monkey's ability to identify a novel from a familiar stimulus over a delay and resembles closely clinical tests that are used to assess memory function in geriatric patients. The task was presented in three stages: acquisition, delays and lists. As a group, aged monkeys were impaired relative to the young adult group on all three conditions. However, within the aged group, individual cases of efficient performance were observed. Error analyses of item positions of the lists condition revealed the absence of enhanced performance for items presented at the end of a list by aged animals, suggesting an abnormal sensitivity to proactive interference. The finding of a recognition impairment with age is in parallel with studies of normal human aging and lends support to the notion that the rhesus monkey is a suitable animal model of human aging.     

Analysis of alpha-2 adrenergic agonist effects on the delayed nonmatch-to-sample performance of aged rhesus monkeys

The administration of alpha-2 adrenergic agonists to aged monkeys has been shown to ameliorate their cognitive deficits on the delayed response (DR) task, a test of spatial working memory (3,5). The present experiment tested whether the alpha-2 agonists, clonidine and guanfacine, would also improve working memory for object feature recognition, as tested by the delayed nonmatch-to-sample (DNMS) task. Five aged monkeys were trained on DNMS and were found to have mild performance deficits comparable to those reported previously for monkeys of similar age (32). However, during the subsequent two years of drug testing, the animals' baseline performance steadily improved, and conditions had to be made progressively more difficult to produce errors in performance. Clonidine and guanfacine significantly altered the DNMS performance of the aged monkeys, but drug-induced improvement was not as robust for DNMS as it was for DR. Clonidine produced a triphasic dose/response curve: Impairment was observed at both very low and high doses, while modest improvement was seen in the middle dose range (average maximal improvement of 21 ± 2.4%). Although improvement could occasionally be replicated for some doses, the clonidine dose/response curves were remarkably inconsistent in the middle dose range. Similarly, doses of guanfacine which had previously produced optimal improvement on the DR task, produced only small but significant improvement in DNMS performance (average improvement of 11 ± 3% for the 0.00011–0.000011 mg/kg dose range). By the end of the drug study, three monkeys were performing well (70–75% correct) with lists of 20 objects and over 400 sec delays, and one monkey performed better than 85% correct with lists of 40 objects and over 800 sec delays. Given the high level of performance of the aged monkeys on the DNMS task, it is unclear whether the weak improvement produced by alpha-2 agonists on the DNMS as compared to the DR task is due to ceiling effects, or rather results from alpha-2 adrenergic mechanisms being more important for spatial working memory than memory for object features. Future experiments utilizing a more difficult object working memory task will be necessary to resolve this issue.     

Visual discrimination and reversal learning in the aged monkey (Macaca mulatta)

Visual discrimination and reversal learning were assessed in young adult (10-12 years old, n = 4) and aged (23-27 years old, n = 5) female rhesus monkeys. Performance was comparable across age groups in many tasks, suggesting that the acquisition of stimulus-reward associations remains largely intact in the aged monkey. Most older subjects, however, required more training than any young animal to learn an initial pattern discrimination. In combination with previous findings from the same groups of monkeys, these data suggest that deficits in attending to the relevant stimulus features in novel testing procedures may contribute to poor performance in aged subjects across a variety of learning and memory tasks. In addition, preliminary findings from a discrimination probe procedure raise the possibility that aged subjects may adopt alternate testing strategies that compensate for some aspects of age-dependent cognitive dysfunction.     

Object recognition versus object discrimination: comparison between human infants and infant monkeys.

Human infants (12-32 months old) and adults learned a delayed nonmatching-to-sample (DNMS) task and single- and multiple-pair discrimination tasks using nonverbal procedures previously used with monkeys. Infants learned discriminations rapidly and at a young age (12 months), but they required prolonged training and maturation before learning the DNMS task. Adults learned all tasks rapidly. After learning the DNMS task to criterion, memory performance declined systematically in an inverse relation to age. The dissociation in ability of infants on the DNMS versus discrimination tasks closely resembles the dissociation previously reported with infant monkeys (Bachevalier & Mishkin, 1984). Results from both infant humans and monkeys support a neurocognitive maturational model.     

Protein kinase C activity is associated with prefrontal cortical decline in aging

Aging is associated with deficiencies in the prefrontal cortex, including working memory impairment and compromised integrity of neuronal dendrites. Although protein kinase C (PKC) is implicated in structural plasticity, and overactivation of PKC results in working memory impairments in young animals, the role of PKC in prefrontal cortical impairments in the aged has not been examined. This study provides the first evidence that PKC activity is associated with prefrontal cortical dysfunction in aging. Pharmacological inhibition of PKC with chelerythrine rescued working memory impairments in aged rats and enhanced working memory in aged rhesus monkeys. Improvement correlated with age, with older monkeys demonstrating a greater degree of improvement following PKC inhibition. Furthermore, PKC activity within the prefrontal cortex was inversely correlated with the length of basal dendrites of prefrontal cortical neurons, as well as with working memory performance in aged rats. Together these findings indicate that PKC is dysregulated in aged animals and that PKC inhibitors may be useful in the treatment of cognitive deficits in the elderly.     

Cognitive function in aged ovariectomized female rhesus monkeys

To determine whether ovariectomy exacerbates age-related cognitive decline, the performance of 6 aged monkeys that had been ovariectomized early in life (OVX-Aged) was compared to that of 8 age-matched controls with intact ovaries (INT-Aged) and that of 5 young controls with intact ovaries (INT-Young) in tasks of visual recognition memory, object and spatial memory, and executive function. The OVX-Aged monkeys were marginally more impaired than the INT-Aged monkeys on the delayed nonmatching-to-sample with a 600-s delay. In contrast, they performed significantly better than the INT-Aged controls on the spatial condition of the delayed recognition span test. The hypothesis that prolonged estrogenic deprivation may exaggerate the age-related decline in visual recognition memory will require additional support. However, the findings suggest that long-term ovariectomy may protect against the development with aging of spatial memory deficits.     

Long-term effects of neonatal damage to the hippocampal formation and amygdaloid complex on object discrimination and object recognition in rhesus monkeys (Macaca mulatta)

Rhesus monkeys with neonatal aspiration lesions of the hippocampal formation or the amygdaloid complex were tested on concurrent discrimination learning (24-hr intertrial interval [ITI]) at 3 months, on object recognition memory (delayed nonmatching-to-sample [DNMS]) at 10 months, and retested on both tasks at 6-7 years of age. Neonatal amygdaloid damage mildly impaired acquisition at the 24-hr ITI and the performance test of DNMS at both ages. In contrast, early hippocampal lesions impaired performance only on the longest lists of 10 items in DNMS in adult monkeys. Thus, early amygdala lesions appeared to have resulted in a greater object memory loss than early hippocampal lesions. However, in light of recent findings from lesion studies in adult monkeys, the object memory impairment after early amygdaloid lesions is better accounted for by damage to the entorhinal and perirhinal cortex than by damage to the amygdaloid nuclei.     

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.     

Impairment in abstraction and set shifting in aged rhesus monkeys

Understanding the nature of changes in cognition with aging has increased in importance as the number of individuals over the age of 65 years grows. To date, studies have demonstrated that age-related changes occur most extensively in the cognitive domains of memory and executive function. Whereas a large number of studies have been conducted about the effects of aging on memory, far less have explored the effects of aging on the so called "executive function" which include abilities essential for successful performance of higher level activities of daily living. As part of our ongoing effort to better characterize these changes, we assessed executive function in a non-human primate model of normal human aging using the Conceptual Set Shifting Task (CSST). This recently developed task assesses abstraction, concept formation and set shifting in the monkey in a way analogous to the Wisconsin Card Sorting Test (WCST) in humans. Relative to young adult monkeys, aged monkeys evidenced significant difficulty in both acquisition and performance on this task, and moreover, demonstrated a high degree of perseverative responding. The pattern of performance displayed by the aged monkeys suggests an age-related decline in prefrontal cortex (PFC) functioning.     

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.     

Memory Deficits in Aged Cebus Monkeys and Facilitation With Central Cholinomimetics

Cebus monkeys of 3 different age groups were trained to perform an automated behavioral task (delayed response), intended to measure recent memory ability. In in initial study, the aged monkeys (18 years and older) exhibit prprogressively greater performance impairments (relative to young monkeys) as they were required to remember the location of a visual stimulus for increasingly longer durations (0 to 20 sec). This deficits replicated previously published results from aged Rhesus monkeys and appeared similar to the primary memory deficits reported in elderly humans and demented patients. In subsequent studies, the effects of three different cholinomimetics were evaluated for their ability to improve the aged monkey's performance on this task. Each monkey was tested under several acute doses of the cholinergic precursor, choline, the anticholinesterase, physostigmine, and the cholinergic muscarinic receptor agonist, arecoline. The results revealed clear differences in the ability of these drugs to improve performance on this task. Choline exerted no apparent effects in the aged monkeys at any dose tested. Physostigmine clearly enhanced performance in certain aged monkeys, but the optimal dose varied dramatically between subjects, replicating previously published results with aged Rhesus monkeys and humans. Arecoline produced clear improvement within a restricted dose range, with little variation in optimal dose between subjects. In addition to demonstrating differences in the effects of different cholinomimetics on memory performance in aged primates, these data also suggest a possible rationale for future investigations. Assuming that each of these drugs primarily affected cholinergic function in the manner conventionally attributed, these data suggest that, within the cholinergic system, the more directly one stimulates the receptor, the more one might expect robust and consistent effects on memory performance in aged subjects.     

Memory Deficits in Aged Cebus Monkeys and Facilitation With Central Cholinomimetics

Cebus monkeys of 3 different age groups were trained to perform an automated behavioral task (delayed response), intended to measure recent memory ability. In in initial study, the aged monkeys (18 years and older) exhibit prprogressively greater performance impairments (relative to young monkeys) as they were required to remember the location of a visual stimulus for increasingly longer durations (0 to 20 sec). This deficits replicated previously published results from aged Rhesus monkeys and appeared similar to the primary memory deficits reported in elderly humans and demented patients. In subsequent studies, the effects of three different cholinomimetics were evaluated for their ability to improve the aged monkey's performance on this task. Each monkey was tested under several acute doses of the cholinergic precursor, choline, the anticholinesterase, physostigmine, and the cholinergic muscarinic receptor agonist, arecoline. The results revealed clear differences in the ability of these drugs to improve performance on this task. Choline exerted no apparent effects in the aged monkeys at any dose tested. Physostigmine clearly enhanced performance in certain aged monkeys, but the optimal dose varied dramatically between subjects, replicating previously published results with aged Rhesus monkeys and humans. Arecoline produced clear improvement within a restricted dose range, with little variation in optimal dose between subjects. In addition to demonstrating differences in the effects of different cholinomimetics on memory performance in aged primates, these data also suggest a possible rationale for future investigations. Assuming that each of these drugs primarily affected cholinergic function in the manner conventionally attributed, these data suggest that, within the cholinergic system, the more directly one stimulates the receptor, the more one might expect robust and consistent effects on memory performance in aged subjects.     

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.     

Aged monkeys exhibit behavioral deficits indicative of widespread cerebral dysfunction

To determine whether the decline of behavioral abilities with aging in monkeys is selective or widespread, we examined 18 monkeys ranging from 3 to 34 years of age on a wide variety of tests with the ultimate goal of correlating behavioral deficits with age-related changes in the brain. In our initial study we found impaired visual recognition ability in the aged monkeys (43). In the present study, we assessed the same animals on tests of spatial memory, visual habit formation, visuospatial orientation, visually guided reaching, motor skill learning, and reaction time, these categories having been chosen to test the integrity of different cerebral systems. There were three major findings. First, age-related impairments were observed in nearly all test categories, though often not on easy versions of the tests, suggesting that the deficits observed were in the specific abilities measured and not an artifact of lowered motivation or other general disability. Second, the behavioral decline began in the late teens for certain spatial abilities but did not affect other abilities until the late 20's, suggesting that although the cerebral dysfunction eventually becomes widespread, the cerebral systems underlying spatial abilities are compromised by aging earlier than others. Finally, the finding of correlations between scores of aged animals primarily within test categories as opposed to across categories suggests that different animals have different patterns of cerebral involvement.     

Effects of combined and separate removals of rostral dorsal superior temporal sulcus cortex and perirhinal cortex on visual recognition memory in rhesus monkeys

The dorsal bank of the superior temporal sulcus (STSd) bears anatomical relations similar to those of perirhinal cortex, an area critical for visual recognition memory. To examine whether STSd makes a similar contribution to visual recognition memory, performance on visual delayed nonmatching-to-sample (DNMS) was assessed in rhesus monkeys with combined or separate ablations of the perirhinal cortex and STSd as well as in unoperated controls. Consistent with previous findings, ablations of perirhinal cortex produced deficits nearly as severe as that found after rhinal (i.e., entorhinal plus perirhinal) cortex lesions. However, combined lesions of perirhinal cortex and STSd produced a deficit no greater than that produced by perirhinal cortex ablation alone, and lesions of STSd alone were without effect on DNMS. We conclude that STSd is not critically involved in visual recognition memory.     

The aging hippocampus: a multi-level analysis in the rat

In the current experiment we conducted a multi-level analysis of age-related characteristics in the hippocampus of young adult (3 months), middle-aged (12 months), and old (24 months) Fisher 344xBrown Norway hybrid (FBNF1) rats. We examined the relationships between aging, hippocampus, and memory using a combination of behavioral, non-invasive magnetic resonance imaging and spectroscopy, and postmortem neuroanatomical measures in the same rats. Aging was associated with functional deficits on hippocampus-dependent memory tasks, accompanied by structural alterations observed both in vivo (magnetic resonance imaging-hippocampal volume) and postmortem (dentate gyrus neuronal density and neurogenesis). Neuronal metabolic integrity, assessed by levels of N-acetylaspartate with magnetic resonance spectroscopy, was however, preserved. Further, our results suggest that neurogenesis (doublecortin) seems to be related to both performance deficits on hippocampus-dependent tasks and hippocampal volume reduction. The observed pattern of age-related alterations closely resembles that previously reported in humans and suggests FBNF1 rats to be a useful model of normal human aging.     

Executive system dysfunction in the aged monkey: Spatial and object reversal learning

As part of an effort to characterize age-related cognitive changes in executive system function in a nonhuman primate model of human aging, the performance of seven rhesus monkeys, 20 to 28 years of age, was compared to that of five young adult monkeys, 6 to 11 years of age, on spatial and object reversal tasks. No differences in performance were found between the two groups in the initial learning of either task. On spatial reversals, aged monkeys were impaired relative to young adults, but there was no difference in overall performance between the groups on object reversals. Central to this article, a perseverative tendency was noted in the aged group on both spatial and object reversal tasks. Changes in executive system dysfunction may represent an important aspect of age-related cognitive decline.     

老年学习记忆减退大鼠脑线粒体DNA缺失

目的和方法:测定老年大鼠脑mtDNA缺失型(以下简称缺失型),缺失mtDNA比例,探讨mtDNA缺失与老年性学习记忆减退的关系,为研究其分子机制提供基础资料。用Morris水迷宫将老年大鼠(24个月)筛选为老年学习记忆正常和学习记忆减退两组。用稀释PCR法测定大鼠大脑皮质、海马和小脑缺失型mtDNA比例。结果:青老年鼠大脑皮质、海马和小脑均存在mtDNA缺失,片段为4834bp;青年鼠的缺失比例约为0.00018%。老年记忆正常鼠上述3个脑区缺失型mtDNA缺失比例分别是青年鼠的6倍、6倍和2倍;老年记忆减退大鼠上述脑区的缺失型mtDNA比例分别是老年记忆正常鼠的2倍、1.8倍和3倍。结论:衰老时脑组织缺失型mtDNA增多,而老年学习记忆减退鼠较老年学习记忆正常鼠mtDNA缺失进一步成倍增加,表明相关脑区的mtDNA缺失在老年学习记忆减退的细胞分子机制中发挥重要作用。