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.     

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.     

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.     

Aged Non-Human Primates: An Animal Model of Age-Associated Neurodegenerative Disease

Aged non-human primates develop age-associated behavioral and brain abnormalities similar to those that occur in aged humans and, to a greater extent, in individuals with Alzheimer's disease. Declines in performance on cognitive and memory tasks begin at the monkey equivalent of late-middle life. As occurs in elderly humans, significant differences have been demonstrated in levels of performance between animals within older age groups. The brains of old monkeys show degenerative changes in neurons, abnormal axons and neurites (particularly in telencephalic areas), and deposits of amyloid in senile plaques and around blood vessels. Moreover, in some older animals, decrements occur in markers of specific neurotransmitter circuits, including the basal forebrain cholinergic system. It has been suggested that alterations in these cholinergic neurons contribute to the memory deficits that occur in older individuals. Because axotomy-induced retrograde degeneration of these neurons can be prevented by the administration of nerve growth factor, we have begun studies to determine whether administration of nerve growth factor improves performance of aged animals on memory tasks. This review describes the complementary nature of studies of non-human primates and human subjects, illustrating how these investigations can clarify factors that influence behavior and brain biology in age-associated diseases.     

Neuropeptide effects on memory in aged monkeys

The effects of several neuropeptides were evaluated using a non-human primate model of age-related memory impairments. Several doses of ACTH4-10, lysine vasopressin, arginine vasopressin, oxytocin and somatostatin were each tested in several aged monkeys. Because data from a large number of non-drug control sessions was collected before, during and after this study, it was possible to define the normal range of control performance for each monkey and statistically determine whether a change in performance under any single dose of drug reflected a significant change from the particular monkey's normal baseline performance. Although none of the neuropeptides produced consistent group effects, evaluations of individual subjects against their own baseline performance revealed reliable changes at certain doses. Arginine vasopressin appeared to produce the best overall effects with three of the five monkeys exhibiting reliable changes in performance from baseline. These same three monkeys also responded positively to the lysine form. Oxytocin impaired memory in three of the six aged monkeys tested over a wide range of doses. Three of six aged monkeys performed better under ACTH4-10 compared to baseline; however, in two of these cases only a single dose was effective. The performance of only one subject was improved under somatostatin, and this was at a single dose only. The data reported here provide evidence for neuropeptides producing behavioral improvement in non-human primates using an appetitive task, eliminating a popular criticism that the data in this literature has depended too heavily on the testing of rodents in shock-motivated tasks. Additionally, the improvements observed in this study involve a behavior that it naturally impaired by age and one which has many operational similarities and some empirical relevance to measures of recent memory in humans. However, these positive findings must be tempered by the lack of robust effects and high individual variation observed.     

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.     

Tetrahydroaminoacridine, 3,4 diaminopyridine and physostigmine: direct comparison of effects on memory in aged primates

The effects of tetrahydroaminoacridine (THA), 3,4 diaminopyridine (3,4 DAP) and physostigmine were evaluated for their ability to reduce memory impairments in aged, test-sophisticated cebus monkeys (18 to 26 years old). Several doses of each drug were tested (PO) in each of ten different monkeys, allowing for direct and extensive comparison of each drug's efficacy in this model. The results of this comparative test revealed several potentially interesting findings: (1) all drugs produced improvement in a portion of the monkeys tested; (2) as in many past tests with aged monkeys and humans, wide variations in most effective dose, per subject, were observed; (3) different monkeys responded more effectively to one drug than another; and (4) under these tightly controlled conditions, physostigmine produced the most reliable and robust effects (p less than 0.005), in more monkeys, than did either THA (p less than 0.05) or 3,4 DAP (p less than 0.10).     

Cholinergic modulation of a specific memory function of prefrontal cortex

Deficits in prefrontal cholinergic function are implicated in cognitive impairment in many neuropsychiatric diseases, but acetylcholine's specific role remains elusive. Rhesus monkeys with selective lesions of cholinergic input to prefrontal cortex (PFC) were unimpaired in tests of decision making and episodic memory that require intact PFC, but were severely impaired on a spatial working memory task. These observations are consistent with a specific role for prefrontal acetylcholine in working memory.     

Beneficial effects of nicotine administered prior to a delayed matching-to-sample task in young and aged monkeys

Our earlier studies have demonstrated that administration of low micrograms/kg doses of nicotine to young adult monkeys prior to a delayed matching-to-sample (DMTS) task resulted in a centrally mediated improvement in performance of the task, particularly when delay intervals which most greatly challenged the animal's capabilities were involved. The present study confirmed these findings using a completely computer driven and automated procedure. In addition, performance on the DMTS was observed to be enhanced when animals were again tested 24 h after the dose of nicotine. Further analysis of the data indicated that the majority of enhancement to nicotine could be accounted for by a greatly increased performance at the least preferred stimulus color. Position preference (left vs. right stimulus) was not a factor in nicotine-induced enhancement. Two aged monkeys (34 years old Macaca mulatta) were significantly more difficult to train in the DMTS task and their longest delay capabilities were significantly shorter than the young animals (Macaca fascicularis). Nevertheless, the aged animals were essentially similar in most respects in their responses to nicotine administration. These data are consistent with a role for central nicotinic systems in memory performance and with the ability of nicotine to produce enhancement of selective features of mnemonic strategy in young and old monkeys. Furthermore, it is possible that either model, the aged animal, or the young animal stressed to his mnemonic capability may provide a good model for learning and memory disorders in humans.     

Age-related working memory deficits in the allocentric place discrimination task: possible involvement in cholinergic dysfunction

It is well known that learning and memory ability declines with aging. Age-related long-term changes in learning and memory ability in rats were investigated with the place navigation task and the allocentric place discrimination task (APDT) in a water maze using the same animals for each task. In a working memory place navigation task, aged animals could learn the location of the platform as well as when they were young, although strategy shifts were observed. In contrast, accuracy in the APDT significantly declined from 90% to 65% with aging. This impairment was ameliorated by an acetylcholine esterase inhibitor physostigmine at 22–23 months old. No amelioration was, however, detected in the same animals tested when they further aged to 26–27 months old. These results suggest that the APDT performance is sensitive to age-related memory deficits and that this may be due to the cholinergic dysfunction.     

Scopolamine impairs auditory delayed matching-to-sample performance in monkeys

Information concerning the major neurotransmitters critical for auditory memory is sparse. One possibility is the cholinergic system, important for performance in some tasks requiring visual short-term memory and attention [T.G. Aigner, M. Mishkin, The effects of physostigmine and scopolamine on recognition memory in monkeys, Behav. Neural. Biol. 45 (1986) 81-87; N. Hironaka, K. Ando, Effects of cholinergic drugs on scopolamine-induced memory impairment in rhesus monkeys, Jpn. J. Psychopharmacol. 16 (1996) 103-108; T.M. Myers, G. Galbicka, M.L. Sipos, S. Varadi, J.L. Oubre, M.G. Clark, Effects of anticholinergics on serial-probe recognition accuracy of rhesus macaques (Macaca mulatta), Pharmacol. Biochem. Behav. 73 (2002) 829-834; H. Ogura, T.G. Aigner, MK-801 Impairs recognition memory in rhesus monkeys: comparison with cholinergic drugs, J. Pharmacol. Exp. Ther. 266 (1993) 60-64; D.M. Penetar, J.H. McDonough Jr., Effects of cholinergic drugs on delayed match-to-sample performance of rhesus monkeys, Pharmacol. Biochem. Behav. 19 (1983) 963-967; M.A. Taffe, M.R. Weed, L.H. Gold, Scopolamine alters rhesus monkey performance on a novel neuropsychological test battery, Cogn. Brain Res. 8 (1999) 203-212]. Five rhesus monkeys were trained to perform an auditory go/no-go delayed matching-to-sample (DMTS) task wherein two acoustic stimuli (500ms), separated by variable memory delays (500ms, 2500ms, or 5000ms), were either identical sound presentations, i.e., match trials, or two different sound presentations, i.e., nonmatch trials. Sound stimuli were chosen semi-randomly from a large set sound set ( approximately 900). After reaching a criterion of 80% correct on the behavioral task, monkeys were injected with saline or doses of scopolamine hydrochloride mixed in saline (3 microg, 5 microg, and 10 microg per 1kg of weight), 30 min before training. Scopolamine impaired performance accuracy on match trials in a dose-dependent manner. Blocking muscarinic receptors with scopolamine did not significantly impair motor responses, food motivation, or responses to rewarded sound. These findings support the hypothesis that the cholinergic system is important for auditory short-term memory.     

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 combined acetylcholinesterase inhibition and serotonergic receptor blockade on age-associated memory impairments in rats

We recently reported that post-training administration of serotonergic receptor antagonists attenuated the inhibitory-avoidance memory deficits normally exhibited by aged rats. In the present study, we determined whether a subeffective dose of the serotonergic type-2 receptor antagonist, ketanserin, would augment the facilitative effects produced by the acetylcholinesterase inhibitor, physostigmine, on memory in aged rats using the same task. The drugs were injected intraperitoneally alone, or in combination, immediately following training. Retention testing occurred 24 hours following training. A dose-dependent enhancement of memory was demonstrated as a result of the two treatment conditions (physostigmine 0.01–10.0 μ/kg, ketanserin 1.0 mg/kg + physostigmine 0.001–0.01 μ/kg). The facilitation of memory produced by the combined treatment was observed at doses well below those required to produce a similar effect when each drug was administered alone. The results provide additional evidence for an interaction between the cholinergic and serotonergic neurotransmitter systems in learning and memory, and may have important implications in the treatment of age-related memory impairments.     

Age differences in recognition memory of the rhesus monkey (Macaca mulatta)

Aging is accompanied by a gradual decline in memory in both humans and nonhuman primates. To determine whether the impairment in nonhuman primates extends to recognition memory, which is a sensitive index of the integrity of the limbic system, we trained rhesus monkeys of four different age groups (3-6, 14-17, 20-24, and 25-29 years of age) on a delayed nonmatching-to-sample task with trial-unique objects. After the animals had learned the task, which required recognition of single objects presented ten seconds earlier, memory demands were increased by gradually lengthening delay intervals (to 120 seconds) and list lengths (to ten objects). With increasing age, only marginal impairments in learning the basic task were observed. However, clear age-related differences did emerge when either delays or list lengths were increased, with the oldest group of monkeys demonstrating the greatest impairments. The decline in visual recognition ability in aging monkeys parallels the decline in memory observed with advancing age in humans.     

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.     

Postsynaptic alpha-2 receptor stimulation improves memory in aged monkeys: Indirect effects of yohimbine versus direct effects of clonidine

Very low doses (0.00001 mg/kg) of the alpha-2 adrenergic antagonist, yohimbine, improved working memory performance in a subset of aged monkeys. Improvement appeared to result from increased norepinephrine (NE) release onto postsynaptic alpha-2 adrenoceptors, as the response was blocked by the “postsynaptic” alpha-2 antagonist, SKF104078. Cognitive-enhancing effects of low dose yohimbine treatment may depend on aged animals retaining an intact, endogenous NE system. In contrast to yohimbine, the alpha-2 agonist, clonidine, has improved working memory in all aged animals examined. In the present study, clonidine's beneficial effects were also blocked by the postsynaptic antagonists SKF104078 and SKF104856, suggesting that clonidine acts by directly stimulating postsynaptic alpha-2 adrenoceptors. Beneficial doses of clonidine (0.01 mg/kg) and yohimbine (0.00001 mg/kg) were combined to see if they would produce additive effects on memory enhancement. This strategy was successful in young monkeys with intact NE systems but was not effective in the aged monkeys. These findings demonstrate that drugs that indirectly stimulate postsynaptic alpha-2 receptors by increasing NE release are not as reliable in aged monkeys as directly acting agonists that can replace NE at postsynaptic alpha-2 receptors.     

Human amnesia and animal models of amnesia: performance of amnesic patients on tests designed for the monkey

The performance of amnesic patients was assessed on five tasks, which have figured prominently in the development of animal models of human amnesia in the monkey. The amnesic patients were impaired on four of these tasks (delayed nonmatching to sample, object-reward association, 8-pair concurrent discrimination learning, and an object discrimination task), in correspondence with previous findings for monkeys with bilateral medial temporal or diencephalic lesions. Moreover, performance of the amnesic patients correlated with the ability to verbalize the principle underlying the tasks and with the ability to describe and recognize the stimulus materials. These tasks therefore seem to be sensitive to the memory functions that are affected in human amnesia, and they can provide valid measures of memory impairment in studies with monkeys. For the fifth task (24-hour concurrent discrimination learning), the findings for the amnesic patients did not correspond to previous findings for operated monkeys. Whereas monkeys with medial temporal lesions reportedly learn this task at a normal rate, the amnesic patients were markedly impaired. Monkeys may learn this task differently than humans.     

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.     

The effects of blood sludging upon short-term memory in rats and rhesus monkeys: an evaluation of its role in age-related cognitive declines.

Blood sludging or intravascular erythrocyte aggregation was induced in male, hooded rats by intravenous injection of the high molecular weight polysaccharide, Dextran 500 (mean mol. wt.=370,000). Analysis of blood sludging produced by dextran, using erythrocyte sedimentation rate (ESR), showed profound sludging after doses of 300 mg/kg, which persisted for several days. In contrast to the effects on ESR, a second study showed that Dextran 500 had no effect upon the accuracy of short-term memory (STM) in rats trained on a delayed spatial alternation task. The behavioral effects of dextran under these procedures were limited to a general, dose-dependent suppression of responding. Finally, the incidence and extent of blood sludging in behaviorally impaired, aged Rhesus monkeys was compared with that of normal, young monkeys using ESR. No significant difference in sludging between the two age groups was found. Thus, blood sludging was not associated with the STM impairment present in old monkeys. These results, taken together, do not support the notion that age related cognitive impairments (specificially in STM), are due to gross changes in blood sludging. The clinical reports of salutary effects in senile patients resulting from long-term anticoagulant therapy may therefore be due to factors unrelated to the antisluding properties of those drugs.     

Age-related deficits in component processes of working memory

Working memory deficits in normal aging have been well documented, and studies suggest that high memory load plus the presence of distraction negatively impacts successful memory performance to a greater degree in older individuals. However, characterization of the component processes that are impaired by these task manipulations is not clear. In this behavioral study, younger and older subjects were tested with a delayed-recognition and recall task in which the encoding and delay period were both manipulated. During the encoding period, the subjects were presented with either a single letter or multiple letters at their predetermined forward letter span, and the delay period was either uninterrupted or interrupted with a visual distraction. There was an age-related impairment of working memory recognition accuracy only in the combination of high memory load and distraction. These results suggest that when working memory maintenance systems are taxed, faulty recognition processes may underlie cognitive aging deficits in healthy older individuals.