Cognitive, emotional and behavioral dysfunctions in aging and dementia

Glutamatergic Neurotransmission in Aging and Neurodegenerative Diseases: A Potential Target to Improve Cognitive Impairment in Aging

Aging is characterized by the decline in many of the individual's capabilities. It has been recognized that the brain undergoes structural and functional changes during aging that are occasionally associated with the development of neurodegenerative diseases. In this sense, altered glutamatergic neurotransmission, which involves the release, binding, reuptake, and degradation of glutamate (Glu) in the brain, has been widely studied in physiological and pathophysiological aging. In particular, changes in glutamatergic neurotransmission are exacerbated during neurodegenerative diseases and are associated with cognitive impairment, characterized by difficulties in memory, learning, concentration, and decision-making. Thus, in the present manuscript, we aim to highlight the relevance of glutamatergic neurotransmission during cognitive impairment to develop novel strategies to prevent, ameliorate, or delay cognitive decline. To achieve this goal, we provide a comprehensive review of the changes reported in glutamatergic neurotransmission components, such as Glu transporters and receptors during physiological aging and in the most studied neurodegenerative diseases. Finally, we describe the current therapeutic strategies developed to target glutamatergic neurotransmission.

Dynamic cognitive remediation for a Traumatic Brain Injury (TBI) significantly improves attention, working memory, processing speed, and reading fluency

BACKGROUND

In the U.S. 3.8 million people have a Traumatic Brain Injury (TBI) each year. Rapid brain training exercises to improve cognitive function after a mild TBI are needed.

OBJECTIVE

This study determines whether cognitive remediation by discriminating the direction a test pattern moves relative to a stationary background (movement figure-ground discrimination) improves the vision and cognitive deficits that result from a TBI, providing a paradigm shift in treatment methods.

METHODS

Movement-discrimination neurotraining was used to remediate low-level visual timing deficits in the dorsal stream to determine whether it improved high-level cognitive functions, such as processing speed, reading fluency, and the executive control functions of attention and working memory in four men with a TBI between the ages of 15-68. Standardized tests, as well as Magnetoencephalography (MEG) brain imaging, were administered at the beginning and end of 8-16 weeks of intervention training to evaluate improvements in cognitive skills.

RESULTS

Movement-discrimination cognitive neurotraining remediated both low-level visual timing deficits and high-level cognitive functioning, including selective and sustained attention, reading fluency, processing speed, and working memory for all TBI patients we studied. MEG brain imaging, using the Fast-VESTAL procedure, showed that this movement-discrimination training improved time-locked activity in the dorsal stream, attention, and executive control networks.

CONCLUSIONS

Remediating visual timing deficits in the dorsal stream revealed the causal role of visual movement discrimination training in improving high-level cognitive functions such as focusing and switching attention, working memory, processing speed, and reading. This study found that movement-discrimination training was very rapid and effective in remediating cognitive deficits, providing a new approach that is very beneficial for treating a mild TBI.

D-Serine in the aging hippocampus

Experimental evidences now indicate that memory formation relies on the capacity of neuronal networks to manage long-term changes in synaptic communication. This property is driven by N-methyl-D-aspartate receptors (NMDAR), which requires the binding of glutamate but also the presence of the co-agonist D-serine at the glycine site. Defective memory function and impaired brain synaptic plasticity observed in aging are rescued by partial agonist acting at this site suggesting that this gating process is targeted to induce age-related cognitive defects. This review aims at compelling recent studies characterizing the role of D-serine in changes in functional plasticity that occur in the aging hippocampus since deficits are rescued by D-serine supplementation. The impaired efficacy of endogenous D-serine is not due to changes in the affinity to glycine-binding site but to a decrease in tissue levels of the amino acid resulting from a weaker expression of the producing enzyme serine racemase (SR). Interestingly, neither SR expression, D-serine levels, nor NMDAR activation is affected in aged LOU/C rats, a model of healthy aging in which memory deficits do not occur. These old animals do not develop oxidative stress suggesting that the D-serine-related pathway could be targeted by the age-related accumulation of reactive oxygen species. Accordingly, senescent rats chronically treated with the reducing agent N-acetyl-cysteine to prevent oxidative damage, show intact NMDAR activation linked to preserved D-serine levels and SR expression. These results point to a significant role of D-serine in age-related functional alterations underlying hippocampus-dependent memory deficits, at least within the CA1 area since the amino acid does not appear as critical in changes affecting the dentate gyrus.

[Normal aging and imaging correlations]

Age-related structural, functional and biochemical changes of the brain can be visualized by neuroimaging methods. Physiological aging of the brain has to be clearly distinguished from pathological alterations of the brain for reliable and early diagnoses of neurodegenerative diseases. Concerning the speed of the cerebral aging process, significant inter-individual differences can be observed. In general, aging is associated with a decline of cognitive functions. Simultaneously, a decay of the average brain volume, especially in the frontal lobe accompanies the process of aging. Correspondingly, a strong susceptibility for age-related degeneration has been observed in the fronto-striato-thalamic network. Due to increasing age the white matter is affected by a progressive loss of fiber integrity mirrored in a significant decay of the fractionated anisotropy (FA) measured by diffusion tensor imaging (DTI). Age-related degeneration of the white matter further leads to a growing number of T2 hyperintense white-matter lesions. Aging also influences the cerebral perfusion pattern leading to a perceptible decay of the global cerebral blood flow (CBF) and blood volume (CBV). During life, iron accumulates in the brain, predominantly in the globus pallidus and in the substantia nigra. By 1H-MR spectroscopy, a decrease of N-acetyl-aspartate (NAA) as a correlate for reduced neuronal metabolism is found in the brain of elderly individuals.

Selective Vulnerabilities of N-methyl-D-aspartate (NMDA) Receptors During Brain Aging

N-methyl-D-aspartate (NMDA) receptors are present in high density within the cerebral cortex and hippocampus and play an important role in learning and memory. NMDA receptors are negatively affected by aging, but these effects are not uniform in many different ways. This review discusses the selective age-related vulnerabilities of different binding sites of the NMDA receptor complex, different subunits that comprise the complex, and the expression and functions of the receptor within different brain regions. Spatial reference, passive avoidance, and working memory, as well as place field stability and expansion all involve NMDA receptors. Aged animals show deficiencies in these functions, as compared to young, and some studies have identified an association between age-associated changes in the expression of NMDA receptors and poor memory performance. A number of diet and drug interventions have shown potential for reversing or slowing the effects of aging on the NMDA receptor. On the other hand, there is mounting evidence that the NMDA receptors that remain within aged individuals are not always associated with good cognitive functioning. This may be due to a compensatory response of neurons to the decline in NMDA receptor expression or a change in the subunit composition of the remaining receptors. These studies suggest that developing treatments that are aimed at preventing or reversing the effects of aging on the NMDA receptor may aid in ameliorating the memory declines that are associated with aging. However, we need to be mindful of the possibility that there may also be negative consequences in aged individuals.

Cognitive functioning in healthy older adults aged 64-81: a cohort study into the effects of age, sex, and education

The objective of this study was to determine a possible differential effect of age, education, and sex on cognitive speed, verbal memory, executive functioning, and verbal fluency in healthy older adults. A group of 578 healthy participants in the age range of 64-81 was recruited from a large population study of healthy adults (Maastricht Aging Study). Even in healthy individuals in this restricted age range, there is a clear, age-related decrease in performance on executive functioning, verbal fluency, verbal memory, and cognitive speed tasks. The capacity to inhibit information is affected most. Education had a substantial effect on cognitive functioning: participants with a middle or high level of education performed better on cognitive tests than did participants with a low level of education. Women performed better than men on verbal memory tasks. Therefore, education and sex must be taken into account when examining an older individual's cognitive performance.

Cognitive functioning in spousal caregivers of dementia patients: findings from the prospective MAASBED study

BACKGROUND

Spousal caregivers are usually of the same age as the dementia patient and therefore at risk of age-related cognitive decline. Suboptimal cognitive functioning in caregivers may have profound implications.

METHODS

Fifty-four spousal caregivers of dementia patients from the Memory Clinic of the Academic Hospital Maastricht and the Regional Institute for Community Mental Health Care in the Netherlands were compared with 108 non-caregiving controls. Data were collected on patient and caregiver characteristics and caregiver cognitive functioning. Repeated measures statistical analyses were carried out to investigate the relationship between caregiver cognitive functioning at baseline and patient behavioural problems and caregiver competence during 1 year of follow-up.

RESULTS

Caregivers performed significantly worse on several cognitive domains compared with control subjects. Low performance on a verbal memory task was related to a decrease in caregiver subjective competence and an increase in patient hyperactivity.

CONCLUSION

The results indicate that screening for cognitive impairment of spousal caregivers may be helpful, because suboptimal cognitive functioning may affect the ability to provide adequate care.

Effects of aging on recognition of intentionally and incidentally stored words: An fMRI study

Functional magnetic resonance imaging (fMRI) was used to gain more insight in the mechanism underlying a decline in recognition memory function with age. Twelve young (23-27 years) and 12 older (63-67 years) healthy men performed two types of word encoding tasks, in which words were either incidentally or intentionally encoded for storage in memory. After a 30min delay, participants performed a recognition task. Older participants were less accurate and slower than young on the recognition task. In the both groups, successful retrieval was accompanied by activation in the left inferior frontal gyrus, left precentral gyrus and right cerebellum. Older participants showed additional activity in the bilateral medial prefrontal gyrus and right parahippocampal gyrus. Correlational analyses showed that only the additional parahippocampal activation correlated positively with task performance in the older but not young participants, suggesting that activation in this area served the purpose of functional compensation. The additional activation in the medial prefrontal cortex, on the other hand, was explained in terms of increased conflict, that is, reduced distinction between target and distracter words leading to increased simultaneous activity of both response tendencies. In a comparison between incidentally and intentionally remembered words the young group showed additional activation in the right middle occipital gyrus. This last result was explained in terms of strategic differences between the young and older group.

Influence of cognitive functioning on functional status in an older population: 3- and 6-year follow-up of the Maastricht Aging Study

BACKGROUND

To date, it remains unclear to what extent cognitive competence is related to a change in general functional status in older adults.

OBJECTIVES

To evaluate both the cross-sectional and the longitudinal relation between cognitive functioning and functional status.

METHODS

Sensorimotor speed, memory, and executive functioning were assessed in a large population of healthy adults aged 60 years and older (n = 485) who participated in the Maastricht Aging Study. Data from the baseline (1993-1995), three-year follow-up, and six-year follow-up were used. Functional status was measured using the SF-36, which was coded into a physical and a mental component summary measure.

RESULTS

After adjustment for age, sex, and educational level, a high level of cognitive functioning appeared to be associated with better functional status in the cross-sectional analysis. Longitudinal analyses demonstrated, that cognitive functioning was not a predictor of functional status three or six years later.

CONCLUSIONS

Thus while cognitive functioning is useful clinically for predicting the short-term functional status of an older person, it is not useful for predicting that person's long-term (>3 years) functional status and thus the period of validity of the results of these tests in answering such questions is limited.

Age-related glutamate and glutamine concentration changes in normal human brain: 1H MR spectroscopy study at 4 T

Proton magnetic resonance spectroscopy was performed at 4 T to determine effects of age, region and gender on glutamate and glutamine in the normal human brain. Furthermore, glutamate and glutamine alterations with age were tested for correlations with other cerebral metabolites. Two 8 cm3 volumes were selected in corona radiata and mesial motor cortex in normal subjects (N = 24) between 24 and 68 years old. Older subjects had lower glutamate concentration in the motor cortex compared to younger subjects (p < 0.001). In corona radiata, older subjects demonstrated a trend in higher glutamine compared to younger subjects (p = 0.05). Glutamate in the motor cortex was positively correlated with glutamine, N-acetyl aspartate and creatine. Reduced glutamate and N-acetyl aspartate in the motor cortex is consistent with neuronal loss/shrinkage with age. In conclusion, different patterns in association with normal aging in these brain regions were detected in this study.

Deficits of memory, executive functioning and attention following infarction in the thalamus; a study of 22 cases with localised lesions

The thalamus plays a crucial role in memory, executive functioning and attention. It remains, however, unclear whether thalamic structures have specific roles in each of these functions. We tested 22 cases of thalamic infarction, proven with MR imaging, using experimental and established neuropsychological tests. We performed a lesion-overlap study in standardised stereotactic space of patients sharing a certain deficit, corrected for the lesion distribution of patients without such deficits and determined the regions of interest using an atlas of the human thalamus. We checked for additional, non-thalamic, damage and for deficient comprehension and perception that would preclude interpretation of the results. Non-thalamic damage such as white matter lesions, hippocampal atrophy, sulcal widening and infarctions occur significantly more often in patients aged over 60. The patients with additional damage overlapped to a major degree with those who showed loss of orientation, or lack of comprehension of the test requirements. In the 10 patients judged 'clean', we observed a deficit of episodic long-term memory with relative sparing of intellectual capacities and short-term memory when the mammillo-thalamic tract was damaged. Lesions including the medial dorsal nucleus, midline nuclei and/or intralaminar nuclei accompany executive dysfunctioning. Reduced simple processing speed and attention are associated with age, but not with a particular structure in the thalamus. Complex attention deficits follow damage to the intralaminar nuclei.We conclude that the analysis of structure-function relationships must take into account extra-structure damage which may explain cognitive deficits. Separate thalamic structures are involved in memory, executive functioning and attention.

The nature of the effect of female gonadal hormone replacement therapy on cognitive function in post-menopausal women: a meta-analysis

We reviewed epidemiological and experimental studies of female gonadal hormone replacement therapy (HRT) on cognitive function in post-menopausal women and carried out meta-analyses. In healthy ageing women, HRT has small and inconsistent effects that include enhancement of verbal memory, abstract reasoning and information processing. Epidemiological studies show larger effects than experimental studies, which is not related to sample size. Important confounds may be that women who start using HRT are healthier than women who do not. Also, controlling for socio-economic status diminishes the effect of HRT. The effects of HRT may depend on the age and type of menopause and the therapeutic intervention used, with the most widely used drug, Premarin, having least effect. However, the effects are independent of mood and climacteric symptom alleviation. There is a paucity of experimental studies that include healthy elderly women. The evidence for an estrogen deficiency in women with dementia and cognitive dysfunction is inconsistent. Nevertheless, epidemiological studies suggest that HRT protects against the development of clinically diagnosed Alzheimer's disease. However, poor recall of HRT use by patients and altered physician behaviour may have confounded the effects. Surprisingly, both healthy and demented women with low education seem to benefit most from HRT. Three recent controlled experimental studies using Premarin showed no effects of HRT in preventing further cognitive decline in women who already have Alzheimer's disease. Duration of treatment seems to play an important role, with beneficial effects declining-and even reversing-with longer treatment in women with Alzheimer's disease.Future research should further investigate the cognitive effect of different HRT preparations, serum estrogen levels, and the interactions of HRT with age, menopausal status and existing protective (e.g. education) and risk factors (e.g. smoking and apolipoprotein E genotype) for cognitive decline and Alzheimer's disease.

The evolution of Alzheimer's disease-related cytoskeletal pathology in the human raphe nuclei

The cross-sectional analyses currently available show that the Alzheimer's disease (AD)-related cytoskeletal alterations within the human brain affect variously susceptible areas of the cerebral cortex in a uniform sequence with very little interpatient variability. This sequence has been divided for research and comparative purposes into six stages (cortical NFT/NT-stages I-VI). Among the subcortical nuclei affected in AD are those belonging to the raphe system. Efforts were focused on the lesions present in these nuclei to see in which of the six stages the AD-related cytoskeletal anomalies begin and whether a correlation exists between the AD-related pathology developing within the cerebral cortex and the cytoskeletal damage that occurs in the nuclei of the raphe system. To this end, serial sections from the brainstems of 27 post-mortem cases with stages I-VI of cortical cytoskeletal lesions were examined. The cytoskeletal pathology was visualized using the modified silver iodide-Gallyas staining technique and the antibody AT8. The latter is directed specifically against the abnormally phosphorylated cytoskeletal protein tau. The dorsal raphe nucleus manifests the cytoskeletal lesions early on (stages I-II). The central and linear raphe nuclei, by contrast, do so initially in stages III-IV, and the caudal raphe nuclei register the first changes in stages V-VI. In stages V and VI, the dorsal raphe nucleus displays the most severe cytoskeletal pathology within the raphe system, followed by the central and linear raphe nuclei, whereas the cytoskeletal anomalies in the caudal raphe nuclei are slight. The developing damage within the nuclei of the raphe system correlates with the stages I-VI and, furthermore, progresses in the oral raphe nuclei in close connection with the evolution of the pathological process in cortical projection destinations of these nuclei. As the source of the ascending serotonergic system, the involvement of the oral raphe nuclei may be partially responsible for the early manifestation of the non-cognitive and emotional deficiencies possibly traceable to dysfunctions within the ascending serotonergic system.

Differential effects of aging on binding sites of the activated NMDA receptor complex in mice

The NMDA receptor site has been shown to be vulnerable to the effects of aging. Decreases in binding to the receptor site of up to 50% have been reported in aged animals. The present study was designed to quantitate and compare the effects of aging on multiple binding sites of the NMDA receptor complex in various brain regions. Autoradiography with [3H]glutamate, [3H]CPP, [3H]glycine, [3H]MK801 and [3H]TCP was performed on brain sections from 3, 10 and 28-30 month old C57B1/6 mice. The percent declines between 3 and 28-30 months of age in [3H]-glutamate (15-35% declines) and [3H]CPP (20-42% declines) binding were similar within most cortical regions and the caudate nucleus but [3H]glutamate binding showed less change (0-11% declines) than [3H]CPP (13-27% declines) in the occipital/temporal cortex and hippocampal regions. [3H]MK801 and [3H]TCP binding, stimulated by 10 microM glutamate, exhibited intermediate aging changes between the glycine and NMDA sites, both in percent decline (3-28% and 0-26%, respectively) and in the number of brain regions involved. [3H]Glycine binding, stimulated by 10 microM glutamate, showed no significant overall effect of age (declines ranged from 0-34%). [3H]CPP binding was significantly more affected than [3H]glycine binding in many regions. These results suggest that aging has heterogeneous effects on different sites on the NMDA receptor complex throughout the brain and on NMDA receptor agonist versus antagonist binding in selected brain regions.

Age-related changes in excitatory amino acid receptors in two mouse strains

The present study examined the binding of [3H]-L-glutamate to NMDA receptors, [3H]-kainate to kainate receptors, and [3H]-AMPA to AMPA/quisqualate receptors in the brains of C57Bl and BALB/c mice as a function of increasing age. Significant decreases in binding to NMDA receptors occurred with increasing age (3 to 30 months) in a majority of cortical and hippocampal brain regions from the C57Bl and BALB/c strains of mice. Significant decreases in binding to kainate and AMPA receptors were found in the inner frontal and parietal cortices and stratum lacunosum/moleculare of CA1 in both strains. These regions also exhibited the greatest percent decline in NMDA binding sites with aging. The loss of NMDA receptors in the stratum lacunosum/moleculare of CA1 was greater in the BALB/c mice than the C57Bl strain. These results demonstrate that a few brain regions have age-associated reductions in all three ionotropic EAA receptors; however, the NMDA receptor appears to be selectively vulnerable to the aging process throughout much of the cerebral cortex and hippocampus.

Age-related decline of psychomotor speed: effects of age, brain health, sex, and education

A cross-sectional study into age-related decline of psychomotor speed is reported. A newly introduced choice response task was used, involving three conditions: simple reaction time (SRT), choice reaction time (CRT), and CRT with stimulus-response incompatibility. Subjects were 247 volunteers, aged 20 to 80 yr. in seven age levels. Although all subjects thought themselves to be normal and healthy, a post hoc division could be made based on biological life events (BLE, mild biological or environmental factors that can hamper optimal brain functioning, such as repeated general anesthesia). Performance was poorer by subjects who had experienced one or more such event: slowing was comparable to the effect of age, especially in the more difficult task conditions. There were significant effects of sex and education, men being consistently faster than women, and more highly educated subjects performing better than subjects with only low or medium education. These findings replicate observations from other test methods. They are also in line with several other studies giving interactions between the effects of aging and physical fitness. This study questions the validity of much research on aging, as the data suggest that a more rigorous health screening for biological life events in subjects recruited from the normal, healthy population can reduce performance effects normally ascribed to aging.

Disruption of circadian regulation by brain grafts that overexpress Alzheimer beta/A4 amyloid

Alzheimer disease patients exhibit irregularities in the patterns of normally circadian (daily) rhythms. Alzheimer-type pathology has been reported in the hypothalamus and in the suprachiasmatic nuclei, the putative site of the circadian oscillator. We examined the relationship between the neuropathology of Alzheimer disease, as modeled by an animal system, and circadian dysregulation by grafting genetically transformed cells that overexpress beta/A4 amyloid into the suprachiasmatic nuclei of adult rats. Grafts of beta/A4-positive cells, but not of control cells, significantly altered the pattern of activity of implanted rats. Although experimental conditions included light-dark cycles that normally tend to drive rats to 24-h rhythms, animals with grafts of beta/A4-positive cells showed abnormally high levels of activity during the light phase in addition to a disrupted circadian pattern. Periodogram analysis demonstrated significant rhythms outside of a circadian range. The body temperature rhythm of these animals was also weak 6 weeks after grafting; however, unlike activity patterns, body temperature regained a circadian period by 8 weeks after cell implantation. These data indicate that disruption of circadian activity is a behavioral measure of the consequences of beta/A4 accumulation in brain implants.

Recall and recognition memory deficits in depression

The aim of the present study was to establish the nature of memory deficits of depressive subjects in word learning tests. A word learning test consisting of 1, 3 or 5 learning trials was used. We found that patients were characterized by inferior memory recall compared to controls when 5 learning trials were given. Patients performed significantly slower than controls on a recognition test but both patients and controls recognized the same number of words. This suggests that the memory deficits that are present in many depressive subjects may be restricted to impaired active retrieval from memory. A second experiment revealed that recognition memory and delayed recall as well as immediate recall were impaired in depressive patients after 1 learning trial. These short-comings vanished after 3 trials, except for immediate recall. These data suggest that not only retrieval but also encoding of information into memory may be impaired in depression, especially in the beginning of a task when demands on cognitive effort are high. The results are discussed in terms of resource allocation and demands on effort that may change in the course of a task.

A comparison between the care of vocally disruptive patients and that of other residents at psychogeriatric wards

This study aimed at examining the relationship between the occurrence of vocally disruptive behaviour and the care of demented patients. The results presented here focus on the contents of the patient's daily life provided by the staff. In all the psychogeriatric wards within one county council, having patients identified as vocally disruptive, 37 patients exhibiting this behaviour and 37 controls were studied. Semistructured continuous observations for 15 hours per patient (7.00 a.m.-10.00 p.m.) were made. Data from a previous report revealed that the vocally disruptive patients were more physically dependent on the staff as compared to controls. However, the physical care provided did not differ significantly. The patients spent 21% of the observed time in caring activities, mainly physical care, 18% of the time they were sleeping or dozing, and 71% of the observed time they were left to themselves and reduced to inactivity and solitude. Regression analysis disclosed that only the amount of activating care in a group was significantly related to the vocally disruptive behaviour. Hence the data support the idea that understimulation is present in the demented patient's life but it can hardly be the cause of the behaviour. However, the environment might contribute to the occurrence and maintenance of vocally disruptive behaviour.