Markers for biogenic amines in the aged rat brain: relationship to decline in spatial learning ability

Age Impacts the Burden That Reference Memory Imparts on an Increasing Working Memory Load and Modifies Relationships With Cholinergic Activity

Rodent aging research often utilizes spatial mazes, such as the water radial-arm-maze (WRAM), to evaluate cognition. The WRAM can simultaneously measure spatial working and reference memory, wherein these two memory types are often represented as orthogonal. There is evidence, however, that these two memory forms yield interference at a high working memory load. The current study systematically evaluated whether the presence of a reference memory component impacts handling of an increasing working memory load. Young and aged female rats were tested to assess whether aging impacts this relationship. Cholinergic projections from the basal forebrain to the hippocampus and cortex can affect cognitive outcomes, and are negatively impacted by aging. To evaluate whether age-related changes in working and reference memory profiles are associated with cholinergic functioning, we assessed choline acetyltransferase activity in these behaviorally-tested rats. Results showed that young rats outperformed aged rats on a task testing solely working memory. The addition of a reference memory component deteriorated the ability to handle an increasing working memory load, such that young rats performed similar to their aged counterparts. Aged rats also had challenges when reference memory was present, but in a different context. Specifically, aged rats had difficulty remembering which reference memory arms they had entered within a session, compared to young rats. Further, aged rats that excelled in reference memory also excelled in working memory when working memory demand was high, a relationship not seen in young rats. Relationships between cholinergic activity and maze performance differed by age in direction and brain region, reflecting the complex role that the cholinergic system plays in memory and attentional processes across the female lifespan. Overall, the addition of a reference memory requirement detrimentally impacted the ability to handle working memory information across young and aged timepoints, especially when the working memory challenge was high; these age-related deficits manifested differently with the addition of a reference memory component. This interplay between working and reference memory provides insight into the multiple domains necessary to solve complex cognitive tasks, potentially improving the understanding of complexities of age- and disease- related memory failures and optimizing their respective treatments.

The moral status of cerebral organoids

Organoids are 3D biological structures constructed from stem cells in vitro. They partially mimic the function of real organs. Although the number of articles detailing this technology has increased in recent years, papers debating their ethical issues are few. In addition, many of such articles outline a mere summary of potential ethical concerns associated with organoids, although some have focused on consciousness assessment or organoid use in cystic fibrosis treatment. This article seeks to evaluate the moral status of cerebral organoids and to determine under which conditions their use should be allowed from a bioethical standpoint. We will present an overview of recent steps in developing highly advanced cerebral organoids, followed by an analysis of their ethics based on three factors: human origin, a specific biological threshold (which, once crossed, grants an entity moral status), and the potential to generate human beings. We will also make practical recommendations for researchers working in this biological field.

Effect of aging and Alzheimer's disease-like pathology on brain monoamines in mice

Aging is the greatest single risk factor of the neurodegenerative disorder Alzheimer's disease (AD). The monoaminergic system, including serotonin (5-HT), dopamine (DA) and noradrenaline (NA) modulates cognition, which is affected in AD. Changes in monoamine levels have been observed in AD, but these can both be age- and/or disease-related. We examined whether brain monoamine levels change as part of physiological aging and/or AD-like disease in APP_SWE/PS1_ΔE9 (APP/PS1) transgenic mice. The neocortex, hippocampus, striatum, brainstem and cerebellum of 6-, 12-, 18- and 24-month-old B6C3 wild-type (WT) mice and of 18-month old APP/PS1 and WT mice were analysed for 5-HT, DA and NA contents by high pressure liquid chromatography (HPLC), along with neocortex from 14-month-old APP/PS1 and WT mice. While, we observed no aging effect in WT mice, we detected region-specific changes in the levels of all monoamines in 18-month-old transgenic compared with WT mice. This included reductions in 5-HT (-30%), DA (-47%) and NA (-32%) levels in the neocortex and increases of 5-HT in the brainstem (+18%). No changes were observed in any of the monoamines in the neocortex from 14-month-old APP/PS1 mice. In combination, these findings indicate that aging alone is not sufficient to affect brain monoamine levels, unlike the APP_SWE/PS1_ΔE9 genotype.

Differences in the Flexibility of Switching Learning Strategies and CREB Phosphorylation Levels in Prefrontal Cortex, Dorsal Striatum and Hippocampus in Two Inbred Strains of Mice

Flexibility in using different learning strategies was assessed in two different inbred strains of mice, the C57BL/6 and DBA/2 strains. Mice were trained sequentially in two different Morris water maze protocols that tested their ability to switch their learning strategy to complete a new task after first being trained in a different task. Training consisted either of visible platform trials (cued training) followed by subsequent hidden platform trials (place training) or the reverse sequence (place training followed by cued training). Both strains of mice showed equivalent performance in the type of training (cued or place) that they received first. However, C57BL/6 mice showed significantly better performances than DBA/2 mice following the switch in training protocols, irrespective of the order of training. After completion of the switched training session, levels of cAMP response element-binding protein (CREB) and phosphorylated CREB (pCREB) were measured in the hippocampus, striatum and prefrontal cortex of the mice. Prefrontal cortical and hippocampal pCREB levels differed by strain, with higher levels found in C57BL/6 mice than in DBA/2 mice. No strain differences were observed in the medial or lateral region of the dorsal striatum. These findings indicate that the engagement (i.e., CREB signaling) of relevant neural structures may vary by the specific demands of the learning strategy, and this is closely tied to differences in the flexibility of C57BL/6 and DBA/2 mice to switch their learning strategies when given a new task.

Interaction of basal forebrain cholinergic neurons with the glucocorticoid system in stress regulation and cognitive impairment

A substantial number of studies on basal forebrain (BF) cholinergic neurons (BFCN) have provided compelling evidence for their role in the etiology of stress, cognitive aging, Alzheimer’s disease (AD), and other neurodegenerative diseases. BFCN project to a broad range of cortical sites and limbic structures, including the hippocampus, and are involved in stress and cognition. In particular, the hippocampus, the primary target tissue of the glucocorticoid stress hormones, is associated with cognitive function in tandem with hypothalamic-pituitary-adrenal (HPA) axis modulation. The present review summarizes glucocorticoid and HPA axis research to date in an effort to establish the manner in which stress affects the release of acetylcholine (ACh), glucocorticoids, and their receptor in the context of cognitive processes. We attempt to provide the molecular interactive link between the glucocorticoids and cholinergic system that contributes to BFCN degeneration in stress-induced acceleration of cognitive decline in aging and AD. We also discuss the importance of animal models in facilitating such studies for pharmacological use, to which could help decipher disease states and propose leads for pharmacological intervention.

Improvement of cognitive deficits in SAMP8 mice by 3-n-butylphthalide

The herbal extract 3-n-butylphthalide (NBP) is used in clinical practice for ischemic patients in China. It has been shown to have various neuroprotective effects both in vitro and in vivo. In the present study, the effects of NBP on learning and memory decline in the senescence-accelerated mouse prone-8 (SAMP8) animal model were investigated. Intragastric NBP administration to 4-month-old SAMP8 mice for 2 months significantly improved spatial learning and memory ability. Moreover, the loss of choline acetyltransferase (ChAT)-positive neurons in the medial septal nucleus and the vertical limb of the diagonal band in SAMP8 mice was slowed down, as was the decline in the protein and mRNA expression of ChAT in the hippocampus, cerebral cortex, and forebrain. These results demonstrated that NBP treatment starting at the age of 4 months protected from the learning/memory deficits with aging of SAMP8 mice, and that this effect might be mediated by preventing the decline of the central cholinergic system.

Memory deficits in aging and neurological diseases

Memory is central to our ability to perform daily life activities and correctly function in society. Improvements in public health and medical treatment for a variety of diseases have resulted in longer life spans; however, age-related memory impairments have been significant sources of morbidity. Loss in memory function is not only associated with aging population but is also a feature of neurodegenerative diseases such as Alzheimer's disease and other psychiatric and neurological disorders. Here, we focus on current understanding of the impact of normal aging on memory and what is known about its mechanisms, and further review pathological mechanisms behind the cause of dementia in Alzheimer's disease. Finally, we discuss schizophrenia and look into abnormalities in circuit function and neurotransmitter systems that contribute to memory impairment in this illness.

Effects of [6]-shogaol on cholinergic signaling in HT22 cells following neuronal damage induced by hydrogen peroxide

Cholinergic neurons play a major role in memory and attention. The dysfunction and death of these neurons, especially in the hippocampus, are thought to contribute to the pathophysiology of memory deficits associated with Alzheimer's disease (AD). Therefore, studying the cholinergic properties and cell survival may help in treating this disease. We investigated the possible effects of [6]-shogaol on cholinergic signaling in HT22 hippocampal neuronal cells. HT22 cells express essential cholinergic markers, including choline acetyltransferase (ChAT) and choline transporter (ChTp). HT22 cells treated with H(2)O(2) for 3h showed an increase in ROS production (35%). These features were partly recovered by [6]-shogaol. Treating H(2)O(2)-treated HT22 cells with [6]-shogaol markedly increased the expression of ChAT and ChTp, an effect similar to that of brain-derived neurotrophic factor (BDNF). Furthermore, K-252a, an inhibitor of the BDNF receptor Trk B, attenuated the effects of both [6]-shogaol and BDNF. These data suggest that [6]-shogaol protects neurons by increasing ChAT and ChTp expression through a BDNF increase and thus may be useful for treating neurodegenerative diseases.

What do we know about aging and spatial cognition? Reviews and perspectives

In order to cope with normal cognitive aging we must understand the patterns and neurofunctional underpinnings of cognitive and behavioral changes throughout adulthood. In this review, we summarize recent advances in our understanding of age-related behavioral differences and changes in brain structure throughout the spatial domain. Although spatial cognition is critically important to everyday life, few studies have examined the relationship between this cognitive function and neural changes in the aged brain. Thus, spatial cognition is considered a key area in which the cognitive neuroscience of aging may expand in the near future. The first section of this review examines the methodologies and studies used to assess differences in spatial cognition during normal cognitive aging in animals and humans. We then relate how each domain of spatial cognition (e.g., visuospatial perception, mental imagery, memory and navigation) is affected by the aging process, and discuss possible links with changes in neural mechanisms. Lastly, we address putative links among the age-related deterioration patterns of the various spatial domains and make suggestions for future research.

Assessment of spatial memory in mice

Improvements in health care have greatly increased life span in the United States. The focus is now shifting from physical well-being to improvement in mental well-being or maintenance of cognitive function in old age. It is known that elderly people suffer from cognitive impairment, even without neurodegeneration, as a part of 'normal aging'. This 'age-associated memory impairment' (AAMI), can have a devastating impact on the social and economic life of an individual as well as the society. Scientists have been experimenting to find methods to prevent the memory loss associated with aging. The major factor involved in these experiments is the use of animal models to assess hippocampal-based spatial memory. This review describes the different types of memory including hippocampal-based memory that is vulnerable to aging. A detailed overview of various behavioral paradigms used to assess spatial memory including the T-maze, radial maze, Morris water maze, Barnes maze and others is presented. The review also describes the molecular basis of memory in hippocampus called as 'long-term potentiation'. The advantages and limitations of the behavioral models in assessing memory and the link to the long-term potentiation are discussed. This review should assist investigators in choosing suitable methods to assess spatial memory in mice.

Involvement of histone acetylation in the regulation of choline acetyltransferase gene in NG108-15 neuronal cells

Post-translational modification of histone such as acetylation of N-terminal of lysine residues influences gene expression by modulating the accessibility of specific transcription factors to the promoter region, and is essential for a wide variety of cellular processes in the development of individual tissues, including the brain. However, few details concerning the acquisition of specific neurotransmitter phenotype have been obtained. In the present study, we investigated the possible involvement of histone acetylation in the gene expression of choline acetyltransferase (ChAT), a specific marker for cholinergic neuron and its function, in NG108-15 neuronal cells as an in vitro model of cholinergic neuron. Treatment with the histone deacetylase (HDAC) inhibitor trichostatin A (TSA), which induces global histone hyper-acetylation of the cells, resulted in marked increase in the expression of ChAT gene in proliferating NG108-15 cells. Furthermore, RT-PCR analysis using primer pairs for individual variants of ChAT mRNA (R1-4, N1, and M type) revealed that M type, not R1-4 and N1 type, ChAT mRNA were mainly transcribed, and chromatin immunoprecipitation assay indicated that the promoter region of M type ChAT gene was highly acetylated, in the dibutyryl cyclic AMP-induced neuronal differentiation of NG108-15 cells. The present findings demonstrate that the acquisition of neurotransmitter phenotype is epigenetically, at least the hyper-acetylation on the core promoter region of ChAT gene, regulated in NG108-15 neuronal cells.

Influence of aging on chondroitin sulfate proteoglycan expression and neural stem/progenitor cells in rat brain and improving effects of a herbal medicine, yokukansan

There is evidence of structural and functional deterioration in the brain, including the prefrontal cortex (PFC) and hippocampus, during the normal aging process in animals and humans. Extracellular matrix-associated glycoproteins, such as chondroitin sulfate proteoglycans (CSPGs), are involved in not only maintaining the structures and functions of adult neurons, but also regulating the proliferation, migration, and neurite outgrowth of neural stem cells in the brain. On the other hand, a herbal medicine, yokukansan (YKS), is used in a variety of clinical situations for treating symptoms associated with age-related neurodegenerative disorders such as Alzheimer's disease, but its pharmacological properties have not been fully understood. The present study was designed to clarify the influence of aging and the improving effects of YKS on the expression of aggrecan, a major molecule of CSPGs, and on the proliferation and migration of neural stem/progenitor cells identified by bromodeoxyuridine (BrdU) incorporation in the PFC and hippocampus including the dentate gyrus. Aged rats (24 months old) showed a significant increase in aggrecan expression throughout the PFC and in the hippocampus particularly in the CA3 subfield, but not the dentate gyrus compared to young rats (5 months old), evaluated by the immunohistochemical method. YKS treatment decreased the age-related increase in aggrecan expression as well as normal expression in young rats. Aged rats also showed a decreased number of BrdU-labeled cells in the PFC and hippocampus, and these decreases were improved by YKS treatment, which also increased the numbers in young rats. These results suggest that aging influences the microenvironment for adult and immature neurons in the brain, which may affect the proliferation and migration of neural stem/progenitor cells, and YKS has pharmacological potency for these age-related events. These findings help to understand the physiology and pathology of the aged brain and provide an anti-aging strategy for the brain.

Estrogens and age-related memory decline in rodents: what have we learned and where do we go from here?

The question of whether ovarian hormone therapy can prevent or reduce age-related memory decline in menopausal women has been the subject of much recent debate. Although numerous studies have demonstrated a beneficial effect of estrogen and/or progestin therapy for certain types of memory in menopausal women, recent clinical trials suggest that such therapy actually increases the risk of cognitive decline and dementia. Because rodent models have been frequently used to examine the effects of age and/or ovarian hormone deficiency on mnemonic function, rodent models of age-related hormone and memory decline may be useful in helping to resolve this issue. This review will focus on evidence suggesting that estradiol modulates memory, particularly hippocampal-dependent memory, in young and aging female rats and mice. Various factors affecting the mnemonic response to estradiol in aging females will be highlighted to illustrate the complications inherent to studies of estrogen therapy in aging females. Avenues for future development of estradiol-based therapies will also be discussed, and it is argued that an approach to drug development based on identifying the molecular mechanisms underlying estrogenic modulation of memory may lead to promising future treatments for reducing age-related mnemonic decline.

Expression of GDNF transgene in astrocytes improves cognitive deficits in aged rats

Glial cell line-derived neurotrophic factor (GDNF) was assayed for its neurotrophic effects against the neuronal atrophy that causes cognitive deficits in old age. Aged Fisher 344 rats with impairment in the Morris water maze received intrahippocampal injections at the dorsal CA1 area of either a lentiviral vector encoding human GDNF or the same vector encoding human green fluorescent protein as a control. Recombinant lentiviral vectors constructed with human cytomegalovirus promotor and pseudotyped with lyssavirus Mokola glycoprotein specifically transduced the astrocytes in vivo. Astrocyte-secreted GDNF enhanced neuron function as shown by local increases in synthesis of the neurotransmitters acetylcholine, dopamine and serotonin. This neurotrophic effect led to cognitive improvement of the rats as early as 2 weeks after gene transduction. Spatial learning and memory testing showed a significant gain in cognitive abilities due to GDNF exposure, whereas control-transduced rats kept their performance at the chance level. These results confirm the broad spectrum of the neurotrophic action of GDNF and open new gene therapy possibilities for reducing age-related neurodegeneration.

Stem cell review series: role of neurogenesis in age-related memory disorders

Neuroplasticity is characterized by growth and branching of dendrites, remodeling of synaptic contacts, and neurogenesis, thus allowing the brain to adapt to changes over time. It is maintained in adulthood but strongly repressed during aging. An age-related decline in neurogenesis is particularly pronounced in the two adult neurogenic areas, the subventricular zone and the dentate gyrus. This age-related decline seems to be attributable mainly to limited proliferation, associated with an age-dependent increase in quiescence and/or a lengthening of the cell cycle, and is closely dependent on environmental changes. Indeed, when triggered by appropriate signals, neurogenesis can be reactivated in senescent brains, thus confirming the idea that the age-related decrease in new neuron production is not an irreversible, cell-intrinsic process. The coevolution of neurogenesis and age-related memory deficits--especially regarding spatial memory--during senescence supports the idea that new neurons in the adult brain participate in memory processing, and that a reduction in the ability to generate new neurons contributes to the appearance of memory deficits with advanced age. Furthermore, the age-related changes in hippocampal plasticity and function are under environmental influences that can favor successful or pathological aging. A better understanding of the mechanisms that regulate neurogenesis is necessary to develop new therapeutic tools to cure or prevent the development of memory disorders that may appear during the course of aging in some individuals.

The length of hippocampal cholinergic fibers is reduced in the aging brain

Cholinergic deficits occur in the aged hippocampus and they are significant in Alzheimer's disease. Using stereological and biochemical approaches, we characterized the cholinergic septohippocampal pathway in old (24 months) and young adult (3 months) rats. The total length of choline acetyltransferase (ChAT)-positive fibers in the dorsal hippocampus was significantly decreased by 32% with aging (F((1,9))=20.94, p=0.0014), along with the levels of synaptophysin, a presynaptic marker. No significant changes were detected in ChAT activity or in the amounts of ChAT protein, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), tropomyosin related kinase receptor (Trk) A, TrkB, or p75 neurotrophin receptor (p75(NTR)) in the aged dorsal hippocampus. The number and size of ChAT-positive neurons and the levels of ChAT activity, NGF and BDNF were not statistically different in the septum of aged and young adult rats. This study suggests that substantial synaptic loss and cholinergic axonal degeneration occurs during aging and reinforces the importance of therapies that can protect axons and promote their growth in order to restore cholinergic neurotransmission.

Neurocognitive aging: prior memories hinder new hippocampal encoding

Normal aging is often accompanied by impairments in forming new memories, and studies of aging rodents have revealed structural and functional changes to the hippocampus that might point to the mechanisms behind such memory loss. In this article, we synthesize recent neurobiological and neurophysiological findings into a model of the information-processing circuit of the aging hippocampus. The key point of the model is that small concurrent changes during aging strengthen the auto-associative network of the CA3 subregion at the cost of processing new information coming in from the entorhinal cortex. As a result of such reorganization in aged memory-impaired individuals, information that is already stored would become the dominant pattern of the hippocampus to the detriment of the ability to encode new information.

Lifelong corticosterone level determines age-related decline in neurogenesis and memory

Ageing is accompanied by an alteration of spatial memory, a decline in hippocampal neurogenesis and a dysregulation of the hypothalamic-pituitary axis (HPA) leading to elevated levels of circulating corticosterone. However, the role of the HPA axis in age-related decline in cognitive functions and in neurogenesis decline remains unclear. We found that suppression of glucocorticoids secretion from midlife to the rest of the animals' life increases neurogenesis in old animals and prevents the emergence of age-related memory disorders. Reciprocally, aged rats with a chronic upregulation of the HPA axis exhibit not only spatial memory impairments but also very low levels of hippocampal cell proliferation and survival. Altogether, these results indicate that the extent of lifetime exposure to glucocorticoids determines the extent of age-related decline in hippocampal neurogenesis and consequently age-related cognitive dysfunctions.

Inhibition of hippocampal function in mild cognitive impairment: targeting the cholinergic hypothesis

Mild cognitive impairment (MCI) is a condition with an increased risk of developing Alzheimer's disease. Chief complaint and diagnostic criterion in subjects with mild cognitive impairment is memory failure. We hypothesized that cholinergic malfunction may underlie memory impairment in these subjects and applied a low dosage of an acetylcholinesterase inhibitor and modulator of nicotinic acetylcholine receptors, galantamine (4 mg bid), for 7 days. We used neuropsychological tests to investigate attention, cognitive flexibility, verbal and visual short-term and working memory, susceptibility to interference and episodic memory and functional magnetic resonance imaging to assess spatial navigation both prior to and after treatment. Late episodic learning and delayed recall improved on treatment as did recruitment of the hippocampal region during spatial navigation. Performance in all other neuropsychological measures remained unchanged. We show that an increase of cholinergic neurotransmission in subjects with MCI specifically improves hippocampal function and thus that a cholinergic deficit is functionally relevant in subjects with MCI. Malfunction of the cholinergic system may be tackled pharmacologically via the inhibition of acetylcholinesterase even when the impairment is slight.

Impaired maze performance in aged rats is accompanied by increased density of NMDA, 5-HT1A, and α-adrenoceptor binding in hippocampus

Using quantitative receptor autoradiography, we assessed binding site densities and distribution patterns of glutamate, GABA(A), acetylcholine (ACh), and monoamine receptors in the hippocampus of 32-month-old Fischer 344/Brown Norway rats. Prior to autoradiography, the rats were divided into two groups according to their retention performance in a water maze reference memory task, which was assessed 1 week after 8 days of daily maze training. The animals of the inferior group showed less long-term retention of the hidden-platform task but did not differ from superior rats in their navigation performance during place training and cued trials. The decreased retention performance in the group of inferior learners was primarily accompanied by increased alpha(1)-adrenoceptors in all hippocampal subregions under inspection (CA1-CA4 and dentate gyrus), while elevated alpha(2)-adrenoceptor binding was observed in the CA1 region and DG. Furthermore, inferior learners had higher NMDA binding in the CA2 and CA4 and increased 5-HT(1A) binding sites in the CA2, CA3, and CA4 region. No significant differences between inferior and superior learners were evident with regard to AMPA, kainate, GABA(A), muscarinergic M(1), dopamine D(1), and 5-HT(2) binding densities in any hippocampal region analyzed. These results show that increased NMDA, 5-HT(1A), and alpha-adrenoceptor binding in the hippocampus is associated with a decline in spatial memory. The increased receptor binding observed in the group of old rats with inferior maze performance might be the result of neural adaptation triggered by age-related changes in synaptic connectivity and/or synaptic activity.

Sexually dimorphic effects of hippocampal cholinergic deafferentation in rats

To determine whether the basal forebrain-hippocampal cholinergic system supports sexually dimorphic functionality, male and female Long-Evans rats were given either selective medial septum/vertical limb of the diagonal band (MS/VDB) cholinergic lesions using the neurotoxin 192 IgG-saporin or a control surgery and then postoperatively tested in a set of standard spatial learning tasks in the Morris water maze. Lesions were highly specific and effective as confirmed by both choline acetyltransferase/parvalbumin immunostaining and acetylcholinesterase histochemistry. Female controls performed worse than male controls in place learning and MS/VDB lesions failed to impair spatial learning in male rats, both consistent with previous findings. In female rats, MS/VDB cholinergic lesions facilitated spatial reference learning. A subsequent test of learning strategy in the water maze revealed a female bias for a response, relative to a spatial, strategy; MS/VDB cholinergic lesions enhanced the use of a spatial strategy in both sexes, but only significantly so in males. Together, these results indicate a sexually dimorphic function associated with MS/VDB-hippocampal cholinergic inputs. In female rats, these neurons appear to support sex-specific spatial learning processes.

Aged and Adult Rats Compared in Acquisition and Extinction of Escape From the Water Maze: Focus on Individual Differences

Individual differences in water maze and open-field performance of aged and adult rats were compared in a cross-sectional study. Three- and 24-month-old rats were classified into superior, moderate, and inferior groups on the basis of escape latencies during hidden platform acquisition and were compared regarding water maze acquisition and extinction, and open-field behavior. Unexpectedly, subgroup differences were invariant across age: The inferior and superior maze learners differed in (a) thigmotactic swimming during water maze acquisition and extinction and (b) open-field rearings. Thus, although aging has a detrimental effect on water maze acquisition and extinction, the degree of impairment might be partly determined by individual novelty-induced rearing activity and thigmotactic swimming at adult ages.

Pregnenolone sulfate enhances neurogenesis and PSA-NCAM in young and aged hippocampus

Age-dependent cognitive impairments have been correlated with functional and structural modifications in the hippocampal formation. In particular, the brain endogenous steroid pregnenolone-sulfate (Preg-S) is a cognitive enhancer whose hippocampal levels have been linked physiologically to cognitive performance in senescent animals. However, the mechanism of its actions remains unknown. Because neurogenesis is sensitive to hormonal influences, we examined the effect of Preg-S on neurogenesis, a novel form of plasticity, in young and old rats. We demonstrate that in vivo infusion of Preg-S stimulates neurogenesis and the expression of the polysialylated forms of NCAM, PSA-NCAM, in the dentate gyrus of 3- and 20-month-old rats. These influences on hippocampal plasticity are mediated by the modulation of the gamma-aminobutyric acid receptor complex A (GABA(A)) receptors present on hippocampal neuroblasts. In vitro, Preg-S stimulates the division of adult-derived spheres suggesting a direct influence on progenitors. These data provide evidence that neurosteroids represent one of the local secreted signals controlling hippocampal neurogenesis. Thus, therapies which stimulate neurosteroidogenesis could preserve hippocampal plasticity and prevent the appearance of age-related cognitive disturbances.

A humanin derivative, S14G-HN, prevents amyloid-?-induced memory impairment in mice

Humanin (HN) is a 24-amino acid peptide that protects neuronal cells from death caused by Alzheimer's disease (AD)-related genes and amyloid-beta (Abeta). Multiple studies have revealed its biochemical and neuroprotective characteristics in vitro; however, little has been known regarding whether HN is effective in vivo in AD model systems. We examined the effect of S14G-HN, a 1,000-fold more potent derivative of HN in vitro, on amnesia induced by Abeta25-35 in mice. The Y-maze test revealed that at least 50 pmol of S14G-HN by intracerebroventricular injection prevented Abeta-induced impairment of short-term/spatial working memory; however, 5 nmol of S14A-HN, a neuroprotection-defective mutant in vitro, did not prevent Abeta-induced amnesia. These results are in agreement with the structure-function correlation shown previously in vitro. In the water-finding task, S14G-HN prevented prolongation of finding latency (the time to find water) observed in Abeta-amnesic mice, indicating that S14G-HN also blocked Abeta-induced impairment of latent learning. In accordance with these observations, immunohistochemical analysis showed that S14G-HN sustained the number of cholinergic neurons in the basal forebrain and the striata nearly to the normal level. Furthermore, genistein, a specific inhibitor of tyrosine kinases, blocked recovery from scopolamine-induced amnesia by S14G-HN, suggesting that certain tyrosine kinase(s) are involved in the inhibitory function of S14G-HN in vivo. Taking these findings together, we conclude that S14G-HN has rescue activity against memory impairment caused by AD-related insults in vivo by activating the same intracellular neuroprotective machinery as elucidated previously in vitro.

Delayed non-matching to position performance in aged hybrid Fischer 344 x brown Norway rats: a longitudinal study

In this study, the effects of aging on the performance in a delayed non-matching to position (DNMTP) task were investigated longitudinally in hybrid Fischer 344 x Brown Norway rats. The rats were first trained to perform the task. Subsequently, their performance was assessed monthly from 28 to 34 months of age. The measures of responding on the DNMTP schedule did not decrease in the course of the study. After the last DNMTP test, choline acetyltransferase (ChAT) activity and glial fibrillary acidic protein (GFAP) content were measured in frontal cortex and hippocampus. We found that higher levels of GFAP in the frontal cortex, but not hippocampus, were associated with a poorer performance in the DNMTP task. Our findings support the notion that repeated testing prevents the age-related decline in cognitive functions that has been reported in cross-sectional studies. Pathology of the frontal cortex seems to predict a faster rate of forgetting in aging rats.

Within-subject memory decline in middle-aged rats: effects of intraseptal tacrine

A longitudinal design was used to examine spatial working memory performance in aging Long-Evans rats on a 12-arm, delayed-non-match-to-sample radial maze task. Compared to performance at 12-13 months of age, the same rats exhibited a significant performance deficit at 15-16 months of age across all retention intervals (1.5-10h). All rats exhibited some degree of decline, and no rat performed as well as they had 3 months earlier. This early onset deficit may relate to the degree of difficulty required to perform accurately in a task that maximizes both spatial information processing and flexible working memory representations. Following our observation, rats were implanted with a chronic cannula aimed at the medial septal nucleus. Acute intraseptal tacrine treatments (0.0-25 micrograms/0.5 microl) did not significantly affect any index of performance. Rats exhibited further memory decline over the course of testing (up to 20 months of age). Detection of early onset dysfunction could allow for experimental analysis of underlying mechanisms and therapeutic strategies early in the course of age-related changes.

Behavioural parameters in aged rats are related to LTP and gene expression of ChAT and NMDA-NR2 subunits in the striatum

Striatal parameters were assessed for their relevance to age-related behavioural decline. Forty aged rats (28-30 months) were tested in the water maze and open field. Of these, seven superior and seven inferior learners were compared with each other in terms of levels of in vitro short- and long-term potentiation (STP and LTP), and gene expression of choline acetyltransferase (ChAT) as well as of the NMDA-NR2A-C subunits assessed by quantitative RT-PCR. Results revealed that the superior as compared with the inferior learners had higher levels of ChAT mRNA in the striatum. For the superior group, ChAT mRNA was correlated with escape on to the cued platform in the water maze, whereas level of LTP was predictive of place learning in the water maze and rearing activity in the open field. For the inferior group, expression of NR2A and NR2B was positively correlated with place learning and probe trial performance in the water maze. The results show that individual differences in various behaviours of aged rats were accounted for by variability in striatal parameters, i.e. LTP, ChAT and NMDA-NR2 subunit mRNA. Notably, the correlations found were heterogeneous amid the groups, e.g. variability in place learning was explained by variability in levels of LTP in the superior learners, but in levels of NR2A-B mRNA in the inferior group.

Reduced cortical noradrenergic neurotransmission is associated with increased neophobia and impaired spatial memory in aged rats

In the present study, young (5-month-old (mo)) and aged (24 mo) adult male Fischer-344 (F344) rats were assigned to experimental groups based upon their performance of a reference memory task in the Morris water maze and reactivity to a novel palatable taste in a gustatory neophobia task. Levels of norepinephrine (NE) and its metabolite 3-methoxy-4-hydroxy-phenylglycol (MHPG) were assayed via high performance liquid chromatography (HPLC) in brain regions associated with the locus coeruleus (LC)-hippocampus-cortex system and A1/A2-hypothalamic system. Binding of ligands specific for alpha-1, alpha-2, beta-1, and beta-2 receptors was assessed in hippocampus and cortex with receptor autoradiography. Impaired acquisition and retention of the water maze task and gustatory neophobia in aged rats was primarily associated with decreased NE activity in cingulate cortex (CC) as indicated by a significant reduction in the MHPG/NE ratio coupled with increased NE content. No significant changes in adrenergic receptor binding were detected in any region sampled. The results suggest that an aging-related reduction in cortical NE neurotransmission is associated with the expression of increased neophobia and deficits in spatial learning and memory performance occurring with advanced age in rats.

Water maze learning and forebrain mRNA expression of the neural cell adhesion molecule L1

L1 and NCAM, two cell adhesion molecules of the immunoglobulin superfamily, have been implicated in the formation of neural circuits, synaptic plasticity, and cognitive function. In this study, we sought to investigate whether differences in the steady-state levels of L1 and NCAM expression in specific brain regions could account for individual differences in learning abilities. Using adult male Wistar rats, we evaluated mRNA levels of L1, NCAM, and the NCAM180 isoform in different brain regions (hippocampus, thalamus, striatum, prefrontal and frontal cortices) immediately after submitting rats to a massed training protocol in the water maze. The results showed that untrained and trained rats exhibited similar levels of mRNA for these molecules, which supports the view that training did not influence their immediate level of expression. However, in most of the brain regions we investigated (with the exception of prefrontal and frontal cortices), L1 mRNA levels were positively correlated with the latency to find the hidden platform in the water maze task and with posttraining plasma corticosterone levels. However, no correlations were observed for total NCAM or NCAM180 mRNA in the brain regions examined in this study. Given that animals with a slower spatial acquisition curve exhibited more anxiety-like responses, including thigmotactic behavior in the water maze and increased corticosterone levels, and that recent genetic studies indicate a role for L1 in anxiety, the current findings suggest a relationship among L1, anxiety, and cognitive processes.

Effects of hippocampal cholinergic deafferentation on learning strategy selection in a visible platform version of the water maze

Recent evidence has suggested that the relative levels of acetylcholine (ACh) between brain structures may be an important factor in the choice of behavioral strategy in settings in which either hippocampal or dorsal striatal brain systems can be employed both effectively and independently (McIntyre and Gold. 1999. Soc Neurosci Abs 25:1388). The current investigation used the neurotoxin 192 IgG-saporin to deplete the hippocampus of ACh selectively, while leaving ACh in other brain regions, including dorsal striatum, intact. Rats were then trained on a version of the Morris water maze, in which behavioral strategies attributed to the hippocampus and dorsal striatum are placed in direct competition. It was predicted that rats with hippocampal ACh depletion would display a cue bias. Contrary to this prediction, depleting hippocampal ACh did not bias against and, in fact, promoted use of a hippocampal place strategy in this task, as indicated by choice in competition tests and performance on hidden platform training trials. These data add to a growing literature demonstrating that the septohippocampal cholinergic system is not required for accurate spatial learning and suggest a complex role for basal forebrain projections in processing information about the spatial environment.

Spatial memory performances of aged rats in the water maze predict levels of hippocampal neurogenesis

Neurogenesis occurs within the adult dentate gyrus of the hippocampal formation and it has been proposed that the newly born neurons, recruited into the preexistent neuronal circuits, might be involved in hippocampal-dependent learning processes. Age-dependent spatial memory impairments have been related to an alteration in hippocampal plasticity. The aim of the current study was to examine whether cognitive functions in aged rats are quantitatively correlated with hippocampal neurogenesis. To this end, we took advantage of the existence of spontaneous individual differences observed in aged subjects in a hippocampal-dependent task, the water maze. We expected that the spatial memory capabilities of aged rats would be related to the levels of hippocampal neurogenesis. Old rats were trained in the water maze, and, 3 weeks after training, rats were injected with 5-bromo-2'-deoxyuridine (BrdUrd, 50 or 150 mg/kg) to label dividing cells. Cell proliferation was examined one day after the last BrdUrd injection, whereas cell survival and differentiation were determined 3 weeks later. It is shown that a quantitative relationship exists between learning and the number of newly generated neurons. Animals with preserved spatial memory, i.e., the aged-unimpaired rats, exhibited a higher level of cell proliferation and a higher number of new neurons in comparison with rats with spatial memory impairments, i.e., the aged-impaired rats. In conclusion, the extent of memory dysfunction in aged rats is quantitatively related to the hippocampal neurogenesis. These data reinforce the assumption that neurogenesis is involved in memory processes and aged-related cognitive alterations.

Hilar neuropeptide Y interneuron loss in the aged rat hippocampal formation

Neuropeptide Y-immunoreactive (NPY-I) interneurons in the dentate gyrus are vulnerable to various insults, including septohippocampal cholinergic deafferentation. The present study examined whether a loss of NPY-I neurons occurs during aging, when the functional integrity of the septohippocampal pathway is thought to be compromised. Sets of male Long Evans rats (consisting of young and aged rats, with and without spatial learning impairments assessed by the Morris water maze) were examined. Light microscopic analysis revealed that hilar NPY-I neuronal number in matched dorsal sections was significantly decreased in aged compared to young rats. Ultrastructural analysis disclosed that the microenvironment (the types of processes apposed to the plasmalemmal surface) of NPY-I neurons also differed significantly between young and aged rats. In particular, a subgroup of NPY-I neurons, distinguished by a higher percentage of unmyelinated axon coverage of the plasmalemmal surface, was present in young, but not aged, rats. Neither the number nor the microenvironment of NPY-I neurons significantly differed between aged animals that were impaired versus unimpaired in spatial learning performance. To our knowledge these findings represent the first report of an age-associated decline in the number of a specific, neurochemically identified neuronal subpopulation within the hippocampal formation. Additionally, they closely parallel observations in 192 IgG-saporin-lesioned animals, suggesting that a distinct subgroup of NPY-I interneurons is particularly dependent on the viability of septohippocampal cholinergic innervation for its survival. Since neuronal loss was not correlated with performance, this alteration by itself does not appear to be sufficient to produce learning impairment.

Place cell rigidity correlates with impaired spatial learning in aged rats

In humans and in animals, some aged individuals are severely impaired in learning and memory capacity whereas others perform as well as young adults. In the present study, the spatial memory capacity of young and aged rats was characterized by the Morris water maze task, and then firing patterns of hippocampal "place cells" were assessed as the animals explored a familiar environment and a geometrically-altered version of the environment. Spatial representations of hippocampal cells in young and memory-intact aged rats changed upon exposure to the altered environment. In contrast, spatial representations of many cells in aged, memory-impaired rats were unaffected by the environmental alteration. Furthermore, combining all groups, the extent to which spatial representations distinguished the familiar and altered environments predicted learning capacity in the water maze. These findings suggest that a major component of memory impairment in aging may be the failure of the hippocampus to encode subtle differences in contextual information that differ across multiple experiences, such as the sequence of training trials in the water maze.

Impact of aging on hippocampal function: plasticity, network dynamics, and cognition

Aging is associated with specific impairments of learning and memory, some of which are similar to those caused by hippocampal damage. Studies of the effects of aging on hippocampal anatomy, physiology, plasticity, and network dynamics may lead to a better understanding of age-related cognitive deficits. Anatomical and electrophysiological studies indicate that the hippocampus of the aged rat sustains a loss of synapses in the dentate gyrus, a loss of functional synapses in area CA1, a decrease in the NMDA-receptor-mediated response at perforant path synapses onto dentate gyrus granule cells, and an alteration of Ca(2+) regulation in area CA1. These changes may contribute to the observed age-related impairments of synaptic plasticity, which include deficits in the induction and maintenance of long-term potentiation (LTP) and lower thresholds for depotentiation and long-term depression (LTD). This shift in the balance of LTP and LTD could, in turn, impair the encoding of memories and enhance the erasure of memories, and therefore contribute to cognitive deficits experienced by many aged mammals. Altered synaptic plasticity may also change the dynamic interactions among cells in hippocampal networks, causing deficits in the storage and retrieval of information about the spatial organization of the environment. Further studies of the aged hippocampus will not only lead to treatments for age-related cognitive impairments, but may also clarify the mechanisms of learning in adult mammals.

Cholinergic system regulation of spatial representation by the hippocampus

The role of the basal forebrain cholinergic system in hippocampal spatial representation was explored by examining the effects of immunotoxic lesions of the septo-hippocampal cholinergic neurons on the firing patterns of hippocampal place cells as rats explored familiar and novel environments. In a highly familiar environment, the basic qualities and stability of place fields were unaffected by the lesion. When first exposed to a set of novel environmental cues without otherwise disorienting the animals, place cells in both normal and lesioned animals responded with similar alterations in their firing patterns. Upon subsequent repetitive exposures to the new environment, place cells of normal rats developed a spatial representation distinct from that of the familiar environment. By contrast, place cells of lesioned animals reconverged in the direction of the representation associated with the familiar environment. These results suggest that cholinergic input may determine whether new visual information or a stored representation of the current environment will be actively processed in the hippocampal network.

Improvement of spatial learning and memory after adenovirus-mediated transfer of the nerve growth factor gene to aged rat brain

Adenovirus-mediated transfer of the nerve growth factor gene promotes significant recovery of age-related cholinergic neuronal deficits in aged rats, but the effects of such treatment on cognitive dysfunction remain unclear. Herein we report a beneficial effect of first-generation adenovirus-mediated nerve growth factor gene transfer (AdNGF) on the spatial learning and memory of aged rats. The NGF protein was detected by enzyme-linked immunosorbent assay in cerebrospinal fluid as early as 3 days after gene transfer and was expressed for at least 30 days. Escape latency in the Morris water maze hidden-platform test was significantly improved on day 8 postinoculation in memory-impaired rats treated with AdNGF as well as at later testing intervals. Ultimately, the escape latency values for the AdNGF group become indistinguishable from those for aged rats with normal learning capacity. Immunohistochemical analysis of septal cholinergic neurons for choline acetyltransferase (ChAT) showed significant increases in both the number and somal distribution of ChAT-positive cells after inoculation of memory-impaired rats with AdNGF. Improvement in memory performance was positively correlated with increases in both NGF concentration in cerebrospinal fluid (r = 0.73, p = 0.005) and the number of ChAT-staining cells (r = 0.77, p = 0.0022). We conclude that AdNGF can improve cognitive function in memory-impaired aged rats and, with refinements in vector-driven expression of the transgene, may prove suitable for use in humans.

Environmental influences on brain neurotrophins in rats

Environmental factors can have profound influences on the brain. Enriching environments with physical, social and sensory stimuli are now established to be beneficial to brain development and ageing. A multitude of responses from cellular and molecular mechanisms to macroscopic changes in neural morphology and neurogenesis have been considered in the context for evidences that environmental inputs can regulate brain plasticity in the rat at all stages of life. Data from our laboratory have revealed that enriched environment increased nerve growth factor (NGF) gene expression and protein levels in the hippocampus, and this may contribute to events underlying environmentally induced neural plasticity. Because neurotrophic factors are essential for neural development and survival, they are likely to be involved in the cerebral consequences modified by enriched experiences.

The molarless condition in aged SAMP8 mice attenuates hippocampal Fos induction linked to water maze performance

The involvement of dysfunctional teeth in senile hippocampal activity was evaluated by examining, in aged SAMP8 mice, the effect of cutting off the upper molars (molarless condition) on hippocampal induction of the protein product, Fos, of the immediate early gene, c-fos, and on spatial performance in a water maze. The molarless condition caused a reduction in the number of Fos-positive cells in the hippocampal CA1 region, in which Fos immunoreactivity was localized in the cell nuclei. This effect was more pronounced the longer the molarless condition persisted. The suppression of both learning ability and Fos induction in the CA1 induced by the molarless condition was considerably reduced by restoring the lost molars with artificial crowns. Taken together with the plethora of research showing a relationship between stress, aging and hippocampal function and our past findings [Brain Res. 1999; 826: 148-53; Behav. Brain Res. 2000;108: 145-55; Exp. Gerontol. 2001; 36:283-95], the present results suggest the detrimental effects of a reduction in chewing on hippocampal processing in aged SAMP8 mice that would be linked with stress induced by the molarless condition.

Nontropic actions of neurotrophins: subcortical nerve growth factor gene delivery reverses age-related degeneration of primate cortical cholinergic innervation

Normal aging is associated with a significant reduction in cognitive function across primate species. However, the structural and molecular basis for this age-related decline in neural function has yet to be defined clearly. Extensive cell loss does not occur as a consequence of normal aging in human and nonhuman primate species. More recent studies have demonstrated significant reductions in functional neuronal markers in subcortical brain regions in primates as a consequence of aging, including dopaminergic and cholinergic systems, although corresponding losses in cortical innervation from these neurons have not been investigated. In the present study, we report that aging is associated with a significant 25% reduction in cortical innervation by cholinergic systems in rhesus monkeys (P < 0.001). Further, these age-related reductions are ameliorated by cellular delivery of human nerve growth factor to cholinergic somata in the basal forebrain, restoring levels of cholinergic innervation in the cortex to those of young monkeys (P = 0.89). Thus, (i) aging is associated with a significant reduction in cortical cholinergic innervation; (ii) this reduction is reversible by growth-factor delivery; and (iii) growth factors can remodel axonal terminal fields at a distance, representing a nontropic action of growth factors in modulating adult neuronal structure and function (i.e., administration of growth factors to cholinergic somata significantly increases axon density in terminal fields). These findings are relevant to potential clinical uses of growth factors to treat neurological disorders.

Perinatal asphyxia results in changes in presynaptic bouton number in striatum and cerebral cortex-a stereological and behavioral analysis

Deficits in cognitive function have been related to quantitative changes in synaptic population, particularly in the cerebral cortex. Here, we used an established model of perinatal asphyxia that induces morphological changes, i.e. neuron loss in the cerebral cortex and striatum, as well as behavioural deficits. We hypothesized that perinatal asphyxia may lead to a neurodegenerative process resulting in cognitive impairment and altered presynaptic bouton numbers in adult rats. We studied cognitive performance at 18 months and presynaptic bouton numbers at 22 months following perinatal asphyxia. Data of the spatial Morris water escape task did not reveal clear memory or learning deficits in aged asphyctic rats compared to aged control rats. However, a memory impairment in aged rats versus young rats was observed, which was more pronounced in asphyctic rats. We found an increase in presynaptic bouton density in the parietal cortex, whereas no changes were found in striatum and frontal cortex in asphyctic rats. An increase of striatal volume was observed in asphyctic rats, leading to an increase in presynaptic bouton numbers in this area. These findings stress the issue that volume measurements have to be taken into account when determining presynaptic bouton density. Furthermore, perinatal asphyxia led to region-specific changes in presynaptic bouton numbers and it worsened the age-related cognitive impairment. These results suggest that perinatal asphyxia induced neuronal loss, which is compensated for by an increase in presynaptic bouton numbers.

The tractable contribution of synapses and their component molecules to individual differences in learning

Though once of central importance to psychologists and neurophysiologists alike, the elucidation of neural substrates for individual differences in learning no longer attracts a broad research effort and occupies a place of largely historical interest to the contemporary disciplines. The decline in interest in this subject ensued in part from the perception, arrived at decades ago, that individual differences in learning were not quantified as easily as had once been presumed. Furthermore, the dominant hypotheses in the field defied testing within the constraints imposed by the complex and largely inaccessible vertebrate nervous system. Using a 'model systems' approach where the individual cells and synaptic interactions that comprise a neural network can be identified, we have returned to this question and have established a framework by which we can begin to discern the basis for much of the variability between individuals in their capacity to learn. In the marine mollusc Hermissenda, we have found that a common influence on transmitter exocytosis is expressed homogeneously throughout the nervous system regardless of transmitter system or receptor class. Though uniformly expressed within an individual, this influence on synaptic efficacy is differentially expressed between animals. Importantly, the basal efficiency of exocytosis expressed in an individual nervous system is strongly correlated with the degree to which activity-dependent forms of neuronal/synaptic facilitation can be induced in that nervous system, and predicts the capacity for the intact animal to learn a Pavlovian association. Furthermore, we have established that a decline in basal synaptic efficacy in aged animals, arising from chronic presynaptic Ca(2+) 'leak', may contribute to age-related learning impairments. Because certain fundamental components of the exocytotic cascade are conserved widely across cell types, transmitter systems and species, the principles that we describe may have broad implications for understanding normal variability in learning, but also, in the development of specific strategies to compensate for mild learning deficits and age-related cognitive decline.

Immunohistochemical and neurochemical correlates of learning deficits in aged rats

This study examined whether cholinergic and monoaminergic dysfunctions in the brain could be related to spatial learning capabilities in 26-month-old, as compared to three-month-old, Long-Evans female rats. Performances were evaluated in the water maze task and used to constitute subgroups with a cluster analysis statistical procedure. In the first experiment (histological approach), the first cluster contained young rats and aged unimpaired rats, the second one aged rats with moderate impairment and the third one aged rats with severe impairment. Aged rats showed a reduced number of choline acetyltransferase- and p75(NTR)-positive neurons in the nucleus basalis magnocellularis, and choline acetyltransferase-positive neurons in the striatum. In the second experiment (neurochemical approach), the three clusters comprised young rats, aged rats with moderate impairment and aged rats with severe impairment. Alterations related to aging consisted of reduced concentration of acetylcholine, norepinephrine and serotonin in the striatum, serotonin in the occipital cortex, dopamine and norepinephrine in the dorsal hippocampus, and norepinephrine in the ventral hippocampus. In the first experiment, there were significant correlations between water maze performance and the number of; (i) choline acetyltransferase- and p75(NTR)-positive neurons in the nucleus basalis magnocellularis; (ii) choline acetyltransferase-positive neurons in the striatum and; (iii) p75(NTR)-positive neurons in the medial septum. In the second experiment, water maze performance was correlated with the concentration of; (i) acetylcholine and serotonin in the striatum; (ii) serotonin and norepinephrine in the dorsal hippocampus; (iii) norepinephrine in the frontoparietal cortex and; (iv) with other functional markers such as the 5-hydroxyindoleacetic acid/serotonin ratio in the striatum, 3,4-dihydroxyphenylacetic acid/dopamine ratio in the dorsal hippocampus, 5-hydroxyindoleacetic acid/serotonin and homovanillic acid/dopamine ratios in the frontoparietal cortex, and 3,4-dihydroxyphenylacetic acid/dopamine ratio in the occipital cortex. The results indicate that cognitive deficits related to aging might involve concomitant alterations of various neurochemical systems in several brain regions such as the striatum, the hippocampus or the cortex. It also seems that these alterations occur in a complex way which, in addition to the loss of cholinergic neurons in the basal forebrain, affects dopaminergic, noradrenergic and serotonergic processes.

Impairment of spatial memory and changes in astroglial responsiveness following loss of molar teeth in aged SAMP8 mice

In order to evaluate the mechanism(s) responsible for senile impairment of cognitive function as a result of reduced mastication, the effects of the loss of the molar teeth (molarless condition) on the hippocampal expression of glial fibrous acidic protein (GFAP) and on spatial memory in young adult and aged SAMP8 mice were studied using immunohistochemical and behavioral techniques. Aged molarless mice showed a significantly reduced learning ability in a water maze test compared with age-matched control mice, while there was no difference between control and molarless young adult mice. Immunohistochemical analysis showed that the molarless condition enhanced the age-dependent increase in the density and hypertrophy of GFAP-labeled astrocytes in the CA1 region of the hippocampus. These effects increased the longer the molarless condition persisted. When the extracellular K+ concentration ([K+]o) was increased from 4 to 40 mM for hippocampal slices in vitro, the mean increase in the membrane potential was about 57 mV for fine, delicate astrocytes, the most frequently observed type of GFAP-positive cell in the young adult mice, and about 44 mV for the hypertrophic astrocytes of aged mice. However, there was no significant difference in resting membrane potential between these cell types. The data suggest that an impairment of spatial memory and changes in astroglial responsiveness occur following the loss of molar teeth in aged SAMP8 mice.

Changes in brain nerve growth factor levels and nerve growth factor receptors in rats exposed to environmental enrichment for one year

This study examined the effects of long-term differential rearing on levels of brain nerve growth factor, its receptors, and their relationships to cognitive function. Adult rats (two months old) were placed into either enriched or standard housing conditions where they remained for 12 months. Animals from the enriched condition group had significantly higher levels of nerve growth factor in hippocampus, visual and entorhinal cortices compared with animals housed in isolated condition. Immunohistochemical analysis of brain tissue from the medial septal area revealed higher staining intensity and fibre density with both the low-affinity and the high-affinity nerve growth factor receptors. Enriched rats performed better than isolated rats in acquisition of spatial learning and had lower locomotion scores in the open field. These results provide further evidence that experimental stimulation results in increased production of trophic factors and structural reorganization in specific brain regions known to be involved in cognitive function.

Differential effects of nucleus basalis lesions in young adult and aging rats

To characterize age-related changes in frontal cortical plasticity, we assessed maze learning and frontal cortical pharmacology in young adult, middle-aged, and aged rats. Rats received either ibotenic acid or sham lesions of the nucleus basalis magnocellularis (NBM) and were then trained on a radial maze task. After training, we assessed [3H]desmethylimipramine (DMI), [3H]muscimol, [3H]AMPA, and [3H]QNB binding using quantitative autoradiography. Both middle-aged and aged rats were impaired on the radial maze task. DMI binding was increased in both middle-aged and aged rats, while QNB binding was decreased in aged rats. While lesions impaired maze performance at all ages, middle-aged and aged rats showed more profound lesion-induced deficits. Lesions increased GABA, and AMPA receptor binding in young adult rats only. These lesion-induced changes may reflect a compensatory response that is lost with advancing age.