Degenerative Changes in Forebrain Cholinergic Nuclei Correlate with Cognitive Impairments in Aged Rats

Septohippocampal cholinergic system at the intersection of stress and cognition: Current trends and translational implications

Deficits in hippocampus-dependent memory processes are common across psychiatric and neurodegenerative disorders such as depression, anxiety and Alzheimer's disease. Moreover, stress is a major environmental risk factor for these pathologies and it exerts detrimental effects on hippocampal functioning via the activation of hypothalamic-pituitary-adrenal (HPA) axis. The medial septum cholinergic neurons extensively innervate the hippocampus. Although, the cholinergic septohippocampal pathway (SHP) has long been implicated in learning and memory, its involvement in mediating the adaptive and maladaptive impact of stress on mnemonic processes remains less clear. Here, we discuss current research highlighting the contributions of cholinergic SHP in modulating memory encoding, consolidation and retrieval. Then, we present evidence supporting the view that neurobiological interactions between HPA axis stress response and cholinergic signalling impact hippocampal computations. Finally, we critically discuss potential challenges and opportunities to target cholinergic SHP as a therapeutic strategy to improve cognitive impairments in stress-related disorders. We argue that such efforts should consider recent conceptualisations on the dynamic nature of cholinergic signalling in modulating distinct subcomponents of memory and its interactions with cellular substrates that regulate the adaptive stress response.

Intermittent fasting disrupts hippocampal-dependent memory and norepinephrine content in aged male and female mice

Intermittent fasting (IMF) is associated with many health benefits in animals and humans. Yet, little is known if an IMF diet affects mood and cognitive processing. We have previously identified that IMF in diet-induced obese males increases norepinephrine and dopamine content in the hypothalamus and increases arcuate neuropeptide Y (NPY) gene expression more than in ad libitum control males. This suggests that IMF may improve cognition through activation of the hindbrain norepinephrine neuronal network and reverse the age-dependent decline in NPY expression. Less is known about the association between anxiety and IMF. Although, in humans, IMF during Ramadan may alleviate anxiety. Here, we address the impact of IMF on anxiety-like behavior using the open field test, hippocampal-dependent memory using the Y-maze and spatial object recognition, and hippocampal-independent memory using novel object recognition in middle-aged male and female (12 mo) and aged male and female (18 mo) mice. Using ELISA, we determined norepinephrine (NE) content in the dorsal hippocampus (DH) and prefrontal cortex (PFC). We also investigated gene expression in the arcuate nucleus (ARC), the lateral hypothalamus (LH), and the locus coeruleus (LC). In IMF-treated females at both ages, we observed an improvement in spatial navigation although an impairment in spatial object orientation. IMF-treated females (12 mo) had a reduction and IMF-treated males (12 mo) displayed an improvement in novel object recognition memory. IMF-treated females (18 mo) exhibited anxiolytic-like behavior and increased locomotion. In the DH, IMF-treated males (12 mo) had a greater amount of NE content and IMF-treated males (18 mo) had a reduction. In the ARC, IMF-treated males (12 mo) exhibited an increase in Agrp and Npy and a decrease in Adr1a. In the ARC, IMF-treated males (18 mo) exhibited an increase in Npy and a decrease in Adr1a; females had a trending decrease in Cart. In the LH at 12 months, IMF-treated males had a decrease in Npy5r, Adr1a, and Adr1b; both males and females had a reduction in Npy1r. In the LH, IMF-treated females (18 mo) had a decrease in Hcrt. In the LC at both ages, mice largely exhibited sex effects. Our findings indicate that IMF produces alterations in mood, cognition, DH NE content, and ARC, LH, and LC gene expression depending on sex and age.

Cognitive Aging and the Primate Basal Forebrain Revisited: Disproportionate GABAergic Vulnerability Revealed

Basal forebrain (BF) projections to the hippocampus and cortex are anatomically positioned to influence a broad range of cognitive capacities that are known to decline in normal aging, including executive function and memory. Although a long history of research on neurocognitive aging has focused on the role of the cholinergic basal forebrain system, intermingled GABAergic cells are numerically as prominent and well positioned to regulate the activity of their cortical projection targets, including the hippocampus and prefrontal cortex. The effects of aging on noncholinergic BF neurons in primates, however, are largely unknown. In this study, we conducted quantitative morphometric analyses in brains from young adult (6 females, 2 males) and aged (11 females, 5 males) rhesus monkeys (Macaca mulatta) that displayed significant impairment on standard tests that require the prefrontal cortex and hippocampus. Cholinergic (ChAT+) and GABAergic (GAD67+) neurons were quantified through the full rostrocaudal extent of the BF. Total BF immunopositive neuron number (ChAT+ plus GAD67+) was significantly lower in aged monkeys compared with young, largely because of fewer GAD67+ cells. Additionally, GAD67+ neuron volume was greater selectively in aged monkeys without cognitive impairment compared with young monkeys. These findings indicate that the GABAergic component of the primate BF is disproportionally vulnerable to aging, implying a loss of inhibitory drive to cortical circuitry. Moreover, adaptive reorganization of the GABAergic circuitry may contribute to successful neurocognitive outcomes.SIGNIFICANCE STATEMENT A long history of research has confirmed the role of the basal forebrain in cognitive aging. The majority of that work has focused on BF cholinergic neurons that innervate the cortical mantle. Codistributed BF GABAergic populations are also well positioned to influence cognitive function, yet little is known about this prominent neuronal population in the aged brain. In this unprecedented quantitative comparison of both cholinergic and GABAergic BF neurons in young and aged rhesus macaques, we found that neuron number is significantly reduced in the aged BF compared with young, and that this reduction is disproportionately because of a loss of GABAergic neurons. Together, our findings encourage a new perspective on the functional organization of the primate BF in neurocognitive aging.

Structural degeneration of the nucleus basalis of Meynert in mild cognitive impairment and Alzheimer's disease - Evidence from an MRI-based meta-analysis

Recent models of Alzheimer's disease (AD) suggest that neuropathological changes of the medial temporal lobe, especially entorhinal cortex, are preceded by degenerations of the cholinergic Nucleus basalis of Meynert (NbM). Evidence from imaging studies in humans, however, is limited. Therefore, we performed an activation-likelihood estimation meta-analysis on whole brain voxel-based morphometry (VBM) MRI data from 54 experiments and 2581 subjects in total. It revealed, compared to healthy older controls, reduced gray matter in the bilateral NbM in AD, but only limited evidence for such an effect in patients with mild cognitive impairment (MCI), which typically precedes AD. Both patient groups showed less gray matter in the amygdala and hippocampus, with hints towards more pronounced amygdala effects in AD. We discuss our findings in the context of studies that highlight the importance of the cholinergic basal forebrain in learning and memory throughout the lifespan, and conclude that they are partly compatible with pathological staging models suggesting initial and pronounced structural degenerations within the NbM in the progression of AD.

Cognitive Deficits in Aging Related to Changes in Basal Forebrain Neuronal Activity

Aging is a physiological process accompanied by a decline in cognitive performance. The cholinergic neurons of the basal forebrain provide projections to the cortex that are directly engaged in many cognitive processes in mammals. In addition, basal forebrain neurons contribute to the generation of different rhythms in the EEG along the sleep/wakefulness cycle. The aim of this review is to provide an overview of recent advances grouped around the changes in basal forebrain activity during healthy aging. Elucidating the underlying mechanisms of brain function and their decline is especially relevant in today's society as an increasingly aged population faces higher risks of developing neurodegenerative diseases such as Alzheimer's disease. The profound age-related cognitive deficits and neurodegenerative diseases associated with basal forebrain dysfunction highlight the importance of investigating the aging of this brain region.

Adolescent Binge-Type Ethanol Exposure in Rats Mirrors Age-Related Cognitive Decline by Suppressing Cholinergic Tone and Hippocampal Neurogenesis

Heavy alcohol consumption followed by periods of abstinence (i.e., binge drinking) during adolescence is a concern for both acute and chronic health issues. Persistent brain damage after adolescent intermittent ethanol exposure in rodents, a model of binge drinking, includes reduced hippocampal neurogenesis and a loss of neurons in the basal forebrain that express the cholinergic phenotype. The circuit formed between those regions, the septohippocampal pathway, is critical for learning and memory. Furthermore, this circuit is also altered during the aging process. Thus, we examined whether pathology in septohippocampal circuit and impairments in spatial behaviors are amplified during aging following adolescent intermittent ethanol exposure. Female and male rats were exposed to intermittent intragastric gavage of water (control) or 20% ethanol (dose of 5 g/kg) for a 2 days on/off cycle from postnatal days 25-55. Either 2 (young adult) or 12-14 (middle-age) months post exposure, rats were tested on two spatial tasks: spontaneous alternation and novel object in place. Acetylcholine efflux was assessed in the hippocampus during both tasks. There was no adolescent ethanol-induced deficit on spontaneous alternation, but middle-aged male rats displayed lower alternation rates. Male rats exposed to ethanol during adolescence had blunted behavioral evoked acetylcholine during spontaneous alternation testing. All ethanol-exposed rats displayed suppression of the cholinergic neuronal phenotype. On the novel object in place task, regardless of sex, ethanol-exposed rats performed significantly worse than control-treated rats, and middle aged-rats, regardless of sex or ethanol exposure, were significantly impaired relative to young adult rats. These results indicate that male rats display earlier age-related cognitive impairment on a working memory task. Furthermore, male rats exposed to ethanol during adolescence have blunted behavior-evoked hippocampal acetylcholine efflux. In addition, middle-aged and ethanol-exposed rats, regardless of sex, are impaired at determining discrete spatial relationship between objects. This type of pattern separation impairment was associated with a loss of neurogenesis. Thus, binge-type adolescent ethanol exposure does affect the septohippocampal circuit, and can accelerate age-related cognitive impairment on select spatial tasks.

Age-associated decline in septum neuronal activation during spatial learning in homing pigeons (Columba livia)

The relationship between hippocampal aging and spatial-cognitive decline in birds has recently been investigated. However, like its mammalian counterpart, the avian hippocampus does not work in isolation and its relationship to the septum is of particular interest. The current study aimed to investigate the effects of age on septum (medial and lateral) and associated nucleus of the diagonal band (NDB) neuronal activation (as indicated by c-Fos expression) during learning of a spatial, delayed non-match-to-sample task conducted in a modified radial arm maze. The results indicated significantly reduced septum, but not NDB, activation during spatial learning in older pigeons. We also preliminarily investigated the effect of age on the number of cholinergic septum and NDB neurons (as indicated by expression of choline acetyltransferase; ChAT). Although underpowered to reveal a statistical effect, the data suggest that older pigeons have substantially fewer ChAT-expressing cells in the septum compared to younger pigeons. The data support the hypothesis that reduced activation of the septum contributes to the age-related, spatial cognitive impairment in pigeons.

Age-related positivity effect on behavioural responses of dogs to human vocalisations

Age-related changes in the brain can alter how emotions are processed. In humans, valence specific changes in attention and memory were reported with increasing age, i.e. older people are less attentive toward and experience fewer negative emotions, while processing of positive emotions remains intact. Little is yet known about this "positivity effect" in non-human animals. We tested young (n = 21, 1-5 years) and old (n = 19, >10 years) family dogs with positive (laugh), negative (cry), and neutral (hiccup, cough) human vocalisations and investigated age-related differences in their behavioural reactions. Only dogs with intact hearing were analysed and the selected sound samples were balanced regarding mean and fundamental frequencies between valence categories. Compared to young dogs, old individuals reacted slower only to the negative sounds and there was no significant difference in the duration of the reactions between groups. The selective response of the aged dogs to the sound stimuli suggests that the results cannot be explained by general cognitive and/or perceptual decline. and supports the presence of an age-related positivity effect in dogs, too. Similarities in emotional processing between humans and dogs may imply analogous changes in subcortical emotional processing in the canine brain during ageing.

Aging with alcohol-related brain damage: Critical brain circuits associated with cognitive dysfunction

Alcoholism is associated with brain damage and impaired cognitive functioning. The relative contributions of different etiological factors, such as alcohol, thiamine deficiency and age vulnerability, to the development of alcohol-related neuropathology and cognitive impairment are still poorly understood. One reason for this quandary is that both alcohol toxicity and thiamine deficiency produce brain damage and cognitive problems that can be modulated by age at exposure, aging following alcohol toxicity or thiamine deficiency, and aging during chronic alcohol exposure. Pre-clinical models of alcohol-related brain damage (ARBD) have elucidated some of the contributions of ethanol toxicity and thiamine deficiency to neuroinflammation, neuronal loss and functional deficits. However, the critical variable of age at the time of exposure or long-term aging with ARBD has been relatively ignored. Acute thiamine deficiency created a massive increase in neuroimmune genes and proteins within the thalamus and significant increases within the hippocampus and frontal cortex. Chronic ethanol treatment throughout adulthood produced very minor fluctuations in neuroimmune genes, regardless of brain region. Intermittent "binge-type" ethanol during the adolescent period established an intermediate neuroinflammatory response in the hippocampus and frontal cortex, that can persist into adulthood. Chronic excessive drinking throughout adulthood, adolescent intermittent ethanol exposure, and thiamine deficiency all led to a loss of the cholinergic neuronal phenotype within the basal forebrain, reduced hippocampal neurogenesis, and alterations in the frontal cortex. Only thiamine deficiency results in gross pathological lesions of the thalamus. The behavioral impairment following these types of treatments is hierarchical: Thiamine deficiency produces the greatest impairment of hippocampal- and prefrontal-dependent behaviors, chronic ethanol drinking ensues mild impairments on both types of tasks and adolescent intermittent ethanol exposure leads to impairments on frontocortical tasks, with sparing on most hippocampal-dependent tasks. However, our preliminary data suggest that as rodents age following adolescent intermittent ethanol exposure, hippocampal functional deficits began to emerge. A necessary requirement for the advancement of understanding the neural consequences of alcoholism is a more comprehensive assessment and understanding of how excessive alcohol drinking at different development periods (adolescence, early adulthood, middle-aged and aged) influences the trajectory of the aging process, including pathological aging and disease.

Design and development of novel p-aminobenzoic acid derivatives as potential cholinesterase inhibitors for the treatment of Alzheimer's disease

Based on the quantitative structure-activity relationship (QSAR), some novel p-aminobenzoic acid derivatives as promising cholinesterase enzyme inhibitors were designed, synthesized, characterized and evaluated to enhance learning and memory. The in vitro enzyme kinetic study of the synthesized compounds revealed the type of inhibition on the respective acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) enzymes. The in vivo studies of the synthesized compounds exhibited significant reversal of cognitive deficits in the animal models of amnesia as compared to standard drug donepezil. Further, the ex vivo studies in the specific brain regions like the hippocampus, hypothalamus, and prefrontal cortex regions also exhibited AChE inhibition comparable to standard donepezil. The in silico molecular docking and dynamics simulations studies of the most potent compound 22 revealed the consensual interactions at the active site pocket of the AChE.

Effects of Intraventricular Encapsulated Hngf-Secreting Fibroblasts in Aged Rats

Exogenous NGF administered into the central nervous system (CNS) has been reported to improve cognitive function in aged rats. However, concerns have been expressed about the risks involved with supplying NGF to the CNS. In this study, baby hamster kidney cells (BHK) genetically modified to secrete human NGF (hNGF) were encapsulated in semipermeable membranes and implanted intraventricularly. ChAT/LNGFR-positive basal forebrain neurons were shown to atrophy and degenerate with age, especially in cognitively impaired rats. The encapsulated BHK-NGF cells produced less than 10% of doses previously reported to be effective, but this was sufficient to increase the size of ChAT/LNGFR-positive basal forebrain neurons in the aged and learning-impaired rats to the size of the neurons in young healthy rats. The hNGF from these encapsulated cells also improved performance in a repeated-acquisition version of the Morris water maze spatial learning task in learning-impaired 20.6- and 26.7- mo-old rats. Furthermore, there was no evidence that these doses of hNGF impaired Morris water maze performance in the youngest 3.3-5.4 mo rats, and analyses of mortality rates, body weights, somatosensory thresholds, potential hyperalgesia, and activity levels, suggested that these levels of exogenous hNGF are not toxic or harmful to aged rats. These results suggest that CNS-implanted semipermeable membranes, containing genetically modified xenogeneic cells continuously producing these levels of hNGF, attenuate age-related cognitive deficits in nonimmunosuppressed aged rats, and that both the surgical implantation procedure and long-term exposure to low doses of hNGF appear safe in aged rats.

Severity of spatial learning impairment in aging: Development of a learning index for performance in the Morris water maze

The Morris water maze task was originally designed to assess the rat's ability to learn to navigate to a specific location in a relatively large spatial environment. This article describes new measures that provide information about the spatial distribution of the rat's search during both training and probe trial performance. The basic new measure optimizes the use of computer tracking to identify the rat's position with respect to the target location. This proximity measure was found to be highly sensitive to age-related impairment in an assessment of young and aged male Long-Evans rats. Also described is the development of a learning index that provides a continuous, graded measure of the severity of age-related impairment in the task. An index of this type should be useful in correlational analyses with other neurobiological or behavioral measures for the study of individual differences in functional/biological decline in aging.

Age-Dependent Effect of β-Amyloid Toxicity on Basal Forebrain Cholinergic Neurons and Inflammation in the Rat Brain

Beta-amyloid (Aβ) accumulation, neuroinflammation, basal forebrain cholinergic loss and hippocampal degeneration are well-described pathologies associated with Alzheimer's disease (AD). However, the role that age plays in the susceptibility of the brain to these AD pathologies and the relationships between them is still not well understood. This study investigated the age-related response to intracerebroventricular injection of Aβ(25-35) in 3-, 6- and 9-month-old rats. Aβ toxicity resulted in an age-related increase in cholinergic loss and microglial activation in the basal forebrain along with neuronal loss in the hippocampal CA3 subfield. Performance in the Morris water maze revealed impairments in long-term reference memory in 6-month-old Aβ administered animals, which was not seen in 3-month-old animals. These results support a role of Aβ administration in inducing age-dependent cholinergic loss and neuroinflammation, and additionally provide evidence for a more age-appropriate model of adult-onset Aβ toxicity demonstrating pathological changes that reflect the early stages of AD pathogenesis including neuroinflammation, cholinergic loss and beginning stages of memory impairment.

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.

Neurokinin3 receptor as a target to predict and improve learning and memory in the aged organism

Impaired learning and memory performance is often found in aging as an early sign of dementia. It is associated with neuronal loss and reduced functioning of cholinergic networks. Here we present evidence that the neurokinin3 receptors (NK3-R) and their influence on acetylcholine (ACh) release may represent a crucial mechanism that underlies age-related deficits in learning and memory. Repeated pharmacological stimulation of NK3-R in aged rats was found to improve learning in the water maze and in object-place recognition. This treatment also enhanced in vivo acetylcholinergic activity in the frontal cortex, hippocampus, and amygdala but reduced NK3-R mRNA expression in the hippocampus. Furthermore, NK3-R agonism incurred a significantly higher increase in ACh levels in aged animals that showed superior learning than in those that were most deficient in learning. Our findings suggest that the induced activation of ACh, rather than basal ACh activity, is associated with superior learning in the aged. To test whether natural variation in NK3-R function also determines learning and memory performance in aged humans, we investigated 209 elderly patients with cognitive impairments. We found that of the 15 analyzed single single-nucleotide ploymorphism (SNPs) of the NK3-R-coding gene, TACR3, the rs2765 SNP predicted the degree of impairment of learning and memory in these patients. This relationship could be partially explained by a reduced right hippocampus volume in a subsample of 111 tested dementia patients. These data indicate the NK3-R as an important target to predict and improve learning and memory performance in the aged organism.

Age-related changes in rostral basal forebrain cholinergic and GABAergic projection neurons: relationship with spatial impairment

Both cholinergic and GABAergic projections from the rostral basal forebrain contribute to hippocampal function and mnemonic abilities. While dysfunction of cholinergic neurons has been heavily implicated in age-related memory decline, significantly less is known regarding how age-related changes in codistributed GABAergic projection neurons contribute to a decline in hippocampal-dependent spatial learning. In the current study, confocal stereology was used to quantify cholinergic (choline acetyltransferase [ChAT] immunopositive) neurons, GABAergic projection (glutamic decarboxylase 67 [GAD67] immunopositive) neurons, and total (neuronal nuclei [NeuN] immunopositive) neurons in the rostral basal forebrain of young and aged rats that were first characterized on a spatial learning task. ChAT immunopositive neurons were significantly but modestly reduced in aged rats. Although ChAT immunopositive neuron number was strongly correlated with spatial learning abilities among young rats, the reduction of ChAT immunopositive neurons was not associated with impaired spatial learning in aged rats. In contrast, the number of GAD67 immunopositive neurons was robustly and selectively elevated in aged rats that exhibited impaired spatial learning. Interestingly, the total number of rostral basal forebrain neurons was comparable in young and aged rats, regardless of their cognitive status. These data demonstrate differential effects of age on phenotypically distinct rostral basal forebrain projection neurons, and implicate dysregulated cholinergic and GABAergic septohippocampal circuitry in age-related mnemonic decline.

Accelerated aging of the GABAergic septohippocampal pathway and decreased hippocampal rhythms in a mouse model of Alzheimer's disease

Patients with Alzheimer's disease (AD) display altered functioning of cortical networks, including altered patterns of synchronous activity and a serious deficit in cholinergic septohippocampal (SH) innervation. However, the mechanisms underlying these alterations and the implication of the GABAergic SH component in AD are largely unknown. In addition, the GABAergic septohippocampal pathway (SHP) is believed to regulate synchronous hippocampal activity by controlling the activity of interneurons. Here we show, using well-characterized pathway tracing experiments, that innervation of the GABAergic SHP decreases during normal aging. Furthermore, in an AD mouse model (hAPP(Sw,Ind); J20 mice), the GABAergic SHP shows a dramatic and early onset of this decrease in 8-mo-old mice. This decline is not caused by neuronal loss, but by the reduced number and complexity of GABAergic SH axon terminals. Finally, we demonstrate that hippocampal θ and γ rhythm power spectra are markedly diminished in 8-mo-old behaving mice expressing mutated hAPP. In addition to the well-known loss of cholinergic input to the hippocampus in AD, these data suggest that the altered patterns of synchronous activity seen in patients with AD could be caused by the loss of GABAergic SH axons, which modulate hippocampal network activities.

Effect of bite-raised condition on the hippocampal cholinergic system of aged SAMP8 mice

Occlusal disharmony induces chronic stress, which results in learning deficits in association with the morphologic changes in the hippocampus, e.g., neuronal degeneration and increased hypertrophied glial fibrillary acidic protein-positive cells. To investigate the mechanisms underlying impaired hippocampal function resulting from occlusal disharmony, we examined the effects of the bite-raised condition on the septohippocampal cholinergic system by assessing acetylcholine release in the hippocampus and choline acetyltransferase immunoreactivity in the medial septal nucleus in aged SAMP8 mice that underwent the bite raising procedure. Aged bite-raised mice showed decreased acetylcholine release in the hippocampus and a reduced number of choline acetyltransferase-immunopositive neurons in the medial septal nucleus compared to age-matched control mice. These findings suggest that the bite-raised condition in aged SAMP8 mice enhances the age-related decline in the septohippocampal cholinergic system, leading to impaired learning.

Galanthamine plus estradiol treatment enhances cognitive performance in aged ovariectomized rats

We hypothesize that beneficial effects of estradiol on cognitive performance diminish with age and time following menopause due to a progressive decline in basal forebrain cholinergic function. This study tested whether galanthamine, a cholinesterase inhibitor used to treat memory impairment associated with Alzheimer's disease, could enhance or restore estradiol effects on cognitive performance in aged rats that had been ovariectomized in middle-age. Rats were ovariectomized at 16-17 months of age. At 21-22 months of age rats began receiving daily injections of galanthamine (5mg/day) or vehicle. After one week, half of each group also received 17ß-estradiol administered subcutaneously. Rats were then trained on a delayed matching to position (DMP) T-maze task, followed by an operant stimulus discrimination/reversal learning task. Treatment with galanthamine+estradiol significantly enhanced the rate of DMP acquisition and improved short-term delay-dependent spatial memory performance. Treatment with galanthamine or estradiol alone was without significant effect. Effects were task-specific in that galanthamine+estradiol treatment did not significantly improve performance on the stimulus discrimination/reversal learning task. In fact, estradiol was associated with a significant increase in incorrect responses on this task after reversal of the stimulus contingency. In addition, treatments did not significantly affect hippocampal choline acetyltransferase activity or acetylcholine release. This may be an effect of age, or possibly is related to compensatory changes associated with long-term cholinesterase inhibitor treatment. The data suggest that treating with a cholinesterase inhibitor can enhance the effects of estradiol on acquisition of a DMP task by old rats following a long period of hormone deprivation. This could be of particular benefit to older women who have not used hormone therapy for many years and are beginning to show signs of mild cognitive impairment. Potential mechanisms for these effects are discussed.

Growth hormone modulates hippocampal excitatory synaptic transmission and plasticity in old rats

Alterations in the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPA-R) and N-methyl-D-aspartate receptor (NMDA-R) have been documented in aged animals and may contribute to changes in hippocampal-dependent memory. Growth hormone (GH) regulates AMPA-R and NMDA-R-dependent excitatory transmission and decreases with age. Chronic GH treatment mitigates age-related cognitive decline. An in vitro CA1 hippocampal slice preparation was used to compare hippocampal excitatory transmission and plasticity in old animals treated for 6-8 months with either saline or GH. Our findings indicate that GH treatment restores NMDA-R-dependent basal synaptic transmission in old rats to young adult levels and enhances both AMPA-R-dependent basal synaptic transmission and long-term potentiation. These alterations in synaptic function occurred in the absence of changes in presynaptic function, as measured by paired-pulse ratios, the total protein levels of AMPA-R and NMDA-R subunits or in plasma or hippocampal levels of insulin-like growth factor-I. These data suggest a direct role for GH in altering age-related changes in excitatory transmission and provide a possible cellular mechanism through which GH changes the course of cognitive decline.

Increased interactions between PKA and NF-κB signaling in the hippocampus following loss of cholinergic input

Neuropsychiatric disorders such as depression are frequently associated with Alzheimer's disease (AD) and the degeneration of cholinergic basal forebrain neurons and reductions in acetylcholine that occur in AD have been identified as potential mediators of these secondary neuropsychiatric symptomologies. Indeed, removal of cholinergic innervation to the hippocampus via selective immunolesions of septohippocampal cholinergic neurons induces dysfunction of the hypothalamic-pituitary-adrenocortical (HPA) axis and decreases glucocorticoid receptor expression (GR). A subsequent study showed that loss of cholinergic input decreases the activity of the catalytic subunit of protein kinase A (PKAc) and lessens the interaction of protein kinase A (PKA) with GR. Because cross-coupling between nuclear factor-κB (NF-κB) p65 and GR depends on PKA signaling, the present study was conducted to evaluate the status of NF-κB as well as interactions of PKA with NF-κB in the hippocampus following cholinergic denervation. Expression of cytosolic NF-κB p65 was diminished and IκB was degraded in the hippocampus of cholinergic immunolesioned rats compared to the controls. Immunolesions also increased NF-κB p65 Ser276 phosphorylation, as well as interactions between PKAc and NF-κB p65. These results indicate that loss of cholinergic input to the hippocampus results in decreased PKA activity and increased NF-κB activity. Such altered signaling may contribute to psychiatric symptoms, including depression, in patients with AD.

Neurokinin3-R agonism in aged rats has anxiolytic-, antidepressant-, and promnestic-like effects and stimulates ACh release in frontal cortex, amygdala and hippocampus

Neurokinin-3 receptors (NK(3)-R) are localized in brain regions which have been implicated in processes governing learning and memory as well as emotionality. The effects of acute subcutaneous (s.c.) senktide (0.2 and 0.4 mg/kg), a NK(3)-R agonist, were tested in aged (23-25 month old) Wistar rats: (a) in an episodic-like memory test, using an object discrimination task (this is the first study to test for deficits in episodic-like memory in aged rats, since appropriate tests have only recently became available); (b) on parameters of anxiety in an open field test, (c) on indices of depression in the forced swimming test and (d) on the activity of cholinergic neurons of the basal forebrain, using in vivo microdialysis and HPLC. Neither the saline-, nor senktide-treated aged animals, exhibited episodic-like memory. However, the senktide-, but not the vehicle-treated group, exhibited object memory for spatial displacement, a component of episodic memory. Senktide injection also had anxiolytic- and antidepressant-like effects. Furthermore, the active doses of senktide on behavior increased ACh levels in the frontal cortex, amygdala and hippocampus, suggesting a relationship between its cholinergic and behavioral actions. The results indicate cholinergic modulation by the NK(3)-R in conjunction with a role in the processing of memory and emotional responses in the aged rat.

Spatial memory alterations by activation of septal 5HT 1A receptors: no implication of cholinergic septohippocampal neurons

INTRODUCTION

In rats, activation of medial septum (MS) 5-HT(1A) receptors with the 5-HT(1A)/5-HT(7) receptor agonist 8-OH-DPAT disrupts encoding and consolidation, but not retrieval of a spatial memory in the water maze task. These findings might be explained by an action of 8-OH-DPAT on 5-HT(1A) receptors located on cholinergic neurons which the drug could transiently hyperpolarise. If so, selective damage of these neurons should mimic the effects of 8-OH-DPAT, or, at least, synergistically interfere with them.

METHODS

To test this hypothesis, rats were subjected to intraseptal infusions of 8-OH-DPAT (or phosphate-buffered saline) during acquisition of a water maze task before and/or after 192 IgG-saporin-induced MS cholinergic lesion (vs. sham-operated).

RESULTS

We confirmed that only pre-acquisition intraseptal 8-OH-DPAT infusions prevented learning and subsequent drug-free retrieval of the platform location in intact rats and found that (1) the cholinergic lesion did not prevent recall of the platform location, and (2) the impairing effects of 8-OH-DPAT were similar in sham-operated and lesioned rats, whether naïve or not, to the task before lesion surgery.

CONCLUSIONS

An action of 8-OH-DPAT on only MS cholinergic neurons is not sufficient to account for the drug-induced memory impairments. A concomitant 8-OH-DPAT-induced hyperpolarisation of cholinergic and/or GABAergic and/or glutamatergic neurons (intact rats), or of only GABAergic and/or glutamatergic ones after cholinergic lesion, might be necessary to obliterate task acquisition, confirming that, in the MS, (1) the three neuronal populations could cooperate to process hippocampal-dependent information, and (2) non-cholinergic septohippocampal neurons might be more important than cholinergic ones in serotonin-induced modulation of hippocampus-dependent memory processing.

Neurobiological aspects of Alzheimer's disease

INTRODUCTION

The molecular pathogenesis of Alzheimer's disease (AD) includes a variety of risk factors, extracellular deposition of β-amyloid, accumulation of intracellular neurofibrillary tangles, oxidative neuronal damage and inflammatory cascades. Although amyloid-β-containing senile plaques and phospho-tau-containing neurofibrillary tangles are hallmark lesions of AD, neither is specific to nor even a marker of the disease. From a biochemical point of view the most consistent finding is a decreased level of choline acetyltransferase. In recent years, cumulative evidence has been gained on the involvement of neuronal lipoprotein activity, and on the role of cholesterol and other lipids in pathogenesis. Although basic research has made remarkable progress in the past two decades, currently available drugs are only able to improve cognitive symptoms temporarily and no treatment can reverse, stop or even slow this inexorable neurodegenerative process.

AREAS COVERED

The various neurobiological events associated with development of AD and the multiple treatment approaches for combating this disorder.

EXPERT OPINION

AD is a complex multifactorial disorder and thus a single target or pathogenic pathway is unlikely to be identified. Developing therapeutic interventions demands a greater understanding of the processes and the differential involvement of the various mediators. Effective therapeutics are urgently needed, and it is hoped that anti-amyloid strategies will offer a significant step towards a causal therapy.

Cholinergic influences on cortical development and adult neurogenesis

In this review, we focus on immature neurons and their regulation by the cholinergic system, both during cortical development as well as during adult neurogenesis. We discuss various studies that indicate roles for acetylcholine in precursor development and neuronal differentiation. Cholinergic neurons projecting from the basal forebrain innervate the cerebral cortex during critical periods of neuronal development. Acetylcholine stimulation may help to promote a favourable environment for neuronal maturation. Afferents and their cortical target cells interact and are likely to influence each other during the establishment and refinement of connections. Intracortical cholinergic interneurons similarly have a local effect on cortical circuits. Reduced cholinergic innervation during development hence leads to reduced cortical thickness and dendritic abnormalities. Acetylcholine is also likely to play a critical role in neuronal plasticity, as shown in the visual and barrel cortices. Spontaneous nicotinic excitation is also important during a brief developmental window in the first postnatal weeks leading to waves of neural activity, likely to have an effect on neurite extension, target selection and synaptogenesis. In the hippocampus such activity plays a role in the maturation of GABAergic synapses during the developmental shift from depolarizing to hyperpolarizing transmission. The cholinergic system also seems likely to regulate hippocampal neurogenesis in the adult, positively promoting proliferation, differentiation, integration and potentially survival of newborn neurons.

Estrogen therapy and cognition: a review of the cholinergic hypothesis

The pros and cons of estrogen therapy for use in postmenopausal women continue to be a major topic of debate in women's health. Much of this debate focuses on the potential benefits vs. harm of estrogen therapy on the brain and the risks for cognitive impairment associated with aging and Alzheimer's disease. Many animal and human studies suggest that estrogens can have significant beneficial effects on brain aging and cognition and reduce the risk of Alzheimer's-related dementia; however, others disagree. Important discoveries have been made, and hypotheses have emerged that may explain some of the inconsistencies. This review focuses on the cholinergic hypothesis, specifically on evidence that beneficial effects of estrogens on brain aging and cognition are related to interactions with cholinergic projections emanating from the basal forebrain. These cholinergic projections play an important role in learning and attentional processes, and their function is known to decline with advanced age and in association with Alzheimer's disease. Evidence suggests that many of the effects of estrogens on neuronal plasticity and function and cognitive performance are related to or rely upon interactions with these cholinergic projections; however, studies also suggest that the effectiveness of estrogen therapy decreases with age and time after loss of ovarian function. We propose a model in which deficits in basal forebrain cholinergic function contribute to age-related changes in the response to estrogen therapy. Based on this model, we propose that cholinergic-enhancing drugs, used in combination with an appropriate estrogen-containing drug regimen, may be a viable therapeutic strategy for use in older postmenopausal women with early evidence of mild cognitive decline.

Blockade of GABA(B) receptors completely reverses age-related learning impairment

Impaired cognitive functions are well-described in the aging process. GABA(B) antagonists can facilitate learning and memory in young subjects, but these agents have not been well-characterized in aging. Here we show a complete reversal of olfactory discrimination learning deficits in cognitively-impaired aged Fischer 344 rats using the GABA(B) antagonist CGP55845, such that drug treatment restored performance to that on par with young and cognitively-unimpaired aged subjects. There was no evidence that this improved learning was due to enhanced olfactory detection abilities produced by the drug. These results highlight the potential of targeting GABA(B) receptors to ameliorate age-related cognitive deficits and demonstrate the utility of olfactory discrimination learning as a preclinical model for testing novel therapies to improve cognitive functions in aging.

Enhanced calcium buffering in F344 rat cholinergic basal forebrain neurons is associated with age-related cognitive impairment

Alterations in neuronal Ca(2+) homeostasis are important determinants of age-related cognitive impairment. We examined the Ca(2+) influx, buffering, and electrophysiology of basal forebrain neurons in adult, middle-aged, and aged male F344 behaviorally assessed rats. Middle-aged and aged rats were characterized as cognitively impaired or unimpaired by water maze performance relative to young cohorts. Patch-clamp experiments were conducted on neurons acutely dissociated from medial septum/nucleus of the diagonal band with post hoc identification of phenotypic marker mRNA using single-cell RT-PCR. We measured whole cell calcium and barium currents and dissected these currents using pharmacological agents. We combined Ca(2+) current recording with Ca(2+)-sensitive ratiometric microfluorimetry to measure Ca(2+) buffering. Additionally, we sought changes in neuronal firing properties using current-clamp recording. There were no age- or cognition-related changes in the amplitudes or fractional compositions of the whole cell Ca(2+) channel currents. However, Ca(2+) buffering was significantly enhanced in cholinergic neurons from aged cognitively impaired rats. Moreover, increased Ca(2+) buffering was present in middle-aged rats that were not cognitively impaired. Firing properties were largely unchanged with age or cognitive status, except for an increase in the slow afterhyperpolarization in aged cholinergic neurons, independent of cognitive status. Furthermore, acutely dissociated basal forebrain neurons in which choline acetyltransferase mRNA was detected had the electrophysiological profiles of identified cholinergic neurons. We conclude that enhanced Ca(2+) buffering by cholinergic basal forebrain neurons may be important during aging.

Age-dependent changes in calretinin immunoreactivity and its protein level in the gerbil hippocampus

Calretinin (CR)-immunoreactive interneurons are well known as the interneuron specific interneurons in the hippocampus. CR-immunoreactive neurons form cellular network and regulate the activity of other GABAergic inhibitory interneurons in the hippocampus. In the present study, we investigated age-related changes in CR-immunoreactive neurons and protein levels in the gerbil hippocampus during normal aging. In all subregions of the gerbil hippocampus, the number of CR-immunoreactive neurons was significantly decreased in the postnatal month 6 (PM 6) group compared to that in the PM 1 group. Thereafter, CR-immunoreactive neurons were decreased with age. In addition, the number of CR-immunoreactive cells in the subgranular zone were significantly decreased in the PM 6 group. We also observed that CR protein levels were decreased gradually with age. These results indicate that both CR immunoreactivity and its protein level were decreased with age in the gerbil hippocampus during normal aging.

Donepezil treatment restores the ability of estradiol to enhance cognitive performance in aged rats: evidence for the cholinergic basis of the critical period hypothesis

Recent studies suggest that the ability of estradiol to enhance cognitive performance diminishes with age and/or time following loss of ovarian function. We hypothesize that this is due, in part, to a decrease in basal forebrain cholinergic function. This study tested whether donepezil, a cholinesterase inhibitor, could restore estradiol effects on cognitive performance in aged rats that had been ovariectomized as young adults. Rats were ovariectomized at 3 months of age, and then trained on a delayed matching to position (DMP) T-maze task, followed by a configural association (CA) operant condition task, beginning at 12-17 or 22-27 months of age. Three weeks prior to testing, rats started to receive either donepezil or vehicle. After one week, half of each group also began receiving estradiol. Acclimation and testing began seven days later and treatment continued throughout testing. Estradiol alone significantly enhanced DMP acquisition in middle-aged rats, but not in aged rats. Donepezil alone had no effect on DMP acquisition in either age group; however, donepezil treatment restored the ability of estradiol to enhance DMP acquisition in aged rats. This effect was due largely to a reduction in the predisposition to adopt a persistent turn strategy during acquisition. These same treatments did not affect acquisition of the CA task in middle-aged rats, but did have small but significant effects on response time in aged rats. The data are consistent with the idea that estrogen effects on cognitive performance are task specific, and that deficits in basal forebrain cholinergic function are responsible for the loss of estradiol effect on DMP acquisition in aged ovariectomized rats. In addition, the data suggest that enhancing cholinergic function pharmacologically can restore the ability of estradiol to enhance acquisition of the DMP task in very old rats following long periods of hormone deprivation. Whether donepezil has similar restorative effects on other estrogen-sensitive tasks needs to be explored.

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.

Age-related alterations of Nestin-immunoreactive neurons in rat basal forebrain with aged memory deficit

Age-related and aged memory deficit changes in Nestin-immunoreactive (Nestin-IR) neurons were studied following recent evidence of distinct Nestin-IR neurons within adult rat basal forebrain. Morris water maze task assessed spatial learning capacity of 3- and 24-month rats (aged-impaired and aged-unimpaired groups). Nestin-IR neuron distributional and morphological features were investigated by immunohistochemistry and positive neuronal number calculation. Nestin-IR neuron number declined with aging, especially aged-impaired. Significant negative correlations existed between average escape latencies and Nestin-IR neuron number in medial septum-diagonal band of Broca (MS-DBB). Correlations of rostral portion [medial septum (MS) and vertical limb diagonal band (vDB)] were higher than caudal portion [horizontal limb diagonal band (hDB)]. Aged-impaired showed reduced complexity of Nestin-IR neuron dendrite arborization and dendritic length. Nestin-IR astrocyte-like cells appeared scattered among Nestin-IR neurons on some aged-impaired slices. In conclusion, aged-impaired rats showed worse cognitive spatial performance and less Nestin-IR neuronal number compared to aged-unimpaired. Nestin-IR neuronal loss and morphological changes are some pathological characteristics of rat aged basal forebrain and may be important in neurobiological mechanisms of brain aging and aged memory deficit.

Functional convergence of dopaminergic and cholinergic input is critical for hippocampus-dependent working memory

Although Parkinson's disease is a movement disorder, in many patients cognitive dysfunction is an important clinical sign. It is not yet clear whether this is attributable solely to a decrease in dopamine levels, or whether other neurotransmitter systems might be involved as well. In the present study, the importance of the mesocorticolimbic dopamine pathway and a possible convergence with forebrain cholinergic projections to neocortex and hippocampus in the regulation of learning and memory abilities were investigated by using specific lesion paradigms in one or both systems. Lesioning of dopaminergic neurons in the ventral tegmental area resulted in an impaired performance in the reference memory task, whereas the execution of the working memory tasks appeared to be unaffected in the Morris water maze. Analysis of the swim paths revealed that the dopamine-depleted animals were capable of adapting a search strategy on a given testing day but failed to transfer this information to the next day, suggesting a deficit in information storage and/or recall. In contrast, cholinergic lesions alone were without effect in all test paradigms. However, when both dopamine and acetylcholine were depleted, animals were also impaired in the working memory task, indicating that a functional convergence of the inputs from these systems was critical for acquisition of spatial memory. Interestingly, such an additional acquisition deficit appeared only after hippocampal cholinergic depletion regardless of a concurrent disruption of basalo cortical cholinergic afferents. Thus, further analyses of cholinergic alterations may prove useful in better understanding the cognitive symptoms in Parkinson's disease.

Specific mechanism for blood inflow stimulation in brain area prone to Alzheimer's disease lesions

The present study describes the specific two-stage mechanism that intensifies blood supply to the brain area comprising amygdala, hippocampus, olfactory bulb, entorhinal cortex, and neocortex (AHBC). Cholinergic neurons from the nuclei of basal forebrain induce vasodilatory effect through release of acetylcholine. In physiological aging the efficacy of this neuronal system declines, while intensive formation of amyloidogenic peptides starts. These peptides at low, picomolar concentrations activate alpha7 nicotinic acetylcholine receptors, thus enhancing angiogenesis and in so doing restoring blood supply to the AHBC area.

Calcium buffering systems and calcium signaling in aged rat basal forebrain neurons

Disturbances of neuronal Ca2+ homeostasis are considered to be important determinants of age-related cognitive impairment. Cholinergic neurons of the basal forebrain (BF) are principal targets of decline associated with aging and dementia. During the last several years, we have attempted to link these concepts in a rat model of 'normal' aging. In this review, we will describe some changes that we have observed in Ca2+ signaling of aged BF neurons and the reversal of one of these changes by dietary caloric restriction. Our evidence supports a scenario in which subtle changes in the properties of voltage-gated Ca2+ channels result in increased Ca2+ influx during aging. This increased Ca2+, in turn, triggers an increase in rapid Ca2+ buffering in the somatic compartment of aged BF neurons. However, this nominal 'compensation', along with other changes in Ca2+ handling machinery (notably mitochondria) alters the Ca2+ signal with age in a way that is dependent on the magnitude of the Ca2+ load. By combining whole-cell patch clamp electrophysiology, ratiometric Ca2+-sensitive microfluorimetry and single-cell reverse transcription-polymerase chain reaction, we have determined that age-related rapid buffering changes are present in identified cholinergic BF neurons and that these changes can be prevented by a caloric restriction dietary regimen. Because caloric restriction extends lifespan and retards the progression of age-related dysfunction, these findings suggest that increased Ca2+ buffering in cholinergic neurons may be relevant to cognitive decline during normal aging. Importantly, calcium homeostatic mechanisms of BF cholinergic neurons are amenable to dietary interventions that could promote cognitive health during aging.

Combined damage to entorhinal cortex and cholinergic basal forebrain neurons, two early neurodegenerative features accompanying Alzheimer's disease: effects on locomotor activity and memory functions in rats

In Alzheimer's disease (AD), cognitive decline is linked to cholinergic dysfunctions in the basal forebrain (BF), although the earliest neuronal damage is described in the entorhinal cortex (EC). In rats, selective cholinergic BF lesions or fiber-sparing EC lesions may induce memory deficits, but most often of weak magnitude. This study investigated, in adult rats, the effects on activity and memory of both lesions, alone or in combination, using 192 IgG-saporin (OX7-saporin as a control) and L-N-methyl-D-aspartate to destroy BF and EC neurons, respectively. Rats were tested for locomotor activity in their home cage and for working- and/or reference-memory in various tasks (water maze, Hebb-Williams maze, radial maze). Only rats with combined lesions showed diurnal and nocturnal hyperactivity. EC lesions impaired working memory and induced anterograde memory deficits in almost all tasks. Lesions of BF cholinergic neurons induced more limited deficits: reference memory was impaired in the probe trial of the water-maze task and in the radial maze. When both lesions were combined, performance never improved in the water maze and the number of errors in the Hebb-Williams and the radial mazes was always larger than in any other group. These results (i) indicate synergistic implications of BF and EC in memory function, (ii) suggest that combined BF cholinergic and fiber-sparing EC lesions may model aspects of anterograde memory deficits and restlessness as seen in AD, (iii) challenge the cholinergic hypothesis of cognitive dysfunctions in AD, and (iv) contribute to open theoretical views on AD-related memory dysfunctions going beyond the latter hypothesis.

Aging causes partial loss of basal forebrain but no loss of pontine reticular cholinergic neurons

Cholinergic degeneration occurs in several neurodegenerative diseases. To investigate whether normal aging causes selective neurodegeneration, we compared counts of cholinergic neurons in the medial septum/vertical limb of the diagonal band and pedunculopontine and laterodorsal tegmental nuclei of the brainstem in young and aged Long-Evans rats characterized for their spatial learning ability in the Morris water maze. A subset of aged rats (aged-unimpaired) learned the spatial learning task as young rats, whereas another group (age-impaired) showed poorer learning than young animals. In the medial septum/diagonal band, there was a significant loss (-23%, P < 0.02) of cholinergic neurons in aged-impaired animals compared with young subjects. In the brainstem, there were no significant differences in cholinergic cell number in any group. This selective loss of cholinergic neurons may, in part, account for the cognitive deficits observed in aging and, considering previous findings in this model, may be related to oxidative stress.

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.

Nerve growth factor in treatment and pathogenesis of Alzheimer's disease

The etiology of Alzheimer's disease (AD) is still unknown. In addition, this terrible neurodegenerative disease will increase exponentially over the next two decades due to longer lifespan and an aging "baby-boomer" generation. All treatments currently approved for AD have moderate efficacy in slowing the rate of cognitive decline in patients, and no efficacy in halting progression of the disease. Hence, there is an urgent need for new drug targets and delivery methods to slow or reverse the progression of AD. One molecule that has received much attention in its potential therapeutic role in AD is nerve growth factor (NGF). This review will demonstrate data from humans and animals which promote NGF as a potential therapeutic target by (1) outlining the hypothesis behind using NGF for the treatment of AD, (2) reviewing both the normal and AD altered signaling pathways and effects of NGF in the central nervous system (CNS), and (3) examining the results of NGF treatment obtained from animal models of AD and AD patients.

Muscarinic receptor-mediated GTP-Eu binding in the hippocampus and prefrontal cortex is correlated with spatial memory impairment in aged rats

The present study examined muscarinic receptor/G-protein coupling in the hippocampus and the prefrontal cortex of young and aged Long-Evans rats characterized for spatial learning ability in the Morris water maze. In a highly sensitive time-resolved fluorometry GTP-Eu binding assay, muscarinic-mediated GTP-Eu binding was severely blunted in hippocampus (-32%) and prefrontal cortex (-34%) as a consequence of aging. Furthermore, the magnitude of decreased muscarinic-mediated GTP-Eu binding was significantly correlated with the severity of spatial learning impairment in hippocampus and prefrontal cortex of aged rats and was specifically decreased in the subset of aged rats that were spatial learning impaired when compared to the aged unimpaired and the young rats. Western blot data indicated a preservation of the membrane-bound M1 receptor and the Galphaq/11 protein in both brain regions. These data demonstrate that muscarinic signaling is severely impaired as a consequence of normal aging in a manner that is closely associated with age-related cognitive decline.

Maturation and maintenance of cholinergic medial septum neurons require glucocorticoid receptor signaling

Glucocorticoids have been shown to influence trophic processes in the nervous system. In particular, they seem to be important for the development of cholinergic neurons in various brain regions. Here, we applied a genetic approach to investigate the role of the glucocorticoid receptor (GR) on the maturation and maintenance of cholinergic medial septal neurons between P15 and one year of age by using a mouse model carrying a CNS-specific conditional inactivation of the GR gene (GRNesCre). The number of choline acetyltransferase and p75NTR immuno-positive neurons in the medial septum (MS) was analyzed by stereology in controls versus mutants. In addition, cholinergic fiber density, acetylcholine release and cholinergic key enzyme activity of these neurons were determined in the hippocampus. We found that in GRNesCre animals the number of medial septal cholinergic neurons was significantly reduced during development. In addition, cholinergic cell number further decreased with aging in these mutants. The functional GR gene is therefore required for the proper maturation and maintenance of medial septal cholinergic neurons. However, the loss of cholinergic neurons in the medial septum is not accompanied by a loss of functional cholinergic parameters of these neurons in their target region, the hippocampus. This pinpoints to plasticity of the septo-hippocampal system, that seems to compensate for the septal cell loss by sprouting of the remaining neurons.

Exacerbated loss of cell survival, neuropeptide Y-immunoreactive (IR) cells, and serotonin-IR fiber lengths in the dorsal hippocampus of the aged flinders sensitive line “depressed” rat: Implications for the pathophysiology of depression?

Impairment of hippocampal neurogenesis has been proposed to provide a cellular basis for the development of major depression. Studies have shown that serotonin (5-HT) and neuropeptide Y (NPY) may be involved in stimulating cell proliferation in the dentate gyrus. The Flinders-sensitive line (FSL) rat represents a genetic model of depression with characterized 5-HT and NPY abnormalities in the hippocampus. Consequently, it could be hypothesized that hippocampal neurogenesis in the FSL rat would be impaired. The present study examined the relationship among 1) number of BrdU-immunoreactive (IR) cells, 2) NPY-IR cells in the dentate gyrus, and 3) length of 5-HT-IR fibers in the dorsal hippocampus, as well as volume and number of 5-HT-IR cells in the dorsal raphé nucleus, in adult and aged FSL rats and control Flinders-resistant line (FRL) rats. Surprisingly, adult FSL rats had significantly more BrdU-IR and NPY-IR cells compared with adult FRL rats. However, aging caused an exacerbated loss of these cell types in the FSL strain compared with FRL. The aged FSL rats also had shortened 5-HT-IR fibers in the dorsal hippocampus, indicative of an impaired 5-HT innervation of this area, compared with FRL. These results suggest that, for "depressed" FSL rats, compared with FRL rats, aging is associated with an excacerbated loss of newly formed cells in addition to NPY-IR cells and 5-HT-IR dendrites in the hippocampus. These observations may be of relevance to the depression-like behavior of the FSL rat and, by inference, to the pathophysiology of depression.

Input from the medial septum regulates adult hippocampal neurogenesis

Neural progenitors in the subgranular zone of the hippocampal formation form a continuously proliferating cell population, generating new granule neurons throughout adult life. Between 10 days and 1 month after their formation, many of the newly generated cells die. The present study investigated whether a partial lesion of one of the main nuclei projecting to the hippocampus, the medial septum (MS), affects survival and differentiation of cells during this critical period. Rats were injected with BrdU and 5 days later excitotoxic lesion of the MS was applied by infusion of either 30 or 60 nmol of N-methyl-D-aspartate (NMDA). One week after the lesion, quantification of immunopositive cells revealed that the number of GABAergic cells was significantly reduced in both lesioned groups, whereas a decline in cholinergic cell number was observed only after injection of 60 nmol of NMDA. The partial septohippocampal denervation significantly reduced hippocampal neurogenesis. Survival of newly generated neurons was decreased by approximately 40%. The MS lesion did not affect proliferation of hippocampal progenitors. The present study points out the importance of a functional septohippocampal pathway for the regulation of hippocampal neurogenesis and highlights the potential role of GABA as a mediator in this phenomenon.

ELISA methods to measure cholinergic markers and nerve growth factor receptors in cortex, hippocampus, prefrontal cortex, and basal forebrain from rat brain

The central cholinergic system has a fundamental role in normal cognitive function, and in diseases that exhibit cognitive dysfunction. The purpose of this study was to design ELISA methods to measure proteins that have essential functions in the central cholinergic system. We were particularly interested in quantifying proteins that respond directly or indirectly to nerve growth factor (NGF). ELISAs offer advantages over Western blot analyses and other methods, such as increased sensitivity, decreased assay variability, increased efficiency, and decreased cost. We developed indirect ELISA methods for: choline acetyltransferase (ChAT); the vesicular acetylcholine transporter (VAChT); the high affinity choline transporter (HACT/CHT); TrkA, the high affinity NGF receptor; the p75 neurotrophin receptor (p75(NTR)). A sandwich ELISA was developed to measure tyrosine-phosphorylated TrkA in brain lysates. We used these ELISAs to compare levels of the above proteins in important memory-related brain regions--basal forebrain, hippocampus, cortex, and prefrontal cortex--from old and young rats. We identified age-related differences in the levels of the aforementioned proteins (e.g., VAChT and HACT/CHT in hippocampus). Thus, these ELISA methods should be particularly useful for comparing the effects of age, disease, drugs, and toxicants on brain levels of key cholinergic and growth factor-related proteins.

Glutamic acid decarboxylase isoforms are differentially distributed in the septal region of the rat

The septal region of the brain consists of a heterogeneous population of GABAergic neurons that play an important role in the generation of hippocampal theta rhythms. While GABAergic neurons employ two isoforms of the enzyme glutamic acid decarboxylase (GAD) for the synthesis of GABA, distribution of GAD isoforms has not been investigated in the septum. Immunohistochemical techniques were used to investigate the expression of GAD enzymes in medial and lateral septum. GAD65 and GAD67 immunohistochemistry revealed dense fibers and punctuated immunoreactivity in septal regions. While few GAD65-positive neuronal somas were detected in medial septum, a significantly higher number of immunoreactive neurons were detected in lateral septum. GAD65- and GAD67-positive neurons in the lateral septum exhibit higher complexity of dendritic arborizations than in the medial septum where staining was mainly restricted to the soma. Presumptive axon terminals (puncta) showed abundant immunoreactivity predominantly for GAD65 isoforms in all septal regions. This suggests that septal GABAergic neurons differentially express GAD enzymes thereby potentially reflecting functional differences. Differences found between medial and lateral septal GABAergic neuronal populations are in agreement with the concept that medial and lateral septum are brain structures with highly different connectivity and function despite anatomical proximity.