Reduced retrograde labelling with fluorescent tracer accompanies neuronal atrophy of basal forebrain cholinergic neurons in aged rats

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.

Enhanced TrkA signaling impairs basal forebrain-dependent behavior

Basal forebrain cholinergic neurons (BFCNs) modulate cognitive functions such as attention, learning and memory. The NGF/TrkA pathway plays an important role in the development and function of BFCNs, although two mouse models conditionally deleting TrkA expression in the central nervous system (CNS) have shown contradictory results. To shed light into this discrepancy, we used a mouse model with a gain-of-function in TrkA receptor signaling. Our results indicate that enhanced TrkA signaling did not alter hippocampal cholinergic innervation, general locomotion or anxiety-related behaviors, but it increases ChAT expression, the number of cholinergic neurons at early postnatal stages and, mutant mice showed impaired motor learning and memory functions. These data demonstrate that proper functioning of the cholinergic system in CNS requires a balanced NGF/TrkA signaling.

Age-related changes in basal forebrain afferent activation in response to food paired stimuli

The basal forebrain contains a phenotypically-diverse assembly of neurons, including those using acetylcholine as their neurotransmitter. This basal forebrain cholinergic system projects to the entire neocortical mantle as well as subcortical limbic structures that include the hippocampus and amygdala. Basal forebrain pathology, including cholinergic dysfunction, is thought to underlie the cognitive impairments associated with several age-related neurodegenerative conditions, including Alzheimer's disease. Basal forebrain dysfunction may stem, in part, from a failure of normal afferent regulation of cholinergic and other neurons in this area. However, little is understood regarding how aging, alone, affects the functional regulation of basal forebrain afferents in the context of motivated behavior. Here, we used neuronal tract-tracing combined with motivationally salient stimuli in an aged rodent model to examine how aging alters activity in basal forebrain inputs arising from several cortical, limbic and brainstem structures. Young rats showed greater stimulus-associated activation of basal forebrain inputs arising from prelimbic cortex, nucleus accumbens and the ventral tegmental area compared with aged rats. Aged rats also showed increased latency to respond to palatable food presentation compared to young animals. Changes in activation of intrinsic basal forebrain cell populations or afferents were also observed as a function of age or experimental condition. These data further demonstrate that age-related changes in basal forebrain activation and related behavioral and cognitive functions reflect a failure of afferent regulation of this important brain region.

Fast Blue and Cholera Toxin-B Survival Guide for Alpha-Motoneurons Labeling: Less Is Better in Young B6SJL Mice, but More Is Better in Aged C57Bl/J Mice

Fast Blue (FB) and Cholera Toxin-B (CTB) are two retrograde tracers extensively used to label alpha-motoneurons (α-MNs). The overall goals of the present study were to (1) assess the effectiveness of different FB and CTB protocols in labeling α-MNs, (2) compare the labeling quality of these tracers at standard concentrations reported in the literature (FB 2% and CTB 0.1%) versus lower concentrations to overcome tracer leakage, and (3) determine an optimal protocol for labeling α-MNs in young B6SJL and aged C57Bl/J mice (when axonal transport is disrupted by aging). Hindlimb muscles of young B6SJL and aged C57Bl/J mice were intramuscularly injected with different FB or CTB concentrations and then euthanized at either 3 or 5 days after injection. Measurements were performed to assess labeling quality via seven different parameters. Our results show that tracer protocols of lower concentration and shorter labeling durations were generally better in labeling young α-MNs, whereas tracer protocols of higher tracer concentration and longer labeling durations were generally better in labeling aged α-MNs. A 0.2%, 3-day FB protocol provided optimal labeling of young α-MNs without tracer leakage, whereas a 2%, 5-day FB protocol or 0.1% CTB protocol provided optimal labeling of aged α-MNs. These results inform future studies on the selection of optimal FB and CTB protocols for α-MNs labeling in normal, aging, and neurodegenerative disease conditions.

Effects of aging on the cholinergic innervation of the rat ventral tegmental area: A stereological study

Dopamine neurons in the ventral tegmental area (VTA) play a main role in processing both rewarding and aversive stimuli, and their response to salient stimuli is significantly shaped by afferents originating in the brainstem cholinergic nuclei. Aging is associated with a decline in dopaminergic activity and reduced response to positive reinforcement. We have used stereological techniques to examine, in adult and aged rats, the dopaminergic neurons and the cholinergic innervation of the VTA, and the cholinergic populations of the pedunculopontine tegmental (PPT) and laterodorsal tegmental (LDT) nuclei, which are the only source of cholinergic inputs to the VTA. In the VTA, there were no age-related variations in the number and size of tyrosine hydroxylase (TH)-immunoreactive neurons, but the density of cholinergic varicosities was reduced in aged rats. The total number of choline acetyltransferase (ChAT)-immunoreactive neurons in the PPT and LDT was unchanged, but their somas were hypertrophied in aged rats. Our results suggest that dysfunction of the cholinergic system might contribute for the age-associated deterioration of the brain reward system.

Basal forebrain volume reliably predicts the cortical spread of Alzheimer's degeneration

Alzheimer's disease neurodegeneration is thought to spread across anatomically and functionally connected brain regions. However, the precise sequence of spread remains ambiguous. The prevailing model used to guide in vivo human neuroimaging and non-human animal research assumes that Alzheimer's degeneration starts in the entorhinal cortices, before spreading to the temporoparietal cortex. Challenging this model, we previously provided evidence that in vivo markers of neurodegeneration within the nucleus basalis of Meynert (NbM), a subregion of the basal forebrain heavily populated by cortically projecting cholinergic neurons, precedes and predicts entorhinal degeneration. There have been few systematic attempts at directly comparing staging models using in vivo longitudinal biomarker data, and none to our knowledge testing if comparative evidence generalizes across independent samples. Here we addressed the sequence of pathological staging in Alzheimer's disease using two independent samples of the Alzheimer's Disease Neuroimaging Initiative (n1 = 284; n2 = 553) with harmonized CSF assays of amyloid-β and hyperphosphorylated tau (pTau), and longitudinal structural MRI data over 2 years. We derived measures of grey matter degeneration in a priori NbM and the entorhinal cortical regions of interest. To examine the spreading of degeneration, we used a predictive modelling strategy that tests whether baseline grey matter volume in a seed region accounts for longitudinal change in a target region. We demonstrated that predictive spread favoured the NbM→entorhinal over the entorhinal→NbM model. This evidence generalized across the independent samples. We also showed that CSF concentrations of pTau/amyloid-β moderated the observed predictive relationship, consistent with evidence in rodent models of an underlying trans-synaptic mechanism of pathophysiological spread. The moderating effect of CSF was robust to additional factors, including clinical diagnosis. We then applied our predictive modelling strategy to an exploratory whole-brain voxel-wise analysis to examine the spatial specificity of the NbM→entorhinal model. We found that smaller baseline NbM volumes predicted greater degeneration in localized regions of the entorhinal and perirhinal cortices. By contrast, smaller baseline entorhinal volumes predicted degeneration in the medial temporal cortex, recapitulating a prior influential staging model. Our findings suggest that degeneration of the basal forebrain cholinergic projection system is a robust and reliable upstream event of entorhinal and neocortical degeneration, calling into question a prevailing view of Alzheimer's disease pathogenesis.

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.

Retrograde axonal transport of BDNF and proNGF diminishes with age in basal forebrain cholinergic neurons

Basal forebrain cholinergic neurons (BFCNs) are critical for learning and memory and degenerate early in Alzheimer's disease (AD). BFCNs depend for their survival and function on nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), which are retrogradely transported from BFCN targets. Age is the greatest risk factor for developing AD, yet the influence of age on BFCN axonal transport is poorly understood. To model aging, embryonic rat basal forebrain or cortical neurons were cultured in microfluidic chambers. Senescence-associated beta-galactosidase staining indicated an aging phenotype only in BFCNs cultured for 18+ days in vitro. BDNF axonal transport impairments were observed exclusivley in aged BFCNs. BFCNs displayed robust proNGF transport, which also diminished with in vitro age. The expression of NGF receptor tropomyosin-related kinase-A and BDNF receptor tropomyosin-related kinase-B also decreased significantly with in vitro age in BFCNs only. These results suggest a unique vulnerability of BFCNs to age-induced transport deficits. These deficits, coupled with the reliance of BFCNs on neurotrophin transport, may explain their vulnerability to age-related neurodegenerative disorders like AD.

Area postrema undergoes dynamic postnatal changes in mice and humans

The postnatal period in mammals represents a developmental epoch of significant change in the autonomic nervous system (ANS). This study focuses on postnatal development of the area postrema, a crucial ANS structure that regulates temperature, breathing, and satiety, among other activities. We find that the human area postrema undergoes significant developmental changes during postnatal development. To characterize these changes further, we used transgenic mouse reagents to delineate neuronal circuitry. We discovered that, although a well-formed ANS scaffold exists early in embryonic development, the area postrema shows a delayed maturation. Specifically, postnatal days 0-7 in mice show no significant change in area postrema volume or synaptic input from PHOX2B-derived neurons. In contrast, postnatal days 7-20 show a significant increase in volume and synaptic input from PHOX2B-derived neurons. We conclude that key ANS structures show unexpected dynamic developmental changes during postnatal development. These data provide a basis for understanding ANS dysfunction and disease predisposition in premature and postnatal humans.

NGF in Early Embryogenesis, Differentiation, and Pathology in the Nervous and Immune Systems

The physiology of NGF is extremely complex, and although the study of this neurotrophin began more than 60 years ago, it is far from being concluded. NGF, its precursor molecule pro-NGF, and their different receptor systems (i.e., TrkA, p75NTR, and sortilin) have key roles in the development and adult physiology of both the nervous and immune systems. Although the NGF receptor system and the pathways activated are similar for all types of cells sensitive to NGF, the effects exerted during embryonic differentiation and in committed mature cells are strikingly different and sometimes opposite. Bearing in mind the pleiotropic effects of NGF, alterations in its expression and synthesis, as well as variations in the types of receptor available and in their respective levels of expression, may have profound effects and play multiple roles in the development and progression of several diseases. In recent years, the use of NGF or of inhibitors of its receptors has been prospected as a therapeutic tool in a variety of neurological diseases and injuries. In this review, we outline the different roles played by the NGF system in various moments of nervous and immune system differentiation and physiology, from embryonic development to aging. The data collected over the past decades indicate that NGF activities are highly integrated among systems and are necessary for the maintenance of homeostasis. Further, more integrated and multidisciplinary studies should take into consideration these multiple and interactive aspects of NGF physiology in order to design new therapeutic strategies based on the manipulation of NGF and its intracellular pathways.

Lifelong environmental enrichment in rats: impact on emotional behavior, spatial memory vividness, and cholinergic neurons over the lifespan

We assessed lifelong environmental enrichment effects on possible age-related modifications in emotional behaviors, spatial memory acquisition, retrieval of recent and remote spatial memory, and cholinergic forebrain systems. At the age of 1 month, Long-Evans female rats were placed in standard or enriched rearing conditions and tested after 3 (young), 12 (middle-aged), or 24 (aged) months. Environmental enrichment decreased the reactivity to stressful situations regardless of age. In the water maze test, it delayed the onset of learning deficits and prevented age-dependent spatial learning and recent memory retrieval alterations. Remote memory retrieval, which was altered independently of age under standard rearing conditions, was rescued by enrichment in young and middle-aged, but unfortunately not aged rats. A protected basal forebrain cholinergic system, which could well be one out of several neuronal manifestations of lifelong environmental enrichment, might have contributed to the behavioral benefits of this enrichment.

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.

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.

Changes with aging in the dopaminergic and noradrenergic innervation of rat neocortex

In normal aging, the mammalian cortex undergoes significant remodeling. Although neuromodulation by dopamine and noradrenaline in the cortex is known to be important for proper cognitive function, little is known on how cortical noradrenergic and dopaminergic presynaptic boutons are affected in normal aging. Using rats we investigated whether these two neurotransmitter systems undergo structural reorganization in aging, and if these changes correlated with cognitive loss. Young and aged rats were tested for cognitive performance using the Morris water maze. Following the behavioral characterization, the animals were sacrificed and the cortical tissue was processed for immunofluorescence using antibodies directed against tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) to detect and discriminate noradrenergic and dopaminergic varicosities. We observed a significant increase in dopaminergic varicosities in lamina V of the anterior cingulate cortex (ACC) of aged cognitively unimpaired rats when compared to young and aged-impaired animals. In laminae II and III of the ACC, we observed a significant decrease of dopaminergic varicosities in aged-impaired animals when compared to young or aged cognitively unimpaired animals. Changes in noradrenergic varicosities never reached statistical significance in any group or brain region. The data suggests that the remodeling of mesocortical dopaminergic fibers may participate in age-associated cognitive decline.

Dynein dysfunction induces endocytic pathology accompanied by an increase in Rab GTPases: a potential mechanism underlying age-dependent endocytic dysfunction

Growing evidence suggests that endocytic dysfunction is intimately involved in early stage Alzheimer disease pathology, such as the accumulation of beta-amyloid precursor protein in enlarged early endosomes. However, it remains unclear how endocytic dysfunction is induced in an age-dependent manner. Cytoplasmic dynein, a microtubule-based motor protein, interacts with another microtubule-associated protein, dynactin. The resulting dynein-dynactin complex mediates minus end-directed vesicle transport, including endosome trafficking. We have previously shown that the interaction between dynein-dynactin complexes is clearly attenuated in aged monkey brains, suggesting that dynein-mediated transport dysfunction exists in aged brains. Our immunohistochemical analyses revealed that age-dependent endocytic pathology was accompanied by an increase in Rab GTPases in aged monkey brains. Here, we demonstrated that siRNA-induced dynein dysfunction reproduced the endocytic pathology accompanied by increased Rab GTPases seen in aged monkey brains and significantly disrupted exosome release. Moreover, it also resulted in endosomal beta-amyloid precursor protein accumulation characterized by increased beta-site cleavage. These findings suggest that dynein dysfunction may underlie age-dependent endocytic dysfunction via the up-regulation of Rab GTPases. In addition, this vicious circle may worsen endocytic dysfunction, ultimately leading to Alzheimer disease pathology.

Ceruloplasmin in neurodegenerative diseases

Two decades ago, patients lacking circulating serum ceruloplasmin (Cp) presented with neurodegeneration associated with brain iron accumulation. These patients, with mutations in the MCO (multi-copper oxidase), Cp, revealed an essential role for Cp in iron homoeostasis. The patients were diagnosed in adulthood with CNS (central nervous system) disease and progressed rapidly, making understanding the mechanism of disease imperative. We now know that (i) Cp regulates the efficiency of iron efflux, (ii) Cp stabilizes ferroportin membrane expression, (iii) GPI (glycosylphosphatidylinositol)-linked Cp is the predominant form expressed in brain, (iv) Cp functions as a ferroxidase and regulates the oxidation of Fe(2+) to Fe(3+), (v) Cp does not bind to transferrin directly, and (vi) Cp is one member of a family of mammalian MCOs, which includes hephaestin. It is still unclear how an absence of Cp results in neurodegeneration: is the iron accumulation a primary or secondary injury? Although it is attractive to invoke an iron-mediated oxidative stress mechanism for the neuronal injury and degeneration in aceruloplasminaemia, our data suggest limited redox injury in the brains of mice lacking MCO. In fact, we propose a role for neuronal iron starvation with associated astrocyte and microglial iron overload. With the defect in aceruloplasminaemia being one of inefficient iron efflux from macrophages, we believe that the iron is trapped in a compartment not readily available to participate in oxyradical injury. It is likely that different mechanisms of neuronal cell protection are offered by astrocytes and microglia, and, once these cells are damaged, neuronal survival is compromised.

Long-term reversal of cholinergic neuronal decline in aged non-human primates by lentiviral NGF gene delivery

Spontaneous atrophy of basal forebrain cholinergic neurons occurs with aging in the non-human primate brain. Short-term reversal of this atrophy has been reported following ex vivo nerve growth factor (NGF) gene delivery, but long-term effects of in vivo NGF gene delivery in the aged primate brain have not to date been examined. We tested the hypothesis that long-term lentiviral NGF intraparenchymal gene delivery would reverse age-related cholinergic decline, without induction of adverse effects previously observed following sustained intracerebroventricular growth factor protein exposure. Three aged rhesus monkeys underwent intraparenchymal lentiviral NGF gene delivery to the cholinergic basal forebrain. 1 year later, cholinergic neuronal numbers were quantified stereologically and compared to findings in four controls, non-treated aged monkeys and four young adult monkeys. Safety was assessed on several variables related to growth factor exposure. We now report that lentiviral gene delivery of NGF to the aged primate basal forebrain sustains gene expression for at least 1 year, and significantly restores cholinergic neuronal markers to levels of young monkeys. Aging resulted in a significant 17% reduction (p<0.05) in the number of neurons labeled for the cholinergic marker p75 among basal forebrain neurons. Lentiviral NGF gene delivery induced significant (p<0.05) and nearly complete recovery of p75-labeled neuronal numbers in aged subjects to levels observed in young monkeys. Similarly, the size of cholinergic neurons in aged monkeys was significantly reduced by 16% compared to young subjects (p<0.05), and lentiviral NGF delivery to aged subjects induced complete recovery of neuronal size. Intraparenchymal NGF gene delivery over a one-year period did not result in systemic leakage of NGF, activation of inflammatory markers in the brain, pain, weight loss, Schwann cell migration, or formation of anti-NGF antibodies. These findings indicate that extended trophic support to neurons in the non-human primate brain reverses age-related neuronal atrophy. These findings also support the safety and feasibility of lentiviral NGF gene transfer for potential testing in human clinical trials to protect degenerating cholinergic neurons in Alzheimer's disease.

Accelerated release of exosome-associated GM1 ganglioside (GM1) by endocytic pathway abnormality: another putative pathway for GM1-induced amyloid fibril formation

Exosomes are extracellularly released small vesicles that are derived from multivesicular bodies formed via the endocytic pathway. We treated pheochromocytoma PC12 cells with chloroquine, an acidotropic agent, which potently perturbs membrane trafficking from endosomes to lysosomes. Chloroquine treatment increased the level of GM1 ganglioside in cell media only when the cells were exposed to KCl for depolarization, which is known to enhance exosome release from neurons. In the sucrose-density-gradient fractionation of cell media, GM1 ganglioside was exclusively recovered with Alix, a specific marker of exosomes, in the fractions with the density corrresponding to that of exosomes. Notably, amyloid-beta assembly was markedly accelerated when incubated with the exosome fraction prepared from the culture media of PC12 cells treated with chloroquine and KCl. Furthermore, amyloid-beta assembly was significantly suppressed by the co-incubation with an antibody specific to GM1-bound amyloid-beta, an endogenous seed for amyloid formation of Alzheimer's disease. Together with our previous finding that chloroquine treatment induces the accumulation of GM1 ganglioside in early endosomes, results of this study suggest that endocytic pathway abnormality accelerates the release of exosome-associated GM1 ganglioside following its accumulation in early endosomes. Furthermore, this study also suggests that extracellular amyloid fibril formation is induced by not only GM1 gangliosides accumulated on the surface of the cells but also those released in association with exosomes.

Microfluorimetry defines early axonal damage in a rat model of optic neuritis: a novel method targeting early CNS autoimmunity

Autoimmune optic neuritis is a common early manifestation of multiple sclerosis (MS), yet early therapeutic interventions for MS often have high ocular toxicity associated with increased risks for glaucoma, cataract, or retinopathy. This need to discover better early treatment options prompted our development of a sensitive and reliable means to quantify the broad range of pathologies that potentially develop very early in autoimmune optic neuritis. Tissue microfluorimetry was used to measure seven established markers for human MS pathology in normal and autoimmune optic nerves 13 days after antigen exposure, in a Brown Norway rat model of myelin oligodendrocyte glycoprotein (MOG) peptide (35-55)-induced autoimmune optic neuritis. Optic neuritis rats demonstrated early and significant pathologic changes in five established indices for neuroinflammation, immune infiltration, and demyelination that accurately modeled pathologies characteristic of MS. Two indices of MS-like axon damage advanced significantly within 13 days of antigen exposure. Fluorimetrically measured immunoreactivity (-ir) was significantly decreased for paranodin (PN, the requisite axonal paranodal junction protein) and significantly increased for amyloid precursor protein (APP), indicating loss of paranodal junctions and impaired fast axonal transport, respectively. Measurements showing decreased PN-ir with increased APP-ir quantitatively defined a pattern of early axonal damage in autoimmune optic neuritis.

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.

Age-related spatial cognitive impairment is correlated with increase of synaptotagmin 1 in dorsal hippocampus in SAMP8 mice

The age-related decline of learning and memory is a common phenomenon in humans and animals, even though the underlying mechanism is not yet known. In the present study, we propose that synaptotagmin 1 (Syt 1) might be a synaptic protein involved in the loss of learning and memory with aging. To test this hypothesis, the age-related spatial cognitive ability of 36 P8 mice (15 mice aged 4 months, 11 mice aged 8 months and 10 mice aged 13 months) was measured in a Morris water maze. After the behavioral test, both the protein and mRNA levels of Syt 1 were determined in the dorsal hippocampus by means of immunocytochemistry and reverse transcriptase polymerase chain reaction (RT-PCR), respectively. In the Morris water maze, the latency of the 4-month mice to find the submerged platform was significantly shorter than that of the older mice, while there were no significant differences between the 8- and 13-month-old mice in this respect. Compared to the 4-month-old mice, the Syt 1 protein in the 13-month-old mice was significantly increased in almost all layers of each subfield of the hippocampus. The average level of Syt 1 mRNA in the dorsal hippocampus of the P8 mice had not changed with aging. The latency of the 13-month-old P8 mice tested in the Morris water maze was positively correlated with the Syt 1 immunoreactivity in four circuit-specific regions in the dorsal hippocampus. Interestingly, the latency in the Morris water maze was also positively correlated with the level of Syt 1 mRNA in the dorsal hippocampus in individual aged P8 mouse. These results suggest that increased Syt 1 in the dorsal hippocampus in aged mice might be responsible for the age-related impairment of learning and memory.

The significance of the cholinergic system in the brain during aging and in Alzheimer's disease

Acetylcholine is widely distributed in the nervous system and has been implicated to play a critical role in cerebral cortical development, cortical activity, controlling cerebral blood flow and sleep-wake cycle as well as in modulating cognitive performances and learning and memory processes. Cholinergic neurons of the basal forebrain complex have been described to undergo moderate degenerative changes during aging, resulting in cholinergic hypofunction that has been related to the progressing memory deficits with aging. Basal forebrain cholinergic cell loss is also a consistent feature of Alzheimer's disease, which has been suggested to cause, at least partly, the cognitive deficits observed, and has led to the formulation of the cholinergic hypotheses of geriatric memory dysfunction. Impaired cortical cholinergic neurotransmission may also contribute to beta-amyloid plaque pathology and increase phosphorylation of tau protein the main component of neurofibrillar tangles in Alzheimer's disease. Understanding the molecular mechanisms underlying the interrelationship between cortical cholinergic dysfunction, beta-amyloid formation and deposition, and tau pathology in Alzheimer's disease, would allow to derive potential treatment strategies to pharmacologically intervene in the disease-causing signaling cascade.

Down regulation of trk but not p75NTR gene expression in single cholinergic basal forebrain neurons mark the progression of Alzheimer's disease

Dysfunction of cholinergic basal forebrain (CBF) neurons of the nucleus basalis (NB) is a cardinal feature of Alzheimer's disease (AD) and correlates with cognitive decline. Survival of CBF neurons depends upon binding of nerve growth factor (NGF) with high-affinity (trkA) and low-affinity (p75(NTR)) neurotrophin receptors produced within CBF neurons. Since trkA and p75(NTR) protein levels are reduced within CBF neurons of people with mild cognitive impairment (MCI) and mild AD, trkA and/or p75(NTR) gene expression deficits may drive NB degeneration. Using single cell expression profiling methods coupled with custom-designed cDNA arrays and validation with real-time quantitative PCR (qPCR) and in situ hybridization, individual cholinergic NB neurons displayed a significant down regulation of trkA, trkB, and trkC expression during the progression of AD. An intermediate reduction was observed in MCI, with the greatest decrement in mild to moderate AD as compared to controls. Importantly, trk down regulation is associated with cognitive decline measured by the Global Cognitive Score (GCS) and the Mini-Mental State Examination (MMSE). In contrast, there is a lack of regulation of p75(NTR) expression. Thus, trk defects may be a molecular marker for the transition from no cognitive impairment (NCI) to MCI, and from MCI to frank AD.

Basal forebrain cholinergic dysfunction in Alzheimer's disease--interrelationship with beta-amyloid, inflammation and neurotrophin signaling

Alzheimer's disease, the most common neurodegenerative disorder of senile dementia, is characterized by two major morpho-pathological hallmarks. Deposition of extracellular neuritic, beta-amyloid peptide-containing plaques (senile plaques) in cerebral cortical regions of Alzheimer patients is accompanied by the presence of intracellular neurofibrillary tangles in cerebral pyramidal neurons. Basal forebrain cholinergic dysfunction is also a consistent feature of Alzheimer's disease, which has been suggested to cause, at least partly, the cognitive deficits observed in patients with Alzheimer's disease. Impaired cortical cholinergic neurotransmission may also contribute to beta-amyloid plaque pathology in Alzheimer's disease by affecting expression and processing of the beta-amyloid precursor protein (APP). Vice versa, low level of soluble beta-amyloid has been observed to inhibit cholinergic synaptic function. Deposition of beta-amyloid plaques in Alzheimer's disease is also accompanied by a significant plaque-associated glial up-regulation of interleukin-1, which has been attributed to affect expression and metabolism of APP and to interfere with cholinergic transmission. Understanding the molecular mechanisms underlying the interrelationship between cortical cholinergic dysfunction, beta-amyloid formation and deposition, as well as local inflammatory upregulation, would allow to derive potential treatment strategies to pharmacologically intervene in the disease-causing signaling cascade.

Responses of Acetylcholine Release and Regional Blood Flow in the Hippocampus during Walking in Aged Rats

Walking produced increases in both the extracellular acetylcholine level and regional blood flow in the hippocampus in aged rats 26-29 months old. The present results in aged rats were compared with our previous data in young adult rats (Nakajima et al., 2003), and it was found that both the acetylcholine and blood flow responses in the hippocampus were well maintained in aged rats.

Chronic alcohol intoxication in rats leads to a strong but transient increase in NGF levels in distinct brain regions

Nerve growth factor (NGF), a member of the neurotrophin family, is an essential mediator of neuronal activity and synaptic plasticity of basal forebrain cholinergic neurons. In this study NGF-protein levels were determined in areas of the basal forebrain cholinergic system, its projection areas as well as the striatum and the cerebellum after long-term exposure (6 and 9 months) to ethanol and a phase of withdrawal in male Sprague-Dawley rats. 6-month alcohol treatment led to an increase of NGF to 650-850% of controls in the basal forebrain and the septum and to a 210-485% increase in the cholinergic projection areas (anterior cortex, hippocampus and olfactory bulb). After 9 months exposure to ethanol, a decrease of NGF by 16% in the frontal cortex was observed compared to controls. In the other brain regions no differences in NGF expression were detectable at this time-point. These results support the idea of an endogenous neuroprotective mechanism acting through a transient NGF induction followed by a decrease in NGF-levels during the course of further neuronal degeneration.

Nerve growth factor restores the expression of vasopressin and vasoactive intestinal polypeptide in the suprachiasmatic nucleus of aged rats

Aging leads to a decrease in the number of neurons expressing vasopressin (VP) and vasoactive intestinal polypeptide (VIP) in the suprachiasmatic nucleus (SCN) of the rat. Similar results were observed following prolonged alcohol consumption and withdrawal. In the latter circumstances, the administration of nerve growth factor (NGF) restored the synthesis and expression of those neuropeptides despite the absence of TrkA receptors in SCN neurons. Thus, we decided to test whether the administration of NGF would improve the expression of neuropeptides in the SCN of aged rats. For this purpose, NGF was delivered intraventricularly to aged rats over a period of 14 days. The somatic volume and the total number of VP- and VIP-immunostained SCN neurons were estimated by applying stereological methods. No age-related variations were found regarding the volume of the neuronal cell bodies. Yet, a striking reduction in the number of VP- and VIP-immunoreactive neurons was detected in aged animals and found to be completely retrieved by NGF. This finding shows that exogenous NGF administered to aged rats restores the neurochemical phenotype of the SCN. This might occur either through direct signaling of SCN neurons via p75NTR or through enhancement of the cholinergic input to the SCN.

The role of nerve growth factor receptors in cholinergic basal forebrain degeneration in prodromal Alzheimer disease

Dysfunction of nerve growth factor (NGF) and its high (TrkA) and low (p75NTR) affinity receptors has been suggested to underlie the selective degeneration of the nucleus basalis (NB) cholinergic cortical projection neurons in end stage Alzheimer disease (AD). Whether the NGF system is dysfunctional during the prodromal stages of AD has only recently been evaluated. Surprisingly, the number of choline acetyltransferase-containing neurons remains stable despite a significant reduction in NGF receptor-positive cells in people with mild cognitive impairment (MCI), suggesting a phenotypic NGF receptor downregulation but not a frank loss of NB neurons during prodromal AD. Moreover, there is a loss of cortical TrkA in the face of stable p75NTR and increased proNGF levels, the precursor molecule of mature NGF, in early AD. Depending upon the cellular context these changes may result in increased pro-apoptotic signaling, cell survival, or a defect in retrograde transport mechanisms. Alterations in NGF and its receptors within the cholinotrophic NB system in early AD suggest that NGF-mediated cell signaling is required for the longterm survival of these neurons. Therapeutic neurotrophic intervention might delay or prevent NB neuron degeneration and preserve cholinergic cortical function during prodromal AD.

The role of NGF uptake in selective vulnerability to cell death in ageing sympathetic neurons

We have examined the hypothesis that differences in nerve growth factor (NGF) uptake and transport determine vulnerability to age-related neurodegeneration. Neurons projecting to cerebral blood vessels (CV) in aged rats are more vulnerable to age-related degeneration than those projecting to the iris. Uptake of NGF was therefore examined in sympathetic neurons projecting from the superior cervical ganglion (SCG) to CV and iris in young and old rats by treating the peripheral processes of these neurons with different doses of I125-NGF. Total uptake of I125-NGF was reduced in old CV-projecting, but not iris-projecting, neurons. Numbers of radiolabelled neurons projecting to each target were counted in sectioned ganglia. The data showed age-related reductions in numbers of labelled neurons projecting to CV, but no change in numbers of neurons projecting to the iris. Calculation of uptake of I125-NGF per neuron unexpectedly showed no major age-related differences in either of the two neuron populations. However, uptake per neuron was considerably lower for young and old CV-projecting, compared to iris-projecting, SCG neurons. We hypothesized that variations in NGF uptake might affect neuronal survival in old age. Counts of SCG neurons using a physical disector following retrograde tracing with Fluorogold confirmed the selective vulnerability of CV-projecting neurons by showing a significant 37% loss of these neurons in the period between 15 and 24 months. In contrast, there was no significant loss of iris-projecting neurons. We conclude that vulnerability to, or protection from, age-related neurodegeneration and neuronal cell death are associated with life-long low, or high, levels of NGF uptake, respectively.

Reduction of cortical TrkA but not p75(NTR) protein in early-stage Alzheimer's disease

Degeneration of cholinergic nucleus basalis (NB) cortical projection neurons is associated with cognitive decline in late-stage Alzheimer's disease (AD). NB neuron survival is dependent on coexpression of the nerve growth factor (NGF) receptors p75(NTR) and TrkA, which bind NGF in cortical projection sites. We have shown previously a significant reduction of NB perikarya expressing p75(NTR) and TrkA protein during the early stages of AD. Whether there is a concomitant reduction in cortical levels of these receptors during the progression of AD is unknown. p75(NTR) and TrkA protein was evaluated by quantitative immunoblotting in five cortical regions (anterior cingulate, superior frontal, superior temporal, inferior parietal, and visual cortex) of individuals clinically diagnosed with no cognitive impairment (NCI), mild cognitive impairment (MCI), mild/moderate AD, or severe AD. Cortical p75(NTR) levels were stable across the diagnostic groups. In contrast, TrkA levels were reduced approximately 50% in mild/moderate and severe AD compared with NCI and MCI in all regions except visual cortex. Mini-Mental Status Examination scores correlated with TrkA levels in anterior cingulate, superior frontal, and superior temporal cortex. The selective reduction of cortical TrkA levels relative to p75(NTR) may have important consequences for cholinergic NB function during the transition from MCI to AD.

Aging and subcellular localization of m2 muscarinic autoreceptor in basalocortical neurons in vivo

By using immunohistochemical approaches at the light and electron microscopic levels, we have shown that aging modifies the subcellular distribution of the m2 muscarinic autoreceptor (m2R) differentially at somato-dendritic postsynaptic sites and at axonal presynaptic sites in cholinergic basalocortical neurons, in vivo. In cholinergic perikarya and dendrites of the nucleus basalis magnocellularis (NBM), aging is associated with a decrease of the density of m2R at the plasma membrane and in the cytoplasm, suggesting a decrease of the total number of m2R in the somato-dendritic field. In contrast, the number of substance P receptors per somato-dendritic surface was not affected. In the frontal cortex (FC), we have shown a decrease of cytoplasmic m2R density also leading to a decrease of the number of m2R per surface of varicosities but with no change of the density of m2R at the membrane. Our results suggest that the decrease of m2R in the somato-dendritic field of the NBM, but not a modification of the number of presynaptic m2 autoreceptors at the plasma membrane in the FC, could contribute to the decrease of the efficacy of cholinergic transmission observed with aging in the rat.

Evaluation of the neurorestorative effects of the murine beta-nerve growth factor infusions in old rat with cognitive deficit

The nerve growth factor (NGF) is known to participate in the regulation of the expression levels and activity of the choline acetyltransferase (ChAT) in the nervous system. This enzyme is sensitive to the degenerative changes found in Alzheimer's disease (AD). We compared the effectiveness of intraparenchymal (ip) and intracerebroventricular (icv) administration of the murine beta-NGF (beta-NGFm) produced in our laboratories, through the determination of the expression levels and activity of the ChAT, and the evaluation of behavioral recovery in aged rat with cognitive deficit. Our results indicated that icv infusion of beta-NGFm stimulates the expression levels of ChAT gene in the striatum of old rats. Remarkable losses in the ChAT activity were observed in the septum and striatum of old rats. Exogenous administration of beta-NGFm produced a significant increase of ChAT activity in these brain regions differentially according to the administration pathway. The behavioral studies demonstrated that the administration pathway is an important factor in order to obtain the best results for a neurorestorative treatment.

Age-related vulnerability to diencephalic amnesia produced by thiamine deficiency: the role of time of insult

Age is a risk factor for the development of many neurological disorders, including alcohol-related neurological disorders. A rodent model of Wernicke-Korsakoff Syndrome (WKS), acute pyrithiamine-induced thiamine deficiency (PTD), produces diencephalic damage and impairments of memory similar to what is seen in WKS patients. Advanced age increases the vulnerability to the cascade of acute and some chronic neurological events caused by PTD treatment. Interactions between PTD treatment and age at the time of treatment (3, 10, or 21 months), in addition to the effects of an increased recovery period, were examined relative to spatial memory impairment and neuropathology in Fischer 344 rats. Although acute neurological disturbances and medial thalamic brain lesions were more prevalent in middle-aged and senescent rats exposed to PTD treatment, relative to young rats, behavioral data did not support the view that PTD and aging have synergistic effects. In addition, both advanced age and PTD treatment result in a loss of basal forebrain cholinergic neurons, though there was no interaction. Despite the fact that no convincing evidence was found for an effect of extended recovery time on neuropathology measures, young rats given an extensive recovery period displayed less working memory impairment. In summary, these data provide evidence for an increased susceptibility of the aged rat to the acute neurological consequences and diencephalic pathology associated with PTD treatment and indicated a similar vulnerability of the middle-aged rat. However, the synergistic interaction between aging and PTD treatment in thalamic tissue loss did not express behaviorally.

Basal forebrain neurons modulate the synthesis and expression of neuropeptides in the rat suprachiasmatic nucleus

We tested the hypothesis that efferents from the nucleus basalis magnocellularis (NBM) play a direct role in the regulation of neuropeptide synthesis and expression by neurons of the rat suprachiasmatic nucleus (SCN). Adult male rats in which the NBM was destroyed with quinolinic acid, either unilaterally or bilaterally, were compared with rats injected with physiological saline and with control rats. The estimators used to assess the effects of cholinergic deafferentation on the neuroanatomy and neurochemistry of the SCN were the total number of SCN neurons, the total number and somatic size of SCN neurons producing vasopressin (VP) and vasoactive intestinal polypeptide (VIP), and the respective mRNA levels. Bilateral destruction of the NBM did not produce cell death in the SCN, but caused a marked reduction in the number and somatic size of SCN neurons expressing VP and VIP, and in the mRNA levels of these peptides. The decrease in the number of VP- and VIP-producing neurons provoked by unilateral lesions was less striking than that resulting from bilateral lesions. It was, however, statistically significant in the ipsilateral hemisphere, but not in the contralateral hemisphere. The results show that the reduction of cholinergic inputs to the SCN impairs the synthesis, and thereby decreases the expression of neuropeptides by SCN neurons, and that the extent of the decline correlates with the amount of cholinergic afferents destroyed. This supports the notion that acetylcholine plays an important, and direct role in the regulation of the metabolic activity of SCN neurons.

Ptychopetalum olacoides, a traditional Amazonian "nerve tonic", possesses anticholinesterase activity

The cholinergic hypothesis of Alzheimer disease (AD) has provided the rationale for the current pharmacotherapy of this disease, in an attempt to downgrade the cognitive decline caused by cholinergic deficits. Nevertheless, the search for potent and long-acting acetylcholinesterase (AChE) inhibitors that exert minimal side effects to AD patients is still an ongoing effort. Amazonian communities use traditional remedies prepared with Ptychopetalum olacoides (PO, Olacaceae) roots for treating various central nervous system conditions, including those associated with aging. The fact that PO ethanol extract (POEE) has been found to facilitate memory retrieval in the step down procedure in young and aged mice prompt us to evaluate its effects on AChE activity in memory relevant brain areas. POEE significantly inhibited AChE activity in vitro in a dose- and time-dependent manner in rat frontal cortex, hippocampus and striatum; a significant inhibition was also found in these same brain areas of aged (14 months) mice after acute administration of POEE (100 mg/kg ip). We propose that such AChE inhibitory activity is a neurochemical correlate of a number of therapeutic properties traditionally claimed for P. olacoides, particularly those associated with cognition.

Exogenous NGF restores endogenous NGF distribution in the brain of the cognitively impaired aged rat

Alzheimer's disease and normal aging may impair retrograde transport of nerve growth factor (NGF) from cortical areas to basal forebrain cholinergic neurons. We demonstrate a relationship between performance in a spatial reference memory task and NGF distribution in the aged rat brain. In addition, exogenous NGF restored endogenous NGF distribution in cognitively impaired aged rats. These data suggest that NGF administration restores utilization of endogenous growth factor in the brain of cognitively impaired aged rats.

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

Evidence for reduced accumulation of exogenous neurotrophin by aged sympathetic neurons

The present study investigated the potential for neurotrophin uptake by cerebrovascular axons and subsequent accumulation in the aged superior cervical ganglion (SCG) following a two week intracerebroventricular infusion of nerve growth factor (NGF). In the SCG from aged rats, NGF protein levels declined significantly compared with the SCG from young adult rats. Following NGF infusion, perivascular axons from both young adult and aged rats showed intense NGF immunostaining. In addition, significant increases in NGF protein were shown using enzyme-linked immunosorbent assay (ELISA) and in counts of NGF immunopositive cell bodies in the SCG when compared with age-matched controls. NGF accumulation in ganglia from aged rats, however, was significantly less when compared with ganglia from young adult rats. The results of the present study suggest that NGF protein is significantly reduced in aged ganglia with the neurons retaining some capacity to take up and transport exogenous neurotrophin. Even so, the potential for NGF accumulation is dramatically reduced in aged rats when compared with that of young adult rats. While previous results have shown robust NGF-induced neurotransmitter responses by sympathetic neurons from the aged animal, the present finding of reduced accumulation of NGF in aged sympathetic neurons suggests an age-related difference in the utilization or transport of NGF.

Aging impairs amygdala-hippocampus interactions involved in hippocampal LTP

Aging impairs amygdala-hippocampus interactions involved in hippocampal LTP. NEUROBIOL. AGING. We have recently shown that the stimulation of the basolateral nucleus of the amygdala (BLA) is able to prolong early-LTP (<4h) into late-LTP (>4h) in the dentate gyrus. To study whether aging affects this interaction, aged (24-27 months) rats were used, classified as cognitively impaired (I), or non-impaired (N) by means of their results in the Morris water maze. Paired pulses (30-90 ms interval) showed no differences among age groups. Among young controls, the early-LTP induced in the dentate gyrus by stimulation of the perforant path (PP) was prolonged in a late-LTP when the BLA was stimulated 15 min later. In aged-impaired rats the stimulation of the PP induced a reduced LTP, decaying to baseline in less than 2 h. BLA stimulation was without effect. Aged non-impaired rats showed an early-LTP identical to that of young animals; however, stimulation of the BLA showed no effect. These results suggest that deficient synaptic plasticity and memory functions in aged animals might be caused, in part by impaired mechanisms of heterosynaptic reinforcement.

Regulation of cerebral cortical blood flow by the basal forebrain cholinergic fibers and aging

This article reviews the study of neural vasodilator mechanisms of the cerebral cortex by basal forebrain cholinergic nerve fibers and their age-related function in rats. During the last decade, we have demonstrated a neural regulatory system of cerebral blood flow in rats involving intracerebral cholinergic vasodilator nerve fibers originating in the basal forebrain and projecting to the cerebral cortex. Activation of these cholinergic vasodilator fibers results in the release of acetylcholine (ACh) within the cortex, activation of both nicotinic and muscarinic ACh receptors, and vasodilatation without coupling to glucose metabolic rates. This cholinergic vasodilator system has been shown to decline with age in rats mainly due to age-related declines of nicotinic ACh receptor activity. However, muscarinic ACh receptor activity and the release of ACh into the extracellular space in the cortex are well maintained during aging. The present age-related decline of the intracerebral cholinergic vasodilator system found in rats seems to affect cognitive function during aging, although this cholinergic vasodilator system has not yet been demonstrated in humans.

Nerve growth factor signaling, neuroprotection, and neural repair

Nerve growth factor (NGF) was discovered 50 years ago as a molecule that promoted the survival and differentiation of sensory and sympathetic neurons. Its roles in neural development have been characterized extensively, but recent findings point to an unexpected diversity of NGF actions and indicate that developmental effects are only one aspect of the biology of NGF. This article considers expanded roles for NGF that are associated with the dynamically regulated production of NGF and its receptors that begins in development, extends throughout adult life and aging, and involves a surprising variety of neurons, glia, and nonneural cells. Particular attention is given to a growing body of evidence that suggests that among other roles, endogenous NGF signaling subserves neuroprotective and repair functions. The analysis points to many interesting unanswered questions and to the potential for continuing research on NGF to substantially enhance our understanding of the mechanisms and treatment of neurological disorders.

Spike responses of neurons in the motor area of the cortex of elderly rabbits to specific stimuli

The spike responses of neurons in the motor area of the cortex to tactile and electrocutaneous stimulation of the forelimb were studied in elderly (aged 6-7 years) rabbits. In comparison with young rabbits, the cortex of adult animals contained fewer cells responding to afferent stimulation. The activatory responses of neurons in elderly animals showed smaller increases in the spike frequency from the baseline level. Long-latency, slow activatory responses, which were not characteristic of cortical neurons in young animals, appeared; the pattern of these responses could be partially corrected by administration of acetylcholine in the vicinity of the neurons being recorded. The parameters of inhibitory responses were enzyme of the significantly different in animals of different ages.

The acetylcholine release enhancer linopirdine induces Fos in neocortex of aged rats

Centrally acting cholinergic agents induce the immediate early gene c-fos in the rat brain resulting in transient increases of Fos protein, most notably in the cerebral cortex. In this study we have monitored by Fos immunohistochemistry the effect of the acetylcholine release enhancer linopirdine (DUP996) on the immediate early gene c-fos in brains of 3 months and 30 months old rats. In young rats linopirdine had only a marginal effect on Fos expression. In contrast, in aged rats linopirdine caused widespread expression of Fos throughout neocortex. In somatosensory cortex, the induction of the c-fos gene by linopirdine was nearly completely blocked by atropine and scopolamine and strongly attenuated by the NMDA receptor blockers CPP and MK-801. The results suggest that the age-related decline in acetylcholine release in rodents can be partially compensated for by administration of linopirdine.

Effects of age on cholinergic vasodilation of cortical cerebral blood vessels in rats

The present study examined the age-related changes in the cholinergic vasodilative system originating in the nucleus basalis of Meynert (NBM) and projecting to the cerebral cortex using Wistar rats of three different ages; young adult (4-7 months), old (24-25 months), and very old (32-42 months) rats. The vasodilative responses in frontal and parietal cortices, measured by laser Doppler flowmetry, induced by electrical stimulation of NBM without blood pressure response were well maintained in old rats, but declined significantly in very old rats. Extracellular acethylcholine (ACh) release in both cortices collected by a microdialysis technique showed both basal levels and response to NBM stimulation to be well maintained in both old and very old rats. The vasodilative cerebral blood flow response elicited by stimulation of the muscarinic ACh receptors, using their agonist, arecoline, was also well maintained in old and very old rats. Considering the present data and our previous finding that the cerebral cortical vasodilative response to activation of the nicotinic ACh receptors using their agonist, nicotine, was markedly reduced in very old rats (Neurosci. Lett., 228 (1997) 203), it was concluded that the age-related decline of nicotinic ACh receptor activity was a cause of the decline of the vasodilative responses elicited by NBM stimulation in very old rats. This result suggests that a reduction of the cholinergic vasodilative system in very old rats due to decreased activity of the nicotinic ACh receptor may cause insufficient blood flow in the cortex when the cortical neurons require.