Amyloid precursor protein in the cerebral cortex is rapidly and persistently induced by loss of subcortical innervation

Visual hallucinations in Parkinson's disease: spotlight on central cholinergic dysfunction

Visual hallucinations are a common non-motor feature of Parkinson's disease and have been associated with accelerated cognitive decline, increased mortality and early institutionalization. Despite their prevalence and negative impact on patient outcomes, the repertoire of treatments aimed at addressing this troubling symptom is limited. Over the past two decades, significant contributions have been made in uncovering the pathological and functional mechanisms of visual hallucinations, bringing us closer to the development of a comprehensive neurobiological framework. Convergent evidence now suggests that degeneration within the central cholinergic system may play a significant role in the genesis and progression of visual hallucinations. Here, we outline how cholinergic dysfunction may serve as a potential unifying neurobiological substrate underlying the multifactorial and dynamic nature of visual hallucinations. Drawing upon previous theoretical models, we explore the impact that alterations in cholinergic neurotransmission has on the core cognitive processes pertinent to abnormal perceptual experiences. We conclude by highlighting that a deeper understanding of cholinergic neurobiology and individual pathophysiology may help to improve established and emerging treatment strategies for the management of visual hallucinations and psychotic symptoms in Parkinson's disease.

The Effect of Lidocaine on the Experimental Model of Streptozotocin-Induced Alzheimer's Disease

Introduction

Alzheimer's disease (AD) is a neurodegenerative disease caused by the accumulation of amyloid plaques in the cerebral cortex and hippocampus. In this study, the effects of local anesthetic lidocaine on neurodegeneration markers and memory were investigated for the first time in streptozotocin-induced rat AD model.

Methods

Streptozotocin (STZ) was administered intracerebroventricularly (ICV) into Wistar rats to develop AD model. For lidocaine group (n=14), lidocaine (5 mg/kg) was administered intraperitoneally (IP) in addition to STZ injection. Control group animals (n=9) were treated with saline for 21 days. Morris Water Maze (MWM) test was performed to evaluate memory after the injections were completed. Also, the serum levels of TAR DNA-binding protein-43 (TDP-43), amyloid precursor protein (APP), β-secretase 1, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), response element binding protein (CREB), c-FOS were measured using ELISA test and compared between groups.

Results

Lidocaine group animals showed lower escape latency and time in quadrant scores in MWM inferring better memory performance. Furthermore, lidocaine administration caused a significant decline in TDP-43 levels. However, the expression of APP and β-secretase were significantly higher in AD and lidocaine groups compared to control group. Moreover, lidocaine group markedly had higher serum NGF, BDNF, CREB, and c-FOS levels compared to those in the AD group.

Conclusion

In addition to neuroprotective effects in STZ-induced AD model, Lidocaine also appears to improve memory. This effect might be associated with increased levels of several growth factors and associated intracellular molecules. The therapeutic role of lidocaine in the pathophysiology of AD should be studied in the future.

Synapses, Microglia, and Lipids in Alzheimer's Disease

Alzheimer's disease (AD) is characterised by synaptic dysfunction accompanied by the microscopically visible accumulation of pathological protein deposits and cellular dystrophy involving both neurons and glia. Late-stage AD shows pronounced loss of synapses and neurons across several differentially affected brain regions. Recent studies of advanced AD using post-mortem brain samples have demonstrated the direct involvement of microglia in synaptic changes. Variants of the Apolipoprotein E and Triggering Receptors Expressed on Myeloid Cells gene represent important determinants of microglial activity but also of lipid metabolism in cells of the central nervous system. Here we review evidence that may help to explain how abnormal lipid metabolism, microglial activation, and synaptic pathophysiology are inter-related in AD.

Improving Anti-Neurodegenerative Benefits of Acetylcholinesterase Inhibitors in Alzheimer's Disease: Are Irreversible Inhibitors the Future?

Decades of research have produced no effective method to prevent, delay the onset, or slow the progression of Alzheimer's disease (AD). In contrast to these failures, acetylcholinesterase (AChE, EC 3.1.1.7) inhibitors slow the clinical progression of the disease and randomized, placebo-controlled trials in prodromal and mild to moderate AD patients have shown AChE inhibitor anti-neurodegenerative benefits in the cortex, hippocampus, and basal forebrain. CNS neurodegeneration and atrophy are now recognized as biomarkers of AD according to the National Institute on Aging-Alzheimer's Association (NIA-AA) criteria and recent evidence shows that these markers are among the earliest signs of prodromal AD, before the appearance of amyloid. The current AChE inhibitors (donepezil, rivastigmine, and galantamine) have short-acting mechanisms of action that result in dose-limiting toxicity and inadequate efficacy. Irreversible AChE inhibitors, with a long-acting mechanism of action, are inherently CNS selective and can more than double CNS AChE inhibition possible with short-acting inhibitors. Irreversible AChE inhibitors open the door to high-level CNS AChE inhibition and improved anti-neurodegenerative benefits that may be an important part of future treatments to more effectively prevent, delay the onset, or slow the progression of AD.

Preclinical Models of Alzheimer's Disease: Relevance and Translational Validity

The only drugs currently approved for the treatment of Alzheimer's Disease (AD) are four acetylcholinesterase inhibitors and the NMDA antagonist memantine. Apart from these drugs, which have minimal to no clinical benefit, the 40-year search for effective therapeutics to treat AD has resulted in a clinical failure rate of 100% not only for compounds that prevent brain amyloid deposition or remove existing amyloid plaques but also those acting by a variety of other putative disease-associated mechanisms. This indicates that the preclinical data generated from current AD targets to support the selection, optimization, and translation of new chemical entities (NCEs) and biologics to clinical trials is seriously compromised. While many of these failures reflect flawed hypotheses or a lack of adequate characterization of the preclinical pharmacodynamic and pharmacokinetic (PD/PK) properties of lead NCEs-including their bioavailability and toxicity-the conceptualization, validation, and interrogation of the current animal models of AD represent key limitations. The overwhelming majority of these AD models are transgenic, based on aspects of the amyloid hypothesis and the genetics of the familial form of the disease. As a result, these generally lack construct and predictive validity for the sporadic form of the human disease. The 170 or so transgenic models, perhaps the largest number ever focused on a single disease, use rodents, mainly mice, and in addition to amyloid also address aspects of tau causality with more complex multigene models including other presumed causative factors together with amyloid. This overview discusses the current animal models of AD in the context of both the controversies surrounding the causative role of amyloid in the disease and the need to develop validated models of cognitive function/dysfunction that more appropriately reflect the phenotype(s) of human aged-related dementias. © 2019 by John Wiley & Sons, Inc.

Targeting norepinephrine in mild cognitive impairment and Alzheimer's disease

The Alzheimer's disease (AD) epidemic is a looming crisis, with an urgent need for new therapies to delay or prevent symptom onset and progression. There is growing awareness that clinical trials must target stage-appropriate pathophysiological mechanisms to effectively develop disease-modifying treatments. Advances in AD biomarker research have demonstrated changes in amyloid-beta (Aβ), brain metabolism and other pathophysiologies prior to the onset of memory loss, with some markers possibly changing one or two decades earlier. These findings suggest that amyloid-based therapies would optimally be targeted at the earliest clinically detectable stage (such as mild cognitive impairment (MCI)) or before. Postmortem data indicate that tau lesions in the locus coeruleus (LC), the primary source of subcortical norepinephrine (NE), may be the first identifiable pathology of AD, and recent data from basic research in animal models of AD indicate that loss of NE incites a neurotoxic proinflammatory condition, reduces Aβ clearance and negatively impacts cognition - recapitulating key aspects of AD. In addition, evidence linking NE deficiency to neuroinflammation in AD also exists. By promoting proinflammatory responses, suppressing anti-inflammatory responses and impairing Aβ degradation and clearance, LC degeneration and NE loss can be considered a triple threat to AD pathogenesis. Remarkably, restoration of NE reverses these effects and slows neurodegeneration in animal models, raising the possibility that treatments which increase NE transmission may have the potential to delay or reverse AD-related pathology. This review describes the evidence supporting a key role for noradrenergic-based therapies to slow or prevent progressive neurodegeneration in AD. Specifically, since MCI coincides with the onset of clinical symptoms and brain atrophy, and LC pathology is already present at this early stage of AD pathogenesis, MCI may offer a critical window of time to initiate novel noradrenergic-based therapies aimed at the secondary wave of events that lead to progressive neurodegeneration. Because of the widespread clinical use of drugs with a NE-based mechanism of action, there are immediate opportunities to repurpose existing medications. For example, NE transport inhibitors and NE-precursor therapies that are used for treatment of neurologic and psychiatric disorders have shown promise in animal models of AD, and are now prime candidates for early-phase clinical trials in humans.

Intensive protein synthesis in neurons and phosphorylation of beta-amyloid precursor protein and tau-protein are triggering factors of neuronal amyloidosis and Alzheimer's disease

Recently the studies of Alzheimer’s disease have become particularly actual and have attracted scientists from all over the world to this problem as a result of dissemination of this dangerous disorder. The reason for such pathogenesis is not known, but the final image, for the first time obtained on microscopic brain sections from patients with this disease more than a hundred years ago, is well known to clinicists. This is the deposition of Ab amyloid in the brain tissue of senile plaques and fibrils. Many authors suppose that the deposition of beta-amyloid provokes secondary neuronal changes which are the reason of neuron death. Other authors associate the death of neurons with hyperphosphorylation of tau-proteins which form neurofibrillar coils inside nerve cells and lead to their death. For creation of methods of preclinical diagnostics and effective treatment of Alzheimer’s disease novel knowledge is required on the nature of triggering factors of sporadic isoforms of Alzheimer’s disease, on cause-effect relationships of phosphorylation of amyloid precursor protein with formation of pathogenic beta-amyloids, on the relationship with these factors of hyperphosphorylation of tau-protein and neuron death. In this review we analyze the papers describing the increasing of intensity of biosynthesis in neurons in normal conditions and under the stress, the possibility of development of energetic unbalanced neurons and activation of their protective systems. Phosphorylation and hyperphosphorylation of tau-proteins is also tightly connected with protective mechanisms of cells and with processes of evacuation of phosphates, adenosine mono-phosphates and pyrophosphates from the region of protein synthesis. Upon long and high intensity of protein synthesis the protective mechanisms are overloaded and the complementarity of metabolitic processes is disturbed. This results in dysfunction of neurons, transport collapse, and neuron death.

Road to Alzheimer's disease: the pathomechanism underlying

Alzheimer's disease (AD), the most common cause of dementia, results from the interplay of various deregulated mechanisms triggering a complex pathophysiology. The neurons suffer from and slowly succumb to multiple irreversible damages, resulting in cell death and thus memory deficits that characterize AD. In spite of our vast knowledge, it is still unclear as to when the disease process starts and how long the perturbations continue before the disease manifests. Recent studies provide sufficient evidence to prove amyloid β (Aβ) as the primary cause initiating secondary events, but Aβ is also known to be produced under normal conditions and to possess physiological roles, hence, the questions that remain are: What are the factors that lead to abnormal Aβ production? When does Aβ turn into a pathological molecule? What is the chain of events that follows Aβ? The answers are still under debate, and further insight may help us in creating better diagnostic and therapeutic options in AD. The present article attempts to review the current literature regarding AD pathophysiology and proposes a pathophysiologic cascade in AD.

2,4-Dinitrophenol blocks neurodegeneration and preserves sciatic nerve function after trauma

Preventing the harm caused by nerve degeneration is a major challenge in neurodegenerative diseases and in various forms of trauma to the nervous system. The aim of the current work was to investigate the effects of systemic administration of 2,4-dinitrophenol (DNP), a compound with newly recognized neuroprotective properties, on sciatic-nerve degeneration following a crush injury. Sciatic-nerve injury was induced by unilateral application of an aneurysm clip. Four groups of mice were used: uninjured, injured treated with vehicle (PBS), injured treated with two intraperitoneal doses of DNP (0.06 mg DNP/kg every 24 h), and injured treated with four doses of DNP (every 12 h). Animals were sacrificed 48 h post injury and both injured and uninjured (contralateral) sciatic nerves were processed for light and electron microscopy. Morphometric, ultrastructural, and immunohistochemical analysis of injured nerves established that DNP prevented axonal degeneration, blocked cytoskeletal disintegration, and preserved the immunoreactivity of amyloid precursor protein (APP) and Neuregulin 1 (Nrg1), proteins implicated in neuronal survival and myelination. Functional tests revealed preservation of limb function following injury in DNP-treated animals. Results indicate that DNP prevents nerve degeneration and suggest that it may be a useful small-molecule adjuvant in the development of novel therapeutic approaches in nerve injury.

Do cholinergic therapies have disease-modifying effects in Alzheimer's disease?

The most widely studied and used therapies for Alzheimer's disease (AD) are based on improving cholinergic function in the central nervous system. The acetylcholine-esterase inhibitors (ChEIs) tacrine, donepezil, rivastigmine, and galantamine are all approved, and the latter three are widely used for the symptomatic treatment of mild to moderate AD. Recent research has found that these drugs may act by a variety of other mechanisms including inhibition of butylcholinesterase, regulation of nicotinic receptors, decreasing amyloid precursor protein (APP) and A beta production, and regulation of tau phosphorylation that may influence disease progression. There is also emerging evidence from clinical trials that the ChEIs may delay cognitive and functional progression. Other cholinergic drugs such as muscarinic agonists have been explored, and although they are not approved, there is robust preclinical evidence for a beneficial, perhaps disease-modifying effect. This review summarizes evidence suggesting that these drugs may do more than improve symptoms; they may delay biological progression of the disease.

ADAM-10 over-expression increases cortical synaptogenesis

Cortical cholinergic, glutamatergic and GABAergic terminals become upregulated during early stages of the transgenic amyloid pathology. Abundant evidence suggests that sAPP alpha, the product of the non-amyloidogenic alpha-secretase pathway, is neurotrophic both in vitro and when exogenously applied in vivo. The disintegrin metalloprotease ADAM-10 has been shown to have alpha-secretase activity in vivo. To determine whether sAPP alpha has an endogenous biological influence on cortical presynaptic boutons in vivo, we quantified cortical cholinergic, glutamatergic and GABAergic presynaptic bouton densities in either ADAM-10 moderate expressing (ADAM-10 mo) transgenic mice, which moderately overexpress ADAM-10, or age-matched non-transgenic controls. Both early and late ontogenic time points were investigated. ADAM-10 mo transgenic mice display significantly elevated cortical cholinergic, glutamatergic and GABAergic presynaptic bouton densities at the early time point (8 months). Only the cholinergic presynaptic bouton density remains significantly elevated in late-staged ADAM-10 mo transgenic animals (18 months). To confirm that the observed elevations were due to increased levels of endogenous murine sAPP alpha, exogenous human sAPP alpha was infused into the cortex of non-transgenic control animals for 1 week. Exogenous infusion of sAPP alpha led to significant elevations in the cholinergic, glutamatergic and GABAergic cortical presynaptic bouton populations. These results are the first to demonstrate an in vivo influence of ADAM-10 on neurotransmitter-specific cortical synaptic plasticity and further confirm the neurotrophic influence of sAPP alpha on cortical synaptogenesis.

Septal grafts restore cognitive abilities and amyloid precursor protein metabolism

Cortical cholinergic loss and amyloidogenic processing of the beta-amyloid precursor protein (APP), may functionally interact in Alzheimer's disease. However, it is still unknown whether biological restoration of regulatory cholinergic inputs affects APP metabolism in vivo. Rats immunolesioned with 192 IgG-saporin exhibited severe acquisition deficits in place navigation that were paralleled by a dramatic loss of terminal cholinergic innervation and by marked changes in the regional expression of APP-like immunoreactivity. Moreover, in these animals, we observed a drastic reduction of soluble APP (sAPP) and a concomitant increase of the unsoluble, membrane-bound fraction (mAPP). Notably, at about 6 months post-surgery, lesioned animals implanted with reinnervating cholinergic-rich septal tissue grafts exhibited fairly normal spatial navigation abilities, as well as cortical and hippocampal APP levels that were restored up to normal or near-normal values. APP levels correlated significantly with lesion- or graft-induced changes in cholinergic innervation density, and both these measures correlated with performance in the spatial navigation task. Thus, integrity of ascending cholinergic inputs may be required to prevent amyloidogenic processing of APP in vivo and to modulate cognitive performance.

Amyloid-beta in Alzheimer disease: the null versus the alternate hypotheses

For nearly 20 years, the primary focus for researchers studying Alzheimer disease has been centered on amyloid-beta, such that the amyloid cascade hypothesis has become the "null hypothesis." Indeed, amyloid-beta is, by the current definition of the disease, an obligate player in pathophysiology, is toxic to neurons in vitro, and, perhaps most compelling, is increased by all of the human genetic influences on the disease. Therefore, targeting amyloid-beta is the focus of considerable basic and therapeutic interest. However, an increasingly vocal group of investigators are arriving at an "alternate hypothesis" stating that amyloid-beta, while certainly involved in the disease, is not an initiating event but rather is secondary to other pathogenic events. Furthermore and perhaps most contrary to current thinking, the alternate hypothesis proposes that the role of amyloid-beta is not as a harbinger of death but rather a protective response to neuronal insult. To determine which hypothesis relates best to Alzheimer disease requires a broader view of disease pathogenesis and is discussed herein.

Attenuation of the blood flow response to physostigmine in the rat cortex deafferented from the basal forebrain

Previous functional investigations in rats failed to demonstrate that the classical cholinesterase inhibitor, physostigmine, can compensate for cortical cholinergic deficit induced by deafferentation from the nucleus basalis magnocellularis (NBM). As these studies were carried out shortly after NBM lesion (1-2 weeks), we sought to determine whether compensatory effects of physostigmine would appear at a longer postlesion time (3-5 weeks). Cerebral blood flow was used as a quantitative measure of brain function. At 3-5 weeks after unilateral NBM lesion, interhemispheric comparisons in resting conditions showed that the cortical cholinergic deficit was still present and that blood flow was lower in cortical areas on the lesion side, similarly to what was observed after 1-2 weeks, while basal blood flow in intact hemispheres remained unchanged. In contrast, under physostigmine, blood flow became significantly lower in deafferented cortical areas at 3-5 weeks postlesion time, whereas there were no significant interhemispheric differences in the short term. Comparisons with saline-infused rats showed reduced blood flow responses to physostigmine in forebrain regions, e.g. in the parietal cortex from 83% to 25% at 1-2 and 3-5 weeks postlesion, respectively. These changes cannot be ascribed to a global loss of reactivity, since responses in brainstem regions (medulla, cerebellum) remained unchanged statistically. The results demonstrate a reduced responsiveness to physostigmine at the longer postlesion time, and support the existence of a cholinosensitive mechanism antagonizing NBM influence. This mechanism may limit the activating effects of cholinergic agonists in the forebrain after NBM deafferentation.

Altered synaptic function in Alzheimer's disease

Alzheimer's disease is the leading cause of dementia in the elderly, presenting itself clinically by progressive loss of memory and learning. Since synaptic density correlates more closely with cognitive impairment than any other pathological lesion observable in the disease pathology, an increased understanding of the mechanisms behind synaptic disconnection is of vital importance. Our lab investigated the neurotransmitter-specific status of distinct cortical presynaptic bouton populations in various transgenic mouse models of the Alzheimer's-like amyloid pathology in order to assess their involvement throughout the progression of the pathology. These studies have revealed that the amyloid pathology appears to progress in a neurotransmitter-specific manner where the cholinergic terminals appear most vulnerable, followed by the glutamatergic terminals and finally by the somewhat more resilient GABAergic terminals. This review will discuss additional studies which also provide evidence of a neurotransmitter-specific pathology as well as comment on the potential explanations for the observed vulnerabilities, touching upon metabolic demand, trophic support and receptor mediated activation.

Immunohistochemical study of the hnRNP A2 and B1 in the hippocampal formations of brains with Alzheimer's disease

To elucidate the post-transcriptional regulation in the subjects with Alzheimer's disease (AD), we employed immunohistochemical techniques and examined the expression of the heterogeneous nuclear ribonucleoprotein (hnRNP) A2 and B1 in the hippocampus with neurofibrillary tangle (NFT) neuropathology. In the mildly affected subjects (Braak stages I and II), the most intense A2 immunoreactivity was observed in the CA3 to CA1 neurons. In the moderately (Braak stages III and IV) and severely affected subjects (Braak stages V and VI), the CA1 region demonstrated a decrease in the number of A2 immunoreactive neurons and in immunoreactivity in the remaining neurons, while within the CA4 to CA2 in the severely affected subjects, the majority of neurons showed increased A2 immunoreactivity. An intense B1 immunoreactivity was observed throughout the CA subfields. In the CA1 subfield of the moderately affected subjects and in the extensive hippocampal regions of the severely affected subjects, a decrease in B1 immunoreactivity was observed. Double-immunolabeling studies demonstrated that tangle-bearing neurons reduced A2 and B1 immunoreactivity. Our study suggests that hnRNP A2 and B1 display different responses in the AD hippocampus, and further suggests that the post-transcriptional regulation is disturbed in neurons of the AD hippocampus.

Amyloid-beta in Alzheimer's disease: the horse or the cart? Pathogenic or protective?

While the pathogenesis of Alzheimer's disease (AD) is unclear, amyloid-beta plaques remain major lesions in the brain of individuals with AD. Likewise, amyloid-beta is one of the best-studied proteins relating to the pathogenesis of AD. Indeed, the pathological diagnosis of AD tends to be congruous with the quantity of amyloid-beta. However, it is important to recognize that pathological diagnosis merely represents the association of a pattern of pathological changes with a clinical phenotype. Therefore, it should be acknowledged that, although amyloid-beta detection and semiquantification have some diagnostic utility, the simple presence of amyloid plaques, as with proteinaceous accumulations in essentially all neurodegenerative diseases, does not presume aetiology. Thus, in this review, we discuss the role of amyloid-beta in the pathogenesis of AD and provide an alternative view to the widely accepted dogma.

Developmental origins of the age-related decline in cortical cholinergic function and associated cognitive abilities

Ontogenetic abnormalities in the regulation of the cortical cholinergic input system are hypothesized to mediate early-life cognitive limitations (ECL) that later escalate, based on reciprocal interactions between a dysregulated cholinergic system and age-related neuronal and vascular processes, to mild cognitive impairment (MCI) and, subsequently, for a majority of subjects, senile dementia. This process is speculated to begin with the disruption of trophic factor support of the basal forebrain ascending cholinergic system early in life, leading to dysregulation of cortical cholinergic transmission during the initial decades of life and associated limitations in cognitive capacities. Results from neurochemical and behavioral experiments support the possibility that aging reveals the vulnerability of an abnormally regulated cortical cholinergic input system. The decline of the cholinergic system is further accelerated as a result of interactions with amyloid precursor protein metabolism and processing, and with cerebral microvascular abnormalities. The determination of the developmental variables that render the cortical cholinergic input system vulnerable to age-related processes represents an important step toward the understanding of the role of this neuronal system in the age-related decline in cognitive functions.

Interactions between beta-amyloid and central cholinergic neurons: implications for Alzheimer's disease

Alzheimer's disease is an age-related neurodegenerative disorder that is characterized by a progressive loss of memory and deterioration of higher cognitive functions. The brain of an individual with Alzheimer's disease exhibits extracellular plaques of aggregated beta-amyloid protein (Abeta), intracellular neurofibrillary tangles that contain hyperphosphorylated tau protein and a profound loss of basal forebrain cholinergic neurons that innervate the hippocampus and the neocortex. Abeta accumulation may trigger or contribute to the process of neurodegeneration. However, the mechanisms whereby Abeta induces basal forebrain cholinergic cell loss and cognitive impairment remain obscure. Physiologically relevant concentrations of Abeta-related peptides have acute, negative effects on multiple aspects of acetylcholine (ACh) synthesis and release, without inducing toxicity. These data suggest a neuromodulatory influence of the peptides on central cholinergic functions. Long-term exposure to micromolar Abeta induces cholinergic cell toxicity, possibly via hyperphosphorylation of tau protein. Conversely, activation of selected cholinergic receptors has been shown to alter the processing of the amyloid precursor protein as well as phosphorylation of tau protein. A direct interaction between Abeta and nicotinic ACh receptors has also been demonstrated. This review addresses the role of Abeta-related peptides in regulating the function and survival of central cholinergic neurons and the relevance of these effects to cholinergic deficits in Alzheimer's disease. Understanding the functional interrelations between Abeta peptides, cholinergic neurons and tau phosphorylation will unravel the biologic events that precede neurodegeneration and may lead to the development of more effective pharmacotherapies for Alzheimer's disease.

Noradrenergic mechanisms in neurodegenerative diseases: a theory

A deficiency in the noradrenergic system of the brain, originating largely from cells in the locus coeruleus (LC), is theorized to play a critical role in the progression of a family of neurodegenerative disorders that includes Parkinson's disease (PD) and Alzheimer's disease (AD). Consideration is given here to evidence that several neurodegenerative diseases and syndromes share common elements, including profound LC cell loss, and may in fact be different manifestations of a common pathophysiological process. Findings in animal models of PD indicate that the modification of LC-noradrenergic activity alters electrophysiological, neurochemical and behavioral indices of neurotransmission in the nigrostriatal dopaminergic system, and influences the response of this system to experimental lesions. In models related to AD, noradrenergic mechanisms appear to play important roles in modulating the activity of the basalocortical cholinergic system and its response to injury, and to modify cognitive functions including memory and attention. Mechanisms by which noradrenaline may protect or promote recovery from neural damage are reviewed, including effects on neuroplasticity, neurotrophic factors, neurogenesis, inflammation, cellular energy metabolism and excitotoxicity, and oxidative stress. Based on evidence for facilitatory effects on transmitter release, motor function, memory, neuroprotection and recovery of function after brain injury, a rationale for the potential of noradrenergic-based approaches, specifically alpha2-adrenoceptor antagonists, in the treatment of central neurodegenerative diseases is presented.

Will preventing protein aggregates live up to its promise as prophylaxis against neurodegenerative diseases?

Protein aggregation and misfolding characterize most age-related neurodegenerative diseases including Alzheimer, Parkinson and Huntington diseases. Protein aggregation has generally been assumed to be responsible for neurodegeneration in these disorders due to association and genetics. However, protein aggregation may, in fact, be an attempt to protect neurons from the stress resulting from the disease etiology. In this review, we weigh the evidence of whether removal of amyloids, aggregates and neuronal inclusions represent a reasonable strategy for protecting neurons.

The impact of Aβ-plaques on cortical cholinergic and non-cholinergic presynaptic boutons in alzheimer's disease-like transgenic mice

A previous study in our laboratory, involving early stage, amyloid pathology in 8-month-old transgenic mice, demonstrated a selective loss of cholinergic terminals in the cerebral and hippocampal cortices of doubly transgenic (APP(K670N,M671L)+PSl(M146L)) mice, an up-regulation in the single mutant APP(K670N,M671L) mice and no detectable change in the PSl(M146L) transgenics [J Neurosci 19 (1999) 2706]. The present study investigates the impact of amyloid plaques on synaptophysin and vesicular acetylcholine transporter (VAChT) immunoreactive bouton numbers in the frontal cortex of the three transgenic mouse models previously described. When compared as a whole, the frontal cortices of transgenic and control mice show no observable differences in the densities of synaptophysin-immunoreactive boutons. An individual comparison of layer V of the frontal cortex, however, shows a significant increase in density in transgenic models. Analysis of the cholinergic system alone shows significant alterations in the VAChT-immunoreactive bouton densities as evidenced by an increased density in the single (APP(K670N,M671L)) transgenics and a decreased density in the doubly transgenics (APP(K670N,M671L)+PSl(M146L)). In investigating the impact of plaque proximity on bouton density at early stages of the amyloid pathology in our doubly (APP(K670N,M671L)+PSl(M146L)) transgenic mouse line, we observed that plaque proximity reduced cholinergic pre-synaptic bouton density by 40%, and yet increased synaptophysin-immunoreactive pre-synaptic bouton density by 9.5%. Distance from plaques (up to 60 microm) seemed to have no effect on bouton density; however a significant inverse relationship was visible between plaque size and cholinergic pre-synaptic bouton density. Finally, the number of cholinergic dystrophic neurites surrounding the truly amyloid, Thioflavin-S(+) plaque core, was disproportionately large with respect to the incidence of cholinergic boutons within the total pre-synaptic bouton population. Confocal and electron microscopic observations confirmed the preferential infiltration of dystrophic cholinergic boutons into fibrillar amyloid aggregates. We therefore hypothesize that extracellular Abeta aggregation preferentially affects cholinergic terminations prior to progression onto other neurotransmitter systems. This is supported by the observable presence of non-cholinergic sprouting, which may be representative of impending neuritic degeneration.

Upregulation and antiapoptotic role of endogenous Alzheimer amyloid precursor protein in dorsal root ganglion neurons

The amyloid precursor protein (APP) is a transmembrane protein whose abnormal processing is associated with the pathogenesis of Alzheimer's disease. In this study, we examined the expression and role of cell-associated APP in primary dorsal root ganglion (DRG) neurons. When dissociated DRG cells prepared from mouse embryos were treated with nerve growth factor (NGF), neuronal APP levels were transiently elevated. DRG neurons treated with an antibody against cell surface APP failed to mature and underwent apoptosis. When NGF was withdrawn from the cultures after a 36-h NGF treatment, virtually all neurons underwent apoptosis by 48 h. During the course of apoptosis, some neurons with intact morphology contained increased levels of APP immunoreactivity, whereas the APP levels were greatly reduced in apoptotic neurons. Furthermore, affected neurons contained immunoreactivities for activated caspase-3, a caspase-cleaved APP fragment (APPDeltaC31), and Abeta. Downregulation of endogenous APP expression by treatment with an APP antisense oligodeoxynucleotide significantly increased the number of apoptotic neurons in NGF-deprived DRG cultures. Furthermore, overexpression of APP by adenovirus vector-mediated gene transfer reduced the number of apoptotic neurons deprived of NGF. These results suggest that endogenous APP is upregulated to exert an antiapoptotic effect on neurotrophin-deprived DRG neurons and subsequently undergoes caspase-dependent proteolysis.

Neuroplasticity in Alzheimer's disease

Ramon y Cajal proclaimed in 1928 that "once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. In the adult centers the nerve paths are something fixed, ended and immutable. Everything must die, nothing may be regenerated. It is for the science of the future to change, if possible, this harsh decree." (Ramon y Cajal, 1928). In large part, despite the extensive knowledge gained since then, the latter directive has not yet been achieved by 'modern' science. Although we know now that Ramon y Cajal's observation on CNS plasticity is largely true (for lower brain and primary cortical structures), there are mechanisms for recovery from CNS injury. These mechanisms, however, may contribute to the vulnerability to neurodegenerative disease. They may also be exploited therapeutically to help alleviate the suffering from neurodegenerative conditions.

Fimbria-fornix lesion does not affect APP levels and amyloid deposition in the hippocampus of APP+PS1 double transgenic mice

The deposition of amyloid beta peptides (Abeta) and cholinergic dysfunction are two characteristic features of Alzheimer's disease. Several studies have suggested that a compromised cholinergic transmission can increase the amount of amyloid precursor protein (APP) in the denervated cortex (or hippocampus); however, whether this will increase Abeta production is unknown. To investigate the relation between cholinergic neurotransmission and APP metabolism, and the possible role of cholinergic dysfunction in the development of amyloid neuropathology, we lesioned the fimbria-fornix pathway in APP+PS1 double transgenic mice, at 5 and 7 months of age. Three months and 11 months postlesion, the mice were sacrificed for biochemical and histopathological analyses. The fimbria-fornix transection resulted in a substantial depletion of cholinergic markers in the hippocampus at both time points. Three months postlesion, hippocampal APP and Abeta levels were not significantly changed. At 11 months postlesion, the fimbria-fornix lesion did not result in an alteration in either the hippocampal Abeta levels or the extent of Abeta deposition, as assessed by amyloid plaque counts and image analysis of Abeta load in the 18-month-old APP+PS1 mice. Our findings indicate that APP metabolism in mice may be dissociated from cholinergic neurotransmission rather than related as previously suggested in other mammalian species.

Identification and comparative analysis of differentially expressed proteins in rat striatum following 6-hydroxydopamine lesions of the nigrostriatal pathway: up-regulation of amyloid precursor-like protein 2 expression

During neurodegenerative processes, cascades of degeneration and subsequent regeneration are triggered. However, the molecular nature of the factors involved in the neurodegeneration of the CNS remains largely unknown. In this study, the variations of protein expression in the striatum of adult Sprague-Dawley rats following 6-hydroxydopamine lesions were investigated, in order to better understand the molecular events occurring in the denervated target tissue. The rat striatum, ipsilateral to the lesion was analysed by two-dimensional gel electrophoresis followed by matrix assisted laser desorption/ionization-time of flight mass spectrometry. Seven proteins were up-regulated (188.1-750% compared to control) in response to the lesion: amyloid precursor-like protein 2 (APLP2), kininogen, glucokinase, tropomyosin alpha chain, type brain-1 and calpactin I light chain; whilst four proteins, neural epidermal growth factor-like 2, minichromosome maintenance 6, and thyroid hormone receptor beta-2, were down-regulated (to between 36% and 59% of levels in sham-operated controls). Three proteins that did not match with available data in the SWISS-PROT protein database were also determined. Immunohistochemical analysis demonstrated colocalization of APLP2 and tyrosine hydroxylase in the nigral neurons. Moreover, reduction of APLP2-positive neurons in the substantia nigra pars compacta as well as the increases in the substantia nigra pars reticulata and in the striatum were observed. Furthermore, the conditioned medium of the Chinese hamster ovary cells over-expressing APLP2-751 (chondroitin sulphate-modified), but not APLP2-763 (nonchondroitin sulphate-modified), was able to increase the number of the tyrosine hydroxylase-positive neurons in fetal mesencephalic cultures. These results suggest that the expression of APLP2, a protein that has been thought to be associated with Alzheimer's disease, is up-regulated in the striatum following dopaminergic denervation. They also support the view that chondroitin sulphate-modified APLP2 protein may play an important role in the dopaminergic nigrostriatal system.

The state versus amyloid-beta: the trial of the most wanted criminal in Alzheimer disease

Investigators studying the primary culprit responsible for Alzheimer disease have, for the past two decades, primarily focused on amyloid-beta (Abeta). Here, we put Abeta on trial and review evidence amassed by the prosecution that implicate Abeta and also consider arguments and evidence gathered by the defense team who are convinced of the innocence of their client. As in all trials, the arguments provided by the prosecution and defense revolve around the same evidence, with opposing interpretations. Below, we present a brief synopsis of the trial for you, the jury, to decide the verdict. Amyloid-beta: guilty or not-guilty?

Distinct subsets of nucleus basalis neurons exhibit similar sensitivity to excitotoxicity

Excitotoxic lesions in the magnocellular nucleus basalis (MBN) lead to a significant damage of cholinergic neurons concomitant with increased amyloid precursor protein (APP) expression in the cerebral cortex. However, the sensitivity of non-cholinergic neurons to excitotoxicity, and changes of APP expression in the damaged MBN are still elusive. Hence, we performed multiple-labeling immunocytochemistry for choline-acetyltransferase (ChAT), neuron-specific nuclear protein (NeuN) and APP 4, 24, and 48 h after NMDA infusion in the MBN. Whereas all cholinergic neurons were immunoreactive for NeuN, this neuronal marker also labeled a population of ChAT-immunonegative non-cholinergic neurons. Both neuron populations exhibited a similar degree of sensitivity to NMDA excitotoxicity that became evident as early as 4 h post-lesion. Cholinergic MBN neurons showed abundant APP immunoreactivity (approximately 90%), while only a fraction (approximately 20-30%) of non-cholinergic neurons expressed the protein. Remarkably, cholinergic but not non-cholinergic neurons retained their APP immunoreactivity after NMDA infusion. In conclusion, cholinergic MBN neurons are not preferentially sensitive to short-term excitotoxicity, but are one of the major sources of APP in the basal forebrain.

Differential expression of Bcl-2 and APP immunoreactivity after intrastriatal injection of MPP+ in the rat

While there is growing evidence that Bcl-2 proto-oncogene and beta-amyloid precursor proteins (APP) are neuroprotective in function, our recent studies have demonstrated that Bcl-2 and APP may be co-expressed and co-regulated in retinal neurons or glia under normal or experimental conditions. Whether Bcl-2 and APP are functionally coupled in other neuronal systems is not clear. This issue was investigated further in the present experiments by examining the expression pattern of two molecules after unilateral intrastriatal injection of 1-methyl-4-phenyl-pyridinium (MPP(+)), a neurotoxic metabolite that selectively damages dopaminergic neurons. One hour to 2 months after MPP(+) injection into rat striatum, a significant increase in Bcl-2 expression was observed in distinct populations of neurons, astrocyte-like and OX-42-positive cells not only in traumatic regions but also in remote areas including the ipsilateral cortex and substantia nigra (SN). No detectable change was observed in the striatum, cortex or SN on the contralateral side of the brain. The immunoreactive pattern and time-dependent APP increase was similar to that of Bcl-2 in the severely injured striatum and cortex. However, an up-regulation of Bcl-2 expression, but not APP, appears in dopaminergic neurons in the ipsilateral SN pars compacta where there was retrograde degeneration. In contrast, APP immunoreactivity was decreased in the hippocampus following intrastriatal injury, whereas, no alteration in Bcl-2 expression was detected. The differential changes in Bcl-2 and APP expression in nigral neurons and some other brain tissues suggest that these proteins may not be co-regulated by a common mechanism, at least in certain neuronal pathways.

Expression of amyloid precursor protein, tau and presenilin RNAs in rat hippocampus following deafferentation lesions

In this study, entorhinal cortex lesions and/or medial septal area cholinergic lesions were used in the rat to mimic some of the principal and earliest affects in Alzheimer's disease, namely hippocampal deafferentation. We wished to test the hypothesis that deafferentation lesions cause changes in the regulation of three proteins that are known to be important in Alzheimer's disease pathology, namely amyloid precursor protein, presenilin and tau. Expression of amyloid precursor protein mRNA was increased in several subfields of hippocampus when examined 1 week after entorhinal cortex lesion, but was reduced, compared to sham operated controls, after medial septal area cholinergic lesions. Cholinergic lesions were combined with entorhinal cortex lesions and produced no change in APP mRNA levels compared to controls. No significant changes were observed in the parietal cortex after entorhinal cortex or cholinergic lesions either alone or in combination. Tau mRNA level in hippocampus was unchanged after lesions. Presenilin-1 mRNA was expressed in the hippocampus at very low levels, and appeared to be increased following entorhinal cortex lesion. Our results support the hypothesis that amyloid precursor protein expression in hippocampal neurons is differentially affected by glutamatergic and cholinergic afferent input, and that presenilin-1, but not tau, may be subject to the same type of control in vivo.

The functions of the amyloid precursor protein gene

The amyloid precursor protein (APP) gene and its protein products have multiple functions in the central nervous system and fulfil criteria as neuractive peptides: presence, release and identity of action. There is increased understanding of the role of secretases (proteases) in the metabolism of APP and the production of its peptide fragments. The APP gene and its products have physiological roles in synaptic action, development of the brain, and in the response to stress and injury. These functions reveal the strategic importance of APP in the workings of the brain and point to its evolutionary significance.

Amyloid beta peptide levels and its effects on hippocampal acetylcholine release in aged, cognitively-impaired and -unimpaired rats

Excessive extracellular deposition of amyloid beta (Abeta) peptide in neuritic plaques and degeneration of forebrain cholinergic neurones, which innervate the hippocampus and the neocortex, are the invariant characteristic features of Alzheimer's disease (AD). Studies of the pathological changes that characterize AD, together with several other lines of evidence, indicate that Abeta accumulation in vivo may initiate and/or contribute to the process of neurodegeneration observed in the AD brain. However, the underlying mechanisms by which Abeta peptide influences/causes degeneration of the basal forebrain cholinergic neurones in AD brains remain obscure. We reported earlier that nM concentrations of Abeta-related peptides, under acute conditions, can potently inhibit K+-evoked endogenous acetylcholine (ACh) release from the hippocampus and the cortex but not from striatum in young adult rats (J. Neurosci. 16 (1996) 1034). In the present study, to determine whether the effects of Abeta peptides alter with normal aging and/or cognitive state, we have measured Abeta1-40 levels and the effects of exogenous Abeta1-40 on hippocampal ACh release in young adult as well as aged cognitively-unimpaired (AU) and -impaired (AI) rats. Endogenous levels of Abeta(1-40) in the hippocampus are significantly increased in aged rats. Additionally, 10 nM Abeta1-40 potently inhibited endogenous ACh release from the hippocampus of the three groups of rats, but the time-course of the effects clearly indicate that the cholinergic neurones of AI rats are more sensitive to Abeta peptides than either AU or young adult rats. These results, together with earlier reports, suggest that the processing of the precursor protein of Abeta peptide alters with normal aging and the response of the cholinergic neurones to the peptide possibly varies with the cognitive status of the animals.

Increased amyloid precursor protein expression and serotonergic sprouting following excitotoxic lesion of the rat magnocellular nucleus basalis: neuroprotection by Ca(2+) antagonist nimodipine

In the present study plastic neural responses to N-methyl-D-aspartate-induced excitotoxic lesions and the neuroprotective effects of the L-type voltage-dependent Ca(2+) channel antagonist nimodipine were investigated in the rat magnocellular nucleus basalis. Assessment of spontaneous behaviour in the elevated plus maze and small open-field paradigms on day 5 and day 14 post-surgery indicated anxiety and persistent hypoactivity of N-methyl-D-aspartate-lesioned rats, as compared with sham-operated controls. Nimodipine administration significantly alleviated the behavioural deficits. Quantitative histochemical analysis of acetylcholinesterase-positive fibre innervation of the somatosensory cortex and determination of the numbers of choline-acetyltransferase-positive proximal fibre branches of cholinergic projection neurons in the magnocellular nucleus basalis demonstrated a severe cholinergic deficit as a consequence of the excitotoxic lesion 14 days post-surgery. Nimodipine pre-treatment significantly attenuated the loss of cortical cholinergic innervation and preserved the functional integrity of cholinergic projection neurons in the magnocellular nucleus basalis. Double-labelling immunocytochemistry demonstrated increased amyloid precursor protein expression in shrinking and presumably apoptotic choline-acetyltransferase-positive neurons, whereas surviving cholinergic nerve cells were devoid of excessive amyloid precursor protein immunoreactivity. Moreover, as a consequence of N-methyl-D-aspartate infusion, rim-like accumulation of amyloid precursor protein-positive astrocytes was visualized in a penumbra-like zone of the excitotoxic injury. Furthermore, abundant sprouting of serotonergic projection fibres invading the damaged magnocellular nucleus basalis subdivision was demonstrated. Pharmacological blockade by the Ca(2+) antagonist nimodipine significantly attenuated both neuronal and glial amyloid precursor protein immunoreactivity and serotonergic fibre sprouting following N-methyl-D-aspartate infusion. The present data characterize plastic endogenous glial and neuronal responses in the magnocellular nucleus basalis model of acute excitotoxic brain damage. The increased amyloid precursor protein expression may indicate effective means of intrinsic neuroprotection, as secreted amyloid precursor protein isoforms are suggested to play a role in neuronal rescue following excitotoxic injury. From a pharmacological point of view, extensive sprouting of serotonergic projections in the damaged magnocellular nucleus basalis may also counteract N-methyl-D-aspartate excitotoxicity via serotonin-induced inhibition of Ca(2+) currents and membrane hyperpolarization. Hence, lesion-induced changes in spontaneous animal behaviour, such as anxiety and novelty-induced hypoactivity, may well be attributed to the considerable re-distribution of serotonergic projections in the basal forebrain. In conclusion, our present data emphasize a role of neuron-glia and neurotransmitter-system interactions in functional recovery after acute excitotoxic brain injury, and the efficacy of L-type Ca(2+) channel blockade by the selective 1,4-dihydropyridine antagonist nimodipine.

Characterization of the neurotrophic interaction between nerve growth factor and secreted alpha-amyloid precursor protein

The expression and secretion of amyloid precursor protein (beta APP) is increased in rat cerebral cortices that have been denervated by subcortical lesions of the nucleus basalis of Meynert. The physiological role of the secreted beta APP in response to this injury has not been established. We have previously shown that secreted beta APP produced by alpha-secretase activity (sAPP(alpha)) potentiates the neuritogenic activity of nerve growth factor (NGF) in vitro on naive PC12 cells. In this investigation, we have further characterized the neurotrophic interaction of NGF and sAPP(alpha) using differentiated PC12 cells and rat primary cortical neurons. NGF required the expression of beta APP to maintain a neuronal phenotype. Reduction of endogenous beta APP expression by introduction of antisense oligonucleotides in the presence of NGF resulted in loss of neurites from differentiated PC12 cells but no apparent cell death. Addition of exogenous sAPP(alpha) (60--200 pM) potentiated the protective activity of NGF in serum-deprived differentiated PC12 cells as determined by retention of neurites and cell viability. In addition, exogenous sAPP(alpha) increased neuron viability in both short-term (3 days) cortical neuron cultures grown in the absence of serum and in long-term (9 days) cultures grown with serum. Disruption of the insulin signaling pathway by reduction of IRS-1 expression inhibited the ability of sAPP(alpha) to potentiate neurotrophic activity. These observations suggest that sAPP(alpha) acts as an injury-induced neurotrophic factor that interacts with NGF to enhance neuronal viability using the insulin signaling pathway.

A plasticity-based theory of the pathogenesis of Alzheimer's disease

Amyloid plaques (APs) and neurofibrillary tangles (NFTs) are the two diagnostic markers of Alzheimer's disease (AD). The neuropsychological features of AD are closely correlated with the distribution of the NFTs and therefore favor a disease process revolving around neurofibrillary degeneration. The genetics, however, favor a disease process revolving around the APs, principally because mutations in the amyloid precursor protein (A beta PP) are sufficient to cause AD. The inability to reconcile these two aspects of AD has prevented the formulation of a unified theory of pathogenesis. It is interesting to note that all genetic causes and risk factors of AD can increase the physiological burden of neuroplasticity. My hypothesis is that the resultant intensification of the plasticity burden leads to an initially adaptive upregulation of tau phosphorylation and A beta PP turnover, to the subsequent formation of NFTs and APs as independent consequences of excessive plasticity-related cellular activity, and to the eventual loss of neurons, dendrites, and synapses as the ultimate expression of plasticity failure. The two pathological markers of AD are therefore independent manifestations of a more fundamental process through which the many different genotypes of AD trigger an identical clinical and neuropathological phenotype.

A beta peptides and calcium influence secretion of the amyloid protein precursor from chick sympathetic neurons in culture

The major constituent of amyloid plaques in the Alzheimer disease (AD) brain is the amyloid protein (A beta). A beta has been shown to be neurotoxic to cells, but the exact mechanism of its effects are still not known. Most studies have focussed on A beta neurotoxicity, but little is known about the effect of A beta peptides on cellular protein metabolism and secretion. To examine the effect of A beta peptides on APP secretion, chick sympathetic neurons were metabolically labeled with [(35)S]methionine and the amounts of radiolabeled APP and A beta quantitated. Several A beta peptides (A beta(25-35), [pyroglu(3)]A beta(3-40), and [pyroglu(11)]A beta(11-40)) inhibited secretion of [(35)S]APP and increased cell-associated [(35)S]APP. There was also a 2-2.5-fold increase in secretion of several other proteins when cells were incubated with A beta(25-35). However, the amount of A beta secreted into the medium was decreased. Treatment of cells with the calcium ionophore A23187 caused a 1.5-fold increase in secreted [(35)S]APP and a decrease in cell-associated [(35)S]APP. Although L-type voltage-dependent calcium channels (VDCC) have been implicated in A beta toxicity, the effect of L-type VDCC on APP secretion has not previously been examined. The L-type VDCC antagonists nifedipine and diltiazem both increased [(35)S]APP secretion into the medium but did not influence the effect of A beta on [(35)S]APP secretion. These studies suggest that A beta interferes with the secretory pathway of APP. Insofar as secreted APP has been proposed to have a neuroprotective function, the accumulation of A beta in the AD brain could decrease secreted APP and thereby indirectly increase A beta toxicity.

A positive-feedback model for the loss of acetylcholine in Alzheimer's disease

We describe a two-component positive-feedback system that could account for the large reduction of acetylcholine that is characteristic of patients with Alzheimer's disease (AD). One component is beta-amyloid-induced apoptosis of cholinergic cells, leading to a decrease in acetylcholine. The other component is an increase in the concentration of beta-amyloid in response to a decrease in acetylcholine. We describe each mechanism with a differential equation, and then solve the two equations numerically. The solution provides a description of the time course of the reduction of acetylcholine in AD patients that is consistent with epidemiological data. This model may also provide an explanation for the significant, but lesser, decrease of other neurotransmitters that is characteristic of AD.

Differentiated cerebrovascular effects of physostigmine and tacrine in cortical areas deafferented from the nucleus basalis magnocellularis suggest involvement of basalocortical projections to microvessels

Cholinesterase inhibitors used to treat Alzheimer's disease according to the principle of cholinergic replacement therapy have proved to be less beneficial than expected. The present study was designed to investigate the cerebrovascular response to physostigmine and tacrine in the experimental model of lesioning of the nucleus basalis magnocellularis (NBM), a model involving a cholinergic deficit. Regional cerebral blood flow was measured by the [14C]iodoantipyrine tissue sampling technique in conscious rats infused with i.v. physostigmine (0.2 mg/kg/h), tacrine (8 mg/kg/h), or saline, 3-5 weeks after unilateral lesion of the NBM with ibotenic acid. Physostigmine and tacrine dose-dependently increased blood flow in most cortical and subcortical regions compared to the control group. However, physostigmine caused smaller blood flow increases in several areas, mostly cortical, of the lesioned compared to the intact hemisphere. The converse was observed with tacrine. A facilitated circulatory response appeared in cortical areas deafferented from the NBM, especially in the frontal cortex. These results provide evidence for distinct NBM-dependent components of the cortical cerebrovascular effects of physostigmine and tacrine. They suggest the involvement of different cellular postsynaptic targets of the NBM. The physostigmine-type effects could involve direct projects onto an inhibitory cortical interneuron supersensitized by deafferentation. This arrangement may explain why physostigmine and perhaps other cholinergic agonists are unable to specifically compensate for a deficit in NBM functioning. The tacrine-type effects presumably involve projections to the microvasculature, including perivascular astrocytes. The neurovascular junction would be sensitized by deafferentation from the NBM. Our data suggest that the regulatory mechanisms of blood flow originating in the NBM might constitute a target of neurodegenerative processes of Alzheimer's disease.

Cholinergic deafferentation of the rabbit cortex: a new animal model of Abeta deposition

Brain deposition of the amyloid beta-peptide (Abeta) is a critical step in the pathogenesis of Alzheimer's disease (AD) and human cerebral amyloid angiopathy (CAA). A small fraction of AD and CAA cases are caused by gene mutations leading to increased production and deposition of Abeta, but for the majority, there is no known direct genetic cause. We have hypothesized that Abeta deposition in these sporadic cases occurs as a result of cortical cholinergic deafferentation. Here we show that cortical cholinergic deafferentation, induced in rabbits by a selective immunotoxin, leads to Abeta deposition in cerebral blood vessels and perivascular neuropil. Biochemical measurements confirmed that lesioned animals had 2.5- and 8-fold elevations of cortical Abeta40 and Abeta42, respectively. Cholinergic deafferentation may be one factor that can contribute to Abeta deposition.

Altered expression of amyloid precursors proteins after traumatic brain injury in rats: in situ hybridization and immunohistochemical study

The expression of alternatively spliced mRNAs for amyloid precursor protein (APP) isoforms and their translation products were examined in the rat cerebral cortex 1, 3, 6, and 12 h and 1, 3, and 7 days (n = 4-5 in each group) after fluid-percussion brain injury. In situ hybridization studies demonstrated that the expression of APP695 mRNA decreased in and around the damaged area of the cerebral cortex exposed to fluid-percussion injury 1 h after the insult. On the other hand, APP751/770 mRNAs were increased in the regions surrounding the damaged cortical areas 1 day after the injury. An increase of immunoreactive APP was detected in the regions around the damaged cortical areas 3 h after traumatic injury and maintained for the following 3 days. The APP immunoreactivity in the damaged cortices declined to the level of sham-operated animals by post-experimental day 7. Using an anti-amyloid beta (Abeta) protein (17-24) antibody, no deposits of immunoreactive Abeta (17-24) were observed in any of the samples examined in these experiments. These results suggest that the induction of Kunitz-type protease inhibitor (KPI) domain-containing APP mRNAs and the increased accumulation of APP are involved in the physiological and neuropathological responses of brains under various neurodegenerative conditions, including head trauma.

Cognitive changes and modified processing of amyloid precursor protein in the cortical and hippocampal system after cholinergic synapse loss and muscarinic receptor activation

A number of in vitro studies have shown that activation of muscarinic receptors by cholinergic agonists stimulates the nonamyloidogenic, alpha-secretase-processing pathway of amyloid precursor protein (APP). To determine whether increased cholinergic neurotransmission can modify the APP processing in vivo, we administered a muscarinic receptor agonist (RS86) to normal or aged rats and rats with severe basal forebrain cholinergic deficits (induced by 192 IgG-saporin). The levels of the cell-associated APP in neocortex, hippocampus, and striatum, as well as the secreted form of APP (APPs) in cerebrospinal fluid, were examined by Western blots. Additionally, we investigated the association between the altered APP levels and behavioral deficits caused by cholinergic lesions. We found that treatment with muscarinic receptor agonist resulted in decreased APP levels in neocortex and hippocampus and increased levels of APPs in cerebrospinal fluid. Regulation of APP processing by the muscarinic agonist treatment occurred not only in normal rats, but also in aged and cholinergic denervated rats that model this aspect of Alzheimer's disease. Interestingly, we found that elevation of APP in neocortex correlated with the cognitive deficits in water-maze testing of rats with cholinergic dysfunction. These data indicate that increased cholinergic neurotransmission can enhance nonamyloidogenic APP processing in intact and lesioned rats and that APP may be involved in cognitive performance.

Reduced cerebrospinal fluid levels of alpha-secretase-cleaved amyloid precursor protein in aged rats: correlation with spatial memory deficits

The amyloid precursor protein undergoes proteolysis at several sites to yield a number of functionally relevant peptides, including beta-amyloid and the soluble amyloid precursor protein derivatives alpha-soluble amyloid precursor protein and beta-soluble amyloid precursor protein. beta-Amyloid is the primary constituent of senile plaques associated with Alzheimer's disease, while a-soluble amyloid precursor protein promotes synaptogenesis and plays a role in neuroprotective processes. We tested for age-related alterations in these amyloid precursor protein proteolytically derived peptides by measuring the levels of alpha-soluble amyloid precursor protein, total soluble amyloid precursor proteins (alpha- and beta-soluble amyloid precursor protein combined) and beta-amyloid in cerebrospinal fluid from three-, 13- and 23-month-old Fischer-344 rats. Western blot analysis using selective antibodies revealed 50% less total soluble amyloid precursor protein and a-soluble amyloid precursor protein in cisternal cerebrospinal fluid from 23-month-old rats compared with three- and 13-month-old animals. Mass spectrometric analysis indicated, however, that beta-amyloid in cerebrospinal fluid was not different between the three age groups. In a second group of young (five to six months of age) and aged (24-25 months of age) rats, spatial working and reference memory were assessed in a water maze followed by collection of cerebrospinal fluid. As a group, the aged rats consistently performed below the young rats in both working and reference memory tests. The aged rats also had 49% less cerebrospinal fluid alpha-soluble amyloid precursor protein than did their younger counterparts. There was a positive correlation (r= 0.52-0.57, P < 0.001) between performance in spatial memory tasks and cerebrospinal fluid alpha-soluble amyloid precursor protein in these young and aged rats. These results suggest that there is a positive association between cerebrospinal fluid levels of alpha-soluble amyloid precursor protein and cognitive performance in rats, and that alpha-soluble amyloid precursor protein may be involved in the spatial learning and memory changes that accompany ageing.