Central cholinergic functions in human amyloid precursor protein knock-in/presenilin-1 transgenic mice

Rodent Models of Alzheimer's Disease: Past Misconceptions and Future Prospects

Alzheimer's disease (AD) is a progressive neurodegenerative disease with no effective treatments, not least due to the lack of authentic animal models. Typically, rodent models recapitulate the effects but not causes of AD, such as cholinergic neuron loss: lesioning of cholinergic neurons mimics the cognitive decline reminiscent of AD but not its neuropathology. Alternative models rely on the overexpression of genes associated with familial AD, such as amyloid precursor protein, or have genetically amplified expression of mutant tau. Yet transgenic rodent models poorly replicate the neuropathogenesis and protein overexpression patterns of sporadic AD. Seeding rodents with amyloid or tau facilitates the formation of these pathologies but cannot account for their initial accumulation. Intracerebral infusion of proinflammatory agents offer an alternative model, but these fail to replicate the cause of AD. A novel model is therefore needed, perhaps similar to those used for Parkinson's disease, namely adult wildtype rodents with neuron-specific (dopaminergic) lesions within the same vulnerable brainstem nuclei, 'the isodendritic core', which are the first to degenerate in AD. Site-selective targeting of these nuclei in adult rodents may recapitulate the initial neurodegenerative processes in AD to faithfully mimic its pathogenesis and progression, ultimately leading to presymptomatic biomarkers and preventative therapies.

Posttraumatic stress disorder-like induction elevates β-amyloid levels, which directly activates corticotropin-releasing factor neurons to exacerbate stress responses

Recent studies have found that those who suffer from posttraumatic stress disorder (PTSD) are more likely to experience dementia as they age, most often Alzheimer's disease (AD). These findings suggest that the symptoms of PTSD might have an exacerbating effect on AD progression. AD and PTSD might also share common susceptibility factors such that those who experience trauma-induced disease were already more likely to succumb to dementia with age. Here, we explored these two hypotheses using a mouse model of PTSD in wild-type and AD model animals. We found that expression of human familial AD mutations in amyloid precursor protein and presenilin 1 leads to sensitivity to trauma-induced PTSD-like changes in behavioral and endocrine stress responses. PTSD-like induction, in turn, chronically elevates levels of CSF β-amyloid (Aβ), exacerbating ongoing AD pathogenesis. We show that PTSD-like induction and Aβ elevation are dependent on corticotropin-releasing factor (CRF) receptor 1 signaling and an intact hypothalamic-pituitary-adrenal axis. Furthermore, we show that Aβ species can hyperexcite CRF neurons, providing a mechanism by which Aβ influences stress-related symptoms and PTSD-like phenotypes. Consistent with Aβ causing excitability of the stress circuitry, we attenuate PTSD-like phenotypes in vivo by lowering Aβ levels during PTSD-like trauma exposure. Together, these data demonstrate that exposure to PTSD-like trauma can drive AD pathogenesis, which directly perturbs CRF signaling, thereby enhancing chronic PTSD symptoms while increasing risk for AD-related dementia.

Cholinergic Depletion in Alzheimer's Disease Shown by [ (18) F]FEOBV Autoradiography

Rationale. Alzheimer's Disease (AD) is a neurodegenerative condition characterized in part by deficits in cholinergic basalocortical and septohippocampal pathways. [¹⁸F]Fluoroethoxybenzovesamicol ([¹⁸F]FEOBV), a Positron Emission Tomography ligand for the vesicular acetylcholine transporter (VAChT), is a potential molecular agent to investigate brain diseases associated with presynaptic cholinergic losses. Purpose. To demonstrate this potential, we carried out an [¹⁸F]FEOBV autoradiography study to compare postmortem brain tissues from AD patients to those of age-matched controls. Methods. [¹⁸F]FEOBV autoradiography binding, defined as the ratio between regional grey and white matter, was estimated in the hippocampus (13 controls, 8 AD) and prefrontal cortex (13 controls, 11 AD). Results. [¹⁸F]FEOBV binding was decreased by 33% in prefrontal cortex, 25% in CA3, and 20% in CA1. No changes were detected in the dentate gyrus of the hippocampus, possibly because of sprouting or upregulation toward the resilient glutamatergic neurons of the dentate gyrus. Conclusion. This is the first demonstration of [¹⁸F]FEOBV focal binding changes in cholinergic projections to the cortex and hippocampus in AD. Such cholinergic synaptic (and more specifically VAChT) alterations, in line with the selective basalocortical and septohippocampal cholinergic losses documented in AD, indicate that [¹⁸F]FEOBV is indeed a promising ligand to explore cholinergic abnormalities in vivo.

Pathophysiological amyloid concentrations induce sustained upregulation of readthrough acetylcholinesterase mediating anti-apoptotic effects

Cholinergically differentiated SH-SY5Y neuroblastoma cells were treated with a pathophysiologically relevant, low (300 nM), and a high (3 μM) dose of amyloid beta 1-42 (Abeta) or 42-1 (revAbeta). At early (1 and 4h) and late (24h) time points, the pro- and anti-apoptotic factors--caspase-3 and p53, and B-cell lymphoma 2 protein (Bcl-2), respectively--were assessed together with lactate dehydrogenase (LDH) release as measure of cell viability and ATP levels as marker of mitochondrial activity. The low peptide dose significantly increased Bcl-2 and, time-delayed, caspase-3 and ATP levels, but barely impacted on LDH release, while the high concentration remarkably depressed Bcl-2 levels, depleted ATP and led to increased LDH release. We also monitored acetylcholinesterase (AChE) enzymatic activity and splice variant levels (tailed and readthrough AChE; AChE-T and AChE-R), and assessed choline acetyltransferase (ChAT) and high-affinity choline uptake (HACU). The low Abeta concentration drastically upregulated AChE-R and increased both ChAT and HACU, while the high dose caused cholinergic toxicity. We believe this study offers the first insight into the highly concentration-dependent effects of Abeta on cholinergic dynamics. In particular, it highlights the rescuing role of AChE-R as being, together with mitochondrial activity, involved in cholinergic adaptation to low doses of Abeta.

APP physiological and pathophysiological functions: insights from animal models

The amyloid precursor protein (APP) has been under intensive study in recent years, mainly due to its critical role in the pathogenesis of Alzheimer's disease (AD). β-Amyloid (Aβ) peptides generated from APP proteolytic cleavage can aggregate, leading to plaque formation in human AD brains. Point mutations of APP affecting Aβ production are found to be causal for hereditary early onset familial AD. It is very likely that elucidating the physiological properties of APP will greatly facilitate the understanding of its role in AD pathogenesis. A number of APP loss- and gain-of-function models have been established in model organisms including Caenorhabditis elegans, Drosophila, zebrafish and mouse. These in vivo models provide us valuable insights into APP physiological functions. In addition, several knock-in mouse models expressing mutant APP at a physiological level are available to allow us to study AD pathogenesis without APP overexpression. This article will review the current physiological and pathophysiological animal models of APP.

Amyloid precursor protein regulates Cav1.2 L-type calcium channel levels and function to influence GABAergic short-term plasticity

Amyloid precursor protein (APP) has been strongly implicated in the pathogenesis of Alzheimer's disease (AD). Although impaired synaptic function is believed to be an early and causative event in AD, how APP physiologically regulates synaptic properties remains poorly understood. Here, we report a critical role for APP in the regulation of L-type calcium channels (LTCC) in GABAergic inhibitory neurons in striatum and hippocampus. APP deletion in mice leads to an increase in the levels of Ca(v)1.2, the pore-forming subunit of LTCCs, and subsequent increases in GABAergic calcium currents (I(Ca(2+))) that can be reversed by reintroduction of APP. Upregulated levels of Ca(v)1.2 result in reduced GABAergic paired-pulse inhibition and increased GABAergic post-tetanic potentiation in both striatal and hippocampal neurons, indicating that APP modulates synaptic properties of GABAergic neurons by regulating Ca(v)1.2. Furthermore, APP physically interacts with Ca(v)1.2, suggesting a mechanism in which loss of APP leads to an inappropriate accumulation and aberrant activity of Ca(v)1.2. These results provide a direct link between APP and calcium signaling and might help explain how altered APP regulation leads to changes in synaptic function that occur with AD.

Neurotransmitters and energy metabolites in amyloid-bearing APP(SWE)xPSEN1dE9 Mouse Brain

Alzheimer's disease is characterized by amyloid peptide formation and deposition, neurofibrillary tangles, synaptic loss and central cholinergic dysfunction, dysfunction of energy metabolism, and dementia; however, the interactions between these hallmarks remain poorly defined. We studied a well characterized mouse model of amyloid deposition, the doubly transgenic APP(SWE)xPSEN1dE9 mouse. At 10 to 14 months of age, these mice had high levels of amyloid peptides (6.6 microg/g wet weight) and widespread amyloid plaques. Extracellular levels of acetylcholine (ACh) were determined by microdialysis in the hippocampus and were comparable with nontransgenic mice from the same colony. In the open field, both mouse strains responded with a 3-fold increase of hippocampal ACh release. Exploratory behavior of the transgenic mice appeared normal. Infusion of scopolamine evoked 5- to 6-fold increases of ACh levels in both mouse strains. High-affinity choline uptake and cholinesterase activities were identical in both mouse lines. Extracellular levels of glucose and glycerol were similar in control and transgenic mice, whereas lactate levels were slightly (p = 0.06) and glutamate levels significantly (p = 0.02) lower in transgenic mice. Exploration caused increases of glucose and lactate, whereas infusion of scopolamine (1 microM) increased glucose but not lactate. Glutamate levels were increased by scopolamine, whereas glycerol remained constant under all the conditions. We conclude that amyloid peptide production and plaque deposition causes minor changes in cholinergic function and energy metabolites in transgenic mice in vivo. Amyloid peptide formation and/or deposition may not be sufficient for long-term cholinergic or metabolic dysfunction.

Intracellular Aß triggers neuron loss in the cholinergic system of the APP/PS1KI mouse model of Alzheimer's disease

Loss of cholinergic neurons in the Nucleus Basalis of Meynert in Alzheimer's disease (AD) patients was one of the first discoveries of neuron loss in AD. Despite an intense focus on the cholinergic system in AD, the reason for this cholinergic neuron loss is yet unknown. In the present study we examined Abeta-induced pathology and neuron loss in the cholinergic system of the bigenic APP/PS1KI mouse model. Expression of the APP transgene was found in ChAT-positive neurons of motor nuclei accompanied by robust intracellular Abeta accumulation, whereas no APP expressing neurons and thus no intracellular Abeta accumulation were found in neither the forebrain or pons complexes, nor in the caudate putamen. This expression pattern was used as a model system to study the effect of intra- and extracellular Abeta accumulation on neuron loss in the cholinergic system. Stereological quantification revealed a loss of ChAT-positive neurons in APP/PS1KI mice only in the motor nuclei Mo5 and 7N accumulating intracellular Abeta. This study supports the hypothesis of intracellular Abeta accumulation as an early pathological alteration contributing to cell death in AD.

Impaired muscarinic regulation of excitatory synaptic transmission in the APPswe/PS1dE9 mouse model of Alzheimer's disease

Cholinergic hypothesis and amyloid cascade hypothesis are mainly proposed for Alzheimer's disease; however, the relationship between these hypotheses is poorly understood. To address the question of whether amyloid beta-peptide pathology affects cholinergic neurotransmission, we examined the effect of a cholinesterase inhibitor, physostigmine, on field excitatory postsynaptic potentials (EPSPs) evoked by single-pulse stimulation in the CA1 region of the hippocampus of various APPswe/PS1dE9 transgenic mice with different degrees of amyloid beta-peptide pathology. Reduced field EPSPs by physostigmine in transgenic mice at 3 months of age, when the mice had negligible amyloid beta-peptide levels and no amyloid beta-peptide deposits, were indistinguishable from those in age-matched wild-type mice. In contrast, reduced field EPSPs by physostigmine in transgenic mice at 5 months of age, when the mice had low amyloid beta-peptide levels and subtle amyloid beta-peptide deposits, were significantly lower than those in age-matched wild-type mice. Next, we characterized acetylcholine receptors, which play important roles in cholinergic neurotransmission, because physostigmine resulted in increased acetylcholine levels in the synaptic cleft. Different reductions of field EPSPs by physostigmine between transgenic and wild-type mice at 5 months of age were not affected by a nicotinic receptor antagonist, mecamylamine; however, reduced field EPSPs by physostigmine in both transgenic and wild-type mice were restored to basal levels by a muscarinic receptor antagonist, atropine. These results indicate that cholinergic modulation of glutamatergic transmission is already impaired at the onset of the formation of amyloid beta-peptide deposits, and muscarinic receptor dysfunction is one of the causes of this impairment.

Alzheimer disease models and human neuropathology: similarities and differences

Animal models aim to replicate the symptoms, the lesions or the cause(s) of Alzheimer disease. Numerous mouse transgenic lines have now succeeded in partially reproducing its lesions: the extracellular deposits of Abeta peptide and the intracellular accumulation of tau protein. Mutated human APP transgenes result in the deposition of Abeta peptide, similar but not identical to the Abeta peptide of human senile plaque. Amyloid angiopathy is common. Besides the deposition of Abeta, axon dystrophy and alteration of dendrites have been observed. All of the mutations cause an increase in Abeta 42 levels, except for the Arctic mutation, which alters the Abeta sequence itself. Overexpressing wild-type APP alone (as in the murine models of human trisomy 21) causes no Abeta deposition in most mouse lines. Doubly (APP x mutated PS1) transgenic mice develop the lesions earlier. Transgenic mice in which BACE1 has been knocked out or overexpressed have been produced, as well as lines with altered expression of neprilysin, the main degrading enzyme of Abeta. The APP transgenic mice have raised new questions concerning the mechanisms of neuronal loss, the accumulation of Abeta in the cell body of the neurons, inflammation and gliosis, and the dendritic alterations. They have allowed some insight to be gained into the kinetics of the changes. The connection between the symptoms, the lesions and the increase in Abeta oligomers has been found to be difficult to unravel. Neurofibrillary tangles are only found in mouse lines that overexpress mutated tau or human tau on a murine tau -/- background. A triply transgenic model (mutated APP, PS1 and tau) recapitulates the alterations seen in AD but its physiological relevance may be discussed. A number of modulators of Abeta or of tau accumulation have been tested. A transgenic model may be analyzed at three levels at least (symptoms, lesions, cause of the disease), and a reading key is proposed to summarize this analysis.

Cofilin-mediated neurodegeneration in Alzheimer's disease and other amyloidopathies

Transport defects may arise in various neurodegenerative diseases from failures in molecular motors, microtubule abnormalities, and the chaperone/proteasomal degradation pathway leading to aggresomal-lysosomal accumulations. These defects represent important steps in the neurodegenerative cascade, although in many cases, a clear consensus has yet to be reached regarding their causal relationship to the disease. A growing body of evidence lends support to a link between neurite transport defects in the very early stages of many neurodegenerative diseases and alterations in the organization and dynamics of the actin cytoskeleton initiated by filament dynamizing proteins in the ADF/cofilin family. This article focuses on cofilin, which in neurons under stress, including stress induced by the amyloid-beta (Abeta) 1-42 peptide, undergoes dephosphorylation (activation) and forms rod-shaped actin bundles (rods). Rods inhibit transport, are sites of amyloid precursor protein accumulation, and contribute to the pathology of Alzheimer's disease. Because rods form rapidly in response to anoxia, they could also contribute to synaptic deficits associated with ischemic brain injury (e.g., stroke). Surprisingly, cofilin undergoes phosphorylation (inactivation) in hippocampal neurons treated with Abeta1-40 at high concentrations, and these neurons undergo dystrophic morphological changes, including accumulation of pretangle phosphorylated-tau. Therefore, extremes in phosphoregulation of cofilin by different forms of Abeta may explain much of the Alzheimer's disease pathology and provide mechanisms for synaptic loss and plaque expansion.

The place of choline acetyltransferase activity measurement in the "cholinergic hypothesis" of neurodegenerative diseases

The so-called "cholinergic hypothesis" assumes that degenerative dysfunction of the cholinergic system originating in the basal forebrain and innervating several cortical regions and the hippocampus, is related to memory impairment and neurodegeneration found in several forms of dementia and in brain aging. Biochemical methods measuring the activity of the key enzyme for acetylcholine synthesis, choline acetyltransferase, have been used for many years as a reliable marker of the integrity or the damage of the cholinergic pathways. Stereologic counting of the basal forebrain cholinergic cell bodies, has been additionally used to assess neurodegenerative changes of the forebrain cholinergic system. While initially believed to mark relatively early stages of disease, cholinergic dysfunction is at present considered to occur in advanced dementia of Alzheimer's type, while its involvement in mild and prodromal stages of the disease has been questioned. The issue is relevant to better understand the neuropathological basis of the diseases, but it is also of primary importance for therapy. During the last few years, indeed, cholinergic replacement therapies, mainly based on the use of acetylcholinesterase inhibitors to increase synaptic availability of acetylcholine, have been exploited on the assumption that they could ameliorate the progression of the dementia from its initial stages. In the present paper, we review data from human studies, as well as from animal models of Alzheimer's and Down's diseases, focusing on different ways to evaluate cholinergic dysfunction, also in relation to the time point at which these dysfunctions can be demonstrated, and on some discrepancy arising from the use of different methodological approaches. The reviewed literature, as well as some recent data from our laboratories on a mouse model of Down's syndrome, stress the importance of performing biochemical evaluation of choline acetyltransferase activity to assess cholinergic dysfunction both in humans and in animal models.

Adaptation to excess acetylcholine by downregulation of adrenoceptors and muscarinic receptors in lungs of acetylcholinesterase knockout mice

The acetylcholinesterase knockout mouse has elevated acetylcholine levels due to the complete absence of acetylcholinesterase. Our goal was to determine the adaptive changes in lung receptors that allow these animals to tolerate excess neurotransmitter. The hypothesis was tested that not only muscarinic receptors but also alpha(1)-adrenoceptors and beta-adrenoceptors are downregulated, thus maintaining a proper balance of receptors and accounting for lung function in these animals. The quantity of alpha(1A), alpha(1B), alpha(1D), beta(1), and beta(2)-adrenoceptors and muscarinic receptors was determined by binding of radioligands. G-protein coupling was assessed using pseudo-competition with agonists. Phospholipase C activity was measured by an enzymatic assay. Cyclic AMP (cAMP) content was measured by immunoassay. Muscarinic receptors were decreased to 50%, alpha(1)-adrenoceptors to 23%, and beta-adrenoceptors to about 50% of control. Changes were subtype specific, as alpha(1A), alpha(1B), and beta(2)-adrenoceptors, but not alpha(1D)-adrenoceptor, were decreased. In contrast, receptor signaling into the cell as measured by coupling to G proteins, cAMP content, and PI-phospholipase C activity was the same as in control. This shows that the nearly normal lung function of these animals was explained by maintenance of a correct balance of adrenoceptors and muscarinic receptors. In conclusion, knockout mice have adapted to high concentrations of acetylcholine by downregulating receptors that bind acetylcholine, as well as by downregulating receptors that oppose the action of muscarinic receptors. Tolerance to excess acetylcholine is achieved by reducing the levels of muscarinic receptors and adrenoceptors.

Amyolid precursor protein mediates presynaptic localization and activity of the high-affinity choline transporter

The key pathological features of Alzheimer's disease include synaptic dysfunction, profound changes in the cholinergic system, and deposition of beta-amyloid peptides generated by proteolytic processing of the amyloid-beta precursor protein (APP). However, the pathways linking APP with synaptic activity and cholinergic neuronal function are poorly understood. We report here that APP is essential in regulating the presynaptic expression and activity of the high-affinity choline transporter (CHT), a molecule that mediates the rate-limiting step of cholinergic synaptic transmission in both the neuromuscular junction and central cholinergic neurons. Loss of APP leads to aberrant localization of CHT at the neuromuscular synapses and reduced CHT activity at cholinergic projections. At the cellular level, we show that APP and CHT can be found in Rab5-positive endosomal compartments and that APP affects CHT endocytosis. Furthermore, we demonstrate that APP interacts with CHT through the C-terminal domain, providing support for a specific and direct regulation of CHT by APP through protein-protein interactions. These results identify a physiological activity of APP in cholinergic neurons, and our data indicate that deregulation of APP function may contribute to cholinergic impairment and AD pathogenesis.

Phenserine

Phenserine, a derivative of physostigmine, was first described as an inhibitor of acetylcholinesterase (AChE) and was shown to improve cognition in various experimental paradigms in rodents and dogs. It was clinically tested for Alzheimer's disease, with moderate success in initial Phase II studies. Phenserine deserves attention for an additional quality of action: in addition to inhibiting AChE, it modulates the amount of beta-amyloid precursor protein (APP) in neuronal cell culture by reducing APP translation. This effect probably involves interaction of phenserine with a regulatory element in the 5'-untranslated region of the APP gene that controls APP expression. Phenserine apparently reduces translational efficiency of APP mRNA into protein, a process that may involve an interaction with iron and/or an iron-responsive element. As a consequence, phenserine reduces beta-amyloid peptide (Abeta) formation in vitro and in vivo. Phenserine is also unique because of differing actions of its enantiomers: (-)-phenserine is the active enantiomer for inhibition of AChE, whereas (+)-phenserine ('posiphen') has weak activity as an AChE inhibitor and can be dosed much higher. Both enantiomers are equipotent in downregulating APP expression. (+)-Posiphen may be a promising drug, either alone or in combination with (-)-phenserine, to attenuate the progression of Alzheimer's disease.

Effects of beta-amyloid protein on M1 and M2 subtypes of muscarinic acetylcholine receptors in the medial septum-diagonal band complex of the rat: relationship with cholinergic, GABAergic, and calcium-binding protein perikarya

Cortical cholinergic dysfunction has been correlated with the expression and processing of beta-amyloid precursor protein. However, it remains unclear as to how cholinergic dysfunction and beta-amyloid (Abeta) formation and deposition might be related to one another. Since the M1- and M2 subtypes of muscarinic acetylcholine receptors (mAChRs) are considered key molecules that transduce the cholinergic message, the purpose of the present study was to assess the effects of the injected Abeta peptide on the number of M1mAchR- and M2mAChR-immunoreactive cells in the medial septum-diagonal band (MS-nDBB) complex of the rat. Injections of Abeta protein into the retrosplenial cortex resulted in a decrease in M1mAChR and M2mAChR immunoreactivity in the MS-nDBB complex. Quantitative analysis revealed a significant reduction in the number of M1mAChR- and M2mAChR-immunoreactive cells in the medial septum nucleus (MS) and in the horizontal nucleus of the diagonal band of Broca (HDB) as compared to the corresponding hemisphere in control animals and with that seen in the contralateral hemisphere, which corresponds to the PBS-injected side. Co-localization studies showed that the M1mAChR protein is localized in GABA-immunoreactive cells of the MS-nDBB complex, in particular those of the MS nucleus, while M2mAChR protein is localized in both the cholinergic and GABAergic cells. Moreover, GABAergic cells containing M2mAChR are mainly localized in the MS nucleus, while cholinergic cells containing M2mAChR are localized in the MS and the HDB nuclei. Our findings suggest that Abeta induces a reduction in M1mAChR- and M2mAChR-containing cells, which may contribute to impairments of cholinergic and GABAergic transmission in the MS-nDBB complex.

Reduction in CHT1-mediated choline uptake in primary neurons from presenilin-1 M146V mutant knock-in mice

The memory loss in Alzheimer's disease (AD) has been linked to cholinergic hypoactivity. Mutations in presenilin-1 (PS-1) may regulate cholinergic signaling, although their precise roles in cholinergic neurotransmission in AD are unsettled. Neuronal uptake of choline via the high affinity choline transporter (CHT1) is essential for cholinergic neurotransmission. CHT1 is a Na+-dependent, hemicholinium-3 (HC-3)-sensitive choline transporter. Although cholinergic neurons in the nucleus basalis of Meynert are a major source of cholinergic projections for the cerebral cortex, it is unclear whether cortical neurons exhibit intrinsic CHT1 activity that is altered in AD. We now report that primary cortical neurons express intrinsic and biologically active CHT1, and that, in these neurons, CHT1-mediated choline uptake activity is significantly reduced in PS-1 M146V mutant knock-in mice. Further kinetic studies using HC-3 binding and cell surface biotinylation assays showed that the PS-1 mutation inhibits CHT1 mediated choline uptake by reducing the ligand binding affinity of CHT1 without significantly altering levels of CHT1 expression in the plasma membrane. Since human neocortex has recently been shown to possess intrinsic cholinergic innervation, our results indicate that alterations in CHT1-mediated high affinity choline uptake in cortical neurons may contribute to Alzheimer's dementia.

Excessive hippocampal acetylcholine levels in acetylcholinesterase-deficient mice are moderated by butyrylcholinesterase activity

Central cholinergic systems are involved in a plethora of brain functions and are severely and selectively damaged in neurodegenerative diseases such as Alzheimer's disease and dementia with Lewy bodies. Cholinergic dysfunction is treated with inhibitors of acetylcholinesterase (AChE) while the role of butyrylcholinesterase (BChE) for brain cholinergic function is unclear. We have used in vivo microdialysis to investigate the regulation of hippocampal acetylcholine (ACh) levels in mice that are devoid of AChE (AChE-/- mice). Extracellular ACh levels in the hippocampus were 60-fold elevated in AChE-/- mice compared with wild-type (AChE+/+) animals. In AChE-/- mice, calcium-free conditions reduced hippocampal ACh levels by 50%, and infusion of tetrodotoxin by more than 90%, indicating continuous ACh release. Infusion of a selective AChE inhibitor (BW284c51) caused a dose-dependent, up to 16-fold increase of extracellular ACh levels in AChE+/+ mice but did not change ACh levels in AChE-/- mice. In contrast, infusion of a selective inhibitor of BChE (bambuterol) caused up to fivefold elevation of ACh levels in AChE-/- mice, but was without effect in AChE+/+ animals. These results were corroborated with two other specific inhibitors of AChE and BChE, tolserine and bis-norcymserine, respectively. We conclude that lack of AChE causes dramatically increased levels of extracellular ACh in the brain. Importantly, in the absence of AChE, the levels of extracellular ACh in the brain are controlled by the activity of BChE. These results point to a potential usefulness of BChE inhibitors in the treatment of central cholinergic dysfunction in which brain AChE activity is typically reduced.

Impairment of muscarinic transmission in transgenic APPswe/PS1dE9 mice

We assessed the integrity of cholinergic neurotransmission in parietal cortex of young adult (7 months) and aged (17 months) transgenic APPswe/PS1dE9 female mice compared to littermate controls. Choline acetyltransferase and acetylcholinesterase activity declined age-dependently in both genotypes, whereas both age- and genotype-dependent decline was found in butyrylcholinesterase activity, vesicular acetylcholine transporter density, muscarinic receptors and carbachol stimulated binding of GTP gamma S in membranes as a functional indicator of muscarinic receptor coupling to G-proteins. Notably, vesicular acetylcholine transporter levels and muscarinic receptor-G-protein coupling were impaired in transgenic mice already at the age of 7 months compared to wild type littermates. Thus, brain amyloid accumulation in this mouse model is accompanied by a serious deterioration of muscarinic transmission already before the mice manifest significant cognitive deficits.

Alzheimer's disease and anaesthesia: implications for the central cholinergic system

Alzheimer's disease (AD) is associated with a loss of cholinergic neurons resulting in profound memory disturbances and irreversible impairment of cognitive function. The central cholinergic system is involved in the action of general anaesthetic agents. Anaesthetic modulation of cholinergic transmission has profound effects on brain function via a cascade of synaptic and postsynaptic events by binding both nicotinic and muscarinic receptors. During general anaesthesia, decrease in acetylcholine release and depression of cholinergic transmission facilitates the desirable effects of general anaesthetics, such as loss of consciousness, pain, voluntary movements and memory. From this point of view, patients with AD, characterized by a compromised neuronal transmission, represent particular cases in which the choice of anaesthesia drugs may have a negative effect on the postoperative outcome. A future challenge may be the identification of brain targets of general anaesthetics which do not expose patients to postoperative cognitive dysfunction, nor interfere with prognosis of brain degenerative disease.

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.

Alzheimer's disease-like neuropathology of gene-targeted APP-SLxPS1mut mice expressing the amyloid precursor protein at endogenous levels

Most transgenic mice used for preclinical evaluation of potential disease-modifying treatments of Alzheimer's disease develop major histopathological features of this disease by several-fold overexpression of the human amyloid precursor protein. We studied the phenotype of three different strains of gene-targeted mice which express the amyloid precursor protein at endogenous levels. Only further crossing with transgenic mice overexpressing mutant human presenilin1 led to the deposition of extracellular amyloid, accompanied by the deposition of apolipoprotein E, an astrocyte and microglia reaction, and the occurrence of dilated cholinergic terminals in the cortex. Features of neurodegeneration, however, were absent. The pattern of plaque development and deposition in these mice was similar to that of amyloid precursor protein overproducing strains if crossed to presenilin1-transgenics. However, plaque development started much later and developed slowly until the age of 18 months but then increased more rapidly.

Choline transporters, cholinergic transmission and cognition

Cholinergic projections to the cortex and hippocampus mediate fundamental cognitive processes. The capacity of the high-affinity choline uptake transporter (CHT) to import choline from the extracellular space to presynaptic terminals is essential for normal acetylcholine synthesis and therefore cholinergic transmission. The CHT is highly regulated, and the cellular mechanisms that modulate its capacity show considerable plasticity. Recent evidence links changes in CHT capacity with the ability to perform tasks that tax attentional processes and capacities. Abnormal regulation of CHT capacity might contribute to the cognitive impairments that are associated with neurodegenerative and neuropsychiatric disorders. Therefore, the CHT might represent a productive target for the development of new pharmacological treatments for these conditions.

Amyloid beta peptide-induced cholinergic fibres loss in the cerebral cortex of the rat is modified by diet high in lipids and by age

The influence of diet and age on the effects of intracerebral injection of beta-amyloid peptide (Abeta1-40) in vehicle phosphate-buffered saline (PBS) and on the effects of vehicle alone on cholinergic fibres of the cerebral cortex was studied in rats. The experiments were carried in two groups of animals: one group of young adult rats and a second group of aged rats. Each group of animals, depending on the diet received, was divided into high-cholesterol, high-fat, and a control diet group. In order to evaluate the interaction of Abeta/PBS-cholesterol and of Abeta/PBS-fat, animals without dietary manipulation receiving Abeta and PBS injection were used as controls. High-cholesterol fed animals showed a statistically significant reduction of 49.62% in the number of cholinergic fibres at the Abeta injection site as compared with that at PBS injection site, while the high-fat and control animals showed a significant reduction of 28.13 and 26.81%, respectively. In all diet groups, the loss of cholinergic fibres caused by Abeta as compared to that caused by PBS injection was significantly greater in aged rats in comparison with that observed in the young animals. Furthermore, the results of a multivariate linear regression model revealed that the greatest reduction in cholinergic fibres was in the high-cholesterol fed animals (35 fibres/mm) as compared with that seen in the high-fat and control animals. A significantly greater reduction was also observed at Abeta injection site (28 fibres/mm) as compared with that caused by PBS injection, and a reduction of 16 cholinergic fibres per mm was found in aged animals as compared to that seen in young adult rats. These results show that high-cholesterol diet enhances the toxicity of Abeta peptide and that this is also age-dependent. Therefore, this study increases the evidences of the role of cholesterol in the pathology of Alzheimer's disease (AD).