Beta-amyloid peptides as direct cholinergic neuromodulators: a missing link?

Sex-dependent Actions of Amyloid Beta Peptides on Hippocampal Choline Carriers of Postnatal Rats

It is suggested that amyloid beta peptides (Abeta) play a role in the pathogenesis of Alzheimer disease but their physiological function is still unknown. However, low pM-nM concentrations mediate a hypofunction of a basal forebrain cholinergic system without marked signs of neurotoxicity. In this study, we compared in vitro effects of soluble nonaggregated human Abeta 1-40 and 1-42 either on synaptosomal hemicholinium-3 sensitive choline carriers or on membrane fluidity in hippocampi of male and female Wistar rats aged 7 and 14 days or 2-3 months. The results indicate age- and sex-dependent effects mediated by peptides at nM concentrations but no significant differences between both fragments. Namely, opposite actions were observed in 14-day (the increase in the choline uptake and membrane fluidity) when compared to 7-day old and adult males (the mild drops). Lineweaver-Burk plot analysis revealed that the enhancement of the high-affinity choline transport in 14-day old males occurs via alterations in K (M )and the change was accompanied by a mild increase in the specific binding of [3H]hemicholinium-3. On the other hand, no age-dependent differences were found in females. Rat Abeta 1-40 mediated similar effects on 14-day old rats as the corresponding human fragment. Moreover, higher levels of soluble peptides were detected in immature when compared to mature male brains by means of competitive ELISA. Our study indicates that Abeta could play a role in postnatal sexual differentiation of hippocampal cholinergic system.

Cholinergic and glutamatergic drugs in Alzheimer's disease therapy

The pathology and clinical symptoms of Alzheimer's disease are well known and include plaques, tangles, cell loss and dysfunction. The target of current treatments is to improve neuronal dysfunction and produce symptomatic benefits based on a clear understanding of neurotransmitter biochemistry. The purpose of this review is to examine the scientific background to currently available treatments, discuss the clinical experience of employing these drugs in Alzheimer's disease patients and review the socioeconomic influences on their use in the future.

Pharmacological and molecular enhancement of learning in aging and Alzheimer's disease

When animals learn hippocampus-dependent associative and spatial tasks such as trace eyeblink conditioning and the water maze, CA1 hippocampal neurons become more excitable as a result of reductions in the post-burst, slow afterhyperpolarization. The calcium-activated potassium current that mediates this afterhyperpolarization is activated by the calcium influx that occurs when a series of action potentials fire and serves as a modulator of neuronal firing frequency. As a result, spike frequency accommodation is also reduced after learning. Neuronal calcium buffering processes change and/or voltage-dependent calcium currents increase during aging; leading to enhancements in the slow afterhyperpolarization, increased spike frequency accommodation and age-associated impairments in learning. We describe a series of studies done to characterize this learning-specific enhancement in intrinsic neuronal excitability and its converse in aging brain. We have also combined behavioral pharmacology and biophysics in experiments demonstrating that compounds that increase neuronal excitability in CA1 pyramidal neurons also enhance learning rate of hippocampus-dependent tasks, especially in aging animals. The studies reviewed here include those using nimodipine, an L-type calcium current blocker that tends to cross the blood-brain barrier; metrifonate, a cholinesterase inhibitor; CI1017, a muscarinic cholinergic agonist; and galantamine, a combined cholinesterase inhibitor and nicotinic agonist. Since aging is the chief risk factor for Alzheimer's disease, a disease that targets the hippocampus and associated brain regions and markedly impairs hippocampus-dependent learning, these compounds have potential use as treatments for this disease. Galantamine has been approved by the USDA for this purpose. Finally, we have extended our studies to the TG2576 transgenic mouse model of Alzheimer's disease (AD), that overproduces amyloid precursor protein (APP) and increases levels of toxic beta-amyloid in the brain. Not only do these mice show deficits in hippocampus-dependent learning as they age, but their hippocampal neurons show a reduced capacity to increase their levels of intrinsic excitability with reductions in the slow afterhyperpolarization after application of the muscarinic agonist carbachol. These TG2576 APP overproducing mice were crossed with BACE1 knockout mice, that do not produce beta-amyloid because cleavage of APP by the beta-site APP cleaving enzyme 1 (BACE1) is a critical step in its formation. Not only was hippocampus-dependent learning rescued in the bigenic TG2576-BACE1 mice, but the capacity of hippocampal neurons to show normal enhancements of intrinsic excitability was restored. The series of studies reviewed here support our hypothesis that enhancement in intrinsic excitability by reductions in calcium-activated potassium currents in hippocampal neurons is an important cellular mechanism for hippocampus-dependent learning.

α7 Nicotinic acetylcholine receptor as a target to rescue deficit in hippocampal LTP induction in β-amyloid infused rats

Continuous intracerebroventricular infusion of beta-amyloid peptide 1-40 (Abeta(1-40)) in animal models induces learning and memory impairment associated with dysfunction of the cholinergic neuronal system, which has been considered to be a pathological model of Alzheimer's disease [Nitta, A., Itoh, A., Hasegawa, T., Nabeshima, T., 1994. Beta-amyloid protein-induced Alzheimer's disease animal model. Neurosci. Lett. 170, 63-66.]. Here, using a real-time optical recording technique, we demonstrate that basal synaptic transmission and several forms of synaptic plasticity, including long-term potentiation (LTP), post-tetanic potentiation (PTP) and paired-pulse facilitation (PPF) are deficient at the Schaffer collateral-CA1 synapse in hippocampal slices from Abeta-infused brain. Throughout this study, an effort was made to address whether the alpha7 nicotinic acetylcholine receptor (alpha7nAChR), which is believed to be a primary target of Abeta [Wang, H.Y., Lee, D.H., Davis, C.B., Shank, R.P., 2000a. Amyloid peptide Abeta (1-42) binds selectively and with picomolar affinity to alpha 7 nicotinic acetylcholine receptors. J. Neurochem. 75, 1155-1161.], is responsible for the deficits in synaptic plasticity observed in the Abeta-infused rats. First, we found that Abeta-infusion markedly depressed the response of alpha7nAChR to a selective alpha7nAChR agonist [3-(2,4-dimethoxybenzylidene)-anabaseine] (DMXB). Second, blockade of alpha7nAChR with either methyllycaconitine (MLA) or alpha-bungarotoxin (alpha-BTX) in control rats inhibited LTP induction, suggesting that the activation of alpha7nAChR is required for LTP induction. Finally, pre-treatment of the slices from Abeta-infused rats with 10 microM DMXB rescued CA1 synapses from the deficit in LTP and PPF. These results suggest that Abeta-impaired LTP and PPF arise as a consequence of dysfunctional alpha7nAChR, and that alpha7nAChR may be an important target to help ameliorate AD patient cognitive deficits.

Effects of ginseng saponins on beta-amyloid-induced amnesia in rats

We have previously demonstrated that ginseng saponins (GS) can reverse the inhibitory effect of beta-amyloid on acetylcholine (ACh) release in the hippocampal slices. The present study was carried out to examine whether GS has any beneficial effects against amnesia induced by beta-amyloid peptides in vivo. Intracerebroventricular injection of 50 microg, but not 10 microg, beta-amyloid fragment(25-35) markedly impaired the performance of rats in avoiding a shock prod, confirming the amnesiac effect of beta-amyloid. Chronically treating the rats with GS (orally, 5 days before icv beta-amyloid injection and 7 days afterward) resulted in a dose-related improvement against beta-amyloid-induced amnesia; a significant reversion was observed at the highest GS dose (80 mg/kg/day). Post-treatment analysis on K(+)-evoked [(3)H]-ACh release from the hippocampal slices showed that beta-amyloid-treatment significantly reduced ACh release from that of the control group. However, pre-treatment with GS completely protected the animal against beta-amyloid-induced reduction of hippocampal ACh release. In contrast, treating the animals with the same optimal dose of GS and duration but only after icv beta-amyloid injection was found to be ineffective in obliterating beta-amyloid's amnesiac effect. Taken together, these observations indicated that GS pre-treatment can functionally prevent the beta-amyloid-induced memory loss possibly by minimizing the inhibitory effect of beta-amyloid on hippocampal cholinergic transmission.

Galantamine increases excitability of CA1 hippocampal pyramidal neurons

Galantamine is a third generation cholinesterase inhibitor and an allosteric potentiating ligand of nicotinic acetylcholine receptors. It enhances learning in aging rabbits and alleviates cognitive deficits observed in patients with Alzheimer's disease. We examined galantamine's effect on CA1 neurons from hippocampal slices of young and aging rabbits using current-clamp, intracellular recording techniques. Galantamine (10-200 microM) dose-dependently reduced the postburst afterhyperpolarization and the spike-frequency accommodation of CA1 neurons from both young and aging animals. These reductions were partially, but significantly, reversed by the addition of the muscarinic receptor antagonist, atropine (1 microM), to the perfusate. In contrast, the nicotinic acetylcholine receptor antagonist, alpha-bungarotoxin (10 nM), had no effect; i.e. alpha-bungarotoxin did not reverse the afterhyperpolarization and accommodation reductions. The allosteric potentiating ligand effect was examined by stimulating the Schaffer collateral and measuring the excitatory postsynaptic potentials for 30 min during bath application of galantamine. Galantamine (200 microM) significantly enhanced the excitatory postsynaptic potential amplitude and area over time. These effects were blocked by 10 nM alpha-bungarotoxin, supporting a role for galantamine as an allosteric potentiating ligand. We did not observe a facilitation of the excitatory postsynaptic potentials with 1 microM galantamine. However, when the excitatory postsynaptic potential was pharmacologically isolated by adding 10 microM gabazine (GABA(A) receptor antagonist) to the perfusate, 1 microM galantamine potentiated the subthreshold excitatory postsynaptic potentials into action potentials. We propose that the learning enhancement observed in aging animals and the alleviation of cognitive deficits associated with Alzheimer's disease after galantamine treatment may in part be due to the enhanced function of both nicotinic and muscarinic excitatory transmission on hippocampal pyramidal neurons.

A model for dynamic regulation of choline acetyltransferase by phosphorylation

Choline acetyltransferase (ChAT) synthesizes the neurotransmitter acetylcholine (ACh) and is a phenotypic marker for cholinergic neurons. Cholinergic neurons in brain are involved in cognitive function, attentional processing and motor control, and decreased ChAT activity is found in several neurological disorders including Alzheimer's disease. Dysregulation of ChAT and cholinergic communication is also associated with some spontaneous point-mutations in ChAT that alter its substrate binding kinetics, or by disruption of signaling pathways that could regulate protein kinases for which ChAT is a substrate. It has been identified recently that the catalytic activity and subcellular distribution of ChAT, and its interaction with other cellular proteins, can be modified by phosphorylation of the enzyme by protein kinase-C and Ca2+/calmodulin-dependent protein kinase II; these kinases appear also to mediate some of the effects of beta-amyloid peptides on cholinergic neuron functions, including the effects on ChAT. This review outlines a new model for the regulation of cholinergic transmission at the level of the presynaptic terminal that is mediated by hierarchically-regulated, multi-site phosphorylation of ChAT.

Early changes in Aβ levels in the brain of APPswe transgenic mice—Implication on synaptic density, α7 neuronal nicotinic acetylcholine- and N-methyl-d-aspartate receptor levels

Tg 2576 (APPswe) mice develop age-related amyloid deposition as well as behavioural- and electrophysiological changes in the brain. In this study, APPswe mice were investigated from 7 to 90 days of age. We observed high Abeta levels in the cortex of APPswe mice at 7 days of age, suggesting that these mice produce Abeta from birth. A positive correlation between Abeta and synaptophysin levels, followed by changes in ERK MAPK activity, indicated that Abeta causes altered synaptic function and an increase in the number of synaptic terminals. In addition, alterations in [(125)I]alphabungarotoxin- and [(3)H]MK-801 binding sites were also observed in APPswe mice compared to controls. In conclusion, over-expression of Abeta early in life causes changes in synaptophysin levels and number of [(125)I]alphabungarotoxin- and [(3)H]MK-801 binding sites. The results may provide important information about the onset and consequences of Abeta pathology in this transgenic mouse model.

Hippocampal long-term potentiation suppressed by increased inhibition in the Ts65Dn mouse, a genetic model of Down syndrome

Although many genetic disorders are characterized by cognitive failure during development, there is little insight into the neurobiological basis for the abnormalities. Down syndrome (DS), a disorder caused by the presence of three copies of chromosome 21 (trisomy 21), is characterized by impairments in learning and memory attributable to dysfunction of the hippocampus. We explored the cellular basis for these abnormalities in Ts65Dn mice, a genetic model for DS. Although basal synaptic transmission in the dentate gyrus was normal, there was severe impairment of long-term potentiation (LTP) as a result of reduced activation of NMDA receptors. After suppressing inhibition with picrotoxin, a GABA(A) receptor antagonist, NMDA receptor-mediated currents were normalized and induction of LTP was restored. Several lines of evidence suggest that inhibition in the Ts65Dn dentate gyrus was enhanced, at least in part, because of presynaptic abnormalities. These findings raise the possibility that similar changes contribute to abnormalities in learning and memory in people with DS and, perhaps, in other developmental disorders with cognitive failure.

β-Amyloid peptide25–35 depresses excitatory synaptic transmission in the rat basolateral amygdala “in vitro”

The effects of beta-amyloid peptide25-35 on resting membrane potential, spontaneous and evoked action potential and synaptic activity have been studied in basolateral amygdaloid complex on slices obtained from adult rats. Intracellular recordings reveal that perfusion with beta-amyloid peptide25-35 at concentrations of 400 nM and less did not generate any effect on resting membrane potential. However, concentrations in the range of 800-1200 nM produced an unpredictable effect, depolarization and/or hyperpolarization, which were blocked by tetrodotoxin or 6-cyano-7-nitroquinoxaline-2,3-dione+D-(-)-2-amino-5-phosphonopentanoic acid together with bicuculline. Excitatory and inhibitory evoked responses mediated by glutamic acid or gamma-aminobutyric acid decreased in amplitude after beta-amyloid peptide25-35 perfusion. Additionally, results obtained using the paired-pulse protocol offer support for a presynaptic mode of action. To determine which type of receptors and/or channels are involved in the presynaptic mechanism of action, a specific blocker of alpha-7 nicotinic receptors (methyllycaconitine citrate) or L-type calcium channel blockers (calcicludine or nifedipine) were used. beta-amyloid petide25-35 decreased excitatory postsynaptic potentials amplitude in control conditions and also in slices permanently perfused with methyllycaconitine citrate. However, this effect was blocked in slices perfused with calcicludine or nifedipine suggesting the involvement of the L-type calcium channels. On the whole, these experiments provide evidence that beta-amyloid peptide25-35 affects neurotransmission in basolateral amygdala and its action is mediated through L-type calcium channels.

Early dysregulation of hippocampal proteins in transgenic rats with Alzheimer's disease-linked mutations in amyloid precursor protein and presenilin 1

The response of the hippocampal proteome to expression of mutant proteins present in familial forms of Alzheimer's disease (AD) was studied using transgenic rats. These animals carry both the amyloid precursor protein Swedish and 717 mutation (APP(SW+717)) as well as the presenilin 1 Finnish mutation (PS1(FINN)). This transgenic rat model displays intracellular amyloid beta (Abeta) in neurons of the neocortex and the hippocampus (CA2 and CA3). The hippocampus was selected as it is one of the first brain regions affected in AD and is involved in the processing of short-term memory and spatial memory. Applying a proteomic approach, we demonstrate that the expression of APP(SW+717) and PS1(FINN) transgenes causes changes in expression of hippocampal proteins, some of which have been previously linked to learning and memory formation. The protein alterations documented here occur in the absence of plaque formation and prior to the onset of cognitive deficits later observed in these transgenic rats. This indicates that molecular changes take place in the hippocampal neurons in response to expression of mutant proteins APP(SW+717) and PS1(FINN), which precede the occurrence of overt extracellular accumulation of extracellular amyloid. The implications of these findings on our understanding of the early stages of AD are discussed.

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.

Biophysical alterations of hippocampal pyramidal neurons in learning, ageing and Alzheimer's disease

A series of behavioral, electrophysiological, and molecular biochemical experiments are reviewed indicating that when animals learn hippocampus-dependent tasks, output neurons in the CA1 and CA3 hippocampal subfields show reductions in the slow, post-burst afterhyperpolarization (AHP). The slow AHP is mediated by an apamin-insensitive calcium-activated potassium current. A reduction in the slow AHP makes hippocampal neurons more excitable and facilitates NMDA receptor-mediated response and temporal summation. During normal aging and in a mouse model of Alzheimer's disease (AD), the slow AHP is increased, making neurons less excitable and making learning more difficult. The subgroup of aging animals that are able to learn demonstrates the capacity to increase neuronal excitability by reducing the size of the slow AHP. Similarly, in a mouse model of AD, mice that are able to learn normally after a genetic alteration have a normal capacity for increasing hippocampal neuron excitability by reducing their slow AHP. We suggest that reduction in the slow AHP is basic to learning in young and aging animals. Inability to modulate the slow AHP contributes to learning deficits that occur during aging and early stages of AD.

beta-Amyloid directly inhibits human alpha4beta2-nicotinic acetylcholine receptors heterologously expressed in human SH-EP1 cells

Amyloid-beta (Abeta) accumulation and aggregation are thought to contribute to the pathogenesis of Alzheimer's disease (AD). In AD, there is a selective decrease in the numbers of radioligand binding sites corresponding to the most abundant nicotinic acetylcholine receptor (nAChR) subtype, which contains human alpha4 and beta2 subunits (halpha4beta2-nAChR). However, the relationships between these phenomena are uncertain, and effects of Abeta on halpha4beta2-nAChR function have not been investigated in detail. We first confirmed expression of halpha4 and hbeta2 subunits as messenger RNA in transfected, human SHEP1 cells by reverse transcription-polymerase chain reaction and mRNA fluorescence in situ hybridization analyses. Immunoprecipitation Western analyses confirmed alpha4 and beta2 subunit protein expression and co-assembly. Whole cell current recording demonstrated heterologous expression in SH-EP1-halpha4beta2 cells of functional halpha4beta2-nAChRs with characteristic responses to nicotinic agonists or antagonists. Nicotine-induced whole cell currents were suppressed by Abeta(1-42) in a dose-dependent manner. Functional inhibition was selective for Abeta(1-42) compared with the functionally inactive, control peptide Abeta(40-1).Abeta(1-42)-mediated inhibition of halpha4beta2-nAChR function was non-competitive, voltage-independent, and use-independent. Pre-loading of cells with guanyl-5'-yl thiophosphate failed to prevent Abeta(1-42)-induced inhibition, suggesting that down-regulation of halpha4beta2-nAChR function by Abeta(1-42) is not mediated by nAChR internalization. Sensitivity to Abeta(1-42) antagonism at 1 nm was evident for halpha4beta2-nAChRs, but not for heterologously expressed human alpha7-nAChRs, although both nAChR subtypes were functionally inhibited by 100 nm Abeta(1-42), with the magnitude of functional block being higher for 100 nm Abeta(1-42) acting on halpha7-nAChRs. These findings suggest that halpha4beta2-nAChRs are sensitive and perhaps pathophysiologically relevant targets for Abeta neurotoxicity in AD.

Alterations in cholinergic and non-cholinergic neurotransmitter receptor densities in transgenic Tg2576 mouse brain with beta-amyloid plaque pathology

Cholinergic deficits in Alzheimer's disease are accompanied by a number of alterations in other transmitter systems including glutamate, noradrenaline and serotonin, suggesting the involvement also of other neurotransmitter systems in the pathogenesis of the disease. To address the question whether beta-amyloid may contribute to these deficits, brain tissue from transgenic Tg2576 mice with Alzheimer plaque pathology at ages of 5 (still no significant plaque load) and 17 months (moderate to high cortical beta-amyloid plaque load) were examined for a number of cholinergic and non-cholinergic markers. Transgenic mice with no significant plaque load demonstrated reduced hemicholinium-3 (HCh-3) binding to choline uptake sites in anterior brain regions as compared to non-transgenic littermates, while in aged transgenic mice with high number of plaque deposits decreased HCh-3 binding levels were accompanied by increased vesicular acetylcholine transporter binding in selected cortical brain regions. In aged transgenic mice GABA(A), NMDA, AMPA, kainate, and beta-adrenergic as well 5-HT(1A)- and 5-HT(2A)-receptor binding levels were hardly affected, whereas alpha(1)- and alpha(2)-adrenoceptor binding was increased in selected cerebral cortical regions as compared to non-transgenic littermates. The development of changes in both cholinergic and non-cholinergic markers in transgenic Tg2576 mouse brain already before the onset of progressive plaque deposition provides in vivo evidence of a modulatory role of soluble beta-amyloid on cortical neurotransmission and may be referred to the deficits in learning and memory observed in these mice also before significant plaque load.

BACE1 Deficiency Rescues Memory Deficits and Cholinergic Dysfunction in a Mouse Model of Alzheimer's Disease

beta-site APP cleaving enzyme 1 (BACE1) is the beta-secretase enzyme required for generating pathogenic beta-amyloid (Abeta) peptides in Alzheimer's disease (AD). BACE1 knockout mice lack Abeta and are phenotypically normal, suggesting that therapeutic inhibition of BACE1 may be free of mechanism-based side effects. However, direct evidence that BACE1 inhibition would improve cognition is lacking. Here we show that BACE1 null mice engineered to overexpress human APP (BACE1(-/-).Tg2576(+)) are rescued from Abeta-dependent hippocampal memory deficits. Moreover, impaired hippocampal cholinergic regulation of neuronal excitability found in the Tg2576 AD model is ameliorated in BACE1(-/-).Tg2576(+) bigenic mice. The behavioral and electrophysiological rescue of deficits in BACE1(-/-).Tg2576(+) mice is correlated with a dramatic reduction of cerebral Abeta40 and Abeta42 levels and occurs before amyloid deposition in Tg2576 mice. Our gene-based approach demonstrates that lower Abeta levels are beneficial for AD-associated memory impairments, validating BACE1 as a therapeutic target for AD.

The Cholinergic Lesion of Alzheimer's Disease: Pivotal Factor or Side Show?

A profound loss of cortical cholinergic innervation is a nearly invariant feature of advanced Alzheimer's disease (AD). The temporal course of this lesion and its relationship to other aspects of the disease have not yet been fully clarified. Despite assertions to the contrary, a review of the evidence suggests that a perturbation of cholinergic innervation is likely to be present even in the very early stages of AD. This cholinergic lesion is unlikely to be a major determinant of the clinical symptoms or of the neuropathological lesions. Nonetheless, it almost certainly contributes to the severity of the cognitive and behavioral deficits, especially in the areas of memory and attention. The cholinergic lesion may also influence the progression of the neuropathological process through complex interactions with amyloidogenesis, tau phosphorylation and neuroplasticity.

Changes in cortical acetyl-CoA metabolism after selective basal forebrain cholinergic degeneration by 192IgG-saporin

The aim of the present study was to reveal whether reduced cortical cholinergic input affects the acetyl-CoA metabolism in cholinoceptive cortical target regions which may play a causative role for the deficits in cerebral glucose metabolism observed in Alzheimer's disease. The effect of cortical cholinergic denervation produced by a single intracerebroventricular application of the cholinergic immunotoxin 192IgG-saporin, on activities of pyruvate dehydrogenase and adenosine triphosphate (ATP)-citrate lyase as well as on the level of synaptoplasmic and mitochondrial acetyl-CoA and acetylcholine release in cortical target regions was studied. Cholinergic lesion produced 83%, 72% and 32% decreases in the activities of choline acetyltransferase, acetylcholinesterase and ATP-citrate lyase in nerve terminals isolated from rat brain cortex, respectively, but no change in pyruvate dehydrogenase activity. Spontaneous and Ca2+-evoked acetylcholine release from synaptosomes was inhibited by 76% and 73%, respectively, following immunolesion. The lesion-induced 39% decrease of acetyl-CoA level in synaptosomal mitochondria was accompanied by 74% increase in synaptoplasmic fraction. Levels of acetyl-CoA and CoASH assayed in fraction of whole brain mitochondria from lesioned cortex were 61% and 48%, respectively, higher as compared to controls. The data suggest a preferential localization of ATP-citrate lyase in cholinergic nerve terminals, where it may contribute to the transport of acetyl-CoA from the mitochondrial to the cytoplasmic compartment. They provide evidence on differential distribution of acetyl-CoA in subcellular compartments of cholinergic and non-cholinergic nerve terminals. There are also indications that cholinergic activity affects acetyl-CoA level and its intracellular distribution in glial and other non-cholinergic cortical cells.

Long-term effects of BIBN-99, a selective muscarinic M2 receptor antagonist, on improving spatial memory performance in aged cognitively impaired rats

Aged Long-Evans rats were screened for spatial memory deficits using the Morris water maze task. Rats found to have impaired performance on the task (aged-impaired, AI) were then treated with a selective muscarinic M2 receptor antagonist, 5,11-dihydro-8-chloro-11-[[4-[3-[(2,2-dimethyl-1-oxopentyl)ethylamino]propyl]-1-piperidinyl]acetyl]-6H-pyrido[2,3-b][1,4]benzodiazepin-6-one (BIBN-99; 0.5 mg/kg, s.c.), for 3 successive days while receiving additional water maze training. BIBN-99 significantly improved performance in the task during the 3 days of drug treatment. Treatment was then ceased for the remainder of the study and rats were tested again in the water maze on days 10, 17, and 24. Compared to vehicle-treated rats, enhanced performance was observed in the AI rats that had previously been treated with BIBN-99. These results indicate that BIBN-99 enhances spatial learning in AI animals and that enhanced (or long-term) memory persists in the absence of the drug. In a second experiment, a 2-month delay was imposed in between the original water maze screening and the drug treatment regime. Again, BIBN-99 significantly improved performance in AI rats. This latter study suggests that reference memory does not decay, even in an AI animal that had displayed poor learning following original water maze screening. Together, these studies help provide further insight into possible mechanism(s) of reference memory and its potential clinical usefulness.

Beta-amyloid regulation of presynaptic nicotinic receptors in rat hippocampus and neocortex

Alteration by beta-amyloid (Abeta) of signaling via nicotinic acetylcholine receptors (nAChRs) has been implicated in the early stages of Alzheimer's disease. nAChRs function both post- and presynaptically in the nervous system; however, little is known about the functional consequence of the interaction of Abeta with these receptors, particularly those on presynaptic nerve terminals. In view of the strong correlation between loss of synaptic terminals and dementia, together with the reduction in nAChRs in Alzheimer's disease, the possibility exists that presynaptic nAChRs may be targets for Abeta. To explore this possibility, we assessed the effect of Abeta peptides on nicotine-evoked changes in presynaptic Ca2+ level via confocal imaging of isolated presynaptic nerve endings from rat hippocampus and neocortex. Abeta1-42 appeared to inhibit presynaptic nAChR activation by nicotine. Surprisingly, picomolar Abeta1-42 was found to directly evoke sustained increases in presynaptic Ca2+ via nAChRs, revealing that the apparent inhibitory action of Abeta1-42 was the result of an occlusion of nicotine to further stimulate the receptors. The direct effect of Abeta was found to be sensitive to alpha-bungarotoxin, mecamylamine, and dihydro-beta-erythroidine, indicating involvement of alpha7-containing nAChRs and non-alpha7-containing nAChRs. Prior depolarization strongly attenuated subsequent Abeta-evoked responses in a manner dependent on the amplitude of the initial presynaptic Ca2+ increase, suggesting that nerve activity or Ca2+ channel density may control the impact of Abeta on presynaptic nerve terminal function. Together, these results suggest that the sustained increases in presynaptic Ca2+ evoked by Abeta may underlie disruptions in neuronal signaling via nAChRs in the early stages of Alzheimer's disease.

Toward an early diagnosis and treatment of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease. There has been a rapid increase in the knowledge of epidemiology, genetics, risk factors, and underlying neuropathological mechanisms, but still there is no cure for AD. Recent promising studies with functional imaging using positron emission tomography (PET) and magnetic resonance imaging reveal that disease processes can be detected when very early subjective symptoms of AD are manifest. Recently the PET ligand PIB was reported to bind in vivo to beta-amyloid in the brains of AD patients. Also cerebrospinal fluid markers including tau, phosphotau, and A beta 1-42 are probably important early biological markers that will provide an early diagnosis of AD. An obvious impairment in central cholinergic transmitter function and its close relation to cognitive function led to the development of the acetylcholinesterase inhibitors that now are used as symptomatic therapy. A drug interfering with the glutaminergic brain transmitter system, the NMDA antagonist memantine, has recently been approved for the treatment of patients with severe AD. In order to stop or reverse disease progression, different AD treatment strategies are of great interest. Epidemiological studies support the hypothesis that long-term treatment with estrogen, antioxidants, anti-inflammatory drugs, and cholesterol-lowering agents could protect against the development of AD. Treatment with these drugs in manifest AD has been less promising. The use of nerve growth factors was limited by severe side effects. Much evidence supports the key role of beta-amyloid in the pathogenesis of AD. Compounds such as amyloid beta-sheet breakers, cholesterol-lowering drugs, estrogen, nicotine, zinc and copper chelators, inhibitors of beta- and gamma-secretases, and immunization to reduce the amyloid burden in transgenic mice overexpressing beta-amyloid all have their advocates. The latter exciting strategy turned out to cause meningoencephalitis in 6% of AD patients so treated. One patient from the trial has died showing less beta-amyloid burden in brain than expected and patients with serum beta-amyloid plaque reactive antibodies had less cognitive decline after 1 year than AD patients without antibodies. There is a great optimism for early diagnosis and effective treatment of AD in the future.

Alpha 7 nicotinic acetylcholine receptors mediate beta-amyloid peptide-induced tau protein phosphorylation

The Alzheimer's disease pathogenic peptide, beta-amyloid42 (A beta 42), induces tau protein phosphorylation. Because hyperphosphorylated tau is a consistent component of neurofibrillary tangles, a pathological hallmark of Alzheimer's disease, we investigated the signaling molecules involved in A beta 42-induced tau phosphorylation. We show that A beta 42 elicited rapid and reversible tau protein phosphorylation on three proline-directed sites (Ser-202, Thr-181, and Thr-231) in systems enriched in alpha 7 nicotinic acetylcholine receptors (alpha 7nAChR) including serum-deprived human SK-N-MC neuroblastoma cells and hippocampal synaptosomes. Although alpha 7nAChR agonists induced similar phosphorylation, pretreatment with antisense-alpha 7nAChR oligonucleotides (in cells) or alpha 7nAChR antagonists (in cells and synaptosomes) attenuated A beta-induced tau phosphorylation. Western analyses showed that the mitogen-activated kinase cascade proteins, ERKs and c-Jun N-terminal kinase (JNK-1), were concomitantly activated by A beta 42, and their respective kinase inhibitors suppressed A beta-induced tau phosphorylation. More importantly, recombinant-activated ERKs and JNK-1 could differentially phosphorylate tau protein in vitro. Thus, the alpha 7nAChR may mediate A beta-induced tau protein phosphorylation via ERKs and JNK-1.

Potential roles of Alzheimer precursor protein A4 and beta-amyloid in survival and function of aged spinal motor neurons after axonal injury

To study the potential role of Alzheimer precursor protein A4 (APP) and beta-amyloid (A/beta) on aging motor neuron survival, expression of APP, A/beta, and choline acetyltransferase (ChaT) were investigated in aged rats after either distal axotomy or root avulsion injury. Approximately 45% in number of total aged spinal motor neuron were normally APP-positive. A/beta-positive neurites were observed normally in the spinal ventral horn of aged rats. After distal axotomy, without apparent neurodegeneration such as cell loss and decreased ChaT-immunoreactivity, increased levels of APP expression were observed in the spinal cords of aged rats post-injury. In contrast, after avulsion, expression of APP and A/beta were downregulated in the spinal ventral horn of aged rats, and marked loss of spinal motor neurons and downregulated ChaT expression were observed. Our data indicate that APP and A/beta might play beneficial roles in neuronal survival of aged spinal motor neurons after axonal injury.

Impaired modulation of GABAergic transmission by muscarinic receptors in a mouse transgenic model of Alzheimer's disease

It has long been recognized that muscarinic acetylcholine receptors (mAChRs) are crucial for the control of cognitive processes, and drugs that activate mAChRs are helpful in ameliorating cognitive deficits of Alzheimer's disease (AD). On the other hand, GABAergic transmission in prefrontal cortex (PFC) plays a key role in "working memory" via controlling the timing of neuronal activity during cognitive operations. To test whether the muscarinic and gamma-aminobutyric acid (GABA) system are interconnected in normal cognition and dementia, we examined the muscarinic regulation of GABAergic transmission in PFC of an animal model of AD. Transgenic mice overexpressing a mutant gene for beta-amyloid precursor protein (APP) show behavioral and histopathological abnormalities resembling AD and, therefore, were used as an AD model. Application of the mAChR agonist carbachol significantly increased the spontaneous inhibitory postsynaptic current (sIPSC) frequency and amplitude in PFC pyramidal neurons from wild-type animals. In contrast, carbachol failed to increase the sIPSC amplitude in APP transgenic mice, whereas the carbachol-induced increase of the sIPSC frequency was not significantly changed in these mutants. Similar results were obtained in rat PFC slices pretreated with the beta-amyloid peptide (Abeta). Inhibiting protein kinase C (PKC) blocked the carbachol enhancement of sIPSC amplitudes, implicating the PKC dependence of this mAChR effect. In APP transgenic mice, carbachol failed to activate PKC despite the apparently normal expression of mAChRs. These results show that the muscarinic regulation of GABA transmission is impaired in the AD model, probably due to the Abeta-mediated interference of mAChR activation of PKC.

Activation of muscarinic receptors inhibits beta-amyloid peptide-induced signaling in cortical slices

Deposition of fibrillar aggregates of the beta-amyloid peptide (Abeta) is a key pathologic feature during the early stage of Alzheimer's disease. The initial neuronal responses to Abeta in cortical circuits and the regulation of Abeta-induced signaling remain unclear. In this study, we found that exposure of cortical slices to Abeta(1-42) or Abeta(25-35) induced a marked increase in the activation of protein kinase C (PKC) and Ca(2+)/calmodulin-dependent kinase II (CaMKII), two enzymes critically involved in a variety of cellular functions. Activation of M1 muscarinic receptors, but not nicotinic receptors, significantly inhibited the Abeta activation of PKC and CaMKII. Increasing inhibitory transmission mimicked the M1 effect on Abeta, whereas blocking GABA(A) receptors eliminated the M1 action. Moreover, electrophysiological evidence shows that application of Abeta to cortical slices induced action potential firing and enhanced excitatory postsynaptic currents, whereas muscarinic agonists potently increased inhibitory postsynaptic currents. These results suggest that Abeta activates PKC and CaMKII through enhancing excitatory activity in glutamatergic synaptic networks. Activation of M1 receptors inhibits Abeta signaling by enhancing the counteracting GABA(ergic) inhibitory transmission. Thus the muscarinic reversal of the Abeta-induced biochemical and physiological changes provides a potential mechanism for the treatment of Alzheimer's disease with cholinergic enhancers.

In vivo muscarinic 2 receptor imaging in cognitively normal young and older volunteers

The precise effects of normal aging on the cholinergic system are unknown, as both in vitro and PET studies have shown conflicting results. In vivo determination of muscarinic receptor distribution and density has been hampered by both poor subtype selectivity and/or blood-brain barrier permeability of known ligands. Previous in vitro and in vivo work with the F-18 labeled muscarinic agonist, 3-(3- (3-[(18)F]Flouropropyl)thio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine ((18)FP-TZTP) suggested the use of (18)FP-TZTP to selectively quantify M2 receptors in humans. In this study, we used (18)FP-TZTP to infer M2 receptor avidity in the brains of 15 healthy younger subjects (mean age = 28.3 +/- 5.5 years) and 20 healthy older subjects (mean age = 62.1 +/- 7.7 years). Corrections for subject motion during the 120-min acquisition and partial voluming (PVC) were performed. A one-tissue compartment model was used to estimate the volumes of distribution (V(T)) of (18)FP-TZTP. Within both groups of subjects, volumes of distribution (K(1)/k(2)) in cortical, subcortical, and cerebellar areas were consistent with M2 receptor topography. Compared to younger subjects older subjects had significantly higher means and standard deviations for the volumes of distribution of (18)FP-TZTP throughout much of the cerebellum, cortex, and subcortex (Global Gray V(T) = 742 +/- 163 in older subjects and 645 +/- 74 in younger subjects, P < 0.03). Across all subjects (18)FP-TZTP, regional, and Global Gray distribution volumes were significantly correlated to age (Global Gray V(T,) r = 0.41, P < 0.01). A lower concentration of acetylcholine in the synapse of some older subjects is one possible explanation for the data.

Beta-amyloid and cholinergic neurons

It is generally accepted that the crucial events in the pathogeny of Alzheimer's disease (AD) are the increased accumulation of amyloidogenic peptides derived from amyloid precursor protein and the harmful actions of these peptides on neurons, which bring about neurodegeneration. The enhanced beta-amyloid accumulation is known to be caused by mutations of specific genes in patients who suffer from the familial (hereditary) form of AD but who represent just a minor group within the total population of AD patients. The reasons for beta-amyloid accumulation are not known in the much larger group of patients with the sporadic form of the disease. A biochemical feature common to either form of the disease is the preferential atrophy and degeneration of cholinergic neurons, which is probably responsible for much of the cognitive decline characteristic of the disease. We present an overview of recent investigations on the interactions between beta-amyloid and cholinergic neurons.

Differential physiologic responses of alpha7 nicotinic acetylcholine receptors to beta-amyloid1-40 and beta-amyloid1-42

The beta-amyloid peptides (Abeta), Abeta(1-40) and Abeta(1-42), have been implicated in Alzheimer's disease (AD) pathology. Although Abeta(1-42) is generally considered to be the pathological peptide in AD, both Abeta(1-40) and Abeta(1-42) have been used in a variety of experimental models without discrimination. Here we show that monomeric or oligomeric forms of the two Abeta peptides, when interact with the neuronal cation channel, alpha7 nicotinic acetylcholine receptors (alpha7nAChR), would result in distinct physiologic responses as measured by acetylcholine release and calcium influx experiments. While Abeta(1-42) effectively attenuated these alpha7nAChR-dependent physiology to an extent that was apparently irreversible, Abeta(1-40) showed a lower inhibitory activity that could be restored upon washings with physiologic buffers or treatment with alpha7nAChR antagonists. Our data suggest a clear pharmacological distinction between Abeta(1-40) and Abeta(1-42).

Guinea pigs as a nontransgenic model for APP processing in vitro and in vivo

Alzheimer's disease (AD) is characterized, amongst others, by the appearance of vascular and parenchymal beta-amyloid deposits in brain. Such aggregates are mainly composed of beta-amyloid peptides, which are derived by proteolytic processing of a larger amyloid precursor protein (APP). APP is highly conserved among mammalian species, but experimental studies in rodents are often hampered by the humble APP-processing in the amyloidogenic pathway and by the inability of rodent beta-amyloid peptides to form higher molecular aggregates such as soluble oligomers and insoluble beta-amyloid plaques. Thus, there is need for in vitro and in vivo model systems that allow identification of factors that increase amyloidogenic APP processing and accelerate beta-amyloid plaque formation and testing the potency of pharmacological manipulations to ameliorate beta-amyloid load in brain. Transgenic mice that overexpress human APP containing AD-associated mutations that favor the amyloidogenic pathway of APP processing represent such a model. However, mutations of the APP gene are not frequent in AD and, therefore, the mechanisms of beta-amyloid plaque formation, the composition of beta-amyloid plaques, and the accompanying tissue response in brain of these animals may be different from that in AD. In contrast, guinea pigs express beta-amyloid peptides of the human sequence and appear to represent a more physiological model to examine the long-term effects of experimental manipulations on APP processing and beta-amyloid plaque formation in vivo. Additionally, APP processing in guinea pig primary neuronal cultures has been shown to be similar to cultures of human origin. In this article we highlight the advantages and limitations of using guinea pigs as experimental models to study APP processing.

Phosphorylation of 69-kDa choline acetyltransferase at threonine 456 in response to amyloid-beta peptide 1-42

Choline acetyltransferase synthesizes acetylcholine in cholinergic neurons. In the brain, these neurons are especially vulnerable to effects of beta-amyloid (A beta) peptides. Choline acetyltransferase is a substrate for several protein kinases. In the present study, we demonstrate that short term exposure of IMR32 neuroblastoma cells expressing human choline acetyltransferase to A beta-(1-42) changes phosphorylation of the enzyme, resulting in increased activity and alterations in its interaction with other cellular proteins. Using mass spectrometry, we identified threonine 456 as a new phosphorylation site in choline acetyltransferase from A beta-(1-42)-treated cells and in purified recombinant ChAT phosphorylated in vitro by calcium/calmodulin-dependent protein kinase II (CaM kinase II). Whereas phosphorylation of choline acetyltransferase by protein kinase C alone caused a 2-fold increase in enzyme activity, phosphorylation by CaM kinase II alone did not alter enzyme activity. A 3-fold increase in choline acetyltransferase activity was found with coordinate phosphorylation of threonine 456 by CaM kinase II and phosphorylation of serine 440 by protein kinase C. This phosphorylation combination was observed in choline acetyltransferase from A beta-(1-42)-treated cells. Treatment of cells with A beta-(1-42) resulted in two phases of activation of choline acetyltransferase, the first within 30 min and associated with phosphorylation by protein kinase C and the second by 10 h and associated with phosphorylation by both CaM kinase II and protein kinase C. We also show that choline acetyltransferase from A beta-(1-42)-treated cells co-immunoprecipitates with valosin-containing protein, and mutation of threonine 456 to alanine abolished the A beta-(1-42)-induced effects. These studies demonstrate that A beta-(1-42) can acutely regulate the function of choline acetyltransferase, thus potentially altering cholinergic neurotransmission.

Amyloid beta peptide induces tau phosphorylation and loss of cholinergic neurons in rat primary septal cultures

The neuropathological features associated with Alzheimer's disease (AD) brain include the presence of extracellular neuritic plaques composed of amyloid beta protein (Abeta), intracellular neurofibrillary tangles containing phosphorylated tau protein and the loss of basal forebrain cholinergic neurons which innervate regions such as the hippocampus and the cortex. Studies of the pathological changes that characterize AD and several other lines of evidence indicate that Abeta accumulation in vivo may initiate phosphorylation of tau protein, which by disrupting neuronal network may trigger the process of neurodegeneration observed in AD brains. However, the underlying cause of degeneration of the basal forebrain cholinergic neurons and their association, if any, to Abeta peptides or phosphorylated tau remains mostly unknown. In the present study, using rat primary septal cultures, we have shown that aggregated Abeta peptides, in a time (18-96 h)- and concentration (0.7-60 microM)-dependent manner, induce toxicity and decrease choline acetyltransferase enzyme activity in cultured neurons. Using immunocytochemistry and immunoblotting, we have also demonstrated that Abeta treatment can significantly increase the phosphorylation of tau protein in septal cultures. At the cellular level, hyperphosphorylated tau is mostly apparent in the somatodendritic compartment of the neurons. Abeta peptide (10 microM), in addition to tau phosphorylation, also activates mitogen-activated protein kinase and glycogen synthase kinase-3beta, the two kinases which are known to be involved in the formation of hyperphosphorylated tau in the AD brain. Exposure to specific inhibitors of the mitogen-activated protein kinase (i.e. PD98059) or glycogen synthase kinase-3beta (i.e. LiCl) attenuated the hyperphosphorylation of the tau protein in cultured neurons. Given the evidence that tau phosphorylation can induce cell loss by disrupting neuronal cytoskeleton, it is likely that aggregated Abeta peptide triggers degeneration of septal neurons, including those expressing the cholinergic phenotype, by phosphorylation of the tau protein activated by mitogen-activated protein kinase and glycogen synthase kinase-3beta. These results, taken together, suggest that cultured septal cholinergic neurons are vulnerable to Abeta-mediated toxicity and tau phosphorylation may play an important role in Abeta-induced neurodegeneration.

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

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

Amyloid beta-peptide 25-35 reduces [3H]acetylcholine release in retinal neurons. Involvement of metabolic dysfunction

Cholinergic pathways serve important functions in learning and memory processes. The loss of basal forebrain cholinergic neurons and the presence of senile plaques composed by amyloid beta-peptide (A beta) are found in post-mortem brains of Alzheimer's disease (AD) patients. However, the role of A beta in the cholinergic dysfunction observed in AD is not yet clarified. In this study, we observed that the release of [3H]acetylcholine evoked by K(+)-depolarization was significantly lower in cells treated with A beta 25-35 peptide, than in untreated cells or in cells exposed to the reverse sequence peptide A beta 35-25. The levels of pyruvate, the substrate for pyruvate dehydrogenase, the enzyme involved in acetyl coenzyme A synthesis in the brain, which is rate-limiting for the synthesis of acetylcholine, were significantly decreased, about 40%, in A beta treated cells. A beta 25-35 did not affect choline acetyltransferase activity or [3H]choline uptake. 2-[3H]-deoxyglucose uptake was decreased when cells were exposed to A beta 25-35 or to A beta 1-40. Taken together these data suggest that an impairment of glycolysis, and the consequent decrease in pyruvate levels, may be responsible for the decrement of acetylcholine release observed in A beta treated cells, thus sustaining the hypothesis that the cholinergic dysfunction, observed in AD patients, might be associated with extracellular A beta accumulation.

Nicotinic receptors in aging and dementia

Activation of neuronal nicotinic acetylcholine receptors (nAChRs) has been shown to maintain cognitive function following aging or the development of dementia. Nicotine and nicotinic agonists have been shown to improve cognitive function in aged or impaired subjects. Smoking has also been shown in some epidemiological studies to be protective against the development of neurodegenerative diseases. This is supported by animal studies that have shown nicotine to be neuroprotective both in vivo and in vitro. Treatment with nicotinic agonists may therefore be useful in both slowing the progression of neurodegenerative illnesses, and improving function in patients with the disease. While increased nicotinic function has been shown to be beneficial, loss of cholinergic markers is often seen in patients with dementia, suggesting that decreased cholinergic function could contribute to both the cognitive deficits, and perhaps the neuronal degeneration, associated with dementia. In this article we will review the literature on each of these areas. We will also present hypotheses that might address the mechanisms underlying the ability of nAChR function to protect against neurodegeneration or improve cognition, two potentially distinct actions of nicotine.

Impairment of cholinergic neurotransmission in adult and aged transgenic Tg2576 mouse brain expressing the Swedish mutation of human beta-amyloid precursor protein

To address the question of whether beta-amyloid peptides also affect cholinergic neurotransmission in vivo, brain tissue from transgenic Tg2576 mice with Alzheimer plaque pathology at ages ranging from 7 to 24 months were examined by immuno- and histochemical staining for choline acetyltransferase (ChAT) and acetycholinesterase (AChE), by assaying cholinergic enzyme activities and high-affinity choline uptake as well muscarinic and nicotinic cholinergic receptor binding levels by quantitative autoradiography. Cortical and hippocampal activities of AChE and ChAT were not different between transgenic mice and non-transgenic littermates regardless of the postnatal ages examined. However, high-affinity choline uptake was reduced in the hippocampus of 21-month-old transgenic mice. In brains of 8-month-old transgenic mice which do not yet demonstrate cortical beta-amyloids, reduced binding levels of cortical and hippocampal M1-muscarinic cholinergic receptors were observed, which were still reduced in 17-month-old transgenic mouse brains with high plaque load as compared to non-transgenic littermates. M2-muscarinic cholinergic receptor binding was hardly affected in brains from 8-month-old transgenic mice, but in 17-month-old transgenic mice reduced cortical and hippocampal binding levels were observed as compared to non-transgenic controls. Decreased cortical nicotinic cholinergic receptor binding was detected in 17-month-old transgenic mice. The development of changes in cholinergic synaptic markers in transgenic Tg2576 mouse brain before the onset of progressive plaque deposition provides in vivo evidence of a modulatory role of soluble beta-amyloid on cholinergic neurotransmission and may be referred to the deficits in learning and memory also observed in these mice before significant plaque load.

Alzheimer's disease and the basal forebrain cholinergic system: relations to beta-amyloid peptides, cognition, and treatment strategies

Alzheimer's disease (AD) is the most common form of degenerative dementia and is characterized by progressive impairment in cognitive function during mid- to late-adult life. Brains from AD patients show several distinct neuropathological features, including extracellular beta-amyloid-containing plaques, intracellular neurofibrillary tangles composed of abnormally phosphorylated tau, and degeneration of cholinergic neurons of the basal forebrain. In this review, we will present evidence implicating involvement of the basal forebrain cholinergic system in AD pathogenesis and its accompanying cognitive deficits. We will initially discuss recent results indicating a link between cholinergic mechanisms and the pathogenic events that characterize AD, notably amyloid-beta peptides. Following this, animal models of dementia will be discussed in light of the relationship between basal forebrain cholinergic hypofunction and cognitive impairments in AD. Finally, past, present, and future treatment strategies aimed at alleviating the cognitive symptomatology of AD by improving basal forebrain cholinergic function will be addressed.

Intracellular A-beta amyloid, a sign for worse things to come?

In this review the authors discuss the possible neuropathological role of intracellular amyloid-beta accumulation in Alzheimer's disease (AD) pathology. There is abundant evidence that at early stages of the disease, prior to A-beta amyloid plaque formation, A-beta peptides accumulate intraneuronally in the cerebral cortex and the hippocampus. The experimental evidence would indicate that intracellular amyloid-beta could originate both by intracellular biosynthesis and also from the uptake of amyloidogenic peptides from the extracellular milieu. Herein the aspects of the possible impact of intracellular amyloid-beta in human AD pathology are discussed, as well as recent observations from a rat transgenic model with a phenotype of intracellular accumulation of A-beta fragments in neurons of the hippocampus and cortex, without plaque formation. In this model, the intracellular amyloid-beta phenotype is accompanied by increased MAPK/ERK activity and tau hyperphosphorylation. Finally, the authors discuss the hypothesis that, prior to plaque formation, intracellular A-beta accumulation induces biochemical and pathological changes in the brain at the cellular level priming neurons to further cytotoxic attack of extracellular amyloidogenic peptides.

Beta-amyloid enhances glial glutamate uptake activity and attenuates synaptic efficacy

Although amyloid beta-protein (A beta) has long been implicated in the pathogenesis of Alzheimer's disease, little is known about the mechanism by which A beta causes dementia. A beta leads to neuronal cell death in vivo and in vitro, but recent evidence suggests that the property of the amnesic characteristic of Alzheimer's disease can be explained by a malfunction of synapses rather than a loss of neurons. Here we show that prolonged treatment with A beta augments the glutamate clearance ability of cultured astrocytes and induces a dramatic decrease in glutamatergic synaptic activity of neurons cocultured with the astrocytes. Biotinylation assay revealed that the enhancement of glutamate uptake activity was associated with an increase in cell-surface expression of GLAST, a subtype of glial glutamate transporters, without apparent changes in the total amount of GLAST. This phenomenon was blocked efficiently by actin-disrupting agents. Thus, A beta-induced actin-dependent GLAST redistribution and relevant synaptic malfunction may be a cellular basis for the amnesia of Alzheimer's disease.

Elevated levels of Abeta1-40 and Abeta1-42 do not alter the binding sites of nicotinic receptor subtypes in the brain of APPswe and PS1 double transgenic mice

The binding sites of nicotinic acetylcholine receptor (nAChR) subtypes were measured in the parietal cortex and hippocampus of transgenic mice carrying mutant human APPswe and presenilin 1 (PS1) genes (APPswe/PS1 mice) between the ages of 3 weeks and 17 months. Soluble and insoluble beta-amyloid peptide (Abeta1-40 and Abeta1-42) levels were investigated in parallel. No significant differences in binding sites of [(3)H]cytisine (alpha4beta2 nAChRs) and [(125)I]alpha-bungarotoxin (alpha7 nAChRs) were observed in APPswe/PS1 mice and wild-type control mice at any age studied. At three weeks of age, soluble Abeta1-40 was detectable in the parietal cortex and hippocampus of APPswe/PS1 mice, whereas Abeta1-42 was detectable from 12 months of age. A pronounced increase in insoluble Abeta1-42 was observed between 3 weeks and 17 months compared with that of insoluble Abeta1-40 in both brain regions, indicating a shift that favors accumulation of Abeta1-42 in older APPswe/PS1 mice. The findings indicate that elevated Abeta levels in the brains of APPswe/PS1 mice do not alter the number of alpha4beta2 and alpha7 receptors, the two major brain nAChR subtypes.

Region-specific alterations in insulin-like growth factor receptor type I in the cerebral cortex and hippocampus of aged rats

In the present study, we investigated age-related changes in IGF-I receptor localization in the cerebral cortex and hippocampus of Sprague-Dawley rats using immunohistochemistry. In the cerebral cortex of adult rats, weakly stained cells were seen in layers II-III and layer V/VI in several cortical regions. In aged rats, there was a significant increase in IGF-I receptor immunoreactivity in the pyramidal cells in the same cortical regions. In the hippocampus of adult rats, several moderately stained neurons were seen in CA1-3 areas and the dentate gyrus. Levels of IGF-I receptor protein increased substantially with age in the CA3 area of the hippocampus. Our first morphological data concerning the differential regulation of IGF-I receptors in aged cerebral cortex and hippocampus may provide insights into age-related changes in trophic support as well as basic knowledge required for the study of neurodegenerative diseases such as Alzheimer's disease.

Assessment of choline uptake for the synthesis and release of acetylcholine in brain slices by a dynamic autoradiographic technique using [11C]choline

The uptake of choline for the synthesis and release of acetylcholine was investigated in brain slices by dynamic positron autoradiography using [11C]choline. Brain slices (330 microm) were incubated with [11C]choline in oxygenated Krebs-Ringer medium at 34 degrees C and serial two-dimensional time-resolved images of the uptake and release of radioactivity were recorded on Storage Phosphor screens. [11C]choline uptake increased with the period of incubation and was 1.9 times higher in the striatum than cerebral cortex. The uptake in the striatum was significantly diminished by hemicholinium-3 (HC-3), an inhibitor of high-affinity choline uptake. Pretreatment of brain slices with 50 mM K(+) for 20 min enhanced the uptake in striatum. The uptake of [11C]choline in brain slices was saturable using nonlabeled choline. Two uptake systems, a high-affinity and a low-affinity system, were confirmed to exist by kinetic analysis using Lineweaver-Burk plots. The 11C radioactivity that had accumulated in the striatum disappeared on treatment with veratridine, a depolarization agent, in the presence of HC-3. This pattern of disappearance was consistent with that of the appearance of unlabeled and labeled acetylcholine in the medium. These results indicate that this method is useful for obtaining information regarding the uptake of choline for the synthesis and release of acetylcholine in live brain tissues.

Selective nicotinic receptor consequences in APP(SWE) transgenic mice

The nicotinic (nAChRs) and muscarinic (mAChRs) acetylcholine receptors and acetylcholinesterase (AChE) activity were studied in the brains of APP(SWE) transgenic mice (Tg+) and age-matched nontransgenic controls (Tg-) that were between 4 and 19 months of age. A significant increase in the binding of 125I-labeled alpha-bungarotoxin (alpha7 nAChRs) was observed in most brain regions analyzed in 4-month-old Tg+ mice, preceding learning and memory impairments and amyloid-beta (Abeta) pathology. The enhanced alpha7 receptor binding was still detectable at 17-19 months of age. Increase in [3H]cytisine binding (alpha4beta2 nAChRs) was measured at 17-19 months of age in Tg+ mice, at the same age when the animals showed heavy Abeta pathology. No significant changes in [3H]pirenzepine (M1 mAChRs) or [3H]AFDX 384 (M2 mAChRs) binding sites were found at any age studied. The upregulation of the nAChRs probably reflects compensatory mechanisms in response to Abeta burden in the brains of Tg+ mice.

No spatial working memory deficit in beta-amyloid-exposed rats. A longitudinal study

Two experiments are described assessing whether long-term intraventricular or intrahippocampal administration of beta-amyloid protein 1-40 (beta A1-40) affects spatial working memory in rats monitored in a longitudinal study using the open-field water maze. A delayed matching-to-position procedure (DMTP) was employed in which platform locations were semi-randomly altered between days but were kept constant over the four trials on each day. Intertrial intervals (ITIs) were either 30 s or 1 h between Trials 1 and 2 (all other intervals = 30 s), with Trial 2 performance being an index for spatial working memory. Animals were trained before and tested repeatedly at various intervals after application of various compounds (see below) in five successive test sessions (TSs). In Experiment 1, beta A1-40 was applied after a challenge with long-term oral exposure to aluminium (Al; as 0.1% sulfate in drinking water). This in itself did not affect spatial working memory at any delay, despite of the more than 6 months of intake. beta A1-40 administered alone via intracerebroventricular (icv) minipumps (20 micrograms in 250 microliters) led to a small increase in latencies to find the platform, which recovered to control levels 3 months after minipumps were exhausted. Application of beta A1-40 in Al-exposed animals led to a subtle and progressive decline in working memory. This deterioration was reversed by the nootropic compound nefiracetam, which had no effect on the Al only group. In Experiment 2, well-trained rats were bilaterally implanted with intra-hippocampal minipumps containing beta A1-40 or reverse sequence beta A40-1. This did not impair spatial working memory in the DMTP task, measured either directly after minipumps were exhausted, or 2 weeks later. When intraperitoneally (i.p.) injected with a low concentration of the muscarinic antagonist scopolamine (0.2 mg/kg), a dose that was not effective alone, animals in the beta A1-40 group were amnesic. These data suggest that intra-hippocampal beta A1-40 administration alters cholinergic transmission, but these alterations may be mild and thus do not lead to obvious working memory deficits in a DMTP task in well-trained animals.

Up-regulation of the inflammatory cytokines IFN-gamma and IL-12 and down-regulation of IL-4 in cerebral cortex regions of APP(SWE) transgenic mice

Alzheimer's disease (AD) is a progressive neurodegenerative disorder, of which the pathogenesis is thought to involve increased beta-amyloid (Abeta) deposition and abnormal immunological responses. To elucidate the mechanisms involved in Abeta-mediated inflammation, we used immunocytochemistry and in situ hybridization to study the potential role of the cytokines interferon-gamma (IFN-gamma), interleukin (IL)-12 and IL-4 in transgenic mice APP(SWE) (Tg2576) that overexpress the human beta-amyloid precursor protein gene. Cytokine and cytokine mRNA expression was detected in brain sections from cortical regions at various postnatal ages ranging from 3 to 19 months. High levels of IFN-gamma and IL-12 mRNA expression, as well as their protein production, appeared early at 9 months and peaked at 17-19 months in Tg2576 mice. Significantly increased transcripts of IFN-gamma and IL-12 genes were found in the reactive microglia and astrocytes surrounding beta-amyloid deposits. In accordance with the kinetics of mRNA levels, the expression of IFN-gamma and IL-12 at the protein level was positively correlated with age and reached a maximum in 17-19-month-old mice. Both findings suggest a role for the pro-inflammatory cytokines IFN-gamma and IL-12 in early disease development and are consistent with microglial activation related to beta-amyloid formation. In contrast, transcription and production of IL-4 in brain sections was almost undetectable in transgenic mice up to post-natal ages of 17-19 months. These results suggest a major pro-inflammatory role for IL-12 and IFN-gamma in Tg2576 transgenic mice that may provide the association between beta-amyloid plaque formation and microglial and astrocyte activation in these animals. These observations call for further studies on the potential role of anti-inflammatory therapeutic strategies for AD.

Cholinergic changes in the APP23 transgenic mouse model of cerebral amyloidosis

Alzheimer's Disease (AD) is a neurodegenerative disorder that is characterized by extracellular deposits of amyloid-beta peptide (Abeta) and a severe depletion of the cholinergic system, although the relationship between these two events is poorly understood. In the neocortex, there is a loss of cholinergic fibers and receptors and a decrease of both choline acetyltransferase (ChAT) and acetylcholinesterase enzyme activities. The nucleus basalis of Meynert (NBM), which provides the major cholinergic input to the neocortex, undergoes profound neuron loss in AD. In the present study, we have examined the cholinergic alterations in amyloid precursor protein transgenic mice (APP23), a mouse model of cerebral beta-amyloidosis. In aged APP23 mice, our results reveal modest decreases in cortical cholinergic enzyme activity compared with age-matched wild-type mice. Total cholinergic fiber length was more severely affected, with 29 and 35% decreases in the neocortex of aged APP23 mice compared with age-matched wild-type mice and young transgenic mice, respectively. However, there was no loss of cholinergic basal forebrain neurons in these aged APP23 mice, suggesting that the cortical cholinergic deficit in APP23 mice is locally induced by the deposition of amyloid and is not caused by a loss of cholinergic basal forebrain neurons. To study the impact of cholinergic basal forebrain degeneration on cortical amyloid deposition, we performed unilateral NBM lesions in adult APP23 mice. Three to 8 months after lesioning, a 38% reduction in ChAT activity and significant cholinergic fiber loss were observed in the ipsilateral frontal cortex. There was a 19% decrease in Abeta levels of the ipsilateral compared with contralateral frontal cortex with no change in the ratio of Abeta40 to Abeta42. We conclude that the severe cholinergic deficit in AD is caused by both the loss of cholinergic basal forebrain neurons and locally by cerebral amyloidosis in the neocortex. Moreover, our results suggest that disruption of the basal cholinergic forebrain system does not promote cerebral amyloidosis in APP23 transgenic mice.

Alzheimer's disease: its diagnosis and pathogenesis

A hypothesis has been presented that links many of the identified and putative risk factors for AD and suggests a mechanism for their action. Crawford (1996, 1998) proposes an association between AD and cerebral blood flow (CBF) by citing evidence that many of the factors that are linked with an increased risk of AD also decrease CBF (e.g., old age, depression, underactivity, head trauma). Similarly, it is suggested factors that increase CBF are associated with a decreased risk of AD (e.g., education, exercise, smoking, NSAIDs). Although the authors acknowledge that reduced CBF is not sufficient to cause AD, the reported positive and negative associations provide tantalizing evidence for a common mode of action for many of the equivocal risk factors reported to date. This hypothesis is also consistent with other data that links microvascular damage and impaired blood flow (de la Torre, 1997, 2000) and low education with increased cerebrovascular disease (Del Ser et al., 1999). Gaining a better understanding of the interaction between AD and vascular disease is of great importance. Not only will it provide insights into the pathogenesis of AD, but it may also provide us with a rare opportunity for the treatment and possible prevention of AD. A great many risk factors for vascular disease have been identified and intervention programs have successfully reduced the incidence of heart disease and stroke. The potential exists to provide the same level of success with AD.

Nicotinic cholinergic modulation: galantamine as a prototype

Nicotinic acetylcholine receptor pharmacology is becoming increasingly important in the clinical symptomatology of neurodegenerative diseases in general and of cognitive and behavioral aspects in particular. In addition, the concept of allosteric modulation of nicotinic acetylcholine receptors has become a research focus for the development of therapeutic agents. In this review the scientific evidence for changes in nicotinic acetylcholine receptors in Alzheimer's disease is described. Within this context, the pharmacology of galantamine, a recently approved drug for cognition enhancement in Alzheimer's disease, is reviewed along with preclinical studies of its efficacy on learning and memory. Galantamine modestly inhibits acetylcholinesterase and has an allosteric potentiating ligand effect at nicotinic receptors. The data collected in this review suggest that the unique combination of acetylcholinesterase inhibition and nicotinic acetylcholine receptor modulation offers potentially significant benefits over acetylcholinesterase inhibition alone in facilitating acetylcholine neurotransmission.