beta-Amyloid(1-42) binds to alpha7 nicotinic acetylcholine receptor with high affinity. Implications for Alzheimer's disease pathology

Progressive Worsening of Adaptive Functions in Down Syndrome May Be Mediated By the Complexing of Soluble Aβ Peptides With the α7 Nicotinic Acetylcholine Receptor: Therapeutic Implications

In persons with Down syndrome, soluble Abeta peptides, which result from the processing of the amyloid precursor protein, appear in the brain decades before the extracellular deposition of neuritic plaques. These soluble amyloidogenic peptides accumulate intraneuronally and can be secreted extracellularly. Their appearance has been reported in the brains of fetuses with Down syndrome. The extra gene dosage effect associated with trisomy 21 results in abnormalities of the processing of amyloid precursor protein in persons with Down syndrome. Abeta peptides, especially Abeta1-42, have been shown to form tight complexes with the alpha7 nicotinic acetylcholine receptor, interfering with transduction of the acetylcholine signal by this nicotinic receptor subtype. Furthermore, the selective binding of Abeta peptides by this nicotinic acetylcholine receptor subtype is associated with cytotoxicity. The alpha7 nicotinic acetylcholine receptor has unique electrophysiologic properties and plays a prominent role in normal psychophysiologic processes (eg, sensory inhibition) and cognition. In persons with Down syndrome there is a decrease in the ability to perform instrumental activities of daily living that worsen with aging. The progressive worsening of adaptive functions and cognition in persons with Down syndrome may be, at least in part, mediated by interference with this receptor by soluble Abeta peptides. In view of this complex formed by soluble Abeta peptides and the alpha7 nicotinic acetylcholine receptor, cholinergic interventions that have been developed for Alzheimer disease, including selective nicotinic ones, should be explored in Down syndrome. Ideally, selective cholinergic interventions would slow the progression of the worsening of adaptive function and emergence of dementia in persons with Down syndrome.

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.

Nicotinic acetylcholine receptor-mediated neuroprotection by donepezil against glutamate neurotoxicity in rat cortical neurons

Donepezil is a potent and selective acetylcholinesterase (AChE) inhibitor developed for the treatment of Alzheimer's disease. To elucidate whether donepezil shows neuroprotective action in addition to amelioration of cognitive deficits, we examined the effects of donepezil on glutamate-induced neurotoxicity using primary cultures of rat cortical neurons. A 10-min exposure of cultures to glutamate followed by a 1-h incubation with glutamate-free medium caused a marked loss of viability, as determined by Trypan blue exclusion. Glutamate neurotoxicity was prevented by 24-h pretreatment of donepezil in a concentration-dependent manner. Among AChE inhibitors examined, donepezil and certain AChE inhibitors such as tacrine and galanthamine showed potent neuroprotective action, although physostigmine did not affect glutamate neurotoxicity. Neuroprotective action of donepezil was antagonized by mecamylamine, a nicotinic acetylcholine receptor (nAChR) antagonist, but not by scopolamine, a muscarinic acetylcholine receptor antagonist. Furthermore, both dihydro-beta-erythroidine, an alpha4beta2-neuronal nAChR antagonist, and methyllycaconitine, an alpha7-nAChR antagonist, each also significantly antagonized the effect of donepezil. Next, we examined the effects of donepezil on glutamate-induced apoptosis. Exposure of 100 microM glutamate to cortical neurons for 24 h induced apoptotic neuronal death and nuclear fragmentation. Donepezil for 24 h before and 24 h during glutamate exposure prevented nuclear fragmentation and glutamate-induced apoptosis. These results suggest that donepezil not only protects cortical neurons against glutamate neurotoxicity via alpha4beta2- and alpha7-nAChRs but also prevents apoptotic neuronal death.

Antibody to beta-amyloid protein increases acetylcholine in the hippocampus of 12 month SAMP8 male mice

Amyloid beta protein (Abeta) is the primary constituent of plaque seen in Alzheimer's disease. Abeta is proposed to play an etiological role in Alzheimer's disease and to be a cause of the decrease in the level of acetylcholine in the hippocampus. The SAMP8 strain of mouse develops age-related increases in Abeta and deficits in learning and memory by 12 months of age. We examined in 12 month old SAMP8 mice the effects of giving antibody to Abeta by septal or intracerebroventricular (ICV) injection on acetylcholine levels in the hippocampus. Antibody to Abeta increased acetylcholine in the hippocampus over 100% after ICV injection and over 200% after septal injection. Injection of rabbit serum, antibody directed towards mouse IgG, or a blocking antibody directed towards human interleukin-1beta were without effect. These results suggest that antagonism of Abeta increases acetylcholine concentrations in the hippocampus, an area important for learning and memory.

A review of rivastigmine: a reversible cholinesterase inhibitor

BACKGROUND

Rivastigmine tartrate is a reversible cholinesterase inhibitor indicated for the symptomatic treatment of mild to moderate dementia. It was approved by the US Food and Drug Administration for the treatment of Alzheimer's disease (AD) on April 21, 2000.

OBJECTIVE

The purpose of this review was to summarize the background on dementia of the Alzheimer type and the pharmacokinetic properties, efficacy and tolerability profiles, clinical applications, adverse effects (AEs), drug interactions, and pharmacoeconomics of rivastigmine.

METHODS

A literature search was conducted using MEDLINE (1995-2002), EMBASE Geriatrics and Gerontology (1995-2002), the National Institutes of Health Alzheimer's Disease Education and Resource Center Combined Health Information Database, and Google. Search terms included rivastigmine, Exelon, ENA 713, and ENA-713. The bibliographies of retrieved articles also were searched for relevant articles.

RESULTS

In clinical trials, rivastigmine has improved or maintained cognitive function, global function (ie, activities of daily living [ADLs]), and behavior in patients with mild to moderate AD for up to 52 weeks. AEs are generally mild to moderate and primarily affect the gastrointestinal (GI) tract. Clinically significant drug interactions with rivastigmine have thus far not been reported. Treatment with rivastigmine for up to 2 years may reduce the cost of caring for patients with AD. Cost savings are minimal during the first year, particularly for those with mild disease, but increase during the second year of treatment. Cost savings occur earlier for those with moderate AD. Most savings are realized from a delay in the need for institutionalization.

CONCLUSIONS

Rivastigmine has been shown to improve or maintain patients' performance in 3 major domains: cognitive function, global function (ADLs), and behavior. The efficacy and tolerability of rivastigmine have been proved by numerous clinical trials, with the most prominent AE being GI irritation.

Nicotine enhances the depressive actions of A beta 1-40 on long-term potentiation in the rat hippocampal CA1 region in vivo

Hippocampal long-term potentiation (LTP) is a form of synaptic plasticity used as a cellular model of memory. Beta amyloid (A beta) is involved in Alzheimer's disease (AD), a neurode-generative disorder leading to cognitive deficits. Nicotine is also claimed to act as a cognitive enhancer. A beta is known to bind with high affinity to the alpha 7-nicotinic acetylcholine receptor (nAChR). Here we have investigated the effect of intracerebroventricular (i.c.v.) injection of the endogenous peptide A beta 1-40 on LTP in area CA1 of urethananesthetized rats. We also examined the effect of A beta 12-28 (i.c.v.), which binds with high affinity to the alpha 7-nAChR and the specific alpha 7-nAChR antagonist methyllycaconitine (MLA) on LTP. We found that A beta 12-28 had no effect on LTP, whereas MLA depressed significantly LTP, suggesting that activation of the alpha 7-nAChR is a requirement for LTP. Within the in vivo environment, where other factors may compete with A beta 12-28 for binding to alpha 7-nAChR, it does not appear to modulate LTP. To determine if the depressive action of A beta 1-40 on LTP could be modulated by nicotine, these agents were also co-applied. Injection of 1 or 10 nmol A beta 1-40 caused a significant depression of LTP, whereas nicotine alone (3 mg/kg) had no effect on LTP. Co-injection of nicotine with A beta 1-40 1 h prior to LTP induction caused a further significant depression of LTP compared with A beta 1-40 alone. These results demonstrate that nicotine enhances the deficit in LTP produced by A beta 1-40. This then suggests that nicotine may exacerbate the depressive actions of A beta on synaptic plasticity in AD.

Astrocytes accumulate A beta 42 and give rise to astrocytic amyloid plaques in Alzheimer disease brains

beta-Amyloid(1-42) (A beta 42), a major component of amyloid plaques, accumulates within pyramidal neurons in the brains of individuals with Alzheimer's disease (AD) and Down syndrome. In brain areas exhibiting AD pathology, A beta 42-immunopositive material is observed in astrocytes. In the present study, single- and double-label immunohistochemistry were used to reveal the origin and fate of this material in astrocytes. Our findings suggest that astrocytes throughout the entorhinal cortex of AD patients gradually accumulate A beta 42-positive material and that the amount of this material correlates positively with the extent of local AD pathology. A beta 42-positive material within astrocytes appears to be of neuronal origin, most likely accumulated via phagocytosis of local degenerated dendrites and synapses, especially in the cortical molecular layer. The co-localization of neuron-specific proteins, alpha 7 nicotinic acetylcholine receptor and choline acetyltransferase, in A beta 42-burdened, activated astrocytes supports this possibility. Our results also suggest that some astrocytes containing A beta 42-positive deposits undergo lysis, resulting in the formation of astrocyte-derived amyloid plaques in the cortical molecular layer in brain regions showing moderate to advanced AD pathology. These astrocytic plaques can be distinguished from those arising from neuronal lysis by virtue of their smaller size, their nearly exclusive localization in the subpial portion of the molecular layer of the cerebrocortex, and by their intense glial fibrillary acidic protein immunoreactivity. Overall, A beta 42 accumulation and the selective lysis of A beta 42-burdened neurons and astrocytes appear to make a major contribution to the observed amyloid plaques in AD brains.

Roles of amyloid precursor protein and its fragments in regulating neural activity, plasticity and memory

Amyloid-beta precursor protein (APP) is a membrane-spanning protein with a large extracellular domain and a much smaller intracellular domain. It is the source of the amyloid-beta (Abeta) peptide found in neuritic plaques of Alzheimer's disease (AD) patients. Because Abeta shows neurotoxic properties, and because familial forms of AD promote Abeta accumulation, a massive international research effort has been aimed at understanding the mechanisms of Abeta generation, catabolism and toxicity. APP, however, is an extremely complex molecule that may be a functionally important molecule in its full-length configuration, as well as being the source of numerous fragments with varying effects on neural function. For example, one fragment derived from the non-amyloidogenic processing pathway, secreted APPalpha (sAPPalpha), is neuroprotective, neurotrophic and regulates cell excitability and synaptic plasticity, while Abeta appears to exert opposing effects. Less is known about the neural functions of other fragments, but there is a growing interest in understanding the basic biology of APP as it has become recognized that alterations in the functional activity of the APP fragments during disease states will have complex effects on cell function. Indeed, it has been proposed that reductions in the level or activity of certain APP fragments, in addition to accumulation of Abeta, may play a critical role in the cognitive dysfunction associated with AD, particularly early in the course of the disease. To test and modify this hypothesis, it is important to understand the roles that full-length APP and its fragments normally play in neuronal structure and function. Here we review evidence addressing these fundamental questions, paying particular attention to the contributions that APP fragments play in synaptic transmission and neural plasticity, as these may be key to understanding their effects on learning and memory. It is clear from this literature that APP fragments, including Abeta, can exert a powerful regulation of key neural functions including cell excitability, synaptic transmission and long-term potentiation, both acutely and over the long-term. Furthermore, there is a small but growing literature confirming that these fragments correspondingly regulate behavioral learning and memory. These data indicate that a full account of cognitive dysfunction in AD will need to incorporate the actions of the full complement of APP fragments. To this end, there is an urgent need for a dedicated research effort aimed at understanding the behavioral consequences of altered levels and activity of the different APP fragments as a result of experience and disease.

Altered auditory-evoked potentials in mice carrying mutated human amyloid precursor protein and presenilin-1 transgenes

Transgenic mice carrying human APPswe and PS1-A264E transgenes (A/P mice) have elevated levels of the highly fibrillogenic amyloid Abeta(1-42) (Abeta) and develop amyloid plaques around the age of 9 months. Our aim was to find whether the gradual accumulation of Abeta in these mice can be detected with long-term recording of auditory-evoked potentials. The A/P double-mutant mice had impaired auditory gating and a tendency toward increased latency of the cortical N35 response, but these changes were not age-dependent between 7 and 11 months of age. In a control experiment that included also APP and PS1 single-mutant mice, the A/P double-mutant mice had weaker auditory gating than either APP or PS1 mice. In contrast, increased N35 latency was found in both A/P and APP mice compared with nontransgenic or PS1 mice. The Abeta40 and Abeta42 levels were robustly increased in A/P mice and Abeta40 moderately increased also in APP mice. Plaques were deposited only in A/P mice. We conclude that the impaired auditory gating is associated with the overproduction Abeta42 but does not reflect its amount. In contrast, increased N35 latency is related to the APP genotype independent of Abeta42 production.

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).

Tyrosine nitration of a synaptic protein synaptophysin contributes to amyloid beta-peptide-induced cholinergic dysfunction

Amyloid beta (Abeta) is a critical factor involved in the pathogenesis of Alzheimer's disease (AD). We have previously demonstrated that continuous intracerebroventricular infusion of Abeta1-40 induced a time-dependent expression of the inducible nitric oxide (NO) synthase (iNOS) and an overproduction of NO in the rat hippocampus. The pathophysiological significance of the overproduction of NO on brain function was manifested by an impairment of nicotine-evoked acetylcholine(ACh) release and memory deficits.(4) Molecular mechanisms by which NO participates in the Abeta-induced brain dysfunction, however, remain to be determined. Here we show that chronic Abeta1-40 infusion caused a robust peroxynitrite formation and subsequent tyrosine nitration of proteins in the hippocampus. Immunoprecipitation and Western blot analyses further revealed that synaptophysin, a synaptic protein, was a main target of tyrosine nitration. Chronic infusion of Abeta1-40 resulted in an impairment of nicotine-evoked ACh release as analyzed by microdialysis. Daily treatment with the iNOS inhibitor aminoguanidine (AG) or the peroxynitrite scavenger uric acid (UA) prevented the tyrosine nitration of synaptophysin as well as the impairment of nicotine-evoked ACh release induced by Abeta. Our findings suggest that the tyrosine nitration of synaptophysin is related to Abeta-induced impairment of ACh release.

A beta-induced TNF-alpha expression and acetylcholine action in mouse glial cells

The brains in patients with Alzheimer's disease show chronic inflammatory responses characterized by activated glial cells and increased expression of cytokines. It is of interest to determine whether acetylcholine (ACh) affects Abeta-induced cytokine expression in the glial cells. Since it has been shown that alpha7 subunits of nicotinic ACh receptors are expressed in glial cells and that Abeta(1-42) binds to alpha7, we examined the effects of cholinergic agonists, carbachol, nicotine and oxotremorine-M, on Abeta-induced TNF-alpha expression in mouse glial cells. We did not observe any regulatory effects of ACh on Abeta-induced TNF-alpha transcription in the glial cells. We discuss the pathophysiological roles of ACh in glial cells in the brains of patients with Alzheimer's disease.

Amyloid beta(1-42) peptide alters the gating of human and mouse alpha-bungarotoxin-sensitive nicotinic receptors

The beta-amyloid(1-42) peptide (Abeta(1-42)), a major constituent of the Alzheimer's disease amyloid plaque, specifically binds to the neuronal alpha-bungarotoxin (alpha-BuTx)-sensitive alpha7 nicotinic acetylcholine receptor (alpha7 nAChR). Accordingly, Abeta1-42 interferes with the function of alpha7 nAChRs in chick and rodent neurons. To gain insights into the human disease, we studied the action of Abeta(1-42) on human alpha7 nAChRs expressed in Xenopus oocytes. In voltage-clamped oocytes expressing the wild-type receptor, Abeta(1-42) blocked ACh-evoked currents. The block was non-competitive, required over 100 s to develop and was partially reversible. In oocytes expressing the mutant L248T receptor, Abeta(1-42) activated methyllycaconitine-sensitive currents in a dose-dependent manner. Peptide-evoked unitary events, recorded in outside-out patches, showed single-channel conductances and open duration comparable to ACh-evoked events. Abeta(1-42) had no effect on the currents evoked by glutamate, GABA or glycine in oocytes expressing human or mouse receptors for these transmitters. Muscle nAChRs are also alpha-BuTx-sensitive and we therefore investigated whether they respond to Abeta(1-42). In human kidney BOSC 23 cells expressing the fetal or adult mouse muscle nAChRs, Abeta(1-42) blocked ACh-evoked whole-cell currents, accelerating their decay. Outside-out single-channel recordings showed that the block was due to a reduced channel open probability and enhanced block upon ACh application. We also report that the inverse peptide Abeta(42-1), but not Abeta(40-1), partially mimicked the effects of the physiological Abeta(1-42) peptide. Possible implications for degenerative neuronal and muscular diseases are discussed.

Dentate gyrus volume is reduced before onset of plaque formation in PDAPP mice: a magnetic resonance microscopy and stereologic analysis

High-resolution magnetic resonance microscopy (MRM) was used to determine regional brain volumetric changes in a mouse model of Alzheimer's disease. These transgenic (Tg) mice overexpress human mutant amyloid precursor protein (APP) V717F under control of platelet-derived growth factor promoter (PDAPP mice), and cortical and hippocampal beta-amyloid (Abeta) deposits accumulate in heterozygotes after 8-10 mos. We used MRM to obtain 3D volumetric data on mouse brains imaged in their skulls to define genotype- and age-related changes. Hippocampal, cerebellar, and brain volumes and corpus callosum length were quantified in 40-, 100-, 365-, and 630-day-old mice. Measurements taken at age 100 days, before Abeta deposition, revealed a 12.3% reduction of hippocampus volume in Tg mice compared with WT controls. This reduction persisted without progression to age 21 mos. A significant 18% increase in hippocampal volume occurred between 40 and 630 days in WT mice, and no corresponding significant increase occurred in Tg mice. Cavalieri volume estimates of hippocampal subfields from 100-day-old Tg mice further localized a 28% volume deficit in the dentate gyrus. In addition, corpus callosum length was reduced by approximately 25% in Tg mice at all ages analyzed. In summary, reduced hippocampal volume and corpus callosum length can be detected by MRM before Abeta deposition. We conclude that overexpression of APP and amyloid may initiate pathologic changes before the appearance of plaques, suggesting novel targets for the treatment of Alzheimer's disease and further reinforcing the need for early diagnosis and treatment.

Loss of nicotinic receptors induced by beta-amyloid peptides in PC12 cells: possible mechanism involving lipid peroxidation

The mechanisms involved in the loss of nicotinic acetylcholine receptors (nAChRs), seen in brains of patients with Alzheimer's disease (AD) and in cultured cells treated by beta-amyloid peptides (A betas), remain elusive. We give results to show that lipid peroxidation induced directly by A beta might be involved in the deficits of nAChRs. In the study, PC12 cells were treated by addition of 5 microM of A beta(25-35) and A beta(1-40), respectively, with or without a antioxidant, vitamin E. Besides significantly decreased MTT (3-(4,5-dimethylthiazol-2-yl)-2,5,diphenyltetrazolium bromide) reduction, an increased lipid peroxidation was detected in the cells, but no protein oxidation. Significant reductions in [(3)H]epibatidine and [(125)I]alpha-bungarotoxin binding sites and in the protein levels of the alpha 3 and alpha 7 nAChR subunits were observed in the cells treated with A betas. Furthermore, A beta(25-35) decreased the level of ubiquinone-9 in PC12 cells, but did not change the amount of cholesterol, providing further evidence for lipid peroxidation. Interestingly, when PC12 cells were pretreated by antioxidant before the addition of A betas, the lipid peroxidation and the decreased ubiquinone resulted from A betas were prohibited. The decreases of nAChR binding sites and subunit proteins resulted from A betas were mostly prevented by the pretreatment with antioxidant. These findings suggest that lipid peroxidation stimulated by A betas might be a mechanism for the loss of nAChRs associated with the pathogenesis of AD.

Receptor for advanced glycation endproducts: a multiligand receptor magnifying cell stress in diverse pathologic settings

Receptor for Advanced Glycation Endproducts (RAGE) is a member of the immunoglobulin superfamily of cell surface molecules capable of interacting with a broad spectrum of ligands, including advanced glycation endproducts (AGEs), amyloid fibrils, S100/calgranulins and amphoterin. The biology of RAGE is dictated by the accumulation of these ligands at pathologic sites, leading to upregulation of the receptor and sustained RAGE-dependent cell activation eventuating in cellular dysfunction. Although RAGE is not central to the initial pathogenesis of disorders in which it ultimately appears to be involved, such as diabetes, amyloidoses, inflammatory conditions and tumors (each of these conditions leading to accumulation of RAGE ligands), the receptor functions as a progression factor driving cellular dysfunction and exaggerating the host response towards tissue destruction, rather than restitution of homeostasis. These observations suggest that RAGE might represent a therapeutic target in a diverse group of seemingly unrelated disorders linked only by a multiligand receptor with an unusually wide and diverse repertoire of ligands, namely, RAGE.

Activation of phospholipases A2 and D of a human neuroblastoma cell line (LA-N-2) by N-dodecyl-L-lysine amide (compound 24), a putative G protein activator: characteristics of inhibition by (-)-nicotine

Compound 24, an alkyl-substituted amino acid amide, previously found to activate pertussis toxin-sensitive G proteins in cell membranes and membrane protein fractions, was used as a tool to determine the mechanism/location of nicotine inhibition of amyloid beta peptide-stimulated phospholipase A2 and D activities in a human neuroblastoma cell line, LA-N-2, in vitro. In contrast to our previous findings with amyloid beta peptide, these phospholipase activations by compound 24 were not inhibited by (-)-nicotine, cholera toxin or tetanus toxin pretreatment. The contrasting activation of these phospholipases by amyloid beta peptide and compound 24 are discussed.

Unconventional ligands and modulators of nicotinic receptors

Evidence gathered from epidemiologic and behavioral studies have indicated that neuronal nicotinic receptors (nAChRs) are intimately involved in the pathogenesis of a number of neurologic disorders, including Alzheimer's disease, Parkinson's disease, and schizophrenia. In the mammalian brain, neuronal nAChRs, in addition to mediating fast synaptic transmission, modulate fast synaptic transmission mediated by the major excitatory and inhibitory neurotransmitters glutamate and GABA, respectively. Of major interest, however, is the fact that the activity of the different subtypes of neuronal nAChR is also subject to modulation by substances of endogenous origin such as choline, the tryptophan metabolite kynurenic acid, neurosteroids, and beta-amyloid peptides and by exogenous substances, including the so-called nicotinic allosteric potentiating ligands, of which galantamine is the prototype, and psychotomimetic drugs such as phencyclidine and ketamine. The present article reviews and discusses the effects of unconventional ligands on nAChR activity and briefly describes the potential benefits of using some of these compounds in the treatment of neuropathologic conditions in which nAChR function/expression is known to be altered.

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.

Consistent immunohistochemical detection of intracellular beta-amyloid42 in pyramidal neurons of Alzheimer's disease entorhinal cortex

We compared the effects of three pretreatment immunohistochemical techniques (no pretreatment, pepsin predigestion and heat pretreatment (HEAT)) for detecting intracellular beta-amyloid42 (Abeta42) in pyramidal neurons of formalin-fixed Alzheimer's disease (AD) cortices (n = 25). Although all three protocols immunostained Abeta42 in amyloid plaques using four commercially-obtained Abeta42 specific antibodies, only the HEAT protocol consistently detected prominent intracellular Abeta42 in pyramidal neurons. This suggests that the Abeta42 present in amyloid plaques may be structurally distinct from that located within the neurons perhaps due to differential binding proteins coupling or a consequence of formalin fixation. Detection of an abundant intracellular Abeta42 in neurons may provide alternate explanations for the origin of dense-core amyloid plaques in AD cortices other than the conventional chronic extracellular Abeta42 deposition hypothesis.

Similarities between Alzheimer's disease and vascular dementia

Alzheimer's disease (AD) and vascular dementia (VaD) are the two most common forms of dementia. In Europe, 800,000 people have a diagnosis of VaD out of 3.7 million people with clinical dementia. These two dementia types share many common pathological, symptomatic and neurochemical features, and cholinergic treatments that have demonstrated robust, broad-ranging and long-term efficacy in AD are now being assessed for the treatment of dementia related to cerebrovascular disease (CVD). There has been recent recognition that dementia in the elderly is a continuum of pathologies, with pure AD and VaD representing the two extremes, and 'mixed' dementia (AD with CVD) in between and perhaps comprising the majority of cases. 'Mixed' dementia is rarely diagnosed in the clinic, however, as the majority of diagnostic procedures are biased toward a diagnosis of AD. Here, the risk factors, pathophysiological mechanisms and clinical symptoms of AD and VaD are described, identifying their overlap as well as some of the differences in both cognitive and noncognitive symptoms. Important findings indicating the high prevalence of 'mixed' dementia in the clinical dementia population are also discussed. In particular, evidence of a causal connection between stroke or CVD and AD is addressed. Regarding effective therapeutic management of dementia patients, further concerted epidemiological study of these related dementia types should aid in clinical decisions on the applicability of cholinergic treatments.

Janus Kinase 2, an Early Target of α7 Nicotinic Acetylcholine Receptor-mediated Neuroprotection against Aβ-(1–42) Amyloid

The molecular mechanisms of alpha7 nicotinic acetylcholine receptor (nAChR)-mediated neuroprotection remain unclear. In this study we provide evidence that nicotine stimulation of alpha7 nAChR transduces signals to phosphatidylinositol 3-kinase and Akt via Janus kinase 2 (JAK2) in a cascade, which results in neuroprotection. Exposure to beta-amyloid results in the activation of the apoptotic enzyme caspase-3 and cleavage of the DNA-repairing enzyme poly-(ADP-ribose) polymerase. This cascade is inhibited by nicotine through JAK2 activation, and these effects are blocked by preincubation with the JAK2-specific inhibitor AG-490. We also found that pretreatment of cells with angiotensin II blocks the nicotine-induced activation of JAK2 via the AT(2) receptor and completely prevents alpha7 nAChR-mediated neuroprotective effects further suggesting a pivotal role for JAK2. These findings identify novel mechanisms of receptor interactions relevant to neuronal viability and suggest novel therapeutic strategies to optimize neuroprotection.

Inhibition of amyloid-beta-induced cell death in human brain pericytes in vitro

Amyloid-beta protein (A beta) deposition in the cerebral vascular walls is one of the key features of Alzheimer's disease and hereditary cerebral hemorrhage with amyloidosis-Dutch type (HCHWA-D). A beta(1-40) carrying the 'Dutch' mutation (HCHWA-D A beta(1-40)) induces pronounced degeneration of cultured human brain pericytes. In this study, we aimed to identify inhibitors of A beta-induced toxicity in human brain pericytes. The toxic effect of HCHWA-D A beta(1-40) on human brain pericytes was inhibited by co-incubation with catalase, but not with superoxide dismutase, glutathione or vitamin E analogue Trolox. Catalase interacts with A beta, both in cell cultures and in cell-free assays, and has a prominent effect on the amount and conformational state of A beta binding to the cell surface of human brain pericytes. This activity of catalase is likely based on its ability to bind and slowly degrade A beta and not by its usual capacity to convert hydrogen peroxide. Our data confirm that assembly of A beta at the cell surface of human brain pericytes is a crucial step in A beta-induced cellular degeneration of human brain pericytes. Inhibition of fibril formation at the cell surface could be an important factor in therapy aimed at reducing cerebral amyloid angiopathy.

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.

Furin at the cutting edge: from protein traffic to embryogenesis and disease

Furin catalyses a simple biochemical reaction--the proteolytic maturation of proprotein substrates in the secretory pathway. But the simplicity of this reaction belies furin's broad and important roles in homeostasis, as well as in diseases ranging from Alzheimer's disease and cancer to anthrax and Ebola fever. This review summarizes various features of furin--its structural and enzymatic properties, intracellular localization, trafficking, substrates, and roles in vivo.

Inhibition by naloxone stereoisomers of beta-amyloid peptide (1-42)-induced superoxide production in microglia and degeneration of cortical and mesencephalic neurons

Previously we reported that naloxone stereoisomers, in an opioid receptor-independent manner, attenuated the inflammation-mediated degeneration of dopaminergic neurons by inhibition of the activation of microglia, the resident immune cells in the brain. Recently we discovered that beta-amyloid peptide Abeta (1-42) exhibited enhanced neurotoxicity toward both cortical and mesencephalic neurons through the activation of microglia and production of superoxide. The purpose of this study was to determine whether naloxone isomers had any effect on Abeta (1-42)-induced neurodegeneration. Pretreatment of either cortical or mesencephalic neuron-glia cultures with 1 to 10 microM (-)-naloxone, prior to treatment for up to 11 days with 0.1 to 3 microM Abeta (1-42), afforded significant neuroprotection as judged by neurotransmitter uptake, immunocytochemical analysis, and cell counting. More importantly, (+)-naloxone, the ineffective enantiomer of (-)-naloxone in binding opioid receptors, was equally effective in affording neuroprotection. Mechanistically, inhibition of Abeta (1-42)-induced production of superoxide in microglia underlay the neuroprotective effect of naloxone stereoisomers. Moreover, neuroprotection and inhibition of Abeta (1-42)-induced superoxide production was also achieved with naloxone methiodide, a charged analog with quaternary amine, suggesting that the site of action for naloxone isomers is at the cell surface of microglia. These results demonstrated that naloxone isomers, through mechanisms unrelated to the opioid receptors, were capable of inhibiting Abeta (1-42)-induced microglial activation and degeneration of both cortical and mesencephalic neurons. Combined with our previous observations with inflammagen-induced neurodegeneration, naloxone analogs, especially (+)-naloxone, may have potential therapeutic efficacy for the treatment of Alzheimer's and Parkinson'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.

beta -Amyloid peptide activates alpha 7 nicotinic acetylcholine receptors expressed in Xenopus oocytes

The alpha7 nicotinic acetylcholine receptor is highly expressed in hippocampus and in cholinergic projection neurons from the basal forebrain, structures that are particularly vulnerable to the ravages of Alzheimer's disease. Previous work suggests that beta-amyloid peptide can interact with alpha7 nicotinic acetylcholine receptors, although the nature of this interaction has not been well characterized. To test whether beta-amyloid peptide can activate alpha7 nicotinic acetylcholine receptors, we expressed these receptors in Xenopus oocytes and performed two-electrode voltage clamp recordings, characterizing the response to beta-amyloid peptide 1-42 applied at concentrations ranging from 1 pm to 100 nm. In alpha7-expressing oocytes, beta-amyloid peptide 1-42 elicits inward currents at low concentrations (1-100 pm), whereas at higher concentrations (nm), less effective receptor activation is observed, indicative of receptor desensitization. Preincubation with the alpha7-selective agents, the antagonist methyllycaconatine, and the agonist 4-OH-GTS-21 blocked beta-amyloid peptide-induced receptor activation. beta-amyloid peptide 1-42 at low concentrations was able to activate the L250T mutant alpha7 receptor. The endogenous Ca(2+)-activated chloride current in Xenopus oocytes is recruited upon receptor activation since replacing Ca(2+) with Ba(2+) in the recording solution reduced current amplitude. Thus, when beta-amyloid peptide activation of alpha7 receptors occurs, these currents are comprised, at least in part, of Ca(2+).

Amyloid beta-peptide induces cholinergic dysfunction and cognitive deficits: a minireview

Amyloid beta-peptide (Abeta) plays a critical role in the development of Alzheimer's disease (AD). Much progress has been made in understanding this age-related neurodegenerative disorder, thus an insight into the cellular actions of Abeta and resulting functional consequences may contribute to preventive and therapeutic approaches for AD. In this review, recent evidence of Abeta-induced brain dysfunction, particularly of cholinergic impairment and memory deficits is summarized. Moreover, proposed mechanisms for Abeta-induced neurotoxicity such as oxidative stress, ion-channel formation, and Abeta-receptor interaction are discussed.

Intracellular accumulation of beta-amyloid(1-42) in neurons is facilitated by the alpha 7 nicotinic acetylcholine receptor in Alzheimer's disease

Amyloid beta(1-42), a major component of amyloid plaques, binds with exceptionally high affinity to the alpha 7 nicotinic acetylcholine receptor and accumulates intracellularly in neurons of Alzheimer's disease brains. In this study, we investigated the possibility that this binding plays a key role in facilitating intraneuronal accumulation of amyloid beta(1-42). Consecutive section immunohistochemistry and digital imaging were used to reveal the spatial relationship between amyloid beta(1-42) and the alpha 7 receptor in affected neurons of Alzheimer's disease brains. Results showed that neurons containing substantial intracellular accumulations of amyloid beta(1-42) invariably express relatively high levels of the alpha 7 receptor. Furthermore, this receptor is highly co-localized with amyloid beta(1-42) within neurons of Alzheimer's disease brains. To experimentally test the possibility that the binding interaction between exogenous amyloid beta(1-42) and the alpha 7 receptor facilitates internalization and intracellular accumulation of amyloid beta(1-42) in Alzheimer's disease brains, we studied the fate of exogenous amyloid beta(1-42) and its interaction with the alpha 7 receptor in vitro using cultured, transfected neuroblastoma cells that express elevated levels of this receptor. Transfected cells exhibited rapid binding, internalization and accumulation of exogenous amyloid beta(1-42), but not amyloid beta(1-40). Furthermore, the rate and extent of amyloid beta(1-42) internalization was related directly to the alpha 7 receptor protein level, since (1) the rate of amyloid beta(1-42) accumulation was much lower in untransfected cells that express much lower levels of this receptor and (2) internalization was effectively blocked by alpha-bungarotoxin, an alpha 7 receptor antagonist. As in neurons of Alzheimer's disease brains, the alpha 7 receptor in transfected cells was precisely co-localized with amyloid beta(1-42) in prominent intracellular aggregates. Internalization of amyloid beta(1-42) in transfected cells was blocked by phenylarsine oxide, an inhibitor of endocytosis. We suggest that the intraneuronal accumulation of amyloid beta(1-42) in Alzheimer's disease brains occurs predominantly in neurons that express the alpha 7 receptor. In addition, internalization of amyloid beta(1-42) may be facilitated by the high-affinity binding of amyloid beta(1-42) to the alpha 7 receptor on neuronal cell surfaces, followed by endocytosis of the resulting complex. This provides a plausible explanation for the selective vulnerability of neurons expressing the alpha 7 receptor in Alzheimer's disease brains and for the fact that amyloid beta(1-42) is the dominant amyloid beta peptide species in intracellular accumulations and amyloid plaques.

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.

The inhibitory and facilitatory actions of amyloid-beta peptides on nicotinic ACh receptors and AMPA receptors

The present study investigated the effects of amyloid-beta peptides on nicotinic ACh receptors (Torpedo, alpha 4 beta 2, and alpha 7 receptors) and AMPA receptors expressed in Xenopus oocytes by monitoring whole-cell membrane currents. Ten-minutes treatment with amyloid-beta(1-42) (1 microM) inhibited Torpedo ACh receptor currents, reaching 53% of original levels 30 min after treatment. Amyloid-beta(1-40) inhibited the currents in a dose-dependent manner (0.1-10 microM) during treatment, gradually reversing after treatment. Amyloid-beta(1-40) and amyloid-beta(1-42) (0.1 microM) depressed alpha 4 beta 2 receptor currents to each 69% and 62% of original levels at 10-min treatment and lesser depression was obtained with alpha 7 receptors. Amyloid-beta(1-42) (0.1 microM) did not significantly inhibit AMPA receptor currents, but amyloid-beta(1-40) (0.1 microM) potentiated the currents to 145-191% of original levels. Amyloid-beta peptides, thus, exert their diverse actions on nicotinic ACh receptors and AMPA receptors, and the inhibitory actions on nicotinic ACh receptors may account for the deterioration of learning and memory in Alzheimer's disease.

Impairment of hippocampal CA1 heterosynaptic transformation and spatial memory by beta-amyloid(25-35)

In Alzheimer's disease, the cholinergic damage (reduced neurotransmission) and cognitive impairment occur long before beta-amyloid (Abeta) plaque formation. It has not been established whether the link between soluble Abeta and cholinergic functions contributes to synaptic dysfunction that underlies the cognitive impairment. Here, we report that Abeta(25-35), an active form of Abeta, inhibited long-term synaptic modification that depends on the associative activation of cholinergic and GABAergic inputs when bilaterally injected intracerebroventricularly (icv; 200 microg/site). The Abeta microinjections did not affect single-pulse-evoked glutamatergic and GABAergic synaptic transmission onto the hippocampal CA1 pyramidal cells, while cholinergic intracellular theta; was dramatically reduced by the Abeta(25-35) injection. Spatial memory of the water maze task was also impaired by the bilateral icv Abeta(25-35) injections, while bilateral microinjections of the same dose of Abeta(35-25) was ineffective in affecting the long-term synaptic modification evoked by associative activation of cholinergic and GABAergic inputs, the cholinergic intracellular theta;, or producing memory impairments. Thus restoring the synaptic plasticity involved in this associative activation of cholinergic and GABAergic inputs may offer an important therapeutic target in the treatment of early Abeta-induced memory decline.

Role of p75 neurotrophin receptor in the neurotoxicity by beta-amyloid peptides and synergistic effect of inflammatory cytokines

The neurodegenerative changes in Alzheimer's disease (AD) are elicited by the accumulation of beta-amyloid peptides (Abeta), which damage neurons either directly by interacting with components of the cell surface to trigger cell death signaling or indirectly by activating astrocytes and microglia to produce inflammatory mediators. It has been recently proposed that the p75 neurotrophin receptor (p75(NTR)) is responsible for neuronal damage by interacting with Abeta. By using neuroblastoma cell clones lacking the expression of all neurotrophin receptors or engineered to express full-length or various truncated forms of p75(NTR), we could show that p75(NTR) is involved in the direct signaling of cell death by Abeta via the function of its death domain. This signaling leads to the activation of caspases-8 and -3, the production of reactive oxygen intermediates and the induction of an oxidative stress. We also found that the direct and indirect (inflammatory) mechanisms of neuronal damage by Abeta could act synergistically. In fact, TNF-alpha and IL-1beta, cytokines produced by Abeta-activated microglia, could potentiate the neurotoxic action of Abeta mediated by p75(NTR) signaling. Together, our results indicate that neurons expressing p75(NTR), mostly if expressing also proinflammatory cytokine receptors, might be preferential targets of the cytotoxic action of Abeta in AD.

Cerebellar diffuse amyloid plaques are derived from dendritic Abeta42 accumulations in Purkinje cells

beta-amyloid(1-42) (Abeta42)-rich amyloid plaques (APs) may be derived from destroyed neurons that were burdened with extensive intracellular Abeta42 accumulations. Since most cells that accumulate Abeta42 express the alpha7 nicotinic acetylcholine receptor (alpha7nAChR), we examined the relationship between the intracellular accumulation of Abeta42 and the expression of the alpha7nAChR in cells from the cerebellum of sporadic Alzheimer's disease (AD) patients. Abeta42, but not Abeta40 or Abeta43, accumulates intracellularly in Purkinje, Golgi II, stellate and basket cells in the AD cerebellum, all of which express the alpha7nAChR. Abeta42 deposits were also prominent within dendrites of Purkinje cells, especially at points of their bifurcation that were often occluded with this material. Diffuse APs appeared to represent the remnants of destroyed Abeta42-laden segments of Purkinje cell dendritic trees. Similarly, the accumulation of Abeta42 and early loss of Golgi II cells in AD cerebella correlated directly to their high level of alpha7nAChR expression. Furthermore, the presence and relative abundance of neuron-derived Abeta42/alpha7nAChR-positive materials within Bergman glia may be indicative of the stage of AD. These data are consistent with a role for the alpha7nAChR in mediating intracellular Abeta42 accumulation and also support the notion that the intracellular and intradendritic accumulation of Abeta42 may eventually result in cell lysis and the formation of APs.

Nicotine and neurodegeneration in ageing

Impairment in cholinergic systems is a highly consistent finding in human dementia. Among cholinergic markers, marked decreases in nicotine binding have been most consistently observed in the telencephalic regions of demented patients and are thought to contribute to the cognitive deficits associated with ageing and age-related neurodegenerative diseases. New evidence that the cholinergic system has a specific pathogenic role in the neurodegenerative alterations of aged and, especially, demented patients is fast accumulating. Both in vivo and in culture, nicotine protects striatal, hippocampal and cortical neurons against the neurotoxicity induced by excitotoxic amino acids as well as the toxicity caused by beta-amyloid, the major component of senile plaques. Further support for the implication of nicotinic receptors in brain ageing is come from recent studies on transgenic animals lacking nicotinic receptor subtypes, which shed light on the mechanisms of nicotine neuroprotection and neurotoxicity.

Regulation of the activity of hippocampal stratum oriens interneurons by alpha7 nicotinic acetylcholine receptors

GABAergic interneurons have been shown to be a major target of cholinergic inputs to the hippocampus. Because these interneurons project to pyramidal neurons as well as other interneurons, activation of the cholinergic system is likely to produce a complex modulation of local inhibitory activity. To better understand the role of post-synaptic alpha7 nicotinic acetylcholine receptors in the hippocampus, we have characterized the effects of nicotinic agents on local interneurons of the rat CA1 stratum oriens in terms of activation, desensitization, and region of axonal termination. Fast application of acetylcholine onto stratum oriens interneurons during whole-cell recordings from hippocampal slices activated the majority of cells tested, and these responses were mediated almost entirely by alpha7 nicotinic acetylcholine receptors. Anatomical reconstructions showed no clear relationship between the acetylcholine responsivity of interneurons and the regions to which their axons project. Currents mediated by alpha7 receptors declined markedly during repetitive activation in the theta rhythm range (4-12 Hz) when activated by either pressure application or synaptic release of acetylcholine. However, the decay of alpha7 receptor-mediated currents was unaffected by treatment with the cholinesterase inhibitor neostigmine (10 nM-10 microM), suggesting that hydrolysis of acetylcholine is not a rate-limiting step in the termination of these responses. From these findings we suggest that nicotinic receptor activity in this region has an extensive and complex impact on local inhibitory circuits that is mediated by activation of several classes of intrinsic GABAergic cells. In addition, desensitization of the alpha7 nicotinic acetylcholine receptor is likely to contribute to the decay of individual responses to pressure application of agonist, and may also act in a cumulative fashion to impair the ability of these receptors to support repetitive activity during trains of activation. If applicable to alpha7 receptor responses in vivo, we suggest it may be difficult to enhance these responses for therapeutic purposes with cholinesterase inhibitors.

alpha7 nicotinic acetylcholine receptors on GABAergic interneurons evoke dendritic and somatic inhibition of hippocampal neurons

GABAergic interneurons in the hippocampus express high levels of alpha7 nicotinic acetylcholine receptors, but because of the diverse roles played by hippocampal interneurons, the impact of activation of these receptors on hippocampal output neurons (i.e., CA1 pyramidal cells) is unclear. Activation of hippocampal interneurons could directly inhibit pyramidal neuron activity but could also produce inhibition of other GABAergic cells leading to disinhibition of pyramidal cells. To characterize the inhibitory circuits activated by these receptors, exogenous acetylcholine was applied directly to CA1 interneurons in hippocampal slices, and the resulting postsynaptic responses were recorded from pyramidal neurons or interneurons. Inhibitory currents mediated by GABA(A) receptors were observed in 27/131 interneuron/pyramidal cell pairs, but no instances of disinhibition of spontaneous inhibitory events or GABA(B) receptor-mediated responses were observed. Two populations of bicuculline-sensitive GABA(A) receptor-mediated currents could be distinguished based on their kinetics and amplitude. Anatomical reconstructions of the interneurons in a subset of connected pairs support the hypothesis that these two populations correspond to inhibitory synapses located either on the somata or dendrites of pyramidal cells. In 11 interneuron/interneuron cell pairs, one presynaptic neuron was observed that produced strong inhibitory currents in several nearby interneurons, suggesting that disinhibition of pyramidal neurons may also occur. All three types of inhibitory responses (somatic-pyramidal, dendritic-pyramidal, and interneuronal) were blocked by the alpha7 receptor-selective antagonist methyllycaconitine. These data suggest activation of these functionally distinct circuits by alpha7 receptors results in significant inhibition of both hippocampal pyramidal neurons as well as interneurons.

Amyloid beta protein forms ion channels: implications for Alzheimer's disease pathophysiology

Amyloid beta protein (AbetaP) is the major constituent of senile plaques associated with Alzheimer's disease (AD). However, its mechanistic role in AD pathogenesis is poorly understood. Globular and nonfibrillar AbetaPs are continuously released during normal metabolism. Using techniques of atomic force microscopy, laser confocal microscopy, electrical recording, and biochemical assays, we have examined the molecular conformations of reconstituted globular AbetaPs as well as their real-time and acute effects on neuritic degeneration. Atomic force microscopy (AFM) of AbetaP1-42 shows globular structures that do not form fibers in physiological-buffered solution for up to 8 h of continuous imaging. AFM of AbetaP1-42 reconstituted in a planar lipid bilayer reveals multimeric channel-like structures. Consistent with these AFM resolved channel-like structures, biochemical analysis demonstrates that predominantly monomeric AbetaPs in solution form stable tetramers and hexamers after incorporation into lipid membranes. Electrophysiological recordings demonstrate the presence of multiple single channel currents of different sizes. At the cellular level, AbetaP1-42 allows calcium uptake and induces neuritic abnormality in a dose- and time-dependent fashion. At physiological nanomolar concentrations, rapid neuritic degeneration was observed within minutes; at micromolar concentrations, neuronal death was observed within 3-4 h. These effects are prevented by zinc (an AbetaP channel blocker) and by the removal of extracellular calcium, but are not prevented by antagonists of putative AbetaP cell surface receptors. Thus, AbetaP channels may provide a direct pathway for calcium-dependent AbetaP toxicity in AD.

Ultrastructural distribution of the alpha7 nicotinic acetylcholine receptor subunit in rat hippocampus

Acetylcholine (ACh) is an important neurotransmitter in the mammalian brain; it is implicated in arousal, learning, and other cognitive functions. Recent studies indicate that nicotinic receptors contribute to these cholinergic effects, in addition to the established role of muscarinic receptors. In the hippocampus, where cholinergic involvement in learning and memory is particularly well documented, alpha7 nicotinic acetylcholine receptor subunits (alpha7 nAChRs) are highly expressed, but their precise ultrastructural localization has not been determined. Here, we describe the results of immunogold labeling of serial ultrathin sections through stratum radiatum of area CA1 in the rat. Using both anti-alpha7 nAChR immunolabeling and alpha-bungarotoxin binding, we find that alpha7 nAChRs are present at nearly all synapses in CA1 stratum radiatum, with immunolabeling present at both presynaptic and postsynaptic elements. Morphological considerations and double immunolabeling indicate that GABAergic as well as glutamatergic synapses bear alpha7 nAChRs, at densities approaching those observed for glutamate receptors in CA1 stratum radiatum. Postsynaptically, alpha7 nAChRs often are distributed at dendritic spines in a perisynaptic annulus. In the postsynaptic cytoplasm, immunolabeling is associated with spine apparatus and other membranous structures, suggesting that alpha7 nAChRs may undergo dynamic regulation, with insertion into the synapse and subsequent internalization. The widespread and substantial expression of alpha7 nAChRs at synapses in the hippocampus is consistent with an important role in mediating and/or modulating synaptic transmission, plasticity, and neurodegeneration.

Brain aging research at the close of the 20th century: from bench to bedside

Remarkable and continued growth in the field of brain aging research has been fueled by a confluence of factors. Developments in molecular biology, imaging, and genetics coupled with the imperative caused by the aging of the population has created fertile ground for improved understanding of the interaction between brain function and behavior. Aging changes in neurochemical systems may account for the spectrum of cognitive and behavioral states of successfully aged pen sons, but may also contribute to enhanced vulnerability to depressive or dementing illness. In particular, the refinement of in vivo imaging approaches to investigating the structure and function of the aging brain has provided the opportunity to strengthen our knowledge of the biological substrate of the aging brain and neuropsychiatrie disorders, and translate these into therapeutics.

Beta-amyloid activates the mitogen-activated protein kinase cascade via hippocampal alpha7 nicotinic acetylcholine receptors: In vitro and in vivo mechanisms related to Alzheimer's disease

Alzheimer's Disease (AD) is the most common of the senile dementias, the prevalence of which is increasing rapidly, with a projected 14 million affected worldwide by 2025. The signal transduction mechanisms that underlie the learning and memory derangements in AD are poorly understood. beta-Amyloid (Abeta) peptides are elevated in brain tissue of AD patients and are the principal component of amyloid plaques, a major criterion for postmortem diagnosis of the disease. Using acute and organotypic hippocampal slice preparations, we demonstrate that Abeta peptide 1-42 (Abeta42) couples to the mitogen-activated protein kinase (MAPK) cascade via alpha7 nicotinic acetylcholine receptors (nAChRs). In vivo elevation of Abeta, such as that exhibited in an animal model for AD, leads to the upregulation of alpha7 nAChR protein. alpha7 nAChR upregulation occurs concomitantly with the downregulation of the 42 kDa isoform of extracellular signal-regulated kinase (ERK2) MAPK in hippocampi of aged animals. The phosphorylation state of a transcriptional mediator of long-term potentiation and a downstream target of the ERK MAPK cascade, the cAMP-regulatory element binding (CREB) protein, were affected also. These findings support the model that derangement of hippocampus signal transduction cascades in AD arises as a consequence of increased Abeta burden and chronic activation of the ERK MAPK cascade in an alpha7 nAChR-dependent manner that eventually leads to the downregulation of ERK2 MAPK and decreased phosphorylation of CREB protein.

Allosteric mechanisms in normal and pathological nicotinic acetylcholine receptors

Recent chemical and advanced structural studies on site-directed and naturally occurring pathological mutants of individual members of the multigene family of nicotinic acetylcholine receptors have yielded structure-function relationships supporting indirect 'allosteric' interactions between the acetylcholine-binding sites and the ion channel in signal transduction.