A beta 25-35-induced depression of long-term potentiation in area CA1 in vivo and in vitro is attenuated by verapamil

The interactive effects of verapamil and CB1 cannabinoid receptor antagonist/inverse agonist, AM251 on passive avoidance learning and memory in rat

There are reports regarding the effects of intracellular Ca2+ and synthesis and release of endocannabinoids. The secretion of endocannabinoids depends on the L-type calcium channel. The present study evaluated the involvement of the cannabinoid CB1 receptors in the effect of L-type calcium channel blocker verapamil on passive avoidance learning (PAL) in adult male rats. In this study, we examined the effects of an acute administration of the cannabinoid CB1 receptors antagonist/inverse agonist AM251 following a chronic administration of the Ca2+ channel blocker verapamil on PAL. Male Wistar rats were administered verapamil (10, 25 and 50 mg/kg) or saline intraperitoneally (i.p) daily for 13 days (n = 10/group). After this treatment period, a learning test (acquisition) was performed, and a retrieval test was performed the following day. The results indicated that chronic systemic administration of verapamil (in a dose-dependent manner) impaired memory acquisition and retrieval. Pre-training acute administration of a selective CB1 antagonist/inverse agonist, AM251 (5 mg/kg, i.p.) did not change memory acquisition and retrieval. Co-administration of the verapamil and AM251 significantly reversed verapamil-induced amnesia, suggesting a functional interaction between AM251 and verapamil. The results indicated the interactive effects of cannabinoid CB1 receptors and L-type calcium channel in passive avoidance learning and AM251 can counter the effects of verapamil on memory.

Differential Effect of Amyloid Beta1-40 on Short-term and Long-term Plasticity in Dentate Gyrus of a Rat Model of Alzheimer Disease

Introduction:

Synaptic plasticity is inappropriately affected by neurodegenerative diseases, including Alzheimer Disease (AD). In this study, we examined the effect of intrahippocampal amyloid-beta (Aβ1-40) on dentate gyrus Long-term Potentiation (LTP) and presynaptic short-term plasticity in a rat model of AD.

Methods:

The experimental groups in this research included the control with no treatment, sham-operated receiving the vehicle (normal saline), and Aβ-lesioned groups. For modeling AD, aggregated Aβ1-40 (10 μg/2 μl on each side) was injected into the hippocampal CA1. Three weeks later, Population Spike (PS) amplitude and slope ratios were determined at different Inter-pulse Intervals (IPI) of 10, 20, 30, and 50 ms as a valid indicator of the short-term presynaptic facilitation and/or depression. In addition, PS amplitude and slope were taken as an index of long-term synaptic plasticity after application of High-frequency Stimulation (HFS) to induce LTP in the medial perforant-dentate gyrus pathway.

Results:

No significant differences were noted amongst the experimental groups regarding fEPSP slope and paired-pulse indices as indicators of short-term plasticity. In contrast, fEPSP slope and PS amplitude significantly decreased following the application of HFS in Aβ-injected group. In addition, there was no significant difference between the control and sham-operated groups regarding the mentioned parameters.

Conclusion:

Findings of this study clearly demonstrated that microinjection of Aβ1-40 into the CA1 could impair LTP in dentate gyrus but could not modify short-term plasticity.

Acute Effects of Two Different Species of Amyloid-β on Oscillatory Activity and Synaptic Plasticity in the Commissural CA3-CA1 Circuit of the Hippocampus

Recent evidence indicates that soluble amyloid-β (Aβ) species induce imbalances in excitatory and inhibitory transmission, resulting in neural network functional impairment and cognitive deficits during early stages of Alzheimer's disease (AD). To evaluate the in vivo effects of two soluble Aβ species (Aβ_25-35 and Aβ_1-40) on commissural CA3-to-CA1 (cCA3-to-CA1) synaptic transmission and plasticity, and CA1 oscillatory activity, we used acute intrahippocampal microinjections in adult anaesthetized male Wistar rats. Soluble Aβ microinjection increased cCA3-to-CA1 synaptic variability without significant changes in synaptic efficiency. High-frequency CA3 stimulation was rendered inefficient by soluble Aβ intrahippocampal injection to induce long-term potentiation and to enhance synaptic variability in CA1, contrasting with what was observed in vehicle-injected subjects. Although soluble Aβ microinjection significantly increased the relative power of γ-band and ripple oscillations and significantly shifted the average vector of θ-to-γ phase-amplitude coupling (PAC) in CA1, it prevented θ-to-γ PAC shift induced by high-frequency CA3 stimulation, opposite to what was observed in vehicle-injected animals. These results provide further evidence that soluble Aβ species induce synaptic dysfunction causing abnormal synaptic variability, impaired long-term plasticity, and deviant oscillatory activity, leading to network activity derailment in the hippocampus.

Impairments of Synaptic Plasticity Induction Threshold and Network Oscillatory Activity in the Hippocampus Underlie Memory Deficits in a Non-Transgenic Mouse Model of Amyloidosis

In early Alzheimer disease (AD) models synaptic failures and upstreaming aberrant patterns of network synchronous activity result in hippocampal-dependent memory deficits. In such initial stage, soluble forms of Amyloid-β (Aβ) peptides have been shown to play a causal role. Among different Aβ species, Aβ_25-35 has been identified as the biologically active fragment, as induces major neuropathological signs related to early AD stages. Consequently, it has been extensively used to acutely explore the pathophysiological events related with neuronal dysfunction induced by soluble Aβ forms. However, the synaptic mechanisms underlying its toxic effects on hippocampal-dependent memory remain unresolved. Here, in an in vivo model of amyloidosis generated by intracerebroventricular injections of Aβ_25-35 we studied the synaptic dysfunction mechanisms underlying hippocampal cognitive deficits. At the synaptic level, long-term potentiation (LTP) of synaptic excitation and inhibition was induced in CA1 region by high frequency simulation (HFS) applied to Schaffer collaterals. Aβ_25-35 was found to alter metaplastic mechanisms of plasticity, facilitating long-term depression (LTD) of both types of LTP. In addition, aberrant synchronization of hippocampal network activity was found while at the behavioral level, deficits in hippocampal-dependent habituation and recognition memories emerged. Together, our results provide a substrate for synaptic disruption mechanism underlying hippocampal cognitive deficits present in Aβ_25-35 amyloidosis model.

Verapamil and Alzheimer's Disease: Past, Present, and Future

Verapamil is a phenylalkylamine class calcium channel blocker that for half a century has been used for the treatment of cardiovascular diseases. Nowadays, verapamil is also considered as a drug option for the treatment of several neurological and psychiatric disorders, such as cluster headache, bipolar disorders, epilepsy, and neurodegenerative diseases. Here, we review insights into the potential preventive and therapeutic role of verapamil on Alzheimer’s disease (AD) based on limited experimental and clinical data. Pharmacological studies have shown that verapamil has a wide therapeutic spectrum, including antihypertensive, anti-inflammatory, and antioxidative effects, regulation of the blood-brain barrier function, due to its effect on P-glycoprotein, as well as adjustment of cellular calcium homeostasis, which may result in the delay of AD onset or ameliorate the symptoms of patients. However, the majority of the AD individuals are on polypharmacotherapy, and the interactions between verapamil and other drugs need to be considered. Therefore, for an appropriate and successful AD treatment, a personalized approach is more than necessary. A well-known narrow pharmacological window of verapamil efficacy may hinder this approach. It is therefore important to note that the verapamil efficacy may be conditioned by different factors. The onset, grade, and brain distribution of AD pathological hallmarks, the time-sequential appearances of AD-related cognitive and behavioral dysfunction, the chronobiologic and gender impact on calcium homeostasis and AD pathogenesis may somehow be influencing that success. In the future, such insights will be crucial for testing the validity of verapamil treatment on animal models of AD and clinical approaches.

Verapamil attenuates scopolamine induced cognitive deficits by averting oxidative stress and mitochondrial injury - A potential therapeutic agent for Alzheimer's Disease

Alzheimer's disease (AD) is a multifactorial disorder where amyloid beta (Aβ) plaques, Ca2+ dysregulation, excessive oxidative stress, mitochondrial dysfunction and synaptic loss operate synergistically to bring about cholinergic deficits and dementia. New therapeutic interventions are gaining prominence as the morbidity and mortality of AD increases exponentially every year. Treating AD with antihypertensive drugs is thought to be a promising intervention; however, its mechanism of action of ameliorating AD needs further investigation. In this context, the present study explores the protective effect of verapamil, an antihypertensive agent of Ca2+ channel blocker (CCB) class against scopolamine-induced in vitro neurotoxicity and in vivo cognitive impairment. Supplementation of verapamil was found to attenuate oxidative stress by preventing mitochondrial injury, and augment the expression of genes involved in the cholinergic function (mACR1), synaptic plasticity (GAP43, SYP) and Ca2+-dependent memory-related genes (CREB1, CREBBP, BDNF). Further, verapamil treatment in mice attenuated the cognitive and behavioural deficits induced by scopolamine as measured by the elevated plus maze and passive avoidance test (P < 0.05). Thus, the present study demonstrates the neuroprotective effect of verapamil against the pathogenesis of AD such as oxidative stress, mitochondrial dysfunction and cognitive decline. These observations emphasize the importance of ‛Ca2+ dysregulation' and ‛mitochondrial dysfunction' theories in AD and recommends the supplementation of compounds that regulate Ca2+ homeostasis and mitochondrial function in susceptible AD individuals.

Antihypertensive agents in Alzheimer's disease: beyond vascular protection

Introduction: Midlife hypertension has been consistently linked with increased risk of cognitive decline and Alzheimer's disease (AD). Observational studies and randomized trials show that the use of antihypertensive therapy is associated with a lesser incidence or prevalence of cognitive impairment and dementia. However, whether antihypertensive agents specifically target the pathological process of AD remains elusive.Areas covered: This review of literature provides an update on the clinical and preclinical arguments supporting anti-AD properties of antihypertensive drugs. The authors focused on validated all classes of antihypertensive treatments such as angiotensin-converting enzyme inhibitors (ACEi), angiotensin receptor blockers (ARB), calcium channel blockers (CCB), β-blockers, diuretics, neprilysin inhibitors, and other agents. Three main mechanisms can be advocated: action on the concurrent vascular pathology, action on the vascular component of Alzheimer's pathophysiology, and action on nonvascular targets.Expert opinion: In 2019, while there is no doubt that hypertension should be treated in primary prevention of vascular disease and in secondary prevention of stroke and mixed dementia, the place of antihypertensive agents in the secondary prevention of 'pure' AD remains an outstanding question.

Synaptic plasticity modulation by circulating peptides and metaplasticity: Involvement in Alzheimer's disease

Synaptic plasticity is a cellular process involved in learning and memory whose alteration in its two main forms (Long Term Depression (LTD) and Long Term Potentiation (LTP)), is observed in most brain pathologies, including neurodegenerative disorders such as Alzheimer's disease (AD). In humans, AD is associated at the cellular level with neuropathological lesions composed of extracellular deposits of β-amyloid (Aβ) protein aggregates and intracellular neurofibrillary tangles, cellular loss, neuroinflammation and a general brain homeostasis dysregulation. Thus, a dramatic synaptic environment perturbation is observed in AD patients, involving changes in brain neuropeptides, cytokines, growth factors or chemokines concentration and diffusion. Studies performed in animal models demonstrate that these circulating peptides strongly affect synaptic functions and in particular synaptic plasticity. Besides this neuromodulatory action of circulating peptides, other synaptic plasticity regulation mechanisms such as metaplasticity are altered in AD animal models. Here, we will review new insights into the study of synaptic plasticity regulatory/modulatory mechanisms which could influence the process of synaptic plasticity in the context of AD with a particular attention to the role of metaplasticity and peptide dependent neuromodulation.

A novel antibody targeting sequence 31-35 in amyloid β protein attenuates Alzheimer's disease-related neuronal damage

Amyloid β protein (Aβ) plays a critical role in pathogenesis of Alzheimer's disease (AD). Our previous studies indicated that the sequence 31-35 in Aβ molecule is an effective active center responsible for Aβ neurotoxicity in vivo and in vitro. In the present study, we prepared a novel antibody specifically targeting the sequence 31-35 of amyloid β protein, and investigated the neuroprotection of the anti-Aβ_31-35 antibody against Aβ_1-42 -induced impairments in neuronal viability, spatial memory, and hippocampal synaptic plasticity in rats. The results showed that the anti-Aβ_31-35 antibody almost equally bound to both Aβ_31-35 and Aβ_1-42 , and pretreatment with the antibody dose-dependently prevented Aβ_1-42 -induced cytotoxicity on cultured primary cortical neurons. In behavioral study, intracerebroventricular (i.c.v.) injection of anti-Aβ_31-35 antibody efficiently attenuated Aβ_1-42 -induced impairments in spatial learning and memory of rats. In vivo electrophysiological experiments further indicated that Aβ_1-42 -induced suppression of hippocampal synaptic plasticity was effectively reversed by the antibody. These results demonstrated that the sequence 31-35 of Aβ may be a new therapeutic target, and the anti-Aβ_31-35 antibody could be a novel immunotheraputic approach for the treatment of AD. © 2016 Wiley Periodicals, Inc.

A novel acetylcholinesterase inhibitor and calcium channel blocker SCR-1693 improves Aβ25-35-impaired mouse cognitive function

RATIONALE

The mechanism involved in AD is complex, which has prompted to develop compounds that could simultaneously interact with several potential targets. Here, we report a new synthesized compound SCR-1693 which is designed to target both AChE and calcium channels that are potential for AD therapy.

OBJECTIVES

We investigated the effects of SCR-1693 on AChE and calcium channels, the effects of neuroprotection and anti-amnesia in icv-Aβ25-35-injected mice, and the potential mechanisms.

METHODS

AChE activity assay, intracellular Ca(2+) content and calcium currents measurement, and Aβ25-35-induced cellular death determine were performed for validation of designed targets and neuroprotection of SCR-1693. Mice were orally administrated with SCR-1693 once daily after an Aβ25-35 injection. The Morris water maze and Y-maze test, and hippocampal protein detection were conducted on days 5-10, day 11, and day 8. The pyramidal neuron number, hippocampal AChE activity, and synaptic transmission were measured on day 12.

RESULTS

SCR-1693 acted as a selective, reversible, and noncompetitive inhibitor of AChE, and a nonselective voltage-gated calcium channel blocker. SCR-1693 also inhibited the increase of AChE activity in the mouse hippocampus. SCR-1693 was more effective than donepezil and memantine in preventing Aβ25-35-induced long-term and short-term memory impairment, maintaining the basal transmission of Schaffer collateral-CA1 synapses, and sustaining LTP in mouse hippocampus. SCR-1693 attenuated Aβ25-35-induced death of SH-SY5Y cell and the loss of hippocampal pyramidal neurons, and regulated Aβ25-35-induced signal cascade in neurons.

CONCLUSIONS

All these findings indicated that SCR-1693, as a double-target-direction agent, is a considerable candidate for AD therapy.

Wnt-5a prevents Aβ-induced deficits in long-term potentiation and spatial memory in rats

Although the neurotoxicity of amyloid β (Aβ) protein in Alzheimer's disease (AD) has been reported widely, the exact molecular mechanism underlying the Aβ-induced synaptic dysfunction and memory impairment remains largely unclear. Growing evidence indicates that wingless-type (Wnt) signaling plays an important role in neuronal development, synapse formation and synaptic plasticity. In the present study, we investigated the neuroprotective action of Wnt-5a against the synaptic damage and memory deficit induced by Aβ25-35 by using in vivo electrophysiological recording and Morris water maze (MWM) test. We found that intracerebroventricular (i.c.v.) injection of Aβ25-35 alone did not affect the baseline field excitatory postsynaptic potentials (fEPSPs) and the paired-pulse facilitation (PPF) in the hippocampal CA1 region of rats, but significantly suppressed high frequency stimulation (HFS) induced long-term potentiation (LTP); pretreatment with Wnt-5a prevented the Aβ25-35-induced suppression of hippocampal LTP in a dose-dependent manner; soluble Frizzled-related protein (sFRP), a specific Wnt antagonist, effectively attenuated the protective effects of Wnt-5a. In MWM test, Aβ25-35 alone significantly disrupted spatial learning and memory ability of rats, while pretreatment with Wnt-5a effectively prevented the impairments induced by Aβ25-35. These results in the present study demonstrated for the first time the neuroprotective effects of Wnt-5a against Aβ-induced in vivo synaptic plasticity impairment and memory disorder, suggesting that Wnt signaling pathway is one of the important targets of Aβ neurotoxicity and Wnt-5a might be used as one of the putative candidates for the therapeutic intervention of AD.

The protective effect of Borago Officinalis extract on amyloid β (25-35)-induced long term potentiation disruption in the dentate gyrus of male rats

Alzheimer's disease (AD) begins with impairment in synaptic functions before developing into later neurodegeneration and neural loss. In the present study we have examined the protective effects of Borago Officinalis (borage) extract on amyloid β (Aβ)--Induced long term potentiation (LTP) disruption in hippocampal dentate gyrus (DG). Wistar male rats received intrahippocampal (IHP) injection of the Aβ (25-35) and borage extract throughout gestation (100 mg/kg). LTP in perforant path- DG synapses was assessed using electrophysiology method and field excitatory post- synaptic potential (fEPSP) slope and population spike (PS) amplitude were measured by 400 Hz tetanization. Finally, the total thiol content of hippocampus was measured using colorimetric reaction based on the Ellman's method. The results showed that Aβ (25-35) significantly decreased fEPSP slope and SP amplitude comparing with the control and sham group, whereas borage extract administration increased these parameters compared to the Aβ group. Aβ induced a remarkable decrease in total thiol content of hippocampus and borage prevented the decrease of the hippocampal total sulfhydryl (SH) groups. This data suggest that Aβ (25-35) can effectively inhibit LTP in the granular cells of the DG in hippocampus, and borage supplementation reverse the synaptic plasticity in DG following Aβ treatment and that borage consumption may lead to an improvement of AD-induced cognitive dysfunction.

Calcium channel blockers and dementia

Degenerative dementia is mainly caused by Alzheimer's disease and/or cerebrovascular abnormalities. Disturbance of the intracellular calcium homeostasis is central to the pathophysiology of neurodegeneration. In Alzheimer's disease, enhanced calcium load may be brought about by extracellular accumulation of amyloid-β. Recent studies suggest that soluble forms facilitate influx through calcium-conducting ion channels in the plasma membrane, leading to excitotoxic neurodegeneration. Calcium channel blockade attenuates amyloid-β-induced neuronal decline in vitro and is neuroprotective in animal models. Vascular dementia, on the other hand, is caused by cerebral hypoperfusion and may benefit from calcium channel blockade due to relaxation of the cerebral vasculature. Several calcium channel blockers have been tested in clinical trials of dementia and the outcome is heterogeneous. Nimodipine as well as nilvadipine prevent cognitive decline in some trials, whereas other calcium channel blockers failed. In trials with a positive outcome, BP reduction did not seem to play a role in preventing dementia, indicating a direct protecting effect on neurons. An optimization of calcium channel blockers for the treatment of dementia may involve an increase of selectivity for presynaptic calcium channels and an improvement of the affinity to the inactivated state. Novel low molecular weight compounds suitable for proof-of-concept studies are now available.

The neuroprotection of Rattin against amyloid β peptide in spatial memory and synaptic plasticity of rats

Rattin, a specific derivative of humanin in rats, shares the ability with HN to protect neurons against amyloid β (Aβ) peptide-induced cellular toxicity. However, it is still unclear whether Rattin can protect against Aβ-induced deficits in cognition and synaptic plasticity in rats. In the present study, we observed the effects of Rattin and Aβ31-35 on the spatial reference memory and in vivo hippocampal Long-term potentiation of rats by using Morris water maze test and hippocampal field potential recording. Furthermore, the probable molecular mechanism underlying the neuroprotective roles of Rattin was investigated. We showed that intra-hippocampal injection of Rattin effectively prevented the Aβ31-35-induced spatial memory deficits and hippocampal LTP suppression in rats; the Aβ31-35-induced activation of Caspase-3 and inhibition of STAT3 in the hippocampus were also prevented by Rattin treatment. These findings indicate that Rattin treatment can protect spatial memory and synaptic plasticity of rats against Aβ31-35-induced impairments, and the underlying protective mechanism of Rattin may be involved in STAT3 and Caspases-3 pathways. Therefore, application of Rattin or activation of its signaling pathways in the brain might be beneficial to the prevention of Aβ-related cognitive deficits.

Amyloid Beta peptides differentially affect hippocampal theta rhythms in vitro

Soluble amyloid beta peptide (A β ) is responsible for the early cognitive dysfunction observed in Alzheimer's disease. Both cholinergically and glutamatergically induced hippocampal theta rhythms are related to learning and memory, spatial navigation, and spatial memory. However, these two types of theta rhythms are not identical; they are associated with different behaviors and can be differentially modulated by diverse experimental conditions. Therefore, in this study, we aimed to investigate whether or not application of soluble A β alters the two types of theta frequency oscillatory network activity generated in rat hippocampal slices by application of the cholinergic and glutamatergic agonists carbachol or DHPG, respectively. Due to previous evidence that oscillatory activity can be differentially affected by different A β peptides, we also compared Aβ 25-35 and Aβ 1-42 for their effects on theta rhythms in vitro at similar concentrations (0.5 to 1.0  μ M). We found that Aβ 25-35 reduces, with less potency than Aβ 1-42, carbachol-induced population theta oscillatory activity. In contrast, DHPG-induced oscillatory activity was not affected by a high concentration of Aβ 25-35 but was reduced by Aβ 1-42. Our results support the idea that different amyloid peptides might alter specific cellular mechanisms related to the generation of specific neuronal network activities, instead of exerting a generalized inhibitory effect on neuronal network function.

Melatonin protects against amyloid-β-induced impairments of hippocampal LTP and spatial learning in rats

Alzheimer's disease (AD), the most prevalent neurodegenerative disease in the elderly, leads to progressive loss of memory and cognitive deficits. Amyloid-β protein (Aβ) in the brain is thought to be the main cause of memory loss in AD. Melatonin, an indole hormone secreted by the pineal gland, has been reported to produce neuroprotective effects. We examined whether melatonin could protect Aβ-induced impairments of hippocampal synaptic plasticity, neuronal cooperative activity, and learning and memory. Rats received bilateral intrahippocampal injection of Aβ1-42 or Aβ31-35 followed by intraperitoneal application of melatonin for 10 days, and the effects of chronic melatonin treatment on in vivo hippocampal long-term potentiation (LTP) and theta rhythm and Morris water maze performance were examined. We showed that intrahippocampal injection of Aβ1-42 or Aβ31-35 impaired hippocampal LTP in vivo, while chronic melatonin treatment reversed Aβ1-42- or Aβ31-35-induced impairments in LTP induction. Intrahippocampal injection of Aβ31-35 impaired spatial learning and decreased the power of theta rhythm in the CA1 region induced by tail pinch, and these synaptic, circuit, and learning deficits were rescued by chronic melatonin treatment. These results provide evidence for the neuroprotective action of melatonin against Aβ insults and suggest a strategy for alleviating cognition deficits of AD.

P/Q-type calcium channel modulators

P/Q-type calcium channels are high-voltage-gated calcium channels contributing to vesicle release at synaptic terminals. A number of neurological diseases have been attributed to malfunctioning of P/Q channels, including ataxia, migraine and Alzheimer's disease. To date, only two specific P/Q-type blockers are known: both are peptides deriving from the spider venom of Agelenopsis aperta, ω-agatoxins. Other peptidic calcium channel blockers with activity at P/Q channels are available, albeit with less selectivity. A number of low molecular weight compounds modulate P/Q-type currents with different characteristics, and some exhibit a peculiar bidirectional pattern of modulation. Interestingly, there are a number of therapeutics in clinical use, which also show P/Q channel activity. Because selectivity as well as the exact mode of action is different between all P/Q-type channel modulators, the interpretation of clinical and experimental data is complicated and needs a comprehensive understanding of their target profile. The situation is further complicated by the fact that information on potency varies vastly in the literature, which may be the result of different experimental systems, conditions or the splice variants of the P/Q channel. This review attempts to provide a comprehensive overview of the compounds available that affect the P/Q-type channel and should help with the interpretation of results of in vitro experiments and animal models. It also aims to explain some clinical observations by implementing current knowledge about P/Q channel modulation of therapeutically used non-selective drugs. Chances and challenges of the development of P/Q channel-selective molecules are discussed.

Behavioural and cellular effects of exogenous amyloid-β peptides in rodents

A better understanding of Alzheimer's disease (AD) and the development of disease modifying therapies are some of the biggest challenges of the 21st century. One of the core features of AD are amyloid plaques composed of amyloid-beta (Aβ) peptides. The first hypothesis proposed that cognitive deficits are linked to plaque-development and transgenic mice have been generated to study this link, thereby providing a good model to develop new therapeutic approaches. Since later it was recognised that in AD patients the cognitive deficit is rather correlated to soluble amyloid levels, consequently, a new hypothesis appeared associating the earliest amyloid toxicity to these soluble species. The purpose of this review is to give a summary of behavioural and cellular data obtained after soluble Aβ peptide administration into rodents' brain, thereby showing that this model is a valid tool to investigate AD pathology when no plaques are present. Additionally, this method offers an excellent, efficient model to test compounds which could act at such early stages of the disease.

Effects of transmitters and amyloid-beta peptide on calcium signals in rat cortical astrocytes: Fura-2AM measurements and stochastic model simulations

Background

To better understand the complex molecular level interactions seen in the pathogenesis of Alzheimer's disease, the results of the wet-lab and clinical studies can be complemented by mathematical models. Astrocytes are known to become reactive in Alzheimer's disease and their ionic equilibrium can be disturbed by interaction of the released and accumulated transmitters, such as serotonin, and peptides, including amyloid- peptides (A). We have here studied the effects of small amounts of A25–35 fragments on the transmitter-induced calcium signals in astrocytes by Fura-2AM fluorescence measurements and running simulations of the detected calcium signals.

Methodology/Principal Findings

Intracellular calcium signals were measured in cultured rat cortical astrocytes following additions of serotonin and glutamate, or either of these transmitters together with A25–35. A25–35 increased the number of astrocytes responding to glutamate and exceedingly increased the magnitude of the serotonin-induced calcium signals. In addition to A25–35-induced effects, the contribution of intracellular calcium stores to calcium signaling was tested. When using higher stimulus frequency, the subsequent calcium peaks after the initial peak were of lower amplitude. This may indicate inadequate filling of the intracellular calcium stores between the stimuli. In order to reproduce the experimental findings, a stochastic computational model was introduced. The model takes into account the major mechanisms known to be involved in calcium signaling in astrocytes. Model simulations confirm the principal experimental findings and show the variability typical for experimental measurements.

Conclusions/Significance

Nanomolar A25–35 alone does not cause persistent change in the basal level of calcium in astrocytes. However, even small amounts of A25–35, together with transmitters, can have substantial synergistic effects on intracellular calcium signals. Computational modeling further helps in understanding the mechanisms associated with intracellular calcium oscillations. Modeling the mechanisms is important, as astrocytes have an essential role in regulating the neuronal microenvironment of the central nervous system.

Beta-like hippocampal network activity is differentially affected by amyloid beta peptides

Alzheimer disease (AD) patients show alterations in both neuronal network oscillations and the cognitive processes associated to them. Related to this clinical observation, it has been found that amyloid beta protein (Abeta) differentially affects some hippocampal network activities, reducing theta and gamma oscillations, without affecting sharp waves and ripples. Beta-like oscillations is another cognitive-related network activity that can be evoked in hippocampal slices by several experimental manipulations, including bath application of kainate and increasing extracellular potassium. Here, we tested whether or not different Abeta peptides differentially affect beta-like oscillatory patterns. We specifically tested the effects of fresh dissolved Abeta(25-35) and oligomerized Abeta(1-42) and found that kainate-induced oscillatory network activity was affected, in a slightly concentration dependent-manner, by both fresh dissolved (mostly monomeric) Abeta(25-35) and oligomeric Abeta(1-42). In contrast, potassium-induced oscillatory activity, which is reduced by oligomeric Abeta(1-42), is not affected by monomeric Abeta(25-35) at any of the concentrations tested. Our results support the idea that different amyloid peptides might alter specific cellular mechanisms related to the generation of specific neuronal network activities, instead of a generalized inhibitory effect of Abeta peptides on neuronal network function.

Beta-amyloid protein (25-35) disrupts hippocampal network activity: role of Fyn-kinase

Early cognitive deficit characteristic of early Alzheimer's disease seems to be produced by the soluble forms of beta-amyloid protein. Such cognitive deficit correlates with neuronal network dysfunction that is reflected as alterations in the electroencephalogram of both Alzheimer patients and transgenic murine models of such disease. Correspondingly, recent studies have demonstrated that chronic exposure to betaAP affects hippocampal oscillatory properties. However, it is still unclear if such neuronal network dysfunction results from a direct action of betaAP on the hippocampal circuit or it is secondary to the chronic presence of the protein in the brain. Therefore, we aimed to explore the effect of acute exposure to betaAP(25-35) on hippocampal network activity both in vitro and in vivo, as well as on intrinsic and synaptic properties of hippocampal neurons. We found that betaAP(25-35), reversibly, affects spontaneous hippocampal population activity in vitro. Such effect is not produced by the inverse sequence betaAP(35-25) and is reproduced by the full-length peptide betaAP(1-42). Correspondingly betaAP(25-35), but not the inverse sequence betaAP(35-25), reduces theta-like activity recorded from the hippocampus in vivo. The betaAP(25-35)-induced disruption in hippocampal network activity correlates with a reduction in spontaneous neuronal activity and synaptic transmission, as well as with an inhibition in the subthreshold oscillations produced by pyramidal neurons in vitro. Finally, we studied the involvement of Fyn-kinase on the betaAP(25-35)-induced disruption in hippocampal network activity in vitro. Interestingly, we found that such phenomenon is not observed in slices obtained from Fyn-knockout mice. In conclusion, our data suggest that betaAP acutely affects proper hippocampal function through a Fyn-dependent mechanism. We propose that such alteration might be related to the cognitive impairment observed, at least, during the early phases of Alzheimer's disease.

The functional neurophysiology of the amyloid precursor protein (APP) processing pathway

Amyloid beta (Abeta) peptides derived from proteolytic cleavage of amyloid precursor protein (APP) are thought to be a pivotal toxic species in the pathogenesis of Alzheimer's disease (AD). Furthermore, evidence has been accumulating that components of APP processing pathway are involved in non-pathological normal function of the CNS. In this review we aim to cover the extensive body of research aimed at understanding how components of this pathway contribute to neurophysiological function of the CNS in health and disease. We briefly outline changes to clinical neurophysiology seen in AD patients before discussing functional changes in mouse models of AD which range from changes to basal synaptic transmission and synaptic plasticity through to abnormal synchronous network activity. We then describe the various neurophysiological actions that are produced by application of exogenous Abeta in various forms, and finally discuss a number or other neurophysiological aspects of the APP pathway, including functional activities of components of secretase complexes other than Abeta production.

[Gly(14)]-humanin rescues long-term potentiation from amyloid beta protein-induced impairment in the rat hippocampal CA1 region in vivo

The novel neuroprotective action of Humanin (HN), especially its derivative [Gly(14)]-humanin (HNG), against Alzheimer's disease (AD)-related insults has been reported. However, it is still short of electrophysiological evidence for the protection of HN on synaptic plasticity, and the molecular mechanisms that underlie the neuroprotective function of HN remain largely unknown. The present study examined the effects of intracerebroventricular (i.c.v.) injection of HNG on amyloid beta (Abeta), a main constituent of senile plaques in the AD brain, induced suppression of long-term potentiation (LTP) in the rat hippocampal CA1 region in vivo and investigated the possible mechanism of HNG in LTP protection. We found that application of Abeta fragments 25-35 (Abeta25-35) and 31-35 (Abeta31-35) significantly inhibited high frequency stimulation-induced LTP, while HNG effectively prevented the suppression of LTP induced by Abeta fragments in a dose-dependent manner. After pretreatment with Genistein, a tyrosine kinase inhibitor, the protective action of HNG on LTP was nearly completely abolished. Therefore, the present study demonstrated for the first time that HNG could protect against the neurotoxic Abeta-induced hippocampal LTP impairment and the tyrosine kinase pathway was involved in the neuroprotective action of HNG, suggesting that HNG might be one of the promising candidates for the treatment of AD in the future.

Calcium dysregulation in Alzheimer's disease: from mechanisms to therapeutic opportunities

Calcium is involved in many facets of neuronal physiology, including activity, growth and differentiation, synaptic plasticity, and learning and memory, as well as pathophysiology, including necrosis, apoptosis, and degeneration. Though disturbances in calcium homeostasis in cells from Alzheimer's disease (AD) patients have been observed for many years, much more attention was focused on amyloid-beta (Abeta) and tau as key causative factors for the disease. Nevertheless, increasing lines of evidence have recently reported that calcium dysregulation plays a central role in AD pathogenesis. Systemic calcium changes accompany almost the whole brain pathology process that is observed in AD, including synaptic dysfunction, mitochondrial dysfunction, presenilins mutation, Abeta production and Tau phosphorylation. Given the early and ubiquitous involvement of calcium dysregulation in AD pathogenesis, it logically presents a variety of potential therapeutic targets for AD prevention and treatment, such as calcium channels in the plasma membrane, calcium channels in the endoplasmic reticulum membrane, Abeta-formed calcium channels, calcium-related proteins. The review aims to provide an overview of the current understanding of the molecular mechanisms involved in calcium dysregulation in AD, and an insight on how to exploit calcium regulation as therapeutic opportunities in AD.

Memory deficits and neurochemical changes induced by C-reactive protein in rats: implication in Alzheimer's disease

RATIONALE

C-reactive protein (CRP), an acute phase protein that is released in response to inflammatory stimuli, is implicated in Alzheimer's disease (AD). However, the role of CRP in memory deficits associated with AD remains unclear.

OBJECTIVE

Experiments were carried out to determine whether CRP impaired memory and altered neurochemical measures associated with AD.

METHODS

The effects of intra-cerebroventricular administration of CRP or beta-amyloid peptide 25-35 (Abeta(25-35)) on memory performance were evaluated using rat Morris water-maze and step-through passive avoidance tests; the levels of inflammatory cytokines (interleukin-1beta (IL-1beta), IL-6, and tumor necrosis factor (TNF-alpha)), endogenous CRP, and markers of the endogenous production of Abeta, including amyloid precursor protein (APP), presenilins (PS-1 and PS-2), and beta-site of APP cleaving enzyme (BACE), were also determined in brain regions using real-time reverse transcriptase polymerase chain reaction (RT-PCR) and Western blotting analysis.

RESULTS

Treatment with CRP (25.6 microg/rat) or Abeta(25-35) (10 microg/rat) 2 weeks ahead produced impairment of long-term memory in both animal tests. Real-time RT-PCR revealed increases in messenger RNA levels of APP, IL-1beta, IL-6, TNF-alpha, and CRP in the cerebral cortex and hippocampus and those of PS-1 and PS-2 in the cerebral cortex produced by treatment with CRP or Abeta(25-35). Immunoblotting analysis showed that while expression of APP was increased in both the cerebral cortex and the hippocampus, expression of IL-1beta, BACE, and TNF-alpha was increased only in the hippocampus.

CONCLUSIONS

The results suggest that CRP contributes to memory loss and early phase of pathogenesis of AD. CRP can be a novel target for therapeutic intervention of AD.

Role of amyloid-beta glycine 33 in oligomerization, toxicity, and neuronal plasticity

The aggregation of the amyloid-beta (Abeta) peptide plays a pivotal role in the pathogenesis of Alzheimer's disease, as soluble oligomers are intimately linked to neuronal toxicity and inhibition of hippocampal long-term potentiation (LTP). In the C-terminal region of Abeta there are three consecutive GxxxG dimerization motifs, which we could previously demonstrate to play a critical role in the generation of Abeta. Here, we show that glycine 33 (G33) of the central GxxxG interaction motif within the hydrophobic Abeta sequence is important for the aggregation dynamics of the peptide. Abeta peptides with alanine or isoleucine substitutions of G33 displayed an increased propensity to form higher oligomers, which we could attribute to conformational changes. Importantly, the oligomers of G33 variants were much less toxic than Abeta(42) wild type (WT), in vitro and in vivo. Also, whereas Abeta(42) WT is known to inhibit LTP, Abeta(42) G33 variants had lost the potential to inhibit LTP. Our findings reveal that conformational changes induced by G33 substitutions unlink toxicity and oligomerization of Abeta on the molecular level and suggest that G33 is the key amino acid in the toxic activity of Abeta. Thus, a specific toxic conformation of Abeta exists, which represents a promising target for therapeutic interventions.

Protective effect of S14G-humanin against beta-amyloid induced LTP inhibition in mouse hippocampal slices

Synaptic dysfunction induced by amyloid-beta protein (Abeta) has been shown to play a critical role in cognitive deficits of Alzheimer's disease (AD). Currently, however there is no clinical causative therapy for the disease. S14G-humanin (HNG) is best known for its strong neuroprotective ability against AD-related insults in vitro, and several in vivo studies have shown its effectiveness in ameliorating the cognitive impairment, but the precise mechanism of HNG on neuroprotection still remains to be elucidated. The present study examined the effects of HNG on Abeta-induced inhibition of hippocampal long-term potentiation (LTP) in mouse hippocampal slices. The results disclosed that soluble Abeta(25-35) significantly inhibited the induction of early-phase LTP (E-LTP) and late-phase LTP (L-LTP) in the hippocampal CA1 region without affecting the basal synaptic transmission, while HNG significantly ameliorated such inhibition of E-LTP and L-LTP in a dose-dependent manner. In addition, the reduction of phosphorylated CREB trigged by Abeta(25-35) was restored by HNG during L-LTP induction, possibly attributing to the improvement of the L-LTP inhibition. Collectively, our findings add to the evidence that soluble Abeta-induced LTP inhibition may represent an early pathological event of AD, and demonstrate for the first time that HNG may improve LTP inhibition by subneurotoxic concentration of soluble Abeta, suggesting that HNG may have therapeutic potential for Abeta-induced synaptic dysfunction closely associated with cognitive deficits in the early stage of AD.

Calcium signaling in neurodegeneration

Calcium is a key signaling ion involved in many different intracellular and extracellular processes ranging from synaptic activity to cell-cell communication and adhesion. The exact definition at the molecular level of the versatility of this ion has made overwhelming progress in the past several years and has been extensively reviewed. In the brain, calcium is fundamental in the control of synaptic activity and memory formation, a process that leads to the activation of specific calcium-dependent signal transduction pathways and implicates key protein effectors, such as CaMKs, MAPK/ERKs, and CREB. Properly controlled homeostasis of calcium signaling not only supports normal brain physiology but also maintains neuronal integrity and long-term cell survival. Emerging knowledge indicates that calcium homeostasis is not only critical for cell physiology and health, but also, when deregulated, can lead to neurodegeneration via complex and diverse mechanisms involved in selective neuronal impairments and death. The identification of several modulators of calcium homeostasis, such as presenilins and CALHM1, as potential factors involved in the pathogenesis of Alzheimer's disease, provides strong support for a role of calcium in neurodegeneration. These observations represent an important step towards understanding the molecular mechanisms of calcium signaling disturbances observed in different brain diseases such as Alzheimer's, Parkinson's, and Huntington's diseases.

Amyloid beta-protein fragments 25-35 and 31-35 potentiate long-term depression in hippocampal CA1 region of rats in vivo

Amyloid beta-protein (Abeta) is thought to be responsible for the deficit of learning and memory in Alzheimer's disease (AD), possibly through interfering with synaptic plasticity in the brain. It has been reported that Abeta fragments suppress the long-term potentiation (LTP) of synaptic transmission. However, it is unclear whether Abeta fragments can regulate long-term depression (LTD), an equally important form of synaptic plasticity in the brain. The present study investigates the effects of Abeta fragments on LTD induced by low frequency stimulation (LFS) in the hippocampus in vivo. Our results showed that (1) prolonged 1-10 Hz of LFS all effectively elicited LTD, which could persist for at least 2 h and be reversed by high frequency stimulation (HFS); (2) the effectiveness of LTD induction depended mainly on the number of pulses but not the frequency of LFS; (3) pretreatment with Abeta fragment 25-35 (Abeta(25-35), 12.5 and 25 nmol) did not change baseline field excitatory postsynaptic potentials but dose-dependently potentiated LTD; (4) Abeta fragment 31-35 (Abeta(31-35)), a shorter Abeta fragment than Abeta(25-35), also dose-dependently strengthened LFS-induced hippocampal LTD. Thus, the present study demonstrates the enhancement of hippocampal LTD by Abeta in in vivo condition. We propose that Abeta-induced potentiation of LTD, together with the suppression of LTP, will result in the impairment of cognitive function of the brain.

Increased intraneuronal resting [Ca2+] in adult Alzheimer's disease mice

Neurodegeneration in Alzheimer's disease (AD) has been linked to intracellular accumulation of misfolded proteins and dysregulation of intracellular Ca2+. In the current work, we determined the contribution of specific Ca2+ pathways to an alteration in Ca2+ homeostasis in primary cortical neurons from an adult triple transgenic (3xTg-AD) mouse model of AD that exhibits intraneuronal accumulation of beta-amyloid proteins. Resting free Ca2+ concentration ([Ca2+](i)), as measured with Ca2+-selective microelectrodes, was greatly elevated in neurons from 3xTg-AD and APP(SWE) mouse strains when compared with their respective non-transgenic neurons, while there was no alteration in the resting membrane potential. In the absence of the extracellular Ca2+, the [Ca2+](i) returned to near normal levels in 3xTg-AD neurons, demonstrating that extracellular Ca2+contributed to elevated [Ca2+](i). Application of nifedipine, or a non-L-type channel blocker, SKF-96365, partially reduced [Ca2+](i). Blocking the ryanodine receptors, with ryanodine or FLA-365 had no effect, suggesting that these channels do not contribute to the elevated [Ca2+](i). Conversely, inhibition of inositol trisphosphate receptors with xestospongin C produced a partial reduction in [Ca2+](i). These results demonstrate that an elevation in resting [Ca2+](i), contributed by aberrant Ca2+entry and release pathways, should be considered a major component of the abnormal Ca2+ homeostasis associated with AD.

Alzheimer amyloid beta-peptide A-beta25-35 blocks adenylate cyclase-mediated forms of hippocampal long-term potentiation

Progressive memory loss and deposition of amyloid beta (Abeta) peptides throughout cortical regions are hallmarks of Alzheimer's disease (AD). Several studies in mice and rats have shown that overexpression of amyloid precursor protein (APP) or pretreatment with Abeta peptide fragments results in the inhibition of hippocampal long-term potentiation (LTP) as well as impairments in learning and memory of hippocampal-dependent tasks. For these studies we have investigated the effects of the Abeta(25-35) peptide fragment on LTP induced by adenylate cyclase stimulation followed immediately by application of Mg(++)-free aCSF ("chemLTP"). Treatment of young adult slices with the Abeta(25-35) peptide had no significant effect on basal synaptic transmission in area CA1, but treatment with the peptide for 20 min before inducing chemLTP with isoproterenol (ISO; 1 microM) or forskolin (FSK;10 microM) + Mg(++)-free aCSF resulted in complete blockade of LTP. In contrast, normal ISO-chemLTP was observed after treatment with the control peptide Abeta(35-25). The ability of the Abeta(25-35) peptide fragment to block this and other forms of synaptic plasticity may help elucidate the mechanisms underlying hippocampal deficits observed in animal models of AD and/or AD individuals.

Soluble beta-amyloid[25–35] reversibly impairs hippocampal synaptic plasticity and spatial learning

Beta-amyloid is a peptide that appears to be responsible for cognitive impairments in patients with Alzheimer's disease. Recent research shows that soluble oligomers of beta-amyloid affect synaptic activity and learning, well before any amyloid has aggregated into plaques. Here we show that injection of 3x10 nmol amyloid [25-35] i.c.v. transiently impairs learning of a radial arm maze and the induction of hippocampal long-term potentiation. Furthermore, hippocampal field potentials had been recorded over a period of 21 days and were found to be reduced from day 9 to day 15 (P<0.001), after which the reduction had reversed to baseline. In the spatial 8-arm learning task, animals had to learn which 3 out of 8 arms had been baited. A significant impairment of working and long-term memory was observed at day 12-20 (P<0.001), but not at days 3-11 or 20-28. Long-term potentiation induction in the hippocampus area CA1 was also impaired at day 12-20 (P<0.001), but not at other days. A scrambled peptide sequence version of amyloid did not have any effect. These results emphasise that soluble amyloid fragments already have detrimental effects on brain function well before aggregation occurs. They also show that these effects are reversible, and therefore most likely do not involve neuronal death. The neurodegeneration seen in Alzheimer's disease brains is most likely a downstream effect, linked to processes such as immune response activation and free radical production. These results suggest that treatment at very early stages of Alzheimer's disease could prevent later irreversible neuronal degeneration.

IL-4 attenuates the neuroinflammation induced by amyloid-beta in vivo and in vitro

It has been shown that Abeta inhibits long-term potentiation (LTP) in the rat hippocampus and this is accompanied by an increase in hippocampal concentration of IL-1beta. Abeta also increases microglial activation, which is the likely cell source of IL-1beta. Because IL-4 attenuates the effects of IL-1beta in hippocampus, and microglial activation is inhibited by minocycline, we assessed the ability of both IL-4 and minocycline to modulate the effects of Abeta on LTP and IL-1beta concentration. Following treatment with Abeta, IL-4 or minocycline, rats were assessed for their ability to sustain LTP in perforant path-granule cell synapses. We report that the Abeta-induced inhibition of LTP was associated with increases in expression of MHCII, JNK phosphorylation and IL-1beta concentration, and that these changes were attenuated by treatment of rats with IL-4 and minocycline. We also report that Abeta-induced increases in expression of MHCII and IL-1beta were similarly attenuated by IL-4 and minocycline in glial cultures prepared from neonatal rats. These data suggest that glial cell activation and the consequent increase in IL-1beta concentration mediate the inhibitory effect of Abeta on LTP and indicate that IL-4, by down-regulating glial cell activation, antagonizes the effects of Abeta.

The HMG-CoA reductase inhibitor, atorvastatin, attenuates the effects of acute administration of amyloid-β1–42 in the rat hippocampus in vivo

One response of the brain to stressors is to increase microglial activation with the consequent production of proinflammatory cytokines like interleukin-1beta (IL-1beta), which has been shown to exert an inhibitory effect on long-term potentiation (LTP) in the hippocampus. It has been consistently shown, particularly in vitro, that amyloid-beta (Abeta) peptides increase activation of microglia, while its inhibitory effect on LTP is well documented, and associated with the Abeta-induced increase in IL-1beta. Here we set out to establish whether the Abeta-induced inhibition of LTP in perforant path-granule cell synapses, was coupled with evidence of microglial activation and to assess whether atorvastatin, which is used primarily in the treatment of hyperlipidaemia but which possesses anti-inflammatory properties, might modulate the effect of Abeta on LTP. We report that intracerebroventricular injection of Abeta increased expression of several markers of microglial activation, and in parallel, inhibited LTP in dentate gyrus. The data show that atorvastatin abrogated the Abeta-induced microglial activation and the associated deficit in LTP. On the basis of the evidence presented, we propose that the action of atorvastatin is mediated by its ability to increase production of the anti-inflammatory cytokine, interleukin-4, which we report mimics several of the actions of atorvastatin in the rat hippocampus.

Impairments of hippocampal synaptic plasticity induced by aggregated beta-amyloid (25–35) are dependent on stimulation-protocol and genetic background

The aggregation of beta-amyloid to plaques in the brain is one of the hallmarks of Alzheimer disease (AD). Numerous studies have tried to elucidate to what degree amyloid peptides play a role in the neurodegenerative developments seen in AD. While most studies report an effect of amyloid on neural activity and cognitive abilities of rodents, there have been many inconsistencies in the results. This study investigated to what degree the different genetic backgrounds affect the outcome of beta-amyloid fragment (25-35) on synaptic plasticity in vivo in the rat hippocampus. Two strains, Wistar and Lister hooded rats, were tested. In addition, the effects of a strong (600 stimuli) and a weak stimulation protocol (100 stimuli) on impairments of LTP were analysed. Furthermore, since the state of amyloid aggregation appears to play a role in the induction of toxic processes, it was tested by dual polarisation interferometry to what degree and at what speed beta-amyloid (25-35) can aggregate in vitro. It was found that 100 nmol beta-amyloid (25-35) injected icv did impair LTP in Wistar rats when using the weak but not the strong stimulation protocol (P < 0.001). One-hundred nano mole of the reverse sequence amyloid (35-25) had no effect. LTP in Lister Hooded rats was not impaired by amyloid at any stimulation protocol. The aggregation studies showed that amyloid (25-35) aggregated within hours, while amyloid (35-25) did not. These results show that the genetic background and the stimulation protocol are important variables that greatly influence the experimental outcome. The fact that amyloid (25-35) aggregated quickly and showed neurophysiological effects, while amyloid (35-25) did not aggregate and did not show any effects indicates that the state of aggregation plays an important role in the physiological effects.

Amyloid peptides mediate hypoxic increase of L-type Ca2+ channels in central neurones

Prolonged hypoxia, encountered in individuals suffering from various cardiorespiratory diseases, enhances the likelihood of subsequently developing Alzheimer's disease (AD). However, the underlying mechanisms are unknown, as are the mechanisms of neurodegeneration of amyloid beta peptides (AbetaPs), although the latter involves disruption of Ca2+ homeostasis. Here, immunohistochemistry demonstrated that hypoxia increased production of AbetaPs, an effect which was prevented by inhibition of either beta or gamma secretase, the enzymes required for liberation of AbetaP from its precursor protein. Whole-cell patch clamp recordings showed that hypoxia selectively increased functional expression of L-type Ca2+ channels. This was prevented by inhibition of either beta or gamma secretase, indicating that hypoxic channel up-regulation is dependent upon AbetaP formation. Our results indicate for the first time that hypoxia promotes AbetaP formation in central neurons, and show that this leads to abnormally high selective expression of L-type Ca2+ channels whose blockade has previously been shown to be neuroprotective in AD models. These findings provide a cellular basis for understanding the increased incidence of AD following prolonged hypoxia.

Non-fibrillar β-amyloid abates spike-timing-dependent synaptic potentiation at excitatory synapses in layer 2/3 of the neocortex by targeting postsynaptic AMPA receptors

Cognitive decline in Alzheimer's disease (AD) stems from the progressive dysfunction of synaptic connections within cortical neuronal microcircuits. Recently, soluble amyloid beta protein oligomers (Abeta(ol)s) have been identified as critical triggers for early synaptic disorganization. However, it remains unknown whether a deficit of Hebbian-related synaptic plasticity occurs during the early phase of AD. Therefore, we studied whether age-dependent Abeta accumulation affects the induction of spike-timing-dependent synaptic potentiation at excitatory synapses on neocortical layer 2/3 (L2/3) pyramidal cells in the APPswe/PS1dE9 transgenic mouse model of AD. Synaptic potentiation at excitatory synapses onto L2/3 pyramidal cells was significantly reduced at the onset of Abeta pathology and was virtually absent in mice with advanced Abeta burden. A decreased alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/N-methyl-D-aspartate (NMDA) receptor-mediated current ratio implicated postsynaptic mechanisms underlying Abeta synaptotoxicity. The integral role of Abeta(ol)s in these processes was verified by showing that pretreatment of cortical slices with Abeta((25-35)ol)s disrupted spike-timing-dependent synaptic potentiation at unitary connections between L2/3 pyramidal cells, and reduced the amplitude of miniature excitatory postsynaptic currents therein. A robust decrement of AMPA, but not NMDA, receptor-mediated currents in nucleated patches from L2/3 pyramidal cells confirmed that Abeta(ol)s perturb basal glutamatergic synaptic transmission by affecting postsynaptic AMPA receptors. Inhibition of AMPA receptor desensitization by cyclothiazide significantly increased the amplitude of excitatory postsynaptic potentials evoked by afferent stimulation, and rescued synaptic plasticity even in mice with pronounced Abeta pathology. We propose that soluble Abeta(ol)s trigger the diminution of synaptic plasticity in neocortical pyramidal cell networks during early stages of AD pathogenesis by preferentially targeting postsynaptic AMPA receptors.

Development of beta-amyloid-induced neurodegeneration in Alzheimer's disease and novel neuroprotective strategies

Alzheimer's disease (AD) is a form of dementia in which people develop rapid neurodegeneration, complete loss of cognitive abilities, and are likely to die prematurely. At present, no treatment for AD is known. One of the hallmarks in the development of AD is the aggregation of amyloid protein fragments in the brain, and much evidence points towards beta-amyloid fragments being one of the main causes of the neurodegenerative processes. This review summarises the present concepts and theories on how AD develops, and lists the evidence that supports them. A cascade of biochemical events is initiated that ultimately leads to neuronal death involving an imbalance of intracellular calcium homeostasis via activation of calcium channels, intracellular calcium stores, and subsequent production of free radicals by calcium-sensitive enzymes. Secondary processes include inflammatory responses that produce more free radicals and the induction of apoptosis. Recently, several new strategies have been proposed to try to ameliorate the neurodegenerative developments associated with AD. These include the activation of neuronal growth factor receptors and insulin-like receptors, both of which have neuroprotective properties. Furthermore, the role of cholesterol and potential protective properties of cholesterol-lowering drugs are under intense investigation. Other promising strategies include the inhibition of beta- and gamma-secretases which produce beta-amyloid, activation of proteases that degrade beta-amyloid, glutamate receptor selective drugs, antioxidants, and metal chelating agents, all of which prevent formation of plaques. Novel drugs that act at different levels of the neurodegenerative processes show great promise to reduce neurodegeneration. They could help to prolong the time of unimpaired cognitive abilities of people who develop AD, allowing them to lead an independent life.

Amyloid β-protein fragment 31–35 suppresses long-term potentiation in hippocampal CA1 region of rats in vivo

Effects of fragment 31-35 of amyloid beta-protein (AbetaP31-35) on the baseline synaptic transmission, shown as fEPSPs, and the long-term potentiation (LTP) induced by high-frequency stimuli (HFS) were investigated in vivo in the hippocampal CA1 region of rats; a longer fragment of AbetaP, i.e., AbetaP25-35, which had been generally accepted as the active center in AbetaP, was also tested comparatively along with AbetaP31-35. The results showed that: (1) the baseline fEPSPs induced by test stimuli were not changed by i.c.v. injection of AbetaP31-35, while application of either AbetaP31-35 or AbetaP25-35 with the same molar concentration (50 nmol) significantly and similarly suppressed the HFS-induced LTP; (2) higher concentration of AbetaP31-35 or longer time of AbetaP exposure exhibited stronger suppression on LTP, indicating a dose- and time-dependent trends; (3) no significant effects could be found on the paired-pulse facilitation (PPF) following AbetaP31-35 injection; (4) pretreatment with verapamil (2.5 mg/kg, i.p., 1 h prior to HFS), a blocker of L-type Ca2+ channels, did not affect the baseline fEPSPs, while it exhibited a significant suppression on LTP induced by HFS; and (5) surprisingly enough, coapplication with verapamil and AbetaP31-35 exhibited a similar suppression on LTP just as both of these two agents were used alone. These results indicate that: (1) AbetaP31-35, similar to AbetaP25-35, possesses potent suppressive effects on hippocampal LTP in vivo, supporting our proposal that the fragment AbetaP31-35 might be to date the shortest active sequence in full-length of AbetaP molecule; (2) AbetaP31-35-induced LTP suppression is not mediated by affecting the presynaptic processes; and (3) L-type Ca2+ channels might be one of the main pathways by which AbetaP31-35 insults LTP.

Calcium-regulated signaling pathways: role in amyloid beta-induced synaptic dysfunction

Amyloid beta (Abeta) peptides have been shown to impair synaptic function, especially long-term synaptic plasticity, in transgenic mouse models of Alzheimer's disease (AD) and in acute hippocampal preparations. In the transgenic mice overexpressing mutant forms of human amyloid precursor protein (APP), the deficits in hippocampal long-term potentiation (LTP) occur prior to synaptic loss and cell death, suggesting early functional changes at these synapses. Recent studies demonstrate that Abeta-induced synaptic dysfunction is linked with altered Ca2+ signaling in hippocampal neurons. While reducing Ca2+ influx through NMDA receptors, Abeta peptides elevate intracellular Ca2+ concentration by enhancing Ca2+ influx from voltage-gated Ca2+ channels or nonselective cation channels, or by stimulating Ca2+ release from intracellular stores. Interestingly, acute application of Abeta or APP overexpression inhibits activity-dependent regulation of several protein kinase pathways that require Ca2+ influx via NMDA receptors for activation, including Ca2+/calmodulin-dependent protein kinase II, protein kinase A, and extracellular regulated kinases (Erk). On the other hand, activation of Ca2+-dependent protein phosphatase 2B (calcineurin) is implicated in Abeta inhibition of LTP. Thus, multiple Ca2+-regulated signaling pathways are involved in the synaptic action of Abeta, and malfunction of these pathways may underlie the synaptic dysfunction in early AD.

Agonists of peroxisome proliferator-activated receptor-γ attenuate the Aβ-mediated impairment of LTP in the hippocampus in vitro

The data we present here suggest that agonists of peroxisome proliferator-activated receptor-gamma (PPARgamma) can attenuate the effects of beta-amyloid peptide (Abeta). Alzheimer's disease is associated with elevated levels of Abeta, and enhanced expression of PPARgamma. In this study, we determined that application of Abeta([1-40]) could impair hippocampal post-tetanic potentiation (PTP) and long-term potentiation (LTP) in vitro. We investigated the effects of PPARgamma agonists; troglitazone, ciglitazone and 15-deoxy-delta(12,14) prostaglandin J2 (PGJ2) on synaptic transmission and plasticity in area CA1. Both ciglitazone and PGJ2 increased baseline synaptic transmission significantly, without altering paired-pulse facilitation. PGJ2 produced a significant reduction in LTP, whereas ciglitazone and troglitazone had no significant effect. In addition, prior application of each ligand attenuated the previously observed Abeta([1-40])-mediated impairment of LTP. The effect of troglitazone on the Abeta([1-40])-mediated impairment of LTP was not reversed by the PPARgamma antagonist, GW-9662. These findings demonstrate that PPARgamma agonists attenuate the effects of Abeta on LTP, and support the potential use of these agents to alleviate the symptoms of Alzheimer's disease. We also suggest that PPARgamma agonists may regulate expression of hippocampal LTP in vitro.

Effect of donepezil and lercanidipine on memory impairment induced by intracerebroventricular streptozotocin in rats

Intracerebroventricular (ICV) injection of streptozotocin (STZ) causes cognitive impairment in rats. ICV STZ is known to impair cholinergic neurotransmission by decreasing choline acetyltransferase (ChAT) levels, glucose and energy metabolism in brain and synthesis of acetyl CoA. However, no reports are available regarding the cholinesterase inhibitors in this model. In aging brain, reduced energy metabolism increases glutamate release, which is blocked by L-type calcium channel blockers. These calcium channel blockers have shown beneficial effects on learning and memory in various models of cognitive impairment. The present study was designed to investigate the influence of chronic administration of donepezil (cholinesterase inhibitor, 1 and 3 mg/kg) and lercanidipine (L-type calcium channel blocker, 0.3 and 1 mg/kg) on cognitive impairment in male Sprague-Dawley rats injected twice with ICV STZ (3 mg/kg) bilaterally on days 1 and 3. ICV STZ injected rats developed a severe deficit in learning and memory indicated by deficits in passive avoidance paradigm and elevated plus maze as compared to control rats. Cholinesterase activity in brain was significantly increased in ICV STZ injected rats. Donepezil dose-dependently inhibited cholinesterase activity and improved performance in memory tests at both the doses. Lercanidipine (0.3 mg/kg) showed significant improvement in memory. When administered together, the effect of combination of these two drugs on memory and cholinesterase activity was higher than that obtained with either of the drugs when used alone.

Synaptic transmission and synchronous activity is disrupted in hippocampal slices taken from aged TAS10 mice

Synaptic transmission was studied in hippocampal slices from aged (12-14 months of age) TAS10 mice overexpressing the human form of the amyloid precursor protein harboring the Swedish mutation. A significant deficit in the input-output relationship of glutamatergic synapses in the CA3-CA1 Schaffer collateral pathway was observed, while synaptic transmission in the medial perforant pathway of the dentate gyrus was comparatively preserved. Despite this deficit, relative levels of short- and long-term synaptic plasticity in the CA1 region were similar to those observed in wildtype slices. Specifically, paired pulse facilitation, frequency facilitation (at frequencies of 1, 5, and 10 Hz), and long-term potentiation induced by a theta burst stimulation paradigm were all normal in the CA3-CA1 synapses of TAS10 hippocampal slices. However, synchronized network activity induced by bath application of 4-aminopyridine (4-AP) was compromised. Thus, the frequency of synchronous events induced by 100 microM 4-AP was significantly lower in TAS10 hippocampal slices (inter-event interval: WT, 2.4+/-0.6 s; TAS10, 6.9+/-1.7 s). To study gamma-aminobutyric acid (GABA)ergic synaptic transmission NBQX (20 microM) and D-AP5 (50 microM) were added in order to isolate bicuculline-sensitive GABA-mediated synchronous network activity. The GABAergic network activity was not significantly different from wildtype in terms of frequency. This study suggests that the deficit in glutamatergic synaptic transmission observed in the TAS10 hippocampal slices, may be coupled with alterations in synchronous network activity, which in turn would lead to deficient information processing.