Ebook: Handbook of Animal Models in Alzheimer's Disease
Animals have been used to model diseases or test new treatments since around 300 BC, and undoubtedly our ability to model disease in animals – including transgenic animals – has provided major breakthroughs in all fields of biomedical research. Due to their complexity and plurality of pathology and symptomatology, the study of neurodegenerative diseases relies heavily on animal models. These models have been developed in many species in the attempt to undercover the complex nature of the disease mechanisms involved. The ultimate goal is to test promising therapies and to manage, prevent or cure neurodegenerative disease. But because most animal models in this area do not reproduce the full phenotypical disease spectrum and the etiology and clinical presentation of neurodegenerative diseases differ from one patient to the next, the testing of these diseases in animal models often translates poorly to indices of efficacy when applied to the clinical population. Written by experts in the field with these advances and challenges in mind, this handbook provides an updated overview of the animal models being developed and used to study complex disease dynamics. The first part of the book presents an overview of animal models of various species and includes a review of new invertebrate animal models to study neurodegeneration. The second section presents the use of animal models to pinpoint disease mechanisms, and the last part of the handbook examines the various therapeutic interventions being used in models of neurodegenerative disease.
References of experimental use of animals to model diseases, novel experimental procedures, or test novel therapeutics date all the way back to 304–258 BCE. It is undisputable that our ability to model disease in animals has provided major breakthroughs in all fields of biomedical research and has been vastly accelerated by the development of transgenic animals.
The study of neurodegenerative diseases is highly reliant on animal models due to their complexity and plurality of pathology and symptomatology. Today we have amassed a multitude of animal models, developed through genetic, chemical, and/or lesions in multiple species with the goal of faithfully mimicking these diseases and uncover the complex nature of disease-associated mechanisms. Ultimately, the goal is to test promising therapies and manage, prevent, or cure neurodegenerative disease.
The field of neurodegenerative diseases faces unique challenges in this application. First, most animal models in this area, unlike in linear diseases, do not reproduce the full phenotypical disease spectrum. Second, for a given neurodegenerative disease, the etiology and the clinical presentation differ from one patient to the next. As such, while the current models are well suited for the study of specific pathology-driven mechanisms, more notably amyloid-β, tau, or alpha-synuclein, pharmacological testing in animal models of neurodegenerative disease often translates into poorer indices of efficacy when applied to the clinical population. With these advances and challenges in mind, this handbook, written by experts in the field of neurodegeneration, provides a rich and updated overview of a wide range of animal models that are being developed and used to study complex disease dynamics, including but also beyond pathology-associated mechanisms, with the ultimate goal to discover the neuroprotective therapeutics of the future through more accurate translation of basic to clinical outputs.
The first section of this handbook presents an overview of animal models of various species, ranging from higher mammals such as primates or dogs, to knowledge gathered for more prevalent rodent genetically-based models, as well as promising models developed in the rabbit to study metabolic endpoints and therapeutic strategies for AD. Last but not least, this first section includes the review of newer invertebrate animal models, such as Drosophila to study neurodegeneration. Invertebrate models provide high-throughput potential, with highly manipulable genetics and functional output that places these models in promising standing within the field.
The second section of this handbook presents the use of animal models to pinpoint disease mechanisms. Pathology driven mechanisms are well represented but not limiting. As we are learning that “bottom-up”, overexpression based transgenic models do not provide an accurate representation of therapeutic effectiveness, we have also focused on more “top-down” models, for example those based on metabolic pathological endpoints that exclude pathology as the primary driver of neurodegenerative disease.
Lastly, this handbook concludes with a representation of various therapeutic interventions that are being used in models of neurodegenerative disease. Critical insight on effectiveness of clinically tested therapies in addition to novel, untested ones are aimed at providing both, the necessary critical due diligence when treatments fail – “where have we gone wrong” – “How can we do better” and a glimpse of hope for the future.
The contributions in this book are based on articles previously published by IOS Press in the Journal of Alzheimer's Disease, and have in most cases been revised and updated.
Non-human primates have served as subjects for studies of the cognition-enhancing potential of novel pharmacological agents for over 25 years. Only recently has a greater appreciation of the translational applicability of this model been realized. Though most Old-World monkeys do not appear to acquire an Alzheimer's-like syndrome in old age, their value resides in the brain physiology they have in common with humans. Paradigms like the delayed matching-to-sample task engender behavior that models aspects of working memory that are substrates for the actions of cognition-enhancing drugs. Our studies have provided information relevant to factors that limit the effectiveness of clinical trial design for compounds that potentially improve cognition. For example, cognition-enhancing compounds from different pharmacological classes, when administered to monkeys, can exhibit remarkable pharmacodynamic effects that outlast the presence of the drug in the body. Studies with non-human primates also can provide information regarding dose ranges and individual subject sensitivity experienced in the clinic. Components of working memory are differentially sensitive to drug effects and may be characterized by different dose ranges for certain compounds, even within the same task. Examples are provided that underscore the possible idiosyncrasies of drug action in the pharmacology of cognition –which could be of critical importance in the design of clinical trials.
Aged dogs (beagles) develop losses in executive function, learning and memory. The severity of decline in these cognitive domains represents a spectrum that captures normal aging, mild cognitive impairment and early/mild Alzheimer disease (AD) in humans. In parallel, dogs naturally accumulate several types of neuropathology (although not all) consistent with human brain aging and AD including cortical atrophy, neuron loss, loss of neurogenesis, beta-amyloid (Aβ) plaques, cerebral amyloid angiopathy, mitochondrial dysfunction and oxidative damage. Many of these neuropathological features correlate with the extent of cognitive decline in a brain region-dependent manner. Dogs are ideally suited for longitudinal studies and we provide a summary of the beneficial effects of an antioxidant diet, of behavioral enrichment and of Aβ immunotherapy. In addition, combinatorial treatment approaches can be a powerful strategy for improving brain function through enhancement of multiple molecular pathways.
Current mouse models of Alzheimer's disease (AD) are restricted to the expression of AD-related pathology associated with specific mutations present in early-onset familial AD and thus represent <5% of AD cases. To date there are no mouse lines that model late-onset/age-related AD, the feature which accounts for the vast majority of cases. As such, based on current mutation-associated models, the chronology of events that lead to the disease in the aged population is difficult to establish. However, published data show that senescence-accelerated mouse (SAMP8), as a model of aging, display many features that are known to occur early in the pathogenesis of AD such as increased oxidative stress, amyloid-β alterations, and tau phosphorylation. Therefore, SAMP8 mice may be an excellent model for studying the earliest neurodegenerative changes associated with AD and provide a more encompassing picture of human disease, a syndrome triggered by a combination of age-related events. Here, the neurochemical, neuropathological, and behavioral alterations, characterized in SAMP8 mice are critically reviewed and discussed in relation to the potential use of this mouse model in the study of AD pathogenesis.
Neurodegenerative diseases are characterized by ‘hot spots’ of degeneration. The regions of primary vulnerability vary between different neurodegenerative diseases. Within these regions, some neurons are lost whereas others that are morphologically indiscriminable survive. The enigma of this selective vulnerability is tightly linked to two fundamental problems in the neurosciences. Firstly, it is not understood how many neuronal cell types make up the mammalian brain; estimates are in the order of more than thousand. Secondly, the mechanisms by which some nerve cells undergo functional impairment followed by degeneration while others do not, remain elusive. Understanding the basis for this selective vulnerability has significant implications for understanding the pathogenesis of disease and for developing treatments. Here, we review what is known about selective vulnerability in Alzheimer's disease, frontotemporal dementia and Parkinson's disease. We suggest, since transgenic animal models of disease reproduce aspects of selective vulnerability, that these models offer a valuable system for future investigations into the physiological basis of selective vulnerability.
Pioneering autopsy studies revealed a possible link between coronary artery disease, cholesterol and Alzheimer's disease (AD). In the cholesterol-fed rabbit model of human coronary artery disease, we identified numerous neuropathologic features of AD including central accumulation of amyloid-β (Aβ) and cognitive deficits compared to rabbits fed unaltered diet. Removing cholesterol from the diet or treatment with cholesterol-lowering medications reversed the severity of AD-like alterations. This fostered the rationale for testing a cholesterol-lowering statin medication for benefit in the treatment of AD. Further studies suggested that the cholesterol-fed rabbit was a viable model for AD, but the severity of the neuropathology produced exhibited gender-related differences. Furthermore the induction of AD-like neuropathology by dietary cholesterol was found to depend on the quality of water the animal was drinking. Cholesterol-fed rabbits drinking distilled water showed minimal central changes, whereas animals drinking distilled water supplemented with low levels of copper were severely affected. It was clear that cholesterol caused the over-production of Aβ in the brain and copper influenced its clearance to the blood. Emerging data suggest that low-density lipoprotein receptor-related protein-1 (LRP) on brain capillaries clears Aβ from brain and that excess circulating copper negatively influences this process.
Supplementing a rabbit's diet with 2% cholesterol alone or with a trace amount of copper created neuropathological changes that resembled those seen in Alzheimer's disease (AD). AD model rabbits were impaired in eyeblink classical conditioning; a form of learning severely impaired in AD. Our aim was to replicate AD rabbit model neuropathology, test eyeblink conditioning in this model, and determine if galantamine (Razadyne™) would ameliorate impaired conditioning. In Experiment 1 rabbit chow with 2% cholesterol and drinking water with 0.12 mg/liter copper sulfate were administered for 10 weeks. Control rabbits received normal food and water. Rabbit brains were probed for neuropathology. AD model rabbits had significant neuronal loss in frontal cortex, hippocampus and cerebellum. Changes in neurons in the hippocampus were consistent with neurofibrillary degeneration and cytoplasmic immunoreactivity for β-amyloid and tau. In Experiment 2 AD model rabbits were injected daily with vehicle or 3.0 mg/kg galantamine and tested on 750 ms trace and delay eyeblink conditioning. Galantamine improved eyeblink conditioning significantly over vehicle. The AD rabbit model has validity from neuropathological to cognitive levels and offers a promising addition to the available animal models of AD. Galantamine ameliorated impaired eyeblink conditioning, extending the validity of the AD rabbit model to treatment modalities.
Alzheimer disease (AD) is the most common form of senile dementia, and a cure is desperately needed. The amyloid-β 42 (Aβ42) has been suggested to play a central role in the pathogenesis of AD. However, the mechanism by which Aβ42 causes AD remains unclear. To understand the pathogenesis and to develop therapeutic avenues, it is crucial to generate animal models of Aβ42 toxicity in genetically tractable organisms. Drosophila is a well-established model system for which abundant genetic tools are available, and recently emerges as a model for human neurodegenerative diseases. We and others have established transgenic flies that express human Aβ42 in the nervous system. These Aβ42 flies developed age-dependent short-term memory impairment and neurodegeneration accompanied by Aβ42 deposits. Here we will summarize key features of transgenic Drosophila models of Aβ42 toxicity and a number of insights into disease mechanisms as well as therapeutic implications gained from these models.
Accumulating evidence indicates that ovarian hormones regulate a wide variety of non-reproductive functions in the central nervous system by interacting with several molecular and cellular processes. A growing animal literature using both adult and aged rodent models indicates that 17β-estradiol (E2), the most potent of the biologically relevant estrogens, and progesterone (P4), a major naturally occurring progestogen, facilitate some forms of learning and memory, in particular those that involve hippocampal-dependent tasks. A recently developed triple-transgenic mouse (3xTg-AD) has been widely used as an animal model of Alzheimer's disease (AD), as this mouse exhibits an age-related and progressive neuropathological phenotype that includes both plaque and tangle pathology mainly restricted to hippocampus, amygdala and cerebral cortex. In this report, we examine recent studies that compare the effects of ovarian hormones on synaptic transmission and synaptic plasticity in adult and aged rodents. A better understanding of the non-reproductive functions of ovarian hormones has farreaching implications for hormone therapy to maintain health and function within the nervous system throughout aging.
Transgenic mice expressing human tau containing the P301L tau mutation (JNPL3; tau mice) develop motor neuron loss, paralysis and death between 7 and 12 months. Surprisingly, at 5 and 7 months of age, tau transgenic mice were superior to other genotypes in the rotarod task, and had near perfect scores on the balance beam and coat hanger tests. One tau transgenic mouse was performing at a superior level in the rotarod one day prior to developing paralysis. Cognitive function was also normal in the tau mice evaluated in the radial arm water maze and the Y-maze tasks. We also crossed the tau transgenic mice with Tg2576 amyloid-β protein precursor (AβPP) transgenic mice. Although AβPP mice were deficient in the radial arm maze task, AβPP + tau mice were not impaired, implying a benefit of the tau transgene. Some mice were homozygous for the retinal degeneration mutation (rd/rd) and excluded from the genotype analysis. Only the water maze task discriminated the rd/rd mice from nontransgenic mice. In conclusion, it seems that the modest tau overexpression or the presence of mutant tau in the JNPL3 tau mice may provide some benefit with respect to motor and cognitive performance before the onset of paralysis.
Cell cycle proteins are elevated in the brain of patients and in transgenic models of Alzheimer's disease (AD), suggesting that aberrant cell cycle re-entry plays a key role in this disorder. However, the precise relationship between cell cycle reactivation and the AD hallmarks, amyloid-beta (Aβ) plaques and tau-laden neurofibrillary tangles, remains unclear. We sought to determine whether cell cycle reactivation initiates in direct response to Aβ and tau accumulation or whether it occurs as a downstream consequence of neuronal death pathways. Therefore, we used a triple transgenic mouse model of AD (3xTg-AD) that develops plaques and tangles, but does not exhibit extensive neuronal loss, whereas to model hippocampal neuronal death a tetracycline-regulatable transgenic model of neuronal ablation (CaM/Tet-DTA mice) was used. Cell-cycle proteins activation was determined in these two models of neurodegeneration, using biochemical and histological approaches. Our findings indicate that Cdk4, PCNA and phospho-Rb are significantly elevated in CaM/Tet-DTA mice following neuronal death. In contrast, no significant activation of cell-cycle proteins occurs in 3xTg-AD mice versus non-transgenic controls. Taken together, our data indicate that neuronal cell cycle reactivation is not a prominent feature induced by Aß or tau pathology, but rather appears to be triggered by acute neuronal loss.
Little is known about how amyloid-β (Aβ) is deposited in relation to the complex ultrastructure of the brain. Here we combined serial section immunoelectron microscopy with 3D reconstruction to elucidate the spatial relationship between Aβ deposits and ultrastructurally identified cellular compartments. The analysis was performed in a transgenic mouse model with mutant presenilin-1, and mutant amyloid-β protein precursor (AβPP) and tau transgenes (3xTg-AD mice) and in aged dogs that develop Aβ plaques spontaneously. Reconstructions based on serial ultrathin sections of hippocampus (mice) or neocortex (dogs) that had been immunolabeled with Aβ (Aβ1–42) antibodies showed that the organization of extracellular Aβ deposits is more complex than anticipated from light microscopic analyses. In both species, deposits were tightly associated with plasma membranes of pyramidal cell bodies and major dendrites. The deposits typically consisted of thin sheets as well as slender tendrils that climbed along the large caliber dendritic stems of pyramidal neurons. No preferential association was observed between Aβ deposits and thin dendritic branches or spines, nor was there any evidence of preferential accumulation of Aβ around synaptic contacts or glial processes. Our data suggest that plaque formation is a precisely orchestrated process that involves specialized domains of dendrosomatic plasma membranes.
Epidemiological, animal, and cellular studies suggest that abnormalities in cholesterol metabolism are a risk factor for Alzheimer's disease (AD), potentially by increasing amyloid-β (Aβ) peptide levels. Accumulation of Aβ in the brain is suggested to play a key role in the neurodegenerative processes by triggering the hyperphosphorylation of tau and the neuronal death that develop in the course of AD. However, the mechanisms by which cholesterol increases Aβ levels are still ill-defined. Work from our laboratory using the cholesterol-fed rabbit model system indicates that hypercholesterolemia increases Aβ through multiple mechanisms that affect production, degradation and clearance of this peptide. We also have found that the oxidized cholesterol metabolite, 27-hydroxycholesterol, also increases Aβ levels in organotypic slices from rabbit hippocampus. Our results suggest that multiple signaling pathways are involved in hypercholesterolemia-induced AD pathology and suggest 27-hydroxycholesterol as the link between circulating cholesterol and AD-like pathology in the brain.
Background: Obesity, type 2 diabetes mellitus (T2DM), and non-alcoholic steatohepatitis (NASH) can lead to cognitive impairment and neurodegeneration. Experimental high fat diet (HFD) induced obesity with T2DM causes neurodegeneration with brain insulin resistance. Objective: Since ceramides are neurotoxic, cause insulin resistance, and are increased in T2DM, we investigated their potential role in neurodegeneration. Methods: C57BL/6 mice were pair-fed HFD or control diets for 4-20 weeks. Pro-ceramide genes and biochemical indices of neurodegeneration were measured. In vitro experiments directly examined neurodegenerative effects of ceramides. Results: Chronic HFD feeding gradually increased body weight, but after 16 weeks, liver weight surged (P<0.001) due to triglyceride accumulation (P<0.001), and brain weight declined (P<0.0001). HFD increased pro-ceramide gene expression in liver (P<0.05-P<0.001), but not brain. Temporal lobes of HFD fed mice had increased ubiquitin (P<0.001) and 4-hydroxynonenal (P<0.05 or P<0.01), and decreased tau, β-actin, and choline acetyltransferase levels (P<0.05-P<0.001) with development of NASH. Ceramide treatment of neuronal cultures caused cell death, oxidative stress, mitochondrial dysfunction, and insulin resistance. Conclusions: In obesity, T2DM, or NASH, excess hepatic production of neurotoxic ceramides that readily cross the blood-brain barrier causes cognitive impairment with brain insulin resistance via a liver-brain axis of neurodegeneration.
Sporadic Alzheimer's disease has been proposed to start with an insulin-resistant brain state (IRBS). We investigated the effect of IRBS induced by intracerebroventricularly (icv) administered streptozotocin (STZ) on behavior, glycogen synthase kinase-3 (GSK)alpha/beta content, and the formation of AD-like morphological hallmarks Abeta-amyloid and tau protein in amyloid precursor protein (APP) tg2576 mice. Nine-month-old tg mice were investigated 6 months after a single icv injection of STZ or placebo. Spatial cognition was analyzed using the Morris water maze test. Soluble and aggregated Abeta40/42 fragments, total and phosphorylated tau protein, and GSK-3alpha/beta were determined by ELISA. Cerebral (immuno)histological analyses were performed. In tg mice, STZ treatment increased mortality, reduced spatial cognition, and increased cerebral aggregated Abeta fragments, total tau protein, and congophilic amyloid deposits. These changes were associated with decreased GSK-3alpha/beta ratio (phosphorylated/total). A linear negative correlation was detected between Abeta42 and cognition, and between GSK-3alpha/beta ratio and aggregated Abeta40+42. No marked necrotic and apoptotic changes were observed. In conclusion, IRBS may aggravate AD-like changes such as behavioral and increase the formation of pathomorphological AD hallmarks via GSK-3alpha/beta pathway in APP-overexpressing mice.
Although the exact cause of Alzheimer's disease remains elusive, many possible risk factors and pathological alterations have been used in the elaboration of in vitro and in vivo models of this disease in rodents, including intracerebral (ICV) infusion of streptozotocin (STZ). Using this model, we evaluated spatial cognitive deficit and neurochemical hippocampal alterations, particularly astroglial protein markers such as glial fibrillary acidic protein (GFAP) and S100B, glutathione content, NO production and cerebrospinal fluid (CSF) S100B; in addition, prevention of these alterations by aminoguanidine administration was evaluated. Results confirm a spatial cognitive deficit and nitrative stress in this dementia model as well as specific astroglial alterations, particularly S100B accumulation in the hippocampus and decreased CSF S100B. The hippocampal astroglial activation occurred independently of the significant alteration in GFAP content. Moreover, all these alterations were completely prevented by aminoguanidine administration, confirming the neuroprotective potential of this compound, but suggesting that nitrative stress and/or glycation may be underlying these alterations. Findings contribute to the understanding of diseases accompanied by cognitive deficits and the STZ-model of dementia.
The success of strategies that therapeutically exploit immune responses to self antigens such as those expressed on melanoma cells to treat patients suffering from melanoma is well established. In transgenic Alzheimer's disease (AD) animal models, elimination of β-amyloid (Aβ) pathology using Aβ immunization galvanized the AD research community a decade ago. Using an animal model of AD with closer parallels to human AD, the cholesterolemic rabbit model, we tested immunotherapeutic strategies that could maximize humoral immune responses while minimizing proinflammatory responses. The finding that some patients with AD enrolled in a clinical trial to inoculate against A experienced a misdirected polarization of Th cells reminds us that our knowledge of T cell biology, immune regulation, and the precise functional properties of adjuvants is incomplete. In this article, we review this knowledge and consider the advantages of the rabbit for immunological studies. The langomorph species is proximate to primates on the phylogenetic scale, its amino acid sequence of A is identical to the human A sequence, and the rabbit model system is extensively characterized on a form of associative learning with parallels in normal aging in rabbits and humans that is severely impaired in human AD. Cholesterol-fed rabbits treated with Aβ immunotherapy generate high titer anti-Aβ responses. The cholesterol-fed rabbit model of AD with its close parallels to human genetics and physiology, along with its validity from molecular to cognitive levels as a model of human AD, provides a promising vehicle for development of immunotherapies.
Therapeutic approaches to the treatment of Alzheimer's disease are focused primarily on the amyloid-β peptide which aggregates to form amyloid deposits in the brain. The amyloid hypothesis states that amyloid is the precipitating factor that results in the other pathologies of Alzheimer's disease. One such therapy that has attracted significant attention is anti-amyloid-β immunotherapy. First described in 1999, immunotherapy uses anti-amyloid-β antibodies to lower brain amyloid levels. Active and passive immunization were shown to lower brain amyloid levels and improve cognition in multiple transgenic mouse models. Mechanisms of action were studied in these mice and revealed a complex set of mechanisms that depended on the type of antibody used. When active immunization advanced to clinical trials a subset of patients developed meningoencephalitis, an event not predicted in mouse studies. It was suspected that a T-cell response due to the type of adjuvant used was the cause. Passive immunization has also advanced to Phase III clinical trials on the basis of successful transgenic mouse studies. Reports from the active immunization clinical trial indicated that, similarly to effects observed in mouse studies, amyloid levels in brain were reduced.
We recently found that moderate consumption of two un-related red wines generate from different grape species, a Cabernet Sauvignon and a muscadine wine that are characterized by distinct component composition of polyphenolic compounds, significantly attenuated the development of Alzheimer's disease (AD)-type brain pathology and memory deterioration in a transgenic AD mouse model. Interestingly, our evidence suggests that the two red wines attenuated AD phenotypes through independent mechanisms. In particular, we previously found that treatment with Cabernet Sauvignon reduced the generation of AD-type beta-amyloid (Aβ) peptides. In contrast, evidence from our present study suggests that muscadine treatment attenuates Aβ neuropathology and Aβ-related cognitive deterioration in Tg2576 mice by interfering with the oligomerization of Aβ molecules to soluble high-molecular-weight Aβ oligomer species that are responsible for initiating a cascade of cellular events resulting in cognitive decline. Collectively, our observations suggest that distinct polyphenolic compounds from red wines may be bioavailable at the organism level and beneficially modulate AD phenotypes through multiple Aβ-related mechanisms. Results from these studies suggest the possibility of developing a “combination” of dietary polyphenolic compounds for AD prevention and/or therapy by modulating multiple Aβ-related mechanisms.
Nerve Growth Factor (NGF) has a great potential for the treatment of Alzheimer's disease. However, the therapeutic administration of NGF represents a significant challenge, due to the difficulty to deliver relevant doses to the brain, in a safe and non-invasive way.
We previously demonstrated the efficacy of a non invasive delivery of NGF to the brain in animal models, by an intranasal route. Recently, topical eye application of NGF was proposed, as an option for the delivery of NGF to the brain. Here, we compare the efficacy of the two delivery routes of hNGF-61, a recombinant traceable form of human NGF, in the mouse neurodegeneration model AD11. The intranasal administration appeared to be significantly more effective than the ocular one, in rescuing the neurodegenerative phenotypic hallmarks in AD11 mice. The ocular administration of hNGF-61 showed a more limited efficacy, even at higher doses. In addition, we used the rescue of the shrinkage of superior cervical ganglia as a parameter to indicate leakage of hNGF-61 in the peripheral blood circulation. We show that only through the intranasal route there are no effects on the peripheral target, suggesting that the ocular administration may be not sufficient to prevent the onset of peripheral side effects, such as pain. Thus, NGF nasal drops represent a viable and effective option to successfully deliver therapeutic NGF to the brain in a non-invasive manner.
The non-competitive NMDA receptor antagonist memantine, currently prescribed for the treatment of Alzheimer's disease, is assumed to prevent the excitotoxicity implicated in neurodegenerative processes. Here, we investigated the actions of memantine on hippocampal function and signalling. In behavioural experiments using the water maze, we observed that memantine (at 2 mg/kg) reversed scopolamine-induced learning deficits in mice. When acutely applied to mouse hippocampal slices, memantine caused a significant upward shift in the population spike input-output relationship at 10 and 100 μM, and a corresponding downward shift in latency, indicative of overall enhanced synaptic transmission. This action was blocked by the muscarinic antagonist scopolamine (10 μM) but not by the NMDA antagonist MK-801 (10 μM) or the GABA antagonist bicuculline (20 μM). Further, memantine occluded potentiation induced by 50 nM carbachol (CCh), while enhancing inhibitory actions of CCh at 1 μM, suggesting additive actions. As anticipated for an NMDA antagonist, 100 μM (but not 10 μM) memantine also inhibited tetanus-induced long-term potentiation (LTP), and NMDA-induced Ca2+ signals were blocked in cultured hippocampal neurones at 10 μM (by 88%). Overall, our data suggest actions of memantine beyond NMDA receptor antagonism, including stimulating effects on cholinergic signalling via muscarinic receptors. These interactions with the cholinergic system are likely to contribute to memantine's therapeutic potential.
Since acute and chronic administration of the acetylcholinesterase inhibitors (AChE-Is), namely of donepezil, improves cognitive functions in patients afflicted by mild to moderate dementia and reverses memory deficits in experimental models of learning and memory, it seemed interesting to assess the effects of chronic donepezil treatment on cognitive functions in adult rats with forebrain cholinergic depletion. Lesions were performed by means of intracerebroventricular injections of the immunotoxin 192 IgG-saporin. The cognitive functions of lesioned animals treated or not with donepezil were compared with those of intact animals. Cholinergic depletion affected working memory functions, weakened procedural competencies, affected the acquisition of localizing knowledge, and evoked remarkable compulsive and perseverative behaviors. In lesioned animals, chronic donepezil treatment ameliorated localizatory capabilities, performances linked to cognitive flexibility and procedural abilities. Furthermore, it attenuated compulsive deficits. The present data indicate positive effects of chronic donepezil treatment on specific cognitive performances, suggesting that an aimed use of AChE-Is, targeting some symptoms more than others, may be beneficial in the case of cholinergic hypofunction. The animal model used in the present research may provide an efficient method for analyzing cognition-enhancing drugs before clinical trials.