Preserved fronto-striatal plasticity and enhanced procedural learning in a transgenic mouse model of Alzheimer's disease overexpressing mutant hAPPswe

β-amyloid's neurotoxic mechanisms as defined by in vitro microelectrode arrays: a review

Alzheimer's disease is characterized by the aggregation of β-amyloid, a pathological feature believed to drive the neuronal loss and cognitive decline commonly seen in the disease. Given the growing prevalence of this progressive neurodegenerative disease, understanding the exact mechanisms underlying this process has become a top priority. Microelectrode arrays are commonly used for chronic, non-invasive recording of both spontaneous and evoked neuronal activity from diverse in vitro disease models and to evaluate therapeutic or toxic compounds. To date, microelectrode arrays have been used to investigate β-amyloids' toxic effects, β-amyloids role in specific pathological features and to assess pharmacological approaches to treat Alzheimer's disease. The versatility of microelectrode arrays means these studies use a variety of methods and investigate different disease models and brain regions. This review provides an overview of these studies, highlighting their disparities and presenting the status of the current literature. Despite methodological differences, the current literature indicates that β-amyloid has an inhibitory effect on synaptic plasticity and induces network connectivity disruptions. β-amyloid's effect on spontaneous neuronal activity appears more complex. Overall, the literature corroborates the theory that β-amyloid induces neurotoxicity, having a progressive deleterious effect on neuronal signaling and plasticity. These studies also confirm that microelectrode arrays are valuable tools for investigating β-amyloid pathology from a functional perspective, helping to bridge the gap between cellular and network pathology and disease symptoms. The use of microelectrode arrays provides a functional insight into Alzheimer's disease pathology which will aid in the development of novel therapeutic interventions.

Choline supplementation in early life improves and low levels of choline can impair outcomes in a mouse model of Alzheimer's disease

Maternal choline supplementation (MCS) improves cognition in Alzheimer's disease (AD) models. However, the effects of MCS on neuronal hyperexcitability in AD are unknown. We investigated the effects of MCS in a well-established mouse model of AD with hyperexcitability, the Tg2576 mouse. The most common type of hyperexcitability in Tg2576 mice are generalized EEG spikes (interictal spikes [IIS]). IIS also are common in other mouse models and occur in AD patients. In mouse models, hyperexcitability is also reflected by elevated expression of the transcription factor ∆FosB in the granule cells (GCs) of the dentate gyrus (DG), which are the principal cell type. Therefore, we studied ΔFosB expression in GCs. We also studied the neuronal marker NeuN within hilar neurons of the DG because reduced NeuN protein expression is a sign of oxidative stress or other pathology. This is potentially important because hilar neurons regulate GC excitability. Tg2576 breeding pairs received a diet with a relatively low, intermediate, or high concentration of choline. After weaning, all mice received the intermediate diet. In offspring of mice fed the high choline diet, IIS frequency declined, GC ∆FosB expression was reduced, and hilar NeuN expression was restored. Using the novel object location task, spatial memory improved. In contrast, offspring exposed to the relatively low choline diet had several adverse effects, such as increased mortality. They had the weakest hilar NeuN immunoreactivity and greatest GC ΔFosB protein expression. However, their IIS frequency was low, which was surprising. The results provide new evidence that a diet high in choline in early life can improve outcomes in a mouse model of AD, and relatively low choline can have mixed effects. This is the first study showing that dietary choline can regulate hyperexcitability, hilar neurons, ΔFosB, and spatial memory in an animal model of AD.

Choline supplementation in early life improves and low levels of choline can impair outcomes in a mouse model of Alzheimer's disease

Maternal choline supplementation (MCS) improves cognition in Alzheimer's disease (AD) models. However, effects of MCS on neuronal hyperexcitability in AD are unknown. We investigated effects of MCS in a well-established mouse model of AD with hyperexcitability, the Tg2576 mouse. The most common type of hyperexcitability in Tg2576 mice are generalized EEG spikes (interictal spikes; IIS). IIS also are common in other mouse models and occur in AD patients. Im mouse models, hyperexcitability is also reflected by elevated expression of the transcription factor ΔFosB in the granule cells (GCs) of the dentate gyrus (DG), which are the principal cell type. Therefore we studied ΔFosB expression in GCs. We also studied the the neuronal marker NeuN within hilar neurons of the DG because other studies have reduced NeuN protein expression is a sign of oxidative stress or other pathology. This is potentially important because hilar neurons regulate GC excitability. Tg2576 breeding pairs received a diet with a relatively low, intermediate or high concentration of choline. After weaning, all mice received the intermediate diet. In offspring of mice fed the high choline diet, IIS frequency declined, GC ΔFosB expression was reduced, and NeuN expression was restored. Using the novel object location task, spatial memory improved. In contrast, offspring exposed to the relatively low choline diet had several adverse effects, such as increased mortality. They had the weakest hilar NeuN immunoreactivity and greatest GC ΔFosB protein expression. However, their IIS frequency was low, which was surprising. The results provide new evidence that a diet high in choline in early life can improve outcomes in a mouse model of AD, and relatively low choline can have mixed effects. This is the first study showing that dietary choline can regulate hyperexcitability, hilar neurons, ΔFosB and spatial memory in an animal model of AD.

Abnormal functional connectivity of the frontostriatal circuits in type 2 diabetes mellitus

Background

Type 2 diabetes mellitus (T2DM) is a metabolic disorder associated with an increased incidence of cognitive and emotional disorders. Previous studies have indicated that the frontostriatal circuits play a significant role in brain disorders. However, few studies have investigated functional connectivity (FC) abnormalities in the frontostriatal circuits in T2DM.

Objective

We aimed to investigate the abnormal functional connectivity (FC) of the frontostriatal circuits in patients with T2DM and to explore the relationship between abnormal FC and diabetes-related variables.

Methods

Twenty-seven patients with T2DM were selected as the patient group, and 27 healthy peoples were selected as the healthy controls (HCs). The two groups were matched for age and sex. In addition, all subjects underwent resting-state functional magnetic resonance imaging (rs-fMRI) and neuropsychological evaluation. Seed-based FC analyses were performed by placing six bilateral pairs of seeds within a priori defined subdivisions of the striatum. The functional connection strength of subdivisions of the striatum was compared between the two groups and correlated with each clinical variable.

Results

Patients with T2DM showed abnormalities in the FC of the frontostriatal circuits. Our findings show significantly reduced FC between the right caudate nucleus and left precentral gyrus (LPCG) in the patients with T2DM compared to the HCs. The FC between the prefrontal cortex (left inferior frontal gyrus, left frontal pole, right frontal pole, and right middle frontal gyrus) and the right caudate nucleus has a significant positive correlation with fasting blood glucose (FBG).

Conclusion

The results showed abnormal FC of the frontostriatal circuits in T2DM patients, which might provide a new direction to investigate the neuropathological mechanisms of T2DM.

Dysfunction of striatal MeCP2 is associated with cognitive decline in a mouse model of Alzheimer's disease

Rationale: Cerebral Methyl-CpG binding Protein 2 (MeCP2) is involved in several psychiatric disorders that are concomitant with cognitive dysfunction. However, the regulatory function of striatal MeCP2 and its association with Alzheimer's disease (AD) has been largely neglected due to the absence of amyloid plaque accumulation in the striatal region until the later stages of AD progression. Considerable evidence indicates that neuropsychiatric symptoms related to cognitive decline are involved with striatal dysfunction. To this respect, we investigated the epigenetic function of striatal MeCP2 paralleling the pathogenesis of AD. Methods: We investigated the brain from amyloid precursor protein (APP)/presenilin1 (PS1) transgenic mice and postmortem brain samples from normal subjects and AD patients. The molecular changes in the brain, particularly in the striatal regions, were analyzed with thioflavin S staining, immunohistochemistry, immunoblotting, and MeCP2 chromatin immunoprecipitation sequencing (ChIP-seq). The cognitive function of APP/PS1 mice was assessed via three behavioral tests: 3-chamber test (3CT), Y-maze test (YMT), and passive avoidance test (PA). A multi-electrode array (MEA) was performed to analyze the neuronal activity of the striatum in APP/PS1 mice. Results: Striatal MeCP2 expression was increased in the younger (6 months) and older (10 months) ages of APP/PS1 mice, and the genome-wide occupancy of MeCP2 in the younger APP/PS1 showed dysregulated binding patterns in the striatum. Additionally, we confirmed that APP/PS1 mice showed behavioral deficits in multiple cognitive behaviors. Notably, defective cognitive phenotypes and abnormal neuronal activity in old APP/PS1 mice were rescued through the knock-down of striatal MeCP2. Conclusion: We found that the MeCP2-mediated dysregulation of the epigenome in the striatum is linked to the defects in cognitive behavior and neuronal activity in the AD animal model, and that this alteration is initiated even in the very early stages of AD pathogenesis. Together, our data indicates that MeCP2 may be a potential target for the diagnosis and treatment of AD at asymptomatic and symptomatic stages.

Place vs. Response Learning: History, Controversy, and Neurobiology

The present article provides a historical review of the place and response learning plus-maze tasks with a focus on the behavioral and neurobiological findings. The article begins by reviewing the conflict between Edward C. Tolman's cognitive view and Clark L. Hull's stimulus-response (S-R) view of learning and how the place and response learning plus-maze tasks were designed to resolve this debate. Cognitive learning theorists predicted that place learning would be acquired faster than response learning, indicating the dominance of cognitive learning, whereas S-R learning theorists predicted that response learning would be acquired faster, indicating the dominance of S-R learning. Here, the evidence is reviewed demonstrating that either place or response learning may be dominant in a given learning situation and that the relative dominance of place and response learning depends on various parametric factors (i.e., amount of training, visual aspects of the learning environment, emotional arousal, et cetera). Next, the neurobiology underlying place and response learning is reviewed, providing strong evidence for the existence of multiple memory systems in the mammalian brain. Research has indicated that place learning is principally mediated by the hippocampus, whereas response learning is mediated by the dorsolateral striatum. Other brain regions implicated in place and response learning are also discussed in this section, including the dorsomedial striatum, amygdala, and medial prefrontal cortex. An exhaustive review of the neurotransmitter systems underlying place and response learning is subsequently provided, indicating important roles for glutamate, dopamine, acetylcholine, cannabinoids, and estrogen. Closing remarks are made emphasizing the historical importance of the place and response learning tasks in resolving problems in learning theory, as well as for examining the behavioral and neurobiological mechanisms of multiple memory systems. How the place and response learning tasks may be employed in the future for examining extinction, neural circuits of memory, and human psychopathology is also briefly considered.

Acrobatic exercise recovers object recognition memory impairment in hypoxic-ischemic rats

Neonatal hypoxia-ischemia (HI) can lead to cognitive impairments and motor dysfunction. Acrobatic exercises (AE) were proposing as therapeutic option to manage HI motor deficits, however, the cognitive effects after this treatment are still poorly understood. Therefore, we evaluated the effects of AE protocol on memory impairments and brain plasticity markers after Rice-Vannucci HI rodent model. Wistar rats on the 7th postnatal day (PND) were submitted to HI model and after weaning (PND22) were trained for 5 weeks with AE protocol, then subsequently submitted to cognitive tests. Our results showed recovery in novel object recognition (NOR) memory, but not, spatial Morris Water Maze (WM) memory after AE treatment in HI rats. BDNF and synaptophysin neuroplasticity markers indicate plastic alterations in the hippocampus and striatum, with maintenance of synaptophysin despite the reduction of total volume tissue, besides, hippocampal HI-induced ipsilateral BDNF increased, and striatum contralateral BDNF decreased were noted. Nevertheless, the exercise promoted functional recovery and seems to be a promising strategy for HI treatment, however, future studies identifying neuroplastic pathway for this improvement are needed.

Behavioral facilitation after hippocampal lesion: A review

When parts of the brain suffer from damage, certain functional deficits or impairments are the expected and typical outcome. A myriad of examples show such negative consequences, which afford the daily tasks of neurologists, neuropsychologists, and also behavioral neuroscientists working with experimental brain lesions. Compared to lesion-induced deficits, examples for functional enhancements or facilitation after brain lesions are rather rare and usually not well studied. Here, the mammalian hippocampus seems to provide an exception, since substantial evidence shows that its damage can have facilitatory behavioral effects under certain conditions. This review will address these effects and their possible mechanisms. It will show that facilitatory effects of hippocampal lesions, although mostly studied in rats, can be found in many mammalian species, that is, they are apparently not species-specific. Furthermore, they can be found with various lesion techniques, from tissue ablation, to neurotoxic damage, and from damage of hippocampal structure itself to damage of fiber systems innervating it. The major emphasis of this review, however, lies on the behavioral effects and their interpretations. Thus, facilitatory effects can be found in several learning paradigms, especially active avoidance, and some forms of Pavlovian and instrumental conditioning. These will be discussed in light of pertinent theories of hippocampal function, such as inhibition, spatial cognition, and multiple memory systems theories, which state that facilitatory effects of hippocampal lesions may reflect the loss of interference between hippocampal spatial and striatal procedural cognition. Using the example of the rat sequential reaction time task, it will also be discussed how such lesions can have direct and indirect consequences on certain behavioral readouts. A final note will advocate considering possible functional facilitation also in neurologic patients, especially those with hippocampal damage, since such a strategy might provide new avenues for therapeutic treatments.

Mice Lacking GPR88 Show Motor Deficit, Improved Spatial Learning, and Low Anxiety Reversed by Delta Opioid Antagonist

BACKGROUND

GPR88 is an orphan G protein coupled receptor highly enriched in the striatum, and previous studies have focused on GPR88 function in striatal physiology. The receptor is also expressed in other brain areas, and here we examined whether GPR88 function extends beyond striatal-mediated responses.

METHODS

We created Gpr88 knockout mice and examined both striatal and extrastriatal regions at molecular and cellular levels. We also tested striatum-, hippocampus-, and amygdala-dependent behaviors in Gpr88(-/-) mice using extensive behavioral testing.

RESULTS

We found increased G protein coupling for delta opioid receptor (DOR) and mu opioid, but not other Gi/o coupled receptors, in the striatum of Gpr88 knockout mice. We also found modifications in gene transcription, dopamine and serotonin contents, and dendritic morphology inside and outside the striatum. Behavioral testing confirmed striatal deficits (hyperactivity, stereotypies, motor impairment in rotarod). In addition, mutant mice performed better in spatial tasks dependent on hippocampus (Y-maze, novel object recognition, dual solution cross-maze) and also showed markedly reduced levels of anxiety (elevated plus maze, marble burying, novelty suppressed feeding). Strikingly, chronic blockade of DOR using naltrindole partially improved motor coordination and normalized spatial navigation and anxiety of Gpr88(-/-) mice.

CONCLUSIONS

We demonstrate that GPR88 is implicated in a large repertoire of behavioral responses that engage motor activity, spatial learning, and emotional processing. Our data also reveal functional antagonism between GPR88 and DOR activities in vivo. The therapeutic potential of GPR88 therefore extends to cognitive and anxiety disorders, possibly in interaction with other receptor systems.

Tissue-Plasminogen Activator Attenuates Alzheimer's Disease-Related Pathology Development in APPswe/PS1 Mice

Alzheimer's disease (AD) is the leading cause of dementia among elderly population. AD is characterized by the accumulation of beta-amyloid (Aβ) peptides, which aggregate over time to form amyloid plaques in the brain. Reducing soluble Aβ levels and consequently amyloid plaques constitute an attractive therapeutic avenue to, at least, stabilize AD pathogenesis. The brain possesses several mechanisms involved in controlling cerebral Aβ levels, among which are the tissue-plasminogen activator (t-PA)/plasmin system and microglia. However, these mechanisms are impaired and ineffective in AD. Here we show that the systemic chronic administration of recombinant t-PA (Activase rt-PA) attenuates AD-related pathology in APPswe/PS1 transgenic mice by reducing cerebral Aβ levels and improving the cognitive function of treated mice. Interestingly, these effects do not appear to be mediated by rt-PA-induced plasmin and matrix metalloproteinases 2/9 activation. We observed that rt-PA essentially mediated a slight transient increase in the frequency of patrolling monocytes in the circulation and stimulated microglia in the brain to adopt a neuroprotective phenotype, both of which contribute to Aβ elimination. Our study unraveled a new role of rt-PA in maintaining the phagocytic capacity of microglia without exacerbating the inflammatory response and therefore might constitute an interesting approach to stimulate the key populations of cells involved in Aβ clearance from the brain.

Delta Opioid Receptors: Learning and Motivation

Delta opioid receptor (DOR) displays a unique, highly conserved, structure and an original pattern of distribution in the central nervous system, pointing to a distinct and specific functional role among opioid peptide receptors. Over the last 15 years, in vivo pharmacology and genetic models have allowed significant advances in the understanding of this role. In this review, we will focus on the involvement of DOR in modulating different types of hippocampal- and striatal-dependent learning processes as well as motor function, motivation, and reward. Remarkably, DOR seems to play a key role in balancing hippocampal and striatal functions, with major implications for the control of cognitive performance and motor function under healthy and pathological conditions.

Optimization of touchscreen-based behavioral paradigms in mice: implications for building a battery of tasks taxing learning and memory functions

Although many clinical pathological states are now detectable using imaging and biochemical analyses, neuropsychological tests are often considered as valuable complementary approaches to confirm diagnosis, especially for disorders like Alzheimer’s or Parkinson’s disease, and schizophrenia. The touchscreen-based automated test battery, which was introduced two decades ago in humans to assess cognitive functions, has recently been successfully back-translated in monkeys and rodents. We focused on optimizing the protocol of three distinct behavioral paradigms in mice: two variants of the Paired Associates Learning (PAL) and the Visuo-Motor Conditional Learning (VMCL) tasks. Acquisition of these tasks was assessed in naive versus pre-trained mice. In naive mice, we managed to define testing conditions allowing significant improvements of learning performances over time in the three aforementioned tasks. In pre-trained mice, we observed differential acquisition rates after specific task combinations. Particularly, we identified that animals previously trained in the VMCL paradigm subsequently poorly learned the sPAL rule. Together with previous findings, these data confirm the feasibility of using such behavioral assays to evaluate the power of different models of cognitive dysfunction in mice. They also highlight the risk of interactions between tasks when rodents are run through a battery of different cognitive touchscreen paradigms.

Epigenetic suppression of neuroligin 1 underlies amyloid-induced memory deficiency

Amyloid-induced microglial activation and neuroinflammation impair central synapses and memory function, although the mechanism remains unclear. Neuroligin 1 (NLGN1), a postsynaptic protein found in central excitatory synapses, governs excitatory synaptic efficacy and plasticity in the brain. Here we found, in rodents, that amyloid fibril-induced neuroinflammation enhanced the interaction between histone deacetylase 2 and methyl-CpG-binding protein 2, leading to suppressed histone H3 acetylation and enhanced cytosine methylation in the Nlgn1 promoter region and decreased NLGN1 expression, underlying amyloid-induced memory deficiency. Manipulation of microglia-associated neuroinflammation modulated the epigenetic modification of the Nlgn1 promoter, hippocampal glutamatergic transmission and memory function. These findings link neuroinflammation, synaptic efficacy and memory, thus providing insight into the pathogenesis of amyloid-associated diseases.

Impaired hippocampal acetylcholine release parallels spatial memory deficits in Tg2576 mice subjected to basal forebrain cholinergic degeneration

The Alzheimer's disease (AD) mouse model Tg2576 overexpresses an AD associated mutant variant of human APP and accumulates amyloid beta (Aβ) in an age-dependent manner. Using the selective cholinergic immunotoxin mu p75-saporin (SAP), we induced a partial basal forebrain cholinergic degeneration (BFCD) in 3 months old male Tg2576 mice to co-express cholinergic degeneration with Aβ overexpression as these characteristics constitutes key hallmarks of AD. At 9 months, SAP lesioned Tg2576 mice were cognitively impaired in two spatial paradigms addressing working memory and mid to long-term memory. Conversely, there was no deterioration of cognitive functioning in sham lesioned Tg2576 mice or wild type littermates (wt) receiving the immunotoxin. At 10 months of age, release of acetylcholine (ACh) was addressed by microdialysis in conscious mice. Scopolamine-induced increases in hippocampal ACh efflux was significantly reduced in SAP lesioned Tg2576 mice compared to sham lesioned Tg2576 mice. Intriguingly, there was no significant difference in ACh efflux between wt treatment groups. Following SAP treatment, choline acetyltransferase activity was reduced in the hippocampus and frontal cortex and the reduction was comparable between groups. Our results suggest that partial BFCD acts collectively with increased levels of Aβ to induce cognitive decline and to compromise cholinergic release. Tg2576 mice with BFCD may constitute a new and suitable AD mouse model to study the interrelations between cholinergic deficits and amsyloid deposition.

Age-dependent changes in hippocampal synaptic transmission and plasticity in the PLB1Triple Alzheimer mouse

Several genetically engineered models exist that mimic aspects of the pathological and cognitive hallmarks of Alzheimer's disease (AD). Here we report on a novel mouse model generated by targeted knock-in of transgenes containing mutated human amyloid precursor protein (APP) and microtubule-associated protein tau genes, inserted into the HPRT locus and controlled by the CaMKIIα regulatory element. These mice were crossed with an asymptomatic presenilin1A246E overexpressing line to generate PLB1Triple mice. Gene expression analysis and in situ hybridization confirmed stable, forebrain-specific, and gene-dose-dependent transgene expression. Brain tissue harvested from homozygous, heterozygous, and wild-type cohorts aged between 3 and 24 months was analyzed immunohistochemically and electrophysiologically. Homozygous PLB1Triple offspring presented with mostly intracellular cortical and hippocampal human APP/amyloid, first detected reliably at 6 months. Human tau was already uncovered at 3 months (phospho-tau at 6 months) and labeling intensifying progressively with age. Gene-dose dependence was confirmed in age-matched heterozygous females that accumulated less tau and amyloid protein. General excitability of hippocampal neurones was not altered in slices from PLB1Triple mice up to 12 months, but 2-year-old homozygous PLB1Triple mice had smaller synaptically evoked postsynaptic potentials compared with wild types. Synaptic plasticity (paired-pulse depression/facilitation and long-term potentiation) of synaptic CA1 pyramidal cell responses was deficient from 6 months of age. Long-term depression was not affected at any age or in any genotype. Therefore, despite comparatively subtle gene expression and protein build-up, PLB1Triple mice develop age-dependent progressive phenotypes, suggesting that aggressive protein accumulation is not necessary to reconstruct endophenotypes of AD.

Impaired hippocampus-dependent and facilitated striatum-dependent behaviors in mice lacking the δ opioid receptor

Pharmacological data suggest that delta opioid receptors modulate learning and memory processes. In the present study, we investigated whether inactivation of the delta opioid receptor modifies hippocampus (HPC)- and striatum-dependent behaviors. We first assessed HPC-dependent learning in mice lacking the receptor (Oprd1(-/-) mice) or wild-type (WT) mice treated with the delta opioid antagonist naltrindole using novel object recognition, and a dual-solution cross-maze task. Second, we subjected mutant animals to memory tests addressing striatum-dependent learning using a single-solution response cross-maze task and a motor skill-learning task. Genetic and pharmacological inactivation of delta opioid receptors reduced performance in HPC-dependent object place recognition. Place learning was also altered in Oprd1(-/-) animals, whereas striatum-dependent response and procedural learning were facilitated. Third, we investigated the expression levels for a large set of genes involved in neurotransmission in both HPC and striatum of Oprd1(-/-) mice. Gene expression was modified for several key genes that may contribute to alter hippocampal and striatal functions, and bias striatal output towards striatonigral activity. To test this hypothesis, we finally examined locomotor effects of dopamine receptor agonists. We found that Oprd1(-/-) and naltrindole-treated WT mice were more sensitive to the stimulant locomotor effect of SKF-81297 (D1/D5), supporting the hypothesis of facilitated striatonigral output. These data suggest, for the first time, that delta receptor activity tonically inhibits striatal function, and demonstrate that delta opioid receptors modulate learning and memory performance by regulating the HPC/striatum balance.

Early impairment in a water-finding test in a longitudinal study of the Tg2576 mouse model of Alzheimer's disease

Behavioral assessments of mouse models of neurodegenerative disorders are useful for investigating the molecular basis of the pathologies of the diseases. Here, we investigated the utility of a water-finding test using a video tracking system as a tool for evaluating cognitive deficits in Alzheimer's disease model mice. Transgenic mice expressing mutant amyloid precursor protein that incorporated the Swedish mutation (Tg2576 mice) were tested for behavioral alterations at 3, 5, 6, or 10 months of age. Tg2576 mice, which are widely used as a model of Alzheimer's disease, exhibited significant cognitive deficits in the water-finding test as early as 5 months of age. The impairments progressively worsened at 6 and 10 months of age. In addition, we analyzed spontaneous physical activities, such as locomotor activity, in the home-cage environment with an automated video analysis system (HomeCageScan). Our longitudinal study revealed that spontaneous behavior was altered in the Tg2576 mice, starting at the age of 10 months. Impairment in the Morris water maze (MWM) task was also first observed in the Tg2576 mice at the age of 10 months. These results indicated that the ability to perform the water-finding test was more susceptible to age-related cognitive deterioration in Tg2576 mice than the MWM test. We therefore propose that the water-finding test is a rapid and sensitive method that can be used to assess cognitive and/or behavioral deficits in mouse models of Alzheimer's disease.

APP transgenic mice for modelling behavioural and psychological symptoms of dementia (BPSD)

The discovery of gene mutations responsible for autosomal dominant Alzheimer's disease has enabled researchers to reproduce in transgenic mice several hallmarks of this disorder, notably Aβ accumulation, though in most cases without neurofibrillary tangles. Mice expressing mutated and wild-type APP as well as C-terminal fragments of APP exhibit variations in exploratory activity reminiscent of behavioural and psychological symptoms of Alzheimer dementia (BPSD). In particular, open-field, spontaneous alternation, and elevated plus-maze tasks as well as aggression are modified in several APP transgenic mice relative to non-transgenic controls. However, depending on the precise murine models, changes in open-field and elevated plus-maze exploration occur in either direction, either increased or decreased relative to controls. It remains to be determined which neurotransmitter changes are responsible for this variability, in particular with respect to GABA, 5HT, and dopamine.

Cognitive and neural determinants of response strategy in the dual-solution plus-maze task

Response strategy in the dual-solution plus maze is regarded as a form of stimulus-response learning. In this study, by using an outcome devaluation procedure, we show that it can be based on both action-outcome and stimulus-response habit learning, depending on the amount of training that the animals receive. Furthermore, we show that deactivation of the dorso-medial and the dorso-lateral striatum with Botulinum neurotoxin A, mimicked or abolished, respectively, the effects of practice on the sensitivity of the response strategy to outcome devaluation. These findings have relevant implications for the understanding of the learning mechanisms underlying different overt behaviors in this widely used maze task.

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.

Significant structural but not physiological changes in cortical neurons of 12-month-old Tg2576 mice

Amyloid-beta (Abeta) plays a key role in the etiology of Alzheimer's disease, and pyramidal cell dendrites exposed to Abeta exhibit dramatic structural alterations, including reduced dendritic spine densities. To determine whether such structural alterations lead to electrophysiological changes, whole-cell patch clamp recordings with biocytin filling were used to assess both the electrophysiological and morphological properties of layer 3 pyramidal cells in frontal cortical slices prepared from 12-month-old Tg2576 amyloid precursor protein (APP) mutant vs. wild-type (Wt) mice. Tg2576 cells exhibited significantly increased dendritic lengths and volumes and decreased spine densities, while the total number of spines was not different from Wt. Tg2576 and Wt cells did not differ with regard to passive membrane, action potential firing or glutamatergic spontaneous excitatory postsynaptic current properties. Thus, overexpression of mutated APP in young Tg2576 mice leads to significant changes in neuronal morphological properties which do not have readily apparent functional consequences.

Parallel striatal and hippocampal systems for landmarks and boundaries in spatial memory

How the memory systems centered on the hippocampus and dorsal striatum interact to support behavior remains controversial. We used functional MRI while people learned the locations of objects by collecting and replacing them over multiple trials within a virtual environment comprising a landmark, a circular boundary, and distant cues for orientation. The relative location of landmark and boundary was occasionally changed, with specific objects paired with one or other cue, allowing dissociation of learning and performance relative to either cue. Right posterior hippocampal activation reflected learning and remembering of boundary-related locations, whereas right dorsal striatal activation reflected learning and remembering of landmark-related locations. Within the right hippocampus, anterior processing of environmental change (spatial novelty) was dissociated from posterior processing of location. Behavioral studies show that landmark-related learning obeys associative reinforcement, whereas boundary-related learning is incidental [Doeller CF, Burgess N (2008) Proc Natl Acad Sci USA 105:5909-5914]. The distinct incidental hippocampal processing of boundaries is suggestive of a "geometric module" or "cognitive map" and may explain the hippocampal support of incidental/observational learning in "declarative" or "episodic" memory versus the striatal support of trial-and-error learning in "procedural" memory. Finally, the hippocampal and striatal systems appear to combine "bottom-up," simply influencing behavior proportional to their activations, without direct interaction, with "top-down" ventromedial prefrontal involvement when both are similarly active.

Alzheimer disease models and human neuropathology: similarities and differences

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

Impairment of spatial memory consolidation in APP(751SL) mice results in cue-guided response

APP(751SL) mice of 5-6- and 7-8-month-old and their wild-type littermates were submitted to one-session learning in a water-maze with three levels of training (4, 12 or 22 trials). Training consisted in finding a submerged platform with a fixed location and marked by a cue. During testing two platforms were presented: one consistent with the spatial location allowing place-response (PR) and the other signaled by the cue enabling cued-response (CR). When testing occurred 24h after training, wild-type and 5-6-month-old APP(751SL) mice exhibited a shift in response strategy as a function of training level, by executing CR when trained with 4 trials and PR when trained with 12 trials, but 7-8-month-old APP(751SL) mice executed only CR. However, they displayed PR when tested 1h after 12- and 22-trial, suggesting a consolidation deficit. Zif268 imaging showed plasticity impairment of the hippocampal-dependent memory system but not of the dorsolateral caudate nucleus. Moreover, in these APP(751SL) mice, the deficit selectively affecting hippocampal function cannot be directly related to the onset of beta-amyloid deposits.

Motor impulsivity in APP-SWE mice: a model of Alzheimer's disease

Among transgenic mouse models of Alzheimer's disease, APP-SWE mice have been shown to develop beta-amyloid plaques and to exhibit progressive impairment of cognitive function. Human Alzheimer's disease, however, also includes secondary clinical manifestations, spanning from hyperactivity to agitation. The aim of this study was a better characterization of motor impulsivity in APP-SWE mice, observed at 12 months of age, when levels of soluble beta-amyloid are elevated and beta-amyloid neuritic plaques start to appear. Mice were tested for spatial learning abilities in the Morris water maze (seven daily sessions, four trials per day). The distance traveled to reach the hidden platform showed a learning curve in both groups. This profile, however, was somewhat delayed in APP-SWE mice, thus confirming slightly impaired spatial capacities. To evaluate motor impulsivity, animals were trained to nose-poke for a food reward, which was delivered after a waiting interval that increased over days (15-60 s). Further nose-poking during this signaled waiting interval resulted in food-reward loss and electric-shock punishment. APP-SWE mice received an increased quantity of punishment and were able to earn fewer food rewards, suggesting inability to wait already at the lowest delay. After the animals were killed, prefrontal cortex samples were assessed for neurochemical parameters. Serotonin turnover was elevated in the prefrontal cortex of APP-SWE mice compared with controls. The results clearly confirm cognitive deficits, and are consistent with the hypothesis of reduced behavioral-inhibition abilities. Together with recent findings, APP-SWE mice emerge as a suitable animal model, characterized by a number of specific behavioral alterations, resembling primary and secondary symptoms of human Alzheimer's disease.

Altered navigational strategy use and visuospatial deficits in hAPP transgenic mice

Navigation deficits are prominent in Alzheimer's disease (AD) patients and transgenic mice expressing familial AD-mutant hAPP and A beta peptides. To determine the impact of strategy use on these deficits, we assessed hAPP and nontransgenic mice in a cross maze that can be solved by allocentric (world-based) or egocentric (self-based) strategies. Most nontransgenic mice used allocentric strategies, whereas half of hAPP mice were egocentric. At 3 months, all mice learned the cross maze rapidly; at 6 months, only allocentric hAPP mice were impaired. At 3 and 6 months, hAPP mice had reduced hippocampal Fos expression, which correlated with cross maze learning in older mice. Striatal pCREB expression was unaltered in hAPP mice, suggesting striatal sparing. We conclude that egocentric strategy use may be an earlier indicator of hAPP/A beta-induced hippocampal impairment than spatial learning deficits. Persistent use of allocentric strategies when egocentric strategies are available is maladaptive when there is hippocampal damage. Interventions promoting flexibility in selecting learning strategies might help circumvent otherwise debilitating navigational deficits caused by AD-related hippocampal dysfunction.

Of mice and men: more neurobiology in dementia

PURPOSE OF REVIEW

An increasing number of genetically modified mouse models are designed and used in the field of Alzheimer disease research. This review aims to offer a general view of the existing transgenic mouse lines and to discuss their relevance and limitations.

RECENT FINDINGS

Potential therapeutic targets have been identified in rodent models of Alzheimer disease. Although important steps towards obtaining a safe vaccine to prevent amyloid plaque formation have been made, further evaluations and the use of intermediate models are considered a necessity.

SUMMARY

More than 18 million people worldwide are suffering from Alzheimer disease, the most common dementing disorder in humans. Transgenic lines have been created in order to understand the underlying mechanisms of Alzheimer disease and to find a cure. None of the available models completely recapitulates the characteristics of human pathology, but they provide valuable information on different pathogenic pathways involved. New therapeutic approaches and improvement of current strategies can be obtained from the use of Alzheimer animal models.

Plaques, tangles, and memory loss in mouse models of neurodegeneration

Within the past decade, our understanding of the pathogenic mechanisms in Alzheimer's disease (AD) has dramatically advanced because of the development of transgenic mouse models that recapitulate the key pathological and behavioral phenotypes of the disease. These mouse models have allowed investigators to test detailed questions about how pathology develops and to evaluate potential therapeutic approaches that could slow down the development of this disease. In this review, we discuss the status of transgenic mouse models and review the complex relationship between pathology and behavior in the development of neuropathological syndromes in AD.

Progressive cognitive decline in a transgenic mouse model of Alzheimer's disease overexpressing mutant hAPPswe

The possibility of detecting progressive changes in cognitive function reflecting the spatio-temporal pattern of beta-amyloid peptide (Abeta) deposition was investigated in Tg2576 mice overexpressing the human mutant amyloid precursor protein (hAPP). Here, we show that at 7 months of age, Tg2576 mice exhibited a selective deficit in hippocampus-based operations including a defective habituation of object exploration, a lack of reactivity to spatial novelty and a disruption of allothetic orientation in a cross-shaped maze. At 14 months of age, Tg2576 mice displayed a more extended pattern of behavioral abnormalities, because they failed to react to object novelty and exclusively relied on motor-based orientation in the cross-shaped maze. However, an impaired reactivity to spatial and object novelty possibly reflecting age-related attention deficits also emerged in aged wild-type mice. These findings further underline that early cognitive markers of AD can be detected in Tg2576 mice before Abeta deposition occurs and suggest that as in humans, cognitive deterioration progressively evolves from an initial hippocampal syndrome to global dementia because of the combined effect of the neuropathology and aging.

The role of beta-amyloid protein in synaptic function: implications for Alzheimer's disease therapy

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive and irreversible loss of memory and other cognitive functions. Substantial evidence based on genetic, neuropathological and biochemical data has established the central role of beta-amyloid protein (betaAP) in this pathology. Although the precise etiology of AD is not well understood yet, strong evidence for some of the molecular events that lead to progressive brain dysfunction and neurodegeneration in AD has been afforded by identification of biochemical pathways implicated in the generation of betaAP, development of transgenic models exhibiting progressive disease pathology and by data on the effects of betaAP at the neuronal network level. However, the mechanisms by which betaAP causes cognitive decline have not been determined, nor is it clear if the degree of dementia correlates in time with the degree of neuronal loss. Hence, it is of interest to understand the biochemical processes involved in the mechanisms of betaAP-induced neurotoxicity and the mechanisms involved in electrophysiological effects of this protein on different parameters of synaptic transmission and on neuronal firing properties. In this review we analyze recent evidence suggesting a complex role of betaAP in the molecular events that lead to progressive loss of function and eventually to neurodegeneration in AD as well as the therapeutic implications based on betaAP metabolism inhibition.

Intact spatial learning in adult Tg2576 mice

Tg2576 mice, a transgenic model of amyloid pathology associated with Alzheimer's disease (AD), develop measurable levels of soluble amyloid beta1-40 and 1-42 by 6 months of age and amyloid plaque deposition in cortex, hippocampus and amygdala by 10 months of age. To investigate whether non-hippocampal learning strategies would predominate coincident with the age-related increase in Abeta load in the hippocampal region, we measured learning strategies in the T-maze and a redundant cued version of the water maze. Each of these tasks can be solved using either hippocampal or non-hippocampal learning strategies and has proved sensitive to hippocampal disruption in other settings. The results revealed subtle differences in T-maze and water maze performance in Tg2576 mice compared to controls. Surprisingly, however, Tg2576 mice were not impaired relative to non-transgenic littermates on any measures of hippocampal dependent behavior assessed in these tasks. These data suggest that the medial temporal lobe retains considerable function in 15-month-old Tg2576 mice despite significant Abeta pathology.