Early-stage attenuation of phase-amplitude coupling in the hippocampus and medial prefrontal cortex in a transgenic rat model of Alzheimer's disease

How to design optimal brain stimulation to modulate phase-amplitude coupling?

Objective.Phase-amplitude coupling (PAC), the coupling of the amplitude of a faster brain rhythm to the phase of a slower brain rhythm, plays a significant role in brain activity and has been implicated in various neurological disorders. For example, in Parkinson's disease, PAC between the beta (13-30 Hz) and gamma (30-100 Hz) rhythms in the motor cortex is exaggerated, while in Alzheimer's disease, PAC between the theta (4-8 Hz) and gamma rhythms is diminished. Modulating PAC (i.e. reducing or enhancing PAC) using brain stimulation could therefore open new therapeutic avenues. However, while it has been previously reported that phase-locked stimulation can increase PAC, it is unclear what the optimal stimulation strategy to modulate PAC might be. Here, we provide a theoretical framework to narrow down the experimental optimisation of stimulation aimed at modulating PAC, which would otherwise rely on trial and error.Approach.We make analytical predictions using a Stuart-Landau model, and confirm these predictions in a more realistic model of coupled neural populations.Main results.Our framework specifies the critical Fourier coefficients of the stimulation waveform which should be tuned to optimally modulate PAC. Depending on the characteristics of the amplitude response curve of the fast population, these components may include the slow frequency, the fast frequency, combinations of these, as well as their harmonics. We also show that the optimal balance of energy between these Fourier components depends on the relative strength of the endogenous slow and fast rhythms, and that the alignment of fast components with the fast rhythm should change throughout the slow cycle. Furthermore, we identify the conditions requiring to phase-lock stimulation to the fast and/or slow rhythms.Significance.Together, our theoretical framework lays the foundation for guiding the development of innovative and more effective brain stimulation aimed at modulating PAC for therapeutic benefit.

Impaired Hippocampal Reactivation Preceding Robust Aβ Deposition in a Model of Alzheimer's Disease

Current therapeutic strategies for Alzheimer's disease (AD) target amyloid-beta (Aβ) fibrils and high molecular weight protofibrils associated with plaques, but other bioactive species may directly contribute to neural systems failure in AD. Employing hippocampal electrophysiological recordings and dynamic calcium imaging across the sleep-wake cycle in young mice expressing human Aβ and Aβ oligomers, we reveal marked impairments of hippocampal function long before amyloid plaques predominate. In slow wave sleep (SWS), Aβ increased the proportion of hypoactive cells and reduced place-cell reactivation. During awake behavior, Aβ impaired theta-gamma phase-amplitude coupling (PAC) and drove excessive synchronization of place cell calcium fluctuations with hippocampal theta. Remarkably, the on-line impairment of hippocampal theta-gamma PAC correlated with the SWS impairment of place-cell reactivation. Together, these results identify toxic effects of Aβ on memory encoding and consolidation processes before robust plaque deposition and support targeting soluble Aβ-related species to treat and prevent AD.

Dynamic prediction of goal location by coordinated representation of prefrontal-hippocampal theta sequences

Prefrontal (PFC) and hippocampal (HPC) sequences of neuronal firing modulated by theta rhythms could represent upcoming choices during spatial memory-guided decision-making. How the PFC-HPC network dynamically coordinates theta sequences to predict specific goal locations and how it is interrupted in memory impairments induced by amyloid beta (Aβ) remain unclear. Here, we detected theta sequences of firing activities of PFC neurons and HPC place cells during goal-directed spatial memory tasks. We found that PFC ensembles exhibited predictive representation of the specific goal location since the starting phase of memory retrieval, earlier than the hippocampus. High predictive accuracy of PFC theta sequences existed during successful memory retrieval and positively correlated with memory performance. Coordinated PFC-HPC sequences showed PFC-dominant prediction of goal locations during successful memory retrieval. Furthermore, we found that theta sequences of both regions still existed under Aβ accumulation, whereas their predictive representation of goal locations was weakened with disrupted spatial representation of HPC place cells and PFC neurons. These findings highlight the essential role of coordinated PFC-HPC sequences in successful memory retrieval of a precise goal location.

Septohippocampal cholinergic system at the intersection of stress and cognition: Current trends and translational implications

Deficits in hippocampus-dependent memory processes are common across psychiatric and neurodegenerative disorders such as depression, anxiety and Alzheimer's disease. Moreover, stress is a major environmental risk factor for these pathologies and it exerts detrimental effects on hippocampal functioning via the activation of hypothalamic-pituitary-adrenal (HPA) axis. The medial septum cholinergic neurons extensively innervate the hippocampus. Although, the cholinergic septohippocampal pathway (SHP) has long been implicated in learning and memory, its involvement in mediating the adaptive and maladaptive impact of stress on mnemonic processes remains less clear. Here, we discuss current research highlighting the contributions of cholinergic SHP in modulating memory encoding, consolidation and retrieval. Then, we present evidence supporting the view that neurobiological interactions between HPA axis stress response and cholinergic signalling impact hippocampal computations. Finally, we critically discuss potential challenges and opportunities to target cholinergic SHP as a therapeutic strategy to improve cognitive impairments in stress-related disorders. We argue that such efforts should consider recent conceptualisations on the dynamic nature of cholinergic signalling in modulating distinct subcomponents of memory and its interactions with cellular substrates that regulate the adaptive stress response.

Aberrant Cortical Activity in 5xFAD Mice in Response to Social and Non-Social Olfactory Stimuli

BACKGROUND

Neuroimaging studies investigating the behavioral and psychological symptoms of dementia (BPSD)- such as apathy, anxiety, and depression- have linked some of these symptoms with altered neural activity. However, inconsistencies in operational definitions and rating scales, limited scope of assessments, and poor temporal resolution of imaging techniques have hampered human studies. Many transgenic (Tg) mouse models of Alzheimer's disease (AD) exhibit BPSD-like behaviors concomitant with AD-related neuropathology, allowing examination of how neural activity may relate to BPSD-like behaviors with high temporal and spatial resolution.

OBJECTIVE

To examine task-dependent neural activity in the medial prefrontal cortex (mPFC) of AD-model mice in response to social and non-social olfactory stimuli.

METHODS

We previously demonstrated age-related decreases in social investigation in Tg 5xFAD females, and this reduced social investigation is evident in Tg 5xFAD females and males by 6 months of age. In the present study, we examine local field potential (LFP) in the mPFC of awake, behaving 5xFAD females and males at 6 months of age during exposure to social and non-social odor stimuli in a novel olfactometer.

RESULTS

Our results indicate that Tg 5xFAD mice exhibit aberrant baseline and task-dependent LFP activity in the mPFC- including higher relative delta (1-4 Hz) band power and lower relative power in higher bands, and overall stronger phase-amplitude coupling- compared to wild-type controls.

CONCLUSIONS

These results are consistent with previous human and animal studies examining emotional processing, anxiety, fear behaviors, and stress responses, and suggest that Tg 5xFAD mice may exhibit altered arousal or anxiety.

The Rehabilitation Potential of Neurostimulation for Mild Traumatic Brain Injury in Animal and Human Studies

Neurostimulation carries high therapeutic potential, accompanied by an excellent safety profile. In this review, we argue that an arena in which these tools could provide breakthrough benefits is traumatic brain injury (TBI). TBI is a major health problem worldwide, with the majority of cases identified as mild TBI (mTBI). MTBI is of concern because it is a modifiable risk factor for dementia. A major challenge in studying mTBI is its inherent heterogeneity across a large feature space (e.g., etiology, age of injury, sex, treatment, initial health status, etc.). Parallel lines of research in human and rodent mTBI can be collated to take advantage of the full suite of neuroscience tools, from neuroimaging (electroencephalography: EEG; functional magnetic resonance imaging: fMRI; diffusion tensor imaging: DTI) to biochemical assays. Despite these attractive components and the need for effective treatments, there are at least two major challenges to implementation. First, there is insufficient understanding of how neurostimulation alters neural mechanisms. Second, there is insufficient understanding of how mTBI alters neural function. The goal of this review is to assemble interrelated but disparate areas of research to identify important gaps in knowledge impeding the implementation of neurostimulation.

Gamma-patterned sensory stimulation reverses synaptic plasticity deficits in rat models of early Alzheimer's disease

Non-invasive sensory stimulation in the range of the brain's gamma rhythm (30-100 Hz) is emerging as a new potential therapeutic strategy for the treatment of Alzheimer's disease (AD). Here, we investigated the effect of repeated combined exposure to 40 Hz synchronized sound and light stimuli on hippocampal long-term potentiation (LTP) in vivo in three rat models of early AD. We employed a very complete model of AD amyloidosis, amyloid precursor protein (APP)-overexpressing transgenic McGill-R-Thy1-APP rats at an early pre-plaque stage, systemic treatment of transgenic APP rats with corticosterone modelling certain environmental AD risk factors and, importantly, intracerebral injection of highly disease-relevant AD patient-derived synaptotoxic beta-amyloid and tau in wild-type animals. We found that daily treatment with 40 Hz sensory stimulation for 2 weeks fully abrogated the inhibition of LTP in all three models. Moreover, there was a negative correlation between the magnitude of LTP and the level of active caspase-1 in the hippocampus of transgenic APP animals, which suggests that the beneficial effect of 40 Hz stimulation was dependent on modulation of pro-inflammatory mechanisms. Our findings support ongoing clinical trials of gamma-patterned sensory stimulation in early AD.

Early classification of Alzheimer's disease phenotype based on hippocampal electrophysiology in the TgF344-AD rat model

The hippocampus plays a vital role in navigation, learning, and memory, and is affected in Alzheimer's disease (AD). This study investigated the classification of AD-transgenic rats versus wild-type littermates using electrophysiological activity recorded from the hippocampus at an early, presymptomatic stage of the disease (6 months old) in the TgF344-AD rat model. The recorded signals were filtered into low frequency (LFP) and high frequency (spiking activity) signals, and machine learning classifiers were employed to identify the rat genotype (TG vs. WT). By analyzing specific frequency bands in the low frequency signals and calculating distance metrics between spike trains in the high frequency signals, accurate classification was achieved. Gamma band power emerged as a valuable signal for classification, and combining information from both low and high frequency signals improved the accuracy further. These findings provide valuable insights into the early stage effects of AD on different regions of the hippocampus.

Disrupted Maturation of Prefrontal Layer 5 Neuronal Circuits in an Alzheimer's Mouse Model of Amyloid Deposition

Mutations in genes encoding amyloid precursor protein (APP) and presenilins (PSs) cause familial forms of Alzheimer's disease (AD), a neurodegenerative disorder strongly associated with aging. It is currently unknown whether and how AD risks affect early brain development, and to what extent subtle synaptic pathology may occur prior to overt hallmark AD pathology. Transgenic mutant APP/PS1 over-expression mouse lines are key tools for studying the molecular mechanisms of AD pathogenesis. Among these lines, the 5XFAD mice rapidly develop key features of AD pathology and have proven utility in studying amyloid plaque formation and amyloid β (Aβ)-induced neurodegeneration. We reasoned that transgenic mutant APP/PS1 over-expression in 5XFAD mice may lead to neurodevelopmental defects in early cortical neurons, and performed detailed synaptic physiological characterization of layer 5 (L5) neurons from the prefrontal cortex (PFC) of 5XFAD and wild-type littermate controls. L5 PFC neurons from 5XFAD mice show early APP/Aβ immunolabeling. Whole-cell patch-clamp recording at an early post-weaning age (P22-30) revealed functional impairments; although 5XFAD PFC-L5 neurons exhibited similar membrane properties, they were intrinsically less excitable. In addition, these neurons received smaller amplitude and frequency of miniature excitatory synaptic inputs. These functional disturbances were further corroborated by decreased dendritic spine density and spine head volumes that indicated impaired synapse maturation. Slice biotinylation followed by Western blot analysis of PFC-L5 tissue revealed that 5XFAD mice showed reduced synaptic AMPA receptor subunit GluA1 and decreased synaptic NMDA receptor subunit GluN2A. Consistent with this, patch-clamp recording of the evoked L23>L5 synaptic responses revealed a reduced AMPA/NMDA receptor current ratio, and an increased level of AMPAR-lacking silent synapses. These results suggest that transgenic mutant forms of APP/PS1 overexpression in 5XFAD mice leads to early developmental defects of cortical circuits, which could contribute to the age-dependent synaptic pathology and neurodegeneration later in life.

The hippocampus associated GABAergic neural network impairment in early-stage of Alzheimer's disease

Alzheimer's disease (AD) is the commonest neurodegenerative disease with slow progression. Pieces of evidence suggest that the GABAergic system is impaired in the early stage of AD, leading to hippocampal neuron over-activity and further leading to memory and cognitive impairment in patients with AD. However, the precise impairment mechanism of the GABAergic system on the pathogenesis of AD is still unclear. The impairment of neural networks associated with the GABAergic system is tightly associated with AD. Therefore, we describe the roles played by hippocampus-related GABAergic circuits and their impairments in AD neuropathology. In addition, we give our understand on the process from GABAergic circuit impairment to cognitive and memory impairment, since recent studies on astrocyte in AD plays an important role behind cognition dysfunction caused by GABAergic circuit impairment, which helps better understand the GABAergic system and could open up innovative AD therapy.

Toward the Identification of Neurophysiological Biomarkers for Alzheimer's Disease in Down Syndrome: A Potential Role for Cross-Frequency Phase-Amplitude Coupling Analysis

Cross-frequency coupling (CFC) mechanisms play a central role in brain activity. Pathophysiological mechanisms leading to many brain disorders, such as Alzheimer's disease (AD), may produce unique patterns of brain activity detectable by electroencephalography (EEG). Identifying biomarkers for AD diagnosis is also an ambition among research teams working in Down syndrome (DS), given the increased susceptibility of people with DS to develop early-onset AD (DS-AD). Here, we review accumulating evidence that altered theta-gamma phase-amplitude coupling (PAC) may be one of the earliest EEG signatures of AD, and therefore may serve as an adjuvant tool for detecting cognitive decline in DS-AD. We suggest that this field of research could potentially provide clues to the biophysical mechanisms underlying cognitive dysfunction in DS-AD and generate opportunities for identifying EEG-based biomarkers with diagnostic and prognostic utility in DS-AD.

The TgF344-AD rat: behavioral and proteomic changes associated with aging and protein expression in a transgenic rat model of Alzheimer's disease

Animal models of Alzheimer's Disease (AD) are attractive tools for preclinical, prodromal drug testing. The TgF344-AD (Tg) rat exhibits cognitive deficits and 5 major hallmarks of AD. Here we show that spatial water maze (WMZ) memory deficits and proteomic differences in dorsal CA1 were present in young Tg rats. Aged learning-unimpaired (AU) and aged learning-impaired (AI) proteome associated changes were identified and differed by sex. Levels of phosphorylated tau, reactive astrocytes and microglia were significantly increased in aged Tg rats and correlated with the WMZ learning index (LI); in contrast, no significant correlation was present between amyloid plaques or insoluble Aβ levels and LI. Neuroinflammatory markers were also significantly correlated with LI and increased in female Tg rats. The anti-inflammatory marker, triggering receptor expressed on myeloid cells-2 (TREM2), was significantly reduced in aged impaired Tg rats and correlated with LI. Identifying and understanding mechanisms that allow for healthy aging by overcoming genetic drivers for AD, and/or promoting drivers for successful aging, are important for developing successful therapeutics against AD.

Neural Oscillation Disorder in the Hippocampal CA1 Region of Different Alzheimer's Disease Mice

BACKGROUND

Alzheimer's disease (AD) is a well-known neurodegenerative disease that gradually induces neural network dysfunction and progressive memory deficits. Neural network activity is represented by rhythmic oscillations that influence local field potentials (LFPs). However, changes in hippocampal neural rhythmic oscillations in the early stage of AD remain largely unexplored.

OBJECTIVE

This study investigated neural rhythmic oscillations in 3-month-old APP/PS1 and 5x- FAD mice to assess early neural connectivity in AD.

METHODS

LFPs were recorded from the hippocampal CA1 region with implanted microelectrode arrays while the mice were in the awake resting stage. Welch fast Fourier transforms, continuous wavelet transforms, and phase-amplitude coupling analyses were used to compute the power density of different frequency bands and phase-amplitude modulation indices in the LFPs.

RESULTS

Our results showed impaired theta, low gamma, and high gamma frequency band power in APP/PS1 and 5xFAD mice during the awake resting stage. AD mice also showed decreased delta, alpha, and beta frequency band power. Impaired theta-low gamma and theta-high gamma phaseamplitude coupling were observed in 5xFAD mice.

CONCLUSION

This study revealed neural network activity differences in oscillation power and cross-frequency coupling in the early stage of AD, providing a new perspective for developing biomarkers for early AD diagnosis.

Alterations in theta-gamma coupling and sharp wave-ripple, signs of prodromal hippocampal network impairment in the TgF344-AD rat model

Alzheimer's disease (AD) is a severe neurodegenerative disorder caused by the accumulation of toxic proteins, amyloid-beta (Aβ) and tau, which eventually leads to dementia. Disease-modifying therapies are still lacking, due to incomplete insights into the neuropathological mechanisms of AD. Synaptic dysfunction is known to occur before cognitive symptoms become apparent and recent studies have demonstrated that imbalanced synaptic signaling drives the progression of AD, suggesting that early synaptic dysfunction could be an interesting therapeutic target. Synaptic dysfunction results in altered oscillatory activity, which can be detected with electroencephalography and electrophysiological recordings. However, the majority of these studies have been performed at advanced stages of AD, when extensive damage and cognitive symptoms are already present. The current study aimed to investigate if the hippocampal oscillatory activity is altered at pre-plaque stages of AD. The rats received stereotactic surgery to implant a laminar electrode in the CA1 layer of the right hippocampus. Electrophysiological recordings during two consecutive days in an open field were performed in 4-5-month-old TgF344-AD rats when increased concentrations of soluble Aβ species were observed in the brain, in the absence of Aβ-plaques. We observed a decreased power of high theta oscillations in TgF344-AD rats compared to wild-type littermates. Sharp wave-ripple (SWR) analysis revealed an increased SWR power and a decreased duration of SWR during quiet wake in TgF344-AD rats. The alterations in properties of SWR and the increased power of fast oscillations are suggestive of neuronal hyperexcitability, as has been demonstrated to occur during presymptomatic stages of AD. In addition, decreased strength of theta-gamma coupling, an important neuronal correlate of memory encoding, was observed in the TgF344-AD rats. Theta-gamma phase amplitude coupling has been associated with memory encoding and the execution of cognitive functions. Studies have demonstrated that mild cognitive impairment patients display decreased coupling strength, similar to what is described here. The current study demonstrates altered hippocampal network activity occurring at pre-plaque stages of AD and provides insights into prodromal network dysfunction in AD. The alterations observed could aid in the detection of AD during presymptomatic stages.

Disruption of hippocampal neuronal circuit function depends upon behavioral state in the APP/PS1 mouse model of Alzheimer's disease

The Alzheimer's disease-associated peptide amyloid-beta (Aβ) has been associated with neuronal hyperactivity under anesthesia, but clinical trials of anticonvulsants or neural system suppressors have, so far, failed to improve symptoms in AD. Using simultaneous hippocampal calcium imaging and electrophysiology in freely moving mice expressing human Aβ, here we show that Aβ aggregates perturbed neural systems in a state-dependent fashion, driving neuronal hyperactivity in exploratory behavior and slow wave sleep (SWS), yet suppressing activity in quiet wakefulness (QW) and REM sleep. In exploratory behavior and REM sleep, Aβ impaired hippocampal theta-gamma phase-amplitude coupling and altered neuronal synchronization with theta. In SWS, Aβ reduced cortical slow oscillation (SO) power, the coordination of hippocampal sharp wave-ripples with both the SO and thalamocortical spindles, and the coordination of calcium transients with the sharp wave-ripple. Physostigmine improved Aβ-associated hyperactivity in exploratory behavior and hypoactivity in QW and expanded the range of gamma that coupled with theta phase, but exacerbated hypoactivity in exploratory behavior. Together, these findings show that the effects of Aβ alone on hippocampal circuit function are profoundly state dependent and suggest a reformulation of therapeutic strategies aimed at Aβ induced hyperexcitability.

Hippocampus-prefrontal cortex inputs modulate spatial learning and memory in a mouse model of sepsis induced by cecal ligation puncture

AIMS

Sepsis-associated encephalopathy (SAE) often leads to cognitive impairments. However, the pathophysiology of SAE is complex and unclear. Here, we investigated the role of hippocampus (HPC)-prefrontal cortex (PFC) in cognitive dysfunction in sepsis induced by cecal ligation puncture (CLP) in mice.

METHODS

The neural projections from the HPC to PFC were first identified via retrograde tracing and viral expression. Chemogenetic activation of the HPC-PFC pathway was shown via immunofluorescent staining of c-Fos-positive neurons in PFC. Morris Water Maze (MWM) and Barnes maze (BM) were used to evaluate cognitive function. Western blotting analysis was used to determine the expression of glutamate receptors and related molecules in PFC and HPC.

RESULTS

Chemogenetic activation of the HPC-PFC pathway enhanced cognitive dysfunction in CLP-induced septic mice. Glutamate receptors mediated the effects of HPC-PFC pathway activation in CLP mice. The activation of the HPC-PFC pathway resulted in significantly increased levels of NMDAR, AMPAR, and downstream signaling molecules including CaMKIIa, pCREB, and BDNF in PFC. However, inhibition of glutamate receptors using 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo (F)quinoxaline (NBQX), which is an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR inhibitor), or D-2-amino-5-phosphonopentanoate (D-AP5), which is an NMDA receptor antagonist abolished this increase.

CONCLUSION

Our study reveals the important role of the HPC-PFC pathway in improving cognitive dysfunction in a mouse model of CLP sepsis and provides a novel pathogenetic mechanism for SAE.

Females exhibit higher GluA2 levels and outperform males in active place avoidance despite increased amyloid plaques in TgF344-Alzheimer's rats

Alzheimer's disease (AD) is a progressive neurodegenerative disease that is most prevalent in females. While estrogen provides neuroprotection in females, sex mediated differences in the development of AD pathology are not fully elucidated. Therefore, comparing events between sexes in early-stage AD pathology may reveal more effective therapeutic targets of intervention. To address sex differences, we analyzed early-stage 9-month male and female TgF344-AD (Tg-AD) rats, an AD model carrying the APPswe and Presenilin 1 (PS1ΔE9) mutations that develops progressive age-dependent AD pathology similar to humans. Tg-AD females significantly outperformed Tg-AD males in the active place avoidance (aPAT) test that assesses hippocampal-dependent spatial learning and memory. However, comparisons between Tg-AD male or female rats and their WT counterparts showed significant deficits for female but not male rats. Nevertheless, Tg-AD females experienced significantly less hippocampal neuronal loss with higher GluA2 subunit levels than Tg-AD males. Unexpectedly, Tg-AD females displayed higher levels of hippocampal amyloid plaques than Tg-AD males. Thus, we propose that GluA2 may provide a neuroprotective function for Tg-AD females in our rat model by mitigating cognitive impairment independently of amyloid plaques. Elucidating this protective mechanism in AD could lead to new targets for early intervention.

The Role of Aberrant Neural Oscillations in the Hippocampal-Medial Prefrontal Cortex Circuit in Neurodevelopmental and Neurological Disorders

The hippocampus (HPC) and medial prefrontal cortex (mPFC) have well-established roles in cognition, emotion, and sensory processing. In recent years, interests have shifted towards developing a deeper understanding of the mechanisms underlying interactions between the HPC and mPFC in achieving these functions. Considerable research supports the idea that synchronized activity between the HPC and the mPFC is a general mechanism by which brain functions are regulated. In this review, we summarize current knowledge on the hippocampal-medial prefrontal cortex (HPC-mPFC) circuit in normal brain function with a focus on oscillations and highlight several neurodevelopmental and neurological disorders associated with aberrant HPC-mPFC circuitry. We further discuss oscillatory dynamics across the HPC-mPFC circuit as potentially useful biomarkers to assess interventions for neurodevelopmental and neurological disorders. Finally, advancements in brain stimulation, gene therapy and pharmacotherapy are explored as promising therapies for disorders with aberrant HPC-mPFC circuit dynamics.

Venular amyloid accumulation in transgenic Fischer 344 Alzheimer’s disease rats

Strong evidence demonstrates a significant association between cerebral amyloid angiopathy (CAA) and Alzheimer’s disease (AD). For this reason, interest in understanding the underlying vascular pathologies that contribute to AD remain. CAA research has primarily focused on arterioles and capillaries, overlooking the draining venules. Therefore, this study sought to examine venular amyloid pathology and its relationship to arteriolar amyloidosis throughout AD progression in the TgF344-AD rat model. Antibodies targeting the amyloid-beta peptide (Aβ) sequence suggest morphological differences between arteriolar and venular amyloid. Mass spectrometric analyses of isolated cortical parenchymal plaques, arteriolar and venular amyloid demonstrated presence of Aβ in all three samples, as well as proteins known to be associated with AD. Histopathological analysis indicates a significant age effect for both arteriolar and venular amyloid accumulation, with accumulation initiated in the somatosensory cortex followed by the motor and cingulate cortex. Lastly, significant arteriolar amyloid accumulates relative to venular amyloid deposition in AD progression. Overall, understanding venular and arteriolar amyloid pathology provides insight into the complex connection between CAA and AD.

Network synchronization deficits caused by dementia and Alzheimer's disease serve as topographical biomarkers: a pilot study

Mild cognitive impairment (MCI) is known as an early stage of cognitive decline. Amnestic MCI (aMCI) is considered as the preliminary stage of dementia which may progress to Alzheimer’s disease (AD). While some aMCI patients may stay in this condition for years, others might develop dementia associated with AD. Early detection of MCI allows for potential treatments to prevent or decelerate the process of developing dementia. Standard methods of diagnosing MCI and AD employ structural (imaging), behavioral (cognitive tests), and genetic or molecular (blood or CSF tests) techniques. Our study proposes network-level neural synchronization parameters as topographical markers for diagnosing aMCI and AD. We conducted a pilot study based on EEG data recorded during an olfactory task from a group of elderly participants consisting of healthy individuals and patients of aMCI and AD to assess the value of different indicators of network-level phase and amplitude synchronization in differentiating the three groups. Significant differences were observed in the percent phase locking value, theta-gamma phase-amplitude coupling, and amplitude coherence between the groups, and classifiers were developed to differentiate the three groups based on these parameters. The observed differences in these indicators of network-level functionality of the brain can help explain the underlying processes involved in aMCI and AD.

Early alterations in brain glucose metabolism and vascular function in a transgenic rat model of Alzheimer's disease

Alteration in brain metabolism predates clinical onset of Alzheimer's Disease (AD). Realizing its potential as an early diagnostic marker, however, requires understanding how early AD metabolic dysregulation manifests on non-invasive brain imaging. We presently utilized magnetic resonance imaging and spectroscopy to map glucose and ketone metabolic profiles and image cerebrovascular function in a rat model of early stage AD - 9-month-old TgF344-AD (TgAD) rats - and their age-matched non-transgenic (nTg) littermates. Compared to the nTg rats, TgAD rats displayed attenuation in global cerebral and hippocampal vasoreactivity to hypercapnia, by 49±17% and 58±19%, respectively, while their functional hyperemia to somatosensory stimulation diminished by 69±5%. To assess brain glucose uptake, rats were fasted overnight and then challenged with an intravenous infusion of 2-deoxy-D-glucose (2DG). Compared to their non-transgenic littermates, TgAD rats exhibited 99±10% and 52±5% smaller glucose uptake in the entorhinal cortex and the hippocampus, respectively. Moreover, hippocampal glucose uptake reduction in male TgAD rats compared to the nTg was 54±36% greater than the reduction seen in female TgAD rats. TgAD rats also showed a 59±42% increase in total choline level in the hippocampus, suggesting increased membrane turnover. In combination with our earlier findings of impaired electrophysiological metrics at this early stage of AD pathology progression, our findings suggest that subtle neuronal function alterations that would be difficult to assess in a clinical population may be accompanied by MRI-detectable changes in brain glucose metabolism and cerebrovascular function.

Neuronal Oscillatory Signatures in the Developing Mouse Visual Cortex After Short-Term Monocular Deprivation

Development and maturation in cortical networks depend on neuronal activity. For stabilization and pruning of connections, synchronized oscillations play a crucial role. A fundamental mechanism that enables coordinated activity during brain functioning is formed of synchronized neuronal oscillations in low- (delta and theta) and high- (gamma) frequency bands. The relationship between neural synchrony, cognition, and the perceptual process has been widely studied, but any possible role of neural synchrony in amblyopia has been less explored. We hypothesized that monocular deprivation (MD) during early postnatal life would lead to changes in neuronal activity that would be demonstrated by changes in phase-amplitude coupling (PAC) and altered power in specific oscillatory frequency. Our results demonstrate that functional connectivity in the visual cortex is altered by MD during adolescence. The amplitude of high-frequency oscillations is modulated by the phase of low-frequency oscillations. Demonstration of enhanced delta–gamma and theta–gamma PAC indicates that our results are relevant for a broad range of nested oscillatory markers. These markers are inherent to neuronal processing and are consistent with the hypothesized increase in the intrinsic coupling that arises from neural oscillatory phase alignment. Our results reveal distinct frequency bands exhibit altered power and coherence variations modulated by experience-driven plasticity.

Dentate Granule Cells Are Hyperexcitable in the TgF344-AD Rat Model of Alzheimer's Disease

The dentate gyrus is both a critical gatekeeper for hippocampal signal processing and one of the first brain regions to become dysfunctional in Alzheimer's disease (AD). Accordingly, the appropriate balance of excitation and inhibition through the dentate is a compelling target for mechanistic investigation and therapeutic intervention in early AD. Previously, we reported an increased long-term potentiation (LTP) magnitude at medial perforant path-dentate granule cell (MPP-DGC) synapses in slices from both male and acutely ovariectomized female TgF344-AD rats compared with wild type (Wt) as early as 6 months of age that is accompanied by an increase in steady-state postsynaptic depolarization during the high-frequency stimulation used to induce plasticity. Subsequently, we found that heightened function of β-adrenergic receptors (β-ARs) drives the increase in the LTP magnitude, but the increase in steady-state depolarization was only partially due to β-AR activation. As we previously reported no detectable difference in spine density or presynaptic release probability, we entertained the possibility that DGCs themselves might have modified passive or active membrane properties, which may contribute to the significant increase in charge transfer during high-frequency stimulation. Using brain slice electrophysiology from 6-month-old female rats acutely ovariectomized to eliminate variability due to fluctuating plasma estradiol, we found significant changes in passive membrane properties and active membrane properties leading to increased DGC excitability in TgF344-AD rats. Specifically, TgF344-AD DGCs have an increased input resistance and decreased rheobase, decreased sag, and increased action potential (AP) spike accommodation. Importantly, we found that for the same amount of depolarizing current injection, DGCs from TgF344-AD compared with Wt rats have a larger magnitude voltage response, which was accompanied by a decreased delay to fire the first action potential, indicating TgF344-AD DGCs membranes are more excitable. Taken together, DGCs in TgF344-AD rats are more excitable, which likely contributes to the heightened depolarization during high-frequency synaptic activation.

Neurophysiological effects of human-derived pathological tau conformers in the APPKM670/671NL.PS1/L166P amyloid mouse model of Alzheimer's disease

Alzheimer’s Disease (AD) is a neurodegenerative disease characterized by two main pathological hallmarks: amyloid plaques and intracellular tau neurofibrillary tangles. However, a majority of studies focus on the individual pathologies and seldom on the interaction between the two pathologies. Herein, we present the longitudinal neuropathological and neurophysiological effects of a combined amyloid-tau model by hippocampal seeding of human-derived tau pathology in the APP.PS1/L166P amyloid animal model. We statistically assessed both neurophysiological and pathological changes using linear mixed modelling to determine if factors such as the age at which animals were seeded, genotype, seeding or buffer, brain region where pathology was quantified, and time-post injection differentially affect these outcomes. We report that AT8-positive tau pathology progressively develops and is facilitated by the amount of amyloid pathology present at the time of injection. The amount of AT8-positive tau pathology was influenced by the interaction of age at which the animal was injected, genotype, and time after injection. Baseline pathology-related power spectra and Higuchi Fractal Dimension (HFD) score alterations were noted in APP.PS1/L166P before any manipulations were performed, indicating a baseline difference associated with genotype. We also report immediate localized hippocampal dysfunction in the electroencephalography (EEG) power spectra associated with tau seeding which returned to comparable levels at 1 month-post-injection. Longitudinal effects of seeding indicated that tau-seeded wild-type mice showed an increase in gamma power earlier than buffer control comparisons which was influenced by the age at which the animal was injected. A reduction of hippocampal broadband power spectra was noted in tau-seeded wild-type mice, but absent in APP.PS1 animals. HFD scores appeared to detect subtle effects associated with tau seeding in APP.PS1 animals, which was differentially influenced by genotype. Notably, while tau histopathological changes were present, a lack of overt longitudinal electrophysiological alterations was noted, particularly in APP.PS1 animals that feature both pathologies after seeding, reiterating and underscoring the difficulty and complexity associated with elucidating physiologically relevant and translatable biomarkers of Alzheimer’s Disease at the early stages of the disease.

Parvalbumin neuroplasticity compensates for somatostatin impairment, maintaining cognitive function in Alzheimer's disease

BACKGROUND

Patient-to-patient variability in the degree to which β-amyloid, tau and neurodegeneration impact cognitive decline in Alzheimer's disease (AD) complicates disease modeling and treatment. However, the underlying mechanisms leading to cognitive resilience are not resolved. We hypothesize that the variability in cognitive function and loss relates to neuronal resilience of the hippocampal GABAergic network.

METHODS

We compared TgF344-AD and non-transgenic littermate rats at 9, 12, and 15 months of age. Neurons, β-amyloid plaques and tau inclusions were quantified in hippocampus and entorhinal cortex. Somatostatin (SST) and parvalbumin (PVB) interneurons were traced to examine hippocampal neuroplasticity and cognition was tested in the Barnes maze.

RESULTS

The 9-month-old TgF344-AD rats exhibited loss of neurons in the entorhinal cortex and hippocampus. Hippocampal neuronal compensation was observed in 12-month TgF344-AD rats, with upregulation of GABAergic interneuronal marker. By 15 months, the TgF344-AD rats had robust loss of excitatory and inhibitory neurons. β-Amyloid and tau pathology accumulated continuously across age. SST interneurons exhibited tau inclusions and atrophy from 9 months, whereas PVB interneurons were resilient until 15 months. The hippocampal PVB circuit underwent neuroplastic reorganization with increased dendritic length and complexity in 9- and 12-month-old TgF344-AD rats, before atrophy at 15 months. Strikingly, 12-month-old TgF344-AD rats were resilient in executive function and cognitive flexibility. Cognitive resilience in TgF344-AD rats occurred as maintenance of function between 9 and 12 months of age despite progressive spatial memory deficits, and was sustained by PVB neuroplasticity.

CONCLUSIONS

Our results demonstrate the inherent neuronal processes leading to cognitive maintenance, and describe a novel finding of endogenous cognitive resilience in an AD model.

Theta and gamma oscillatory dynamics in mouse models of Alzheimer's disease: A path to prospective therapeutic intervention

Understanding the neural basis of cognitive deficits, a key feature of Alzheimer's disease (AD), is imperative for achieving the therapy of the disease. Rhythmic oscillatory activities in neural systems are a fundamental mechanism for diverse brain functions, including cognition. In several neurological conditions like AD, aberrant neural oscillations have been shown to play a central role. Furthermore, manipulation of brain oscillations in animals has confirmed their impact on cognition and disease. In this article, we review the evidence from mouse models that shows how synchronized oscillatory activity is intricately linked to AD machinery. We primarily focus on recent reports showing abnormal oscillatory activities at theta and gamma frequencies in AD condition and their influence on cellular disturbances and cognitive impairments. A thorough comprehension of the role that neuronal oscillations play in AD pathology should pave the way to therapeutic interventions that can curb the disease.

Emerging Electroencephalographic Biomarkers to Improve Preclinical to Clinical Translation in Alzheimer’s Disease

Continually emerging data indicate that sub-clinical, non-convulsive epileptiform activity is not only prevalent in Alzheimer’s disease (AD) but is detectable early in the course of the disease and predicts cognitive decline in both humans and animal models. Epileptiform activity and other electroencephalographic (EEG) measures may hold powerful, untapped potential to improve the translational validity of AD-related biomarkers in model animals ranging from mice, to rats, and non-human primates. In this review, we will focus on studies of epileptiform activity, EEG slowing, and theta-gamma coupling in preclinical models, with particular focus on its role in cognitive decline and relevance to AD. Here, each biomarker is described in the context of the contemporary literature and recent findings in AD relevant animal models are discussed.

Transcorneal Electrical Stimulation Induces Long-Lasting Enhancement of Brain Functional and Directional Connectivity in Retinal Degeneration Mice

To investigate neuromodulation of functional and directional connectivity features in both visual and non-visual brain cortices after short-term and long-term retinal electrical stimulation in retinal degeneration mice. We performed spontaneous electrocorticography (ECoG) in retinal degeneration (rd) mice following prolonged transcorneal electrical stimulation (pTES) at varying currents (400, 500 and 600 μA) and different time points (transient or day 1 post-stimulation, 1-week post-stimulation and 2-weeks post-stimulation). We also set up a sham control group of rd mice which did not receive any electrical stimulation. Subsequently we analyzed alterations in cross-frequency coupling (CFC), coherence and directional connectivity of the primary visual cortex and the prefrontal cortex. It was observed that the sham control group did not display any significant changes in brain connectivity across all stages of electrical stimulation. For the stimulated groups, we observed that transient electrical stimulation of the retina did not significantly alter brain coherence and connectivity. However, for 1-week post-stimulation, we identified enhanced increase in theta-gamma CFC. Meanwhile, enhanced coherence and directional connectivity appeared predominantly in theta, alpha and beta oscillations. These alterations occurred in both visual and non-visual brain regions and were dependent on the current amplitude of stimulation. Interestingly, 2-weeks post-stimulation demonstrated long-lasting enhancement in network coherence and connectivity patterns at the level of cross-oscillatory interaction, functional connectivity and directional inter-regional communication between the primary visual cortex and prefrontal cortex. Application of electrical stimulation to the retina evidently neuromodulates brain coherence and connectivity of visual and non-visual cortices in retinal degeneration mice and the observed alterations are largely maintained. pTES holds strong possibility of modulating higher cortical functions including pathways of cognition, awareness, emotion and memory.

Phase-amplitude modulation during critical period plasticity in mouse visual cortex

Much of our understanding of experience-dependent plasticity originates from the level of single cells and synapses through the well-established techniques of whole-cell recording and calcium imaging. The study of cortical plasticity of neural oscillatory networks remains largely unexplored. Cross-frequency coupling has become an emerging tool to study the underlying mechanisms for synchronization and interaction between local and global processes of cortical networks. The phase of low-frequency oscillations modulates the amplitude of high-frequency oscillations through a phase-amplitude coupling. Recent studies found that gamma-band oscillations associate with critical period plasticity. The existence of such mechanisms in ocular dominance plasticity is yet to be fully demonstrated. In this study, in-vivo electrophysiological methods for recording local field potentials in the primary visual cortex (V1) of anesthetized mice are employed. Our results reveal the mechanisms of neuronal oscillatory activities for the experience-dependent plasticity of developing visual cortical circuits.

Neuromodulation of Hippocampal-Prefrontal Cortical Synaptic Plasticity and Functional Connectivity: Implications for Neuropsychiatric Disorders

The hippocampus-prefrontal cortex (HPC-PFC) pathway plays a fundamental role in executive and emotional functions. Neurophysiological studies have begun to unveil the dynamics of HPC-PFC interaction in both immediate demands and long-term adaptations. Disruptions in HPC-PFC functional connectivity can contribute to neuropsychiatric symptoms observed in mental illnesses and neurological conditions, such as schizophrenia, depression, anxiety disorders, and Alzheimer's disease. Given the role in functional and dysfunctional physiology, it is crucial to understand the mechanisms that modulate the dynamics of HPC-PFC communication. Two of the main mechanisms that regulate HPC-PFC interactions are synaptic plasticity and modulatory neurotransmission. Synaptic plasticity can be investigated inducing long-term potentiation or long-term depression, while spontaneous functional connectivity can be inferred by statistical dependencies between the local field potentials of both regions. In turn, several neurotransmitters, such as acetylcholine, dopamine, serotonin, noradrenaline, and endocannabinoids, can regulate the fine-tuning of HPC-PFC connectivity. Despite experimental evidence, the effects of neuromodulation on HPC-PFC neuronal dynamics from cellular to behavioral levels are not fully understood. The current literature lacks a review that focuses on the main neurotransmitter interactions with HPC-PFC activity. Here we reviewed studies showing the effects of the main neurotransmitter systems in long- and short-term HPC-PFC synaptic plasticity. We also looked for the neuromodulatory effects on HPC-PFC oscillatory coordination. Finally, we review the implications of HPC-PFC disruption in synaptic plasticity and functional connectivity on cognition and neuropsychiatric disorders. The comprehensive overview of these impairments could help better understand the role of neuromodulation in HPC-PFC communication and generate insights into the etiology and physiopathology of clinical conditions.

Frequency selective neuronal modulation triggers spreading depolarizations in the rat endothelin-1 model of stroke

Ischemia is one of the most common causes of acquired brain injury. Central to its noxious sequelae are spreading depolarizations (SDs), waves of persistent depolarizations which start at the location of the flow obstruction and expand outwards leading to excitotoxic damage. The majority of acute stage of stroke studies to date have focused on the phenomenology of SDs and their association with brain damage. In the current work, we investigated the role of peri-injection zone pyramidal neurons in triggering SDs by optogenetic stimulation in an endothelin-1 rat model of focal ischemia. Our concurrent two photon fluorescence microscopy data and local field potential recordings indicated that a ≥ 60% drop in cortical arteriolar red blood cell velocity was associated with SDs at the ET-1 injection site. SDs were also observed in the peri-injection zone, which subsequently exhibited elevated neuronal activity in the low-frequency bands. Critically, SDs were triggered by low- but not high-frequency optogenetic stimulation of peri-injection zone pyramidal neurons. Our findings depict a complex etiology of SDs post focal ischemia and reveal that effects of neuronal modulation exhibit spectral and spatial selectivity.

The Medial Septum as a Potential Target for Treating Brain Disorders Associated With Oscillopathies

The medial septum (MS), as part of the basal forebrain, supports many physiological functions, from sensorimotor integration to cognition. With often reciprocal connections with a broad set of peers at all major divisions of the brain, the MS orchestrates oscillatory neuronal activities throughout the brain. These oscillations are critical in generating sensory and emotional salience, locomotion, maintaining mood, supporting innate anxiety, and governing learning and memory. Accumulating evidence points out that the physiological oscillations under septal influence are frequently disrupted or altered in pathological conditions. Therefore, the MS may be a potential target for treating neurological and psychiatric disorders with abnormal oscillations (oscillopathies) to restore healthy patterns or erase undesired ones. Recent studies have revealed that the patterned stimulation of the MS alleviates symptoms of epilepsy. We discuss here that stimulus timing is a critical determinant of treatment efficacy on multiple time scales. On-demand stimulation may dramatically reduce side effects by not interfering with normal physiological functions. A precise pattern-matched stimulation through adaptive timing governed by the ongoing oscillations is essential to effectively terminate pathological oscillations. The time-targeted strategy for the MS stimulation may provide an effective way of treating multiple disorders including Alzheimer's disease, anxiety/fear, schizophrenia, and depression, as well as pain.

Prodromal neuroinflammatory, cholinergic and metabolite dysfunction detected by PET and MRS in the TgF344-AD transgenic rat model of AD: a collaborative multi-modal study

Mouse models of Alzheimer's disease (AD) are valuable but do not fully recapitulate human AD pathology, such as spontaneous Tau fibril accumulation and neuronal loss, necessitating the development of new AD models. The transgenic (TG) TgF344-AD rat has been reported to develop age-dependent AD features including neuronal loss and neurofibrillary tangles, despite only expressing APP and PSEN1 mutations, suggesting an improved modelling of AD hallmarks. Alterations in neuronal networks as well as learning performance and cognition tasks have been reported in this model, but none have combined a longitudinal, multimodal approach across multiple centres, which mimics the approaches commonly taken in clinical studies. We therefore aimed to further characterise the progression of AD-like pathology and cognition in the TgF344-AD rat from young-adults (6 months (m)) to mid- (12 m) and advanced-stage (18 m, 25 m) of the disease. Methods: TgF344-AD rats and wild-type (WT) littermates were imaged at 6 m, 12 m and 18 m with [¹⁸F]DPA-714 (TSPO, neuroinflammation), [¹⁸F]Florbetaben (Aβ) and [¹⁸F]ASEM (α7-nicotinic acetylcholine receptor) and with magnetic resonance spectroscopy (MRS) and with (S)-[¹⁸F]THK5117 (Tau) at 15 and 25 m. Behaviour tests were also performed at 6 m, 12 m and 18 m. Immunohistochemistry (CD11b, GFAP, Aβ, NeuN, NeuroChrom) and Tau (S)-[¹⁸F]THK5117 autoradiography, immunohistochemistry and Western blot were also performed. Results: [¹⁸F]DPA-714 positron emission tomography (PET) showed an increase in neuroinflammation in TG vs wildtype animals from 12 m in the hippocampus (+11%), and at the advanced-stage AD in the hippocampus (+12%), the thalamus (+11%) and frontal cortex (+14%). This finding coincided with strong increases in brain microgliosis (CD11b) and astrogliosis (GFAP) at these time-points as assessed by immunohistochemistry. In vivo [¹⁸F]ASEM PET revealed an age-dependent increase uptake in the striatum and pallidum/nucleus basalis of Meynert in WT only, similar to that observed with this tracer in humans, resulting in TG being significantly lower than WT by 18 m. In vivo [¹⁸F]Florbetaben PET scanning detected Aβ accumulation at 18 m, and (S)-[¹⁸F]THK5117 PET revealed subsequent Tau accumulation at 25m in hippocampal and cortical regions. Aβ plaques were low but detectable by immunohistochemistry from 6 m, increasing further at 12 and 18 m with Tau-positive neurons adjacent to Aβ plaques at 18 m. NeuroChrom (a pan neuronal marker) immunohistochemistry revealed a loss of neuronal staining at the Aβ plaques locations, while NeuN labelling revealed an age-dependent decrease in hippocampal neuron number in both genotypes. Behavioural assessment using the novel object recognition task revealed that both WT & TgF344-AD animals discriminated the novel from familiar object at 3 m and 6 m of age. However, low levels of exploration observed in both genotypes at later time-points resulted in neither genotype successfully completing the task. Deficits in social interaction were only observed at 3 m in the TgF344-AD animals. By in vivo MRS, we showed a decrease in neuronal marker N-acetyl-aspartate in the hippocampus at 18 m (-18% vs age-matched WT, and -31% vs 6 m TG) and increased Taurine in the cortex of TG (+35% vs age-matched WT, and +55% vs 6 m TG). Conclusions: This multi-centre multi-modal study demonstrates, for the first time, alterations in brain metabolites, cholinergic receptors and neuroinflammation in vivo in this model, validated by robust ex vivo approaches. Our data confirm that, unlike mouse models, the TgF344-AD express Tau pathology that can be detected via PET, albeit later than by ex vivo techniques, and is a useful model to assess and longitudinally monitor early neurotransmission dysfunction and neuroinflammation in AD.

The Theta Rhythm of the Hippocampus: From Neuronal and Circuit Mechanisms to Behavior

This review focuses on the neuronal and circuit mechanisms involved in the generation of the theta (θ) rhythm and of its participation in behavior. Data have accumulated indicating that θ arises from interactions between medial septum-diagonal band of Broca (MS-DbB) and intra-hippocampal circuits. The intrinsic properties of MS-DbB and hippocampal neurons have also been shown to play a key role in θ generation. A growing number of studies suggest that θ may represent a timing mechanism to temporally organize movement sequences, memory encoding, or planned trajectories for spatial navigation. To accomplish those functions, θ and gamma (γ) oscillations interact during the awake state and REM sleep, which are considered to be critical for learning and memory processes. Further, we discuss that the loss of this interaction is at the base of various neurophatological conditions.

Covert strokes prior to Alzheimer's disease onset accelerate peri-lesional pathology but not cognitive deficits in an inducible APP mouse model

It is estimated that up to 1 in 3 healthy middle-aged adults will have had a covert stroke during their lifetime. Furthermore, post-stroke, survivors are more than twice as likely to develop dementia. In the present study, we aimed to model the impact of focal subclinical ischemia prior to the onset of AD pathogenesis in a preclinical model. We utilized endothelin-1 to induce ischemia in an iducible transgenic mouse model of Alzheimer's disease, APPsi:tTA, allowing for temporal control of APP gene expression. We induced the focal subclinical ischemic events in the absence of APP expression, thus prior to AD onset. T2 structural magnetic resonance imaging confirmed the volume and location of focal subclinical ischemic lesions to the medial prefrontal cortex. Following recovery from surgery and 7 weeks of APP expression, we found that two subclinical ischemic lesions resulted in a significant localized increase in amyloid load and in microglial activation proximal to the lesion. However, no differences were found in astrogliosis. A battery of behaviour tests was conducted, in which no significant differences were detected in activities of daily living and cognitive function between stroked and sham cohorts. Overall, our results demonstrated that APP expression was the sole driving force behind behavioural deficits. In conclusion, our results suggest that a history of two subclinical strokes prior to AD onset does not worsen early disease trajectory in a mouse model.

Altered Cortical and Hippocampal Excitability in TgF344-AD Rats Modeling Alzheimer's Disease Pathology

Current findings suggest that accumulation of amyloid-β (Aβ) and hyperphosphorylated tau in the brain disrupt synaptic function in hippocampal-cortical neuronal networks leading to impairment in cognitive and affective functions in Alzheimer's disease (AD). Development of new disease-modifying AD drugs are challenging due to the lack of predictive animal models and efficacy assays. In the present study we recorded neural activity in TgF344-AD rats, a transgenic model with a full array of AD pathological features, including age-dependent Aβ accumulation, tauopathy, neuronal loss, and cognitive impairments. Under urethane anesthesia, TgF344-AD rats showed significant age-dependent decline in brainstem-elicited hippocampal theta oscillation and decreased theta-phase gamma-amplitude coupling comparing to their age-matched wild-type counterparts. In freely-behaving condition, the power of hippocampal theta oscillation and gamma power during sharp-wave ripples were significantly lower in TgF344-AD rats. Additionally, these rats showed impaired coherence in both intercortical and hippocampal-cortical network dynamics, and increased incidence of paroxysmal high-voltage spindles, which occur during awake, behaviorally quiescent state. TgF344-AD rats demonstrated impairments in sensory processing, having diminished auditory gating and 40-Hz auditory evoked steady-state response. The observed differences in neurophysiological activities in TgF344-AD rats, which mirror several abnormalities described in AD patients, may be used as promising markers to monitor disease-modifying therapies.

Regional differences in Alzheimer's disease pathology confound behavioural rescue after amyloid-β attenuation

Failure of Alzheimer’s disease clinical trials to improve or stabilize cognition has led to the need for a better understanding of the driving forces behind cognitive decline in the presence of active disease processes. To dissect contributions of individual pathologies to cognitive function, we used the TgF344-AD rat model, which recapitulates the salient hallmarks of Alzheimer’s disease pathology observed in patient populations (amyloid, tau inclusions, frank neuronal loss, and cognitive deficits). scyllo-Inositol treatment attenuated amyloid-β peptide in disease-bearing TgF344-AD rats, which rescued pattern separation in the novel object recognition task and executive function in the reversal learning phase of the Barnes maze. Interestingly, neither activities of daily living in the burrowing task nor spatial memory in the Barnes maze were rescued by attenuating amyloid-β peptide. To understand the pathological correlates leading to behavioural rescue, we examined the neuropathology and in vivo electrophysiological signature of the hippocampus. Amyloid-β peptide attenuation reduced hippocampal tau pathology and rescued adult hippocampal neurogenesis and neuronal function, via improvements in cross-frequency coupling between theta and gamma bands. To investigate mechanisms underlying the persistence of spatial memory deficits, we next examined neuropathology in the entorhinal cortex, a region whose input to the hippocampus is required for spatial memory. Reduction of amyloid-β peptide in the entorhinal cortex had no effect on entorhinal tau pathology or entorhinal-hippocampal neuronal network dysfunction, as measured by an impairment in hippocampal response to entorhinal stimulation. Thus, rescue or not of cognitive function is dependent on regional differences of amyloid-β, tau and neuronal network dysfunction, demonstrating the importance of staging disease in patients prior to enrolment in clinical trials. These results further emphasize the need for combination therapeutic approaches across disease progression.

Electroencephalographic Cross-Frequency Coupling as a Sign of Disease Progression in Patients With Mild Cognitive Impairment: A Pilot Study

Mild cognitive impairment (MCI) refers to mild objective cognitive deficits and is associated with the later development of Alzheimer’s disease (AD). However, not all patients with MCI convert to AD. EEG spectral power has shown promise as a marker of progression, but brain oscillations in different frequencies are not isolated entities. Coupling between different frequency bands, so-called cross-frequency coupling (CFC), has been associated with memory function and may further contribute to our understanding of what characterizes patients with MCI who progress to AD. In the current study, we wanted to investigate the changes in gamma/theta CFC in patients with AD and MCI compared to HC and in patients with pMCI compared to patients with sMCI. Furthermore, we wanted to investigate the association with cognitive test scores. EEGs were included at baseline for 15 patients with AD, 25 patients with MCI, and 36 older HC, and the participants were followed for up to 3 years. To investigate CFC, we calculated the modulation index (MI), which has been shown to be less affected by noisy data compared to other techniques. We found that patients with pMCI showed a significantly lower global gamma/theta CFC compared to patients with sMCI. In addition, global gamma/theta CFC was significantly correlated with Addenbrooke’s Cognitive Examination (ACE) score (p-value = 0.030, rho = 0.527). Although not significant, patients with AD and MCI showed a lower gamma/theta CFC compared to HC. These findings suggest that gamma/theta CFC is important for proper cognitive functioning and that a decrease in gamma/theta CFC in patients with MCI may be a sign of progression. Gamma/theta CFC may therefore serve as a progression marker in MCI, but larger studies are needed to validate these findings.

Functional Alterations in the Olfactory Neuronal Circuit Occur before Hippocampal Plasticity Deficits in the P301S Mouse Model of Tauopathy: Implications for Early Diagnosis and Translational Research in Alzheimer's Disease

Alzheimer's disease (AD) is characterized by neuronal loss and impaired synaptic transmission, ultimately leading to cognitive deficits. Early in the disease, the olfactory track seems most sensitive to tauopathy, while most plasticity studies focused on the hippocampal circuits. Functional network connectivity (FC) and long-term potentiation (LTP), considered as the plasticity substrate of learning and memory, were longitudinally assessed in mice of the P301S model of tauopathy following the course (time and location) of progressively neurodegenerative pathology (i.e., at 3, 6, and 9 months of age) and in their wild type (WT) littermates. Using in vivo local field potential (LFP) recordings, early (at three months) dampening in the gamma oscillatory activity and impairments in the phase-amplitude theta-gamma coupling (PAC) were found in the olfactory bulb (OB) circuit of P301S mice, which were maintained through the whole course of pathology development. In contrast, LFP oscillatory activity and PAC indices were normal in the entorhinal cortex, hippocampal CA1 and CA3 nuclei. Field excitatory postsynaptic potential (fEPSP) recordings from the Shaffer collateral (SC)-CA1 hippocampal stratum pyramidal revealed a significant altered synaptic LTP response to high-frequency stimulation (HFS): at three months of age, no significant difference between genotypes was found in basal synaptic activity, while signs of a deficit in short term plasticity were revealed by alterations in the fEPSPs. At six months of age, a slight deviance was found in basal synaptic activity and significant differences were observed in the LTP response. The alterations in network oscillations at the OB level and impairments in the functioning of the SC-CA1 pyramidal synapses strongly suggest that the progression of tau pathology elicited a brain area, activity-dependent disturbance in functional synaptic transmission. These findings point to early major alterations of neuronal activity in the OB circuit prior to the disturbance of hippocampal synaptic plasticity, possibly involving tauopathy in the anomalous FC. Further research should determine whether those early deficits in the OB network oscillations and FC are possible mechanisms that potentially promote the emergence of hippocampal synaptic impairments during the progression of tauopathy.

Neural oscillations and brain stimulation in Alzheimer's disease

Aging is associated with alterations in cognitive processing and brain neurophysiology. Whereas the primary symptom of amnestic mild cognitive impairment (aMCI) is memory problems greater than normal for age and education, patients with Alzheimer's disease (AD) show impairments in other cognitive domains in addition to memory dysfunction. Resting-state electroencephalography (rsEEG) studies in physiological aging indicate a global increase in low-frequency oscillations' power and the reduction and slowing of alpha activity. The enhancement of slow and the reduction of fast oscillations, and the disruption of brain functional connectivity, however, are characterized as major rsEEG changes in AD. Recent rodent studies also support human evidence of age- and AD-related changes in resting-state brain oscillations, and the neuroprotective effect of brain stimulation techniques through gamma-band stimulations. Cumulatively, current evidence moves toward optimizing rsEEG features as reliable predictors of people with aMCI at risk for conversion to AD and mapping neural alterations subsequent to brain stimulation therapies. The present paper reviews the latest evidence of changes in rsEEG oscillations in physiological aging, aMCI, and AD, as well as findings of various brain stimulation therapies from both human and non-human studies.

Alzheimer's Disease: The Link Between Amyloid-β and Neurovascular Dysfunction

While prevailing evidence supports that the amyloid cascade hypothesis is a key component of Alzheimer's disease (AD) pathology, many recent studies indicate that the vascular system is also a major contributor to disease progression. Vascular dysfunction and reduced cerebral blood flow (CBF) occur prior to the accumulation and aggregation of amyloid-β (Aβ) plaques and hyperphosphorylated tau tangles. Although research has predominantly focused on the cellular processes involved with Aβ-mediated neurodegeneration, effects of Aβ on CBF and neurovascular coupling are becoming more evident. This review will describe AD vascular disturbances as they relate to Aβ, including chronic cerebral hypoperfusion, hypertension, altered neurovascular coupling, and deterioration of the blood-brain barrier. In addition, we will describe recent findings about the relationship between these vascular defects and Aβ accumulation with emphasis on in vivo studies utilizing rodent AD models.

Neural oscillations during cognitive processes in an App knock-in mouse model of Alzheimer's disease pathology

Multiple animal models have been created to gain insight into Alzheimer's disease (AD) pathology. Among the most commonly used models are transgenic mice overexpressing human amyloid precursor protein (APP) with mutations linked to familial AD, resulting in the formation of amyloid β plaques, one of the pathological hallmarks observed in AD patients. However, recent evidence suggests that the overexpression of APP by itself can confound some of the reported observations. Therefore, we investigated in the present study the AppNL-G-Fmodel, an App knock-in (App-KI) mouse model that develops amyloidosis in the absence of APP-overexpression. Our findings at the behavioral, electrophysiological, and histopathological level confirmed an age-dependent increase in Aβ1-42 levels and plaque deposition in these mice in accordance with previous reports. This had apparently no consequences on cognitive performance in a visual discrimination (VD) task, which was largely unaffected in AppNL-G-F mice at the ages tested. Additionally, we investigated neurophysiological functioning of several brain areas by phase-amplitude coupling (PAC) analysis, a measure associated with adequate cognitive functioning, during the VD task (starting at 4.5 months) and the exploration of home environment (at 5 and 8 months of age). While we did not detect age-dependent changes in PAC during home environment exploration for both the wild-type and the AppNL-G-F mice, we did observe subtle changes in PAC in the wild-type mice that were not present in the AppNL-G-F mice.

Combinatorial Treatment Using Umbilical Cord Perivascular Cells and Aβ Clearance Rescues Vascular Function Following Transient Hypertension in a Rat Model of Alzheimer Disease

Supplemental Digital Content is available in the text.

Transient hypertension is a risk factor for Alzheimer disease (AD), but the effects of this interaction on brain vasculature are understudied. Addressing vascular pathology is a promising avenue to potentiate the efficacy of treatments for AD. We used arterial spin labeling magnetic resonance imaging to longitudinally assess brain vascular function and immunohistopathology to examine cerebrovascular remodeling and amyloid load. Hypertension was induced for 1 month by administration of l-N^G-nitroarginine-methyl-ester in TgF344-AD rats at the prodromal stage. Following hypertension, nontransgenic rats showed transient cerebrovascular changes, whereas TgF344-AD animals exhibited sustained alterations in cerebrovascular function. Human umbilical cord perivascular cells in combination with scyllo-inositol, an inhibitor of Aβ oligomerization, resulted in normalization of hippocampal vascular function and remodeling, in contrast to either treatment alone. Prodromal stage hypertension exacerbates latter AD pathology, and the combination of human umbilical cord perivascular cells with amyloid clearance promotes cerebrovascular functional recovery.

Resting State Networks in the TgF344-AD Rat Model of Alzheimer's Disease Are Altered From Early Stages

A better and non-invasive characterization of the preclinical phases of Alzheimer's disease (AD) is important to advance its diagnosis and obtain more effective benefits from potential treatments. The TgF344-AD rat model has been well characterized and shows molecular, behavioral and brain connectivity alterations that resemble the silent period of the pathology. Our aim was to longitudinally investigate functional brain connectivity in established resting-state networks (RSNs) obtained by independent component analysis (ICA) in a cohort of TgF344-AD and control rats every 3 months, from 5 to 18 months of age, to cover different stages of the disease. Before each acquisition, working memory performance was evaluated by the delayed non match-to-sample (DNMS) task. Differences in the temporal evolution were observed between groups in the amplitude and shape of the somatosensorial and sensorimotor networks but not in the whole default mode network (DMN). Subsequent high dimensional ICA analysis showed early alterations in the anterior DMN subnetwork activity of TgF344-AD rats compared to controls. Performance of DNMS task was positively correlated with somatosensorial network at 5 months of age in the wild-type (WT) animals but not in the Tg-F344 rats. At different time points, DMN showed negative correlation with cognitive performance in the control group while in the transgenic group the correlation was positive. In addition, behavioral differences observed at 5 months of age correlated with alterations in the posterior DMN subnetwork. We have demonstrated that functional connectivity using ICA represents a useful biomarker also in animal models of AD such as the TgF344AD rats, as it allows the identification of alterations associated with the progression of the disease, detecting differences in specific networks even at very early stages.

Early Electrophysiological Disintegration of Hippocampal Neural Networks in a Novel Locus Coeruleus Tau-Seeding Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a progressive, neurodegenerative disease characterized by loss of synapses and disrupted functional connectivity (FC) across different brain regions. Early in AD progression, tau pathology is found in the locus coeruleus (LC) prior to amyloid-induced exacerbation of clinical symptoms. Here, a tau-seeding model in which preformed synthetic tau fibrils (K18) were unilaterally injected into the LC of P301L mice, equipped with multichannel electrodes for recording EEG in frontal cortical and CA1-CA3 hippocampal areas, was used to longitudinally quantify over 20 weeks of functional network dynamics in (1) power spectra; (2) FC using intra- and intersite phase-amplitude theta-gamma coupling (PAC); (3) coherence, partial coherence, and global coherent network efficiency (Eglob) estimates; and (4) the directionality of functional connectivity using extended partial direct coherence (PDC). A sustained leftward shift in the theta peak frequency was found early in the power spectra of hippocampal CA1 networks ipsilateral to the injection site. Strikingly, hippocampal CA1 coherence and Eglob measures were impaired in K18-treated animals. Estimation of instantaneous EEG amplitudes revealed deficiency in the propagation directionality of gamma oscillations in the CA1 circuit. Impaired PAC strength evidenced by decreased modulation of the theta frequency phase on gamma frequency amplitude further confirms impairments of the neural CA1 network. The present results demonstrate early dysfunctional hippocampal networks, despite no spreading tau pathology to the hippocampus and frontal cortex. The ability of the K18 seed in the brainstem LC to elicit such robust functional alterations in distant hippocampal structures in the absence of pathology challenges the classic view that tau pathology spread to an area is necessary to elicit functional impairments in that area.