Age-dependent dysregulation of locus coeruleus firing in a transgenic rat model of Alzheimer's disease

Effects of social isolation on locus coeruleus opioid receptor expression and affective behavior

Social isolation is a stressor that impairs homeostatic neuroendocrine functions and is associated with the development of several mood disorders characterized by persistent negative affect. Persistent feelings of loneliness have been growing public health concerns for several years and were greatly exacerbated by the onset of the COVID-19 pandemic. The problem has grown so severe the U.S. Surgeon General recently declared loneliness to be an epidemic health concern that is associated with poor mental and somatic health outcomes. Therefore, identifying mechanisms of neuroadaptation that contribute to the development of persistent negative affect is a critical step in the identifying better treatments for mood disorders. One region of the brain that becomes dysregulated in neuropsychiatric disease is the locus coeruleus. It is innervated by multiple stress-related peptidergic afferents, including those that release endogenous opioids to affect behavior. It is a major contributor to the behavioral limb of the stress response, but its role in the neurobiology of social behavior is understudied. Here we show that in laboratory rats, six weeks of social isolation leads to increased neophobia, reduced sociality, and passive stress coping. These behavioral changes are also associated with downregulation of the δ-opioid receptor and upregulation of the κ-opioid receptor in locus coeruleus. These findings suggest that extended social isolation promotes dysregulation of several opioid receptor subtypes in a brain structure that has an important role in regulating affective behavior, implicating them as potential targets for the treatment of neuropsychiatric disease associated with social isolation and loneliness.

Loss of excitatory inputs and decreased tonic and evoked activity of locus coeruleus neurons in aged P301S mice

Tau pathology in the locus coeruleus (LC) is associated with several neurodegenerative conditions including Alzheimer's disease and frontotemporal dementia. Phosphorylated tau accumulates in the LC and results in inflammation, synaptic loss, and eventually cell death as the disease progresses. Loss of LC neurons and noradrenergic innervation is thought to contribute to the symptoms of cognitive decline later in disease. While loss and degeneration of LC neurons has been well studied, less is known about changes in LC physiology at advanced stages of tau pathology that precedes neurodegeneration. In this study, we investigated the ex vivo electrophysiological properties of LC neurons in male and female mice from the P301S mouse model of tauopathy at 9 months of age, a time-point when significant tau accumulation, cell death, and cognitive impairments are observed. We found a reduction in excitatory inputs and changes in excitatory post-synaptic current kinetics in male and female P301S. There was also a decrease in spontaneous discharge of LC neurons and an increase in AP threshold in P301S mice of both sexes. Finally, we observed a decrease in excitability and increase in rheobase current in P301S mice. Despite the decrease in LC activity in ex vivo slices, we did not identify differences in total tissue norepinephrine (NE) or NE metabolites in prefrontal cortex or hippocampus. Together these findings demonstrate reductions in the activity and excitability of LC neurons at late stages of tau accumulation. However, compensatory mechanisms may maintain normal NE levels in LC projection regions in vivo.

tDCS cranial nerve Co-stimulation: Unveiling brainstem pathways involved in trigeminal nerve direct current stimulation in rats

BACKGROUND

The effects of transcranial direct current stimulation (tDCS) are generally thought to result from the polarization of cortical neurons by the weak electric fields it creates. However, recent evidence suggests that some tDCS effects may be mediated through co-stimulation of peripheral or cranial nerves, particularly the trigeminal nerve (TN). The TN projects to key brainstem nuclei that regulate neurotransmitter release throughout the central nervous system, but the specific pathways involved are not yet well understood.

METHODS

In this study, we examined the effects of acute transcutaneous TN direct current stimulation (TN-DCS) on tonic (i.e. mean spike rate) and phasic (number of bursts, spike rate per burst, burst duration, and inter-burst interval) activities while simultaneously recording single-neuron activity across three brainstem nuclei in rats: the locus coeruleus (LC; phasic and tonic activities), dorsal raphe nucleus (DRN; tonic activity), and median raphe nucleus (MnRN; tonic activity).

RESULTS

TN-DCS significantly modulated tonic activity in the LC and DRN, with interactions between amplitude, polarity, and time affecting mean spike rates. It also influenced phasic activity in the LC, altering burst number, duration, and inter-burst intervals. In contrast, MnRN tonic activity was unchanged. Blocking TN with xylocaine eliminated the effects on tonic activity in both the LC and DRN.

CONCLUSIONS

These results suggest that tDCS may modulate the TN, altering DRN and LC activity. Differential changes in tonic and phasic LC activity highlight their roles in TN-DCS effects on the cortex. This research offers insights to improve tDCS efficacy and understanding.

Fading Blue: Exploring the Causes of Locus Coeruleus Damage Across the Lifespan

Locus Coeruleus (LC) is a brain nucleus that is involved in a variety of key functions (ranging from attention modulation to sleep-wake cycle regulation, to memory encoding); its proper function is necessary both during brain development and for brain integrity maintenance, and both at the microscale and macroscale level. Due to their specific intrinsic and extrinsic features, LC cells are considered particularly susceptible to damage concerning a variety of insults. This explains LC involvement in degenerative diseases not only in adults (in the context of neurodegenerative disease, mainly), but also in children (in relation to early hypoxic damage and Down's Syndrome, among others). In this narrative review, we dissect the potential mechanisms through which LC is affected in different diseases, with a special emphasis on the high rate of activity it is subjected to and the oxidative stress associated with it. Further research aimed at deepening our understanding of these mechanisms is needed to enable the development of potential strategies in the future that could slow down LC degeneration in subjects predisposed to specific brain disorders.

Anything but small: Microarousals stand at the crossroad between noradrenaline signaling and key sleep functions

Continuous sleep restores the brain and body, whereas fragmented sleep harms cognition and health. Microarousals (MAs), brief (3- to 15-s-long) wake intrusions into sleep, are clinical markers for various sleep disorders. Recent rodent studies show that MAs during healthy non-rapid eye movement (NREM) sleep are driven by infraslow fluctuations of noradrenaline (NA) in coordination with electrophysiological rhythms, vasomotor activity, cerebral blood volume, and glymphatic flow. MAs are hence part of healthy sleep dynamics, raising questions about their biological roles. We propose that MAs bolster NREM sleep's benefits associated with NA fluctuations, according to an inverted U-shaped curve. Weakened noradrenergic fluctuations, as may occur in neurodegenerative diseases or with sleep aids, reduce MAs, whereas exacerbated fluctuations caused by stress fragment NREM sleep and collapse NA signaling. We suggest that MAs are crucial for the restorative and plasticity-promoting functions of sleep and advance our insight into normal and pathological arousal dynamics from sleep.

Loss of excitatory inputs and decreased tonic and evoked activity of locus coeruleus neurons in aged P301S mice

Tau pathology in the locus coeruleus (LC) is associated with several neurodegenerative conditions including Alzheimer's disease and frontotemporal dementia. Phosphorylated tau accumulates in the LC and results in inflammation, synaptic loss, and eventually cell death as the disease progresses. Loss of LC neurons and noradrenergic innervation is thought to contribute to the symptoms of cognitive decline later in disease. While loss and degeneration of LC neurons has been well studied, less is known about changes in LC physiology at advanced stages of tau pathology that precedes neurodegeneration. In this study, we investigated the ex vivo electrophysiological properties of LC neurons in male and female mice from the P301S mouse model of tauopathy at 9 months of age, a time-point when significant tau accumulation, cell death, and cognitive impairments are observed. We found a reduction in excitatory inputs and changes in excitatory post-synaptic current kinetics in male and female P301S. There was also a decrease in spontaneous discharge of LC neurons and an increase in AP threshold in P301S mice of both sexes. Finally, we observed a decrease in excitability and increase in rheobase current in P301S mice. Despite the decrease in LC activity in slice, we did not identify differences in total tissue norepinephrine (NE) or NE metabolites in prefrontal cortex or hippocampus. Together these findings demonstrate reductions in the activity and excitability of LC neurons at late stages of tau accumulation. However, compensatory mechanisms may maintain normal NE levels in LC projection regions in vivo.

Diurnal Modulation of Locus Coeruleus Noradrenergic Neurons in Anesthetized Adult Male Rats

The locus coeruleus (LC) is the primary source of noradrenaline (NA) in brain and its activity is essential for learning, memory, stress, arousal, and mood. LC-NA neuron activity varies over the sleep-wake cycle, with higher activity during wakefulness, correlating with increased CSF NA levels. Whether spontaneous and burst firing of LC-NA neurons during active and inactive periods is controlled by mechanisms independent of wakefulness and natural sleep, is currently unknown. Here, using multichannel in vivo electrophysiology under anesthesia, we assessed LC-NA neuron firing in adult male Fisher 344 rats at two different times of day- ZT4- the inactive period (light phase) and ZT16-the active period (dark phase)- independent of contributions from behavioral arousal and natural sleep. In the dark phase, LC-NA neurons exhibit increased average firing rate during baseline compared to the light phase. Using a relatively weak foot shock paradigm, we observed distinct populations of LC-NA neurons with some increasing, and others decreasing, their firing rate compared to baseline. Additionally, while spike frequency during spontaneous and evoked bursts is consistent across the dark-light phase, units recorded during the dark phase have more frequent bursts with a longer duration than those during the light phase. Our findings show that independent of wake state, LC-NA neurons exhibit intrinsic diurnal activity, and that the variability of response to foot shock stimulation demonstrates a physiological heterogeneity of LC-NA neurons that is just beginning to be appreciated.

NEW & NOTEWORTHY

Multichannel in vivo electrophysiology assesses activity of large populations of NA neurons within an intact LC. Recording activity under anesthesia eliminates influence of behavior and sleep on LC-NA neuron physiology. Our data show that LC-NA neurons have heightened firing and burst activity during the dark phase, suggesting a hardwired diurnal rhythm. Additionally, LC-NA neurons have variable evoked firing highlighting heterogeneity, consistent with a contemporary view that LC physiology is more complex than previously appreciated.

Research on Alzheimer's Disease (AD) Involving the Use of In vivo and In vitro Models and Mechanisms

BACKGROUND

Alzheimer's Disease (AD) is a neurodegenerative disorder characterized by the progressive formation of extracellular amyloid plaques, intracellular neurofibrillary tangles, inflammation, and impaired antioxidant systems. Early detection and intervention are vital for managing AD effectively.

OBJECTIVES

This review scrutinizes both in-vivo and in-vitro screening models employed in Alzheimer's disease research. in-vivo models, including transgenic mice expressing AD-related mutations, offer profound insights into disease progression and potential therapeutic targets. A thorough understanding of these models and mechanisms will facilitate the development of novel therapies and interventions for Alzheimer's disease. This review aims to provide an overview of the current experimental models in AD research, assess their strengths and weaknesses as model systems, and underscore the future prospects of experimental AD modeling.

METHODS

We conducted a systematic literature search across multiple databases, such as Pub- Med, Bentham Science, Elsevier, Springer Nature, Wiley, and Research Gate. The search strategy incorporated pertinent keywords related to Alzheimer's disease, in-vivo models, in-vitro models, and screening mechanisms. Inclusion criteria were established to identify studies focused on in-vivo and in-vitro screening models and their mechanisms in Alzheimer's disease research. Studies not meeting the predefined criteria were excluded from the review.

RESULTS

A well-structured experimental animal model can yield significant insights into the neurobiology of AD, enhancing our comprehension of its pathogenesis and the potential for cutting-edge therapeutic strategies. Given the limited efficacy of current AD medications, there is a pressing need for the development of experimental models that can mimic the disease, particularly in pre-symptomatic stages, to investigate prevention and treatment approaches. To address this requirement, numerous experimental models replicating human AD pathology have been established, serving as invaluable tools for assessing potential treatments.

CONCLUSION

In summary, this comprehensive review underscores the pivotal role of in-vivo and in-vitro screening models in advancing our understanding of Alzheimer's disease. These models offer invaluable insights into disease progression, pathological mechanisms, and potential therapeutic targets. By conducting a rigorous investigation and evaluation of these models and mechanisms, effective screening and treatment methods for Alzheimer's disease can be devised. The review also outlines future research directions and areas for enhancing AD screening models.

Effects of Age and Atomoxetine on Olfactory Perception and Learning and Underlying Plasticity Mechanisms in Rats

The locus coeruleus (LC) plays a vital role in cognitive function through norepinephrine release. Impaired LC neuronal health and function is linked to cognitive decline during ageing and Alzheimer's disease. This study investigates age-related alterations in olfactory detection and discrimination learning, along with its reversal, in Long-Evans rats, and examines the effects of atomoxetine (ATM), a norepinephrine uptake inhibitor, on these processes. Adult (6-9 months) and aged (22-24 months) Long-Evans rats underwent odour detection threshold experiments with saline and two doses of ATM (0.3 and 1 mg/kg). Reward-based odour discrimination learning included simple, difficult and reversal learning tasks. LC neuron density, dopamine beta-hydroxylase and norepinephrine transporter expression in the piriform cortex (PC) and orbitofrontal cortex were measured. Reversal learning and olfactory threat extinction were used to measure behavioural flexibility. Immunohistochemistry and western blotting were used to analyse phosphorylated cAMP response element binding protein (pCREB) and cFos expression and ex vivo electrophysiology assessed long-term depression (LTD) in the PC. Whereas adult and aged cohorts showed similar odour detection and discrimination learning, fewer aged rats acquired reversal learning successfully. ATM improved reward-based odour discrimination in adults but hindered learning reversal. A delayed CREB phosphorylation in the posterior PC associated with atomoxetine administration possibly underlies learning enhancement. ATM resulted in less freezing behaviour in a threat conditioning and extinction paradigm at moderate, but not at higher doses. ATM administration ex vivo prevented PC LTD. These findings highlight the intricate effects of atomoxetine, influenced by target structures, and suggest potential interactions with other neurotransmitters. Our results contribute to understanding the impact of ageing and norepinephrine enhancers on cognitive processes.

Locus coeruleus signal intensity and emotion regulation in agitation in Alzheimer's disease

Hyperphosphorylated tau accumulation is seen in the noradrenergic locus coeruleus from the earliest stages of Alzheimer's disease onwards and has been associated with symptoms of agitation. It is hypothesized that compensatory locus coeruleus-noradrenaline system overactivity and impaired emotion regulation could underlie agitation propensity, but to our knowledge this has not previously been investigated. A better understanding of the neurobiological underpinnings of agitation would help the development of targeted prevention and treatment strategies. Using a sample of individuals with amnestic mild cognitive impairment and probable mild Alzheimer's disease dementia from the German Center for Neurodegenerative Diseases (DZNE)-Longitudinal Cognitive Impairment and Dementia (DELCODE) study cohort (N = 309, aged 67-96 years, 51% female), we assessed cross-sectional relationships between a latent factor representing the functional integrity of an affect-related executive regulation network and agitation point prevalence and severity scores. In a subsample of individuals with locus coeruleus MRI imaging data (N = 37, aged 68-93 years, 49% female), we also investigated preliminary associations between locus coeruleus MRI contrast ratios (a measure of structural integrity, whole or divided into rostral, middle, and caudal thirds) and individual affect-related regulation network factor scores and agitation measures. Regression models controlled for effects of age and clinical disease severity and, for models including resting-state functional MRI connectivity variables, grey matter volume and education years. Agitation point prevalence showed a positive relationship with a latent factor representing the functional integrity (and a negative relationship with a corresponding structural measure) of the affect-related executive regulation network. Locus coeruleus MRI contrast ratios were positively associated with agitation severity (but only for the rostral third, in N = 13) and negatively associated with the functional affect-related executive regulation latent factor scores. Resting-state functional connectivity between a medial prefrontal cortex region and the left amygdala was related to locus coeruleus MRI contrast ratios. These findings implicate the involvement of locus coeruleus integrity and emotion dysregulation in agitation in Alzheimer's disease and support the presence of potential compensatory processes. At the neural level, there may be a dissociation between mechanisms underlying agitation risk per se and symptom severity. Further studies are needed to replicate and extend these findings, incorporating longitudinal designs, measures of autonomic function and non-linear modelling approaches to explore potential causal and context-dependent relationships across Alzheimer's disease stages.

VTA dopamine neurons are hyperexcitable in 3xTg-AD mice due to casein kinase 2-dependent SK channel dysfunction

Alzheimer's disease (AD) patients exhibit neuropsychiatric symptoms that extend beyond classical cognitive deficits, suggesting involvement of subcortical areas. Here, we investigated the role of midbrain dopamine (DA) neurons in AD using the amyloid + tau-driven 3xTg-AD mouse model. We found deficits in reward-based operant learning in AD mice, suggesting possible VTA DA neuron dysregulation. Physiological assessment revealed hyperexcitability and disrupted firing in DA neurons caused by reduced activity of small-conductance calcium-activated potassium (SK) channels. RNA sequencing from contents of single patch-clamped DA neurons (Patch-seq) identified up-regulation of the SK channel modulator casein kinase 2 (CK2), which we corroborated by immunohistochemical protein analysis. Pharmacological inhibition of CK2 restored SK channel activity and normal firing patterns in 3xTg-AD mice. These findings identify a mechanism of ion channel dysregulation in VTA DA neurons that could contribute to behavioral abnormalities in AD, paving the way for novel treatment strategies.

Examining resilience to Alzheimer's disease through the lens of monoaminergic neuromodulator systems

The monoaminergic nuclei are thought to be some of the earliest sites of Alzheimer's disease (AD) pathology in the brain, with tau-containing pretangles appearing in these nuclei decades before the onset of clinical impairments. It has increasingly been recognized that monoamine systems represent a critical target of investigation towards understanding the progression of AD and designing early detection and treatment approaches. This review synthesizes evidence across animal studies, human neuropathology, and state-of-the-art neuroimaging and daily life assessment methods in humans, which demonstrate robust relationships between monoamine systems and AD pathophysiology and behavior. Further, the review highlights the promise of multimethod, multisystem approaches to studying monoaminergic mechanisms of resilience to AD pathology.

Structural and functional remodeling of neural networks in β-amyloid driven hippocampal hyperactivity

Early detection of Alzheimer's disease (AD) is essential for improving the patients outcomes and advancing our understanding of disease, allowing for timely intervention and treatment. However, accurate biomarkers are still lacking. Recent evidence indicates that hippocampal hyperexcitability precedes the diagnosis of AD decades ago, can predict cognitive decline. Thus, could hippocampal hyperactivity be a robust biomarker for early-AD, and what drives hippocampal hyperactivity in early-AD? these critical questions remain to be answered. Increasing clinical and experimental studies suggest that early hippocampal activation is closely associated with longitudinal β-amyloid (Aβ) accumulation, Aβ aggregates, in turn, enhances hippocampal activity. Therefore, in this narrative review, we discuss the role of Aβ-induced altered intrinsic neuronal properties as well as structural and functional remodeling of glutamatergic, GABAergic, cholinergic, noradrenergic, serotonergic circuits in hippocampal hyperactivity. In addition, we analyze the available therapies and trials that can potentially be used clinically to attenuate hippocampal hyperexcitability in AD. Overall, the present review sheds lights on the mechanism behind Aβ-induced hippocampal hyperactivity, and highlights that hippocampal hyperactivity could be a robust biomarker and therapeutic target in prodromal AD.

Diversity of ancestral brainstem noradrenergic neurons across species and multiple biological factors

The brainstem region, locus coeruleus (LC), has been remarkably conserved across vertebrates. Evolution has woven the LC into wide-ranging neural circuits that influence functions as broad as autonomic systems, the stress response, nociception, sleep, and high-level cognition among others. Given this conservation, there is a strong possibility that LC activity is inherently similar across species, and furthermore that age, sex, and brain state influence LC activity similarly across species. The degree to which LC activity is homogenous across these factors, however, has never been assessed due to the small sample size of individual studies. Here, we pool data from 20 laboratories (1,855 neurons) and show diversity across both intrinsic and extrinsic factors such as species, age, sex and brain state. We use a negative binomial regression model to compare activity from male monkeys, and rats and mice of both sexes that were recorded across brain states from brain slices ex vivo or under different anesthetics or during wakefulness in vivo. LC activity differed due to complex interactions of species, sex, and brain state. The LC became more active during aging, independent of sex. Finally, in contrast to the foundational principle that all species express two distinct LC firing modes ("tonic" or "phasic"), we discovered great diversity within spontaneous LC firing patterns. Different factors were associated with higher incidence of some firing modes. We conclude that the activity of the evolutionarily-ancient LC is not conserved. Inherent differences due to age and species-sex-brain state interactions have implications for understanding the role of LC in species-specific naturalistic behavior, as well as in psychiatric disorders, cardiovascular disease, immunology, and metabolic disorders.

tDCS Cranial Nerve Co-Stimulation: Unveiling Brainstem Pathways Involved in Trigeminal Nerve Direct Current Stimulation in Rats

The effects of transcranial direct current stimulation (tDCS) are typically attributed to the polarization of cortical neurons by the weak electric fields it generates in the cortex. However, emerging evidence indicates that certain tDCS effects may be mediated through the co-stimulation of peripheral or cranial nerves, particularly the trigeminal nerve (TN), which projects to critical brainstem nuclei that regulate the release of various neurotransmitters throughout the central nervous system. Despite this, the specific pathways involved remain inadequately characterized. In this study, we examined the effects of acute transcutaneous TN direct current stimulation (TN-DCS) on tonic (i.e. mean spike rate and spike rate over time) and phasic (number of bursts, spike rate per burst, burst duration, and inter-burst interval) activities while simultaneously recording single-neuron activity across three brainstem nuclei in rats: the locus coeruleus (LC), dorsal raphe nucleus (DRN), and median raphe nucleus (MnRN). We found that TN-DCS significantly modulated tonic activity in the LC, with notable interactions between stimulation amplitude, polarity, and time epoch affecting mean spike rates. Similar effects were observed in the DRN regarding tonic activity. Further, phasic activity in the LC was influenced by TN-DCS, with changes in burst number, duration, and inter-burst intervals linked to stimulation parameters. Conversely, MnRN tonic activity following TN-DCS remained unchanged. Importantly, xylocaine administration to block TN abolished the effects on tonic activities in both the LC and DRN. These results suggest that tDCS effects may partially arise from indirect modulation of the TN, leading to altered neuronal activity in DRN and LC. Besides, the differential changes in tonic and phasic LC activities underscore their complementary roles in mediating TN-DCS effects on higher cortical regions. This research bears significant translational implications, providing mechanistic insights that could enhance the efficacy of tDCS applications and deepen our understanding of its neurophysiological effects.

Entorhinal cortex vulnerability to human APP expression promotes hyperexcitability and tau pathology

Preventative treatment for Alzheimer's Disease (AD) is dire, yet mechanisms underlying early regional vulnerability remain unknown. In AD, one of the earliest pathophysiological correlates to cognitive decline is hyperexcitability, which is observed first in the entorhinal cortex. Why hyperexcitability preferentially emerges in specific regions in AD is unclear. Using regional, cell-type-specific proteomics and electrophysiology in wild-type mice, we uncovered a unique susceptibility of the entorhinal cortex to human amyloid precursor protein (hAPP). Entorhinal hyperexcitability resulted from selective vulnerability of parvalbumin (PV) interneurons, with respect to surrounding excitatory neurons. This effect was partially replicated with an APP chimera containing a humanized amyloid-beta sequence. EC hyperexcitability could be ameliorated by co-expression of human Tau with hAPP at the expense of increased pathological tau species, or by enhancing PV interneuron excitability in vivo. This study suggests early interventions targeting inhibitory neurons may protect vulnerable regions from the effects of APP/amyloid and tau pathology.

Entorhinal cortex vulnerability to human APP expression promotes hyperexcitability and tau pathology

Preventative treatment for Alzheimer's Disease is of dire importance, and yet, cellular mechanisms underlying early regional vulnerability in Alzheimer's Disease remain unknown. In human patients with Alzheimer's Disease, one of the earliest observed pathophysiological correlates to cognitive decline is hyperexcitability. In mouse models, early hyperexcitability has been shown in the entorhinal cortex, the first cortical region impacted by Alzheimer's Disease. The origin of hyperexcitability in early-stage disease and why it preferentially emerges in specific regions is unclear. Using cortical-region and cell-type-specific proteomics coupled with ex vivo and in vivo electrophysiology, we uncovered differential susceptibility to human-specific amyloid precursor protein (hAPP) in a model of sporadic Alzheimer's. Unexpectedly, our findings reveal that early entorhinal hyperexcitability may result from intrinsic vulnerability of parvalbumin (PV) interneurons, rather than the suspected layer II excitatory neurons. This vulnerability of entorhinal PV interneurons is specific to hAPP, as it could not be recapitulated with increased murine APP expression. However, partial replication of the findings could be seen after introduction of a murine APP chimera containing a humanized amyloid-beta sequence. Surprisingly, neurons in the Somatosensory Cortex showed no such vulnerability to adult-onset hAPP expression. hAPP-induced hyperexcitability in entorhinal cortex could be ameliorated by enhancing PV interneuron excitability in vivo. Co-expression of human Tau with hAPP decreased circuit hyperexcitability, but at the expense of increased pathological tau species. This study suggests early disease interventions targeting non-excitatory cell types may protect regions with early vulnerability to pathological symptoms of Alzheimer's Disease and downstream cognitive decline.

Atrophy links lower novelty-related locus coeruleus connectivity to cognitive decline in preclinical AD

INTRODUCTION

Animal research has shown that tau pathology in the locus coeruleus (LC) is associated with reduced norepinephrine signaling, lower projection density to the medial temporal lobe (MTL), atrophy, and cognitive impairment. We investigated the contribution of LC-MTL functional connectivity (FCLC-MTL) on cortical atrophy across Braak stage regions and its impact on cognition.

METHODS

We analyzed functional magnetic resonance imaging and amyloid beta (Aβ) positron emission tomography data from 128 cognitively normal participants, associating novelty-related FCLC-MTL with longitudinal atrophy and cognition with and without Aβ moderation.

RESULTS

Cross-sectionally, lower FCLC-MTL was associated with atrophy in Braak stage II regions. Longitudinally, atrophy in Braak stage 2 to 4 regions related to lower baseline FCLC-MTL at elevated levels of Aβ, but not to other regions. Atrophy in Braak stage 2 regions mediated the relation between FCLC-MTL and subsequent cognitive decline.

DISCUSSION

FCLC-MTL is implicated in Aβ-related cortical atrophy, suggesting that LC-MTL connectivity could confer neuroprotective effects in preclinical AD.

HIGHLIGHTS

Novelty-related functional magnetic resonance imaging (fMRI) LC-medial temporal lobe (MTL) connectivity links to longitudinal Aβ-dependent atrophy. This relationship extended to higher Braak stage regions with increasing Aβ burden. Longitudinal MTL atrophy mediated the LC-MTL connectivity-cognition relationship. Our findings mirror the animal data on MTL atrophy following NE signal dysfunction.

The Effects of Locus Coeruleus Optogenetic Stimulation on Global Spatiotemporal Patterns in Rats

Whole-brain intrinsic activity as detected by resting-state fMRI can be summarized by three primary spatiotemporal patterns. These patterns have been shown to change with different brain states, especially arousal. The noradrenergic locus coeruleus (LC) is a key node in arousal circuits and has extensive projections throughout the brain, giving it neuromodulatory influence over the coordinated activity of structurally separated regions. In this study, we used optogenetic-fMRI in rats to investigate the impact of LC stimulation on the global signal and three primary spatiotemporal patterns. We report small, spatially specific changes in global signal distribution as a result of tonic LC stimulation, as well as regional changes in spatiotemporal patterns of activity at 5 Hz tonic and 15 Hz phasic stimulation. We also found that LC stimulation had little to no effect on the spatiotemporal patterns detected by complex principal component analysis. These results show that the effects of LC activity on the BOLD signal in rats may be small and regionally concentrated, as opposed to widespread and globally acting.

Spatiotemporal patterns of locus coeruleus integrity predict cortical tau and cognition

Autopsy studies indicated that the locus coeruleus (LC) accumulates hyperphosphorylated tau before allocortical regions in Alzheimer's disease. By combining in vivo longitudinal magnetic resonance imaging measures of LC integrity, tau positron emission tomography imaging and cognition with autopsy data and transcriptomic information, we examined whether LC changes precede allocortical tau deposition and whether specific genetic features underlie LC's selective vulnerability to tau. We found that LC integrity changes preceded medial temporal lobe tau accumulation, and together these processes were associated with lower cognitive performance. Common gene expression profiles between LC-medial temporal lobe-limbic regions map to biological functions in protein transport regulation. These findings advance our understanding of the spatiotemporal patterns of initial tau spreading from the LC and LC's selective vulnerability to Alzheimer's disease pathology. LC integrity measures can be a promising indicator for identifying the time window when individuals are at risk of disease progression and underscore the importance of interventions mitigating initial tau spread.

Obesity during preclinical Alzheimer's disease development exacerbates brain metabolic decline

Alzheimer's disease (AD) is the most common form of dementia. Obesity in middle age increases AD risk and severity, which is alarming given that obesity prevalence peaks at middle age and obesity rates are accelerating worldwide. Midlife, but not late-life obesity increases AD risk, suggesting that this interaction is specific to preclinical AD. AD pathology begins in middle age, with accumulation of amyloid beta (Aβ), hyperphosphorylated tau, metabolic decline, and neuroinflammation occurring decades before cognitive symptoms appear. We used a transcriptomic discovery approach in young adult (6.5 months old) male and female TgF344-AD rats that overexpress mutant human amyloid precursor protein and presenilin-1 and wild-type (WT) controls to determine whether inducing obesity with a high-fat/high-sugar "Western" diet during preclinical AD increases brain metabolic dysfunction in dorsal hippocampus (dHC), a brain region vulnerable to the effects of obesity and early AD. Analyses of dHC gene expression data showed dysregulated mitochondrial and neurotransmission pathways, and up-regulated genes involved in cholesterol synthesis. Western diet amplified the number of genes that were different between AD and WT rats and added pathways involved in noradrenergic signaling, dysregulated inhibition of cholesterol synthesis, and decreased intracellular lipid transporters. Importantly, the Western diet impaired dHC-dependent spatial working memory in AD but not WT rats, confirming that the dietary intervention accelerated cognitive decline. To examine later consequences of early transcriptional dysregulation, we measured dHC monoamine levels in older (13 months old) AD and WT rats of both sexes after long-term chow or Western diet consumption. Norepinephrine (NE) abundance was significantly decreased in AD rats, NE turnover was increased, and the Western diet attenuated the AD-induced increases in turnover. Collectively, these findings indicate obesity during prodromal AD impairs memory, potentiates AD-induced metabolic decline likely leading to an overproduction of cholesterol, and interferes with compensatory increases in NE transmission.

Anxiety and Alzheimer's disease pathogenesis: focus on 5-HT and CRF systems in 3xTg-AD and TgF344-AD animal models

Dementia remains one of the leading causes of morbidity and mortality in older adults. Alzheimer's disease (AD) is the most common type of dementia, affecting over 55 million people worldwide. AD is characterized by distinct neurobiological changes, including amyloid-beta protein deposits and tau neurofibrillary tangles, which cause cognitive decline and subsequent behavioral changes, such as distress, insomnia, depression, and anxiety. Recent literature suggests a strong connection between stress systems and AD progression. This presents a promising direction for future AD research. In this review, two systems involved in regulating stress and AD pathogenesis will be highlighted: serotonin (5-HT) and corticotropin releasing factor (CRF). Throughout the review, we summarize critical findings in the field while discussing common limitations with two animal models (3xTg-AD and TgF344-AD), novel pharmacotherapies, and potential early-intervention treatment options. We conclude by highlighting promising future pharmacotherapies and translational animal models of AD and anxiety.

Entorhinal cortex vulnerability to human APP expression promotes hyperexcitability and tau pathology

Preventative treatment for Alzheimer's Disease is of dire importance, and yet, cellular mechanisms underlying early regional vulnerability in Alzheimer's Disease remain unknown. In human patients with Alzheimer's Disease, one of the earliest observed pathophysiological correlates to cognitive decline is hyperexcitability1. In mouse models, early hyperexcitability has been shown in the entorhinal cortex, the first cortical region impacted by Alzheimer's Disease2-4. The origin of hyperexcitability in early-stage disease and why it preferentially emerges in specific regions is unclear. Using cortical-region and cell-type- specific proteomics and patch-clamp electrophysiology, we uncovered differential susceptibility to human-specific amyloid precursor protein (hAPP) in a model of sporadic Alzheimer's. Unexpectedly, our findings reveal that early entorhinal hyperexcitability may result from intrinsic vulnerability of parvalbumin interneurons, rather than the suspected layer II excitatory neurons. This vulnerability of entorhinal PV interneurons is specific to hAPP, as it could not be recapitulated with increased murine APP expression. Furthermore, the Somatosensory Cortex showed no such vulnerability to adult-onset hAPP expression, likely resulting from PV-interneuron variability between the two regions based on physiological and proteomic evaluations. Interestingly, entorhinal hAPP-induced hyperexcitability was quelled by co-expression of human Tau at the expense of increased pathological tau species. This study suggests early disease interventions targeting non-excitatory cell types may protect regions with early vulnerability to pathological symptoms of Alzheimer's Disease and downstream cognitive decline.

Connectome-based modelling of neurodegenerative diseases: towards precision medicine and mechanistic insight

Neurodegenerative diseases are the most common cause of dementia. Although their underlying molecular pathologies have been identified, there is substantial heterogeneity in the patterns of progressive brain alterations across and within these diseases. Recent advances in neuroimaging methods have revealed that pathological proteins accumulate along specific macroscale brain networks, implicating the network architecture of the brain in the system-level pathophysiology of neurodegenerative diseases. However, the extent to which 'network-based neurodegeneration' applies across the wide range of neurodegenerative disorders remains unclear. Here, we discuss the state-of-the-art of neuroimaging-based connectomics for the mapping and prediction of neurodegenerative processes. We review findings supporting brain networks as passive conduits through which pathological proteins spread. As an alternative view, we also discuss complementary work suggesting that network alterations actively modulate the spreading of pathological proteins between connected brain regions. We conclude this Perspective by proposing an integrative framework in which connectome-based models can be advanced along three dimensions of innovation: incorporating parameters that modulate propagation behaviour on the basis of measurable biological features; building patient-tailored models that use individual-level information and allowing model parameters to interact dynamically over time. We discuss promises and pitfalls of these strategies for improving disease insights and moving towards precision medicine.

Noradrenergic and cholinergic systems take centre stage in neuropsychiatric diseases of ageing

Noradrenergic and cholinergic systems are among the most vulnerable brain systems in neuropsychiatric diseases of ageing, including Alzheimer's disease, Parkinson's disease, Lewy body dementia, and progressive supranuclear palsy. As these systems fail, they contribute directly to many of the characteristic cognitive and psychiatric symptoms. However, their contribution to symptoms is not sufficiently understood, and pharmacological interventions targeting noradrenergic and cholinergic systems have met with mixed success. Part of the challenge is the complex neurobiology of these systems, operating across multiple timescales, and with non-linear changes across the adult lifespan and disease course. We address these challenges in a detailed review of the noradrenergic and cholinergic systems, outlining their roles in cognition and behaviour, and how they influence neuropsychiatric symptoms in disease. By bridging across levels of analysis, we highlight opportunities for improving drug therapies and for pursuing personalised medicine strategies.