Background:Chronic microglial activation is a key characteristic of Alzheimer's disease (AD). In mouse models of AD, microglial activation is considered associated with microglial cell population expansion. Objective:To elucidate species- and gene-dosage-dependent differences and similarities in microglial cell population expansion in response to amyloid-β (Aβ) plaque pathology. Methods:The total number of Iba1+ microglia in the neocortex of hemizygous APPswe/PS1ΔE9 (APP/PS1) mice and hemi- and homozygous TgF344-AD rats, carrying the same human mutations, was estimated by use of stereological techniques. Furthermore, microglial cluster formation was assessed. Proliferation was assessed in the mouse model. Results:A significant two-fold increase in microglia was observed in the neocortex of hemizygous APP/PS1 mice at 18 months and of homozygous TgF344-AD rats at 17 months. In comparison, the number of microglia in hemizygous TgF344-AD rats, remained constant from 4 to 17 months. Microglial clusters formed prior to the increase in microglial numbers in both species. The clusters were typically small, surrounding the smaller-sized Aβ plaques, occasionally also containing recently proliferated microglia. The Aβ plaque loads were comparable in hemizygous TgF344-AD rats and APP/PS1 mice, and two-three-fold higher in homozygous TgF344-AD rats. The microglial population remained constant across ages in wild types in both species. Conclusions:Transgenic mouse and rat AD models show significant differences in microglial population expansion, with a more restrained expansion in the rat. However, in both species and regardless of gene-dosage, population expansion is preceded by microglial clustering around Aβ plaques, indicating that cluster-formation is a key event in AD neuropathology.

Desynchronization of circadian rhythms is a hallmark of major depressive disorder (MDD). Agomelatine is an atypical antidepressant that acts as a melatonin receptor agonist and serotonin receptor antagonist. It has shown efficacy in alleviating symptoms of MDD with a favorable side effect profile. In the brain, agomelatine also modulates the glutamatergic system and in the present study, we investigated the effects of chronic mild stress (CMS) and agomelatine treatment on metabotropic glutamate receptor 5 (mGluR5) and synaptic vesicle glycoprotein 2 A (SV2A) binding in the medial prefrontal cortex (mPFC) and hippocampus (HP) in postmortem brain tissue derived from male rats using autoradiography. To account for diurnal influences, assessments were conducted at two time points: light-on (ZT6) and light-off (ZT18). The sucrose consumption test classified animals into four groups: Control, anhedonic-like, agomelatine responders, and non-responders. CMS increased mGluR5 binding in the prelimbic cortex of the mPFC during the light-on phase, an effect that was normalized by agomelatine treatment in responder rats. Agomelatine also reduced mGluR5 binding in the infralimbic cortex of the mPFC. No changes in mGluR5 binding were detected during the light-off phase or in the HP at either time point. Presynaptic density, assessed by SV2A levels, remained unchanged across all groups and time points. These findings reveal significant region-specific and diurnal alterations in mGluR5, emphasizing the role of time-of-day dependent timing in regulating mGluR5 and its association with depressive-like behaviors. Furthermore, the selective normalization of mGluR5 by agomelatine in responders reinforces its potential as a targeted therapeutic approach for MDD.

BackgroundAnimal models of Alzheimer’s disease (AD) are essential tools for investigating disease pathophysiology and conducting preclinical drug testing. In this study, we examined neuronal and glial alterations in the hippocampus and medial prefrontal cortex (mPFC) of young TgF344-AD rats and correlated these changes with cognitive decline and amyloid-β plaque load.MethodsWe compared TgF344-AD and non-transgenic littermate rats aged 7–8 months of age. We systematically quantified β-amyloid plaques, astrocytes, microglia, four different subtypes of GABAergic interneurons (calretinin-, cholecystokinin-, parvalbumin-, and somatostatin-positive neurons), and newly generated neurons in the hippocampus. Spatial learning and memory were assessed using the Barnes maze test.ResultsYoung TgF344-AD rats had a large number of amyloid plaques in both the hippocampus and mPFC, together with a pronounced increase in microglial cell numbers. Astrocytic activation was significant in the mPFC. Cholecystokinin-positive cell numbers were decreased in the hippocampus of transgenic rats, but calretinin-, parvalbumin-, and somatostatin-positive cell numbers were not altered. Adult neurogenesis was not affected by genotype. TgF344-AD rats had spatial learning and memory impairments, but this cognitive deficit did not correlate with amyloid plaque number or cellular changes in the brain. In the hippocampus, amyloid plaque numbers were negatively correlated with cholecystokinin-positive neuron and microglial cell numbers. In the mPFC, amyloid plaque number was negatively correlated with the number of astrocytes.ConclusionPronounced neuropathological changes were found in the hippocampus and mPFC of young TgF344-AD rats, including the loss of hippocampal cholecystokinin-positive interneurons. Some of these neuropathological changes were negatively correlated with amyloid-β plaque load, but not with cognitive impairment.

Background Chronic microglial activation is a key characteristic of Alzheimer's disease (AD). In mouse models of AD, microglial activation is considered associated with microglial cell population expansion. Objective To elucidate species- and gene-dosage-dependent differences and similarities in microglial cell population expansion in response to amyloid-β (Aβ) plaque pathology. Methods The total number of Iba1 + microglia in the neocortex of hemizygous APP swe /PS1 ΔE9 ( APP/PS1) mice and hemi- and homozygous TgF344-AD rats, carrying the same human mutations, was estimated by use of stereological techniques. Furthermore, microglial cluster formation was assessed. Proliferation was assessed in the mouse model. Results A significant two-fold increase in microglia was observed in the neocortex of hemizygous APP/PS1 mice at 18 months and of homozygous TgF344-AD rats at 17 months. In comparison, the number of microglia in hemizygous TgF344-AD rats, remained constant from 4 to 17 months. Microglial clusters formed prior to the increase in microglial numbers in both species. The clusters were typically small, surrounding the smaller-sized Aβ plaques, occasionally also containing recently proliferated microglia. The Aβ plaque loads were comparable in hemizygous TgF344-AD rats and APP/PS1 mice, and two-three-fold higher in homozygous TgF344-AD rats. The microglial population remained constant across ages in wild types in both species. Conclusions Transgenic mouse and rat AD models show significant differences in microglial population expansion, with a more restrained expansion in the rat. However, in both species and regardless of gene-dosage, population expansion is preceded by microglial clustering around Aβ plaques, indicating that cluster-formation is a key event in AD neuropathology.

Depression affects 20% of the general population and is a leading cause of disability worldwide, with a strong female prevalence. Current pharmacotherapies have significant limitations, and one third of patients are unresponsive. Male Wistar-Kyoto rats exposed to chronic mild stress (CMS) were recently proposed as a model to study antidepressant resistance. However, sex differences and interindividual vulnerability to stress are yet unexplored in this model. We aimed to investigate these in the context of the behavioural impact of CMS in the sucrose preference test, elevated plus maze (EPM), forced swim test (FST), open field test and daily locomotor activity rhythms, in male and female WKY rats exposed or not to a 4-week CMS protocol. CMS-exposed animals were clustered through K-means into subgroups based on the EPM and FST results. In both sexes, one subgroup behaved similarly to non-stressed animals and was labelled stress-non vulnerable; the second exhibited less open arms exploration in the EPM and higher immobility in the FST and was named stress-vulnerable. Vulnerable males presented phase delay in daily locomotor activity following CMS, but no significant rhythm could be determined in females. CMS-exposed males of both groups showed hyperlocomotion in reaction to novelty and slower weight gain through the course of CMS, while CMS-exposed females showed smaller sucrose intake. Unexpectedly, CMS did not affect sucrose preference. Our findings strengthen the view that in models of psychiatric pathologies based on stress exposure it is important to consider the effect of sex and to differentiate the non vulnerable and vulnerable subpopulations.

Exposure to severe, uncontrollable and long-lasting stress is a strong risk factor for the development of numerous mental and somatic disorders. Animal studies document that chronic stress can alter neuronal morphology and functioning in limbic brain structures such as the prefrontal cortex. Mitochondria are intracellular powerhouses generating chemical energy for biochemical reactions of the cell. Recent findings document that chronic stress can lead to changes in mitochondrial function and metabolism. Here, we studied putative mitochondrial damage in response to chronic stress in neurons of the medial prefrontal cortex. We performed a systematic quantitative ultrastructural analysis to examine the consequences of 9-weeks of chronic mild stress on mitochondria number and morphology in the infralimbic cortex of adult male rats. In this preliminary study, we analyzed 4,250 electron microscopic images and 67000 mitochondria were counted and examined in the brains of 4 control and 4 stressed rats. We found significantly reduced number of mitochondria in the infralimbic cortex of the stressed animals, but we could not detect any significant alteration in mitochondrial morphology. These data support the concept that prolonged stress can lead to mitochondrial loss. This in turn may result in impaired energy production. Reduced cellular energy may sensitize the neurons to additional injuries and may eventually trigger the development of psychopathologies.

Neurovascular coupling ensures rapid and precise delivery of O-2 and nutrients to active brain regions. Chronic stress is known to disturb neurovascular signaling with grave effects on brain integrity. We hypothesized that stress-induced neurovascular disturbances depend on stress susceptibility. Wistar male rats were exposed to 8 weeks of chronic mild stress. Stressed rats with anhedonia-like behavior and with preserved hedonic state were identified from voluntary sucrose consumption. In brain slices from nonstressed, anhedonic, and hedonic rats, neurons and astrocytes showed similar intracellular Ca2+ responses to neuronal excitation. Parenchymal arterioles in brain slices from nonstressed, anhedonic, and hedonic rats showed vasodilation in response to neuronal excitation. This vasodilation was dependent on inward rectifying K+ channel (K(ir)2) activation. In hedonic rats, this vasodilation was transient and followed by vasoconstriction insensitive to K(ir)2 channel inhibition with 100 mu M BaCl2. Isolated arteries from hedonic rats showed increased contractility. Elevation of bath K+ relaxed isolated middle cerebral arteries in a concentration-dependent and K(ir)2-dependent manner. The vasorelaxation to 20-24 mM K+ was reduced in arteries from hedonic rats. The expression of voltage-gated K+ channels, K(v)7.4, was reduced in the cerebral arteries from hedonic rats, whereas the expression of arterial inward-rectifying K+ channels, K(ir)2.1 was similar to that of nonstressed and anhedonic rats. We propose that preserved hedonic state is associated with increased arterial contractility caused by reduced hyperpolarizing contribution of K(v)7.4 channels leading to biphasic cerebrovascular responses to neuronal excitation. These findings reveal a novel potential coping mechanism associated with altered neurovascular signaling.

BACKGROUND:Modulation of serotonergic signaling by treatment with selective serotonin reuptake inhibitors (SSRIs) has been suggested to mitigate amyloid-β (Aβ) pathology in Alzheimer's disease, in addition to exerting an anti-depressant action.OBJECTIVE:To investigate the efficacy of chronic treatment with the SSRI paroxetine, in mitigating Aβ pathology and Aβ plaque-induced microgliosis in the hippocampus of 18-month-old APPswe/PS1ΔE9 mice.METHODS:Plaque-bearing APPswe/PS1ΔE9 and wildtype mice were treated with paroxetine per os at a dose of 5 mg/kg/day, from 9 to 18 months of age. The per os treatment was monitored by recording of the body weights and serum paroxetine concentrations, and by assessment of the serotonin transporter occupancy by [3H]DASB-binding in wildtype mice. Additionally, 5,7-dihydroxytryptamine was administered to 9-month-old APPswe/PS1ΔE9 mice, to examine the effect of serotonin depletion on Aβ pathology. Aβ pathology was evaluated by Aβ plaque load estimation and the Aβ42/Aβ40 ratio by ELISA.RESULTS:Paroxetine treatment led to > 80% serotonin transporter occupancy. The treatment increased the body weight of wildtype mice, but not of APPswe/PS1ΔE9 mice. The treatment had no effect on the Aβ plaque load (p = 0.39), the number and size of plaques, or the Aβ plaque-induced increases in microglial numbers in the dentate gyrus. Three months of serotonin depletion did not significantly impact the Aβ plaque load or Aβ42/Aβ40 ratio in APPswe/PS1ΔE9 mice at 12 months.CONCLUSION:Our results show that chronic treatment with the SSRI paroxetine does not mitigate Aβ pathology and Aβ plaque-induced microgliosis in the hippocampus of APPswe/PS1ΔE9 mice.

BACKGROUND Alzheimer's disease (AD) is a neurodegenerative disorder and the most common cause of dementia worldwide. Despite decades of investigation, the etiology of AD is not fully understood, although emerging evidence suggest that chronic environmental and psychological stress plays a role in the mechanisms and contributes to the risk of developing AD. Thus, dissecting the impact of stress on the brain could improve our understanding of the pathological mechanisms. OBJECTIVE We aimed to study the effect of chronic stress on the hippocampal proteome in male APPPS1 transgenic mice and wildtype (WT) littermates. METHODS APPPS1 and WT mice were subjected to 4 weeks of chronic stress followed by 3 weeks of continued diurnal disruption. Hippocampal tissue was used for proteomics analysis using label-free quantitative DIA based LC-MS/MS analysis. RESULTS We identified significantly up- and downregulated proteins in both APPPS1 and WT mice exposed to chronic stress compared to the control groups. Via interaction network mapping, significant proteins could be annotated to specific pathways of mitochondrial function (oxidative phosphorylation and TCA cycle), metabolic pathways, AD pathway and synaptic functions (long term potentiation). In WT mice, chronic stress showed the highest impact on complex I of the oxidative phosphorylation pathway, while in APPPS1 mice this pathway was compromised broadly by chronic stress. CONCLUSION Our data shows that chronic stress and amyloidosis additively contribute to mitochondrial damage in hippocampus. Although these results do not explain all effects of chronic stress in AD, they add to the scientific knowledge on the topic.

Depression is one of the most prevalent mental diseases worldwide. Patients with psychiatric diseases often have a history of childhood neglect, indicating that early-life experiences predispose to psychiatric diseases in adulthood. Two strong models were used in the present study: the maternal separation/early deprivation model (MS) and the chronic mild stress model (CMS). In both models, we found changes in the expression of a number of genes such as Creb and Npy. Strikingly, there was a clear regulation of expression of four genes involved in the AP-1 complex: c-Fos, c-Jun, FosB, and Jun-B. Interestingly, different expression levels were observed depending on the model, whereas the combination of the models resulted in a normal level of gene expression. The effects of MS and CMS on gene expression were associated with distinct histone methylation/acetylation patterns of all four genes. The epigenetic changes, like gene expression, were also dependent on the specific stressor or their combination. The obtained results suggest that single life events leave a mark on gene expression and the epigenetic signature of gene promoters, but a combination of different stressors at different life stages can further change gene expression through epigenetic factors, possibly causing the long-lasting adverse effects of stress.

Major depressive disorder is a leading cause of disability worldwide. Because conventional therapies are ineffective in many patients, novel strategies are needed to overcome treatment‐resistant depression (TRD). Limiting factors of successful drug development in the last decades were the lack of (1) knowledge of pathophysiology, (2) translational animal models and (3) objective diagnostic biomarkers. Here, we review novel drug targets and drug candidates currently investigated in Phase I–III clinical trials. The most promising approaches are inhibition of glutamatergic neurotransmission by NMDA and mGlu5 receptor antagonists, modulation of the opioidergic system by κ receptor antagonists, and hallucinogenic tryptamine derivates. The only registered drug for TRD is the NMDA receptor antagonist, S‐ketamine, but add‐on therapies with second‐generation antipsychotics, certain nutritive, anti‐inflammatory and neuroprotective agents seem to be effective. Currently, there is an intense research focus on large‐scale, high‐throughput omics and neuroimaging studies. These results might provide new insights into molecular mechanisms and potential novel therapeutic strategies.

Depression-associated cognitive impairments are among the most prevalent and persistent symptoms during remission from a depressive episode and a major risk factor for relapse. Consequently, development of antidepressant drugs, which also alleviate cognitive impairments, is vital. One such potential antidepressant is vortioxetine that has been postulated to exhibit both antidepressant and pro-cognitive effects. Hence, we tested vortioxetine for combined antidepressant and pro-cognitive effects in male Long-Evans rats exposed to the chronic mild stress (CMS) paradigm. This well-established CMS paradigm evokes cognitive deficits in addition to anhedonia, a core symptom of depression. Learning and memory performance was assessed in the translational touchscreen version of the paired-associates learning task. To identify the mechanistic underpinning of the neurobehavioural results, transcriptional profiling of genes involved in the stress response, neuronal plasticity and genes of broad relevance in neuropsychiatric pathologies were assessed. Vortioxetine substantially relieved the anhedonic-like state in the CMS rats and promoted acquisition of the cognitive test independent of hedonic phenotype, potentially due to an altered cognitive strategy. Minor alterations in gene expression profiling in prefrontal cortex and hippocampus were found. In summary, our findings suggest that vortioxetine exhibits an antidepressant effect as well as behavioural changes in a translational learning task.

Neuropsychiatric disturbances (NPDs) are considered hallmarks of Alzheimer's disease (AD). Nevertheless, treatment of these symptoms has proven difficult and development of safe and effective treatment options is hampered by the limited understanding of the underlying pathophysiology. Thus, robust preclinical models are needed to increase knowledge of NPDs in AD and develop testable hypotheses and novel treatment options. Abnormal activity of the hypothalamic-pituitary-adrenal (HPA) axis is implicated in many psychiatric symptoms and might contribute to both AD and NPDs development and progression. We aimed to establish a mechanistic preclinical model of NPD-like behavior in the APPPS1 mouse model of AD and wildtype (WT) littermates. In APPPS1 and WT mice, we found that chronic stress increased anxiety-like behavior and altered diurnal locomotor activity suggestive of sleep disturbances. Also, chronic stress activated the HPA axis, which, in WT mice, remained heightened for additional 3 weeks. Chronic stress caused irregular expression of circadian regulatory clock genes (BMAL1, PER2, CRY1 and CRY2) in both APPPS1 and WT mice. Interestingly, APPPS1 and WT mice responded differently to chronic stress in terms of expression of serotonergic markers (5-HT1A receptor and MAOA) and inflammatory genes (IL-6, STAT3 and ADMA17). These findings indicate that, although the behavioral response to chronic stress might be similar, the neurobiochemical response was different in APPPS1 mice, which is an important insight in the efforts to develop safe and effective treatments options for NPDs in AD patients. Further work is needed to substantiate these findings.

Major depressive disorder (MDD) is a severe illness imposing an increasing social and economic burden worldwide. Numerous rodent models have been developed to investigate the pathophysiology of MDD. One of the best characterized and most widely used models is the chronic mild stress (CMS) model which was developed more than 30 years ago by Paul Willner. More than 2000 published studies used this model, mainly to assess novel compounds with potential antidepressant efficacy. Most of these studies examined the behavioral consequences of stress and concomitant drug intervention. Much fewer studies focused on the CMS-induced neurobiological changes. However, the stress-induced cellular and molecular changes are important as they may serve as potential translational biomarkers and increase our understanding of the pathophysiology of MDD. Here, we summarize current knowledge on the structural and molecular alterations in the brain that have been described using the CMS model. We discuss the latest neuroimaging and postmortem histopathological data as well as molecular changes including recent findings on microRNA levels. Different chronic stress paradigms occasionally deliver dissimilar findings, but the available experimental data provide convincing evidence that the CMS model has a high translational value. Future studies examining the neurobiological changes in the CMS model in combination with clinically effective antidepressant drug intervention will likely deliver further valuable information on the pathophysiology of MDD.

The events that direct the shift from asymptomatic to symptomatic Alzheimer’s disease (AD) have yet to be fully characterized, however the consensus is that neuropsychiatric disturbances (NPDs) play a significant role in this. Most AD patients (80‐97%) experience at least one NPD at least once during their disease, nevertheless recognizing and treating NPDs remain a major challenge due to the poor understanding of the underlying pathology. Inflammation is thought to contribute to NPD development and ultimately exacerbate AD pathology. In AD, sleep disturbances drive neuroinflammation processes, precede clinical symptoms, and increase the risk of developing AD. Neuroinflammation can also be associated with more server NPDs and is linked to faster cognitive decline and worse prognosis. Contrary to memory and learning, which is reasonably well modelled in transgenic animals, robust preclinical models for NPDs are still lacking. We hypothesis that circadian disruption combined with chronic mild stress can introduce sleep disturbances, alter phenotypic behavior, and exacerbate AD‐related neuropathology in a common mouse model of AD.

Alzheimer’s disease (AD) is the most common form of dementia worldwide. It is mostly known for its devastating effect on memory and learning but behavioral alterations commonly known as neuropsychiatric disturbances (NPDs) are also characteristics of the disease. These include apathy, depression-like behavior, and sleep disturbances, and they all contribute to an increased caregiver burden and earlier institutionalization. The interaction between NPDs and AD pathology is not well understood, but the consensus is that they contribute to disease progression and faster decline. Consequently, recognizing and treating NPDs might improve AD pathology and increase the quality of life for both patients and caregivers. In this review article, we examine previous and current literature on apathy, depressive symptoms, and sleep disturbances in AD patients and preclinical AD mechanistic models. We hypothesize that tau accumulation, beta-amyloid (Aβ) aggregation, neuroinflammation, mitochondrial damage, and loss of the locus coeruleus (LC)-norepinephrine (NE) system all collectively impact the development of NPDs and contribute synergistically to AD pathology. Targeting more than one of these processes might provide the most optimal strategy for treating NPDs and AD. The development of such clinical approaches would be preceded by preclinical studies, for which robust and reliable mechanistic models of NPD-like behavior are needed. Thus, developing effective preclinical research models represents an important step towards a better understanding of NPDs in AD.