Altered neurochemical profile in the McGill-R-Thy1-APP rat model of Alzheimer's disease: a longitudinal in vivo 1 H MRS study

Sex Modifies the Impact of Type 2 Diabetes Mellitus on the Murine Whole Brain Metabolome

Type 2 diabetes mellitus (T2DM) leads to the development of cardiovascular diseases, cognitive impairment, and dementia. There are sex differences in the presentation of T2DM and its associated complications. We sought to determine the impact of sex and T2DM on the brain metabolome to gain insights into the underlying mechanisms of T2DM-associated cognitive complications. Untargeted metabolomic analysis was performed, using liquid chromatography-mass spectrometry, on whole brain tissue from adult male and female db/db mice (a T2DM model) compared to wild-type (WT) C57Bl6/J mice. Regardless of sex, T2DM increased free fatty acids and decreased acylcarnitines in the brain. Sex impacted the number (103 versus 65 in males and females, respectively), and types of metabolites shifted by T2DM. Many choline-containing phospholipids were decreased by T2DM in males. Female-specific T2DM effects included changes in neuromodulatory metabolites (γ-aminobutyric acid, 2-linoleoyl glycerol, N-methylaspartic acid, and taurine). Further, there were more significantly different metabolites between sexes in the T2DM condition as compared to the WT controls (54 vs. 15 in T2DM and WT, respectively). T2DM alters the murine brain metabolome in both sex-independent and sex-dependent manners. This work extends our understanding of brain metabolic sex differences in T2DM, cognitive implications, and potential sex-specific metabolic therapeutic targets.

Imaging Methods Applicable in the Diagnostics of Alzheimer's Disease, Considering the Involvement of Insulin Resistance

Alzheimer's disease (AD) is an incurable neurodegenerative disease and the most frequently diagnosed type of dementia, characterized by (1) perturbed cerebral perfusion, vasculature, and cortical metabolism; (2) induced proinflammatory processes; and (3) the aggregation of amyloid beta and hyperphosphorylated Tau proteins. Subclinical AD changes are commonly detectable by using radiological and nuclear neuroimaging methods such as magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET), and single-photon emission computed tomography (SPECT). Furthermore, other valuable modalities exist (in particular, structural volumetric, diffusion, perfusion, functional, and metabolic magnetic resonance methods) that can advance the diagnostic algorithm of AD and our understanding of its pathogenesis. Recently, new insights into AD pathoetiology revealed that deranged insulin homeostasis in the brain may play a role in the onset and progression of the disease. AD-related brain insulin resistance is closely linked to systemic insulin homeostasis disorders caused by pancreas and/or liver dysfunction. Indeed, in recent studies, linkages between the development and onset of AD and the liver and/or pancreas have been established. Aside from standard radiological and nuclear neuroimaging methods and clinically fewer common methods of magnetic resonance, this article also discusses the use of new suggestive non-neuronal imaging modalities to assess AD-associated structural changes in the liver and pancreas. Studying these changes might be of great clinical importance because of their possible involvement in AD pathogenesis during the prodromal phase of the disease.

Nicotinamide as potential biomarker for Alzheimer's disease: A translational study based on metabolomics

Introduction: The metabolic routes altered in Alzheimer's disease (AD) brain are poorly understood. As the metabolic pathways are evolutionarily conserved, the metabolic profiles carried out in animal models of AD could be directly translated into human studies. Methods: We performed untargeted Nuclear Magnetic Resonance metabolomics in hippocampus of McGill-R-Thy1-APP transgenic (Tg) rats, a model of AD-like cerebral amyloidosis and the translational potential of these findings was assessed by targeted Gas Chromatography-Electron Impact-Mass Spectrometry in plasma of participants in the German longitudinal cohort AgeCoDe. Results: In rat hippocampus 26 metabolites were identified. Of these 26 metabolites, nine showed differences between rat genotypes that were nominally significant. Two of them presented partial least square-discriminant analysis (PLS-DA) loadings with the larger absolute weights and the highest Variable Importance in Projection (VIP) scores and were specifically assigned to nicotinamide adenine dinucleotide (NAD) and nicotinamide (Nam). NAD levels were significantly decreased in Tg rat brains as compared to controls. In agreement with these results, plasma of AD patients showed significantly reduced levels of Nam in respect to cognitively normal participants. In addition, high plasma levels of Nam showed a 27% risk reduction of progressing to AD dementia within the following 2.5 years, this hazard ratio is lost afterwards. Discussion: To our knowledge, this is the first report showing that a decrease of Nam plasma levels is observed couple of years before conversion to AD, thereby suggesting its potential use as biomarker for AD progression.

Altered glutamate-glutamine and amide proton transfer-weighted values in the hippocampus of patients with amnestic mild cognitive impairment: A novel combined imaging diagnostic marker

Background and purpose

Early diagnosis of amnestic mild cognitive impairment (aMCI) and timely management to delay the onset of Alzheimer's disease (AD) would benefit patients. Pathological metabolic changes of excitatory/inhibitory neurotransmitters and abnormal protein deposition in the hippocampus of aMCI may provide a new clue to imaging diagnosis. However, the diagnostic performance using these hippocampal metabolite measurements is still unclear. We aimed to quantify right hippocampal glutamate-glutamine (Glx) and gamma-aminobutyric acid (GABA) levels as well as protein-based amide proton transfer-weighted (APTw) signals of patients with aMCI and investigate the diagnostic performance of these metabolites.

Methods

In this cross-sectional study, 20 patients with aMCI and 20 age- and gender-matched healthy controls (HCs) underwent MEGA Point Resolved Spectroscopy (MEGA-PRESS) and APTw MR imaging at 3 T. GABA+, Glx, and APTw signals were measured in the right hippocampus. The GABA+ levels, Glx levels, Glx/GABA+ ratios, and APTw values were compared between the HCs and aMCI groups using the Mann-Whitney U test. Binary logistic regression and receiver operating characteristic (ROC) curve analyses were used to evaluate MEGA-PRESS and APTw parameters' diagnostic performance.

Results

Compared with HCs, patients with aMCI had significantly lower Glx levels in the right hippocampus (7.02 ± 1.41 i.u. vs. 5.81 ± 1.33 i.u., P = 0.018). No significant changes in the GABA+ levels were observed in patients with aMCI (HCs vs. aMCI: 2.54 ± 0.28 i.u. vs. 2.47 ± 0.36 i.u., P = 0.620). In addition, Glx/GABA+ ratios between the two groups (HCs vs. aMCI: 2.79 ± 0.60 vs. 2.37 ± 0.55, P = 0.035) were significantly different. Compared with HCs, patients with aMCI showed higher APTw values in the right hippocampus (0.99 ± 0.26% vs. 1.26% ± 0.28, P = 0.006). The ROC curve analysis showed that Glx, GABA+, Glx/GABA+, and APTw values had an area under the curve (AUC) of 0.72, 0.55, 0.70, and 0.75, respectively, for diagnosing aMCI. In the ROC curve analysis, the AUC of the combination of the parameters increased to 0.88, which is much higher than that observed in the univariate analysis (P < 0.05).

Conclusion

The combination of right hippocampal Glx levels and APTw values improved the diagnostic performance for aMCI, indicating it as a promising combined imaging diagnostic marker. Our study provided a potential imaging diagnostic strategy of aMCI, which may promote early detection of aMCI and facilitate timely intervention to delay the pathological progress toward AD.

Spatio-temporal metabolic rewiring in the brain of TgF344-AD rat model of Alzheimer's disease

Brain damage associated with Alzheimer's disease (AD) occurs even decades before the symptomatic onset, raising the need to investigate its progression from prodromal stages. In this context, animal models that progressively display AD pathological hallmarks (e.g. TgF344-AD) become crucial. Translational technologies, such as magnetic resonance spectroscopy (MRS), enable the longitudinal metabolic characterization of this disease. However, an integrative approach is required to unravel the complex metabolic changes underlying AD progression, from early to advanced stages. TgF344-AD and wild-type (WT) rats were studied in vivo on a 7 Tesla MRI scanner, for longitudinal quantitative assessment of brain metabolic profile changes using MRS. Disease progression was investigated at 4 time points, from 9 to 18 months of age, and in 4 regions: cortex, hippocampus, striatum, and thalamus. Compared to WT, TgF344-AD rats replicated common findings in AD patients, including decreased N-acetylaspartate in the cortex, hippocampus and thalamus, and decreased glutamate in the thalamus and striatum. Different longitudinal evolution of metabolic concentration was observed between TgF344-AD and WT groups. Namely, age-dependent trajectories differed between groups for creatine in the cortex and thalamus and for taurine in cortex, with significant decreases in Tg344-AD animals; whereas myo-inositol in the thalamus and striatum showed greater increase along time in the WT group. Additional analysis revealed divergent intra- and inter-regional metabolic coupling in each group. Thus, in cortex, strong couplings of N-acetylaspartate and creatine with myo-inositol in WT, but with taurine in TgF344-AD rats were observed; whereas in the hippocampus, myo-inositol, taurine and choline compounds levels were highly correlated in WT but not in TgF344-AD animals. Furthermore, specific cortex-hippocampus-striatum metabolic crosstalks were found for taurine levels in the WT group but for myo-inositol levels in the TgF344-AD rats. With a systems biology perspective of metabolic changes in AD pathology, our results shed light into the complex spatio-temporal metabolic rewiring in this disease, reported here for the first time. Age- and tissue-dependent imbalances between myo-inositol, taurine and other metabolites, such as creatine, unveil their role in disease progression, while pointing to the inadequacy of the latter as an internal reference for quantification.

Taurine Supplementation as a Neuroprotective Strategy upon Brain Dysfunction in Metabolic Syndrome and Diabetes

Obesity, type 2 diabetes, and their associated comorbidities impact brain metabolism and function and constitute risk factors for cognitive impairment. Alterations to taurine homeostasis can impact a number of biological processes, such as osmolarity control, calcium homeostasis, and inhibitory neurotransmission, and have been reported in both metabolic and neurodegenerative disorders. Models of neurodegenerative disorders show reduced brain taurine concentrations. On the other hand, models of insulin-dependent diabetes, insulin resistance, and diet-induced obesity display taurine accumulation in the hippocampus. Given the possible cytoprotective actions of taurine, such cerebral accumulation of taurine might constitute a compensatory mechanism that attempts to prevent neurodegeneration. The present article provides an overview of brain taurine homeostasis and reviews the mechanisms by which taurine can afford neuroprotection in individuals with obesity and diabetes. We conclude that further research is needed for understanding taurine homeostasis in metabolic disorders with an impact on brain function.

Altered hippocampal astroglial metabolism is associated with aging and preserved spatial learning and memory

An age-related decrease in hippocampal metabolism correlates with cognitive decline. Hippocampus-dependent learning and memory requires glutamatergic neurotransmission supported by glutamate-glutamine (GLU-GLN) cycling between neurons and astrocytes. We examined whether GLU-GLN cycling in hippocampal subregions (dentate gyrus and CA1) in Fischer 344 rats was altered with age and cognitive status. Hippocampal slices from young adult, aged cognitively-unimpaired (AU) and aged cognitively-impaired (AI) rats were incubated in artificial cerebrospinal fluid (aCSF) containing 1-13C-glucose to assess neural metabolism. Incorporation of 13C-glucose into glutamate and glutamine, measured by mass spectroscopy/liquid chromatography tandem mass spectroscopy, did not significantly differ between groups. However, when 13C-acetate, a preferential astrocytic metabolite, was used, a significant increase in 13C-labeled glutamate was observed in slices from AU rats. Taken together, the data suggest that resting state neural metabolism and GLU-GLN cycling may be preserved during aging when sufficient extracellular glucose is available, but that enhanced astroglial metabolism can occur under resting state conditions. This may be an aging-related compensatory change to maintain hippocampus-dependent cognitive function.

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 relationship between measurement of in vivo brain glutamate and markers of iron metabolism: A proton magnetic resonance spectroscopy study in healthy adults

Fundamental human studies which address associations between glutamate and iron metabolism are needed. Basic research reports associations between glutamate and iron metabolism. Human studies report sex differences in iron metabolism and glutamate concentrations, which suggest that these relationships may differ by sex. We hypothesised associations would be apparent between in vivo glutamate and peripheral markers of iron metabolism, and these associations would differ by sex. To test this, we recruited 40 healthy adults (20 men, 20 women) and measured (a) standard clinical biomarker concentrations for iron metabolism and (b) an in vivo proxy for glutamate concentration, glutamate with glutamine in relation to total creatine containing metabolites using proton magnetic resonance spectroscopy studies with a two-dimensional chemical shift imaging slice, with voxels located in bilateral dorsolateral prefrontal cortices, anterior cingulate cortices and frontal white matter. Only the female group reported significant associations between peripheral markers of iron metabolism and Glx:tCr concentration: (a) right dorsolateral prefrontal cortex Glx:tCr associated positively with serum transferrin (r = .60, p = .006) and negatively with transferrin saturation (r = -.62, p = .004) and (b) right frontal white matter Glx:tCr associated negatively with iron concentration (r = -.59, p = .008) and transferrin saturation (r = -.65, p = .002). Our results support associations between iron metabolism and our proxy for in vivo glutamate concentration (Glx:tCr). These associations were limited to women, suggesting a stronger regulatory control between iron and glutamate metabolism. These associations support additional fundamental research into the molecular mechanisms of this regulatory control.

Ameliorative effect of deoxyvasicine on scopolamine-induced cognitive dysfunction by restoration of cholinergic function in mice

BACKGROUND

Aerial parts of Peganum harmala Linn is used as a traditional medical herb for treatment of amnesia in Uighur medicine in China. Deoxyvasicine (DVAS) is one of the chief active ingredients in P. harmala, it possesses strong acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibitory activities in vitro, but the therapeutic effect and mechanisms on amnesia in vivo are unclear.

PURPOSE

The objective of this study was to investigate the improvement effect of DVAS from P. harmala in learning and memory deficits of scopolamine-induced mice and elucidate the underlying mechanisms involved.

METHODS

Mice were pretreated with DVAS (5, 15 and 45 mg/kg) and huperzine-A (0.2 mg/kg) by gavage for 7 days, and subsequently were daily intraperitoneally injected with scopolamine (1 mg/kg) to induce learning and memory deficits and behavioral performance was assessed by Morris water maze. To further evaluate the potential mechanisms of DVAS in improving learning and memory capabilities, pathological change, levels of various biochemical markers and protein expressions related to cholinergic system, oxidative stress, and neuroinflammation were examined.

RESULTS

The results showed that DVAS could alleviate learning and memory deficits in scopolamine-treated mice. DVAS could regulate cholinergic function by inhibiting AChE and activating choline acetyltransferase (ChAT) activities and protein expressions. DVAS could induce brain-derived neurotrophic factor and protect hippocampal pyramidal cells against neuronal damage. DVAS also enhanced antioxidant defense via increasing the antioxidant enzyme level and activity of glutathione peroxidase, and anti-inflammatory function through suppressing tumor necrosis factor-α. Additionally, DVAS could regulate the neurotransmitters by elevating acetylcholine, 5-hydroxytryptamine, γ-aminobutyric acid and reducing 5-hydroxyindole-3-acetic acid and glutamic acid.

CONCLUSION

Results illustrated that DVAS may be a promising candidate compound against amnesia via restoration of cholinergic function, regulating neurotransmitters, attenuating neuroinflammation and oxidative stress.

Experimental Pharmacology in Transgenic Rodent Models of Alzheimer's Disease

This Mini Review discusses the merits and shortfalls of transgenic (tg) rodents modeling aspects of the human Alzheimer’s disease (AD) pathology and their application to evaluate experimental therapeutics. It addresses some of the differences between mouse and rat tg models for these investigations. It relates, in a condensed fashion, the experience of our research laboratory with the application of anti-inflammatory compounds and S-adenosylmethionine (SAM) at the earliest stages of AD-like amyloid pathology in tg mice. The application of SAM was intended to revert the global brain DNA hypomethylation unleashed by the intraneuronal accumulation of amyloid-β-immunoreactive material, an intervention that restored levels of DNA methylation including of the bace1 gene. This review also summarizes experimental pharmacology observations made in the McGill tg rat model of AD-like pathology by applying “nano-lithium” or a drug with allosteric M1 muscarinic and sigma 1 receptor agonistic properties (AF710B). Extremely low doses of lithium (up to 400 times lower than used in the clinic) had remarkable beneficial effects on lowering pathology and improving cognitive functions in tg rats. Likewise, AF710B treatment, even at advanced stages of the pathology, displayed remarkable beneficial effects. This drug, in experimental conditions, demonstrated possible “disease-modifying” properties as pathology was frankly diminished and cognition improved after a month of “wash-out” period. The Mini-Review ends with a discussion on the predictive value of similar experimental pharmacological interventions in current rodent tg models. It comments on the validity of some of these approaches for early interventions at preclinical stages of AD, interventions which may be envisioned once definitive diagnosis of AD before clinical presentation is made possible.

In vivo molecular imaging of neuroinflammation in Alzheimer's disease

It has become increasingly evident that neuroinflammation plays a critical role in the pathophysiology of Alzheimer's disease (AD) and other neurodegenerative disorders. Increased glial cell activation is consistently reported in both rodent models of AD and in AD patients. Moreover, recent genome wide association studies have revealed multiple genes associated with inflammation and immunity are significantly associated with an increased risk of AD development (e.g. TREM2). Non‐invasive in vivo detection and tracking of neuroinflammation is necessary to enhance our understanding of the contribution of neuroinflammation to the initiation and progression of AD. Importantly, accurate methods of quantifying neuroinflammation may aid early diagnosis and serve as an output for therapeutic monitoring and disease management. This review details current in vivo imaging biomarkers of neuroinflammation being explored and summarizes both pre‐clinical and clinical results from molecular imaging studies investigating the role of neuroinflammation in AD, with a focus on positron emission tomography and magnetic resonance spectroscopy (MRS).

Decreased Taurine and Creatine in the Thalamus May Relate to Behavioral Impairments in Ethanol-Fed Mice: A Pilot Study of Proton Magnetic Resonance Spectroscopy

Minimal hepatic encephalopathy (MHE) is highly prevalent, observed in up to 80% of patients with liver dysfunction. Minimal hepatic encephalopathy is defined as hepatic encephalopathy with cognitive deficits and no grossly evident neurologic abnormalities. Clinical management may be delayed due to the lack of in vivo quantitative methods needed to reveal changes in brain neurobiochemical biomarkers. To gain insight into the development of alcoholic liver disease–induced neurological dysfunction (NDF), a mouse model of late-stage alcoholic liver fibrosis (LALF) was used to investigate changes in neurochemical levels in the thalamus and hippocampus that relate to behavioral changes. Proton magnetic resonance spectroscopy of the brain and behavioral testing were performed to determine neurochemical alterations and their relationships to behavioral changes in LALF. Glutamine levels were higher in both the thalamus and hippocampus of alcohol-treated mice than in controls. Thalamic levels of taurine and creatine were significantly diminished and strongly correlated with alcohol-induced behavioral changes. Chronic long-term alcohol consumption gives rise to advanced liver fibrosis, neurochemical changes in the nuclei, and behavioral changes which may be linked to NDF. Magnetic resonance spectroscopy represents a sensitive and noninvasive measurement of pathological alterations in the brain, which may provide insight into the pathogenesis underlying the development of MHE.

Increased astroglial activity and reduced neuronal function across brain in AβPP-PS1 mouse model of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease associated with progressive loss of cognitive function, personality, and behavior. The present study evaluates neuronal and astroglial metabolic activity, and neurotransmitter cycle fluxes in AβPP-PS1 mouse model of AD by using ¹H-[¹³C]-nuclear magnetic resonance (NMR) spectroscopy together with an infusion of either [1,6-¹³C₂]glucose or [2-¹³C]acetate. The levels of N-acetyl-aspartate (NAA) and glutamate were found to be decreased in the cerebral cortex and hippocampus in AβPP-PS1 mice, when compared with wild type controls. The cerebral metabolic rate of acetate oxidation was increased in the hippocampus and cerebral cortex of AβPP-PS1 mice suggesting enhanced astroglial activity in AD. AβPP-PS1 mice exhibit severe reduction in glutamatergic and gamma-amino butyric acid (GABA)ergic neuronal metabolic activity and neurotransmitter cycling fluxes in the hippocampus, cerebral cortex, and striatum as compared with controls. These data suggest that metabolic activity of excitatory and inhibitory neurons is compromised across brain in AβPP-PS1 mouse model of AD.

Analogous β-Carboline Alkaloids Harmaline and Harmine Ameliorate Scopolamine-Induced Cognition Dysfunction by Attenuating Acetylcholinesterase Activity, Oxidative Stress, and Inflammation in Mice

The analogous β-carboline alkaloids, harmaline (HAL) and harmine (HAR), possess a variety of biological properties, including acetylcholinesterase (AChE) inhibitory activity, antioxidant, anti-inflammatory, and many others, and have great potential for treating Alzheimer’s disease (AD). However, studies have showed that the two compounds have similar structures and in vitro AChE inhibitory activities but with significant difference in bioavailability. The objective of this study was to comparatively investigate the effects of HAL and HAR in memory deficits of scopolamine-induced mice. In the present study, mice were pretreated with HAL (2, 5, and 10 mg/kg), HAR (10, 20, and 30 mg/kg) and donepezil (5 mg/kg) by intragastrically for 7 days, and were daily intraperitoneal injected with scopolamine (1 mg/kg) to induce memory deficits and then subjected to behavioral evaluation by Morris water maze. To further elucidate the underlying mechanisms of HAL and HAR in improving learning and memory, the levels of various biochemical factors and protein expressions related to cholinergic function, oxidative stress, and inflammation were examined. The results showed that HAL and HAR could effectively ameliorate memory deficits in scopolamine-induced mice. Both of them exhibited an enhancement in cholinergic function by inhibiting AChE and inducing choline acetyltransferase (ChAT) activities, and antioxidant defense via increasing the antioxidant enzymes activities of superoxide dismutase and glutathione peroxidase, and reducing maleic diadehyde production, and anti-inflammatory effects through suppressing myeloperoxidase, tumor necrosis factor α, and nitric oxide as well as modulation of critical neurotransmitters such as acetylcholine (ACh), choline (Ch), L-tryptophan (L-Trp), 5-hydroxytryptamine (5-HT), γ-aminobutyric acid (γ-GABA), and L-glutamic acid (L-Glu). Furthermore, the regulations of HAL on cholinergic function, inflammation, and neurotransmitters were more striking than those of HAR, and HAL manifested a comparable antioxidant capacity to HAR. Remarkably, the effective dosage of HAL (2 mg/kg) was far lower than that of HAR (20 mg/kg), which probably due to the evidently differences in the bioavailability and metabolic stability of the two analogs. Taken together, all these results revealed that HAL may be a promising candidate compound with better anti-amnesic effects and pharmacokinetic characteristics for the treatments of AD and related diseases.

Structural Magnetic Resonance Imaging Markers of Alzheimer's Disease and Its Retranslation to Rodent Models

The importance of imaging biomarkers has been acknowledged in the diagnosis and in the follow-up of Alzheimer's disease (AD), one of the major causes of dementia. Next to the molecular biomarkers and PET imaging investigations, structural MRI approaches provide important information about the disease progression and about the pathomechanism. Furthermore,a growing body of literature retranslates these imaging biomarkers to various rodent models of the disease. The goal of this review is to provide an overview of the macro- and microstructural imaging biomarkers of AD, concentrating on atrophy measures and diffusion MRI alterations. A survey is also given of the imaging approaches used in rodent models of dementias that can promote drug development.

Preclinical Magnetic Resonance Imaging and Spectroscopy Studies of Memory, Aging, and Cognitive Decline

Neuroimaging provides for non-invasive evaluation of brain structure and activity and has been employed to suggest possible mechanisms for cognitive aging in humans. However, these imaging procedures have limits in terms of defining cellular and molecular mechanisms. In contrast, investigations of cognitive aging in animal models have mostly utilized techniques that have offered insight on synaptic, cellular, genetic, and epigenetic mechanisms affecting memory. Studies employing magnetic resonance imaging and spectroscopy (MRI and MRS, respectively) in animal models have emerged as an integrative set of techniques bridging localized cellular/molecular phenomenon and broader in vivo neural network alterations. MRI methods are remarkably suited to longitudinal tracking of cognitive function over extended periods permitting examination of the trajectory of structural or activity related changes. Combined with molecular and electrophysiological tools to selectively drive activity within specific brain regions, recent studies have begun to unlock the meaning of fMRI signals in terms of the role of neural plasticity and types of neural activity that generate the signals. The techniques provide a unique opportunity to causally determine how memory-relevant synaptic activity is processed and how memories may be distributed or reconsolidated over time. The present review summarizes research employing animal MRI and MRS in the study of brain function, structure, and biochemistry, with a particular focus on age-related cognitive decline.

Assessment of brain metabolite correlates of adeno-associated virus-mediated over-expression of human alpha-synuclein in cortical neurons by in vivo (1) H-MR spectroscopy at 9.4 T

In this study, we used proton-localized spectroscopy ((1) H-MRS) for the acquisition of the neurochemical profile longitudinally in a novel rat model of human wild-type alpha-synuclein (α-syn) over-expression. Our goal was to find out if the increased α-syn load in this model could be linked to changes in metabolites in the frontal cortex. Animals injected with AAV vectors encoding for human α-syn formed the experimental group, whereas green fluorescent protein expressing animals were used as the vector-treated control group and a third group of uninjected animals were used as naïve controls. Data were acquired at 2, 4, and 8 month time points. Nineteen metabolites were quantified in the MR spectra using LCModel software. On the basis of 92 spectra, we evaluated any potential gender effect and found that lactate (Lac) levels were lower in males compared to females, while the opposite was observed for ascorbate (Asc). Next, we assessed the effect of age and found increased levels of GABA, Tau, and GPC+PCho. Finally, we analyzed the effect of treatment and found that Lac levels (p = 0.005) were specifically lower in the α-syn group compared to the green fluorescent protein and control groups. In addition, Asc levels (p = 0.05) were increased in the vector-injected groups, whereas glucose levels remained unchanged. This study indicates that the metabolic switch between glucose-lactate could be detectable in vivo and might be modulated by Asc. No concomitant changes were found in markers of neuronal integrity (e.g., N-acetylaspartate) consistent with the fact that α-syn over-expression in cortical neurons did not result in neurodegeneration in this model. We acquired the neurochemical profile longitudinally in a rat model of human wild-type alpha-synuclein (α-syn) over-expression in cortical neurons. We found that Lactate levels were reduced in the α-syn group compared to the control groups and Ascorbate levels were increased in the vector-injected groups. No changes were found in markers of neuronal integrity consistent with the fact that α-syn over-expression did not result in frank neurodegeneration.

Stereological estimation of neuron number and plaque load in the hippocampal region of a transgenic rat model of Alzheimer's disease

The main hallmarks of Alzheimer's disease (AD) are senile plaques, neurofibrillary tangles and neuronal death. The McGill-R-Thy1-APP rat is one of the few transgenic rat models of AD that displays progressive amyloid pathology. This study aimed to further characterise this rat model, focusing on the pathological changes in the hippocampal formation and the parahippocampal region. These structures, that are important for episodic memory and spatial navigation, are affected in the early stages of the disease. This study used unbiased stereology to investigate possible neuronal loss in the CA1, subiculum and entorhinal cortex of 18-month-old homozygous McGill-R-Thy1-APP rats, and also quantified the plaque load in all the areas of the hippocampal formation and parahippocampal region from 9 to 18 months old. A significant reduction of neurons at 18 months was only seen in the subiculum. The first plaque pathology was seen at 9 months in the subiculum. Although the quantified plaque load was variable between animals, the pattern of spatiotemporal progression was similar for all animals. The spread of plaque pathology mainly affected anatomically connected regions. Overall, the plaque pathology observed in the transgenic rats was similar to the early phases of amyloid beta (Aβ)-deposition described in human patients. The findings here thus indicate that the McGill-R-Thy1-APP rat could be a good model of the Aβ pathology in AD, but less so with respect to neuron loss.

Neuronal marker recovery after Simvastatin treatment in dementia in the rat brain: in vivo magnetic resonance study

The aim of study was to search for new biomarkers with a magnetic resonance technique to identify the early stages of dementia, induced by D-galactose, and evaluate Simvastatin therapy. Localized proton magnetic resonance spectroscopy measurements showed a significant decrease in the concentration of N-acetylaspartate+N-acetylaspartylglutamate and myo-inositol in the D-galactose group compared to the control group, and, conversely, an increase of N-acetylaspartate+N-acetylaspartylglutamate in the D-galactose/Simvastatin group. Using a saturation transfer experiment, with phosphorus magnetic resonance spectroscopy, we observed a significant elevation of the forward rate constant of the creatine kinase reaction in the brains of the D-galactose group compared to controls, and subsequently, a significant reduction of this reaction in the D-galactose/Simvastatin group. Spatial learning and memory were evaluated using the modified Morris water maze test. The dynamics of the learning process represented by the learning index revealed a significant reduction in learning in the D-galactose group, but the deficits as a consequence of the D-galactose effects were recovered in the D-galactose/Simvastatin group, in which the learning dynamics resembled those of the control group. By determining the thiobarbituric acid reactive substances and total coenzyme Q9 in plasma, we have shown that long-term administration of D-galactose created conditions for oxidative stress, and that the administration of Simvastatin decreased oxidative stress in plasma. Volumetry analyses from the hippocampal area show a reduction in the segmented area in the D-galactose group, compared with the control group, and an enlarged area in the hippocampus in the d-galactose/Simvastatin group.

Age-related alterations in the metabolic profile in the hippocampus of the senescence-accelerated mouse prone 8: a spontaneous Alzheimer's disease mouse model

Alzheimer's disease (AD), the most common age-dependent neurodegenerative disorder, produces a progressive decline in cognitive function. The metabolic mechanism of AD has emerged in recent years. In this study, we used multivariate analyses of gas chromatography-mass spectrometry measurements to determine that learning and retention-related metabolic profiles are altered during aging in the hippocampus of the senescence-accelerated mouse prone 8 (SAMP8). Alterations in 17 metabolites were detected in mature and aged mice compared to young mice (13 decreased and 4 increased metabolites), including metabolites related to dysfunctional lipid metabolism (significantly increased cholesterol, oleic acid, and phosphoglyceride levels), decreased amino acid (alanine, serine, glycine, aspartic acid, glutamate, and gamma-aminobutyric acid), and energy-related metabolite levels (malic acid, butanedioic acid, fumaric acid, and citric acid), and other altered metabolites (increased N-acetyl-aspartic acid and decreased pyroglutamic acid, urea, and lactic acid) in the hippocampus. All of these alterations indicated that the metabolic mechanisms of age-related cognitive impairment in SAMP8 mice were related to multiple pathways and networks. Lipid metabolism, especially cholesterol metabolism, appears to play a distinct role in the hippocampus in AD.

3.0 T MRI arterial spin labeling and magnetic resonance spectroscopy technology in the application of Alzheimer's disease

The purpose of this study was to investigate the changes in the cerebral blood flow (CBF) and spectrum variables in the posterior cingulate region of patients with AD under the detection of arterial spin labeling (ASL) and magnetic resonance spectroscopy (MRS). A total of 20 AD patients (8 males and 12 females; mean age, 64.84±8.82 years) and 20 healthy controls (9 males and 11 females; mean age, 64.94±7.93 years) were included in this study for analysis. All images were obtained using a 3.0-T MR imager and an 8-channel head array receiving coil. MRS measurements were conducted exploring variables of metabolite ratios. Statistical analyses were conducted with the SPSS 11.0 statistical software package. Findings in the present study revealed a significant difference in the mean MMSE scores between the AD group and the healthy control group (16.21±4.01 vs. 27.35±1.01, P<0.01). Compared with the healthy control group, CBF in the bilateral frontal region showed a significant decrease in the AD group (right frontal: 83.5±7.2 vs. 110±11.5, P<0.05; left frontal: 85.6±8.1 vs. 108.7±12.2, P<0.05, respectively), and a similar association was also observed in the TL, TPJ, parietal, and hippocampal regions (all P<0.05). MRS imaging in the posterior cingulate region showed a significant reduction in the NAA/Cr ratio in the AD group (1.43±0.1 vs. 1.49±0.0, P<0.05). Additionally, we found that the MI/Cr and Cho/Cr ratios were higher than normal controls in patients with AD (all P<0.05). Our results suggested that 3D ASL detection combined with MRS in studying AD could show the regional hypo-perfusion with the decrease of CBF and the abnormal metabolic changes of the posterior cingulate cortex.

Glutamate synthesis has to be matched by its degradation - where do all the carbons go?

The central process in energy production is the oxidation of acetyl-CoA to CO2 by the tricarboxylic acid (TCA, Krebs, citric acid) cycle. However, this cycle functions also as a biosynthetic pathway from which intermediates leave to be converted primarily to glutamate, GABA, glutamine and aspartate and to a smaller extent to glucose derivatives and fatty acids in the brain. When TCA cycle ketoacids are removed, they must be replaced to permit the continued function of this essential pathway, by a process termed anaplerosis. Since the TCA cycle cannot act as a carbon sink, anaplerosis must be coupled with cataplerosis; the exit of intermediates from the TCA cycle. The role of anaplerotic reactions for cellular metabolism in the brain has been studied extensively. However, the coupling of this process with cataplerosis and the roles that both pathways play in the regulation of amino acid, glucose, and fatty acid homeostasis have not been emphasized. The concept of a linkage between anaplerosis and cataplerosis should be underscored, because the balance between these two processes is essential. The hypothesis that cataplerosis in the brain is achieved by exporting the lactate generated from the TCA cycle intermediates into the blood and perivascular area is presented. This shifts the generally accepted paradigm of lactate generation as simply derived from glycolysis to that of oxidation and might present an alternative explanation for aerobic glycolysis. Intermediates leave the tricarboxylic acid cycle and must be replaced by a process termed anaplerosis that must be coupled to cataplerosis. We hypothesize that cataplerosis is achieved by exporting the lactate generated from the cycle into the blood and perivascular area. This shifts the paradigm of lactate generation as solely derived from glycolysis to that of oxidation and might present an alternative explanation for aerobic glycolysis.

Decreased γ-aminobutyric acid levels in the parietal region of patients with Alzheimer's disease

PURPOSE

To determine whether there are in vivo differences of γ-aminobutyric acid (GABA) levels in frontal and parietal regions of Alzheimer's disease (AD) patients, compared with healthy controls using magnetic resonance spectroscopy ((1) H-MRS).

MATERIALS AND METHODS

Fifteen AD patients and fifteen age- and gender-matched healthy controls underwent (1) H-MRS of the frontal and parietal lobes using the "MEGA-Point Resolved Spectroscopy Sequence" (MEGA-PRESS) technique, and cognitive levels of subjects were evaluated using Mini-Mental State Examination (MMSE) tests. MRS data were processed using the Gannet program. Because the signal detected by MEGA-PRESS includes contributions from GABA, macromolecules and homocarnosine, it is labeled as "GABA+" rather than GABA. Differences of GABA+/Cr ratios between AD patients and controls were tested using covariance analysis, adjusting for gray matter fraction. The relationship between GABA+/Cr and MMSE scores was also analyzed.

RESULTS

Significant lower GABA+/Cr ratios were found in the parietal region of AD patients compared with controls (P = 0.041). In AD patients, no significant correlations between GABA+/Cr and MMSE scores were found in either the frontal (r = -0.164; P = 0.558) or parietal regions (r = 0.025; P = 0.929).

CONCLUSION

Decreased GABA+/Cr levels were present in the parietal region of patients with AD in vivo, suggesting that abnormalities of the GABAergic system may be present in the pathogenesis of AD.

Neuronal and astrocytic metabolism in a transgenic rat model of Alzheimer's disease

Regional hypometabolism of glucose in the brain is a hallmark of Alzheimer's disease (AD). However, little is known about the specific alterations of neuronal and astrocytic metabolism involved in homeostasis of glutamate and GABA in AD. Here, we investigated the effects of amyloid β (Aβ) pathology on neuronal and astrocytic metabolism and glial-neuronal interactions in amino acid neurotransmitter homeostasis in the transgenic McGill-R-Thy1-APP rat model of AD compared with healthy controls at age 15 months. Rats were injected with [1-(13)C]glucose and [1,2-(13)C]acetate, and extracts of the hippocampal formation as well as several cortical regions were analyzed using (1)H- and (13)C nuclear magnetic resonance spectroscopy and high-performance liquid chromatography. Reduced tricarboxylic acid cycle turnover was evident for glutamatergic and GABAergic neurons in hippocampal formation and frontal cortex, and for astrocytes in frontal cortex. Pyruvate carboxylation, which is necessary for de novo synthesis of amino acids, was decreased and affected the level of glutamine in hippocampal formation and those of glutamate, glutamine, GABA, and aspartate in the retrosplenial/cingulate cortex. Metabolic alterations were also detected in the entorhinal cortex. Overall, perturbations in energy- and neurotransmitter homeostasis, mitochondrial astrocytic and neuronal metabolism, and aspects of the glutamate-glutamine cycle were found in McGill-R-Thy1-APP rats.

Modeling Alzheimer's disease in transgenic rats

Alzheimer's disease (AD) is the most common form of dementia. At the diagnostic stage, the AD brain is characterized by the accumulation of extracellular amyloid plaques, intracellular neurofibrillary tangles and neuronal loss. Despite the large variety of therapeutic approaches, this condition remains incurable, since at the time of clinical diagnosis, the brain has already suffered irreversible and extensive damage. In recent years, it has become evident that AD starts decades prior to its clinical presentation. In this regard, transgenic animal models can shed much light on the mechanisms underlying this "pre-clinical" stage, enabling the identification and validation of new therapeutic targets. This paper summarizes the formidable efforts to create models mimicking the various aspects of AD pathology in the rat. Transgenic rat models offer distinctive advantages over mice. Rats are physiologically, genetically and morphologically closer to humans. More importantly, the rat has a well-characterized, rich behavioral display. Consequently, rat models of AD should allow a more sophisticated and accurate assessment of the impact of pathology and novel therapeutics on cognitive outcomes.

Glutamate metabolism is impaired in transgenic mice with tau hyperphosphorylation

In neurodegenerative diseases including Alzheimer's disease and frontotemporal dementia, the protein tau is hyperphosphorylated and eventually aggregates to develop neurofibrillary tangles. Here, the consequences of tau hyperphosphorylation on both neuronal and astrocytic metabolism and amino-acid neurotransmitter homeostasis were assessed in transgenic mice expressing the pathogenic mutation P301L in the human tau gene (pR5 mice) compared with nontransgenic littermate controls. Mice were injected with the neuronal and astrocytic substrate [1-(13)C]glucose and the astrocytic substrate [1,2-(13)C]acetate. Hippocampus and cerebral cortex extracts were analyzed using (1)H and (13)C nuclear magnetic resonance spectroscopy, gas chromatography-mass spectrometry and high-performance liquid chromatography. The glutamate level was reduced in the hippocampus of pR5 mice, accompanied by reduced incorporation of (13)C label derived from [1-(13)C]glucose in glutamate. In the cerebral cortex, glucose utilization as well as turnover of glutamate, glutamine, and GABA, were increased. This was accompanied by a relative increase in production of glutamate via the pyruvate carboxylation pathway in cortex. Overall, we revealed that astrocytes as well as glutamatergic and GABAergic neurons in the cortex of pR5 mice were in a hypermetabolic state, whereas in the hippocampus, where expression levels of mutant human tau are the highest, glutamate homeostasis was impaired.