Effect of high-fat diet on metabolic indices, cognition, and neuronal physiology in aging F344 rats

Variable bioenergetic sensitivity of neurons and astrocytes to insulin and extracellular glucose

Energy flow within cellular elements of the brain is a well-orchestrated, tightly regulated process, however, details underlying these functions at the single-cell level are still poorly understood. Studying hypometabolism in aging and neurodegenerative diseases may benefit from experimentation on unicellular bioenergetics. Here, we examined energy status in neurons and astrocytes using mixed hippocampal cultures and PercevalHR, an ATP:ADP nanosensor. We assessed exposures of several compounds including KCl, glutamate, FCCP, insulin, and glucose. A mitochondrial stress test was performed, and PercevalHR's fluorescence was corrected for pH using pHrodo. Results demonstrate that PercevalHR can reliably report on the energetic status of two cell types that communicate in a mixed-culture setting. While KCl, glutamate, and FCCP showed clear changes in PercevalHR fluorescence, insulin and glucose responses were found to be more subtle and sensitive to extracellular glucose. These results may highlight mechanisms that mediate insulin sensitivity in the brain.

Affective and Cognitive Impairments in Rodent Models of Diabetes

Diabetes and related acute and long-term complications have a profound impact on cognitive, emotional, and social behavior, suggesting that the central nervous system (CNS) is a crucial substrate for diabetic complications. When anxiety, depression, and cognitive deficits occur in diabetic patients, the symptoms and complications related to the disease worsen, contributing to lower quality of life while increasing health care costs and mortality. Experimental models of diabetes in rodents are a fundamental and valuable tool for improving our understanding of the mechanisms underlying the close and reciprocal link between diabetes and CNS alterations, including the development of affective and cognitive disorders. Such models must reproduce the different components of this pathological condition in humans and, therefore, must be associated with affective and cognitive behavioral alterations. Beyond tight glycemic control, there are currently no specific therapies for neuropsychiatric comorbidities associated with diabetes; animal models are, therefore, essential for the development of adequate therapies. To our knowledge, there is currently no review article that summarizes changes in affective and cognitive behavior in the most common models of diabetes in rodents. Therefore, in this review, we have reported the main evidence on the alterations of affective and cognitive behavior in the different models of diabetes in rodents, the main mechanisms underlying these comorbidities, and the applicable therapeutic strategy.

The Effect of Fat Intake with Increased Omega-6-to-Omega-3 Polyunsaturated Fatty Acid Ratio in Animal Models of Early and Late Alzheimer's Disease-like Pathogenesis

This work aims to clarify the effect of dietary polyunsaturated fatty acid (PUFA) intake on the adult brain affected by amyloid pathology. McGill-R-Thy1-APP transgenic (Tg) rat and 5xFAD Tg mouse models that represent earlier or later disease stages were employed. The animals were exposed to a control diet (CD) or an HFD based on corn oil, from young (rats) or adult (mice) ages for 24 or 10 weeks, respectively. In rats and mice, the HFD impaired reference memory in wild-type (WT) animals but did not worsen it in Tg, did not cause obesity, and did not increase triglycerides or glucose levels. Conversely, the HFD promoted stronger microglial activation in Tg vs. WT rats but had no effect on cerebral amyloid deposition. IFN-γ, IL-1β, and IL-6 plasma levels were increased in Tg rats, regardless of diet, while CXCL1 chemokine levels were increased in HFD-fed mice, regardless of genotype. Hippocampal 3-nitrotyrosine levels tended to increase in HFD-fed Tg rats but not in mice. Overall, an HFD with an elevated omega-6-to-omega-3 ratio as compared to the CD (25:1 vs. 8.4:1) did not aggravate the outcome of AD regardless of the stage of amyloid pathology, suggesting that many neurobiological processes relevant to AD are not directly dependent on PUFA intake.

CAR Protects Females from Diet-Induced Steatosis and Associated Metabolic Disorders

Non-Alcoholic Fatty Liver Disease (NAFLD) is the most common cause of chronic liver disease worldwide, affecting 70-90% of obese individuals. In humans, a lower NAFLD incidence is reported in pre-menopausal women, although the mechanisms affording this protection remain under-investigated. Here, we tested the hypothesis that the constitutive androstane nuclear receptor (CAR) plays a role in the pathogenesis of experimental NAFLD. Male and female wild-type (WT) and CAR knock-out (CAR-/-) mice were subjected to a high-fat diet (HFD) for 16 weeks. We examined the metabolic phenotype of mice through body weight follow-up, glucose tolerance tests, analysis of plasmatic metabolic markers, hepatic lipid accumulation, and hepatic transcriptome. Finally, we examined the potential impact of HFD and CAR deletion on specific brain regions, focusing on glial cells. HFD-induced weight gain and hepatic steatosis are more pronounced in WT males than females. CAR-/- females present a NASH-like hepatic transcriptomic signature suggesting a potential NAFLD to NASH transition. Transcriptomic correlation analysis highlighted a possible cross-talk between CAR and ERα receptors. The peripheral effects of CAR deletion in female mice were associated with astrogliosis in the hypothalamus. These findings prove that nuclear receptor CAR may be a potential mechanism entry-point and a therapeutic target for treating NAFLD/NASH.

Falling Short: The Contribution of Central Insulin Receptors to Gait Dysregulation in Brain Aging

Insulin resistance, which manifests as a reduction of insulin receptor signaling, is known to correlate with pathological changes in peripheral tissues as well as in the brain. Central insulin resistance has been associated with impaired cognitive performance, decreased neuronal health, and reduced brain metabolism; however, the mechanisms underlying central insulin resistance and its impact on brain regions outside of those associated with cognition remain unclear. Falls are a leading cause of both fatal and non-fatal injuries in the older population. Despite this, there is a paucity of work focused on age-dependent alterations in brain regions associated with ambulatory control or potential therapeutic approaches to target these processes. Here, we discuss age-dependent alterations in central modalities that may contribute to gait dysregulation, summarize current data supporting the role of insulin signaling in the brain, and highlight key findings that suggest insulin receptor sensitivity may be preserved in the aged brain. Finally, we present novel results showing that administration of insulin to the somatosensory cortex of aged animals can alter neuronal communication, cerebral blood flow, and the motivation to ambulate, emphasizing the need for further investigations of intranasal insulin as a clinical management strategy in the older population.

Sensitivity of the S1 neuronal calcium network to insulin and Bay-K 8644 in vivo: Relationship to gait, motivation, and aging processes

Neuronal hippocampal Ca²⁺ dysregulation is a critical component of cognitive decline in brain aging and Alzheimer's disease and is suggested to impact communication and excitability through the activation of a larger after hyperpolarization. However, few studies have tested for the presence of Ca²⁺ dysregulation in vivo, how it manifests, and whether it impacts network function across hundreds of neurons. Here, we tested for neuronal Ca²⁺ network dysregulation in vivo in the primary somatosensory cortex (S1) of anesthetized young and aged male Fisher 344 rats using single‐cell resolution techniques. Because S1 is involved in sensory discrimination and proprioception, we tested for alterations in ambulatory performance in the aged animal and investigated two potential pathways underlying these central aging‐ and Ca²⁺‐dependent changes. Compared to young, aged animals displayed increased overall activity and connectivity of the network as well as decreased ambulatory speed. In aged animals, intranasal insulin (INI) increased network synchronicity and ambulatory speed. Importantly, in young animals, delivery of the L‐type voltage‐gated Ca²⁺ channel modifier Bay‐K 8644 altered network properties, replicating some of the changes seen in the older animal. These results suggest that hippocampal Ca²⁺ dysregulation may be generalizable to other areas, such as S1, and might engage modalities that are associated with locomotor stability and motivation to ambulate. Further, given the safety profile of INI in the clinic and the evidence presented here showing that this central dysregulation is sensitive to insulin, we suggest that these processes can be targeted to potentially increase motivation and coordination while also reducing fall frequency with age.

Resveratrol attenuates behavioural impairment associated with learning and memory in HFD-STZ induced diabetic rats

BACKGROUND AND PURPOSE

Literature have indicated that a high-fat diet (HFD) is a common risk factor for type 2 diabetes mellitus (T2DM) and its associated cognitive-impairments. Mounting evidence supports that, in the diabetic animal model, resveratrol (RSV, SIRT1-modulator) can regulate the fasting glucose and antioxidant levels, as well as the lipid profile, and may alleviate the cognitive-dysfunction associated with diabetes.

EXPERIMENTAL APPROACH

Albino rats were fed 60% HFD-STZ (45mg/kg,i.p, single dose) to induce T2DM so that the experimental T2DM animal model could be used. After 14 weeks of the animals being in a confirmed diabetic condition, they were divided into various groups and treated with metformin(200mg/kg,i.p.) and RSV(50 and 100 mg/kg,i.p.) for four weeks. A multimodal approach involving oxidative-nitroso-stress, SIRT1, TGF-β1 levels, inflammation, cholinergic activity (serum, hippocampus, cerebral cortex), and a battery of behavioural studies associated with learning-memory were performed during and after the experimental-protocol.

KEY RESULTS

The administration of RSV significantly attenuated the increased glucose levels (pre, and post-prandial), impaired glucose tolerance, HbA1c, and decreased the body weights of the T2DM rats. Moreover, RSV ameliorated the impaired learning and memory-associated with increased SIRT1 and the decreased TGF-β1, TNF-α, oxidative-nitroso-stress and cholinergic activities in the serum and the brains of the T2DM-animals.

CONCLUSION AND IMPLICATION

Our investigations demonstrate that SIRT1-modulation can inter-play with TGF-β1 signalling, as well as mitigate hyperglycaemia and subsequent learning-memory impairments, in the T2DM-animals. Moreover, our study showed that novel therapeutic-targets, including TGF-β1, may add to our knowledge of RSV when used in the treatment of impaired memory-associated with diabetes.

Hibiscus sabdariffa extract improves hepatic steatosis, partially through IRS-1/Akt and Nrf2 signaling pathways in rats fed a high fat diet

Non-alcoholic fatty liver disease (NAFLD) has become a major world-wide health problem and is characterized by lipid accumulation in the liver induced by high fat diet (HFD) consumption. It is usually associated with inflammation, oxidative stress, and insulin resistance. Roselle extract (Hibiscus sabdariffa) is an herb which is used in traditional medicine. However, further study is necessary to represent the mechanism of NAFLD and find new preventive strategies. This study aims to investigate the protective effects of roselle extract on NAFLD rat models. Male Sprague-Dawley rats (n = 35) were divided into 5 groups, control, HFD, HFD + Simvastatin (HFD + SIM), HFD + 250 mg/kg BW, and HFD + 500 mg/kg BW of roselle extract (HFD + R250 and HFD + R500, respectively). The results showed that roselle extract reduced hepatic lipid contents, de novo lipogenesis enzymes, microsomal triglyceride transfer protein, inflammatory cytokines, malondialdehyde, and increased antioxidant properties, transporter related with lipoprotein uptake, and insulin signal proteins. Comparing to SIM, the HFD + R500 group exhibited the greater benefit in terms of anti-hepatic steatosis, antioxidant properties, and an ability to improve insulin resistance. This study demonstrates that roselle extract improved antioxidant properties and attenuated hepatic steatosis, liver inflammation, oxidative stress, and insulin resistance in HFD-induced NAFLD in rats, which could be used for NAFLD prevention.

Sex Differences in Cognition Across Aging

Sex and gender differences are seen in cognitive disturbances in a variety of neurological and psychiatry diseases. Men are more likely to have cognitive symptoms in schizophrenia whereas women are more likely to have more severe cognitive symptoms with major depressive disorder and Alzheimer's disease. Thus, it is important to understand sex and gender differences in underlying cognitive abilities with and without disease. Sex differences are noted in performance across various cognitive domains - with males typically outperforming females in spatial tasks and females typically outperforming males in verbal tasks. Furthermore, there are striking sex differences in brain networks that are activated during cognitive tasks and in learning strategies. Although rarely studied, there are also sex differences in the trajectory of cognitive aging. It is important to pay attention to these sex differences as they inform researchers of potential differences in resilience to age-related cognitive decline and underlying mechanisms for both healthy and pathological cognitive aging, depending on sex. We review literature on the progressive neurodegenerative disorder, Alzheimer's disease, as an example of pathological cognitive aging in which human females show greater lifetime risk, neuropathology, and cognitive impairment, compared to human males. Not surprisingly, the relationships between sex and cognition, cognitive aging, and Alzheimer's disease are nuanced and multifaceted. As such, this chapter will end with a discussion of lifestyle factors, like education and diet, as modifiable factors that can alter cognitive aging by sex. Understanding how cognition changes across age and contributing factors, like sex differences, will be essential to improving care for older adults.

Visceral adiposity, inflammation, and hippocampal function in obesity

The 'apple-shaped' anatomical pattern that accompanies visceral adiposity increases risk for multiple chronic diseases, including conditions that impact the brain, such as diabetes and hypertension. However, distinguishing between the consequences of visceral obesity, as opposed to visceral adiposity-associated metabolic and cardiovascular pathologies, presents certain challenges. This review summarizes current literature on relationships between adipose tissue distribution and cognition in preclinical models and highlights unanswered questions surrounding the potential role of tissue- and cell type-specific insulin resistance in these effects. While gaps in knowledge persist related to insulin insensitivity and cognitive impairment in obesity, several recent studies suggest that cells of the neurovascular unit contribute to hippocampal synaptic dysfunction, and this review interprets those findings in the context of progressive metabolic dysfunction in the CNS. Signalling between cerebrovascular endothelial cells, astrocytes, microglia, and neurons has been linked with memory deficits in visceral obesity, and this article describes the cellular changes in each of these populations with respect to their role in amplification or diminution of peripheral signals. The picture emerging from these studies, while incomplete, implicates pro-inflammatory cytokines, insulin resistance, and hyperglycemia in various stages of obesity-induced hippocampal dysfunction. As in the parable of the five blind wanderers holding different parts of an elephant, considerable work remains in order to assemble a model for the underlying mechanisms linking visceral adiposity with age-related cognitive decline.

Long-term obesity is associated with depression and neuroinflammation

OBJECTIVE

Obesity is characterized by a state of chronic, low-intensity systemic inflammation frequently associated with insulin resistance and dyslipidemia.

METHODS

Given that chronic inflammation has been implicated in the pathogenesis of mood disorders, we investigated if chronic obesity that was initiated early in life - lasting through adulthood - could be more harmful to memory impairment and mood fluctuations such as depression.

RESULTS

Here we show that pre-pubertal male rats (30 days old) treated with a high-fat diet (40%) for 8-months gained ~50% more weight when compared to controls, exhibited depression and anxiety-like behaviors but no memory impairment. The prefrontal cortex of the obese rats exhibited an increase in the expression of genes related to inflammatory response, such as NFKb, MMP9, CCl2, PPARb, and PPARg. There were no alterations in genes known to be related to depression.

CONCLUSION

Long-lasting obesity with onset in prepuberal age led to depression and neuroinflammation but not to memory impairment.

B Vitamins Supplementation Can Improve Cognitive Functions and May Relate to the Enhancement of Transketolase Activity in A Rat Model of Cognitive Impairment Associated with High-fat Diets

OBJECTIVE

To determine whether B vitamin treatment was sufficient to reduce cognitive impairment associated with high-fat diets in rats and to modulate transketolase (TK) expression and activity.

METHODS

To test this, we separated 50 rats into five groups that were either fed a standard chow diet (controls) or a high-fat diet (experimental groups H0, H1, H2, and H3). H0 group animals received no additional dietary supplementation, while H1 group animals were administered 100 mg/kg body weight (BW) thiamine, 100 mg/kg BW riboflavin, and 250 mg/kg BW niacin each day, and group H2 animals received daily doses of 100 mg/kg BW pyridoxine, 100 mg/kg BW cobalamin, and 5 mg/kg BW folate. Animals in the H3 group received the B vitamin regimens administered to both H1 and H2 each day.

RESULTS

Over time, group H0 exhibited greater increases in BW and fat mass relative to other groups. When spatial and memory capabilities in these animals were evaluated via conditioned taste aversion (CTA) and Morris Water Maze (MWM), we found B vitamin treatment was associated with significant improvements relative to untreated H0 controls. Similarly, B vitamin supplementation was associated with elevated TK expression in erythrocytes and hypothalamus of treated animals relative to those in H0 (P<0.05).

CONCLUSION

Together, these findings suggest B vitamin can modulate hypothalamic TK activity to reduce the severity of cognitive deficits in a rat model of obesity. As such, B vitamin supplementation may be a beneficial method for reducing cognitive dysfunction in clinical settings associated with high-fat diets.

Aging mitigates the severity of obesity-associated metabolic sequelae in a gender independent manner

BACKGROUND

Understanding gender-associated bias in aging and obesity-driven metabolic derangements has been hindered by the inability to model severe obesity in female mice.

METHODS

Here, using chow- or high fat diet (HFD)-feeding regimens at standard (TS) and thermoneutral (TN) housing temperatures, the latter to model obesity in female mice, we examined the impact of gender and aging on obesity-associated metabolic derangements and immune responsiveness. Analysis included quantification of: (i) weight gain and adiposity; (ii) the development and severity of glucose dysmetabolism and non-alcoholic fatty liver disease (NAFLD); and (iii) induction of inflammatory pathways related to metabolic dysfunction.

RESULTS

We show that under chow diet feeding regimen, aging was accompanied by increased body weight and white adipose tissue (WAT) expansion in a gender independent manner. HFD feeding regimen in aged, compared to young, male mice at TS, resulted in attenuated glucose dysmetabolism and hepatic steatosis. However, under TS housing conditions only aged, but not young, HFD fed female mice developed obesity. At TN however, both young and aged HFD fed female mice developed severe obesity. Independent of gender or housing conditions, aging attenuated the severity of metabolic derangements in HFD-fed obese mice. Tempered severity of metabolic derangements in aged mice was associated with increased splenic frequency of regulatory T (Treg) cells, Type I regulatory (Tr1)-like cells and circulating IL-10 levels and decreased vigor of HFD-driven induction of inflammatory pathways in adipose and liver tissues.

CONCLUSION

Our findings suggest that aging-associated altered immunological profile and inflammatory vigor may play a dominant role in the attenuation of obesogenic diet-driven metabolic dysfunction.

Nutritional Status and Diet Style Affect Cognitive Function in Alcoholic Liver Disease

Malnutrition and cognitive dysfunction are typical features of alcoholic liver disease (ALD) and are correlated with the development of complications. The aim of this study is to explore the effect of nutritional state and diet on cognitive function in ALD. A total of 43 patients with compensated alcoholic cirrhosis were enrolled, and a neuropsychological test was assessed according to body mass index (BMI, <22 and ≥22). In the ALD animal study, mice were divided into five groups (n = 9/group; normal liquid, 5% EtOH + regular liquid, 5% EtOH + high-carbohydrate liquid, 5% EtOH + high-fat liquid, and 5% EtOH + high-protein liquid diet) and fed the same calories for eight weeks. To assess cognitive function, we performed T-maze studies weekly before/after alcohol binging. In cognitive function (BMI < 22/≥22), language score of Korea mini-mental state (7.4 ± 1.4/7.9 ± 0.4), Boston naming (11.7 ± 2.7/13.0 ± 1.8), forward digit span (6.7 ± 1.8/7.5 ± 1.6), Korean color word stroop (24.2 ± 26.5/43.6 ± 32.4), and interference score (33.9 ± 31.9/52.3 ± 33.9) revealed significant differences. In the T-maze test, alcohol significantly delayed the time to reach food, and binge drinking provided a temporary recovery in cognition. The alcohol-induced delay was significantly reduced in the high-carbohydrate and high-fat diet groups. Synaptic function exhibited no changes in all groups. Cognitive dysfunction is affected by nutritional status and diet in ALD.

Lactobacillus plantarum DR7 improved brain health in aging rats via the serotonin, inflammatory and apoptosis pathways

Aging processes affect the brain in many ways, ranging from cellular to functional levels which lead to cognitive decline and increased oxidative stress. The aim of this study was to investigate the potentials of Lactobacillus plantarum DR7 on brain health including cognitive and memory functions during aging and the impacts of high fat diet during a 12-week period. Male Sprague-Dawley rats were separated into six groups: (1) young animals on normal diet (ND, (2) young animals on a high fat diet (HFD), (3) aged animals on ND, (4) aged animals on HFD, (5) aged animals on HFD and L. plantarum DR7 (10⁹ cfu/day) and (6) aged animals receiving HFD and lovastatin. To induce ageing, all rats in group 3 to 6 were injected sub-cutaneously at 600 mg/kg/day of D-galactose daily. The administration of DR7 has reduced anxiety accompanied by enhanced memory during behavioural assessments in aged-HFD rats (P<0.05). Hippocampal concentration of all three pro-inflammatory cytokines were increased during aging but reduced upon administration of both statin and DR7. Expressions of hippocampal neurotransmitters and apoptosis genes showed reduced expressions of indoleamine dioxygenase and P53 accompanied by increased expression of TPH1 in aged- HFD rats administered with DR7, indicating potential effects of DR7 along the pathways of serotonin and oxidative senescence. This study provided an insight into potentials of L. plantarum DR7 as a prospective dietary strategy to improve cognitive functions during aging. This study provided an insight into potentials of L. plantarum DR7 as a prospective dietary strategy to improve cognitive functions during aging.

Molecular elevation of insulin receptor signaling improves memory recall in aged Fischer 344 rats

As demonstrated by increased hippocampal insulin receptor density following learning in animal models and decreased insulin signaling, receptor density, and memory decline in aging and Alzheimer's diseases, numerous studies have emphasized the importance of insulin in learning and memory processes. This has been further supported by work showing that intranasal delivery of insulin can enhance insulin receptor signaling, alter cerebral blood flow, and improve memory recall. Additionally, inhibition of insulin receptor function or expression using molecular techniques has been associated with reduced learning. Here, we sought a different approach to increase insulin receptor activity without the need for administering the ligand. A constitutively active, modified human insulin receptor (IRβ) was delivered to the hippocampus of young (2 months) and aged (18 months) male Fischer 344 rats in vivo. The impact of increasing hippocampal insulin receptor expression was investigated using several outcome measures, including Morris water maze and ambulatory gait performance, immunofluorescence, immunohistochemistry, and Western immunoblotting. In aged animals, the IRβ construct was associated with enhanced performance on the Morris water maze task, suggesting that this receptor was able to improve memory recall. Additionally, in both age-groups, a reduced stride length was noted in IRβ-treated animals along with elevated hippocampal insulin receptor levels. These results provide new insights into the potential impact of increasing neuronal insulin signaling in the hippocampus of aged animals and support the efficacy of molecularly elevating insulin receptor activity in vivo in the absence of the ligand to directly study this process.

Acetylator Genotype-Dependent Dyslipidemia in Rats Congenic for N-Acetyltransferase 2

Recent reports suggest that arylamine N-acetyltransferases (NAT1 and/or NAT2) serve important roles in regulation of energy utility and insulin sensitivity. We investigated the interaction between diet (control vs. high-fat diet) and acetylator phenotype (rapid vs. slow) using previously established congenic rat lines (in F344 background) that exhibit rapid or slow Nat2 (orthologous to human NAT1) acetylator genotypes. Male and female rats of each genotype were fed control or high-fat (Western-style) diet for 26 weeks. We then examined diet- and acetylator genotype-dependent changes in body and liver weights, systemic glucose tolerance, insulin sensitivity, and plasma lipid profile. Male and female rats on the high fat diet weighed approximately 10% more than rats on the control diet and the percentage liver to body weight was consistently higher in rapid than slow acetylator rats. Rapid acetylator rats were more prone to develop dyslipidemia overall (i.e., higher triglyceride; higher LDL; and lower HDL), compared to slow acetylator rats. Total cholesterol (TC)-to-HDL ratios were significantly higher and HDL-to-LDL ratios were significantly lower in rapid acetylator rats. Our data suggest that rats with rapid systemic Nat2 (NAT1 in humans) genotype exhibited higher dyslipidemia conferring risk for metabolic syndrome and cardiovascular dysfunction.

Elevating Insulin Signaling Using a Constitutively Active Insulin Receptor Increases Glucose Metabolism and Expression of GLUT3 in Hippocampal Neurons

Insulin signaling is an integral component of healthy brain function, with evidence of positive insulin-mediated alterations in synaptic integrity, cerebral blood flow, inflammation, and memory. However, the specific pathways targeted by this peptide remain unclear. Previously, our lab used a molecular approach to characterize the impact of insulin signaling on voltage-gated calcium channels and has also shown that acute insulin administration reduces calcium-induced calcium release in hippocampal neurons. Here, we explore the relationship between insulin receptor signaling and glucose metabolism using similar methods. Mixed, primary hippocampal cultures were infected with either a control lentivirus or one containing a constitutively active human insulin receptor (IRβ). 2-NBDG imaging was used to obtain indirect measures of glucose uptake and utilization. Other outcome measures include Western immunoblots of GLUT3 and GLUT4 on total membrane and cytosolic subcellular fractions. Glucose imaging data indicate that neurons expressing IRβ show significant elevations in uptake and rates of utilization compared to controls. As expected, astrocytes did not respond to the IRβ treatment. Quantification of Western immunoblots show that IRβ is associated with significant elevations in GLUT3 expression, particularly in the total membrane subcellular fraction, but did not alter GLUT4 expression in either fraction. Our work suggests that insulin plays a significant role in mediating neuronal glucose metabolism, potentially through an upregulation in the expression of GLUT3. This provides further evidence for a potential therapeutic mechanism underlying the beneficial impact of intranasal insulin in the clinic.

Effects of Maternal Resveratrol on Maternal High-Fat Diet/Obesity with or without Postnatal High-Fat Diet

To examine the effects of maternal resveratrol in rats borne to dams with gestational high-fat diet (HFD)/obesity with or without postnatal high-fat diet. We first tested the effects of maternal resveratrol intake on placenta and male fetus brain in rats borne to dams with gestational HFD/obesity. Then, we assessed the possible priming effect of a subsequent insult, male offspring were weaned onto either a rat chow or a HFD. Spatial learning and memory were assessed by Morris water maze test. Blood pressure and peripheral insulin resistance were examined. Maternal HFD/obesity decreased adiponectin, phosphorylation alpha serine/threonine-protein kinase (pAKT), sirtuin 1 (SIRT1), and brain-derived neurotrophic factor (BDNF) in rat placenta, male fetal brain, and adult male offspring dorsal hippocampus. Maternal resveratrol treatment restored adiponectin, pAKT, and BDNF in fetal brain. It also reduced body weight, peripheral insulin resistance, increased blood pressure, and alleviated cognitive impairment in adult male offspring with combined maternal HFD and postnatal HFD. Maternal resveratrol treatment restored hippocampal pAKT and BDNF in rats with combined maternal HFD and postnatal HFD in adult male offspring dorsal hippocampus. Maternal resveratrol intake protects the fetal brain in the context of maternal HFD/obesity. It effectively reduced the synergistic effects of maternal HFD/obesity and postnatal HFD on metabolic disturbances and cognitive impairment in adult male offspring. Our data suggest that maternal resveratrol intake may serve as an effective therapeutic strategy in the context of maternal HFD/obesity.

27-Hydroxycholesterol Induces Aberrant Morphology and Synaptic Dysfunction in Hippocampal Neurons

Hypercholesterolemia is a risk factor for neurodegenerative diseases, but how high blood cholesterol levels are linked to neurodegeneration is still unknown. Here, we show that an excess of the blood-brain barrier permeable cholesterol metabolite 27-hydroxycholesterol (27-OH) impairs neuronal morphology and reduces hippocampal spine density and the levels of the postsynaptic protein PSD95. Dendritic spines are the main postsynaptic elements of excitatory synapses and are crucial structures for memory and cognition. Furthermore, PSD95 has an essential function for synaptic maintenance and plasticity. PSD95 synthesis is controlled by the REST-miR124a-PTBP1 axis. Here, we report that high levels of 27-OH induce REST-miR124a-PTBP1 axis dysregulation in a possible RxRγ-dependent manner, suggesting that 27-OH reduces PSD95 levels through this mechanism. Our results reveal a possible molecular link between hypercholesterolemia and neurodegeneration. We discuss the possibility that reduction of 27-OH levels could be a useful strategy for preventing memory and cognitive decline in neurodegenerative disorders.

White matter inflammation and cognitive function in a co-morbid metabolic syndrome and prodromal Alzheimer's disease rat model

BACKGROUND

Metabolic syndrome, the development of which is associated with high-caloric Western diet (HCD) intake, represent a risk factor for mild cognitive impairment (MCI) and dementia including Alzheimer's disease (AD) later in life. This study aimed to investigate the effect of diet-induced metabolic disturbances on white matter neuroinflammation and cognitive function in a transgenic (TG) Fischer 344 rat carrying a human β-amyloid precursor protein (APP) gene with Swedish and Indiana mutations (APP21 TG), a model of pre-AD and MCI.

METHODS

TG and wildtype (WT) rats received either a HCD with 40% kJ from fat supplemented with 20% corn syrup drink or a standard diet for 12 weeks. Body weight, caloric intake, and blood pressure were measured repeatedly. End-point changes in glucose and lipid metabolism were also assessed. Open field task was used for assessment of activity; Morris water maze was used to assess spatial learning and memory. Cerebral white matter microglia and astrocytes, hippocampal neurons, and neuronal synapses were examined using immunohistochemistry.

RESULTS

Rats maintained on the HCD developed significant obesity, visceral adiposity, dyslipidemia, and hyperinsulinemia, but did not become hypertensive. Impaired glucose tolerance was observed only in WT rats on the HCD. Total microglia number, activated OX-6+ microglia, as well as GFAP+ astrocytes located predominantly in the white matter were greater in the APP21 TG rat model in comparison to WT rats. HCD-driven metabolic perturbations further exacerbated white matter microgliosis and microglia cell activation in the APP21 TG rats and led to detectable changes in spatial reference memory in the comorbid prodromal AD and metabolic syndrome group compared to WT control rats. Neuronal density in the CA1 subregion of the hippocampus was not different between the experimental groups. Synaptic density in the CA1 and CA3 hippocampal subregions was lower in the TG rats compared to WT rats; however, there was no additional effect of the co-morbidity on this measure.

CONCLUSIONS

These results suggest that white matter neuroinflammation might be one of the possible processes of early interaction of metabolic syndrome with MCI and pre-AD and could be one of the early brain pathologies contributing to cognitive deficits observed in mild cognitive impairment and dementia, including AD cases.

Can krill oil be of use for counteracting neuroinflammatory processes induced by high fat diet and aging?

Most neurodegenerative diseases, such as Alzheimer's and Parkinson's disease, demonstrate preceding or on-going inflammatory processes. Therefore, discovering effective means of counteracting detrimental inflammatory mediators in the brain could help alter aging-related disease onset and progression. Fish oil and marine-derived omega-3, long-chain polyunsaturated fatty acids (LC n-3) have shown promising anti-inflammatory effects both systemically and centrally. More specifically, krill oil (KO), extracted from small Antarctic crustaceans, is an alternative type of LC n-3 with reported health benefits including improvement of spatial memory and learning, memory loss, systemic inflammation and depression symptoms. Similar to the more widely studied fish oil, KO contains the long chain fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) which are essential for basic brain functions. Moreover, the phospholipid bound nature of fatty acids found in KO improves bioavailability and efficiency of absorption, thus supporting the belief that KO may offer a superior method of dietary n-3 delivery. Finally, KO contains astaxanthin, an antioxidant capable of reducing potentially excessive oxidative stress and inflammation within the brain. This review will discuss the potential benefits of KO over other marine-based LC n-3 on brain inflammation and cognitive function in the context of high fat diets and aging.

Neuroprotective effect of levetiracetam in mouse diabetic retinopathy: Effect on glucose transporter-1 and GAP43 expression

AIMS

Retinopathy is a neurodegenerative complication associating diabetes mellitus. Diabetic retinopathy (DR) is the primary reason of visual loss during early adulthood. DR has a complicated multifactorial pathophysiology initiated by hyperglycaemia-induced ischaemic neurodegenerative retinal changes, followed by vision-threatening consequences. The main therapeutic modalities for DR involve invasive delivery of intravitreal antiangiogenic agents as well as surgical interventions. The current work aimed to explore the potential anti-inflammatory and retinal neuroprotective effects of levetiracetam.

MAIN METHODS

This study was performed on alloxan-induced diabetes in mice (n: 21). After 10 weeks, a group of diabetic animals (n: 7) was treated with levetiracetam (25 mg/kg) for six weeks. Retinal tissues were dissected and paraffin-fixed for examination using (1) morphometric analysis with haematoxylin and eosin (HE), (2) immunohistochemistry (GLUT1, GFAP and GAP43), and (3) RT-PCR-detected expression of retinal inflammatory and apoptotic mediators (TNF-α, IL6, iNOS, NF-κB and Tp53).

KEY FINDINGS

Diabetic mice developed disorganized and debilitated retinal layers with upregulation of the gliosis marker GFAP and downregulation of the neuronal plasticity marker GAP43. Additionally, diabetic retinae showed increased transcription of NF-κB, TNF-α, IL6, iNOS and Tp53. Levetiracetam-treated mice showed downregulation of retinal GLUT1 with relief and regression of retinal inflammation and improved retinal structural organization.

SIGNIFICANCE

Levetiracetam may represent a potential neuroprotective agent in DR. The data presented herein supported an anti-inflammatory role of levetiracetam. However, further clinical studies may be warranted to confirm the effectiveness and safety of levetiracetam in DR patients.

A Crosstalk Between Brain Cholesterol Oxidation and Glucose Metabolism in Alzheimer's Disease

In Alzheimer’s disease (AD), both cholesterol and glucose dysmetabolism precede the onset of memory deficit and contribute to the disease’s progression. It is indeed now believed that oxidized cholesterol in the form of oxysterols and altered glucose uptake are the main triggers in AD affecting production and clearance of Aβ, and tau phosphorylation. However, only a few studies highlight the relationship between them, suggesting the importance of further extensive studies on this topic. Recently, a molecular link was demonstrated between cholesterol oxidative metabolism and glucose uptake in the brain. In particular, 27-hydroxycholesterol, a key linker between hypercholesterolemia and the increased AD risk, is considered a biomarker for reduced glucose metabolism. In fact, its excess increases the activity of the renin-angiotensin system in the brain, thus reducing insulin-mediated glucose uptake, which has a major impact on brain functioning. Despite this important evidence regarding the role of 27-hydroxycholesterol in regulating glucose uptake by neurons, the involvement of other cholesterol oxidation products that have been clearly demonstrated to be key players in AD cannot be ruled out. This review highlights the current understanding of the potential role of cholesterol and glucose dysmetabolism in AD progression, and the bidirectional crosstalk between these two phenomena.

Broadening the definition of brain insulin resistance in aging and Alzheimer's disease

It has been >20 years since studies first revealed that the brain is insulin sensitive, highlighted by the expression of insulin receptors in neurons and glia, the presence of circulating brain insulin, and even localized insulin production. Following these discoveries, evidence of decreased brain insulin receptor number and function was reported in both clinical samples and animal models of aging and Alzheimer's disease, setting the stage for the hypothesis that neuronal insulin resistance may underlie memory loss in these conditions. The development of therapeutic insulin delivery to the brain using intranasal insulin administration has been shown to improve aspects of memory or learning in both humans and animal models. However, whether this approach functions by compensating for poorly signaling insulin receptors, for reduced insulin levels in the brain, or for reduced trafficking of insulin into the brain remains unclear. Direct measures of insulin's impact on cellular physiology and metabolism in the brain have been sparse in models of Alzheimer's disease, and even fewer studies have analyzed these processes in the aged brain. Nevertheless, recent evidence supports the role of brain insulin as a mediator of glucose metabolism through several means, including altering glucose transporters. Here, we provide a review of contemporary literature on brain insulin resistance, highlight the rationale for improving memory function using intranasal insulin, and describe initial results from experiments using a molecular approach to more directly measure the impact of insulin receptor activation and signaling on glucose uptake in neurons.

Acute psychosocial stress in mid-aged male rats causes hyperthermia, cognitive decline, and increased deep sleep power, but does not alter deep sleep duration

Aging is associated with altered sleep architecture and worsened hippocampus-dependent cognition, highly prevalent clinical conditions that detract from quality of life for the elderly. Interestingly, exposure to psychosocial stress causes similar responses in young subjects, suggesting that age itself may act as a stressor. In prior work, we demonstrated that young animals show loss of deep sleep, deficits in cognition, and elevated body temperature after acute stress exposure, whereas aged animals are hyporesponsive on these measures. However, it is unclear if these age-altered stress responses occur in parallel over the course of aging. To address this, here we repeated the experiment in mid-aged animals. We hypothesized that mid-aged stress responses would be intermediate between those of young and aged subjects. Sixteen mid-aged (12 months) male F344 rats were implanted with EEG/EMG emitters to monitor sleep architecture and body temperature, and were trained on the Morris water maze for 3 days. On the fourth day, half of the subjects were restrained for 3 hours immediately before the water maze probe trial. Sleep architecture and body temperature were measured during the ensuing inactive period, and on the following day, endpoint measures were taken. Restrained mid-aged animals showed resistance to deep sleep loss, but demonstrated stress-induced water maze probe trial performance deficits as well as postrestraint hyperthermia. Taken in the context of prior work, these data suggest that age-related loss of sleep architecture stress sensitivity may precede both cognitive and body temperature-related stress insensitivity.

Loss of odor-induced c-Fos expression of juxtaglomerular activity following maintenance of mice on fatty diets

Diet-induced obesity (DIO) decreases the number of OMP+ olfactory sensory neurons (OSN) in the olfactory epithelium by 25% and reduces correlate axonal projections to the olfactory bulb (OB). Whether surviving OSNs have equivalent odor responsivity is largely unknown. Herein, we utilized c-fos immediate-early gene expression to map neuronal activity and determine whether mice weaned to control (CF), moderately-high fat (MHF), or high-fat (HF) diet for a period of 6 months had changes in odor activation. Diet-challenged M72-IRES-tau-GFP mice were exposed to either a preferred M72 (Olfr160) ligand, isopropyl tiglate, or clean air in a custom-made Bell-jar infusion chamber using an alternating odor exposure pattern generated by a picosprizer™. Mice maintained on fatty diets weighed significantly more and cleared glucose less efficiently as determined by an intraperitoneal glucose tolerance test (IPGTT). The number of juxtaglomerular cells (JGs) decreased following maintenance of the mice on the MHF diet for cells surrounding the medial but not lateral M72 glomerulus within a 4 cell-column distance. The percentage of c-fos + JGs surrounding the lateral M72 glomerulus decreased in fat-challenged mice whereas those surrounding the medial glomerulus were not affected by diet. Altogether, these results show an asymmetry in the responsiveness of the 'mirror image' glomerular map for the M72 receptor that shows greater sensitivity of the lateral vs. medial glomerulus upon exposure to fatty diet.

Luteolin Ameliorates Cognitive Impairments by Suppressing the Expression of Inflammatory Cytokines and Enhancing Synapse-Associated Proteins GAP-43 and SYN Levels in Streptozotocin-Induced Diabetic Rats

Luteolin, a flavonoid isolated from Cirsium japonicum, has antioxidant, anti-inflammatory and neuroprotective activities. Our previous studies brought a prospect that luteolin benefited diabetic rats with cognitive impairments. In this study, we examined whether luteolin could suppress the inflammatory cytokines, thus increasing synapse-associated proteins in streptozotocin (STZ)-induced diabetes in rat models. The model rats underwent luteolin treatment for 8 consecutive weeks, followed by assessment of cognitive performances with MWM test. Nissl staining was employed to assess the neuropathological changes in the hippocampus and the effects of luteolin on diabetic rats. With animals sacrificed, expressions of inflammatory cytokines including interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) and synapse-associated proteins including growth-associated protein-43 (GAP-43) and synaptophysin (SYN) were determined. The results affirmed improvement of behavioral performances in the MWM test, downexpression of glycation end products (AGEs) in the plasma and the receptor for advanced glycation end products in the hippocampus, inhibition of IL-1β and TNF-α in both the hippocampus and plasma in diabetic rats. Furthermore, luteolin treatment upregulated the expressions of GAP-43 and SYN in the hippocampus. Thus, luteolin could ameliorate the cognitive dysfunctions in STZ-induced diabetic rat model.

[Long-term high-fat diet inhibits hippocampal expression of insulin receptor substrates and accelerates cognitive deterioration in obese rats]

OBJECTIVE

To assess the effect of long-term high-fat diet on the expressions of insulin receptor substrates in the hippocampus and spatial learning and memory ability of obese rats.

METHODS

A total of 100 4-week-old male SD rats were randomly divided into two groups and fed with common diet (CD group, n=40) or high-fat diet (HFD group, n=60) for 16 weeks. At 4, 8, 12, 16 and 20 weeks, 8 rats were randomly selected from each group for testing their spatial learning and memory function using Morris water maze. After the tests, the rats were sacrificed for measurement of the metabolic parameters and detection of the expressions of insulin receptor substrate-1 (IRS-1) and IRS-2 mRNAs in the CA1 region of the hippocampus.

RESULTS

Compared with those in CD group, the rats in HFD group showed a prolonged escape latency, longer swimming distance, faster average swimming speed, and shorter stay in the platformat 12 weeks. In HFD group, the serum levels of total cholesterol, triglyceride, low-density lipoprotein cholesterol, and fasting insulin were all significantly increased (P<0.05) and the level of high-density lipoprotein cholesterol decreased (P<0.01) in comparison with those in CD group at each of the time points. No significant difference was found in fast glucose levels between the two groups (P>0.05), but the expressions of IRS-1 and IRS-2 mRNAs were significantly decreased in HFD group at 12 weeks (P<0.05).

CONCLUSION

In obese rats, long-term feeding with high-fat diet leads to insulin resistance, which interferes with hippocampal expression of insulin receptor substrates and insulin metabolism to cause impairment of the cognitive function and accelerate cognitive deterioration.

[Long-term high-fat diet inhibits hippocampal expression of insulin receptor substrates and accelerates cognitive deterioration in obese rats]

OBJECTIVE

To assess the effect of long-term high-fat diet on the expressions of insulin receptor substrates in the hippocampus and spatial learning and memory ability of obese rats.

METHODS

A total of 100 4-week-old male SD rats were randomly divided into two groups and fed with common diet (CD group, n=40) or high-fat diet (HFD group, n=60) for 16 weeks. At 4, 8, 12, 16 and 20 weeks, 8 rats were randomly selected from each group for testing their spatial learning and memory function using Morris water maze. After the tests, the rats were sacrificed for measurement of the metabolic parameters and detection of the expressions of insulin receptor substrate-1 (IRS-1) and IRS-2 mRNAs in the CA1 region of the hippocampus.

RESULTS

Compared with those in CD group, the rats in HFD group showed a prolonged escape latency, longer swimming distance, faster average swimming speed, and shorter stay in the platformat 12 weeks. In HFD group, the serum levels of total cholesterol, triglyceride, low-density lipoprotein cholesterol, and fasting insulin were all significantly increased (P<0.05) and the level of high-density lipoprotein cholesterol decreased (P<0.01) in comparison with those in CD group at each of the time points. No significant difference was found in fast glucose levels between the two groups (P>0.05), but the expressions of IRS-1 and IRS-2 mRNAs were significantly decreased in HFD group at 12 weeks (P<0.05).

CONCLUSION

In obese rats, long-term feeding with high-fat diet leads to insulin resistance, which interferes with hippocampal expression of insulin receptor substrates and insulin metabolism to cause impairment of the cognitive function and accelerate cognitive deterioration.

Expression of a Constitutively Active Human Insulin Receptor in Hippocampal Neurons Does Not Alter VGCC Currents

Memory and cognitive decline are the product of numerous physiological changes within the aging brain. Multiple theories have focused on the oxidative, calcium, cholinergic, vascular, and inflammation hypotheses of brain aging, with recent evidence suggesting that reductions in insulin signaling may also contribute. Specifically, a reduction in insulin receptor density and mRNA levels has been implicated, however, overcoming these changes remains a challenge. While increasing insulin receptor occupation has been successful in offsetting cognitive decline, alternative molecular approaches should be considered as they could bypass the need for brain insulin delivery. Moreover, this approach may be favorable to test the impact of continued insulin receptor signaling on neuronal function. Here we used hippocampal cultures infected with lentivirus with or without IRβ, a constitutively active, truncated form of the human insulin receptor, to characterize the impact continued insulin receptor signaling on voltage-gated calcium channels. Infected cultures were harvested between DIV 13 and 17 (48 h after infection) for Western blot analysis on pAKT and AKT. These results were complemented with whole-cell patch-clamp recordings of individual pyramidal neurons starting 96 h post-infection. Results indicate that while a significant increase in neuronal pAKT/AKT ratio was seen at the time point tested, effects on voltage-gated calcium channels were not detected. These results suggest that there is a significant difference between constitutively active insulin receptors and the actions of insulin on an intact receptor, highlighting potential alternate mechanisms of neuronal insulin resistance and mode of activation.

A maternal high-fat diet during pregnancy and lactation, in addition to a postnatal high-fat diet, leads to metabolic syndrome with spatial learning and memory deficits: beneficial effects of resveratrol

We tested the hypothesis that high-fat diet consumption during pregnancy, lactation, and/or post weaning, altered the expression of molecular mediators involved in hippocampal synaptic efficacy and impaired spatial learning and memory in adulthood. The beneficial effect of resveratrol was assessed. Dams were fed a rat chow diet or a high-fat diet before mating, during pregnancy, and throughout lactation. Offspring were weaned onto either a rat chow or a high-fat diet. Four experimental groups were generated, namely CC, HC, CH, and HH (maternal chow diet or high-fat diet; postnatal chow diet or high-fat diet). A fifth group fed with HH plus resveratrol (HHR) was generated. Morris water maze test was used to evaluate spatial learning and memory. Blood pressure and IPGTT was measured to assess insulin resistance. Dorsal hippocampal expression of certain biochemical molecules, including sirtuin 1, ERK, PPARγ, adiponectin, and BDNF were measured. Rats in HH group showed impaired spatial memory, which was partly restored by the administration of resveratrol. Rats in HH group also showed impaired glucose tolerance and increased blood pressure, all of which was rescued by resveratrol administration. Additionally, SIRT1, phospho-ERK1/2, and phospho-PPARγ, adiponectin and BDNF were all dysregulated in rats placed in HH group; administration of resveratrol restored the expression and regulation of these molecules. Overall, our results suggest that maternal high-fat diet during pregnancy and/or lactation sensitizes the offspring to the adverse effects of a subsequent high-fat diet on hippocampal function; however, administration of resveratrol is demonstrated to be beneficial in rescuing these effects.

High-fat diet and aging interact to produce neuroinflammation and impair hippocampal- and amygdalar-dependent memory

More Americans are consuming diets higher in saturated fats and refined sugars than ever before, and based on increasing obesity rates, this is a growing trend among older adults as well. While high saturated fat diet (HFD) consumption has been shown to sensitize the inflammatory response to a subsequent immune challenge in young adult rats, the inflammatory effect of HFD in the already-vulnerable aging brain has not yet been assessed. Here, we explored whether short-term (3 days) consumption of HFD would serve as a neuroinflammatory trigger in aging animals, leading to cognitive deficits. HFD impaired long-term contextual (hippocampal dependent) and auditory-cued fear (amygdalar dependent) memory in aged, but not young adult rats. Short-term memory performance for both tasks was intact, suggesting that HFD impairs memory consolidation processes. Microglial markers of activation Iba1 and cd11b were only increased in the aged rats, while MHCII was further amplified by HFD. Furthermore, these HFD-induced long-term memory impairments were accompanied by IL-1β protein increases in both the hippocampus and amygdala in aged rats. Central administration of IL-1RA in aged rats following conditioning mitigated both contextual and auditory-cued fear memory impairments caused by HFD, strongly suggesting that IL-1β plays a critical role in these effects. Voluntary wheel running, known to have anti-inflammatory effects in the hippocampus, rescued hippocampal-dependent but not amygdalar-dependent memory impairments caused by HFD. Together, these data suggest that short-term consumption of HFD can lead to memory deficits and significant brain inflammation in the aged animal, and strongly suggest that appropriate diet is crucial for cognitive health.

Novel calcium-related targets of insulin in hippocampal neurons

Both insulin signaling disruption and Ca²⁺ dysregulation are closely related to memory loss during aging and increase the vulnerability to Alzheimer's disease (AD). In hippocampal neurons, aging-related changes in calcium regulatory pathways have been shown to lead to higher intracellular calcium levels and an increase in the Ca²⁺-dependent afterhyperpolarization (AHP), which is associated with cognitive decline. Recent studies suggest that insulin reduces the Ca²⁺-dependent AHP. Given the sensitivity of neurons to insulin and evidence that brain insulin signaling is reduced with age, insulin-mediated alterations in calcium homeostasis may underlie the beneficial actions of insulin in the brain. Indeed, increasing insulin signaling in the brain via intranasal delivery has yielded promising results such as improving memory in both clinical and animal studies. However, while several mechanisms have been proposed, few have focused on regulation on intracellular Ca²⁺. In the present study, we further examined the effects of acute insulin on calcium pathways in primary hippocampal neurons in culture. Using the whole-cell patch-clamp technique, we found that acute insulin delivery reduced voltage-gated calcium currents. Fura-2 imaging was used to also address acute insulin effects on spontaneous and depolarization-mediated Ca²⁺ transients. Results indicate that insulin reduced Ca²⁺ transients, which appears to have involved a reduction in ryanodine receptor function. Together, these results suggest insulin regulates pathways that control intracellular Ca²⁺ which may reduce the AHP and improve memory. This may be one mechanism contributing to improved memory recall in response to intranasal insulin therapy in the clinic.

Long-Term Consumption of High-Fat Diet in Rats: Effects on Microglial and Astrocytic Morphology and Neuronal Nitric Oxide Synthase Expression

Obesity in humans is associated with cognitive decline and elevated risk of neurodegenerative diseases of old age. Variations of high-fat diet are often used to model these effects in animal studies. However, we previously reported improvements in markers of memory and learning, as well as larger hippocampi and higher metabolite concentrations in Wistar rats fed high-fat, high-carbohydrate diet (HFCD, 60 % energy from fat, 28 % from carbohydrates) for 1 year; this diet leads to mild ketonemia (Setkowicz et al. in PLoS One 10:e0139987, 2015). In the present study, we follow up on this cohort to assess glial morphology and expression of markers related to gliosis. Twenty-five male Wistar rats were kept on HFCD and twenty-five on normal chow. At 12 months of age, the animals were sacrificed and processed for immunohistochemical staining for astrocytic (glial fibrillary acidic protein), microglial (Iba1), and neuronal (neuronal nitric oxide synthetase, nNOS) markers in the hippocampus. We have found changes in immunopositive area fraction and cellular complexity, as studied by a simplified Sholl procedure. To our knowledge, this study is the first to apply this methodology to the study of glial cells in HFCD animals. GFAP and Iba1 immunoreactive area fraction in the hippocampi of HFCD-fed rats were decreased, while the mean number of intersections (an indirect measure of cell complexity) was decreased in GFAP-positive astrocytes, but not in Iba1-expressing microglia. At the same time, nNOS expression was lowered after HFCD in both the cortex and the hippocampus.

High-Fat Diet and Voluntary Chronic Aerobic Exercise Recover Altered Levels of Aging-Related Tryptophan Metabolites along the Kynurenine Pathway

Tryptophan metabolites regulate a variety of physiological processes, and their downstream metabolites enter the kynurenine pathway. Age-related changes of metabolites and activities of associated enzymes in this pathway are suggestable and would be potential intervention targets. Blood levels of serum tryptophan metabolites in C57BL/6 mice of different ages, ranging from 6 weeks to 10 months, were assessed using high-performance liquid chromatography, and the enzyme activities for each metabolic step were estimated using the ratio of appropriate metabolite levels. Mice were subjected to voluntary chronic aerobic exercise or high-fat diet to assess their ability to rescue age-related alterations in the kynurenine pathway. The ratio of serum kynurenic acid (KYNA) to 3-hydroxylkynurenine (3-HK) decreased with advancing age. Voluntary chronic aerobic exercise and high-fat diet rescued the decreased KYNA/3-HK ratio in the 6-month-old and 8-month-old mice groups. Tryptophan metabolites and their associated enzyme activities were significantly altered during aging, and the KYNA/3-HK ratio was a meaningful indicator of aging. Exercise and high-fat diet could potentially recover the reduction of the KYNA/3-HK ratio in the elderly.

In vivo imaging of prodromal hippocampus CA1 subfield oxidative stress in models of Alzheimer disease and Angelman syndrome

Hippocampus oxidative stress is considered pathogenic in neurodegenerative diseases, such as Alzheimer disease (AD), and in neurodevelopmental disorders, such as Angelman syndrome (AS). Yet clinical benefits of antioxidant treatment for these diseases remain unclear because conventional imaging methods are unable to guide management of therapies in specific hippocampus subfields in vivo that underlie abnormal behavior. Excessive production of paramagnetic free radicals in nonhippocampus brain tissue can be measured in vivo as a greater-than-normal 1/T₁ that is quenchable with antioxidant as measured by quench-assisted (Quest) MRI. Here, we further test this approach in phantoms, and we present proof-of-concept data in models of AD-like and AS hippocampus oxidative stress that also exhibit impaired spatial learning and memory. AD-like models showed an abnormal gradient along the CA1 dorsal-ventral axis of excessive free radical production as measured by Quest MRI, and redox-sensitive calcium dysregulation as measured by manganese-enhanced MRI and electrophysiology. In the AS model, abnormally high free radical levels were observed in dorsal and ventral CA1. Quest MRI is a promising in vivo paradigm for bridging brain subfield oxidative stress and behavior in animal models and in human patients to better manage antioxidant therapy in devastating neurodegenerative and neurodevelopmental diseases.-Berkowitz, B. A., Lenning, J., Khetarpal, N., Tran, C., Wu, J. Y., Berri, A. M., Dernay, K., Haacke, E. M., Shafie-Khorassani, F., Podolsky, R. H., Gant, J. C., Maimaiti, S., Thibault, O., Murphy, G. G., Bennett, B. M., Roberts, R. In vivo imaging of prodromal hippocampus CA1 subfield oxidative stress in models of Alzheimer disease and Angelman syndrome.

The BMP2 nuclear variant, nBMP2, is expressed in mouse hippocampus and impacts memory

The novel nuclear protein nBMP2 is synthesized from the BMP2 gene by translational initiation at an alternative start codon. We generated a targeted mutant mouse, nBmp2NLStm, in which the nuclear localization signal (NLS) was inactivated to prevent nuclear translocation of nBMP2 while still allowing the normal synthesis and secretion of the BMP2 growth factor. These mice exhibit abnormal muscle function due to defective Ca2+ transport in skeletal muscle. We hypothesized that neurological function, which also depends on intracellular Ca2+ transport, could be affected by the loss of nBMP2. Age-matched nBmp2NLStm and wild type mice were analyzed by immunohistochemistry, behavioral tests, and electrophysiology to assess nBMP2 expression and neurological function. Immunohistochemical staining of the hippocampus detected nBMP2 in the nuclei of CA1 neurons in wild type but not mutant mice, consistent with nBMP2 playing a role in the hippocampus. Mutant mice showed deficits in the novel object recognition task, suggesting hippocampal dysfunction. Electrophysiology experiments showed that long-term potentiation (LTP) in the hippocampus, which is dependent on intracellular Ca2+ transport and is thought to be the cellular equivalent of learning and memory, was impaired. Together, these results suggest that nBMP2 in the hippocampus impacts memory formation.

Potential Neuroprotective Effects of Adiponectin in Alzheimer's Disease

The adipocyte-secreted protein adiponectin (APN) has several protective functions in the peripheral tissues including insulin sensitizing, anti-inflammatory and anti-oxidative effects that may benefit neurodegenerative diseases such as Alzheimer's disease (AD). In addition, dysregulation of cerebral insulin sensitivities and signaling activities have been implicated in AD. Emerging insights into the mechanistic roles of adiponectin and AD highlight the potential therapeutic effects for AD through insulin signaling.

A maternal diet high in saturated fat impairs offspring hippocampal function in a sex-specific manner

While a maternal diet high in saturated fat is likely to affect foetal brain development, whether the effects are the same for male and female offspring is unclear. As a result, we randomly assigned female, Sprague-Dawley rats to either a control, or high-fat diet (HFD; 45% of calories from saturated fat) for 10 weeks. A range of biometrics were collected, and hippocampal function was assessed at both the tissue level (by measuring synaptic plasticity) and at the behavioural level (using the Morris water maze; MWM). Subsequently, a subset of animals was bred and remained on their respective diets throughout gestation and lactation. On post-natal day 21, offspring were weaned and placed onto the control diet; biometrics and spatial learning and memory were then assessed at both adolescence and young adulthood. Although the HFD led to changes in the maternal generation consistent with an obese phenotype, no impairments were noted at the level of hippocampal synaptic plasticity, or MWM performance. Unexpectedly, among the offspring, a sexually dimorphic effect upon MWM performance became apparent. In particular, adolescent male offspring displayed a greater latency to reach the platform during training trials and spent less time in the target quadrant during the probe test; notably, when re-examined during young adulthood, the performance deficit was no longer present. Overall, our work suggests the existence of sexual dimorphism with regard to how a maternal HFD affects hippocampal-dependent function in the offspring brain.

27-Hydroxycholesterol impairs neuronal glucose uptake through an IRAP/GLUT4 system dysregulation

Ismail et al. show that 27-hydroxycholesterol, a peripheral cholesterol metabolite capable of passing the blood–brain barrier, reduces brain glucose uptake by upregulating the renin-angiotensin system and inhibiting GLUT4. This alteration affects memory processes and is likely to have implications on neurodegenerative diseases.

Hypercholesterolemia is associated with cognitively deteriorated states. Here, we show that excess 27-hydroxycholesterol (27-OH), a cholesterol metabolite passing from the circulation into the brain, reduced in vivo brain glucose uptake, GLUT4 expression, and spatial memory. Furthermore, patients exhibiting higher 27-OH levels had reduced ¹⁸F-fluorodeoxyglucose uptake. This interplay between 27-OH and glucose uptake revealed the engagement of the insulin-regulated aminopeptidase (IRAP). 27-OH increased the levels and activity of IRAP, countered the IRAP antagonist angiotensin IV (AngIV)–mediated glucose uptake, and enhanced the levels of the AngIV-degrading enzyme aminopeptidase N (AP-N). These effects were mediated by liver X receptors. Our results reveal a molecular link between cholesterol, brain glucose, and the brain renin-angiotensin system, all of which are affected in some neurodegenerative diseases. Thus, reducing 27-OH levels or inhibiting AP-N maybe a useful strategy in the prevention of the altered glucose metabolism and memory decline in these disorders.

Calcium Hypothesis of Alzheimer's disease and brain aging: A framework for integrating new evidence into a comprehensive theory of pathogenesis

This article updates the Calcium Hypothesis of Alzheimer's disease and brain aging on the basis of emerging evidence since 1994 (The present article, with the subtitle "New evidence for a central role of Ca²⁺ in neurodegeneration," includes three appendices that provide context and further explanations for the rationale for the revisions in the updated hypothesis-the three appendices are as follows: Appendix I "Emerging concepts on potential pathogenic roles of [Ca²⁺]," Appendix II "Future studies to validate the central role of dysregulated [Ca²⁺] in neurodegeneration," and Appendix III "Epilogue: towards a comprehensive hypothesis.") (Marx J. Fresh evidence points to an old suspect: calcium. Science 2007; 318:384-385). The aim is not only to re-evaluate the original key claims of the hypothesis with a critical eye but also to identify gaps in knowledge required to validate relevant claims and delineate additional studies and/or data that are needed. Some of the key challenges for this effort included examination of questions regarding (1) the temporal and spatial relationships of molecular mechanisms that regulate neuronal calcium ion (Ca²⁺), (2) the role of changes in concentration of calcium ion [Ca²⁺] in various subcellular compartments of neurons, (3) how alterations in Ca²⁺ signaling affect the performance of neurons under various conditions, ranging from optimal functioning in a healthy state to conditions of decline and deterioration in performance during aging and in disease, and (4) new ideas about the contributions of aging, genetic, and environmental factors to the causal relationships between dysregulation of [Ca²⁺] and the functioning of neurons (see Appendices I and II). The updated Calcium Hypothesis also includes revised postulates that are intended to promote further crucial experiments to confirm or reject the various predictions of the hypothesis (see Appendix III).

Carnosine ameliorates cognitive deficits in streptozotocin-induced diabetic rats: Possible involved mechanisms

Diabetic patients are at increased risk to develop cognitive deficit and senile dementia. This study was planned to assess the benefits of chronic carnosine administration on prevention of learning and memory deterioration in streptozotocin (STZ)-diabetic rats and to explore some of the involved mechanisms. Rats were divided into 5 groups: i.e., control, carnosine100-treated control, diabetic, and carnosine-treated diabetics (50 and 100mg/kg). Carnosine was injected i.p. at doses of 50 or 100mg/kg for 7 weeks, started 1 week after induction of diabetes using streptozotocin. Treatment of diabetic rats with carnosine at a dose of 100mg/kg at the end of the study lowered serum glucose, improved spatial recognition memory in Y maze, improved retention and recall in elevated plus maze, and prevented reduction of step-through latency in passive avoidance task. Furthermore, carnosine at a dose of 100mg/kg reduced hippocampal acetylcholinesterase (AChE) activity, lowered lipid peroxidation, and improved superoxide dismutase (SOD) activity and non-enzymatic antioxidant defense element glutathione (GSH), but not activity of catalase. Meanwhile, hippocampal level of nuclear factor-kappaB (NF-κB), tumor necrosis factor α (TNF-α), and glial fibrillary acidic protein (GFAP) decreased and level of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) and heme oxygenase 1 (HO-1) increased upon treatment of diabetic group with carnosine at a dose of 100mg/kg. Taken together, chronic carnosine treatment could ameliorate learning and memory disturbances in STZ-diabetic rats through intonation of NF-κB/Nrf2/HO-1 signaling cascade, attenuation of astrogliosis, possible improvement of cholinergic function, and amelioration of oxidative stress and neuroinflammation.

Calcium's role as nuanced modulator of cellular physiology in the brain

Neuroscientists studying normal brain aging, spinal cord injury, Alzheimer's disease (AD) and other neurodegenerative diseases have focused considerable effort on carefully characterizing intracellular perturbations in calcium dynamics or levels. At the cellular level, calcium is known for controlling life and death and orchestrating most events in between. For many years, intracellular calcium has been recognized as an essential ion associated with nearly all cellular functions from cell growth to degeneration. Often the emphasis is on the negative impact of calcium dysregulation and the typical worse-case-scenario leading inevitably to cell death. However, even high amplitude calcium transients, when executed acutely, can alter neuronal communication and synaptic strength in positive ways, without necessarily killing neurons. Here, we focus on the evidence that calcium has a subtle and distinctive role in shaping and controlling synaptic events that underpin neuronal communication and that these subtle changes in aging or AD may contribute to cognitive decline. We emphasize that calcium imaging in dendritic components is ultimately necessary to directly test for the presence of age- or disease-associated alterations during periods of synaptic activation.

Impact of Single or Repeated Dose Intranasal Zinc-free Insulin in Young and Aged F344 Rats on Cognition, Signaling, and Brain Metabolism

Novel therapies have turned to delivering compounds to the brain using nasal sprays, bypassing the blood brain barrier, and enriching treatment options for brain aging and/or Alzheimer's disease. We conducted a series of in vivo experiments to test the impact of intranasal Apidra, a zinc-free insulin formulation, on the brain of young and aged F344 rats. Both single acute and repeated daily doses were compared to test the hypothesis that insulin could improve memory recall in aged memory-deficient animals. We quantified insulin signaling in different brain regions and at different times following delivery. We measured cerebral blood flow (CBF) using MRI and also characterized several brain metabolite levels using MR spectroscopy. We show that neither acute nor chronic Apidra improved memory or recall in young or aged animals. Within 2 hours of a single dose, increased insulin signaling was seen in ventral areas of the aged brains only. Although chronic Apidra was able to offset reduced CBF with aging, it also caused significant reductions in markers of neuronal integrity. Our data suggest that this zinc-free insulin formulation may actually hasten cognitive decline with age when used chronically.

Inside the Diabetic Brain: Role of Different Players Involved in Cognitive Decline

Diabetes mellitus is the most common metabolic disease, and its prevalence is increasing. A growing body of evidence, both in animal models and epidemiological studies, has demonstrated that metabolic diseases like obesity, insulin resistance, and diabetes are associated with alterations in the central nervous system (CNS), being linked with development of cognitive and memory impairments and presenting a higher risk for dementia and Alzheimer's disease. The rising prevalence of diabetes together with its increasing earlier onset suggests that diabetes-related cognitive dysfunction will increase in the near future, causing substantial socioeconomic impact. Decreased insulin secretion or action, dysregulation of glucose homeostasis, impairment in the hypothalamic-pituitary-adrenal axis, obesity, hyperleptinemia, and inflammation may act independently or synergistically to disrupt neuronal homeostasis and cause diabetes-associated cognitive decline. However, the crosstalk between those factors and the mechanisms underlying the diabetes-related CNS complications is still elusive. During the past few years, different strategies (neuroprotective and antioxidant drugs) have emerged as promising therapies for this complication, which still remains to be preventable or treatable. This Review summarizes fundamental past and ongoing research on diabetes-associated cognitive decline, highlighting potential contributors, mechanistic mediators, and new pharmacological approaches to prevent and/or delay this complication.

High-fat diet impairs spatial memory and hippocampal intrinsic excitability and sex-dependently alters circulating insulin and hippocampal insulin sensitivity

BACKGROUND

High-fat diets promoting obesity/type-2 diabetes can impair physiology and cognitive performance, although sex-dependent comparisons of these impairments are rarely made. Transient reductions in Ca(2+)-dependent afterhyperpolarizations (AHPs) occur during memory consolidation, enhancing intrinsic excitability of hippocampal CA1 pyramidal neurons. In rats fed standard diets, insulin can enhance memory and reduce amplitude and duration of AHPs.

METHODS

Effects of chronic high-fat diet (HFD) on memory, circulating insulin, and neuronal physiology were compared between young adult male and female Long-Evans rats. Rats fed for 12 weeks (from weaning) a HFD or a control diet (CD) were then tested in vivo prior to in vitro recordings from CA1 pyramidal neurons.

RESULTS

The HFD significantly impaired spatial memory in both males and females. Significant sex differences occurred in circulating insulin and in the insulin sensitivity of hippocampal neurons. Circulating insulin significantly increased in HFD males but decreased in HFD females. While the HFD significantly reduced hippocampal intrinsic excitability in both sexes, CA1 neurons from HFD females remained insulin-sensitive but those from HFD males became insulin-insensitive.

CONCLUSIONS

Findings consistent with these have been characterized previously in HFD or senescent males, but the effects observed here in young females are unique. Loss of CA1 neuronal excitability, and sex-dependent loss of insulin sensitivity, can have significant cognitive consequences, over both the short term and the life span. These findings highlight needs for more research into sex-dependent differences, relating systemic and neural plasticity mechanisms in metabolic disorders.