Impairment of long-term potentiation in the CA1, but not dentate gyrus, of the hippocampus in Obese Zucker rats: role of calcineurin and phosphorylated CaMKII

Synaptic Plasticity and Cognitive Ability in Experimental Adult-Onset Hypothyroidism

Adult-onset hypothyroidism impairs normal brain function. Research on animal models of hypothyroidism has revealed critical information on how deficiency of thyroid hormones impacts the electrophysiological and molecular functions of the brain, which leads to the well known cognitive impairment in untreated hypothyroid patients. Currently, such information can only be obtained from experiments on animal models of hypothyroidism. This review summarizes important research findings that pertain to understanding the clinical cognitive consequences of hypothyroidism, which will provide a better guiding path for therapy of hypothyroidism. SIGNIFICANCE STATEMENT: Cognitive impairment occurs during adult-onset hypothyroidism in both humans and animal models. Findings from animal studies validate clinical findings showing impaired long-term potentiation, decreased CaMKII, and increased calcineurin. Such findings can only be gleaned from animal experiments to show how hypothyroidism produces clinical symptoms.

High-fat diet consumption promotes adolescent neurobehavioral abnormalities and hippocampal structural alterations via microglial overactivation accompanied by an elevated serum free fatty acid concentration

Mounting evidence suggests that high-fat diet (HFD) consumption increases the risk for depression, but the neurophysiological mechanisms involved remain to be elucidated. Here, we demonstrated that HFD feeding of C57BL/6J mice during the adolescent period (from 4 to 8 weeks of age) resulted in increased depression- and anxiety-like behaviors concurrent with changes in neuronal and myelin structure in the hippocampus. Additionally, we showed that hippocampal microglia in HFD-fed mice assumed a hyperactive state concomitant with increased PSD95-positive and myelin basic protein (MBP)-positive inclusions, implicating microglia in hippocampal structural alterations induced by HFD consumption. Along with increased levels of serum free fatty acids (FFAs), abnormal deposition of lipid droplets and increased levels of HIF-1α protein (a transcription factor that has been reported to facilitate cellular lipid accumulation) within hippocampal microglia were observed in HFD-fed mice. The use of minocycline, a pharmacological suppressor of microglial overactivation, effectively attenuated neurobehavioral abnormalities and hippocampal structural alterations but barely altered lipid droplet accumulation in the hippocampal microglia of HFD-fed mice. Coadministration of triacsin C abolished the increases in lipid droplet formation, phagocytic activity, and ROS levels in primary microglia treated with serum from HFD-fed mice. In conclusion, our studies demonstrate that the adverse influence of early-life HFD consumption on behavior and hippocampal structure is attributed at least in part to microglial overactivation that is accompanied by an elevated serum FFA concentration and microglial aberrations represent a potential preventive and therapeutic target for HFD-related emotional disorders.

Sex differences in response to obesity and caloric restriction on cognition and hippocampal measures of autophagic-lysosomal transcripts and signaling pathways

BACKGROUND

Obesity rates in the U.S. continue to increase, with nearly 50% of the population being either obese or morbidly obese. Obesity, along with female sex, are leading risk factors for sporadic Alzheimer's Disease (AD) necessitating the need to better understand how these variables impact cellular function independent of age or genetic mutations. Animal and clinical studies both indicate that autophagy-lysosomal pathway (ALP) dysfunction is among the earliest known cellular systems to become perturbed in AD, preceding cognitive decline, yet little is known about how obesity and sex affects these cellular functions in the hippocampus, a brain region uniquely susceptible to the negative effects of obesity. We hypothesized that obesity would negatively affect key markers of ALP in the hippocampus, effects would vary based on sex, and that caloric restriction would counteract obesity effects.

METHODS

Female and male mice were placed on an obesogenic diet for 10 months, at which point half were switched to caloric restriction for three months, followed by cognitive testing in the Morris watermaze. Hippocampus was analyzed by western blot and qPCR.

RESULTS

Cognitive function in female mice responded differently to caloric restriction based on whether they were on a normal or obesogenic diet; male cognition was only mildly affected by caloric restriction and not obesity. Significant male-specific changes occurred in cellular markers of autophagy, including obesity increasing pAkt, Slc38a9, and Atg12, while caloric restriction reduced pRPS6 and increased Atg7. In contrast females experienced changes due to diet/caloric restriction predominately in lysosomal markers including increased TFE3, FLCN, FNIP2, and pAMPK.

CONCLUSIONS

Results support that hippocampal ALP is a target of obesity and that sex shapes molecular responses, while providing insight into how dietary manipulations affect learning and memory based on sex.

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.

Cardiometabolic health, menopausal estrogen therapy and the brain: How effects of estrogens diverge in healthy and unhealthy preclinical models of aging

Research in preclinical models indicates that estrogens are neuroprotective and positively impact cognitive aging. However, clinical data are equivocal as to the benefits of menopausal estrogen therapy to the brain and cognition. Pre-existing cardiometabolic disease may modulate mechanisms by which estrogens act, potentially reducing or reversing protections they provide against cognitive decline. In the current review we propose mechanisms by which cardiometabolic disease may alter estrogen effects, including both alterations in actions directly on brain memory systems and actions on cardiometabolic systems, which in turn impact brain memory systems. Consideration of mechanisms by which estrogen administration can exert differential effects dependent upon health phenotype is consistent with the move towards precision or personalized medicine, which aims to determine which treatment interventions will work for which individuals. Understanding effects of estrogens in both healthy and unhealthy models of aging is critical to optimizing the translational link between preclinical and clinical research.

Up-regulation of CB1 cannabinoid receptors located at glutamatergic terminals in the medial prefrontal cortex of the obese Zucker rat

The present study describes a detailed neuroanatomical distribution map of the cannabinoid type 1 (CB₁) receptor, along with the biochemical characterization of the expression and functional coupling to their cognate G_i/o proteins in the medial prefrontal cortex (mPCx) of the obese Zucker rats. The CB₁ receptor density was higher in the prelimbic (PL) and infralimbic (IL) subregions of the mPCx of obese Zucker rats relative to their lean littermates which was associated with a higher percentage of CB₁ receptor immunopositive excitatory presynaptic terminals in PL and IL. Also, a higher expression of CB₁ receptors and WIN55,212-2-stimulated [³⁵S]GTPγS binding was observed in the mPCx but not in the neocortex (NCx) and hippocampus of obese rats. Low-frequency stimulation in layers II/III of the mPCx induced CB₁ receptor-dependent long-term synaptic plasticity in IL of area obese Zucker but not lean rats. Overall, the elevated 2-AG levels, up-regulation of CB₁ receptors, and increased agonist-stimulated [³⁵S]GTPγS binding strongly suggest that hyperactivity of the endocannabinoid signaling takes place at the glutamatergic terminals of the mPCx in the obese Zucker rat. These findings could endorse the importance of the CB₁ receptors located in the mPCx in the development of obesity in Zucker rats.

Peripheral versus central insulin and leptin resistance: Role in metabolic disorders, cognition, and neuropsychiatric diseases

Insulin and leptin are classically regarded as peptide hormones that play key roles in metabolism. In actuality, they serve several functions in both the periphery and central nervous system (CNS). Likewise, insulin and leptin resistance can occur both peripherally and centrally. Metabolic disorders such as diabetes and obesity share several key features including insulin and leptin resistance. While the peripheral effects of these disorders are well-known (i.e. cardiovascular disease, hypertension, stroke, dyslipidemia, etc.), the CNS complications of leptin and insulin resistance have come into sharper focus. Both preclinical and clinical findings have indicated that insulin and leptin resistance are associated with cognitive deficits and neuropsychiatric diseases such as depression. Importantly, these studies also suggest that these deficits in neuroplasticity can be reversed by restoration of insulin and leptin sensitivity. In view of these observations, this review will describe, in detail, the peripheral and central functions of insulin and leptin and explain the role of insulin and leptin resistance in various metabolic disorders, cognition, and neuropsychiatric diseases.

Biological evaluation of naturally occurring bulbocodin D as a potential multi-target agent for Alzheimer's disease

In recent years, with the in-depth understanding of the pathogenesis of Alzheimer's disease (AD) and the development of advanced pharmacological technology, "multi-target" strategy has recently attracted the wide interest of scientists. The purpose of this study was to investigate the protective effect and mechanism of bulbocodin D for AD in vitro. In this study, we established the oxidative stress model of SH-SY5Y cells mediated by H₂O₂ and the inflammatory model of BV2 cells stimulated by LPS. Western blot was used to analysis the expression of mitochondrial apoptosis and inflammation related proteins. Moreover, predicted binding modes of bulbocodin D with AChE and GSK3β were performed by molecular docking analysis. We found that bulbocodin D could reduce cell apoptosis, reduce ROS production in SH-SY5Y cell, and inhibit the secretion of inflammatory cytokines in BV2 cell. Furthermore, western blot results showed that bulbocodin D could regulate mitochondrial apoptotic pathway and MAPKs pathway. In addition, bulbocodin D can reduce the aggregation of Aβ. We also found that bulbocodin D had a good inhibitory effect on AChE and GSK3β. In summary, bulbocodin D might be a potential multi-target agent for Alzheimer's disease.

A novel hippocampus metabolite signature in diabetes mellitus rat model of diabetic encephalopathy

Diabetic encephalopathy (DE) is one of the chronic complications of diabetes. Even then, the molecular mechanism underlying DE remains unexplored. In this study, we have made an attempt to investigate the metabolic changes associated with the streptozocin (STZ)-induced cognitive dysfunction in the hippocampus of the rat model, a classical rodent model for DE, with the help of Gas Chromatography-Mass Spectrometry-based method. The STZ injections led to the rise of mean blood glucose levels in the diabetes mellitus (DM) group of rats as compared to the control (CON) group of rats throughout the experiment. However, we did not find any significant difference between the blood glucose levels of the DM & the CON groups of rats before the STZ injection. The results indicated a behavioral and morphological cognitive dysfunction in the DM groups of rats. The metabolomic investigation of these DE rats demonstrated a lower level of N-acetylaspartate and dihydroxyacetone phosphate accompanied by a higher level of homocysteine and glutamate as against the CON group of rats. The outcome of this study may unravel the underlying pathophysiological mechanism of DE. Also, the metabolomic data from this study may provide a platform for the development of DE biomarkers.

Insulin resistance and hippocampal dysfunction: Disentangling peripheral and brain causes from consequences

In the periphery insulin plays a critical role in the regulation of metabolic homeostasis by stimulating glucose uptake into peripheral organs. In the central nervous system (CNS), insulin plays a critical role in the formation of neural circuits and synaptic connections from the earliest stages of development and facilitates and promotes neuroplasticity in the adult brain. Beyond these physiological roles of insulin, a shared feature between the periphery and CNS is that decreases in insulin receptor activity and signaling (i.e. insulin resistance) contributes to the pathological consequences of type 2 diabetes (T2DM) and obesity. Indeed, clinical and preclinical studies illustrate that CNS insulin resistance elicits neuroplasticity deficits that lead to decreases in cognitive function and increased risk of neuropsychiatric disorders. The goals of this review are to provide an overview of the literature that have identified the neuroplasticity deficits observed in T2DM and obesity, as well as to discuss the potential causes and consequences of insulin resistance in the CNS, with a particular focus on how insulin resistance impacts hippocampal neuroplasticity. Interestingly, studies that have examined the effects of hippocampal-specific insulin resistance illustrate that brain insulin resistance may impair neuroplasticity independent of peripheral insulin resistance, thereby supporting the concept that restoration of brain insulin activity is an attractive therapeutic strategy to ameliorate or reverse cognitive decline observed in patients with CNS insulin resistance such as T2DM and Alzheimer's Disease.

Cellular and Molecular Differences Between Area CA1 and the Dentate Gyrus of the Hippocampus

A distinct feature of the hippocampus of the brain is its unidirectional tri-synaptic pathway originating from the entorhinal cortex and projecting to the dentate gyrus (DG) then to area CA3 and subsequently, area CA1 of the Ammon's horn. Each of these areas of the hippocampus has its own cellular structure and distinctive function. The principal neurons in these areas are granule cells in the DG and pyramidal cells in the Ammon's horn's CA1 and CA3 areas with a vast network of interneurons. This review discusses the fundamental differences between the CA1 and DG areas regarding cell morphology, synaptic plasticity, signaling molecules, ability for neurogenesis, vulnerability to various insults and pathologies, and response to pharmacological agents.

Leptin resistance and hippocampal behavioral deficits

The adipocyte-derived hormone leptin is an important regulator of body weight and metabolism through activation of brain leptin receptors expressed in regions such as the hypothalamus. Beyond these well described and characterized activities of leptin in the hypothalamus, it is becoming increasingly clear that the central activities of leptin extend to the hippocampus. Indeed, leptin receptors are expressed in the hippocampus where these receptors are proposed to mediate various aspects of hippocampal synaptic plasticity that ultimately impact cognitive function. This concept is supported by studies demonstrating that leptin promotes hippocampal-dependent learning and memory, as well as studies indicating that leptin resistance is associated with deficits in hippocampal-dependent behaviors and in the induction of depressive-like behaviors. The effects of leptin on cognitive/behavioral plasticity in the hippocampus may be regulated by direct activation of leptin receptors expressed in the hippocampus; additionally, leptin-mediated activation of synaptic networks that project to the hippocampus may also impact hippocampal-mediated behaviors. In view of these previous observations, the goal of this review will be to discuss the mechanisms through which leptin facilitates cognition and behavior, as well as to dissect the loci at which leptin resistance leads to impairments in hippocampal synaptic plasticity, including the development of cognitive deficits and increased risk of depressive illness in metabolic disorders such as obesity and type 2 diabetes mellitus (T2DM).

Withania somnifera leaf alleviates cognitive dysfunction by enhancing hippocampal plasticity in high fat diet induced obesity model

Background

Sedentary lifestyle, psychological stress and labor saving devices in this current society often disrupts the energy gain and expenditure balance leading to obesity. High caloric diet is associated with the high prevalence of cognitive dysfunction and neuropsychiatric disorders in addition to cardiovascular and metabolic abnormalities. The present study was aimed to elucidate the potential beneficial effect of dry leaf powder of Withania somnifera (Ashwagandha) in preventing the cognitive decline associated with diet induced obesity.

Methods

Experiments were performed on four groups of young adult female rats: [Low fat diet (LFD) rats fed on regular low fat chow, High fat diet (HFD) rats on feed containing 30% fat by weight, Low fat diet extract (LFDE) rats given regular chow and dry leaf powder of Ashwagandha 1 mg/g of body weight (ASH) and high fat diet extract (HFDE) rats fed on diet containing high fat and dry leaf powder of ASH. All the rats were kept on their respective diet regimen for 12 weeks.

Results

ASH treated rats showed significant improvement in their working memory and locomotor coordination during behavioral studies as compared to HFD rats. At the molecular level, ASH treatment was observed to restore the levels of BDNF and its receptor TRKB as well as the expression of other synaptic regulators, which are highly implicated in synaptic plasticity. Further, ASH triggered the activation of PI3/AKT pathway of cell survival and plasticity by enhancing the levels of phosphorylated Akt-1 and immediate early genes viz. c-Jun and c-fos.

Conclusions

ASH could be a key regulator in maintaining the synaptic plasticity in HFD induced obesity and can serve as a nootropic candidate against obesity induced cognitive impairments.

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.

Hippocampal insulin resistance and cognitive dysfunction

Clinical studies suggest a link between type 2 diabetes mellitus (T2DM) and insulin resistance (IR) and cognitive dysfunction, but there are significant gaps in our knowledge of the mechanisms underlying this relationship. Animal models of IR help to bridge these gaps and point to hippocampal IR as a potential mediator of cognitive dysfunction in T2DM, as well as in Alzheimer disease (AD). This Review highlights these observations and discusses intervention studies which suggest that the restoration of insulin activity in the hippocampus may be an effective strategy to alleviate the cognitive decline associated with T2DM and AD.

Are microRNAs the Molecular Link Between Metabolic Syndrome and Alzheimer's Disease?

Alzheimer's disease (AD) is the most common cause of dementia in people over 65 years of age. At present, treatment options for AD address only its symptoms, and there are no available treatments for the prevention or delay of the disease process. Several preclinical and epidemiological studies have linked metabolic risk factors such as hypertension, obesity, dyslipidemia, and diabetes to the pathogenesis of AD. However, the molecular mechanisms that underlie this relationship are not fully understood. Considering that less than 1% of cases of AD are attributable to genetic factors, the identification of new molecular targets linking metabolic risk factors to neuropathological processes is necessary for improving the diagnosis and treatment of AD. The dysregulation of microRNAs (miRNAs), small non-coding RNAs that regulate several biological processes, has been implicated in the development of different pathologies. In this review, we summarize some of the relevant evidence that points to the role of miRNAs in metabolic syndrome (MetS) and AD and propose that miRNAs may be a molecular link in the complex relationship between both diseases.

Insulin treatment restores glutamate (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor function in the hippocampus of diabetic rats

Type 1 diabetes is associated with cognitive dysfunction. Cognitive processing, particularly memory acquisition, depends on the regulated enhancement of expression and function of glutamate receptor subtypes in the hippocampus. Impairment of memory was been detected in rodent models of type 1 diabetes induced by streptozotocin (STZ). This study examines the functional properties of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and the expression of synaptic molecules that regulate glutamatergic synaptic transmission in the hippocampus of STZ-diabetic rats. The AMPA receptor-mediated miniature excitatory postsynaptic currents (mEPSCs) and single-channel properties of synaptosomal AMPA receptors were examined after 4 weeks of diabetes induction. Results show that amplitude and frequency of mEPSCs recorded from CA1 pyramidal neurons were decreased in diabetic rats. In addition, the single-channel properties of synaptic AMPA receptors from diabetic rat hippocampi were different from those of controls. These impairments in synaptic currents gated by AMPA receptors were accompanied by decreased protein levels of AMPA receptor subunit GluR1, the presynaptic protein synaptophysin, and the postsynaptic anchor protein postsynaptic density protein 95 in the hippocampus of diabetic rats. Neural cell adhesion molecule (NCAM), an extracellular matrix molecule abundantly expressed in the brain, and the polysialic acid (PSA) attached to NCAM were also downregulated in the hippocampus of diabetic rats. Insulin treatment, when initiated at the onset of diabetes induction, reduced these effects. These findings suggest that STZ-induced diabetes may result in functional deteriorations in glutamatergic synapses in the hippocampus of rats and that these effects may be reduced by insulin treatment.

Cellular and metabolic alterations in the hippocampus caused by insulin signalling dysfunction and its association with cognitive impairment during aging and Alzheimer's disease: studies in animal models

A growing body of animal and epidemiological studies suggest that metabolic diseases such as obesity, insulin resistance, metabolic syndrome and type 2 diabetes mellitus are associated with the development of cognitive impairment, dementia and Alzheimer's disease, particularly in aging. Several lines of evidence suggest that insulin signalling dysfunction produces these metabolic alterations and underlie the development of these neurodegenerative diseases. In this article, we address normal insulin function in the synapse; we review and discuss the physiopathological hallmarks of synaptic insulin signalling dysfunction associated with metabolic alterations. Additionally, we describe and review the major animal models of obesity, insulin resistance, metabolic syndrome and type 2 diabetes mellitus. The comprehensive knowledge of the molecular mechanisms behind the association of metabolic alterations and cognitive impairment could facilitate the early detection of neurodegenerative diseases in patients with metabolic alterations, with treatment that focus on neuroprotection. It could also help in the development of metabolic-based therapies and drugs for using in dementia and Alzheimer's disease patients to alleviate their symptoms in a more efficient and comprehensive way.

Regular treadmill exercise prevents sleep deprivation-induced disruption of synaptic plasticity and associated signaling cascade in the dentate gyrus

STUDY OBJECTIVES

Evidence suggests that regular exercise can protect against learning and memory impairment in the presence of insults such as sleep deprivation. The dentate gyrus (DG) area of the hippocampus is a key staging area for learning and memory processes and is particularly sensitive to sleep deprivation. The purpose of this study was to determine the effect of regular exercise on early-phase long-term potentiation (E-LTP) and its signaling cascade in the presence of sleep deprivation.

EXPERIMENTAL DESIGN

Rats were exposed to 4 weeks of regular treadmill exercise then subsequently sleep-deprived for 24h using the modified multiple platform model before experimentation. We tested the effects of exercise and/or sleep deprivation using electrophysiological recording in the DG to measure synaptic plasticity; and Western blot analysis to quantify the levels of key signaling proteins related to E-LTP.

MEASUREMENTS AND RESULTS

Regular exercise prevented the sleep deprivation-induced impairment of E-LTP in the DG area as well as the sleep deprivation-associated decrease in basal protein levels of phosphorylated and total α calcium/calmodulin-dependent protein kinase II (P/total-CaMKII) and brain-derived neurotrophic factor (BDNF). High frequency stimulation (HFS) to the DG area was used to model learning stimuli and increased the P-CaMKII and BDNF levels in normal animals: yet failed to change these levels in sleep-deprived rats. However, HFS in control and sleep-deprived rats increased the levels of the phosphatase calcineurin. In contrast, exercise increased BDNF and P-CaMKII levels in exercised/sleep-deprived rats.

CONCLUSIONS

Regular exercise appears to exert a protective effect against sleep deprivation-induced spatial memory impairment by inducing hippocampal signaling cascades that positively modulate basal and stimulated levels of key effectors such as P-CaMKII and BDNF, while attenuating increases in the protein phosphatase calcineurin.

Effects of diabetes on hippocampal neurogenesis: links to cognition and depression

Diabetes often leads to a number of complications involving brain function, including cognitive decline and depression. In addition, depression is a risk factor for developing diabetes. A loss of hippocampal neuroplasticity, which impairs the ability of the brain to adapt and reorganize key behavioral and emotional functions, provides a framework for understanding this reciprocal relationship. The effects of diabetes on brain and behavioral functions in experimental models of type 1 and type 2 diabetes are reviewed, with a focus on the negative impact of impaired hippocampal neurogenesis, dendritic remodeling and increased apoptosis. Mechanisms shown to regulate neuroplasticity and behavior in diabetes models, including stress hormones, neurotransmitters, neurotrophins, inflammation and aging, are integrated within this framework. Pathological changes in hippocampal function can contribute to the brain symptoms of diabetes-associated complications by failing to regulate the hypothalamic-pituitary-axis, maintain learning and memory and govern emotional expression. Further characterization of alterations in neuroplasticity along with glycemic control will facilitate the development and evaluation of pharmacological interventions that could successfully prevent and/or reverse the detrimental effects of diabetes on brain and behavior.

Impairment of long-term potentiation in the hippocampus of alcohol-treated OLETF rats

Type 2 diabetes and chronic heavy alcohol consumption each have been known to be associated with the impairment of hippocampus-dependent cognitive functions. Although both conditions often coexist clinically and there is accumulated evidence of a relationship between the two, the combined effect on hippocampal long-term potentiation (LTP) has not yet been investigated. We compared the effect of type 2 diabetes itself with that of type 2 diabetes with chronic heavy alcohol consumption on the hippocampal LTP using Otsuka Long-Evans Tokushima Fatty (OLETF) rat model, which resembles the characteristics of human type 2 diabetes. Ten of 16-week-old male OLETF rats were randomized into two treatment groups according to weight: the OLETF-Alcohol (O-A, n=5) and the OLETF-Control (O-C, n=5). The rats in the O-A group were fed Lieber-DeCarli Regular EtOH over a 10-week period and the amount of alcohol consumption was 8.42±2.52g/kg/day. To ensure the effect of poor glycemic control on LTP, intraperitoneal glucose tolerance test was performed after a 10-week treatment. The hippocampal LTP was measured by extracellular field excitatory post-synaptic potentials at Shaffer collateral (SC) synapses in the CA1 region. Although the O-A group showed significantly lower fasting and postprandial glucose (P<0.01 and P=0.02, respectively), the hippocampal LTP was more significantly attenuated in the O-A group than the O-C group (P=0.032). The results of this study suggested that chronic heavy alcohol consumption could potentiate the impairment of hippocampal LTP in individuals with impaired glucose tolerance or early type 2 diabetes, even though it did not aggravate, but did improve glycemic control. Clinical attention to chronic heavy drinking will be required in preventing cognitive impairment in individuals with type 2 diabetes.

Diabetes as a chronic metabolic stressor: causes, consequences and clinical complications

Diabetes mellitus is an endocrine disorder resulting from inadequate insulin release and/or reduced insulin sensitivity. The complications of diabetes are well characterized in peripheral tissues, but there is a growing appreciation that the complications of diabetes extend to the central nervous system (CNS). One of the potential neurological complications of diabetes is cognitive deficits. Interestingly, the structural, electrophysiological, neurochemical and anatomical underpinnings responsible for cognitive deficits in diabetes are strikingly similar to those observed in animals subjected to chronic stress, as well as in patients with stress-related psychiatric illnesses such as major depressive disorder. Since diabetes is a chronic metabolic stressor, this has led to the suggestion that common mechanistic mediators are responsible for neuroplasticity deficits in both diabetes and depression. Moreover, these common mechanistic mediators may be responsible for the increase in the risk of depressive illness in diabetes patients. In view of these observations, the aims of this review are (1) to describe the neuroplasticity deficits observed in diabetic rodents and patients; (2) to summarize the similarities in the clinical and preclinical studies of depression and diabetes; and (3) to highlight the diabetes-induced neuroplasticity deficits in those brain regions that have been implicated as important pathological centers in depressive illness, namely, the hippocampus, the amygdala and the prefrontal cortex.

Synaptic plasticity in the pathophysiology and treatment of bipolar disorder

Emerging evidence suggests that synaptic plasticity is intimately involved in the pathophysiology and treatment of bipolar disorder (BPD). Under certain conditions, over-strengthened and/or weakened synapses at different circuits in the brain could disturb brain functions in parallel, causing manic-like or depressive-like behaviors in animal models. In this chapter, we summarize the regulation of synaptic plasticity by medications, psychological conditions, hormones, and neurotrophic factors, and their correlation with mood-associated animal behaviors. We conclude that increased serotonin, norepinephrine, dopamine, brain-derived neurotrophic factor (BDNF), acute corticosterone, and antidepressant treatments lead to enhanced synaptic strength in the hippocampus and also correlate with antidepressant-like behaviors. In contrast, inhibiting monoaminergic signaling, long-term stress, and pathophysiological concentrations of cytokines weakens glutamatergic synaptic strength in the hippocampus and is associated with depressive-like symptoms.

In vivo expression of ganglionic long-term potentiation in superior cervical ganglia from hypertensive aged rats

Sustained increase in central sympathetic outflow to ganglia may provide the repeated high frequency presynaptic activity required for induction of long-term potentiation in sympathetic ganglia (gLTP), which is known to be involved in the manifestation of a neurogenic form of hypertension, namely stress-hypertension. Aging is often viewed as a progressive decline in physiological competence with a corresponding impaired ability to adapt to stressful stimuli. Old animals have exaggerated sympathetic activity as well as increased morbidity and mortality during prolonged exposure to stressful stimuli. Using the superior cervical ganglion (SCG) as a model for sympathetic ganglia, electrophysiological and biochemical evidence show that mildly hypertensive aged rats (22-month old) have expressed gLTP in vivo. This is suggested by a number of lines of evidence. Firstly, a shift in input/output (I/O) curve of ganglia from aged rats to the left side of I/O curve of ganglia from 6-month old (adult) rats indicating expression of gLTP. Secondly, failure of in vitro high frequency stimulation to induce gLTP in ganglia isolated from aged rats, which indicates occlusion due to saturation, which, in turn, suggests in vivo expression of gLTP in these ganglia. Thirdly, in vitro inhibition of basal ganglionic transmission by blockers of gLTP (5-HT(3) antagonists) is observed in ganglia isolated from aged rats, but not in those from adult rats. Finally, immunoblot analysis revealed that protein levels of signaling molecules such as calcium-calmodulin kinase II (CaMKII; phosphorylated and total), which normally increase during expression of LTP, are elevated in ganglia isolated from aged rats compared to those from adult ones. Protein levels of calcineurin, which dephosphorylates P-CaMKII, were reduced in ganglia isolated from aged rats, probably as a support mechanism to allow prolonged phosphorylation of CaMKII. Our findings suggest in vivo expression of gLTP in sympathetic ganglia of aged animals, which may contribute to the moderate hypertension often seen in aged subjects.

Adult-onset hypothyroidism facilitates and enhances LTD: Reversal by chronic nicotine treatment

Chronic nicotine treatment reverses hypothyroidism-induced impairment of hippocampus-dependent spatial memory and long-term potentiation (LTP). We investigated the effect of hypothyroidism on long-term depression (LTD) and possible protection by nicotine. Following paired pulse stimulation, LTD was expressed in hippocampal area CA1 of anesthetized thyroidectomized, euthyroid (sham control), nicotine-treated and nicotine-treated thyroidectomized (hypothyroid) rats. In hypothyroid rats, a significantly higher LTD magnitude was seen compared with that in control rats. A brief train of stimuli (5 pulses at 100 Hz), which did not affect synaptic transmission in control rats, induced a robust LTD in hypothyroid rats suggesting facilitation of LTD expression in these rats. Chronic nicotine treatment (1 mg/kg, 2x day) of hypothyroid rats reversed hypothyroidism-induced enhancement and facilitation of LTD. Western blot analysis of the NMDA receptor subunits in the membranous fractions of hippocampal area CA1 neurons revealed that hypothyroidism reduced NR1 and increased NR2B without affecting NR2A protein levels. These changes in NMDA receptors in hypothyroid rats were reversed by chronic nicotine treatment. Hypothyroidism did not alter BDNF or nicotinic acetylcholine receptor (nAChR) levels. However, nicotine treatment increased protein levels of these molecules in both euthyroid and hypothyroid rats. Our results suggest that alterations in the levels of NMDA receptor subunits may account for the facilitation and enhancement of LTD in hypothyroidism.