Chronic exogenous corticosterone administration generates an insulin-resistant brain state in rats

The Expression of Insulin in the Central Nervous System: What Have We Learned So Far?

After being discovered over a century ago, insulin was long considered to be a hormone exclusively produced by the pancreas. Insulin presence was later discovered in the brain, which was originally accounted for by its transport across the blood-brain barrier. Considering that both insulin mRNA and insulin were detected in the central nervous system (CNS), it is now known that this hormone is also synthesized in several brain regions, including the hypothalamus, hippocampus, cerebral and cerebellar cortex, and olfactory bulb. Although many roles of insulin in the CNS have been described, it was initially unknown which of them could be attributed to brain-derived and which to pancreatic insulin or whether their actions in the brain overlap. However, more and more studies have been emerging lately, focusing solely on the roles of brain-derived insulin. The aim of this review was to present the latest findings on the roles of brain-derived insulin, including neuroprotection, control of growth hormone secretion, and regulation of appetite and neuronal glucose uptake. Lastly, the impairment of signaling initiated by brain-derived insulin was addressed in regard to memory decline in humans.

Co-administration of bromocriptine and corticosterone produces short- and long-lasting reduction in intake of high-fat food in male rats

Dopaminergic and glucocorticoid activity has been associated with reduced food consumption; however, their possible synergic action has not yet been studied. With the aim of examining the effect of the co-administration of the dopamine receptor D2 agonist bromocriptine and corticosterone on palatable food intake, male Wistar rats were administered either bromocriptine (1 mg/kg), corticosterone (2 mg/kg), bromocriptine + corticosterone (1 mg + 2 mg/kg) or a vehicle, with a fifth group used as a control. In all cases, substances were administered 30 min before exposure to standard food or palatable food, the latter high in carbohydrates [high carbohydrate food (HCF), 75%] or high-fat food (HFF, 67%). Food consumption and body weight were recorded daily. Results showed higher consumption of standard food but lower consumption of HCF and HFF in the groups that received bromocriptine, alone or in combination. In general, lower total kcal intake was observed in the bromocriptine and bromocriptine + corticosterone groups during the period of pharmacological treatment and following re-exposure to palatable food. The low HFF intake in the bromocriptine + corticosterone group persisted 10 days after the pharmacological treatment was interrupted. This effect suggests plastic changes in either the mechanisms involved in the incentive value of palatable food - particularly foods with high-fat content - or those that regulate lipid metabolism. Our findings suggest that homeostatic and reward mechanisms could be influenced by the co-participation of the dopaminergic and hypothalamic-pituitary-adrenal systems, and the macronutrient content of food.

Clinical Evidence of Antidepressant Effects of Insulin and Anti-Hyperglycemic Agents and Implications for the Pathophysiology of Depression-A Literature Review

Close connections between depression and type 2 diabetes (T2DM) have been suggested by many epidemiological and experimental studies. Disturbances in insulin sensitivity due to the disruption of various molecular pathways cause insulin resistance, which underpins many metabolic disorders, including diabetes, as well as depression. Several anti-hyperglycemic agents have demonstrated antidepressant properties in clinical trials, probably due to their action on brain targets based on the shared pathophysiology of depression and T2DM. In this article, we review reports of clinical trials examining the antidepressant effect of these medications, including insulin, metformin, glucagon like peptide-1 receptor agonists (GLP-1RA), and peroxisome proliferator-activated receptor (PPAR)-γ agonists, and briefly consider possible molecular mechanisms underlying the associations between amelioration of insulin resistance and improvement of depressive symptoms. In doing so, we intend to suggest an integrative perspective for understanding the pathophysiology of depression.

Potential role of insulin on the pathogenesis of depression

The regulation of insulin on depression and depression-like behaviour has been widely reported. Insulin and activation of its receptor can promote learning and memory, affect the hypothalamic-pituitary-adrenal axis (HPA) balance, regulate the secretion of neurotrophic factors and neurotransmitters, interact with gastrointestinal microbiome, exert neuroprotective effects and have an impact on depression. However, the role of insulin on depression remains largely unclear. Therefore, in this review, we summarized the potential role of insulin on depression. It may provide new insight for clarifying role of insulin on the pathogenesis of depression.

P2X7 receptor mediates NLRP3 inflammasome activation in depression and diabetes

The increasing prevalence of depression and diabetes mellitus has become a major public health problem worldwide. Studies have shown that people with diabetes are at a high risk of being diagnosed with depression, and diabetes complicates depression treatment by promoting the deterioration of glycemic control, reducing self-care ability and quality of life, and causing severe functional disability and early mortality. Moreover, health deterioration dramatically increases the financial cost of social and health care system. Thus, how to treat depression, diabetes, and diabetes complicated by depression has become one of the world’s urgent concerns. The activation of nod-like receptor family pyrin domain containing 3 (NLRP3) is closely related to mental illness. This finding provides a new perspective for studying depression. NLRP3 plays an important role in the development of diabetes. In this review, we elaborate the definition and epidemiology of depression, diabetes, and diabetic depression and introduce the functional characteristics of an NLRP3 inflammasome and upstream P2X7 receptor. Moreover, related research on NLRP3 inflammasomes and P2X7 receptors is summarized and used as a reference for confirming that the excessive activation of P2X7- NLRP3 leads to the increased release of inflammatory cytokines, such as IL-1β, in depression and diabetes. We provide insights into the P2X7–NLRP3–IL-1β pathway as an important pathological mechanism and novel therapeutic target in diabetes and depression. Given that the P2X7–NLRP3–IL-1β pathway may play an important role in diabetes confounded by comorbid depression, the possibility of intervention with baicalin is proposed.

Shared cerebral metabolic pathology in non-transgenic animal models of Alzheimer's and Parkinson's disease

Parkinson's disease (PD) and Alzheimer's disease (AD) are the most common chronic neurodegenerative disorders, characterized by motoric dysfunction or cognitive decline in the early stage, respectively, but often by both symptoms in the advanced stage. Among underlying molecular pathologies that PD and AD patients have in common, more attention is recently paid to the central metabolic dysfunction presented as insulin resistant brain state (IRBS) and altered cerebral glucose metabolism, both also explored in animal models of these diseases. This review aims to compare IRBS and alterations in cerebral glucose metabolism in representative non-transgenic animal PD and AD models. The comparison is based on the selectivity of the neurotoxins which cause experimental PD and AD, towards the cellular membrane and intracellular molecular targets as well as towards the selective neurons/non-neuronal cells, and the particular brain regions. Mitochondrial damage and co-expression of insulin receptors, glucose transporter-2 and dopamine transporter on the membrane of particular neurons as well as astrocytes seem to be the key points which are further discussed in a context of alterations in insulin signalling in the brain and its interaction with dopaminergic transmission, particularly regarding the time frame of the experimental AD/PD pathology appearance and the correlation with cognitive and motor symptoms. Such a perspective provides evidence on IRBS being a common underlying metabolic pathology and a contributor to neurodegenerative processes in representative non-transgenic animal PD and AD models, instead of being a direct cause of a particular neurodegenerative disorder.

Poor Diet, Stress, and Inactivity Converge to Form a "Perfect Storm" That Drives Alzheimer's Disease Pathogenesis

North American incidence of Alzheimer's disease (AD) is expected to more than double over the coming generation. Although genetic factors surrounding the production and clearance of amyloid-β and phosphorylated tau proteins are known to be responsible for a subset of early-onset AD cases, they do not explain the pathogenesis of the far more prevalent sporadic late-onset variant of the disease. It is thus likely that lifestyle and environmental factors contribute to neurodegenerative processes implicated in the pathogenesis of AD. Herein, we review evidence that (1) excess sucrose consumption induces AD-associated liver pathologies and brain insulin resistance, (2) chronic stress overdrives activity of locus coeruleus neurons, leading to loss of function (a common event in neurodegeneration), (3) high-sugar diets and stress promote the loss of neuroprotective sex hormones in men and women, and (4) Western dietary trends set the stage for a lithium-deficient state. We propose that these factors may intersect as part of a "perfect storm" to contribute to the widespread prevalence of neurodegeneration and AD. In addition, we put forth the argument that exercise and supplementation with trace lithium can counteract many of the deleterious consequences associated with excessive caloric intake and perpetual stress. We conclude that lifestyle and environmental factors likely contribute to AD pathogenesis and that simple lifestyle and dietary changes can help counteract their effects.

Excessive corticosterone induces excitotoxicity of hippocampal neurons and sensitivity of potassium channels via insulin-signaling pathway

Corticosterone (CORT) is a kind of corticosteroid produced by cortex of adrenal glands. Hypothalamic-pituitary-adrenal (HPA) axis hyperfunction leads to excessive CORT, which is associated with depression. Few studies have investigated the role of CORT in voltage-gated ion channels and its upstream signaling pathway in central nervous system. In this study, we investigated the mechanism of excessive CORT resulting in brain impairment on voltage-gated ion channels, and its upstream signaling effectors in hippocampal CA1 neurons. The action potential (AP) and voltage-gated potassium currents were determined by using whole-cell patch-clamp. Insulin and CORT improved the neuronal excitability. Independent effects existed in transient potassium channel (I_A) and delay rectifier potassium channel (I_K). The inhibition of potassium currents, I_A in our experiment, could increase neuronal excitability. CORT led to the excitotoxicity of hippocampal neurons via phosphatidylinositol 3 kinase (PI3K)-mediated insulin-signaling pathway. Therefore, the stimulation of excessive CORT induces excitotoxicity of hippocampal neurons and sensitivity of potassium channels via PI3K-mediated insulin-signaling pathway, which indicates one possible way of depression treatment.

Maternal separation blunted spatial memory formation independent of peripheral and hippocampal insulin content in young adult male rats

This study explores the effects of maternal separation as a chronic early life stress (ELS) on pancreatic islets insulin content and secretion, and their potential relationship with the hippocampus insulin content and spatial memory in young adulthood. Male rat offspring were divided into two groups: stress (STR) and non-stress (non-STR) groups. The animals of the STR group were separated from their mothers during postnatal days (PND) 1 to 21. During the weaning time, that is, PND-0 to PND-21, the body weight and length of the pups were measured. Blood samples were collected on PND-1, 21, 29 and 34 and during young adulthood (53±2 days) to determine plasma corticosterone and insulin levels. The young adult animals were also tested for spatial memory. One day after the memory test, the animals were decapitated and their pancreases were removed to measure the islets insulin content and secretion. Finally, the animals' hippocampi were isolated to determine their insulin content and insulin receptor protein amounts. During the period of weaning, the body weight and length of pups belonging to the STR group were significantly lower as compared to those in the non-STR group. Maternal separation did not change the plasma levels of insulin but increased plasma corticosterone levels from PND-21 to young adulthood and also reduced the islets insulin content but did not affect insulin secretion and the hippocampus insulin content and insulin receptor protein amount. Although, at the end of the memory tests, rats of the STR group reached the escape box at almost the same time and distance and with the same errors as rats of the non-STR group, the distance traveled to reach the escape box showed a steep reduction in the non-STR group as compared to the STR group after the first trial. Moreover, as compared to the STR group, the non-STR group showed an increasing trend for direct strategy to find the escape box. The islets insulin content and secretion, and the plasma insulin concentration were not significantly correlated with the hippocampus insulin content. From the results of the present study, it appears that the main behavioral effect of the maternal separation stress in the spatial memory task was to impair the strategy used by the animals to reach the escape box. This may indicate that maternal separation stress affects brain regions other than the hippocampus. Moreover, due to the reduction of the body weight and length of offspring belonging to the STR group, it should be further considered that both maternal separation and early life malnutrition are directly (and mechanistically) linked to cognitive alterations later in life in ways that are not dependent on peripheral and hippocampal insulin content.

The locus coeruleus neurotoxin, DSP4, and/or a high sugar diet induce behavioral and biochemical alterations in wild-type mice consistent with Alzheimers related pathology

Alzheimer's disease (AD) is the sixth leading cause of death in the United States where it is estimated that one in three seniors dies with AD or another dementia. Are modern lifestyle habits a contributing factor? Increased carbohydrate (sugar) consumption, stress and disruption of sleep patterns are quickly becoming the norm rather than the exception. Interestingly, seven months on a non-invasive high sucrose diet (20% sucrose in drinking water) has been shown to induce behavioral, metabolic and pathological changes consistent with AD in wild-type mice. As chronic stress and depression are associated with loss of locus coeruleus (LC) noradrenergic neurons and projections (source of anti-inflammatory and trophic factor control), we assessed the ability for a selective LC neurotoxin (DSP4) to accelerate and aggravate a high-sucrose mediated AD-related phenotype in wild-type mice. Male C57/Bl6 mice were divided into four groups: 1) saline injected, 2) DSP4 injected, 3) high sucrose drinking water (20%) or 4) DSP4 injected and high sucrose drinking water. We demonstrate that high sucrose consumption and DSP4 treatment promote an early-stage AD-related phenotype after only 3-4 months, as evidenced by elevated fecal corticosterone, increased despair, spatial memory deficits, increased AChE activity, elevated NO production, decreased pGSK3β and increased pTau. Combined treatment appears to accelerate and aggravate pathological processes consistent with Alzheimer disease and dementia. Developing a simple model in wild-type mice will highlight environmental and lifestyle factors that need to be addressed to slow, prevent or even reverse the rising trend in dementia patient numbers and cost.

Chronic restraint stress induces hippocampal memory deficits by impairing insulin signaling

Chronic stress is a psychologically significant factor that impairs learning and memory in the hippocampus. Insulin signaling is important for the development and cognitive function of the hippocampus. However, the relation between chronic stress and insulin signaling at the molecular level is poorly understood. Here, we show that chronic stress impairs insulin signaling in vitro and in vivo, and thereby induces deficits in hippocampal spatial working memory and neurobehavior. Corticosterone treatment of mouse hippocampal neurons in vitro caused neurotoxicity with an increase in the markers of autophagy but not apoptosis. Corticosterone treatment impaired insulin signaling from early time points. As an in vivo model of stress, mice were subjected to chronic restraint stress. The chronic restraint stress group showed downregulated insulin signaling and suffered deficits in spatial working memory and nesting behavior. Intranasal insulin delivery restored insulin signaling and rescued hippocampal deficits. Our data suggest that psychological stress impairs insulin signaling and results in hippocampal deficits, and these effects can be prevented by intranasal insulin delivery.

Banxia-houpu decoction restores glucose intolerance in CUMS rats through improvement of insulin signaling and suppression of NLRP3 inflammasome activation in liver and brain

ETHNOPHARMACOLOGICAL RELEVANCE

Banxia-houpu decoction is a famous formula in traditional Chinese medicine (TCM) with the powerful anti-depressant activity.

AIM OF THE STUDY

This study aimed to investigate the effect of Banxia-houpu decoction on glucose intolerance associated with anhedonia in chronic unpredictable mild stress (CUMS) rats, then to explore its underlying pharmacological mechanisms.

MATERIALS AND METHODS

After 6-week CUMS procedure, male Wistar rats were given Banxia-houpu decoction (3.29 and 6.58g/kg, intragastrically) for 6 weeks. Sucrose solution consumption test was employed to evaluate the anhedonia behavior. Oral glucose tolerance test (OGTT) was used to determine glucose tolerance. Serum levels of corticosterone, corticotropin-releasing factor (CRF), insulin and interleukin-1 beta (IL-1β) were measured by commercial enzyme-linked immunosorbent assay kits, respectively. Furthermore, the key proteins for insulin signaling, as well as nod-like receptor family pyrin domain containing 3 (NLRP3) inflammasome, were analyzed by Western blot in periphery liver and brain regions hypothalamus, hippocampus and prefrontal cortex, respectively.

RESULTS

Banxia-houpu decoction significantly increased sucrose solution consumption and decreased serum corticosterone and CRF levels in CUMS rats, further demonstrating its antidepressant activity. More importantly, Banxia-houpu decoction improved glucose tolerance in OGTT in this animal model. Furthermore, it protected against CUMS-induced insulin signaling impairment in the liver, as well as hypothalamus and prefrontal cortex in rats. Although without significant effect on serum IL-1β levels, Banxia-houpu decoction inhibited NLRP3 inflammasome activation in the liver, hypothalamus, hippocampus and prefrontal cortex of CUMS rats, respectively.

CONCLUSIONS

The present study demonstrates that Banxia-houpu decoction suppresses NLRP3 inflammasome activation and improves insulin signaling impairment in both periphery liver and brain regions in CUMS rats, possibly contributing to its anti-depressive effect with glucose tolerance improvement. These results may provide the evidence that Banxia-houpu decoction is a potential antidepressant with the advantage to reduce the risk of comorbid depression with type 2 diabetes mellitus.

Light and water are not simple conditions: fine tuning of animal housing in male C57BL/6 mice

While animal housing conditions are highly controlled and standardized between different laboratories, there are still many subtle differences that unavoidably influence the host organisms and, consequently, interlaboratory reproducibility. Here, we investigated the physiological and immunological consequences between two light/dark cycle (LDC) lengths (14-h/10-h vs. 12-h/12-h LDC) and two commonly used forms of drinking water (acidified drinking water (AW) versus normal tap water (NW)) in single-housed (SH) mice. Our results indicate that SH mice bred under a 12-h/12-h LDC and NW at the supplier's facility showed increased basal morning plasma corticosterone (CORT) levels even 4 weeks after arrival at our animal facility employing a 14-h/10-h LDC and AW. This effect was even more pronounced two weeks after arrival and had abated after 8 weeks. In agreement, increased plasma adrenocorticotropic hormone (ACTH), adrenal in vitro ACTH sensitivity, as well as relative and absolute adrenal weight normalized during this 8-week exposure to the novel and unfamiliar 14-h/10-h LDC and AW. Employment of a 12-h/12-h LDC in our facility completely abrogated the CORT-elevating effects of the 14-h/10-h LDC, despite these animals drinking AW. When both the water and light conditions were matched to those at the supplier's facility, we observed a further reduction in adrenal weight, increased thymus weight, and decreased pro-inflammatory cytokine secretion of isolated and anti-CD3/28-stimulated mesenteric lymph node cells. In summary, our results indicate that prolonged alteration of both the light phase and drinking water represent severe and long-lasting stressors for laboratory rodents. These findings are of general interest for all scientists obtaining their experimental animals from conventional suppliers.

Brain glucose metabolism in an animal model of depression

An increasing number of data support the involvement of disturbances in glucose metabolism in the pathogenesis of depression. We previously reported that glucose and glycogen concentrations in brain structures important for depression are higher in a prenatal stress model of depression when compared with control animals. A marked rise in the concentrations of these carbohydrates and glucose transporters were evident in prenatally stressed animals subjected to acute stress and glucose loading in adulthood. To determine whether elevated levels of brain glucose are associated with a change in its metabolism in this model, we assessed key glycolytic enzymes (hexokinase, phosphofructokinase and pyruvate kinase), products of glycolysis, i.e., pyruvate and lactate, and two selected enzymes of the tricarboxylic acid cycle (pyruvate dehydrogenase and α-ketoglutarate dehydrogenase) in the hippocampus and frontal cortex. Additionally, we assessed glucose-6-phosphate dehydrogenase activity, a key enzyme in the pentose phosphate pathway (PPP). Prenatal stress increased the levels of phosphofructokinase, an important glycolytic enzyme, in the hippocampus and frontal cortex. However, prenatal stress had no effect on hexokinase or pyruvate kinase levels. The lactate concentration was elevated in prenatally stressed rats in the frontal cortex, and pyruvate levels remained unchanged. Among the tricarboxylic acid cycle enzymes, prenatal stress decreased the level of pyruvate dehydrogenase in the hippocampus, but it had no effect on α-ketoglutarate dehydrogenase. Like in the case of glucose and its transporters, also in the present study, differences in markers of glucose metabolism between control animals and those subjected to prenatal stress were not observed under basal conditions but in rats subjected to acute stress and glucose load in adulthood. Glucose-6-phosphate dehydrogenase activity was not reduced by prenatal stress but was found to be even higher in animals exposed to all experimental conditions, i.e., prenatal stress, acute stress, and glucose administration. Our data indicate that glycolysis is increased and the Krebs cycle is decreased in the brain of a prenatal stress animal model of depression.

β-endorphin degradation and the individual reactivity to traumatic stress

Reactivity to traumatic stress varies between individuals and only a minority of those exposed to trauma develops stress-induced psychopathologies. Currently extensive effort is made to unravel the specific mechanisms predisposing to vulnerability vs. resilience to stress. We investigated in rats the role of β-endorphin metabolism in vulnerability to acute traumatic stress. Responders (showing extreme anxiety; n=7) and resilient non-responders (not differing from the non-stressed individuals; n=8) to traumatic foot-shock stress were compared for their blood levels of stress hormones as well as brain levels and activity of two opioid-degrading enzymes. β-endorphin is a substrate to insulin degrading enzyme, which also degrades insulin. Therefore, the effects of insulin application on behavioral and hormonal responses and on β-endorphin degradation were tested. Pre- and post-stress levels of serum corticosterone, and post-stress plasma β-endorphin concentration differentiated between the responders and the non-responders. In brain, responders showed enhanced degradation rates of β-endorphin, assessed by Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS), in hippocampal and amygdalar slices as compared to non-responders. Application of insulin to the amygdala, prior to exposure to traumatic stress, reduced post-stress anxiety and serum corticosterone levels only in the responders. In parallel, amygdalar β-endorphin degradation rate was also reduced by insulin. These results suggest that slowing down β-endorphin degradation rate may constitute an integral part of the normal stress-response, upon a failure of which an extreme anxiety develops. Modulation of opioid degradation may thus present a potential novel target for interference with extreme anxiety.

A subchronic application period of glucocorticoids leads to rat cognitive dysfunction whereas physostigmine induces a mild neuroprotection

The cholinergic neurotransmitter system and prolonged glucocorticoid-induced stress can affect cognitive functions in opposite ways. While pharmacological enhancement of cholinergic neurotransmission is known to induce neuroprotective effects, chronic glucocorticoids impair cognitive functions. Up to now, there is no consensus as to whether a subchronic stress period of several days would affect cognitive function. The goal of this study was to investigate whether or not repeated applications of physostigmine over 3 days lead to protective effects on rat spatial cognitive abilities in contrast to the deteriorating effect on rat cognitive function after corticosterone treatment. Furthermore, we wanted to investigate in what extent this cognition-modulating effect is associated with rat cerebral acetylcholinergic system. Male adult rats (n = 40) were randomly divided into four groups with n = 10 per group: (I) placebo-, (II) corticosterone- (15 mg/day), (III) physostigmine- (0.014 mg/day), and (IV) physostigmine + corticosterone-treated rats. Body mass and plasma corticosterone concentrations were measured. Psychometric investigations were conducted using a Morris water maze before and after a subchronic treatment. In cerebral tissue, ACh and acetylcholinesterase (AChE) content and ACh receptor density were determined. Tissue corticosterone concentration was measured in cerebral cortex, hippocampus, and adrenal glands. In corticosterone-treated rats, reduced spatial cognitive abilities were associated with a significant increase in plasma (+25%) and cerebral corticosterone levels (+350%) parallelled by a significant reduction in adrenal gland concentrations (-84%) as compared to placebo. Repeated physostigmine injections improved rats' spatial memory and increased cerebral ACh and AChE content (p < 0.05). When physostigmine was administered at the same time as corticosterone (group IV), it was not able to reverse the corticosterone effect. A significant correlation was detected between cerebral AChE and corticosterone concentrations as well as between AChE and psychometric parameters. We conclude that subchronic exogenous corticosterone administration induces memory dysfunction whereas physostigmine exerts cognitive-enhancing effects if given for 3 days. An apparently existing interaction between glucocorticoid excess and ACh metabolism is discussed.