Role of insulin and insulin receptor in learning and memory

Why Alzheimer's is a disease of memory: the attack on synapses by A beta oligomers (ADDLs)

Individuals with early-stage Alzheimer's disease (AD) suffer from profound failure to form new memories. A novel molecular mechanism with implications for therapeutics and diagnostics is now emerging in which the specificity of AD for memory derives from disruption of plasticity at synapses targeted by neurologically active A beta oligomers (1). We have named these oligomers "ADDLs" (for pathogenic A beta-Derived Diffusible Ligands). ADDLs constitute metastable alternatives to the disease-defining A beta fibrils deposited in amyloid plaques. In AD brain, ADDLs accumulate primarily as A beta 12mers (2) (approximately 54 kDa) and can be found in dot-like clusters distinct from senile plaques (3). Oligomers of equal mass have been reported to occur in tgmouse AD models where they emerge concomitantly with memory failure (4), consistent with ADDL inhibition of LTP (1). In cell biology studies, ADDLs act as pathogenic gain-of-function ligands that target particular synapses, binding to synaptic spines at or near NMDA receptors (5,6). Binding produces ectopic expression of the memory-linked immediate early gene Arc. Subsequent ADDL-induced abnormalities in spine morphology and synaptic receptor composition (7) are predicted consequences of Arc overexpression, a pathology associated with memory dysfunction in tg-Arc mice. Significantly, the attack on synapses provides a plausible mechanism unifying memory dysfunction with major features of AD neuropathology; recent findings show that ADDL binding instigates synapse loss, oxidative damage, and AD-type tau hyperphosphorylation. Acting as novel neurotoxins that putatively account for memory loss and neuropathology, ADDLs present significant targets for disease-modifying therapeutics in AD.

Type 2 diabetes and risk of cognitive impairment and dementia

Diabetes is a major public health burden. Even a modest effect of diabetes on cognitive function has significant public health implications. Several lines of mechanistic evidence implicate a role of insulin and glucose metabolism on risk of developing dementia, including Alzheimer's disease. Population-based studies have shown that those with type 2 diabetes mellitus have an increased risk of cognitive impairment, dementia, and neurodegeneration. There are many mechanisms through which diabetes could increase risk of dementia, including glycemia, insulin resistance, oxidative stress, advanced glycation endproducts, inflammatory cytokines, and microvascular and macrovascular disease. This paper presents a review of the evidence on diabetes and increased risk of dementia and cognitive impairment, a discussion of different possible mechanisms, and remaining gaps in our knowledge.

Insulin and ghrelin: peripheral hormones modulating memory and hippocampal function

Peptide hormones, initially identified in the periphery and best known for regulation of food intake and appetite, have increasingly been shown to regulate brain functions not only within the hypothalamus but elsewhere. The hippocampus, in particular, expresses receptors for many hormones. Both insulin and ghrelin are now known to enhance hippocampal memory processes; in addition, insulin acts to increase local hippocampal metabolism and regulate synaptic plasticity, while administration of ghrelin has been shown to promote dendritic spine synaptic formation and to increase anxiety. While insulin's effects appear to be specifically within the hippocampus, ghrelin may act at a range of sites within the limbic system.

The effect of glucose administration on the recollection and familiarity components of recognition memory

Previous research has demonstrated that glucose administration facilitates long-term memory performance. The aim of the present research was to evaluate the effect of glucose administration on different components of long-term recognition memory. Fifty-six healthy young individuals received (a) a drink containing 25 g of glucose or (b) an inert placebo drink. Recollection and familiarity components of recognition memory were measured using the 'remember-know' paradigm. The results revealed that glucose administration led to significantly increased proportion of recognition responses based on recollection, but had no effect on the proportion of recognition responses made through participants' detection of stimulus familiarity. Consequently, the data suggest that glucose administration appears to facilitate recognition memory that is accompanied by recollection of contextual details and episodic richness. The findings also suggest that memory tasks that result in high levels of hippocampal activity may be more likely to be enhanced by glucose administration than tasks that are less reliant on medial temporal lobe structures.

Anti-Apoptotic Effect of Insulin in the Control of Cell Death and Neurologic Deficit after Acute Spinal Cord Injury in Rats

Recent studies confirmed that the new cell survival signal pathway of Insulin-PI3K-Akt exerted cyto-protective actions involving anti-apoptosis. This study was undertaken to investigate the potential neuroprotective effects of insulin in the pathogenesis of spinal cord injury (SCI) and evaluate its therapeutic effects in adult rats. SCI was produced by extradural compression using modified Allen's stall with damage energy of 40 g-cm force. One group of rats was subjected to SCI in combination with the administration of recombinant human insulin dissolved in 50% glucose solution at the dose of 1 IU/kg day, for 7 days. At the same time, another group of rats was subjected to SCI in combination with the administration of an equal volume of sterile saline solution. Functional recovery was evaluated using open-field walking, inclined plane tests, and motor evoked potentials (MEPs) during the first 14 days post-trauma. Levels of protein for B-cell lymphoma/leukemia-2 gene (Bcl-2), Caspase-3, inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2) were quantified in the injured spinal cord by Western blot analysis. Neuronal apoptosis was detected by TUNEL, and spinal cord blood flow (SCBF) was measured by laser-Doppler flowmetry (LDF). Ultimately, the data established the effectiveness of insulin treatment in improving neurologic recovery, increasing the expression of anti-apoptotic bcl-2 proteins, inhibiting caspase-3 expression decreasing neuronal apoptosis, reducing the expression of proinflammatory cytokines iNOS and COX-2, and ameliorating microcirculation of injured spinal cord after moderate contusive SCI in rats. In sum, this study reported the beneficial effects of insulin in the treatment of SCI, with the suggestion that insulin should be considered as a potential therapeutic agent.

Amyloid beta oligomers induce impairment of neuronal insulin receptors

Recent studies have indicated an association between Alzheimer's disease (AD) and central nervous system (CNS) insulin resistance. However, the cellular mechanisms underlying the link between these two pathologies have not been elucidated. Here we show that signal transduction by neuronal insulin receptors (IR) is strikingly sensitive to disruption by soluble Abeta oligomers (also known as ADDLs). ADDLs are known to accumulate in AD brain and have recently been implicated as primary candidates for initiating deterioration of synapse function, composition, and structure. Using mature cultures of hippocampal neurons, a preferred model for studies of synaptic cell biology, we found that ADDLs caused a rapid and substantial loss of neuronal surface IRs specifically on dendrites bound by ADDLs. Removal of dendritic IRs was associated with increased receptor immunoreactivity in the cell body, indicating redistribution of the receptors. The neuronal response to insulin, measured by evoked IR tyrosine autophosphorylation, was greatly inhibited by ADDLs. Inhibition also was seen with added glutamate or potassium-induced depolarization. The effects on IR function were completely blocked by NMDA receptor antagonists, tetrodotoxin, and calcium chelator BAPTA-AM. Downstream from the IR, ADDLs induced a phosphorylation of Akt at serine473, a modification associated with neurodegenerative and insulin resistance diseases. These results identify novel factors that affect neuronal IR signaling and suggest that insulin resistance in AD brain is a response to ADDLs, which disrupt insulin signaling and may cause a brain-specific form of diabetes as part of an overall pathogenic impact on CNS synapses.

Lentivirus-mediated downregulation of hypothalamic insulin receptor expression

Regulation of feeding behavior and energy balance are among the central effects of insulin. For example, intracerebroventricular administration of insulin decreases food intake and body weight, whereas antisense oligodeoxynucleotide downregulation of insulin receptors (IRs) produces hyperphagia. To further examine the role of IRs in the central actions of insulin, we designed an IR antisense lentiviral vector (LV-IRAS) and injected this vector into the third ventricle to selectively decrease IR expression in the rat hypothalamus. Three weeks after LV-IRAS administration, the expression of IRs in the hypothalamus was significantly decreased, whereas no changes were observed in hippocampal IR levels. LV-IRAS administration decreased insulin-stimulated phosphorylation of hypothalamic IRs and translocation of the insulin-sensitive glucose transporter GLUT4 in the hypothalamus; no changes in IR signaling were observed in the hippocampus of LV-IRAS-treated rats. Lentivirus-mediated downregulation of IR expression and signaling produced significant increases in body weight, as well as increases in fat mass that were selective for the subcutaneous compartment. Conversely, lean muscle mass and water mass were not affected in LV-IRAS-treated rats compared to rats treated with control virus. Changes in peripheral adiposity were associated with increases in basal hypothalamic leptin signaling in the absence of changes in leptin receptor expression in LV-IRAS rats. Collectively, these data illustrate the important functional relationships between hypothalamic insulin and leptin signaling in the regulation of body composition and provide insight into the mechanisms through which decreases in IR expression and signaling dysregulates leptin activity, thereby promoting increases in peripheral adiposity.

Intra CA1 insulin microinjection improves memory consolidation and retrieval

Although the brain was considered as an insulin-insensitive organ, recent studies have shown that insulin receptors exist in the brain and insulin modulates some of the brain tasks. Insulin and its receptor are found in specific areas of CNS with a variety of region-specific functions different from its direct glucose regulation in the periphery. The hippocampus and cerebral cortex distributed insulin/insulin receptor has been shown to be involved in brain cognitive functions. The improving effect of insulin on spatial memory acquisition has been shown. In the present study, the effect of insulin microinjection into the CA1 region of rat hippocampus on spatial memory consolidation and retrieval has been investigated. Insulin in 12 MU (but not in 0.5 and 6 MU) improved both memory retrieval and consolidation.

Evolutionary origins of insulin resistance: a behavioral switch hypothesis

BACKGROUND

Insulin resistance, which can lead to a number of diseases including type 2 diabetes and coronary heart disease, is believed to have evolved as an adaptation to periodic starvation. The "thrifty gene" and "thrifty phenotype" hypotheses constitute the dominant paradigm for over four decades. With an increasing understanding of the diverse effects of impairment of the insulin signaling pathway, the existing hypotheses are proving inadequate.

PRESENTATION OF THE HYPOTHESIS

We propose a hypothesis that insulin resistance is a socio-ecological adaptation that mediates two phenotypic transitions, (i) a transition in reproductive strategy from "r" (large number of offspring with little investment in each) to "K" (smaller number of offspring with more investment in each) and (ii) a transition from "stronger to smarter" or "soldier to diplomat" i.e. from relatively more muscle dependent to brain dependent lifestyle. A common switch could have evolved for the two transitions since the appropriate environmental conditions for the two transitions are highly overlapping and interacting.

TESTING THE HYPOTHESIS

Gestational insulin resistance diverts more energy through the placenta, resulting in increased investment per offspring. On the other hand, insulin resistance is associated with reduced ovulation. The insulin signaling pathway is also related to longevity. Insulin resistance diverts more nutrients to the brain as compared to muscle. Also, hyperinsulinemia has direct positive effects on cognitive functions of the brain. The hypothesis gets support from known patterns in human clinical data and recent research on the molecular interactions in the insulin signaling pathway. Further we state many predictions of the hypothesis that can be tested experimentally or epidemiologically.

IMPLICATIONS OF THE HYPOTHESIS

The hypothesis can bring about a significant change in the line of treatment as well as public health policies for the control of metabolic syndrome.

Protective effects of insulin on polychlorinated biphenyls-induced disruption of actin cytoskeleton in hippocampal neurons

Insulin receptors are widely distributed in the brain, and insulin improves learning and memory in some brain injury. Insulin elevates LIM kinase 1 (LIMK-1) activity and induces actin polymerization in some cells, while actin cytoskeleton dynamics mediated via LIMK-1/cofilin signal pathway is considered important to learning and memory formation. Our previous studies have shown that polychlorinated biphenyls (PCBs) disrupt the actin cytoskeleton by inhibiting LIMK-1/cofilin signaling pathway in the cultured hippocampal neurons. To determine potential neuronal protective effects by insulin, we administered insulin to the cultured hippocampal neurons after exposure to PCBs mixture Aroclor 1254 (A 1254). We found that insulin antagonized a loss of filamentous actin and the cytotoxicity induced by A 1254. Similarly, insulin restored the decrease of LIMK-1 and cofilin phosphorylation induced by A 1254. We concluded that insulin could protect neurons, probably partly by ameliorating filamentous actin cytoskeleton disruption mediated via the activation of LIMK-1/cofilin signal pathway in cultured hippocampal neurons after exposure to A 1254. The above protective effects in hippocampal neuron may have important implications in the treatment of PCBs-induced neurotoxicity and the mechanism by which insulin improves learning and memory. (c) 2007 Wiley Periodicals, Inc. Environ Toxicol 22: 152-158, 2007.

MAPK-dependent actin cytoskeletal reorganization underlies BK channel activation by insulin

Numerous brain regions are enriched with insulin and insulin receptors, and several lines of evidence indicate that insulin is an important modulator of neuronal function. Indeed, recent studies have demonstrated that insulin inhibits hippocampal epileptiform-like activity, in part by activating large-conductance Ca2+-activated potassium (BK) channels. Moreover, the mitogen-activated protein kinase (MAPK) signalling cascade has been found to couple insulin to BK channel activation. However, the cellular events downstream of MAPK that underlie this action of insulin are unknown. Here we demonstrate that in hippocampal neurons, BK channel activation by insulin is blocked by actin filament stabilization, suggesting that this process is dependent on the actin cytoskeleton. Stabilizing actin filaments also markedly attenuated the ability of insulin to inhibit the aberrant hippocampal synaptic activity evoked following Mg2+ removal. Insulin also promoted rapid reorganization of fluorescently labelled polymerized actin filaments; an action that was prevented by inhibitors of MAPK activation. Moreover, in parallel studies, insulin increased the level of phospho-MAPK immunostaining in hippocampal neurons. These data are consistent with BK channel activation by insulin involving MAPK-dependent alterations in actin dynamics. This process may have important implications for the role of insulin in regulating hippocampal excitability.

Insulin expression in the brain and pituitary cells of tilapia (Oreochromis niloticus)

While the presence of immunoreactive insulin in the central nervous system of many vertebrate species is well known, the origin of brain insulin is still debated. In this study, we applied RT-PCR, quantitative RT-PCR (qRT-PCR), and Northern hybridization to examine expression of the insulin gene in different tissues of an adult teleost fish, the Nile Tilapia (Oreochromis niloticus). We found that the insulin gene is transcribed at a high level in Brockmann bodies (pancreatic islet organs) and at a low level in the brain and pituitary gland. In the brain, insulin transcripts were detected in all areas by qRT-PCR and in situ hybridization. The highest level of insulin mRNA was found in the hypothalamus. The level of insulin transcription in the pituitary gland was 6-fold higher than that in the brain and 4.6-fold higher than that in the hypothalamus. Furthermore, insulin mRNA and immunoreactive insulin-like protein was detected in the pituitary gland using in situ hybridization, immunohistochemistry, and Western blot analysis. Our results indicate that in adult tilapia insulin expression is not restricted to the endocrine pancreatic cells, but also occurs in endocrine cells of the pituitary gland and in the neuronal cells of the brain, suggesting that the brain/pituitary gland might represent extrapancreatic origin of insulin production.

Corticosterone impairs insulin-stimulated translocation of GLUT4 in the rat hippocampus

BACKGROUND

Exposure to stress levels of glucocorticoids produces physiological responses that are characteristic of type 2 diabetes, such as peripheral insulin resistance and impairment in insulin-stimulated trafficking of glucose transporter 4 (GLUT4) in muscle and fat. In the central nervous system, stress produces neuroanatomical and neurochemical changes in the hippocampus that are associated with cognitive impairments.

METHODS

In view of these observations, the current studies examined the effects of short-term (1 week) exposure of stress levels of glucocorticoids upon insulin receptor (IR) expression and signaling, including GLUT4 translocation, in the rat hippocampus.

RESULTS

One week of corticosterone (CORT) treatment produced insulin resistance in response to peripheral glucose challenge. In the hippocampus, IR expression was unchanged in CORT-treated rats as compared with vehicle-treated rats. However, insulin-stimulated phosphorylation of the IR, total Akt levels and total GLUT4 levels were reduced in CORT-treated rats when compared to controls. In addition, insulin-stimulated translocation of hippocampal GLUT4 to the plasma membrane was completely abolished in CORT-treated rats.

CONCLUSIONS

These results demonstrate that in addition to eliciting peripheral insulin resistance, short-term CORT administration impairs insulin signaling in the rat hippocampus, effects that may contribute to the deleterious consequences of hypercortisolemic/hyperglycemic states observed in type 2 diabetes.

Higher fasting serum insulin is associated with increased resting energy expenditure in nondiabetic schizophrenia patients

BACKGROUND

Insulin has emerged as an important determinant of food intake, energy expenditure, and weight control. This study examined the relationship between fasting serum insulin level and resting energy expenditure (REE) in a cross-sectional sample of nondiabetic schizophrenia patients.

METHODS

Subjects were recruited from an urban community mental health clinic. Each subject underwent a series of anthropometric measures and an indirect calorimetry measure. A fasting blood sample was taken for plasma glucose, serum insulin, and lipid profile.

RESULTS

Seventy-one subjects (54 male, 17 female) were included in the study. There was a significant positive relationship between REE and fasting serum insulin level (r = .39, p = .001). Stepwise multiple regression analysis was performed with various characteristics such as age, race, antipsychotic agent used, fat-free mass, BMI, waist circumference, waist-hip ratio, physical activity level, and fasting serum insulin as candidate predictors for REE. Only fat-free mass and insulin were able to enter into the regression model, which indicates that higher fat-free mass and higher fasting serum insulin level predict increased REE.

CONCLUSIONS

A higher fasting serum insulin level is associated with an increased REE, which may prevent further weight gain in nondiabetic patients with schizophrenia.

Plasma C-peptide and cognitive performance in older men without diabetes

BACKGROUND

Emerging evidence suggests that type 2 diabetes may be related to diminished cognition, but little data are available directly regarding the role of insulin levels.

OBJECTIVE

The objective of this prospective cohort study was to examine the relation of insulin secretion to cognitive function among men without diabetes.

SETTING

The study setting was the Physicians' Health Study-U.S. male physicians.

PARTICIPANTS

Three hundred sixty-seven men who provided blood samples in 1982, when they had no lifetime history of diabetes and ranged in age from 47-65 years (mean age: 57 years).

MEASUREMENTS

The authors assayed plasma C-peptide, reflecting insulin secretion, in the stored blood samples. Beginning in 2001, an average 18 years after blood collection, the authors administered telephone interviews, including tests of general cognition (Telephone Interview of Cognitive Status [TICS]), verbal memory, and category fluency. The authors used regression models to estimate mean differences in cognitive performance across levels of C-peptide controlling for a wide variety of potential confounding factors.

RESULTS

On the TICS, men in the top tertile of C-peptide performed significantly worse than those in the bottom (multivariable-adjusted mean difference: -1.01 points, 95% confidence interval: -1.78 to -0.24); this apparent impact of C-peptide on cognition was equivalent to the cognitive differences the authors observed between men 6 years apart in age. Performance on the global score (combining results from all the individual tests) and verbal memory score (combining results from four tests of verbal memory) appeared lower among men in the highest C-peptide tertile, but results were not statistically significant.

CONCLUSION

Higher midlife insulin secretion may be related to decreased later-life cognitive function, even among men without diabetes.

The effect of intrahippocampal insulin microinjection on spatial learning and memory

Insulin is best known for its action on peripheral target tissues such as the adipocyte, muscle and liver to regulate glucose homeostasis. Insulin and its receptor are found in specific area of CNS with a variety of region-specific functions different from its direct glucose regulation in the periphery. The hippocampus and cerebral cortex distributed insulin/insulin receptor has been shown to be involved in brain cognitive functions. Previous studies about the effect of insulin on memory are controversial. In the present study, the effect of insulin microinjection into CA1 region of rat hippocampus on water maze performance has been investigated. Insulin had a discrepant effect dose dependently. The spatial learning and memory were impaired with lower dose of insulin, had not changed with intermediate doses, while they improved with higher doses. These results suggest that insulin may have a dose-dependent effect on spatial learning and memory.

Insulin promotes diacylglycerol kinase activation by different mechanisms in rat cerebral cortex synaptosomes

The mechanism by which insulin increases diacylglycerol kinase (DAGK) activity has been studied in cerebral cortex (CC) synaptosomes from adult (3-4 months of age) rats. The purpose of this study was to identify the role of phospholipases C and D (PLC and PLD) in DAGK activation by insulin. Neomycin, an inhibitor of PLC phosphatidylinositol-bisphosphate (PIP2) specific; ethanol, an inhibitor of phosphatidic acid (PA) formation by the promotion of a transphosphatidyl reaction of phosphatidylcholine phospholipase D (PC-PLD); and DL propranolol, an inhibitor of phosphatidate phosphohydrolase (PAP), were used in this study. Insulin (0.1 microM) shielded an increase in PA synthesis by [32P] incorporation using [gamma-32P]ATP as substrate and endogenous diacylglycerol (DAG) as co-substrate. This activated synthesis was strongly inhibited either by ethanol or DL propranolol. Pulse chase experiments also showed a PIP2-PLC activation within 1 min exposure to insulin. When exogenous unsaturated 18:0-20:4 DAG was present, insulin increased PA synthesis significantly. However, this stimulatory effect was not observed in the presence of exogenous saturated (di-16:0). In the presence of R59022, a selective DAGK inhibitor, insulin exerted no stimulatory effect on [32P]PA formation, suggesting a strong relationship between increased PA formation by insulin and DAGK activity. These data indicate that the increased synthesis of PA by insulin could be mediated by the activation of both a PC-PLD pathway to provide DAG and a direct DAGK activation that is associated to the use of 18:0-20:4 DAG species. PIP2-PLC activation may contribute at least partly to the insulin effect on DAGK activity.

Endogenous insulin signaling protects cultured neurons from oxygen-glucose deprivation-induced cell death

Curiosity surrounding the physiological relevance of neural insulin signaling has gradually developed since the discovery that nervous tissue contains both the hormone and its receptor. Similar to other receptor tyrosine kinases, ligand interaction with the insulin receptor (IR) activates a variety of intracellular signaling pathways, particularly those relevant to cellular survival. Consequently, one explanation for the presence of the insulin pathway in the brain may involve participation in the response to neuronal injury. To investigate this possibility, the present study began by examining the effect of oxygen-glucose deprivation (OGD), a well-characterized in vitro model of ischemia, on ligand-binding, surface expression, and function of the IR in cultured rat neurons that were prepared under serum-free conditions. Reduced insulin-binding was observed following OGD, although surface expression of the receptor was not altered. However, OGD did significantly decrease the ability of insulin to stimulate phosphorylation of the transmembrane IR beta-subunit, without affecting protein expression of this subunit. Subsequent experiments focused on the manner in which pharmacologically manipulating IR function affected neuronal viability after OGD. Application of the IR sensitizer metformin moderately improved neuronal viability, while the specific IR tyrosine kinase inhibitor tyrphostin A47 was able to dramatically decrease viability; both compounds acted without affecting IR surface expression. Our study suggests that not only does the IR appear to play an important role in neuronal survival, but also that neurons may actively maintain IRs on the cell surface to compensate for the OGD-induced decrease in the ability of insulin to phosphorylate its receptor.

Cardiovascular disease and Alzheimer's disease: common links

Growing evidence supports a strong and likely causal association between cardiovascular disease (CVD), and its risk factors, with incidence of cognitive decline and Alzheimer's disease. Individuals with subclinical CVD are at higher risk for dementia and Alzheimer's. Several cardiovascular risk factors are also risk factors for dementia, including hypertension, high LDL cholesterol, low HDL cholesterol and especially diabetes. Moderate alcohol appears to be protective for both CVD and dementia. In contrast, inflammatory markers predict cardiovascular risk, but not dementia, despite biological plausibility for such a link. The substantial overlap in risk factors points to new avenues for research and prevention.

Higher fasting serum insulin levels are associated with a better psychopathology profile in acutely ill non-diabetic inpatients with schizophrenia

OBJECTIVE

Recent studies have suggested a beneficial role of insulin on brain function and psychological well-being. This study was undertaken to examine whether fasting serum insulin levels are associated with the psychopathology profile in a cross-sectional sample of acutely ill non-diabetic inpatients with schizophrenia.

METHODS

Subjects were recruited from a county hospital. Each subject underwent a psychopathology assessment with the Positive and Negative Syndrome Scale (PANSS). A fasting blood sample was taken to measure serum insulin, plasma glucose and lipids.

RESULTS

Twenty-six subjects (7 females, 19 males) were included in the study. Pearson correlation analysis showed significant inverse relationships between serum insulin level and PANSS-Total, Positive Symptom subscale, and General Psychopathology subscale scores (r=-0.41, p=0.037; r=-0.49, p=0.010; r=-0.45, p=0.023, respectively). However, there was no significant relationship between serum insulin level and PANSS-Negative Symptom subscale score (r=-0.13, p=0.53). Partial correlation analysis showed that the inverse relationships between serum insulin levels and PANSS-Total, Positive Symptom subscale, and General Psychopathology subscale scores became even stronger after controlling for potential confounding variables including age, gender, race, family history of mental illness, age of illness onset and body-mass index (BMI).

CONCLUSIONS

Higher fasting serum insulin levels are associated with a better psychopathology profile in acutely ill non-diabetic inpatients with schizophrenia. It is speculated that insulin might improve clinical symptoms of schizophrenia by interacting with dopamine and other neurotransmitter systems.

Enhancement of insulin-induced PI3K/Akt/GSK-3beta and ERK signaling by neuronal nicotinic receptor/PKC-alpha/ERK pathway: up-regulation of IRS-1/-2 mRNA and protein in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells treated with nicotine (10 microm for 24 h), phosphorylation of Akt, glycogen synthase kinase-3beta (GSK-3beta) and extracellular signal-regulated kinase (ERK)1/2 induced by insulin (100 nm for 10 min) was enhanced by approximately 62%, without altering levels of these protein kinases. Nicotine produced time (> 12 h)- and concentration (EC(50) 3.6 and 13 microm)-dependent increases in insulin receptor substrate (IRS)-1 and IRS-2 levels by approximately 125 and 105%, without altering cell surface density of insulin receptors. In these cells, insulin-induced tyrosine phosphorylation of IRS-1/IRS-2 and recruitment of phosphoinositide 3-kinase (PI3K) to IRS-1/IRS-2 were augmented by approximately 63%. The increase in IRS-1/IRS-2 levels induced by nicotine was prevented by nicotinic acetylcholine receptor (nAChR) antagonists, the Ca(2+) chelator 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetra-acetic acid tetrakis-acetoxymethyl ester, cycloheximide or actinomycin D. Nicotine increased IRS-1 and IRS-2 mRNA levels by approximately 57 and approximately 50%, and this was prevented by conventional protein kinase C (cPKC) inhibitor Gö6976, or ERK kinase inhibitors PD98059 and U0126. Nicotine phosphorylated cPKC-alpha, thereby increasing phosphorylation of ERK1/ERK2, as demonstrated by using Gö6976, PD98059 or U0126. Selective activation of cPKC-alpha by thymeleatoxin mimicked these effects of nicotine. Thus, stimulation of nAChRs up-regulated expression of IRS-1/IRS-2 via Ca(2+)-dependent sequential activation of cPKC-alpha and ERK, and enhanced insulin-induced PI3K/Akt/GSK-3beta and ERK signaling pathways.

Apolipoprotein E, cholesterol metabolism, diabetes, and the convergence of risk factors for Alzheimer's disease and cardiovascular disease

High fat diets and sedentary lifestyles are becoming major concerns for Western countries. They have led to a growing incidence of obesity, dyslipidemia, high blood pressure, and a condition known as the insulin-resistance syndrome or metabolic syndrome. These health conditions are well known to develop along with, or be precursors to atherosclerosis, cardiovascular disease, and diabetes. Recent studies have found that most of these disorders can also be linked to an increased risk of Alzheimer's disease (AD). To complicate matters, possession of one or more apolipoprotein E epsilon4 (APOE epsilon4) alleles further increases the risk or severity of many of these conditions, including AD. ApoE has roles in cholesterol metabolism and Abeta clearance, both of which are thought to be significant in AD pathogenesis. The apparent inadequacies of ApoE epsilon4 in these roles may explain the increased risk of AD in subjects carrying one or more APOE epsilon4 alleles. This review describes some of the physiological and biochemical changes that the above conditions cause, and how they are related to the risk of AD. A diversity of topics is covered, including cholesterol metabolism, glucose regulation, diabetes, insulin, ApoE function, amyloid precursor protein metabolism, and in particular their relevance to AD. It can be seen that abnormal lipid, cholesterol and glucose metabolism are consistently indicated as central in the pathophysiology, and possibly the pathogenesis of AD. As diagnosis of mild cognitive impairment and early AD are becoming more reliable, and as evidence is accumulating that health conditions such as diabetes, obesity, and coronary artery disease are risk factors for AD, appropriate changes to diets and lifestyles will likely reduce AD risk, and also improve the prognosis for people already suffering from such conditions.

Constitutive activity of glycogen synthase kinase-3beta: positive regulation of steady-state levels of insulin receptor substrates-1 and -2 in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells, 12-h treatment with 1-20 mM LiCl, an inhibitor of glycogen synthase kinase-3 (GSK-3), increased Ser(9) phosphorylation of GSK-3beta by approximately 44%, while decreasing insulin receptor substrate-1 (IRS-1) and IRS-2 protein levels by approximately 38 and approximately 62% in a concentration-dependent manner. Treatment with SB216763 (0.1-30 microM for 12 h), a selective inhibitor of GSK-3, lowered IRS-1 and IRS-2 levels by approximately 38 and approximately 48%, while increasing beta-catenin protein level by approximately 47%, due to the prevention of GSK-3-induced degradation of beta-catenin by SB216763. Insulin (100 nM for 24 h) increased Ser(9) phosphorylation of GSK-3beta by approximately 104%, while decreasing IRS-1 and IRS-2 levels by approximately 41 and approximately 72%; the insulin-induced Ser(9) phosphorylation of GSK-3beta, as well as down-regulations of IRS-1 and IRS-2 levels were restored to the control levels of nontreated cells at 24 h after the washout of the insulin (100 nM for 12 h)-treated cells. Either clasto-lactacystin beta-lactone or lactacystin (an inhibitor of proteasome) prevented LiCl- or SB216763-induced decreases of IRS-1 and IRS-2 levels by approximately 100 and approximately 69%, respectively. In contrast, calpastatin (an inhibitor of calpain) and leupeptin (an inhibitor of lysosome) failed to prevent the decreases of IRS-1 and IRS-2 levels caused by LiCl or SB216763. LiCl or SB216763 lowered IRS-2 mRNA level, with no effect on IRS-1 mRNA level. These results suggest that constitutive activity of GSK-3beta in quiescent cells positively maintains steady-state levels of IRS-1 and IRS-2 via regulating proteasomal degradation and/or synthesis of IRS-1 and IRS-2 proteins.

Risk of dementia in diabetes mellitus: a systematic review

The relation between diabetes and major types of dementia is controversial. This systematic review examines the incidence of dementia in people with diabetes mellitus. We identified 14 eligible longitudinal population-based studies of variable methodological quality. The incidence of "any dementia" was higher in individuals with diabetes than in those without diabetes in seven of ten studies reporting this aggregate outcome. This high risk included both Alzheimer's disease and vascular dementia (eight of 13 studies and six of nine studies respectively). Detailed data on modulating and mediating effects of glycaemic control, microvascular complications, and comorbidity (eg, hypertension and stroke) were generally absent. The findings of mechanistic studies suggest that vascular disease and alterations in glucose, insulin, and amyloid metabolism underlie the pathophysiology, but which of these mechanisms are clinically relevant is unclear. Further high quality studies need to be initiated, with objective diabetes assessment, together with reliable methods to establish the contribution of vascular disease and other comorbidity to dementia.

Phosphatidic acid and diacylglycerol generation is regulated by insulin in cerebral cortex synaptosomes from adult and aged rats

Insulin receptor associated with the cerebral cortex (CC) has been shown to be involved in brain cognitive functions. Furthermore, deterioration of insulin signaling has been associated with age-related brain degeneration. We have reported previously that aging stimulates phospholipase D/phosphatidate phosphohydrolase 2 (PLD/PAP2) pathway in CC synaptosomes from aged rats, generating a differential availability of their reaction products: diacylglycerol (DAG) and phosphatidic acid (PA). The aim of this work was to determine the effect of aging on DAG kinase (DAGK), as an alternative pathway for PA generation, and to evaluate the effect of insulin on PLD/PAP2 pathway and DAGK. PLD, PAP2, and DAGK activities were measured using specific radiolabeled substrates in CC synaptosomes from adult (4 months old) and aged rats (28 months old). In adult animals, in the presence of the tyrosine phosphatase inhibitor (sodium o-vanadate), insulin stimulated PLD activity at 5 min incubation. DAGK activity was also increased at the same time of incubation and PAP2 was inhibited. In aged animals, PLD activity was not modified by the presence of insulin plus vanadate, PAP2 was inhibited, and DAGK was stimulated by the hormone. Insulin, vanadate, and the combination of both induced protein tyrosine phosphorylation in adult CC synaptosomes. Aged rats showed a lower level of protein phosphorylation with respect to adult rats. Our results show that insulin modulates PA and DAG availability through the regulation of PLD/PAP2 and DAGK pathways in adult rat CC synaptosomes. Additionally, we demonstrated that PA and DAG generation is regulated differentially by insulin during aging.

Insulin and cholesterol pathways in neuronal function, memory and neurodegeneration

Insulin and cholesterol play important roles in basic metabolic processes in peripheral tissues. Both insulin and cholesterol can also act as signalling molecules in the central nervous system that participate in neuronal function, memory and neurodegenerative diseases. A high-cholesterol diet improves spatial memory in experimental animals. β-Amyloid, the toxic peptide in neurons of AD (Alzheimer's disease) patients, binds cholesterol and catalyses its oxidation to 7β-hydroxycholesterol, a highly toxic oxysterol that is a potent inhibitor of α-PKC (α-protein kinase C), an enzyme critical in memory consolidation and synaptic plasticity and implicated in AD. Oxidized cholesterol also can act as a second messenger for insulin. Oxidized low-density lipoprotein inhibits insulin-dependent phosphorylation of the signalling kinases ERK (extracellular-signal-regulated kinase) and PKB/Akt. In sporadic AD patients, insulin levels are decreased, suggesting links between AD and diabetes. Insulin signalling is also important in synaptic plasticity. Insulin receptors are up-regulated and undergo translocation after spatial learning. Insulin modulates the activity of excitatory and inhibitory receptors including the glutamate and γ-aminobutyric acid receptors and activates two biochemical pathways: the shc-ras-mitogen-activated protein kinase pathway and the PI3K (phosphoinositide 3-kinase)/PKC pathway, both of which are involved in memory processing. These findings point to a convergence at the biochemical level between pathways involved in AD and those important for normal memory.

Insulin modulates hippocampal activity-dependent synaptic plasticity in a N-methyl-d-aspartate receptor and phosphatidyl-inositol-3-kinase-dependent manner

Insulin and its receptor are both present in the central nervous system and are implicated in neuronal survival and hippocampal synaptic plasticity. Here we show that insulin activates phosphatidylinositol 3-kinase (PI3K) and protein kinase B (PKB), and results in an induction of long-term depression (LTD) in hippocampal CA1 neurones. Evaluation of the frequency-response curve of synaptic plasticity revealed that insulin induced LTD at 0.033 Hz and LTP at 10 Hz, whereas in the absence of insulin, 1 Hz induced LTD and 100 Hz induced LTP. LTD induction in the presence of insulin required low frequency synaptic stimulation (0.033 Hz) and blockade of GABAergic transmission. The LTD or LTP induced in the presence of insulin was N-methyl-d-aspartate (NMDA) receptor specific as it could be inhibited by alpha-amino-5-phosphonopentanoic acid (APV), a specific NMDA receptor antagonist. LTD induction was also facilitated by lowering the extracellular Mg(2+) concentration, indicating an involvement of NMDA receptors. Inhibition of PI3K signalling or discontinuing synaptic stimulation also prevented this LTD. These results show that insulin modulates activity-dependent synaptic plasticity, which requires activation of NMDA receptors and the PI3K pathway. The results obtained provide a mechanistic link between insulin and synaptic plasticity, and explain how insulin functions as a neuromodulator.

Electrophysiological and behavioral phenotype of insulin receptor defective mice

The olfactory bulb expresses one of the highest levels of insulin found in the brain. A high level of expression of the concomitant insulin receptor (IR) kinase is also retained in this brain region, even in the adult. We have previously demonstrated in a heterologous system that insulin modulates the voltage-dependent potassium channel, Kv1.3, through tyrosine phosphorylation of three key residues in the amino and carboxyl terminus of the channel protein. Phosphorylation also induces current suppression of the Kv1.3-contributed current in cultured olfactory bulb neurons (OBNs) of rodents. In order to explore the behavioral importance of this kinase-induced modulation of the channel for the olfactory ability of the animal, mice with a targeted-gene deletion of the insulin receptor were electrophysiologically and behaviorally characterized. Mice heterozygous for the insulin receptor kinase (IR+/-) gene performed the same as wild-type (+/+) mice when challenged with a traditional, non-learning-based task to test gross anosmia. There was also no significant difference across the two genotypes in tests designed to measure exploratory behavior or in a battery of systems physiology experiments designed to assess metabolic energy usage (locomotion, ingestive behaviors, weight, oxygen consumption, and respiratory quotient). Object memory recognition tests suggest that IR+/- mice have an impairment in recognition of familiarized objects; IR+/- mice demonstrate poor performance for both short-term (1 h) and long-term (24 h) memory tests in comparison to that of wild-type mice. Electrophysiological experiments indicate that mitral cell neurons cultured from both heterozygous and homozygous-null mice (IR+/- and IR-/-) have an decreased peak current amplitude compared with that recorded for wild-type (+/+) animals matched for days in vitro (DIV). These data indicate that the loss of one allele of the IR kinase gene modifies the electrical phenotype of the mitral cell neurons in the olfactory bulb without a change in gross olfactory ability. Given our findings that there are no significant changes in metabolic balance of the IR (+/-) mice but some impairment in memory retention, future experiments testing for specific olfactory behaviors or functional deficits in IR-/+ mice models of diabetes will need to either be tasks that do not require learning or will require a different model (such as diet-induced diabetes) that may evoke a stronger phenotype.

A biochemical and functional characterization of diet-induced brain insulin resistance

While considerable research has examined diminished insulin responses within peripheral tissues, comparatively little has been done to examine the effects of this metabolic disruption upon the CNS. The present study employed biochemical and electrophysiological assays of acutely prepared brain slices to determine whether neural insulin resistance is a component of the metabolic syndrome observed within the fructose-fed (FF) hamster. The tyrosine phosphorylation levels of the insulin receptor (IR) and insulin receptor substrate 1 (IRS-1) in response to insulin were significantly reduced within FF hamsters. Also, insulin-mediated phosphorylation of both residues necessary for activation of the serine-threonine kinase Akt/PKB, a key effector of insulin signaling, was markedly decreased. Elevated levels of the protein tyrosine phosphatase 1B, which dephosphorylates the IR and IRS-1, were also observed within the cerebral cortex and hippocampus of FF hamsters. Examination of whether a nutritionally induced compromise of neural insulin signaling altered synaptic function revealed a significant attenuation of insulin-induced long-term depression, but no effect upon either paired-pulse facilitation or electrically induced long-term potentiation. Collectively, our results demonstrate, for the first time, that nutritionally induced insulin resistance significantly affects the neural insulin signaling pathway, and suggest that brain insulin resistance may contribute to cognitive impairment.

PKC site mutations reveal differential modulation by insulin of NMDA receptors containing NR2A or NR2B subunits

Insulin modulates N-methyl-d-aspartate (NMDA) receptors in the CNS and potentiates currents of recombinant NMDA receptors in a subunit-specific manner in Xenopus oocytes. Previously we identified two sites in the NR2B C-terminus as targets for direct phosphorylation by C-type protein kinases (PKCs). Mutating these sites reduced insulin potentiation of currents by one half, reflecting the PKC-mediated portion of the NR2B insulin effect. The PKC-proline rich tyrosine kinase (Pyk2)-Src family kinase pathway may also mediate insulin potentiation. A dominant negative Pyk2 mutant significantly reduced insulin potentiation when co-expressed with NR2B-containing receptors, suggesting that Pyk2 and downstream Src-family tyrosine kinases are involved, along with PKCs, in insulin potentiation of NR2B. The NR2A C-terminus contains two residues homologous to the NR2B PKC targets. Mutating both these sites eliminated insulin potentiation of NR2A-containing receptors, while co-expression of dominant negative Pyk2 had no effect. Together, these data indicate that PKCs alone mediate the NR2A insulin effect. When tested individually for importance in insulin potentiation, the two PKC sites showed an additive effect in potentiation of NR2A-containing receptors. Insulin modulation of NR2A-containing receptors is mediated solely by PKCs, whereas insulin modulation of NR2B-containing receptors is mediated by PKCs and tyrosine kinases (PTKs).

Insulin affects the neuronal response in the medial temporal lobe in humans

In recent years, clear evidence has accumulated that insulin affects central nervous functions. Besides controlling metabolic processes such as energy homeostasis by the regulation of food intake through hypothalamic receptors, the peptide hormone also appears to be capable of modulating cognitive functions. Experimental and clinical evidence for insulin supports effects on learning and memory. This study explores the impact of insulin on neuronal activity using a picture encoding task in a functional magnetic resonance imaging approach. Ten subjects performed two independent scanning sessions, each session divided into one part of four baseline runs and a second part of four runs during either insulin or saline was infused. A hyperinsulinemic- euglycemic clamp technique was applied to keep the blood glucose concentrations normal during insulin infusion. Contrast images between the two parts revealed identical activation patterns during baseline and saline conditions while during the insulin condition a higher level of activation was detected within the fusiform gyrus in both hemispheres. Shorter reaction times during the insulin condition underlined the cognitive benefit. For the first time, we were able to demonstrate that insulin enhances neuronal activity within the medio-temporal lobe and increased performance in humans under in-vivo conditions.

Increased levels of insulin and insulin-like growth factor-1 hybrid receptors and decreased glycosylation of the insulin receptor alpha- and beta-subunits in scrapie-infected neuroblastoma N2a cells

We have previously shown that ScN2a cells (scrapie-infected neuroblastoma N2a cells) express 2-fold- and 4-fold-increased levels of IR (insulin receptor) and IGF-1R (insulin-like growth factor-1 receptor) respectively. In addition, the IR alpha- and beta-subunits are aberrantly processed, with apparent molecular masses of 128 and 85 kDa respectively, as compared with 136 and 95 kDa in uninfected N2a cells. Despite the 2-fold increase in IR protein, the number of (125)I-insulin-binding sites was slightly decreased in ScN2a cells [Ostlund, Lindegren, Pettersson and Bedecs (2001) Brain Res. 97, 161-170]. In order to determine the cellular localization of IR in ScN2a cells, surface biotinylation was performed, showing a correct IR trafficking and localization to the cell surface. The present study shows for the first time that neuroblastoma N2a cells express significant levels of IR-IGF-1R hybrid receptors, and in ScN2a cells the number of hybrid receptors was 2-fold higher than that found in N2a cells, potentially explaining the apparent loss of insulin-binding sites due to a lower affinity for insulin compared with the homotypic IR. Furthermore, the decreased molecular mass of IR subunits in ScN2a cells is not caused by altered phosphorylation or proteolytic processing, but rather by altered glycosylation. Enzymic deglycosylation of immunoprecipitated IR from N2a and ScN2a cells with endoglycosidase H, peptide N-glycosidase F and neuraminidase all resulted in subunits with increased electrophoretic mobility; however, the 8-10 kDa shift remained. Combined enzymic or chemical deglycosylation using anhydrous trifluoromethane sulphonic acid treatment ultimately showed that the IR alpha- and beta-subunits from ScN2a cells are aberrantly glycosylated. The increased formation of IR-IGF-1R hybrids in ScN2a cells may be part of a neuroprotective response to prion infection. The degree and functional significance of aberrantly glycosylated proteins in ScN2a cells remain to be determined.

Hypothalamic PI3K and MAPK differentially mediate regional sympathetic activation to insulin

The action of insulin in the central nervous system produces sympathetic nervous system activation (also called sympathoactivation), although the neuronal intracellular mechanisms that mediate this are unclear. We hypothesized that PI3K and MAPK, the major pathways involved in insulin receptor signaling, mediate sympathetic nerve responses to insulin. Intracerebroventricular administration of insulin in rat increased multifiber sympathetic nerve activity to the hindlimb, brown adipose tissue (BAT), adrenal gland, and kidney. Ex vivo biochemical studies of mediobasal hypothalamic tissue revealed that insulin stimulated the association of insulin receptor substrate-1 with the p85alpha subunit of PI3K and also tyrosine phosphorylation of p42 and p44 subunits of MAPK in the hypothalamus. In order to determine whether PI3K and/or MAPK were involved in insulin-mediated sympathoactivation, we tested the effect of specific inhibitors of PI3K (LY294002 and wortmannin) and MAPK (PD98059 and U0126) on regional sympathetic responses to insulin. Interestingly, regional sympathoactivation to insulin was differentially affected by blockade of PI3K and MAPK. Inhibition of PI3K specifically blocked insulin-induced sympathoactivation to the hindlimb, while inhibition of MAPK specifically blocked insulin-induced sympathoactivation to BAT. Sympathoactivation to corticotrophin-releasing factor, however, was not affected by inhibition of PI3K and MAPK. These data demonstrate that PI3K and MAPK are specific and regionally selective mediators of the action of insulin on the sympathetic nervous system.

Insulin activates native and recombinant large conductance Ca(2+)-activated potassium channels via a mitogen-activated protein kinase-dependent process

Evidence is accumulating that, in addition to regulating peripheral energy metabolism, insulin is an important modulator of neuronal function. Indeed, high levels of insulin and insulin receptors are expressed in several brain regions including the hippocampus. We have shown previously that insulin inhibits aberrant synaptic activity in hippocampal neurons via activation of large conductance Ca(2+)-activated K+ (BK) channels. In this study, we have examined further the effects of insulin on native hippocampal and recombinant (hSlo) BK channels expressed in human embryonic kidney (HEK) 293 cells. Pipette or bath application of insulin evoked a rapid increase in hippocampal BK channel activity, an action caused by activation of insulin receptors because insulin-like growth factor 1 (IGF-1) failed to mimic insulin action. In parallel studies, insulin, applied via the pipette or bath, also activated hSlo channels expressed in HEK293 cells. Although phosphoinositide 3-kinase is a key component of insulin and IGF-1 receptor signaling pathways, activation of this lipid kinase does not underlie the effects of insulin because neither 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) nor wortmannin inhibited or reversed insulin action. However, specific inhibitors of mitogen-activated protein kinase (MAPK) activation, 2'-amino-3'-methoxyflavone (PD98059) or 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)-butadiene (U0126), attenuated insulin action, indicating that a MAPK-dependent mechanism underlies this process. Furthermore, insulin activation of this pathway enhances BK channel activity by shifting the Ca(2+)-sensitivity such that BK channels are active at more hyperpolarized membrane potentials. Because postsynaptic BK channels are important regulators of neuronal hyperexcitability, insulin-induced activation of BK channels, via stimulation of a MAPK-dependent pathway, may be an important process for regulating hippocampal function under normal and pathological conditions.

Insulin and neurodegenerative disease: shared and specific mechanisms

Insulin has functions in the brain and dysregulation of these functions may contribute to the expression of late-life neurodegenerative disease. We provide a brief summary of research on the influence of insulin on normal brain function. We then review evidence that perturbation of this role may contribute to the symptoms and pathogenesis of various neurodegenerative disorders, such as Alzheimer's disease, vascular dementia, Parkinson's disease, and Huntington's disease. We conclude by considering whether insulin dysregulation contributes to neurodegenerative disorders through disease-specific or general mechanisms.

Insulin, C-peptide, hyperglycemia, and central nervous system complications in diabetes

Diabetes is an increasingly common disorder which causes and contributes to a variety of central nervous system (CNS) complications which are often associated with cognitive deficits. There appear to be two types of diabetic encephalopathy. Primary diabetic encephalopathy is caused by hyperglycemia and impaired insulin action, which evolves in a diabetes duration-related fashion and is associated with apoptotic neuronal loss and cognitive decline. This appears to be particularly associated with insulin-deficient diabetes. Secondary diabetic encephalopathy appears to arise from hypoxic-ischemic insults due to underlying microvascular disease or as a consequence of hypoglycemia. This type of cerebral diabetic complication is more common in the type 2 diabetic population. Here, we will review the clinical and experimental data supporting this conceptual division of diabetic CNS complications and discuss the underlying metabolic, molecular, and functional aberrations.

Brain insulin and feeding: a bi-directional communication

Insulin and specific insulin receptors are found widely distributed in the central nervous system (CNS) networks related in particular to energy homeostasis. This review highlights the complex regulatory loop between dietary nutrients, brain insulin and feeding. It is well documented that brain insulin has a negative, anorexigenic effect on food intake. At present, a specific role for brain insulin on cognitive functions related to feeding is emerging. The balance between orexigenic and anorexigenic pathways in the hypothalamus is crucial for the maintenance of energy homeostasis in animals and humans. The ingestion of nutrients triggers neurochemical events that signal nutrient and energy availability in the CNS, down regulate stimulators, activate anorexigenic factors, including brain insulin, and result in reduced eating. The effects of insulin in the CNS are under a multilevel control of food-intake peripherally and in the CNS, via the metabolic, endocrine and neural modifications induced by nutrients. Single meals as well as glucose and serotonin are able to regulate insulin release directly in the hypothalamus and may be of importance for its biological effects. Central mechanisms operating in glucose-induced insulin release show some analogy with the mechanisms operating in the pancreas. Leptin and melanocortins, peptides that down regulate food intake and are largely affected by nutrients, are highly interactive with insulin in the CNS probably via the neurotransmitter serotonin. In the hypothalamus, insulin and leptin share a common signaling pathway involved in food intake, namely the insulin receptor substrate, phosphatidylinositol 3-kinase pathway. Over or under-feeding, unbalanced single meals or diets, in particular diets enriched in fat, modify the amount of insulin actively transported into the brain, the release of brain insulin, the expression of insulin messenger RNA and potentially disrupt insulin signaling in the CNS. This impairment may result in disorders related to feeding behavior and energy homeostasis leading to profound dysregulations, obesity or diabetes.

Type 2 diabetes and atrophy of medial temporal lobe structures on brain MRI

AIM/HYPOTHESIS

Type 2 diabetes increases the risk not only of vascular dementia but also of Alzheimer's disease. The question remains whether diabetes increases the risk of Alzheimer's disease by diabetic vasculopathy or whether diabetes influences directly the development of Alzheimer neuropathology. In vivo, hippocampal and amygdalar atrophy on brain MRI are good, early markers of the degree of Alzheimer neuropathology. We investigated the association between diabetes mellitus, insulin resistance and the degree of hippocampal and amygdalar atrophy on magnetic resonance imaging (MRI) accounting for vascular pathology.

METHODS

Data was obtained in a population-based study of elderly subjects without dementia between 60 to 90 years of age. The presence of diabetes mellitus and, in non-diabetic subjects, insulin resistance was assessed for 506 participants in whom hippocampal and amygdalar volumes on MRI were measured. We assessed the degree of vascular morbidity by rating carotid atherosclerosis, and brain white matter lesions and infarcts on MRI.

RESULTS

Subjects with diabetes mellitus had more hippocampal and amygdalar atrophy on MRI compared to subjects without diabetes mellitus. Furthermore, increasing insulin resistance was associated with more amygdalar atrophy on MRI. The associations were not due to vascular morbidity being more pronounced in persons with diabetes mellitus.

CONCLUSIONS/INTERPRETATION

Type 2 diabetes is associated with hippocampal and amygdalar atrophy, regardless of vascular pathology. This could suggest that Type 2 diabetes directly influences the development of Alzheimer neuropathology.

Insulin inhibits extracellular regulated kinase 1/2 phosphorylation in a phosphatidylinositol 3-kinase (PI3) kinase-dependent manner in Neuro2a cells

Insulin signalling is well studied in peripheral tissue, but not in neuronal tissue. To gain more insight into neuronal insulin signalling we examined protein kinase B (PKB) and extracellular regulated kinase 1 and 2 (ERK1/2) regulation in serum-deprived Neuro2a cells. Insulin phosphorylated PKB in a dose-dependent manner but reduced phosphorylation of ERK1/2. Both processes were phosphatidylinositol 3-kinase (PI3K) dependent. Interestingly, blockade of PI3K in combination with insulin induced phosphorylation of ERK1/2. The phosphorylation of ERK1/2 could be blocked with a specific inhibitor of mitogen-activated protein/ERK kinase (MEK), suggesting that it was mediated through the highly conserved Ras-Raf-MEK-ERK1/2 pathway. Prolonged exposure to high concentrations of insulin resulted in a desensitized PI3K-PKB route. The insulin-induced inhibition of ERK1/2 phosphorylation was also diminished when the PI3K-PKB route was desensitized. Blockade of PI3K in combination with insulin, however, still resulted in an unaltered MEK-dependent phosphorylation of ERK1/2. We conclude that PI3K is an important integrator of insulin signalling in Neuro2a cells as it regulates activation of PKB and inhibition of ERK1/2, and is sensitive to the duration of the insulin stimulus.

Insulin inhibits rat hippocampal neurones via activation of ATP-sensitive K+ and large conductance Ca2+-activated K+ channels

In this study, we have used a combination of immunocytochemical and Ca(2+) imaging techniques to determine the functional localisation of insulin receptors as well as the potential role for insulin in modulating hippocampal synaptic activity. Comparison of insulin receptor and MAP2 labelling demonstrated extensive insulin receptor immunoreactivity on the soma and dendrites of cultured hippocampal neurones. Dual labelling with synapsin 1 also showed punctate insulin receptor labelling associated with synapses. In functional studies, insulin inhibited spontaneous Ca(2+) oscillations evoked in cultured hippocampal neurones following Mg(2+) removal. This action of insulin was mimicked by the ATP-sensitive K(+) (K(ATP)) channel opener diazoxide or the large conductance Ca(2+)-activated K(+) (BK) channel activator NS-1619. Furthermore, application of the K(ATP) channel blocker glybenclamide or the BK channel inhibitors iberiotoxin or charybdotoxin attenuated the actions of insulin, whereas prior incubation with a combination of glybenclamide and iberiotoxin completely blocked insulin action. The ability of insulin to modulate the Ca(2+) oscillations was reduced by the inhibitors of MAPK activation PD 98059 and U0126, but not by the PI 3-kinase inhibitors LY 294002 or wortmannin, indicating that a MAPK-driven process underlies insulin action. In conclusion, insulin inhibits spontaneous Ca(2+) oscillations via a process involving MAPK-driven activation of BK and K(ATP) channels. This process may be a useful therapeutic target for the treatment of epilepsy and certain neurodegenerative diseases.

Distinct effects of ketone bodies on down-regulation of cell surface insulin receptor and insulin receptor substrate-1 phosphorylation in adrenal chromaffin cells

Treatment (>/=24 h) of cultured bovine adrenal chromaffin cells with ketoacidosis-related concentrations (>/=3 mM) of acetoacetate (but not beta-hydroxybutyrate, acetone, and acidic medium) caused a time- and concentration-dependent reduction of cell surface (125)I-insulin binding by ~38%, with no change in the K(d) value. The reduction of (125)I-insulin binding returned to control nontreated level at 24 h after the washout of acetoacetate-treated cells. Acetoacetate did not increase the internalization rate of cell surface insulin receptor (IR), as measured in the presence of brefeldin A, an inhibitor of cell surface vesicular exit from the trans-Golgi network. Acetoacetate (10 mM for 24 h) lowered cellular levels of the immunoreactive IR precursor molecule (approximately 190 kDa) and IR by 22 and 28%, respectively. Acetoacetate decreased IR mRNA levels by approximately 23% as early as 6 h, producing their maximum plateau reduction at 12 and 24 h. The half-life of IR mRNA was shortened by acetoacetate from 13.6 to 9.5 h. Immunoprecipitation followed by immunoblot analysis revealed that insulin-induced (100 nM for 10 min) tyrosine-phosphorylation of insulin receptor substrate-1 (IRS-1) was attenuated by 56% in acetoacetate-treated cells, with no change in IRS-1 level. These results suggest that chronic treatment with acetoacetate selectively down-regulated the density of cell surface functional IR via lowering IR mRNA levels and IR synthesis, thereby retarding insulin-induced activation of IRS-1.

Brain insulin: regulation, mechanisms of action and functions

1. While many questions remained unanswered, it is now well documented that, contrary to earlier views, insulin is an important neuromodulator, contributing to neurobiological processes, in particular energy homeostasis and cognition. A specific role on cognitive functions related to feeding is proposed, and it is suggested that brain insulin from different sources might be involved in the above vital functions in health and disease. 2. A molecule identical to pancreatic insulin, and specific insulin receptors, are found widely distributed in the central nervous system networks related to feeding, reproduction, or cognition. 3. The actions of insulin in the central nervous system may be under both multilevel and multifactorial controls. The amount of blood insulin reaching the brain, brain insulin stores and secretion, potential local biosynthesis and degradation of the peptide, and insulin receptors and signal transduction can be affected by metabolic factors induced by nutrients, hormones, neurotransmitters, and regulatory peptides, peripherally or in the central nervous system. 4. Glucose and serotonin regulate insulin directly in the hypothalamus and may be of importance for its biological effects. Central mechanisms regulating glucose-induced insulin secretion show some analogy with the mechanisms operating in the pancreas. 5. A cross-talk between insulin and leptin receptors has been observed in the brain, and a regulation of central insulin actions, potentially via serotonin modulation, by leptin, galanin, melancortins, and neuropeptide Y (NPY) is suggested. 6. A more complete knowledge of the biological role of insulin in brain function and dysfunction, and of the regulatory mechanisms involved in these processes, constitutes a real advancement in the understanding of the pathophysiology of metabolic and mental diseases and could lead to important medical benefits.

Birth insult interacts with stress at adulthood to alter dopaminergic function in animal models: possible implications for schizophrenia and other disorders

Altered subcortical dopaminergic activity is thought to be involved in the pathophysiology of several disorders including schizophrenia, substance abuse and attention deficit hyperactivity disorder. Epidemiological studies have implicated perinatal insults, particularly obstetric complications involving fetal or neonatal hypoxia, as etiological risk factors for schizophrenia. This suggests the possibility that perinatal hypoxia might have lasting effects on dopaminergic function. In animal models, dopaminergic systems appears to be particularly vulnerable to a wide range of perinatal insults, resulting in persistent alterations in function of mesolimbic and mesostriatal pathways. This review summarizes recent work characterizing long-term changes in dopaminergic function and biochemistry in models of Caesarean section (C-section) birth and of C-section birth with added global anoxia in the rat and guinea pig. C-section birth and C-section with anoxia appear to be two distinct hypoxic birth insults, with somewhat differing patterns of lasting effects on dopamine systems. In addition, birth insult alters the manner in which dopaminergic function is regulated by stress at adulthood. The possible relevance of these finding to effects of human birth procedures is discussed.

Molecular Mechanisms of Migraine

Migraine is a common complex disorder that affects a large portion of the population and thus incurs a substantial economic burden on society. The disorder is characterized by recurrent headaches that are unilateral and usually accompanied by nausea, vomiting, photophobia, and phonophobia. The range of clinical characteristics is broad and there is evidence of comorbidity with other neurological diseases, complicating both the diagnosis and management of the disorder. Although the class of drugs known as the triptans (serotonin 5-HT(1B/1D) agonists) has been shown to be effective in treating a significant number of patients with migraine, treatment may in the future be further enhanced by identifying drugs that selectively target molecular mechanisms causing susceptibility to the disease.Genetically, migraine is a complex familial disorder in which the severity and susceptibility of individuals is most likely governed by several genes that may be different among families. Identification of the genomic variants involved in genetic predisposition to migraine should facilitate the development of more effective diagnostic and therapeutic applications. Genetic profiling, combined with our knowledge of therapeutic response to drugs, should enable the development of specific, individually-tailored treatment.

Different Kv2.1/Kv9.3 heteromer expression during brain and lung post-natal development in the rat

The Kv2.1/Kv9.3 heteromer generates an O2 sensitive potassium channel and induces a slow deactivation that has important consequences for brain and lung physiology. We examined the developmental regulation of Kv2.1 and Kv9.3 mRNAs in brain and lung. Both genes followed parallel expression patterns in brain, increasing progressively through post-natal life. In lung, however, the expression of the two genes followed opposite trends: Kv2.1 transcripts decreased, while Kv9.3 mRNA increased. The Kv9.3/Kv2.1 ratio shows that while in brain the expression of both genes followed a similar pattern, the relative abundance of Kv9.3 increased steadily through post-natal life in lung. Furthermore, there is selective regulation of gene expression during the suckling-weaning transition. Our results suggest that different Kv9.3/Kv2.1 ratios could have physiological implications in both organs during post-natal development, and that diet composition and selective tissue-specific insulin regulation modulate the expression of Kv2.1 and Kv9.3.