Transport of insulin across the blood-brain barrier: saturability at euglycemic doses of insulin

Brain insulin dysregulation: implication for neurological and neuropsychiatric disorders

Arduous efforts have been made in the last three decades to elucidate the role of insulin in the brain. A growing number of evidences show that insulin is involved in several physiological function of the brain such as food intake and weight control, reproduction, learning and memory, neuromodulation and neuroprotection. In addition, it is now clear that insulin and insulin disturbances particularly diabetes mellitus may contribute or in some cases play the main role in development and progression of neurodegenerative and neuropsychiatric disorders. Focusing on the molecular mechanisms, this review summarizes the recent findings on the involvement of insulin dysfunction in neurological disorders like Alzheimer's disease, Parkinson's disease and Huntington's disease and also mental disorders like depression and psychosis sharing features of neuroinflammation and neurodegeneration.

Type 2 diabetes and cognitive impairment: linking mechanisms

This manuscript provides a brief review of current concepts in the mechanisms potentially linking type-2-diabetes (T2D) with cognitive impairment. Existing epidemiologic studies, imaging studies, autopsy studies, and clinical trials provide insights into the mechanisms linking T2D and cognitive impairment. There seems to be little dispute that T2D can cause cerebrovascular disease and thus cause vascular cognitive impairment (VCI). Whether T2D can cause late onset Alzheimer's disease (LOAD) remains to be elucidated. Many epidemiologic studies show an association between T2D and cognitive impairment, but the association with VCI seems to be stronger compared to LOAD, suggesting that cerebrovascular disease may be the main mechanism linking T2D and cognitive impairment. Imaging studies show an association between T2D and imaging markers of LOAD, but these observations could still be explained by cerebrovascular mechanisms. Autopsy studies are few and conflicting, with some suggesting a predominantly cerebrovascular mechanism, and others providing support for a neurodegenerative mechanism. Thus far, the evidence from clinical trials is mixed in supporting a causal association between T2D and cognitive impairment, and most clinical trials that can answer this question are yet to be reported or finished. Given the epidemic of T2D in the world, it is important to elucidate whether the association between T2D and cognitive impairment, particularly LOAD, is causal, and if so, what the mechanisms are.

Insulin in the brain: there and back again

Insulin performs unique functions within the CNS. Produced nearly exclusively by the pancreas, insulin crosses the blood-brain barrier (BBB) using a saturable transporter, affecting feeding and cognition through CNS mechanisms largely independent of glucose utilization. Whereas peripheral insulin acts primarily as a metabolic regulatory hormone, CNS insulin has an array of effects on brain that may more closely resemble the actions of the ancestral insulin molecule. Brain endothelial cells (BECs), the cells that form the vascular BBB and contain the transporter that translocates insulin from blood to brain, are themselves regulated by insulin. The insulin transporter is altered by physiological and pathological factors including hyperglycemia and the diabetic state. The latter can lead to BBB disruption. Pericytes, pluripotent cells in intimate contact with the BECs, protect the integrity of the BBB and its ability to transport insulin. Most of insulin's known actions within the CNS are mediated through two canonical pathways, the phosphoinositide-3 kinase (PI3)/Akt and Ras/mitogen activated kinase (MAPK) cascades. Resistance to insulin action within the CNS, sometimes referred to as diabetes mellitus type III, is associated with peripheral insulin resistance, but it is possible that variable hormonal resistance syndromes exist so that resistance at one tissue bed may be independent of that at others. CNS insulin resistance is associated with Alzheimer's disease, depression, and impaired baroreceptor gain in pregnancy. These aspects of CNS insulin action and the control of its entry by the BBB are likely only a small part of the story of insulin within the brain.

Impact of postprandial glycaemia on health and prevention of disease

Postprandial glucose, together with related hyperinsulinemia and lipidaemia, has been implicated in the development of chronic metabolic diseases like obesity, type 2 diabetes mellitus (T2DM) and cardiovascular disease (CVD). In this review, available evidence is discussed on postprandial glucose in relation to body weight control, the development of oxidative stress, T2DM, and CVD and in maintaining optimal exercise and cognitive performance. There is mechanistic evidence linking postprandial glycaemia or glycaemic variability to the development of these conditions or in the impairment in cognitive and exercise performance. Nevertheless, postprandial glycaemia is interrelated with many other (risk) factors as well as to fasting glucose. In many studies, meal-related glycaemic response is not sufficiently characterized, or the methodology with respect to the description of food or meal composition, or the duration of the measurement of postprandial glycaemia is limited. It is evident that more randomized controlled dietary intervention trials using effective low vs. high glucose response diets are necessary in order to draw more definite conclusions on the role of postprandial glycaemia in relation to health and disease. Also of importance is the evaluation of the potential role of the time course of postprandial glycaemia.

Insulin in the brain: sources, localization and functions

Historically, insulin is best known for its role in peripheral glucose homeostasis, and insulin signaling in the brain has received less attention. Insulin-independent brain glucose uptake has been the main reason for considering the brain as an insulin-insensitive organ. However, recent findings showing a high concentration of insulin in brain extracts, and expression of insulin receptors (IRs) in central nervous system tissues have gathered considerable attention over the sources, localization, and functions of insulin in the brain. This review summarizes the current status of knowledge of the peripheral and central sources of insulin in the brain, site-specific expression of IRs, and also neurophysiological functions of insulin including the regulation of food intake, weight control, reproduction, and cognition and memory formation. This review also considers the neuromodulatory and neurotrophic effects of insulin, resulting in proliferation, differentiation, and neurite outgrowth, introducing insulin as an attractive tool for neuroprotection against apoptosis, oxidative stress, beta amyloid toxicity, and brain ischemia.

Protection of exenatide for retinal ganglion cells with different glucose concentrations

Exendin-4 is a peptide resembling glucagon-like peptide-1 (GLP-1), which has protective effects on nerve cells. However, the effects of Exendin-4 on retinal ganglion cells (RGC) are still under clear. The purpose of the present study is to demonstrate that exenatide prevents high- or low-glucose-induced retinal ganglion cell impairment. We observed the expression of GLP-1R in RGC-5 cells by immunofluorescence and Western blot. To investigate the effect of exenatide on RGC-5 cells incubated different glucose concentrations, CCK-8 measured the survival rates and electron microscopy detected cellular injury. The expression levels of Bcl-2 and Bax were analyzed by immunocytochemistry and Western blot. Exenatide protects RGC-5 from high- or low-glucose-induced cellular injury and the optimum concentration was 0.5μg/ml. Exenatide can inhibit high- or low-glucose-induced mitochondrial changes. Exenatide protects RGC-5 from high- or low-glucose-induced Bax increased and Bcl-2 decreased. Furthermore, the protective effect of exenatide could be inhibited by Exendin (9-39). These findings indicate that exenatide shows a neuroprotective effect for different glucose concentrations-induced RGC-5 cells injury. Exenatide could protect RGC-5 cells from degeneration or death, which may protect retinal function and have a potential value for patients with diabetic retinopathy.

Consequences of Aberrant Insulin Regulation in the Brain: Can Treating Diabetes be Effective for Alzheimer's Disease

There is an urgent need for new ways to treat Alzheimer’s disease (AD), the most common cause of dementia in the elderly. Current therapies are modestly effective at treating the symptoms, and do not significantly alter the course of the disease. Over the years, a range of epidemiological and experimental studies have demonstrated interactions between diabetes mellitus and AD. As both diseases are leading causes of morbidity and mortality in the elderly and are frequent co-morbid conditions, it has raised the possibility that treating diabetes might be effective in slowing AD. This is currently being attempted with drugs such as the insulin sensitizer rosiglitazone. These two diseases share many clinical and biochemical features, such as elevated oxidative stress, vascular dysfunction, amyloidogenesis and impaired glucose metabolism suggesting common pathogenic mechanisms. The main thrust of this review will be to explore the evidence from a pathological point of view to determine whether diabetes can cause or exacerbate AD. This was supported by a number of animal models of AD that have been shown to have enhanced pathology when diabetic conditions were induced. The one drawback in linking diabetes and insulin to AD has been the postmortem studies of diabetic brains demonstrating that AD pathology was not increased; in fact decreased pathology has often been reported. In addition, diabetes induces its own distinct features of neuropathology different from AD. There are common pathological features to be considered including vascular abnormalities, a major feature arising from diabetes; there is increasing evidence that vascular abnormalities can contribute to AD. The most important common mechanism between insulin-resistant (type II) diabetes and AD could be impaired insulin signaling; a form of toxic amyloid can damage neuronal insulin receptors and affect insulin signaling and cell survival. It has even been suggested that AD could be considered as “type 3 diabetes” since insulin can be produced in brain. Another common feature of diabetes and AD are increased advanced glycation endproduct-modified proteins are found in diabetes and in the AD brain; the receptor for advanced glycation endproducts plays a prominent role in both diseases. In addition, a major role for insulin degrading enzyme in the degradation of Aβ peptide has been identified. Although clinical trials of certain types of diabetic medications for treatment of AD have been conducted, further understanding the common pathological processes of diabetes and AD are needed to determine whether these diseases share common therapeutic targets.

Insulin resistance in the brain: an old-age or new-age problem?

Life expectancy is rising however with more people living longer there is a concomitant rise in the incidence of dementia. In addition to age-related cognitive decline there is a higher risk of going on to develop vascular dementia and Alzheimer's disease associated with aspects of modern lifestyle. Most worryingly, recent data reports accelerated cognitive decline in adolescents associated with poor diet (high fat and calorie intake). Thus the increase in dementia in 'old-age' may have as much to do with 'new-age' lifestyle as it does with normal ageing. It would seem wise therefore to investigate the molecular connections between lifestyle and cognitive decline in more detail. Epidemiological evidence suggests an increased risk of developing dementia (including Alzheimer's disease) in individuals with obesity and type 2 diabetes but also in those with poor insulin sensitivity without diabetes, implicating a mechanistic link between adiposity, insulin sensitivity and dementia. Insulin receptors are expressed in the brain and physiological roles for insulin in the CNS are starting to be delineated. Indeed disrupted neuronal insulin action may underlie the link between diabetes and neurodegenerative disorders. This review discusses the difficulties in quantifying insulin sensitivity of the brain and why it is vital that we develop technology for this purpose so that we can establish its role in this 'new-age' dementia. This has particular relevance to the design and interpretation of clinical trials in progress to assess potential benefits of insulin and insulin sensitisers on prevention of cognitive decline.

Drug delivery to the brain in Alzheimer's disease: consideration of the blood-brain barrier

The successful treatment of Alzheimer's disease (AD) will require drugs that can negotiate the blood-brain barrier (BBB). However, the BBB is not simply a physical barrier, but a complex interface that is in intimate communication with the rest of the central nervous system (CNS) and influenced by peripheral tissues. This review examines three aspects of the BBB in AD. First, it considers how the BBB may be contributing to the onset and progression of AD. In this regard, the BBB itself is a therapeutic target in the treatment of AD. Second, it examines how the BBB restricts drugs that might otherwise be useful in the treatment of AD and examines strategies being developed to deliver drugs to the CNS for the treatment of AD. Third, it considers how drug penetration across the AD BBB may differ from the BBB of normal aging. In this case, those differences can complicate the treatment of CNS diseases such as depression, delirium, psychoses, and pain control in the AD population.

The central insulin system and energy balance

Insulin acts throughout the body to reduce circulating energy and to increase energy storage. Within the brain, insulin produces a net catabolic effect by reducing food intake and increasing energy expenditure; this is evidenced by the hypophagia and increased brown adipose tissue sympathetic nerve activity induced by central insulin infusion. Reducing the activity of the brain insulin system via administration of insulin antibodies, receptor antisense treatment, or receptor knockdown results in hyperphagia and increased adiposity. However, despite decades of research into the role of central insulin in food intake, many questions remain to be answered, including the underlying mechanism of action.

Insulin resistance and pathological brain ageing

Sir Harold Himsworth's prescient observations 75 years ago have recently been expanded to include a clear relationship between insulin resistance and central nervous system function. Insulin is a master regulator of corporeal ageing in all known species, determining the rate and expression of ageing in multiple body systems. Thus, it is not surprising that insulin also plays an important role in brain ageing and cognitive decline that is associated with pathological brain ageing. Brain ageing is accompanied by reduced insulin effectiveness, either by an inadequate cellular response to insulin or by insulin deficiency attributable to reduced insulin transport across the blood-brain barrier. Age-associated brain insulin abnormalities may contribute to cognitive decline in ageing, as have been documented in older adults with Type 2 diabetes mellitus and hypertension. With more extreme pathology, brain insulin resistance may be associated with neurogenerative diseases such as Alzheimer's disease, and the condition which precedes Alzheimer's disease, known as amnestic mild cognitive impairment. In the following review, we discuss the mechanisms through which insulin resistance may induce or potentiate pathological brain ageing and thereby create a neurobiological environment that promotes neurodegeneration and associated cognitive decline. This topic is timely, given that insulin resistance-associated conditions such as diabetes and obesity have reached epidemic proportions. The prevalence of such chronic conditions, in combination with a rapidly ageing population, may result in a corresponding increase in the prevalence of Alzheimer's disease and other cognitive disorders. Fortunately, insulin resistance-associated conditions are amenable to both pharmacologic and lifestyle interventions that may reduce the deleterious impact of insulin resistance on the ageing brain.

Reprint of: 'Brain insulin signaling: A key component of cognitive processes and a potential basis for cognitive impairment in type 2 diabetes'

Understanding of the role of insulin in the brain has gradually expanded, from initial conceptions of the brain as insulin-insensitive through identification of a role in regulation of feeding, to recent demonstration of insulin as a key component of hippocampal memory processes. Conversely, systemic insulin resistance such as that seen in type 2 diabetes is associated with a range of cognitive and neural deficits. Here we review the evidence for insulin as a cognitive and neural modulator, including potential effector mechanisms, and examine the impact that type 2 diabetes has on these mechanisms in order to identify likely bases for the cognitive impairments seen in type 2 diabetic patients.

Influence of insulin on glutamine synthetase in the Müller glial cells of retina

Glutamine synthetase (GS), a Müller cell specific enzyme in the retina, is the key enzyme involve in glutamate metabolism. The goal of this study was to investigate the expression and regulation of GS by insulin in the cultured rat retinal Müller cells. Immunocytochemical and immunoblotting experiments showed that the cultured Müller cells express GS protein under normal cell culture conditions. Insulin treatments decreased the GS expression both in a time and dose dependent manner. Insulin also decreased the hydrocortisone induced GS expression. Furthermore, we investigated the expression and regulation of two other Müller cell specific enzymes known to be involved in glutamate metabolism, the mitochondrial branched chain aminotransferase (BCATm) and pyruvate carboxylase (PC). Immunoblotting experiments showed that Müller cells expressed both BCATm and PC. Treatments of cells with hydrocortisone or insulin did not influence the BCATm expression level. Hydrocortisone treatment of cells increased the PC expression but this induced expression was suppressed by insulin treatment. Müller cells expressed insulin receptor proteins (IRβ and IRS-1) and insulin activation induced the phosphotyrosine level of insulin receptor proteins. Moreover, hydrocortisone did not influence the expression or activation of these receptor proteins. The data suggests that insulin modulates the GS synthesis and may influence glutamate metabolism in the cultured retinal Müller cells but not by influencing the insulin signaling pathway.

Possible implications of insulin resistance and glucose metabolism in Alzheimer's disease pathogenesis

Type 2 diabetes mellitus (DM) appears to be a significant risk factor for Alzheimer disease (AD). Insulin and insulin-like growth factor-1 (IGF-1) also have intense effects in the central nervous system (CNS), regulating key processes such as neuronal survival and longevity, as well as learning and memory. Hyperglycaemia induces increased peripheral utilization of insulin, resulting in reduced insulin transport into the brain. Whereas the density of brain insulin receptor decreases during age, IGF-1 receptor increases, suggesting that specific insulin-mediated signals is involved in aging and possibly in cognitive decline. Molecular mechanisms that protect CNS neurons against β-amyloid-derived-diffusible ligands (ADDL), responsible for synaptic deterioration underlying AD memory failure, have been identified. The protection mechanism does not involve simple competition between ADDLs and insulin, but rather it is signalling dependent down-regulation of ADDL-binding sites. Defective insulin signalling make neurons energy deficient and vulnerable to oxidizing or other metabolic insults and impairs synaptic plasticity. In fact, destruction of mitochondria, by oxidation of a dynamic-like transporter protein, may cause synapse loss in AD. Moreover, interaction between Aβ and τ proteins could be cause of neuronal loss. Hyperinsulinaemia as well as complete lack of insulin result in increased τ phosphorylation, leading to an imbalance of insulin-regulated τ kinases and phosphatates. However, amyloid peptides accumulation is currently seen as a key step in the pathogenesis of AD. Inflammation interacts with processing and deposit of β-amyloid. Chronic hyperinsulinemia may exacerbate inflammatory responses and increase markers of oxidative stress. In addition, insulin appears to act as 'neuromodulator', influencing release and reuptake of neurotransmitters, and improving learning and memory. Thus, experimental and clinical evidence show that insulin action influences cerebral functions. In this paper, we reviewed several mechanisms by which insulin may affect pathophysiology in AD.

Brain insulin signaling: a key component of cognitive processes and a potential basis for cognitive impairment in type 2 diabetes

Understanding of the role of insulin in the brain has gradually expanded, from initial conceptions of the brain as insulin-insensitive through identification of a role in regulation of feeding, to recent demonstration of insulin as a key component of hippocampal memory processes. Conversely, systemic insulin resistance such as that seen in type 2 diabetes is associated with a range of cognitive and neural deficits. Here we review the evidence for insulin as a cognitive and neural modulator, including potential effector mechanisms, and examine the impact that type 2 diabetes has on these mechanisms in order to identify likely bases for the cognitive impairments seen in type 2 diabetic patients.

Type 2 diabetes and late-onset Alzheimer's disease

BACKGROUND/AIMS

To confirm in a cohort recruited in 1999-2001 our finding in a cohort recruited in 1992-1994 relating type 2 diabetes (T2D) to late-onset Alzheimer's disease (LOAD).

METHODS

Participants were 1,488 persons aged 65 years and older without dementia at baseline from New York City. T2D was ascertained by self-report. Dementia and LOAD were ascertained by standard research procedures. Proportional hazard regression was used for analyses relating T2D and LOAD.

RESULTS

The prevalence of T2D was 17%. There were 161 cases of dementia and 149 cases of LOAD. T2D was related to dementia (hazard ratio = 1.7; 95% confidence interval = 1.4-2.9) and LOAD (1.6; 1.0-2.6) after adjustment for age, sex, education, ethnic group and apolipoprotein E ε4. This association was weaker when only AD - excluding cases of mixed dementia - was considered (hazard ratio = 1.3; 95% confidence interval = 0.8-2.2).

CONCLUSION

T2D is associated with LOAD. Cerebrovascular disease may be an important mediator.

Expression of cholinergic, insulin, vitamin D receptors and GLUT 3 in the brainstem of streptozotocin induced diabetic rats: effect of treatment with vitamin D₃

Complications arising from diabetes mellitus include cognitive deficits, neurophysiological and structural changes in the brain. The current study investigated the expression of cholinergic, insulin, Vitamin D receptor and GLUT 3 in the brainstem of streptozotocin-induced diabetic rats. Radioreceptor binding assays and gene expression were done in the brainstem of male Wistar rats. Our results showed that B(max) of total muscarinic, muscarinic M3 receptors was increased and muscarinic M1 receptor was decreased in diabetic rats compared to control. A significant increase in gene expression of muscarinic M3, α7 nicotinic acetylcholine, insulin, Vitamin D₃ receptors, acetylcholine esterase, choline acetyl transferase and GLUT 3 were observed in the brainstem of diabetic rats. Immunohistochemistry studies of muscarinic M1, M3 and α7 nicotinic acetylcholine receptors confirmed the gene expression at protein level. Vitamin D₃ and insulin treatment reversed diabetes-induced alterations to near control. This study provides an evidence that diabetes can alter the expression of cholinergic, insulin, Vitamin D receptors and GLUT 3 in brainstem. We found that Vitamin D₃ treatment could modulate the Vitamin D receptors and plays a pivotal role in maintaining the glucose transport and expressional level of cholinergic receptors in the brainstem of diabetic rats. Thus, our results suggest a therapeutic role of Vitamin D₃ in managing neurological disorders associated with diabetes.

Diet intervention and cerebrospinal fluid biomarkers in amnestic mild cognitive impairment

OBJECTIVE

To compare the effects of a 4-week high-saturated fat/high-glycemic index (HIGH) diet with a low-saturated fat/low-glycemic index (LOW) diet on insulin and lipid metabolism, cerebrospinal fluid (CSF) markers of Alzheimer disease, and cognition for healthy adults and adults with amnestic mild cognitive impairment (aMCI).

DESIGN

Randomized controlled trial.

SETTING

Veterans Affairs Medical Center clinical research unit.

PARTICIPANTS

Forty-nine older adults (20 healthy adults with a mean [SD] age of 69.3 [7.4] years and 29 adults with aMCI with a mean [SD] age of 67.6 [6.8] years).

INTERVENTION

Participants received the HIGH diet (fat, 45% [saturated fat, > 25%]; carbohydrates, 35%-40% [glycemic index, > 70]; and protein, 15%-20%) or the LOW diet (fat, 25%; [saturated fat, < 7%]; carbohydrates, 55%-60% [glycemic index, < 55]; and protein, 15%-20%) for 4 weeks. Cognitive tests, an oral glucose tolerance test, and lumbar puncture were conducted at baseline and during the fourth week of the diet.

MAIN OUTCOME MEASURES

The CSF concentrations of β-amyloid (Aβ42 and Aβ40), tau protein, insulin, F2-isoprostanes, and apolipoprotein E, plasma lipids and insulin, and measures of cognition.

RESULTS

For the aMCI group, the LOW diet increased CSF Aβ42 concentrations, contrary to the pathologic pattern of lowered CSF Aβ42 typically observed in Alzheimer disease. The LOW diet had the opposite effect for healthy adults, ie, decreasing CSF Aβ42, whereas the HIGH diet increased CSF Aβ42. The CSF apolipoprotein E concentration was increased by the LOW diet and decreased by the HIGH diet for both groups. For the aMCI group, the CSF insulin concentration increased with the LOW diet, but the HIGH diet lowered the CSF insulin concentration for healthy adults. The HIGH diet increased and the LOW diet decreased plasma lipids, insulin, and CSF F2-isoprostane concentrations. Delayed visual memory improved for both groups after completion of 4 weeks of the LOW diet.

CONCLUSION

Our results suggest that diet may be a powerful environmental factor that modulates Alzheimer disease risk through its effects on central nervous system concentrations of Aβ42, lipoproteins, oxidative stress, and insulin.

Unconditioned and conditioned effects of intravenous insulin and glucose on heart rate variability in healthy men

We examined whether an injection of intravenous insulin and intravenous glucose would affect frequency-domain measures of heart rate variability (HRV), i.e., the high-frequency (HF-) band and the ratio of the low frequency (LF-) to the HF-band in healthy humans. Using a classical conditioning protocol, we also assessed whether the measures of HRV are subject to classical conditioning. Thirty healthy men were divided into three groups, given a conditioned stimulus (CS) and an intravenous injection of either insulin (0.05IU/kg) in Group 1, glucose (15%, 0.5g/kg) in Group 2, or placebo (physiological saline [0.9%]) in Group 3 during the 4-day acquisition phase. All subjects were given an olfactory CS (rosewood-peppermint smell) and placebo injection on day 5 (test). Due to their high inter-individual variability, HF and LF/HF-ratio were analysed by intragroup comparisons, using a pre-injection baseline interval (min -15 to -5), and three functional post-injection intervals: a) the interval to the maximum insulin level, i. e. insulin peak (min 0-5) in Groups 1 and 2, b) the interval to the maximum of insulin-induced hypoglycaemia (min 20-25) in Group 1, and c) the end of the session (min 70-75). On days 1 to 4, we found significant increases of the HF-band from baseline to interval min 0-5 in Group 1, and an even more pronounced increase in the glucose-treated Group 2. At the test (Day 5), both experimental groups responded with an HF-increase in the interval of the former insulin peak, and also at the other measurement intervals, reflecting some general increase of vagal activity remaining as a conditioned response. On days 1 to 4, the HF-band was positively correlated with the change of peripheral insulin levels in Group 1, reaching statistical significance on days 3 and 4. This pattern only emerged in tendency on Day 4 in Group 2. In conclusion, insulin triggers an increase in parasympathetic tone at maximum hyperinsulinaemia, and our data support the notion that this response pattern can become classically conditioned.

Insulin detemir is not transported across the blood-brain barrier

Insulin detemir has a different profile of action on the central nervous system (CNS) than human insulin. It has been hypothesized that this is caused by an altered ability of insulin detemir to cross the blood-brain barrier (BBB). Here, we measured the permeability of the BBB to insulin detemir. We labeled insulin detemir with radioactive iodine (I-Det) and examined its ability to cross the BBB of the mouse. Permeation was assessed after intravenous injection and by brain perfusion in the presence or absence of excess insulin detemir. The ability of insulin detemir to inhibit human insulin transport across the BBB was also assessed. I-Det did not cross the BBB either after intravenous injection or when studied by brain perfusion, a method which removes or reduces the influence of circulating proteins. Unlabeled detemir was about 10 times less potent than human insulin at inhibiting the transport of radioactive human insulin across the BBB. The altered CNS profile of insulin detemir may be caused by its poor access to CNS receptors and by a block of human insulin from crossing the BBB.

Diabetes and insulin in regulation of brain cholesterol metabolism

The brain is the most cholesterol-rich organ in the body, most of which comes from in situ synthesis. Here we demonstrate that in insulin-deficient diabetic mice, there is a reduction in expression of the major transcriptional regulator of cholesterol metabolism, SREBP-2, and its downstream genes in the hypothalamus and other areas of the brain, leading to a reduction in brain cholesterol synthesis and synaptosomal cholesterol content. These changes are due, at least in part, to direct effects of insulin to regulate these genes in neurons and glial cells and can be corrected by intracerebroventricular injections of insulin. Knockdown of SREBP-2 in cultured neurons causes a decrease in markers of synapse formation and reduction of SREBP-2 in the hypothalamus of mice using shRNA results in increased feeding and weight gain. Thus, insulin and diabetes can alter brain cholesterol metabolism, and this may play an important role in the neurologic and metabolic dysfunction observed in diabetes and other disease states.

GLP-1 receptor stimulation reduces amyloid-beta peptide accumulation and cytotoxicity in cellular and animal models of Alzheimer's disease

Type 2 (T2) diabetes mellitus (DM) has been associated with an increased incidence of neurodegenerative disorders, including Alzheimer's disease (AD). Several pathological features are shared between diabetes and AD, including dysfunctional insulin signaling and a dysregulation of glucose metabolism. It has therefore been suggested that not only may the two conditions share specific molecular mechanisms but also that agents with proven efficacy in one may be useful against the other. Hence, the present study characterized the effects of a clinically approved long-acting analogue, exendin-4 (Ex-4), of the endogenous insulin releasing incretin, glucagon-like peptide-1 (GLP-1), on stress-induced toxicity in neuronal cultures and on amyloid-beta protein (Abeta) and tau levels in triple transgenic AD (3xTg-AD) mice with and without streptozocin (STZ)-induced diabetes. Ex-4 ameliorated the toxicity of Abeta and oxidative challenge in primary neuronal cultures and human SH-SY5Y cells in a concentration-dependent manner. When 11 to 12.5 month old female 3xTg AD mice were challenged with STZ or saline, and thereafter treated with a continuous subcutaneous infusion of Ex-4 or vehicle, Ex-4 ameliorated the diabetic effects of STZ in 3xTg-AD mice, elevating plasma insulin and lowering both plasma glucose and hemoglobin A1c (HbA1c) levels. Furthermore, brain levels of Abeta protein precursor and Abeta, which were elevated in STZ 3xTg-AD mice, were significantly reduced in Ex-4 treated mice. Brain tau levels were unaffected following STZ challenge, but showed a trend toward elevation that was absent following Ex-4 treatment. Together, these results suggest a potential value of Ex-4 in AD, particularly when associated with T2DM or glucose intolerance.

Transport of prion protein across the blood-brain barrier

The cellular form of the prion protein (PrP(c)) is necessary for the development of prion diseases and is a highly conserved protein that may play a role in neuroprotection. PrP(c) is found in both blood and cerebrospinal fluid and is likely produced by both peripheral tissues and the central nervous system (CNS). Exchange of PrP(c) between the brain and peripheral tissues could have important pathophysiologic and therapeutic implications, but it is unknown whether PrP(c) can cross the blood-brain barrier (BBB). Here, we found that radioactively labeled PrP(c) crossed the BBB in both the brain-to-blood and blood-to-brain directions. PrP(c) was enzymatically stable in blood and in brain, was cleared by liver and kidney, and was sequestered by spleen and the cervical lymph nodes. Circulating PrP(c) entered all regions of the CNS, but uptake by the lumbar and cervical spinal cord, hypothalamus, thalamus, and striatum was particularly high. These results show that PrP(c) has bidirectional, saturable transport across the BBB and selectively targets some CNS regions. Such transport may play a role in PrP(c) function and prion replication.

Brain insulin infusion does not augment the counterregulatory response to hypoglycemia or glucoprivation

Although high dosages of insulin can cause hypoglycemia, several studies suggest that increased insulin action in the head may paradoxically protect against severe hypoglycemia by augmenting the sympathoadrenal response to hypoglycemia. We hypothesized that a direct infusion of insulin into the third ventricle and/or the mediobasal hypothalamus (MBH) would amplify the sympathoadrenal response to hypoglycemia. Nine-week-old male rats had insulin (15 mU) or artificial cerebrospinal fluid (aCSF, control) infused bilaterally into the MBH or directly into the third ventricle. During the final 2 hours of the brain insulin or aCSF infusions, the counterregulatory response to either a hyperinsulinemic hypoglycemic (approximately 50 mg/dL) clamp or a 600-mg/kg intravenous bolus of 2-deoxyglucose (2DG) was measured. 2-Deoxyglucose was used to induce a glucoprivic response without peripheral insulin infusion. In response to insulin-induced hypoglycemia, epinephrine rose more than 60-fold, norepinephrine rose more than 4-fold, glucagon rose 8-fold, and corticosterone rose almost 2-fold; but these increments were not different in aCSF vs insulin treatment groups with either intracerebroventricular or bilateral MBH insulin protocols. Intracerebroventricular insulin infusion stimulated insulin signaling as noted by a 5-fold increase in AKT phosphorylation. In the absence of systemic insulin infusion, 2DG-induced glucopenia resulted in an equal counterregulatory response with brain aCSF and insulin infusions. Under the conditions studied, although insulin infusion acted to stimulate hypothalamic insulin signaling, neither intrahypothalamic nor intracerebroventricular insulin infusion augmented the counterregulatory response to hypoglycemia or to 2DG-induced glucoprivation. Therefore, it is proposed that the previously noted acute actions of insulin to augment the sympathoadrenal response to hypoglycemia are likely mediated via mechanisms exterior to the central nervous system.

Endocytosis at the blood–brain barrier: From basic understanding to drug delivery strategies

The blood-brain barrier (BBB) protects the central nervous system (CNS) from potentially harmful xenobiotics and endogenous molecules. Anatomically, it comprises the brain microvasculature whose functionality is nevertheless influenced by associated astrocyte, pericyte and neuronal cells. The highly restrictive paracellular pathway within brain microvasculature restricts significant CNS penetration to only those drugs whose physicochemical properties afford ready penetration into hydrophobic cell membranes or are capable of exploiting endogenous active transport processes such as solute carriers or endocytosis pathways. Endocytosis at the BBB is an essential pathway by which the brain obtains its nutrients and affords communication with the periphery. The development of strategies to exploit these endocytic pathways for the purposes of drug delivery to the CNS is still an immature field although some impressive results have been documented with the targeting of particular receptors. This current article initially provides an overview of general endocytosis processes and pathways showing evidence of their functional existence within the BBB. Subsequent sections provide, in an entity-specific manner, comprehensive reviews on BBB transport investigations of endocytosis involving: transferrin and the targeting of the transferrin receptor; hormones; cytokines; cell penetrating peptides; microorganisms and toxins, and nanoparticles aimed at more effectively delivering drugs to the CNS.

Blood-brain barrier and feeding: regulatory roles of saturable transport systems for ingestive peptides

The two main ways for peptides in the peripheral body to enter the brain are by either saturable transport or passive diffusion across the blood-brain barrier (BBB). Saturable transport systems have the advantage of being responsive to physiological and pathological stimuli. Since saturable systems can regulate peptide entry into the brain, they have the potential to play controlling roles in feeding behavior. For therapeutic applications, however, saturable systems have the disadvantage of functioning as a threshold to limit access of large amounts of peptides into the brain. This pharmacological problem presumably would not be encountered for peptides crossing the BBB by passive diffusion, a process dependent on physicochemical properties. Thus, the gatekeeper function of the BBB can be expanded to a primary governing role, especially for entry of ingestive peptides subject to their respective saturable transport systems.

The blood-brain barrier: connecting the gut and the brain

The BBB prevents the unrestricted exchange of substances between the central nervous system (CNS) and the blood. The blood-brain barrier (BBB) also conveys information between the CNS and the gastrointestinal (GI) tract through several mechanisms. Here, we review three of those mechanisms. First, the BBB selectively transports some peptides and regulatory proteins in the blood-to-brain or the brain-to-blood direction. The ability of GI hormones to affect functions of the BBB, as illustrated by the ability of insulin to alter the BBB transport of amino acids and drugs, represents a second mechanism. A third mechanism is the ability of GI hormones to affect the secretion by the BBB of substances that themselves affect feeding and appetite, such as nitric oxide and cytokines. By these and other mechanisms, the BBB regulates communications between the CNS and GI tract.

Nitric oxide isoenzymes regulate lipopolysaccharide-enhanced insulin transport across the blood-brain barrier

Insulin transported across the blood-brain barrier (BBB) has many effects within the central nervous system. Insulin transport is not static but altered by obesity and inflammation. Lipopolysaccharide (LPS), derived from the cell walls of Gram-negative bacteria, enhances insulin transport across the BBB but also releases nitric oxide (NO), which opposes LPS-enhanced insulin transport. Here we determined the role of NO synthase (NOS) in mediating the effects of LPS on insulin BBB transport. The activity of all three NOS isoenzymes was stimulated in vivo by LPS. Endothelial NOS and inducible NOS together mediated the LPS-enhanced transport of insulin, whereas neuronal NOS (nNOS) opposed LPS-enhanced insulin transport. This dual pattern of NOS action was found in most brain regions with the exception of the striatum, which did not respond to LPS, and the parietal cortex, hippocampus, and pons medulla, which did not respond to nNOS inhibition. In vitro studies of a brain endothelial cell (BEC) monolayer BBB model showed that LPS did not directly affect insulin transport, whereas NO inhibited insulin transport. This suggests that the stimulatory effect of LPS and NOS on insulin transport is mediated through cells of the neurovascular unit other than BECs. Protein and mRNA levels of the isoenzymes indicated that the effects of LPS are mainly posttranslational. In conclusion, LPS affects insulin transport across the BBB by modulating NOS isoenzyme activity. NO released by endothelial NOS and inducible NOS acts indirectly to stimulate insulin transport, whereas NO released by nNOS acts directly on BECs to inhibit insulin transport.

Insulin rescues amyloid beta-induced impairment of hippocampal long-term potentiation

Cerebral accumulation of amyloid beta-protein (Abeta) is generally believed to play a critical role in the pathogenesis of Alzheimer's disease (AD). Recent evidence suggests that Abeta-induced synaptic dysfunction is one of earliest pathogenic events observed in AD. Here we report that synthetic Abeta(1-42) strongly inhibited the induction of long-term potentiation (LTP) in the CA1 region of rat hippocampal slices. To ascertain which Abeta(1-42) sequences contribute to the impairment of LTP, we compared actions of several Abeta fragments and found that the sequence within 25-35 region of Abeta mainly contributes to the expression of LTP impairment. Importantly, we show that insulin and insulin-like growth factor-1 significantly inhibit Abeta oligomer formation, particularly dimers and trimers, and ameliorate the synthetic Abeta-induced suppression of LTP. Furthermore, dithiothreitol was found to be capable of significantly preventing the inhibitory effect of insulin on Abeta oligomer formation. In contrast, hemoglobin promotes Abeta oligomer formation and enhances Abeta-mediated inhibition of LTP induction. These results suggest that insulin may have utility in treating the earliest stages of Abeta-induced synaptic dysfunction in AD patients.

Nanomedicine in the diagnosis and therapy of neurodegenerative disorders

Neurodegenerative and infectious disorders including Alzheimer's and Parkinson's diseases, amyotrophic lateral sclerosis, and stroke are rapidly increasing as population's age. Alzheimer's disease alone currently affects 4.5 million Americans, and more than $100 billion is spent per year on medical and institutional care for affected people. Such numbers will double in the ensuing decades. Currently disease diagnosis for all disorders is made, in large measure, on clinical grounds as laboratory and neuroimaging tests confirm what is seen by more routine examination. Achieving early diagnosis would enable improved disease outcomes. Drugs, vaccines or regenerative proteins present "real" possibilities for positively affecting disease outcomes, but are limited in that their entry into the brain is commonly restricted across the blood-brain barrier. This review highlights how these obstacles can be overcome by polymer science and nanotechnology. Such approaches may improve diagnostic and therapeutic outcomes. New developments in polymer science coupled with cell-based delivery strategies support the notion that diseases that now have limited therapeutic options can show improved outcomes by advances in nanomedicine.

Imaging on the binding of FITC-insulin with insulin receptors in cortical neurons of rat

It has been identified that insulin was present in the central nervous system (CNS) with some types of action there, and it exerted important actions within the brain and functions as neuropeptide. Insulin should bind with insulin receptors (IR) to perform its functions, so it is important to study the binding of insulin with IR in neurons. A direct imaging method was developed by fluorescence microscopy. HepG2 cells were firstly selected to be the model for methodological study, the results showed that insulin could bind with IR at the membrane of the studied cells after incubated 1 minute with the cells. In order to show the binding of insulin with IR in neurons, the cultured cortical neurons of rat were selected as representative. It was found that insulin could bind with IR at the membrane of the neurons, and IR distribute not only on the somas, but also on the neurites. Using fluorescent imaging to directly detect the binding of insulin with IR in neurons could be promising for further study of insulin functions in brain. It is rarely reported the direct imaging on the binding of insulin with IR of neurons by microcopy system in live cells.

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.

Diabetic retinopathy: seeing beyond glucose-induced microvascular disease

Diabetic retinopathy remains a frightening prospect to patients and frustrates physicians. Destruction of damaged retina by photocoagulation remains the primary treatment nearly 50 years after its introduction. The diabetes pandemic requires new approaches to understand the pathophysiology and improve the detection, prevention, and treatment of retinopathy. This perspective considers how the unique anatomy and physiology of the retina may predispose it to the metabolic stresses of diabetes. The roles of neural retinal alterations and impaired retinal insulin action in the pathogenesis of early retinopathy and the mechanisms of vision loss are emphasized. Potential means to overcome limitations of current animal models and diagnostic testing are also presented with the goal of accelerating therapies to manage retinopathy in the face of ongoing diabetes.

Delivery of 125I-cobrotoxin after intranasal administration to the brain: a microdialysis study in freely moving rats

In order to determine the contribution of intranasal (i.n.) administration to the uptake of large molecular weight (MW) substances into central nervous system (CNS), concentration in brain of the centrally acting polypeptide cobrotoxin (NT-I) versus time profiles were studied using dual-probe microdialysis in awake free-moving rats. NT-I, radiolabeled with sodium (125)I-Iodide ((125)I-NT-I), was administered at the dose of 105 microg/kg intravenously and intranasally in the same set of rat (n=15). The (125)I-NT-Inasal preparations were formulated with borneol/menthol eutectic mixture (+BMEM) as an absorption enhancer and without (-BMEM). After application, the dialysates sampled simultaneously from olfactory bulb and cerebellar nuclei were measured in a gamma-counter for radioactivity. The real concentrations of NT-I were recalculated by in vivo recoveries of microdialysis probes. The results showed that the area under the curve (AUC) value in cerebellar nuclei (2283.51+/-34.54 min ng/ml) following i.n. administration (+BMEM) was significantly larger than those (AUC(olfactory)=1141.92+/-26.42 min ng/ml; AUC(cerebellar)=1364.62+/-19.35 min ng/ml) after intravenous (i.v.) bolus, respectively. A prolonged time values to peak concentrations after i.n. application (+BMEM) were observed compared with those following i.v. administration. Also, following i.n. application (+BMEM) the measured time value to peak concentration in cerebellar nuclei (85 min) was statistically longer than that in olfactory bulb (75 min), which could be plausibly an indication for NT-I delivery into brain via nose-brain pathway in the presence of absorption enhancer. i.n. administration (-BMEM) had little or no ability of NT-I delivering into brain. In conclusion, i.n. administration (+BMEM) significantly enhanced brain transport of NT-I with uneven distribution in discrete regions of brain compared with i.v. administration. Additionally, multi-probe microdialysis technique should be considerably valuable in brain delivery studies.

Blood-brain barrier and energy balance

The blood-brain barrier (BBB) plays a critical role in the transduction of signals between the central nervous system and peripheral tissues. It does so through several mechanisms, including the direct transport of peptides and regulatory proteins such as insulin and leptin. Another mechanism that may be important is the secretion by brain endothelial cells of substances that affect feeding, such as proinflammatory cytokines and NO. We have recently shown that the BBB is capable of receiving an input from one side and secreting a substance into the other. Additionally, BBB secretions can be modulated by substances that affect feeding, such as adiponectin and lipopolysaccharide.

Pancreatic signals controlling food intake; insulin, glucagon and amylin

The control of food intake and body weight by the brain relies upon the detection and integration of signals reflecting energy stores and fluxes, and their interaction with many different inputs related to food palatability and gastrointestinal handling as well as social, emotional, circadian, habitual and other situational factors. This review focuses upon the role of hormones secreted by the endocrine pancreas: hormones, which individually and collectively influence food intake, with an emphasis upon insulin, glucagon and amylin. Insulin and amylin are co-secreted by B-cells and provide a signal that reflects both circulating energy in the form of glucose and stored energy in the form of visceral adipose tissue. Insulin acts directly at the liver to suppress the synthesis and secretion of glucose, and some plasma insulin is transported into the brain and especially the mediobasal hypothalamus where it elicits a net catabolic response, particularly reduced food intake and loss of body weight. Amylin reduces meal size by stimulating neurons in the hindbrain, and there is evidence that amylin additionally functions as an adiposity signal controlling body weight as well as meal size. Glucagon is secreted from A-cells and increases glucose secretion from the liver. Glucagon acts in the liver to reduce meal size, the signal being relayed to the brain via the vagus nerves. To summarize, hormones of the endocrine pancreas are collectively at the crossroads of many aspects of energy homeostasis. Glucagon and amylin act in the short term to reduce meal size, and insulin sensitizes the brain to short-term meal-generated satiety signals; and insulin and perhaps amylin as well act over longer intervals to modulate the amount of fat maintained and defended by the brain. Hormones of the endocrine pancreas interact with receptors at many points along the gut-brain axis, from the liver to the sensory vagus nerve to the hindbrain to the hypothalamus; and their signals are conveyed both neurally and humorally. Finally, their actions include gastrointestinal and metabolic as well as behavioural effects.

Insulin degrading enzyme is localized predominantly at the cell surface of polarized and unpolarized human cerebrovascular endothelial cell cultures

Insulin degrading enzyme (IDE) is expressed in the brain and may play an important role there in the degradation of the amyloid beta peptide (Abeta). Our results show that cultured human cerebrovascular endothelial cells (HCECs), a primary component of the blood-brain barrier, express IDE and may respond to exposure to low levels of Abeta by upregulating its expression. When radiolabeled Abeta is introduced to the medium of cultured HCECs, it is rapidly degraded to smaller fragments. We believe that this degradation is largely the result of the action of IDE, as it can be substantially blocked by the presence of insulin in the medium, a competitive substrate of IDE. No inhibition is seen when an inhibitor of neprilysin, another protease that may degrade Abeta, is present in the medium. Our evidence suggests that the action of IDE occurs outside the cell, as inhibitors of internalization fail to affect the rate of the observed degradation. Further, our evidence suggests that degradation by IDE occurs on the plasma membrane, as much of the IDE present in HCECs was biotin-labeled by a plasma membrane impermeable reagent. This activity seems to be polarity dependent, as measurement of Abeta degradation by each surface of differentiated HCECs shows greater degradation on the basolateral (brain-facing) surface. Thus, IDE could be an important therapeutic target to decrease the amount of Abeta in the cerebrovasculature.

Insulin resistance, inflammation, and cognition in Alzheimer's Disease: Lessons for multiple sclerosis

Insulin resistance (reduced ability of insulin to stimulate glucose utilization) is common in North American and Europe, where as many as one third of all older adults suffer from prodromal or clinical type 2 diabetes mellitus. It has long been known that insulin-resistant conditions adversely affect general health status. A growing body of findings suggests that insulin contributes to normal brain functioning and that peripheral insulin abnormalities increase the risk for memory loss and neurodegenerative disorders such as Alzheimer's disease. Potential mechanisms for these effects include insulin's role in cerebral glucose metabolism, peptide regulation, modulation of neurotransmitter levels, and modulation of many aspects of the inflammatory network. An intriguing question is whether insulin abnormalities also influence the pathophysiology of multiple sclerosis (MS), an autoimmune disorder characterized by elevated inflammatory biomarkers, central nervous system white matter lesions, axonal degeneration, and cognitive impairment. MS increases the risk for type 1 diabetes mellitus. Furthermore, the lack of association between MS and type 2 diabetes may suggest that insulin resistance affects patients with MS and the general population at the same alarming rate. Therefore, insulin resistance may exacerbate phenomena that are common to MS and insulin-resistant conditions, such as cognitive impairments and elevated inflammatory responses. Interestingly, the thiazolidinediones, which are used to treat patients with type 2 diabetes, have been proposed as potential therapeutic agents for both Alzheimer's disease and MS. The agents improve insulin sensitivity, reduce hyperinsulinemia, and exert anti-inflammatory actions. Ongoing studies will determine whether thiazolidinediones improve cognitive functioning for patients with type 2 diabetes or Alzheimer's disease. Future studies are needed to examine the effects of thiazolidinediones on patients with MS.

Reciprocal interactions of insulin and insulin-like growth factor I in receptor-mediated transport across the blood-brain barrier

Although the blood-brain barrier limits free passage of peptides and proteins from the peripheral circulation to the central nervous system, specific transport systems for insulin and IGF-I have been identified. To further determine whether insulin and IGF-I share the same transport system, and if not, whether the two transport systems interact with each other, we performed multiple-time regression analysis in mice after iv injection and in situ brain perfusion of these peptides. Insulin and IGF-I caused reciprocal inhibition of each other's transport, although the effect of insulin was detected only by the in situ brain perfusion system. The interaction took place mainly at the step of cell surface binding as seen in cultured rat brain endothelium 4 brain microvessel endothelial cells. Further studies in 3T3 cells stably overexpressing the insulin receptor showed that the sharing of the transport systems was only partial. We conclude that insulin and IGF-I are mainly transported by their own transport systems, but a small amount can enter the brain by their "noncognate" transporters. The redundancy of their transport systems illustrates the regulatory function of the blood-brain barrier and reflects the importance of blood-borne insulin and IGF-I in the central nervous system.

[The hypothalamic control of feeding and thermogenesis: implications on the development of obesity]

The worldwide increase in the prevalence of obesity is becoming one of the most important clinical-epidemiological phenomena of the present days. Environmental factors such as changes in life-style and feeding behavior associated with poorly characterized genetic determinants are though to play the most important roles in the pathogenesis of this disease. During the last ten years, since the discovery of leptin, great advances were obtained in the characterization of the hypothalamic mechanisms involved in the control of food intake and thermogenesis. Such advances are unveiling a complex and integrated system and are opening a wide perspective for the finding of novel therapeutic targets for the treatment of this harming condition. This review will present some of the most recent findings in this field. It will be focused on the actions of leptin and insulin in the hypothalamus and will explore the hypothesis that hypothalamic resistance to the action of these hormones may play a role in the development of obesity and may act as a molecular link between obesity, type 2 diabetes mellitus and other clinical conditions on which insulin resistance plays an important pathogenetic role.

Alterations in blood glucose levels under hyperinsulinemia affect accumbens dopamine

Dopaminergic systems have been implicated in diabetes and obesity. Notwithstanding, the most basic relationship between dopamine and plasma insulin as well as glucose levels yet remains unknown. The present experiments were designed to investigate the effects of acute hyperinsulinemia on basal dopamine levels in the nucleus accumbens of the rat under chloral hydrate anesthesia using acute microdialysis in combination with the hyperinsulinemic-glycemic clamping procedure. In Experiment 1, each rat was infused with one of the three concentrations of insulin (2.4, 4.8, or 9.6 mU/kg per min) while plasma glucose levels were maintained at euglycemia (approximately 5.5 mmol/L). Dopamine, dihydroxyphenylacetic acid and homovanillic acid were not significantly different from baseline during either the clamp or post-clamp periods for all insulin concentrations. In Experiment 2, rats were infused with the highest concentration of insulin (9.6 mU/kg per min) and plasma glucose levels were maintained at either hypoglycemia (approximately 3 mmol/L) or hyperglycemia (approximately 14 mmol/L). Dopamine was elevated at 100 min (+113% above basal levels) and 120 min (+117%) in the hypoglycemic condition and at 120 min (+121%) in the hyperglycemic condition. In the hyperglycemic post-clamp period, homovanillic acid was decreased below basal levels (approximately -32%). These results together suggest that short-term blood glucose deviations coupled with acute hyperinsulinemia affect the mesoaccumbens dopamine system.

Differential BBB interactions of three ingestive peptides: obestatin, ghrelin, and adiponectin

Endogenous compounds, including ingestive peptides, can interact with the blood-brain barrier (BBB) in different ways. Here we used in vivo and in vitro techniques to examine the BBB permeation of the newly described satiety peptide obestatin. The fate of obestatin in blood and at the BBB was contrasted with that of adiponectin. By the sensitive multiple time-regression method, obestatin appeared to have an extremely fast influx rate to the brain whereas adiponectin did not cross the BBB. HPLC analysis, however, showed the obestatin result to be spurious, reflecting rapid degradation. Absence of BBB permeation by obestatin and adiponectin was in contrast to the saturable transport of human ghrelin reported previously. As a positive control, ghrelin showed saturable binding and endocytosis in RBE4 cerebral microvessel endothelial cells. By comparison, obestatin lacked specific binding and endocytosis, and the small amount internalized showed rapid intracellular degradation before the radioactivity was released by exocytosis. The differential interactions of obestatin, adiponectin, and ghrelin with the BBB illustrate their distinctive physiological interactions with the CNS.

Efficacy of rosiglitazone in a genetically defined population with mild-to-moderate Alzheimer's disease

Mild-to-moderate AD patients were randomized to placebo or rosiglitazone (RSG) 2, 4 or 8 mg. Primary end points at Week 24 were mean change from baseline in AD Assessment Scale-Cognitive (ADAS-Cog) and Clinician's Interview-Based Impression of Change Plus Caregiver Input global scores in the intention-to-treat population (N=511), and results were also stratified by apolipoprotein E (APOE) genotype (n=323). No statistically significant differences on primary end points were detected between placebo and any RSG dose. There was a significant interaction between APOE epsilon4 allele status and ADAS-Cog (P=0.014). Exploratory analyses demonstrated significant improvement in ADAS-Cog in APOE epsilon4-negative patients on 8 mg RSG (P=0.024; not corrected for multiplicity). APOE epsilon4-positive patients did not show improvement and showed a decline at the lowest RSG dose (P=0.012; not corrected for multiplicity). Exploratory analyses suggested that APOE epsilon4 non-carriers exhibited cognitive and functional improvement in response to RSG, whereas APOE epsilon4 allele carriers showed no improvement and some decline was noted. These preliminary findings require confirmation in appropriate clinical studies.

Aggregation of vascular risk factors and risk of incident Alzheimer disease

BACKGROUND

The prevalence of Alzheimer disease (AD) is increasing in the elderly, and vascular risk factors may increase its risk.

OBJECTIVE

To explore the association of the aggregation of vascular risk factors with AD.

METHODS

The authors followed 1,138 individuals without dementia at baseline (mean age 76.2) for a mean of 5.5 years. The presence of vascular risk factors was related to incident possible and probable AD.

RESULTS

Four risk factors (diabetes, hypertension, heart disease, and current smoking) were associated with a higher risk of AD (p < 0.10) when analyzed individually. The risk of AD increased with the number of risk factors (diabetes + hypertension + heart disease + current smoking). The adjusted hazards ratio of probable AD for the presence of three or more risk factors was 3.4 (95% CI: 1.8, 6.3; p for trend < 0.0001) compared with no risk factors. Diabetes and current smoking were the strongest risk factors in isolation or in clusters, but hypertension and heart disease were also related to a higher risk of AD when clustered with diabetes, smoking, or each other.

CONCLUSIONS

The risk of Alzheimer disease (AD) increased with the number of vascular risk factors. Diabetes and current smoking were the strongest risk factors, but clusters including hypertension and heart disease also increased the risk of AD. These associations are unlikely to be explained by misclassification of the outcome, given strong associations when only probable AD is considered.

Provocation of kainic acid receptor mRNA changes in the rat paraventricular nucleus by insulin-induced hypoglycaemia

Hypoglycaemia induced by insulin injection is a powerful stimulus to the hypothalamic-pituitary-adrenal (HPA) axis and drives the secretion of corticotropin-releasing hormone and vasopressin from the neurones in the paraventricular nucleus (PVN), as well as the downstream hormones, adrenocorticotropic hormone and corticosterone. In some brain regions, hypoglycaemia also provokes increases in extracellular fluid concentrations of glutamate. Regulation of glutamatergic mechanisms could be involved in the control of the HPA axis during hypoglycaemic stress and one potential site of regulation might be at the receptors for glutamate, which are expressed in the PVN. Insulin (2.0 IU/kg, i.p.) or saline was administered to adult male Sprague-Dawley rats and the animals were sacrificed 30 min, 180 min and 24 h after injection. The amount of several kainic acid-preferring glutamate receptor mRNAs (i.e. KA2, GluR5 and GluR6) were assessed in the PVN by in situ hybridisation histochemistry. Injection of insulin induced a rapid fall in plasma glucose concentrations, which was mirrored by an increase in plasma corticosterone concentrations. KA2 and GluR5 mRNAs are highly expressed within the rat PVN, and responded to hypoglycaemia with robust increases in expression that endured beyond the period of hypoglycaemia itself. However, GluR6 mRNA is expressed in the areas adjacent to the PVN and hypoglycaemic stress failed to alter expression of this mRNA. These experiments suggest that kainic acid-preferring glutamate receptors are responsive to changes in plasma glucose concentrations and may participate in the activation of the PVN neurones during hypoglycaemic stress.

Modulation of memory by insulin and glucose: neuropsychological observations in Alzheimer's disease

Converging evidence has identified a potential association among Alzheimer's disease, glucose metabolism, insulin activity, and memory. Notably, type 2 diabetes, which is characterized by insulin resistance, may modulate the risk of Alzheimer's disease, and patients with Alzheimer's disease may have a greater risk for glucoregulatory impairments than do healthy older adults. In animal studies, it has been shown that raising blood glucose levels acutely can facilitate memory, in part, by increasing cholinergic activity, which is greatly diminished in patients with Alzheimer's disease. Other studies have confirmed that glucose administration can facilitate memory in healthy humans and in patients with Alzheimer's disease. Interestingly, glucose effects on memory appear to be modulated by insulin sensitivity (efficiency of insulin-mediated glucose disposal). Of course, the acute effects of glucose administration should be distinguished from the effects of chronic hyperglycemia (diabetes), which has been associated with cognitive impairments, at least in older adults. The relationship of insulin and memory has been more difficult to characterize. In animals, systemic insulin administration has been associated with memory deficits, likely due, in part, to hypoglycemia that occurs when exogenous insulin is not supplemented with glucose to maintain euglycemia. In healthy adults and patients with Alzheimer's disease, raising plasma insulin levels while maintaining euglycemia can improve memory; however, raising plasma glucose while suppressing endogenous insulin secretion may not improve memory, suggesting that adequate levels of insulin and glucose are necessary for memory facilitation. Clinical studies have corroborated findings that patients with Alzheimer's disease are more likely than healthy older adults to have reduced insulin sensitivity, and further suggest that apolipoprotein E genotype may modulate the effects of insulin on glucose disposal, memory facilitation, and amyloid precursor protein processing. Collectively, these findings support an association among Alzheimer's disease, impaired glucose metabolism, and reduced insulin sensitivity.

Why study transport of peptides and proteins at the neurovascular interface

The blood-brain barrier (BBB) is an immense neurovascular interface. In neurodegenerative, ischemic, and traumatic disorders of the central nervous system (CNS), the BBB may hinder the delivery of many therapeutic peptides and proteins to the brain and spinal cord. Fortunately, the mistaken dogma that peptides and proteins do not cross the BBB has been corrected during the past two decades by the accumulating evidence that peptides and proteins in the periphery exert potent effects in the CNS. Not only can peptides and proteins serve as carriers for selective therapeutic agents, but they themselves may directly cross the BBB after delivery into the bloodstream. Their passage may be mediated by simple diffusion or specific transport, both of which can be affected by interactions in the blood compartment (outside the BBB) and within the endothelial cells (at the BBB level). Although the majority of current delivery strategies focuses on modification of the molecule to be delivered, understanding the mechanisms of transport will eventually facilitate regulation of the BBB directly. We review the different aspects of interactions and discuss recent advances in the cell biology of peptide/protein transport across the BBB. Better understanding of the nature and regulation of the transport systems at the BBB will provide a new direction to enhance the interactions of peripheral peptides and proteins with the CNS.