Insulin and the insulin receptor in experimental models of learning and memory

Insulin resistance and amyloidogenesis as common molecular foundation for type 2 diabetes and Alzheimer's disease

Characterized as a peripheral metabolic disorder and a degenerative disease of the central nervous system respectively, it is now widely recognized that type 2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) share several common abnormalities including impaired glucose metabolism, increased oxidative stress, insulin resistance and amyloidogenesis. Several recent studies suggest that this is not an epiphenomenon, but rather these two diseases disrupt common molecular pathways and each disease compounds the progression of the other. For instance, in AD the accumulation of the amyloid-beta peptide (Abeta), which characterizes the disease and is thought to participate in the neurodegenerative process, may also induce neuronal insulin resistance. Conversely, disrupting normal glucose metabolism in transgenic animal models of AD that over-express the human amyloid precursor protein (hAPP) promotes amyloid-peptide aggregation and accelerates the disease progression. Studying these processes at a cellular level suggests that insulin resistance and Abeta aggregation may not only be the consequence of excitotoxicity, aberrant Ca(2+) signals, and proinflammatory cytokines such as TNF-alpha, but may also promote these pathological effectors. At the molecular level, insulin resistance and Abeta disrupt common signal transduction cascades including the insulin receptor family/PI3 kinase/Akt/GSK3 pathway. Thus both disease processes contribute to overlapping pathology, thereby compounding disease symptoms and progression.

Blueberry polyphenols attenuate kainic acid-induced decrements in cognition and alter inflammatory gene expression in rat hippocampus

Cognitive impairment in age-related neurodegenerative diseases such as Alzheimer's disease may be partly due to long-term exposure and increased susceptibility to inflammatory insults. In the current study, we investigated whether polyphenols in blueberries can reduce the deleterious effects of inflammation induced by central administration of kainic acid by altering the expression of genes associated with inflammation. To this end, 4-month-old male Fischer-344 (F344) rats were fed a control, 0.015% piroxicam (an NSAID) or 2% blueberry diet for 8 weeks before either Ringer's buffer or kainic acid was bilaterally micro-infused into the hippocampus. Two weeks later, following behavioral evaluation, the rats were killed and total RNA from the hippocampus was extracted and used in real-time quantitative RT-PCR (qRT-PCR) to analyze the expression of inflammation-related genes. Kainic acid had deleterious effects on cognitive behavior as kainic acid-injected rats on the control diet exhibited increased latencies to find a hidden platform in the Morris water maze compared to Ringer's buffer-injected rats and utilized non-spatial strategies during probe trials. The blueberry diet, and to a lesser degree the piroxicam diet, was able to improve cognitive performance. Immunohistochemical analyses of OX-6 expression revealed that kainic acid produced an inflammatory response by increasing the OX-6 positive areas in the hippocampus of kainic acid-injected rats. Kainic acid up-regulated the expression of the inflammatory cytokines IL-1beta and TNF-alpha, the neurotrophic factor IGF-1, and the transcription factor NF-kappaB. Blueberry and piroxicam supplementations were found to attenuate the kainic acid-induced increase in the expression of IL-1beta, TNF-alpha, and NF-kappaB, while only blueberry was able to augment the increased IGF-1 expression. These results indicate that blueberry polyphenols attenuate learning impairments following neurotoxic insult and exert anti-inflammatory actions, perhaps via alteration of gene expression.

A novel, rapid, inhibitory effect of insulin on alpha1beta2gamma2s gamma-aminobutyric acid type A receptors

In the CNS, GABA and insulin seem to contribute to similar processes, including neuronal survival; learning and reward; and energy balance and food intake. It is likely then that insulin and GABA may interact, perhaps at the GABA(A) receptor. One such interaction has already been described [Q. Wan, Z.G. Xiong, H.Y. Man, C.A. Ackerley, J. Braunton, W.Y. Lu, L.E. Becker, J.F. MacDonald, Y.T. Wang, Recruitment of functional GABA(A) receptors to postsynaptic domains by insulin, Nature 388 (1997) 686-690]; in it a micromolar concentration of insulin causes the insertion of GABA(A) receptors into the cell membrane, increasing GABA current. I have discovered another effect of insulin on GABA(A) currents. Using a receptor isoform, alpha(1)beta(2)gamma(2s) that is the likely main neuronal GABA(A) isoform expressed recombinantly in Xenopus oocytes, insulin inhibits GABA-induced current when applied simultaneously with low concentrations of GABA. Insulin will significantly inhibit currents induced by EC(30-50) concentrations of GABA by about 38%. Insulin is potent in this effect; IC(50) of insulin was found to be about 4.3 x 10(-10) M. The insulin effect on the GABA dose responses looked like that of an antagonist similar to bicuculline or beta-carbolines. However, an effect of phosphorylation on the GABA(A) receptor from the insulin receptor signal transduction pathway cannot yet be dismissed.

Seeing the light: insulin receptors and the CNS

Although insulin clearly affects brain function, the role of insulin receptor (IR) signaling in the establishment and function of circuits in vivo remains largely unknown. In this issue of Neuron, Chiu et al. show a role for IRs in regulating synapse density and dendritic plasticity required for visual responses in Xenopus.

Rosiglitazone attenuates the cognitive deficits induced by high fat diet feeding in rats

The present study was designed to test the hypothesis that insulin resistance plays a role in high fat diet feeding induced cognitive deficits. Rats consuming the high fat diet exhibited characteristic features of insulin resistance viz. mild hyperglycemia, hypertriglyceridemia, hypercholesterolemia, and hyperinsulinemia. Further, these rats showed a severe deficit in learning and memory. In contrast, rosiglitazone at the dose of 5 mg/kg, p.o. for 7 days prior to biochemical and behavioral testing significantly lowered the plasma glucose, triglycerides, cholesterol, and insulin levels. These animals also performed better on Morris water maze task, suggesting improved spatial memory. Our data demonstrate that the insulin sensitizers can overcome the cognitive deficits arising from high fat diet feeding, which may be in part mediated through the development of peripheral insulin resistance.

Involvement of nitric oxide in insulin induced memory improvement

Although brain was considered as an insulin-insensitive organ, recently it has appeared that insulin has some interesting effects on some brain regions like hippocampus. It has been known that intra-hippocampally administered insulin can improve learning and memory. Knowing that insulin can stimulate nitric oxide (NO) synthesis via eNOS activation and also that NO synthase (NOS) inhibitors can affect learning and memory, the aim of this study was to assess if NO is involved in insulin induced memory improvement. Wistar male rats were intra-CA1 cannulated and the effect of post-training and pre-probe trial intra-hippocampal administration of N-nitro-L-arginine methyl ester (L-NAME) (5, 10, 30 microg), insulin+L-NAME+/-L-arginine were assessed in a single-day testing version of Morris water maze (MWM) task. Our results show that, l-NAME can prevent insulin induced memory improvement. This drug had no effect on escape latency of a non-spatial visual discrimination task. Therefore, it seems that endogenous nitric oxide has a role in spatial learning and memory improvement caused by insulin.

CASY-1, an ortholog of calsyntenins/alcadeins, is essential for learning in Caenorhabditis elegans

Calsyntenins/alcadeins are type I transmembrane proteins with two extracellular cadherin domains highly expressed in mammalian brain. They form a tripartite complex with X11/X11L and APP (amyloid precursor protein) and are proteolytically processed in a similar fashion to APP. Although a genetic association of calsyntenin-2 with human memory performance has recently been reported, physiological roles and molecular functions of the protein in the nervous system are poorly understood. Here, we show that CASY-1, the Caenorhabditis elegans ortholog of calsyntenins/alcadeins, is essential for multiple types of learning. Through a genetic screen, we found that casy-1 mutants show defects in salt chemotaxis learning. casy-1 mutants also show defects in temperature learning, olfactory adaptation, and integration of two sensory signals. casy-1 is widely expressed in the nervous system. Expression of casy-1 in a single sensory neuron and at the postdevelopmental stage is sufficient for its function in salt chemotaxis learning. The fluorescent protein-tagged ectodomain of CASY-1 is released from neurons. Moreover, functional domain analyses revealed that both cytoplasmic and transmembrane domains of this protein are dispensable, whereas the ectodomain, which contains the LG/LNS-like domain, is critically required for learning. These results suggest that learning is modulated by the released ectodomain of CASY-1.

Age-related insulin resistance in hypothalamus and peripheral tissues of orexin knockout mice

AIMS/HYPOTHESIS

Orexin/hypocretin is a hypothalamic neuropeptide that regulates motivated behaviours, such as feeding and arousal, and, importantly, is also involved in energy homeostasis. The aim of this study was to reveal the role of orexin in the regulation of insulin sensitivity for glucose metabolism.

METHODS

Orexin knockout mice fasted overnight underwent oral glucose tolerance testing and insulin tolerance testing. The impact of orexin deficiency on insulin signalling was studied by Western blotting to measure levels of Akt phosphorylation and its upstream and downstream molecules in the hypothalamus, muscle and liver in orexin knockout mice.

RESULTS

We found that orexin deficiency caused the age-related development of impaired glucose tolerance and insulin resistance in both male mice without obesity and female mice with mild obesity, fed a normal chow diet. When maintained on a high-fat diet, these abnormalities became more pronounced exclusively in female orexin knockout mice that developed severe obesity. Insulin signalling through Akt was disrupted in peripheral tissues of middle-aged (9-month-old) but not young adult (2-to-3-month-old) orexin knockout mice fed a normal chow diet. Moreover, basal levels of hypothalamic Akt phosphorylation were abnormally elevated in orexin knockout mice at every age studied, and insulin stimulation failed to increase the level of phosphorylation. Similar abnormalities were observed with respect to GSK3beta phosphorylation in the hypothalamus and peripheral tissues of middle-aged orexin knockout mice.

CONCLUSIONS/INTERPRETATION

Our results demonstrate a novel role for orexin in hypothalamic insulin signalling, which is likely to be responsible for preventing the development of peripheral insulin resistance with age.

Impairment of hippocampal neurogenesis in streptozotocin-treated diabetic rats

OBJECTIVES

Adult neurogenesis in dentate gyrus (DG) is an evolutionarily preserved trait in most mammals examined thus far. Neuronal proliferation and subsequent integration of new neurons into the hippocampal circuit are regulated processes that can have profound effects on an animal's behaviour. A streptozotocin model of type I diabetes, characterized by low insulin and high plasma glucose levels, affects not only body's overall metabolism but also brain activity.

MATERIALS AND METHODS

Neurogenesis was measured within the DG of the hippocampus using immunohistochemical markers Ki67, Doublecortin, Calbindin (CaBP) and bromodeoxyuridine (BrdU).

RESULTS

Cell proliferation, measured with the endogenous marker Ki67, was reduced by 45%, and cell survival, measured with BrdU, was reduced by 64% of the control. Combined effects on proliferation and survival produced dramatically lower neuronal production. Among the surviving cells only 33% matured normally as judged by the co-labelling of BrdU and CaBP.

CONCLUSION

Such a reduction lowered the number of surviving cells with neuronal phenotype by over 80% of the control values and this is expected to cause a significant functional impairment of learning and memory in diabetic animals. These results may shed light on causes of diabetic neuropathology and provide an explanation for the memory deficiencies seen in some diabetic patients.

Brain insulin, energy and glucose homeostasis; genes, environment and metabolic pathologies

The central nervous system is essential in maintaining energy and glucose homeostasis. In both animals and humans, efficient cerebral insulin signalling is a pivotal control element in these pathophysiological processes. The action of insulin in the brain is under a multilevel control via metabolic, endocrine and neural signals induced by nutrients, integrated mainly by the hypothalamus. Of particular interest is the interaction of insulin with the anabolic and catabolic neuroregulators. The anorexic peptides insulin, leptin and the neurotransmitter serotonin share common signalling pathways involved in food intake, in particular the insulin receptor substrate, phosphatidylinositol-3-kinase (PI3K) pathway. The dialogue of neurotransmitters and peptides via this signalling pathway is potentially of major importance in the pathophysiology of the brain in general and specifically in the regulation of feeding behaviour. At this time, a new concept in the aetiopathology of type 2 diabetes is immerging. This concept proposes that the combination of defective pancreatic beta-cell function and insulin resistance not only in classical insulin target tissues but in every tissue, contributes to the onset of the disease. It highlights the importance of the disruption of cerebral insulin signal transmission and its direct relation to metabolic diseases. Impaired brain insulin signalling, a link coupling obesity to diabetes, may be related to either genetic factors, or environmental factors such as stress, over or under-feeding and unbalanced diets: such factors may work either independently or in concert. Current approaches used for the prevention and treatment of type 2 diabetes are not adequately effective. Most of the anti-diabetic therapies induce many adverse effects, in particular obesity, and thus may initiate a vicious cycle of problems. In order to develop new, more efficient, preventive and therapeutic strategies for metabolic pathologies, there is an urgent need for increased understanding of the complexity of insulin signalling in the brain and on the interactive, central and peripheral effects of insulin.

Determinants and associations of homocysteine and prothrombotic risk factors in Kuwaiti patients with cerebrovascular accident

OBJECTIVE

The objective of this study was to evaluate the determinants and associations of some prothrombotic risk factors in patients with cerebrovascular accidents (CVAs).

SUBJECTS AND METHODS

In this case-control study, plasma total homocysteine (tHcy), lupus anticoagulant, protein C, protein S, activated protein C resistance (APC-R) and antithrombin were measured in 102 patients (60 males and 42 females) and 167 controls (87 males, 80 females). Serum vitamin B(12), folate, red cell folate, creatinine, lipid profile and glucose were also determined. Glomerular filtration rate (GFR) was calculated.

RESULTS

13 (22%) of the 60 male patients, and 16 (39%) of the 42 female patients had hyperhomocysteinemia. Median (interquartile range) tHcy was higher in male patients [11.22 micromol/l (9.60-15.40)] than female patients [10.05 micromol/l (8.72-17.54)]. On binary logistic regression analysis, the significant (p < 0.05) determinants of tHcy were urea, creatinine and GFR. Comparing patients with control subjects showed that tHcy, age, fasting glucose, urea, serum creatinine, white blood cell count, protein S, APC-R and factor VIII were significantly higher, while protein C, factor II, total cholesterol, high-density lipoprotein cholesterol and low-density lipoprotein cholesterol were significantly lower in patients. Lupus anticoagulant was not associated with tHcy and not detected in patients and controls. Low concentrations of vitamins B(12) and folate were not associated with tHcy. Logistic regression analysis showed a significant association of tHcy with CVA (OR = 9.55; p = 0.047) in males in the presence of other traditional CVA risk factors but tHcy is not independently associated with CVA in females.

CONCLUSION

Hyperhomocysteinemia is common in Kuwaiti patients with CVA and tHcy probably interacts with prothrombotic factors (protein C, APC-R and factor VIII) to increase CVA risk. The main determinants, age and GFR markers, should be kept in mind when determining the risk associated with tHcy.

Insulin and IGF-1 enhance the expression of the neuronal monocarboxylate transporter MCT2 by translational activation via stimulation of the phosphoinositide 3-kinase-Akt-mammalian target of rapamycin pathway

MCT2 is the main neuronal monocarboxylate transporter essential for facilitating lactate and ketone body utilization as energy substrates. Our study reveals that treatment of cultured cortical neurons with insulin and IGF-1 led to a striking enhancement of MCT2 immunoreactivity in a time- and concentration-dependent manner. Surprisingly, neither insulin nor IGF-1 affected MCT2 mRNA expression, suggesting that regulation of MCT2 protein expression occurs at the translational rather than the transcriptional level. Investigation of the putative signalling pathways leading to translation activation revealed that insulin and IGF-1 induced p44- and p42 MAPK, Akt and mTOR phosphorylation. S6 ribosomal protein, a component of the translational machinery, was also strongly activated by insulin and IGF-1. Phosphorylation of p44- and p42 MAPK was blocked by the MEK inhibitor PD98058, while Akt phosphorylation was abolished by the PI3K inhibitor LY294002. Phosphorylation of mTOR and S6 was blocked by the mTOR inhibitor rapamycin. In parallel, it was observed that LY294002 and rapamycin almost completely blocked the effects of insulin and IGF-1 on MCT2 protein expression, whereas PD98059 and SB202190 (a p38K inhibitor) had no effect on insulin-induced MCT2 expression and only a slight effect on IGF-1-induced MCT2 expression. At the subcellular level, a significant increase in MCT2 protein expression within an intracellular pool was observed while no change at the cell surface was apparent. As insulin and IGF-1 are involved in synaptic plasticity, their effect on MCT2 protein expression via an activation of the PI3K-Akt-mTOR-S6K pathway might contribute to the preparation of neurons for enhanced use of nonglucose energy substrates following altered synaptic efficacy.

A potential role for the hippocampus in energy intake and body weight regulation

Recent research and theory point to the possibility that hippocampal-dependent learning and memory mechanisms translate neurohormonal signals of energy balance into adaptive behavioral outcomes involved with the inhibition of food intake. The present paper summarizes these findings and ideas and considers the hypothesis that excessive caloric intake and obesity may be produced by dietary and other factors that are known to alter hippocampal functioning.

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.

Glycogen synthase kinase-3beta: homologous regulation of cell surface insulin receptor level via controlling insulin receptor mRNA stability in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells, 48 h-treatment with 20 mmol/L LiCl, 1 mmol/L valproic acid, 30 micromol/L SB216763, 30 micromol/L SB415286, or 100 nmol/L insulin, a condition that inhibits constitutive active glycogen synthase kinase-3 (GSK-3), decreased cell surface (125)I-insulin binding capacity by approximately 39%, without altering the K(d) value; LiCl, SB216763 or insulin decreased insulin receptor (IR) and IR precursor levels, attenuating insulin-induced Tyr-autophosphorylation of IR. LiCl increased inhibitory Ser9-phosphorylation of GSK-3beta at 6 h, decreasing (125)I-insulin binding at 24 h. SB216763-induced (125)I-insulin binding reduction (IC(50) = 3 micromol/L) was preceded by beta-catenin level increase by SB216763 (EC(50) = 11 micromol/L), a hallmark of GSK-3 inhibition. Insulin-induced rapid (> 1 min) Ser9-phosphorylation of GSK-3beta (Nemoto et al. 2006) was followed by approximately 48% decrease of IR level. LiCl did not stimulate endocytosis, nor proteolysis of IR. LiCl destabilized IR mRNA (t(1/2) = 9.3 vs. 6.5 h), decreasing IR mRNA level by approximately 47%, without altering IR gene transcription. Decreases of (125)I-insulin binding and IR level, as well as increased Ser9-phosphorylation of GSK-3beta were restored to the control levels by washing the test compound-treated cells. Thus, GSK-3beta regulates IR level via controlling IR mRNA stability.

Common pathological processes in Alzheimer disease and type 2 diabetes: a review

Alzheimer disease (AD) and type 2 diabetes mellitus (T2DM) are conditions that affect a large number of people in the industrialized countries. Both conditions are on the increase, and finding novel treatments to cure or prevent them are a major aim in research. Somewhat surprisingly, AD and T2DM share several molecular processes that underlie the respective degenerative developments. This review describes and discusses several of these shared biochemical and physiological pathways. Disturbances in insulin signalling appears to be the main common impairment that affects cell growth and differentiation, cellular repair mechanisms, energy metabolism, and glucose utilization. Insulin not only regulates blood sugar levels but also acts as a growth factor on all cells including neurons in the CNS. Impairment of insulin signalling therefore not only affects blood glucose levels but also causes numerous degenerative processes. Other growth factor signalling systems such as insulin growth factors (IGFs) and transforming growth factors (TGFs) also are affected in both conditions. Also, the misfolding of proteins plays an important role in both diseases, as does the aggregation of amyloid peptides and of hyperphosphorylated proteins. Furthermore, more general physiological processes such as angiopathic and cytotoxic developments, the induction of apoptosis, or of non-apoptotic cell death via production of free radicals greatly influence the progression of AD and T2DM. The increase of detailed knowledge of these common physiological processes open up the opportunities for treatments that can prevent or reduce the onset of AD as well as T2DM.

Brain insulin system dysfunction in streptozotocin intracerebroventricularly treated rats generates hyperphosphorylated tau protein

The intracerebroventricular (icv) application of streptozotocin (STZ) in low dosage was used in 3-month-old rats to explore brain insulin system dysfunction. Three months following STZ icv treatment, the expression of insulin-1 and -2 mRNA was significantly reduced to 11% in hippocampus and to 28% in frontoparietal cerebral cortex, respectively. Insulin receptor (IR) mRNA expression decreased significantly in frontoparietal cerebral cortex and hippocampus (16% and 33% of control). At the protein/activity level, different abnormalities of protein tyrosine kinase activity (increase in hippocampus), total IR beta-subunit (decrease in hypothalamus) and phosphorylated IR tyrosine residues (increase) became apparent. The STZ-induced disturbance in learning and memory capacities was not abolished by icv application of glucose transport inhibitors known to prevent STZ-induced diabetes mellitus. The discrepancy between reduced IR gene expression and increase in both phosphorylated IR tyrosine residues/protein tyrosine kinase activity may indicate imbalance between phosphorylation/dephosphorylation of the IR beta-subunit causing its dysfunction. These abnormalities may point to a complex brain insulin system dysfunction after STZ icv application, which may lead to an increase in hyperphosphorylated tau-protein concentration. Brain insulin system dysfunction is discussed as possible pathological core in the generation of hyperphosphorylated tau protein as a morphological marker of sporadic Alzheimer's disease.

Reversal of Memory Deficits by Atorvastatin and Simvastatin in Rats

The present study was undertaken to investigate the beneficial effects of Atorvastatin and Simvastatin in cognitive dysfunctions of rats. Alprazolam, Scopolamine and high fat diet (HFD) induced amnesia served as interoceptive memory models where as, Water-maze and Elevated plus-maze served as exteroceptive models. A total of 38 groups of rats were used in this investigation. Escape latency time (ELT) recorded during acquisition trials conducted from day 1 to day 4, in water maze was taken as an index of acquisition, where as mean time spent in target quadrant during retrieval trial on day 5, was taken as the index of retrieval (memory). On elevated plus-maze, transfer latency (TL) measured on 1st d served as the index of acquisition and TL recorded on 2nd d was taken as the index of retrieval (memory). Alprazolam (0.5 mg kg(-1) intraperitoneally), Scopolamine (0.4 mg kg(-1) intraperitoneally) and HFD treated (for 90 days) rats exhibited amnesia as reflected by impairment in learning ability as well as memory, when tested on both, water maze and elevated plus maze. Atorvastatin (5 mg kg(-1) orally) as well as Simvastatin (5 mg kg(-1) orally) significantly attenuated Alprazolam, Scopolamine and HFD induced amnesia. These results highlight the ameliorative role of statins in experimental amnesia with possible involvement of their cholesterol dependent as well as cholesterol independent actions.

Hippocampal expression analyses reveal selective association of immediate-early, neuroenergetic, and myelinogenic pathways with cognitive impairment in aged rats

Although expression of some genes is known to change during neuronal activity or plasticity, the overall relationship of gene expression changes to memory or memory disorders is not well understood. Here, we combined extensive statistical microarray analyses with behavioral testing to comprehensively identify genes and pathways associated with aging and cognitive dysfunction. Aged rats were separated into cognitively unimpaired (AU) or impaired (AI) groups based on their Morris water maze performance relative to young-adult (Y) animals. Hippocampal gene expression was assessed in Y, AU, and AI on the fifth (last) day of maze training (5T) or 21 d posttraining (21PT) and in nontrained animals (eight groups total, one array per animal; n = 78 arrays). ANOVA and linear contrasts identified genes that differed from Y generally with aging (differed in both AU and AI) or selectively, with cognitive status (differed only in AI or AU). Altered pathways/processes were identified by overrepresentation analyses of changed genes. With general aging, there was downregulation of axonal growth, cytoskeletal assembly/transport, signaling, and lipogenic/uptake pathways, concomitant with upregulation in immune/inflammatory, lysosomal, lipid/protein degradation, cholesterol transport, transforming growth factor, and cAMP signaling pathways, primarily independent of training condition. Selectively, in AI, there was downregulation at 5T of immediate-early gene, Wnt (wingless integration site), insulin, and G-protein signaling, lipogenesis, and glucose utilization pathways, whereas Notch2 (oligodendrocyte development) and myelination pathways were upregulated, particularly at 21PT. In AU, receptor/signal transduction genes were upregulated, perhaps as compensatory responses. Immunohistochemistry confirmed and extended selected microarray results. Together, the findings suggest a new model, in which deficient neuroenergetics leads to downregulated neuronal signaling and increased glial activation, resulting in aging-related cognitive dysfunction.

Intrahippocampal insulin improves memory in a passive-avoidance task in male wistar rats

The main impacts of insulin favor the peripheral organs. Although it functions as a neuropeptide, insulin possesses also some central effects. The aim of this study was to determine the effect of intrahippocampal infusion of insulin on passive avoidance learning in healthy male rats. Thirty male wistar rats were divided into three groups (n=10 each). The experimental group had posttraining insulin infusion into the CA1 region of dorsal hippocampus, after which they were compared with sham (saline) and control (intact) groups. Insulin treated animals had greater latency to enter the dark compartment in compare with saline treated (p=0.023) or control groups (p=0.017). Upon our results, we concluded that intrahippocampal injections of insulin may enhance memory for a simple learning task which supports the concept that insulin possibly plays an endogenous role in memory formation.

Insulin resistance of hypothalamic arcuate neurons in neonatally overfed rats

Rats exposed to early postnatal overfeeding by rearing in small litters become hyperphagic, hyperleptinemic, and hyperinsulinemic throughout later life. Medial arcuate neurons are involved in body weight regulation. They were tested in brain slices of control and small-litter rats concerning differences in responses to insulin. Insulin induced suppression of firing in controls, whereas in small-litter rats inhibition was significantly reduced and activation increased. This could be observed in juvenile as well as adult rats. A gamma-aminobutyric acid type A receptor antagonist did not change the responses. Thus, negative feedback to the satiety signal insulin on medial arcuate neurons is reduced in neonatally overfed small-litter rats. This can be regarded as insulin resistance, which is induced during early development and persists in later life.

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.

Intranasal insulin improves memory in humans: superiority of insulin aspart

There is compelling evidence that intranasal administration of regular human insulin (RH-I) improves memory in humans. Owing to the reduced tendency of its molecules to form hexamers, the rapid-acting insulin analog insulin aspart (ASP-I) is more rapidly absorbed than RH-I after subcutaneous administration. Since after intranasal insulin administration, ASP-I may also be expected to access the brain, we examined whether intranasal ASP-I has stronger beneficial effects on declarative memory than RH-I in humans. Acute (40 IU) and long-term (4 x 40 IU/day over 8 weeks) effects of intranasally administered ASP-I, RH-I, and placebo on declarative memory (word lists) were assessed in 36 healthy men in a between-subject design. Plasma insulin and glucose levels were not affected. After 8 weeks of treatment, however, word list recall was improved compared to placebo in both the ASP-I (p<0.01) and the RH-I groups (p<0.05). ASP-I-treated subjects performed even better than those of the RH-I-treated group (p<0.05). Our results indicate that insulin-induced memory improvement can be enhanced by using ASP-I. This finding may be especially relevant for a potential clinical administration of intranasal insulin in the treatment of memory disorders like Alzheimer's disease.

Intranasal insulin to improve memory function in humans

BACKGROUND

Compelling evidence indicates that central nervous insulin enhances learning and memory and in particular benefits hippocampus-dependent (i.e., declarative) memory. Intranasal administration of insulin provides an effective way of delivering the compound to the central nervous system, bypassing the blood-brain barrier and avoiding systemic side effects.

METHODS

Here we review a series of recent studies on the effects of intranasally administered insulin on memory functions in humans. In accordance with the beneficial effects of intravenously administered insulin on hippocampus-dependent declarative memory observed in hyperinsulinemic-euglycemic clamp studies, intranasal insulin administration similarly improves this type of memory, but in the absence of adverse peripheral side effects.

RESULT AND CONCLUSION

Considering that cerebrospinal fluid insulin levels are reduced in patients suffering from Alzheimer's disease, these results may be of considerable relevance for future clinical applications of insulin in the treatment of memory disorders.

Central insulin resistance as a trigger for sporadic Alzheimer-like pathology: an experimental approach

A growing body of evidence implicates impairments in brain insulin signaling in early sporadic Alzheimer disease (sAD) pathology. However, the most widely accepted hypothesis for AD aetiology stipulates that pathological aggregations of the amyloid beta (Abeta) peptide are the cause of all forms of Alzheimer's disease. Streptozotocin-intracerebroventricularly (STZ-icv) treated rats are proposed as a probable experimental model of sAD. The current work reviews evidence obtained from this model indicating that central STZ administration induces brain pathology and behavioural alterations resembling those in sAD patients. Recently, alterations of the brain insulin system resembling those in sAD have been found in the STZ-icv rat model and are associated with tau protein hyperphosphorylation and Abeta-like aggregations in meningeal vessels. In line with these findings the hypothesis has been proposed that insulin resistance in the brain might be the primary event which precedes the Abeta pathology in sAD.

Effects of Glucose, Insulin, and Supernatant from Pancreatic β-cells on Brain–Pancreas Relative Protein in Rat Hippocampus

Brain-pancreas relative protein (BPRP) is a novel protein that mainly expresses in brain and pancreas. In our previous study, we found that various stressors significantly decreased the expression of BPRP in pancreas in vivo, accompanied by changes in insulin and glucose levels, and that expression of BPRP in pancreas also decreased significantly in diabetic rats induced by Streptozocin (STZ). All these findings suggest that BPRP may be a glucose or insulin-sensitive protein. However, how the changes in insulin or glucose levels influence the expression of BPRP in hippocampus requires further study. Here, we investigated the effects of insulin or glucose on the expression of BPRP in primary cultured hippocampal neurons. We supplied hippocampal neurons with glucose, insulin, or supernatant from pancreatic beta-cells, which secrete insulin into the supernatant. Our data showed that insulin had beneficial effect on the viability while no significant effect on the expression of BPRP in hippocampal neurons. On the contrary, 40 mM glucose or free glucose culture significantly decreased the expression of BPRP, while had no significant effect on the viability and apoptosis of hippocampal neurons. Further study showed that levels of insulin in the supernatant collected from pancreatic beta-cells medium changed over days, and that supernatant increased the viability of hippocampal neurons, while it had no obvious effect on the expression of BPRP in hippocampal neurons. These results suggest that BPRP may be a glucose-sensitive protein.

Consumption of a mid-morning snack improves memory but not attention in school children

Muthayya, S., T. Thomas, K. Srinivasan, K. Rao, A. V. Kurpad, J.-W. Van Klinken, G. Owen and E.A. de Bruin: Consumption of a mid-morning snack improves memory but not attention in school children. Physiol Behav 00(0) 000-000, 2006.--This study aimed to determine whether consumption of a mid-morning snack with appropriate energy compensation through a smaller breakfast or lunch, resulted in improved cognitive performance of 7-9 year old children with a low and high socioeconomic status (LSES and HSES, n=35 and 34 respectively). The children were each randomly assigned to three iso-caloric dietary interventions: control (standard breakfast, no snack and standard lunch), intervention A (small breakfast, snack, and standard lunch) and intervention B (standard breakfast, snack, and small lunch), using a cross-over design. The children were tested on three different days, each one week apart. Computerised tests of cognitive performance, consisting of memory, sustained attention and psychomotor speed, were performed during four sessions, i.e., prior to breakfast, after breakfast, after a mid-morning snack and after lunch. Having a mid-morning snack resulted in a smaller decline in immediate and delayed memory in LSES but not in HSES children. Having a snack did not influence sustained attention and psychomotor speed in either LSES or HSES children. This study shows that a more evenly distributed energy intake throughout the morning by consuming a mid-morning snack improves memory performance in school-age LSES children even when the total amount of energy consumed during the morning is not altered.

The insulin/PI 3-kinase pathway regulates salt chemotaxis learning in Caenorhabditis elegans

The insulin-like signaling pathway is known to regulate fat metabolism, dauer formation, and longevity in Caenorhabditis elegans. Here, we report that this pathway is also involved in salt chemotaxis learning, in which animals previously exposed to a chemoattractive salt under starvation conditions start to show salt avoidance behavior. Mutants of ins-1, daf-2, age-1, pdk-1, and akt-1, which encode the homologs of insulin, insulin/IGF-I receptor, PI 3-kinase, phosphoinositide-dependent kinase, and Akt/PKB, respectively, show severe defects in salt chemotaxis learning. daf-2 and age-1 act in the ASER salt-sensing neuron, and the activity level of the DAF-2/AGE-1 pathway in this neuron determines the extent and orientation of salt chemotaxis. On the other hand, ins-1 acts in AIA interneurons, which receive direct synaptic inputs from sensory neurons and also send synaptic outputs to ASER. These results suggest that INS-1 secreted from AIA interneurons provides feedback to ASER to generate plasticity of chemotaxis.

GABAA receptor antagonists prevent abnormalities in leptin, insulin and amylin actions on paraventricular hypothalamic neurons of overweight rats

The hypothalamic regulatory system of body weight which develops in rats during critical periods of early postnatal life seems to express plastic changes depending on nutrition at that time. Adult rats previously exposed to early postnatal overnutrition by raising them in small litters become persistently predisposed to overweight, hyperphagia and hyperleptinaemia. The hypothesis was raised that feeding-related peptides could be involved through altered effects on neuronal activity of the regulatory systems of such rats. This was studied on brain slices of small-litter rats and normal-weight controls between days 60 and 120 of life. Neurons of the medial parvocellular part of the paraventricular nucleus were significantly activated by the adiposity signals leptin, insulin and amylin in controls. This is a kind of negative feedback, because activation of these neurons is known to be followed in vivo by increased energy expenditure. GABAergic mechanisms seem to affect these neuronal responses because the activating effects of insulin and amylin were reduced in the presence of a GABA(A) receptor antagonist. In overweight small-litter rats, however, the neuronal responses to the adiposity signals were significantly changed; activating effects were reduced and inhibitory effects increased. By means of blockade of GABA(A) receptors, significant alterations in the neuronal responses to leptin, insulin and amylin in small-litter rats were prevented. Responses to the peptides were reversed and now resembled those of controls. In conclusion, changes in neuronal wiring with GABAergic interneurons seem to contribute to a persistently reduced negative feedback of adiposity signals in early postnatally overfed rats.

Effects of sex and insulin/insulin-like growth factor-1 signaling on performance in an associative learning paradigm in Caenorhabditis elegans

Learning is an adaptive change in behavior in response to environmental stimuli. In mammals, there is a distinct female bias to learn skills that is still unprecedented in other animal taxa. Here we have investigated the biological determinants of performance in an associative learning paradigm in the nematode Caenorhabditis elegans. Using an assay of chemotactic reactions associated with food deprivation, wild-type male worms show inferior learning ability relative to hermaphrodites. Sex-based learning difference is therefore an ancient evolutionary feature appearing even in relatively simple animals. C. elegans mutants with reduced insulin/IGF-1 signaling also exhibit a greatly reduced learning ability in this assay. In addition, hyperactivation of insulin/IGF-1 signaling through loss-of-function mutations in the PTEN phosphatase daf-18, a negative regulator of insulin/IGF-1 signaling, enhances learning ability beyond that of wild type. According to our epistasis analysis, the effect of DAF-2 on learning acts via phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) production, but not the DAF-16 FOXO transcription factor. This implies that the signaling pathway from DAF-2 affecting this learning paradigm branches between PIP(3) production and DAF-16. However, learning capacity of nematodes is lowered by loss-of-function mutations in daf-16, suggesting involvement of noninsulin/IGF-1 signaling-dependent DAF-16 activation in learning. Potentially, sex and insulin/IGF-1 signaling affect performance in this learning assay via effects on the neurobiology of learning.

Regional variations in NMDA receptor downregulation in streptozotocin-diabetic rat brain

Insulin insufficiency has multiple actions on the CNS. Three weeks after streptozotocin-induced diabetes in rats, we found a preferential downregulation of l-[(3)H]glutamate-labeled NMDA receptors in primary sensory cortical regions. Layers I-III of the parietal cortex and superficial piriform cortex were the most sensitive followed by other cortical regions and, in turn, deeper brain structures. These effects on NMDA receptors can potentially explain some of the known CNS effects of diabetes.

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.

Canine cognitive deficit correlates with diffuse plaque maturation and S100beta (-) astrocytosis but not with insulin cerebrospinal fluid level

Like humans, canines develop with aging beta-amyloid (Abeta) plaques and a progressive cognitive deficit on tasks similar to those used in diagnosis and follow-up of Alzheimer's disease. Owing to that, dogs are quite unique to investigate the early events taking place in the diffuse Abeta plaque maturation and its relationship with cognitive deficit. The aim of the present investigation was to study the link between the diffuse Abeta plaque maturation and the astro- and microglial reactivity. The involvement of insulin and beta-subunit of S100 protein (S100beta) overexpression in the process was also investigated. Abeta plaques were measured and counted in prefrontal cortex of 16 pet dogs of different breeds, weight and sex, classified as control and with a light or severe cognitive deficit. A correlation between canine graded cognitive deficit, diffuse plaque maturation, and S100beta (-) astrocytosis, but not with cerebrospinal fluid insulin level, was found that may reflect the very early events of Abeta deposition in Alzheimer's disease.

Rosiglitazone attenuates learning and memory deficits in Tg2576 Alzheimer mice

The thiazolidinediones, such as rosiglitazone, increase peripheral insulin sensitivity and their use is proposed for the treatment of Alzheimer's disease. However, the mechanisms underlying the potential beneficial effects of rosiglitazone in Alzheimer's disease remain unclear. In previous studies, we observed that Tg2576 Alzheimer mice develop peripheral insulin resistance with age and have much higher serum corticosterone levels than wild-type mice when fasted overnight. We further showed that both of these defects can be ameliorated by rosiglitazone administration. Here, we report that during behavioral testing which involves repetitive overnight fasting, Tg2576 mice administered rosiglitazone exhibited better spatial learning and memory abilities and had lower serum corticosterone levels than untreated Tg2576 mice. When untreated Tg2576 mice were administered metyrapone, a drug that blocks glucocorticoid production, their spatial learning and memory abilities and serum corticosterone levels were similar to those of rosiglitazone-treated mice. We further report here that rosiglitazone attenuated reductions in insulin-degrading enzyme (IDE) mRNA and activity, and reduced amyloid beta-peptide (Abeta)42 levels without affecting amyloid deposition, in the brains of Tg2576 mice. These results demonstrate that rosiglitazone attenuates learning and memory deficits in Tg2576 mice and suggest that the effects of the drug on learning and memory, brain IDE levels, and brain Abeta42 levels in the mice may be due to its glucocorticoid-lowering actions.

Alzheimer-like changes in protein kinase B and glycogen synthase kinase-3 in rat frontal cortex and hippocampus after damage to the insulin signalling pathway

The insulin-resistant brain state is related to late-onset sporadic Alzheimer's disease, and alterations in the insulin receptor (IR) and its downstream phosphatidylinositol-3 kinase signalling pathway have been found in human brain. These findings have not been confirmed in an experimental model related to sporadic Alzheimer's disease, for example rats showing a neuronal IR deficit subsequent to intracerebroventricular (i.c.v.) treatment with streptozotocin (STZ). In this study, western blot analysis performed 1 month after i.c.v. injection of STZ showed an increase of 63% in the level of phosphorylated glycogen synthase kinase-3alpha/beta (pGSK-3alpha/beta) protein in the rat hippocampus, whereas the levels of the unphosphorylated form (GSK-3alpha/beta) and protein kinase B (Akt/PKB) remained unchanged. Three months after STZ treatment, pGSK-3alpha/beta and Akt/PKB levels tended to decrease (by 8 and 9% respectively). The changes were region specific, as a different pattern was found in frontal cortex. Structural alterations were also found, characterized by beta-amyloid peptide-like aggregates in brain capillaries of rats treated with STZ. Similar neurochemical changes and cognitive deficits were recorded in rats treated with i.c.v. 5-thio-d-glucose, a blocker of glucose transporter (GLUT)2, a transporter that is probably involved in brain glucose sensing. The IR signalling cascade alteration and its consequences in rats treated with STZ are similar to those found in humans with sporadic Alzheimer's disease, and our results suggest a role for GLUT2 in Alzheimer's pathophysiology.

Revenge of the “Sit”: How lifestyle impacts neuronal and cognitive health through molecular systems that interface energy metabolism with neuronal plasticity

Exercise, a behavior that is inherently associated with energy metabolism, impacts the molecular systems important for synaptic plasticity and learning and memory. This implies that a close association must exist between these systems to ensure proper neuronal function. This review emphasizes the ability of exercise and other lifestyle implementations that modulate energy metabolism, such as diet, to impact brain function. Mechanisms believed to interface metabolism and cognition seem to play a critical role with the brain derived neurotrophic factor (BDNF) system. Behaviors concerned with activity and metabolism may have developed simultaneously and interdependently during evolution to determine the influence of exercise and diet on cognition. A look into our evolutionary past indicates that our genome remains unchanged from the times of our hunter-gatherer ancestors, whose active lifestyle predominated throughout almost 100% of humankind's existence. Consequently, the sedentary lifestyle and eating behaviors enabled by the comforts of technologic progress may be reaping "revenge" on the health of both our bodies and brains. In the 21st century we are confronted by the ever-increasing incidence of metabolic disorders in both the adult and child population. The ability of exercise and diet to impact systems that promote cell survival and plasticity may be applicable for combating the deleterious effects of disease and ageing on brain health and cognition.

Increased risk of Alzheimer's disease in Type II diabetes: insulin resistance of the brain or insulin-induced amyloid pathology?

Type II diabetes mellitus (DM2) is associated with an increased risk of cognitive dysfunction and dementia. The increased risk of dementia concerns both Alzheimer's disease and vascular dementia. Although some uncertainty remains into the exact pathogenesis, several mechanisms through which DM2 may affect the brain have now been identified. First, factors related to the ‘metabolic syndrome’, a cluster of metabolic and vascular risk factors (e.g. dyslipidaemia and hypertension) that is closely linked to DM2, may be involved. A number of these risk factors are predictors of cerebrovascular disease, accelerated cognitive decline and dementia. Secondly, hyperglycaemia may be involved, through adverse effects of potentially ‘toxic’ glucose metabolites on the brain and its vasculature. Thirdly, insulin itself may be involved. Insulin can directly modulate synaptic plasticity and learning and memory, and disturbances in insulin signalling pathways in the periphery and in the brain have recently been implicated in Alzheimer's disease and brain aging. Insulin also regulates the metabolism of β-amyloid and tau, the building blocks of amyloid plaques and neurofibrillary tangles, the neuropathological hallmarks of Alzheimer's disease. In this paper, the evidence for the association between DM2 and dementia and for each of these underlying mechanisms will be reviewed, with emphasis on the role of insulin itself.

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.

New twist on neuronal insulin receptor signaling in health, disease, and therapeutics

Long after the pioneering studies documenting the existence of insulin (year 1967) and insulin receptor (year 1978) in brain, the last decade has witnessed extraordinary progress in the understanding of brain region-specific multiple roles of insulin receptor signalings in health and disease. In the hypothalamus, insulin regulates food intake, body weight, peripheral fat deposition, hepatic gluconeogenesis, reproductive endocrine axis, and compensatory secretion of counter-regulatory hormones to hypoglycemia. In the hippocampus, insulin promotes learning and memory, independent of the glucoregulatory effect of insulin. Defective insulin receptor signalings are associated with the dementia in normal aging and patients with age-related neurodegenerative diseases (e.g., Alzheimer's disease); the cognitive impairment can be reversed with systemic administration of insulin in the euglycemic condition. Intranasal administration of insulin enhances memory and mood and decreases body weight in healthy humans, without causing hypoglycemia. In the hypothalamus, insulin-induced activation of the phosphoinositide 3-kinase pathway followed by opening of ATP-sensitive K+ channel has been shown to be related to multiple effects of insulin. However, the precise molecular mechanisms of insulin's pleiotropic effects still remain obscure. More importantly, much remains unknown about the quality control mechanisms ensuring correct conformational maturation of the insulin receptor, and the cellular mechanisms regulating density of cell surface functional insulin receptors.

A neural signaling triumvirate that influences ageing and age-related disease: insulin/IGF-1, BDNF and serotonin

The ageing process and its associated diseases all involve perturbed energy metabolism, oxidative damage, and an impaired ability of the organism and its cells to cope with adversity. We propose that some specific signaling pathways in the brain may be important determinants of health during ageing. Among such specific signaling modalities are those activated in neurons by insulin-like growth factors (IGFs), brain-derived neurotrophic factor (BDNF) and serotonin. This triumvirate may be particularly important because of their cooperative influence on energy metabolism, food intake, stress responses and cardiovascular function. The health benefits to the periphery and central nervous system of dietary restriction and exercise may be mediated by this triumvirate of signals in the brain. At the molecular level, BDNF, serotonin and IGFs can all stimulate the production of proteins involved in cellular stress adaptation, growth and repair, neurogenesis, learning and memory and cell survival. The importance of this triumvirate is emphasized when it is seen that their general roles in energy metabolism, stress adaptation and disease resistance are conserved among diverse organisms consistent with important roles in the ageing process.