Immunoreactive insulin levels are elevated in the cerebrospinal fluid of genetically obese Zucker rats

Excision of the endothelial blood-brain barrier insulin receptor does not alter spatial cognition in mice fed either a chow or high-fat diet

Insulin is transported across the blood-brain barrier (BBB) endothelium to regulate aspects of metabolism and cognition. Brain insulin resistance often results from high-fat diet (HFD) consumption and is thought to contribute to spatial cognition deficits. To target BBB insulin function, we used Cre-LoxP genetic excision of the insulin receptor (InsR) from endothelial cells in adult male mice. We hypothesized that this excision would impair spatial cognition, and that high-fat diet consumption would exacerbate these effects. Excision of the endothelial InsR did not impair performance in two spatial cognition tasks, the Y-Maze and Morris Water Maze, in tests held both before and after 14 weeks of access to high-fat (or chow control) diet. The HFD increased body weight gain and induced glucose intolerance but did not impair spatial cognition. Endothelial InsR excision tended to increase body weight and reduce sensitivity to peripheral insulin, but these metabolic effects were not associated with impairments to spatial cognition and did not interact with HFD exposure. Instead, all mice showed intact spatial cognitive performance regardless of whether they had been fed chow or a HFD, and whether the InsR had been excised or not. Overall, the results indicate that loss of the endothelial InsR does not impact spatial cognition, which is in line with pharmacological evidence that other mechanisms at the BBB facilitate insulin transport and allow it to exert its pro-cognitive effects.

Brain insulin signalling in metabolic homeostasis and disease

Insulin signalling in the central nervous system regulates energy homeostasis by controlling metabolism in several organs and by coordinating organ crosstalk. Studies performed in rodents, non-human primates and humans over more than five decades using intracerebroventricular, direct hypothalamic or intranasal application of insulin provide evidence that brain insulin action might reduce food intake and, more importantly, regulates energy homeostasis by orchestrating nutrient partitioning. This Review discusses the metabolic pathways that are under the control of brain insulin action and explains how brain insulin resistance contributes to metabolic disease in obesity, the metabolic syndrome and type 2 diabetes mellitus.

The regulation of food intake by insulin in the central nervous system

Food intake and energy expenditure are regulated by peripheral signals providing feedback on nutrient status and adiposity to the central nervous system. One of these signals is the pancreatic hormone, insulin. Unlike peripheral administration of insulin, which often causes weight gain, central administration of insulin leads to a reduction in food intake and body weight when administered long-term. This is a result of feedback processes in regions of the brain that regulate food intake. Within the hypothalamus, the arcuate nucleus (ARC) contains subpopulations of neurones that produce orexinergic neuropeptides agouti-related peptide (AgRP)/neuropeptide Y (NPY) and anorexigenic neuropeptides, pro-opiomelanocortin (POMC)/cocaine- and amphetamine-regulated transcript (CART). Intracerebroventricular infusion of insulin down-regulates the expression of AgRP/NPY at the same time as up-regulating expression of POMC/CART. Recent evidence suggests that insulin activity within the amygdala may play an important role in regulating energy balance. Insulin infusion into the central nucleus of the amygdala (CeA) can decrease food intake, possibly by modulating activity of NPY and other neurone subpopulations. Insulin signalling within the CeA can also influence stress-induced obesity. Overall, it is evident that the CeA is a critical target for insulin signalling and the regulation of energy balance.

The impact of insulin on the serotonergic system and consequences on diabetes-associated mood disorders

The idea that insulin could influence emotional behaviours has long been suggested. However, the underlying mechanisms have yet to be solved and there is no direct and clear-cut evidence demonstrating that such action involves brain serotonergic neurones. Indeed, initial arguments in favour of the association between insulin, serotonin and mood arise from clinical or animal studies showing that impaired insulin action in type 1 or type 2 diabetes causes anxiety- and depressive symptoms along with blunted plasma and brain serotonin levels. The present review synthesises the main mechanistic hypotheses that might explain the comorbidity between diabetes and depression. It also provides a state of knowledge of the direct and indirect experimental evidence that insulin modulates brain serotonergic neurones. Finally, it highlights the literature suggesting that antidiabetic drugs present antidepressant-like effects and, conversely, that serotonergic antidepressants impact glucose homeostasis. Overall, this review provides mechanistic insights into how insulin signalling alters serotonergic neurotransmission and related behaviours bringing new targets for therapeutic options.

Molecular Mechanisms of Hypothalamic Insulin Resistance

Insulin exists in the central nervous system, where it executes two important functions in the hypothalamus: the suppression of food intake and the improvement of glucose metabolism. Recent studies have shown that both are exerted robustly in rodents and humans. If intact, these functions exert beneficial effects on obesity and diabetes, respectively. Disruption of both occurs due to a condition known as hypothalamic insulin resistance, which is caused by obesity and the overconsumption of saturated fat. An enormous volume of literature addresses the molecular mechanisms of hypothalamic insulin resistance. IKKβ and JNK are major players in the inflammation pathway, which is activated by saturated fatty acids that induce hypothalamic insulin resistance. Two major tyrosine phosphatases, PTP-1B and TCPTP, are upregulated in chronic overeating. They dephosphorylate the insulin receptor and insulin receptor substrate proteins, resulting in hypothalamic insulin resistance. Prolonged hyperinsulinemia with excessive nutrition activates the mTOR/S6 kinase pathway, thereby enhancing IRS-1 serine phosphorylation to induce hypothalamic insulin resistance. Other mechanisms associated with this condition include hypothalamic gliosis and disturbed insulin transport into the central nervous system. Unveiling the precise molecular mechanisms involved in hypothalamic insulin resistance is important for developing new ways of treating obesity and type 2 diabetes.

Using the cerebrospinal fluid to understand ingestive behavior

The cerebrospinal fluid (CSF) offers a window into the workings of the brain and blood-brain barrier (BBB). Molecules that enter into the central nervous system (CNS) by passive diffusion or receptor-mediated transport through the choroid plexus often appear in the CSF prior to acting within the brain. Other molecules enter the CNS by passing through the BBB into the brain's interstitial fluid prior to appearing in the CSF. This pattern is also often observed for molecules synthesized by neurons or glia within the CNS. The CSF is therefore an important conduit for the entry and clearance of molecules into/from the CNS and thereby constitutes an important window onto brain activity and barrier function. Assessing the CSF basally, under experimental conditions, or in the context of challenges or metabolic diseases can provide powerful insights about brain function. Here, we review important findings made by our labs, as influenced by the late Randall Sakai, by interrogating the CSF.

Insulin Regulates Hepatic Triglyceride Secretion and Lipid Content via Signaling in the Brain

Hepatic steatosis is common in obesity and insulin resistance and results from a net retention of lipids in the liver. A key mechanism to prevent steatosis is to increase secretion of triglycerides (TG) packaged as VLDLs. Insulin controls nutrient partitioning via signaling through its cognate receptor in peripheral target organs such as liver, muscle, and adipose tissue and via signaling in the central nervous system (CNS) to orchestrate organ cross talk. While hepatic insulin signaling is known to suppress VLDL production from the liver, it is unknown whether brain insulin signaling independently regulates hepatic VLDL secretion. Here, we show that in conscious, unrestrained male Sprague Dawley rats the infusion of insulin into the third ventricle acutely increased hepatic TG secretion. Chronic infusion of insulin into the CNS via osmotic minipumps reduced the hepatic lipid content as assessed by noninvasive (1)H-MRS and lipid profiling independent of changes in hepatic de novo lipogenesis and food intake. In mice that lack the insulin receptor in the brain, hepatic TG secretion was reduced compared with wild-type littermate controls. These studies identify brain insulin as an important permissive factor in hepatic VLDL secretion that protects against hepatic steatosis.

Hypothalamic Phosphodiesterase 3B Pathway Mediates Anorectic and Body Weight-Reducing Effects of Insulin in Male Mice

BACKGROUND

Insulin action in the hypothalamus plays a critical role in the regulation of energy homeostasis, yet the intracellular signaling mechanisms mediating insulin action are incompletely understood. Although phosphodiesterase 3B (PDE3B) mediates insulin action in the adipose tissue and it is highly expressed in the hypothalamic areas implicated in energy homeostasis, its role, if any, in mediating insulin action in the hypothalamus is unknown. We tested the hypothesis that insulin action in the hypothalamus is mediated by PDE3B.

METHODS

Using enzymatic assay, we examined the effects of peripheral or central administration of insulin on hypothalamic PDE3B activity in adult mice. Western blotting and immunohistochemistry also examined p-Akt and p-STAT3 levels in the hypothalamus. Effects of leptin on these parameters were also compared. We injected cilostamide, a PDE3 inhibitor, prior to central injection of insulin and examined the 12- to 24-hour food intake and 24-hour body weight. Finally, we examined the effect of cilostamide on insulin-induced proopiomelanocortin (Pomc), neurotensin (Nt), neuropeptide Y (Npy) and agouti-related peptide (Agrp) gene expression in the hypothalamus by qPCR.

RESULTS

Peripheral or central injection of insulin significantly increased PDE3B activity in the hypothalamus in association with increased p-Akt levels but without any change in p-STAT3 levels. However, leptin-induced increase in PDE3B activity was associated with an increase in both p-Akt and p-STAT3 levels in the hypothalamus. Prior administration of cilostamide reversed the anorectic and body weight-reducing effects as well as stimulatory effect of insulin on hypothalamic Pomc mRNA levels. Insulin did not alter Nt, Npy and Agrp mRNA levels.

CONCLUSION

Insulin induction of hypothalamic PDE3B activity and the reversal of the anorectic and body weight-reducing effects and stimulatory effect of insulin on hypothalamic Pomc gene expression by cilostamide suggest that activation of PDE3B is a novel mechanism of insulin signaling in the hypothalamus.

Insulin transport into the brain and cerebrospinal fluid

The pancreatic hormone insulin plays a well-described role in the periphery, based principally on its ability to lower circulating glucose levels via activation of glucose transporters. However, insulin also acts within the central nervous system (CNS) to alter a number of physiological outcomes ranging from energy balance and glucose homeostasis to cognitive performance. Insulin is transported into the CNS by a saturable receptor-mediated process that is proposed to be dependent on the insulin receptor. Transport of insulin into the brain is dependent on numerous factors including diet, glycemia, a diabetic state and notably, obesity. Obesity leads to a marked decrease in insulin transport from the periphery into the CNS and the biological basis of this reduction of transport remains unresolved. Despite decades of research into the effects of central insulin on a wide range of physiological functions and its transport from the periphery to the CNS, numerous questions remain unanswered including which receptor is responsible for transport and the precise mechanisms of action of insulin within the brain.

Modulation of olfactory sensitivity and glucose-sensing by the feeding state in obese Zucker rats

The Zucker fa/fa rat has been widely used as an animal model to study obesity, since it recapitulates most of its behavioral and metabolic dysfunctions, such as hyperphagia, hyperglycemia and insulin resistance. Although it is well established that olfaction is under nutritional and hormonal influences, little is known about the impact of metabolic dysfunctions on olfactory performances and glucose-sensing in the olfactory system of the obese Zucker rat. In the present study, using a behavioral paradigm based on a conditioned olfactory aversion, we have shown that both obese and lean Zucker rats have a better olfactory sensitivity when they are fasted than when they are satiated. Interestingly, the obese Zucker rats displayed a higher olfactory sensitivity than their lean controls. By investigating the molecular mechanisms involved in glucose-sensing in the olfactory system, we demonstrated that sodium-coupled glucose transporters 1 (SGLT1) and insulin dependent glucose transporters 4 (GLUT4) are both expressed in the olfactory bulb (OB). By comparing the expression of GLUT4 and SGLT1 in OB of obese and lean Zucker rats, we found that only SGLT1 is regulated in genotype-dependent manner. Next, we used glucose oxidase biosensors to simultaneously measure in vivo the extracellular fluid glucose concentrations ([Gluc]ECF) in the OB and the cortex. Under metabolic steady state, we have determined that the OB contained twice the amount of glucose found in the cortex. In both regions, the [Gluc]ECF was 2 fold higher in obese rats compared to their lean controls. Under induced dynamic glycemia conditions, insulin injection produced a greater decrease of [Gluc]ECF in the OB than in the cortex. Glucose injection did not affect OB [Gluc]ECF in Zucker fa/fa rats. In conclusion, these results emphasize the importance of glucose for the OB network function and provide strong arguments towards establishing the OB glucose-sensing as a key factor for sensory olfactory processing.

Effect of insulin-induced hypoglycaemia on the central nervous system: evidence from experimental studies

Insulin-induced hypoglycaemia (IIH) is a major acute complication in type 1 as well as in type 2 diabetes, particularly during intensive insulin therapy. The brain plays a central role in the counter-regulatory response by eliciting parasympathetic and sympathetic hormone responses to restore normoglycaemia. Brain glucose concentrations, being approximately 15-20% of the blood glucose concentration in humans, are rigorously maintained during hypoglycaemia through adaptions such as increased cerebral glucose transport, decreased cerebral glucose utilisation and, possibly, by using central nervous system glycogen as a glucose reserve. However, during sustained hypoglycaemia, the brain cannot maintain a sufficient glucose influx and, as the cerebral hypoglycaemia becomes severe, electroencephalogram changes, oxidative stress and regional neuronal death ensues. With particular focus on evidence from experimental studies on nondiabetic IIH, this review outlines the central mechanisms behind the counter-regulatory response to IIH, as well as cerebral adaption to avoid sequelae of cerebral neuroglycopaenia, including seizures and coma.

Reversal of diet-induced obesity increases insulin transport into cerebrospinal fluid and restores sensitivity to the anorexic action of central insulin in male rats

Diet-induced obesity (DIO) reduces the ability of centrally administered insulin to reduce feeding behavior and also reduces the transport of insulin from the periphery to the central nervous system (CNS). The current study was designed to determine whether reversal of high-fat DIO restores the anorexic efficacy of central insulin and whether this is accompanied by restoration of the compromised insulin transport. Adult male Long-Evans rats were initially maintained on either a low-fat chow diet (LFD) or a high-fat diet (HFD). After 22 weeks, half of the animals on the HFD were changed to the LFD, whereas the other half continued on the HFD for an additional 8 weeks, such that there were 3 groups: 1) a LFD control group (Con; n = 18), 2) a HFD-fed, DIO group (n = 17), and 3) a HFD to LFD, DIO-reversal group (DIO-rev; n = 18). The DIO reversal resulted in a significant reduction of body weight and epididymal fat weight relative to the DIO group. Acute central insulin administration (8 mU) reduced food intake and caused weight loss in Con and DIO-rev but not DIO rats. Fasting cerebrospinal fluid insulin was higher in DIO than Con animals. However, after a peripheral bolus injection of insulin, cerebrospinal fluid insulin increased in Con and DIO-rev rats but not in the DIO group. These data provide support for previous reports that DIO inhibits both the central effects of insulin and insulin's transport to the CNS. Importantly, DIO-rev restored sensitivity to the effects of central insulin on food intake and insulin transport into the CNS.

CNS control of glucose metabolism: response to environmental challenges

Over the last 15 years, considerable work has accumulated to support the role of the CNS in regulating postprandial glucose levels. As discussed in the first section of this review, the CNS receives and integrates information from afferent neurons, circulating hormones, and postprandially generated nutrients to subsequently direct changes in glucose output by the liver and glucose uptake by peripheral tissues. The second major component of this review focuses on the effects of external pressures, including high fat diet and changes to the light:dark cycle on CNS-regulating glucose homeostasis. We also discuss the interaction between these different pressures and how they contribute to the multifaceted mechanisms that we hypothesize contribute to the dysregulation of glucose in type 2 diabetes mellitus (T2DM). We argue that while current peripheral therapies serve to delay the progression of T2DM, generating combined obesity and T2DM therapies targeted at the CNS, the primary site of dysfunction for both diseases, would lead to a more profound impact on the progression of both diseases.

From conditioned hypoglycemia to obesity: following the data

While a graduate student in the late 1960s I trained rats to lower their blood glucose in response to an arbitrary cue, a phenomenon called conditioned hypoglycemia. Over many years as my colleagues and I attempted to understand the underlying physiology of conditioned insulin secretion and conditioned hypoglycemia, it became clear that there were many implications that were highly important, including the entry of insulin into the brain, the existence of insulin receptors in certain brain areas, neural reflexes that project to insulin-secreting B-cells in the pancreas, the entrainment of those reflexes to improve the efficiency of meal-taking, and the possibility of adiposity signals from the body to the brain that influence behavior and metabolism. This article summarizes how we tackled each of these areas of research.

Insulin increases central apolipoprotein E levels as revealed by an improved technique for collection of cerebrospinal fluid from rats

Cerebrospinal fluid (CSF) provides an invaluable analytical window to the central nervous system (CNS) because it reflects the dynamically changing complement of CNS constituents. We describe an improved method for sampling CSF in rats that is easy to perform. It has a 96% success rate of CSF collection and consistently yields large volumes (150-200 μl) of CSF. The blood contamination rate is also low (6%) as determined by both visual inspection and the lack of molecular detection of apolipoprotein B, a plasma-derived protein, which is absent in the CNS. This improved method of CSF sampling can have broad applicability in physiological and pharmacological evaluation for diverse CNS targets. We used this technique to provide proof of principle by examining the effect of intraperitoneal insulin on the level of apolipoprotein E (apoE) in the CSF. Insulin (0.5 and 1 U/kg) led to a significant increase of insulin in both plasma and CSF at 2 h after intraperitoneal administration and decreased blood glucose for at least 2h. ApoE concentrations in CSF, but not in plasma, were also significantly increased, and its time-course was inversely correlated with the alterations in blood glucose over 2 h. These results provide a pharmacological validation of the novel CSF sampling and validation procedure for sampling rat CSF.

Altered hypothalamic function in diet-induced obesity

Energy homeostasis involves a complex network of hypothalamic and extra-hypothalamic neurons that transduce hormonal, nutrient and neuronal signals into responses that ultimately match caloric intake to energy expenditure and thereby promote stability of body fat stores. Growing evidence suggests that rather than reflecting a failure to regulate caloric intake, common forms of obesity involve fundamental changes to this homeostatic system that favor the defense of an elevated level of body adiposity. This article reviews emerging evidence that during high-fat feeding, obesity pathogenesis involves fundamental alteration of hypothalamic systems that regulate food intake and energy expenditure.

Central control of body weight and appetite

CONTEXT

Energy balance is critical for survival and health, and control of food intake is an integral part of this process. This report reviews hormonal signals that influence food intake and their clinical applications.

EVIDENCE ACQUISITION

A relatively novel insight is that satiation signals that control meal size and adiposity signals that signify the amount of body fat are distinct and interact in the hypothalamus and elsewhere to control energy homeostasis. This review focuses upon recent literature addressing the integration of satiation and adiposity signals and therapeutic implications for treatment of obesity.

EVIDENCE SYNTHESIS

During meals, signals such as cholecystokinin arise primarily from the GI tract to cause satiation and meal termination; signals secreted in proportion to body fat such as insulin and leptin interact with satiation signals and provide effective regulation by dictating meal size to amounts that are appropriate for body fatness, or stored energy. Although satiation and adiposity signals are myriad and redundant and reduce food intake, there are few known orexigenic signals; thus, initiation of meals is not subject to the degree of homeostatic regulation that cessation of eating is. There are now drugs available that act through receptors for satiation factors and which cause weight loss, demonstrating that this system is amenable to manipulation for therapeutic goals.

CONCLUSIONS

Although progress on effective medical therapies for obesity has been relatively slow in coming, advances in understanding the central regulation of food intake may ultimately be turned into useful treatment options.

The integrative role of CNS fuel-sensing mechanisms in energy balance and glucose regulation

The incidences of both obesity and type 2 diabetes mellitus are rising at epidemic proportions. Despite this, the balance between caloric intake and expenditure is tremendously accurate under most circumstances. Growing evidence suggests that nutrient and hormonal signals converge and directly act on brain centers, leading to changes in fuel metabolism and, thus, stable body weight over time. Growing evidence also suggests that these same signals act on the central nervous system (CNS) to regulate glucose metabolism independently. We propose that this is not coincidental and that the CNS responds to peripheral signals to orchestrate changes in both energy and glucose homeostasis. In this way the CNS ensures that the nutrient demands of peripheral tissues (and likely of the brain itself) are being met. Consequently, dysfunction of the ability of the CNS to integrate fuel-sensing signals may underlie the etiology of metabolic diseases such as obesity and diabetes.

TNF-alpha acts in the hypothalamus inhibiting food intake and increasing the respiratory quotient--effects on leptin and insulin signaling pathways

Acting in the hypothalamus, tumor necrosis factor-alpha (TNF-alpha) produces a potent anorexigenic effect. However, the molecular mechanisms involved in this phenomenon are poorly characterized. In this study, we investigate the capacity of TNF-alpha to activate signal transduction in the hypothalamus through elements of the pathways employed by the anorexigenic hormones insulin and leptin. High dose TNF-alpha promotes a reduction of 25% in 12h food intake, which is an inhibitory effect that is marginally inferior to that produced by insulin and leptin. In addition, high dose TNF-alpha increases body temperature and respiratory quotient, effects not reproduced by insulin or leptin. TNF-alpha, predominantly at the high dose, is also capable of activating canonical pro-inflammatory signal transduction in the hypothalamus, inducing JNK, p38, and NFkappaB, which results in the transcription of early responsive genes and expression of proteins of the SOCS family. Also, TNF-alpha activates signal transduction through JAK-2 and STAT-3, but does not activate signal transduction through early and intermediary elements of the insulin/leptin signaling pathways such as IRS-2, Akt, ERK and FOXO1. When co-injected with insulin or leptin, TNF-alpha, at both high and low doses, partially impairs signal transduction through IRS-2, Akt, ERK and FOXO1 but not through JAK-2 and STAT-3. This effect is accompanied by the partial inhibition of the anorexigenic effects of insulin and leptin, when the low, but not the high dose of TNF-alpha is employed. In conclusion, TNF-alpha, on a dose-dependent way, modulates insulin and leptin signaling and action in the hypothalamus.

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.

Effect of diazoxide on brain capillary insulin receptor binding and food intake in hyperphagic obese Zucker rats

Insulin is believed to act as a central adiposity signal by binding to hypothalamic and other brain insulin receptors. Entry of circulating insulin into the brain is accomplished by a saturable receptor-mediated transendothelial transport system and is believed to be impaired in hyperinsulinemic, insulin-resistant, and hyperphagic obese Zucker rats. Theoretically, if hyperinsulinemia is decreased simultaneously while brain capillary insulin binding is increased, uptake of insulin into the brain can be enhanced leading to reduced food intake. To test this hypothesis, we administered diazoxide (DZ, 150 mg/kg/day), an inhibitor of glucose-mediated insulin secretion, or vehicle (control) to 7-week-old female obese and lean Zucker rats for 4 weeks (n = 24-28/subgroup-strain). Animals were assigned to either fasted (FD) or free-fed (FF) protocol for determination of plasma and cerebrospinal fluid (CSF) insulin and brain capillary insulin binding at the end of 4 weeks. DZ obese consumed fewer calories (P<0.01) and gained less weight than control obese (P<0.01), whereas DZ lean had similar amounts of caloric intake and weight gain compared with lean controls. DZ obese had lower fasting and random plasma glucose than control obese (P<0.05). FD and FF DZ-treated obese and lean rats had lower plasma insulin than their respective obese (P<0.01) and lean (P<0.01) controls. FD and FF DZ-treated obese rats demonstrated higher CSF insulin (P<0.05) and CSF/ plasma insulin ratio (P<0.01) than their controls, while only FF DZ lean animals showed higher CSF/plasma insulin ratio (P<0.01) than their controls (P<0.05). This was associated with enhanced brain capillary insulin binding in FD and FF DZ-treated obese (P<0.01) and lean (P<0.05) animals compared with their respective controls. It was concluded that DZ treatment in obese Zucker rats caused a decrease in insulin secretion and partially reversed impaired insulin binding to brain capillaries, leading to enhanced brain insulin uptake, and resulted in reduced food intake and weight gain observed in these animals.

Hypothalamic monoamines and insulin in relation to feeding in the genetically obese Zucker rat as revealed by microdialysis

Dynamic changes in VMH and PVN monoamines and immunoreactive insulin (IRI) were investigated by microdialysis in freely-moving genetically obese Zucker rats in order to relate possible disturbances to the impaired regulation of food intake of this model. Serotonin (5-HT), 5-HIAA and dopamine (DA) increased at the beginning of spontaneous meals while DOPAC decreased. Although similar in normal and obese rats, these changes were much more dramatic in the latter, as if more "signal" for satiety were necessary at the VMH-PVN level. Glucoprivic feeding or satiety are induced in normal rats by intravenous infusions of insulin or insulin+glucose respectively. The Zucker rat is resistant to these treatments. The monoaminergic changes brought about by these infusions were similar in obese and normal rats (decreases in 5-HT and DA and increases in 5-HIAA and DOPAC), but the occurrence of meals, in the obese, showed a superim-position of monoaminergic changes resembling those related to spontaneous feeding. The monoaminergic effects of insulin must therefore be dissociated from its effects on feeding. Hypothalamic insulin itself might be the brain signal. At the beginning of meals presented for the first time, VMH-PVN IRI increased earlier and with a smaller magnitude in the obese. When the rats were accustomed to scheduled meals, a similar anticipatory increase in IRI was found in both obese and lean rats. This suggests that brain insulin is more than a satiety signal. In addition, in response to an i.v. insulin infusion, IRI increased twice as much in obese rats despite lower basal levels. Whatever the origin of hypothalamic insulin, the larger response of the obese Zucker rat, known to be insulin resistant, may reflect the inefficiency of the peptide in reducing feeding and body weight in this pathological model.

Basal and hyperinsulinemia-induced immunoreactive hypothalamic insulin changes in lean and genetically obese Zucker rats revealed by microdialysis

Lean and genetically obese Zucker rats were implanted with permanent intravenous catheters and a guide cannula was aimed at the region of the ventromedial (VMH) and paraventricular (PVN) nuclei to measure immunoreactive insulin collected by means of microdialysis. Preliminary experiments assessed the validity of a novel assay of insulin in microdialysates by a sensitized radioimmunoassay technique. This method was then used to measure basal levels of insulin and those induced by i.v. infusion of 0.5 U of insulin over 30 min in both lean and obese rats. Basal hypothalamic immunoreactive insulin levels were lower in the obese rats than in the lean Zucker rats. When insulin was infused i.v. for 30 min, hypothalamic immunoreactive insulin showed an increase in the 30-60 min sample, which was twice as great in the obese rats. Two facts suggest that the insulin found in the microdialysates was of cerebral, not vascular origin: the short latency in the response and the finding that the response was greater in obese rats.

Food intake and estradiol effects on insulin binding in brain and liver

Three groups of ovariectomized rats were treated for 6 days: 1) estradiol benzoate (100 micrograms/kg) (SC) and fed ad lib; 2) vehicle-injected controls fed the same amount of food as eaten by estradiol-treated rats; 3) vehicle-injected, free-feeding controls. Specific binding of insulin to liver and hypothalamus slices was measured by quantitative film autoradiography. Estradiol-treated rats lost weight (p < 0.001) and had elevated plasma insulin (p < 0.01). Liver insulin binding in rats with estradiol treatment was greater (p < 0.01) than in rats without estradiol, but was less (p < 0.05) than in controls fed the same food levels as consumed by the estradiol-treated rats. Therefore, with equal food intake, estradiol decreased liver insulin binding. Insulin binding in the dorsomedial, ventromedial, and arcuate nuclei of the hypothalamus was unchanged by food intake or estradiol, however. Thus, altered insulin binding in the arcuate, ventromedial, or dorsomedial nuclei of the hypothalamus is probably not involved in the effects of insulin or estradiol on food intake.

Effect of diet-induced obesity and experimental hyperinsulinemia on insulin uptake into CSF of the rat

We examined the hypothesis that the uptake of plasma insulin into cerebrospinal fluid (CSF) is saturable in two rat models. Dietary obese and control female Osborne Mendel rats received 24-h infusions of vehicle or insulin. CSF insulin levels in cafeteria- and chow-fed rats were comparable at all levels of plasma insulin (4.5 +/- 2.8, 7.6 +/- 2.4, and 23.9 +/- 6.4 microU/ml in cafeteria diet vs. 4.5 +/- 0.9, 6.8 +/- 1.1, and 17.0 +/- 4.0 microU/ml in chow rats). CSF insulin uptake as a percentage of plasma insulin decreased with increased plasma insulin in both groups. A similar relationship was observed in Wistar rats receiving 6-day infusions of vehicle or insulin (plasma insulin = 55 +/- 12 vs. 365 +/- 98 microU/ml; CSF/plasma insulin ratio = 0.022 +/- .007 vs. 0.013 +/- .006, respectively). Hyperinsulinemic Wistar rats did not demonstrate decreased brain capillary insulin binding vs. vehicle-infused controls. The results suggest that a saturable transport process contributes insulin transport into CSF in normal rats and that this process is not altered by moderate diet-induced obesity or hyperinsulinemia per se.

The cellular and physiological actions of insulin in the central nervous system

Insulin is a peptide hormone involved in the regulation of glucose homeostasis. Its synthesis and function in the peripheral tissues have been extensively studied and well understood. In contrast, demonstration of insulin in the brain has raised questions concerning its origin and physiological significance. In spite of extensive studies, the source of insulin present in the brain has not yet been conclusively identified. Evidence exists in support of both peripheral and central origins of this hormone in the brain. Recognized physiological effects of insulin in the central nervous system (CNS) include regulation of food intake, control of glucose uptake and trophic actions on neuronal and glial cells. These actions of insulin are mediated by insulin receptor resembling closely that in peripheral tissues and coupled with tyrosine kinase signal transduction pathway. In this review we will discuss theories concerning the origin of insulin in the CNS. In addition, we will present current information on both cellular and physiological effects of this hormone in the brain.

Glucose utilization and insulin binding in discrete brain areas of obese rats

The present study was carried out to determine whether genetically obese Zucker rats present changes in brain glucose utilization and/or insulin binding when compared to their lean counterparts. Glucose utilization in the whole brain, determined by measurement of 2-deoxy(1-3H)glucose-6-phosphate, was significantly lower in obese than in lean Zucker rats. In order to precise the structure involved, we then used quantitative autoradiography methods after either (1-14C) 2-deoxyglucose injection or 125I-insulin incubation. In obese rats, local cerebral glucose utilization (LCGU) was significantly decreased in the external plexiform layer (-37%, p < 0.05), in the lateral hypothalamus (-23%, p < 0.05), and in the basolateral amygdaloid nucleus (-30%, p < 0.05). In contrast, no difference in specific insulin binding was found between the two genotypes in any of the areas studied. These results are consistent with some data showing a decrease of LCGU in hyperinsulinemic rats. All together, these data show perturbations of glucose utilization, particularly in structures linked to the regulation of body weight and food intake in obese Zucker rats.

Intraventricular neuropeptide Y injections stimulate food intake in lean, but not obese Zucker rats

We examined the effect of acute third intraventricular (IVT) injections of either saline or NPY (0.95, 3.0, 9.5, or 30.0 micrograms in 1 microliter) on the 1-, 4-, and 22-hour postinjection food and water intake of female obese (fa/fa), heterozygous lean (Fa/fa), and homozygous lean (Fa/Fa) Zucker rats. None of the doses of NPY had an effect on either food or water intake of fa/fa rats. A significant increase of food intake was seen in Fa/Fa rats at 1 and 4 hours after the 3.0 micrograms injection of NPY and at 1, 4, and 22 hours after the 9.5 micrograms injection of NPY. Both 3.0 and 9.5 micrograms of NPY also stimulated 1- and 4-hour postinjection food intake of Fa/fa rats, although this effect was significant only at 4 hours after the 3.0 micrograms dose. NPY had a less reliable effect on water intake; 3.0 micrograms of NPY stimulated 1-hour postinjection water intake of Fa/fa rats and 4-hour postinjection water intake of Fa/Fa rats. These results indicate that lean, but not obese Zucker rats, respond by eating more to centrally administered NPY. This deficit is similar to the effects seen with IVT insulin injections and may be a result of a common receptor-mediated mechanism.

Insulin in the cerebrospinal fluid

The presence, distribution and specific localization of insulin and its receptors in the central nervous system (CNS) have been described in numerous reports. Insulin in the CNS appears to be similar to pancreatic insulin by biochemical and immunological criteria. While the presence of insulin in the cerebrospinal fluid (CSF)--an essential neurohumoral transport system--has been widely reported, the available information is fragmented and therefore it is difficult to determine the significance of insulin in the CSF and to establish future research directions. This paper presents an integrative view of the studies concerning insulin in the CSF of various species including the human. Evidence suggests that insulin in the CSF and brain may be the result of local synthesis in the CNS, and uptake from the peripheral blood through the blood-brain barrier and circumventricular organs. The passage of insulin from the peripheral blood through the blood-brain barrier may be mediated by a specific transport system coupled to insulin receptors in cerebral microvessels. The transfer of insulin from the peripheral blood through the circumventricular organs is not specific and may depend on simple diffusion. Slow access of insulin to brain interstitial fluid adjacent to the blood-brain barrier and circumventricular organs may be followed by selective transport to other brain sites and into the ventricular-subarachnoideal CSF. It has been hypothesized that the choroid plexuses, which constitute the blood-CSF interface, might be a nonspecific pathway for rapid insulin transport into the CSF. Insulin may also pass from the CSF into the peripheral blood via absorption into the arachnoid villi. This evidence indicates that insulin may be transported in both directions between the CSF-brain and the peripheral blood. Evidence also suggests that the presence of insulin in the CSF is of pivotal importance for its neurophysiological or neuropathophysiological significance.

Food intake and meal patterns in rhesus monkeys: significance of chronic hyperinsulinemia

To investigate the role of plasma insulin on food intake, we have examined the effect of naturally occurring chronic hyperinsulinemia on the feeding behavior of male rhesus monkeys. Two groups of monkeys, a group with normal fasting insulin concentrations (52.4 +/- 2.2 microU/ml) (mean +/- SE) and a hyperinsulinemic group (148.6 +/- 14.5 microU/ml), were selected to be similar in weight, 13.0 +/- 1.0 and 15.3 +/- 0.5 kg, respectively, prior to study. Food intake and feeding patterns were recorded and analyzed. No differences in either daily caloric intake, 815.2 +/- 27.4 versus 890.0 +/- 64.2 kcal (p less than 0.32), or feeding patterns were found. The number of meals taken per day did not differ between the two groups, 8.7 +/- 1.7 versus 6.7 +/- 1.1 (p less than 0.35), nor did meal size differ, 129 +/- 16.5 versus 110.5 +/- 16.3 (p less than 0.45). We conclude that chronic endogenous hyperinsulinemia as it occurs naturally in some obese rhesus monkeys has no significant effect on daily feeding behavior.

Long-term maternal-fetal exposure to high-low insulin concentrations alter liver but not brain insulin receptors

We investigated the effects of long-term (5 to 6 days) in vivo exposure to hyperinsulinemia and hypoinsulinemia on maternal and fetal rat (18 to 20 days' gestation; term approximately 21 days) brain and liver insulin receptors. Further we studied the in vitro effects of long-term insulin exposure on cultured rabbit neuronal cell insulin receptors. Long-term glucose infusions to maternal rats resulted in maternal and fetal hyperglycemia and mild hyperinsulinemia, which decreases the liver insulin receptor abundance of the mother but increased that of the fetus. Streptozotocin-induced maternal diabetes resulted in maternal and fetal hyperglycemia with hypoinsulinemia (or near-normal insulin levels) and increased maternal and fetal liver insulin receptors. The maternal and fetal brain insulin receptor abundance failed to change at high or low insulin concentrations. Similarly long-term insulin (5 micrograms/ml) exposure failed to alter cultured neuronal cell insulin receptors as well. We conclude that perturbations in maternal and fetal circulating insulin concentrations fail to affect the brain insulin receptors. This protective phenomenon may be critical in maintaining the normal neuronal energy balance and functioning during certain disease states such as diabetes mellitus and hyperinsulinism.

Interaction of intracerebroventricular insulin and glucose in the regulation of the activity of sympathetic efferent nerves to brown adipose tissue in lean and obese Zucker rats

The effects of injection of insulin and glucose into the third cerebral ventricle on the firing rate of the sympathetic efferent nerves to intercapsular brown adipose tissue was investigated in anaesthetized lean and obese Zucker rats. Injection of insulin resulted in a dose-dependent (70-480 pmol) inhibition of nerve firing rate, whereas in combination with glucose (140 pmol of insulin and 139 nmol of glucose), insulin strongly potentiated the increase in firing rate seen with glucose alone. Although basal levels of nerve firing rates were lower in the obese rat, responses to insulin, glucose, and insulin plus glucose were qualitatively similar to those seen in the lean rat. These results are consistent with the hypothesis that insulin acts in the central nervous system as a physiological signal in the control of thermogenesis after feeding, and that this effector system is intact in the obese rat.

Insulin and insulin-like growth factor receptors in the nervous system

Insulin and the insulin-like growth factors (I and II) are homologous peptides essential to normal metabolism as well as growth. These peptide hormones are present in the brain, and, based on biosynthetic labeling studies as well as evidence for local gene expression, they are synthesized by nervous tissue as well as being taken up by the brain from the peripheral circulation. Furthermore, the presence of insulin and IGF receptors in the brain, on both neuronal and glial cells, also suggests a role for these peptides in the nervous system. Thus, these ligands affect brain electrical activity, either as neurotransmitters or as neuromodulators, altering the release and re-uptake of other neurotransmitters. The insulin and IGF-I and -II receptors found in the brain exhibit a lower molecular weight than corresponding receptors on peripheral tissues, primarily caused by alterations in glycosylation. Despite these alterations, both brain insulin and IGF-I receptors exhibit tyrosine kinase activity in cell-free systems, as do their peripheral counterparts. Brain insulin and IGF-I receptors are developmentally regulated, with the highest levels appearing in fetal or perinatal life. However, the altered glycosylation of brain receptors does not appear until late in fetal development. The receptors are widely distributed in the brain, but especially enriched in the circumventricular organs, choroid plexus, hypothalamus, cerebellum, and olfactory bulb. These studies on the insulin and IGF receptor in brain, add strong support to the suggestion that insulin and IGFs are important neuroactive substances, regulating growth, development, and metabolism in the brain.

Studies on the sympathetic efferent nerves of brown adipose tissue of lean and obese Zucker rats

Previous studies have suggested that the sympathetic tone to brown adipose tissue (BAT) is reduced in the genetically obese (fa/fa) rat. The following experiments were designed to examine with electrophysiological techniques the activity of the sympathetic nerve innervating the interscapular BAT. The spontaneous activity of the efferent nerves was reduced in the obese (fa/fa) rat compared with the lean control. The activity of the nerve showed a linear relationship with changes in core temperature in both genotypes. Electrical stimulation of the ventromedial hypothalamus resulted in similar heat increments in BAT temperature for lean and obese, but this was associated with a smaller increase in nerve firing in the obese rat. Intracerebroventricular administration of glucose enhanced the nerve activity, whereas 2-deoxy-D-glucose reduced the nerve activity in both lean and obese rats. These data suggest that the sympathetic tone is suppressed in the genetically obese rat, but the response to temperature and central glucose metabolism is intact.

Experimental obesity: a homeostatic failure due to defective nutrient stimulation of the sympathetic nervous system

The basic hypothesis of this review is that studies on models of experimental obesity can provide insight into the control systems regulating body nutrient stores in humans. In this homeostatic or feedback approach to analysis of the nutrient control system, we have examined the afferent feedback signals, the central controller, and the efferent control elements regulating the controlled system of nutrient intake, storage, and oxidation. The mechanisms involved in the beginning and ending of single meals must clearly be related to the long-term changes in fat stores, although this relationship is far from clear. Changes in total nutrient storage in adipose tissue can arise as a consequence of changes in the quantity of nutrients ingested in one form or another or a decrease in the utilization of the ingested nutrients. A change in energy intake can be effected by increased size of individual meals, increased number of meals in a 24-hour period, or a combination of these events. Similarly, a decrease in utilization of these nutrients can develop through changes in resting metabolic energy expenditure which are associated with one of more of the biological cycles such as protein metabolism, triglyceride for glycogen synthesis and breakdown, or maintenance of ionic gradients for Na+ + K+ across cell walls. In addition, differences in energy expenditure related to the thermogenesis of eating or to the level of physical activity may account for differences in nutrient utilization.

Circadian regulation of feeding in rats: suprachiasmatic versus ventromedial hypothalamic nuclei

The role of the suprachiasmatic nuclei as a major component of a specific circadian system controlling feeding periodicity is reviewed. Evidence is presented supporting the assumption that the ventromedial hypothalamus and the suprachiasmatic nucleus may act as a constant (tonic) regulator and a circadian modulator respectively of feeding in rats. It is concluded that a specific circadian system differing from the metabolic control mechanism superimposes the circadian periodicity of feeding. A model is put forward for the possible functional relationships between circadian and metabolic (homeostatic) control mechanisms of feeding in rats.

Blood-brain barrier transcytosis of insulin in developing rabbits

Previous studies with isolated brain microvessels have suggested that blood insulin is selectively transported through the brain capillary, i.e. the blood-brain barrier (BBB), by receptor-mediated transcytosis. The purpose of the present study is to demonstrate in vivo the uptake of circulating 125I-insulin by brain using thaw-mount autoradiography. However, metabolism of systemic 125I-insulin to 125I-tyrosine would allow for brain uptake of 125I-tyrosine and this would preclude interpretation of the autoradiogram. Therefore, the present studies were performed in developing rabbits, since plasma protein degradation of peptides is greatly reduced in developing animals. 125I-insulin was infused via the carotid artery at a rate of 0.25 ml/min for 1, 5, or 10 min, and the mean brain uptake, relative to a [3H]albumin reference, was 99.3 +/- 5.5%, 110.1 +/- 4.3%, and 143.6 +/- 7.9%, respectively. This uptake was saturable by simultaneously infusing unlabeled insulin. Thaw-mount autoradiography of rabbit brain after a 10-min infusion of 125I-insulin revealed silver grains in the pericapillary space and well within the brain parenchyma. HPLC analysis of acid-ethanol extracts of rabbit blood after a 10-min infusion showed virtually all of the 125I-radioactivity co-migrated with a known insulin standard on a reverse-phase column, indicating minimal degradation of infused 125I-insulin. HPLC analysis of brain radioactivity showed the major peak co-migrated with 125I-insulin and this peak was precipitated by an anti-insulin antiserum. The correlation of the transport data, the autoradiography, and the HPLC analysis support the model that brain insulin originates from blood via receptor-mediated transport of the peptide at the BBB.

Decreased binding to hypothalamic insulin receptors in young genetically obese rats

[125I]insulin binding to partially purified hypothalamic membranes is reduced during prolonged starvation, and changes in hypothalamic insulin binding capacity correlate well with spontaneous variations in energy balance in ground squirrels. To determine whether an insulin binding impairment in the central nervous system can be observed during the early expression of genetic obesity, both obese (fa/fa) and phenotypically lean (Fal-) Zucker rats were studied at 6 weeks of age. Hypothalamic tissue from fa/fa rats bound significantly less hormone than that from the lean animals, but binding was not changed in tissue from cerebral cortex. It is concluded that a defect in insulin binding to hypothalamic receptors in Zucker fatty rats may contribute to the development of weight gain in these animals.

Intraventricular insulin reduces food intake and body weight of lean but not obese Zucker rats

Porcine insulin (2 mU/rat/day) and its saline vehicle were infused into the third cerebral ventricle of female lean or obese Zucker rats using 14-day osmotic minipumps. Lean rats receiving saline (N = 6) gained 14 +/- 3 g over the 14 days, whereas lean rats receiving insulin (N = 7) lost 12 +/- 4 g over the same interval (p less than 0.01). The average total food intake of the insulin-infused group was decreased by 14% (p less than 0.05) as compared with that of the saline-infused group. The decreased caloric consumption was adequate to account for the body weight loss. Insulin infusion had no effect on food intake or body weight of the obese rats relative to their saline-infused controls (change in body weight: saline (N = 5), -14 +/- 23 g; insulin (N = 7), +3 +/- 14 g). These results suggest that genetically obese Zucker rats have reduced sensitivity to insulin in the central nervous system. We propose that this phenomenon may participate in the development and maintenance of hyperphagia and obesity in these animals.

Peripheral sympathectomy and adrenal medullectomy do not alter cerebrospinal fluid norepinephrine

Despite a blood-brain barrier for norepinephrine, the concentration of norepinephrine in plasma and cerebrospinal fluid has been observed to be similar. This relationship between plasma and cerebrospinal fluid norepinephrine levels suggest that peripheral sympathetic neurons innervating blood vessels to brain and spinal cord may contribute significantly to cerebrospinal fluid norepinephrine levels, and questions the validity of cerebrospinal fluid norepinephrine as an index of central nervous system noradrenergic activity. We demonstrate that extensive destruction of the peripheral sympathetic nervous system and the adrenal medulla has no effect on rat cerebrospinal fluid norepinephrine. It is therefore unlikely that peripheral sources of norepinephrine contribute significantly to cerebrospinal fluid norepinephrine levels.

A technique for repeated sampling of CSF from the anesthetized rat

A method for repeatedly sampling cerebrospinal fluid (CSF) from anesthetized rats is described. The technique reliably and quickly yields blood-free samples of CSF and requires supplies that are commonly available. Samples as large as 250 microliter can be collected in a few minutes. There is no apparent malaise even when CSF is withdrawn once every three days for two weeks. This technique offers an alternative to surgical cannulation for sampling of CSF from the rat.

Brain insulin binding is decreased in Wistar Kyoto rats carrying the 'fa' gene

We have previously reported that insulin binding is decreased in the olfactory bulb of both heterozygous (Fa/fa) and obese (fa/fa) Zucker rats. In the present study, we measured insulin binding in membranes prepared from the olfactory bulb, cerebral cortex, and hypothalamus of control (Fa/Fa) Wistar Kyoto rats; "fatty" (fa/fa) Wistar Kyoto rats; and phenotypically lean (Fa/?) Wistar Kyoto rats. Insulin binding was decreased in all brain regions, as well as the liver of the obese Wistar Kyoto fa/fa rats. Additionally, insulin binding was decreased in the liver and brain membranes from the Fa/? Wistar Kyoto rats. As most of the Fa/? rats were probably carriers of one 'fa' gene, but the population was only slightly hyperinsulinemic, we conclude that--as in the Zucker rat--it is the presence and expression of the 'fa' gene rather than downregulation which results in the decreased insulin binding. Thus, regulation of the brain insulin receptor appears to be independent of plasma or cerebrospinal fluid insulin levels.

Dependence of food intake on acute and chronic ventricular administration of insulin

Several lines of evidences indicate that insulin affords short- and long-term neuroendocrine signals to modulate ingestive behavior. To further study a possible role of insulin in the control of food intake, male Wistar rats were subjected to various intra-third cerebro-ventricular applications of saline and insulin. Infusion of 2.0 mIU/rat of insulin at 1100 and 1900 decreased food intake in a 23.5 hr test period. Infusion of 0.5 mIU/rat of insulin between 1100 and 1200 decreased nighttime food intake during the 1st and 2nd days. Infusion of 2.0 mIU/rat/24 hr of insulin from osmotic minipumps decreased nighttime food intake throughout the active pump period and the effect persisted into the post-pump period. The results support the notion that insulin is involved in the regulation of food intake in the rat.

Genetically obese Zucker rats have abnormally low brain insulin content

The concentration of immunoreactive insulin (IRI) extracted from the olfactory bulb, hypothalamus, hippocampus, cerebral cortex, amygdala, midbrain, and hindbrain was significantly lower in obese (fa/fa) and heterozygous (Fa/fa) Zucker rats in comparison to lean (Fa/Fa) Zucker rats. This deficit in brain IRI content was most severe in the hypothalamus and olfactory bulb and was independent of severe obesity since the marked reduction of brain IRI content was also found in heterozygous rats which possessed only one copy of the fa allele. These results demonstrate that in the 2-3 month-old female Zucker rat, the fa allele is associated with defective regulation of insulin in the brain.