Comparative measurements of glucose, beta-hydroxybutyrate, acetoacetate, and insulin in blood and cerebrospinal fluid during starvation

Characterization of histone lysine β-hydroxybutyrylation in bovine tissues, cells, and cumulus-oocyte complexes

Besides their canonical roles as energy sources, short-chain fatty acids act as metabolic regulators of gene expression through histone posttranslational modifications. Ketone body β-hydroxybutyrate (BHB) causes a novel epigenetic modification, histone lysine β-hydroxybutyrylation (Kbhb), which is associated with genes upregulated in starvation-responsive metabolic pathways. Dairy cows increase BHB in early lactation, and the effects of this increase on cellular epigenomes are unknown. We searched for and identified that Kbhb is present in bovine tissues in vivo and confirmed that this epigenetic mark is responsive to BHB in bovine and human fibroblasts cultured in vitro in a dose-dependent manner. Maturation of cumulus-oocyte complexes with high concentrations of BHB did not affect the competence to complete meiotic maturation or to develop until the blastocyst stage. BHB treatment strongly induced H3K9bhb in cumulus cells, but faintly in oocytes. RNA-seq analysis in cumulus cells indicated that BHB treatment altered the expression of 345 genes. The downregulated genes were mainly involved in glycolysis and ribosome assembly pathways, while the upregulated genes were involved in mitochondrial metabolism and oocyte development. The genes and pathways altered by BHB will provide entry points to carry out functional experiments aiming to mitigate metabolic disorders and improve fertility in cattle.

Insulin and endocannabinoids in the mesolimbic system

Easy access to palatable food and an abundance of food-related cues exacerbate non-homeostatic feeding. The metabolic and economical sequelae of non-homeostatic feeding outweigh those of homeostatic feeding and contribute significantly to the global obesity pandemic. The mesolimbic dopamine system is the primary central circuit that governs the motivation to consume food. Insulin and endocannabinoids (eCBs) are two major, presumably opposing, players in regulating homeostatic and non-homeostatic feeding centrally and peripherally. Insulin is generally regarded as a postprandial satiety signal, whereas eCBs mainly function as pre-prandial orexinergic signals. In this review, we discuss the effects of insulin and eCB-mediated actions within the mesolimbic pathways. We propose that insulin and eCBs have regional- and time course-dependent roles. We discuss their mechanisms of actions in the ventral tegmental area and nucleus accumbens, as well as how their mechanisms converge to finely tune dopaminergic activity and food intake.

Ketogenic Diet: an Endocrinologist Perspective

PURPOSE OF REVIEW

Obesity and its related comorbidities make up a large part of healthcare expenditures. Despite a wide array of options for treatment of obesity, rates of sustained weight loss continue to be low, leading patients to seek alternative treatment options. Although the first medically utilized ketogenic diet was described nearly 100 years ago, it has made a resurgence as a treatment option for obesity. Despite increased popularity in the lay public and increased use of ketogenic dietary strategies for metabolic therapy, we are still beginning to unravel the metabolic impact of long-term dietary ketosis.

RECENT FINDINGS

There are a number of recent trials that have highlighted the short- and long-term benefits of ketogenic diet on weight, glycemic control, and other endocrine functions including reproductive hormones. This review is a summary of available data on the effectiveness and durability of the ketogenic diet when compared to conventional interventions. Ketogenic dietary strategies may play a role in short-term improvement of important metabolic parameters with potential for long-term benefit. However, response may vary due to inter-individual ability to maintain long-term carbohydrate restriction.

Is insulin intoxication still the perfect crime? Analysis and interpretation of postmortem insulin: review and perspectives in forensic toxicology

Insulin is an anabolic hormone essential to glucose homeostasis. Insulin therapy, comprising human insulin (HI) or biosynthetic analogs, is critical for the management of type-1 diabetes and many of type-2 diabetes. However, medication error including non-adapted dose and confusion of insulin type, and misuse, such as massive self-administration or with criminal intent, can have lethal consequences. The aim of this paper is to review the state of knowledge of insulin analysis in biological samples and of the interpretation of insulin concentrations in the situation of insulin-related death investigations. Analytic aspects are considered, as quantification can be strongly impacted by methodology. Immunoanalysis, the historical technique, has a prominent role due to its sensitivity and ease of implementation. Recently, liquid chromatography coupled to mass spectrometry has provided indispensable selectivity in forensic contexts, distinguishing HI, analogs, and degradation products. We review the numerous antemortem (dose, associated pathology, injection-to-death interval, etc.) and postmortem parameters (in corpore degradation, in vitro degradation related to hemolysis, etc.) involved in the interpretation of insulin concentration. The interest and limitations of various alternative matrices providing a valuable complement to blood analysis are discussed. Vitreous humor is one of the most interesting, but the low diffusion of insulin in this matrix entails very low concentrations. Injection site analysis is relevant for identifying which type of insulin was administered. Muscle and renal cortex are matrices of particular interest, although additional studies are required. A table containing most case reports of fatal insulin poisoning published, with analytical data, completes this review. A logic diagram is proposed to highlight analytical issues and the main parameters to be considered for the interpretation of blood concentrations. Finally, it remains a challenge to provide reliable biological data and solid interpretation in the context of death related to insulin overdose. However, the progress of analytical tools is making the "perfect crime" ever more difficult to commit.

Utility of Postmortem Vitreous Beta-Hydroxybutyrate Testing for Distinguishing Sudden from Prolonged Deaths and for Diagnosing Ketoacidosis

A retrospective, cross-sectional analysis of vitreous beta-hydroxybutyrate (BHB) on 967 forensic cases over a two-year period was conducted. Cases were sorted into six categories of death: (i) sudden traumatic/non-natural (ST), (ii) sudden natural (SN), (iii) prolonged traumatic/non-natural (PT), (iv) prolonged natural (PN), (v) diabetic ketoacidosis (DKA), and (vi) alcoholic ketoacidosis (AKA). The mean BHB for all cases was 1.67 mmol/L (17.4 mg/dL; range: 0.11-18.02 mmol/L). The numbers of DKA, AKA, PN, PT, SN, and ST deaths were 21, 5, 155, 258, 275, and 253, respectively. Their mean vitreous BHBs were as follows: 11.04 mmol/L (DKA), 8.88 mmol/L (AKA), 1.56 mmol/L (PN), 1.55 mmol/L (PT), 1.26 mmol/L (SN), and 1.38 mmol/L (ST). There was a statistically significant difference between the mean BHBs of the PN and SN death groups (p < 0.001), as well as between those of the PT and ST death groups (p = 0.004). Given the overlapping ranges seen between the prolonged and sudden death groups, the identified differences did not hold clinical significance. In addition, we sought to determine a threshold value for vitreous BHB to definitely diagnose cases of ketoacidosis. BHB threshold concentrations between 2.5 and 5 mmol/L produced sensitivities >92% and specificities >96%. A receiver operator characteristic curve found 3.43 mmol/L to be the optimal cutoff value, demonstrating a specificity of 98.3% and a sensitivity of 96.2%.

Modulation of cerebral ketone metabolism following traumatic brain injury in humans

Adaptive metabolic response to injury includes the utilization of alternative energy substrates - such as ketone bodies (KB) - to protect the brain against further damage. Here, we examined cerebral ketone metabolism in patients with traumatic brain injury (TBI; n = 34 subjects) monitored with cerebral microdialysis to measure total brain interstitial tissue KB levels (acetoacetate and β-hydroxybutyrate). Nutrition - from fasting vs. stable nutrition state - was associated with a significant decrease of brain KB (34.7 [10th-90th percentiles 10.7-189] µmol/L vs. 13.1 [6.5-64.3] µmol/L, p < 0.001) and blood KB (668 [168.4-3824.9] vs. 129.4 [82.6-1033.8] µmol/L, p < 0.01). Blood KB correlated with brain KB (Spearman's rho 0.56, p = 0.0013). Continuous feeding with medium-chain triglycerides-enriched enteral nutrition did not increase blood KB, and provided a modest increase in blood and brain free medium chain fatty acids. Higher brain KB at the acute TBI phase correlated with age and brain lactate, pyruvate and glutamate, but not brain glucose. These novel findings suggest that nutritional ketosis was the main determinant of cerebral KB metabolism following TBI. Age and cerebral metabolic distress contributed to brain KB supporting the hypothesis that ketones might act as alternative energy substrates to glucose. Further studies testing KB supplementation after TBI are warranted.

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.

Serial changes in plasma ketone concentrations in patients with acute brain injury

Objective Acute brain injury (ABI) is a catastrophic event, leading to disruption of the normal cerebral metabolic pathways and a subsequent cerebral energy deficit. Ketones (beta-hydroxybutyrate (BHB) and acetoacetate) may represent an alternative metabolic substrate with the potential to improve cerebral energy supply and decrease injury. The purpose of this study was to evaluate baseline ketone concentrations in the ABI population. Methods Thirty-eight patients with ABI were enrolled into the study and followed for up to 7 days. We collected arterial blood samples immediately after admission and daily to measure the levels of BHB and acetoacetate. Where possible, matching cerebrospinal fluid (CSF) specimens were also collected. Results During the study period, plasma BHB levels were increased initially but normalized by day 3 while acetoacetate levels remained within the normal range. The change in BHB was significant. There were 30 observations in 10 patients where BHB could be measured in both blood and CSF. When the data were averaged over patients there was a weak correlation between blood and CSF BHB (Spearman's ρ = 0.62, p = 0.054). Conclusion Blood ketone concentrations remain low within the ABI population. An external source of ketones will be required to increase blood concentrations to clinically relevant levels.

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.

Regulation of gonadotropin secretion by monitoring energy availability

Nutrition is a principal environmental factor influencing fertility in animals. Energy deficit causes amenorrhea, delayed puberty, and suppression of copulatory behaviors by inhibiting gonadal activity. When gonadal activity is impaired by malnutrition, the signals originating from an undernourished state are ultimately conveyed to the gonadotropin-releasing hormone (GnRH) pulse generator, leading to suppressed secretion of GnRH and luteinizing hormone (LH). The mechanism responsible for energetic control of gonadotropin release is believed to involve metabolic signals, sensing mechanisms, and neuroendocrine pathways. The availabilities of blood-borne energy substrates such as glucose, fatty acids, and ketone bodies, which fluctuate in parallel with changes in nutritional status, act as metabolic signals that regulate the GnRH pulse generator activity and GnRH/LH release. As components of the specific sensing system, the ependymocytes lining the cerebroventricular wall in the lower brainstem integrate the information derived from metabolic signals to control gonadotropin release. One of the pathways responsible for the energetic control of gonadal activity consists of noradrenergic neurons from the solitary tract nucleus in the lower brainstem, projecting to the paraventricular nucleus of the hypothalamus. Further studies are needed to elucidate the mechanisms underlying energetic control of reproductive function.

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.

Arcuate nucleus injection of an anti-insulin affibody prevents the sympathetic response to insulin

Accumulating evidence suggests that insulin acts within the hypothalamus to alter sympathetic nerve activity (SNA) and baroreflex function. Although insulin receptors are widely expressed across the hypothalamus, recent evidence suggests that neurons of the arcuate nucleus (ARC) play an important role in the sympathoexcitatory response to insulin. The purpose of the present study was to determine whether circulating insulin acts directly in the ARC to elevate SNA. In anesthetized male Sprague-Dawley rats (275-425 g), the action of insulin was neutralized by microinjection of an anti-insulin affibody (1 ng/40 nl). To verify the efficacy of the affibody, ARC pretreatment with injection of the anti-insulin affibody completely prevented the increase in lumbar SNA produced by ARC injection of insulin. Next, ARC pretreatment with the anti-insulin affibody attenuated the lumbar sympathoexcitatory response to intracerebroventricular injection of insulin. Third, a hyperinsulinemic-euglycemic clamp increased lumbar, but not renal, SNA in animals that received ARC injection of a control affibody. However, this sympathoexcitatory response was absent in animals pretreated with the anti-insulin affibody in the ARC. Injection of the anti-insulin affibody in the adjacent ventromedial hypothalamus did not alter the sympathoexcitatory response to insulin. The ability of the anti-insulin affibody to prevent the sympathetic effects of insulin cannot be attributed to a general inactivation or nonspecific effect on ARC neurons as the affibody did not alter the sympathoexcitatory response to ARC disinhibition by gabazine. Collectively, these findings suggest that circulating insulin acts within the ARC to increase SNA.

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.

Consumption of a high-fat diet induces central insulin resistance independent of adiposity

Plasma insulin enters the CNS where it interacts with insulin receptors in areas that are related to energy homeostasis and elicits a decrease of food intake and body weight. Here, we demonstrate that consumption of a high-fat (HF) diet impairs the central actions of insulin. Male Long-Evans rats were given chronic (70-day) or acute (3-day) ad libitum access to HF, low-fat (LF), or chow diets. Insulin administered into the 3rd-cerebral ventricle (i3vt) decreased food intake and body weight of LF and chow rats but had no effect on HF rats in either the chronic or the acute experiment. Rats chronically pair-fed the HF diet to match the caloric intake of LF rats, and with body weights and adiposity levels comparable to those of LF rats, were also unresponsive to i3vt insulin when returned to ad libitum food whereas rats pair-fed the LF diet had reduced food intake and body weight when administered i3vt insulin. Insulin's inability to reduce food intake in the presence of the high-fat diet was associated with a reduced ability of insulin to activate its signaling cascade, as measured by pAKT. Finally, i3vt administration of insulin increased hypothalamic expression of POMC mRNA in the LF- but not the HF-fed rats. We conclude that consumption of a HF diet leads to central insulin resistance following short exposure to the diet, and as demonstrated by reductions in insulin signaling and insulin-induced hypothalamic expression of POMC mRNA.

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.

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.

Pyruvate carboxylase deficiency: clinical and biochemical response to anaplerotic diet therapy

A six-day-old girl was referred for severe hepatic failure, dehydratation, axial hypotonia, and both lactic acidosis and ketoacidosis. Biotin-unresponsive pyruvate carboxylase deficiency type B was diagnosed. Triheptanoin, an odd-carbon triglyceride, was administrated as a source for acetyl-CoA and anaplerotic propionyl-CoA. Although this patient succumbed to a severe infection, during the six months interval of her anaplerotic and biochemical management, the following important observations were documented: (1) the immediate reversal (less than 48 h) of major hepatic failure with full correction of all biochemical abnormalities, (2) on citrate supplementation, the enhanced export from the liver of triheptanoin's metabolites, namely 5 carbon ketone bodies, increasing the availability of these anaplerotic substrates for peripheral organs, (3) the demonstration of the transport of C5 ketone bodies-representing alternative energetic fuel for the brain-across the blood-brain barrier, associated to increased levels of glutamine and free gamma-aminobutyric acid (f-GABA) in the cerebrospinal fluid. Considering that pyruvate carboxylase is a key enzyme for anaplerosis, besides the new perspectives brought by anaplerotic therapies in those rare pyruvate carboxylase deficiencies, this therapeutic trial also emphasizes the possible extended indications of triheptanoin in various diseases where the citric acid cycle is impaired.

Reduced anorexic effects of insulin in obesity-prone rats fed a moderate-fat diet

Rats prone to develop diet-induced obesity (DIO) have reduced central sensitivity to many metabolic and hormonal signals involved in energy homeostasis. High-fat diets produce similar defects in diet-resistant (DR) rats. To test the hypothesis that genotype and diet exposure would similarly affect central insulin signaling, we assessed the anorectic effects of 8 mU third ventricular (iv3t) insulin before and after 4 wk intake of a 31% fat, high-energy (HE) diet intake in outbred (OutB) rats. Rats were retrospectively designated as DR or DIO by their low or high weight gains on HE diet. Before the HE diet, iv3t insulin reduced 4-h and 24-h chow intake by 53% and 69% in DR rats but by only 17% and 27% in DIO rats, respectively. Also, the anorectic response to iv3t insulin in OutB rats was inversely correlated (r = 0.72, P = 0.002) with subsequent 4-wk weight gain on the HE diet. Similarly, in selectively bred (SB) chow-fed DR rats, 8 mU iv3t insulin reduced 4-h and 24-h intake by 21% and 22%, respectively, but had no significant effect in SB DIO rats. Four-week HE diet intake reduced 4-h and 24-h insulin-induced anorexia by 45% in OutB DR rats and completely abolished it in SB DR rats. Reduced insulin responsiveness was unassociated with differences in arcuate nucleus insulin receptor mRNA expression between DIO and DR rats or between rats fed chow or HE diet. These data suggest that DIO rats have a preexisting reduction in central insulin signaling, which might contribute to their becoming obese on the HE diet. However, since the HE diet reduced central insulin sensitivity in DR rats but did not make them obese, it is likely that other brain areas are involved in insulin's anorectic action or that other pathways contribute to the development and maintenance of obesity.

Effects of the ketogenic diet in the glucose transporter 1 deficiency syndrome

The ketogenic diet (KD), established to treat intractable childhood epilepsy, has emerged as the principal treatment of GLUT1 deficiency syndrome (OMIM 606777). This defect of glucose transport into the brain results in hypoglycorrhachia causing epilepsy, developmental delay, and a complex motor disorder in early childhood. Ketones provided by a high-fat, low-carbohydrate diet serve as an alternative fuel to the brain. Glucose, lactate, lipids, and ketones in blood and cerebrospinal fluid were investigated in five GLUT1-deficient patients before and on the KD. Hypoglycorrhachia was detected in the non-ketotic and ketotic state. In ketosis, lactate concentrations in the cerebrospinal fluid increased moderately. The CSF/blood ratio for acetoacetate was higher compared to beta-hydroxybutyrate. Free fatty acids did not enter the brain in significant amounts. Blood concentrations of essential fatty acids determined in 18 GLUT1-deficient patients on the KD were sufficient in all age groups. The effects of the KD in GLUT1 deficiency syndrome, particularly the course of blood lipids, are discussed in an illustrative case. In this syndrome, the KD effectively restores brain energy metabolism. Ketosis does not influence impaired GLUT1-mediated glucose transport into brain: hypoglycorrhachia, the biochemical hallmark of the disease, can be identified in GLUT1-deficient patients on a KD. The effects of ketosis on the concentrations of glucose, lactate, ketones, and fatty acids in blood and cerebrospinal fluid in this entity are discussed in view of previous data on ketosis in man.

Calorie restriction of a high-carbohydrate diet elevates the threshold of PTZ-induced seizures to values equal to those seen with a ketogenic diet

The purpose of this study was to evaluate the contributions of ketonemia, caloric restriction, and carbohydrates to seizure protection in rats fed selected diets. Male Sprague-Dawley rats were fed experimental diets of two basic types, one high in carbohydrates and restricted to 90, 65, or 50% of the normal daily caloric requirement and the other a normal rodent chow diet restricted to 90 or 65% of the daily caloric requirement. After consuming their respective diets for 20 days, animals were subjected to tail-vein infusion of pentylenetetrazole (PTZ) to determine seizure threshold, taken as the dose required to evoke the first clonic reaction. Seizure thresholds were compared to those of rats fed control diets of either normal rodent chow fed ad libitum or a standard high-fat (ketogenic) diet calorie-restricted to 90% of daily caloric requirement, all animals age- and weight-matched at the time of diet onset. All diets were balanced for vitamins and minerals and contained at least 10% protein (by weight). Seizure threshold and ketonemia were elevated in both experimental diets in approximate proportion to the degree of calorie restriction. Animals fed the most severely restricted high-carbohydrate diet (50%) had seizure thresholds equal to those fed the ketogenic diet but had significantly lower ketonemia.

Caloric restriction inhibits seizure susceptibility in epileptic EL mice by reducing blood glucose

PURPOSE

Caloric restriction (CR) involves underfeeding and has long been recognized as a dietary therapy that improves health and increases longevity. In contrast to severe fasting or starvation, CR reduces total food intake without causing nutritional deficiencies. Although fasting has been recognized as an effective antiseizure therapy since the time of the ancient Greeks, the mechanism by which fasting inhibits seizures remains obscure. The influence of CR on seizure susceptibility was investigated at both juvenile (30 days) and adult (70 days) ages in the EL mouse, a genetic model of multifactorial idiopathic epilepsy.

METHODS

The juvenile EL mice were separated into two groups and fed standard lab chow either ad libitum (control, n=18) or with a 15% CR diet (treated, n=17). The adult EL mice were separated into three groups; control (n=15), 15% CR (n=6), and 30% CR (n=3). Body weights, seizure susceptibility, and the levels of blood glucose and ketones (beta-hydroxybutyrate) were measured over a 10-week treatment period. Simple linear regression and multiple logistic regression were used to analyze the relations among seizures, glucose, and ketones.

RESULTS

CR delayed the onset and reduced the incidence of seizures at both juvenile and adult ages and was devoid of adverse side effects. Furthermore, mild CR (15%) had a greater antiepileptogenic effect than the well-established high-fat ketogenic diet in the juvenile mice. The CR-induced changes in blood glucose levels were predictive of both blood ketone levels and seizure susceptibility.

CONCLUSIONS

We propose that CR may reduce seizure susceptibility in EL mice by reducing brain glycolytic energy. Our preclinical findings suggest that CR may be an effective antiseizure dietary therapy for human seizure disorders.

Fasting studies in cerebrospinal fluid and blood in children with epilepsy of unknown origin

Alterations in the cerebral energy supply are likely to cause cerebral function disturbances. Fasting is a suitable method for studying the energy metabolism. As the cerebrospinal fluid (CSF) compartment reflects the brain metabolism, data in CSF might give information about the metabolism of fuel substrates in brain. We compared the biochemical data on several fuel-related components in blood and CSF at the end of a 40-hours fast of epileptic children with unknown origin of epilepsy (aged 6-15 years) with the values of a reference group of children. In children with primary generalized epilepsy no abnormalities were found. In children with complex partial epilepsy many significant abnormalities were found, such as low blood lactate and alanine and low CSF ketones and CSF blood ratio for ketones. The possible significance of the observed abnormalities are discussed.

Chronic intrahypothalamic infusions of insulin or insulin antibodies alter body weight and food intake in the rat

In Experiment 1, one-week infusion of insulin (0.15, 1.5, or 15.0 microU/hr) into the ventromedial hypothalamus (VMH) of rats reduced body weight (BW) and nighttime food intake (FI). While 0.15 microU/h decreased daytime FI, 1.5 microU/h increased daytime FI and 15.0 microU/h left daytime FI unchanged. Total daily FI was decreased by the two highest doses. In Experiment 2, intra-VMH infusion of specific insulin antibodies (1.5 microUeq/h) increased BW and FI, while C-peptide antibodies were ineffective. In Experiment 3a, intracerebroventricular infusions of insulin failed to decrease FI and BW comparably to similar intrahypothalamic infusions. In Experiment 3b, intra-VMH insulin was infused via cannulae that bypassed the cerebral ventricles. The decrease in FI and BW was comparable to that observed when insulin was infused via cannulae that penetrated a ventricle. Histology from animals used in Experiments 1-3 indicates that optimum sites for insulin-induced changes in BW and FI in the hypothalamus lie in an area that includes portions of the paraventricular, arcuate, dorsomedial, and ventromedial nuclei.

Food intake and serum insulin responses to intraventricular infusions of insulin and IGF-I

Previous studies reported that intracerebroventricular (ICV) infusion of insulin decreased food intake in rats and baboons. Insulin can bind to insulin-like growth factor I (IGF-I) receptors and mimic the response of IGF-I. Our objective was to determine the effects of ICV infused-insulin or IGF-I on food intake in sheep. In the present study, a 6-day ICV infusion of insulin (123 ng/kg of body weight/day) but not of IGF-I (123 ng/kg of body weight/day) decreased food intake by 40% (p less than 0.003) and body weight (p less than 0.015) compared with control sheep. In addition, sheep that received ICV insulin or IGF-I had only half the concentration of insulin in serum as compared with controls. Our results support the hypothesis that ICV insulin does not decrease food intake through IGF-I receptors. Nevertheless, apparently both insulin and IGF-I in the brain can influence the concentration of insulin in blood.

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.

CSF concentration and CSF/blood ratio of fuel related components in children after prolonged fasting

In order to obtain information about blood and cerebrospinal fluid (CSF) concentrations, and CSF/blood ratio data of fuel related substrates at the end of a prolonged fast in children, we have selected biochemical data from fasting test procedures in 11 control children aged 3-5 yr, fasted 24 h, and 58 control children aged 6-15 yr, fasted 40 h. There was a good correlation between blood and CSF concentrations for glucose, acetoacetate and beta-hydroxybutyrate. The relation with age and sex has been analyzed only in the older children. CSF and blood values for glucose are positively related with age, and both ketones are negatively related with age. Lactate, pyruvate and alanine concentrations in blood and CSF are not related with age, except for CSF pyruvate. With respect to the CSF/blood ratio for the above mentioned components, only the value for acetoacetate is sex and age related. The calculated median caloric values for the sum of glucose, lactate, pyruvate and ketones in CSF are independent of age at the end of a 40-h fast. The diminished glucose contribution on the CSF caloric homeostasis in younger children is fully compensated by the ketone bodies.

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.

The role of glucose, insulin and glucagon in the regulation of food intake and body weight

Glucose and related pancreatic hormones play a major role in the metabolism of monogastric mammals yet their influence on hunger and/or satiety is, as yet, poorly understood. Glucose, insulin and glucagon rise during a meal and gradually decline to baseline levels shortly after a meal. A sudden drop in plasma glucose as well as insulin have been reported just prior to the onset of a meal but the functional significance of this is not yet clear. Systemic injections of glucose have no acute satiety effects but intraduodenal and intrahepatic infusions reduce food intake and free-feeding and deprived animals respectively. Treatments which decrease cellular glucose utilization directly (2-DG) or indirectly (insulin) increase food intake while exogenous glucagon (which produces hyperglycemia) decreases it. There is considerable evidence that some or all of these effects may be due to a direct central action of glucose, 2-DG, insulin, and glucagon on brain mechanisms concerned with the regulation of hunger and satiety although influences on peripheral "glucoreceptors" have been demonstrated as well. The functional significance of glucoprivic feeding is, however, questioned. The feeding response to 2-DG and related compounds is capricious, and its temporal course does not parallel the hyperglycemic reaction which presumably reflects cellular glucopenia. Moreover, numerous brain lesions which increase, decrease, or have no effect on ad lib intake and often have no effect on the response to deprivation have been shown to severely impair or abolish feeding responses to systemic injections of 2-DG that produce severe central as well as peripheral glucopenia. Feeding responses to insulin are intact after most of these lesions, suggesting that this hormone may influence food intake in a fundamentally different fashion. The mechanism of insulin action is not understood--the classic feeding response is obtained only with doses that are pharmacological when compared to normal plasma levels and there is increasing evidence that lower doses may have opposite, inhibitory effects on food intake and body weight. Relatively small doses of glucagon decrease food intake (although opposite facilitatory effects have been reported after even smaller doses) but the effect does not appear to be due to hepatic mobilization of glucose as initially assumed. Decreases in food intake after intracranial injections of very small doses suggest a direct central action.

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.

The concentration of blood components related to fuel metabolism during prolonged fasting in children

In order to study the relationship between sex, age and glucose, and the concentrations of various fuel related blood substrates in children during prolonged fasting, we have selected data of fasting procedures in 13 control children aged 3-5 yr, fasted 24 h, and 58 control children aged 6-15 yr, fasted 40 h. Compared to the blood results after overnight fast, glucose is decreased, and lactate, pyruvate, ketones and non-esterified fatty acids (NEFA's) are all clearly increased at the end of fast. The concentrations of alanine and triglycerides remain unchanged. The relation with sex, age and glucose has only been analyzed in the older children group. A sex-dependency is indicated for the ketones. Ketones are negatively related with age. NEFA's pyruvate and alanine are not age-related, whereas glucose, lactate and triglycerides are moderately age-dependent. Ketones are negatively related with glucose, whereas pyruvate, NEFA's and triglycerides are not glucose-related. Lactate and alanine are weakly related to glucose. The data demonstrate diminished glucose homeostasis and increased ketogenesis in younger children compared to older ones during prolonged fasting.

Metabolic changes during maturation of male monkeys: possible signals for onset of puberty

There is a close relationship between the metabolic status of a maturing animal and the timing of puberty onset. However, the signals linking metabolic status to the maturation of the reproductive axis remain unknown. We looked for metabolic differences before and after puberty by comparing plasma profiles of insulin, glucose, amino acids, beta-hydroxybutyrate, and glycerol between juvenile and adult monkeys in fed and fasted states. Thirteen juvenile and 13 adult male crab-eating macaques (Macaca fascicularis) were fed a mixed meal, and blood samples were collected at intervals between 1.5 and 52 h after the meal. Plasma insulin concentrations decreased in a similar manner in both groups during the first 16 h of fasting. By 20 h after a meal, basal insulin levels were significantly lower (P less than 0.025) in juveniles compared with adults and remained so until the end of the fast. Circulating levels of glucose were similar in juveniles and adults immediately after a meal and then decreased significantly (P less than 0.025) in juveniles by 28 h of fasting and in adults by 52 h of fasting. Plasma concentrations of all large neutral amino acids (i.e., tyrosine, tryptophan, phenylalanine, valine, leucine, and isoleucine, LNAA) except tryptophan decreased more precipitously in juveniles than in adults during the first 20 h of fasting. However, the ratios of tyrosine to other LNAA and tryptophan to other LNAA were similar in juveniles and adults at all times. beta-Hydroxybutyrate concentrations were low in both groups until 24 h after a meal, at which time plasma levels increased more rapidly and attained higher values in juveniles compared with adults.(ABSTRACT TRUNCATED AT 250 WORDS)

The anorectic effect of satietin is unrelated to carbohydrate metabolism

The effect of satietin and amphetamine on the carbohydrate metabolism of free fed and food deprived rats was studied. Rats deprived of food for 96 hours maintained normal glucose and glucagon blood levels but the blood concentration of insulin dropped from 232.02 +/- 23.93 to 12.48 +/- 0.71 pmol/l. Amphetamine (500 micrograms/animal, intracerebroventricularly) left in normally fed rats the blood concentration of glucose, insulin and glucagon unchanged. The same treatment, however, increased the insulin concentration in the blood of food deprived rats from 11.37 +/- 4.43 to 73.47 +/- 8.29 pmol/l. Glucose and glucagon, as well as insulin levels remained unchanged in both normally fed and food deprived rats when treated with satietin (20 micrograms/rat, intracerebroventricularly). It was concluded that the anorectic effect of satietin is unrelated to carbohydrate metabolism.

Reference values of blood components related to fuel metabolism in children after an overnight fast

The interrelation between blood components, involved in fuel metabolism, and age, sex and glucose was studied in 72 control children (26 girls and 46 boys, aged between 3 and 15 yr) after an overnight fast (14 h). Glucose, lactate, pyruvate, triglycerides and cholesterol are age-independent. Alanine is positively correlated, whereas beta-hydroxybutyrate, acetoacetate and NEFA's are negatively correlated with age. With respect to blood sugar, acetoacetate, NEFA's and cholesterol are glucose-independent. Lactate, pyruvate, alanine and triglycerides are positively correlated with glucose, and beta-hydroxybutyrate--and total ketone bodies--are negatively correlated with glucose. Except for triglycerides, no differences in the concentrations of the above mentioned blood substrates are seen between boys and girls. These data demonstrate that after an overnight fast lipolysis and ketogenesis already are active in young children, probably related to inadequate gluconeogenesis and improvement of carbohydrate regulation with age.

Satietin; a 50,000 dalton glycoprotein in human serum with potent, long-lasting and selective anorectic activity

Satietin, a 50,000 dalton anorectic glycoprotein was isolated from human serum. Its isoelectric point is 7.0. It contains 14-15% amino acids and 70-75% carbohydrates. Its biological activity survives digestion with proteases and boiling. Satietin is a highly potent anorectic substance. The intracerebroventricular administration of 10-20 micrograms satietin suppresses food intake in rats during the first day of feeding after deprivation of food for 96 hours to half of the amount eaten by untreated controls (ID50). The onset of the effect can be detected within 30 minutes, the peak effect is reached within an hour. The effect lasts 24-30 hours. Satietin acts both at intravenous and subcutaneous administration (ID50 = 0.5-0.75 mg/kg) in rats deprived of food for 96 hours. The peak effect is reached within an hour and lasts over 24 hours. In contrast to the anorectic drugs in clinical use and to the endogenous anorectic substances (like cholecystokinin and calcitonin) satietin proved to be highly selective in suppressing food intake. Considering that satietin is widely distributed in the world of vertebrates, its concentration in the blood is amazingly high, its site of effect is in the central nervous system and it induces satiety without having any other detectable central or peripheral effect, the hypothesis was forwarded that satietin may play the role of a rate limiting blood-borne satiety signal in the negative feedback of food intake, i.e. serving as the essential chemical link connecting the gastrointestinal tract and the brain in the regulation of feeding.

Reduction of food intake and body weight by chronic intraventricular insulin infusion

This study examined the effect of chronic infusions of insulin in one of three doses (5, 7.5 or 10 mU/day) into the third ventricle, on food and water intake and body weight in the rat. Solutions were infused via osmotic minipumps at a rate of 1 microliter/hour for seven days. The two highest doses of insulin produced a dose-related suppression of food intake and weight loss, which was greater than the effect produced by 5 mU/day or a control infusion of Ringers solution. The effect of 5 mU/day on food and water intake and body weight was similar to the effect of the control infusion. All groups treated with insulin decreased food intake during the day and night, although only differences in nighttime food intake were statistically significant. Ten mU/day also produced a significantly greater reduction in water intake than each of the other solutions. Weight loss in the animals infused with insulin could not be explained by a decrease in caloric intake alone. Food intake returned to normal in all groups by the end of a seven day post-infusion period, with recovery being slowest among the animals receiving the highest doses of insulin. All animals recovered body weight at approximately the same rate. These results provide further evidence for the view that brain insulin plays a role in the regulation of food intake and body weight.

Insulin in the central nervous system

In summary, before hypothesizing synthesis of insulin in nonpancreatic tissues, one must determine with some accuracy the insulin concentrations in tissues such as the brain of various species or in IM-9 lymphocytes, or of non-guinea pig insulin in guinea pig tissues. If the concentrations are no more than a few percentage points of the levels initially reported by the NIH laboratory (Havrankova et al., 1978, 1979; Rosenzweig et al., 1980a,b), then some explanation should be given for the erroneously high concentrations that they earlier reported. If the very much lower concentrations that we have reported (Eng and Yalow, 1979, 1980, 1981, 1982; Bauman et al., 1982) are the true levels, then attempts to demonstrate synthesis in extrapancreatic tissues either by amino acid incorporation or by the methodology that has been described by Giddings et al. (1982) are doomed to failure. Our observations that transfer from the periphery can result in insulin levels in the brains of small-brained but not of large-brained animals comparable to or even, on occasion, higher than plasma levels when plasma levels are falling can account for our earlier observations (Eng and Yalow, 1979, 1980) that in rat but not in dog or rabbit brain insulin concentrations may be comparable to plasma levels. Furthermore, the absence of mechanisms in nonendocrine cells for the complex processing of insulin precursors to the 6000-dalton peptide and the absence of proinsulin in the extracts of the variety of tissues reported from the NIH laboratory suggest that the insulin found in these extracts was ultimately derived from pancreatic insulin.

Regulation of food intake and body weight by insulin

A feedback system for the regulation of food intake and body weight, consisting of two elements is proposed. One is related to the quantitiy and quality of the food ingested. It consists of neural afferents, psychosocial conditioning factors, and peptide signals from the gastrointestinal tract released by specific nutrient intake. The other is also sensitive to nutrient intake, but importantly modulated by relative adiposity. We present evidence to suggest that insulin serves as the key feedback signal to the central nervous system to serve this second function (body adiposity signal). Insulin has been found in cerebrospinal fluid where its concentration is increased by systemic infusions of glucose or insulin and is proportional to its concentration in plasma. When insulin (10 and 100 μU/kg/day) is infused into the lateral cerebral ventricles of free feeding baboons a dose dependent suppression of food intake and body weight is found. Intravenous infusion of 25% and 50% of total calories as glucose elevates endogenous insulin concentrations and suppresses food intake. These findings suggest that the amount of insulin secreted per day and more modulates food intake to maintain a constant body weight.

Relationship between plasma and cerebrospinal fluid insulin levels of dogs

Several experiments are reported in which insulin or glucose was administered intravenously to anesthetized dogs. Plasma and cerebrospinal fluid levels of glucose and immunoreactive insulin were determined at several intervals before and after the administrations. Intravenous insulin (0.2 U/kg) administered as either a pulse or a 1-h infusion caused a large increase of plasma insulin, but a relatively small increase of cerebrospinal fluid insulin. When endogenous insulin was elevated by the administration of glucose (100 mg/kg), cerebrospinal fluid insulin changed only slightly. A significant correlation was found between steady-state plasma and CSF endogenous insulin levels. The results are interpreted to indicate that the level of insulin in the cerebrospinal fluid reflects basal plasma level plus an integral over time of the insulin response to challenge. The implications of such a system are discussed.