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

A method for third ventricular cannulation of small passerine birds

This paper describes a method for chronically cannulating the third ventricle of the white-crowned sparrow, a small passerine bird, without damaging the midsagittal sinus. The method is reliable and chronic over at least 1 month. The technique was verified by assessing the effects of angiotensin II (ANG II) on inducing drinking behavior. All birds rapidly recovered from the surgery and tolerated repeated injections spaced over 1 month. Animals were injected with saline or 0.01, 0.5, 1.0, and 10.0 micrograms of ANG II, respectively. The intermediate dose of 1.0 microgram was maximally effective and caused a significant increase of water intake over the test hour. Lower and higher doses were less effective. This method for cannulating the third ventricle of small passerine birds should prove beneficial in future neurobiological applications.

Intracerebroventricular injection of insulin or glucose alters insulin-like growth factor II (IGF-II) concentrations in specific hypothalamic nuclei

Peripherally administered insulin has been shown to alter content and gene expression of hypothalamic insulin-like growth factor II (IGF-II) in a region specific manner (Lauterio, TJ. et al., Endocrinology, 126 (1990) 392-398. The objective of this experiment was to determine whether central administration of insulin can modulate hypothalamic IGF-II peptide content. Male Sprague-Dawley rats were implanted with lateral ventricular cannulae and allowed to recover from surgery one week prior to injection. At that point animals were remotely injected with one of the following: (1) synthetic cerebral spinal fluid vehicle (sCSF); (2) 2 mIU porcine insulin (I); (3) glucose (100 mg%) only. Animals were decapitated 30 min after injections and brains were quickly removed, frozen and dissected into specific hypothalamic regions for IGF-II analysis by RIA. Insulin increased IGF-II content in the ventromedial hypothalamic region by 80% (P < 0.001) and paraventricular nucleus by 30% (P < 0.01) compared to sCSF or glucose treatment. Arcuate nucleus and neurointermediary lobe pituitary IGF-II content was decreased with insulin treatment compared to controls (P < 0.01). Insulin had no effect on IGF-II concentrations in the dorsomedial or lateral hypothalamic regions or in the supraoptic and suprachiasmatic nuclei. Peripheral concentrations of glucose, insulin and IGF-II were unaffected by any treatment. Results show that insulin which reaches the brain can alter IGF-II levels in specific regions of the hypothalamus and suggests a possible role for IGF-II in insulin mediated changes in metabolism or hypothalamic hormone secretion.

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.

Naltrexone, serotonin receptor subtype antagonists, and glucoprivic intake: 2. Insulin

Opiate antagonist inhibition of deprivation-induced intake and 2-deoxy-D-glucose (2DG) hyperphagia is significantly enhanced by the 5-hydroxytryptamine3 (5-HT3) antagonist, ICS-205,930. Interactions between opiate antagonists and either 5-HT or 5-HT2 antagonists produced smaller effects. The present study evaluated whether insulin (5 U/kg) hyperphagia was affected by methysergide (0.5-5 mg/kg), ritanserin (0.25-2.5 mg/kg), and ICS-205,930 (0.5-5 mg/kg) alone or in combination with naltrexone (2.5-10 mg/kg). Whereas ICS-205,930 stimulated insulin hyperphagia across the 6-h time course, ritanserin and, to a lesser degree, methysergide reduced insulin hyperphagia. Naltrexone marginally (19-33%) reduced insulin hyperphagia. Pairing naltrexone with either ICS-205,930 or ritanserin significantly suppressed insulin hyperphagia after 2 h. Pairing naltrexone with each of the serotonin antagonists significantly enhanced insulin hyperphagia after 4 and 6 h. These data suggest that 5-HT2 and 5-HT3 receptor subtypes interact with opioid systems to modulate insulin hyperphagia. Given that central insulin reduces food intake and body weight, the interaction between serotonergic and opioid systems may occur peripherally.

Effect of delayed treatment with nerve growth factor on choline acetyltransferase activity in the cortex of rats with lesions of the nucleus basalis magnocellularis: dose requirements

Rats received bilateral ibotenic acid-lesions of the nucleus basalis magnocellularis. Starting two weeks after the lesion, cytochrome c (0.3 micrograms/rat/day) or 0.01, 0.1, 1 or 10 micrograms/rat/day human recombinant nerve growth factor (NGF) was infused into the lateral ventricle. The highest dose of NGF reduced the weight gain of the animals. Six weeks, but not two weeks of treatment with 10 micrograms/rat/day NGF increased choline acetyltransferase (ChAT) activity in the frontal cortex, parietal cortex and hippocampus, predominantly on the side of the ventricular cannula. The 1 microgram/rat/day dose only increased ChAT activity in the frontal cortex on the infused side. Six weeks of treatment with 10 micrograms/rat/day NGF increased the size, but not the number of NGF-receptor-immunoreactive neurons in the nucleus basalis. This treatment did not affect the levels of dopamine, norepinephrine and serotonin in any of the brain regions studied. These data suggest that prolonged treatment with relatively high doses of NGF is necessary to increase ChAT activity in cortical regions of nucleus basalis-lesioned rats. This treatment will also increase ChAT activity in the intact septohippocampal system, but does not affect the levels of several non-cholinergic neurotransmitters.

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.

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.

Effect of chronic insulin administration on food intake and body weight in rats

Insulin was chronically administered to rats to determine its effect on the daily changes in food intake and body weight. Animals received regular insulin via 14-day osmotic minipumps in doses of 0.0, 0.5, 1.0, 3.0, and 5.0 IU/day treated either with (+GLU) or without glutamic acid (-GLU). Previous studies have shown that glutamic acid prevents insulin aggregation in the minipumps to provide a more stable flow rate. Food intake and body weights were measured each day of treatment. Chronic insulin treatment was ineffective in promoting changes in animals receiving any dose of insulin except the highest dose. Animals receiving 5.0 IU/day insulin + GLU experienced a transient hyperphagia and weight gain followed by a suppression in food intake and body weight by Day 4 of treatment. Effects were attenuated in animals receiving insulin -GLU. Plasma insulin concentrations on Day 14 were similar for all doses, suggesting a compensation took place either in insulin degradation or endogenous insulin production. Results indicate that glutamic acid treatment enhances the effects of chronic insulin administration via osmotic minipumps.

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.

Mediation of insulin hyperphagia by specific central opiate receptor antagonists

The hyperphagic properties of insulin (10 U/kg, s.c.) were transiently (2h) and dose-dependently inhibited (30%) by central pretreatment with naltrexone (20-50 micrograms, i.c.v.). The irreversible mu opioid antagonist, beta-funaltrexamine (B-FNA, 20 micrograms, i.c.v.) significantly inhibited insulin hyperphagia by 28-54% over the 6-h time course. In contrast, insulin hyperphagia was only transiently (2 h) inhibited (27-30%) by either the irreversible mu 1 antagonist, naloxonazine (50 micrograms, i.c.v.) or the selective kappa antagonist, nor-binaltorphamine (NorBNI, 20 micrograms, i.c.v.). The delta-antagonistic actions of [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, 40 micrograms, i.c.v.) failed to affect insulin hyperphagia. These data suggest that the mu 2 opioid receptor subtype modulates insulin hyperphagia.

Intraventricular insulin reduces food intake and body weight of marmots during the summer feeding period

The study presented below describes experiments that investigate the ability of insulin to inhibit food intake in awake, active marmots during the summer season. Our results suggest that increasing intraventricular insulin concentration during the summer active feeding period will cause a decrease in food intake and body weight of marmots. When infused with insulin into their lateral ventricles (Alzet #2002 minipumps), animals had significantly lower food intake as compared to their food intake during the control period. In addition, these animals lost body weight during the period of the insulin infusion. We suggest that during the summer when marmots are not hibernating and are actively feeding, brain insulin levels may play a role in regulating food intake.

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.

Neuroendocrine mechanisms involved in regulation of body weight, food intake and metabolism

Body weight regulation is the result of food intake and energy expenditure. The central nervous system (CNS), and in particular, the hypothalamus, controls food intake as well as metabolism, the latter mainly by autonomic effects on the islet of Langerhans, hepatocytes and adipocytes. Body weight, more precisely body fat content, is probably controlled by a feedback mechanism in which insulin, released from the B cell of the islet of Langerhans, plays a key role. The islet of Langerhans is an intricate neuroendocrine unit in which the release of glucagon, insulin, and somatostatin from A, B, and D cells, respectively, is controlled by the CNS via a rich autonomic innervation. In addition, the endocrine cells of the pancreas influence each other by paracrine actions. The CNS control of the islets shapes the plasma insulin and blood glucose profiles during the circadian cycle and thereby regulates the nutrient flow to the different tissues in the body. Thus, the CNS structures involved in regulation of body weight and food intake control also metabolism. The mechanisms contributing to match food intake and the needs of metabolism are discussed.

Maternal hormone manipulations and the development of obesity in rats

Pregnant rats were injected with either insulin, corticosterone, thyroxin, or saline during the third trimester (week) of pregnancy. Offspring from these groups had equivalent body weights at birth and at weaning. However, beginning at approximately seven weeks of age, male offspring in the insulin condition gained weight at significantly higher rates than their counterparts in the other three conditions. This increase in body weight was accompanied by a significant increase in carcass lipid content. These effects were not observed in female offspring.

[Regulation of food intake]

Regulation of food intake is commonly treated as a negative feedback-loop. Hunger and/or appetite lead man and animals to ingest food. The subsequent meal-contingent activation of pre- and postabsorptive mechanisms then leads to satiety. The activation of oral and gastrointestinal chemo- and mechanoreceptors is important on the preabsorptive site. The gastrointestinal hormone cholecystokinin may also have a physiological satiety effect. Preabsorptive satiety mechanisms are influenced by the rate of gastrointestinal transit. The pancreatic hormone glucagon, which is released during meal taking, and various metabolites contribute to the postabsorptive regulation of food intake through activation of hepatic chemoreceptors, which are connected to the brain via predominantly vagal afferents. In addition, glucoreceptors in the brain, in particular in the nucleus of the solitary tract, contribute to food intake regulation by monitoring blood glucose concentration or, more specifically, glucose utilization. The nucleus of the solitary tract, which relays vagal afferents from gut and liver and also gustatory afferents, projects to the hypothalamus and to other forebrain structures. In this neural network the informations from the periphery are integrated by various neurotransmitters and neuropeptides, but the exact role of the substances involved is not fully understood yet. Body weight and, hence, body fat presumably affects feeding through modulation of a postabsorptive mechanism.

Effects of glucose, 2-deoxyglucose, phlorizin, and insulin on food intake of lean and fatty rats

Glucose, 2-deoxyglucose, phlorizin, and insulin were injected into the third ventricle of lean and fatty rats, and food intake recorded hourly for the next 6 h. In the lean rats, there was a significant but unimpressive decrease in food intake after the intraventricular injection of glucose, but there was no effect of glucose in the fatty rat. Phlorizin in the lowest dose (10 micrograms) increased the food intake in lean animals at 1 and 2 h, and all three doses increased it significantly at 6 h after intraventricular injection. The fatty rat, in contrast, showed no response to phlorizin. 2-Deoxyglucose showed a dose-related stimulation of food intake in the lean rats at 1, 2, 3, and 6 h after injection. In the fatty rat, there was no significant effect on food intake at any dose. The intraventricular injection of insulin had no effect on food intake in either the lean or fatty rats. These studies indicate that glucose-responding systems in the region of the third ventricle are defective in the fatty rat to signals that normally increase or decrease food intake in lean animals.

Response to chronic insulin administration: effect of area postrema ablation

The effects of daily administration of protamine zinc insulin (PZI) on plasma insulin and glucose levels and on food intake and body weight of rats with lesions of the area postrema and adjacent caudal-medial portions of the nucleus of the solitary tract (APX rats) were examined. Prior to insulin treatment, APX rats weighted less and had lower plasma immunoreactive insulin (IRI) levels than nonlesioned controls but did not differ from controls in plasma glucose levels. Five daily injections of 5 U/kg PZI raised plasma IRI and lowered plasma glucose levels similarly for both lesioned and nonlesioned rats. When injected with increasing doses of PZI over a 30-day period, both lesioned and nonlesioned rats showed increases of food intake and rate of weight gain in response to 8 U/kg PZI. These data indicate that APX does not affect either physiological or behavioral responses to chronic peripheral insulin administration.

The regulation of food intake by peptides

Historically, nutrients and related metabolic signals were considered to control the onset and offset of meals. Recent research has focused upon the roles of peptides found in the gastrointestinal tract and brain as alternate controllers of these processes. During a meal, the gut secretes a variety of peptides as part of the digestive process. Some of these substances, acting as hormonal or as local signals, may also provide information which is relayed to the central nervous system, causing eating to stop and producing the sense of satiety. When administered to animals or people before a meal, exogenous cholecystokinin (CCK), the most studied of the putative satiety peptides, reduces food intake in a dose-dependent manner. Recent findings support the concept that endogenous CCK acts during meals to limit meal size, and evidence is reviewed suggesting a possible pathophysiological role for CCK in bulimia. Adiposity is also regulated via peptide hormones, especially insulin. Insulin is secreted in direct proportion to adiposity, and blood-borne insulin gains access to brain areas important in the regulation of feeding. The administration of insulin into the brain causes reduced eating and weight loss.

Differential actions of central alloxan upon opioid and nonopioid antinociception in rats

Spontaneous or induced diabetes, as well as glucose loading, reduce opiate antinociception, presumably through induction of hyperglycemia. While peripheral administration of alloxan is a potent pancreatic beta-cell toxin, intracerebroventricular (ICV) alloxan reduces glucoprivic feeding in the absence of hyperglycemia, presumably through interactions with specific brain glucoreceptors. Our laboratory demonstrated that opioid-mediated 2-deoxy-D-glucose (2DG) antinociception is significantly reduced by central pretreatment with alloxan, and that this deficit is reversed by coadministration with 3M-D-glucose. The present study compared ICV and intravenous (IV) routes of alloxan (200 micrograms) upon morphine (1-10 mg/kg, SC) analgesia on the tail-flick and jump tests in rats, and evaluated these effects in terms of concomitant changes induced by ICV alloxan upon nonopioid-mediated continuous cold-water swim (CCWS: 2 degrees C for 3.5 min) antinociception. Two weeks following central, but not peripheral pretreatment with alloxan, morphine (2.5 and 5.0 mg/kg, SC) antinociception was markedly (30-56%) reduced on both nociceptive tests. In contrast, central pretreatment with alloxan respectively reduced (30 min) and subsequently potentiated (60 and 90 min) CCWS antinociception on the jump test. Alterations in antinociception by central alloxan occurred in the absence of changes in basal nociceptive thresholds, hypothermia or hyperglycemia. These data suggest that central alloxan may be acting upon either specific, but unidentified brain glucoreceptors and/or a glucoprivic control mechanism.

Insulin binding in the hypothalamus of lean and genetically obese Zucker rats

Recent reports have suggested that the obesity and hyperphagia of the genetically obese Zucker rat may be related to defective insulin action or binding in the hypothalamus. We used quantitative autoradiography to determine if insulin binding is altered in specific hypothalamic nuclei associated with food intake. Insulin binding was measured in the arcuate (ARC), dorsomedial (DMN), and ventromedial (VMN) hypothalamic nuclei of 3-4-month-old lean (Fa/Fa) and genetically obese (fa/fa) Zucker rats. A consistently reproducible 15% increase in the total specific binding of 0.1 nM [125I]-insulin was found in the ARC of the obese genotype. A slight increase in insulin binding in the DMN was also found. No difference in specific insulin binding was found between genotypes in the VMN. Nonlinear least squares analysis of competitive binding studies showed that the Kd of the ARC insulin binding site was 33% higher in the lean rats than in the obese rats, indicating an increased affinity for insulin. No difference in site number (Bmax) was found in the ARC, DMN or VMN, and no evidence was found for reduced insulin binding in the hypothalamus of the obese (fa/fa) genotype. The results suggest that hyperphagia and obesity of the obese (fa/fa) Zucker rat genotype may be associated with increased insulin binding in the arcuate nucleus.

Acetylation alters the feeding response to MSH and beta-endorphin

The effects on food intake of the N-acetylation of MSH and beta-endorphin have been examined following their injection into the third ventricle. Desacetyl-MSH and alpha-MSH were injected into fasted rats, and beta-endorphin and N-acetyl-beta-endorphin into fed rats. Desacetyl-MSH had no effect on food intake following ICV injection into food-deprived rats at any dose between 100 and 2500 pmoles. Alpha-MSH, the N-acetylated form of MSH, on the other hand, showed a highly significant inhibition of food intake in food-deprived rats with doses of 100 and 250 pmoles but no effect with the higher doses. With beta-endorphin, there was a dose-related biphasic effect. One hour after injection of beta-endorphin (2500 pmole) food intake was inhibited whereas the lowest dose, 100 pmole, significantly stimulated it. By 3 hours, the 2 lowest doses of beta-endorphin both significantly stimulated food intake, but the highest dose remained inhibitory. By 6 hours all doses of beta-endorphin stimulated food intake compared to the vehicle-treated animals. In contrast, N-acetylation of beta-endorphin eliminated all effects on food intake following injection into the third ventricle. These data suggest that N-acetylation of products formed by the processing of POMC can markedly alter their biological properties.

Vanadate stimulates in vivo glucose uptake in brain and arrests food intake and body weight gain in rats

Vanadate, administered via drinking fluid (0.2-0.8 mg/ml in 80 mM NaCl), attenuated food intake and strongly suppressed body weight gain in normally-fed or 20-hour food-deprived rats. At 0.8 mg/ml for 4 days, oral vanadate significantly stimulated the rate of hexose uptake by brain tissue. When microinjected into the lateral cerebral ventricle at a dose of 82 nmol, vanadate strongly and specifically suppressed food intake and body weight gain in 20-hour food deprived rats previously maintained on tap water. This inhibitory effect was reversed by coadministration of 3-O-methyl glucose. Collectively, the results suggest that vanadate is capable of blocking food intake by a specific effect in the central nervous system that involves stimulation of local glucose uptake.

Nutrient balance and obesity: an approach to control of food intake in humans

This article has examined the regulated systems that control nutrient balance. From this analysis, the following conclusions may be suggested: 1. Each nutrient is regulated separately in a feedback system. 2. The control of glucose is regulated by the size of the glycogen stores; the size of the fat depots, by the rate of hepatic fatty acid oxidation; and protein, by the size of the protein depots. 3. Obesity can occur as a result of hyperphagia or from repartitioning the deposition of nutrients. In either case, there is a relative or absolute reduction in the activity of the sympathetic nervous system, requiring adequate levels of circulating corticosteroids.

Effect of starvation on insulin receptors in rat brain

To determine the effect of starvation on brain insulin receptors, rats were fed 4 g of chow/day for 14 days and then P2 fraction membranes were prepared from different brain regions. Compared to the fed state, there was an 18% reduction of insulin binding in olfactory bulbs from starved animals, but no change in the cerebellum, frontal cortex, amygdala, medial hypothalamus or lateral hypothalamus. A 15% reduction of olfactory bulb insulin binding was obtained by totally starving animals for four days. When membrane content was measured using the plasma membrane marker Na/K ATPase, insulin binding decreased by 26% and 14% in olfactory bulb membranes from starved and totally starved animals, respectively. The starvation-induced change in olfactory bulb binding was due to a loss of binding sites and not a decrease in binding affinity. Non-specific catabolism of protein and a change in the composition of membranes following starvation were excluded as causes for this effect. As streptozotocin induced diabetes had no effect on brain insulin binding, it was concluded that hypoinsulinaemia associated with starvation had not caused the reduction in olfactory bulb binding. Under similar conditions of starvation and diabetes, insulin binding in liver plasma membranes increased 26% and 38%, respectively. At 8 and 14 days of starvation, the reductions in olfactory bulb insulin binding and body weight were similar. On refeeding for three days, there was no increase in insulin binding, although body weight increased 7%. On refeeding for eight days, olfactory bulb insulin and body weight had returned to near normal.(ABSTRACT TRUNCATED AT 250 WORDS)

The psychoneuroendocrinology of diabetes mellitus in rodents

The metabolic-endocrine state of diabetes mellitus affects the brain and behavior of diabetic animals. Feeding, paradoxical sleep, analgesia, submissive behavior, and avoidance behavior, are generally increased in diabetic compared with nondiabetic rodents. In contrast, sexual behavior, aggressive behavior and sensitivity to the behavioral effects of amphetamine are decreased in diabetic rodents. This review examines behavioral changes in diabetes mellitus within the context of known disease-linked alterations in hypothalamo-pituitary relationships and brain monoamine metabolism.

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.

Chronic intracerebroventricular infusion of insulin failed to alter brain insulin-binding sites, food intake, and body weight

The present study was performed to explore the role of exogenous insulin in CSF in the control of energy balance in the rat. For this purpose, adult male Sprague-Dawley rats carrying an indwelling cannula in the right lateral cerebral ventricle were infused for a maximum of 10 days with insulin (Actrapid) at various rates (starting at 0, 45, 85, 170, and 600 ng/day) or anti-insulin antibody (IgG fraction; diluted 1:10 wt/vol) with an osmotic minipump. All those treatments did not modify the growing rates; neither total daily food intake nor the circadian rhythm of food intake was further modified. The chronic insulin infusion starting at 600 ng/day resulted in a chronic significant increase in CSF insulin levels without changing the plasma insulin level. It failed to alter specific insulin binding sites to Triton X-100 solubilized microsomal membranes from various brain areas (cerebral cortex, olfactory bulbs, and lateral and medial hypothalami) at the end of the 5- or 10-day period of insulin infusion. Purification of insulin receptors on a wheat germ agglutinin did not reveal any further effect of insulin. From these results, it seems unlikely that the input to the brain insulin-effector systems could arise from CSF insulin.

Insulin responses and glucose levels in plasma and cerebrospinal fluid during fasting and refeeding in the rat

The present experiments were designed to investigate the rate of penetration of insulin from the plasma into the cerebrospinal fluid (CSF) during 24 hr of fasting and refeeding in the light phase. The results show that under these conditions basal CSF-immunoreactive insulin (IRI) levels were positively correlated with plasma IRI levels. Basal plasma IRI fell during a fast but was similar to prefast control after one day of refeeding. Although CSF-IRI levels rose during glucose infusion, CSF-IRI was not elevated by glucose during a fast. During refeeding, CSF-IRI responses returned toward control, prefeeding values. This study suggests a decreased transport of insulin from plasma to CSF during fasting. The lower CSF-IRI levels achieved under these conditions may determine meal size by allowing larger meals after a fast.

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.

Intrahypothalamic injection of insulin decreases firing rate of sympathetic nerves

Injection of picomolar quantities of insulin into the ventromedial hypothalamus of rats significantly reduced the firing rate of sympathetic nerves that supply interscapular brown adipose tissue. The minimal firing rate was reached in 2 min, and the effect was gone within 4 min. The effect of insulin was dose-related and did not occur when comparable volumes of physiological saline were injected into the ventromedial hypothalamus. Destruction of neurons in the ventromedial hypothalamus by injection of kainic acid abolished the inhibitory effects of insulin. These data suggest that insulin may play a role in modulating the sympathetic firing rate to thermogenically important tissues.

Brain uptake of ketones in rats with differing susceptibility to dietary obesity

The effect of a low- and high-fat diet on the transport of 3-hydroxybutyrate and glucose across the blood brain barrier has been measured in two strains of animals that have a marked difference in the degree of obesity that develops when they eat a high fat diet. The S 5B/Pl rats are resistant to dietary obesity whereas the Osborne-Mendel rats readily develop obesity when eating a high-fat diet. The transport of 3-hydroxybutyrate and glucose across the blood-brain barrier was measured as the ratio of radioactive compound (3-hydroxybutyrate or glucose) to radioactively labeled water by the technique of Oldendorf. The uptake of 3-hydroxybutyrate was significantly higher in the S 5B/Pl rats than in the Osborne-Mendel rats when they were eating either the low-fat diet or the high-fat diet. In addition, there was a significant increase in the transport of 3-hydroxybutyrate in the animals of both strains when eating the high-fat diet as compared to the low-fat diet. However, there was no difference in the transport of glucose between the two strains of rats whether they ate a low-fat or high-fat diet. These data are consistent with the hypothesis that resistance to dietary obesity is associated with increased transport of 3-hydroxybutyrate across the blood-brain barrier.

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.

Localization of 125I-insulin binding sites in the rat hypothalamus by quantitative autoradiography

In vitro autoradiography and computer video densitometry were used to localize and quantify binding of 125I-insulin in the hypothalamus of the rat brain. Highest specific binding was found in the arcuate, dorsomedial, suprachiasmatic, paraventricular and periventricular regions. Significantly lower binding was present in the ventromedial nucleus and median eminence. The results are consistent with the hypothesis that insulin modulates the neural regulation of feeding by acting at sites in the hypothalamus.

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.

Effect of intra-third ventricular administration of insulin on food intake after food deprivation

It has been suggested that insulin may participate as a signal in the overall control of feeding. To further study this possible role of insulin, food-deprived male Wistar rats were subjected to intra-third cerebro-ventricular infusions of insulin. Infusion of 0.5 and 2.0 mIU/rat at 0745 hr, and 2.0 mIU/rat of insulin at 1900 hr in 24.5 hr food-deprived rats, and 2.0 mIU/rat of insulin at 2200 hr in 4 hr food-deprived rats did not significantly affect food intake. Infusion of the high dose of 8.0 mIU/rat of insulin at 2200 hr in 4 hr food-deprived rats significantly decreased food intake with a long-delayed and long-lasting effect. This and previous evidence suggest that intra-third ventricular administration of small amounts of insulin induce a decrease of food intake only in non-food-deprived rats.

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.

Peptides as central regulators of feeding

During the past decade there has been an increased awareness of the role peptides play as neuromodulators. In this article we review the available data on peptides as central regulators of food ingestion. We stress the possible problems of non-specific effects. We stress that whereas many peptides decrease feeding after central injection, only two families of peptides have been shown to increase feeding after central injection. These are the opioid family and the pancreatic polypeptide-neuropeptide Y family. The putative role of corticotropin releasing factor as the mediator of norepinephrine and serotonin effects on feeding is discussed.