Thomas A. Lutz

Lesion of the area postrema/nucleus of the solitary tract (AP/NTS) attenuates the anorectic effects of amylin and calcitonin gene-related peptide (CGRP) in rats.

The area postrema/nucleus of the solitary tract (AP/NTS) region plays an important role in the control of food intake since it receives peripheral satiety signals via splanchnic and vagal afferents. Due to the lack of the blood brain barrier in this region, blood borne signals can directly be monitored in the AP/NTS. Furthermore, receptors for anorectic peptides such as amylin or calcitonin gene-related peptide (CGRP) have been found in the AP/NTS. It was therefore the aim of the present study to investigate the role of the AP/NTS region in mediating the anorectic effects of these peptides. Thermal ablation of the AP/NTS resulted in a significant reduction of the anorectic effects of IP injected amylin (5 microg/kg) and CGRP (5 microg/kg) in food deprived rats. The anorectic actions of CCK and BBS were also reduced by the AP/NTS lesion which agrees with previous studies. We conclude that the AP/NTS region is an important brain site for mediating the anorectic effects of amylin and CGRP. It remains to be clarified whether this effect is due to amylin and CGRP action on receptors within the AP/NTS region or peripheral receptors on afferent nerves projecting to the AP/NTS.

Gastric Bypass Increases Energy Expenditure in Rats

BACKGROUND & AIMS Mechanisms underlying weight loss maintenance after gastric bypass are poorly understood. Our aim was to examine the effects of gastric bypass on energy expenditure in rats. METHODS Thirty diet-induced obese male Wistar rats underwent either gastric bypass (n = 14), sham-operation ad libitum fed (n = 8), or sham-operation body weight-matched (n = 8). Energy expenditure was measured in an open circuit calorimetry system. RESULTS Twenty-four-hour energy expenditure was increased after gastric bypass (4.50 +/- 0.04 kcal/kg/h) compared with sham-operated, ad libitum fed (4.29 +/- 0.08 kcal/kg/h) and sham-operated, body weight-matched controls (3.98 +/- 0.10 kcal/kg/h, P < .001). Gastric bypass rats showed higher energy expenditure during the light phase than sham-operated control groups (sham-operated, ad libitum fed: 3.63 +/- 0.04 kcal/kg/h vs sham-operated, body weight-matched: 3.42 +/- 0.05 kcal/kg/h vs bypass: 4.12 +/- 0.03 kcal/kg/h, P < .001). Diet-induced thermogenesis was elevated after gastric bypass compared with sham-operated, body weight-matched controls 3 hours after a test meal (0.41% +/- 1.9% vs 10.5% +/- 2.0%, respectively, P < .05). The small bowel of gastric bypass rats was 72.1% heavier because of hypertrophy compared with sham-operated, ad libitum fed rats (P < .0001). CONCLUSIONS Gastric bypass in rats prevented the decrease in energy expenditure after weight loss. Diet-induced thermogenesis was higher after gastric bypass compared with body weight-matched controls. Raised energy expenditure may be a mechanism explaining the physiologic basis of weight loss after gastric bypass.

The anorectic effect of a chronic peripheral infusion of amylin is abolished in area postrema/nucleus of the solitary tract (AP/NTS) lesioned rats

OBJECTIVE: Neurons in the area postrema/nucleus of the solitary tract (AP/NTS) region mediate amylins anorectic effect elicited by a single intraperitoneal (i.p.) injection of a low dose (5 μg/kg). Here, we tested if a sustained elevation in amylin levels which was achieved by chronic amylin infusion reduces food intake by acting in the AP/NTS region or, possibly, at other brain sites. Further, we tested the role of the AP/NTS region in mediating the anorectic effects of high doses of amylin and its receptor agonist salmon calcitonin (sCT) after an acute single injection.DESIGN: Amylin (2 μg/kg/h) was chronically infused i.p. by osmotic minipumps in AP/NTS-lesioned (AP-X) or sham-lesioned (SHAM) rats. For the acute experiments, amylin or sCT was injected i.p. at doses of 0.5 (only sCT), 5 or 50 μg/kg. Food intake was measured by a computerized system. Body weight was assessed by manually weighing the rats.RESULTS: Amylin significantly reduced cumulative food intake for about 7 days in SHAM but not in AP-X rats. Amylins effect in SHAM rats was mainly due to a reduction of the size of nocturnal meals (eg average meal size during the first four dark phases; SHAM, NaCl 4.1±0.6 vs amylin 2.6±0.4 g; n=6, P<0.05; AP-X, 2.6±0.3 vs 3.7±0.3) while light phase food intake was unaffected. Body weight gain over the whole 14 day infusion period was reduced by amylin in SHAM (NaCl 61±6 vs amylin 46±4 g; P<0.05) but not in AP-X rats (54±4 vs 62±4). After single injection, the anorectic effect of high doses of amylin and sCT (50 μg/kg) was attenuated, but not abolished, in AP-X rats.CONCLUSION: We conclude that, under our experimental conditions, neurons in the AP/NTS region are necessary for chronically elevated peripheral amylin to reduce food intake in rats. High doses of amylin, however, may be able to overrun these receptors and reduce feeding by acting at other brain sites.

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.

Gastric bypass reduces fat intake and preference

Roux-en-Y gastric bypass is the most effective therapy for morbid obesity. This study investigated how gastric bypass affects intake of and preference for high-fat food in an experimental (rat) study and within a trial setting (human). Proportion of dietary fat in gastric bypass patients was significantly lower 6 yr after surgery compared with patients after vertical-banded gastroplasty (P = 0.046). Gastric bypass reduced total fat and caloric intake (P < 0.001) and increased standard low-fat chow consumption compared with sham controls (P < 0.001) in rats. Compared with sham-operated rats, gastric bypass rats displayed much lower preferences for Intralipid concentrations > 0.5% in an ascending concentration series (0.005%, 0.01%, 0.05%, 0.1%, 0.5%, 1%, 5%) of two-bottle preference tests (P = 0.005). This effect was demonstrated 10 and 200 days after surgery. However, there was no difference in appetitive or consummatory behavior in the brief access test between the two groups (P = 0.71) using similar Intralipid concentrations (0.005% through 5%). Levels of glucagon-like peptide-1 (GLP-1) were increased after gastric bypass as expected. An oral gavage of 1 ml corn oil after saccharin ingestion in gastric bypass rats induced a conditioned taste aversion. These findings suggest that changes in fat preference may contribute to long-term maintained weight loss after gastric bypass. Postingestive effects of high-fat nutrients resulting in conditioned taste aversion may partially explain this observation; the role of GLP-1 in mediating postprandial responses after gastric bypass requires further investigation.

Reduction of food intake in rats by intraperitoneal injection of low doses of amylin

The effect of amylin injected IP on food intake in rats of different age (7-9 weeks, 3 months, 15-18 months) was investigated. The possible site of amylin action was investigated using vagotomized rats. Lastly, the influence of food composition on amylins effect was investigated. In 12-h food-deprived old rats, food intake was decreased significantly by amylin (1-10 micrograms/kg) when injected at the beginning of the dark phase. Although the anorectic effect of amylin occurred somewhat earlier at 10 micrograms/kg, no clear dose-response relationship was observed. The anorectic effect was most marked in the first 2 h after amylin injection and was compensated over 24 h. Amylin (1 and 5 micrograms/kg) did not reduce food intake in undeprived old rats. In young rats, amylin (0.1-1 microgram/kg) dose-dependently reduced food intake if rats were food-deprived for 24 h, but not when deprived for 12 h. Dissection of the common hepatic vagus branch did not block the anorectic effect of amylin (age of rats: 3 months). The effect tended to last longer in vagotomized rats. The anorectic effect of amylin did not depend on the presence of carbohydrates in the diet. Water intake was not affected by amylin in water-deprived rats. In conclusion, the anorectic effect of amylin was observed at much lower doses (minimal effective dose: 0.5 microgram/kg) than reported before. These doses are similar to anorectic doses of cholecystokinin, a physiological peripheral satiety agent.

Amylinergic control of food intake

Amylin is a pancreatic B-cell hormone that plays an important role in the regulation of nutrient fluxes. As such, amylin reduces food intake in laboratory animals and man, slows gastric emptying and it reduces postprandial glucagon secretion. Amylin deficiency which occurs concomitantly to insulin deficiency in diabetes mellitus, may therefore contribute to some of the major derangements associated with this disorder (hyperphagia, excessive glucagon secretion, accelerated rate of gastric emptying). The described actions of amylin all seem to depend on a direct effect of amylin on the area postrema (AP). As to amylins satiating effect, the physiological relevance of this action is underlined by studies involving specific amylin antagonists and amylin-deficient mice. In the AP, amylin seems to modulate the anorectic signal elicited by CCK. Subsequent to AP activation, the amylin signal is conveyed to the forebrain via distinct relay stations. Within the lateral hypothalamic area, amylin diminishes the expression of orexigenic neuropeptides such as orexin and MCH. Whether these effects contribute to amylins short term satiating action remains to be determined. Recent studies suggest that amylin may also play a role as a long-term, lipostatic signal, especially when other feedback systems to the brain are deficient. Obese, leptin-resistant Zucker rats which are hyperinsulinemic and hyperamylinemic, were chronically infused with the amylin antagonist AC 187. AC 187 significantly elevated food intake in obese Zucker rats while having no effect in lean controls. This indicates that at least under certain conditions, chronic blockade of endogenous amylin action may lead to an increase in food intake and/or body weight. As mentioned, the site and mechanism of action for peripheral amylin to reduce food intake seems to be well established. It is less clear how centrally administered amylin reduces food intake although it is well known that 3rd ventricular administration of amylin produces a very strong and long-lasting anorectic action. Amylin receptors have been described in various hypothalamic nuclei but the endogenous ligand of these receptors remains to be investigated. The same holds true as to the physiological relevance of the anorectic effect seen after central amylin administration.

Site-specific effects of ghrelin on the neuronal activity in the hypothalamic arcuate nucleus

The recently discovered hormone ghrelin, which is secreted from the stomach during fasting and hypoglycemia opposes the homeostatic functions of leptin by increasing food intake and decreasing energy expenditure. The hypothalamic arcuate nucleus (Arc) mediates the effects of leptin and contains a high density of ghrelin receptors. The leptin- and ghrelin-responsive network involves the hypothalamic neuropeptide Y/alpha-melanocyte stimulating hormone (NPY/alpha-MSH) system. In the rat, neurons expressing the orexigenic peptide NPY are mainly located in the ventromedial Arc (ArcM), while pro-opiomelanocortin (POMC) neurons, synthesizing the anorectic peptide alpha-MSH, predominate in the ventrolateral Arc (ArcL). In extracellular single unit recordings from in vitro slice preparations of the Arc, superfusion of ghrelin (10(-8) M) exerted predominantly excitatory effects on ArcM neurons (73%, n=93), while a high number ArcL neurons were inhibited in response to ghrelin (42%, n=43). The excitatory effect of ghrelin on neuronal activity was postsynaptic since it was unaffected by synaptic blockade (low Ca(2+)/high Mg(2+) solution). In contrast, the inhibitory response in the ArcL was abolished by the blockade of synaptic interactions indicating a presynaptic mechanism. These results indicate that circulating ghrelin may oppose the actions of leptin by directly activating NPY-neurons of the ArcM and by indirectly inhibiting POMC neurons of the ArcL.

Overview of Animal Models of Obesity

The focus of this overview is on the animal models of obesity most commonly utilized in research. The models include monogenic models in the leptin pathway, polygenic diet‐dependent models, and, in particular for their historical perspective, surgical and chemical models of obesity. However, there are far too many models to consider all of them comprehensively, especially those caused by selective molecular genetic approaches modifying one or more genes in specific populations of cells. Further, the generation and use of inducible transgenic animals (induced knock‐out or knock‐in) is not covered, even though they often carry significant advantages compared to traditional transgenic animals, e.g., influences of the genetic modification during the development of the animals can be minimized. The number of these animal models is simply too large to be covered in this unit. Curr. Protoc. Pharmacol. 58:5.61.1‐5.61.18.

Peptide YY directly inhibits ghrelin-activated neurons of the arcuate nucleus and reverses fasting-induced c-Fos expression.

The hypothalamic arcuate nucleus (Arc) monitors and integrates hormonal and metabolic signals involved in the maintenance of energy homeostasis. The orexigenic peptide ghrelin is secreted from the stomach during negative status of energy intake and directly activates neurons of the medial arcuate nucleus (ArcM) in rats. In contrast to ghrelin, peptide YY (PYY) is released postprandially from the gut and reduces food intake when applied peripherally. Neurons in the ArcM express ghrelin receptors and neuropeptide Y receptors. Thus, PYY may inhibit feeding by acting on ghrelin-sensitive Arc neurons. Using extracellular recordings, we (1) characterized the effects of PYY on the electrical activity of ghrelin-sensitive neurons in the ArcM of rats. In order to correlate the effect of PYY on neuronal activity with the energy status, we (2) investigated the ability of PYY to reverse fasting-induced c-Fos expression in Arc neurons of mice. In addition, we (3) sought to confirm that PYY reduces food intake under our experimental conditions. Superfusion of PYY reversibly inhibited 94% of all ArcM neurons by a direct postsynaptic mechanism. The PYY-induced inhibition was dose-dependent and occurred at a threshold concentration of 10–8M. Consistent with the opposite effects of ghrelin and PYY on food intake, a high percentage (50%) of Arc neurons was activated by ghrelin and inhibited by PYY. In line with this inhibitory action, peripherally injected PYY partly reversed the fasting-induced c-Fos expression in Arc neurons of mice. Similarly, refeeding of food-deprived mice reversed the fasting-induced activation in the Arc. Furthermore, peripherally injected PYY reduced food intake in 12-hour fasted mice. Thus the activity of Arc neurons correlated with the feeding status and was not only reduced by feeding but also by administration of PYY in non-refed mice. In conclusion, our current observations suggest that PYY may contribute to signaling a positive status of energy intake by inhibiting Arc neurons, which are activated under a negative status of energy intake by signals such as ghrelin.