Fear extinction is impaired in aged rats

Normal aging is accompanied by broad loss of cognitive function in humans and rodents, including declines in cognitive flexibility. In extinction, a conditional stimulus (CS) that was previously paired with a footshock is presented alone. This procedure reliably reduces conditional freezing behavior in young adult rats. Here, we aimed to investigate how normal aging affects extinction learning. Using young (3 months) and aged (20 months) male and female Long Evans rats, we compared extinction (using 20 CS-alone presentations) to a no extinction control (equal exposure to the conditioning chamber without CS presentations) following delay fear conditioning. We found that young animals in the extinction group showed a decrease in freezing following extinction; aged animals did not. We next examined changes in neural activity using expression of the immediate early gene zif268. In young animals, extinction corresponded with decreased expression of zif268 in the basolateral amygdala and anterior retrosplenial cortex; this was not observed in aged animals. Further, aged animals showed increased zif268 expression in each region examined, suggesting that dysfunction in neural activity precedes cognitive deficits. These results demonstrate that aging impacts both extinction learning and neural activity.

Adolescent activity-based anorexia has a substantial and prolonged impact on social behavior in young adult female rats

Activity-based anorexia (ABA) is a rodent model of anorexia nervosa (AN) that induces several key components of AN, including voluntary reduction in food intake, reduced body weight, hyperactivity, and alterations to the hypothalamic-pituitary-adrenal (HPA) axis. Previous research has demonstrated persistently increased anxiety-like behavior in the elevated plus maze (EPM), a test measuring avoidance of novel and open areas in adult female rats that experienced ABA during adolescence and are weight-restored in adulthood. Whether the same behavioral effects of two bouts of adolescent ABA emerge in response to different anxiety-provoking stimuli, however, has not been explored. We used the social partition (SP), novelty suppressed feeding (NSF), marble burying, and EPM tests to explore whether two bouts of adolescent ABA have persistent effects on anxiety-like behavior in weight restored young adult female rats. One-way ANOVA analyses revealed that female rats that experienced two bouts of ABA during adolescence had increased anxiety-like behavior in the EPM and SP tests in young adulthood following weight restoration compared with controls. These data demonstrate that the enduring behavioral effects of two bouts of adolescent ABA are specific to particular anxiety-provoking stimuli and suggest that adolescent ABA has enduring effects on social relationships.

Sex-specific effects of ketogenic diet after pre-exposure to a high-fat, high-sugar diet in rats

BACKGROUND AND AIMS

The objectives were to evaluate the relationship between ketogenic diets, the ketone body beta-hydroxybutyrate (BHB), parameters known to increase risk for cardiovascular and metabolic diseases in both sexes, using a pre-clinical model of obesity.

METHODS AND RESULTS

Rats had access to a diet high in fat and sugar (HFS) for 12 weeks. After HFS, they switched to chow (HFS-CH) or ketogenic diet (HFS-KD) for 3 weeks to model a dietary intervention. Body weight, adiposity, and food intake were measured. Glucose tolerance and corticosterone response to stress were measured after HFS, then again after the intervention. Both sexes increased body weight, food intake, and adiposity compared to control (CTL) while on HFS. HFS females showed impaired glucose tolerance. HFS males developed a dampened corticosterone to stress, whereas HFS females developed an exacerbated response. The effects of HFS on adiposity and corticosterone were reversed in HFS-CH males. These same improvements were observed in HFS-CH females, although they still had impaired glucose tolerance. HFS-KD males showed some improvements, however, they still had higher body weight and adiposity than CTL. The same pattern was observed in females. These beneficial effects of KD correlated with plasma BHB levels in females but not in males.

CONCLUSIONS

These data model effects reported in clinical literature and serve as a valuable translational tool to further test causal mechanisms that lead to desirable outcomes of KD. These sex-specific relationships are important, as KD could potentially affect endocrine mechanisms differently in males and females.

Dual functions of CNS inflammation in food intake and metabolic regulation

Western diet (WD) consumption induces chronic mild inflammation in the hypothalamus. However, metabolic consequences of increased hypothalamic inflammatory cytokines remain unclear. This research first aimed to examine whether increased proinflammatory cytokines in the brain influenced feeding or metabolism. Rats that received an intracerebroventricular third ventricle injection (i3vt) of 0.5 pg TNFα daily for six days consumed significantly more calories than saline-injected rats, with no differences between treatment groups in terms of body weight, blood triglycerides nor glucose regulation. Continuously infusing TNFα for three weeks decreased hepatic fatty acid synthase (FAS) and increased body weight and the epididymal adipose sterol regulatory element-binding protein 1c (SREBP-1c) gene expression. Differences were not due to food intake nor voluntary wheel running activity. The second aim of this research was to examine whether inhibition of inflammation signaling in the brain at early stage of switching from chow to WD would affect diet-induced obesity development. WD-fed rats with i3vt NFκB inhibitor had greater caloric intake than rats given i3vt saline. These studies suggest elevated inflammatory cytokines in the brain induce food intake acutely and favor fat storage and weight gain in the long term. However, in the early stage of WD consumption, hypothalamic inflammatory signaling inhibits caloric intake and may serve as a warning signal of energy imbalance.

Dietary Slowly Digestible Starch Triggers the Gut-Brain Axis in Obese Rats with Accompanied Reduced Food Intake

SCOPE

Slowly digestible starch (SDS), as a functional carbohydrate providing a slow and sustained glucose release, may be able to modulate food intake through activation of the gut-brain axis.

METHODS AND RESULTS

Diet-induced obese rats were used to test the effect on feeding behavior of high-fat (HF) diets containing an SDS, fabricated to digest into the ileum, as compared to rapidly digestible starch (RDS). Ingestion of the HF-SDS diet over an 11-week period reduced daily food intake, through smaller meal size, to the same level as a lean body control group, while the group consuming the HF-RDS diet remained at a high food intake. Expression levels (mRNA) of the hypothalamic orexigenic neuropeptide Y (NPY) and Agouti-related peptide (AgRP) were significantly reduced, and the anorexigenic corticotropin-releasing hormone (CRH) was increased, in the HF-SDS fed group compared to the HF-RDS group, and to the level of the lean control group.

CONCLUSION

SDS with digestion into the ileum reduced daily food intake and paralleled suppressed expression of appetite-stimulating neuropeptide genes associated with the gut-brain axis. This novel finding suggests further exploration involving a clinical study and potential development of SDS-based functional foods as an approach to obesity control.

Reprint of "Repeated adolescent activity-based anorexia influences central estrogen signaling and adulthood anxiety-like behaviors in rats"

Anorexia nervosa (AN) typically presents in adolescence and is highly comorbid with anxiety and depression, which often persist after elimination of AN symptomology. The activity-based anorexia (ABA) paradigm allows for evaluation of behavioral and neuroendocrine consequences of AN-like behaviors, including voluntary anorexia, hyperactivity, and disruption of the hypothalamic-pituitary-gonadal (HPG) and the hypothalamic pituitary adrenal (HPA) axis. Because ABA in adolescent females results in increased anxiety-like behavior in adulthood and the estrogen signaling system has been shown to play a role in anxiety and food intake, we investigated the role of ovarian hormones in adolescent ABA-treated rats, and long-term effects of mid- and late adolescent ABA exposure on behavior and estrogen signaling. While previous research demonstrated that two bouts of ABA during adolescence resulted in decreased time in the open arm of the elevated plus maze (EPM) and increased activity of the HPA axis in response to a novel stressor, here we show that one bout of ABA in mid-or late-adolescence did not result in the same behavioral outcome. Two exposures to ABA during adolescence were necessary to produce long-term anxiety-like behavior on the EPM. Finally, removal of ovarian hormones by ovariectomy (OVX) prior to puberty did not attenuate long-term behavioral consequences of ABA in adolescence, and estrogen receptor β (ERβ) expression level in the amygdala of ABA rats was significantly lower than control subjects. Taken together, these studies identify enduring effects of ABA in adolescent females that may be mediated by ABA-induced changes to CNS ERβ signaling that increase anxiety-like behaviors.

Western diets induce blood-brain barrier leakage and alter spatial strategies in rats

Western diet (WD) intake induces obesity and metabolic dysfunction. The present study examined the effects of WD on hippocampal-dependent cognitive functioning and blood-brain barrier (BBB) permeability as a function of exposure duration, obesity phenotype, and peripheral markers of energy regulation. The use of hippocampal-dependent "place" or hippocampal-independent "response" strategies in a Y maze was assessed in male rats following 10, 40, and 90 days of WD exposure in diet-induced obese (DIO) rats, in diet resistant (DR) rats that are relatively insensitive to the obesogenic properties of WD, and in chow-fed controls. Insulin, glucose, and BBB permeability throughout several loci in the hippocampus, striatum, and cerebellum were evaluated in relation to duration of WD exposure, obesity phenotype, and type of strategy used. DIO rats had increased body weight and adiposity throughout the study, and elevated 10-day glucose and 90-day insulin levels. Throughout the study, chow-fed and DR rats reliably relied on a place strategy. DIO rats, in contrast, favored a response strategy at the 10- and 90-day time points. BBB leakage was observed in the dorsal striatum and multiple subregions of the hippocampus of DIO, but not DR or chow-fed rats. Increased ventral hippocampal BBB permeability and blood glucose levels were associated with reduced place strategy use. These data indicate that WD-induced BBB leakage is dependent on duration of diet exposure as well as obesity phenotype, and implicates BBB leakage and impaired glucoregulation in behavioral strategy and cognitive performance.

Diet-induced obesity and insulin resistance spur tumor growth and cancer cachexia in rats bearing the Yoshida sarcoma

Obesity and insulin resistance are associated with increased risk of cancer and cancer mortality. However, it is currently unknown whether they contribute to the development of cancer cachexia, a syndrome that contributes significantly to morbidity and mortality in individuals with cancer. The present experiment addresses the question of whether preexisting obesity and insulin resistance alter tumor growth and cancer cachexia symptoms in Yoshida sarcoma bearing male rats. Obesity and insulin resistance were induced through 5 weeks of high-fat (HF) diet feeding and insulin resistance was confirmed by intraperitoneal glucose tolerance testing. Chow-fed animals were used as a control group. Following the establishment of insulin resistance, HF- and chow-fed animals were implanted with fragments of the Yoshida sarcoma or received a sham surgery. Tumor growth rate was greater in HF-fed animals, resulting in larger tumors. In addition, cancer cachexia symptoms developed in HF-fed animals but not chow-fed animals during the 18-day experiment. These results support a stimulatory effect of obesity and insulin resistance on tumor growth and cancer cachexia development in Yoshida sarcoma-bearing rats. Future research should investigate the relationship between obesity, insulin resistance, and cancer cachexia in human subjects.

Chronic exendin-4 treatment prevents the development of cancer cachexia symptoms in male rats bearing the Yoshida sarcoma

Cancer cachexia is the syndrome of weight loss, loss of appetite, and wasting of skeletal muscle and adipose tissue experienced by many individuals with cancer. Currently, few effective treatment and prevention strategies are available for these patients, due in part to a poor understanding of the mechanisms contributing to cachexia. Insulin resistance has been associated with cancer cachexia in epidemiological, human, and animal research. The present experiment was designed to examine the ability of Exendin-4, a GLP-1 agonist and insulin sensitizing agent, to prevent the development of cachexia symptoms in male Sprague Dawley rats bearing the Yoshida sarcoma. Following tumor implantation or sham surgery, rats were treated daily with saline or Exendin-4 (3 μg/kg body weight/day) and were monitored for tumor growth and cachexia symptoms for 21-23 days. As a result of large variability in treatment effects, data were analyzed separately for animals with large and small tumors. Exendin-4 treatment reduced tumor growth and prevented the development of cancer cachexia symptoms in animals with small, but not large, tumors. In addition, insulin levels were preserved in Exendin-4-treated tumor-bearing animals. The results of this experiment demonstrate a novel preventative therapy for cancer cachexia and a novel use of Exendin-4. Further research is necessary to determine the mechanisms through which Exendin-4 exerts these potent effects.

Inter-relationships among diet, obesity and hippocampal-dependent cognitive function

Intake of a Western diet (WD), which is high in saturated fat and sugar, is associated with deficits in hippocampal-dependent learning and memory processes as well as with markers of hippocampal pathology. In the present study, rats were trained to asymptote on hippocampal-dependent serial feature negative (FN) and hippocampal-independent simple discrimination problems. Performance was then assessed following 7 days on ad libitum chow and after 10, 24, 40, 60, and 90 days of maintenance on WD, on ketogenic (KETO) diet, which is high in saturated fat and low in sugar and other carbohydrates, or continued maintenance on chow (CHOW). Confirming and extending previous findings, diet-induced obese (DIO) rats fed WD showed impaired FN performance, increased blood-brain barrier (BBB) permeability, and increased fasting blood glucose levels compared to CHOW controls and to diet-resistant (DR) rats that did not become obese when maintained on WD. For rats fed the KETO diet, FN performance and BBB integrity were more closely associated with level of circulating ketone bodies than with obesity phenotype (DR or DIO), with higher levels of ketones appearing to provide a protective effect. The evidence also indicated that FN deficits preceded and predicted increased body weight and adiposity. This research (a) further substantiates previous findings of WD-induced deficits in hippocampal-dependent FN discriminations, (b) suggests that ketones may be protective against diet-induced cognitive impairment, and (c) provides evidence that diet-induced cognitive impairment precedes weight gain and obesity.

Altered Hypothalamic Signaling and Responses to Food Deprivation in Rats Fed a Low-Carbohydrate Diet

OBJECTIVE

To model how consuming a low-carbohydrate (LC) diet influences food intake and body weight.

RESEARCH METHODS AND PROCEDURES

Food intake and body weight were monitored in rats with access to chow (CH), LC-high-fat (HF), or HF diets. After 8 weeks, rats received intracerebroventricular injections of a melanocortin agonist (melanotan-II) and antagonist (SHU9119), and feeding responses were measured. At sacrifice, plasma hormones and hypothalamic expression of mRNA for proopiomelanocortin (POMC), melanocortin-4 receptor, neuropeptide Y (NPY), and agouti related protein (AgRP) were assessed. A second set of rats had access to diet (chow or LC-HF) for 4 weeks followed by 24 h food deprivation on two occasions, after which food intake and hypothalamic POMC, NPY, and AgRP mRNA expression were measured.

RESULTS

HF rats consumed more food and gained more weight than rats on CH or LC-HF diets. Despite similar intakes and weight gains, LC-HF rats had increased adiposity relative to CH rats. LC-HF rats were more sensitive to melanotan-II and less sensitive to SHU9119. LC-HF rats had increased plasma leptin and ghrelin levels and decreased insulin levels, and patterns of NPY and POMC mRNA expression were consistent with those of food-deprived rats. LC-HF rats did not show rebound hyperphagia after food deprivation, and levels NPY, POMC, and AgRP mRNA expression were not affected by deprivation.

DISCUSSION

Our results demonstrate that an LC diet influences multiple systems involved in the controls of food intake and body weight. These data also suggest that maintenance on an LC-HF diet affects food intake by reducing compensatory responses to food deprivation.

Glucose tolerance in response to a high-fat diet is improved by a high-protein diet

Consumption of a high-fat (HF) diet results in insulin resistance and glucose intolerance. Weight loss is often recommended to reverse these metabolic alterations and the use of a high-protein (HP), low-carbohydrate diet is encouraged. In lean rats, consumption of a HP diet improves glycemic control. However, it is unknown whether this diet has a similar effectiveness in rodents with impaired glucose tolerance. Rats were fed a HF or a chow (CH) diet for 6 weeks and then switched to a HP diet or a CH or pair-fed (PF) to the amount of kcals consumed per day by the HP group. Following the diet switch, body weight gain was attenuated as compared to HF rats, and similar between HP, CH, and PF rats. Despite similar weight progression, HP and PF rats had a significant decrease in body fat after 2 weeks, as compared to HF rats. In contrast, CH rats did not show this effect. Glucose tolerance was attenuated more quickly in HP rats than in CH or PF rats. These results indicate that a HP diet may be more effective than a balanced diet for improving glycemic control in overweight individuals.

The role of insulin resistance in the development of muscle wasting during cancer cachexia

BACKGROUND

Cancer cachexia is a complex syndrome associated with multiple metabolic abnormalities. Insulin resistance is present in many cancer patients and may be one mechanism through which muscle wasting occurs.

METHODS AND RESULTS

The present review examines evidence in support of a role for insulin resistance in the development of muscle wasting during cancer cachexia and identifies areas for future research. Patients suffering from cancer cachexia tend to exhibit insulin resistance and improvements in insulin resistance have the potential to improve cachexia symptoms. In addition, evidence suggests that insulin resistance may occur prior to the onset of cachexia symptoms.

CONCLUSIONS

Further investigation of the role of insulin resistance in cancer cachexia is needed. The use of translational research in this area is strongly encouraged, and has important implications for clinical research and the treatment and prevention of cancer cachexia.

Repeated gastric distension alters food intake and neuroendocrine profiles in rats

The consumption of a large food bolus leads to stomach distension. Gastric distension potently signals the termination of a meal by stimulating gastric mechanoreceptors and activating neuroendocrine circuitry. The ability to terminate a meal is altered in disorders such as bulimia nervosa (BN), binge-eating disorder (BED) and certain subtypes of obesity in which large quantities of food are frequently ingested. When a large meal is consumed, the stomach is rapidly stretched. We modeled this rapid distension of the stomach in order to determine if the neuroendocrine abnormalities present in these disorders, including increased gastric capacit3y, leptin dysregulation, and alterations in neuropeptide Y (NPY), and proopiomelanocortin (POMC) expression, were influenced by the rapid stretch aspect of repeatedly consuming a large meal. To test the effects of repeated gastric distension (RGD) on neuroendocrine factors involved in energy homeostasis, a permanent intra-gastric balloon was implanted in rats, and briefly inflated daily for 4 weeks. Though body weights and daily food intakes remained equivalent in RGD and control rats, a significant delay in the onset of feeding was present during the first and second, but not the third and fourth weeks of inflations. Despite equivalent body weights and daily caloric consumption, RGD animals had significantly decreased leptin levels (p<0.05), and tended to have increased fasting arcuate NPY levels (p=0.08), which were suppressed more than control animals following food intake (control and RGD decreases from baseline were 184.95% and 257.42%, respectively). NPY expression in the nucleus of the solitary tract followed a similar pattern. These data demonstrate that the act of regularly distending the stomach can have effects on the regulation of energy balance that are independent from those related to caloric consumption, and may be related to disorders such as BN, BED, and certain types of obesity in which meal termination is impaired.

Insulin sensitivity and glucose tolerance are altered by maintenance on a ketogenic diet

Low-carbohydrate, ketogenic diets (KD) are frequently implemented in efforts to reduce or maintain body weight, although the metabolic effects of long-term exposure to this type of diet remain controversial. This study assessed the responsivity to peripheral and central insulin, glucose tolerance, and meal-induced effects of consuming a KD in the rat. After 8 wk of consuming chow or KD, caloric intake after peripheral or central insulin and insulin and glucose levels after a glucose challenge were assessed. In a separate group of rats, glucose and insulin responses to either a low- or high-carbohydrate test meal were measured. Finally, rats maintained on KD were switched back to a chow diet, and insulin sensitivity and glucose tolerance were evaluated to determine whether the effects of KD were reversible. Maintenance on KD resulted in decreased sensitivity to peripheral insulin and impaired glucose tolerance. Furthermore, consumption of a high-carbohydrate meal in rats that habitually consumed KD induced significantly greater insulin and glucose levels for an extended period of time, as compared with chow-fed controls. Responsivity to central insulin was heightened in KD rats and associated with increased expression levels of insulin receptor mRNA. Finally, returning to a chow diet rapidly reversed the effects of KD on insulin sensitivity and glucose tolerance. These data suggest that maintenance on KD negatively affects glucose homeostasis, an effect that is rapidly reversed upon cessation of the diet.

Adolescent activity-based anorexia increases anxiety-like behavior in adulthood

Activity-based anorexia is a paradigm that induces increased physical activity, reduced food intake, and heightened activity of the hypothalamic-pituitary-adrenal axis in adult rats. To investigate whether experience with activity-based anorexia produced enduring effects on brain and behavior, female adolescent rats experienced activity-based anorexia during adolescence and were tested in adulthood for anxiety-like behavior on an elevated plus maze and in an open field. Analysis of elevated plus maze and open field behavior in adulthood revealed that rats that experienced activity-based anorexia during adolescence, but not rats that were simply food restricted, displayed increased anxiety-like behavior in adulthood. Plasma corticosterone and expression levels of corticotropin-releasing hormone mRNA in the hypothalamic paraventricular nucleus and in the central nucleus of the amygdala were significantly elevated in adult rats that had undergone activity-based anorexia in adolescence in response to the open field exposure, as compared to control rats. These data demonstrate enduring effects of adolescent activity-based anorexia on anxiety-like behavior and neuroendocrine factors critical in stress responsivity in adulthood. Furthermore, we demonstrate that activity-based anorexia during adolescence serves as a model whereby prolonged anxiety is induced, allowing for evaluation of the behavioral and neural correlates of mediating anxiety-like behaviors in adulthood.

Sensitivity to the anorectic effects of leptin is retained in rats maintained on a ketogenic diet despite increased adiposity

BACKGROUND

Rats maintained on a ketogenic diet (KD; 80% fat, 15% protein, 5% carbohydrate) have increased adiposity and leptin as compared to chow-fed controls (CH; 16% fat, 19% protein, 65% carbohydrate), although body weights and daily caloric intakes do not differ.

METHODS

Rats maintained on a KD or CH were assessed for responsivity to intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) leptin. Hypothalamic gene expression was evaluated to determine the effects of KD on proopiomelanocortin (POMC) mRNA expression and components of the leptin-signaling system.

RESULTS

Caloric intake by KD rats was decreased at a lower dose of i.p. leptin (100 microg) than was required to reduce intake by CH rats (leptin, caloric intake was reduced in KD rats as compared to intake following i.p. saline; p < 0.05). In a separate experiment to evaluate responsivity to i.c.v. leptin, the minimal dose of leptin required to significantly reduce 24-hour caloric intake did not differ between the groups. In the arcuate nucleus, POMC mRNA was elevated after a lower dose of i.c.v. leptin in KD rats (5 microg) than was required to increase POMC mRNA expression in CH rats (15 microg) or reduce caloric intake in either group. Finally, evaluation of the level of phosphorylated STAT3 (pSTAT3) in the arcuate and SOCS3 mRNA in the hypothalamus revealed significantly more pSTAT3-positive cells and increased SOCS3 mRNA expression at baseline for KD rats, compared to CH, neither of which was further increased following i.p. leptin administration.

CONCLUSION

These data demonstrate that despite increased adiposity, leptin and markers of leptin resistance, responsivity to the anorectic effects of exogenous leptin is retainable during maintenance on a KD.

Effects of consuming a high carbohydrate diet after eight weeks of exposure to a ketogenic diet

BACKGROUND

Ketogenic diets have been utilized for weight loss and improvement in metabolic parameters. The present experiments examined the effects of returning to a chow diet after prolonged ingestion of a ketogenic diet.

METHODS

Rats were maintained on chow (CH) or a ketogenic diet (KD) for 8 weeks, after which the KD rats were given access to chow only (KD:CH) for 8 additional weeks. Caloric intake, body weight, and plasma leptin, insulin and ghrelin were measured before and after the dietary switch.

RESULTS

After 8 weeks of consuming a ketogenic diet, KD rats had increased adiposity and plasma leptin levels, and reduced insulin, as compared to CH controls. One week after the diet switch, fat pad weight and leptin levels remained elevated, and were normalized to CH controls within 8 weeks of the dietary switch. Switching from KD to chow induced a transient hypophagia, such that KD:CH rats consumed significantly fewer calories during the first week after the dietary switch, as compared to calories consumed by CH rats. This hypophagia was despite significantly increased plasma ghrelin in KD:CH rats. Finally, KD:CH rats developed hyperphagia over time, and during weeks 6-8 after the diet switch consumed significantly more calories per day than did CH-fed controls and gained more weight than CH-fed controls.

CONCLUSION

Collectively, these data demonstrate that returning to a carbohydrate-based diet after a period of consuming a ketogenic diet has post-diet effects on caloric intake, body weight gain, and insulin levels.

Central and peripheral effects of chronic food restriction and weight restoration in the rat

Previous studies have demonstrated that some endocrine consequences of long-term caloric restriction persist after weight restoration in human subjects. Here we evaluate effects of chronic food restriction in rats that were restricted to 70% of control kcal for 4 wk and subsequently weight restored. Measures were taken from rats at 80% (chronically restricted; CR), 90% (partially weight restored; PR), 100% (fully weight restored; FR), and after 4 wk at 100% body weight of controls (extended weight restored; ER). Plasma insulin and leptin were decreased, and ghrelin was increased in CR compared with controls. Leptin and ghrelin normalized with weight restoration at PR, FR, and ER; however, baseline insulin was not normalized until the ER state. Hypothalamic mRNA expression levels for proopiomelanocortin (POMC), agouti-related protein (AgRP), and neuropeptide Y (NPY) revealed significantly less POMC mRNA expression in CR and PR rats, and significantly less arcuate NPY mRNA in PR and FR. In the dorsomedial hypothalamus, CR, PR, and FR rats had significantly increased NPY expression that was not normalized until the ER state. In response to a test meal, insulin and ghrelin release patterns were altered through the FR stage, and ghrelin remained affected at ER. Collectively, these data demonstrate that mere weight restoration is not sufficient to normalize hypothalamic gene expression levels and endocrine responses to a meal, and that meal-related ghrelin responses persist despite weight restoration for up to 4 wk.

Insulin, glucose, and pancreatic polypeptide responses to a test meal in restricting type anorexia nervosa before and after weight restoration

Prolonged malnutrition in individuals with anorexia nervosa (AN) has been associated with alterations in endocrine function that may play a sustaining role in the disorder. We hypothesized that abnormalities in endocrine responses to ingestion of a meal in AN are reversible and depend on weight restoration. We measured meal-induced endocrine responses in AN subjects at three time points during hospitalization: before refeeding (n = 13, mean BMI 16.7 kg/m(2)), after 2 wk of refeeding (mean BMI 18.0 kg/m(2)), and in the weight-restored state (mean BMI 20.3 kg/m(2)). Control subjects (n = 13, BMI 19-24.9 kg/m(2)) were tested once. Tests were 2.5-h sessions in which blood was drawn every 15 min before, during, and after a approximately 650-kcal test breakfast. Relative to controls, peak levels of glucose were depressed and peak levels of insulin in response to ingestion of the test meal were delayed, with response patterns in the third trial most similar to controls. Pancreatic polypeptide (PP) levels were increased in AN relative to controls regardless of weight status. The delay in insulin release and elevated PP levels did not correct with short-term refeeding and may contribute to the high relapse rates and maintenance of AN.

Energy balance and hypothalamic effects of a high-protein/low-carbohydrate diet

Diets high in fat or protein and extremely low in carbohydrate are frequently reported to result in weight loss in humans. We previously reported that rats maintained on a low-carbohydrate-high fat diet (LC-HF) consumed similar kcals/day as chow (CH)-fed rats and did not differ in body weight after 7 weeks. LC-HF rats had a 45% decrease in POMC expression in the ARC, decreased plasma insulin, and increased plasma leptin and ghrelin. In the present study we assessed the effects of a low-carbohydrate-high-protein diet (HP: 30% fat, 65% protein, and 5% CHO) on body weight, caloric intake, plasma hormone levels and hypothalamic gene expression. Male rats (n=16) were maintained on CH or HP for 4 weeks. HP rats gained significantly less weight than CH rats (73.4+/-9.4 and 125.0+/-8.2 g) and consumed significantly less kcals/day (94.8+/-1.5 and 123.6+/-1.1). Insulin was significantly reduced in HP rats (HP: 1.8+/-0.6 vs. CH: 4.12+/-0.8 ng/ml), there were no differences between groups in plasma leptin and plasma ghrelin was significantly elevated in HP rats (HP: 127.5+/-45 vs. CH: 76.9+/-8 pg/ml). Maintenance on HP resulted in significantly increased ARC POMC (HP: 121+/-10.0 vs. 100+/-5.9) and DMH NPY (HP: 297+/-82.1 vs. CH: 100+/-37.7) expression compared to CH controls. These data suggest that the macronutrient content of diets differentially influences hypothalamic gene expression in ways that can affect overall intake.

Lateral ventricular ghrelin and fourth ventricular ghrelin induce similar increases in food intake and patterns of hypothalamic gene expression

The gut peptide ghrelin has been shown to stimulate food intake after both peripheral and central administration, and the hypothalamic arcuate nucleus has been proposed to be the major site for mediating this feeding stimulatory action. Ghrelin receptors are widely distributed in the brain, and hindbrain ghrelin administration has been shown to potently stimulate feeding, suggesting that there may be other sites for ghrelin action. In the present study, we have further assessed potential sites for ghrelin action by comparing the ability of lateral and fourth ventricular ghrelin administration to stimulate food intake and alter patterns of hypothalamic gene expression. Ghrelin (0.32, 1, or 3.2 nmol) in the lateral or fourth ventricle significantly increased food intake in the first 4 h after injection, with no ventricle-dependent differences in degree or time course of hyperphagia. One nanomole of ghrelin into either the lateral or fourth ventricle resulted in similar increases in arcuate nucleus neuropeptide Y mRNA expression. Expression levels of agouti-related peptide or proopiomelanocortin mRNA were not affected by ghrelin administration. These data demonstrate that ghrelin can affect food intake and hypothalamic gene expression through interactions at multiple brain sites.

Peptide YY(3-36) inhibits gastric emptying and produces acute reductions in food intake in rhesus monkeys

Peptide YY3-36 [PYY(3-36)], a gastrointestinal peptide that is released into the circulation in response to ingesting a meal, has recently been suggested to play a role in controlling food intake. PYY(3-36) has been reported to inhibit food intake following peripheral administration in rodents and in human subjects. To more fully characterize the potential feeding actions of PYY(3-36), we examined the ability of a dose range of PYY(3-36) (0.3-3.0 nmol/kg) to affect liquid gastric emptying and daily 6-h food intake in male rhesus monkeys. Intramuscular PYY(3-36) produced a dose-related inhibition of saline gastric emptying that was maximal at a dose of 3 nmol/kg. Intramuscular PYY(3-36) administered before daily 6-h food access produced significant feeding reductions at doses of 1 and 3 nmol/kg. Analyses of the patterns of food intake across the 6-h period of food access revealed that PYY(3-36) increased the latency to the first meal and reduced average meal size without altering meal number. Although single doses of PYY(3-36) reduced intake, a suppressive effect on food intake was not sustained over multiple administrations across successive days. Together, these data suggest that PYY(3-36) has the ability to reduce food intake in acute test situations in nonhuman primates. Whether this is a physiological action of the endogenous peptide remains to be determined.

Physiology: does gut hormone PYY3-36 decrease food intake in rodents?

Batterham et al. report that the gut peptide hormone PYY3-36 decreases food intake and body-weight gain in rodents, a discovery that has been heralded as potentially offering a new therapy for obesity. However, we have been unable to replicate their results. Although the reasons for this discrepancy remain undetermined, an effective anti-obesity drug ultimately must produce its effects across a range of situations. The fact that the findings of Batterham et al. cannot easily be replicated calls into question the potential value of an anti-obesity approach that is based on administration of PYY3-36.

Gastrointestinal satiety signals II. Cholecystokinin

During a meal, ingested nutrients accumulate in the stomach, with a significant portion passing on to the small intestine. The gastrointestinal presence of ingested nutrients initiates a range of physiological responses that serve to facilitate the overall digestive process. Thus peptides and transmitters are released, and various neural elements are activated that coordinate gastrointestinal secretion and motility and can eventually lead to meal termination or satiety. Among the range of gastrointestinal peptides released by ingested nutrients is the brain/gut peptide CCK. CCK plays a variety of roles in coordinating gastrointestinal activity and has been demonstrated to be an important mediator for the control of meal size.

CNS glucagon-like peptide-1 receptors mediate endocrine and anxiety responses to interoceptive and psychogenic stressors

Responses to stressors serve to adjust physiology and behavior to increase short-term survival at the potential expense of increasing susceptibility to disease over the long term. We show that glucagon-like peptide-1 (7-36) amide (GLP-1) increases levels of the stress-activated hormones ACTH and corticosterone when administered directly into the rat brain and increases levels of anxiety as measured by the elevated plus maze. The endocrine response is preferentially activated by GLP-1 administration in the paraventricular nucleus of the hypothalamus, whereas the anxiety response is preferentially activated by administration in the central nucleus of the amygdala. Furthermore, GLP-1 antagonists block increases in stress hormones associated with the toxin LiCl and both the endocrine and anxiety responses to vertical heights. Although diverse neural circuits must necessarily process disparate stressors, the current data implicate a role for the GLP-1 system as a critical mediator of multiple stress responses.

The diverse roles of specific GLP-1 receptors in the control of food intake and the response to visceral illness

Intracerebroventricular administration of glucagon-like peptide-1 (7-36) amide (GLP-1) reduces food intake and produces symptoms of visceral illness, such as a conditioned taste aversion (CTA). The central hypothesis of the present work is that separate populations of GLP-1 receptors mediate the anorexia and taste aversion associated with GLP-1 administration. To test this hypothesis, we first compared the ability of various doses of GLP-1 to induce anorexia or CTA when administered into either the lateral or fourth ventricle. Lateral and fourth ventricular GLP-1 resulted in reduction of food intake at similar doses, whereas only lateral ventricular GLP-1 resulted in a CTA. Such data indicate that both hypothalamic and caudal brainstem GLP-1 receptors are likely to participate in the ability of GLP-1 to reduce food intake. We also hypothesized that the site that must mediate the ability of GLP-1 to induce visceral illness is in the central nucleus of the amygdala (CeA). Administration of 0.2 or 1.0 microg of GLP-1 (7-36) but not the inactive GLP-1 (9-36) resulted in a strong CTA with no accompanying anorexia. In addition, bilateral CeA administration of 2.5 microg of a GLP-1 receptor antagonist before intraperitoneal administration of the toxin lithium chloride resulted in a diminished CTA. Together, these data indicate that separate GLP-1 receptor populations mediate the multiple responses to GLP-1. These results indicate that GLP-1 is a flexible system that can be activated under various circumstances to alter the ingestion of nutrients and/or produce other visceral illness responses, depending on the ascending pathways of the GLP-1 system that are recruited.