Principles for interpreting interactions among the multiple systems that influence food intake

Neither GLP-1 receptors nor GFRAL neurons are required for aversive or anorectic response to DON (vomitoxin)

Deoxynivalenol (DON), a type B trichothecene mycotoxin contaminating grains, promotes nausea, emesis and anorexia. With DON exposure, circulating levels of intestinally derived satiation hormones, including glucagon-like peptide 1 (GLP-1) are elevated. To directly test whether GLP-1 signaling mediates the effects of DON, we examined the response of GLP-1 or GLP-1R-deficient mice to DON injection. We found comparable anorectic and conditioned taste avoidance learning responses in GLP-1/GLP-1R deficient mice compared to control littermates, suggesting that GLP-1 is not necessary for the effects of DON on food intake and visceral illness. We then used our previously published data from translating ribosome affinity purification with RNA sequencing (TRAP-seq) analysis of area postrema neurons that express the receptor for the circulating cytokine growth differentiation factor (GDF15), growth differentiation factor a-like (GFRAL). Interestingly, this analysis showed that a cell surface receptor for DON, calcium sensing receptor (CaSR), is heavily enriched in GFRAL neurons. Given that GDF15 potently reduces food intake and can cause visceral illness by signaling through GFRAL neurons, we hypothesized that DON may also signal by activating CaSR on GFRAL neurons. Indeed, circulating GDF15 levels are elevated after DON administration but both GFRAL knockout and GFRAL neuron-ablated mice exhibited similar anorectic and conditioned taste avoidance responses compared to WT littermates. Thus, GLP-1 signaling and GFRAL signaling and neurons are not required for DON-induced visceral illness or anorexia.

Effects of Combined Oxytocin and Beta-3 Receptor Agonist (CL 316243) Treatment on Body Weight and Adiposity in Male Diet-Induced Obese Rats

Previous studies have indicated that oxytocin (OT) reduces body weight in diet-induced obese (DIO) rodents through reductions in energy intake and increases in energy expenditure. We recently demonstrated that hindbrain [fourth ventricular (4V)] administration of OT evokes weight loss and elevates interscapular brown adipose tissue temperature (T IBAT ) in DIO rats. What remains unclear is whether OT can be used as an adjunct with other drugs that directly target beta-3 receptors in IBAT to promote BAT thermogenesis and reduce body weight in DIO rats. We hypothesized that the combined treatment of OT and the beta-3 agonist, CL 316243, would produce an additive effect to decrease body weight and adiposity in DIO rats by reducing energy intake and increasing BAT thermogenesis. We assessed the effects of 4V infusions of OT (16 nmol/day) or vehicle (VEH) in combination with daily intraperitoneal injections of CL 316243 (0.5 mg/kg) or VEH on food intake, T IBAT , body weight and body composition. OT and CL 316243 alone reduced body weight by 7.8 ± 1.3% (P < 0.05) and 9.1 ± 2.1% (P < 0.05), respectively, but the combined treatment produced more pronounced weight loss (15.5 ± 1.2%; P < 0.05) than either treatment alone. These effects were associated with decreased adiposity, adipocyte size, energy intake and increased uncoupling protein 1 (UCP-1) content in epididymal white adipose tissue (EWAT) (P < 0.05). In addition, CL 316243 alone (P < 0.05) and in combination with OT (P < 0.05) elevated T IBAT and IBAT UCP-1 content and IBAT thermogenic gene expression. These findings are consistent with the hypothesis that the combined treatment of OT and the beta-3 agonist, CL 316243, produces an additive effect to decrease body weight. The findings from the current study suggest that the effects of the combined treatment on energy intake, fat mass, adipocyte size and browning of EWAT were not additive and appear to be driven, in part, by transient changes in energy intake in response to OT or CL 316243 alone as well as CL 316243-elicited reduction of fat mass and adipocyte size and induction of browning of EWAT.

Corticotropin-releasing hormone and the sympathoadrenal system are major mediators in the effects of peripherally administered exendin-4 on the hypothalamic-pituitary-adrenal axis of male rats

Glucagon-like peptide-1 (GLP-1) and the GLP-1 receptor agonist, exendin-4 (Ex-4), potently stimulate hypothalamic-pituitary-adrenal (HPA) axis activity after either central or peripheral administration. Because several GLP-1 derivative drugs, including synthetic Ex-4, are currently in use to treat patients with type II diabetes mellitus, the characterization of Ex-4 effects on the HPA axis is highly relevant. Herein, the roles of CRH and AVP on these effects were investigated by administering the antagonists astressin and d(CH2)5Tyr(Me)AVP, respectively. The role of the sympathoadrenal system (SAS) was explored in bilateral adrenal enucleated and guanethidine-treated rats, whereas primary pituitary cell cultures were used to study direct effects on the corticotropes. Astressin completely abrogated (P < .05) the effects of Ex-4 central administration on ACTH secretion but only slightly reduced (by 35%) the ACTH response to Ex-4 peripheral administration. Moreover, astressin significantly (P < .05) decreased the corticosterone response to centrally but not peripherally administered Ex-4, suggesting different mechanisms depending on the route of administration. Pretreatment with d(CH2)5Tyr(Me)AVP failed to diminish either the ACTH or corticosterone response to Ex-4 and no direct effect of Ex-4 or GLP-1 was observed on pituitary cell cultures. In contrast, a significant (P < .05) reduction in the corticosterone response elicited by Ex-4 peripheral administration was observed in enucleated and guanethidine-treated rats, indicating a role of the SAS in the glucocorticoid stimulatory effects of Ex-4. Our data demonstrate that the effects of Ex-4 on the HPA axis are partially mediated by CRH and the sympathoadrenal system, and stress the relevance of Ex-4 as a corticosterone secretagogue.

GLP-1 receptor signaling is not required for reduced body weight after RYGB in rodents

Exaggerated GLP-1 and PYY secretion is thought to be a major mechanism in the reduced food intake and body weight after Roux-en-Y gastric bypass surgery. Here, we use complementary pharmacological and genetic loss-of-function approaches to test the role of increased signaling by these gut hormones in high-fat diet-induced obese rodents. Chronic brain infusion of a supramaximal dose of the selective GLP-1 receptor antagonist exendin-9-39 into the lateral cerebral ventricle significantly increased food intake and body weight in both RYGB and sham-operated rats, suggesting that, while contributing to the physiological control of food intake and body weight, central GLP-1 receptor signaling tone is not the critical mechanism uniquely responsible for the body weight-lowering effects of RYGB. Central infusion of the selective Y2R-antagonist BIIE0246 had no effect in either group, suggesting that it is not critical for the effects of RYGB on body weight under the conditions tested. In a recently established mouse model of RYGB that closely mimics surgery and weight loss dynamics in humans, obese GLP-1R-deficient mice lost the same amount of body weight and fat mass and maintained similarly lower body weight compared with wild-type mice. Together, the results surprisingly provide no support for important individual roles of either gut hormone in the specific mechanisms by which RYGB rats settle at a lower body weight. It is likely that the beneficial effects of bariatric surgeries are expressed through complex mechanisms that require combination approaches for their identification.

The anorectic effect of GLP-1 in rats is nutrient dependent

GLP-1-induced insulin secretion from the β-cell is dependent upon glucose availability. The purpose of the current study was to determine whether CNS GLP-1 signaling is also glucose-dependent. We found that fasting blunted the ability of 3(rd) cerebroventricularly (i3vt)-administered GLP-1 to reduce food intake. However, fasted animals maintained the anorexic response to melanotan II, a melanocortin receptor agonist, indicating a specific effect of fasting on GLP-1 action. We also found that i3vt administration of leptin, which is also decreased with fasting, was not able to potentiate GLP-1 action in fasted animals. However, we did find that CNS glucose sensing is important in GLP-1 action. Specifically, we found that i3vt injection of 2DG, a drug that blocks cellular glucose utilization, and AICAR which activates AMPK, both blocked GLP-1-induced reductions in food intake. To examine the role of glucokinase, an important CNS glucose sensor, we studied glucokinase-heterozygous knockout mice, but found that they responded normally to peripherally administered GLP-1 and exendin-4. Interestingly, oral, but not i3vt or IP glucose potentiated GLP-1's anorectic action. Thus, CNS and peripheral fuel sensing are both important in GLP-1-induced reductions in food intake.

A potential role for hypothalamomedullary POMC projections in leptin-induced suppression of food intake

Melanocortin-3/4 receptor ligands administered to the caudal brain stem potently modulate food intake by changing meal size. The origin of the endogenous ligands is unclear, because the arcuate nucleus of the hypothalamus and the nucleus of the solitary tract (NTS) harbor populations of proopiomelanocortin (POMC)-expressing neurons. Here we demonstrate that activation of hypothalamic POMC neurons leads to suppression of food intake and that this suppression is prevented by administration of a melanocortin-3/4 receptor antagonist to the NTS and its vicinity. Bilateral leptin injections into the rat arcuate nucleus produced long-lasting suppression of meal size and total chow intake. These effects were significantly blunted by injection of SHU-9119 into the fourth ventricle, although SHU-9119 increased meal size and food intake during the first, but not the second, 14-h observation period. Leptin effects on meal size and food intake were abolished throughout the 40-h observation period by injection of SHU-9119 into the NTS at a dose that by itself had no effect. Neuron-specific tracing from the arcuate nucleus with a Cre-inducible tract-tracing adenovirus in POMC-Cre mice showed the presence of labeled axons in the NTS. Furthermore, density of alpha-melanocyte-stimulating hormone-immunoreactive axon profiles throughout the NTS was decreased by approximately 70% after complete surgical transection of connections with the forebrain in the chronic decerebrate rat model. The results further support the existence of POMC projections from the hypothalamus to the NTS and suggest that these projections have a functional role in the control of food intake.

Septal co-infusions of glucose with the benzodiazepine agonist chlordiazepoxide impair memory, but co-infusions of glucose with the opiate morphine do not

We have found repeatedly that medial septal (MS) infusions of glucose impair memory when co-infused with the gamma-amino butyric acid (GABA) agonist muscimol. The present experiments sought to determine whether the memory-impairing effects of this concentration of glucose would generalize to another GABA(A) receptor agonist and to an agonist from another neurotransmitter system that is known to impair memory. Specifically, we determined whether the dose of glucose that produces memory deficits when combined with muscimol in the MS would also impair memory when co-infused with the GABA(A) receptor modulator chlordiazepoxide (CDP) or the opiate morphine. Male Sprague-Dawley rats were given MS co-infusions and then 15 min later tested for spontaneous alternation or given shock avoidance training (retention tested 48 h later). The results showed that MS infusions of the higher dose of glucose with morphine did not produce memory deficits, whereas, the performance of rats given MS co-infusions of CDP with glucose was impaired. These findings suggest that the memory-impairing effects of brain glucose administration may involve an interaction with the GABA(A) receptor.

Zero net flux estimates of septal extracellular glucose levels and the effects of glucose on septal extracellular GABA levels

Although hippocampal infusions of glucose enhance memory, we have found repeatedly that septal glucose infusions impair memory when gamma-aminobutyric acid (GABA) receptors are activated. For instance, hippocampal glucose infusions reverse the memory-impairing effects of co-infusions of the GABA agonist muscimol, whereas septal glucose infusions exacerbate memory deficits produced by muscimol. One potential explanation for these deleterious effects of glucose in the septum is that there are higher levels of endogenous extracellular fluid glucose concentrations in the septum than in the hippocampus. Another hypothesis is that septal glucose infusions impair memory by increasing septal GABA synthesis or release, which is possible because elevating glucose increases GABA levels in other brain regions. To test these hypotheses, Experiment 1 quantified extracellular fluid glucose levels in the septum and hippocampus using zero net flux in vivo microdialysis procedures in conscious, freely moving rats. Experiment 2 determined whether septal infusions of glucose would increase GABA concentrations in dialysates obtained from the septum. The results of Experiment 1 indicated that extracellular fluid glucose levels in the hippocampus and septum are comparable. The results of Experiment 2 showed that co-infusions of glucose with muscimol, at doses that did not affect memory on their own, decreased percent alternation memory scores. However, none of the infusions significantly affected GABA levels. Collectively, these findings suggest that the memory-impairing effects of septal infusions of glucose are not likely due to regional differences in basal extracellular fluid glucose concentrations and are not mediated via an increase in septal GABA release.

The memory-impairing effects of septal GABA receptor activation involve GABAergic septo-hippocampal projection neurons

Septal infusions of the gamma-aminobutyric acid (GABA)(A) agonist muscimol impair memory, and the effect likely involves the hippocampus. GABA(A) receptors are present on the perikarya of cholinergic and GABAergic septo-hippocampal (SH) projections. The current experiments determined whether GABAergic SH projections are involved in the memory-impairing effects of septal GABA(A) receptor activation. Experiment 1 tested whether combining septal co-infusions of subeffective doses of muscimol with scopolamine, a drug that selectively influences GABA SH projections, would produce memory deficits. Experiment 2 tested whether hippocampal infusions of a GABA(A) receptor antagonist would block the effects of septal muscimol infusions. Fifteen minutes prior to assessing spontaneous alternation (SA) or training in a multiple trial inhibitory avoidance (CMIA) task, male Sprague-Dawley rats were given septal infusions of vehicle, muscimol, scopolamine, or co-infusions of muscimol with scopolamine, or septal infusions of vehicle or muscimol combined with hippocampal infusions of vehicle or bicuculline. Septal co-infusions of muscimol with scopolamine significantly impaired SA and CMIA. Hippocampal bicuculline infusions blocked deficits produced by septal muscimol infusions in SA and attenuated deficits produced in CMIA. Combined, these findings suggest that GABAergic SH projections are involved in the memory-impairing effects of septal GABA receptor activation.

The central melanocortin system directly controls peripheral lipid metabolism

Disruptions of the melanocortin signaling system have been linked to obesity. We investigated a possible role of the central nervous melanocortin system (CNS-Mcr) in the control of adiposity through effects on nutrient partitioning and cellular lipid metabolism independent of nutrient intake. We report that pharmacological inhibition of melanocortin receptors (Mcr) in rats and genetic disruption of Mc4r in mice directly and potently promoted lipid uptake, triglyceride synthesis, and fat accumulation in white adipose tissue (WAT), while increased CNS-Mcr signaling triggered lipid mobilization. These effects were independent of food intake and preceded changes in adiposity. In addition, decreased CNS-Mcr signaling promoted increased insulin sensitivity and glucose uptake in WAT while decreasing glucose utilization in muscle and brown adipose tissue. Such CNS control of peripheral nutrient partitioning depended on sympathetic nervous system function and was enhanced by synergistic effects on liver triglyceride synthesis. Our findings offer an explanation for enhanced adiposity resulting from decreased melanocortin signaling, even in the absence of hyperphagia, and are consistent with feeding-independent changes in substrate utilization as reflected by respiratory quotient, which is increased with chronic Mcr blockade in rodents and in humans with loss-of-function mutations in MC4R. We also reveal molecular underpinnings for direct control of the CNS-Mcr over lipid metabolism. These results suggest ways to design more efficient pharmacological methods for controlling adiposity.

Leptin modulation of peripheral controls of meal size

Leptin reduces food intake through a specific effect on meal size. Investigations into how this within meal effect of leptin is mediated have demonstrated that leptin increases the ability of within meal inhibitory feedback signaling to limit intake and activate neurons within the nucleus of the solitary tract (NTS). Leptin's effects on neural activation can be demonstrated both as an increase in c-fos activation and as increase in electrophysiological activity in response to peripheral stimuli. Leptin can exert these effects through interactions at hypothalamic sites and activation of a descending pathway. NPY has opposite effect suggesting a role for reduced NPY signaling in the actions of leptin. Forebrain ventricular administration of a melanocortin agonist does not mimic the actions of leptin. As well as modulating within meal signaling through a descending pathway leptin, NPY and melanocortins could work directly at hindbrain integrative sites suggesting the possibility of distributed controls of meal size by anorexigenic and orexigenic signaling.

Apolipoprotein A-IV interacts synergistically with melanocortins to reduce food intake

Apolipoprotein (apo) A-IV is an anorexigenic gastrointestinal peptide that is also synthesized in the hypothalamus. The goal of these experiments was to determine whether apo A-IV interacts with the central melanocortin (MC) system in the control of feeding. The third ventricular (i3vt) administration of a subthreshold dose of apo A-IV (0.5 microg) potentiated i3vt MC-induced (metallothionein-II, 0.03 nmol) suppression of 30-min feeding in Long-Evans rats. A subthreshold dose of the MC antagonist (SHU9119, 0.1 nmol, i3vt) completely attenuated the anorectic effect of i3vt apo A-IV (1.5 microg). The i3vt apo A-IV significantly elevated the expression of c-Fos in neurons of the paraventricular nucleus of the hypothalamus, but not in the arcuate nucleus or median eminence. In addition, c-Fos expression was not colocalized with proopiomelanocortin-positive neurons. These data support a synergistic interaction between apo A-IV and melanocortins that reduces food intake by acting downstream of the arcuate.

Gastrointestinal hormones and food intake

Despite dramatic fluctuations in calorie intake, animals maintain a very stable body weight. The reason is that energy intake and expenditure are precisely matched. Long-term regulation of energy balance is dependent on the coordination and interpretation of signals such as those given by insulin and leptin indicating sufficient long-term energy stores as well as short-term, meal-related signals such as those given by cholecystokinin (CCK). Within the last 30 years, our knowledge of short-term signals has increased dramatically. Throughout the cephalo-caudal axis of the gastrointestinal system, discrete enteroendocrine cells respond to both mechanical and chemical stimulation. Meal-associated hormone release is dependent on the concentration and composition of the nutrients ingested. Released signals are transmitted neurally through vagal afferents or humorally as circulating ligands for specific receptor populations in the periphery and central nervous system. These signals are interpreted by the CNS and manifested as a behavioral modification of feeding. This review will present past and recent literature in support of gut hormones and their roles as mediators of satiety. Evidence from pharmacologic and physiologic studies involving both humans and rodents will be presented, along with a short section outlining the knowledge gained through the use of murine knockout models. Last, the contribution of satiety hormones as likely mediators of the effectiveness seen following obesity surgery will be reviewed. Although traditionally thought of as short-term, meal-related signals, enhanced, chronic hormone secretion and signaling resulting from gut reconstruction as seen with gastric bypass surgery most likely contributes to the superior efficacy of surgery as a treatment for obesity.

The utility of animal models of human energy homeostasis

The prevalence of obesity among adults and children has increased steadily over the last few years worldwide, reaching epidemic proportions. Particularly alarming is the link between obesity and the development of chronic disorders such as heart disease, type 2 diabetes, hypertension and some cancers (Bjorntorp, 1997). Environmental causes of obesity are thought to include a sedentary lifestyle and an abundance of highly palatable energy-dense foods (Hill et al. 2003). Genetic factors also contribute to susceptibility to obesity, although the genetic basis of most human obesities is thought to be polygenic (Comuzzie & Allison, 1998; Barsh et al. 2000). The present paper considers some of the animal models used to infer aspects of human obesity, with an emphasis upon their usefulness.