A physiological role in satiety is proposed for glucagon-like peptide-1 (GLP-1), secreted by the distal intestine in response to ingested nutrients. Here we report that in rats, ip injection of the GLP-1 receptor (GLP-1-R) antagonist exendin 9-39 (Ex9) elicited hyperphagia, but only at times of day when intake is otherwise low. Furthermore, ip administration of Ex9 attenuated satiety induced by either a voluntarily consumed sucrose meal (by 100%) or an intragastric glucose load (by 40%). To determine whether these effects involve blockade of GLP-1-R in brain or at a peripheral site, we injected Ex9 either centrally (into the third ventricle) or peripherally (ip) prior to GLP-1 injected either centrally or peripherally. Anorexia induced by peripheral GLP-1 was fully blocked by peripheral, but not central, pretreatment with Ex9, whereas the opposite was true for anorexic effect of central GLP-1. Thus, ip Ex9 appears to attenuate satiety via peripheral GLP-1-R blockade. Finally, anorexia induced by ip injection of exendin-4 (a GLP-1-R agonist) was due to both reduced meal size and increased duration between meals. We conclude that GLP-1 released from the intestine in response to ingested nutrients is a physiologically active satiety signal.

Centrally administered glucagon-like peptide-1 (GLP-1) supresses food intake. Here we demonstrate that GLP-1-producing (PPG) neurons in the nucleus tractus solitarii (NTS) are the predominant source of endogenous GLP-1 within the brain. Selective ablation of NTS PPG neurons by viral expression of diphtheria toxin subunit A (DTA) substantially reduced active GLP-1 concentrations in brain and spinal cord. Contrary to expectations, this loss of central GLP-1 had no significant effect on ad libitum feeding of mice, affecting neither daily chow intake nor body weight or glucose tolerance. Only after bigger challenges to homeostasis were PPG neurons necessary for food intake control. PPG-ablated mice increased food intake following a prolonged fast and after a liquid diet preload. Consistent with our ablation data, acute inhibition of hM4Di-expressing PPG neurons did not affect ad libitum feeding, however, it increased post-fast refeeding intake and blocked stress-induced hypophagia. Additionally, chemogenetic PPG neuron activation through hM3Dq caused a strong acute anorectic effect. We conclude that PPG neurons are not involved in primary intake regulation, but form part of a secondary satiation/satiety circuit, activated by both psychogenic stress and large meals. Given their hypophagic capacity, PPG neurons might be an attractive drug target in obesity treatment.

Central glucagon-like peptide 1 receptor (GLP-1R) stimulation suppresses food intake, and hindbrain GLP-1 neurons project to numerous feeding-relevant brain regions. One such region is the nucleus accumbens (NAc), which plays a role in reward and motivated behavior. Using immunohistochemical and retrograde tracing techniques in rats, we identified a robust projection from GLP-1 neurons in the nucleus of the solitary tract to the NAc. We hypothesized that activation of NAc GLP-1Rs suppresses feeding. When injected into the NAc core of rats at doses subthreshold for effect when administered to the lateral ventricle, GLP-1 significantly reduced food intake relative to vehicle at 1, 2, and 24 h posttreatment. The same doses had no effect when injected into the NAc shell. NAc core treatment with ventricle-subthreshold doses of the GLP-1R antagonist exendin (9–39) caused significant hyperphagia at 2 h posttreatment, suggesting that endogenous stimulation of NAc core GLP-1Rs plays a role in limiting food intake. It has been suggested that GLP-1 can cause nausea, but we found that NAc core administration of GLP-1 did not cause a conditioned taste aversion to saccharin, suggesting that the anorexic effect of NAc core GLP-1 is not caused by malaise. Finally, we observed that NAc core injection of GLP-1 significantly increased c-Fos expression in the NAc core. We conclude that that GLP-1Rs in the NAc play a physiologic role in food intake control, and suggest that the GLP-1 projection to NAc core may link satiation signal processing in the hindbrain with forebrain processing of food reward.

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Plasma ghrelin levels are responsive to short- and long-term nutrient fluctuation, rapidly decreasing with food consumption and increasing with food deprivation or weight loss. We hypothesized a vagal contribution to both responses. Nutrient-related ghrelin suppression may be mediated by gastrointestinal load-related vagal afferent activity, or depend upon vagal efferent input to the foregut, where most ghrelin is produced. Similarly, the deprivation-induced ghrelin rise could require state-related vagal afferent or efferent activity. Here, we examined the role of the vagus nerve in the regulation of plasma ghrelin by sampling blood from rats with subdiaphragmatic vagotomy and from sham-operated controls over 48 h of food deprivation, and before and after gastric gavage of liquid diet. Vagotomy affected neither baseline ghrelin levels nor the suppression of ghrelin by a nutrient load. The food deprivation-induced elevation of plasma ghrelin levels ( approximately 160% of baseline), however, was completely prevented by subdiaphragmatic vagotomy. In a separate experiment, the deprivation-related rise in plasma ghrelin was substantially reduced by atropine methyl nitrate treatment, indicating that the response to fasting is driven by increased vagal efferent tone. The dissociation between nutrient load- and deprivation-related ghrelin responses indicates that the regulation of circulating ghrelin levels involves separate mechanisms operating through anatomically distinct pathways.