We hypothesized that anorexia induced by novelty stress caused by exposure to a novel environment may be due to activation of corticotropin-releasing factor (CRF) and subsequently mediated by decreasing peripheral ghrelin concentration via serotonin (5-HT) and melanocortin-4 receptors (MC4R). Each mouse was transferred from group-housed cages to individual cages to establish the novelty stress. We observed the effect of changes in feeding behavior in a novel environment using the method of transferring group-housed mice to individual cages. We investigated the effect of an intracerebroventricular injection of antagonists/agonists of CRF1/2 receptors (CRF1/2Rs), 5-HT(1B)/(2C) receptors (5-HT(1B)/(2C)R), and MC4R to clarify the role of each receptor on the decrease in food intake. Plasma ghrelin levels were also measured. The novelty stress caused a reduction in food intake that was abolished by administering a CRF1R antagonist. Three hours after the novelty stress, appetite reduction was associated with reduced levels of neuropeptide Y/agouti-related peptide mRNA, increased levels of proopiomelanocortin mRNA in the hypothalamus, and a decrease in plasma ghrelin level. Administering a CRF1R antagonist, a 5-HT(1B)/(2C)R antagonist, an MC4R antagonist, exogenous ghrelin, and an enhancer of ghrelin secretion, rikkunshito, resolved the reduction in food intake 3 h after the novelty stress by enhancing circulating ghrelin concentrations. We showed that anorexia during a novelty stress is a process in which CRF1R is activated at the early stage of appetite loss and is subsequently activated by a 5-HT(1B)/(2C)R and MC4R stimulus, leading to decreased peripheral ghrelin concentrations.

This study was conducted to clarify the role of serotonin (5-hydroxytryptamine, 5-HT) 2C receptor (5-HT2CR) signaling during novelty-induced hypophagia in aged mice. Male C57BL/6J mice [6-week-old (young) and 79-80-week-old (aged) mice] were exposed to a novel environment, and its effects on feeding behavior, stress hormones, and appetite-related factors were examined. Exposure of aged mice to a novel environment suppressed food intake and increased corticosterone secretion. These responses were marked compared with those in young mice. The expression in hypothalamic corticotropin-releasing factor (CRF), pituitary CRF1R and proopiomelanocortin mRNA in aged mice exposed to a novel environment was increased or tended to increase, compared to control mice. 5-HT2CR antagonist, SB242084 or rikkunshito administration attenuated the decrease in food intake and increased stress hormone levels in aged mice exposed to the environmental change. The 5-HT2CR mRNA expression in paraventricular nucleus was significantly enhanced, when aged mice was exposure to the novel environment. Thus, novelty-induced hypophagia in aged mice resulted, at least in part, from up-regulated hypothalamic 5-HT2CR function. In conclusion, 5-HT2CR signaling enhancement and the subsequent activation of the CRF neuron were involved in novelty-induced hypophagia in aged mice, and the 5-HT2CR antagonists offer a promising therapeutic option for depression.

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The combination of depression and anorexia may influence morbidity and progressive physical disability in the elderly. Gender differences exist in hypothalamic-pituitary-adrenal axis activation following stress exposure. The objective of this study was to investigate gender differences in feeding behavior under novelty stress in aged mice. Food intake measurement, immunohistochemical assessment, and mRNA expression analysis were conducted to investigate the role of serotonin 2C receptor (5-HT(2C)R) and its relationship with ghrelin in stress-induced suppression of feeding behavior in aged mice. After exposure to novelty stress, a 21-fold increase in plasma corticosterone and remarkable suppression of food intake were observed in aged male mice. Furthermore, a 5-HT(2C)R agonist suppressed food intake in aged male mice. Novelty stress induced a 7-fold increase in 5-HT(2C)R and c-Fos co-expressing cells in the paraventricular nucleus of the hypothalamus in aged male mice but caused no change in aged female mice. Plasma acylated ghrelin levels decreased in stressed aged male mice and administration of the 5-HT(2C)R antagonist inhibited this decrease. The 5-HT(2C)R antagonist also reversed the suppression of food intake in estrogen receptor α agonist-treated aged male mice. Therefore, conspicuously suppressed feeding behavior in novelty stress-exposed aged male mice may be mediated by 5-HT(2C)R hypersensitivity, leading to hypoghrelinemia. The hypersensitivity may partly be due to estrogen receptor activation in aged male mice.

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Psychological stress due to social isolation is known to cause abnormal feeding behaviors, but the influences of gender and aging on subchronic stress-induced changes in feeding behaviors are unknown. Thus, we examined the changes in body weight, food intake, and orexigenic ghrelin-related factors during 2 weeks of isolation stress in young and aged mice. Food intake increased significantly in young mice in the isolation group compared with the group-housed control throughout the experimental period. This isolation-induced increase in food intake was not observed in aged mice. In young mice, there were no significant differences in body weight between the isolated group and group-housed control up to 2 weeks. However, aged male mice exhibited significant weight loss at 2 weeks and a similar tendency was observed in aged female mice. Young male mice, but not female mice, had significantly increased (2.2-fold) plasma acylated ghrelin levels after 1 week of isolation compared with the group-housed control. A significant but lower increase (1.3-fold) was also observed in aged male mice. Hypothalamic preproghrelin gene expression decreased significantly with isolation in young male mice, whereas it increased significantly in female mice. The expression levels of NPY and AGRP in the hypothalamus, which are transmitted by elevated peripheral ghrelin signals, increased significantly in isolated young male mice, whereas the AGRP expression levels decreased significantly in young female mice. Isolation caused no significant differences in the expression levels of these genes in aged mice. In isolation, young female mice exhibited markedly increased dark- and light-phase locomotor activities compared with male mice, whereas male and female aged mice exhibited no obvious increases in activity immediately after the dark phase started. We conclude that the gender-specific homeostatic regulatory mechanisms required to maintain body weight operated during subchronic psychological stress in young mice but not in aged mice.

BackgroundPhysical or psychological stress causes functional disorders in the upper gastrointestinal tract. This study aims to elucidate the ameliorating effect of exogenous acylated ghrelin or rikkunshito, a Kampo medicine which acts as a ghrelin enhancer, on gastric dysfunction during acute restraint stress in mice.MethodsFasted and postprandial motor function of the gastric antrum was wirelessly measured using a strain gauge force transducer and solid gastric emptying was detected in mice exposed to restraint stress. Plasma corticosterone and ghrelin levels were also measured. To clarify the role of ghrelin on gastrointestinal dysfunction in mice exposed to stress, exogenous acylated ghrelin or rikkunshito was administered, then the mice were subjected to restraint stress.Key ResultsMice exposed to restraint stress for 60 min exhibited delayed gastric emptying and increased plasma corticosterone levels. Gastric motility was decreased in mice exposed to restraint stress in both fasting and postprandial states. Restraint stress did not cause any change in plasma acylated ghrelin levels, but it significantly increased the plasma des-acyl ghrelin levels. Administration of acylated ghrelin or rikkunshito improved the restraint stress-induced delayed gastric emptying and decreased antral motility. Ameliorating effects of rikkunshito on stress-induced gastric dysfunction were abolished by simultaneous administration of a ghrelin receptor antagonist.Conclusions & InferencesPlasma acylated/des-acyl ghrelin imbalance was observed in acute restraint stress. Supplementation of exogenous acylated ghrelin or enhancement of endogenous ghrelin signaling may be useful in the treatment of decreased gastric function caused by stress.

We investigated the effects of rikkunshito (RKT), a ghrelin signal enhancer, on the decrease in food intake after exposure to novelty stress in mice. RKT administration (500 mg/kg, per os) improved the decrease in 6 h cumulative food intake. In control mice, the plasma acylated ghrelin levels significantly increased by 24 h fasting. In contrast, the acylated ghrelin levels did not increase by fasting in mice exposed to the novelty stress. RKT administration to the novelty stress mice showed a significant increase in the acylated ghrelin levels compared with that in the distilled-water-treated control mice. Food intake after administering serotonin 2B (5-HT2B) receptor antagonists was evaluated to clarify the role of 5-HT2B receptor activation in the decrease in feeding behavior after novelty stress. SB215505 and SB204741, 5-HT2B receptor antagonists, significantly improved the decrease in food intake after exposure to novelty stress. A component of RKT, isoliquiritigenin, prevented the decrease in 6 h cumulative food intake. Isoliquiritigenin showed 5-HT2B receptor antagonistic activity in vitro. In conclusion, the results suggested that RKT improves the decrease in food intake after novelty stress probably via 5-HT2B receptor antagonism of isoliquiritigenin contained in RKT.