Aim/hypothesis. By acting in the brain, insulin suppresses food intake. However, little is known with regard to insulin signalling in the hypothalamus in insulin-resistant states. Methods. Western blotting, immunohistochemistry and polymerase chain reaction assays were combined to compare in vivo hypothalamic insulin signalling through the PI3-kinase and MAP kinase pathways between lean and obese Zucker rats. Results. Intracerebroventricular insulin infusion reduced food intake in lean rats to a greater extent than that observed in obese rats, and pre-treatment with PI3-kinase inhibitors prevented insulin-induced anorexia. The relative abundance of IRS-2 was considerably higher than that of IRS-1 in hypothalamus of both lean and obese rats. Insulin-stimulated phosphorylation of IR, IRS-1/2, the associations of PI 3-kinase to IRS-1/2 and phosphorylation of Akt in hypothalamus were decreased in obese rats compared to lean rats. These effects seem to be mediated by increased phosphoserine content of IR, IRS-1/2 and decreased protein levels of IRS-1/2 in obese rats. In contrast, insulin stimulated the phosphorylation of MAP kinase equally in lean and obese rats. Conclusion/interpretation. This study provides direct measurements of insulin signalling in hypothalamus, and documents selective resistance to insulin signalling in hypothalamus of Zucker rats. These findings provide support for the hypothesis that insulin could have anti-obesity actions mediated by the PI3-kinase pathway, and that impaired insulin signalling in hypothalamus could play a role in the development of obesity in this animal model of insulin-resistance. [Diabetologia (2003[Diabetologia ( ) 46:1629[Diabetologia ( -1640 Keywords Obesity, insulin resistance, hypothalamus, anorexia, signal transduction, phosphatidylinositol 3-kinase/antagonists & inhibitors/*metabolism, mitogen-activated protein kinase. Abbreviations: ERK, extracellular signal-regulated kinase; Grb2, growth factor receptor binding protein 2; IR, insulin receptor; IRS, insulin receptor substrate; MAPK, mitogen-activated protein kinase; PI 3-kinase, phosphatidylinositol 3-kinase; PKC, Protein kinase C; Shc, Src-homology and collagen homology; SHP2, Src-homology phosphatase 2; TNF-α, Tumor-necrosis factor-α.

Insulin and angiotensin II (AngII) may act through overlapping intracellular pathways to promote cardiac myocyte growth. In this report insulin and AngII signaling, through the phosphatidylinositol 3-kinase (PI 3-kinase) and MAPK pathways, were compared in cardiac tissues of control and obese Zucker rats. AngII induced Janus kinase 2 tyrosine phosphorylation and coimmunoprecipitation with insulin receptor substrate 1 (IRS-1) and IRS-2 as well as an increase in tyrosine phosphorylation of IRS and its association with growth factor receptor-binding protein 2. Simultaneous treatment with both hormones led to marked increases in the associations of IRS-1 and -2 with growth factor receptor-binding protein 2 and in the dual phosphorylation of ERK1/2 compared with the administration of AngII or insulin alone. In contrast, an acute inhibition of both basal and insulin-stimulated PI 3-kinase activity was induced by both hormones. Insulin stimulated the phosphorylation of MAPK equally in lean and obese rats. Conversely, insulin-induced phosphorylation of Akt in heart was decreased in obese rats. Pretreatment with losartan did not change insulin-induced activation of ERK1/2 and attenuated the reduction of Akt phosphorylation in the heart of obese rats. Thus, the imbalance between PI 3-kinase-Akt and MAPK signaling pathways in the heart may play a role in the development of cardiovascular abnormalities observed in insulin-resistant states, such as in obese Zucker rats.

The hypothalamic serotonergic system is involved in the regulation of food ingestion and energy metabolism. Since disturbances of both energy intake and expenditure can contribute to obesity, the objective of the present study was to evaluate the serotonergic response stimulated by food ingestion in two different models of obesity: the hyperphagic Zucker and the hypophagic and hypometabolic, monosodium glutamate (MSG) obese Wistar rat. For this we used microdialysis to examine the release of 5-hydroxytryptamine (serotonin, 5HT) and 5-hydroxyindoleacetic acid (5HIAA) in the lateral hypothalamus. Daily intake of MSG-obese rats was 40% lower while that of Zucker obese rats was 60% higher than that of the respective lean controls. In overnight-fasted animals, 20-min microdialysate samples were collected before (basal release) and during a 2-h period of access to a balanced palatable food mash. The animals began to eat during the first 20 min of food access, and food consumption was similar among the four groups in all six individual 20-min periods recorded. Ingestion of food increased 5HT release in all groups. In MSG-obese and lean Wistar rats, 5HT levels were similarly elevated during the whole experimental period. In the Zucker strain, 5HT increments of basal release tended to be higher in obese than in lean rats at 20 and 40 min, and a significantly higher increment was observed at 60 min after food access (40 and 135% for lean and obese, respectively). The area under the curve relating serotonin levels to the 120 min of food availability was significantly higher in Zucker obese (246.7 +/- 23.3) than MSG-obese (152.7 +/- 13.4), lean Wistar (151.9 +/- 11.1), and lean Zucker (173.5 +/- 24.0) rats. The present observation, of a food-induced serotonin release in the lateral hypothalamus of lean Wistar and Zucker rats, evidences that 5HT in the lateral hypothalamus is important in the normal response to feeding. In obese animals, the serotonin response was similar to (in the hypophagic-hypometabolic MSG model) or even higher than (in the hyperphagic Zucker model) that seen in the respective lean controls. This result indicates that the energy homeostasis disturbances of both these obesity models may not be ascribed to an impairment of the acute lateral hypothalamic serotonin response to a dietary stimulus.

We have previously shown that adult female rats exposed to intra-uterine malnutrition were normophagic, although obese and resistant to insulininduced hypophagia. The present study aimed at examining aspects of another important catabolic component of energy homeostasis control, the hypothalamic serotonergic function, which inhibits feeding and stimulates energy expenditure. Pregnant dams were fed ad libitum or were restricted to 50 % of ad libitum intake during the first 2 weeks of pregnancy. Control and restricted 4-month-old progeny were studied. The restricted rats had increased body adiposity with normal daily food intake but failed to respond with hypophagia to an intracerebroventricular injection of serotonin (5-hydroxytryptamine; 5-HT). Stimulation, by food ingestion, of extracellular levels of serotonin in medial hypothalamus microdialysates was more pronounced and lasted longer in the restricted than in the control rats. In the restricted group, hypothalamic levels of 5-HT 2C receptor protein tended to be reduced (P¼ 0·07) while the levels of 5-HT 1B receptor and serotonin transporter proteins were significantly elevated (36 and 79 %, respectively). In conclusion, female rats undernourished in utero had normophagic obesity as adults but had an absence of serotonininduced hypophagia and low hypothalamic levels of the 5-HT 2C receptor. Compensatory adaptations for the functional serotonergic impairment were evidenced, such as an enhanced release of serotonin in response to a meal allied to up-regulated hypothalamic 5-HT 1B and transporter expression. Whether these compensations will persist in later life warrants further investigation. Moreover, it cannot be ruled out that the serotonergic component of energy expenditure was already impaired, thus contributing to the observed body-fat phenotype.Intra-uterine malnutrition: Serotonin: Hypothalamus: Obesity: Control of feeding The hypothalamus is the major centre of interaction of the numerous anabolic and catabolic factors involved in the control of energy balance. The serotonergic system plays a key role in these mechanisms, inhibiting food intake and stimulating energy expenditure (1 -4) . Serotonin (5-hydroxytryptamine; 5-HT) release is stimulated by the ingestion of food in hypothalamic sites related to feeding control, a finding that points to the physiological relevance of the system (5 -10) . Moreover, several investigations indicate that alterations of central serotonergic activity are present in obese rodents and humans (6,10 -12) and drugs that act as serotonin receptor agonists or serotonin reuptake inhibitors are used in the treatment of obesity (13,14) . Obesity, with its associated conditions, such as CVD, hypertension, insulin resistance, type 2 diabetes and the metabolic syndrome, represents nowadays a major health issue worldwide. Malnutrition during intra-uterine life has been linked to the development of obesity in later life.The 'thrifty phenotype' hypothesis suggests that the adaptive strategies developed during the period of poor nu...

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