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The proportion of saturated fatty acids/unsaturated fatty acids in the diet seems to act as a physiological regulation on obesity, cardiovascular diseases, and diabetes. Differently composed fatty acid diets may induce satiety of the hypothalamus in different ways. However, the direct effect of the different fatty acid diets on satiety in the hypothalamus is not clear.
Three experiments in mice were conducted to determine whether: different compositions of fatty acids affects gene mRNA expression of the hypothalamus over time; different types of fatty acids administered into the stomach directly affect gene mRNA expression of the hypothalamus; and fat composition changes in the diet affects gene mRNA expression of the hypothalamus.
The type of fat in cases of purified fatty acid administration directly into the stomach may cause changes of gene expressions in the hypothalamus. Gene expression by dietary fat may be regulated by calorie amount ingested rather than weight amount or type of fat.
Therefore, the calorie density factor of the diet in regulating hypothalamic gene in food intake may be detrimental, although the possibility of type of fat cannot be ruled out.
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The prevalence of obesity has been rapidly increasing worldwide over the last several decades and has become a major health problem in developed countries. The brain, especially the hypothalamus, plays a key role in the control of food intake by sensing metabolic signals from peripheral organs and modulating feeding behaviors. To accomplish these important roles, the hypothalamus communicates with other brain areas such as the brainstem and reward-related limbic pathways. The adipocyte-derived hormone leptin and pancreatic β-cell-derived insulin inform adiposity to the hypothalamus. Gut hormones such as cholecystokinin, peptide YY, pancreatic polypeptide, glucagon-like peptide 1, and oxyntomodulin transfer satiety signals to the brain and ghrelin relays hunger signals. The endocannabinoid system and nutrients are also involved in the physiological regulation of food intake. In this article, we briefly review physiological mechanisms of appetite regulation.
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Metformin, an oral biguanide insulin-sensitizing agent, is well known to decrease appetite. Although there is evidence that metformin could affect the brain directly, the exact mechanism is not yet known.
To evaluate whether metformin induces anorexia via the hypothalamus, various concentrations of metformin were injected into the lateral ventricle of rats through a chronically implanted catheter and food intake was measured for 24 hours. The hypothalamic neuropeptides associated with regulation of food intake were also analyzed following 1 hour of intracerebroventricular (ICV) injections of metformin.
An ICV injection of metformin decreased food intake in a dose-dependent manner in unrestrained conscious rats. Hypothalamic phosphorylated AMP-activated protein kinase (pAMPK) increased by 3 µg with metformin treatment, but there was no further increase in pAMPK with increases in metformin dosage. The hypothalamic phosphorylated signal transducer and activator of transcription 3 (pSTAT3) increased by 3 µg with metformin treatment, but, there was no further increase in pSTAT3 level following increases of metformin dosage. Hypothalamic proopiomelanocortin was elevated with metformin treatment, while neuropeptide Y was not significantly changed.
Our results suggest that metformin induces anorexia via direct action in the hypothalamus and the increase in pSTAT3, at least in part, is involved in the process. However, hypothalamic pAMPK appears not to contribute to metformin-induced appetite reduction in normal rats. Further studies exploring new pathways connecting metformin and feeding regulation are needed.
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The hypothalamus, the center for body weight regulation, can sense changes in blood glucose level based on ATP-sensitive potassium (KATP) channels in the hypothalamic neurons. We hypothesized that a lack of glucose sensing in the hypothalamus affects the regulations of appetite and body weight.
To evaluate this hypothesis, the responses to glucose loading and high fat feeding for eight weeks were compared in Kir6.2 knock-out (KO) mice and control C57BL/6 mice, because Kir6.2 is a key component of the KATP channel.
The hypothalamic neuropeptide Y (NPY) analyzed one hour after glucose injection was suppressed in C57BL/6 mice, but not in Kir6.2 KO mice, suggesting a blunted hypothalamic response to glucose in Kir6.2 KO mice. The hypothalamic NPY expression at a fed state was elevated in Kir6.2 KO mice and was accompanied with hyperphagia. However, the retroperitoneal fat mass was markedly decreased in Kir6.2 KO mice compared to that in C57BL/6 mice. Moreover, the body weight and visceral fat following eight weeks of high fat feeding in Kir6.2 KO mice were not significantly different from those in control diet-fed Kir6.2 KO mice, while body weight and visceral fat mass were elevated due to high fat feeding in C57BL/6 mice.
These results suggested that Kir6.2 KO mice showed a blunted hypothalamic response to glucose loading and elevated hypothalamic NPY expression accompanied with hyperphagia, while visceral fat mass was decreased, suggesting resistance to diet-induced obesity. Further study is needed to explain this phenomenon.
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