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1 Department of Internal Medicine, University of Campinas, Campinas, SP, Brazil
* To whom correspondence should be addressed. E-mail: lavelloso{at}fcm.unicamp.br.
Insulin and leptin act in the hypothalamus providing robust anorexigenic signals. The exposition of homoeothermic animals to cold environment leads to increased feeding accompanied by sustained low levels of insulin and leptin. In the present study, the initial and intermediate steps of the insulin signaling cascade were evaluated in the hypothalamus of cold exposed Wistar rats. By immunohistochemistry most insulin receptor (IR) and insulin receptor substrate 2 (IRS2) immunoreactivity localized to the arcuate nucleus. Basal levels of tyrosine phosphorylation of IR and IRS2 were increased in cold exposed rats as compared to rats maintained at room temperature. However, following an acute, peripheral infusion of exogenous insulin, significantly lower increases of IR and IRS2 tyrosine phosphorylation were detected in the hypothalamus of cold exposed rats. Insulin-induced association of p85/PI3kinase with IRS2, 473-serine phosphorylation of Akt and tyrosine phosphorylation of ERK were significantly reduced in the hypothalamus of cold exposed rats. To test the hypothesis of functional impairment of insulin signaling in hypothalamus, ICV cannulated rats were acutely treated with insulin, and food ingestion was measured over a 12 h period. Cold exposed animals presented a significantly lower insulin-induced reduction in food consumption as compared to animals maintained at room temperature. Hence, the present studies reveal that animals exposed to cold are resistant, both at the molecular and functional levels to the actions of insulin in hypothalamus.
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