The decline of leptin (Ob protein) concentrations during fasting is implicated as a signal for increasing the expression of the orexigenic peptide neuropeptide Y (NPY) in the hypothalamus. To test the hypothesis that the effects of food intake on arcuate nucleus NPY activation are mediated by leptin, we performed simultaneous triple in situ hybridization colocalization studies to determine whether the subset of NPY neurons that are activated by fasting preferentially expresses the long form of the leptin receptor (Ob-Rb). Thus, mRNAs encoding NPY and pro-opiomelanocortin (POMC) were colocalized in the arcuate nucleus of fed and fasted rats by fluorescence in situ hybridization in combination with isotopic in situ hybridization for Ob-Rb mRNA. In fed animals, 47% of arcuate nucleus neurons containing NPY mRNA also contained Ob-Rb mRNA, compared with 79% of POMC neurons (P < 0.01). After a 2-day fast, the number of arcuate nucleus neurons with NPY mRNA increased 50% (P < 0.05); the number of these that coexpressed Ob-Rb increased twofold (P = 0.013). Furthermore, Ob-Rb mRNA hybridization in individual NPY neurons increased by 64% (P < 0.02). In contrast, the number of POMC neurons that coexpressed Ob-Rb was unchanged. A significant interpretation of these findings is that the NPY neurons that do not express detectable levels of Ob-Rb mRNA are not activated by fasting, whereas the NPY neurons that are activated by fasting are the ones that express Ob-Rb. These data demonstrate a significant physiological difference between NPY neurons that express Ob-Rb and those that do not. The results support the conclusion that the effect of food intake on NPY neurons is mediated by the direct action of leptin via Ob-Rb receptors expressed by these NPY cells. The results also indicate that expression of Ob-Rb is a defining phenotypic characteristic of the subset of arcuate nucleus NPY neurons that are activated by fasting and play a central role in the adaptive response to negative energy balance.

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