Photoperiod regulates arcuate nucleus POMC, AGRP, and leptin receptor mRNAin Siberian hamster hypothalamus

Siberian hamsters decreased body weight by 30% during 18 wk in short day (S D) vs. long day (LD) controls. Subsequent imposed food deprivation (FD; 24 h) caused a further 10% decrease. In the hypothalamic arcuate nucleus (ARC) , SDs reduced proopiomelanocortin (POMC) gene expression and agouti-related protein (AGRP) mRNA was elevated, changes that summate to reduced cataboli c drive through the melanocortin receptors. There was no effect of photoper iod on neuropeptide Y (NPY), melanin concentrating hormone, orexin, or cort icotropin-releasing factor mRNAs. Superimposed FD increased AGRP gene expre ssion and caused a localized elevation of NPY mRNA in the ARC. Both adipose tissue leptin and ARC leptin receptor (OB-Rb) mRNAs were downregulated in SDs, whereas FD increased OB-Rb gene expression. Thus OB-Rb mRNA is differe ntially regulated by acute and chronic changes in plasma leptin in this spe cies. In a separate experiment in LDs, AGRP gene expression was increased b y 24 or 48 h FD, whereas POMC mRNA was downregulated in the caudal ARC. AGR P and NPY mRNAs were extensively coexpressed in the ARC, and their differen tial regulation by photoperiod and FD is suggestive of transcript-specific regulation at the level of individual neurons.