Obesity alters circadian expressions of molecular clock genes in the brainstem

Major components of energy homeostasis, including feeding behavior and glucose and lipid metabolism, are subject to circadian rhythms. Recent studies have suggested that dysfunctions of molecular clock genes are involved in the development of obesity and diabetes. To examine whether metabolic states per se alter the circadian clock in the central nervous system (CNS), we analyzed the daily mRNA expression profiles of core clock genes in the caudal brainstem nucleus of the solitary tract (NTS). In lean C57BL/6 mice, transcript levels of the core clock genes (Npas2, Bmal1, Per1, Per2 and Rev-erbα) clearly showed 24-h rhythmicity. On the other hand, the expression profiles of Bmal1 and Rev-erbα were attenuated in mice with high fat diet-induced obesity as well as genetically obese KK-A^y and ob/ob mice. Clock expression levels were increased in mice with high fat diet-induced obesity and Cry1 expression levels were decreased in KK-A^y and ob/ob mice. In addition, peroxisome proliferator-activated receptor α (PPARα), which reportedly increases the BMAL1 transcriptional level, was up-regulated in the NTS of these murine models of obesity and insulin resistance, suggesting involvement of PPARα in the attenuation of circadian rhythms in the NTS in obese states. Furthermore, a circadian expression profile of a downstream target of clock genes, the large conductance Ca²⁺-activated K⁺channel, was disturbed in the NTS of these murine obesity models. These perturbations might contribute to neuronal dysfunction in obese states. This is the first report showing that obesity perturbs the circadian expressions of core clock genes in the CNS.