Diurnal Regulation of the Orexin/Hypocretin System in Mice

A prominent feature of the hypothalamic neuropeptides orexins/hypocretins is their role in the regulation of sleep-wake behavior. While there is strong evidence for a diurnal (i.e. 24-h) rhythmicity of the expression of prepro-orexin (PPO) and its cleavage products, orexin A and B, it is not known whether orexin receptors are also subject to diurnal regulation. Here we ask whether besides the regulation of PPO the expression of the orexin receptor subtypes OX₁R and OX₂R varies over 24 hours in the mouse brain. The mRNA levels of PPO, OX₁R, and OX₂R as well as of various clock genes were analyzed over 24 hours in the hypothalamus, cortex, and adrenal glands of male mice using qPCR. We found a significant diurnal regulation of the mRNA levels of PPO as well as both orexin receptor subtypes in the brain, while no regulation was observed in adrenal glands. While in the cortex the mRNA levels of both OX₁R and OX₂R showed a significant diurnal regulation, in the hypothalamus, only the OX₂R mRNA expression was subject to a diurnal rhythm. The expression of both orexin receptor subtypes significantly correlated with that of clock genes. Remarkably, the expression pattern of OX₂R showed a strong and highly significant correlation with that of the clock gene Bmal1 in the cortex and hypothalamus. These results suggest that the rhythmic expression of orexin receptors is linked to clock gene expression and that OX₂R may potentially play a role in the timing of sleep-wake behavior.