Ghrelin receptor-knockout mice display alterations in circadian rhythms of activity and feeding under constant lighting conditions

Ghrelin is an orexigenic hormone produced by the stomach. Ghrelin, however, may also be a modulator of the circadian system given that ghrelin receptors are expressed in the master clock, the suprachiasmatic nucleus (SCN) and several outputs of this region. To investigate this, we performed analyses of running wheel activity and neuronal activation in wild type (WT) and growth hormone secretagogue receptor-knockout (GHSR-KO) mice under various lighting conditions. GHSR-KO and WT mice were maintained under constant dark (DD) or constant light (LL) with ad libitum access to food before being placed on a schedule of temporally restricted access to food (4 h/day) for 2 weeks. There were no differences between KO and WT mice in free-running period under DD, but GHSR-KO mice required more days to develop a high level of food anticipatory activity, and this was lower than that observed in WT mice. Under LL, GHSR-KO mice showed greater activity overall, lengthening of their circadian period, and more resistance to the disorganisational effects of LL. Furthermore, GHSR-KO mice showed greater activity overall, and greater activity in anticipation of a scheduled meal under LL. These behavioral effects were not correlated with changes in the circadian expression of the Fos, Per1 or Per2 proteins under any lighting conditions. These results suggest that the ghrelin receptor plays a role in modulating the activity of the circadian system under normal conditions and under restricted feeding schedules, but does so through mechanisms that remain to be determined. Ghrelin is an orexigenic stomach hormone, and may also be a modulator of the circadian system. In constant darkness, GHSR KO mice required more days to develop a high level of food anticipatory activity, but, in constant light, showe