Feeding Time Entrains the Olfactory Bulb Circadian Clock in Anosmic PER2::LUC Mice

Circadian rhythms in many brain regions and peripheral organs can be entrained by daily feeding schedules. The set of feeding-related signals that entrain peripheral clocks are tissue specific and include nutrients, metabolic hormones and temperature. Signals that entrain neural circadian clocks to mealtime have yet to be established for any brain region. The olfactory bulb (OB) contains a robust circadian clock that can cycle independently of the suprachiasmatic nucleus (SCN) master pacemaker. We used PER2::LUC mice to evaluate the suitability of the OB for analysis of inputs that mediate entrainment of neural clocks by feeding schedules. Explants of SCN and OB from mice fed ad-libitum exhibited robust circadian rhythms of bioluminescence for three or more days in vitro, with rhythm peaks occurring late in the day and night, respectively. Mice restricted to a 4 h meal/day in the light period exhibited food anticipatory activity and a 6.3 h advance of the OB PER2::LUC rhythm. The cumulative phase shift of the OB rhythm increased gradually in OB explants harvested after 2-7 days of restricted feeding. In mice anosmic after nasal irrigation with zinc sulfate and fed ad-libitum for one week, OB phases were desynchronized. Group synchrony in anosmic mice was restored by restricted feeding. The OB circadian clock is food-entrainable, entrains gradually to a mid-day meal, and requires neither olfaction nor circadian signaling from olfactory sensory neurons. The OB can be used as a model system for analysis of input pathways by which circadian clocks in the brain entrain to daily mealtimes.