Circadian food anticipatory activity across the seasons: The relationship between feeding schedules and photoperiod in mice

Daily feeding schedules induce circadian rhythms of food anticipatory activity (FAA) by entrainment of circadian oscillators outside of the master light-entrainable pacemaker in the suprachiasmatic nucleus (SCN). Efforts to localize these food-entrainable oscillators (FEOs) and specify molecular mechanisms have been complicated by the wide range of non-circadian factors that can modulate expression of food-motivated behaviours. Here, we examine the effect of photoperiod (duration of the daily light period) on FAA induced in mice by restricting food to a 4h daily meal in the light period, the usual rest phase in nocturnal rodents.To express FAA in the light period, FEOs must compete with SCN clock outputs, which normally suppress activity and promote sleep at this time of day. Photoperiod modifies both the period () and amplitude of the SCN pacemaker, as indicated by aftereffects of long and short days on  and on the phase shift response to light pulses in constant dark (DD). Exposure to long days is thought to reduce SCN amplitude, and would be expected to permit greater FAA to a daytime meal. To test this prediction, mice were entrained to a 16h light:8h dark (L16) or L8 cycle, with or without running discs, and then maintained in DD for 2 weeks. Mice previously entrained to L16 exhibited a shorter  and smaller phase shift to light in DD, confirming an effect of photoperiod on the SCN pacemaker. After re-entrainment to L16 or L8, food was restricted to the last 4h of the light period. FAA was enhanced in L16 in mice with running discs, but the difference was reversed in mice without running discs. Additional groups of mice were entrained to L18, L16, L12 or L8, and the 4h daily meal was centered in the light period. Prior to restricted feeding, photoperiod modified parameters of the light-entrained rhythms as expected. During restricted feeding, there was no systematic effect of photoperiod on FAA. After restricted feeding, an aftereffect of photoperiod on  in DD was absent. Centering of daily mealtime in the light period may block the effect of photoperiod on the SCN pacemaker, and thereby eliminate the potential impact of day length on the expression of FAA to daytime meals.