Competing clocks: metabolic status moderates signals from the master circadian pacemaker

Food anticipation and its behavioural manifestation, food anticipatory activity (FAA), require entrainment of a suprachiasmatic nucleus (SCN) independent circadian mechanism, a food entrainable oscillator (FEO), with an unknown neural substrate. While the FEO entrains to fasting/feeding related cues and drives FAA independently of the pacemaker in the SCN, it must disengage the suppressing signals of the SCN for the behavioural and physiological manifestations of meal anticipation to occur outside of the typical period of arousal. This review provides a brief overview of mammalian circadian biology before summarizing experimental data demonstrating several mechanisms by which this may occur, including: reducing activation of SCN cells receiving retinal input, transient disorganization of SCN outputs, and reduced sensitivity to SCN signals in hypothalamic sites responsible for integrating homeostatic and circadian information. Further investigation of these mechanisms will be key to elucidating pharmacological or behavioural interventions that suppress the negative psychological and health effects of light-driven circadian rhythms in humans, specifically those with work schedules that do not conform to the solar day.