Clock mechanisms for seasonal adaptation: Morning and evening oscillators in the suprachiasmatic nucleus

The circadian behavioral rhythms show seasonal changes not only in animals in the field but also in humans in urban environments. In nocturnal rodents the activity time is compressed or decompressed in response to day lengths. A two-oscillator model has been proposed to explain this photoperiodic response; an evening (E) oscillator is entrained to a dusk signal and drives activity onset, while a morning (M) oscillator is entrained to a dawn signal and controls the end of the active period. However, the location and the oscillation mechanism for these oscillators remain to be elucidated. Recent progress in understanding molecular circadian clock mechanisms and bioluminescent reporters for clock gene expressions enabled us to monitor the E and M clocks’ tick separately. We measured clock gene Per1 expression levels continuously from individual cells as well as tissue explants of the hypothalamic suprachiasmatic nucleus (SCN) using transgenic mice carrying a luciferase reporter gene (Per1-luc mice) which were kept in different photoperiods. We found that there are two regionally specific oscillatory cell groups in the SCN that regulate activity onset and end separately in response to photoperiods, which we assume correspond to E and M oscillators. In addition, a third group of oscillatory cells was identified in the SCN only in a long photoperiod. These oscillatory cell networks change dynamically their clock gene expression pattern depending on environmental lights. Similar mechanisms may underlie seasonal changes in sleep time and melatonin rhythms of humans.