Abstract
Photoperiodic time measurement by the circadian clock is one of the strategies by which animals know the time of year. In nocturnal rodents, circadian behavioral rhythms are photoperiod-dependent. The activity bands are compressed and decompressed in long and short photoperiods, respectively. Two separate oscillators, evening (E) and morning (M) ones, have been proposed to explain photoperiodic responses of behavioral rhythms, the location of which is not yet identified. We exposed transgenic mice carrying a Per1-luciferase reporter gene to three different photoperiods, and compared circadian parameters of behavioral rhythms and Per1 expression rhythms in single cells as well as tissues of serial coronal slices of the suprachiasmatic nucleus (SCN). Furthermore, those of horizontal slice were examined. Irrespective of photoperiods, the circadian peak in the posterior SCN was phase-locked to the end of activity, suggesting the localization of M oscillator, whereas that in the anterior SCN, to the activity onset, suggesting that of E oscillator. In the longest photoperiod, two Per1 peaks were detected only in the anterior SCN. Single cell analyses revealed two separate cell groups in the anterior SCN. The long photoperiod seem to separate the third component in the anterior SCN responding light-on from that responding light-off. We further analyze the three oscillating cell networks, two in the anterior and one in posterior SCN, which couple differentially to dawn and dusk using horizontal slices with intact rostro-caudal connections. [J Physiol Sci. 2007;57 Suppl:S138]
