The rat circadian clockwork and its photoperiodic entrainment during development.

The mammalian circadian pacemaker is located in the suprachiasmatic nucleus (SCN), which is composed of dorsomedial (dm) and ventrolateral (vl) regions. The molecular clockwork responsible for the SCN rhythmicity consists of clock genes and their transcriptional-translational feedback loops. The rat SCN rhythmicity and clockwork are affected by the photoperiod. The aim of this study was to elucidate development of the rat SCN rhythmicity, namely of the rhythmicity of the dm- and vl-SCN and of expression of clock genes and to ascertain when the photoperiod starts to affect the SCN rhythmicity. Rhythmicity of the dm-SCN, measured as the rhythm in spontaneous c-FOS production, developed earlier than that of the vl-SCN, which was measured as the rhythm in c-FOS photoinduction. However, photoperiodic affection of the rhythmicity occurred earlier in the vl-SCN than in the dm-SCN. From the 4 clock genes (Per1, Per2, Cry1 and Bmal1) studied, the expression of Bmal1 and Per1 was rhythmic already in 1-day-old rats; at this age, the Per2 mRNA rhythm just started to form and no rhythm in Cry1 expression was detected. After the second postnatal day, all 4 genes were expressed in a rhythmic manner. Thereafter, the rhythms matured gradually via increasing amplitude. Per1 and Per2 mRNA rhythms started to be affected by the photoperiod at the 10th postnatal day. The data suggest that the rhythms in clock genes expression in the rat SCN develop mostly postnatally. The molecular clockwork may start to be photoperiod-dependent around the 10th postnatal day.