Aims: 

Light entrains circadian rhythms to the 24-h period of solar day through shifting the phase of the endogenous circadian clock in suprachiasmatic nuclei of hypothalamus (SCN). Light sensitivity of the circadian core clock is controlled by the core clock mechanism itself. Aim of our study was to determine when during postnatal development expression of clock genes Period1 (Per1) and Period2 (Per2), i.e. parts of the molecular mechanism underlying circadian rhythmicity, and of c-Fos, an immediate early gene, became light sensitive and when clock begins to control the photosensitivity.

Methods: 

Adult rats and pups at postnatal day 1 (P1), P3 or P10 maintained at LD 12:12 were released into darkness at circadian time (CT) 0. Animals were exposed to a 30min light pulse during the fist (CT15) or second (CT21) part of subjective night or during the subjective day (CT7) and sampled 30min, 1h and 2h after the start of each pulse together with untreated controls. Levels of Per1, Per2 and c-Fos mRNA in the SCN were assessed by in-situ hybridization.

Results: 

At P1, photoinduction of Per1 and c-Fos mRNA occurred during the day as well as during the night. At P3, the photoinduction was present during the night and not during the day, hence, clock started to control it. Development of Per2 mRNA photoinduction seems to be different from that of Per1 mRNA.

Conclusions: 

Development of the gating mechanism of light sensitivity of circadian clock is gradual and non-parallel.