Insight into molecular core clock mechanism of embryonic and early postnatal rat suprachiasmatic nucleus

Rhythmicity of the rat suprachiasmatic nucleus (SCN), a site of the circadian clock, develops prenatally. A molecular clockwork responsible for the rhythmicity consists of clock genes and their negative and positive transcriptional-translational feedback loops. The aim of the present study was to discover the development of the clockwork during ontogenesis. Daily profiles of Per1, Per2, Cry1, Bmal1, and Clock mRNA in the SCN of fetuses at the embryonic day (E)19 and of newborn rats at the postnatal day (P)3 and P10 were assessed by the in situ hybridization method. In addition, daily profiles of PER1, PER2, and CRY1 proteins at E19 were assessed by immunohistochemistry. As early as at E19, all the studied clock genes were already expressed in the SCN. However, no SCN rhythm in their expression was detected; Per1, Cry1, and Clock mRNA levels were low, whereas Bmal1 mRNA levels were high and Per2 mRNA levels were medium. Moreover, no rhythms of PER1, PER2, and CRY1 were detectable, as no immunoreactive cells were present at E19. At P3, rhythms in Per1, Per2, Cry1, and Bmal1, but not in Clock mRNA, were expressed in the SCN. The rhythm matured gradually; at P10, the amplitude of Per1, Per2, and Bmal1 mRNA rhythms was more pronounced than at P3. Altogether, the data show a gradual development of both the positive and negative elements of the molecular clockwork, from no detectable rhythmicity at E19 to highly developed rhythms at P10.