In Vivo Initiation of Clock Gene Expression Rhythmicity in Fetal Rat Suprachiasmatic Nuclei

The mammalian suprachiasmatic nuclei (SCN) and their intrinsic rhythmicity develop gradually during ontogenesis. In the rat, the SCN forms between embryonic day (E) 14 and E17, with gestation terminating at E21–22. Overt SCN rhythmicity is already present in the late embryonic stage. The aim of the present study was to determine when the fetal SCN clock develops in vivo and whether overt rhythmicity results from a functional fetal clock. To achieve this goal, the prenatal development of rhythmic expression of clock genes was measured with a more sensitive method for detection of the clock gene expression than previously. Fetal SCN were collected at 3 h intervals during the 24 h period on E19 and E21 by laser dissection and expression of clock genes (Per2, Nr1d1 and Bmal1) and genes related to cellular activity (c-fos, Avp and Vip) was measured by qRT PCR. At E19, the expression of canonical clock genes Per2 and Bmal1 was not rhythmic; however, the expression of all other studied genes followed clear circadian rhythms. At E21, Per2 and Bmal1 expression exhibited low amplitude but significant rhythmicity. From E19 to E21, the levels of the non-rhythmic transcripts (Per2 and Bmal1) decreased; however, the levels of the rhythmic transcripts (Nr1d1, c-fos, Avp and Vip) increased. In summary, these data demonstrate that at E19, rhythms in Per2 and Bmal1 expression were absent in the fetal SCN; however, the expression of Nr1d1 and other genes related to cellular activity was driven rhythmically. Therefore, at the early stage in vivo, the developing fetal SCN clock could theoretically be entrained by oscillation of Nr1d1 which may be driven by the maternal rather than fetal circadian system.