Circadian rhythms (CR) are changes in physiological variables with a period close to 24 h., synchronized by environment signals. The fetal sheep presents CR of plasma prolactin and cerebospinal fluid vasopressin at 135 days of gestational age (GA, term 147 days) In adults, CR are driven by the SCN. We investigated whether the maternal and fetal sheep SCN behave as circadian oscillators. To that effect, we measured expression of c-fos (marker of cellular activity) at 135 days's GA in the SCNs of 6 maternal sheep and their fetuses (n=9) at 1200h (day) and 2400h (night). The sheep. were sacrificed with pentobarbitone and the fetuses were delivered by hysterotomy. The maternal and fetal brain were perfused with 4% paraformaldehyde in phosphate buffer 0.1 M, pH 7.4. The brains were removed, post-fixed and the SCN cut in 40μm frozen sections. Alternated sections were stained with cresyl violet of with anti-Fos antibody (Oncogene Science), developed with avidin-biotin and diaminobenzidine. We measured the density of Fos immunoreactive (Fos +) neurons bys mm2 in the SCN. Result were mean ± SE; ANOVA.

The density of Fos + neurons in SCN was higher in the fetus than in the mothers. In both, the density was higher during the day than at night (fetal SCN: 535.5±61.9, n=5 vs. 140.5±9.4, n=4, P<0.05; maternal SCN: 30.2±5.1, n=3 vs 0.3±2, n=3 neurons/mm2). In the fetal SCN, Fos + neurons were localized in the dorso medial zone at day time and in the ventro lateral zone at night. We did not observe preferential distribution of Fos + neurons in the maternal SCN. Our results show that maternal and fetal SCN act as circadian oscillators in the sheep. Both oscillators have similar phases. The different number and distribution of Fos+ neurons suggest a functional difference the adult and fetal SCN. (Financed by FONDECYT 92-607 and OMS 92/CHI/LID2).