DAY-NIGHT CHANGES IN C-FOS EXPRESSION IN THE FETAL SHEEP SUPRACHIASMATIC NUCLEUS AT LATE-GESTATION

The suprachiasmatic nucleus (SCN) is a circadian oscillator in mammals and shows day-night changes in metabolic activity. To investigate whether the fetal sheep SCN behaves as a circadian oscillator, day-night changes in c-fos expression, a marker of neuronal activity, were measured. Eight fetal sheep were sacrificed at 135 days gestation-four at day-time (1200 hours) and four at night-time (2400 hours). Fetal brains were fixed, removed and cut in 40-mu m serial coronal sections. Alternate sections were incubated with anti-Fos antibody (1:500) and Fos expression was revealed with extra-avidin-peroxidase and 3,3'-diaminobenzidine or stained with cresyl violet. The number of Fos-immunoreactive (Fos-ir) neurons per mm(2) in the rostral, intermediate and caudal regions of the fetal sheep SCN was counted. Fetuses sacrificed in the day-time showed a higher number of Fos-ir neurons per mm(2) (mean+/-s.e, 516.7+/-60.1) in the three regions of the SCN than fetuses sacrificed at night-time (140.5+/-21.8). In addition, at night-time Fos-ir neurons were mainly localized to the ventrolateral area of the SCN. These findings demonstrate day-night changes in molecular activity consistent with the presence of a circadian oscillator in the fetal sheep SCN.