Increased Fos Immunoreactivity in Suprachiasmatic Nucleus before Luteinizing Hormone Surge in Estrogen-Treated Ovariectomized Female Rats

Abstract

Background/Aims: The suprachiasmatic nucleus (SCN) is thought to control the timing of luteinizing hormone (LH) surges. The present study was designed to examine temporal patterns of Fos expression in the dorsomedial and ventrolateral parts of the SCN (SCNdm and SCNvl) of female rats during an LH surge. It also included examination of temporal changes in plasma LH levels and temporal changes in Fos levels in the anteroventral periventricular nucleus (AVPV) and gonadotropin-releasing hormone (GnRH) neurons. Methods: Ovariectomized rats injected with 20 µg estradiol benzoate (EB) or vehicle were sacrificed at various times from Zeitgeber time (ZT) 8:00 to 16:30 h (ZT8–16.5; ZT0 = lights on; ZT12 = lights off) on the 2nd day after the injection. Immunohistochemical analyses for Fos and GnRH and enzyme-linked immunosorbent assays for LH were then performed. Results: In both the SCNdm and SCNvl of EB rats, the number of Fos-immunoreactive cells significantly increased between ZT9.5–10.5 and ZT11–12. On the other hand, in EB rats there were significant peaks of LH levels and Fos levels in GnRH neurons and the AVPV between ZT11–12 and ZT13–14. There was no significant difference in the number of Fos-immunoreactive cells between EB and control rats in either the SCNdm or SCNvl at ZT9.5–10.5, or in the SCNdm at ZT11–12, whereas the SCNvl of EB rats contained more Fos-immunoreactive cells than that of control rats at ZT11–12. Conclusion: These results suggest that in female rats during an LH surge, a peak in the Fos level in the SCN precedes peaks in Fos levels in the AVPV and GnRH neurons.