Effects of photoperiod and hypothalamic knife cuts on the timing of FSH surges in hamsters

The timing of the proestrous surge of follicle-stimulating hormone (FSH) was examined in female hamsters with hypothalamic knife cuts that prevented reproductive responses to photoperiod. All animals received either a horizontal knife cut aimed between the suprachiasmatic nuclei (SCN) and the paraventricular nuclei (PVN), or sham surgery, and were housed in long (16 h of light/24 h) or short (6 h of light/24 h) photoperiods. Following exposure to either photo-period for 11-12 weeks, a subset of the animals was fitted with an indwelling jugular cannula. Blood samples were taken hourly over a 24-h period and plasma levels of FSH were determined by RIA. Knife cuts placed ventral to or through the ventral portions of the PVN prevented short day-induced anestrus. On the day of proestrus, peak elevations of FSH in cycling animals with knife cuts in both photoperiods, as well as in sham-operated females in long days, occurred 4-5 h before lights out. In contrast, sham-operated anestrous females in short days showed peak elevations of FSH approximately 3-4 h after lights out. The present results support the view that neural connections between the SCN and the PVN mediate the effects of short days on reproductive physiology, including changes in the timing of the FSH surge.