Role of Light in the Timing of Ovulation in Rats: Evidence that Dawn Light Advances and Dusk Light Delays Ovulation

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Abstract
To investigate the nature of the light-dark signals that time the preovulatory LH surge and ovulation in the rat, we observed acute effects of altering photoperiod length on the timing of ovulation. Virgin female rats were exposed for 2 weeks to daily 10 h photoperiods and 14 h dark periods. Following this entrainment period, experimental groups were exposed to altered photoperiods and the time (hour of the day) of ovulation during the second estrous cycle was determined by observing tubal ova during laparotomy. Ovulation occurred at 0300-0600 h in rats remaining in the control lighting (lights on 0700 h; lights off 1700 h). When the entire 10 h photoperiod was advanced or delayed, the mean time of ovulation was significantly advanced or delayed, respectively. Addition of 5 h of light (dawn light) to the beginning of the control (10 h) photoperiod significantly advanced ovulation. Although deletion of 5 h of light (dusk light) from the end of the control photoperiod advanced ovulation in most rats, this effect was not statistically significant (P>0.05). Addition of 5 h of dusk light to (or deletion of 5 h of dawn light from) the control photoperiod significantly delayed ovulation. Addition or deletion of equal amounts of light to or from both ends of the photoperiod did not alter ovulation. The results indicate that the rhythm which controls the time of ovulation (i.e., LH secretion), like other previously studied cricadian rhythms, is phase advanced by dawn light and phase delayed by dusk light.