Light and reproduction in the rat: effect of lighting schedule on ovulation blockade.

The purpose of this study was to define the critical period of luteinizing hormone (LH) release under 3 different light-dark (L-D) ratios--12:12 14:10 and 16:8. This was done to determine what aspect of the light-dark cycle acts as the cue. In each lighting schedule pentobarbital (30 mg/kg) was administered to between 10 and 20 rats at 1 2 3 or 4 PM on the day of proestrus. The animals were killed the next day and examined for the presence of ova. Although the time the lights went on and off in the 3 lighting schedules was different the time of maximum ovulation blockage always occurred at 14 hours after midnight (midpoint of dark) or 2 hours after noon (midpoint of light) which corresponds to 2 PM in each group. When animals were studied in 12 hours of light per day with the lights going on and off 3 hours earlier than in the other control groups the peak of ovulation blockade also occurred 3 hours earlier. It appears that the onset of the "critical period" for LH release is not timed by the onset of light on dark but by the midpoint of either dark or light.