TIMING OF OVULATION IN RATS DURING EXPOSURE TO CONTINUOUS LIGHT: EVIDENCE FOR A CIRCADIAN RHYTHM OF LUTEINIZING HORMONE SECRETION

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Abstract
SUMMARY: In order to determine whether the timing of ovulation in rats was controlled by an endogenous circadian rhythm, the hour of ovulation was determined by observing tubal ova during laparotomy in adult rats exposed to full animal room illumination (150 lux) during daily photoperiods of 14 h (full LD), continuous 150 lux illumination (full LL), daily dim (0·2 lux) photoperiods of 14 h (dim LD), continuous 0·2 lux illumination (dim LL) or continuous darkness (DD). Rats in all groups except those exposed to full LL continued normal cyclic ovulation. By the second oestrous cycle, most rats in the full LL group failed to ovulate, even though they showed characteristic cyclic changes in the vaginal smear pattern. The hour at which ovulation occurred was similar in rats exposed to full LD, dim LD or DD but was delayed in rats exposed to full LL or dim LL; the longer the period of exposure, the greater was the delay. For a given length of exposure, ovulation was delayed more in full LL than in dim LL. The full LL used in this study produced persistent vaginal oestrus within 40 days, whereas the dim LL did not. The delayed ovulation in rats exposed to dim LL was associated with a delayed preovulatory surge of LH. These results are consistent with the hypothesis that the timing of the preovulatory surge of LH and ovulation are controlled by an endogenous circadian rhythm, which in most rats has a periodicity in continuous light of slightly longer than 24 h.