Evidence of a molecular clock in the ovine ovary and the influence of photoperiod

The influence of the central circadian clock on reproductive timing is well established. Much less is known about the role of peripheral oscillators such as those in the ovary. We investigated the influence of photoperiod and timing of the LH surge on expression of circadian clock genes and genes involved in steroidogenesis in ovine ovarian stroma. Seventy-two Suffolk cross ewes were divided into two groups, and their estrous cycles were synchronized. Progestagen sponge removal was staggered by 12 hours between the groups such that expected LH peak would occur midway through either the light or dark phase of the photoperiodic cycle. Four animals from each group were killed, and their ovaries were harvested beginning 36 hours after sponge removal, at 6-hour intervals for 48 hours. Blood was sampled every 3 hours for the period 24 to 48 hours after sponge removal to detect the LH surge. The interval to peak LH did not differ between the groups (36.2 ± 1.2 and 35.6 ± 1.1 hours, respectively). There was an interaction between group and the time of sponge removal on the expression of the core clock genes ARNTL, PER1, CRY1, CLOCK, and DBP (P < 0.01, P < 0.05, P < 0.01, P < 0.01, and P < 0.01, respectively). As no significant interaction between group and time of day was detected, the datasets were combined. Statistically significant rhythmic oscillation was observed for ARNTL, CLOCK, CRY1 (P < 0.01, respectively), PTGS2, DBP, PTGER2, and CYP17A1 (P < 0.05, respectively), confirming the existence of a time-sensitive functionality within the ovary, which may influence steroidogenesis and is independent of the ovulatory cycle.