Phase control of the ovarian circadian clock by the central clock

Abstract

The suprachiasmatic nucleus (SCN) works as the central circadian oscillator that controls phases of circadian oscillators in peripheral tissues, although the mechanism of the control is not well known. Current understanding is that the timing of ovulation in rats is determined by the luteinizing hormone (LH) surge from the pituitary, itself timed by the SCN through gonadotropin releasing hormone surge. In this study we tried to reveal how the phase of ovarian circadian oscillator is controlled. To monitor phases of circadian oscillator we used transgenic rats in which luciferase is rhythmically expressed under the control of the Period1 promoter. First, we asked whether ovarian clocks were influenced by signals from the pituitary. To determine whether the phase was set by neural or endocrine signals we surgically denervated or heterotopically transplanted ovaries with or without encapsulation in dialysis membrane. Our results indicated that endocrine signals are sufficient to transmit circadian phase information to the ovary. We next evaluated LH and follicle stimulating hormone as potential endocrine signals for phase control. Peak phase of Per1-luc rhythm was shifted by the pituitary hormones when applied to the cultured granulosa cells. These results raises the possibility that ovulation is timed by interaction between circadian clocks in the hypothalamus/pituitary and those in the ovary. [J Physiol Sci. 2008;58 Suppl:S13]