A circadian egg timer gates ovulation

Since the pioneering work of Everett and Sawyer, the idea that pituitary gonadotrophins provide the critical timing cue for ovulation has remained unquestioned [1]. It is widely accepted that the timing of ovulation depends entirely on the timing of luteinizing hormone (LH) secretion, itself driven by neuroendocrine releasing factors controlled by the circadian clock in the suprachiasmatic nucleus (SCN) [2,3]. As a consequence, there has been little investigation of a role for the ovary in this process. However, we and others have demonstrated the presence of endogenous circadian clocks in the rat ovary [4–6]. Here we describe a circadian rhythm of ovarian sensitivity to LH that determines the ovulatory response to gonadotrophins. It is plausible that the circadian clock in the ovary may set the responsiveness of the ovarian follicle to the LH surge. Our results significantly alter the classic view that gonadotrophins provide the only timing cue for ovulation. They suggest that the ovary itself plays a major role in the process and provide a new perspective that will inform future research on infertility and ovarian physiology. We blocked endogenous gonadotrophin secretion and assessed ovulation in response to timed exogenous LH treatments as a measure of phasic ovarian sensitivity. We suppressed endogenous gonadotrophin secretion with cetrorelix pamoate depot (CET), a highly selective and long-lasting GnRH receptor antagonist [7] (see Figure S1A in supplemental data, published with this article online). We first analyzed the pattern of ovarian sensitivity between the evening of diestrus and the afternoon of proestrus. Cycling rats maintained under a 12:12 L:D cycle (lights on 05:00h) were injected at ZT11 (Zeitgeber Time; ZT0 = lights on) on diestrus with CET (1 mg/0.1 m; i.m.). Beginning 7h later, groups of rats were treated with equine LH (eLH; 600 IU; see Figure S1B) at 3h intervals during the subsequent 18h (ZT18 and 21 on diestrus; ZT0, 3, 6, 9 and 12 on proestrus). Rats injected with eLH during the middle of the dark portion of the L: D cycle on diestrus ovulated more frequently and produced significantly more oocytes than did animals injected during the middle of the day (Figure 1A). The number of oocytes released between ZT6 and ZT9 increased and remained elevated through the end of the light phase on proestrus (ZT12; Figure 1A). Separate groups of cycling rats maintained under the same light–dark cycle were injected with CET at ZT5 on proestrus. Beginning 7h after CET …