Ovulation requires the activation on proestrus of M₁ muscarinic receptors in the left ovary.

We analyzed the effects of chemically blocking type 1 muscarinic receptors (M1R) on either the left or right ovary on ovulation rate, number of ova shed and steroid hormones levels. M1R were unilaterally blocked in ovary with the M1R selective antagonist pirenzepine (PZP). PZP was delivered into the bursa ovarica of the left or right ovary of adult rats at 13:00 h on proestrus day. PZP treatment in the left but not in the right ovary blocked ovulation. PZP did not modify the number of ova shed, nor progesterone or 17β-estradiol serum levels. The surge of luteinizing hormone levels was diminished while that of follicle-stimulating hormone did not change in animals treated with PZP in the left ovary. Interestingly, treatment with either synthetic luteinizing hormone-releasing hormone or human chorionic gonadotropin 1 h after PZP administration in the left ovary restored ovulation in both ovaries. The presence of M1R protein in the theca cells of the ovarian follicles as well as in cells of the corpus luteum was detected on proestrus day. These results suggest that M1R activation in the left ovary is required for pre-ovulatory gonadotropin-releasing hormone (GnRH) secretion and ovulation. Furthermore, these results also suggest that M1R in the left ovary might be regulating ovulation asymmetrically through a stimulatory neural signal relayed to the hypothalamus via the vagus nerve to induce the GnRH secretion which then triggers ovulation.