Effects of unilateral or bilateral superior ovarian nerve section in prepubertal rats on the ovulatory response to gonadotrophin administration

Abstract

The modulating effects of ovarian innervation reaching the ovary through the suspensory ovarian nerve on the reactivity of the ovaries to gonadotrophins were analysed. Juvenile rats (32 days old), with or without unilateral or bilateral section of the superior ovarian nerve, were injected with 8 iu of pregnant mare serum gonadotrophin (PMSG), 10 iu of human chorionic gonadotrophin (hCG) or with 8 iu of PMSG followed 56 h later with 10 iu of hCG. Treatments were given immediately after surgery or 4 days later, and the rats were killed on the day of first vaginal oestrus. In rats with unilateral section, treatment with PMSG did not induce full ovulatory response by the denervated ovary whether the treatment was applied immediately or 4 days after surgery (0/11 rats treated immediately ovulated vs 5/5 (sham) and 11/12 (control, P < 0.05 Fisher's exact probability test), and 4/19 did when treatment was done 4 days after surgery vs 8/10 (sham) and 11/12 (control, P < 0.05). The rats with bilateral section receiving the same hormonal treatment, PMSG administration, ovulated. The number of ova shed by the left ovary was similar to those of the control, while the right ovary released fewer ova. Stimulation with hCG immediately after unilateral section did not induce ovulation in normal or denervated ovary. When the treatment was applied 4 days after surgery, ovulation was observed only in the innervated ovary. In the rats with bilateral section, hCG injection induced ovulation in both ovaries. In those rats with unilateral section of the superior ovarian nerve, the treatment with PMSG + hCG given immediately after surgery resulted in a compensatory ovulation by the innervated ovary (the number of ova shed/ovulating animal was significantly higher than those released by control or sham-operated animals: left section, number of ova shed by the right ovary 7.6 +/- 0.3 vs 5.5 +/- 0.8 and 4.9 +/- 1 respectively, P < 0.05; right section, number of ova shed by the left ovary 10.2 +/- 0.6 vs 4.4 +/- 1.1 and 7.0 +/- 0.9, P < 0.05), while the denervated one showed a lower ovulation rate as well as a smaller number of ova shed than those by the control animals. When the hormonal replacement was given 4 days after surgery, such compensatory ovulation was observed in the left ovary of those rats with a section of the right nerve (14.3 +/- 2.6 vs 4.4 +/- 1.1 and 6.5 +/- 1.1, P < 0.05). When the PMSG + hCG treatment was applied to animals with bilateral section of the superior ovarian nerve, the ovulation rate by the right ovary was significantly lower than in control and sham-operated treated animals (2/10 vs 11/11 and 6/7, P < 0.05). Because the ovaries receive innervation through the superior ovarian nerve, the ovarian plexus and the vagus nerve, the results obtained in unilateral denervated animals suggest that the innervation of the ovary via the superior ovarian nerve regulates in a stimulatory way the effects of gonadotrophin resulting in ovulation. The ovulation induced by hormonal treatment of rats with bilateral section of the superior ovarian nerve suggests that the effects of bilateral section on ovulation are not the addition of the effects of left and right denervation, implying the existence of a modulatory effect in gonadotrophin action on ovulation via other neural pathways.