Increase of intra-ovarian oocyte release in PMSG-primed immature rats and its inhibition by arginine vasotocin

Twenty-seven day old, immature, female rats were injected subcutaneously with 30 IU pregnant mare serum gonadotrophin (PMSG) (0h). Forty-six hours after PMSG treatment, saline or 1 μg arginine vasotocin (AVT) was given subcutaneously every 2 h for either 8 or 24 h. At 72 h the ovarian and uterine weights were significantly heavier in PMSG-primed ovaries in comparison with those from PMSG-AVT treated rats. Plasma surges of follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin were present in both PMSG- and PMSG-AVT treated rats. Ovaries removed 72 h after the PMSG injection demonstrated a dramatic increase in intra-ovarian oocyte release (IOR). This was primarily due to intra-ovarian rupture of antral and preovulatory follicles, the latter being followed by luteinization. After 72 h, ovaries from the PMSG-AVT treated rats demonstrated 56% of intact antral follicles which showed preovulatory size and typical preovulatory changes; IOR occurred in only a few preovulatory follicles. It is concluded that IOR of antral and preovulatory follicles is a likely consequence of the LH peak induced by PMSG-treatment in the controls. The inhibition of IOR in PMSG-AVT treated ovaries may be the result of a delayed ovulation.