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After ovulation, the fertile life of oocytes is short. In the present study, the fertile life of oocytes was studied in relation to the resumption of meiosis. Early on the day of pro-oestrus, meiotic resumption was advanced in rats by a brief infusion of LH; ovulation was induced 8 h later by Ovalyse, a GnRH analogue; rats were mated 13 h after receiving Ovalyse, that is at about the time of ovulation. Rats in which meiosis was not advanced were injected with Ovalyse and mated at various intervals after ovulation. Rats were killed 13 h after mating or on day 20 of pregnancy. In rats in which meiosis was not advanced that were mated around ovulation, fetal survival was about 90%. In rats with meiosis advanced by 8 h and mated around ovulation, only 44% of the ovulated oocytes with advanced meiosis developed into healthy fetuses; mortality before and after implantation was 37 and 19%, respectively, Rats in which meiosis was not advanced that were mated 8–9 h after ovulation had similar fetal survival and similar mortality before and after implantation. Thus ageing of the oocyte may occur either before, or after, ovulation. Preovulatory ageing is related to the resumption of meiosis.
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Summary. Sodium pentobarbital (Nembutal) is often used to block the pro-oestrous luteinizing hormone (LH) surge in rats. Nembutal is also known to lower body temperature. This study was designed to investigate whether Nembutal affected the time course of meiosis and timing of ovulation induced by exogenous hormones, and whether the possible effects of Nembutal on these processes were related to temperature. Gonadotrophin-releasing hormone (GnRH), the GnRH-analogue Ovalyse, or rat luteinizing hormone (LH) were administered to trigger resumption of meiosis and ovulation; Nembutal (35 mg kg−1 body weight) or saline was given 10 or 60 min later. Plasma profiles of LH were measured and Graafian follicles were studied histologically for meiotic progress and ovulation. Nembutal suppressed the spontaneous surge of LH at pro-oestrus and caused a long-lasting decrease in body temperature. If 1000 ng GnRH was given 2 h before the pro-oestrous LH surge, most of the oocytes had extruded a polar body 10 h later and most follicles had ovulated 14 h later. Nembutal given 1 h after GnRH delayed extrusion of the polar body and ovulation by about 2 h. Nembutal caused a similar delay in ovulation when it was administered after 100 ng of Ovalyse, and it also delayed meiosis when given after 1000 ng of LH. This effect of Nembutal was prevented if body temperature was maintained at 37°C. The delaying effect of Nembutal on meiosis and ovulation induced by exogenous GnRH or LH is related to a long-lasting decrease in body temperature.
Keywords: Nembutal; temperature; meiosis; ovulation; rat
