177 DOWN-REGULATION OF P450 AROMATASE AND GLUCOSE TRANSPORTER 4 mRNAs EXPRESSION IN SHEEP OVARIAN FOLLICLES AFTER ULTRASHORT NUTRITIONAL FLUSHING

Abstract

Nutritional supplementation before breeding (flushing) has become a common practice and is a reliable method to increase lambing and twining rates in sheep. We have shown that ultrashort flushing (USF: 1 day long) acutely increases ovulation rate without affecting the body condition or animal weight. A short nutritional stimulus would necessarily be acting either directly at the ovarian level or through hormones and factors other than gonadotropins. The possible mechanisms of action of USF on ovulation rate is the deregulation of the feedback loop between the ovaries and gonadotropin secretion, where nutrition causes a direct inhibition of follicular oestradiol production leading to compensatory secretion of FSH that stimulates folliculogenesis, or direct stimulation of follicular development through hormones and factors other than gonadotropins. We hypothesised that USF enhances the number of follicles selected for ovulation due to either a decrease in the expression of P450 aromatase or alternatively by advancing the maturation of the ovarian follicles, which would be reflected in an increase in LH receptor (LHr) and 3β-hydroxysteroid dehydrogenase (3β-HSD) mRNA expression, an increase in glucose transporter 4 (GLUT4) mRNA expression, or both. To test these hypotheses, the oestrus cycle of 30 ewes was synchronized with progestin intravaginal sponges and prostaglandins. Animals were given the USF on the day of progestin withdrawal. The ovaries were removed surgically before treatment (time 0; n = 6), or at 12, 24, and 48 h after being treated with either glycerol or water (n = 4 per treatment by time category). The ovarian follicles were dissected out and counted. Follicles larger than 3 mm in diameter were pooled and the mRNA extracted for specific P450aromatase, 3β-HSD, LHr, and glucose transporter 4 determinations. USF increased the number of follicles larger than 3 mm at 48 h after treatment (P < 0.05). No effect was observed in the number of small follicles. In the two largest follicles, aromatase and GLUT4 mRNA abundance decreased (P < 0.01) 12 h after treatment. There was no effect of treatment on mRNA for LH receptor or 3β-HSD. These results demonstrate a reduction in aromatase expression in potentially ovulatory follicles 12 h after flushing. The decrease in aromatase may favour a transient increase in FSH concentrations 12 h after flushing that would allow the stimulation and selection of supplementary ovulatory follicles.