Nutritional regulation of negative feedback homeostasis in the hyothalamo-pituitary-ovarian axis of sheep and its effect on folliculogenesis

What an animal eats has a major effect on its fertility. More specifically, if female sheep are fed a good diet just before they are mated they will produce more sets of twins than similar ewes fed poor diets. The most remarkable feature of this phenomenon is that the period of good feeding needs to last for only 3 or 4 days. The reason for the effect of good nutrition on the number of twins is put down to a stimulation of egg production in the few days leading up to mating. Why this happens is not at all clear and this research will investigate how this happens. Published studies from our team and other researchers in the field point to the increased energy content of the diet one of the most important reasons for the this effect. In sheep, the main source of this extra energy is plant carbohydrate and as a result we have turned our attention to the role of carbohydrate in our attempts to unravel the biological mechanism behind this phenomenon. Our interest in carbohydrates is based on published information mainly from Australian researchers into the so called 'lupin effect'. In short, the Australians discovered that feeding sheep a supplement of lupin grain for as little as 3 days just before mating will reliably and consistently stimulate the birth of more twins in supplemented flocks. So what is so special about lupins? Well, they have a very high amount of carbohydrate. Lupins contain other things beside carbohydrate. For example, lupins also contain lots of protein. We have successfully mimicked the effect of lupins feeding by giving animals pure carbohydrate as glucose or as propylene glycol. It is our theory that high levels of carbohydrate in the diet act very quickly probably within 3 or 4 days to produce more eggs. The purpose of this project is to test this theory and explain how it works. In order to test our theory we need to study how quickly the food an animal eats affects its egg production. In order to do this we will develop an in vivo model to test the normal speed of response of the ovarian control systems that regulate egg production. We will then examine how high carbohydrate diets modify the response time of these control systems and their effects on egg production. We expect that this research will increase our understanding of how diet affects fertility, which is of particular interest to farmers, veterinarians and medical practitioners.

Nutrition is a key environmental factor affecting reproduction but our understanding its mechanisms is poor and therefore this mechanism remains one of the major unsolved questions in reproductive physiology. Extra nutrition just before mating has long been known to stimulate extra ovulations in sheep and it is well established that feeding lupins for a period of 5 days will stimulate folliculogenesis and produce ovulations. Recent data from our laboratory has shown that the stimulatory effect of diet on folliculogenesis can be demonstrated after only 3 days of improved nutrition. This has led us to formulate a new hypothesis. The hypothesis that will be tested in this project is that dietary supplementation with high energy feed stimulates folliculogenesis by rapidly perturbing negative feedback homeostasis to induce compensatory increases in folliculogenesis. We propose to investigate this hypothesis using an in vivo model of negative feedback modulation; compensatory ovarian hypertrophy (COH) induced by hemi-ovariectomy. We have shown that this model produces a short-term perturbation in negative feedback in the hypothalamo-pituitary-ovarian axis that is corrected within 24h to restore homeostasis. We propose to characterise this model and to then use the model to test our hypothesis. The major endpoints of these studies are (i) the patterns of plasma FSH, oestradiol and inhibin A and (ii) the patterns of expression of protein (western blotting) and mRNA (real time PCR) in Graafian follicles at selected times in a 30 hour period around the time of dietary change and/or hemi-ovariectomy. The project will concentrate on enzymes, receptors and growth factors with known roles in folliculogenesis with the added bonus from the use of real time PCR, to detect novel genes that respond to these treatments. We expect that this research will increase our understanding of the mechanism of the nutritional stimulation off folliculogenesis and of follicle selection.