This review summarizes highlights of our experiments investigating mechanisms, mediators and sites by which endotoxin disrupts reproductive neuroendocrine activity and interferes with the estrous cycle of sheep. Endotoxin, or lipopolysaccharide (LPS), is a commonly used model for immune and inflammatory stress. When administered to ovary-intact ewes, endotoxin interrupts the follicular phase of the cycle by interfering with several steps in the preovulatory chain of endocrine events. One such step is the development of high frequency LH pulses, which provide an essential stimulus for the preovulatory increase in estradiol secretion from the ovarian follicle. Follow-up experiments in ovariectomized ewes demonstrate that endotoxin inhibits pulsatile LH secretion at both the hypothalamic and pituitary levels, suppressing pulsatile GnRH secretion and reducing pituitary responsiveness to GnRH. This disruption of GnRH and LH pulsatility is mediated by pathways that include the synthesis of prostaglandins and cortisol, both of which are increased by endotoxin. It is postulated that a prostaglandin-mediated pathway disrupts the cycle during immune and inflammatory stress, whereas a separate cortisol-mediated pathway reinforces this disruption and also participates more generally in suppressing cyclicity during other stressful situations that activate the hypothalamo-pituitary-adrenal axis.

Exposure of anoestrous ewes to rams induces an increase in LH secretion, eventually leading to ovulation. This technique therefore is an effective, low-cost and hormone-free way of mating sheep outside the breeding season. However, the use of this technique is limited by the variability of the ewes' responses. In this study, our objective was to understand more completely the origins of this variability and to determine the relative roles of breed, the point in time during anoestrus and the depth of anoestrus on the response to the 'ram effect'. In the first experiment, the pattern of anoestrus on the basis of the concentration of progesterone determined weekly, was determined in four breeds including two less seasonal (Mé rinos d'Arles and Romane), one highly seasonal (Mouton Vendé en) and one intermediate (Iˆle-de-France) breeds. Anoestrus was longer and deeper in Mouton Vendé en and Iˆle-deFrance than in Romane or Mé rinos d'Arles. In the second experiment, we used the same four breeds and tested their hypophyseal response to a challenge with a single dose of 75 ng gonadotrophin-releasing hormone (GnRH) in early, mid and late anoestrus, and then we examined their endocrine and ovarian responses to the 'ram effect'. Most (97%) ewes responded to GnRH and most (93%) showed a short-term increase in LH pulsatility following the 'ram effect'. The responses in both cases were higher in females that went on to ovulate, suggesting that the magnitude of the hypophyseal response to a GnRH challenge could be a predictor of the response to the 'ram effect'. As previously observed, the best ovarian response was in Mé rinos d'Arles at the end of anoestrus. However, there was no relationship between the proportion of females in the flock showing spontaneous ovulation and the response to the 'ram effect' of anoestrous ewes from the same flock.

Bacterial endotoxin (lipopolysaccharide), a commonly used model of immune/inflammatory stress, inhibits reproductive neuroendocrine activity and concurrently induces a profound stimulation of the hypothalamo-pituitary-adrenal axis. We employed two approaches to test the hypothesis that enhanced secretion of cortisol mediates endotoxin-induced suppression of pulsatile GnRH and LH secretion in the ovariectomized ewe. First, we mimicked the endotoxin-induced increase in circulating cortisol by delivering the glucocorticoid in the absence of the endotoxin challenge. Within 1-2 h, experimentally produced increments in circulating cortisol suppressed pulsatile LH secretion in a dose-dependent fashion. Second, we blocked the endotoxin-induced stimulation of cortisol secretion using the drug metyrapone, which inhibits the 11-beta hydroxylase enzyme necessary for cortisol biosynthesis. In the absence of a marked stimulation of cortisol secretion, endotoxin still profoundly inhibited pulsatile GnRH and LH secretion. We conclude that, although enhanced cortisol secretion may contribute to endotoxin-induced suppression of reproductive neuroendocrine activity, the marked stimulation of the glucocorticoid is not necessary for this response. Our findings are consistent with the hypothesis that immune/inflammatory stress inhibits reproductive neuroendocrine activity via more than one inhibitory pathway, one involving enhanced secretion of cortisol and the other(s) being independent of this glucocorticoid.

There is ample evidence on the importance of maternal nutrition during pregnancy on fetal and offspring development. In ruminant females, the pool of oocytes is complete and definitive before birth, based on the resting reserve of primordial follicles established during fetal life, which represent the lifespan supply for the female’s fertilisable oocytes, whereas in males, although the production of spermatozoa is a continuous process throughout post-pubertal life. Sertoli cells, which play a central role in the development of a functional testis, proliferate during pre- and post-natal life, coordinating testicular development. Both male and female fertility may, therefore, be affected by the maternal environment, but studies on the effects of developmental nutritional conditions on reproductive function and fertility, both in males and females, are relatively scarce. In humans, intrauterine growth retardation has been associated with abnormal ovarian development, characterised by a decreased volume of primordial follicles in the ovarian cortical tissue in girls, and a higher incidence of cryptorchidism in boys, with subsequent low sperm counts in adulthood. Age at puberty and gonadotropin and inhibin B plasma concentrations are also affected. Animal studies suggest both in males and females that maternal undernutrition during pregnancy may affect pituitary response to GnRH and gonadal development and function, depending on the timing and magnitude of the undernutrition. Excess nutrition, which is often associated with intrauterine growth retardation in domestic species, induces effects on the onset of puberty and both testicular and ovarian function, maybe through the observed reduction in fetal growth. This review addresses the influence of maternal nutrition on offspring reproductive function using examples in humans and animals, with particular focus on ruminants.