Bioscientifica Proceedings

Abstract unavailable© 1981 Journals of Reproduction & Fertility Ltd...

Recent studies have demonstrated that an important component of the positive feedback response to oestradiol in mammals is an action within the central nervous system to induce a large surge in the secretion of gonadotrophin-releasing hormone (GnRH). This oestradiol-induced neuroendocrine signal for ovulation has been best characterized in ewes. The GnRH surge is high in amplitude; the amount secreted increases on average more than 40 times above the pre-surge baseline value. ...

This article reviews experiments performed to investigate the importance of thyroid hormones to the expression of the seasonal reproductive cycle of ewes. Thyroidectomy was found to block the transition from the breeding season to anoestrus and to cause ewes to exhibit oestrous cycles all year round. Mechanistically, thyroidectomy produced this effect by preventing the seasonal increase in responsiveness to the negative feedback action of oestradiol on episodic GnRH secretion,...

Stress influences the activity of the reproductive system at several sites. One of the most significant effects is at level of the GnRH secretory system to reduce GnRH pulsatility and thus LH pulsatility. This in turn reduces the oestradiol signal that stimulates the GnRH–LH surge in the follicular phase. Three sequential phases have been identified in the induction of the GnRH–LH surge by oestradiol: (i) activation, (ii) transmission and (iii) surge secretion. There...