Neuroendocrine regulation of seasonal breeding cycles in the ewe

Seasonal reproduction has emerged as an important topic of investigation which bridges the areas of endocrinology, neurobiology, biological rhythms, and fertility regulation. Among the species in which this topic has been studied, the sheep has proven to be especially useful for investigating the neuroendocrine mechanisms which underlie the seasonal switches in fertility. This is due to a number of attributes. One of these is the large blood volume of sheep which permits extensive sampling for characterizing pulsatile patterns of hormone secretion. Another attribute is the large size of its brain and pituitary gland which facilitates complex neurosurgical procedures such as those required for collecting hypothalamic-pituitary portal blood for measurement of hypophysiotropic substances. Based on work in sheep, it has become evident that pulsatile secretion of luteinizing hormone (LH) from the anterior pituitary is driven by an episodic discharge of gonadotropin-releasing hormone (GnRH) from the hypothalamus, and that seasonal reproduction results largely from alterations in the pulsatile pattern of LH release. By using the sheep as a model, it should now be possible to elucidate the neural mechanisms which underlie these seasonal alterations in the reproductive neuroendocrine axis.