Mechanisms regulating gonadotropin-releasing hormone (GnRH) secretion in Shiba goats

摘要

The purpose of this study was to examine the role of hypothalamic gonadotropin-releasing hormone (GnRH) in the regulation of reproductive endocrine system in Shiba goats. A pulsatile pattern of GnRH discharge into the pituitary portal circulation was clearly observed and each GnRH pulse was associated with an luteinizing hormone (LH) pulse. A characteristic increase in multiple-unit activity (MUA volley) synchronized with the pulsatile secretion of LH was recorded from electrodes chronically implanted in the medial basal hypothalamus. In intact goats, it was revealed, by recording the MUA volley throughout the estrous cycle, that GnRH pulse generator activity was suppressed in the luteal phase and stimulated in the follicular phase. Using ovariectomized goats as a model it was suggested that progesterone played a predominant role to hold the activity of the GnRH pulse generator with a synergetic action of basal levels of estradiol which appeared to play a rather permissive role during the lutealphase in cycling goats. In cycling and ovariectomized goats GnRH pulse generator activity was slightly suppressed during the LH surge, suggesting that the positive feedback effects of estradiol on GnRH/LH secretion appear to be exerted through a neuronalmechanism that is intrinsically different from the GnRH pulse generator.