Role of gonadotrophin releasing hormone baseline concentrations in the control of pituitary gonadotrophin and ovarian steroid secretion in the pseudopregnant rat.

To study the effect of moderately elevated gonadotrophin releasing hormone (GnRH) baseline concentrations during the luteal and the follicular phase, pseudopregnant rats were infused s.c. with GnRH at several doses for 5 days. These rats were also treated with oestradiol or sham-treated during the last 3 days of GnRH treatment. GnRH infusions started on day 7 or day 3 of the luteal phase of the ovulatory cycle; in the rat, the luteal phase or pseudopregnancy lasts about 10 days. Luteinizing hormone (LH) and follicle stimulating hormone (FSH) responses were induced by i.v. injection of GnRH on days 12 (after expected luteolysis) or on day 8 (before expected luteolysis). In normal rats the LH and FSH responses induced by GnRH on day 12 were higher than on day 8 (approximately 160 and approximately 50% respectively). In GnRH-infused rats the LH and FSH responses were not increased. In these rats the luteal phase was extended (the plasma progesterone concentrations remained high) and the onset of the follicular phase was postponed (plasma oestrogen concentrations did not increase). Oestradiol increased the day 12 LH and FSH responses; this effect of oestradiol was suppressed by GnRH infusion. On day 8, exogenous oestradiol also increased the LH and FSH responses, but again the effect of oestradiol was suppressed when the animals were concomitantly infused with GnRH. These data may suggest that in the rat, GnRH baseline concentrations participate in the neuroendocrine system controlling gonadotrophin secretion and hence the ovulatory cycle.