ROLE OF GONADOTROPIN-RELEASING-HORMONE BASE-LINE CONCENTRATIONS IN THE CONTROL OF PITUITARY GONADOTROPIN AND OVARIAN-STEROID SECRETION IN THE PSEUDOPREGNANT RAT

To study the effect of moderately elevated gonadotrophin releasing hor mone (GnRH) baseline concentrations during the luteal and the follicul ar phase, pseudopregnant rats were infused s.c. with GnRH at several d oses for 5 days, These rats were also treated with oestradiol or sham- treated during the last 3 days of GnRH treatment, GnRH infusions start ed on day 7 or day 3 of the luteal phase of the ovulatory cycle; in th e rat, the luteal phase or pseudopregnancy lasts about 10 days, Lutein izing hormone (LH) and follicle stimulating hormone (FSH) responses we re induced by i.v. injection of GnRH on day 12 (after expected luteoly sis) or on day 8 (before expected luteolysis). In normal rats the LH a nd FSH responses induced by GnRH on day 12 were higher than on day 8 ( similar to 160 and similar to 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 hi gh) and the onset of the follicular phase was postponed (plasma oestro gen 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 FS H responses, but again the effect of oestradiol was suppressed when th e animals were concomitantly infused with GnRH. These data may suggest that in the rat, GnRH baseline concentrations participate in the neur oendocrine system controlling gonadotrophin secretion and hence the ov ulatory cycle.