A possible role of corpora lutea with regard to oestrogen-induced changes in pituitary responsiveness to LH-RH.

The effect of oestradiol benzoate (OeB; 3 micrograms sc) on LH-RH-induced LH-secretory responses was investigated in: (1) long-term ovariectomized (OVX) rats; (2) intact (i.e. sham-OVX) and 24 h-OVX persistent oestrous (PO) rats (intact PO rats do not possess corpora lutea); (3) intact and 24 h-OVX cyclic (dioestrous) rats (intact cyclic rats possess "not-activated" corpora lutea), and (4) intact and 24 h-OVX phase-2-pseudopregnant (phase-2-PSP) rats (phase 2 of PSP lasts from day 2 to day 10. Experiments were performed on days 7--8. Intact phase-2-PSP rats possess "activated" corpora lutea). A "positive" effect of OeB was observed in: (1) long-term OVX rats; (2) intact and 24 h-OVX PO rats; (3) 24 h-OVX cyclic rats and (4) 24 h-OVX phase-2-PSP rats. In intact cyclic and phase-2-PSP rats OeB was completely ineffective. From these observations it is concluded that only in the absence of corpora lutea OeB can exert a "positive" effect on LH-RH-induced LH-secretory responses. Ovariectomy alone, however, may have a negative impact on LH-RH-induced LH-secretory responses. This holds for PO- and phase-3-PSP rats (phase 3 of PSP begins once the corpora lutea come to lysis, i.e. on or shortly before day 10, and lasts until the end of PSP), but not for phase-2-PSP rats. In cyclic rats, on the other hand, ovariectomy performed on dioestrus-1 "only" interferes with the normal development of pituitary responsiveness to LH-RH and this can be nullified by OeB. Our observations suggest that (1) relatively long-acting products of corpora lutea "stabilize" (phase-2-PSP rats), c.q. limit the (almost certainly oestrogen-dependent) development of pituitary responsiveness to LH-RH (cyclic and phase-3-PSP rats); (2) in ovariectomized rats the influence of these products vanishes and that (3) this results in a situation in which OeB can become effective. It is also suggested that in the course of the ovulatory cycle and phase 3 of PSP a similar disappearance of corpora luteal products occurs "spontaneously" as a result of the process of luteolysis with as a consequence that the pituitary responsiveness to LH-RH, under the influence of endogenous oestrogens, develops continuously until, on pro-oestrus, an LH-surge occurs. Evidence is presented that the corpora luteal products, postulated in this paper, are not identical to progesterone.