In cyclic rats, apoptosis of luteal cells during structural luteolysis occurs cyclically at the transition from pro-oestrus to oestrus in response to the preovulatory prolactin surge. This finding indicates that cyclic changes in apoptosis during luteolysis are dependent on prolactin surge cyclicity. In this study, the effects of prolactin on structural luteolysis were studied under different experimental conditions in relation to the phase of the oestrous cycle. In rats treated with prolactin at metoestrus and dioestrus, apoptosis did not occur in regressing corpora lutea, whereas in rats treated with prolactin on the morning of pro-oestrus, a 12.3-fold and 3.4-fold increase were observed in the number of apoptotic cells in regressing corpora lutea of the current and previous oestrous cycles, respectively. However, when the preovulatory prolactin surge and hence the subsequent apoptotic burst were blocked, prolactin treatment at the dioestrus phase induced a 13-fold increase in the number of apoptotic cells and significant changes in the volume of the corpus luteum (38% decrease) and the number of steroidogenic cells per corpus luteum (70% decrease). The results of this study indicate that the responsiveness of the regressing corpus luteum to the pro-apoptotic effects of prolactin are dependent on the phase of the oestrous cycle and on the presence or absence of an apoptotic burst in response to the preovulatory prolactin surge on the evening of pro-oestrus. Steroidogenic cells surviving to the apoptotic burst during the transition from pro-oestrus to oestrus became refractory to the lytic effect of prolactin. Furthermore, these cells also responded to the luteotrophic effects of prolactin, reaching full morphological luteinization, as indicated by the rescue of regressing cyclic corpora lutea during pregnancy.
F Gaytan, C Bellido, C Morales and JE Sanchez-Criado
F Gaytan, E Tarradas, C Morales, C Bellido and JE Sanchez-Criado
The ovulatory process in cyclic rats was studied after prostanoid synthesis was blocked using indomethacin. Animals were injected at 12:00 h in pro-oestrus with 1.0 mg indomethacin or vehicle (olive oil) and killed at 18:30 h in pro-oestrus, at 02:00, 09:00 and 19:00 h in oestrus and at 09:00 h in metoestrus. Additional rats injected with 0.5 or 4.0 mg indomethacin were killed at 09:00 h in oestrus. No differences in either morphology or serum LH concentrations were found between vehicle or indomethacin-treated rats at 18:30 h in pro-oestrus. However, from 02:00 h in oestrus onward, the process of follicle rupture was altered considerably in indomethacin-treated rats, irrespective of the dose. Early vascular changes, observed in control rats at the apex of the follicle, were absent in indomethacin-treated rats. In some follicles, disruption of the theca layers, invasion of the perifollicular tissue by granulosa cells and follicular fluid, and release of the oocyte to the ovarian interstitium were observed at 02:00 h in oestrus. A small number of follicles ruptured at the ovarian surface. Furthermore, invasion of interstitial tissue, rupture of blood vessel walls, production of emboli of granulosa cells and follicular fluid, and inflammatory reactions were observed in oestrus and metoestrus. The results of the present study demonstrate uncontrolled proteolytic activity, and indicate that abnormal follicle rupture (but not inhibition of follicle rupture) is responsible for ovulation failure in indomethacin-treated rats.
G Hernandez, JG Hernandez-Jimenez, P Guelmes, JE Sanchez-Criado, C Bellido, Martinez-Morales JR, L Prieto, F Marin, C Glidewell-Kenney, FJ Lopez and R Alonso
The effects of LY117018-HCl (LY; a benzothiophene similar to raloxifene) were examined on various reproductive parameters in female rats. Four-day cyclic rats were treated (10:00 h on dioestrus) with LY (0.01, 0.1, 0.5, 1, 2, 4 or 16 mg kg(-1) p.o.) and assessed for ovulation at oestrus. LY inhibited ovulation at doses as low as 0.5 mg kg(-1), and ovulation did not occur at doses of 4 and 16 mg kg(-1). LY (16 mg kg(-1)) reduced wet uterine mass and LH concentrations at the time of the expected ovulatory surge. Ovulation induced by hCG in pentobarbital-treated rats was not altered by LY treatment, indicating normal ovarian sensitivity to gonadotrophins. LY, however, completely blocked the effects of oestradiol (under either negative or positive feedback modes) on LH secretion in ovariectomized rats. GnRH secretion into hypophyseal portal blood during pro-oestrus was not affected by treatment with LY, whereas the concentrations of serum LH remained reduced. Finally, treatment with LY markedly reduced pituitary sensitivity to GnRH during pro-oestrus, as it completely blocked GnRH-induced LH secretion. These results demonstrate that LY inhibits oestradiol action in the uterus and prevents ovulation in normal cyclic rats. LY-induced inhibition of ovulation is not caused by an alteration of the ovarian response to gonadotrophins or an impairment of GnRH secretion at the hypothalamus, but by a reduction in the sensitivity of gonadotrophs to the stimulatory effects of GnRH during pro-oestrus.