Pituitary responsiveness to LHRH during pregnancy in the rat: effect of progesterone

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Abstract
The LH and FSH responses to a standard infusion of LHRH were studied on days 8, 12, 15, 17, 19, 21 and 22 after conception as well as on day 23, i.e. after parturition. Groups of rats were also killed on days 8, 15, 19, 22 and 23 and on the day of pro-oestrus of the 4-day cycle for the assay of progesterone, 20α-dihydroprogesterone (DHP), oestradiol-17β, LH and FSH. Finally, the post-partum surges of LH and FSH were compared with those at pro-oestrus in 4-day cyclic rats. The LH and FSH responses to LHRH were relatively low on days 8 and 12, twice as high on days 15, 17 and 19, had increased further on day 21 and reached maximal values on day 22. The gonadotrophin responses were low on day 23. The post-partum surges of LH and FSH were much higher than the pro-oestrous surges. Pituitary contents of LH and FSH were higher than on pro-oestrus of the 4-day cycle. On day 23, however, the pituitary contents had declined by 60–80%. No apparent relationship was found between plasma concentrations of LH and FSH and LHRH responsiveness during pregnancy. Concentrations of oestradiol-17β on day 22 were higher than on all other days of pregnancy, but lower than on pro-oestrus of the 4-day cycle. Concentrations of progesterone were high until day 19 and low on days 22 and 23; the concentration of DHP was low until day 19 and very high on days 22 and 23. It was concluded that the increase of the gonadotrophin response to LHRH half-way through pregnancy, in contrast to the increase at the end of pregnancy, cannot be explained by an increase in the concentration of oestradiol-17β, while the pituitary LH and FSH contents play, if any, only a permissive role. The effect of s.c. implants of progesterone on various hormone parameters during the last week of pregnancy was also investigated. It was found that progesterone not only delayed parturition but also prevented the rise of the gonadotrophin response at the end of pregnancy. Concentrations of oestradiol-17β and DHP were also lower on day 23 than in untreated rats, while the pituitary gonadotrophin contents were higher. It is suggested that delay of luteolysis (in the present study by progesterone treatment) at the end of pregnancy prevents the occurrence of a series of interrelated events: a rise in oestradiol-17β production, a rise in the gonadotrophin responses to LHRH, parturition and the generation of the post-partum gonadotrophin surges. J. Endocr. (1987) 115, 247–254