MATERNAL PITUITARY GONADOTROPH FUNCTION IN RELATION TO GNRH RECEPTOR AND LH-BETA MESSENGER-RNA CONTENT DURING PREGNANCY IN EWES

To investigate the mechanism controlling the fall in maternal pituitar y responsiveness to GnRH, LH synthesis and pituitary GnRH receptor con tent during pregnancy, maternal pituitaries were collected from sheep on days 35, 45, 60, 90, 110, 125 and 135 of pregnancy. Circulating ste roids and gonadotrophins were determined in blood samples collected fr om these ewes immediately before death. Pituitary blocks from each ewe were perifused with either medium alone (control) or medium supplemen ted with oestradiol, oestradiol plus progesterone or oestradiol plus R U486, for 150 min before administration of two 15 s GnRH pulses 90 min apart. The amounts of mRNA encoding LH beta and GnRH receptor were de termined in pituitary tissue fragments snap-frozen in liquid N-2 at th e time of collection from the ewes. While basal LH secretion fell duri ng pregnancy, pituitary responsiveness to GnRH remained high (up to se ven times basal LH concentrations). After day 90, the first GnRH pulse elicited LH peaks equivalent to the LH peaks produced by the second G nRH pulse. Therefore, GnRH self-priming was not evident possibly becau se the pituitaries were constantly primed by increased concentrations of maternal oestradiol. Around day 90, circulating concentrations of p rogesterone rose from 7.8 +/- 1.5 to 12.2 +/- 3.8 ng ml(-1). Up to day 60, oestradiol in the perifusion buffer had stimulatory effects on LH secretion although this was reduced by RU486. By day 125, the content of mRNA encoding LH beta had declined during pregnancy to 7% of the c ontent on day 35, although the content of mRNA encoding GnRH receptor remained unchanged. From these data, there appears to be a transitiona l period at around day 90 of gestation when pituitary sensitivity to s teroids in vitro is lost together with detectable GnRH self-priming. I n conclusion, the marked decline in pituitary amounts of mRNA encoding LH beta, but not in GnRH responsiveness or expression of GnRH recepto r, after day 45 of pregnancy suggests that the principal effect of pre gnancy on gonadotroph function is mediated via a mechanism other than reduced pituitary amounts of GnRH receptors. Two possible mechanisms a re (1) a reduction in GnRH output leading to lowered LH synthesis, or (2) the presence of an inhibitory factor with a short half-life in the maternal circulation.