GONADOTROPIN-DEPENDENT AND GONADOTROPIN-INDEPENDENT DEVELOPMENT OF INHIBIN SUBUNIT MESSENGER-RIBONUCLEIC-ACID LEVELS IN THE MOUSE OVARY

The inhibins and activins are dimeric growth factors with important re gulatory functions during development. In this study, changes in inhib in subunit messenger RNA (mRNA) levels were measured in the ovary duri ng early postnatal development in the normal mouse and the hypogonadal (hpg) mouse, which lacks circulating gonadotropins. Levels of inhibin alpha-, beta(A)-, and beta(B)-subunit mRNAs were measured relative to beta-actin using a semiquantitative reverse transcription-polymerase chain reaction technique. Transcripts encoding all three subunits were present at birth; the alpha-subunit was the most abundant, followed b y beta(A)-subunit (6% of alpha) and beta(B)-subunit (0.4% of alpha). A fter birth, levels of all three subunit transcripts increased between 6- and 10-fold. Changes in inhibin beta(A)- and beta(B)-subunit levels were most marked around 7 days, the period of secondary follicle deve lopment, whereas alpha-subunit transcript levels increased constantly after day 1 to reach a peak at 10 days, when mature secondary follicle s are present. In hpg mice, levels of ovarian inhibin alpha-subunit mR NA levels were normal at all ages up to 15 days. In contrast, inhibin beta(A)-subunit mRNA levels were normal at birth in hpg mice, but did not increase after that up to day 15. Levels of beta(B)-subunit mRNA w ere significantly lower than normal on day 1 in hpg mice and also fail ed to show a significant increase up to 15 days. These results show th at inhibin subunit mRNA levels are differentially regulated during ova rian development in the mouse. Normal expression of beta-subunits is c ompletely gonadotropin dependent around the period of late primary to secondary follicle development The inhibin alpha-subunit, in contrast, is expressed at a high level during development independent of gonado tropin stimulation.