The loss of ovarian follicles through atresia, which accounts for greater than 99% of postnatal female germ cell depletion, is mediated through apoptotic cell death. Although recent data have shown that apoptosis in granulosa cells of ovarian follicles can be hormonally manipulated, the molecular mechanisms responsible for transducing external signals remain to be elucidated. Herein we have characterized changes in the expression of the bcl-2 protooncogene (an inhibitor of apoptosis), the bax gene (an inducer of apoptosis), and the bcl-x gene (which encodes both bcl-xlong, an inhibitor of apoptosis, and bcl-xshort, an inducer of apoptosis) during gonadotropin-stimulated follicular development in vivo and during atresia of antral follicles incubated in vitro. Complementary DNA fragments corresponding to rat bcl-2, rat bax, and rat bcl-x coding sequences were obtained by the reverse transcription-polymerase chain reaction (RT-PCR) technique using total RNA prepared from immature rat ovaries. Northern blot analysis of steady-state bcl-2, bax, and bcl-x messenger RNA (mRNA) levels in total RNA prepared from ovaries of immature rats before and after in vivo priming with 10 IU equine CG (eCG) revealed that eCG-induced follicular growth and survival were associated with a relatively constitutive level of bcl-2 and bcl-x expression but markedly reduced levels of bax mRNA (29 +/- 5% of saline-treated control animals). Using the RT-PCR technique coupled with Southern blot hybridization analysis to distinguish the long vs. short forms of bcl-x (which differ in size by 189 base pairs), it was determined that bcl-xlong was the predominant message expressed by granulosa cells of eCG-primed ovaries. The eCG-mediated decrease in bax expression, coupled with a maintenance of bcl-2 and bcl-xlong mRNA levels, were associated with a pronounced reduction in the extent of granulosa cell apoptosis. In contrast, the induction of apoptosis in a homogeneous population of healthy antral follicles by incubation in vitro without tropic support was associated with an significant increase in bax mRNA levels to 220 +/- 10% of those detected in freshly isolated, healthy follicles. Moreover, bcl-xlong message levels were significantly reduced in follicles incubated for 24 h to 69 +/- 4% of those levels detected in freshly isolated, healthy follicles. However, no significant change in the level of bcl-2 mRNA was detected.(ABSTRACT TRUNCATED AT 250 WORDS)