Changes in Pituitary and Plasma Follicle Stimulating Hormone Concentrations During the Rat Estrous Cycle1

Pituitary FSH content (Steelman- Pohley assay) in rats exhibiting 4- and 5- day cycles was elevated at 10 AM on the day of proestrus. The content decreased to minimal values by 3:30 PM on that day, coinciding with an increase of FSH in the plasma. Both pituitary and plasma FSH were low during estrus. These changes in pituitary and plasma FSH are similar to those previously reported for LH. Barbital administered before 1 PM on proestrus, or anestrogen-antagonist, MER-25, administered at 10 AM on the day before proestrus, each prevented ovulation and also inhibited the depletions in pituitary FSH and LH contents that normally occur between proestrus and estrus. The striking similarity of changes in FSH to changes in LH suggest that the 2 gonadotropins may share, at least partially, a common releasing or regulatory mechanism. (Endocrinology83: 433, 1968)