Effects of Dexamethasone on LH Release and Ovulation in the Cyclic Rat12

A study was made of the effects of dexamethasone on ovulation and LH release (radioimmunoassay) in 4-day cycling rats. Subcutaneous injections of dexamethasone given in the earlier stages of the cycle (early diestrus) resulted in a 24-hr delay in ovulation by extending the diestrous period from 2 days to 3 days. Injection given in the later stages of the cycle (late diestrus, early proestrus) extended the cycle to 5 days by adding an extra day of vaginal co nification, with ovulation occurring on the second day, i.e., 24 hr later than expected in a normal 4-day cycle. The latter injection schedule was used in all subsequent experiments reported in this paper. Small dosages of LH induced ovulation in animals “blocked” by dexamethasone, suggesting thatlittle, if any, action of dexamethasone was exerted directly at the ovarian level. Dexamethasone blocked ovulation by inhibiting the normal preovulatory LH surge on the afternoon of proestrus. This inhibition of LH release was thought to be due to a decreased synthesis and release of LH at the pituitary level since 1) in dexamethasone-treated animals the responsiveness of the pituitary to various dosages of LH-RH given at 1500 proestrus was greatly depressed, and 2) sinecontrol and dexamethasonetreated animals showed no difference in pituitary LH concentration or content either before or after a large dose of LH-RH. Dexamethasone was not acting by inhibiting adrenal steroid production since it was just as effective in blocking ovulationin adrenalectomized rats as in intact animals. These results suggest that under our experimental conditions, dexamethasone blocks ovulation by inhibiting the synthesis and release of pituitary LH. (Endocrinology94: 1397, 1974)