Pituitary and hypothalamic action of progestagens in castrated female rats

Different doses of progesterone, D-norgestrel and delta 15-D-norgestrel were injected subcutaneously for 7 days to female rats castrated 7 days prior to the experiments. One day after the last steroid injection a i.p. LH-RH test was performed, blood samples were collected by heart puncture and serum levels of r-LH and r-FSH were estimated by radioimmunoassay. LH-RH serum levels were determined in progestagen treated control groups by an anterior pituitary cell culture bioassay. Dose dependent inhibitory effects of steroids on LH-RH stimulated gonadotropin release were observed and a dissociation of r-LH and r-FSH serum levels was found. The modulatory influence of progestagens used in this study on LH-RH provoked gonadotropin release correlated with their biological efficiency evaluated in the Clauberg test. Progesterone, D-norgestrel and delta 15-D-norgestrel treatment had no significant effects on basal r-LH levels, but LH-RH provoked r-LH levels were higher than in the control animals. Progesterone and D-norgestrel suppressed basal as LH-RH stimulated r-FSH levels, whereas delta 15-D-norgestrel showed a complex influence on r-FSH secretion. A progestagen stimulated rise of LH-RH serum levels was found and indicated an additional effect of these hormones at the hypothalamic level. These data suggest the action of progestagens at various levels of the hypothalamus-pituitary axis.