Behavioral and neuroendocrine effects of long-term progesterone treatment in the rat

Progesterone and estradiol, alone or in combination, were administered to ovariectomized rats for 2 weeks. Progesterone alone had no effect on body weight, luteinizing hormone (LH) secretion, receptivity or brain cytoplasmic progestin receptors (CPR). Progesterone in combination with estradiol significantly attenuated estrogen suppression of weight gain and estrogen stimulation of receptivity, the LH afternoon surge and induction of CPR, but did not affect the negative feedback of estrogen on morning LH levels. The decrease in CPR after 2 weeks of progesterone is very similar in magnitude to the decrease observed following acute treatment, suggesting that, unlike neurotransmitter agonists and glucocorticoids, progesterone does not cause down regulation of its receptors in the brain following chronic treatment.