Effects of estrogen and estrogen receptor messenger RNA levels in young and middle-aged female rats: comparison of medial preoptic area and mediobasal hypothalamus.

We examined the effects of estrogen on estrogen receptor (ER) mRNA in the rat hypothalamus during aging. Two-month-old (young) and 9-month-old (middle-aged) rats were ovariectomized (OVX) and after 10 days given an injection of estradiol benzoate (20 micrograms/rat). The young rats were sacrificed 4 h, 24 h and 72 h, and the middle-aged rats 72 h after estrogen injection. Control rats received a sesame oil injection. The medial preoptic area (MPO) and mediobasal hypothalamus (MBH) were dissected and extracted to obtain total RNAs. The levels of ER mRNA were evaluated by northern blotting. We found that, in the MBH, estrogen treatment decreased ER mRNA in young but not in middle-aged rats. In addition, ER mRNA levels in middle-aged OVX rats were lower than in young OVX rats. By contrast, estrogen failed to decrease ER mRNA in the MPO of young OVX rats. The ER mRNA levels in the MPO of middle-aged OVX rats were significantly higher than in any other groups. The present study indicates that the estrogenic regulation of ER mRNA is different between the MBH and the MPO, and impaired in the MBH, but not in the MPO of middle-aged rats.