Effects of ovariectomy and steroid replacement on hypothalamic LHRH content in aging female rats.

To examine the role of age on the hypothalamic LHRH response to ovariectomy (ovx) and steroid replacement, young cycling (3-4 months) and old constant estrous (18-20 months) rats were ovariectomized. Two weeks later, rats were treated for 5 days with estradiol benzoate (E, 5 micrograms/kg), progesterone (P, 10 mg/kg), E + P (5 micrograms E + 10 mg/kg) or corn oil, after which time they were killed for determination of hypothalamic LHRH content. The young and old ovx rats had similar levels of LHRH in the medial basal (MBH) and anterior (AH) hypothalamus, but E treatment was only effective in increasing MBH-LHRH content in the young animals. There was no significant effect of P alone or in combination with E. In the second experiment, similar results were seen using a single dose of E (1 microgram/rat) in young and old ovx rats. In addition, the radioimmunoassay of LHRH using two different antibodies binding to different portions of LHRH gave similar results in young and old rats, suggesting that the LHRH peptide was being processed similarly in the two age groups. In conclusion, it appears that hypothalamic LHRH content of young and old ovx rats does not differ with age despite a marked attenuation of serum LH levels with age. The hypothalamus from old rats, however, is less responsive to steroid stimulation of LHRH content.