Attenuation during aging of the postovariectomy rise in median eminence catecholamines

The effects of aging on the responsiveness of the neuroendocrine system to ovariectomy and estrogen replacement were determined. Neuroendocrine responsiveness in young (6-month-old) cyclic rats was compared to that in old (24-month-old) constant estrous rats. Seven neuroendocrine parameters were examined: median eminence (ME) concentrations of norepinephrine (NE), dopamine (DA) and luteinizing hormone-releasing factor (LRF) and circulating concentrations of LH, FSH, prolactin (PRL) and 17 beta-estradiol (E2). These concentrations were measured in intact and ovariectomized rats and in ovariectomized rats treated for 3 weeks with E2. 3 weeks after ovariectomy of young rats the ME concentrations of NE (38.0 +/- 10.4 pg/microgram, n = 15) and DA (201 +/- 32 pg/microgram, n = 15) were significantly higher (p less than 0.05) than the respective levels in intact proestrous controls (18.7 +/- 1.9 pg/microgram, n = 18) and 89.4 +/- 12.4 pg/microgram, n = 18). In old rats there was no significant change in ME concentration of NE after ovariectomy. ME DA levels in old rats did rise after castration by 62%; however, this rise was only half of that in young animals (125%). The increases in DA after ovariectomy could be completely reversed by E2 in both young and old rats. ME concentration of LRF was reduced after castration to a similar extent in young (76%) and old (81%) rats, and these effects were partially E2-reversible in both age groups. The postovariectomy increases in LH (485%) and FSH (665%) in young rats were greater than the respective increases in old rats (169) and 191%). In contrast, the changes in PRL concentration following ovariectomy and E2 replacement were similar in magnitude in both age groups. The present results indicate that the neuroendocrine responsiveness to ovariectomy is altered during aging. This alteration in responsiveness is selective, in that changes were demonstrable in only 2 out of 7 parameters measured.