Estradiol-induced diurnal changes in lactotroph proliferation and their hypothalamic regulation in ovariectomized rats.

Pentobarbital anesthesia during the proestrous afternoon delays proliferation of lactotrophs of the anterior pituitary from estrus to diestrus 1 in cycling female rats. We determined whether estradiol treatment induced diurnal changes in rates of lactotroph proliferation in ovariectomized rats, and if so, examined whether hypothalamic neural activity was involved in the occurrence of the estradiol-induced diurnal changes. Dispersed anterior pituitary cells were obtained from ovariectomized rats bearing estradiol implants that had been treated with 5-bromo-2'-deoxyuridine (BrdU) 3 h before decapitation. BrdU-labeling indices representative of the proliferation rate of lactotrophs were determined by double immunofluorescence staining for BrdU and PRL. After treatment of ovariectomized rats with estradiol on day 0, BrdU-labeling indices of lactotrophs as determined by injecting BrdU at 1000 h increased markedly with time peaking on days 4-7. Levels of BrdU-labeling indices at 1000 h on day 4 were 2.8-fold higher than those at 2200 h on day 3 or 4 after estradiol treatment. However, levels of BrdU-labeling indices at 1000 h on day 14 were 35% lower than those at the same time on day 4 and did not differ from those at 2200 h on day 13 or 14. In addition, a difference in BrdU-labeling indices as observed between 1000 and 2200 h on day 4 in the ovariectomized rats was not detected in estradiol-treated orchidectomized male rats. Serial determinations of BrdU-labeling indices throughout day revealed that the difference in BrdU-labeling indices between 1000 and 2200 h on day 4 in the ovariectomized rats reflected estradiol-induced diurnal changes that were characterized by a peak between 0700-0900 h and a nadir between 1900-2200 h. Pentobarbital injected at 0900 or 2100 h on day 3 decreased slightly high levels of BrdU-labeling indices at 0800 h on day 4. However, pentobarbital injection at 1345 h on day 3, which was effective in blocking estradiolinduced surges of LH and PRL secretion, suppressed markedly the high levels at 0800 h on day 4. In these pentobarbital-blocked rats, the diurnal changes in BrdU-labeling indices whose peak would normally have occurred at 0700-0900 h on day 4 were delayed not by the time corresponding to the duration of pentobarbital anesthesia but exactly by 24 h. These results suggest that 1) hypothalamic and sexually dependent diurnal changes in lactotroph proliferation can be induced by short-term estradiol treatment in ovariectomized rats as well as in cycling rats, and 2) estradiol treatment for 14 days rather prevents the diurnal changes in lactotroph proliferation.