Dynamic changes in hypothalamic angiotensin II levels and release in association with progesterone-induced...

To test the hypothesis that brain angiotensin II (Ang II) may be involved in the preovulatory release of LH on proestrus, we evaluated the pattern of changes in hypothalamic Ang II levels and release in ovariectomized (ovx) rats treated sequentially with estrogen and progesterone. This is an experimental paradigm that reliably evokes dynamic changes in hypothalamic LHRH levels in association with LH hypersecretion, simulating the LH surge on proestrus. Rats were ovx and after 4 weeks received estradiol benzoate followed by progesterone 2 days later at 1000 h. We observed that in these progesterone-treated rats, serum LH levels were low until 1400 h, but thereafter, the rate of LH secretion increased and remained elevated at 1600 h when the experiment was terminated. In these rats, hypothalamic Ang II levels increased abruptly at 1330 h and returned rapidly to baseline levels before the onset of LH surge. Also, a similar pattern in hypothalamic Ang II levels occurred at 1500 h with the rise and peak serum LH levels in the late afternoon. In the second experiment, Ang II levels in the cerebrospinal fluid (CSF) of rats similarly pretreated with ovarian steroids were evaluated. Again, CSF Ang II levels rose abruptly to a peak at 1330 h and returned to baseline range preceding the expected rise in serum LH. Thereafter, no further change in CSF Ang II levels was detected during the period of LH hypersecretion. In the third experiment, perfusates were collected from a push-pull cannula aimed at the paraventricular nucleus in ovx rats similarly treated with ovarian steroids. A peak of Ang II was observed at 1330 h and a later peak at 1430 h. A comparison with LH profiles indicated that these peaks in Ang II levels were evident before and during the LH surge. Thus, in three separate experiments, the results showed that rapid dynamic changes in hypothalamic Ang II levels and release occur in association with the progesterone-induced LH surge in estrogen-primed ovx rats. These findings support the previous observations that Ang II can stimulate LHRH and LH release. Since similar, temporally correlated changes occur in hypothalamic neuropeptide Y and LHRH, the peptides involved in the induction of LH surge, these results are in agreement with the hypothesis that Ang II-expressing neurons may play an important role in the hypothalamic circuitry responsible for stimulation of LH surge in ovarian steroid-treated ovx rats.