Endocrine Abstracts

Annexin-1 (Anx-1), a 37 kDa protein, is a member of the super-family of Ca²⁺and phospholipid-binding annexin proteins and acts as a mediator of glucocorticoid (GC) action in the host defence and neuroendocrine systems. In the anterior pituitary Anx-1 is expressed mainly by folliculostellate (FS) cells and mediates the early-delayed feedback inhibition exerted by GCs on the release of ACTH and other pituitary hormones.

The stress responsiveness of the female rat hypothalamo-pituitary-adrenal system is known to vary with the estrus cycle, with an increased CORT release in response to stress at proestrus (Viau 1991). The anterior pituitary Anx-l levels vary significantly with the estrus cycle in the rat with a peak at estrus declining to a trough at proestrus. The aim of this study was to investigate the effect of ovarian hormones (17β-estradiol and progesterone) on the anterior pituitary Anx-1 content of female rats. Adult female Sprague Dawley rats were bilaterally ovariectomised or sham operated and allowed to recover for 7 days. Following the recovery period they received a subcutaneous injection of; vehicle, 17β-estradiol (1μg/100 g) or progesterone (1 mg/100 g) at 24 h and 12 h before sacrifice. The anterior pituitary Anx1 content was determined by SDS-PAGE and western blotting.

Ovariectomy produced a significant decrease in the anterior pituitary Anx-1 content indicating that the maintenance of anterior pituitary Anx-l levels is dependant upon an ovarian factor. Administration of a physiological dose of 17β-estradiol for 12 h or 24 h prior to sacrifice produced a 9-fold increase in anterior pituitary Anx1 in both the sham-operated and ovariectomised animals, whereas the administration of progesterone had no effect.

These results suggest that the estradiol peak on the afternoon of proestrus leads to the peak in anterior pituitary Anx-1 levels seen on the morning of estrus, and are consistent with the hypothesis that the cyclical variation in anterior pituitary Anx-1 levels is related to the change in stress responsiveness during the rat estrus cycle.