The percentage of pituitary gonadotropes with immunoreactive oestradiol receptors increases in the follicular phase of the ovine oestrous cycle

During the oestrous cycle, there is an alteration in gonadotrope responsiveness to gonadotropin releasing hormone (GnRH). One cellular mechanism that may be involved in these changes at the pituitary level is the hormonal regulation of oestrogen receptor (ER) expression. Using double-label immunohistochemistry, we examined the proportion of gonadotropes, lactotropes and somatotropes with immunoreactive (ir) oestrogen receptor alpha (ERα) in pituitary sections from ewes at three stages of the ovine oestrous cycle (n = 8 per group). The percentage of ERα positive cells that also stained positive for luteinizing hormone (LH) increased in the transition from the luteal phase to the follicular phase (n = 8), with no further increase at the time of oestrous (n = 8). In the pituitaries from the luteal phase sheep, only a small number (15%) of lactotropes and 4% of somatotropes were found to contain ir-ERα and there were no alterations across the oestrous cycle. When we examined pituitaries from ovariectomized (OVX) ewes treated (i.m.) with either oestradiol benzoate (50 μg) or oil vehicle for 2, 4, 6 or 16 h (n = 4 per group), there was no effect of treatment. In fact, the percentage of gonadotropes that were ERα-positive in OVX ewes was similar to that observed in the pituitaries from the follicular phase ewes, both of which display a high frequency of pulsatile GnRH secretion. We conclude that the number of gonadotropes that contain ir-ERα increases in the follicular phase of the oestrous cycle and this may enhance the responsiveness of these cells to oestrogen and GnRH. We suggest that this may be due to increased pulsatile GnRH input rather than rising oestrogen levels.