Relationship between serum gonadotropins and pituitary immunoreactive gonadotropins and steroid receptors during the first FSH increase of the estrous cycle and following steroid treatment in heifers

The objectives were to determine the effects of (i) time during the first FSH increase of the estrous cycle (time-course study) and (ii) exogenous steroid treatment (steroid feedback study) on the relationship between circulating serum gonadotropins, and the proportions of pituitary cells immunoreactive for gonadotropins and steroid receptors during the estrous cycle in heifers. Pituitaries were collected from heifers (n = 40) slaughtered at 13 h (n = 8), 30 h (n = 24) and 66 h (n = 8) after estrous onset, corresponding to before, during and after the first FSH increase of the estrous cycle. Heifers slaughtered during the FSH increase (at 30 h) either received no treatment (n = 8), or were treated (n = 16) with estradiol benzoate and/or progesterone before slaughter. During the time-course study, the proportion of pituitary cells immunoreactive for FSH increased (P < 0.05) during the first transient FSH increase reflecting serum concentrations. The proportion of pituitary cells immunoreactive for LH was unaltered, a reflection of serum LH concentrations. The proportion of estrogen receptors (ER)-α, but not ER-β, was decreased (P < 0.05) at 30 h compared with at either 13 or 66 h. During the steroid feedback study, exogenous progesterone with or without estradiol suppressed (P < 0.05) the proportions of pituitary cells immunoreactive for gonadotropins, serum FSH concentrations and LH pulse frequency. Steroid treatment did not alter the proportion of pituitary cells positive for estrogen receptors (α and β). While progesterone receptors (PR) were not detected in the anterior pituitary by immunohistochemistry during the early estrous cycle or in response to steroid treatment, quantitative real-time PCR revealed that mRNA for progesterone receptors was expressed at very low levels. The expression of pituitary PR mRNA was decreased (P < 0.05) at 30 and 66 h compared with 13 h, and was suppressed (P < 0.05) following steroid treatments. Alterations in pituitary steroid receptors are implicated in the differential regulation of gonadotropin secretion during the first transient FSH rise, but not in response to exogenous steroids. The time-course study and steroid feedback responses support the hypothesis that LH pulse frequency is tightly linked to regulation of GnRH pulse frequency. Serum FSH is regulated by its own synthesis, as reflected by pituitary FSH content and perhaps by alterations in pituitary sensitivity to circulating steroids by changes in steroid receptor content.