title: Estradiol up-regulation of pituitary progesterone binding is required for progesterone inhibition of luteinizing hormone release.
abstract: This study examines the regulation of progesterone receptor (PR) in the inhibition of pituitary luteinizing hormone (LH) secretion. Ovariectomized ewes underwent hypothalamic-pituitary disconnection, were pulsed with gonadotropin-releasing hormone (GnRH) and received 1 of 4 treatments: estradiol alone (E), estradiol priming before progesterone (E+P), E removed and replaced with P (E-P), or no steroids (C). P treatment for 24 h, with E or following E-priming, reduced LH pulse amplitude by 55% (p<0.05). E alone did not affect LH release. E increased pituitary cytosolic P binding capacity fourfold over controls (p<0.01) and P further increased binding to eight times controls (p<0.01). Pituitary PR mRNA increased to 149 and 171% of C in E and E+P groups, respectively (p<0.05), but E removal resulted in PR mRNA levels not different from controls. Pituitary receptors for GnRH were tripled by E alone compared to C (p<0.01), whereas P alone or with E had no effect. These data suggest an E-induced, direct pituitary inhibition of LH secretion by P and that this effect of P is associated with E-enhanced binding of P in the pituitary. Additionally, the direct pituitary effects of P on LH secretion cannot be accounted for by influences on GnRH receptor numbers.
authors.affiliation: Department of Entomology, University of Arizona, Tucson, AZ.
authors.forename: R L
authors.initials: RL
authors.lastname: Girmus
authors.forename: A M
authors.initials: AM
authors.lastname: Dunn
authors.forename: T M
authors.initials: TM
authors.lastname: Nett
authors.forename: E
authors.initials: E
authors.lastname: Esquivel
authors.forename: M E
authors.initials: ME
authors.lastname: Wise