ESTRADIOL UP-REGULATION OF PITUITARY PROGESTERONE BINDING IS REQUIREDFOR PROGESTERONE INHIBITION OF LUTEINIZING-HORMONE RELEASE

This study examines the regulation of progesterone receptor (PR) in th e inhibition of pituitary luteinizing hormone (LH) secretion. Ovariect omized ewes underwent hypothalamic-pituitary disconnection, were pulse d with g,onadotropin-releasing hormone (GnRH) and received 1 of 4 trea tments: estradiol alone (E), estradiol priming before progesterone (EP), E removed and replaced with P (E-P), or no steroids (C). P treatme nt 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 pit uitary cytosolic P binding capacity fourfold over controls (p < 0.01) and P further increased binding to eight times controls (p < 0.01). Pi tuitary PR mRNA increased to 149 and 171% of C in E and E+P groups, re spectively (p < 0.05), but E removal resulted in PR mRNA levels not di fferent 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 effec t. 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 eff ects of P on LH secretion cannot be accounted for by influences on GnR H receptor numbers.