Positive Feedback Effect of Progesterone on Luteinizing Hormone (LH) Release in Cyclic Female Rhesus Monkeys

Abstract

The synergistic effects of progesterone (P) and estrogen on LH release were examined in nine monkeys with regular menstrual cycles. During six menstrual cycles, animals were subjected, in alternate cycles, to each of the following treatments in random order beginning at 0830 on the third day of the cycle: 1) 30 <g estradiol benzoate (EB) sc, 2) 30 jug EB sc, followed by 2.5 mg P sc 24 h later, or 3) 330 <g EB sc. During the seventh menstrual cycle, all animals were assigned again to either treatment 1 or 2. Blood samples were obtained at 3- to 24-h intervals, and serum concentrations of LH, estradiol, and P were determined by RIA. The high dose of EB induced the characteristic negative feedback response of serum LH, followed by the positive feedback response which peaked 36–54 h after EB, while the low dose of EB induced initial small decreases of LH within 24 h after EB, followed by slight increases of LH during the second and third days after EB. P administration after the low dose of EB induced two dis...