Factors Influencing the Progesterone-Induced Luteinizing Hormone Surge in Rhesus Monkeys: Diurnal Influence and Time Interval After Estrogen 2

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Abstract
In the rhesus monkey, progesterone (P) given after a small dose of estradiol benzoate (EB) induces a luteinizing hormone (LH) surge with a short latency and short duration (Terasawa et al., 1982). In the present study, effects of P injection in relation to the interval after EB and to the time of day were investigated. Nine long-term ovariectomized female rhesus monkeys (which were implanted with an estradiol-17 beta (E2) capsule 2 wk prior to the experiments) were injected with EB (10 micrograms) and P (2.5 mg). In order to determine the period of estrogen priming necessary to induce the facilitatory effects of P, P was injected at 30, 24, 12 or 0 h after EB in the first four experiments. The time of EB injections was fixed at 0830 h. To determine whether there is any diurnal influence on the action of P, the EB injection was moved to 2030 h, followed by P 30 h later. Administration of P 30 h after EB induced a typical LH surge with peak latency (6.7 +/- 0.5 h) and duration (16.0 +/- 1.4 h) in all animals. Similarly, P injection 24 h after EB induced a LH surge with peak latency (7.4 +/- 0.4 h) and duration (18.0 +/- 1.2 h) in all animals. The amplitudes of the P-induced LH surges at both 30 h and 24 h after EB were also similar (P 30 h; 65.0 +/- 18.7 ng/ml, P 24 h; 59.8 +/- 24.4 ng/ml). In contrast, administration of P 12 h or 0 h after EB resulted in a LH surge in only 4 and 3 of 9 animals, respectively, and these numbers of responders were significantly less (P 12 h: P=0.0147, P 0 h: P=0.0045) than those of P 30 h and P 24 h. The amplitudes of the LH surge in animals responding to P 12 h and P 0 h after EB were much smaller than those of P 30 and P 24 h after EB (P less than 0.01), although the peak latency and duration of the response were similar. Overall responses to P 12 h and P 0 h after EB were significantly (P less than 0.005) different from those of P 30 h and P 24 h after EB. A 12-h shift of the injection time of both EB and P did not alter the LH response. Reversed timing of EB and P injections induced a LH surge with peak latency, 7.7 +/- 0.5 h; duration, 16.3 +/- 1.7 h; and amplitude, 59.2 +/- 20.7 ng/ml. These results indicate that 1) P injection 24 h or 30 h after EB reliably induces a LH surge with a short latency and duration, 2) P injection 12 h or 0 h after EB is not effective in inducing a LH surge, and 3) there is no diurnal influence on the P-induced LH surge. Therefore, the time interval of estrogen priming required for progesterone action in the rhesus monkey is similar to that required in rodents for LH release as well as for lordosis behavior.(ABSTRACT TRUNCATED AT 400 WORDS)