Estrous Behavior and Circadian Discharge of Luteinizing Hormone in the Prepubertal Gilt in Response to Exogenous Estrogen

The influence of varying doses of estradiol benzoate (EB) on the induction of estrus and luteinizing hormone (LH) discharge was studied in crossbred prepubertal gilts of 135 to 150 days of age. Five gilts were assigned randomly to each of 6 groups and treated with 0, 10, 20, 100 or 200 μg EB/kg body weight (BW) at 1200 h or with 10 μg/kg at 2400 h. The characteristics of the estrous and endocrine responses of the prepubertal gilts to EB were compared to the responses of 4 ovariectomized, adult gilts treated with EB. Dose of EB administered to prepubertal gilts had no influence on the interval from injection to estrus. Amount of EB administered, however, was correlated positively (r=0.82, P<0.001) with the duration of estrus. Increasing the dose of EB tended to increase (0.05<P<0.10) the proportion of prepubertal gilts in estrus. Ovariectomized adult gilts treated with 10 μg EB/kg BW were in estrus for a duration similar to that of prepubertal gilts treated with that dose of EB. All doses of EB induced at least one surge of LH in each prepubertal gilt. As the dose of EB was increased, a greater proportion of prepubertal gilts showed 2 surges of LH at a 24-h interval. Dose had no influence either on the magnitude or the duration of LH surges. In all gilts, whether prepubertal or mature, surges of LH occurred at either 2400 or 0600 h. Thus, gilts treated with EB at either 2400 or 1200 h were asynchronous with respect to time after treatment until LH peak but were synchronous relative to the time during the day of LH surge. Ovariectomized adult gilts treated with EB had a single surge of LH that was similar in magnitude and duration to the first surge of LH observed in prepubertal gilts. These studies demonstrate that: 1) treatment with EB will induce estrus and LH surges in prepubertal and ovariectomized gilts similar to the proestrous rise of ovarian estrogens in the cyclic gilt, 2) dose of EB influences the duration of estrus but not the interval to estrus, 3) the administration of EB causes multiple surges of LH occurring at 24-h intervals in the prepubertal but not ovariectomized, adult gilt, 4) daily surges of LH terminate at 60 to 84 h following treatment regardless of dose of EB, and 5) the surges of LH tend to occur at certain hours of the days and may be unrelated to the time of treatment with EB.