Longitudinal study of the surge of gonadotropins induced by exogenous hormones in prepuberal gilts.

The transition at puberty to adult hormonal patterns has been hypothesized to involve a shift in the sensitivity of the hypothalamic-hypophysial axis to hormonal stimulation. Prepuberal gilts were treated with GnRH or estradiol-benzoate (EB) to compare responsiveness of the pituitary at 40, 80, 120, and 160 days of age. Mean BW at each was 15.6, 36.3, 56.4, and 70.7 kg, respectively. Gilts were injected with GnRH (625 ng/kg; iv) or EB (600 micrograms/kg; im) or with vehicles. Blood samples were collected at 10-min intervals for 3 h after injection of GnRH or vehicle or 4-h intervals for 96 h after injection with EB or vehicle. Plasma concentrations of LH and FSH were measured by RIA, and data were analyzed by a general linear model for split plot design. Pretreatment basal concentrations of LH and FSH decreased between 40 and 160 days of age from 1.7 to less than 0.7 ng/ml and from 29.9-4.1 ng/ml, respectively. GnRH-induced surges of LH became more synchronous relative to time of injection and decreased in maximum concentration with age. In the youngest gilts, multiple surges of LH were released after a single injection of GnRH. The magnitude of the initial release of LH decreased with age from 5.6-2.7 ng/ml. Gilts treated with EB had two or more surges of LH at all ages. However, the concentration of the surge was greatest and occurred synchronously 12 h earlier in the 160-day-old gilts than at other ages. Therefore, sensitivity to EB was enhanced both in response time and amount of LH released. Concentrations of FSH were highly variable among gilts, and treatment with GnRH or EB induced an adult-like release of gonadotropins only at 160 days of age. Maturation of the surge mechanism involved at least two distinct alterations for LH. First there was a reduction in the amount of LH released in response to GnRH after day 40. Second an increase in the precision (timing) and magnitude of the surge of LH in response to EB was seen as gilts approached puberty.