PUBERTAL DEVELOPMENT OF THE GILT: CHARACTERIZATION OF GONADOTROPIC AND GONADAL HORMONES AND THE NEGATIVE AND POSITIVE REGULATION OF LH AND FSH

The maturation of the hypothalamic-pituitary-ovarian axis and the development of positive and negative regulation of FSH and LH were characterized in the gilt. In experiment 1, profiles of plasma luteinizing hormone (LH), follicle stimulating hormone (FSH), estradiol 17(beta) (E(,2)) and progesterone (P) from 3 to 228 days of age were compared between gilts ovariectomized at birth (OVX) and intact gilts (I) with or without (E(,2)) implants for the first 28 days of life. LH (ng/ml) decreased from 1.34 (+OR-) .04 from 3 to 29 days of age to minimal values from days 31 to 40 (.56 (+OR-) .02) and then increased linearly from 47 to 75 days of age (.81 (+OR-) .06). LH and FSH in OVX gilts were elevated (P \u3c .01) above intact gilts from 96 to 228 days of age (1.20 (+OR-) .05 vs .93 (+OR-) .07 ng/ml LH, 9.13 (+OR-) .65 vs 6.09 (+OR-) .42 ng/ml FSH, respectively). FSH (ng/ml) decreased from 8.77 (+OR-) .18 between 3 and 29 days of age to lower values between 31 and 40 days of age (6.72 (+OR-) .22) and then increased between 47 and 75 days of age (7.47 (+OR-) .37). The presence of E(,2) implants for 28 days did not affect LH but decreased FSH (P \u3c .01). Plasma progesterone was not influenced by age or treatment, nor was E(,2) after implant removal. In experiment 2, estradiol benzoate (EB, 20 ug/kg BW) was administered to OVX and I gilts at five different ages. LH and FSH responses were monitored prior to EB (day 0) and on days 1, 2 and 3. Following EB at 1, 4, 8, 16 or 20 weeks of age, plasma E(,2) was elevated on day 1 at all ages. LH was suppressed on day 1 as compared to day 0 (P \u3c .01) in 8-, 16- and 20-week-old gilts. FSH was suppressed on day 1 through 8 weeks of age (P \u3c .01). No positive gonadotropin response to EB was observed through 8 weeks of age. At 16 and 20 weeks of age, intact gilts responded with increases in LH and FSH on day 2 as compared to day 0 ((DELTA)LH, 2.26 and 3.59 ng/ml, (DELTA)FSH 1.25 and 3.96 ng/ml for 16 and 20 week age groups), while LH and FSH in OVX gilts decreased (P \u3c .01). The present data indicate that: (1) E(,2) negative feedback regulation of FSH is established by 1 week of age but not until 4 to 8 weeks of age for LH; (2) ovarian negative feedback on LH and FSH secretion becomes operational sometime between 75 and 96 days of age; (3) E(,2) positive feedback on LH and FSH is established between 8 and 16 weeks of age and does not develop in gilts ovariectomized at birth