EXOGENOUS ESTRADIOL REDUCES INHIBITION OF LUTEINIZING-HORMONE BY ESTRADIOL IN PREPUBERTAL HEIFERS

The hypothesis that high levels of exogenous estradiol administered to heifers during the prepubertal period would decrease subsequent negative feedback of estradiol on luteinizing hormone (LH) secretion was tested. Fourteen prepubertal heifers were ovariectomized on Day 0. Ovariectomized heifers received either no further treatment (OVX, n = 4), a single estradiol implant on Day 0 (OVXE, n = 5), or the single implant on Day 0 and two additional implants between Days 16 and 30 (OVXE+E, n = 5). Ten ovary-intact heifers received either no treatment (INT, n = 5) or were administered the two estradiol implants between Days 16 and 30 (INT+5, n = 5). Comparison of LH secretion in OVXE to OVXE+E, and in INT to INT+E resulted in significant time-by-treatment interactions (p < 0.05 for both). As pubertal age approached, mean concentration of LH (p < 0.05) and fulse frequency (p < 0.05) increased more rapidly in OVXE+E and INT+E than in OVXE and INT, respectively. Amplitude of LH pulses was unaffected by treatment. When data were standardized to day of puberty in INT and INT+E heifers, mean LH concentration and LH pulse frequency increased as puberty approached in both groups. These data confirm earlier reports indicating that secretion of LH increases gradually as puberty approachesin heifers. It was concluded that administration of estradiol during the prepubertal period hastened the decline in the subsequent negative feedback of estradiol. Precocious puberty was not induced in ovary-intact females.