Effects of estradiol on secretion of LH, hypothalamic function and testicular development in bull calves

Abstract Two experiments were conducted in order to determine the effects of estradiol (E 2 ) on the development of the hypothalamic-pituitary-testicular axis in bull calves. In experiment 1, calves were assigned randomly to one of the following groups: 1) intact, 2) intact E 2 -treated, 3) castrated, or 4) castrated E 2 -treated. Treatments began when the calves were 7.5 wk of age and continued for 16.5 wk. Samples of blood were collected once a week from 3 to 14 wk of age and every 10 min for 6 hr at 8, 12 and 16 wk of age. Concentrations of E 2 in plasma decreased between 3 and 4 wk of age and were further reduced by castration. Maximum concentrations of E 2 (24.3 pg/ml) were observed 72 hr after insertion of E 2 implants, however, plasma E 2 stablized at 5–9 pg/ml by 2 wk after insertion of implants. Treatment with E 2 eliminated the pulsatile secretion of LH in intact and castrated calves and retarded testicular growth. In experiment 2, calves were assigned to a control (n=4) or E 2 -treated (n=6) group. Implants of E 2 were inserted at 7.5 wk of age. At 24 wk of age, calves were bled and then sacrificed to collect hypothalamic and pituitary tissues. Age-related changes in testicular weight and secretion of LH were blocked by E 2 . Neither the morphology nor the intensity of immunostaining of GnRH nerve cell bodies in the preoptic area (POA) were affected by E 2 . However, the density of GnRH fibers and beads in the stalk median eminence (SME), and concentrations of pituitary GnRH receptors were greater (P 2 -treated compared to control calves. In addition, concentrations of norepinephrine (NE) in the SME were lower in E 2 -treated calves when compared to controls. Based on these observations, it is concluded that administration of E 2 at 7.5 wk of age causes profound alterations in hypothalamic function including, changes in metabolism of NE and suppression of GnRH release.