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Changes in the Pituitary-Gonadal Axis Associated with Puberty in Holstein Bulls¹

The Pennsylvania State University³, University Park 16802

² Reprint requests to R. P. Amann, Anim. Reprod. Lab., Colorado State Univ., Ft. Collins, CO 80523

Abstract

The temporal pattern of the endocrine changes associated with puberty were studied using 52 bulls born in October or April. Blood samples were taken weekly and at 30-min intervals for 5 h every 4 wk. Bulls were castrated at one of six 4-wk intervals between 12 and 32 wk and blood samples were taken. Season of birth affected concentrations of testosterone (greater for spring-born) in intact bulls, but not luteinizing hormone (LH) or follicle stimulating hormone (FSH). The concentration of FSH increased about 30% between 4 and 32 wk, without evidence of pulsatile discharge. Basal concentration of LH was low and pulsatile discharges were infrequent at 4 or 8 wk. At 12, 16 and 20 wk, however, basal LH concentration was elevated and LH discharges were at <2-h intervals. Testosterone concentration did not rise until 18 to 20 wk, but then continued to rise; LH discharge was suppressed concomitantly. Bulls castrated at 16 or 20 wk had higher concentrations of LH in their blood both before and shortly after castration than older bulls, but by 21 d after castration values for bulls of all ages were similar. It was concluded that elimination of an unidentified suppressive factor allows frequent discharges of LH between 12 an 16 wk, but the testes do not respond by secreting more testosterone until 18 to 20 wk. By 24 wk, the testes are secreting more testosterone and pituitary production of LH is restored to a lower level; LH discharges decline in frequency and basal LH level declines. The high frequency discharges of LH between 12 and 20 wk are postulated to induce responsiveness of Leydig cells to LH and, thus, enable elevation of intratesticular testosterone to levels necessary for Sertoli cell differentiation and initiation of spermatogenesis.

¹ Authorized for publication as Paper No. 6455 in the Journal Series of the Pennsylvania Agr. Exp. Sta. Appreciation is expressed to Dr. L. C. Griel, Jr. for performing castrations and providing veterinary services; S. K.Curtis, G. R. Klinefelter and M. T. O'Connor for their assistance with sample collection; J. M. Hudson and G. V. Hudson, of Colorado State Univ. for assistance in hormonal analyses; and Y. Akin and A. Nakpil for help with statistical analyses. Supported by the Pennsylvania Agr. Exp. Sta. and the Animal Reprod. Lab., Colorado State Univ.

³ Dairy Breeding Research Center, Dept. of Dairy and Anim. Sci.

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