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Differential effects of gonadectomy on sensitivity to testosterone of brain centres associated with gonadotrophin negative feedback and with mating behaviour in rams



Differential effects of gonadectomy on sensitivity to testosterone of brain centres associated with gonadotrophin negative feedback and with mating behaviour in rams



Journal of Endocrinology 104(1): 69-75



Castrated sheep were used to study the effects of gonadectomy on sensitivity to testosterone of brain centers associated with gonadotropin negative feedback and with mating behavior. In the 1st experiment, serum LH [luteinizing hormone] and FSH concentrations were determined in intact rams, recently castrated (2 days and 3 wk) and long-term castrated animals (> 2 yr, wethers) during i.v. testosterone infusion at physiological and supraphysiological levels. In intact rams, testosterone infusions effectively suppressed serum LH while FSH levels were suppressed only after prolonged infusion at the supraphysiological dose. Recently castrated sheep, which had higher gonadotropin levels than intact rams, were less sensitive to testosterone feedback. Neither rate of testosterone infusion had any effect on the raised gonadotropin levels in wethers. In a 2nd experiment, gonadotropin concentrations and mating behavior were determined in wethers bearing subdermal polydimethylsiloxane implants of testosterone, dihydrotestosterone and estradiol. Testosterone implants stimulated mating behavior in all wethers but suppressed gonadotropins in only a proportion (3 of 7) of the animals. both estradiol and dihydrotestosterone suppressed LH and FSH in all wethers, while estradiol, but not dihydrotestosterone, also stimulated mating behavior, Apparently, testosterone imposes continuing negative feedback on gonadotropin secretion and that changes in the gonadotropin regulatory system, which lead eventually to a loss in sensitivity to testosterone feedback, develop soon after gonadectomy. The results also provide the 1st direct evidence that longterm gonadectomy in male sheep has differential effects on sensitivity to testosterone of brain centers associated with gonadotropin negative feedback and with mating behavior. A loss in sensitivity to testosterone feedback in castrated animals may involve a lesion in 5.alpha.-reductase, the enzyme required for conversion of testosterone to dihydrotestosterone.

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Accession: 001336222

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PMID: 3918134

DOI: 10.1677/joe.0.1040069



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