The negative feedback effects of testicular steroids are predominantly at the hypothalamus in the ram

A. J. Tilbrook, D. M. de Kretser, J. T. Cummins, I. J. Clarke

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This study aimed to delineate the hypothalamic and/or pituitary actions of testosterone and its primary metabolites 5a-dihydrotestosterone and estradiol (E) in adult castrated rams (wethers) during the breeding season. In Exp 1, wethers were treated for a week with twice daily injections (im) of peanut oil, 8, 16 or 32 mg/day testosterone propionate (TP) or dihydro-testosterone benzoate (DHTB)or an sc silastic implant containing 1 or 3 cm E. TP decreased plasma LH concentrations, increased (P < 0.05) LH interpuise interval, did not have consistent effects on LH pulse amplitude, and had minimal effects on plasma FSH concentrations. DHTB decreased LH andFSH concentrations and increased (P < 0.05) LH interpuise interval. E reduced (P < 0.05) plasma LH and FSH concentrations and increased LH interpuise interval but had no effects on LH pulse amplitude. In Exp 2, hypothalamo-pituitary disconnected wethers given 125 ng GnRH every 2 h, were treated with either peanut oil, 32 mg/day TP or DHTB or 3 cm E. None of the treatments affected plasma LH orFSH concentrations or LH pulse amplitude. Exp 3 investigated the effects on GnRH of treatment of wethers either with peanut oil or TP. TP reduced GnRH concentrations (P < 0.05) and pulse amplitude (P < 0.01) and increased interpulse interval (P < 0.05). These dataprovide evidence that, during the breeding season, the principal site of negative feedback of testicular steroids in the ram is the hypothalamus, resulting in decreased GnRH secretion; feedback effects at the pituitary are minimal.

Original languageEnglish
Pages (from-to)3080-3092
Number of pages13
Issue number6
Publication statusPublished - 1 Jan 1991

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