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The Negative Feedback Effects of Testicular Steroids Are Predominantly at the Hypothalamus in the Ram^*

Victorian Institute of Animal Science, Department of Agriculture Werribee Victoria, 3030, Australia
Department of Anatomy, Monash University Clayton Victoria, 3168, Australia
Prince Henry’s Institute of Medical Research P.O. Box 118, South Melbourne, Victoria, 3001, Australia

Address all correspondence and requests for reprints to: Dr. Alan J. Tilbrook, Victorian Institute of Animal Science, Department of Agriculture, Princes Highway, Werribee, Victoria, 3030, Australia.

Abstract

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 dihydrotestosterone benzoate (DHTB) or an sc silastic implant containing 1 or 3 cm E. TP decreased plasma LH concentrations, increased (P < 0.05) LH interpulse interval, did not have consistent effects on LH pulse amplitude, and had minimal effects on plasma FSH concentrations. DHTB decreased LH and FSH concentrations and increased (P < 0.05) LH interpulse interval. E reduced (P < 0.05) plasma LH and FSH concentrations and increased LH interpulse 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 or FSH 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 data provide 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. (Endocrinology 129: 3080–3092, 1991)

Footnotes

^* This work was funded by the National Health and Medical Research Council of Australia.

Received March 12, 1991.

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