Role of peripheral and central aromatization in the control of gonadotrophin secretion in the male sheep

Both testosterone and its aromatized metabolite, oestradiol-17 beta, are kn own to act centrally on the secretion of GnRH, but the major site of aromat ization is not clear as aromatase activities are found in numerous tissues including brain and testis. Here, we rested the importance of central aroma tization of testosterone using a non-steroidal aromatase inhibitor, fadrozo le. To distinguish between testicular and non-testicular sites, five intact and five testosterone-infused castrated rams (600 mu g kg(-1) per 24 h for 3 days) were given foul injections of fadrozole (i.m; 500 mu g kg (1)) at 48, 52, 64 and 68 h relative to the start of testosterone infusion. Control rams (II = 5) received vehicle only. Fadrozole treatment decreased plasma oestradiol-17 beta concentrations and increased the LH pulse frequency in b oth intact rams and testosterone-treated castrates, suggesting that non-tes ticular sites of aromatization are important in the control of pulsatile LH secretion. To test the importance of central aromatization, intact rams (n = 5) were infused into the third ventricle with vehicle (artificial cerebr ospinal fluid) or with fadrozole (20 and 200 mu g kg(-1) per day). After tw o weeks, the same two doses of fadrozole were infused intravenously instead of intracerebrally. Central infusion of fadrozole did not affect plasma oe stradiol concentrations but increased LH pulse frequency. Only the highest dose increased LW pulse frequency when infused intravenously. In conclusion , central aromatization is involved in the control of pulsatile LH secretio n in male sheep.