CSIRO PUBLISHING www.publish.csiro.au/journals/rfd Reproduction, Fertility and Development, 2004, 16, 709–715 Distribution of aromatase activity in brain and peripheral tissues of male sheep: effect of nutrition T. P. Sharma A,C , D. Blache A , C. E. Roselli B and G. B. Martin A,D A School of Animal Biology, Faculty of Natural and Agricultural Sciences, University of Western Australia, Crawley, WA 6009, Australia. B Department of Physiology and Pharmacology, Oregon Health Sciences University, Portland, OR 97239, USA. C Present address: WatfordVeterinary Clinic, Watford, ON NOM2SO, Canada. D To whom correspondence should be addressed. email: gmartin@agric.uwa.edu.au Abstract. Conversion of testosterone to oestradiol plays a major role in the feedback inhibition of gonadotrophin secretion in male sheep but little is known of the distribution or control of aromatase activity among central and peripheral tissues. Changes in activity at those sites may mediate alterations in the effectiveness of negative feedback following, for example, a change in nutrition. Using a tritiated-water assay, we quantified aromatase in several tissues in mature male sheep, assessed their contribution to oestradiol production, and tested whether activity at each site was affected by a nutritional treatment that stimulates gonadotrophin secretion. Among the brain tissues, the preoptic area had the highest concentration of activity, followed by the hypothalamus, amygdala and cortex. Among the peripheral tissues, liver and testis had the highest activity and, due to their mass, they are the major sources of circulating oestradiol. Pituitary, muscle, kidney and adipose tissues had very low aromatase levels. The nutritional stimulus increased activity in testis but not in liver or brain. We conclude that changes in aromatase activity do not mediate the effects of nutrition on steroid feedback, but aromatisation in testis, liver and brain is important in the endocrine regulation of reproduction in the mature ram. Introduction In addition to testosterone, the testes produce oestradiol in quantities that are sufficient to exert important biological effects. In the sheep, the plasma concentrations of oestra- diol in the testicular vein are 10–20-fold higher than those in the jugular vein (Pope et al. 1990; Setchell et al. 1991; Hötzel et al. 1998), and concentrations in the general circulation are sufficient to inhibit the secretion of gonadotrophin-releasing hormone (GnRH), luteinising hormone (LH) and follicle- stimulating hormone (FSH) by the hypothalamic–pituitary axis and therefore play a major role in negative feedback (D’Occhio et al. 1983; Schanbacher 1984; Sanford and Robaire 1990; Tilbrook et al. 1991; Blache et al. 1997; Scott et al. 1997). Oestrogen produced in the testis must be the result of aromatisation of androgens in that tissue, but there is also some aromatisation in other tissues, including the cen- tral nervous system, where the process contributes to negative feedback (Sharma et al. 1999). There are two other lines of evidence for major sources of oestradiol outside the testes in mature male sheep: (1) after castration, circulating concentra- tions of oestradiol only fall to about half those of testis-intact animals (D’Occhio et al. 1983; Pope et al. 1990), and (2) when castrated rams are injected with testosterone, plasma oestradiol concentrations can be double those observed in testis-intact rams (Sharma et al. 1999). There is very little specific information about the distri- bution of aromatase among peripheral and central tissues in sheep and the present study addresses that issue. The expression of aromatase activity in any tissue is likely to be controlled actively and therefore to respond to environmental factors. We have considered this with respect to changes in the feedback control over gonadotrophin secretion by chal- lenging the system with an increase in the level of nutrition, a stimulus that increases the testicular production of both testosterone and oestradiol (Hötzel et al. 1998). This extra steroid should exert a greater effect on the hypothalamic sites that mediate negative feedback and reduce the frequency of secretion of GnRH pulses, thereby neutralising the effect of the nutritional stimulus. However, this does not happen— instead, a high plane of nutrition increases GnRH/LH pulse frequency three-fold and the effect is sustained for 3–4 weeks, apparently due to a reduction in the effectiveness of the pro- cess of negative feedback (Tjondronegoro et al. 1996; Beckett et al. 1997). An alternative hypothesis is that nutrition interacts with steroid feedback by influencing the metabolism of testosterone by aromatase within brain or other tissues. If this were the case, an increase in nutrition would decrease aromatase activity in brain tissues, thereby decreasing the conversion of testosterone to oestradiol-17β, reducing the © CSIRO 2004 10.1071/RD04018 1031-3613/04/070709