Aromatase gene expression in the stallion

Adult stallion secretes very high estrogen levels in its testicular vein and semen, and the responsible enzyme cytochrome P450 aromatase (P450 arom) is known to be present mainly in Leydig cells. We studied in further details the distribution of equine aromatase in various adult tissues including the brain (hypothalamic area), liver, kidney, small intestine, muscle, bulbourethral gland and testes. The aromatase mRNA was essentially detected by RT-PCR in testis (169±14 amol of aromatase mRNA per μg of total RNA) and was barely detectable in brain, or below 0.1 amol/μg RNA in other tissues. This range of expression was confirmed by ELISA (50±7 pg/μg total protein) in the testis, and by immunoblot, evidencing a 53 kDA specific protein band in testis and brain only. The corresponding aromatase activity was well detected, by 3 H 2 O release from 1β, 2β 3 H-androstenedione, in testis and brain (200±23 and 25±6 pmol/min per mg, respectively) and below 3 pmol product formed/min per mg in other tissues. This study indicates that the testis, among the tissues analyzed, is the major source of aromatase in the adult stallion, and that the aromatase gene expression is specifically enhanced at this level, and is responsible for the high estrogen synthesis observed. Moreover, the study of aromatase in one colt testis has shown lower levels of transcripts, protein and enzyme activity, evidencing that aromatase is regulated during the development and may serve as a useful marker of testicular function. As the second organ where aromatase mRNA and activity are both well detected is brain, this study also underlines the possible role of neurosteroids in stallion on behaviour, brain function or central endocrine control.