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Effects of administration of testosterone, dihydrotestosterone, oestrogen and fadrozole, an aromatase inhibitor, on sex skin colour in intact male rhesus macaques

For defining the mechanism of control of sex skin colour in male rhesus macaques (Macaca mulatta) by hormones, a spectrocolorimeter was used to monitor skin redness after administration of testosterone, dihydrotestosterone (a non-aromatizable androgen), oestradiol or fadrozole (an aromatase inhibitor that blocks the conversion of testosterone to oestrogen). Skin blood flow was measured by laser doppler. Eight 9–14 kg, 5–9 year old intact male rhesus macaques were given hormone, fadrozole or vehicle treatments in a cross-over experimental design. Baseline blood flow and colour measurements were taken in four paired tattoo defined areas on the back and legs of each animal (one pair in non-sex skin, three pairs in sex skin). Colour and blood flow measurements were taken 3–4 days after the first dose and, thereafter, once a week for 3–6 weeks. Measurements taken after treatments were compared with baseline and intra-animal comparisons were made between treatment and vehicle for each animal. In all animals after administration of 4 mg testosterone kg^–1 (long-acting), redness in the sex skin areas increased (P= 0.032) by day 3 and returned to baseline values by day 7. Administration of 1 mg oestradiol kg^–1 day^–1 for 4 days caused increased redness in all animals (P = 0.007) similar in magnitude to that caused by testosterone. Administration of 0.1 mg dihydrotestosterone kg^–1 day^–1 for 4 days resulted in a nonsignificant decrease in redness (P = 0.09) on days 3–7. Treatment with fadrozole (0.25–0.5 mg kg^–1 day^–1) for 3 weeks caused sex skin to become significantly less red during treatment (P = 0.014). There was no significant change in redness in non-sex skin areas during any treatment. Sex skin blood flow increased in animals treated with testosterone, correlating with increased redness (R = 0.906), while blood flow in non-sex skin was unchanged. Increased redness after treatment with testosterone and oestrogen, no change in redness with treatment with dihydrotestosterone and a decrease in redness after treatment with fadrozole support the conclusion that oestrogen controls sex skin redness, and testosterone acts indirectly through conversion to oestrogen to cause increased sex skin redness in male rhesus macaques.