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Geestelijke Gezondheidszorg Delfland, Institute of Mental Health, PO Box 5016, 2600 GA Delft, The Netherlands
¹ Radboud University Nijmegen Medical Centre, Department of Neurology, Laboratory of Pediatrics and Neurology, Nijmegen, The Netherlands
² Division of Human Nutrition, Wageningen University, Wageningen, The Netherlands
³ Department of Endocrinology, Andrology Unit, Vrije Universiteit University Medical Center, Amsterdam, The Netherlands

(Requests for offprints should be addressed to E J Giltay; Email: giltay{at}dds.nl)

There is a close relationship between the brain and the endocrine system. The brain expresses receptors for sex steroids and is capable of metabolizing these hormones. We explored (1) sex differences in homovanillic acid (HVA), a metabolite of the neurotransmitter dopamine, and (2) the effects of cross-sex steroid administration in transsexual subjects. First, we compared plasma HVA levels between 38 male and 34 female healthy volunteers (not using hormone replacement therapy) of a mean age of 72 years (range 65–84 years). Secondly, we measured plasma HVA levels in 15 male-to-female transsexuals treated with 100 µg ethinyl estradiol/day and 100 mg cyproterone acetate/day for 4 months, and in 17 female-to-male transsexuals treated with testosterone esters (250 mg/2 weeks i.m. for 4 months). Plasma HVA levels were lower in elderly men than in elderly postmenopausal women (geometric mean 25.4 nmol/l (percentile (P)₁₀ 4.9; P₉₀ 69.8) vs 39.0 nmol/l (19.0; 76.1); P=0.027). In transsexuals before cross-sex hormone administration, genetic males also had lower plasma levels of HVA than genetic females (geometric mean 14.8 nmol/l (P₁₀ 7.0; P₉₀ 35.0) vs 34.3 nmol/l (21.8; 61.4); P<0.001). Cross-sex hormone administration did not affect plasma HVA in either group (P>0.5). The pretreatment sex difference in plasma HVA was unaffected after 4 months of cross-sex hormone administration (P=0.003). The sex difference in plasma HVA was not reversed by cross-sex hormone administration in transsexuals, and was also preserved in elderly subjects. This indicated that differences in dopamine gene expression were largely unaffected by exposure to sex hormone levels in adulthood, but must rather be explained by a sex difference in genetic factors or by the organizing effects of sex hormones during early development.