The aim of this study is to prove the influence of the sex steroid hormones estrogen, progesterone and testosterone on the serotonin transporter (5-HTT) binding using positron emission tomography (PET) and the selective radioligand [11C]DASB. Specifically, the 5-HTT binding will be quantified before and after hormone therapy underwent by 10 male-to-female (MtF) and 10 female-to-male (FtM) transsexuals urging for hormone treatment. The high-level, long-term administration of opsite sex steroid hormones in transsexuals provide the unique opportunity to investigate the influence of sex steroid hormones on the serotonergic system. Since the serotonin transporter serves as a primary target molecule for antidepressant treatment, the results of the study will be of benefit for the assessment of the clinical relevance of estrogen and testosterone as modulatory and neuroactive agents.

Inclusion Criteria:

• somatic health
• no previous sex hormone medication
• willingness to sign the written informed consent

Exclusion Criteria:

• severe diseases
• steroid hormone treatment within 6 months prior inclusion
• treatment with psychotropic agents such as selective serotonin reuptake inhibitors (SSRIs)
• any implant or stainless steel graft
• positive urine pregnancy test in women at the screening visit at each PET day