""Differential Brain Activation in Exclusively Homosexual and Heterosexual Men Produced by an SSRI

Two recent studies have shown a relationship between “sexual orientation” and size of anterior hypothalamic nuclei; others that hypothalamic serotonin is involved in male sexual arousal and behavior. We hypothesized that serotonergic differences might parallel sexual orientation differences in the hypothalamus. Accordingly, we administered 40 mg of fluoxetine (in a counterbalanced design with placebo) as a challenge to the serotonergic systems of homosexual and heterosexual men who differed maximally in their sexual orientation, and measured cerebral metabolic changes with FDG-PET. In accord with our hypothesis, differential effects were observed in the hypothalamus, as well as in other brain areas. In the hypothalamus, the homosexual group exhibited a significantly smaller reduction in glucose metabolism in response to fluoxetine than the heterosexual group. The fact that metabolic differences were produced by a pharmacologic challenge to the serotonergic system, might be reflective of underlying neurochemical differences between homosexual and heterosexual men.