Researchers at the University of Chicago have shown for the first time that strong sexual orientation among men appears to be connected with brain metabolism. The results of their study are reported in a paper to be presented Nov. 12 at the annual Society for Neuroscience conference in New Orleans.

In the paper, the researchers report on a study that shows differences between exclusively homosexual and exclusively heterosexual men in glucose metabolism in the hypothalamus and other brain areas following the administration of fluoxetine, a drug commonly called Prozac.

“The important distinction among the group of men we studied is that they were all exclusively homosexual or heterosexual. They had not had any sexual experiences or reported any sexual fantasies outside their preference,” said Howard Moltz, Professor Emeritus in Psychology at the University and an author of the paper.

The paper, entitled, “Differential Cerebral Glucose Metabolic Patterns in Exclusively Homosexual and Exclusively Heterosexual Men Produced by a Selective Serotonin Reuptake Inhibitor,” will be presented by Leann Kinnunen, a recent Ph.D. graduate in Psychology from the University.

For their study, the scientists first talked with approximately 80 sexually active men in their 20s and 30s. Among that group, they choose eight exclusively homosexual and eight exclusively heterosexual men for further study. Other men in the larger group experienced fantasies or had had sexual experiences with both men and women.

“We know from studies on rats that the hypothalamus is strongly connected with sexual activity. We also know from those studies that the neurotransmitter serotonin is associated with sexual behavior and arousal,” Moltz said.

The research team theorized that hypothalamic differences may reflect biological differences between men with strong sexual orientation differences. In order to determine whether such differences might exist, they gave the men doses of Prozac, a drug that selectively inhibits the reuptake of serotonin in the brain. At another time point, the researchers also administered a placebo to the men. Neither the researchers nor the subjects were aware of which substance they were given.

They were tested 90 minutes after having received the drug or the placebo. The subjects were also administered radioactive glucose, which provides a marker for a Positron Emission Tomography (PET) brain scan to det­ermine where in the brain the drug acted to inhibit the reuptake of serotonin. The heterosexual men had a much stronger response in the hypothalamus to the Prozac than did the homosexual men, a finding that suggested that men with strong sexual preferences have differences in the ways in which the neurotransmitter serotonin works in their brains.

“The remarkably strong association seen in this study between hypothalamic physiology and sexual orientation underlines the promise of functional brain imaging for elucidating the neuroanatomy and neurophysiology underlying human sexual behavior and functioning,” said Heino F. L. Meyer-Bahlburg, professor of clinical psychology at Columbia University.

Other studies that have attempted to determine if there are biological differences between homosexual and heterosexual men have been inconclusive, possibly because the sample included men who were not as exclusively oriented as were the men in the Chicago study, Moltz said.