NEW YORK (Reuters Health) May 08 – Lesbian women appear to process two putative pheromones in a manner that more closely resembles heterosexual males than heterosexual females, according to findings documented by positron emission tomography (PET) and MRI imaging during exposure to the agents, Swedish investigators report.

Dr. Ivanka Savic and her associates at the Karolinska University Hospital performed PET and MRI as subjects were smelling the progesterone derivative 4,16-androstadien-3-one (AND), which is found in human sweat at concentrations 10 times as high in men than in women, and the estrogen-like steroid estra-1,3,5(10),16-tetraen-3-ol (EST), which is detected in the urine of pregnant women.

The research team previously found that homosexual men responded more like heterosexual women than heterosexual men in PET scans of regional cerebral blood flow when smelling the two pheromones.

Areas of the preoptic and ventromedial hypothalamic nuclei were activated among homosexual men when smelling AND, a “pattern of activation that was reciprocal in heterosexual men.” Homosexual men responded to EST with activated olfactory regions, similar to reactions observed in heterosexual women.

For their current study, published in the May 8 online issue of the Proceedings of the National Academy of Sciences, Dr. Savic’s group performed PET and MRI for 12 lesbian women, 12 heterosexual women, and 12 heterosexual men as they smelled each of the two pheromones, odorless air, and four other ordinary odors.

The main finding was that “lesbian women differed from heterosexual women in that they did not activate the preoptic hypothalamus with AND.” The researchers also found that “lesbian women processed AND and EST more congruently with heterosexual men than heterosexual women.”

The lesbian women showed activation of the olfactory regions with both AND and EST, whereas among heterosexual women, only EST involved the olfactory regions, while AND showed activation of the preoptic hypothalamus.

Lesbians also exhibited partial activation of the anterior hypothalamus upon exposure to EST, which is the area of primary activation when heterosexual men smell EST.

None of the other odors exhibited differential activation in any of the study groups.

Dr. Savic’s team concludes: “The data support the notion of a coupling between hypothalamic neuronal circuits and sexual preferences and encourage further evaluation of the possible neurobiology of homosexuality and human sexuality in general.”

Proc Natl Acad Sci USA 2006