Of mice and men who like men

I’ve always been fascinated by the biological origin of human homosexuality. Growing up, I put my faith in genetics and evolution, so I was convinced that there must be some elegant explanation for this seemingly non-procreative sexual orientation.

Research in humans has been inconclusive and controversial due to the multiple factors that feed into human attraction, but recently scientists have begun to address the question in our genetic cousin and standard animal model, the mouse.

In the newest issue of the journal Nature, a team led by Yan Liu in Beijing reports a role for serotonin in the sexual preferences of mice.

Usually, male mice show a preference for mounting females over other males (yes, they sometimes mount other males). But when Liu knocked out serotonin-sensitive neurons, serotonin-producing enzymes, or serotonin itself, the male mice lost this preference: They mounted males and females at the same frequency.

In addition, Liu could restore the preference for females by injecting extra serotonin precursor into those serotonin-deficient male mice. Interestingly, the same treatment in unmodified males effected a decrease in male-on-female mounting.

These findings are just another piece in the complex puzzle of human and mammalian (homo)sexuality. Previous studies have claimed that male homosexuality runs in families and between siblings/twins, or that there might be a heritable “gay gene” on the X chromosome, suggesting a genetic component. Other studies have said that hormonal differences during fetal development can cause differences in the brains of homosexual men, possibly in the hypothalamus.

But can we add low serotonin to one of the potential causes of being gay? It’s unlikely.

First, the experimental mice seemed only to lose preferential distinction between sexes. They did not really gain a homosexual preference, and I would even hesitate to label them as bisexual.

Also, as far as we know, sexual attraction in humans is more complicated than in mice, given the strong influences of our social environment, so it is difficult to make a direct comparison.

Finally, while there is some evidence that homosexual men may respond differently to SSRI antidepressants (which increase available serotonin) and that reduced libido is a common side effect of SSRIs, there has not been any evidence of SSRIs having an effect on sexual preference.

And that is really where the question transcends scientific curiosity and carries societal implications. Some gay rights advocates argue that a definitive biological origin would prove that being gay is not a choice. At the same time, a biological origin would invite cures or screens to remove homosexuality.

Now that I’ve grown up some, I’ve become less interested in this quest for the Homo Grail. I’d still like to see what scientists discover next, but I believe people should have the self-driven freedom to choose, change or realize their sexuality, regardless of biological determination.

I still think you’re on the right track, baby, whether or not you were born this way.

Perry Tsai is a Sexual Health Columnist for the Daily Tar Heel. He is a second year medical student from New Orleans, La. Contact him at perrytsai@gmail.com.