Fruit flies that develop homosexual tendencies when drunk may help reveal how alcohol loosens human sexual inhibitions, claim researchers.

Kyung-An Han and her colleagues at Pennsylvania State University in University Park used a voyeuristic chamber dubbed the "Flypub" to observe the influence of alcohol on the sexual behaviour of male Drosophila fruit flies.

The researchers got the flies drunk on the fumes of an ethanol-doused cotton pad placed at the base of the chamber, and filmed them using a camera held above the Flypub's transparent ceiling.

Male Drosophila will normally only court females, following them and vibrating their wings in a courtship "song", before attempting to copulate.

The first time they were exposed to alcohol, groups of male flies became noticeably intoxicated but kept themselves to themselves. But with repeated doses of alcohol on successive days, homosexual courtship became common.

From the third day onwards, the flies were forming "courtship chains" of amorous males.

Blocked reward

Han argues that the drunken flies provide a good model to explore how alcohol affects human sexual behaviour. While the ability of alcohol to loosen human inhibitions is well known, it is difficult for scientists to study.

Han’s team used flies that were genetically modified so they cannot release dopamine in the brain unless the temperature exceeds 30 ºC, to test if the effects of alcohol were dependent on this brain chemical. Indeed they did show that the effect of alcohol on sexual behaviour depends on the presence of this neurotransmitter.

That makes sense, says Ulrike Heberlein, who studies the genetics of alcohol-induced behaviour at the University of California, San Francisco. She says dopamine is central to the neural reward circuits that evolved to motivate animals to seek food and sex, but which are also stimulated by drugs of abuse.

But do fruit flies really provide a good model for what happens in the inebriated human brain? Heberlein, who works on both flies and mice, believes they do. "What is cool is that there is such a similarity," she says. "I am surprised by the parallels."

Journal reference: PLoS ONE (DOI: 10.1371/journal.pone.0001391)

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