Chronic boozing sends male flies chasing after any and every potential mate.

From the annals of insect biology comes a cautionary tale for those recovering from their post-New Year's celebration: heavy boozing has been shown to send male fruitflies, like their human counterparts, into a lusty fog.

Fruit flies get amorous under the influence of constant booze.

In the flies, hypersexuality caused by chronic alcohol exposure has the effect of making the males chase anything with wings - other males included. Although sexual preference in humans is obviously a complex phenomenon not replicated by the fly work, the findings could be used to further establish a fly model system for the study of alcoholism, observers say.

Although it may seem a bit of a stretch to study alcoholism in fruitflies, intoxicated insects bear many similarities to intoxicated humans, says Ulrike Heberlein, who studies alcohol and cocaine responses in fruitflies at the University of California, San Francisco.

As the concentration of ethanol in the body rises, flies begin to become uncoordinated and oblivious to their surroundings: they get tipsy. "They bump into each other. They bump into the walls," says Heberlein.

Add more alcohol and the flies become sedated. Add still more and the soused flies die. Remarkably, even the concentrations of ethanol that induce these behaviours are nearly the same in flies and humans, says Heberlein. Flies also develop a tolerance to alcohol, and can develop withdrawal-like symptoms.

Combine these features with the genetic information and tools available for flies, and you can begin to address questions that can't be answered by studying humans, says Robert Anholt, a geneticist at North Carolina State University in Raleigh. "In human genetic association studies, the only thing you can reliably detect is genes that have relatively large effects," says Anholt. "In flies you have far better resolution because you can grow many flies cheaply and quickly."

©Kyung-An Han laboratory, Penn State
Love chain: male fruit flies chase each other in a circle.

Gender bender

With this in mind, Kyung-An Han, a neurobiologist at Pennsylvania State University in University Park, and her colleagues tested the effects of chronic alcohol exposure on sexual behaviour in the fruitfly Drosophila melanogaster . The researchers noted that male flies repeatedly exposed to ethanol vapour became less discriminate in their mate selection. The buzzed flies often courted fellow males, pursuing them around the cage while serenading with a traditional fruitfly courtship song played on vibrating wings1.

Eventually, the lusty flies devolve into a courting frenzy. "You get a chain of males chasing each other," says Heberlein, who was not associated with the study but has observed similar behaviour in her own unpublished work. In contrast, alcohol had little effect on mating in female fruitflies, which normally do not court their mates.

The findings suggest that the flies do not fundamentally change their sexual orientation, but rather get over-sexed. "Multiple alcohol exposures makes them essentially hypersexual," says Heberlein. The mind-dulling effects of alcohol might also make it more of a challenge for male fruitflies to distinguish the gender of other flies in the crowd.

Although the drunken dipterans were more amorous, their rates of successful copulation declined after getting tipsy, the researchers found - a trend that has long been observed in humans. Anholt notes that William Shakespeare even described the phenomenon in his play Macbeth when he wrote that alcohol "provokes the desire, but it takes away the performance".

Released inhibitions

Preliminary work suggests that the link between sex and alcohol may be the neurotransmitter dopamine. Han and her colleagues found that lowering dopamine concentrations in drunken flies reduced male-to-male courtship. But dopamine is associated with general activity levels, and it is unclear whether the reduced dopamine levels may have simply caused the flies to become a bit sluggish.

The results are interesting and could prove useful for understanding how other animals respond to ethanol, says Nigel Atkinson, a geneticist at the University of Texas in Austin. The next step is to work out whether alcohol specifically affects sexual behaviour, or generally hits levels of alertness.

One way to test this would be to find out whether boozed animals have a more reactive startle reflex. If they jump more readily in response to passing shadows, the effects may not be specific to courtship, he says.

Heberlein agrees, but notes that a link between sex and alcohol would be intuitive. "Drugs and sex act on the same circuitry in the mammalian brain," says Heberlein. "The so-called reward centre did not evolve for abusing drugs - it evolved for natural rewards such as food and sex."

* References
1. Lee, H. G. , Kim, Y. C. , Dunning, J. S. & Han, K. A. PLoS ONE 3, e1391 (2008).