A lack of sex drives flies to drink

If you’re a single male, this situation may be familiar: you spend the evening trying to pick up women at a bar, but after enough rejections, you end up drowning your sorrows. It turns out that humans are not the only species that turns to demon rum when we can’t get laid. A study in Science this week suggests that male fruit flies deprived of sex will drink alcohol to stimulate a natural reward system.

The researchers began by splitting a population of male Drosophila into two groups. The fortunate flies each got six-hour sex marathons with receptive virgin females once a day for four days. The males that weren’t so lucky were subjected to one-hour sessions of constant sexual rejection by females three times a day for four days.

Then, males from each treatment headed to the bar. In this case, the local watering hole was a jar where they had unrestricted access to both a solution with no alcohol and one that contained 15 percent ethanol. Consistent with what we might expect from humans, the flies that couldn’t get laid preferred the alcoholic drink to the ethanol-free solution by a huge margin.

But the flies that preferred the alcohol had been exposed to two unfortunate experiences; not only were they constantly rejected, they were also denied sex. Was it the sexual deprivation or the rejection that drove them to drink? The researchers compared the preferences of this first group of males to a group that experienced no sex but also no rejection (these guys were exposed to yet another group of unlucky flies: decapitated virgin females that couldn’t court or mate). These two groups of males drank about the same amount of alcohol, suggesting that it’s the sexual deprivation, not the rejection, that drives the boozing behavior.

Don’t feel too bad for the sexless males, because they were eventually given the chance to mate. And once they got some action, their preferences changed: they no longer sought out the alcoholic solution.

So why does a lack of sex affect flies’ drinking habits? The researchers hypothesized that a molecule called neuropeptide F (NPF), which is related to reward processing in the brain, might be to blame. The researchers found that NPF levels were much lower in males that were deprived of sex, compared to mated males. Furthermore, by tinkering with the flies’ genetics, they discovered that interfering with NPF receptors increases the preference for alcohol in mated males. This suggests that sexual behavior affects the transcription of NPF, and NPF levels are tied to alcohol preference.

The researchers were also able to show that the NPF is a natural reward system that connects these two stimulating behaviors. After confirming that both sex and alcohol consumption are rewarding (no surprise there), they artificially activated the NPF receptor system, and found that alcohol consumption dropped. The activation of the NPF system, in itself, is actually rewarding to flies. If males are mating, the NPF system is being stimulated, so males don't seek out other rewards like booze.   In males that aren’t getting any action, NPF production drops off, provoking the flies to seek out a pleasantly alcoholic haze.

While boozehounds everywhere will probably see this study as validation, we have a way to go before we understand reward processing in humans. We do have a variant of NPF, called neuropeptide Y, but we don’t yet know that it links sex and alcohol in the same way as NPF does in Drosophila. So for now, don’t blame sex (or your lack of it) for that nasty hangover.

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