Male fruit flies that don't get sex drink more alcohol. In addition, they have lower levels of a protein linked to their reward system in their brains, a new study shows. The discovery could help scientists understand how the reward system works in flies and possibly how it is related to addiction in humans.
In the study published March 15 in Science, the authors identify a brain circuit in the Drosophilia that is defined by a specific brain protein called neuropeptide F (NPF). The amount of this protein in the brain correlates to the male flies’ sexual satisfaction as well as their ethanol consumption.
In a series of experiments, Heberlein and colleagues observed the flies’ preferences toward the natural reward of sex and the drug reward of alcohol. Past research had shown that flies have addictive-like behaviors when given ethanol and that they can develop a taste for alcohol-infused food, even if made unpalatable. The researchers, however, extended that study to show how social experiences—such as sexual rejection—shapes fruit flies' behavior, in this case the amount of alcohol consumed.
In the first test, the researchers measured how the flies respond to sexual rejection. One group of flies were allowed to mate as much as possible with virgin females for a six-hour session each day for four days. The other group faced sexual rejection by non-virgin females for an hour a day over four days. In response to this rejection, the males no longer wanted to mate, even when they shared a space with virgin females.
Furthermore, these males drank more alcohol than their sexually satisfied counterparts. In the experiment, both satisfied and rejected males were placed in a container with two capillaries—one containing a sugary drink and the other a cocktail of sugar and 15% ethanol. The sexually satisfied males drank equally from both tubes; the sexually deprived males drank more from the ethanol-infused one.
Next, to understand the brain chemicals involved in these behaviors, Heberlein and colleagues analyzed the flies' levels of NPF. Research has shown that a homolog protein, neuropeptide Y, is related to alcohol consumption and stress in mammals. Looking at the brains of the both groups of flies, the team found that sex increased the quantities of NPF in the fly brains; sex deprivation lowered the level.
In the next experiment, the scientists influenced the flies’ preference for alcohol by activating or suppressing the cells that produce NPF. Activation reduced the preference for alcohol; suppression increased it. Finally, the team activated a rejected fly's NPF brain cells and found that it reduced his drinking habit. This suggests that sexual experience, activity in the NPF system, and alcohol consumption are all linked in the fruit fly’s brain circuitry.
In the end, Heberlein says this study is "really about more than alcohol and sex." It's getting at the "nuts and bolts, the molecules and circuits, of what reward really means," and that could have implications for understanding the root cause of diseases that drive humans to alcohol, such as depression, post-traumatic stress disorder, and anxiety.
1. Shohat-Ophir, G., et. al. 2012. Sexual deprivation increases ethanol intake in Drosophila. Science 335:1351–1355.