In fruit flies, hunger shapes alcohol-reward memory

Most evenings, a bar on your way home is just a bar. Skip lunch, though, and the same sign can suddenly pull you inside. Researchers at Brown University have now traced part of that mechanism in the brain, using the common fruit fly. Their study, published in Current Biology, links hunger to the formation of “reward” memories that play a role in alcohol addiction.

The fruit fly is a practical model because its brain is sophisticated enough to show complex behaviour, yet small enough to observe neuron by neuron. The team, led at Brown's Carney Institute for Brain Science, first mapped and isolated neurons that respond to hunger. In food-deprived flies, those neurons activated a chemical called octopamine, described by the researchers as the fly version of norepinephrine, the fight-or-flight neurotransmitter in humans. Octopamine is also activated by thirst and pain. Until now, research on reward has focused mainly on molecules such as dopamine, and the authors argue that internal states like hunger, thirst and pain deserve as much attention.

The key experiment is precise. Silencing or activating a single octopamine neuron, called VUMa3, changed how soon flies found alcohol rewarding and how strongly they preferred it, but only when the flies were hungry. Kaun summarises the finding this way: hunger recruits a new neuron to communicate with the circuits that link cues in the environment to the rewarding feeling of alcohol. Her example is an everyday one. A bar you usually walk past without a second thought can call to you “when you're hungry after skipping lunch,” and the sign that “didn't entice you when you weren't hungry” can trigger a craving.

The researchers go further, but carefully. Sherer, a postdoctoral researcher and first author of the study, suggests that people who grow up very hungry or very stressed “might have a higher propensity” to develop alcohol addiction. The study itself was carried out in flies, not people, so this remains a hypothesis. The team says the work could bring scientists one step closer to understanding how external stressors such as food insecurity might shape the internal reward systems involved in alcohol and drug addiction.

For people working on addiction, the gain is a specific target: a single identified neuron and a circuit that only switches on under certain internal conditions. The authors say the insight could inform strategies for treatment and prevention, including preventing relapse. For now, though, these are results in flies, and the study gives no timeline for treatments. The next question is whether comparable circuits exist in humans, and that remains open.