Hands up if you get grumpy when you are hungry! Don’t feel alone; this happens to many of us. Not eating properly throughout the day is often cited as contributing to “having a bad day”. A woman in New York recently pushed a bystander into the subway tracks, stating that she was “angry and hungry” when the crime was committed1. Hungry people also take more risks. Previous studies observed people playing gambling or financial investment “games”, and found that the hungry participants were far more likely to make risky decisions2; in real-life they’d have lost heaps of money! A bit closer to home, we might think of this situation a bit like driving dangerously on your way home after work just to get to your evening meal. Curiously, this happens to animals too. Anecdotal evidence suggests that hungry predators are more likely to hunt dangerous prey, which increases their own risk of getting hurt. For survival this makes pretty good sense – if you’re starving, it’s worth taking risks so you don’t starve to death. But what we’ve not yet figured out is how the brain encodes and weighs relative risk when risk-taking happens in the context of hunger or other potentially life-threatening situations.
New Neuroscience research has come one step closer to figuring out how this works. This research used the fruit fly, because their brains are very simple, and so individual nerve cells that make the decisions about both food and risk can be monitored. As their name suggests, fruit flies love fruit (actually they feed on the yeast that live on the fruit), and they are strongly attracted to it. On the opposite side of things, fruit flies are incredibly sensitive to carbon dioxide (CO2). It gets emitted from flies when they are stressed, as a warning signal to others3. The first reaction for flies that sense CO2 is to fly away. But here’s where things get a bit tricky: rotting fruit, which is full of the yeast that flies love, also emits CO2 as part of the rotting process. So, what’s a poor fly to do when the yummy thing ‘smells’ dangerous? Their brain comes to the rescue, of course.
In this study4, fruit flies were put into a container that had rotting fruit and nasty CO2, and the researchers asked: if the flies are hungry, are they more likely to go for the fruit even if there is CO2 around? You can probably guess what happened – the hungry flies went for it. Somehow, their hungry brains were able to ignore the danger signal from the CO2 in order to get the much-needed food. Makes sense: if the hungry brain wants you to take a risk to get some food, it will dampen your ability to identify risk in the first place (that really didn’t seem like a dangerous lane change).
But how did the brain make this decision? The fruit fly model was really handy here because researchers already knew which nerve cells transmitted the information about the risk of CO2. So all they had to do was look to see whether these cells acted differently in hungry and non-hungry flies. Alas, things are never quite this simple in the brain. When the researchers looked they found that the activity of the CO2-sensing cells was the same in both hungry and non-hungry flies.
So, could there be some other neural cells – ones that are responsive to hunger signals – in this circuit? The authors found that a cell called a Kenyon cell (named after FC Kenyon in 1896, who studied bees!), appears to be involved. These cells have been already described for their function in learning, but this new study found that they were also involved in the circuit that transmitted the CO2 risk message. In order to work-out the function of these Kenyon cells, the researchers asked: if we artificially disable these cells can we make the animals unable to perceive risk associated with CO2 ? The answer to their question was yes, which means that they found the cells that modulate the activity of the CO2-sensing cells.
But where does the hunger angle come in? For this, let’s return to thinking about the whole circuit with the CO2-sensing cells and the Kenyon cells that can modulate their activity. If this scenario we are building is correct, and hunger is important to how the circuit works, then changing the activity of the Kenyon cells should only have an effect on the CO2-sensing cells if the flies are hungry. And this is exactly what they found. The CO2-sensing cells only listen to the Kenyon cells in certain situations – the activity of the circuit is context-dependent. When the researchers disabled the Kenyon cells in fed (non-hungry) flies, CO2 avoidance was completely normal – the CO2 sensing cells did not listen to the Kenyon cells, and instead went about their business of telling the flies to stay away from the CO2. But when researchers disabled the Kenyon cells in hungry flies, CO2 avoidance was completely blocked, suggesting that the CO2-sensing cells only listen to the Kenyon cells when the fly is hungry. How hunger signals in the brain can control this circuit is still unclear.
Our human brains have risk sensing neural circuits composed of thousands of nerve cells, so they’re not as simple as the fly’s. But the fly experiments show that altering the activity of these circuits may be an effective way to reduce or moderate risk-taking behaviour.
2. Symmonds, M., Emmanuel, J., Drew, M., Batterham, R., & Dolan, R. (2010). Metabolic State Alters Economic Decision Making under Risk in Humans. PLoS ONE, 5 (6) DOI: 10.1371/journal.pone.0011090
3. G.S. Suh, A.M. Wong, A.C. Hergarden, J.W. Wang, A.F. Simon, S. Benzer, R. Axel, D.J. Anderson (2004) A single population of olfactory sensory neurons mediates an innate avoidance behaviour in Drosophila Nature, 431: pp. 854–859
4. Bracker LB et al (2013) “Essential role of the mushroom body in context dependent CO2 avoidance in Drosophila.” Current Biology, 23: 1228–1234.