Thirsty fruit flies see water as a reward, but the neural circuits that mediate this are different from the circuits that mediate reward for sugar or other foods, reports a paper published online this week at Nature Neuroscience. This suggests a similarity to the reward circuits of mammals and indicates that there may be an evolutionarily conserved system that mediates different types of reward.
Psychologists have divided reward in animals into wanting, learning and liking components, and these can be assessed using feeding behaviours in a laboratory setting. Wanting describes an animal’s desire to seek a food as resource; learning assigns the resource value to food itself. Finally, an animal is considered to like a substance if it is accepted as palatable.
Scott Waddell and colleagues find that water-deprived fruit flies seek out water despite the fact that, under normal conditions, water-sated fruit flies avoid it and favour a dry environment. They also report that this water-seeking behaviour is mediated by neurons in the fly brain, which release the neurotransmitter dopamine. They find that these neurons are a distinct population, separate from the neuronal circuits that process other rewarding components, such as positive reinforcements from eating foods such as sugar.
The authors find that flies can also dissociate wanting water from liking it; they displayed a distinct water-seeking behaviours as well as a water-liking behaviours (extending their proboscis, an extendable mouthpart used for eating palatable food). This study demonstrates that fruit flies, like mammals, can make a psychophysical distinction between wanting, value learning and liking a given reward.