A paper in Nature from Prof Scott Waddell’s team in the Centre for Neural Circuits and Behaviour has shown that the neural reward system of the fruit fly Drosophila is much closer to that of mammals than was previously thought. Octopamine, an analogue of noradrenaline, has historically been believed to be the reward signal in insects. However recent studies have now implicated dopamine.
The study used transgenic Drosophila in which dopamine and octopamine neurons could be controlled with temperature. Surprisingly blocking synaptic transmission from octopamine neurons did not impair the formation of appetitive memories using a nutritious sugar reward. However, octopamine neurons were critical for forming memory with a sweet-tasting but non-nutritious sugar. These results suggested that a pathway other than octopamine neurons is involved in forming nutritional memories and that octopamine only reinforces sweet taste memories.
Further experiments from graduate student Christopher Burke and postdoctoral fellow Wolf Huetteroth found that memories could be formed by replacing sugar with direct activation of octopamine or dopamine releasing neurons. Strikingly, octopamine neurons only formed short-lived memory whereas dopamine neurons formed long-lasting memory. Moreover, manipulating both systems revealed that octopamine signals act through the rewarding dopamine neurons.
Therefore this study shows that dopamine represents reward in the insect brain, similar to its established role in mammals.
Please visit the Nature website to read the full paper.