Behavioural and neurogenetic evidence for emotion primitives in the fruit fly Drosophila from an open field test

Emotions are based on evolutionarily conserved building blocks ('emotion primitives') that are present across mammals. Whether these building blocks are found in insects is largely undefined. Here, we employ the open field test (OFT) in Drosophila and focus on wall-following and total walking activity as behavioural indicators of emotion-like states. Wildtype and transgenic flies were subjected to various conditions prior to behavioural testing. The results indicate that wall-following and total walking activity are independent measures that are modulated, with generally increased wall-following observed under aversive stimuli and decreased values under positive conditions, independent of the time of day. The effects of genetic manipulation of selected neuromodulatory systems support their role in modulating emotion-like states in Drosophila. Activation of reward-related neurons decreased wall-following while inhibition increased it. Moreover, treatment with ethanol or diazepam produced predictable changes in behaviour. Our findings suggest that Drosophila shares conserved core emotion-like building blocks with vertebrates, and show that wall-following and total walking activity in the OFT can serve as a reliable indicator of emotional valence and arousal in the fruit fly. This positions Drosophila as a powerful genetic model to dissect the neuronal and neurochemical substrates of emotion primitives.