Whole brain monoamine detection and manipulation in a stalk-eyed fly

Understanding the physiological mechanisms that influence conflict resolution is of great importance because the outcome of contests over limited resources such as mates, territories, and food has significant fitness consequences. Male stalk-eyed flies (Teleopsis dalmanni) compete over territory and mates and provide an excellent model system to study aggression. To investigate potential effects of serotonin (5-HT) on aggressive behavior in these flies, we developed a dissection and sample preparation method sufficiently sensitive to measure monoamine concentrations from whole brain samples of small insects. This new method allows the detection of monoamines from a single fly brain using high performance liquid chromatography with electrochemical detection. The method allows for the detection and quantification of octopamine (OA), 5-hydroxytryptophan (5-HTP), dopamine (DA), 5-hydroxyindoleacetic acid (5-HIAA), tyramine (TA), and serotonin (5-HT) and provides a means for assessing changes in stalk-eyed fly brain monoamine concentrations in response to drug administration in food media. We successfully elevated 5-HT levels approximately 8-fold that of control levels in stalk-eyed fly brains by oral administration of the 5-HT precursor 5-HTP. Furthermore, in size-matched competitions for a food resource, flies that had elevated 5-HT in response to 5-HTP pretreatment exhibited a high probability of winning the contests. These results suggest that 5-HT enhances aggression in the stalk-eyed fly and highlight the potential of our method for testing putative roles of monoamines in modulating self and rival assessment in conflict resolution.