Serotonergic Modulation of a Visual Microcircuit in Drosophila melanogaster

Serotonin (5-HT, 5-hydroxytryptamine) and other neuromodulators tune circuit activity to impart behavioral flexibility and adaptation. We demonstrate that Drosophila optic lobe neurons involved in the initial steps of visual processing are subject to serotonergic neuromodulation. The visual processing neurons L2 and T1 express 5-HT2B and 5-HT1-type receptors, respectively. Serotonin increases calcium in L2, but not T1 neurons, and serotonin potentiates the L2 neuron response to luminance increases. While we did not detect serotonin receptor expression in L1 neurons, they also undergo serotonin-induced calcium changes, likely through electrical coupling between L1 and L2 neurons. We also found that L2 and T1 form reciprocal synapses onto serotonergic projections in the medulla, forming a potential feedback loop. Together, these results describe a serotonergic microcircuit for regulating the first steps of visual processing in Drosophila.