Epstein, JacobCritical period plasticity (CPP) allows sensory circuits to adapt to the environment during the development of the brain. In the fruit fly, Drosophila melanogaster, the olfactory circuitry undergoes CPP in the antennal lobe (AL), the first place where odor information is processed. Sensory neurons in the fly’s antenna project their axons to the AL which is composed of globular neuropil called glomeruli. Within the glomeruli, the sensory neurons synapse onto projection neurons (PNs). CPP in the AL is characterized by changes in glomerular volume and projection neuron activity. The neurotransmitter serotonin is known to play an important role in odor processing but its involvement in Drosophila olfactory CPP remains unexplored. Here we use RNAi silencing of the 5-HT1b serotonergic receptor (5-HT1bR) and volumetric analysis of AL glomeruli to assess this receptor’s role in olfactory CPP. We show that 5-HT1bR is necessary for CPP in a pair of serotonergic neurons innervating the AL but not in the mushroom body, a further downstream area of olfactory processing, where the receptor is highly expressed. This research uncovers a role for 5-HT regulation in the plasticity of the developing brain in addition to its known role in odor processing.en-USBiologyNeuroscienceOlfactionOther