Classifying Drosophila Olfactory Projection Neuron Boutons by Quantitative Analysis of Electron Microscopic Reconstruction

Abstract

In Drosophila melanogaster , olfactory projection neurons (PNs) convey odor information from the peripheral olfactory center to higher brain regions. Recent single-cell sequencing studies reveal a large diversity of PN subtypes, while their structural diversity remains unknown. Herein, we volume reconstructed 89 PN axonal boutons under electron microscopy and quantitively analyzed the internal neurotransmitter release site, presynaptic active zones (PAZs), and neuromodulator container, dense-core vesicles (DCVs). The ultrastructure-based cluster reveals three morphological distinct bouton subtypes: complex boutons, unilobed boutons, and simple boutons. The complex boutons contain the most PAZs and DCVs, which suggests that they are of the highest capability of releasing neurotransmitters and neuromodulators. By expressing membrane-target Horseradish Peroxidase in a subset of boutons, we found that DCVs are not uniformly distributed in all boutons but preferentially distributed in certain GH146-positive subtypes. Our study demonstrates that morphologically distinct PN bouton subtypes display different PAZ and DCV distribution features, which may determine their capacity of releasing neurotransmitters and neuromodulators, and provides ultrastructure evidence of the olfactory information processing of PN in Drosophila.