In Drosophila melanogaster, olfactory projection neurons (PNs) convey odor information from peripheral olfactory center, antenna lobe, to central olfactory center, mushroom body (MB), and lateral horn (LH). In MB, the mechanisms underlining the transformation from coarse-coding PNs to sparse-coding MB intrinsic Kenyon cells (KCs) remain an open question. Here, we used HRP-labeled electron microscopy (EM) to volume reconstruct 89 PN axonal boutons in a reference area of the input region, the calyx of MB. The results showed that the number of presynaptic active zones (PAZs), neurotransmitter release site, is in positive linear correlation with the surface area of PN axonal boutons, while the number of dense core vesicles (DCVs), vesicles that containing neuropeptides, monoamines, or neurotrophic factors, is weakly related to the morphology of PN axonal boutons. Further analysis illustrated that DCVs preferentially exist in PN axonal boutons labeled by GH146-GAL4, a most widely used genetic marker for studying olfactory PNs. Our data suggest that synapses are uniformly distributed on the surface of all PN boutons, thus the neurotransmission capability of a PN axonal bouton could be predicted by its size, and PN subtypes release neuropeptides, monoamines, or neurotrophic factors, as well as classical neurotransmitters, to mediate the PN-KC transformation.
bioRxiv Subject Collection: Neuroscience