Synaptic circuitry of identified neurons in the antennal lobe of Drosophila melanogaster

Abstract

In Drosophila melanogaster olfactory sensory neurons (OSNs) establish synapses with projection neurons (PNs) and local interneurons within antennal lobe (AL) glomeruli. Substantial knowledge regarding this circuitry has been obtained by functional studies, whereas ultrastructural evidence of synaptic contacts is scarce. To fill this gap, we studied serial sections of three glomeruli using electron microscopy. Ectopic expression of a membrane-bound peroxidase allowed us to map synaptic sites along PN dendrites. Our data prove for the first time that each of the three major types of AL neurons is both pre-and postsynaptic to the other two types, as previously indicated by functional studies. PN dendrites carry a large proportion of output synapses, with approximately one output per every three input synapses. Detailed reconstructions of PN dendrites showed that these synapses are distributed unevenly, with input and output sites partially segregated along a proximal-distal gradient and the thinnest branches carrying solely input synapses. Moreover, our data indicate synapse clustering, as we found evidence of dendritic tiling of PN dendrites. PN output synapses exhibited T-shaped presynaptic densities, mostly arranged as tetrads. In contrast, output synapses from putative OSNs showed elongated presynaptic densities in which the T-bar platform was supported by several pedestals and contacted as many as 20 postsynaptic profiles. We also discovered synaptic contacts between the putative OSNs. The average synaptic density in the glomerular neuropil was about two synapses/mm 3 . These results are discussed with regard to current models of olfactory glomerular microcircuits across species. J. Comp. Neurol. 524:1920-1956, 2016. INDEXING TERMS: Drosophila melanogaster; olfactory system; glomerulus; projection neuron; ultrastructure; synaptic microcircuitsThe neural circuitry of the antennal lobe (AL) of the vinegar fly Drosophila melanogaster is one of the principal models for the study of how chemosensory information is processed in the brain. Thus it has been investigated intensively with a variety of anatomical, physiological, and genetic methods (Stocker, 1994;Vosshall and Stocker, 2007;Wilson, 2011). Very little is known, however, about the synaptic circuitry, synapse structure, numbers, and identity of pre-and postsynaptic components of this brain center.The olfactory sensory neurons (OSNs) located in the antennae and maxillary palps send axons to the AL, where they form synapses with different types of interneurons within functional and morphological units called glomeruli (Stocker, 1994;Couto et al., 2005). These generic structural features of glomerular anatomy are preserved across species (Hildebrand and Shepherd, 1997;Ache and Young, 2005;Martin et al., 2011). Olfactory glomeruli are spherical structures of synaptic neuropilThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work...

Neurogenetic dissection of the Drosophila innate olfactory processing center

“…We rarely observed complete polarisation in LHONs, many of which had some presynaptic labelling in their dendrites (see Figure S1C for illustrative example of cell-type PV5a1). This is similar to mPN1s which have been shown to form dendrodendritic synapses (Kazama and Wilson, 2009;Rybak et al, 2016) . For one likely dopaminergic cell-type isolated we could not determine polarity and did not further analyse these neurons ( Figure S1D).…”

Active Zone Scaffold Protein Ratios Tune Functional Diversity across Brain Synapses

“…Information processing is tuned by local interneurons (LNs) (Chou et al, 2010; Nagel et al, 2015). BRP levels in the AL were high at the cortex of individual AL glomeruli (Figure 1C, see magnified image to the right), a position typical for ORN terminals (Mosca and Luo, 2014; Rybak et al, 2016). Syd-1, however, was distributed rather homogeneously over individual glomeruli and the whole AL.…”

“…BRP-short-GFP was expressed in ORNs and imaged in parallel to the STED channels to identify ORN-derived AZs (Figure 4A, blue label). The ORN-derived synapses form elongated AZs, at which multiple T bars are fused and arranged opposite to multiple postsynaptic profiles (“dyad synapses”) (Mosca and Luo, 2014; Rybak et al, 2016). Elon-gated BRP staining patterns were thus expected, as BRP is the major building block of T bars.…”

Presynaptic LRP4 promotes synapse number and function of excitatory CNS neurons

“…Putative ORN terminals were identified based on morphology (Rybak et al, 2016; Tobin et al, 2017) and quantified as described (Mosca and Luo, 2014). Terminal perimeter was measured using ImageJ (NIH, Bethesda, MD) and used to calculate T-bar density.…”