Nonspiking Interneurons in the Drosophila Antennal Lobe Exhibit Spatially Restricted Activity.

Inhibitory interneurons are important for neuronal circuit function. They regulate sensory inputs and enhance output discriminability (Olsen et al., 2010; Olsen and Wilson, 2008; Root et al., 2008). Often, the identities of interneurons can be determined by location and morphology, which can have implications for their functions (Wachowiak and Shipley, 2006). While most interneurons fire traditional action potentials, many are nonspiking. These can be seen in insect olfaction (Husch et al., 2009; Laurent and Davidowitz, 1994; Tabuchi et al., 2015) and the vertebrate retina (Gleason et al., 1993). Here, we present the novel observation of nonspiking inhibitory interneurons in the antennal lobe (AL) of the adult fruit fly, Drosophila melanogaster These neurons have a morphology where they innervate a patchwork of glomeruli. We used electrophysiology to determine if their nonspiking characteristic is due to a lack of sodium current. We then used immunohistochemsitry and in situ hybridization to show this is likely achieved through translational regulation of the voltage gated sodium channel gene, para Using in vivo calcium imaging, we explored how these cells respond to odors, finding regional isolation in their responses' spatial patterns. Further, their response patterns were dependent on both odor identity and concentration. Thus, we surmise these neurons are electrotonically compartmentalized such that activation of the neurites in one region does not propagate across the whole antennal lobe. We propose these neurons may be the source of intraglomerular inhibition in the AL and may contribute to regulation of spontaneous activity within glomeruli. Significance Statement These findings are a novel discovery of nonspiking interneurons specifically in the olfactory system of adult Drosophila melanogaster The role of the nonspiking characteristic of similar interneurons in other species is not fully understood. Further, the sources of specific regulatory mechanisms such as intraglomerular inhibition in the fly are unclear. The characterization of nonspiking interneurons in Drosophila begins to explain these mechanisms and provides an avenue for further study into the roles of similar cells across species.