Connectivity of serotonin neurons reveals a constrained inhibitory subnetwork within the olfactory system

Neuromodulators flexibly adjust how sensory information is processed by acting upon a diverse set of receptors expressed by different neuronal types within a given network. The morphological and physiological traits of individual inhibitory local interneuron (LN) types, as well as their connectivity within sensory networks, enable each LN type to support different computations and are therefore ideal targets for modulatory neurons to have widespread impacts on network activity. In this study, we combined detailed connectivity analyses, serotonin receptor expression, neurophysiology, and computational modeling to demonstrate the functional impact of serotonin on a constrained LN network in the olfactory system of Drosophila. This subnetwork is composed of three LN types and we describe each of their distinctive morphology, connectivity, biophysical properties and odor response properties. We demonstrate that each LN type expresses different combinations of serotonin receptors and that serotonin differentially impacts the excitability of each LN type. Finally, by applying these serotonin-induced changes in excitability to a firing rate model that simulates the impact of inhibition exerted by each LN type, we predict a role for serotonin in adjusting the dynamic range of antennal lobe output neurons and in noise reduction in odor representations. Thus, a single modulatory system can differentially impact LN types that subserve distinct roles within the olfactory system.NEW & NOTEWORTHY This study combines detailed synapse resolution connectivity analysis with mapping of serotonin receptor expression to reveal an inhibitory subnetwork that is the target of serotonergic neurons within the olfactory system of Drosophila. Using neurophysiology and computational modeling, the differential effects of serotonin on this inhibitory subnetwork are posited to provide a mechanism for noise reduction in odor representations.