Chronic exposure to odors at naturally occurring concentrations triggers limited plasticity in early stages of Drosophila olfactory processing

Abstract

ABSTRACTIn insects and mammals, chronic exposure to odors at high concentrations in early life alters olfactory function, but the role of odor experience-dependent plasticity in more naturalistic contexts is less clear. We investigated olfactory plasticity in the Drosophila antennal lobe by exposing flies to odors at concentrations that are typically encountered in natural odor sources. These stimuli also strongly and selectively activated only a single class of olfactory receptor neuron (ORN) input, facilitating the investigation of input-specific plasticity. Overall, chronic exposure to three such odors elicited limited plasticity in the odor responses of second-order projection neurons (PNs). Exposure to some odors elicited mild increases in PN responses to weak stimuli, extending the lower bound of the dynamic range of PN signaling. When present, plasticity was observed broadly in multiple PN types and thus was not selective for PNs receiving direct input from the chronically active ORNs. Chronic E2-hexenal exposure did not affect PN intrinsic properties, local inhibitory innervation, ORN responses, or ORN-PN synaptic strength, but modestly increased broad lateral excitation evoked by some odors. These results show that PN odor coding is only mildly affected by strong persistent activation of a single olfactory input and highlight the stability of early stages of insect olfactory processing to significant perturbations in the sensory environment.