Plasticity is widely studied across different sensory systems and behavioral paradigms, but the underlying mechanisms are varied and incompletely understood. Previous work in the fruit fly Drosophila melanogaster reported changes in odor preference and walking behavior after chronic odor exposure during early adulthood. Here, we investigated the hypothesis that changes in behavior reflect changes in how odors are encoded in the first two layers of the fly olfactory circuit. We chronically exposed flies to naturalistic odor stimuli that selectively and robustly activate a single olfactory receptor neuron (ORN) class. We then performed targeted intracellular recordings from genetically identified second-order olfactory projection neurons (PNs) that either receive direct input from the activated ORN class, or receive indirect activity (via local lateral circuitry), during chronic odor exposure. In addition, we used existing reagents to create a novel optical method to characterize ORN-PN synaptic strength. We find that the fly antennal lobe is resistant to plasticity, with a few exceptions. Of the odors we tested, we find that rearing in trans-2-hexenal, a leaf aldehyde that selectively activates ab4a ORNs, weakly enhanced odor responses in some PNs. The effects of rearing on PNs were not explained by ORN odor responses or changes in ORN-PN synaptic strength. We find evidence that lateral excitation may increase across glomeruli following rearing, suggesting that some odors may alter PN responses globally. We discuss possible reasons for differences between our observations and prior work on olfactory plasticity in this circuit, which has been conducted primarily in the context of exposures to much higher, non-naturalistic concentrations of odor. Our results point to the stability of insect sensory circuits in the face of large perturbations in the sensory environment.
During our optical stimulation experiments, we find that driving Chrimson expression may abolish odor responses in some ORNs. We include sample data highlighting this observation in a population of pb1a olfactory neurons. Lastly, we include antennal local field potential recordings in response to a variety of odor concentrations to help guide future experiments seeking isointense odor panels.</p
