Disruption of GABA[sub]A in the Insect Antennal Lobe Generally Increases Odor Detection and Discrimination Thresholds

Studies of olfactory function show that disruption of GABA[sub]A receptors within the insect antennal lobe (AL) disrupts discrimination of closely related odors, suggesting that local processing within the AL specifically enhances fine odor discrimination. It remains unclear, however, how extensively AL function has been disrupted in these circumstances. Here we psychophysically characterize the effect of GABA[sub]A blockade in the AL of the moth Manduca sexta. We used 2 GABA[sub]A antagonists and 3 Pavlovian-based behavioral assays of olfactory function. In all cases, we used matched saline-injected controls in a blind study. Using a stimulus generalization assay, we found that GABA[sub]A disruption abolished the differential response to related odors, suggesting that local processing mediates fine odor discrimination. We then assessed the effect of GABA[sub]A antagonist on discrimination thresholds. Moths were differentially conditioned to respond to one odor (reinforced conditioned stimulus [CS+]) but not a second (unreinforced conditioning stimulus [CS-]) then tested for a significant differential conditioned response between them across a series of increasing concentrations. Here, GABA[sub]A blockade disrupted discrimination of both similar and dissimilar odor pairs as indicated by generally increased discrimination thresholds. Finally, using a detection threshold assay, we established that GABA[sub]A blockade also increases detection thresholds. Because detection is a prerequisite of discrimination, this later finding suggests that disrupted discrimination may be due to impairment of the ability to detect. We conclude that the loss of ability to detect and subsequently discriminate is attributable to a loss of ability of the AL to provide a clear neural signal from background.