Modulation of odor-evoked signals in the mushroom body by addition of a second functional OSN. (a) Representative G-CaMP activity in PN terminals in mushroom body elicited by three odorants (10-2 dilution except ethyl acetate, 10-4 dilution) and paraffin oil (solvent) in Or42a+Or42b-functional larvae compared to Or42a-functional larvae (all but the ethyl acetate image are reprinted from Figure 5c). Top image shows intrinsic mushroom body G-CaMP fluorescence with overlaid numbers indicating the location of subdomains in Figure 5a. Bottom four images show false color-coded image of mushroom body taken 600 ms after stimulus onset, and represented as ΔF/F (%) (scale at the right). (b) Responses of subdomain 2 to high concentrations of ethyl butyrate are decreased in Or42a+Or42b-functional larvae compared to those in Or42a-functional larvae. Responses to a dilution series of ethyl butyrate, 2-Heptanone (10-2 odor dilution), ethyl acetate (10-4 dilution), and paraffin oil are calculated as the average ΔF/F over 1 s after odor stimulus onset (mean ± SEM). Purple: Or42a-functional larvae (n = 8). Light blue: responses from Or42a+Or42b-functional larvae, n = 5 except paraffin oil (n = 6), 10-4 and 10-2 dilutions of ethyl butyrate (n = 6), 10-2 dilution of 2-Heptanone (n = 6), and 10-4 dilution of ethyl acetate (n = 6). Responses that differ significantly between the two genotypes are indicated with an asterisk (*p < 0.01, Student's t-test). (c) Schematic model of gain control in the larval olfactory system. In single-OSN-functional animals (left), low concentrations of odor cause moderate activation of the single OSN and its PN, leading to attraction to the odor source (magenta trajectory to the right). High concentrations of odor fail to activate the LNs (green) and cause strong activation of the PN and corresponding behavioral avoidance of the odor. In wild-type animals, low odor concentrations activate a single OSN and its PN, leading to odor attraction. At high odor concentration, two additional OSNs are recruited and the LN network is activated, preventing PN activity from reaching saturation and maintaining stable attraction to the odor.