, 1998). The average peak increase in fluorescence rose smoothly with stimulation frequency in the presence and absence of GABA, but the frequency dependence of vesicle release differed in the two conditions. In comparison to control conditions, the presence of GABA severely attenuated spH signals at low stimulation frequencies but had little effect at high frequencies ( Figures 7A–7C), including when total spike number was kept constant ( Figure S7).

Thus, the presynaptic terminals of ePNs in the LH contain machinery that allows GABA to modulate vesicle release in the manner of a high-pass PFI-2 filter ( Figure 7C). To examine whether iPNs could supply modulatory GABA to ePN terminals, we expressed a QUAS-spH transgene under GH146-QF control in ePNs and a UAS-dTRPA1 transgene under Mz699-GAL4 control click here in iPNs. dTRPA1 is a transient receptor potential channel whose Ca2+ conductance gates open at temperatures >25°C ( Hamada et al., 2008), thus stimulating iPN activity. We shifted flies between holding temperatures of 25°C and 32°C while imaging spH fluorescence

during electrical stimulation of ePN axons. Like the direct application of GABA ( Figure 7C), the thermal activation of iPNs had a frequency-dependent effect on ePN synaptic output ( Figure 7D): transmission at 130 Hz was unaffected by iPN activity, whereas transmission at 40 Hz was roughly cut in half ( Figure 7D). Thus, iPN projections to the LH regulate the transmission characteristics of ePN terminals. To simulate the impact of the inhibitory high-pass filter on odor discrimination, we passed the ePN activity vectors of 110 odors (Hallem and Carlson, 2006 and Hallem et al., 2004) through a filter with the empirically derived transmission characteristics (Figure 7C). Because iPN activity scales with the overall drive to the olfactory system (Figure 5G), the strength of the filter was adjusted linearly with the number of glomerular channels an odor activates. We assumed that the maximal blocking effect, corresponding to the transmission curve

in 50 μM GABA (Figure 7C), is achieved when ePN spike rates in 22 of the 24 characterized glomeruli exceed 30 Hz (Figure 5G). Comparisons of all 5,995 possible pairwise distances between the filtered Fossariinae vectors with their 5,995 unfiltered counterparts showed that inhibition shifts the distributions of both Euclidean and cosine distances toward larger values (Figures 8A–8D). Replotting the data from Figure 2 against these increased ePN distances preserved the shape of the distance-discrimination function, only displacing it to the right (Figure S8). Knowledge of the transmission characteristics of the inhibitory high-pass filter should enable a prediction of WT performance from the measured behavior of flies lacking the distance-enhancing effect of the filter.