However, when pooling more trials, one can easily see the inhibitory effect of the stimulus as a consistent gap in firing that outlasts the stimulus by roughly 100 ms (Figure 7B). For the analysis of the inhibitory effect, we constructed PSTHs using 20 ms time bins. This example cell had an average spontaneous firing rate of 11.9 spikes/s, which decreased by 93% to 0.8 spikes/s upon stimulation of AON axons (Figure 7C). Across experiments, light stimulation of AON axons led to a reduction of firing by 58% ± 31% (p < 0.01), which recovered with a time constant of 189 ms (n = 20; Figure 7D top). No such effect was observed in noninjected control animals (n = 12; Figure 7D bottom).

We also tested the effects of AON activation on odor-evoked responses in MCs. We used a custom-built olfactometer to deliver up to three different odors to anesthetized rats with Trametinib manufacturer ChR2 expression in AON. Light stimuli were delivered 3.5 s after onset of odor stimulus (Figure 7E). In units that showed increased firing rate upon odor stimulation, brief light pulses rapidly suppressed firing, which recovered upon termination of light stimuli (Figure 7E). On average, AON

stimulation suppressed odor-evoked responses by 66% ± 33% (n = 9 cells from five animals; p < 0.01 compared to prestimulus firing rate; Figure 7F). The degree of suppression was not different from that observed for spontaneous firing (p Proteasome inhibition > 0.5). Because MCs have a tendency to fire at specific phases of the breathing cycle (Figure 7G) (Macrides and Chorover, 1972), we asked whether the effect of AON activation will depend on the phase in which it arrives (-)-p-Bromotetramisole Oxalate in the breathing cycle. For this analysis, we split the

data from the experiments on spontaneous MC activity into two separate histograms: one for all stimuli that arrived at the preferred half of the cycle (where MCs tend to fire, Figure 7H) and one for the stimuli that arrived at the nonpreferred half of the cycle (Figure 7I). Because the baseline for these histograms is not flat (reflecting the breathing dependent modulation of MC activity), it is harder to visualize the effect of stimulation. We therefore generated control histograms that are aligned by a “sham” stimulus at 1Hz (Figures 7H and 7I, middle panels). The subtraction of these sham histograms from the AON stimulus aligned histograms shows the net effect on firing rate (Figures 7H and 7I, bottom panels). AON stimulation was able to inhibit MC firing in both halves of the breathing cycle in the population data (Figures 7J and 7K). The integrated effect over 500 ms was significant in both conditions. Light stimulation reduced firing by 36% ± 27% (p < 0.01, n = 9) when it coincided with the high firing phase, and by 39% ± 30% (p < 0.01, n = 9) when it coincided with the low firing phase.