Figure 3. Optogenetic activation of cholinergic OB inputs enhances sensory-evoked excitation. A, Spike raster and rate histogram of MTC spiking during inhalation of clean air and optical stimulation (blue shaded area) in six repeated trials. The spike rate was calculated per 50 ms bin. Inhalation-evoked spike rates increase during optical stimulation and return to baseline within 5–10 s after stimulation ceases. The top trace (sniff) shows artificial inhalation as measured by a pressure sensor connected to the nasopharyngeal cannula. B, Plot of inhalation-evoked firing rates during air inhalation, averaged for the nine inhalations just before (no stim) and after (stim) optical stimulation (n = 20 units). Data were analyzed and plotted as in Figure 2. C, Sniff-triggered spike histogram of MTC spikes aligned to the start of inhalation of clean air before (blue) and after (red) optical stimulation, normalized to the maximum bin in the no-stimulation condition. Bin width, 100 ms. The histogram is compiled from all units, with firing rate normalized separately for each unit. Note that the relative increase in spike rate at the peak bin is larger than the relative increase of the baseline bins immediately after inhalation. Inset, Sniff-triggered histogram normalized to the maximum and minimum bin for both conditions independently, showing no change in spiking dynamics after OB stimulation. D, Odorant-evoked MTC spiking is enhanced by optical OB stimulation. This example MTC shows a moderately increased firing rate in response to odorant presentation in baseline conditions (top) and strongly increased odorant-evoked firing rates during optical stimulation (bottom). The histogram is the average of five trials. E, Plot of odorant-evoked changes in MTC spiking (Δ spikes/sniff) in the absence of (no stim) and during (stim) optogenetic stimulation of cholinergic afferents to the OB (n = 92 units). F, Odorant response magnitudes (Δ spikes/s) plotted for baseline (blue) and optical stimulation (red) as a function of cell identity, sorted in order of magnitude of excitatory response in baseline conditions. Note that all units show an increase or no change in odorant-evoked excitation, including those that are suppressed during odorant presentation. Shown is the same dataset as in E. G, Spike histograms for two additional units showing that optical OB stimulation increases spike rate even for neurons that show a null (Unit 3, left) or suppressive (Unit 4, right) response to odorant. H, Time course of effects of optical stimulation on odorant-evoked spike rate, averaged across all units. The blue bar shows time of optical stimulation and simultaneous odorant presentation. The darker trace shows mean change in odorant-evoked spike rate between trials with and without light stimulation, measured after each inhalation (at 1 Hz); the shaded area indicates variance (SEM) around mean. The lighter trace (spont) shows light-evoked change in the spontaneous firing rate in the absence of inhalation, reproduced from Figure 2C. I, Sniff-triggered spike histogram of MTC spikes during odorant presentation in baseline conditions (blue) and during optical OB stimulation (red), normalized and plotted as in C. Inset, Sniff-triggered histogram normalized to the maximum and minimum bin for both conditions independently.