5, n = 9) probably due to the prolonged inhibitory effect of AON stimulation. Using 50 ms bins,

we were unable to find evidence for fast excitation that was observed in the in vitro experiments. We therefore constructed PSTHs using 1 ms bins. By comparing these PSTHs to randomly aligned PSTHs, we found significant fast excitation in 9 out of 20 cells (see Experimental Procedures). An example of this excitation is find more shown in Figure 8. While only inhibition was seen with 50 ms bins (Figure 8A), a very brief and precise excitation was evident with finer binning (Figure 8B). Excitation in this cell was manifested as a 1 bin (1 ms) of increased probability of firing from 1% to 8.9%, with a latency of 5 ms (Figure 8C). This latency was markedly Selleck PD0325901 different from the latency to the photoelectric artifact that always coincided with the first bin of light stimulation (Figure 8D). On average, AON axon stimulation increased firing probability 9.5 ± 3.3 times with a latency of 6 ± 1.8 ms (n = 9). Average population PSTHs pooling data from the nine cells that were excited by the AON

fibers and of the whole population, are shown in Figure 8E. Figure 8F shows the nine cell histogram at an enlarged scale. The duration of the excitatory response in the average PSTH mostly reflects the variability in the latency among the cells. Indeed, if responses were aligned on the peaks of each cell’s excitation, the average PSTH exhibited a narrow peak of less than 5 ms (Figure 8G).

Importantly, no excitation was evident in any of the control cells (n = 11, Figure 8H). We did not find any evidence of rapid excitation in odor-evoked responses. These results reveal that activation of AON axons in vivo leads to an immediate and brief increase in firing probability of MCs, followed by a longer lasting inhibition. We used optogenetic methods to selectively activate feedback axons to the OB, and determine their cellular targets and their functional effects on bulbar output neurons. The major findings of Mephenoxalone our study are that: (1) AON axons have a dual effect on MCs: fast, brief depolarization and more prolonged hyperpolarization, (2) the fast depolarization is likely to be due to direct monosynaptic excitation, (3) the inhibitory effect of AON activation on MCs is mediated through GCs as well as glomerular layer interneurons, and (4) as a result of these synaptic effects, activation of AON axons could impose precisely timed spikes on output neurons, followed by suppression of spikes for tens of milliseconds. Broadly similar results, but with some interesting specific differences, have been reported for feedback projections from the piriform cortex in independent work (Boyd et al., 2012). Cortical inputs to the OB are diverse (Price and Powell, 1970; Pinching and Powell, 1972; Davis et al.