Very few labeled cell bodies were detected in the OB (Figure S1), and even these are likely newborn GCs migrating from the rostral migratory stream—such cells take more than 3 weeks to release GABA (Bardy et al., 2010) and should not contribute significantly toward direct release upon light stimulation. To confirm functional expression of ChR2 in AON neurons, we obtained whole-cell patch-clamp recordings

from AON neurons in acute slices from infected animals. Stimulation with blue light (whole field illumination, Topoisomerase inhibitor 5–10 mW/mm2) depolarized AON neurons sufficiently to evoke action potentials (Figure 1C). In fixed brain tissue, EYFP-positive axon terminals were clearly visible in the granule cell layer and the glomerular layer in both the ipsilateral and contralateral OB (Figure 1D).

The fluorescence intensity of EYFP per area unit was not uniform across the different layers of the OB, with greater intensities in the granule cell layer and the bottom part of the glomerular layer; fluorescence intensities were distinctly lower in the external plexiform layer, where most of the dendrodendritic synapses between MCs/TCs and GCs are located (Figure 1E). Contralateral projections to the glomerular layer had lower intensity than those in ipsilateral glomerular layer, even when normalized to their corresponding granule cell layer intensities (1.02 ± 0.09 versus 0.62 ± 0.13, n = 3, p < 0.05; Screening Library datasheet Figure 1F). These differences in average fluorescence intensities reflected the difference in density of fibers rather than expression levels of ChR2-EYFP, because the fluorescence intensity per area unit of single fibers in ipsilateral and contralateral OB were 1.00 ± 0.21 and 1.07 ± 0.28, respectively (n = 3 experiments, > 50 axons per experiment; errors are SD; Figures 1G and 1H), and not significantly different (p > 0.1). We examined synaptic responses of MCs to AON stimulation using whole cell recordings in acute OB slices that were made 2 to 4 weeks postinjection (Figure 2A). Excitatory and inhibitory synaptic currents were recorded in the voltage-clamp

mode at −70mV and 0mV, respectively, in response to a pair of 10 ms light pulses 100 ms apart. Although responses to pairs of stimuli are shown in the figures, all analysis mafosfamide reported below were done for the response to the first of the pair of stimuli. Light stimulation, unexpectedly, elicited excitatory as well as inhibitory synaptic currents in MCs, with inhibition being the dominant component (Figure 2B). All evoked currents were blocked by ionotropic glutamate receptor blockers (10 μM CNQX+ 100 μM APV; excitation blocked by 92.6% ± 4%, n = 3; inhibition blocked by 94.7% ± 3.1%, n = 4; p < 0.01; Figure 2C). Excitatory postsynaptic current (EPSC) amplitudes ranged from 5.8 to 29.1 pA and averaged 18.5 ± 6.6 pA (n = 15).