There is evidence for preferential ipsilateral versus contralateral projections from different subregions of the AON (Reyher et al., 1988; Brunjes et al., 2005), and future studies targeting ChR2 expression to specific subregions may reveal functional specializations. Based on previous studies, we expected to selleck chemicals llc see disynaptic inhibition in MCs when AON axons were stimulated. Unexpectedly, we found that MCs receive not only inhibition but also direct excitation. A synaptic origin of this excitation is supported by

the following observations: (1) the reversal potential of EPSCs was close to 0mV, as expected for ionotropic glutamatergic currents; (2) light-evoked currents are blocked by ionotropic glutamatergic

receptor blockers; and (3) the currents persist when polysynaptic activity is minimized with TTX. Additional experiments also offer strong support for direct excitation from the AON. First, selleck compound the latency of these events was the same as the latency of EPSCs in all other cells examined in our study (Figure S2). Second, EPSCs persisted even in the absence of MC primary tufts in the glomerular layer, or even in the complete absence of the glomerular layer itself—ruling out a sole contribution from ETCs, which are the only identified local source of excitation for MCs. Our experiments with the low-affinity γ-DGG also indicate that the excitation is due to synapses made directly on MCs, and not through extrasynaptic activation of MC glutamate receptors, which mediates dendrodendritic self-excitation (Nicoll and Jahr, 1982; Christie and Westbrook, 2006; Pimentel and Margrie, 2008). Because the glomerular layer is also dispensable for this excitation of MCs by AON, and there is negligible innervation of AON axons in the EPL where MC

lateral dendrites are, the likely locus of MC excitation is the cell body layer. Independent of the exact mechanism of depolarization, AON axons are able to evoke time-locked spikes in MCs at least under some conditions. The direct excitation followed by disynaptic inhibition establishes a small time window within which MCs can emit spikes, reminiscent of the action of many feedforward circuits throughout the brain (Pouille and Scanziani, 2001; Isaacson and Scanziani, 2011). Robust inhibition is evoked in MCs following activation of AON axons, leading to a pause in firing for tens of milliseconds. The latency of inhibition, as well as its indirect blockade through glutamatergic receptor antagonists, confirms its disynaptic origin. At least part of the inhibition arises through GCs, which receive monosynaptic excitation from AON. GC-mediated inhibition has most often been studied using sensory inputs in the OB, either by directly stimulating OSNs or by stimulating MCs (Isaacson and Strowbridge, 1998; Schoppa et al., 1998; Egger and Urban, 2006).