Optogenetic identification of target-defined OB projection neurons after retrograde expression of ChR2. A, In vivo extracellular recording from an MTC in the mouse OB after injection of 2.1.DIO.hChR2(H134R)-EYFP into aPC of a PCdh21-Cre mouse. Top, Three trials of 50 light pulses (10 ms duration, 10 Hz, 200 μm optical fiber, ∼1 mW total power at the fiber tip) elicits a single spike following each pulse with near 100% reliability. Bottom, Pulse-triggered spike histogram showing spike latencies of 8–10 ms with little jitter. B, In vitro whole-cell recording from a mitral cell in an OB slice after retrograde ChR2 expression in MTCs (PCdh21-Cre mouse, as in A). Top trace shows responses to a train of light pulses (5 ms duration, 5 Hz, 1 mm optical fiber, ∼24 mW total power), with each pulse eliciting a spike. Lower trace shows overlay of the response to each pulse in the train, showing short latency to depolarization onset and low jitter in spike time. C, In vivo whole-cell recording from an MTC after retrograde ChR2 expression, as in A and B. Left trace shows continuous recording of response to train of 10 pulses (11 mW total power) delivered as in B. In this cell, the light pulse does not always evoke a spike; right traces show overlay of responses to pulses that fail to evoke a spike. Failed responses consist of a depolarization with short latency (2–3 ms) and no detectable jitter in onset time. Higher intensities (≤25 mW total power) or longer durations elicited spikes to each pulse (data not shown).