Figure 5. In vivo photostimulation of callosally projecting auditory cortex neurons.

ChR2 expression in rat auditory cortex neurons can be used to tag and identify this neuronal population during in vivo recordings. (A) Callosally projecting neurons in the rat auditory cortex were labeled with ChR2 via retrogradely HSV1-mediated transfection; (B) Light-evoked activity of three well-isolated single units recorded simultaneously is shown as rasters. Each unit is color-coded (green, blue, red). After blocking fast glutamate (AMPA) receptors (left) with the selective antagonist NBQX, only the shortest latency unit (green, right) continued to show light-activated activity. This indicates that activity in the other two units was indirect, i.e. synaptic, and therefore blocked by NBQX. (C) Population histogram showing that ability to follow light flashes at higher repetition rate (5 Hz, ISI = 200 ms, 10 ms LED pulses) cleanly separated recordings into two classes, which we interpret as direct (ChR2-positive; dark green: single units responding after NBQX application) and synaptic (ChR2-negative; dark gray: single units not responding after NBQX simulation; light gray: multiunit recordings). The x-axis shows the spiking response reliability, computed as the fraction of trials in which the firing rate in 40 ms after the start of the second pulse of 5 Hz LED trains was greater than in the 40 ms immediately preceding the pulse. (D) Callosally projecting neurons (ChR2+) are non-responsive to white noise stimulation, showing different response reliability from the average population (Other). Reliability was computed as the fraction of trials in which the firing rate in 40 ms after sound onset was greater than in 40 ms immediately preceding the sound. Error bars show standard error of the mean.