Figure 2. Optogenetically triggered firing in Cajal-Retzius cells. A, Camera lucida tracing of whole-cell, biocytin-filled, EYFP-expressing neurons of stratum lacunosum-moleculare reveals the typical features of Cajal-Retzius (l-m, stratum lacunosum-moleculare). B, Short flashes of blue light induce action currents in cell-attached configuration (c-a, top traces; five sweeps superimposed) and reveal inward currents after breakthrough into whole-cell voltage-clamp mode (w-c, Vh = −60 mV, bottom traces; five sweeps superimposed). Notice the beginning of the current already during the flash. The empty square indicates latencies measured from the start of the light flash (vertical dotted blue line) to the peak of the action current. The dotted line is for reference. C, Similarly, flashes of blue light trigger firing in the whole-cell current-clamp configuration (five traces superimposed). The empty circle highlights the measurement of the latency from the beginning of the light flash (vertical dotted blue line) to the peak of the action potential. The dotted line indicates the 0 mV level. D, Firing pattern of ChR-expressing cells. Notice the sag in the hyperpolarization response (−25 pA step, 1 s) and the train of action potentials terminated by depolarization block (70 pA, 1 s), which is characteristic of Cajal-Retzius cells. The dotted line indicates −62 mV. E, Summary plot of the latency of the firing measured under cell-attached (c-a, empty squares) and whole-cell configuration (w-c, empty circles). Both mean ± SE and individual data points are shown. F, Unreliable stimulation by trains of flashes. Notice the decreasing probability of firing coupled with increasing latencies during a 10 Hz train of flashes delivered to a Cajal-Retzius cell (recoded in cell-attached mode: c-a, four traces superimposed). Action currents are expanded in the insets, and the blue dotted lines indicate the start of the flash. Same cell after passing in whole-cell mode (w-c); notice the decreasing amplitude of the photocurrents during the repeated stimulation. Black dotted line for reference.