Figure 3. Interneurons are synaptically but not directly activated by optical stimuli.

(a) Left, example of high-frequency firing activity evoked in a putative IN in response to injection a suprathreshold current stimulus (250 pA, 1 s). Right, a 1-s light stimulus directly centered on the same interneuron in the presence of 5 μM NBQX and 10 μM GBZ did not evoke any significant response. (b) Representative trace showing synaptic activity elicited in a putative IN in response to a photostimulus in the absence of synaptic blockers. Shaded rectangles include portions of the trace magnified in bottom insets. The first depolarizing deflection of V_m (shown in left inset) occurred at a relatively high latency from the stimulus onset (9 ms), triggering one occasional early spike. An array of postsynaptic potentials (PSPs, magnified in right inset) were elicited throughout the remainder of the stimulus. (c) When the flash was delivered at a more depolarized V_m (−50 mV), the synaptic response was sufficient to reach the firing threshold, resulting in robust spiking activity.