First, we tested the functionality of CINs in our in vitro preparation by performing whole-cell recordings. Under the current-clamp configuration, YFP-positive neurons (putative CINs) displayed the characteristic spontaneous, tonic firing at a rate of ~10 Hz [the average of the membrane potential value cycles between action potentials was -28 mV (Fig. 1A)]. Under voltage clamp, delivery of a single optical (100 ms) or a brief (4 ms) pulse, elicited an inward current that lasted for the length of the pulse (n = 6, Fig. 1A). Furthermore, delivery of a 4 msec blue light pulse under current clamp conditions induced a single action potential on YFP-positive cells (Fig. 1A). Mean latency between the start of the light pulse and the start of the action potential was 5.0 ± 0.36 msec (n = 5, confirming that optical excitation of ChR2 reliably drives generation of action potentials on accumbal CINs). Additionally, histological analyses confirmed expression of ChR2-eYFP (Fig. 1B).
