Figure 2.

Identification of striatal-projecting neurons during extracellular recording. A, CAV2-Cre injected in the posterior striatum (pStr) of LSL-ChR2 mice travels retrogradely and causes expression of ChR2 in striatal-projecting neurons. ChR2-expressing neurons were identified by their responses to laser light during extracellular recording. To validate approaches for distinguishing between ChR2-expressing neurons and neurons that respond to light because of synaptic excitation, we measured changes in light-evoked responses after blocking synaptic transmission with NBQX. B, Top row, Response of an example cortical neuron to a 100 ms pulse of blue laser light before and after application of NBQX. Middle row, This neuron responds reliably to a 5 Hz train of 10 ms laser light pulses before and after NBQX. Bottom row, This neuron responds within 10 ms of light onset (left). Spike shape was consistent before and after NBQX injection (right). C, Example cortical neuron that responds to light (top) but cannot follow a train of laser pulses (middle). The light-evoked responses of this neuron disappear after NBQX application. D, Example cortical neuron that responds to a train of laser pulses (middle) but has a long latency response (bottom). The light-evoked responses of this neuron disappear after NBQX application. E, Neurons collected during sound response characterization recordings were classified as striatal-projecting if they displayed fast, reliable responses to light (blue points). Laser-responsive neurons that did not meet these criteria were excluded (gray points). Dotted gray line represents a threshold requiring neurons to reach 1/2 of their maximum laser-evoked firing rate within 10 ms. F, Neurons collected during NBQX control experiments that continued to respond after application of NBQX display fast, reliable responses to light (blue points). Laser-responsive neurons which failed to respond after application of NBQX did not meet the criteria used to operationally define striatal-projecting neurons.