The retrograde tracer, Neuro-DiI, was injected bilaterally into the NAc. A, A representative coronal section confirming the injection sites within the NAc. Arrowheads indicate tracer deposit. The double arrowheads indicate the anterior commissure. Scale bar, 1 mm. B, Representative images showing that numerous Neuro-DiI-labeled VTA neurons are also TH-positive. Shown are dual-channel fluorescent images for DiI (Red), TH (Green), and overlay (Yellow). The arrowheads indicate cells labeled with DiI and stained for TH (Right). Scale bars, 500 μm (Left) and 50 μm (Right). C, Representative images showing a DiI-labeled VTA neuron that was selected for electrophysiology. Upper panel, Red fluorescent image. Bottom panel, DIC image. Scale bar, 20 μm. D, A bar graph summarizing the mean increase by GDNF (200 ng/ml) in the firing rate of Neuro-DiI labeled VTA neurons. *p < 0.05. n = 8 cells.