AAV5-mediated ChR2 is absent in CINs of D1/A2a-cre mice. A–E, Different section showing absence of Cre-dependent ChR2 expression from CINs. A, ChR2 expression of D1-SPNs in D1-Cre mice (arrowheads). B, Example CIN. C, Serotonin 5-HT1B receptor clustered on somata of CIN and other neurons. D, Cell nuclei stained with DAPI. Scale bar: 20 µm. E, Overlay of E–H. F–J, Magnification of rectangle from A–E. F, ChR2 expressed in the cell membrane (arrowheads) and spines (arrows) of D1-Cre SPNs. G, CINs. H, Serotonin 5-HT1B receptor. I, Cell nuclei stained with DAPI. J, Overlay showing ChAT-positive cells negative for ChR2 (arrowheads). Serotonin receptor 5-HT1B is clustered on somata of CINs and SPNs. Scale bar: 10 µm.