Figure 1. Conditional knock out of Slc32a1 from direct or indirect pathway MSNs abolishes GABAergic output.

a. Cre expression driven by Drd1a (D1-Cre, top) and Drd2 (D2-Cre, bottom) BACs was visualized via activation of TdTomato in a reporter mouse. Red fluorescence reveals expression throughout striatum and in axons in expected target nuclei of direct and indirect pathway MSNs (SNr and GP, respectively). Green fluorescence reflects expression of a GAD67-GFP that reports GABAergic neurons. As seen in the red channel, there is diffuse cortical expression of Cre in the D1-Cre mice; however, this occurs in non-GABAergic neurons as noted by the lack of overlap with GFP fluorescence (see Supplementary Figure 2 for complete anlaysis). Scale bar: 500 µm.

b. AAV DIO-ChR2-mCherry injected into the striatum of a mouse carrying D2-Cre and D2-GFP transgenes shows ChR2-mCherry labeling in GFP⁺ cells, indicating pathway specific conditional expression of the virally encoded protein. ChR2-mCherry-expressing somata are marked with an asterisk and represent over 2/3 of the GFP⁺ MSNs in the area of dense infection. ChR2-mCherry was never observed in D2-GFP⁻ MSNs in these mice. Scale bar: 10 µm.

c. Voltage-clamp recordings from ChR2-mCherry⁻ MSNs demonstrate GABAergic synaptic currents evoked by 2 ms-long pulses of 473 nm light that stimulates neighboring ChR2-mCherry⁺ MSNs. Example currents from MSNs in D1-Cre (left) and D2-Cre (right) mice that were either homozygous (gray, Slc32a1^f/f) or heterozygous (black, Slc32a1^f/+) for the Slc32a1 conditional allele are shown. GABAergic currents are inward due to high intracellular Cl⁻ concentration. Insets, graphs of average peak current amplitudes evoked in animals of each genotype. * indicates p<0.05 for comparison of Slc32a1^f/+ and Slc32a1^f/f data. Error bars: SEM