For histological analysis, brains were removed and immersion-fixed overnight at room temperature. Brains were then dehydrated in graded ethanol, cleared in xylene, and paraffin-embedded [14]. Seven micrometer-thick serial coronal sections were cut and mounted on glass slides, which were dried overnight at 42°C. Sections were deparaffinized, rehydrated through a graded series of ethanol, and washed in water. For immunostaining [15], sections were incubated with blocking solution (2% horse serum, 1% bovine serum albumin, and 0.1% Triton X-100 in phosphate-buffered saline, pH 7.5) and then with primary antibodies at 4°C overnight followed by secondary antibodies in blocking solution at room temperature for 2 hours. The primary antibodies used were rabbit anti-TH antibody (1:500; Pel-freez, Rogers, AR http://www.pelfreez-bio.com, P40101) and mouse anti-human nuclear antibody (1:300; Chemicon Billerica, MA http://www.millipore.com, MAB 1281). The secondary antibodies were rhodamine-conjugated rat-absorbed donkey anti-rabbit IgG (Jackson ImmunoResearch Laboratories, West Grove, PA http://www.jacksonimmuno.com; 1:200) and fluorescein isothiocyanate-conjugated pig anti-mouse IgG (Vector Laboratories Burlingame, CA http://www.vectorlabs.com; 1:200). Nuclei were counterstained with 4′,6-diamidino-2-phenylindole using proLong Gold antifade reagent (Invitrogen, Carlsbad, CA http://www.invitrogen.com). To quantitatively analyze double-labeled neurons in the striatum,
