library_construction_protocol: scRNA-seq was performed to profile the striatum of C57BL6J mice at 18 months of age. The brain was immediately dissected out and was sectioned into 400 mm slices in ice-cold artificial cerebrospinal fluid (ACSF) oxygenated with 95% O2 and 5% CO2 and immediately transferred to an oxygenated recovery solution. The cortical, hippocampal, or striatal tissue was dissected out in ice-cold ACSF and cut into small pieces. The tissue was digested in ACSF containing 1 mg/ml pronase at 34 °C for 30 min. The digested tissue was dissociated in ice-cold oxygenated ACSF containing 1% fetal bovine serum. The dissociated cells were filtered through a 20 mm filter and washed with ice-cold ACSF. To remove myelin, the cell pellet was resuspended in PBS and processed with debris removal kit (Miltenyi Biotec). Isolated cells were diluted to 1000 cells/ml in 0.04% BSA/PBS and processed with 10X Genomics platform.  The library preparation and sequencing were performed following 10X Genomics Chromium protocols. Single cell suspension from mouse brain tissues were prepared using 10x Genomics Chromium Single Cell 3' Reagent Kits v2 and v3 (10X Genomics, Pleasanton, CA) according to manufacture's protocol. Quantity and quality of cDNA were assessed by Agilent 2100 expert High Sensitivity DNA Assay. cDNA samples were sequenced on NovaSeq 6000 or NextSeq 500 platform at UCLA Technology Center for Genomics and Bioinformatics.