Fine morphological changes of MSN dendritic arborization induced by DA denervation were characterized by single-neuronal injections of Lucifer yellow applied to PFA-fixed brain sections from the Drd1a-tdTomato/Drd2-EGFP transgenic mice, using a method previously described7273. Sharp heat-pulled glass micropipettes filled with a 4% solution of Lucifer yellow (catalog no. L453; Life Technologies, Carlsbad, CA, USA) and containing a silver electrode connected to a computer-controlled microelectrode amplifier (Multiclamp 700 A, Axon Instruments) were inserted into 250 μm-thick brain section kept in ice-cold PBS (0.1 M), under an epifluorescence microscope (model no. E600FM, Nikon, Tokyo). After the insertion of the micropipette into the soma of visually identified MSNs located in the dorsal striatum, a negative direct current of 5 nA was administered for 20 minutes during which MSNs were filled with the negatively charged Lucifer yellow tracer. A 488 nm filter was used to visualize GFP contained in D2 MSNs whereas a 568 nm filter was used to detect the presence of tdTomato into D1 MSNs. Each neurons being injected was carefully inspected with both filters in order to determine content in GFP
