Cell surface and intracellular AMPAR subunit levels were determined using a protein crosslinking assay. Rats were taken directly from home cages and killed. A 2mm coronal section containing the NAc was obtained using a brain matrix (ASI Instruments, Inc., Warren, MI). A tissue punch was used to obtain the core (final diameter ~1.75mm) and then a scalpel was used to cut a crescent-shaped piece of tissue (shell) medial to the core punch (see Fig. 3B for a schematic of the dissection). Tissue samples were quickly minced with a scalpel and then incubated for 30 min at 4 ° C with 2mM bis(sulfosuccinimidyl)suberate (BS3; Pierce Biotechnology, Rockford, IL). BS3 is a bifunctional crosslinking reagent that does not cross membranes. Therefore, it selectively modifies surface-expressed proteins, increasing their apparent molecular weight and enabling them to be separated from unmodified intracellular proteins by SDS-PAGE and quantified by immunoblotting (see Ferrario et al., 2010 (link) for detailed procedures). Blots were processed exactly as described previously using primary antibodies to GluA1 (PA1-37776; Affinity Bioreagents, Rockford, IL), GluA2 (AB1768; Millipore, Billerica, MA) or GluA3 (3437; Cell Signaling Technology, Danvers, MA) and chemiluminescence detection. Values (diffuse densities) for surface, intracellular and total (surface + intracellular) protein levels were normalized to a loading control (GADPH; CB1001; Calbiochem, La Jolla, CA). For each of these measures (surface, intracellular and total protein) in each experiment, values from the cocaine group are expressed as percent of the mean value for a saline group run in parallel and killed on the same day.