One hour after re-exposure of mice to Context A (Context A -A reactivation or Context B (Context A-B reactivation), mice were intracardially perfused with ice-cold 4% paraformaldehyde in phosphate buffer (pH 7.4, 50 mL per mouse), brains were collected, and immunohistochemistry for GFP, Npas4, and cFos was performed on 50 μm free-floating sections (26 (link)) with primary antibodies against cFos (1:2,000, guinea pig, 226 308, Synaptic Systems), Npas4 (1:10,000, rabbit, Yingxi Lin (27 (link))), or GFP (1:4000, chicken, Abcam ab13970). Brains of all mice were similarly collected and processed for analyzes of virus spread using anti-mCherry antibodies (1:1,000, rabbit, Abcam ab167453 or 1:16,000, chicken, ab205402). Secondary antibodies were obtained from Vector (1:300, biotinylated-anti chicken, anti-mouse, or anti-rabbit IgG). Sections were mounted using Vectashield (Vector) and observed with a confocal laser-scanning microscope (Olympus Fluoview FV10i) (8 , 28 (link)). For light microscopy, signals were visualized with diaminobenzidine (Sigma) (28 (link)).