One thing you should do is to permeabilize well, by including 0.1% to 0.2% Triton X-100 in all washes and incubations. Another recommendation is to extend the labeling times to overnight at 4oC for both primary antibody and secondary antibody. Finally, label the section free-floating, rather than adhered to a slide, to gain penetration for the antibody from both sides.
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