Reduction of background autofluorescence in brain sections following immersion in sodium borohydride

Autofluorescence of aldehyde-fixed neural tissue often obscures perikaria and fine processes labeled with fluorescent anterograde or retrograde tracers. In particular, this autofluorescence hinders the detectability of fine axonal projections labeled with the convenient anterograde tracer, tetramethylrhodamine dextranamine. Background fluorescence was notably reduced by immersion of free-floating brain tissue sections in a solution of sodium borohydride (NaBH4, 0.1%), a chemical which is known to neutralize Schiffs bases through reduction of amine-aldehyde compounds into non-fluorescent salts. The reversible and renewable immersion technique was most effective in paraformaldehyde-fixed tissue where the preservation quality was improved such that labeled axons remained detectable for more than 1 year after initial preparation.