Poly-L-ornithine enhances the uptake of horseradish peroxidase

Poly-L-ornithine (ORN) in concentrations of 0.1-0.5% when added to 10-25% concentrations of horseradish peroxidase (HRP) improves significantly the ease of detection of labeled cells with the light microscope. Increased uptake of the HRP marker is demonstrated first by the 3-4-fold elevation in the number of retrogradely labeled cells, and second, by the fact that labeled somata are readily detected as early as 5 h and as late as 75-100 h postinjection with ORN-HRP. ORN-HRP similarly increases the number of presumed axon terminals labeled by anterograde transport. The enhanced uptake of HRP induced by ORN facilitates light microscopic analysis of HRP labeled cell bodies and terminals in 4 ways: (1) revealing connections refractory to HRP alone, (2) potentially allowing cotemporal visualization of both short and long fiber paths, (3) permitting reduction in injection volume for more refined studies and (4) increasing the reliability of labeling.