Uptake of horseradish peroxidase by axons of passage and its modification by poly-L-ornithine and dimethylsulphoxide.

It has been shown that transected axons of passage in the optic tract can take up horseradish peroxidase (HRP) and transport it back to the cell bodies. This effect is considerably reduced if the HRP is injected from a small-tipped micropipette 6-12 h after insertion into the injection site. HRP is not taken up by uninjured axons of passage. The use of poly-L-ornithine and dimethylsulphoxide dramatically increases the numbers of labeled cells. These numbers are undiminished after 6-12 h waiting periods. From this and from calculation it is clear that much of this effect must occur via uninjured axons. These substances should not be used in any situation where it is important not to label axons of passage. If other HRP conjugates and additives have the same effect as those described here, the results of experiments with these substances should be interpreted with caution.