Observations on labeling of neuronal cell bodies, axons, and terminals after injection of horseradish peroxidase into rat brain

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Abstract
We observed with light microscopy the evolution of neuronal labeling phenomena in rats with two hours to eight days survival following injection of horseradish peroxidase (HRP) into the neostriatum. We compared the results of neostriatal injection with those injection of the overlying neocortex. The following conclusions were reached.The zone of diffuse background HRP around the injection site began to shrink after 18 hours. The extent of the early zone of diffusion was marked from 18 hours to two days by a universal granular labeling of neurons. This granular cell labeling appears to be the result of earlier diffuse cell labeling rather than retrograde axonal transport.Retrograde labeling of cells takes place from terminals and from damaged axons. Anterograde labeling of what appear to be terminals and fine preterminal axons takes place only from cells in the injection area, and not from either intact or damaged axons of passage. Usually the axons engaged in such retrograde or anterograde transport are not visibly labeled. Labeling of cells and terminals is generally optimal at 12 to 24 hours, but corticostriatal cells in layer V of the neocortex are seen best at 24 to 36 hours. The area from which retrograde labeling of cells takes place is larger than that from which anterograde labeling of terminals occurs.Massive intra‐axonal movement of HRP away from the injection site occurs within a few hours in axons of passage as well as those originating there. This axonal labeling fades in most axons by 24 hours, but certain axons remain darkly labeled. The latter are probably damaged, since labeled axons which appear to be degenerating are later (3–5 days) seen in the same place, and Fink‐Heimer staining of adjacent sections also shows degenerating axons in these regions. The early massive labeling of axons can lead to short‐lived labeling of complete axonal arborizations within the range of this intra‐axonal movement of HRP. But it seems to have no relation to the longer‐lived labeling of distant cells and terminals, which presumably results from active axonal transport. This suggests that HRP entering axons of passage is not captured by the axonal transport mechanisms.