A study of the dynamics of retrograde transport and accumulation of horseradish peroxidase in injured neurons.

The phenomenon of retrograde intraaxonal transport of extracellular markers introduced at the level of the axon terminal has been suggested as a possible mechanism of communication between the axon terminal and the neuron cell body. We tested the hypothesis that communication after axotomy might consist of a change in the rate of uptake or of transport of material by injured neurons. Small lesions were made with a needle in one retinal quadrant of chicks and immediately afterwards horseradish peroxidase (HRP) was injected into the vitreous body of the eye. The amount of HRP accumulated by some of the neurons of the isthmo-optic nucleus (ION) which project to the damaged area was clearly different from that of nearby cells which project to the non-damaged portions of the retina. The uninjured cells accumulated enzyme marker beginning at 3.5 h after injection. The injured neurons did not accumulate significant amounts of HRP until between 4 and 6 h after injection. Between 6.75 h and 18 h the injured cells in the ION accumulated greater amounts of HRP than cells in other regions, but by 24 h the cells of the ION in the region of injury contained distinctly less label. This pattern of enzyme accumulation was confirmed by counts of the number of HRP-positive granules within cells of chicks fixed 4, 11.75, 12.25, 27.6 and 72 h after injury. In another series of experiments, the axon terminals of the ION were first exposed to HRP, and 1 h later some of the axons were damaged with a needle. In these cases, there was no difference between the injured and control neurons in the time of first appearance of labeled cells in the ION within the first 4 h after injection of HRP. These findings suggest that injury initially results in a decrease in the uptake of the marker rather than a decrease in the rate of retrograde transport. The amount of marker found in the injured neurons later is greater than that found in the control neurons. This subsequent difference may represent an increase in the rate of uptake, transport, or both or a decrease in the rate of degradation of HRP within the cell body as a response to injury of the axon.