Uptake and retrograde axonal transport of horseradish peroxidase in normal and axotomized motor neurons during postnatal development.

The axonal uptake and somatopetal transport of horseradish peroxidase (HRP) was studied during early postnatal development of facial neurons in mice and rats. HRP injected systemically or locally into the muscles of the vibrissae, diffused into the region of the immature neuromuscular junction and was incorporated into vesicles in the axon terminals on the first and third postnatal days, at a time when synaptic vesicles were already present. HRP later was found in the nerve cell bodies of the facial nucleus in the brain stem indicating a somatopetal transport of the tracer in axons. The response of facial neurons to nerve transection changed from rapid neuronal death to prolonged survival between the 6th and 10th postnatal day. HRP was transferred to nerve cell bodies after topical application to the proximal stump of transected facial nerves in rats 3 days-of-age. In the perikaryon it was localized to vesicles and vacuoles with no signs of leakage into the cytoplasm. In the light of our findings different hypotheses for the mechanism of the neuronal death in the immature animals are discussed.