Absence of 'superfast' axonal transport in rat sciatic nerve

Axonal transport of labelled protein was studied in rat sciatic nerve by analyzing nerve segments at intervals after injection of L-[3H]leucine into the lumbar spinal cord. Some nerves were sectioned before injection so that material in transit accumulated proximal to the section. The segments distal to the section served as controls for incorporation into the nerve of blood-borne label. An analysis of TCA-soluble and TCA-insoluble activity in cut and intact nerve segments was also made. No evidence was found for the existence of a 'superfast' component of axonal transport (velocity 2000 mm/day). Results showed that the most rapidly transported protein derived from the neuron soma had a conventional 'fast' velocity of 350-420 mm/day. There was no transport of TCA-soluble material. It is suggested that 'superfast' transport, detected in mice by other investigators, is an artefact resulting from failure to control for incorporation of circulating label into the sciatic nerve.