Neurofilament and tubulin transport slows along the course of mature motor axons.

The rate of axonal transport of tubulin and of neurofilament proteins was measured in mature, non-growing, axons of rat lumbar ventral roots and sciatic nerves. A progressive decrease in the velocity of transport was observed along the course of the axons. The rate of advance of the 'front' of labeled neurofilaments decreased from 1.40 +/- 0.07 mm/day proximally to 0.64 +/- 0.04 mm/day distally. The rate for tubulin decreased from 3.10 +/- 0.13 proximally to 1.01 +/- 0.04 mm/day distally. These results demonstrate that a gradient of the velocity of cytoskeletal transport along axons is not a specific adaptation for radial growth of axons, as previously suggested. Local degradation of excess cytoskeletal proteins may permit axons to accommodate a net slowing of cytoskeletal transport without accumulation of neurofilaments or microtubules. Alternatively, the transport kinetics of radiolabeled proteins may be influenced by mixing of labeled and unlabeled cytoskeletal proteins within the axoplasm; in this case, a slowing of the movement of the labeled proteins may not reflect net slowing of transport of the total pool of cytoskeletal proteins.