Fast axonal transport of membrane protein and intra-axonal diffusion of free leucine in a neuron of Aplysia.

We injected radioactive leucine into a neuron soma of Aplysia, and found that the leucine and synthesized membrane proteins moved in the axon, but soluble proteins did not. The movement of membrane proteins showed typical characteristics of fast axonal transport, whereas that of leucine could be explained totally due to intra-axonal diffusion by the following observations: the migration profiles of the leucine closely coincided with the theoretical diffusion; the migration continued even when the fast axonal transport of membrane proteins was stopped by local cooling of the axon; and a damming phenomenon, an indication of axonal transport, seen in membrane proteins was not observed when the axon was occluded or when the axonal transport was blocked by the local cooling.