Analysis of the mechanism of fast axonal transport by intracellular injection of potentially inhibitory macromolecules: evidence for a possible role of actin filaments.

Although actin is thought to participate in several types of cell motility other than muscle contraction, no direct evidence has linked it to the force-generating mechanism for fast axonal transport. We have obtained evidence for the involvement of actin by microinjecting, into the serotonergic giant cerebral neuron of Aplysia, two preparations that have been shown to depolymerize actin filaments. One is a fraction of rabbit serum containing a heat-labile gamma globulin that affects actin polymerization in a manner similar to that of cytochalasin and several proteins that are thought to regulate the length of actin filaments. The other is bovine pancreatic DNase I which binds to actin stoichiometrically. Both preparations substantially decreased the transport of storage vesicles containing [3H]serotonin. Phalloidin, a toxic fungal peptide that binds to actin filaments but stabilizes rather than depolymerizes them, did not inhibit transport. We have not yet determined whether the inhibition od transport occurs during export of [3H]serotonin from the cell body into the axon or during translocation along the axon. Nevertheless, these observations provide a promising experimental indication that actin is involved in fast axonal transport.