Axonal transport of vesicles carrying (3H)serotonin in the metacerebral neuron of Aplysia californica.

The axon of each unipolar metacerebral neuron bifurcates into two large branches within the cerebral ganglion quite close to the cell body, and each branch exists from the ganglion in a separate nerve, either the lip nerve or the cerebrobuccal connective. Because the serotonergic vesicle appears to be more stable to isolation than the cholinergic vesicle and because serotonin, like other transmitter molecules with primary amino groups, can be fixed into tissue with aldehyde fixatives, we were able to show that serotonergic vesicles, originating from the cell body, are transported in both branches of the axon by fast axonal transport. We also described some experiments suggesting that the membranes of the serotonergic vesicles being transported were newly synthesized in the cell body and contained vesicle-specific glycoproteins. This is important because it permits us to estimate the degree of the neurons' biosynthetic activity involved in the formation of these vesicles. Finally, we took advantage of the axon geometry of the metacerebral cell to perturb the dynamics of the transport system. Cutting one branch abruptly increases the number of vesicles traveling along the uncut axon. Using this manipulation, we found that when twice the amount was being transported in the uncut nerve, [3H]serotonin was distributed more distally, suggesting that on the average, vesicles were displaced twice as far per unit time. We therefore proposed a mechanism for axonal transport which involves the binding of vesicles to stable tracks as a rate-limiting step. When attached, vesicles move at a constant, fast rate.