A quantitative study of anterograde and retrograde axonal transport of exogenous proteins in olfactory nerve C-fibers.

ABSTRACT

The pike olfactory nerve which consists of a homogeneous population of C-fibers of 0.25 micron diameter or less was used to study quantitatively both anterograde and retrograde axoplasmic transport of wheat germ agglutinin and horseradish peroxidase. It was found that even in these extremely thin axons anterograde and retrograde transport takes place. Activity distribution profiles (transport profiles) for retrograde transport were established and found to be similar to the typical profiles of anterograde transport as they consisted of a small rapidly moving peak and a saddle region followed by the bulk of the material which moved more slowly. Horseradish peroxidase activity profiles were obtained both after injection into the synaptic region and after injection into the perikaryal region. From these transport profiles a maximal velocity of 25 mm/d (19 degrees C) for the leading peak and of about 7 mm/d for the slower component could be determined. There is no significant difference between the velocities for anterograde and retrograde transport. In the case of wheat germ agglutinin, only injection into the synaptic region resulted in typical transport profiles (retrograde transport) with a peak and saddle region. The maximum velocities of retrograde transport were about the same as for horseradish peroxidase [26 mm/d and 7 mm/d (19 degrees C)]. The electron microscopic analysis of horseradish peroxidase revealed that after injection into the olfactory bulb it was taken up into the neurons where it was found mainly in multivesicular bodies (0.5 micron diameter). In longitudinal sections of the nerve similar but slightly more elongated organelles (diameter 0.25 micron, length 0.4 micron) were found in those segments in which the slowly moving bulk of the peroxidase activity was located. The number of these organelles decreased with distance from the site of injection. The horseradish peroxidase transported within the leading peak could not be assigned to specific structures although several electron microscopic-histochemical methods were applied. It was concluded that anterograde and retrograde transport occur simultaneously in these axons, and that, therefore, even the large organelles, each of which almost fills the axon, must be able to pass each other. This would necessitate that the axons are able to transiently enlarge their diameter considerably.