Influence of temperature on slow flow in populations of regenerating axons with different elongation velocities

The slow intra-axonal transport mechanisms may be subdivided into 4 elementary processes: (a) protein synthesis; (b) release of the slow material from the cell body; (c) transport of the molecules along the axon; and (d) deposition of the material from the moving phase into the immobile axonal constituents. These 4 components of the slow flow mechanisms are differently affected by temperature and react differently to axonal regeneration. They have been studied at temperatures ranging from 10 to 35 degrees C, in 3 different populations of regenerating C-fibers of the garfish olfactory nerve. These 3 classes of axons have very different elongation properties as determined by elongation rates ranging from a minimum of 0.3 mm/day at 14 degrees C to a maximum of 12.4 mm/day at 35 degrees C. From this study several properties of the slow intra-axonal transport mechanism can be underlined: (a) in regenerating nerves the rate of slow flow is multiplied by a factor of 3.3 as compared to the rate characteristic of an intact nerve. A similar increase was measured in every regenerating phase regardless of the specific elongation velocity of the growing axons. This accelerated rate of slow flow has been measured previously in degenerating axons detached from their perikarya: therefore the rate increase appears to be triggered by the injury rather than the needs of the neuron; (b) protein synthesis represents one of the main steps controlling axonal elongation. Neurons with the most active level of protein synthesis grow with the highest elongation velocity; (c) the somal releasing times decrease exponentially with temperature but are identical to the values measured in intact neurons; (d) finally the height of the slow flow peak decreases exponentially as a function of the distance, as seen previously in intact nerves, but a significantly larger amount of radioactivity reaches the end of the nerve. It can be concluded that slow flow is intimately linked with the regeneration process. The rate of slow flow and more specifically of Scb represents the upper limit that can be reached by the elongation velocities of regenerating fibers when all the growth conditions are maximal. When the regenerating conditions are not at their peak, the axonal elongation proceeds at velocities which can be very significantly slower than the rate of slow flow and might be determined by the level of protein synthesis in the perikarya. Axonal elongation, however, can never proceed at a rate faster than slow flow.