Evidence that axoplasmic transport of trophic factors is involved in the regulation of peripheral nerve fields in salamanders

Abstract

1. We have compared the effects of partial denervation with those of colchicine-induced block of axoplasmic flow, on the peripheral fields of nerves innervating the hind limb of salamanders.2. Acute application of colchicine solution (0.03-0.10 M) to spinal nerve 16 results in a dose-dependent increase in skin and muscle fields of the adjacent nerves (15 and 17). The time course, magnitude and distribution of the response to 0.10 M colchicine is not distinguishable from that of the compensatory sprouting of nerves 15 and 17 that occurs after section of nerve 16.3. In contrast to the situation with nerve section, sprouting of adjacent nerves occurred after colchicine applications which produced no behavioural deficit, no change in the peripheral field of the treated nerve, and no interference with impulse conduction in it; nor was there subsequent degeneration in the nerve.4. The same concentrations of colchicine reduced the axoplasmic flow of catecholamines and cholinesterase; treated nerves contained fewer microtubules than untreated controls.5. A similar application of colchicine solution to nerve 15 prevented it from sprouting in response to the stimulus provided by section of nerve 16.6. We conclude that nerve terminals are continuously supplied by axoplasmic flow with a trophic factor concerned with the regulation of nerve fields. When the supply of this factor is reduced, adjacent nerves sprout and invade the territory of the treated nerve. In addition, the ability of nerves to sprout is itself dependent upon the maintenance of axoplasmic flow.