An in vivo assay of neurotropic activity.

Previous studies in this laboratory indicate that diffusible factors from distal stumps of transected peripheral nerves exert a neurotropic effect on regenerating nerve fibers in vivo. The present study was performed (a) to strengthen this hypothesis, and (b) to begin to determine the distribution and identity of putative neurotropic factors in peripheral nerves. Rat sciatic nerves were transected and inserted into the inlet end of a hollow 6 mm-long Y-shaped implant made of medical-grade Silastic tubing. To one of the outlet ends was attached an Elvax pellet impregnated with whole homogenate from a single sciatic nerve. The other outlet was attached to a pellet not containing homogenate. Homogenates assayed were obtained from distal stumps of transected nerves derived 14 days postoperatively or from unoperated sciatic nerves. After 3.5 weeks, nerve fiber bundles consisting of myelinated regenerating axons were only present in implant forks attached to pellets containing denervated nerve homogenate. This activity was heat- and trypsin-sensitive. These data (a) strengthen the hypothesis that diffusible factors from distal nerve stumps of transected nerves can support nerve fiber regeneration, (b) indicate that those factors are protein and/or protein dependent, and (c) suggest that unoperated peripheral nerve (at the levels assayed) either do not contain such factors or contain substances which inhibit such factors.