THE INCREASE IN B-50 GAP-43 IN REGENERATING RAT SCIATIC-NERVE OCCURS PREDOMINANTLY IN UNMYELINATED AXON SHAFTS - A QUANTITATIVE ULTRASTRUCTURAL-STUDY/

The growth-associated protein B-50/GAP-43 is thought to play a crucial role in axonal growth. We investigated, by quantitative immunoelectro n microscopy, whether there are differences in the subcellular distrib ution of B-50 in unmyelinated and myelinated axons of intact and regen erating sciatic nerves. Adult rats received an unilateral sciatic nerv e crush and were euthanized 8 days later. Nerve pieces proximal from t he crush site were embedded, and B-50 was visualized by specific B-50 antibodies and immunogold detection in ultrathin sections. The density of B-50 at the plasma membrane of unmyelinated axon shafts was signif icantly increased in the ipsilateral regenerating nerve in comparison to that of the contralateral intact nerve. In contrast, there was no s ignificant difference in the B-50 density at the axolemma of myelinate d regenerating and intact axon shafts. In the contralateral intact ner ve, more B-50 was associated with the axolemma of unmyelinated axons t han with the plasma membrane of myelinated axons. The density of axopl asmic B-50 was similar in intact unmyelinated and myelinated axon shaf ts, but was higher in regenerating nerve than in intact nerve. This su ggests that enhanced axonal transport of B-50 occurs during axon outgr owth. Our study demonstrates a differential subcellular distribution o f B-50 in unmyelinated and myelinated axon shafts in both the intact a nd regenerating sciatic nerve, indicating a differential inducible cap acity for remodeling of the axon shafts. (C) 1995 Wiley-Liss, Inc.