The expression of growth-associated protein GAP-43 mRNA in spinal cord and dorsal root ganglion (DRG) neurons has been studied using an enzyme linked in situ hybridization technique in neonatal and adult rats. High levels of GAP-43 mRNA are present at birth in the majority of spinal cord neurons and in all dorsal root ganglion cells. This persists until postnatal day 7 and then declines progressively to near adult levels (with low levels of mRNA in spinal cord motor neurons and 2000 - 3000 DRG cells expressing high levels) at postnatal day 21. A re-expression of GAP-43 mRNA in adult rats is apparent, both in sciatic motor neurons and the majority of L4 and L5 dorsal root ganglion cells, 1 day after sciatic nerve section. High levels of the GAP-43 mRNA in the axotomized spinal motor neurons persist for at least 2 weeks but decline 5 weeks after sciatic nerve section, with the mRNA virtually undetectable after 10 weeks. The initial changes after sciatic nerve crush are similar, but by 5 weeks GAP-43 mRNA in the sciatic motor neurons has declined to control levels. In DRG cells, after both sciatic nerve section or crush, GAP-43 mRNA re-expression persists much longer than in motor neurons. There was no re-expression of GAP-43 mRNA in the dorsal horn of the spinal cord after peripheral nerve lesions. Our study demonstrates a similar developmental regulation in spinal cord and DRG neurons of GAP-43 mRNA. We show moreover that failure of re-innervation does not result in a maintenance of GAP-43 mRNA in axotomized motor neurons.

The relative contribution of intrinsic growth capacity versus extrinsic growth-promoting factors in determining the capacity of transected dorsal root axons to regenerate long distances was studied. L4 dorsal root axons regenerating into 4-cm peripheral nerve grafts on transected dorsal roots were counted. Few dorsal root myelinated axons regenerated to the distal end of the grafts by 10 weeks unless the sciatic nerve was also crushed. Regeneration of unmyelinated axons was also increased by peripheral lesions. Crush or transection of the dorsal roots without grafting did not alter GAP-43 mRNA expression in L4 dorsal root ganglion (DRG) cells. Grafting a peripheral nerve onto the cut end of an L4 dorsal root doubled the number of DRG cells expressing high levels of GAP-43 mRNA after a delay of several weeks. Peripheral nerve crush at the time of nerve grafting resulted in a very rapid rise in GAP-43 mRNA expression, which then declined to a steady level, twice that of controls, by 7 weeks. Thus, the rapid increase in the number of DRG neurons expressing high levels of GAP-43 mRNA after peripheral but not central axotomy correlates with the regeneration of central axons through nerve grafts. Because GAP-43 mRNA is slowly upregulated in a subpopulation of sensory neurons in response to exposure of their central axons to a peripheral nerve environment, environments favourable for axonal growth may act by increasing the intrinsic growth response of neurons. Lack of intrinsic growth capacity may contribute to the failure of dorsal root axons to regenerate into the spinal cord.

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Injury to the central processes of primary sensory neurons produces less profound changes in the expression of growth-related molecules and less vigorous axonal regeneration than does injury to their peripheral processes. The left L4, L5, and L6 dorsal roots of deeply anaesthetized adult Sprague-Dawley rats were severed and reanastomosed, and in some animals, the ipsilateral sciatic nerve was crushed to increase the expression of growth-related molecules. After between 28 days and three months, the sciatic nerve of most animals was injected with transganglionic tracers and the animals were killed 2-3 days later. Other animals were perfused for electron microscopy. Very few regenerating axons entered the spinal cord of the rats without sciatic nerve injuries. Labelled axons, however, were always found in the spinal cord of rats with sciatic nerve injuries. They often entered the cord around blood vessels, ran rostrally within the superficial dorsal horn, and avoided the degenerating white matter. The animals with a conditioning sciatic nerve crush had many more myelinated axons around the dorsal root entry zone (DREZ) and on the surface of the cord. Thus, a conditioning lesion of their peripheral processes increased the ability of the central processes of myelinated A fibres to regenerate, including to sites (such as lamina II) they do not normally occupy. Astrocytes, oligodendrocytes, and meningeal fibroblasts in and around the DREZ may have inhibited regeneration in that region, but growth of the axons into the deep grey matter and degenerated dorsal column was also blocked.

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