Differential expression of growth-associated protein (GAP-43) mRNA in rat primary sensory neurons after peripheral nerve lesion: a non-radioactive in situ hybridisation study.

An alkaline phosphatase-labelled anti-sense oligodeoxynucleotide probe specific for growth-associated protein messenger RNA (GAP-43 mRNA) was used for non-radioactive in situ hybridisation histochemistry to follow relative changes in GAP-43 mRNA content in lumbar primary sensory neurons (L4-6) after unilateral ligation of the sciatic nerve. In normal dorsal root ganglia (DRG) 16% of neurons expressed GAP-43 mRNA, and these cells belonged to a sub-group of intermediate-sized (32-50 microns diameter) and large (> 50 microns) neurons. The hybridisation signal detected in these cells was weak to moderate. One day after nerve ligature a significant increase in the number of GAP-43 mRNA expressing neurons in the ipsilateral DRG was detected involving particularly the very small (12-20 microns) cells, and small cell population (20-32 microns), though the hybridisation signal was less pronounced in this latter cell group. A significant increase in the cellular content of GAP-43 mRNA was detected in both cell groups when compared to the normal DRG by 2 days after the lesion. At later times (4, 7, and 10 days postinjury) the intermediate-sized and large cell subpopulations also showed an increase in the number of GAP-43 mRNA positive neurons, followed by a significant rise in their content of GAP-43 mRNA. However, they did not reach the same intensity of hybridisation signal as seen in the small and very small neurons. All DRG neurons showed a maximum of GAP-43 mRNA expression by 10 days postsurgery. At longer times there was a slight decrease in the content of GAP-43 mRNA towards 14 days postinjury, but mRNA levels remained elevated up to 28 days after nerve ligature, the longest time point examined in this study. The different onset and levels of GAP-43 gene expression in the rat primary sensory neurons after lesion of their peripheral branch axons further characterize the different subclasses of these cells and may reflect their different involvement in the plastic changes following peripheral nerve injury.