EVIDENCE FOR GAP-43 WITHIN DESCENDING SPINAL AXONS IN THE NORTH-AMERICAN OPOSSUM, DIDELPHIS-VIRGINIANA

We have shown previously that GAP-43, a growth associated protein char acteristically present in growing and regenerating axons, is relativel y abundant in the spinal cord of adult opossums. In the present study, we combined the orthograde transport of the fluorescent marker Fluoro -Ruby with immunofluorescence for GAP-43 to determine if any of it is present within descending spinal axons. When Fluoro-Ruby was injected into the red nucleus and midbrain tegmentum, the medial pontine or med ullary reticular formation, the medullary raphe or the lateral vestibu lar nucleus, axons were labeled in the expected areas of the spinal co rd, but in most cases none showed evidence for GAP-33. In two of the f our cases with rubral injections, however, a few labeled axons within the rubrospinal tract showed GAP-43 immunofluorescence, and in one cas e with an injection of the gigantocellular reticular nucleus and adjac ent raphe, labeled axons within lamina IX immunostained for the protei n. Since serotoninergic neurons are present within the gigantocellular reticular nucleus and adjacent raphe, and axons of the same phenotype are abundant within lamina IX, we asked whether serotoninergic axons contain GAP-43, When sections of the spinal cord were immunostained fo r both serotonin and GAP-43, many axons within lamina IX showed eviden ce for both substances. Such axons appeared to contact presumptive mot oneurons. In cases with Fluoro-Ruby injections of the forelimb motor c ortex, labeled axons were present within the pyramidal tract, and some of them showed evidence for GAP-43.