Connective integration of hippocampal grafts in excitotoxic hippocampal lesions in adult rats: An anterograde axonal tracing study

Exchange of nerve connections between developing neural grafts and adult recipient brains is enhanced for grafts placed in excitotoxic lesions, which spare recipient brain afferent axons in otherwise neuron-depleted lesion areas. In previous studies of hippocampal grafts placed in such lesions, we have used anterograde axonal degeneration, histochemical Timm staining and acetylcholinesterase to demonstrate host-graft interconnectivity. In this study, we have now used three anterograde axonal tracers, Phaseoulus vulgaris-leukoaglutinin (PHA-L), biocytin and biotinylated dextran amine (BDA), which allow individual fibers to be traced. Adult male rats with 1-week-old axon-sparing ibotenic acid lesions of the dorsal CA3 region or fascia dentata were grafted into the respective lesions with suspensions of fetal (E18-19) CA3 cells or a block of neonatal fascia dentata tissue. One to twelve months later, recipients were injected with Phaseoulus vulgaris-leukoaglutinin or biocytin in the hippocampus contralateral to the graft to trace the possible ingrowth and distribution within the transplants of host commissural axons, or into the transplants with biotinylated dextran amine in order to trace outgrowing graft fibers. In rats with succesfull host Phaseoulus vulgaris-leukoaglutinin or biocytin injections, the CA3 and fascia dentata transplants were innervated by labelled host commissural fibers. In the dentate transplants, most commissural fibers projected as normally to the inner part of the molecular layer, with fewer aberrant fibers extending more superficially into the molecular layer. Following injections into the fascia dentata and CA3 grafts of biotinylated dextran amine, labelled graft fibers were traced into the ipsilateral host dentate hilus, CA3 and CAI. From some CA3 containing grafts, a few labelled fibers were also observed passing through the host fimbria-fornix to the lateral septum on the grafted side. A few fibers were projected as far as to the most septal levels of the contralateral CA1.