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 adu lt recipient brains is enhanced for grafts placed in excitotoxic lesio ns, which spare recipient brain afferent axons in otherwise neuron-dep leted lesion areas. In previous studies of hippocampal grafts placed i n such lesions, we have used anterograde axonal degeneration, histoche mical 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 a nd 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 t he 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 b iocytin in the hippocampus contralateral to the graft to trace the pos sible ingrowth and distribution within the transplants of host commiss ural 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 commissur al fibers. In the dentate transplants, most commissural fibers project ed as normally to the inner part of the molecular layer, with fewer ab errant fibers extending more superficially into the molecular layer. F ollowing injections into the fascia dentata and CA3 grafts of biotinyl ated dextran amine, labelled graft fibers were traced into the ipsilat eral host dentate hilus, CA3 and CAI. From some CA3 containing grafts, a few labelled fibers were also observed passing through the host fim bria-fornix to the lateral septum on the grafted side. A few fibers we re projected as far as to the most septal levels of the contralateral CA1.