Transplantation of rat synapsin-EGFP-labeled embryonic neurons into the intact and ischemic CA1 hippocampal region: distribution, phenotype, and axodendritic sprouting.

A major limitation of neural transplantation studies is assessing the degree of host-graft interaction. In the present study, rat hippocampal/cortical embryonic neurons (E18) were infected with a lentivirus encoding enhanced green fluorescent protein (GFP) under control of the neuron-specific synapsin promoter, thus permitting robust identification of labeled neurons after in vivo grafting. Two weeks after transient forebrain ischemia or sham-surgery, GFP-expressing neurons were transplanted into CA1 hippocampal regions in immunosuppressed adult Wistar rats. The survival, distribution, phenotype, and axonal projections of GFP-immunoreactive (IR) positive transplanted neurons were evaluated in the sham-operated and ischemia- damaged CA1 hippocampal regions 4, 8, and 12 weeks after transplantation. In both experimental groups, we observed that the main phenotype of host-derived afferents projecting towards grafted GFP-IR neurons as well as transplant-derived GFP-IR efferents were glutamatergic in both animal groups. GFP axonal projections were seen in the nucleus accumbens, septal nuclei, and subiculum-known target areas of CA1 pyramidal neurons. Compared to sham-operated animals, GFP-IR neurons grafted into the ischemia-damaged CA1 migrated more extensively throughout a larger volume of host tissue, particularly in the stratum radiatum. Moreover, enhanced axonal sprouting and neuronal plasticity of grafted cells were evident in the hippocampus, subiculum, septal nuclei, and nucleus accumbens of the ischemia-damaged rats. Our study suggests that the adult rat brain retains some capacity to direct newly sprouting axons of transplanted embryonic neurons to the correct targets and that this capacity is enhanced in previously ischemia-injured forebrain.