Development of neural connections between visual cortex and transplanted lateral geniculate nucleus in rats

The development of neural connections between transplanted lateral geniculate nucleus (LGN) and host visual cortex (VC) was studied in slice preparations obtained from rat brain in which a fetal (embryonic day 15-17) rat LGN was transplanted to the white matter underlying the VC of a neonate rat (postnatal day 0-1). Placing a fluorescent dye (DiI) in the transplant of the fixed slices revealed that retrogradely labeled cortical cells projecting to the transplant were broadly distributed through layers II to VI at 1 week after transplantation. Three weeks after transplantation, these cells were virtually confined to layer VI. Likewise, anterograde labeling showed that cells in the transplant sent axons up to layer I with a few branches at 1 week after transplantation, while the axons were found to terminate at layer IV with many arborizations at 3 weeks after transplantation. These observations were supported by electrophysiological studies. Analysis of the antidromic responses of the cortical cells to stimulation of the transplant showed that the efferent cells projecting to the transplant were broadly distributed in layers II-VI at 1 week after transplantation, while they were virtually restricted to layer VI at 3 weeks after transplantation. Current source-density analysis of the field potentials and intracellular analysis of the synaptic potentials in the cortical cells demonstrated that geniculocortical connections were broadly established in layers II-VI at 1 week after transplantation, and were localized to layer IV and VI at 3 weeks after transplantation. These results suggest that the development of neural connections between transplanted LGN and host VC is characterized by an initial broad distribution of afferent and efferent connections without laminar specificity, and by later selection of appropriate connections to yield lamina-specific connections comparable to those in normal adult VC.