Evidence for excitatory amino acid neurotransmitters in the geniculo-cortical pathway and local projections within rat primary visual cortex.

To examine the organization of axon collaterals of neurons that selectively take up and transport excitatory amino acids, we have used retrograde tracing with D-[3H]Aspartate after injections into different layers of rat primary visual cortex. The results show cells in the lateral geniculate nucleus retrogradely labeled from the cortex. Additional topographically precise input to the thalamic recipient layer 4 originates from neurons in the visual cortex lying in layers 2/3, 5 and 6. These inputs are reciprocated by point-to-point projections from layer 4. Layer 2/3 cells project to layers 5 and 6 in columnar fashion. Putative excitatory input to layer 2/3 originates from a vertical column of cells in layer 5 and the middle of layer 6. In addition layer 2/3 receives input via horizontal collaterals of topographically distant upper layer neurons, from more widespread projections in lower layer 6, and from very widespread projections of cells at the layer 5/6 border. Cells in the depth of layer 5 also distribute collaterals within layers 5 and 6. Our findings provide anatomical evidence that the geniculo-cortical pathway in the mammalian visual system may use excitatory amino acid transmitters. In addition, the results support the notion that most long range connections that link distant points of the topographic map are excitatory.