Spatial orientation of horizontal cell axon terminals in the carp retina

In flatmounts of the carp (Cyprinus carpio) retina, 646 horizontal cells were singly marked by intracellular Lucifer yellow CH (LY) in the presence of dopamine or amphetamine, agents which were useful for restricting LY to single injected cells. Most axon terminals of cone-connected horizontal cells have a tendency to orient either radially or tangentially in the retinal field with respect to the optic disc as a center. Although the fluorescent cellular (dendritic field) area greatly varied depending upon the cell type (L-, RG- and YRB-type), the lengths of the axonal processes (axon plus terminal) were all comparable (400-600 microns). A few cells (4.1% of cells with visible axons) possessed a bifurcate axon with two axon terminals. Axons were not observed on rod-connected horizontal cells. The cellular area and the axonal length of L- and RG-type cells appeared to be smaller in the central than in the intermediate region of the retinal field.