AMPA Receptors on Horizontal Cells in the Rabbit Retina

Abstract: Purpose: Glutamate is the principal excitatory neurotransmitter in the retina. A– type horizontal cells and the somatic dendrites of B–type horizontal cells receive glutamate input from cones. In contrast, the B–type axon terminals receive glutamate input from rods. The post–synaptic responses of second order neurons are mediated by different glutamate receptors. We wanted to determine which glutamate receptors are expressed at the axon terminals of B–type horizontal cells.

Methods: B–type horizontal cells were intracellular injected with Lucifer Yellow and Neurobiotin in DAPI–incubated isolated rabbit retina. Tissues were then immunolabeled with antibodies against AMPA receptors (GluR2 and GluR4) and mGluR6. The relationship of glutamate receptors to the horizontal cell dendritic tips was analyzed by confocal microscopy.

Results: In the rabbit retina, dye injection revealed a network of overlapping axon terminals which are extensively coupled. The dendritic tips of rod bipolar cells express mGluR6 receptors which occupy a central position in the invagination of the rod spherule. In triple label material, the fine dendrites of the B–type axon terminals wrap closely around the central mGluR6 labeled processes. Immunolabeling with antibodies against GluR2 and GluR4 revealed punctate labeling in the outer plexiform layers. Small clusters of GluR2 and GluR4 receptors were found on the axon terminal dendrites. Triple labeling showed that GluR2 and GluR4 receptors were often colocalized on the B–type horizontal cell axon terminals.

Conclusions:In the rabbit retina, the rod driven input to B– type horizontal cell axon terminals may be mediated by AMPA receptors containing GluR2 and Glur4 subunits.