Alpha subunit of Go localizes in the dendritic tips of ON bipolar cells

The metabotropic glutamate receptor (mGluR6), expressed by rod bipolar cells and ON cone bipolar cells, activates a trimeric guanine nucleotide-binding protein (G-protein) that ultimately closes a cation channel. The G-protein remains unidentified, but the alpha subunit of Go (Go(alpha)) has been suggested as a candidate because it is present in rod bipolar cells. However, the precise subcellular distribution of Go within the rod bipolar cell, and its distribution among cone bipolar cells was not determined. This information is important in assessing the hypothesis that Go couple mGluR6 to its effector. Here I report the distribution of Go (alpha subunit) by immunostaining in several mammalian retinas. The overall distribution is conserved across mammalian species: strongest in the dendrites of ON bipolar cells, moderate in their somas, weak in their axons, and absent from their terminals. Go(alpha) is also present in some amacrine somas and processes. In monkey fovea, where rods and rod bipolar cells are absent, Go(alpha) is present in about half of the bipolar somas which occupy the upper tiers of the bipolar layer, and are therefore identified as ON cone bipolar cells. Ultrastructurally, in monkey and cat, Go(alpha) is present in the dendritic tips of rod bipolar cells and ON cone bipolar cells, which are identified by their invaginating contacts. It is absent from OFF cone bipolar dendrites, which are identified by their flat contacts. It is also absent from axons entering the inner plexiform layer, and their terminals. In the primary dendrites, stain for Go(alpha) mainly associates with the plasma membrane, but in the dendritic tips it is also present in the cytosol. Apparently, Go(alpha) is expressed by the same bipolar cells that also express mGluR6, and is concentrated at the same subcellular location. Thus, Go(alpha) could serve to couple mGluR6 to later stages of its signaling cascade.