Immunocytochemical evidence that monkey rod bipolar cells use GABA.

Certain bipolar cells in most species immunostain for GABA or its synthesizing enzyme glutamic acid decarboxylase. However, it is unknown whether they actually release GABA and, if so, from which cellular compartment and by what release mechanism. We investigated these questions in monkey retina where rod bipolar cells immunostain for GABA. We found that rod bipolar cells immunostain for one isoform of GAD (GAD65) in their somas, dendrites and axon terminals. Near the fovea, the somatic stain of rod bipolar cells is weaker than that of horizontal cells but, at the periphery, it is stronger. Staining for the vesicular GABA transporter in monkey rod bipolar cells is negative. However, staining for the GABA transporter GAT3 is positive in the soma and primary dendrites (but not in the axon terminals). Staining for GAT3 is also positive in horizontal cells. Double staining of rod bipolar cells and the alpha subunit of the GABAA receptor reveals scarce GABAA puncta that appose rod bipolar dendrites. We conclude that monkey rod bipolar cells use GABA and discuss the possibility that they tonically release GABA from their dendrites using a reverse action of GAT3.