Bipolar Cell Type-Specific Expression and Conductance of Alpha-7 Nicotinic Acetylcholine Receptors in the Mouse Retina

PURPOSE. Motion detection is performed by a unique neural network in the mouse retina. Starburst amacrine cells (SACs), which release acetylcholine and gamma-aminobutyric acid (GABA) into the network, are key neurons in the motion detection pathway. Although GABA contributions to the network have been extensively studied, the role of acetylcholine is minimally understood. Acetylcholine receptors are present in a subset of bipolar, amacrine, and ganglion cells. We focused on alpha 7-nicotinic acetylcholine receptor (alpha 7-nAChR) expression in bipolar cells, and investigated which types of bipolar cells possess alpha 7-nAChRs. METHODS. Retinal slice sections were prepared from C57BL/6J and Gus8.4-GFP mice. Specific expression of alpha 7-nAChRs in bipolar cells was examined using alpha-bungarotoxin (alpha BgTx)-conjugated Alexa dyes co-labeled with specific bipolar cell markers. Whole-cell recordings were conducted from bipolar cells in retinal slice sections. A selective alpha 7-nAChR agonist, PNU282987, was applied by a puff and responses were recorded. RESULTS. alpha BgTx fluorescence was observed primarily in bipolar cell somas. We found that alpha 7-nAChRs were expressed by the majority of type 1, 2, 4, and 7 bipolar cells. Whole-cell recordings revealed that type 2 and 7 bipolar cells depolarized by PNU application. In contrast, alpha 7-nAChRs were not detected in most of type 3, 5, 6, and rod bipolar cells. CONCLUSIONS. We found that alpha 7-nAChRs are present in bipolar cells in a type-specific manner. Because these bipolar cells provide synaptic inputs to SACs and direction selective ganglion cells, alpha 7-nAChRs may play a role in direction selectivity by modulating these bipolar cells' outputs.