Morphological and physiological analysis of type-5 and other bipolar cells in the Mouse Retina

•We show three subsets of type-5 ON bipolar cells in the mouse retina by morphological and physiological analysis.•We demonstrate strong and weak HCN1 expression across subsets of type-5 ON bipolar cells.•We demonstrate Na+ current in three types of retinal bipolar cells including type-5 cells.•Combined with previous work, we show that Na+ and HCN currents play a role in temporal tuning of visual signaling.

Retinal bipolar cells are second-order neurons in the visual system, which initiate multiple image feature-based neural streams. Among more than ten types of bipolar cells, type-5 cells are thought to play a role in motion detection pathways. Multiple subsets of type-5 cells have been reported; however, detailed characteristics of each subset have not yet been elucidated. Here, we found that they exhibit distinct morphological features as well as unique voltage-gated channel expression. We have conducted electrophysiological and immunohistochemical analysis of retinal bipolar cells. We defined type-5 cells by their axon terminal ramification in the inner plexiform layer between the border of ON/OFF sublaminae and the ON choline acetyltransferase (ChAT) band. We found three subsets of type-5 cells: XBCs had the widest axon terminals that stratified at a close approximation of the ON ChAT band as well as exhibiting large voltage-gated Na+ channel activity, type-5-1 cells had compact terminals and no Na+ channel activity, and type-5-2 cells contained umbrella-shaped terminals as well as large voltage-gated Na+ channel activity. Hyperpolarization-activated cyclic nucleotide-gated (HCN) currents were also evoked in all type-5 bipolar cells. We found that XBCs and type-5-2 cells exhibited larger HCN currents than type-5-1 cells. Furthermore, the former two types showed stronger HCN1 expression than the latter. Our previous observations (Ichinose et al., 2014) match the current study: low temporal tuning cells that we named 5S corresponded to 5-1 in this study, while high temporal tuning 5f cells from the previous study corresponded to 5-2 cells. Taken together, we found three subsets of type-5 bipolar cells based on their morphologies and physiological features.

Graphical abstractA neurobiotin injected, immunolabeled XBC bipolar cell and type 5 bipolar cell stratifying near the ON-ChAT (choline acetyltransferase) band in the mouse retina. The graph shows step-pulse evoked Na+ currents from voltage clamp recordings.Figure optionsDownload full-size imageDownload high-quality image (157 K)Download as PowerPoint slide