An unique subtype of BCs provides excitatory input to both ON and OFF synaptic pathways from both rods and cones in the retina

Abstract

Purpose : To characterize structurally and functionally a newly identified subtype of bipolar cells of mammalian retina.

Methods : We developed a transgenic-viral approach to transcellularly label presynaptic neurons of RGCs by expressing the WGA (wheat germ agglutinin)-Flp recombinase fused protein in Cre-specific RGCs of Cre:FRT-EGFP mice. WGA-Flp can be transported into neurons presynaptic to transduced RGCs and activates mEGFP expression in these cells. These mEGFP-positive neurons were imaged and analyzed for cell type identification. Using this method, we identified an unique subtype of BCs previously not described. We used additional antibody staining to further characterize the morphological and biochemical features of these cells. We also measured GCaMP6 signaling to record the light evoked activity of these BCs.

Results : The newly identified cells have dendritic processes ramified in the OPL and axonal processes ramified in the IPL. They express mGluR6 at their dendritic terminals, ribbons and vesicular glutamate transporter 1 at their axonal terminals. They have increased intracellular calcium concentration in responding to light, indicating they are ON BCs. Unlike other BCs, the axonal plexus of these BCs closely resembles the dendritic plexus of Aii-ACs, which ramify throughout the IPL and form large varicosities at their axonal terminals. Therefore, we name them Aii-BCs. In addition, Aii-BCs synapse with both rods and cone, which is different from classic rod and cone BCs. Furthermore, Aii-BCs are found in mouse, marmoset and human retina, indicating a cell type preserved during evolution.

Conclusions : The morphological, biochemical and physiological features of the Aii-BCs place them in a singular position as the only subtype of BCs receiving inputs from both rods and cones and having excitatory outputs to both ON and OFF pathways.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

A. An Aii-BC and 3D rendering. B. An Aii-AC (left) and an Aii-BC (right). C. Ribbons (red) inside axonal varicosities (green) (C1). A 3D rendering (C2).

 

D. An Aii-BC (green) labeled with VGluT1 (red). E. VGluT1 (red) inside varicosities (E1). A 3D rendering (E2). F. Ribbons (red) and VGluT1 (blue) inside axonal varicosities (green) (F1). A 3D rendering (F2). G. Aii-BCs labeled by anti-GlyT1 (green) and anti-mGluR6 (red). H. mGluR6 (red) on the dendritic terminals of Aii-BC (green) (H1). A 3D rendering (H2).