Tiered Cross-Class Bipolar Cell Gap Junctional Coupling in the Rabbit Retina

Retinal circuit_

This abstract was presented today at the Association for Research in Vision and Opthalmology (ARVO) meetings in Seattle, Washington by J. Scott Lauritzen, John V. Hoang, Crystal Sigulinsky, Bryan W. Jones, James R. Anderson, Carl B. Watt, Shoeb Mohammed and Robert E. Marc.

Purpose:  Photon fluxes from sunlight to starlight span up to 10 log units (Rodieck, 1998 Sinauer), yet individual retinal cell class responses saturate within a 2.5 log unit dynamic range (Barlow & Levick, 1969 J Physiol; Diamond & Copenhagen, 1995 J Physiol; Enroth-Cugell & Shapely, 1973 J Physiol; Thibos & Werblin, 1968 J Physiol). This photic input segmentation is referred to as range fractionation, but it is not understood how individual cell classes coordinate their restricted responses to create smooth percepts of the entire visual range. Rod and cone pathways segment the scotopic and photopic ranges, yet pure within-photopic division must  occur as well, consistent with morphological diversity among OFF (CBa) and ON (CBb) cone bipolar cells. Gap junctions provide a vehicle for sign-conserved membrane potential sharing, and thus could link individual response ranges into a unified system. We mined ≈ 400 bipolar cells (BCs) the rabbit retinal connectome (RC1) for neural substrates capable of subserving cross-class cellular coordination required for photopic range fractionation.

Methods:  Bipolar cell (BC) networks in the ultrastructural rabbit retinal connectome RC1 were annotated with the Viking viewer, and explored via graph visualization of connectivity and 3D rendering (Anderson et al., 2011 J Microscopy). Small molecule signals embedded in RC1, e.g. GABA (y), glycine (G), and L-glutamate (E), combined with morphological reconstruction and connectivity analysis allow for robust BC classification. Gap junctions were validated by TEM re-imaging at 0.3 nm resolution.

Results:  All major classes of OFF and ON cone BCs homocellularly couple (eg. CBa1::CBa1, CBb5::CBb5). Moreover, within OFF and ON channels, BCs of a specific class form gap junctions with BCs in vertically adjacent inner plexiform layer (IPL) sublaminae (ie. CBa1::CBa2, CBb3::CBb4, CBb4::CBb5, CBb5::CBb6), thus constructing tiered BC coupling patterns across IPL strata. The net result is that all BC classes either directly or indirectly couple into multi-tiered sheets across the IPL.

Conclusions:  Cone BC tiered cross-class gap junctions pervade the IPL and segment into parallel ON and OFF domains. This could share membrane potentials in a sign-conserved fashion across individual BC classes with restricted dynamic ranges while preserving ON-OFF antagonism, thus linking the functionally fractionated photopic range into a unified system.

Support:  NIH EY02576 (RM), NIH EY015128 (RM), NSF 0941717 (RM), NIH EY014800 Vision Core (RM), RPB award to Moran Eye Center, RPB Career Development Award (BWJ), Thome Foundation grant for AMD Research (BWJ).