Ganglion Cell Responses to Photopic Stimuli Are Suppressed in a Cone-Specific Connexin36 Knockout

Abstract

Purpose:: There are two canonical pathways for the transduction of signals from rod photoreceptors to retinal ganglion cells. In the first, rod bipolar cells synapse with AII amacrines which then form inhibitory chemical synapses with OFF cone bipolar cells and electrical synapses (gap junctions) with ON cone bipolar cells. In the second, rod signals enter cones directly via gap junctions. The relative contributions of each pathway have not been clearly defined.

Methods:: We have constructed a knockout mouse line in which Cx36, a connexin expressed by both AII amacrines and cone photoreceptors, is selectively removed from the cones.A conditional allele was produced by flanking the second exon of Cx36 with loxP sites (Cx36fl). To generate cone-specific Cx36 knockouts, we crossed Cx36fl mice with a transgenic line expressing cre-recombinase driven by both the S and M cone opsin promoters. Homozygous Cx36fl -cre-positive and -cre negative littermates were analyzed by immunofluorescence microscopy. To evaluate if Cx36 is the sole mediator of rod-cone coupling we cloned and cone-specifically expressed the Glycine transporter1B in a Cx36 null and heterozygous background.

Results:: Cre-positive animals showed dramatically reduced staining for Cx36 in the outer plexiform layer (OPL) with no obvious alteration of staining within the inner plexiform layer (IPL), consistent with the notion that Cx36 was selectively removed from cones. Analysis of glycine diffusion patterns between rods and cones showed that eliminating Cx36 abolishes rod-cone coupling. Light-evoked ganglion cell responses were obtained by extracellular recording from a flattened retina-eyecup preparation. Dark adapted retinas were stimulated with a series of 0.5 second full-field illuminations of increasing intensity. The normalized intensity-response profiles from ON ganglion cells revealed that the highest sensitivity responses were unaffected by the cone-specific loss of Cx36. However, a suppression of ganglion responses was evident at mesopic and photopic light intensities.

Conclusions:: Taken at face value, these data suggest that Cx36 mediates rod-cone coupling and that rod signals and rod-cone coupling contribute to ganglion cell responses at higher light levels.

Keywords: retinal connections, networks, circuitry • gap junctions/coupling • retina