Connexins in the Mammalian Retina

Abstract

Gap junctions are particularly numerous in the retina and found in every major retinal cell type. They provide the primary connections in certain retinal pathways and form the substrate for signal averaging in others. At least four neuronal connexins are found in the mammalian retina, and different cell types express specific connexins with distinct properties. Cones are coupled via Cx36 gap junctions, which are thought to improve the cone signal-to-noise ratio. In contrast, rod-cone coupling, perhaps via heterotypic gap junctions, forms the basis for an alternate processing pathway that is active at intermediate light intensities. Horizontal cells are extensively coupled to provide spatial averaging over a wide area. Modulation of these junctions changes the spatial profile of horizontal cell feedback to photoreceptors. In rabbit and cat retina, A-type horizontal cells are coupled via massive Cx50 gap junctions, whereas the axon-bearing or B-type horizontal cells of the mouse and rabbit retina have different coupling properties and may express Cx57. In the inner retina, the primary output of the rod bipolar cell is to AII amacrine cells, which form a well-coupled network via Cx36 gap junctions. The AII network is prominent in all mammalian retinas and seems to provide for signal averaging in the noisy high-gain rod pathway. In addition, the signaling pathway from AII amacrine cells to ON cone bipolar cells passes via gap junctions, some of which may be heterotypic Cx36–Cx45 gap junctions. Many other types of amacrine cells and ganglion cells are also coupled via Cx36 or Cx45 gap junctions. Ganglion cell coupling produces synchronized spike activity between neighboring cells of the same type. The prevalence of gap junctions in the retina may occur because signal averaging and noise reduction are important strategies in the early stages of visual processing. Finally, while several retinal connexins have now been described, the connexins expressed in some coupled cells have not yet been identified. This suggests there are additional neuronal connexins (or perhaps pannexins) still to be identified in the mammalian retina.