GAP-JUNCTIONS IN THE VERTEBRATE RETINA

The vertebrate retina is a highly laminated assemblage of specialized neuronal types, many of which are coupled by gap junctions. With one i nteresting exception, gap junctions are not directly responsible for t he 'vertical' transmission of visual information from photoreceptors t hrough bipolar and ganglion cells to the brain. Instead, they mediate 'lateral' connections, coupling neurons of a single type or subtype in to an extended, regular array or mosaic in the plane of the retina. Su ch mosaics have been studied by several microscopic techniques, but ne w evidence for their coupled nature has recently been obtained by intr acellular injection of biotinylated tracers, which can pass through ga p junctional assemblies that do not pass Lucifer Yellow. This evidence adds momentum to an existing paradigm shift towards a population-base d view of the retina, which can now be envisaged both as an array of s emi-autonomous vertical processing modules, each extending right throu gh the retina, and as a multi-layered stack of interacting planar mosa ics, bearing some resemblance to a set of interleaved neural networks. Junctional conductance across mosaics of horizontal cells is known to be controlled dynamically with a circadian rhythm, and other dynamica lly-regulated conductance changes are also likely to make important co ntributions to signal processing. The retina is an excellent system in which to study such changes because many aspects of its structure and function are already well understood. In this review, we summarize th e microscopic appearance, coupling properties and functions of gap jun ctions for each cell type of the neural retina, the regulatory propert ies that could be provided by selective expression of different connex in proteins, and the evidence for gap junctional coupling in retinal d evelopment. (C) 1995 Wiley-Liss, Inc.