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Direction selectivity, a key feature of visual perception, originates in the retina and is transmitted by bistratified ganglion cells that, in the rabbit retina, exhibit a particular coupling pattern. We intracellularly labeled ganglion cells in different transgenic mouse lines, allowing a morphological classification of bistratified ganglion cells, an analysis of their coupling pattern, and the molecular identification of the connexins responsible for the coupling. Based on dendritic characteristics including co-fasciculation with the dendrites of cholinergic starburst amacrine cells, we were able to distinguish three types of bistratified ganglion cells. Two of these co-fasciculate with starburst amacrine cells and exhibit a specific homologous coupling pattern. Connexin45 (Cx45) appears to be the major component of the gap junctional channels because tracer coupling is absent in Cx45-deficient animals whereas it persists in Cx36-deficient animals. It is speculated that the transjunctional voltage dependence of Cx45 channels could support the transmission of direction selectivity.

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Gap junctions are channel-forming structures in contacting plasma membranes that allow direct metabolic and electrical communication between almost all cell types in the mammalian brain. At least 20 connexin genes and 3 pannexin genes probably code for gap junction proteins in mice and humans. Gap junctions between murine neurons (also known as electrical synapses) can be composed of connexin 36, connexin 45 or connexin 57 proteins, depending on the type of neuron. Furthermore, pannexin 1 and 2 are likely to form electrical synapses. Here, we discuss the roles of connexin and pannexin genes in the formation of neuronal gap junctions, and evaluate recent functional analyses of electrical synapses that became possible through the characterization of mouse mutants that show targeted defects in connexin genes.

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Astrocytes form extensive gap junctions with other astrocytes and with oligodendrocytes. Junctional communication between CNS glia is likely of critical importance because loss of the gap junction channel-forming proteins, connexins Cx32 and Cx47, result in severe demyelination. However, CNS glia express at least six connexins, and the cellular origins and relationships of these proteins have not been determined. We produced a Cx29 reporter mouse in which the connexin coding sequence was replaced with a histological marker, which was used to demonstrate that Cx29, Cx32, and Cx47 are expressed specifically in oligodendrocytes. To determine the relationships between astrocyte and oligodendrocyte connexins, we used double- and triple-immunofluorescence microscopy using semithin sections (<1 microm) of adult mouse spinal cord. Astrocytes form two distinct classes of gap junctions with each other; those composed of Cx26 and those composed of Cx43 and Cx30. In addition, astrocytes establish two classes of intercellular channels with oligodendrocytes, heterotypic Cx26-Cx32 channels and heterotypic Cx30/Cx43-Cx47 channels that may also be heteromeric. In contrast, Cx29 does not colocalize with any of the other five connexins. The data provide the first in vivo demonstration of heterotypic intercellular channels and reveal an unexpected complexity in the composition of glial gap junctions.

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Gap junctions serve many important roles in various tissues, but their abundance and diversity in neurons is only beginning to be understood. The tracer Neurobiotin has revealed many different networks interconnected by gap junctions in retina. We compared the relative permeabilities of five different retinal gap junctions by measuring their permeabilities to a series of structurally related tracers. When large tracers were injected, the staining of coupled cells fell off more rapidly in some networks than others relative to Neurobiotin controls. Three distinctly different permeability profiles were found, suggesting that multiple neuronal connexin types were present. The most permeant to large molecules were gap junctions from A-type horizontal cells. The permeability of gap junctions of two types of amacrine cell were not distinguishable from those from B-type horizontal cells. The lowest permeability was found for gap junctions between cone bipolar cells and the AII amacrine cells to which they are coupled. Because only a single neural connexin type has been identified in retina, our results suggest more types remain to be found. To determine whether the unitary permeability of channels is altered by channel modulators, we reduced permeability with octanol and a cAMP analog. Although net permeability was substantially diminished, the proportion by which it declined was constant across tracer size. This suggests that these agents act only to close channels rather than alter individual channel permeabilities. This tracer series can therefore be used to contrast permeability properties of gap junctions in intact circuits, even at the level of individual channels.

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