Specificity of cell-cell coupling in rat optic nerve astrocytes in vitro

Intercellular coupling was studied in cultured rat optic nerve astrocytes individually characterized by A2B5 antibody staining. The presence of cell coupling was assessed by injecting single cells with the low molecular weight fluorescent dye Lucifer yellow and noting dye passage into adjacent cells; cell coupling was also studied by analyzing the decay phase of current transients recorded in response to small voltage steps using whole-cell patch-clamp recording. Cell coupling was restricted to A2B5- astrocytes, the majority of which had a flat fibroblast-like appearance and was never observed in A2B5+ stellate-shaped astrocytes. Furthermore, A2B5- astrocytes showed coupling only to A2B5- and never to A2B5+ astrocytes. Analysis of current transients provided an additional indicator for cell coupling. Astrocytes that showed dye coupling to at least one neighboring cell required the sum of two exponential functions to fit current transients, whereas a single exponential function sufficed to fit transients in cells that were not dye coupled. The specificity of cell coupling in cultured rat optic nerve astrocytes suggests that predominantly A2B5- astrocytes comprise a coupled glial syncytium; this physiological feature of these cells may be a specialized adaptation for "spatial buffering," the transport of K+ away from areas of focal extracellular accumulation. On the other hand, A2B5+ astrocytes form an uncoupled subpopulation of rat optic nerve glial cells that may serve different functions.