A quantitative analysis of glial cell coupling in the retina of the axolotl (Ambystoma mexicanum).

The strength of gap junctional coupling of radial glial cells (Müller cells) in the isolated axolotl retina was assessed by monitoring the spread of dye between cells, and by injecting current into one cell and recording the voltage response in surrounding cells. Dye injected into one Müller cell spread to surrounding Müller cells, and could be detected up to 130 micron away, i.e. over 4 times the mean Müller cell spacing of 30 micron. Injecting 1 nA of current into a Müller cell evoked responses of 7 mV in that cell, 1 mV in next neighbour cells, and 0.2 mV in cells at 60 micron distance. Analysis of these data indicates an electrical space constant for the Müller cell network of 15 micron, and predicts that isolated cells should have a resistance of 11.4 M omega. Müller cells isolated by papain dissociation of the retina were found, by whole-cell patch-clamping, to have a mean resistance of 12.4 M omega. These results on lateral coupling are combined with data showing that over 90% of the Müller cell potassium conductance is in the vitreal endfoot of these cells to provide a fairly complete electrical description of the radial glial cell network in the retina. Gap junctional coupling of Müller cells increases by 60% the 'spatial buffering' that these glial cells can carry out to reduce localized rises in extracellular potassium concentration. The location of the majority of the Müller cell potassium conductance in the cell endfoot ensures that laterally buffered K+ is deposited in the vitreous, rather than depolarizing surrounding retinal neurones.