Electrical coupling between glial cells in the rat retina

The strength of electrical coupling between retinal glial cells was quantified with simultaneous whole-cell current-clamp recordings from astrocyte-astrocyte, astrocyte-Müller cell, and Müller cell-Müller cell pairs in the acutely isolated rat retina. Experimental results were fit and space constants determined using a resistive model of the glial cell network that assumed a homogeneous two-dimensional glial syncytium. The effective space constant (the distance from the point of stimulation to where the voltage falls to 1/e) equaled 12.9, 6.2, and 3.7 μm, respectively for astrocyte-astrocyte, astrocyte-Müller cell, and Müller cell-Müller cell coupling. The addition of 1 mM Ba²⁺ had little effect on network space constants, while 0.5 mM octanol shortened the space constants to 4.7, 4.4, and 2.6 μm for the three types of coupling. For a given distance separating cell pairs, the strength of coupling showed considerable variability. This variability in coupling strength was reproduced accurately by a second resistive model of the glial cell network (incorporating discrete astrocytes spaced at varying distances from each other), demonstrating that the variability was an intrinsic property of the glial cell network. Coupling between glial cells in the retina may permit the intercellular spread of ions and small molecules, including messengers mediating Ca²⁺ wave propagation, but it is too weak to carry significant K⁺ spatial buffer currents.