Electrical impedance of isolated retina and its changes during spreading depression

The electrical impedance of isolated chick and toad retinas were measured in the absence of, and during, spreading depression, using sinusoidal measuring currents ranging from 10 to 50,000 Hz. Data obtained in the absence of the reaction indicate an electric heterogeneity of the tissue and suggest that at least two groups of cells are responsible for the observed frequency distribution. Spreading depression is accompanied by impedance changes that depend on the frequency of the measuring current and composition of the Ringer's solution. In chick retinas immersed in standard solution, impedance magnitude (Z) as well as phase angle (0) change in phase, displaying an initial increase followed by a longer decrease; in the toad, these changes depend on the measuring frequency: at 10 Hz they are similar to the ones observed in chick retinas, whereas at intermediate and high frequencies they are out of phase. In low Cl- + sucrose Ringer's solution a decrease of impedance is observed in both chick and toad retinas. In low Cl- + Na2SO4 + sucrose Ringer's solution a decrease of impedance also predominates but the Z and 0 curves are biphasic and complex, having time courses which are frequency-dependent. In addition, these changes are different in chick and toad preparations. The experiments show that the impedance changes during the retinal reaction might be related both to extracellular as well as cellular currents. The extracellular current changes could be due to decrease of ionic content and volume of extracellular space, the latter variations being drastically reduced when retinas are immersed in low Cl- solutions. The results also suggest changes of current flow through cellular components which are frequency-dependent: at low frequencies the preferential path being probably through neuronal, and at high frequencies, through glial cells.