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In several neural systems, glial cells appear to take up and release gamma-aminobutyric acid (GABA) upon depolarization. We have studied the release of [3H]GABA from Muller (glial) cells in the rat retina by a double isotope-labeling technique in which Muller cells are preloaded with 3H-GABA while a population of neurons is prelabeled with [14C]glycine. By autoradiography, we have confirmed that [3H]GABA is taken up by the radially oriented Muller cells, whereas [3H]glycine is accumulated by a subset of amacrine cells (neurons). Using the double- labeling procedure, we have examined the effects of two depolarizing agents, high K+ and veratridine, and the GABA mimetic, ethylenediamine, on transmitter release from glial cells and neurons simultaneously. We found the following. (1) Depolarization with 56 mM K+ released both [3H]GABA and [14C]glycine. About 70 to 80% of this release was blocked in Ca2+-free medium. (2) Veratridine (10 microM) also released both of the transmitters. This release was strongly inhibited by 100 nM tetrodotoxin or 1mM procaine. Under Ca2+-free conditions, less than 20% isotope release was observed. (3) Ethylenediamine released [3H]GABA readily, whereas little [14C]glycine release was observed. Removal of Ca2+ had no significant effect on transmitter release. Furthermore, in Na+-free medium ethylenediamine failed to induce [3H] GABA or [14C]glycine release. These results suggest that high K+ and veratridine release [3H]GABA from Muller cells by a Ca2+-dependent process. Ethylenediamine, on the other hand, appears to induce [3H]GABA release by a Ca2+-independent, carrier-mediated exchange mechanism.

Release of [3H]gamma-aminobutyric acid from glial (Muller) cells of the rat retina: effects of K+, veratridine, and ethylenediamine