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

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 Müller (glial) cells in the rat retina by a double isotope-labeling technique in which Müller 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 Müller 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 Müller 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.