The release of gamma-aminobutyric acid from horizontal cells of the goldfish (Carassius auratus) retina.

Isolated horizontal cells from goldfish retinas were prepared by enzymatic dissociation using papain and separated from other cells by velocity sedimentation. In the intact retina, H1 horizontal cells possess a high-affinity mechanism for accumulating gamma-aminobutyric acid (GABA). This property is retained in isolated cells, which also release the accumulated GABA in response to depolarization by elevated external K+. L-Glutamic acid and its analogues are highly effective at micromolar concentrations in eliciting the release of preloaded GABA from isolated cells. At saturating concentrations, L-aspartic acid stimulates about one-third as much release as L-glutamic acid. In contrast, the D-isomers of glutamate and aspartate are ineffective. In the intact retina, micromolar concentrations of L-glutamic acid analogues are also capable of eliciting GABA release from H1 horizontal cells. Release of the accumulated GABA from isolated H1 cells is largely independent of external Ca2+ concentrations. In the intact retina, H1 horizontal cells also possess a K+-stimulated GABA release mechanism that is independent of the Ca2+ concentrations in the medium. In addition, there appears to be a small but significant amount of [3H]GABA release that may be Ca2+ dependent. Under our conditions, [3H]GABA release from isolated cells is unaffected by external Na+ concentrations between 20 and 120 mM. However, concentrations of 10 mM or less significantly diminishes this release, with 70% curtailed in Na+-free solutions. Our results, together with morphological observations by a number of other investigators, suggest that there may be two distinct mechanisms for GABA release from goldfish H1 horizontal cells: one being a conventional vesicular mechanism which is Ca2+ dependent, while the other is Na+ driven and Ca2+ independent. H1 horizontal cells in the intact goldfish retina release the accumulated GABA in response to brief incubations in darkness, which is known to be the natural stimulus that depolarizes these neurones.