Factors controlling the release of GABA from goldfish retinal horizontal cells

Cloning, immunolocalization, and functional expression of a GABA transporter from the retina of the skate

Termination of GABA signals within the retina occurs through high-affinity reuptake of the released neurotransmitter by GABA transporters (GATs) present in neurons and glia surrounding the release site. In the present work, we have cloned a novel GAT from the retina of the skate (Raja erinacea). The clone codes for a 622 amino acid protein whose sequence has highest similarity to the GABA/β-alanine transporter of the electric ray (Torpedo marmorata) (88% identity) and the GAT-3 isolated from rat brain (75% identity). The protein was expressed inXenopusoocytes and characterized using the two-electrode voltage-clamp technique. Application of GABA induced a dose-dependent inward current, with 8 μM GABA producing a half-maximal response. The current required the presence of extracellular sodium and was unaffected by the GABA receptor blocker picrotoxin or the GAT-1 specific antagonist NO-711. The high homology between the cloned skate GABA transporter and the GAT-3 equivalents of other species, coupled with the strikingly similar pharmacological profile to GAT-3s of other species, lead us to conclude that we had cloned the GAT-3 homologue for the skate. Polyclonal antibodies specific to GAT-3 and the previously cloned skate GAT-1 transporter were used to examine the distribution of GAT-3 and GAT-1 immunoreactivity in the retina and in isolated cells of the skate. Antibodies for both transporters showed labeling in the outer and inner plexiform layers, and staining extended from the outer to inner limiting membranes. Both GAT-1 and GAT-3 antibodies labeled enzymatically isolated Müller cells, while bipolar cells and horizontal cells did not appear to express either transporter. These results imply that GAT-1 and GAT-3 are both present in Müller cells of the skate retina where they are likely involved in regulating extracellular concentrations of GABA.

The mismatch problem for GABAergic amacrine cells in goldfish retina: resolution and other issues

GABAergic neurons in the vertebrate retina have received intensive study. Yet there are several notable examples of a "mismatch" among the cytochemical markers used to identify GABAergic neurons. The mismatch between [3H]GABA uptake autoradiography and all other indicators of GABAergic neurons as they pertain to amacrine cells in goldfish retina is examined in this overview. The discrepancies can be accounted for largely by barriers to diffusion presented by significant GABA uptake sinks at the inner and outer margins of the retina and by the differential subcellular distribution of the various markers for GABAergic neurons. Also, conditions producing a redistribution of [3H]-GABA and endogenous GABA stores within the retina are described and discussed.

Sodium-dependent suppression of gamma-aminobutyric-acid-gated chloride currents in internally perfused frog sensory neurones

1. The effects of the Na+ electrochemical potential gradient on gamma-aminobutyric acid (GABA)-induced Cl- currents (ICl) in frog sensory neurones were studied, using a suction pipette technique with which internal perfusion can be accomplished under current- and voltage-clamp conditions. 2. Under current clamp, the depolarizing response to GABA decreased in the presence of external Na+. A similar external Na+-dependent reduction in the GABA-induced inward ICl was observed under voltage clamp. The reversal potential of GABA-induced ICl (EGABA) was nearly equal to the Cl- equilibrium potential (ECl), irrespective of the presence or absence of external Na+. 3. Varying the Na+ influx by changing the holding membrane potential (VH) altered the GABA response: the GABA-induced ICl decreased progressively as VH became more negative. 4. The effects of changing the external and internal Na+ concentrations ([Na+]o and [Na+]i) on the GABA-induced ICl were also studied. Increasing [Na+]o at a constant [Na+]i reduced this current while increasing [Na+]i at a fixed [Na+]o facilitated it. 5. A high temperature coefficient of about 3 was estimated with respect to the percentage reduction in GABA-induced ICl due to [Na+]o. 6. These results indicate that the [Na+]o-dependent suppression of GABA-induced ICl was mediated chiefly by the uptake of GABA subserved by a Na-GABA co-transport mechanism. 7. GABA dose-response measurements were made with and without external Na+. The [Na+]o-induced suppression was more pronounced in relative amount at lower concentrations and in absolute amount at intermediate concentrations. Analysis of these data indicates, however, that the Na+-coupled GABA influx kept increasing at GABA concentrations high enough to nearly saturate GABA-induced ICl, and the same saturating level was observed as in the Na+-free case. This indicates that the electrogenic co-transport current was much smaller so that our measurements of GABA-induced ICl' were contaminated very little. Thus, the present method based on recording of GABA-induced ICl was legitimate for the analysis of the Na-GABA co-transport. 8. By analysing the [Na+]o-dependent suppression of GABA-induced ICl, the stoichiometric ratio of the underlying co-transport was estimated to be one: one Na+ ion per GABA molecule. 9. The ICl induced by GABA agonists such as beta-alanine, taurine, l-GABOB (l-gamma-amino-beta-hydroxybutyric acid) and muscimol was not affected by the amount of external Na+ present, suggesting difference in the affinity between receptor and transport carrier.