GABA release from Xenopus retina does not correlate with horizontal cell membrane potential.

The relationship between horizontal cell membrane potential and the release of GABA was explored in the retina of Xenopus laevis. The intracellularly recorded membrane potential of horizontal cells was monitored while the retina was exposed to different concentrations of depolarizing agents. The dose-response curves obtained revealed a rise from 5 to 95% maximum depolarization in 0.5-1.5 log unit concentration change. The molar concentrations that elicited a 20 mV depolarization were 40 mM (potassium), 0.8 mM (glutamate), 0.8 mM (glycine), 5 microM (kainate) and 1.3 microM (quisqualate). Autoradiography revealed that radiolabel was accumulated almost exclusively by horizontal cells when isolated retinas were incubated in medium containing 1 microM [3H]GABA. Thus, retinal release of radioactivity was used as a measure of [3H]GABA release from horizontal cells. Endogenous GABA released from retinas was measured using high performance liquid chromatography and was taken to reflect both amacrine and horizontal cell GABA pools. The release of both [3H]GABA and endogenous GABA was stimulated by glutamate, kainate and potassium, but not by glycine or quisqualate. Similar dose-response curves for GABA release and for depolarization were obtained in the case of potassium and kainate but not for glutamate. Potassium-evoked release either of endogenous GABA or [3H]GABA was both calcium- and sodium-dependent, whereas kainate- or glutamate-evoked GABA release was sodium-dependent but calcium-independent. The results indicate that depolarization per se is not necessarily associated with transmitter release in Xenopus retinal horizontal cells. It is suggested that the action of a given neurotransmitter upon the efflux of GABA from horizontal cells may depend on the degree to which it modifies the sodium conductance of the horizontal cell.