Accumulation of gamma-aminobutyric acid by horizontal cells isolated from the goldfish retina.

In the goldfish retina, H1 horizontal cells, which receive input predominantly from red sensitive cone photoreceptors, possess a single high-affinity uptake mechanism for gamma-aminobutyric acid (GABA). This GABA uptake is enhanced by light stimulation, which hyperpolarizes the H1 cells. The regulation of this uptake mechanism was examined in isolated horizontal cells by measuring the accumulation of exogenously supplied 3H-GABA. Solutions containing elevated external K+ or glutamate were used to quantitatively depolarize the cells to reveal that the potential-sensitive GABA uptake is maximal under hyperpolarizing conditions and minimal with depolarization. The driving force for GABA uptake is derived from the Na+ electrochemical gradient, with approximately 2 Na+ ions being cotransported with each molecule of GABA. The results presented suggest that the uptake mechanism permits the synaptic concentration of GABA to be regulated by the membrane potential of the H1 horizontal cells. This, then permits the presynaptic horizontal cell to modulate the synaptic concentration of transmitter in this tonically active synapse.