Abstract
Horizontal cells (HCs) appear to release, and also to be sensitive to, GABA. The external GABA concentration is increased with depolarization of the HC membrane via an electrogenic GABA transporter. This extracellular GABA opens a GABAA-gated Cl- channel in the HC membrane. Since the equilibrium potential for Cl- (ECl) is near -20 mV, GABA released by the HC further depolarizes the HC. The GABA transporter and the GABAA receptor thus constitute a positive feedback loop in the HC membrane. This loop can slow down the kinetics of the light responses in HCs. GABA released via the GABA transporter can affect the GABAA receptor, probably because diffusion from the extracellular space is normally restricted by the intact retinal structure. We therefore used retinal slices rather than isolated HCs to maintain that structure. To measure single-cell currents in the slice, HCs were electrically uncoupled by including cAMP in the patch pipette. Under these conditions, bath application of GABA elicited two currents: (1) a picrotoxin-blocked current reversing near ECl, probably mediated by GABAA receptors, and (2) a picrotoxin-insensitive current similar to that elicited by cis-4-hydroxynipecotic acid (NIP) shown in other preparations to act at the GABA transporter. Under physiological conditions, the HC membrane potential is controlled by two major conductances, the GABAA-gated Cl- conductance described above, and the glutamate-gated conductance modulated by photoreceptor input. A bright light flash eliminates the glutamate-gated conductance, leaving only the GABA-gated Cl- conductance to control the membrane. With the Cl- conductance a significant fraction of the overall membrane conductance the GABAergic positive feedback loop can decrease the response kinetics. We increased the ambient extracellular GABA concentration by adding 50 microM GABA to the extracellular medium. This increased the ambient Cl- conductance, but the transporter still modulated Cl- conductance because responses to light stimuli were significantly slowed. The slowdown of the HC response could be reversed by interrupting the loop in two ways: (1) picrotoxin opened the loop and speeded the responses by uncoupling the GABA concentration from control of the membrane conductance, and (2) NIP opened the loop by uncoupling the extracellular GABA concentration from the Cl- conductance and therefore the membrane potential.(ABSTRACT TRUNCATED AT 400 WORDS)
