The Journal of Neurophysiology Vol. 83 No. 6 June 2000, pp. 3473-3482
Copyright ©2000 by the American Physiological Society

I4AA-Sensitive Chloride Current Contributes to the Center Light Responses of Bipolar Cells in the Tiger Salamander Retina

Cullen Eye Institute, Baylor College of Medicine, Houston, Texas 77030

Gao, Fan, Bruce R. Maple, and Samuel M. Wu. I4AA-Sensitive Chloride Current Contributes to the Center Light Responses of Bipolar Cells in the Tiger Salamander Retina. J. Neurophysiol. 83: 3473-3482, 2000. Light-evoked currents in depolarizing and hyperpolarizing bipolar cells (DBCs and HBCs) were recorded under voltage-clamp conditions in living retinal slices of the larval tiger salamander. Responses to illumination at the center of the DBCs' and HBCs' receptive fields were mediated by two postsynaptic currents: I_C, a glutamate-gated cation current with a reversal potential near 0 mV, and I_Cl, a chloride current with a reversal potential near 60 mV. In DBCs I_C was suppressed by L-2-amino-4-phosphonobutyric acid (L-AP4), and in HBCs it was suppressed by 6,7-dinitroquinoxaline-2,3-dione (DNQX). In both DBCs and HBCs I_Cl was suppressed by imidazole-4-acetic acid (I4AA), a GABA receptor agonist and GABA_C receptor antagonist. In all DBCs and HBCs examined, 10 µM I4AA eliminated I_Cl and the light-evoked current became predominately mediated by I_C. The addition of 20 µM L-AP4 to the DBCs or 50 µM DNQX to HBCs completely abolished I_C. Focal application of glutamate at the inner plexiform layer elicited chloride currents in bipolar cells by depolarizing amacrine cells that release GABA at synapses on bipolar cell axon terminals, and such glutamate-induced chloride currents in DBCs and HBCs could be reversibly blocked by 10 µM I4AA. These experiments suggest that the light-evoked, I4AA-sensitive chloride currents (I_Cl) in DBCs and HBCs are mediated by narrow field GABAergic amacrine cells that activate GABA_C receptors on bipolar cell axon terminals. Picrotoxin (200 µM) or (1,2,5,6-tetrahydropyridine-4yl) methyphosphinic acid (TPMPA) (2 other GABA_C receptor antagonists) did not block (but enhanced and broadened) the light-evoked I_Cl, although they decreased the chloride current induced by puff application of GABA or glutamate. The light response of narrow field amacrine cells were not affected by I4AA, but were substantially enhanced and broadened by picrotoxin. These results suggest that there are at least two types of GABA_C receptors in bipolar cells: one exhibits stronger I4AA sensitivity than the other, but both can be partially blocked by picrotoxin. The GABA receptors in narrow field amacrine cells are I4AA insensitive and picrotoxin sensitive. The light-evoked I_Cl in bipolar cells are mediated by the more strongly I4AA-sensitive GABA_C receptors. Picrotoxin, although acting as a partial GABA_C receptor antagonist in bipolar cells, does not suppress I_Cl because its presynaptic effects on amacrine cell light responses override its antagonistic postsynaptic actions.