Light-evoked responses of bipolar cells in a mammalian retina

We recorded light-evoked responses from rod and cone bipolar cells using pa rch-clamp techniques in a slice preparation of the rat retina. Rod bipolar cells responded to light with a sustained depolarization (ON response) foll owed at light offset by a slight hyperpolarization. ON and OFF cone bipolar cells were encountered, both with diverse temporal properties. The respons es of rod bipolar cells were composed primarily of two components, a nonspe cific cation current and a chloride current. The chloride current was reduc ed greatly in axotomized cells and could be suppressed by coapplication of the GABA, antagonist bicuculline and the GABA(C) antagonist (1,2,5,6-tetrah ydropyridine-4-yl) methylphosphinic acid. This suggests that it largely ref lects feedback from GABAergic amacrine cells. The response latency of intac t rod bipolar cells was shorter than that of the axotomized cells, and the sensitivity curve covered more than twice the dynamic range. Application of the GABA receptor antagonists partially mimicked the effects of axotomy. T hese findings suggest that functional properties of the axon terminal syste m-notably synaptic feedback from amacrine cells-play an important role in d efining the response properties of mammalian bipolar cells.