Analysis of synaptic inputs to on-off amacrine cells of the carp retina

To elucidate the synaptic transmission between bipolar cells and amacrine cells, the effect of polarization of a bipolar cell on an amacrine cell was examined by simultaneous intracellular recordings from both cells in the isolated carp retina. When either an ON or OFF bipolar cell was depolarized by an extrinsic current step, an ON-OFF amacrine cell was transiently depolarized at the onset of the current but no sustained polarization during the current was detected. The current hyperpolarizing the OFF bipolar cell also produced the transient depolarization of the amacrine cell at the termination of the current. These responses had a latency of approximately 10 ms. The amplitude of the current-evoked responses changed gradually with current intensity within the range used in these experiments. They were affected by polarization of the amacrine cell membrane; the amplitude of the current-evoked responses as well as the light-evoked responses was increased when the amacrine cell membrane was hyperpolarized, while the amplitude was decreased when the cell was depolarized. These results confirm directly that ON-OFF amacrine cells receive excitatory inputs from both ON and OFF bipolar cells: the ON transient is due to inputs from ON bipolar cells, and the OFF transient to inputs from OFF bipolar cells. The steady polarization of bipolar cells is converted into transient signals during the synaptic process.