1. In this report we describe extracellular recordings made from directionally selective (DS) ganglion cells in the rabbit retina during perfusion with 2-amino-4-phosphonobutyric acid (APB) to block ON channels through the retina. 2. Application of 100 microM APB selectively and reversibly abolished the responses of ON ganglion cells in the rabbit retina. In addition, 100 microM APB completely and reversibly blocked ON component responses of ON-OFF DS ganglion cells to both stationary and moving stimuli. These results are consistent with the idea that APB blocks ON pathways through the retina. 3. Under ON pathway blockade with APB, OFF component responses of ON-OFF DS ganglion cells remained DS. DS OFF responses retained the same preferred direction as the pre-APB ON-OFF responses and could be driven using either normal or reversed contrast stimuli. 4. Extracellular responses of ON DS ganglion cells were completely blocked by APB. Under APB, these cells showed no response to stationary or moving stimuli. 5. Application of the gamma-aminobutyric acid-A (GABAA) antagonist 2-(3-Carboxypropyl)-3-amino-6-(4-methoxyphenyl)pyridazinium bromide (SR95531) reversibly abolished directional selectivity of ON DS and ON-OFF DS ganglion cells in the rabbit retina. This finding is consistent with previous data for picrotoxin. 6. Application of SR95531 during ON channel blockade by APB caused OFF component responses of ON-OFF DS ganglion cells to lose their directional selectivity. Under these conditions, OFF responses to movement in the preferred and null directions became virtually identical. 7. These results indicate that simultaneous ON and OFF layer input is not required to generate directional responses in ON-OFF DS ganglion cells. In addition, it appears that a GABAA-dependent mechanism for directional selectivity may operate independently in the two separate dendritic layers of the ON-OFF DS ganglion cell.
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