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J.M. Ackert, S.A. Bloomfield; Application of Picrotoxin Reveals a Direction Selective OFF Response in ON Direction Selective Ganglion Cells in Rabbit Retina . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3106.
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© ARVO (1962-2015); The Authors (2016-present)
To study the light–evoked responses of ON direction selective (DS) ganglion cells in the rabbit retina during blockade of GABAergic inhibition and/or the ON bipolar cell pathway.
Dual, simultaneous extracellular recordings were obtained from pairs of neighboring ON DS ganglion cells visualized with transcleral infrared illumination in a flattened retinal preparation of the albino rabbit. Responses were examined before and after application of 50 µm picrotoxin (PTX) and/or 50 µM L–AP4.
ON–DS ganglion cells were readily identified by their characteristic somatic labeling with the vital dye Azure B. We have shown previously that application of PTX abolished the direction selectivity of ON DS cells and reduced the synchronized firing of neighbors in response to stimulus movement in the preferred direction. Interestingly, we found that PTX revealed a late response that occurred about 140 msec after the offset of a stationary full–field stimulus. To determine whether this emerging late response was from the ON pathway, we applied L–AP4. As expected, application of L–AP4 abolished the ON response of ON DS cells. However, the late response was unaffected, indicating that it reflected signals carried by the OFF pathway. In experiments in which we applied both PTX and L–AP4 simultaneously, the OFF response could be readily isolated. Under these conditions, we found that the surviving OFF response showed direction selectivity to a moving slit of light. However, the preferred direction for the OFF response was never the same as that for the ON response under control conditions. That is, the preferred direction for the OFF response was opposite to that of the ON response or along an orthogonal axis.
Our results indicate the presence of both ON and OFF inputs to ON DS cells. The OFF response emerged only after application of PTX, suggesting that it is normally "hidden" by a GABAergic inhibition. Further, the OFF response showed direction selectivity that survived the addition of PTX, indicating a generating mechanism that is different from that of the ON response. These results indicate an unexpected complexity in the synaptic inputs to ON DS cells in the rabbit retina.
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