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K.Y. Wong, E.D. Cohen, A.R. Adolph, J.E. Dowling; Glutamate Transporter Currents in OFF Bipolar Cells in the Giant Danio . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1006.
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Purpose: Glutamate transporters have been reported to mediate the light-evoked response of ON bipolar cells in the teleost retina (Grant and Dowling, 1995). Glutamate transporter immunoreactivity has been detected in OFF bipolar cells in salamander (Eliasof et al., 1998), but their functions in these cells have not been investigated. In this study, we looked for glutamate transporter-mediated currents in OFF bipolar cells in the teleost Danio aequipinnatus. Methods: Whole-cell recordings were made from bipolar cells in slices. Using multi-barrel pipettes positioned near the outer plexiform layer, kainate and D-aspartate were puffed onto each cell to selectively activate AMPA receptors and glutamate transporters, respectively. Cobalt, picrotoxin and strychnine were added to the Ringer's solution to block synaptic transmission, and D-AP5 was added to block any NMDA receptor responses to D-aspartate. Lucifer Yellow was added to the internal solution to stain the cells recorded. Results: A total of 69 cells were recorded. 13 of them responded to only kainate, and all had axon terminals in the OFF sublamina only. 38 cells responded to only D-aspartate, with reversal potential close to ECl. 33 of these had terminals in the ON sublamina only, and 5 in both ON and OFF sublaminae. 18 cells responded to both agonists, with the D-aspartate response reversing near ECl. 4 of these cells had terminals in the OFF sublamina only, and 14 in both ON and OFF sublaminae. To determine the origin of the D-aspartate response in the cells that responded to both kainate and D-aspartate, the puffer pipette was moved to different positions around each cell. In all 5 cells tested, the strongest response was obtained when the transporter substrate was puffed close to the dendrites. Conclusions: We have found kainate-responding bipolar cells that also respond to D-aspartate. This response is localized close to the dendrites of these cells, suggesting that it may contribute to their response to light. Since a majority of these cells have axon terminals in both ON and OFF sublaminae, and the glutamate transporter response reverses close to ECl (as in the ON bipolar cells), it is possible that they depolarize at the onset as well as the offset of a light stimulus. We will test this idea by studying these cells' light-evoked responses.
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