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Retinal Bipolar Cell Input Mechanisms in Giant Danio. III. ON-OFF Bipolar Cells and Their Color-Opponent Mechanisms

Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts

Submitted 18 March 2004; accepted in final form 2 March 2005

Whole cell patch recording was performed from morphologically identified cone-driven ON-OFF bipolar cells (Cabs) in giant danio retinal slices to study their glutamate receptors and light-evoked responses. Specific agonists were puffed in the presence of cobalt, picrotoxin, and strychnine to identify glutamate receptors on these cells. Most Cabs responded to both the -amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor agonist kainate and the excitatory amino acid transporter (EAAT) substrate D-aspartate, and both responses were localized to the dendrites. Kainate generated depolarizations whereas D-aspartate had E_rev close to E_Cl and generated hyperpolarizations, indicating that the AMPA/kainate receptors are sign-preserving, whereas the EAATs are sign-inverting. In response to white light, some Cabs gave ON bipolar cell-like responses whereas others gave OFF bipolar cell-like ones, but many cells' responses had both ON and OFF bipolar cell components. In response to appropriately colored center-selective stimuli, many Cabs responded to short and long wavelengths with opposite polarities and were thus double color-opponent. The depolarizing components of the responses to white or colored stimuli were suppressed by the EAAT blocker DL-threo--benzyloxyaspartate (TBOA), whereas the hyperpolarizing components were reduced by the AMPA/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). These results are consistent with the hypothesis that both EAATs and AMPA/kainate receptors are involved in the generation of light-evoked responses in Cabs and that they confer these cells with ON and OFF bipolar cell properties, respectively. Cabs can generate double color-opponent center responses by receiving inputs from certain cones through EAATs and from other cones through AMPA/kainate receptors.

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