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K.Y. Wong, E.D. Cohen, A.R. Adolph, J.E. Dowling; Fish retinal ON bipolar cells respond to cones via mGluR6 and EAATs, and to rods via mGluR6 . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1335.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Grant and Dowling (1995) found an excitatory amino acid transporter (EAAT)–like receptor on teleost ON bipolar cells in addition to the well–known mGluR6 receptor. They proposed rods signal via mGluR6 whereas cones drive the EAAT–like receptor. However, ERG studies (e.g. DeMarco et al. 1991; Ren and Li 2003) showed that the mGluR6 agonist L–AP4 nearly abolishes both scotopic and photopic b–waves, challenging Grant and Dowling’s hypothesis. We re–examined the input mechanisms of ON bipolars using the Giant Danio. Methods: A white stimulus was used except where noted. ERGs were recorded from eyecups, using normal Ringer’s or a Ringer's containing picrotoxin, strychnine and TTX (PST) to isolate the photoreceptor–to–ON bipolar cell synapse. Whole–cell patch clamping was used to study cone–driven and mixed ON bipolars (Cb’s and Mb’s) identified morphologically. To characterize the glutamate receptors on each cell, D–aspartate (an EAAT substrate) and L–AP4 were puffed in the presence of Co2+, picrotoxin and strychnine. For light response experiments, Co2+ in the Ringer’s was replaced with Ca2+. Results: 1) ERG. Confirming previous studies, the photopic b–wave was largely removed when L–AP4 was applied in normal Ringer's. However, in the presence of PST, most of the b–wave survived L–AP4 or the mGluR6 antagonist CPPG. Adding the EAAT inhibitor TBOA blocked the remaining b–wave. Conversely, TBOA alone removed most but not all of the photopic b–wave. Under scotopic conditions, L–AP4 or CPPG nearly completely blocked rod–driven b–waves. 2) Patch clamp. Cb’s responded to D–aspartate but not to L–AP4, and TBOA blocked all of their light responses. Mb’s responded to both D–aspartate and L–AP4, and complete elimination of their light responses required TBOA plus CPPG. Responses of Mb’s to rod–selective green stimuli were reduced by CPPG but enhanced by TBOA, whereas their responses to cone–selective red stimuli were reduced by both agents. Mb’s could respond to light under conditions where rods were saturated and EAATs were blocked. Conclusions: With inhibitory interactions blocked by PST, the ERG results indicate a minor contribution of mGluR6 to the photopic b–wave. Thus, most of the photopic b–wave is EAAT–dependent, suggesting L–AP4’s near elimination of the b–wave in earlier studies probably results from polysynaptic effects. Consistent with these observations, Cb’s respond to cones via only EAATs, whereas Mb’s respond to cones with EAATs and mGluR6. Both the ERG and the patch–clamp data suggest that rod inputs (presumably onto Mb’s exclusively) use mainly mGluR6.
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