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Xue Liu, Arlene A. Hirano, Steven Barnes, Nicholas C. Brecha; Voltage-gated Calcium Channels in Horizontal Cells Mediate Inhibitory Feedback to Photoreceptors. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2570.
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© ARVO (1962-2015); The Authors (2016-present)
Horizontal cells (HCs) send inhibitory feedback to photoreceptors (PRs) but the mechanisms underlying this inhibition are not fully understood. Immunohistochemical (IHC) studies show that the proteins responsible for Ca-dependent vesicular fusion are present in HCs. The goal of this study is to determine the contribution of voltage-gated calcium channels (VGCCs) in HCs to feedback.
Rat retinal slices loaded with Fluo-4 were imaged on a Zeiss Pascal confocal microscope. Brief superfusion with 30 mM K+ activated PR L-type Ca channels. 50 µM kainate (KA) and 50 µM NBQX were used to alter the feedback signal from HCs. 1.5 µM ω-conotoxin GVIA (CTX) and 400 nM ω-agatoxin IVA (ATX) were used to block N- and P/Q-type Ca channels, respectively. 20 µM dopamine D1 receptor (D1R) agonist SKF-38393 and 20 µM D2 receptor (D2R) antagonist spiperone were used to modulate dopamine pathways. IHC was used to study the localization of Ca channel subunits.
IHC analysis showed that L-, N- and P/Q-type Ca channels are present in HCs. N-type channels in particular were found to be localized to the tips of the HC processes. Consistent with previous reports, KA inhibited high K+-evoked Ca2+ signals in PRs by ~36%, whereas NBQX enhanced Ca2+ signals by ~27%. When the N-type Ca channel blocker CTX was applied, high K+-evoked Ca2+ signals were potentiated by ~28%. The P/Q-type channel blocker ATX also enhanced Ca2+ signals by ~40%. HC L-type Ca channels cannot be tested. Since high K+-evoked dopamine release might modulate Ca channels in PRs directly via D2Rs on PRs and indirectly via D1Rs on HCs, we tested these pathways. The D1R agonist SKF-38393 and the D2R antagonist spiperone were applied and had no modulatory effect on PR Ca2+ signals.
These results show that block of N- and P/Q-type Ca channels in HCs diminishes feedback to PRs. This suggests that signaling from HCs to PRs is at least in part dependent on VGCCs, consistent with Ca-dependent vesicular release from HCs mediating inhibitory feedback.
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