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Chase Hellmer, Bozena E. Fyk-Kolodziej, Tomomi Ichinose; . Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):3226.
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© ARVO (1962-2015); The Authors (2016-present)
Retinal bipolar cells (BCs) encode various aspects of visual signaling separately among the different subtypes. BCs transmit visual signals primarily by transient and/or sustained graded potentials, although how they shape these signals is not well understood. Previous studies show that Ca2+ influx desensitizes visual signals in some depolarizing BCs (DBCs). We investigated whether desensitization shapes mGluR6 signaling in a BC subtype-dependent manner in the mouse retina.
Whole-cell recordings were made from DBCs of mouse retinal slices. CPPG (mGluR6 antagonist) puff-evoked responses were recorded either in voltage-clamp or current-clamp mode. EGTA (0.5 mM, weak Ca2+ chelator) or BAPTA (10 mM, strong Ca2+ chelator) was included in the pipette solution. Network effects were blocked with GABA and glycine receptor antagonists. Morphological subtypes were identified by including sulforhodamine B and neurobiotin in the pipette along with ChAT staining by immunolabeling. The peak amplitude, charge transfer, and rising time were measured and a transient-to-sustained ratio was calculated.
Recordings were taken from 50+ DBCs. Five subtypes of cone BCs (CBCs) and two subsets of rod BCs (RBCs) were identified by morphological and physiological analysis. In a subset of RBCs, CPPG-evoked currents were transient with EGTA but sustained in another. With BAPTA, the currents were sustained in all RBCs, indicating that intracellular Ca2+ caused desensitization in a subset of RBCs. Intracellular Ca2+ also decreased the rise time of one subset of RBC. In XBC and a subset of type-5 , evoked currents were small with EGTA whereas large sustained currents were evoked with BAPTA, indicating signal desensitization by intracellular Ca2+. In type-6, -7 and a subset of type-5 cells, CPPG-evoked currents were large and sustained both with EGTA and BAPTA. Finally, voltage responses of RBCs yielded only sustained signaling.
Intracellular Ca2+ desensitized mGluR6 currents in a subset of RBCs and a subset of type-5 and XBC CBCs. By contrast, the desensitization was not seen in type-6, type-7 and another subset of type-5 CBCs. Moreover, the voltage responses of RBCs indicate a role for voltage gated channels in shaping mGluR6 signaling. Our results suggest that each subtype of bipolar cell utilizes distinct mechanisms to shape visual signaling.
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