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Qi Lu, Jin-Juan Cui, Zhuo-Hua Pan; Characterization Of Voltage-gated Calcium Currents In Type 4 Cone Bipolar Cells In The Mouse Retina. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1163.
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Voltage-gated calcium channels play an important role in controlling many neuronal functions, including electrical excitability and neurotransmitter release. Retinal bipolar cells have been known to possess both T- and L-type voltage-gated calcium currents. Previous immunotaining and electrophysiological recording studies suggest a heterogeneous expression of both T- and L-type calcium channels among different bipolar cells. The detailed molecular and electrophysiological properties of the calcium currents in specific bipolar cell types, however, still remain largely unclear. In this study, we investigated the molecular and electrophysiological properties of voltage-gated calcium currents in type 4 cone bipolar cells in the mouse retina.
Calcium currents were recorded by whole-cell patch-clamp recordings in retinal slices under the elevated extracellular calcium (10 mM). Type 4 cone bipolar cells were identified based on their GFP fluorescence in a type 4 cone bipolar cell-specific GFP-transgenic mouse line, 5HT2a-GFP or their terminal stratification by dye filling through recording electrodes. The calcium currents were also recorded from a CaV1.4 mutant mouse line.
T-type calcium currents were observed in type 4 cone bipolar cells when the cells were depolarized from the holding potential of -80 mV. The currents were activated at a threshold potential around -55 mV and reach to their peak around -40 mV. The currents display slow activation and inactivation, similar to the properties of CaV3.3 calcium channels. The type 4 cone bipolar cells also show large L-type calcium currents. No significant L-type calcium currents were observed from the cells when their axon terminals were lost during slice preparation, suggesting that the L-type calcium currents were mainly located at the axon terminals. The L-type calcium currents were activated at a threshold potential around -40 mV and reach to their peak around -30 mV. The currents were not found to be significantly reduced in the CaV1.4 mutant mice.
Type 4 retinal cone bipolar cells show both T- and L-type voltage-gated calcium currents. The T-type calcium currents display slow activation and inactivation similar to that of CaV3.3 calcium channels. The L-type calcium currents are mainly located at the axon terminals and do not appears to be primarily formed by CaV1.4 calcium channels.
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