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W. N. Grimes, J. S. Diamond; Voltage Sensitive Channels and Intracellular Calcium Dynamics in A17 Amacrine Cells. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5966.
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© ARVO (1962-2015); The Authors (2016-present)
Reciprocal inhibitory feedback from A17 amacrine cells (A17s) to rod bipolar cells(RBCs) shapes the time course of visual signaling. Recent work has shown that calcium influx triggering this feedback is mediated by calcium-permeable AMPA receptors(CP-AMPARs) and occurs independently of voltage-gated calcium channels (VGCCs; Chavez, 2006). Here we determine the identity and location of voltage-sensitive channels and measure intracellular calcium dynamics in A17s, in an effort to develop a general model of A17 physiology.
Simultaneous optical measurements of intracellular calcium signals and voltage-clamp recordings were made in A17s (in rat slices) filled through the patch electrode with calcium indicator dyes (Fluo-4 and Fluo-5F). Standard electrophysiology techniques were used to elicit and measure calcium influx through VGCCs. Increases in intracellular calcium were visualized via two-photon microscopy. Resting intracellular calcium levels were measured following techniques described by Maravall (2000).
Depolarizing voltage steps elicit currents mediated by L-type VGCCs and calcium influx that, when visualized, were localized to the soma and to the same dendritic varicosities that receive AMPA receptor-mediated synaptic input. VGCC-mediated currents were blocked by cadmium (100 µM) and isradipine (10 µM); depolarization-evoked fluorescence transients in varicosities and somata exhibited similar voltage-dependence and antagonist sensitivity. We also detected small, rapidly activating and inactivating, TTX-sensitive currents in A17s that likely reflect a small population of voltage-sensitive sodium channels.
CP-AMPARs and VGCCs appear co-localized in dendritic varicosities, suggesting that calcium signaling in these small structures is highly compartmentalized. Further experiments are required to determine the physiological role of VGCCs in A17 varicosities. Possible roles for putative sodium channels in propagating membrane depolarizations through A17 dendrites remains to be determined.
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