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M. Kamermans, D. Endeman; Activation of Chloride Currents in Cones Reduces Feedback From Horizontal Cells to Cones in the Goldfish Retina. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3286.
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The calcium-dependent chloride current (ICl(Ca)) is one of the largest currents in photoreceptors. It activates at depolarized potentials, above -40 mV, in close relation to the intracellular concentration of free Ca2+. Despite its large size, the consequences of activation of this current for information processing by photoreceptors remain largely unknown. Therefore we have examined the effect of ICl(Ca) on the size of light induced feedback responses from horizontal cells to cones.
Using voltage-clamp techniques in the isolated goldfish retina under a number of pharmacological conditions.
We used two protocols to manipulate ICl(Ca) in cones. In the first protocol, the cell was hyperpolarized for an extensive period to deactivate the ICl(Ca). In the other protocol, the cell was depolarized for an extensive period to activate this current. In these two conditions, we determined the IV relation of the cell. When subtracted from each other a linear current which reversed around ECl remained. This current was absent in the presence of niflumic acid (an aspecific blocker of the ICl(Ca)). We measured feedback responses in cones saturated by a small spot of light by stimulating the surround with steps of light after prolonged hyperpolarization or depolarization. At all the potentials tested the feedback induced shift of the calcium current in cones was significantly reduced when the ICl(Ca) was activated. Next we tested whether activation of another Cl-current in cones would lead to a similar modulation of feedback. We found that application GABA down regulated feedback from horizontal cells to cones while picrotoxin did the opposite.
The present work shows that the effectiveness of feedback from horizontal cells to cones depends on the size of Cl-currents in cones. Both Cl-currents will be activated under different conditions. ICl(Ca) will be activated when cones are depolarized over a prolonged time. This pathway thus makes that feedback is strong to horizontal cells that are hyperpolarized for a prolonged time whereas it will be weak to cones that are depolarized for a prolonged time. The GABA-gated pathway will most likely modulate the strength of feedback during light dark adaptation with feedback large in the light adapted state and small in the dark adapted state.
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