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W. B. Thoreson, N. Babai, T. M. Bartoletti; Feedback From Horizontal Cells to Rods. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5796. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Negative feedback from horizontal cells (HCs) to cones is well established, but many investigators have concluded that there is no feedback from HCs onto rods, largely because of the absence of a depolarizing rollback in response to annular illumination. However, feedback from HCs to cones involves small changes in cone calcium currents (ICa) that do not always generate a significant depolarizing rollback in the cone light response. We therefore re-examined the question of feedback from HCs to rods.
Simultaneous whole cell recordings were obtained from rods and HCs in the salamander retinal slice preparation to test effects of HC membrane potential changes on rod ICa. We also tested effects of HC hyperpolarization by light on ICa recorded from rods lacking outer segments. Retinas were superfused with bicarbonate-buffered medium. HEPES (10 mM) was added in some experiments.
Hyperpolarizing the HC membrane potential from 0 to -40, -70, and -90 mV caused a progressive hyperpolarizing activation shift and increased the amplitude of ICa recorded from synaptically connected rods. ICa was unaffected by changes in HC membrane potential in paired recordings without synaptic communication. ICa was also unaffected by changes in post-synaptic membrane potential during paired recordings from rods and bipolar cells. To test whether light produced similar effects, we recorded from rods lacking outer segments and found that light-evoked hyperpolarization of surrounding HCs caused a leftward activation shift in ICa. Compared to rods held at -70 mV, light flashes evoked a net inward current in rods held at the dark resting potential of -40 mV, consistent with a feedback-mediated increase in ICa. As found in cones, HEPES blocked the leftward shift in rod ICa caused by direct or light-evoked HC hyperpolarization.
Similar to cones, negative feedback from HCs causes a leftward activation shift and increase in amplitude of rod ICa thereby increasing ICa during light. These effects were blocked by HEPES suggesting they involve similar mechanisms as HC to cone feedback. The light-evoked increase in rod ICa resulting from HC feedback will influence spatio-temporal processing at the first synapse in vision.
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