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Masaaki Ishii, Yu Cao, Antoine Chaffiol, Christophe Ribelayga, Stuart C. Mangel; Mechanisms that Maintain Horizontal Cells in a Depolarized State and Light Responsive as the Ambient Illumination Gradually Changes During the Day. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4305.
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If horizontal cells (HCs) contribute to cone-bipolar cell (cBC) surround responses following prolonged (>30 min) light adaptation by releasing GABA onto cBC dendrites, which express Cl-permeable GABAA receptors (GABAARs), then HCs should be depolarized under light-adapted conditions to release GABA. However, it has been assumed that HCs, which hyperpolarize to brief light stimuli, are hyperpolarized under light-adapted conditions due to reduced glutamate input from cones. We therefore studied whether 1) HCs express greater levels of GABAARs and the Cl cotransporter NKCC, which mediates GABA-evoked depolarizations, following prolonged light, compared to dark, adaptation and 2) HCs are depolarized following both prolonged light and dark adaptation.
Rabbits were light (photopic)- or dark (low scotopic)-adapted for 1 h or retinas were incubated in Ringer for 1 h under light-adapted conditions with test drugs. Retinal sections were processed in an identical manner for immunostaining with anti-GABAAR (β2/3) and anti-NKCC antibodies. Double labeling with the HC-specific anti-calbindin antibody revealed GABAARs and NKCC distributions. HC light responses and membrane potential were compared following prolonged light and dark adaptation.
Intense GABAAR and NKCC antibody labeling were observed on HC dendrites in the day following prolonged light adaptation, but were minimal in the day following dark adaptation and following light adaptation after pre-treatment with a dopamine D1R antagonist. HCs were relatively depolarized (-35 mV) at rest and light responsive following 30 min of light or dark adaptation. Slowly increasing the background illumination following 60 min of darkness maintained HCs in a depolarized state. Picrotoxin (GABAA/C antagonist) and bumetanide (NKCC inhibitor) hyperpolarized HCs by ~22 mV vs. ~3 mV following prolonged light vs. dark adaptation, respectively.
The results indicate that 1) HCs are depolarized during a gradual increase in background illumination and following both prolonged light and dark adaptation and 2) NKCC and GABAAR expression and activity of HCs are greater following prolonged light, compared to dark, adaptation. Because the Cl conductance of HCs is ~3-fold greater than the glutamate-gated Na conductance (Yang et al., 1999), HCs may be dominated by a GABA-gated Cl conductance following light adaptation and a glutamate-gated Na conductance following dark adaptation, so that HCs remain in a depolarized state, releasing GABA, during the gradual changes in background illumination that normally occur throughout the day.
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