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W. Shen; Effect of Light Adaptation on Glycine Sensitivity in Horizontal Cells . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2007.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Glycine activates distal neurons through feedback from interplexiform cells in proximal retina. The function and mechanism of the glycine feedback are still under investigation. Horizontal cells (HCs) are local feedback neurons involved in regulation of contrast and sensitivity in distal retinas. It has been found that glycine could depolarize HCs by activating glycine receptors (GlyRs). Therefore, modulation of GlyRs might directly affect the physiological function of HCs. This study investigated the influence of dark and the light adaptation on GlyRs in HCs. The experiments aimed at exploring how calcium channels, glutamate receptors and intracellular events affect local GlyRs in HCs. Methods: Whole cell voltage-clamp experiments were performed on isolated and slice-prepared tiger salamander retinal HCs. Glycine currents (Igly) were recorded at -50mV, around dark membrane potential of HCs, in dark and after a brief light adaptation. In some experiments, Fluo-4 calcium indicator was loaded into the cells through a recording electrode in order to detect [Ca2+]i changes. Results: HCs were recorded in slice preparation under dark-adaptation. Applying 50µM glycine produced a small and sluggish Igly, which was largely enhanced after a train of light adaptation. The brief light adaptation also provided an inward current on HCs, suggesting that HCs were depolarized. Calcium imaging study showed that the light adaptation also gradually increased [Ca2+]i in HCs. The enhancement of Igly could be suppressed by intracellular application of 10mM BAPTA, a faster calcium buffer, suggested that [Ca2+]i was the major factor in regulation of GlyRs in HCs during the light adaptation. The increase [Ca2+]i could be due to calcium influx through voltage-gated calcium channels and glutamate receptors. To further examine this issue, Igly was studied in isolated HCs. 30µM Igly was increased about 35% by a pre-pulse voltage step and applying 500nM Bay K, a L-type calcium channel enhancer. The effect was fully blocked by 25µM Nifedipine, a L-type calcium channel inhibitor. The Igly was also increased around 50% when 50µM glutamate was present. BAPTA blocked 90% of the effect, suggesting that glutamate might increase [Ca2+]i that enhanced Igly in HCs. Conclusions: The experiments suggest that brief light adaptation increases local [Ca2+]i, thereby synergistically potentiating the Igly. Both glutamate receptors and L-type calcium channels could initiate the [Ca2+]i elevation. In consequence, the effect of glycine feedback is more significant in HCs under mesopic condition of the retina. Thus, light adaptation can reset HC in control of contrast and sensitivity.
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