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C. Ribelayga, S.C. Mangel; Light Responses of Rabbit A–Type Horizontal Cells Exhibit a Diurnal Rhythm. . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1120.
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Purpose: A circadian clock regulates the light responses of fish cone horizontal cells so that cone input predominates during the day and rod input predominates during the night (Wang and Mangel, 1996; Ribelayga et al., 2002, 2004). To determine whether the light responses of horizontal cells in the mammalian retina depend on the time of day, rabbit A–type (axonless) horizontal cells were studied during the day and night. Methods: Rabbits were maintained for at least 14 days on a 12 hr light/12 hr dark cycle before the start of experiments. Surgery was performed under dim red light. At different times of the day or night, retinas were superfused in darkness for 60 min, following which horizontal cells were impaled without the aid of any light flashes. Responses of the cells to dim full–field white light flashes (ranging from –10 log Io to –5 log Io, with unattenuated intensity Io = 2.0 mW/cm2) were then recorded. Following recording, Neurobiotin tracer was iontophoresed for 15 min. The morphology of the injected cell was revealed by the tracer and the extent of tracer coupling was used as an index of gap junctional coupling. Results: During the day, dark–adapted A–type horizontal cells had a resting potential of – 32 +/– 2 (SEM) mV (n = 13) and a light response threshold of –6 log Io. At –5 log Io, the average amplitude of the light response was 5.39 +/– 0.63 mV (n = 13). The extent of tracer coupling averaged 2257 cells +/– 650 (n = 4). During the night under dark–adapted conditions, no differences were observed compared to the day with respect to resting potential (– 35 +/– 3 mV; n = 10) and tracer coupling (2981 +/– 563 cells; n = 4). However, A–type horizontal cells were ∼100 times more sensitive to light at night than during the day. That is, the light response threshold was –8 log Io and the average amplitude of the response at –5 log Io was 13.85 +/– 2.34 mV (n = 10). Light sensitization (–3 log Io; 500 ms; 0.125 Hz; applied during the entire experiment) dramatically reduced the extent of tracer coupling both during the day (382 +/– 112; n = 2) and night (776 +/– 490; n = 6). Finally, light sensitization of the retina at night decreased the light sensitivity of horizontal cells 100–fold (threshold = –6 log Io). Conclusions: Our observations indicate that dark–adapted A–type horizontal cells are ∼100 times more sensitive to light at night than during the day, suggesting that their light responses are likely regulated by a circadian (24 h) clock so that the cells receive rod input at night. These results demonstrate for the first time that the light responses of a neuron in the dark–adapted mammalian retina depend on the time of day.
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