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Katherine Fehlhaber, Alapakkam Sampath; Intracellular calcium acts on two separate targets to reduce the gain mGluR6 signaling in rod bipolar cells. Invest. Ophthalmol. Vis. Sci. 2013;54(15):6148. doi: https://doi.org/.
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Light-evoked responses in rod bipolar cells are generated by a metabotropic signaling cascade in their dendrites. Although this cascade has not been fully elucidated, intracellular Ca2+ is known to reduce the gain of rod bipolar cell responses. However the biophysical basis for modulation is not well understood. To determine how Ca2+ modulates the mGluR6 cascade, we measured rod bipolar cell response to steps of background light while buffering the rod bipolar cell cytoplasm with the Ca2+ buffer, BAPTA.
In dark-adapted mouse retinal slices, whole-cell voltage clamp recordings were made from rod bipolar cells (Vm = -60mV) as full-field 1-sec steps of light were delivered from an LED. The BAPTA pipette intracellular solution was identical to the control pipette solution, with the exception of the addition of 5mM BAPTA.
The internal dialysis of 5mM BAPTA into rod bipolar cells did not influence the magnitude or time course of responses to background lights of less than 7 Rh*/rod/sec. At brighter light intensities, BAPTA produced two changes to the rod bipolar cell step response: (1) increased the slope of the early rising phase, and (2) slowed the response time course and reduced the magnitude of adaptation during the 1-sec step of light. These changes scaled with the intensity of the background light.
This work suggests that Ca2+ normally slows the response to the peak, perhaps through direct interactions with TRPM1 channels. Additionally, Ca2+ reduces the extent of adaptation and slows the time to reach steady-state during exposure to background light. These Ca2+-dependent mechanisms operate on different time scales, and perhaps collectively allow rod bipolar cells to remain maximally responsive as background light varies.
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