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M. Rampino, S. Nawy; Examining the Role of Phospholipase C in mGluR6- Mediated Signaling in Retinal on Bipolar Cells. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1020.
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Light onset results in the opening of nonselective cation channels in On bipolar cells. This effect is a result of inactivation of the metabotropic glutamate receptor mGluR6. However, it is unclear how signaling downstream of mGluR6 mediates channel opening. The cleavage of PIP2 by PLC is known to regulate channel function in many systems, through either depletion of PIP2 or generation of its hydrolysis products, DAG and IP3. This study seeks to elucidate the role of PIP2-PLC signaling in modulating channel function in On bipolar cells.
Patch-clamp recordings were performed in rod bipolar cells in acute mouse retinal slices. Pharmacological simulation of dark and light conditions was used to mimic light responses of rod bipolar cells under voltage clamp conditions. The simulated light response was elicited by displacing 4 µM bath-applied L-AP4 with 500 ms puffs of the mGluR antagonist LY351495 at a concentration of 500 µM. The PIP2-PLC pathway was manipulated using various drugs that were applied to cells through the patch pipette.
Application of the PLC inhibitor U73122 (10 µM) resulted in a decrease of the simulated light response by approximately 50%, while the inactive analog of this drug caused no change. To determine if accumulation of membrane PIP2 caused response run-down, we applied the non-hydrolyzable PIP2 analog diC8 PIP2. We saw no change in response amplitude, indicating that depletion of one or both of the PIP2 hydrolysis products rather than accumulation of PIP2, was responsible for the observed effect of the PLC inhibitor. Additionally, prevention of PIP2 synthesis using wortmannin, an inhibitor of PI4Kinase, caused run-down over time, supporting a mechanism by which slow depletion of DAG and/or IP3 cause a decrease in current.
Our data suggests PLC activity contributes to the regulation of the transduction channel function in rod bipolar cells, and that the effector of this regulation is downstream of PLC, perhaps either IP3 or DAG.
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