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M. Kamermans, C. Joselevitch, J. Klooster; Rod–driven light–responses in mixed–input bipolar cells . Invest. Ophthalmol. Vis. Sci. 2004;45(13):2198.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To describe the properties of the photoreceptor input to depolarizing mixed–input bipolar cells (dMBC) in the goldfish retina. This abstract focuses on the rod input. Methods: Whole cell patch–clamp recordings from mixed input depolarizing bipolar cells in goldfish retinal slices and immunohistochemistry. Results: Nawy and Copenhagen (1990) showed that in dMBC two conductances (one sensitive to DL–AP4 and one not) are modulated by glutamate. The result is that the glutamate–induced response does not have a reversal potential. At present is it generally believed that the rods drive the DL–AP4–sensitive conductance whereas the cones modulate a Cl– channel associated with a glutamate transporter, EAAT5. We found that light–induced responses of dMBCs in goldfish near rod threshold do not show a reversal potential. Bath application of DL–AP4 completely blocks these light responses. In the same cells, however, a local puff of DL–AP4 at the dendrites of the dMBC induces a current with a reversal potential around 0 mV, which is consistent with the work of Nawy and Copenhagen (1990). Our results indicate that a local puff of L–AP4 activates only part of the machinery involved in the generation of rod–driven light responses. Immunohistochemical data indicate that other metabotropic glutamate receptors are expressed on the dendrites of the bipolar cells and on the synaptic terminals of the rods. Also EAAT5, which is thought to mediate cone–driven light responses in these cells (Grant and Dowling, 1995), is found in bipolar cell dendrites invaginating in rods spherules. These results indicate that either rods modulate two separate DL–AP4–sensitive inputs on dMBCs or that one metabotropic glutamate receptor in these BCs modulates two targets. Conclusions: The rod input to dMBCs in goldfish retina does not drive a sole mechanism that modulates a conductance with a reversal potential around 0 mV. Additional mechanisms are present such that the light responses of the dMBC do not have a reversal potential. As a consequence, the amplitude of the rod driven light responses becomes almost independent of the membrane potential of the dMBC. This might be an advantage for near threshold responses, when variations in membrane potential of the dMBC could induce a large variability in the amplitude of rod–driven light responses.
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