Processing and Integration Mechanisms of Visual Transmission in the Retinal Network
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Visual signals are transmitted in the retinal network through the principal pathway consisting of photoreceptors, bipolar cells, and ganglion cells. The key processing of visual transmission is segregation of light and dark signals into ON and OFF responses at the level of bipolar cells. This segregation is interesting because glutamate transmitter is used as a common transmitter but generates opposite ON and OFF responses, depending on bipolar cell types. We identified and characterized mGluR6 metabotropic glutamate receptor subtype in the retina. mGluR6 is selectively and exclusively expressed at postsynaptic sites of ON bipolar cells, and mGluR6 deficiency by gene targeting causes a loss of ON responses without affecting OFF responses. Two distinct types of glutamate receptors, metabotropic and ionotropic glutamate receptors, are thus effectively used for segregating light and dark signals into ON and OFF responses in the retinal network.