What causes cation channels in the photoreceptor cells to close during the light phase of phototransduction?

In the cone pathway glutamate: ... Glutamate that is released from the photoreceptors in the dark binds to metabotropic glutamate receptors (mGluR6), which, through a G-protein coupling mechanism, causes non-specific cation channels in the cells to close, thus hyperpolarizing the bipolar cell.

Explanation:

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