mGluR6 is metabotropic glutamate receptor subtype 6, a receptor protein involved in transmission from photoreceptors to ON retinal bipolar cells. Reduced glutamate release in light is coupled through mGluR6 to a depolarizing response. The receptor and the downstream TRPM1 channel are distinct components.[ref]
TRPM1 is required for the depolarizing light response in retinal ON-bipolar cells by Catherine W. Morgans and others. Abstract: receptor-controlled depolarizing current
A 1995 gene knockout experiment tested that role directly. Mice lacking mGluR6 lost the measured ON responses while retaining OFF responses to light. Gross retinal organization and projections showed no obvious changes; light-conditioned avoidance behaviour remained.[ref]
The preserved OFF response is an illuminating internal comparison: failure of one transmission route does not establish failure of all visual signalling. Likewise, detecting an intact layer of neurons does not establish that its molecular signalling machinery works. The 2009 TRPM1 experiments later helped distinguish the receptor stage from the channel that carries the depolarizing current.[ref]
TRPM1 is required for the depolarizing light response in retinal ON-bipolar cells by Catherine W. Morgans and others. Figures 3–4: electroretinography and bipolar-cell recordings
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