Light-sensitive cation channel in retinal ON bipolar cells (TRPM1)

The light-sensitive cation channel in retinal bipolar cells, specifically identified as TRPM1 (transient receptor potential melastatin 1), is a key ion channel expressed in the dendrites of ON bipolar cells in the retina. It opens in response to light-induced reduction in glutamate release from photoreceptors, allowing influx of cations such as sodium and calcium, which depolarizes the ON bipolar cell.[1][2][3] This channel is regulated by the metabotropic glutamate receptor mGluR6 (GRM6) via a G-protein signaling cascade: in darkness, glutamate activates mGluR6, closing the channel through G-protein inhibition, hyperpolarizing the cell; light decreases glutamate, reopening the channel for depolarization.[3][5] TRPM1 thus inverts the photoreceptor's hyperpolarizing light response into a depolarizing signal in ON bipolar cells, enabling parallel ON and OFF pathways for contrast detection and visual information processing.[1][2][6] Mutations in TRPM1 cause complete congenital stationary night blindness (cCSNB), where the ON pathway fails, impairing vision in low light without progressive degeneration.[3] No approved drugs directly target this channel, but its role in visual transduction positions it as a potential therapeutic target for retinal disorders, though challenges include specificity to avoid disrupting retinal circuitry.[3] The channel's structure features a non-selective permeability to monovalent and divalent cations, contributing to graded postsynaptic potentials without action potentials in bipolar cells.[1][5] It integrates with horizontal and amacrine cell feedback for center-surround receptive fields, enhancing spatial contrast.[2][3]

Other names

mGluR6-associated cation channelON bipolar cell cation channel