TRPM1: a vertebrate TRP channel responsible for retinal ON bipolar function

The transient receptor potential (TRP) channels affect essential functions widely in sensory systems of various species, both invertebrates and vertebrates. The channel protein encoded by the trp gene, the first identified TRP superfamily molecule, is known to mediate the Drosophila light response. A vertebrate TRP channel playing a crucial role in the visual system has not yet been discovered, although numerous studies have revealed primal functions of TRP superfamily molecules in various sensory systems other than vision. In the retina, which is the entry tissue in the vertebrate visual pathway, the transduction cation channel in ON bipolar cells has been elusive, despite intensive investigation by many researchers over a long period of time. Recent studies finally revealed that TRPM1, the first member of the melanoma-related transient receptor potential (TRPM) subfamily to be discovered, is a visual transduction channel in retinal ON bipolar cells. This review covers the significant discoveries on the physiological function and regulatory mechanism of the TRPM1 channel in retinal ON bipolar cells and the association of human TRPM1 mutations with congenital stationary night blindness.