An essential step in intricate visual processing is the segregation of visual signals into ON and OFF pathways by retinal bipolar cells (BCs). Glutamate released from photoreceptors modulates the photoresponse of ON BCs via metabotropic glutamate receptor 6 (mGluR6) and G protein (Go) that regulates a cation channel. However, the cation channel has not yet been unequivocally identified. Here, we report a mouse TRPM1 long form (TRPM1-L) as the cation channel. We found that TRPM1-L localization is developmentally restricted to the dendritic tips of ON BCs in colocalization with mGluR6. TRPM1 null mutant mice completely lose the photoresponse of ON BCs but not that of OFF BCs. In the TRPM1-L-expressing cells, TRPM1-L functions as a constitutively active nonselective cation channel and its activity is negatively regulated by Go in the mGluR6 cascade. These results demonstrate that TRPM1-L is a component of the ON BC transduction channel downstream of mGluR6 in ON BCs.egregation of visual signals into ON and OFF pathways originates in BCs, the second-order neurons in the retina (1, 2). ON and OFF BCs express metabotropic glutamate receptors, mGluR6, and ionotropic glutamate receptors (iGluRs), respectively, on their dendrites (3-5). Reduction of glutamate released from photoreceptors by light stimulation depolarizes ON BCs and hyperpolarizes OFF BCs (6-8) mediated through respective glutamate receptors. The mGluR6 couples to a heterotrimeric G protein complex, Go (9, 10). Signals require Goα, which ultimately closes a downstream nonselective cation channel in ON BCs (6, 9, 11-13). However, this transduction cation channel in ON BCs has not been identified, despite intensive investigation.In our screen to identify functionally important molecules in the retina, we found that TRPM1 is predominantly expressed in retinal BCs. Most members of the TRP superfamily, which are found in a variety of sense organs, are non-voltage-gated cation channels (14-16). The founding member of the TRP family was discovered as a key component of the light response in Drosophila photoreceptors (17). TRPM1, also known as melastatin, was the first member of the melanoma-related transient receptor potential (TRPM) subfamily to be discovered (18,19). TRPM1 is alternatively spliced, resulting in the production of a long form (TRPM1-L) and a short N-terminal form devoid of transmembrane segments (TRPM1-S) (18,20). Although mouse TRPM1-S was previously identified as melastatin, mouse TRPM1-L has not been identified (18). The distinct physiological and biological functions of TRPM1 still remain elusive, although some recent evidences including us suggested that TRPM1 might contribute to retinal BC function (21-23). Here, we show that TRPM1-L is the transduction cation channel of retinal ON BCs in the downstream of mGluR6 cascade.

Reliable synaptic transmission depends not only on the release machinery and the postsynaptic response mechanism but also on removal or degradation of transmitter from the synaptic cleft. Accumulating evidence indicates that postsynaptic and glial excitatory amino acid transporters (EAATs) contribute to glutamate removal. However, the role of presynaptic EAATs is unclear. Here, we show in the mouse retina that glutamate is removed from the synaptic cleft at the rod to rod bipolar cell (RBC) synapse by presynaptic EAATs rather than by postsynaptic or glial EAATs. The RBC currents evoked by electrical stimulation of rods decayed slowly after pharmacological blockade of EAATs. Recordings of the evoked RBC currents from EAAT subtype-deficient mice and the EAAT-coupled anion current reveal that functional EAATs are localized to rod terminals. Model simulations suggest that rod EAATs are densely packed near the release site and that rods are equipped with an almost self-sufficient glutamate recollecting system.

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