The expression and function of TRPV4 channels in primate retinal ganglion cells and bipolar cells

The transient receptor potential vanilloid 4 (TRPV4) channel may be opened by mechanical stimuli to mediate Ca2+ and Na+ influxes, and it has been suggested to mediate glaucoma retinopathy. However, it has been mostly unclear how TRPV4 activities affect the function of primate retinal ganglion cells (RGCs). We studied RGCs and bipolar cells (BCs) in the peripheral retina of the old-world primate using whole-cell current-clamp and voltage-clamp recordings, immunomarkers and confocal microscopy. RGCs were distinguished from displaced amacrine cells (ACs) by the absence of GABA and glycine immunoreactivity and possession of an axon and a large soma in the RGC layer. Strong TRPV4 signal was concentrated in medium to large somas of RGCs, and some TRPV4 signal was found in BCs (including PKC alpha-positive rod BCs), as well as the end feet, soma and outer processes of Muller cells. TRPV4 immunoreactivity quantified by the pixel intensity histogram revealed a high-intensity component for the plexiform layers, a low-intensity component for the soma layers of ACs and Muller cells, and both components in the soma layers of RGCs and BCs. In large RGCs, TRPV4 agonists 4 alpha-phorbol 12,13 didecanoate (4 alpha PDD) and GSK1016790A reversibly enhanced the spontaneous firing and shortened the delay of voltage-gated Na+ (Nav) currents under current-clamp conditions, and under voltage-clamp conditions, 4 alpha PDD largely reversibly increased the amplitude and frequency of spontaneous excitatory postsynaptic currents. In BCs, changes in the membrane tension induced by either applying pressure or releasing the pressure both activated a transient cation current, which reversed at similar to -10 mV and was enhanced by heating from 24 degrees C to 30 degrees C. The pressure for the half-maximal effect was similar to 18 mmHg. These data indicate that functional TRPV4 channels are variably expressed in primate RGCs and BCs, possibly contributing to pressure-related changes in RGCs in glaucoma.