Fig. 4. Opening TRPV4 enhances the spontaneous firing in parasol ganglion cells.
a to f show an RGC, which was recorded for action potentials under loose-patch mode (c and d) and for light-evoked currents under voltage-clamp mode (e and f) from a flat mount retina. The cell was filled with neurobiotin during recording. Confocal micrographs (a and b) morphologically identify the cell as an ON parasol cell. The x-y view (a) and y-z view (b) of the 3D reconstructed cell images reveal a soma of 25 μm in diameter and a dendritic arbor of 254 × 218 μm ramified round 65% of the IPL depth. Current responses evoked by the light steps of a duration of 2.5 s reverse near −15 mV (e and f) and are inward cation currents at ECl (−61 mV), and the light-evoked current (e) was enhanced by 250 μM TBOA (a glutamate transporter inhibitor) after 2 minutes of bath application of the drug and fully abolished after 8 minutes, which indicate that the activities are dominated by ON bipolar cell inputs. TRPV4 agonists 4αPDD 0.4 μM (c and g) and GSK 0.4 μM (d and g) applied in the bath show similar effects on RGCs (g), which significantly and reversibly increase the spontaneous firing rate (g, n = 5 experiments/cells, two-tail t-test, p < 0.001 for both drugs). In a and b, the arrow shows the axon and scale bars are 20 μm. Vh-holding potential