Cover Photograph: Seeing near visual threshold requires optimizing the signal-to-noise ratio of single-photon responses. Most mammalian species exhibit exquisite night vision that is initiated by G-protein coupled signaling cascades in both the retinal rod photoreceptors and the second order rod bipolar cells. In this issue (pages 438–447), Pahlberg and Sampath describe how these and other signaling mechanisms set the threshold for seeing. In rods, the absorption of a photon by rhodopsin leads to an amplifying cascade of events that alters the membrane potential, thereby reducing glutamate release onto the dendrites of rod bipolar cells. Rod bipolar cells sense glutamate release from rods using metabotropic glutamate receptors, mGluR6, whose activity leads to the closure of TRPM1 cation channels. The properties of these signaling cascades are crucial for setting high visual sensitivity, the rod phototransduction cascade amplifying the single-photon response and the rod bipolar signaling cascade suppressing noise from rods not absorbing light.
Cover by J. Pahlberg and A. P. Sampath