Our laboratory studies phototransduction, the molecular mechanism by which illumination is detected by the light-sensitive outer segments of photoreceptors. Increases in illumination cause hyperpolarization of rods and cones, decreases cause depolarization, and constant illumination leads to adaptation. Photoreceptors need to respond quickly to changes in illumination. Consequently, they have high metabolic rates, and mutations that affect phototransduction enzymes often have dramatic effects on the viability of photoreceptors. In rod photoreceptors light is absorbed by the visual pigment, rhodopsin, within the outer segment.
Photoactivated rhodopsin stimulates a G protein, transducin.
This closes cGMP-gated cation channels in the plasma membrane, hyperpolarizes the photoreceptor, and slows glutamate release at the photoreceptor synaptic terminal. In this way, the rod detects light and that information is passed to the inner retina. Each step in this phototransduction pathway is associated with a biochemical mechanism that inactivates it. For example, rhodopsin kinase inactivates photoactivated rhodopsin by phosphorylating it, and transducin inactivates itself by hydrolyzing the GTP bound to it. Although rod photoreceptors have been studied extensively, less is known about the molecular basis of cone photoreceptor function.