rhodopsine

Rhodopsine, or rhodopsin, is the light-sensitive pigment in the retinal rod cells of many vertebrates. In vertebrates it is a G protein-coupled receptor composed of an opsin protein bound to the chromophore 11-cis-retinal, embedded in the disc membranes of the rod outer segments. When a photon is absorbed, the 11-cis-retinal isomerizes to all-trans-retinal, activating the pigment.

The active pigment, rhodopsin (R*), then activates the G protein transducin, which stimulates a phosphodiesterase that decreases intracellular cGMP. The drop in cGMP closes cGMP-gated ion channels, causing the rod cell to hyperpolarize and reduce neurotransmitter release. This creates a signal transmitted to bipolar and other retinal neurons.

Rhodopsin has peak sensitivity near 500 nm and supports scotopic, or low-light, vision. It is less useful for color perception, which occurs in cone pathways. The pigment is regenerated in the retinal pigment epithelium, where all-trans-retinal is converted back to 11-cis-retinal; a supply of vitamin A is essential. Vitamin A deficiency can cause night blindness.

Rhodopsine is widespread in animals; vertebrates rely mainly on rod rhodopsin for low-light vision, while cones express different opsins for color. The study of rhodopsin photochemistry and signaling helped establish the framework of GPCR signaling and visual transduction, a landmark in neuroscience.