Rodopsyna

Rodopsyna, also known as rhodopsin, is a light-sensitive visual pigment located in the rod photoreceptor cells of the vertebrate retina. It is a G protein-coupled receptor formed by the apoprotein opsin bound to the chromophore 11-cis-retinal through a Schiff base linkage. The chromophore absorbs a photon and photoisomerizes to all-trans-retinal, converting rhodopsin into its active form, metarhodopsin II (R*).

Activation of rhodopsin triggers a phototransduction cascade. Active rhodopsin stimulates the G protein transducin (Gt), which in turn activates phosphodiesterase 6 (PDE6). PDE6 lowers the intracellular concentration of cyclic GMP, causing closure of cGMP-gated ion channels. This reduces inward cation current, leading to hyperpolarization of the rod cell and a decrease in the release of the neurotransmitter glutamate at the synapse with bipolar cells, thereby signaling light detection.

Rhodopsin is regenerated through the visual cycle. All-trans-retinal is released from opsin and transported to the retinal pigment epithelium, where it is converted back to 11-cis-retinal and then recombined with opsin to reform rhodopsin, restoring light sensitivity.

Localization and significance. Rod opsin is highly concentrated in the rod outer segment discs, supporting high-sensitivity vision under dim lighting (scotopic vision). Mutations in the RHO gene can cause inherited retinal diseases such as retinitis pigmentosa and congenital stationary night blindness.

Discovery and impact. Rhodopsin played a central role in understanding visual photochemistry and GPCR signaling; research on its light-induced changes contributed to Nobel Prize–winning work on the visual system.