Metarhodopsin

Metarhodopsin is a photochemical intermediate in the visual system that forms when light activates the visual pigment rhodopsin. In vertebrate rods, absorption of a photon triggers isomerization of 11-cis-retinal to all-trans-retinal, yielding metarhodopsin II, the active signaling state. Metarhodopsin II activates the heterotrimeric G protein transducin, which then stimulates phosphodiesterase to lower cGMP levels, causing closure of cGMP-gated ion channels and hyperpolarization of the cell.

Metarhodopsin II is unstable and decays through intermediate species, including metarhodopsin I, while the all-trans retinal is released from opsin. The released retinal is reduced to all-trans retinol and transported to the retinal pigment epithelium, where it is recycled back to 11-cis-retinal. The 11-cis-retinal recombines with opsin to regenerate rhodopsin, completing the visual cycle. Additional transient photoproducts such as bathorhodopsin and lumirhodopsin are part of the early photochemical sequence before metarhodopsin I and II form.

In invertebrates, the term metarhodopsin is also used to describe the photoproduct of rhodopsin; in many insect photoreceptors, it functions as part of the signaling state in the phototransduction cascade. While the exact mechanisms and spectral properties differ from vertebrates, the core concept remains that light converts rhodopsin into an active intermediate that is subsequently deactivated and regenerated.