Abstract
Visual pigment proteins belong to the superfamily of G protein-coupled receptors and are the light-sensitive molecules in rod and cone photoreceptor cells. The protein moiety is known as opsin and the ligand in the dark is 11-cis retinal, which serves as both the photon detector and an inverse agonist. While much is known about properties of the rod pigment rhodopsin, much less is understood about cone visual pigments. Being able to identify ligands that effect opsins give an insight into structure-activity relationships. The action of some ligands indicates that there are differences between not only rod and cone opsins but also among the different classes of cone opsins. Furthermore, inverse agonists of cone opsins may have potential therapeutic uses under conditions when the native 11-cis retinal ligand is absent. A method for determining the effects of ligands on rod and cone opsin activity is described.