RECENT CHEMICAL STUDIES RELATED TO VISION

Vision is initiated through absorption of electromagnetic radiation (photons) in a specific energy range (visual spectrum) by specialized neurons (photoreceptor cells) in the retina. Here these cells produce a special photosensitive protein (visual pigment), consisting of a membrane-bound apoprotein (opsin) with a covalently linked (protonated Schiff base) chromophoric group (Vitamin A aldehyde or retinal, in the 11-cis geometry). The photoreceptor cell transforms the photon flux into a graded membrane potential (receptor potential). The latter functions to signal neuronal circuitry in the retina, which initiates processing and integration of the incoming signals before they are transferred to ganglion cells and transmitted to the brain. The photoreceptor cells contain complex biochemical machinery to receive, transduce and amplify the light signal into the receptor potential. This machinery is compartmentalized in a highly specialized cell-segment, the outer segment in vertebrates or the rhabdomere in invertebrates, which is fully stacked with photoreceptormembranes, harbouring the visual pigment. Over the last four decades, worldwide research into the biochemical mechanism producing the receptor potential has already revealed many fascinating aspects thanks to a very multidisciplinary approach and collaboration, employing biophysical, biochemical, cell-biological and chemical techniques. These have mainly focussed on the vertebrate rod photoreceptor cell, since this cell and its visual pigment rhodopsin are most easily accessible in relatively large amounts. To date, globally, the following scheme has been disclosed. Upon light absorption the chromophore of rhodopsin isomerizes from ll-cis to all-trans, which induces a cascade of comformational changes in the protein, eventually releasing all-trans retinal (photolysis). This photolytic cascade is characterized by several discrete, spectrally defined, intermediate stages (photointermediates), one of which (Rh*) exposes specific sites (signal expression) to be recognized by the next element in the transduction chain, a GDPiGTP-binding protein (G-protein or transducin; T). The resulting Rh*-T complex catalyzes GDP-GTP exchange on the T,-subunit, which is thereupon released from the complex, finds its way to a c-GMP phosphodiesterase (PDE) and activates the latter close to a diffusion-limited maximal activity by interacting with and dissociating or displacing the inhibitory PDE,-subunit from the catalytic PDE,,-subunits. The resulting local drop in c-GMP concentration leads to a prolonged nonconductive state of c-GMP sensitive cation channels in the plasma membrane, which are selective for Na+ and CaZ+ ions. Consequently, the inward Na’ current (’dark current’) is reduced and the corresponding hyperpolarization of the cell produces the receptor potential. The entire process takes only several hundred ms at very low light intensities, down to several tens of ms a saturating light levels. Furthermore, it produces very high amplification factors: one Rh* can reduce the dark current (usually 1&30 PA) by several percent. Densensitization mechanisms have been less detailed as yet. At the receptor level they include decay of Rh” into opsin and all-trans retinal, as well as phosphorylation of Rh* by a special kinase, which allows binding of a specific 48 kDa protein (S-antigen or arrestin) thereby inhibiting binding and further activation of T. At the transductor and effector levels they involve inactivation of GTP-T, by the intrinsic GTP-ase activity, modulation of PDE,, activity by allosterically bound c-GMP and by other PDE,,, entities as well as supplementation of c-GMP from various binding sites and by action of a guanylate cyclase. In the invertebrates several aspects are less well established yet, but their vision acts on the same principle, except that the effector enzyme probably is a phospholipase C producing an increase in inositolphosphates, which release Ca” ions from intracellular stores. The invertebrate photoreceptor also behaves more like the usual neuron, as its photoresponse originates in depolarization due to an increase in cell membrane permeability. In vertebrates, regeneration of photosensitive pigment is a multi-cellular affair. The released all-trans retinal is reduced by a specific oxidoreductase to all-trans retinol (vitamin A), which migrates to the retinal pigment epithelium, where it is stored as the ester. Eventually it is recycled to 11-cis retinal to recombine with opsin in the outer segment. For interand intra-cellular transport of these retinoids specific binding-proteins are present. Renewal of the whole system occurs by continuous biosynthesis of the outer segment components followed by photoreceptormembrane biogenesis at the base of the outer segment. In order to maintain a relatively constant length, the growth at the base is compensated for by removal of the outer segment tip in a circadian pattern through phagocytosis and internal degradation by the retinal pigment epithelium cells. In invertebrates, these processes may be all con-