Flash photolysis of rhodopsin in the cat retina

Rhodopsin, light-sensor of vision

The light sensor of vertebrate scotopic (low-light) vision, rhodopsin, is a G-protein-coupled receptor comprising a polypeptide chain with bound chromophore, 11-cis-retinal, that exhibits remarkable physicochemical properties. This photopigment is extremely stable in the dark, yet its chromophore isomerises upon photon absorption with 70% efficiency, enabling the activation of its G-protein, transducin, with high efficiency. Rhodopsin's photochemical and biochemical activities occur over very different time-scales: the energy of retinaldehyde's excited state is stored in <1 ps in retinal-protein interactions, but it takes milliseconds for the catalytically active state to form, and many tens of minutes for the resting state to be restored. In this review, we describe the properties of rhodopsin and its role in rod phototransduction. We first introduce rhodopsin's gross structural features, its evolution, and the basic mechanisms of its activation. We then discuss light absorption and spectral sensitivity, photoreceptor electrical responses that result from the activity of individual rhodopsin molecules, and recovery of rhodopsin and the visual system from intense bleaching exposures. We then provide a detailed examination of rhodopsin's molecular structure and function, first in its dark state, and then in the active Meta states that govern its interactions with transducin, rhodopsin kinase and arrestin. While it is clear that rhodopsin's molecular properties are exquisitely honed for phototransduction, from starlight to dawn/dusk intensity levels, our understanding of how its molecular interactions determine the properties of scotopic vision remains incomplete. We describe potential future directions of research, and outline several major problems that remain to be solved.

Rhodopsin regeneration in the normal and in the detached/replaced retina of the skate

The bleaching and regeneration of rhodopsin in the skate retina was studied by means of fundus reflectometry, both in the normal eyecup preparation and after the retina had been detached and then replaced on the surface of the pigment epithelium (RPE). After bleaching virtually all the rhodopsin in the retinal test area of the normal eyecup, more than 90% of the photopigment was reformed after about 2 hr in darkness; over most of this time course, rhodopsin density rose linearly at a rate of 0.875% min-1 with a half-time of 55 min. Detaching the retina from its pigment epithelium resulted in a number of abnormalities, both structural and functional. Histological examination of the detached/replaced (D/R) retina showed striking alterations in the structural integrity of the RPE cells at their interface with the neural retina. The cells appeared vacuolated and misshapen, and the apical processes of the RPE, which normally ensheath the receptor outer segments, were shredded and free of their association with the visual cells. These morphological changes, as well as dilution of the IRBP content of the subretinal space caused by separation of the tissues, appear to be the main factors contributing to the functional abnormalities in rhodopsin kinetics. But despite these abnormalities and the persistent detachment, the rate of regeneration and the amount of rhodopsin reformed after bleaching were reduced by less than 50% of their normal values. The fact that a significant fraction of the bleached rhodopsin was regenerated under these conditions indicates that 11-cis retinal formed in the RPE was able to traverse a much greater than normal subretinal space to reach the opsin-bearing photoreceptor membranes.

Experimental stray light in retinal densitometry

The effects of stray light upon retinal pigment densitometric measurements were evaluated by adding specified amounts of stray light to fundus reflections. To allow for interpolations and extrapolations, a computer simulation was devised and validated against the empirical data. The results demonstrate that measured density is greatly decreased by increases in stray light but the time constant of pigment regeneration is little affected.

Rhodopsin levels and retinal function in cats during recovery from vitamin A deficiency

Extended vitamin A deficiency in the cat led to an abnormal appearance in the tapetal fundus with the formation of a dark brown streak centered on the area centralis. At this time rod sensitivity, as measured by the b-wave of the electroretinogram, was reduced by more than two log units; the level of rod visual pigment was reduced by about 90% throughout the paracentral retinal region and was essentially absent from the area centralis. Following oral supplementation with vitamin A there was a rapid partial recovery of both rhodopsin levels and rod sensitivity. Further recovery continued over more than 18 days to levels that were not substantially below normal. This recovery was absent from the area centralis, in which measured visual pigment levels remained very low. In supplemented cats, the brown color in the fundus faded but there remained a small hyper-reflective zone at the area centralis. Morphological examination of the central retina in a supplemented cat showed an outer nuclear layer reduced to one or two rows in the small zone with low rhodopsin levels. Cone but not rod photoreceptors were present in this zone and they appeared to lack outer segments. During recovery, the increase in rod sensitivity was approximately linearly related to the recovery of rhodopsin levels. Thus, in these conditions reduction in sensitivity resulting from previous vitamin A deficiency was limited by the ability of the photoreceptors to absorb incident quanta. The time course of the recovery of rhodopsin and sensitivity suggests that at least two processes were involved. The faster of these may be the regeneration of rhodopsin from existing opsin molecules in the outer segments, while the slower may depend on the renewal of the outer segments themselves.

Variable expressivity in fundus albipunctatus

A healthy, 14-year-old girl presented with nyctalopia, good vision, and multiple, irregular, yellowish lesions of the fundus. Dark adaptometry showed prolonged cone and rod branches, elevated thresholds, and the cone-rod transition occurring after 50 minutes in darkness. Her scotopic electroretinogram (ERG) b-wave attained normal amplitudes after 45 minutes of dark adaptation. The half-time for regeneration of rhodopsin after an extensive bleach was 16 minutes, four times longer than normal, and the maximum density difference measured by fundus reflectometry was at the lower limit of the normal range. Although photopigment kinetics were significantly faster than observed in other reported cases of fundus albipunctatus, it appears likely that there is a wide spectrum of functional and funduscopic abnormalities in this disorder. However, fundus appearance, adaptometric findings, and rhodopsin determinations serve to distinguish fundus albipunctatus from other flecked retina diseases.

Rhodopsin topography and rod-mediated function in cats with the retinal degeneration of taurine deficiency

Cats on a taurine-deficient diet were studied with imaging fundus reflectometry and full-field electroretinography. The pattern of rhodopsin loss and the natural history of the disease were determined from maps of the rhodopsin distribution in the central and nasal retina of cats with different degrees of severity of the retinopathy. Rhodopsin loss is first detectable in a focal region of the central retina. Subsequently, there are decreases in rhodopsin in the paracentral and nasal midperipheral retina. The horizontal streak of high rhodopsin levels is preferentially reduced in this retinopathy. The b-wave amplitude of the rod-dominated ERG is markedly reduced in cats with only mildly decreased levels of rhodopsin in the peripheral retina. In an affected cat with moderate rhodopsin loss in the central retina but minimal loss nasally, a light-microscopic study of the retina showed that there was disorganization and shortening of rod outer segments and loss of rod photoreceptor cells in the areas of reduced rhodopsin levels.

Human rhodopsin measurement using a T.V.-based imaging fundus reflectometer

An imaging fundus reflectometer for in vivo mapping of rhodopsin levels is described. The instrument is based on a high-sensitivity television camera attached to a Zeiss fundus camera, which enables areas of retina of angular subtense 25 degrees to be examined at a resolution of about 1 degree. Digital processing techniques are used to average the video signal spatially and temporally and to analyse the spatial information. Measurements with an artificial eye indicate that performance is comparable to that of photomultiplier-based systems. Rhodopsin levels and regeneration data for a normal human subject are presented; these are consistent with published values. The map of visual pigment levels derived from these normal data is contrasted with that for a subject with a patchy retinal dysfunction (autosomal dominant retinitis pigmentosa).

Densitometric measurement of human cone photopigment kinetics

We measured the radiance response function for steady state bleaching lights and the regeneration of the cone visual photopigments using the continuous recording densitometer described by v. Norren and v.d. Kraats. Measurements made on 5 deuteranopes, 1 protanope and 2 color-normal observers were similar. The radiance response function was steeper than the function predicted by a simple first-order kinetic equation. For a measured density (ca 0.3) we evaluated whether high stray light (ca 47.5%) and high two-way optical density (ca 1.3) could account for the deviation from the prediction of a first-order equation. Such a model was rejected because these parameters predicted different estimates of the time course of regeneration for different test wavelengths (554 and 605 nm). Statistical analysis of the regeneration data revealed a highly significant non-linearity. A model in which the rate of regeneration increases as the proportion of bleached photopigment decreases is required to explain both the radiance function and the regeneration data.

The products of photoreversing rhodopsin bleaching by microsecond flashes in the isolated vertebrate retina

Bleaching experiments were carried out at room temperature on rhodopsin in isolated rat, frog and rat retinas with blue and orange laser flashes of very high energy and duration less than 3 microseconds. Blue flashes bleached a maximum of about 50% at the highest energies; orange ones bleached about 30% at intermediate energies but the value decreased to below 20% as the energy increased. This bleaching behaviour can be explained in terms of a kinetic model which assumes that bleaching is photoreversed during the flashes and which incorporates the relevant properties of rhodopsin, isorhodopsin, bathorhodopsin and lumirhodopsin.

Rhodopsin kinetics in the cat retina

The bleaching and regeneration of rhodopsin in the living cat retina was studied by means of fundus reflectometry. Bleaching was effected by continuous light exposures of 1 min or 20 min, and the changes in retinal absorbance were measured at 29 wavelengths. For all of the conditions studied (fractional bleaches of from 65 to 100%), the regeneration of rhodopsin to its prebleach levels required greater than 60 min in darkness. After the 1-min exposures, the difference spectra recorded during the first 10 min of dark adaptation were dominated by photoproduct absorption, and rhodopsin regeneration kinetics were obscured by these intermediate processes. Extending the bleaching duration to 20 min gave the products of photolysis an opportunity to dissipate, and it was possible to follow the regenerative process over its full time-course. It was not possible, however, to fit these data with the simple exponential function predicted by first-order reaction kinetics. Other possible mechanisms were considered and are presented in the text. Nevertheless, the kinetics of regeneration compared favorably with the temporal changes in log sensitivity determined electrophysiologically by other investigators. Based on the bleaching curve for cat rhodopsin, the photosensitivity was determined and found to approximate closely the value obtained for human rhodopsin; i.e., the energy Ec required to bleach 1-e-1 of the available rhodopsin was 7.09 log scotopic troland-seconds (corrected for the optics of the cat eye), as compared with approximately 7.0 in man.