A study of the effects of bleaching on the width and index of refraction of frog rod outer segments

Optical Assessment of Photoreceptor Function Over the Macula

Purpose

The purpose of this study was to develop next-generation functional photoreceptor imaging using ultrahigh-speed swept-source optical coherence tomography (UHS-SS-OCT) and split-spectrum amplitude-decorrelation optoretinography (SSADOR) algorithm. The advancement enables rapid surveying of large retinal areas, promising non-contact, objective, and quantifiable measurements of macular visual function.

Methods

We designed and built a UHS-SS-OCT prototype instrument using a wavelength tunable laser with 1 MHz A-scan rate. The functional scanning protocol records 5 repeated volumes in 3 seconds. A flash pattern selectively exposes the imaged retina area. SSADOR quantifies photoreceptor light response by extracting optical coherence tomography (OCT) signal changes within the photoreceptor outer segment before and after the flash.

Results

The study prospectively enrolled 16 eyes from 8 subjects, demonstrating the ability to measure photoreceptor light response over a record field of view (3 × 3 mm2) with high topographical resolution (approximately 100 µm). The measured SSADOR signal corresponds to the flashed pattern, whose amplitude also correlates with flash strength, showing consistency and reproducibility across subjects.

Conclusions

The integration of high-performance UHS-SS-OCT and SSADOR enables characterizing photoreceptor function over a clinically meaningful field of view, while maintaining a workflow that can be integrated into routine clinical tests and trials. The new approach allows detecting changes in photoreceptor light response with high sensitivity and can detect small focal impairments.

Translational Relevance

This innovative advance can enable us to detect early photoreceptor abnormalities, as well as help to stage and monitor degenerative retinal diseases, potentially providing a surrogate visual function marker for retinal diseases and accelerating therapeutic development through a safe and efficient outcome endpoint.

In vivo functional imaging of human cone photoreceptors

We evaluate a novel non-invasive optical technique for observing fast physiological processes, in particular phototransduction, in single photoreceptor cells in the living human eye. The method takes advantage of the interference of multiple reflections within the outer segments (OS) of cones. This self-interference phenomenon is highly sensitive to phase changes such as those caused by variations in refractive index and scatter within the photoreceptor cell. A high-speed (192 Hz) flood-illumination retina camera equipped with adaptive optics (AO) is used to observe individual photoreceptors, and to monitor changes in their reflectance in response to visible stimuli ("scintillation"). AO and high frame rates are necessary for resolving individual cones and their fast temporal dynamics, respectively. Scintillation initiates within 5 to 10 ms after the onset of the stimulus flash, lasts 300 to 400 ms, is observed at visible and near-infrared (NIR) wavelengths, and is highly sensitive to the coherence length of the imaging light source. To our knowledge this is the first demonstration of in vivo optical imaging of the fast physiological processes that accompany phototransduction in individual photoreceptors.

Light-induced infrared light scattering signal in the retina: effect of phosphodiesterase inhibitors

Visible light changes IR light scatter through a toad retina. This signal presents three components: at low light intensity (100-400 bleached rhodopsins/rod) an early decrease in IR light scatter, of small amplitude, with time to peak of 1-6 s; at intermediate light intensity (1200-16,000 bleached rhodopsins/rod) a slow increase in IR light scatter, with time to peak of 10-30 s; at high light intensity (50,000-160,000 bleached rhodopsins/rod) a last increase in IR light scatter, with time to peak of 1 min. Light sensitivity, amplitude and time to peak of the last two components are increased by inhibitors (3-isobutyl-1-methyl-xanthine and papaverine) of the cyclic 3'5' guanosine monophosphate phosphodiesterase.

Structural features of the terminal loop region of frog retinal rod outer segment disk membranes: I. Organization of lipid components

We have applied thin sectioning and freeze-fracture techniques to investigate the terminal loop structure of photoreceptive disks in frog retinal rod outer segments. Our studies of this region demonstrate a highly curved terminal loop bilayer that is continuous with both lamellar bilayers of the disk, and equivalent to them in dimensions and staining properties. Rhodopsin, however, appears to be excluded from this region of high curvature.

Vertebrate receptor optics and orientation

Vertebrate photoreceptors act as optical waveguides. They exhibit directionality, non-uniform distribution of energy within and immediately about the receptor, etc. Photolabile pigment absorption favors light traveling axially down the receptor. Clearly these properties influence response of the transducer. Retinal receptors, rods and cones, are aligned normally with a point approximating the center of the exit pupil of the eye. Taken together, these findings suggest that a prime role of receptor optics is to favor acceptance of the pertinent visual stimulus passing through the pupillary aperture and to inhibit stray light noise contained in the integrating sphere-like eyes. Many intriguing problems remain to be resolved. It is necessary to relate the properties of the receptor as a waveguide to determinations of directional sensitivity of the retina (Stiles-Crawford effects), and in particular, to understand mechanisms leading to fine receptor alignment. There must also be a pathology of receptor orientation--a science still in its infancy.

The carbohydrates in frog retinal rod outer segments

Frog retinal rod outer segments appear to contain uncharacterized chemical components whose mass is roughly equivalent to 12--51% of the rhodopsin mass. Available data suggest that such components include soluble proteins and complex polysaccharides, and that hyaluronic acid accounts for a substantial fraction of this mass. Electron microscopic histochemical staining studies suggest that these polysaccharide components are located within the ROS disks. The oligosaccharide moieties of rhodopsin also appear localized within the disks. The interdisk cytoplasm may contain carbohydrates, but their quantity and identity are uncertain. Rhodopsin oligosaccharides as well as some fraction of the intradisk polysaccharide appear to have extended saccharide chains preferentially oriented perpendicular to the surface of the disk membrane. Possible roles for these polysaccharides in disk development and photoexcitation are discussed. The immediate need for complete rod outer segment chemical composition data is emphasized.

Use of the waveguide parameter V to determine the difference in the index of refraction between the rat rod outer segment and the interstitial matrix

The difference in the index of refraction between the receptor outer segment and the interstitial matrix has been determined to be 0.06 by making use of the waveguide properties of rat rod photoreceptors. This was done by determining the cutoff wavelength for the transition between the bilobe or annulus modal patterns and the single-lobe H E 11 modal pattern, and by measuring the diameter of the transmitting cell. These measurements were made on bleached receptors. Enoch, Scandrett, and Tobey had previously shown that bleaching only slightly alters the outer-segment index of refraction and diameter in the frog. Assuming interspecies transfer, and using Sidman's prior estimate of bleached rat rod outer-segment index of refraction, then the index of refraction of the interstitial matrix of the albino rat is estimated to be 1.3476. This estimate was made using two different media in contact with the retinal preparation, a miscible mountant tissue culture medium believed to be compatible with the rat retina, and an encapsulating nonmiscible silicone oil which allows only ocular media from the same eye in contact with the outer segments.