Nrl-knockout mice deficient in Rpe65 fail to synthesize 11-cis retinal and cone outer segments.

PURPOSE: To define rod and cone function further in terms of visual cycle mechanism, the retinal phenotype resulting from Rpe65 (retinoid isomerase I) deficiency in Nrl(-)(/)(-) mice having a single class of photoreceptors resembling wild-type cones was characterized and outcomes of retinoid supplementation evaluated. METHODS: Rpe65(-)(/)(-)/Nrl(-)(/)(-) mice were generated by breeding Rpe65(-)(/)(-) and Nrl(-)(/)(-) strains. Retinal histology, protein expression, retinoid content, and electroretinographic (ERG) responses were evaluated before and after treatment with 11-cis retinal by intraperitoneal injection. Results Retinas of young Rpe65(-)(/-)/Nrl(-)(/-) mice exhibited normal lamination, but lacked intact photoreceptor outer segments at all ages examined. Rpe65, Nrl, and rhodopsin were not detected, and S-opsin and M/L-opsin levels were reduced. Retinyl esters were the only retinoids present. In contrast, Nrl(-)(/)(-) mice exhibited decreased levels of retinaldehydes and retinyl esters, and elevated levels of retinols. ERG responses were elicited from Rpe65(-)(/-)/Nrl(-)(/-) mice only at the two highest intensities over a 4-log-unit range. Significant retinal thinning and outer nuclear layer loss occurred in Rpe65(-)(/-)/Nrl(-)(/-) mice with aging. Administration of exogenous 11-cis retinal did not rescue retinal morphology or markedly improve ERG responses. CONCLUSIONS: The findings provide clarification of reported cone loss of function in Rpe65(-)(/-)/Nrl(-)(/-) mice, now showing that chromophore absence results in destabilized cone outer segments and rapid retinal degeneration. The data support the view that rod-dominant retinas do not have a cone-specific mechanism for 11-cis retinal synthesis and have potential significance for therapeutic strategies for rescue of cone-rich retinal regions affected by disease in the aging human population.

Functional observations in vitamin A deficiency: diagnosis and time course of recovery.

AIMS: To describe the effects of vitamin A deficiency (VAD) on retinal function and the subsequent recovery following treatment in three patients with systemic conditions (two with Crohn disease; one secondary to IgE syndrome). METHODS: Electrophysiological testing (including pattern electroretinogram, PERG; electroretinogram, ERG; visual-evoked potential) established the diagnosis of VAD. Repeat testing was carried out in two patients to monitor the time course of recovery following intramuscular vitamin A injection. The third patient had repeat recordings following 13 months of oral supplementation. RESULTS: All three patients initially displayed a characteristic absence of rod function associated with VAD. In addition, delayed and reduced amplitude cone ERGs, loss of short wavelength cone (S-cone) function and subnormal macular function were observed in two patients. Restoration of rod and generalised cone function was rapid in the two patients who received intramuscular injection, with normalisation of some electrophysiological responses after only 3 days. Normal S-cone amplitudes and cone latencies were reached within 12 days of vitamin A injection. Macular function returned to within normal limits by 12 days postinjection in one patient, but remained mildly subnormal in the second patient. Full recovery was present after 13 months oral supplementation in the third patient. CONCLUSIONS: Novel observations regarding dark-adapted cone function, S-cone function, and PERG are presented. The differences between the effects of VAD on rod and cone function, and their rate of recovery, may reflect differences in the visual cycle between the two photoreceptor classes. The importance of rapidly and accurately diagnosing VAD, a treatable condition, is noted.

Disclosing disease mechanisms with a spatio-temporal summation paradigm.

BACKGROUND: We develop the logic for a stimulus that can evaluate cone-dependent spatial summation and detail the modelling and interpretation of thresholds obtained with this stimulus. METHODS: Fifteen observers participated, including two young normals tested extensively in control experiments, and a clinical trial based on four observers with age-related macular degeneration (AMD), four age-similar controls and five young observers. Monocular spatial summation functions were measured with contrast-modulated Gabor targets that approximated the optimal visual contrast detector. Thresholds were returned from a yes/no adaptive psychophysical algorithm. By fine titration along the size domain it was demonstrated that the spatial summation of normal observers can be adequately described by a two-component model. A reduced set of variables are proposed for clinical applications and the model was applied to data derived using these variables in persons with AMD and age-similar controls. RESULTS: We do not find a significant age-related loss of contrast sensitivity in our normal group. On the other hand, persons with early AMD exhibited a 0.41 log unit loss of sensitivity (P=0.04) from age-similar controls, without any change in their maximum summation area (A(max)). CONCLUSIONS: The nature of the spatial summation is consistent with the interpretation that early AMD produces a decrease in cone input to post-receptoral mechanisms in the absence of neural remodelling.

Inhibition of the visual cycle in vivo by 13-cis retinoic acid protects from light damage and provides a mechanism for night blindness in isotretinoin therapy.

Isotretinoin (13-cis retinoic acid) is frequently prescribed for severe acne [Peck, G. L., Olsen, T. G., Yoder, F. W., Strauss, J. S., Downing, D. T., Pandya, M., Butkus, D. & Arnaud-Battandier, J. (1979) N. Engl. J. Med. 300, 329-333] but can impair night vision [Fraunfelder, F. T., LaBraico, J. M. & Meyer, S. M. (1985) Am. J. Ophthalmol. 100, 534-537] shortly after the beginning of therapy [Shulman, S. R. (1989) Am. J. Public Health 79, 1565-1568]. As rod photoreceptors are responsible for night vision, we administered isotretinoin to rats to learn whether night blindness resulted from rod cell death or from rod functional impairment. High-dose isotretinoin was given daily for 2 months and produced systemic toxicity, but this caused no histological loss of rod photoreceptors, and rod-driven electroretinogram amplitudes were normal after prolonged dark adaptation. Additional studies showed, however, that even a single dose of isotretinoin slowed the recovery of rod signaling after exposure to an intense bleaching light, and that rhodopsin regeneration was markedly slowed. When only a single dose was given, rod function recovered to normal within several days. Rods and cones both showed slow recovery from bleach after isotretinoin in rats and in mice. HPLC analysis of ocular retinoids after isotretinoin and an intense bleach showed decreased levels of rhodopsin chromophore, 11-cis retinal, and the accumulation of the biosynthetic intermediates, 11-cis and all-trans retinyl esters. Isotretinoin was also found to protect rat photoreceptors from light-induced damage, suggesting that strategies of altering retinoid cycling may have therapeutic implications for some forms of retinal and macular degeneration.

Photopic and scotopic light detection in patients with seasonal affective disorder and control subjects.

BACKGROUND: Retinal sensitivity may play a role in the pathogenesis of seasonal affective disorder (SAD) and response to light therapy. METHODS: Using a dark adaptation procedure, SAD patients and normal control subjects were tested in the winter and summer, with patients retested after light treatment. The eyes were preadapted to bright light followed by 30 min in darkness, during which subjects detected a dim signal titrated around the detection threshold. Photopic (cone-mediated) and scotopic (rod-mediated) components of the data were identified by nonlinear exponential curve fits to successive threshold estimates. RESULTS: Patients (n = 24) showed significantly lower cone and rod thresholds in the summer than winter, while control subjects (n = 12) showed a similar trend. Relative to the control subjects, however, patients were supersensitive in winter (lower cone final threshold, faster rod recovery). Clinical responders to morning light showed a small summer-like increase in cone sensitivity, whereas nonresponders became subsensitive. In comparison to darker-eyed patients, blue-eyed patients showed a larger summertime increase in cone sensitivity and a similar trend after response to morning light. CONCLUSIONS: Heightened retinal sensitivity with increased light exposure, and supersensitivity of patients relative to control subjects in winter, may play roles in the pathogenesis of winter depression and the action of therapeutic light.

Foveal dysfunction and central visual field loss in glaucoma.

OBJECTIVE: To determine whether foveal function distal to the ganglion cell layer is an independent predictor of central visual field function in glaucoma. SETTING: University affiliated hospital and private practice. PARTICIPANTS: Twenty-seven eyes (27 patients) with normal-pressure glaucoma, 10 eyes (10 patients) with primary open-angle glaucoma, and 47 eyes of 47 matched normal volunteers. INTERVENTION AND MAIN OUTCOME MEASURES: Foveal cone electroretinogram (ERG) amplitude, relative optic cup to disc area and their relations to Humphrey full-threshold 30-2 visual field central 4-point mean total deviation (C4MTD) and pattern deviation (C4MPD). RESULTS: Foveal cone ERG amplitude was subnormal in 14 (37.8%) of the 37 glaucomatous eyes and lower in the glaucoma group compared with normal eyes (P<.01). The C4MTD and C4MPD were lower in glaucomatous eyes with subnormal amplitudes compared with those with normal amplitudes (P<.01 and P<.05, respectively). Amplitude was directly correlated with C4MTD (P<.01) and C4MPD (P<.01). Relative optic cup to disc area was inversely correlated with C4MTD (P<.001) and C4MPD (P<.001). Partial correlation analysis revealed that amplitude and relative optic cup to disc area were independent predictors of C4MTD and C4MPD. CONCLUSION: Foveal function distal to the ganglion cell layer and optic disc cupping independently predict central visual field function in glaucoma.