Rhodopsin bleaching signals in essential night blindness

Photoreceptor and postreceptor responses in congenital stationary night blindness

PURPOSE

To investigate photoreceptor and postreceptor retinal function in patients with congenital stationary night blindness (CSNB).

METHODS

Forty-one patients with CSNB (ages 0.19-32 years) were studied. ERG responses to a series of full-field stimuli were obtained under scotopic and photopic conditions and were used to categorize the CSNB patients as complete (cCSNB) or incomplete (iCSNB). Rod and cone photoreceptor (R(ROD), S(ROD), R(CONE), S(CONE)) and rod-driven postreceptor (V(MAX), log σ) response parameters were calculated from the a- and b-waves. Cone-driven responses to 30 Hz flicker and ON and OFF responses to a long duration (150 ms) flash were also obtained. Dark-adapted thresholds were measured. Analysis of variance was used to compare data from patients with cCSNB, patients with iCSNB, and controls.

RESULTS

We found significant reduction in saturated photoreceptor amplitude (R(ROD), R(CONE)) but normal photoreceptor sensitivity (S(ROD), S(CONE)) in both CSNB groups. Rod-driven postreceptor response amplitude (V(MAX)) and sensitivity (log σ) were significantly reduced in CSNB. Log σ was significantly worse in cCSNB than in iCSNB; this was the only scotopic parameter that differed between the two CSNB groups. Photopic b-wave amplitude increased monotonically with stimulus strength in CSNB patients rather than showing a normal photopic hill. The amplitude of the 30-Hz flicker response was reduced compared with controls, more so in iCSNB than in cCSNB. The mean dark-adapted threshold was significantly elevated in CSNB, more so in cCSNB than in iCSNB.

CONCLUSIONS

These results are evidence of normal photoreceptor function (despite the low saturated photoresponse amplitude) and anomalous postreceptor retinal circuitry.

Retinal abnormalities associated with the G90D mutation in opsin

Several mutations in the opsin gene have been associated with congenital stationary night blindness, considered to be a relatively nonprogressive disorder. In the present study, we examined the structural and functional changes induced by one of these mutations, i.e., substitution of aspartic acid for glycine at position 90 (G90D). Transgenic mice were created in which the ratio of transgenic opsin transcript to endogenous was 0.5:1, 1.7:1, or 2.5:1 and were studied via light and electron microscopy, immunocytochemistry, electroretinography (ERG), and spectrophotometry. Retinas with transgenic opsin levels equivalent to one endogenous allele (G0.5) appeared normal for a period of about 3-4 months, but at later ages there were disorganized, shortened rod outer segments (ROS), and a loss of photoreceptor nuclei. Higher levels of G90D opsin expression produced earlier signs of retinal degeneration and more severe disruption of photoreceptor morphology. Despite these adverse effects, the mutation had a positive effect on the retinas of rhodopsin knockout (R-/-) mice, whose visual cells fail to form ROS and rapidly degenerate. Incorporation of the transgene in the null background (G+/-/R-/- or G+/+/R-/-) led to the development of ROS containing G90D opsin and prolonged survival of photoreceptors. Absorbance spectra measured both in vitro and in situ showed a significant reduction of more than 90% in the amount of light-sensitive pigment in the retinas of G+/+/R-/- mice, and ERG recordings revealed a >1 log unit loss in sensitivity. However, the histological appearances of the retinas of these mice show no significant loss of photoreceptors and little change in the lengths of their outer segments. These findings suggest that much of the ERG sensitivity loss derives from the reduced quantal absorption that results from a failure of G90D opsin to bind to its chromophore and form a normal complement of light-sensitive visual pigment.

Rod pathways: the importance of seeing nothing

Anatomical and physiological studies of the mammalian retina have revealed two primary pathways available for the transmission of rod signals to the ganglion cells: one via ON rod bipolars, amacrine II cells, and ON and OFF cone bipolars, which is exquisitely designed for the transmission of single-photon absorption events; and a second via rod-cone gap junctions, and ON and OFF cone bipolars, which is designed for the transmission of multiple photon-absorption events at higher light levels. Psychophysical and electroretinographic (ERG) studies in normal observers and in two rare types of observer, who are devoid of either rod or cone function, support an analogous duality in the human visual system, the clearest signature of which is a loss of flicker visibility and ERG amplitude at frequencies near 15 Hz that results from destructive interference between sensitive 'slow' and insensitive 'fast' rod signals. The slow rod signal is most probably derived from the ON rod bipolar pathway and the fast signal from the rod-cone gap junction and cone pathways. Evidence has emerged recently for a third, insensitive rod pathway between rods and OFF cone bipolars, but it has so far only been observed clearly in rodents.

Parameters of transplant-mediated pupilloconstriction in rats with unilateral olivary pretectal lesions

Embryonic retina, transplanted to the midbrain of neonatal rats, innervates the host brain and mediates a pupilloconstrictor response in the host eye. We hypothesise that there is a dynamic interaction between normal host and transplant photic input to the pupilloconstrictor centres. This study aims to characterise the nature of these interactions and determine the impact of experimental lesions on this reflex system. Pupillary diameter in normal rats is regulated by integration of intensity levels of the light delivered to the two eyes. The integration occurs at the primary input nucleus, the olivary pretectal nucleus, and at the output nucleus, the Edinger-Westphal nucleus. We have examined the pattern of integration of inputs delivered through the host eye and a retinal transplant placed over the midbrain at birth. Restriction of the site of integration to one olivary pretectal nucleus by ablating the contralateral nucleus causes a substantial enhancement of the transplant-mediated response and a major reduction in the host eye-mediated response. Although the pattern of change is quite similar to that seen between the two eyes of a normal animal after a similar lesion, the magnitude of improvement of the transplant response is much greater. The integration that occurs between transplant and host inputs is dynamic, and the efficacy of the transplant can be modulated by the competing host input. These results have implications for the use of neural transplants in degenerative diseases and might also offer insight into the limited functional recovery that occurs after neurological injury.

Clinical applications of fundus reflection densitometry

Fundus reflection densitometry or retinal densitometry is a non-invasive technique to examine the visual photopigment kinetics in living eyes. The technique is based on the comparison of the reflected light from the fundus in a fully light adapted eye (when all visual photopigment has been bleached) with the reflected light following complete dark adaptation (when the retina contains its maximum amount of visual photopigment). The technique provides a measure of the density of visual photopigment, its time constant of regeneration, its distribution and spectral characteristics if measured at a series of wavelengths. Fundus reflection densitometry in the human eye was introduced 40 years ago. Presently, it is the only available technique from which direct and objective insight can be obtained into visual photopigment. This knowledge is particularly relevant in eyes where abnormalities of photoreceptor function are suspected. This paper summarizes the current knowledge of fundus reflection densitometry in the diseased and in the aging human retina, gathered over the last 30 years. Considerable improvements of the instrument for clinical purposes have been obtained, and are also discussed.

The pupil photostress test

PURPOSE

An objective, pupillographic photostress test was developed to assess the initial sensitivity loss and its rate of recovery in healthy eyes and in eyes affected by optic neuropathy.

METHODS

Pupillary light reflexes were recorded with an infrared pupillometer after a binocular photostress that subtended 30 degrees of the central visual field. The initial loss of light sensitivity (in log units) and its recovery with time were derived by analyzing subsequent increases in pupil contraction amplitude to a series of alternating light stimuli. The effects of photostress duration and intensity were studied in ten control subjects. Photostress recovery also was compared between eyes of nine patients with unilateral optic neuropathy.

RESULTS

In the ten control subjects, increasing photostress duration caused significantly increased sensitivity loss at time zero after the photostress (analysis of variance), but did not significantly affect the exponential recovery rate. Intense photostress brightness caused more initial sensitivity loss than did dimmer photostress lights. In addition, half-time of sensitivity recovery was prolonged after the brightest photostress. In nine patients with unilateral optic neuropathy, a 120-second photostress caused significantly less initial sensitivity loss in the affected eye than in the healthy fellow eye.

CONCLUSIONS

The pupil photostress test objectively can measure the eye's loss of light sensitivity after a photostress and its exponential rate of recovery. Loss of optic nerve function diminishes the initial stress-induced loss of light sensitivity and does not significantly affect the poststress rate of recovery in the patients tested.

Models of the normal and abnormal rod system

A framework is presented for using threshold data to test hypotheses about the action of a disease, a chemical agent, or a developmental process. A model of the normal rod system, based on models from the physiological and psychophysical literature, is presented. Hypotheses about the alteration of the rod system are specified in this model. The approach is illustrated with a class of hypotheses that places the decrease in sensitivity with retinal disease at the rod receptors and with data from patients with retinitis pigmentosa and congenital stationary night blindness. The implications for models of the normal rod system are considered.

Rod densitometry in congenital stationary night blindness

Rod densities were measured in ten patients with congenital stationary night blindness (CSNB) using the Utrecht densitometer. In five patients rod density was within the range observed in a group of control subjects. One of these patients reached a normal rod two-way density after a prolonged regeneration time. In five patients with different hereditary patterns of CSNB we found decreased amounts of rhodopsin. These observations deviate from the findings on four CSNB patients hitherto described in the literature, where normal concentrations and kinetics of rhodopsin were found. We observed no relationship between the age of CSNB patients and mode of rhodopsin kinetics. No inconsistencies between rod densitometry and electro-retinography were revealed. The different electroretinography patterns were not helpful in predicting the densitometric responses in CSNB. The present results included, five of fourteen CSNB patients ever studied with the technique of retinal densitometry show abnormal rhodopsin kinetics. We propose a schema of CSNB subtyping based on the different rhodopsin kinetics.

Rod densitometry in night blindness: a review and two puzzling cases. Rod densitometry in night blindness

Since the non-invasive technque of retinal densitometry became available in 1955, rhodopsin kinetics could be studied in vivo. It was obvious that with this new tool investigators focussed attention on the aetiology of night blindness in various diseases. A brief review about the clinical developments in the past two decades is given. Also three case-reports are presented, which suggest that in some cases of congenital stationary night blindness (CSNB) the night blindness might arise from the absence of rhodopsin. This is contrary to the standing opinion and present problems regarding the integrity of the retina.

Photoreceptor responses of patients with congenital stationary night blindness

The present study examines the hypothesis that the electroretinogram (ERG) a-wave of patients with congenital stationary night blindness with myopia (CSNB) is solely cone mediated. The subjects include four patients with x-linked recessive inheritance, two patients with no family history, and thirteen age-matched normal subjects. Because normal subjects have relatively large b-waves (which can influence the measurement of the PIII component of the a-wave), the main response parameter studied was the slope of the a-wave. The results for the CSNB patients show that in the light adapted condition, the ratio of the responses to photopically balanced blue and red flashes was nearly unity; whereas, in the dark-adapted condition, the ratio was ~3 times greater. The ratios for the normal subjects were similar to those of the CSNB patients. These results lead to the conclusion that, in patients with CSNB, the ERG a-wave is not solely mediated by cones.

Pupillary constriction to darkness

Patients with congenital achromatopsia and congenital stationary night blindness have been known to show a transient pupillary constriction to darkness. We examined 50 normal subjects and 108 patients with retinal and optic nerve dysfunction to see if any had an initial pupillary constriction to darkness. We used a new infrared television apparatus. Four patients with congenital stationary night blindness, four with achromatopsia, two with bilateral optic neuritis, and one with dominant optic atrophy showed the phenomenon. In the patients who showed this unusual pupillary response to darkness it was the first observable event every time the lights were turned off. The constriction could usually be seen with a handlight, and it was similar in latency to the normal pupillary dilatation to darkness. Pupillary constriction to darkness is a clinically valuable sign that can be used in the detection of congenital retinal disease in children with poor vision.

Discrete visual defects in pearl mutant mice

The mutant mouse pearl, characterized by its hypopigmentation, has a specific functional defect in a sensory system--the retina. The intact pearl mouse has reduced sensitivity in the dark-adapted condition. Normal sensitivity is restored by isolation and superfusion of the retina with bicarbonate-buffered Ringer solution, suggesting that the retinal expression of the pearl mutation depends on a diffusible substance. The pearl phenotype is described as a possible model for human congenital stationary night blindness.

Paradoxical pupil in congenital achromatopsia

Children with congenital achromatopsia possess an interesting paradoxical pupillary constriction to darkness that has not been previously described. This paper describes three children in whom this paradoxical pupillary reaction was documented with infrared pupillometry. The literature on the pupil in achromatopsia is reviewed and a tentative testable model of the reaction is hypothesized.

Congenital stationary nightblindness

Images

Dark adaptation and visual pigment regeneration in human cones

Foveal threshold elevation and red-green cone pigment regeneration have been studied in the dark after a wide range of bleaches in normal man with a view to probing the limits of the application of the Dowling-Rushton relation: i.e., the direct proportionality between log threshold elevation and fraction of unregenerated pigment. Cone pigment regeneration (and threshold recovery) is much faster after short bleaches than expected from the kinetics of a simple monomolecular reaction. Recovery is faster after a fixed (short) duration bleach the weaker it is. Except for the first 30 s after relatively weak bleaches and the entire recovery after a very brief (<0.001 s) saturating bright flash which bleaches a little more than 50%, the results are accurately fit by the Dowling-Rushton relation over the entire range tested with only one arbitrary constant (the proportionality factor). Theory predicts too low threshold in comparison with what is obtained, for both of these exceptions