The increase in sensitivity following light illumination in frog photoreceptors.

Dark-adaptation of photoreceptors was studied by recording fast PIII responses of the isolated bull frog retina superfused with Conway's solution containing 5 mM sodium aspartate. When a dark-adapted retina is illuminated by an adapting light, the amplitude of the response decreases. Initially on turning off this light, the amplitude increases over that of the dark-adapted response. This phenomenon, “hypersensitivity” is thought to be due to an increase in sensitivity of red rods. The hypersensitivity occurs following several minutes illumination with relatively weak light if [Ca²⁺ out is low. Intense light and higher concentration of Ca²⁺ inhibit the hypersensitivity. Possible mechanisms for the hypersensitivity are discussed.     

Opsin distribution and synthesis in degenerating photoreceptors of rd mutant mice

The distribution of opsin and the capacity of photoreceptors to synthesize opsin was studied in retinas of mice bearing the rd (retinal degeneration) mutation and compared to control normal mice. Opsin was localized by means of pre-embedding and post-embedding immunocytochemistry and electron microscopy. Cones were identified with anti-cone antibodies and peanut agglutinin lectin which labels cone matrix sheaths. Opsin synthesis was measured by incorporation of [35S]methionine into opsin which was detected by immunoblots. Immunocytochemistry revealed that degeneration of rod outer segments was accompanied by accumulation of opsin in the plasma membrane enveloping the inner segment, nuclei and synaptic terminals. Rod photoreceptors degenerated faster than cones. By post-natal day 19 (P19), 87% of the remaining inner segments were cones. Opsin synthesis in rd mice could no longer be measured after P15. However, opsin molecules could be detected both by immunocytochemistry and immunoblotting up to P30. Between P20 and P30 all detectable opsin was localized in the plasma membrane which envelopes nuclei and synaptic terminals. Unlabeled surviving nuclei after P30 are probably cones. The opsin which is detected in rod inner segment plasma membranes might be derived, by back diffusion, from degenerating outer segments and represent molecules synthesized at an earlier time. Alternatively opsin accumulation might be a result of low levels of opsin synthesis, undetectable by [35S]methionine incorporation, which continues in the absence of outer segments.     

Genetic regulation of light damage to photoreceptors

Albino mice of different inbred strains have been exposed to constant fluorescent lighting at an illuminance level of 115-130 ft-c for intervals of 1-6 weeks. Under these conditions the photoreceptors in retinas of albino BALB/cByJ mice rapidly degenerate, whereas the photoreceptors in retinas of albino C57BL/6J-c2J mice are remarkably more resistant to light damage. F1 heterozygotes produced from these two strains display an intermediate degree of light-induced degeneration. These findings demonstrate that phenotypically identical populations with different genetic constitutions can show markedly different sensitivities to light, and that genetic factors must now be included as a determinant for the severity of light damage.     

Use of therapeutic ultrasound to restore failed trabeculectomies

We treated 20 recently postoperative eyes with nonfunctioning trabeculectomies at the site of the failed bleb with two to four ultrasound applications, each of five seconds duration, at an intensity level of 10 kW/cm2. Fifteen of the eyes demonstrated a long-term decrease in intraocular pressure to 21 mm Hg or below. The mean intraocular pressure of all patients fell from a preultrasound mean (+/- S.D.) of 32 +/- 8 mm Hg to 20 +/- 10 mm Hg at a mean time of 236 +/- 204 days postultrasound. Complications included an immediate postultrasound rise in pressure over 10 mm Hg in 13 eyes, and the immediate development of a cataract in one patient.     

Masking-induced sensitivity changes in Limulus photoreceptors

Limulus photoreceptor sensitivity was investigated at various times during the response to a 400 msec flash of light. This procedure is the same as that used psychophysically when masking by light is studied and masking functions were obtained that were very similar to typical psychophysical masking functions. However, even at this most distal level, sensitivity was not determined by any one variable alone. Instead, sensitivity was dependent on receptor potential amplitude and receptor saturation and receptor dynamics.     

Activation of endoplasmic reticulum stress in degenerating photoreceptors of the rd1 mouse

PURPOSE: Endoplasmic reticulum (ER) stress has been implicated in a wide variety of neurodegenerative disorders of the central nervous system (CNS). This study was designed to elucidate the role of ER stress in photoreceptor apoptosis in the rd1 mouse. METHODS: Photoreceptor apoptosis in the rd1 mouse was detected by terminal dUTP transferase nick-end labeling (TUNEL). Protein expressions of ER stress sensors, including glucose-regulated protein-78 (GRP78/BiP), caspase-12, phospho-eukaryotic initiation factor 2alpha (eIF2alpha), and phospho-pancreatic ER kinase (PERK), were examined by immunofluorescence and Western blot assays. RESULTS: Accompanying photoreceptor apoptosis in the rd1 mouse, the protein expressions of GRP78/BiP, caspase-12, phospho-eIF2alpha, and phospho-PERK were upregulated in a time-dependent manner. The upregulation of these proteins coincided with or preceded photoreceptor apoptosis. At the peak of their expression, these proteins were primarily located in the photoreceptor inner segments, the outer nuclear layer, or both. CONCLUSIONS: ER stress plays an important role in photoreceptor apoptosis in the rd1 mouse. Therefore, ER stress modulators may be strong candidates as therapeutic agents in the treatment of retinal degenerative diseases.     

Dark and light adaptation of pineal photoreceptors

Dark and light adaptation of pineal photoreceptors was studied in the isolated pineal organ of the rainbow trout, Salmo gairdneri. After intracellular recording, the photoreceptors were iontophoretically injected with Lucifer yellow CH or with horseradish peroxidase for morphological characterization. Pineal photoreceptor cells responded to light with a hyperpolarization whose amplitude was graded with intensity. Following a 30-60 s bleach, receptor responsiveness was greatly reduced with a gradual recovery in the dark. Recovery of membrane potential was complete within 2-4 min in the dark. In response to flashes the hyperpolarizing response increased in darkness in amplitude and duration over a period of more than 30 min and the voltage-intensity curves continuously shifted to lower intensities. After exposure to strong light the time-course of dark adaptation, determined with a threshold criterion, was monophasic and receptor sensitivity increased by at least 5-6 log units. The results show that pineal photoreceptors exhibited the full characteristics of dark adaptation processes previously ascribed to cells proximal to the receptors, i.e. to ganglion cells. Exposure to steady illumination of different intensities induced graded and sustained hyperpolarizations of the receptor membrane potential. The incremental voltage range of responses to test flashes superimposed on the backgrounds was reduced. Voltage-intensity curves were shifted to higher intensities with increasing background illumination indicating that adaptation occurred over a range of about 2.5 log units before the receptors saturated.     

Speed, sensitivity, and stability of the light response in rod and cone photoreceptors: facts and models

The light responses of rod and cone photoreceptors in the vertebrate retina are quantitatively different, yet extremely stable and reproducible because of the extraordinary regulation of the cascade of enzymatic reactions that link photon absorption and visual pigment excitation to the gating of cGMP-gated ion channels in the outer segment plasma membrane. While the molecular scheme of the phototransduction pathway is essentially the same in rods and cones, the enzymes and protein regulators that constitute the pathway are distinct. These enzymes and regulators can differ in the quantitative features of their functions or in concentration if their functions are similar or both can be true. The molecular identity and distinct function of the molecules of the transduction cascade in rods and cones are summarized. The functional significance of these molecular differences is examined with a mathematical model of the signal-transducing enzymatic cascade. Constrained by available electrophysiological, biochemical and biophysical data, the model simulates photocurrents that match well the electrical photoresponses measured in both rods and cones. Using simulation computed with the mathematical model, the time course of light-dependent changes in enzymatic activities and second messenger concentrations in non-mammalian rods and cones are compared side by side.     

Light adaptation and sensitivity controlling mechanisms in vertebrate photoreceptors

The human visual system can discriminate increment and decrement light stimuli over a wide range of ambient illumination; from moonlight to bright sunlight. Several mechanisms contribute to this property but the major ones reside in the retina and more specifically within the photoreceptors themselves. Numerous studies in retinae from cold- and warm-blooded vertebrates have demonstrated the ability of the photoreceptors to respond in a graded manner to light increments and decrements even if these are applied during a background illumination that is expected to saturate the cells.

In all photoreceptors regardless of type and species, three cellular mechanisms have been identified that contribute to background desensitization and light adaptation. These gain controlling mechanisms include; response-compression due to the non-linearity of the intensity-response function, biochemical modulation of the phototransduction process and pigment bleaching. The overall ability of a photoreceptor to adapt to background lights reflects the relative contribution of each of these mechanisms and the light intensity range over which they operate. In rods of most species, response-compression tends to dominate these mechanisms at light levels too weak to cause significant pigment bleaching and therefore, rods exhibit saturation. In contrast, cones are characterized by powerful background-induced modulation of the photo-transduction process at moderate to bright background intensities where pigment bleaching becomes significant. Therefore, cones do not exhibit saturation even when the level of ambient illumination is raised by 6-7 log units.     

Directional sensitivity of photoreceptors for different degrees of coherence and directions of polarization of the incident light

In two experiments the dependence of the Stiles-Crawford effect (SCE) on the degree of spatial coherence and direction of polarization of monochromatic (633 nm) light was measured. Within the accuracy of the measurements no dependence was found.     

Loss of BCL-XL in rod photoreceptors: Increased susceptibility to bright light stress

PURPOSE: BCL-X(L), an anti-apoptotic member of the BCL-2 family proteins and a cell death/survival checkpoint regulator, was shown to be upregulated in bright light-stressed mouse photoreceptors during an investigation of bright light-induced protein expression. To investigate the significance of BCL-X(L) upregulation in the bright light damage model, the Bcl-x gene was disrupted specifically in mouse rod photoreceptors, and the effect of Bcl-x disruption was characterized on retinal apoptosis, function, and morphology. METHODS: Rod-specific Bcl-x knockout mice, generated by mating mouse opsin promoter-controlled Cre mice with floxed Bcl-x mice, were subjected to bright light stress. Retinal apoptosis in the bright light-stressed conditional Bcl-x knockout mice was characterized with TUNEL, DNA fragmentation, and nuclear staining assays. Photoreceptor structural and functional integrity in the bright light-stressed conditional Bcl-x knockout mice was determined by measuring photoreceptor outer nuclear layer (ONL) thickness and electroretinography amplitudes. RESULTS: Disruption of Bcl-x in rod photoreceptors caused increased photoreceptor apoptosis, decreased retinal function, and decreased ONL thickness in bright light-stressed mice. CONCLUSIONS: The loss of BCL-X(L) increased rod photoreceptor susceptibility to bright light stress. Although the biochemical mechanism(s) of BCL-X(L) in photoreceptor death or survival has not been investigated extensively, results of the present study suggest that BCL-X(L), a cell survival/death checkpoint regulator, is involved in photoreceptor survival under bright light stress.     

Effects of oxygen and bFGF on the vulnerability of photoreceptors to light damage

PURPOSE. To test whether tissue oxygen levels affect the vulnerability of photoreceptors to damage by bright continuous light (BCL). METHODS. Albino rats were raised in standard conditions of cyclic light (12-hour light, 12-hour darkness) with the light level at 5 to 10 lux or 40 to 65 lux. They were then exposed to BCL (1000-1400 lux), either continuously for 48 hours or for the day or night components of the 48-hour period. During BCL, some rats were kept in room air (normoxia, 21% oxygen), some in hypoxia (10%), and some in hyperoxia (70%). Their retinas were examined for cell death, for the expression of basic fibroblast growth factor (bFGF), and for response to light (electroretinogram, ERG). RESULTS. The death of retinal cells induced by BCL was confined to photoreceptors. Within the retina, the severity of death was inversely related to the level of bFGF immunolabeling in the somas of the outer nuclear layer (ONL) before exposure. The death of photoreceptors was accompanied by an upregulation of bFGF protein levels in the ONL and by a decline in the ERG. Both hypoxia and hyperoxia during BCL reduced the photoreceptor death, bFGF upregulation, and ERG decline caused by BCL. The protective effects of hyperoxia and hypoxia were evident during both the day and night halves of the daily cycle. Hypoxia or hyperoxia alone did not upregulate bFGF or ciliary neurotrophic factor (CNTF) expression in the retina. CONCLUSIONS. Photoreceptors are protected from light damage by hypoxia and hyperoxia during exposure. The protection provided by oxygen levels operates during both day and night. The protection is not mediated by an upregulation of bFGF or CNTF.     

Slow optical changes in human photoreceptors induced by light

PURPOSE: The basic assumption of fundus reflection densitometry is that changes in reflectance are solely determined by photolysis and regenerating visual pigments. This study was undertaken to investigate small but systematic deviations from this rule. METHODS: Spectral reflectance changes (450-740 nm) of the fovea were measured during light and dark adaptation over a period of 66 minutes in five healthy subjects. The directional properties of the fundus reflection were examined with a custom-built scanning laser ophthalmoscope (SLO) at 514, 633, and 790 nm. The same instrument was also used to find the spatial distribution of the reflectance changes. RESULTS: In addition to fast changes consistent with visual pigment, slower reflectance changes (lasting 10-20 minutes) were observed at all wavelengths including 740 nm. Because visual pigment does not absorb at 740 nm, a second mechanism must be involved. Factor analysis generated two factors (i.e., spectral curves) that explained more than 97% of the variations in the time course of the spectral reflectance. Total reflectance was modeled by means of an existing model for fundus reflection, and it was found that the first factor strongly resembled the rapid changes in absorption of the cone pigments. The second factor seems linked to slow changes in cone reflectance. Measurements with the SLO showed a clear increase in directionally dependent reflectance from 6 to 30 minutes in the dark. This was observed only in the central 6 degrees of the retina. CONCLUSIONS: The characteristics of the slow reflectance changes all point to the cone photoreceptors as the origin. Most likely, alterations in the index of refraction between the interphotoreceptor matrix and photoreceptors lie at the base of this hitherto unknown phenomenon.     

Activation of electrogenic Na-Ca exchange by light in fly photoreceptors

Illumination of white-eyed Musca photoreceptors following hypoxia or the application of ruthenium red (RR, a known blocker of Ca2+ uptake into intracellular organelles) induced a transient after depolarization (TA). The TA was enhanced when external [Ca2+] was reduced; it was abolished when external [Na+] was reduced to a level that affected the receptor potential to a small degree. The TA was enhanced or depressed when the activity of Na/K pump, which controls the Na+ gradient, was enhanced or depressed respectively. This effect was observed even when the receptor potential was not affected. All of the above observations are consistent with the hypothesis that the TA is triggered by a light-induced increase in the concentration of intracellular free Ca2+ which appear to be very high, following treatments with hypoxia or RR. The high sensitivity of the TA to Na+ and Ca2+ gradients across the photoreceptors membrane strongly suggests that the TA is due to a transient activation of an electrogenic Na-Ca exchange mechanism which depolarizes the cell.     

Comparison of the responses to light and to GABA of cells postsynaptic to barnacle photoreceptors (I-cells)

We tested the hypothesis that gamma-aminobutyric acid (GABA) is the transmitter released by barnacle photoreceptors onto postsynaptic cells (I-cells). GABA was applied to I-cells either by superfusion or by ejecting it with pressure from a pipette positioned close to the I-cell's soma. The I-cell's response to GABA was compared with its response to light (i.e. to the photoreceptors' transmitter) by recording intracellularly from its soma. Bath-applied (100 microns to 10 mM) and pressure-applied GABA (10 mM in pipette) hyperpolarizes I-cells by increasing their conductance, as does the photoreceptors' transmitter. The response to pressure-applied GABA consists of two components; both persist when Co2+ or Cd2+ are added to the saline to block synaptic transmission in the preparation, indicating that GABA affects the I-cell directly rather than affecting a presynaptic cell. GABA hyperpolarizes the I-cell when applied to the cell over the soma and ipsilateral arbor or over the contralateral arbor. The I-cells' responses to GABA and to light both depend on extracellular K+ and are affected by changes in intracellular and extracellular Cl-. However, picrotoxin and beta-guanidinopropionic acid block the response to pressure-applied GABA but do not block the response to light even at an order of magnitude higher concentration. Thus, GABA is not likely to be the transmitter that causes the hyperpolarizing response of the I-cell. It may be a neuromodulator or the transmitter of an unknown input to the I-cell.