The bleaching and regeneration of rhodopsin in the cat

Altered fundus appearance resulting from autofluorescence imaging with the confocal scanning laser ophthalmoscope (cSLO) in cats

PURPOSE

This paper is to report that imaging the tapetal fundus of cats with the 488 nm laser of the Spectralis(®) HRA+OCT (Heidelberg Engineering Inc., Heidelberg, Germany) can result in a pale appearance of the imaged area.

ANIMALS STUDIED AND PROCEDURES

Wild-type and Rdy kittens (CRX mutant heterozygotes-CRX(Rdy+/-) ) (8-20 weeks of age) and adult cats (1-4 years of age) were imaged by confocal scanning laser ophthalmoscope (cSLO) and spectral domain optical coherence tomography (SD-OCT) using the Spectralis(®) HRA+OCT. Color fundus photography (RetCam II(®) , Clarity Medical Systems, Inc., Pleasanton, CA) was performed after imaging using the Spectralis(®) HRA+OCT.

RESULTS

Following retinal cSLO imaging using the 488 nm laser (autofluorescence imaging) in both wild-type kittens and adult cats, the imaged region appeared paler than the adjacent retina that had not been imaged. This change was probably due to retinal bleaching and was fully reversible. Imaging CRX(Rdy+/-) kittens or adults, which had very reduced levels of visual pigments, did not induce the altered fundus appearance.

CONCLUSIONS

Those using autofluorescence imaging by cSLO should be aware that it can induce a characteristic pale appearance of the tapetal fundus in the imaged area of normal cats.

Mechanistic modeling of vertebrate spatial contrast sensitivity and acuity at low luminance

The validity of the Barten theoretical model for describing the vertebrate spatial contrast sensitivity function (CSF) and acuity at scotopic light levels has been examined. Although this model (which has its basis in signal modulation transfer theory) can successfully describe vertebrate CSF, and its relation to underlying visual neurophysiology at photopic light levels, significant discrepancies between theory and experimental data have been found at scotopic levels. It is shown that in order to describe scotopic CSF, the theory must be modified to account for important mechanistic changes, which occur as cone vision switches to rod vision. These changes are divided into photon management factors [changes in optical performance (for a dilated pupil), quantum efficiency, receptor sampling] and neural factors (changes in spatial integration area, neural noise, and lateral inhibition in the retina). Predictions of both scotopic CSF and acuity obtained from the modified theory were found to be in good agreement with experimental values obtained from the human, macaque, cat, and owl monkey. The last two species have rod densities particularly suited for scotopic conditions.

A mechanistic inter-species comparison of spatial contrast sensitivity

The validity of the Rovamo-Barten modulation transfer function model for describing spatial contrast sensitivity in vertebrates was examined using published data for the human, macaque, cat, goldfish, pigeon and rat. Under photopic conditions, the model adequately described overall contrast sensitivity for changes in both stimulus luminance and stimulus size for each member of this diverse range of species. From this examination, optical, retinal and post-retinal neural processes subserving contrast sensitivity were quantified. An important retinal process is lateral inhibition and values of its associated point spread function (PSF) were obtained for each species. Some auxiliary contrast sensitivity data obtained from the owl monkey were included for these calculations. Modeled values of the lateral inhibition PSF were found to correlate well with ganglion cell receptive field surround size measurements obtained directly from electrophysiology. The range of vertebrates studied was then further extended to include the squirrel monkey, tree shrew, rabbit, chicken and eagle. To a first approximation, modeled estimates of lateral inhibition PSF width were found to be inversely proportional to the square root of ganglion cell density. This finding is consistent with a receptive field surround diameter that changes in direct proportion to the distance between ganglion cells for central vision. For the main species examined, contrast sensitivity is considerably less than that for the human. Although this is due in part to a reduction in the performance of both optical and retinal mechanisms, the model indicates that poor cortical detection efficiency plays a significant role.

Role of photoreceptor-specific retinol dehydrogenase in the retinoid cycle in vivo

The retinoid cycle is a recycling system that replenishes the 11-cis-retinal chromophore of rhodopsin and cone pigments. Photoreceptor-specific retinol dehydrogenase (prRDH) catalyzes reduction of all-trans-retinal to all-trans-retinol and is thought to be a key enzyme in the retinoid cycle. We disrupted mouse prRDH (human gene symbol RDH8) gene expression by targeted recombination and generated a homozygous prRDH knock-out (prRDH-/-) mouse. Histological analysis and electron microscopy of retinas from 6- to 8-week-old prRDH-/- mice revealed no structural differences of the photoreceptors or inner retina. For brief light exposure, absence of prRDH did not affect the rate of 11-cis-retinal regeneration or the decay of Meta II, the activated form of rhodopsin. Absence of prRDH, however, caused significant accumulation of all-trans-retinal following exposure to bright lights and delayed recovery of rod function as measured by electroretinograms and single cell recordings. Retention of all-trans-retinal resulted in slight overproduction of A2E, a condensation product of all-trans-retinal and phosphatidylethanolamine. We conclude that prRDH is an enzyme that catalyzes reduction of all-trans-retinal in the rod outer segment, most noticeably at higher light intensities and prolonged illumination, but is not an essential enzyme of the retinoid cycle.

Dark adaptation and the retinoid cycle of vision

Following exposure of our eye to very intense illumination, we experience a greatly elevated visual threshold, that takes tens of minutes to return completely to normal. The slowness of this phenomenon of "dark adaptation" has been studied for many decades, yet is still not fully understood. Here we review the biochemical and physical processes involved in eliminating the products of light absorption from the photoreceptor outer segment, in recycling the released retinoid to its original isomeric form as 11-cis retinal, and in regenerating the visual pigment rhodopsin. Then we analyse the time-course of three aspects of human dark adaptation: the recovery of psychophysical threshold, the recovery of rod photoreceptor circulating current, and the regeneration of rhodopsin. We begin with normal human subjects, and then analyse the recovery in several retinal disorders, including Oguchi disease, vitamin A deficiency, fundus albipunctatus, Bothnia dystrophy and Stargardt disease. We review a large body of evidence showing that the time-course of human dark adaptation and pigment regeneration is determined by the local concentration of 11-cis retinal, and that after a large bleach the recovery is limited by the rate at which 11-cis retinal is delivered to opsin in the bleached rod outer segments. We present a mathematical model that successfully describes a wide range of results in human and other mammals. The theoretical analysis provides a simple means of estimating the relative concentration of free 11-cis retinal in the retina/RPE, in disorders exhibiting slowed dark adaptation, from analysis of psychophysical measurements of threshold recovery or from analysis of pigment regeneration kinetics.

Recovery of the human photopic electroretinogram after bleaching exposures: estimation of pigment regeneration kinetics

We used a fibre electrode in the lower conjunctival sac of the human eye to record the a-wave of the photopic electroretinogram elicited in response to dim red flashes, delivered in the presence of a rod-saturating blue background, before and after exposure of the eye to bright white illumination that bleached a significant fraction of cone photopigment. Responses were recorded from two normal subjects whose pupils were maximally dilated. A range of intensities of bleaching light were used, from 500 to 3000 photopic cd m(-2), and exposures were made sufficiently long in duration to achieve a steady-state bleach. In addition, responses were also recorded following shorter durations of exposures to the highest intensity (3000 cd m(-2)); these durations ranged from 5 to 60 s. The amplitude of the a-wave response to dim flashes was reduced following the exposures, with brighter or longer exposures causing greater reduction. The amplitude then recovered within about 4 min to the prebleach level. The amplitudes measured at ca 15 ms after the flash were used to derive the effective intensity of the flashes, thereby quantifying the fraction of photopigment available at the time of delivery of each flash. Recovery from all exposures in both subjects followed a common time course, which could be described well by a model of pigment kinetics based on rate-limited regeneration, where the initial rate of recovery following a total bleach was ca 50% of the total pigment per minute, and the residual pigment level for half the maximal rate was ca 20% of the total pigment. The same parameters, together with a fixed photosensitivity, could account for the steady-state pigment levels seen at each bleaching intensity, and also for the fraction of pigment bleached following exposures of different duration at the highest intensity. The dim-flash ERG thus provides a novel method for assessing pigment regeneration in vivo. Our finding that pigment regeneration follows rate-limited kinetics may explain previous reports of pigment regeneration deviating from first order kinetics. We present a model of regeneration in which the rate limit arises from a limitation in the delivery of 11-cis-retinoid to the photoreceptor outer segments.

Spatial and temporal properties of cat horizontal cells after prolonged dark adaptation

We studied the change of spatial and temporal response properties for cat horizontal (H-) cells during prolonged dark adaptation. H-cell responses were recorded intracellularly in the optically intact, in vivo eye. Spatial and temporal properties were first measured for light-adapted H-cells, followed by a period of dark adaptation, after which the same measurements were repeated. During dark adaptation threshold sensitivity was measured at regular intervals. Stable, long lasting recordings allowed us to measure changes of sensitivity and receptive field characteristics for adaptation periods up to 45 min. Although cat H-cells showed no signs of dark suppression or light sensitization, they remained insensitive in the scotopic range, even after prolonged dark adaptation. Absolute thresholds were in the low mesopic range. The sensitization was brought about by a shift from cone to rod input, and by substantial increases of both spatial and temporal integration upon dark adaptation. The length constant in the light-adapted state was on average about 4 deg. After dark adaptation it was up to a factor of three larger, with a median ratio of 1.85. Response delays, latencies and durations for (equal amplitude) threshold flash responses substantially increased during dark adaptation.

Horizontal cell sensitivity in the cat retina during prolonged dark adaptation

The effects of dark adaptation on the response properties of ganglion cells have been documented extensively in the cat retina. To pinpoint the different retinal mechanisms that underlie these effects, we studied the response characteristics of cat horizontal (H) cells during prolonged dark adaptation. H-cell responses were recorded intracellularly in the optically intact, in vivo eye. To disentangle rod and cone contributions, sensitivity changes during dark adaptation were tracked with white light and with monochromatic lights that favored either rod or cone excitation. Stable, long-lasting recordings allowed us to measure changes of sensitivity for adaptation periods up to 45 min. Thresholds for white light and 503-nm monochromatic light decreased steadily and in parallel. The maximum increase of sensitivity, after extinguishing a photopic adaptation light, was 1.8 log units only, reached after about 35 min. Sensitivity for 581-nm lights also increased steadily, but at a shallower slope. The steady increase of sensitivity was concomitant with a linear shift in resting membrane potential and with an increase in relative rod contribution to the threshold responses. Even though small-amplitude responses were rod dominated after prolonged dark adaptation, sensitivity to rod signals remained relatively low, compared to sensitivity of cone responses or to the absolute sensitivity of ganglion cells. This suggests that the cone-H-cell pathway plays no role in the dark-adapted cat retina.

Visual resolution and sensitivity in a nocturnal primate (galago) measured with visual evoked potentials

Visual resolution and contrast sensitivity were examined in anesthetized, paralyzed galagos using visual evoked potentials (VEPs) resulting from stimulation with phase-reversed sinewave gratings. Spatial frequency vs contrast response functions were band-pass with peak sensitivity at 0.2-0.4 c/deg and a high frequency cut-off between 1.6 and 3 c/deg. Peak contrast sensitivities (estimated from extrapolation of contrast response functions) varied across animals from 10 to 170. Variation of the stimulus modulation rate showed that best responses occurred at 1 Hz with an upper limit of 6-16 Hz. As in other primates, an oblique effect was seen in 6 of 8 animals. The contrast sensitivity function (CSF) determined from cortical VEPs agrees well with the CSFs of cells in the lateral geniculate nucleus, but peak sensitivity and spatial frequency are slightly lower than found for the behavioral CSF. Overall visual performance resembled closely that of another nocturnal species, the cat.

Some properties of solubilized human rhodopsin

This investigation involved an examination of some properties of solubilized human rhodopsin. In confirmation of previous work, the spectral maximum was found to be at 493 nm at temperatures 5-10 degrees C below 37.5 degrees C. An increase in temperature to 37.5 degrees C produced only a minor shift of 2-4 nm toward the blue. The opsin displayed the classic and typical stereospecificity of vertebrate visual pigments, regenerating a pigment at 493 nm with 11-cis retinal and an isopigment at 483 nm with 9-cis retinal. No regeneration occurred with either all-trans or 13-cis retinal. The chromophoric photosensitivity of human rhodopsin and of its 11-cis regenerated pigment was found to be the same at 13.2 X 10(-17) cm2; that of the isopigment, at 4.5 X 10(-17) cm2. The long-lived photoproduct of human rhodopsin at 475 nm (metarhodopsin-III) was found to be especially interesting because of its protracted growth following a brief (20 sec) light exposure of the pigment and because of its long decay time even at 27 degrees C and higher. This property (growth and decay of metarhodopsin-III) was studied at temperatures ranging from 1.9 to 37.5 degrees C. Though NH2OH (4.6 X 10(-3) M) was found to speed the decay of metarhodopsin-III, it did not prevent its presence during decay for minutes after the 20-sec bleach. It is clear that the human metarhodopsin-III is indeed a long-lived intermediate of bleaching and evidence from the literature, which is cited, suggests that this product persists for significant periods of time in the retinas of mammals, including that of man. This fact suggests the possible physiological role of metarhodopsin-III in some aspects of vertebrate vision.

Spatial consequences of bleaching adaptation in cat retinal ganglion cells

1. Experiments were conducted to study the effects of localized bleaching on the centre responses of rod-driven cat retinal ganglion cells. 2. Stimulation as far as 2 degrees from the bleaching site yielded responses which were reduced nearly as much as those generated at the bleaching site. Bleaching in the receptive field middle reduced responsiveness at a site 1 degrees peripheral more than bleaching at that peripheral site itself. 3. The effectiveness of a bleach in reducing centre responsiveness is related to the sensitivity of the region in which the bleach is applied. 4. Response reduction after a 0.2 degree bleach followed the same temporal pattern for concentric test spots of from 0.2 to 1.8 degrees in diameter, implying a substantially uniform spread of adaptation within these bounds. 5. A linear trade-off between fraction of rhodopsin and area bleached over a range of 8:1 yields the same pattern of response reduction, implying that the non-linear nature of bleaching adaptation is a property of the adaptation pool rather than independent photoreceptors.

Flash photolysis of rhodopsin in the cat retina

The bleaching of rhodopsin by short-duration flashes of a xenon discharge lamp was studied in vivo in the cat retina with the aid of a rapid, spectral-scan fundus reflectometer. Difference spectra recorded over a broad range of intensities showed that the bleaching efficacy of high-intensity flashes was less than that of longer duration, steady lights delivering the same amount of energy. Both the empirical results and those derived from a theoretical analysis of flash photolysis indicate that, under the conditions of these experiments, the upper limit of the flash bleaching of rhodopsin in cat is approximately 90%. Although the fact that a full bleach could not be attained is attributable to photoreversal, i.e., the photic regeneration of rhodopsin from its light-sensitive intermediates, the 90% limit is considerably higher than the 50% (or lower) value obtained under other experimental circumstances. Thus, it appears that the duration (approximately 1 ms) and spectral composition of the flash, coupled with the kinetic parameters of the thermal and photic reactions in the cat retina, reduce the light-induced regeneration of rhodopsin to approximately 10%.

"Rapid regeneration" in the cat retina: a case for spreading depression

Fundus reflectometry of the cat retina showed that under certain circumstances a rapid increase in density may follow intense bleaching exposures. The spectral characteristics of the density changes indicated that neither rhodopsin nor its bleach products could be responsible for this effect. The poor condition of the animals in which the phenomenon was observed and its conspicuous absence in the majority of the experimental runs suggested that the effect was associated with a process other than the resynthesis of rhodopsin. It was shown that an extrareceptoral event, spreading depression (SD) of the retina, is the most likely source of the rapid spectral change. The well-known tissue alterations associated with SD were induced in the retina independently of pigment density change. The resultant difference spectra resembled those produced when the rapid density increase occurred spontaneously. It seems likely that the abnormal physiological condition of those cats in which the phenomenon is more frequently observed primes the retina for the light-induced generation of spreading depression.

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.

Recovery of cat retinal ganglion cell sensitivity following pigment bleaching

1. The threshold illuminance for small spot stimulation of on-centre cat retinal ganglion cells was plotted vs. time after exposure to adapting light sufficiently strong to bleach significant amounts of rhodopsin. 2. When the entire receptive field of an X- or Y-type ganglion cell is bleached by at most 40%, recovery of the cell's rod-system proceeds in two phases: an early relatively fast one during which the response appears transient, and a late, slower one during which responses become more sustained. Log threshold during the later phase is well fit by an exponential in time (tau = 11.5-38 min). 3. After bleaches of 90% of the underlying pigment, threshold is cone-determined for as long as 40 min. Rod threshold continues to decrease for at least 85 min after the bleach. 4. The rate of recovery is slower after strong than after weak bleaches; 10 and 90% bleaches yield time constants for the later phase of 11.5 and 38 min, respectively. This contrasts with an approximate time constant of 11 min for rhodopsin regeneration following any bleach. 5. The relationship between the initial elevation of log rod threshold extrapolated from the fitted exponential curves and the initial amount of pigment bleached is monotonic, but nonlinear. 6. After a bleaching exposure, the maintained discharge is initially very regular. The firing rate first rises, then falls to the pre-bleach level, with more extended time courses of change in firing rate after stronger exposures. The discharge rate is restored before threshold has recovered fully. 7. The change in the response vs. log stimulus relationship after bleaching is described as a shift of the curve to the right, paired with a decrease in slope of the linear segment of the curve.

Kinetics of bleaching and regeneration of rhodopsin in abnormal (RCS) and normal albino rats in vivo

1. Rhodopsin concentration has been measured by the method of densitometry in retinae of rats with inherited retinal dystrophy (RCS) raised in darkness and compared with that of normal rats similarly reared. 2. In both RCS and normal rats the fraction of rhodopsin bleached is always directly proportional to the photon content of the light, I.t, where I is the light intensity in effective quanta (500 nm) cm-2 sec-1 and t is the duration of the bleaching exposure in seconds. 3. Rhodopsin photosensitivity for bleaching is slightly higher in RCS rats than in normals (2.3 (10)-16 cm2 chromophore-1 compared with 1.3 (10)-16 cm2 chromophore-1). 4. Rhodopsin regeneration in the dark in both RCS and normal rats cannot be described by the kinetics of a simple monomolecular chemical reaction. 5. Following 5 min bleaches, the regeneration rate becomes slower as the preceding bleach is made stronger. Regeneration in the dark is significantly faster in the RCS rats than in the normal ones. 6. In normal rats, after a full bleach, rhodopsin regenerates back to the dark-adapted level within 3--4 hr. In RCS rats rhodopsin regenerates to reach a plateau level, below the previous dark-adapted level, that lasts for several hours. 7. The faction of total rhodopsin that can regenerate gradually declines with age until in 70 days old RCS rats no rhodopsin regeneration can be measured by the ensitometer. However, total rhodopsin density (fully bleached-dark-adapted) is still close to normal.

Cone signals in the cat's retina

1. The discharges of ganglion cells in the cat's retina were recorded under conditions intended to isolate the cone system.2. Stiles' two-colour threshold technique permitted the photopic system to be studied when at its highest sensitivity. The absolute sensitivity of a ganglion cell, expressed in equivalent photons of lambda(max) at the cornea per impulse discharged, was about 2500 times less when driven by cones than when driven by rods. This ratio improves to around 200 when allowance is made for the much smaller fraction absorbed by cones of photons incident on the cornea.3. The number of extra impulses discharged in response to a brief flash was approximately proportional to the number of photons in the flash, up to a limit.4. There was a region in the middle of the receptive field within which the area of a test spot and its illumination for threshold varied inversely. A flash extending over the peripheral part of the receptive field raised threshold above its minimum, presumably as a result of surround antagonism. Assessed from area-threshold curves, the balance of centre-surround antagonism in the photopic receptive field did not seem to depend upon background illumination.5. The threshold for a small (0.2 degrees ) flash confined to the middle of the receptive field was independent of background illumination until the background exceeded a particular level, the ;dark light' (I(o)). In different units this ranged about a mean of 7.89 log photons (560 nm equivalent) deg(-2) sec(-1). For backgrounds that exceeded I(o), threshold followed approximately Weber's law up to the highest illuminations that could be produced.6. With test flashes that filled the centre of the receptive field, the Weber fraction (test flash illumination/background illumination) in some units fell below 1%.7. Changes in the time course and latency of response accompanied the changes in sensitivity caused by alterations in background illumination. Responses of both X- and Y-cells became more transient and faster.8. The loss of sensitivity to a test flash brought about by a steady background light depended upon the size of that light. Sensitivity varied inversely with background area within a central region that matched closely the summing area for test flashes.

Dark adaptation and receptive field organisation of cells in the cat lateral geniculate nucleus

The receptive fields of LGN cells were investigated with stationary light and dark spot and annulus stimuli. Stimulus size and background intensity were varied while stimulus/background contrast was kept constant. The speed of dark adaptation vaired considerably from cell to cell. Dark adaptation made responses more sustained in all neurones and eliminated the oscillatory on-responses evoked under some conditions in the light-adapted cells. Dark adaptation led also to a disappearance of early phasic inhibition in on-responses, and increased response rise time and latency. The power of surround responses to inhibit centre responses decreased slightly at low levels of light adaptation in LGN cells but much less than in retinal ganglion cells. Some other traces of changing retinal surround effects also appeared inthe LGN on dark adaptation. For example, the functional size of receptive fields increased at low levels of illuminance as has been observed in retinal ganglion cells and the receptive fields as estimated from response peaks were larger than those estimated from sustained components.

Surround contribution to light adaptation in cat retinal ganglion cells

1. The sensitivity of a cat's retinal ganglion cell to a small, dim, spot flashed upon the middle of the receptive field depends upon the size of a concentric steady background: sensitivity is reduced monotonically with background area. All backgrounds which equal or exceed in size the central summing area of the ganglion cell produce an equivalent reduction of sensitivity, even though only backgrounds which extend outside the central summing area depress the maintained discharge. 2. If a small background lies upon the middle of the receptive field, and the test spot is made intense enough to evoke a strong response, steady illumination of the periphery may make the response larger. 3. This change in response is not due to an enhancement of centre sensitivity by the surround, but is readily understood if steady illumination of the periphery adapts out the surround's antagonism of the centre's response to the test flash. 4. The failure of steady stimulation of the surround to alter centre sensitivity implies that signals from the surround subtract from, or add to, those from the centre.