"Self-screening" of rhodopsin in rod outer segments

Space-time codependence of retinal ganglion cells can be explained by novel and separable components of their receptive fields

Reverse correlation methods such as spike‐triggered averaging consistently identify the spatial center in the linear receptive fields (RFs) of retinal ganglion cells (GCs). However, the spatial antagonistic surround observed in classical experiments has proven more elusive. Tests for the antagonistic surround have heretofore relied on models that make questionable simplifying assumptions such as space–time separability and radial homogeneity/symmetry. We circumvented these, along with other common assumptions, and observed a linear antagonistic surround in 754 of 805 mouse GCs. By characterizing the RF's space–time structure, we found the overall linear RF's inseparability could be accounted for both by tuning differences between the center and surround and differences within the surround. Finally, we applied this approach to characterize spatial asymmetry in the RF surround. These results shed new light on the spatiotemporal organization of GC linear RFs and highlight a major contributor to its inseparability.

Distinct subcomponents of mouse retinal ganglion cell receptive fields are differentially altered by light adaptation

The remarkable dynamic range of vision is facilitated by adaptation of retinal sensitivity to ambient lighting conditions. An important mechanism of sensitivity adaptation is control of the spatial and temporal window over which light is integrated. The retina accomplishes this by switching between parallel synaptic pathways with differing kinetics and degrees of synaptic convergence. However, the relative shifts in spatial and temporal integration are not well understood - particularly in the context of the antagonistic spatial surround. Here, we resolve these issues by characterizing the adaptation-induced changes to spatiotemporal integration in the linear receptive field center and surround of mouse retinal ganglion cells. While most ganglion cells lose their antagonistic spatial surround under scotopic conditions, a strong surround is maintained in a subset. We then applied a novel technique that allowed us to analyze the receptive field as a triphasic temporal filter in the center and a biphasic filter in the surround. The temporal tuning of the surround was relatively maintained across adaptation conditions compared to the center, which greatly increased its temporal integration. Though all phases of the center's triphasic temporal response slowed, some shifted significantly less. Additionally, adaptation differentially shifted ON and OFF pathway temporal tuning, reducing their asymmetry under scotopic conditions. Finally, spatial integration was significantly increased by dark adaptation in some cells while it decreased it in others. These findings provide novel insight into how adaptation adjusts visual information processing by altering fundamental properties of ganglion cell receptive fields, such as center-surround antagonism and space-time integration.

The neural retina in retinopathy of prematurity

Retinopathy of prematurity (ROP) is a neurovascular disease that affects prematurely born infants and is known to have significant long term effects on vision. We conducted the studies described herein not only to learn more about vision but also about the pathogenesis of ROP. The coincidence of ROP onset and rapid developmental elongation of the rod photoreceptor outer segments motivated us to consider the role of the rods in this disease. We used noninvasive electroretinographic (ERG), psychophysical, and retinal imaging procedures to study the function and structure of the neurosensory retina. Rod photoreceptor and post-receptor responses are significantly altered years after the preterm days during which ROP is an active disease. The alterations include persistent rod dysfunction, and evidence of compensatory remodeling of the post-receptor retina is found in ERG responses to full-field stimuli and in psychophysical thresholds that probe small retinal regions. In the central retina, both Mild and Severe ROP delay maturation of parafoveal scotopic thresholds and are associated with attenuation of cone mediated multifocal ERG responses, significant thickening of post-receptor retinal laminae, and dysmorphic cone photoreceptors. These results have implications for vision and control of eye growth and refractive development and suggest future research directions. These results also lead to a proposal for noninvasive management using light that may add to the currently invasive therapeutic armamentarium against ROP.

Connexin 36 and rod bipolar cell independent rod pathways drive retinal ganglion cells and optokinetic reflexes

Rod pathways are a parallel set of synaptic connections which enable night vision by relaying and processing rod photoreceptor light responses. We use dim light stimuli to isolate rod pathway contributions to downstream light responses then characterize these contributions in knockout mice lacking rod transducin-α (Trα), or certain pathway components associated with subsets of rod pathways. These comparisons reveal that rod pathway driven light sensitivity in retinal ganglion cells (RGCs) is entirely dependent on Trα, but partially independent of connexin 36 (Cx36) and rod bipolar cells. Pharmacological experiments show that rod pathway-driven and Cx36-independent RGC ON responses are also metabotropic glutamate receptor 6-dependent. To validate the RGC findings in awake, behaving animals we measured optokinetic reflexes (OKRs), which are sensitive to changes in ON pathways. Scotopic OKR contrast sensitivity was lost in Trα(-/-) mice, but indistinguishable from controls in Cx36(-/-) and rod bipolar cell knockout mice. Mesopic OKRs were also altered in mutant mice: Trα(-/-) mice had decreased spatial acuity, rod BC knockouts had decreased sensitivity, and Cx36(-/-) mice had increased sensitivity. These results provide compelling evidence against the complete Cx36 or rod BC dependence of night vision's ON component. Further, the findings suggest the parallel nature of rod pathways provides considerable redundancy to scotopic light sensitivity but distinct contributions to mesopic responses through complicated interactions with cone pathways.

Screening hypochromism in molecular aggregates and biopolymers

An improved model of the screening effect is suggested. Mutual ≪shielding≫ of chromophores from light due to competition for the incident photon can take place in molecular aggregates and macromolecules. From a common point of view, it could be interpreted as in ≪interaction≫ of the absorption dipol moment transitions. Screening leads to a decrease in the extinction coefficient. The largest decrease is observed in the maximum of the absorption band. This is demonstrated with chromophores of adenine, tyrosine, tryptophan, retinol, porphyrin and anthracene. The model enables prediction of hypochromic spectra or evaluation of the quantity of chromophores in an aggregate or macromolecule.

The effect of photopigment optical density on the color vision of the anomalous trichromat

We present a theoretical model to estimate the influence of photopigment optical density (OD) on the color vision of anomalous trichromats. Photopigment spectral sensitivities are generated using the Lamb (1995) template, which we correct for OD and pre-receptoral filters. Sixteen hyperspectral images (Foster, Nascimento, & Amano, 2004; Nascimento, Ferreira, & Foster, 2002) are analyzed, and the signals produced in the post-receptoral channels calculated. In the case of anomalous trichromats whose two longer-wavelength cones have peak sensitivities that lie close together in the spectrum, color vision can be substantially enhanced if the cones differ in optical density by a realistic amount.

Dark light, rod saturation, and the absolute and incremental sensitivity of mouse cone vision

Visual thresholds of mice for the detection of small, brief targets were measured with a novel behavioral methodology in the dark and in the presence of adapting lights spanning ∼8 log(10) units of intensity. To help dissect the contributions of rod and cone pathways, both wild-type mice and mice lacking rod (Gnat1(-/-)) or cone (Gnat2(cpfl3)) function were studied. Overall, the visual sensitivity of mice was found to be remarkably similar to that of the human peripheral retina. Rod absolute threshold corresponded to 12-15 isomerized pigment molecules (R*) in image fields of 800 to 3000 rods. Rod "dark light" (intrinsic retinal noise in darkness) corresponded to that estimated previously from single-cell recordings, 0.012 R* s(-1) rod(-1), indicating that spontaneous thermal isomerizations are responsible. Psychophysical rod saturation was measured for the first time in a nonhuman species and found to be very similar to that of the human rod monochromat. Cone threshold corresponded to ∼5 R* cone(-1) in an image field of 280 cones. Cone dark light was equivalent to ∼5000 R* s(-1) cone(-1), consistent with primate single-cell data but 100-fold higher than predicted by recent measurements of the rate of thermal isomerization of mouse cone opsins, indicating that nonopsin sources of noise determine cone threshold. The new, fully automated behavioral method is based on the ability of mice to learn to interrupt spontaneous wheel running on the presentation of a visual cue and provides an efficient and highly reliable means of examining visual function in naturally behaving normal and mutant mice.

Visual cycle modulation in neurovascular retinopathy

Rats with oxygen-induced retinopathy (OIR) model the pediatric retinal disease retinopathy of prematurity (ROP). Recent findings in OIR rats imply a causal role for the rods in the ROP disease process, although only experimental manipulation of rod function can establish this role conclusively. Accordingly, a visual cycle modulator (VCM) - with no known direct effect on retinal vasculature - was administered to "50/10 model" OIR Sprague-Dawley rats to test the hypotheses that it would 1) alter rod function and 2) consequently alter vascular outcome. Four litters of pups (N=46) were studied. For two weeks, beginning on postnatal day (P) 7, the first and fourth litters were administered 6 mg kg(-1) N-retinylacetamide (the VCM) intraperitoneally; the second and third litters received vehicle (DMSO) alone. Following a longitudinal design, retinal function was assessed by electroretinography (ERG) and the status of the retinal vessels was monitored using computerized fundus photograph analysis. Rod photoreceptor and post-receptor response amplitudes were significantly higher in VCM-treated than in vehicle-treated rats; deactivation of phototransduction was also significantly more rapid. Notably, the arterioles of VCM-treated rats showed significantly greater recovery from OIR. Presuming that the VCM did not directly affect the retinal vessels, a causal role for the neural retina - particularly the rod photoreceptors - in OIR was confirmed. There was no evidence of negative alteration of photoreceptor function consequent to VCM treatment. This finding implicates the rods as a possible therapeutic target in neurovascular diseases such as ROP.

Histamine reduces flash sensitivity of on ganglion cells in the primate retina

PURPOSE. In Old World primates, the retina receives input from histaminergic neurons in the posterior hypothalamus. They are a subset of the neurons that project throughout the central nervous system and fire maximally during the day. The contribution of these neurons to vision, was examined by applying histamine to a dark-adapted, superfused baboon eye cup preparation while making extracellular recordings from peripheral retinal ganglion cells. METHODS. The stimuli were 5-ms, 560-nm, weak, full-field flashes in the low scotopic range. Ganglion cells with sustained and transient ON responses and two cell types with OFF responses were distinguished; their responses were recorded with a 16-channel microelectrode array. RESULTS. Low micromolar doses of histamine decreased the rate of maintained firing and the light sensitivity of ON ganglion cells. Both sustained and transient ON cells responded similarly to histamine. There were no statistically significant effects of histamine in a more limited study of OFF ganglion cells. The response latencies of ON cells were approximately 5 ms slower, on average, when histamine was present. Histamine also reduced the signal-to-noise ratio of ON cells, particularly in those cells with a histamine-induced increase in maintained activity. CONCLUSIONS. A major action of histamine released from retinopetal axons under dark-adapted conditions, when rod signals dominate the response, is to reduce the sensitivity of ON ganglion cells to light flashes. These findings may relate to reports that humans are less sensitive to light stimuli in the scotopic range during the day, when histamine release in the retina is expected to be at its maximum.

The neurovascular retina in retinopathy of prematurity

The continuing worldwide epidemic of retinopathy of prematurity (ROP), a leading cause of childhood visual impairment, strongly motivates further research into mechanisms of the disease. Although the hallmark of ROP is abnormal retinal vasculature, a growing body of evidence supports a critical role for the neural retina in the ROP disease process. The age of onset of ROP coincides with the rapid developmental increase in rod photoreceptor outer segment length and rhodopsin content of the retina with escalation of energy demands. Using a combination of non-invasive electroretinographic (ERG), psychophysical, and image analysis procedures, the neural retina and its vasculature have been studied in prematurely born human subjects, both with and without ROP, and in rats that model the key vascular and neural parameters found in human ROP subjects. These data are compared to comprehensive numeric summaries of the neural and vascular features in normally developing human and rat retina. In rats, biochemical, anatomical, and molecular biological investigations are paired with the non-invasive assessments. ROP, even if mild, primarily and persistently alters the structure and function of photoreceptors. Post-receptor neurons and retinal vasculature, which are intimately related, are also affected by ROP; conspicuous neurovascular abnormalities disappear, but subtle structural anomalies and functional deficits may persist years after clinical ROP resolves. The data from human subjects and rat models identify photoreceptor and post-receptor targets for interventions that promise improved outcomes for children at risk for ROP.

Development of rod function in term born and former preterm subjects

PURPOSE

To provide an overview of some of our electroretinographic (ERG) and psychophysical studies of normal development of rod function and their application to retinopathy of prematurity (ROP).

METHODS

ERG responses to full-field stimuli were recorded from dark adapted subjects. Rod photoreceptor sensitivity (SROD) was calculated by fit of a biochemical model of the activation of phototransduction to the ERG a-wave. Dark adapted psychophysical thresholds for detecting 2 degrees spots in parafoveal (10 degrees eccentric) and peripheral (30 degrees eccentric) retina were measured and the difference between the thresholds, Delta10-30, was examined as a function of age. SROD and Delta10-30 in term born and former preterm subjects were compared.

RESULTS

In term born infants, (1) the normal developmental increase in SROD changes proportionately with the amount of rod visual pigment, rhodopsin, and (2) rod-mediated function in central retina is immature compared with that in peripheral retina. In subjects born prematurely, deficits in SROD persist long after active ROP has resolved. Maturation of rod-mediated thresholds in the central retina is prolonged by mild ROP.

CONCLUSIONS

Characterization of the development of normal rod and rod-mediated function provides a foundation for understanding ROP.

Modelling the Rayleigh match

We use the photopigment template of Baylor et al. (1987) to define the set of Rayleigh matches that would be satisfied by a photopigment having a given wavelength of peak sensitivity (λmax) and a given optical density (OD). For an observer with two photopigments in the region of the Rayleigh primaries, the observer's unique match is defined by the intersection of the sets of matches that satisfy the individual pigments. The use of a template allows us to illustrate the general behavior of Rayleigh matches as the absorption spectra of the underlying spectra are altered. In a plot of the Y setting against the red–green ratio (R), both an increase in λmax and an increase in optical density lead to an anticlockwise rotation of the locus of the matches satisfied by a given pigment. Since both these factors affect the match, it is not possible to reverse the analysis and define uniquely the photopigments corresponding to a specific Rayleigh match. However, a way to constrain the set of candidate photopigments would be to determine the trajectory of the change of match as the effective optical density is altered (by, say, bleaching or field size).

From candelas to photoisomerizations in the mouse eye by rhodopsin bleaching in situ and the light-rearing dependence of the major components of the mouse ERG

To quantify the rate at which light in a ganzfeld produces photoisomerizations in mouse rods in situ, we measured the rate of rhodopsin bleaching in eyes of recently euthanized mice with fully dilated pupils. The amount of rhodopsin declined as a first-order (exponential) function of the duration of the exposure at the luminance of 920 scot cd m(-2): the rate constants of bleaching were 8.3 x 10(-6) and 2.8 x 10(-5) s(-1) (scot cd(-1)m2)(-1) for C57B1/6 and 129P3/J mice, respectively. When the approximately 3-fold difference in effective areas of the pupils of the mice are taken into consideration, the bleaching rates for both strains become essentially the same, 2.6 x 10(-6) fraction rhodopsin (scot Td s)(-1). Assuming 7 x 10(7) rhodopsin molecules per rod, this bleaching rate yields the result that a flash of 1 scot Td s produces 181 photoisomerizations per rod, a value close to that derived from analysis of the collecting area of the rod for axially propagating light. We measured the electroretinograms of mice of the two strains reared under controlled illumination conditions (2 and 100 lux), and compared their properties, using the calibrations to determine the absolute sensitivities of the b-wave and a-waves. The intensity that produces a half-saturating rod b-wave response is 0.3-0.6 photoisomerizations rod(-1), and the amplification constant of the rod a-wave is 5-6 s(-2) photoisomerization(-1), with little dependence on the strain.

Recovery of the rod photoresponse in infant rats

We tested the hypothesis that the kinetics of recovery of the rod photoresponse differ between mature and immature rods. A paired flash paradigm was used. The effect of a test flash on the ERG a-wave response to a probe flash presented 60 to 2 s after the test flash was studied. The functions summarizing the interaction between the test and probe flash did not differ significantly between infants and adults if the stimuli were equated for estimated proportion of rhodopsin isomerized/rod/flash. The kinetics of rod cell recovery are likely the same in infants and adults.

Augmented rod bipolar cell function in partial receptor loss: an ERG study in P23H rhodopsin transgenic and aging normal rats

Physiological consequences of early stages of photoreceptor degeneration were examined in heterozygous P23H rhodopsin transgenic (Tg) and in aging normal Sprague-Dawley rats. Rod photoreceptor and rod bipolar (RB) cell function were estimated with maximum value and sensitivity parameters of P3 and P2 components of the electroretinogram. In both Tg and aging normal rats, the age-related rate of decline of P3 amplitude was steeper than that of the P2 amplitude. Tg rats showed greater than normal sensitivity of the rods. A new model of distal RB pathway connectivity suggested photoreceptor loss could not be the sole cause of physiological abnormalities; there was an additional increase of post-receptoral sensitivity. We propose that changes at rod-RB synapses compensate for the partial loss of rod photoreceptors in senescence and in early stages of retinal degeneration.

UV- and midwave-sensitive cone-driven retinal responses of the mouse: a possible phenotype for coexpression of cone photopigments

Molecular biological, histological and flicker electroretinographic results have established that mice have two cone photopigments, one peaking near 350 nm (UV-cone pigment) and a second near 510 nm [midwave (M)-cone pigment]. The goal of this investigation was to measure the action spectra and absolute sensitivities of the UV-cone- and M-cone-driven b-wave responses of C57BL/6 mice. To achieve this goal, we suppressed rod-driven signals with steady or flashed backgrounds and obtained intensity-response relations for cone-driven b-waves elicited by narrowband flashes between 340 and 600 nm. The derived cone action spectra can be described as retinal1 pigments with peaks at 355 and 508 nm. The UV peak had an absolute sensitivity of approximately 8 nV/(photon microm2) at the cornea, approximately fourfold higher than the M peak. In an attempt to isolate UV-cone-driven responses, it was discovered that an orange conditioning flash (lambda > 530 nm) completely suppressed ERG signals driven by both M pigment- and UV pigment-containing cones. Analysis showed that the orange flash could not have produced a detectable response in the UV-cone pathway were their no linkage between M pigment- and UV pigment-generated signals. Because cones containing predominantly the UV and M pigments have been shown to be located largely in separate parts of the mouse retina (), the most probable linkage is coexpression of M pigment in cones primarily expressing UV pigment. New histological evidence supports this interpretation (). Our data are consistent with an upper bound of approximately 3% coexpression of M pigment in the cones that express mostly the UV pigment.

The rod sensitivity of dark adapted human infants

The determinants of infants' low scotopic visual sensitivity are controversial. Some interpret infants' scotopic vision as indicative of immature rod photoreceptor function while others attribute infantile sensitivities mainly to post receptoral immaturities. To date the rod photoreceptor sensitivity of human infants has not actually been measured. In the work reported herein, electroretinographic a-wave responses, which represent the rod photoresponse, were recorded from dark adapted 10-week old infants and adult control subjects. Rod isolated a-waves indicate that 10-week-old infants' rods are less sensitive than adults'. Thus, any explanation of infants' scotopic visual sensitivity must take into account this fundamental property of infants' rods, low sensitivity.

The quantity of rhodopsin in young human eyes

The rhodopsin content of 20 eyes of infants and children ages 27 weeks gestation to 8 years (11 donors) was assayed and compared to the rhodopsin content of adults (36 eyes; 19 donors). Infants have significantly lower rhodopsin contents than adults. On average the rhodopsin content of young infants is about a third of adults. Previously reported full-field b-wave sensitivity of young infants is about 0.5 log units, that is about a third, less than adults. Thus, as previously found in infant rats, photon capture by rhodopsin appears to limit the dark adapted sensitivity of young human infants.

Development of the light response in neonatal mammalian rods

The sensitivity to light is low in many neonatal mammals when compared with that in the adult. In human infants at one month of age, for example, the dark-adapted sensitivity for detection of large stimuli is 50 times lower than in the adult, and in rats the overall sensitivity of the neonatal retina is also low compared with the adult. This low sensitivity in the neonate has been attributed to a number of factors, but the possibility that the photoreceptors themselves might be an important limitation on the overall visual sensitivity has not so far been clearly established. Here we record the light response of single neonatal rat rods and find that the sensitivity is considerably lower than in the adult. The response to a single photoisomerization is normal in the neonate, and the sensitivity deficit can therefore be attributed to a low level of functional rhodopsin. Opsin, the protein component of rhodopsin, must be present in normal amounts, as the sensitivity can be restored to adult levels by treating the retina with 9-cis retinal, an active homologue of the native chromophore 11-cis retinal. The low sensitivity of photoreceptors in the neonate can therefore be attributed mainly to a low concentration of 11-cis retinal in the developing retina.

Blue light hazard in rat

Rats have been extensively used in light damage studies. Retinal damage threshold for white light were found at 1-10 J/cm2, and the action spectrum resembled the absorption spectrum of visual pigment. We wished to answer the question whether a different class of light damage, the "blue light hazard", with white light damage thresholds at about 300 J/cm2, and an action spectrum peaking in the ultra-violet, could also be demonstrated in rat. To that purpose 5 deg patches of retina were exposed to white xenon light with exposure times between 10 sec and 1 hr. We found that for funduscopic threshold damage the product of irradiance and exposure time was constant at a level of 315 J/cm2. Thereafter, the action spectrum was measured by exposing rat eyes to narrow band spectral lights. Threshold irradiant dose ranged from 4 J/cm2 at 379 nm to 2000 J/cm2 at 559 nm. Thus, susceptibility for damage sharply increased towards the ultra-violet, just like in earlier monkey studies. We conclude that in similar experimental conditions susceptibility to photic injury in rat is comparable to that in primates. Rat is the first species for which two different action spectra of photochemical damage have been established.

Psychophysical estimates of ocular media density of human infants

The ocular media densities of eleven 10-week-old infants and of 10 adults were estimated from dark adapted thresholds measured using a two alternative forced choice preferential looking method. The estimated media density at 400 nm derived from these thresholds ranged from 0.55 to 1.00 (median 0.75) log units for infants, and from 1.35 to 1.69 (median 1.46) log units for young adults. This indicates media density increases significantly (0.71 log units) between infancy and adulthood. These data allow more complete specification of the retinal stimulus for quantitative analysis of infants' retinal function.

The development of scotopic retinal function in human infants

Scotopic retinal function undergoes age-related changes early in human infancy. Electroretinographic psychophysical, and pupillographic responses have been used in the study of normal development. Various components of the electroretinographic responses index distal and proximal retinal function. Changes in pupillary diameter, measurable in infants under carefully selected conditions, represent rhodopsin regeneration in the infants under carefully selected conditions, represent rhodopsin regeneration in the photoreceptor outer segment. From psychophysical data, inferences can be drawn about scotopic retinal control of visual performance. These data constrain theories about the determinants of sensitivity, about the flow of signals from the distal, rhodopsin-bearing, outer segments to the proximal retina, and about modulations of straight-through flow by feedback or inhibitory circuits. The results indicate that the post natal development of human scotopic function is due mainly to reorganization of processes central to the photoreceptors.

Visual detection by the rod system in goldfish of different sizes

New rods are continually generated and inserted across the entire differentiated retina in juvenile and adult goldfish; no other retinal cells share this characteristic. How does the preferential addition of rods affect visual function? To examine the relation between continued rod addition and visual sensitivity, we measured absolute threshold in fish of different sizes. Twenty-nine fish were trained in a classical conditioning paradigm, and psychometric functions were obtained for each of them for detection of a 532 nm light 5 sec in duration, 140 deg in angular subtense, presented while the fish was fully dark adapted. We found that absolute threshold (expressed in terms of retinal photon density) was lower in larger fish, but by a very small amount; on average, large fish (15.4 +/- 0.5 cm standard body length) were 1.45 times more sensitive than small fish (4.3 +/- 0.3 cm). Morphometric analysis showed that the planimetric density of rods in goldfish retina increases at a similar rate between small and large fish, while the density of retinal ganglion cells declines between small and large fish (by a factor of 3.8). The ratio of rods to ganglion cells (a possible indicator of neural convergence) increased, but by a factor that is too large to reconcile with the psychophysical results (5.3 x). The results suggest that absolute visual threshold in the goldfish is closely related to the density of rods in the retina.

A note on the action spectrum of human rod vision

A template representing the spectral distribution of the absorption coefficients of human rhodopsin was fitted to each of 59 individual action spectra of human rod vision (from one of three populations) by an optimization routine. Curve-fitting parameters included peak wavenumber, optical density at this wavenumber and (for those from the population neither aphakic nor constrained to ages where the standard lens transmissivity curve is supposed valid), density of the latter at the wavenumber of peak lens absorption. The average peak wavenumber of each population differed significantly from that of the other two. Either the standard curve of lens absorption (even with peak lens density as a curve-fitting parameter) is inappropriate for correcting the normal spectrum or the rhodopsins in the retinas of these populations do not all have identical wave-numbers of peak absorbance.