The ageing photoreceptor

With age many retinal neurons are lost. In humans the rod photoreceptor population in the perimacular region is subject to approximately 30% loss over life. Those that remain have been reported to suffer from extensive convolutions and localized swellings of their outer segments abnormally increasing their disc content and outer segment length. Here we examine quantitatively age-related changes in rat rod photoreceptors. The rat retina is approximately 97% rod dominated. Here, aged rods showed significant reductions in outer segment length. The discs in their outer segments had a similar density, irrespective of whether they were young or old, however, in aged animals a higher proportion were misregistered. Surprisingly, in all of the tissue examined, we found no evidence for any convolution of outer segments or localized swelling as reported in humans, rather all remained straight. There are methodological differences between the research reported here and that undertaken on human retinae. There are also major differences in overall retinal architecture between humans and rodents that could contribute to differences in the aging process of individual cells. If it is the case that individual photoreceptors age differently in rodents compared to humans, it may pose significant problems for the use of this animal model in studies of ageing and age related outer retinal disease.

Pathways mediating resolution in the primate retina

In recent years there has been a dramatic increase in knowledge of the anatomy of the primate retina that relates to the pathways involved in visual resolution. The density of cones at the fovea has been shown to have a surprising degree of individual variability and the cone distribution is asymmetric about the fovea with a greater density of cones in nasal than temporal retina. Information about the fine detail of our visual world is carried to the dorsal lateral geniculate nucleus of the thalamus in two parallel pathways. These pathways originate from two morphologically distinct types of ganglion cell; the M-ganglion cells project to the magnocellular layers, P-ganglion cells to the parvocellular layers. Different roles for the two types of ganglion cell in mediating spatial visual resolution have been proposed. These cannot be determined using available physiological and anatomical data.

Regenerative capacity of retinal cells and the maintenance of their differentiation

Mechanisms underlying cell type stability and the capacity of retinal cells for transdifferentiation are discussed. It is shown that cells of amphibian pigmented epithelium can be transformed into retina or lens cells depending on the inducing cell type: the influence of retina enables them to be transformed into retina, the influence of lens epithelium, to lens cells (lentoids or lenses). This led to an attempt to discover the molecular character of cell action by means of transfilter induction in early gastrula ectoderm of Xenopus laevis. The results show that the induced cell types correspond to the main inducing cell type, around which a range of neighbouring cell types is produced; this has been shown for five different cell types. The inducing factors involved seem to show qualitative differences. It is probable that they play a stabilizing role in the maintenance of the differentiated state of tissues, since temporary dissociation into cells leads eye tissues to transdifferentiate into other types. Such molecular factors can play a significant role in the maintenance of the type of differentiation and also in conversion into other cell types. These mechanisms of maintenance are not restricted to interactions between molecules and cells, since membranes on the surface of the retina and pigmented epithelium contribute to their shaping and consequently to the stability of the cell type.

Time course analysis of gene expression during light-induced photoreceptor cell death and regeneration in albino zebrafish

Constant intense light causes apoptosis of rod and cone photoreceptors in adult albino zebrafish. The photoreceptors subsequently regenerate from proliferating inner nuclear layer (INL) progenitor cells that migrate to the outer nuclear layer (ONL) and differentiate into rods and cones. To identify gene expression changes during this photoreceptor regeneration response, a microarray analysis was performed at five time points during the light treatment. The time course included an early time point during photoreceptor death (16 h), later time points during progenitor cell proliferation and migration (31, 51, and 68 h) and a 96 h time point, which likely corresponds to the initial photoreceptor differentiation. Mean expression values for each gene were calculated at each time point relative to the control (0 h light exposure) and statistical analysis by one-way ANOVA identified 4567 genes exhibiting significant changes in gene expression along the time course. The genes within this data set were clustered based on their temporal expression patterns and proposed functions. Quantitative real-time PCR validated the microarray expression profiles for selected genes, including stat3 whose expression increased markedly during the light exposure. Based on immunoblots, both total and activated Stat3 protein expression also increased during the light treatment. Immunolocalization of Stat3 on retinal tissue sections demonstrated increased expression in photoreceptors and Müller glia by 16 h of light exposure. Some of the Stat3-positive Müller cells expressed PCNA at 31 h, suggesting that Stat3 may play a role in signaling a subset of Müller cells to proliferate during the regeneration response.

Oxygen distribution and consumption in the macaque retina

The oxygen distribution in the retina of six anesthetized macaques was investigated as a model for retinal oxygenation in the human retina in and adjacent to the fovea. Po2 was measured as a function of retinal depth under normal physiological conditions in light and dark adaptation with O2 microelectrodes. Oxygen consumption (Qo2) of the photoreceptors was extracted by fitting a steady-state diffusion model to Po2 measurements. In the perifovea, the Po2 was 48 ± 13 mmHg (mean and SD) at the choroid and fell to a minimum of 3.8 ± 1.9 mmHg around the photoreceptor inner segments in dark adaptation, rising again toward the inner retina. The Po2 in the inner half of the retina in darkness was 17.9 ± 7.8 mmHg. When averaged over the outer retina, photoreceptor Qo2 (called Qav) was 4.6 ± 2.3 ml O2·100 g−1·min−1 under dark-adapted conditions. Illumination sufficient to saturate the rods reduced Qav to 72 ± 11% of the dark-adapted value. Both perifoveal and foveal photoreceptors received most of their O2 from the choroidal circulation. While foveal photoreceptors have more mitochondria, the Qo2 of photoreceptors in the fovea was 68% of that in the perifovea. Oxygenation in macaque retina was similar to that previously found in cats and other mammals, reinforcing the relevance of nonprimate animal models for the study of retinal oxygenation, but there was a smaller reduction in Qo2 with light than observed in cats, which may have implications for understanding the influence of light under some clinical conditions.

Vesicular glutamate transporter 1 is required for photoreceptor synaptic signaling but not for intrinsic visual functions

Glutamatergic neurotransmission requires vesicular glutamate transporters (VGLUTs) to sequester glutamate into synaptic vesicles. Generally, VGLUT1 and VGLUT2 isoforms show complementary expression in the CNS and retina. However, little is known about whether isoform-specific expression serves distinct pathways and physiological functions. Here, by examining visual functions in VGLUT1-null mice, we demonstrate that visual signaling from photoreceptors to retinal output neurons requires VGLUT1. However, photoentrainment and pupillary light responses are preserved. We provide evidence that melanopsin-containing, intrinsically photosensitive retinal ganglion cells (RGCs), signaling via VGLUT2 pathways, support these non-image-forming functions. We conclude that VGLUT1 is essential for transmitting visual signals from photoreceptors to second- and third-order neurons, but VGLUT1 is not necessary for intrinsic visual functions. Furthermore, melanopsin and VGLUT2 expression in a subset of RGCs immediately after birth strongly supports the idea that intrinsic vision can function well before rod- and cone-mediated signaling has matured.

Molecular mechanisms characterizing cone photoresponses

In the vertebrate retina, rods mediate twilight vision and cones mediate daylight vision. Their photoresponse characteristics are different. The light-sensitivity of a cone is 10(2)-10(3) times lower than that of a rod. In addition, the photoresponse time course is much faster in cones. The mechanism characterizing cone photoresponses has not been known mainly because of the difficulty in isolating cones in large quantities to perform biochemistry. Recently, we developed a method to purify cones from carp retina using a density gradient, which made it possible to analyze the differences in the molecular mechanism of phototransduction between rods and cones. The results showed that signal amplification in cones is less effective, which explains the lower light-sensitivity of cones. The results also showed that visual pigment phosphorylation, a quenching mechanism of light-activated visual pigment, is much more rapid in cones than in rods. The rapid phosphorylation in cones is attributed to a very high total kinase activity in cones. Because of this high activity, cone pigment is readily phosphorylated even at very high bleaching levels, which probably explains why cone photoresponses recover quickly. Based on these findings, the molecular mechanisms of the differences in the photoresponse characteristics between rods and cones are outlined.

Preservation of visual cortical function following retinal pigment epithelium transplantation in the RCS rat using optical imaging techniques

The aim of this study was to determine the extent of cortical functional preservation following retinal pigment epithelium (RPE) transplantation in the Royal College of Surgeons (RCS) rat using single-wavelength optical imaging and spectroscopy. The cortical responses to visual stimulation in transplanted rats at 6 months post-transplantation were compared with those from age-matched untreated dystrophic and non-dystrophic rats. Our results show that cortical responses were evoked in non-dystrophic rats to both luminance changes and pattern stimulation, whereas no response was found in untreated dystrophic animals to any of the visual stimuli tested. In contrast, a cortical response was elicited in most of the transplanted rats to luminance changes and in many of those a response was also evoked to pattern stimulation. Although the transplanted rats did not respond to high spatial frequency information we found evidence of preservation in the cortical processing of luminance changes and low spatial frequency stimulation. Anatomical sections of transplanted rat retinas confirmed the capacity of RPE transplantation to rescue photoreceptors. Good correlation was found between photoreceptor survival and the extent of cortical function preservation determined with optical imaging techniques. This study determined the efficacy of RPE transplantation to preserve visual cortical processing and established optical imaging as a powerful technique for its assessment.

A numerical study of red-green colour opponent properties in the primate retina

It remains an important question whether neural function is mediated entirely by its tailored circuitry. A persistent debate in retinal colour vision is whether the centre and the surround of a ganglion cell receptive field receive dominant inputs either from L or M cones in an antagonistic manner (the selective wiring model) or mixed inputs (the mixed wiring model). Despite many anatomical, physiological and psychophysical experiments, a decisive conclusion has not been reached. An in-depth examination of what the pure mixed wiring mechanisms predicts is therefore important. These two models make different predictions both for the fovea and for the peripheral retina. Recently, a dynamic cellular model of the primate fovea was developed [Momiji et al. (2006) Vis. Res., 46, 365-381]. Unlike earlier models, it explicitly incorporates spatial non-uniformities, such as the random arrangement of L and M cones. Here, a related model is developed for the peripheral retina by incorporating anatomically reasonable degrees of convergence between cones, bipolar cells and ganglion cells. These two models, in which selective wiring mechanisms are absent, are applied to describe both foveal and peripheral colour vision. In numerical simulations, peripheral ganglion cells are less colour sensitive than foveal counterparts, but none-the-less display comparative sensitivities. Furthermore, peripheral colour sensitivity increases with temporal frequency, relative to foveal sensitivity. These results are congruent with recent physiological experiments.

Role of neuronal activity and kinesin on tract tracing by manganese-enhanced MRI (MEMRI)

MEMRI offers the exciting possibility of tracing neuronal circuits in living animals by MRI. Here we use the power of mouse genetics and the simplicity of the visual system to test rigorously the parameters affecting Mn2+ uptake, transport and trans-synaptic tracing. By measuring electrical response to light before and after injection of Mn2+ into the eye, we determine the dose of Mn2+ with the least toxicity that can still be imaged by MR at 11.7 T. Using mice with genetic retinal blindness, we discover that electrical activity is not necessary for uptake and transport of Mn2+ in the optic nerve but is required for trans-synaptic transmission of this tracer to distal neurons in this pathway. Finally, using a kinesin light chain 1 knockout mouse, we find that conventional kinesin is a participant but not essential to neuronal transport of Mn2+ in the optic tract. This work provides a molecular and physiological framework for interpreting data acquired by MEMRI of circuitry in the brain.

An overview on the toxic morphological changes in the retinal pigment epithelium after systemic compound administration

Many medications that are administered systemically for nonocular conditions may evoke ocular toxicological complications. Therefore, the eye is routinely investigated histopathologically in preclinical in vivo toxicity studies. The retinal pigment epithelium is a likely target for systemically administered compounds, since the underlying choroid is highly vascularized. The specialized pigment epithelium has numerous functions that all maintain the integrity and function of photoreceptors. Consequently, toxic effects on the pigment epithelium will eventually affect the neural retina. The potential of pigment epithelial cells to respond to toxic injury is limited, but a standardized terminology to describe its morphological changes does not exist in the scientific literature. Detailed morphologic analysis, however, might allow early detection of retinotoxicity and may provide evidence on the underlying pathomechanism. We here review toxic effects on the pigment epithelium focusing in particular on the morphology of toxic cell injury. Morphological changes comprise hypertrophy, intracytoplasmic accumulation of cellular components, loss of cell polarity, degeneration, metaplasia, and formation of subretinal membranes. Some of these changes are reversible whereas others are permanent, leading to impaired function of the pigment epithelium and eventually to photoreceptor loss and retinal atrophy.

Scotopic and photopic visual thresholds and spatial and temporal discrimination evaluated by behavior of mice in a water maze

Methods that allow specific manipulations of the mouse genome have made it possible to alter specific aspects of photoreceptor function within the mouse retina. Mice with photoreceptors that have altered photosensitivities and altered photoresponse kinetics are now available. Methods are needed that can show how those perturbations in photoreceptor response characteristics translate into perturbations in visual sensitivity and perception. We have adapted a previously described method to evaluate visual threshold, spatial discrimination and temporal discrimination in mice swimming in a water maze. In this report we describe the sensitivities of rod-mediated and cone-mediated vision using GNAT1-/-and GNAT2-/- mice. Cone-mediated vision is approximately 10,000 times less sensitive than rod-mediated vision in mice. We also demonstrate that mice can distinguish striped from solid objects in the water maze and that they can distinguish flickering from continuous illumination.

Tokay gecko photoreceptors achieve rod-like physiology with cone-like proteins

The retinal photoreceptors of the nocturnal Tokay gecko (Gekko gekko) consist exclusively of rods by the criteria of morphology and key features of their light responses. Unlike cones, they display robust photoresponses and have relatively slow recovery times. Nonetheless, the major and minor visual pigments identified in gecko rods are of the cone type by sequence and spectroscopic behavior. In the ongoing search for the molecular bases for the physiological differences between cones and rods, we have characterized the molecular biology and biochemistry of the gecko rod phototransduction cascade. We have cloned cDNAs encoding all or part of major protein components of the phototransduction cascade by RT-PCR with degenerate oligonucleotides designed to amplify cone- or rod-like sequences. For all proteins examined we obtained only cone-like and never rod-like sequences. The proteins identified include transducin alpha (Galphat), phosphodiesterase (PDE6) catalytic and inhibitory subunits, cyclic nucleotide-gated channel (CNGalpha) and arrestin. We also cloned cDNA encoding gecko RGS9-1 (Regulator of G Protein Signaling 9, splice variant 1), which is expressed in both rods and cones of all species studied but is typically found at 10-fold higher concentrations in cones, and found that gecko rods contain slightly lower RGS9-1 levels than mammalian rods. Furthermore, we found that the levels of GTPase accelerating protein (GAP) activity and cyclic GMP (cGMP) phosphodiesterase activity were similar in gecko and mammalian rods. These results place substantial constraints on the critical changes needed to convert a cone into a rod in the course of evolution: The many features of phototransduction molecules conserved between those expressed in gecko rods and those expressed in cones cannot explain the physiological differences, whereas the higher levels of RGS9-1 and GAP activity in cones are likely among the essential requirements for the rapid photoresponses of cones.

Absence of normal photic integration in the circadian visual system: response to millisecond light flashes

Light is the most prominent synchronizing stimulus for circadian rhythms. The circadian visual system responds in accordance with the energy content of photic stimuli longer than a few seconds. Here, as few as three flashes (2 ms each delivered to hamsters over 5 or 60 min at circadian time 19) elicited large phase advances. Ten or more flashes were required to induce FOS protein in the suprachiasmatic nucleus (SCN), and such induction occurred throughout the entire SCN, as well as outside the nucleus. High-density flash stimulation (0.5 s interflash interval) was ineffective, but response increased as the interval increased up to 4 s. In an irradiance response test, phase shifts appeared to be all-or-none with threshold irradiance between 140 and 1070 microW/cm2, implying lack of stimulus energy summation. Nevertheless, an irradiance ineffective when delivered as 10 flashes induced phase shifts when given as 100 flashes, but the response was substantially smaller than elicited by 10 flashes, each with approximately 1 log unit more irradiance. The results also show reduced sensitivity of flash-induced FOS response in the intergeniculate leaflet compared with the SCN, contrary to studies using longer light stimuli. Masking was robust and prolonged in response to 10 flashes. The data demonstrate that the circadian visual system responds markedly to brief, intense light stimuli without normal photic integration. This may involve a second input pathway different from that mediating the effects of longer, dimmer photic stimuli.

An intron control region differentially regulates expression of thyroid hormone receptor beta2 in the cochlea, pituitary, and cone photoreceptors

The Thrb gene, encoding thyroid hormone receptor beta (TRbeta), serves key roles in endocrine regulation and the development of the senses of hearing and color vision. The versatile functions of this gene depend upon its expression of distinct receptor isoforms by differential promoter activation. The TRbeta2 isoform has a particularly specialized distribution including in the anterior pituitary and cochlea. TRbeta2 is also found in immature cone photoreceptors where it has a unique role in programming the expression pattern of opsin photopigments that mediate color vision. Given the importance of precise, tissue-specific expression for the function of TRbeta2, we investigated the genomic control elements that direct this expression in vivo using lacZ reporter transgenes in mice. The TRbeta2 promoter region is sufficient for cochlear expression, whereas a complex intron control region is necessary for pituitary and retinal expression. In the retina, the intron region directs peak expression in the embryo in postmitotic, immature cones. The retinal control region is further subdivided into domains that specify and amplify expression, respectively, indicating that timely, cone-specific expression reflects an integrated response to complex signals. The mammalian Thrb gene has therefore incorporated several mechanisms into a multifunctional intron control region that regulates developmental induction of the distant promoter. This specialized genomic organization underlies the unique expression pattern and functions of TRbeta2.

Parasympathetic control of the pupillary light response in the red-eared slider turtle (Pseudemys scripta elegans)

OBJECTIVE

We investigated effects of both vecuronium bromide, a nicotinic cholinergic antagonist, and atropine, a muscarinic cholinergic antagonist, on the pupil of the turtle to determine whether responses to light are controlled by parasympathetic innervations acting on the iris.

ANIMAL STUDIED

Three red-eared slider turtles, Pseudemys scripta elegans.

PROCEDURE

Turtles were secured to immobilize their head movements and then inserted into a light-integrating sphere. Both pupils were monitored through small apertures by digital video cameras. Pupil diameters were measured manually with a digital caliper. During each trial, drugs (0.4%) were topically applied, four times at 15 min intervals, to the corneas of each eye. One eye was randomly selected for treatment of the drug while the other, treated with saline (0.9% NaCl), was used as control. Pupil sizes under adaptation to light were tracked after drug or saline applications.

RESULTS

Mean pupillary diameters of eyes treated with vercuronium bromide increased by 28%, reaching peak size in 90 min. Onset of response occurred 20 min after drug application and then increased at a rate having a time constant of 26 min. Recovery began at 120 min after initial application. Atropine had no effect on pupil size. No systemic side effects by drugs were observed in turtles.

CONCLUSIONS

Although atropine does not cause mydriasis, vecuronium bromide does. These results suggest that the parasympathetic system in turtles acts through acetylcholine onto nicotinic receptors to stimulate pupillary light constriction.

Photoreceptor waveguides and effective retinal image quality

Individual photoreceptor waveguiding suggests that the entire retina can be considered as a composite fiber-optic element relating a retinal image to a corresponding waveguided image. In such a scheme, a visual sensation is produced only when the latter interacts with the pigments of the outer photoreceptor segments. Here the possible consequences of photoreceptor waveguiding on vision are studied with important implications for the pupil-apodization method commonly used to incorporate directional effects of the retina. In the absence of aberrations, it is found that the two approaches give identical predictions for an effective retinal image only when the pupil apodization is chosen twice as narrow as suggested by the traditional Stiles-Crawford effect. In addition, phase variations in the retinal field due to ocular aberrations can delicately alter a waveguided image, and this may provide plausible justification for an improved visual sensation as compared with what should be expected on the grounds of a retinal image only.

Significant photoreceptor rescue by treatment with a combination of antioxidants in an animal model for retinal degeneration

The purpose of this study was to investigate the presence of oxidative DNA damage in the photoreceptors of the rd1 mouse, an animal model for retinitis pigmentosa, and to determine if antioxidants could delay the progress of photoreceptor cell death. Retinas of rd1 mice and congenic wild type controls were examined for DNA oxidation and fragmentation. To study the rescue effect of antioxidants on retinal degeneration, rd1 retinas were studied in vitro and in vivo using lutein, zeaxanthin, alpha lipoic acid and reduced l-glutathione. For the in vitro studies, antioxidants were added to the culture medium. For the in vivo studies, postnatal day (PN3) pups of rd1 mice were fed antioxidants either individually or in combination and control rd1 animals received vehicle alone. Histological evaluation was performed using hematoxylin/eosin and avidin staining, as well as terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay. Many of the rd1 rod photoreceptors at PN11 displayed oxidative DNA damage and TUNEL positive reaction which co-localized in a subset of rod photoreceptors. Avidin-labeled rod photoreceptors were more abundant than the TUNEL positive photoreceptors of the rd1 mouse, indicating that oxidative DNA damage precedes fragmentation. The number of TUNEL positive and avidin positive cells was considerably decreased upon treatment with the combination of the antioxidants. Rescue of rd1 photoreceptors was significant at PN18 and PN17, respectively, in the in vitro and in vivo studies. In conclusion individual antioxidants had no significant rescue effect but the combination slowed down the rd1 rod photoreceptor degeneration, indicating an additive or synergistic effect.

Reduced synaptic vesicle density and aberrant synaptic localization caused by a splice site mutation in the Rs1h gene

X-linked retinoschisis (XLRS) is a common form of inherited macular degeneration caused by mutations in the RS1 gene. Whereas the role of RS1 has been implicated in the synaptic structure as well as layer organization in the retina, the pathological effect of a defective RS1 gene on the synaptic interaction between photoreceptor cells and second-order neurons has not been thoroughly investigated. In this study, we perform a detailed characterization of the retinal synaptic phenotypes caused by a splice site mutation in the murine RS1 homolog (Rs1htmgc1). Electron microscopic analysis showed that presynaptic terminals of photoreceptor cells contain a lower areal density of synaptic vesicles in the Rs1htmgc1 retina. Examination of the synaptic interactions in the outer plexiform layer also revealed ectopic localization of photoreceptor cell presynaptic markers and elongation of neurites from postsynaptic neurons (bipolar and horizontal cells), which are observed in other mouse models with defective photoreceptor cell molecules. Consistent with these synaptic abnormalities, ERG analysis of young Rs1htmgc1 mice revealed attenuation of the b-wave with preservation of the a-wave. These results demonstrate that RS1H has functional significance in the morphology and function of the synapse between photoreceptors and second-order neurons. A developmental study from postnatal day (P) 15 through P19 showed that synaptic interactions form normally, and structural abnormalities occur after completion of synaptic formation suggesting that RS1H is important for the maintenance of this synaptic interaction. Thus, Rs1htmgc1 mice may serve as a new genetic model for human XLRS and other synaptic disorders.

Involvement of a Go-type G-protein coupled to guanylate cyclase in the phototransduction cGMP cascade of molluscan simple photoreceptors

Simple photoreceptors, namely photoresponsive neurons, designated as A-P-1, Es-1, Ip-2 and Ip-1, exist in the sea slug Onchidium ganglion. Previous works has shown that, of these, Ip-2 and Ip-1 respond to light with a hyperpolarizing receptor potential, caused by the opening of light-dependent, cGMP-gated K+ channels, whereas A-P-1 and Es-1 are depolarized by light, owing to the closing of the same K+ channels. The present study of Ip-2 or Ip-1 cells was undertaken to identify the G-proteins that couple light to the activation of guanylate cyclase (GC), thereby leading to the opening of K+ channels and the consequent hyperpolarizing photocurrents. The specific channel blocker, 4-aminopyridine (4-AP), and a GC inhibitor, LY-83583, both suppressed this hyperpolarizing photocurrent. N-ethylmaleimide and GDP-beta-S also inhibited this photocurrent, consistent with the involvement of G-proteins. Mastoparan an activator of both Go- and Gi-type G-proteins, induced an outward current. Furthermore, benzalkonium chloride (C(16)BAC), a selective activator of Go, dose-dependently generated an outward current similar to that induced by mastoparan. Both of these outward currents were susceptible to 4-AP, LY-83583 and N-ethylmaleimide. Taken together, these results suggest that phototransduction in Ip-2 or Ip-1 cells is triggered by a Go-type G-protein coupled to GC. Thus, this new cGMP cascade contrasts with the conventional phototransduction cGMP cascade mediated by the Gt-type G-protein coupled to phosphodiesterase, seen in the vertebrate photoreceptors and the above A-P-1 or Es-1 cells.