Light-dependent changes in cytoplasmic aspartate aminotransferase activity in rod spherules of the rat retina. A cytochemical study.

The cytoplasmic activity of the glutamate synthesizing enzyme aspartate aminotransferase (c-AAT) has been investigated on the ultrastructure level in rod spherules of light and dark adapted rat retinae using cytochemistry. Although most rod terminals react negatively, in a subpopulation of rods a weak activity, which is observed in light adapted retinae, is markedly increased under dark conditions. This indicates, that in addition to cones, some rods might use glutamate as their transmitter as well.     

真鲷视网膜结构及其视觉特性研究：Ⅱ视网膜光感受细胞超微结构研究

对真鲷光感受细胞的超微结构进行观察,结果表明：视杆外段膜盘为游离膜盘,视锥外段膜盘则为连续的膜结构,视锥和视杆均含有连接纤毛和辅助外段。花萼状突起起源于内段。椭体内充满线粒体,无球状小体。双锥椭圆体并生膜为六层,视锥内段无鳍状突起,视锥突触带,在明适应视网膜中数量增多,在暗适应视网中数量减少,视杆突触带在这两种适应网膜中数量不变,每一杆小球只有一个突触带,而锥小足有４－６个突触带。     

Elevation of cyclic AMP activates an actin-dependent contraction in teleost retinal rods

Agents which elevate cyclic AMP (cAMP) cause teleost retinal rods to contract. We have characterized this cAMP effect and have evaluated the role of the cytoskeleton in cyclic nucleotide-induced contraction, using actin and microtubule inhibitors. The necklike myoid region of the rod contracts in the dark and elongates in the light. If long, light-adapted rods are cultured with cAMP analogs and IBMX, rods contract to their short dark-adapted position. Cyclic nucleotide- induced rod contraction occurs in constant light, requires a phosphodiesterase inhibitor, and is specific to cAMP (db cyclic GMP, 8- bromocyclic GMP, 5'AMP, and adenosine have no effect on rod myoid length). Cyclic AMP effects on rod length are consistent with observations from several species that cAMP levels are higher in dark- adapted than in light-adapted retinas. Since rod myoids contain paraxially aligned actin filaments and microtubules, we have used the motility inhibitors cytochalasin D and cold and nocodazole to investigate the roles of these cytoskeletal elements in rod contraction. Cyclic nucleotide-induced contraction is not inhibited when myoid microtubules are disrupted with cold and nocodazole treatments, but contraction is blocked if myoid actin filaments are disrupted with cytochalasin D. Thus, we conclude that actin filaments, but not microtubules, are required for rod contraction. We propose that rod contraction in vivo is triggered by a rise of cytoplasmic cAMP at onset of darkness and that this contraction is mediated by an actin- dependent mechanism.     

Rod outer segments are designed for optimum photon detection

The proposal that rod outer segment length is optimal with respect to photon absorption and noise control is extended and tested in a number of species. We find good agreement with our optimality criterion in duplex retinae where rods act as detectors of one or a few photons, but not in all rod retinae nor in those which are exposed to significant photic environmental noise.     

Rod sensitivity of neonatal mouse and rat

We have measured the sensitivity of rod photoreceptors isolated from overnight dark-adapted mice of age P12 (neonate) through P45 (adult) with suction-pipette recording. During this age period, the dark current increased roughly in direct proportion to the length of the rod outer segment. In the same period, the flash sensitivity of rods (reciprocal of the half-saturating flash intensity) increased by approximately 1.5-fold. This slight developmental change in sensitivity was not accentuated by dark adapting the animal for just 1 h or by increasing the ambient luminance by sixfold during the prior light exposure. The same small, age-dependent change in rod sensitivity was found with rat. After preincubation of the isolated retina with 9-cis-retinal, neonatal mouse rods showed the same sensitivity as adult rods, suggesting the presence of a small amount of free opsin being responsible for their lower sensitivity. The sensitivity of neonate rods could also be increased to the adult level by dark adapting the animal continuously for several days. By comparing the sensitivity of neonate rods in darkness to that of adult rods after light bleaches, we estimated that approximately 1% of rod opsin in neonatal mouse was devoid of chromophore even after overnight dark adaptation. Overall, we were unable to confirm a previous report that a 50-fold difference in rod sensitivity existed between neonatal and adult rats.     

Taurine,amino acid transmitters,and related molecules in the retina of the Australian lungfish Neoceratodus forsteri: a light-microscopic immunocytochemical and electron-microscopic study

The morphology of the retina of the Australian lungfish Neoceratodus forsteri was investigated by means of light- and electron microscopy, whilst immunocytochemical studies were performed to determine the cellular distributions of the major amino acid neurotransmitters and other amino acids. The distributions of glycine and GABA were similar to those previously described for teleost, amphibian and mammalian retinae. Labelling was abundant in amacrine cells, whilst GABA was also present in one layer of horizontal cells and some bipolar cells. Taurine was present in both rods and cones, but, unlike the mammalian or avian retina, was absent from other cellular structures, including glial elements. Unexpectedly, the photoreceptor terminals lacked an apparent content of the excitatory amino acid transmitter glutamate. The glutamate that was present in the rods and cones occupied a crescentic arc corresponding to the location of glycogen-rich paraboloids. Asparagine was also present in rods, albeit in the modified mitochondria that formed the elipsoids of the rod inner segments. Arginine, the precursor for formation of nitric oxide, was present in glial cells, and in the paraboloids of both rods and cones.     

Retinal response in the post metamorphic American eel (Anguilla rostrata) (Pisces,Teleostei)

Retinas of light and dark adapted post metamorphic American eelAnguilla rostrata were examined. The retinal epithelial pigment migrates vitreally in light and sclerally in darkness. Two layers of rods and a layer of single cones are present. Some cones elongate slightly in the dark and contract in the light. The cone synaptic ribbons show no difference between the light and dark adapted stages. It appears that this eel stage is capable of functioning in bright and dim environments.     

Rod sensitivity during Xenopus development

We have measured the sensitivity of rod photoreceptors from overnight-dark-adapted Xenopus laevis through developmental stages 46-66 into adulthood by using suction-pipette recording. The dark current increased gradually from approximately 5 pA at stage 46 to approximately 20 pA at stage 57, compared with an adult (metamorphosed) current of approximately 35 pA. This increase in dark current largely paralleled the progressive increase in length and diameter of the rod outer segment (ROS). Throughout stages 46-66, the dark current increased approximately linearly with ROS surface area. At stage 53, there was a steep (approximately 10-fold) increase in the rod flash sensitivity, accompanied by a steep increase in the time-to-peak of the half-saturated flash response. This covariance of sensitivity and time-to-peak suggested a change in the state of adaptation of rods at stage 53 and thereafter. When the isolated retina was preincubated with 11-cis-retinal, the flash sensitivity and the response time-to-peak of rods before stage 53 became similar to those at or after stage 53, suggesting that the presence of free opsin (i.e., visual pigment without chromophore) in rods before stage 53 was responsible for the adapted state (low sensitivity and short time-to-peak). By comparing the response sensitivity before stage 53 to the sensitivity at/after stage 53 measured from rods that had been subjected to various known bleaches, we estimated that 22-28% of rod opsin in stage 50-52 tadpoles (i.e., before stage 53) was devoid of chromophore despite overnight dark-adaptation. When continuously dark adapted for 7 d or longer, however, even tadpoles before stage 53 yielded rods with similar flash sensitivity and response time-to-peak as those of later-stage animals. In conclusion, it appears that chromophore regeneration is very slow in tadpoles before stage 53, but this regeneration becomes much more efficient at stage 53. A similar delay in the maturity of chromophore regeneration may partially underlie the low sensitivity of rods observed in newborn mammals, including human infants.     

Spectral sensitivity of the cod,Gadus morhua L.

The spectral sensitivity of the cod was determined under both dark adapted and light adapted conditions in the laboratory. Cod were trained by cardiac conditioning to detect a difference in radiance between an image of spots and the background radiance of a screen. Thresholds for this response were measured for a range of different wavelengths, and expressed as quantum adjusted values. Electroretino‐graphic studies were also performed on the eyes of cod, and spectral sensitivity curves prepared. Under dark adapted conditions both the behavioural and e.r.g. derived curves showed greatest sensitivity in the blue/green at 490 nm, matching the absorption curve for rhodopsin. A secondary peak in the behaviourally derived curve in the green/yellow at 550 nm indicated that a population of yellow cones may be implicated with the rods in scotopic vision. Under light adapted conditions the behavioural curves showed a shift to the blue, perhaps indicating an adaption to the high red content of the illuminating source. The e.r.g. curve showed greatest sensitivity to blue/green, as in the scotopic experiments but with an enhanced response at 550 nm, indicating greater cone activity. It is suggested that there is complex interaction between rods and cones in the cod retina, both types of receptor being active over a wide range of light intensities.     

Origin and functional impact of dark noise in retinal cones

Spontaneous fluctuations in the electrical signals of the retina's photoreceptors impose a fundamental limit on visual sensitivity. While noise in the rods has been studied extensively, relatively little is known about the noise of cones. We show that the origin of the dark noise in salamander cones varies with cone type. Most of the noise in long wavelength-sensitive (L) cones arose from spontaneous activation of the photopigment, which is a million-fold less stable than the rod photopigment rhodopsin. Most of the noise in short wavelength-sensitive (S) cones arose in a later stage of the transduction cascade, as the photopigment was relatively stable. Spontaneous pigment activation effectively light adapted L cones in darkness, causing them to have a smaller and briefer dim flash response than S cones.     

THE INFLUENCE OF LIGHT ADAPTATION ON SUBSEQUENT DARK ADAPTATION OF THE EYE

The course of dark adaptation of the human eye varies with the intensity used for the light adaptation which precedes it. Preadaptation to intensities below 200 photons is followed only by rod adaptation, while preadaptation to intensities above 4000 photons is followed first by cone adaptation and then by rod adaptation. With increasing intensities of preadaptation, cone dark adaptation remains essentially the same in form, but covers an increasing range of threshold intensities. At the highest preadaptation the range of the subsequent cone dark adaptation covers more than 3 log units. Rod dark adaptation appears in two types—a rapid and a delayed. The rapid rod dark adaptation is evident after preadaptations to low intensities corresponding to those usually associated with rod function. The delayed rod dark adaptation shows up only after preadaptation to intensities which are hundreds of times higher than those which produce the maximal function of the rods in flicker, intensity discrimination, and visual acuity. The delayed form remains essentially constant in shape following different intensities of preadaptation. However, its time of appearance increases with the preadaptation intensity; after the highest preadaptation, it appears only after 12 or 13 minutes in the dark. These two modes of rod dark adaptation are probably the expression of two methods of formation of visual purple in the rods after its bleaching by the preadaptation lights.     

Weber and noise adaptation in the retina of the toad Bufo marinus

Responses to flashes and steps of light were recorded intracellularly from rods and horizontal cells, and extracellularly from ganglion cells, in toad eyecups which were either dark adapted or exposed to various levels of background light. The average background intensities needed to depress the dark-adapted flash sensitivity by half in the three cell types, determined under identical conditions, were 0.9 Rh*s-1 (rods), 0.8 Rh*s-1 (horizontal cells), and 0.17 Rh*s-1 (ganglion cells), where Rh* denotes one isomerization per rod. Thus, there is a range (approximately 0.7 log units) of weak backgrounds where the sensitivity (response amplitude/Rh*) of rods is not significantly affected, but where that of ganglion cells (1/threshold) is substantially reduced, which implies that the gain of the transmission from rods to the ganglion cell output is decreased. In this range, the ganglion cell threshold rises approximately as the square root of background intensity (i.e. in proportion to the quantal noise from the background), while the maintained rate of discharge stays constant. The threshold response of the cell will then signal light deviations (from a mean level) of constant statistical significance. We propose that this type of ganglion cell desensitization under dim backgrounds is due to a post-receptoral gain control driven by quantal fluctuations, and term it noise adaptation in contrast to the Weber adaptation (desensitization proportional to the mean background intensity) of rods, horizontal cells, and ganglion cells at higher background intensities.     

Duplex Retina in the Mesopelagic Deep-Sea Teleost Lestidiops affinis (Ege, 1930)

The eye of the deep-sea teleost Lestidiops affinis has been examined primarily by light microscopy and found to possess a duplex retina consisting of two main divisions, a pure-cone and a pure-rod region, with a narrow zone of transition, possessing both cones and rods, joining the two. The pure-cone region is located in the temporal (caudal) part of the retina subserving binocular vision in the rostral direction. It has an area temporalis retinae with particularly long and densely packed single cones arranged in a regular hexagonal mosaic. Joined (double or twin) cones have not been recognized with certainty in the pure-cone region. The pure-rod region, comprising the larger part of the retina, contains rods grouped in bundles separated by retinal pigment epithelium (RPE) processes with pigmented cores. The synaptic endings of the rods are arranged in separate clusters in the outer plexiform layer, there being apparently a separate rod pedicle cluster beneath (vitread to) each rod bundle. Structural comparisons with certain other deep-sea teleosts suggest the likely presence of a retinal tapetum in L. affinis, i.e. each single cone or rod bundle is situated in a reflecting pit formed by the RPE, with a discrete reflector apposed to the tip of each cone outer segment and the tips of the outer segments of each square-cut rod bundle.     

Light-adaptive role of nitric oxide in the outer retina of lower vertebrates: a brief review

The role of nitric oxide (NO) as a novel neurochemical mechanism controlling light adaptation of the outer retina is discussed by considering mainly published results. The emphasis is on the retinae of fishes and amphibia, but some data from the mammalian (rabbit) retinae have also been included for completeness. In the fish retina, application of NO donors in the dark caused light-adaptive photomechanical movements of cones. The normal effect of light adaptation in inducing cone contractions was suppressed by pretreatment of retinae with an NO scavenger. NO donors modulated horizontal cell activity by uncoupling the cells' lateral gap junctional interconnections and enhancing negative feedback to cones, again consistent with a light-adaptive role of NO. Direct evidence for light adaptation-induced release of NO has been obtained in fish (carp) and rabbit retinae. The results strongly suggest that control of retinal light adaptation is, under multiple neurochemical control, with NO and dopamine having an interactive role.     

Kinetics of light-sensitive channels in vertebrate photoreceptors

We have studied the ion channels mediating the light response of vertebrate rod photoreceptors by analysing fluctuations in the current across the rod membrane, using the whole cell patch-clamp technique on rods isolated from the axolotl retina. Light decreases the membrane current fluctuations. Noise analysis reveals two components to this decrease: a low frequency component due to biochemical noise in the transduction mechanism, and a high frequency component we attribute to the random opening and closing of the ion channels in the dark. The probability of any one channel being open in the dark is low. The spectrum of the high frequency component of the current fluctuations indicates that the current through an open channel is 4 X 10(-15)A, that the mean channel open time is 2 ms, and that about 10000 channels are open in each rod in the dark. The effect of light is to reduce the opening rate constant of these channels, with no effect on the closing rate constant.     

Amyloid-β and Proinflammatory Cytokines Utilize a Prion Protein-Dependent Pathway to Activate NADPH Oxidase and Induce Cofilin-Actin Rods in Hippocampal Neurons

Neurites of neurons under acute or chronic stress form bundles of filaments (rods) containing 1∶1 cofilin∶actin, which impair transport and synaptic function. Rods contain disulfide cross-linked cofilin and are induced by treatments resulting in oxidative stress. Rods form rapidly (5–30 min) in >80% of cultured hippocampal or cortical neurons treated with excitotoxic levels of glutamate or energy depleted (hypoxia/ischemia or mitochondrial inhibitors). In contrast, slow rod formation (50% of maximum response in ∼6 h) occurs in a subpopulation (∼20%) of hippocampal neurons upon exposure to soluble human amyloid-β dimer/trimer (Aβd/t) at subnanomolar concentrations. Here we show that proinflammatory cytokines (TNFα, IL-1β, IL-6) also induce rods at the same rate and within the same neuronal population as Aβd/t. Neurons from prion (PrP^C)-null mice form rods in response to glutamate or antimycin A, but not in response to proinflammatory cytokines or Aβd/t. Two pathways inducing rod formation were confirmed by demonstrating that NADPH-oxidase (NOX) activity is required for prion-dependent rod formation, but not for rods induced by glutamate or energy depletion. Surprisingly, overexpression of PrP^C is by itself sufficient to induce rods in over 40% of hippocampal neurons through the NOX-dependent pathway. Persistence of PrP^C-dependent rods requires the continuous activity of NOX. Removing inducers or inhibiting NOX activity in cells containing PrP^C-dependent rods causes rod disappearance with a half-life of about 36 min. Cofilin-actin rods provide a mechanism for synapse loss bridging the amyloid and cytokine hypotheses for Alzheimer disease, and may explain how functionally diverse Aβ-binding membrane proteins induce synaptic dysfunction.     

Two temporal phases of light adaptation in retinal rods

Vertebrate rod photoreceptors adjust their sensitivity as they adapt during exposure to steady light. Light adaptation prevents the rod from saturating and significantly extends its dynamic range. We examined the time course of the onset of light adaptation in bullfrog rods and compared it with the projected onset of feedback reactions thought to underlie light adaptation on the molecular level. We found that adaptation developed in two distinct temporal phases: (1) a fast phase that operated within seconds after the onset of illumination, which is consistent with most previous reports of a 1-2-s time constant for the onset of adaptation; and (2) a slow phase that engaged over tens of seconds of continuous illumination. The fast phase desensitized the rods as much as 80-fold, and was observed at every light intensity tested. The slow phase was observed only at light intensities that suppressed more than half of the dark current. It provided an additional sensitivity loss of up to 40-fold before the rod saturated. Thus, rods achieved a total degree of adaptation of approximately 3,000-fold. Although the fast adaptation is likely to originate from the well characterized Ca(2+)-dependent feedback mechanisms regulating the activities of several phototransduction cascade components, the molecular mechanism underlying slow adaptation is unclear. We tested the hypothesis that the slow adaptation phase is mediated by cGMP dissociation from noncatalytic binding sites on the cGMP phosphodiesterase, which has been shown to reduce the lifetime of activated phosphodiesterase in vitro. Although cGMP dissociated from the noncatalytic binding sites in intact rods with kinetics approximating that for the slow adaptation phase, this hypothesis was ruled out because the intensity of light required for cGMP dissociation far exceeded that required to evoke the slow phase. Other possible mechanisms are discussed.     

Light-Induced Translocation of RGS9-1 and Gβ5L in Mouse Rod Photoreceptors

The transducin GTPase-accelerating protein complex, which determines the photoresponse duration of photoreceptors, is composed of RGS9-1, Gβ5L and R9AP. Here we report that RGS9-1 and Gβ5L change their distribution in rods during light/dark adaptation. Upon prolonged dark adaptation, RGS9-1 and Gβ5L are primarily located in rod inner segments. But very dim-light exposure quickly translocates them to the outer segments. In contrast, their anchor protein R9AP remains in the outer segment at all times. In the dark, Gβ5L''s interaction with R9AP decreases significantly and RGS9-1 is phosphorylated at S⁴⁷⁵ to a significant degree. Dim light exposure leads to quick de-phosphorylation of RGS9-1. Furthermore, after prolonged dark adaptation, RGS9-1 and transducin Gα are located in different cellular compartments. These results suggest a previously unappreciated mechanism by which prolonged dark adaptation leads to increased light sensitivity in rods by dissociating RGS9-1 from R9AP and redistributing it to rod inner segments.     

Kinetics of turn-offs of frog rod phototransduction cascade

The time course of the light-induced activity of phototrandsuction effector enzyme cGMP-phosphodiesterase (PDE) is shaped by kinetics of rhodopsin and transducin shut-offs. The two processes are among the key factors that set the speed and sensitivity of the photoresponse and whose regulation contributes to light adaptation. The aim of this study was to determine time courses of flash-induced PDE activity in frog rods that were dark adapted or subjected to nonsaturating steady background illumination. PDE activity was computed from the responses recorded from solitary rods with the suction pipette technique in Ca²⁺-clamping solution.     

Ca2+ fluxes and channel regulation in rods of the albino rat

By use of microelectrodes, changes in the receptor current and the Ca2+ concentration were measured in the rod layer of the rat retina after stimulation by flashes or steady light. Thereby light induced Ca2+ sources, and sinks along a rod were determined in dependence of time. Thus, the Ca2+ fluxes across the plasma membrane of a mammalian rod could be studied in detail. By light stimulation, Ca2+ sources are evoked along the outer segment only. Immediately after a saturating flash, a maximum of Ca2+ efflux is observed which decays exponentially with tau = 0.3 s at 37 degrees C (4.2 s at 23 degrees C). During regeneration of the dark current, the outer segment acts as a Ca2+ sink, indicating a restoration of the Ca(2+)-depleted outer segment. These findings agree with earlier reports on amphibian rods. Further experiments showed that the peak Ca2+ efflux and tau are temperature dependent. The peak amplitude also depends on the external Ca2+ concentration. In contrast to the reports on amphibian rods, only a part of the Ca2+ ions extruded from the outer segment is directly restored. Surprisingly, during steady light the Ca2+ efflux approaches a permanent residual value. Therefore, in course of a photoresponse, Ca2+ must be liberated irreversibly from internal Ca2+ stores. There is certain evidence that the inner segment acts as a Ca2+ store. Our results show that the Ca2+ fraction of the ions carrying the dark current is proportional to the extracellular Ca2+ concentration. This indicates that the Ca2+ permeability of the plasma membrane of the rod outer segment is independent of the Ca2+ concentration.