This study defined the amplitude and phase characteristics of rod- and cone-isolated flicker electroretinograms (ERGs) and determined how these responses summate to generate the nonreceptor-specific ERG. Full-field ERGs were obtained from six normally sighted subjects (age 26 to 44 years) using a four-primary LED-based photostimulator and standard recording techniques. The four primaries were either modulated sinusoidally in phase to achieve simultaneous rod and cone activation (ERGR+C; nonreceptor-specific) or in different phases to achieve rod-isolated (ERGR) and cone-isolated (ERGC) responses by means of triple silent substitution. ERGs were measured at two mean luminance levels (2.4 and 24 cd/m2), two contrasts (20 and 40%), and four temporal frequencies (2–15 Hz). Fundamental amplitude and phase for each condition were derived by Fourier analysis. Response amplitude and phase depended on the stimulus conditions (frequency, mean luminance, and contrast), however, for all conditions: 1) response phase decreased monotonically as stimulus frequency increased; 2) response amplitude tended to decrease monotonically as stimulus frequency increased, with the exception of the 24 cd/m2, 40% contrast ERGR+C that was sharply V-shaped; 3) ERGR phase was delayed (32 to 210 deg) relative to the ERGC phase; 4) ERGR amplitude was typically equal to or lower than the ERGC amplitude, with the exception of the 2.4 cd/m2, 40% contrast condition; and 5) the pattern of ERGR+C responses could be accounted for by a vector summation model of the rod and cone pathway signals. The results show that the ERGR+C amplitude and phase can be predicted from ERGR and ERGC amplitude and phase. For conditions that elicit ERGR and ERGC responses that have approximately equal amplitude and opposite phase, there is strong destructive interference between the rod and cone responses that attenuates the ERGR+C. Conditions that elicit equal amplitude and opposite phase rod and cone responses may be particularly useful for evaluating rod–cone interactions.

Rod and cone photoreceptors possess ribbon synapses that assist in the transmission of graded light responses to second-order bipolar and horizontal cells of the vertebrate retina. Proper functioning of the synapse requires the juxtaposition of presynaptic release sites immediately adjacent to postsynaptic receptors. In this review, we focus on the synaptic, cytoskeletal, and extracellular matrix proteins that help to organize photoreceptor ribbon synapses in the outer plexiform layer. We examine the proteins that foster the clustering of release proteins, calcium channels, and synaptic vesicles in the presynaptic terminals of photoreceptors adjacent to their postsynaptic contacts. Although many proteins interact with one another in the presynaptic terminal and synaptic cleft, these protein–protein interactions do not create a static and immutable structure. Instead, photoreceptor ribbon synapses are remarkably dynamic, exhibiting structural changes on both rapid and slow time scales.

The effect of light adaptation on the period of photocurrent saturation induced by a bright stimulating flash was examined in rod photoreceptors of the larval-stage tiger salamander (Ambystoma tigrinum). Using suction electrodes, photocurrent responses to brief flashes were recorded from single, isolated rods in the presence and absence of steady background illumination. Background light decreased the saturation period (T) measured at fixed flash intensity (fixed If) and in this respect light-adapted the saturating response. Effects of the background on responses to weak (i.e. subsaturating) and bright flashes were compared with changes in a parameter, where ΔT is the decrease in saturation period, and where TR* is the slope of the line that relates T and ln If in a given state of adaptation. Dark- and light-adapted responses to flash intensities and , respectively, exhibited similar absolute peak photocurrent and falling-phase kinetics when and satisfied the relation, , where Ib is the background intensity. It is argued that ψ approximates the relative PDE*/R* gain of transduction, i.e. the relative peak level of activated cGMP phosphodiesterase (PDE*) produced by a given, small amount of photoactivated visual pigment (R*). Interpreted on this view, the results imply that light adaptation derives largely from a decrease in PDE*/R* gain, rather than from the stimulation of guanylate cyclase activity. The data are consistent with the possibility that modulation of the lifetime of PDE* underlies the background dependence of ψ.

An eletrical potential recorded from the cornea, the a-wave of the ERG, is evaluated as a measure of human photoreceptor activity by comparing its behavior to a model derived from in vitro recordings from rod photoreceptors. The leading edge of the ERG exhibits both the linear and nonlinear behavior perdicted by this model. The capability for recording the electrical activity of humans photoreceptors in in vivo opens new avenues for assessing normal and abnormal receptor activity in humans. Furthermore, the quantitative model of the receptor response can be used to isolate the inner retinal contribution, Granit's PII, to the gross ERG. Based on this analysis, the practice of using the trough-to-peak amplitude of the b-wave as a proxy for the amplitude of the inner nuclear layer activity is evaluated.

To improve the usefulness of the ERG in identifying the sites and mechanisms of adaptation, development, and disease processes, a theoretical framework based upon Granit's analysis of the ERG was evaluated. The framework assumes that the ERG is the sum of two potentials, one, P3, generated by the receptors and the other, P2, generated by the cells of the INL. Hood and Birch (1990a, b) demonstrated that the leading edge of the a-wave can be quantitatively described by a model used to describe the response from single rod receptors. This model provides P3(t), a theoretical receptor response as a function of time, for any given flash intensity. The ERGs from normal observers and patients with retinal diseases were analyzed in this framework, first by deriving P2 by computer subtracting the predicted P3(t) responses. This analysis was successful and a computational model of the ERG was then derived. The model of P2(t) was constructed with linear filters and a static nonlinearity and using P3(t) as the input. The ERG for any given flash intensity is then P3(t) + P2(t) The model describes (1) the change both in implicit times and in trough-to-peak b–wave amplitudes with flash intensity for the normal, dark-adapted observers; and (2) the changes in b–wave implicit times and amplitudes for three patients with retinal diseases.

Among the implications drawn from these analyses were as follows: (1) The fits of the Naka-Rushton equation to trough-to-peak b–wave amplitudes must be interpreted with great care. (2) When the INL is affected by retinal disease, the b–wave may be a very poor reflection of INL activity. (3) The implicit time of the b–wave can provide a measure of receptor sensitivity.

The effects of cobalt ions on 502-nm rod- and 575-nm cone-driven components of the b-wave of the electroretinogram were studied in the isolated frog retina. Addition of 100–150 μM cobalt initially caused a suppression of rod-driven responses and an enhancement of cone-driven responses. In the continued presence of cobalt, however, the rod-driven responses gradually recovered and the cone-driven responses became suppressed. These concentrations of cobalt had no effect on the rod- and cone-driven mass receptor potentials which were isolated in the presence of 4 mM glutamate. At higher concentrations of cobalt (1 mM or greater), both rod- and cone-driven b-wave responses were eliminated and there was no recovery in the continued presence of cobalt. The results suggest that cobalt has markedly different, time-dependent effects on signal transmission from rods and cones to second-order cells.

The sensitivity of rod- and cone-driven responses was studied in the isolated frog retina during the period of rapid dark adaptation following a conditioning flash which bleached a negligible amount of visual pigment. Following a conditioning flash, cone-driven b-wave responses were first enhanced and then depressed. The time courses of the enhancement and subsequent depression of cone-drive responses varies greatly with the intensity and wavelength of the conditioning flash, but were identical when the conditioning' flashes were matched for equal excitation of 502 nm rods. These changes in cone-driven response sensitivity were correlated with the desensitization and recovery of rods following the conditioning flash. When signal transmission from rods to second-order cells was interrupted by the addition of L-glutamate, the conditioning flash did not produce the above-described enhancement and subsequent depression of long-wavelength receptor potential responses. The suppression of cone-driven response therefore appears to be due to a synaptically mediated influence from 502 nm rods which is maximal when the rods are in the dark-adapted state, with little or no contribution from 433 nm rods, and no involvement of the pigment epithelium.

Glutamate, when puffed near the synaptic terminals, elicits a current in rod photoreceptors. The current is strongly dependent upon both the intracellular and extracellular chloride concentration: its reversal potential follows the predicted Nernst potential for a chloride permeable channel. The glutamate-elicited current also requires the presence of extracellular sodium. This glutamate-elicited current is pharmacologically like a glutamate transporter: it is elicited, in order of efficacy, by L-glutamate, L-aspartate, L-cysteate, D-aspartate, and D-glutamate, all shown to activate glutamate transport in other systems. Furthermore, it is reduced by the glutamate transport antagonists dihydrokainate (DHKA) and D, L-threo-3–hydroxyaspartate (THA). THA, when applied alone, elicits a current similar to that elicited by glutamate. The current cannot be activated by the glutamate receptor agonists kainate, quisqualate, NMDA and APB, nor can it be blocked by the glutamale receptor antagonists CNQX and APV. Thus, the current does not appear to be mediated by a conventional glutamate receptor. Taken together, the ionic dependence and pharmacology of this current suggest that it is generated by glutamate transporter coupled to a chloride channel.

La dynamique des fils, et en particulier le problème de la "corde vibrante", a de nombreux champs d'application et met en jeu de multiples phénomènes complexes que seule une modélisation rigoureuse en déplacements finis peut faire apparaître. Les difficultés auxquelles on se heurte, avant tout liées à la cinématique, sont celles rencontrées en mécanique des poutres, cadre général dans lequel on se place.Le mouvement étant tridimensionnel, on détaille le paramétrage permettant de rendre compte facilement de la torsion matérielle. On s'attache ensuite à construire les équations du mouvement pour un fil linéairement élastique dont la tension dépend aussi de la torsion.Ces équations exhibent de nombreux couplages dont les mécanismes peuvent être mis en évidence par projection sur les modes spatiaux dans le cas d'extrémités fixes. Le système d'équations modales obtenu fait apparaître des règles de sélection précises portant sur la distribution spatiale des modes, la modulation des parties temporelles n'intervenant qu'en terme d'efficacité sur les couplages.

To better understand the evolution of spatial and color vision, the number and spatial distributions of cones, rods, and optic nerve axon numbers were assessed in seven New World primates (Cebus apella, Saimiri ustius, Saguinus midas niger, Alouatta caraya, Aotus azarae, Calllithrix jacchus, and Callicebus moloch). The spatial distribution and number of rods and cones was determined from counts of retinal whole mounts. Optic axon number was determined from optic nerve sections by electron microscopy. These data were amassed with existing data on retinal cell number and distribution in Old World primates, and the scaling of relative densities and numbers with respect to retinal area, eye and brain sizes, and foveal specializations were evaluated. Regular scaling of all cell types was observed, with the exceptionally large, rod-enriched retina of the nocturnal owl monkey Aotus azarae, and the unusually high cone density of the fovea of the trichromatic howler monkey Alouatta caraya presenting interesting variations on this basic plan. Over all species, the lawful scaling of rods, cones, and retinal ganglion cell number is hypothesized to result from a conserved sequence of cell generation that defends retinal acuity and sensitivity over a large range of eye sizes.

Absorbance spectra of rods and some cones were measured by microspectrophotometry in 22 fish species from the brackish-water of the Baltic Sea, and when applicable, in the same species from the Atlantic Ocean (3 spp.), the Mediterranean Sea (1 sp.), or Finnish fresh-water lakes (9 spp.). The main purpose was to study whether there were differences suggesting spectral adaptation of rod vision to different photic environments during the short history (<104 years) of postglacial isolation of the Baltic Sea and the Finnish lakes. Rod absorbance spectra of the Baltic subspecies/populations of herring (Clupea harengus membras), flounder (Platichthys flesus), and sand goby (Pomatoschistus minutus) were all long-wavelength-shifted (9.8, 1.9, and 5.3 nm, respectively, at the wavelength of maximum absorbance, λmax) compared with their truly marine counterparts, consistent with adaptation for improved quantum catch, and improved signal-to-noise ratio of vision in the Baltic light environment. Judged by the shape of the spectra, the chromophore was pure A1 in all these cases; hence the differences indicate evolutionary tuning of the opsin. In no species of fresh-water origin did we find significant opsin-based spectral shifts specific to the Baltic populations, only spectral differences due to varying A1/A2 chromophore ratio in some. For most species, rod λmax fell within a wavelength range consistent with high signal-to-noise ratio of vision in the spectral conditions prevailing at depths where light becomes scarce in the respective waters. Exceptions were sandeels in the Baltic Sea, which are active only in bright light, and all species in a “brown” lake, where rod λmax lay far below the theoretically optimal range.

Opsins, like many other G-protein-coupled receptors, sustain constitutive activity in the absence of ligand. In partially bleached rods and cones, opsin's activity closes cGMP-gated channels and produces a state of “pigment adaptation” with reduced sensitivity to light and accelerated flash response kinetics. The truncated retinal analogue, β-ionone, further desensitizes partially bleached green-sensitive salamander rods, but enables partially bleached red-sensitive cones to recover dark-adapted physiology. Structural differences between rod and cone opsins were proposed to explain the effect. Rods and cones, however, also contain different transducins, raising the possibility that G-protein type determines the photoreceptor-specific effects of β-ionone. To test the two hypotheses, we applied β-ionone to partially bleached blue-sensitive rods and cones of salamander, two cells that couple the same cone-like opsin to either rod or cone transducin, respectively. Immunocytochemistry confirmed that all salamander rods contain one form of transducin, whereas all cones contain another. β-Ionone enhanced pigment adaptation in blue-sensitive rods, but it also did so in blue- and UV-sensitive cones. Furthermore, all recombinant salamander rod and cone opsins, with the exception of the red-sensitive cone opsin, activated rod transducin upon the addition of β-ionone. Thus opsin structure determines the identity of β-ionone as an agonist or an inverse agonist and in that respect distinguishes the red-sensitive cone opsin from all others.

The dynamics of interactions between rod and L-cone driven signals were studied psychophysically in two deuteranopic observers. Flicker detection thresholds for different ratios of rod to L-cone modulation were measured at temporal frequencies between 1 and 15 Hz. A model, which assumes that rod and L-cone driven signals are vector added, can describe the threshold data adequately. We found that up to about 8–10 Hz temporal frequency, rod and L-cone signals interact additively, whereas at higher frequencies the interaction is subtractive. Rod and L-cone signal strengths depend similarly on temporal frequency and are maximal between 3 and 5 Hz. The phase difference between rod and L-cone signals increases linearly with temporal frequency, indicating that their responses have a delay difference of about 20 to 30 ms, consistent with involvement of the faster rod pathway. The data would suggest a nearly complete additivity of the rod and cone driven signals when using flashed stimuli. But, literature data showed only partial additivity of the two, suggesting that different postreceptoral mechanisms are involved in the two tasks.

Vertebrate photoreceptors respond to light with changes in membrane conductance that reflect the activity of cyclic-nucleotide gated channels (CNG channels). The functional features of these channels differ in rods and cones; to understand the basis of these differences we cloned CNG channels from the retina of striped bass, a fish from which photoreceptors can be isolated and studied electrophysiologically. Through a combination of experimental approaches, we recovered and sequenced three full-length cDNA clones. We made unambiguous assignments of the cellular origin of the clones through single photoreceptor RT-PCR. Synthetic peptides derived from the sequence were used to generate monospecific antibodies which labeled intact, unfixed photoreceptors and confirmed the cellular assignment of the various clones. In rods, we identified the channel α subunit gene product as 2040 bp in length, transcribed into two mRNA 1.8 kb and 2.9 kb in length and translated into a single 96-kDa protein. In cones we identified both α (CNGA3) and β (CNGB3) channel subunits. For α, the gene product is 1956 bp long, the mRNA 3.4 kb, and the protein 74 kDa. For β, the gene product is 2265 bp long and the mRNA 3.3 kb. Based on deduced amino acid sequence, we developed a phylogenetic map of the evolution of vertebrate rod and cone CNG channels. Sequence comparison revealed channels in striped bass, unlike those in mammals, are likely not N-linked-glycosylated as they are transported within the photoreceptor. Also bass cone channels lack certain residues that, in mammals, can be phosphorylated and, thus, affect the cGMP sensitivity of gating. On the other hand, functionally critical residues, such as positively charged amino acids within the fourth transmembrane helix (S4) and the Ca2+-binding glutamate in the pore loop are absolutely the same in mammalian and nonmammalian species.

Phosducin (Pd) is a 28-kD phosphoprotein whose expression in retina appears limited to photoreceptor cells. Pd binds to the β,γ subunits of transducin (Gt). Their binding affinity is markedly diminished by Pd phosphorylation. While Pd has long been regarded as a candidate for the regulation of Gt, the molecular details of Pd function remain unclear. This gap in understanding is due in part to a lack of precise information concerning the total amount and subcellular localization of rod Pd. While earlier studies suggested that Pd was a rod outer segment (ROS) protein, recent findings have demonstrated that Pd is distributed throughout the rod. In this report, the subcellular distribution and amounts of rat Pd are quantified with immunogold electron microscopy. After light or dark adaptation, retinal tissues were fixed in situ and prepared for ultrathin sectioning and immunogold labeling. Pd concentrations were analyzed over the entire length of the rod. The highest Pd labeling densities were found in the rod synapse. Less intense Pd staining was observed in the ellipsoid and myoid regions, while minimal labeling densities were found in the ROS and the rod nucleus. In contrast with rod Gt, no evidence was found for light-dependent movement of Pd between inner and outer segments. There is a relative paucity of Pd in the ROS as compared with the large amounts of Gt found there. This does not support the earlier idea that Pd could modulate Gt activity by controlling its concentration. On the other hand, the presence of Pd in the nucleus is consistent with its possible role as a regulator of transcription. The functions of Pd in the ellipsoid and myoid regions remain unclear. The highest concentration of Pd was found at the rod synapse, consistent with a suggested role for Pd in the regulation of synaptic function.

The retinas of anchovies have two unique photoreceptor types: “bifid” and “long” cones (Fineran & Nicol, 1976). The outer segments of these cells contain multiple layers of membranes (lamellae) oriented longitudinally (axially). This orientation is distinct from that in all other vertebrate rods and cones, where the lamellae are stacked transversely with their planes perpendicular to the incident light path. Although the common arrangement provides optimal absorption for normally incident light rays, it is also insensitive to the rays' direction of vibration (i.e. their polarization). In contrast, the two mutually perpendicular sets of axially oriented lamellae segregated into bifid and long cones could function as the principal analyzers for linearly polarized light, as previously hypothesized (Fineran & Nicol, 1976, 1978). Here, we report on a microspectrophotometric study that shows (1) the presence of two spectrally distinct visual pigments in the three photoreceptor types of the bay anchovy retina; these are typical vertebrate pigments in that they bleach, when exposed to light, and have absorption spectra like all other vitamin A1-based visual pigments; (2) that the rods and cones exhibit dichroic absorption of light in accordance with their lamellar orientation, and (3) that the two cone types of the retina contain a spectrally indistinguishable pigment with peak absorbance (λmax) around 540 nm, while the rods contain a rhodopsin-like pigment with λmax near 500 nm. Compared to other vertebrates, anchovies are remarkable for using a monochromatic cone system with unusual specializations supportive of a polarization detection system.

Most molecular studies on the visual system in fish have been performed on freshwater teleosts such as goldfish and zebrafish where cones and rods appear simultaneously. Many marine fishes have long larval phase in the upper pelagic zone before transformation into a juvenile and a benthic life style. The retina at the larval stages consists of only single cone cells; later during metamorphosis double cones and rods develop. The flatfish Atlantic halibut (Hippoglossus hippoglossus) is a typical example of a marine species with such a two-step retina development. In this study, we have cloned five different opsins from Atlantic halibut larvae and juvenile retinas. Sequence comparisons with other opsins and phylogenetic analysis show that the five genes belong to the opsins of long-wavelength sensitive (L); middle-wavelength sensitive, MCone and MRod; and short-wavelength sensitive, SBlue and SUltraviolet, respectively. In situ hybridization analysis reveals expression in double cone (L and MCone), single cone (SBlue and SUltraviolet), and rod (MRod) types of photoreceptor cells in juvenile halibut retina. The visual system in Atlantic halibut seems therefore to have all four types of cone photoreceptors in addition to rod photoreceptors. This work shows for the first time molecular isolation of a complete set of retinal visual pigment genes from a marine teleost and describes the first cloning of an ultraviolet-sensitive opsin type from a marine teleost.

The present study has examined the spatial and temporal expression patterns of various proteins associated with the structure and function of mature photoreceptor outer segments in the developing ferret's retina using immunocytochemistry and RT-PCR. One set of proteins, including rod opsin, arrestin, and recoverin, was detected progressively in photoreceptors as they became postmitotic, being expressed well before the differentiation of outer segments. A second set of proteins, including β- and γ-transducin, cGMP-phosphodiesterase, phosducin, rhodopsin kinase, rod cGMP-gated cation channel protein, and peripherin, displayed a contrasting temporal onset and pattern of spatial emergence. These latter proteins first became detectable either shortly before or coincident with outer segment formation, and were expressed simultaneously in both older and younger photoreceptor cells. A third set, the short wavelength-sensitive (SWS) and medium wavelength-sensitive (MWS) cone opsin proteins, was the last to be detected, but materialized in a spatio-temporal pattern reminiscent of the neurogenetic gradient of the cones. These different spatial and temporal patterns indicate that cellular maturation must play a primary role in regulating the onset of expression of some of these proteins, while extrinsic signals must act to coordinate the expression of other proteins across photoreceptors of different ages.

Vertebrate vision starts with photoisomerization of the 11-cis-retinal chromophore to all-trans-retinal. Biosynthesis of 11-cis-retinal is required to maintain vision. A key enzyme catalyzing the oxidation of 11-cis-retinol is 11-cis-retinol dehydrogenase (11-cis-RDH), which is encoded by the RDH5 gene. 11-cis-RDH is expressed in the RPE and not in the neural retina. The consequences of a lack of 11-cis-RDH were studied in a family with fundus albipunctatus. We identified the causative novel RDH5 mutation, Arg157Trp, that replaces an amino acid residue conserved among short-chain alcohol dehydrogenases. Three-dimensional structure modeling and in vitro experiments suggested that this mutation destabilizes proper folding and inactivates the enzyme. Studies using RPE membranes indicated the existence of an alternative oxidizing system for the production of 11-cis-retinal. In vivo visual consequences of this null mutation showed complex kinetics of dark adaptation. Rod and cone resensitization was extremely delayed following full bleaches; unexpectedly, the rate of cone recovery was slower than rods. Cones showed a biphasic recovery with an initial rapid component and an elevated final threshold. Other unanticipated results included normal rod recovery following 0.5% bleach and abnormal recovery following bleaches in the 2–12% range. These intermediate bleaches showed rapid partial recovery of rods with transitory plateaux. Pathways in addition to 11-cis-RDH likely provide 11-cis-retinal for rods and cones and can maintain normal kinetics of visual recovery but only under certain constraints and less efficiently for cone than rod function.

The neotenic tiger salamander retina is a major model system for the study of retinal physiology and circuitry, yet there are unresolved issues regarding the organization of the photoreceptors and the photoreceptor mosaic. The rod and cone subtypes in the salamander retina were identified using a combination of morphological and immunocytochemical markers for specific rod and cone opsin epitopes. Because the visual pigment mechanisms present in the tiger salamander retina are well characterized and the antibodies employed in these studies are specific for particular rod and cone opsin epitopes, we also were able to identify the spectral class of the various rod and cone subtypes. Two classes of rods corresponding to the “red” and “green” rods previously reported in amphibian retinas were identified. In serial semithin section analyses, rods and cones comprised 62.4 ± 1.4% and 37.6 ± 1.4% of all photoreceptors, respectively. One rod type comprising 98.0 ± 0.7% of all rods showed the immunological and morphological characteristics of “red” rods, which are maximally sensitive to middle wavelengths. The second rod subtype comprised 2.0 ± 0.7% of all rods and possessed the immunological and morphological characteristics of “green” rods, which are maximally sensitive to short wavelengths. By morphology four cone types were identified, showing three distinct immunological signatures. Most cones (84.8 ± 1.5% of all cones), including most large single cones, the accessory and principal members of the double cone, and some small single cones, showed immunolabeling by antisera that recognize long wavelength-sensitive cone opsins. A subpopulation of small single cones (8.4 ± 1.7% of all cones) showed immunolabeling for short wavelength-sensitive cone opsin. A separate subpopulation of single cones which included both large and small types (6.8 ± 1.4% of all cones) was identified as the UV-Cone population and showed immunolabeling by antibodies that recognize rod opsin epitopes. Analysis of flatmounted retinas yielded similar results. All photoreceptor types appeared to be distributed in all retinal regions. There was no obvious crystalline organization of the various photoreceptor subtypes in the photoreceptor mosaic.