Membrane voltage was recorded in rod photoreceptors in retina isolated from macaque monkey. The size of the single photon response and the magnitude of membrane voltage fluctuations were assessed in dark- and light-adapted retina. The “dark light” rate ID, defined as the rate of spontaneous photopigment isomerizations that would produce a variance equivalent to that of the noise measured in the dark, was calculated after matched filtering. The average value of 0.08 s−1 fell at the higher end of psychophysical estimates of dark light in human observers. In light-adapted rods the photon response decreased in amplitude and duration, and the magnitude of the voltage fluctuations increased with increasing background light intensity. The signal-to-noise ratio (SNR) for single rods was defined as the ratio of the peak amplitude of the photon response to the standard deviation of the noise fluctuations. The signal-to-noise ratio for dark-adapted rods SNRD was about 7. With increasing background intensity I, the SNR fell as SNRD(1 + I/ID)−1/2. This function may account for the increment thresholds measured with small brief test flashes in human psychophysical experiments.

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.

We evaluated the generality of two models of vertebrate phototransduction. The approach was to quantitatively optimize each model to the full waveform of high-quality, dark-adapted (DA), salamander rod flash responses. With the optimal parameters, each model was then used to account for signature, qualitative features of rod responses from three experimental paradigms (stimulus/response, “S/R suite”): (1) step responses; (2) the intensity dependence of the period of photocurrent saturation (Tsatvs. ln(I)); and (3) light-adapted (LA) incremental flash sensitivity as a function of background intensity. The first model was the recent successful model of Nikonov et al. (1998). The second model replaced the instantaneous Ca2+ buffering used in the Nikonov et al. model with a dynamic buffer. The results showed that, in the absence of the dynamic Ca2+ buffer, the Nikonov et al. model does not have sufficient flexibility to provide a good fit to the flash responses, and, using the same parameters, reproduce the salient features of the S/R suite—critical features at step onset and offset are absent; the Tsat function has too shallow a slope; and the model cannot generate the empirically observed I-range of Weber–Fechner LA behavior. Some features could be recovered by changing parameters, but only at the expense of the fit to the reference (Ref) data. When the dynamic buffer is added, the model is able to achieve an acceptable fit to the Ref data while reproducing several features of the S/R suite, including an empirically observed Tsat function, and an extended range of LA flash sensitivity adhering to Weber's law. The overall improved behavior of the model with a dynamic Ca2+ buffer indicates that it is an important mechanism to include in a working model of phototransduction, and that, despite the slow kinetics of amphibian rods, Ca2+ buffering should not be simulated as an instantaneous process. However, neither model was able to capture all the features with the same parameters yielding the optimal fit to the Ref data. In addition, neither model could maintain a good fit to the Ref data when five key biochemical parameters were held at their current known values. Moreover, even after optimization, a number of important parameters remained outside their empirical estimates. We conclude that other mechanisms will need to be added, including additional Ca2+-feedback mechanisms. The present research illustrates the importance of a hybrid qualitative/quantitative approach to model development, and the limitations of modeling restricted sets of data.

The purpose of the present study was to examine the contributions made by cat posterior parietal cortex to the analyses of the relative position of objects in visual space. Two cats were trained on a landmark task in which they learned to report the position of a landmark object relative to a right or left food-reward chamber. Subsequently, three pairs of cooling loops were implanted bilaterally in contact with visuoparietal cortices forming the crown of the middle suprasylvian gyrus (MSg; architectonic area 7) and the banks of the posterior-middle suprasylvian sulcus (pMS sulcal cortex) and in contact with the ventral-posterior suprasylvian (vPS) region of visuotemporal cortex. Bilateral deactivation of pMS sulcal cortex resulted in a profound impairment for all six tested positions of the landmark, yet bilateral deactivation of neither area 7 nor vPS cortex yielded any deficits. In control tasks (visual orienting and object discrimination), there was no evidence for any degree of attentional blindness or impairment of form discrimination during bilateral deactivation of pMS cortex. Therefore, we conclude that bilateral cooling of pMS cortex, but neither area 7 nor vPS cortex, induces a specific deficit in spatial localization as examined with the landmark task. These observations have significant bearing on our understanding of visuospatial processing in cat, monkey, and human cortices.

In many vertebrate retinas the outer segments of rod photoreceptors have multiple incisures, that is, there are numerous indentations in the highly curved membrane forming the edge of their disks and in the plasma membrane enclosing the entire stack of disks. Immunofluorescent localization of tubulin in amphibian photoreceptors yielded a novel series of thin, parallel, fluorescent lines in rod outer segments that extended their full length and coincided with their multiple incisures. Electron-microscopic examination of amphibian retinas revealed the structures responsible for this fluorescence: longitudinally oriented microtubules were associated with incisures at heights throughout rod outer segments. These microtubules were located between the disk rims and the overlying plasma membrane, in the small cytoplasmic compartment at the mouth of incisures; the microtubules and membranes were separated from each other by distances that were uniform, as though interconnected by filaments described in other studies. Thus, in amphibian rod outer segments the incisures mark the site of a cytoskeletal system containing longitudinal microtubules distinct from those of the ciliary axoneme, linked by filaments to the adjacent membranes. This cytoskeleton is expected to be important for the normal structure, function, and renewal of rod outer segments. In amphibian cone outer segments, which do not have incisures, the only anti-tubulin immunofluorescence and the only microtubules were at the axoneme. These findings may help elucidate the diverse properties of rods and cones in many vertebrate retinas and could prove relevant for human retinal degenerations.

Although the retinotectal projection of goldfish has long been known to have a high degree of retinotopic order, the structural basis for this in terms of the precise positioning of axonal arbors from neighboring retinal ganglion cells has not been determined. In studying this, a small number of neighboring retinal ganglion cells was selectively labeled by a microinjection of DiI into the retina. Following axoplasmic transport for several days, the tectum was removed and flat-mounted for fluorescence microscopy. The injection labeled a small number of axons and their terminal arbors which ranged in size from 108 × 134 μm to 394 × 331 μm with a mean of 233 × 219 μm. This mean size corresponds to about 1/15 of the length of one tectal axis. Although individual arbors labeled from one small retinal injection were always observed near the same retinotopic position, they were almost never coextensive. Overlap between pairs of arbors along the lines of projection perpendicular to the tectal surface averaged 57% of the area of a single arbor. These results indicate that neighboring retinal ganglion cells do not converge onto the same locus but instead project as a continuous retinotopic array of partially overlapping terminal fields.

Previous studies have shown that the expression of glutamate receptor subunits is developmentally regulated and have been implicated in processes of cell differentiation during postnatal life. The tissue localization and developmental pattern of the glutamate receptor 2 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) receptor were investigated by means of immunohistochemistry and immunoblotting. Labeling of amacrine and ganglion cells and the inner plexiform layer appeared early during development, while glutamate receptor 2 subunit expression in the outer plexiform layer started after the first postnatal week. The distribution of labeling within the inner plexiform layer changed from nonorganized to laminated appearance prior to eye-opening. There was an increasing number of positive amacrine and ganglion cell somata during the first 2 weeks, but their number decreased considerably as the retina matured and were seen at least up to 35 days of postnatal development. Little labeling was found in the ganglion cell layer and in the inner plexiform layer of late postnatal and adult retina. Labeling in the outer plexiform layer and of bipolar cell somata appeared to increase in the developing retina. Glur2 labeling of these cells and the outer plexiform layer became discernible during the second postnatal week, and this labeling was present in the adult as well. Immunoblotting showed that GluR2 protein levels were similar at postnatal days 7 and 10, but slightly decreased between the second and fourth postnatal weeks. Our data imply that the immunological expression of glutamate receptor 2 subunit in the inner plexiform layer decreases as a function of age, and is correlated with developmental event(s) in the postnatal retina.

Acetylcholine (ACh) in the vertebrate retina affects the response properties of many ganglion cells, including those that display directional selectivity. Three β and eight α subunits of neuronal nicotinic acetylcholine receptors (nAChRs) have been purified and antibodies have been raised against many of them. Here we describe biochemical and immunocytochemical studies of nAChRs in the rabbit retina. Radioimmunoassay and Western blot analysis demonstrated that many of the nAChRs recognized by a monoclonal antibody (mAb210) contain β2 subunits, some of which are in combination with α3 and possibly other subunits. MAb210-immunoreactive cells in the inner nuclear layer (INL) were 7–14 μm in diameter and were restricted to the innermost one or two tiers of cells, although occasional cells were found in the middle of the INL. At least 60% of the cells in the ganglion cell layer (GCL) in the visual streak displayed mAb210 immunoreactivity; these neurons ranged from 7–18 μm in diameter. The dendrites of cells in both the INL and GCL could sometimes be followed until they entered one of two dense, poorly defined, bands of processes in the inner plexiform layer (IPL) that overlap the arbors of the cholinergic starburst cells. Parvalbumin and serotonin-positive neurons did not exhibit nAChR immunoreactivity. Although the level of receptor expression appeared to be low, mAb210 immunoreactivity was observed in some of the ChAT-positive (starburst) amacrine cells.

The perceived direction of a grating moving behind an elongated aperture is biased towards the aperture's long axis. This “barber pole” illusion is a consequence of integrating one-dimensional (1D) or grating and two-dimensional (2D) or terminator motion signals. In humans, we recorded the ocular following responses to this stimulus. Tracking was always initiated at ultra-short latencies (≈ 85 ms) in the direction of grating motion. With elongated apertures, a later component was initiated 15–20 ms later in the direction of the terminator motion signals along the aperture's long axis. Amplitude of the later component was dependent upon the aperture's aspect ratio. Mean tracking direction at the end of the trial (135–175 ms after stimulus onset) was between the directions of the vector sum computed by integrating either terminator motion signals only or both grating and terminator motion signals. Introducing an elongated mask at the center of the “barber pole” did not affect the latency difference between early and later components, indicating that this latency shift was not due to foveal versus peripheral locations of 1D and 2D motion signals. Increasing the size of the foveal mask up to 90% of the stimulus area selectively reduced the strength of the grating motion signals and, consequently, the amplitude of the early component. Conversely, reducing the contrast of, or indenting the aperture's edges, selectively reduced the strength of terminator motion signals and, consequently, the amplitude of the later component. Latencies were never affected by these manipulations. These results tease apart an early component of tracking responses, driven by the grating motion signals and a later component, driven by the line-endings moving at the intersection between grating and aperture's borders. These results support the hypothesis of a parallel processing of 1D and 2D motion signals with different temporal dynamics.

In addition to the well-studied AII amacrine cell, there is another amacrine cell type participating in the rod pathway of the mammalian retina. In cat, this cell is called the A17 amacrine cell, and in rabbits, it is called the indoleamine-accumulating amacrine cell (S1 and S2); however, the presence of the corresponding cell type has not yet been described in detail for the rat retina. To this end, we injected amacrine cells with Neurobiotin in vertical retinal slices. After histological processing, we were able to reconstruct the morphology of a wide-field amacrine cell which showed characteristics of A17 and S1/S2 amacrine cells. The rat wide-field amacrine cells exhibited the same stratification pattern, their dendrites bore varicosities and ramified in sublamina 5 of the inner plexiform layer (IPL), and they were dye-coupled to other amacrine cells. To determine whether those amacrine cells shared electrophysiological characteristics as well, we performed whole-cell patch-clamp recordings and examined their voltage-activated currents and neurotransmitter-induced currents. We never observed voltage-gated Na+ currents and spike-like potentials upon depolarization by current injection in these cells. We identified GABA- and glycine-sensitive Cl− currents that could be blocked by bicuculline and strychnine, respectively. We also observed kainate- and AMPA-activated currents, which could be inhibited by the application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Finally, a 400-ms full-field light stimulus was used to characterize the light responses of A17 amacrine cells. The light ON-induced inward current could be suppressed by the application of 2,3-Dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulphonamide (NBQX), while the majority of the light OFF-induced current was inhibited by bicuculline and reduced to a smaller extent by NBQX. CPP, an NMDA blocker, had no effect on the light response of rat A17 amacrine cells.

It has been observed that deep-foraging marine mammals have visual pigments that are blue shifted in terms of their wavelength of maximal absorbance (λmax) when compared to analogous pigments from terrestrial mammals. The mechanisms underlying the spectral tuning of two of these blue-shifted pigments have recently been elucidated and depend on three amino acid substitutions (83Asn, 292Ser, and 299Ser) in dolphin rhodopsin, but only one amino acid substitution (308Ser) in the dolphin long-wavelength-sensitive pigment. The objective of this study was to investigate the molecular basis for changes in the spectral sensitivity of rod visual pigments from seven distantly related marine mammals. The results show a relationship between blue-shifted rhodopsins (λmax ≤ 490 nm), deep-diving foraging behavior, and the substitutions 83Asn and 292Ser. Species that forage primarily near the surface in coastal habitats have a rhodopsin with a λmax similar to that of terrestrial mammals (500 nm) and possess the substitutions 83Asp and 292Ala, identical to rhodopsins from terrestrial mammals.

The retinas of adult teleost fish can regenerate neurons following a chemical or mechanical injury. Previous studies have demonstrated that mechanical excision of fish retina induces a hyperplasia within the retinal sheet, including the formation of a proliferative blastema from whence new retinal cells are produced to fill the excision site. The current study was designed to address two issues regarding injury-induced retinal hyperplasia: (1) Retinas of adult zebrafish can regenerate following a surgical excision, but compared to other fish they contain very few proliferative cells: Might retinal injury in adult zebrafish therefore induce minimal, or perhaps no, hyperplasia? (2) The fate of injury-induced, proliferative retinal cells outside surgical excision sites has yet to be determined. Do such cells produce retinal neurons? Evidence is presented that mechanical injury to the adult zebrafish retina induces a dramatic increase in the number of proliferative cells both within and external to the lesion site, and some of these cells apparently migrate within the radial dimension of the retina. Evidence is also presented that injury-induced proliferative cells outside a lesion site can produce retinal neurons—including cone photoreceptors, interplexiform cells, and amacrine cells—that are incorporated into the extant retina. The results suggest that the adult zebrafish retina contains a latent population of cells that is induced to proliferate following retinal injury, and that these cells might represent a novel avenue for pluripotent neurogenesis within the intact adult teleost retina.

Retinal horizontal cells display large receptive fields as a result of extensive electrical coupling via gap junctions. There is abundant evidence that these gap junctions are dynamically regulated by changes in the adaptational state of the retina. The neuromodulator dopamine appears to play a major role in regulating gap junctional conductances of horizontal cells. Emerging evidence indicates that nitric oxide (NO) also acts as a neuromodulator in the retina and, more specifically, regulates the coupling between horizontal cells. In the present study, we examined the effects of a nitric oxide, and its secondary messenger cGMP, on electrical and tracer coupling between A-type and between B-type horizontal cells in the rabbit retina. Application of the NO donors S-nitroso-N-acetylpenicillamine (SNAP) or sodium nitroprusside (SNP) significantly reduced the coupling between horizontal cells as evidenced by a decrease in their space constants, annulus-to-small spot response ratios, and the extent of tracer coupling following injection with Neurobiotin. Further, application of SNP eliminated the increase in coupling of horizontal cells normally seen with exposure to dim background illumination. Application of 8-bromo-cGMP produced effects similar to those of the NO donors, consistent with the idea that the uncoupling actions of NO were mediated via a cGMP cascade.

Arrestins participate in the termination of phototransduction in both vertebrates and invertebrates. However, the visual arrestins of invertebrates and vertebrates differ significantly from one another in that the invertebrate visual arrestins become phosphorylated rapidly in response to light while those in the photoreceptors of vertebrates do not. In an effort to understand the functional relevance of arrestin phosphorylation, we examined this process in the photoreceptors of the horseshoe crab Limulus polyphemus. We report that Limulus visual arrestin can be phosphorylated at three sites near its C-terminus and show that arrestin molecules phosphorylated on one, two, and three sites are normally present in both light- and dark-adapted photoreceptors. Light adaptation increases the amount of arrestin phosphorylated at three sites.