Visual Streak Research Articles

Measurement of Regional Choroidal Blood Flow in Rabbits and Monkeys Using Fluorescent Microspheres

To develop a quantitative measure of regional variation in choroidal blood flow (ChBF). Five million 15-microm fluorescent microspheres were injected into the left ventricles of 4 rabbits and 3 monkeys. The fixed globes were bleached, flat mounted, and photomicrographed. After image analysis to locate each microsphere, regional densities and blood flow were determined. Regional variation in ChBF was clearly evident. In the rabbit, a high density of spheres was seen in the visual streak. This was surrounded by a middle peripheral area of low sphere density and a far peripheral region of moderately high density. In the monkeys, sphere density was markedly greater in the macula compared with the periphery. Contour plots produced lines of constant flow that were oval and extended farther nasally than temporally in the monkeys. The ratio of central to peripheral ChBF was much greater in the monkeys than in the rabbits. Quantitative assessment of regional ChBF can be performed using a modification of the fluorescent microsphere impaction method. This method of determining regional ChBF will be useful for studying the vascular effects of pharmacologic agents and for characterizing animal models of human disease involving the outer retina.

Retinal ganglion cell activity from the multifocal electroretinogram in pig: optic nerve section, anaesthesia and intravitreal tetrodotoxin

Non-invasive recordings of the retinal activity have an important role to play in the diagnosis of retinal pathologies. The detection of diseases that involve retinal ganglion cells (RGCs), such as optic atrophy and glaucoma, may be improved by isolating the RGC contribution from the multifocal electroretinogram (mfERG). In this study, mfERGs were performed on 20 pigs, 1-6 weeks following unilateral retrobulbar optic nerve section (ONS). The stimuli were 103 non-scaled high-contrast hexagons from which summed and individual mfERG responses were obtained in experimental and control fellow eyes under conditions of ketamine (n = 11) or isoflurane anaesthesia (n = 9). The effect of intravitreal injection of tetrodotoxin (TTX; n = 6) was also investigated. The summed mfERG responses showed a first positive peak (P1) with a short latency (21 ms) followed by two smaller peaks (P2 and P3) of longer latency (46 and 65 ms, respectively). While P2 and P3 amplitude were highly correlated with the time post-optic nerve section (ONS) (P2: r(2) = 0.669; P = 0.007; P3: r(2) = 0.651; P = 0.005), P1 was not (r(2) = 0.193; P = 0.38). P1 and P2 showed no implicit time variation as a function of retinal location, while P3 implicit time varied along the axis of the visual streak, generating a naso-temporal asymmetry. However, the P3 implicit time did not vary consistently with distance away from the optic nerve head. Intravitreal injections of TTX reduced P2 and P3 in the control eyes, consistent with the effect of ONS, and also induced a series of regular oscillations lasting up to 200 ms post stimulus. Under isoflurane anaesthesia, all components of the mfERG ifn experimental and control eyes were, at all time points post-ONS, of similar amplitude and without naso-temporal asymmetry, suggesting a reduced participation of RGCs under these anaesthesic conditions. These data clearly demonstrate that it is possible to isolate the RGC contribution from non-invasive multifocal electroretinography.

Horizontal cells in the retina of a diurnal rodent, the agouti (Dasyprocta aguti)

The morphology and distribution of normally placed and displaced A horizontal cells were studied in the retina of a diurnal hystricomorph rodent, the agouti Dasyprocta aguti. Cells were labeled with anti-calbindin immunocytochemistry. Dendritic-field size reaches a minimum in the visual streak, of about 9,000 microm(2), and increases toward the retinal periphery both in the dorsal and ventral regions. There is a dorsoventral asymmetry, with dorsal cells being larger than ventral cells at equal distances from the streak. The peak value for cell density of 281 +/- 28 cells/mm(2) occurs in the center of the visual streak, decreasing toward the dorsal and ventral retinal periphery, paralleling the increase in dendritic-field size. Along the visual streak, the decline in cell density is less pronounced, remaining between 100-200 cells/mm(2) in the temporal and nasal periphery. Displaced horizontal cells are rare and occur in the retinal periphery. They tend to be smaller than normally placed horizontal cells in the ventral region, whilst no systematic difference was observed between the two cell groups in the dorsal region. Mosaic regularity was studied using nearest-neighbor analysis and the Ripley function. When mosaic regularity was determined removing the displaced horizontal cells, there was a slight increase in the conformity ratio, but the bivariate Ripley function indicated some repulsive dependence between the two mosaics. Both results were near the level of significance. A similar analysis performed in the capybara retina, a closely related hystricomorph rodent bearing a higher density of displaced horizontal cells than found in the agouti, suggested spatial independence between the two mosaics, normally placed versus displaced horizontal cells.

MfERG in normal and lesioned rabbit retina

To evaluate and describe the cone function in the normal and lesioned rabbit retina using the multifocal electroretinogram (mfERG). Twelve animals underwent a two-port vitrectomy with subsequent retinectomy in one eye. The area of removed retina was located in the nasal part of the visual streak, and measured approximately 1-2 disc diameters. Both eyes were investigated with mfERG preoperatively and up to 13 weeks postoperatively. A Burian-Allen bipolar contact lens with built-in infrared emitters was used to visualize the retina during the recordings. The averages of the trace array amplitudes in the lower nasal and temporal quadrants were calculated and statistically analyzed at the different time intervals. All eyes were examined histologically with hematoxylin and eosin staining. The retina could be visualized during the mfERG examinations. Postoperatively, up to 3 weeks, amplitudes were reduced over the entire stimulated area in retinectomized eyes (2.20 microV+/-1.22 SD) as compared with preoperative examinations (3.40 microV+/-1.00 SD). After 7 weeks the amplitudes in the quadrant including the retinectomized area remained low (2.62 microV+/-1.02 SD), whereas they were higher than at earlier postoperative examinations in the lower unlesioned temporal quadrant (3.56 microV+/-0.71 SD) with a statistical difference between the quadrants. At 13 weeks this was even more pronounced. In unoperated eyes, the area corresponding to the visual streak displayed significantly higher amplitudes than the area superior to the myelinated streak, corresponding well to the cone distribution. High amplitudes were also detected in the area of the myelinated nerve fibers and optic nerve head, most likely as a result of scattering light. In histological sections, the retinectomized area had a diameter of 1-3 mm. This study shows that the mfERG technique can be used as a tool in experimental retinal research involving the rabbit eye, where retinal lesions down to at least 1 mm can be detected. One difficulty involves scattering light emanating from the relatively large optic disc and the myelinated nerve fibers, which makes the use of a mfERG system, in which the fundus can be visualized during stimulation, mandatory.

Photoreceptors and visual pigments in the retina of the fully anadromous green sturgeon (Acipenser medirostrus) and the potamodromous pallid sturgeon (Scaphirhynchus albus)

Green sturgeon and pallid sturgeon photoreceptors were studied with scanning electron microscopy (SEM), microspectrophotometry and, in the case of the green sturgeon, retinal whole-mounts. The retinas of both species contain both rods and cones: cones comprise between 23% (whole-mount) and 36% (SEM) of the photoreceptors. The cone population of both species is dominated by large single cones, but a rare small single cone is also present. In both species, most rods have long outer segments of large diameter. A rod with a relatively thin outer segment is present in the pallid sturgeon retina. Mean cone packing density for the entire green sturgeon retina is 4,690+/-891 cones/mm2, with the dorsal retina 14% more dense than the ventral. There is evidence for a horizontal visual streak just above and including the optic disc. Mean rod packing density is 16,006+/-1,668 rods/mm2 for the entire retina, and fairly uniform throughout. Both species have rods with peak absorbance near 540 nm, as well as short-wavelength-sensitive cones (green: 464.5+/-0.7 nm; pallid: 439.7+/-3.5 nm); middle-wavelength-sensitive cones (green: 538.0+/-1.4 nm; pallid: 537.0+/-1.7 nm); and long-wavelength-sensitive cones (green: 613.9+/-3.0 nm; pallid: 617.8+/-7.6 nm).

Comparison of electrically evoked cortical potential thresholds generated with subretinal or suprachoroidal placement of a microelectrode array in the rabbit**Supported in part by The Charles D Kelman, MD Postdoctoral Scholar Award 2003 (YY); Boston VA Hospital (V523P-7278); Research to Prevent Blindness, New York City, NY and Kentucky Research Challenge Trust Fund (HJK).

The aim of the study was to directly compare the threshold electrical charge density of the retina (retinal threshold) in rabbits for the generation of electrical evoked potentials (EEP) by delivering electrical stimulation with a custom-made microelectrode array (MEA) implanted into either the subretinal or suprachoroidal space. Nine eyes of seven Dutch-belted rabbits were studied. The electroretinogram (ERG), visual evoked potentials (VEP) and EEP were recorded. Electrodes for the VEP and EEP were placed on the dura mater overlying the visual cortex. The EEP was recorded following electrical stimulation of the MEA placed either subretinally beneath the visual streak of the retina or in the suprachoroidal space in the rabbit eye. An ab externo approach was used for placement of the MEA. Liquid perfluorodecaline (PFCL; 0.4 ml) was placed within the vitreous cavity to flatten the neurosensory retina on the MEA after subretinal implantation. The retinal threshold for generation of an EEP was determined for each MEA placement by three consecutive measurements consisting of 100 computer-averaged recordings. Animals were sacrificed at the conclusion of the experiment and the eyes were enucleated for histological examination. The retinal threshold to generate an EEP was 9 ± 7 nC (0.023 ± 0.016 mC cm−2) within the subretinal space and 150 ± 122 nC (0.375 ± 0.306 mC cm−2) within the suprachoroidal space. Histology showed disruption of the outer retina with subretinal but not suprachoroidal placement. The retinal threshold to elicit an EEP is significantly lower with subretinal placement of the MEA compared to suprachoroidal placement (P < 0.05). The retinal threshold charge density with a subretinal MEA is well below the published charge limit of 1 mC cm−2, which is the level below which chronic stimulation of the retina is considered necessary to avoid tissue damage (Shannon 1992 IEEE Trans. Biomed. Eng. 39 424–6).

The Pax6 isoform bearing an alternative spliced exon promotes the development of the neural retinal structure

The vertebrate retina has an area where visual cells are closely packed for proper vision that is known as a fovea, an area centralis or a visual streak. The molecular mechanism that regulates the formation of these structures and visual cell gradients is unknown. The transcription factor Pax6 is a master regulator of eye development. A Pax6 isoform that contains an exon 5a-encoded 14 amino acid insertion in its paired domain, Pax6(+5a), has different DNA-binding properties compared with the Pax6(-5a) isoform. Little is known about the functional significance of Pax6(+5a). Here, we show that Pax6(+5a) is expressed especially in the retinal portion where visual cells accumulate during eye development and, when overexpressed, induces a remarkable well-differentiated retina-like structure. Pax6(+5a) proteins that bear point mutations that are found in patients with foveal hypoplasia are unable to induce these ectopic retina-like structures. We propose that Pax6(+5a) induces a developmental cascade in the prospective fovea, area centralis or visual streak region that leads to the formation of a retinal architecture bearing densely packed visual cells.

Microvasculature of the Retina, Ciliary Processes and Choroid in the North American Raccoon (Procyon lotor) Eye: A Scanning Electron Microscopic Study of Corrosion Casts

We have studied the vasculature of the retina, ciliary processes and choroid in the North American raccoon (Procyon lotor), a nocturnal mammal, using light and scanning electron microscopic examination of corrosion casts. We carried out an identical study in the crab-eating macaque (Macaca fascicularis), which forages only during the daytime, in order to compare the ocular vasculature with that of nocturnal mammals. Our observations in raccoons demonstrated a photoreceptor layer associated with rich lymph and a poorly vascularized retina. The meridian region of the eye, which lies in the horizontal plane and pass around the optic disc, had a markedly sparse capillary network. This horizontal sparse vascular band may correspond to a visual streak. Ciliary process capillaries were delicate, and formed a well-developed and compact network. Choriocapillaries were quite thin and formed a coarse capillary network. This contrasted with the dense retinal and well-extended choroidal capillary networks noted in the macaques. Our findings suggest that the sparse retinal capillary network in raccoons is extremely beneficial for photon capture, thereby allowing the raccoon to see well at night, as the retinal vessels restrict the inflow of photons toward the photoreceptors. The well-developed lymph probably compensates for the sparse retinal capillaries and choriocapillaries and nourishes the retina in the nocturnal raccoon.

Retinal specialisations in the dogfish &lt;i&gt;Centroscymnus coelolepis&lt;/i&gt; from the Mediterranean deep-sea

The present work attempted to study the importance of vision in Centroscymnuscoelolepis , the most abundant shark in the Mediterranean beyond a depthof 1000 m, by using anatomical and histological data. C.coelolepis exhibited large lateral eyes with a large pupil, spherical lens and a tapetum lucidum that gave the eye a strong greenish-golden “eye shine”. In the outer retinal layer, a uniform population of rod-like photoreceptors was observed while in the vitreal retina a thick inner plexiform layer comprised up to 30% of the whole retinal thickness. The cell distribution of the ganglion cell layer formed a thin elongated visual streak in the central plane of the eye that provided a horizontal panoramic field of view. A specialised area of higher visual acuity was located caudally at 32-44o from the geometric centre of the retina and 5-10o above the horizontal plane of the eye. This position indicated that the visual axis pointed in a slightly outward-forward direction with respect to the fish body axis. A non-uniform distribution of large ganglion cells was also found in the horizontal plane of the retina that practically coincided with the distribution of the total cell population in the ganglion cell layer. This is the first time that this type of retinal specialisation has been observed in the elasmobranchs. These characteristics indicate that the retina of C.coelolepis is designed not only to increase sensitivity in the horizontal field of view, as was also observed in other sharks, but also to improve motion detection in the same plane. The visual capacities evolved by C.coelolepis make this species adapted for discriminating the horizontal gradation of light that exists in the mesopelagic environment. Similarly, the large ganglion cell distribution observed in its retina seems to be related to its predatory behaviour, since it allows this shark to perceive the movement of bioluminescent prey against a totally dark background.

Transretinal electrical stimulation by an intrascleral multichannel electrode array in rabbit eyes

A new method of stimulating the retina electrically, called suprachoroidal transretinal stimulation (STS), was shown to be effective in eliciting electrically evoked cortical potentials (EEPs) in Royal College of Surgeons (RCS) rats. Before extending this technique to patients, it is important to determine its safety and feasibility in eliciting EEPs from medium-size animal (rabbits). The purpose of this study was to determine the safety and efficacy of the surgical procedures used to implant an multichannel electrode array into a scleral pocket, and to determine whether the implanted electrodes can stimulate the retina effectively. These acute experiments were conducted on six rabbits. An array of eight gold microelectrodes, embedded in polyimide, was implanted into a scleral pocket over the visual streak area. The size of the microarray was 2 x 4 x 0.180 mm. The reference electrode was implanted into the vitreous. The electrode array and reference electrodes were connected to a stimulator to deliver monophasic current pulses. Cortical responses were recorded with a stainless steel electrode implanted into each rabbit's skull over the visual cortex. After the experiment, the eyes and electrodes were examined histologically. The surgical procedures for electrode implantation were accomplished without serious complications. EEPs were recorded after monophasic electrical pulse stimulation from each electrode. The mean threshold for EEPs was 55.0+/-10.0 microA with a 0.5-ms duration inward current pulse. The charge delivered at threshold was about 27.5 nC, and the charge density was about 56.0 microC/cm2. Histopathological examination of the retinal tissue around the area of stimulation did not show damage at the light microscope level with the electrical parameters used. Our technique for STS with an intrascleral microelectrode array is safe in rabbit eyes, and EEPs were elicited by current densities that did not induce tissue damage. These results suggest that STS via intrascleral multichannel electrodes is a feasible method for stimulating the retina.

Vision in the deep sea.

The deep sea is the largest habitat on earth. Its three great faunal environments--the twilight mesopelagic zone, the dark bathypelagic zone and the vast flat expanses of the benthic habitat--are home to a rich fauna of vertebrates and invertebrates. In the mesopelagic zone (150-1000 m), the down-welling daylight creates an extended scene that becomes increasingly dimmer and bluer with depth. The available daylight also originates increasingly from vertically above, and bioluminescent point-source flashes, well contrasted against the dim background daylight, become increasingly visible. In the bathypelagic zone below 1000 m no daylight remains, and the scene becomes entirely dominated by point-like bioluminescence. This changing nature of visual scenes with depth--from extended source to point source--has had a profound effect on the designs of deep-sea eyes, both optically and neurally, a fact that until recently was not fully appreciated. Recent measurements of the sensitivity and spatial resolution of deep-sea eyes--particularly from the camera eyes of fishes and cephalopods and the compound eyes of crustaceans--reveal that ocular designs are well matched to the nature of the visual scene at any given depth. This match between eye design and visual scene is the subject of this review. The greatest variation in eye design is found in the mesopelagic zone, where dim down-welling daylight and bio-luminescent point sources may be visible simultaneously. Some mesopelagic eyes rely on spatial and temporal summation to increase sensitivity to a dim extended scene, while others sacrifice this sensitivity to localise pinpoints of bright bioluminescence. Yet other eyes have retinal regions separately specialised for each type of light. In the bathypelagic zone, eyes generally get smaller and therefore less sensitive to point sources with increasing depth. In fishes, this insensitivity, combined with surprisingly high spatial resolution, is very well adapted to the detection and localisation of point-source bioluminescence at ecologically meaningful distances. At all depths, the eyes of animals active on and over the nutrient-rich sea floor are generally larger than the eyes of pelagic species. In fishes, the retinal ganglion cells are also frequently arranged in a horizontal visual streak, an adaptation for viewing the wide flat horizon of the sea floor, and all animals living there. These and many other aspects of light and vision in the deep sea are reviewed in support of the following conclusion: it is not only the intensity of light at different depths, but also its distribution in space, which has been a major force in the evolution of deep-sea vision.

Photoreceptor topography in the duplex retina of the paddlefish (Polyodon spathula)

Retinal whole-mount preparations from the eyes of the North American paddlefish, Polyodon spathula, were examined with a combination of bright field and differential interference contrast microscopy. The entire retina was mapped and population counts of rod and cone photoreceptors were made at regular intervals throughout the retina. The retina is dominated by rods, but a significant percentage (ca. 38%) of the photoreceptors are cones. Mean cone packing density for the entire retina is 6,402+/-1,216 cones/mm2. There is a small (16%) but statistically significant difference between cone packing density in the dorsal retina (6,674+/-1,168 cones/mm2) and the ventral retina (5,745+/-1,076 cones/mm2). There is no region of unusually high cone concentration that might be construed as a fovea or a visual streak. Mean rod packing density for the entire retina is 10,271+/-1,205 rods/mm2. Except in the far periphery, where rods are less numerous, the density of rods is fairly uniform throughout the retina. The data are discussed with regard to paddlefish habitat and behavior.

Localization of NMDA receptor subunits and mapping NMDA drive within the mammalian retina

Glutamate is a major neurotransmitter in the retina and other parts of the central nervous system, exerting its influence through ionotropic and metabotropic receptors. One ionotropic receptor, the N-methyl-D-aspartate(NMDA) receptor, is central to neural shaping, but also plays a major role during neuronal development and in disease processes. We studied the distribution pattern of different subunits of the NMDA receptor within the rat retina including quantifying the pattern of labelling for all the NRI splice variants, the NR2A and NR2B subunits. The labelling pattern for the subunits was confined predominantly in the outer two-thirds of the inner plexiform layer. We also wanted to probe NMDA receptor function using an organic cation, agmatine (AGB); a marker for cation channel activity. Although there was an NMDA concentration-dependent increase in AGB labelling of amacrine cells and ganglion cells, we found no evidence of functional NMDA receptors on horizontal cells in the peripheral rabbit retina, nor in the visual streak where the type A horizontal cell was identified by GABA labelling. Basal AGB labelling within depolarizing bipolar cells was also noted. This basal bipolar cell AGB labelling was not modulated by NMDA and was completely abolished by the use of L-2-amino-4-phosphono-butyric acid,which is known to hyperpolarize retinal depolarizing bipolar cells. AGB is therefore not only useful as a probe of ligand-gated drive, but can also identify neurons that have constitutively open cationic channels. In combination,the NMDA receptor subunit distribution pattern and the AGB gating experiments strongly suggests that this ionotropic glutamate receptor is functional in the cone-driven pathway of the inner retina.

The type 1 polyaxonal amacrine cells of the rabbit retina: a tracer-coupling study.

The type 1 polyaxonal (PA1) cell is a distinct type of axon-bearing amacrine cell whose soma commonly occupies an interstitial position in the inner plexiform layer; the proximal branches of the sparse dendritic tree produce 1-4 axon-like processes, which form an extensive axonal arbor that is concentric with the smaller dendritic tree (Dacey, 1989; Famiglietti, 1992a,b). In this study, intracellular injections of Neurobiotin have revealed the complete dendritic and axonal morphology of the PA1 cells in the rabbit retina, as well as labeling the local array of PA1 cells through homologous tracer coupling. The dendritic-field area of the PA1 cells increased from a minimum of 0.15 mm2 (0.44-mm equivalent diameter) on the visual streak to a maximum of 0.67 mm2 (0.92-mm diameter) in the far periphery; the axonal-field area also showed a 3-fold variation across the retina, ranging from 3.1 mm2 (2.0-mm diameter) to 10.2 mm2 (3.6-mm diameter). The increase in dendritic- and axonal-field size was accompanied by a reduction in cell density, from 60 cells/mm2 in the visual streak to 20 cells/mm2 in the far periphery, so that the PA1 cells showed a 12 times overlap of their dendritic fields across the retina and a 200-300 times overlap of their axonal fields. Consequently, the axonal plexus was much denser than the dendritic plexus, with each square millimeter of retina containing approximately 100 mm of dendrites and approximately 1000 mm of axonal processes. The strong homologous tracer coupling revealed that approximately 45% of the PA1 somata were located in the inner nuclear layer, approximately 50% in the inner plexiform layer, and approximately 5% in the ganglion cell layer. In addition, the Neurobiotin-injected PA1 cells sometimes showed clear heterologous tracer coupling to a regular array of small ganglion cells, which were present at half the density of the PA1 cells. The PA1 cells were also shown to contain elevated levels of gamma-aminobutyric acid (GABA), like other axon-bearing amacrine cells.

Variations in the off-axis refractive state in the eye of the Vietnamese leaf turtle (Geoemyda spengleri).

Lower-field myopia has been described for various vertebrates as an adaptation that permits the animal to keep the ground in focus during foraging, and, at the same time, to look out for distant objects, such as predators, in the upper visual field. Off-axis measurements with infrared photoretinoscopy in the eye of Geoemyda spengleri revealed a constant refractive state in the horizontal plane of the visual field but variable refraction in the vertical plane. In the three turtles investigated, the refractions increased continuously from the ventral to the dorsal visual field over a range of 35, 40 and 56 D, respectively. While this finding confirms the presence of an adaptive change of the refractive state equivalent to lower field myopia, subsequent measurements with a rotated retinoscope showed that at least part of the variation in the ventral field was attributed to astigmatism. The reason for this astigmatism is unknown. Anatomical investigation of the retina revealed that the constant refractive values in the horizontal plane corresponded to a stripe of increased ganglion cell density. A maximum density of 4,200 ganglion cells mm(-2) was counted in the centre of this visual streak.

A Strong Correlation Exists between the Distribution of Retinal Ganglion Cells and Nose Length in the Dog

The domestic dog, Canis lupus familiaris, is a subspecies of the gray wolf, Canis lupus, with almost identical mitochondrial DNA. The dog is the most diverse species on earth, with skull length varying between 7 and 28 cm whereas the wolf skull is around 30 cm long. However, eye size in dogs does not appear to vary as much. For example, small dogs such as the chihuahua appear to have very large eyes in proportion to the skull. Our aim was to examine eye size and retinal cell numbers and distribution to determine whether the dog eye exhibits as much variation as the skull. We found a correlation between eye radius and skull dimensions. However, the most surprising finding was that the distribution of ganglion cells in the eye varied tremendously from a horizontally aligned visual streak of fairly even density across the retina (as seen in the wolf) to a strong area centralis with virtually no streak (for example, as observed in a pug from the current series). This variation in ganglion cell density within a single species is quite unique. Intriguingly, the ratio of peak ganglion cell density in the area centralis to visual streak was highly negatively correlated with skull length (r = –0.795, n = 22) and positively correlated with cephalic index (r = 0.687, n = 22). The orientation of eyelid aperture was also correlated with cephalic index (r = 0.648, n = 22). Therefore, the genetic manipulation of selective breeding, which has produced an abnormal shortening of the skull and eyelids with less lateral apertures, has also produced a considerably more pronounced area centralis in the dog.

Rabbit retinal ganglion cell survival after optic nerve section and its effect on the inner plexiform layer

Structural modifications of the inner retina were studied after optic nerve section (ONS) in the rabbit. Retinal ganglion cells (RGC) were labelled by injection of Fast Blue into the optic nerve, and counted under fluorescent light in control retina and retina 7, 14, 21 and 26 days post-axotomy. Studies on retinal cross-sections were also performed. For this purpose, retinal sections were stained with haematoxylin–eosin and immunohystochemistry for α1 and β2/β3 sub-units of the GABA A receptors. One week after axotomy, there was no significant loss in the number of ganglion cells with respect to control counts (1086±173 cells mm −2 in the visual streak and 119±46 cells mm −2 in the periphery, mean± sd, n=5). At 14 days post-axotomy, 271±46 cells mm −2 remained in the visual streak and 33±6 cells mm −2 in the periphery, corresponding to a mean survival of 27%. The number of ganglion cells decreased further on the following days, reaching 7·54% 1 month after ONS. A significant reduction in the thickness of the inner plexiform and ganglion cell layers was also observed in retinal cross-sections. Immunocytochemical studies show a remarkable disorganization of the layer stratification in the inner plexiform layer (IPL). We conclude that after ONS, RGC death occurs mainly between 7 and 14 days post-axotomy and a progressive death up to 26 days, causing a decrease in the thickness of the IPL and subsequent disorganization of its layers.

Morphological analysis of the hyperpolarization-activated cyclic nucleotide-gated cation channel 1 (HCN1) immunoreactive bipolar cells in the rabbit retina.

Hyperpolarization-activated cation currents (I(h)) have been identified in neurons in the central nervous system, including the retina. There is growing evidence that these currents, mediated by the hyperpolarization-activated cyclic nucleotide-gated cation channel (HCN), may play important roles in visual processing in the retina. This study was conducted to identify and characterize HCN1-immunoreactive (IR) bipolar cells by immunocytochemistry, quantitative analysis, and electron microscopy. The HCN1-IR bipolar cells were a subtype of OFF-type cone bipolar cells and comprised 10% of the total number of cone bipolar cells. The axons of the HCN1-IR cone bipolar cells ramified narrowly in the border of strata 1 and 2 of the inner plexiform layer (IPL). These cells formed a regular distribution, with a density of 1,825 cells/mm(2) at a position 1 mm ventral to the visual streak, falling to 650 cells/mm(2) in the ventral periphery. Double-labeling experiments demonstrated that their axons stratified narrowly within and slightly proximal to the OFF-starburst amacrine cell processes. In the IPL, they were presynaptic to amacrine cell processes. The most frequent postsynaptic dyads formed of HCN1-IR bipolar cell axon terminals are pairs composed of both amacrine cell processes. These results suggest that these HCN1-IR cone bipolar cells might be the same as the DAPI-Ba1 bipolar population, and might therefore be involved in a direction-selective mechanism, providing inputs to the OFF-starburst amacrine cells and/or the OFF-plexus of the ON-OFF ganglion cells.

Topography of Photoreceptors and Retinal Ganglion Cells in the Spotted Hyena (Crocuta crocuta)

The spatial distributions of photoreceptors and retinal ganglion cells were examined in the spotted hyena (Crocuta crocuta). Two populations of cones were identified by immunocytochemical labeling. The hyena retina contains approximately 2.3 million middle- to long-wavelength sensitive (M/L) cones that reach peak densities of about 7,500/mm² in the vicinity of the optic nerve head. A sparser population of short-wavelength sensitive (S) cones, totaling about 0.3 million, was also detected. There is a striking disparity in the spatial distributions of the two cone types with S cones achieving peak density in a region located well below the optic nerve head. The differences in the spatial distributions of the two cone types have implications both for visual sensitivity and for color vision. Hyena rods outnumber cones by about 100:1 with rod density falling off modestly along a central-peripheral gradient. Ganglion cells were identified in retinal wholemounts by Nissl staining patterns. Their distribution defines a prominent visual streak with highest spatial packing (approx. 4,200/mm²) in an area centralis that is located in the temporal retina. The total number of ganglion cells is estimated at about 260,000. Using standard assumptions the maximum spatial resolution of the spotted hyena is calculated to be about 8.4 cycles/degree, a value similar to estimates obtained for other terrestrial carnivores.

Low oxygen consumption in the inner retina of the visual streak of the rabbit.

The oxygen requirements of different retinal layers are of interest in understanding the vulnerability of the retina to hypoxic damage in retinal diseases with an ischemic component. Here, we report the first measurements of retinal oxygen consumption in the visual streak of the rabbit retina, the region with the highest density of retinal neurons, and compare it with that in the less-specialized region of the retina underlying the vascularized portion of the rabbit retina. Oxygen-sensitive microelectrodes were used to measure oxygen tension as a function of retinal depth in anesthetized animals. Measurements were performed in the region of the retina containing overlying retinal vessels and in the center of the visual streak. Established mathematical analyses of the intraretinal oxygen distribution were used to quantify the rate of oxygen consumption in the inner and outer retina and the relative oxygen contributions from the choroidal and vitreal sides. Outer retinal oxygen consumption was higher in the visual streak than in the vascularized area (means +/- SE, 284 +/- 20 vs. 210 +/- 23 nl O2.min(-1) x cm(-2), P = 0.026, n = 10). However, inner retinal oxygen consumption in the visual streak was significantly lower than in the vascular area (57 +/- 4.3 vs. 146 +/- 12 nl O2 x min(-1) x cm(-2), P < 0.001). We conclude that despite the higher processing requirements of the inner retina in the visual streak, it has a significantly lower oxygen consumption rate than the inner retina underlying the retinal vasculature. This suggests that the oxygen uptake of the inner retina is regulated to a large degree by the available oxygen supply rather than the processing requirements of the inner retina alone.