Frog Retina Research Articles

Why do green rods of frog and toad retinas look green?

Amphibian “green” rods express a blue-sensitive cone visual pigment, and should look yellow. However,when observing them axially under microscope one sees them as green. We used single-cell microspectrophotometry (MSP) to reveal the basis of the perceived color of these photoreceptors. Conventional side-on MSP recording of the proximal cell segments reveals no selective longwave absorbing pigment explaining the green color. End-on MSP recording shows, in addition to the green rod visual pigment, an extra 2- to 4-fold attenuation being almost flat throughout the visible spectrum. This attenuation is absent in red (rhodopsin) rods, and vanishes in green rods when the retina is bathed in high-refractive media, and at wide illumination aperture. The same treatments change the color from green to yellow. It seems that the non-visual pigment attenuation is a result of slender green rod myoids operating as non-selective light guides. We hypothesize that narrow myoids, combined with photomechanical movements of melanin granules, allow a wide range of sensitivity regulation supporting the operation of green rods as blue receptors at mesopic-to low-photopic illumination levels.End-on transmittance spectrum of green rods looks similar to the reflectance spectrum of khaki military uniforms. So their greenness is the combined result of optics and human color vision.

Effects of picrotoxin on light adapted frog electroretinogram are not due entirely to its action in proximal retina.

In order to evaluate the site of action of picrotoxin (antagonist of ionotropic GABA receptors) on the electroretinographic (ERG) b- and d-waves, in this study we compared its effects on the intensity-response function of the ERG waves in intact light adapted frog eyecup preparations with its effects in eyecups, where the activity of proximal neurons was blocked by 1 mMN-methyl-d-aspartate (MNDA). Picrotoxin markedly enhanced the b- and d-wave amplitude and slowed the time course of the responses at all stimulus intensities in the intact eyecups. Perfusion with NMDA alone caused significant enhancement of the b-wave amplitude and diminution of the d-wave amplitude without altering their time course in the entire intensity range. When picrotoxin was applied in combination with NMDA, an enhancement of the b-wave amplitude and slowing of its time course were observed at all stimulus intensities. The increase of the b-wave amplitude was significantly higher than that seen in NMDA group. Combined application of picrotoxin and NMDA caused an enhancement of the d-wave amplitude at the lower stimulus intensities and its diminution at the higher ones, while the d-wave time course was delayed over the entire intensity range. The results obtained indicate that a part of picrotoxin effects on the amplitude and time course of the photopic ERG b- and d-waves are due to its action in the distal frog retina.

Noggin Expression in the Adult Retina Suggests a Conserved Role during Vertebrate Evolution.

Vertebrates share common mechanisms in the control of development and in the maintenance of neural and retinal function. The secreted factor Noggin, a BMP inhibitor, plays a crucial role in neural induction during embryonic development. Moreover, we have shown its involvement in retinal differentiation of pluripotent cells. Here we show Noggin expression in the adult retina in three vertebrate species. Four Noggin genes are present in zebrafish (Danio rerio; ZbNog1, 2, 3, 5), three in frog (Xenopus laevis; XenNog1, 2 and 4), and one in mouse (Mus musculus; mNog). Quantitative RT-PCR experiments show the presence of ZbNog3 and ZbNog5 mRNAs, but not ZbNog1 and ZbNog2, in the adult zebrafish retina. All three genes are expressed in the frog retina, and mNog in the mouse. Immunohistochemistry data show that Noggin proteins are predominantly localized in the Golgi apparatus of photoreceptors and in the fibers of the outer plexiform layer. Lower expression levels are also found in inner plexiform layer fibers, in ganglion cells, in the ciliary marginal zone, and in retinal pigmented epithelium. Our results show that Noggin has a specific cellular and sub-cellular expression in the adult vertebrate retina, which is conserved during evolution. In addition to its established role during embryonic development, we postulate that Noggin also exerts a functional role in the adult retina.

Ionotropic purinergic receptors P2X in frog and turtle retina: Glial and neuronal localization

Purinergic signaling is represented in both the peripheral and central nervous system (CNS), and in particular in the retina, which may be regarded as a part of the CNS. While purigenic signaling is relatively well studied in mammalian retinas, little is known about it in retinas of lower vertebrates. The aim of present study was to investigate, using immunocytochemistry, the distribution of purinoreceptors P2X in retinas of frog and turtle, which are appropriate models of the brain neuron-to-glia interactions. The results showed widespread expression of all seven ionotropic purinoreceptors (P2X1–P2X7) in both frog and turtle retinas. They were predominantly expressed in Müller cells, the principal glial cells in the retina. All structures typical of Müller cells: the outer and the inner limiting membranes, the cells bodies in the inner nuclear layer, the radial processes in the inner plexiform layer (IPL), and the so called endfeet (frog) or the orthogonal arrays of particles (turtle) in the ganglion cells layer were immunostained. Colocalizations between P2X1–P2X7 and the glial cell marker Vimentin proved that the immunostaining was in the Müller cells. In addition to the glial staining, neuronal staining was also seen as fine puncta in the inner plexiform layer and by small dots and patches in the outer plexiform layer. Some cell bodies of horizontal, amacrine and ganglion cells were also stained. The results obtained imply that the purinergic P2X receptors may significantly contribute to the neuron-to-glia signaling in retinas of the lower vertebrates.

In vivo confocal intrinsic optical signal identification of localized retinal dysfunction.

The purposes of this study were to investigate the physiological mechanism of stimulus-evoked fast intrinsic optical signals (IOSs) recorded in dynamic confocal imaging of the retina, and to demonstrate the feasibility of in vivo confocal IOS mapping of localized retinal dysfunctions. A rapid line-scan confocal ophthalmoscope was constructed to achieve in vivo confocal IOS imaging of frog (Rana pipiens) retinas at cellular resolution. In order to investigate the physiological mechanism of confocal IOS, comparative IOS and electroretinography (ERG) measurements were made using normal frog eyes activated by variable-intensity stimuli. A dynamic spatiotemporal filtering algorithm was developed to reject the contamination of hemodynamic changes on fast IOS recording. Laser-injured frog eyes were employed to test the potential of confocal IOS mapping of localized retinal dysfunctions. Comparative IOS and ERG experiments revealed a close correlation between the confocal IOS and retinal ERG, particularly the ERG a-wave, which has been widely used to evaluate photoreceptor function. IOS imaging of laser-injured frog eyes indicated that the confocal IOS could unambiguously detect localized (30 μm) functional lesions in the retina before a morphological abnormality is detectable. The confocal IOS predominantly results from retinal photoreceptors, and can be used to map localized photoreceptor lesion in laser-injured frog eyes. We anticipate that confocal IOS imaging can provide applications in early detection of age-related macular degeneration, retinitis pigmentosa, and other retinal diseases that can cause pathological changes in the photoreceptors.

CAMP controls rod photoreceptor sensitivity via multiple targets in the phototransduction cascade

In early studies, both cyclic AMP (cAMP) and cGMP were considered as potential secondary messengers regulating the conductivity of the vertebrate photoreceptor plasma membrane. Later discovery of the cGMP specificity of cyclic nucleotide–gated channels has shifted attention to cGMP as the only secondary messenger in the phototransduction cascade, and cAMP is not considered in modern schemes of phototransduction. Here, we report evidence that cAMP may also be involved in regulation of the phototransduction cascade. Using a suction pipette technique, we recorded light responses of isolated solitary rods from the frog retina in normal solution and in the medium containing 2 µM of adenylate cyclase activator forskolin. Under forskolin action, flash sensitivity rose more than twofold because of a retarded photoresponse turn-off. The same concentration of forskolin lead to a 2.5-fold increase in the rod outer segment cAMP, which is close to earlier reported natural day/night cAMP variations. Detailed analysis of cAMP action on the phototransduction cascade suggests that several targets are affected by cAMP increase: (a) basal dark phosphodiesterase (PDE) activity decreases; (b) at the same intensity of light background, steady background-induced PDE activity increases; (c) at light backgrounds, guanylate cyclase activity at a given fraction of open channels is reduced; and (d) the magnitude of the Ca2+ exchanger current rises 1.6-fold, which would correspond to a 1.6-fold elevation of [Ca2+]in. Analysis by a complete model of rod phototransduction suggests that an increase of [Ca2+]in might also explain effects (b) and (c). The mechanism(s) by which cAMP could regulate [Ca2+]in and PDE basal activity is unclear. We suggest that these regulations may have adaptive significance and improve the performance of the visual system when it switches between day and night light conditions.

Frog retinal ganglion cells projecting to the tectum layer F release acetylcholine as co-mediator

Neurons may release more than one classical neurotransmitter (co-mediator). It has been demonstrated in a recent study that a burst of action potentials in frog retina ganglion cells induces an after-burst increase (phasic potentiation) of the retinotectal transmission that lasts tens of seconds. This increase is mediated by presynaptic nicotinic acetylcholine receptors that are activated by the endogenous acetylcholine released during the burst of action potentials of the retinotectal fiber. The objective of the present study was to find out the origin of acetylcholine release. We show that reduction of the retinotectal transmission to the subthreshold level by application of moderate concentrations of kynurenic acid or CNQX had no effect on the phasic nicotinic potentiation of the retinotectal transmission. This demonstrates that the retinotectal terminals are the source of acetylcholine – responsible for the phasic potentiation of retinotectal transmission. The acetylcholine is thus co-released with glutamate.

An X-ray diffraction study on a single rod outer segment from frog retina

X-ray diffraction patterns were recorded from isolated single rod outer segments of frog. The outer segments in Ringer's solution were exposed to a 6 µm microbeam (15 keV) at the BL40XU beamline, SPring-8. The diffraction pattern demonstrated a remarkable regularity in the stacking and flatness of the disk membranes. The electron density profile calculated from the intensity of up to tenth-order reflections showed a pair of bilayers that comprise a disk membrane. The structure of the disk membrane and the changes in the profile on swelling generally agreed with previous reports. Radiation damage was significant with an irradiation of 5 × 10(5) Gy which is much lower than the known damaging dose on proteins at the liquid-nitrogen temperature.

All‐Trans Retinal Mediates Light‐Induced Oxidation in Single Living Rod Photoreceptors†

All-trans retinal is a potent photosensitizer that is released in photoreceptor outer segments by the photoactivated visual pigment following the detection of light. Photoreceptor outer segments also contain high concentrations of polyunsaturated fatty acids, and are thus particularly susceptible to oxidative damage such as that initiated by light via a photosensitizer. Upon its release, all-trans retinal is reduced within the outer segment to all-trans retinol, through a reaction requiring metabolic input in the form of NADPH. The phototoxic potential of physiologically generated all-trans retinal was examined in single living rod photoreceptors obtained from frog (Rana pipiens) retinas. Light-induced oxidation was measured with fluorescence imaging using an oxidation-sensitive indicator dye from the shift in fluorescence between the intact and oxidized forms. Light-induced oxidation was highest in metabolically compromised rod outer segments following photoactivation of the visual pigment rhodopsin, and after a time interval, sufficiently long to ensure the release of all-trans retinal. Furthermore, light-induced oxidation increased with the concentration of exogenously added all-trans retinal. The results show that the all-trans retinal generated during the detection of light can mediate light-induced oxidation. Its removal through reduction to all-trans retinol protects photoreceptor outer segments against light-induced oxidative damage.

Using myc genes to search for stem cells in the ciliary margin of the Xenopus retina

The ciliary marginal zone (CMZ) of fish and frog retinas contains cells that proliferate throughout postembryonic development as the retina grows with increasing body size, indicating the presence of stem cells in this region. However, neither the location nor the molecular identity of retinal stem cells has been identified. Here, we show in Xenopus that c-myc and n-myc are sequentially expressed both during development and in the post-embryonic retina. The c-myc+/n-myc- cells near the extreme periphery of the CMZ cycle more slowly and preferentially retain DNA label compared to their more central cmyc+/n-myc+ neighbors which cycle rapidly and preferentially dilute DNA label. During retinal development c-myc is functionally required earlier than n-myc, and n-myc expression depends on earlier c-myc expression. The expression of c-myc but not n-myc in the CMZ depends on growth factor signaling. Our results suggest that c-myc+/n-myc- cells in the far peripheral CMZ are candidates for a niche-dependent population of retinal stem cells that give rise to more centrally located and rapidly dividing n-myc+ progenitors of more limited proliferative potential. Analysis of homologues of these genes in the zebrafish CMZ suggests that the transition from c-myc to n-myc expression might be conserved in other lower vertebrates whose retinas growth throughout life.

Investigation of the hyper-reflective inner/outer segment band in optical coherence tomography of living frog retina

This study is to test anatomic correlates, including connecting cilium (CC) and inner segment (IS) ellipsoid, to the hyper-reflective band visualized by optical coherence tomography (OCT) and commonly attributed to the photoreceptor inner/outer segment (IS/OS) junction. A line-scan OCT (LS-OCT) was constructed to achieve sub-cellular resolution (lateral: ≈ 2 μm; axial: ≈ 4 μm) of excised living frog retinas. An electro-optic phase modulator was employed for rapid and vibration-free phase modulation. Comparison of normalized distance measurements between LS-OCT images and histological images revealed that the dominant source of the signal reported as the IS/OS OCT band actually originates from the IS.

Morphological versus functional photoreceptor viability of retinal pigment epithelium tears

Editor, Retinal pigment epithelium tears (RPE tears) are a serious complication in patients with age-related macular degeneration (AMD) leading to a loss of photoreceptors. Previously, we have shown that the photoreceptors can be rescued and function restored by autologous transplantation of RPE and choroid in these patients (Caramoy et al. 2011a). We have also described previously that the morphological signs of viable photoreceptors can be seen in the area of pigment epithelium tears up to 325 days (Caramoy et al. 2011b), however little is known about the function of these photoreceptors in the area without retinal pigment epithelium. In a retrospective study, we examined the morphology of RPE tears using spectral-domain optical coherence tomography (SD-OCT) (Spectralis HRA + OCT; Heidelberg Engineering, Heidelberg, Germany) in relation to its function in microperimetry (MP1; Nidek Technologies, Padova, Italy). Mean age was 76 ± 7.9 years (range 63–85 years) and mean best corrected visual acuity was 0.54 ± 0.37 LogMAR. Among the seven patients, three were men and four were women. Four patients showed unstable fixation. Relatively unstable and stable fixations were seen in two patients and in one patient, respectively. In the patient with stable fixation, the fovea was still supported by pigment epithelium. The fixation stability (percentage of fixation within the central 2° and 4°) was 42 ± 29.65% and 75 ± 19.61%, respectively. Among these patients, morphological signs of viable photoreceptors were seen in four of seven patients, that is junction of inner and outer photoreceptor segments, external limiting membrane and nonatrophic outer nuclear layer. Time from RPE tear to OCT and microperimetry in these patients was <30 days in two patients, 21 and 109 days in the other two. These signs were not seen in three of seven patients because of retinal atrophy. Time from RPE tear to OCT and microperimetry in these three patients was <30 days in two patients and 717 days in one patient. Subretinal fluid was seen in the RPE denuded area in five of seven patients. Independent of these morphological signs of viable photoreceptors and independent of subretinal fluid presence, all patients showed an absolute scotoma in the RPE denuded area (Fig. 1). Mean retinal sensitivity among this region was 0.07 ± 0.1 dB. Microperimetry and spectral-domain optical coherence tomography of a pigment epithelium tear. A dense absolute scotoma is seen in the region without retinal pigment epithelium support in microperimetry (A). However, the corresponding spectral-domain optical coherence tomography shows signs of viable photoreceptors (B and C). Identifying signs of viable photoreceptors in the wrinkled RPE region was challenging. The retina in this region was thin; its anatomical layers could not be visualized closely. In this region, mean retinal sensitivity was 0.89 ± 1.20 dB. Unlike the RPE denuded area, which exhibited a dense absolute scotoma, the region of wrinkled RPE was more heterogenous. It consisted of areas of relative and absolute scotoma. The results from our study underlined the importance of the RPE regarding visual function. In normal retina, the RPE is essential in the visual cycle towards regenerating visual pigments in cones and rods. Kühne (1878) found in bleached frog retina that rhodopsin could only regenerate partially, if the retina was detached from the retinal pigment epithelium. Weinstein et al. (1967) have also shown that no rhodopsin regeneration is observed in rat retina perfused with enriched medium detached from retinal pigment epithelium. Photoreceptors lose their functions after separation from the pigment epithelium as shown by microperimetry. But they still show signs of viability in SD-OCT. These morphological signs seem to be correct as shown by the above-mentioned recovery of photoreceptors after transplantation of RPE and choroid. These findings emphasize the importance of RPE for photoreceptors towards regenerating visual pigments, which are crucial for the visual function.

Fate Restriction and Multipotency in Retinal Stem Cells

Stem cells have the capacity to both self-renew and generate postmitotic cells. Long-term tracking of individual clones in their natural environment constitutes the ultimate way to validate postembryonic stem cells. We identify retinal stem cells (RSCs) using the spatiotemporal organization of the fish retina and follow the complete offspring of a single cell during the postnatal life. RSCs generate two tissues of the adult fish retina, the neural retina (NR) and the retinal-pigmented epithelium (RPE). Despite their common embryonic origin and tight coordination during continuous organ growth, we prove that NR and RPE are maintained by dedicated RSCs that contribute in a fate-restricted manner to either one or the other tissue. We show that in the NR, RSCs are multipotent and generate all neuron types and glia. The clonal origin of these different cell types from a multipotent NSC has far-reaching implications for cell type and tissue homeostasis.

Presynaptic nicotinic potentiation of a frog retinotectal transmission evoked by discharge of a single retina ganglion cell

It was demonstrated in our previous studies of the frog retinotectal transmission that retinotectal synaptic potentials are enhanced by a factor of 1.5 due to the tonic presynaptic nicotinic potentiation, caused by the ambient level of the acetylcholine in the frog tectum. Furthermore, the results of those studies have indicated that the mechanism of the nicotinic potentiation is only partially exploited, because the application of the cholinergic agonist had increased the retinotectal transmission more than 2 times above the level of the tonic potentiation. The purpose of the present study was to explore this additional potentiation. We have shown that: (1) Bursts of 4–10 action potentials of a frog retina ganglion cell gave rise to an increase (phasic potentiation) of the retinotectal transmission 1.4–2.2 times, depending on the burst strength, that lasted tens of seconds. (2) This increase has been mediated through the presynaptic nicotinic acetylcholine receptors activated by the endogenous acetylcholine released into the tectum during relatively strong bursts of the retina ganglion cell. (3) Two types of the nicotinic acetylcholine receptors are co-localized in the presynaptic terminals of the individual retinotectal input to the tectum layer F – high-affinity (tonic) and low-affinity (phasic) nicotinic receptors.

Two-photon excited autofluorescence imaging of freshly isolated frog retinas

The purpose of this study was to investigate cellular sources of autofluorescence signals in freshly isolated frog (Rana pipiens) retinas. Equipped with an ultrafast laser, a laser scanning two-photon excitation fluorescence microscope was employed for sub-cellular resolution examination of both sliced and flat-mounted retinas. Two-photon imaging of retinal slices revealed autofluorescence signals over multiple functional layers, including the photoreceptor layer (PRL), outer nuclear layer (ONL), outer plexiform layer (OPL), inner nuclear layer (INL), inner plexiform layer (IPL), and ganglion cell layer (GCL). Using flat-mounted retinas, depth-resolved imaging of individual retinal layers further confirmed multiple sources of autofluorescence signals. Cellular structures were clearly observed at the PRL, ONL, INL, and GCL. At the PRL, the autofluorescence was dominantly recorded from the intracellular compartment of the photoreceptors; while mixed intracellular and extracellular autofluorescence signals were observed at the ONL, INL, and GCL. High resolution autofluorescence imaging clearly revealed mosaic organization of rod and cone photoreceptors; and sub-cellular bright autofluorescence spots, which might relate to connecting cilium, was observed in the cone photoreceptors only. Moreover, single-cone and double-cone outer segments could be directly differentiated.

A Simplified Mass-Transfer Model for Visual Pigments in Amphibian Retinal-Cone Outer Segments

When radiolabeled precursors and autoradiography are used to investigate turnover of protein components in photoreceptive cone outer segments (COSs), the labeled components—primarily visual pigment molecules (opsins)—are diffusely distributed along the COS. To further assess this COS labeling pattern, we derive a simplified mass-transfer model for quantifying the contributions of advective and diffusive mechanisms to the distribution of opsins within COSs of the frog retina. Two opsin-containing regions of the COS are evaluated: the core axial array of disks and the plasmalemma. Numerical solutions of the mass-transfer model indicate three distinct stages of system evolution. In the first stage, plasmalemma diffusion is dominant. In the second stage, the plasmalemma density reaches a metastable state and transfer between the plasmalemma and disk region occurs, which is followed by an increase in density that is qualitatively similar for both regions. The final stage consists of both regions slowly evolving to the steady-state solution. Our results indicate that autoradiographic and cognate approaches for tracking labeled opsins in the COS cannot be effective methodologies for assessing new disk formation at the base of the COS.

The ins and outs of cholesterol in the vertebrate retina

The vertebrate retina has multiple demands for utilization of cholesterol and must meet those demands either by synthesizing its own supply of cholesterol or by importing cholesterol from extraretinal sources, or both. Unlike the blood-brain barrier, the blood-retina barrier allows uptake of cholesterol from the circulation via a lipoprotein-based/receptor-mediated mechanism. Under normal conditions, cholesterol homeostasis is tightly regulated; also, cholesterol exists in the neural retina overwhelmingly in unesterified form, and sterol intermediates are present in minimal to negligible quantities. However, under certain pathological conditions, either due to an inborn error in cholesterol biosynthesis or as a consequence of exposure to selective inhibitors of enzymes in the cholesterol pathway, the ratio of sterol intermediates to cholesterol in the retina can rise dramatically and persist, in some cases resulting in progressive degeneration that significantly compromises the structure and function of the retina. Although the relative contributions of de novo synthesis versus extraretinal uptake are not yet known, herein we review what is known about these processes and the dynamics of cholesterol in the vertebrate retina and indicate some future avenues of research in this area.

Rod phosphodiesterase-6 PDE6A and PDE6B Subunits Are Enzymatically Equivalent

Phosphodiesterase-6 (PDE6) is the key effector enzyme of the phototransduction cascade in rods and cones. The catalytic core of rod PDE6 is a unique heterodimer of PDE6A and PDE6B catalytic subunits. The functional significance of rod PDE6 heterodimerization and conserved differences between PDE6AB and cone PDE6C and the individual properties of PDE6A and PDE6B are unknown. To address these outstanding questions, we expressed chimeric homodimeric enzymes, enhanced GFP (EGFP)-PDE6C-A and EGFP-PDE6C-B, containing the PDE6A and PDE6B catalytic domains, respectively, in transgenic Xenopus laevis. Similar to EGFP-PDE6C, EGFP-PDE6C-A and EGFP-PDE6C-B were targeted to the rod outer segments and concentrated at the disc rims. PDE6C, PDE6C-A, and PDE6C-B were isolated following selective immunoprecipitation of the EGFP fusion proteins. All three enzymes, PDE6C, PDE6C-A, and PDE6C-B, hydrolyzed cGMP with similar K(m) (20-23 μM) and k(cat) (4200-5100 s(-1)) values. Likewise, the K(i) values for PDE6C, PDE6C-A, and PDE6C-B inhibition by the cone- and rod-specific PDE6 γ-subunits (Pγ) were comparable. Recombinant cone transducin-α (Gα(t2)) and native rod Gα(t1) fully and potently activated PDE6C, PDE6C-A, and PDE6C-B. In contrast, the half-maximal activation of bovine rod PDE6 required markedly higher concentrations of Gα(t2) or Gα(t1). Our results suggest that PDE6A and PDE6B are enzymatically equivalent. Furthermore, PDE6A and PDE6B are similar to PDE6C with respect to catalytic properties and the interaction with Pγ but differ in the interaction with transducin. This study significantly limits the range of mechanisms by which conserved differences between PDE6A, PDE6B, and PDE6C may contribute to remarkable differences in rod and cone physiology.

Immunocytochemical study of NR1, NR2A and NR2B subunits of NMDA receptor in frog retina ( Rana ridibunda)

The NMDA receptors are ionotropic glutamate receptors that are involved in a variety of functions in the nervous system and in particular in the retina. They are composed of NR1 and NR2 subunits. The NMDA receptors have been fairly well studied in the retina of mammals, however, there is only limited information concerning these receptors in the retinas of lower vertebrates. The aim of the present study was to investigate immunocytochemically the NR1, NR2A and NR2B subunits of the NMDA receptors in the frog retina. Six primary antibodies were used. Three of them were directed to different splice variants of the NR1 subunit and the remaining three variants directed to NR2 subunits. All antibodies showed well expressed labeling in the frog retina. The labels had a punctate character and were located mainly in the inner and the outer plexiform layers. The results obtained indicate that the NR1, NR2A and NR2B subunits of NMDA receptor may participate in the glutamatergic neurotransmission from photoreceptors to second order retinal neurons, as well as from bipolar cells to third order retinal neurons. It has been proposed that in the frog retina, several subtypes of NMDA receptors exist each involved with different functions.

Parallel optical monitoring of visual signal propagation from the photoreceptors to the inner retina layers

Understanding of visual signal processing can benefit from simultaneous measurement of different types of retinal neurons working together. In this Letter, we demonstrate that intrinsic optical signal (IOS) imaging of frog retina slices allows simultaneous observation of stimulus-evoked responses propagating from the photoreceptors to the inner neurons. High-resolution imaging revealed robust IOSs at the photoreceptor, the inner plexiform, and the ganglion cell layers. While IOSs of the photoreceptor layer were mainly confined to the area directly stimulated by the visible light, IOSs of the inner retinal layers spread from the stimulus site into relatively large areas with a characteristic near-to-far time course.