Functional architecture of the turtle retina

Invaginating Presynaptic Terminals in Neuromuscular Junctions, Photoreceptor Terminals, and Other Synapses of Animals

Typically, presynaptic terminals form a synapse directly on the surface of postsynaptic processes such as dendrite shafts and spines. However, some presynaptic terminals invaginate-entirely or partially-into postsynaptic processes. We survey these invaginating presynaptic terminals in all animals and describe several examples from the central nervous system, including giant fiber systems in invertebrates, and cup-shaped spines, electroreceptor synapses, and some specialized auditory and vestibular nerve terminals in vertebrates. We then examine mechanoreceptors and photoreceptors, concentrating on the complex of pre- and postsynaptic processes found in basal invaginations of the cell. We discuss in detail the role of vertebrate invaginating horizontal cell processes in both chemical and electrical feedback mechanisms. We also discuss the common presence of indenting or invaginating terminals in neuromuscular junctions on muscles of most kinds of animals, and especially discuss those of Drosophila and vertebrates. Finally, we consider broad questions about the advantages of possessing invaginating presynaptic terminals and describe some effects of aging and disease, especially on neuromuscular junctions. We suggest that the invagination is a mechanism that can enhance both chemical and electrical interactions at the synapse.

Specificity of the horizontal cell-photoreceptor connections in the zebrafish (Danio rerio) retina

Horizontal cells (HCs) are involved in establishing the center-surround receptive field organization of photoreceptor and bipolar cells. In many species, HCs respond differentially to colors and may play a role in color vision. An earlier study from our laboratory suggested that four types of HCs exist in the zebrafish retina: three cone HCs (H1, H2 and H3) and one rod HC. In this study, we describe their photoreceptor connections. Cones are arranged in a mosaic in which rows of alternating blue (B)- and ultraviolet (UV)-sensitive single cones alternate with rows of red (R)- and green (G)-sensitive double cones; the G cones are adjacent to UV cones and B cones adjacent to R cones. Two small-field (H1 and H2) and two large-field (H3 and rod HC) cells were observed. The cone HC dendritic terminals connected to cones with single boutons, doublets, or rosettes, whereas the rod HCs connected to rods with single boutons. The single boutons/doublets/rosettes of cone HCs were arranged in double rows separated by single rows for H1 cells, in pairs and singles for H2 cells, and in a rectilinear pattern for H3 cells. These connectivity patterns suggest that H1 cells contact R, G, and B cones, H2 cells G, B, and UV cones, and H3 cells B and UV cones. These predictions were confirmed by applying the DiI method to SWS1-GFP retinas whose UV cones express green fluorescent protein. Each rod HC was adjacent to the soma or axon of a DiI-labeled cone HC and connected to 50-200 rods.

Twelve chromatically opponent ganglion cell types in turtle retina

The turtle retina has been extensively used for the study of chromatic processing mechanisms. Color opponency has been previously investigated with trichromatic paradigms, but behavioral studies show that the turtle has an ultraviolet (UV) channel and a tetrachromatic visual system. Our laboratory has been working in the characterization of neuronal responses in the retina of vertebrates using stimuli in the UV-visible range of the electromagnetic spectrum. In the present investigation, we recorded color-opponent responses from turtle amacrine and ganglion cells to UV and visible stimuli and extended our previous results that UV color-opponency is present at the level of the inner nuclear layer. We recorded from 181 neurons, 36 of which were spectrally opponent. Among these, there were 10 amacrine (5%), and 26 ganglion cells (15%). Morphological identification of color-opponent neurons was possible for two ganglion cell classes (G17 and G22) and two amacrine cell classes (A22 and A23b). There was a variety of cell response types and a potential for complex processing of chromatic stimuli, with intensity- and wavelength-dependent response components. Ten types of color opponency were found in ganglion cells and by adding previous results from our laboratory, 12 types of opponent responses have been found. The majority of the ganglion cells were R+UVBG- and RG+UVB-color-opponents but there were other less frequent types of chromatic opponency. This study confirms the participation of a UV channel in the processing of color opponency in the turtle inner retina and shows that the turtle visual system has the retinal mechanisms to allow many possible chromatic combinations.

AMPA and Kainate Receptors in Turtle Retina: An Immunocytochemical Study

1. Glutamate is one of the main neurotransmitters in the retina. Its effects are mediated by a large number of ionotropic and metabotropic receptors.2. The distribution of ionotropic AMPA receptor subunits GluR1-4, kainate receptor subunits GluR5-7 and KA2, as well as delta receptors 1-2 was studied in turtle retina. Indirect immunofluorescence was used to localize the different receptor subunits viewed using light microscopy.3. Results show that all subunits, with exception of GluR1 and GluR5, are widely distributed in the turtle retina.4. They are mainly located in the both plexiform layers of the retina where punctate staining, a sign for synaptic localization, is observed.5. The vast majority of the subunits possess specific pattern of staining that allow to suppose that they are involved in different retinal circuits.6. It can be assumed that the GluR2/3 and GluR6/7 subunits are expressed on the dendrites of a subpopulation of bipolar cells that are immunopositive for alpha-isoform of protein kinase C (PKCalpha). The GluR2/3 and GluR6/7 subunits are most probably used by the same PKCalpha immunopositive bipolar cells in their synaptic contacts with the third-order retinal neurons, the amacrine and ganglion cells.

The temporal structure of transient ON/OFF ganglion cell responses and its relation to intra-retinal processing

A subpopulation of transient ON/OFF ganglion cells in the turtle retina transmits changes in stimulus intensity as series of distinct spike events. The temporal structure of these event sequences depends systematically on the stimulus and thus carries information about the preceding intensity change. To study the spike events' intra-retinal origins, we performed extracellular ganglion cell recordings and simultaneous intracellular recordings from horizontal and amacrine cells. Based on these data, we developed a computational retina model, reproducing spike event patterns with realistic intensity dependence under various experimental conditions. The model's main features are negative feedback from sustained amacrine onto bipolar cells, and a two-step cascade of ganglion cell suppression via a slow and a fast transient amacrine cell. Pharmacologically blocking glycinergic transmission results in disappearance of the spike event sequence, an effect predicted by the model if a single connection, namely suppression of the fast by the slow transient amacrine cell, is weakened. We suggest that the slow transient amacrine cell is glycinergic, whereas the other types release GABA. Thus, the interplay of amacrine cell mediated inhibition is likely to induce distinct temporal structure in ganglion cell responses, forming the basis for a temporal code.

Photoreceptor cell types in the retina of the tuatara (Sphenodon punctatus) have cone characteristics

The tuatara Sphenodon punctatus, restricted to a few New Zealand offshore islands and now strictly protected, belongs to the Rhynchocephalia, the smallest order of extant reptiles. Earlier light microscopical studies on the retina of this species described photoreceptors with both rod- and cone-like features and the presence of a fovea. A limited amount of retinal material from S. punctatus has now allowed us to prepare the first-ever electron microscopic observations on the eye of this reptile. We were able to distinguish three types of photoreceptor, all with fine structural features characteristic of cone cells. Large single cones as well as double cones had open discs in their outer segments and straight axons with pedicle-type terminals. An additional cone type, characterized by somewhat more slender inner and outer segments, vitreally-displaced cell bodies and oblique axons, resembled short-wavelength cones known from other sauropsids. No cells with rod characteristics could be confirmed in the samples, although they might occur in retinal regions not available for this study. We conclude that the tuatara has cone-like photoreceptors, which-as in other crepuscular or nocturnal reptiles-have acquired rod-like features. The phenotypic adaptations notwithstanding, the set of photoreceptor types is quite typical of the reptilian eye and in some respects reminiscent of those seen in lizards and turtles.

Evolution of vertebrate colour vision

Recent years have witnessed a growing interest in learning how colour vision has evolved. This trend has been fuelled by an enhanced understanding of the nature and extent of colour vision among contemporary species, by a deeper understanding of the paleontological record and by the application of new tools from molecular biology. This review provides an assessment of the progress in understanding the evolution of vertebrate colour vision. In so doing, we offer accounts of the evolution of three classes of mechanism important for colour vision--photopigment opsins, oil droplets and retinal organisation--and then examine details of how colour vision has evolved among mammals and, more specifically, among primates.

Contribution of connexin26 to electrical feedback inhibition in the turtle retina

The first synaptic integration in the neuronal cascade of vision in vertebrates includes a feedback from horizontal cells to cones by a mechanism yet not fully understood. Recent observations in teleosts suggested an electrical feedback mechanism mediated by connexin26 (Cx26) hemichannels at the terminal dendrites of horizontal cells. By using reverse transcription-polymerase chain reaction and immunoblotting from retinal homogenate, we detected Cx26 mRNA transcripts in the turtle retina and demonstrated that they were translated into protein. Cx26 immunoreactivity was especially prominent in the outer plexiform layer. Subcellularly, immunoreactivity was located mainly between horizontal cell axon terminals and in horizontal cell dendrites forming the lateral elements at the ribbon synaptic complex of the cone pedicle. The label was confined to the horizontal cell membrane flanking the ribbon and was not found on the opposing photoreceptor membrane. No gap junctions at this location are known, so immunosignaling suggested the presence of hemichannels. Their relevance to the feedback mechanism was investigated by intracellular recordings from horizontal cells during application of the hemichannel blocker carbenoxolone. Carbenoxolone hyperpolarized the dark membrane potential by about 25 mV, decreased the amplitudes of responses to full-field light flashes, and suppressed the feedback-induced depolarizing inflexion in the response profile. These physiological findings are compatible with the involvement of hemichannels in the feedback between horizontal cells and cones and support the anatomical findings. Together, these data suggest the presence of an electrical feedback mechanism in the turtle retina, which therefore might be a common mechanism at the first visual synapse in vertebrates.

Structure and function of photoreceptor and second-order cell mosaics in the retina of Xenopus

The structure, physiology, synaptology, and neurochemistry of photoreceptors and second-order (horizontal and bipolar) cells of Xenopus laevis retina is reviewed. Rods represent 53% of the photoreceptors; the majority (97%) are green light-sensitive. Cones belong to large long-wavelength-sensitive (86%), large short-wavelength-sensitive (10%), and miniature ultraviolet wavelength-sensitive (4%) groups. Photoreceptors release glutamate tonically in darkness, hyperpolarize upon light stimulation and their transmitter release decreases. Photoreceptors form ribbon synapses with second-order cells where postsynaptic elements are organized into triads. Their overall adaptational status is regulated by ambient light conditions and set by the extracellular dopamine concentration. The activity of photoreceptors is under circadian control and is independent of the central body clock. Bipolar cell density is about 6000 cells/mm2 They receive mixed inputs from rods and cones. Some bipolar cell types violate the rule of ON-OFF segregation, giving off terminal branches in both sublayers of the inner plexiform layer. The majority of them contain glutamate, a small fraction is GABA-positive and accumulates serotonin. Luminosity-type horizontal cells are more frequent (approximately 1,000 cells/mm2) than chromaticity cells (approximately 450 cells/mm2). The dendritic field size of the latter type was threefold bigger than that of the former. Luminosity cells contact all photoreceptor types, whereas chromatic cells receive their inputs from the short-wavelength-sensitive cones and rods. Luminosity cells are involved in generating depolarizing responses in chromatic horizontal cells by red light stimulation which form multiple synapses with blue-light-sensitive cones. Calculations indicate that convergence ratios in Xenopus are similar to those in central retinal regions of mammals, predicting comparable spatial resolution.

Structure-function correlation in transient amacrine cells of goldfish retina: basic and multifractal analyses of dendritic trees in distinct synaptic layers

Amacrine cells generating light-evoked transient ON-OFF responses were stained by intracellular injection of horseradish peroxidase after determining their input-output (voltage response vs. light intensity) profiles. Ten cells specifically having bistratified dendritic trees were analyzed. The cross-sectional area of the dendrites in each sublamina (a and b) of the inner plexiform layer was initially measured. Although some variability was observed, there was no statistically significant overall difference in the cross-sectional areas of the dendritic trees in sublaminae a and b. Also, the amplitudes of the ON and OFF responses, generated by a midrange criterion stimulus, could not be correlated with the cross-sectional areas or the number of branches of the dendrites in sublaminae b and a, respectively. On the other hand, determination of the generalized fractal spectra revealed that the negative (up to -3) and zero-order fractal dimensions of the dendritic trees in sublamina a were consistently higher than those for sublamina b. Furthermore, there was a positive correlation between response amplitude and some part of the generalized fractal dimension in the respective parts of the dendritic trees. It is concluded that dendritic tree characteristics differ in the two halves of the inner plexiform layer and that these can be related to the cells' light-evoked response amplitudes. Furthermore, generalized fractal analysis appears to be a useful method for correlating structure and function in retinal amacrine cells with complex dendritic trees.

Immunocytochemical and electrophysiological characterization of GABA receptors in the frog and turtle retina

The expression of GABA receptors (GABARs) was studied in frog and turtle retinae. Using immunocytochemical methods, GABA(A)Rs and GABA(C)Rs were preferentially localized to the inner plexiform layer (IPL). Label in the IPL was punctate indicating a synaptic clustering of GABARs. Distinct, but weaker label was also present in the outer plexiform layer. GABA(A)R and GABA(C)R mediated effects were studied by recording electroretinograms (ERGs) and by the application of specific antagonists. Bicuculline, the GABA(A)R antagonist, produced a significant increase of the ERG. Picrotoxin, when co-applied with saturating doses of bicuculline, caused a further increase of the ERG due to blocking of GABA(C)Rs. The putative GABA(C)R antagonist Imidazole-4-acidic acid (I4AA) failed to antagonize GABA(C)R mediated inhibition and, in contrast, appeared rather as an agonist of GABARs.

The mammalian photoreceptor mosaic-adaptive design

Unlike in birds and cold-blooded vertebrates' retinas, the photoreceptors of mammalian retinas were long supposed to be morphologically uniform and difficult to distinguish into subtypes. A number of new techniques have now begun to overcome the previous limitations. A hitherto unexpected variability of spectral and morphological subtypes and topographic patterns of distribution in the various retinas are being revealed. We begin to understand the design of the photoreceptor mosaics, the constraints of evolutionary history and the ecological specialization of these mosaics in all the mammalian subgroups. The review discusses current cytological identification of mammalian photoreceptor types and speculates on the likely "bottleneck-scenario" for the origin of the basic design of the mammalian retina. It then provides a brief synopsis of current data on the photoreceptors in the various mammalian orders and derives some trends for phenomena such as rod/cone dualism, spectral range, preservation or loss of double cones and oil droplets, photopigment co-expression and mono- and tri-chromacy. Finally, we attempt to demonstrate that, building on the limits of an ancient rod dominant (probably dichromatic) model, mammalian retinas have developed considerable radiation. Comparing the nonprimate models with the intensively studied primate model should provide us with a deeper understanding of the basic design of the mammalian retina.

Choline acetyltransferase is found in terminals of horizontal cells that label with GABA, nitric oxide synthase and calcium binding proteins in the turtle retina

In this study, we discriminated the various types of horizontal cell in the turtle retina on their content of neuroactive substances. Double label immunocytochemistry was performed on sectioned and wholemount retina using antisera to neural- and endothelial-nitric oxide synthase (nNOS, and eNOS), calretinin (CR), calbindin (CB), gamma-aminobutyric acid (GABA) and choline acetyltransferase (ChAT). H1 cells and their axon terminals label with CR, CB and GABA. Only H1 axon terminals label with eNOS. H2 cells contain CB, CR, nNOS and GABA maybe in their dendrites. H3 cells label only with nNOS. The localization of nNOS in the H2 and H3 cells is a novel finding. None of these antibodies labels H4 cells. The photoreceptor subtypes have been differentiated by different intensity of labeling with CB. The accessory member of the double cone is less intensely labeled with CB than the principal member and rods and blue cones do not appear to label at all. ChAT-IR is located in terminal boutons of H1 and H2 horizontal cells and H1 axon terminals and these boutons contact rods and all spectral types of cones. Clearly, GABA is present in H1 horizontal cells and may be used in neurotransmission between horizontal cells and possibly for feedback pathways to photoreceptors. The evidence of nNOS immunoreactivity in H2 and H3 horizontal cells, combined with available physiological evidence, suggests that NO may be involved in electrical coupling and/or modulation of synaptic input to these types of cells. Furthermore, our results raise the possibility that cholinergic synaptic transmission may occur from horizontal cell processes to photoreceptors in the outer plexiform layer of the turtle retina.

Multirecording of Ca(2+) signals from inner retinal neurons evoked by light stimulation of photoreceptors

We simultaneously monitored changes of intracellular free Ca(2+) concentration ([Ca(2+)](i)) following different light stimuli from different inner retinal neurons of the turtle retina slice preparation. [Ca(2+)](i) increased with an increase of the light stimulus intensity. Some of the cells also showed color opponent Ca(2+) signals. 2-Amino-4-phosphonobutyric acid (APB) blocked in particular [Ca(2+)](i) increases and picrotoxin enhanced the observed [Ca(2+)](i) changes. These data support the idea that the observed [Ca(2+)](i) changes result from light stimulation and subsequent retinal processing. Similar Ca(2+) signals were observed when the release of Ca(2+) from internal stores was blocked with caffeine and thapsigargin. These results indicate that retinal Ca(2+) signals evoked by light stimulation depend to a large extent on voltage-dependent Ca(2+) influx and might therefore reflect signal processing.

Localization of nNOS in photoreceptor, bipolar and horizontal cells in turtle and rat retinas

Neuronal nitric oxide synthase (nNOS), an enzyme that synthesizes NO, has been found in the outer retina using light microscopic immunocytochemistry, but its subcellular localization is unknown. We used electron immunocytochemistry to examine nNOS-like immunoreactivity (nNOS-LI) in the outer plexiform layer of turtle and rat retinas. In turtle, nNOS-LI was present in some bipolar and horizontal cell processes at photoreceptor ribbon synapses and at basal junctions between photoreceptors. In rat, nNOS-LI was present in some rod bipolar and B-type horizontal cell axon terminals at rod ribbon synapses. These results indicate that in vertebrates, NO is produced by all of the major nerve cell types in the outer retina at specific synaptic contacts.

UV-sensitive input to horizontal cells in the turtle retina

Microspectrophotometry, electroretinography and behavioural studies have indicated that ultraviolet (UV) light contributes to functional vision in various vertebrate species. Based on behavioural evidence, this was also suggested for turtle vision. In order to reveal the interactions underlying detection of UV light in the distal retina, we recorded intracellularly the photoresponses of cones and horizontal cells in retinas of Pseudemys scripta elegans and Mauremys caspica and calculated the action spectra of these cells under different conditions of adaptation. In the dark-adapted retina, all three types of horizontal cells; luminosity-type, red/green chromaticity-type and yellow/blue chromaticity-type exhibited increased sensitivity in the UV region of the spectrum. However, chromatic adaptation indicated that only the yellow/blue chromaticity-type horizontal cells received excitatory input from UV-sensitive cones with peak sensitivity approximately 360 nm. The enhanced UV sensitivity of luminosity-type horizontal cells probably reflected the beta-band of the long-wavelength sensitive visual pigment as indicated by the action spectra of dark-adapted L-cones. It is suggested that the enhanced UV sensitivity of red/green chromaticity-type horizontal cells reflects the beta-band of the medium-wavelength sensitive visual pigment. Transmission measurements of the optical media (cornea, lens and vitreous) indicated that UV vision can be functional under normal circumstances.

Synaptic inputs to identified color-coded amacrine and ganglion cells in the turtle retina

Previous studies have proposed models of the specific synaptic circuitry responsible for color processing in the turtle retina. To determine the accuracy of these models of the circuits underlying color opponency in the inner retina of the turtle (Pseudemys scripta), we have studied the physiology, morphology, and synaptic connectivity of identified amacrine and ganglion cells. These cells were first characterized electrophysiologically and were then stained with horseradish peroxidase. Postembedding electron immunocytochemistry for gamma-aminobutyric acid (GABA) and glycine was used to reveal the neurochemical identity of their synaptic inputs. The red-ON/green, blue-OFF small-field ganglion cell, classified as G24, branched primarily in strata S1, S4, and S5 of the inner plexiform layer (IPL). Ganglion cell G24 showed a complex receptive field organized into a red-ON center surrounded by an inhibitory region, which, in turn, was surrounded by a second excitatory region. Only the center responses were color opponent. The red-OFF/green, blue-ON large-field, stellate amacrine cell, classified as A23b, stratified exclusively in stratum S2, near the S2/S3 border. The color-coded center was surrounded by a luminosity, red-sensitive surround. Synaptic input to G24 and A23b was dominated by amacrine cells (89% and 87%, respectively). G24 received significant input from amacrine cell profiles with GABA (13% of total) as well as glycine (11% of total) immunoreactivity, mostly in the proximal stratum S5 of the IPL (64% and 67% of the total GABA- and glycine-immunoreactive input, respectively). Bipolar cell synaptic input was also found predominantly in S4 and S5 (89%). In contrast, we found no glycine-immunoreactive input to A23b, and the density of the GABA-immunoreactive amacrine cell synaptic input revealed a central (15%) to peripheral (3%) gradient within the dendritic tree. The results of the present study support the previous models of the synaptic circuitry responsible for color-opponent signal processing in the inner retina of the turtle.

Uniqueness of the S-cone pedicle in the human retina and consequences for color processing

The purpose of this study was to investigate more fully the shape and content of ribbons and synapses to second-order neurons in the short-wavelength cone (S-cone, blue cone) pedicle and to learn more concerning the uniqueness of the S-cone system in the primate retina. A piece of well-fixed peripheral human retina (10 mm, 35 degrees nasal to the fovea) was serially thick sectioned in the tangential plane from the level of the outer segments to the tops of the cone pedicles. Then serial electron microscope (EM) sections were collected through the whole depth of the pedicle-occupying region into the neuropil of the outer plexiform layer (OPL). The resultant EM micrograph montages of a large field of cone pedicles were perused, and S-cone pedicles were identified. Serial micrographs of a single S-cone pedicle, picked out of the montages, were digitized and reconstructed by computer three-dimensional methods. The S-cone pedicle arose from a slightly oblique axon and projected 0.5-1 microm more vitread in the OPL than other cone pedicles. It was bilobed in shape, with synaptic invaginations and ribbons in both lobes. No cone-contacting telodendria projected from the S-cone pedicle itself, but a small number of neighboring cones sent telodendria to its surface to make small gap junctions. Neighboring rod spherules also made small gap junctions. Four robust bipolar cell dendrites, most likely from S-cone-specific bipolar cells, made synapses at ribbons and basal (distal) junctions. A small number of other bipolar cell dendrites made narrow-cleft basal junction only. The majority of lateral elements were thought to be from HII horizontal cells, and a minority from HI horizontal cells. We conclude that the S-cone pedicle has a unique morphology and connectivity to second-order neurons that makes it quite different from the other two longer wavelength cone systems, and we speculate on the consequences for color processing in the visual system in general.

Effects of horizontal cell network architecture on signal spread in the turtle outer retina. Experiments and simulations

In the Pseudemys turtle retina five functionally distinct, electrically coupled networks of horizontal cells distribute signals in the outer plexiform layer. These networks differ significantly in their architecture, as determined by intracellular labeling with Neurobiotin after physiological recording and identification. The density of H1 horizontal cells is highest, ranging around 1800 cells/mm2 at approximately 2.3 mm eccentricity. H1 horizontal cell somata are connected via 6-10 thin, short dendrites. The H1 horizontal cell axon terminal network is composed of thick axon terminals, forming a three-dimensional, sheath-like structure. Networks of coupled H2 and H3 horizontal cells have cell densities of around 210 cells/mm2 and 350 cells/mm2, respectively, at the same eccentricity of 2.3 mm. Cell bodies are connected with 6-12 long, thin dendrites. Here we report for the first time H4 horizontal cell networks. Cell density is approximately 970 cells/mm2 at 2 mm eccentricity, and cell bodies are connected with 6-10 thin, short dendrites. General properties of passive voltage spread were compared for three of these horizontal cell networks using NeuronC. Realistic network architectures were obtained by digitizing the intracellularly labeled networks, respectively. One network obtained from coupled H1 horizontal cell bodies, one from coupled H1 horizontal cell axon terminals, and one from H2 horizontal cells were simulated. These three realistic networks were compared with an artificial, electrically coupled regular triangular network. Passive signal spread in these networks strongly depended on the exact network architecture using otherwise identical parameters. Changes in coupling strength affected signal spread in these networks differently. As in the experimental situation, changes in synaptic conductance influenced signal spread. Some principal effects of extensively coupled horizontal cells on photoreceptor signal processing were simulated with one type of photoreceptor connected by telodendria, synapsing onto an underlying triangular network and receiving feedback synapses. Under certain conditions, spatial information is coded in single photoreceptors. This was also the case in the experimental situation. In the simulation, spatial filter adjustment for optimal spatial coding in photoreceptors can be achieved by changing coupling strength in the horizontal cell network.