Structural basis for on-and off-center responses in retinal bipolar cells

Ultrastructural Characteristics and Synaptic Connectivity of Photoreceptors in the Simplex Retina of Little Skate (Leucoraja erinacea)

The retinas of the vast majority of vertebrate species are termed "duplex," that is, they contain both rod and cone photoreceptor neurons in different ratios. The retina of little skate (Leucoraja erinacea) is a rarity among vertebrates because it contains only a single photoreceptor cell type and is thus "simplex." This unique retina provides us with an important comparative model and an exciting opportunity to study retinal circuitry within the context of a visual system with a single photoreceptor cell type. What is perhaps even more intriguing is the fact that the Leucoraja retina is able use that single photoreceptor cell type to function under both scotopic and photopic ranges of illumination. Although some ultrastructural characteristics of skate photoreceptors have been examined previously, leading to a general description of them as "rods" largely based on outer segment (OS) morphology and rhodopsin expression, a detailed study of the fine anatomy of the entire cell and its synaptic connectivity is still lacking. To address this gap in knowledge, we performed serial block-face electron microscopy imaging and examined the structure of skate photoreceptors and their postsynaptic partners. We find that skate photoreceptors exhibit unusual ultrastructural characteristics that are either common to rods or cones in other vertebrates (e.g., outer segment architecture, synaptic ribbon number, terminal extensions), or are somewhere in between those of a typical vertebrate rod or cone (e.g., number of invaginating contacts, clustering of multiple ribbons over a single synaptic invagination). We suggest that some of the ultrastructural characteristics we observe may play a role in the ability of the skate retina to function across scotopic and photopic ranges of illumination. Our findings have the potential to reveal as yet undescribed principles of vertebrate retinal design.

The molecular mechanisms that underlie neural network assembly

Neural networks are groups of interconnected neurons, which collectively give rise to emergent neural activities and functions that cannot be explained by the activity of single neurons. How neural networks are assembled is poorly understood. While all aspects of neuronal development are essential for the assembly of a functional neural network, we know little about high-level principles that govern neural network assembly beyond the basic steps of neuronal development. In this review, I use vertebrate spinal motor columns, Drosophila larval motor circuit, and the lamination in the vertebrate inner retina to highlight the spatial codes, temporal codes, and cell adhesion codes for neural network assembly. Nevertheless, these examples only show preliminary connections between neural network development and their functions. Much needs to be done to understand the molecular mechanisms that underlie the assembly of functional neural networks.

Neuromorphic System for Edge Information Encoding: Emulating Retinal Center-Surround Antagonism by Li-Ion-Mediated Highly Interactive Devices

Center-surround antagonism, a key mechanism in the retina, contributes to the encoding of edge contrast rather than of the overall information on a visual image. Here, a neuromorphic system consisting of multiple ionic devices is built, where each device has a lithium cobalt oxide channel arranged on a common lithium phosphorus oxynitride electrolyte. Because of the migration of Li ions between the channels through the electrolyte, the devices are highly interactive, as is seen with retinal neurons. On the basis of the excitation of single devices and device-to-device inhibition, the system successfully emulates the antagonistic center-surround receptive field and the Mach band effect in which perceived contrast is enhanced at the edges between dark and bright regions. Furthermore, a two-dimensional array system is simulated to implement edge detection for real images. This scheme enables computer vision tasks with simple and effective operations, owing to the intrinsic properties of the materials employed.

Mutation of wrb, a Component of the Guided Entry of Tail-Anchored Protein Pathway, Disrupts Photoreceptor Synapse Structure and Function

PURPOSE

Tail-anchored (TA) proteins contain a single hydrophobic domain at the C-terminus and are posttranslationally inserted into the ER membrane via the GET (guided entry of tail-anchored proteins) pathway. The role of the GET pathway in photoreceptors is unexplored. The goal of this study was to characterize the zebrafish pinball wizard mutant, which disrupts Wrb, a core component of the GET pathway.

METHODS

Electroretinography, optokinetic response measurements (OKR), immunohistochemistry, and electron microscopy analyses were employed to assess ribbon synapse function, protein expression, and ultrastructure in 5-day-old zebrafish larvae. Expression of wrb was investigated with real-time qRT-PCR and in situ hybridization.

RESULTS

Mutation of wrb abolished the OKR and greatly diminished the ERG b-wave, but not the a-wave. Ribeye and SV2 were partially mislocalized in both photoreceptors and hair cells of wrb mutants. Fewer contacts were seen between photoreceptors and bipolar cells in wrb-/- mutants. Expression of wrb was observed throughout the nervous system and Wrb localized to the ER and synaptic region of photoreceptors. Morpholino knockdown of the cytosolic ATPase trc40, which targets TA proteins to the ER, also diminished the OKR. Overexpression of wrb fully restored contrast sensitivity in mutants, while overexpression of mutant wrbR73A, which cannot bind Trc40, did not.

CONCLUSIONS

Proteins Wrb and Trc40 are required for synaptic transmission between photoreceptors and bipolar cells, indicating that TA protein insertion by the TRC pathway is a critical step in ribbon synapse assembly and function.

ON-OFF Interactions in the Retina: Role of Glycine and GABA

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells and it apparently remains as signals propagate to higher brain visual centers. A fundamental question in visual neuroscience is how these two parallel pathways function: are they independent from each other or do they interact somehow? In the latter case, what kinds of mechanisms are involved and what are the consequences from this cross-talk? This review summarizes current knowledge about the types of interactions between the ON and OFF channels in nonmammalian and mammalian retina. Data concerning the ON-OFF interactions in distal retina revealed by recording of single bipolar cell activity and electroretinographic ON (b-wave) and OFF (d-wave) responses are presented. Special emphasis is put on the ON-OFF interactions in proximal retina and their dependence on the state of light adaptation in mammalian retina. The involvement of the GABAergic and glycinergic systems in the ON-OFF crosstalk is also discussed.

Role of melatonin and its receptors in the vertebrate retina

Melatonin is a chemical signal of darkness that is produced by retinal photoreceptors and pinealocytes. In the retina, melatonin diffuses from the photoreceptors to bind to specific receptors on a variety of inner retinal neurons to modify their activity. Potential target cells for melatonin in the inner retina are amacrine cells, bipolar cells, horizontal cells, and ganglion cells. Melatonin inhibits the release of dopamine from amacrine cells and increases the light sensitivity of horizontal cells. Melatonin receptor subtypes show differential, cell-specific patterns of expression that are likely to underlie differential functional modulation of specific retinal pathways. Melatonin potentiates rod signals to ON-type bipolar cells, via activation of the melatonin MT2 (Mel1b) receptor, suggesting that melatonin modulates the function of specific retinal circuits based on the differential distribution of its receptors. The selective and differential expression of melatonin receptor subtypes in cone circuits suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons and thus promote dark adaptation.

Changes to tear cytokines of type 2 diabetic patients with or without retinopathy

PURPOSE

To investigate changes in cytokine levels in tears of type 2 diabetics with or without retinopathy.

METHODS

Tears were collected from 15 type 2 diabetics without retinopathy (DNR), 15 patients with retinopathy (DR), and 15 age and gender matched non-diabetic controls. Tear concentrations of 27 cytokines were measured by multiplex bead immunoassay. Cytokine differences between groups, ratios of type-1 T helper (Th1)/type-2 T helper (Th2) cytokines and anti-angiogenic/pro-angiogenic cytokines were analyzed statistically.

RESULTS

The most abundant cytokine detected in tears was interferon-induced protein-10 (IP-10). In comparison with controls, IP-10 and monocyte chemoattracant protein-1 (MCP-1) levels were significantly elevated in DR (p=0.016 and 0.036, respectively) and DNR groups (p=0.021 and 0.026, respectively). Interleukin-1 (IL-1) receptor antagonist (IL-1ra) levels were significantly increased in DNR (p=0.016). Th1/Th2 cytokines interferon-gamma (IFN-γ)/IL-5 and IL-2/IL-5 ratios were significantly increased in DR compared to controls (p=0.037 and 0.031, respectively). Anti-angiogenic/angiogenic cytokines IFN-γ/MCP-1 and IL-4/MCP-1 ratios in DR and DNR were significantly decreased compared to controls (p<0.05). IL-4/IL-8 and IL-12p70/IL-8 ratios were also significantly decreased in DR compared to controls (p=0.02 and 0.045, respectively). No significant correlation was demonstrated between tear cytokine concentrations and glycosylated hemoglobin (HbA1c) or fasting plasma glucose (FPG).

CONCLUSIONS

Diabetic tears exhibited elevated levels of IP-10 and MCP-1. The Th1/Th2 cytokine balance may shift to a predominantly Th1 state in DR patients. Pro-angiogenic cytokines are more highly represented than anti-angiogenic cytokines in the tears of diabetic patients.

Caspase-dependent apoptosis of ON bipolar cells in the goldfish marginal retina

Progenitor cells proliferate at the retinal margin of adult goldfish, so that the developmental stages of maturing retinal cells are arranged in temporal sequence along the periphero-central axis. We studied the caspase-dependent apoptosis of ON bipolar cells, which were labeled by an antibody to protein kinase C (PKC). In both primary and secondary growth zones of the marginal retina, PKC+ cell somata were of two distinct sizes, large and small. A caspase inhibitor (zVAD-fmk), injected into the vitreous, increased the number of both large and small PKC+ somata (times approximately 2 and 1.1, respectively) in both growth zones. These data show that about one-third of immature ON bipolar cells were removed by caspase-dependent apoptosis before being incorporated into the mature retina.

Labelling and targeted ablation of specific bipolar cell types in the zebrafish retina

Background

Development of a functional retina depends on regulated differentiation of several types of neurons and generation of a highly complex network between the different types of neurons. In addition, each type of retinal neuron includes several distinct morphological types. Very little is known about the mechanisms responsible for generating this diversity of retinal neurons, which may also display specific patterns of regional distribution.

Results

In a screen in zebrafish, using a trapping vector carrying an engineered yeast Gal4 transcription activator and a UAS:eGFP reporter cassette, we have identified two transgenic lines of zebrafish co-expressing eGFP and Gal4 in specific subsets of retinal bipolar cells. The eGFP-labelling facilitated analysis of axon terminals within the inner plexiform layer of the adult retina and showed that the fluorescent bipolar cells correspond to previously defined morphological types. Strong regional restriction of eGFP-positive bipolar cells to the central part of the retina surrounding the optic nerve was observed in adult zebrafish. Furthermore, we achieved specific ablation of the labelled bipolar cells in 5 days old larvae, using a bacterial nitroreductase gene under Gal4-UAS control in combination with the prodrug metronidazole. Following prodrug treatment, nitroreductase expressing bipolar cells were efficiently ablated without affecting surrounding retina architecture, and recovery occurred within a few days due to increased generation of new bipolar cells.

Conclusion

This report shows that enhancer trapping can be applied to label distinct morphological types of bipolar cells in the zebrafish retina. The genetic labelling of these cells yielded co-expression of a modified Gal4 transcription activator and the fluorescent marker eGFP. Our work also demonstrates the potential utility of the Gal4-UAS system for induction of other transgenes, including a bacterial nitroreductase fusion gene, which can facilitate analysis of bipolar cell differentiation and how the retina recovers from specific ablation of these cells.

Synaptic vesicle dynamics in mouse rod bipolar cells

To better understand synaptic signaling at the mammalian rod bipolar cell terminal and pave the way for applying genetic approaches to the study of visual information processing in the mammalian retina, synaptic vesicle dynamics and intraterminal calcium were monitored in terminals of acutely isolated mouse rod bipolar cells and the number of ribbon-style active zones quantified. We identified a releasable pool, corresponding to a maximum of 7 s. The presence of a smaller, rapidly releasing pool and a small, fast component of refilling was also suggested. Following calcium channel closure, membrane surface area was restored to baseline with a time constant that ranged from 2 to 21 s depending on the magnitude of the preceding Ca2+ transient. In addition, a brief, calcium-dependent delay often preceded the start of onset of membrane recovery. Thus, several aspects of synaptic vesicle dynamics appear to be conserved between rod-dominant bipolar cells of fish and mammalian rod bipolar cells. A major difference is that the number of vesicles available for release is significantly smaller in the mouse rod bipolar cell, both as a function of the total number per neuron and on a per active zone basis.

The circadian clock in the retina controls rod-cone coupling

Although rod and cone photoreceptor cells in the vertebrate retina are anatomically connected or coupled by gap junctions, a type of electrical synapse, rod-cone electrical coupling is thought to be weak. Using tracer labeling and electrical recording in the goldfish retina and tracer labeling in the mouse retina, we show that the retinal circadian clock, and not the retinal response to the visual environment, controls the extent and strength of rod-cone coupling by activating dopamine D(2)-like receptors in the day, so that rod-cone coupling is weak during the day but remarkably robust at night. The results demonstrate that circadian control of rod-cone electrical coupling serves as a synaptic switch that allows cones to receive very dim light signals from rods at night, but not in the day. The increase in the strength and extent of rod-cone coupling at night may facilitate the detection of large dim objects.

Novel processes invaginate the pre-synaptic terminal of retinal bipolar cells

Mixed-rod cone bipolar (Mb) cells of goldfish retina have large synaptic terminals (10 microm in diameter) that make 60-90 ribbon synapses mostly onto amacrine cells and rarely onto ganglion cells and, in return, receive 300-400 synapses from gamma-aminobutyric acid (GABA)-ergic amacrine cells. Tissue viewed by electron microscopy revealed the presence of double-membrane-bound processes deep within Mb terminals. No membrane specializations were apparent on these invaginating processes, although rare vesicular fusion was observed. These invaginating dendrites were termed "InDents". Mb bipolar cells were identified by their immunoreactivity for protein kinase C. Double-label immunofluorescence with other cell-type-specific labels eliminated Müller cells, efferent fibers, other Mb bipolar cells, dopaminergic interplexiform cells, and somatostatin amacrine cells as a source of the InDents. Confocal analysis of double-labeled tissue clearly showed dendrites of GABA amacrine cells, backfilled ganglion cells, and dendrites containing PanNa immunoreactivity extending into and passing through Mb terminals. Nearly all Mb terminals showed evidence for the presence of InDents, indicating their common presence in goldfish retina. No PanNa immunoreactivity was found on GABA or ganglion cell InDents, suggesting that a subtype of glycine amacrine cell contained voltage-gated Na channels. Thus, potassium and calcium voltage-gated channels might be present on the InDents and on the Mb terminal membrane opposed to the InDents. In addition to synaptic signaling at ribbon and conventional synapses, Mb bipolar cells may exchange information with InDents by an alternative signaling mechanism.

OFF ganglion cells cannot drive the optokinetic reflex in zebrafish

Whereas the zebrafish retina has long been an important model system for developmental and genetic studies, little is known about the responses of the inner retinal neurons. Here we report single-unit ganglion cell recordings from 5- to 6-day-old zebrafish larvae. In wild-type larvae we identify at least five subtypes of ganglion cell responses to full-field illumination, with ON-OFF and ON-type cells predominating. In the nrc mutant retina, in which the photoreceptor terminals develop abnormally, we observe normal OFF responses but abnormal ON-OFF responses and no ON responses. Previously characterized as blind, these mutants lack an optokinetic reflex (OKR), but in another behavioral assay nrc mutant fish have near-normal responses to the offset of light and slow and sluggish responses to the onset of light. Pharmacological block of the ON pathway mimics most of the nrc visual defects. We conclude that the abnormal photoreceptor terminals in nrc mutants predominantly perturb the ON pathway and that the ON pathway is necessary to drive the OKR in larval zebrafish.

Bipolar cells in the “grouped retina” of the elephantnose fish (Gnathonemus petersii)

To elucidate the specific properties of retinae with grouped photoreceptors the neural pathways in the outer and inner plexiform layer were studied. Photoreceptor bundles in this species consist of more than 100 rods and up to 50 cones, and are usually regarded as functional units. Golgi impregnation in thick and thin sections and light microscopy were used to identify bipolar cell types linking photoreceptors to amacrine and/or ganglion cells. Nine different types were distinguished based on their dendritic morphology and the position of the axon terminal in the inner plexiform layer. Small cells have dendritic fields smaller than the diameter of a photoreceptor bundle and are contacted mostly by cones. The dendritic field size of bushy cells matches that of a photoreceptor bundle; they are contacted mainly by rods. Flat cells receive about equal input from rods and cones; their dendritic field size exceeds the bundle diameter. Within the three major classes there are subtypes addressing three sublaminae of the inner plexiform layer, the proximal On-centre region (sl b), the distal Off-centre region (sl a) and a central sublayer (sl c) probably with transient activity. These observations suggest that cone vision has a spatial acuity better than the “bundle grain”. In rod dominated vision the resolution matches that of the bundles; for this pathway, the hypothesis of the bundle as a functional unit is confirmed. The mesopic flat cell pathway has a resolution inferior to the “bundle grain”; it may therefore be dedicated to movement detection.

A novel connection between rods and ON cone bipolar cells revealed by ectopic metabotropic glutamate receptor 7 (mGluR7) in mGluR6-deficient mouse retinas

Since the discovery of direct chemical synapses between rod photoreceptor and OFF cone bipolar cells in mouse retinas, whether the ON cone bipolar cell also receive direct chemical input from rod has been a pending question. In finding that metabotropic glutamate receptor 7 (mGluR7) was uniquely expressed in dendrites of ON cone bipolar cells in the mGluR6-deficient mouse retina, we used this ectopic mGluR7 immunoreactivity as a specific marker for the ON cone bipolar to search for its rod connection. Here, we show that a certain type of ON cone bipolar cell forms ribbon-associated synapses not only with cones, but also rods. This finding was verified in the wild-type mouse retina by three-dimensional reconstruction of bipolar cells from serial electron micrographs. These ON cone bipolars were further identified as corresponding to type 7 of mouse bipolar cell described by Ghosh et al. (2004) and also to the green fluorescent protein (GFP)-labeled type 7 bipolars in the alpha-gustducin-GFP transgenic mouse. Our findings suggest that, in mice, rod signals bifurcate into a third ON and OFF pathway in addition to the two known routes to cone bipolar cells: (1) via rod chemical synapse --> rod bipolar --> AII amacrine --> ON and OFF cone bipolar cells; (2) via rod-cone gap junction --> cone chemical synapse --> ON and OFF cone bipolar cells; and (3) via rod chemical synapse --> ON and OFF cone bipolar cells. This third novel pathway is thought to transmit fast and moderately light-sensitive rod signals, functioning to smooth out the intensity changes at the scotopic-mesopic interface.

Dopamine modulates voltage-activated potassium currents in zebrafish retinal on bipolar cells

We report a study of the characterization of voltage-activated potassium (K+) currents in retinal ON bipolar cells in zebrafish. At single-channels levels, the open probability of the K+ channels increased when the membrane potential was increased. The maximal open proportion was 0.76+/-0.05 under our testing conditions. In whole-cell recordings, the K+ current displayed two exponential components with the activation time constants of 11-22 msec (tau1) and 0.8-4 msec (tau2). Dopamine modulated the K+ current. Dopamine reduced the time constant tau2 when the membrane potential was depolarized to high voltages. A decrease in K+ current was seen when dopamine D1 receptors were selectively activated by SKF38393 or when the D1 receptor-coupled G-proteins were activated by GTP-gamma-S. The activation of adenylate cyclase by forskolin or the increase of intracellular cAMP concentrations by 8-Br-cAMP or Sp-cAMPS also resulted in a decrease in K+ current. Together, the data suggest that dopamine modulates the K+ current via D1 receptor-coupled G-protein pathways.

Calbindin and calretinin immunoreactivity in the retina of adult and larval sea lamprey

The presence of calretinin and calbindin immunoreactivity is studied in the retina of larval and adult lamprey and their respective distributions are compared. Calretinin distribution is also studied in the retina of transforming stages. Western blot analysis in brain extracts showed a 29-kDa band with both polyclonal anti-calbindin and anti-calretinin antibodies. Calbindin and calretinin immunoreactivity has shown a partially different distribution. In the adult retina large and small bipolar cells, with respectively stratified or diffuse axons, the inner row of horizontal cells and ganglion cells and/or some amacrine cells were labeled with anti-calretinin antibody. The anti-calbindin antibody labels the same cell types except most of ganglion cells, but the label was less conspicuous. Therefore, the possible existence of these two calcium-binding proteins in the central nervous system of the sea lamprey could be discussed. In the differentiated central retina of larval lampreys, numerous calretinin immunoreactive bipolar and ganglion cells were observed, while, in the lateral retina, only ganglion cells were labeled, accordingly with the lack of differentiation of other neural cell types. CR-ir bipolar cells appeared in the retina by the stage 5 of transformation, i.e. about the time when differentiation of photoreceptors occurs. The comparison of the distribution of calretinin and calbindin between adult and larval central retina of lampreys shows striking differences that could be related to the different functionality of eyes in these two stages of the life cycle of lampreys. In addition, this is the first report on the presence of calcium-binding proteins in the larval and transforming lamprey retina, on the presence of calretinin- and calbindin-immunoreactive horizontal cells in adult lamprey retinas and on the differential stratification of bipolar cell terminals.

Targeting of amacrine cell neurites to appropriate synaptic laminae in the developing zebrafish retina

Cellular mechanisms underlying the precision by which neurons target their synaptic partners have largely been determined based on the study of projection neurons. By contrast, little is known about how interneurons establish their local connections in vivo. Here, we investigated how developing amacrine interneurons selectively innervate the appropriate region of the synaptic neuropil in the inner retina, the inner plexiform layer (IPL). Increases (ON) and decreases (OFF) in light intensity are processed by circuits that are structurally confined to separate ON and OFF synaptic sublaminae within the IPL. Using transgenic zebrafish in which the majority of amacrine cells express fluorescent protein, we determined that the earliest amacrine-derived neuritic plexus formed between two cell populations whose somata, at maturity, resided on opposite sides of this plexus. When we followed the behavior of individual amacrine cells over time, we discovered that they exhibited distinct patterns of structural dynamics at different stages of development. During cellular migration, amacrine cells exhibited an exuberant outgrowth of neurites that was undirected. Upon reaching the forming IPL, neurites extending towards the ganglion cell layer were relatively more stable. Importantly, when an arbor first formed, it preferentially ramified in either the inner or outer IPL corresponding to the future ON and OFF sublaminae, and maintained this stratification pattern. The specificity by which ON and OFF amacrine interneurons innervate their respective sublaminae in the IPL contrasts with that observed for projection neurons in the retina and elsewhere in the central nervous system.

Retinal bipolar cell input mechanisms in giant danio. III. ON-OFF bipolar cells and their color-opponent mechanisms

Whole cell patch recording was performed from morphologically identified cone-driven on-off bipolar cells (Cabs) in giant danio retinal slices to study their glutamate receptors and light-evoked responses. Specific agonists were puffed in the presence of cobalt, picrotoxin, and strychnine to identify glutamate receptors on these cells. Most Cabs responded to both the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor agonist kainate and the excitatory amino acid transporter (EAAT) substrate D-aspartate, and both responses were localized to the dendrites. Kainate generated depolarizations whereas D-aspartate had E(rev) close to E(Cl) and generated hyperpolarizations, indicating that the AMPA/kainate receptors are sign-preserving, whereas the EAATs are sign-inverting. In response to white light, some Cabs gave on bipolar cell-like responses whereas others gave off bipolar cell-like ones, but many cells' responses had both on and off bipolar cell components. In response to appropriately colored center-selective stimuli, many Cabs responded to short and long wavelengths with opposite polarities and were thus double color-opponent. The depolarizing components of the responses to white or colored stimuli were suppressed by the EAAT blocker DL-threo-beta-benzyloxyaspartate (TBOA), whereas the hyperpolarizing components were reduced by the AMPA/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). These results are consistent with the hypothesis that both EAATs and AMPA/kainate receptors are involved in the generation of light-evoked responses in Cabs and that they confer these cells with on and off bipolar cell properties, respectively. Cabs can generate double color-opponent center responses by receiving inputs from certain cones through EAATs and from other cones through AMPA/kainate receptors.

The Organization of the Inner Retina in a Pure-Rod Deep-Sea Fish

The pure rod retina of a deep-sea eel species was used as a model system for the study of the differentiation of horizontal, bipolar, amacrine and ganglion cells. We wanted to test the hypothesis that the functional organization of the inner retina is less complex than in species with duplex, rod- and cone-containing retinae. We used immunocytochemistry, backfilling ganglion cells with fluorescent dextranes and microinjection of Lucifer Yellow, to visualize the micromorphology of the various cell types in a confocal microscope. The pure rod retina contains a single type of horizontal cell. The inner plexiform layer is 10-15 microm thick and shows three main sublayers. Bipolar terminals are found in all sublayers, but the majority are found in the inner sublamina b (PKC-immunoreactive cells, that in fish with duplex retinae receive a mixed rod-cone input). The neurochemical diversity of amacrine cells in terms of immunoreactivity does not differ from other teleosts; this similarity includes the pattern of dendritic stratification and ramification as revealed by microinjection. Ten different types of ganglion cells are distinguished based on the sizes of their perikaryon and dendritic field, and the stratification pattern in the inner plexiform layer. This is similar to the situation in catfish with retinae containing a single type of cone in addition to a majority of rods. In this respect, the differences between pure rod retinae and duplex retinae containing a single cone type were less obvious than hypothesized. In the deep-sea eel, the density of dendritic ramification in amacrine and ganglion cells was strongly reduced. This may be functionally related to the fact that vision in the deep sea environment relies exclusively on bioluminescence and is represented by burst-like emissions of point sources. This requires a mode of retinal signal processing that is less complex than in duplex retinae and involves a lower density of dendritic branching and synapses.

Reelin expression in the retina and optic tectum of developing common brown trout

Reelin (RELN) is an extracellular matrix protein largely related with laminar organization in several brain areas. The development of RELN immunoreactivity in the retina and the optic tectum of the brown trout are analyzed with a monoclonal (142) antibody against RELN whose suitability has been ascertained by western blot. In the retina of embryos and alevins, RELN immunoreactivity is detected in cells of the ganglion cell layer (GCL) and inner nuclear layer (INL), and in the inner plexiform layer (IPL), where it appears as "diffuse" material confined to the ON-sublayer. In juveniles, RELN expression becomes restricted to a stripe of cells in the INL. RELN-immunoreactive (RELN-ir) cells are absent from the outer nuclear layer (ONL) at any developmental stage. The developmental pattern of RELN expression in the trout retina shows many similarities with that of amniotes: (a) RELN expression parallels the vitreal to scleral progression of differentiation of the retina and, within each cell layer, RELN immunoreactivity appears confined to a subpopulation of postmitotic cells; (b) at early stages RELN expression is exclusively observed in the central retina and as maturation progresses from the center to the periphery, more RELN-ir cells are observed following the same spatial pattern. Differences with amniotes are noted regarding the absence of RELN expression in the GCL and INL in adulthood, and in the ONL at any developmental stage. In the optic tectum (OT) of trout, as in amniotes, RELN immunoreactivity increases within specific cell layers as lamination proceeds, and decreases when it is complete, except in the stratum opticum (SO), where RELN-ir cells are observed throughout life. Time-course expression of RELN in the OT suggests a role in the early modeling of synaptic contacts and the accommodation of new retinal arriving axons throughout life.

Laminar circuit formation in the vertebrate retina

Neuronal function depends on the accurate wiring between pre- and postsynaptic cells. Determining the mechanisms underlying precision in neuronal connectivity is challenging because of the complexity of the nervous system. In diverse parts of the nervous system, regions of synaptic contact are organized into distinct parallel layers, or laminae, that are correlated with distinct functions. Such an arrangement enables the development of synapse specificity to be more readily investigated. Here, we present an overview of the developmental mechanisms that are thought to underlie the formation of synaptic layers in the vertebrate retina, a highly laminated CNS structure. We will contrast the roles of activity-dependent and activity-independent mechanisms in establishing functionally discrete sublaminae in the inner retina, where circuits involving many subtypes of retinal neurons are assembled precisely. In addition, we will discuss new optical imaging approaches for elucidating how retinal synaptic lamination occurs in vivo.

Identification and morphological classification of horizontal, bipolar, and amacrine cells within the zebrafish retina

Horizontal, bipolar, and amacrine cells in the zebrafish retina were morphologically characterized using DiOlistic techniques. In this method, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-coated microcarriers are shot at high speed onto the surfaces of living retinal slices where the DiI then delineates axons, somata, and dendrites of isolated neurons. Zebrafish retinal somata were 5-10 microm in diameter. Three horizontal cell types (HA-1, HA-2, and HB) were identified; dendritic tree diameters averaged 25-40 microm. HA somata were round. Cells classified as HA-2 were larger than HA-1 cells and possessed an axon. HB somata were flattened, without an axon, although short fusiform structure(s) projected from the soma. Bipolar cells were separated into 17 morphological types. Dendritic trees ranged from 10 to 70 microM. There were six B(on) types with axon boutons only in the ON sublamina of the inner plexiform layer (IPL), and seven B(off) types with axon boutons or branches only in the OFF sublamina. Four types of bistratified bipolar cells displayed boutons in both ON and OFF layers. Amacrine cells occurred in seven types. A(off) cells (three types) were monostratified and ramified in the IPL OFF sublamina. Dendritic fields were 60-150 microM. A(on) pyriform cells (three types) branched in the ON sublamina. Dendritic fields were 50-170 microM. A(diffuse) cells articulated processes in all IPL strata. Dendritic fields were 15-90 microM. These findings are important for studies examining signal processing in zebrafish retina and for understanding changes in function resulting from mutations and perturbations of retinal organization.

A circadian clock regulates the process of ERG b- and d-wave dominance transition in dark-adapted zebrafish

In zebrafish, during dark adaptation following bright light adaptation, the dominance of electroretinogram (ERG) b- and d-waves switches. In the early dark adaptation, when visual sensitivity is cone-dominant, both the b- and d-waves are readily recorded. In the late dark adaptation, along with the increase of rod sensitivity, the b-wave becomes dominant whereas the d-wave is gradually lost. The time for the ERG b- and d-wave dominance transition varies between the day and night. The transition requires a longer amount of time in the night and early morning than in the afternoon. This pattern of timing for ERG b- and d-wave dominance transition persists in constant light and can be reversed after exposure to a reversed light-dark cycle. The data suggest that the transition of the dominance of ERG b- and d-waves is regulated by an endogenous circadian clock.

ON-pathway disturbance in two siblings

PURPOSE

To present two clinical cases diagnosed with predominant cone dystrophy and demonstrating early disturbance in the on-centre bipolar cells (ON-pathway).

METHODS

Electrophysiological findings are presented in two siblings with predominant cone dystrophy. The subjects showed no remarkable ophthalmoscopic or fluorescein angiographic retinal changes, but demonstrated progressive visual disturbance during their 20s.

RESULTS

The electroretinograms (ERGs) showed reduced dark-adapted responses but the positive component of the photopic ERG was absent. Response to 30 Hz flicker was severely reduced. Electroretinograms elicited by long-duration stimuli showed a loss of the b-wave, and the off-response was slightly reduced. In both patients, multifocal ERGs (m-ERGs) were more reduced within the central 10 degrees, where the ON-pathway is normally a major contributor.

CONCLUSION

We conclude that these patients may be affected by an abnormality of the synapses of the cone receptors and that their decrease in vision might, at least initially, be due to selective ON-pathway dysfunction.

Microcircuits for night vision in mouse retina

Because the mouse retina has become an important model system, we have begun to identify its specific neuron types and their synaptic connections. Here, based on electron micrographs of serial sections, we report that the wild-type mouse retina expresses the standard rod pathways known in other mammals: (1) rod --> cone (via gap junctions) to inject rod signals into the cone bipolar circuit; and (2) rod --> rod bipolar --> AII amacrine --> cone bipolar --> ganglion cell. The mouse also expresses another rod circuit: a bipolar cell with cone input also receives rod input at symmetrical contacts that express ionotropic glutamate receptors (Hack et al., 1999, 2001). We show that this rod-cone bipolar cell sends an axon to the outer (OFF) strata of the inner plexiform layer to form ribbon synapses with ganglion and amacrine cells. This rod-cone bipolar cell receives direct contacts from only 20% of all rod terminals. However, we also found that rod terminals form gap junctions with each other and thus establish partial syncytia that could pool rod signals for direct chemical transmission to the OFF bipolar cell. This third rod pathway probably explains the rod responses that persist in OFF ganglion cells after the well known rod pathways are blocked (Soucy et al., 1998).

Selective synaptic distribution of AMPA and kainate receptor subunits in the outer plexiform layer of the carp retina

The subunit composition of ionotropic glutamate receptors (GluRs) is extremely diverse and responsible for the diversity of postsynaptic responses to the release of glutamate, which is the major excitatory neurotransmitter in the retina. To understand the functional consequences of this diversity, it is necessary to reveal the synaptic localization and subunit composition of GluRs. We have used immuno light and electron microscopy to localize AMPA and kainate (GluR1, GluR2/3, GluR4, GluR5-7) subunits in identified carp retinal neurons contributing to the outer plexiform layer. GluR1 could not be detected within the outer plexiform layer. Rod and cone horizontal cells all express only GluR2/3 at the tips of their invaginating dendrites. These receptors are also inserted into the membrane of spinules, light-dependent protrusions of the horizontal cell dendrites, flanking the synaptic ribbon of the cone synapse. Bipolar cells express GluR2/3, GluR4, and GluR5-7 at their terminal dendrites invaginating cone pedicles and rod spherules. Colocalization data suggest that each subunit is expressed by a distinct bipolar cell type. The majority of bipolar cells expressing these receptors seem to be of the functional OFF-type; however, in a few instances, GluR2/3 could also be detected on dendrites of bipolar cells that, based on their localization within the cone synaptic complex, appeared to be of the functional ON-type. The spatial arrangement of the different subunits within the cavity of the cone pedicle appeared not to be random: GluR2/3 was found predominantly at the apex of the cavity, GluR4 at its base and GluR5-7 dispersed between the two.

Synapse formation is arrested in retinal photoreceptors of the zebrafish nrc mutant

We describe the effects of a recessive mutation on visual behavior, the electroretinogram (ERG), and photoreceptor structure in zebrafish. At 6 d post-fertilization (dpf), no optokinetic reflex could be elicited in no optokinetic response c (nrc) mutant animals under any test condition. The animals exhibited ERG responses at 5-7 dpf that were markedly abnormal and could be categorized into two groups. The first showed an initial negative a-wave followed by a delayed positive b-wave of small amplitude. Often a second ERG-like response was recorded after the initial b-wave. The second group showed only a large negative a-wave; an initial b-wave was not evident. In most recordings additional oscillatory waves varying in number, amplitude, and time course were observed. Multiple responses at the cessation of long-duration flashes were also observed. Light and electron microscopy revealed that the cone photoreceptor pedicles of nrc fish were highly abnormal. Although the appropriate number of synaptic ribbons formed in these terminals, they "floated" in the terminal, unassociated with postsynaptic processes or arciform densities. The few processes invaginating the nrc pedicles resembled those of horizontal cells. Invaginating bipolar cell processes were rare, but basal contacts were observed on pedicle surfaces. The severity of the mutation did not change between 6 and 8 dpf, showing that there is neither a delay in development nor a degeneration of the terminals; rather, nrc pedicle development appears arrested. Bipolar cell terminals in the inner plexiform layer made normal ribbon synapses; thus, the mutation appears to affect only the outer retina.

On- and off-responses of the photopic electroretinograms in X-linked juvenile retinoschisis

PURPOSE

To examine the physiologic condition of the middle retinal layer of patients with X-linked juvenile retinoschisis (xlRS) by studying the on- and off-responses of the photopic electroretinograms (ERGs).

METHODS

Eleven unrelated Japanese men (mean age; 24.9 +/- 7.6 years) who were clinically diagnosed with xlRS and molecularly confirmed as having XLRS1 mutations were investigated. For the photopic ERGs, the a-, b- and d-wave amplitudes elicited by long duration stimuli were recorded, and the responses from the xlRS patients were compared to those recorded from normal subjects (n = 14, mean age, 27.5 +/- 4.5 years). We also examined the relationship between the photopic ERG responses and the genotype.

RESULTS

No significant difference was found between the a- and d-wave amplitudes in the xlRS patients (34.2 +/- 8.7 microV, 52.5 +/- 10.4 microV, respectively), and those in normal subjects (40.4 +/- 10.3 microV, 44.7 +/- 6.3 microV, respectively). The mean b-wave amplitude in the xlRS patients was significantly smaller (10.5 +/- 7.7 microV) than the mean of normal subjects (46.4 +/- 10.2 microV) (P < 0.0001). No significant correlation was found between the ERG responses and the locus of the mutation.

CONCLUSION

The photopic ERG demonstrated considerable impairment of the on-pathway arising from an abnormality of the on-bipolar cells or possibly secondary to Müller cell abnormality in xlRS.

Distribution of the calcium-binding proteins parvalbumin, calbindin D-28k and calretinin in the retina of two teleosts

Using monoclonal antibodies against parvalbumin (PV) and calbindin (CB), and a polyclonal antiserum against calretinin (CR), the expression patterns of these proteins in the retina of the tench and rainbow trout were studied at light microscopic level in in toto preparations and radial sections. Parvalbumin was present in subpopulations of small amacrine cells in both species, but these cells were more abundant and had a clear centre-periphery gradient distribution in the tench. Using the McAB 300 monoclonal antibody against CB, glial cells such as Müller cells, astrocytes in the nerve fibre layer, and sparse large cells close to the entrance of the optic nerve were observed in both species. Moreover, this antibody strongly labelled H1 horizontal cells and their thick axon terminals in the tench retina, whereas only a small population of amacrine cells was stained in the trout. Calretinin was expressed in different types of ganglion cells and numerous neurones located in the inner plexiform layer in both species, but was more abundant and more strongly stained in the trout retina, where some bipolar cells were easily distinguishable. A comparison to current results in other vertebrate species is offered.

Axonal stratification patterns and glutamate-gated conductance mechanisms in zebrafish retinal bipolar cells

1. Whole-cell patch recording and puff pipette techniques were used to identify glutamate receptor mechanisms on bipolar cell (BC) dendrites in the zebrafish retinal slice. Recorded neurons were stained with Lucifer Yellow, to correlate glutamate responses with BC morphology. 2. BC axon terminals (ATs) consisted of swellings or varicosities along the axon, as well as at its end. AT stratification patterns identified three regions in the inner plexiform layer (IPL): a thick sublamina a, with three bands of ATs, a narrow terminal-free zone in the mid-IPL, and a thin sublamina b, with two bands of ATs. BCs occurred with ATs restricted to sublamina a(Group a), sublamina b(Group b) or with ATs in both sublaminae (Group a/b). 3. OFF-BCs belonged to Group a or Group a/b. These cells responded to glutamate or kainate with a CNQX-sensitive conductance increase. Reversal potential (Erev) ranged from -0.6 to +18 mV. Bipolar cells stimulated sequentially with both kainate and glutamate revealed a population of glutamate-insensitive, kainate-sensitive cells in addition to cells sensitive to both agonists. 4. ON-BCs responded to glutamate via one of three mechanisms: (a) a conductance decrease with Erev approximately 0 mV, mimicked by L-(+)-2-amino-4-phosphonobutyric acid (APB) or trans-1-amino-1, 3-cyclopentanedicarboxylic acid (trans-ACPD), (b) a glutamate-gated chloride conductance increase (IGlu-like) characterized by Erev >= ECl (where ECl is the chloride equilibrium potential) and partial blockade by extracellular Li+/Na+ substitution or (c) the activation of both APB and chloride mechanisms simultaneously to produce a response with outward currents at all holding potentials. APB-like responses were found only among BCs in Group b, with a single AT ramifying deep within sublamina b; whereas, cells expressing IGlu-like currents had one or more ATs, and occurred within Groups b or a/b. 5. Multistratified cells (Group a/b) were common and occurred with either ON- or OFF-BC physiology. OFF-BCs typically had one or more ATs in sublamina a and only one AT in sublamina b. In contrast, multistratified ON-BCs had one or more ATs in sublamina b and a single AT ramifying deep in sublamina a. Multistratified ON-BCs expressed the IGlu-like mechanism only. 6. Visual processing in the zebrafish retina involves at least 13 BC types. Some of these BCs have ATs in both the ON- and OFF-sublaminae, suggesting a significant role for ON- and OFF-inputs throughout the IPL.

Characterization of spontaneous excitatory synaptic currents in newt retinal bipolar cells

The kinetics of glutamate concentration in the synaptic cleft is an important determinant of synaptic function. To elucidate peak concentration of glutamate released from a single vesicle in the cleft, spontaneous excitatory postsynaptic currents (sEPSCs) in Off-bipolar cells from the sliced newt retina were analyzed using whole-cell patch clamp recording and the computer simulation. The sEPSCs were blocked by an AMPA/kainate (KA) antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and prolonged by cyclothiazide. However, an N-methyl-D-aspartate (NMDA) antagonist, D-2-amino-5-phosphonopentanoic acid (D-AP5), was ineffective. These suggest that sEPSCs in Off-bipolar cells are mediated exclusively by AMPA/KA receptors. sEPSCs simulated by a detailed kinetic model of AMPA receptor best approximated the data, when peak glutamate concentration was 10 microM. Therefore, it was concluded that peak concentration of glutamate released from a single vesicle would be elevated to approximately 10 microM at the newt Off-bipolar dendrite.

The eyes of deep-sea fish. II. Functional morphology of the retina

Three different aspects of the morphological organisation of deep-sea fish retinae are reviewed: First, questions of general cell biological relevance are addressed with respect to the development and proliferation patterns of photoreceptors, and problems associated with the growth of multibank retinae, and with outer segment renewal are discussed in situations where there is no direct contact between the retinal pigment epithelium and the tips of rod outer segments. The second part deals with the neural portion of the deep-sea fish retina. Cell densities are greatly reduced, yet neurohistochemistry demonstrates that all major neurotransmitters and neuropeptides found in other vertebrate retinae are also present in deep-sea fish. Quantitatively, convergence rates in unspecialised parts of the retina are similar to those in nocturnal mammals. The differentiation of horizontal cells makes it unlikely that species with more than a single visual pigment are capable of colour vision. In the third part, the diversity of deep-sea fish retinae is highlighted. Based on the topography of ganglion cells, species are identified with areae or foveae located in various parts of the retina, giving them a greatly improved spatial resolving power in specific parts of their visual fields. The highest degree of specialisation is found in tubular eyes. This is demonstrated in a case study of the scopelarchid retina, where as many as seven regions with different degrees of differentiation can be distinguished, ranging from an area giganto cellularis, regions with grouped rods to retinal diverticulum.

Syncytial integration by a network of coupled bipolar cells in the retina

A model system for syncytial integration is the outer vertebrate retina, where graded signals or electrotonic potentials interact laterally via gap junctions to form an integrated response that is relayed by chemical synapses to the next layer of interconnected cells. Morphological and physiological experiments confirm that bipolar cells form quasisyncytial lattices, and so this review will aim to address two important issues: the function of coupling in visual information processing and the construction of a robust mathematical model that can adequately simulate signal spread in the bipolar cell syncytium. It is shown that the role of coupling in bipolar cells differs from that associated in the presynaptic networks, namely, loss in spatial resolution in order to increase the signal-to-noise ratio. The intrinsic membrane properties of bipolar cells which give rise to voltage-dependent currents are inactive over the normal in vivo operating range of membrane potential and may be shunted as a direct result of electrotonic coupling, suppressing any possibility of action potential propagation in the bipolar cell syncytium. It is therefore speculated that the mechanisms underlying processing of information in bipolar networks are dependent on the structure of bipolar cells and in particular, on the presence of gap junctions. It is proposed that a three-dimensional model which incorporates the spatial properties of each bipolar cell in the network in the form of a leaky cable is the most likely model to simulate signal spread in the bipolar cell syncytium in vivo. This is because discrete network models represent each bipolar cell in the syncytium as isopotential units without any spatial structure, and thus are unable to reproduce the temporal characteristics of electrotonic potential spread within the central receptive field of bipolar cells.

ON cone bipolar cells in rat express the metabotropic receptor mGluR6

The rod bipolar cell and about five types of ON cone bipolar cells depolarize to light by employing a sign-reversing metabotropic glutamate receptor. Glutamate responses are similar in both rod bipolar and cone bipolar cells, but the receptor mediating this response (mGluR6) was so far demonstrated only in rod bipolar cells. To test if ON cone bipolar cells also express mGluR6, we immunoreacted rat retina with an antibody specific for mGluR6, and studied the staining from serial ultrathin sections. We demonstrate that mGluR6 is indeed expressed in the dendritic tips of cone bipolar cells, the majority of which receive a ribbon synapse, and thus probably are ON cone bipolar cells. We further show that half of the dendritic tips contacting the cones stain for mGluR6, thus implying that all ON cone bipolar cell types express mGluR6.

Immunocytochemical localization of the GABA(C) receptor rho subunits in the cat, goldfish, and chicken retina

Polyclonal antibodies against the N-terminus of the rat rho1 subunit were used to study the distribution of gamma-aminobutyric acid C (GABA(C)) receptors in the cat, goldfish, and chicken retina. Strong punctate immunoreactivity was present in the inner plexiform layer (IPL) of all three species. The punctate labelling suggests a clustering of the GABA(C) receptors at synaptic sites. Weak label was also found in the outer plexiform layer (OPL) and over the cell bodies of bipolar cells. Double immunostaining of vertical sections with an antibody against protein kinase C (PKC) showed the punctate immunofluorescence to colocalize with bipolar cell axon terminals. In the goldfish retina, the axon terminals of Mb1 bipolar cells were enclosed by rho-immunoreactive puncta. In the chicken retina, several distinct strata within the IPL showed a high density of rho-immunoreactive puncta. The results suggest a high degree of sequence homology between the rho subunits of different vertebrate species, and they show that the retinal localization of GABA(C) receptors is similar across different species.

Glutamate responses of bipolar cells in a slice preparation of the rat retina

Whole-cell currents from >70 voltage-clamped bipolar cells were recorded in a slice preparation of the rat retina. The recorded cells were identified and classified by intracellular staining with Lucifer yellow. Glutamate, the specific agonists (+/-)-2-amino-4-phosphonobutyric acid (AP-4) and kainate (KA), and the antagonist 6-cyanoquinoxaline-2,3-dione (CNQX) were applied. The cells could be isolated from presynaptic influences by the co-application of bicuculline, strychnine, and cobalt ions. Responses to AP-4 were elicited only from bipolar cells with axons stratifying in the inner part of the inner plexiform layer (IPL). AP-4 caused an outward current in these cells attributable to the closure of nonspecific cation channels. Responses to kainate representing a direct action of the drug on the recorded cells were observed only in bipolar cells with axons stratifying in the outer part of the IPL. KA caused a CNQX-sensitive inward current in these cells, associated with openings of nonspecific cation channels. The results predict that cone bipolar (CB) cells with axons terminating in the outer IPL are OFF-bipolars, whereas those with axons terminating in the inner IPL are ON-bipolars. Most of the cells expressed GABA-gated Cl- conductances. In rod bipolar and in some CB cells, only part of the GABA-induced currents could be blocked by the application of bicuculline, suggesting the presence of GABAc receptors in addition to GABAA receptors.

Pharmacology of the skate electroretinogram indicates independent ON and OFF bipolar cell pathways

Organization of afferent information into parallel ON and OFF pathways is a critical feature of the vertebrate visual system. All afferent visual information in the vertebrate retina reaches the inner plexiform layer (IPL) via bipolar cells. It is at the bipolar cell level that separation of ON and OFF information first appears for afferent information from cones. This may also hold true for the rod pathway of cold-blooded vertebrates, but not for mammals. The all-rod retina of the skate presents an opportunity to examine such pathways in a retina having but a single class of photoreceptor. Immunocytochemical evidence suggests that both ON and OFF bipolar cells are present in the skate retina. We examined the pharmacology of the skate electroretinogram (ERG) to test the hypothesis that independent ON and OFF bipolar cell pathways are functional as rod afferent pathways from outer to inner plexiform layer in the skate. 100 microM 2-amino-4-phosphonobutyric acid (APB) reversibly blocked the skate ERG b-wave. A small d-wave-like OFF component of the ERG revealed by DC recording of response to a prolonged (10 s) flash of light was reduced or blocked by 5 mM kynurenic acid (KYN). We found that addition of 200 microM picrotoxin to the Ringer's solution revealed prominent ON and OFF components of the skate ERG while reducing the c-wave. These ON and OFF components were reversibly blocked by 100 microM APB and 5 mM KYN, respectively. Reversible block of the OFF component by KYN was also accomplished in the presence of 500 microM N-methyl-DL-aspartate. From these findings, we conclude that ON and OFF bipolar cells are likely to be functional as parallel afferent interplexiform pathways in the all-rod retina of the skate.

A calbindin-immunoreactive cone bipolar cell type in the rabbit retina

We have studied the distribution of the calcium-binding protein calbindin in the adult rabbit retina by using a commercially available antibody and immunocytochemical methods. The most heavily labeled cells are A-type horizontal cells, but B-type horizontal cells are also lightly labeled by this antibody. Among the horizontal cells, there is a mosaic of small, well-labeled somata, which we have identified as a subset of ON cone bipolar cells. In addition, some wide-field amacrine cells and a few large ganglion cells are also labeled for calbindin. The calbindin bipolar cells form a regular mosaic with a peak density of approximately 1,700 cells/mm2, falling to 550 cells/mm2 in the periphery. They account for about one-twelfth of cone bipolar cells, and they are narrowly stratified deep in sublamina 4 of the inner plexiform layer immediately above the rod bipolar terminals. Double-label experiments using an antibody to protein kinase C (PKC) indicate that the calbindin bipolar cells are completely distinct from the population of rod bipolar cells. Rod bipolar cells outnumber the calbindin cone bipolar cells by a factor of four to five. Further double-label experiments show that the calbindin bipolar cells are also labeled for recoverin. The calbindin bipolar cells are well coupled to AII amacrine cells, and they account for roughly 23% of the AII coupled bipolar cells. This suggests that there are three to four additional ON cone bipolar cell types that are coupled to AII amacrine cells. The calbindin cone bipolar cell described in this paper shares many characteristics with a reconstructed cone bipolar cell that forms the most gap junctions with AII amacrine cells (Strettoi et al. [1994] J. Comp. Neurol. 347:139-149). We conclude that these different methodologies provide complementary descriptions of the same cone bipolar cell type. The calbindin antibody defines a subset of cone bipolar cells in the rabbit retina. The cells in this subset are almost certainly the deepest of the cone bipolar cells. The tight stratification of the calbindin cone bipolar cell suggests that the inner plexiform layer is stratified according to depth, with narrow functional divisions within the broad partition of sublamina b, where ON signals are processed. The strength of coupling between the calbindin cone bipolar cells and AII amacrine cells suggests this pathway plays a major role under scotopic conditions.

Light- and electron-microscopic study of substance P-immunoreactive neurons in the guinea pig retina

Substance P (SP) immunoreactivity in the guinea pig retina was studied by light and electron microscopy. The morphology and distribution of SP-immunoreactive neurons was defined by light microscopy. The SP-immunoreactive neurons formed one population of amacrine cells whose cell bodies were located in the proximal row of the inner nuclear layer. A single dendrite emerged from each soma and descended through the inner plexiform layer toward the ganglion cell layer. SP-immunoreactive processes ramified mainly in strata 4 and 5 of the inner plexiform layer. SP-immunoreactive amacrine cells were present at a higher density in the central region around the optic nerve head and at a lower density in the peripheral region of the retina. The synaptic connectivity of SP-immunoreactive amacrine cells was identified by electron microscopy. SP-labeled amacrine cell processes received synaptic inputs from other amacrine cell processes in all strata of the inner plexiform layer and from bipolar cell axon terminals in sublamina b of the same layer. The most frequent postsynaptic targets of SP-immunoreactive amacrine cells were the somata of ganglion cells and their dendrites in sublamina b of the inner plexiform layer. Amacrine cell processes were also postsynaptic to SP-immunoreactive neurons in this sublamina. No synaptic outputs onto the bipolar cells were observed.

The organization of the turtle inner retina. I. ON- and OFF-center pathways

Intracellular recordings and dye injections of Lucifer yellow, horseradish peroxidase, or Neurobiotin were made in bipolar, amacrine, and ganglion cells of the Pseudemys turtle retina. By using a standard light-stimulation protocol in a sample of 375 labeled neurons, we were able to identify morphological and physiological characteristics of 11 types of bipolar cell, 37 types of amacrine cell, and 24 types of ganglion cell. To make sense of these data, we have chosen to group the 72 essentially different neuron types into traditional, functionally significant pathways. In this paper we look at the neuronal types in the inner plexiform layer (IPL) in terms of their contribution to generalized luminosity responses such as sustained ON- or OFF-center and transient ON-OFF ganglion cells; in the companion paper (J. Ammermüller, J.F. Muller, and H. Kolb, 1995, J. Comp. Neurol. 358:35-62) we look at them in terms of their involvement in color opponency and directional selectivity. A functional organization of the turtle IPL into OFF sublaminae (strata 1 and 2) and ON sublaminae (strata 3, 4, and 5), as has been described for other vertebrate retinas, was quite clear for two varieties of OFF-center bipolar cells (B4 and B5) and for all four types of sustained ON-center bipolar cell (B1, B2, B6, and B7). Thus, we found no sustained ON-center bipolar cell terminating in strata 1 and 2. We did, however, see three varieties of sustained OFF-center bipolar cells (B3, B9, and B10) having axon terminals in strata 3-5 (the ON sublamina) in addition to their terminations in stratum 1 or 2 (the OFF sublamina). Monostratified sustained ON- and OFF-center amacrine and ganglion cells rigidly obeyed the border of ON and OFF sublaminae. However, multistratified and diffuse sustained amacrine and ganglion cells could be either ON-center or OFF-center, and they did not strictly obey the border: such ON-center cells always had processes in one of the ON sublaminae (strata 3-5), and the equivalent OFF-center cells always had processes in one of the OFF sublaminae (strata 1 and 2). Monostratified transient amacrine and ganglion cells were concentrated in the middle of the IPL (around stratum 3), whereas bi-, tri-, or multistratified transient amacrine or ganglion cells always had processes in both the ON and the OFF sublaminae.(ABSTRACT TRUNCATED AT 400 WORDS)

Contrast-dependent responses in the human visual system: childhood through adulthood

Visual evoked potentials (VEPs) to temporal modulation of spatial patterns, recorded from humans ranging in age from 4-42 years, demonstrated that contrast-dependent responses exist in early childhood and change dramatically throughout childhood. Bright or dark isolated-check stimuli were used to emphasize contributions from ON or OFF pathways to the VEP. (ON and OFF pathways constitute one major pair of parallel subsystems, which process brightness [positive-contrast] and darkness [negative-contrast] information, respectively.) The developmental effects observed for each pathway were similar in magnitude and time course, suggesting maturation of a common physiological mechanism dependent on spatial contrast. Children's responses were more variable and larger than those of adults, and exhibited a relative phase lag. In addition, we recorded transient VEPs to a conventional contrast-reversing checkerboard pattern. The latency of the major positive wave (P100) was found to decrease, while the latency of the initial positive wave (P60) was found to increase, with increasing age. We propose a vector-summation model, which posits a relative decrease in cortical excitation with increasing age, to explain our major findings.

Functional morphologies of retinal ganglion cells in the turtle

Retinal ganglion cells in the turtle, Pseudemys scripta elegans, were examined by intracellular recording with a protocol of stationary and moving lights. Responses were apportioned among OFF, ON, and ON-OFF categories, and directional selectivity. Cells were injected with Neurobiotin, then later conjugated with avidin-horseradish peroxidase in standard procedure. Morphological analysis of the stained cells included measurements of soma and dendritic field sizes, dendritic stratification, number of cell processes, dendritic branchings, and dendritic symmetry ratios. ON and ON-OFF cells are at least bistratified, sometimes tristratified, in both sublaminae A and B whether directionally selective or not. OFF cells, in contrast, are monostratified, or at least confined to sublamina A. Morphological parameters of somal and dendritic field areas, branch point densities, and dendritic field asymmetries do not predict directional selectivity. Membrane polarization accompanying moving stimulation is discussed in terms of shunting inhibition and recording site.

Interaction between enkephalin and GABA in the chicken retina: further analyses of coexisting relationships

Previous studies have indicated an interactive relationship between enkephalin and gamma-aminobutyric acid (GABA) in the vertebrate retina. Among these studies are those that have demonstrated the colocalization of enkephalin and GABA in retinal amacrine cells. In the present study, enkephalin immunocytochemistry was combined with either autoradiography of tritiated GABA high-affinity uptake or GABA immunocytochemistry to further investigate the coexistence of GABA in enkephalin-amacrine cells of the chicken retina. A regional analysis revealed that the percentage colocalization of GABA high-affinity uptake in enkephalin-amacrine cells did not vary appreciably throughout the retina. Overall, 15.2% of enkephalin-amacrine cells exhibited high-affinity GABA uptake. Double-label immunofluorescence histochemistry revealed that 15.1% of enkephalin-amacrine cells express endogenous GABA-like immunoreactivity. These double-labelled cells were observed throughout central and peripheral regions of the retina. In each of the double-label analyses, only less intensely labelled enkephalin-amacrine cells expressed markers of GABA activity. The two double-label analyses reveal almost identical percentages of coexistence of GABA markers in chicken enkephalin-amacrine cells and therefore, provide supportive evidence for the GABAergic nature of these cells. These results suggest a functional diversity in the population of chicken enkephalin-amacrine cells and imply the possibility of multiple signalling through amacrine cells which contain enkephalin and GABA.