Organizational motifs for ground squirrel cone bipolar cells

Mechanisms of simultaneous linear and nonlinear computations at the mammalian cone photoreceptor synapse

Neurons enhance their computational power by combining linear and nonlinear transformations in extended dendritic trees. Rich, spatially distributed processing is rarely associated with individual synapses, but the cone photoreceptor synapse may be an exception. Graded voltages temporally modulate vesicle fusion at a cone's ~20 ribbon active zones. Transmitter then flows into a common, glia-free volume where bipolar cell dendrites are organized by type in successive tiers. Using super-resolution microscopy and tracking vesicle fusion and postsynaptic responses at the quantal level in the thirteen-lined ground squirrel, Ictidomys tridecemlineatus, we show that certain bipolar cell types respond to individual fusion events in the vesicle stream while other types respond to degrees of locally coincident events, creating a gradient across tiers that are increasingly nonlinear. Nonlinearities emerge from a combination of factors specific to each bipolar cell type including diffusion distance, contact number, receptor affinity, and proximity to glutamate transporters. Complex computations related to feature detection begin within the first visual synapse.

Digital reconstruction and quantitative morphometric analysis of bipolar cells in live rat retinal slices

Bipolar cells convey signals from photoreceptors in the outer retina to amacrine and ganglion cells in the inner retina. In mammals, there are typically 10–15 types of cone bipolar cells and one type of rod bipolar cell. Different types of cone bipolar cells are thought to code and transmit different features of a complex visual stimulus, thereby generating parallel channels that uniquely filter and transform the photoreceptor outputs. Differential synaptic connectivity and expression of ligand‐ and voltage‐gated ion channels are thought to be important mechanisms for processing and filtering visual signals. Whereas the biophysical basis for such mechanisms has been investigated more extensively in rat retina, there is a lack of quantitative morphological data necessary for advancing the structure–function correlation in this species, as recent connectomics investigations have focused on mouse retina. Here, we performed whole‐cell recordings from cone and rod bipolar cells in rat retinal slices, filled the cells with fluorescent dyes, and acquired image stacks by multiphoton excitation microscopy. Following deconvolution, we performed digital reconstruction and morphometric analysis of 25 cone and 14 rod bipolar cells. Compared to previous descriptions, the extent and complexity of branching of the axon terminal was surprisingly high. By precisely quantifying the level of stratification of the axon terminals in the inner plexiform layer, we have generated a reference system for reliable classification of individual cells in future studies focused on correlating physiological and morphological properties. The implemented workflow can be extended to the development of morphologically realistic compartmental models for these neurons.

The mammalian rod synaptic ribbon is essential for Cav channel facilitation and ultrafast synaptic vesicle fusion

Rod photoreceptors (PRs) use ribbon synapses to transmit visual information. To signal ‘no light detected’ they release glutamate continually to activate post-synaptic receptors. When light is detected glutamate release pauses. How a rod’s individual ribbon enables this process was studied here by recording evoked changes in whole-cell membrane capacitance from wild-type and ribbonless (Ribeye-ko) mice. Wild-type rods filled with high (10 mM) or low (0.5 mM) concentrations of the Ca²⁺-buffer EGTA created a readily releasable pool (RRP) of 87 synaptic vesicles (SVs) that emptied as a single kinetic phase with a τ<0.4 ms. The lower concentration of EGTA accelerated Ca_v channel opening and facilitated release kinetics. In contrast, ribbonless rods created a much smaller RRP of 22 SVs, and they lacked Ca_v channel facilitation; however, Ca²⁺ channel-release coupling remained tight. These release deficits caused a sharp attenuation of rod-driven scotopic light responses. We conclude that the synaptic ribbon facilitates Ca²⁺-influx and establishes a large RRP of SVs.

Ground squirrel - A cool model for a bright vision

The great evolutionary biologist, Theodosius Dobzhansky, once said, "Nothing in biology makes sense except in the light of evolution." Vision, no doubt, is a poster child for the work of evolution. If it has not already been said, I would humbly add that "Nothing in biology makes sense except in the context of metabolism." Marrying these two thoughts together, when one chooses an animal model for vision research, the ground squirrel jumps out immediately for its unique cone dominant retina, which has evolved for its diurnal lifestyle, and for hibernation-an adaptation to unique metabolic challenges encountered during its winter sojourn.

Cell types and cell circuits in human and non-human primate retina

This review summarizes our current knowledge of primate including human retina focusing on bipolar, amacrine and ganglion cells and their connectivity. We have two main motivations in writing. Firstly, recent progress in non-invasive imaging methods to study retinal diseases mean that better understanding of the primate retina is becoming an important goal both for basic and for clinical sciences. Secondly, genetically modified mice are increasingly used as animal models for human retinal diseases. Thus, it is important to understand to which extent the retinas of primates and rodents are comparable. We first compare cell populations in primate and rodent retinas, with emphasis on how the fovea (despite its small size) dominates the neural landscape of primate retina. We next summarise what is known, and what is not known, about the postreceptoral neurone populations in primate retina. The inventories of bipolar and ganglion cells in primates are now nearing completion, comprising ~12 types of bipolar cell and at least 17 types of ganglion cell. Primate ganglion cells show clear differences in dendritic field size across the retina, and their morphology differs clearly from that of mouse retinal ganglion cells. Compared to bipolar and ganglion cells, amacrine cells show even higher morphological diversity: they could comprise over 40 types. Many amacrine types appear conserved between primates and mice, but functions of only a few types are understood in any primate or non-primate retina. Amacrine cells appear as the final frontier for retinal research in monkeys and mice alike.

Sensory Processing at Ribbon Synapses in the Retina and the Cochlea

In recent years, sensory neuroscientists have made major efforts to dissect the structure and function of ribbon synapses which process sensory information in the eye and ear. This review aims to summarize our current understanding of two key aspects of ribbon synapses: 1) their mechanisms of exocytosis and endocytosis and 2) their molecular anatomy and physiology. Our comparison of ribbon synapses in the cochlea and the retina reveals convergent signaling mechanisms, as well as divergent strategies in different sensory systems.

Diurnal rodents as pertinent animal models of human retinal physiology and pathology

This presentation will survey the retinal architecture, advantages, and limitations of several lesser-known rodent species that provide a useful diurnal complement to rats and mice. These diurnal rodents also possess unusually cone-rich photoreceptor mosaics that facilitate the study of cone cells and pathways. Species to be presented include principally the Sudanian Unstriped Grass Rat and Nile Rat (Arvicanthis spp.), the Fat Sand Rat (Psammomys obesus), the degu (Octodon degus) and the 13-lined ground squirrel (Ictidomys tridecemlineatus). The retina and optic nerve in several of these species demonstrate unusual resilience in the face of neuronal injury, itself an interesting phenomenon with potential translational value.

Diverse Cell Types, Circuits, and Mechanisms for Color Vision in the Vertebrate Retina

Synaptic interactions to extract information about wavelength, and thus color, begin in the vertebrate retina with three classes of light-sensitive cells: rod photoreceptors at low light levels, multiple types of cone photoreceptors that vary in spectral sensitivity, and intrinsically photosensitive ganglion cells that contain the photopigment melanopsin. When isolated from its neighbors, a photoreceptor confounds photon flux with wavelength and so by itself provides no information about color. The retina has evolved elaborate color opponent circuitry for extracting wavelength information by comparing the activities of different photoreceptor types broadly tuned to different parts of the visible spectrum. We review studies concerning the circuit mechanisms mediating opponent interactions in a range of species, from tetrachromatic fish with diverse color opponent cell types to common dichromatic mammals where cone opponency is restricted to a subset of specialized circuits. Distinct among mammals, primates have reinvented trichromatic color vision using novel strategies to incorporate evolution of an additional photopigment gene into the foveal structure and circuitry that supports high-resolution vision. Color vision is absent at scotopic light levels when only rods are active, but rods interact with cone signals to influence color perception at mesopic light levels. Recent evidence suggests melanopsin-mediated signals, which have been identified as a substrate for setting circadian rhythms, may also influence color perception. We consider circuits that may mediate these interactions. While cone opponency is a relatively simple neural computation, it has been implemented in vertebrates by diverse neural mechanisms that are not yet fully understood.

Gene delivery to cone photoreceptors by subretinal injection of rAAV2/6 in the mouse retina

Adeno-associated virus (AAV) has been studied as a safe delivery tool for gene therapy of retinal blinding diseases such as Leber's congenital amaurosis (LCA). The tropism of recombinant AAV (rAAV) including its specificity and efficiency in targeting retinal cell types has been studied with native or engineered capsids, along with specific promoters. However, one of the rAAV serotypes, rAAV2/6, has not been well-studied based on a report of low infection efficiency in the retina. We investigated the tropism of several rAAVs by subretinal injection in the adult mouse and found that rAAV2/6 predominantly infected cone photoreceptors including the main spectral type. Our data suggest that subretinal injection with rAAV2/6 may provide both an efficacious and specific means of gene delivery to cone photoreceptors in murine retinas.

Compartment models for the electrical stimulation of retinal bipolar cells

Bipolar cells of the retina are among the smallest neurons of the nervous system. For this reason, compared to other neurons, their delay in signaling is minimal. Additionally, the small bipolar cell surface combined with the low membrane conductance causes very little attenuation in the signal from synaptic input to the terminal. The existence of spiking bipolar cells was proven over the last two decades, but until now no complete model including all important ion channel types was published. The present study amends this and analyzes the impact of the number of model compartments on simulation accuracy. Characteristic features like membrane voltages and spike generation were tested and compared for one-, two-, four- and 117-compartment models of a macaque bipolar cell. Although results were independent of the compartment number for low membrane conductances (passive membranes), nonlinear regimes such as spiking required at least a separate axon compartment. At least a four compartment model containing the functionally different segments dendrite, soma, axon and terminal was needed for understanding signaling in spiking bipolar cells. Whereas for intracellular current application models with small numbers of compartments showed quantitatively correct results in many cases, the cell response to extracellular stimulation is sensitive to spatial variation of the electric field and accurate modeling therefore demands for a large number of short compartments even for passive membranes.

The morphological characterization of orientation-biased displaced large-field ganglion cells in the central part of goldfish retina

The vertebrate retina has about 30 subtypes of ganglion cells. Each ganglion cell receives synaptic inputs from specific types of bipolar and amacrine cells ramifying at the same depth of the inner plexiform layer (IPL), each of which is thought to process a specific aspect of visual information. Here, we identified one type of displaced ganglion cell in the goldfish retina which had a large and elongated dendritic field. As a population, all of these ganglion cells were oriented in the horizontal axis and perpendicular to the dorsal-ventral axis of the goldfish eye in the central part of retina. This ganglion cell has previously been classified as Type 1.2. However, the circuit elements which synapse with this ganglion cell are not yet characterized. We found that this displaced ganglion cell was directly tracer-coupled only with homologous ganglion cells at sites containing Cx35/36 puncta. We further illustrated that the processes of dopaminergic neurons often terminated next to intersections between processes of ganglion cells, close to where dopamine D1 receptors were localized. Finally, we showed that Mb1 ON bipolar cells had ribbon synapses in the axonal processes passing through the IPL and made ectopic synapses with this displaced ganglion cell that stratified into stratum 1 of the IPL. These results suggest that the displaced ganglion cell may synapse with both Mb1 cells using ectopic ribbon synapses and OFF cone bipolar cells with regular ribbon synapses in the IPL to function in both scotopic and photopic light conditions.

Developmental and adult characterization of secretagogin expressing amacrine cells in zebrafish retina

Calcium binding proteins show stereotypical expression patterns within diverse neuron types across the central nervous system. Here, we provide a characterization of developmental and adult secretagogin-immunolabelled neurons in the zebrafish retina with an emphasis on co-expression of multiple calcium binding proteins. Secretagogin is a recently identified and cloned member of the F-hand family of calcium binding proteins, which labels distinct neuron populations in the retinas of mammalian vertebrates. Both the adult distribution of secretagogin labeled retinal neurons as well as the developmental expression indicative of the stage of neurogenesis during which this calcium binding protein is expressed was quantified. Secretagogin expression was confined to an amacrine interneuron population in the inner nuclear layer, with monostratified neurites in the center of the inner plexiform layer and a relatively regular soma distribution (regularity index > 2.5 across central–peripheral areas). However, only a subpopulation (~60%) co-labeled with gamma-aminobutyric acid as their neurotransmitter, suggesting that possibly two amacrine subtypes are secretagogin immunoreactive. Quantitative co-labeling analysis with other known amacrine subtype markers including the three main calcium binding proteins parvalbumin, calbindin and calretinin identifies secretagogin immunoreactive neurons as a distinct neuron population. The highest density of secretagogin cells of ~1800 cells / mm² remained relatively evenly along the horizontal meridian, whilst the density dropped of to 125 cells / mm² towards the dorsal and ventral periphery. Thus, secretagogin represents a new amacrine label within the zebrafish retina. The developmental expression suggests a possible role in late stage differentiation. This characterization forms the basis of functional studies assessing how the expression of distinct calcium binding proteins might be regulated to compensate for the loss of one of the others.

Noninvasive imaging of the thirteen-lined ground squirrel photoreceptor mosaic

Ground squirrels are an increasingly important model for studying visual processing, retinal circuitry, and cone photoreceptor function. Here, we demonstrate that the photoreceptor mosaic can be longitudinally imaged noninvasively in the 13-lined ground squirrel (Ictidomys tridecemlineatus) using confocal and nonconfocal split-detection adaptive optics scanning ophthalmoscopy using 790 nm light. Photoreceptor density, spacing, and Voronoi analysis are consistent with that of the human cone mosaic. The high imaging success rate and consistent image quality in this study reinforce the ground squirrel as a practical model to aid drug discovery and testing through longitudinal imaging on the cellular scale.

Mechanism of High-Frequency Signaling at a Depressing Ribbon Synapse

Ribbon synapses mediate continuous release in neurons that have graded voltage responses. While mammalian retinas can signal visual flicker at 80-100 Hz, the time constant, τ, for the refilling of a depleted vesicle release pool at cone photoreceptor ribbons is 0.7-1.1 s. Due to this prolonged depression, the mechanism for encoding high temporal frequencies is unclear. To determine the mechanism of high-frequency signaling, we focused on an "Off" cone bipolar cell type in the ground squirrel, the cb2, whose transient postsynaptic responses recovered following presynaptic depletion with a τ of ∼0.1 s, or 7- to 10-fold faster than the τ for presynaptic pool refilling. The difference in recovery time course is caused by AMPA receptor saturation, where partial refilling of the presynaptic pool is sufficient for a full postsynaptic response. By limiting the dynamic range of the synapse, receptor saturation counteracts ribbon depression to produce rapid recovery and facilitate high-frequency signaling.

Ocular Stem Cell Research from Basic Science to Clinical Application: A Report from Zhongshan Ophthalmic Center Ocular Stem Cell Symposium

Stem cells hold promise for treating a wide variety of diseases, including degenerative disorders of the eye. The eye is an ideal organ for stem cell therapy because of its relative immunological privilege, surgical accessibility, and its being a self-contained system. The eye also has many potential target diseases amenable to stem cell-based treatment, such as corneal limbal stem cell deficiency, glaucoma, age-related macular degeneration (AMD), and retinitis pigmentosa (RP). Among them, AMD and glaucoma are the two most common diseases, affecting over 200 million people worldwide. Recent results on the clinical trial of retinal pigment epithelial (RPE) cells from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) in treating dry AMD and Stargardt’s disease in the US, Japan, England, and China have generated great excitement and hope. This marks the beginning of the ocular stem cell therapy era. The recent Zhongshan Ophthalmic Center Ocular Stem Cell Symposium discussed the potential applications of various stem cell types in stem cell-based therapies, drug discoveries and tissue engineering for treating ocular diseases.

Seasonal and post-trauma remodeling in cone-dominant ground squirrel retina

With a photoreceptor mosaic containing ∼85% cones, the ground squirrel is one of the richest known mammalian sources of these important retinal cells. It also has a visual ecology much like the human's. While the ground squirrel retina is understandably prominent in the cone biochemistry, physiology, and circuitry literature, far less is known about the remodeling potential of its retinal pigment epithelium, neurons, macroglia, or microglia. This review aims to summarize the data from ground squirrel retina to this point in time, and to relate them to data from other brain areas where appropriate. We begin with a survey of the ground squirrel visual system, making comparisons with traditional rodent models and with human. Because this animal's status as a hibernator often goes unnoticed in the vision literature, we then present a brief primer on hibernation biology. Next we review what is known about ground squirrel retinal remodeling concurrent with deep torpor and with rapid recovery upon re-warming. Notable here is rapidly-reversible, temperature-dependent structural plasticity of cone ribbon synapses, as well as pre- and post-synaptic plasticity throughout diverse brain regions. It is not yet clear if retinal cell types other than cones engage in torpor-associated synaptic remodeling. We end with the small but intriguing literature on the ground squirrel retina's remodeling responses to insult by retinal detachment. Notable for widespread loss of (cone) photoreceptors, there is surprisingly little remodeling of the RPE or Müller cells. Microglial activation appears minimal, and remodeling of surviving second- and third-order neurons seems absent, but both require further study. In contrast, traumatic brain injury in the ground squirrel elicits typical macroglial and microglial responses. Overall, the data to date strongly suggest a heretofore unrecognized, natural checkpoint between retinal deafferentiation and RPE and Müller cell remodeling events. As we continue to discover them, the unique ways by which ground squirrel retina responds to hibernation or injury may be adaptable to therapeutic use.

Fixation strategies for retinal immunohistochemistry

Immunohistochemical and ex vivo anatomical studies have provided many glimpses of the variety, distribution, and signaling components of vertebrate retinal neurons. The beauty of numerous images published to date, and the qualitative and quantitative information they provide, indicate that these approaches are fundamentally useful. However, obtaining these images entailed tissue handling and exposure to chemical solutions that differ from normal extracellular fluid in composition, temperature, and osmolarity. Because the differences are large enough to alter intercellular and intracellular signaling in neurons, and because retinae are susceptible to crush, shear, and fray, it is natural to wonder if immunohistochemical and anatomical methods disturb or damage the cells they are designed to examine. Tissue fixation is typically incorporated to guard against this damage and is therefore critically important to the quality and significance of the harvested data. Here, we describe mechanisms of fixation; advantages and disadvantages of using formaldehyde and glutaraldehyde as fixatives during immunohistochemistry; and modifications of widely used protocols that have recently been found to improve cell shape preservation and immunostaining patterns, especially in proximal retinal neurons.

Cone bipolar cells in the retina of the microbat Carollia perspicillata

We studied the retinal cone bipolar cells of Carollia perspicillata, a microchiropteran bat of the phyllostomid family. Microchiroptera are strongly nocturnal, with small eyes and rod-dominated retinae. However, they also possess a significant cone population (2-4%) comprising two spectral types, which are hence the basis for daylight and color vision. We used antibodies against the calcium-binding protein recoverin and the carbohydrate epitope 15 (CD15) as reliable markers for certain cone bipolar cells. Dye injections of recoverin- or CD15-prelabeled cone bipolar cells in vertical slices revealed the morphology of the axon terminal system of individual bipolar cells. Seven distinct cone bipolar cell types were identified. They differed in the morphology and stratification level of their axon terminal system in the inner plexiform layer and in immunoreactivity for recoverin and/or CD15. Additional immunocytochemical markers were used to assess the functional ON/OFF subdivision of the inner plexiform layer. In line with the extended thickness of the ON sublayer of the inner plexiform layer in the microbat retina, more ON than OFF cone bipolar cell types were found, namely, four versus three. Most likely, in the bats' predominantly dark environment, ON signals have greater importance for contrast perception. We conclude that the microbat retina conforms to the general mammalian blueprint, in which light signals of intensities above rod sensitivity are detected by cones and transmitted to various types of ON and OFF cone bipolar cells.

Short-wavelength cone-opponent retinal ganglion cells in mammals

In all of the mammalian species studied to date, the short-wavelength-sensitive (S) cones and the S-cone bipolar cells that receive their input are very similar, but the retinal ganglion cells that receive synapses from the S-cone bipolar cells appear to be quite different. Here, we review the literature on mammalian retinal ganglion cells that respond selectively to stimulation of S-cones and respond with opposite polarity to longer wavelength stimuli. There are at least three basic mechanisms to generate these color-opponent responses, including: (1) opponency is generated in the outer plexiform layer by horizontal cells and is conveyed to the ganglion cells via S-cone bipolar cells, (2) inputs from bipolar cells with different cone inputs and opposite response polarity converge directly on the ganglion cells, and (3) inputs from S-cone bipolar cells are inverted by S-cone amacrine cells. These are not mutually exclusive; some mammalian ganglion cells that respond selectively to S-cone stimulation seem to utilize at least two of them. Based on these findings, we suggest that the small bistratified ganglion cells described in primates are not the ancestral type, as proposed previously. Instead, the known types of ganglion cells in this pathway evolved from monostratified ancestral types and became bistratified in some mammalian lineages.

Kainate receptors mediate synaptic input to transient and sustained OFF visual pathways in primate retina

Visual signals are segregated into parallel pathways at the first synapse in the retina between cones and bipolar cells. Within the OFF pathways of mammals, the selective expression of AMPA or kainate-type glutamate receptors in the dendrites of different OFF-bipolar cell types is thought to contribute to formation of distinct temporal channels. AMPA receptors, with rapid recovery from desensitization, are proposed to transmit high temporal frequency signals, whereas kainate receptors (KARs) are presumed to encode lower temporal frequencies. Here we studied the glutamate receptors expressed by OFF-bipolar cells in slice preparations of macaque monkey retina, where the low (midget/parvocellular) and high-frequency (parasol/magnocellular) temporal channels are well characterized. We found that all OFF-bipolar types receive input primarily through KARs and that KAR antagonists block light-evoked input to both OFF-midget and OFF-parasol ganglion cells. KAR subunits were differentially expressed in OFF-bipolar types; the diffuse bipolar (DB) cells, DB2 and DB3b, expressed GluK1 and showed transient responses to glutamate and the KAR agonist, ATPA. In contrast, flat midget bipolar, DB1, and DB3a cells lacked GluK1 and showed relatively sustained responses. Finally, we found that the KAR accessory protein, Neto1, is expressed at the base of cone pedicles but is not colocalized with the GluK1 subunit. In summary, the results indicate that transient signaling in the OFF pathway of macaques is not dependent on AMPA receptors and that heterogeneity of KARs and accessory proteins may contribute to the formation of parallel temporal channels.

Receptive field properties of bipolar cell axon terminals in direction-selective sublaminas of the mouse retina

Retinal bipolar cells (BCs) transmit visual signals in parallel channels from the outer to the inner retina, where they provide glutamatergic inputs to specific networks of amacrine and ganglion cells. Intricate network computation at BC axon terminals has been proposed as a mechanism for complex network computation, such as direction selectivity, but direct knowledge of the receptive field property and the synaptic connectivity of the axon terminals of various BC types is required in order to understand the role of axonal computation by BCs. The present study tested the essential assumptions of the presynaptic model of direction selectivity at axon terminals of three functionally distinct BC types that ramify in the direction-selective strata of the mouse retina. Results from two-photon Ca(2+) imaging, optogenetic stimulation, and dual patch-clamp recording demonstrated that 1) CB5 cells do not receive fast GABAergic synaptic feedback from starburst amacrine cells (SACs); 2) light-evoked and spontaneous Ca(2+) responses are well coordinated among various local regions of CB5 axon terminals; 3) CB5 axon terminals are not directionally selective; 4) CB5 cells consist of two novel functional subtypes with distinct receptive field structures; 5) CB7 cells provide direct excitatory synaptic inputs to, but receive no direct GABAergic synaptic feedback from, SACs; and 6) CB7 axon terminals are not directionally selective, either. These findings help to simplify models of direction selectivity by ruling out complex computation at BC terminals. They also show that CB5 comprises two functional subclasses of BCs.

Kainate receptor subunit diversity underlying response diversity in retinal off bipolar cells

Postsynaptic kainate receptors mediate excitatory synaptic transmission over a broad range of temporal frequencies. In heterologous systems, the temporal responses of kainate receptors vary when different channel-forming and auxiliary subunits are co-expressed but how this variability relates to the temporal differences at central synapses is incompletely understood. The mammalian cone photoreceptor synapse provides advantages for comparing the different temporal signalling roles of kainate receptors, as cones release glutamate over a range of temporal frequencies, and three functionally distinct Off bipolar cell types receive cone signals at synapses that contain either AMPA or kainate receptors, all with different temporal properties. A disadvantage is that the different receptor subunits are not identified. We used in situ hybridization, immunocytochemistry, and pharmacology to identify the kainate receptor and auxiliary subunits in ground squirrel (Ictidomys tridecimlineatus) cb1a/b, cb2, and cb3a/b Off bipolar cell types. As expected, the types showed distinct subunit expression patterns. Kainate receptors mediated ∼80% of the synaptic response in cb3a/b cells and were heteromers of GluK1 and GluK5. Cb3a/b cells contained message for GluK1 and GluK5, and also GluK3 and the auxiliary subunit Neto1. The synaptic responses in cb1a/b cells were mediated by GluK1-containing kainate receptors that behaved differently from the receptors expressed by cb3a/b cells. AMPA receptors mediated the entire synaptic response in cb2 cells and the remaining synaptic response in cb3a/b cells. We conclude that GluK1 is the predominant kainate receptor subunit in cb1 and cb3 Off bipolar cells. Different temporal response properties may result from selective association with GluK3, GluK5, or Neto1.

Three distinct blue-green color pathways in a mammalian retina

In mammalian retinae, the first steps in the process of discrimination of color are mediated by color-opponent neurons that respond with opposite polarity to signals from short (S, blue) and longer wavelength (M, green or L, red) cones. Primates also contain a second system that is different from M and L cones. Although pathways responding to the onset of S-cone stimulation (S-ON) are well known, the existence of bipolar cells and retinal ganglion cells that respond to the offset of S-cone stimulation (S-OFF) has been controversial. We have recorded from and stained three different types of S/M color-opponent ganglion cells in the rabbit retina that are distinguished by the polarity of their responses to S-cone stimulation, the stratification pattern of their dendrites, and the distinct mechanisms underlying their color-opponent responses. We describe an S-ON and an S-OFF pathway formed by amacrine cells inverting the S-ON signal. Most importantly, we also provide both anatomical and physiological evidence for a direct S-OFF pathway dependent on an S-OFF cone bipolar cell. The results indicate a greater diversity of pathways for processing of signals from S-cones than previously suspected.

Cre-mediated recombination efficiency and transgene expression patterns of three retinal bipolar cell-expressing Cre transgenic mouse lines

PURPOSE

Retinal bipolar cells, comprising multiple types, play an essential role in segregating visual information into multiple parallel pathways in the retina. The ability to manipulate gene expression in specific bipolar cell type(s) in the retina is important for understanding the molecular basis of their normal physiological functions and diseases/disorders. The Cre/LoxP recombination system has become an important tool for allowing gene manipulation in vivo, especially with the increasing availability of cell- and tissue-specific Cre transgenic mouse lines. Detailed in vivo examination of the Cre/LoxP recombination efficiency and the transgene expression patterns for cell- and tissue-specific Cre transgenic mouse lines is essential for evaluating their utility. In this study, we investigated the Cre-mediated recombination efficiency and transgene expression patterns of retinal bipolar cell-expressing Cre transgenic lines by crossing with a Cre reporter mouse line and through Cre-dependent recombinant adeno-associated virus (rAAV) vector-mediated transgene delivery.

METHODS

Three retinal bipolar cell-expressing Cre-transgenic mouse lines, 5-HTR2a-cre, Pcp2-cre, and Chx10-cre, were crossed with a strong Cre reporter mouse line that expresses a red fluorescent protein variant, tdTomato. rAAV2 vectors carrying a double-floxed inverted open-reading frame sequence encoding channelrhodopsin-2-mCherry (ChR2-mCherry) driven by a ubiquitous neuronal EF1α or a ubiquitous CMV promoter with a rAAV2 capsid mutation (Y444F) were injected into the intravitreal space of the eyes. Immunohistochemistry using retinal bipolar cell type-specific markers was performed to examine Cre-mediated recombination efficiency and the transgene expression patterns in bipolar cells in retinal whole mounts and vertical sections.

RESULTS

For the 5-HTR2a-cre and Pcp2-cre mouse lines, the expression pattern of the Cre-mediated recombination by crossing the reporter line largely resembled the expression pattern of Cre. The bipolar cells showing Cre-mediated recombination in the 5-HTR2a-cre line and the Pcp2-cre line were predominantly type 4 cone bipolar cells and rod bipolar cells, respectively. For the Chx10-cre mouse line, the expression pattern of the Cre-mediated recombination by crossing the reporter line was different from that of Cre. The Cre-mediated transgene expression in retinal bipolar cells in the Chx10-cre line was not observed by crossing with the reporter mouse line but through Cre-dependent rAAV vector delivery. A rAAV2 vector with the combination of a CMV promoter and the Y444F capsid mutation achieved Cre-dependent transgene expression in retinal bipolar cells.

CONCLUSIONS

The retinal bipolar cell-expressing Cre-transgenic lines and the Cre-dependent rAAV vector reported in this study could be valuable tools for gene targeting and manipulation in retinal bipolar cells in mice.

OFF bipolar cells express distinct types of dendritic glutamate receptors in the mouse retina

Parallel representations of the visual world are already established at the very first synapse of the visual system. Cone photoreceptors, which hyperpolarize in response to light, forward the visual signal onto distinct types of ON and OFF cone bipolar cells (BCs). In the case of OFF BCs, the glutamatergic cone input is integrated by ionotropic glutamate receptors, giving rise to a sign-preserving mode of synaptic transmission. The combination of glutamate receptor (GluR) subunits, i.e. AMPA or kainate subunits, importantly contributes to shaping the OFF bipolar cells' distinct response properties. The mouse is one of the few mammals in which the (most likely) complete set of (five) retinal OFF BC types is identified. However, it is not clear which GluR subtypes are expressed by the different mouse OFF BC types. We addressed this question by combining immunolabeling, electrical whole-cell recordings and pharmacology, and present evidence that the different types of OFF BCs express distinct types of glutamate receptors: Type 1 BCs exclusively expressed AMPA receptors, whereas type 2 and type 3a BCs expressed kainate receptors of different subunit compositions. Additionally, we found that two OFF BC types (3b and 4) very likely express both AMPA and kainate receptors but, interestingly, the two receptor subunits were not co-localized at the same dendritic site. The complex, BC type-specific expression pattern of GluRs we describe here supports their essential role in establishing parallel pathways at the first synapse of the mouse visual system.