The spatial distribution of glutamatergic inputs to dendrites of retinal ganglion cells

A presynaptic source drives differing levels of surround suppression in two mouse retinal ganglion cell types

In early sensory systems, cell-type diversity generally increases from the periphery into the brain, resulting in a greater heterogeneity of responses to the same stimuli. Surround suppression is a canonical visual computation that begins within the retina and is found at varying levels across retinal ganglion cell types. Our results show that heterogeneity in the level of surround suppression occurs subcellularly at bipolar cell synapses. Using single-cell electrophysiology and serial block-face scanning electron microscopy, we show that two retinal ganglion cell types exhibit very different levels of surround suppression even though they receive input from the same bipolar cell types. This divergence of the bipolar cell signal occurs through synapse-specific regulation by amacrine cells at the scale of tens of microns. These findings indicate that each synapse of a single bipolar cell can carry a unique visual signal, expanding the number of possible functional channels at the earliest stages of visual processing.

Degenerated Cones in Cultured Human Retinas Can Successfully Be Optogenetically Reactivated

Biblical references aside, restoring vision to the blind has proven to be a major technical challenge. In recent years, considerable advances have been made towards this end, especially when retinal degeneration underlies the vision loss such as occurs with retinitis pigmentosa. Under these conditions, optogenetic therapies are a particularly promising line of inquiry where remaining retinal cells are made into "artificial photoreceptors". However, this strategy is not without its challenges and a model system using human retinal explants would aid its continued development and refinement. Here, we cultured post-mortem human retinas and show that explants remain viable for around 7 days. Within this period, the cones lose their outer segments and thus their light sensitivity but remain electrophysiologically intact, displaying all the major ionic conductances one would expect for a vertebrate cone. We optogenetically restored light responses to these quiescent cones using a lentivirus vector constructed to express enhanced halorhodopsin under the control of the human arrestin promotor. In these 'reactivated' retinas, we show a light-induced horizontal cell to cone feedback signal in cones, indicating that transduced cones were able to transmit their light response across the synapse to horizontal cells, which generated a large enough response to send a signal back to the cones. Furthermore, we show ganglion cell light responses, suggesting the cultured explant's condition is still good enough to support transmission of the transduced cone signal over the intermediate retinal layers to the final retinal output level. Together, these results show that cultured human retinas are an appropriate model system to test optogenetic vision restoration approaches and that cones which have lost their outer segment, a condition occurring during the early stages of retinitis pigmentosa, are appropriate targets for optogenetic vision restoration therapies.

Particle-Mediated Gene Transfection and Organotypic Culture of Postmortem Human Retina

Purpose

Particle-mediated gene transfer has been used in animal models to study the morphology and connectivity of retinal ganglion cells. The aim of the present study was to apply this method to transfect ganglion cells in postmortem human retina.

Methods

Postmortem human eyes from male and female donors aged 40 to 76 years old were obtained within 15 hours after death. In addition, two marmoset retinas were obtained immediately after death. Ganglion cells were transfected with an expression plasmid for the postsynaptic density 95 protein conjugated to green or yellow fluorescent protein. Retinas were cultured for 3 days, fixed and then processed with immunohistochemical markers to reveal their stratification in the inner plexiform layer.

Results

The retinas maintained their morphology and immunohistochemical properties for at least 3 days in culture. Bipolar and ganglion cell morphology was comparable to that observed in noncultured tissue. The quality of transfected cells in human retina was similar to that in freshly enucleated marmoset eyes. Based on dendritic field size and stratification, at least 11 morphological types of retinal ganglion cell were distinguished.

Conclusions

Particle-mediated gene transfer allows efficient targeting of retinal ganglion cells in cultured postmortem human retina.

Translational Relevance

The translational value of this methodology lies in the provision of an in vitro platform to study structural and connectivity changes in human eye diseases that affect the integrity and organization of cells in the retina.

Brn3a and Brn3b knockout mice display unvaried retinal fine structure despite major morphological and numerical alterations of ganglion cells

Ganglion cells (GCs), the retinal output neurons, receive synaptic inputs from bipolar and amacrine cells in the inner plexiform layer (IPL) and send information to the brain nuclei via the optic nerve. Although GCs constitute less than 1% of the total retinal cells, they occur in numerous types and are the first neurons formed during retinal development. Using Brn3a and Brn3b mutant mice in which the alkaline phosphatase gene was knocked-in (Badea et al. [Neuron] 2009;61:852-864; Badea and Nathans [Vision Res] 2011;51:269-279), we studied the general effects after gene removal on the retinal neuropil together with the consequences of lack of development of large numbers of GCs onto the remaining retinal neurons of the same class. We analyzed the morphology, number, and general architecture of various neuronal types presynaptic to GCs, searching for changes secondary to the decrement in the number of their postsynaptic partners, as well as the morphology and distribution of retinal astrocytes, for their strong topographical relation to GCs. We found that, despite GC losses, retinal organization in Brn3 null mice is remarkably similar to that of wild-type controls. J. Comp. Neurol. 527:187-211, 2019. © 2016 Wiley Periodicals, Inc.

A Self-Regulating Gap Junction Network of Amacrine Cells Controls Nitric Oxide Release in the Retina

Neuromodulators regulate circuits throughout the nervous system, and revealing the cell types and stimulus conditions controlling their release is vital to understanding their function. The effects of the neuromodulator nitric oxide (NO) have been studied in many circuits, including in the vertebrate retina, where it regulates synaptic release, gap junction coupling, and blood vessel dilation, but little is known about the cells that release NO. We show that a single type of amacrine cell (AC) controls NO release in the inner retina, and we report its light responses, electrical properties, and calcium dynamics. We discover that this AC forms a dense gap junction network and that the strength of electrical coupling in the network is regulated by light through NO. A model of the network offers insights into the biophysical specializations leading to auto-regulation of NO release within the network.

Neural Mechanisms of Motion Processing in the Mammalian Retina

Visual motion on the retina activates a cohort of retinal ganglion cells (RGCs). This population activity encodes multiple streams of information extracted by parallel retinal circuits. Motion processing in the retina is best studied in the direction-selective circuit. The main focus of this review is the neural basis of direction selectivity, which has been investigated in unprecedented detail using state-of-the-art functional, connectomic, and modeling methods. Mechanisms underlying the encoding of other motion features by broader RGC populations are also discussed. Recent discoveries at both single-cell and population levels highlight the dynamic and stimulus-dependent engagement of multiple mechanisms that collectively implement robust motion detection under diverse visual conditions.

Cellular and Molecular Analysis of Dendritic Morphogenesis in a Retinal Cell Type That Senses Color Contrast and Ventral Motion

As neuronal dendrites develop, they acquire cell-type-specific features including characteristic size, shape, arborization, location and synaptic patterns. These features, in turn, are major determinants of type-specific neuronal function. Because neuronal diversity complicates the task of relating developmental programs to adult structure and function, we analyzed dendritic morphogenesis in a single retinal ganglion cell (RGC) type in mouse called J-RGC. We documented the emergence of five dendritic features that underlie J-RGC physiology: (1) dendritic field size, which approximate receptive field size; (2) dendritic complexity, which affects how J-RGCs sample space; (3) asymmetry, which contributes to direction-selectivity; (4) restricted lamination within the inner plexiform layer (IPL), which renders J-RGCs responsive to light decrements; and (5) distribution of synaptic inputs, which generate a color-opponent receptive field. We found dendritic growth in J-RGCs is accompanied by a refinement in dendritic self-crossing. Asymmetry arises by a combination of selective pruning and elaboration, whereas laminar restriction results from biased outgrowth toward the outermost IPL. Interestingly, asymmetry develops in a protracted dorsoventral wave, whereas lamination does so in a rapid centrifugal wave. As arbors mature, they acquire excitatory and inhibitory synapses, with the latter forming first and being concentrated in proximal dendrites. Thus, distinct mechanisms operate in different spatiotemporal dimensions of J-RGC dendritic patterning to generate the substrate for specific patterns of synaptogenesis. Finally, we asked whether the defining molecular signature of J-RGCs, the adhesion molecule JAM-B, regulates morphogenesis, and showed that it promotes dendro-dendritic interactions. Our results reveal multiple mechanisms that shape a dendritic arbor.SIGNIFICANCE STATEMENT Visual perception begins in the retina, where distinct types of retinal ganglion cells (RGCs) are tuned to specific visual features such as direction of motion. The features to which each RGC type responds are determined largely by the number and type of synaptic inputs it receives, and these, in turn, are greatly influenced by the size, shape, arborization pattern, and location of its dendrites. We analyzed dendritic morphogenesis in a functionally characterized RGC type, the J-RGC, demonstrating distinct mechanisms that operate in different dimensions to generate the dendritic scaffold and synaptic patterns for feature detection. Our work elucidates cellular and molecular mechanisms that shape dendritic arbors and synaptic distribution, enabling J-RGC connectivity and thus, function.

Identification of synaptic pattern of NMDA receptor subunits upon direction-selective retinal ganglion cells in developing and adult mouse retina

Direction selectivity of the retina is a unique mechanism and critical function of eyes for surviving. Direction-selective retinal ganglion cells (DS RGCs) strongly respond to preferred directional stimuli, but rarely respond to the opposite or null directional stimuli. These DS RGCs are sensitive to glutamate, which is secreted from bipolar cells. Using immunocytochemistry, we studied with the distributions of N-methyl-d-aspartate (NMDA) receptor subunits on the dendrites of DS RGCs in the developing and adult mouse retina. DS RGCs were injected with Lucifer yellow for identification of dendritic morphology. The triple-labeled images of dendrites, kinesin II, and NMDA receptor subunits were visualized using confocal microscopy and were reconstructed from high-resolution confocal images. Although our results revealed that the synaptic pattern of NMDA receptor subunits on dendrites of DS RGCs was not asymmetric in developing and adult mouse retina, they showed the anatomical connectivity of NMDA glutamatergic synapses onto DS RGCs and the developmental formation of the direction selectivity in the mouse retina. Through the comprehensive interpretation of the direction-selective neural circuit, this study, therefore, implies that the direction selectivity may be generated by the asymmetry of the excitatory glutamatergic inputs and the inhibitory inputs onto DS RGCs.

Survey of retinal ganglion cell morphology in marmoset

In primate retina, the midget, parasol, and small bistratified cell populations form the large majority of ganglion cells. In addition, there is a variety of low-density wide-field ganglion cell types that are less well characterized. Here we studied retinal ganglion cells in the common marmoset, Callithrix jacchus, using particle-mediated gene transfer. Ganglion cells were transfected with an expression plasmid for the postsynaptic density 95-green fluorescent protein. The retinas were processed with established immunohistochemical markers for bipolar and/or amacrine cells to determine ganglion cell dendritic stratification. In total over 500 ganglion cells were classified based on their dendritic field size, morphology, and stratification in the inner plexiform layer. Over 17 types were distinguished, including midget, parasol, broad thorny, small bistratified, large bistratified, recursive bistratified, recursive monostratified, narrow thorny, smooth monostratified, large sparse, giant sparse (melanopsin) ganglion cells, and a group that may contain several as yet uncharacterized types. Assuming each characterized type forms a hexagonal mosaic, the midget and parasol cells account for over 80% of all ganglion cells in the central retina but only ∼50% of cells in the peripheral (>2 mm) retina. We conclude that the fovea is dominated by midget and parasol cells, but outside the fovea the ganglion cell diversity in marmoset is likely as great as that reported for nonprimate retinas. Taken together, the ganglion cell types in marmoset retina resemble those described previously in macaque retina with respect to morphology, stratification, and change in proportion across the retina.

Changes in ganglion cells during retinal degeneration

Inherited retinal degeneration such as retinitis pigmentosa (RP) is associated with photoreceptor loss and concomitant morphological and functional changes in the inner retina. It is not known whether these changes are associated with changes in the density and distribution of synaptic inputs to retinal ganglion cells (RGCs). We quantified changes in ganglion cell density in rd1 and age-matched C57BL/6J-(wildtype, WT) mice using the immunocytochemical marker, RBPMS. Our data revealed that following complete loss of photoreceptors, (∼3months of age), there was a reduction in ganglion cell density in the peripheral retina. We next examined changes in synaptic inputs to A type ganglion cells by performing double labeling experiments in mice with the ganglion cell reporter lines, rd1-Thy1 and age-matched wildtype-Thy1. Ribbon synapses were identified by co-labelling with CtBP2 (RIBEYE) and conventional synapses with the clustering molecule, gephyrin. ON RGCs showed a significant reduction in RIBEYE-immunoreactive synapse density while OFF RGCs showed a significant reduction in the gephyrin-immmunoreactive synapse density. Distribution patterns of both synaptic markers across the dendritic trees of RGCs were unchanged. The change in synaptic inputs to RGCs was associated with a reduction in the number of immunolabeled rod bipolar and ON cone bipolar cells. These results suggest that functional changes reported in ganglion cells during retinal degeneration could be attributed to loss of synaptic inputs.

Dendritic mitochondria reach stable positions during circuit development

Mitochondria move throughout neuronal dendrites and localize to sites of energy demand. The prevailing view of dendritic mitochondria as highly motile organelles whose distribution is continually adjusted by neuronal activity via Ca(2+)-dependent arrests is based on observations in cultured neurons exposed to artificial stimuli. Here, we analyze the movements of mitochondria in ganglion cell dendrites in the intact retina. We find that whereas during development 30% of mitochondria are motile at any time, as dendrites mature, mitochondria all but stop moving and localize stably to synapses and branch points. Neither spontaneous nor sensory-evoked activity and Ca(2+) transients alter motility of dendritic mitochondria; and pathological hyperactivity in a mouse model of retinal degeneration elevates rather than reduces motility. Thus, our findings indicate that dendritic mitochondria reach stable positions during a critical developmental period of high motility, and challenge current views about the role of activity in regulating mitochondrial transport in dendrites.

Synapse Loss and Dendrite Remodeling in a Mouse Model of Glaucoma

It has been hypothesized that synaptic pruning precedes retinal ganglion cell degeneration in glaucoma, causing early dysfunction to retinal ganglion cells. To begin to assess this, we studied the excitatory synaptic inputs to individual ganglion cells in normal mouse retinas and in retinas with ganglion cell degeneration from glaucoma (DBA/2J), or following an optic nerve crush. Excitatory synapses were labeled by AAV2-mediated transfection of ganglion cells with PSD-95-GFP. After both insults the linear density of synaptic inputs to ganglion cells decreased. In parallel, the dendritic arbors lost complexity. We did not observe any cells that had lost dendritic synaptic input while preserving a normal or near-normal morphology. Within the temporal limits of these observations, dendritic remodeling and synapse pruning thus appear to occur near-simultaneously.

Connectivity between the OFF bipolar type DB3a and six types of ganglion cell in the marmoset retina

Parallel visual pathways originate at the first synapse in the retina, where cones make connections with cone bipolar cells that in turn contact ganglion cells. There are more ganglion cell types than bipolar types, suggesting that there must be divergence from bipolar to ganglion cells. Here we analyze the contacts between an OFF bipolar type (DB3a) and six ganglion cell types in the retina of the marmoset monkey (Callithrix jacchus). Ganglion cells were transfected via particle-mediated gene transfer of an expression plasmid for the postsynaptic density 95-green fluorescent protein (PSD95-GFP), and DB3a cells were labeled via immunohistochemistry. Ganglion cell types that fully or partially costratified with DB3a cells included OFF parasol, OFF midget, broad thorny, recursive bistratified, small bistratified, and large bistratified cells. On average, the number of DB3a contacts to parasol cells (18 contacts per axon terminal) is higher than that to other ganglion cell types (between four and seven contacts). We estimate that the DB3a output to OFF parasol cells accounts for at least 30% of the total DB3a output. Furthermore, we found that OFF parasol cells receive approximately 20% of their total bipolar input from DB3a cells, suggesting that other diffuse bipolar types also provide input to OFF parasol cells. We conclude that DB3a cells preferentially contact OFF parasol cells but also provide input to other ganglion cell types.

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.

General features of the retinal connectome determine the computation of motion anticipation

Motion anticipation allows the visual system to compensate for the slow speed of phototransduction so that a moving object can be accurately located. This correction is already present in the signal that ganglion cells send from the retina but the biophysical mechanisms underlying this computation are not known. Here we demonstrate that motion anticipation is computed autonomously within the dendritic tree of each ganglion cell and relies on feedforward inhibition. The passive and non-linear interaction of excitatory and inhibitory synapses enables the somatic voltage to encode the actual position of a moving object instead of its delayed representation. General rather than specific features of the retinal connectome govern this computation: an excess of inhibitory inputs over excitatory, with both being randomly distributed, allows tracking of all directions of motion, while the average distance between inputs determines the object velocities that can be compensated for.

DOI:http://dx.doi.org/10.7554/eLife.06250.001

The retina is a structure at the back of the eye that converts light into nerve impulses, which are then processed in the brain to produce the images that we see. It normally takes about one-tenth of a second for the retina to send a signal to the brain after an object first moves into view. This is about the same time it takes a tennis ball to travel several meters during a tennis match, yet we are still able to see where the moving tennis ball is in real time. This is because a process called ‘motion anticipation’ is able to compensate for the delay in processing the position of a moving object. However, it was not known precisely how motion anticipation occurs.

Inside the retina, cells called photoreceptors detect light and ultimately send signals (via some intermediate cell types) to nerve cells known as retinal ganglion cells. These signals can either excite a retinal ganglion cell to cause it to send an electrical signal to the brain, or inhibit it, which temporarily prevents electrical activity. Each cell receives signals from several photoreceptors, which each connect to a different site along branch-like structures called dendrites that project out of the retinal ganglion cells.

Johnston and Lagnado have now investigated how motion anticipation occurs in the retina by using electrical recordings of the activity in the retinas of goldfish combined with computer simulations of this activity. This revealed inhibitory signals, sent from photoreceptors to retinal ganglion cells via a type of intermediate cell (called amacrine cells), play a key role in motion anticipation. The ability to track motion effectively in all directions requires more inhibitory signals to be sent to the dendrites of a retinal ganglion cell than excitatory signals. These two types of input must also be randomly distributed across the cell. Furthermore, it is the density of these input sites on a dendrite that determines how well the retina can compensate for the motion of a fast-moving object. The building blocks required for motion anticipation in the retina are also found in visual areas higher in the brain. Therefore, further work may reveal that higher visual areas also use this mechanism to predict the future location of moving objects.

DOI:http://dx.doi.org/10.7554/eLife.06250.002

Short-term increases in transient receptor potential vanilloid-1 mediate stress-induced enhancement of neuronal excitation

Progression of neurodegeneration in disease and injury is influenced by the response of individual neurons to stressful stimuli and whether this response includes mechanisms to counter declining function. Transient receptor potential (TRP) cation channels transduce a variety of disease-relevant stimuli and can mediate diverse stress-dependent changes in physiology, both presynaptic and postsynaptic. Recently, we demonstrated that knock-out or pharmacological inhibition of the TRP vanilloid-1 (TRPV1) capsaicin-sensitive subunit accelerates degeneration of retinal ganglion cell neurons and their axons with elevated ocular pressure, the critical stressor in the most common optic neuropathy, glaucoma. Here we probed the mechanism of the influence of TRPV1 on ganglion cell survival in mouse models of glaucoma. We found that induced elevations of ocular pressure increased TRPV1 in ganglion cells and its colocalization at excitatory synapses to their dendrites, whereas chronic elevation progressively increased ganglion cell Trpv1 mRNA. Enhanced TRPV1 expression in ganglion cells was transient and supported a reversal of the effect of TRPV1 on ganglion cells from hyperpolarizing to depolarizing, which was also transient. Short-term enhancement of TRPV1-mediated activity led to a delayed increase in axonal spontaneous excitation that was absent in ganglion cells from Trpv1(-/-) retina. In isolated ganglion cells, pharmacologically activated TRPV1 mobilized to discrete nodes along ganglion cell dendrites that corresponded to sites of elevated Ca(2+). These results suggest that TRPV1 may promote retinal ganglion cell survival through transient enhancement of local excitation and axonal activity in response to ocular stress.

The effect of photoreceptor degeneration on ganglion cell morphology

Retinitis pigmentosa refers to a family of inherited photoreceptor degenerations resulting in blindness. During and after photoreceptor loss, neurons of the inner retina are known to undergo plastic changes. Here, we have investigated in detail whether ganglion cells are altered at late stages of degeneration, well after the total loss of photoreceptors. We used mice, rd1-Thy1, that carry a mutation in the β-subunit of phosphodiesterase 6 and a fluorescent protein that labels a subset of ganglion cells and B6-Thy1 control mice. Retinal wholemounts from mice aged 3-11 months were processed for immunohistochemistry and analyzed. Ganglion cells were classified based on soma area, dendritic field size, and branching of dendrites. The dendritic fields of some ganglion cells were further analyzed for their length, area and quantity of branching points. There was a decrease in size and level of branching of A2, B1, and D type ganglion cells in the degenerated retina at 11 months of age. In contrast, C1 ganglion cells remained unchanged. In addition, there was a shift in the proportion of ganglion cells ramifying in the different layers of the inner plexiform layer. Careful analysis of the dendrites of ganglion cells revealed some projecting to new, more distal regions of the inner plexiform layer. We propose that these changes in ganglion cell morphology could impact the function of individual cells as well as the retinal circuitry in the degenerated retina.

Retrograde plasticity and differential competition of bipolar cell dendrites and axons in the developing retina

Most neurons function in the context of pathways that process and propagate information through a series of stages, e.g., from the sensory periphery to cerebral cortex. Because activity at each stage of a neural pathway depends on connectivity at the preceding one, we hypothesized that during development, axonal output of a neuron may regulate synaptic development of its dendrites (i.e., retrograde plasticity). Within pathways, neurons often receive input from multiple partners and provide output to targets shared with other neurons (i.e., convergence). Converging axons can intermingle or occupy separate territories on target dendrites. Activity-dependent competition has been shown to bias target innervation by overlapping axons in several systems. By contrast, whether territorial axons or dendrites compete for targets and inputs, respectively, has not been tested. Here, we generate transgenic mice in which glutamate release from specific sets of retinal bipolar cells (BCs) is suppressed. We find that dendrites of silenced BCs recruit fewer inputs when their neighbors are active and that dendrites of active BCs recruit more inputs when their neighbors are silenced than either active or silenced BCs with equal neighbors. By contrast, axons of silenced BCs form fewer synapses with their targets, irrespective of the activity of their neighbors. These findings reveal that retrograde plasticity guides BC dendritic development in vivo and demonstrate that dendrites, but not territorial axons, in a convergent neural pathway engage in activity-dependent competition. We propose that at a population level, retrograde plasticity serves to maximize functional representation of inputs.

The retinal projectome reveals brain-area-specific visual representations generated by ganglion cell diversity

BACKGROUND

Visual information is transmitted to the vertebrate brain exclusively via the axons of retinal ganglion cells (RGCs). The functional diversity of RGCs generates multiple representations of the visual environment that are transmitted to several brain areas. However, in no vertebrate species has a complete wiring diagram of RGC axonal projections been constructed. We employed sparse genetic labeling and in vivo imaging of the larval zebrafish to generate a cellular-resolution map of projections from the retina to the brain.

RESULTS

Our data define 20 stereotyped axonal projection patterns, the majority of which innervate multiple brain areas. Morphometric analysis of pre- and postsynaptic RGC structure revealed more than 50 structural RGC types with unique combinations of dendritic and axonal morphologies, exceeding current estimates of RGC diversity in vertebrates. These single-cell projection mapping data indicate that specific projection patterns are nonuniformly specified in the retina to generate retinotopically biased visual maps throughout the brain. The retinal projectome also successfully predicted a functional subdivision of the pretectum.

CONCLUSIONS

Our data indicate that RGC projection patterns are precisely coordinated to generate brain-area-specific visual representations originating from RGCs with distinct dendritic morphologies and topographic distributions.

Interplay of cell-autonomous and nonautonomous mechanisms tailors synaptic connectivity of converging axons in vivo

Neurons receive input from diverse afferents but form stereotypic connections with each axon type to execute their precise functions. Developmental mechanisms that specify the connectivity of individual axons across populations of converging afferents are not well-understood. Here, we untangled the contributions of activity-dependent and independent interactions that regulate the connectivity of afferents providing major and minor input onto a neuron. Individual transmission-deficient retinal bipolar cells (BCs) reduced synapses with retinal ganglion cells (RGCs), but active BCs of the same type sharing the dendrite surprisingly did not compensate for this loss. Genetic ablation of some BC neighbors resulted in increased synaptogenesis by the remaining axons in a transmission-independent manner. Presence, but not transmission, of the major BC input also dissuades wiring with the minor input and with synaptically compatible but functionally mismatched afferents. Cell-autonomous, activity-dependent and nonautonomous, activity-independent mechanisms thus together tailor connectivity of individual axons among converging inner retinal afferents.

Identification of a pathway from the retina to koniocellular layer K1 in the lateral geniculate nucleus of marmoset

Three well characterized pathways in primate vision (midget-parvocellular, parasol-magnocellular, bistratified-koniocellular) have been traced from the first synapse in the retina, through the visual thalamus (lateral geniculate nucleus, LGN), to the visual cortex. Here we identify a pathway from the first synapse in the retina to koniocellular layer K1 in marmoset monkeys (Callithrix jacchus). Particle-mediated gene transfer of an expression plasmid for the postsynaptic density 95-green fluorescent protein (PSD95-GFP) was used to label excitatory synapses on retinal ganglion cells and combined with immunofluorescence to identify the presynaptic bipolar cells. We found that axon terminals of one type of diffuse bipolar cell (DB6) provide dominant synaptic input to the dendrites of narrow thorny ganglion cells. Retrograde tracer injections into the LGN and photofilling of retinal ganglion cells showed that narrow thorny cells were preferentially labeled when koniocellular layer K1 was targeted. Layer K1 contains cells with high sensitivity for rapid movement, and layer K1 sends projections to association visual areas as well as to primary visual cortex. We hypothesize that the DB6-narrow thorny-koniocellular pathway contributes to residual visual functions ("blindsight") that survive injury to primary visual cortex in adult or early life.

A general principle governs vision-dependent dendritic patterning of retinal ganglion cells

Dendritic arbors of retinal ganglion cells (RGCs) collect information over a certain area of the visual scene. The coverage territory and the arbor density of dendrites determine what fraction of the visual field is sampled by a single cell and at what resolution. However, it is not clear whether visual stimulation is required for the establishment of branching patterns of RGCs, and whether a general principle directs the dendritic patterning of diverse RGCs. By analyzing the geometric structures of RGC dendrites, we found that dendritic arbors of RGCs underwent a substantial spatial rearrangement after eye-opening. Light deprivation blocked both the dendritic growth and the branch patterning, suggesting that visual stimulation is required for the acquisition of specific branching patterns of RGCs. We further showed that vision-dependent dendritic growth and arbor refinement occurred mainly in the middle portion of the dendritic tree. This nonproportional growth and selective refinement suggest that the late-stage dendritic development of RGCs is not a passive stretching with the growth of eyes, but rather an active process of selective growth/elimination of dendritic arbors of RGCs driven by visual activity. Finally, our data showed that there was a power law relationship between the coverage territory and dendritic arbor density of RGCs on a cell-by-cell basis. RGCs were systematically less dense when they cover larger territories regardless of their cell type, retinal location, or developmental stage. These results suggest that a general structural design principle directs the vision-dependent patterning of RGC dendrites.

Spatial distribution of excitatory synapses on the dendrites of ganglion cells in the mouse retina

Excitatory glutamatergic inputs from bipolar cells affect the physiological properties of ganglion cells in the mammalian retina. The spatial distribution of these excitatory synapses on the dendrites of retinal ganglion cells thus may shape their distinct functions. To visualize the spatial pattern of excitatory glutamatergic input into the ganglion cells in the mouse retina, particle-mediated gene transfer of plasmids expressing postsynaptic density 95-green fluorescent fusion protein (PSD95-GFP) was used to label the excitatory synapses. Despite wide variation in the size and morphology of the retinal ganglion cells, the expression of PSD95 puncta was found to follow two general rules. Firstly, the PSD95 puncta are regularly spaced, at 1–2 µm intervals, along the dendrites, whereby the presence of an excitatory synapse creates an exclusion zone that rules out the presence of other glutamatergic synaptic inputs. Secondly, the spatial distribution of PSD95 puncta on the dendrites of diverse retinal ganglion cells are similar in that the number of excitatory synapses appears to be less on primary dendrites and to increase to a plateau on higher branch order dendrites. These observations suggest that synaptogenesis is spatially regulated along the dendritic segments and that the number of synaptic contacts is relatively constant beyond the primary dendrites. Interestingly, we also found that the linear puncta density is slightly higher in large cells than in small cells. This may suggest that retinal ganglion cells with a large dendritic field tend to show an increased connectivity of excitatory synapses that makes up for their reduced dendrite density. Mapping the spatial distribution pattern of the excitatory synapses on retinal ganglion cells thus provides explicit structural information that is essential for our understanding of how excitatory glutamatergic inputs shape neuronal responses.

Spatial relationships between GABAergic and glutamatergic synapses on the dendrites of distinct types of mouse retinal ganglion cells across development

Neuronal output requires a concerted balance between excitatory and inhibitory (I/E) input. Like other circuits, inhibitory synaptogenesis in the retina precedes excitatory synaptogenesis. How then do neurons attain their mature balance of I/E ratios despite temporal offset in synaptogenesis? To directly compare the development of glutamatergic and GABAergic synapses onto the same cell, we biolistically transfected retinal ganglion cells (RGCs) with PSD95CFP, a marker of glutamatergic postsynaptic sites, in transgenic Thy1­YFPγ2 mice in which GABA_A receptors are fluorescently tagged. We mapped YFPγ2 and PSD95CFP puncta distributions on three RGC types at postnatal day P12, shortly before eye opening, and at P21 when robust light responses in RGCs are present. The mature I_GABA/E ratios varied among ON-Sustained (S) A-type, OFF-S A-type, and bistratified direction selective (DS) RGCs. These ratios were attained at different rates, before eye-opening for ON-S and OFF-S A-type, and after eye-opening for DS RGCs. At both ages examined, the I_GABA/E ratio was uniform across the arbors of the three RGC types. Furthermore, measurements of the distances between neighboring PSD95CFP and YFPγ2 puncta on RGC dendrites indicate that their local relationship is established early in development, and cannot be predicted by random organization. These close spatial associations between glutamatergic and GABAergic postsynaptic sites appear to represent local synaptic arrangements revealed by correlative light and EM reconstructions of a single RGC's dendrites. Thus, although RGC types have different I_GABA/E ratios and establish these ratios at separate rates, the local relationship between excitatory and inhibitory inputs appear similarly constrained across the RGC types studied.

Directional summation in non-direction selective retinal ganglion cells

Retinal ganglion cells receive inputs from multiple bipolar cells which must be integrated before a decision to fire is made. Theoretical studies have provided clues about how this integration is accomplished but have not directly determined the rules regulating summation of closely timed inputs along single or multiple dendrites. Here we have examined dendritic summation of multiple inputs along On ganglion cell dendrites in whole mount rat retina. We activated inputs at targeted locations by uncaging glutamate sequentially to generate apparent motion along On ganglion cell dendrites in whole mount retina. Summation was directional and dependent13 on input sequence. Input moving away from the soma (centrifugal) resulted in supralinear summation, while activation sequences moving toward the soma (centripetal) were linear. Enhanced summation for centrifugal activation was robust as it was also observed in cultured retinal ganglion cells. This directional summation was dependent on hyperpolarization activated cyclic nucleotide-gated (HCN) channels as blockade with ZD7288 eliminated directionality. A computational model confirms that activation of HCN channels can override a preference for centripetal summation expected from cell anatomy. This type of direction selectivity could play a role in coding movement similar to the axial selectivity seen in locust ganglion cells which detect looming stimuli. More generally, these results suggest that non-directional retinal ganglion cells can discriminate between input sequences independent of the retina network.

Visual information is coded by the output of retinal ganglion cells. Through evolution retinal ganglion cells acquired unique properties that allowed them to transmit to the brain such signals as direction of movement. The quest for the cellular mechanism of the detection of movement by retinal ganglion cells has been the holy grail of research on direction selectivity. In this study we have found a mechanism that allows individual non-direction selective On retinal ganglion cells to code sequences of excitatory inputs moving either away or toward the soma. We observed that inputs moving away from the soma resulted in enhanced, supralinear EPSP summation. Evidence from computational modeling suggests that expression of a specific set of voltage-dependent channels in dendrites introduces nonlinearities that could give a ganglion cell the ability to code looming motion. We predict that in a retinal network, such a directional tuning mechanism, together with asymmetric presynaptic inhibition, could be the building block for the development of more complex detection of visual motion.

Stereological and ultrastructural quantification of the afferent synaptome of individual neurons

Determining the number and placement of synaptic inputs along the distinct plasma membrane domains of neurons is essential for explaining the basis of neuronal activity and function. We detail a strategy that combines juxtacellular labeling, neuronal reconstructions and stereological sampling of inputs at the ultrastructural level to define key elements of the afferent ‘synaptome’ of a given neuron. This approach provides unbiased estimates of the total number and somato-dendritic distribution of synapses made with individual neurons. These organizational properties can be related to the activity of the same neurons previously recorded in vivo, for direct structure–function correlations at the single-cell level. The approach also provides the quantitative data required to develop biologically realistic models that simulate and predict neuronal activity and function.

Diversity of retinal ganglion cells identified by transient GFP transfection in organotypic tissue culture of adult marmoset monkey retina

The mammalian retina has more diversity of neurons than scientists had once believed in order to establish complicated vision processing. In the monkey retina, morphological diversity of retinal ganglion cells (RGCs) besides dominant midget and parasol cells has been suggested. However, characteristic subtypes of RGCs in other species such as bistratified direction-selective ganglion cells (DSGC) have not yet been identified. Increasing interest has been shown in the common marmoset (Callithrix jacchus) monkey as a "super-model" of neuroscientific research. Here, we established organotypic tissue culture of the adult marmoset monkey retina with particle-mediated gene transfer of GFP to survey the morphological diversity of RGCs. We successfully incubated adult marmoset monkey retinas for 2 to 4 days ex vivo for transient expression of GFP. We morphologically examined 121 RGCs out of more than 3240 GFP-transfected cells in 5 retinas. Among them, we identified monostratified or broadly stratified ganglion cells (midget, parasol, sparse, recursive, thorny, and broad thorny ganglion cells), and bistratified ganglion cells (recursive, large, and small bistratified ganglion cells [blue-ON/yellow-OFF-like]). By this survey, we also found a candidate for bistratified DSGC whose dendrites were well cofasciculated with ChAT-positive starburst dendrites, costratified with ON and OFF ChAT bands, and had honeycomb-shaped dendritic arbors morphologically similar to those in rabbits. Our genetic engineering method provides a new approach to future investigation for morphological and functional diversity of RGCs in the monkey retina.

Intrinsic physiological properties of rat retinal ganglion cells with a comparative analysis

Mammalian retina contains 15–20 different retinal ganglion cell (RGC) types, each of which is responsible for encoding different aspects of the visual scene. The encoding is defined by a combination of RGC synaptic inputs, the neurotransmitter systems used, and their intrinsic physiological properties. Each cell's intrinsic properties are defined by its morphology and membrane characteristics, including the complement and localization of the ion channels expressed. In this study, we examined the hypothesis that the intrinsic properties of individual RGC types are conserved among mammalian species. To do so, we measured the intrinsic properties of 16 morphologically defined rat RGC types and compared these data with cat RGC types. Our data demonstrate that in the rat different morphologically defined RGC types have distinct patterns of intrinsic properties. Variation in these properties across cell types was comparable to that found for cat RGC types. When presumed morphological homologs in rat and cat retina were compared directly, some RGC types had very similar properties. The rat A2 cell exhibited patterns of intrinsic properties nearly identical to the cat alpha cell. In contrast, rat D2 cells (ON-OFF directionally selective) had a very different pattern of intrinsic properties than the cat iota cell. Our data suggest that the intrinsic properties of RGCs with similar morphology and suspected visual function may be subject to variation due to the behavioral needs of the species.

The spatial structure of a nonlinear receptive field

Understanding a sensory system implies the ability to predict responses to a variety of inputs from a common model. In the retina, this includes predicting how the integration of signals across visual space shapes the outputs of retinal ganglion cells. Existing models of this process generalize poorly to predict responses to new stimuli. This failure arises in part from properties of the ganglion cell response that are not well captured by standard receptive-field mapping techniques: nonlinear spatial integration and fine-scale heterogeneities in spatial sampling. Here we characterize a ganglion cell's spatial receptive field using a mechanistic model based on measurements of the physiological properties and connectivity of only the primary excitatory circuitry of the retina. The resulting simplified circuit model successfully predicts ganglion-cell responses to a variety of spatial patterns and thus provides a direct correspondence between circuit connectivity and retinal output.

Spontaneous activity promotes synapse formation in a cell-type-dependent manner in the developing retina

Spontaneous activity is thought to regulate synaptogenesis in many parts of the developing nervous system. In vivo evidence for this regulation, however, is scarce and comes almost exclusively from experiments in which normal activity was reduced or blocked completely. Thus, whether spontaneous activity itself promotes synaptogenesis or plays a purely permissive role remains uncertain. In addition, how activity influences synapse dynamics to shape connectivity and whether its effects among neurons are uniform or cell-type-dependent is unclear. In mice lacking the cone-rod homeobox gene (Crx), photoreceptors fail to establish normal connections with bipolar cells (BCs). Here, we find that retinal ganglion cells (RGCs) in Crx⁻/⁻ mice become rhythmically hyperactive around the time of eye opening as a result of increased spontaneous glutamate release from BCs. This elevated neurotransmission enhances synaptogenesis between BCs and RGCs, without altering the overall circuit architecture. Using live imaging, we discover that spontaneous activity selectively regulates the rate of synapse formation, not elimination, in this circuit. Reconstructions of the connectivity patterns of three BC types with a shared RGC target further revealed that neurotransmission specifically promotes the formation of multisynaptic appositions from one BC type without affecting the maintenance or elimination of connections from the other two. Although hyperactivity in Crx⁻/⁻ mice persists, synapse numbers do not increase beyond 4 weeks of age, suggesting closure of a critical period for synaptic refinement in the inner retina. Interestingly, despite their hyperactivity, RGC axons maintain normal eye-specific territories and cell-type-specific layers in the dorsal lateral geniculate nucleus.

Physiological response of mouse retinal ganglion cells to electrical stimulation: effect of soma size

An epiretinal prosthesis aims to restore functional vision by stimulating electrically the retinal ganglion cells (RGCs) in patients affected by photoreceptor degenerative diseases, such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP). During retinal degeneration, photoreceptor death is followed by pronounced remodeling and rewiring of inner retinal cells. Despite these changes, a considerable population of RGCs remain receptive to prosthetic stimulation. To target selectively a localized subset of RGCs, an improved understanding of the anatomical and physiological properties of these cells is required. Additionally, potential alterations in electrical excitability produced by the retinal degeneration needs to be assessed. This study investigates the effect of RGC soma size on the threshold for action potential (spike) generation and its implications for the rescue of visual function.

Amacrine and bipolar inputs to midget and parasol ganglion cells in marmoset retina

Retinal ganglion cells receive excitatory synapses from bipolar cells and inhibitory synapses from amacrine cells. Previous studies in primate suggest that the strength of inhibitory amacrine input is greater to cells in peripheral retina than to foveal (central) cells. A comprehensive study of a large number of ganglion cells at different eccentricities, however, is still lacking. Here, we compared the amacrine and bipolar input to midget and parasol ganglion cells in central and peripheral retina of marmosets (Callithrix jacchus). Ganglion cells were labeled by retrograde filling from the lateral geniculate nucleus or by intracellular injection. Presumed amacrine input was identified with antibodies against gephyrin; presumed bipolar input was identified with antibodies against the GluR4 subunit of the AMPA receptor. In vertical sections, about 40% of gephyrin immunoreactive (IR) puncta were colocalized with GABAA receptor subunits, whereas immunoreactivity for gephyrin and GluR4 was found at distinct sets of puncta. The density of gephyrin IR puncta associated with ganglion cell dendrites was comparable for midget and parasol cells at all eccentricities studied (up to 2 mm or about 16 degrees of visual angle for midget cells and up to 10 mm or >80 degrees of visual angle for parasol cells). In central retina, the densities of gephyrin IR and GluR4 IR puncta associated with the dendrites of midget and parasol cells are comparable, but the average density of GluR4 IR puncta decreased slightly in peripheral parasol cells. These anatomical results indicate that the ratio of amacrine to bipolar input does not account for the distinct functional properties of parasol and midget cells or for functional differences between cells of the same type in central and peripheral retina.

Synaptic Pattern of KA1 and KA2 upon the Direction-Selective Ganglion Cells in Developing and Adult Mouse Retina

The detection of image motion is important to vision. Direction-selective retinal ganglion cells (DS-RGCs) respond strongly to stimuli moving in one direction of motion and are strongly inhibited by stimuli moving in the opposite direction. In this article, we investigated the distributions of kainate glutamate receptor subtypes KA1 and KA2 on the dendritic arbors of DS-RGCs in developing (5, 10) days postnatal (PN) and adult mouse retina to search for anisotropies. The distribution of kainate receptor subtypes on the DS-RGCs was determined using antibody immunocytochemistry. To identify their characteristic morphology, DS-RGCs were injected with Lucifer yellow. The triple-labeled images of dendrites, kinesin II, and receptors were visualized by confocal microscopy and were reconstructed from high-resolution confocal images. We found no evidence of asymmetry in any of the kainate receptor subunits examined on the dendritic arbors of both the On and Off layers of DS-RGCs in all periods of developing and adult stage that would predict direction selectivity.

Synaptic inputs to two types of koniocellular pathway ganglion cells in marmoset retina

The retinal connectivity of the diverse group of cells contributing to koniocellular visual pathways (widefield ganglion cells) is largely unexplored. Here we examined the synaptic inputs onto two koniocellular-projecting ganglion cell types named large sparse and broad thorny cells. Ganglion cells were labeled by retrograde tracer injections targeted to koniocellular layer K3 in the lateral geniculate nucleus in marmosets (Callithrix jacchus) and subsequently photofilled. Retinal preparations were processed with antibodies against the C-terminal binding protein 2, the AMPA receptor subunit GluR4, and against CD15 to identify bipolar (excitatory) and/or antibodies against gephyrin to identify amacrine (inhibitory) input. Large sparse cells are narrowly stratified close to the ganglion cell layer. Broad thorny ganglion cells are broadly stratified in the center of the inner plexiform layer. Bipolar input to large sparse cells derives from DB6 and maybe other ON bipolar types, whereas that to broad thorny cells derives from ON and OFF bipolar cell types. The total number of putative synapses on broad thorny cells is higher than the number on large sparse cells but the density of inputs (between 2 and 5 synapses per 100 μm(2) dendritic area) is similar for the two cell types, indicating that the larger number of synapses on broad thorny cells is attributable to the larger membrane surface area of this cell type. Synaptic input density is comparable to previous values for midget-parvocellular and parasol-magnocellular pathway cells. This suggests functional differences between koniocellular, parvocellular, and magnocellular pathways do not arise from variation in synaptic input densities.

Coordinated increase in inhibitory and excitatory synapses onto retinal ganglion cells during development

Background

Neuronal output is shaped by a balance of excitation and inhibition. How this balance is attained in the central nervous system during development is not well understood, and is complicated by the fact that, in vivo, GABAergic and glycinergic synaptogenesis precedes that of glutamatergic synapses. Here, we determined the distributions of inhibitory postsynaptic sites on the dendritic arbors of individual neurons, and compared their developmental patterns with that of excitatory postsynaptic sites. We focused on retinal ganglion cells (RGCs), the output neurons of the retina, which receive excitatory input from bipolar cells and inhibitory input from amacrine cells. To visualize and map inhibitory postsynaptic sites, we generated transgenic mice in which RGCs express fluorescently tagged Neuroligin 2 (YFP-NL2) under the control of the Thy1 promoter. By labeling RGC dendrites biolistically in YFP-NL2-expressing retinas, we were able to map the spatial distribution and thus densities of inhibitory postsynaptic sites on the dendritic arbors of individual large-field RGCs across ages.

Results

We demonstrate that YFP-NL2 is present at inhibitory synapses in the inner plexiform layer by its co-localization with gephyrin, the γ2 subunit of the GABA_A receptor and glycine receptors. YFP-NL2 puncta were apposed to the vesicular inhibitory transmitter transporter VGAT but not to CtBP2, a marker of presynaptic ribbons found at bipolar cell terminals. Similar patterns of co-localization with synaptic markers were observed for endogenous NL2. We also verified that expression of YFP-NL2 in the transgenic line did not significantly alter spontaneous inhibitory synaptic transmission onto RGCs. Using these mice, we found that, on average, the density of inhibitory synapses on individual arbors increased gradually until eye opening (postnatal day 15). A small centro-peripheral gradient in density found in mature arbors was apparent at the earliest age we examined (postnatal day 8). Unexpectedly, the adult ratio of inhibitory/excitatory postsynaptic sites was rapidly attained, shortly after glutamatergic synaptogenesis commenced (postnatal day 7).

Conclusion

Our observations suggest that bipolar and amacrine cell synaptogenesis onto RGCs appear coordinated to rapidly attain a balanced ratio of excitatory and inhibitory synapse densities prior to the onset of visual experience.

Localization of presynaptic inputs on dendrites of individually labeled neurons in three dimensional space using a center distance algorithm

The spatial distribution of synaptic inputs on the dendritic tree of a neuron can have significant influence on neuronal function. Consequently, accurate anatomical reconstructions of neuron morphology and synaptic localization are critical when modeling and predicting physiological responses of individual neurons. Historically, generation of three-dimensional (3D) neuronal reconstructions together with comprehensive mapping of synaptic inputs has been an extensive task requiring manual identification of putative synaptic contacts directly from tissue samples or digital images. Recent developments in neuronal tracing software applications have improved the speed and accuracy of 3D reconstructions, but localization of synaptic sites through the use of pre- and/or post-synaptic markers has remained largely a manual process. To address this problem, we have developed an algorithm, based on 3D distance measurements between putative pre-synaptic terminals and the post-synaptic dendrite, to automate synaptic contact detection on dendrites of individually labeled neurons from 3D immunofluorescence image sets. In this study, the algorithm is implemented with custom routines in Matlab, and its effectiveness is evaluated through analysis of primary sensory afferent terminals on motor neurons. Optimization of algorithm parameters enabled automated identification of synaptic contacts that matched those identified by manual inspection with low incidence of error. Substantial time savings and the elimination of variability in contact detection introduced by different users are significant advantages of this method.

Regular mosaic of synaptic contacts among three retinal neurons

Retinal bipolar, amacrine, and ganglion cells contact each other within precisely defined synaptic laminae, but the spatial distribution of contacts between the cells is generally treated as random. Here we show that not to be the case. Excitatory inputs to inner retinal neurons were visualized by introduction of a plasmid coding for the postsynaptic protein PSD95-GFP. Our initial finding was that synapses on the dendrites of retinal ganglion cells are regularly spaced, at 2-3-μm intervals, along the dendrites. Thus, the presence of a PSD95 punctum creates a nearby zone from which other inputs appear to be excluded. Despite their great variation in size and different morphologies, the spacing is similar for the arbors of different retinal ganglion cell types. Regular spacing was also observed for the starburst amacrine cells. This regularity is mirrored in the spacing of axonal varicosities of the stratified bipolar cells, which have a regular, nonrandom interval consistent with that of the PSD95 puncta on ganglion cells. Thus, for each level of the inner plexiform layer all three cell types participate in a single 2D mosaic of synaptic contacts. These findings raise a new set of questions: How does the self-avoidance of synaptic sites along an individual dendrite arise and how is it physically maintained? Why is a regular spacing of inputs important for the computational function of the cells? Finally, which of the three players, if any, is developmentally responsible for the initial establishment of the pattern?

Organotypic tissue culture of adult rodent retina followed by particle-mediated acute gene transfer in vitro

Background

Organotypic tissue culture of adult rodent retina with an acute gene transfer that enables the efficient introduction of variable transgenes would greatly facilitate studies into retinas of adult rodents as animal models. However, it has been a difficult challenge to culture adult rodent retina. The purpose of this present study was to develop organotypic tissue culture of adult rodent retina followed by particle-mediated acute gene transfer in vitro.

Methodology/Principal Findings

We established an interphase organotypic tissue culture for adult rat retinas (>P35 of age) which was optimized from that used for adult rabbit retinas. We implemented three optimizations: a greater volume of Ames' medium (>26 mL) per retina, a higher speed (constant 55 rpm) of agitation by rotary shaker, and a greater concentration (10%) of horse serum in the medium. We also successfully applied this method to adult mouse retina (>P35 of age). The organotypic tissue culture allowed us to keep adult rodent retina morphologically and structurally intact for at least 4 days. However, mouse retinas showed less viability after 4-day culture. Electrophysiologically, ganglion cells in cultured rat retina were able to generate action potentials, but exhibited less reliable light responses. After transfection of EGFP plasmids by particle-mediated acute gene transfer, we observed EGFP-expressing retinal ganglion cells as early as 1 day of culture. We also introduced polarized-targeting fusion proteins such as PSD95-GFP and melanopsin-EYFP (hOPN4-EYFP) into rat retinal ganglion cells. These fusion proteins were successfully transferred into appropriate locations on individual retinal neurons.

Conclusions/Significance

This organotypic culture method is largely applicable to rat retinas, but it can be also applied to mouse retinas with a caveat regarding cell viability. This method is quite flexible for use in acute gene transfection in adult rodent retina, replacing molecular biological bioassays that used to be conducted in isolated cultured cells.

NMDA receptor contributions to visual contrast coding

In the retina, it is not well understood how visual processing depends on AMPA- and NMDA-type glutamate receptors. Here we investigated how these receptors contribute to contrast coding in identified guinea pig ganglion cell types in vitro. NMDA-mediated responses were negligible in ON alpha cells but substantial in OFF alpha and delta cells. OFF delta cell NMDA receptors were composed of GluN2B subunits. Using a novel deconvolution method, we determined the individual contributions of AMPA, NMDA, and inhibitory currents to light responses of each cell type. OFF alpha and delta cells used NMDA receptors for encoding either the full contrast range (alpha), including near-threshold responses, or only a high range (delta). However, contrast sensitivity depended substantially on NMDA receptors only in OFF alpha cells. NMDA receptors contribute to visual contrast coding in a cell-type-specific manner. Certain cell types generate excitatory responses using primarily AMPA receptors or disinhibition.

Regulation of dendrite arborization by substrate stiffness is mediated by glutamate receptors

Brain injury or disease can initiate changes in local or global stiffness of brain tissue. While stiffness of the extracellular environment is known to affect the morphology and function of many cell types, little is known about how the dendrites of neurons respond to changes in brain stiffness. To assess how extracellular stiffness affects dendrite morphology, we took biomaterial and biomedical engineering approaches. We cultured mixed and pure hippocampal neurons on hydrogels composed of polyacrylamide (PA) of varying stiffnesses to mimic the effects of extracellular matrix stiffness on dendrite morphology. The majority of investigations of cortical and spinal cord neurons on soft hydrogels examined branching at early time points (days in vitro (DIV) 2-7), an important distinction from our study, where we include later time points that encompass the peak of branching (DIV 10-12). At DIV 12, dendrite branching was altered by stiffness for both pure and mixed neuronal cultures. Furthermore, we treated hippocampal cultures with glutamate receptor antagonists and with astrocyte-conditioned media. Blocking AMPA and NMDA receptors affected the changes in dendrite branching seen at varying rigidities. Moreover, extracellular factors secreted by astrocytes also change dendrite branching seen at varying rigidities. Thus, astrocytes and ionotropic glutamate receptors contribute to mechanosensing.

Synaptic input of ON-bipolar cells onto the dopaminergic neurons of the mouse retina

In the retina, dopamine fulfills a crucial role in neural adaptation to photopic illumination, but the pathway that carries cone signals to the dopaminergic amacrine (DA) cells was controversial. We identified the site of ON-cone bipolar input onto DA cells in transgenic mice in which both types of catecholaminergic amacrine (CA) cells were labeled with green fluorescent protein or human placental alkaline phosphatase (PLAP). In confocal Z series of retinal whole mounts stained with antibodies to tyrosine hydroxylase (TH), DA cells gave rise to varicose processes that descended obliquely through the scleral half of the inner plexiform layer (IPL) and formed a loose, tangential plexus in the middle of this layer. Comparison with the distribution of the dendrites of type 2 CA cells and examination of neurobiotin-injected DA cells proved that their vitreal processes were situated in stratum S3 of the IPL. Electron microscope demonstration of PLAP activity showed that bipolar cell endings in S3 established ribbon synapses onto a postsynaptic dyad in which one or both processes were labeled by a precipitate of lead phosphate and therefore belonged to DA cells. In places, the postsynaptic DA cell processes returned a reciprocal synapse onto the bipolar endings. Confocal images of sections stained with antibodies to TH, kinesin Kif3a, which labels synaptic ribbons, and glutamate or GABA(A) receptors, confirmed that ribbon-containing endings made glutamatergic synapses onto DA cells processes in S3 and received from them GABAergic synapses. The presynaptic ON-bipolar cells most likely belonged to the CB3 (type 5) variety.

Synaptic inputs onto small bistratified (blue-ON/yellow-OFF) ganglion cells in marmoset retina

The inner plexiform layer of the retina contains functional subdivisions, which segregate ON and OFF type light responses. Here, we studied quantitatively the ON and OFF synaptic input to small bistratified (blue-ON/yellow-OFF) ganglion cells in marmosets (Callithrix jacchus). Small bistratified cells display an extensive inner dendritic tier that receives blue-ON input from short-wavelength-sensitive (S) cones via blue cone bipolar cells. The outer dendritic tier is sparse and is thought to receive yellow-OFF input from medium (M)- and long (L)-wavelength-sensitive cones via OFF diffuse bipolar cells. In total, 14 small bistratified cells from different eccentricities were analyzed. The cells were retrogradely labeled from the koniocellular layers of the lateral geniculate nucleus and subsequently photofilled. Retinal preparations were processed with antibodies against the C-terminal binding protein 2, the AMPA receptor subunit GluR4, and/or gephyrin to identify bipolar and/or amacrine input. The results show that the synaptic input is evenly distributed across the dendritic tree, with a density similar to that reported previously for other ganglion cell types. The population of cells showed a consistent pattern, where bipolar input to the inner tier is about fourfold greater than bipolar input to the outer tier. This structural asymmetry of bipolar input may help to balance the weight of cone signals from the sparse S cone array against inputs from the much denser M/L cone array.

Genetically-directed, cell type-specific sparse labeling for the analysis of neuronal morphology

Background

In mammals, genetically-directed cell labeling technologies have not yet been applied to the morphologic analysis of neurons with very large and complex arbors, an application that requires extremely sparse labeling and that is only rendered practical by limiting the labeled population to one or a few predetermined neuronal subtypes.

Methods and Findings

In the present study we have addressed this application by using CreER technology to non-invasively label very small numbers of neurons so that their morphologies can be fully visualized. Four lines of IRES-CreER knock-in mice were constructed to permit labeling selectively in cholinergic or catecholaminergic neurons [choline acetyltransferase (ChAT)-IRES-CreER or tyrosine hydroxylase (TH)-IRES-CreER], predominantly in projection neurons [neurofilament light chain (NFL)-IRES-CreER], or broadly in neurons and some glia [vesicle-associated membrane protein2 (VAMP2)-IRES-CreER]. When crossed to the Z/AP reporter and exposed to 4-hydroxytamoxifen in the early postnatal period, the number of neurons expressing the human placental alkaline phosphatase reporter can be reproducibly lowered to fewer than 50 per brain. Sparse Cre-mediated recombination in ChAT-IRES-CreER;Z/AP mice shows the full axonal and dendritic arbors of individual forebrain cholinergic neurons, the first time that the complete morphologies of these very large neurons have been revealed in any species.

Conclusions

Sparse genetically-directed, cell type-specific neuronal labeling with IRES-creER lines should prove useful for studying a wide variety of questions in neuronal development and disease.

Distinct expressions of contrast gain control in parallel synaptic pathways converging on a retinal ganglion cell

Visual neurons adapt to increases in stimulus contrast by reducing their response sensitivity and decreasing their integration time, a collective process known as 'contrast gain control.' In retinal ganglion cells, gain control arises at two stages: an intrinsic mechanism related to spike generation, and a synaptic mechanism in retinal pathways. Here, we tested whether gain control is expressed similarly by three synaptic pathways that converge on an OFF alpha/Y-type ganglion cell: excitatory inputs driven by OFF cone bipolar cells; inhibitory inputs driven by ON cone bipolar cells; and inhibitory inputs driven by rod bipolar cells. We made whole-cell recordings of membrane current in guinea pig ganglion cells in vitro. At high contrast, OFF bipolar cell-mediated excitatory input reduced gain and shortened integration time. Inhibitory input was measured by clamping voltage near 0 mV or by recording in the presence of ionotropic glutamate receptor (iGluR) antagonists to isolate the following circuit: cone --> ON cone bipolar cell --> AII amacrine cell --> OFF ganglion cell. At high contrast, this input reduced gain with no effect on integration time. Mean luminance was reduced 1000-fold to recruit the rod bipolar pathway: rod --> rod bipolar cell --> AII cell --> OFF ganglion cell. The spiking response, measured with loose-patch recording, adapted despite essentially no gain control in synaptic currents. Thus, cone bipolar-driven pathways adapt differently, with kinetic effects confined to the excitatory OFF pathway. The ON bipolar-mediated inhibition reduced gain at high contrast by a mechanism that did not require an iGluR. Under rod bipolar-driven conditions, ganglion cell firing showed gain control that was explained primarily by an intrinsic property.

Developmental patterning of glutamatergic synapses onto retinal ganglion cells

Background

Neurons receive excitatory synaptic inputs that are distributed across their dendritic arbors at densities and with spatial patterns that influence their output. How specific synaptic distributions are attained during development is not well understood. The distribution of glutamatergic inputs across the dendritic arbors of mammalian retinal ganglion cells (RGCs) has long been correlated to the spatial receptive field profiles of these neurons. Thus, determining how glutamatergic inputs are patterned onto RGC dendritic arbors during development could provide insight into the cellular mechanisms that shape their functional receptive fields.

Results

We transfected developing and mature mouse RGCs with plasmids encoding fluorescent proteins that label their dendrites and glutamatergic postsynaptic sites. We found that as dendritic density (dendritic length per unit area of dendritic field) decreases with maturation, the density of synapses along the dendrites increases. These changes appear coordinated such that RGCs attain the mature average density of postsynaptic sites per unit area (areal density) by the time synaptic function emerges. Furthermore, stereotypic centro-peripheral gradients in the areal density of synapses across the arbor of RGCs are established at an early developmental stage.

Conclusion

The spatial pattern of glutamatergic inputs onto RGCs arises early in synaptogenesis despite ensuing reorganization of dendritic structure. We raise the possibility that these early patterns of synaptic distributions may arise from constraints placed on the number of contacts presynaptic neurons are able to make with the RGCs.