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. J. Comp. Neurol. 517:655–669, 2009. © 2009 Wiley‐Liss, Inc.     

Off-alpha and OFF-beta ganglion cells in cat retina: II. Neural circuitry as revealed by electron microscopy of HRP stains

An OFF-center alpha and an OFF-center beta ganglion cell in cat retina, which had been recorded from and intracellularly stained with horseradish peroxidase (HRP) were examined by serial section electron microscopy. We counted synapses and identified presynaptic neurons to the HRP-stained cells in 20 μm radial slices through the centers of their dendritic trees. Presynaptic amacrine and bipolar cells were identified on cytological criteria known from previous studies. The OFF-beta cell with a 62 μm dendritic arbor, restricted to S1 and S2 (sublamina a) of the inner plexiform layer (IPL), received 38% bipolar and 62% amacrine cell synapses. The bipolar input was from both cb1 and cb2 cone bipolar types. Input from three distinct amacrine cell types occurred upon the dendrites, namely from: (1) AII amacrine lobular appendages, (2) large pale amacrine profiles (possibly A2 or A3 cells), and (3) small, dark amacrine types (possibly A8 cells). Large pale amacrine profiles (possibly A13) were found on the cell body and apical dendrite in sublamina b of the IPL. In addition, several amacrine profiles synapsed directly on the sides and base of the cell body in the ganglion cell layer. We estimate that the complete dendritic tree of this beta cell received about 1,000 synapses contributed by 12–14 bipolar cells, 7–10 AII amacrines and 28–41 other amacrine cells. The OFF-alpha cell had a dendritic tree size of 680 × 920 μm. A 250 μm length of two major dendrites stratifying narrowly in S2 of the IPL was reconstructed. Amacrine cells provided most of the synaptic input (80%). This input came from: (1) AII amacrine lobular appendages, (2) amacrines exhibiting large, pale synaptic profiles (possibly A2 or A3 cells), (3) pale amacrines with large mitochondria and a few neurotubules (unknown type), and (4) densely neurotubule-filled amacrine profiles (possibly A19 cells). A large pale amacrine cell type (possibly A13) provided synaptic input to the cell body as a serial synaptic intermediary with rod bipolar cells. Cone bipolar synapses were from only one type of cone bipolar, the cb2 type and formed 20% of the total synaptic input. We estimate that a minimum of 142 bipolar cells, 256 AII amacrine cells and 1,011 other amacrine cells, altogether providing 6,000–10,000 synapses, converged on the dendritic tree of this OFF-alpha cell. © 1993 Wiley-Liss, Inc.     

Retinal ganglion cell inputs to the koniocellular pathway

To understand the transmission of sensory signals in visual pathways we studied the morphology and central projection of ganglion cell populations in marmoset monkeys. Retinal ganglion cells were labeled by photofilling following injections of retrograde tracer in the lateral geniculate nucleus (LGN), or by intracellular injection with neurobiotin. Ganglion cell morphology was analyzed using hierarchical cluster analysis. In addition to midget and parasol ganglion cells, this method distinguished three main clusters of wide-field cells that correspond to small bistratified, sparse, and broad thorny cells identified previously. The small bistratified and sparse cells occupy neighboring positions on the hierarchical (linkage distance) tree. These cell types are presumed to carry signals originating in short-wavelength sensitive (S or "blue") cones in the retina. The linkage distance from these putative S-cone pathway ganglion cells to other wide-field cells is similar to the linkage distance from midget cells to parasol cells, suggesting that S-cone cells form a distinct functional subgroup of ganglion cells. Small bistratified cells and large sparse cells were the most commonly labeled wide-field cells following LGN injections in koniocellular layer K3. This is consistent with physiological evidence that the role of this layer includes transmission of S-cone signals to the visual cortex. Other wide-field cell types were also labeled following injections including K3, and other koniocellular LGN layers; these cell types may correspond to "non-blue koniocellular" receptive fields recorded in physiological studies.     

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.     

Selective synaptic connections in the retinal pathway for night vision

The mammalian retina encodes visual information in dim light using rod photoreceptors and a specialized circuit: rods→rod bipolar cells→AII amacrine cell. The AII amacrine cell uses sign-conserving electrical synapses to modulate ON cone bipolar cell terminals and sign-inverting chemical (glycinergic) synapses to modulate OFF cone cell bipolar terminals; these ON and OFF cone bipolar terminals then drive the output neurons, retinal ganglion cells (RGCs), following light increments and decrements, respectively. The AII amacrine cell also makes direct glycinergic synapses with certain RGCs, but it is not well established how many types receive this direct AII input. Here, we investigated functional AII amacrine→RGC synaptic connections in the retina of the guinea pig (Cavia porcellus) by recording inhibitory currents from RGCs in the presence of ionotropic glutamate receptor (iGluR) antagonists. This condition isolates a specific pathway through the AII amacrine cell that does not require iGluRs: cone→ON cone bipolar cell→AII amacrine cell→RGC. These recordings show that AII amacrine cells make direct synapses with OFF Alpha, OFF Delta and a smaller OFF transient RGC type that co-stratifies with OFF Alpha cells. However, AII amacrine cells avoid making synapses with numerous RGC types that co-stratify with the connected RGCs. Selective AII connections ensure that a privileged minority of RGC types receives direct input from the night-vision pathway, independent from OFF bipolar cell activity. Furthermore, these results illustrate the specificity of retinal connections, which cannot be predicted solely by co-stratification of dendrites and axons within the inner plexiform layer.     

Midget ganglion cells of the parafovea of the human retina: a study by electron microscopy and serial section reconstructions.

In this study we used serial section electron microscopy and three-dimensional reconstructions to examine four midget ganglion cells of the human retina. The four cells were located in the parafoveal retina 2.5 mm or 8 degrees from the foveal center. Both type a (with dendritic trees in distal inner plexiform layer) and type b (with dendritic trees in proximal inner plexiform layer) midget ganglion cells have been studied. These cells have dendritic trees of 7-9 microns diameter, and their complete dendritic trees in the neuropil of the inner plexiform layer can be analyzed, as well as the bipolar cell axon terminals having synaptic input, by a study of 100-150 serial ultrathin sections. Type a midget ganglion cells appear to be in a one-to-one relationship with flat midget bipolar cell axon terminals ending in distal inner plexiform layer. Type b midget ganglion cells are in a one-to-one synaptic relationship with invaginating midget bipolar cell axon terminals in proximal inner plexiform layer. The midget bipolar cells primarily involved with the midget ganglion cells do not contact other ganglion cell dendrites. In other words, midget bipolar cells appear to be in exclusive contact with single midget ganglion cells in the human retina. The midget ganglion cells receive most of their input from their associated midget bipolar cells in the form of ribbon synapses at dyads or monads (55-81 ribbons total), although ribbonless synapses are seen occasionally. In all four midget ganglion cells reconstructed, one or two other bipolar cell axon terminals, presumed to be from wide-field bipolar types, provide 1-3 ribbon synapses each. The number of amacrine synapses upon a midget ganglion cell's dendritic tree is approximately equal to the number of bipolar ribbon inputs (43%-56% bipolar ribbons: 44%-57% amacrine synapses). We assume from our knowledge of response characteristics of ganglion cells in other mammalian retinas (Nelson et al., '78: J. Neurophysiol. 41:427-483), that the type a midget ganglion cell and its exclusive connectivity with a flat midget bipolar cell forms a single cone connected OFF-center pathway, whereas the type b midget ganglion cell with its exclusive connectivity to an invaginating midget bipolar cell forms a single cone connected ON-center pathway, through the retina to the brain.     

Asymmetry of on- and off-pathways of blue-sensitive cones of the retina of macaques

Macaque retinal ganglion cells whose receptive-field center recieves input from blue-sensitive cones show an overt asymmetry of the frequency of ON-center and OFF-center varieties, an asymmetry not present in ganglion cells whose center receives input from the other two cone types. A similar asymmetry of ON/OFF responses is found in the local electrotetinogram (d-wave) mediated by signals from blue-sensitive cones. ‘Blue-ON-center’ ganglion cells have larger receptive-field centers and shorter conduction latencies than other opponent-color varieties, suggesting an appreciable degree of receptor convergence and presumably large cell bodies. Intracellular stainings of these neurons with Procion Yellow show that they correspond to diffuse stratified (Parasol) ganglion cells whose flat-topped dendritic arborization stratifies in the sclerad half of the inner plexiform layer. In view of the known characteristics of macaque bipolar cells and of the ON/OFF asymmetry, it is proposed that these ganglion cells are postsynaptic to cone-specific flat bipolars possibly mediating sign-inverting synaptic contacts. The results also indicate a reversal, for the blue-cone pathway, of the ON/OFF lamination of the inner plexiform layer that has recently been described in other species.     

Synaptic inputs to physiologically identified retinal X-cells in the cat.

The cat retina contains a number of different classes of ganglion cells, each of which has a unique set of receptive field properties. The mechanisms that underlie the functional differences among classes, however, are not well understood. All of the afferent input to retinal ganglion cells are from bipolar and amacrine cell terminals in the inner plexiform layer, suggesting that the physiological differences among cat retinal ganglion cells might be due to differences in the proportion of input that they receive from these cell types. In this study, we have combined in vivo intracellular recording and labeling with subsequent ultrastructural analysis to determine directly the patterns of synaptic input to physiologically identified X-cells in the cat retina. Our primary aim in these analyses was to determine whether retinal X-cells receive a characteristic pattern of bipolar and amacrine cell input, and further, whether the functional properties of this cell type can be related to identifiable patterns of synaptic input in the inner plexiform layer. We reconstructed the entire dendritic arbor of an OFF-center X-cell and greater than 75% of the dendritic tree of an ON-center X-cell and found that 1) both ON- and OFF-center X-cells are contacted with approximately the same frequency by bipolar and amacrine cell terminals, 2) each of these input types is distributed widely over their dendritic fields, and 3) there is no significant difference in the pattern of distribution of bipolar and amacrine cell synapses onto the dendrites of either cell type. Comparisons of the inputs to the ON- and the OFF-center cell, however, did reveal differences in the complexity of the synaptic arrangements found in association with the two neurons; a number of complex synaptic arrangements, including serial amacrine cell synapses, were found exclusively in association with the dendrites of the OFF-center X-cell. Most models of retinal X-cell receptive fields, because their visual responses share a number of features with those of bipolar cells, have attributed X-cell receptive field properties to their bipolar cell inputs. The data presented here, the first obtained from analyzing the inputs to the entire dendritic arbors of retinal X-cells, demonstrate that these retinal ganglion cells receive nearly one-half of their input from amacrine cells. These results clearly indicate that further data concerning the functional consequences of amacrine cell input are needed to understand more fully visual processing in the X-cell pathway.     

Differential output of the high-sensitivity rod photoreceptor: AII amacrine pathway

In the mammalian retina, the scotopic threshold of ganglion cells is in part dependent on how rod inputs are summed by their presynaptic cone bipolar cells. For ON cone bipolar cells, there are two anatomical routes for rod signals: 1) cone photoreceptors receive inputs via gap junctions with the surrounding, more numerous rods; and 2) ON cone bipolar cells receive highly convergent input via gap junctions with AII amacrine cells, which each receive input from hundreds of rods. Rod-cone coupling is thought to be utilized at higher photon fluxes relative to the AII-ON cone bipolar pathway due to the impedance mismatch of a single small rod driving a larger cone. Furthermore, it is widely held that the convergence of high-gain chemical synapses onto AIIs confers the highest sensitivity to ON cone bipolar cells and ganglion cells. A lack of coupling between one or more types of ON cone bipolar cells and AIIs would obviate this high-sensitivity pathway and explain the existence of ganglion cells with elevated scotopic thresholds. To investigate this possibility, we examined Neurobiotin and glycine diffusion from AIIs to bipolar cells and found that approximately one-fifth of ON cone bipolar cells are not coupled to AIIs. Unlike AII-AII coupling, which changes with ambient background intensity, the fraction of noncoupled ON cone bipolar cells was unaltered by dark or light adaptation. These data suggest that one of five morphologically distinct ON cone bipolar cell types is not coupled to AIIs and suggest that AII-ON cone bipolar coupling is modulated differently from AII-AII coupling.     

Characterization and synaptic connectivity of melanopsin-containing ganglion cells in the primate retina

Melanopsin is a photopigment expressed in retinal ganglion cells, which are intrinsically photosensitive and are also involved in retinal circuits arising from rod and cone photoreceptors. This circuitry, however, is poorly understood. Here, we studied the morphology, distribution and synaptic input to melanopsin-containing ganglion cells in a New World monkey, the common marmoset (Callithrix jacchus). The dendrites of melanopsin-containing cells in marmoset stratify either close to the inner nuclear layer (outer stratifying), or close to the ganglion cell layer (inner stratifying). The dendritic fields of outer-stratifying cells tile the retina, with little overlap. However, the dendritic fields of outer-stratifying cells largely overlap with the dendritic fields of inner-stratifying cells. Thus, inner-stratifying and outer-stratifying cells may form functionally independent populations. The synaptic input to melanopsin-containing cells was determined using synaptic markers (antibodies to C-terminal binding protein 2, CtBP2, for presumed bipolar synapses, and antibodies to gephyrin for presumed amacrine synapses). Both outer-stratifying and inner-stratifying cells show colocalized immunoreactive puncta across their entire dendritic tree for both markers. The density of CtBP2 puncta on inner dendrites was about 50% higher than that on outer dendrites. The density of gephyrin puncta was comparable for outer and inner dendrites but higher than the density of CtBP2 puncta. The inner-stratifying cells may receive their input from a type of diffuse bipolar cell (DB6). Our results are consistent with the idea that both outer and inner melanopsin cells receive bipolar and amacrine input across their dendritic tree.     

Synaptic connections of DB3 diffuse bipolar cell axons in macaque retina

In primate retinas, the dendrites of DB3 diffuse bipolar cells are known to receive inputs from cones. The goal of this study was to describe the synaptic connections of DB3 bipolar cell axons in the inner plexiform layer. DB3 bipolar cells in midperipheral retina were labeled with antibodies to calbindin, and their axons were analyzed in serial, ultrathin sections by electron microscopy. Synapses were found almost exclusively at the axonal varicosities of DB3 axon terminals. There were 2.14 synaptic ribbons per varicosity. There were 33 varicosities per DB3 cell, giving an average of 71 ribbons per axon terminal. Because there were 1.5 postsynaptic ganglion cell dendrites per DB3 axonal varicosity, we estimate that there is at least 1 synapse per varicosity onto a parasol ganglion cell dendrite. There were 3.4 input synapses from amacrine cells per axonal varicosity. Among these were feedback synapses to the DB3 bipolar cell axon varicosities, which were made by 47% of the postsynaptic amacrine cell processes. Some of the feedback synapses could be from amacrine cells immunoreactive for cholecystokinin precursor or choline acetyltransferase, because both types of amacrine cells costratify with parasol cells and are known to be presynaptic to bipolar cells. AII amacrine cells were both presynaptic and postsynaptic to DB3 axons, a finding consistent with the large rod input to parasol ganglion cells reported in physiological experiments. DB3 bipolar cell axons also made frequent contacts with neighboring DB3 axons, and gap junctions were always found at these sites.     

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.     

Wide-field amacrine cell inputs to ON parasol ganglion cells in macaque retina

Parasol cells are one of the major types of primate retinal ganglion cells. The goal of this study was to describe the synaptic inputs that shape the light responses of the ON type of parasol cells, which are excited by increments in light intensity. A connectome from central macaque retina was generated by serial blockface scanning electron microscopy. Six neighboring ON parasol cells were reconstructed, and their synaptic inputs were analyzed. On average, they received 21% of their input from bipolar cells, excitatory local circuit neurons receiving input from cones. The majority of their input was from amacrine cells, local circuit neurons of the inner retina that are typically inhibitory. Their contributions to the neural circuit providing input to parasol cells are not well-understood, and the focus of this study was on the presynaptic wide-field amacrine cells, which provided 17% of the input to ON parasol cells. These are GABAergic amacrine cells with long, relatively straight dendrites, and sometimes also axons, that run in a single, narrow stratum of the inner plexiform layer. The presynaptic wide-field amacrine cells were reconstructed, and two types were identified based on their characteristic morphology. One presynaptic amacrine cell was identified as semilunar type 2, a polyaxonal cell that is electrically coupled to ON parasol cells. A second amacrine was identified as wiry type 2, a type known to be sensitive to motion. These inputs likely make ON parasol cells more sensitive to stimuli that are rapidly changing outside their classical receptive fields.     

ON cone bipolar cell axonal synapses in the OFF inner plexiform layer of the rabbit retina

Analysis of the rabbit retinal connectome RC1 reveals that the division between the ON and the OFF inner plexiform layer (IPL) is not structurally absolute. ON cone bipolar cells make noncanonical axonal synapses onto specific targets and receive amacrine cell synapses in the nominal OFF layer, creating novel motifs, including inhibitory crossover networks. Automated transmission electron microscopic imaging, molecular tagging, tracing, and rendering of ～400 bipolar cells reveals axonal ribbons in 36% of ON cone bipolar cells, throughout the OFF IPL. The targets include γ‐aminobutyrate (GABA)‐positive amacrine cells (γACs), glycine‐positive amacrine cells (GACs), and ganglion cells. Most ON cone bipolar cell axonal contacts target GACs driven by OFF cone bipolar cells, forming new architectures for generating ON–OFF amacrine cells. Many of these ON–OFF GACs target ON cone bipolar cell axons, ON γACs, and/or ON–OFF ganglion cells, representing widespread mechanisms for OFF to ON crossover inhibition. Other targets include OFF γACs presynaptic to OFF bipolar cells, forming γAC‐mediated crossover motifs. ON cone bipolar cell axonal ribbons drive bistratified ON–OFF ganglion cells in the OFF layer and provide ON drive to polarity‐appropriate targets such as bistratified diving ganglion cells (bsdGCs). The targeting precision of ON cone bipolar cell axonal synapses shows that this drive incidence is necessarily a joint distribution of cone bipolar cell axonal frequency and target cell trajectories through a given volume of the OFF layer. Such joint distribution sampling is likely common when targets are sparser than sources and when sources are coupled, as are ON cone bipolar cells. J. Comp. Neurol. 521:977–1000, 2013. © 2012 Wiley Periodicals, Inc.     

Organisation of koniocellular‐projecting ganglion cells and diffuse bipolar cells in the primate fovea

The roles of the midget and parasol pathways as the anatomical foundation for high‐acuity vision at the fovea are well established. There is also evidence for the presence of other (non‐midget, non‐parasol) ganglion cell types in the foveal retina, but it is not established whether these cells receive input from cone photoreceptors in the central few degrees of the visual field, i.e. the region most important for conscious visual perception. To address this question, we targeted injections of retrograde tracer to the koniocellular layers in the posterior aspect of the lateral geniculate nucleus, where the central visual field is represented, in marmoset monkeys (Callithrix jacchus). Labeled ganglion cells were photofilled to reveal their dendritic morphology. Potential inputs to foveal koniocellular cells from diffuse bipolar cells were investigated in vertical sections through the fovea of marmoset and macaque (Macaca fascicularis) monkey retinas using immunohistochemistry. Forty koniocellular‐projecting ganglion cells were analysed. We used an established model of marmoset foveal topography to show that all these koniocellular‐projecting cells receive cone inputs from the central‐most 6°, with about half the cells receiving input from below 2° eccentricity, in the rod‐free central bouquet of cones at the foveola. In addition, all diffuse bipolar types investigated were present in the fovea at stratification depths similar to those of their counterparts in the peripheral retina. We conclude that the diverse visual representations established for koniocellular pathways in the peripheral retina are also a feature of the fovea, suggesting that koniocellular pathways contribute to foveal vision.     

Melanopsin retinal ganglion cells receive bipolar and amacrine cell synapses

Melanopsin is a novel opsin synthesized in a small subset of retinal ganglion cells. Ganglion cells expressing melanopsin are capable of depolarizing in response to light in the absence of rod or cone input and are thus intrinsically light sensitive. Melanopsin ganglion cells convey information regarding general levels of environmental illumination to the suprachiasmatic nucleus, the intergeniculate leaflet, and the pretectum. Typically, retinal ganglion cells communicate information to central visual structures by receiving input from retinal photoreceptors via bipolar and amacrine cells. Because melanopsin ganglion cells do not require synaptic input to generate light-induced signals, these cells need not receive synapses from other neurons in the retina. In this study, we examined the ultrastructure of melanopsin ganglion cells in the mouse retina to determine the type (if any) of synaptic input these cells receive. Melanopsin immunoreaction product was associated primarily with the plasma membrane of (1) perikarya in the ganglion cell layer, (2) dendritic processes in the inner plexiform layer (IPL), and (3) axons in the optic fiber layer. Melanopsin-immunoreactive dendrites in the inner (ON) region of the IPL were postsynaptic to bipolar and amacrine terminals, whereas melanopsin dendrites stratifying in the outer (OFF) region of the IPL received only amacrine terminals. These observations suggested that rod and/or cone signals may be capable of modifying the intrinsic light response in melanopsin-expressing retinal ganglion cells.     

Morphology and connectivity of the small bistratified A8 amacrine cell in the mouse retina

Amacrine cells comprise ～30 morphological types in the mammalian retina. The synaptic connectivity and function of a few γ‐aminobutyric acid (GABA)ergic wide‐field amacrine cells have recently been studied; however, with the exception of the rod pathway‐specific AII amacrine cell, the connectivity of glycinergic small‐field amacrine cells has not been investigated in the mouse retina. Here, we studied the morphology and connectivity pattern of the small‐field A8 amacrine cell. A8 cells in mouse retina are bistratified with lobular processes in the ON sublamina and arboreal dendrites in the OFF sublamina of the inner plexiform layer. The distinct bistratified morphology was first visible at postnatal day 8, reaching the adult shape at P13, around eye opening. The connectivity of A8 cells to bipolar cells and ganglion cells was studied by double and triple immunolabeling experiments by using various cell markers combined with synaptic markers. Our data suggest that A8 amacrine cells receive glutamatergic input from both OFF and ON cone bipolar cells. Furthermore, A8 cells are coupled to ON cone bipolar cells by gap junctions, and provide inhibitory input via glycine receptor (GlyR) subunit α1 to OFF cone bipolar cells and to ON A‐type ganglion cells. Measurements of spontaneous glycinergic postsynaptic currents and GlyR immunolabeling revealed that A8 cells express GlyRs containing the α2 subunit. The results show that the bistratified A8 cell makes very similar synaptic contacts with cone bipolar cells as the rod pathway‐specific AII amacrine cell. However, unlike AII cells, A8 amacrine cells provide glycinergic input to ON A‐type ganglion cells. J. Comp. Neurol. 523:1529–1547, 2015. © 2015 Wiley Periodicals, Inc.     

Thorny ganglion cells in marmoset retina: Morphological and neurochemical characterization with antibodies against calretinin

In primates, over 17 morphological types of retinal ganglion cell have been distinguished by their dendritic morphology and stratification, but reliable markers for specific ganglion cell populations are still rare. The calcium binding protein calretinin is known to be expressed in the inner nuclear and the ganglion cell layer of marmoset retina, however, the specific cell type(s) expressing calretinin in the ganglion cell layer are yet to be determined. Here, we identified calretinin positive retinal ganglion cells in the common marmoset Callithrix jacchus. Double labeling with the ganglion cell marker RBPMS demonstrated that the large majority (80%) of the calretinin positive cells in the ganglion cell layer are ganglion cells, and 20% are displaced amacrine cells. The calretinin positive ganglion cells made up on average 12% of the total ganglion cell population outside of the foveal region and their proportion increased with eccentricity. Prelabeling with antibodies against calretinin and subsequent intracellular injection with DiI revealed that the large majority of the injected cells (n = 74) were either narrow thorny or broad thorny ganglion cells, 14 cells were displaced amacrine cells. Narrow thorny cells were further distinguished into outer and inner stratifying cells. In addition, weakly labeled cells with a large soma were identified as parasol ganglion cells. Our results show that three types of thorny ganglion cells in marmoset retina can be identified with antibodies against calretinin. Our findings are also consistent with the idea that the proportion of wide‐field ganglion cell types increases in peripheral retina.     

Microcircuitry of bipolar cells in cat retina

We have studied 15 bipolar neurons from a small patch (14 X 120 micron) of adult cat retina located within the area centralis. From electron micrographs of 189 serial ultrathin sections, the axon of each bipolar cell was substantially reconstructed with its synaptic inputs and outputs by means of a computer-controlled reconstruction system. Based on differences in stratification, cytology, and synaptic connections, we identified eight different cell types among the group of 15 neurons: one type of rod bipolar and seven types of cone bipolar neurons. These types correspond to those identified by the Golgi method and by intracellular recording. Those bipolar cell types for which we reconstructed three or four examples were extremely regular in form, size, and cytology, and also in the quantitative details of their synaptic connections. They appeared quite as specific in these respects as invertebrate "identified" neurons. The synaptic patterns observed for each type of bipolar neuron were complex but may be summarized as follows: the rod bipolar axon ended in sublamina b of the inner plexiform layer and provided major input to the AII amacrine cell. The axons of three types of cone bipolar cells also terminated in sublamina b and provided contacts to dendrites of on-beta and other ganglion cells. All three types, but especially the Cb1, received gap junction contacts from the AII amacrine cell. Axons of four types of cone bipolar cells terminated in sublamina a of the inner plexiform layer and contacted dendrites of off-beta and other ganglion cells. One of these cone bipolar cell types, CBa1, made reciprocal chemical contacts with the lobular appendage of the AII amacrine cell. These results show that the pattern of cone bipolar cell input to beta (X) and probably alpha (Y) ganglion cells is substantially more complex than had been suspected. At least two types of cone bipolar contribute to each type of ganglion cell where only a single type had been anticipated. In addition, many of the cone bipolar cell pathways in the inner plexiform layer are available to the rod system, since at least four types of cone bipolar receive electrical or chemical inputs from the AII amacrine cell. This may help to explain why, in a retina where rods far outnumber the cones, there should be so many types of cone bipolar cells.     

Ectopic retinal ON bipolar cell synapses in the OFF inner plexiform layer: Contacts with dopaminergic amacrine cells and melanopsin ganglion cells

A key principle of retinal organization is that distinct ON and OFF channels are relayed by separate populations of bipolar cells to different sublaminae of the inner plexiform layer (IPL). ON bipolar cell axons have been thought to synapse exclusively in the inner IPL (the ON sublamina) onto dendrites of ON‐type amacrine and ganglion cells. However, M1 melanopsin‐expressing ganglion cells and dopaminergic amacrine (DA) cells apparently violate this dogma. Both are driven by ON bipolar cells, but their dendrites stratify in the outermost IPL, within the OFF sublamina. Here, in the mouse retina, we show that some ON cone bipolar cells make ribbon synapses in the outermost OFF sublayer, where they costratify with and contact the dendrites of M1 and DA cells. Whole‐cell recording and dye filling in retinal slices indicate that type 6 ON cone bipolars provide some of this ectopic ON channel input. Imaging studies in dissociated bipolar cells show that these ectopic ribbon synapses are capable of vesicular release. There is thus an accessory ON sublayer in the outer IPL. J. Comp. Neurol. 517:226‐244, 2009. © 2009 Wiley‐Liss, Inc.