Synaptic input to small bistratified (blue-ON) ganglion cells in the retina of a new world monkey, the marmoset Callithrix jacchus.

Small bistratified (blue-ON) ganglion cells in the primate retina are involved in processing short wavelength sensitive cone signals. These ganglion cells stratify in both the ON- and OFF-sublamina of the inner plexiform layer. We investigated the origin of synaptic input to the small bistratified ganglion cell in the retina of a New World primate, the marmoset Callithrix jacchus. Two small bistratified cells from peripheral retina were intracellularly filled with Lucifer Yellow, subsequently photoconverted and processed for electron microscopy. Serial ultrathin sections were cut through portions of each cell, and these were analysed in the electron microscope. The majority of synaptic input (about 84%) to both the inner and outer tier of dendrites was from amacrine cells. Both dendritic tiers also received bipolar cell input. These findings are consistent with predictions from physiological studies that synaptic input to the inner and outer tier of small bistratified cells should be excitatory. However, the tiny fraction of total input supplied from bipolar cells to the outer tier is not consistent with the strong excitatory OFF response in cells of this pathway.     

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.     

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.     

Morphological analysis of the blue cone pathway in the retina of a New World monkey,the marmoset Callithrix jacchus

We have studied the components of the short wavelength-sensitive (SWS or “blue”) cone pathway in the retina of a New World primate, the marmoset Callithrix jacchus. Of particular interest was the small bistratified ganglion cell, which has been identified in macaque monkey to be the morphological substrate of the blue-ON cell (Dacey and Lee [1994] Nature 367:731–735). Small bistratified cells were intracellularly filled with Neurobiotin in an in vitro retinal wholemount preparation. Their morphology, size, and level of dendritic stratification were similar to their homologues in macaque and human retina. A number of different antibodies were applied to vertical cryostat sections, some of which were cut through the processes of injected small bistratified or parasol ganglion cells. We used antibodies against cholecystokinin (CCK) to label blue cone bipolar cells, and antibodies against the human SWS cone photopigment to label SWS cones. Double-labelled preparations showed that blue cone bipolar cell dendrites contact SWS cone pedicles, and the inner dendrites of the small bistratified cell are costratified with the axon terminals of blue cone bipolar cells. A monoclonal antibody against calbindin was used to label a subpopulation of bipolar cells that stratifies in the outer half of the inner plexiform layer. The axon terminals of these bipolar cells occasionally cross the outer dendrites of small bistratified cells and show extensive costratification with the dendrites of OFF parasol cells. We conclude that an SWS cone pathway with similar connectivity is a preserved feature of the primate visual system. J. Comp. Neurol. 379:211–225, 1997. © 1997 Wiley-Liss, Inc.     

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.     

Characterization of small-field bistratified amacrine cells in macaque retina labeled by antibodies against synaptotagmin-2

Macaque retinae were immunostained with monoclonal antibodies directed against the protein synaptotagmin‐2 (Syt2). Syt2 was localized in a population of small‐field amacrine cells, whose cell bodies formed a regular mosaic within the inner nuclear layer, indicating they represent a single amacrine cell type. The labeled amacrine cells had a bistratified appearance with a dense dendritic plexus in the OFF‐layer and only a few lobular processes extending into the ON‐layer of the inner plexiform layer, similar to A8 amacrine cells described in cat and human retina. Syt2‐labeled cells were immunoreactive for glycine but lacked immunoreactivity for γ‐aminobutyric acid (GABA), suggesting they use glycine as their neurotransmitter. The density of these cells increases from ～200/mm² in peripheral retina to ～1,400/mm² in central retina. Their bipolar cell input was studied by immunolabeling experiments using various bipolar cell markers combined with CtBP2, a marker of presynaptic ribbons. Our data show that Syt2‐labeled amacrine cells receive input from both OFF and ON cone bipolar cells, as well as from rod bipolar cells. The OFF input is dominated by the diffuse bipolar cell DB1 (44%) and the OFF midget bipolar cell (38%). Here we describe a population of bistratified small‐field amacrine cells closely resembling A8 amacrine cells and their cone‐dominated bipolar cell input. J. Comp. Neurol. 521:709–724, 2013. © 2012 Wiley Periodicals, Inc.     

Cone signals in monostratified and bistratified amacrine cells of adult zebrafish retina

Strata within the inner plexiform layer (IPL) of vertebrate retinas are suspected to be distinct signaling regions. Functions performed within adult zebrafish IPL strata were examined through microelectrode recording and staining of stratified amacrine types. The stimulus protocol and analysis discriminated the pattern of input from red, green, blue, and UV cones as well as the light‐response waveforms in this tetrachromatic species. A total of 36 cells were analyzed. Transient depolarizing waveforms at ON and OFF originated with bistratified amacrine types, whose dendritic planes branched either in IPL sublaminas a & b, or only within sublamina a. Monophasic‐sustained depolarizing waveforms originated with types monostratified in IPL s4 (sublamina b). OFF responses hyperpolarized at onset, depolarized at offset, and in some cases depolarized during mid‐stimulus. These signals originated with types monostratified in s1 or s2 (sublamina a). Bistratified amacrines received depolarizing signals only from red cones, at both ON and OFF, while s4 stratified ON cells combined red and green cone signals. The s1/s2 stratified OFF cells utilized hyperpolarizing signals from red, red and green, or red and blue cones at ON, but only depolarizing red cone signals at OFF. ON and OFF depolarizing transients from red cones appear widely distributed within IPL strata. “C‐type” physiologies, depolarized by some wavelengths, hyperpolarized by others, in biphasic or triphasic spectral patterns, originated with amacrine cells monostratified in s5. Collectively, cells in this stratum processed signals from all cone types. J. Comp. Neurol. 525:1532–1557, 2017. © 2016 Wiley Periodicals, Inc.     

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. J. Comp. Neurol. 524:1839–1858, 2016. © 2015 Wiley Periodicals, Inc.     

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.     

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.     

Regulation of neonatal development of retinal ganglion cell dendrites by neurotrophin‐3 overexpression

The morphology of dendrites constrains and reflects the nature of synaptic inputs to neurons. The visual system has served as a useful model to show how visual function is determined by the arborization patterns of neuronal processes. In retina, light ON and light OFF responding ganglion cells selectively elaborate their dendritic arbors in distinct sublamina, where they receive, respectively, inputs from ON and OFF bipolar cells. During neonatal maturation, the bilaminarly distributed dendritic arbors of ON‐OFF retinal ganglion cells (RGCs) are refined to more narrowly localized monolaminar structures characteristic of ON or OFF RGCs. Recently, brain‐derived neurotrophic factor (BDNF) has been shown to regulate this laminar refinement, and to enhance the development of dendritic branches selectively of ON RGCs. Although other related neurotrophins are known to regulate neuronal process formation in the central nervous system, little is known about their action in maturing retina. Here, we report that overexpression of neurotrophin‐3 (NT‐3) in the eye accelerates RGC laminar refinement before eye opening. Furthermore, NT‐3 overexpression increases dendritic branch number but reduces dendritic elongation preferentially in ON‐OFF RGCs, a process that also occurs before eye opening. NT‐3 overexpression does affect dendritic maturation in ON RGCs, but to a much less degree. Taken together, our results suggest that NT‐3 and BDNF exhibit overlapping effects in laminar refinement but distinct RGC‐cell‐type specific effects in shaping dendritic arborization during postnatal development. J. Comp. Neurol. 514:449–458, 2009. © 2009 Wiley‐Liss, Inc.     

Biocytin wide-field bipolar cells in rabbit retina selectively contact blue cones

The biocytin wide-field bipolar cell in rabbit retina has a broad axonal arbor in layer 5 of the inner plexiform layer and a wide dendritic arbor that does not contact all cones in its dendritic field. The purpose of our study was to identify the types of cones that this cell contacts. We identified the bipolar cells by selective uptake of biocytin, labeled the cones with peanut agglutinin, and then used antibodies against blue cone opsin and red-green cone opsin to identify the individual cone types. The biocytin-labeled cells selectively contacted cones whose outer segments stained for blue cone opsin and avoided cones that did not. We conclude that the biocytin wide-field bipolar cell is an ON blue cone bipolar cell in the rabbit retina and is homologous to the blue cone bipolar cells that have been previously described in primate, mouse, and ground squirrel retinas.     

Bipolar cell-photoreceptor connectivity in the zebrafish (Danio rerio) retina

Bipolar cells convey luminance, spatial, and color information from photoreceptors to amacrine and ganglion cells. We studied the photoreceptor connectivity of 321 bipolar cells in the adult zebrafish retina. 1,1′‐Dioctadecyl‐3,3,3′,3′‐tetramethylindocarbocyanine perchlorate (DiI) was inserted into whole‐mounted transgenic zebrafish retinas to label bipolar cells. The photoreceptors that connect to these DiI‐labeled cells were identified by transgenic fluorescence or their positions relative to the fluorescent cones, as cones are arranged in a highly ordered mosaic: rows of alternating blue‐ (B) and ultraviolet‐sensitive (UV) single cones alternate with rows of red‐ (R) and green‐sensitive (G) double cones. Rod terminals intersperse among cone terminals. As many as 18 connectivity subtypes were observed, 9 of which—G, GBUV, RG, RGB, RGBUV, RGRod, RGBRod, RGBUVRod, and RRod bipolar cells—accounted for 96% of the population. Based on their axon terminal stratification, these bipolar cells could be further subdivided into ON, OFF, and ON–OFF cells. The dendritic spread size, soma depth and size, and photoreceptor connections of the 308 bipolar cells within the nine common connectivity subtypes were determined, and their dendritic tree morphologies and axonal stratification patterns compared. We found that bipolar cells with the same axonal stratification patterns could have heterogeneous photoreceptor connectivity whereas bipolar cells with the same dendritic tree morphology usually had the same photoreceptor connectivity, although their axons might stratify on different levels. J. Comp. Neurol. 520:3786–3802, 2012. © 2012 Wiley Periodicals, Inc.     

Inner S-cone bipolar cells provide all of the central elements for S cones in macaque retina

Synaptic terminals of cones (pedicles) are presynaptic to numerous processes that arise from the dendrites of many types of bipolar cell. One kind of process, a central element, reaches deeply into invaginations of the cone pedicle just below an active zone associated with a synaptic ribbon. By reconstruction from serial electron micrographs, we show that L- and M-cone pedicles in macaque fovea are presynaptic to approximately 20 central elements that arise from two types of inner (invaginating) bipolar cell, midget and diffuse. In contrast, S-cone pedicles, with more synaptic ribbons, active zones/ribbon, and central elements/active zone, are presynaptic to approximately 33 central elements. Moreover, all of these arise from one type of bipolar cell, previously described by others, here termed an inner S-cone bipolar cell. Each provides approximately 16 central elements. Thirty-three is twice 16; correspondingly, these bipolar cells are twice as numerous as S cones. (Specifically, each S cone is presynaptic to four inner S-cone bipolar cells; in turn, each bipolar cell provides central elements to two S cones.) These bipolar cells are presynaptic to an equal number of small-field bistratified ganglion cells, giving cell numbers in 2G:2B:1S ratios. Each ganglion cell receives input from two or more inner S-cone bipolar cells and thereby collects signals from three or more S cones. This convergence, along with chromatic aberration of short-wavelength light, suggests that S-cone contributions to this ganglion cell's coextensive blue-ON/yellow-OFF receptive field are larger than opponent L/M-cone contributions via outer diffuse bipolar cells and that opponent L/M-cone signals are conveyed mainly by inner S-cone bipolar cells.     

Immunocytochemical localization of kainate-selective glutamate receptor subunits GluR5, GluR6, and GluR7 in the cat retina

Localizations of the kainate-selective glutamate receptor subunits GluR5, 6, and 7 were studied in the cat retina by light and electron microscopic immunocytochemistry. GluR5 immunoreactivity was observed in the cell bodies and dendrites of numerous cone bipolar cells and ganglion cells. The labeled cone bipolar cells make basal or flat contacts with cone pedicles in the outer plexiform layer, leading to their identification as OFF-center bipolar cells. Reaction product within the inner plexiform layer was observed in processes of ganglion cells at their sites of input from cone bipolar cells. Staining for GluR6 was localized to A- and B-type horizontal cells, numerous amacrine cells, and an occasional cone bipolar cell. The larger ganglion cells were also immunoreactive. As with other GluR molecules, labeling was usually confined to one of the two postsynaptic elements at a cone bipolar dyad contact. Immunoreactivity for GluR7 was very limited and was seen only in a few amacrine and displaced amacrine cells. Findings of this study are consistent with a major role for kainate receptors in mediating OFF pathways in the outer retina with participation in both OFF and ON pathways in the inner retina.     

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.     

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.     

The M6 cell: A small-field bistratified photosensitive retinal ganglion cell

We have identified a novel, sixth type of intrinsically photosensitive retinal ganglion cell (ipRGC) in the mouse—the M6 cell. Its spiny, highly branched dendritic arbor is bistratified, with dendrites restricted to the inner and outer margins of the inner plexiform layer, co-stratifying with the processes of other ipRGC types. We show that M6 cells are by far the most abundant ganglion cell type labeled in adult pigmented Cdh3-GFP BAC transgenic mice. A few M5 ipRGCs are also labeled, but no other RGC types were encountered. Several distinct subnuclei in the geniculate complex and the pretectum contain labeled retinofugal axons in the Cdh3-GFP mouse. These are presumably the principle central targets of M6 cells (as well as M5 cells). Projections from M6 cells to the dorsal lateral geniculate nucleus were confirmed by retrograde tracing, suggesting they contribute to pattern vision. M6 cells have low levels of melanopsin expression and relatively weak melanopsin-dependent light responses. They also exhibit strong synaptically driven light responses. Their dendritic fields are the smallest and most abundantly branched of all ipRGCs. They have small receptive fields and strong antagonistic surrounds. Despite deploying dendrites partly in the OFF sublamina, M6 cells appear to be driven exclusively by the ON pathway, suggesting that their OFF arbor, like those of certain other ipRGCs, may receive ectopic input from passing ON bipolar cells axons in the OFF sublayer.     

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.     

Identification of AⅡ amacrine,displaced amacrine,and bistratified ganglion cell types in human retina with antibodies against calretinin

Antibodies against calretinin are markers for one type of rod pathway interneuron (AⅡ amacrine cell) in the retina of some but not all mammalian species. The AⅡ cells play a crucial role in night‐time (scotopic) vision and have been proposed as a target for optogenetic restoration of vision in retinal disease. In the present study we aimed to characterize the AⅡ cells in human retina. Postmortem human donor eyes were obtained with ethical approval and processed for calretinin immunofluorescence. Calretinin‐positive somas in the inner nuclear and the ganglion cell layer were filled with the lipophilic dye DiI. The large majority (over 80%) of calretinin‐immunoreactive cells is located in the inner nuclear layer, is immunopositive for glycine transporter 1, and shows the typical morphology of AⅡ amacrine cells. In addition, a small proportion of calretinin‐positive cells in the inner nuclear layer and in the ganglion cell layer is glutamic acid decarboxylase‐positive and shows the morphology of widefield amacrine cells (stellate, semilunar, and thorny amacrine cells). About half of the calretinin cells in the ganglion cell layer are bistratified ganglion cells resembling the small bistratified (presumed blue‐ON/yellow‐OFF) and the G17 ganglion cell previously described in primates. We conclude that in human retina, antibodies against calretinin can be used to identify AⅡ amacrine cells in the inner nuclear layer as well as widefield amacrine and small bistratified ganglion cells in the ganglion cell layer. J. Comp. Neurol. 524:39–53, 2016. © 2015 Wiley Periodicals, Inc.