Rod-cone crossover connectome of mammalian bipolar cells

The basis of cross-suppression between rod and cone channels has long been an enigma. Using rabbit retinal connectome RC1, we show that all cone bipolar cell (BC) classes inhibit rod BCs via amacrine cell (AC) motifs (C1–6); that all cone BC classes are themselves inhibited by AC motifs (R1–5, R25) driven by rod BCs. A sparse symmetric AC motif (CR) is presynaptic and postsynaptic to both rod and cone BCs. ON cone BCs of all classes drive inhibition of rod BCs via motif C1 wide-field GABAergic ACs (γACs) and motif C2 narrow field glycinergic ON ACs (GACs). Each rod BC receives ≈10 crossover AC synapses and each ON cone BC can target ≈10 or more rod BCs via separate AC processes. OFF cone BCs mediate monosynaptic inhibition of rod BCs via motif C3 driven by OFF γACs and GACs and disynaptic inhibition via motifs C4 and C5 driven by OFF wide-field γACs and narrow-field GACs, respectively. Motifs C4 and C5 form halos of 60–100 inhibitory synapses on proximal dendrites of AI γACs. Rod BCs inhibit surrounding arrays of cone BCs through AII GAC networks that access ON and OFF cone BC patches via motifs R1, R2, R4, R5 and a unique ON AC motif R3 that collects rod BC inputs and targets ON cone BCs. Crossover synapses for motifs C1, C4, C5, and R3 are 3–4× larger than typical feedback synapses, which may be a signature for synaptic winner-take-all switches. J. Comp. Neurol. 527:87–116, 2019. © 2016 The Authors The Journal of Comparative Neurology Published by 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.     

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.     

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.     

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.     

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.     

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.     

Synaptic connections of the narrow-field, bistratified rod amacrine cell (AII) in the rabbit retina.

The synaptic connections of the narrow-field, bistratified rod amacrine cell (AII) in the inner plexiform layer (IPL) of the rabbit retina were reconstructed from electron micrographs of continuous series of thin sections. The AII amacrine cell receives a large synaptic input from the axonal endings of rod bipolar cells in the most vitreal region of the IPL (sublamina b, S5) and a smaller input from axonal endings of cone bipolar cells in the scleral region of the IPL (sublamina a, S1-S2). Amacrine input, localized at multiple levels in the IPL, equals the total number of synapses received from bipolar cells. The axonal endings of cone bipolar cells represent the major target for the chemical output of the AII amacrine cell: these synapses are established by the lobular appendages in sublamina a (S1-S2). Ganglion cell dendrites represent only 4% of the output of the AII amacrine and most of them are also postsynaptic to the cone bipolars which receive AII input. The AII amacrine is not presynaptic to other amacrine cells. Finally, the AII amacrine makes gap junctions with the axonal arborizations of cone bipolars that stratify in sublamina b (S3-S4) as well as with other AII amacrine cells in S5. Therefore, in the rabbit retina 1) the rod pathway consists of five neurons arranged in series: rod-->rod bipolar-->AII amacrine-->cone bipolar-->ganglion cell; 2) it seems unlikely that a class of ganglion cells exists that is exclusively devoted to scotopic functions. In ventral, midperipheral retina, about nine rod bipolar cells converge onto a single AII amacrine, but one of them establishes a much higher proportion of synaptic contacts than the rest. Conversely, each rod bipolar cell diverges onto four AII amacrine cells, but one of them receives the largest fraction of synapses. Thus, within the pattern of convergence and divergence suggested by population studies, preferential synaptic pathways are established.     

The absence of Complexin 3 and Complexin 4 differentially impacts the ON and OFF pathways in mouse retina

Complexins (Cplxs) regulate the speed and Ca₂₊‐sensitivity of synaptic vesicle fusion. It has been shown that all four known Cplxs are present at mouse retinal synapses – at conventional amacrine cell synapses (Cplx 1 to Cplx 3) and at photoreceptor and bipolar cell ribbon synapses (Cplx 3 and Cplx 4) [ K. Reim et al. (2005) J. Cell Biol., 169 , 669‐680]. Electroretinographic recordings in Cplx 3/Cplx 4 double‐knockout (DKO) mice showed perturbed transmission in the outer plexiform layer, and possible changes in the inner plexiform layer [ K. Reim et al. (2009) J. Cell Sci., 122 , 1352–1361]. In the present study, we examined the effects of the absence of Cplx 3 and Cplx 4 on ganglion cell responses. We report that the lack of Cplx 3 and Cplx 4 differentially impacts the ON and OFF pathways. Under photopic conditions, the responses in the cone OFF pathway are largely unaffected, whereas the responses in the cone ON pathway are diminished in Cplx 3/Cplx 4 DKO mice. Under scotopic conditions, both ON and OFF response rates are reduced and high‐sensitivity OFF responses are missing in Cplx 3/Cplx 4 DKO mice. The electrophysiological findings are corroborated by new immunocytochemical findings. We now show that rod spherules contain only Cplx 4. However, both Cplx 3 and Cplx 4 co‐localize in cone pedicles. In the inner plexiform layer, Cplx 3 is present in rod bipolar cell terminals and in amacrine cell processes. Most importantly, Cplx 3 is localized in the lobular appendages of AII amacrine cells, the sites of signal transmission from the primary rod pathway into the OFF pathway in the inner plexiform layer.     

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.     

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.     

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. J. Comp. Neurol. 518:2035–2050, 2010. © 2010 Wiley‐Liss, Inc.     

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.     

Synaptic contacts between an identified type of ON cone bipolar cell and ganglion cells in the mouse retina

We surveyed the potential contacts between an identified type of bipolar cell and retinal ganglion cells in the mouse. By crossing two existing mouse strains (line 357 and line GFP-M), we created a double transgenic strain in which GFP is expressed by all members of a single type of ON cone bipolar cell and a sparse, mixed population of retinal ganglion cells. The GFP-expressing bipolar cells appear to be those termed CB4a of Pignatelli & Strettoi [(2004) J. Comp. Neurol., 476, 254-266] and type 7 of Ghosh et al. [(2004) J. Comp. Neurol., 469, 70-82 and J. Comp. Neurol., 476, 202-203]. The labelled ganglion cells include examples of most or all types of ganglion cells present in the mouse. By studying the juxtaposition of their processes in three dimensions, we could learn which ganglion cell types are potential synaptic targets of the line 357 bipolar cell. Of 12 ganglion cell types observed, 10 types could be definitively ruled out as major synaptic targets of the line 357 bipolar cells. One type of monostratified ganglion cell and one bistratified cell tightly cofasciculate with axon terminals of the line 357 bipolar cells. Double labelling for kinesin II demonstrates colocalization of bipolar cell ribbons at the sites of contact between these two types of ganglion cell and the line 357 bipolar cells.     

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.     

Dendritic co-stratification of ON and ON-OFF directionally selective ganglion cells with starburst amacrine cells in rabbit retina.

The morphology, dendritic branching patterns, and dendritic stratification of retinal ganglion cells have been studied in Golgi-impregnated, whole-mount preparations of rabbit retina. Among a large number of morphological types identified, two have been found that correspond to the morphology of ON and ON-OFF directionally selective (DS) ganglion cells identified in other studies. These cells have been characterized in the preceding paper in terms of their cell body size, dendritic field size, and branching pattern. In this paper, the two kinds of DS ganglion cell are compared in terms of their levels of dendritic stratification. They are compared with each other and also with examples of class III.1 cells, defined in the preceding paper with reference to our previous studies. Studies employing computer-aided, 3D reconstruction of dendritic trees, as well as analysis of a pair of ON DS and ON-OFF DS ganglion cells with overlapping dendritic trees show that the two types of DS ganglion cell partly co-stratify in the middle of sublamina b (stratum 4). The report that some ON DS ganglion cells extend a few dendrites into sublamina a is confirmed. The study of pairs of ON-OFF DS ganglion cells and starburst amacrine cells with overlapping dendritic trees reveals a precise co-stratification of these two cell types, and many points of close apposition of starburst boutons with ON-OFF DS ganglion cell dendrites in both sublaminae of the inner plexiform layer (IPL). This is confirmed by high-resolution light microscopy and by electron microscopy. It is possible to conclude, therefore, that ON DS are also partly co-stratified with type b starburst (cholinergic) amacrine cells, and are apparently also partly co-stratified with type a starburst amacrine cells, when occasional dendrites rise to that level. The co-stratification of the two kinds of DS ganglion cell is consistent with the sharing of some inputs in common, including some cone bipolar cell inputs. The co-stratification of both with starburst amacrine cells agrees with the physiological demonstration of the powerful pharmacological effects upon ON and ON-OFF DS ganglion cells reported for cholinergic agonists. The major difference in the dendritic stratification of bistratified ON-OFF DS ganglion cells and generally unistratified ON DS ganglion cells is consistent with the bisublaminar organization of ON and OFF pathways in the IPL. The problem of occasional branches of ON DS cells in sublamina a is discussed in terms of a threshold for OFF responses.(ABSTRACT TRUNCATED AT 400 WORDS)     

Melatonin receptors are anatomically organized to modulate transmission specifically to cone pathways in the retina of Xenopus laevis

Melatonin receptors have been identified in several retinal cell types, including photoreceptors, horizontal cells, amacrine cells, and ganglion cells. Recent reports suggest that melatonin potentiates signaling from rods to inner retinal neurons. However, the organization of the melatonin receptors mediating this action in the outer plexiform layer (OPL) is not clear. To assess melatonin receptor localization in the OPL, double-label confocal immunohistochemistry for Mel1a or Mel1b melatonin receptors was performed in combination with markers for cone photoreceptors (calbindin, XAP-1) and ON bipolar cells (guanine nucleotide binding protein alpha, Goα) on the retina of Xenopus laevis. Both Mel1a and Mel1b receptors were specifically associated with processes contacting the pedicles of cones, but localized to processes from different sets of second-order neurons. Mel1a receptors localized to the large axonal processes of horizontal cells, while Mel1b receptors localized to the dendrites of OFF bipolar cells. Both receptors also localized to third-order amacrine and ganglion cells and their processes in the inner plexiform layer. This study indicates that Mel1a and Mel1b melatonin receptors are expressed specifically in the Xenopus OPL to modulate transmission from cones to horizontal cells and OFF bipolar cells, respectively; they are second-order neurons that predominantly contact ribbon synapses and display OFF responses to light. When combined with results from recent physiological studies, the current results suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons across species, although the precise mechanisms by which melatonin enhances this transmission are likely to vary in a species-dependent manner.     

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.     

Components and properties of the G3 ganglion cell circuit in the rabbit retina

Each point on the retina is sampled by about 15 types of ganglion cell, each of which is an element in a circuit also containing specific types of bipolar cell and amacrine cell. Only a few of these circuits are well characterized. We found that intracellular injection of Neurobiotin into a specific ganglion cell type targeted by fluorescent markers also stained an asymmetrically branching ganglion cell. It was also tracer‐coupled to an unusual type of amacrine cell whose dendrites were strongly asymmetric, coursing in a narrow bundle from the soma in the dorsal direction only. The dendritic field of the ganglion cell stratifies initially in sublamina b (the ON layers), but with few specializations and branches, and then more extensively in sublamina a (the OFF layers) at the level of the processes of the coupled amacrine cell. Intersections of the ganglion and amacrine cell processes contain puncta immunopositive for Cx36. Additionally, we found that the dopaminergic amacrine cell makes contact with both the ganglion cell and the amacrine cell, and that a bipolar cell immunopositive for calbindin synapses onto the sublamina b processes of the ganglion cell. Dopamine D₁ receptor activation reduced tracer flow to the amacrine cells. We have thus targeted and characterized two poorly understood retinal cell types and placed them with two other cell types in a substantial portion of a new retinal circuit. This unique circuit comprised of pronounced asymmetries in the ganglion cell and amacrine cell dendritic fields may result in a substantial orientation bias. J. Comp. Neurol. 513:69–82, 2009. © 2008 Wiley‐Liss, Inc.     

Connections of two types of flat cone bipolars in the rabbit retina

The synaptic connections of two types of cone bipolar cells in the rabbit retina were studied with the electron microscope after labeling in vitro with 4′,6-diamidino-2-phenylindole (DAPI), intracellular injection with Lucifer Yellow, and photooxidation (Mills and Massey [1992] J. Comp. Neurol. 321:133). Both types of bipolars belong to the flat variety, because they make basal junctions with a group of four to ten neighboring cone pedicles. One cell type has an axonal arborization that occupies strata 1 through 3 of the inner plexiform layer (IPL). At ribbon synaptic junctions, it is presynaptic to ganglion cell dendrites and to reciprocal dendrites belonging to narrow-field bistratified (AII) amacrine cells. In addition, it contacts and is contacted by other amacrine cell processes of unknown origin. The other cell type has an axonal arborization entirely confined to stratum 2 of the IPL; it is pre- or postsynaptic to a pleomorphic population of amacrine cell processes, and, in particular, it receives input from the lobular appendages of AII. Thus, these two bipolar types probably belong to the off-variety because they make basal junctions with cone photoreceptors and send their axon to sublamina α of the IPL, which is occupied by the dendrites of off-ganglion cells. They are also part of the rod pathway because they receive input from AII amacrine cells. © 1996 Wiley-Liss, Inc.