Synaptology of physiologically identified ganglion cells in the cat retina: A comparison of retinal X- and Y-cells

It has long been known that a number of functionally different types of ganglion cells exist in the cat retina, and that each responds differently to visual stimulation. To determine whether the characteristic response properties of different retinal ganglion cell types might reflect differences in the number and distribution of their bipolar and amacrine cell inputs, we compared the percentages and distributions of the synaptic inputs from bipolar and amacrine cells to the entire dendritic arbors of physiologically characterized retinal X- and Y- cells. Sixty-two percent of the synaptic input to the Y-cell was from amacrine and bipolar cells. We found no significant difference in the distributions of bipolar or amacrine cell inputs to X- and Y- cells, or ON-center and OFF-center cells, either as a function of dendritic branch order or distance from the origin of the dendritic arbor. While, on the basis of these data, we cannot exclude the possibility that the difference in the proportion of bipolar and amacrine cell input contributes to the functional differences between X- and Y- cells, the magnitude of this difference, and the similarity in the distributions of the input from the two afferent cell types, suggest that mechanisms other than a simple predominance of input from amacrine or bipolar cells underlie the differences in their response properties. More likely, perhaps, is that the specific response features of X- and Y- cells originated in differences in the visual responses of the bipolar and amacrine cells that provides their input, or in the complex synaptic arrangements found among amacrine and bipolar cell terminals and dendrites of specific types of retinal ganglion cells. © 1994 Wiley-Liss, Inc.     

Electron microscopic analysis of amacrine and bipolar cell inputs on Y-, X- and W-cells in the cat retina

In the cat retina, bipolar and amacrine cell inputs were analyzed electron microscopically in 5 ganglion cells (two Y-cells, two X-cells and one W-cell) that were well-isolated and had clear morphological features. For Y- and X-cells, subtypes of a and b were further identified according to the sublamina of the inner plexiform layer in which their dendrites extended. Y-a and Y-b ganglion cells had large somas, thick axons, and several thick dendrites that branched extensively with a large dendritic field. X-a and X-b cells had medium-sized somas, medium-sized axons and extremely narrow dendritic fields. The W-cell studied had a medium-sized soma, a medium-sized axon, and extremely thin dendrites that extended widely. For each of the 5 ganglion cells, ultrathin serial sections were made to study relative occurrence of amacrine and bipolar synapses in whole length of dendrites. About 50% of the terminals were bipolar in the Y-a and Y-b cell dendrites, 36-38% in the X-a and X-b cell dendrites, whereas only 19.7% were bipolar in the W cell dendrites. Bipolar terminals tended to make synaptic contacts with the distal dendrites of Y- and W-cells.     

The inner plexiform layer of the vertebrate retina: a quantitative and comparative electron microscopic analysis

The inner plexiform layer of human, monkey, cat, rat, rabbit, ground squirrel, frog and pigeon retinas was studied by electron microscopy. All showed the same qualitative synaptic arrangements: bipolar cells made dyad ribbon synapses onto amacrine and ganglion cells; amacrine cells made conventional synaptic contacts onto bipolar, ganglion cells; amacrine cells montage of electron micrographs through the full thickness of the inner plexiform layer were made for each species and were scored for synaptic contacts. Both absolute and relative quantitative differences were found between species. The ratio of amacrine cell (conventional) synapses to bipolar cell (ribbon) synapses, the absolute number of amacrine cell synapses and the number of inter-amacrine cell synapses were all found to be higher in those animals which are known to have relatively complex retinal ganglion cell receptive field properties. It is suggested that the amacrine cell is involved in mediating complex visual transformations in certain vertebrate retinas.     

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.     

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.     

Light and electron microscopy of the ground squirrel retina: Functional considerations

Light and electron microscopy of Golgi-impregnated ground squirrel retinas have revealed a range of morphological subtypes of bipolar, amacrine, and ganglion cells. There are at least seven subtypes of bipolar cells. Those subtypes in which the somata were high (sclerad) in the inner nuclear layer (3 subtypes) had axon terminals low (vitread) in the inner plexiform layer, and those with somata low in the inner nuclear layer (4 subtypes) had axon terminals high in the inner plexiform layer. The bipolar subtypes with high axon terminals made flat contacts with receptor cells, whereas all but one of the bipolar subtypes with low axon terminals made ribbon-related contacts with receptor cells. There are at least five subtypes of amacrine cells. The two subtypes which the Golgi method revealed most frequently were a broad-field, unistratified neuron with a dendritic spread in excess of 1,000 m?m and a narrow-field, diffuse neuron with a dendritic spread of about 30 m?m. The broad-field, unistratified cell had the lowest proportion of amacrine vs. bipolar cell synaptic input of the amacrine subtypes (43%), whereas the narrow-field, diffuse cell had one of the greatest proportions of amacrine cell input (96%). There are at least 15 subtypes of ganglion cells. The proportion of synaptic inputs to these cells ranged from 21% to 100% amacrine cell synapses. An attempt has been made to relate this new knowledge of retinal circuitry to the physiological output of the ganglion cells.     

Intraretinal differentiation in the synaptic organization of the inner plexiform layer of the pigeon retina

The inner plexiform layer at ten retinal loci in pigeon was examined by electron microscopy. Photomontages of the entire depth of the inner plexiform layer at each locus were analyzed with respect to the number of amacrine and bipolar synapses, their respective ratios, synaptic densities, percent amacrine synapses in serial configuration, synaptic layering patterns, and the effect of staining procedures on these quantities. The results show that the pigeon retina is not homogeneous regarding the structural complexity of the inner plexiform layer, but may be divided into four general areas in decreasing order of complexity: red field, temporal yellow field, nasal yellow field, and the area centralis. Significant differences in the amacrine synapse to bipolar synapse ration and amacrine synaptic density were observed across the retina, while bi-polar synaptic density and the percent of serial synapses were rather constant. Amacrine synapses displayed a layering pattern which was consistent throughout the retina; while bipolar synapses showed two patterns. It was further observed that the density of amacrine and bipolar synapses bears little relationship to the density of amacrine and bipolar cells in the immediately overlying inner nuclear layer. This suggests that the various retinal loci may be characterized by different proportions of the morphological types of amacrine and bipolar cells present in the pigeon retina. Based on recent studies which have shown that a relationship exists between the complexity of ganglion cell receptive fields and the synaptic complexity of the inner plexiform layer, it is suggested that the ganglion cells of pigeon would show a physiological differentiation among retinal loci consistent with the observed differences in the anatomical complexity of the inner plexiform layer.     

Synaptic organization of starburst amacrine cells in rabbit retina: analysis of serial thin sections by electron microscopy and graphic reconstruction

The synaptic organization of starburst amacrine cells was studied by electron microscopy of individual or overlapping pairs of Golgi-impregnated cells. Both type a and type b cells were analyzed, the former with normally placed somata and dendritic branching in sublamina a, and the latter with somata displaced to the ganglion cell layer and branching in sublamina b. Starburst amacrine cells were thin-sectioned horizontally, tangential to the retinal surface, and electron micrographs of each section in a series were taken en montage. Cell bodies and dendritic trees were reconstructed graphically from sets of photographic montages representing the serial sections. Synaptic inputs from cone bipolar cells and amacrine cells are distributed sparsely and irregularly all along the dendritic tree. Sites of termination include the synaptic boutons of starburst amacrine cells, which lie at the perimeter of the dendritic tree in the "distal dendritic zone." In central retina, bipolar cell input is associated with very small dendritic spines near the cell body in the "proximal dendritic zone." The proximal dendrites of type a and type b cells generally lie in planes or "strata" of the inner plexiform layer (IPL), near the margins of the IPL. The boutons and varicosities of starburst amacrine cells, distributed int he distal dendritic zone, lie in the "starburst substrata," which occupy a narrow middle region in each of the two sublaminae, a and b, in rabbit retina. As a consequence of differences in stratification, proximal and distal dendritic zones are potentially subject to different types of input. Type b starburst amacrines do not receive inputs from rod bipolar terminals, which lie mainly in the inner marginal zone of the IPL (stratum 5), but type a cells receive some input from the lobular presynaptic appendages of rod amacrine cells in sublamina a, at the border of strata 1 and 2. There is good correspondence between boutons or varicosities and synaptic outputs of starburst amacrine cells, but not all boutons gave ultrastructural evidence of presynaptic junctions. The boutons and varicosities may be both pre- and postsynaptic. They are postsynaptic to cone bipolar cell and amacrine cell terminals, and presynaptic primarily to ganglion cell dendrites. In two pairs of type b starburst amacrine cells with overlapping dendritic fields, close apposition of synaptic boutons was observed, raising the possibility of synaptic contact between them. The density of the Golgi-impregnation and other technical factors prevented definite resolution of this question. No unimpregnated profiles, obviously amacrine in origin, were found postsynaptic to the impregnated starburst boutons.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

ON-OFF amacrine cells in cat retina

We studied the morphology, photic responses, and synaptic connections of ON-OFF amacrine cells in the cat retina by penetrating them with intracellular electrodes, staining them with horseradish peroxidase, and examining them with the electron microscope. In a sample of seven cells, we found two different morphological types: the A19, which ramifies narrowly in stratum 2 (sublamina a ) of the inner plexiform layer, and the A22, which ramifies mostly in stratum 4 (sublamina b ) but extends some dendrites to sublamina a . Both of these cell types have axon-like processes that extend >800 μm from the conventional dendritic arbor. ON-OFF amacrine cells in our sample had receptive fields (1.7 ± 0.3 mm diameter) that were broader than their dendritic arbors (425 ± 35 μm diameter) and that extended over the region of axon-like processes. In addition, we found many features in common with ON-OFF amacrine cells in poikilotherm vertebrates: a broad receptive field without surround antagonism, two sizes of spike-like events, narrow dynamic range (1 log unit intensity), and excitatory postsynaptic potentials at light on and light off. Two A19 amacrine cells were examined in the electron microscope: most synaptic inputs (93 and 76%, respectively) to either cell were from amacrine cells, with minor inputs from cone bipolar cells. Synaptic outputs were to bipolar, amacrine, and ganglion cells, including the OFF-α cell. © 1996 Wiley-Liss, Inc.     

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.     

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.     

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.     

Synaptic Contacts and the Transient Dendritic Spines of Developing Retinal Ganglion Cells

The dendrites of ganglion cells in the mammalian retina become extensively remodelled during synapse formation in the inner plexiform layer. In particular, after birth in the cat, many short spiny protrusions are lost from the dendrites of ganglion cells during the time when ribbon, presumably bipolar, synapses appear in the inner plexiform layer and when conventional, presumed amacrine, synapses increase significantly in number. It has therefore been postulated that these transient spines may be the initial or preferred substrates for competitive interactions between amacrine or bipolar cell terminals that subsequently result in the formation of appropriate synapses onto the ganglion cells. If so, the majority of synapses made onto developing ganglion cells should be found on these dendritic spines. To test this hypothesis, we determined the synaptic connectivity of identified ganglion cells in the postnatal cat retina during the period of peak spine loss and synapse formation. The dendritic trees of ganglion cells were intracellularly filled with Lucifer yellow that was subsequently photo-oxidized into an electron-dense product suitable for electron microscopy. In serial reconstructions of the dendrites of a postnatal day 11 (P11) alpha ganglion cell and a P14 beta ganglion cell, conventional and ribbon synapses were found predominantly on dendritic shafts. Only three out of a total of 341 dendritic spines from the two cells received direct presynaptic input, all of which were conventional synapses. Thus, our observations suggest that the transient dendritic spines are not the preferred postsynaptic sites as previously suspected. However, it is possible that these structures play a different role in synaptogenesis, such as mediating interactions between retinal neurons that may lead to cell-cell recognition, a necessary step prior to synapse formation at the appropriate target sites (Cooper and Smith, Soc. Neurosci. Abstr. , 14 , 893, 1988).     

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.     

Localization of aspartate-like immunoreactivity in the retina of the turtle (Pseudemys scripta).

Aspartate has been reported to be a putative excitatory neurotransmitter in the retina, but little detailed information is available concerning its anatomical distribution. We used an antiserum directed against an aspartate-albumin conjugate to analyze the anatomy, dendritic stratification, and regional distribution of cell types with aspartate-like immunoreactivity in the turtle retina. The results showed dramatic differences in immunoreactivity in the peripheral versus the central retina. Strong aspartate-like immunoreactivity was shown in the peripheral retina, with many well-labeled processes in the inner plexiform layer. Many bipolar, horizontal, amacrine, and ganglion cells, some photoreceptors, and some unidentified cells were strongly immunoreactive in the peripheral retina. In contrast, although the central retina showed well-labeled horizontal cells, there was only light labeling in the inner plexiform layer with weakly immunoreactive amacrine and ganglion cells and no labeled bipolar cells. There were several strongly immunoreactive efferent nerve fibers which left the optic nerve head and arborized extensively in the retina. At the electron microscopic level, electron-dense reaction product was associated with synaptic vesicles at bipolar and amacrine cell synapses in the inner plexiform layer. These results suggest that aspartate may be involved in many diverse synaptic interactions in both the outer plexiform layer and the inner plexiform layer of the turtle 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.     

On and off sublaminae in the lateral geniculate nucleus of the ferret

Like the retinal ganglion cells from which they receive their input, most relay neurons in the lateral geniculate nucleus have ON- or OFF-center receptive fields with antagonistic surrounds. In the cat, neurons with these two types of receptive fields are anatomically intermingled, even though the ON and OFF systems are functionally segregated. In the ferret, there is a sublamination of the retinal input to lateral geniculate nucleus laminae A and A1. We have investigated the function of this sublamination by making microelectrode recordings and have found that each sublamina consists of geniculate neurons of a single center type.     

Class I and class II ganglion cells of rabbit retina: a structural basis for X and Y (brisk) cells

Morphological studies of rabbit retina have identified ganglion cells resembling "alpha" cells but none resembling cat "beta" cells. Four distinct types of class I cell, similar to alpha cells, were identified, each narrowly stratified and differing from the other three principally in the level of dendritic branching. These four levels of dendritic branching flank the two starburst/cholinergic amacrine cell substrata that mark the middle of sublaminae a and b. Compared with the other three, class Ia2 cells are the largest in cell body and dendritic field size, are sometimes homotypically dye coupled, and have slightly broader dendritic stratification. Class Ia2 and the slightly smaller class Ib2 cells form a paramorphic pair. Compared with class I cells, class II cells have smaller dendritic fields; a greater tendency to "tufted" dendritic branching, as shown in the companion paper; branching at one of three levels of the IPL; and similarly narrow stratification. Class IIa and class Ia1 cells branch at the same level, as do class IIb1 and class Ib1 cells. Class IIb2 cells branch slightly nearer the ganglion cell layer than class Ib2 cells and costratify with "blue-ON" cone bipolar cells. The class IIa and IIb2 cells form a paramorphic pair, whereas class IIb1 cells appear to be unpaired. The four types of class I cell probably correspond to ON- and OFF-center brisk-transient, fast-movement and slow-movement cells, whereas the three types of class II cell probably correspond to ON- and OFF-center brisk-sustained and color-coded ON-center X cells.     

Connectivity of glycine immunoreactive amacrine cells in the cat retina

The synaptic relationships of glycine immunoreactive amacrine cells in the cat retina were studied through the use of postembedding immunogold techniques. Glycine immunoreactive amacrine cells were found to synapse extensively with other amacrines and ganglion cells, particularly in strata 1-3 of the inner plexiform layer. This contrasts with GABA immunoreactive amacrine cells which provide major input to bipolar cells in strata 3-5. Glycine containing amacrine terminals exhibited diversity with respect to the morphology of their synaptic vesicles. The three types of terminals which could be distinguished were characterized by small pleomorphic (32-35 nm), medium-sized flattened (38-45 nm), or larger rounded (48-55 nm) vesicles. Comparison of retinal sections processed for glycine immunoreactivity with adjacent sections stained for GABA reactivity revealed a colocalization of glycine and GABA in 3% of the cells in the amacrine layer and approximately 40% of the cells in the ganglion cell layer. The amacrine terminals in which glycine and GABA were colocalized typically contained the small pleomorphic type of vesicles.