‘Starburst’ amacrine cells and cholinergic neurons: mirror-symmetric ON and OFF amacrine cells of rabbit retina

Golgi-impregnated 'starburst' amacrine cells share significant morphological features with cholinergic neurons in rabbit retina. They are mirror-symmetrical about the a/b (OFF/ON) sublaminar border of the inner plexiform layer. Type a starburst amacrines have cell bodies in the amacrine cell layer and dendrites in sublamina a, while type b cells have their cell bodies in the ganglion cell layer and dendrites in sublamina b of the inner plexiform layer (IPL). The two levels of narrow dendritic stratification are precisely those demonstrated by Masland and Mills for cholinergic amacrine cells. The morphological evidence indicates that the duality of ON and OFF pathways is served separately by type b (displaced) and type a starburst amacrine cells, respectively.     

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)     

Starburst amacrine cells in cat retina are associated with bistratified, presumed directionally selective, ganglion cells

Starburst amacrine cells of cat retina are similar in form, though more delicate and less profusely branched, when compared to the starburst/cholinergic amacrine cells of rabbit retina, as identified in Golgi preparations. In both species, type a cells branch in the middle of sublamina a of the inner plexiform layer (IPL), but type b (displaced) starburst amacrine cells of cat branch near the a/b sublaminar border (stratum 3) of the IPL, not in the middle of sublamina b (stratum 4), as do those of rabbit. Nevertheless, in each species, this starburst substratum in sublamina b coincides with the sublamina b-level branching of a bistratified ganglion cell, which in rabbit retina shows directionally selective responses. It is proposed that starburst amacrine cells of cat retina are cholinergic and, as in rabbit retina, make selective connections with on-off directionally selective ganglion cells.     

NADPH diaphorase histochemistry in the rabbit retina

NADPH diaphorase activity has been shown by histochemical staining to co-localize with markers for selective neurotransmitter candidates in various regions of the rat brain. The rabbit retina was therefore examined to determine if the technique stains a selective population of retinal neurons as well. Whole retinas of adult, male, pigmented rabbits are incubated with a specific reaction mixture containing nitro blue tetrazolium as the electron acceptor. Dark blue reaction product is deposited in two populations of cell bodies near the inner border of the inner nuclear layer (INL). One cell type is larger and more darkly stained than the second. The larger cells have 2-4 tapering primary dendrites which branch sparsely in the inner plexiform layer (IPL) and which can be traced for up to 500 microns. The second cell type has smaller and more lightly stained somata. In retinal cross sections, a dense layer of varicose fibers is seen in the middle (sublamina 3) of the IPL; these fibers arise at least in part from the larger, darkly stained cell bodies. A less dense plexus of fibers is stained at the outer margin (sublamina 1) of the IPL, and occasional varicosities are seen in the inner sublaminas (4 and 5) of the IPL. NADPH diaphorase histochemistry, therefore, selectively stains at least two subtypes of amacrine cells in rabbit retina. Although a definite identification of the transmitter content of these cells cannot be made, diaphorase histochemistry provides, in the retina, a remarkably convenient method for achieving Golgi-like images of morphologically distinct neuronal populations.     

Neuropeptide Y-like immunoreactive amacrine cells in the retina of Bufo marinus

Neuropeptide Y-like immunoreactive (NPY-LI) amacrine cells of the Bufo marinus retina were morphologically characterized, and their retinal distribution was established using immunohistochemistry on retinal wholemount preparations and sectioned material. The somas of NPY-LI amacrine cells were situated in the innermost part of the inner nuclear layer and their dendrites branched primarily in the scleral sublamina of the inner plexiform layer. A subgroup of the NPY-LI cells had dendrites in both the scleral and vitreal sublamina. All immunoreactive cells had large dendritic fields (average 0.5 mm2) that resulted in a high dendritic overlap across the retina. NPY-LI amacrine cells were evenly distributed across the retina, with an average density of 30 cells/mm2, although higher densities were observed at regions adjacent to the ciliary margin. The dendritic field size of the NPY-LI cells, together with the previously characterized substance P-like immunoreactive (SP-LI) amacrine cells, indicates that they belong to the class of wide-field amacrine cells. However, unlike the SP-LI neurons whose dendrites branch in the vitreal sublamina of the inner plexiform layer, the dendrites of the majority of the NPY-LI neurons branch in the scleral sublamina.     

Light microscopic localization of putative glycinergic neurons in the larval tiger salamander retina by immunocytochemical and autoradiographical methods

Putative glycinergic neurons in the larval tiger salamander retina were localized by a comparative analysis of high affinity 3H-glycine uptake and glycine-like immunoreactivity (Gly-IR) at the light microscopic level. Commonly labeled neurons include at least three types of amacrine cell (Type IAd, Type IAb, Type IIAd; distinguished by soma location and dendritic ramification), cell bodies in the ganglion cell layer (GCL), and rarely observed Type II (inner) bipolar cells. With the increased resolution provided by Gly-IR, we identified a Type IAa amacrine cell, two types of Type IIAd amacrine cells, and Gly-IR interplexiform cells. Gly-IR axons in longitudinal sections of the optic nerve indicate the presence of Gly-IR ganglion cells. The percentage of labeled somas in the inner nuclear layer (INL) compared to all cells in each layer was similar for the two methods: 30-40% in INL 2 (middle layer of somas), 30-40% in INL 3 (inner layer of somas), and about 5% in the GCL. Labeled processes were found throughout the full thickness of the inner plexiform layer (IPL), but with a much denser band in the proximal half (sublamina b). The only major difference between the two methods (3H-glycine uptake vs. Gly-IR) was that Type I (outer) bipolar cells were labeled only by 3H-glycine uptake; these cells were more lightly labeled with silver grains than cell bodies in either INL 2 or INL 3. Postembed labeling of 1 micron Durcupan plastic sections for Gly-IR showed the same pattern, but with much higher resolution, as obtained with 10 micron cryostat sections. This study indicates extensive colocalization of labeling by both probes in INL 2, INL 3, the IPL, and the GCL. We conclude that Gly-IR can serve as a valid and reliable marker for glycine-containing neurons in this retina and suggest that glycine serves as a transmitter for several morphologically distinct types of amacrine cell, an interplexiform cell, and perhaps a small percentage of Type II bipolar cells and ganglion cells.     

Neuropeptide Y-like immunoreactive amacrine cells in the retina ofBufo marinus

Neuropeptide Y-like immunoreactive (NPY-LI) amacrine cells of theBufo marinus retina were morphologically characterized, and their retinal distribution was established using immunohistochemistry on retinal wholemount preparations and sectioned material. The somas of NPY-LI amacrine cells were situated in the innermost part of the inner nuclear layer and their dendrites branched primarily in the scleral sublamina of the inner plexiform layer. A subgroup of the NPY-LI cells had dendrites in both the scleral and vitreal sublamina. All immunoreactive cells had large dendritic fields (average 0.5 mm²) that resulted in a high dendritic overlap across the retina. NPY-LI amacrine cells were evenly distributed across the retina, with an average density of 30 cells/mm², although higher densities were observed at regions adjacent to the ciliary margin. The dendritic field size of the NPY-LI cells, together with the previously characterized substance P-like immunoreactive (SP-LI) amacrine cells, indicates that they belong to the class of wide-field amacrine cells. However, unlike the SP-LI neurons whose dendrites branch in the vitreal sublamina of the inner plexiform layer, the dendrites of the majority of the NPY-LI neurons branch in the scleral sublamina.     

Localization of GABA, glycine, glutamate and tyrosine hydroxylase in the human retina.

A light microscope study using postembedding immunocytochemistry techniques to demonstrate the common neurotransmitter candidates gamma-aminobutyric acid (GABA), glycine, glutamate, and tyrosine hydroxylase for dopamine has been done on human retina. By using an antiserum to GABA, we found GABA-immunoreactivity (GABA-IR) to be primarily in amacrine cells lying in the inner nuclear layer (INL) or displaced to the ganglion cell layer (GCL). A few stained cells in the INL, which are probably interplexiform cells, were observed to project thin processes towards the outer plexiform layer (OPL). There were heavily stained bands of immunoreactivity in strata 1, 3 and 5 of the inner plexiform layer (IPL). An occasional ganglion cell was also GABA-IR. By using an antiserum to glycine, stained cells were observed at all levels of the INL. Most of these were amacrines, but a few bipolar cells were also glycine-IR. Displaced amacrine cells and large-bodied cells, which are probably ganglion cells, stained in the GCL. The bipolar cells that stained appeared to include both diffuse and midget varieties. The AII amacrine cell of the rod pathway was clearly stained in our material but at a lower intensity than two other amacrine cell types tentatively identified as A8 and A3 or A4. Again, there was stratified staining in the IPL, with strata 2 and 4 being most immunoreactive. An antiserum to glutamate revealed that most of the neurons of the vertical pathways in the human retina were glutamate-IR. Rod and cone photoreceptor synaptic endings labeled as did the majority of bipolar and ganglion cells. The rod photoreceptor stained more heavily than the cone photoreceptor in our material. While both midget and diffuse cone bipolar cell types were clearly glutamate-IR, rod bipolars were not noticeably stained. The most strongly staining glutamate-IR processes of the IPL lay in the outer half, in sublamina a. The antiserum to tyrosine hydroxylase (TOH) revealed two different amacrine cell types. Strongly immunoreactive cells (TOH1) had their cell bodies in the INL and their dendrites ramified in a dense plexus in stratum 1 of the IPL. Fine processes arising from their cell bodies or from the stratum 1 plexus passed through the INL to reach the OPL but did not produce long-ranging ramifications therein. The less immunoreactive amacrines (TOH2) lay in the INL, the center of the IPL or the GCL and emitted thick dendrites that were monostratified in stratum 3 of the IPL.     

Development of serotoninergic chick retinal neurons

Numerous neurotransmitters have been studied in detail in the developing retina. Almost all known neurotransmitters and neuromodulators were demonstrated in vertebrate retinas using formaldehyde-induced fluorescence, uptake autoradiography or immunohistochemistry procedures. Serotoninergic (5HT) amacrine neurons were described in the inner nuclear layer (INL) of the retina with their dendrites spreading within the inner plexiform layer (IPL). The present work describes the morphological pattern of development of serotoninergic amacrine neurons with a stratified dendritic branching pattern in the chick retina from embryonic day 12 to postnatal day 7. Serotoninergic-bipolar neurons are also described. 5HT-amacrine neurons have round or pear-shaped somata and primary dendritic trees oriented toward the IPL that runs through the INL, showing several varicosities. Secondary dendrites then go through the INL, without any collateral branch. At the outer and inner margin of the IPL the primary and secondary dendrites originate an outer and an inner serotoninergic network, respectively. When the primary dendritic tree reaches the IPL it deflects laterally in sublayer 1—the outer serotoninergic network. Tertiary branches then arise from the secondary dendrite and deflect in the innermost sublayer of the IPL— the inner serotoninergic network. The final pattern of branching of 5HT amacrine cells was present at embryonic day 14 and was completely developed at hatching. Serotoninergic (5HT) bipolar neurons were also present in the INL at hatching. They are weakly immunoreactive and are probably a subset of bipolar cells that accumulate serotonin from the intersynaptic cleft and are not ‘‘true’’ 5HT neurons.     

Localization of neuropeptide-immunoreactive neurons in the human retina.

Light microscopic immunocytochemistry was utilized to localize populations of neurons in the human retina immunoreactive for the following neuroactive peptides: substance P (SP), vasoactive intestinal polypeptide (VIP), somatostatin (SOM) and LANT-6-(H-Lys-Asn-Pro-Tyr-Ile-Leu-OH), a hexapeptide which is identical to the C-terminal half of neurotensin except for the amino acid substitutions Lys/Arg and Asn/Arg. The majority of SP immunoreactive cells were amacrine cells whose pear-shaped or oval cell bodies (about 8 microns in diameter) were situated in the proximal parts of the inner nuclear layer. A small number of SP-stained somas (about 10-15 microns in diameter) were located in the ganglion cell layer and were designated as those of displaced amacrine cells. The SP-immunoreactive processes were distributed in sublamina 1, 3 and 5 with the most dense plexus being found in sublamina 3 of the inner layer. VIP-positive cell bodies (8-9 microns) were oval or pear-shaped and were situated in the innermost cell rows of inner nuclear layer. The majority of fine VIP-immunoreactive processes extended to sublamina 3 with only a few branches distributing in sublamina 1 of the inner plexiform layer. The SOM-stained cell bodies (10-11 microns) were round and were situated in the innermost cell rows of inner nuclear layer. SOM-positive processes were observed in sublamina 1 and 2 of the inner plexiform layer. The LANT-6 immunoreactive cell bodies (12-22 microns) were either oval-, round- or pyriform-shaped and were situated in ganglion cell layer.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dendritic morphology of interstitial amacrine cells with monostratified dendrites in different-sized carp retinas.

The dendritic morphology of a class of interstitial (IS) amacrine cells in retinas of different-sized carp (body length, 9.1-32.3 cm) was investigated by identifying their fluorescent nuclei pre-loaded with 4,6-diamidino-2-phenylindole (DAPI), followed by iontophoretic injection of Lucifer yellow (LY) in isolated and formaldehyde-fixed flat-mounts under microscopic control. The LY-injected fusiform or pyriform cell bodies were found to locate at the middle of the inner plexiform layer (IPL) or immediately beneath the amacrine cell layer, and their dendrites monostratified in sublamina b of the IPL. The pyriform cells had a short stem from which extended 4-5 stout dendrites, while the fusiform cells extended similar dendrites from the soma. The dendrites of both types of cell were decorated with spines and a few long axon-like processes. The pyriform cells were found more frequently in smaller retinas than in larger retinas, suggesting that the former may migrate proximally during retinal growth. The dendritic field sizes of these IS amacrine cells were wider as the fish became larger, while the dendritic morphology, analyzed by the Sholl's branching model, was very similar in smaller and larger retinas. The results indicate that the IS amacrine cells do not add dendrites, but that their dendritic trees simply expand during retinal growth.     

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.     

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.     

Electron microscopical observations on the indoleamine-accumulating neurons and their synaptic connections in the retina of the cat

The distribution of indoleamine-accumulating amacrine cells and their synaptic connections in the retina of the cat were analyzed in the fluorescence, phase-contrast, and electron microscopes. The findings were compared to recently characterized morphological subclasses of amacrine cells. The indoleamine-accumulating neurons were visualized after labeling with an exogenous indoleamine, 5, 6-dihydroxytryptamine. The intravitreal injection of the labeling drug was preceded by treatment with the neurotoxic dopamine-analogue, 6-hydroxydopamine, in order to destroy the otherwise interfering dopaminergic processes. The analysis in the fluorescence and phase-contrast microscopes confirmed earlier reports that the indoleamine-accumulating cell bodies and processes have a distribution consistent with that of amacrine cells. A stratified branching pattern of the indoleamine-accumulating processes in the outer half of the inner plexiform layer was discovered. In the inner half of that layer the branching pattern is diffuse. In the electron microscope the indoleamine-accumulating neurons were seen to have synapses of the conventional type. Their main synaptic contacts are reciprocal synapses with rod bipolar terminals in sublamina b of the inner plexiform layer. They also have synapses with flat cone bipolar terminals in sublamina a, and occasionally with amacrine cells and ganglion cells throughout the inner plexiform layer. Synapses with invaginating cone bipolar terminals, in sublamina b, appear to be rare. The synaptic arrangement with reciprocal synapses with rod bipolar terminals is similar to that of the recently reported AI amacrine cells. It is also similar to that of the indoleamine-accumulating neurons in the retinae of other mammals investigated earlier.     

Light- and electron-microscopic analysis of vasoactive intestinal polypeptide-immunoreactive amacrine cells in the guinea pig retina

Vasoactive intestinal polypeptide (VIP) is a neuroactive substance that is expressed in both nonmammalian and mammalian retinas. This study investigated the morphology and synaptic connections of VIP-containing neurons in the guinea pig retina by immunocytochemistry, by using antisera against VIP. Specific VIP immunoreactivity was localized to a population of wide-field and regularly spaced amacrine cells with processes ramifying mainly in strata 1 and 2 of the inner plexiform layer (IPL). Double-label immunohistochemistry demonstrated that all VIP-immunoreactive cells possessed gamma-aminobutyric acid immunoreactivity. The synaptic connectivity of VIP-immunoreactive amacrine cells was identified in the IPL by electron microscopy. The VIP-labeled amacrine cell processes received synaptic input from other amacrine cell processes and bipolar cell axon terminals in strata 1 to 3 of the IPL. The most frequent postsynaptic targets of VIP-immunoreactive amacrine cells were other amacrine cell processes in strata 1 to 3 of the IPL. Synaptic outputs to bipolar cells were also observed in strata 1 to 3 of the IPL. In addition, ganglion cell dendrites were also postsynaptic to VIP-immunoreactive neurons in the sublamina a of the IPL. These studies show that one type of VIP-immunoreactive amacrine cells make contact predominantly with other amacrine cell processes. This finding suggests that VIP-containing amacrine cells may influence inner retinal circuitry, thus mediating visual processing.     

Cholinergic and GABAergic neurons occur in both the distal and proximal turtle retina.

Turtle retinas were processed immunocytochemically and histochemically to detect the presence of choline acetyltransferase (ChAT), acetylcholinesterase (AChE), and glutamate decarboxylase (GAD). We observed cholinergic and gamma-aminobutyric acid (GABA)ergic neurons in the proximal retina, as expected, and in the distal retina as well. ChAT immunoreactivity in the distal retina was observed within the axons and pedicles of numerous cone photoreceptors, suggesting that a population of turtle cone photoreceptors uses ACh as a neurotransmitter. Type L2 horizontal cells were immunoreactive for GAD, and their dendrites invaginated into cone pedicles. AChE histochemistry revealed processes within the outer plexiform layer which formed a loosely organized lattice. In the proximal retina, labeling for ChAT and GAD was similar to that reported by previous investigators. Processes from ChAT-labeled amacrine cells in the inner nuclear layer formed a stratum within the distal inner plexiform layer (IPL) (at 16-21% relative IPL depth), and processes from ChAT-labeled amacrines in the ganglion cell layer formed a proximal ChAT stratum (at 55-58% relative IPL depth). In addition, six AChE-labeled bands and five GAD-labeled bands were observed within the IPL of stained retinas. Therefore, we determined that the two broadest AChE-labeled bands and the two broadest GAD-labeled bands overlapped the two labeled ChAT strata. The evidence for cholinergic and GABAergic processes in both the inner plexiform layer and the outer plexiform layer, combined with electrophysiological evidence from other investigators, raises the possibility that distal retinal neurons may be involved in the encoding of directional information.     

Morphology and distribution of neurons immunoreactive for substance P in the turtle retina

Immunocytochemical staining procedures with the HRP-complexed antibody to substance P have been carried out on the turtle retina. Examination by light microscopy of wholemount retinas has allowed us to evaluate the morphology and distribution of the substance P immunoreactive cell types. Two amacrine cell types and two or more ganglion cell types are stained in our hands. Type A amacrines are tri-stratified wide-field amacrines. They have their major dendrites in S1 and S3 of the inner plexiform layer and they emit fine dendrites from the major dendrites that end in varicose boutons in S5 on and around cell bodies in the ganglion cell layer. Some of the dendrites in S1 radiate out in axon-like fashion for 1 mm across the retina. The type B amacrine cells are small to medium-field in dendritic extent. They have smaller cell bodies than type A and a single or, at most, two primary dendrites that pass directly to S3 before branching profusely into an intricate net-like dendritic field. The ganglion cells that are stained with substance P antibodies appear to be of several types but their exact morphologies are in doubt because only portions of their major dendrites are stained. Substance P immunoreactive axons are clearly seen to project from the cell bodies to the optic nerve head and axons are stained in the optic nerve itself. The substance P-stained ganglion cells occur in an irregular distribution that reaches a peak density in an elongated band parallel to and 1 mm below the visual streak. The type B amacrine cells reach a maximum density in the visual streak and are distributed in a highly regular mosaic decreasing in density in elliptical isodensity contours from the visual streak. In contrast the type A amacrine cells are rare or absent in the streak, being located in an irregular mosaic in peripheral retina.     

Choline acetyltransferase is expressed by non-starburst amacrine cells in the ground squirrel retina

We have used immunostaining techniques to reveal a new type of amacrine cell that is immunoreactive for choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme, in the Ground Squirrel (Spermophilus beecheyi) retina. Cryostat sections and double immunostained wholemount preparations were examined by confocal microscopy. This new ChAT type III cell is distinct in morphology and neurotransmitter content from the well know 'starburst' amacrine cells (types I and II) that are so well represented in the ground squirrel retina [J. Comp. Neurol. 365 (1996) 173-216]. The type III cell colocalizes glycine with the acetylcholine and does not appear to be GABAergic or exhibit calcium-binding proteins like the well-known starburst type. As well, type III cells do not occur as a mirror-symmetric pair with normally placed and displaced varieties. The type III cell is probably a small field amacrine type branching broadly in upper sublamina b of the inner plexiform layer, and is most likely A6 of the Ground Squirrel retina [J. Comp. Neurol. 365 (1996) 173-216]. Type III cells are ideally placed in the architecture of the Ground Squirrel retina to influence ON directionally selective ganglion cell types.     

Polyaxonal amacrine cells of rabbit retina: morphology and stratification of PA1 cells.

Polyaxonal amacrine cells are a new class of amacrine cell bearing one to six branching, axon-like processes, closely resembling the axons of Golgi type II cells found elsewhere in the central nervous system. Of the four types of polyaxonal amacrine cell that we have recognized in rabbit retina, three have been described previously in brief communications, and one is the subject of this paper. Type 1 polyaxonal (PA1) amacrine cells have larger cell bodies than most amacrine cells in Golgi preparations, averaging about 13 microns in diameter. These are typically positioned interstitially in the middle of the inner plexiform layer (IPL), although some are also found in the amacrine and ganglion cell layers. Axons and dendrites are broadly stratified in the middle of the IPL, in the vicinity of the a/b sublaminar border. Sparsely branching dendrites have a conventional appearance, branching at a narrow angle, and giving rise to smaller daughter branches, which taper gradually toward their termination. An unusual feature of the dendrites is the zig-zag course of some terminal branches. Clusters of small, pedunculated spines are common on proximal dendrites, and spines are virtually absent on axons. Axons emerge from proximal dendrites within 50 microns of the soma, and more rarely from the soma, in a tapering initial segment, commonly interrupted by one or two large swellings. Subsequent branching is at a wide angle, and the fine caliber is maintained in the transition from parent to daughter branches. The uniform thickness of the axonal branches is interrupted at intervals by boutons en passant. Although the extent of the dendritic tree is large, exceeding 500 microns in radial extent from the cell body, for cells a few millimeters distant from the visual streak, the axonal tree is much larger, and its radial extent is measured in millimeters. PA1 amacrine cells are believed to be polarized in their functional organization, with a primarily recipient dendritic tree and a primarily transmissive axonal tree. PA1 amacrine cells co-stratify with nab cone bipolar cells and with certain small tufted amacrine and ganglion cells at the a/b sublaminar border. The co-stratification of both axons and dendrites at the a/b sublaminar border of the IPL suggests that PA1 amacrine cells are important modulators of neural activity in the middle of the IPL, affecting both ON and OFF responses, and perhaps ON-OFF cells selectively.     

Morphological classification of retinal ganglion cells in mice

Mice have been used for extensive studies on optic nerves and retinal ganglion cells, but mouse retinal ganglion cells have not been classified morphologically. In the present study, normally placed retinal ganglion cells and displaced retinal ganglion cells in pigmented and albino mice were classified morphologically using horseradish peroxidase. These cells were classified into three types according to the sizes of the soma and the dendritic field: type I cells, large soma and large dendritic field; type II cells, small-to-medium soma and small dendritic field; and type III cells, small-to-medium soma and large dendritic field. Some ganglion cells had both symmetric and asymmetric cells. Each type was further subdivided according to the termination level of dendrites in the inner plexiform layer and the dendritic branching pattern. Except for type III displaced ganglion cells, dendrites of the normally placed ganglion cells and the displaced ganglion cells ramify in the outer two-fifths of the inner plexiform layer (sublamina a) or the inner three-fifths of the inner plexiform layer (sublamina b). Type III displaced ganglion cells ramify only in sublamina a. Dendrites of some normally placed type I ganglion cells ramify in both sublaminae. Displaced biplexiform cells were observed, the dendrites of which ramify in both the inner and the outer plexiform layers. All cell types were found in both mouse strains. © 1995 Wiley-Liss, Inc.