Polyaxonal amacrine cells of rabbit retina: size and distribution of PA1 cells.

Type 1 polyaxonal (PA1) amacrine cells have been identified previously in rabbit retina, and their morphological characteristics have been described in detail in the preceding paper. Like other polyaxonal amacrine cells they bear distinct dendritic and axonal branching systems, the latter of which originates in two to six thin, branching axons which emerge from or near to the cell body. Unlike other types of polyaxonal amacrine cells, however, their branching is stratified at the a/b sublaminar border and their cell bodies are most often displaced interstitially in the inner plexiform layer (IPL). This report emphasizes quantitative features of the population of PA1 cells, documented in Golgi-impregnated and Nissl-stained retinas, and provides further evidence in Nissl preparations for the amacrine-cell nature of polyaxonal amacrine cells. The cell bodies of Golgi-impregnated PA1 amacrine cells are relatively large: 12-15 microns in equivalent diameter over the range extending from the visual streak 6 mm into ventral retina. Over the same range, dendritic trees are 400-800 microns in equivalent diameter, but they are much smaller than the axonal arborizations, which extend up to and perhaps beyond 2 mm from the cell body. Interstitial cell bodies appropriate to PA1 cells have been identified in Nissl-stained, whole-mounted rabbit retinas. In the plane of the retina, these are comparable in area to smaller medium-size ganglion cells, but their very pale Nissl staining, high nuclear/cytoplasmic ratio, and absence of nucleolar staining are all characteristics of amacrine cells. Interstitial displacement of presumed PA1 cells is rare in the visual streak, and the frequency of interstitial cells reaches a peak between 1 and 2 mm ventral to the streak. Counts in Nissl-stained retinas and estimates from nearest neighbor analyses in these and in Golgi-impregnated retinas indicate a density of PA1 cells in the range of 15-16 cells/mm2 at about 2 mm ventral to the streak, when an estimated 25% shrinkage of the material is taken into account. Dendritic field overlap, based upon this estimate, is calculated to be about fourfold, while a lower bound to estimates of the overlap of axonal arborizations is nearly an order of magnitude higher. Many similarities are noted in a qualitative and quantitative comparison of PA1 amacrine cells in rabbit and monkey retinas. In assessing the contribution of the structural organization of PA1 amacrine cells to their possible functional role(s), it is notable that their appearance conforms not to amacrine cells as commonly viewed, but to a more conventional model of neuronal dynamic polarization.(ABSTRACT TRUNCATED AT 400 WORDS)     

Polyaxonal amacrine cells of rabbit retina: PA2, PA3, and PA4 cells. Light and electron microscopic studies with a functional interpretation.

Polyaxonal (PA) amacrine cells are a new class of amacrine cell bearing one to six branching, axon-like processes that emerge from the cell body or dendritic trees within 50 microns of the cell body. These slender processes of uniform caliber branch at right angles and in many respects closely resemble the axons of Golgi type II cells found elsewhere in the brain. Of the four types of polyaxonal amacrine cell that we have recognized in rabbit retina, two have been described previously in brief communications. One of these, the PA1 amacrine cell with its interstitially displaced cell body, located in the inner plexiform layer (IPL), has been analyzed extensively in two preceding reports. This paper concerns PA2, PA3, and PA4 amacrine cells. Type 2 polyaxonal (PA2) amacrine cells, identified in Golgi preparations of whole-mounted rabbit retinas, have smaller cell bodies (9-14 microns) than the other three types and these are always displaced to the ganglion cell layer (GCL) or the inner border of the inner plexiform layer (IPL). The dendritic fields of PA2 cells are also smaller than those of other PA amacrine cells, and most of their sparse dendritic branching is narrowly stratified at the border of strata (S) 4 and 5. Some members of this more heterogeneous amacrine cell "type" are bistratified, however, and more highly branched with terminal branches rising to end in S1. PA2 amacrine cells bear a scattering of small dendritic spines and may also exhibit complex dendritic appendages arising at the ends of terminal branches in proximal regions of the dendritic tree. PA2 cells emit one to three axons from the proximal dendritic tree, and about half of the cells bear a single axon. Type 3 polyaxonal (PA3) amacrine cells resemble PA1 cells in the large size of their cells bodies (11-16 microns) and dendritic fields, but differ from the latter in placement of cell bodies, which is in the GCL, and dendritic and axonal stratification, which is multistratified, ranging from S4 to S1, with a concentration in S3 or S4 and a variable contribution to S1. PA3 cells differ from PA1 cells in several other respects, including dendritic branching which occurs at higher frequency and is biased toward temporal retina, and in characteristic bristling dendritic spines, clustered in the intermediate regions of the dendritic tree, that are longer, more variable in appearance and more tightly clustered than the small, uniform spines of PA1 cells that are clustered on proximal dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)     

Morphology and physiology of the polyaxonal amacrine cells in the rabbit retina.

We examined the morphology and physiological response properties of the axon-bearing, long-range amacrine cells in the rabbit retina. These so-called polyaxonal amacrine cells all displayed two distinct systems of processes: (1) a dendritic field composed of highly branched and relatively thick processes and (2) a more extended, often sparsely branched axonal arbor derived from multiple thin axons emitted from the soma or dendritic branches. However, we distinguished six morphological types of polyaxonal cells based on differences in the fine details of their soma/dendritic/axonal architecture, level of stratification within the inner plexiform layer (IPL), and tracer coupling patterns. These morphological types also showed clear differences in their light-evoked response activity. Three of the polyaxonal amacrine cell types showed on-off responses, whereas the remaining cells showed on-center responses; we did not encounter polyaxonal cells with off-center physiology. Polyaxonal cells respected the on/off sublamination scheme in that on-off cells maintained dendritic/axonal processes in both sublamina a and b of the IPL, whereas processes of on-center cells were restricted to sublamina b. All polyaxonal amacrine cell types displayed large somatic action potentials, but we found no evidence for low-amplitude dendritic spikes that have been reported for other classes of amacrine cell. The center-receptive fields of the polyaxonal cells were comparable to the diameter of their respective dendritic arbors and, thus, were significantly smaller than their extensive axonal fields. This correspondence between receptive and dendritic field size was seen even for cells showing extensive homotypic and/or heterotypic tracer coupling to neighboring neurons. These data suggest that all polyaxonal amacrine cells are polarized functionally into receptive dendritic and transmitting axonal zones.     

Postnatal development of tyrosine hydroxylase immunoreactive amacrine cells in the rabbit retina: I. Morphological characterization.

The present and accompanying (Casini, G., and N.C. Brecha, J. Comp. Neurol. 326:302-313, 1992) papers investigate the postnatal development of tyrosine hydroxylase (TH)-immunoreactive (IR) amacrine cells in the rabbit retina. This study is focused on a detailed analysis of the patterns of cellular growth and differentiation of TH-IR amacrine cells, which serve as a model to gain insights into the mechanisms underlying developmental changes associated with the maturation of amacrine cells. Faintly staining TH-IR neurons are present in the proximal inner nuclear layer of newborn retinas. They are characterized by a large nucleus and usually a single primary process lacking varicosities. At postnatal day (PND) 6, TH-IR processes display more complex morphological characteristics, including a few varicosities, and second- and third-order ramifications. Growth cones are often seen. At PNDs 10 and 12 (eye opening), TH-IR cells have general morphological characteristics similar to adult TH-IR amacrines. They display 2-5 primary processes, which start forming a complex network of fibers in lamina 1 of the inner plexiform layer (IPL). TH-IR processes are also present in lamina 3 and rarely in lamina 5 of the IPL. Many fibers ending in growth cones are observed. In addition, very rare, thin TH-IR fibers are present in the outer plexiform layer. At PND 19, TH-IR fibers form a complex, dense network in lamina 1 of the IPL, and loose networks in laminae 3 and 5. Growth cones are not observed at this age. At PND 26, a few "ring-like" structures formed by TH-IR fibers in lamina 1 of the IPL are observed for the first time. In adult retinas, the "ring-like" structures are more numerous than at PND 26. A second, rare type of TH-IR cell ("type B") is encountered in all retinal regions beginning at PND 10. These cells are characterized by weak immunostaining and a small soma size. The present findings show that a significant differentiation of TH-IR neurons occurs during the first 10-12 PNDs. Eye opening is an important period for the maturation of TH-IR amacrines and, more generally, for the maturation of the IPL.     

Postnatal development of tyrosine hydroxylase immunoreactive amacrine cells in the rabbit retina: II. Quantitative analysis.

Tyrosine hydroxylase (TH)-immunoreactive (IR) amacrine cells of the rabbit retina mature during the first four postnatal weeks, and their cellular development is described in the preceding paper (Casini, G., and N.C. Brecha, J. Comp. Neurol. 326:283-301, 1992). The present investigation is a quantitative analysis of the postnatal development of the TH-IR amacrine cell population. TH-IR amacrine cells gradually increase in size from birth (soma area of 44.7 +/- 12.4 microns2, mean +/- standard deviation) to adulthood (144.2 +/- 28.0 microns2). Cell density slightly increases from postnatal day (PND) 0 (41.9 +/- 9.5 cells/mm2) to PND 6 (47.2 +/- 7.2 cells/mm2), then markedly decreases from PND 6 to adulthood (17.8 +/- 5.3 cells/mm2) as a consequence of retinal growth. TH-IR cell number almost doubles from PND 0 (about 4,100 cells/retina) to adulthood (about 7,850 cells/retina). The increase in the total number of TH-IR amacrine cells can be explained by the generation of new TH-IR cells in the inner nuclear layer, a delay in the expression of the TH phenotype after neurogenesis by cells committed to be dopaminergic, or the acquisition of this dopaminergic phenotype by uncommitted cells. The development of the TH-IR amacrine cell mosaic was assessed by an evaluation of the distribution of nearest neighbor distances of TH-IR cells. There is a poor correlation between this distribution and a theoretical nonrandom distribution before PND 12. After this age, the nearest neighbor distance distribution shifts towards a nonrandom distribution, and is similar to that of the TH-IR amacrine cell population in the adult retina. The establishment of the TH-IR amacrine cell population mosaic is likely to be achieved through different interacting events, including intrinsic (e.g., genetic) factors, environmental influences, and nonuniform retinal growth. Overall, the population parameters analyzed in the present study approach adult values about the time of eye opening (PND 12) and they reach adult values by PND 26.     

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)     

Vasoactive intestinal polypeptide-containing cells in the rabbit retina: immunohistochemical localization and quantitative analysis

Vasoactive intestinal polypeptide (VIP) possesses neuroactive properties in the nervous system. In this study we characterized VIP immunoreactive neurons in the rabbit retina to provide a basis for a better understanding of the role of this peptide in retinal functions and to further define the morphology of wide-field amacrine cells. VIP immunoreactivity was demonstrated in colchicine-treated retinas. Immunolabeling was observed in amacrine cells located in the proximal inner nuclear layer and, occasionally, in the ganglion cell layer and inner plexiform layer (IPL). Varicose fibers were distributed in laminae 1, 3, and 5 of the IPL. The distribution of VIP immunoreactive cells showed a peak of approximately 50 cells/mm2 in the visual streak and minimum values of approximately 12 cells/mm2 in the peripheral retina. The total number of VIP immunopositive neurons was estimated to be about 11,000. Cell body diameters in the visual streak (8-9 microns) were slightly smaller than those measured in the dorsal or in the ventral retina (9-10 microns). The distribution of nearest neighbor distances (approximately 109 microns in the visual streak and approximately 99 microns in the peripheral retina) showed that VIP immunoreactive neurons were nonrandomly spaced. Labeled neurons emitted one to three thick primary processes, arborizing in secondary processes and collaterals rich in varicosities; these processes often crossed among different IPL laminae. Arborization fields of individual cells overlapped extensively. In the dorsal retina, estimated areas of single arborization fields were larger and processes had lower branching frequency than in the visual streak and in the ventral retina. On the whole, VIP immunoreactive amacrine cells gave rise to a loose meshwork of fibers in the IPL. These characteristics indicate VIP is contained in a class of wide-field amacrine cells and is likely to be involved in widespread regulatory or modulatory functions rather than in the direct transmission of visual information through the retina.     

Dendritic maturation of displaced putative cholinergic amacrine cells in the rabbit retina

The dendritic trees of Cb, cholinergic, amacrine cells in the ganglion cell layer of the developing rabbit retina are revealed by intracellular injection with Lucifer yellow to have the adult dendritic branching pattern at birth. It is demonstrated that these cells maintain a constant number of dendritic branches throughout postnatal development and that their dendritic trees increase in size by the growth and subsequent elongation of all branches. Proximal and distal dendrites increase in length by almost the same proportions between birth and adulthood. Although the adult pattern of dendritic branching of Cb amacrine cells is established by birth, dendrites in the young possess numerous short appendages (1-5 microns in length) resembling the "dendritic spines" of immature cat retinal ganglion cells. Some of these structures remain on the dendrites of adult cells but the majority are lost at the end of the third postnatal week. As dendritic spines disappear, the dendrites of Cb amacrine cells, especially the distal portion of the tree, acquire numerous varicosities. At each stage after P10, the gain in the number of varicosities greatly exceeds the loss in spines; this is not consistent with the hypothesis that all varicosities are retracted dendritic spines. The rapid increase in the number of varicosities on distal dendrites of Cb amacrine cells during the first 3 postnatal weeks coincides with the maturation of amacrine cell physiological responses. There is no distinct centroperipheral gradient in the postnatal dendritic maturation (acquisition of varicosities, loss of spines, attainment of the adult number of branches) of Cb amacrine cells from the visual streak to the peripheral retina. However, the area of their dendritic tree increases relatively more in the retinal periphery compared to that in the visual streak.     

‘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)     

Glycinergic amacrine cells of the rat retina

Physiological studies of neurons of the inner retina, e.g., of amacrine cells, are now possible in a mammalian retinal slice preparation. The present anatomical study characterizes glycinergic amacrine cells of the rat retina and thus lays the ground for such future physiological and pharmacological experiments. Rat retinae were immunolabeled with antibodies against glycine and the glycine transporter-1 (GLYT-1), respectively. Glycine immunoreactivity was found in approximately 50% of the amacrine and 25% of the bipolar cells. GLYT-1 immunoreactivity was restricted to glycinergic amacrine cells. They were morphologically characterized by the intracellular injection of Lucifer Yellow followed by GLYT-1 immunolabeling. Eight different types of glycinergic amacrine cells could be distinguished. They were all small-field amacrine cells with bushy dendritic trees terminating at different levels within the inner plexiform layer. The well-known AII amacrine cell was encountered most frequently. From our measurements of the dendritic field sizes and the density of glycinergic cells, we estimate that there are enough glycinergic amacrine cells available to make sure that all eight types and possibly more tile the retina regularly with their dendritic fields. J. Comp. Neurol. 401:34–46, 1998. © 1998 Wiley-Liss, Inc.     

Characterization of secretagogin‐immunoreactive amacrine cells in marmoset retina

The retina contains at least 30 different types of amacrine cells but not many are well characterized. In the present study the calcium‐binding protein secretagogin was localized in a population of regular and displaced amacrine cells in the retina of the common marmoset Callithrix jacchus. Irrespective of their soma location, the dendrites of secretagogin amacrine cells occupy strata 2, 3, and 4 of the inner plexiform layer, between the two bands formed by cholinergic amacrine cells. Segretagogin amacrine cells are also immunopositive to antibodies against glutamic acid decarboxylase, suggesting that they use γ‐aminobutyric acid (GABA) as their neurotransmitter. The spatial density of secretagogin amacrine cells decreases from a peak of about 400 cells/mm² near 1 mm eccentricity to less than 100 cells/mm² in peripheral retina; these densities account for about 1% of amacrine cells in the inner nuclear layer and for up to 27% of displaced amacrine cells. The cell bodies form a regular mosaic, suggesting that they constitute a single amacrine cell population. Secretagogin cells have varicose dendrites, which are decorated with small spines. Intracellular injection of DiI into secretagogin cells revealed an average dendritic field diameter of 170 μm and an average coverage factor of 3.2. In summary, secretagogin cells in marmoset retina are medium‐field amacrine cells that share their stratification pattern with narrow‐field amacrine cells and their neurotransmitter with wide‐field amacrine cells. They may mediate spatial inhibition spanning the centralmost (on and off) bands of the inner plexiform layer. J. Comp. Neurol. 522:435–455, 2014. © 2013 Wiley Periodicals, Inc.     

Development of cholinergic amacrine cells is visual activity-dependent in the postnatal mouse retina

In the present study, we used immunocytochemistry to study the temporal and spatial arrangement of mouse cholinergic amacrine cells during postnatal retinal development under normal light/dark cycles and during visual deprivation. Choline acetyltransferase (ChAT)-immunolabeled cells were detected in the neuroblastic layer (NBL) and in the ganglion cell layer (GCL) at postnatal day 0 (P0). Between P3-5, two characteristic cholinergic bands were clearly identified in the inner plexiform layer (IPL). The signal intensity of somas and processes progressively increased over the first 2 postnatal weeks. Around eye opening at P12, cholinergic neurons were mature-like. This early developmental process was not altered by visual deprivation. After eye opening, the space between the two cholinergic bands increased continuously and the spatial regularity index changed constantly, indicating that the cholinergic neurons possibly underwent refinement during later postnatal development. The changes occurring following eye opening were retarded by visual deprivation. The morphologies of photoreceptors, horizontal cells, recoverin-positive OFF-cone bipolar cells, rod bipolar cells, dopaminergic amacrine cells, and Müller cells appeared normal. Their stratification in the outer plexiform layer (OPL) and the IPL was not affected by visual deprivation. However, glial cells grew vertically across the entire thickness of dark-reared retinas. Our results suggest that the development of cholinergic neurons before eye opening is independent of the lighting conditions. Their development after eye opening is greatly impeded by visual deprivation. This visual activity-dependent phase of development may be a critical period for the maturation and synaptic wiring of cholinergic amacrine cells in the mammalian retina.     

Immunocytochemical staining of cholinergic amacrine cells in rabbit retina

Cholinergic neurons of rabbit retina were labelled with an antibody against choline acetyltransferase, the synthesizing enzyme for acetylcholine. Two populations of cells are immunoreactive. Type a cell bodies lie in the inner nuclear layer (INL), their dendrites branching narrowly in sublamina a of the inner plexiform layer (IPL), while type b cell bodies lie in the ganglion cell layer (GCL) with dendrites branching in sublamina b of the IPL. The irregular networks of clustered immunoreactive dendrites are similar, but not identical, in the two sublaminae. Type b cells are more numerous than type a cells in central retina. No axons were stained. It appears that the immunoreactive neurons are normally placed and displaced starburst/cholinergic amacrine cells.     

Axon-bearing amacrine cells of the macaque monkey retina

A new and remarkable type of amacrine cell has been identified in the primate retina. Application of the vital dye acridine orange to macaque retinas maintained in vitro produced a stable fluorescence in the somata of apparently all retinal neurons in both the inner nuclear and ganglion cell layers. Large somata (approximately 15-20 microns diam) were also consistently observed in the approximate center of the inner plexiform layer (IPL). Intracellular injections of horseradish peroxidase (HRP) made under direct microscopic control showed that the cells in the middle of the IPL constitute a single, morphologically distinct amacrine cell subpopulation. An unusual and characteristic feature of this cell type is the presence of multiple axons that arise from the dendritic tree and project beyond it to form a second, morphologically distinct arborization within the IPL; these cells have thus been referred to as axon-bearing amacrine cells. The dendritic tree of the axon-bearing amacrine cell is highly branched (approximately 40-50 terminal dendrites) and broadly stratified, spanning the central 50% of the IPL so that the soma is situated between the outermost and innermost branches. Dendritic field size increases from approximately 200 microns in diameter within 2 mm of the fovea to approximately 500 microns in the retinal periphery. HRP injections of groups of neighboring cells revealed a regular intercell spacing (approximately 200-300 microns in the retinal periphery), suggesting that dendritic territories uniformly cover the retina. One to four axons originate from the proximal dendrites as thin (less than 0.5 microns), smooth processes. The axons increase in diameter (approximately 1-2 microns) as they course beyond the dendritic field and bifurcate once or twice into secondary branches. These branches give rise to a number of thin, bouton-bearing collaterals that extend radially from the dendritic tree for 1-3 mm without much further branching. The result is a sparsely branched and widely spreading axonal tree that concentrically surrounds the smaller, more highly branched dendritic tree. The axonal tree is narrowly stratified over the central 10-20% of the IPL; it is approximately ten times the diameter of the dendritic tree, resulting in a 100 times greater coverage factor. The clear division of an amacrine cell's processes into distinct dendritic and axonal components has recently been observed in other, morphologically distinct amacrine cell types of the cat and monkey retina and therefore represents a property common to a number of functionally distinct cell types. It is hypothesized that the axon-bearing amacrine cells, like classical neurons,     

Tracer coupling pattern of amacrine and ganglion cells in the rabbit retina

We examined the tracer coupling pattern of more than 15 morphological types of amacrine and ganglion cells in the rabbit retina. Individual cells were injected intracellularly with the biotinylated tracer Neurobiotin, which was then allowed to diffuse across gap junctions to label neighboring neurons. We found that homologous and/or heterologous tracer coupling was common for most proximal neurons. In fact, the starburst amacrine cell was the only amacrine cell type that showed no evidence of coupling. The remaining types of amacrine cell were coupled exclusively to other amacrines, either homologously or, more often, through a combination of homologous and heterologous junctions. In only one case did we visualize labeled ganglion cells following injection of Neurobiotin into an amacrine cell. In contrast, injection of Neurobiotin into ganglion cells almost always resulted in the labeling of amacrine cells. Taken together, these results suggest a directionality to the movement of tracer across gap junctions connecting amacrine and ganglion cells. We found that the coupling pattern for a given morphological type of cell was generally stereotypic and consistent across retinas. The notable exceptions to this finding were alpha ganglion cells and cells with morphology corresponding to that of on-off direction selective ganglion cells. In both cases, individual cells showed either extensive coupling to both amacrine and ganglion cells or no coupling at all. A notable finding was that, in every case, the neighboring cells within a tracer-coupled array were always within one gap junction of the injected neuron. Furthermore, in many cases, the array formed by the somata of tracer-coupled cells was almost perfectly coincident with the dendritic arbor of the injected cell. Thus, our results indicate that whereas coupling is extensive within the proximal retina, individual cells partake in coupled networks that are stereotypic and highly circumscribed. J. Comp. Neurol. 383:512-528, 1997. © 1997 Wiley-Liss, Inc.     

Morphological analysis of disabled-1-immunoreactive amacrine cells in the guinea pig retina

Disabled-1 (Dab1) is an adapter molecule in a signaling pathway, stimulated by reelin, that controls cell positioning in the developing brain. It localizes to selected neurons in the nervous system, including the retina, and Dab1-like immunoreactivity is present in AII amacrine cells in the mouse retina. This study was conducted to characterize Dab1-labeled cells in the guinea pig retina in detail using immunocytochemistry, quantitative analysis, and electron microscopy. Dab1 immunoreactivity is present in a class of amacrine cell bodies located in the inner nuclear layer adjacent to the inner plexiform layer (IPL). These cells give rise to processes that ramify the entire depth of the IPL. Double-labeling experiments demonstrated that these amacrine cells make contacts with the axon terminals of rod bipolar cells and that their processes make contacts with each other via connexin 36 in sublamina b of the IPL. In addition, all Dab1-labeled amacrine cells showed glycine transporter 1 immunoreactivity, indicating that they are glycinergic. The density of Dab1-labeled AII amacrine cells decreased from about 3,750 cells/mm(2) in the central retina to 1,725 cells/mm(2) in the peripheral retina. Dab1-labeled amacrine cells receive synaptic inputs from the axon terminals of rod bipolar cells in stratum 5 of the IPL. From these morphological features, Dab1-labeled cells of the guinea pig retina resemble the AII amacrine cells described in other mammalian species. Thus, the rod pathway of the guinea pig retina follows the general mammalian scheme and Dab1 antisera can be used to identify AII amacrine cells in the mammalian retina.     

Tracer coupling of intrinsically photosensitive retinal ganglion cells to amacrine cells in the mouse retina

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are a subtype of ganglion cell in the mammalian retina that expresses the photopigment melanopsin and drives non-image-forming visual functions. Three morphological subtypes of ipRGCs (M1, M2, and M3) have been described based on their dendritic stratifications in the inner plexiform layer (IPL), but the question of their potential interactions via electrical coupling remains unsettled. In this study, we have addressed this question in the mouse retina by, injecting the tracer Neurobiotin into ipRGCs that had been genetically labelled with the fluorescent protein, tdTomato. We confirmed the presence of the M1-M3 subtypes of ipRGCs based on their distinct dendritic stratifications. All three subtypes were tracer coupled to putative amacrine cells situated within the ganglion cell layer (GCL) but not the inner nuclear layer (INL). The cells tracer coupled to the M1 and M2 cells were shown to be widefield GABA-immunoreactive amacrine cells. We found no evidence of homologous tracer coupling of ipRGCs or heterologous coupling to other types of ganglion cells.     

Ontogeny of cholinergic amacrine cells in the oppossum (Didelphis aurita) retina.

Immunocytochemistry for choline acetyltransferase (ChAT), the synthesizing enzyme for acetylcholine, was used to determine the onset and to follow the maturation of the cholinergic cells in the retina of a marsupial, the South American opossum (Didelphis aurita). ChAT-immunoreactivity was first detected in amacrine cells in the ganglion cell layer by postnatal day 15 (P15) and in the inner nuclear layer by P35. Much later, at P50 a second sub-population of ChAT-immunoreactive cell bodies was evident in the inner nuclear layer. Processes from ChAT-immunoreactive amacrine cells were detected in the two bands of the inner plexiform layer before synaptogenesis. In the adult retina, these two bands correspond to sublamina 2 and 4 of the inner plexiform layer. In flat whole-mounted preparations, cholinergic cell density was 263±13 cells/mm[2] in the ganglion cell layer and it was estimated a total of 24,000 cholinergic neurons. ChAT-immunoreactive somata showed a random pattern of distribution.