The morphology of the bipolar cells, amacrine cells and ganglion cells in the retina of the turtle Pseudemys scripta elegans

Functional connexin35 increased in the myopic chicken retina

Our previous research showed that increased phosphorylation of connexin (Cx)36 indicated extended  coupling of AII amacrine cells (ACs) in the rod-dominant mouse myopic retina. This research will determine whether phosphorylation at serine 276 of Cx35-containing gap junctions increased in the myopic chicken, whose retina is cone-dominant. Refractive errors and ocular biometric dimensions of 7-days-old chickens were determined following 12 h and 7 days induction of myopia by a −10D lens. The expression pattern and size of Cx35-positive plaques were examined in the early (12 h) and compensated stages (7 days) of lens-induced myopia (LIM). At the same time, phosphorylation at serine 276 (functional assay) of Cx35 in strata 5 (S5) of the inner plexiform layer was investigated. The axial length of the 7 days LIM eyes was significantly longer than that of non-LIM controls (P < 0.05). Anti-phospho-Ser276 (Ser276-P)-labeled plaques were significantly increased in LIM retinas at both 12 h and 7 days. The density of Ser276-P of Cx35 was observed to increase after 12 h LIM. In the meanwhile, the areas of existing Cx35 plaques did not change. As there was more phosphorylation of connexin35 at Ser276 at both the early and late stages (12 h) and 7 days of LIM chicken retinal activity, the coupling with ACs could be increased in myopia development of the cone-dominated chicken retina.

Distribution of calcium-binding proteins in the pigeon visual thalamic centers and related pretectal and mesencephalic nuclei. Phylogenetic and functional determinants

Multichannel processing of environmental information constitutes a fundamental basis of functioning of sensory systems in the vertebrate brain. Two distinct parallel visual systems - the tectofugal and thalamofugal exist in all amniotes. The vertebrate central nervous system contains high concentrations of intracellular calcium-binding proteins (CaBPrs) and each of them has a restricted expression pattern in different brain regions and specific neuronal subpopulations. This study aimed at describing the patterns of distribution of parvalbumin (PV) and calbindin (CB) in the visual thalamic and mesencephalic centers of the pigeon (Columba livia). We used a combination of immunohistochemistry and double labeling immunofluorescent technique. Structures studied included the thalamic relay centers involved in the tectofugal (nucleus rotundus, Rot) and thalamofugal (nucleus geniculatus lateralis, pars dorsalis, GLd) visual pathways as well as pretectal, mesencephalic, isthmic and thalamic structures inducing the driver and/or modulatory action to the visual processing. We showed that neither of these proteins was unique to the Rot or GLd. The Rot contained i) numerous PV-immunoreactive (ir) neurons and a dense neuropil, and ii) a few CB-ir neurons mostly located in the anterior dorsal part and associated with a light neuropil. These latter neurons partially overlapped with the former and some of them colocalized both proteins. The distinct subnuclei of the GLd were also characterized by different patterns of distribution of CaBPrs. Some (nucleus dorsolateralis anterior, pars magnocellularis, DLAmc; pars lateralis, DLL; pars rostrolateralis, DLAlr; nucleus lateralis anterior thalami, LA) contained both CB- and PV-ir neurons in different proportions with a predominance of the former in the DLAmc and DLL. The nucleus lateralis dorsalis of nuclei optici principalis thalami only contained PV-ir neurons and a neuropil similar to the interstitial pretectal/thalamic nuclei of the tectothalamic tract, nucleus pretectalis and thalamic reticular nucleus. The overlapping distribution of PV and CB immunoreactivity was typical for the pretectal nucleus lentiformis mesencephali and the nucleus ectomamillaris as well as for the visual isthmic nuclei. The findings are discussed in the light of the contributive role of the phylogenetic and functional factors determining the circuits׳ specificity of the different CaBPr types.

ON cone bipolar cell axonal synapses in the OFF inner plexiform layer of the rabbit retina

Analysis of the rabbit retinal connectome RC1 reveals that the division between the ON and the OFF inner plexiform layer (IPL) is not structurally absolute. ON cone bipolar cells make noncanonical axonal synapses onto specific targets and receive amacrine cell synapses in the nominal OFF layer, creating novel motifs, including inhibitory crossover networks. Automated transmission electron microscopic imaging, molecular tagging, tracing, and rendering of ~400 bipolar cells reveals axonal ribbons in 36% of ON cone bipolar cells, throughout the OFF IPL. The targets include γ-aminobutyrate (GABA)-positive amacrine cells (γACs), glycine-positive amacrine cells (GACs), and ganglion cells. Most ON cone bipolar cell axonal contacts target GACs driven by OFF cone bipolar cells, forming new architectures for generating ON-OFF amacrine cells. Many of these ON-OFF GACs target ON cone bipolar cell axons, ON γACs, and/or ON-OFF ganglion cells, representing widespread mechanisms for OFF to ON crossover inhibition. Other targets include OFF γACs presynaptic to OFF bipolar cells, forming γAC-mediated crossover motifs. ON cone bipolar cell axonal ribbons drive bistratified ON-OFF ganglion cells in the OFF layer and provide ON drive to polarity-appropriate targets such as bistratified diving ganglion cells (bsdGCs). The targeting precision of ON cone bipolar cell axonal synapses shows that this drive incidence is necessarily a joint distribution of cone bipolar cell axonal frequency and target cell trajectories through a given volume of the OFF layer. Such joint distribution sampling is likely common when targets are sparser than sources and when sources are coupled, as are ON cone bipolar cells.

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

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

Ultrastructure of the distal retina of the adult zebrafish, Danio rerio

The organization, morphological characteristics, and synaptic structure of photoreceptors in the adult zebrafish retina were studied using light and electron microscopy. Adult photoreceptors show a typical ordered tier arrangement with rods easily distinguished from cones based on outer segment (OS) morphology. Both rods and cones contain mitochondria within the inner segments (IS), including the large, electron-dense megamitochondria previously described (Kim et al.) Four major ultrastructural differences were observed between zebrafish rods and cones: (1) the membranes of cone lamellar disks showed a wider variety of relationships to the plasma membrane than those of rods, (2) cone pedicles typically had multiple synaptic ribbons, while rod spherules had 1-2 ribbons, (3) synaptic ribbons in rod spherules were ∼2 times longer than ribbons in cone pedicles, and (4) rod spherules had a more electron-dense cytoplasm than cone pedicles. Examination of photoreceptor terminals identified four synaptic relationships at cone pedicles: (1) invaginating contacts postsynaptic to cone ribbons forming dyad, triad, and quadrad synapses, (2) presumed gap junctions connecting adjacent postsynaptic processes invaginating into cone terminals, (3) basal junctions away from synaptic ribbons, and (4) gap junctions between adjacent photoreceptor terminals. More vitread and slightly farther removed from photoreceptor terminals, extracellular microtubule-like structures were identified in association with presumed horizontal cell processes in the OPL. These findings, the first to document the ultrastructure of the distal retina in adult zebrafish, indicate that zebrafish photoreceptors have many characteristics similar to other species, further supporting the use of zebrafish as a model for the vertebrate visual system.

Displaced amacrine cells of the mouse retina

The aim of this study was to characterize and classify the displaced amacrine cells in the mouse retina. Amacrine cells in the ganglion cell layer were injected with fluorescent dyes in flat-mounted retinas. Dye-filled displaced amacrine cells were classified according to dendritic field size, horizontal and vertical stratification patterns, and general morphology. We identified 10 different morphological types of displaced amacrine cell. Six of the cell types identified here are novel cell types that have not been described previously in the mouse retina, to the best of our knowledge. The displaced amacrine cells included four types of medium-field cells, with dendritic field diameters of 200-500 microm, and six types of wide-field cells, with dendritic fields extending over 500 microm. Narrow-field displaced amacrine cells, with dendritic field diameters smaller than 200 microm, were not encountered. The most frequently labeled displaced amacrine cell type was the starburst amacrine cell. At least three cell types identified here have nondisplaced counterparts in the inner nuclear layer as well. Displaced amacrine cells display a rich variety of stratification and branching patterns, which surely reflect the wide range of their functional roles in the processing of visual signals in the inner retina.

Dimensional differences among the groups of retinal ganglion cells according to the retinal zones in chicks

The populations of retinal ganglion cell (RGC) groups (Groups I, II, III, IV) were similar each other between the central and intermediate zones, but the population in the peripheral zone were clearly different from those in the central and intermediate zones due to increase of Group III and IV cells and decrease of Group I cells. The dimensions of somal area and dendritic field of Group I cells increased very gradually toward the peripheral zone, but those of other three Groups grew steeply in the peripheral zone. The correlation index between somal area and dendritic field of RGCs showed high coefficient in the central (r=0.73) and intermediate (r=0.77) zones, but lowered clearly in the peripheral zone (r=0.64) due to increase of Group III cells, which showed nonlinear relation between somal area and dendritic field.

Early neural activity and dendritic growth in turtle retinal ganglion cells

Early neural activity, both prenatal spontaneous bursts and early visual experience, is believed to be important for dendritic proliferation and for the maturation of neural circuitry in the developing retina. In this study, we have investigated the possible role of early neural activity in shaping developing turtle retinal ganglion cell (RGC) dendritic arbors. RGCs were back-labelled from the optic nerve with horseradish peroxidase (HRP). Changes in dendritic growth patterns were examined across development and following chronic blockade or modification of spontaneous activity and/or visual experience. Dendrites reach peak proliferation at embryonic stage 25 (S25, one week before hatching), followed by pruning in large field RGCs around the time of hatching. When spontaneous activity is chronically blocked in vivo from early embryonic stages (S22) with curare, a cholinergic nicotinic antagonist, RGC dendritic growth is inhibited. On the other hand, enhancement of spontaneous activity by dark-rearing (Sernagor & Grzywacz (1996)Curr. Biol., 6, 1503-1508) promotes dendritic proliferation in large-field RGCs, an effect that is counteracted by exposure to curare from hatching. We also recorded spontaneous activity from individual RGCs labelled with lucifer yellow (LY). We found a tendency of RGCs with large dendritic fields to be spontaneously more active than small-field cells. From all these observations, we conclude that immature spontaneous activity promotes dendritic growth in developing RGCs.

Receptive field structure-function correlates in developing turtle retinal ganglion cells

Mature retinal ganglion cells (RGCs) have distinct morphologies that often reflect specialized functional properties such as On and Off responses. But the structural correlates of many complex receptive field (RF) properties (e.g. responses to motion) remain to be deciphered. In this study, we have investigated whether motion anisotropies (non-homogeneities) characteristic of embryonic turtle RGCs arise from immature dendritic arborization in these cells. To test this hypothesis, we have looked at structure-function correlates of developing turtle RGCs from Stage 23 (S23) when light responses emerge, until 15 weeks post-hatching (PH). Using whole cell patch clamp recordings, RGCs were labelled with Lucifer Yellow (LY) while recording their responses to moving edges of light. Comparison of RF and dendritic arbor layouts revealed a weak correlation. To obtain a larger structural sample of developing RGCs, we have looked at dendritic morphology in RGCs retrogradely filled with the tracer horseradish peroxidase (HRP) from S22 (when RGCs become spontaneously active, shortly before they become sensitive to light) until two weeks PH. We found that there was intense dendritic growth from S22 onwards, reaching peak proliferation at S25 (a week before hatching), while RGCs are still exhibiting significant motion anisotropies. Based on these observations, we suggest that immature anisotropic RGC RFs must originate from sparse synaptic inputs onto RGCs rather than from the immaturity of their growing dendritic trees.

Morphological features of chick retinal ganglion cells

On average, in chicks, the total number of retinal ganglion cells is 4.9 x 10(6) and the cell density is 10400 cells/mm2. Two high-density areas, namely the central area (CA) and the dorsal area (DA), are located in the central and dorsal retinas, respectively, in post-hatching day 8 (P8) chicks (19000 cells/mm2 in the CA; 12800 cells/mm2 in the DA). Thirty percent of total cells in the ganglion cell layer are resistant to axotomy of the optic nerve. The distribution of the axotomy resistant cells shows two high-density areas in the central and dorsal retinas, corresponding to the CA (5800 cells/mm2) and DA (3200 cells/mm2). The number of presumptive ganglion cells in P8 chicks is estimated to be 4 x 10(6) (8600 cells/mm2 on average) and the density is 13500 and 10200 cells/mm2 in the CA and DA, respectively, and 4300 cell/mm2 in the temporal periphery (TP). The somal area of presumptive ganglion cells is small in the CA and DA (mean (+/- SD) 35.7 +/- 9.1 and 40.0 +/- 11.3 microm2, respectively) and their size increases towards the periphery (63.4 +/- 29.7 microm2 in the TP), accompanied by a decrease in cell density. Chick ganglion cells are classified according to dendritic field, somal size and branching density of the dendrites as follows: group Ic, Is, IIc, IIs, Ills, IVc. The density of branching points of dendrites is approximately 10-fold higher in the complex type (c) than in the simple type (s) in each group. The chick inner plexiform layer is divided into eight sublayers according to the dendritic strata of retinal ganglion cells and 26 stratification patterns are discriminated.

Identification and morphological classification of horizontal, bipolar, and amacrine cells within the zebrafish retina

Horizontal, bipolar, and amacrine cells in the zebrafish retina were morphologically characterized using DiOlistic techniques. In this method, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-coated microcarriers are shot at high speed onto the surfaces of living retinal slices where the DiI then delineates axons, somata, and dendrites of isolated neurons. Zebrafish retinal somata were 5-10 microm in diameter. Three horizontal cell types (HA-1, HA-2, and HB) were identified; dendritic tree diameters averaged 25-40 microm. HA somata were round. Cells classified as HA-2 were larger than HA-1 cells and possessed an axon. HB somata were flattened, without an axon, although short fusiform structure(s) projected from the soma. Bipolar cells were separated into 17 morphological types. Dendritic trees ranged from 10 to 70 microM. There were six B(on) types with axon boutons only in the ON sublamina of the inner plexiform layer (IPL), and seven B(off) types with axon boutons or branches only in the OFF sublamina. Four types of bistratified bipolar cells displayed boutons in both ON and OFF layers. Amacrine cells occurred in seven types. A(off) cells (three types) were monostratified and ramified in the IPL OFF sublamina. Dendritic fields were 60-150 microM. A(on) pyriform cells (three types) branched in the ON sublamina. Dendritic fields were 50-170 microM. A(diffuse) cells articulated processes in all IPL strata. Dendritic fields were 15-90 microM. These findings are important for studies examining signal processing in zebrafish retina and for understanding changes in function resulting from mutations and perturbations of retinal organization.

Neurotransmitter actions on transient amacrine and ganglion cells of the turtle retina

We obtained intracellular recordings from transient, On-Off amacrine and ganglion cells of the turtle retina. We tested the ability of neurotransmitter agonists and antagonists to modify the responses to light stimuli. The metabotropic glutamate agonist, 2-amino-phosphonobutyric acid (APB), selectively blocked On responses, whereas the amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) receptor antagonist, GYKI, blocked both On and Off responses. Although GYKI appeared to block excitation completely, suggesting an absence of N-methyl-d-aspartate (NMDA)-mediated responses, it was found that in the presence of ionotropic gamma-aminobutyric acid (GABA) blockers, the excitatory postsynaptic potential (EPSP) was prolonged. The late component of the EPSP was blocked by the NMDA antagonist, D-2-amino-5-phosphopentanoic acid (D-AP5). Picrotoxin (PTX) and bicuculline (BCC) induced a mean hyperpolarization of -6.4 mV, suggesting a direct effect of GABA on transient amacrine and ganglion cells, since antagonism of a GABA-mediated inhibition of release of glutamate by bipolars would depolarize third-order neurons. The acetylcholine agonist, carbachol, or the nicotinic agonist, epibatidine, depolarized all On-Off neurons. This action was blocked by d-tubocurarine. Cholinergic inputs to On-Off neurons increase their excitability without altering the pattern of light responsiveness.

Optimization of Golgi methods for impregnation of brain tissue from humans and monkeys

Golgi impregnation is unique in its ability to display the dendritic trees of large numbers of individual neurons. However, its reputation for inconsistency leaves many investigators reluctant to embrace this methodology, particularly for the study of formalin-fixed human brain tissue. After reviewing the literature, testing a variety of technical variations, and discussing the procedure with experienced practitioners, we have concluded that much of the unpredictability can be removed by matching the Golgi technique to the conditions that were used for fixation of the tissue. Briefly fixed tissues worked best with the rapid Golgi technique, which includes osmium during the initial chromation step, and with the Golgi-Cox method, which includes mercuric chloride during chromation. For tissues that have been fixed for several years or even for several decades, superior results are obtained with the Golgi-Kopsch technique, using multiple changes of a chromation solution that contains paraformaldehyde. In the Golgi-Kopsch technique, pH should be used to monitor the reduction of Cr6+ to Cr3+, which is a crucial determinant of successful chromation. With any Golgi technique, agitation throughout the impregnation helps to avoid precipitates and to improve the quality of impregnation. When the appropriate method is chosen, Golgi impregnation is a useful technique for the neuropathologist.

Morphologic analysis and classification of ganglion cells of the chick retina by intracellular injection of lucifer yellow and retrograde labeling with DiI

Retinal ganglion cells (RGCs) of chicks were labeled by using the techniques of intracellular filling with Lucifer Yellow and retrograde axonal labeling with carbocyanine dye (DiI). Labeled RGCs were morphologically analyzed and classified into four major groups: Group I cells (57.1%) with a small somal area (77.5 microm(2) on average) and narrow dendritic field (17,160 microm(2) on average), Group II cells (28%) with a middle-sized somal area (186 microm(2)) and middle-sized dendritic field (48,800 microm(2)), Group III cells (9.9%) with a middle-sized somal area (203 microm(2)) and wide dendritic field (114,000 microm(2)), and Group IV cells (5%) with a large somal area (399 microm(2)) and wide dendritic field (117,000 microm(2)). Of the four groups, Groups I and II were further subdivided into two types, simple and complex, on the basis of dendritic arborization: Groups Is, Ic, and Groups IIs, IIc. However, Group III and IV showed either a simple or complex type, Group IIIs and Group IVc, respectively. The density of branching points of dendrites was approximately 10 times higher in the complex types (18,350, 6,190, and 3,520 points/mm(2) in Group Ic, IIc, and IVc, respectively) than in the simple types (1,890, 640, and 480 points/mm(2) in Group Is, IIs, and IIIs). The branching density of Group I cells was extremely high in the central zone. The chick inner plexiform layer was divided into eight sublayers by dendritic strata of RGCs and 26 stratification patterns were discriminated. The central and peripheral retinal zones were characterized by branching density of dendrites and composition of RGC groups, respectively.

Electrophysiological evidence for push-pull interactions in the inner retina of turtle

The responses of the inner retinal neurons of turtle to light spots of sizes were studied in an attempt to reveal characteristics that may reflect possible interactions of the neural circuits underlying the center and surround responses. For the ON-OFF cells, the responses were also analyzed to observe whether interference or augmentation of these responses occur. The intracellular recordings revealed several such interactions, observed either in the form of altered spike activity or as changes in the transiency of the light responses. The ON-responding amacrine cell presented in this study became more sustained, while for the ON-OFF amacrine cells larger light spots tended to make the responses more transient and both the ON and OFF components became more pronounced. The spiking activity of the OFF-type ganglion cell shifted in relation to the light stimulus and the number of spikes observed upon presentation of larger spots increased. We suggest that the surround circuits activated by increasing light spots may substantially influence and reorganize not only the overall center-surround balance, but also the center response of the cells. Although it cannot be excluded that intrinsic membrane properties also influence these processes to some extent, it is more likely that lateral inhibition and disinhibitory mechanisms play the leading role in this process.

The computation of directional selectivity in the retina occurs presynaptic to the ganglion cell

Directional selectivity is a response that is greater for a visual stimulus moving in one (PREF) direction than for the opposite (NULL) direction, and its computation in the vertebrate retina is a classical issue in functional neurophysiology. To date, most quantitative experimental studies have relied on extracellular responses for identifying properties of the directionally selective circuit. Here I describe an intracellular analysis using whole-cell patch recordings of the synaptic events underlying the spike response in directionally selective ganglion cells of the turtle retina. These quantitative measurements allowed me to distinguish among various explicit classes of circuit models that can, in principle, account for ganglion cell directional selectivity. I found that ganglion cell directional selectivity is due to an excitatory input that itself is directionally selective, and that the crucial shunting inhibition implicated in this computation must act on cells presynaptic to the ganglion cell.

Functional architecture of synapses in the inner retina: segregation of visual signals by stratification of bipolar cell axon terminals

We correlated the morphology of salamander bipolar cells with characteristics of their light responses, recorded under voltage-clamp conditions. Twelve types of bipolar cells were identified, each displaying a unique morphology and level(s) of axon terminal stratification in the inner plexiform layer (IPL) and exhibiting light responses that differed with respect to polarity, kinetics, the relative strengths of rod and cone inputs, and characteristics of spontaneous EPSCs (sEPSCs) and IPSCs. In addition to the well known segregation of visual information into ON and OFF channels along the depth of the IPL, we found an overlying mapping of spectral information in this same dimension, with cone signals being transmitted predominantly to the central IPL and rod signals being sent predominantly to the margins of the IPL. The kinetics of bipolar cell responses correlated with this segregation of ON and OFF and of rod and cone information in the IPL. At light offset, rod-dominated cells displayed larger slow cationic current tails and smaller rapid overshoot responses than did cone-dominated cells. sEPSCs were generally absent in depolarizing bipolar cells but present in all hyperpolarizing bipolar cells (HBCs) and larger in rod-dominated HBCs than in cone-dominated HBCs. Inhibitory chloride currents, elicited both at light onset and light offset, tended to be larger for cone-dominated cells than for rod-dominated cells. This orderly segregation of visual signals along the depth of the IPL simplifies the integration of visual information in the retina, and it begins a chain of parallel processing in the visual system.

Axonal stratification patterns and glutamate-gated conductance mechanisms in zebrafish retinal bipolar cells

1. Whole-cell patch recording and puff pipette techniques were used to identify glutamate receptor mechanisms on bipolar cell (BC) dendrites in the zebrafish retinal slice. Recorded neurons were stained with Lucifer Yellow, to correlate glutamate responses with BC morphology. 2. BC axon terminals (ATs) consisted of swellings or varicosities along the axon, as well as at its end. AT stratification patterns identified three regions in the inner plexiform layer (IPL): a thick sublamina a, with three bands of ATs, a narrow terminal-free zone in the mid-IPL, and a thin sublamina b, with two bands of ATs. BCs occurred with ATs restricted to sublamina a(Group a), sublamina b(Group b) or with ATs in both sublaminae (Group a/b). 3. OFF-BCs belonged to Group a or Group a/b. These cells responded to glutamate or kainate with a CNQX-sensitive conductance increase. Reversal potential (Erev) ranged from -0.6 to +18 mV. Bipolar cells stimulated sequentially with both kainate and glutamate revealed a population of glutamate-insensitive, kainate-sensitive cells in addition to cells sensitive to both agonists. 4. ON-BCs responded to glutamate via one of three mechanisms: (a) a conductance decrease with Erev approximately 0 mV, mimicked by L-(+)-2-amino-4-phosphonobutyric acid (APB) or trans-1-amino-1, 3-cyclopentanedicarboxylic acid (trans-ACPD), (b) a glutamate-gated chloride conductance increase (IGlu-like) characterized by Erev >= ECl (where ECl is the chloride equilibrium potential) and partial blockade by extracellular Li+/Na+ substitution or (c) the activation of both APB and chloride mechanisms simultaneously to produce a response with outward currents at all holding potentials. APB-like responses were found only among BCs in Group b, with a single AT ramifying deep within sublamina b; whereas, cells expressing IGlu-like currents had one or more ATs, and occurred within Groups b or a/b. 5. Multistratified cells (Group a/b) were common and occurred with either ON- or OFF-BC physiology. OFF-BCs typically had one or more ATs in sublamina a and only one AT in sublamina b. In contrast, multistratified ON-BCs had one or more ATs in sublamina b and a single AT ramifying deep in sublamina a. Multistratified ON-BCs expressed the IGlu-like mechanism only. 6. Visual processing in the zebrafish retina involves at least 13 BC types. Some of these BCs have ATs in both the ON- and OFF-sublaminae, suggesting a significant role for ON- and OFF-inputs throughout the IPL.

Calcium released from intracellular stores inhibits GABAA-mediated currents in ganglion cells of the turtle retina

We studied spiking neurons isolated from turtle retina by the whole cell version of the patch clamp. The studied cells had perikaryal diameters > 15 microns and fired multiple spikes in response to depolarizing current steps, indicating they were ganglion cells. In symmetrical [Cl-], currents elicited by puffs of 100 microM gamma-aminobutyric acid (GABA) were inward at a holding potential of -80 mV. All of the GABA-evoked current was blocked by SR95331 (20 microM), indicating that it was mediated by a GABAA receptor. The GABA-evoked currents were unaltered by eliciting a transmembrane calcium current either just before or during the response to GABA. On the other hand caffeine (10 mM), which induces Ca2+ release from intracellular stores, inhibited the GABA-evoked current on average by 30%. The caffeine effect was blocked by introducing the calcium buffer bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) into the cell but was unaffected by replacing [Ca2+]o with equimolar cobalt. Thapsigargin (10 microM), an inhibitor of intracellular calcium pumps, and ryanodine (20 microM), which depletes intracellular calcium stores, both markedly reduced a caffeine-induced inhibition of the GABA-evoked current. Another activator of intracellular calcium release, inositol trisphosphate (IP3; 50 microM), also progressively reduced the GABA-induced current when introduced into the cell. Dibutyryl adenosine 3'5'-cyclic monophosphate (cAMP; 0.5 mM), a membrane-permeable analogue of cAMP, did not reduce GABA-evoked currents, suggesting that cAMP-dependent kinases are not involved in suppressing GABAA currents, whereas calmidazolium (30 microM) and cyclosporin A (20 microM), which inhibit Ca/calmodulin-dependent phosphatases, did reduce the caffeine-induced inhibition of the GABA-evoked current. Alkaline phosphatase (150 micrograms/ml) and calcineurin (300 micrograms/ml) had a similar action to caffeine or IP3. Antibodies directed against the ryanodine receptor or the IP3 receptor reacted with the great majority of neurons in the ganglion cell layer. We found that these two antibodies colocalized in large ganglion cells. In summary, intracellular calcium plays a role in reducing the currents elicited by GABA, acting through GABAA receptors. The modulatory action of calcium on GABA responses appears to work through one or more Ca-dependent phosphatases.

Amacrine cells of the mammalian retina: neurocircuitry and functional roles

Since amacrine cells are important interneurons of the inner retina and their activity may be detected in certain waveforms of the electroretinogram, this paper reviews their morphologies, classification, mosaics, neurotransmitter content, neural circuitry and physiological responses to light. Nine different amacrine cell types of cat, rabbit and human retinas are presently quite well studied in terms of the aforementioned aspects and are described in detail in this paper.

Synaptic inputs to identified color-coded amacrine and ganglion cells in the turtle retina

Previous studies have proposed models of the specific synaptic circuitry responsible for color processing in the turtle retina. To determine the accuracy of these models of the circuits underlying color opponency in the inner retina of the turtle (Pseudemys scripta), we have studied the physiology, morphology, and synaptic connectivity of identified amacrine and ganglion cells. These cells were first characterized electrophysiologically and were then stained with horseradish peroxidase. Postembedding electron immunocytochemistry for gamma-aminobutyric acid (GABA) and glycine was used to reveal the neurochemical identity of their synaptic inputs. The red-ON/green, blue-OFF small-field ganglion cell, classified as G24, branched primarily in strata S1, S4, and S5 of the inner plexiform layer (IPL). Ganglion cell G24 showed a complex receptive field organized into a red-ON center surrounded by an inhibitory region, which, in turn, was surrounded by a second excitatory region. Only the center responses were color opponent. The red-OFF/green, blue-ON large-field, stellate amacrine cell, classified as A23b, stratified exclusively in stratum S2, near the S2/S3 border. The color-coded center was surrounded by a luminosity, red-sensitive surround. Synaptic input to G24 and A23b was dominated by amacrine cells (89% and 87%, respectively). G24 received significant input from amacrine cell profiles with GABA (13% of total) as well as glycine (11% of total) immunoreactivity, mostly in the proximal stratum S5 of the IPL (64% and 67% of the total GABA- and glycine-immunoreactive input, respectively). Bipolar cell synaptic input was also found predominantly in S4 and S5 (89%). In contrast, we found no glycine-immunoreactive input to A23b, and the density of the GABA-immunoreactive amacrine cell synaptic input revealed a central (15%) to peripheral (3%) gradient within the dendritic tree. The results of the present study support the previous models of the synaptic circuitry responsible for color-opponent signal processing in the inner retina of the turtle.

Immunocytochemical and histochemical localization of nitric oxide synthase in the turtle retina

Recent interest in nitric oxide and its relationship to cGMP has produced many attempts to anatomically localize the enzyme synthesizing nitric oxide, nitric oxide synthase. In the retina, numerous previous studies have used the NADPH-diaphorase enzyme activity of nitric oxide synthase as a histochemical method to localize nitric oxide synthase. However, all NADPH-diaphorase activity is not necessarily nitric oxide synthase, because several enzymes have similar biochemical activity. Additionally, various histochemical methods have been used to demonstrate NADPH-diaphorase activity, which makes comparisons between studies difficult. The purpose of this study was twofold. First, we wanted to examine the histochemical labeling of NADPH-diaphorase in the turtle retina to allow comparisons to previous studies. Second, we wanted to compare the histochemical localization of NADPH-diaphorase activity to the immunocytochemical localization of nitric oxide synthase in the turtle retina. Our histochemical localization of NADPH-diaphorase activity and our localization of nitric oxide synthase-like immunoreactivity in the turtle retina both produced similar results. Both the histochemistry and immunocytochemistry consistently labeled photoreceptor inner segments, at least three amacrine cell types, and processes in the inner plexiform layer. In optimized double-labeled preparations, all cells with NADPH-diaphorase activity were also positive for nitric oxide synthase-like immunoreactivity, although some somata in the ganglion cell layer only had nitric oxide synthase-like immunoreactivity. The immunocytochemical localization of nitric oxide synthase in photoreceptors, amacrine cells, and putative ganglion cells indicates that nitric oxide may function at several levels of visual processing in the turtle retina.

Possible roles of spontaneous waves and dendritic growth for retinal receptive field development

Several models of cortical development postulate that a Hebbian process fed by spontaneous activity amplifies orientation biases occurring randomly in early wiring, to form orientation selectivity. These models are not applicable to the development of retinal orientation selectivity, since they neglect the polarization of the retina's poorly branched early dendritic trees and the wavelike organization of the retina's early noise. There is now evidence that dendritic polarization and spontaneous waves are key in the development of retinal receptive fields. When models of cortical development are modified to take these factors into account, one obtains a model of retinal development in which early dendritic polarization is the seed of orientation selectivity, while the spatial extent of spontaneous waves controls the spatial profile of receptive fields and their tendency to be isotropic.

The number and distribution of bipolar to ganglion cell synapses in the inner plexiform layer of the anuran retina

The main route of information flow through the vertebrate retina is from the photoreceptors towards the ganglion cells whose axons form the optic nerve. Bipolar cells of the frog have been so far reported to contact mostly amacrine cells and the majority of input to ganglion cells comes from the amacrines. In this study, ganglion cells of frogs from two species (Bufo marinus, Xenopus laevis) were filled retrogradely with horseradish peroxidase. After visualization of the tracer, light-microscopic cross sections showed massive labeling of the somata in the ganglion cell layer as well as their dendrites in the inner plexiform layer. In cross sections, bipolar output and ganglion cell input synapses were counted in the electron microscope. Each synapse was assigned to one of the five equal sublayers (SLs) of the inner plexiform layer. In both species, bipolar cells were most often seen to form their characteristic synaptic dyads with two amacrine cells. In some cases, however, the dyads were directed to one amacrine and one ganglion cell dendrite. This type of synapse was unevenly distributed within the inner plexiform layer with the highest occurrence in SL2 both in Bufo and Xenopus. In addition, SL4 contained also a high number of this type of synapse in Xenopus. In both species, we found no or few bipolar to ganglion cell synapses in the marginal sublayers (SLs 1 and 5). In Xenopus, 22% of the bipolar cell output synapses went onto ganglion cells, whereas in Bufo this was only 10%. We conclude that direct bipolar to ganglion cell information transfer exists also in frogs although its occurrence is not as obvious and regular as in mammals. The characteristic distribution of these synapses, however, suggests that specific type of the bipolar and ganglion cells participate in this process. These contacts may play a role in the formation of simple ganglion cell receptive fields.

Immunocytochemical localization of the high-affinity glutamate transporter, EAAC1, in the retina of representative vertebrate species

The glutamate transporter, EAAC1, was localized immunocytochemically in goldfish, salamander, turtle, chicken, and rat retinas, using affinity-purified oligopeptide antibodies. Immunoreactive (IR) EAAC1 was present in the inner plexiform layer of all species, and in cell bodies of bipolar, amacrine, and ganglion cells of most species, but absent from photoreceptors and Müller's glial cells. Western blots revealed an IR-EAAC1 band at 70 kDa. Staining was abolished by preabsorption with EAAC1 peptide.

Activation of metabotropic glutamate receptors decreases a high-threshold calcium current in spiking neurons of the Xenopus retina

Two types of spiking neuron were identified among acutely dissociated neurons from the Xenopus retina by their responses to a depolarizing current step: single spikers and multiple spikers. In culture, multiple spikers had perikaryal diameters > 15 microns, whereas single spikers had smaller somata, 5-10 microns in diameter. Using a conventional whole-cell patch-clamp technique, both T- and L-type calcium currents were identified in multiply spiking cells whereas only an L-type current was present in singly spiking cells. The metabotropic glutamate receptor (mGluR) agonist trans-(1S-3R)-1-amino-1,3-cyclopentane-dicarboxylic acid (trans-ACPD) significantly decreased the L-type calcium current by 46 +/- 3% (mean +/- S.E.M.) in both types of cell but had only a minor effect on the T-type current in multiply spiking neurons. In the presence of 50 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 100 microM quisqualate (a potent mGluR1/5 agonist) decreased the L-type calcium current by 47 +/- 9% but had no effect on the T-type current. The selective mGluR4/6/7 agonist (+/-) 2-amino-4-phosphonobutyric acid (L-AP4, 100 microM), and the mGluR2/3 agonist (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (L-CCG1, 100 microM) decreased the L-type calcium current by 12 +/- 3% and 14 +/- 2%, respectively. The inhibition of calcium current by trans-ACPD was reduced when the patch pipette contained the G-protein inhibitor, GDP beta S. The presence of the G-protein activator GTP gamma S in the patch pipette irreversibly reduced the L-type calcium current, but was without effect on the T-type current. Heparin applied intracellularly significantly reduced the inhibitory effect of quisqualate, indicating an involvement of the inositol triphosphate (IP3) pathway in the mGluR-induced reduction of calcium current. Replacement of internal EGTA with BAPTA significantly reduced the inhibitory effect of quisqualate. In contrast, internal application of cAMP did not prevent an inhibition of calcium current by quisqualate. Thus, the mechanism by which calcium current is inhibited by mGluR seems not to involve an intracellular cAMP cascade. Our findings indicate that activation of mGluR1/5 results in the inhibition of a high-threshold calcium current. This process is mediated by the activation of a G-protein and is consistent with inhibition occurring by an IP3-stimulated release of internal calcium.

A reliable method for Golgi staining of retina and brain slices

Although the classical Golgi method is a powerful means for structural analysis of the brain, it is generally considered to be an unpredictable technique making anatomists wary of using it. Often, even when successful staining has occurred, deposits of silver chromate crystals on the surface of the tissue obscure examination. This paper describes a simple procedure for Golgi impregnation of retina and brain slices so that good, even staining is obtained and crystal formation is avoided. The most outstanding feature of the method is the consistency of results. This consistency is due to two factors: (1) the accurate determination of the optimal chromation by measuring the rise of pH in the solutions and (2) the uniform penetration of dichromate and silver nitrate to the specimen by using a freely hanging, sandwiching technique. We suggest that the method described here can be applied to other parts of the nervous system and will be a reliable way to identify and better classify new cell types.

Large retinal ganglion cells in the channel catfish (Ictalurus punctatus): three types with distinct dendritic stratification patterns form similar but independent mosaics

Retinal ganglion cells in the channel catfish (Ictalurus punctatus) were retrogradely labelled, and those with the largest somata and thickest primary dendrites were categorized by their levels of dendritic stratification. Three types were found, each forming a mosaic making up approximately 1% of the ganglion cell population. Using a system based on established sublaminar terminology, we call these the alpha-a (alpha a), alpha-b (alpha b), and alpha-c (alpha c) ganglion cell mosaics. Cells of the alpha a mosaic had large, sparsely branched trees in sublamina a at 10-30% of the depth of the inner plexiform layer (IPL), sclerad to those of all other large ganglion cells. Some alpha a somata were displaced into the IPL or inner nuclear layer (INL) but belonged to the same mosaic as their orthotopic counterparts. Cells of the alpha b mosaic had dendrites that branched a little more and arborized in sublamina b at 50-60% of the IPL depth. Many also sent fine branches into sublamina a, and some were fully bistratified in a and b. The alpha c cells arborized in the most vitread sublamina, sublamina c, at 80-95% of the IPL depth. The soma areas of the three types in the largest retina studied ranged between 139 microns 2 and 670 microns 2 with significant differences in the order alpha a > alpha c > or = alpha b. Analyses based on nearest-neighbour distance (NND) and on spatial auto- and cross-correlograms showed that each mosaic was statistically regular and independent of the others. Mosaic spacings were similar for each type, giving mean NNDs of 242-279 microns in the largest retina and 153-159 microns in a smaller one. Correspondences between these mosaics, previously defined large ganglion cell types in catfish, and other mosaic-forming large ganglion cells in fish and frogs are discussed along with their implications for neuronal classification, function, development, and evolution.

The organization of the turtle inner retina. I. ON- and OFF-center pathways

Intracellular recordings and dye injections of Lucifer yellow, horseradish peroxidase, or Neurobiotin were made in bipolar, amacrine, and ganglion cells of the Pseudemys turtle retina. By using a standard light-stimulation protocol in a sample of 375 labeled neurons, we were able to identify morphological and physiological characteristics of 11 types of bipolar cell, 37 types of amacrine cell, and 24 types of ganglion cell. To make sense of these data, we have chosen to group the 72 essentially different neuron types into traditional, functionally significant pathways. In this paper we look at the neuronal types in the inner plexiform layer (IPL) in terms of their contribution to generalized luminosity responses such as sustained ON- or OFF-center and transient ON-OFF ganglion cells; in the companion paper (J. Ammermüller, J.F. Muller, and H. Kolb, 1995, J. Comp. Neurol. 358:35-62) we look at them in terms of their involvement in color opponency and directional selectivity. A functional organization of the turtle IPL into OFF sublaminae (strata 1 and 2) and ON sublaminae (strata 3, 4, and 5), as has been described for other vertebrate retinas, was quite clear for two varieties of OFF-center bipolar cells (B4 and B5) and for all four types of sustained ON-center bipolar cell (B1, B2, B6, and B7). Thus, we found no sustained ON-center bipolar cell terminating in strata 1 and 2. We did, however, see three varieties of sustained OFF-center bipolar cells (B3, B9, and B10) having axon terminals in strata 3-5 (the ON sublamina) in addition to their terminations in stratum 1 or 2 (the OFF sublamina). Monostratified sustained ON- and OFF-center amacrine and ganglion cells rigidly obeyed the border of ON and OFF sublaminae. However, multistratified and diffuse sustained amacrine and ganglion cells could be either ON-center or OFF-center, and they did not strictly obey the border: such ON-center cells always had processes in one of the ON sublaminae (strata 3-5), and the equivalent OFF-center cells always had processes in one of the OFF sublaminae (strata 1 and 2). Monostratified transient amacrine and ganglion cells were concentrated in the middle of the IPL (around stratum 3), whereas bi-, tri-, or multistratified transient amacrine or ganglion cells always had processes in both the ON and the OFF sublaminae.(ABSTRACT TRUNCATED AT 400 WORDS)

The organization of the turtle inner retina. II. Analysis of color-coded and directionally selective cells

Color coding and directional selectivity (DS) of retinal neurons were studied in the Pseudemys turtle by using similar intracellular recording and staining techniques as in the preceding paper (J. Ammermüller and H. Kolb, 1995, J. Comp. Neuronal. 358:1-34). Color-coded responses were elicited by red (621 or 694 nm), green (525 or 514 nm), and blue (455 nm) light flashes. In addition to red/green and yellow/blue types of chromaticity horizontal cells, in our sample of 305 identified cells we found that 17% of bipolar cells, 6.5% of amacrine cells, and 18% of ganglion cells exhibit color-coded responses. DS responses were found in 37% of the tested ganglion cells and 41% of the tested amacrine cells. Two morphologically identified bipolar cell types, B10 and B11, were red-ON/blue-OFF and red-OFF/green, blue-ON, respectively. Of five identified amacrine cell types, three were red-OFF/blue-ON center (A1, A3, A23b), one was red-OFF/green-ON center (A32), and one (A33) was double color-opponent of red-ON/blue-OFF center:red-OFF/blue-ON surround. Five ganglion cell types had variously color-coded centers (G14 and G24) or surrounds (G3 and G18), including one type, G6, that was double color-opponent (red-OFF/green-ON center:red-ON/green-OFF surround). Responses to colors were found primarily in sustained responses of bipolar and ganglion cells. However, in amacrine cells, transient components of the response also showed color dependence. Red-OFF-center responses were found in ganglion cells that were in a position to make connections at the strata 2/3 border with the red-OFF bipolar cell (B11); red-ON-center responses occurred in ganglion cells with branches in stratum 4 of the IPL where the red-ON-center bipolar (B10) ended. Blue-ON-center signals appeared to be processed mainly in strata 1-2/3, and blue-OFF-center signals in strata 3-5 of the IPL, with contributions of amacrine cells and bipolar cells. Labeled DS amacrine cells could be identified as A9, A20, and A22, and ganglion cells as G19, G20, and G24. The latter type (G24) showed DS and color coding. All response types (ON-center, OFF-center, ON-OFF) were encountered. DS amacrine cells were monostratified near the middle of the IPL, whereas DS ganglion cells were mono-, bi-, and multistratified, although all DS ganglion cells had one feature in common: they had dendrites in stratum 1 of the IPL.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of kainic acid and piperidine dicarboxylic acid on displaced bipolar cells in the turtle retina

An immunoreaction against glutamate was used to visualize photoreceptors, bipolar, and ganglion cells in the turtle retina. Incubation of the retina prior to fixation in kainic acid (9 microM) led to selective loss of glutamate-like immunoreactivity in OFF-centre bipolar cells, as judged by the loss of staining in the distal half of the inner plexiform layer. In addition, displaced bipolar cells and ganglion cells lost their immunoreactivity. Incubation of the retina in 2,3-cis piperidine dicarboxylate (1 mM) did not result in noticeable glutamate depletion in any cell but enhanced labelling in displaced bipolar cells. These findings suggest that all displaced bipolar cells in the turtle retina are depolarized by kainic acid and hyperpolarized by 2,3-cis piperidine dicarboxylate.

Short-term effects of dopamine on photoreceptors, luminosity- and chromaticity-horizontal cells in the turtle retina

The effects of dopamine on luminosity-type horizontal cells have been documented in different vertebrate retinas, both in vivo and in vitro. Some of these effects may reflect direct action of dopamine onto these cells, but indirect effects mediated by presynaptic neurons cannot be ruled out. Furthermore, direct effects of dopamine on horizontal cells may affect other, postsynaptic neurons in the outer plexiform layer. To test these possibilities, we studied the effects of dopamine on photoreceptors and all types of horizontal cells in the turtle (Pseudemys scripta elegans) retina. Receptive-field properties, responsiveness to light, and time course of light responses were monitored with intracellular recordings. Dopamine at a concentration of 40 microM exerted effects with two different time courses. "Short-term" effects were fully developed after 3 min of dopamine application and reversed within 30 min of washout of the drug. "Long-term" effects were fully developed after about 7-10 min and could not be washed out during the course of our experiments. Only the "short-term" effects were studied in detail in this paper. These were expressed in a reduction of the receptive-field size of all types of horizontal cells studied; L1 and L2 luminosity types as well as Red/Green and Yellow/Blue chromaticity types. The L1 horizontal cells did not exhibit signs of reduced responsiveness to light under dopamine, while in the L2 cells and the two types of chromaticity cells responsiveness decreased. None of the rods, long-wavelength-sensitive, or medium-wavelength-sensitive cones exhibited any apparent reduction in their receptive-field sizes or responsiveness to light. The present results suggest that the "short-term" effects of dopamine are not mediated by photoreceptors and are probably due to direct action of dopamine on horizontal cells.

Functional morphologies of retinal ganglion cells in the turtle

Retinal ganglion cells in the turtle, Pseudemys scripta elegans, were examined by intracellular recording with a protocol of stationary and moving lights. Responses were apportioned among OFF, ON, and ON-OFF categories, and directional selectivity. Cells were injected with Neurobiotin, then later conjugated with avidin-horseradish peroxidase in standard procedure. Morphological analysis of the stained cells included measurements of soma and dendritic field sizes, dendritic stratification, number of cell processes, dendritic branchings, and dendritic symmetry ratios. ON and ON-OFF cells are at least bistratified, sometimes tristratified, in both sublaminae A and B whether directionally selective or not. OFF cells, in contrast, are monostratified, or at least confined to sublamina A. Morphological parameters of somal and dendritic field areas, branch point densities, and dendritic field asymmetries do not predict directional selectivity. Membrane polarization accompanying moving stimulation is discussed in terms of shunting inhibition and recording site.

Ultrastructural and immunocytochemical analysis of the circuitry of two putative directionally selective ganglion cells in turtle retina

Two well-stained, horseradish peroxidase-filled varieties of putative ON-OFF directionally selective ganglion cells, G14a and G15, that project to the dorsolateral optic tectum (Guiloff and Kolb [1992a] Vis. Neurosci. 8:295-313) were studied qualitatively and quantitatively. Both were bistratified ganglion cells with one tier of dendrites in the OFF sublamina and the other in the ON sublamina of the inner plexiform layer (IPL). The cells were serially sectioned and examined for synaptic inputs by electron microscopy. Portions of the dendritic trees were also analyzed after postembedding immunocytochemistry for neurotransmitter candidates gamma aminobutyric acid (GABA), glycine, choline acetyltransferase (ChAT), and glutamate in presynaptic neurons. Both G14a and G15 are dominated by amacrine cell inputs and have only minor bipolar cell involvement. Probably at least two different types of bipolar cell are presynaptic. Both ganglion cells receive some GABA-positive (GABA+) amacrine inputs and G14a receives ChAT+ amacrine inputs. Glycine+ and glutamate+ inputs could not be detected in either cell. The GABA+ inputs appeared to be regionally arranged in the dendritic trees. The general distribution of amacrine and bipolar inputs to the two tiers of dendrites in both cell types appeared to be asymmetrical, both along the radial extent of the dendritic trees and within the depth of the IPL. Our data support some aspects of the current models for directional selectivity. We suggest candidate bipolar and amacrine cells that could have input to these ganglion cells. Since many of the putative presynaptic amacrine cells coincide with directionally selective types recorded and stained by other authors, we propose that in turtle retina directional selectivity arises in neurons presynaptic to the ganglion cells.

Complexity and scaling properties of amacrine, ganglion, horizontal, and bipolar cells in the turtle retina

In the present study we have evaluated the complexity and scaling properties of the morphology of retinal neurons using fractal dimension as a quantitative parameter. We examined a large number of cells from Pseudemys scripta and Mauremys caspica turtles that had been labeled using Golgi-impregnation techniques, intracellular injection of Lucifer Yellow followed by photooxidation, intracellular injection of rhodamine conjugated horseradish peroxidase, or intracellular injection of Lucifer Yellow or horseradish peroxidase alone. The fractal dimensions of two-dimensional projections of the cells were calculated using a box counting method. Discriminant analysis revealed fractal dimension to be a significant classification parameter among several other parameters typically used for placing turtle retinal neurons in different cell classes. The fractal dimension of amacrine cells was significantly correlated with dendritic field diameters, while the fractal dimensions of ganglion cells did not vary with dendritic field span. There were no significant differences between the same cell types in two different turtle species, or between the same types of neurons in the same species after labeling with different techniques. The application of fractal dimension, as a quantitative measure of complexity and scaling properties and as a classification criterion of neuronal types, appears to be useful and may have wide applicability to other parts of the central nervous system.

Anatomical characterization of retinal ganglion cells that project to the nucleus of the basal optic root in the turtle (Pseudemys scripta elegans)

There is little detailed information about retinal ganglion cells which project to specific central targets in the brain. The present study examined retinal ganglion cells projecting to the nucleus of the basal optic root, a major accessory retinal target in the turtle. These ganglion cells were first selectively labeled using retrograde transport of rhodamine injected stereotaxically into the nucleus of the basal optic root. The number and distribution of the retrogradely labeled cells in the retina was then determined. Some of these retrogradely labeled cells were then injected intracellularly with Lucifer Yellow, photoconverted using diaminobenzidine, and drawn in detail using a camera lucida attachment. There were approximately 1500 ganglion cells in each retina which projected to the nucleus of the basal optic root, of which 20% had cell bodies displaced to the inner nuclear layer. More than 50% of the total population was concentrated in the visual streak region. All ganglion cells projecting to the nucleus of the basal optic root, both normal and displaced, had monostratified dendritic arborizations in stratum 1 of the inner plexiform layer. About 41% of these ganglion cells had elongated dendritic arborizations with distinct orientations, which may suggest a correlation of morphology and function. There were similarities in the overall appearance, and in the type and stratification of the dendritic arborizations of all ganglion cells projecting to the nucleus of the basal optic root. These anatomical similarities are consistent with the previously demonstrated similarities in physiology and may reflect a common role for these ganglion cells in visual processing.

GABA and glycine in retinal amacrine cells: combined Golgi impregnation and immunocytochemistry

Golgi-impregnated amacrine cells in the all-cone lizard retina (Anolis carolinensis) were characterized on the bases of dendritic and somatic criteria. Four major cell categories, comprising 23 types were identified: three non-stratified, 13 monostratified, five bistratified, and two tristratified types. Four of the cell types comprised two to four subtypes based on stratification of their dendrites within the inner plexiform layer (IPL). Golgi impregnation strongly favoured monostratified amacrine cells with cell bodies at the proximal margin of the inner nuclear layer. The neurotransmitter content of each of the 23 amacrine cell types was examined by combined Golgi-immunocytochemistry after morphological classification. Putative neurotransmitters examined included gamma-aminobutyric acid (GABA), glycine (GLY) and aspartate (ASP). Seventeen cell types showed GABA-immunoreactivity (IR), three cell types showed GLY-IR, and four cell types showed neither GABA-IR nor GLY-IR. No cell types showed ASP-IR. Each cell type had a characteristic neurochemical signature, with the exception of one monostratified cell type that showed three different neurochemical signatures. Postembedding immunocytochemistry on conventionally processed retinas confirmed the localization of glutamic acid decarboxylase, the synthetic enzyme for GABA, to cells similar to several of the GABA-IR Golgi-stained types. Postembedding immunocytochemistry for tyrosine hydroxylase (the synthetic enzyme for catecholamines) and GABA on serial sections demonstrated colocalization of GABA and a catecholamine, probably dopamine, in a bistratified amacrine cell type. We conclude that GABA-IR amacrine cell types are more numerous and morphologically heterogeneous than GLY-IR amacrine cells. The morphological heterogeneity and, with one exception, exclusivity of GABA-IR and GLY-IR amacrine cell types indicate that both neurotransmitters play a variety and different functional roles in the lizard inner retina.

Synaptic microcircuitry of bipolar and amacrine cells with serotonin-like immunoreactivity in the retina of the turtle, Pseudemys scripta elegans

Although serotonin is thought to be a neurotransmitter in a number of retinal systems, much of the precise synaptic connectivity of serotonergic neurons is unknown. To address this issue, we used an antiserum directed against serotonin to label serotonergic bipolar and amacrine cells in the turtle retina. Light-microscopic analysis of labeled amacrine and bipolar cells indicated that both had bistratified dendritic arborizations primarily in stratum 1 and in strata 4/5 of the inner plexiform layer. Ultrastructural analysis of the neurocircuitry of these cells indicated that the processes of labeled bipolar cells in the outer plexiform layer made basal junction contacts with photoreceptor terminals. Only in rare instances did labeled bipolar cells processes invaginate near photoreceptor ribbon synapses. Processes of labeled bipolar cells received both conventional and small ribbon synaptic contacts in the outer plexiform layer. Bipolar cell processes in stratum 1 of the inner plexiform layer synapsed onto either amacrine/amacrine or amacrine/ganglion cell dyads, and made rare ribbon synaptic contacts onto labeled amacrine cell processes. Synaptic inputs to serotonergic bipolar cells in stratum 1 were from unlabeled bipolar and amacrine cells. Bipolar cell contacts in strata 4/5 were similar to those in stratum 1, but were fewer in number and no bipolar cell inputs were seen. Labeled amacrine cell output in both strata was onto other unlabeled amacrine cells and ganglion cells; but synaptic outputs to unlabeled bipolar cells were only seen in strata 4/5. In both strata 1 and 4/5, synaptic inputs to labeled amacrine cells were from both unlabeled amacrine cells and labeled bipolar cells. The serotonergic amacrine cells had many more synaptic interactions in stratum 1 than in strata 4/5 which supports the role of serotonergic bipolar cells in the OFF pathway of retinal processing. Interactions between serotonergic bipolar and amacrine cells may play an important role in visual processing.

Goldfish bipolar cells and axon terminal patterns: a Golgi study

The morphology and axon terminal arrangement of Golgi stained goldfish bipolar cells were examined to understand better the organization of bipolar cells in the inner plexiform layer (IPL) of the retina. Fifteen morphological bipolar cell types were identified, representing two major cell classes: mixed input cells that receive input from rod and cone photoreceptors, and cone bipolar cells that receive input from cones only. Mixed input bipolar cells comprised six types, including two new types, characterized by large somas and terminals. The terminals of mixed input bipolar cells terminated strictly within sublamina a or b of the IPL. Cone bipolar cells comprised nine subtypes, including seven new types, characterized by small somas and from one to four small terminal bulbs along the length of the axon, each having a characteristic termination depth in the IPL. The cone bipolar cell system had a complex multilaminar organization of terminals in the IPL, but maintained a high degree of anatomical symmetry about sublamina a and b. Cone bipolar cells could be divided into three groups: cells terminating within sublamina a and having an anatomically symmetrical counterpart terminating in sublamina b; cells with anatomically similar terminals in both sublamina a and b; and cells having no anatomically symmetrical counterpart or having anatomically dissimilar terminals in sublamina a and b. Based on bipolar cell terminal arrangement, we suggest that each bipolar cell type probably has a unique set of synaptic targets in the IPL, and that several bipolar cell types may be involved in functionally equivalent circuits at more than one level in the IPL.

Ultrastructural and functional connectivity of intracellularly stained neurones in the vertebrate retina: correlative analyses

A variety of intracellular recording and staining techniques has been used to establish structure-function and, in some cases, structure-function-neurochemical correlations in fish, turtle, and cat retinae. Cone photoreceptor-horizontal cell connectivity has been studied extensively in the cyprinid fish retina by intracellular staining with horseradish peroxidase (HRP) and subsequent electron microscopy. The available data suggest that horizontal cell dendrites around the ridge of the synaptic ribbon are postsynaptic, whilst finger-like extensions ("spinules") of lateral dendrites function as inhibitory feedback terminals. An interesting feature of this interaction is its plasticity: the feedback pathway is suppressed in the dark and becomes potentiated by light adaptation of the retina. Intracellular recordings and stainings of ganglion cells in both turtle and cat retinae have been possible. Prelabelling of ganglion cells by retrograde transport of rhodamine from the tectum allows ganglion cells to be stained under visual control, and their synaptic inputs determined by electron microscopy. Such studies have been extended to double labelling by using autoradiography or postembedding immunohistochemistry to identify the neurotransmitter content of the labelled cell and/or the neurotransmitter(s) converging upon it. It is envisaged that further applications of intracellular staining followed by double- or even triple-labelling will continue to enhance greatly our understanding of the functional architecture of the vertebrate retina.

Immunocytochemical staining with antibodies against protein kinase C and its isozymes in the turtle retina

An LM immunocytochemical study has investigated the patterns of staining in turtle retina with monoclonal antibodies to the alpha, beta and gamma isozymes of protein kinase C. The protein kinase C-gamma antibody reveals cells in the ganglion cell layer, occasional amacrine cells and faint banding in strata 2 and 4 of the inner plexiform layer. The protein kinase C-beta antibody stains primarily amacrine cells that have dendrites running in strata 2, in 4 close to the 3/4 border and on the 4/5 border of the inner plexiform layer. Protein kinase C-alpha immunoreactivity is seen in a population of bipolar cells. The latter are characterized by stained axon terminals in strata 3 and 4 of the inner plexiform layer. A type of amacrine cell, different from those seen with the other antibodies, is also immunoreactive to protein kinase C-alpha. EM immunocytochemistry (using a polyclonal antibody) reveals protein kinase C immunoreactivity in photoreceptor cells, bipolar cells, amacrine cells and ganglion cells. In photoreceptors protein kinase C immunoreactivity occurs as patchy staining associated with vesicles and the plasmalemma in pedicles and telodendria. Some varieties of bipolar cell display protein kinase C reaction product throughout the entire cell. Their dendrites contact photoreceptor pedicles at wide-cleft basal junctions and ribbon and non-ribbon related narrow cleft junctions. A few lateral elements per cone or rod pedicle are always protein kinase C-immunoreactive. Amacrine and ganglion cells typically show small clumps of protein kinase C immunoreactivity around vesicles and close to the postsynaptic membranes. Synaptic boutons of some varieties of amacrine cell stain more uniformly. Protein kinase C-immunoreactive bipolar cells are most commonly presynaptic in stratum 4 of the inner plexiform layer, while protein kinase C-immunoreactive amacrine cells are both pre- and postsynaptic throughout strata 1, 2, 3 and 4. Stratum 5 appears to be almost devoid of protein kinase C-immunoreactive neural profiles.

Neurons immunoreactive to choline acetyltransferase in the turtle retina

Light microscopic immunocytochemistry using anti-choline acetyltransferase (ChAT) was performed to stain putative cholinergic amacrine cells in turtle retina. ChAT-immunoreactive somata lie in the inner nuclear (INL) and ganglion cell (GCL) layers. Three types of amacrine cells were found according to the location of their somata and their dendritic stratification pattern in the inner plexiform layer (IPL). Type I amacrines lie in the row of cells closest to the INL/IPL limits and they branch along the s1/s2 border of the IPL. Type II amacrines are displaced to the GCL and they ramify along the s3/s4 border of the IPL. Type III amacrines lie in the middle of the INL, 2-3 rows away from the IPL limits and their dendrites appear to be bi- or tri-stratified in s1 and s3-s4 of the IPL. The turtle ChAT-IR amacrines are thus similar to the types described in chicken retina. A regular, non-random mosaic formed by stained type II amacrine cells was observed in the GCL. Their density in mid-central retina was 750 cells/mm2, tapering off to 393 cells/mm2 in peripheral retina. Our study indicates that a pair of cholinergic amacrine cell types in turtle retina is arranged in mirror-image symmetry contributing to sublamina "a" and sublamina "b" of the IPL, like in other vertebrate retinas.

Glutamate antagonists that block hyperpolarizing bipolar cells increase the release of dopamine from turtle retina

Some neurochemical features of the neuronal circuitry regulating dopamine release were examined in the retina of the turtle, Pseudemys scripta elegans. Glutamate antagonists that block hyperpolarizing bipolar cells, such as 2,3 piperidine dicarboxylic acid (PDA), produced dose-dependent dopamine release. In contrast, the glutamate agonist 2-amino-4-phosphonobutyric acid (APB), which blocks depolarizing bipolar cell responses with high specificity, had no effect on the release of dopamine. The gamma-aminobutyric acid (GABA) antagonist, bicuculline, also produced potent dose-dependent release of dopamine. The release of dopamine produced by PDA was blocked by exogenous GABA and muscimol, suggesting that the PDA-mediated release process was polysynaptic and involved a GABAergic synapse interposed between the bipolar and dopaminergic amacrine cells. The only other agents that produced dopamine release were chloride-free media and high extracellular K+; in particular, kainic acid and glutamate itself were ineffective. These results suggest that the primary neuronal chain mediating dopamine release in the turtle retina is: cone----hyperpolarizing bipolar cell----GABAergic amacrine cell----dopaminergic amacrine cell.

Push-pull modulation of ganglion cell responses of carp retina by amacrine cells

The responses of a ganglion and an amacrine cell were recorded simultaneously in the carp retina. Sinusoidal current injected into amacrine cells modulated ganglion cell discharges either in phase (excitation) or in opposite phase (inhibition). ON-center ganglion cells received excitatory inputs and OFF-center ganglion cells received inhibitory inputs from ON-center amacrine cells. They received inputs of opposite polarity from OFF-center amacrine cells. Namely, inputs from ON-center and OFF-center amacrine cells augment the responses of ON-center and OFF-center ganglion cells in a push-pull manner.