Synaptic inputs to physiologically defined turtle retinal ganglion cells

Characterization of glycinergic synapses in vertebrate retinas

Glycine is one of the essential neurotransmitters modulating visual signals in retina. Glycine activates Cl(-) permeable receptors that conduct either inhibitory or excitatory actions, depending on the Cl(-) electrical-chemical gradient (E (Cl)) positive or negative to the resting potential in the cells. Interestingly, both glycine-induced inhibitory and excitatory responses are present in adult retinas, and the effects are confined in the inner and outer retinal neurons. Glycine inhibits glutamate synapses in the inner plexiform layer (IPL), resulting in shaping light responses in ganglion cells. In contrast, glycine excites horizontal cells and On-bipolar dendrites in the outer plexiform layer (OPL). The function of glycinergic synapse in the outer retina represents the effect of network feedback from a group of centrifugal neurons, glycinergic interplexiform cells. Moreover, immunocytochemical studies identify glycine receptor subunits (alpha1, alpha2, alpha3 and beta) in retinas, forming picrotoxin-sensitive alpha-homomeric and picrotoxin-insensitive alpha/beta-heteromeric receptors. Glycine receptors are modulated by intracellular Ca(2+) and protein kinas C and A pathways. Extracellular Zn(2+) regulates glycine receptors in a concentration-dependent manner, nanomolar Zn(2+) enhancing glycine responses, and micromolar Zn(2+) suppressing glycine responses in retinal neurons. These studies describe the function and mechanism of glycinergic synapses in retinas.

The temporal structure of transient ON/OFF ganglion cell responses and its relation to intra-retinal processing

A subpopulation of transient ON/OFF ganglion cells in the turtle retina transmits changes in stimulus intensity as series of distinct spike events. The temporal structure of these event sequences depends systematically on the stimulus and thus carries information about the preceding intensity change. To study the spike events' intra-retinal origins, we performed extracellular ganglion cell recordings and simultaneous intracellular recordings from horizontal and amacrine cells. Based on these data, we developed a computational retina model, reproducing spike event patterns with realistic intensity dependence under various experimental conditions. The model's main features are negative feedback from sustained amacrine onto bipolar cells, and a two-step cascade of ganglion cell suppression via a slow and a fast transient amacrine cell. Pharmacologically blocking glycinergic transmission results in disappearance of the spike event sequence, an effect predicted by the model if a single connection, namely suppression of the fast by the slow transient amacrine cell, is weakened. We suggest that the slow transient amacrine cell is glycinergic, whereas the other types release GABA. Thus, the interplay of amacrine cell mediated inhibition is likely to induce distinct temporal structure in ganglion cell responses, forming the basis for a temporal code.

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.

Fundamental GABAergic amacrine cell circuitries in the retina: nested feedback, concatenated inhibition, and axosomatic synapses

Presynaptic gamma-aminobutyrate-immunoreactive (GABA+) profiles were mapped in the cyprinid retina with overlay microscopy: a fusion of electron and optical imaging affording high-contrast ultrastructural and immunocytochemical visualization. GABA+ synapses, deriving primarily from amacrine cells (ACs), compose 92% of conventional synapses and 98% of the input to bipolar cells (BCs) in the inner plexiform layer. GABA+ AC synapses, the sign-inverting elements of signal processing, are deployed in micronetworks and distinctive synaptic source/target topologies. Nested feedback micronetworks are formed by three types of links (BC --> AC, reciprocal BC <-- AC, and AC --> AC synapses) arranged as nested BC<--> [AC --> AC] loops. Circuits using nested feedback can possess better temporal performance than those using simple reciprocal feedback loops. Concatenated GABA+ micronetworks of AC --> AC and AC --> AC --> AC chains are common and must be key elements for lateral spatial, temporal, and spectral signal processing. Concatenated inhibitions may represent exceptionally stable, low-gain, sign-conserving devices for receptive field construction. Some chain elements are GABA immunonegative (GABA-) and are, thus, likely glycinergic synapses. GABA+ synaptic baskets target the somas of certain GABA+ and GABA- cells, resembling cortical axosomatic synapses. Finally, all myelinated intraretinal profiles are GABA+, suggesting that some efferent systems are sources of GABAergic inhibition in the cyprinid retina and may comprise all axosomatic synapses. These micronetworks are likely the fundamental elements for receptive field shaping in the inner plexiform layer, although few receptive field models incorporate them as functional components. Conversely, simple feedback and feedforward synapses may often be chimeras: the result of an incomplete view of synaptic topology.

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.

Synaptic inputs to retrogradely labeled ganglion cells in the retina of the cane toad, Bufo marinus

The entire population of ganglion cells in the retina of the toad Bufo marinus was labeled by retrograde transport of a lysine-fixable biotinylated dextran amine of 3000 molecular weight. Synaptic connections between bipolar, amacrine, and ganglion cells in the inner plexiform layer were quantitatively analyzed, with emphasis on synaptic inputs to labeled ganglion cell dendrites. Synapses onto ganglion cell dendrites comprised 47% of a total of 1234 identified synapses in the inner plexiform layer. Approximately half of the bipolar or amacrine cell synapses were directed onto ganglion cell dendrites, while the rest were made mainly onto amacrine cell dendrites. Most of the synaptic inputs to ganglion cell dendrites derived from amacrine cell dendrites (84%), with the rest from bipolar cell terminals. Synaptic inputs to ganglion cell dendrites were distributed relatively uniformly throughout all sublaminae of the inner plexiform layer. The present study provides unambiguous identification of ganglion cell dendrites including very fine processes, enabling a detailed analysis of the types and distribution of synaptic inputs from the bipolar and amacrine cell to the ganglion cells. The retrograde tracing technique used in the present study will prove to be a useful tool for identifying synaptic inputs to ganglion cell dendrites from neurochemically identified bipolar and amacrine cell types in the retina.

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)

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.

A geometrical description of horizontal cell networks in the turtle retina

Networks of physiologically identified H2 horizontal cells in the turtle retina were labeled by intracellular injection of Neurobiotin. We obtained a quantitative description of the neighbourhood relations in the dye-coupled cell mosaics by using the somata as centers for the Voronoi-Delaunay construction. Computational models simulating the experimental data are presented.

Extremely high titre polyclonal antisera against small neurotransmitter molecules: rapid production, characterisation and use in light- and electron-microscopic immunocytochemistry

We have produced polyclonal antibodies against the small amino acid neurotransmitters, GABA, glutamate, glycine and taurine, with a simple new technique using antigens co-adsorbed with an adjuvant peptide to gold particles, which causes rapid and massive immune responses in all animals that we have studied. These antibodies are all of extremely high titre; they are typically used in immunocytochemistry at dilutions from 1 in 250,000 to 1 in 1,000,000 which represents an increase in titre of at least two orders of magnitude compared to standard antibody production techniques. Such very high dilutions result in minimal background labeling and a high signal-to-noise ratio when applied to sections of aldehyde-fixed, epoxy resin-embedded tissues at both light- and electron-microscopic levels. Each antibody displays minimal cross-reactivity with other neurotransmitter molecules. We suggest that our technique may be broadly applicable for raising antibodies against a wide variety of antigens of interest to neuroscientists, particularly those that normally elicit weak immune responses. The technique may also assist in clonal expansion prior to generation of monoclonal antibodies and may be viable, with modifications, for use in human immunisations.

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.

Ganglion cell types of the turtle retina that project to the optic tectum: Intracellular HRP injections of retrogradely, rhodamine-marked cell bodies

The turtle retina has been shown to have a variety of different morphological ganglion cell types as well as distinct physiological ganglion cell types. The major projection of the retina to the brain in nonmammalian vertebrates is to the optic tectum. In this study, we address the question of which retinal ganglion cell types project to the optic tectum in the turtle. Fluorescent rhodamine-labeled microspheres were used to trace the retinal ganglion cell projection to the superficial layers of the optic tectum. The fluorescent ganglion cell somata, retrogradely marked by transport from the contralateral optic tectum, were impaled with micropipettes containing rhodamine-horseradish peroxidase solution and this dye was iontophoresed into the cells under visual control. Most of the morphological ganglion cell types described in Golgi studies (Kolb, 1982; Kolb et al., 1988) were stained. Thus, the small cell types G1, G2, G3, G5, G6, and G7; the medium-sized types G10, G11, G12, G13, and G14; and the large-sized types G15, G16, G19, G20, and G21 project to the optic tectum in the turtle. We have added a new type, G2a, which proves to have some differences from the original G2 in branching pattern. We were unable to stain the small type G4, the medium-sized types G8 and G9, and the large cell types G17 and G18: this suggests that they might not project to the superficial layers of the dorsolateral optic tectum, at least, in the turtle.