The DAPI-3 amacrine cells of the rabbit retina

Cholinergic feedback to bipolar cells contributes to motion detection in the mouse retina

Retinal bipolar cells are second-order neurons that transmit basic features of the visual scene to postsynaptic partners. However, their contribution to motion detection has not been fully appreciated. Here, we demonstrate that cholinergic feedback from starburst amacrine cells (SACs) to certain presynaptic bipolar cells via alpha-7 nicotinic acetylcholine receptors (α7-nAChRs) promotes direction-selective signaling. Patch clamp recordings reveal that distinct bipolar cell types making synapses at proximal SAC dendrites also express α7-nAChRs, producing directionally skewed excitatory inputs. Asymmetric SAC excitation contributes to motion detection in On-Off direction-selective ganglion cells (On-Off DSGCs), predicted by computational modeling of SAC dendrites and supported by patch clamp recordings from On-Off DSGCs when bipolar cell α7-nAChRs is eliminated pharmacologically or by conditional knockout. Altogether, these results show that cholinergic feedback to bipolar cells enhances direction-selective signaling in postsynaptic SACs and DSGCs, illustrating how bipolar cells provide a scaffold for postsynaptic microcircuits to cooperatively enhance retinal motion detection.

Network Architecture of Gap Junctional Coupling among Parallel Processing Channels in the Mammalian Retina

Gap junctions are ubiquitous throughout the nervous system, mediating critical signal transmission and integration, as well as emergent network properties. In mammalian retina, gap junctions within the Aii amacrine cell-ON cone bipolar cell (CBC) network are essential for night vision, modulation of day vision, and contribute to visual impairment in retinal degenerations, yet neither the extended network topology nor its conservation is well established. Here, we map the network contribution of gap junctions using a high-resolution connectomics dataset of an adult female rabbit retina. Gap junctions are prominent synaptic components of ON CBC classes, constituting 5%-25% of all axonal synaptic contacts. Many of these mediate canonical transfer of rod signals from Aii cells to ON CBCs for night vision, and we find that the uneven distribution of Aii signals to ON CBCs is conserved in rabbit, including one class entirely lacking direct Aii coupling. However, the majority of gap junctions formed by ON CBCs unexpectedly occur between ON CBCs, rather than with Aii cells. Such coupling is extensive, creating an interconnected network with numerous lateral paths both within, and particularly across, these parallel processing streams. Coupling patterns are precise with ON CBCs accepting and rejecting unique combinations of partnerships according to robust rulesets. Coupling specificity extends to both size and spatial topologies, thereby rivaling the synaptic specificity of chemical synapses. These ON CBC coupling motifs dramatically extend the coupled Aii-ON CBC network, with implications for signal flow in both scotopic and photopic retinal networks during visual processing and disease.SIGNIFICANCE STATEMENT Electrical synapses mediated by gap junctions are fundamental components of neural networks. In retina, coupling within the Aii-ON CBC network shapes visual processing in both the scotopic and photopic networks. In retinal degenerations, these same gap junctions mediate oscillatory activity that contributes to visual impairment. Here, we use high-resolution connectomics strategies to identify gap junctions and cellular partnerships. We describe novel, pervasive motifs both within and across classes of ON CBCs that dramatically extend the Aii-ON CBC network. These motifs are highly specific with implications for both signal processing within the retina and therapeutic interventions for blinding conditions. These findings highlight the underappreciated contribution of coupling motifs in retinal circuitry and the necessity of their detection in connectomics studies.

Cell types and cell circuits in human and non-human primate retina

This review summarizes our current knowledge of primate including human retina focusing on bipolar, amacrine and ganglion cells and their connectivity. We have two main motivations in writing. Firstly, recent progress in non-invasive imaging methods to study retinal diseases mean that better understanding of the primate retina is becoming an important goal both for basic and for clinical sciences. Secondly, genetically modified mice are increasingly used as animal models for human retinal diseases. Thus, it is important to understand to which extent the retinas of primates and rodents are comparable. We first compare cell populations in primate and rodent retinas, with emphasis on how the fovea (despite its small size) dominates the neural landscape of primate retina. We next summarise what is known, and what is not known, about the postreceptoral neurone populations in primate retina. The inventories of bipolar and ganglion cells in primates are now nearing completion, comprising ~12 types of bipolar cell and at least 17 types of ganglion cell. Primate ganglion cells show clear differences in dendritic field size across the retina, and their morphology differs clearly from that of mouse retinal ganglion cells. Compared to bipolar and ganglion cells, amacrine cells show even higher morphological diversity: they could comprise over 40 types. Many amacrine types appear conserved between primates and mice, but functions of only a few types are understood in any primate or non-primate retina. Amacrine cells appear as the final frontier for retinal research in monkeys and mice alike.

Identification of Synaptic Patterns of NMDA Receptor Subtypes Upon Direction-Selective Rabbit Retinal Ganglion Cells

PURPOSE

The objective of this study was to identify anisotropies that contribute to the directional preference of direction-selective retinal ganglion cells (DS RGCs) in the rabbit retina. We investigated the distributions of N-methyl-d-aspartate receptor 1 (NMDAR1), NMDAR2A and NMDAR2B receptor subunits in the dendritic arbors of rabbit DS RGCs.

METHODS

The distributions of the NMDAR subunits on the DS RGCs were determined using immunocytochemistry. DS RGCs were injected with Lucifer yellow, and the cells were identified by their characteristic morphology. The triple-labeled images of dendrites, kinesin II and NMDARs were visualized using confocal microscopy and were reconstructed from high-resolution confocal images.

RESULTS

We found no evidence of asymmetry in any of the NMDAR subunits examined on the dendritic arbors of both the ON and OFF layers of DS RGCs.

CONCLUSIONS

Our results indicate that direction selectivity appears to lie in the neuronal circuitry afferent to the DS RGCs.

The first stage of cardinal direction selectivity is localized to the dendrites of retinal ganglion cells

Inferring the direction of image motion is a fundamental component of visual computation and essential for visually guided behavior. In the retina, the direction of image motion is computed in four cardinal directions, but it is not known at which circuit location along the flow of visual information the cardinal direction selectivity first appears. We recorded the concerted activity of the neuronal circuit elements of single direction-selective (DS) retinal ganglion cells at subcellular resolution by combining GCaMP3-functionalized transsynaptic viral tracing and two-photon imaging. While the visually evoked activity of the dendritic segments of the DS cells were direction selective, direction-selective activity was absent in the axon terminals of bipolar cells. Furthermore, the glutamate input to DS cells, recorded using a genetically encoded glutamate sensor, also lacked direction selectivity. Therefore, the first stage in which extraction of a cardinal motion direction occurs is the dendrites of DS cells.

Survey on amacrine cells coupling to retrograde-identified ganglion cells in the mouse retina

PURPOSE

Retinal amacrine cells (ACs) may make inhibitory chemical synapses and potentially excitatory gap junctions on ganglion cells (GCs). The total number and subtypes of ACs coupled to the entire GC population were investigated in wild-type and three lines of transgenic mice.

METHODS

GCs and GC-coupled ACs were identified by the previously established LY-NB (Lucifer yellow-Neurobiotin) retrograde double-labeling technique, in conjunction with specific antibodies and confocal microscopy.

RESULTS

GC-coupled ACs (NB-positive and LY-negative) comprised nearly 11% of displaced ACs and 4% of conventional ACs in wild-type mice, and were 9% and 4% of displaced ACs in Cx45(-/-) and Cx36/45(-/-) mice, respectively. Their somas were small in Cx36/45(-/-) mice, but variable in other strains. They were mostly γ-aminobutyric acid (GABA)-immunoreactive (IR) and located in the GC layer. They comprised only a small portion in the AC subpopulations, including GABA-IR, glycine-IR, calretinin-IR, 5-HT-accumulating, and ON-type choline acetyltransferase (ChAT) ACs in wild-type and ChAT transgenic mice (ChAT- tdTomato). In the distal 80% of the inner plexiform layer (IPL), dense GC dendrites coexisted with rich glycine-IR and GABA-IR. In the inner 20% of the IPL, sparse GC dendrites presented with a major GABA band and sparse glycine-IR.

CONCLUSIONS

Various subtypes of ACs may couple to GCs. ACs of the same immunoreactivity may either couple or not couple to GCs. Cx36 and Cx45 dominate GC-AC coupling except for small ACs. The overall potency of GC-AC coupling is moderate, especially in the proximal 20% of the IPL, where inhibitory chemical signals are dominated by GABA ACs.

The retinal hypercircuit: a repeating synaptic interactive motif underlying visual function

Abstract

The vertebrate retina generates a stack of about a dozen different movies that represent the visual world as dynamic neural images or movies. The stack is embodied as separate strata that span the inner plexiform layer (IPL). At each stratum, ganglion cell dendrites reach up to read out inhibitory interactions between three different amacrine cell classes that shape bipolar-to-ganglion cell transmission. The nexus of these five cell classes represents a functional module, a retinal ‘hypercircuit’, that is repeated across the surface of each of the dozen strata that span the depth of the IPL. Individual differences in the characteristics of each cell class at each stratum lead to the unique processing characteristics of each neural image throughout the stack. This review shows how the interactions between the morphological and physiological characteristics of each cell class generate many of the known retinal visual functions including motion detection, directional selectivity, local edge detection, looming detection, object motion and looming detection.

Dendritic morphology and tracer-coupling pattern of physiologically identified transient uniformity detector ganglion cells in rabbit retina

Transient uniformity detectors (UDs) are a unique type of retinal ganglion cell (RGC) whose maintained firing is transiently suppressed by all types of visual stimuli. In this study, we have characterized the dendritic morphology and tracer-coupling pattern of UDs that were labeled by loose-seal electroporation of Neurobiotin following functional identification in the isolated rabbit retina. The UDs have a bistratified dendritic tree, branching near the margins of the inner plexiform layer in stratum 1 (part of the OFF sublamina) and stratum 4/5 (part of the ON sublamina). Characteristically, many of the distal dendrites in the OFF arbor do not terminate there but dive recurrently back to the ON arbor. As a consequence, the ON dendritic arbor is usually twice as large as the OFF dendritic arbor in area. The UDs sometimes show homologous tracer coupling to neighboring RGCs with the same morphology, and from this material, we estimate that the UDs have a threefold dendritic field overlap and a maximum density of ~100 cells/mm2 on the peak visual streak, accounting for ~2% of RGCs in rabbit retina. The UDs also show strong heterologous tracer coupling to a novel type of amacrine cell that costratifies with the ON arbor of the UD. Consistent with their unistratified medium-field morphology, these St4/5 amacrine cells appear to be GABAergic: their somata are immunopositive for GABA but immunonegative for glycine and glycine transporter 1. We compare the dendritic morphology of the UDs to that of other types of bistratified RGCs described in rabbit retina and note that the stratification levels and distinctive recurrent dendrites closely resemble those of the "ON bistratified diving" RGCs. This raises the possibility that there are two types of RGCs with distinctive physiological properties that have almost identical bistratified dendritic morphologies.

Cellular mechanisms for direction selectivity in the retina

Direction selectivity represents a fundamental computation found across multiple sensory systems. In the mammalian visual system, direction selectivity appears first in the retina, where excitatory and inhibitory interneurons release neurotransmitter most rapidly during movement in a preferred direction. Two parallel sets of interneuron signals are integrated by a direction-selective ganglion cell, which creates a direction preference for both bright and dark moving objects. Direction selectivity of synaptic input becomes amplified by action potentials in the ganglion cell dendrites. Recent work has elucidated direction-selective mechanisms in inhibitory circuitry, but mechanisms in excitatory circuitry remain unexplained.

Expression of alpha 7 nicotinic acetylcholine receptors by bipolar, amacrine, and ganglion cells of the rabbit retina

Cholinergic agents affect the light responses of many ganglion cells (GCs) in the mammalian retina by activating nicotinic acetylcholine receptors (nAChRs). Whereas retinal neurons that express beta2 subunit-containing nAChRs have been characterized in the rabbit retina, expression patterns of other nAChR subtypes remain unclear. Therefore, we evaluated the expression of alpha7 nAChRs in retinal neurons by means of single-, double-, and triple-label immunohistochemistry. Our data demonstrate that, in the rabbit retina, several types of bipolar cells, amacrine cells, and cells in the GC layer express alpha7 nAChRs. At least three different populations of cone bipolar cells exhibited alpha7 labeling, whereas glycine-immunoreactive amacrine cells comprised the majority of alpha7-positive amacrine cells. Some GABAergic amacrine cells also displayed alpha7 immunoreactivity; alpha7 labeling was never detected in rod bipolar cells or rod amacrine cells (AII amacrine cells). Our data suggest that activation of alpha7 nAChRs by acetylcholine (ACh) or choline may affect glutamate release from several types of cone bipolar cells, modulating GC responses. ACh-induced excitation of inhibitory amacrine cells might cause either inhibition or disinhibition of other amacrine and GC circuits. Finally, ACh may act on alpha7 nAChRs expressed by GCs themselves.

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.

Modeling Starburst cells' GABA(B) receptors and their putative role in motion sensitivity

Neal and Cunningham (Neal, M. J., and J. R. Cunningham. 1995. J. Physiol. (Lond.). 482:363-372) showed that GABA(B) agonists and glycinergic antagonists enhance the light-evoked release of retinal acetylcholine. They proposed that glycinergic cells inhibit the cholinergic Starburst amacrine cells and are in turn inhibited by GABA through GABA(B) receptors. However, as recently shown, glycinergic cells do not appear to have GABA(B) receptors. In contrast, the Starburst amacrine cell has GABA(B) receptors in a subpopulation of its varicosities. We thus propose an alternate model in which GABA(B)-receptor activation reduces the release of ACh from some dendritic compartments onto a glycinergic cell, which then feeds back and inhibits the Starburst cell. In this model, the GABA necessary to make these receptors active comes from the Starburst cell itself, making them autoreceptors. Computer simulations of this model show that it accounts quantitatively for the Neal and Cunningham data. We also argue that GABA(B) receptors could work to increase the sensitivity to motion over other stimuli.

Diversity of glycine receptors in the mouse retina: Localization of the α4 subunit

Glycine and gamma-aminobutyric acid (GABA) are the major inhibitory neurotransmitters in the retina. Approximately half of the amacrine cells release glycine at their synapses with bipolar, other amacrine, and ganglion cells. Whereas the retinal distributions of glycine receptor (GlyR) subunits alpha1, alpha2, and alpha3 have been mapped, the role of the alpha4 subunit in retinal circuitry remains unclear. A rabbit polyclonal antiserum was raised against a peptide that comprises the C-terminal 14 amino acids of the mouse GlyR alpha4 subunit. Using immunocytochemistry, we localized the alpha4 subunit in the inner plexiform layer (IPL) in brightly fluorescent puncta, which represent postsynaptically clustered GlyRs. This was shown by double-labeling sections for GlyR alpha4 and synaptic markers (bassoon, gephyrin). Double-labeling sections for GlyR alpha4 and the other GlyR alpha subunits shows that they are mostly clustered at different synapses; however, approximately 30% of the alpha4-containing synapses also express the alpha2 subunit. We also studied the pre- and postsynaptic partners at GlyR alpha4-containing synapses and found that displaced (ON-) cholinergic amacrine cells prominently expressed the alpha4 subunit. The density of GlyR alpha4-expressing synapses in wildtype, Glra1(ot/ot), and Glra3(-/-) mouse retinas did not differ significantly. Thus, there is no apparent compensation of the loss of alpha1 or alpha3 subunits by an upregulation of alpha4 subunit gene expression; however, the alpha2 subunit is moderately upregulated.

Compartmental localization of gamma-aminobutyric acid type B receptors in the cholinergic circuitry of the rabbit retina

Although many effects of gamma-aminobutyric acid (GABA) on retinal function have been attributed to GABA(A) and GABA(C) receptors, specific retinal functions have also been shown to be mediated by GABA(B) receptors, including facilitation of light-evoked acetylcholine release from the rabbit retina (Neal and Cunningham [1995] J. Physiol. 482:363-372). To explain the results of a rich set of experiments, Neal and Cunningham proposed a model for this facilitation. In this model, GABA(B) receptor-mediated inhibition of glycinergic cells would reduce their own inhibition of cholinergic cells. In turn, muscarinic input from the latter to the glycinergic cells would complete a negative-feedback circuitry. In this study, we have used immunohistochemical techniques to test elements of this model. We report that glycinergic amacrine cells are GABA(B) receptor negative. In contrast, our data reveal the localization of GABA(B) receptors on cholinergic/GABAergic starburst amacrine cells. High-resolution localization of GABA(B) receptors on starburst amacrine cells shows that they are discretely localized to a limited population of its varicosities, the majority of likely synaptic-release sites being devoid of detectable levels of GABA(B) receptors. Finally, we identify a glycinergic cell that is a potential muscarinic receptor-bearing target of GABA(B)-modulated acetylcholine release. This target is the DAPI-3 cell. We propose, based on these data, a modification of the Neal and Cunningham model in which GABA(B) receptors are on starburst, not glycinergic amacrine cells.

Identification and characterization of an aquaporin 1 immunoreactive amacrine-type cell of the mouse retina

Using immunocytochemistry, a type of amacrine cell that is immunoreactive for aquaporin 1 was identified in the mouse retina. AQP1 immunoreactivity was found in photoreceptor cells of the outer nuclear layer (ONL) and in a distinct type of amacrine cells of the inner nuclear layer (INL). AQP1-immunoreactive (IR) amacrine cell somata were located in the INL and their processes extended through strata 3 and 4 of the inner plexiform layer (IPL) with thin varicosities. The density of the AQP1-IR amacrine cells increased from 100/mm(2) in the peripheral retina to 350/mm(2) in the central retina. The AQP1-IR amacrine cells comprise 0.5% of the total amacrine cells. The AQP1-IR amacrine cell bodies formed a regular mosaic, which suggested that they represent a single type of amacrine cell. Double labeling with AQP1 and glycine, gamma-aminobutyric acid (GABA) or GAD(65) antiserum demonstrated that the AQP1-IR amacrine cells expressed GABA or GAD(65) but not glycine. Their synaptic input was primarily from other amacrine cell processes. They also received synaptic inputs from a few cone bipolar cells. The primary synaptic targets were ganglion cells, followed by other amacrine cells and cone bipolar cells. In addition, gap junctions between an AQP1-IR amacrine process and another amacrine process were rarely observed. In summary, a GABAergic amacrine cell type labeled by an antibody against AQP1 was identified in the mouse retina and was found to play a possible role in transferring a certain type of visual information from other amacrine or a few cone bipolar cells primarily to ganglion cells.

AII amacrine cells in the distal inner nuclear layer of the mouse retina

We serendipitously found a distal Disabled-1 (Dab1)-immunoreactive cell in retina of the C57BL/6J black mouse. The somata of these cells are located in the outermost part of the inner nuclear layer (INL). Their processes extend toward the outer plexiform layer (OPL), receiving synaptic inputs from horizontal and interplexiform cells. In the current study, we name this cell the "distal Dab1-immunoreactive cell." Double-labeling experiments demonstrate that the distal Dab1-immunoreactive cell is not a horizontal cell. Rather, the distal Dab1 cell appears to be a misplaced AII cell, by being glycine transporter-1-immunoreactive and by resembling the latter cell in an electron microscopic analysis. A distal Dab1 cell had been reported in the FVB/N mouse retina, a model of retinitis pigmentosa (Park et al. [2004] Cell Tissue Res 315:407-412). However, here, we found this distal Dab1-immunoreactive cell in the adult and normal developing mouse retinas. Hence, we show that such cells do not require the loss of photoreceptors as suggested previously (Park et al. [2004] Cell Tissue Res 315:407-412). Instead, two other pieces of data suggest an alternative explanation sources for distal Dab1 cells. First, we find a correlation between the number of these cells in the left and right eyes Second, developmental analysis shows that the distal Dab1-immunoreactive cell is first observed shortly after birth. At the same time, AII cells emerge, extending their neurites into the inner retina. These data suggest that distal Dab1-immunoreactive cells are misplaced AII amacrine cells, resulting from genetically modulated anomalies owing to migration errors.

Quantitative analysis of neuronal morphologies in the mouse retina visualized by using a genetically directed reporter

An alkaline phosphatase (AP) reporter has been used to visualize detailed morphologies for all major classes of retinal neurons in the adult mouse. The analysis was performed on retinas in which AP expression was activated by Cre-mediated DNA recombination in a small fraction of cells. Recombination was controlled pharmacologically and, to a first approximation, appears to have occurred randomly. The morphologies of 794 inner retinal neurons have been analyzed by measuring arbor area, stratification level, and neurite branching patterns. When analyzed in this multidimensional parametric space, the cells can be clustered into subgroups by visual inspection and by using the Ward's and K-means algorithms. One application of this cell morphology data set and cluster analysis is as a standard for comparison with the retinas of genetically altered mice. This work illustrates the utility and feasibility of genetically directed marking methods for large-scale surveys of neuronal morphology.

Diversity of glycine receptors in the mouse retina: localization of the alpha2 subunit

Gamma-aminobutyric acid (GABA) and glycine are the major inhibitory neurotransmitters in the retina, glycine being produced in approximately half of all amacrine cells. Whereas retinal cell types expressing the glycine receptor (GlyR) alpha1 and alpha3 subunits have been mapped, the role of the alpha2 subunit in retinal circuitry remains unclear. By using immunocytochemistry, we localized the alpha2 subunit in the inner plexiform layer (IPL) in brightly fluorescent puncta, which represent postsynaptically clustered GlyRs. This was shown by doubly labeling sections for GlyR alpha2 and bassoon (a presynaptic marker) or gephyrin (a postsynaptic marker). Synapses containing GlyR alpha2 were rarely found on ganglion cell dendrites but were observed on bipolar cell axon terminals and on amacrine cell processes. Recently, an amacrine cell type has been described that is immunopositive for glycine and for the vesicular glutamate transporter vGluT3. The processes of this cell type were presynaptic to GlyR alpha2 puncta, suggesting that vGluT3 amacrine cells release glycine. Double labeling of sections for GlyR alpha1 and GlyR alpha2 subunits showed that they are clustered at different synapses. In sections doubly labeled for GlyR alpha2 and GlyR alpha3, approximately one-third of the puncta were colocalized. The most abundant GlyR subtype in retina contains alpha3 subunits, followed by those containing GlyR alpha2 and GlyR alpha1 subunits.

The type 1 polyaxonal amacrine cells of the rabbit retina: A tracer-coupling study

The type 1 polyaxonal (PA1) cell is a distinct type of axon-bearing amacrine cell whose soma commonly occupies an interstitial position in the inner plexiform layer; the proximal branches of the sparse dendritic tree produce 1–4 axon-like processes, which form an extensive axonal arbor that is concentric with the smaller dendritic tree (Dacey, 1989; Famiglietti, 1992a,b). In this study, intracellular injections of Neurobiotin have revealed the complete dendritic and axonal morphology of the PA1 cells in the rabbit retina, as well as labeling the local array of PA1 cells through homologous tracer coupling. The dendritic-field area of the PA1 cells increased from a minimum of 0.15 mm2 (0.44-mm equivalent diameter) on the visual streak to a maximum of 0.67 mm2 (0.92-mm diameter) in the far periphery; the axonal-field area also showed a 3-fold variation across the retina, ranging from 3.1 mm2 (2.0-mm diameter) to 10.2 mm2 (3.6-mm diameter). The increase in dendritic- and axonal-field size was accompanied by a reduction in cell density, from 60 cells/mm2 in the visual streak to 20 cells/mm2 in the far periphery, so that the PA1 cells showed a 12 times overlap of their dendritic fields across the retina and a 200–300 times overlap of their axonal fields. Consequently, the axonal plexus was much denser than the dendritic plexus, with each square millimeter of retina containing ∼100 mm of dendrites and ∼1000 mm of axonal processes. The strong homologous tracer coupling revealed that ∼45% of the PA1 somata were located in the inner nuclear layer, ∼50% in the inner plexiform layer, and ∼5% in the ganglion cell layer. In addition, the Neurobiotin-injected PA1 cells sometimes showed clear heterologous tracer coupling to a regular array of small ganglion cells, which were present at half the density of the PA1 cells. The PA1 cells were also shown to contain elevated levels of γ-aminobutyric acid (GABA), like other axon-bearing amacrine cells.

AII amacrine cells in the mammalian retina show disabled-1 immunoreactivity

Disabled 1 (Dab1) is an adapter molecule in a signaling pathway, stimulated by Reelin, which controls cell positioning in the developing brain. It has been localized to AII amacrine cells in the mouse and guinea pig retinas. This study was conducted to identify whether Dab1 is commonly localized to AII amacrine cells in the retinas of other mammals. We investigated Dab1-labeled cells in human, rat, rabbit, and cat retinas in detail by immunocytochemistry with antisera against Dab1. Dab1 immunoreactivity was found in certain populations of amacrine cells, with lobular appendages in the outer half of the inner plexiform layer (IPL) and a bushy, smooth dendritic tree in the inner half of the IPL. Double-labeling experiments demonstrated that all Dab1-immunoreactive amacrine cells were immunoreactive to antisera against calretinin or parvalbumin (i.e., other markers for AII amacrine cells in the mammalian retina) and that they made contacts with the axon terminals of the rod bipolar cells in the IPL close to the ganglion cell layer. Furthermore, all Dab1-labeled amacrine cells showed glycine transporter-1 immunoreactivity, indicating that they are glycinergic. The peak density was relatively high in the human and rat retinas, moderate in the cat retina, and low in the rabbit retina. Together, these morphological and histochemical observations clearly indicate that Dab1 is commonly localized to AII amacrine cells and that antiserum against Dab1 is a reliable and specific marker for AII amacrine cells of diverse mammals.

Inhibitory network properties shaping the light evoked responses of cat alpha retinal ganglion cells

Cat retinal ganglion cells of the Y (or alpha) type respond to luminance changes opposite those preferred by their receptive-field centers with a transient hyperpolarization. Here, we examine the spatial organization and synaptic basis of this light response by means of whole-cell current-clamp recordings made in vitro. The hyperpolarization was largest when stimulus spots approximated the size of the receptive-field center, and diminished substantially for larger spots. The hyperpolarization was largely abolished by bath application of strychnine, a blocker of glycinergic inhibition. Picrotoxin, an antagonist of ionotropic GABA receptors, greatly reduced the attenuation of the hyperpolarizing response for large spots. The data are consistent with a model in which (1) the hyperpolarization reflects inhibition by glycinergic amacrine cells of bipolar terminals presynaptic to the alpha cells, and perhaps direct inhibition of the alpha cell as well; and (2) the attenuation of the hyperpolarization by large spots reflects surround inhibition of the glycinergic amacrine by GABAergic amacrine cells. This circuitry may moderate nonlinearities in the alpha-cell light response and could account for some excitatory and inhibitory influences on alpha cells known to arise from outside the classical receptive field.

Morphological analysis of the hyperpolarization-activated cyclic nucleotide-gated cation channel 1 (HCN1) immunoreactive bipolar cells in the rabbit retina

Hyperpolarization-activated cation currents (I(h)) have been identified in neurons in the central nervous system, including the retina. There is growing evidence that these currents, mediated by the hyperpolarization-activated cyclic nucleotide-gated cation channel (HCN), may play important roles in visual processing in the retina. This study was conducted to identify and characterize HCN1-immunoreactive (IR) bipolar cells by immunocytochemistry, quantitative analysis, and electron microscopy. The HCN1-IR bipolar cells were a subtype of OFF-type cone bipolar cells and comprised 10% of the total number of cone bipolar cells. The axons of the HCN1-IR cone bipolar cells ramified narrowly in the border of strata 1 and 2 of the inner plexiform layer (IPL). These cells formed a regular distribution, with a density of 1,825 cells/mm(2) at a position 1 mm ventral to the visual streak, falling to 650 cells/mm(2) in the ventral periphery. Double-labeling experiments demonstrated that their axons stratified narrowly within and slightly proximal to the OFF-starburst amacrine cell processes. In the IPL, they were presynaptic to amacrine cell processes. The most frequent postsynaptic dyads formed of HCN1-IR bipolar cell axon terminals are pairs composed of both amacrine cell processes. These results suggest that these HCN1-IR cone bipolar cells might be the same as the DAPI-Ba1 bipolar population, and might therefore be involved in a direction-selective mechanism, providing inputs to the OFF-starburst amacrine cells and/or the OFF-plexus of the ON-OFF ganglion cells.

Type 1 nitrergic (ND1) cells of the rabbit retina: Comparison with other axon-bearing amacrine cells

NADPH diaphorase (NADPHd) histochemistry labels two types of nitrergic amacrine cells in the rabbit retina. Both the large ND1 cells and the small ND2 cells stratify in the middle of the inner plexiform layer, and their overlapping processes produce a dense plexus, which makes it difficult to trace the morphology of single cells. The complete morphology of the ND1 amacrine cells has been revealed by injecting Neurobiotin into large round somata in the inner nuclear layer, which resulted in the labelling of amacrine cells whose proximal morphology and stratification matched those of the ND1 cells stained by NADPHd histochemistry. The Neurobiotin-injected ND1 cells showed strong homologous tracer coupling to surrounding ND1 cells, and double-labelling experiments confirmed that these coupled cells showed NADPHd reactivity. The ND1 amacrine cells branch in stratum 3 of the inner plexiform layer, where they produce a sparsely branched dendritic tree of 400-600 microm diameter in ventral peripheral retina. In addition, each cell gives rise to several fine beaded processes, which arise either from a side branch of the dendritic tree or from the tapering of a distal dendrite. These axon-like processes branch successively within the vicinity of the dendritic field before extending, with little or no further branching, for 3-5 mm from the soma in ventral peripheral retina. Consequently, these cells may span one-third of the visual field of each eye, and their spatial extent appears to be greater than that of most other types of axon-bearing amacrine cells injected with Neurobiotin in this study. The morphology and tracer-coupling pattern of the ND1 cells are compared with those of confirmed type 1 catecholaminergic cells, a presumptive type 2 catecholaminergic cell, the type 1 polyaxonal cells, the long-range amacrine cells, a novel type of axon-bearing cell that also branches in stratum 3, and a type of displaced amacrine cell that may correspond to the type 2 polyaxonal cell.

Characterization of an amacrine cell type of the mammalian retina immunoreactive for vesicular glutamate transporter 3

Immunocytochemical staining of vertical sections through rat, mouse, and macaque monkey retinae with antibodies against the vesicular glutamate transporter vesicular glutamate transporter 3 (vGluT3) showed a sparse population of amacrine cells. The labeled cells had similar appearances in the three species and probably represent homologous types. They were studied in detail in the rat retina. The thin varicose dendrites of vGluT3 amacrine cells formed a convoluted dendritic tree of approximately 100 microm in diameter that was bistratified in the center of the inner plexiform layer. The dendrites of vGluT3 cells were squeezed between the two strata of cholinergic dendrites. The density of vGluT3 cells was measured in retinal wholemounts and increased from 200/mm2 in peripheral retina to 400/mm2 in central retina, accounting for about 1% of all amacrine cells in the rat retina. The vGluT3 cells had a two- to threefold dendritic overlap, and their cell bodies formed a regular mosaic, suggesting they represent a single type of amacrine cell. The vGluT3 amacrine cells expressed glycine and glycine transporter 1 (GlyT1) but not the vesicular glycine transporter (vesicular inhibitory amino acid transporter). They also expressed glutamate; hence, there is the possibility that, comparable to cholinergic amacrine cells, they are "dual transmitter" amacrine cells. The synaptic input of vGluT3 cells was studied by electron microscopy. They received input from bipolar cells at ribbon synapses and from other amacrine cells at conventional synapses. The types of bipolar cells possibly involved with vGluT3 cells were demonstrated by double labeling sections for vGluT3 and the calcium-binding protein CaB5. The axon terminals of type 3 and 5 bipolar cells costratified with vGluT3 dendrites, and it is possible that vGluT3 cells have ON and OFF light responses.

Inner retinal neurons display differential responses to N-methyl-D-aspartate receptor activation

The N-methyl-D-aspartate (NMDA) responses of neurons from within the inner rabbit retina were mapped using a channel permeable cation, 1-amino-4-guanidobutane (agmatine, AGB). Serial sections were subsequently probed with immunoglobulins targeting AGB, glutamate, gamma-aminobutyric acid (GABA), and glycine to visualize the NMDA responses of neurochemical subpopulations of neurons. Most inner retinal subpopulations of neurons demonstrated an NMDA concentration-dependent increase in activation. This NMDA-induced activation displayed a distinct pattern, with the most sensitive class to least sensitive class ranking being GC > GABA cAC > GABA/Gly cAC > Gly cAC > GABA dAC (GC, ganglion cells; AC, amacrine cells; c, conventional; d, displaced; Gly, glycine). The variable NMDA response may reflect differences in NMDA receptor subunit disposition or differences in receptor density. In addition to the variable NMDA activation pattern, we found that virtually all ganglion cells (87%) showed NMDA-gated AGB entry, compared with only 58% of amacrine cells. We conclude that a large cohort of amacrine cells do not possess functional NMDA receptors. In addition to most ganglion cells being activated by NMDA, a large subpopulation displayed the highest sensitivity to NMDA application. The functional significance of this finding is that the ganglion cell population will be the first neuronal class to be susceptible to glutamate-induced neurotoxicity mediated through the NMDA receptor. The addition of betaxolol significantly reduced NMDA-mediated AGB entry into most neuronal groups (ganglion cells, GABA, and glycine amacrine cells), with the greatest effect being on ganglion cells. Betaxolol had no significant effect on NMDA-gated entry of AGB on the GABA/Gly amacrine cell population.

Identification of cholinoceptive glycinergic neurons in the mammalian retina

The light-evoked release of acetylcholine (ACh) affects the responses of many retinal ganglion cells, in part via nicotinic acetylcholine receptors (nAChRs). nAChRs that contain beta2alpha3 neuronal nicotinic acetylcholine receptors have been identified and localized in the rabbit retina; these nAChRs are recognized by the monoclonal antibody mAb210. We have examined the expression of beta2alpha3 nAChRs by glycinergic amacrine cells in the rabbit retina and have identified different subpopulations of nicotinic cholinoceptive glycinergic cells using double and triple immunohistochemistry with quantitative analysis. Here we demonstrate that about 70% of the cholinoceptive amacrine cells in rabbit retina are glycinergic cells. At least three nonoverlapping subpopulations of mAb210 glycine-immunoreactive cells can be distinguished with antibodies against calretinin, calbindin, and gamma-aminobutyric acid (GABA)(A) receptors. The cholinergic cells in rabbit retina are thought to synapse only on other cholinergic cells and ganglion cells. Thus, the expression of beta2alpha3 nAChRs on diverse populations of glycinergic cells is puzzling. To explore this finding, the subcellular localization of beta2alpha3 was studied at the electron microscopic level. mAb210 immunoreactivity was localized on the dendrites of amacrines and ganglion cells throughout the inner plexiform layer, and much of the labeling was not associated with recognizable synapses. Thus, our findings indicate that ACh in the mammalian retina may modulate glycinergic circuits via extrasynaptic beta2alpha3 nAChRs.

Light and electron microscopic analysis of aquaporin 1-like-immunoreactive amacrine cells in the rat retina

Aquaporin 1 (AQP1; also known as CHIP, a channel-forming integral membrane protein of 28 kDa) is the first protein to be shown to function as a water channel and has been recently shown to be present in the rat retina. We previously showed (Kim et al. [1998] Neurosci Lett 244:52-54) that AQP1-like immunoreactivity is present in a certain population of amacrine cells in the rat retina. This study was conducted to characterize these cells in more detail. With immunocytochemistry using specific antisera against AQP1, whole-mount preparations and 50-microm-thick vibratome sections were examined by light and electron microscopy. These cells were a class of amacrine cells, which had symmetric bistratified dendritic trees ramified in stratum 2 and in the border of strata 3 and 4 of the inner plexiform layer (IPL). Their dendritic field diameters ranged from 90 to 230 microm. Double labeling with antisera against AQP1 and gamma-aminobutyric acid or glycine demonstrated that these AQP1-like-immunoreactive amacrine cells were immunoreactive for glycine. Their most frequent synaptic input was from other amacrine cell processes in both sublaminae a and b of the IPL, followed by a few cone bipolar cells. Their primary targets were other amacrine cells and ganglion cells in both sublaminae a and b of the IPL. In addition, synaptic output onto bipolar cells was rarely observed in sublamina b of the IPL. Thus, the AQP1 antibody labels a class of glycinergic amacrine cells with small to medium-sized dendritic fields in the rat retina.

Distribution and synaptic connectivity of neuropeptide Y-immunoreactive amacrine cells in the rat retina

Neuropeptide Y (NPY) is a potent bioactive peptide that is widely expressed in the nervous system, including the retina. Here we show that specific NPY immunoreactivity was localized to amacrine and displaced amacrine cells in the rat retina. Immunoreactive cells had a regular distribution across the retina and an overall cell density of 280 cells/mm(2) in the inner nuclear layer (INL) and 90 cells/mm(2) in the ganglion cell layer (GCL). In the INL, most immunoreactive cells were characterized by small cell bodies and fine processes that appeared to ramify primarily in stratum 1 of the inner plexiform layer (IPL). A few cells in the INL also ramified in stratum 3 of the IPL. In the GCL, small to medium immunoreactive cells appeared to ramify primarily in stratum 5 of the IPL. A few immunoreactive processes, originating from somata in the INL and processes in the IPL, ramified in the OPL. NPY-immunoreactive cells contained GABA immunoreactivity, and some amacrine cells also contained tyrosine hydroxylase immunoreactivity. NPY-immunostained processes were most frequently presynaptic to nonimmunostained amacrine and ganglion cell processes and postsynaptic to nonimmunostained amacrine cell processes and cone bipolar cell axonal terminals. These findings indicate that NPY immunoreactivity is present in two populations of amacrine cells, one located in the INL and the other in the GCL, and that these cells mainly form synaptic contacts with other amacrine cells. These observations suggest that NPY-immunoreactive cells participate in multiple circuits mediating visual information processing in the inner retina.

Synaptic input to an ON parasol ganglion cell in the macaque retina: a serial section analysis

A labeled ON parasol ganglion cell from a macaque retina was analyzed in serial, ultrathin sections. It received 13% of its input from diffuse bipolar cells. These directed a large proportion of their output to amacrine cells but received a relatively small proportion of their amacrine cell input via feedback synapses. In these respects, they were similar to the DB3 bipolar cells that make synapses onto OFF parasol cells. Bipolar cell axons that contacted the ON parasol cell in stratum 4 of the inner plexiform layer always made synapses onto the dendrite, and therefore, the number of bipolar cell synapses onto these ganglion cells could be estimated reliably by light microscopy in the future. Amacrine cells provided the majority of inputs to the ON parasol cell. Only a few of the presynaptic amacrine cell processes received inputs from the same bipolar cells as the parasol cells, and most of the presynaptic amacrine cell processes did not receive any inputs at all within the series. These findings suggest that most of the inhibitory input to the ON parasol cell originates from other areas of the retina. Amacrine cells presynaptic to the parasol ganglion cell interacted very infrequently with other neurons in the circuit, and therefore, they would be expected to act independently, for the most part.

Amino acids and their transporters in the retina

This review provides an overview of the distributions, properties and roles of amino acid transport systems in normal and pathological retinal tissues and discusses the roles of specific identified transporters in the mammalian retina. The retina is used in this context as a vehicle for describing neuronal and glial properties, which are in some, but not all cases comparable to those found elsewhere an the brain. Where significant departures are noted, these are discussed in the context of functional specialisations of the retina and its relationship to adjacent supporting tissues such as the retinal pigment epithelium. Specific examples are given where immunocytochemical labelling for amino acid transporters may yield inaccurate results, possibly because of activity-dependent conformation changes of epitopes in these proteins which render the epitopes more or less accessible to antibodies.

Distribution of GABA and glycine receptors on bipolar and ganglion cells in the mammalian retina

The amino acids GABA and glycine mediate synaptic transmission via specific neurotransmitter receptors. Molecular cloning studies have shown that there is a great diversity of GABA and glycine receptors. In the present article, the distribution of GABA and glycine receptors on identified bipolar and ganglion cell types in the mammalian retina is reviewed. Immunofluorescence obtained with antibodies against GABA and glycine receptors is punctate. Electron microscopy shows that the puncta represent a cluster of receptors at synaptic sites. Bipolar cell types were identified with immunohistochemical markers. Double immunofluorescence with subunit-specific antibodies was used to analyze the distribution of receptor clusters on bipolar axon terminals. The OFF cone bipolar cells seem to be dominated by glycinergic input, whereas the ON cone bipolar and rod bipolar cells are dominated by GABAergic input. Ganglion cells were intracellularly injected with Neurobiotin, visualized with Streptavidin coupled to FITC, and subsequently stained with subunit specific antibodies. The distribution and density of receptor clusters containing the alpha1 subunit of the GABA(A) receptor and the alpha1 subunit of the glycine receptor, respectively, were analyzed on midget and parasol cells in the marmoset (a New World monkey). Both GABA(A) and glycine receptors are distributed uniformly along the dendrites of ON and OFF types of parasol and midget ganglion cells, indicating that functional differences between these subtypes of ganglion cells are not determined by GABA or glycinergic input.

Endogenous dopaminergic regulation of horizontal cell coupling in the mammalian retina

Horizontal cells in an isolated wholemount preparation of the mouse retina were injected with Lucifer yellow and neurobiotin to characterize both the pattern of gap junctional connectivity and its regulation by dopamine. The injected horizontal cells had a uniform morphology of a round cell body, a compact dendritic tree, and an axon, which could sometimes be traced to an expansive terminal system. The dendro-dendritic gap junctions between neighboring cells mediated both weak Lucifer yellow dye coupling and strong neurobiotin tracer coupling. The extent of the tracer coupling was decreased by either exogenous dopamine (100 microM) or cyclic adenosine monophosphate (cAMP) analogs and was significantly increased by the D1 antagonist SCH 23390 (10 microM). These results provide the first evidence in the mammalian retina that the gap junctions between horizontal cells are endogenously regulated by dopamine, which acts through D1 receptors to increase the intracellular cAMP. It has been proposed that the gap junctional coupling between horizontal cells is mediated by connexin 32 (Cx32), but the pattern and dopaminergic regulation of horizontal cell coupling were unaffected in Cx32-knockout mice, ruling out the possible involvement of Cx32. Every tracer-coupled horizontal cell showed calbindin immunoreactivity, and vice versa, providing strong evidence that the horizontal cells in the mouse retina comprise a single cell type. Like the axonless horizontal cells in other mammalian retinas, the axon-bearing horizontal cells in the mouse retina are coupled by gap junctions that are permeable to Lucifer yellow and dopamine sensitive, suggesting that the mouse horizontal cells have hybrid properties to compensate for the absence of axonless horizontal cells.

Neuronal coupling in the central nervous system: lessons from the retina

The retina is a model system for studying gap junctional intercellular communication in the CNS. The cellular coupling can be graphically visualized in retinal whole mounts by injecting small cationic tracers into microscopically identified neurons; the pattern of tracer coupling shown by each type of retina neuron is highly stereotyped, with many types of amacrine cells and ganglion cells showing complex patterns of both homologous and heterologous coupling. Parallel physiological studies have demonstrated that the gap junctions can be modulated dynamically by neurotransmitters and by the level of ambient illumination. Taken together, the numerous structural and functional studies on gap junctions in the retina provide powerful support for the concept that electrical synapses are complex components of neuronal circuits, having many of the attributes normally ascribed to chemical synapses.

Neurotransmitter coupling through gap junctions in the retina

Although all bipolar cells in the retina probably use the excitatory transmitter glutamate, approximately half of the cone bipolar cells also contain elevated levels of the inhibitory transmitter glycine. Some types of cone bipolar cells make heterologous gap junctions with rod amacrine cells, which contain elevated levels of glycine, leading to the hypothesis that the bipolar cells obtain their glycine from amacrine cells. Experimental support for this hypothesis is now provided by three independent lines of evidence. First, the glycine transporter GLYT1 is expressed by the glycine-containing amacrine cells but not by the glycine-containing bipolar cells, suggesting that only the amacrine cells are functionally glycinergic. Second, the gap-junction blocker carbenoxolone greatly reduces exogenous 3H-glycine accumulation into the bipolar cells but not the amacrine cells. Moreover, when the endogenous glycine stores in both cell classes are depleted by incubating the retina with a glycine-uptake inhibitor, carbenoxolone blocks the subsequent glycine replenishment of the bipolar cells but not the amacrine cells. Third, intracellular injection of rod amacrine cells with the gap-junction permeant tracer Neurobiotin secondarily labels a heterogenous population of cone bipolar cells, all of which show glycine immunoreactivity. Taken together, these findings indicate that the elevated glycine in cone bipolar cells is not derived by high-affinity uptake or de novo synthesis but is obtained by neurotransmitter coupling through gap junctions with glycinergic amacrine cells. Thus transmitter content may be an unreliable indicator of transmitter function for neurons that make heterologous gap junctions.

Glycinergic amacrine cells of the rat retina

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

Extreme diversity among amacrine cells: implications for function

We report a quantitative survey of the population of amacrine cells present in the retina of the rabbit. The cells' dendritic shape and level of stratification were visualized by a photochemical method in which a fluorescent product was created within an individual cell by focal irradiation of that cell's nucleus. A systematically random sample of 261 amacrine cells was examined. Four previously known amacrine cells were revealed at their correct frequencies. Our central finding is that the heterogeneous collection of other amacrine cells is broadly distributed among at least 22 types: only one type of amacrine cell makes up more than 5% of the total amacrine cell population. With these results, the program of identification and classification of retinal neurons begun by Cajal is nearing completion. The complexity encountered has implications both for the retina and for the many regions of the central nervous system where less is known.