Bistratified ganglion cells of rabbit retina: neural architecture for contrast-independent visual responses

Morphological identification and systematic classification of mammalian retinal ganglion cells. I. Rabbit retinal ganglion cells

Retinal ganglion cells (RGCs) convey visual signals to 50 regions of the brain. For reasons of interest and convenience, they constitute an excellent system for the study of brain structure and function. There is general agreement that, absent a complete "parts list," understanding how the nervous system processes information will remain an elusive goal. Recent studies indicate that there are 30-50 types of ganglion cell in mouse retina, whereas only a few years ago it was still written that mice and the more visually oriented lagomorphs had less than 20 types of RGC. More than 30 years ago, I estimated that rabbits have about 40 types of RGC. The present study indicates that this number is much too low. I have employed the old but powerful method of Golgi-impregnation to rabbit retina, studying the range of component neurons in this already well-studied retinal system. Close quantitative and qualitative analyses of 1,142 RGCs in 26 retinas take into account cell body and dendritic field size, level(s) of dendritic stratification in the retina's inner plexiform layer, and details of dendritic branching. Ninety-one morphologies are recognized. Of these, at least 32 can be correlated with physiologically studied RGCs, dye-injected for morphological analysis. It is unlikely that rabbits have 91 types of RGC, but is argued here that this number lies between 60 and 70. The present study provides a "yardstick" for measuring the output of future molecular studies that may be more definitive in fixing the number of RGC types in rabbit retina.

Synaptic inputs from identified bipolar and amacrine cells to a sparsely branched ganglion cell in rabbit retina

There are more than 30 distinct types of mammalian retinal ganglion cells, each sensitive to different features of the visual environment. In rabbit retina, they can be grouped into four classes according to their morphology and stratification of their dendrites in the inner plexiform layer (IPL). The goal of this study was to describe the synaptic inputs to one type of Class IV ganglion cell, the third member of the sparsely branched Class IV cells (SB3). One cell of this type was partially reconstructed in a retinal connectome developed using automated transmission electron microscopy (ATEM). It had slender, relatively straight dendrites that ramify in the sublamina a of the IPL. The dendrites of the SB3 cell were always postsynaptic in the IPL, supporting its identity as a ganglion cell. It received 29% of its input from bipolar cells, a value in the middle of the range for rabbit retinal ganglion cells studied previously. The SB3 cell typically received only one synapse per bipolar cell from multiple types of presumed OFF bipolar cells; reciprocal synapses from amacrine cells at the dyad synapses were infrequent. In a few instances, the bipolar cells presynaptic to the SB3 ganglion cell also provided input to an amacrine cell presynaptic to the ganglion cell. There was apparently no crossover inhibition from narrow-field ON amacrine cells. Most of the amacrine cell inputs were from axons and dendrites of GABAergic amacrine cells, likely providing inhibitory input from outside the classical receptive field.

Neural architecture of the "transient" ON directionally selective (class IIb1) ganglion cells in rabbit retina, partly co-stratified with starburst amacrine cells

Recent physiological studies coupled with intracellular staining have subdivided ON directionally selective (DS) ganglion cells of rabbit retina into two types. One exhibits more "transient" and more "brisk" responses (ON DS-t), and the other has more "sustained' and more "sluggish" responses (ON DS-s), although both represent the same three preferred directions and show preference for low stimulus velocity, as reported in previous studies of ON DS ganglion cells in rabbit retina. ON DS-s cells have the morphology of ganglion cells previously shown to project to the medial terminal nucleus (MTN) of the accessory optic system, and the MTN-projecting, class IVus1 cells have been well-characterized previously in terms of their dendritic morphology, branching pattern, and stratification. ON DS-t ganglion cells have a distinctly different morphology and exhibit heterotypic coupling to amacrine cells, including axon-bearing amacrine cells, with accompanying synchronous firing, while ON DS-s cells are not coupled. The present study shows that ON DS-t cells are morphologically identical to the previously well-characterized, "orphan" class IIb1 ganglion cell, previously regarded as a member of the "brisk-concentric" category of ganglion cells. Its branching pattern, quantitatively analyzed, is similar to that of the morphological counterparts of X and Y cells, and very different from that of the ON DS-s ganglion cell. Close analysis of the dendritic stratification of class IIb1 ganglion cells together with fiducial cells indicates that they differ from that of the ON DS-s cells. In agreement with one of the three previous studies, class IIb1/ON DS-t cells, unlike class IVus1/ON DS-s ganglion cells, in the main do not co-stratify with starburst amacrine cells. As the present study shows, however, portions of their dendrites do deviate from the main substratum, coming within range of starburst boutons. Parsimony favors DS input from starburst amacrine cells both to ON DS-s and to ON DS-t ganglion cells, given the similarity of their DS responses, but further studies will be required to substantiate the origin of the DS responses of ON DS-t cells. Previously reported OFF DS responses in ON DS-t cells, unmasked by pharmacological agents, and mediated by gap junctions with amacrine cells, suggests an unusual trans-sublaminar organization of directional selectivity in the inner plexiform layer, connecting sublamina a and sublamina b.

Short-wavelength cone-opponent retinal ganglion cells in mammals

In all of the mammalian species studied to date, the short-wavelength-sensitive (S) cones and the S-cone bipolar cells that receive their input are very similar, but the retinal ganglion cells that receive synapses from the S-cone bipolar cells appear to be quite different. Here, we review the literature on mammalian retinal ganglion cells that respond selectively to stimulation of S-cones and respond with opposite polarity to longer wavelength stimuli. There are at least three basic mechanisms to generate these color-opponent responses, including: (1) opponency is generated in the outer plexiform layer by horizontal cells and is conveyed to the ganglion cells via S-cone bipolar cells, (2) inputs from bipolar cells with different cone inputs and opposite response polarity converge directly on the ganglion cells, and (3) inputs from S-cone bipolar cells are inverted by S-cone amacrine cells. These are not mutually exclusive; some mammalian ganglion cells that respond selectively to S-cone stimulation seem to utilize at least two of them. Based on these findings, we suggest that the small bistratified ganglion cells described in primates are not the ancestral type, as proposed previously. Instead, the known types of ganglion cells in this pathway evolved from monostratified ancestral types and became bistratified in some mammalian lineages.

Characterization of multiple bistratified retinal ganglion cells in a purkinje cell protein 2-Cre transgenic mouse line

Retinal ganglion cells are categorized into multiple classes, including multiple types of bistratified ganglion cells (BGCs). The recent use of transgenic mouse lines with specific type(s) of ganglion cells that are labeled by fluorescent markers has facilitated the morphological and physiological studies of BGCs, particularly the directional-selective BGCs. The most important benefit from using transgenic animals is the capability to perform in vivo gene manipulation. In particular, the Cre/LoxP recombination system has become a powerful tool, allowing gene deletion, overexpression, and ectopic expression in a cell type-specific and temporally controlled fashion. The key to this tool is the availability of Cre mouse lines with cell or tissue type-specific expression of Cre recombinase. In this study we characterized the Cre-positive retinal ganglion cells in a PCP2 (Purkinje cell protein 2)-cre mouse line. We found that all of the Cre-positive retinal ganglion cells were BGCs. Based on morphological criteria, we determined that they can be grouped into five types. The On- and Off-dendrites of three of these types stratified outside of the cholinergic bands and differed from directional selective ganglion cells (DSGCs) morphologically. These cells were negative for Brn-3b and positive for both calretinin and CART retina markers. The remaining two types were identified as putative On-Off and On-DSGCs. This Cre mouse line could be useful for further studies of the molecular and functional properties of BGCs in mice.

Properties of the ON bistratified ganglion cell in the rabbit retina

The identity of the types of different neurons in mammalian retinae is now close to being completely known for a few mammalian species; comparison reveals strong homologies for many neurons across the order. Still, there remain some cell types rarely encountered and inadequately described, despite not being rare in relative frequency. Here we describe in detail an additional ganglion cell type in rabbit that is bistratified with dendrites in both sublaminae, yet spikes only at light onset and has no response bias to the direction of moving bars. This ON bistratified ganglion cell type is most easily distinguished by the unusual behavior of its dendritic arbors. While dendrites that arborize in sublamina b terminate at that level, those that ascend to arborize in sublamina a do not normally terminate there. Instead, when they reach the approximate radius of the dendrites in sublamina b, they dive sharply back down to ramify in sublamina b. Here they continue to course even further away from the soma at the same level as the branches wholly contained in sublamina b, thereby forming an annulus of secondary ON dendrites in sublamina b. This pattern of branching creates a bistratified dendritic field of approximately equal area in the two sublaminae initially, to which is then added an external annulus of dendrites only in sublamina b whose origin is entirely from processes descending from sublamina a. It is coupled to a population of wide-field amacrine cells upon which the dendrites of the ganglion cell often terminate.

Differential retinal origins of separate anatomical channels for pattern and motion vision in rabbit

The most conspicuous feature of the rabbit retina is the visual streak that extends along the horizontal azimuth from the nasal margin to the temporal limit of the retina. We believe the streak processes movement vision and that the temporal region (area centralis) is responsible for pattern perception. Both anatomical and behavioural experiments were used to test this hypothesis. Behavioural measures of pattern vision in normal and chiasma-sectioned rabbits revealed both to have the same visual acuity. Using OKN as a measure of movement vision, normal rabbits showed both a directional and velocity-tuned response. The chiasma-sectioned rabbits, with only uncrossed fibre projections remaining, showed a total loss of movement detection. The injection of HRP into the vitreal chamber of one eye in normal rabbits revealed extensive uptake throughout the contralateral thalamus. In the ipsilateral thalamus, there was uptake solely from the ipsilateral retinal projection to a restricted wafer of the lateral geniculate nucleus (LGN). The chiasma cut rabbits showed a very different distribution of HRP in the thalamus. The uptake was restricted to a thin wafer of the LGN, with no contralateral uptake. Thus, the thalamic projections from the retinal area centralis were strictly segregated from the thalamic target areas for the visual streak without any overlap. These findings provide strong evidence for separate retinal origins with anatomically separate pathways for pattern and movement vision in the rabbit.

A novel type of complex ganglion cell in rabbit retina

The 15-20 physiological types of retinal ganglion cells (RGCs) can be grouped according to whether they fire to increased illumination in the receptive-field center (ON cells), decreased illumination (OFF cells), or both (ON-OFF cells). The diversity of RGCs has been best described in the rabbit retina, which has three types of ON-OFF RGCs with complex receptive-field properties: the ON-OFF direction-selective ganglion cells (DSGCs), the local edge detectors, and the uniformity detectors. Here we describe a novel type of bistratified ON-OFF RGC that has not been described in either physiological or morphological studies of rabbit RGCs. These cells stratify in the ON and OFF sublaminae of the inner plexiform layer, branching at about 30% and 60% depth, between the ON and OFF arbors of the bistratified DSGCs. Similar to the ON-OFF DSGCs, these cells respond with transient firing to both bright and dark spots flashed in the receptive field but, unlike the DSGCs, they show no directional preference for moving stimuli. We have termed these cells "transient ON-OFF" RGCs. Area-response measurements show that both the ON and the OFF spike responses have an antagonistic receptive-field organization, but with different spatial extents. Voltage-clamp recordings reveal transient excitatory inputs at light ON and light OFF; this excitation is strongly suppressed by surround stimulation, which also elicits direct inhibitory inputs to the cells at light ON and light OFF. Thus the receptive-field organization is mediated both within the presynaptic circuitry and by direct feed-forward inhibition.

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.

Morphologies of mouse retinal ganglion cells expressing transcription factors Brn3a, Brn3b, and Brn3c: analysis of wild type and mutant cells using genetically-directed sparse labeling

The mammalian retina contains more than 50 distinct neuronal types, which are broadly classified into several major classes: photoreceptor, bipolar, horizontal, amacrine, and ganglion cells. Although some of the developmental mechanisms involved in the differentiation of retinal ganglion cells (RGCs) are beginning to be understood, there is little information regarding the genetic and molecular determinants of the distinct morphologies of the 15-20 mammalian RGC cell types. Previous work has shown that the transcription factor Brn3b/Pou4f2 plays a major role in the development and survival of many RGCs. The roles of the closely related family members, Brn3a/Pou4f1 and Brn3c/Pou4f3 in RGC development are less clear. Using a genetically-directed method for sparse cell labeling and sparse conditional gene ablation in mice, we describe here the sets of RGC types in which each of the three Brn3/Pou4f transcription factors are expressed and the consequences of ablating these factors on the development of RGC morphologies.

Uniformity detector retinal ganglion cells fire complex spikes and receive only light-evoked inhibition

Retinal ganglion cells convey information by increasing their firing in response to an optimal visual stimulus or "trigger feature." However, one class of ganglion cell responds to changes in the visual scene by decreasing its firing. These cells, termed uniformity detectors in the rabbit retina, are encountered only rarely and the synaptic mechanisms underlying their unusual responses have not been investigated. In this study, patch-clamp recordings of uniformity detectors show that the action potentials underlying the maintained firing arise within "complex spikes." Both ON and OFF visual stimuli elicit only inhibitory synaptic input, the immediate effect of which is to suppress the maintained firing. However, this inhibition also alters the properties of the "renascent" spiking by increasing the amplitude of the spikes within each burst, suggesting that the effect may increase the efficacy of spike propagation and transmission.