Ulrike Grünert

Retinal ganglion cell density and cortical magnification factor in the primate.

The question of whether the large area occupied by the primate fovea in the visual cortex (V1) is the result of a selective amplification of the central visual field, or whether it merely reflects the ganglion cell density of the retina, has been a subject of debate for many years. Measurements of the ganglion cell densities are made difficult by lateral displacements of cells around the fovea and the occurrence of amacrine cells in the ganglion cell layer. We have now identified and counted these amacrine cells by GABA immunocytochemistry and by retrograde degeneration of ganglion cells. By reconstructing the fovea from vertical and horizontal serial sections, we were able to measure the densities of cones, cone pedicles and ganglion cells within the same retina. We found 3-4 ganglion cells for every foveal cone. This ratio decreased to one ganglion cell per cone at an eccentricity of 15-20 deg (3-4 mm) and in peripheral retina there are more cones than ganglion cells. The ganglion cell density changes by a factor of 1000-4000 between peripheral and central retina. A comparable gradient has been reported for the representation of the peripheral and central visual field in V1. We suggest that ganglion cell density can fully account for the cortical magnification factor and there is no need to postulate a selective amplification of the foveal representation.

The rod bipolar cell of the mammalian retina

Three approaches to study the function of mammalian rod bipolar cells are described. Extracellular recordings from the intact cat eye under light- and dark-adapted conditions showed that in dark-adapted retina all light responses can be blocked by 2-amino-4-phosphonobutyrate (APB). Immunocytochemical staining with an antibody against protein kinase C (PKC) labeled rod bipolar cells in all mammalian retinae tested. When rat retinae were dissociated, PKC immunoreactivity was also found in isolated bipolar cells and could be used for their identification as rod bipolars. Patch-clamp recordings were performed from such dissociated rod bipolar cells and their responses to APB were measured. APB closed a nonselective cation channel in the cell membrane. The actions of GABA and glycine were also tested and both opened chloride channels in dissociated rod bipolar cells. These results suggest that rod bipolar cells are depolarized by a light stimulus and that GABA as well as glycine modulate their light responses.

Parasol (Pα) ganglion-cells of the primate fovea: Immunocytochemical staining with antibodies against GABAA-receptors

Retinae of macaque monkeys were immuno-stained with antibodies against GABAA-receptors. In peripheral retina most ganglion cells were immunoreactive. In central retina, around the fovea, staining in the ganglion cell layer was selective and only 5-8% of all ganglion cells were labelled: these had the largest cell bodies and their dendrites occupied a broad stratum in the middle of the inner plexiform layer. From comparison with Golgi-stained ganglion cells it is concluded that the entire population of parasol (P alpha)-cells at the fovea was labelled. The mosaic and sampling properties of parasol cells were determined by combining dendritic field measurements of Golgi-stained cells with their density when immuno-stained. There is convergence of 30-50 cones onto each foveal parasol ganglion cell. The dendritic fields of both ON- and OFF-parasol cells provide complete retinal coverage. The Nyquist limits of their mosaics are 4 min of arc.

Localization of GABAA receptors in the rabbit retina

The distribution of gamma-aminobutyric acidA (GABAA) receptors in the rabbit retina is investigated and compared with the distribution of GABAergic neurons using immunocytochemical methods. Antibodies against the α1, β2/3, and γ2 subunits of the GABAA receptor label subpopulations of bipolar, amacrine and ganglion cells. Double labeling experiments show that the γ2 subunit is colocalized with the α1 and the β2/3 subunits in bipolar, amacrine and ganglion cells. Electron microscopy reveals that in the outer plexiform layer, GABAA receptor immunoreactivity is present on dendrites of cone bipolar cells adjacent to the cone pedicles. Bipolar cell dendrites are also receptor-positive at synapses from interplexiform cells. Some receptor immunoreactivity is found intracellularly in processes of horizontal cells. In the inner plexiform layer, GABAA receptor immunoreactivity is present on both rod bipolar and cone bipolar axon terminals at putative GABAergic input sites. Amacrine and ganglion cell processes in sublamina a and b are also labeled.

Differential expression of glycine receptor subunits in the retina of the rat: a study using immunohistochemistry and in situ hybridization

Immunohistochemistry and in situ hybridization were used to study the distribution of glycine receptor (GlyR) subunits and the GlyR-associated protein gephyrin in the rat retina. Monoclonal antibodies against the alpha and beta subunits of the GlyR and gephyrin showed a strong punctate labeling pattern in the inner plexiform layer. Glycine receptor mRNAs were found in the inner nuclear layer and the ganglion cell layer. The alpha 1 subunit mRNA is predominantly present in the outer half of the INL and on some but not all ganglion cells. GlyR alpha 2 subunit mRNA is predominantly present in the inner half of the INL and on nearly all cells in the ganglion cell layer. GlyR alpha 3-, GlyR beta-, and gephyrin-mRNAs are present in the entire INL and in cells in the ganglion cell layer. The differential expression of glycine receptor subunits indicates a functional diversity of glycine receptors in the retina.

The Synaptic Complex of Cones in the Fovea and in the Periphery of the Macaque Monkey Retina

Parallel pathways for visual information processing start at the first synapse of the retina, at the cone pedicle. At least eight different types of bipolar cells receive direct synaptic input from an individual cone. The present study explores whether enough synaptic sites are available at the cone pedicle to supply all these bipolar cells. Monkey retinae were optimally fixed for electron microscopy. Serial horizontal sections were cut through the cone pedicle layer in a piece close to the fovea (eccentricity: 0.75 mm) and in a peripheral piece (eccentricity: 5-6 mm). The ribbon synapses (triads) at the cone pedicle base were analysed. The average number of synaptic ribbons per cone pedicle increased from 21.4 +/- 1.6 (n = 26) in central retina to 41.8 +/- 3 (n = 14) in peripheral retina. Five central and five peripheral pedicles were reconstructed and the invaginating bipolar cell dendrites forming the central elements of the triads were characterized. Close to the fovea an average of 18 invaginating bipolar cell dendrites was found, in peripheral retina the average was 90. Pedicles of foveal cones have one invaginating central process per ribbon, pedicles of peripheral cones have two. It is possible that midget bipolar cell dendrites occupy the majority of triads in the fovea, while in peripheral retina both midget and diffuse bipolar cell dendrites share the triads.

The cGMP-gated channel of rod outer segments is not localized in bipolar cells of the mammalian retina

It has recently been suggested (Nature, 346 (1990) 269-271) that ON-bipolar cells express the same biochemical cascade and guanosine 3,5-cyclic monophosphate (cGMP)-gated cation channel as rod outer segments. An antibody directed against the cGMP-gated channel of bovine rod outer segments was applied to cryostat sections of rat and cat retinae. No immunocytochemical labelling was found in bipolar cells. Therefore, if those cells express a cGMP-gated channel, it must be immunologically different to the 63 kDa protein constituting the cGMP-gated channel of the outer segment.

Immunohistochemical localization of GABAA receptors in the scotopic pathway of the cat retina

The distribution of GABAA receptors in the inner plexiform layer of cat retina was studied using monoclonal antibodies against the β2/β3 subunits. A dense band of receptor labeling was found in the inner region of the inner plexiform layer where the rod bipolar axons terminate. Three forms of evidence indicate that the GABAA receptor labeling is on the indoleamine-accumulating, GABAergic amacrine cell that is synaptically interconnected with the rod bipolar cell terminal. (1) Electron microscopy showed that the anti-GABAA receptor antibody (62-3G1) labeled profiles that were postsynaptic to rod bipolar axons and made reciprocal synapses. (2) Indoleamine uptake (and the subsequent autofluorescence) combined with GABAA receptor immunohistochemistry showed co-localization of the two markers in half of the receptor-positive amacrine cells. (3) Double labeling demonstrated that half of the receptor-positive somata also contained GABA. These results indicate that a GABAergic amacrine cell interconnected with the rod bipolar cell, most likely the so-called A17 amacrine cell, itself bears GABAA receptors.

Cholinergic amacrine cells of the rat retina express the δ-subunit of the GABAA-receptor

Abstract Antibodies directed against the δ-subunit of the GABA A -receptor were applied to cryostat sections of rat retinae. Two narrow bands of the inner plexiform layer were strongly immunoreactive. Some cell bodies in both the amacrine- and ganglion-cell layer were weakly immunoreactive. The position of the labelled bands and the distribution of the cell bodies was strongly reminiscent of the cholinergic amacrine cells. In order to show directly that cholinergic amacrine cells express the δ-subunit of the GABA A -receptor, double immunofluorescence with an antibody against choline acetyltransferase (ChAT) and with antibodies against the δ-subunit was performed on the same cryostat sections. This showed the labelled cells to be cholinergic amacrine cells.

Density of Bipolar Cells in the Macaque Monkey Retina

The inner nuclear layer (INL) of the mammalian retina is composed of the cell bodies of bipolar, horizontal, amacrine, interplexiform and Muller cells. Of these, bipolar cells are responsible for the vertical flow of information from the photoreceptor matrix to the inner plexiform layer and hence to the ganglion cells. We have begun an analysis of the neuronal composition of the INL of the macaque monkey with emphasis on the different subpopulations of bipolar cells which are present there. In this chapter we describe the density and distribution of rod bipolar cells within the central 12 mm and provide an estimate of the total number of bipolar cells within one millimeter of the fovea. The results can be used to estimate limits to the spatial resolving power of rod and cone mediated vision in the central retina, and may thus have relevance for psychophysical estimates of these values.