A unique morphological subtype of horizontal cell in the rabbit retina with orientation-sensitive response properties

Horizontal cells of the rabbit retina are non-selectively connected to the cones

Mammalian horizontal cells have generally been assumed to be spectrally non-selective in their cone contacts until recently, when specific contacts have been found for some species. The rabbit retina is frequently studied as a representative of dichromatic mammalian retinae. These are the reasons for elucidating the connections of the two types of horizontal cells (A-HCs and B-HCs) with the green-sensitive and blue-sensitive cones of the rabbit retina. Individual A-HCs and B-HCs were revealed by Lucifer Yellow injections, the total cone population overlying them was stained using peanut agglutinin, and the blue cones among these were identified by the antiserum JH 455 against blue cone opsin. Both A-HCs and B-HCs indiscriminately contact the two cone types available. This holds for the green cone-dominated dorsal retina and the blue cone-dominated ventral retina. No evidence was found for a third, potentially blue cone-selective, horizontal cell type [postulated by Famiglietti, E. V. (1990) Brain Res., 535, 174-179].

The horizontal cells of artiodactyl retinae: a comparison with Cajal's descriptions

The morphology of horizontal cells in ox, sheep, and pig retinae as observed after Lucifer Yellow injections are described and compared with the descriptions of Golgi-stained cells by Ramón y Cajal (1893). Horizontal cells in the retinae of less domesticated species, wild pig, fallow and sika deer, mouflon, and aurochs were also examined. All these retinae have two types of horizontal cell; their morphologies are in common, although with some familial differences. Their basic appearance is as Cajal described; except in one important respect, a single axon-like process could not be identified on the external horizontal cells. It is concluded that external horizontal cells of artiodactyls correspond to the axonless (A-type) cells of other mammals. Cajal's internal horizontal cells have a single axon which contacts rods. This type corresponds to the B-type cells of other mammalian retinae. Artiodactyl A- and B-type horizontal cells differ from those of many other mammals in that the B-type dendritic tree is robust and the A-type dendritic tree is delicate. Historically, this morphological difference between orders of mammals has led to some confusion. The comparisons presented here suggest that the morphological types of primate horizontal cells can be integrated into a general mammalian classification.

A comparison of receptive field and tracer coupling size of horizontal cells in the rabbit retina

The large receptive fields of retinal horizontal cells are thought to reflect extensive electrical coupling via gap junctions. It was shown recently that the biotinylated tracers, biocytin and Neurobiotin, provide remarkable images of coupling between many types of retinal neuron, including horizontal cells. Further, these demonstrations of tracer coupling between horizontal cells rivaled the size of their receptive fields, suggesting that the pattern of tracer coupling may provide some index of the extent of electrical coupling. We studied this question by comparing the receptive field and tracer coupling size of dark-adapted horizontal cells recorded in the superfused, isolated retina-eyecup of the rabbit. Both the edge-to-edge receptive field and space constants (lambda) were computed for each cell using a long, narrow slit of light displaced across the retinal surface. Cells were subsequently labeled by iontophoretic injection of Neurobiotin. The axonless A-type horizontal cells showed extensive, homologous tracer coupling in groups greater than 1000 covering distances averaging about 2 mm. The axon-bearing B-type horizontal cells were less extensively tracer coupled, showing homologous coupling of the somatic endings in groups of about 100 cells spanning approximately 400 microns and a separate homologous coupling of the axon terminal endings covering only about 275 microns. Moreover, we observed a remarkable, linear relationship between the size of the receptive fields of each of the three horizontal cell endings and the magnitude of their tracer coupling. Our findings suggest that the extent of tracer coupling provides a strong, linear index of the magnitude of electrical current flow, as derived from receptive-field measures, across groups of coupled horizontal cells. These data thus provide the first direct evidence that the receptive-field size of horizontal cells is related to the extent of their coupling via gap junctions.

Development of A-type (axonless) horizontal cells in the rabbit retina

The development of A-type horizontal cells (HC) was studied in the rabbit retina between embryonic day (E)24 and adulthood [the day of birth was called postnatal day (P)1 and corresponds to E31-32]. The cells were visualized by several methods 1) by immunolabeling with antibodies to neurofilament 70,000 (NF-70kD), 2) by immunolabeling with antibodies to a calcium binding protein (CaBP-28kD), 3) by two different methods of silver impregnation, and 4) by histochemical demonstration of NADH-diaphorase activity. Most methods labeled A-type HC only in the dorsal retina; thus, our study is restricted to HC of this region. HC densities were determined at each developmental stage. The cells were drawn at scale, and size, quotient of symmetry, and topographical orientation of dendritic trees were studied by image analysis. The growth of HC dendritic fields was correlated with data on the postnatal local retinal expansion, which is known to be driven by the intraocular pressure (after cessation of retinal cell proliferation at P9). This expansion was evaluated in an earlier paper (Reichenbach et al. [1993] Vis. Neurosci. 10:479-498) by using local subpopulations of Müller cells as "markers" of distinct topographic regions of the retinae. After E24, when the final number of HC is established, we can discriminate three distinct developmental stages of A-type HC. During the first stage, between E24 and E27, the young cells are often vertically oriented and may extend their first short dendrites within (the primordia of) both plexiform layers. The irregular HC mosaic at E24 shows a significant difference to all other stages. The second stage begins after birth when the dendritic trees of the cells are already restricted to the outer plexiform layer. Between P3 and P9, their dendritic trees enlarge more than the surrounding retinal tissue expands, and the coverage factor almost doubles from 2.5 to 4.4. The third stage occurs after P9 when the growth rate of dendritic tree areas corresponds to that of the local retinal tissue expansion caused by "passive stretching" of the postmitotic tissue, and the coverage factor remains constant. This is compatible with the view that mature synaptic connections of A-type HC are mostly established after the first week of life and are then maintained.

Expression of neurofilament proteins by horizontal cells in the rabbit retina varies with retinal location

Classical neurofibrillar staining methods and immunocytochemistry with antibodies to the light, medium and heavy chain subunits of the neurofilament triplet have been used for in situ and in vitro investigation of the organization of neurofilaments in A- and B-type horizontal cells of the adult rabbit retina. Surprisingly, their expression and organization within a cell is dependent on its location along the dorso-ventral axis of the retina. A-type horizontal cells in superior retina consistently stained with a wide variety of neurofibrillar methods to reveal neurofibrillar bundles, which immunocytochemistry showed to contain all three neurofilament subunits. A-type horizontal cells in inferior retina were uniformly refractory to neurofibrillar staining, although they expressed all three subunits. However, there was less of the light and medium subunits; the organization of the filaments into bundles (neurofibrils) is minimal. B-type horizontal cells could not be stained with any neurofibrillar method and were not recognizable by in situ immunocytochemistry. However, B-type cells could be seen to express all three subunits in vitro, but the expression of the light and medium subunits was weak. There was only a slight difference between B-type cells taken from superior and inferior retina. Combined with the results of recent transfection studies, these findings suggest that the amount of the light neurofilament subunit present in a horizontal cell determines its content of neurofibrillar bundles, and that rabbit horizontal cells may contain more neurofilament protein, particularly of the heavy subunit, than is used for neurofilament formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution and coverage of A- and B-type horizontal cells stained with Neurobiotin in the rabbit retina

Both A- and B-type horizontal cells in the rabbit retina were labeled by brief in vitro incubations of the isolated retina in the blue fluorescent dye 4,6-diamino-2-phenylindole. Intracellular injection of Lucifer Yellow into the somata revealed the morphology of the individual cells. Dye-coupling with Lucifer Yellow was seen only between A-type horizontal cells. By contrast, injection of the tracer Neurobiotin showed dye-coupling between both A- and B-type horizontal cells. There also appeared to be coupling between the axon terminals of B-type horizontal cells. The extensive dye-coupling seen following injection of Neurobiotin into a single horizontal cell soma can be used to obtain population counts of each cell type. Staining of large numbers of each cell type across the retina showed that each type increased in number and declined in dendritic diameter as the visual streak was approached, such that relatively constant coverage across the retina was maintained. In the visual streak, A-type horizontal cells numbered 555 cells/mm2 and averaged 120 microns in diameter, compared to 1375 cells/mm2 and 100 microns for B-type horizontal cells. In the periphery, the A- and B-types numbered 250 cells/mm2 and 400 cells/mm2, respectively. The average diameters of the dendritic trees at these locations were 225 microns for the A-type and 175 microns for the B-type. Coverage across the retina averaged almost six for A-type horizontal cells and 8-10 for the B-type. A-type horizontal cells in the visual streak whose elliptical dendritic fields were shown by Bloomfield (1992) to correlate physiologically with orientation bias were shown to be dye-coupled to cells with symmetrical dendritic fields.

Morphology and birth dates of horizontal cells in the retina of a marsupial

Most eutherian (placental) mammals have two horizontal cell types; however, one type only has been seen in rodents. In order to assess whether one type of horizontal cell or two is a basic mammalian feature, we have examined the morphology of horizontal cells in a marsupial, the quokka wallaby, by Golgi staining or horseradish peroxidase labelling. The birth dates of horizontal cells have also been determined by 3H-thymidine/autoradiography. There are two types of horizontal cell in the wallaby retina. One type has no axon and corresponds to the axonless cell in eutherian species; the other has shorter dendrites, an axon, and an axonal arbor, corresponding to the eutherian short-axon cell. As in eutherian mammals, the dendrites of each horizontal cell type lie in the outer plexiform layer (OPL) and contact cones and the axonal arbor of the short-axon cell contacts rods. The dendrites of the axonless cells are long, with an average length of 250 microns, and each cell has one, sometimes two, short, stubby processes, which branch off a dendrite, traverse the inner nuclear layer, and reach the inner plexiform layer. The dendritic field of these cells is elongated, and dendrites show a preferential orientation at right angles to the trajectory of overlying ganglion cell axons. Short-axon cells have a morphology similar to that seen in other species, although the axonal arbor is relatively small. Both types of horizontal cell are generated in the first phase of retinal cell generation.