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The distribution of endothelial nitric oxide synthase immunoreactivity (eNOS-IR) was investigated in the retinas of all phylogenetic vertebrate classes by using a monoclonal eNOS antibody. Confocal light microscopy showed immunoreactive labeling in Müller cells of fish, frog, salamander, turtle, chicken, rat, ground squirrel, and monkey retina. In vascularized retinas (rat, monkey), astrocytes and some blood vessels were also stained. Furthermore, eNOS-IR was localized to axon terminals of turtle and fish horizontal cells. These observations are the first to show the presence of eNOS-IR in Muller glia and horizontal cell structures of the vertebrate retina. Supported by grants EY03323 and EY04855 and a grant from Research to Prevent Blindness to the Department of Ophthalmology, Moran Eye Center.

Light microscopic immunocytochemistry using anti-choline acetyltransferase (ChAT) was performed to stain putative cholinergic amacrine cells in turtle retina. ChAT-immunoreactive somata lie in the inner nuclear (INL) and ganglion cell (GCL) layers. Three types of amacrine cells were found according to the location of their somata and their dendritic stratification pattern in the inner plexiform layer (IPL). Type I amacrines lie in the row of cells closest to the INL/IPL limits and they branch along the s1/s2 border of the IPL. Type II amacrines are displaced to the GCL and they ramify along the s3/s4 border of the IPL. Type III amacrines lie in the middle of the INL, 2–3 rows away from the IPL limits and their dendrites appear to be bi- or tri-stratified in s1 and s3-s4 of the IPL. The turtle ChAT-IR amacrines are thus similar to the types described in chicken retina. A regular, non-random mosaic formed by stained type II amacrine cells was observed in the GCL. Their density in mid-central retina was 750 cells/mm 2 , tapering off to 393 cells/mm 2 in peripheral retina. Our study indicates that a pair of cholinergic amacrine cell types in turtle retina is arranged in mirror-image symmetry contributing to sublamina “a” and sublamina “b” of the IPL, like in other vertebrate retinas.

In this paper we describe the morphology of the different spectral types of photoreceptor pedicles in the outer plexiform layer (OPL) of the Pseudemys turtle as studied by light (LM) and electron microscopy (EM). Tangential serial thick sections were cut from the oil droplet region to the level where the axons emerge from cell bodies and then serial thin sections through the axons and the entire pedicles were collected and examined. Thus photoreceptor pedicles could be identified by tracing cells from their oil droplets to their synapses in the OPL. Double cone pedicles consisted of closely applied pairs with the principal member's pedicle wrapping around the accessory member's pedicle. Each pedicle was approx. 104 microns 2 in area and contained 12 and 8 synaptic ribbons respectively. Rods comprised 8% of the pedicles in the field, were small (84 microns 2), contained closely packed synaptic vesicles, and on average, 9 long ribbons. Single red and green cone pedicles could not be told apart without following them from their oil droplets, however, both were about the same size (106-127 microns 2) and contained 10-12 ribbons. Blue cone pedicles were small and round (80 microns 2) and arose from short oblique axons giving off from large, greenish, clear oil droplet-containing cell bodies (13% of the cone population). The least common pedicle types (5% of the cone population) were identified tentatively as UV cones because they originated from small, clear oil droplet-containing cell bodies. UV cones had spherical pedicles, elongated in the vertical axis, that arose from extremely long, angled axons. Their very small pedicles (64 microns 2) exhibited characteristic "horns" that projected from the top sides of the pedicle. Both putative UV and blue cone pedicles ended more vitread in the OPL than other pedicles and contained only 5-6 and 8-10 ribbons respectively. Understanding the ultrastructural features that distinguish the different types of photoreceptor pedicle will allow us to begin a study of spectral connections to second order neutrons in the turtle OPL in the future.

Neurons involved in the transfer of rod signals to the ganglion cells in the retina of the cat have been recorded from and stained with horseradish peroxidase (HRP) and their synaptic connections determined by electron microscopy. The single morphological type of rod bipolar cell responds with a sustained hyperpolarization to light and in turn drives at least five morphologically different types of amacrine cells, each of which has a unique response pattern. Two amacrines respond with either a transient (AII) or a sustained (A17) depolarization to light, while three amacrines give transient (A8) or sustained (A6, A13) hyperpolarizations. Circuitry whereby rod signals reach both on-centre and off-centre ganglion cells is discussed.

Quantitative light microscopic techniques have been applied to photoreceptors, ganglion cells and dopaminergic amacrine cells in the retina of the North American opossum Didelphis virginiana. This marsupial has a retina with a central areal specialization where ganglion cells reach a maximum density of 2900/mm2 and cone photoreceptors are concentrated. However, the retina is very much rod dominated and there is a ring of peak rod density approximately 2 mm around the area centralis (a.c.). At the a.c. the cone to rod ratio is 1:50 while in peripheral retina it is 1:120. Dopaminergic amacrines, revealed by induced fluorescence (Fa/Glu) techniques are extremely numerous in opossum retina reaching a peak density at the a.c. and distributed in a high density plateau covering much of tapetal superior retina. Opossum retina is compared to mammalian retinas, particularly cats, with the aim of understanding the evolution of mammalian retinal circuits.

The horizontal cells of the rabbit retina have been studied by light microscopy of Golgi-impregnated whole-mount retinas. The two types of horizontal cell of the rabbit retina are similar to the horizontal cells of the cat retina in most respects. However, the majority of the A-type horizontal cells of the rabbit have asymmetrical dendritic fields compared to the circular, symmetrical dendritic fields of this cell type in the cat. The A-type horizontal cells of the superior edge of the linear visual streak in the rabbit retina are the most strikingly asymmetric and most of them are elongated and oriented in a direction approximately parallel to the linear visual streak. Like H1 axon terminals of the turtle retina the oriented, elongated A-type horizontal cells of the rabbit visual streak region may play a role in the neurocircuitry which underlies orientation sensitive ganglion cells.

Neuronal types contributing to the inner plexiform layer of the cat retina are described based primarily on light microscopy of Golgi-impregnated retinal whole-mounts. Cells have been characterized on morphological criteria that include dendritic branching patterns, dendritic tree sizes, cell body sizes and stratification of processes in the inner plexiform layer. Nine different types of bipolar cell, 22 different types of amacrine cell and 23 different types of ganglion cell can be distinguished using one or more of these morphological criteria. The significance of the different morphological types of cells is discussed, particularly in relationship to the functional bisublamination of the cat inner plexiform layer.

After intracellular recording, bipolar cells of the cat retina have been stained with HRP and their contacts in the outer and inner plexiform layers examined by electron microscopy. Rod bipolars and cone bipolar cb6 make invaginating, ribbon related contacts with photoreceptors, hyperpolarize in response to light, and have axons terminating in layer b of the IPL. The axon terminal of cb2 ends in layer a of the IPL and its basal contacts with cones mediate hyperpolarizing light-responses. Cone bipolar cb5 is a center-depolarizing type with an axon ending in layer b but its cone contacts are at semi-invaginating basal junctions. Except for the amacrine-contacting rod bipolar cell, all cone bipolar types synapse with both amacrine and ganglion cells in the inner plexiform layer. In addition cb5 contacts AII amacrine cells with large gap junctions, and is physiologically rod dominated.