Regional difference in the dendritic morphology of dopamine cells in carp retina

New aspects of dopaminergic interplexiform cell organization in the goldfish retina

Dopaminergic interplexiform cells (DA-IPCs) in the goldfish retina have been reexamined by light and electron microscopic immunocytochemistry with antisera against dopamine (DA) or tyrosine hydroxylase (TH). Successful immunostaining with a specific anti-DA antiserum offers further direct support for DA-IPCs. Anti-DA immunocytochemistry in combination with [3H]-DA autoradiography shows 92% colocalization of the two markers, indicating that [3H]-DA autoradiography is a reliable technique for identification of DA-IPCs. Incubations with anti-TH antiserum show that immunoreactive DA-IPCs have a homogeneous distribution, with an average frequency of 71 +/- 8 cells/mm2 in retinas of 14-15 cm long goldfish. Their arrangement is distinctly nonrandom. Electron microscopy of TH-immunoreactive cell processes confirms that horizontal cell axons synapse onto DA-IPCs and adds the following junctional arrangements to the circuit diagram of the DA-IPC: 1) adjacent serial synapses between DA-IPCs, external horizontal cells, and putative glycinergic interplexiform cells, 2) junctional appositions between DA-IPCs and photoreceptor cells, 3) junctional appositions between neighbouring DA-IPCs, and 4) the "gap junctional complex," typically consisting of a DA-IPC process juxtaposed with a gap junction between horizontal cell axons. The gap junction is flanked by clusters of small, round vesicles and groups of electron-dense structures resembling intermediate filaments. These morphological results support the functional involvement of DA-IPCs in adaptive retinomotor movements and in horizontal cell gap junction modulation and/or dynamics. They also suggest particular interaction between the dopaminergic and the glycinergic IPC system in the outer plexiform layer of goldfish retina.

Intracellular injection in fixed slices in combination with neuroanatomical tracing techniques and electron microscopy to determine multisynaptic pathways in the brain

Intracellular Lucifer Yellow filling in fixed tissue has been recently introduced as a novel neuroanatomical approach to reveal the detailed morphology of individual neurons in isolated preparations of the central nervous system. Since dye injections are performed under visual control, the method is characterized by a high degree of inherent staining selectivity, thus circumventing the element of randomness often considered to be the crux of classical golgi-impregnation techniques. Moreover, the opportunity to optically monitor the injection procedure renders fixed slice preparations highly advantageous to be used in combination with retrograde fluorescent tracing. Subsequently, dye-filled neurons may be subjected to a simple photoconversion procedure leading to the intracellular formation of a stable polymer thus obtaining permanent specimens for light microscopy purposes. Due to the osmiophilic nature of the precipitate the photoconverted material is equally suitable for correlated electron microscopy, thus enabling the analysis of neuronal microcircuitry. At the ultrastructural level, sources of afferent input to identified projection neurons may be revealed by lesion-induced anterograde degeneration of synaptic terminals, therefore enabling the direct demonstration of multisynaptic links. Finally, morphologically identified neurons may be immunocytochemically characterized at the pre- and postembedding levels. It is therefore suggested that their methodological versatility and relative technical ease render intracellular fixed-slice injections a promising complement to the catalogue of anatomical techniques.

Localization and function of dopamine in the adult vertebrate retina

Dopamine (DA) has satisfied many of the criteria for being a major neurochemical in vertebrate retinae. It is synthesized in amacrine and/or interplexiform cells (depending on species) and released upon membrane depolarization in a calcium-dependent way. Strong evidence suggests that it is normally released within the retina during light adaptation, although flickering and not so much steady light stimuli have been found to be most effective in inducing endogenous dopamine release. DA action is not restricted to those neurones which appear to be in "direct" contact with pre-synaptic dopaminergic terminals. Neurones that are several microns away from such terminals can also be affected, presumably by short diffusion of the chemical. DA thus affects the activity of many cell types in the retina. In photoreceptors, it induces retinomotor movements, but inhibits disc shedding acting via D2 receptors, without significantly altering their electrophysiological responses. DA has two main effects upon horizontal cells: it uncouples their gap junctions and, independently, enhances the efficacy of their photoreceptor inputs, both effects involving D1 receptors. In the amphibian retina, where horizontal cells receive mixed rod and cone inputs, DA alters their balance in favour of the cone input, thus mimicking light adaptation. Light-evoked DA release also appears to be responsible for potentiating the horizontal cell-->cone negative feed-back pathway responsible for generation of multi-phasic, chromatic S-potentials. However, there is little information concerning action of DA upon bipolar and amacrine cells. DA effects upon ganglion cells have been investigated in mammalian (cat and rabbit) retinae. The results suggest that there are both synaptic and non-synaptic D1 and D2 receptors on all physiological types of ganglion cell tested. Although the available data cannot readily be integrated, the balance of evidence suggests that dopaminergic neurones are involved in the light/dark adaptation process in the mammalian retina. Studies of the DA system in vertebrate retinae have contributed greatly to our understanding of its role in vision as well as DA neurobiology generally in the central nervous system. For example, the effect of DA in uncoupling horizontal cells is one of the earliest demonstrations of the uncoupling of electrotonic junctions by a neurally released chemical. The many other, diverse actions of DA in the retina reviewed here are also likely to become model modes of neurochemical action in the nervous system.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphology and retinal distribution of tyrosine hydroxylase-like immunoreactive amacrine cells in the retina of developing Xenopus laevis

The development of neurons immunoreactive to tyrosine hydroxylase (TH-IR) in the retina of Xenopus laevis was investigated from stage 53 tadpoles to adult, by using an antibody against tyrosine hydroxylase. At all developmental stages, most of the immunoreactive somata were located in the inner nuclear layer, and a few in the ganglion cell layer. Immunoreactive processes arborized in the scleral and vitreal sublaminae of the inner plexiform layer, indicating that these cells were bistratified amacrine cells. However, occasionally a few immunoreactive processes were observed projecting to the outer plexiform layer, suggesting the presence of TH-IR interplexiform cells. The number of immunoreactive amacrine cells in the inner nuclear layer per retina increased from 204 at stage 53 tadpole to 735 in adult, while the number of immunoreactive amacrine cells in the ganglion cell layer did not change significantly over the same period. Retinal area increased from 1.95 mm2 at stage 53 to 23.40 mm2 in the adult, and correspondingly cell density in the inner nuclear layer decreased from 104/mm2 to 31/mm2. At all stages there was an increasing density towards the ciliary margin, but this gradient decreased with age. The soma size of immunoreactive amacrine cells increased with age, and was consistently larger in the central than in the peripheral retina. Dendritic field size was estimated to increase 13-fold, from stage 53 to adult. This study shows that tyrosine hydroxylase-like immunoreactive amacrine cells are generated continuously throughout life, that after metamorphosis the retina grows more by stretching than by cell generation at the ciliary margin, and that the increase of dendritic field size is proportional to the increase in retinal surface area.

Dendritic morphology of a class of interstitial and normally placed amacrine cells revealed by intracellular Lucifer yellow injection in carp retina

The dendritic morphology of a class of interstitial amacrine (ISA) and normally placed amacrine cells was investigated in carp retina. We first identified their fluorescent nuclei after preloading with 4,6-diamidino-2-phenylindole (DAPI) in living or aldehyde-fixed retinal wholemounts and then injected them iontophoretically with Lucifer Yellow (LY) under microscopic control. Although DAPI appeared to be accumulated nonspecifically by amacrine and ganglion cells, ISA cell nuclei were discriminated by focusing between the amacrine and ganglion cell layers. The fusiform cell bodies of LY-injected cells were located in the middle of the inner plexiform layer (IPL), and the 4-5 stout primary dendrites were monostratified in sublamina b of the IPL and decorated with spines and long thin processes. The average spatial properties of these cells were approximately: density, 6.0 cells/mm2; intersomatic distance, 400 microns; dendritic field size, 0.27 mm2; dendritic coverage, 1.6. The dendritic interconnections were made of tip-to-tip or tip-to-side contacts between dendrites and between a dendrite and thin process, forming many closed loops. The ISA cells belong to a morphological type Fnb. A class of normally placed amacrine cells with dendritic morphology similar to that of the ISA cells was also found by LY injection in wholemounts. These cells belong to a morphological type Fna, with dendrites monostratified in sublamina a of the IPL in a 0.35-mm2 dendritic field. The ISA (Fnb) and Fna cells appear to represent a matched pair of cell types that are similar in structure but complementary in function.

The dopaminergic amacrine cell

The detailed morphology of the dopaminergic amacrine cell type has been characterized in the macaque monkey retina by intracellular injection of horseradish peroxidase (HRP). This cell type was recognized by its large soma in an in vitro, wholemount preparation of the retina stained with the fluorescent dye, acridine orange. HRP-fills revealed a large, sparsely branching, spiny dendritic tree and a number of extremely thin, axon-like processes that arose from the soma and proximal dendrites. The axon-like processes were studded with distinct varicosities and were traced for up to 3 mm beyond the dendritic tree. The true lengths of the axon-like processes were greater than 3 mm, however, because the HRP reaction product consistently diminished before an endpoint was reached. Both the dendrites and the axon-like processes were narrowly stratified close to the outer border of the inner plexiform layer, although in a few cases single axon-like processes projected into the outer nuclear and outer plexiform layers. The HRP-filled amacrines appeared equivalent to a subpopulation of neurons that are intensely immunoreactive for tyrosine hydroxylase (TH). TH-immunoreactive cells showed a nearly identical soma size and dendritic field size range, the same pattern of dendritic branching and spiny morphology, and also gave rise to distinct axon-like processes from both the soma and proximal dendrites. To test this correspondence more directly, the large acridine stained cells were injected with Lucifer Yellow and the retina was subsequently processed for TH immunoreactivity using diaminobenzidine as the chromagen. In all cases Lucifer Yellow injected cells also showed intense TH immunoreactivity. Spatial densities of the TH amacrine cells were therefore used to calculate coverage factors for the dendritic trees and for the axon-like components of the HRP-filled cells. The axon-like processes showed a coverage factor of at least 300, about 100 times that of the dendritic fields. This great overlap could be directly observed in TH-immunoreacted retinal wholemounts as a dense plexus of fine, varicose processes. The density of the TH plexus is greater than the density predicted from the lengths (1-3 mm) of the HRP-filled axon-like processes however, and suggests that the axon-like processes have an actual length of about 4-5 mm.(ABSTRACT TRUNCATED AT 400 WORDS)

Interplexiform cells of the goldfish retina

Dopaminergic and glycinergic interplexiform cells (IPCs) in the goldfish retina were impregnated by using two new Golgi protocols. The two cell types have markedly different morphological characteristics: Dopaminergic IPCs have primary dendrites that descend into and stratify in the inner plexiform layer, where they give rise to processes that project to the outer plexiform layer. Conversely, glycinergic IPCs have primary dendrites that ascend to the outer plexiform layer and from this dendritic arbor, many processes then project into the inner plexiform layer. The apparent coverage of dopaminergic IPCs is almost four times that of glycinergic IPCs. Even so, the coverage of each glycinergic IPC in the outer plexiform layer allows it to provide an accurate copy of the S-space to the inner plexiform layer. Considering the known GABAergic and glycinergic synaptologies in the inner plexiform layer, the glycinergic IPC must form a major element in the retinal circuitry of the goldfish.

Dendritic morphology and retinal distribution of tyrosine hydroxylase-like immunoreactive amacrine cells in Bufo marinus

Tyrosine hydroxylase-like immunoreactive (TH-IR) amacrine cells (ACs) in the retina of metamorphosing and adult Bufo marinus were visualized, and their retinal distribution established, using immunohistochemistry on retinal wholemount and sectioned material. The somata of TH-IR ACs were located in the innermost part of the inner nuclear layer (INL). Their dendrites branched predominantly in the scleral sublamina of the inner plexiform layer (IPL), with sparse branching also in the vitreal sublamina. In the retinae of metamorphosing animals 592 +/- 113 (mean +/- S.D.) immunoreactive cells and in adult 5,670 +/- 528 cells were found. Usually 1, 2 or 3 stem dendrites arose from the somata of TH-IR cells which branched 2 or 3 times. In the adult retinae the dendritic field sizes of immunoreactive cells were in the range of 0.059 +/- 0.012 mm2, which resulted in a considerable dendritic overlap across the retina. TH-IR cells were unevenly distributed over the retina, with 72 cells/mm2 in the central temporal retina, 45-50 cells/mm2 along the naso-temporal axis of the retina and 25 cells/mm2 in the dorsal and ventral peripheral retina. The average density was 36 +/- 6 cells/mm2. A considerable number of TH-IR cells (range 52-133, n = 4) were displaced into the ganglion cell layer (GCL) of the retina. The mean soma sizes of immunoreactive cells were significantly higher in the low density (95 +/- 13 microns 2) than in the high cell density areas (86 +/- 12 microns 2).(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide Y-like immunoreactive amacrine cells in the retina of Bufo marinus

Neuropeptide Y-like immunoreactive (NPY-LI) amacrine cells of the Bufo marinus retina were morphologically characterized, and their retinal distribution was established using immunohistochemistry on retinal wholemount preparations and sectioned material. The somas of NPY-LI amacrine cells were situated in the innermost part of the inner nuclear layer and their dendrites branched primarily in the scleral sublamina of the inner plexiform layer. A subgroup of the NPY-LI cells had dendrites in both the scleral and vitreal sublamina. All immunoreactive cells had large dendritic fields (average 0.5 mm2) that resulted in a high dendritic overlap across the retina. NPY-LI amacrine cells were evenly distributed across the retina, with an average density of 30 cells/mm2, although higher densities were observed at regions adjacent to the ciliary margin. The dendritic field size of the NPY-LI cells, together with the previously characterized substance P-like immunoreactive (SP-LI) amacrine cells, indicates that they belong to the class of wide-field amacrine cells. However, unlike the SP-LI neurons whose dendrites branch in the vitreal sublamina of the inner plexiform layer, the dendrites of the majority of the NPY-LI neurons branch in the scleral sublamina.

Dendritic morphology of retinal dopamine cells in carp of different sizes

The dendritic morphology of dopamine (DA) cells in the inner plexiform layer of the retina of different-sized carp (8.6-33.6 cm in body length) was investigated by identifying their fluorescent cell bodies in isolated, aldehyde-fixed flatmounts and injecting them iontophoretically with Lucifer yellow (LY) under microscopic control. DA cells for LY injection were chosen in an intermediate region between the optic disc and the retinal margin, and their spatial parameters (cell density, nearest neighbor distance, dendritic field size and dendritic coverage) were compared between retinas of different sizes to explore whether this region is simply stretched by tissue expansion during retinal growth. It was shown that as the fish grow larger the dendritic field size of DA cells increases while the cell density is sharply reduced and the dendritic coverage gradually decreases. Differences in the dendritic coverage of DA cells between certain fish groups with different sizes were statistically significant. Further, the irregularity in cell distribution appeared to increase as the fish became larger. These results suggest that the dendritic tree of individual DA cells in the intermediate region is developed not only by tissue expansion but also by other unknown factors.

Dopamine cells and rod bipolar cells contain protein kinase C-like immunoreactivity in some vertebrate retinas

The localization of cells immunoreactive to a monoclonal antibody against protein kinase C (PKC) and to polyclonal antibodies against tyrosine hydroxylase (TH) was investigated in the retina of fish (carp, goldfish, dace and catfish), frog, turtle, chick and some mammalians (guinea pig, rat, cat and rabbit) by means of fluorescence microscopy. PKC-like immunoreactivity was found in dopamine (DA) or TH-like immunoreactive (IR) cells in all the species examined and also in rod bipolar cells in the fish (except for catfish), and in presumed rod bipolar cells in the other animals (except for frog and turtle). In the catfish, frog and turtle retinas, no PKC-like IR bipolar cells were found. In the rat retina, some other amacrine cells in addition to TH-like IR amacrine cells were reactive to the anti-PKC antibody. It is of interest that PKC-like immunoreactivity is commonly found in DA cells and probably in rod bipolar cells in most animal species, although the functional significance is unknown at present.