Synaptic circuitry of serotonin-synthesizing and serotonin-accumulating amacrine cells in the retina of the cane toad, Bufo marinus

The synaptic connections of amacrine cells synthesizing or accumulating serotonin in the retina of the cane toad, Bufo marinus, were studied by using preembedding double-labeling electron-microscopic immunocytochemistry. The binding sites of an anti-serotonin antibody were revealed by the diaminobenzidine reaction, whilst a colloidal gold-conjugated secondary antibody was used to detect an antibody to phenylalanine hydroxylase. Since the latter antibody recognizes tryptophan 5-hydroxylase, one of the synthesizing enzymes for serotonin, as well as tyrosine hydroxylase, the rate-limiting enzyme for catecholamine synthesis, the double labeling of the present study enabled us to identify three groups of labeled profiles at the ultrastructural level. The profiles of serotonin-synthesizing amacrine cells contained both diaminobenzidine reaction product and colloidal gold particles, whilst those of serotonin-accumulating and dopaminergic amacrine cells contained only diaminobenzidine reaction product or colloidal gold particles, respectively. The synapses of serotonin-synthesizing or serotonin-accumulating amacrine cells were distributed all through the inner plexiform layer of the retina. The profiles of serotonin-synthesizing amacrine cells predominantly received synapses from, and made synapses onto, unlabeled amacrine cell dendrites. They also received synapses from, and made synapses onto, bipolar cell terminals. They also made synapses onto presumed ganglion cell dendrites. However, the profiles of serotonin-accumulating cells made synapses only with unlabeled amacrine cell processes. There were close contacts between the profiles of serotonin-synthesizing and serotonin-accumulating amacrine cells. No synaptic relationships were observed between dopaminergic and serotonin-synthesizing or serotonin-accumulating amacrine cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The contralaterally projecting neurons of the isthmic nucleus in five anuran species: a retrograde tracing study with HRP and cobalt

The morphology of projection neurons of the isthmic nucleus was studied in Rana esculenta, R. nigromaculata, Bufo marinus, B. bufo gargarizans, and Xenopus laevis from a comparative anatomical point of view. The main point of this work was to provide an anatomical basis for electrophysiological studies. Neurons projecting to the ipsilateral optic tectum were labeled by retrograde transport of horseradish peroxidase and cobaltous lysine complex injected into the optic tectum. Contralaterally projecting cells were filled by injecting the tracer substances into the crossed isthmotectal tract. Cells of the anterior nonrim cortex and the rostral part of the medulla project to the ipsilateral tectum. A band of cells in the middle of the medulla, a few cells in the caudal part of the medulla, and most of the neurons in the rim cortex project to the contralateral tectum. Five types of neurons were distinguished in the rim cortex of R. esculenta. Most of them have piriform perikarya and their dendrites arborize in the rim neuropil. In the medulla of the isthmic nucleus of R. esculenta, seven types of neurons were distinguished. Most of these neurons also exist in the other species. Medullary cells are piriform, fusiform, or multipolar, and are variable in size and in dendritic arborization. The isthmic neurons of the two Ranae and Bufo species are similar. The dominant cell types in Xenopus are multipolar with extensive dendritic arborization, which occupies more space in the nucleus than in the other species. Neurons with narrow dendritic trees may represent a system of fine resolution, and those neurons with extensive dendritic arborization may belong to a coarser system.

Neuropeptide Y-immunoreactive neurons in the retina of two Australian lizards

Wholemounts and sectioned retina from adults of two lizard species, Pogona vitticeps and Varanus gouldii, were studied by immunohistochemistry for neuropeptide Y (NPY)-like immunoreactivity. In both species the morphology of two classes of amacrine cells (types A and B) were described. Cell somata were located mostly in the inner nuclear layer (INL) but were occasionally displaced into the ganglion cell layer (GCL). In the Pogona retina, type A cells had large somata and dendritic arbor that branched in sublamina (S) 1 and 2/3 of the inner plexiform layer (IPL). Type B amacrine cells had smaller somata and dendritic arbor branching mostly in S5 of the IPL. In the Varanus retina, the levels of dendritic branching of types A and B amacrine cells in the IPL were similar to those in Pogona although branching in the middle of the IPL occurred at S3. NPY-immunoreactive cells with small somata and narrow to medium sized dendritic fields were predominant. Unclassified cells also displayed NPY-like immunoreactivity; however, their dendritic morphology could not be determined due to the faint and inconsistent staining. In transverse retinal sections three bands of NPY-like immunoreactivity were evident in the IPL of both species, to which the unclassified cells also contributed. In both species type A cells were most numerous. Total NPY-immunoreactive cells were estimated to be 8,600 in Pogona and 32,860 in Varanus. In both species types A and B cells were non-uniformly distributed across the retina. The most apparent non-uniformity in distribution was observed in type A cells in Varanus. Peak cell density was found across the horizontal meridian of the retina from where cell density decreased towards the dorsal and ventral retinal margins. The results of this study provide evidence for the presence of NPY-immunoreactive amacrine cells in the lizard retina of which two types were morphologically characterized. Cross-species comparisons were also made among NPY-immunoreactive amacrine cells, and their possible function/s discussed.

Co-localization of serotonin and GABA in neurons of the Xenopus laevis retina

Serotonin-synthesizing neurons in the retina of Xenopus laevis have been identified using anti-phenylalanine hydroxylase (PH) antibody which recognizes tryptophan 5-hydroxylase, the rate-limiting enzyme for serotonin synthesis. Double-labelling experiments, using anti-PH antibody and anti-serotonin antibody/5,7-dihydroxytryptamine (5,7-DHT) uptake, have shown that some serotonin-like immunoreactive/5,7-DHT-labelled neurons exhibit PH-like immunoreactivity (PH-LI) (serotonin-synthesizing neurons), but the others do not (serotonin-accumulating neurons). In the present study, triple-labelling experiments were performed using 5,7-DHT uptake and antibodies raised against GABA and PH, to determine the possible co-localization of y-aminobutyric acid (GABA) in serotonin-synthesizing and/or -accumulating neurons in the Xenopus retina. All 5,7-DHT-labelled bipolar cells lacked PH-LI; all of them were immunoreactive to GABA. In contrast, all 5,7-DHT-labelled large amacrine cells exhibited PH-LI, but none of them expressed GABA-LI. Small amacrine cells labelled with 5,7-DHT but not PH-LI exhibited GABA-LI, whilst the small amacrine cells with PH-LI lacked GABA-LI. These observations indicate that GABA is co-localized in serotonin-accumulating amacrine and bipolar cells, whereas serotonin-synthesizing large and small amacrine cells do not contain GABA-LI.

Synaptic contacts of serotonin-like immunoreactive and 5,7-dihydroxytryptamine-accumulating neurons in the anuran retina

The synapses of serotonin-like immunoreactive retinal neurons were studied in Bufo marinus and Xenopus laevis and those of 5,7-dihydroxytryptamine-labelled cells in Xenopus. Immunoreactivity to serotonin was mostly confined to amacrine cells. Synapses formed by profiles of labelled cells were almost uniformly distributed in the inner plexiform layer in both species. Interamacrine synapses were the most frequent, and in some cases two labelled amacrine cell profiles made a gap junction. Some of the labelled amacrine cells synapsed on to presumed ganglion cell dendrites and onto bipolar cell terminals. Labelled bipolar cell terminals synapsed on to non-labelled amacrine cell dendrites and received inputs both from labelled and non-labelled amacrine cells. Labelled bipolar cell profiles were not observed in the outer plexiform layer. After preloading and photoconversion of 5,7-dihydroxytryptamine in the Xenopus retina, labelled bipolar cell dendrites in the outer plexiform layer were observed to be postsynaptic to cone pedicles and less frequently to rods and horizontal cells. In the inner plexiform layer, synapse types formed by labelled bipolar cells were similar to those with serotonin immunoreactivity. The frequency of synapses formed by 5,7-dihydroxytryptamine-labelled amacrine cells increased, compared with serotonin immunocytochemistry. Labelled amacrine cells synapsed mostly with non-labelled amacrine cells, although the ratio of contacts formed by two labelled profiles increased. Synapses from labelled amacrine cell dendrites to non-labelled bipolar cell terminals and from non-labelled bipolar cell terminals to labelled amacrine cell profiles increased in number, while those from labelled amacrine cells to presumed ganglion cell dendrites decreased. The quantitative data obtained by the two approaches enabled us to propose different neuronal circuits for serotonin-synthesizing and -accumulating neurons of the Xenopus retina.

Morphology and distribution of Müller cells in the retina of the toad Bufo marinus

We have previously shown that an antibody against neuron-specific enolase (NSE) selectively labels Müller cells (MCs) in the anuran retina (Wilhelm et al. 1992). In the present study the light- and electron-microscopic morphology of MCs and their distribution were described in the retina of the toad, Bufo marinus, using the above antibody. The somata of MCs were located in the proximal part of the inner nuclear layer and were interconnected with each other by their processes. The MCs were uniformly distributed across the retina with an average density of 1500 cells/mm2. Processes of MCs encircled the somata of photoreceptor cells isolating them from each other by glial sheath, except for those of the double cones. Some of the photoreceptor pedicles remained free of glial sheath. Electron-microscopic observations confirmed that MC processes provide an extensive scaffolding across the neural retina. At the outer border of the ganglion cell layer these processes formed a non-continuous sheath. The MC processes traversed through the ganglion cell layer and spread beneath it between the neuronal somata and the underlying optic axons. These processes formed a continuous inner limiting membrane separating the optic fibre layer from the vitreous tissue. Neither astrocytic nor oligodendrocytic elements were found in the optic fibre layer. The significance of the uniform MC distribution and the functional implications of the observed pattern of MC scaffolding are discussed.

Immunocytochemical identification of serotonin-synthesizing neurons in the vertebrate retina: a comparative study

Serotonin-synthesizing neurons in the retinas of goldfish, axolotl, turtle, chick, rabbit and cat were identified using double labelling with anti-serotonin and anti-phenylalanine hydroxylase antibodies. The latter antibody recognizes tryptophan 5-hydroxylase, one of the synthesizing enzymes for serotonin. Neurons labelled by both markers were considered to be serotonin-synthesizing neurons, while those only with serotonin-immunoreactivity were assumed to be serotonin-accumulating neurons. In the goldfish and chick retinas, all serotonin-immunoreactive amacrine cells (S1) were positive for phenylalanine hydroxylase. In the axolotl and turtle retinas, all the S1 amacrine cells, and only 52.8% and 40.5% of S2 amacrine cells were double-labelled. Although serotonin-immunoreactive bipolar cells were observed in the turtle and chick retinas, the synthesizing enzyme for serotonin could not be detected in these cells. In the rabbit and cat retinas, tryptophan hydroxylase could not be revealed in any cell type with immunocytochemistry. In control experiments SLI neurons in the raphe nuclei of the brain stem always exhibited PH-LI in all species examined, including mammals, indicating that our anti-PH antibody is able to recognize tryptophan hydroxylase across vertebrate classes. These results indicate that the majority of serotonin-immunoreactive amacrine cells are able to synthesize serotonin and are the source of endogenous serotonin in the non-mammalian retina, while some serotonin-immunoreactive amacrine and bipolar cells possibly only accumulate serotonin. We also suggest that serotonin may not be a primary neurotransmitter in the serotonin-accumulating bipolar and amacrine cells of the non-mammalian retina.

Quantitative analysis of GABA-immunoreactive synapses in the inner plexiform layer of the Bufo marinus retina: identification of direct output to ganglion cells and contacts with dopaminergic amacrine cells

We have recently reported that about 50% of amacrine cells and some of the bipolar and ganglion cells are GABA-immunoreactive in the retina of Bufo marinus. Synapses formed by these elements in the inner plexiform layer were studied. GABA-immunoreactive amacrine cell processes were found most frequently in synaptic contact with non-immunoreactive amacrine cells. Double-label experiments showed that some of these non-GABA-immunoreactive elements contain tyrosine hydroxylase immunoreactivity. Another source of input to the GABA-immunoreactive amacrine cells were the bipolar cells; some of which were GABA-immunoreactive. GABA-immunoreactive amacrine cells synapsed also onto bipolar cell terminals, and ganglion cell dendrites that were identified by the retrograde transport of horseradish peroxidase from the optic nerve. Synapses between GABA-immunoreactive amacrine cells and bipolar and ganglion cells were non-uniformly distributed in the inner plexiform layer. Synaptic contacts with bipolar cells were more frequent in the OFF-sublamina, and those with ganglion cell dendrites in the ON-sublamina. These results demonstrate that GABA-immunoreactive amacrine cells (1) preferentially synapse with OFF-responding bipolar and ON-centre ganglion cells in the through-pathway, (2) synapse with tyrosine hydroxylase-immunoreactive amacrine cells in both the OFF- and ON-sublaminae, and (3) synapse directly with GABA-immunoreactive ganglion cells. The synapses between GABA-immunoreactive amacrine and GABA-immunoreactive ganglion cells may inhibit the centrally projecting inhibitory ganglion cells, causing disinhibition in the visual centres.

Immunocytochemical localization of parvalbumin- and neurofilament triplet protein immunoreactivity in the cat retina: colocalization in a subpopulation of AII amacrine cells

Using antibodies against parvalbumin and neurofilament triplet protein, colocalization of these two neuronal markers was revealed in all of type A horizontal cells and alpha ganglion cells and in a small number of AII amacrine cells of the cat retina. Besides the double-labeled neurons, parvalbumin alone was present in type B horizontal cells, in small numbers of starburst- and A13-like amacrine cells and in the somata of unidentified ganglion cells. The processes of the double- or single-labeled amacrine cells did not have a continuous retinal cover. Although the parvalbumin- and neurofilament-immunolabeled amacrine cells belonged to groups of neurons with well-defined cell morphologies, their neurochemical features differed from other AII, starburst and A13 amacrine cells. The presence of these cells may be due to an accidental expression of an unusual combination of neurochemical features during retinal development. It is also possible that these cells support the functioning of ganglion cells with rarely occurring complex receptive fields.

Neuron-specific enolase-like immunoreactivity in the vertebrate retina: selective labelling of Müller cells in Anura

Neuron-specific enolase (NSE) immunocytochemistry was carried out in retinae of goldfish, axolotl, clawed frog, cane toad, lizard, chick, guinea-pig, rabbit, rat, cat and human. With the exception of Anura, strong immunoreactivity was seen in the large ganglion, amacrine cells and horizontal cells of the retina in all of the other species. Photoreceptors were found to be labelled in the rat and human retina and only one cone type in rabbit. Photoreceptor pedicles and ellipsoids were stained in the goldfish and the somata and inner segments of some photoreceptors in axolotl. In the axolotl retina, besides neurons, Müller cells (MCs) were also immunolabelled. In the retina of the cane toad and the clawed frog MCs were the only stained elements. Similarly in other parts of the central nervous system of the cane toad, glial elements of the optic tectum and spinal cord were immunoreactive. In contrast, in the peripheral nervous system, neurons of the 1st sympathetic ganglion and the 2nd dorsal root ganglion were labelled. In double-labelling experiments, glial fibrillary acidic protein and NSE showed colocalisation both in the glial elements of the optic tectum and spinal cord and in MCs of the retina of the cane toad.

Large serotonin-like immunoreactive amacrine cells in the retina of developing Xenopus laevis

The earliest appearance of serotonin-like immunoreactivity (SLI) in different cell types and the development of large SLI amacrine cells were studied in the retina of Xenopus laevis from stage 33/34 to adult. Intense SLI was first found in the somas of large amacrine cells at stage 39. The somas of small amacrine cells showed weak SLI at stage 41, followed by bipolar cells at stage 43. The number of large SLI amacrine cells in the inner nuclear layer of the retina increased from 57 at stage 40 to 774 in adult. Over the same period, retinal area increased from 0.19 mm2 to 24.57 mm2 with an accompanying decrease of cell density from 301/mm2 to 32/mm2. in adult animals large SLI amacrine cells were non-uniformly distributed. Peak cell density of 50-60/mm2 was located in the center of the ventrotemporal quadrant and a trough of 8-15/mm2 in the dorsal periphery of the retina. Peak cell density region of the adult retina corresponded to part of the retina formed at early developmental stages where the rate of cell generation of large SLI amacrine cells was higher. These observations indicate that (1) SLI is expressed first by large amacrine cells, followed by small amacrine and bipolar cells; (2) large SLI amacrine cells are generated continuously throughout life, (3) the non-uniform retinal distribution of large cells results from a spatio-temporally differential cell generation at the ciliary margin.

Serotonin synthesis and accumulation by neurons of the anuran retina

Serotonin-synthesizing and serotonin-accumulating neurons were studied in the retinas of Xenopus laevis and Bufo marinus. All previously identified cell types exhibiting serotonin-like immunoreactivity (SLI) were labeled by intravitreal injection of 5,7-dihydroxytryptamine (5,7-DHT). They included two amacrine cell types (large and small) in both species, and one bipolar cell type in Xenopus. Incubation of retinas in culture medium in the ambient light reduced SLI in amacrine cells and enhanced the labeling in bipolar cells. After incubation, some photoreceptor cell bodies and large numbers of outer segments also displayed SLI in both species. Incubation with the serotonin-uptake inhibitor, fluoxetine, reduced immunolabeling in bipolar cells and outer segments to the level in the untreated retinas. Both large SLI and 5,7-DHT-accumulating amacrine cells in Xenopus and Bufo were labeled with an antibody raised against phenylalanine hydroxylase (PH), which binds to tryptophan 5-hydroxylase, one of the synthesizing enzymes for serotonin. Small SLI and 5,7-DHT-accumulating amacrine cells in both species represented two populations, one with and the other without PH-like immunoreactivity (PH-LI). The anti-PH antibody failed to label any SLI or 5,7-DHT-accumulating bipolar cells in Xenopus. These observations indicate that all large and some small SLI amacrine cells in the retinas of Xenopus and Bufo synthesize serotonin, while other small SLI amacrine, bipolar and photoreceptor cell bodies, and outer segments only accumulate serotonin.

Microtubule-associated protein 2 (MAP2)-immunoreactive neurons in the retina of Bufo marinus: colocalisation with tyrosine hydroxylase and serotonin in amacrine cells

Neuron populations in the retina of the toad, Bufo marinus, were labelled with a monoclonal antibody raised against microtubule-associated protein 2 (MAP2). A subpopulation of cones, probably corresponding to the blue-sensitive small single cones, large diameter amacrine cells in the most proximal row of the inner nuclear layer and some large ganglion cells in the ganglion cell layer were labelled. Double labelling experiments were carried out to establish the colocalisation of MAP2 with known putative transmitter substances of the anuran amacrine cells. MAP2 was colocalised in a subpopulation of serotonin-immunoreactive and in all tyrosine hydroxylase-immunoreactive amacrine cells. The results indicate, that the MAP2 content in the neurons of the anuran retina can be correlated with other well-defined neurochemical and/or physiological properties.

The topographic organization of the retinal ganglion cell layer of the lizard Ctenophorus nuchalis

The numbers of neurons of the ganglion cell layer (GCL) and their distribution in the retina of an Australian lizard Ctenophorus nuchalis were investigated. Retinal wholemounts and sections were prepared for light microscopic and optic nerves for electron microscopic study. Counts of cell numbers in the GCL from wholemounts varied from 200,000 to 380,000. Neurons in the GCL were non-uniformly distributed, forming a high cell density streak along the naso-temporal axis of the retina. Neurons of the GCL formed 2 to 9 layers in the visual streak and a single layer in the rest of the retina. The number of neurons of the GCL in this area was estimated at about 2,100,000. Although the visual streak represented only 16% of the total retinal surface area, it contained about 90% of all neurons of the GCL. Optic axon counts yielded 147,000 myelinated and 2,643,000 unmyelinated fibres. The estimated optic fibre number of 2,790,000 was 18.2% less than the total number of neurons counted from sections in the GCL of the same eye. The unexpected high number of neurons in the area of the visual streak indicates that cell numbers obtained only from wholemount preparations may vastly underestimate the total neuron numbers in the GCL of the lizard retina.

The generation and changing retinal distribution of displaced amacrine cells in Bufo marinus from metamorphosis to adult

The generation and retinal distribution of displaced amacrine cells (DAs) were studied from metamorphosis to adult in the cane toad Bufo marinus. Displaced amacrine cells were identified by inducing chromatolytic changes in ganglion cells (GCs) following optic nerve section. Cells that did not chromatolyse in the ganglion cell layer (GCL) of the retina were regarded as DAs. The number of DAs increased considerably from an estimated 10,000 at metamorphosis to 211,000 in the adult toad, and was accompanied by a substantial decrease of average cell density. In contrast to the reported 6:1 cell density gradient of all cells of the GCL in adult toad (Nguyen and Straznicky 1989) only a shallow 1.6:1 density gradient of DAs from the visual streak to the dorsal and ventral retinal margins was detected. Consequently, the incidence of DAs increased from 15% of all cells of the GCL in the visual streak to 30% in the dorsal and ventral peripheral retina. These results indicate that the ratio of the newly generated DAs and GCs at the ciliary margin must be changing during development. More GCs are generated before and around metamorphosis then DAs, in contrast to the relative increase of the percentage of DAs generated after metamorphosis. The possible control of the numbers of DAs in the GCL is discussed.

A neurofilament protein antibody selectively labels a large ganglion cell type in the human retina

An antibody (SMI-32) raised against the non-phosphorylated form of the neurofilament protein triplet (NFP) revealed immunoreactivity in the soma and dendritic arborization of a group of large ganglion cells in the human retina. In addition, a population of smaller somas was also faintly labeled with this antibody in the ganglion cell layer. The completely stained cells amounted to 44,000 and were non-uniformly distributed across the retina with a peak density of 100 cells/mm2 in the retinal periphery. The soma sizes increased about two-fold and dendritic field sizes about 3-fold with retinal eccentricity. The immunoreactive dendrites branched in the vitread sublamina of the inner plexiform layer. The dendritic branching pattern of these cells indicated that they correspond to the previously described shrub cells. Antibodies against NFP and neuropeptide Y showed colocalization of these markers in all of the completely stained cells.

Synaptic contacts of tyrosine hydroxylase-immunoreactive elements in the inner plexiform layer of the retina of Bufo marinus

Tyrosine hydroxylase (TH) immunocytochemistry was utilized to quantify dopaminergic synapses in the inner plexiform layer of the retina of Bufo marinus. Since dopaminergic cells have bistratified dendritic arborisation in the inner plexiform layer, attention was given to the segregation of synapses between the scleral and the vitreal sublaminae. Light-microscopically, a more elaborate dendritic branching was observed in the scleral than in the vitreal sublamina. In contrast, about 55% of synapses occurred in the vitreal one fifth of the inner plexiform layer, 30% in the scleral fifth, and 15% in the intermediate laminae. Input sources and output targets showed only minor quantitative differences between sublaminae 1 and 5. TH-immunoreactive processes were found in presynaptic (62.8%) and postsynaptic (37.2%) positions. Synapses to the stained dendrites derived from bipolar (40.4%) and amacrine (59.6%) cells, whereas outputs from the TH-positive processes were directed to amacrine cells (56.8%) and to small and medium-sized dendrites (35.4%); at least some of these can be considered as ganglion cell dendrites. TH-positive profiles neither formed synapses with each other nor were presynaptic to bipolar cell terminals. Junctional appositions of the immunoreactive profiles were occasionally seen on non-stained amacrine and ganglion cell dendrites in the scleral sublamina of the inner plexiform layer and on optic axons in the optic fibre layer. Although dopaminergic cells are mainly involved in amacrine-amacrine interactions, inputs from bipolar terminals and outputs to ganglion cell dendrites were also substantial, suggestive of a role also in vertical information processing.

GABA-like immunoreactive neurons in the retina of Bufo marinus: evidence for the presence of GABA-containing ganglion cells

gamma-Aminobutyric acid (GABA)-like immunoreactive (IR) neurons in the retina of the cane toad Bufo marinus were revealed using immunohistochemistry on retinal wholemount preparation and sectioned material. GABA-IR neurons included horizontal, bipolar and amacrine cells in the inner nuclear layer and small to medium sized cells in the ganglion cell layer. A few IR axons were seen in the optic fiber layer of the retina. Following the injection of the carbocyanine dye, DiI into the optic tectum ganglion cells were retrogradely filled. A small population of DiI-filled ganglion cells (2.8%) was found to be GABA-IR. GABA-IR neurons in the ganglion cell layer without DiI label were considered to be displaced amacrine cells of which 45.3% were GABA positive. It is proposed that GABA-containing ganglion cells may form an inhibitory projection to visual centers of the anuran brain.

Retinal projections in the cane toad, Bufo marinus

The location and extent of retinorecipient areas in the cane toad, Bufo marinus, were established by anterograde transport of cobaltic-lysine complex from the cut optic nerve. Most of the labeled optic axons travelled in the marginal optic tract, while others were in the axial optic tract, and/or the basal optic tract. Retinal projections terminated in both contralateral and ipsilateral targets. In addition to the optic tectum, the main visual center, retinorecipient areas included the suprachiasmatic nucleus, rostral visual nucleus, neuropil of Bellonci, corpus geniculatum thalamicum, ventrolateral thalamic nucleus (dorsal part), posterior thalamic neuropil, uncinate neuropil, pretectal nucleus lentiformis mesencephali and basal optic nucleus. While all of these retinorecipient areas receive optic fibers from both eyes, the ipsilateral retinal projections were observed to be generally sparser than those from the contralateral retina. A sparse optic fiber projection covers the surface of the ipsilateral optic tectum and is most prominent rostromedially and caudolaterally. The position and the extent of each of the retinorecipient areas were determined in relation to a three-dimensional coordinate system. Morphometric analysis showed that 85.3% of the retinorecipient area is in the contralateral optic tectum, 10.4% in contralateral non-tectal areas, 1.6% in the ipsilateral optic tectum and 2.7% in ipsilateral non-tectal areas. The presence of an ipsilateral tectal projection and the well defined pretectal visual neuropil complex may be related to the highly developed visual behavior and visual acuity of Bufo marinus.

Tyrosine hydroxylase-immunoreactive elements in the distal retina of Bufo marinus: a light and electron microscopic study

Tyrosine hydroxylase-immunoreactive elements in the distal retina of Bufo marinus were investigated using light and electron microscopic immunocytochemistry. At the light microscopic level, immunoreactive somas were seen in the proximal part of the inner nuclear layer, and immunoreactive processes projected both to the inner and outer plexiform layers. In some instances stained axon-like processes traveled from the inner plexiform layer, across the inner nuclear layer to the distal retina. Immunolabeled elements formed basket-like structures around the photoreceptor inner segments. At the ultrastructural level immunostained fibers were observed in close contact with the necks, lateral walls, bases and the outer surfaces of rod outer segments. Synaptic specializations were neither observed at rod contacts nor at other possible contact sites such as bipolar dendrites and horizontal cell somata and processes in the outer plexiform layer. In contrast, synaptic specializations between immunolabeled profiles and amacrine, bipolar and ganglion cells were regularly present in the inner plexiform layer. These findings suggest that a population of dopaminergic interplexiform cells is present in the Bufo retina and sends axon-like processes towards the distal retina. It is assumed that dopamine is probably released non-synaptically from the immunolabeled terminals in the distal retina influencing rods directly, by which the quality of photopic vision is enhanced in the anuran retina.

Retinal distribution of ganglion cells which project to the ipsilateral optic tectum in Bufo marinus

The retinotectal projection in anura is mainly crossed, although a small proportion of optic axons projects to the ipsilateral tectum. Using the fluorescent carbocyanide dye, DiI, we mapped the retinal topography of ganglion cells which project to the ipsilateral tectum in adult Bufo marinus. DiI was injected into particular locations in the right tectum. After 10 days survival both the right and the left retinals were wholemounted and the number and retinal position of retrogradely filled ganglion cells were determined. The contralateral and ipsilateral cells were visuotopically distributed in the retina in the majority of experiments. However, in two cases cells were located in visuotopically disparate parts of the retina. The ipsilateral cells represented 3.7% of contralaterally projecting cells in the temporal retina, 0.1% in the nasal and dorsal retina and 0.6% of the ventral retina. The density of ipsilaterally projecting ganglion cells varied from a top of 25 cells/mm2 in the temporal retina, 9 cells/mm2 in the nasal, 3 cells/mm2 in the dorsal to 11 cells/mm2 in the ventral retina. The diversity of size and shape of retrogradely filled ganglion cells indicated that the ipsilateral population corresponded to a heterogeneous class of ganglion cell types. The functional significance of the direct ipsilateral retinotectal projection of the anuran visual system has yet to be elucidated. However, in light of the involvement of the indirect ipsilateral retinotectal projection in binocular vision, the direct pathway is likely to be associated with a retino-tecto-spinal circuit subserving postural adjustment to visually derived stimulation.

Substance P-immunoreactive neurons in the retina of two lizards

The dendritic morphology and retinal distribution of substance P(SP)-immunoreactive neurons was determined in two Australian lizard species Pogona vitticeps and Varanus gouldii, by using immunohistochemistry on retinal wholemounts and sectioned materials. In both species, two classes of SP-immunoreactive neurons were described in the inner nuclear layer (INL) and classified as amacrine cells (types A and B). Type A amacrine cells had large somata and wide-field, bistratified dendrites branching in sublaminas 1 and 5 of the inner plexiform layer (IPL). Their morphology and retinal distribution differed between the two species. Type B amacrine cells in both species had small somata and small-field dendritic branching. A population of SP-immunoreactive neurons with classical ganglion cell morphology were identified in the ganglion cell layer (GCL). Immunostained ganglion cells occurred in larger numbers of Varanus gouldii than in Pogona vitticeps. In both species type B SP cells were the most numerous and were estimated to be about 60,000-70,000. They were distributed non-uniformly with a high density band across the horizontal meridian of the retina, from where the density decreased towards the dorsal and ventral retinal margins. In both species type A amacrine cells occurred in small numbers distributed sparsely in the peripheral retina. The faint immunostaining of SP-immunoreactive neurons in the GCL, did not allow us to reliably determine their numbers and retinal distribution. The functional significance of SP-immunoreactive amacrine and ganglion cells in the lizard retina remains to be determined.

NADPH-diaphorase positive neurons in the retina of Bufo marinus: selective staining of bipolar and amacrine cells

Unfixed retinal tissues of adult Bufo marinus were reacted for NADPH-diaphorase histochemistry that resulted in selective staining of populations of bipolar and amacrine cells. The number of stained bipolar cells in the retina was estimated to be 233,600 +/- 38,900 (mean +/- S.D.). These cells were non-uniformly distributed across the retina with the highest density of 4,308 cells/mm2 along the visual streak decreasing to lowest density of 1,378 cells/mm2 at the dorsal and ventral poles of the retina. The stained bipolar cells represented a morphologically uniform population. The number of labelled amacrine cells in the retina amounted to 3,116 +/- 405 including 251 cells that were displaced into the ganglion cell layer. Labelled amacrine cells were also unevenly distributed; with no stained cells in the retinal centre from where the density increased steadily to 72 cells mm2 in the retinal periphery. NADPH-diaphorase positive amacrine cells were classified on the basis of their dendritic arborization pattern into three morphological types: 1) narrow-field cells both at the periphery and the centre of the retina; 2) wide-field amacrine cells, and 3) amacrines with eccentric medium-sized dendritic field. The results of this study provide evidence for the presence of NADPH-diaphorase containing neurons in the anuran retina. Furthermore, this is the first report on the selective staining of bipolar cells for NADPH-diaphorase in the vertebrate retina.

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.

The morphology and distribution of photoreceptors in the retina of Bufo marinus

The number, cell morphology and retinal distribution of rods and cones were determined in the retina of the toad, Bufo marinus. Adult animals were sacrificed, both eyes were removed and prepared for either tangential section across the outer segments of the photoreceptor layer, or transverse section across the whole retina. Cone densities increased from an average of 7000/mm2 in the peripheral to a maximum of 25,000/mm2 in the central retina. The high cone densities extended across the naso-temporal axis of the retina corresponding to the position of the visual streak in the ganglion cell layer. The total number of cones in the retina was estimated to be 1.1 million. Rod density of 21,000/mm2 in the central retina decreased to 17,000/mm2 at 1.5-4 mm eccentricity, and then increased to 29,000/mm2 in the peripheral retina. The total number of rods amounted to about 2 million. The mean of the cross-sectional area of rod outer segments was 11.2 +/- 1.5 microns 2 (mean +/- SD) in the highest and 17.9 +/- 4.7 microns 2 in the lowest density areas of the retina. The length of the rod outer segments extended from 28 microns in the ventral peripheral retina to a maximum of 89 microns in the dorsal retina, dorsal to the visual streak of the ganglion cell layer. The results of the present study showed a differential retinal distribution of photoreceptors, with a peak density in the retinal centre and a higher density along the naso-temporal axis of the eye. We conclude that the area of high photoreceptor density, matched by high neuron densities of the INL and GCL, corresponds to the site of acute vision of the Bufo retina.

Morphology and distribution of serotonin-like immunoreactive amacrine cells in the retina of Bufo marinus

Using an antibody against serotonin (5-hydroxytryptamine, 5-HT), serotonin-like immunoreactive (serotonin-IR) neurons were demonstrated in the retina of adult Bufo marinus. All immunoreactive neurons were identified as amacrine cells (ACs). The dendrites of serotonin-IR ACs branched diffusely and densely throughout all levels of the inner plexiform layer (IPL) of the retina. The great majority of these cell somata were located in the vitread part of the inner nuclear layer (INL) and a few of them (ranging from 9-29 cells) were displaced into the ganglion cell layer (GCL). On the basis of the soma sizes, two populations of serotonin-IR ACs, large (type A) and small (type B), were distinguished. 6-Hydroxydopamine (6-OHDA) injected into the eye abolished immunoreactivity in the recently reported tyrosine hydroxylase (TH)-IR ACs (Zhu & Straznicky, 1990), whereas serotonin-IR ACs remained unaffected. The number of serotonin-IR cells per retina ranged from 23, 750-27, 390, with a ratio of 1:1.6 to 1:1.9 between type A and B cells. Both cell types were distributed nonuniformly across the retina. Cell densities were slightly lower in the peripheral (96 cells/mm2) than in the central (164 cells/mm2) retina. Linear regression analysis confirmed the presence of a decreasing density gradient from the retinal center to the retinal margin for both small and large cell types. The analysis of the nearest neighbor distances showed that the retinal distribution of serotonin-IR ACs was orderly. These results have been taken to indicate that 5-HT-IR cells correspond to a population of serotonin-containing ACs.(ABSTRACT TRUNCATED AT 250 WORDS)

The changing distribution of neurons in the inner nuclear layer from metamorphosis to adult: a morphometric analysis of the anuran retina

The generation and changing distribution of neurons of the inner nuclear layer (INL) in the retina of two anuran species, Bufo marinus and Xenopus laevis, were studied from metamorphosis to adult. Morphometric studies were undertaken at six developmental stages in Bufo and four in Xenopus. The number and thickness of neurons in the INL were established in 29 predetermined retinal locations from serial sections of the eyes cut vertically or horizontally. The total number of neurons in the INL increased from metamorphosis to adult from 826,000 +/- 185 to 18,760,000 +/- 562 (mean +/- SD) in Bufo and from 308,000 +/- 25 to 877,000 +/- 31 in Xenopus. Over the same period the surface area of the INL increased about 50-fold from 2 mm2 to 96 mm2 in Bufo and 5-fold from 2.5 mm2 to 13 mm2 in Xenopus. In Bufo the difference between the highest cell number (central-temporal retina) and the lowest cell number in a sample area (dorsal and ventral peripheral retina) was 2.1:1 at metamorphosis. This ratio increased to 3.4:1 in the adult. Both the cell number and cell density per sample area in the INL was found to be higher along the nasotemporal meridian of the eye overlying the visual streak of the ganglion cell layer (GCL) of the retina. The retinal distribution of neurons in the INL did not change significantly during postmetamorphic growth in Xenopus. At metamorphosis a 1.7:1 difference was found between the highest neuron number (retinal ciliary margin) and lowest neuron number (retinal centre) decreasing to 1.5:1 in the adult. Retinae were labelled with 3H-thymidine in 15 mm Bufos and examined 2, 6, 12 and 18 weeks later. Higher rates of cell addition to the nasal and temporal poles of the INL were found compared with that at the dorsal and ventral poles. The retinal radial growth at the ciliary margin of the dorsal, ventral, nasal and temporal poles between the time of isotope injection and 18 weeks survival was found to be uneven; more radial elongation occurred at the nasal, dorsal and ventral poles and less at the temporal pole. These observations suggest that (a) the neuron distribution of the INL in adult animals approximates that of the GCL and (b) the visual streak-like area of the INL in Bufo develops by a sustained differential cell addition at the temporal and nasal poles of the retina.

Neuropeptide Y- and substance P-like immunoreactive amacrine cells in the retina of the developing Xenopus laevis

The development of neuropeptide Y-like (NPY-LI) and substance P-like (SP-LI) immunoreactive neurons was studied in retinas of Xenopus laevis from young tadpole through to adult animals. In adult retina these neuropeptides are present in wide-field amacrine cells located in the inner nuclear layer and the ganglion cell layer of the retina. Retinal wholemount preparations and sectioned material showed that immunoreactive cells appeared during early larval life and NPY-LI occurred earlier than SP-LI cells. The primary dendritic branching of NPY-LI neurons appeared from early larval life whilst SP-LI was evident in dendrites from mid-larval stages. In postmetamorphic animals the numbers of immunoreactive cells increased in proportion to retinal area growth with a relatively constant cell density of about 35 cells/mm2 for SP-LI and 45 cells/mm2 for NPY-LI. The maturation of dendritic morphology of both NPY- and SP-LI amacrine cells appeared later in larval development than the appearance of immunoreactivity in cell somas. However, the sequence of expression of NPY- or SP-LI and their dendritic maturation was different for the two classes of amacrine cells. It is suggested that the maturation of dendritic fields of amacrine cells is complete just prior to metamorphosis, consistent with the postmetamorphic onset of electrophysiological features of ganglion cells attributed to amacrine cells.

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)

Morphological classification and retinal distribution of large ganglion cells in the retina of Bufo marinus

The retrograde transport of horseradish peroxidase (HRP) and cobaltic-lysine complex (CLC) was used to morphologically characterize large ganglion cells (GCs) and to determine their distribution in retinal wholemounts and in sectioned material in the retina of Bufo marinus. Large GCs, amounting to about 0.5% of total GC population, were defined to be those with very large dendritic field sizes varying between 0.1 mm2 to 0.6 mm2 and cell soma sizes of between 100 microns 2 to 400 microns 2. These cells were subdivided into 3 major groups, Types I, II and III, on the basis of their dendritic field sizes, aborization patterns and the strata of dendritic branching within the inner plexiform layer (IPL). The majority of large neurons (about 90%) were classified as Type I GCs with symmetrical dendritic arbor. These cells had either bistratified branching in the scleral and vitreal sublamina of the IPL (65% of Type I Cells) or unistratified branching in the scleral (26%) or in the vitreal (9%) sublamina. Their dendritic field sizes increased linearly from the retinal centre from 0.13 mm +/- 0.02 mm2 (mean and S.D.) to 0.58 +/- 0.11 mm2 in the retinal periphery. Type II GCs (about 9% of large GC population) were characterized by an asymmetrical dendritic aborization directed towards the ciliary margin with unistratified branching in the scleral sublamina of the IPL. The mean dendritic field sizes of these cells were 0.26 +/- 0.09 mm2. Type III GCs, the least frequent (about 1%) category of large GCs had sparsely branching, elongated dendritic branching aligned approximately parallel with the nasotemporal axis of the retina. The unistratified dendritic branches of these neurons were located in the vitreal sublamina of the IPL with a mean dendritic field size of 0.42 +/- 0.11 mm2. The dendritic field sizes of Types II and III GCs did not increase with retinal eccentricity. Type I GCs were distributed unevenly across the retina, the density being greatest in the visual streak, along the nasotemporal meridian of the retina. The dendritic field sizes of these cells increased towards the retinal periphery, resulting in a constant dendritic field coverage factor across the retina. Each retinal point was covered by the dendritic fields of 4-5 adjacent GCs. In contrast, Types II and III GCs had only discontinuous dendritic coverage. The identification of morphological types of large GCs with previously described functional classes of GCs in the anuran retina is discussed.

The morphological characterization and distribution of displaced ganglion cells in the anuran retina

The number, dendritic morphology, and retinal distribution of displaced ganglion cells were studied in two anuran species, Xenopus laevis and Bufo marinus. Horseradish peroxidase or cobaltic lysine complex was applied to the cut end of the optic nerve, and the size, shape, and retinal position of retrogradely filled ganglion cells displaced into the inner nuclear layer were determined in retinal wholemount and sectioned material. Approximately 1% of ganglion cells in Xenopus and 0.1% in Bufo were found to be displaced. In both species, many of the previously described orthotopic ganglion cell types (Straznicky & Straznicky, 1988; Straznicky et al., 1990) were present among displaced ganglion cells. In Xenopus more displaced ganglion cells were found in the retinal periphery than in the retinal center, and they formed 3 or 4 distinct bands around the optic nerve head. In Bufo the incidence of displaced ganglion cells was higher along the visual streak than in the dorsal and ventral peripheral retina. These results indicate that the distribution of displaced ganglion cells approximates the retinal distribution of orthotopic ganglion cells. One of the likely mechanisms to account for this developmental paradox may be that the formation of the inner plexiform layer, adjacent to the ciliary margin, acts as a mechanical barrier by preventing the entry of some of the late developing ganglion cells into the ganglion cell layer.

Biplexiform ganglion cells in the retina of Xenopus laevis

A few cells were seen on retinal wholemounts of Xenopus frogs whose dendrites branched in both the inner and the outer plexiform layers after the application of retrograde tracers to the cut optic nerve. The somas of these cells were displaced into the inner nuclear layer of the retina, and occasionally in orthotopic position. These observations provide convincing evidence that previously described biplexiform cells are ganglion cells and that these cells are regular components of the vertebrate retina.

Retino-retinal projections in three anuran species

In juvenile and adult Xenopus laevis, in adult Bufo marinus and Rana esculenta frogs retino-retinal projections were traced by filling the central stump of one optic nerve, cut 2-3 mm from the eye, with horse-radish peroxidase (HRP) or cobaltic-lysine complex (CLC). The presence of retino-retinal projections was confirmed in all 3 species both in the juvenile and the adult. Up to 12 ganglion cells per retina were found to be filled retrogradely with HRP together with optic axons filled anterogradely with CLC. These findings suggest that (1) a small proportion of ganglion cells project, erroneously, to the opposite retina and (2) this erroneous retino-retinal projection persists throughout the whole lifespan of the animals.

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 identified retinal ganglion cells in Xenopus laevis: a comparison between the results of horseradish peroxidase and cobaltic-lysine retrograde labelling

Following the application either of cobaltic-lysine complex or a 30% solution of horseradish peroxidase (HRP) in a sealed tube to the cut end of the optic nerve of young adult Xenopus frogs, the dendritic morphology of large ganglion cells was studied in retinal wholemount preparations, and compared with that in animals of the same size as revealed by the short time administration of HRP crystals. In the former two groups of animals, after 24 h survival, the size of the dendritic arborization of characterized ganglion cell types, Types I and III, were found to be significantly larger (61-79% and 180-187%, respectively) than those which survived 3 days after the administration of HRP crystals. These findings suggest that the very fine dendritic branches of large ganglion cells may remain unlabelled after a short-time exposure to HRP.

The development and the topographic organization of the retinal ganglion cell layer in Bufo marinus

The number and distribution of neurons in the retinal ganglion cell layer were studied from the metamorphic climax to adulthood in the toad Bufo marinus. Retinal wholemounts stained with cresyl violet showed that total neuron numbers increased from 55,000 at metamorphic climax to about 950,000 in adult animals. During the same time the entire retinal area increased 46-fold from an average 3.4 mm2 to 157 mm2. The morphological character of the neurons and their density across the retina changed during development. In metamorphosing animals, the neurons of the ganglion cell layer had a uniform appearance and their density increased slightly from the centre to the dorsal ciliary margin. After metamorphosis a high neuron density area, the visual streak, evolved in the retinal centre, resulting in the formation of a 6 to 1 density gradient from the visual streak out to the dorsal and ventral retinal poles in adult animals. Optic fibre numbers in juvenile and adult optic nerves were estimated to be 330,000 and 745,000, respectively, corresponding to similar ganglion cell numbers. One optic nerve was sectioned in a few animals and 4 weeks later the number of intact neurons--assumed to be displaced amacrine cells (DA)--was estimated. They amounted to 80,000 in juvenile and 189,000 in adult animals or about 20% of the total neuron population of the retinal ganglion cell layer, the remaining 80% being GC. A 1.7 to 1 density gradient of DA from the visual streak out to the dorsal and ventral retinal periphery was established.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinal ganglion cell death induced by unilateral tectal ablation in Xenopus

The survival of retinal ganglion cells (GCs) in the left eye was studied on retinal wholemounts from 2-33 weeks after the surgical removal of the right tectum in juvenile Xenopus. Two to five weeks after tectal removal, about 76% of neurons of the retinal ganglion cell (GC) layer showed signs of retrograde degeneration: swelling of their somata and chromatolysis. Neurons that were not affected by the operation were taken to be either displaced amacrine cells (DAs) or GCs not projecting to the tectum. A portion of GCs showing retrograde degeneration became pyknotic and died within the period of 2-16 weeks after operation. Counts of surviving GCs 20-33 weeks after tectal removal amounted to about 55% of the corresponding neuron number in the right intact retina of the same animal. No discernible GC loss was observed in animals where only the optic fibers were cut at their entry point to the tectum indicating that axotomy alone, followed by rapid regrowth to the target, does not adversely influence the survival of GCs. In long-surviving animals, the left optic nerve was exposed to cobaltic-lysine complex and the position of filled optic axons within the brain determined. Optic axons whose tectal target had been removed were seen to cross over to the left intact tectum via the posterior and pretectal commissures. Aberrant projections were detected to the ipsilateral tectum and the diencephalic periventricular grey in addition to an increased projection to the accessory optic nucleus. It is concluded that the removal of the tectum, the main target of optic fiber projection, induces a very substantial GC death. Since only a portion of optic fibers were able to grow to alternative targets, the surviving GCs may have also included those with main projection areas to the diencephalic visual centers.

Morphological characterization of substance P-like immunoreactive amacrine cells in the anuran retina

Using substance P immunohistochemistry it was possible to demonstrate a class of morphologically homogeneous group of neurons in the inner nuclear layer (INL) of the retina of two anuran species: Xenopus laevis and Bufo marinus. The number of cells with substance P-like immunoreactivity (SP-LI) was about 250 and 800 in juvenile and 600 and 2500 in adult Xenopus and Bufo, respectively, SP-LI cells had a small soma with one primary dendrite having up to four slender branches, located in the vitreal sublamina of the inner plexiform layer (IPL). Mean dendritic field sizes were 0.12 and 0.30 mm2 in juvenile and 0.29 and 0.65 mm2 in adult Xenopus and Bufo, respectively. The density of SP-LI cells was 40/mm2 in juvenile and 24/mm2 in adult Xenopus compared with 20/mm2 in juvenile and 13/mm2 in adult Bufo. Nearest neighbour distance measurements indicated that SP-LI cells were randomly distributed across the entire retina in both species. The location and the morphology of SP-LI cells indicated that they correspond to a subclass of wide-field amacrine cells, similar to types 20 and 21 described by Golgi techniques in the cat.

Neuropeptide Y-like immunoreactivity in neurons of the human retina

The distribution of neuropeptide Y-like immunoreactivity (NPY-LI) was investigated in wholemounts and in transverse sections of the human retina. NPY-LI was localized to the soma and axonal processes of large ganglion cells (GCs) and to the soma and dendritic arborization of amacrine cells (ACs). NPY-LI GCs were unevenly distributed across the retina, the highest density of 875 cells/mm2 was found in the fovea centralis and the lowest density of 15 cells/mm2 in the peripheral retina. The total number of NPY-LI GCs in the retina was estimated to be about 85,000. The soma sizes of NPY-LI GCs increased from 116 microns 2 +/- 23 (s.d.) in the retinal centre to 251 microns 2 +/- 57 in the retinal periphery. The soma size of NPY-LI ACs was in the range of 40 and 50 microns 2. In transverse sections NPY-LI was seen to be localized to the optic fibre layer, to the somata of GCs, to the scleral sublamina of the inner plexiform layer (AC dendrites) and to the innermost part of the inner nuclear layer (somata of ACs). The gradients of soma sizes and retinal distribution of NPY-LI GCs were taken as an indication that they correspond to the class of large to very large GCs, previously identified in the human retina by Golgi impregnation.

Lack of axon regeneration of isthmic neurons in juvenile Xenopus

In young adult Xenopus laevis frogs the axons of isthmic neurons projecting to the contralateral tectum were severed at the postoptic commissure and the survival of such neurons was studied between 2 and 26 weeks after the operation. Pyknotic neurons were first observed in the isthmic nucleus 2 weeks after axotomy. The continued neuron loss resulted in the decrease from 2920 in intact animals to 1520 surviving isthmic neurons in animals 26 weeks after the operation. These findings suggest that (i) severed axons of the isthmic neurons are not able to regenerate to their natural target, and (ii) isthmic neurons require continuous contact with appropriate tectal target for their survival.

Morphological classification of retinal ganglion cells in adult Xenopus laevis

Retrograde transport of horseradish peroxidase (HRP) was used to characterise the soma and dendritic arborization of retinal ganglion cells in adult Xenopus laevis toad. HRP was administered to the cut end of the optic nerve and the morphological characteristics of HRP-filled ganglion cells were analysed in retinal wholemount preparations using computer assisted morphometry. Ganglion cells were classified according to their soma size, dendritic branching pattern, dendritic field and the number of shaft dendrites. Ganglion cells were divided into 3 major classes on the basis of soma sizes and extent of dendritic field: large (soma size, mean 258.04 micron 2 +/- 52.03 SD; dendritic field size 0.104 mm2 +/- 0.23), medium size (126.7 micron 2 +/- 37.01; 0.041 mm2 +/- 0.013) and small (87.3 micron 2 +/- 22.69; 0.0061 mm2 +/- 0.0035). A more detailed analysis allowed 12 morphologically distinct subgroups to be identified (Types I-XII). Quantitative studies showed that large cells comprise about 1%, medium size about 8-9% and the small cells over 90% of total ganglion cell population. The number of large and medium size ganglion cells corresponded well with the number of myelinated optic fibres and the number of small neurons with the number of unmyelinated optic fibres in the optic nerve. Large ganglion cells were correlated with Class 4 and 5, medium size ganglion cells with Class 3 and small ganglion cells with Class 1 and 2 functionally characterized ganglion cells in the frog retina (Maturana et al. 1960). The retinal distribution of large ganglion cells appear to suggest certain similarities to mammalian alpha type ganglion cells.

The ultrastructural organization of the isthmic nucleus in Xenopus

The isthmic nucleus (IN) of the frog brain forms a linkage, relaying visual information from one tectum to the other. It receives afferent input from the tectum of the same side and projects bilaterally to both tecta. The ultrastructural features of the tecto-isthmic synaptic connections were studied in young postmetamorphic Xenopus frogs. Most synaptic profiles in the isthmic nucleus have spheroidal vesicles and an asymmetric zone of apposition. Frequently, synaptic glomeruli consisting of up to 8 terminal boutons surrounding a shaft dendrite were observed. The synaptic density in the rostral IN was slightly higher than in the middle or caudal portions. Partial deafferentation by transection of the tecto-isthmic pathway or total deafferentation by removal of the tectum was followed by a widespread degeneration of terminals in the ipsilateral IN. In the former case, the density of synapses in the IN decreased initially by about 64%, and then increased by 30 days after operation to about 50% of the normal synaptic density. After tectal removal, all the terminal boutons in the isthmic neuropil degenerated by 3 days after operation. These studies, along with recent findings, indicate that most, if not all, of the afferent fibres to IN are of tectal origin.

The effects of tectal lesion on the survival of isthmic neurones in Xenopus

The isthmic nucleus (IN) is a visual relay centre of the frog brain. It receives afferent projection from the optic tectum of the same side and projects bilaterally to both tecta. In young postmetamorphic Xenopus frogs, the survival of neurones in the IN on both sides was studied following the complete removal of the right tectum. In 6- to 8-week-old frogs, the right tectum was surgically removed and the operated animals allowed to survive for 1 to 13 weeks after operation. In selected animals, 3 days before the intended sacrifice, the postoptic commissure was transected and the cut isthmotectal fibres filled with horseradish peroxidase (HRP). In serial paraffin sections of the midbrain, the numbers of surviving and dying (pyknotic) neurones in the left and right IN were counted. The soma size of viable isthmic neurones and the volume of both IN were measured. Pyknotic neurones were seen between 1 and 6 weeks after operation in both the left and right IN, although the rate of cell loss was much greater in the latter. Virtually all the neurones of the right IN degenerated by 6 weeks after tectal ablation. In contrast, approximately 60% of neurones of the left IN survived. HRP histochemistry showed labelled isthmic neurones both in the left and right IN up to 3 weeks after operation. Thereafter, HRP-labelled neurones appeared only in the left IN. These observations indicate that the removal of the natural target of isthmic neurones brings about severe neurone death.(ABSTRACT TRUNCATED AT 250 WORDS)

The formation of the area centralis of the retinal ganglion cell layer in the chick

In adult domestic chickens, the neurones in the retinal ganglion cell layer are very unevenly disposed such that there is a sixfold increase in neurone density from the retinal edge to the retinal centre. The formation of the high ganglion-cell-density area centralis was studied on chick retinal wholemounts from the 8th day of incubation (E8) to 4 weeks after hatching (4WAH). The density of viable neurones and the number and the distribution of pyknotic neurones in the ganglion cell layer were estimated across the whole retina. Between E8 and E10, the distribution of neurones in the ganglion cell layer was anisodensitic with 53,000 mm-2 in the centre compared to 34,000 mm-2 in the periphery of the retina. Thereafter, a progressively steeper gradient of neurone density developed, which decreased from 24,000 mm-2 in the retinal centre to 6000 mm-2 at the retinal periphery by 4WAH. Neuronal pyknosis in the ganglion cell layer was observed between E9 and E17. From E11 onwards, consistently more pyknotic neurones were found in the peripheral than in the central retina. It was estimated that over the period of cell death approximately twice as many neurones died per unit area in the retinal periphery than in the centre. Retinal area measurements and estimation of neurone densities in the ganglion cell layer after the period of neurone generation and neurone death indicated differential retinal expansion, with more expansion in the peripheral than in the central retina. These observations allow us to conclude that the formation of the area centralis of the chick retina involves (1) slightly higher cell generation in the retinal centre, (2) higher rate of cell loss in the retinal periphery and (3) differential retinal expansion.

The thoracic sympathetic neurons of the chick: normal development and the effects of nerve growth factor

The generation and degeneration of sympathetic neurons in the third thoracic ganglion (segment 19) of the chick were studied between embryonic days (E) 7-18 using 3H-Thymidine autoradiography and routine cell counts. Cumulative radiolabelling experiments indicated that few sympathetic neurons were generated on E6-7. 10% of the sympathetic neurons were generated on E8 and a further 20% on E9. The final 70% of neurons completed the mitotic cycle between E10-12. Cell counts demonstrated that the neuronal population increased from 10,166 +/- 423 (mean +/- SEM) to 22,291 +/- 767 between E8-10 and remained stable up to E14. The population subsequently declined by 37%, to 14,157 +/- 831, by E18. Pyknotic neurons were found at all stages of development, but were most apparent between E7-15. The effects of Nerve Growth Factor (NGF) on the number of both surviving and pyknotic neurons in the ganglion were also examined. E9 embryos treated with NGF from E5-8 showed a 57% increase in the number of sympathetic neurons. This increase therefore occurred prior to the decline in neuronal number and was not accompanied by a decrease in the number of visibly pyknotic neurons. It is therefore possible that early NGF treatment increases the number of sympathetic neurons through a mechanism other than the attenuation of cell death.

The formation of axonal projections of the mesencephalic trigeminal neurones in chick embryos

Horseradish peroxidase-wheat germ agglutinin conjugate (HRP) was injected into masticatory and eye muscles of 6- to 15-day-old chick embryos and posthatched chicks to establish the timetable of axonal outgrowth and distribution of central and peripheral terminations of mesencephalic trigeminal neurones (MTN). HRP-labelled MTN first appeared on the 10th day of incubation and by the 14th day most of MTN became labelled, indicating that axonic outgrowth to peripheral targets occurred between the 10th and 14th days of incubation. Peripheral targets included the pterygoideus lateralis and medialis, protractor quadratus, pseudotemporalis superficialis and profundus, and adductor mandibulae of jaw-closing muscles. HRP-filled central axonic processes of MTN were identified first on the 13th day of incubation and they terminated exclusively in the motor nucleus of the trigeminal nerve. Between the 10th and 13th days of incubation, at the peak of naturally occurring cell death in the MTN pool, a consistently lower percentage of neurones could be labelled with HRP than in older embryos and in posthatched chicks. This finding suggests that many MTN die before establishing contact with peripheral targets.

Naturally occurring and induced ganglion cell death. A retinal whole-mount autoradiographic study in Xenopus

The retina in frogs grows continuously throughout the whole life of the animal by the addition of rings of cells at the ciliary margin. Naturally occurring neuron death cannot, consequently, be established by counting surviving neurons. A new approach, retinal whole-mount auto-radiography was introduced in this study to estimate cell loss occurring in the ganglion cell layer over a long period of time. 3H-thymidine injection at stage 53 (midlarval stage) labels a ring of cells, thereby marking the extent of retina formed up to the time of isotope administration. In the present study the number of neurons in the ganglion cell layer within the autoradiographically identified central retinal sector was estimated from midlarval stage to 6 months after metamorphosis in Xenopus laevis. The mean neuron number in the central retinal sector formed up to stage 53 was 17,420 and this was reduced by 20% to 13,515 by 6 months after metamorphosis. Optic nerve section at the time of isotope injection and subsequent regeneration brought about a reduction of the number of surviving neurons in the part of the retina formed up to stage 53 to 7,720, or to about 57% of the normal neuron number in an equivalent retinal area of an intact eye of the same age. A further reduction to 20% of normal neuron population was observed in retinae where the optic nerve failed to regenerate. The surviving neurons are assumed to be amacrine cells. The bulk of natural neuron loss in the retinal centre occurs during premetamorphic stages while little further loss takes place in the next 6 months suggesting that the underlying mechanism is a fine tuning of the developing retinal projections.

The development of the trigeminal (V) motor nucleus in normal and tubocurare treated chick embryos

The generation of cells and the naturally occurring neuronal death was studied in the trigeminal motor neuron pool in normal and tubocurare treated chick embryos between the 5th and 18th days of incubation. 3H-thymidine autoradiography revealed that the generation time extends from the 2nd to the 5th day of incubation, wherein about 50% of trigeminal motoneurons are born on the 3rd day. Maximum neuron number was found on the 7th day of incubation which steadily decreased to about 50% of the originally generated neurons by the 13th day. Nuclear pyknosis occurred from the 6th to the 13th day of incubation with a peak of neuron loss on the 7th day. Tubocurare, administered daily from the 5th day of incubation rescued most of the generated motoneurons which would otherwise have died. Cell nuclear area measurements in the motoneuron pool of the tubocurare treated animals showed a marked hypertrophy accompanying the increased neuronal survival. These observations indicate that tubocurare treatment prevents naturally occurring neuron death and causes significant nuclear hypertrophy within the trigeminal motoneuron pool innervating special, branchial arch derived muscles. Thus these neurons respond to tubocurare treatment in a manner similar to motoneurons of the spinal cord.

The development of the neurons of the glossopharyngeal (IX) and vagal (X) sensory ganglia in chick embryos

The timetable of cell generation, neuronal death and neuron numbers in the fused proximal glossopharyngeal (IX) and vagal (X) ganglion and distal IX and X ganglia were studied in normal and nerve growth factor (NGF) treated chick embryos. 3H-thymidine was injected between the 3rd and 7th days of incubation and embryos sacrificed on the 11th day. Neurons in the distal IX and X ganglia were generated between the 2nd and 5th days of incubation, the peak mitotic activity occurring on the 4th and 3rd days, respectively. Neurons of the proximal IX and X ganglion were generated between the 4th and 7th days, with maximum neuron generation on the 5th day of incubation. Counts of neurons in the 3 ganglia between the 5th and 18th days of incubation showed a maximum of 22,000 on the 8th day in the proximal IX and X ganglion and this decreased to 12,000 by the 13th day. In the distal IX ganglion, the neuron number decreased by 44% from 4,500 on the 6th day to 2,500 by the 11th day. A similar decrease of 43% was found in the distal X ganglion, the neuron number falling from 11,500 on the 7th day to 6,500 by the 11th day of incubation. Neuronal cell death in these ganglia extended from the 5th to the 12th day of incubation, maximum cell death occurring at or after the cessation of mitotic activity. NGF administration from the 5th to the 11th day of incubation did not have a measurable effect on the neurons of proximal IX and X and distal IX ganglia, but increased neuronal survival by 30% in the distal X ganglion.(ABSTRACT TRUNCATED AT 250 WORDS)

Nerve growth factor treatment does not prevent dorsal root ganglion cell death induced by target removal in chick embryos

In chick embryos, on the 3rd day of incubation, the developing right wing bud was removed. One group of the operated embryos was treated with a daily dose of 20 micrograms purified nerve growth factor (NGF) from the 5th day of incubation and sacrificed on the 12th day. The other group was sacrificed on the 12th day of incubation and served as control. NGF was also administered to intact, unoperated embryos for comparison. The size of the dorsal root ganglia in segments 13-16 innervating the wings, were estimated and the number of surviving dorsal root ganglion cells counted both on the right (operated) and left (intact) sides. Although NGF brought about an increase in the size of the ganglia and an increase in the number of dorsal root ganglion cells bilaterally, it was not able to prevent excessive cell death of dorsal root ganglion cells on the operated side. The number of surviving neurons in the dorsal root ganglia on the operated side in embryos with or without NGF administration was only about 30-50% of the number of the intact side. These results show that cell death induced by target removal cannot be offset by NGF administration. It is concluded that NGF may act as a growth promoting agent for developing sensory neurons but other peripheral trophic factor/s are also needed for the maintenance and survival of dorsal root ganglion cells.