Retinal neurochemistry of three elasmobranch species: an immunohistochemical approach

New Species Can Broaden Myelin Research: Suitability of Little Skate, Leucoraja erinacea

Although myelinated nervous systems are shared among 60,000 jawed vertebrates, studies aimed at understanding myelination have focused more and more on mice and zebrafish. To obtain a broader understanding of the myelination process, we examined the little skate, Leucoraja erinacea. The reasons behind initiating studies at this time include: the desire to study a species belonging to an out group of other jawed vertebrates; using a species with embryos accessible throughout development; the availability of genome sequences; and the likelihood that mammalian antibodies recognize homologs in the chosen species. We report that the morphological features of myelination in a skate hatchling, a stage that supports complex behavioral repertoires needed for survival, are highly similar in terms of: appearances of myelinating oligodendrocytes (CNS) and Schwann cells (PNS); the way their levels of myelination conform to axon caliber; and their identity in terms of nodal and paranodal specializations. These features provide a core for further studies to determine: axon-myelinating cell communication; the structures of the proteins and lipids upon which myelinated fibers are formed; the pathways used to transport these molecules to sites of myelin assembly and maintenance; and the gene regulatory networks that control their expressions.

Early development of GABAergic cells of the retina in sharks: an immunohistochemical study with GABA and GAD antibodies

We studied the ontogeny and organization of GABAergic cells in the retina of two elasmobranches, the lesser-spotted dogfish (Scyliorhinus canicula) and the brown shyshark (Haploblepharus fuscus) by using immunohistochemistry for gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD). Both antibodies revealed the same pattern of immunoreactivity and both species showed similar organization of GABAergic cells. GABAergic cells were first detected in neural retina of embryos at stage 26, which showed a neuroepithelial appearance without any layering. In stages 27-29 the retina showed similar organization but the number of neuroblastic GABAergic cells increased. When layering became apparent in the central retina (stage-30 embryos), GABAergic cells mainly appeared organized in the outer and inner retina, and GABAergic processes and fibres were seen in the primordial inner plexiform layer (IPL), optic fibre layer and optic nerve stalk. In stage-32 embryos, layering was completed in the central retina, where immunoreactivity appeared in perikarya of the horizontal cell layer, inner nuclear layer and ganglion cell layer, and in numerous processes coursing in the IPL, optic fibre layer and optic nerve. From stage 32 to hatching (stage 34), the layered retina extends from centre-to-periphery, recapitulating that observed in the central retina at earlier stages. In adults, GABA/GAD immunoreactivity disappears from the horizontal cell layer except in the marginal retina. Our results indicate that the source of GABA in the shark retina can be explained by its synthesis by GAD. Such synthesis precedes layering and synaptogenesis, thus supporting a developmental role for GABA in addition to act as neurotransmitter and neuromodulator.

Muscarinic signaling influences the patterning and phenotype of cholinergic amacrine cells in the developing chick retina

Background

Many studies in the vertebrate retina have characterized the differentiation of amacrine cells as a homogenous class of neurons, but little is known about the genes and factors that regulate the development of distinct types of amacrine cells. Accordingly, the purpose of this study was to characterize the development of the cholinergic amacrine cells and identify factors that influence their development. Cholinergic amacrine cells in the embryonic chick retina were identified by using antibodies to choline acetyltransferase (ChAT).

Results

We found that as ChAT-immunoreactive cells differentiate they expressed the homeodomain transcription factors Pax6 and Islet1, and the cell-cycle inhibitor p27^kip1. As differentiation proceeds, type-II cholinergic cells, displaced to the ganglion cell layer, transiently expressed high levels of cellular retinoic acid binding protein (CRABP) and neurofilament, while type-I cells in the inner nuclear layer did not. Although there is a 1:1 ratio of type-I to type-II cells in vivo, in dissociated cell cultures the type-I cells (ChAT-positive and CRABP-negative) out-numbered the type-II cells (ChAT and CRABP-positive cells) by 2:1. The relative abundance of type-I to type-II cells was not influenced by Sonic Hedgehog (Shh), but was affected by compounds that act at muscarinic acetylcholine receptors. In addition, the abundance and mosaic patterning of type-II cholinergic amacrine cells is disrupted by interfering with muscarinic signaling.

Conclusion

We conclude that: (1) during development type-I and type-II cholinergic amacrine cells are not homotypic, (2) the phenotypic differences between these subtypes of cells is controlled by the local microenvironment, and (3) appropriate levels of muscarinic signaling between the cholinergic amacrine cells are required for proper mosaic patterning.

The centrifugal visual system of vertebrates: a comparative analysis of its functional anatomical organization

The present review is a detailed survey of our present knowledge of the centrifugal visual system (CVS) of vertebrates. Over the last 20 years, the use of experimental hodological and immunocytochemical techniques has led to a considerable augmentation of this knowledge. Contrary to long-held belief, the CVS is not a unique property of birds but a constant component of the central nervous system which appears to exist in all vertebrate groups. However, it does not form a single homogeneous entity but shows a high degree of variation from one group to the next. Thus, depending on the group in question, the somata of retinopetal neurons can be located in the septo-preoptic terminal nerve complex, the ventral or dorsal thalamus, the pretectum, the optic tectum, the mesencephalic tegmentum, the dorsal isthmus, the raphé, or other rhombencephalic areas. The centrifugal visual fibers are unmyelinated or myelinated, and their number varies by a factor of 1000 (10 or fewer in man, 10,000 or more in the chicken). They generally form divergent terminals in the retina and rarely convergent ones. Their retinal targets also vary, being primarily amacrine cells with various morphological and neurochemical properties, occasionally interplexiform cells and displaced retinal ganglion cells, and more rarely orthotopic ganglion cells and bipolar cells. The neurochemical signature of the centrifugal visual neurons also varies both between and within groups: thus, several neuroactive substances used by these neurons have been identified; GABA, glutamate, aspartate, acetylcholine, serotonin, dopamine, histamine, nitric oxide, GnRH, FMRF-amide-like peptides, Substance P, NPY and met-enkephalin. In some cases, the retinopetal neurons form part of a feedback loop, relaying information from a primary visual center back to the retina, while in other, cases they do not. The evolutionary significance of this variation remains to be elucidated, and, while many attempts have been made to explain the functional role of the CVS, opinions vary as to the manner in which retinal activity is modified by this system.

Cloning, immunolocalization, and functional expression of a GABA transporter from the retina of the skate

Termination of GABA signals within the retina occurs through high-affinity reuptake of the released neurotransmitter by GABA transporters (GATs) present in neurons and glia surrounding the release site. In the present work, we have cloned a novel GAT from the retina of the skate (Raja erinacea). The clone codes for a 622 amino acid protein whose sequence has highest similarity to the GABA/β-alanine transporter of the electric ray (Torpedo marmorata) (88% identity) and the GAT-3 isolated from rat brain (75% identity). The protein was expressed inXenopusoocytes and characterized using the two-electrode voltage-clamp technique. Application of GABA induced a dose-dependent inward current, with 8 μM GABA producing a half-maximal response. The current required the presence of extracellular sodium and was unaffected by the GABA receptor blocker picrotoxin or the GAT-1 specific antagonist NO-711. The high homology between the cloned skate GABA transporter and the GAT-3 equivalents of other species, coupled with the strikingly similar pharmacological profile to GAT-3s of other species, lead us to conclude that we had cloned the GAT-3 homologue for the skate. Polyclonal antibodies specific to GAT-3 and the previously cloned skate GAT-1 transporter were used to examine the distribution of GAT-3 and GAT-1 immunoreactivity in the retina and in isolated cells of the skate. Antibodies for both transporters showed labeling in the outer and inner plexiform layers, and staining extended from the outer to inner limiting membranes. Both GAT-1 and GAT-3 antibodies labeled enzymatically isolated Müller cells, while bipolar cells and horizontal cells did not appear to express either transporter. These results imply that GAT-1 and GAT-3 are both present in Müller cells of the skate retina where they are likely involved in regulating extracellular concentrations of GABA.

Identification of 5-HT(3A) and 5-HT(3B) receptor subunits in mammalian retinae: potential pre-synaptic modulators of photoreceptors

Although serotonin (5-HT) is found in the mammalian retina only at low levels, considerable evidence suggests that it plays a role in visual processing. Pharmacological experiments indicate that numerous receptors for 5-HT are present in the mammalian retina. One of these is the ionotropic 5-HT(3) receptor. So far, two subunits for this receptor have been identified in the nervous system, 5-HT(3A) and 5-HT(3B). Co-expression of these subunits in Xenopus oocytes is sufficient to reconstitute native 5-HT(3) receptor properties. Thus, it is believed that a native neuronal 5-HT(3) receptor is multimeric similar to the related acetylcholine receptor family. To determine whether this receptor is expressed in the mammalian retina, we first performed reverse transcription polymerase chain reaction and first demonstrated the presence of transcripts for both the 5-HT(3A) and 5-HT(3B) receptor subunits. Then using a well-characterized polyclonal antiserum against the 5-HT(3A) receptor subunit, we demonstrated 5-HT(3A) receptor immunoreactivity (IR) in the rabbit, rat, and human retina. This IR was localized specifically to the rod photoreceptor terminals in all three species, suggesting that this receptor may modulate the rod signaling pathway by controlling the output at the rod terminals.

Confirmation of the retinopetal/centrifugal nature of the tyrosine hydroxylase-immunoreactive fibers of the retina and optic nerve in the weaver mouse

The number of tyrosine hydroxylase-immunoreactive fibers in the nerve fiber layer is increased in the retina of the weaver compared to control mice (Dev. Brain Res. 121 (2000) 113). To confirm the retinopetal/centrifugal nature of these fibers, a newly devised whole-mounted optic nerve technique allowed us to determine, during development, their first appearance within the optic nerve (post-natal day 12) compared to retina (post-natal day 13). One such fiber was also observed looping in the retina of a monkey fetus.

Paradoxical increase of tyrosine hydroxylase-immunoreactive retinopetal fibers in the weaver mouse

Weaver mice undergo apoptosis of the granule cell precursors of the cerebellum and nonapoptotic death of mesencephalic dopaminergic cells during post-natal development. In contrast, the number of retinal dopaminergic cells was transiently increased in weaver compared to control mice [C. Savy, E. Martin-Martinelli, A. Simon, C. Duyckaerts, C. Verney, C. Adelbrecht, R. Raisman-Vozari, J. Nguyen-Legros, Altered development of dopaminergic cells in the retina of weaver mice, J. Comp. Neurol. 1999;412:656-668]. While re-examining the retinas, we observed, in the nerve fiber layer, retinopetal tyrosine hydroxylase-immunoreactive fibers, which were dramatically increased in number throughout development and adulthood in the weaver compared to control mice.

Identification of the cellular source of laminin beta2 in adult and developing vertebrate retinae

The interphotoreceptor matrix (IPM) is a specialized extracellular matrix that surrounds the inner and outer segments of photoreceptors. This matrix contains molecules that may be important in directing photoreceptor differentiation and survival. For example, one molecule that we have previously identified as a component of the IPM, laminin beta2 (formerly known as s-laminin), is implicated in the differentiation of rod photoreceptor cells. Developmentally, laminin beta2 is present before rod birth in a position that is consistent with a role in directing rod differentiation; it is found, in both the rat and skate, in the ventricular space that ultimately becomes the IPM. In this study, we identify the source of laminin beta2 in the adult and developing retina. Both immunohistochemistry in the adult skate retina and in situ hybridizations in the adult rat retina reveal that laminin beta2 is produced by Müller cells. In addition, in the skate but not the rat retina, retinal pigment epithelial cells may be an alternative source of laminin beta2. During development, however, laminin beta2 is present before the birth of Müller glial cells; at this stage of development, laminin beta2 RNA is present within the neuroepithelial layer in a pattern that is consistent with its production by neuroepithelial cells.

Enrichment of glutamate-like immunoreactivity in the retinotectal terminals of the viper Vipera aspis: an electron microscope quantitative immunogold study

A post-embedding immunogold study was carried out to estimate the immunoreactivity to glutamate in retinal terminals, P axon terminals and dendrites containing synaptic vesicles in the superficial layers of the optic tectum of Vipera. Retinal terminals, identified following either intraocular injection of tritiated proline, horseradish peroxidase (HRP) or short-term survivals after retinal ablation, were observed to be highly glutamate-immunoreactive. A detailed quantitative analysis showed that about 50% of glutamate immunoreactivity was localized over the synaptic vesicles, 35.8% over mitochondria and 14.2% over the axoplasmic matrix. The close association of immunoreactivity with the synaptic vesicles could indicate that Vipera retino-tectal terminals may use glutamate as their neurotransmitter. P axon terminals and dendrites containing synaptic vesicles, strongly gamma-aminobutyric (GABA)-immunoreactive, were shown to be also moderately glutamate-immunoreactive, but two to three times less than retinal terminals. Moreover, in P axon terminals, the glutamate immunoreactivity was denser over mitochondria than over synaptic vesicles, possibly reflecting the 'metabolic' pool of glutamate, which serves as a precursor in the formation of GABA.

Dopaminergic and GABAergic retinal cell populations in mammals

A number of modern techniques now allow histologists to characterize subpopulations of retinal neurons by their neurotransmitters. The morphologies and connections of these chemically defined neurons can be analyzed precisely at both light and electron microscope levels and lead to a better understanding of retinal circuitry. The dopaminergic neurons form a loose population of special wide-field amacrine cells bearing intraretinal axons within the inner plexiform layer. One subtype, the interplexiform cell, sends an axon to the outer plexiform and outer nuclear layers. The number of interplexiform cells is variable throughout mammalian species. The GABAergic neurons form a dense and heterogeneous population of amacrine cells branching at all levels of the inner plexiform layer. The presence of GABA in horizontal cells seems to be species-dependent. Close relationships occur between dopaminergic and GABAergic cells. GABA antagonizes a number of dopaminergic actions by inhibiting both the release and synthesis of dopamine. This inhibition can be supported by GABA synapses onto dopaminergic cells, but GABA can also diffuse to its targets. Finally, GABA is also contained and synthesized in dopaminergic cells. This colocalization might be the basis of an intracellular modulation of dopamine by GABA.

Two classes of bipolar cell in the retina of the skate Raja erinacea

We have used immunoreactions against serotonin and protein kinase C to visualize two distinct classes of bipolar cell in the all-rod retina of the skate, Raja erinacea. To enhance the immunoreaction in serotonin-accumulating bipolar cells, prior to fixation, some retinas were incubated in Ringer's solution containing serotonin and pargyline. We found the somata of serotonin-accumulating bipolar cells to be located slightly distal to the midline of the inner nuclear layer. With increasing eccentricity from the visual streak, the size of the perikarya increases, concomitant with a decline in density of their distribution. Dendrites emanate from stout primary stalks and branch out before reaching the outer plexiform layer. Axons are bistratified within the inner plexiform layer with ramifications at the border of strata 1 and 2 and in stratum 4. The overall morphology of serotonin-accumulating bipolar cells is similar to that of serotonin-accumulating OFF bipolar cells of other non-mammalian vertebrates. Protein kinase C immunoreactive cells display the typical appearance of rod bipolar cells. Somata of protein kinase C immunoreactive bipolar cells are spindle-shaped and located distal to the serotonin-accumulating bipolar cells. Dendrites of these bipolars do not ramify before reaching the outer plexiform layer. Thin axons of protein kinase C immunoreactive bipolar cells end in large, club-shaped terminals in stratum 5 of the inner plexiform layer, bearing a striking similarity to axon terminals of mammalian ON rod bipolar cells. Our findings suggest that the all-rod retina of the skate contains at least two distinct vertical pathways including an OFF bipolar cell pathway in addition to a classical rod ON bipolar pathway.

A retinal dark-light switch: a review of the evidence

We propose that there exists within the avian, and perhaps more generally in the vertebrate retina, a two-state nonadapting flip-flop circuit, based on reciprocal inhibitory interactions between the photoreceptors, releasing melatonin, the dopaminergic amacrine cells, and amacrine cells which contain enkephalin-, neurotensin-, and somatostatin-like immunoreactivity (the ENSLI amacrine cells). This circuit consists of two loops, one based on the photoreceptors and dopaminergic amacrine cells, and the other on the dopaminergic and ENSLI amacrine cells. In the dark, the photoreceptors and ENSLI amacrine cells are active, with the dopaminergic amacrine cells inactive. In the light, the dopaminergic amacrine cells are active, with the photoreceptors and ENSLI amacrine cells inactive. The transition from dark to light state occurs over a narrow (< 1 log unit) range of low light intensities, and we postulate that this transition is driven by a graded, adapting pathway from photoreceptors, releasing glutamate, to ON-bipolar cells to dopaminergic amacrine cells. The properties of this pathway suggest that, once released from the reciprocal inhibitory controls of the dark state, dopamine release will show graded, adapting characteristics. Thus, we postulate that retinal function will be divided into two phases: a dopamine-independent phase at low light intensities, and a dopamine-dependent phase at higher light intensities. Dopamine-dependent functions may show two-state properties, or two-state properties on which are superimposed graded, adapting characteristics. Functions dependent upon melatonin, the enkephalins, neurotensin, and somatostatin may tend to show simpler two-state properties. We propose that the dark-light switch may have a role in a range of light-adaptive phenomena, in signalling night-day transitions to the suprachiasmatic nucleus and the pineal, and in the control of eye growth during development.

GABA transport and calcium dynamics in horizontal cells from the skate retina

1. Changes in intracellular calcium concentration [Ca2+]i in response to extracellularly applied gamma-aminobutyric acid (GABA) were studied in isolated horizontal cells from the all-rod skate retina. 2. Calcium measurements were made using fura-2 AM, both with and without whole-cell voltage clamp. Superfusion with GABA, in the absence of voltage clamp, resulted in an increase in [Ca2+]i; the threshold for detection was approximately 50 microM GABA, and a maximal response was elicited by 500 microM GABA. 3. The rise in [Ca2+]i was not mimicked by baclofen nor was it blocked by phaclofen, picrotoxin or bicuculline. However, the GABA-induced [Ca2+]i increase was completely abolished when extracellular sodium was replaced with N-methyl-D-glucamine. 4. With the horizontal cell voltage clamped at -70 mV, GABA evoked a large inward current, but there was no concomitant change in [Ca2+]i. Nifedipine, which blocks L-type voltage-gated Ca2+ channels, suppressed the GABA-induced increase in [Ca2+]i. These findings suggest that the calcium response was initiated by GABA activation of sodium dependent electrogenic transport, and that the resultant depolarization led to the opening of voltage-gated Ca2+ channels, and a rise in [Ca2+]i. 5. The GABA-induced influx of calcium appears not to have been the sole source of the calcium increase. The GABA-induced rise in [Ca2+]i was reduced by dantrolene, indicating that internal Ca2+ stores contributed to the GABA-mediated Ca2+ response. 6. These observations demonstrate that activation of the GABA transporter induces changes in [Ca2+]i which may have important implications for the functional properties of horizontal cells.

The occurrence of dopaminergic interplexiform cells correlates with the presence of cones in the retinae of fish

Using light-microscopic immunocytochemistry against tyrosine hydroxylase, we have investigated the morphology of dopaminergic cells in 23 species of fishes representing various systematic classes and subclasses and which live in very different habitats. We have, for the first time, observed teleosts with dopaminergic amacrine cells. Thus, in both bony and cartilaginous fishes, dopaminergic cells are differentiated as interplexiform and amacrine cells. The differentiation of dopaminergic cells into amacrine or interplexiform cells in fishes correlates with the absence or presence of cones. In pure-rod retinae, they occur as amacrine cells, and in mixed rod/cone retinae, they occur as interplexiform cells. We conclude therefore that the differentiation of retinal dopaminergic cells in fish does not depend on the evolutionary or systematic classification of a given species. Rather, it is correlated with the occurrence of rods and/or cones, and thus linked more closely to the habitat. We argue that, in fish, the presence of cones and cone-specific horizontal cells may be responsible for inducing dopaminergic cells to differentiate as interplexiform cells. Possible functions of dopamine in all-rod retinae, which may not require adaptation, may include neuromodulation in the inner plexiform layer for the sensitization of the rod pathway, the shaping of biological rhythms, and the control of eye growth.

Afferent and efferent connections of the habenula in the larval sea lamprey (Petromyzon marinus L.): an experimental study

The habenula is an integrative center between the striatum and the limbic and motor systems. With the aim of achieving further understanding of the evolution of this structure in vertebrates, we carried out an experimental study of the afferent and efferent connections of the habenula of larval sea lamprey. Experimental procedures included in vivo and in vitro transport after injections of horseradish peroxidase (HRP) into the habenula, telencephalon, pineal organ, dorsal thalamus, and posterior tubercle as well as carbocyanine dye tracing (DiI). The combined results of these experiments showed that the pattern of habenular connections is very simple. Most afferents appear to originate from the lobus subhippocampalis and neighboring area, whereas the only efferents found coursed in the fasciculus retroflexus to the neuropil of the nucleus interpeduncularis. This neuropil comprises a commissural region in the rostral mesencephalon, two long bilateral areas extending in the basal mesencephalon and medulla oblongata to the trigeminal level, and, finally, a caudal commissural zone. The conspicuous habenular commissure contains interhemispheric fibers that appear to form occasional contacts within the habenulae. The lamprey habenula also receives a few immunocytochemically identified fibers (somatostatinergic, catecholaminergic, and serotoninergic fibers) from other sources.

Are the dopaminergic cells of the lamprey retina interplexiform cells? A dopamine, tyrosine hydroxylase and dopamine beta-hydroxylase immunocytochemical study

The distribution of dopaminergic cells in the retina of sea lamprey Petromyzon marinus L. was studied with antisera against dopamine and tyrosine hydroxylase. Both antisera consistently labelled cells and processes located in the inner nuclear and inner plexiform layers. No dopamine-immunoreactive processes were observed in the external retina. The ellipsoids of photoreceptors and small bodies close to the external limiting membrane were also labelled with the tyrosine hydroxylase antiserum. It is concluded that the dopaminergic cells of the lamprey retina are amacrine cells. In addition, the presence of dopamine-beta-hydroxylase-like-immunoreactive fibres of central origin is reported for the first time.

Serotonin-like immunoreactivity in the adult and developing retina of the leopard frog Rana pipiens

Recent work in nonmammalian vertebrate retinas has suggested that other cell types besides the generally accepted amacrine cells may contain serotonin. We have used immunocytochemical methods to study serotonin-like immunoreactivity (5-HTLI) in the retina of the developing and mature frog Rana pipiens. In the adult, two types of serotonin immunoreactive (5-HT-ir) cells were found in the inner nuclear layer (INL) of the retina. Additionally, a large population of cells in the retinal ganglion cell layer (RGCL) had 5-HTLI. These cells were grouped into three types based on their soma size and their primary dendritic branching pattern. The optic nerve fiber layer was also intensely stained with serotonin antisera although staining intensity decreased progressively as the fibers approached the optic nerve head. Severing the optic nerve resulted in 5-HT-ir elements that extended up the optic nerve shaft from the lesion site toward the retina. Both regional and temporal changes in the pattern of 5-HTLI were seen. In middle regions of retina, approximately 30% of the cells in the RGCL were 5-HT-ir. Nasal and temporal regions of central retina had significantly fewer 5-HT-ir cells. Early in development only scattered cells in the RGCL were 5-HT-ir. As the animals matured there was an increase in both the proportion and the staining intensity of these cells. Our results suggest that in studying the function and development of the visual system in this animal, the role of serotonin must be examined.

The effects of lowered extracellular sodium on gamma-aminobutyric acid (GABA)-induced currents of Muller (glial) cells of the skate retina

1. The effects of external sodium on GABA-induced chloride currents were examined with whole-cell voltage-clamp recordings obtained from enzymatically dissociated solitary Muller cells in culture. Our goal was to determine whether a sodium-dependent GABA uptake mechanism influences the GABAa-mediated responses of skate Muller cells. 2. At low concentrations of GABA (0.01 to 0.5 microM), removal of sodium from the external solution resulted in a marked increase in the ligand-gated currents mediated by activation of GABAa receptors. The enhancement by lowered sodium was greatest at hyperpolarizing potentials and decreased progressively as the cell was depolarized. 3. The reversal potential for the GABA-induced response was not significantly altered by the removal of sodium, suggesting that sodium ions did not directly contribute to the GABAa-mediated current. 4. Lowering external sodium had no effect on the currents induced by the GABAa-agonist muscimol, consistent with its much lower affinity for the GABA transport carrier. 5. Application of the GABA uptake blocker nipecotic acid also abolished the effects of lowered sodium. 6. These findings suggest that the effects of lowered external sodium resulted from a decrease in the uptake of GABA into the Muller cells, thus raising the effective concentration of GABA acting upon the GABAa receptors.

Synaptic organization of dopaminergic amacrine cells in the larval tiger salamander retina

The ultrastructural features and synaptic interactions of tyrosine hydroxylase-like-immuno-reactive amacrine cells in the larval tiger salamander retina were examined using routine immunoelectron microscopy. The somas of tyrosine hydroxylase-like-immunoreactive amacrine cells were immunostained evenly throughout their cytoplasm. Their nuclei were generally unstained and possessed indented nuclear membranes. The processes of tyrosine hydroxylase-like-immunoreactive amacrine cells were homogeneously stained with the exception of their mitochondria, whose morphology was often disrupted by the staining procedure. Tyrosine hydroxylase-like-immunoreactive amacrine cell processes were characterized by an occasional dense-cored vesicle(s), in addition to a generally homogeneous population of small, round, agranular synaptic vesicles. They formed conventional synaptic junctions that were characterized by symmetrical synaptic membrane densities. A total of 168 synapses were observed that involved tyrosine hydroxylase-like-immunoreactive amacrine cell processes. A large percentage (79.8%) of these synaptic arrangements were found in sublayer 1 of the inner plexiform layer, while substantially lower percentages were observed in sublayers 3 (9.5%) and 5 (10.7%). They served as pre- and postsynaptic elements 63.1 and 36.9% of the time, respectively. Tyrosine hydroxylase-like-immunoreactive amacrine cell processes were presynaptic to amacrine cell processes (36.9% of total synaptic involvement) and processes that lack synaptic vesicles and whose origin remains uncertain (26.2%). They received synaptic input primarily from amacrine cell processes (31.0%). Tyrosine hydroxylase-like-immunoreactive amacrine cell processes also received a few ribbon synapses from bipolar cells (5.9%). Each of these synaptic relationships were observed in each of sublayers 1, 3 and 5 of the inner plexiform layer, with the majority of each arrangement being found in sublayer 1.

S-laminin expression in adult and developing retinae: a potential cue for photoreceptor morphogenesis

The development of the neural retina follows a stereotyped time course that begins with an undifferentiated neuroepithelium populated by multipotential progenitor cells and ends with a highly differentiated tissue containing diverse cell types. The identities of the factors that guide this differentiation have remained elusive; a likely location for such factors, however, is the extracellular environment. Here, we show that the extracellular matrix component s-laminin is present in the neural retina, that s-laminin expression parallels the differentiation of rod photoreceptors, that photoreceptors interact with s-laminin in vitro, and that antibodies to s-laminin profoundly reduce the appearance of cells that express rhodopsin in vitro. These data suggest that s-laminin plays a role in the differentiation of the neural retina and provide evidence that the composition of the extracellular matrix may be an important determinant of retinal differentiation.

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)

Immunohistochemical localization of retinal serotonin cells in the lamprey (Lampetra fluviatilis)

Light-microscopic immunocytochemistry was used to study serotonin (5HT)-containing retinal cells in the lamprey (Lampetra fluviatilis). Observations of sections and flatmounted retinas enabled us to distinguish four principal types of 5HT-immunoreactive neurons, on the basis of the localization of their somata and the arrangement of their processes in the inner plexiform layer, (IPL). Type 1 cell bodies (9 micron mean diameter) were numerous and were found in the innermost row of the inner nuclear layer (INL). They sent their processes into a dense plexus in sublamina a of the IPL. Type 2 cell bodies (12 micron mean diameter) were observed near the inner limiting membrane, their processes forming a plexus in sublamina b of the IPL. Most of the type 3 cells were bistratified, their cell bodies (similar in dimension to type 1) were located in the INL and their dendrites projected to both plexuses. Type 4 cell bodies (15 micron mean diameter) were observed in the middle of the IPL and could be compared with the interstitial described elsewhere. Their processes probably ended in the 5HT plexus of sublamina b but because of their sinuous course in the IPL, we could not affirm this fact. Most of 5HT immunoreactive cells were thought to be amacrine cells, but the presence of some thin processes emerging either from the soma or the primary dendrite, principally in type 1 and 2 cells, raises the possibility that some ganglion cells could be 5HT immunoreactive. The organization of the 5HT processes into two plexuses located in sublaminae a and b of the IPL resemble the functional ON and OFF pathway seen in the other vertebrates.

Effects of gamma-aminobutyric acid on skate retinal horizontal cells: evidence for an electrogenic uptake mechanism

In the retinae of many vertebrates, there are classes of horizontal cell that probably utilize gamma-aminobutyric acid (GABA) as a neurotransmitter. As with other amino acid transmitter agents, the postsynaptic action of GABA is thought to be terminated by uptake into neurons and glia surrounding the release site. The present study examined whether an uptake system for GABA could be detected in isolated skate horizontal cells by means of electrophysiological methods. Pressure ejection of GABA onto voltage-clamped horizontal cells produced an inward current that showed no sign of desensitization regardless of the GABA concentration. The dose-response relationship followed simple Michaelis-Menten kinetics, with a half-maximal response elicited at approximately 110 microM. Nipecotic acid produced a similar current and reduced the responses to GABA when introduced in the bath solution prior to the GABA pulse. On the other hand, application of 500 microM muscimol or 1 mM baclofen, GABAA and GABAB receptor agonists, respectively, were completely without effect. The GABA-induced current was not blocked by superfusion with 500 microM bicuculline, 500 microM picrotoxin, or 500 microM phaclofen. However, the responses to GABA were abolished when the cells were superfused in Ringer's solution in which choline or lithium had been substituted for sodium, and were reduced when the extracellular chloride concentration was decreased from 266 mM to 16 mM. Current-voltage data showed a maximal response to GABA when the cells were held at or below their resting potential. At more depolarized levels, the inward current became progressively smaller until, near +50 mV, it could no longer be detected; over the range tested (-90 to +50 mV), the response never reversed into an outward current. These findings suggest that the GABA-induced currents in skate horizontal cells are mediated by an electrogenic uptake mechanism.

Synaptic and voltage-gated currents in interplexiform cells of the tiger salamander retina

We have correlated the membrane properties and synaptic inputs of interplexiform cells (IPCs) with their morphology using whole-cell patch-clamp and Lucifer yellow staining in retinal slices. Three morphological types were identified: (a) a bistratified IPC with descending processes ramifying in both sublaminas a and b of the inner plexiform layer (IPL), and an ascending process that branched in the outer plexiform layer (OPL) and originated from the soma, (b) another bistratified IPC with descending processes ramifying in both sublaminas a and b, and an ascending process that branched in the OPL and originated directly from IPC processes in the IPL, and (c) a monostratified IPC with a descending process ramifying over large lateral extents within the most distal stratum of the IPL, and sending an ascending process to the OPL with little branching. Similar voltage-gated currents were measured in all three types including: (a) a transient inward sodium current, (b) an outward potassium current, and (c) an L-type calcium current. All cells generated multiple spikes with frequency increasing monotonically with the magnitude of injected current. The IPCs that send their descending processes into both sublaminas of the IPL (bistratified) receive excitatory synaptic inputs at both light ON and OFF that decay with a time constant of approximately 1.3 s. Slowly decaying excitation at both ON and OFF suggests that bistratified IPCs may spike continuously in the presence of a dynamic visual environment.

Presumptive catecholaminergic ganglion cells in the pigeon retina

An antiserum directed against tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of dopamine, was used to study the pigeon retina. Labeled cells were observed in both the inner nuclear layer (INL) and ganglion cell layer (GCL). Two populations of TH-immunoreactive neurons were observed in the INL. Some of these cells were 7-10 microns in diameter and gave rise to processes that arborized in three layers of the inner plexiform layer (IPL). These cells appeared similar to the dopaminergic amacrine cells described previously (Marc, 1988). Other labeled cells in the INL were 12-20 microns in diameter and were recognizable as a previously described subpopulation of TH-immunoreactive displaced ganglion cells (Britto et al., 1988). A population of labeled cells was observed in the GCL. Counts of these cells in two retinae revealed 5000 and 7000 cells, respectively. They ranged in size from 8-15 microns in diameter in the central retina and from 8-20 microns in diameter in the peripheral retina. The density of labeled cells was highest in the central retina and red field and lowest in the retinal periphery. The difference in cell size and cell density as a function of eccentricity is characteristic of the total population of ganglion cells in the avian retina (Ehrlich, 1981; Hayes, 1982). Some of the TH-positive cells in the GCL could be classified as ganglion cells for two reasons: (1) The axons of many of the TH-positive cells in the GCL were TH-immunoreactive as well and could be followed to the optic nerve head. (2) The injection of rhodamine-labeled microspheres into the nucleus geniculatus lateralis, pars ventralis (GLv), resulted in the retrograde labeling of many of the TH-positive cells in the contralateral retina.

Sequential course of uptake of intravitreal 5,7-dihydroxytryptamine by carp retinal cells

The sequential course of uptake by retinal cells of intravitreally injected 5,7-dihydroxytryptamine (5,7-DHT) together with dopamine (DA) was investigated in juvenile carp retinas, which were removed at various intervals (1-24 h) after injection. The cells taken up 5,7-DHT were visualized immunohistochemically with anti-serotonin (5-HT) antibody and FITC-conjugated IgG. After a mixture of 5,7-DHT and DA (2.5, 10 or 20 micrograms each) was given, large-sized indoleamine (IA) amacrine cells first (1-4 h), and then small-sized indoleamine-accumulating amacrine amacrine (IAA) cells (4-12 h), bipolar cells (8-12 h) and in some cases photoreceptor cells (12-24 h) were sequentially observed, and finally the immunoreactive structures almost disappeared around 24 h after injection. When the mixture of 5,7-DHT and DA (10 micrograms each) was injected into the eyes of reserpinized fish, the same sequential uptake of 5,7-DHT was seen in a faster time course, but additionally various classes of retinal cells (horizontal, ganglion and Müller cells) became visible as irregular clusters. However, DA cells were never visualized at any stages of all the experiments, indicating that DA cells do not take up 5,7-DHT in the carp retina, which was further confirmed by double labeling of 5-HT- and tyrosine hydroxylase-like immunoreactive cells. Double labeling also revealed that 5,7-DHT-accumulating bipolar cells appear to represent a subclass different from that of protein kinase C-like immunoreactive bipolar cells.

Synaptic organization of serotonin-like immunoreactive amacrine cells in the larval tiger salamander retina

Immunoelectron microscopy was used to investigate the ultrastructural features and synaptic relationships of serotonin-like immunoreactive amacrine cells in the larval tiger salamander retina. Serotonin-positive somas exhibited an evenly distributed peroxidase reaction product throughout their cytoplasm. Their nuclei were unstained and possessed indented nuclear membranes. Serotonin-immunoreactive processes were generally stained throughout with the exception of their mitochondria, whose morphology was often disrupted by the staining reaction. They were further characterized by an occasional dense-cored vesicle/s in addition to a generally homogeneous population of small, round, clear synaptic vesicles. Serotonin-immunoreactive amacrine cell processes formed conventional synapses that were characterized by symmetrical synaptic membrane densities. A total of 222 synaptic arrangements were observed that involved the immunostained processes of serotonin-amacrine cells. As presynaptic elements, they primarily contacted amacrine cells processes (37.8%). They also provided substantial synaptic input to processes that lacked synaptic vesicles (16.2%) and whose origin was unidentified. Serotonin-processes provided a far fewer number of synaptic contacts onto the processes of bipolar cells (1.4%) and the somas of cells in the amacrine cell layer (0.5%). As postsynaptic elements, they received synaptic inputs from amacrine cells (27.9%) and bipolar cells (16.2%). With the exception of their synapses onto bipolar cells and the somas of cells in the amacrine cell layer, each of the synaptic relationships of serotonin-amacrine cells was observed in each of sublayers 1-5 of the inner plexiform layer.

Structural and functional properties of two types of horizontal cell in the skate retina

Two morphologically distinct types of horizontal cell have been identified in the all-rod skate retina by light- and electron-microscopy as well as after isolation by enzymatic dissociation. The external horizontal cell is more distally positioned in the retina and has a much larger cell body than does the internal horizontal cell. However, both external and internal horizontal cells extend processes to the photoreceptor terminals where they end as lateral elements adjacent to the synaptic ribbons within the terminal invaginations. Whole-cell voltage-clamp studies on isolated cells similar in appearance to those seen in situ showed that both types displayed five separate voltage-sensitive conductances: a TTX-sensitive sodium conductance, a calcium current, and three potassium-mediated conductances (an anomalous rectifier, a transient outward current resembling an A current, and a delayed rectifier). There was, however, a striking difference between external and internal horizontal cells in the magnitude of the current carried by the anomalous rectifier. Even after compensating for differences in the surface areas of the two cell types, the sustained inward current elicited by hyperpolarizing voltage steps was a significantly greater component of the current profile of external horizontal cells. A difference between external and internal horizontal cells was seen also in the magnitudes of their TEA-sensitive currents; larger currents were usually obtained in recordings from internal horizontal cells. However, the currents through these K+ channels were quite small, the TEA block was often judged to be incomplete, and except for depolarizing potentials greater than or equal to +20 mV (i.e., outside the normal operating range of horizontal cells), this current did not provide a reliable indicator of cell type. The fact that two classes of horizontal cell can be distinguished by their electrophysiological responses, as well as by their morphological appearance and spatial distribution in the retina, suggests that they may play different roles in the processing of visual information within the retina.

Glutamate-immunoreactivity in the retina and optic tectum of goldfish

Glutamate was immunohistochemically localized in the goldfish retina and tectum at the light and electron microscopic (E.M.) levels using double affinity purified antisera against glutaraldehyde conjugated L-glutamate. In retina, glutamate-immunoreactivity (Glu+) was observed in cone inner segments, cone pedicles, bipolar cells, a small number of amacrine cells and the majority of cells in the ganglion cell layer. The latter were shown to be ganglion cells by simultaneous retrograde labeling. Centrally, Glu+ was observed in axons in the optic nerve and tract, and in stratum opticum and stratum fibrosum et griseum superficialis (SFGS) of the tectum. The Glu+ in the optic pathway disappeared four days after optic denervation and was restored by regeneration without affecting the Glu+ of intrinsic tectal neurons. In tectum, Glu+ was also observed in torus longitudinalis granule cells, toral terminals in stratum marginale, some pyramidal neurons in the SFGS, multipolar and fusiform neurons in stratum griseum centrale, large multipolar and pyriform projection neurons in stratum album centrale, and many periventricular neurons. Glu+ was also localized within unidentified puncta throughout the tectum and within radially oriented dendrites of periventricular neurons. At the E.M. level, a variety of Glu+ terminals were observed. Glu+ toral terminals formed axospinous synapses with dendritic spines of pyramidal neurons. Ultrastructurally identifiable Glu+ putative optic terminals formed synapses with either Glu+ or Glu- dendritic profiles, and with Glu- vesicle-containing profiles, presumed to be GABAergic. These findings are consistent with the hypothesis that a number of intrinsic and projection neurons in the goldfish retinotectal system, including most ganglion cells, may use glutamate as a neurotransmitter.

Localization of serotoninlike-immunoreactive amacrine cells in the larval tiger salamander retina

Light microscopic immunocytochemistry was used to study the populations of serotoninlike-immunoreactive cells in the larval tiger salamander retina. Of 1,135 serotonin-immunostained cells observed in transverse cryosections, 87% were identified as amacrine cells, whereas 13% were tentatively designated as displaced amacrine cells. The somas of the vast majority of serotonin-amacrine cells were situated in the innermost cell row of the inner nuclear layer. Only a few serotonin-immunostained amacrine cell somas were observed in the second row of cells from the inner nuclear layer. Serotonin-immunoreactive processes generally appeared as a diffuse plexus distributed evenly throughout all levels of the inner plexiform layer. As determined in whole-mount preparations, serotonin-amacrine cells were divisible into two populations on the basis of the diameters of their somas. Large cells (45%) ranged from 16 to 19 microns in diameter with the vast majority measuring 17-18 microns. Smaller and sometimes less intensely stained cells ranged from 14 to 16 microns in diameter with the large majority measuring 15 microns. The diameters of serotonin-displaced amacrine cells ranged from 19 to 22 microns with the large majority measuring 20 microns in diameter. An examination of whole-mount retinas revealed that serotonin-immunoreactive amacrine and displaced amacrine cells were distributed throughout the center and the periphery of the retina. The density of serotonin-amacrine cells (large and small combined) was calculated to be 173 +/- 4.5 (mean +/- standard error) cells per mm2.

gamma-Aminobutyric acid (GABA)-induced currents of skate Muller (glial) cells are mediated by neuronal-like GABAA receptors

Radial glia (Muller cells) of the vertebrate retina appear to be intimately involved in regulating the actions of amino acid neurotransmitters. One of the amino acids thought to be important in mediating retinal information flow is gamma-aminobutyric acid (GABA). The findings of this study indicate that enzymatically isolated skate Muller cells are depolarized by GABA and the GABAA agonist muscimol and that the actions of these agents are reduced by bicuculline and picrotoxin. Membrane currents induced by GABA under voltage clamp were dose dependent, were associated with an increase in membrane conductance, and showed marked desensitization when the concentration of GABA exceeded 2.5 microM. The responses had a reversal potential close to that calculated for chloride, indicating that the currents were generated by ions passing through channels. These data support the view that skate Muller cells possess functional GABAA receptors. The presence of such receptors on retinal glia may have important implications for the role of Muller cells in maintaining the constancy of the extracellular milieu, for neuron-glia interactions within the retina, and for theories concerning the generation of the electroretinogram.

GABA as a potential transmitter in lizard photoreceptors: immunocytochemical and biochemical evidence

The retina of the desert night lizard, Xantusia vigilis, was examined for immunoreactivity to antibodies against gamma-aminobutyric acid and L-glutamate decarboxylase. At the electron microscopic level it was found that a distinct population of the photoreceptor cells was immunoreactive to both antibodies. Computer-assisted reconstruction of serial sections positively identified the immunoreactive receptors as cones. These cones constituted 15% of the photoreceptors in the retinal sections, and they were morphologically distinct. The mean diameter of the labeled cone synaptic pedicles was 5.8 micron whereas that of the unlabeled pedicles was 7.9 micron, a statistically significant difference. L-glutamate decarboxylase was extracted from the lizard brain, positively identified radiometrically, and shown by immunodiffusion to crossreact with the antibody used for localization. The authors suggest that the immunoreactive cones synthesize and accumulate gamma-aminobutyric acid. Whether or not it is used by those cones as a neurotransmitter should be tested directly.

Bipolar cells in the turtle retina are strongly immunoreactive for glutamate

Strong glutamate immunoreactivity was observed by both light and electron microscopy in bipolar cells of the turtle (Pseudemys scripta elegans) retina after postembedding immunohistochemistry. Virtually all bipolar cells showed strong labeling, on average 18 times that of the Müller (glial) cells. The data suggest that both on- and off-center bipolar cells are glutamatergic. Photoreceptors were also labeled, but with a labeling intensity about half that of the bipolar cells. Other types of retinal neurons showed less immunoreactivity, except for a small population of strongly labeled amacrine cells.

Localization of putative GABAergic neurons in the larval tiger salamander retina by immunocytochemical and autoradiographic methods

Putative GABAergic neurons in the larval tiger salamander retina were localized by a comparative analysis of glutamate decarboxylase immunoreactivity (GAD-IR), GABA-like immunoreactivity (GABA-IR), and high-affinity 3H-GABA uptake at the light microscopical level. Preliminary data showed that all GAD-IR neurons were double labeled for GABA-IR. However, because the weak somatic labeling with GAD-IR, we could not determine if the converse were true. Neurons commonly labeled with GABA-IR and 3H-GABA uptake include horizontal cells, type I (outer) and type II (inner) bipolar cells, type I (inner) and type II (outer) amacrine cells, and cell bodies in the ganglion cell layer (GCL). In addition, interplexiform cells were identified with GABA-IR. The presence of GABA-IR ganglion cells was indicated by GABA-IR fibers in the optic fiber layer and optic nerve as well as by a GABA-IR cell in the GCL that included a labeled axon. The percentage of labeled somas in the inner nuclear layer (INL) compared to all cells in each layer was similar for the two methods: 30% in INL 1 (outer layer of somas), 15% in INL 2 (middle layer), 43-52% in INL 3 (inner layer), and about 21-26% in the GCL. Labeled processes were found in three bands in the inner plexiform layer, with the densest band located in the most proximal part. Postembedding labeling of 1-micron Durcupan resin sections for GABA-IR showed the same general pattern as obtained with 10-microns cryostat sections, with additional staining, however, of type II (inner) bipolar cell Landolt's clubs. Extensive colocalization of labeling was indicated, and we conclude that GABA-IR can serve as a valid and reliable marker for GABA-containing neurons in this retina and suggest that GABA serves as a transmitter for horizontal cells, several types of amacrine cell, a type of interplexiform cell, and perhaps a small percentage of type I and type II bipolar cells and ganglion cells.

The action of gamma-aminobutyric acid on the horizontal cells of the skate retina

The effects of gamma-aminobutyric acid (GABA) were studied in the superfused retina of the skate. Intracellular recordings were made from horizontal cells. After application of 500 microM GABA there was a depolarization of the membrane potential, a decrease in the light-evoked amplitude of the response and an increase in the duration of the waveform.

Neuropharmacological analysis of the role of indoleamine-accumulating amacrine cells in the rabbit retina

In order to elucidate the role of putative indoleaminergic amacrine cells in visual processing, we have employed pharmacological agents specific for the two classes of serotonin receptor, 5-HT2 and 5-HT1, which have been identified in both the retina and brain. Perfusion of the rabbit retina with 5-HT2 selective antagonists decreases the ON-excitation of all classes of ganglion cell as well as the spontaneous activity of these cells. The effect on OFF-responses depends on the cell type: OFF-excitation of center-surround brisk and sluggish cells is increased or not affected by these drugs, but OFF excitation of ON/OFF direction selective cells is reduced. No disruption of the trigger features of direction selective or orientation selective cells was discovered, suggesting that indoleaminergic amacrine cells do not play a role in the generation of the complex properties of these cells. 5-HT1 receptors are heterogeneous and classified as a, b, or c subtypes. Since no selective antagonists are available for these sites, we have employed specific agonists. The most specific of these are for the 5-HT1A receptor. Perfusion with these agonists had physiological effects similar to those with perfusion of 5-HT2 antagonists. Thus, we have suggested that these two classes of serotonin receptors mediate opponent processes in the neural pathway. Since indoleaminergic cells make reciprocal synaptic connections with rod bipolar cell terminals, which are depolarizing in the rabbit retina, we hypothesize that 5-HT2 receptors facilitate the synaptic transmission from the depolarizing rod bipolar cell thus facilitating ON-excitation in the retinal network while 5-HT1A receptors mediate an inhibitory process. Similar self-opponent processing appears to take place in the hypothalamic and hippocampal serotonergic systems as well as in the dopaminergic retinal system. Thus, it is likely that this organization is a general feature of monoamine signal transmission in the central nervous system.

Catecholaminergic subpopulation of retinal displaced ganglion cells projects to the accessory optic nucleus in the pigeon (Columba livia)

In birds, displaced ganglion cells (DGCs) constitute the exclusive source of retinal input to the nucleus of the basal optic root (nBOR) of the accessory optic system. Tyrosine-hydroxylase (TH) immunoreactivity was examined in the pigeon retina after injections of rhodamine-labeled microspheres into the nBOR. A population of about 400 DGCs was observed in each case to exhibit both TH immunoreactivity and rhodamine bead fluorescence. This corresponded to about 10-15% of the total number of identified DGCs in each retina. Double-labeled cells were medium- to large-size (12 to 20 microns in the largest axis) and were always located at the border between the inner nuclear and the inner plexiform layers. Their dendrites could be followed horizontally in lamina 1 of the inner plexiform layer for up to 300 microns from the cell body. The distribution of double-labeled DGCs appeared to be mostly peripheral, matching the overall distribution of identified DGCs. Larger DGCs (21-28 microns) were never seen to contain TH immunoreactivity. Examination of brain sections revealed plexuses of thin varicose TH-positive axons in all subdivisions of the nBOR. Unilateral enucleation produced an almost complete elimination of TH immunoreactivity in the contralateral nucleus. Such results suggest the existence of a population of catecholaminergic DGCs projecting into the accessory optic system of the pigeon. They also support the emerging hypothesis concerning the neurotransmitter heterogeneity of ganglion cells in the vertebrate retina.

Putative serotonergic bipolar and amacrine cells in the chicken retina

Four populations of putative serotonergic cells could be detected in the chicken retina by histofluorescence and immunohistochemistry. Numerous (10,000/mm2) small (6 micron diameter) bipolar cells were located towards the middle of the inner nuclear layer, as were sparser (1000/mm2) larger (12 micron diameter) amacrine cells. Very sparse large (greater than 30 micron diameter) and more numerous small (12 micron diameter) ganglion cells were also detected. Prominent fibre plexuses were detected in the inner plexiform layer, close to the inner nuclear and ganglion cell layers, and appeared to be formed by the processes of the bipolar cells, amacrine cells and at least the large ganglion cells. Exogenous serotonin (5-HT) was detected in the chicken retina. From the effects of neurotoxins on 5-HT levels and 5-HT-like immunoreactivity (5-HTLI), most of this appeared to be associated with the amacrine cells. 5-HTLI bipolar cells were selectively destroyed by intravitreal injections of 5-10 nmol of kainic acid, while 5-HTLI amacrine cells were destroyed by N-methyl-D,L-aspartic acid and 5,7-dihydroxytryptamine. The sensitivity of the bipolar cells to kainic acid indicates that they are OFF-cells.

Distribution of substance P-like immunoreactive retinal ganglion cells and their pattern of termination in the optic tectum of chick (Gallus gallus)

Substance P-like immunoreactive (SP-LI) neurons were identified within the inner nuclear layer and ganglion cell layer of the chick retina. The SP-LI cells in the inner nuclear layer consisted of several subtypes of neurons, differing in soma size and dendritic arborization. In the ganglion cell layer a population of moderately labelled SP-LI neurons was also present. About 6-9 microns in diameter and spaced 50-80 microns apart, they formed a regular array across the entire retina, with a density of about 400 cells/mm2 in the superior temporal retina, declining to less than 100 cells/mm2 in the peripheral retina. The total number of SP-LI cells in the ganglion cell layer was approximately 75,000. Individual axons could be followed toward the optic nerve head. Lesions near the optic nerve head resulted in axotomy of ganglion cells within a limited portion of the retina. Two days of postaxotomy there were numerous SP-LI swellings in the proximal segments of axotomized axons. SP-LI neurons in the axotomized zone were larger, more numerous, and showed increased staining of their processes. Fourteen days following a retinal lesion, there was depletion of all SP-LI cells in the ganglion cell layer within the axotomized zone, but the SP-LI neurons in the inner nuclear layer were not noticeably affected. Following a localized injection of rhodamine-coupled latex beads into the optic tectum, a population of retinal ganglion cells (RGCs) in the contralateral retina was retrogradely labelled. Many of these cells also exhibited SP-like immunoreactivity. Examination of the optic tectum indicated the presence of SP-LI fibres in laminae 2-13 (nomenclature of Cajal: Histologie du Systeme Nerveux. Vol. 2. Paris: Maloine, '11), with immunoreactive terminal regions present mainly in laminae 2-4, 7, and 9-13. SP-LI cell bodies were found predominantly in laminae 10-12 and 13. Fourteen days following a retinal lesion, SP-LI processes and terminals were depleted from laminae 2 and 3. Immunoreactive cells and processes in the remaining laminae of the optic tectum were not noticeably altered. The present report confirms the existence of SP-LI retinal ganglion cells in the chick retina and demonstrates their contribution to lamina specific SP-LI arborization in the optic tectum.

Distribution and morphology of dopaminergic amacrine cells in the retina of the turtle (Pseudemys scripta elegans)

A light microscopical study of the cell types that stain by immunohistochemistry for the synthesizing enzyme for dopamine, tyrosine hydroxylase, has been performed on the retina of the turtle Pseudemys scripta elegans. The immunostain can be localized to a single morphological type of amacrine cell. The cells are like A28 cells of a Golgi classification. They have medium sized dendritic fields that range in diameter from 200 to 700 micron with eccentricity from the visual streak. The amacrines have a tri-stratified dendritic tree with tiers of fine, curved dendrites ramifying in strata S1, lower S2 and the S4/5 border of the inner plexiform layer. We, like others, can find no good evidence that these cells are interplexiform cells. The dopaminergic amacrine cells have a low frequency (approximately 1300-1500 total cells in 130 mm2 retina), with their highest density occurring in the visual streak (60 cells per mm2). The density profiles fall in elliptical isodensity rings from the visual streak towards the peripheral retina. At all points on the retina the dendritic fields maintain a constant coverage factor independent of eccentricity. A comparison of the dopaminergic amacrine cells in the turtle and other vertebrate retinae is made.

Dendritic morphology of indoleamine cells revealed by intracellular injection of lucifer yellow in fixed carp retina

The dendritic morphology of indoleamine amacrine cells in carp retina was investigated by identifying their fluorescent cell bodies by preloading with noradrenaline followed by iontophoretic injection of Lucifer Yellow in isolated and aldehyde-fixed preparations under microscopic control. Although two subpopulations of serotonin-like immunoreactive amacrine cells (small and large in soma size) were found, small cells were not seen in aldehyde-fixed preparations. Cells preloaded with noradrenaline corresponded to large immunoreactive cells and were labeled with Lucifer Yellow. The cell bodies labeled were located at the innermost level of the inner nuclear layer, and gave rise to three to five primary dendrites which branched frequently and were found mainly in sublamina a of the inner plexiform layer. These cells examined in an intermediate region between the optic disc and the retinal periphery were pyriform in soma shape while dendritic fields were found or oval covering an area of 0.18 +/- 0.05 mm2 (510 +/- 80 microns in diameter). Cell density in this region was about 32 cells/mm2 and, therefore, their dendritic field coverage was approximately 6.0.

Gamma-aminobutyric acid- and glutamic acid decarboxylase-immunoreactive neurons in the retina of different vertebrates

The localization of gamma-aminobutyric acid (GABA)- and L-glutamate 1 carboxy-lyase (GAD)-immunoreactive neurons was compared in the skate, frog, pigeon, chicken, rabbit, and man. Horizontal cells show both GABA and GAD immunoreactivity in the skate, frog, and bird. Certain amacrine cells show GABA and GAD immunoreactivity in all species. The distribution of GABA- and GAD-immunoreactive cell bodies and cell processes was very similar, if not identical, in the skate and man. In the other species, cell populations with GAD immunoreactivity also showed GABA immunoreactivity. However, in the bird, frog, and rabbit, the GABA-immunoreactive amacrine cells were at least twice as numerous as the GAD-immunoreactive cells. In birds, the distributions of the GAD and GABA immunoreactivities were different in the sublayers of the inner plexiform layer. The reason for the difference is currently unknown. GABA-immunoreactive bipolar-like cells were seen in the frog.

Neuropeptide Y (NPY) immunoreactive neurons in the retina of different species

Neurons displaying Neuropeptide Y (NPY) immunoreactivity were found among amacrine cells in the retina of baboon, pig, cat, pigeon, chicken, frog, trout, carp and goldfish. The immunoreactive cell bodies were located in the middle and the innermost cell rows of the inner nuclear layer with processes forming one, two or three more or less well-defined sublayers in the inner plexiform layer. The location and the density of the sublayers varied with the species investigated. In the frog retina, bipolar-like cell bodies were found in the middle of the inner nuclear layer as well as sparsely occurring ovoid cell bodies in the ganglion cell layer. Like the amacrine cells, these cells emitted processes ramifying in three sublayers in the inner plexiform layer.