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

Ptf1a triggers GABAergic neuronal cell fates in the retina

Background

In recent years, considerable knowledge has been gained on the molecular mechanisms underlying retinal cell fate specification. However, hitherto studies focused primarily on the six major retinal cell classes (five types of neurons of one type of glial cell), and paid little attention to the specification of different neuronal subtypes within the same cell class. In particular, the molecular machinery governing the specification of the two most abundant neurotransmitter phenotypes in the retina, GABAergic and glutamatergic, is largely unknown. In the spinal cord and cerebellum, the transcription factor Ptf1a is essential for GABAergic neuron production. In the mouse retina, Ptf1a has been shown to be involved in horizontal and most amacrine neurons differentiation.

Results

In this study, we examined the distribution of neurotransmitter subtypes following Ptf1a gain and loss of function in the Xenopus retina. We found cell-autonomous dramatic switches between GABAergic and glutamatergic neuron production, concomitant with profound defects in the genesis of amacrine and horizontal cells, which are mainly GABAergic. Therefore, we investigated whether Ptf1a promotes the fate of these two cell types or acts directly as a GABAergic subtype determination factor. In ectodermal explant assays, Ptf1a was found to be a potent inducer of the GABAergic subtype. Moreover, clonal analysis in the retina revealed that Ptf1a overexpression leads to an increased ratio of GABAergic subtypes among the whole amacrine and horizontal cell population, highlighting its instructive capacity to promote this specific subtype of inhibitory neurons. Finally, we also found that within bipolar cells, which are typically glutamatergic interneurons, Ptf1a is able to trigger a GABAergic fate.

Conclusion

Altogether, our results reveal for the first time in the retina a major player in the GABAergic versus glutamatergic cell specification genetic pathway.

Circadian phototransduction and the regulation of biological rhythms

The vertebrate circadian system that controls most biological rhythms is composed of multiple oscillators with varied hierarchies and complex levels of organization and interaction. The retina plays a key role in the regulation of daily rhythms and light is the main synchronizer of the circadian system. To date, the identity of photoreceptors/photopigments responsible for the entrainment of biological rhythms is still uncertain; however, it is known that phototransduction must occur in the eye because light entrainment is lost with eye removal. The retina is also rhythmic in physiological and metabolic activities as well as in gene expression. Retinal oscillators may act like clocks to induce changes in the visual system according to the phase of the day by predicting environmental changes. These oscillatory and photoreceptive capacities are likely to converge all together on selected retinal cells. The aim of this overview is to present the current knowledge of retinal physiology in relation to the circadian timing system.

Structure and function of photoreceptor and second-order cell mosaics in the retina of Xenopus

The structure, physiology, synaptology, and neurochemistry of photoreceptors and second-order (horizontal and bipolar) cells of Xenopus laevis retina is reviewed. Rods represent 53% of the photoreceptors; the majority (97%) are green light-sensitive. Cones belong to large long-wavelength-sensitive (86%), large short-wavelength-sensitive (10%), and miniature ultraviolet wavelength-sensitive (4%) groups. Photoreceptors release glutamate tonically in darkness, hyperpolarize upon light stimulation and their transmitter release decreases. Photoreceptors form ribbon synapses with second-order cells where postsynaptic elements are organized into triads. Their overall adaptational status is regulated by ambient light conditions and set by the extracellular dopamine concentration. The activity of photoreceptors is under circadian control and is independent of the central body clock. Bipolar cell density is about 6000 cells/mm2 They receive mixed inputs from rods and cones. Some bipolar cell types violate the rule of ON-OFF segregation, giving off terminal branches in both sublayers of the inner plexiform layer. The majority of them contain glutamate, a small fraction is GABA-positive and accumulates serotonin. Luminosity-type horizontal cells are more frequent (approximately 1,000 cells/mm2) than chromaticity cells (approximately 450 cells/mm2). The dendritic field size of the latter type was threefold bigger than that of the former. Luminosity cells contact all photoreceptor types, whereas chromatic cells receive their inputs from the short-wavelength-sensitive cones and rods. Luminosity cells are involved in generating depolarizing responses in chromatic horizontal cells by red light stimulation which form multiple synapses with blue-light-sensitive cones. Calculations indicate that convergence ratios in Xenopus are similar to those in central retinal regions of mammals, predicting comparable spatial resolution.

Posttranslational mechanisms control the timing of bHLH function and regulate retinal cell fate

During central nervous system development, neurons are often born in a precise temporal sequence. Basic helix-loop-helix (bHLH) transcription factors are required for the development of specific subpopulations of neurons, but how they contribute to their ordered genesis is unclear. We show that the ability of bHLH factors to regulate the development of distinct neuronal subtypes in the Xenopus retina depends upon the timing of their function. In addition, we find that the timing of bHLH function can be regulated posttranslationally, so that bHLH factors with overlapping expression can function independently. Specifically, XNeuroD function in the retina can be inhibited by glycogen synthase kinase 3beta (GSK3beta), while Xath5 function can be inhibited by Notch. Thus, the potential of bHLH factors to regulate the development of neuronal subtypes depends upon the context in which they function.

Amacrine cells of the anuran retina: morphology, chemical neuroanatomy, and physiology

Amacrine cells are third-order retinal interneurons, projecting their processes into the inner plexiform layer. Historically, they were not considered as neurons first. By the middle of the 20th century, their neuronal nature was confirmed, and their enormous diversity established. Amacrine cells have been most successfully subdivided into morphological categories based on two parameters: diameter of the dendritic field and ramification pattern in the inner plexiform layer. Works combining anatomy, physiology, and neurochemistry are scarce and in the case of the anuran retina, the situation is even worse. Correlation between morphology, neurochemistry, and physiology is little studied. Here we try to build up a database and pinpoint some of the missing data. Obtaining those could help to better understand retinal function. Sporadic attempts did not make it possible to develop a comprehensive catalog of morphologically distinct amacrine cell types in the anuran retina. The number of morphologically identified amacrine cells currently stands at 16. The list of neurochemically identified distinct cell types can be given as follows: five types GABA-containing cell types with secondary markers and at least one without; two glycinergic cell types and one interplexiform cell where glycine colocalizes with somatostatin; one dopaminergic amacrine cell and also a variant of this with interplexiform morphology; two types of serotoninergic cells; three NADPHdiaphorase-positive cells, one substance P-positive cell type without identified second marker; one CCK-positive cell type without identified second marker and the calbindin positive cells (at least one but potentially more types). This adds up to 19 cell types, out of which two are interplexiform in character. This is more than that could be identified by purely morphological means. Out of Cajal's original 13 amacrine cell types described in the frog retina, 5 parallel unequivocally with neurons defined by neurochemistry. Three others have one close match each, but their exact identity is uncertain. The remaining amacrine cells have more than one potential matches. At the same time, on one hand the amacrine cell named two-layered by Cajal so far has no match among the neurochemically identified amacrine cells. On the other hand, the interplexiform subtype of the dopaminergic cell, the somatostatin-containing glycinergic interplexiform cell, the starburst cell, and the bistratified neuropeptide Y-immunoreactive cell have no match among Cajal's cells. All in all, the number of known amacrine and interplexiform cells now stands at at least 21 in the anuran retina. Physiological characterization of amacrine cells shows that their general features seem to be rather similar to those described in tiger salamander retina. In Xenopus retina, morphologically and physiologically identified amacrine cells responded to light stimulation most frequently with ON-OFF characteristics. Immunhistochemical identification of the recorded and dye injected cells showed that amacrine cells of the "same physiological type" might have different morphology. In other words, amacrine cells with different morphology can respond similarly to illumination. Even so, small differences between almost identical responses may reflect that the cell they stem from indeed belongs to different cell types.

Symphony of rhythms in the Xenopus laevis retina

The photoreceptor layer in the retina of Xenopus laevis harbors a circadian clock. Many molecular components known to drive the molecular clock in other organisms have been identified in Xenopus, such as XClock, Xper2, and Xcrys, demonstrating phylogenetic conservation. This model system displays a wide array of rhythms, including melatonin release, ERG rhythms, and retinomotor movements, suggesting that the ocular clock is important for proper retinal function. A flow-through culture system allows measurements of retinal rhythms such as melatonin release in vitro over time from a single eyecup. This system is suited for pharmacological perturbations of the clock, and has led to important observations regarding the circadian control of melatonin release, the roles of light and dopamine as entraining agents, and the circadian mechanisms regulating retinomotor movements. The development of a transgenic technique in Xenopus allows precise and reliable molecular perturbations. Since it is possible to follow rhythms in eyecups obtained from adults or tadpoles, the combination of the flow-through culture system and the transgenic technique leads to the fast generation of transgenic tadpoles to monitor the effects of molecular perturbations on the clock.

Calretinin is present in serotonin- and gamma-aminobutyric acid-positive amacrine cell populations in the retina of Xenopus laevis

In the frog retina most bipolar cells, sparsely distributed amacrine cells and some ganglion cells contain calretinin (CaR). Double-label immunocytochemistry shows that in the Xenopus retina many calretinin positive amacrine cells are also gamma-aminobutyric acid (GABA)-immunoreactive (IR), none colocalizes glycine or dopamine but some contain serotonin (SER). The CaR-IR cells represent 8-9% of all GABA-IR amacrine cells. Only 4.6% of the CaR-positive cells contain SER. The SER-positive cells are present in two sizes in the anuran retina: the large cells never contain CaR but some of the small cells do. To further characterize the CaR+/SER+ amacrine cell population, immunolabeling for tryptophan hydroxylase (TrOH), the rate-limiting enzyme for SER synthesis, was performed. The results showed that large TrOH-containing cells are always negative for CaR. However, the small SER-synthesizing amacrine cells are invariably CaR-positive. Thus the anuran retina contains three neurochemically distinct SER-positive amacrine cell types, one of which (the small SER-synthesizing cell type) is also CaR-IR.

Anatomical and electrophysiological evidence for GABAergic bipolar cells in tiger salamander retina

Our previous work showed that about 12% of bipolar cells in salamander retina synthesize and take up gamma-aminobutyric acid (GABA), are GABA transporter (GAT)-immunoreactive, and respond with a GAT current to extracellularly applied GABA, suggesting that these bipolar cells use GABA, in addition to glutamate, as a neurotransmitter. Further support for this idea was obtained in this study by use of immunogold electron microscopy and whole-cell patch clamp electrophysiology. Ultrastructural analysis showed that amacrine cell and ganglion cell processes were postsynaptic to GABA-immunoreactive synapses made by bipolar cell axon terminals. Whole-cell recordings were obtained from amacrine and ganglion cells in response to activation of bipolar cells by puffing KCl at their dendrites in the outer plexiform layer. Inhibitory postsynaptic currents were observed in several third order neurons, even after blocking the excitatory postsynaptic responses, generated in the inner plexiform layer, with a combined application of NMDA and non-NMDA receptor antagonists, AP-5 and CNQX. These ultrastructural and electrophysiological data support our previous neurochemical results, and suggest that the retinal through-information pathway in salamander includes both inhibitory GABAergic as well as excitatory glutamatergic synaptic mechanisms.

gamma-aminobutyric acid transporter-mediated current from bipolar cells in tiger salamander retinal slices

About 10% of bipolar cells in salamander retina synthesize and take up gamma-aminobutyric acid (GABA), and may use GABA as a neurotransmitter. As GABA uptake is electrogenic, bipolar cells expressing GABA transporters (GATs) should give transport current (IGAT) to extracellular GABA. Using whole-cell patch recording, 28 bipolar cells responded to 30-200 microM GABA puffed to the axon terminals with a picrotoxin (PTX)-sensitive chloride current (ICI) only. Another three bipolar cells had, in addition to ICI, a PTX-resistant, sodium-dependent current that was completely and reversibly blocked by NO-711, an IGAT inhibitor, indicating that this component was an IGAT. This finding provides further support for a subset of GABAergic bipolar cells in the salamander retina.

Calretinin-immunoreactive elements in the retina and optic tectum of the frog, Rana esculenta

The frog retina contains numerous 28 kDa calbindin positive elements in its every layer. At the same time, parvalbumin has been observed only in a few elements in the visual system of amphibian species, whilst calretinin immunoreactivity could be detected in salamander retina and the optic tectum of tench. However, the presence and distribution of calretinin have been described to date neither in the retina nor in the other parts of the visual system of anurans. Therefore, the aim of this study is to describe the calretinin immunoreactive elements in the retina and the optic tectum of the frog and to establish whether or not the expression of this calcium-binding protein is transmitter-related and/or cell type specific in these parts of the central nervous system. In the retina, numerous bipolar cells showed calretinin immunoreactivity. The axon terminals of the bipolar cells branched in both the OFF and ON sublayers of the inner plexiform layer. The few labeled amacrine cells were larger than 10 microns in diameter. Over 50% of the cells in the ganglion cell layer contained calretinin. The labeled cells in the ganglion cell layer were of usually 16-22 microns in diameter, although a few smaller cells were also seen. Accordingly, many optic fibers were also labeled. In colocalization experiments, gamma-aminobutyric acid and calretinin were found in partially overlapping amacrine cell populations, cells with the former marker being much more numerous. At the same time, all the gamma-aminobutyric acid positive bipolar cells also contained calretinin. Most of the calretinin positive neurons in the ganglion cell layer however were only single-labeled. Axons of ganglion cells terminated in B, C and F sublayers of layer 9 in the optic tectum. Local tectal neuron populations in layer 4, 6, 8 and 9 were also labeled and a few calretinin positive cells were detected also in layer 2. Approximately 10% of the tectal cells were found to be immunoreactive for calretinin. Layer 4 and 6 cells were mostly large pear-shaped neurons while cells in the 8th layer were small pear-shaped and ganglion cells labeled too. Coexistence of gamma-aminobutyric acid and calretinin was characteristic in cells of the upper tectal layers while they were not detected in neurons of deep layers of the tectum. After monocular enucleation, contralateral to the removed eye, calretinin-immunoreactivity disappeared almost completely from F sublayer and became less pronounced in sublayers B and C after 90 days. Calretinin-immunoreactivity remained mostly unchanged in local tectal cells. The results show that, although its function remains undetermined, calretinin is the major EF-hand calcium-binding protein in the frog retina and optic tectum.

Asymmetrical blastomere origin and spatial domains of dopamine and neuropeptide Y amacrine subtypes in Xenopus tadpole retina

Amacrine cells are located almost exclusively in the inner nuclear layer (INL) of the retina, but they express a variety of neurotransmitters. To begin to elucidate the relative roles of the local environment and cell lineage in determining the different neurotransmitter subtypes of amacrine cells, we combined lineage tracing and immunocytochemical techniques to map the spatial distribution and clonal origin of dopamine (DA) and neuropeptide Y (NPY) amacrine cells in Xenopus tadpole retina. At the earliest period of neurotransmitter expression, both DA and NPY amacrine cells were distributed preferentially in center and intermediate annular regions, and in anterior and dorsal quadrants. Most of the DA and NPY cells first emerged as scattered cells and later as clusters (of 2 or more cells) that increased in number and size up to premetamorphic stages. These results suggest that DA and NPY amacrine subtypes may be influenced by environmental cues localized to specific regions of the retina. Lineage analysis showed that the percentage of DA or NPY amacrine cells produced by most blastomere progenitors is significantly different from that predicted by the number of cells in the retina produced by those blastomeres. Only two blastomeres produced over 90% of the DA amacrine cells and only four produced 97% of the NPY amacrine cells. Some retinal progenitors did not contribute at all to these two amacrine subtypes. There also is a marked asymmetry in the blastomere origin of DA and NPY amacrine cells. Two retinal progenitors produced significant numbers of NPY but very few DA amacrine cells. This analysis provides evidence that blastomere origin restricts the developmental choices of retinal progenitors.

Tryptophan hydroxylase is expressed by photoreceptors in Xenopus laevis retina

Serotonin has important roles, both as a neurotransmitter and as a precursor for melatonin synthesis. In the vertebrate retina, the role and the localization of serotonin have been controversial. Studies examining serotonin immunoreactivity and uptake of radiolabeled serotonin have localized serotonin to inner retinal neurons, particularly populations of amacrine cells, and have proposed that these cells are the sites of serotonin synthesis. However, other reports identify other cells, such as bipolars and photoreceptors, as serotonergic neurons. Tryptophan hydroxylase (TPH), the rate-limiting enzyme in the serotonin synthetic pathway, was recently cloned from Xenopus laevis retina, providing a specific probe for localization of serotonin synthesis. Here we demonstrate that the majority of retinal mRNA encoding TPH is present in photoreceptor cells in Xenopus laevis retina. These cells also contain TPH enzyme activity. Therefore, in addition to being the site of melatonin synthesis, the photoreceptor cells also synthesize serotonin, providing a supply of the substrate needed for the production of melatonin.

Glutamate-, GABA-, and GAD-immunoreactivities co-localize in bipolar cells of tiger salamander retina

The presence of inhibitory bipolar cells in salamander retina was investigated by a comparative analysis of the distribution of glutamate- and GABA-immunoreactivities (GLU-IR; GABA-IR) using a postembedding immunocytochemical method. GLU-IR was found in virtually all photoreceptors, bipolar cells and ganglion cells, neuronal elements that transfer information vertically through the retina. GLU-IR also was found in numerous amacrine cells in the mid and proximal inner nuclear layer as well as in the cytoplasm of horizontal cells, while the nucleus of horizontal cells was either lightly labeled or not labeled at all. GLU-IR was found in the outer plexiform layer and intensely in the inner plexiform layer, in which there was no apparent sublamination. Forty-seven percent of Type IB bipolar cells in the distal inner nuclear layer and 13% of the displaced bipolar cells were GABA-IR. All bipolar cells were also GLU-IR, indicating that GABA-IR bipolar cells were a subset of GLU-IR bipolar cells rather than a separate population. About 12% of the Type IB bipolar cells were moderately GABA-IR and likely comprised a GABAergic subtype. GLU-IR levels in the presumed GABAergic bipolar cells were higher than in other purely GLU-IR bipolar cells suggesting that these GABA-IR bipolar cells are glutamatergic as well. All of the displaced bipolar cells were only lightly GABA-IR, indicating that displaced bipolar cells comprise a more homogeneous class of glutamatergic cell than orthotopic bipolar cells. GAD-IR co-localized with GABA-IR in orthotopic but not displaced bipolar cells, further supporting the idea that some orthotopic bipolar cells are GABAergic. A small proportion of bipolar cells in salamander retina contain relatively high levels of both GABA and glutamate. Co-release of these substances by bipolar cells could contribute to the "push-pull" modulation of ganglion cell responses.