Postnatal development of parvalbumin immunoreactive amacrine cells in the rabbit retina

Distinct Developmental Mechanisms Act Independently to Shape Biased Synaptic Divergence from an Inhibitory Neuron

Neurons often contact more than one postsynaptic partner type and display stereotypic patterns of synaptic divergence. Such synaptic patterns usually involve some partners receiving more synapses than others. The developmental strategies generating "biased" synaptic distributions remain largely unknown. To gain insight, we took advantage of a compact circuit in the vertebrate retina, whereby the AII amacrine cell (AII AC) provides inhibition onto cone bipolar cell (BC) axons and retinal ganglion cell (RGC) dendrites, but makes the majority of its synapses with the BCs. Using light and electron microscopy, we reconstructed the morphology and connectivity of mouse retinal AII ACs across postnatal development. We found that AII ACs do not elaborate their presynaptic structures, the lobular appendages, until BCs differentiate about a week after RGCs are present. Lobular appendages are present in mutant mice lacking BCs, implying that although synchronized with BC axonal differentiation, presynaptic differentiation of the AII ACs is not dependent on cues from BCs. With maturation, AII ACs maintain a constant number of synapses with RGCs, preferentially increase synaptogenesis with BCs, and eliminate synapses with wide-field amacrine cells. Thus, AII ACs undergo partner type-specific changes in connectivity to attain their mature pattern of synaptic divergence. Moreover, AII ACs contact non-BCs to the same extent in bipolarless retinas, indicating that AII ACs establish partner-type-specific connectivity using diverse mechanisms that operate in parallel but independently.

Cx36 expression in the AII-mediated rod pathway is activity dependent in the developing rabbit retina

Gap junctions are composed of connexin 36 (Cx36) and play a critical role in the rod photoreceptor signaling pathways of the vertebrate retina. Despite the fact that their connection and modulation in various rod pathways have been extensively studied in adult animals, little is known about the contribution and regulation of gap junctions to the development of the AII amacrine cell (AC)-mediated rod pathway. Using immunohistochemistry and microinjection, this study demonstrates a steady increase in relative Cx36 protein expression in both plexiform layers of the rabbit retina at around the time of eye opening. However, immediately after eye opening, most Cx36 immunoreactive AII ACs show no gap junction coupling pattern to neighboring cells and it is not until the third postnatal week that AII cells begin to exhibit an adult-like tracer-coupling pattern. Moreover, studies using dark-rearing and AMPA receptor blockade during postnatal development both revealed that relative levels of Cx36 immunoreactivity in AII ACs were increased when neural activity was inhibited. Our findings suggest that Cx36 expression in the AII-mediated rod pathway is activity dependent in the developing rabbit retina.

Calcium buffer proteins are specific markers of human retinal neurons

Ca(2+)-buffer proteins (CaBPs) modulate the temporal and spatial characteristics of transient intracellular Ca(2+)-concentration changes in neurons in order to fine-tune the strength and duration of the output signal. CaBPs have been used as neurochemical markers to identify and trace neurons of several brain loci including the mammalian retina. The CaBP content of retinal neurons, however, varies between species and, thus, the results inferred from animal models cannot be utilised directly by clinical ophthalmologists. Moreover, the shortage of well-preserved human samples greatly impedes human retina studies at the cellular and network level. Our purpose has therefore been to examine the distribution of major CaBPs, including calretinin, calbindin-D28, parvalbumin and the recently discovered secretagogin in exceptionally well-preserved human retinal samples. Based on a combination of immunohistochemistry, Neurolucida tracing and Lucifer yellow injections, we have established a database in which the CaBP marker composition can be defined for morphologically identified cell types of the human retina. Hence, we describe the full CaBP make-up for a number of human retinal neurons, including HII horizontal cells, AII amacrine cells, type-1 tyrosine-hydroxylase-expressing amacrine cells and other lesser known neurons. We have also found a number of unidentified cells whose morphology remains to be characterised. We present several examples of the colocalisation of two or three CaBPs with slightly different subcellular distributions in the same cell strongly suggesting a compartment-specific division of labour of Ca(2+)-buffering by CaBPs. Our work thus provides a neurochemical framework for future ophthalmological studies and renders new information concerning the cellular and subcellular distribution of CaBPs for experimental neuroscience.

Developmental changes in the expression level of connexin36 in the rat retina

Connexin36 (Cx36) is the major gap junction forming protein in the brain and the retina; thus, alterations in its expression indicate changes in the corresponding circuitry. Many structural changes occur in the early postnatal retina before functional neuronal circuits are finalized, including those that incorporate gap junctions. To reveal the time-lapse formation of inner retinal gap junctions, we examine the developing postnatal rat retina from birth (P0) to young adult age (P20) and follow the expression of Cx36 in the mRNA and protein levels. We found a continuous elevation in the expression of both the Cx36 transcript and protein between P0 and P20 and a somewhat delayed Cx36 plaque formation throughout the inner plexiform layer (IPL) starting at P10. By using tristratificated calretinin positive (CaR(+)) fibers in the IPL as a guide, we detected a clear preference of Cx36 plaques for the ON sublamina from the earliest time of detection. This distributional preference became more pronounced at P15 and P20 due to the emergence and widespread expression of large (>0.1 μm(2)) Cx36 plaques in the ON sublamina. Finally, we showed that parvalbumin-positive (PV(+)) AII amacrine cell dendrites colocalize with Cx36 plaques as early as P10 in strata 3 and 4, whereas colocalizations in stratum 5 became characteristic only around P20. We conclude that Cx36 expression in the rat IPL displays a characteristic succession of changes during retinogenesis reflecting the formation of the underlying electrical synaptic circuitry. In particular, AII cell gap junctions, first formed with ON cone bipolar cells and later with other AII amacrine cells, accounted for the observed Cx36 expressional changes.

Retinal distribution of Disabled-1 in a diurnal murine rodent, the Nile grass rat Arvicanthis niloticus

We sought to study the expression pattern of Disabled-1 (Dab1; an adaptor protein in the reelin pathway) in the cone-rich retina of a diurnal murine rodent. Expression was examined by western blotting and immunohistochemistry using well-established antibodies against Dab1 and various markers of retinal neurons. Western blots revealed the presence of Dab1 (80 kDa) in brain and retina of the Nile grass rat. Retinal immunoreactivity was predominant in soma and dendrites of horizontal cells as well as in amacrine cell bodies aligned at the INL/IPL border. Dab1(+) neurons in the inner retina do not stain for parvalbumin, calbindin, protein kinase C-alpha, choline acetyltransferase, glutamic acid decarboxylase, or tyrosine hydroxylase. They express, however, the glycine transporter GlyT1. They have small ovoid cell bodies (7.1 ± 1.06 μm in diameter) and bistratified terminal plexii in laminas a and b of the IPL. Dab1(+) amacrine cells are evenly distributed across the retina (2600 cells/mm(2)) in a fairly regular mosaic (regularity indexes ≈3.3-5.5). We conclude that retinal Dab1 in the adult Nile grass rat exhibits a dual cell patterning similar to that found in human. It is expressed in horizontal cells as well as in a subpopulation of glycinergic amacrine cells undetectable with antibodies against calcium-binding proteins. These amacrine cells are likely of the AII type.

Amacrine cell subtypes differ in their intrinsic neurite growth capacity

PURPOSE

Amacrine cell neurite patterning has been extensively studied in vivo, and more than 30 subpopulations with varied morphologies have been identified in the mammalian retina. It is not known, however, whether the complex amacrine cell morphology is determined intrinsically, is signaled by extrinsic cues, or both.

METHODS

Here we purified rat amacrine cell subpopulations away from their retinal neighbors and glial-derived factors to ask questions about their intrinsic neurite growth ability. In defined medium strongly trophic for amacrine cells in vitro, we characterized survival and neurite growth of amacrine cell subpopulations defined by expression of specific markers.

RESULTS

We found that a series of amacrine cell subtype markers are developmentally regulated, turning on through early postnatal development. Subtype marker expression was observed in similar fractions of cultured amacrine cells as was observed in vivo, and was maintained with time in culture. Overall, amacrine cell neurite growth followed principles very similar to those in postnatal retinal ganglion cells, but embryonic retinal ganglion cells demonstrated different features, relating to their rapid axon growth. Surprisingly, the three subpopulations of amacrine cells studied in vitro recapitulated quantitatively and qualitatively the varied morphologies they have in vivo.

CONCLUSIONS

Our data suggest that cultured amacrine cells maintain intrinsic fidelity to their identified in vivo subtypes, and furthermore, that cell-autonomous, intrinsic factors contribute to the regulation of neurite patterning.

Types of parvalbumin-containing retinotectal ganglion cells in mouse

The calcium-binding protein parvalbumin (PV) occurs in the retinal ganglion cells (RGCs) of various vertebrate species. In the present study, we aimed to identify the types of PV-containing RGCs that project to the superior colliculus (SC) in the mouse. We injected retrograde tracer dextran into the mouse SC to label RGCs. PV-containing RGCs were first identified by immunocytochemistry and then neurons double-labeled with dextran and PV were iontophoretically injected with a lipophilic dye, DiI. Subsequently, confocal microscopy was used to characterize the morphologic classification of the PV-immunoreactive (IR) retinotectal ganglion cells on the basis of dendritic field size, branching pattern, and stratification within the inner plexiform layer. Among the 8 different types of PV-containing RGCs in the mouse retina, we found all 8 types of RGCs projecting to the SC. The RGCs were heterogeneous in morphology. The combined approach of using tracer injection and a single cell injection after immunocytochemistry on a particular protein will provide valuable data to further understand the functional features of the RGCs which constitute the retinotectal pathway.

DARPP-32-like immunoreactivity in AII amacrine cells of rat retina

Previous studies demonstrated that the dopamine- and adenosine 3',5'-monophosphate-regulated phosphatase inhibitor known as "DARPP-32" is present in rat, cat, monkey, and human retinas. We have followed up these studies by asking what specific cell subtypes contain DARPP-32. Using a polyclonal antibody directed against a peptide sequence of human DARPP-32, we immunostained adult rat retinas that were either transretinally sectioned or flat mounted and found DARPP-32-like immunoreactivity in some cells of the amacrine cell layer across the entire retinal surface. We report here, based on the shape and spatial distribution of these cells, their staining by an anti-parvalbumin antibody, and their juxtaposition with processes containing tyrosine hydroxylase, that DARPP-32-like immunoreactivity is present in AII amacrine cells of rat retina. These results suggest that the response of AII amacrine cells to dopamine is not mediated as simply as previously supposed.

AII amacrine cells in the distal inner nuclear layer of the mouse retina

We serendipitously found a distal Disabled-1 (Dab1)-immunoreactive cell in retina of the C57BL/6J black mouse. The somata of these cells are located in the outermost part of the inner nuclear layer (INL). Their processes extend toward the outer plexiform layer (OPL), receiving synaptic inputs from horizontal and interplexiform cells. In the current study, we name this cell the "distal Dab1-immunoreactive cell." Double-labeling experiments demonstrate that the distal Dab1-immunoreactive cell is not a horizontal cell. Rather, the distal Dab1 cell appears to be a misplaced AII cell, by being glycine transporter-1-immunoreactive and by resembling the latter cell in an electron microscopic analysis. A distal Dab1 cell had been reported in the FVB/N mouse retina, a model of retinitis pigmentosa (Park et al. [2004] Cell Tissue Res 315:407-412). However, here, we found this distal Dab1-immunoreactive cell in the adult and normal developing mouse retinas. Hence, we show that such cells do not require the loss of photoreceptors as suggested previously (Park et al. [2004] Cell Tissue Res 315:407-412). Instead, two other pieces of data suggest an alternative explanation sources for distal Dab1 cells. First, we find a correlation between the number of these cells in the left and right eyes Second, developmental analysis shows that the distal Dab1-immunoreactive cell is first observed shortly after birth. At the same time, AII cells emerge, extending their neurites into the inner retina. These data suggest that distal Dab1-immunoreactive cells are misplaced AII amacrine cells, resulting from genetically modulated anomalies owing to migration errors.

Postnatal development of somatostatin 2A (sst2A) receptors expression in the rabbit retina

In the retina, somatostatin (SRIF) acts as a neuromodulator by interacting with specific SRIF subtype (sst) receptors. Aim of this investigation was to determine the cellular localization of the sst2A receptor isoform in the postnatal rabbit retina. Receptor immunoreactivity was localized using the antiserum K-230, directed to the C-terminus of the human sst2A receptor. In the postnatal rabbit retina, sst2A receptors were abundantly expressed without significant regional differences. They were localized predominantly to rod bipolar cells, identified with a protein kinase C (PKC) antibody, to amacrine cells, some of which also containing tyrosine hydroxylase (TH), and to presumed rare horizontal cells. Quantitative analysis showed that sst2A-immunoreactive (-IR) bipolar and amacrine cells reached their maximum density and absolute number at the time of eye opening, when the expression pattern of sst2A receptors was similar to that in adult retinas. In the adult retina, 68% of the PKC-IR rod bipolars and 34% of the TH-IR amacrine cells were observed to also express sst2A receptors. The appearance of sst2A receptor immunolabeling prior to eye opening and the developmental profile of sst2A receptor expression are compatible with a role of SRIF in the maturation of retinal circuitries. The partial expression of sst2A receptors in PKC-IR rod bipolar cells and in TH-IR amacrine cells may suggest some type of heterogeneity within these cell populations.

Disabled-1 is expressed in type AII amacrine cells in the mouse retina

The organization of several laminated structures in the brain is controlled by a signaling pathway activated by Reelin, a large glycoprotein secreted by pioneer neurons in the developing brain. Reelin binds to transmembrane receptors, including VLDLR and ApoER2, and stimulates tyrosine phosphorylation of Disabled-1 (Dab1), which associates with an NPxY motif present in the cytoplasmic domain of the receptors. Disruption of reelin, dab1, or both the vldr and apoer2 genes results in similar cell positioning defects in laminated brain regions including the cerebellum, hippocampus, and cerebral cortex. Although retinal ganglion cells express reelin during development, there is no obvious disruption of cell positioning in the retina of reeler mice. Here, we examine the expression pattern of Dab1 as a first step toward understanding the function of the Reelin signaling pathway in neural retina. Immunohistochemical analysis of the adult retina revealed that Dab1 is expressed in a specific type of amacrine cell. These cells display a narrow dendritic field and they project to two distinct sublaminae within the inner plexiform layer. Dab1 co-localizes with the high-affinity glycine transporter, indicating that these amacrine cells are glycinergic. Cells that express Dab1 are surrounded by dopaminergic fibers originating from wide-field amacrine cells. These features are characteristic of type AII amacrine cells described in other mammalian species. Analysis of the retina at several stages of development revealed that Dab1 is expressed shortly after birth during the time at which AII amacrine cells extend neurites and form synaptic connections in the inner retina. This raises the possibility that the Reelin/Dab1 signaling pathway contributes to formation of intraretinal circuitry in the neural retina.

Neurotrophins and development of the rod pathway: an elementary deduction

The rodent retina is a particularly attractive model for the study of neuronal developmental processes since considerable neurogenesis, cellular migration, phenotypic differentiation of retinal cell types and synaptogenesis occurs postnatally. In addition, the retina is readily accessible to surgical intervention, pharmacological manipulation, and local suppression of gene expression-tools that can be utilized to study mechanisms underlying the development of retinal neurons and their interconnections that form distinct functional circuits. Here, I review our studies describing the ontogeny of a specific retinal interneuron, the AII amacrine cell, an integral element in the rod (scotopic) pathway. Specifically, we used a number of approaches to examine the potential role of neurotrophic factors on the morphological and neurochemical differentiation of the AII.

Parvalbumin immunoreactivity is enhanced by brain-derived neurotrophic factor in organotypic cultures of rat retina

The rodent retina undergoes considerable postnatal neurogenesis and phenotypic differentiation, and it is likely that diffusible neurotrophic factors contribute to this development and to the subsequent formation of functional retinal circuitry. Accordingly, perturbation of specific neurotrophin ligand-receptor interactions has provided valuable information as to the fundamental processes underlying this development. In the present studies we have built upon our previous observation that suppression of expression of trk(B), the high-affinity receptor for brain-derived neurotrophic factor (BDNF), in the postnatal rat retina results in the alteration of a specific interneuron in the rod pathway-the parvalbumin (PV)-immunoreactive AII amacrine cell. Here, we isolated retinas from newborn rats and maintained them in organotypic culture for up to 14 days (approximating the time of eye opening, in vivo) in the presence of individual neurotrophins [BDNF or nerve growth factor (NGF)]. We then examined histological sections of cultures for PV immunoreactivity. In control cultures, only sparse PV-immunostained cells were observed. In cultures supplemented with NGF, numerous lightly immunostained somata were present in the inner nuclear layer (INL) at the border of the inner plexiform layer (IPL). Many of these cells had rudimentary dendritic arborizations in the IPL. Cultures supplemented with BDNF displayed numerous well-immunostained somata at the INL/IPL border that gave rise to elaborate dendritic arborizations that approximated the morphology of mature AII amacrine cells in vivo. These observations indicate that neurotrophins have specific effects upon the neurochemical and, perhaps, morphological differentiation of an important interneuron in a specific functional retinal circuit.

Morphologic maturation of tachykinin peptide-expressing cells in the postnatal rabbit retina

Tachykinin (TK) peptides, which include substance P, neurokinin A, two neurokinin A-related peptides and neurokinin B, are widely present in the nervous system, including the retina, where they act as neurotransmitters/modulators as well as growth factors. In the present study, we investigated the maturation of TK-immunoreactive (IR) cells in the rabbit retina with the aim of further contributing to the knowledge of the development of transmitter-identified retinal cell populations. In the adult retina, the pattern of TK immunostaining is consistent with the presence of TK peptides in amacrine, displaced amacrine, interplexiform and ganglion cells. In the newborn retina, intensely immunostained TK-IR somata are located in the ganglion cell layer (GCL) and in the inner nuclear layer (INL) adjacent to the inner plexiform layer (IPL). They are characterized by an oval-shaped cell body originating a single process without ramifications. TK-IR processes are occasionally observed in the IPL and in the outer plexiform layer (OPL). Long TK-IR fiber bundles are observed in the ganglion cell axon layer. TK-IR profiles resembling small somata are rarely observed in the INL adjacent to the OPL. At postnatal day (PND) 2, some TK-IR cells display more complex morphologic features, including processes with secondary ramifications. Long TK-IR processes in the IPL are often seen to terminate with growth cones. Between PND 6 and PND 11 (eye opening), there is a dramatic increase in the number of immunolabeled processes with growth cones both in the IPL and in the OPL and the mature lamination of TK-IR fibers in laminae 1, 3 and 5 of the IPL is established. TK-IR cells attain mature morphological characteristics and the rare, putative TK-IR somata in the distal INL are no longer observed. After eye opening, growth cones are not present and the pattern typical of the adult is reached. These observations indicate that the development of TK-IR cells can be divided into an early phase (from birth to PND 6) in which these cells establish their morphological characteristics, and a later phase (from PND 6 to eye opening) in which they are involved in active growth of their processes and likely in synapse formation. Since TK peptides are thought to play neurotrophic actions in the developing nervous system and they are consistently present in the retina throughout postnatal development, they may also act as growth factors during retinal maturation.