The emergence of GABA-accumulating neurons during retinal histogenesis in the embryonic chick

Silva H, P. de Carvalho R, Ventura ALM, Calaza KC, Silveira MS, Kubrusly RCC, de Melo Reis RA. The Healthy and Diseased Retina Seen through Neuron-Glia Interactions

The retina is the sensory tissue responsible for the first stages of visual processing, with a conserved anatomy and functional architecture among vertebrates. To date, retinal eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, glaucoma, and others, affect nearly 170 million people worldwide, resulting in vision loss and blindness. To tackle retinal disorders, the developing retina has been explored as a versatile model to study intercellular signaling, as it presents a broad neurochemical repertoire that has been approached in the last decades in terms of signaling and diseases. Retina, dissociated and arranged as typical cultures, as mixed or neuron- and glia-enriched, and/or organized as neurospheres and/or as organoids, are valuable to understand both neuronal and glial compartments, which have contributed to revealing roles and mechanisms between transmitter systems as well as antioxidants, trophic factors, and extracellular matrix proteins. Overall, contributions in understanding neurogenesis, tissue development, differentiation, connectivity, plasticity, and cell death are widely described. A complete access to the genome of several vertebrates, as well as the recent transcriptome at the single cell level at different stages of development, also anticipates future advances in providing cues to target blinding diseases or retinal dysfunctions.

Caffeine regulates GABA transport via A1R blockade and cAMP signaling

Caffeine is the most consumed psychostimulant drug in the world, acting as a non-selective antagonist of adenosine receptors A₁R and A_2AR, which are widely expressed in retinal layers. We have previously shown that caffeine, when administered acutely, acts on A₁R to potentiate the NMDA receptor-induced GABA release. Now we asked if long-term caffeine exposure also modifies GABA uptake in the avian retina and which mechanisms are involved in this process. Chicken embryos aged E11 were injected with a single dose of caffeine (30 mg/kg) in the air chamber. Retinas were dissected on E15 for ex vivo neurochemical assays. Our results showed that [³H]-GABA uptake was dependent on Na⁺ and blocked at 4 °C or by NO-711 and caffeine. This decrease was observed after 60 min of [³H]-GABA uptake assay at E15, which is accompanied by an increase in [³H]-GABA release. Caffeine increased the protein levels of A₁R without altering ADORA1 mRNA and was devoid of effects on A_2AR density or ADORA2A mRNA levels. The decrease of GABA uptake promoted by caffeine was reverted by A₁R activation with N6-cyclohexyl adenosine (CHA) but not by A_2AR activation with CGS 21680. Caffeine exposure increased cAMP levels and GAT-1 protein levels, which was evenly expressed between E11-E15. As expected, we observed an increase of GABA containing amacrine cells and processes in the IPL, also, cAMP pathway blockage by H-89 decreased caffeine mediated [³H]-GABA uptake. Our data support the idea that chronic injection of caffeine alters GABA transport via A₁R during retinal development and that the cAMP/PKA pathway plays an important role in the regulation of GAT-1 function.

From neuroepithelial cells to neurons: changes in the physiological properties of neuroepithelial stem cells

The central nervous system, which includes the spinal cord, retina, and brain, is derived from the neural tube. The neural tube is formed of a sheet of cells called the neuroepithelium. During embryonic development, neuroepithelial cells function as neural stem cells: they renew themselves while undergoing interkinetic nuclear movements along the apico-basal axis during the cell cycle, and they produce postmitotic cells that function as newborn neurons. Neuroepithelial cells exhibit a robust increase in nucleoplasmic [Ca(2+)] in response to G protein-coupled receptor activation during S-phase when the nucleus is located in the basal region of the cell. This Ca(2+) rise is caused by the release of Ca(2+) from intracellular Ca(2+) stores, and the Ca(2+) release in turn activates Ca(2+) entry from the extracellular space, which is called capacitative (or store-operated) Ca(2+) entry. The Ca(2+) release and store-operated Ca(2+) entry are essential for DNA synthesis during S-phase. The activity of this store-operated Ca(2+) signaling system declines in parallel with the decreasing proliferative activity of neuroepithelial cells. When exiting the cell cycle, the cells lose the apical process where gap junctions are located. Following the loss of gap junction coupling, the postmitotic cells show a high input resistance, which allows them to be readily depolarized. The Ca(2+) response to the excitatory neurotransmitter glutamate appears and develops during neuronal differentiation. The glutamate-induced Ca(2+) rise increases transiently during natural cell death (apoptosis). The rise in Ca(2+) levels mediated by voltage-gated Ca(2+) channels also develops during neuronal differentiation. Thus, when neuroepithelial cells differentiate into neurons, a transition from a store-operated system to a voltage-operated system occurs in the main Ca(2+) signaling system. This transition may reflect a change in the mode of intercellular communication from a stored Ca(2+)-dependent mode to a plasma membrane potential-dependent mode.

Control of cell proliferation by neurotransmitters in the developing vertebrate retina

In the developing vertebrate retina, precise coordination of retinal progenitor cell proliferation and cell-cycle exit is essential for the formation of a functionally mature retina. Unregulated or disrupted cell proliferation may lead to dysplasia, retinal degeneration or retinoblastoma. Both cell-intrinsic and -extrinsic factors regulate the proliferation of progenitor cells during CNS development. There is now growing evidence that in the developing vertebrate retina, both slow and fast neurotransmitter systems modulate the proliferation of retinal progenitor cells. Classic neurotransmitters, such as GABA (gamma-amino butyric acid), glycine, glutamate, ACh (acetylcholine) and ATP (adenosine triphosphate) are released, via vesicular or non-vesicular mechanisms, into the immature retinal environment. Furthermore, these neurotransmitters signal through functional receptors even before synapses are formed. Recent evidence indicates that the activation of purinergic and muscarinic receptors may regulate the cell-cycle machinery and consequently the expansion of the retinal progenitor pool. Interestingly, GABA and glutamate appear to have opposing roles, inducing retinal progenitor cell-cycle exit. In this review, we present recent findings that begin to elucidate the roles of neurotransmitters as regulators of progenitor cell proliferation at early stages of retinal development. These studies also raise several new questions, including how these neurotransmitters regulate specific cell-cycle pathways and the mechanisms by which retinal progenitor cells integrate the signals from neurotransmitters and other exogenous factors during vertebrate retina development.

Expression of Fgf19 in the developing chick eye

Fibroblast growth factor 19 (FGF19) is a new member of the FGF family of growth factors. Here, we describe the localization of Fgf19 mRNA in the developing chick retina and lens in stages from the Hamburger and Hamilton stage 15 (HH15) to postnatal day 30 (P30). Fgf19 was expressed in a transient manner in postmitotic neuroblasts during the migration from the ventricular surface to their final location. Moreover, from HH31 (embryonic day 7, E7) on, a subset of lined up Fgf19 expressing cells was distributed in the outer region of the presumptive INL. These cells were Pax6 immunoreactive horizontal cells. During the last third of embryogenesis, Fgf19 expression in the retina was progressively down-regulated and was not detected at P30. Also, it was transiently expressed in the equatorial region of the lens.

Butyrylcholinesterase-positive cells of the developing chicken retina that are non-cholinergic and GABA-positive

Butyrylcholinesterase (BChE) is closely related to acetylcholinesterase (AChE), but its function in nervous system development or physiology is unclear. Here, the distribution of BChE was investigated by immunohistochemical methods in the developing chick retina. Using a specific anti-BChE antibody, we detected immunoreactivity associated with different cell types in two nuclear layers and in plexiform layers of the retina. At embryonic day 10 (E10), a transient BChE staining is detected in the inner plexiform layer (IPL) and in radial cells, the latter possibly representing Müller glia. At E12, a subpopulation of amacrine cells appeared, followed by cells in the middle and outer half of the inner nuclear layer. These cells at locations of amacrine, bipolar and horizontal cells represented the predominant three cell types persisting until hatching. The BChE+ amacrine cells were studied in more detail. Their distribution was not significantly different in the central and peripheral retina. Double labelling experiments revealed that BChE+ amacrine cells did not express choline acetyltransferase (ChAT), and, thus, are non-cholinergic. Only a minority of them coexpressed AChE. On the other hand, the majority of them colocalized with anti-GABA immunoreactivity. Taken together, these data support a hitherto unsuspected role of BChE in non-cholinergic cells, possibly in conjunction with GABA.

Muscarinic acetylcholine responses in the early embryonic chick retina

The action of acetylcholine on cytoplasmic Ca2+ concentration ([Ca2+]i) was studied in early embryonic chick retinae. Whole neural retinae were isolated from embryonic day 3 (E3) chicks and loaded with a Ca(2+)-sensitive fluorescent dye (Fura-2). Increases in [Ca2+]i were evoked by the puff application of acetylcholine at concentrations higher than 0.1 microM. The Ca2+ response became larger in a dose-dependent manner up to 10 microM of acetylcholine applied. The rise in [Ca2+]i was not due to the influx of Ca2+ through calcium channels, but to the release of Ca2+ from internal stores. A calcium channel antagonist, nifedipine, which completely blocks the Ca2+ rise caused by depolarization with 100 mM K+, had no effects on the acetylcholine response and the Ca2+ response to acetylcholine occurred even in a Ca(2+)-free medium. The Ca2+ response to acetylcholine was mediated by muscarinic receptors. Atropine of 1 microM abolished the response to 10 microM acetylcholine, whereas d-tubocurarine of 100 microM had no effects. Two muscarinic agonists, muscarine and carbamylcholine (100 microM each), evoked comparable responses with that to 10 microM acetylcholine. The developmental change of the muscarinic response was examined from E3 to E13. The Ca2+ response to 100 microM carbamylcholine was intense at E3-E5, then rapidly declined until E8. The muscarinic Ca2+ mobilization we found in the early embryonic chick retina may be regarded as a part of the "embryonic muscarinic system" proposed by Drew's group, which appears transiently and ubiquitously at early embryonic stages in relation to organogenesis.

Calcium channels and GABA receptors in the early embryonic chick retina

The properties of calcium channels were studied at the period of neurogenesis in the early embryonic chick retina. The whole neural retina was isolated from embryonic day 3 (E3) chick and loaded with a Ca(2+)-sensitive fluorescent dye (Fura-2). The retinal cells were depolarized by puff application of high-K+ solutions. Increases in intracellular Ca2+ concentrations were evoked by the depolarization through calcium channels. The type of calcium channel was identified as L-type by the sensitivity to dihydropyridines. The Ca2+ response was completely blocked by 10 microM nifedipine, whereas it was remarkably enhanced by 5 microM Bay K 8644. Then we sought a factor to activate the calcium channel and found that GABA could activate it by membrane depolarization at the E3 chick retina. Puff application of 100 microM GABA raised intracellular Ca2+ concentrations, and this Ca2+ response to GABA was also sensitive to the two dihydropyridines. Intracellular potential recordings verified clear depolarization by bath-applied 100 microM GABA. The Ca2+ response to GABA was mediated by GABAA receptors, since the GABA response was blocked by 10 microM bicuculline or 50 microM picrotoxin, and mimicked by muscimol but not by baclofen. Neither glutamate, kainate, nor glycine evoked any Ca2+ response. We conclude that L-type calcium channels and GABAA receptors are already expressed before differentiation of retinal cells and synapse formation in the chick retina. A possibility is proposed that GABA might act as a trophic factor by activating L-type calcium channels via GABAA receptors during the early period of retinal neurogenesis.

GABA immunoreactive axons and growth cones in the developing chicken optic nerve and tract

Immunohistochemical studies of the chicken embryo optic tract using an antibody to gamma-aminobutyric acid (GABA) reveal that the tract is initially free of GABA immunoreactive axons. During the second week of incubation, GABA+ axons appear in the tract, chiasm, and optic nerve. The number of GABA+ axons in the optic nerve increases through E18, although few are recognizable after hatching. Detailed staining of GABA+ growth cones confirmed that virtually all the GABA+ axons in the optic nerve were growing toward the retina. Taken together, the findings suggest that the GABA+ axons in the chiasm and nerve are largely a transient extension of the GABA+ optic tract cells, the tectogeniculate projection, or both.

GABA-like immunoreactivity in the developing chick retina: differentiation of GABAergic horizontal cell and its possible contacts with photoreceptors

The morphology of gamma-aminobutyric acid (GABA)-containing horizontal cells was examined in mature and developing chick retinas by GABA immunocytochemistry. In the outer plexiform layer of the mature retina, GABA-immunoreactive components were located in three different sublayers. In the inner (vitreal) layer most positively-stained fibres were laterally oriented processes from horizontal cells. Thick processes were found in the middle layer, and the relatively thin fibres in the outer (scleral) layer showed a concave curvature, suggesting their termination on photoreceptor terminals. By electron microscopy it was found that the principal cone pedicles were usually indented by immunoreactive lateral neurites of horizontal cells but that rod spherules faced only occasionally immunoreactive fibres. Accessory cones and single cones were also not usually indented by immunoreactive fibres. These observations may indicate that horizontal cells regulate the excitation of cone photoreceptors by several different inhibitory mechanisms. During retinal development, horizontal cells begin to extend lateral fibres by the ninth embryonic day, and some GABAergic horizontal cells also possess inwardly extending fibres until embryonic day 11. Between embryonic days 13 and 15, some immunoreactive cells were found among the bipolar cells, suggesting that they were still migrating to their final position. On embryonic day 17, the staining pattern was very similar to that of the mature retina. These results suggest that GABA immunohistochemistry may be an excellent tool for studying horizontal cell differentiation.

Chicken optic tract cells showing GABA-like immunoreactivity: morphological and immunocytochemical studies

A population of cells has been found in the chick optic tract and chiasm exhibiting GABA-like immunoreactivity (GABA+; Granda and Crossland, J. Comp. Neurol. 287:455-469, '89). It is not known, however, whether the cells are neurons. We have studied the GABA+ cells by using morphological and immunocytochemical methods. We found that there are more than 500 cells in each tract. At the light microscopic level, the cells possess processes resembling dendrites and axons. At the electron microscopic level, the organelle content of the cells is similar to that of neurons. The cells are immunoreactive with antibodies to MAP2 and neuron specific enolase, two proteins characteristic of neurons. Taken together the findings indicate that the GABA+ cells of the chick optic tract are neurons, perhaps similar to the interstitial neurons found in the white matter of other vertebrates.

Ontogeny of catecholaminergic and cholinergic cell distributions in the cat's retina

The development of catecholaminergic and cholinergic neurones in the cat's retina has been examined with antibodies against their respective rate-limiting enzymes, tyrosine hydroxylase (TH) and choline acetyl transferase (ChAT). ChAT-immunoreactive (IR) cells were first detected at E (embryonic day) 56 with somata in the ganglion cell layer (GCL) or in the inner cytoblast layer (CBL). At P (postnatal day) 1, two faint bands of ChAT-IR fibres were evident in an inner and outer strata of the inner plexiform layer (IPL) and by P26, the bands were similar to those in the adult. TH immunoreactivity was first detected at E59 in either darkly labelled somata in the inner CBL with processes extending toward the IPL or in lightly labelled somata also located in CBL but with no processes. At P1, most TH-IR cells had prominently labelled dendrites and, by P8, most of the features of the adult cells were evident. Soma size gradients among TH-IR cells were first detected at P8, with cells in temporal retina being larger than those in nasal retina or at the area centralis. The smaller sizes of cells at the area centralis emerged after P26. The smaller sizes of ChAT-IR somata at the area centralis, by contrast, emerged between P8 and P26. The number of both TH-IR and ChAT-IR cells declined from the time they first appeared till adulthood. The decline was smaller among ChAT-IR cells (24%) than among TH-IR cells (68%). In distribution, the differential expansion of the retina appeared to be largely responsible for generating the final adult distribution of ChAT-IR cells. However, during late postnatal development (P26 to adulthood), the density of ChAT-IR cells in the periphery declined more than that of the ganglion cells, suggesting that some ChAT-IR cells may die in the periphery during this time. Prior to P26, the changes in the distribution of TH-IR cells were inconsistent with the pattern of retinal expansion. It is suggested that during this period, regional cell loss and cell addition may account for the changes in distribution of TH-IR cells. Later in development (P26 to adulthood), the changes in the density of TH-IR cells closely conformed to the differential expansion of the retina.