Local differences in GABA release induced by excitatory amino acids during retina development: selective activation of NMDA receptors by aspartate in the inner retina

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.

Retinal exposure to high glucose condition modifies the GABAergic system: Regulation by nitric oxide

Diabetic retinopathy is a severe retinal complication that diabetic patients are susceptible to present. Although this disease is currently characterized as a microvascular disease, there is growing evidence that neural changes occur and maybe precede vascular impairments. Using chicken retina, an avascular tissue with no direct contact with blood vessels and neural retina, this study aimed to evaluate the influence of acute exposure to high glucose concentration in the retinal GABAergic system, and the role of nitric oxide (NO) in this modulation. Therefore, in ex vivo experiments, retinas were incubated in control (10 mM glucose) or high glucose condition (35 mM) for 30 min. By using DAF-FM to evaluate NO production, it was possible to show that high glucose (HG) significantly increased NO levels in the outer nuclear layer, inner nuclear layer (outer and inner portion), and inner plexiform layer. It was also observed that HG increased GABA immunoreactivity (IR) in amacrine and horizontal cells. HG did not change glutamic acid decarboxylase-IR, whereas it decreased GABA Transporter (GAT) 1-IR and increased GAT-3-IR. The co-treatment with 7-NI, an inhibitor of neuronal nitric oxide synthase (nNOS), blocked all changes stimulated by HG exposure. The concomitant exposure with SNAP-5114, a GAT-2/3 inhibitor, blocked the increase in GABA-IR caused by HG incubation. Therefore, our data suggest that hyperglycemia induces GABA accumulation in the cytosol by modulating GABA transporters. This response is dependent on NO production and signaling.

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.

Transporter mediated GABA release in the retina: Role of excitatory amino acids and dopamine

In general, the release of neurotransmitters in the central nervous system is accomplished by a calcium-dependent process which constitutes a common feature of exocytosis, a conserved mechanism for transmitter release in all species. However, neurotransmitters can also be released by the reversal of their transporters. In the retina, a large portion of GABA is released by this mechanism, which is under the control of neuroactive agents, such as excitatory amino acids and dopamine. In this review, we will focus on the transporter mediated GABA release and the role played by excitatory amino acids and dopamine in this process. First, we will discuss the works that used radiolabeled GABA to study the outflow of the neurotransmitter and then the works that took into consideration the endogenous pool of GABA and the topography of GABAergic circuits influenced by excitatory amino acids and dopamine.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.