Early appearance and transient expression of putative amino acid neurotransmitters and related molecules in the developing rabbit retina: an immunocytochemical study

Postnatal developmental expression of glutamine and related amino acids in the rat retinas

PURPOSE

To evaluate postnatal developmental changes in the amounts of retinal glutamate, glutamine and GABA, and in the distribution of retinal glutamine in the rat.

METHODS

Free amino acids were extracted from rat retinas of different postnatal stages, and the concentrations of glutamate, glutamine and GABA were determined by HPLC. Also, anti-glutamine antibody was raised and an immunocytochemistry was performed with paraffin-embedded retinal sections in parallel with free amino acid analyses.

RESULTS

Glutamate occurred in high concentrations at the birth and showed a stable pool, while glutamine and GABA remained low until postnatal day 3 or 5, and gradually increased in the developing rat retinas. Glutamine immunolabeling was observed in the retinal pigment epithelium and in a subpopulation of presumed amacrine cells in the early postnatal days. It was also found in Muller cells and in some ganglion cells or displaced amacrine cells in the ganglion cells layer. Glutamine immunolabeling was transiently observed also in horizontal cells. Finally, the immunolabeling was dominant in the inner and outer plexiform layers in the adult retinas.

CONCLUSIONS

Postnatal developmental increase in the levels of glutamine and GABA might be dependent on the maturation of neurons or glial cells that possess the activity of the key enzymes of each amino acid. It was suggested that an expression of glutamine immunolabeling can be a marker of neurons that utilize glutamine as a precursor for glutamate or GABA, and of Müller cell maturations in postnatal early stage of the retina, while it changes to demonstrate the locations of glutamine cycle in the retina with adult characteristics.

GABA-induced increases in [Ca2+]i in retinal neurons of postnatal rabbits

Previous studies have indicated that gamma-aminobutyric acid (GABA) plays an important trophic role in the synapse formation between horizontal cells and photoreceptors in postnatal rabbit retina. However, the mechanism of the GABA effect has not been identified. Using fluo-3 Ca2+ imaging and confocal laser scanning microscopy we examined the effect of GABA on [Ca2+]i during postnatal retinal development. GABA (100 microM) evoked a fast and transient increase of [Ca2+]i in selected populations of freshly dissociated retinal cells from postnatal rabbits. This increase was apparent on postnatal day 1 and reached a maximum on day 5. Little increase in [Ca2+]i was observed in retinal cells isolated from adult rabbits. GABA receptor antagonists, picrotoxin and bicuculline, significantly reduced the response. The GABAB agonist, baclofen, did not evoke any [Ca2+]i changes. The GABA-induced increase in [Ca2+]i was observed in all retinal layers in neonatal retinal whole-mount explants. In the outer retina, the increase was seen in cone photoreceptors which were specifically labeled with peanut agglutinin (PNA). The GABA-induced increase in [Ca2+]i may provide an important mechanism for regulating cone synaptogenesis in the outer plexiform layer of the postnatal retina.

Retinal ganglion cell dendritic development and its control. Filling the gaps

The way in which central neurons acquire their complex and precise dendrite arbors is of considerable developmental interest. Using retinal ganglion cells (RGCs) as a model, the mechanisms that pattern dendritic development are beginning to emerge. As in other systems, final dendrite phenotype is achieved by a mixture of intrinsic and extrinsic determinants. The extrinsic determinants of RGC dendrite shape reflect the anatomical constraints of producing a paracrystalline mosaic of arbors that laminates the inner plexiform layer of the retina. In this article, the key features of RGC dendrite development are reviewed. The emerging molecular mechanisms behind dendritic laminar segregation and "dendritic competition" are described. The role of afferent extrinsic influences are contrasted with those of retrograde, activity-dependent target influences that may regulate the final maturational phase of dendrite remodeling.

Immunocytochemical evidence for an axonal localization of GABA in the optic nerves of rabbits, rats, and cats

We have examined, by light-microscopic immunocytochemistry, the distribution of GABA in the optic nerves of adult rabbits, rats, and cats. Within the optic nerves, immunoreactivity for GABA was restricted to a small subset of axons; some axons were strongly labelled, others weakly labelled, whilst most axons were unlabelled. Glia and other non-neuronal elements were always unlabelled. Our ability to detect GABA in optic nerve axons of adult mammals contrasts with previous reports that indicate a lack of GABA immunoreactivity in such axons. We suggest that this discrepancy may be due to the sensitivity of our immunocytochemical techniques which enable us to detect low concentrations of GABA.

Effects of glutamate and its analogs on intracellular calcium levels in the developing retina

Stimulation of neuronal cells by the excitatory amino acid, glutamate, often leads to a rise in cytosolic free calcium concentration ([Ca2+]i), which can affect cell survival and differentiation. The early appearance of endogenous glutamate in the embryonic rabbit retina suggests that it may be involved in intercellular signalling during development. Thus, the effect of glutamate on the [Ca2+]i of cells in the fetal and neonatal rabbit retina was examined using Ca2+ imaging techniques, which enabled the responses of large numbers of morphologically identified classes of cell to be compared directly. Ganglion cells and amacrine cells, the first retinal neurons to differentiate, showed a rise in [Ca2+]i in the presence of glutamate from the earliest age studied (embryonic day 20; E20). These responses were mediated by non-NMDA (non-N-methyl-D-aspartate) receptors. NMDA stimulated ganglion cells and amacrine cells only several days later, at about E24. Moreover, whilst most, if not all, putative ganglion cells responded to NMDA, only a subset of putative amacrine cells were sensitive to NMDA throughout development. Photoreceptors, bipolar cells, horizontal cells, and Müller cells differentiate later than the ganglion cells and amacrine cells. Between E20 and birth, cells in the ventricular zone are largely the precursors of these cell types. During this period, 50-60% of ventricular cells responded to glutamate with an increase in [Ca2+]i, upon activation of ionotropic non-NMDA receptors. At no age studied were these ventricular cells, or their differentiated counterparts, stimulated by NMDA. After birth, most cells in the inner nuclear layer were sensitive to non-NMDA receptor agonists, but photoreceptors showed no response. Taken together, the results suggests that NMDA and non-NMDA receptors may adopt separate roles during retinal development, and that non-NMDA receptors, rather than NMDA receptors, may be involved in developmental processes in the ventricular zone.

Immunocytochemical evidence for the presence of high levels of reduced glutathione in radial glial cells and horizontal cells in the rabbit retina

Reduced glutathione is an antioxidant; it is thought to be essential for normal functioning of the central nervous system. We have examined by means of immunocytochemistry, the distribution of reduced glutathione in the retina of the rabbit. Strong immunoreactivity was present in the radial glial cells (Müller cells) and in the horizontal cells. Other neuronal elements contained only low, or no detectable levels of immunoreactivity for reduced glutathione. The presence of an abundance of reduced glutathione in glial cells suggests that glia play a critical role in regulating the content of potentially damaging oxidative species in the central nervous system.

Early functional neural networks in the developing retina

In the adult mammalian retina, the principal direction of information flow is along a vertical pathway from photoreceptors to retinal interneurons to ganglion cells, the output neurons of the retina. We report here, however, that initially in development, at a time when the photoreceptors are not yet even present, there are already functionally defined networks within the retina. These networks are spontaneously active rather than visually driven, and they involve horizontal rather than vertical pathways. By means of optical recording using the calcium-sensitive dye Fura-2, we have found that sets of retinal ganglion cells and amacrine cells, a type of retinal interneuron, undergo synchronized oscillations in intracellular calcium concentration. These oscillations are highly correlated among subgroups of neighbouring cells, and spread in a wave-like fashion tangentially across the retina. Thus, in development of retinal circuitry, the initial patterning of neuronal function occurs in the horizontal domain before the adult pattern of vertical information transfer emerges.

Rapid postmortem changes in the cellular localisation of amino acid transmitters in the retina as assessed by immunocytochemistry

We have assessed by means of immunocytochemistry, the cellular distributions of the amino acid transmitters GABA, glycine and glutamate, and the free-radical scavenger taurine, in the retinae of adult rabbits at various times after death. Within 10 min of death, horizontal cells began to display immunoreactivity for GABA, whilst displaced amacrine cells began to display immunoreactivity for glycine. By 40 min postmortem, GABA was present in glial cells. Glutamate, which is not normally detectable in retinal glia, was detected in such glia by 20 min postmortem. By contrast immunocytochemically detectable glycine did not accumulate in glia. There was a gradual diminution of immunoreactivity for taurine in glial cells and photoreceptors. By 2 h postmortem, most immunoreactivity had disappeared from the retina. We conclude that amino acid transmitters show rapid changes in their distributions immediately after death, which may be related to changes in the patterns of transmitter release and uptake, and changes in degradation mechanisms. The rapid changes in cellular localisation of amino acid immunoreactivity illustrated in this study, indicate that the fixation of nervous tissues must be performed rapidly. Moreover, the massive loss of immunoreactivity by 2 h postmortem suggests that any assays for content of these transmitters at this, and subsequent time-points, will bear little resemblance to the values obtained at the time of death.