Nitric oxide synthase in developing retinas and after optic tract section

The CNTF/LIF signaling pathway regulates developmental programmed cell death and differentiation of rod precursor cells in the mouse retina in vivo

Natural cell death is critical for normal development of the nervous system, but the extracellular regulators of developmental cell death remain poorly characterized. Here, we studied the role of the CNTF/LIF signaling pathway during mouse retinal development in vivo. We show that exposure to CNTF during neonatal retinal development in vivo retards rhodopsin expression and results in an important and specific deficit in photoreceptor cells. Detailed analysis revealed that exposure to CNTF during retinal development causes a sharp increase in cell death of postmitotic rod precursor cells. Importantly, we show that blocking the CNTF/LIF signaling pathway during mouse retinal development in vivo results in a significant reduction of naturally occurring cell death. Using retroviral lineage analysis, we demonstrate that exposure to CNTF causes a specific reduction of clones containing only rods without affecting other clone types, whereas blocking the CNTF/LIF receptor complex causes a specific increase of clones containing only rods. In addition, we show that stimulation of the CNTF/LIF pathway positively regulates the expression of the neuronal and endothelial nitric oxide synthase (NOS) genes, and blocking nitric oxide production by pre-treatment with a NOS inhibitor abolishes CNTF-induced cell death. Taken together, these results indicate that the CNTF/LIF signaling pathway acts via regulation of nitric oxide production to modulate developmental programmed cell death of postmitotic rod precursor cells.

Amyloid beta peptide causes chronic glial cell activation and neuro-degeneration after intravitreal injection

We have previously demonstrated that amyloid beta (Abeta) peptide is acutely toxic to retinal neurones in vivo and that this toxicity is mediated by an indirect mechanism. We have now extended these studies to look at the chronic effect of intravitreal injection of Abeta peptides on retinal ganglion cells (RGC), the projection neurones of the retina and the glial cell response. 5 months after injection of Abeta1-42 or Abeta42-1 there was no significant reduction in RGC densities but there was a significant reduction in the retinal surface area after both peptides. Phosphate-buffered saline (PBS) injection had no effect on retinal size or RGC density. There was a pronounced reduction in the number of large RGCs with a concomitant significant increase in medium and small RGCs. There was no change in cell sizes 5 months after injection with PBS. At 5 months after injection of both peptides, there was marked activation of Muller glial cells and microglia. There was also expression of the major histocompatibility complex (MHC) class II molecule on some of the microglial cells but we saw no evidence of T-cell infiltration into the injected retinas. In order to elucidate potential toxic mechanisms, we have looked at levels of glutamine synthetase and nitric oxide synthase. As early as 2 days after injection we noted that activation of Muller glia was associated with a decrease in glutamine synthetase immuno-reactivity but there was no detectable expression of inducible nitric oxide synthase in any retinal cells. These results suggest that chronic activation of glial cells induced by Abeta peptides may result in chronic atrophy of projection neurones in the rat retina.

Quantitative spatial analysis of the distribution of NADPH-diaphorase-positive neurons in the developing and mature rat retina

NADPH-diaphorase (NADPH-d) histochemistry labels a subpopulation of nitric oxide-synthesizing amacrine cells in the inner nuclear layer of the rat retina. We have studied their morphology and distribution in postnatal and adult rats in whole-mounted retinae. NAPDH-d-positive neurons are detected as early as postnatal day (P)5, especially in the peripheral retina; intense labeling of somata and long lengths of dendrites is obtained between P10 and P18, after which only the somata exhibit NADPH-d activity. The density and number of these cells increase progressively from P7 to P14, with a significantly higher density in the central retina as compared to the periphery. The sociology of these cells was analyzed quantitatively studying the Voronoi domains: a polygon area can be drawn that delineates the territory of the map that is closer to the cell than to any other cell of the map. In addition, we calculated the conformity ratio of Cook, i.e., the mean nearest neighbor distance/standard deviation of all the nearest neighbor distances, in order to reveal whether or not these cells are regularly distributed through the retina. We find that the distribution of the NADPH-d-positive cells tends to be regular throughout the retina: the local coefficient of variation (obtained by comparing the size of each Voronoi polygon area to those of its neighbors) tends to regularity at P14 and remains unaltered through maturity. Therefore, as other cell types, NADPH-d-positive amacrine cells are almost regularly distributed from the time of eye opening and nitric oxide may play a role in the development of retinal circuitry and in the regulation of retinal blood flow.

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.

Neuronal nitric oxide synthase is expressed in the axotomized ganglion cells of the rat retina

This study investigated the expression and cellular localization of neuronal nitric oxide synthase in the rat retina following optic nerve transection (ONT). In the normal rat retina, nNOS immunoreactivity was localized to amacrine cells and displaced amacrine cells. A few bipolar cells were also labeled. In the axotomized retina, ganglion cells showed nNOS immunoreactivity from 3 days after ONT, and these cells increased in number, peaking 5 days after ONT. Quantitative evaluation using immunoblotting confirmed that nNOS expression showed a peak value (255% of control levels) 5 days after ONT and decreased to 137% of controls by 28 days. These findings suggest that axotomized ganglion cells degenerate via NO-mediated excitotoxicity.

Long-term effect of retinal ganglion cell axotomy on the histomorphometry of other cells in the porcine retina

PURPOSE

To determine the effect of retinal ganglion cell axotomy on the thickness of inner plexiform, inner nuclear, and outer plexiform layers, as well as the densities of short- and middle-to-long-wavelength cones, in the porcine retina.

METHODS

Unilateral retinal ganglion cell axotomy was performed in seven domestic pigs by either surgical optic nerve section or peripapillary argon laser photocoagulation. Damage to the retinal vasculature was ruled out with fluorescein angiography. Histologic examination of the retinal tissue was performed nine months later. Cone densities were determined immunohistochemically with the anti-visual pigment antibodies COS-1 and OS-2. Image analysis of semithin retinal cross sections was used to measure the thickness of the retinal layers. The effect of axotomy was quantified by optic nerve axon counts and estimations of retinal ganglion cell counts. The data were compared between the eyes with axotomy and the contralateral normal eye using the nonparametric Wilcoxon rank sum test.

RESULTS

Treatment of the peripapillary retina with the argon laser resulted in a median decrease in axon counts and retinal ganglion cell density estimates of 31%. No optic nerve axons and cells resembling retinal ganglion cells were found in the eyes with transected optic nerves. There was no significant difference in either the thickness of any retinal layers or cone densities between axotomized and normal control eyes.

CONCLUSION

No signs of retrograde transsynaptic degeneration were observed in porcine retinas nine months after retinal ganglion cell axotomy.

Differences in nitric oxide production: a comparison of retinal ganglion cells and retinal glial cells cultured under hypoxic conditions

The aim of this study was to compare the effects of hypoxia on nitric oxide synthase (NOS) expression and the production of NO between isolated retinal ganglion cells (RGCs) and retinal glial cells. Reverse transcription-polymerase chain reaction (RT-PCR) was employed to examine the presence of neuronal NOS mRNA, inducible NOS mRNA, and endothelial NOS mRNAs in the two cell types. RGCs and retinal glial cells were separately cultured under hypoxic (10% O(2)) or control (20% O(2)) conditions. Changes in NOS-mRNA expression were quantified by real-time PCR, and nitrite in the medium was measured up to 96 h of culture. The effects of non-NOS- and iNOS-selective inhibitors on hypoxia-induced release of nitrite in the culture medium were evaluated. RT-PCR revealed the presence of three types of NOSs in the two types of cultured cells. Hypoxic culture conditions significantly changed the expression of all NOS mRNAs in retinal glial cells but not in RGCs. NO production showed significant changes corresponding to those of NOS mRNAs in retinal glial cells but not in RGCs, and both NOS inhibitors significantly reduced hypoxia-induced nitrite release in retinal glial cells. Retinal glial cells but not RGCs may be the major source of NO under hypoxic conditions.

Inhibition of nitric oxide synthase induces increased production of growth-associated protein 43 in the developing retina of the postnatal rat

We investigated the effects of N(G)-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, on retinal development in the postnatal rat by immunocytochemistry and immunoblotting using antisera against neuronal nitric oxide synthase (nNOS) or growth-associated protein 43 (GAP-43). An nNOS-immunoreactive band of 155 kDa and a GAP-43-immunoreactive band of 48 kDa were present in the extracts of both control and L-NAME-treated rat retinas. The intensity of the nNOS-immunoreactive band was much weaker in the treated rats, whereas the intensity of the GAP-43-immunoreactive band of 48 kDa was much stronger in the treated rats. Much stronger GAP-43 immunoreactivity was visible in the inner plexiform layer (IPL) of the treated retinas at P10, P14 and P21. Our findings suggest that NO may play an important role in the maturation of the IPL in the developing rat retina.

In situ localization of nitric oxide synthase and direct evidence of NO production in rat retinal ganglion cells

The expression of isoforms of nitric oxide synthase (NOS), enzymes responsible for NO production, and the synthesis of nitric oxide (NO) in rat retinal ganglion cells (RGCs) during synaptogenesis for various phases of the pre- and postnatal developmental periods were investigated. The retinas from prenatal, lactating, young, and adult rats were fixed in paraformaldehyde. The cryosections or paraformaldehyde-fixed ganglion cells purified from rat pups were immunostained for constitutive isoforms of NOS (n and eNOS) and observed with a confocal laser scanning microscope. Synthesis of NO in the RGCs was achieved by in vitro stimulation with glutamate. The intracellular NO levels were measured in real time using diaminofluorescein-2 diacetate, a fluorescence indicator of NO. Immunohistochemical analysis revealed nNOS and eNOS expressed in retinal ganglion cells during the first 2 postnatal weeks. Cultured RGCs also expressed nNOS and eNOS in vitro. Intracellular NO levels in cultured RGCs showed spontaneous fluctuation during a 20-min observation. The presence of both a non-specific NOS inhibitor, L-NAME, and a specific nNOS inhibitor, 7-NI, significantly inhibited (P<0.001) the increase of intracellular NO 6 and 8 min after the introduction of L-arginine and glutamate to the medium. This study revealed that all constitutive NOS isoforms are expressed in RGCs and demonstrated that NO is produced by nNOS mainly through stimulation by glutamate in cultured RGCs.

The development of NADPH-diaphorase and nitric oxide synthase in the visual system of the cichlid fish, Tilapia mariae

The pattern of NADPH-diaphorase expression was studied in the retina and optic tectum of the cichlid fish Tilapia mariae during the first developmental stages. NADPH-diaphorase activity was seen early, at hatching. In the retina a few cell bodies of the retinal inner nuclear layer showed a faint labeling. Scattered labeled cells were found in the stratum periventriculare of the optic tectum, while the optic nerve was unlabeled. Two days after hatching, the number of labeled neurons increased in the inner nuclear layer and a few stained cell bodies were also scattered in the ganglion cell layer. Both the inner and outer plexiform layers showed a diffuse staining and the optic nerve was devoid of labeling. In the optic tectum several positive cells in the periventricular layer, with their dendritic trees extending in the superficial fibrous layer, were found. In 1-month-old Tilapia, NADPH-diaphorase staining and nitric oxide synthase immunoreactivity were found to overlap in both the retina and optic tectum. The density of NADPH-diaphorase labeled neurons in the inner nuclear layer of the retina and in the stratum periventriculare of the optic tectum was largely reduced in comparison with 2 days posthatching embryos. These findings indicated an early and transient production of nitric oxide in the retina and optic tectum of Tilapia, suggesting a functional role for nitric oxide in the development of visual structures in aquatic vertebrates.

Nitric oxide, microglial activities and neuronal cell death in the lateral geniculate nucleus of glaucomatous rats

The present study was initiated to investigate neuronal degeneration, microglial reactivity and possible roles of NO in the lateral geniculate nucleus (LGN) of glaucomatous rats. An experimental one-eye glaucoma model was created by cauterization of the limbal-derived veins. Neuronal cell viability was studied by immunostaining with antibody against neuronal nuclei. Changes of expressions of nitric oxide synthase I (NOS I), NOS II, ED 1, OX6 and OX42 in the LGN were studied by immunohistochemistry. NADPH-d histochemistry was also employed. In the experimental glaucomatous rats, the number of NeuN labelled neurons was significantly decreased in both the ipsi- and contra-lateral sides of the ventral LGN (vLGN) but not the dorsal LGN (dLGN) at 1 month post-operation and beyond. Expressions of NOS I and NADPH-d were notably increased from 1 week post-operation in the ipsilateral vLGN. In the contralateral side of the vLGN, however, this change was only observed from 1 month post-operation. No NOS II immunoreaction was observed in LGN of both the normal control and glaucomatous rats. Increased microglial reactivity as indicated by OX-42 immunoreactivity was first observed in both sides of the LGN at 1 week post-operation, and this was most significant especially at 1 and 2 months post-operation. The present results suggest that NO and microglial cells may play some important roles in the pathologic processes of neuronal degeneration in the LGN of glaucomatous rats.

Cellular localization of neuronal nitric oxide synthase (NOS-1) in the human and rat retina

The neuronal form of nitric oxide synthase (NOS-1) has been localized to several cell types in the retinas of experimental animals; however, localization in the human retina has not been definitive. By using in situ hybridization and immunohistochemistry, we have compared the cellular expression and localization of NOS-1 in the rat and human retinas. In both rat and human retinas, NOS-1 is expressed in the inner segments of photoreceptors, cells in the inner nuclear layer, particularly amacrine cells, and retinal ganglion cells. In human cones, NOS-1 is abundantly present in the outer segments. In the rat, optic nerve transection caused a loss of cells that were positive for NOS-1 in the ganglion cell layer. Although a retinal ganglion cell localization has not been reported consistently in the literature, our data clearly localize NOS-1 to the retinal ganglion cells of the rat and human retinas.

Expression of beta-amyloid precursor and Bcl-2 proto-oncogene proteins in rat retinas after intravitreal injection of aminoadipic acid

In order to investigate the role of glia in relation to factors that affect the expression of beta-amyloid precursor protein (betaAPP) and B cell lymphoma oncogene protein (Bcl-2) in the central nervous tissue, the patterns of expression of betaAPP and Bcl-2 in developing and mature rat retinas were studied immunocytochemically after intravitreal injection of alpha-aminoadipic acid (alpha-AAA), a glutamate analogue and gliotoxin that is known to cause injury of retinal Müller glial cells. In normal developing retinas, betaAPP and Bcl-2 were expressed primarily but transiently in a small number of neurons in the ganglion cell layer during the first postnatal week. Immunoreactivity of betaAPP and Bcl-2 appeared in the endfeet and proximal part of the radial processes of Müller glial cells from the second postnatal week onwards. In rats that received intravitreal injection of alpha-AAA at birth, there was a loss of immunoreactivity to vimentin, and a delayed expressed on betaAPP or Bcl-2 in Muller glial cells until 3-5 weeks post-injection. Immunoreactive neurons were also observed in the inner retina especially in the ganglion cell layer from 5 to 35 days after injection. A significant reduction in numerical density of cells with large somata in the ganglion cell layer was observed in the neonatally injected retinas at P56, which was accompanied by an increased immunostaining in radial processes of Müller glial cells. In contrast, no detectable changes in the expression of betaAPP and Bcl-2 were observed in retina that received alpha-AAA as adults. These results indicate that the gliotoxin alpha-AAA has long lasting effects on the expression of betaAPP and Bcl-2 in Müller glial cells as well as neurons in the developing but not mature retinas. The loss of vimentin and delayed expression of betaAPP and Bcl-2 in developing Müller glial cells suggests that the metabolic integrity of Müller cells was temporarily compromised, which may have adverse effects on developing neurons that are vulnerable or dependent on trophic support from the Müller glial cells.

NADPH-diaphorase activity in normally developing and intracranially transplanted retinas

The activity and distribution of nicotinamide dinucleotide phosphate diaphorase (NADPH-d), an enzyme that is widely distributed in the central nervous system and involved in the production of the free radical nitric oxide, were investigated histochemically in the normal developing and intracranially transplanted retinas. In the normal rat retina, NADPH-d activity was first detected in cells in the ganglion cells layer (GCL) and blood vessels on the first postnatal day (P0). A small but distinct population of NADPH-d positive cells were observed along the inner border of the inner nuclear layer at P7. NADPH-d positive sublaminae began to appear in the inner plexiform layer during the second postnatal week, and several strongly reactive sublaminae resembling those observed in the adult were observed by the fourth postnatal week. The overall spatio- temporal sequence of development of NADPH-d positive cells in the transplanted retina was similar to that of the normal retina, except a lack of reactive in the inner plexiform layer in more mature transplants as compared with normal retinas of corresponding ages. These results indicate that the time course of development and distribution of NADPH-d cells in early postnatal retina requires signals mainly of intraretinal origin and is independent of influence from the surroundings. While this finding is supportive to the notion that neurons that are rich in NADPH-d are resistant to injury or perturbation, the observation of a lack of well organized NADPH-d reactive sublaminae in the inner plexiform layer in older transplants suggests a possible alteration in the synaptic circuitry in the inner retina with increasing postgrafting survival time.

Role of nitric oxide in photoreceptor survival in embryonic chick retinal cell culture

The presence of nitric oxide synthase (NOS) in chick retina during development has allowed us to study the role of nitric oxide (NO) during retinal differentiation in dissociated chick retinal cell culture from embryonic day 6. We have demonstrated the presence of nicotinamide adenine dinucleotide phosphate diaphorase staining in these cultures after 3 days in vitro (Div), with a maximal intensity after 8 Div, corresponding to embryonic day 14. Immunohistochemistry studies confirmed the presence of the two isoforms of NOS, NOS-I and -III, in dissociated retinal cell cultures at 8 Div. Addition of NG-monomethyl-L-arginine, a NOS inhibitor, to retinal cell cultures prevented NO production but did not modify the appearance and the survival of ganglion and amacrine cells. However, immunohistochemical analysis with distinct markers for photoreceptor cells (rods and cones) showed that inhibition of endogenous NOS in retinal cell cultures prevented the developmental decrease of rod number between 5 and 8 Div, thus supporting the hypothesis that NO may be involved in the cell death of rods during the development of the retina.