Endothelial nitric oxide synthase (eNOS) is localized to Müller cells in all vertebrate retinas

Tert-butylhydroquinone protects the retina from oxidative stress in STZ-induced diabetic rats via the PI3K/Akt/eNOS pathway

This study aims to investigate whether tert-butylhydroquinone protects the retina from oxidative stress in STZ-induced experimental diabetic rats through the activation of phosphinositide 3-kinase (PI3K)/Akt/endothelial nitric oxide synthase (eNOS) pathway.In vitro, NO, reactive oxygen species(ROS), eNOS, p-eNOS Ser1179, Akt, p-Akt Ser473 and L-NAME protein expression was analyzed within rMC-1 cells cultivated within normal control(NC), high glucose (HG) and HG-containing tert-butyl hydroquinone (tBHQ) (5 μM) medium. We confirmed tBHQ's protection through administering inhibitors of PI3K and Akt. In vivo, tBHQ was administered at a ratio of 1% (w/w) to diabetic rats was induced through an STZ injection (65 mg/kg) for a 3-month period, and the retinal expression of eNOS, p-eNOS Ser1179, Akt, and p-Akt Ser473 proteins was measured using Western blotting (WB) assay. We also utilized the TUNEL kit for detecting retinal cell apoptosis. The changes of retinal morphology and visual function were measured by performing hematoxylin-eosin staining (HE staining) and electroretinograms. In vitro, ROS levels were increased in the high glucose group, NO levels were decreased, and the relative expression of Akt/p-Akt Ser473 and eNOs/p-eNOS Ser1179 was reduced. tBHQ abolished these changes, and these effects were suppressed by specific inhibitors. In vivo, tBHQ upregulated retinal protein expression in STZ-induced diabetic rats, reduced retinal apoptotic cell numbers, and partially prevented abnormalities in retinal function and structure caused by diabetes. tBHQ alleviates oxidative stress during diabetic retinopathy by upregulating the PI3K/Akt/eNOS pathway and partially restoring the structure and function of the retina. It may play a role in delaying vision loss caused by diabetic retinopathy.

Light adaptation in the chick retina: Dopamine, nitric oxide, and gap-junction coupling modulate spatiotemporal contrast sensitivity

Adaptation to changes in ambient light intensity, in retinal cells and circuits, optimizes visual functions. In the retina, light-adaptation results in changes in light-sensitivity and spatiotemporal tuning of ganglion cells. Under light-adapted conditions, contrast sensitivity (CS) of ganglion cells is a bandpass function of spatial frequency; in contrast, dark-adaptation reduces CS, especially at higher spatial frequencies. In this work, we aimed to understand intrinsic neuromodulatory mechanisms that underlie retinal adaptation to changes in ambient light level. Specifically, we investigated how CS is affected by dopamine (DA), nitric oxide (NO), and modifiers of electrical coupling through gap junctions, under different conditions of adapting illumination. Using the optokinetic response as a behavioral readout of direction-selective ganglion cell activity, we characterized the spatial CS of chicks under high- and low-photopic conditions and how it was regulated by DA, NO, and gap-junction uncouplers. We observed that: (1) DA D2R-family agonists and a donor of NO increased CS tested in low-photopic illumination, as if observed in the high-photopic light; whereas (2) removing their effects using either DA antagonists or NO- synthase inhibitors mimicked low-photopic CS; (3) simulation of high-photopic CS by DA agonists was abolished by NO-synthase inhibitors; and (4) selectively blocking coupling via connexin 35/36-containing gap junctions, using a "designer" mimetic peptide, increased CS, as does strong illumination. We conclude that, in the chicken retina: (1) DA and NO induce changes in spatiotemporal processing, similar to those driven by increasing illumination, (2) DA possibly acts through stimulating NO synthesis, and (3) blockade of coupling via gap junctions containing connexin 35/36 also drives a change in retinal CS functions. As a noninvasive method, the optokinetic response can provide rapid, conditional, and reversible assessment of retinal functions when pharmacological reagents are injected into the vitreous humor. Finally, the chick's large eyes, and the many similarities between their adaptational circuit functions and those in mammals such as the mouse, make them a promising model for future retinal research.

Diabetic macular oedema: under-represented in the genetic analysis of diabetic retinopathy

Diabetic retinopathy, a complication of both type 1 and type 2 diabetes, is a complex disease and is one of the leading causes of blindness in adults worldwide. It can be divided into distinct subclasses, one of which is diabetic macular oedema. Diabetic macular oedema can occur at any time in diabetic retinopathy and is the most common cause of vision loss in patients with type 2 diabetes. The purpose of this review is to summarize the large number of genetic association studies that have been performed in cohorts of patients with type 2 diabetes and published in English-language journals up to February 2017. Many of these studies have produced positive associations with gene polymorphisms and diabetic retinopathy. However, this review highlights that within this large body of work, studies specifically addressing a genetic association with diabetic macular oedema, although present, are vastly under-represented. We also highlight that many of the studies have small patient numbers and that meta-analyses often inappropriately combine patient data sets. We conclude that there will continue to be conflicting results and no meaningful findings will be achieved if the historical approach of combining all diabetic retinopathy disease states within patient cohorts continues in future studies. This review also identifies several genes that would be interesting to analyse in large, well-defined cohorts of patients with diabetic macular oedema in future candidate gene association studies.

NO signaling in retinal bipolar cells

Nitric oxide (NO) is a neuromodulator involved in physiological and pathological processes in the retina. In the inner retina, a subgroup of amacrine cells have been shown to synthesize NO, but bipolar cells remain controversial as NO sources. This study correlates NO synthesis in dark-adapted retinas, through labeling with the NO marker DAF-FM, with neuronal nitric oxide synthase (nNOS) and inducible NOS expression, and presence of the NO receptor soluble guanylate cyclase in bipolar cells. NO containing bipolar cells were morphologically identified by dialysis of DAF fluorescent cells with intracellular dyes, or by DAF labeling followed by immunohistochemistry for nNOS and other cellular markers. DAF fluorescence was observed in all types of bipolar cells that could be identified, but the most intense DAF fluorescence was observed in bipolar cells with severed processes, supporting pathological NO signaling. Among nNOS expressing bipolar cells, type 9 was confirmed unequivocally, while types 2, 3a, 3b, 4, 5, 7, 8 and the rod bipolar cell were devoid of this enzyme. These results establish specific bipolar cell types as NO sources in the inner retina, and support the involvement of NO signaling in physiological and pathological processes in the inner retina.

Diabetic macular oedema: clinical risk factors and emerging genetic influences

Diabetic macular oedema is the major cause of visual impairment in type 1 and type 2 diabetes. As type 2 diabetes becomes more prevalent worldwide, the prevalence of diabetic macular oedema is also expected to rise. Current management of diabetic macular oedema is challenging, expensive and not optimal in a subset of patients. Therefore, it is important to increase our understanding of the risk factors involved and develop preventative strategies. While clinical risk factors for diabetic macular oedema have been identified, few studies have addressed potential genetic risk factors. Epidemiology and family studies suggest genetic influences are of importance. In this review, we summarise known clinical risk factors, as well as discuss the small number of genetic studies that have been performed for diabetic macular oedema.

Light-evoked S-nitrosylation in the retina

Nitric oxide (NO) synthesis in the retina is triggered by light stimulation. NO has been shown to modulate visual signal processing at multiple sites in the vertebrate retina, via activation of the most sensitive target of NO signaling, soluble guanylate cyclase. NO can also alter protein structure and function and exert biological effects directly by binding to free thiol groups of cysteine residues in a chemical reaction called S-nitrosylation. However, in the central nervous system, including the retina, this reaction has not been considered to be significant under physiological conditions. Here we provide immunohistochemical evidence for extensive S-nitrosylation that takes place in the goldfish and mouse retinas under physiologically relevant light intensities, in an intensity-dependent manner, with a strikingly similar pattern in both species. Pretreatment with N-ethylmaleimide (NEM), which occludes S-nitrosylation, or with 1-(2-trifluromethylphenyl)imidazole (TRIM), an inhibitor of neuronal NO synthase, eliminated the light-evoked increase in S-nitrosylated protein immunofluorescence (SNI) in the retinas of both species. Similarly, light did not increase SNI, above basal levels, in retinas of transgenic mice lacking neuronal NO synthase. Qualitative analysis of the light-adapted mouse retina with mass spectrometry revealed more than 300 proteins that were S-nitrosylated upon illumination, many of which are known to participate directly in retinal signal processing. Our data strongly suggest that in the retina light-evoked NO production leads to extensive S-nitrosylation and that this process is a significant posttranslational modification affecting a wide range of proteins under physiological conditions.

Expression of inducible nitric oxide synthase (iNOS) in microglia of the developing quail retina

Inducible nitric oxide synthase (iNOS), which produce large amounts of nitric oxide (NO), is induced in macrophages and microglia in response to inflammatory mediators such as LPS and cytokines. Although iNOS is mainly expressed by microglia that become activated in different pathological and experimental situations, it was recently reported that undifferentiated amoeboid microglia can also express iNOS during normal development. The aim of this study was to investigate the pattern of iNOS expression in microglial cells during normal development and after their activation with LPS by using the quail retina as model. iNOS expression was analyzed by iNOS immunolabeling, western-blot, and RT-PCR. NO production was determined by using DAR-4M AM, a reliable fluorescent indicator of subcellular NO production by iNOS. Embryonic, postnatal, and adult in situ quail retinas were used to analyze the pattern of iNOS expression in microglial cells during normal development. iNOS expression and NO production in LPS-treated microglial cells were investigated by an in vitro approach based on organotypic cultures of E8 retinas, in which microglial cell behavior is similar to that of the in situ retina, as previously demonstrated in our laboratory. We show here that amoeboid microglia in the quail retina express iNOS during normal development. This expression is stronger in microglial cells migrating tangentially in the vitreal part of the retina and is downregulated, albeit maintained, when microglia differentiate and become ramified. LPS treatment of retina explants also induces changes in the morphology of amoeboid microglia compatible with their activation, increasing their lysosomal compartment and upregulating iNOS expression with a concomitant production of NO. Taken together, our findings demonstrate that immature microglial cells express iNOS during normal development, suggesting a certain degree of activation. Furthermore, LPS treatment induces overactivation of amoeboid microglia, resulting in a significant iNOS upregulation.

Ionotropic purinergic receptors P2X in frog and turtle retina: glial and neuronal localization

Purinergic signaling is represented in both the peripheral and central nervous system (CNS), and in particular in the retina, which may be regarded as a part of the CNS. While purigenic signaling is relatively well studied in mammalian retinas, little is known about it in retinas of lower vertebrates. The aim of present study was to investigate, using immunocytochemistry, the distribution of purinoreceptors P2X in retinas of frog and turtle, which are appropriate models of the brain neuron-to-glia interactions. The results showed widespread expression of all seven ionotropic purinoreceptors (P2X1-P2X7) in both frog and turtle retinas. They were predominantly expressed in Müller cells, the principal glial cells in the retina. All structures typical of Müller cells: the outer and the inner limiting membranes, the cells bodies in the inner nuclear layer, the radial processes in the inner plexiform layer (IPL), and the so called endfeet (frog) or the orthogonal arrays of particles (turtle) in the ganglion cells layer were immunostained. Colocalizations between P2X1-P2X7 and the glial cell marker Vimentin proved that the immunostaining was in the Müller cells. In addition to the glial staining, neuronal staining was also seen as fine puncta in the inner plexiform layer and by small dots and patches in the outer plexiform layer. Some cell bodies of horizontal, amacrine and ganglion cells were also stained. The results obtained imply that the purinergic P2X receptors may significantly contribute to the neuron-to-glia signaling in retinas of the lower vertebrates.

Nitric oxide mediates activity-dependent plasticity of retinal bipolar cell output via S-nitrosylation

Coding a wide range of light intensities in natural scenes poses a challenge for the retina: adaptation to bright light should not compromise sensitivity to dim light. Here we report a novel form of activity-dependent synaptic plasticity, specifically, a "weighted potentiation" that selectively increases output of Mb-type bipolar cells in the goldfish retina in response to weak inputs but leaves the input-output ratio for strong stimuli unaffected. In retinal slice preparation, strong depolarization of bipolar terminals significantly lowered the threshold for calcium spike initiation, which originated from a shift in activation of voltage-gated calcium currents (ICa) to more negative potentials. The process depended upon glutamate-evoked retrograde nitric oxide (NO) signaling as it was eliminated by pretreatment with an NO synthase blocker, TRIM. The NO-dependent ICa modulation was cGMP independent but could be blocked by N-ethylmaleimide (NEM), indicating that NO acted via an S-nitrosylation mechanism. Importantly, the NO action resulted in a weighted potentiation of Mb output in response to small (≤-30 mV) depolarizations. Coincidentally, light flashes with intensity ≥ 2.4 × 10(8) photons/cm(2)/s lowered the latency of scotopic (≤ 2.4 × 10(8) photons/cm(2)/s) light-evoked calcium spikes in Mb axon terminals in an NEM-sensitive manner, but light responses above cone threshold (≥ 3.5 × 10(9) photons/cm(2)/s) were unaltered. Under bright scotopic/mesopic conditions, this novel form of Mb output potentiation selectively amplifies dim retinal inputs at Mb → ganglion cell synapses. We propose that this process might counteract decreases in retinal sensitivity during light adaptation by preventing the loss of visual information carried by dim scotopic signals.

Nitric oxide production and the expression of two nitric oxide synthases in the avian retina

Nitric oxide (NO) is known to exert multiple effects on the function of many retinal neurons and their synapses. Therefore, it is equally important to understand the potential sources of NO within the retina. To explore this, we employ a combination of 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM) based NO detection and immunohistochemistry for the NO synthetic enzymes, neuronal and endothelial nitric oxide synthase (nNOS and eNOS). We find DAF signals in photoreceptors, horizontal cells, amacrine cells, efferent synapses, Müller cells, and cells in the ganglion cell layer (GCL). nNOS immunoreactivity was consistent with the DAF signal with the exception that horizontal cells and Müller cells were not clearly labeled. eNOS-like immunoreactivity (eNOS-LI) was more widespread with photoreceptors, horizontal cells, occasional bipolar cells, amacrine cells, Müller cells, and cells in the GCL all showing labeling. Double labeling with antibodies raised against calretinin, syntaxin, and glutamine synthetase confirmed that horizontal cells, amacrine cells, and Müller cells (respectively) were expressing eNOS-LI. Although little or no nNOS labeling is observed in horizontal cells or Müller cells, the expression of eNOS-LI is consistent with the ability of these cells to produce NO. Together these results suggest that the capability to produce NO is widespread in the chicken retina. We propose that multiple forms of regulation for nNOS and eNOS play a role in the patterning of NO production in the chicken retina.

NR2C and NR2D subunits of NMDA receptors in frog and turtle retina

Glutamate NMDA (N-methyl-D-aspartate) receptors are widely distributed in the central nervous system where they are involved in cognitive processes, motor control and many other functions. They are also well studied in the retina, which may be regarded as a biological model of the nervous system. However, little is known about NR2C and NR2D subunits of NMDA receptors, which have some specific features as compared to other subunits. Consequently the aim of the present study was to investigate their distribution in frog (Rana ridibunda) and turtle (Emys orbicularis) retinas which possess mixed and cone types of retina respectively. The experiments were performed using an indirect immunofluorescence method. Four antibodies directed to NR2C and NR2D subunits of NMDA receptor, as well as three antibodies directed to different splice variants of NR1 subunit, which is known to be obligatory for proper functioning of the receptor, were applied. All antibodies caused well expressed labeling in frog and turtle retinas. The NR2C and NR2D subunits were localized in glial Müller cells, while the NR1 subunit had both neuronal and glial localization. Our results show that glial NMDA receptors differ from neuronal ones in their subunit composition. The functional significance of the NMDA receptors and their NR2C and NR2D subunits, in particular for the neuron-glia interactions, is discussed.

Involvement of oxidative stress and mitochondrial dysfunction in the osmotic swelling of retinal glial cells from diabetic rats

Osmotic swelling of retinal glial (Müller) cells may contribute to the development of edema in diabetic retinopathy. Here, we tested whether oxidative stress and mitochondrial dysfunction are pathogenic factors involved in the osmotic swelling of Müller cells in retinal slices from control and streptozotocin-injected hyperglycemic rats. Hypotonic challenge did not change the size of Müller cell somata from control animals but induced soma swelling in Müller cells of diabetic animals. Administration of a reducing agent blocked the osmotic swelling of Müller cell somata. In retinal tissues from control animals, administration of the reducing agent blocked also the swelling-inducing effects of antagonists of P2Y₁ and adenosine A₁ receptors. In tissues from diabetic animals, inhibition of xanthine oxidase decreased the soma swelling by approximately 50% while inhibition of NADPH oxidase and nitric oxide synthase had no effects. Blockade of mitochondrial oxidative stress by perindopril, as well as of mitochondrial permeability transition by cyclosporin A or minocycline, attenuated the swelling. In addition, activation of mitochondrial K(ATP) channels by pinacidil fully prevented the swelling. The data suggest that oxidative stress produced by xanthine oxidase, as well as the mitochondria, are implicated in the induction of osmotic swelling of Müller cells from diabetic rats.

Evolution of the nitric oxide synthase family in metazoans

Nitric oxide (NO) is essential to many physiological functions and operates in several signaling pathways. It is not understood how and when the different isoforms of nitric oxide synthase (NOS), the enzyme responsible for NO production, evolved in metazoans. This study investigates the number and structure of metazoan NOS enzymes by genome data mining and direct cloning of Nos genes from the lamprey. In total, 181 NOS proteins are analyzed from 33 invertebrate and 63 vertebrate species. Comparisons among protein and gene structures, combined with phylogenetic and syntenic studies, provide novel insights into how NOS isoforms arose and diverged. Protein domains and gene organization--that is, intron positions and phases--of animal NOS are remarkably conserved across all lineages, even in fast-evolving species. Phylogenetic and syntenic analyses support the view that a proto-NOS isoform was recurrently duplicated in different lineages, acquiring new structural configurations through gains and losses of protein motifs. We propose that in vertebrates a first duplication took place after the agnathan-gnathostome split followed by a paralog loss. A second duplication occurred during early tetrapod evolution, giving rise to the three isoforms--I, II, and III--in current mammals. Overall, NOS family evolution was the result of multiple gene and genome duplication events together with changes in protein architecture.

Identification of alternate transcripts of neuronal nitric oxide synthase in the mouse retina

Nitric oxide (NO) is a major signaling molecule in the retina and CNS, with physiological roles in every cell type in the retina. Previous work shows that neuronal nitric oxide synthase (nNOS) is an important source of NO in the vertebrate retina. There are distinct, active alternative transcripts of nNOS observed in many tissues, including testes and brain, that may differ in both localization and enzyme kinetics. The present study characterized nNOS and the NO production from nNOS in the mouse retina in terms of its alternate transcripts, namely, nNOS alpha, nNOS beta, and nNOS gamma. We examined both basal and light-stimulated NO production as imaged using the NO-sensitive dye 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate-FM (DAF-FM), and we compared the NO production with the immunocytochemical localization of nNOS using antisera that recognize nNOS alpha/beta or nNOS alpha/beta/gamma. Western blots suggested the presence of NOS alpha/gamma protein in retina, but not nNOS beta, and we confirmed this at the message level by using a combination of RT-PCR and quantitative real-time PCR. Our findings indicated that the primary source of NO in the mammalian retina is nNOS alpha and that nNOS gamma may contribute to NO production as well.

Effects of carbon monoxide on trout and lamprey vessels

Carbon monoxide (CO) is endogenously produced by heme oxygenase (HO) and is involved in vascular, neural, and inflammatory responses in mammals. However, the biological activities of CO in nonmammalian vertebrates is unknown. To this extent, we used smooth muscle myography to investigate the effects of exogenously applied CO (delivered via a water-soluble CO-releasing molecule, CORM-3) on isolated lamprey ( Petromyzon marinus) dorsal aortas and examined its mechanisms of action on trout ( Oncorhynchus mykiss) efferent branchial (EBA) and celiacomesenteric (CMA) arteries. CORM-3 dose-dependently relaxed all vessels examined. Trout EBA were twofold more sensitive to CORM-3 when precontracted with norepinephrine (NE) than KCl and CORM-3 relaxed five-fold more of the NE- than KCl-induced tension. Glybenclamide (10 μM), an ATP-sensitive potassium channel inhibitor, inhibited NE-induced contraction, but did not affect CORM-3-induced relaxation. NS-2028 (10 μM), a soluble guanylyl cyclase inhibitor, had no effect on a NE-contraction, but inhibited a subsequent CORM-3-induced relaxation. Zinc protopophyrin-IX (ZnPP-IX, 0.3–30 μM), a HO inhibitor, elicited a small, yet dose-dependent and significant, increase in baseline tension but did not have any effect on subsequent NE-induced contractions or a nitric oxide-induced relaxation (via sodium nitroprusside). [ZnPP-IX] greater than 3 μM, however, significantly reduced the predominant vasodilatory response of trout EBA to hydrogen sulfide. These results implicate an active HO/CO pathway in trout vessels having an impact on resting vessel tone and CO-induced vasoactivity that is at least partially mediated by soluble guanylyl cyclase.

Neurally-derived nitric oxide regulates vascular tone in pulmonary and cutaneous arteries of the toad,Bufo marinus

In this study, the role of nitric oxide (NO) in regulation of the pulmocutaneous vasculature of the toad, Bufo marinus was investigated. In vitro myography demonstrated the presence of a neural NO signaling mechanism in both arteries. Vasodilation induced by nicotine was inhibited by the soluble guanylyl cyclase (GC) inhibitor, 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one, and the NO synthase (NOS) inhibitor, Nω-nitro-l-arginine (l-NNA). Removal of the endothelium had no significant effect on the vasodilation. Furthermore, pretreatment with N5-(1-imino-3-butenyl)-l-ornithine (vinyl-l-NIO), a more specific inhibitor of neural NOS, caused a significant decrease in the nicotine-induced dilation. In the pulmonary artery only, a combination of l-NNA and the calcitonin gene-related peptide (CGRP) receptor antagonist, CGRP(8-37), completely blocked the nicotine-induced dilation. In both arteries, the vasodilation was also significantly decreased by glibenclamide, an ATP-sensitive K+(K+ATP) channel inhibitor. Levcromakalim, a K+ATPchannel opener, caused a dilation that was blocked by glibenclamide in both arteries. In the pulmonary artery, NO donor-mediated dilation was significantly decreased by pretreatment with glibenclamide. The physiological data were supported by NADPH-diaphorase histochemistry and immunohistochemistry, which demonstrated NOS in perivascular nerve fibers but not the endothelium of the arteries. These results indicate that the pulmonary and cutaneous arteries of B. marinus are regulated by NO from nitrergic nerves rather than NO released from the endothelium. The nitrergic vasodilation in the arteries appears to be caused, in part, via activation of K+ATPchannels. Thus, NO could play an important role in determining pulmocutaneous blood flow and the magnitude of cardiac shunting.

Glial cells dilate and constrict blood vessels: a mechanism of neurovascular coupling

Neuronal activity evokes localized changes in blood flow. Although this response, termed neurovascular coupling, is widely used to monitor human brain function and diagnose pathology, the cellular mechanisms that mediate the response remain unclear. We investigated the contribution of glial cells to neurovascular coupling in the acutely isolated mammalian retina. We found that light stimulation and glial cell stimulation can both evoke dilation or constriction of arterioles. Light-evoked and glial-evoked vasodilations were blocked by inhibitors of cytochrome P450 epoxygenase, the synthetic enzyme for epoxyeicosatrienoic acids. Vasoconstrictions, in contrast, were blocked by an inhibitor of omega-hydroxylase, which synthesizes 20-hydroxyeicosatetraenoic acid. Nitric oxide influenced whether vasodilations or vasoconstrictions were produced in response to light and glial stimulation. Light-evoked vasoactivity was blocked when neuron-to-glia signaling was interrupted by a purinergic antagonist. These results indicate that glial cells contribute to neurovascular coupling and suggest that regulation of blood flow may involve both vasodilating and vasoconstricting components.

Phylogenesis of constitutively formed nitric oxide in non-mammals

It is widely recognized that nitric oxide (NO) in mammalian tissues is produced from L-arginine via catalysis by NO synthase (NOS) isoforms such as neuronal NOS (nNOS) and endothelial NOS (eNOS) that are constitutively expressed mainly in the central and peripheral nervous system and vascular endothelial cells, respectively. This review concentrates only on these constitutive NOS (cNOS) isoforms while excluding information about iNOS, which is induced mainly in macrophages upon stimulation by cytokines and polysaccharides. The NO signaling pathway plays a crucial role in the functional regulation of mammalian tissues and organs. Evidence has also been accumulated for the role of NO in invertebrates and non-mammalian vertebrates. Expression of nNOS in the brain and peripheral nervous system is widely determined by staining with NADPH (reduced nicotinamide adenine dinucleotide phosphate) diaphorase or NOS immunoreactivity, and functional roles of NO formed by nNOS are evidenced in the early phylogenetic stages (invertebrates and fishes). On the other hand, the endothelium mainly produces vasodilating prostanoids rather than NO or does not liberate endothelium-derived relaxing factor (EDRF) (fishes), and the ability of endothelial cells to liberate NO is observed later in phylogenetic stages (amphibians). This review article summarizes various types of interesting information obtained from lower organisms (invertebrates, fishes, amphibians, reptiles, and birds) about the properties and distribution of nNOS and eNOS and also the roles of NO produced by the cNOS as an important intercellular signaling molecule.

Neuroprotective effect of citicoline on retinal cell damage induced by kainic acid in rats

PURPOSE

To examine whether citicoline has a neuroprotective effect on kainic acid (KA)-induced retinal damage.

METHODS

KA (6 nmol) was injected into the vitreous of rat eyes. Citicoline (500mg/kg, i.p.) was administered to the rats once before and twice a day after KA-injection for 3- and 7-day intervals. The neuroprotective effects of citicoline were estimated by measuring the thickness of the various retinal layers using hematoxylin-eosin (H&E) staining. In addition, immunohistochemistry was conducted to elucidate the expression of endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS).

RESULTS

Morphometric analysis of retinal damage in KA-injected eyes showed significant cell loss in the inner nuclear layer (INL) and inner plexiform layer (IPL) of the retinas at 3 and 7 days after KA injection, but not in the outer nuclear layer (ONL). At 3 days after citicoline treatment, no significant changes were detected in the retinal thickness and immunoreactivities of eNOS and nNOS. The immunoreactivities of eNOS and nNOS increased in the retina at 7 days after the KA injection. However, prolonged treatment for 7 days significantly attenuated the immunoreactivities and the reduction of thickness.

CONCLUSIONS

The results indicate that citicoline has a neuroprotective effect on KA-induced neurotoxicity in the retina.

NO as a mediator during the early development of the cardiovascular system in the zebrafish

As a general pattern innervation of the cardiovascular system appears late during development in vertebrate embryos, and cardiovascular control may be achieved by hormonal activity in early stages. However, very little is known about the onset of NO-responsiveness during development, which in adult vertebrates is known to play a key function in many physiological processes such as control of vascular tone, neurotransmission, macrophage activity, and angiogenesis. Analysis of the effect of NO on the cardiovascular system in zebrafish (Danio rerio) embryos and larvae revealed almost no effect on cardiac activity during chronic exposure to NO-producing chemicals, whereas vascular reactivity was observed in veins and arteries of the zebrafish in early developmental stages (5-6 days post fertilization). Chronic exposure also modified the development of the vascular system. The presence of an NO donor (sodium nitroprusside) did not change the patterning of the vascular bed, but it induced an earlier appearance of some blood vessels in the trunk region of the zebrafish larvae. The data reveal that NO plays an important role in the development of the cardiovascular system and in the ontogeny of the cardiovascular control system in fish.

Constitutive nitric oxide synthase activity is required to trigger ischemic tolerance in mouse retina

Profound morphologic and functional protection against retinal ischemic injury can be achieved if the tissue is 'preconditioned' one day earlier with a brief, noninjurious ischemic challenge. To begin to address the mechanistic basis of this 'ischemic tolerance', we used genetic and pharmacologic approaches to test the hypothesis that nitric oxide (NO) derived from one of the three NO synthase (NOS) isoforms was responsible for triggering the adaptive response to brief preconditioning ischemia. Retinae of adult mice were preconditioned with 5-min preconditioning ischemia and subjected to 45-min injurious ischemia 24 hr later. Some animals were treated with the constitutive NOS inhibitor L-nitroarginine (5 mg/kg, i.p.) 1 hr before preconditioning. Retinal layer thicknesses and cell counts were determined one week postischemia in 5-mum thin sections, and flash electroretinograms were obtained at 1 and 7 days postischemia. We confirmed that ischemic preconditioning afforded morphologic and functional protection in the strains of wild-type mice studied. Histopathologic analyses of inducible NOS (iNOS) knockout mice revealed that ischemic preconditioning was completely effective, whereas ischemic tolerance was not achieved in the retinae of endothelial NOS (eNOS) and neuronal NOS (nNOS) knockout mice. The participation of the constitutive NOS enzymes in preconditioning-induced tolerance was confirmed by the finding that administration of the NOS inhibitor L-NA to wild-type mice prior to ischemic preconditioning blocked the development of ischemic tolerance. These cross-validating genetic and pharmacologic findings indicate that NO derived from both eNOS and nNOS is a required molecular signal in the adaptive response to ischemic preconditioning in the retina.

Nitric Oxide (NO) in normal and hypoxic vascular regulation of the spiny dogfish,Squalus acanthias

Nitric oxide (NO) is a potent vasodilator in terrestrial vertebrates, but whether vascular endothelial-derived NO plays a role in vascular regulation in fish remains controversial. To explore this issue, a study was made of spiny dogfish sharks (Squalus acanthias) in normoxia and acute hypoxia (60 min exposure to seawater equilibrated with 3% oxygen) with various agents known to alter NO metabolism or availability. In normoxia, nitroprusside (a NO donor) reduced blood pressure by 20%, establishing that vascular smooth muscle responds to NO. L-arginine, the substrate for NO synthase, had no hemodynamic effect. Acetylcholine, which stimulates endothelial NO and prostaglandin production in mammals, reduced blood pressure, but also caused marked bradycardia. L-NAME, an inhibitor of all NO synthases, caused a small 10% rise in blood pressure, but cell-free hemoglobin (a potent NO scavenger and hypertensive agent in mammals) had no effect. Acute hypoxia caused a 15% fall in blood pressure, which was blocked by L-NAME and cell-free hemoglobin. Serum nitrite, a marker of NO production, rose with hypoxia, but not with L-NAME. Results suggest that NO is not an endothelial-derived vasodilator in the normoxic elasmobranch. The hypertensive effect of L-NAME may represent inhibition of NO production in the CNS and nerves regulating blood pressure. In acute hypoxia, there is a rapid up-regulation of vascular NO production that appears to be responsible for hypoxic vasodilation.

Nitric oxide modulates the transfer function between cones and horizontal cells during changing conditions of ambient illumination

It has been suggested that nitric oxide (NO) serves as a retinal neuromodulator, adjusting retinal function to changing conditions of adaptation. We tested this hypothesis in the intact turtle retina by recording the photoresponses of L-cones and L1-horizontal cells, while changing retinal NO level and background illumination. Raising the retinal level of NO, by adding an NO donor (sodium nitroprusside) or the precursor for NO synthesis (L-arginine), induced response augmentation in L-cones and L1-horizontal cells. Lowering retinal level of NO by adding L-NAME, an inhibitor of NO synthesis, reduced the amplitudes of the photoresponses in these retinal neurons. The transfer function between L-cones and L1-horizontal cells, constructed from the photoresponses of these cells, was modified by NO and by background lights. The nonlinear transfer function, characteristic of the dark-adapted retina, became linear and of low gain when the retinal NO level was increased or by increasing the level of ambient illumination. In contrast, inhibiting NO synthesis in the light-adapted retina induced nonlinearity in the cone-to-horizontal cell transfer function similar to that seen in the dark-adapted state. NADPH diaphorase histochemistry, conducted on isolated retinal cells, demonstrated activity in cone inner segments and distal process of Müller cells. These findings support the hypothesis that NO synthesis in the distal turtle retina is triggered by background illumination, and that NO acts to adjust the modes of visual information processing in the outer plexiform layer to the conditions required during continuous background illumination.

Endothelial nitric oxide synthase gene is associated with diabetic macular edema in type 2 diabetes

OBJECTIVE

We examined the endothelial nitric oxide (eNOS) gene polymorphisms to assess its possible association with diabetic retinopathy and macular edema.

RESEARCH DESIGN AND METHODS

A total of 226 patients with type 2 diabetes and 186 healthy subjects were studied. Type 2 diabetic patients consisted of 110 patients without retinopathy, 46 patients with nonproliferative diabetic retinopathy, and 71 patients with proliferative diabetic retinopathy. Diabetic macular edema was present in 48 patients. Three polymorphisms of the eNOS gene were determined: T-786C in the promoter region, 27-bp repeat in intron 4, and Glu298Asp in exon 7.

RESULTS

Close linkage disequilibrium was observed between the T-786C polymorphism and the 27-bp repeat, as has been previously reported, but Glu298Asp was not in linkage disequilibrium with the other two polymorphisms. The eNOS gene polymorphisms were not significantly associated with the presence of retinopathy or with retinopathy severity or type 2 diabetes itself. However, by both association study and multiple logistic regression analysis, the T-786C and 27-bp repeat polymorphisms were significantly associated with a risk of developing macular edema with the -786C allele and the "a" allele increasing the risk.

CONCLUSIONS

The present study suggests that the eNOS gene is a novel genetic risk factor for diabetic macular edema. The eNOS gene polymorphisms may contribute to the development of macular edema by impairing basal eNOS expression and resulting in the breakdown of the blood-retina barrier.

Reciprocal Modulation of Calcium Dynamics at Rod and Cone Photoreceptor Synapses by Nitric Oxide

The abundance of nitric oxide (NO) synthesizing enzymes identified in the vertebrate retina highlight the importance of NO as a signaling molecule in this tissue. Here we describe opposing actions of NO on the rod and cone photoreceptor synapse. Depolarization-induced increases of calcium concentration in rods and cones were enhanced and inhibited, respectively, by the NO donor S-nitrosocysteine. NO suppressed calcium current in cones by decreasing the maximum conductance, whereas NO facilitated rod Ca channel activation. NO also activated a nonselective voltage-independent conductance in both rods and cones. Suppression of NO production in the intact retina with NG-nitro-l-arginine favored cone over rod driven postsynaptic signals, as would be expected if NO enhanced rod and suppressed cone synaptic activity. These findings may imply involvement of NO in regulating the strength of rod and cone pathways in the retina during different states of adaptation.

Pharmacological characterization of P2Y receptor subtypes on isolated tiger salamander Müller cells

Müller cells express a variety of neurotransmitter receptors that permit them to "sense" the extracellular environment within the retina. We have used a battery of agonists and antagonists to characterize the purinergic receptor subtypes expressed on isolated tiger salamander Müller cells. Changes in intracellular calcium ion concentration ([Ca(2+)](i)) in Müller cells were measured using the Ca(2+) indicator dye Fura-2 and digital imaging microscopy. ATP, 2-methylthio-ATP, 2-methylthio-ADP, ADP, UTP, UDP, deoxyATP, and 3'-O-(4-benzoyl)benzoyl ATP evoked increases in [Ca(2+)](i) in both the presence and absence of extracellular Ca(2+). Therefore, the increases we observed were likely due to intracellular Ca(2+) release mediated by G-protein-coupled P2Y receptor activation, rather than Ca(2+) influx via P2X receptor channels. The P2Y(1) receptor agonists 2-methylthio-ATP, 2-methylthio-ADP, and ADP evoked increases in [Ca(2+)](i) that were inhibited by the P2Y(1) receptor antagonists adenosine 3'-phosphate 5'-phosphosulfate and 2'-deoxy-N(6)-methyleneadenosine-3',5'-bisphosphate. Responses to ADP were not completely inhibited by the P2Y(1) receptor antagonists. The residual response to ADP could be mediated by P2Y(13) receptors. UTP evoked an increase in [Ca(2+)](i) that was partially inhibited by suramin, suggesting that Müller cells express P2Y(2) and P2Y(4) receptors. The P2Y(6) receptor agonist UDP, and the P2Y(11) receptor agonists deoxyATP, and 3'-O-(4-benzoyl)benzoyl ATP, evoked increases in [Ca(2+)](i) in Müller cells. We conclude that isolated tiger salamander Müller cells express P2Y(1), P2Y(2), P2Y(6), P2Y(11), and possibly P2Y(4) and P2Y(13) receptors. Therefore, the physiological release of ATP, ADP, UTP, and UDP and/or their accumulation in the retina under pathological conditions could stimulate increases in [Ca(2+)](i) in Müller cells.

Physiologic pH changes modulate calcium ion dependence of brain nitric oxide synthase in Carassius auratus

Species of the fish genus Carassius survive prolonged anoxia. Nitric oxide (NO) regulates cerebral blood flow in these fish during normoxic conditions whereas adenosine is the main vasoregulating molecule during anoxia. We investigated the calcium ion dependence of Carassius auratus brain NO synthase (NOS) as a function of pH. The physiological pH decrease from 7.2 to 6.8, which takes place during anoxia, greatly decreases NOS activity. This strong pH dependence is mainly due to variation of the calcium sensitivity of the enzyme. The EC(50) is 0.15 microM at pH 7.2 and 2.1 microM at pH 6.8 for the soluble enzyme. The particulate enzyme is also dependent on pH variations. The reduced sensitivity to calcium ions at acidic pH decreases both NO and H(2)O(2) production, saving the cells by suppression of the formation of potentially toxic nitrogen and oxygen species. Modulation of NOS activity by variation of its calcium affinity within the range of physiological pH constitutes an important and rapid mechanism to control the formation of NO and H(2)O(2) during normoxia-anoxia and anoxia-normoxia transitions.

Aged endothelial nitric oxide synthase knockout mice exhibit higher mortality concomitant with impaired open-field habituation and alterations in forebrain neurotransmitter levels

Endothelial nitric oxide synthase (eNOS) has been implicated in various brain and peripheral pathologies such as renal failure, heart failure or stroke. Consequently, the mortality rate of aged eNOS knockout mice (eNOS-/-) was higher than that of age-matched (18-22 months old) controls. Only seven of the original 14 eNOS-/- animals that participated in the study reached the age of 18 months or older, whereas no control mice died during this life span. In order to assess the behavioral and neurochemical consequences of chronic eNOS deficiency we examined whether the surviving aged eNOS-/- mice showed changes in terms of motor, emotional, exploratory and neurochemical parameters. Aged eNOS-/- mice showed reduced exploratory activity in the open-field with no habituation observable neither within sessions nor after repeated exposures. Pole test performance of eNOS-/- mice was comparable to controls. In the elevated plus-maze eNOS-/- mice did not differ from controls in terms of time spent in and entries into arms, but showed less locomotion on the open arms. The most prominent neurochemical alterations in the forebrains of aged eNOS-/- mice were: (a) increased acetylcholine levels in the neostriatum; (b) decreased noradrenaline concentrations in the ventral striatum; and (c) lower serotonin levels in the frontal cortex and ventral striatum. The present findings suggest that mice which survived chronic eNOS-deficiency into old age, show some behavioral and neurochemical phenotypes distinct from adult eNOS-/- mice.

Physiological mechanisms regulating the expression of endothelial-type NO synthase

Although endothelial nitric oxide synthase (eNOS) is a constitutively expressed enzyme, its expression is regulated by a number of biophysical, biochemical, and hormonal stimuli, both under physiological conditions and in pathology. This review summarizes the recent findings in this field. Shear stress, growth factors (such as transforming growth factor-beta, fibroblast growth factor, vascular endothelial growth factor, and platelet-derived growth factor), hormones (such as estrogens, insulin, angiotensin II, and endothelin 1), and other compounds (such as lysophosphatidylcholine) upregulate eNOS expression. On the other hand, the cytokine tumor necrosis factor-alpha and bacterial lipopolysaccharide downregulate the expression of this enzyme. The growth status of cells, the actin cytoskeleton, and NO itself are also important regulators of eNOS expression. Both transcriptional and posttranscriptional mechanisms are involved in the expressional regulation of eNOS. Different signaling pathways are involved in the regulation of eNOS promoter activity and eNOS mRNA stability. Changes in eNOS expression and activity under pathophysiological conditions and the pharmacological modulation of eNOS expression are subject of a subsequent brief review (part 2) to be published in the next issue of this journal.

Neuroprotective effects of nipradilol on purified cultured retinal ganglion cells

PURPOSE

To investigate effects of nipradilol, a nonselective beta- andbgr;-and selective alpha1-receptor antagonist and a potential nitric oxide releaser, on retinal ganglion cells purified and cultured in a serum-free medium.

METHODS

Retinal ganglion cells were isolated from 2-day-old Sprague-Dawley rats by means of two-step panning. A series of nipradilol (10(-5), 10(-6), 10(-7), 10(-8), 10(-9), and 10(-10)-mol/L) or vehicle solutions were administered to the culture medium for 48 hours, and the survival rate of retinal ganglion cells was evaluated using a newly developed system that evaluates the survival rate in small and large retinal ganglion cells separately. The effects of timolol maleate or bunazosin (10(-5), 10(-6), 10(-7), and 10(-8) -mol/L) solutions on retinal ganglion cells survival were also evaluated. The survival rate was evaluated after 10(-5)-mol/L c-PTIO (2-[4-carboxyphenyl]-4,4,5,5 tetramethylimidazoline-1-oxyl-3-oxide potassium salt), a nitric oxide scavenger, was administered to retinal ganglion cells with 10(-5)-mol/L nipradilol.

RESULTS

Nipradilol significantly increased the survival rate of both small and large retinal ganglion cells in a concentration-dependent manner compared with the controls. The maximum survival rate improvement of small and large retinal ganglion cells was 29.1% and 14.5%, respectively. Although timolol maleate and bunazosin did not affect the survival rate, 10(-5)-mol/L c-PTIO significantly inhibited the nipradilol-induced survival rate improvement by 69.9% in small retinal ganglion cells and by 91.6% in large retinal ganglion cells.

CONCLUSION

Nipradilol improves the survival rate of cultured postnatal rat retinal ganglion cells, and the nitric oxide generated from nipradilol may contribute to this effect.

Ontogeny of NADPH diaphorase/nitric oxide synthase reactivity in the brain of Xenopus laevis

The development of nitric oxide synthase (NOS) expression in the brain of Xenopus laevis tadpoles was studied by means of immunohistochemistry using specific antibodies against NOS and enzyme histochemistry for nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase. Both techniques yielded identical results and were equally suitable for demonstrating the nitrergic system in the brain. The only mismatches were observed in the olfactory nerve and glomeruli and in the terminal nerve; they were intensely labeled with the NADPH-diaphorase technique but failed to stain with NOS immunohistochemistry. As early as stage 33, nitrergic cells were observed in the caudal rhombencephalon within the developing inferior reticular nucleus. At later embryonic stages, different sets of reticular and tegmental neurons were labeled in the middle reticular nucleus and, more conspicuously, in the laterodorsal and pedunculopontine tegmental nuclei. As development proceeded, new nitrergic cell groups gradually appeared in the mesencephalon, diencephalon, and telencephalon. A general caudorostral temporal sequence was observed, both in the whole brain and within each main brain subdivision. The premetamorphic period was mainly characterized by the maturation of the cell populations developed in the embryonic period. During prometamorphosis, the nitrergic system reached an enormous development, and many new cell groups were observed for the first time, in particular in the telencephalon. By the climax of metamorphosis, the pattern of organization of nitrergic cells and fibers observed in the brain was similar to that present in the adult brain. Transient expression of NOS was not detected in any brain region. Our data suggest that nitric oxide plays an important role during brain development of Xenopus. Comparison with the developmental pattern of nitrergic systems in other vertebrates shows that amphibians possess more common features with amniotes than with anamniotes.

Heat stress-activated, calcium-dependent nitric oxide synthase in sponges

The presence of Ca(2+)-dependent, heat-stress-activated nitric oxide synthase (NOS) activity in peculiarly shaped, fusiform, and dendritic sponge cells is described for the first time. The NOS activity was evidenced evaluating the conversion of radioactive citrulline from [(14)C]arginine in intact cells from two different species that are phylogenetically unrelated in the class of Demospongiae: Axinella polypoides and Petrosia ficiformis. The production of nitrogen monoxide (NO) was confirmed by electron paramagnetic resonance analysis, and the histochemistry technique of NADPH diaphorase showed a specific localization of NOS activity in a particular network of dendritic cells in the sponge parenchyma. Sponges are the most primitive metazoan group; their evolution dates back 600 million years. The presence of environmental stress-activated NOS activity in these organisms may prove to be the most ancient NO-dependent signaling network in the animal kingdom.

Nitric oxide/cGMP-induced inhibition of calcium and chloride currents in retinal pericytes

In the CNS, contractile pericytes positioned on endothelium-lined lumens appear to play a role in regulating capillary blood flow. This function may be particularly important in the retina where pericytes are more numerous than in other tissues. Despite the importance of pericytes, knowledge of the effects of vasoactive molecules, such as nitric oxide (NO), on the physiology of these cells is limited. Since it is likely that ion channels play a role in the response of pericytes to signaling molecules from other cells, we used the perforated-patch configuration of the patch-clamp technique to record the whole-cell currents of pericytes located on microvessels freshly isolated from the rat retina. We found that voltage-gated calcium currents and calcium-activated chloride currents were inhibited during exposure to the NO donor, sodium nitroprusside (SNP). 8-Bromo-cyclic guanosine monophosphate (cGMP) mimicked these effects. In contrast, neither SNP nor the cGMP analog significantly affected the potassium or nonspecific cation conductances, which establish the resting membrane potential of retinal pericytes. Consistent with endogenous NO suppressing pericyte channel activity, exposure of isolated microvessels to an inhibitor of NO synthase increased the calcium and chloride currents. Since our experiments indicate that chloride channel activity is dependent, in part, upon the function of voltage-gated calcium channels, we postulate that a NO/cGMP-mediated inhibition of calcium channels reduces calcium influx and, thereby, lessens the opening of the calcium-activated chloride channels. This may be one mechanism by which NO decreases the contractile tone of pericytes.

Regulation of cytoplasmic calcium levels by two nitric oxide receptors

We examined the effects of dissolved nitric oxide (NO) gas on cytoplasmic calcium levels ([Ca(2+)](i)) in C6 glioma cells under anoxic conditions. The maximum elevation (27 +/- 3 nM) of [Ca(2+)](i) was reached at 10 microM NO. A second application of NO was ineffective if the first was >0.5 microM. The NO donor diethylamine/NO mimicked the effects of NO. Acute exposure of the cells to low calcium levels was without effect on the NO-evoked response. Thapsigargin (TG) increased [Ca(2+)](i) and was less effective if cells were pretreated with NO. Hemoglobin inhibited the effects of NO at a molar ratio of 10:1. 8-Bromo-cGMP was without effect on the NO-evoked response. If cells were pretreated with TG or exposed chronically to nominal amounts of calcium, NO decreased [Ca(2+)](i). The results suggest that C6 glioma cells have two receptors for NO. One receptor (NO(A)) elevates [Ca(2+)](i) and resides on the endoplasmic reticulum (ER). The other receptor (NO(B)) decreases [Ca(2+)](i) and resides on the plasmalemma or the ER. The latter receptor dominates when the level of calcium within intracellular stores is diminished.

NADPH diaphorase activity in the rabbit retina is modulated by glutamatergic pathways

NADPH diaphorase activity in the rabbit retina is modulated by the state of visual adaptation. In this study, we tested possible glutamatergic control of this phenomenon. Rabbits were injected intravitreally with agonists and antagonists of glutamate. After adaptation (3 hours) to either room light or darkness, the rabbits were killed and the retinae were prepared for NADPH diaphorase histochemistry. Kainic acid significantly reduced the number of NADPH diaphorase amacrine cells but augmented NADPH diaphorase activity in horizontal cells in both light- and dark-adapted animals. 6,7-Dinitroquinoxaline-2,3(1H,4H)-dione exerted no effect on amacrine cells but eliminated NADPH diaphorase activity in horizontal cells. 2-Amino-4-phosphono butyric acid did not affect NADPH diaphorase activity in horizontal cells but reduced the degree of staining in the neuronal processes of amacrine cells. MK-801 and N-methyl-D-aspartic acid (NMDA) had no effect on NADPH diaphorase activity in horizontal cells. However, MK-801 reduced staining in the neuronal processes of amacrine cells but not in their cell bodies. NMDA effects were expressed in a significant reduction in the number and size of amacrine cells that were NADPH diaphorase positive. These results indicate that activation of NADPH diaphorase in horizontal cells by darkness is mediated by the activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainic acid (KA)-type glutamate receptors. The ON pathway in the retina is probably involved in modulation of NADPH diaphorase in the neuronal processes of amacrine cells. Amacrine cells that are NADPH diaphorase positive contain NMDA-type and AMPA/KA-type receptors and are highly susceptible to NMDA and kainic acid toxicity.

Choline acetyltransferase is found in terminals of horizontal cells that label with GABA, nitric oxide synthase and calcium binding proteins in the turtle retina

In this study, we discriminated the various types of horizontal cell in the turtle retina on their content of neuroactive substances. Double label immunocytochemistry was performed on sectioned and wholemount retina using antisera to neural- and endothelial-nitric oxide synthase (nNOS, and eNOS), calretinin (CR), calbindin (CB), gamma-aminobutyric acid (GABA) and choline acetyltransferase (ChAT). H1 cells and their axon terminals label with CR, CB and GABA. Only H1 axon terminals label with eNOS. H2 cells contain CB, CR, nNOS and GABA maybe in their dendrites. H3 cells label only with nNOS. The localization of nNOS in the H2 and H3 cells is a novel finding. None of these antibodies labels H4 cells. The photoreceptor subtypes have been differentiated by different intensity of labeling with CB. The accessory member of the double cone is less intensely labeled with CB than the principal member and rods and blue cones do not appear to label at all. ChAT-IR is located in terminal boutons of H1 and H2 horizontal cells and H1 axon terminals and these boutons contact rods and all spectral types of cones. Clearly, GABA is present in H1 horizontal cells and may be used in neurotransmission between horizontal cells and possibly for feedback pathways to photoreceptors. The evidence of nNOS immunoreactivity in H2 and H3 horizontal cells, combined with available physiological evidence, suggests that NO may be involved in electrical coupling and/or modulation of synaptic input to these types of cells. Furthermore, our results raise the possibility that cholinergic synaptic transmission may occur from horizontal cell processes to photoreceptors in the outer plexiform layer of the turtle retina.