Nitric oxide inhibits depolarization-induced release of endogenous dopamine in the rabbit retina

Physiology and Pathophysiology of Nitric Oxide in the Retina

The role of nitric oxide (NO) signaling in the retina can be simply termed as "extensive." The picture remains incomplete, but it is now known that NO has many sites of production and action in the retina, both physiological and pathophysiological in nature. Perspectives from retinal neurophysiology and clinical pathology have merged in a number of studies examining NO action, but renewed emphasis is needed to discover the links between the roles of NO in the neurons, glia, and vasculature of the retina. NEUROSCIENTIST 3:357-360, 1997

Endogenous and Synthetic Cannabinoids as Therapeutics in Retinal Disease

The functional significance of cannabinoids in ocular physiology and disease has been reported some decades ago. In the early 1970s, subjects who smoked Cannabis sativa developed lower intraocular pressure (IOP). This led to the isolation of phytocannabinoids from this plant and the study of their therapeutic effects in glaucoma. The main treatment of this disease to date involves the administration of drugs mediating either the decrease of aqueous humour synthesis or the increase of its outflow and thus reduces IOP. However, the reduction of IOP is not sufficient to prevent visual field loss. Retinal diseases, such as glaucoma and diabetic retinopathy, have been defined as neurodegenerative diseases and characterized by ischemia-induced excitotoxicity and loss of retinal neurons. Therefore, new therapeutic strategies must be applied in order to target retinal cell death, reduction of visual acuity, and blindness. The aim of the present review is to address the neuroprotective and therapeutic potential of cannabinoids in retinal disease.

The rat with oxygen-induced retinopathy is myopic with low retinal dopamine

PURPOSE

Dopamine (DA) is a neurotransmitter implicated both in modulating neural retinal signals and in eye growth. Therefore, it may participate in the pathogenesis of the most common clinical sequelae of retinopathy of prematurity (ROP), visual dysfunction and myopia. Paradoxically, in ROP myopia the eye is usually small. The eye of the rat with oxygen-induced retinopathy (OIR) is characterized by retinal dysfunction and short axial length. There have been several investigations of the early maturation of DA in rat retina, but little at older ages, and not in the OIR rat. Therefore, DA, retinal function, and refractive state were investigated in the OIR rat.

METHODS

In one set of rats, the development of dopaminergic (DAergic) networks was evaluated in retinal cross-sections from rats aged 14 to 120 days using antibodies against tyrosine hydroxylase (TH, the rate-limiting enzyme in the biosynthesis of DA). In another set of rats, retinoscopy was used to evaluate spherical equivalent (SE), electoretinography (ERG) was used to evaluate retinal function, and high-pressure liquid chromatography (HPLC) was used to evaluate retinal contents of DA, its precursor levodopamine (DOPA), and its primary metabolite 3,4-dihydroxyphenylacetic acid (DOPAC).

RESULTS

The normally rapid postnatal ramification of DAergic neurons was disrupted in OIR rats. Retinoscopy revealed that OIR rats were relatively myopic. In the same eyes, ERG confirmed retinal dysfunction in OIR. HPLC of those eyes' retinae confirmed low DA. Regression analysis indicated that DA metabolism (evaluated by the ratio of DOPAC to DA) was an important additional predictor of myopia beyond OIR.

CONCLUSIONS

The OIR rat is the first known animal model of myopia in which the eye is smaller than normal. Dopamine may modulate, or fail to modulate, neural activity in the OIR eye, and thus contribute to this peculiar myopia.

Nitric oxide signaling in the retina: what have we learned in two decades?

Two decades after its first detection in the retina, nitric oxide (NO) continues to puzzle visual neuroscientists. While its liberation by photoreceptors remains controversial, recent evidence supports three subtypes of amacrine cells as main sources of NO in the inner retina. NO synthesis was shown to depend on light stimulation, and mounting evidence suggests that NO is a regulator of visual adaptation at different signal processing levels. NO modulates light responses in all retinal neuron classes, and specific ion conductances are activated by NO in rods, cones, bipolar and ganglion cells. Light-dependent gap junction coupling in the inner and outer plexiform layers is also affected by NO. The vast majority of these effects were shown to be mediated by activation of the NO receptor soluble guanylate cyclase and resultant cGMP elevation. This review analyzes the current state of knowledge on physiological NO signaling in the retina.

Dopamine (D1) receptor activation and nitrinergic agents influence somatostatin levels in rat retina

Somatostatin (SRIF) influences the release of two important neuromodulators of retinal circuitry, dopamine (DA) and nitric oxide (NO). The aim of the present study was to examine whether DA and NO modulate SRIF release in rat retina, and the mechanisms involved in their actions. Retinas of adult female Sprague--Dawley rats (250--300 g) were mechanically detached from the eyecup and ex vivo experiments were performed. Retinal explants were incubated in the presence of dopaminergic [DA (10 microM, 100 microM and 200 microM), apomorphine (nonselective D1/D2 agonist, 0.50 mM, 1.0 microM and 10 microM), A68930 (D1 selective agonist, 0.50 microM, 1.0 microM and 10 microM), quinpirole (D2 selective agonist, 0.50 microM, 1.0 microM and 10 microM), SCH 23390 (D1 selective antagonist, 250 nM and 500 nM) and sulpiride (D2 selective antagonist, 100 microM and 200 microM)], and nitrinergic agents [arginine (62.5 microM--5mM), SIN-1 (50 microM, 100 microM and 500 microM) and 8-Br-cGMP (50 microM, 250 microM and 500 microM)]. SRIF levels were quantified using radioimmunoassay (RIA). Dopamine had no effect on SRIF levels. Apomorphine produced a concentration dependent decrease and increase in SRIF levels, suggestive of pre- and postsynaptic effects. A68930 (10 microM) and SCH 23390 (250 nM and 500 nM) mimicked and reversed apomorphine's postsynaptic actions, respectively. Quinpirole had no effect, but blockade of D2 autoreceptors by sulpiride (200 microM) afforded an increase in SRIF levels. Arginine and SIN-1 increased, and 8-Br-cGMP attenuated SRIF levels. These results show that dopamine D1 receptors, and NO/peroxynitrite agents modulate SRIF release in the retina suggesting that the triad SRIF--DA--NO have reciprocal interactions via which they regulate retinal circuitry and vision transduction.

Somatostatin modulates dopamine release in rat retina

The aim of the present study was to determine the possible role of somatostatin as a modulator of dopamine release in rat retina. Basal release of dopamine, and how this is influenced by somatostatin receptor (sst) selective ligands, was examined ex vivo in rat retinal explants. Dopamine levels were quantified by high-pressure liquid chromatography (HPLC) with electrochemical detection. Basal levels of dopamine were measured over 120 min of tissue incubation and found to be 1.17+/-0.35 ng/ml. Somatostatin (10(-6), 10(-5), 10(-4)M) increased dopamine levels in a concentration-dependent manner, while the sst(2) antagonist CYN154806 (10(-4)M) reversed its actions. BIM23014 (sst(2) agonist) increased dopamine levels in a statistically significant manner only at the concentration of 10(-5)M. The sst(1) agonist L797.591 (10(-5), 10(-4)M) also increased dopamine levels, while activation of the sst(3) receptor (sst(3) agonist, L796.778, 10(-4)M) had no effect. These data substantiate a neuromodulatory role for sst(1) and sst(2) somatostatin receptors in the retina and show for the first time somatostatin's influence on dopamine release.

Nitric oxide release is induced by dopamine during illumination of the carp retina: serial neurochemical control of light adaptation

Several lines of indirect evidence have suggested that nitric oxide may play an important role during light adaptation of the vertebrate retina. We aimed to verify directly the effect of light on nitric oxide release in the isolated carp retina and to investigate the relationship between nitric oxide and dopamine, an established neuromodulator of retinal light adaptation. Using a biochemical nitric oxide assay, we found that steady or flicker light stimulation enhanced retinal nitric oxide production from a basal level. The metabotropic glutamate receptor agonist L-amino-4-phosphonobutyric acid, inhibited the light adaptation-induced nitric oxide production suggesting that the underlying cellular pathway involved centre-depolarizing bipolar cell activity. Application of exogenous dopamine to retinas in the dark significantly enhanced the basal production of nitric oxide and importantly, inhibition of endogenous dopaminergic activity completely suppressed the light-evoked nitric oxide release. The effect of dopamine was mediated through the D1 receptor subtype. Imaging of the nitric oxide-sensitive fluorescent indicator 4,5-diaminofluorescein di-acetate in retinal slices revealed that activation of D1 receptors resulted in nitric oxide production from two main spatial sources corresponding to the photoreceptor inner segment region and the inner nuclear layer. The results taken together would suggest that during the progression of retinal light adaptation there is a switch from dopaminergic to nitrergic control, probably to induce further neuromodulatory effects at higher levels of illumination and to enable more efficient spreading of the light adaptive signal.

Motor pattern selection by nitric oxide in the stomatogastric nervous system of the crab

The gas nitric oxide (NO) serves a diversity of functions in the nervous system and plays an important role in the modulation of oscillatory networks. We investigated the actions of intrinsically produced NO on the rhythmically active gastric mill circuit within the stomatogastric ganglion (STG) of the crab, Cancer pagurus. Bath application of different NO blockers exclusively to the STG terminated spontaneously active gastric mill rhythms. Furthermore, a reduction in the activity levels of projection neurons that sustain the gastric mill rhythm was observed, suggesting that NO blockade influences feedback mechanisms that affect projection neuron activity. When STG feedback to these projection neurons was intact, their activity decreased strongly with NO blockers present exclusively in the STG. When either neuronal feedback was eliminated or projection neurons were tonically activated, NO blockade did not terminate the gastric mill rhythm, indicating an indirect ascending control of the projection neurons. Together, our results show that ascending feedback from a motor network is important in shaping network activity and that this feedback is state-dependent and can be modulated to alter the output of the motor network.

Inhibitory modulation of photoreceptor melatonin synthesis via a nitric oxide-mediated mechanism

Nitric oxide (NO) has been suggested to have many physiological functions in the vertebrate retina, including a role in light-adaptive processes. The aim of this study was to examine the influence of the NO-donor sodium nitroprusside (SNP) on the activity of arylalkylamine-N-acetyltransferase (AA-NAT; EC. 2.3.1.87), the activity of which responds to light and reflects the changes in retinal melatonin synthesis--a key feature of light-adaptive responses in photoreceptors. Incubation of dark-adapted and dark-maintained retinas with SNP lead to the NO-specific suppression of AA-NAT activity, with NO suppressing AA-NAT activity to a level similar to that seen in the presence of dopaminergic agonists or light. Increased levels of cGMP appeared to be causally involved in the suppression of AA-NAT activity by SNP, as non-hydrolysable analogues of cGMP and the cGMP-specific phosphodiesterase (PDE) inhibitor zaprinast also significantly suppressed AA-NAT activity, while an inhibitor of soluble guanylate cyclase blocked the effect of SNP. While this chain of events may not be part of the normal physiology of the retina, it could be important in pathological circumstances that are associated with marked increase in levels of cGMP, as is found to be the case in certain forms photoreceptor degeneration, which are produced by defects in cGMP phosphodiesterase activity.

Dopamine and nitric oxide control both flickering and steady-light-induced cone contraction and horizontal cell spinule formation in the teleost (carp) retina: serial interaction of dopamine and nitric oxide

Adaptation to ambient light, which is an important characteristic of the vertebrate visual system, involves cellular and subcellular (synaptic) plasticity of the retina. The present study investigated dopamine (DA) and nitric oxide (NO) as possible neurochemical modulators controlling cone photomechanical movements (PMMs) and horizontal cell (HC) spinules in relation to steady and flickering light adaptation in the carp retina. Haloperidol (HAL; a nonspecific DA receptor blocker) or cPTIO (a NO scavenger) largely inhibited the cone PMMs and HC spinule formation induced by either steady or flickering light. These results suggested that both DA and NO could be involved in the light-adaptation changes induced by either pattern of input and that DA and NO effects may not be completely independent. The possibility that NO and DA interact serially was evaluated pharmacologically by cross-antagonist application (i.e., DA + cPTIO or NO + HAL). When a NO donor was coapplied with HAL to dark-adapted eyecups, normal light-adaptive cone PMMs and HC spinules occurred. In contrast, when DA was applied in the presence of cPTIO, the dark-adapted state persisted. It was concluded 1) that DA and NO are both light-adaptive neurochemicals, released in the retina during either steady or flickering light; 2) that the effects of DA and NO on light-adaptive cone PMMs and HC spinules do not occur in parallel; and 3) that NO and DA act mainly in series, specifically as follows: Light --> DA --> NO --> Cone PMMs + HC spinules.

A critical assessment of the neurodestructive and neuroprotective effects of nitric oxide

Whether nitric oxide is cytodestructive or cytoprotective is of obvious clinical importance. The debate on this subject in the past decade has generated much "heat and light". This paper focuses on the actions of NO on the nervous system and reexamines the controversial issue and the contribution of the authors and their colleagues in the light of recent findings. We also report new findings, critically assesses previous experimental data, and share perspectives on this important subject.

Role of nitric oxide in control of light adaptive cone photomechanical movements in retinas of lower vertebrates: a comparative species study

The possible role of nitric oxide (NO) as a novel light adaptive neuromodulator of cone plasticity (photomechanical movements) in retinae of two contrasting species of fish (freshwater carp and marine bream) and an example of an amphibian (Xenopus laevis) was studied pharmacologically by cytomorphometric measurements. Application of a NO donor [S-nitroso-N-acetyl-d, l-penicillamine] (500-700 microM) to dark-adapted retinae induced contraction of cones with an efficiency (CE) relative to full light adaptation of around 54% in all three species. Pretreatment with a NO scavenger [2-(4-Carboxyphenyl)-4,4,5,5-tetrametylimidazoline-1-oxil-3-oxide] (30-35 microM) produced a consistent significant inhibition of the light adaptation-induced cone contraction (CE = 15-20%). These results strongly suggest the involvement of endogenous NO in the cone contractions that occur in freshwater and marine fish and amphibian retinae as a part of the light adaptation process.

Comparative study of glutamate mediated gamma-aminobutyric acid release from nitric oxide synthase and tyrosine hydroxylase immunoreactive cells of the Cebus apella retina

The effects of excitatory amino acids (EAAs) upon transporter-mediated gamma-aminobutyric acid (GABA) release were investigated in cells containing tyrosine hydroxylase (TH) or nitric oxide synthase (NOS) in retina of the primate Cebus apella. Retinas were treated in vitro with 50 microM Kainate (KA) or 5 mM L-Glutamate (L-Glu), for 30 min at 37 degrees C, in an Mg2+-free Locke's solution with or without Ca2+. The effects of EAAs were measured immunocytochemically by determining the GABA content in TH or NOS-immunoreactive cells in the inner retina, after stimulation. L-Glu and KA induced a Ca2+-independent GABA release from most GABA-immunoreactive cells of the inner retina. Double label experiments indicated that this release occurs in NOS+/GABA+ cells, but not in TH+/GABA+ cells suggesting that these cell subpopulations may be differentiated in some functional aspects.

Paracrine neuroprotective effect of nitric oxide in the developing retina

The retina of newborn rats consists of the ganglion cell layer (GCL), the inner plexiform layer (IPL), the inner nuclear layer (INL) containing amacrine cells and the neuroblastic layer (NBL). In retinal explants, the GCL enters cell death after sectioning of the optic nerve, whereas there is almost no cell death in the NBL. When protein synthesis is inhibited with anisomycin, cell death is blocked in the GCL and induced in the NBL. We tested the roles of nitric oxide (NO) on cell death in the retina in vitro. Either L-arginine, the substrate for NO synthase or the NO donor S:-nitroso-acetylpenicillamine (SNAP) blocked cell death induced by anisomycin in the NBL, but had no effect in the GCL. Sepiapterin, a precursor of the nitric oxide synthase (NOS)-cofactor tetrahydrobiopterin also had a protective effect against anisomycin. The use of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, an inhibitor of soluble form of guanylyl cyclase, showed that anti-apoptotic effect of SNAP is partially mediated by cGMP generated by activation of guanylyl cyclase. NADPH-diaphorase histochemistry stained cells only in the GCL and INL. Thus, the degenerative effect of anisomycin is observed within the NBL, whereas the localization of NOS is restricted to the GCL and INL. The protective effect of both the NO substrate and cofactor upon cell death induced by anisomycin in the NBL, indicates that NO produced by amacrine and ganglion cells is a paracrine modulator of cell death within the retinal tissue.

Light-adaptive role of nitric oxide in the outer retina of lower vertebrates: a brief review

The role of nitric oxide (NO) as a novel neurochemical mechanism controlling light adaptation of the outer retina is discussed by considering mainly published results. The emphasis is on the retinae of fishes and amphibia, but some data from the mammalian (rabbit) retinae have also been included for completeness. In the fish retina, application of NO donors in the dark caused light-adaptive photomechanical movements of cones. The normal effect of light adaptation in inducing cone contractions was suppressed by pretreatment of retinae with an NO scavenger. NO donors modulated horizontal cell activity by uncoupling the cells' lateral gap junctional interconnections and enhancing negative feedback to cones, again consistent with a light-adaptive role of NO. Direct evidence for light adaptation-induced release of NO has been obtained in fish (carp) and rabbit retinae. The results strongly suggest that control of retinal light adaptation is, under multiple neurochemical control, with NO and dopamine having an interactive role.

Role of nitric oxide in the regulation of monoaminergic neurotransmission

Data accumulated in the last decade indicate that nitric oxide (NO) participates in the regulation of neurotransmission in the central nervous system. Due to its physicochemical properties, NO is an ideal mediator of nonsynaptic interactions. The importance of monoaminergic systems in the function of the brain is clearly shown by the number of severe neuropsychiatric diseases (e.g. depression, Parkinson's disease) caused by the impairment of monoaminergic neurotransmission. Because of their neuroanatomical characteristic, monoaminergic systems participate mainly in nonsynaptic interactions. Since NO is a potential nonsynaptic modulator, it may have an important role in the regulation of monoaminergic systems. The aim of the present review is to survey the literature on the effect of NO on dopaminergic, noradrenergic and serotonergic neurotransmission. The potential mechanisms of action are summarized. Since there is no agreement in the literature on the nature of the effect of NO exerted on monoaminergic neurotransmission, and there are contradictory data concerning the mechanisms involved, the possible reasons for this unusual inconsistency are also discussed.

Neuronal nitric oxide synthase immunoreactive neurons in the mammalian retina

The development of immunocytochemistry has led to a better understanding of synaptic transmission carried out by neuroactive substances in the mammalian brain, including the retina. In the mammalian retina, nitric oxide (NO) is widely accepted as a neuromodulator. Histochemistry based on NADPH-d and immunocytochemistry based on nitric oxide synthase (NOS) have been used to identify the presence of nitric oxide in the mammalian retina. Certain types of amacrine cells and a class of displaced amacrine cells have been labeled consistently in all mammalian retinae studied to date. Other cell types showing NADPH-d reactivity or NOS immunoreactivity varied between species. NADPH-d reactive or NOS immunoreactive amacrine cells may serve as a source of NO for amacrine, bipolar, and ganglion cells in the inner retina, whereas interplexiform cells, bipolar cells, and horizontal cells may serve as a source of NO for the outer retina of mammals.

Renewal of photoreceptor outer segments and their phagocytosis by the retinal pigment epithelium

The discovery of disc protein renewal in rod outer segments, in 1960s, was followed by the observation that old discs were ingested by the retinal pigment epithelium. This process occurs in both rods and cones and is crucial for their survival. Photoreceptors completely degenerate in the Royal College of Surgeons mutant rat, whose pigment epithelium cannot ingest old discs. The complete renewal process includes the following sequential steps involving both photoreceptor and pigment epithelium activity: new disc assembly and old disc shedding by photoreceptor cells; recognition and binding to pigment epithelium membranes; then ingestion, digestion, and segregation of residual bodies in pigment epithelium cytoplasm. Regulating factors are involved at each step. While disc assembly is mostly genetically controlled, disc shedding and the subsequent pigment epithelium phagocytosis appear regulated by environmental factors (light and temperature). Disc shedding is rhythmically controlled by an eye intrinsic circadian oscillator using endogenous dopamine and melatonin as light and dark signal, respectively. Of special interest is the regulation of phagocytosis by multiple receptors, including specific phagocytosis receptors and receptors for neuroactive substances released from the neuroretina. The candidates for phagocytosis receptors are presented, but it is acknowledged that they are not completely known. The main neuromodulators are adenosine, dopamine, glutamate, serotonin, and melatonin. Although the transduction mechanisms are not fully understood, attention was brought to cyclic AMP, phosphoinositides, and calcium. The chapter points to the multiplicity of regulating factors and the complexity of their intermingling modes of action. Promising areas for future research still exist in this field.

Effects of brain-derived neurotrophic factor on the development of NADPH-diaphorase/nitric oxide synthase-positive amacrine cells in the rodent retina

Amacrine neurons expressing nitric oxide synthase (NOS) contain brain-derived neurotrophic factor (BDNF) receptors and respond to exogenous BDNF [Klöcker, N., Cellerino, A. & Bähr, M. (1998) J. Neurosci., 18, 1038-1046]. We analysed the effects of BDNF on the development of neurons which express NOS in the mouse and rat retina. Rat pups received a total of three intraocular injections of BDNF at intervals of 48 h, starting at postnatal day 16 (P16), and were killed at P22. The retinas were stained for NADPH-diaphorase, a histological marker of NOS. NOS-expressing neurons were found in both the inner nuclear layer (INL) and the ganglion cell layer (GCL). Two classes of NOS-expressing neurons, type I and type II, had already been distinguished in the INL [Koistinaho, J. & Sagar, S.M. (1995) In Osborne, N.N. & Chader, G.J. (eds), Progress in Retinal and Eye Research, Vol. 15. Oxford University Press, pp. 69-87] and a third one in the GCL. Up-regulation of NADPH-diaphorase activity was observed after BDNF treatment. The number of type I neurons remained stable, whereas the number of type II neurons and NOS-positive neurons in the GCL increased significantly (P < 0.001). Type I and type II neurons were significantly larger in BDNF-treated retinas. Double-labelling experiments revealed that BDNF induces NADPH-diaphorase in dopaminergic neurons and amacrine cells displaced to the GCL, but not in retinal ganglion cells. In mice homozygous for a null mutation of the bdnf gene, the intensity of NADPH-diaphorase labelling in both somata and processes was reduced, but the number of labelled neurons was not dramatically reduced. These findings indicate that BDNF regulates the neurotransmitter phenotype of NOS-expressing amacrine neurons under physiological conditions, but is not required for their survival.

Electrical responses from diabetic retina

Diabetic retinopathy has long been considered to be a retinal manifestation of systemic diabetic angiopathy. Indeed, it is therapeutically true. However, the prolongation of OP peak latency in diabetic eyes without any angiographic evidence of angiopathy leads us to presume that certain neuronal disorders occur early in diabetic eyes. Even though we cannot neglect the possibility that the prolongation of the OP peak latency may derive from undetectable retinal hypoperfusion, it is still far from conventional diabetic angiopathy. Rather, the status should be properly termed "intraretinal diabetic neuropathy" in that the neurones are the disturbed cells to cause visual dysfunction. Thereafter, the OP amplitude diminishes as retinopathy advances, probably depending on the degree of retinal circulatory disturbance. Marked diminution of the OP amplitude predicts rapid progression and poor prognosis of retinopathy. Diabetic retinal pigment epitheliopathy as manifested by one of our non-photic EOG responses is another kind of early ocular involvement of diabetes. Because its mechanisms are not yet known, so far we have not succeeded in correlating it to any kind of subjective visual index. Routine fundus inspection or fluorescent fundus angiography is incapable of detecting the compromised neural retina and/or retinal pigment epithelial integrity and thus the electrophysiology of vision has the edge in ophthalmology.

Neurobiology of retinal dopamine in relation to degenerative states of the tissue

Neurobiology of retinal dopamine is reviewed and discussed in relation to degenerative states of the tissue. The Introduction deals with the basic physiological actions of dopamine on the different neurons in vertebrate retinae with an emphasis upon mammals. The intimate relationship between the dopamine and melatonin systems is also covered. Recent advances in the molecular biology of dopamine receptors is reviewed in some detail. As degenerative states of the retina, three examples are highlighted: Parkinson's disease; ageing; and retinal dystrophy (retinitis pigmentosa). As visual functions controlled, at least in part, by dopamine, absolute sensitivity, spatial contrast sensitivity, temporal (including flicker) sensitivity and colour vision are reviewed. Possible cellular and synaptic bases of the visual dysfunctions observed during retinal degenerations are discussed in relation to dopaminergic control. It is concluded that impairment of the dopamine system during retinal degenerations could give rise to many of the visual abnormalities observed. In particular, the involvement of dopamine in controlling the coupling of horizontal and amacrine cell lateral systems appears to be central to the visual defects seen.

Modulation of glutamate release from rat hippocampal synaptosomes by nitric oxide

We used hippocampal synaptosomes to study the effect of NO originating from NO donors and from the activation of the NO synthase on the Ca2+-dependent release of glutamate due to 4-aminopyridine (4-AP) depolarization. We distinguished between the effects of NO on the exocytotic and on the carrier-mediated release of glutamate, which we found to be related to an increase in cGMP content and to a reduction of the ATP/ADP ratio, respectively. The NO donor hydroxylamine, at concentrations < or = 0.3 mM, inhibited the Ca2+-dependent glutamate release evoked by 4-AP, and addition of the NO donor, NOC-7, had a similar effect, which was reversed by the NO scavenger, carboxy-PTIO. Increasing the activity of NO synthase by addition of L-arginine also led to a decrease in the Ca2+-dependent release of glutamate induced by 4-AP, and this effect was reversed by inhibiting NO synthase with NG-nitro-L-arginine. This depression of the exocytotic release of glutamate was accompanied by an increase in cGMP levels due to the stimulation of soluble guanylyl cyclase by NO, produced either by the NO donors (hydroxylamine <0.3 mM) or by the endogenous NO synthase, but no significant decrease in ATP/ADP ratio was observed. However, at concentrations > or = 0.3 mM, hydroxylamine drastically increased the basal release and completely inhibited the Ca2+-dependent release of glutamate (IC50 = 168 microM). At these higher levels of NO, cGMP levels dropped to about 40% of the maximal values obtained at lower concentrations, and the ATP/ADP ratio decreased to about 50% (at 0.3 mM hydroxylamine). The large increase in the basal release could be partially inhibited by L-trans-2,4-PDC, previously loaded into the synaptosomes, suggesting that the nonexocytotic basal release occurred by reversal of the glutamate carrier. Therefore, the increase in cGMP induced by NO stimulation of the guanylyl cyclase decreases the exocytotic release of glutamate, but higher NO levels reduce the ATP/ADP ratio by inhibiting mitochondrial function, which therefore causes the massive release of cytosolic glutamate through the glutamate carrier.

Effects of nitric oxide on the horizontal cell network and dopamine release in the carp retina

In the teleost retina the intercellular messenger nitric oxide can be synthesized by several cell types including cone photoreceptors and H1 horizontal cells, indicating a modulatory role within the outer plexiform layer, the first stage of the visual information processing. Therefore, the aim of this study was to elucidate the effects of nitric oxide on the physiology of cone horizontal cells in the intact retina. The nitric oxide donor sodium nitroprusside (0.5-2.5 mM) enhanced the light responsiveness of cone horizontal cells and reduced the degree of electrical coupling in the network. Furthermore, the spread of intracellularly injected Lucifer Yellow was restricted. The effects on light responsiveness and electrical coupling were qualitatively mimicked by 8-bromo-cGMP (0.5 mM) and could not be achieved by ferrocyanide (1 mM), the byproduct of nitric oxide liberation from nitroprusside. The effects of NO on the responsiveness of horizontal cells may be due to an action on green- and red-sensitive cones. Nitroprusside (0.1 mM) diminished the K(+)-stimulated release of endogenous dopamine by 50%, whereas the basal dopamine release was not affected, indicating that the effects on electrotonic horizontal cell coupling were not elicited by an NO-induced release of dopamine. With respect to the morphologic plasticity of the cone-horizontal cell synapse the inhibitor of endogenous nitric oxide synthesis L-nitroarginine (0.1 mM) had no influence on the formation or retraction of spinules. These results show that NO affects the responsiveness and coupling of the horizontal cell network in a dopamine-independent way.

Histochemistry of NADPH-diaphorase--a marker for neuronal nitric oxide synthase--in the carp retina

The nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase histochemical technique was used as a marker to assess the distribution of nitric oxide synthase activity in the carp retina. NADPH-diaphorase activity was found to be present in photoreceptors (rods and cones), horizontal cells, amacrine cells, bipolar cells, Müller cells and ganglion cells. Staining was most prominent in the photoreceptor ellipsoids but was not confined to any particular subtype. The density of the staining within the inner plexiform layer (IPL) was determined by image analysis. There was a broad peak of activity in each sublamina of the IPL, but sublamina b appeared to be relatively more heavily stained. The results taken together suggest that the nitric oxide signalling system could have a broader involvement in retinal function than previously thought. Furthermore, nitric oxide may have a novel mode of action in the retina whereby it could be effective on cells (photoreceptors) that also synthesize it.