Modulation of ion channels in rod photoreceptors by nitric oxide

Silva H, P. de Carvalho R, Ventura ALM, Calaza KC, Silveira MS, Kubrusly RCC, de Melo Reis RA. The Healthy and Diseased Retina Seen through Neuron-Glia Interactions

The retina is the sensory tissue responsible for the first stages of visual processing, with a conserved anatomy and functional architecture among vertebrates. To date, retinal eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, glaucoma, and others, affect nearly 170 million people worldwide, resulting in vision loss and blindness. To tackle retinal disorders, the developing retina has been explored as a versatile model to study intercellular signaling, as it presents a broad neurochemical repertoire that has been approached in the last decades in terms of signaling and diseases. Retina, dissociated and arranged as typical cultures, as mixed or neuron- and glia-enriched, and/or organized as neurospheres and/or as organoids, are valuable to understand both neuronal and glial compartments, which have contributed to revealing roles and mechanisms between transmitter systems as well as antioxidants, trophic factors, and extracellular matrix proteins. Overall, contributions in understanding neurogenesis, tissue development, differentiation, connectivity, plasticity, and cell death are widely described. A complete access to the genome of several vertebrates, as well as the recent transcriptome at the single cell level at different stages of development, also anticipates future advances in providing cues to target blinding diseases or retinal dysfunctions.

Potential therapeutic strategies for photoreceptor degeneration: the path to restore vision

Photoreceptors (PRs), as the most abundant and light-sensing cells of the neuroretina, are responsible for converting light into electrical signals that can be interpreted by the brain. PR degeneration, including morphological and functional impairment of these cells, causes significant diminution of the retina's ability to detect light, with consequent loss of vision. Recent findings in ocular regenerative medicine have opened promising avenues to apply neuroprotective therapy, gene therapy, cell replacement therapy, and visual prostheses to the challenge of restoring vision. However, successful visual restoration in the clinical setting requires application of these therapeutic approaches at the appropriate stage of the retinal degeneration. In this review, firstly, we discuss the mechanisms of PR degeneration by focusing on the molecular mechanisms underlying cell death. Subsequently, innovations, recent developments, and promising treatments based on the stage of disorder progression are further explored. Then, the challenges to be addressed before implementation of these therapies in clinical practice are considered. Finally, potential solutions to overcome the current limitations of this growing research area are suggested. Overall, the majority of current treatment modalities are still at an early stage of development and require extensive additional studies, both pre-clinical and clinical, before full restoration of visual function in PR degeneration diseases can be realized.

Genetic network regulating visual acuity makes limited contribution to visually guided eye emmetropization

During postnatal development, the eye undergoes a refinement process whereby optical defocus guides eye growth towards sharp vision in a process of emmetropization. Optical defocus activates a signaling cascade originating in the retina and propagating across the back of the eye to the sclera. Several observations suggest that visual acuity might be important for optical defocus detection and processing in the retina; however, direct experimental evidence supporting or refuting the role of visual acuity in refractive eye development is lacking. Here, we used genome-wide transcriptomics to determine the relative contribution of the retinal genetic network regulating visual acuity to the signaling cascade underlying visually guided eye emmetropization. Our results provide evidence that visual acuity is regulated at the level of molecular signaling in the retina by an extensive genetic network. The genetic network regulating visual acuity makes relatively small contribution to the signaling cascade underlying refractive eye development. This genetic network primarily affects baseline refractive eye development and this influence is primarily facilitated by the biological processes related to melatonin signaling, nitric oxide signaling, phototransduction, synaptic transmission, and dopamine signaling. We also observed that the visual-acuity-related genes associated with the development of human myopia are chiefly involved in light perception and phototransduction. Our results suggest that the visual-acuity-related genetic network primarily contributes to the signaling underlying baseline refractive eye development, whereas its impact on visually guided eye emmetropization is modest.

Voltage- and calcium-gated ion channels of neurons in the vertebrate retina

In this review, we summarize studies investigating the types and distribution of voltage- and calcium-gated ion channels in the different classes of retinal neurons: rods, cones, horizontal cells, bipolar cells, amacrine cells, interplexiform cells, and ganglion cells. We discuss differences among cell subtypes within these major cell classes, as well as differences among species, and consider how different ion channels shape the responses of different neurons. For example, even though second-order bipolar and horizontal cells do not typically generate fast sodium-dependent action potentials, many of these cells nevertheless possess fast sodium currents that can enhance their kinetic response capabilities. Ca2+ channel activity can also shape response kinetics as well as regulating synaptic release. The L-type Ca2+ channel subtype, CaV1.4, expressed in photoreceptor cells exhibits specific properties matching the particular needs of these cells such as limited inactivation which allows sustained channel activity and maintained synaptic release in darkness. The particular properties of K+ and Cl- channels in different retinal neurons shape resting membrane potentials, response kinetics and spiking behavior. A remaining challenge is to characterize the specific distributions of ion channels in the more than 100 individual cell types that have been identified in the retina and to describe how these particular ion channels sculpt neuronal responses to assist in the processing of visual information by the retina.

Retinal photoreceptor apoptosis is associated with impaired serum ionized calcium homeostasis in diabetic retinopathy: An in-vivo analysis

PURPOSE

The aim of this work was to study the association of serum ionized calcium with retinal photoreceptor apoptosis on spectral domain optical coherence tomography (SD-OCT) in diabetic retinopathy (DR).

METHODS

Sixty consecutive cases with Type 2 diabetes mellitus were categorized into three groups: no diabetic retinopathy; non-proliferative DR; proliferative DR. The eye with more severe form of the disease was considered. Twenty healthy controls were also included. Best corrected visual acuity (BCVA) was measured on logMAR scale. Retinal photoreceptor apoptosis was defined as disruption of retinal photoreceptor ellipsoid zone (EZ). Ellipsoid zone disruption was assessed using SD-OCT. Serum levels of total and ionized calcium were measured using standard protocol.

RESULTS

EZ disruption was found to be positively associated with serum total calcium and ionized calcium. Also, EZ disruption was found to be positively associated with logMAR BCVA.

CONCLUSION

Increased serum ionized calcium induces retinal photoreceptor apoptosis resulting in increased EZ disruption in DR.

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

Unbiased View of Synaptic and Neuronal Gene Complement in Ctenophores: Are There Pan-neuronal and Pan-synaptic Genes across Metazoa?

Hypotheses of origins and evolution of neurons and synapses are controversial, mostly due to limited comparative data. Here, we investigated the genome-wide distribution of the bilaterian "synaptic" and "neuronal" protein-coding genes in non-bilaterian basal metazoans (Ctenophora, Porifera, Placozoa, and Cnidaria). First, there are no recognized genes uniquely expressed in neurons across all metazoan lineages. None of the so-called pan-neuronal genes such as embryonic lethal abnormal vision (ELAV), Musashi, or Neuroglobin are expressed exclusively in neurons of the ctenophore Pleurobrachia. Second, our comparative analysis of about 200 genes encoding canonical presynaptic and postsynaptic proteins in bilaterians suggests that there are no true "pan-synaptic" genes or genes uniquely and specifically attributed to all classes of synapses. The majority of these genes encode receptive and secretory complexes in a broad spectrum of eukaryotes. Trichoplax (Placozoa) an organism without neurons and synapses has more orthologs of bilaterian synapse-related/neuron-related genes than do ctenophores-the group with well-developed neuronal and synaptic organization. Third, the majority of genes encoding ion channels and ionotropic receptors are broadly expressed in unicellular eukaryotes and non-neuronal tissues in metazoans. Therefore, they cannot be viewed as neuronal markers. Nevertheless, the co-expression of multiple types of ion channels and receptors does correlate with the presence of neural and synaptic organization. As an illustrative example, the ctenophore genomes encode a greater diversity of ion channels and ionotropic receptors compared with the genomes of the placozoan Trichoplax and the demosponge Amphimedon. Surprisingly, both placozoans and sponges have a similar number of orthologs of "synaptic" proteins as we identified in the genomes of two ctenophores. Ctenophores have a distinct synaptic organization compared with other animals. Our analysis of transcriptomes from 10 different ctenophores did not detect recognized orthologs of synthetic enzymes encoding several classical, low-molecular-weight (neuro)transmitters; glutamate signaling machinery is one of the few exceptions. Novel peptidergic signaling molecules were predicted for ctenophores, together with the diversity of putative receptors including SCNN1/amiloride-sensitive sodium channel-like channels, many of which could be examples of a lineage-specific expansion within this group. In summary, our analysis supports the hypothesis of independent evolution of neurons and, as corollary, a parallel evolution of synapses. We suggest that the formation of synaptic machinery might occur more than once over 600 million years of animal evolution.

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.

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.

Calcium channels in rat horizontal cells regulate feedback inhibition of photoreceptors through an unconventional GABA- and pH-sensitive mechanism

Horizontal cells send inhibitory feedback to photoreceptors, helping form antagonistic receptive fields in the retina, but the neurotransmitter and the mechanisms underlying this signalling are not known. Since the proteins responsible for conventional Ca(2+)-dependent release of GABAergic synaptic vesicles are present in mammalian horizontal cells, we investigated this conventional mechanism as the means by which horizontal cells inhibit photoreceptors. Using Ca(2+) imaging in rat retinal slices, we confirm that horizontal cell depolarization with kainate inhibits and horizontal cell hyperpolarization with NBQX disinhibits the Ca(2+) signals produced by pH-sensitive activation of voltage-gated calcium channels (Ca channels) in photoreceptors. We show that while 100 μm Co(2+) reduces photoreceptor Ca(2+) signals, it disinhibits them at 10 μm, an effect reminiscent of earlier studies where low [Co(2+)] eliminated feedback. The low [Co(2+)] disinhibition is pH sensitive. We localized L-, N- and P/Q-type Ca channels in rat horizontal cells, and showed that both the N-type Ca channel blocker -conotoxin GVIA and the P/Q-type Ca channel blocker -agatoxin IVA increased Ca(2+) signals in photoreceptors in a pH-sensitive manner. Pronounced actions of GABAergic agents on feedback signals to photoreceptors were observed, and are pH sensitive, but are inconsistent with direct inhibition by GABA of photoreceptor [Ca(2+)]. Patch-clamp studies revealed that GABA activates a conductance having high bicarbonate permeability in isolated horizontal cells, suggesting that the commonality of pH sensitivity throughout the results could arise from a GABA autofeedback action in horizontal cells. This could change cleft pH with concomitant inhibitory influences on photoreceptor Ca channels.

Effect of amiloride to retinal toxicity induced by tissue plasminogen activator

Purpose

The effects of amiloride on cellular toxicity caused by tissue plasminogen activator (tPA) in mouse primary retinal cells were investigated.

Methods

Primary retinal cell cultures were maintained using glial conditioned medium. Commercial tPA and L-arginine were added, and the level of cyclic guanosine monophosphate (cyclic-GMP) in the culture supernatant was assessed using an ELISA assay. We measured the cell viability of cultured retinal cells pretreated with three different concentrations of amiloride (1, 10, and 100 µm) in addition to commercial tPA or L-arginine treatment.

Results

After exposing the cultured mouse retinal cells to tPA plus L-arginine or L-arginine alone, cyclic-GMP concentrations were 61.9 ± 5.1 pmole/mL and 63.1 ± 6.1 pmole/mL, respectively. However, the control group had a significantly lower concentration of cyclic-GMP (37.2 ± 3.4 pmole/mL, p < 0.01). The cyclic GMP-dissolved solution did not cause retinal cell death. In the control group and the group treated with 1 µm amiloride and tPA containing L-arginine, the cell viability was 43.7% and 44.5%, respectively. However, cell viability increased to 70.6% with 10 µm amiloride and 78.4% with 100 µm amiloride (p = 0.015).

Conclusions

L-arginine increases intracellular cyclic-GMP and may give rise to retinal cells through this mechanism. In addition, amiloride in concentrations greater than 10 µm protects against L-arginine-induced retinal cell death.

Light-driven calcium signals in mouse cone photoreceptors

Calcium mediates various neuronal functions. The complexity of neuronal Ca²⁺ signaling is well exemplified by retinal cone photoreceptors, which, with their distinct compartmentalization, offer unique possibilities for studying the diversity of Ca²⁺ functions in a single cell. Measuring subcellular Ca²⁺ signals in cones under physiological conditions is not only fundamental for understanding cone function, it also bears important insights into pathophysiological processes governing retinal neurodegeneration. However, due to the proximity of light-sensitive outer segments to other cellular compartments, optical measurements of light-evoked Ca²⁺ responses in cones are challenging. We addressed this problem by generating a transgenic mouse (HR2.1:TN-XL) in which both short- and middle-wavelength-sensitive cones selectively express the genetically encoded ratiometric Ca²⁺ biosensor TN-XL. We show that HR2.1:TN-XL allows recording of light-evoked Ca²⁺ responses using two-photon imaging in individual cone photoreceptor terminals and to probe phototransduction and its diverse regulatory mechanisms with pharmacology at subcellular resolution. To further test this system, we asked whether the classical, nitric oxide (NO)-soluble guanylyl-cyclase (sGC)-cGMP pathway could modulate Ca²⁺ in cone terminals. Surprisingly, NO reduced Ca²⁺ resting levels in mouse cones, without evidence for direct sGC involvement. In conclusion, HR2.1:TN-XL mice offer unprecedented opportunities to elucidate light-driven Ca²⁺ dynamics and their (dys)regulation in cone photoreceptors.

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.

Light modulation, not choroidal vasomotor action, is a regulator of refractive compensation to signed optical blur

BACKGROUND AND PURPOSE

The nitric oxide system has two proposed sites and mechanisms of action within the ocular growth/refractive compensation platform-neuromodulatory effects on retinal physiology, and vascular/smooth muscle effects in the choroid. The relative contribution of these mechanisms are tested here with drugs that perturb the nitric oxide system and with slow flicker modulation of the ON and OFF pathways of the retina.

EXPERIMENTAL APPROACH

Intravitreal injection of saline or 900 nmol N(G) -nitro-L-arginine methyl ester or L-arginine in saline was followed by monocular defocus with ±10 D lens (or no lens), from days 5-9 under standard diurnal (SD) or daytime 1 Hz ramped flicker conditions. Biometric, electrophysiological and histological analyses were conducted.

KEY RESULTS

After 4 days of SD conditions, both drugs enhanced electroretinogram (ERG) b-wave cf. d-wave amplitudes compared with saline and reduced refractive compensation to -10 D lenses. Under flicker conditions compensation to +10 D lenses was suppressed. Choroidal thinning was observed in the drug, no lens groups under SD conditions, whereas choroidal thickening was seen in most groups under flicker conditions, irrespective of refractive outcomes.

CONCLUSIONS AND IMPLICATIONS

As choroidal thickness was not predictive of final refractive compensation across any of the variables of drug, defocus sign or light condition, it is unlikely that choroidal thickness is a primary mechanism underlying refractive compensation across the range of parameters of this study. Rather, the changes in refractive compensation observed under these particular drug and light conditions are more likely due to a neuromodulatory action on retinal ON and OFF pathways.

Endogenous nitric oxide enhances the light-response of cones during light-adaptation in the rat retina

The electroretinogram (ERG) is a non-invasive indicator of retinal function. Light flashes evoke a cornea-negative a-wave followed by a cornea-positive b-wave. Light-adaptation is known to increase the amplitude of cone-dependent b-waves. To identify the underlying mechanism, we recorded rat cone photoresponses in situ, using intravitreally-injected glutamate to block synaptic transmission and intense paired-flash stimuli to isolate cone a-waves. Steady adapting illumination caused a progressive increase in cone a-wave amplitude, which was suppressed in a dose-dependent manner by intravitreal CPTIO, a nitric oxide scavenger. We conclude that light-adaptation causes release of nitric oxide, which enhances the cone photoresponse.

Inhibition of nitric oxide synthase desensitizes retinal ganglion cells to light by diminishing their excitatory synaptic currents under light adaptation

The effect of inhibiting nitric oxide synthase (NOS) on the visual responses of mouse retinal ganglion cells (RGCs) was studied under light adaptation by using patch-clamp recordings. The results demonstrated that NOS inhibitor, l-NAME, reduced the sensitivity of RGCs to light under light adaptation at different ambient light conditions. These observations were seen in all cells that recordings were made from. l-NAME diminished the excitatory synaptic currents (EPSCs), rather than increasing the inhibitory synaptic currents, of RGCs to reduce the sensitivity of RGCs to light. Cones may be the sites that l-NAME acted to diminish the EPSCs of RGCs.

Functional localization of the nitric oxide/cGMP pathway in the salamander retina

Nitric oxide (NO) is a gaseous neuromodulator that has physiological functions in every cell type in the retina. Evidence indicates that NO often plays a role in the processing of visual information in the retina through the second messenger cyclic guanosine monophosphate (cGMP). Despite numerous structural and functional studies of this signaling pathway in the retina, none have examined many of the elements of this pathway within a single study in a single species. In this study, the NO/cGMP pathway was localized to specific regions and cell types within the inner and outer retina. We have immunocytochemically localized nitric oxide synthase, the enzyme that produces NO, in photoreceptor ellipsoids, four distinct classes of amacrine cells, Müller and bipolar cells, somata in the ganglion cell layer, as well as in processes within both plexiform layers. Additionally, we localized NO production in specific cell types using the NO-sensitive dye diaminofluorescein. cGMP immunocytochemistry was used to functionally localize soluble guanylate cyclase that was activated by an NO donor in select amacrine and bipolar cell classes. Analysis of cGMP and its downstream target, cGMP-dependent protein kinase II (PKGII), showed colocalization within processes in the outer retina as well as in somata in the inner retina. The results of this study showed that the NO/cGMP signaling pathway was functional and its components were widely distributed throughout specific cell types in the outer and inner salamander retina.

Retinal Toxicity of Commercial Tissue Plasminogen Activator Is Mediated by the Induction of Nitric Oxide in the Mouse Retinal Primary Cells

PURPOSE

Tissue plasminogen activator (tPA) is an efficient thrombolytic agent, but the dose-dependent retinal toxicity of intravitreal injection of commercial tPA (containing L-arginine) has been reported. Here, we sought to investigate the mechanism of tPA-induced cell death in mouse retinal cell cultures and the role of nitric oxide (NO).

METHODS

Primary retinal cell cultures were maintained using glial conditioned medium (GCM) solution. Mouse retinal cell death was observed by using Hoechst-propidium iodide staining. Mouse retinal cell death was also measured by lactate dehydrogenase (LDH) assay. The formation of NO was measured using Griess reagent.

RESULTS

tPA-induced cell death was detected in mouse retinal cell cultures by Hoechst-propidium iodide staining or LDH assay. L-arginine seems to be the major factor in retinal toxicity of commercial tPA (containing L-arginine). The formation of NO was markedly increased in mouse retinal cell cultures treated with tPA (containing L-arginine) or L-arginine. NO inhibitor reduced the cell death induced by commercially available tPA or L-arginine.

CONCLUSIONS

This study suggests that l-arginine from commercial tPA (containing L-arginine) induces the majority of cell death in mouse retinal cell cultures and that its cytotoxicity may depend on the induction of NO.

Endocannabinoids in the retina: from marijuana to neuroprotection

The active component of the marijuana plant Cannabis sativa, Delta9-tetrahydrocannabinol (THC), produces numerous beneficial effects, including analgesia, appetite stimulation and nausea reduction, in addition to its psychotropic effects. THC mimics the action of endogenous fatty acid derivatives, referred to as endocannabinoids. The effects of THC and the endocannabinoids are mediated largely by metabotropic receptors that are distributed throughout the nervous and peripheral organ systems. There is great interest in endocannabinoids for their role in neuroplasticity as well as for therapeutic use in numerous conditions, including pain, stroke, cancer, obesity, osteoporosis, fertility, neurodegenerative diseases, multiple sclerosis, glaucoma and inflammatory diseases, among others. However, there has been relatively far less research on this topic in the eye and retina compared with the brain and other organ systems. The purpose of this review is to introduce the "cannabinergic" field to the retinal community. All of the fundamental works on cannabinoids have been performed in non-retinal preparations, necessitating extensive dependence on this literature for background. Happily, the retinal cannabinoid system has much in common with other regions of the central nervous system. For example, there is general agreement that cannabinoids suppress dopamine release and presynaptically reduce transmitter release from cones and bipolar cells. How these effects relate to light and dark adaptations, receptive field formation, temporal properties of ganglion cells or visual perception are unknown. The presence of multiple endocannabinoids, degradative enzymes with their bioactive metabolites, and receptors provides a broad spectrum of opportunities for basic research and to identify targets for therapeutic application to retinal diseases.

Nitric oxide synthase in photoreceptive pineal organs of fish

Photoreceptor cells in the fish pineal gland transduce light-dark information differentially into a neuroendocrine melatonin message; distinguishing features are the presence or absence of endogenous oscillators that drive these rhythms. In the present study, we have analysed the presence and distribution of nitric oxide (NO) synthase in both pineal types by NADPH-diaphorase (NADPHd) histochemistry and determined the effects of NO donors on cGMP formation and melatonin production. NADPHd staining was confined to photoreceptor cells in clock-driven pineal organs of zebrafish and goldfish as evidenced by a codistribution with S-antigen-immunoreactivity (-ir) or cyclic GMP-ir and, in the pineal of the trout, to cells that are S-antigen negative. In the trout pineal, but not in the other species, NADPHd staining was clearly codistributed with growth associated protein-43 (GAP-43) immunoreactivity, an antibody that recognizes developing and regenerating neurons in the fish brain. The presence of a functional NO system in photosensory pineal organs is supported by the fact that NO donors like S-nitroso N-acetylpenicillamine (SNAP) elevate intracellular cGMP levels. However, despite the significant rise in cGMP levels nitric oxide donors did neither affect acute light-dependent melatonin formation in the trout pineal nor the rhythmic production of melatonin in the zebrafish pineal.

Nitric oxide signaling pathways at neural level in invertebrates: functional implications in cnidarians

Nitric oxide (NO) is a small molecule with unconventional properties. It is found in organisms throughout the phylogenetic scale, from fungi to mammals, in which it acts as an intercellular messenger of main physiological events, or even as an intracellular messenger in invertebrates. In both vertebrates and invertebrates, NO is involved in many processes, regulated in part by cyclic guanosine monophosphate (cGMP), and reacts with different oxygen molecular species. The presence of NO in the early-diverging metazoan phylum of Cnidaria, of which Hydra represents the first known species having a nervous system, supports a role of this molecule as an ancestral neural messenger with physiological roles that remain to be largely elucidated. Therefore, our novel findings on the presence of NO in Hydra are here integrated in such a comparative frame.

Role of acetylcholine in nitric oxide production in the salamander retina

Although acetylcholine is one of the most widely studied neurotransmitters in the retina, many questions remain about its downstream signaling mechanisms. In this study we initially characterized the cholinergic neurotransmitter system in the salamander retina by localizing a variety of cholinergic markers. We then examined the link between both muscarinic and nicotinic receptor activation and nitric oxide production by using immunocytochemistry for cyclic guanosine monophosphate (cGMP) as an indicator. We found a large increase in cGMP-like immunoreactivity (cGMP-LI) in the inner retina in response to muscarinic (but not nicotinic) receptor activation. Based on the amplification of mRNA transcripts, receptor immunocytochemistry, and the use of selective antagonists, we identified these receptors as M2 muscarinic receptors. Using double-labeling techniques, we established that these increases in cGMP-LI were seen in GABAergic but not cholinergic amacrine cells, and that the increases were blocked by inhibitors of nitric oxide production. The creation of nitric oxide in response to cholinergic receptor activation may provide a mechanism for modulating the well-known mutual interactions of acetylcholine-glycine-GABA in the inner retina. As GABA and glycine are the primary inhibitory neurotransmitters in the retina, signaling pathways that modulate their levels or release will have major implications for the processing of complex stimuli by the retina.

Dopamine D4 receptors regulate intracellular calcium concentration in cultured chicken cone photoreceptor cells: relationship to dopamine receptor-mediated inhibition of cAMP formation

Dopamine is a retinal neuromodulator secreted from amacrine and interplexiform cells. Activation of dopamine D4 receptors on photoreceptor cells reduces a light-sensitive pool of cAMP. The aim of the present study was to evaluate the role of dopamine receptors and cAMP in the regulation of intracellular Ca(2+) concentrations ([Ca(2+)](i)) in photoreceptor cells of chick retina. Retinal cells from 6 day-old chicken embryos were isolated and cultured for 5-7 days prior to experiments. Cone photoreceptors were the predominant cell type in these cultures. Dopamine and agonists of dopamine D4 receptors suppressed K(+)-stimulated uptake of (45)Ca(2+) and [Ca(2+)](i), measured with the Ca(2+)-sensitive fluorescent dye fura-2AM. The effects of the agonists were blocked by dopamine D2/D4 receptor antagonists or by pertussis toxin. 8Br-cAMP, a cell-permeable analog of cAMP, had no effect on inhibition of K(+)-stimulated (45)Ca(2+) influx or [Ca(2+)](i) by dopamine D2/D4 receptor agonists. Quinpirole inhibited the increase in cAMP level elicited by K(+), which requires Ca(2+) influx through voltage-gated Ca(2+) channels, but not that induced by the calcium ionophore A23187. Moreover, dopamine had no effect on either forskolin-stimulated or Ca(2+)/calmodulin-stimulated adenylyl cyclase activity in cell membranes prepared from the cultured cells. These data indicate that the decrease of cAMP elicited by dopamine D4 receptor stimulation may be secondary to decreased [Ca(2+)](i).

Immunological aspects of axon injury in multiple sclerosis

The role of immune-mediated axonal injury in the induction of nonremitting functional deficits associated with multiple sclerosis is an area of active research that promises to substantially alter our understanding of the pathogenesis of this disease and modify or change our therapeutic focus. This review summarizes the current state of research regarding changes in axonal function during demyelination, provides evidence of axonal dysmorphia and degeneration associated with demyelination, and identifies the cellular and molecular effectors of immune-mediated axonal injury. Finally, a unifying hypothesis that links neuronal stress associated with demyelination-induced axonal dysfunction to immune recognition and immunopathology is provided in an effort to shape future experimentation.

Kinetics of synaptic transmission at ribbon synapses of rods and cones

The ribbon synapse is a specialized structure that allows photoreceptors to sustain the continuous release of vesicles for hours upon hours and years upon years but also respond rapidly to momentary changes in illumination. Light responses of cones are faster than those of rods and, mirroring this difference, synaptic transmission from cones is also faster than transmission from rods. This review evaluates the various factors that regulate synaptic kinetics and contribute to kinetic differences between rod and cone synapses. Presynaptically, the release of glutamate-laden synaptic vesicles is regulated by properties of the synaptic proteins involved in exocytosis, influx of calcium through calcium channels, calcium release from intracellular stores, diffusion of calcium to the release site, calcium buffering, and extrusion of calcium from the cytoplasm. The rate of vesicle replenishment also limits the ability of the synapse to follow changes in release. Post-synaptic factors include properties of glutamate receptors, dynamics of glutamate diffusion through the cleft, and glutamate uptake by glutamate transporters. Thus, multiple synaptic mechanisms help to shape the responses of second-order horizontal and bipolar cells.

Does nitric oxide modulate transmitter release at the mammalian neuromuscular junction?

1. Application of the nitric oxide (NO) donor, sodium nitrite and the NO synthase substrate l-arginine had no effect on nerve-evoked transmitter release in the rat isolated phrenic nerve/hemidiaphragm preparation; however, when adenosine A(1) receptors were blocked with the adenosine A(1) receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) prior to application of sodium nitrate or l-arginine, a significant increase in transmitter release was observed. In addition, the NO donor s-nitroso-N-acetylpenicillamine (SNAP) significantly increased transmitter release in the presence of DPCPX. In the present study, we have made the assumption that these NO donors elevate the level of NO in the tissue. Future studies should test other NO-donating compounds and also monitor the NO concentrations in the tissue to ensure that these effects are, in fact, NO induced. 2. Elevation of cGMP in this preparation with the guanylyl cyclase activator 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1) significantly enhanced transmitter release. In the presence of DPCPX and the selective guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), which blocks the production of cGMP, the excitatory effects of sodium nitrite and l-arginine were abolished. 3. These results suggest that NO serves to enhance transmitter release at the rat neuromuscular junction (NMJ) via a cGMP pathway and this facilitation of transmitter release can be blocked with adenosine. Previously, we demonstrated that adenosine inhibits N-type calcium channels. Because NO only affects transmitter release when adenosine A(1) receptors are blocked, we suggest that NO enhances transmitter release by enhancing calcium influx via N-type calcium channels. Further studies are needed to confirm that NO alters transmitter release via cGMP and that this action involves the N-type calcium channel. 4. The results of the present study are consistent with a model of NO neuromodulation that has been proposed for the mammalian vagal-atrial junction. This model suggests that NO acts on NO-sensitive guanylyl cyclase to increase the intracellular levels of cGMP. In turn, cGMP inhibits phosphodiesterase-3, increasing levels of cAMP, which then acts on the N-type calcium channels to enhance calcium influx, leading to an increase in transmitter release. Our only modification to this model for the NMJ is that adenosine serves to block the modulation of transmitter release by NO.

Retrograde endocannabinoid inhibition of goldfish retinal cones is mediated by 2-arachidonoyl glycerol

A functional role for retinal endocannabinoids has not been determined. We characterized retrograde suppression of membrane currents of goldfish cones in a retinal slice. Whole-cell recordings were obtained from cone inner segments under voltage clamp. I(K(V)) was elicited by a depolarizing pulse to +54 mV from a holding potential of -70 mV. A fifty-millisecond puff of saline with 70 mM KCl or Group I mGluR agonist DHPG was applied through a pipette directly at a mixed rod/cone (Mb) bipolar cell body. The amplitude of I(K(V)) decreased 25% compared to the pre-puff control. Retrograde suppression of I(K(V)) was blocked by CB1 receptor antagonist, SR141716A. The FAAH inhibitor URB597 had no effect on the suppression of I(K(V)), whereas nimesulide, a COX-2 inhibitor, prolonged the effects of the K+ puff 10-fold. Orlistat, a blocker of 2-AG synthesis, blocked the effect of the K+ puff. Group I mGluR activation of Gq/11 was demonstrated in that a puff with DHPG decreased I(K(V)) of cones by 32%, an effect blocked by SR141716A. The effect of DHPG was not blocked by the mGluR5 antagonist MPEP, indicating involvement of mGluR1. The suppressive effect of the K+ puff vanished in a Ca2+-free, 2 mM Co2+ saline. TMB-8 or ryanodine, blocked the effect of DHPG, but not that of the K+ puff, showing that calcium influx or release from intracellular stores could mediate retrograde release. We suggest that retrograde suppression of cone I(K(V)) is mediated by Ca2+-dependent release of 2-AG from Mb bipolar cell dendrites by separate mechanisms: (1) voltage-dependent, mimicked by the K+ puff, that may be activated by the depolarizing ON response to light; (2) voltage-independent, occurring under ambient illumination, mediated by tonic mGluR1 activation. The negative feedback of this latter mechanism could regulate tonic glutamate release from cones within narrow limits, regardless of ambient illumination.

Nitric Oxide Synthase and NADPH Diaphorase Distribution in the Bullfrog (Rana catesbeiana) CNS: Pathways and Functional Implications

The gas nitric oxide (NO) is emerging as an important regulator of normal physiology and pathophysiology in the central nervous system (CNS). The distribution of cells releasing NO is poorly understood in non-mammalian vertebrates. Nitric oxide synthase immunocytochemistry (NOS ICC) was thus used to identify neuronal cells that contain the enzyme required for NO production in the amphibian brain and spinal cord. NADPH-diaphorase (NADPHd) histochemistry was also used because the presence of NADPHd serves as a reliable indicator of nitrergic cells. Both techniques revealed stained cells in all major structures and pathways in the bullfrog brain. Staining was identified in the olfactory glomeruli, pallium and subpallium of the telencephalon; epithalamus, thalamus, preoptic area, and hypothalamus of the diencephalon; pretectal area, optic tectum, torus semicircularis, and tegmentum of the mesencephalon; all layers of the cerebellum; reticular formation; nucleus of the solitary tract, octaval nuclei, and dorsal column nuclei of the medulla; and dorsal and motor fields of the spinal cord. In general, NADPHd histochemistry provided better staining quality, especially in subpallial regions, although NOS ICC tended to detect more cells in the olfactory bulb, pallium, ventromedial thalamus, and cerebellar Purkinje cell layer. NOS ICC was also more sensitive for motor neurons and consistently labeled them in the vagus nucleus and along the length of the rostral spinal cord. Thus, nitrergic cells were ubiquitously distributed throughout the bullfrog brain and likely serve an essential regulatory function.

Effect of pH and Metal Ions on the Decomposition Rate of S-nitrosocysteine

S-nitrosothiols (RSNOs) have many biological functions including platelet deactivation, immunosupression, neurotransmission, and host defense. Most of the functions are attributed to nitric oxide (NO) release during S-nitrosothiol decomposition. As the simplest biologically occurring S-nitrosothiol, S-nitrosocysteine (CySNO) has been widely used as an NO donor and has also been incorporated into biomedical polymers. Knowledge of the CySNO decomposition rate is important for assessing the impact of CySNO on various bioengineering applications or biological systems. In this work, spectrophotometer measurements of CySNO decomposition in the presence of metal ions showed that the decomposition rate is highly susceptible to the pH. The maximum decomposition occurs near physiological pH (near 7.4) while in the acidic (pH < 6) and alkaline (pH > 9) condition CySNO is very stable. This demonstrates that blood provides an optimized environment for the decomposition of CySNO leading to the release of NO. The CySNO decomposition rate can also be affected by buffers with different purity levels in the presence and absence of metal ion chelators-although all buffers show the same pH phenomenon of maximizing near physiological pH. An equilibrium model of metal ions as a function of pH provides a plausible explanation for the pH dependence on the experimental decomposition rate.

Effect of nitroxyl on the hamster retinal nitridergic pathway

There is a growing body of evidence on the role of nitric oxide (NO) in retinal physiology. Recently, interest has developed in the functional role of an alternative redox form of NO, namely nitroxyl (HNO/NO(-)), because it is formed by a number of diverse biochemical reactions. The aim of the present report was to comparatively analyze the effect of HNO and NO on the retinal nitridergic pathway in the golden hamster. For this purpose, sodium trioxodinitrate (Angeli's salt) and diethylammonium (Z)-1-(N,N-diethylamino)diazen-1-ium-1,2-diolate (DEA/NO) were used as HNO and NO releasers, respectively. Angeli's salt and DEA/NO significantly decreased nitric oxide synthase activity. In addition, Angeli's salt (but not DEA/NO) significantly decreased l-arginine uptake. DEA/NO significantly increased cGMP accumulation at low micromolar concentrations, while Angeli's salt affected this parameter with a threshold concentration of 200muM. Although Angeli's salt and DEA/NO significantly diminished reduced glutathione and protein thiol levels in a similar way, DEA/NO was significantly more effective than AS in increasing S-nitrosothiol levels. None of these compounds increased retinal lipid peroxidation. These results suggest that HNO could regulate the hamster retinal nitridergic pathway by acting through a mechanism that only partly overlaps with that involved in NO response.

Distribution of soluble guanylyl cyclase in rat retina

The nitric oxide (NO)-cGMP pathway is implicated in modulation of visual information processing in the retina. Despite numerous functional studies of this pathway, information about the retinal distribution of the major downstream effector of NO, soluble guanylyl cyclase (sGC), is very limited. In the present work, we have used immunohistochemistry and multiple labeling to determine the distribution of sGC in rat retina. sGC was present at high levels in inner retina but barely detectable in outer retina. Photoreceptors and horizontal cells, as well as Müller cells, were immunonegative, whereas retinal ganglion cells exhibited moderate staining for sGC. Strong immunostaining was found in subpopulations of bipolar and amacrine cells, but staining was weak in rod bipolar cells, and AII amacrine cells were immunonegative. Double labeling of sGC with neuronal nitric oxide synthase showed that the two proteins are generally located in adjacent puncta in inner plexiform layer, implying paracrine interactions. Our results suggest that the NO-cGMP pathway modulates the neural circuitry in inner retina, preferentially within the cone pathway.

Nitric oxide-induced accumulation of lipofuscin-like materials is caused by inhibition of cathepsin S

To determine whether nitric oxide (NO) is involved in accumulation of lipofuscin-like material (LFM) in retinal pigment epithelial (RPE) cells and if this formation is related to NO-mediated modification of cathepsin S (cat S). RPE cell cultures were fed once every day with porcine photoreceptor outer segments (POS) in the presence of NO-donor [S-nitroso-N-acetylpenicillamine (SNAP) or NOC18] for 2 weeks. LFM autofluorescence within the cells was measured by fluorophotometric flow cytometry (FACS). The activity of purified cat S was measured in the presence of NO-donor with or without dithiothreitol (DTT). The following results were observed. SNAP and NOC18 caused LFM accumulation in RPE cells in a dose-dependent manner, and this accumulation was reversed by the addition of NO-scavengers (hydroxycobalamin, carboxy-PTIO). Purified cat S activities were inhibited by NO-donors without DTT, but in the presence of DTT, NO-donors exhibited no inhibitory effect on its activity. Phagocytic challenge of RPE cells increased cat S activity, which was reduced by the addition of NO donors. These results indicated that cat S activity was inhibited by NO-donors and resulted in LFM accumulation in RPE cells. We conclude that NO-mediated inhibition of cat S was caused through protein modification of cat S and resulted in LFM accumulation.

Nitric oxide synthase in retina and optic nerve head of rat with increased intraocular pressure and effect of timolol

We investigated the expression of nitric oxide synthase (NOS) isoforms -1, -2 and -3 in the retina and optic nerve head (ONH) in an experimental rat model of elevated intraocular pressure (IOP) before and after treatment with timolol, to assess whether its neuroprotective action is associated with the activity of these enzymes. Episcleral vein cauterization in unilateral eyes of Wistar rats was performed to produce elevated IOP. Histological sections of retina and ONH from animals with normal IOP, with elevated IOP, and elevated IOP treated with timolol, were studied by immunohistochemistry with antibodies to NOS-1, NOS-2, and NOS-3. In the control rats, NOS-1 was localized to photoreceptor inner segments, amacrine cells and bipolar cells in the retina, and in astrocytes, pericytes and vascular nitrergic terminals in the ONH. NOS-3 immunostaining localized to the endothelial cells. The rats with elevated IOP showed increased expression of NOS-1 in the plexiform layers of the retina and reactive astrocytes in the ONH. These cells also showed NOS-2 positivity. The rats treated with timolol showed reduced expression of NOS-1 in the retina and ONH. NOS-2 was only detected in a few groups of astrocytes in the ONH. NOS-3 was unchanged in both elevated IOP and timolol-treated groups. These results show that excessive levels of NO synthesized by the NOS-1 and -2 isoforms, considered neurotoxic, might contribute to the progressive lesions of retinal ganglion cell axons. Their reduction after treatment suggests a possible neuroprotective effect of timolol in neurons exposed to excessive amounts of NO.

Neuroprotective effects of nitric oxide synthase inhibitors in spinal cord injury-induced pathophysiology and motor functions: an experimental study in the rat

The role of nitric oxide (NO) in spinal cord injury (SCI)-induced motor dysfunction, breakdown of the blood-spinal cord barrier (BSCB), edema formation, and cell injury was examined using a pharmacological approach. We used three types of nitric oxide synthase (NOS) inhibitors: a nonselective blocker, L-NAME; an irreversible inhibitor of all isoforms of NOS, L-NMMA; and a long-term competitive inhibitor of neuronal NOS with equal potency to inhibit endothelial NOS, L-NNA. The compounds were administered once daily in separate groups of rats for 7 days. On the 8th day, SCI was performed by making a longitudinal incision into the right dorsal horn of the T10-11 segments, and the rats were allowed to survive 5 h after injury. Long-term treatment with L-NNA attenuated SCI-induced NOS upregulation, BSCB breakdown, edema formation, and cell injury, whereas comparatively less neuroprotection is offered by L-NMMA. The magnitude of neuroprotection is much less evident in injured animals that received L-NAME. Interestingly, SCI-induced motor dysfunction measured according to the Tarlov scale showed close correlation with the magnitude of neuroprotection. Thus, an improvement in motor function was seen in animals pretreated with L-NNA, whereas rats treated with L-NAME or L-NMMA did not show any influence on motor dysfunction after SCI. This observation suggests that inhibition of neuronal NOS is important for neuroprotection, and the disturbances in motor function following SCI are associated with the state of spinal cord pathology.

The nitric oxide-cGMP signaling pathway differentially regulates presynaptic structural plasticity in cone and rod cells

Although abundant structural plasticity in the form of axonal retraction, neurite extension, and formation of presynaptic varicosities is displayed by photoreceptors after retinal detachment and during genetic and age-related retinal degeneration, the mechanisms involved are mostly unknown. We demonstrated recently that Ca(2+) influx through cGMP-gated channels in cones and voltage-gated L-type channels in rods is required for neurite extension in vitro (Zhang and Townes-Anderson, 2002). Here, we report that the nitric oxide (NO)-cGMP signaling pathway is active in photoreceptors and that its manipulation differentially regulates the structural plasticity of cone and rod cells. The NO receptor soluble guanylyl cyclase (sGC) was detected immunocytochemically in both cone and rod cells. Stimulation of sGC increased cGMP production in retinal cultures. In cone cells, quantitative analysis showed that NO or cGMP stimulated neuritic sprouting; this stimulatory effect was dependent on both Ca2+ influx through cGMP-gated channels and phosphorylation by protein kinase G (PKG). At the highest levels of cGMP, however, cone outgrowth was no longer increased. In rod photoreceptors, NO or cGMP consistently inhibited neuritic growth in a dose-dependent manner; this inhibitory effect required PKG. When NO-cGMP signaling was inhibited, changes in the neuritic development of cone and rod cells were also observed but in the opposite direction. These results expand the role of cGMP in axonal activity to adult neuritogenesis and suggest an explanation for the neurite sprouting observed in an autosomal recessive form of retinitis pigmentosa that is characterized by high cGMP levels in photoreceptor layers.

Synaptic transmission at retinal ribbon synapses

The molecular organization of ribbon synapses in photoreceptors and ON bipolar cells is reviewed in relation to the process of neurotransmitter release. The interactions between ribbon synapse-associated proteins, synaptic vesicle fusion machinery and the voltage-gated calcium channels that gate transmitter release at ribbon synapses are discussed in relation to the process of synaptic vesicle exocytosis. We describe structural and mechanistic specializations that permit the ON bipolar cell to release transmitter at a much higher rate than the photoreceptor does, under in vivo conditions. We also consider the modulation of exocytosis at photoreceptor synapses, with an emphasis on the regulation of calcium channels.

A pilot study of nitric oxide blood levels in patients with chronic orofacial pain

BACKGROUND

Control of pain is the major goal in the management of chronic orofacial pain (COP) patients. The pathogenesis of COP is currently not well understood. Consequently, the treatment of COP may be suboptimal or even harmful. Based on independent observations, we propose that local elevated levels of nitric oxide (NO) may have a central role in the pathogenesis of COP.

HYPOTHESIS

NO level in the orofacial region of COP patients is elevated. A regional increased level of NO causes excessive vasodilatation. This hyperperfusion is manifested by hyperthermia of the overlying skin, while NO enhances nociception, aggravating orofacial pain. An alternative mechanism involving NO as a neurotransmitter at the CNS level may contribute to orofacial pain, but seems not to account for all the known clinical observations.

METHODS

Two groups of subjects were studied: 5 patients with COP and 59 control subjects. For each subject we collected blood samples for analysis of nitrite\nitrate (or NOx).

RESULTS

(1) NOx blood levels for 5 patients diagnosed with COP was 65.9 microM (SD of 10.4) verses 42.7 microM (SD of 24.2) for 59 control subjects, the difference being statistically significant, t-statistic = -2.12 (P > .05). (2) No statistical difference was found for NOx blood levels for 59 control subjects divided by gender (male vs female), with 23 female controls having NOx blood levels of 42.6 microM (SD of 25.2) and male controls having NOx blood levels of 42.8 microM (SD of 24.0), t-statistic = -0.03, P = .98.

CONCLUSION

This pilot study suggests that NO blood levels may have an association with COP. A better understanding of the mechanism of chronic orofacial pain is expected to lead to more precise diagnostic staging and management of this disorder.

Retinal G-substrate, potential downstream component of NO/cGMP/PKG pathway, is located in subtype of retinal ganglion cells and amacrine cells with protein phosphatases

The aim of this study was to determine the distribution and function of G-substrate, a specific substrate of the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP)-cGMP-dependent protein kinase (PKG) signaling pathway, in normal rat retina and in G-substrate knockout mice. The retinas of adult wild-type rats and mice and G-substrate knockout mice were studied immunohistologically to characterize the upstream and downstream components of the NO-cGMP-PKG pathway. Immunoblot analysis showed that the molecular weight of retinal G-substrate was similar to that of cerebellar G-substrate. In adult rats and mice, retinal G-substrate was located in a subpopulation of amacrine cells and in C38-positive retinal ganglion cells (RGCs) but not in alpha RGCs. In addition, retinal G-substrate was co-expressed with other upstream and downstream signaling components of the NO-cGMP-PKG-G-substrate-phosphatase pathway in the adult retina. Electroretinographic (ERG) analysis demonstrated that there was no significant difference between the ERGs of wild-type and G-substrate knockout mice. These results suggest that retinal G-substrate plays a role as a downstream component of the NO-cGMP-PKG pathway. The co-localization of retinal G-substrate with protein Ser/Thr phosphatases suggests that it acts as an endogenous protein phosphatase inhibitor as in the cerebellum.

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.

Circadian rhythm of Period1 clock gene expression in NOS amacrine cells of the mouse retina

The vertebrate retina contains self-sustained circadian clocks that broadly influence retinal physiology. In the present study, we have examined the relationship of nitric oxide, GABAergic and glycinergic inner retinal neurons with expression of a reporter for the circadian clock gene Period1 (Per1). Using Per1 : :GFP transgenic mice, we found that 72% of brain nitric oxide synthase (bNOS) expressing amacrine cells (NOS amacrine cells) sampled during the daytime were also immunoreactive for Per1-driven GFP. The number of bright GFP(+) NOS(+) cells was greater at Zeitgeber time (ZT) 10 than at 22, and this pattern persisted in retinas from animals which were placed in constant darkness [Circadian time (CT) 10 vs. 22]. Intensities of GFP-IR for individual NOS amacrine cells were analyzed at ZT4, 10, 16 and 22, with the peak value occurring at ZT10. Similar results were obtained from retinas sampled at CT4, 10, 16 and 22 in constant darkness, indicating that an endogenous circadian clock drives the transcription of the Per1 clock gene within NOS amacrine cells. The predominance of Per1 : :GFP(+) amacrine cells (82%), was immunoreactive to glutamate decarboxylase 65, but no Per1 : :GFP(+) amacrine cells colabeled with a glycine transporter 1 antibody. The results demonstrate circadian rhythms in Per1 promoter activation in nitric oxide (NO) and GABA secreting amacrine cells, and suggest that NO and GABA could be controlled by circadian clock mechanisms in the mammalian retina.

Nitric oxide stimulates gamma-aminobutyric acid release and inhibits glycine release in retina

Nitric oxide (NO) modulates the uptake and/or release of neurotransmitters through a variety of cellular mechanisms. However, the pharmacological and biochemical processes underlying these neurochemical effects of NO often remain unclear. In our study, we used immunocytochemical methods to study the effects of NO, cyclic guanosine monophosphate (cGMP), and peroxynitrite on the uptake and release of gamma-aminobutyric acid (GABA) and glycine in the turtle retina. In addition, we examined the involvement of glutamate receptors, calcium, and the GABA transporter in this GABA uptake and release. We also tested for interactions between the GABAergic and glycinergic systems. In general, we show that NO stimulated GABA release and inhibited glycine release. The NO-stimulated GABA release involved calcium-dependent or calcium-independent synaptic release or reversal of the GABA transporter. Some effects of NO on GABA release involved glutamate, cGMP, or peroxynitrite. NO promoted glycine uptake and inhibited its release, and this inhibition of glycine release was influenced by GABAergic modulation. These findings indicate that NO modulates the levels of the inhibitory transmitters GABA and glycine through several specific biochemical mechanisms in different retinal cell types and layers. Thus it appears that some of the previously described reciprocal interactions between GABA and glycine in the retina function through specific NO signaling pathways.

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.

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.

Modulation of Ca(v)1 and Ca(v)2.2 channels induced by nitric oxide via cGMP-dependent protein kinase

The unconventional gaseous transmitter nitric oxide (NO) markedly influences most of mechanisms involved in the regulation of intracellular Ca2+ homeostasis. In excitable cells, Ca2+ signaling mainly depends on the activity of voltage-gated Ca2+ channels (VGCCs). In the present paper, we will review data from our laboratory and others characterizing NO-induced modulation of Ca(v)1 (L-type) and Ca(v)2.2 (N-type) channels. In particular, we will explore experimental evidence indicating that NO's inhibition of channel gating is produced via cGMP-dependent protein kinase and examine some of the numerous cell functions that are potentially influenced by the action of NO on Ca2+ channels.

Functionally intact glutamate-mediated signaling in bipolar cells of the TRKB knockout mouse retina

In the juvenile trkB knockout (trkB−/−) mouse, retina synaptic communication from rods to bipolar cells is severely compromised as evidenced by a complete absence of electroretinogram (ERG)b-wave, even though the inner retina appears anatomically normal (Rohrer et al., 1999). Since it is well known that theb-wave reflects light-dependent synaptic activation of ON bipolar cellsviatheir metabotropic glutamate receptor, mGluR6, we sought to analyze the anatomical and functional integrity of the glutamatergic synapses at these and other bipolar cells in thetrkB−/−mouse. Although rod bipolar cells from wild-type juvenile mice were determined to be immunopositive for trkB, postsynaptic metabotropic and ionotropic glutamate receptor-mediated pathways in ON and OFF bipolar cells were found to be functionally intact, based on patch electrode recordings, using brief applications (“puffs”) of glutamate or its analog, 2-amino-4-phosphonobutyric acid (APB), a selective agonist for mGluR6 receptors. Ionotropic glutamate receptor function was assayed in OFF-cone bipolar and horizontal cells by applying exogenous glutamatergic agonists in the presence of the channel-permeant guanidinium analogue, 1-amino-4-guanidobutane (AGB). Electron-microscopic analysis revealed that the ribbon synapses between rods and postsynaptic rod bipolar and horizontal cells were formed at the appropriate age and appear to be structurally intact, and immunohistochemical analysis did not detect profound defects in the expression of excitatory amino acid transporters involved in glutamate clearance from the synaptic cleft. These data indicate that there does not appear to be evidence for postsynaptic deficits in glutamatergic signaling in the ON and OFF bipolar cells of mice lacking trkB.

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.

The CACNA1F gene encodes an L-type calcium channel with unique biophysical properties and tissue distribution

Glutamate release from rod photoreceptors is dependent on a sustained calcium influx through L-type calcium channels. Missense mutations in the CACNA1F gene in patients with incomplete X-linked congenital stationary night blindness implicate the Ca(v)1.4 calcium channel subtype. Here, we describe the functional and pharmacological properties of transiently expressed human Ca(v)1.4 calcium channels. Ca(v)1.4 is shown to encode a dihydropyridine-sensitive calcium channel with unusually slow inactivation kinetics that are not affected by either calcium ions or by coexpression of ancillary calcium channel beta subunits. Additionally, the channel supports a large window current and activates near -40 mV in 2 mM external calcium, making Ca(v)1.4 ideally suited for tonic calcium influx at typical photoreceptor resting potentials. Introduction of base pair changes associated with four incomplete X-linked congenital night blindness mutations showed that only the G369D alteration affected channel activation properties. Immunohistochemical analyses show that, in contrast with previous reports, Ca(v)1.4 is widely distributed outside the retina, including in the immune system, thus suggesting a broader role in human physiology.