Sodium nitroprusside alters dark voltage and light responses in isolated retinal rods during whole-cell recording

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

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.

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.

Effects of lithium on basal and modulated activities of the particulate and soluble guanylate cyclases in retinal rod outer segments

A large amount of information regarding the kinetics of biochemical reactions involved in visual transduction was derived from electrophysiological studies on dark-adapted rod outer segments. Hodgkin et al. [(1985) J. Physiol. 358, 447-468] observed that when Na was replaced with Li in the perfusion solution bathing the rod outer segment, the dark current slowly declined to zero. This decline was thought to result from a rise in intracellular calcium which was hypothesized to inhibit guanylate cyclase activity and reduce the cyclic GMP concentration. Rod outer segments contain membrane and soluble guanylate cyclase activities, and we show here that Li directly inhibits both types of activities very strongly. Both the basal (at high calcium) and the stimulated (at low calcium) activities of the membrane enzyme were inhibited by Li. Half-maximal inhibition of the stimulated enzyme was at 30 mM Li while for the basal activity it was at 100 mM. Over 80% of the activated enzyme was inhibited at 110 mM Li. The soluble guanylate cyclase activity was stimulated by nitroprusside. One hundred millimolar Li inhibited the basal activity by 20-30%, but the inhibition of the nitroprusside-stimulated (soluble) enzyme was much stronger, resembling that of the activated membrane enzyme. Half-maximal inhibition occurred at 30 mM, and about 80% inhibition was found at 100 mM Li. Stimulation of the soluble enzyme by nitroprusside was independent of calcium in the physiological range. The inhibition of the stimulated enzyme by Li was similarly independent of calcium, except at unphysiologically high concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

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.

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

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

Light adaptation and sensitivity controlling mechanisms in vertebrate photoreceptors

The human visual system can discriminate increment and decrement light stimuli over a wide range of ambient illumination; from moonlight to bright sunlight. Several mechanisms contribute to this property but the major ones reside in the retina and more specifically within the photoreceptors themselves. Numerous studies in retinae from cold- and warm-blooded vertebrates have demonstrated the ability of the photoreceptors to respond in a graded manner to light increments and decrements even if these are applied during a background illumination that is expected to saturate the cells. In all photoreceptors regardless of type and species, three cellular mechanisms have been identified that contribute to background desensitization and light adaptation. These gain controlling mechanisms include; response-compression due to the non-linearity of the intensity-response function, biochemical modulation of the phototransduction process and pigment bleaching. The overall ability of a photoreceptor to adapt to background lights reflects the relative contribution of each of these mechanisms and the light intensity range over which they operate. In rods of most species, response-compression tends to dominate these mechanisms at light levels too weak to cause significant pigment bleaching and therefore, rods exhibit saturation. In contrast, cones are characterized by powerful background-induced modulation of the phototransduction process at moderate to bright background intensities where pigment bleaching becomes significant.Therefore, cones do not exhibit saturation even when the level of ambient illumination is raised by 6-7 log units.

Adenosine receptor blockade and nitric oxide synthase inhibition in the retina: impact upon post-ischemic hyperemia and the electroretinogram

We preformed this study to determine the effect on ocular blood flow and the electroretinogram of either nitric oxide synthase (NOS) inhibition, adenosine receptor blockade or the combination of both after 1 hr of ocular ischemia. Thirty-seven cats under general anesthesia were subjected to 1 hr of complete ischemia in one eye by raising the intraocular pressure above systolic blood pressure. The other eye in each animal served as a non-ischemic control. Arterial blood gas tension, systemic arterial pressure, body temperature, hematocrit, and anesthetic level were controlled in each experiment. Cats were divided into four groups. Group 1 received normal saline injections [intravenous (i.v.) and intravitreal], Group 2 adenosine receptor blockade (0.1 ml of 0.01 M 8-sulfophenyltheophylline intravitreal) and saline i.v., Group 3 NOS inhibition (30 mg/kg l-NG-nitroarginine-methyl-ester i.v.) and saline intravitreal, and Group 4 intravitreal adenosine receptor blockade and NOS inhibition i.v. A subset of Group 3 received l-arginine to investigate the reversibility of NOS inhibition, after the blood flow measurements were completed. Five minutes after the end of ischemia, blood flows in retina and choroid were measured using injections of radioactively labeled microspheres. Electroretinographic (ERG) studies were carried out before treatment, before ischemia, during ischemia, and 1, 2, 3, and 4 hr after ischemia ended. NOS inhibition significantly reduced basal blood flow in the choroid, and in the retina when combined with adenosine receptor blockade. Adenosine receptor blockade completely attenuated post-ischemic hyperemia in the retina, but retinal hyperemia reappeared when adenosine receptor blockade and NOS inhibition were combined. Adenosine receptor blockade had no effect on ERG recovery after ischemia. NOS inhibition led to a reduction of ERG a- and b-wave amplitudes in control eyes, that could be reversed by l-arginine. Nitric oxide (NO) appears to be a significant factor in the regulation of basal blood flow in the choroid. Adenosine appears to be a major mediator of retinal hyperemia after 60 min of ischemia. Since NOS inhibition appeared to have direct effects on ERG wave amplitudes, short-term ERG studies may be of limited use in assessing the role of NO in postischemic recovery of the retina. Our observations correlate well with the emerging role of NO as a neurotransmitter in the retina.

Transient corneal edema induced by nitric oxide synthase inhibition

The aim of the study is to identify nitric oxide synthase (NOS) in the rabbit cornea and further investigate the physiological role of nitric oxide in the rabbit cornea. For histological identification, an immunohistochemical technique using anti-NOS monoclonal antibodies was employed. For the physiological study, we measured the corneal thickness in vivo as an indicator of corneal edema by ultrasonic pachymetry. The measurements were repeated before and after ipsilateral injections of N(G)-nitro-L-arginine methyl ester (L-NAME) or N(G)-nitro-D-arginine methyl ester (D-NAME) or 6-anilino-5,8-quinolinedione (LY-83583) with contralateral injection of vehicle (balanced salt solution) into the anterior chamber of the rabbit. We also monitored intraocular pressure (IOP) by pneumatonometry. Endothelial NOS (eNOS) immunoreactivity was demonstrated both in the corneal epithelium and the endothelium. The corneal thickness significantly increased after L-NAME or LY-83583 without significant rise of IOP, whereas no change was detected after vehicle or D-NAME. These results suggest that NO is spontaneously produced in the corneal endothelium and the NO/cyclic GMP pathway is involved in maintainance of corneal thickness.

Nitric oxide in the eye: multifaceted roles and diverse outcomes

Recent works have highlighted the role of nitric oxide in a wide array of disease entities, including septic shock, hypertension, cerebral ischemia, and chronic degenerative diseases of the nervous system. The functions of nitric oxide appear very diverse, having actions on vascular tone, neurotransmission, immune cytotoxicity, and many others. Nitric oxide is an important mediator of homeostatic processes in the eye, such as regulation of aqueous humor dynamics, retinal neurotransmission and phototransduction. Changes in its generation or actions could contribute to pathological states such as inflammatory diseases (uveitis, retinitis) or degenerative diseases (glaucoma, retinal degeneration). Localization in the eye and biochemical characteristics of nitric oxide will be reviewed. A better understanding of the nitric oxide pathway will be the key to the development of new approaches to the management and treatment of various ocular diseases.

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.

Possible role of nitric oxide on fertile and asthenozoospermic infertile human sperm functions

The capacity of human sperm fertilization is principally dependent on sperm motility and membrane integrity. Oxygen-derived free radicals, such as superoxide anion, are known to impair sperm motility and membrane integrity by inducing membrane lipid peroxidation (LPO). Nitric oxide (NO), a biologically active free radical, has recently been shown to inactivate superoxide and increase intracellular guanosine-3', 5'-cyclic monophosphate (cGMP). The aim of this study is to investigate the effects of NO on human sperm motility, viability, lipid peroxidation and cGMP in fertile and asthenozoospermic infertile individuals in vitro. Semen samples were obtained from 10 fertile volunteers and 10 asthenozoospermic infertile patients. Washed spermatozoa were incubated at 37 degrees C in Ham's F-10 medium with 0, 25, 50, 100, 200, or 400nM sodium nitroprusside (SNP, Na2 [Fe(CN) 5NO] 2H2O), a nitric oxide releaser. Samples were analyzed for viability, determined by eosin-Y dye exclusion method at 0, 1, 2, 3, 5 and 6 h of incubation; motility, determined by the trans-membrane migration method within 2 h of incubation; LPO determined by malondialdehyde (MDA)-thiobarbituric acid method at 3 h of incubation; and the intracellular cGMP, determined by 125I-cGMP radioimmunoassay at 3 h of incubation. The results showed: in both fertile and infertile samples, viability, trans-membrane migration ratio and the levels of intracellular cGMP in 25-100nM SNP-treated spermatozoa were significantly higher than those in control groups, while MDA contents in treated groups were significantly lower than those in controls. However, when concentrations of SNP increased to 200-400nM, the opposite effects were exhibited. The effects of SNP on these processes were biphasic within 25-400nM. The most effective concentration was 100nM. These data suggested that NO is beneficial to sperm viability and motility in both fertile and infertile individuals, and that reduction of lipid peroxidative damage to sperm membranes and increase of intracellular cGMP may be involved in these benefits.

Nitric oxide: a review of its role in retinal function and disease

Nitric oxide synthase (NOS), the enzyme that catalyzes the formation of nitric oxide from L-arginine, exists in three major isoforms, neuronal, endothelial, and immunologic. Neuronal and endothelial isoforms are constitutively expressed, and require calcium for activation. Both of these isoforms can be induced (i.e., new protein synthesis occurs) under appropriate conditions. The immunologic isoform is not constitutively expressed, and requires induction usually by immunologic activation; calcium is not necessary for its activation. Neuronal and immunologic NOS have been detected in the retina. Neuronal NOS may be responsible for producing nitric oxide in photoreceptors and bipolar cells. Nitric oxide stimulates guanylate cyclase of photoreceptor rod cells and increases calcium channel currents. In the retina of cats, NOS inhibition impairs phototransduction as assessed by the electroretinogram. Inducible nitric oxide synthase, found in Müller cells and in retinal pigment epithelium, may be involved in normal phagocytosis of the retinal outer segment, in infectious and ischemic processes, and in the pathogenesis of diabetic retinopathy. Nitric oxide contributes to basal tone in the retinal circulation. To date, findings are conflicting with respect to its role in retinal autoregulation. During glucose and oxygen deprivation, nitric oxide may increase blood flow and prevent platelet aggregation, but it may also mediate the toxic effects of excitatory amino acid release. This reactive, short-lived gas is involved in diverse processes within the retina, and its significance continues to be actively studied.

Calcium store depletion potentiates a phosphodiesterase inhibitor- and dibutyryl cGMP-evoked calcium influx in rat pituitary GH3 cells

A role for cGMP in the control of capacitative Ca2+ influx was identified in rat pituitary GH3 cells. Application of 50 microM - 1 mM of the non-specific phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX), or the specific cGMP-phosphodiesterase inhibitor, zaprinast, induced a dose-dependent increase in the intracellular free Ca2+ concentration [Ca2+]i of the pituitary cell line, as assessed by video ratio imaging using fura-2. Response onset times were identical and response profiles were similar in all cells analysed. Application of 50 microM dibutyryl cGMP to GH3 cells resulted in heterogeneous Ca2+ responses, consisting of single or multiple transients with varying onset times. In all cases, increases in [Ca2+]i were predominantly due to Ca2+ influx, since no responses were detected in low Ca2+ medium, or following pre-incubation of cells with 1 microM verapamil, or nicardipine. Depleting intracellular Ca2+ stores by prior treatment of cells with 1 microM thapsigargin resulted in a dramatic potentiation in the Ca2+ influx mediated by both phosphodiesterase inhibitors and dibutyryl cGMP, suggesting that cGMP modulates a dihydropyridine-sensitive Ca2+ entry mechanism in GH3 cells which is possibly regulated by the state of filling of Ca2+ stores.

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.

Nitric oxide synthase in tiger salamander retina

Previous studies have indicated that nitric oxide, a labile freely diffusible biological messenger synthesized by nitric oxide synthase, may modulate light transduction and signal transmission in the retina. In the present work, the large size of retinal cells in tiger salamander (Ambystoma tigrinum) allowed the utilization of nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry and nitric oxide synthase immunocytochemistry to delineate the cell-specific intracellular localization of nitric oxide synthase. NADPH-diaphorase activity was highly concentrated in the outer retina, in rod and cone inner segment ellipsoids, and between and adjacent to the photoreceptor cell bodies in the outer nuclear layer. Examination of enzymatically isolated retinal cells indicated that outer nuclear layer NADPH-diaphorase activity was localized to the distal processes of the retinal glial (Müller) cells and to putative bipolar cell Landolt clubs. Less intense NADPH-diaphorase activity was seen in the photoreceptor inner segment myoid region, in a small number of inner nuclear layer cells, in cap-like configurations at the distal poles of cells in the ganglion cell layer and surrounding ganglion cell layer somata, and in punctate form within both plexiform layers, the pigment epithelium, and the optic nerve. Nitric oxide synthase-like immunoreactivity was similarly localized, but was also concentrated along a thin sublamina centered within the inner plexiform layer. The potential for nitric oxide generation at multiple retinal sites suggests that this molecule may play a number of roles in the processing of visual information in the retina.

Control of photoreceptor proteins by Ca2+

A decrease of cytoplasmic Ca(2+)-concentration in vertebrate photoreceptor cells after illumination is necessary for light adaptation. Although the mechanisms of adaptation is not completely understood, several Ca(2+)-dependent cellular processes have been discovered. Some involve calcium-binding proteins like recoverin, guanylyl cyclase-activating protein and calmodulin, and their target proteins rhodopsin kinase, guanylyl cyclase, the cGMP-gated channel, and NO synthase. The activity of several enzymes or channels is directly controlled by Ca2+ and does not involve calcium-binding proteins. These proteins are pyrophosphatase, protein kinase C and the cGMP-gated channel.

Fibroblast growth factors alter light responses and dark voltage in retinal rods of the frog (Rana temporaria)

Fibroblast growth factors (FGF-1 and FGF-2) were applied intracellularly via whole-cell patch-clamp electrodes while the membrane voltage was recorded simultaneously. During recording the exchange of substances by diffusion between cytosol and pipette medium affects the cell's function. Under control conditions, the loss of nucleotides is reflected by a slow hyperpolarization of the dark voltage and prolongated light responses. Addition of FGF-1 and FGF-2 to the pipette medium accelerated the time course of the hyperpolarization and intensified the prolongation of the light responses. The depolarization of photoreceptor cells after intracellular application of the nitric oxide (NO)-synthase cofactor nicotinamide adenine dinucleotide phosphate (NADPH) and the stabilization of light response recovery by L-arginine is abolished by FGF-2. FGF-2 was ineffective when it was applied together with the calcium chelator ethylene glycol-bis(2-aminoethylether)tetraacetate (EGTA). The results indicate a possible role of FGF in the regulation of NO and calcium in photoreceptor cells and may explain protective effects of FGF in degenerative processes of photoreceptor cells.

Differentiation of short-wavelength-sensitive cones by NADPH diaphorase histochemistry

NADPH diaphorase (NADPH dehydrogenase; EC 1.6.99.1) histochemistry labels neurons that synthesize the neurotransmitter nitric oxide (NO). In retina, it has been demonstrated that NO can affect the metabolism of cGMP in rod photoreceptors. To investigate potential involvement of NO in cone photoreceptor activity, we utilized NADPH diaphorase histochemistry to study the cone-dominated retina of the tree shrew (Tupaia belangeri). Unexpectedly, our results revealed different NADPH diaphorase activity in the cellular subcompartments of the spectral classes of cone photoreceptors. Although all cones showed intense labeling of inner segment ellipsoids, the short-wavelength-sensitive (SWS or "blue-sensitive") cones and the rods displayed intense staining of the myoid inner segment subcompartment as well. Furthermore, only SWS cones and rods displayed surface labeling of their nuclei. These findings indicate a manner in which SWS cones differ biochemically from other cone types and in which they are more similar to rods. Such differences may underlie some of the unusual functional properties of the SWS cone system, which have been attributed to postreceptoral processes.

Nitric oxide synthesis in retinal photoreceptor cells

Nitric oxide (NO) is known to be synthesized in several tissues and to increase the formation of cyclic GMP through the activation of soluble guanylate cyclases. Since cyclic GMP plays an important role in visual transduction, we investigated the presence of nitric oxide synthesizing activity in retinal rod outer segments. Bovine rod outer segments were isolated intact and separated into membrane and cytosolic fractions. Nitric oxide synthase activity was assayed by measuring the conversion of L-arginine to L-citrulline. Both membrane and cytosolic fractions were active in the presence of calcium and calmodulin. The activity in both fractions was stimulated by the nitric oxide synthase cofactors FAD, FMN, and tetrahydrobiopterin and inhibited by the L-arginine analog, L-monomethyl arginine. The Km for L-arginine was similar, about 5 microM for the enzyme in both fractions. However, the two fractions differed in their calcium/calmodulin dependence: the membrane fraction exhibited basal activity even in the absence of added calcium and calmodulin while the cytosolic fraction was inactive. But the activity increased in both fractions when supplemented with calcium/calmodulin: in membranes from about 40 to 110 fmol/min/mg of protein and in the cytosol from near zero to about 350 fmol/min/mg of protein in assays carried out at 0.3 microM L-arginine. The two enzymes also responded differently to detergent: the activity of the membrane enzyme was doubled by Triton X-100 while that of the cytosolic enzyme was unaffected. These results show that NO is produced by cytosolic and membrane-associated enzymes with distinguishable properties.(ABSTRACT TRUNCATED AT 250 WORDS)

Purified retinal nitric oxide synthase enhances ADP-ribosylation of rod outer segment proteins

Nitric oxide synthase is present in different cell layers of vertebrate retina and seems to have neuromodulatory functions in the outer retina. The enzyme, when purified from a bovine retina extract, has an apparent molecular mass of 160 kDa and resembles the neuronal constitutive NOS type I with respect to Ca(2+)-calmodulin sensitivity, Km value and inhibition by analogues of L-arginine. Retinal NOS is present in a preparation of rod outer segments attached to parts of the inner segments, but not in pure outer segments. We describe the enhancement of specific ADP-ribosylation of outer segment proteins by purified retinal NOS.

Inhibition of nitric oxide synthase alters light responses and dark voltage of amphibian photoreceptors

We studied the effects of competitive inhibitors of nitric oxide synthase (L-NMMA and L-NNA) on dark voltage and flash responses of retinal rods of the frog. Substances were applied intracellularly via whole-cell patch-clamp electrodes while the membrane voltage was recorded simultaneously. During recording the exchange of substances by diffusion between cytosol and pipette medium affects the cell's function. Under control conditions this exchange is reflected by a slow hyperpolarization of the dark voltage with time and a prolongated flash response recovery, which is mainly due to a loss of nucleotides. Application of L-NMMA and L-NNA accelerated the spontaneous hyperpolarization of the membrane voltage during the course of an experiment, while the recovery of the flash responses was slowed down. The effects observed upon intracellular application of NO-synthase inhibitors were opposite to those observed previously upon application of sodium nitroprusside. Sodium nitroprusside was much less effective when the intracellular calcium level was decreased by application of EGTA at the same time. It is reasonable to assume that the observed effects are linked to nitric oxide synthase and to a NO-dependent soluble guanylate cyclase. The results suggest that the activity of NO-synthase in photoreceptor cells has an influence on concentration and metabolic flux of cGMP in photoreceptors, which may be of relevance for flash response recovery and adaptation processes. It is likely that the regulation of the soluble guanylate cyclase requires a physiological level of calcium.

Modulation of ion channels in rod photoreceptors by nitric oxide

Subcellular compartments in the outer retina of the larval tiger salamander were identified as likely sites of production of nitric oxide (NO), a recently recognized intercellular messenger. NADPH diaphorase histochemistry and NO synthase immunocytochemistry labeled photoreceptor ellipsoids and the distal regions of bipolar and glial cells apposing photoreceptor inner segments, suggesting a role for NO in visual processing in the outer retina. We investigated the actions of NO on several rod photoreceptor ion channels. Application of the NO-generating compound S-nitrosocysteine increased Ca2+ channel current and a voltage-independent conductance, but had no affect on voltage-gated K+ or nonspecific cation currents. Given the steep relation between voltage-dependent Ca2+ influx and photoreceptor synaptic output, these results indicate that NO could modulate transmission of the photoresponse to second order cells.

NADPH-diaphorase (nitric oxide synthase)-reactive amacrine cells of rabbit retina: putative target cells and stimulation by light

In the mammalian retina there are two populations of nitric oxide synthase-containing amacrine cells that stain with the nicotinamide adenine dinucleotide phosphate-diaphorase reaction. To determine the response of these neurons to light, immunoreactivity to Fos proteins was used as a marker of synaptic activation. Fos immunoreactivity is absent in dark-adapted retinas, but 70% of large, Type I nicotinamide adenine dinucleotide phosphate-diaphorase-reactive amacrine cells and 5-10% of the smaller but more numerous Type II nicotinamide adenine dinucleotide phosphate-diaphorase-reactive amacrine cells contain Fos proteins after light stimulation. To localize putative cellular targets of nitric oxide in the retina, retinas were stained immunocytochemically for cyclic GMP after the local administration of the nitric oxide donors sodium nitroprusside and S-nitroso-N-acetylpenicillamine. Both compounds induce strong cyclic GMP immunoreactivity in ON cone bipolar cells. The data suggest that the light-induced inward current in ON cone bipolar cells is enhanced by a nitric oxide-cyclic GMP pathway and that the major source of nitric oxide is the nicotinamide adenine dinucleotide phosphate-diaphorase-reactive amacrine cells in the rabbit retina.

The marker for nitric oxide synthase, NADPH-diaphorase, co-localizes with GABA in horizontal cells and cells of the inner retina in the carp retina

NADPH-diaphorase histochemistry selectively stained discrete populations of neurons contributing to the inner plexiform layer of the carp retina. In addition H1 horizontal cells contributing to the outer plexiform layer were labeled. In these cells as in a subpopulation of the labeled cells in the inner retina, NADPH-activity co-localized with GABA immunoreactivity. NADPH-activity is a direct marker for NO synthesis and it is concluded that NO might be an important mediator of light-dependent adaptational processes in the outer retina.

A rich complexity emerges in phototransduction

Over the past two decades there has been an explosive growth in our understanding of phototransduction, leading to the development of a comprehensive scheme for the process. On the basis of this scheme the finer details of the process are being elucidated. Additional protein components and pathways have been identified, successful quantitative models of parts of the process have been developed, and a detailed understanding of the molecular basis of physiological function has begun to emerge. Here we summarize the most recent developments.

L-arginine and nicotinamide adenine dinucleotide phosphate alter dark voltage and accelerate light response recovery in isolated retinal rods of the frog (Rana temporaria)

Effects of intracellularly applied L-arginine and nicotinamide adenine dinucleotide phosphate (NADPH) on the dark voltage and light responses of retinal rods were studied by means of the whole-cell patch-clamp technique. In this mode an exchange of substances by diffusion between cytosol and pipette medium occurs (Pflügers Arch., 411 (1988) 204-211). In retinal rods a loss of nucleotides is reflected by a hyperpolarization of the dark voltage and by a prolongation of the light responses (Vis. Neurosci., 2 (1989) 101-108). Intracellular application of L-arginine prevented the prolongation of the light responses and NADPH accelerated the light response recovery and in addition depolarized the photoreceptor cells. The effects were similar to those observed before upon application of the nitric oxide (NO)-releasing substance sodium nitroprusside (Vis. Neurosci., 9 (1992) 205-209). It is therefore assumed that the observed effects are linked to the NO-synthase and to an activation of a guanylate cyclase by NO. It is concluded that the level of NADPH in photoreceptor cells may affect the metabolic flux of guanosine 3',5'-cyclic monophosphate (cGMP).