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

NADPH diaphorase activity in mammalian retinas is modulated by the state of visual adaptation

NADPH diaphorase histochemistry is commonly used to identify cells containing nitric oxide synthase (NOS), the enzyme catalyzing the production of nitric oxide from L-arginine. NADPH diaphorase activity and NOS immunostaining was demonstrated in different cells of the vertebrate retina; photoreceptors, horizontal cells, amacrine cells, ganglion cells, and Müller cells. However, the physiological role of nitric oxide (NO) in the retina has yet to be elucidated. In this study, we tested the assumption that NADPH diaphorase activity in the retinas of rabbits and rats depended on the state of visual adaptation. In the rabbit, light adaptation enhanced NADPH diaphorase activity in amacrine cells and practically eliminated it in horizontal cells. Dark adaptation induced the opposite effects; the NADPH diaphorase activity was reduced in amacrine cells and enhanced in horizontal cells. Retinas from eyes that were injected intravitreally with L-glutamate exhibited a pattern of NADPH diaphorase activity that was similar to that seen in dark-adapted retinas. In rats, the NADPH diaphorase activity of amacrine and horizontal cells exhibited adaptation dependency similar to that of the rabbit retina. But, the most pronounced effect of dark adaptation in the rat's retina was an enhancement of NADPH diaphorase activity in Müller cells, especially of the endfoot region. Assuming that NADPH diaphorase activity is a marker for NOS, these findings suggest that NO production in the mammalian retina is modulated by the level of ambient illumination and support the notion that NO plays a physiological role in the retina.

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.

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.

Distribution of NADPH-diaphorase staining and light-induced Fos expression in the rat suprachiasmatic nucleus region supports a role for nitric oxide in the circadian system

Nitric oxide serves as a messenger molecule in some neuronal systems that use glutamate as a transmitter and it has been shown that glutamate mediates the transmission of photic signals by retinal ganglion cell axons terminating in the hypothalamic suprachiasmatic nucleus, site of the circadian pacemaker in rodents. Recent experiments have demonstrated that pharmacological treatments which block nitric oxide synthesis by nitric oxide synthase prevent glutamate-induced phase shifts of the cell firing rhythm in suprachiasmatic nucleus slice preparation in vitro; similar treatments were found to inhibit light transmission to the suprachiasmatic nucleus as well as light-induced phase shifts in activity rhythms in vivo, implicating nitric oxide in circadian light signalling in vivo. There is limited information, however, about the presence and function of nitric oxide synthase-containing neurons within retinorecipient regions of the rodent suprachiasmatic nucleus. In the present study we used NADPH-diaphorase histochemistry and immunostaining for the nuclear phosphoprotein Fos to assess the co-distribution of nitric oxide synthase-containing neurons and light-responsive cells in the rat suprachiasmatic nucleus region. A strong convergence between NADPH-diaphorase-stained cell bodies and fibres and cells that expressed Fos in response to photic stimulation was noted in the anterior periventricular nucleus, suprachiasmatic preoptic nucleus, retrochiasmatic area, the inter-suprachiasmatic nucleus region, and the dorsal aspect of the optic chiasm, below the suprachiasmatic nucleus. A similar convergence between NADPH-diaphorase-stained fibres and Fos-immunoreactive cells was noted inside the suprachiasmatic nucleus, but the number of NADPH-diaphorase-stained elements found in this region was substantially low compared with that found in retinorecipient regions bordering the nucleus. In many cases both inside and outside the suprachiasmatic nucleus, the Fos-immunoreactive cells appeared to make direct contact with NADPH-diaphorase-stained cells or fibres, but no co-localization of Fos immunoreactivity and NADPH-diaphorase histochemical activity within individual cells was detected. Extensive co-distribution of NADPH-diaphorase-stained cells and fibres and cells that express Fos in response to photic stimulation in the suprachiasmatic nucleus region is in line with the hypothesis that nitric oxide participates in the mechanism mediating circadian light signalling in the suprachiasmatic nucleus. However, lack of co-localization of the two markers to individual cells rules out the possibility that retinorecipient cells in the suprachiasmatic region synthesize and release nitric oxide when photically-activated. Instead, the results support the possibility that photic stimulation triggers nitric oxide synthesis in nitric oxide synthase-containing neurons located near the photically-activated cells.

Differential properties of two gap junctional pathways made by AII amacrine cells

The retina is sensitive to light stimuli varying over more than 12 log units in intensity. It accomplishes this, in part, by switching between rod-dominated circuits designed for maximum utilization of scarce photons and cone circuits designed for greater acuity. Rod signals are integrated into the cone pathways through AII amacrine cells, which are connected by gap junctions both to other AII amacrine cells and to cone bipolar cells. To determine the relative permeabilities of the two junctional pathways, we have measured the distribution of biotinylated tracers across this heterologous cell assembly after injecting a single AII amacrine cell. We found that neurobiotin (relative molecular mass, 286) passed easily through both types of gap junctions, but that biotin-X cadaverine (relative molecular mass, 442) passed through AII/bipolar cell gap junctions poorly compared to AII/AII gap junctions. Thus, the AII/bipolar cell channel has a lower permeability to large molecules than does the AII/AII amacrine cell channel. The two pathways are also regulated differently. Dopamine and cyclic AMP agonists, known to diminish AII-AII coupling, did not change the relative labelling intensity of AII to bipolar cells. However, nitric oxide and cGMP agonists selectively reduced labelling in bipolar cells relative to AII amacrine cells, perhaps by acting at the bipolar side of this gap junction. This suggests that increased cGMP controls the network switching between rod and cone pathways associated with light adaptation.

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.

Expression of c-fos and c-jun in rat retina following protracted illumination

Illumination produces degeneration of outer photoreceptor segments, a phenomenon that may be reversed after a period of darkness. Neuronal expression of the immediate early genes (IEGs) c-fos and c-jun, both recognized as proto-oncogenes, has been reported after stimulation of different regions in the central nervous system (CNS). We performed a sequential study on Fos and Jun immunoreactivity to investigate the role of IEGs following 8 days of continuous illumination in 30-35-day-old Wistar rats. Retinas were fixed by perfusion in 4% paraformaldehyde, after a period of illumination followed by 0, 2, 7, 10 and 20 days in total darkness. Cryostat sections were immunocytochemically stained using antibodies to Fos and Jun. Fos and Jun immunoreactivities were detected in all photoreceptors evaluated, peaking in nuclei of rats kept in total darkness for 2 days, and becoming negative as from 7 days. Increases in c-fos and c-jun products during the darkness period may play a role in triggering molecular events participating in plastic changes in photoreceptors and/or in the protection for oxidative damage cause by free radicals induced by light irradiation.

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

The effect of nitric oxide donor compounds (sodium nitroprusside, hydroxylamine and S-nitroso-N-acetyl-D,L-penicillamine) on depolarization-induced release of endogenous dopamine in the light-adapted, isolated retina of the rabbit was studied by HPLC. All three compounds had the same effect, reducing the amount of dopamine released by up to 90%. The effect was concentration dependent, saturating at 300 microM; it was blocked by the nitric oxide scavenger, mannitol (50 mM), which by itself had no effect on the basal release of dopamine. GABAA receptors were not involved. Possible cellular mechanisms underlying the findings are discussed. It is suggested that the inhibitory interaction between dopamine and nitric oxide could represent a higher order function in the light adaptation process in the retina.

NADPH-diaphorase expression in neurons and glia of the normal adult rat retina

In the rat retina, nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) staining has been described previously in a population of amacrine cells, most of which were located in the inner nuclear layer. In the present study, a number of parameters such as the nature of the fixative, the time of fixation and photointensification were optimised to obtain the strongest possible reaction for this enzyme. As a result, a very different staining pattern emerged: with short paraformaldehyde fixation, numerous neurons (identified as a combination of ganglion cells and amacrines) were labelled in the ganglion cell layer, NADPH-d-positive amacrine cells (described previously) were seen in the inner nuclear layer and Müller cells were labelled strongly, particularly in the inner retina. Glutaraldehyde fixation of the same duration resulted in the preferential staining of Müller cells while neurons appeared less reactive. Therefore, fixation conditions are a determining factor in the cellular localisation of NADPH-d in the rat retina. By taking fixation into account, future studies should gain more rigorous insights into the possible functions of this enzyme in the vertebrate retina.

Localization of NADPH-diaphorase/nitric oxide synthase in the rat retina: an electron microscopic study

The activity of NADPH-diaphorase (NADPH-d), a marker for nitric oxide synthase (NOS), was examined histochemically in the rat and mice retina. Mice in which the neuronal NOS gene has been disrupted (nNOS- mice) were used for specificity controls. Light microscopically a few amacrine cells were heavily stained. Other cells were stained weakly or not at all. Under the electron microscope, formazan precipitates were detectable on membranes of endoplasmic reticulum, nuclear envelope, mitochondria, and, in a few cases, the Golgi complex. Bipolar, horizontal, and Müller cells, were if at all, sparsely labeled with formazan. Labeled mitochondria were observed in rod endings and in inner segments of photoreceptors. Outer segments of photoreceptors and ganglion cells were completely free of reaction product. The NADPH-d reaction in wild-type mice displayed a similar distribution pattern to that in rats. Retinae of nNOS- mice showed a complete lack of prominent NADPH-d stained (amacrine) cells. None or a very few labeled membranes were seen.

Light-induced c-fos expression in amacrine cells in the rabbit retina

Retinal neurons that express the immediate early gene c-fos after light exposure were characterized by neurotransmitter content using histochemical and immunocytochemical staining. In Northern blots the amount of c-fos mRNA peaked at 30 min, but remained detectable 60 min following light stimulation. Fos proteins were seen in the inner nuclear and ganglion cell layers, and the staining was most intense two and three hours after beginning the light exposure. In the ganglion cell layer 30-40% of Fos-immunoreactive cells were cholinergic displaced amacrine cells and 3-5% were ganglion cells. In the inner nuclear layer 24% of Fos-immunoreactive cells were Type I and 7% Type II NADPH-diaphorase-reactive (nitric oxide synthase) amacrine cells, 11% were tyrosine hydroxylase-containing cells, and 10-15% cholinergic amacrine cells. No Fos immunoreactivity was seen in serotoninergic, somatostatin- or VIP-immunoreactive cells, bipolar, horizontal or photoreceptor cells. Nicotine, kainic acid, NMDA and SCH 38393, a dopamine D1 receptor agonist, induced Fos immunostaining in the inner nuclear and ganglion cell layers, but administration of the corresponding receptor blockers mecamylamine, kynuretic acid, MK-801, haloperidol and SCH 23990 did not prevent light-induced Fos expression.

Immunocytochemical analysis of bipolar cells in the macaque monkey retina

Transfer of visual information from photoreceptors to ganglion cells within the retina is mediated by specialized groups of bipolar cells. At least 10 different morphological types of bipolar cells have been distinguished in Golgi studies of primate retina. In the present study, bipolar cell populations in the macaque monkey retina were identified by their differential immunoreactivity to a spectrum of antibody markers. This enabled their spatial density and photoreceptor connections to be analysed. An antibody against the beta isozyme of protein kinase C (PKCA beta) labelled many cone bipolar cells. Invaginating (presumed ON) cone bipolar cells and rod bipolar cells were preferentially labelled with a monoclonal antibody raised against rabbit olfactory bulb. Flat (presumed OFF) bipolar cells were labelled with an antiserum against the glutamate transporter protein (GLT-1). Different populations of diffuse cone bipolar cells, which contact 5-10 cones, could be distinguished. The GLT-1 antiserum preferentially labelled the flat diffuse bipolar cell type DB2 (Boycott and Wässle, 1991, Eur. J. Neurosci. 3:1069-1088) as well as flat midget bipolar cells. Antibodies to calbindin (CaBP D-28K) labelled the flat diffuse bipolar cell type DB3 and (possibly) the invaginating diffuse bipolar cell type DB5. An antibody against the alpha isozyme of PKC labelled an invaginating diffuse bipolar cell type (DB4) as well as rod bipolar cells. Comparison of the spatial density of cone bipolar cell populations with that of photoreceptors suggests that each bipolar cell class provides a complete coverage of the cone array (each cone is contacted by at least one member of every bipolar cell class). These results support the classification scheme of Boycott and Wässle (1991) by showing that different diffuse bipolar cell classes express different patterns of immunoreactivity, and they reinforce the view that different spatial and temporal components of the signal from the photoreceptor array are processed in parallel within the primate retina.