Localization of nitric oxide synthase, NADPH diaphorase and soluble guanylyl cyclase in adult rabbit retina

Nitric oxide (NO) acts as a neuronal messenger which activates soluble guanylyl cyclase (SGC) in neighboring cells and produces a wide range of physiological effects in the central nervous system (CNS). Using immunocytochemical and histochemical stains, we have characterized the NO/SGC system in the rabbit retina and to a lesser extent, in monkey retina. Based on staining patterns observed with an antibody to nitric oxide synthase (NOS) type I and a histochemical marker for NADPH diaphorase, a metabolic intermediate required for NOS activity, three major classes of neurons appear to generate NO in the rabbit retina. These include two subclasses of sparsely distributed wide field amacrine cells, rod and cone photoreceptors, and a subpopulation of ganglion cells. Equivalent cell populations were labeled in monkey retina. An antibody to SGC (tested only in rabbit retina), labeled large arrays of cone photoreceptors in the outer nuclear layer, both amacrine and bipolar cells in the inner nuclear layer (INL), as well as populations of neurons in the ganglion cell layer. These data suggest that the ability to generate NO is restricted to relatively few neurons in the inner retina and to photoreceptor cells in the outer retina; while presumptive target cells, containing pools of SGC, are widespread and form contiguous fields across the inner and outer nuclear layers (ONL) as well as the ganglion cell layer.     

Müller cells in the retina of the cane toad, Bufo marinus, express neuropeptide Y-like immunoreactivity

We have used light- and electron-microscopic immunohistochemistry to identify the presence of immunoreactivity to neuropeptide Y (NPY) within Müller cells in the retina of the cane toad, Bufo marinus. Müller cells containing NPY-like immunoreactivity (NPY-LI) were identified at the light-microscopic level by the coexistence with immunoreactivity to glial fibrillary acidic protein (GFAP) and at the ultrastructural level by their characteristic relationship to neuron cell bodies and processes. At the light-microscopic level, those cells which contained both NPY-LI and GFAP-LI usually had small cell bodies in the inner nuclear layer, while those cells which contained only NPY-LI were identified as large and small amacrine cells. The radially oriented primary processes in the inner plexiform layer and the vitreal end feet of GFAP-LI Müller cells also expressed NPY-LI. At the ultrastructural level, thin lamellar processes of Müller cells with NPY-LI enclosed some amacrine cell bodies in the inner nuclear layer and amacrine cell dendrites in the inner plexiform layer. These observations suggest that NPY-LI is localized in Müller cells in addition to two types of amacrine cells previously identified in the Bufo retina. This study provides the first evidence that glial elements in the vertebrate retina express NPY-LI.     

Histochemical evaluation of placental dipeptidyl peptidase IV (CD26) in pre-eclampsia: enzyme activity in villous trophoblast indicates an enhanced likelihood of gestational hypertensive disorders

Nitric oxide (NO) acts as a modulator of neuronal transmission in mature neuronal systems, including the retina. Recently, NO has also been suggested to have a trophic function during development. We examined immunocytochemically the distribution of NO-producing cells in developing and transplanted rabbit retinas. An antibody detecting the neuronal isoform of its biosynthetic enzyme, nitric oxide synthase (NOS), was used on normal developing retinas [starting at embryonic day (E) 15] and on rabbit retinal transplants after various survival times (1-139 days after surgery). Weakly stained cell bodies were first observed in the proximal margin of the neuroblastic layer at E 29. Stained processes projecting towards a developing inner plexiform layer were also visible at this time point. Immunoreactive cells were located at later stages in the innermost part of the inner nuclear layer and in the ganglion cell layer, and are likely to correspond mainly to amacrine cells. NOS-labelled cells were also found in retinal transplants. The first NOS-labelled cells appeared, as in normal developing retinas, in ages corresponding to E 29 and were still detected in transplants corresponding to postnatal day 123. NOS-labelled cells were seen in areas between rosettes, where amacrine cells are located. NOS-labelled processes were at times seen to project for long distances, forming very distinct plexuses. NOS-containing amacrine cells thus appear both in the transplants and in developing retinas in the embryonic stages, long before synaptic function involving these cells can be expected, suggesting a role for NO not only in neuromodulation but also in retinal development.     

Nitric oxide modulates cGMP levels in neurons of the inner and outer retina in opposite ways

In the mammalian retina, neuronal nitric oxide synthase (NOS) is mainly localized in subpopulations of amacrine cells. One function of nitric oxide (NO) is to stimulate soluble guanylate cyclases which in turn synthesize cGMP. We used an antibody specific for cGMP to demonstrate cGMP-like immunoreactivity (cG-IR) in bovine, rat, and rabbit retinae and investigated the effects on cGMP levels of both exogenously applied NO and of endogenously released NO. We found that cGMP levels in inner and outer retina were controlled in opposite ways. In the presence of the NO-donors SNP, SIN-1 or SNAP, cG-IR was prominent in neurons of the inner retina, mainly in cone bipolar cells, some amacrine and ganglion cells. Retinae incubated in IBMX showed weak cG-IR in bipolar cells. Glutamate increased cG-IR in the inner retina, presumably by stimulating endogenous NO release, whereas NOS inhibitors or GABA and glycine decreased cG-IR in bipolar cells by reducing NO release. In somata, inner segments and spherules of rod photoreceptors the situation was reversed. cG-IR was undetectable in the presence of NO-donors or glutamate, was moderate in IBMX-treated retinae, but increased strongly in the presence of NOS inhibitors or GABA/glycine. We conclude that NO is released endogenously in the retina. In the presence of NO, cGMP levels are increased in neurons of the inner retina, but are decreased in rods.     

Modulation of quinolinic acid-induced depletion of striatal NADPH diaphorase and enkephalinergic neurons by inhibition of nitric oxide synthase

The present study was designed to examine the role of nitric oxide (NO) in quinolinic acid (QUIN)-induced depletion of rat striatal nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase and enkephalinergic neurons. Intrastriatal injection of QUIN produced a dose-dependent decrease in NADPH diaphorase and enkephalin positive cells, with cell loss being evident following the injection of 6 and 18 nmol QUIN, respectively. To evaluate the role of NO in QUIN-induced toxicity, animals were pretreated with the non-specific nitric oxide synthase (NOS) inhibitor, Nomega-nitro-l-arginine (l-NAME) or the selective neuronal NOS inhibitor, 7-nitro indazole (7-NI). l-NAME (2x250 mg/kg, i.p. 8 h apart) maximally inhibited striatal NOS activity by 85%, while 7-NI (50 mg/kg, i.p.) maximally inhibited striatal NOS activity by 60%. Pretreatment with l-NAME or 7-NI potentiated the loss of NADPH diaphorase neurons resulting from intrastriatal injection of low doses of QUIN (18 nmol). Neither NOS inhibitor had any effect on the loss of striatal NADPH diaphorase neurons induced by a higher dose of QUIN (24 nmol). In contrast, 7-NI partially prevented the QUIN (18 and 24 nmol)-induced loss of enkephalinergic neurons, while l-NAME had no effect. These results indicate that NO formation may play a role in QUIN-induced loss of enkephalinergic neurons, but not in the loss of NADPH diaphorase neurons.     

The major cell populations of the mouse retina

We report a quantitative analysis of the major populations of cells present in the retina of the C57 mouse. Rod and cone photoreceptors were counted using differential interference contrast microscopy in retinal whole mounts. Horizontal, bipolar, amacrine, and Müller cells were identified in serial section electron micrographs assembled into serial montages. Ganglion cells and displaced amacrine cells were counted by subtracting the number of axons in the optic nerve, learned from electron microscopy, from the total neurons of the ganglion cell layer. The results provide a base of reference for future work on genetically altered animals and put into perspective certain recent studies. Comparable data are now available for the retinas of the rabbit and the monkey. With the exception of the monkey fovea, the inner nuclear layers of the three species contain populations of cells that are, overall, quite similar. This contradicts the previous belief that the retinas of lower mammals are "amacrine-dominated", and therefore more complex, than those of higher mammals.     

Inhibition by protein kinase C of the 86Rb+ efflux and vasorelaxation induced by P1075, a K(ATP) channel opener, in rat isolated aorta

The expression of GABA in the human fetal (12-25 weeks of gestation), postnatal (five-month-old), and adult (35-year-old) retinas was investigated by immunohistochemistry. GABA expression was seen as early as 12 weeks in the undifferentiated cells of the inner neuroblast zone; a few optic nerve fiber layer axons were clearly labeled, suggesting that some of the stained cell bodies were prospective ganglion cells, others could be displaced amacrine cells. From 16-17 to 24-25 weeks, intense labeling was found in the amacrine, displaced amacrine, and some ganglion cells. During this time period, horizontal cells (identified by calbindin immunohistochemistry), undergoing migration (periphery) and differentiation (center), expressed GABA prominently. In the postnatal retina, some horizontal cells were moderately labeled, but very weakly in a few cells, in the adult. The Müller cells developed immunoreactivity first weakly at 12 weeks and then moderately from 16-17 weeks onward. The staining was also evident in the postnatal and adult retinas, showing labeled processes of these glial cells. Virtually no axons in the adult optic nerve and nerve fiber layer were stained; the staining was restricted to a few, large ganglion cells and displaced amacrine cells: Some amacrines were also labeled. The possibility that GABA might play a role in horizontal cell differentiation and maturation is highlighted. Other evidences suggest that GABA might play a role in metabolism during retinal development.     

Effect of 7-nitro indazole on quinolinic acid-induced striatal toxicity in the rat

Nitric oxide (NO) is implicated as a mediator of cell death in models of neurodegenerative disease. However, the precise role of NO in neuronal degeneration remains controversial. In the present study we employed 7-nitro indazole (7-NI), reportedly a selective inhibitor of neuronal nitric oxide synthase (nNOS) in vivo, to investigate the possible involvement of NO in quinolinic acid (QA)-induced striatal toxicity in the rat. Intrastriatal injection of QA (30 nmol) caused loss of NADPH diaphorase (48%), NOS (48%) and acetylcholinesterase (AChE; 22%) positive neurones and a loss of NOS activity (78%) in striatal homogenates. 7-NI (30 mg kg-1, i.p. every 4 h for 20 h) did not affect the loss of NADPH diaphorase (52%), NOS (52%) and AChE (16%) positive neurones or the loss of NOS activity (66%) in striatal homogenates. The present study does not support a role for NO in QA-induced striatal toxicity.     

Loss of inwardly rectifying potassium currents by human retinal glial cells in diseases of the eye

We compared the inward K+ currents of Müller glial cells from healthy and pathologically changed human retinas. To this purpose, the whole-cell voltage-clamp technique was performed on noncultured Müller cells acutely isolated from human retinas. Cells originated from retinas of four healthy organ donors and of 24 patients suffering from different vitreoretinal and chorioretinal diseases. Müller cells from organ donors displayed inward K+ currents in the whole-cell mode similar to those found in other species. In contrast, this pattern was clearly changed in the Müller cells from patient retinas. In whole-cell recordings many Müller cells had strongly decreased inward K+ current amplitudes or lost these currents completely. Thus, the mean input resistance of Müller cells from patients was significantly increased to 1,129 +/- 812 M omega, compared to 279 +/- 174 M omega in Müller cells from healthy organ donor retinas. Accordingly, since the membrane potential is mainly determined by the K+ inward conductance in healthy Müller cells, a large amount of Müller cells from patient retinas had a membrane potential which was significantly lower than that of Müller cells from control eyes. The mean membrane potentials were -37 +/- 24 mV and -63 +/- 25 mV for patient and donor Müller cells, respectively. The newly described membrane characteristic changes of Müller cells from patient eyes are assumed to interfere severely with normal retinal function: (1) the retinal K+ homeostasis, which is partly regulated by the Müller cell-mediated spatial buffering, should be disturbed, and (2) the diminished membrane potential should influence voltage-dependent transporter systems of the Müller cells, e.g., the Na(+)-dependent glutamate uptake.     

Distribution of mitochondria within Müller cells--I. Correlation with retinal vascularization in different mammalian species

The distribution of mitochondria within retinal glial (Müller) cells and neurons was studied by electron microscopy, by confocal microscopy of a mitochondrial dye and by immunocytochemical demonstration of the mitochondrial enzyme GABA transaminase (GABA-T). We studied sections and enzymatically dissociated cells from adult vascularized (human, pig and rat) and avascular or pseudangiotic (guinea-pig and rabbit) mammalian retinae. The following main observations were made. (1) Müller cells in adult euangiotic (totally vascularized) retinae contain mitochondria throughout their length. (2) Müller cells from the periphery of avascular retinae display mitochondria only within the sclerad-most end of Müller cell processes. (3) Müller cells from the vascularized retinal rim around the optic nerve head in guinea-pigs contain mitochondria throughout their length. (4) Müller cells from the peripapillar myelinated region ('medullary rays') of the pseudangiotic rabbit retina contain mitochondria up to their soma. In living dissociated Müller cells from guinea-pig retina, there was no indication of low intracellular pH where the mitochondria were clustered. These data support the hypothesis that Müller cells display mitochondria only at locations of their cytoplasm where the local O2 pressure (pO2) exceeds a certain threshold. In contrast, retinal ganglion cells of guinea-pig and rabbit retinae display many mitochondria although the local pO2 in the inner (vitread) retinal layers has been reported to be extremely low. It is probable that the alignment of mitochondria and the expression of mitochondrial enzymes are regulated by different mechanisms in various types of retinal neurons and glial cells.     

Immunocytochemical localization of GABA and glycine in amacrine and displaced amacrine cells of macaque monkey retina

Recent studies have varied widely in the percentages of GABA- and glycine-immunoreactive (GABA+, GLY+) amacrines reported for primate retina. We compared the distributions of GABA+ and GLY+ amacrines and displaced amacrines at seven locations along the horizontal meridian of macaque retina using postembedding immunogold labeling with silver intensification. The percentage of GABA+ amacrine profiles was higher in central retina (50-55%) than peripheral retina (30-40%), whereas the percentage of GLY+ amacrine profiles did not vary much with eccentricity (52-57%). GABA and glycine were colocalized in 5-20% of amacrines, depending on the eccentricity, whereas 5-30% of amacrines were not immunoreactive for either neurotransmitter. GABA+ amacrines were slightly larger than GLY+ amacrines or Müller cells. In the ganglion cell layer, 5-20% of neurons were labeled for either GABA or glycine and were identified as displaced amacrines. Of these, 53% were GABA+ only, 11% were GLY+ only, and 37% were double-labeled. A few large, very lightly labeled GABA+ cells were identified as ganglion cells. Other features that varied with eccentricity included the linear density of GABA+ and GLY+ amacrines, and the ratio of amacrines to Müller cells.     

Prolonged delivery of brain-derived neurotrophic factor by adenovirus-infected Müller cells temporarily rescues injured retinal ganglion cells

In this study, we demonstrate that: (i) injection of an adenovirus (Ad) vector containing the brain-derived neurotrophic factor (BDNF) gene (Ad.BDNF) into the vitreous chamber of adult rats results in selective transgene expression by Müller cells; (ii) in vitro, Müller cells infected with Ad.BDNF secrete BDNF that enhances neuronal survival; (iii) in vivo, Ad-mediated expression of functional BDNF by Müller cells, temporarily extends the survival of axotomized retinal ganglion cells (RGCs); 16 days after axotomy, injured retinas treated with Ad.BDNF showed a 4.5-fold increase in surviving RGCs compared with control retinas; (iv) the transient expression of the BDNF transgene, which lasted approximately 10 days, can be prolonged with immunosuppression for at least 30 days, and such Ad-mediated BDNF remains biologically active, (v) persistent expression of BDNF by infected Müller cells does not further enhance the survival of injured RGCs, indicating that the effect of this neurotrophin on RGC survival is limited by changes induced by the lesion within 10-16 days after optic nerve transection rather than the availability of BDNF. Thus, Ad-transduced Müller cells are a novel pathway for sustained delivery of BDNF to acutely-injured RGCs. Because these cells span the entire thickness of the retina, Ad-mediated gene delivery to Müller cells may also be useful to influence photoreceptors and other retinal neurons.     

Late proliferation of retinal Müller cell progenitors facilitates preferential targeting with retroviral vectors in vitro

During vertebrate neural retina development, the relationship between mitotic activity in progenitor cells and the acquisition of a mature cell phenotype remains an area of controversy. The Müller glial cell has long been recognized as one of the last cell types of the retina to mature, which occurs under the influence of cell-cell interactions. In this report we examine the acquisition of the Müller cell phenotype in relation to mitotic activity. Using immunohistochemical markers, we demonstrate that a gene product characteristic of mature Müller cells, the 2M6 antigen, is expressed in mitotically active cells, even after all the major retina architectural features have been laid down. Furthermore, we show that retroviral infection, a process that requires mitotically active cells, preferentially targets Müller cell progenitors when late embryonic retina is infected in vitro. The two lines of evidence are consistent with a model for Müller cell differentiation that includes a mitotically active progenitor that has already begun to express specific differentiation gene products.     

Elevation of soluble IL-2 receptor and IL-4, and nonelevation of IFN-gamma in sera from patients with allergic asthma

This study is the first demonstration of glial fibrillary acidic protein (GFAP)-immunoreactivity in the retina of the lamprey Lampetra fluviatilis. This immunoreactivity is expressed on one hand, in radial processes and somata which belong to Müller cells and, on the other hand, in horizontal fibers in the intermediate plexus between horizontal cells. The tracing of these fibers to Müller cells or horizontal cells is discussed.     

Nitric oxide synthase in the heterogeneous population of intramural striated muscle fibres of the human membranous urethral sphincter

PURPOSE: Nitric oxide (NO) is known to relax urethral smooth muscle. The role of NO in the control of urethral striated muscle remains unknown. We have investigated the distribution of nitric oxide synthase (NOS) immunoreactivity and its possible relationship with subtypes of intramural striated muscle fibers in the human male membranous urethra. MATERIALS AND METHODS: Whole transverse cryostat sections from seven membranous urethrae were studied using NOS immunohistochemistry and NADPH diaphorase histochemistry. Striated fiber subtypes were demonstrated using immunohistochemistry for troponin T and histochemistry for myofibrillary adenosine triphosphatase (ATPase). Consecutive sections were used to assess the correlation between the distribution of NOS immunoreactivity and the type of striated fibers. RESULTS: NOS immunoreactivity and NADPH diaphorase activity were detected in the sarcolemma of 48.5% of the intramural striated muscle fibers. NOS immunoreactive nerve trunks and fine nerve fibers, a few of which appeared to end on muscle fibers, were present in the striated sphincter. Fast twitch fibers were detected by ATPase staining, and also exhibited positive immunoreactivity for troponin T, constituting 34.6% of the total number of striated fibers. Two populations of slow twitch fibers were identified; one with small diameter (mean: 15.7 microns) and another of larger diameter (mean: 21.7 microns) comparable to that of fast twitch fibers. 86% of the fast twitch fibers and 29% of slow twitch fibers (most of which had larger diameters) exhibited NOS immunoreactivity and NADPH diaphorase activity in the sarcolemma. CONCLUSIONS: The presence of nitrergic nerve fibers in the striated urethral sphincter suggests an involvement in the innervation of urethral striated muscle. Furthermore, the presence of NOS immunoreactivity in the sarcolemma may indicate a role for NO in the regulation of urethral striated muscle metabolism and contraction.     

The Müller (glial) cell in normal and diseased retina: a case for single-cell electrophysiology

In the retina of most vertebrates there exists only one type of macroglia, the Müller cell. Müller cells express voltage-gated ion channels, neurotransmitter receptors and various uptake carrier systems. These properties enable the Müller cells to control the activity of retinal neurons by regulating the extracellular concentration of neuroactive substances such as K+, GABA and glutamate. We show here how electrophysiological recordings from enzymatically dissociated mammalian Müller cells can be used to study these mechanisms. Müller cells from various species have Na(+)-dependent GABA uptake carriers, but only cells from primates have additional GABA receptors that activate Cl- channels. Application of glutamate analogues causes enhanced membrane currents recorded from Müller cells in situ but not from isolated cells. We show that mammalian Müller cells have no ionotropic glutamate receptors but respond to increased K+ release from glutamate-stimulated retinal neurons. This response is involved in extracellular K+ clearance and is mediated by voltage-gated (inwardly rectifying) K+ channels which are abundantly expressed by healthy Müller cells. In various cases of human retinal pathology, currents through these channels are strongly reduced or even extinguished. Another type of voltage-gated ion channels, observed in Müller cells from many mammalian species, are Na+ channels. In Müller cells from diseased human retinae, voltage-dependent Na+ currents were significantly increased in comparison to cells from control donors. Thus, the expression of glial ion channels seems to be controlled by neuronal signals. This interaction may be involved in the pathogenesis of retinal gliosis which inevitably accompanies any degeneration of retinal neurons. In particular, Müller cell proliferation may be triggered by mechanisms requiring the activation of Ca(2+)-dependent K+ channels. Ca(2+)-dependent K+ currents are easily elicitable in Müller cells from degenerating retinae and can be blocked by 1 mM TEA (tetraethylammonium). In purified Müller cell cultures, the application of 1 mM TEA greatly reduces the proliferative activity of the cells. These data clearly show that Müller cells are altered in cases of neuronal degeneration and may be crucially involved in pathogenetic mechanisms of the retina.     

C-reactive protein as an outcome predictor for maintenance hemodialysis patients

We produced the monoclonal antibody RT10F7, characterized its antigenic specificity and expression in the adult and developing retina, in cultured retinal cells and in other parts of the central nervous system. In metabolically-labelled retinal cultures RT10F7 immunoprecipitated a protein of approximately 36,000 mol. wt. In the adult, RT10F7 stained endfeet of Müller cells in the ganglion cell layer, four horizontal bands in the inner plexiform layer, and radial fibres in the outer plexiform layer which terminated at the outer limiting membrane. In the inner nuclear layer, most somata were underlined by Müller processes that wrapped around them, but some cell bodies were immunoreactive for RT10F7 in the cytoplasm. During development, postnatal day 21 was the first age at which the adult pattern of immunoreactivity was present, although a fourth band in the inner plexiform layer was less clear than for the adult. By 14 and eight days after birth, the pattern of RT10F7 immunoreactivity approximated that of the adult; however, only three bands and one band were present, respectively, in the inner plexiform layer. At earlier ages, postnatal days 4, 1 and embryonic ages 19 and 15, the monoclonal antibody stained Müller cell endfeet and radial fibres, from the inner plexiform layer through the neuroblastic layer to the outer limiting membrane. At these ages, the immunoreactivity was more prominent at the level of Müller cell endfeet. The monoclonal antibody stained glia in preparations of dissociated retinal cells maintained in culture but not astrocytes or oligodendrocytes from optic nerve cultures. In brain sections, tanycytes exhibited RT10F7 immunoreactivity. The monoclonal antibody RT10F7 recognized a specific cell type in the retina, the Müller cell. In the adult and developing retina, RT10F7 recognized an antigen that is present primarily in Müller cell processes. This feature allowed us to follow the maturation of the Müller cell and correlate it with developmental events in the retina. RT10F7 is a specific marker for Müller cells in vivo and in vitro and may be useful for studies of function of Müller cells after ablation or after injuries that are known to activate Müller cells.     

Plasma-induced changes in the physiology of mammalian retinal glial cells: role of glutamate

Plasma can leak into the nervous system when the vascular endothelial barrier is compromised. Although this occurs commonly, little is known about the effects of plasma on the function of cells in the central nervous system. In this study, we focused on the responses of glial cells, which, because they ensheathe the blood vessels, are the first cells exposed to leaking plasma. We used the perforated-patch configuration of the patch-clamp technique to assess the effects of plasma on freshly dissociated bovine and human Müller cells, the principal glia of the retina. To monitor the function of Müller cells in situ, we recorded electroretinograms from isolated retinas. We found that plasma activates an electrogenic glutamate transporter and inhibits inward-rectifying K+ channels, as well as a transient outward current. Glutamate, a normal constituent of the blood, mimicked these effects. Unlike our recent findings with serum, which contains molecules generated by the clotting process, plasma neither activated a nonspecific cation conductance nor inhibited the slow P(III) component of the electroretinogram, which is generated by Müller cells responding to light-evoked changes in the extracellular potassium concentration ([K+]o). Taken together, our observations indicate that a leakage of serum into the retina compromises the regulation of [K+]o by Müller cells; however, when plasma enters the retina at sites of a breakdown in the blood-retinal barrier, these glia can maintain K+ homeostasis while reducing the potentially neurotoxic levels of glutamate.     

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.     

Na+ channels are expressed by mammalian retinal glial (Müller) cells

Müller cells constitute the principal glia of the vertebrate retina. Unlike other types of neuroglial cells such as astrocytes and Schwann cells, Müller cells have not yet been demonstrated to express Na+ channels. Here we present first evidence of Müller cell Na+ currents from voltage-clamp studies in enzymatically isolated cells. Some cells from retinae of cats and dogs, but none from rabbit or guinea-pig retinae, revealed fast and rapidly inactivating inward currents in response to depolarizing voltage steps. The currents reversibly disappeared in Na+ free solutions or under tetrodotoxin (TTX, 1 microM). Activation and inactivation characteristics of these currents were strikingly similar to those of neurone-type Na+ channels.