Synthesis of nitric oxide in the bovine 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.

The localization of nitric oxide synthase in the rat eye and related cranial ganglia

Nitric oxide synthase is the biosynthetic enzyme for the free radical neurotransmitter nitric oxide. Using an affinity-purified antiserum, nitric oxide synthase was found to be localized to peripheral ocular nerve fibers, related cranial ganglia, and the retina of the rat. In the eye, nitric oxide synthase-like immunoreactive peripheral nerve fibers were visualized mainly in the choroid and about limbal blood vessels. The anterior uvea was quite sparsely innervated, and the cornea was negative. Many principal neurons in the pterygopalatine ganglion were immunoreactive for nitric oxide synthase while very few cells stained in the superior cervical and trigeminal ganglia. Virtually all nitric oxide synthase-like immunoreactive pterygopalatine cells were also immunostained for vasoactive intestinal polypeptide; nitric oxide synthase also partially co-localized with neuropeptide Y in some of the neurons of this ganglion. Pterygopalatine ganglionectomy significantly reduced the number of peripheral nitric oxide synthase-like immunoreactive nerve fibers in the eye. A variety of immunoreactive retinal cells were seen. Most cells in the inner nuclear layer or ganglion cell layer corresponded morphologically to amacrine cells and displaced amacrine cells. Interplexiform cells and occasional faintly stained cells in the outer portion of the inner nuclear layer also were visualized. Nicotinamide adenine dinucleotide phosphate diaphorase histochemistry generally stained cells of similar distribution but did reveal somewhat more extensive localizations in peripheral ocular tissues, the ciliary ganglion, and the retina, compared with nitric oxide synthase immunohistochemistry. Nitric oxide synthase thus localizes to peripheral ocular nerve fibers, chiefly parasympathetic in nature and derived from the pterygopalatine ganglion, and to several cell types in the retina. Nitric oxide probably acts as a choroidal vasodilator of parasympathetic origin in the eye; the neuropeptide co-localizations in the pterygopalatine ganglion suggest complex neuromodulatory interactions. The retinal localizations imply potential neurotransmitter functions for nitric oxide in this tissue.

Differential regulation of inducible nitric oxide synthase by fibroblast growth factors and transforming growth factor beta in bovine retinal pigmented epithelial cells: inverse correlation with cellular proliferation

Bovine retinal pigmented epithelial (RPE) cells express, after activation with interferon gamma (IFN-gamma) and lipopolysaccharide (LPS), an inducible nitric oxide synthase (NOS). Experiments were done to investigate the effects of the transforming growth factor beta 1, epidermal growth factor, and fibroblast growth factors (FGFs), which are abundant in the retina, on NOS activity. Transforming growth factor beta 1 slightly increases the production of nitrite, an oxidation product of NO, induced by LPS plus IFN-gamma, whereas acidic and basic FGFs markedly inhibit the nitrite release due to LPS/IFN-gamma in a concentration-dependent manner, and epidermal growth factor did not modify LPS/IFN-gamma-induced NOS activity. The growth factors alone did not stimulate nitrite release. We have attempted to elucidate the mechanism of FGF inhibition. Results with heparin, suramin, and tyrphostin suggest involvement of the high-affinity receptor for FGF in its inhibition of LPS/IFN-gamma-stimulated NOS activity. Continued stimulation of RPE cells with LPS/IFN-gamma was essential for the induction of NO synthesis, and maximal inhibition was obtained when FGF was present during stimulation with LPS/IFN-gamma, suggesting that FGF inhibits NOS induction. Furthermore, an antiproliferative action of NO was demonstrated by an inverse correlation between the amounts of nitrite or citrulline produced in response to different stimuli (LPS/IFN-gamma or LPS/IFN-gamma with growth factors) and the level of cellular proliferation. Similar inhibition of growth was obtained when RPE cells were incubated with an NO donor, sydnonimide. Because NO acts as a cytotoxic compound in the retina, FGF, by inhibiting the induction of NOS in RPE cells, may have beneficial effects in protecting the retina from cytokine and endotoxin-mediated tissue damage.

Substance P injected into the hypothalamic supraoptic nucleus causes antidiuresis through the release of arginine-vasopressin in water-loaded and ethanol-anesthetized rats

The effects of injection of substance P (SP) into the hypothalamic supraoptic vasopressinergic nucleus (SON) in water-loaded and ethanol-anesthetized rats were examined. Substance P and its analog [D-Pro2, D-Trp7,9]SP induced marked decreases in urine outflow, with a ED50 value of approx. 0.4 and 0.9 nmol, respectively. The antidiuresis of SP was inhibited by a prior injection of [D-Arg1, D-Trp7,9, Leu11]SP (spantide), an SP-receptor antagonist into the SON. After the injection of SP, urine osmotic pressure was increased by threefold, and the urine level of arginine-vasopressin (AVP) was elevated by 70-fold. The effects of SP and [D-Pro2, D-Trp7,9]SP were completely blocked by pretreatment with an intravenous injection of d(CH2)5-D-Tyr(Et)VAVP, an AVP (V1V2)-receptor antagonist. A prior injection of atropine, a muscarinic receptor antagonist, inhibited the effect of [D-Pro2, D-Trp7,9]SP, but not that of SP. The results suggest that SP, injected into the SON, causes antidiuresis through the release of AVP. A possible mechanism for the antidiureses induced by SP and [D-Pro2, D-Trp7,9]SP is discussed.

Endothelium-dependent relaxation of hypertensive resistance arteries is not impaired under all conditions

Endothelium-dependent relaxation of mesenteric resistance arteries of spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto (WKY) rats was studied. Acetylcholine-induced relaxation of SHR vessels precontracted with 10 microM norepinephrine was endothelium dependent and attenuated compared with WKY vessels. The impaired response of SHR vessels was normalized by inhibition of cyclooxygenase with indomethacin. Blockade of nitric oxide synthetase with NG-nitro L-arginine methyl ester (L-NAME) or inhibition of guanylate cyclase with methylene blue attenuated acetylcholine-induced relaxation of norepinephrine-contracted SHR vessels but had no effect on WKY vessels. When vessels were precontracted with 30 nM arginine vasopressin, acetylcholine induced similar degrees of relaxation in both strains. A similar response was detected when lysine vasopressin was used to induce tone. Indomethacin had no effect on relaxation responses of SHR and WKY vessels precontracted with either form of vasopressin. L-NAME and methylene blue partially inhibited acetylcholine-induced relaxation of vasopressin-contracted vessels from both strains. Acetylcholine added at baseline did not induce contraction of vessels from either strain. It is concluded that endothelium-dependent relaxation of SHR resistance arteries is not impaired under all circumstances. Acetylcholine-induced relaxation may be suppressed in SHR resistance arteries when norepinephrine is used to induce contraction as a result of catecholamine-induced production of an endothelium-derived contracting factor. Vasopressin, on the other hand, does not elicit production of this contracting factor and may enhance the vasorelaxant action of acetylcholine in resistance arteries of both strains via actions on endothelial or vascular smooth muscle cells.

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).

Mono ADP-ribosylation of transducin catalyzed by rod outer segment extract

Transducin is the retinal rod outer segment (ROS)-specific G protein coupling the photoexcited rhodopsin to cyclic GMP-phosphodiesterase. The alpha subunit of transducin is known to be ADP-ribosylated by bacterial toxins. We investigated the possibility that transducin is modified in vitro by an endogenous ADP-ribosyltransferase activity. By using either ROS, cytosolic extract of ROS or purified transducin in the presence of [alpha-32P]nicotinamide adenine dinucleotide (NAD+), the alpha and beta subunits of transducin were found to be radiolabeled. The labeling was decreased by snake venom phosphodiesterase I (PDE I). The modification was shown to be mono ADP-ribosylation by analyses on thin layer chromatography of the PDE I-hydrolyzed products which revealed only 5'AMP residues. In addition we report that sodium nitroprusside activates the ADP-ribosylation of transducin.

Lipopolysaccharide and cytokines induce a macrophage-type of nitric oxide synthase in bovine retinal pigmented epithelial cells

The present study demonstrates that bovine retinal pigmented epithelial cells, which are neuroectodermal in origin, produce nitric oxide (NO) upon treatment with interferon-gamma in the presence of lipopolysaccharide or tumor necrosis factor-alpha. NO production was measured by the accumulation of the stable endproduct NO2-. The biosynthesis of NO requires an induction period of approximately 12 hours and continues for at least 96 hours. The synthesis was abolished by the stereoselective inhibitors of NO synthase, NG-monomethyl-L-arginine and NG-nitro-L-arginine-benzylester. Cycloheximide and dexamethasone blocked cytokine-induced NO production. The results indicate that endotoxin and cytokines are capable of inducing NO synthase of the macrophage type, in retinal pigmented epithelial cells.

Nitroprusside-sensitive and insensitive guanylate cyclases in retinal rod outer segments

Rod outer segments of retina contain guanylate cyclase activity both in the cytosol and membrane fractions. Though the activity in the cytosol is a small fraction of the total activity, it is highly activated by nitroprusside, a nitric oxide generating agent. The membrane guanylate cyclase on the other hand is unaffected by nitroprusside both before and after solubilization. The effects of nitroprusside or nitric oxide on photoreceptor function should therefore be mediated by the cytosolic and not the membrane guanylate cyclase.