A Kainate Receptor Linked to Nitric Oxide Synthesis from Arginine

In slices of young rat cerebellum, the glutamate analogue kainate induced a large accumulation of cyclic GMP, which was inhibited by non-N-methyl-D-aspartate antagonists. Quisqualate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate evoked only small cyclic GMP responses and inhibited the effect of kainate. When tested in cerebellar cell suspensions, glutamate was also a potent antagonist of the cyclic GMP response to kainate. Superoxide dismutase enhanced the response in the isolated cells, whereas haemoglobin and methylene blue were inhibitory. The response in slices was Ca2+ dependent, augmented by arginine, and inhibited by L-NG-monomethylarginine in a manner that could be reversed by additional arginine. It is concluded that stimulation of kainate receptors leads to activation of the enzyme that synthesizes nitric oxide from arginine and that activation of soluble guanylate cyclase by the released nitric oxide accounts for the cyclic GMP generation.     

Excitatory Amino Acid Receptors Coupled to the Nitric Oxide/Cyclic GMP Pathway in Rat Cerebellum During Development

The coupling of excitatory amino acid receptors to the formation of nitric oxide (NO) from arginine during the postnatal development of rat cerebellum was assayed in slice preparations by measuring cyclic GMP accumulation. In the immature tissue, N-methyl-D-aspartate (NMDA) and glutamate were highly efficacious agonists, whereas alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and quisqualate evoked only small responses. The effect of glutamate at all concentrations tested (up to 10 mM) was abolished by the NMDA antagonist, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801). In adult slices, AMPA and quisqualate were much more effective and their effects were inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist for ionotropic non-NMDA receptors, whereas the apparent efficacy of NMDA was greatly reduced. The major changes took place between 8 and 14 days postnatum and, in the case of NMDA, part of the loss of sensitivity appeared to reflect a decline in the ambient levels of glycine with age. Moreover, a component of the response to glutamate in the adult was resistant to MK-801. Cyclic GMP accumulations induced by NMDA and non-NMDA agonists alike were Ca(2+)-dependent and could be antagonized by competitive NO synthase inhibitors in an arginine-sensitive manner, indicating that they are all mediated by NO formation. With one of the inhibitors, L-NG-nitroarginine, a highly potent component (IC50 = 6 nM) evident in slices from rats of up to 8 days old was lost during maturation, indicating that there may be a NO synthase isoform which is prominent only in the immature tissue. Cyclic GMP levels in adult slices under "basal" conditions were reduced markedly by blocking NMDA receptors, by inhibiting action potentials with tetrodotoxin, or by NO synthase inhibition, suggesting that the endogenous transmitter released during spontaneous synaptic activity acts mainly through NMDA receptors to trigger NO formation.     

Nitric Oxide Regulates Substance P Release from Rat Spinal Cord Synaptosomes

Abstract: In order to determine whether nitric oxide (NO) acts directly upon nerve terminals to regulate the synaptic transmission at the level of spinal cord, effects of NO-donors on release of substance P (SP) and glutamic acid (Glu) were investigated by superfusion of synaptosomes prepared from the rat spinal cord. Basal levels of endogenous SP and Glu release were 5.99 ± 2.50 fmol/min/mg of protein and 26.2 ± 4.8 pmol/min/mg of protein, respectively. Exposure to a depolarizing concentration of KCI evoked 2.7- and 3.8-fold increases in SP and Glu release in a calcium-dependent manner, respectively. Sodium nitroprusside (NP) caused a reduction in the depolarization-evoked overflow of SP in a concentration-dependent manner without affecting its basal release, although it failed to affect either basal or evoked release of Glu. The reduction in SP overflow was also observed by the perfusion with S -nitroso- N -acetyl-penicillamine or membrane-permeable cyclic GMP, but not with cyclic AMP. NP caused the concentration-dependent increases in cyclic GMP levels in synaptosomes. Together with reports that excitatory amino acids stimulate NO synthase and release NO in the spinal cord, these data suggest that there may be an interaction between nerve terminals containing Glu and SP, and that NO may directly participate in the regulation of synaptic transmission in SP-containing nerve terminals, which may be mediated through the activation of guanylate cyclase and the increase in cyclic GMP levels.     

Nitric Oxide Synthase Activity Endogenously Modulates NMDA Receptors

Abstract: We tested the possibility that endogenous nitric oxide synthase activity regulated NMDA receptors in primary cultured striatal neurons. We monitored NMDA-induced increase in intra-cellular Ca²⁺ levels with fura-2 ratio imaging, while nitric oxide synthase activity was either increased with l -arginihe (the natural substrate of nitric oxide synthase) or inhibited using nitro- l -arginine (a specific inhibitor of nitric oxide synthase). We found that the NMDA receptor effect was slowly but strongly diminished after an l -arginine (1 m M , 15 min) treatment ( l -arginine preincubation reduced the 100 μM NMDA-induced maximal effect by 30–50%). The l -arginine blockade of NMDA receptors was long-lasting but could be partially reversed by hemoglobin (100 μM , 10 min), which binds nitric oxide. This was not observed when the neurons were treated with l -arginine together with nitro- l -arginine. Our data strongly suggest that physiological nitric oxide synthase activity could regulate NMDA receptors.     

Nitric Oxide-Induced Release of Acetylcholine in the Nucleus Accumbens: Role of Cyclic GMP, Glutamate, and GABA

Abstract: We have previously shown that the basal acetylcholine release in the ventral striatum is under the enhancing influence of endogenous nitric oxide (NO) and that NO donors cause pronounced increases in the acetylcholine release rate. To investigate the role of cyclic GMP, glutamate, and GABA in the NO-induced acetylcholine release, we superfused the nucleus accumbens, (Nac) of the anesthetized rat with various compounds through a push-pull cannula and determined the neurotransmitter released in the perfusate. Superfusion of the Nac with the NO donors diethylamine/NO (DEANO; 100 µmol/L), S-nitroso-N-acetylpenicillamine (SNAP; 200 µmol/L), or 3-morpholinosydnonimine (SIN-1; 200 µmol/L) enhanced the acetylcholine release rate. The guanylyl cyclase inhibitor 1H-(1,2,4)-oxodiazolo(4,3-a)quinoxalin-1-one (ODQ; 10 µmol/L) abolished the effects of DEANO and SIN-1. 6-(Phenylamino)-5,8-quinolinedione (LY-83583; 100 µmol/L), which also inhibits cyclic GMP synthesis, inhibited the releasing effects of DEANO and of SNAP, whereas the effect of SIN-1 on acetylcholine release was not influenced. The DEANO-induced release of acetylcholine was also abolished in the presence of 20 µmol/L 6,6-dinitroquinoxaline-2,3-dione (DNQX) and 10 µmol/L (±)-2-amino-5-phosphonopentanoic acid (AP-5). Simultaneous superfusion with 50 µmol/L quinpirole and 10 µmol/L 7-bromo-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SKF 83566) was ineffective. Superfusion with 500 µmol/L DEANO decreased the release of acetylcholine. The inhibitory effect of 500 µmol/L DEANO was reversed to an enhanced release on superfusion with 20 µmol/L bicuculline. Bicuculline also enhanced the basal release rate. These findings indicate that cyclic GMP mediates the NO-induced release of acetylcholine by enhancing the outflow of glutamate. Dopamine is not involved in this process. Only high concentrations of NO increase the output of GABA, which in turn decreases acetylcholine release. Our results suggest that cells that are able to release glutamate, such as glutamatergic neurons, are the main target of NO in the Nac.     

Polyamines in Human Brain: Regional Distribution and Influence of Aging

Abstract: Depolarization of adult rat forebrain slices with veratrine induced the release of excitatory amino acids (glutamate and aspartate), the synthesis of nitric oxide (NO), and increases in cyclic GMP (cGMP). The NO synthase inhibitors N ^ω-monomethyl- l -arginine and N ^ω-nitro- l -arginine methyl ester decreased the release of NO and the levels of cGMP without affecting the release of excitatory amino acids. In contrast, the antiepileptic drug lamotrigine inhibited the release of excitatory amino acids and of NO, and decreased the levels of cGMP without causing a significant direct inhibition of the NO synthase. Furthermore, the synthesis of NO and the increases in cGMP induced by veratrine were partially blocked by the N -methyl- d -aspartate (NMDA) receptor antagonist MK-801 but not by 6-nitro-7-sulphamobenzo( f )quinoxaline-2,3-dione, a non-NMDA receptor antagonist. Neither of these compounds inhibited directly the NO synthase or the release of excitatory amino acids. Thus, these three types of compound act as an inhibitor of voltage-sensitive sodium channels (lamotrigine), as a receptor antagonist (MK-801), or as direct inhibitors of the NO synthase, to block the pathway leading to increased cGMP after veratrine depolarization. It is likely that some of the pharmacological and therapeutic actions shared by these three types of compound are, at least in part, a consequence of inhibition of the synthesis of NO.     

Serotonin Inhibition of the NMDA Receptor/Nitric Oxide/Cyclic GMP Pathway in Rat Cerebellum: Involvement of 5-Hydroxytryptamine2C Receptors

Abstract: Previous studies have shown that 5-hydroxytryptamine (5-HT) can potently inhibit glutamatergic transmission in rat cerebellum through the activation of multiple 5-HT receptors. The aim of this study was to subclassify the 5-HT₂ receptor mediating inhibition of the cyclic GMP response elicited by N -methyl- d -aspartate in adult rat cerebellar slices. Seven receptor antagonists, endowed with relative selectivities for the 5-HT_2A, 5-HT_2B, and 5-HT_2C subtypes, differentially affected the inhibition by (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane of the cyclic GMP response, suggesting that the receptor involved belongs to the 5-HT_2C subtype.     

Nitric Oxide Regulates Cyclic GMP-Dependent Protein Kinase Phosphorylation in Rat Brain

Abstract: Nitric oxide (NO) acts via soluble guanylyl cyclase to increase cyclic GMP (cGMP), which can regulate various targets including protein kinases. Western blotting showed that type II cGMP-dependent protein kinase (cGK II) is widely expressed in various brain regions, especially in the thalamus. In thalamic extracts, the phosphorylation of several proteins, including cGK II, was increased by exogenous NO or cGMP. In vivo pretreatment with a NO synthase inhibitor reduced the phosphorylation of cGK II, and this could be reversed by exogenous NO or cGMP. Conversely, brainstem electrical stimulation, which enhances thalamic NO release, caused a NO synthase-dependent increase in the phosphorylation of thalamic cGK II. These results indicate that endogenous NO regulates cGMP-dependent protein phosphorylation in the thalamus. The activation of cGKII by NO may play a role in thalamic mechanisms underlying arousal.     

Nitric oxide can differentially modulate striatal neurotransmitter concentrations via soluble guanylate cyclase and peroxynitrite formation

In vivo microdialysis was used to investigate whether nitric oxide (NO) modulates striatal neurotransmitter release in the rat through inducing cyclic GMP formation via soluble guanylate cyclase or formation of peroxynitrite (ONOO(-)). When NO donors, S-nitroso-N-acetyl-DL-penicillamine (SNAP; 1 mM) or (Z)-1-[2-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1- ium-1, 2-diolate (NOC-18; 1 mM), were retrodialysed for 15 min, acetylcholine (ACh), serotonin (5-HT), glutamate (Glu), gamma-aminobutyric acid (GABA), and taurine levels were significantly increased, whereas those of dopamine (DA), dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA) were decreased. Only effects on ACh, 5-HT, and GABA showed calcium dependency. Inhibition of soluble guanylate cyclase by 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ; 100 and 200 microM) dose-dependently reduced NO donor-evoked increases in ACh, 5-HT, Glu, and GABA levels. Coperfusion of SNAP or NOC-18 with an ONOO(-) scavenger, L-cysteine (10 mM) resulted in enhanced concentrations of Glu and GABA. On the other hand, DA concentrations increased rather than decreased, and no reductions in DOPAC and 5-HIAA occurred. This increase in DA and the potentiation of Glu and GABA were calcium-dependent and prevented by ODQ. Similar to NO, infusions of ONOO(-) (10 or 100 microM) decreased DA, DOPAC, and 5-HIAA. Overall, these results demonstrate that NO increases ACh, 5-HT, Glu, and GABA levels primarily through a cyclic GMP-dependent mechanism. For DA, DOPAC, and 5-HIAA, effects are determined by levels of ONOO(-) stimulated by NO donors. When these are high, they effectively reduce extracellular concentrations through oxidation. When they are low, DA concentrations are increased in a cyclic GMP-dependent manner and may act to facilitate Glu and GABA release further. Thus, changes in brain levels of antioxidants, and the altered ability of NO to stimulate cyclic GMP formation during ageing, or neurodegenerative pathologies, may particularly impact on the functional consequences of NO on striatal dopaminergic and glutamatergic function.     

Dexamethasone Up-Regulates a Constitutive Nitric Oxide Synthase in Cerebellar Astrocytes but Not in Granule Cells in Culture

Abstract: Treatment of rat cerebellar astrocyte-enriched primary cultures with dexamethasone enhances the nitric oxide-dependent cyclic GMP formation induced by noradrenaline in a time-(>6 h) and concentration-dependent manner (half-maximal effect at 1 n M ). Stimulation of cyclic GMP formation by the calcium ionophore A23187 is similarly enhanced. In contrast, cyclic GMP accumulation in cells treated with lipopolysaccharide is inhibited by dexamethasone. The potentiating effect of dexamethasone is prevented by the protein synthesis inhibitor cycloheximide and is not due to increased soluble guanylate cyclase activity. Agonist stimulation of [³H]arginine to [³H]citrulline conversion is enhanced by dexamethasone in astrocytes but not in cerebellar granule cells. These results indicate that glucocorticoids may up-regulate astroglial calcium-dependent nitric oxide synthase while preventing expression of inducible nitric oxide synthase and are the first report of a differential long-term regulation of the expression of neuronal and astroglial constitutive nitric oxide synthase activities.     

Nitric Oxide Implication in the Control of Neurosecretion by Chromaffin Cells

Abstract: In this work, we have studied the effects of pure nitric oxide (NO) on the regulation of catecholamine (CA) secretion by chromaffin cells, as well as the possible presence of its synthesizing enzyme l -arginine:NO synthase (NOS) in these cells. Our results show that NO produces a large stimulation of basal CA secretion. This effect was calcium- and concentration-dependent (EC₅₀ = 64 ± 8 µ M ) and was not due to nonspecific damage of the tissue by NO. NO also modulates the CA secretion evoked by nicotine in a dose-dependent manner. Although it has a stimulatory effect on the CA secretion evoked by low doses of nicotine (<3 µ M ; EC₅₀ = 16 ± 3 µ M ), it produces a dose-dependent inhibition of the CA secretion induced by high doses of nicotine (≥30 µ M ; IC₅₀ = 52 ± 6 µ M ). The mechanism by which NO modulates CA secretion seems to be through the increase in the cyclic GMP levels, because there was a close correlation between the CA secretion and the cyclic GMP levels. The presence of a specific activity of NOS in chromaffin cells has been demonstrated by two independent methods: release of [¹⁴C]citruiline from [¹⁴C]arginine and formation of an NO-hemoglobin complex. NOS activity was about 0.5 pmol/min/mg of protein. It was calcium- and mainly calmodulin-dependent and could be specifically blocked by the NOS inhibitor N -methyl- l -arginine. These results suggest that NO could be an important intracellular messenger in the regulation of neurosecretion in chromaffin cells.     

Anomalous Increase in Nitric Oxide Synthase Activity by Certain Nitric Oxide-Generating Compounds in Intact Neuronal Cells

Abstract: It has been shown that nitric oxide (NO) regulates NO synthase (NOS) activity through negative feedback in cytosolic enzyme preparations in various cell types. We compared the effects of the NO-generating compounds S-nitroso-N-acetylpenicillamine (SNAP), 3-morpholinosydnonimine (SIN-1), and sodium nitroprusside (SNP) on NOS activity in intact neuroblastoma N1E-115 cells and in the cytosol obtained from the same cells. Enzyme activity was measured by the conversion of l -[³H]arginine into l -[³H]citrulline. At concentrations that elicit almost complete inhibition of NOS activity in cytosolic enzyme preparations of these cells, SIN-1 and SNP did not cause significant attenuation of enzyme activity measured at 45 min in intact cells. It is surprising that SIN-1 and SNP markedly stimulated l -[³H]citrulline formation in a time- and concentration-dependent manner when cells were incubated with the compounds for >1.5 h. Neither inhibitory nor stimulatory effects of SNAP on NOS were observed in intact N1E-115 cells. This is in contrast to the inhibitory effects of SNAP in cytosolic preparations of the enzyme. The increased NOS activity by SIN-1 or SNP in intact cells was dependent on the presence of extracellular Ca²⁺, suggesting that it might be due to increased Ca²⁺ influx. On the other hand, measurements of the activity of lactate dehydrogenase showed that there was no generalized increase in cell permeability in response to SIN-1 or SNP. There was no agreement in the rank order of potencies of these compounds in activating guanylate cyclase and in affecting NOS activity, both in broken-cell preparations and in intact cells. Thus, modulation of NOS activity by NO-releasing compounds is not dependent on cyclic GMP formation and might not be related in a simple fashion to NO generation. Alternatively, activation of guanylate cyclase and stimulation of NOS activity might require different redox species of NO. Our present findings might be of clinical relevance in relation to long-term use of NO-generating compounds as therapeutic agents.     

Chronic Exposure to Aluminum Impairs Neuronal Glutamate-Nitric Oxide-Cyclic GMP Pathway

Abstract: Humans are exposed to aluminum from environmental sources and therapeutic treatments. However, aluminum is neurotoxic and is considered a possible etiologic factor in Alzheimer's disease and other neurological disorders. The molecular mechanism of aluminum neurotoxicity is not understood. We tested the effects of aluminum on the glutamate-nitric oxide-cyclic GMP pathway in cultured neurons. Neurons were exposed to 50 µ M aluminum in culture medium for short-term (4 h) or long-term (8–14 days) periods, or rats were prenatally exposed, i.e., 3.7% aluminum sulfate in the drinking water, during gestation. Chronic (but not short-term) exposure of neurons to aluminum decreased glutamate-induced activation of nitric oxide synthase by 38% and the formation of cyclic GMP by 77%. The formation of cyclic GMP induced by the nitric oxide-generating agent S -nitroso- N -acetylpenicillamine was reduced by 33%. In neurons from rats prenatally exposed to aluminum but not exposed to it during culture, glutamate-induced formation of cyclic GMP was inhibited by 81%, and activation of nitric oxide synthase was decreased by 85%. The formation of cyclic GMP induced by S -nitroso- N -acetylpenicillamine was not affected. These results indicate that chronic exposure to aluminum impairs glutamate-induced activation of nitric oxide synthase and nitric oxide-induced activation of guanylate cyclase. Impairment of the glutamate-nitric oxide-cyclic GMP pathway in neurons may contribute to aluminum neurotoxicity.     

Nitric oxide: a vasodilatatory mediator in the cephalic aorta of Sepia officinalis (L.) (Cephalopoda)

Evidence is presented that nitric oxide (NO) may regulate blood pressure in cephalopod molluscs. In vitro tests performed on the cephalic aorta of Sepia officinalis (L.) (Cephalopoda) showed that the NO releasers (glyceroltrinitrate, sodium nitroprusside, 3-morpholinylsydnoneimine chloride and KNO₂) induced concentration-dependent vasodilatation of vessel segments (without the tunica adventitia/periadventitia) precontracted by dopamine. These vasodilatatory actions could be totally blocked by oxadiazolo[4,3-a] quinoxalin-1-one, an inhibitor of the NO-sensitive guanylyl cyclase, and partially mimicked by the cyclic guanosine monophosphate (cGMP) analogue 8-bromo cGMP and by the phosphodiesterase inhibitor, zaprinast. The NO-precursor, l-arginine, showed vasodilatatory effects only on segments of the aorta in which the layers containing nerves (tunica adventitia/periadventitia) had been left intact, suggesting that NO synthase may be located within peripheral nerves. Accepted: 11 August 1998     

Up‐regulation of Tyrosinase Gene by Nitric Oxide in Human Melanocytes

Ultraviolet light (UV) radiation causes skin‐tanning, which is thought to be mediated by stimulating the release of melanogenic factors from keratinocytes as well as other cells. Nitric oxide (NO) has been reported to be generated after UV radiation and to stimulate melanocytes as one of the melanogens. In a previous experiment by another group on melanogenesis induced by NO, increases in both tyrosinase activity and tyrosinase protein levels were observed after daily stimulation of NO for 4 days. In the present study, we investigated tyrosinase gene expression within the first 24 hr of NO‐induced melanogenesis. Tyrosinase mRNA expression was found to be induced 2 hr after a single treatment with S‐nitroso‐N‐acetyl‐ l ‐arginine. An increase of tyrosinase activity was also detected time‐dependently within the 24‐hr period, accompanied by an increase of tyrosinase protein levels. The induction of mRNA expression was suppressed by a cyclic guanosine 3′,5′‐monophosphate (cGMP)‐dependent protein kinase (cGMP/PKG) inhibitor. These results suggest that the enhancement of tyrosinase gene expression via the cGMP pathway may be a primary mechanism for NO‐induced melanogenesis.     

Nitric Oxide Mediates the Glutamate-dependent Pathway for Neurotransmission in Sepia officinalis Chromatophore Organs

Chromatophore organs are complex and unique structures responsible for the variety of body coloration patterns used by cephalopods to communicate and camouflage. They are formed by a pigment-containing cytoelastic sacculus, surrounded by muscle fibers directly innervated from the brain. Muscle contraction and relaxation are responsible for expansion and retraction of the pigment-containing cell. Their functioning depends on glutamate and Phe-Met-Arg-Phe-NH₂-related peptides, which induce fast and slow cell expansion, respectively, and 5-hydroxytryptamine, which induces retraction. Apart from these three substances and acetylcholine, which acts presynaptically, no other neuroactive compounds have so far been found to be involved in the neuroregulation of chromatophore physiology, and the detailed signaling mechanisms are still little understood. Herein, we disclose the role of nitric oxide (NO) as mediator in one of the signaling pathways by which glutamate activates body patterning. NO and nitric-oxide synthase have been detected in pigment and muscle fibers of embryo, juvenile, and adult chromatophore organs from Sepia officinalis. NO-mediated Sepia chromatophore expansion operates at slower rate than glutamate and involves cGMP, cyclic ADP-ribose, and ryanodine receptor activation. These results demonstrate for the first time that NO is an important messenger in the long term maintenance of the body coloration patterns in Sepia.     

Increased Nitric Oxide Synthase Activities and l-[3H]Arginine Uptake in Brain Following Portacaval Anastomosis

Abstract: Glutamatergic synaptic dysfunction has been proposed as a causal factor in portal-systemic encephalopathy. Increased in vitro and in vivo glutamate release and decreased glutamate binding to NMDA receptors were previously reported in the brains of portacaval-shunted rats. Such changes could lead to alterations in the second messenger systems coupled to glutamate receptors. As NMDA receptors have been shown to act via the nitric oxide/cyclic GMP second messenger system, we studied the activities of constitutive nitric oxide synthase (NOS), in the brains of rats following portacaval shunting. Results demonstrate that NOS activities are significantly increased in cerebellum (by 54%, p < 0.01), cerebral cortex (by 65%, p < 0.01), hippocampus (by 88%, p < 0.01), and striatum (by 64%, p < 0.01) of shunted rats compared with sham-operated controls. As l -arginine transport is a prerequisite for nitric oxide production, we also studied l -[³H]arginine transport into cerebellar and cerebral cortical synaptosomes prepared from the brains of portacaval-shunted and sham-operated rats. l -[³H]Arginine uptake was significantly increased (by ∼50%, p < 0.01) in both cerebellum and cortex. Increased NOS activities of neuronal and/or astrocytic origin and the resultant increased production of nitric oxide in brain could be the consequence of increased NMDA receptor activation following portacaval shunting. Furthermore, increased nitric oxide production could contribute to the increased cerebral blood flow consistently observed following portacaval shunting.     

Eclosion Hormone Stimulates Cyclic GMP Levels in Manduca sexta Nervous Tissue via Arachidonic Acid Metabolism with Little or No Contribution from the Production of Nitric Oxide

The neuropeptide eclosion hormone acts directly on the nervous system of the tobacco hornworm, Manduca sexta, to trigger ecdysis behavior at the end of each molt. Previous studies have shown that the action of eclosion hormone is mediated via the intracellular messenger cyclic GMP. In the present study we have investigated the mechanisms involved in the eclosion hormone-stimulated increases in cyclic GMP. No stimulation of guanylate cyclase was seen in homogenized nervous tissue, suggesting that eclosion hormone does not directly stimulate a membrane-bound form of guanylate cyclase. Nitric oxide synthase inhibitors, N-methylarginine and nitroarginine, had no effect on eclosion hormone-stimulated cyclic GMP levels. By contrast, 4-bromophenacyl bromide, an inhibitor of arachidonic acid release, and nordihydroguaiaretic acid, an inhibitor of arachidonic acid metabolism, almost completely abolished the eclosion hormone-stimulated cyclic GMP increase. We hypothesize that eclosion hormone receptors are coupled to a lipase, activation of which causes the release of arachidonic acid. Either the arachidonic acid directly stimulates the soluble guanylate cyclase or further metabolism of arachidonic acid yields compounds that activate guanylate cyclase.     

Modulation of In Vivo Striatal Transmitter Release by Nitric Oxide and Cyclic GMP

Abstract: The effects of nitric oxide (NO) and cyclic GMP on in vivo transmitter release in the rat striatum were investigated using microdialysis sampling in urethane-anaesthetised animals. The NO release-inducing substances S -nitrosoacetylpenicillamine (SNAP), S -nitrosoglutathione (SNOG), and sodium nitroprusside (SNP) increased extracellular concentrations of aspartate (Asp), glutamate (Glu), γ-aminobutyric acid (GABA), taurine (Tau), acetylcholine (ACh), and serotonin (5-HT). Dopamine (DA) concentrations were decreased by SNAP but were increased by SNOG and SNP. An NO scavenger, haemoglobin, blocked or reduced the effects of SNAP on transmitter release. However, the control carrier compounds for SNAP, SNOG, and SNP (penicillamine, glutathione, and potassium ferricyanide, respectively, which do not induce release of NO) also increased GABA, Tau, DA, and 5-HT concentrations. When NO gas was given directly by dissolving it in degassed Ringer's solution, DA concentrations decreased significantly, and those of Asp, Glu, GABA, Tau, ACh, and 5-HT increased. These effects of NO gas were all inhibited by coadministration of haemoglobin and for GABA, Tau, ACh, and DA showed some calcium dependency. The cyclic GMP agonists 8-bromo-cyclic GMP and dibutryl-cyclic GMP stimulated dose-dependent increases in Asp, Glu, GABA, Tau, ACh, DA, and 5-HT concentrations. Increased striatal transmitter release in response to NO may therefore be mediated by its stimulatory action on cyclic GMP formation. NO inhibition of DA release may be mediated indirectly through its stimulation of local cholinergic and GABAergic neurones.