A Nitric Oxide Synthase Activity Selectively Stimulated by NMDA Receptors Depends on Protein Kinase C Activation in Mouse Striatal Neurons

Regulation of nitric oxide synthase activity in cortical slices by excitatory amino acids and calcium

Nitric oxide synthase (NOS) activity was determined in adult rat frontal cortex and hippocampus by measuring the conversion of L-[3H]arginine to L-[3H]citrulline. N-methyl-D-aspartate (NMDA), but not kainate or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), stimulated NOS activity. This effect was concentration dependent (EC50 approximately 30 microM) and was inhibited by tetrodotoxin, EGTA, N omega-nitro-L-arginine (NOARG), Mg2+, phencyclidine, and (cis)-4-phosphonomethyl-2-piperidine carboxylate (CGS 19755), but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). NOS activity was increased to an even greater extent by the calcium ionophores ionomycin and A23187 and by depolarization with 50 mM K+. Interestingly, neither caffeine nor 1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), drugs that would be expected to increase intracellular Ca2+ concentration by release of Ca2+ from intracellular ryanodine- and inositol-1,4,5-trisphosphate-sensitive stores, respectively, had any significant effect on NOS activity. It is concluded that NOS can be activated by NMDA binding to a classic NMDA glutamate receptor subtype as well as by depolarization or other agents that increase the influx of extracellular Ca2+. The paradoxical lack of effect of caffeine, as well as the inhibitory effect of tetrodotoxin, are discussed.

Nitric oxide and the cerebral circulation

BACKGROUND

Nitric oxide (NO) is a potent vasodilator that was initially described as the mediator of endothelium-dependent relaxation (endothelium-derived relaxing factor, EDRF). It is now known that NO is produced by a variety of other cell types.

SUMMARY OF REVIEW

Endothelium produces NO (EDRF) under basal conditions and in response to a variety of vasoactive stimuli in large cerebral arteries and the cerebral microcirculation. Endothelium-dependent relaxation is impaired in the presence of several pathophysiological conditions. This impairment may contribute to cerebral ischemia or stroke. Activation of glutamate receptors appears to be a major stimulus for production of NO by neurons. Neuronally derived NO may mediate local increases in cerebral blood flow during increases in cerebral metabolism. NO synthase-containing neurons also innervate large cerebral arteries and cerebral arterioles on the brain surface. Activation of parasympathetic fibers that innervate cerebral vessels produces NO-dependent increases in cerebral blood flow. Increases in cerebral blood flow during hypercapnia also appear to be dependent on production of NO. Astrocytes may release some NO constitutively, but astrocytes and microglia can release relatively large quantities of NO after induction of NO synthase in response to endotoxin or some cytokines. Expression of inducible NO synthase, perhaps in response to local production of cytokines, may exert cytotoxic effects in brain during or after ischemia.

CONCLUSIONS

Because endothelium, neurons, and glia can all produce NO in response to some stimuli, the influence of NO on the cerebral circulation appears to be very important. Under normal conditions, constitutively produced NO influences basal cerebral vascular tone and mediates vascular responses to a diverse group of stimuli. The inducible form of NO synthase produces much greater amounts of NO that may be an important mediator of cytotoxicity in brain.

The formation of reactive oxygen species in a fraction of rat brain by metabolism of nitric oxide

The potential of nitric oxide for the induction of reactive oxygen species (ROS) has been evaluated in a cerebral synaptosomal fraction. Nitric oxide (NO) was generated using S-nitroso-N-acetylpenicillamine (SNAP). NO stimulated ROS production, assayed using a fluorescent probe. This stimulation was largely inhibited by superoxide dismutase but not by catalase, implying that superoxide anion but not hydrogen peroxide, was a critical intermediate. An iron chelator, deferoxamine or hydroxyl radical scavengers failed to prevent stimulation of ROS by SNAP. This suggests the possibility of formation of a novel class of short-lived active oxygen species by NO.

Monitoring of cyclic GMP during cerebellar microdialysis in freely-moving rats as an index of nitric oxide synthase activity

The nitric oxide synthase/cyclic GMP pathway has been studied in vivo in the adult rat cerebellum by monitoring the levels of extracellular cyclic GMP during microdialysis in conscious unrestrained animals. The basal cyclic GMP efflux was concentration-dependently reduced upon local infusion of the nitric oxide synthase inhibitor NG-nitro-L-arginine (10 microM-1 mM). The nitric oxide donor S-nitroso-N-penicillamine, perfused through the dialysis probe at 1 mM, increased by about 200% the extracellular levels of cyclic GMP. The glutamate receptor agonist N-methyl-D-aspartate (500 microM) produced a cyclic GMP response which was abolished by the selective receptor antagonist D-2-amino-5-phosphonovaleric acid (500 microM) or by NG-nitro-L-arginine (10 microM). The elevation of cyclic GMP levels caused by local infusion of 500 microM N-methyl-D-aspartate was also abolished by parenteral administration of the N-methyl-D-aspartate channel blocker dizocilpine (0.4 mg/kg, i.p.). Local perfusion of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (1 mM) increased by about 150% the extracellular levels of cyclic GMP. It is concluded that cyclic GMP collected during in vivo microdialysis reflects nitric oxide synthase activity in the rat cerebellum. The technique may be utilized to investigate the pathophysiology and the pharmacology of the nitric oxide/cyclic GMP pathway in the cerebellum of living animals.

Neurotransmitter release regulated by nitric oxide in PC-12 cells and brain synaptosomes

BACKGROUND

Nitric oxide is a messenger molecule of the nervous system, which is produced by the enzyme nitric oxide synthase, which may regulate cyclic guanosine monophosphate levels and which has been implicated in the control of neurotransmitter release. PC-12 pheochromocytoma cells differentiate to form neuronal cells in culture when they are exposed to nerve growth factor. The levels of cyclic guanosine monophosphate in the cells and their ability to release acetylcholine in response to K(+)-depolarization are both maximal after eight days of treatment with nerve growth factor. We set out to assess a possible role for nitric oxide in the processes that occur in differentiating PC-12 cells.

RESULTS

Nitric oxide synthase is first evident in differentiating PC-12 cells eight days after beginning treatment with nerve growth factor, coinciding with the marked increase in K(+)-depolarization-induced release of acetylcholine. The release of both acetylcholine and dopamine in response to K(+)-depolarization is blocked by inhibitors of nitric oxide synthase and by hemoglobin, which binds nitric oxide. Providing l-arginine, a precursor required for nitric oxide synthesis, reverses the effects of the inhibitors. In synaptosomal preparations from the corpus striatum, inhibitors of nitric oxide synthase prevent the release of glutamate in response to the glutamate derivative N-methyl-d-aspartate but not in response to K(+)-depolarization.

CONCLUSION

Nitric oxide may mediate the release of acetylcholine and dopamine in response to K(+)-depolarization in PC-12 cells and the release of glutamate in response to N-methyl-d-aspartate in striatal synaptosomes. Nitric oxide synthase expression is induced after eight days of treating PC-12 cells with nerve growth factor, coinciding with a marked enhancement of the release of neurotransmitters in response to K(+)-depolarization.

Nitric oxide, superoxide and peroxynitrite: putative mediators of NMDA-induced cell death in cerebellar granule cells

In this study, we analysed the implication of superoxide (O2-.) and nitric oxide (NO.) free radicals and their resulting product peroxynitrite (ONOO-) in the neuronal death induced by the activation of the glutamatergic receptor of the N-methyl-D-aspartate (NMDA) subtype using cultured cerebellar granule cells. The NOl donor SIN-1 (3-morpholinosydnonimine N-ethylcarbamide), at concentrations which produced a much higher guanylate cyclase activation (i.e. NO. concentration) than NMDA, was not neurotoxic and did not increase the NMDA-induced neuronal death. The absence of involvement of NO. in NMDA-induced neuronal death was confirmed by the ineffectiveness of L-NG-nitroarginine (L-Narg) as a neuroprotective compound. Electron paramagnetic resonance (EPR) experiments, using 5,5-dimethyl pyrroline 1-oxide (DMPO) as a spin trap, indicated that NMDA receptor stimulation led to the generation of O2-. from at least 15-30 min. The generation of O2-. by xanthine (XA)-xanthine oxidase (XO) induced a neuronal death similar to that of NMDA. XA-XO-induced neuronal death was suppressed by addition of either superoxide dismutase (SOD) plus catalase (CAT), or DMPO in the incubation medium. In contrast, NMDA-induced neuronal death was widely blocked by DMPO and other spin trap compounds, but not by SOD +/- CAT. XA-XO-induced neuronal death was not potentiated by SIN-1 indicating that ONOO- is not more toxic than O2-. in our neuronal model.

The nitric oxide-cyclic GMP signalling pathway in rat brain

Nitric oxide is a novel signalling molecule in the brain and a potent activator of the cyclic GMP-synthesising enzyme, soluble guanylate cyclase. To determine if stimulation of cyclic GMP formation is a widespread mechanism of nitric oxide signal transduction, we have compared the distribution of the nitric oxide-generating enzyme (nitric oxide synthase) with that of nitric oxide-stimulated cyclic GMP accumulation, throughout the rat brain. The former was done using NADPH diaphorase histochemistry and the latter by cyclic GMP immunohistochemistry following perfusion of the nitric oxide donor, nitroprusside, in vivo. At a gross level, there was generally a good match when the two were compared in adjacent sections. Although the relative staining intensity varied from area to area, in no grey matter region did we observe cyclic GMP accumulation in the absence of nitric oxide synthase staining. In detail, the locations were complementary rather than identical. In some areas, nitric oxide synthase was found in postsynaptic structures and cyclic GMP accumulation in presynaptic elements and fibres; in others, the locations were reversed. Glial cells and their processes also accumulated cyclic GMP in the cerebellum. The results suggest that soluble guanylate cyclase is a major nitric oxide "receptor" throughout the brain. They also support the hypothesis that nitric oxide generated therein primarily functions as a mediator of cell-cell signaling rather than as a conventional second messenger acting within the cells in which it is produced. The types of communication subserved by nitric oxide appear to be extraordinarily diverse.

Endothelium-derived vasoactive factors and regulation of the cerebral circulation

Vasoactive factors produced and released by endothelium exert a powerful influence on vascular tone in the cerebral circulation. Endothelium-derived relaxing factor (EDRF), which has been identified as nitric oxide (NO) or an NO-containing compound, is produced under basal conditions in cerebral blood vessels. EDRF mediates endothelium-dependent relaxation in response to a number of stimuli in the cerebral circulation. The influence of NO on the cerebral circulation appears to be particularly important and complex because both neurons and glia, in addition to endothelium, produce NO in response to some stimuli. Neuronally derived NO may mediate local vasodilation in response to increased neuronal activity. In addition to EDRF, cerebral endothelium may produce other relaxing factors, including prostacyclin, endothelium-derived hyperpolarizing factor, and oxygen-derived free radicals. Several pathophysiological conditions are associated with impaired endothelium-dependent responses that may involve the decreased production of EDRF and release of endothelium-derived contracting factors, such as the cyclooxygenase products of arachidonic acid and endothelin. The release of endothelin, an extremely potent and long-lasting vasoconstrictor peptide, may contribute to vasospasm after subarachnoid hemorrhage.

Inhibiting synthesis of the putative retrograde messenger nitric oxide results in amnesia in a passive avoidance task in the chick

It has been hypothesized that nitric oxide (NO) may act as a 'retrograde messenger' in the CNS, mediating intersynaptic communication in the context of neural plasticity during memory formation. To test this hypothesis the effects of the competitive NO synthase inhibitor N-nitro-L-arginine (NARG) on memory retention has been studied in a one-trial passive avoidance task in the day-old chick. Intracerebral injections before training into the intermediate medial hyperstriatum ventrale (IMHV), an area that is of crucial importance in learning in the chick, produced amnesia in this task when tested at various time points from 30 min to 24 h after training. Time- and dose dependencies of NARG when injected i.p. or i.c. were evaluated. Injection into one IMHV alone (left or right) proved to be sufficient to produce amnesia. Diffusion of NARG into the untreated hemisphere was ruled out by injecting it with L-arginine, which competes with NARG and prevents inhibition of NO synthase. Additional tests showed that the amnestic effect is not due to state-dependent learning, nor to interference of the drug with general motor ability or motivation.

NMDA-dependent superoxide production and neurotoxicity

Neuronal injury resulting from acute brain insults and some neurodegenerative diseases implicates N-methyl-D-aspartate (NMDA) glutamate receptors. The fact that antioxidants reduce some types of brain damage suggests that oxygen radicals may have a role. It has been shown that mutations in Cu/Zn-superoxide dismutase (SOD), an enzyme catalysing superoxide (O2.-) detoxification in the cell, are linked to a familial form of amyotrophic lateral sclerosis (ALS). Here we report that O2.- is produced upon NMDA receptor stimulation in cultured cerebellar granule cells. Electron paramagnetic resonance was used to assess O2.- production that was due in part to the release of arachidonic acid. Activation of kainic acid receptors, or voltage-sensitive Ca2+ channels, did not produce detectable O2.-. We also find that the nitrone DMPO (5,5-dimethyl pyrroline 1-oxide), used as a spin trap, is more efficient than the nitric oxide synthase inhibitor, L-NG-nitro-arginine, in reducing NMDA-induced neuronal death in these cultures.

Non-classical glutamate receptors, blocked by both NMDA and non-NMDA antagonists, stimulate nitric oxide production in neurons

In striatal neurons in primary culture, kainate and domoate stimulated cGMP production, whereas two other analogs of glutamate which act at non-NMDA receptors, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) and quisqualate were ineffective. However, both agonists stimulated cGMP accumulation on neurons pretreated with concanavalin A, a lectin which is known to prevent desensitization of AMPA receptors. We show here that such a treatment also potentiated the kainate-stimulated cGMP production. Responses induced by all agonists of non-NMDA receptors tested were mediated by nitric oxide (NO) production, since they were inhibited by haemoglobin, an NO scavenger, and two competitive inhibitors of NO-synthase L-NG-monomethylarginine and L-NG-nitro-arginine, the effects of both inhibitors being reversed by an excess of L-arginine. The rank order of potency of the agonists tested (domoate > kainate approximately AMPA approximately quisqualate) suggests that a kainate receptor subtype triggers NO production in striatal neurons. Surprisingly, response evoked by maximally effective concentrations of kainate, quisqualate and AMPA on concanavalin A-treated neurons were partially antagonized by two non-competitive antagonists of NMDA receptors, MK-801 and phencyclidine, and by Mg2+ ions, which block NMDA-operated channels. However, in neurons which had not been treated with concanavalin A, kainate-induced NO production was not inhibited by these antagonists. These results suggest that, in addition to kainate receptor subtype, another glutamate receptor subtype which may be composed of both kainate and NMDA receptor subunits and which is desensitized by kainate, AMPA and quisqualate, is involved in NO production.

Nitric oxide-induced blockade of NMDA receptors

We studied the effects of nitric oxide (NO)-producing agents on N-methyl-D-aspartate (NMDA) receptor activation in cultured neurons. 3-Morpholino-sydnonimine (SIN-1) blocked both NMDA-induced currents and the associated increase in intracellular Ca2+. The actions of SIN-1 were reversible and suppressed by hemoglobin. A degraded SIN-1 solution that did not release NO was unable to block NMDA receptors. This showed that the SIN-1 effects were due to NO and not to another breakdown product. Similar results were obtained with 1-nitrosopyrrolidine (an NO-containing drug) and with NO released from NaNO2. Pretreatment with hemoglobin potentiated NMDA-induced effects, demonstrating that endogenous NO modulates NMDA receptors. Since NMDA receptor activation induces NO synthesis, these results suggest a feedback inhibition of NMDA receptors by NO under physiological condition.