Nitric Oxide Synthase Inhibitor 7-nitroindazole Research Articles

Stimulation with N-methyl-D-aspartate or kainic acid increases cyclic guanosine monophosphate-like immunoreactivity in turtle retina: involvement of nitric oxide synthase.

In brain and retina, stimulation with excitatory amino acids (EAA) can generate nitric oxide (NO) and increase levels of cyclic guanosine monophosphate (cGMP). Because nitric oxide synthase (NOS) has been found in retinas of all species examined to date, an NO signal-transduction pathway is likely to be present in all retinas. We tested the hypothesis that stimulation of ionotropic glutamate receptors in turtle retina would result in increases in cGMP through an NOS/NO/cGMP pathway. Following in vitro incubations of turtle eye cups with the glutamate receptor agonists, N-methyl-D-aspartate (NMDA) or kainic acid (KA), we quantified the increases in cGMP-like immunoreactivity (cGMP-LI) by using enzyme-linked immunosorbant assay (ELISA) and localized the increased cGMP-LI by using an antibody against cGMP. Stimulation with NMDA or KA increased cGMP-LI in bipolar and amacrine cells as well as in some somata in the ganglion cell layer. Either KA or NMDA produced statistically significant increases in total retinal cGMP-LI by ELISA. To test the involvement of NO, we used the NOS inhibitors 7-nitroindazole and L-nitroarginine. Both inhibitors blocked virtually all of the KA- or NMDA-stimulated increases in cGMP-LI. These results indicate that activation of ionotropic glutamate receptors can increase cGMP in select retinal neurons. Differences between the agonist-evoked increases of retinal cGMP-LI suggest that there can be specificity in the activation of the NOS/NO/cGMP signal-transduction pathway by glutamate. This suggests that, in addition to short-term electrical changes, activation of ionotropic glutamate receptors also may produce longer term modulatory or metabolic effects involving NO/cGMP.

Evidence that the neuronal nitric oxide synthase inhibitor 7 -nitroindazole inhibits monoamine oxidase in the rat: in vivo effects on extracellular striatal dopamine and 3,4-dihydroxyphenylacetic acid

The present study investigated in vivo the kinetics of the changes in rat striatal extracellular concentrations of dopamine (DA), and its monoamine oxidase (MAO)-derived metabolite 3,4-dihydroxyphenylacetic acid (DOPAC), following administration either of nitric oxide (NO) synthase inhibitors 7-nitroindazole (7-NI) and N ω -nitro- l-arginine methyl ester ( l-NAME) or of the widely used MAO inhibitor pargyline. DA and DOPAC concentrations were determined every 4 min by microdialysis combined with capillary zone electrophoresis coupled with laser-induced fluorescence detection (CZE-LIFD) and by differential normal pulse voltammetry (DNPV), respectively. Administration of 7-NI, both systemic (30 mg/kg, i.p.) or intrastriatal (1 mM through the microdialysis probe), as well as administration of pargyline (75 mg/kg, i.p.), induced simultaneously in the striatum a significant increase in extracellular DA and a significant decrease in extracellular DOPAC. On the other hand, administration of l-NAME (200 mg/kg, i.p.) produced a significant increase in striatal extracellular DA without changes in extracellular DOPAC. These data suggest a possible MAO inhibitory effect of 7-NI which seems to be restricted to this NOS inhibitor. These results may be of special interest for the studies on the functional role of NO in the brain, particularly in dopaminergic transmission.

The effects of the nitric oxide synthase inhibitor 7-nitroindazole on ethanol pharmacokinetics in rats after acute and chronic ethanol administration.

The aim of this work was to study the effects of the nitric oxide synthase (NOS) inhibitors 7-nitroindazole (7-NI) and NG-nitro-L-arginine (L-NOARG) on the effects and pharmacokinetics of ethanol in rats. Ethanol at a dose of 4 g/kg, i.p. induced sleep in rats (sleep time: 117.2+/-30.7 min). Administration of the NOS inhibitors 7-NI (20 mg/kg, i.p.) and L-NOARG (20 mg/kg, i.p.) 30 min before ethanol significantly increased the duration of ethanol-induced sleep. L-NOARG also significantly increased the toxicity of ethanol as evidenced by increased post-experimental lethality. Ethanol at a dose of 2 g/kg (i.p.) did not induce sleep in vehicle-treated rats; however, the combined administration of ethanol (2 g/kg) and 7-NI at doses of 40, 80, and 120 mg/kg caused sleep, for 49.4+/-3.7, 204.0+/-13.3, and 447.5+/-62.8 min, respectively. L-NOARG (20 mg/kg) had no effect on ethanol concentrations in blood after acute ethanol administration (4 g/kg). 7-NI in lower doses (20 and 40 mg/kg) had no effect and in higher doses (80 and 120 mg/kg) significantly slowed ethanol clearance during the 12 h after ethanol administration. The effect of 7-NI (20 mg/kg) on ethanol pharmacokinetics after chronic ethanol administration (inhalation for 18 days) was also studied. The administration of 7-NI immediately after the end of ethanol exposure had a pronounced effect on ethanol pharmacokinetics; in 7-NI-treated rats the fall in ethanol concentrations was significantly slower as compared with vehicle-treated rats. In 7-NI-treated rats, blood-ethanol levels were higher at 3, 6, 9, and 12 h after the end of ethanol exposure.

Possible role for nitric oxide releasing nerves in the regulation of ocular blood flow in the rat

AIMTo investigate the role of nitrergic nerves in the regulation of ocular blood flow.METHODSConscious, lightly restrained rats were treated with either the neuronal nitric oxide synthase inhibitor 7-nitroindazole (7-NI), or...

Is neuronal nitric oxide involved in adjuvant-induced joint inflammation?

Several reports have described a role of macrophagic, endothelial and synoviocytal nitric oxide (NO) in inflammation, immunity and sensory processes in joint diseases. In view of the role of the peripheral nervous system in arthritis and owing to the presence of NO-producing neurons in primary sensory neurons, we have investigated the possible role of neuronal NO during adjuvant-induced joint inflammation in rats. Neural nitric oxide synthase production in sensory ganglia and the spinal cord was investigated by in situ hybridization and immunocytochemistry. Neuronal NO synthase mRNA expression and neuronal NO synthase immunoreactivity increased in lumbar dorsal root ganglia in arthritic rats compared to those of normal rats, whereas neuronal NO synthase mRNA expression decreased in lamina X and lamina I–II of the lumbar spinal cord. The administration of the selective neuronal NO synthase inhibitor 7-nitro indazole, reduced the joint inflammation, whereas the administration of the inducible NO synthase selective inhibitor, aminoguanidine, had no effect on inflammation when administered daily from the third day after adjuvant. These findings could indicate a role for neural NO in adjuvant arthritis.

The neuronal NO-synthase inhibitor 7-nitroindazole and morphine have independent effects on the control of breathing in the anesthetized cat

The neuronal NO-synthase inhibitor 7-nitroindazole and morphine have independent effects on the control of breathing in the anesthetized cat

The nitric oxide synthase inhibitor 7-nitroindazole displays enhanced anxiolytic efficacy without tolerance in rats following subchronic administration

The nitric oxide synthase inhibitor 7-nitroindazole (7-NI) dose-dependently (3.0–30.0 mg/kg) displayed anxiolytic activity, as measured by an increase in open arm exploration time in the elevated plus-maze (EPM), following intraperitoneal (i.p.) administration in rats. Acute administration of 7-NI at 30.0 mg/kg significantly ( P<0.05) increased open arm exploration time by 176% compared to vehicle control, similar to the benzodiazepine diazepam at 1.0 and 3.0 mg/kg (+191 and +200%, respectively). However, 39 h following subchronic 5-day administration of diazepam twice daily (bid) at 3.0 mg/kg, diazepam was devoid of anxiolytic activity at 1.0 mg/kg, as measured by no difference in open arm exploration time compared to vehicle control, while the 3.0 mg/kg dose still produced a significant ( P<0.05) 175% increase in open arm exploration time. In contrast, following subchronic administration of 7-NI (30.0 mg/kg, bid), a significant ( P<0.01) enhancement in open arm exploration time was observed at 30.0 mg/kg (+665% compared to control). Therefore, inhibition of nitric oxide synthase by 7-NI resulted in anxiolysis similar to diazepam following acute administration in the EPM. However, following subchronic administration, unlike diazepam which showed an attenuation of anxiolytic activity, 7-NI displayed enhanced anxiolytic efficacy and was devoid of tolerance.

CGRP and nitric oxide of neuronal origin and their involvement in neurogenic vasodilatation in rat skin microvasculature.

1. Sensory nerves are important for the initiation of neurogenic inflammation and tissue repair. Both calcitonin gene-related peptide (CGRP) and nitric oxide (NO) have been implicated in neurogenic vasodilatation and inflammatory responses. 2. A blister model in the rat hind footpad was used as a site to induce neurogenic vasodilatation in response to antidromic electrical stimulation of the sciatic nerve. Blood flux was monitored with a laser Doppler flow monitor. 3. The quantitative contributions of CGRP and NO to vasodilatation were examined by use of the CGRP receptor antagonist CGRP8-37 and NO synthase inhibitors 7-nitroindazole (7-NI), 3-bromo 7-NI and N(G)-nitro L-arginine methyl ester (L-NAME). The potential modulatory role of endothelin was examined by use of the ET(A) receptor antagonist BQ-123. 4. CGRP8-37 (10 microM) was perfused over the blister base before nerve stimulation and continuously throughout the post-stimulation period, resulting in a significant reduction (41%) in the blood flux vascular response. 5. Pretreatment with the specific neuronal NO synthase inhibitors, 7-NI and 3-bromo 7-NI (10 mg kg(-1), i.v.), and of the non-specific L-NAME (100 microM), resulted in significant inhibition of the blood flux response (36%, 72% and 57% decrease, respectively). In contrast, 7-NI treatment in young rats pretreated with capsaicin had no further effect on the vascular response, suggesting that the source of NO is the sensory nerves. 6. BQ-123 (10 microM) significantly enhanced the stimulation-induced blood flux response (61% increase). When 7-NI was co-administered with either CGRP8-37 or BQ-123, the drug actions were additive, suggesting that there was no interaction between NO and CGRP or endothelin. 7. These data suggest that both NO and CGRP participate in neurogenic vasodilatation in rat skin microvasculature and that this response is modulated by endogenous endothelin.

The Role of the Nitric Oxide (NO) Pathway in the Discriminative Stimuli of Amphetamine and Cocaine

SKI. The role of the nitric oxide (NO) pathway in the discriminative stimuli of amphetamine and cocaine. PHARMACOL BIOCHEM BEHAV 59(3) 703–708, 1998.—To examine the role of the nitric oxide (NO) pathway in the stimulus effects induced by some psychostimulants, separate groups of rats were trained to discriminate between amphetamine (AMPH; 0.5 mg/kg) and saline, or cocaine (COC; 5 mg/kg) and saline using a standard two-lever operant procedure. Substitution studies showed that AMPH and COC generalized for the training drugs in a dose-dependent manner, their ED 50 values being 0.1 mg/kg and 1.2 mg/kg, respectively. The dose–response function of both those psychostimulants did not change in the course of the experiment. Moreover, AMPH and COC induced crosssubstitution effects towards each other. Successive combination tests demonstrated that injection of a fixed dose of the NO synthase (NOS) inhibitor 7-nitro indazole (7-NI; 25 mg/kg) plus different doses of AMPH or COC resulted in a leftward shift in the dose–response curves of those psychostimulants and a decrease in their ED 50 values. On the other hand, pretreatment with the NO donor molsidomine (MOL), injected in a fixed dose of 100 mg/kg before AMPH and COC, shifted the dose–response curves of the psychostimulants to the right and increased their ED 50 values. Our results indicate that NO plays an inhibitory role in the dopamine (DA)-evoked discrimination effects of AMPH and COC in rats.

Blockade of neuronal nitric oxide synthase alters the baroreflex control of heart rate in the rabbit.

In previous studies we used NG-nitro-L-arginine (L-NNA) to investigate the role of nitric oxide (NO) in baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA). L-NNA increased resting mean arterial pressure (MAP), decreased HR, and did not change or slightly decreased RSNA. These changes complicated the assessment of the central effects of NO on the baroreflex control of HR and RSNA. Therefore, in the present study the effects of the relatively selective neuronal NO synthase inhibitor 7-nitroindazole (7-NI) on the baroreflex control of HR and RSNA were investigated in rabbits. Intraperitoneal injection of 7-NI (50 mg/kg) had no effect on resting HR, MAP, or RSNA. 7-NI significantly reduced the lower plateau of the HR-MAP baroreflex curve from 140 +/- 4 to 125 +/- 4 and from 177 +/- 10 to 120 +/- 9 beats/min in conscious and anesthetized preparations, respectively (P < 0.05). In contrast, there was no significant difference in the RSNA-MAP curves before and after 7-NI administration in conscious or anesthetized preparations. These data suggest that blockade of neuronal NO synthase influences baroreflex control of HR but not of RSNA in rabbits.

Nitric oxide signalling is required for the generation of anoxia-induced long-term potentiation in the hippocampus.

The involvement of nitric oxide in anoxia-induced long-term potentiation (anoxic LTP) of synaptic transmission was investigated in CA1 neurons of rat hippocampal slices using intracellular recording techniques in vitro. In response to superfusion of an anoxic artificial cerebral spinal fluid saturated with 95% N2-5% CO2, the excitatory postsynaptic potential (EPSP) generated in hippocampal CA1 neurons by stimulation of the Schaffer collateral/commissural afferent pathway was completely abolished within 10 min of anoxia. On return to reoxygenated medium, the EPSP returned to the control value within 10 min and was subsequently and progressively potentiated to reach a plateau 15-20 min after return to oxygen. This anoxia-induced persistent increase in synaptic transmission lasted for more than 1 h. Application of the nitric oxide synthase inhibitors 7-nitroindazole (7-NI) or L-N(G)-nitroarginine (NOARG) produced no effects on the baseline EPSP amplitude, but effectively attenuated the anoxic LTP. The inhibitory effects of both 7-NI and NOARG on the anoxic LTP were blocked by L-arginine, a substrate for nitric oxide synthase. These results suggest that nitric oxide is required for the generation of anoxia-induced LTP of glutamatergic synaptic transmission in the CA1 region of the rat hippocampus.

Involvement of nitric oxide in 2-deoxy-D-glucose-induced hyperphagia in rats.

The effects of a nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) on 2-deoxy-D-glucose (2-DG)-induced hyperphagia were investigated in rats. L-NAME dose-dependently inhibited 2-DG-induced eating in non-food-deprived rats, although the inactive isomer D-NAME on 2-DG-induced hyperphagia were inhibited by co-administration of L-arginine. The neuronal NO synthase inhibitor 7-nitroindazole also inhibited 2-DG-induced hyperphagia. These results suggest that 2-DG-induced hyperphagia is linked with NO and that brain NO may participate in this hyperphagic model.

7-Nitroindazole, a nitric oxide synthase inhibitor, has anxiolytic-like properties in exploratory models of anxiety.

The action of the novel nitric oxide synthase (NOS) inhibitor 7-nitroindazole (7-NI) was studied in different exploratory models of anxiety. In the rat plus-maze test, 7-NI potently increased time spent on open arms and percentage of open arm visits in a dose dependent manner with the minimal effective dose of 40 mg/kg. 7-NI caused an anxiolytic-like effect in the rat social interaction test. The minimal dose increasing social interaction time was 20 mg/kg. However, the drug also produced a clear sedative effect occurring even at smaller doses (10 mg/kg) in the open field test. 7-NI also showed an anxiolytic-like profile in the mouse light-dark compartment test and in the elevated plus-maze test, but the doses required were higher (80-120 mg/kg) than in rat models. Also, the sedative effect occurred at these doses in open field. We failed to demonstrate any effect of L-arginine either in the rat elevated plus-maze test or in the open field test at doses up to 600 mg/kg IP. These results indicate that there are no major interspecies differences between rats and mice in respect of action of 7-NI. The clear anxiolytic-like action of the nitric oxide synthase inhibitor in four different models shows that nitric oxide is involved in the process of anxiety and that NOS could be a new target in developing anxiolytic drugs.

The neuronal nitric oxide synthase inhibitor 7-nitroindazole also inhibits the monoamine oxidase-B-catalyzed oxidation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine.

The neurodegenerative properties of the parkinsonian inducing agent 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) are thought to result from inhibition of complex I of the mitochondrial respiratory chain by the monoamine oxidase-B (MAO-B) generated 1-methyl-4-phenylpyridinium metabolite MPP+. Treatment with 7-nitroindazole (7-NI) protects rodents and baboons against MPTP's neurotoxicity, presumably as a consequence of its inhibition of neuronal nitric oxide synthase (nNOS). The results reported in the present communication, while not in conflict with the proposed role of nNOS, raise the possibility that the inhibition of MAO-B by 7-NI also may contribute to the observed neuroprotection.

Inhibitory effects of a nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), on 2-deoxy-D-glucose-induced hyperglycemia in rats.

The inhibitory effects of a potent nitric oxide (NO) synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), on 2-deoxy-D-glucose (2-DG)-induced hyperglycemia were investigated in rats. L-NAME significantly inhibited 2-DG-induced hyperglycemia, although N(G)-nitro-D-arginine methyl ester (D-NAME) did not affect it. A similar NO synthase inhibitor, N(G)-monomethyl-L-arginine (L-NMMA), also inhibited 2-DG-induced hyperglycemia. The antagonistic effects of L-NAME are unrelated to the cholinergic system, since the muscarinic receptor antagonist scopolamine did not affect 2-DG-induced hyperglycemia. The neuronal NO synthase inhibitor 7-nitroindazole (7-NI) did not reduce 2-DG-induced hyperglycemia, but rather enhanced it. Our results suggest that NO may be involved in glucose homeostasis and that the inhibitory effects of L-NAME on 2-DG-induced hyperglycemia are not related to muscarinic receptors or neuronal NO synthase.

Nitric oxide synthase inhibitors produce phencyclidine-like behavioral effects in pigeons

The present study assessed the ability of nitric oxide synthase (NOS) inhibitors to produce PCP-like behavioral effects in pigeons. Food-restricted pigeons were trained to discriminate between PCP (1.0 mg/kg i.m.) from saline in a two-key operant procedure. NOS inhibitors 7-nitroindazole (7-NI) and N ω-nitro- l-arginine methyl ester ( l-NAME) produced PCP-like discriminative stimulus effects. 7-NI (17.8 mg/kg, i.m.) completely generalized to PCP. l-NAME (320-1000 mg/kg) produced partial generalization to PCP. d-NAME, the enantiomer of l-NAME, did not produce PCP-appropriate behavior. l-NAME was approximately 200-times more potent i.c.v., but did not fully generalize to PCP. Both NOS inhibitors were effective in producing catalepsy, which is an effect commonly produced by competitive and uncompetitive NMDA antagonists. 7-NI (32 mg/kg) produced catalepsy in all subjects, whereas l-NAME (3200 mg/kg) produced catalepsy in 50% of the subjects. d-NAME did not produce catalepsy. Pretreatment with l-arginine (32–3200 mg/kg) prevented the PCP-like discriminative stimulus and cataleptic effects of 7-NI (17.8-32 mg/kg), demonstrating that 7-NI produced PCP-like effects through blockade of NO synthesis. The current studies reveal that NOS inhibitors induced two behavioral actions, discriminative stimulus effects and catalepsy, that are very selective for NMDA antagonists in pigeons.

Blockade of neuronal nitric oxide synthase protects against excitotoxicity in vivo

Nitric oxide may be a key mediator of excitotoxic neuronal injury in the central nervous system. We examined the effects of the neuronal nitric oxide synthase inhibitor 7-nitroindazole (7-NI) on excitotoxic striatal lesions. 7-NI significantly attenuated lesions produced by intrastriatal injections of NMDA, but not kainic acid or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) 7-NI attenuated secondary striatal excitotoxic lesions produced by the succinate dehydrogenase inhibitor malonate, and the protection was reversed by L-arginine but not by D-arginine, 7-NI produced nearly complete protection against striatal lesions produced by systemic administration of 3-nitropropionic acid (3-NP), another succinate dehydrogenase inhibitor, 7-NI protected against malonate induced decreases in ATP, and increases in lactate, as assessed by 1H magnetic resonance spectroscopy. 7-NI had no effects on spontaneous electrophysiologic activity in the striatum in vivo, suggesting that its effects were not mediated by an interaction with excitatory amino acid receptors. 7-NI attenuated increases in hydroxyl radical, 8-hydroxy-2-deoxyguanosine and 3-nitrotyrosine generation in vivo, which may be a consequence of peroxynitrite formation. The present results implicate neuronal nitric oxide generation in the pathogenesis of both direct and secondary excitotoxic neuronal injury in vivo. As such they suggest that neuronal nitric oxide synthase inhibitors may be useful in the treatment of neurologic diseases in which excitotoxic mechanisms play a role.

Inhibition of Neuronal Nitric Oxide Synthase by 7-Nitroindazole: Effects upon Local Cerebral Blood Flow and Glucose Use in the Rat

The novel nitric oxide synthase inhibitor 7-nitroindazole (7-NI) is relatively specific for the neuronal isoform of the enzyme and in this study we have used this compound to investigate the physiological role of perivascular nitric oxide-containing nerves in the cerebrovascular bed. Following injection of 7-NI (25 or 50 mg/kg, i.p.), cerebral blood flow and glucose utilization were measured in the conscious rat using the fully quantitative [14C]iodoantipyrine and 2-[14C]deoxyglucose techniques, respectively. Neither dose of the drug produced any change in arterial blood pressure, confirming a lack of effect upon the endothelial isoform of the enzyme, although there was a pronounced decrease in heart rate (-28% by 10 min postinjection). Throughout the brain 25 mg/kg 7-NI i.p. resulted in decreases in blood flow of between -20% in the hippocampus and -58% in the substantia nigra. Increasing the dose to 50 mg/kg resulted in a further generalized decrease, to almost -60% in parts of the thalamus and hippocampus, but in every animal this higher dose of 7-NI also produced randomly distributed areas of relative hyperaemia, which were most commonly found in those areas where the most intense hypoperfusion was otherwise in evidence. Despite these changes in blood flow, in all but a very few areas of the brain no significant decrease in glucose use was measured at either of the two doses of 7-NI. Thus despite the greater specificity of 7-NI for neuronal nitric oxide synthase, the cerebrovascular effects of the drug in vivo are very similar to that reported for the arginine analogues. However, these data do suggest that nitric oxide-releasing neurones in the brain may have an important role to play in the regulation of cerebral blood flow.

Increased striatal dopamine efflux in vivo following inhibition of cerebral nitric oxide synthase by the novel monosodium salt of 7‐nitro indazole

The role of nitric oxide (NO) in striatal dopamine release has been controversial. Most NO synthase inhibitors affect more than one isoform of the enzyme and exert vasoconstrictor effects which may also affect striatal dopamine function. We now report on the effect of a soluble monosodium salt of the selective brain NO synthase inhibitor 7-nitro indazole (7-NINA). Using 7-NINA the first study of selective inhibition of the brain isoform of NO synthase on dopamine efflux in rat striatum was undertaken by use of in vivo microdialysis. Perfusion with 7-NINA (1 mM) increased striatal dopamine efflux. The effect of 7-NINA was partially antagonized (67%) by co-perfusion with L-arginine (1 mM), the precursor of NO formation in vivo. This suggests that 7-NINA induces a competitive inhibition of NO synthase activity. These data show that endogenous NO has an inhibitory effect on striatal dopamine efflux in vivo.

Inhibition of NO synthase increases the severity of kainic acid-induced seizures in rodents

The nitric oxide (NO) synthase inhibitor N ω- nitro- l- arginine (NNA) and the putative brain-selective NO synthase inhibitor 7-nitroindazole (7-NI) were used to determine the role of endogenous NO on seizures induced by kainic acid (KA) in rats and KA, pilocarpine, bicuculline, picrotoxin and pentylenetetrazole (PTZ) in mice. Rats given a subconvulsant dose of KA (6 mg/kg i.p.) had seizures after they had been pretreated with NNA (50 mg/kg i.p.). With a higher dose of KA (12 mg/kg i.p.), NNA caused an increase in wild running seizures and mortality. Unlike NNA, 7-NI had no effect on KA-induced seizures. Similarly, NNA but not 7-NI caused a worsening of seizures in mice measured as a shortening of seizure latency and an increase in wild running and mortality. The effect of NNA on seizure latency was completely reversed by the competitive substrate for NO synthase, l-arginine. NNA had no effect on seizure latency following any of the other convulsants and increased mortality following pilocarpine and picrotoxin alone. Our results indicate that NNA may enhance the severity of KA-induced seizures through suppression of NO synthase activity in the vascular endothelium. The resulting impairment of cerebrovascular autoregulation may cause a mismatch between metabolic demand and blood flow during seizures leading to facilitation of spread. The absence of a comparable effect of NNA on other seizure models may indicate differences in the degree to which seizure activity in different models is influenced by the metabolic impairment secondary to decreased blood flow.