Physiological and toxicological actions of nitric oxide in the central nervous system

In Vitro and In Vivo Anti-Inflammatory and Antidepressant-like Effects of Cannabis sativa L. Extracts

Cannabis sativa L. has been widely used by humans for centuries for various purposes, such as industrial, ceremonial, medicinal, and food. The bioactive components of Cannabis sativa L. can be classified into two main groups: cannabinoids and terpenes. These bioactive components of Cannabis sativa L. leaf and inflorescence extracts were analyzed. Mice were systemically administered 30 mg/kg of Cannabis sativa L. leaf extract 1 h before lipopolysaccharide (LPS) administration, and behavioral tests were performed. We conducted an investigation into the oxygen saturation, oxygen tension, and degranulation of mast cells (MCs) in the deep cervical lymph nodes (DCLNs). To evaluate the anti-inflammatory effect of Cannabis sativa L. extracts in BV2 microglial cells, we assessed nitrite production and the expression levels of inducible nitric oxide synthase (iNOS), cyclooxygenase (COX)-2, interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α. The main bioactive components of the Cannabis sativa L. extracts were THCA (a cannabinoid) and β-caryophyllene (a terpene). Cannabis sativa L. leaf extract reduced the immobility time in the forced swimming test and increased sucrose preference in the LPS model, without affecting the total distance and time in the center in the open field test. Additionally, Cannabis sativa L. leaf extract improved oxygen levels and inhibited the degranulation of MCs in DCLNs. The Cannabis sativa L. extracts inhibited IL-1β, IL-6, TNF-α, nitrite, iNOS, and COX-2 expression in BV2 microglia cells. The efficacy of Cannabis sativa L. extracts was suggested to be due to the entourage effect of various bioactive phytochemicals. Our findings indicate that these extracts have the potential to be used as effective treatments for a variety of diseases associated with acute inflammatory responses.

The effects of ketamine and thiopental used alone or in combination on the brain, heart, and bronchial tissues of rats

INTRODUCTION

We compared the side effects of ketamine and thiopental used alone and of a ketamine/thiopental combination dose on the brain,heart, and bronchial tissues of rats.

MATERIAL AND METHODS

Three groups received intraperitoneal injections of 30 mg/kg ketamine (K-30); 15 mg/kg thiopental (T-15); or of both in combination (KTSA). These doses were doubled in another set of study groups (K-60, T-30, and KTA groups, respectively). Optimal anesthesia duration was examined in all groups.

RESULTS

Anesthesia did not occur with 30 mg/kg ketamine or 15 mg/kg thiopental. However, when used alone ketamine and thiopental led to oxidative stress in the striatum, heart, and bronchial tissues. Conversely, combined administration of anesthetics and subanesthetic doses were found not to create oxidative stress in any of these areas. The highest level of adrenaline in blood samples collected from the tail veins was measured in the KTA-60, and the lowest amount in the T-30. Creatine kinase activity was highest in the KTA-60 group (p < 0.001). When we compared for all 5 groups to untreated control group; the creatine kinase-MB activities were significiantly different in K-30, T-15 and T-30 (p < 0.001).

CONCLUSIONS

The studied doses of ketamine led to oxidative stress by increasing the amount of adrenaline. Thiopental increased oxidative stress with decreases in adrenaline. A longer anesthetic effect with minimal adverse events may be achieved by ketamine and thiopental in combination.

Nutrient supplementation with n3 polyunsaturated fatty acids, lutein, and zeaxanthin decrease A2E accumulation and VEGF expression in the retinas of Ccl2/Cx3cr1-deficient mice on Crb1rd8 background

The Age-Related Eye Diseases Study 2 (AREDS2) clinical trial is assessing the effects of higher dietary xanthophyll (lutein and zeaxanthin) and long-chain n3 polyunsaturated fatty acid (LCPUFA) docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) intake on progression to advanced age-related macular degeneration (AMD). This study's purpose was to examine the retinal effects of the AREDS2 formulation on Chemokine (C-C motif) ligand 2 (Ccl2(-/-))/CX3C chemokine receptor 1 (Cx3cr1(-/-)) mice on Crumbs homolog 1 retinal degeneration phenotype 8 (Crb1(rd8)) background (DKO), which develop focal retinal lesions with certain features similar to AMD. DKO and C57BL/6N rd8 background mice (WT) were bred and randomized into 4 groups. Two groups, WT mice on AREDS2 diet (A-WT) and DKO mice on AREDS2 diet (A-DKO), were supplemented daily with 1.76 μmol of lutein, 35.1 μmol of zeaxanthin, 215 μmol EPA, and 107 μmol of DHA, and 2 control groups, WT mice on control diet (C-WT) and DKO mice on control diet (C-DKO), were fed an isocaloric diet. All mice had monthly fundus photos and were killed after 3 mo for biochemical and histologic analyses. After 3 mo, 81% of A-DKO mice had lesion regression compared with 25% of C-DKO mice (P < 0.05). Toxic retinal 2-[2,6-dimethyl-8-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E,7E-octatetra-enyl]-1-(2-hydroxyethyl)-4-[4-methyl-6(2,6,6-trimethyl-1-cyclohexen-1-yl) 1E,3E,5E,7E-hexatrienyl]-pyridinium (A2E) concentrations were significantly lower in A-DKO compared with C-DKO mice. The outer nuclear layer thickness in A-DKO mice was significantly greater than that in C-DKO mice. Retinal expression of inducible nitric oxide synthase (iNos) tumor necrosis factor-α (Tnf-α), Cyclooxygenase-2 (Cox-2), interleukin1beta (IL-1β), and vascular endothelial growth factor (Vegf) was significantly lower in A-DKO compared with C-DKO mice. Xanthophylls and LCPUFAs have antiinflammatory, neuroprotective, and antiangiogenic properties. Our data provide potential mechanisms by which the AREDS2 formula has a protective effect on retinal lesions in DKO mice.

Parkin promotes degradation of the mitochondrial pro-apoptotic ARTS protein

Parkinson's disease (PD) is associated with excessive cell death causing selective loss of dopaminergic neurons. Dysfunction of the Ubiquitin Proteasome System (UPS) is associated with the pathophysiology of PD. Mutations in Parkin which impair its E3-ligase activity play a major role in the pathogenesis of inherited PD. ARTS (Sept4_i2) is a mitochondrial protein, which initiates caspase activation upstream of cytochrome c release in the mitochondrial apoptotic pathway. Here we show that Parkin serves as an E3-ubiquitin ligase to restrict the levels of ARTS through UPS-mediated degradation. Though Parkin binds equally to ARTS and Sept4_i1 (H5/PNUTL2), the non-apoptotic splice variant of Sept4, Parkin ubiquitinates and degrades only ARTS. Thus, the effect of Parkin on ARTS is specific and probably related to its pro-apoptotic function. High levels of ARTS are sufficient to promote apoptosis in cultured neuronal cells, and rat brains treated with 6-OHDA reveal high levels of ARTS. However, over-expression of Parkin can protect cells from ARTS-induced apoptosis. Furthermore, Parkin loss-of-function experiments reveal that reduction of Parkin causes increased levels of ARTS and apoptosis. We propose that in brain cells in which the E3-ligase activity of Parkin is compromised, ARTS levels increase and facilitate apoptosis. Thus, ARTS is a novel substrate of Parkin. These observations link Parkin directly to a pro-apoptotic protein and reveal a novel connection between Parkin, apoptosis, and PD.

Preconditioning with a novel metallopharmaceutical NO donor in anesthetized rats subjected to brain ischemia/reperfusion

Rut-bpy is a novel nitrosyl-ruthenium complex releasing NO into the vascular system. We evaluated the effect of Rut-bpy (100 mg/kg) on a rat model of brain stroke. Forty rats were assigned to four groups (Saline solution [SS], Rut-bpy, SS+ischemia-reperfusion [SS+I/R] and Rut-bpy+ischemia-reperfusion [Rut-bpy+I/R]) with their mean arterial pressure (MAP) continuously monitored. The groups were submitted (SS+I/R and Rut-bpy+I/R) or not (SS and Rut-bpy) to incomplete global brain ischemia by occlusion of the common bilateral carotid arteries during 30 min followed by reperfusion for further 60 min. Thirty minutes before ischemia, rats were treated pairwise by intraperitoneal injection of saline solution or Rut-bpy. At the end of experiments, brain was removed for triphenyltetrazolium chloride staining in order to quantify the total ischemic area. In a subset of rats, hippocampus was obtained for histopathology scoring, nitrate and nitrite measurements, immunostaining and western blotting of the nuclear factor- κB (NF-κB). Rut-bpy pre-treatment decreased MAP variations during the transition from brain ischemia to reperfusion and decreased the fractional injury area. Rut-bpy pre-treatment reduced NF-κB hippocampal immunostaining and protein expression with improved histopathology scoring as compared to the untreated operated control. In conclusion, Rut-bpy improved the total brain infarction area and hippocampal neuronal viability in part by inhibiting NF-κB signaling and helped to stabilize the blood pressure during the transition from ischemia to reperfusion.

The selective and competitive N-methyl-D-aspartate receptor antagonist, (-)-6-phosphonomethyl-deca-hydroisoquinoline-3-carboxylic acid, prevents synaptic toxicity induced by amyloid-β in mice

The toxicity of amyloid β (Aβ) is highly associated with Alzheimer's disease (AD), which has a high incidence in elderly people worldwide. While the current treatment for moderate and severe AD includes blockage of the N-methyl-d-aspartate receptor (NMDAR), the molecular mechanisms of its effect are still poorly understood. Herein, we report that a single i.p. administration of the selective and competitive (NMDAR) antagonist LY235959 reduced Aβ neurotoxicity by preventing the down-regulation of glial glutamate transporters (glutamate-aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1)), the decrease in glutamate uptake, and the production of reactive oxygen species (ROS) induced by Aβ(1-40). Importantly, the blockage of NMDAR restored the Aβ(1-40)-induced synaptic dysfunction and cognitive impairment. However, LY235959 failed to prevent the inflammatory response associated with Aβ(1-40) treatment. Altogether, our data indicate that the acute administration of Aβ promotes oxidative stress, a decrease in glutamate transporter expression, and neurotoxicity. Our results reinforce the idea that NMDAR plays a critical regulatory action in Aβ toxicity and they provide further pre-clinical evidence for the potential role of the selective and competitive NMDAR antagonists in the treatment of AD.

Short-term ethanol administration does not change the total pyramidal neuron number of the rat hippocampus: a stereologic study

In the hippocampus, short-term exposure to ethanol (EtOH) has been shown to inhibit some functions, and nitric oxide (NO) is an important modulator of physiologic processes. In this study, investigators explored the effects of EtOH on the total number of neurons in rat CA regions and the possible neuroprotective role of NO. The role of NO in rats given EtOH was examined with the use of a nonselective inhibitor of NOS (N[G]-nitro-L-arginine methyl ester [L-NAME], D[G]-nitro-Larginine methyl ester [D-NAME]), a central selective inhibitor of NOS (7-NI), and a donor of NO (L-arginine). Toward this end, rats were randomly divided into 6 groups: control (saline 3 g/kg intraperitoneal [ip]), ethanol (ethanol 3 g/kg ip), L-NAME (ethanol 3 g/kg ip + L- NAME 60 mg/kg ip), L-arginine (ethanol 3 g/kg ip + L-arginine 1 g/kg ip), D-NAME (ethanol 3 g/kg ip + D-NAME 60 mg/kg ip), and 7-NI (ethanol 3 g/kg ip + 7-NI 40 mg/kg ip). Blood ethanol concentrations were measured 90 min after EtOH administration. Means (value+/-standard deviation) of total pyramidal neuronal numbers in the right hippocampus were estimated using the optical fractionator counting method. Values were as follows: 446,558+/-6207, 483,517+/-20,311, 464,588+/-30,637, 479,688+/-10,780, 458,294+/-17,770, and 477,281+/-7641 in the control, ethanol, L-arginine, 7-NI, L-NAME, and D-NAME groups, respectively. No significant differences were observed between groups (P>.05). The results of the present study imply that short-term administration of EtOH does not affect total pyramidal neuronal number in the right hippocampus of the rat. Furthermore, NO does not change the effects of short-term EtOH on total pyramidal neuronal number in the right hippocampal CA regions of the rat. Additional studies are needed to clarify the role of NO and NOS inhibition in the effects associated with EtOH given on a short-term basis.

Afferent projections to the mediodorsal and anterior thalamic nuclei in the cat. Anatomical-clinical correlations

Afferent projections to the mediodorsal and anterior thalamic nuclei in the cat were studied by means of stereotaxic injection of neuronal tracers (horseradish peroxidase and fluorochromes). Acetylcholinesterase reaction was studied, as well as horseradish peroxidase and NADPH-diaphorase colocation in neuronal bodies which send and receive projections to and from the mediodorsal thalamic nucleus. Based on the connectivity and histochemistry findings, the possibility that prion agents responsible for fatal familial insomnia spread from the mediodorsal and anterior thalamic nuclei through a retrograde pathway is discussed. The possible pathophysiological implication of nitrergic systems in fatal familial insomnia is also considered.

Redox regulation of heat shock protein expression by signaling involving nitric oxide and carbon monoxide: relevance to brain aging, neurodegenerative disorders, and longevity

Increased free radical generation and decreased efficiency of the reparative/degradative mechanisms both primarily contribute to age-related elevation in the level of oxidative stress and brain damage. Excess formation of reactive oxygen and nitrogen species can cause proteasomal dysfunction and protein overloading. The major neurodegenerative diseases are all associated with the presence of abnormal proteins. Different integrated responses exist in the brain to detect oxidative stress which is controlled by several genes termed vitagenes, including the heat shock protein (HSP) system. Of the various HSPs, heme oxygenase-I (HO-1), by generating the vasoactive molecule carbon monoxide and the potent antioxidant bilirubin, could represent a protective system potentially active against brain oxidative injury. The HO-1 gene is redox regulated and its expression is modulated by redox active compounds, including nutritional antioxidants. Given the broad cytoprotective properties of the heat shock response, there is now strong interest in discovering and developing pharmacological agents capable of inducing the heat shock response. These findings have opened up new neuroprotective strategies, as molecules inducing this defense mechanism can be a therapeutic target to minimize the deleterious consequences associated with accumulation of conformationally aberrant proteins to oxidative stress, such as in neurodegenerative disorders and brain aging, with resulting prolongation of a healthy life span.

Nitric oxide as inflammatory mediator in post-traumatic stress disorder (PTSD): evidence from an animal model

Post-traumatic stress disorder (PTSD) is a severe anxiety disorder that may develop after experiencing or witnessing a traumatic event. Recent clinical evidence has suggested the involvement of neurodegenerative pathology in the illness, particularly with brain imaging studies revealing a marked reduction in hippocampal volume. Of greater significance is that these anatomical changes appear to be positively correlated with the degree of cognitive deficit noted in these patients. Stress-induced increases in plasma cortisol have been implicated in this apparent atrophy. Although not definitive, clinical studies have observed a marked suppression of plasma cortisol in PTSD. The basis for hippocampal neurodegeneration and cognitive decline therefore remains unclear. Stress and glucocorticoids increase glutamate release, which is recognized as an important mediator of glucocorticoid-induced neurotoxicity. Recent preclinical studies have also noted that glutamate and nitric oxide (NO) play a causal role in anxiety-related behaviors. Because of the prominent role of NO in neuronal toxicity, cellular memory processes, and as a neuromodulator, nitrergic pathways may have an important role in stress-related hippocampal degenerative pathology and cognitive deficits seen in patients with PTSD. This paper reviews the preclinical evidence for involvement of the NO-pathway in PTSD, and emphasizes studies that have addressed these issues using time-dependent sensitization - a putative animal model of PTSD.

Sequential changes of nitric oxide levels in the temporal lobes of kainic acid-treated mice following application of nitric oxide synthase inhibitors and phenobarbital

Although studies have indicated a close relationship between nitric oxide (NO) and kainic acid (KA)-induced seizures, the role of NO in seizures is not fully understood. Here, we quantified NO levels in the brain of KA-treated mice using EPR spectrometry to elucidate the role of NO in KA-induced seizures. KA was administered to mice with or without pretreatment with one of the following: N(G)-nitro-l-arginine methyl ester (l-NAME), an NO synthase (NOS) inhibitor that acts on both endothelial NOS (eNOS) and neuronal NOS (nNOS); 7-nitroindazole (7-NI), which acts more selectively on nNOS in vivo; or the anti-epileptic drug, phenobarbital. To accurately assess NO production during seizure activity, we directly measured KA-induced NO levels in the temporal lobe using an electron paramagnetic resonance NO trapping technique. Our results revealed that the both dose- and time-dependent changes of NO levels in the temporal lobe of KA-treated mice were closely related to the development of seizure activity. l-NAME mediated suppression of the KA-induced NO generation led to enhanced severity of KA-induced seizures. In contrast, 7-NI induced only about 50% suppression and had little effect on seizure severity; while phenobarbital markedly reduced both NO production and seizure severity. These results show that KA-induced neuroexcitation leads to profound increases in NO release to the temporal lobe of KA-treated mice and that NO generation from eNOS exerts an anti-convulsant effect.

Effects of nitric oxide on dentate gyrus cell proliferation after seizures induced by pentylenetrazol in the adult rat brain

Epileptic seizures have been shown to increase the proliferation of granule cell precursors in the adult brain, but the underlying mechanisms remain largely unknown. This study examined the effect of nitric oxide (NO) on the proliferation of granule cell precursors in adult rats after pentylenetrazol (PTZ)-induced generalized clonic seizures. Using systemic bromodeoxyuridine (BrdU) to label dividing cells, we found that injection of the neuronal nitric oxide synthase (nNOS) inhibitor 7-nitroindazole (50 mg/kg i.p.) 10 min before PTZ significantly reduced the number of BrdU labeled cells in the dentate gyrus 3, 7, and 14 days after seizures (P < 0.05). Administration of the inducible NOS (iNOS) inhibitor aminoguanidine (100 mg/kg i.p.) also significantly inhibited the proliferation rate of neural precursor cells in the dentate gyrus at various time points after PTZ-induced seizures. Our findings suggest that epileptic seizures lead to increased cell proliferation in the adult rat dentate gyrus through NO-dependent mechanisms. Both the NO originating from nNOS and iNOS may be involved in brain repair after seizures.

Redox regulation in neurodegeneration and longevity: role of the heme oxygenase and HSP70 systems in brain stress tolerance

Efficient functioning of maintenance and repair processes seems to be crucial for both survival and physical quality of life. This is accomplished by a complex network of the so-called longevity assurance processes, which are composed of several genes termed "vitagenes," among these, the heat shock system, a highly conserved mechanism responsible for the preservation and repair of cellular macromolecules, such as proteins, RNAs, and DNA. Recent studies have shown that the heat shock response contributes to establishing a cytoprotective state in a wide variety of human diseases, including ischemia and reperfusion damage, inflammation, cancer, as well as metabolic and neurodegenerative disorders. Recently, the involvement of the heme oxygenase (HO) pathway in antidegenerative mechanisms has received considerable attention, as it has been demonstrated that the expression of HO is closely related to that of amyloid precursor protein. HO induction occurs together with the induction of other heat shock proteins during various physiopathological conditions. The vasoactive molecule carbon monoxide and the potent antioxidant bilirubin, products of HO-catalyzed reaction, represent a protective system potentially active against brain oxidative injury. Given the broad cytoprotective properties of the heat shock response, molecules inducing this defense mechanism appear to be possible candidates for novel cytoprotective strategies. Particularly, manipulation of endogenous cellular defense mechanisms, via the heat shock response, through nutritional antioxidants or pharmacological compounds, may represent an innovative approach to therapeutic intervention in diseases causing tissue damage, such as neurodegeneration. Consistently, by maintaining or recovering the activity of vitagenes, it is feasible to delay the aging process and decrease the occurrence of age-related diseases with resulting prolongation of a healthy life span.

Role of oxidative stress in neurodegeneration: recent developments in assay methods for oxidative stress and nutraceutical antioxidants

Reactive oxygen species (ROS) are produced in the course of normal metabolism and they serve important physiological functions. However, because of their high reactivity, accumulation of ROS beyond the immediate needs of the cell may affect cellular structure and functional integrity, by bringing about oxidative degradation of critical molecules, such as the DNA, proteins, and lipids. Although cells possess an intricate network of defense mechanisms to neutralize excess ROS and reduce oxidative stress, some tissues, especially the brain, are much more vulnerable to oxidative stress because of their elevated consumption of oxygen and the consequent generation of large amounts of ROS. For the same reason, the mitochondrial DNA (mtDNA) of brain cells is highly susceptible to structural alterations resulting in mitochondrial dysfunction. Several lines of evidence strongly suggest that these effects of ROS may be etiologically related to a number of neurodegenerative disorders. Nutraceutical antioxidants are dietary supplements that can exert positive pharmacological effects on specific human diseases by neutralizing the negative effects of ROS. The present communication concentrates on a review of recent concepts and methodological developments, some of them based on the results of work from our own laboratory, on the following aspects: (1) the complex interactions and complementary interrelationships between oxidative stress, mitochondrial dysfunction, and various forms of neural degeneration; (2) fractionation and isolation of substances with antioxidant properties from plant materials, which are extensively used in the human diet and, therefore, can be expected to be less toxic in any pharmacological intervention; (3) recent developments in methodologies that can be used for the assay of oxidative stress and determination of biological activities of exogenous and endogenous antioxidants; and (4) presentation of simple procedures based on polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) of the resulting amplicon for investigations of structural alterations in mtDNA.

Effects of chronic ethanol administration and ethanol withdrawal on cyclic guanosine 3',5'-monophosphate (cGMP) levels in the rat brain

The main objective of the present study is to investigate the possible effects of chronic ethanol consumption and ethanol withdrawal on cyclic guanosine 3', 5'-monophosphate (cGMP) levels in cerebral cortex, striatum, hippocampus and hypothalamus of rat brain. Ethanol was given to female Wistar rats (225-270g) by a liquid diet for 21 days. cGMP levels were measured in respective brain regions using an EIA kit at 7th, 14th and 21st days of ethanol ingestion and at 6th and 24th h of ethanol withdrawal. cGMP levels in cortex, striatum and hippocampus but not hypothalamus were found significantly increased at 14th and 21st days of ethanol consumption. The most prominent increase was observed in striatal tissues (approximately 350%). cGMP levels of striatum and hippocampus were still remaining significantly high at 6th h of ethanol withdrawal. Blood ethanol levels were found as 115.60, 50.0 and 7.0mg/dl just before and after 6 and 24h of ethanol withdrawal, respectively and audiogenic seizures also occurred at 6th h of ethanol withdrawal with an incidence of 75% in individual parallel groups. Our results suggest that changes of cGMP levels in cerebral cortex, striatum and hippocampus might participate in the mechanism of ethanol dependence and withdrawal in rats.

Nitrosative stress and potassium channel-mediated neuronal apoptosis: is zinc the link?

Nitrosative stress has been implicated in a large number of neurological disorders. The molecular mechanisms underlying the neuronal injury associated with this stimulus, however, are not clearly understood. Emerging evidence suggests that the liberation of intracellular zinc as well as over-activation of potassium channels may be two important components of nitrosative stress-induced neuronal death.

Differential toxicity of nitric oxide, aluminum, and amyloid beta-peptide in SN56 cholinergic cells from mouse septum

A characteristic feature of several encephalopathies is preferential impairment of cholinergic neurons. Their particular susceptibility to cytotoxic insults may result from the fact that they utilise acetyl-CoA both for energy production and acetylcholine synthesis. In addition, phenotypic modifications of cholinergic neurons are likely to influence their susceptibility to specific harmful conditions. SN56 cholinergic cells were differentiated by the combination of dibutyryl cAMP and retinoic acid. Al and sodium nitroprusside (SNP, NO donor) exerted direct additive inhibitory effects on mitochondrial aconitase activity. However, NO, Al, or amyloid beta (Abeta)(25-35) caused none or only slight changes of choline O-acetyl transferase (ChAT) and pyruvate dehydrogenase (PDH) activity and relatively small loss of non-differentiated cells (NCs). On the other hand, in differentiated cells (DCs) these neurotoxins brought about marked decreases of these enzyme activities along with greater than in non-differentiated ones increase of cell-death rate. Abeta(35-25) had no effect on these cell parameters. NO and other compounds aggravated detrimental effect of each other particularly in differentiated cells. Thus, differential vulnerability of brain cholinergic neurons to various degenerative signals may result from their phenotype-dependent ratios of acetylcholine to acetyl-CoA synthesising capacities.

Neuropathic pain is associated with alterations of nitric oxide synthase immunoreactivity and catalytic activity in dorsal root ganglia and spinal dorsal horn

Previous experiments have suggested that nitric oxide may play an important role in nociceptive transmission in the spinal cord. To assess the possible roles of neuronal nitric oxide synthase (nNOS) in spinal sensitization after nerve injury, we examined the distribution of nNOS immunoreactivity in dorsal root ganglia (DRGs) and dorsal horn of the corresponding spinal segments. NOS catalytic activity was also determined by monitoring the conversion of [3H]arginine to [3H]citrulline in the lumbar (L4-L6) spinal cord segments and DRGs in rats 21 days after unilateral loose ligation of the sciatic nerve. Behavioral signs of tactile and cold allodynia developed in the nerve-ligated rats within 1 week after surgery and lasted up to 21 days. Immunocytochemical staining revealed a significant increase (approximately 6.7-fold) of nNOS-immunoreactive neurons and fibers in the DRGs L4-L6. No significant changes were detected in the number of nNOS-positive neurons in laminae I-II of the spinal segments L4-L6 ipsilateral to nerve ligation. However, an increased number of large stellate or elongated somata in deep laminae III-V of the L5 segment expressed high nNOS immunoreactivity. The alterations of NOS catalytic activity in the spinal segments L4-L6 and corresponding DRGs closely correlated with nNOS distribution detected by immunocytochemistry. No such changes were detected in the contralateral DRGs or spinal cord of sham-operated rats. The results indicate that marked alterations of nNOS in the DRG cells and in the spinal cord may contribute to spinal sensory processing as well as to the development of neuronal plasticity phenomena in the dorsal horn.

A critical assessment of the neurodestructive and neuroprotective effects of nitric oxide

Whether nitric oxide is cytodestructive or cytoprotective is of obvious clinical importance. The debate on this subject in the past decade has generated much "heat and light". This paper focuses on the actions of NO on the nervous system and reexamines the controversial issue and the contribution of the authors and their colleagues in the light of recent findings. We also report new findings, critically assesses previous experimental data, and share perspectives on this important subject.

Spin trapping agent, phenyl N-tert-butylnitrone, reduces nitric oxide production in the rat brain during experimental meningitis

Phenyl N-tert-butylnitrone (PBN) is a spin trapping agent previously shown to exert a neuroprotective effect in infant rat brain during bacterial meningitis. In the present study, we investigated the effect of systemic PBN administration on nitric oxide (NO) production in a rat model of experimental meningitis induced by lipopolysaccharide (LPS). We assessed the NO concentration in rat brain tissues with an electron paramagnetic resonance (EPR) NO trapping technique. In this model, rats receiving intracisternal LPS administration showed symptoms of meningitis and cerebrospinal fluid (CSF) pleocytosis. The time course study indicated that the concentration of NO in the brain reached the maximum level 8.5 h after injection of LPS, and returned to the control level 24 h after the injection. When various doses of PBN (125-400 mg/kg) were injected intraperitoneally 30 min prior to LPS, NO production in the brain was reduced with increasing PBN dose (250 mg/kg suppressed 80% at 8.5 h after LPS injection), and white blood cells (WBC) in CSF were significantly decreased. We concluded that reduction of NO generation during bacterial meningitis contributes to the neuroprotective effect of PBN in addition to its possible direct scavenging of reactive oxygen intermediate (ROI).

Targeted disruption of inducible nitric oxide synthase protects against obesity-linked insulin resistance in muscle

Inducible nitric oxide synthase (iNOS) is induced by inflammatory cytokines in skeletal muscle and fat. It has been proposed that chronic iNOS induction may cause muscle insulin resistance. Here we show that iNOS expression is increased in muscle and fat of genetic and dietary models of obesity. Moreover, mice in which the gene encoding iNOS was disrupted (Nos2-/- mice) are protected from high-fat-induced insulin resistance. Whereas both wild-type and Nos2-/- mice developed obesity on the high-fat diet, obese Nos2-/- mice exhibited improved glucose tolerance, normal insulin sensitivity in vivo and normal insulin-stimulated glucose uptake in muscles. iNOS induction in obese wild-type mice was associated with impairments in phosphatidylinositol 3-kinase and Akt activation by insulin in muscle. These defects were fully prevented in obese Nos2-/- mice. These findings provide genetic evidence that iNOS is involved in the development of muscle insulin resistance in diet-induced obesity.

Ontogenesis of neurons immunoreactive for nitric oxide synthase in rat forebrain and midbrain

We studied the immunohistochemical localization of neuronal nitric oxide synthase (nNOS) in the developing rat brain on embryonic days 13 (E13), 15 (E15), 17 (E17) and 19 (E19) and postnatal days 0 (P0), 7 (P7) and 14 (P14). A few neurons positive for nNOS were first detected at E15 in the hypothalamus and pons. At E17, many positive cells became detectable in the thalamus. At E19, the positive cells in these three regions were rapidly increased in number, and a few positive neurons were also observed in such regions as the cerebral cortex and striatum. Positive cells in the hypothalamus tended to locate ventrolaterally. Positive neurons, stained very intensely as in adult rats, were seen in the pedunculopontine tegmental nucleus, laterodorsal tegmental nucleus and parafascicular nucleus. Two weeks after birth, positive neurons of larger somata with many processes were distributed widely in the cerebral cortex and hippocampus. The present study indicates that, in the forebrain and midbrain, the distribution pattern of nNOS-containing neurons is fundamentally completed by E19.

Local anesthetics potentiate nitric oxide synthase type 2 expression in rat glial cells

Expression of the calcium-independent nitric oxide synthase (NOS2) contributes to damage in neurologic disease and trauma. The effects of local anesthetics on NOS2 expression have not been examined. The authors tested the effects of four local anesthetics on the expression of NOS2 in immunostimulated rat C6 glioma cells. Incubation with local anesthetics alone did not induce nitrite accumulation; however, the nitrite production induced by stimulation with bacterial endotoxin lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) was increased in a dose-dependent manner by bupivacaine (maximal 3-fold at 360 microM), tetracaine (maximal 7-fold at 360 microM), and lidocaine at higher doses (5-fold increase at 3.3 mM). Significant increases in nitrite production were observed in concentrations of bupivacaine or tetracaine as low as 120 microM, which correspond to 30 microg/mL (.003% weight/volume). In contrast, ropivacaine had little effect on nitrite production (160% of control values) and only at the highest concentration (3.3 mM, corresponding to 890 microg/mL or 0.089% w/v) tested. Increased nitrite production was not caused by cytotoxic effects of the drugs used, as assessed by release of intracellular lactate dehydrogenase. Increased nitrite production was accompanied by increased NOS2 catalytic activity, steady state mRNA levels, and promoter activation. These results demonstrate that submillimolar doses of two commonly used local anesthetics can increase glial NOS2 expression.

The microenvironment of injured and regenerating peripheral nerves

Local events in the milieu of injured peripheral nerve trunks may have an important influence on the likelihood of regenerative success or the development of neuropathic pain. Injury-related changes in the microcirculation of this milieu have provided some evidence that axonal endbulbs, structures that form at the proximal end of transected axons, dump peptides and other molecules into the injury milieu where they may exert local actions, including those on microvessels. During a later phase of nerve repair, macrophage influx and pancellular proliferative events appear to develop in a coordinated fashion. Nitric oxide is probably an important and prominent player in the injured nerve trunk, both at early and later stages of the repair process. A better understanding of the injured peripheral nerve microenvironment may allow therapeutic approaches that can enhance regeneration and diminish pain.

Nitric oxide and substance dependence

The free-radical gas nitric oxide (NO) plays an important role in a diverse range of physiological processes. It is synthesized from the precursor L-arginine by the enzyme NO synthase (NOS), which transforms L-arginine into NO and citrulline. This synthetic pathway exists in the central nervous system (CNS), and NO appears to be a messenger molecule in the CNS, fulfilling most of the criteria of a neurotransmitter. Recent studies indicate that NO may play an important role in dependence on drugs of abuse. The purpose of this review is to address the role of NO in dependence on substances such as opioids, ethanol, psychostimulants and nicotine. Inhibitors of NOS modulate withdrawal from opioids and ethanol, diminishing many signs of withdrawal. In addition, NOS inhibitors suppress signs of withdrawal from nicotine. These data suggest that NO may be involved in the expression of withdrawal signs, and they leave open the possibility that NO may mediate the development of many of these signs. Although preliminary, data to date suggest that glutamate neurotransmission may be related to these beneficial effects of NOS inhibitors on signs of withdrawal. Emerging data further suggest that NO may have a general role in the dependence potential of various classes of drugs of abuse. Thus, modulation of NO systems may be a potential therapeutic target for treatment of substance abuse.

NO synthase and NO-dependent signal pathways in brain aging and neurodegenerative disorders: the role of oxidant/antioxidant balance

Nitric oxide and other reactive nitrogen species appear to play several crucial roles in the brain. These include physiological processes such as neuromodulation, neurotransmission and synaptic plasticity, and pathological processes such as neurodegeneration and neuroinflammation. There is increasing evidence that glial cells in the central nervous system can produce nitric oxide in vivo in response to stimulation by cytokines and that this production is mediated by the inducible isoform of nitric oxide synthase. Although the etiology and pathogenesis of the major neurodegenerative and neuroinflammatory disorders (Alzheimer's disease, amyothrophic lateral sclerosis, Parkinson's disease, Huntington's disease and multiple sclerosis) are unknown, numerous recent studies strongly suggest that reactive nitrogen species play an important role. Furthermore, these species are probably involved in brain damage following ischemia and reperfusion, Down's syndrome and mitochondrial encephalopathies. Recent evidence also indicates the importance of cytoprotective proteins such as heat shock proteins (HSPs) which appear to be critically involved in protection from nitrosative and oxidative stress. In this review, evidence for the involvement of nitrosative stress in the pathogenesis of the major neurodegenerative/ neuroinflammatory diseases and the mechanisms operating in brain as a response to imbalance in the oxidant/antioxidant status are discussed.

Dietary arginine alters time of symptom onset in Huntington's disease transgenic mice

Recent neuroimaging studies reported complex changes in cerebral blood flow (CBF) in early-staged Huntington's disease (HD) patients. Deckel and co-workers [Deckel and Duffy, Brain Res. (in press); Deckel and Cohen, Prog. Neuro-Psychopharmacol. Biol. Psychiatry 24 (2000) 193; Deckel et al., Neurology 51 (1998) 1576; Deckel et al., J. Nucl. Med. 41 (2000) 773] suggested that these findings might be accounted for, in part, by alterations in cerebral nitric oxide (NO) and its byproduct, peroxynitrite. The current experiment tested this hypothesis by altering NO levels via manipulations of dietary L-arginine (ARG), the dietary precursor of NO, in mice transgenic for HD. Seventy-one mice were assigned at 12 weeks of age to one of three isocaloric diets that varied in their content of ARG. These diets included: (a) 0% ARG, (b) 1.2% ARG (i.e. typical mouse chow), or (c) 5% ARG. The 5% ARG diets in HD mice accelerated the time of onset of body weight loss (P<0.05) and motor impairments (P<0.05), and increased resting CBF in HD relative to control (P<0.05). Conversely, the 0% ARG diet demonstrated no loss of body weight and had no changes in CBF relative to controls. However, the 0% ARG HD group continued to show significant deficits on motor testing (P<0. 05). The 1.2% ARG HD group showed reduced body weight loss, better motor functioning, and fewer changes in CBF compared to the 5% ARG HD group. Immunocytochemistry analysis found greater deposition of nitrotyrosine in the cortex, and vasculature, of HD+ mice, 5% and 1. 2%>0% arginine diets. When collapsed across all conditions, CBF inversely correlated (P<0.05) both with the body weight and motor changes suggesting that changes in CBF are associated with behavioral decline in HD mice. Collectively, these findings indicate that dietary consumption of the NO precursor ARG has a measurable, but complex, effect on symptom progression in HD transgenic mice, and implicates NO in the pathophysiology of HD.

Detection of 8-oxoG DNA glycosylase activity and OGG1 transcripts in the rat CNS

The oxoguanine DNA glycosylase (Ogg1) is a DNA repair enzyme that excises 7,8-dihydro-8-oxoguanine present in DNA damaged by oxidative stress. We have investigated the expression of the OGG1 gene in different regions of the rat CNS. Biochemical studies on brain homogenates of adult rats have shown that Ogg1 nicking activity is present at relatively similar levels in the cerebral cortex, the hypothalamus, the pons and the cerebellum. Following in situ hybridization with radiolabeled OGG1 cDNA or specific antisense oligonucleotides, OGG1 transcripts showed a widespread but heterogeneous distribution pattern among distinct brain regions of adult rats: high levels of this transcript were detected in the CA1-CA3 layers and the gyrus dentate of the hippocampal formation, the piriform cortex, the supraoptic nuclei, the olivary complex as well as in the pyramidal cells of layer V of the cortex and the Purkinje cells of the cerebellum. In peripheral organs such as the lungs, the stomach and the spleen, OGG1 transcript is however expressed in specific subpopulations of cells. Using a semi-quantitative reverse transcription - polymerase chain reaction assay on total mRNA from the frontal cortex, OGG1 mRNA was determined to be expressed with relatively the same levels in 1-day-old and 7-day-old rats as well as in adult rats. These results provide evidence for the widespread expression of the OGG1 gene in developing and adult brains.

Basal nitric oxide limits immune, nervous and cardiovascular excitation: human endothelia express a mu opiate receptor

Nitric oxide (NO) is a major signaling molecule in the immune, cardiovascular and nervous systems. The synthesizing enzyme, nitric oxide synthase (NOS) occurs in three forms: endothelial (e), neuronal (n) and inducible (i) NOS. The first two are constitutively expressed. We surmise that in many tissues there is a basal level of NO and that the actions of several signaling molecules initiate increases in cNOS-derived NO to enhance momentary basal levels that exerts inhibitory cellular actions, via cellular conformational changes. It is our contention that much of the literature concerning the actions of NO really deal with i-NOS-derived NO. We make the case that cNOS is responsible for a basal or 'tonal' level of NO; that this NO keeps particular types of cells in a state of inhibition and that activation of these cells occurs through disinhibition. Furthermore, naturally occurring signaling molecules such as morphine, anandamide, interleukin-10 and 17-beta-estradiol appear to exert, in part, their beneficial physiological actions, i.e., immune and endothelial down regulation by the stimulation of cNOS. In regard to opiates, we demonstrate the presence of a human endothelial mu opiate receptor by RT-PCR and sequence determination, further substantiating the role of opiates in vascular coupling to NO release. Taken together, cNOS derived NO enhances basal NO actions, i.e., cellular activation state, and these actions are further enhanced by iNOS derived NO.

Allosteric inhibitors of inducible nitric oxide synthase dimerization discovered via combinatorial chemistry

Potent and selective inhibitors of inducible nitric oxide synthase (iNOS) (EC ) were identified in an encoded combinatorial chemical library that blocked human iNOS dimerization, and thereby NO production. In a cell-based iNOS assay (A-172 astrocytoma cells) the inhibitors had low-nanomolar IC(50) values and thus were >1,000-fold more potent than the substrate-based direct iNOS inhibitors 1400W and N-methyl-l-arginine. Biochemical studies confirmed that inhibitors caused accumulation of iNOS monomers in mouse macrophage RAW 264.7 cells. High affinity (K(d) approximately 3 nM) of inhibitors for isolated iNOS monomers was confirmed by using a radioligand binding assay. Inhibitors were >1,000-fold selective for iNOS versus endothelial NOS dimerization in a cell-based assay. The crystal structure of inhibitor bound to the monomeric iNOS oxygenase domain revealed inhibitor-heme coordination and substantial perturbation of the substrate binding site and the dimerization interface, indicating that this small molecule acts by allosterically disrupting protein-protein interactions at the dimer interface. These results provide a mechanism-based approach to highly selective iNOS inhibition. Inhibitors were active in vivo, with ED(50) values of <2 mg/kg in a rat model of endotoxin-induced systemic iNOS induction. Thus, this class of dimerization inhibitors has broad therapeutic potential in iNOS-mediated pathologies.

Capturing the sublimity of a free radical gas

This paper reviews the work related to nitric oxide (NO) done by the author and his postgraduates and colleagues in the past 7 years in the National University of Singapore. Our work shows that (i) NADPH-d and NO synthase (NOS) are often but not always identical; (ii) NO (as indicated by NADPH-d histochemistry and NOS immunohistochemistry) is generated in some endocrine (thyroid, parathyroid and ultimobranchial glands) and immune (thymus and bursa of Fabricius) organs and the cochlea. It is noted from the above studies that NO could possibly regulate blood flow through the various organs via its presence in the vascular endothelial cells and also via nitrergic neurons innervating the blood vessels. It could also regulate the activity of the secretary cells of these organs by being present in them, as well as acting through nitrergic neurons closely related to them. The paper also examines the Janus-faced nature of NO as a neuroprotective and neurodestructive agent, and the apparent noninvolvement of peroxynitrite and inducible NOS in neuronal death occurring in the red nucleus and nucleus dorsalis after spinal cord hemisection.

Direct evidence of in vivo nitric oxide production and inducible nitric oxide synthase mRNA expression in the brain of living rat during experimental meningitis

To detect endogenous nitric oxide (NO) produced in a rat bacterial meningitis model, the authors applied an electron paramagnetic resonance (EPR) NO-trapping technique. Iron complex with N,N-diethyldithiocarbamate were used as a trapping agent. Experimental meningitis was induced by a mixture of lipopolysaccharide and interferon-gamma. Sequential changes of NO formation under meningitis were observed in rat brain tissue by using X-band (9 GHz) EPR spectroscopy, and endogenous NO was detected in the head of a living rat with a 700-MHz EPR system. Inducible NO synthase mRNA expression in the brain tissues also was proven by using a reverse transcriptase-polymerase chain reaction technique.

Evidence for nitric oxide and nitric oxide synthase activity in proximal stumps of transected peripheral nerves

Nitric oxide may be liberated as an inflammatory mediator within injured peripheral nerve trunks. We evaluated the proximal stumps of injured peripheral nerve stumps that later form neuromas or regenerative nerve sprouts, for evidence of local nitric oxide elaboration and activity. Proximal stumps were created in male Sprague-Dawley rats by sectioning of the sciatic nerve and resection of its distal portions and branches. There was striking physiological evidence of nitric oxide activity at the tips of 48-h and 14-day-old proximal nerve stumps. We detected local nitric oxide-mediated hyperemia of both extrinsic plexus and endoneurial microvessels that was reversible, in a dose-dependent stereospecific fashion, by the broad-spectrum nitric oxide synthase inhibitors, Nomega-nitro-L-arginine-methyl ester or Nomega-nitro-L-arginine, but not by 7-nitroindazole, an inhibitor with relative selectivity for neuronal nitric oxide. Immunohistochemical studies provided evidence for the localization of nitric oxide generators at the same sites. In 48-h but not 14-day stumps increased expression of two isoforms of nitric oxide synthase was detected: endothelial nitric oxide and to a much lesser extent neuronal nitric oxide synthase. Both isoforms appeared in axonal endbulb-like profiles that co-localized with neurofilament immunostaining. Western immunoblots identified a band consistent with endothelial nitric oxide synthase expression. In 14-day stumps with early neuroma formation, but not 48-h stumps, there was staining for immunological nitric oxide synthase in some endoneurial and epineurial macrophages. Total nitric oxide synthase biochemical enzymatic activity, measured by labelled arginine to citrulline conversion, was increased in 14-day but not 48-h stumps. Injured peripheral nerves have evidence of early nitric oxide action, nitric oxide synthase expression and nitric oxide activity in proximal nerve stumps. Nitric oxide may have an important impact on the regenerative milieu.

Mechanism of selective motor neuronal death after exposure of spinal cord to glutamate: involvement of glutamate-induced nitric oxide in motor neuron toxicity and nonmotor neuron protection

In this study, we analyzed the mechanism of selective motor neuronal death, a characteristic of amyotrophic lateral sclerosis, using embryonic rat spinal cord culture. When dissociated cultures were exposed to low-level glutamate (Glu) coadministered with the Glu transporter inhibitor L-trans-pyrrolidine-2,4-decarboxylate (PDC) for 24 hours, motor neurons were selectively injured through N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate receptors. Nitric oxide synthase (NOS) inhibitors attenuated this toxicity, and long-acting nitric oxide (NO) donors damaged motor neurons selectively. Nonmotor neurons survived after exposure to low-dose Glu/PDC, but Glu-induced toxicity was potentiated by coadministration of an NO-dependent guanylyl cyclase inhibitor. In addition, 8-bromo-cyclic GMP, a soluble cyclic GMP analogue, rescued nonmotor neurons, but not motor neurons, exposed to high-dose Glu/PDC. Twenty-four hours' incubation with PDC elevated the number of neuronal NOS-immunoreactive neurons by about twofold compared with controls, and a double-staining study, using the motor neuron marker SMI32, revealed that most of them were nonmotor neurons. These findings suggest that selective motor neuronal death caused by chronic low-level exposure to Glu is mediated by the formation of NO in nonmotor neurons, which inversely protects nonmotor neurons through the guanylyl cyclase-cyclic GMP cascade. Induction of neuronal NOS in nonmotor neurons might enhance both the toxicity of motor neurons and the protection of nonmotor neurons, which could explain the pathology of amyotrophic lateral sclerosis.

Opportunities for the treatment of erectile dysfunction by modulation of the NO axis--alternatives to sildenafil citrate

Erectile function in man depends upon a complex interaction of psychogenic, neurologic, hormonal and vascular factors, and therefore the management of erectile dysfunction (ED) reflects this complexity of control. Therapeutic options include psychological and non-pharmacological approaches as well as drug treatments. The effectiveness of the type-5 cGMP phosphodiesterase inhibitor sildenafil citrate (Viagra) confirms the pivotal role of the NO-cGMP axis in promoting and maintaining erection. Although widely acclaimed, sildenafil leaves many questions unanswered, especially regarding its susceptibility to pharmacokinetic drug interactions, and its safety in patients with ischaemic heart disease and those taking nitrates. In view of the epidemiological link between erectile dysfunction and cardiovascular disease in the elderly, this limitation might have much broader implications. The presently available scientific documentation, although less extensive, indicates that NO donors, such as topically applied nitroglycerin (GTN; for example, 1-2 puffs of an ordinary GTN spray applied to the shaft of the penis), might be a reasonable alternative. Further larger-scale research on the efficacy and tolerability of topical GTN is needed to establish its full therapeutic potential in the treatment of erectile dysfunction.

Nitric oxide synthase inhibitor, aminoguanidine, reduces inflammation and demyelination produced by Theiler's virus infection

This study evaluated effects of the inducible nitric oxide synthase (iNOS) inhibitor, aminoguanidine (AG), on the neuropathology and clinical disease produced by Theiler's murine encephalomyelitis virus (TMEV) DA strain infection. Treatment with AG was started on day 7, 14, 28 or 66 post-inoculation and continued for a minimum of 21 days. Inflammation, demyelination and axonal necrosis were scored in a blinded fashion on spinal cord sections from each mouse. Reduction in inflammation, demyelination and axonal necrosis was observed in mice treated with AG. Apoptosis within the spinal cord parenchyma and perivascular cuffs was significantly decreased. AG treatment resulted in delayed onset of clinical disease.

Tetrahydrobiopterin in the treatment of infantile hypertrophic pyloric stenosis

Evidence is emerging that reduced nitric oxide production may be involved in the pathogenesis of hypertrophic pyloric stenosis. Nitric oxide synthase (NOS) requires tetrahydrobiopterin (BH4) for activity. Four infants with hypertrophic pyloric stenosis were treated with oral BH4 (10 mg/kg/day) for 2.5 days. Although plasma total biopterin increased significantly at 3, 27, and 51 h after BH4 administration, there was no effect on the production of plasma cGMP, nitrite, nitrate, or citrulline. Ultrasound investigations before and after the ingestion of BH4 revealed no changes in the hypertrophic pyloric stenosis. We conclude that oral BH4, in the dose utilized in our investigations, does not modify the cause of hypertrophic pyloric stenosis, presumably because it did not restore nitric oxide production in the nonadrenergic noncholinergic nerves of the enteric nervous system.

Inhibition of nitric oxide synthase enhances peripheral nerve regeneration in mice

We tested the hypothesis that inhibition of nitric oxide synthase (NOS) following transection of the sciatic nerve in the mouse would adversely influence regeneration of myelinated fibers from the proximal stump. NOS was inhibited by N(omega)-nitro-L-arginine-methyl ester (L-NAME; 10 mg/kg i.p.), a broad spectrum NOS inhibitor given twice daily for the first 10 days following nerve transection in Swiss mice. Controls received the inactive enantiomer N(omega)-nitro-D-arginine methyl ester (D-NAME). Regeneration was assessed by serial recordings of the M potential from interosseous muscles of the foot innervated by sciatic-tibial motor fibers and morphometric analysis of myelinated fibers distal to the injury site. Contrary to expectation, M potentials reappeared earlier in the mice treated with L-NAME and were higher in amplitude (reflecting the number of reinnervating motor fibers) at 10 weeks after the injury. In the L-NAME treated mice, the mean axonal diameter of regenerating tibial myelinated fibers was larger and the fiber size histogram was shifted to larger fibers. Inhibition of NOS in a transected peripheral nerve is associated with enhanced regeneration of myelinated fibers. Local elaboration of NO may be toxic to regenerating axons.