Inhibition of nitric oxide synthesis induces a significant reduction in local cerebral blood flow in the rat

nNOS-expressing interneurons control basal and behaviorally evoked arterial dilation in somatosensory cortex of mice

Cortical neural activity is coupled to local arterial diameter and blood flow. However, which neurons control the dynamics of cerebral arteries is not well understood. We dissected the cellular mechanisms controlling the basal diameter and evoked dilation in cortical arteries in awake, head-fixed mice. Locomotion drove robust arterial dilation, increases in gamma band power in the local field potential (LFP), and increases calcium signals in pyramidal and neuronal nitric oxide synthase (nNOS)-expressing neurons. Chemogenetic or pharmocological modulation of overall neural activity up or down caused corresponding increases or decreases in basal arterial diameter. Modulation of pyramidal neuron activity alone had little effect on basal or evoked arterial dilation, despite pronounced changes in the LFP. Modulation of the activity of nNOS-expressing neurons drove changes in the basal and evoked arterial diameter without corresponding changes in population neural activity.

Effects of anesthesia on cerebral blood flow, metabolism, and neuroprotection

Administration of anesthetic agents fundamentally shifts the responsibility for maintenance of homeostasis from the patient and their intrinsic physiological regulatory mechanisms to the anesthesiologist. Continuous delivery of oxygen and nutrients to the brain is necessary to prevent irreversible injury and arises from a complex series of regulatory mechanisms that ensure uninterrupted cerebral blood flow. Our understanding of these regulatory mechanisms and the effects of anesthetics on them has been driven by the tireless work of pioneers in the field. It is of paramount importance that the anesthesiologist shares this understanding. Herein, we will review the physiological determinants of cerebral blood flow and how delivery of anesthesia impacts these processes.

Robust neuroprotective effects of intranasally delivered iNOS siRNA encapsulated in gelatin nanoparticles in the postischemic brain

The therapeutic efficacy of intranasal iNOS siRNA delivery was investigated in the postischemic rat brain after encapsulating on in gelatin nanoparticles (GNPs; diameter 188.0 ± 60.9 nm) cross-linked with 0.0667% glutaraldehyde (GA). Intranasally delivered GNPs were found in extracellular and intracellular compartments of many brain regions, including the olfactory bulb, cerebral cortex, and striatum at 1 hour after infusion and continued to be detected for days. Infarct volumes were markedly suppressed (maximal reduction to 42.1 ± 2.6%) at 2 days after 60 minutes of middle cerebral artery occlusion (MCAO) when iNOS siRNA/GNPs were delivered at 6 hours post-MCAO. In addition, this protective effect was manifested by reductions in neurological and behavioral deficits that were sustained for 2 weeks. Therapeutic potency of iNOS siRNA/GNPs was significantly greater and sustained longer than that of bare siRNA and prolonged and efficient iNOS by iNOS siRNA/GNP is responsible for the robust neuroprotective effect.

Asphyxia and therapeutic hypothermia modulate plasma nitrite concentrations and carotid vascular resistance in preterm fetal sheep

In this study, we tested the hypothesis that cerebral hypoperfusion after asphyxia and induced hypothermia is associated with reduced circulating nitrite levels as an index of nitric oxide synthase (NOS) activity. The preterm fetal sheep at 0.7 gestation (103-104 days, term = 147 days) received 25-minute umbilical cord occlusion, followed by mild whole-body cooling from 30 minutes to 72 hours after occlusion. Occlusion and induced hypothermia were independently associated with reduced carotid vascular conductance (CaVC) from 2 to 72 hours, and with transiently suppressed plasma nitrite levels at 6 hours. There was a significant within-subjects correlation (r(2) = 0.33, P = .002) between CaVC and plasma nitrite values in the first 24 hours after occlusion but not after sham occlusion. These findings suggest that in preterm fetal sheep, changes in NOS activity are an important mediator of changes in carotid vascular tone in the early recovery phase after asphyxia and may help mediate some of the vascular effects of induced hypothermia.

Nitric oxide and cerebrovascular regulation

The cerebrovascular regulation involves highly complex mechanisms to assure that the brain is perfused at all times. These mechanisms depend on all components of the neurovascular units: neurons, glia, and vascular cells. All these cell types can produce nitric oxide (NO), a powerful vasodilator through different NO synthases. Many studies underlined the key role of NO in the maintenance of resting cerebral blood flow (CBF) as well as in the mechanisms that control cerebrovascular tone: autoregulation and neurovascular coupling. However, although the role of NO in the control of CBF has been largely investigated, the complexity of the NO system and the lack of specific NO synthase inhibitors led to still unresolved questions such as the origin of NO and the pathways by which it controls the vascular tone. In this chapter, the role of NO in the regulation of CBF is critically reviewed and discussed in the context of the neurovascular unit and the general principles of cerebrovascular regulation.

Effects of hemodilution after traumatic brain injury in juvenile rats

BACKGROUND

Normovolemic hemodilution (HD) in adult animal studies has shown exacerbation of traumatic brain injury (TBI) lesion volumes. Similar studies in juvenile rats have not been reported and outcomes are likely to be different. This study investigated the effects of normovolemic hemodilution (21% hematocrit) in a juvenile TBI (jTBI) model.

METHODS

Twenty 17-day-old rats underwent moderate cortical contusion impact injury (CCI) and were divided into four groups: CCI/hemodilution (HD) (group HD), CCI/no HD (group C), Sham/HD (group SHD), and Sham/no HD (group S). Regional laser Doppler flowmetry (LDF), edema formation (MRI-T2WI), water mobility assessed using diffusion weighted imaging (MRI-DWI), open field activity tests, and histological analyses were evaluated for lesion characteristics.

RESULTS

Hemodilution significantly increased blood flow in the HD compared to the C group after TBI. T2WI revealed a significantly increased extravascular blood volume in HD at 1, 7, and 14 days post-CCI. Edematous tissue and total contusional lesion volume were higher in HD-treated animals at 1 and 14 days. DWI revealed that HD, SHD, and C groups had elevated water mobility compared to S groups in the ipsilateral cortex and striatum. Histology showed a larger cortical lesion in the C than HD group. Open field activity was increased in HD, C, and SHD groups compared to the S group.

CONCLUSIONS

Hemodilution results in significant brain hyperemia with increased edema formation, extravascular blood volume, and water mobility after jTBI. Hemodilution results in less cortical damage but did not alter behavior. Hemodilution is likely not to be clinically beneficial following jTBI.

Different sources of nitric oxide mediate neurovascular coupling in the lateral geniculate nucleus of the cat

Understanding the link between neuronal responses (NRs) and metabolic signals is fundamental to our knowledge of brain function and it is a milestone in our efforts to interpret data from modern non invasive optical techniques such as fMRI, which are based on the close coupling between metabolic demand of active neurons and local changes in blood flow. The challenge is to unravel the link. Here we show, using spectrophotometry to record oxyhaemoglobin and methemoglobin (surrogate markers of cerebral flow and nitric oxide levels respectively) together with extracellular neuronal recordings in vivo and applying a multiple polynomial regression model, that the markers are able to predict up about 80% of variability in NR. Furthermore, we show that the coupling between blood flow and neuronal activity is heavily influenced by nitric oxide (NO). While NRs show the typical saturating response, blood flow shows a linear behaviour during contrast-response curves, with nitric oxide from different sources acting differently for low and high intensity.

EFFECTS OF HYPERTONIC ARGININE ON CEREBRAL BLOOD FLOW AND INTRACRANIAL PRESSURE AFTER TRAUMATIC BRAIN INJURY COMBINED WITH HEMORRHAGIC HYPOTENSION

Hypertonic saline solutions improve cerebral blood flow (CBF) when used for acute resuscitation from hemorrhagic hypotension accompanying some models of traumatic brain injury (TBI); however, the duration of increased CBF is brief. Because the nitric oxide synthase substrate l-arginine provides prolonged improvement in CBF after TBI, we investigated whether a hypertonic resuscitation fluid containing l-arginine would improve CBF in comparison to hypertonic saline without l-arginine in a model of moderate, paramedian, fluid-percussion TBI followed immediately by hemorrhagic hypotension (mean arterial pressure [MAP] = 60 mm Hg for 45 min). Sprague-Dawley rats were anesthetized with 4.0% isoflurane, intubated and ventilated with 1.5%-2.0% isoflurane in oxygen/air (50:50). After preparation for TBI and measurement of CBF using laser Doppler flowmetry and measurement of intracranial pressure (ICP) using an implanted transducer, rats were subjected to moderate (2.0 atm) TBI, hemorrhaged for 45 min, and randomly assigned to receive an infusion of hypertonic saline (7.5%, 2,400 mOsm total; 6 mL/kg; n = 6) or hypertonic saline with 50, 100, or 300 mg/kg L-arginine (2,400 mOsm; 6 mL/kg; n = 6 in each of the three dose groups) and then monitored for 120 min after the end of infusion. CBF was measured continuously and calculated as a percent of the pre-TBI baseline during the hemorrhage period, after reinfusion of one of the hypertonic arginine solutions, and 30, 60, and 120 min after reinfusion. All four hypertonic solutions initially improved MAP, which, by 120 min after infusion, had decreased nearly to the levels observed during hemorrhage. ICP remained below baseline levels during resuscitation in all groups, although ICP was slightly greater (P = NS) than baseline in the hypertonic saline group. CBF increased similarly in all groups during infusion and then decreased similarly in all groups. At 120 min after infusion, CBF was highest in the group infused with hypertonic saline, but the difference was not significant. We conclude that the improvement of MAP, ICP, and CBF produced by hypertonic saline alone after TBI and hemorrhagic hypotension is not significantly enhanced by the addition of L-arginine at these doses.

Endothelial influences on cerebrovascular tone

The cerebrovascular endothelium exerts a profound influence on cerebral vessels and cerebral blood flow. This review summarizes current knowledge of various dilator and constrictor mechanisms intrinsic to the cerebrovascular endothelium. The endothelium contributes to the resting tone of cerebral arteries and arterioles by tonically releasing nitric oxide (NO•). Dilations can occur by stimulated release of NO•, endothelium-derived hyperpolarization factor, or prostanoids. During pathological conditions, the dilator influence of the endothelium can turn to that of constriction by a variety of mechanisms, including decreased NO• bioavailability and release of endothelin-1. The endothelium may participate in neurovascular coupling by conducting local dilations to upstream arteries. Further study of the cerebrovascular endothelium is critical for understanding the pathogenesis of a number of pathological conditions, including stroke, traumatic brain injury, and subarachnoid hemorrhage.

Effects of selective neuronal nitric oxide synthase inhibition on sleep and wakefulness in the rat

The role played by the unconventional messenger Nitric Oxide (NO) upon the sleep-wake cycle remains controversial. Evidence suggests a positive role of NO on Slow Wave Sleep (SWS) and Paradoxical Sleep (PS) regulation, favoring sleep. However, other studies have found a role of NO upon wakefulness and alertness, inhibiting sleep. Divergences have been explained in part because of the use of different inhibitors of nitric oxide synthases (NOS). The aim of this study is to analyse the effects of a highly selective neuronal NOS inhibitor (3-Bromo7-Nitroindazole) on sleep-wake states in rats. Male Wistar rats were stereotaxically prepared for polysomnography. 3-Bromo-7-Nitroindazole (10, 20, 40 mg/kg, i.p.) dissolved in DMSO 10% filled with saline, or vehicle (DMSO 10% in saline) was administered at the beginning of the light period. Three hours of polygraphic recordings were evaluated for stages of vigilance. Results show dose-dependent effects of 3-Bromo7-Nitroindazole upon sleep: 10 mg/kg decreases duration and number of episodes of deep SWS, increasing duration of light SWS. 20 mg/kg decreased duration of light and deep SWS, while active and quiet wake increased. Deep SWS and PS latency increased. Number of episodes of PS decreased, as well as number of cycles of sleep and time spent asleep. 40 mg/kg reduced duration of deep SWS and increased mean episode duration of light SWS. Therefore, sleep states are affected by selective inhibition of nNOS, reducing in all cases deep SWS. These results support the hypothesis that nitric oxide, produced by nNOS, is involved in sleep processes, favoring sleep.

Nitric oxide from the laterodorsal tegmental neurons: its possible retrograde modulation on norepinephrine release from the axon terminal of the locus coeruleus neurons

Nitric oxide released from the cholinergic neurons in the pons may play important roles in sleep-wake regulation. However, there are few reports demonstrating the mechanisms of nitric oxide release in the cholinergic neurons in the pons. The present study investigated the effects of drug delivery of N-methyl-D-aspartic acid on nitric oxide and the neurotransmitters released in the laterodorsal tegmental nucleus (LDT), one of the major cholinergic cell groups in the pons, in rats by in vivo microdialysis with a view to clarifying nitric oxide functions in the cholinergic system. The application of N-methyl-D-aspartic acid (1 mM) into the LDT induced a significant increase in NO(2)and NO(3) for 40 min (P<0.001). Furthermore the same dose of N-methyl-D-aspartic acid induced a significant increase in cyclic GMP for 30 min (P<0.05), as well as in acetylcholine (P<0.001) and norepinephrine for 15 min (P<0.001). 3-(4-Morpholinyl)-sydonone imine hydrochloride (a nitric oxide donor, 5 mM) also induced significant increase in norepinephrine (P<0.05). Pretreatment with 1 mM 2-amino-5-phosphonopentanoic acid (an antagonist of N-methyl-D-aspartic acid receptor) prevented the N-methyl-D-aspartic acid-induced increase in cyclic GMP (P<0.01), acetylcholine and norepinephrine (P<0.01), while that with 1 mM N(G)-nitro-L-arginine (an inhibitor of nitric oxide synthase) prevented the increase in cyclic GMP (P<0.01) and norepinephrine (P<0.01) but not in acetylcholine. These results suggested that nitric oxide release in the LDT induced by activation of the N-methyl-D-aspartic acid receptor on the cholinergic neurons of the LDT, then through the cyclic GMP system, facilitates norepinephrine release from the terminals of noradrenergic neurons in the locus coeruleus. Based on these findings, we propose a possible role of nitric oxide in the LDT is as a retrograde regulator of norepinephrine release from the locus coeruleus.

Day- and nighttime injection of a nitric oxide synthase inhibitor elicits opposite sleep responses in rats

Previous studies suggest that nitric oxide (NO) may play a role in sleep regulation, particularly in the homeostatic process. The present studies were undertaken to compare the sleep effects of injecting a NO synthase (NOS) inhibitor when homeostatic sleep pressure is naturally highest (light onset) or when it is at its nadir (dark onset) in rats. Sleep, electroencephalogram delta-wave activity during nonrapid eye movement sleep (NREMS), also known as slow-wave activity (SWA), and brain temperature responses to three doses of the NOS inhibitor Nω-nitro-l-arginine methyl ester (l-NAME; 5, 50, and 100 mg/kg) injected intraperitoneally at light or dark onset were examined in rats ( n = 6 to 8). The effects of 5 mg/kg l-NAME were determined in both normal and vagotomized (VX) rats. Light onset administration of 50 mg/kg l-NAME decreased NREMS amounts and suppressed SWA and increased rapid eye movement sleep (REMS) amounts. At dark onset, l-NAME injection also dose dependently suppressed SWA; however, unlike light onset injections, both NREMS and REMS amounts were increased after all three doses. Sleep responses to 5 mg/kg l-NAME were not different in control and VX rats, suggesting that the sleep effects of l-NAME are not mediated through the activation of sensory vagal mechanisms. The present findings suggest that timing of the injection is a major determinant of the sleep responses observed after systemic l-NAME injection in rats.

The pathophysiology of falciparum malaria

Falciparum malaria is a complex disease with no simple explanation, affecting organs where the parasite is rare as well as those organs where it is more common. We continue to argue that it can best be understood in terms of excessive stimulation of normally useful pathways mediated by inflammatory cytokines, the prototype being tumor necrosis factor (TNF). These pathways involve downstream mediators, such as nitric oxide (NO) that the host normally uses to control parasites, but which, when uncontrolled, have bioenergetic failure of patient tissues as their predictable end point. Falciparum malaria is no different from many other infectious diseases that are clinically confused with it. The sequestration of parasitized red blood cells, prominent in some tissues but absent in others with equal functional loss, exacerbates, but does not change, these overriding principles. Recent opportunities to stain a wide range of tissues from African pediatric cases of falciparum malaria and sepsis for the inducible NO synthase (iNOS) and migration inhibitory factor (MIF) have strengthened these arguments considerably. The recent demonstration of bioenergetic failure in tissue removed from sepsis patients being able to predict a fatal outcome fulfils a prediction of these principles, and it is plausible that this will be demonstrable in severe falciparum malaria. Understanding the disease caused by falciparum malaria at a molecular level requires an appreciation of the universality of poly(ADP-ribose) polymerase-1 (PARP-1) and Na(+)/K(+)-ATPase and the protean effects of activation by inflammation of the former that include inactivation of the latter.

Tissue distribution of migration inhibitory factor and inducible nitric oxide synthase in falciparum malaria and sepsis in African children

BACKGROUND

The inflammatory nature of falciparum malaria has been acknowledged since increased circulating levels of tumour necrosis factor (TNF) were first measured, but precisely where the mediators downstream from this prototype inflammatory mediator are generated has not been investigated. Here we report on the cellular distribution, by immunohistochemistry, of migration inhibitory factor (MIF) and inducible nitric oxide synthase (iNOS) in this disease, and in sepsis.

METHODS

We stained for MIF and iNOS in tissues collected during 44 paediatric autopsies in Blantyre, Malawi. These comprised 42 acutely ill comatose patients, 32 of whom were diagnosed clinically as cerebral malaria and the other 10 as non-malarial diseases. Another 2 were non-malarial, non-comatose deaths. Other control tissues were from Australian adults.

RESULTS

Of the 32 clinically diagnosed cerebral malaria cases, 11 had negligible histological change in the brain, and no or scanty intravascular sequestration of parasitised erythrocytes, another 7 had no histological changes in the brain, but sequestered parasitised erythrocytes were present (usually dense), and the remaining 14 brains showed micro-haemorrhages and intravascular mononuclear cell accumulations, plus sequestered parasitised erythrocytes. The vascular walls of the latter group stained most strongly for iNOS. Vascular wall iNOS staining was usually of low intensity in the second group (7 brains) and was virtually absent from the cerebral vascular walls of 8 of the 10 comatose patients without malaria, and also from control brains. The chest wall was chosen as a typical non-cerebral site encompassing a range of tissues of interest. Here pronounced iNOS staining in vascular wall and skeletal muscle was present in some 50% of the children in all groups, including septic meningitis, irrespective of the degree of staining in cerebral vascular walls. Parasites or malarial pigment were rare to absent in all chest wall sections. While MIF was common in chest wall vessels, usually in association with iNOS, it was absent in brain vessels.

CONCLUSIONS

These results agree with the view that clinically diagnosed cerebral malaria in African children is a collection of overlapping syndromes acting through different organ systems, with several mechanisms, not necessarily associated with cerebral vascular inflammation and damage, combining to cause death.

Reduced levels of nitric oxide metabolites in cerebrospinal fluid are associated with equine protozoal myeloencephalitis

Equine protozoal myeloencephalitis (EPM) is a disease of horses that is primarily associated with infection with the apicomplexan Sarcocystis neurona. Infection with this parasite alone is not sufficient to induce the disease, and the mechanism of neuropathogenesis associated with EPM has not been reported. Nitric oxide (NO) functions as a neurotransmitter, a vasodilator, and an immune effector and is produced in response to several parasitic protozoa. The purpose of this work was to determine if the concentration of NO metabolites (NO(x)(-)) in the cerebrospinal fluid (CSF) is correlated with the development of EPM. CSF NO(x)(-) levels were measured before and after transport-stressed, acclimated, or dexamethasone-treated horses (n = 3 per group) were experimentally infected with S. neurona sporocysts. CSF NO(x)(-) levels were also compared between horses that were diagnosed with EPM after natural infection with S. neurona and horses that did not have clinical signs of disease or that showed no evidence of infection with the parasite (n = 105). Among the experimentally infected animals, the mean CSF NO(x)(-) levels of the transport-stressed group, which had the most severe clinical signs, was reduced after infection, while these values were found to increase after infection in the remaining groups that had less severe signs of EPM. Under natural conditions, horses with EPM (n = 65) had a lower mean CSF NO(x)(-) concentration than clinically normal horses with antibodies (Abs) against S. neurona (n = 15) in CSF, and horses that developed ataxia (n = 81) had a significantly lower mean CSF NO(x)(-) concentration than horses that did not have neurologic signs (n = 24). In conclusion, lower CSF NO(x)(-) levels were associated with clinical EPM, suggesting that measurement of CSF NO(x)(-) levels could improve the accuracy of diagnostic tests that are based upon detection of S. neurona-specific Abs in CSF alone and that reduced NO levels could be causally related to the development of EPM.

Nitric oxide synthase inhibitors unmask acetylcholine-mediated constriction of cerebral vessels in the in vitro isolated guinea-pig brain

The control of arterial vascular tone by acetylcholine contributes to the regulation of cerebral blood flow. We analysed the effects of intraluminal application of acetylcholine (1microM) on the cerebral vascular tone by measuring changes in resistance to perfusion pressure in an isolated guinea-pig brain preparation maintained in vitro by arterial perfusion under constant flow. Acetylcholine induced a reproducible, fast-onset dilation that was prevented by the nitric oxide scavenger Methylene Blue (10microM) and by the muscarinic receptor antagonist atropine (0.1microM). Prolonged arterial perfusion with the nitric oxide synthase inhibitors N-nitro-L-arginine (1mM) and N-nitro-L-arginine methyl ester (30-100microM) induced a slowly developing increase of 25.9+/-13. 44mmHg in vascular tone and blocked the acetylcholine-induced vasodilation. In these experimental conditions, the dilation determined by the nitric oxide donor nitroprusside (0.1microM) was unaffected. In five experiments, the blockade of dilation unmasked a slow acetylcholine-mediated vasoconstriction (14.40+/-3.85mmHg) that was antagonized by atropine.The results demonstrate that acetylcholine exerts two simultaneous and opposite effects on guinea-pig cerebral vessels, characterized by a slow direct constriction concealed in physiological conditions by a fast vasodilation mediated through the release of nitric oxide by endothelial cells. Acetylcholine-mediated increase in vascular tone may play a role in aggravating cerebral perfusion when endothelial cell damage occurs during brain ischemia.

Nitric oxide synthase expression in the cerebral cortex of patients with epilepsy

PURPOSE

Nitric oxide (NO), a short-lived radical synthesized from L-arginine by activation of the enzyme nitric oxide synthase (NOS), has been implicated in the pathophysiology of epilepsy by some investigators. However, the current data about NO and NOS in epilepsy are controversial and are derived only from animal models of epilepsy. In this study we investigated possible changes in NOS expression in the cerebral cortex of patients with epilepsy.

METHODS

Qualitative and quantitative parameters of the immunolabeling pattern of the neuronal, endothelial, and inducible isoforms of NOS were analyzed in biopsy material obtained from patients with short and long seizure history and from patients without epilepsy.

RESULTS

The comparative study showed that in the cerebral cortex of patients with epilepsy, particularly in those with a long seizure history, the number and labeling intensity of NOS-positive neurons increased, and that a subpopulation of nonpyramidal GABAergic neurons (type II NOS neurons) was responsible for this phenomenon.

CONCLUSIONS

The fact that NOS upregulation is more evident in patients with a long seizure history suggests that this is a consequence of seizures, acting probably as an adaptative response to the sustained release of excitatory amino acids.

Nitric oxide deficiency contributes to large cerebral infarct size

The purpose of this study was to examine the role played by a deficit in nitric oxide (NO) in contributing to the large cerebral infarcts seen in hypertension. Cerebral infarction was produced in rats by occlusion of the middle cerebral artery (MCA). Studies were performed in Sprague-Dawley (SD) rats subjected to NO synthase blockade (N(G)-nitro-L-arginine [L-NNA], 20 mg x kg(-1) x d(-1) in drinking water) and in spontaneously hypertensive stroke-prone rats (SHRSP). NO released in the brain in response to MCA occlusion was monitored with a porphyrinic microsensor in Wistar-Kyoto rats. The increment in NO released with MCA occlusion was 1.31+/-0.05 micromol/L in L-NNA-treated rats, 1.25+/-0.04 micromol/L in SHRSP, 2. 24+/-0.07 micromol/L in control SD rats, and 2.25+/-0.06 micromol/L in Wistar-Kyoto rats (P<0.0001 for control versus the other groups). Infarct sizes in the L-NNA-treated and control SD rats were 8.50+/-0. 8% and 5.22+/-0.7% of the brain weights, respectively (P<0.05). The basilar arterial wall was significantly thicker in L-NNA-treated rats compared with their controls. We conclude that both the deficit in NO and the greater wall thickness contribute to the larger infarct size resulting from MCA occlusion in SHRSP and in L-NNA-treated rats compared with their respective controls.

Segmental and laminar distributions of nicotinamide adenine dinucleotide phosphate-diaphorase-expressing and neuronal nitric oxide synthase-immunoreactive neurons versus radioassay detection of catalytic nitric oxide synthase activity in the rabbit spinal cord

The distributions of neuronal nitric oxide synthase-immunoreactive neurons and of nicotinamide adenine dinucleotide phosphate-diaphorase activity were studied in the C6, Th2, L1, L5, S2 and S3 segments and laminae in the rabbit spinal cord and compared with the catalytic nitric oxide synthase activity, determined by monitoring the conversion of [3H]arginine to [3H]citrulline in the same segments and laminae. Morphologically, a heterogeneous population of nicotinamide adenine dinucleotide phosphate-diaphorase-expressing and neuronal nitric oxide synthase-immunoreactive neurons was detected in the superficial and deep dorsal horn and the pericentral region in all segments studied, and in the intermediolateral cell column of the thoracic and lumbosacral segments. A disproportionate distribution of both neuronal categories which had a significantly higher number of nicotinamide adenine dinucleotide phosphate-diaphorase-expressing rather than neuronal nitric oxide synthase-immunoreactive cell bodies was found in all segments. The catalytic nitric oxide synthase activity was distributed unequally in the C6, Th2, L1, L5, S2 and S3 segments, with a comparatively low value in the Th2 segment (70 +/- 5.1 d.p.m./microg protein) in comparison with the S3 segment, where the highest level (140 +/- 5.5 d.p.m./microg protein) was found. A close correlation between the number of neuronal nitric oxide synthase-immunoreactive somata and catalytic nitric oxide synthase activity was revealed in the dorsal horn (laminae I-VI). Whereas a low number of neuronal nitric oxide synthase-immunoreactive somata in laminae VII-X was found in the L5, S2 and S3 segments, the values of catalytic nitric oxide synthase activity in the same laminae and segments were found to be exceedingly high. These findings indicate that the occurrence of many neuronal nitric oxide synthase-immunoreactive fibers (mainly axons), and dense, punctate, non-somatic neuronal nitric oxide synthase immunopositivity in the neuropil staining of the same laminae and segments, can substantially enhance catalytic nitric oxide synthase activity.

The role of astrocytes and noradrenaline in neuronal glucose metabolism

In the classical model the energy requirements during neuronal activation are provided by the delivery of additional glucose directly into the extracellular compartment that results from the increase in local cerebral blood flow (rCBF). The present review proposes that astrocytes play a key role in the response to neuronal activation. Arginine for the synthesis of NO, which has a major role in the increase in rCBF, is released from astrocytes in response to stimulation of astrocytic glutamate receptors. The increased delivery of glucose by the blood stream enters astrocytes, where some of it is converted to glycogen. During neuronal activation there is a decrease in extracellular glucose owing to increased utilization followed by a delayed increase; this results from stimulation of astrocytic beta-adrenergic receptors, which leads to a breakdown of glycogen and the export of glucose.

Cerebral hemodynamic effects of phenylephrine and L-arginine after cortical impact injury

OBJECTIVE

To determine the effects of a pressor agent (phenylephrine and L-arginine) on the abnormal cerebral hemodynamics and on neurologic outcome after a severe cortical impact injury in rats.

DESIGN

Prospective, randomized study.

SETTING

University laboratory.

SUBJECTS

Male Long-Evans rats, weighing 300 to 400 g, fasted overnight.

INTERVENTIONS

The animals were anesthetized with isoflurane, and a severe cortical impact injury (velocity, 5 m/sec; deformation, 3 mm) was produced in the right parietal cortex. Five minutes after impact injury, one of the following three treatments were infused: 1 mL saline intravenously for 10 mins, 300 mg/kg L-arginine in 1 mL saline intravenously for 10 mins, or 0.3 microg/kg/min phenylephrine intravenously for 3 hrs. Mean arterial pressure, intracranial pressure (ICP), cerebral perfusion pressure (CPP), and laser Doppler flow (LDF) at the impact site and in the contralateral parietal cortex were monitored for 3 hrs after the impact injury. Histologic examination of the brain was performed at 2 wks after injury in a separate group of L-arginine- and saline-treated animals.

MEASUREMENTS AND MAIN RESULTS

The immediate response to the impact injury was an increase in ICP, and a decrease in mean arterial pressure, CPP, and LDF. In the saline-treated animals, LDF decreased to <25% of the baseline values at the impact site and stayed at that level for the entire 3-hr monitoring period. On the contralateral side, LDF decreased initially and recovered gradually to approximately 50% of the preimpact baseline value. Infusion of both phenylephrine and L-arginine increased LDF back to near-baseline levels. However, phenylephrine increased ICP significantly, whereas ICP with L-arginine did not change. L-arginine treatment reduced the contusion volume from a median value of 5.28 mm3 to 0.63 mm3.

CONCLUSIONS

Phenylephrine increased cerebral blood flow (CBF) by increasing CPP. L-arginine, however, increased CBF without changing CPP. The improvement in CBF was accompanied by a decrease in neurologic injury. Although the pressor agents are used currently to increase CBF after traumatic brain injury, other strategies may also increase CBF without the potential adverse effects of induced hypertension.

Effects of hypoxia and reoxygenation with 21% and 100%-oxygen on cerebral nitric oxide concentration and microcirculation in newborn piglets

Bioelectric sensors for continuous registration of nitric oxide (NO) concentrations in tissues provide a new tool for invasive measurement of this gaseous molecule. This study sought to validate cerebral NO measurements using an amperiometric sensor. A series of experiments in 1- to 3-day-old piglets was carried out to study the response of NO and microcirculation during hypoxia (F(i)O(2) 0.06) and reoxygenation with 100% and 21% oxygen. Two-channel laser Doppler flowmetry was performed in the forebrain cortex. Significant decreases of NO levels were observed immediately after induction of hypoxemia (p < 0.05). During reoxygenation with 21 or 100% O(2) for 30 min, NO increased significantly compared to the values at the end of hypoxia (p < 0.05). The increase of NO levels in the 100% oxygen group was greater than the increase in the 21% oxygen group (p < 0. 05). There were no significant differences between the two groups during the following 3.5 h of observation. A significant increase in CBF was found in the first 2 min of hypoxia (p < 0.05), it then continued to fall to values significantly lower than baseline values at the end of hypoxemia (p < 0.05). During reoxygenation CBF normalised and there were consistent but no significant differences between the two reoxygenation groups. We conclude that NO concentration decreased during the course of hypoxia. Hypoxia-induced cerebral hyperaemia occurred in spite of significantly lower NO concentrations. Reoxygenation with 21 or 100% O(2) restored CBF in both groups similarly, although values were higher after reoxygenation with 100% O(2) compared to air. In fact, reoxygenation with 100% O(2) led to supranormal levels of NO by contrast to 21% O(2).

Acetazolamide-induced cerebral and ocular vasodilation in humans is independent of nitric oxide

Acetazolamide, a carbonic anhydrase inhibitor, is used orally in the treatment of primary and secondary open-angle glaucoma and induces ocular and cerebral vasodilation. Several in vitro studies have shown that carbonic anhydrase pharmacology and the L-arginine-nitric oxide (NO) pathway are closely related. We investigated the role of NO in acetazolamide-induced vasodilation on cerebral and ocular vessels in 12 healthy subjects in the presence or absence of NG-monomethyl-L-arginine (L-NMMA), a NO synthase inhibitor, and in the presence or absence of L-arginine, the precursor of NO. Acetazolamide was administered after pretreatment with either L-NMMA or placebo and either L-arginine or placebo. Pulsatile choroidal blood flow was assessed with laser interferometric measurement of fundus pulsation. In addition, mean blood flow velocity (MFV) in the middle cerebral artery (MCA) and ophthalmic artery (OA) was measured with Doppler sonography. Acetazolamide increased ocular fundus pulsation amplitude (FPA; +27%, P < 0.001) and MFV in the MCA (+38%, P < 0.001) and in the OA (+19%, P = 0.003). Administration of L-NMMA alone reduced FPA (-21%, P < 0.001) and MFV in the MCA (-11%, P = 0. 030) but did not change MFV in the OA. All hemodynamic effects of L-NMMA were reversed by L-arginine. However, neither L-NMMA nor L-arginine altered acetazolamide-induced changes in cerebral or ocular hemodynamic parameters. The present data indicate that acetazolamide-induced hemodynamic changes are not mediated by NO. Which mediators other than NO are involved in the hemodynamic effects as induced by carbonic anhydrase inhibitors remains to be elucidated.

Effects of nitric oxide inhibition on the spread of biotinylated dextran and on extracellular space parameters in the neostriatum of the male rat

Volume transmission in the brain is mediated by the diffusion of neurotransmitters, modulators and other neuroactive substances in the extracellular space. The effects of nitric oxide synthase inhibition on extracellular space diffusion properties were studied using two different approaches, the histological dextran method and the real-time iontophoretic tetramethylammonium method. The spread of biotinylated dextran (mol. wt 3000) in the extracellular space was measured morphometrically following microinjection into the neostriatum of male rats. Two parameters were used to describe the spread of biotinylated dextran in brain tissue, namely, total volume of spread and the mean grey value. The nonspecific nitric oxide synthase inhibitors NG-nitro-L-arginine methyl ester (10-100 mg/kg) and NG-monomethyl-L-arginine acetate (30-200 mg/kg) decreased the total volume of spread of dextran in a dose-dependent manner. 7-Nitroindazole monosodium salt (50-100 mg/kg), a specific neuronal nitric oxide synthase inhibitor, did not change the total volume of spread of dextran. Using the tetramethylammonium method, the extracellular space diffusion properties can be described by the volume fraction (alpha = extracellular space volume/total tissue volume), tortuosity lambda (lambda2 = free diffusion coefficient/apparent diffusion coefficient in tissue), and non-specific uptake kappa' [Nicholson C. and Syková E. (1998) Trends Neurosci. 21, 207-215]. Nitric oxide synthase inhibition by NG-nitro-L-arginine methyl ester (50 mg/kg) had relatively little effect on volume fraction and tortuosity, and no changes were observed after NG-monomethyl-L-arginine acetate (20 mg/kg) or 7-nitroindazole monosodium salt (100 mg/kg) treatment. A substantial increase was found only in non-specific uptake, by 13% after NG-nitro-L-arginine methyl ester and by 16% after NG-monomethyl-L-arginine acetate, which correlates with the decreased total volume of spread of dextran observed with the dextran method. NG-Nitro-L-arginine methyl ester treatment (100 mg/kg) decreased striatal blood flow and increased mean arterial blood pressure. The changes in dextran spread and non-specific uptake can be explained by an increased capillary clearance following the inhibition of endothelial nitric oxide synthase, as neuronal nitric oxide synthase inhibition had no effect. The observed changes after non-specific nitric oxide synthase inhibition may affect the extracellular space concentration of neurotransmitters and modulators, and influence volume transmission pathways in the central nervous system by increased capillary and/or cellular clearance rather than by changes in extracellular space diffusion.

Electroencephalogram changes during inhalation with nitric oxide in the pediatric intensive care patient--a preliminary report

OBJECTIVES

Although endogenous nitric oxide (NO) is an excitatory mediator in the central nervous system, inhaled NO is not considered to cause neurologic side effects because of its short half-life. This study was motivated by a recent case report about neurologic symptoms and our own observation of severe electroencephalogram (EEG) abnormalities during NO inhalation.

DESIGN

Blind, retrospective analyses of EEGs which were registered before, during, and after NO inhalation. EEG was classified in a 5-point rating system by an independent electroencephalographer who was blinded to the patients' clinical histories. Comparisons were made with the previous evaluation documented at recording. Other EEG-influencing parameters such as oxygen saturation, hemodynamics, electrolytes, and pH were evaluated.

SETTING

Pediatric intensive care unit of a tertiary care university children's hospital.

PATIENTS

Eleven ventilated, long-term paralyzed, sedated children (1 mo to 14 yrs) who had EEG or clinical assessment before NO treatment and EEG during NO inhalation. They were divided into two groups according to the NO-indication (e.g., congenital heart defect, acute respiratory distress syndrome).

MEASUREMENTS AND MAIN RESULTS

All 11 patients had an abnormal EEG during NO inhalation. EEG-controls without NO showed remarkable improvement. EEG abnormalities were background slowing, low voltage, suppression burst (n = 2), and sharp waves (n = 2) independent of patients' age, NO-indication, and other EEG-influencing parameters.

CONCLUSIONS

These preliminary data suggest the occurrence of EEG-abnormalities after application of inhaled NO in critically ill children. We found no correlation with other potential EEG-influencing parameters, although clinical state, medication, or hypoxemia might contribute. Comprehensive, prospective, clinical assessment regarding a causal relationship between NO-inhalation and EEG-abnormalities and their clinical importance is needed.

Kainic acid may enhance hippocampal NO generation of awake rats in a seizure stage-related fashion

Temporal changes in kainate-induced seizure activity and hippocampal NO generation were evaluated simultaneously in conscious rats by using in vivo microdialysis and by determining the concentrations of nitrite and nitrate in perfusates. Intraperitoneal injection of kainic acid produced 'wet dog shake', focal seizure of the limbs and neck, hypersalivation, or generalized convulsion. These behavioral changes peaked at 120 min after the drug challenge and lasted for about 100 min. In contrast, the concentrations of NO metabolites, nitrite and nitrate, in the hippocampal perfusates increased rapidly and reached a plateau level at 40 min after the injection, and the level remained high for over 220 min. The increase was more marked in animals presenting severe seizures than those presenting mild ones. Pre-treatment with 25 mg/kg N(G)-nitro-L-arginine methyl ester promoted the severity of kainate-induced seizures, but it suppressed the increase in NO metabolites. These results suggest that kainic acid enhances hippocampal NO generation in a severity-related manner of the induced seizures. The enhanced NO generation upon kainate challenge appears mainly to be involved in seizure suppression.

Effect of MK-801 on seizures induced by exposure to hyperbaric oxygen: comparison with AP-7

The effect of the noncompetitive N-methyl-d-aspartate (NMDA)-receptor antagonist MK-801 on seizures induced by hyperbaric oxygen in relation to changes in cerebral blood flow (CBF) was investigated. Rats were injected with MK-801 (0.005-8 mg/kg) 30 min before exposure to 100% O2 at 5 atm (gauge pressure). MK-801 administration resulted in a biphasic response in seizure latency. Doses of 0.1-4 mg/kg significantly decreased time to EEG and motor seizures, while 8 mg/kg had no effect on seizure latency. MK-801 had no effect on seizure duration. In a dose range 0.1-8 mg/kg MK-801 increased CBF in awake animals, which might be responsible for the decreased seizure latency. The gradual increase in seizure latency with increasing MK-801 doses suggests involvement of an additional factor probably related to the drug's anticonvulsive effect. Unlike MK-801, a competitive NMDA receptor antagonist, AP-7, at a dose 250 mg/kg had no effect on latency to seizures or CBF.

Role of cerebral blood flow in seizures from hyperbaric oxygen exposure

Hyperbaric O2 exposure causes seizures by an unknown mechanism. Cerebral blood flow (CBF) may affect seizure latency, although no studies have demonstrated a direct relationship. Awake rats (male, Sprague-Dawley, 350-450 g), instrumented for measuring electroencephalographic activity (EEG) and CBF (laser-Doppler flowmetry), were exposed to 100% O2 at 4 or 5 atm (gauge pressure) until EEG seizures. Compression with O2 caused vasoconstriction to about 70% of control flow that was maintained for various times. CBF then suddenly, but transiently, increased at a time that was reliably related to seizure latency (r=0.8, p<0.01). Additional animals were treated with agents that have diverse pharmacology and their effects on CBF and latency were measured. Glutamate receptor antagonists MK-801 (1 or 4 mg/kg) and ketamine (20-100 mg/kg) significantly increased CBF by 60-80% and decreased seizure latency from about 17+/-8 min (+/-S.D.) in controls to 5+/-1 and 6+/-2 min, respectively. In opposite, a nitric oxide synthase (NOS) inhibitor, N-nitro-L-arginine (NNA)(25 mg/kg) decreased CBF by about 25% and increased time to seizure to 60+/-16 min. If these effects occur in humans, non-invasive measurement of CBF could potentially improve the safety and reliability of hyperbaric O2 usage in clinical and diving applications. It also appears that the effect of drugs on seizure latency can be explained, at least in part, by their effect on CBF.

Role of nitric oxide in post-ischemic cerebral hyperemia in anesthetized rats

This study was undertaken to determine the extent to which nitric oxide (NO) mechanisms are involved in cerebral hyperemia following global brain ischemia. The vertebral arteries were cauterized through the first alar foramina in anesthetized male Sprague-Dawley rats and followed by 20-min occlusion of the common carotid arteries. Blood flow from the parietal cerebral cortex was measured using laser-Doppler flowmetry. In saline-treated animals, carotid occlusion reduced cerebral blood flow by approximately 95% with a maximal hyperemia of about 400% observed after 15 min of reperfusion. Pre-treatment with the nonspecific NO synthase inhibitor, L-NAME (NG-nitro-L-arginine methyl ester; 2, 10 and 50 mg kg(-1)), produced dose-related depression of post-ischemic hyperemia, whereas D-NAME (10 mg kg(-1)) was inactive. Pre-treatment with L-arginine (300 mg kg(-1), i.v.) prevented L-NAME attenuation of cerebral hyperemia. The selective neuronal NO synthase inhibitor, 7-nitroindazole (30 mg kg(-1)), was without significant depressant effect. These results suggest that NO (largely from vascular endothelium) is instrumental in development of post-ischemic cerebral hyperemia.

Possible role of nitric oxide in autoregulatory response in rat intracerebral arterioles

OBJECTIVE

Cerebral autoregulation is an important regulatory mechanism that maintains a constant cerebral blood flow over a wide range of perfusion pressures. The goal of this study was to determine whether nitric oxide contributes to the autoregulatory response of cerebral arterioles to altered transmural pressure (TMP).

METHODS

Seventy-nine intraparenchymal arterioles (53.6 +/- 3.5 microm mean diameter) isolated from rats were cannulated with micropipettes and pressurized at a TMP of 60 mm Hg (control pressure). Vessel diameters were monitored continuously using a video dimensional analyzer. The autoregulatory diameter responses to varying intraluminal pressures were observed in the presence and absence of a nitric oxide synthase inhibitor, NG-monomethyl-L-arginine (L-NMMA). The effect of L-NMMA-induced constriction on autoregulatory response also was compared with responses after prostaglandin F2alpha and alkalosis-induced constrictions.

RESULTS

Autoregulatory responses were observed over a range from 10 to 90 mm Hg of TMP. Treatment with 10(-4) mol/L L-NMMA constricted arterioles and inhibited the autoregulatory vasodilation to TMP reductions from 60 mm Hg to 10 or 30 mm Hg. In L-NMMA-treated arterioles, elevation in TMP from 60 to 90 mm Hg caused an autoregulatory vasoconstriction. Treatment with alkaline pH 7.65 constricted arterioles to a similar degree as that induced by L-NMMA at 60 mm Hg, and under these conditions, the autoregulatory response remained intact. Arterioles severely constricted with prostaglandin F2alpha showed no significant autoregulatory response.

CONCLUSION

These results suggest that 1) vascular nitric oxide release increases in response to a decrease in TMP from 60 mm Hg, thereby contributing to the autoregulatory vasodilation intrinsic to the vessel during hypotension, 2) arteriolar nitric oxide appears not to be involved in the autoregulatory vasoconstriction induced by elevating TMP from 60 to 90 mm Hg, and 3) a marked increase in vascular tone may affect autoregulatory response.

Does NO regulate the cerebral blood flow response in hypoxia?

OBJECTIVES

To study whether nitric oxide (NO) affects the CBF response to hypoxic and carbon monoxide (CO) hypoxia.

MATERIAL AND METHODS

We incrementally reduced arterial oxygen content in rats, by decreasing the concentration of inspired oxygen (20 rats) or by repeated CO inhalation (20 rats), and measured local CBF using the hydrogen clearance method. Ten animals of each group received 80 mg/kg NO synthase (NOS) inhibitor N-monomethyl-L-arginine intravenously prior to hypoxia, while 10 rats served as controls.

RESULTS

Inhibition of NOS decreased mean CBF by 30% and increased cerebrovascular resistance by 70%. Under hypoxic hypoxia, mean oxygen reactivity of CBF (relative change of CBF to a change of arterial oxygen content) was 7.8%/vol% in control animals and 3.3%/vol% after NOS inhibition (P < 0.02). Under CO hypoxia, mean oxygen reactivity was 7.3%/vol% in control animals and 5.1%/vol% after NOS inhibition (P < 0.05). Inhibition of NOS diminished significantly the cerebral vasodilatory response during hypoxic hypoxia (P < 0.05) but only to a lesser extent during CO hypoxia.

CONCLUSION

These observations suggest that NO is involved in cerebral oxygen vasoreactivity, particularly in severe hypoxia.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Cellular changes induced by chronic nitric oxide inhibition in intact rat basilar arteries revealed by confocal microscopy

BACKGROUND

Cellular aspects of remodelling have not been investigated fully in intact vessels due to lack of appropriate methodology.

OBJECTIVE

To determine the cellular alterations induced by chronic inhibition of nitric oxide (NO) production in intact rat basilar arteries by combined use of perfusion myography and a laser scanning confocal microscope.

METHODS

Wistar-Kyoto rats were treated with 10 mg/kg per day NG-nitro-L-arginine methyl ester (L-NAME) for 3 weeks. Basilar arteries from treated and age-matched Wistar-Kyoto rat controls were mounted on a perfusion myograph, stained with the nuclear dye Hoechst 33342 and fixed under pressure. The segments were mounted on a slide and visualized using the 364 nm line of a laser scanning confocal microscope. MetaMorph software was used to obtain optical sections from the vessel and for morphology determinations.

RESULTS

L-NAME treatment induced hypertension (systolic blood pressure control 129.2+/-2.7 mmHg and SBP L-NAME treatment 176.3+/-5.2 mmHg, P< 0.001). Compared with control rat arteries, arteries from treated rats had a reduced lumen diameter, similar wall thickness and an increased wall: lumen ratio. L-NAME treatment induced specific changes in adventitia, media and intima, namely an increase in number of adventitial cells and in adventitia thickness, a reduction in number of smooth muscle cells with no change in media thickness and reductions in number of endothelial cells, size of nuclei and luminal surface area.

CONCLUSIONS

Hypertension induced by chronic inhibition of NO production is associated with eutrophic remodelling of rat basilar artery. However, within this overall maintenance of constant volume, there are marked cellular changes in adventitia, media and intima. The separate contributions of inhibition of NO production and hypertension to the remodelling process need to be elucidated.

The role of nitric oxide in the cerebrovascular response to hypercapnia

Laser Doppler flowmetry was used to further investigate the role of nitric oxide (NO) in CO2-induced cerebrocortical hyperemia in rats. A second objective was to elucidate the source(s) of the NO involved in the response to hypercapnia. We used the L-arginine analogue N omega-nitro-L-arginine methyl ester (L-NAME) to inhibit NO synthase (NOS) and 7-nitroindazole (7-NI) to selectively inhibit brain or nonendothelial NOS. Rats were anesthetized with a single dose of intraperitoneal (IP) pentobarbital (65 mg/kg) for surgery; 60-90 min later they were ventilated with 1.0% halothane in 30% O2 for 1 h to achieve a steady state. The animals were assigned to one of five groups. A control group (n = 9) was infused with 1 mL of saline. The second group (n = 10) received 20 mg/kg of L-NAME intravenously (IV). A third group (n = 9) also received L-NAME; in addition, cerebrocortical laser Doppler flow (LDF) and mean arterial pressure (MAP) were restored to baseline using the NO donor sodium nitroprusside (SNP). In a fourth group (n = 9), MAP was increased to the level usually seen after L-NAME with an infusion of phenylephrine (0.5-5 micrograms.kg-1.min-1). A fifth group (n = 11) received 7-NI at 40 mg/kg IP. The hypercapnic response of LDF was tested in all groups by adding 5% CO2 to the inspired gas at 30-45 min posttreatment; all changes in LDF were significant. In the control group, hypercapnia induced a 70% +/- 24% increase in LDF. In the L-NAME-treated group, the response was decreased to 36% +/- 22% at a posttreatment LDF that was 25% +/- 13% lower than the pre-L-NAME level. In the group where baseline LDF and MAP were restored with SNP, the CO2 response was 56% +/- 15% (not significant versus control). In the group in which MAP was increased with phenylephrine, the response to hypercapnia was 48% +/- 22% at a posttreatment LDF unchanged from pretreatment. These data suggest that increased vascular tone or the absence of basal NO after NOS inhibition influenced the vasodilator response to hypercapnia. In the 7-NI-treated group the response to hypercapnia was 38% +/- 3%, significantly attenuated at a posttreatment flow only 14% +/- 7% lower than pre-7-NI. We conclude that 1) endothelial NO does not mediate the response to hypercapnia but may have a permissive role in the response and 2) that brain NO may have an important role in response to hypercapnia.

Prenatal development of nicotinamide adenine dinucleotide phosphate-diaphorase activity in the human hippocampal formation

Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry was used to study the development of the neurons metabolizing nitric oxide in the prenatal human hippocampal formation. Strongly reactive non-pyramidal neurons appeared in small numbers in the subplate at 15 weeks, and rapidly increased in this layer, as well as the cortical plate-derived layers between 17 and 24 weeks. The marginal zone also had a few NADPH-d cells at 15 weeks. The pattern of these darkly reactive cells stabilized by 28 weeks, with the somata distributed mostly at the border of the cortex and white matter in the entorhinal cortex and subiculum, or the alveus in Ammon's horn. Moderately stained non-pyramidal neurons appeared in the dentate gyrus by 17 weeks, and increased in this region and Ammon's horn up to 28 weeks. Small, lightly reactive non-pyramidal neurons were first seen by 32 weeks and increased in number by term. They were mainly distributed in layers II/III of the entorhinal cortex and stratum pyramidale of the subiculum and Ammon's horn. NADPH-d positive fibers in the marginal zone were mostly thin and developed between 20 and 28 weeks. In other cortical layers, thick processes from the darkly stained NADPH-d neurons appeared first, then fine fibers appeared more numerous, especially after 28 weeks. NADPH-d processes that arose from non-pyramidal cells were frequently apposed to blood vessels, including those in the hippocampal fissure. In addition, NADPH-d reactivity was also present in pyramidal and granule cells, but this staining was most pronounced between 15 and 24 weeks. The results show three types of distinctly stained NADPH-d interneurons in the fetal human hippocampal formation with different developmental courses and morphology. Also, hippocampal principal neurons transiently express NADPH-d at early fetal ages. Our data correlated with other findings suggest that nitric oxide may play a role in neuronal development in the hippocampal formation by modulating neuronal differentiation and maturation, and regulating blood supply.

Inhibition of nitric oxide generation and lipid peroxidation attenuates hemolysate-induced injury to cerebrovascular endothelium

The mechanisms of hemolysate-induced cerebral injury following subarachnoid hemorrhage are just beginning to be clarified. This study examined the injurious effects of hemolysate on endothelial cells derived from bovine middle cerebral arteries, and evaluated the roles of lipid peroxidation and nitric oxide production in this type of damage. Cultured endothelial cells were grown to confluency on gelatin-coated plates. The cells were characterized as endothelial cells on the basis of morphology. Factor VIII-related antigen staining, and low density lipoprotein (LDL) uptake. Additional cells were grown to confluency on collagen-coated well inserts, and were treated with hemolysate for 24 hours. Prior to hemolysate exposure, cells were treated with: a) an inhibitor of iron-dependent lipid peroxidation (tirilazad mesylate 100 microM), or b) an inhibitor of nitric oxide synthase (either N-nitro-L-arginine: NLA 300 microM, or aminoguanidine: AG at 1.5, 7.5, 15 or 150 microM). Permeability of the tracer, U-14C-sucrose, across the layer of endothelial cells was examined over a 24 hour period. Hemolysate induced a significant increase in the permeability across the endothelial cell layer. Pretreatment with tirilazad mesylate, NLA, or AG attenuated significantly hemolysate-induced changes in the endothelial cell barrier. These findings indicate that free radical generation and lipid peroxidation are critical participants in hemolysate-induced injury to the barrier function of the cerebrovascular endothelium. In addition, the results indicate that endothelial cells provide an adequate source of nitric oxide to damage their own cellular function. Finally, these findings strongly implicate free radical mechanisms in endothelial damage associated with subarachnoid hemorrhage.

Response of spinal cord blood flow to the nitric oxide inhibitor nitroarginine

OBJECTIVE

The extent to which nitric oxide (NO) is involved in the modulation of spinal cord blood flow (SCBF) in the uninjured and injured cord is unknown. To elucidate these questions, the following experiments in anesthetized rats were conducted.

METHODS

Because NO is an unstable free radical with a half-life of seconds, its role can be understood through the study of the NO synthase inhibitor L-NG-nitroarginine (L-NOARG). L-NOARG was administered intravenously for 30 minutes at a dose of 100 or 500 micrograms/kg/min in 12 and 10 uninjured animals, respectively. SCBF fluctuations at C7-T1 were measured using laser doppler flowmetry. In a second set of 12 rats, L-NOARG (500 micrograms/kg/min) was administered 10 minutes before spinal cord injury using a modified aneurysm clip at C7-T1 and continued for 30 minutes thereafter.

RESULTS

In the uninjured animals, L-NOARG was associated with a dose-dependent increase in mean arterial pressure of 20 to 80% above baseline (P = 0.0001), together with a dose-related decrease in SCBF (P = 0.0373). In the injured animals, L-NOARG was associated with a 48% increase in mean arterial pressure. With L-NOARG, the changes in SCBF from baseline after injury were similar to those of noninjured controls (n = 25) and significantly less than injury controls (n = 18) or those receiving phenylephrine (n = 8).

CONCLUSION

NO synthase inhibitors, by reducing available NO, cause systemic vasoconstriction and a decrease in SCBF in the uninjured spinal cord. In the injured spinal cord, the administration of L-NOARG results in a redistribution of blood flow with an augmentation in posttraumatic SCBF at the injury site.

Nitric oxide detection with intracerebral microdialysis: important considerations in the application of the hemoglobin-trapping technique

Nitric oxide (NO.) is involved in processes such as neurotransmission, memory, brain injury, vessel relaxation, etc. To study the functional and pathological roles of NO. in the brain, a reliable method to monitor NO. directly is needed. Since oxyhemoglobin (Hb) has a high affinity for NO. and upon binding is converted quantitatively to methemoglobin (MetHb), spectrophotometry of Hb conversion to MetHb can give a credible measurement of NO. concentration. Although this method is especially promising for in vivo microdialysis, factors can influence the reproducibility and stability, making it difficult to obtain reliable results at low NO. levels. Evaluation of the diffusion rates of NO. and sodium nitroprusside across the microdialysis membrane indicates that NO. readily diffuses through the membrane. By taking into account protein degradation and Hb autoxidation as well as integrating the difference spectra, this assay has a practical differential detection limit of about 7 nM (0.4 pmol) in vivo. We evaluated this method in anesthetized and awake rats by measuring the release of NO. induced by the excitotoxin kainic acid (13 mg/kg, i.p.). A protocol with detailed analytical parameters for NO. monitoring in neurobiological research is given.

Tonic protein kinase C-mediated vasoconstriction is unmasked when nitric oxide synthase is inhibited in cerebral microvessels

Recent evidence indicates that nitric oxide participates in the modulation of vascular tone in a variety of vascular beds, including the parenchymal microvasculature of the brain. The present study examined the role of protein kinase activity in the induction and maintenance of the contractile response when endogenous nitric oxide production is inhibited in parenchymal microvessels of the rat hippocampus. Microvessels in in vitro slices of the hippocampus were monitored using computer-assisted video microscopy. The effects of inhibitors of two kinases, protein kinase C and calcium/calmodulin-dependent protein kinase, on the vasoconstrictor response to NG-nitro-L-arginine (L-NNA) were investigated. The resting luminal diameter of the microvessels examined in this study ranged from 9 to 29 microns. Addition of 100 microM L-NNA to the medium superfusing the slice constricted microvessels by 38.8 +/- 0.6%. The addition of protein kinase inhibitors reversed this constriction in a dose-dependent manner. H-7 (50 microM), a relatively non-selective protein kinase C inhibitor, elicited an 81.4 +/- 10.0% reversal of the L-NNA-induced constriction. Bisindolylmaleimide (5 microM), a selective protein kinase C inhibitor, reversed the constriction by 69.1 +/- 13.7%. KN-62, an inhibitor of calcium/calmodulin-dependent protein kinase II, elicited a smaller yet statistically significant reversal of 17.1 +/- 5.1%. Pretreatment with H-7 or bisindolyl-maleimide blocked the LNNA-induced constriction entirely, while KN-62 did not significantly inhibit the response. These findings indicate that the contractile response observed upon removal of endogenous nitric oxidergic vasodilation is mediated by protein kinase activity, and the contribution of protein kinase C to this effect is greater than that of calcium/calmodulin-dependent protein kinase II. The results suggest that a tonic nitric oxidergic influence serves to mask the potential for protein kinase C-mediated vasoconstriction in cerebral microvessels.

Limited role for nitric oxide in mediating cerebrovascular control of newborn piglets

AIMS

To investigate the effects of the nitric oxide (NO) synthase inhibitor L-nitro-arginine methyl ester (L-NAME) on cerebral blood flow, and its response to alterations in arterial carbon dioxide tension (CBF-CO2 reactivity).

METHODS

Cerebral blood flow was measured six times at varying arterial carbon dioxide tension (PaCO2) using the intravenous 133Xenon clearance technique in eight mechanically ventilated piglets of less than 24 hours postnatal age. After the third measurement L-NAME was administered as a bolus (20 mg/kg) and subsequently infused (10 mg/kg/hour).

RESULTS

PaCO2 ranged between 2.7-8.9 kPa. Cerebral blood flow decreased by 14.0% (95% confidence interval 1.9-27.4) after L-NAME. CBF-CO2 reactivity was 18.4% per kPa (95% CI 14.1-22.2) before L-NAME and 15.2%/kPa (95% CI 11.1-19.3) afterwards; the difference between the CBF-CO2 reactivities was 3.2%/kPa (95% CI -0.4-6.8): these were not significantly different.

CONCLUSIONS

Inhibition of nitric oxide synthesis reduces cerebral blood flow no more than a 0.5-1.0 kPa fall in PaCO2. Nitric oxide is not an important mediator of CBF-CO2 reactivity.

The role of nitric oxide in soman-induced seizures, neuropathology, and lethality

The effects of the inhibitors of endothelial and neuronal nitric oxide (NO) synthases, N-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI), respectively, and the precursor of NO, glyceryl trinitrate, on soman-induced seizures, lethality, and neuropathology were studied in rats. It was found that pretreatment of rats with L-NAME and 7-NI potentiated the severity of motor convulsions and enhanced lethality produced by soman. On the other hand, glyceryl trinitrate, administered transdermally at doses ranging from 2.5-5 mg/day 1 day before soman, decreased seizure susceptibility and lethality in soman-intoxicated animals. This was accompanied by a subsequent reduction of central neuronal damage 24 h after soman treatment. Pretreatment with glyceryl trinitrate also reversed seizure latency produced by 7-NI treatment during soman intoxication. These results indicate that neuronal NO may play a prominent role in seizures by acting as an anticonvulsant and neuroprotectant in soman intoxication.