Prospective Multicenter Study of Changes in MTT after Aneurysmal SAH and Relationship to Delayed Cerebral Ischemia in Patients with Good- and Poor-Grade Admission Status.

BACKGROUND AND PURPOSE: Patients with aneurysmal SAH and good clinical status at admission are considered at a lower risk for delayed cerebral ischemia. Prolonged MTT may be associated with an increased risk. It is unclear whether this is dependent on clinical status. Our purpose was to determine whether increased MTT within 3 days of aneurysmal SAH compared with baseline is associated with a higher risk of delayed cerebral ischemia in patients with good (World Federation of Neurosurgical Societies I-III) versus poor (World Federation of Neurosurgical Societies IV-V) admission status. MATERIALS AND METHODS: This prolonged MTT was a multicenter, prospective cohort investigation of 87 patients with aneurysmal SAH. MTT was measured at admission before aneurysm treatment (MTT1) and following repair (MTT2) within 3 days of admission; MTTdiff was calculated as the difference between MTT2 and MTT1. Changes in MTT across time were assessed with repeated measures analyses. Risk of delayed cerebral ischemia or death was determined with multivariate logistic regression analysis. RESULTS: In patients with a good grade (n = 49), MTT was prolonged in patients who developed delayed cerebral ischemia, with MTTdiff significantly greater (0.82 ± 1.5) compared with those who did not develop delayed cerebral ischemia (-0.14 ± 0.98) (P = .03). Prolonged MTT was associated with a significantly higher risk of delayed cerebral ischemia or death (OR = 3.1; 95% CI, 1.3-7.4; P = .014) on multivariate analysis. In patients with poor grades (n = 38), MTTdiff was not greater in patients who developed delayed cerebral ischemia; MTT1 was significantly prolonged compared with patients with a good grade. CONCLUSIONS: Patients in good clinical condition following aneurysmal SAH but with increasing MTT in the first few days after aneurysmal SAH are at high risk of delayed cerebral ischemia and warrant close clinical monitoring.

Role of von Willebrand factor and ADAMTS-13 in early brain injury after experimental subarachnoid hemorrhage.

Essentials von Willebrand Factor (VWF) and ADAMTS13 may affect early injury after subarachnoid hemorrhage (SAH). Early brain injury was assessed in VWF-/- , ADAMTS13-/- and recombinant (r) ADAMTS13 treated mice. VWF-/- and rADAMTS13 treated mice had less brain injury than ADAMTS13-/- and wild-type mice. Early administration of rADAMTS13 may improve outcome after SAH by reducing early brain injury. SUMMARY: Background Early brain injury is an important determinant of poor functional outcome and case fatality after aneurysmal subarachnoid hemorrhage (SAH), and is associated with early platelet aggregation. No treatment exists for early brain injury after SAH. We investigated whether von Willebrand factor (VWF) is involved in the pathogenesis of early brain injury, and whether ultra-early treatment with recombinant ADAMTS-13 (rADAMTS-13) reduces early brain injury after experimental SAH. Methods Experimental SAH in mice was induced by prechiasmatic injection of non-anticoagulated blood from a littermate. The following experimental SAH groups were investigated: C57BL/6J control (n = 21), VWF-/- (n = 25), ADAMTS-13-/- (n = 23), and C57BL/6J treated with rADAMTS-13 (n = 26). Mice were killed at 2 h after SAH. Primary outcome measures were microglial activation (IBA-1 surface area) and neuronal injury (number of cleaved caspase-3-positive neurons). Results As compared with controls, microglial activation was decreased in VWF-/- mice (mean difference of - 20.0%, 95% confidence interval [CI] - 4.0% to - 38.6%), increased in ADAMTS-13-/- mice (mean difference of + 34.0%, 95% CI 16.2-51.7%), and decreased in rADAMTS-13-treated mice (mean difference of - 22.1%, 95% CI - 3.4% to - 39.1%). As compared with controls (185 neurons, interquartile range [IQR] 133-353), neuronal injury in the cerebral cortex was decreased in VWF-/- mice (63 neurons, IQR 25-78), not changed in ADAMTS-13-/- mice (53 neurons, IQR 26-221), and reduced in rADAMTS-13-treated mice (45 neurons, IQR 9-115). Conclusions Our findings suggest that VWF is involved in the pathogenesis of early brain injury, and support the further study of rADAMTS-13 as a treatment option for early brain injury after SAH.

Construction of a coarse-grain quasi-classical trajectory method. II. Comparison against the direct molecular simulation method.

This work presents the analysis of non-equilibrium energy transfer and dissociation of nitrogen molecules (N2(Σg+1)) using two different approaches: the direct molecular simulation (DMS) method and the coarse-grain quasi-classical trajectory (CG-QCT) method. The two methods are used to study thermochemical relaxation in a zero-dimensional isochoric and isothermal reactor in which the nitrogen molecules are heated to several thousand degrees Kelvin, forcing the system into strong non-equilibrium. The analysis considers thermochemical relaxation for temperatures ranging from 10 000 to 25 000 K. Both methods make use of the same potential energy surface for the N2(Σg+1)-N2(Σg+1) system taken from the NASA Ames quantum chemistry database. Within the CG-QCT method, the rovibrational energy levels of the electronic ground state of the nitrogen molecule are lumped into a reduced number of bins. Two different grouping strategies are used: the more conventional vibrational-based grouping, widely used in the literature, and energy-based grouping. The analysis of both the internal state populations and concentration profiles show excellent agreement between the energy-based grouping and the DMS solutions. During the energy transfer process, discrepancies arise between the energy-based grouping and DMS solution due to the increased importance of mode separation for low energy states. By contrast, the vibrational grouping, traditionally considered state-of-the-art, captures well the behavior of the energy relaxation but fails to consistently predict the dissociation process. The deficiency of the vibrational grouping model is due to the assumption of strict mode separation and equilibrium of rotational energy states. These assumptions result in errors predicting the energy contribution to dissociation from the rotational and vibrational modes, with rotational energy actually contributing 30%-40% of the energy required to dissociate a molecule. This work confirms the findings discussed in Paper I [R. L. Macdonald et al., J. Chem. Phys. 148, 054309 (2018)], which underlines the importance of rotational energy to the dissociation process, and demonstrates that an accurate non-equilibrium chemistry model must accurately predict the deviation of rovibrational distribution from equilibrium.

Construction of a coarse-grain quasi-classical trajectory method. I. Theory and application to N2-N2 system.

This work aims to construct a reduced order model for energy transfer and dissociation in non-equilibrium nitrogen mixtures. The objective is twofold: to present the Coarse-Grain Quasi-Classical Trajectory (CG-QCT) method, a novel framework for constructing a reduced order model for diatom-diatom systems; and to analyze the physics of non-equilibrium relaxation of the nitrogen molecules undergoing dissociation in an ideal chemical reactor. The CG-QCT method couples the construction of the reduced order model under the coarse-grain model framework with the quasi-classical trajectory calculations to directly construct the reduced model without the need for computing the individual rovibrational specific kinetic data. In the coarse-grain model, the energy states are lumped together into groups containing states with similar properties, and the distribution of states within each of these groups is prescribed by a Boltzmann distribution at the local translational temperature. The required grouped kinetic properties are obtained directly by the QCT calculations. Two grouping strategies are considered: energy-based grouping, in which states of similar internal energy are lumped together, and vibrational grouping, in which states with the same vibrational quantum number are grouped together. A zero-dimensional chemical reactor simulation, in which the molecules are instantaneously heated, forcing the system into strong non-equilibrium, is used to study the differences between the two grouping strategies. The comparison of the numerical results against available experimental data demonstrates that the energy-based grouping is more suitable to capture dissociation, while the energy transfer process is better described with a vibrational grouping scheme. The dissociation process is found to be strongly dependent on the behavior of the high energy states, which contribute up to 50% of the dissociating molecules. Furthermore, up to 40% of the energy required to dissociate the molecules comes from the rotational mode, underscoring the importance of accounting for this mode when constructing non-equilibrium kinetic models. In contrast, the relaxation process is governed primarily by low energy states, which exhibit significantly slower transitions in the vibrational binning model due to the prevalence of mode separation in these states.

Management of aneurysmal subarachnoid hemorrhage.

Spontaneous subarachnoid hemorrhage (SAH) affects people with a mean age of 55 years. Although there are about 9/100 000 cases per year worldwide, the young age and high morbidity and mortality lead to loss of many years of productive life. Intracranial aneurysms account for 85% of cases. Despite this, the majority of survivors of aneurysmal SAH have cognitive deficits, mood disorders, fatigue, inability to return to work, and executive dysfunction and are often unable to return to their premorbid level of functioning. The main proven interventions to improve outcome are aneurysm repair in a timely fashion by endovascular coiling rather than neurosurgical clipping when feasible and administration of nimodipine. Management also probably is optimized by neurologic intensive care units and multidisciplinary teams. Improved diagnosis, early aneurysm repair, administration of nimodipine, and advanced neurointensive care support may be responsible for improvement in survival from SAH in the last few decades.

Mechanisms of microthrombi formation after experimental subarachnoid hemorrhage.

Microcirculatory dysfunction may contribute to delayed cerebral ischemia after subarachnoid hemorrhage (SAH). Using a prechiasmatic injection model, this study investigated ultrastructural changes in microvessels in brain parenchyma to determine the nature of the microthromboemboli, the involvement of nitric oxide (NO) and P-selectin in their formation, and relationship to brain injury after SAH. Brains were examined by electron microscopy (EM) and immunohistochemistry. EM demonstrated that mice with SAH had significantly more arterioles filled with lesions consistent with microthrombi (in cortex, 20 ± 5 for SAH, 8 ± 4 saline-injected and 2.4 ± 0.2 for sham). SAH animals also had more constriction of arterioles. The concentration of NO was lower in mice with SAH (44 ± 9 for sham, 46 ± 20 for saline-injected and 24 ± 11 for SAH). The number of microthrombi correlated with the number of apoptotic neuronal cells (R(2)=0.80 in cortex). Cell membrane P-selectin increased in the endothelium of arterioles in mice with SAH (11.4 ± 0.7 for SAH, 6.8 ± 0.9 for sham and 6.1 ± 0.9 for saline-injected controls). This correlated with decreased NO in the brain. In conclusion, SAH causes microthrombosis and constriction of arterioles, which correlates with neuronal cell death. Increased P-selectin and decreased NO suggest a mechanism for microthrombosis and arteriolar constriction.

Maternal transmission of a rare GABRB3 signal peptide variant is associated with autism.

Maternal 15q11-q13 duplication is the most common copy number variant in autism, accounting for ∼1-3% of cases. The 15q11-q13 region is subject to epigenetic regulation, and genomic copy number losses and gains cause genomic disorders in a parent-of-origin-specific manner. One 15q11-q13 locus encodes the GABA(A) receptor ß3 subunit gene (GABRB3), which has been implicated by several studies in both autism and absence epilepsy, and the co-morbidity of epilepsy in autism is well established. We report that maternal transmission of a GABRB3 signal peptide variant (P11S), previously implicated in childhood absence epilepsy, is associated with autism. An analysis of wild-type and mutant ß3 subunit-containing α1ß3γ2 or α3ß3γ2 GABA(A) receptors shows reduced whole-cell current and decreased ß3 subunit protein on the cell surface due to impaired intracellular ß3 subunit processing. We thus provide the first evidence of an association between a specific GABA(A) receptor defect and autism, direct evidence that this defect causes synaptic dysfunction that is autism relevant and the first maternal risk effect in the 15q11-q13 autism duplication region that is linked to a coding variant.

Learning deficits after experimental subarachnoid hemorrhage in rats.

About 50% of subarachnoid hemorrhage (SAH) survivors have cognitive or neurobehavioral dysfunction. The mechanisms are not known. This study characterized behavioral deficits in a rat SAH model, and correlated these changes with histological alterations. SAH was induced by injection of 0.3 ml blood into the prechiasmatic cistern. Cognitive and memory changes were investigated in the Morris water maze. Neuronal cell death was evaluated by fluoro-jade and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining. Vasospasm was assessed on cross-sections of middle and anterior cerebral arteries. Microthromboemboli were quantified by fibrinogen staining. Escape latency and swimming distance were significantly increased in rats with SAH as compared to controls (P<0.05-0.001). SAH rats tended to do poorly on accuracy in spatial and working memory tests. SAH rats had a significantly higher number of fluoro-jade and TUNEL positive neurons in CA1 and CA3 of the hippocampus, cerebral cortex and Purkinje cells in the cerebellum (P<0.05-0.001). The number of microthromboemboli in the cortex and cerebellum were significantly higher after SAH than in controls (P<0.05-0.001). Cognitive deficits were induced by SAH in rats. There was a significant increase in apoptotic neurons in all regions of brain examined. However, cell death in the hippocampus was not sufficient to cause the neurobehavioral deficits observed in the Morris water maze. This suggests that other factors such as dysfunction of neurotransmission or plasticity in hippocampal pathways might contribute to the impairments.

Loss of long-term potentiation in the hippocampus after experimental subarachnoid hemorrhage in rats.

Survivors of aneurysmal subarachnoid hemorrhage (SAH) often suffer from cognitive impairment such as memory loss. However, the underlying mechanisms of these impairments are not known. Long-term potentiation (LTP) of synapses in the hippocampus is generally regarded as a molecular substrate of memory. The purpose of this study was to examine the effect of SAH on LTP in the hippocampal Schaffer collateral (CA3-CA1) pathway in a rat model of SAH. We found SAH caused significant vasospasm of the middle cerebral artery (MCA) compared to saline injected or sham controls (P<0.001). Basic neurotransmission quantified as excitatory post synaptic and spike response from animals with SAH were significantly decreased as compared to naive controls (P<0.05). However, sham operated and saline injected controls showed similar amplitude as naive controls. This suggests that reduction in basic neurotransmission is due to blood in the subarachnoid space. Similarly, analysis of LTP demonstrated that naive, sham and saline controls have a 92+/-16%, 69+/-27% and 71+/-14% increase over the baseline in the average spike amplitude following high frequency stimulation (HFS), respectively. This indicates the presence of LTP (P<0.05). In contrast, the spike amplitude in animals of SAH returned to baseline level within 60 min post HFS indicating the absence of LTP. We conclude that SAH caused vasospasm of the MCA that was associated with disrupted basic neurotransmission and plasticity at CA3-CA1 synapses. These changes might be accountable for the memory loss in humans with SAH.

Achieving synaptically relevant pulses of neurotransmitter using PDMS microfluidics.

Fast synaptic transmission is mediated by post-synaptic ligand-gated ion channels (LGICs) transiently activated by neurotransmitter released from pre-synaptic vesicles. Although disruption of synaptic transmission has been implicated in numerous neurological and psychiatric disorders, effective and practical methods for studying LGICs in vitro under synaptically relevant conditions are unavailable. Here, we describe a novel microfluidic approach to solution switching that allows for precise temporal control over the neurotransmitter transient while substantially increasing experimental throughput, flexibility, reproducibility, and cost-effectiveness. When this system was used to apply ultra-brief ( approximately 400micros) GABA pulses to recombinant GABA(A) receptors, members of the cys-loop family of LGICs, the resulting currents resembled hippocampal inhibitory post-synaptic currents (IPSCs) and differed from currents evoked by longer, conventional pulses, illustrating the importance of evaluating LGICs on a synaptic timescale. This methodology should therefore allow the effects of disease-causing mutations and allosteric modulators to be evaluated in vitro under physiologically relevant conditions.

Vasospasm after aneurysmal subarachnoid hemorrhage: need for further study.

Cerebral vasospasm is the classic cause of delayed neurological deterioration leading to cerebral ischemia and infarction, and thus, poor outcome and occasionally death, after aneurysmal subarachnoid hemorrhage (SAH). Advances in diagnosis and treatment, principally nimodipine, intensive care management, hemodynamic manipulations, and endovascular neuroradiology procedures, have improved the prospects for these patients, but outcomes remain disappointing. A phase 2b clinical trial (CONSCIOUS-1) demonstrated marked prevention of vasospasm with the endothelin antagonist, clazosentan, yet patient outcome was not improved. The most likely explanation is that the study was underpowered to detect the relatively small improvements in outcome that would be seen with prevention of vasospasm, especially when assessed using relatively insensitive measures such as the modified Rankin and Glasgow outcome scales. Other possible explanations for this result are that adverse effects of treatment affected the beneficial effects of the drug. It also is possible that alternative causes of neurological deterioration and poor outcome after SAH, including delayed effects of acute global cerebral ischemia, thromboembolism, microcirculatory dysfunction, and cortical spreading depression, play a role. Clazosentan reduced angiographic vasospasm in a dose-dependent manner in patients with aneurysmal SAH following coiling or clipping of the aneurysm. Reducing the incidence of vasospasm should have an important effect on clinical outcome. A phase 3 clinical trial (CONSCIOUS-2) will focus on quantifying this outcome in patients undergoing aneurysm clipping receiving placebo or 5 mg/h of clazosentan.

A novel GABRG2 mutation associated with febrile seizures.

Mutations in the gene encoding the gamma2 subunit of the gamma-aminobutyric acid type A receptor (GABRG2) have been reported to cause childhood absence epilepsy (CAE), febrile seizures (FS), and generalized epilepsy with FS plus (GEFS+). The authors analyzed GABRG2 in 47 unrelated patients with CAE, FS, and GEFS+ and identified a novel mutation that cosegregated with FS. Electrophysiologic studies demonstrated altered current desensitization and reduced benzodiazepine enhancement in mutant receptors.

Angiographic, hemodynamic and histological characterization of an arteriovenous fistula in rats.

BACKGROUND: Our understanding of the pathogenesis of arteriovenous malformations (AVMs) and arteriovenous fistulas (AVFs) has been limited by the lack of adequate animal models. In this study we evaluate the time course of angiographic, hemodynamic and histopathological changes in an arteriovenous fistula in rats as a potential model. METHODS: An arteriovenous fistula was created by a side-to-end anastomosis of the common carotid artery (CCA) to the external jugular vein (EJV). The animals underwent angiography of the fistula and were sacrificed 1, 7, 21, 42 or 90 days later. Flow and pressure measurements were performed in the CCA and ipsi- and contralateral EJV and detailed histological examination of whole mount sections of the fistula and cranium were done on fixed sections. Immunohistochemistry for CD31, smooth muscle alpha-actin and Ki-67 were performed. FINDINGS: Hemodynamic changes occur immediately after fistula formation creating a stable high flow, low resistant state. This induces a gradual increase in the inner diameter of the EJV and transverse sinus followed by a decrease in size of the transverse sinus. This decrease is associated with increased expression of alpha-actin in the wall of the sinus. The fistula becomes angiographically and histologically stable after 21 days. CONCLUSION: This model describes the time course of hemodynamic and histopathological changes after occur after AVF formation. Stabilization after 21 days makes it an attractive model for mechanistic and therapeutic studies of AVFs.

A longitudinal neuropsychological study of partial brain radiation in adults with brain tumors.

OBJECTIVE: To investigate longitudinal cognitive functioning in patients with brain tumor treated with modern highly conformal fractionated partial brain radiation therapy (RT). METHODS: Seventeen (of 22 initial consecutive patients) adults with primarily low-grade brain neoplasms who underwent either biopsy or tumor resection were tested at pre-RT baseline and at 3, 6, 12, and 24 months after baseline. Participants were classified as RT-treated nonprogressors (n = 12) or progressors (n = 3) based on serial follow-up structural imaging. Two patients received surgery only and served as controls to help minimize surgical, practice, test form, or other potential non-RT effects. Serial neuropsychological assessments were conducted using alternate forms of the Selective Reminding Test, 10/36 Spatial Recall Test, and Symbol Digit Modality Test (oral, written) as well as the Shipley Scale (baseline only), Wechsler Adult Intelligence Scale-Revised Digit Span, Trail Making Test, and the Symptom Checklist-90-Revised Global Severity Index scale. RESULTS: There was evidence of subtle attention and memory improvement in RT-treated nonprogressors throughout the 2-year period, with no evidence of cognitive decline. In contrast, patients with disease progression evidenced more substantial decline in memory and attention. CONCLUSIONS: Partial brain fractionated RT was not associated with adverse neuropsychological effects through the first 2 years following therapy.

Alpha1 and alpha6 subunits specify distinct desensitization, deactivation and neurosteroid modulation of GABA(A) receptors containing the delta subunit.

GABA(A) receptor alpha subunit subtypes have distinct CNS distributions and confer different pharmacological and biophysical properties to alphabetagamma receptor isoforms. However, the alpha subtype-dependent properties of alphabetadelta receptor isoforms that may be targeted to extrasynaptic sites remain poorly understood. We investigated the properties of alpha1beta3delta and alpha6beta3delta receptor currents evoked by concentration jumps using a saturating GABA concentration (1 mM). alpha1beta3delta receptor currents desensitized slowly, deactivated rapidly and displayed voltage-dependence only of peak amplitude. In contrast, alpha6beta3delta receptor currents had voltage-dependent increased desensitization and slower deactivation, but did not show rectification. The neurosteroid THDOC (1 microM) enhanced alpha1beta3delta more than alpha6beta3delta currents, but increased the extent of desensitization and prolonged deactivation for both receptor isoforms. alpha1-alpha6 and alpha6-alpha1 chimeras (spliced in transmembrane domain 1) suggested that differences in deactivation rate and its voltage-dependence correlated with N-terminal domains, while the extent of desensitization and its voltage-dependence correlated with C-terminal domains. Both chimeras showed outward rectification. alpha1 subunit-like THDOC enhancement was observed with the alpha1-alpha6 chimera, but the alpha6-alpha1 chimera did not confer alpha6 subunit-like enhancement, suggesting that multiple alpha1 subunit domains contributed to neurosteroid efficacy. Thus, alpha subunit subtypes may regulate the kinetic and pharmacological properties of tonic neuronal inhibition.

Agonist Trapping by GABAA Receptor Channels.

GABAergic IPSCs have a relatively slow decay (deactivation) that appears to result from GABA(A) receptor channel openings that occur well beyond the predicted duration of free GABA at central synapses. Open and desensitized states have been suggested to prevent dissociation of agonist from the receptor, thus prolonging deactivation. However, simultaneous assessment of GABA binding and channel gating has not been possible. We developed a functional assay for occupancy of the GABA binding site or sites to test the GABA "trapping" hypothesis. Deactivation currents were compared in the absence and presence of bicuculline, a competitive antagonist that also allosterically inhibits GABA(A) receptors. This provided a model-independent, functional test of the hypothesis that GABA is trapped on the receptor during gating: bicuculline could only inhibit the channel if it was open but unbound by GABA. Although bicuculline inhibited spontaneous and neurosteroid-activated GABA(A) receptor currents, it failed to alter the deactivation time course of GABA-activated GABA(A) receptor currents. Protection of deactivation current from bicuculline block indicated that GABA remained bound to the receptors while the channel was open, thus suggesting that all open states, as well as all closed and desensitized states from which channel opening can occur, must be GABA liganded states. Trapping may be specific to agonists, because the positive allosteric modulator diazepam unbound from GABA(A) receptors independent of GABA binding and channel activity.

Mutation of the 9' leucine in the GABA(A) receptor gamma2L subunit produces an apparent decrease in desensitization by stabilizing open states without altering desensitized states.

A conserved leucine near the middle (9' position) of the second transmembrane domain of ligand-gated ion channels has been implicated in both gating and desensitization. Specifically, L9'S and L9'T mutations decreased agonist EC50, decreased apparent desensitization and prolonged deactivation in members of the LGIC superfamily, suggesting that this residue may regulate channel properties including desensitization. GABA(A) receptors desensitize in three phases, but in previous 9' leucine studies, only slow phases of desensitization were resolved. We used excised patches containing alpha1beta3gamma2L or alpha1beta3gamma2L(L9'S) GABA(A) receptors and combined single channel recording and concentration jump techniques to reevaluate the effects of this mutation on desensitization. Although desensitization extent was decreased in mutated channels, desensitization still occurred in three phases, suggesting that desensitized states may be intact. Interestingly, deactivation rate was slowed by the mutation, opposite to that expected if desensitization was attenuated. alpha1beta3gamma2L(L9'S) receptor single channels had increased open durations. Simulations revealed that stabilizing the open state (by decreasing the channel closing rate) could account for multiple macroscopic findings: left-shifted GABA EC50, smaller extent of desensitization, slower desensitization rate, and longer deactivation. We concluded that changes in efficacy can alter macroscopic desensitization without affecting desensitized states per se.

The vasorelaxation of cerebral arteries by carbon monoxide.

Carbon monoxide (CO) is known to increase cerebral blood flow, but the effect of CO on the vascular tone of large cerebral arteries is uncertain. We tested whether CO affects cerebral artery tone by measuring tension generated by ex vivo segments of dog basilar artery upon exposure to CO. In cerebral artery segments contracted with either KCl or prostaglandin F(2alpha), CO caused a concentration-related relaxation beginning with a concentration of 57 microM. Relaxation did not occur if CO was administered in the presence of bubbling carboxygen (95% O(2):5% CO(2)), which reduces greater than 99% of CO from the solution. Furthermore, the CO-induced relaxation of cerebral artery segments was reduced in the presence of the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 microM)or the potassium channel blocker tetraethylammonium (TEA, 1 mM). Neither ODQ nor TEA completely eliminated the relaxation caused by CO and there was no additive effect if ODQ and TEA were administered together. These results suggest that cerebral arteries are directly relaxed by CO and that this relaxation depends upon the activation of guanylyl cyclase and the opening of potassium channels.