Regulation of the cerebral circulation by endothelium

NO Deficiency Compromises Inter- and Intrahemispheric Blood Flow Adaptation to Unilateral Carotid Artery Occlusion

Carotid artery stenosis (CAS) affects approximately 5-7.5% of older adults and is recognized as a significant risk factor for vascular cognitive impairment (VCI). The impact of CAS on cerebral blood flow (CBF) within the ipsilateral hemisphere relies on the adaptive capabilities of the cerebral microcirculation. In this study, we aimed to test the hypothesis that the impaired availability of nitric oxide (NO) compromises CBF homeostasis after unilateral carotid artery occlusion (CAO). To investigate this, three mouse models exhibiting compromised production of NO were tested: NOS1 knockout, NOS1/3 double knockout, and mice treated with the NO synthesis inhibitor L-NAME. Regional CBF changes following CAO were evaluated using laser-speckle contrast imaging (LSCI). Our findings demonstrated that NOS1 knockout, NOS1/3 double knockout, and L-NAME-treated mice exhibited impaired CBF adaptation to CAO. Furthermore, genetic deficiency of one or two NO synthase isoforms increased the tortuosity of pial collaterals connecting the frontoparietal and temporal regions. In conclusion, our study highlights the significant contribution of NO production to the functional adaptation of cerebrocortical microcirculation to unilateral CAO. We propose that impaired bioavailability of NO contributes to the impaired CBF homeostasis by altering inter- and intrahemispheric blood flow redistribution after unilateral disruption of carotid artery flow.

Tea consumption is associated with cognitive impairment in older Chinese adults

OBJECTIVE

To explore the association between tea consumption and cognitive impairment (CoI).

METHODS

4579 adults (≥60 years) from the Weitang Geratric Diseases Study were assessed for characteristics of tea consumption and cognitive function by administering questionnaires and the Abbreviated Mental Test (AMT), respectively. We divided the subjects into normal cognitive function group (AMT score ≥8) and CoI group (AMT score ≤7).   The association between tea consumption and risk of CoI was determined by logistic regression models.

RESULTS

The least-squared means of the AMT scores for the subjects who seldom consumed tea were less favorable than those who habitually consumed tea. An inverse association was found between tea consumption (of any type) and prevalence of CoI (odds ratio = 0.74, 95% confidence interval = 0.57-0.98, P = 0.032). Interestingly, the protective correlation of tea was more obvious in never smokers (odds ratio = 0.63), but vanished in current/former smokers (odds ratio = 1.10). In never smokers, frequency of tea consumption was significantly associated with CoI (P for trend = 0.010).

CONCLUSION

Habitual tea consumption is suggested to be associated with a decreased risk of CoI among elders in Suzhou, and a higher frequency of tea consumption was associated with a lower prevalence of CoI among never smokers.

REVIEW ■ : Nitric Oxide: Actions and Pathological Roles

Nitric oxide is a unique biological messenger molecule. It mediates, in part, the immune functions of mac rophages ; it is produced by endothelial cells to mediate blood vessel relaxation; and it also serves as a neurotransmitter in the central and peripheral nervous system. Endothelial nitric oxide synthase and neuronal nitric oxide synthase are thought to be primarily constitutive, with activation induced by calcium entry into cells in the absence of protein synthesis, whereas the macrophage nitric oxide synthase is inducible with large increases in new nitric oxide synthase protein synthesis after immune stimulation. The molecular targets of nitric oxide are expanding, as are its physiological and pathophysiological roles in the nervous system. Nitric oxide may regulate neurotransmitter release, and it may play a key role in nervous system morpho genesis and synaptic plasticity and regulate gene expression. Under conditions of excessive formation, nitric oxide is emerging as an important neurotoxin in a variety of disorders of the nervous system. The Neuro scientist 1:7-18, 1995

Abnormal excitability and episodic low-frequency oscillations in the cerebral cortex of the tottering mouse

The Ca(2+) channelopathies caused by mutations of the CACNA1A gene that encodes the pore-forming subunit of the human Cav2.1 (P/Q-type) voltage-gated Ca(2+) channel include episodic ataxia type 2 (EA2). Although, in EA2 the emphasis has been on cerebellar dysfunction, patients also exhibit episodic, nonmotoric abnormalities involving the cerebral cortex. This study demonstrates episodic, low-frequency oscillations (LFOs) throughout the cerebral cortex of tottering (tg/tg) mice, a widely used model of EA2. Ranging between 0.035 and 0.11 Hz, the LFOs in tg/tg mice can spontaneously develop very high power, referred to as a high-power state. The LFOs in tg/tg mice are mediated in part by neuronal activity as tetrodotoxin decreases the oscillations and cortical neuron discharge contain the same low frequencies. The high-power state involves compensatory mechanisms because acutely decreasing P/Q-type Ca(2+) channel function in either wild-type (WT) or tg/tg mice does not induce the high-power state. In contrast, blocking l-type Ca(2+) channels, known to be upregulated in tg/tg mice, reduces the high-power state. Intriguingly, basal excitatory glutamatergic neurotransmission constrains the high-power state because blocking ionotropic or metabotropic glutamate receptors results in high-power LFOs in tg/tg but not WT mice. The high-power LFOs are decreased markedly by acetazolamide and 4-aminopyridine, the primary treatments for EA2, suggesting disease relevance. Together, these results demonstrate that the high-power LFOs in the tg/tg cerebral cortex represent a highly abnormal excitability state that may underlie noncerebellar symptoms that characterize CACNA1A mutations.

[Fetal cerebral-umbilical Doppler ratio in prediction of fetal distress in patients with preeclampsia]

BACKGROUND/AIM

The use of color Doppler ultrasonography provides noninvasive observation, confirmation and quantification of pathophysiological processes in fetoplacental circulation in pregnant patients. By blood vessel mapping and the obtained waves spectral analysis it is posible to evaluate vascular resistency of the fetus blood vessels. The aim of the study was to evaluate cerebral-umbilical pulsatility index ratio in fetal circulation in prediction of fetal distress in patients with preeclampsia.

METHODS

By measurement of pulsatility indices in medial cerebral and umbilical arteries in 400 patients with uncomplicated pregnancy, normal values were calculated for fetuses from 15-40 weeks. In our study group 70 patients with preeclampsia were included. Cerebral-umbilical (C/U) ratio was calculated after pulsatility indices in medial cerebral artery and umbilical artery determining by the spectral Doppler analysis of flow velocity waveforms in these vessels. Fetal outcome was analyzed by measurement of the Apgar score at the 5th minute and fetal pH at birth.

RESULTS

The mean C/U ratio values in the third trimester of normal pregnancy were between 1.8 and 1.9. The mean C/U ratio values in the patients complicated with preeclampsia were significantly lower comparing to normal pregnancies (ANOVA, p < 0.05). The mean 5th minute Apgar score in the study group was 6.35 +/- 1.58, and the mean fetal pH at birth was 7.16 +/- 0.15. Linear regression test showed a highly significant correlation between low C/U ratio and fetal pH at birth in patients with preeclampsia (r = 0.49, p < 0.01).

CONCLUSION

The C/U ratio values obtained from spectral Doppler analysis in fetal vessels showed a highly significant correlation with fetal pH at birth in the patients with preeclampsia. The results of our study confirmed the reliability of C/U ratio in estimation of fetal condition in preeclamptic patients. Very low C/U ratio values in patients with preeclampsia indicate that in these fetuses fetal acidosis and fetal distress may be expected.

Cilostazol, a phosphodiesterase inhibitor, attenuates photothrombotic focal ischemic brain injury in hypertensive rats

The aim of this study was to evaluate and compare the effects of anti-platelet agents with different modes of action (cilostazol, aspirin, and clopidogrel) on brain infarction produced by photothrombotic middle-cerebral-artery (MCA) occlusion in male, spontaneously hypertensive rats. Cerebral blood flow (CBF) was measured with laser-Doppler flowmetry in the penumbral cortex. Infarct size was evaluated 24 h after MCA occlusion. The effects of these drugs on infarct size were examined by pretreatment of rats undergoing MCA occlusion. Pretreatment with cilostazol (100 mg/kg) significantly reduced infarct size. In contrast, aspirin (10 mg/kg) and clopidogrel (3 mg/kg) failed to mitigate infarct size, regardless of their apparent inhibitory effects on platelet aggregation. Post-treatment with cilostazol also significantly attenuated the infarct size, associated with improved CBF in the penumbral region. In support of this effect, cilostazol increased nitric oxide (NO) production and prostaglandin-I(2) (PGI(2)) release in cultured human brain microvascular endothelial cells. Cilostazol-induced NO production and PGI(2) release were completely abolished by an NO synthase inhibitor and aspirin, respectively. These findings show that cilostazol reduced brain infarct size due to an improvement in penumbral CBF possibly in association with increased endothelial NO and PGI(2) production.

Role of angiotensin II in the response to endothelin-1 of goat cerebral arteries after ischemia-reperfusion

As angiotensin II may underlie the deleterious effects of some vascular diseases, we have examined the role of this peptide on the cerbrovascular endothelin-1 action after ischemia-reperfusion. In anesthetized goats, 1 hour-occlusion followed by 1 hour-reperfusion of the left middle cerebral artery (MCA) was induced, and then segments 3-mm in length from branches of the right MCA (control) and the left MCA (ischemic) were obtained for isometric tension recording. Endothelin-1 (10(-11)-10(-7) M) produced a contraction that was higher in ischemic than in control arteries, and in control but not in ischemic arteries this contraction was potentiated by angiotensin II (10(-7) M). Losartan (3 x 10(-6) M), antagonist of AT1 receptors, did not affect the response to endothelin-1 in control arteries, but reduced it both in ischemic arteries and angiotensin II-treated control arteries. PD123,319 (3 x 10(-6) M), antagonist of AT2 receptors, or the inhibitor of nitric oxide synthesis L-NAME (10(-4) M) did not alter the arterial effects of endothelin-1. Therefore, angiotensin II may potentiate the constriction to endothelin-1 in normal cerebral arteries by activating AT1 receptors. The observed cerebrovascular increased response to endothelin-1 after ischemia-reperfusion might be related in part to activation of AT1 receptors under this condition.

Role of oxidative stress and AT1 receptors in cerebral vascular dysfunction with aging

Vascular dysfunction occurs with aging. We hypothesized that oxidative stress and ANG II [acting via ANG II type 1 (AT(1)) receptors] promotes cerebral vascular dysfunction with aging. We studied young (5-6 mo), old (17-19 mo), and very old (23 +/- 1 mo) mice. In basilar arteries in vitro, acetylcholine (an endothelium-dependent agonist) produced dilation in young wild-type mice that was reduced by approximately 60 and 90% (P < 0.05) in old and very old mice, respectively. Similar effects were seen using A23187, a second endothelium-dependent agonist. The vascular response to acetylcholine in very old mice was almost completely restored with tempol (a scavenger of superoxide) and partly restored by PJ34, an inhibitor of poly(ADP-ribose) polymerase (PARP). We used mice deficient in Mn-SOD (Mn-SOD(+/-)) to test whether this form of SOD protected during aging but found that age-induced endothelial dysfunction was not altered by Mn-SOD deficiency. Cerebral vascular responses were similar in young mice lacking AT(1) receptors (AT(1)(-/-)) and wild-type mice. Vascular responses to acetylcholine and A23187 were reduced by approximately 50% in old wild-type mice (P < 0.05) but were normal in old AT(1)-deficient mice. Thus, aging produces marked endothelial dysfunction in the cerebral artery that is mediated by ROS, may involve the activation of PARP, but was not enhanced by Mn-SOD deficiency. Our findings suggest a novel and fundamental role for ANG II and AT(1) receptors in age-induced vascular dysfunction.

Roles of glia limitans astrocytes and carbon monoxide in adenosine diphosphate-induced pial arteriolar dilation in newborn pigs

BACKGROUND AND PURPOSE

Astrocytes, neurons, and microvessels together form a neurovascular unit allowing blood flow to match neuronal activity. Adenosine diphosphate (ADP) is an important signaling molecule in the brain, and dilation in response to ADP is astrocyte-dependent in rats and newborn pigs. Carbon monoxide (CO), produced endogenously by catabolism of heme to CO, iron, and biliverdin via heme oxygenase, is an important cell-signaling molecule in the neonatal cerebral circulation. We hypothesize ADP stimulates CO production by glia limitans astrocytes and that this CO causes pial arteriolar dilation.

METHODS

Experiments were performed using anesthetized piglet with closed cranial windows, and freshly isolated piglet astrocytes and microvessels. Astrocyte injury was caused by topical application of L-2-alpha aminoadipic acid (2 mmol/L, 5 hours). Cerebrospinal fluid was collected from under the cranial windows for measurement of ADP-stimulated CO production. CO was measured by gas chromatography-mass spectroscopy analysis.

RESULTS

Before, but not after, astrocyte injury in vivo, topical ADP stimulated both CO production and dilation of pial arterioles. Astrocyte injury did not block dilation to isoproterenol or bradykinin. Chromium mesoporphyrin, an inhibitor of heme oxygenase, also prevented the ADP-induced increase in cerebrospinal fluid CO and pial arteriolar dilation caused by ADP, but not dilation to sodium nitroprusside. ADP also increased CO production by freshly isolated piglet astrocytes and cerebral microvessels, although the increase was smaller in the microvessels.

CONCLUSIONS

These data suggest that glia limitans astrocytes use CO as a gasotransmitter to cause pial arteriolar dilation in response to ADP.

Microcirculatory function monitoring at the bedside--a view from the intensive care

Microcirculatory dysfunction plays a key role in the pathophysiology of various disease states and may consequently impact patient outcome. Until recently, the evaluation of the microcirculation using different measurement techniques has been mostly limited to animal and human research. With technical advances, microcirculatory monitoring nowadays becomes more and more available for application in clinical praxis. Unfortunately, measurements within the microcirculation are mostly limited to easily accessible surfaces, such as skin, muscle and tongue. Due to major differences in the physiologic regulation of microcirculatory blood flow and in metabolism between organs and even within different tissues in one organ, the clinical importance of regional microcirculatory measurements remains to be determined. In addition, technical methods available demonstrate large differences in the measured parameters and sampling volume, making interpretation of data even more difficult. Nonetheless, the monitoring of the microcirculation may, ahead of time, alert physicians that tissue oxygen supply becomes compromised and it may lead to a better understanding of basic pathophysiological aspects of disease. In the present review, we describe available non-invasive microcirculatory measurement techniques which can be applied clinically at the bedside. After a short discussion of physiologic and pathophysiologic basics related to microcirculatory monitoring, the measuring principles, applications, strengths and limitations of different monitoring systems are discussed.

Goat cerebrovascular reactivity to ADP after ischemia-reperfusion. Role of nitric oxide, prostanoids and reactive oxygen species

To analyze the cerebrovascular effects of ischemia-reperfusion, cerebrovascular reactivity to ADP was studied after inducing 60-min occlusion followed by 60-min reperfusion of the left middle cerebral artery (MCA) in anesthetized goats. In 12 goats, at the end of reperfusion, left MCA resistance was decreased by 19%, and reactive hyperemia to 5- and 10-s occlusions as well as the cerebral vasodilatation to ADP (0.03-0.3 microg) but not to sodium nitroprusside (0.3-3 microg) was decreased. In 28 animals, killed at the end of reperfusion, segments 3-mm long were obtained from the left (ischemic) and right (control) MCA, prepared for isometric tension recording, and precontracted with the thromboxane A2 analogue U46619. The relaxation to ADP (10(-8) to 10(-5) M) but not to sodium nitroprusside (10(-8) to 10(-4) M) was lower in ischemic arteries. L-NAME (inhibitor of nitric oxide synthesis, 10(-4) M), charybdotoxin (10(-7) M)+apamin (10(-6) M) (blockers of KCa), or catalase (1000 U/ml) reduced the relaxation to ADP only in control arteries. Charybdotoxin+apamin further augmented the L-NAME-induced reduction in the relaxation to ADP in control arteries. The inhibitor of cyclooxygenase meclofenamate (10(-5) M) increased the relaxation to ADP only in ischemic arteries. The superoxide dismutase mimetic tiron (10(-2) M) increased the ADP-induced relaxation only in ischemic arteries. Therefore, it is suggested that ischemia-reperfusion produces cerebrovascular endothelial dysfunction, which may be associated with decreased nitric oxide bioavailability, decreased release of an EDHF, and increased production of vasoconstrictor prostanoids. All these alterations may be related in part with an increased production of superoxide anion.

Cerebral vascular effects of angiotensin II: new insights from genetic models

Very little is known regarding the mechanisms of action of angiotensin II (Ang II) or the consequences of Ang II-dependent hypertension in the cerebral circulation. We tested the hypothesis that Ang II produces constriction of cerebral arteries that is mediated by activation of AT1A receptors and Rho-kinase. Basilar arteries (baseline diameter approximately 130 microm) from mice were isolated, cannulated and pressurized to measure the vessel diameter. Angiotensin II was a potent constrictor in arteries from male, but not female, mice. Vasoconstriction in response to Ang II was prevented by an inhibitor of Rho-kinase (Y-27632) in control mice, and was reduced by approximately 85% in mice deficient in expression of AT1A receptors. We also examined the chronic effects of Ang II using a model of Ang II-dependent hypertension, mice which overexpress human renin (R+) and angiotensinogen (A+). Responses to the endothelium-dependent agonist acetylcholine were markedly impaired in R+A+ mice (P<0.01) compared with controls, but were restored to normal by a superoxide scavenger (PEG-SOD). A-23187 (another endothelium-dependent agonist) produced vasodilation in control mice, but no response or vasoconstriction in R+A+ mice. In contrast, dilation of the basilar artery in response to a NO donor (NONOate) was similar in R+A+ mice and controls. Thus, Ang II produces potent constriction of cerebral arteries via activation of AT1A receptors and Rho-kinase. There are marked gender differences in cerebral vascular responses to Ang II. Endothelial function is greatly impaired in a genetic model of Ang II-dependent hypertension via a mechanism that involves superoxide.

Reactive oxygen species: influence on cerebral vascular tone

Reactive oxygen species have multiple effects on vascular cells. Defining the sources and the impact of the various reactive oxygen species within the vessel wall has emerged as a major area of study in vascular biology. This review will focus on recent findings related to effects of reactive oxygen species on cerebral vascular tone. Effects of superoxide radical, hydrogen peroxide, and the reactive nitrogen species peroxynitrite are summarized. Although higher concentrations may be important for cerebral vascular biology in disease, relatively low concentrations of reactive oxygen species may function as signaling molecules involved with normal regulation of cerebral vascular tone. The mechanisms by which reactive oxygen species affect vascular tone may be quite complex, and our understanding of these processes is increasing. Additionally, the role of reactive oxygen species as mediators of endothelium-dependent relaxation is addressed. Finally, the consequences of the molecular interactions of superoxide with nitric oxide and arachidonic acid are discussed.

Modulation of cerebral vascular tone by activated glia: involvement of nitric oxide

The ability of activated glia to affect cerebral vascular tone has been evaluated using an in vitro experimental system in which basilar arteries were incubated with glial cultures activated by treatment with lipopolysaccharide (LPS). Vascular tone was measured with an isometric myograph. Contraction in response to high KCl and serotonin was reduced in arteries co-incubated for 24 h with LPS-activated glia, whereas the response to acetylcholine was not modified. The reduced contraction was prevented when the nitric oxide synthase (NOS) inhibitor L-N-nitro-arginine (L-NNA) was added throughout the whole incubation time (activation of glial cells with LPS + co-incubation of glial cells with cerebral arteries). Under these conditions, nitrite levels were drastically reduced. A reduced contraction to KCl was also observed after treatment of the cerebral vessel with sodium nitroprusside. In contrast, L-NNA added to the vessel did not modify the response to contracting stimuli and the expression of endothelial NOS was not modified in cerebral arteries pre-incubated with activated glia. These results suggest that activated glia, which finds an in vivo correlate in several neuropathological conditions, can contribute to changes of vascular tone by modifying the levels of nitric oxide (NO) to which the vessel is exposed.

Responses of cerebral arterioles to ADP: eNOS-dependent and eNOS-independent mechanisms

ADP mediates platelet-induced relaxation of blood vessels and may function as an important intercellular signaling molecule in the brain. We used pharmacological and genetic approaches to examine mechanisms that mediate responses of cerebral arterioles to ADP, including the role of endothelial nitric oxide synthase (eNOS). We examined responses of cerebral arterioles (control diameter approximately 30 microm) in anesthetized wild-type (WT, eNOS+/+) and eNOS-deficient (eNOS-/-) mice using a cranial window. In WT mice, local application of ADP produced vasodilation that was not altered by indomethacin but was reduced by approximately 50% by NG-nitro-L-arginine (L-NNA) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (inhibitors of NOS and soluble guanylate cyclase, respectively). In eNOS-/- mice, responses to ADP were largely preserved, and a significant component of the response was resistant to L-NNA (a finding similar to that in WT mice treated with L-NNA). In the absence of L-NNA, responses to ADP were markedly reduced by charybdotoxin plus apamin [inhibitors of Ca2+-dependent K+ channels and responses mediated by endothelium-derived hyperpolarizing factor (EDHF)] in both WT and eNOS-/- mice. Thus pharmacological and genetic evidence suggests that a significant portion of the response to ADP in cerebral microvessels is mediated by a mechanism independent of eNOS. The eNOS-independent mechanism is functional in the absence of inhibited eNOS and most likely is mediated by an EDHF.

Endothelial dysfunction in a primate model of cerebral vasospasm

OBJECT

Although abnormalities in the control of endothelial vasomotility have been reported in both experimental and clinical studies, the mechanism of the endothelial dysfunction that occurs following subarachnoid hemorrhage (SAH) remains unclear. Because of the absence of previous in vivo studies of endothelial function in cerebral vessels in response to SAH or cerebral vasospasm, the authors investigated endothelium-dependent responses in an established primate model of vasospasm after SAH. Endothelial function was assessed by examining vascular responses to intracarotid injections of various drugs known to act via the endothelium. Drugs that have a rapid total body clearance were selected so that their pharmacological effects would be limited to the cerebral circulation after an intracarotid infusion.

METHODS

Seventeen adult male cynomolgus monkeys were used. Cerebrovascular endothelium-dependent responses were examined in control animals and in animals with SAH 7, 14, and 21 days after placement of a subarachnoid clot around the right middle cerebral artery. Cortical cerebral blood flow (CBF) and cerebrovascular resistance (CVR) were recorded continuously during 5-minute intracarotid infusions of 5% dextrose vehicle, acetylcholine, histamine, bradykinin, or Calcimycin. In control animals the intracarotid infusion of acetylcholine produced a significant (7.8 +/- 9.5%) increase in CBF and a 9.3 +/- 8.7% reduction in CVR in comparison with a control infusion of dextrose vehicle. The responses to acetylcholine disappeared in animals 7 days post-SAH, specifically in the subset of animals in which arteriography confirmed the presence of vasospasm. Infusion of Calcimycin produced no significant changes in CBF or CVR in control animals, but resulted in a significant reduction in CBF and increase in CVR in animals 7 days after SAH and in animals with vasospasm. An infusion of histamine or bradykinin had no significant effect on CBF or CVR.

CONCLUSIONS

An intracarotid infusion of acetylcholine, but not one of histamine, bradykinin, or Calcimycin, produced a measurable physiological response in the normal primate cerebrovasculature. Cerebral vasospasm that occurred after SAH produced a pathophysiological effect similar to the endothelial denudation shown in the in vitro experiments of Furchgott and Zawadzki, in which acetylcholine constricted the vessels via activation of receptors on smooth-muscle cells. Changes in vascular responses to acetylcholine and Calcimycin in animals with vasospasm, compared with control animals, provide evidence that endothelial dysfunction plays a key role in the development and/or sustenance of vasospasm after SAH.

L-Citrulline recycle for synthesis of NO in cerebral perivascular nerves and endothelial cells

Recycle of L-citrulline to form L-arginine in cerebral perivascular nerves has been well described, providing direct evidence that nitric oxide (NO) is synthesized and released from these nerves to act as the transmitter for vasodilation. NO is also synthesized and released from cerebral endothelial cells, involving L-citrulline conversion to L-arginine. Evidence for the presence of enzymes involved in the conversion, however, has not been shown. The presence of nitric oxide synthase (NOS), argininosuccinate synthetase (ASS), and argininosuccinate lyase (ASL), and their coexistence with NADPH-diaphorase (NADPHd), a marker for NOS, in endothelial cells of middle cerebral arteries and the circle of Willis of the pig, therefore, were examined using combined immunohistochemical and histochemical techniques. NOS-, ASS-, and ASL-immunoreactivities were found in almost all endothelial cells of all cerebral arteries examined. All ASS-, ASL-, and NOS-immunoreactive (I) endothelial cells also stained positively for NADPHd, suggesting that ASS, ASL, and NOS were colocalized in endothelial cells of middle cerebral arteries and the circle of Willis. These results provide morphological evidence that cerebral vascular endothelial cells like cerebral perivascular nerves contain enzymes necessary for recycling L-citrulline to L-arginine to synthesize NO via an argininosuccinate (AS) pathway.

Nicotinamide modulates mitochondrial membrane potential and cysteine protease activity during cerebral vascular endothelial cell injury

Microvascular endothelial cell (EC) apoptosis or programmed cell death (PCD) during free radical injury may be involved in the development of cerebral ischemic and degenerative diseases. Yet, the cellular mechanisms that mediate cerebral EC injury require further definition. We therefore used the agent nicotinamide as an investigative tool in EC cultures to examine the role of free radical nitric oxide (NO)-induced PCD. EC injury was evaluated by the trypan blue dye exclusion method, DNA fragmentation, membrane phosphatidylserine (PS) exposure, cysteine protease activity, mitochondrial membrane potential, and mitogen-activated protein kinase phosphorylation. We demonstrate that cerebrovascular PCD consists of two distinct pathways that involve the degradation of genomic DNA and the exposure of membrane PS residues. Each of these pathways is reversible in nature and is controlled independently by caspase 8, caspase 1, and caspase 3. As a cytoprotectant, nicotinamide is novel in the vascular system and functions at two levels. Nicotinamide not only maintains the mitochondrial membrane potential and the prevention of cytochrome c release, but also prevents the induction of caspase-8-, caspase-1- and caspase-3-like activities linked to the DNA repair enzyme poly(ADP-ribose) polymerase through mechanisms that are independent from the MAP kinase systems of p38 and JNK. The work begins to identify therapeutic strategies for the protection of the cerebral vasculature during both acute and chronic degenerative disorders.

Regulation of blood-brain barrier permeability

The blood-brain barrier minimizes the entry of molecules into brain tissue. This restriction arises by the presence of tight junctions (zonulae occludens) between adjacent endothelial cells and a relative paucity of pinocytotic vesicles within endothelium of cerebral arterioles, capillaries, and venules. Many types of stimuli can alter the permeability characteristics of the blood-brain barrier. Acute increases in arterial blood pressure beyond the autoregulatory capacity of cerebral blood vessels, application of hyperosmolar solutions, application of various inflammatory mediators known to be elevated during brain injury, and/or activation of blood-borne elements such as leukocytes can produce changes in permeability of the blood-brain barrier. The second messenger systems that account for increases in permeability of the blood-brain barrier during pathophysiologic conditions, however, remain poorly defined. This review will summarize studies that have examined factors that influence disruption of the blood-brain barrier, and will discuss the contribution of various cellular second messenger pathways in disruption of the blood-brain barrier during pathophysiologic conditions.

Exogenous A beta1-40 reproduces cerebrovascular alterations resulting from amyloid precursor protein overexpression in mice

Transgenic mice overexpressing the amyloid precursor protein (APP) have a profound impairment in endothelium-dependent cerebrovascular responses that is counteracted by the superoxide scavenger superoxide dismutase (SOD). The authors investigated whether the amyloid-beta peptide (A beta) is responsible for the cerebrovascular effects of APP overexpression. Cerebral blood flow (CBF) was monitored by a laser-Doppler flowmeter in anesthetized-ventilated mice equipped with a cranial window. Superfusion of A beta1-40 on the neocortex reduced resting CBF in a dose-dependent fashion (-29% +/- 7% at 5 micromol/L) and attenuated the increase in CBF produced by the endothelium-dependent vasodilators acetylcholine (-41% +/- 8%), bradykinin (-39% +/- 9%), and the calcium ionophore A23187 (-37% +/- 5%). A beta1-40 did not influence the CBF increases produced by the endothelium-independent vasodilators S-nitroso-N-acetylpenicillamine and hypercapnia. In contrast, A beta1-42 did not attenuate resting CBF or the CBF increases produced by endothelium-dependent vasodilators. Cerebrovascular effects of A beta1-40 were reversed by the superoxide scavengers SOD or MnTBAP. Furthermore, substitution of methionine 35 with norleucine, a mutation that blocks the ability of A beta to generate reactive oxygen species, abolished A beta1-40 vasoactivity. The authors conclude that A beta1-40, but not A beta1-42, reproduces the cerebrovascular alterations observed in APP transgenics. Thus, A beta1-40 could play a role in the cerebrovascular alterations observed in Alzheimer's dementia.

The effects of potassium on the rat middle cerebral artery

After traumatic brain injury, extracellular K(+) in brain can dramatically increase. We studied the effects of increased K(+) on the isolated pressurized rat middle cerebral artery (MCA). MCAs (200-250 microm OD) were isolated, cannulated with glass micropipettes, and pressurized. K(+) was increased in the extraluminal bath using three paradigms: (1) isotonic K(+) (K(iso)) where increases in K(+) were offset by decreases in Na(+), (2) hypertonic K(+) (K(hyper)) where K(+) was increased without a concomitant adjustment of Na(+), and (3) K(suc), a solution using K(iso) but with the addition of sucrose to obtain a hypertonic solution. Increases in K(+) in the extraluminal bath produced significant dilations (approximately 20%) at 21 mM K(+) in all three groups (K(iso), K(hyper), and K(suc)). With the K(hyper) and K(suc) groups, the magnitude of the dilation diminished with further increases in K(+). L-NAME (10(-5) M), an inhibitor of nitric oxide synthase, had no effect on the response of the K(hyper) and K(suc) groups at 21 mM but significantly enhanced constrictions of the MCAs above 40 mM K(+) compared to the control. The K(iso) group was not affected by L-NAME at any K(+) concentration and showed profound constrictions above 40 mM K(+). We conclude that changes in the K(+) concentration and osmolality of the extracellular fluid may have profound effects on the cerebral vasculature.

Advances in the vascular pathophysiology of ischemic stroke

The cerebral vascular supply is constructed to protect the cerebral hemispheres and brainstem from the consequences of blood flow cessation. Reversal of blood flow around local obstructions is a feature of the microvascular beds of the striatum and cerebral cortex. Cerebral capillaries of these beds consist of endothelial cells, basal lamina, and astrocyte end-feet that sit in close apposition. The interaction of astrocytes with neurons indicates the close relationship of microvessels to neurons. These relationships are altered when blood flow ceases in the supplying artery. Increased endothelial cell permeability and endocytoses lead to edema formation, and matrix degradation is associated with hemorrhage. Autoregulation is lost. Ischemia initiates leukocyte adhesion receptor expression, which is promoted by cytokine generation from the neuropil and activated monocytes. "Preactivation" may further augment the inflammatory responses to ischemia. The activation of cerebral microvessels by ischemia is heterogeneous, involving alterations in integrin-matrix interactions, leukocyte-endothelial cell adhesion, permeability changes, and the "no-reflow" phenomenon due to platelet activation, fibrin formation, and leukocyte adhesion. Ischemia produces swelling of the microvascular endothelium, and rapid detachment and swelling of the astrocyte end-feet. Ischemic injury targets the microvasculature, where the inflammatory responses are initiated and contribute to tissue injury.

Nitric oxide and the cerebral vascular function

The presence of a cholinergic vasodilator innervation to cerebral circulation is well established. Despite its high endogenous concentration in cerebral blood vessels, acetylcholine (ACh) is not the transmitter for vasodilation. This finding has led to the discovery that nitric oxide (NO), which is coreleased with ACh and neural peptides such as vasoactive intestinal polypeptide (VIP) from the respective cholinergic-nitrergic (nitric oxidergic) nerves and the VIPergic-nitrergic nerves, is the primary transmitter in relaxing smooth muscle. ACh and VIP act presynaptically to inhibit and facilitate, respectively, the release of NO. Release of NO from cerebral vascular endothelial cells is also well established. A similar system for recycling L-citrulline to L-arginine for synthesizing more NO has been demonstrated in both cerebral perivascular nerves and endothelial cells. Neuronal and endothelial NO appears to play an important role in controlling cerebral vascular tone and circulation in health and disease.

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.

Endothelial-mediated dilations following severe controlled cortical impact injury in the rat middle cerebral artery

The mechanisms associated with dysfunction of the cerebral vasculature following head trauma have not yet been fully elucidated. In an attempt to shed more light on the matter, we investigated the endothelial-mediated dilations in the rat middle cerebral artery (MCA) following severe traumatic brain injury (TBI). Rats were subjected to severe controlled cortical impact injury (CCI; 5 m/s, 130 ms duration, 3 mm deformation) over the right parietal cortex. At 24 h postinjury, ipsilateral segments of MCA and corresponding contralateral segments were isolated, mounted in a vessel chamber, and pressurized. The responses to 2 methylthio-ATP (2MeSATP), a selective agonist for the P2Y1 purinoceptors, N(omega)-nitro-L-arginine (L-NAME), an NO synthase inhibitor, and S-nitroso-N-acetylpenicillamine (SNAP), an exogenous NO donor, were determined. 2MeSATP elicited concentration dependent dilations in all MCAs studied. Ipsilateral MCAs harvested following TBI or sham-TBI, showed similar maximum dilations to 2MeSATP [70 +/- 4% (n = 17) and 72 +/- 6% (n = 13), respectively]. However, TBI reduced the concentration of 2MeSATP necessary to elicit one-half of the maximum dilation (EC50) from 15 to 9 nM (p < 0.05). Inhibition of NO synthase with 10(-5) M L-NAME abolished the dilation to 2MeSATP in both TBI and sham-TBI MCAs. The constriction to L-NAME was significantly reduced in TBI MCAs compared to sham vessels. Dilations to SNAP, an NO donor, were not altered by TBI indicating that the mechanisms of dilation involving NO in the vascular smooth muscle were not affected. Unlike other pathological conditions which often diminish endothelial-mediated responses, severe TBI enhanced the sensitivity to 2MeSATP without altering the maximum response.

Acute vasoconstriction after subarachnoid hemorrhage

OBJECTIVE

Decreased cerebral blood flow (CBF) and cerebral ischemia occurring immediately after subarachnoid hemorrhage (SAH) may be caused by acute microvascular constriction. However, CBF can also be influenced by changes in intracranial pressure (ICP) and cerebral perfusion pressure (CPP). The goal of these experiments was to assess the significance of acute vasoconstriction after SAH and its relationship to changes in CBF, ICP, CPP, and extracellular glutamate concentrations.

METHODS

Three experiments were performed using the endovascular filament technique to produce SAH. In the first experiment, CBF, ICP, and CPP were measured for 60 minutes after SAH (n = 21) and were correlated with the 24-hour mortality rate. In the second experiment, rats undergoing SAH (n = 23) or a sham procedure (n = 7) were perfused 60 minutes after SAH for measurement of the circumference and wall thickness of the internal carotid and anterior cerebral arteries and correlation with CBF, ICP, and CPP. In the third experiment (n = 11), extracellular glutamate concentrations determined by hippocampal and cortical microdialysis and high performance liquid chromatography were correlated with physiological changes.

RESULTS

CBF reductions to less than 40% of baseline for 60 minutes after SAH predicted 24-hour mortality with 100% accuracy and were used to define "lethal" SAH. In contrast, ICP and CPP 60 minutes after SAH were not correlated with the mortality rate. The vascular circumference was significantly smaller in lethal than in sublethal SAH or sham-operated rats (P < 0.001). Vessel measurements were correlated with both CBF and hemorrhage size (P < 0.01). Extracellular glutamate concentration increased to 600% of baseline after lethal SAH in both hippocampus and cortex and was inversely correlated with CBF (r = 0.9, P < 0.001) but did not increase after sublethal SAH.

CONCLUSION

Acute vasoconstriction after SAH occurs independently of changes in ICP and CPP and is associated with decreased CBF, larger hemorrhage size, persistent elevations of extracellular glutamate, and poor outcome. Acute vasoconstriction seems to contribute directly to ischemic brain injury after SAH. Further evaluations of pharmacological agents with the potential to reverse acute vasoconstriction may increase CBF and improve outcome.

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.

Increased susceptibility to ischemic brain damage in transgenic mice overexpressing the amyloid precursor protein

We studied the role of the amyloid precursor protein (APP) in ischemic brain damage using transgenic mice overexpressing APP. The middle cerebral artery (MCA) was occluded in FVB/N mice expressing APP695.SWE (Swedish mutation) and in nontransgenic littermates. Infarct volume (cubic millimeters) was assessed 24 hr later in thionin-stained brain sections. The infarct produced by MCA occlusion was enlarged in the transgenics (+32 +/- 6%; n = 12; p < 0. 05; t test). Measurement of APP by ELISA revealed that, although relatively high levels of Abeta were present in the brain of the transgenics (Abeta1-40 = 80 +/- 19 pmol/g; n = 6), there were no differences between ischemic and nonischemic hemispheres (p > 0.05). The reduction in cerebral blood flow produced by MCA occlusion at the periphery of the ischemic territory was more pronounced in APP transgenics (-42 +/- 8%; n = 9) than in controls (-20 +/- 8%; n = 9). Furthermore, the vasodilatation produced by neocortical application of the endothelium-dependent vasodilator acetylcholine (10 microM) was reduced by 82 +/- 5% (n = 8; p < 0.05) in APP transgenics. The data demonstrate that APP overexpression increases the susceptibility of the brain to ischemic injury. The effect is likely to involve the Abeta-induced disturbance in endothelium-dependent vascular reactivity that leads to more severe ischemia in regions at risk for infarction. The cerebral vascular actions of peptides deriving from APP metabolism may play a role in the pathogenic effects of APP.

Hypoxia-induced inhibition of the response to nitroxidergic nerve stimulation in canine cerebral arteries

In isolated canine middle cerebral arteries contracted with prostaglandin F2 alpha, transmural electrical stimulation (TES), nicotine, and substance P produced relaxations. Transmural electrical stimulation- and nicotine-induced endothelium-independent responses are mediated by nitric oxide (NO) liberated from perivascular nerve, whereas substance P-induced relaxations are mediated by endothelium-derived NO. These responses were attenuated by replacement of 95% O2 and 5% CO2 gas (about 550 mm Hg of partial O2 pressure) with 95% N2 and 5% CO2 gas (about 40 mm Hg); inhibition of the response to TES was stabilized 30 minutes later. Reoxygenation partially reversed the response. Relaxations caused by exogenous NO were not influenced by hypoxia. Inhibition by hypoxia of the response to TES was not affected by superoxide dismutase. However, the inhibitory effect was prevented by amiloride and dimethyl-amiloride, Na(+)-H+ exchange inhibitors, or acidosis caused by the addition of HCl. The inhibition by hypoxia was reversed by amiloride. It is concluded that depression by hypoxia of the response mediated by endogenous NO is associated with impaired membrane function caused by restoration of normal intracellular pH by Na(+)-H+ exchanger.

Regional measurements of NO formed in vivo during brain ischemia

Nitric oxide formed in vivo in the rat brain regions of hippocampus, striatum, neocortex and cerebellum was spin trapped and measured ex vivo by cryogenic electron paramagnetic resonance spectroscopy. In non-ischemic control animals the rate of nitric oxide (NO) formation in the individual brain regions ranged from 15 to 42 pmol.g-1.min-1. During exposure to global ischemia for 7 min the generation of NO increased in all parts of the brain. In the hippocampus the rate of NO formation during ischemia increased by 6-fold from a control rate of 19 pmol.g-1.min-1. This increase was attenuated 47% by pretreatment with the NO synthase antagonist 7-nitroindazole, whereas pretreatment with the non-NMDA receptor anatogonist NBQX and the Ca2+ channel blocker NS638 did not influence the NO formation. The data show that short-duration ischemia elicits a significant, NO-synthase-dependent formation of NO in all brain regions.

Effects of a novel inhibitor of guanylyl cyclase on dilator responses of mouse cerebral arterioles

BACKGROUND AND PURPOSE

Nitric oxide-induced vasodilatation is mediated by both cGMP-dependent and -independent mechanisms. Previous studies that examined the role of soluble guanylyl cyclase in cerebral vessels have used methylene blue and LY-83583, compounds that generate superoxide anion and are not specific for inhibition of soluble guanylyl cyclase. We examined the effects of ODQ (1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one), a novel and highly selective inhibitor of soluble guanylyl cyclase, on responses of cerebral arterioles.

METHODS

The effects of ODQ on responses of cerebral arterioles to acetylcholine, nitroprusside, 8-bromo-cGMP, and adenosine were examined in anesthetized mice by means of a cranial window. The effects of two concentrations of ODQ were examined in the absence and presence of superoxide dismutase. The effects of NG-nitro-L-arginine, an inhibitor of nitric oxide synthase, were also tested.

RESULTS

ODQ (3 and 10 mumol/L) produced concentration-dependent inhibition of dilatation of cerebral arterioles (control diameter = 29 +/- 1 microns) (mean +/- SE) in response to acetylcholine and nitroprusside. For example, 10 mumol/L acetylcholine and 1 mumol/L nitroprusside dilated cerebral arterioles by 28 +/- 3% and 44 +/- 2% in the absence and 6 +/- 2% and 7 +/- 1%, respectively, in the presence of 10 mumol/L ODQ (P < .05 versus control). The inhibitory effects of ODQ were not altered by superoxide dismutase. Vasodilatation in response to 8-bromo-cGMP and adenosine was not inhibited by ODQ. NG-Nitro-L-arginine (100 mumol/L), an inhibitor of nitric oxide synthase, inhibited responses to acetylcholine by approximately 80% but tended to enhance responses to nitroprusside.

CONCLUSIONS

Thus, nitric oxide-mediated dilatation of mouse cerebral arterioles is profoundly inhibited by ODQ, an inhibitor of activity of soluble guanylyl cyclase. Cerebral vasodilator responses to adenosine and 8-bromo-cGMP were preserved in the presence of ODQ, indicating that inhibition by ODQ was selective. In contrast to previously used inhibitors of soluble guanylyl cyclase (methylene blue and LY-83583), the effects of ODQ are not mediated by generation of superoxide anion.

Interactions between vasoconstrictors in isolated human cerebral arteries

This study investigates whether different endogeneous vasoconstrictors exert synergistic effects in isolated human cerebral arteries, because potentiation of contractile effects may play a role in the pathogenesis of cerebral vasospasm. Isolated human pial arteries obtained from macroscopically intact tissue during brain tumour operations were mounted onto a wire myograph. Concentration-response curves of 5-hydroxytryptamine (5-HT) were constructed in the absence and presence of threshold concentrations of the thromboxane A2 (TXA)-analog U46619, and endothelin-1 (ET-1). Threshold concentrations of U46619 markedly enhanced the maximum contractile effect of 5-HT. The response to 5-HT remained markedly increased even after washout of U46619. Threshold concentrations of ET-1 increased the maximum response to 5-HT, and markedly shifted the dose-response curve to the left. Even after washout of ET-1, the dose-response curve of 5-HT remained shifted to the left. The increase of the contractile effect of 5-HT in the presence of U46619 did not correlate with the relaxant action of the endothelium-dependent vasodilator carbachol. Thus, synergism between contractile substances such as 5-HT, U46619, or ET-1 is seen in human cerebral arteries, and responses to 5-HT are potentiated even after washout of ET-1 and U46619. The potentiation does not depend on the endothelial function. We conclude that synergistic responses between endogeneous vasoconstrictors such as 5-HT, TXA and ET-1 may be involved in the pathogenesis of cerebral vasospasm after subarachnoid haemorrhage.

Cerebral vasoconstriction in comatose patients resuscitated from a cardiac arrest?

OBJECTIVE

To determine the role of cerebral vasoconstriction in the delayed hypoperfusion phase in comatose patients after cardiac arrest.

DESIGN

Prospective study.

SETTING

Medical intensive care unit in a university hospital.

PATIENTS

10 comatose patients (Glasgow Coma Score +/- 6)successfully resuscitated from a cardiac arrest occurring outside the hospital.

MEASUREMENTS

We measured the pulsatility index (PI) and mean blood flow velocity (MFV) of the middle cerebral artery, the cerebral oxygen extraction ratio and jugular bulb levels of endothelin, nitrate, and cGMP during the first 24 h after cardiac arrest.

RESULTS

The PI decreased significantly from 1.86 +/- 1.02 to 1.05 +/- 0.22 (p = 0.03). The MFV increased significantly from 29 +/- 10 to 62 +/- 25 cm/s (p = 0.003). Cerebral oxygen extraction ratio decreased also from 0.39 +/- 0.13 to 0.24 +/- 0.11 (p = 0.015). Endothelin levels were high but did not change during the study period. Nitrate levels varied widely and showed a slight but significant decrease from 37.1 mumol/l (median; 25th-75th percentiles: 26.8-61.6) to 31.3 mumol/l (22.1-39.6) (p = 0.04). Cyclic guanosine monophosphate levels increased significantly from 2.95 mumol/l (median; 25th-75th percentiles: 2.48-5.43) to 7.5 mumol/l (6.20-14.0) (p = 0.02).

CONCLUSIONS

We found evidence of increased cerebrovascular resistance during the first 24 h after cardiac arrest with persistent high endothelin levels, gradually decreasing nitrate levels, and gradually increasing cGMP levels, This suggests that active cerebral vasoconstriction due to an imbalance between local vasodilators and vasoconstrictors plays a role in the delayed hypoperfusion phase.

Nitric oxide actions in neurochemistry

The discovery of the role of nitric oxide (NO) as a messenger molecule in the central nervous system has radically altered our conception of neuronal communication. Information has been generated at a rapid pace in the last few years regarding the biochemistry, enzymatic activity and molecular characteristics of NO synthase (NOS) and NO synthesis, and the biological consequences of NO production. The full scope of the biological consequences of NO production is unknown but putative cellular targets of NO are rapidly being discovered as well as potential physiological and pathophysiological roles in the nervous system. Roles for NO have been identified in neurotransmitter release, morphogenesis, synaptic plasticity, regulation of gene expression, as well as mediation of inhibitory processes associated with sexual and aggressive behaviour. Inappropriate formation of NO is emerging as an important factor in the neurotoxicity associated with a variety of central nervous system disorders. The current information on the various roles of NO as a neuronal messenger molecule as well as a neurotoxin are reviewed in this article.

Physiological and pathophysiological roles of nitric oxide in the central nervous system

Nitric oxide (NO) is produced by three distinct isoforms of nitric oxide synthases in the central nervous system. Here, the roles of nitric oxide in the central nervous system are reviewed under physiological and pathophysiological conditions. Under physiological conditions, NO plays a role in the regulation of cerebral blood flow and autoregulation, blood flow-metabolism coupling, neurotransmission, memory formation, modulation of neuroendocrine functions, and behavioral activity. Impairment of the NO-mediated cerebrovascular vasodilatation occurs during ischemia-reperfusion, diabetes, hypertension, subararchnoid hemorrhage, and various forms of shock. Enhancement of NO production in the brain occurs during stoke, seizures, and acute and chronic inflammatory and neurodegenerative disorders. The alterations of the expression of the various isoforms of nitric oxide synthases under the above conditions are discussed. Moreover, the molecular mechanisms of NO and peroxynitrite induced cellular injury are delineated. Finally, the current strategies available for selective pharmacological manipulation of individual nitric oxide synthase isoforms are discussed.

Acute effects of ethanol on responses of cerebral arterioles

BACKGROUND AND PURPOSE

Previous studies have suggested that acute exposure of large peripheral arteries to ethanol impairs endothelium-dependent relaxation. The goal of the present study was to determine the acute effects of ethanol exposure on responses of cerebral resistance arterioles in vivo.

METHODS

We prepared a cranial window in rats to expose the cerebral (pial) microcirculation. We measured the diameter of pial arterioles in vivo in response to agonists that presumably stimulate the synthesis/release of nitric oxide from the endothelium (ADP, acetylcholine, and histamine) or neurons (N-methyl-D-aspartate [NMDA]) before and after topical application of various concentrations of ethanol added to the cerebrospinal fluid (20, 40, 60, 80, and 100 mmol/L). In addition, we examined responses of pial arterioles to nitroglycerin before and 1 hour after topical application of ethanol.

RESULTS

Before application of ethanol, ADP, acetylcholine, histamine, NMDA, and nitroglycerin produced dose-related dilatation of pial arterioles. Application of the various concentrations of ethanol did not alter the baseline diameter of pial arterioles. However, application of 80 and 100 mmol/L ethanol inhibited dilatation of pial arterioles in response to agonists that stimulate the synthesis/release of nitric oxide. Dilatation of pial arterioles in response to nitroglycerin was not altered by application of ethanol.

CONCLUSIONS

The findings of the present study suggest that acute exposure of cerebral arterioles to modest-to-moderate concentrations of ethanol (20 to 60 mmol/L) does not alter responses of cerebral arterioles. In contrast, exposure of cerebral arterioles to higher concentrations of ethanol (80 and 100 mmol/L) can produce specific impairment of dilatation to agonists that stimulate the synthesis/release of nitric oxide from endothelium and neurons.

Role of potassium channels in cerebral blood vessels

BACKGROUND

Hyperpolarization of vascular muscle in response to activation of potassium channels is a major mechanism of vasodilatation. In cerebral blood vessels, four different potassium channels have been described: ATP-sensitive potassium channels, calcium-activated potassium channels, delayed rectifier potassium channels, and inward rectifier potassium channels.

SUMMARY OF REVIEW

Activation of ATP-sensitive and calcium activated potassium channels appears to play a major role in relaxation of cerebral arteries and arterioles in response to diverse stimuli, including receptor-mediated agonists, intracellular second messengers, and hypoxia. Both calcium-activated and delayed rectifier potassium channels may contribute to a negative feedback system that regulates tone in large cerebral arteries. The influence of ATP-sensitive and calcium-activated potassium channels is altered in disease states such as hypertension, diabetes, and atherosclerosis.

CONCLUSIONS

Activation of potassium channels is a major mechanism of cerebral vasodilatation. Alteration of activity of potassium channels and impairment of vasodilatation may contribute to the development or maintenance of cerebral ischemia or vasospasm.

Substance P-induced relaxation and hyperpolarization in human cerebral arteries

1. Vascular effects of substance P were studied in human isolated pial arteries removed from 14 patients undergoing cerebral cortical resection. 2. Substance P induced a concentration-dependent relaxation in the presence of indomethacin. No relaxation was seen in arteries where the endothelium had been removed. 3. N omega-nitro-L-arginine (L-NOARG, 0.3 mM) abolished the relaxation in arteries from six patients. The relaxation was only partially inhibited in the remaining eight patients, the reduction of the maximum relaxation being less than 50% in each patient. 4. The L-NOARG-resistant relaxation was abolished when the external K+ concentration was raised above 30 mM. 5. Substance P caused a smooth muscle hyperpolarization (in the presence of L-NOARG and indomethacin), but only when the artery showed an L-NOARG-resistant relaxation. 6. The results indicate that nitric oxide is an important mediator of endothelium-dependent relaxation in human cerebral arteries. Furthermore, another endothelium-dependent pathway, causing hyperpolarization and vasodilatation, was identified in arteries from more than half the population of patients.

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

NO has clearly revolutionized our thinking about aspects of neurotransmission and neuronal signaling. It has also radically altered our thoughts about how synaptic transmission takes place. NO is emerging as an important regulator of a variety of physiological processes; however, under certain conditions of excessive formation, NO is emerging as an important mediator of pathological nervous tissue damage. Understanding the role of NO in these processes will hopefully lead to the development of selective therapeutic agents and to a better understanding of basic processes underlying normal and pathological neuronal functions.

Endothelins: a role in cerebrovascular disease?

Vasoactive factors produced and released by the endothelium exert a powerful influence on vascular tone in the cerebral circulation. Impaired endothelium-dependent responses, such as decreased production of endothelium-derived relaxing factors, and/or release of endothelium-derived contractile factors may give rise to different pathophysiological conditions. Among the endothelium-derived contractile factors the endothelins have recently received particular attention. Endothelin-1 is the major isoform in the endothelin family, which also includes endothelin-2 and endothelin-3. Endothelin-1 is synthesized within the endothelium of cerebral vessels, whereas both endothelin-1 and endothelin-3 in addition have been identified in neurons and glia. Recent electrophysiological work has suggested a neuromodulatory role for these peptides, but at present the general interest is mainly focused on their vasoactive role. Physiological stimuli such as hypoxia, anoxia, and hemodynamic shear stress will stimulate the endothelial endothelin production. In the brain, at least two types of specific subreceptors have been cloned; ETA receptors, exclusively associated with blood vessels and ETB receptors also found on glial, epithelial, and ependymal cells. The endothelins seem so far to be the most potent vasoconstrictors yet identified. The circulating plasma levels of immunoreactive endothelin are low. Since more than 80% of the total amount released from endothelial cells seems to be secreted towards the underlying smooth muscle, endothelins have been ascribed a local vasoregulatory role. Endothelins are believed to be involved in several of our most common cerebrovascular diseases and the present review comments on their possible pathophysiological role in subarachnoid haemorrhage, cerebral ischemia, and migraine.

Astrocyte-derived lipoxygenase product evokes endothelium-dependent relaxation of the basilar artery

The goal of this study was to examine the possible production of vasoactive factors by astrocytes. We consistently observe that rat astroglial cells in suspension produce marked relaxation when added to precontracted rings of intact (but not endothelium-denuded) rabbit basilar artery. The ultimate mediator of this relaxation was endothelium-derived nitric oxide whose synthesis is activated by an as yet unidentified factor(s) produced tonically by astrocytes. The factor is relatively stable, and is not arachidonate, or a product of cyclooxygenase or P450 metabolism. Based upon studies with selective inhibitors, the factor appears to result from 12- or 15-lipoxygenase metabolism, the products of which are known to be vasoactive. In a separate series of experiments, astrocyte-conditioned medium stimulated the production of citrulline from L-arginine by nitric oxide synthase in bovine aortic endothelial cells. The possible significance for central nervous system (CNS) pathophysiology of an astrocyte-derived vasodilator is discussed.

Dilatation of cerebral arterioles in response to N-methyl-D-aspartate: role of CGRP and acetylcholine

The purpose of these experiments was to examine mechanisms by which N-methyl-D-aspartate (NMDA) produces nitric oxide-dependent vasodilatation in brain. Some nitrovasodilators appear to dilate cerebral arterioles, in part, by release of calcitonin gene-related peptide (CGRP) from trigeminal fibers. The first goal of this study was to examine the hypothesis that dilatation of cerebral arterioles in response to NMDA is mediated by activation of receptors for CGRP. Diameters of cerebral arterioles were measured using a closed cranial window in anesthetized rabbits. Topical CGRP (1 and 10 nM) dilated cerebral arterioles by 30 +/- 9 (mean +/- S.E.M.) and 72 +/- 9%, respectively, from a control diameter of 94 +/- 7 microns. This response was inhibited almost completely by the CGRP antagonist CGRP(8-37) (0.5 microM). NMDA (100 and 300 microM) dilated cerebral arterioles by 14 +/- 5 and 38 +/- 7% in the absence and 20 +/- 5% and 30 +/- 6% in the presence, respectively, of CGRP(8-37). Neurons may release acetylcholine in response to activation with NMDA. The second goal of the present study was to examine the hypothesis that dilatation of cerebral arterioles in response to NMDA is mediated by acetylcholine. Topical atropine (2 micrograms/ml) completely inhibited dilatation of cerebral arterioles in response to acetylcholine, but had no effect on vasodilatation in response to NMDA. Thus, vasodilatation of cerebral arterioles in response to NMDA does not appear to be dependent on activation of receptors for CGRP or acetylcholine.

Nitric oxide synthase inhibition and cerebrovascular regulation

There is increasing evidence that nitric oxide (NO) is an important molecular messenger involved in a wide variety of biological processes. Recent data suggest that NO is also involved in the regulation of the cerebral circulation. Thus, NO participants in the maintenance of resting cerebrovascular tone and may play an important role in selected vasodilator responses of the cerebral circulation. Furthermore, evidence has been presented suggesting that NO participates in the mechanisms of cerebral ischemic damage. Despite the widespread attention that NO has captured in recent years and the large number of studies that have been published on the subject, there is considerable controversy regarding the role of this agent in cerebrovascular regulation and in ischemic damage. In this paper the results of investigations on NO and the cerebral circulation are reviewed and the evidence for and against a role of NO is critically examined.

Nitric oxide and the cerebral circulation

BACKGROUND

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

SUMMARY OF REVIEW

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

CONCLUSIONS

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

Nitric oxide synthase inhibition alters cerebral blood flow and oxygen balance in focal cerebral ischemia in rats

BACKGROUND AND PURPOSE

This study investigated whether the nitric oxide synthase inhibitor NG-nitro-L-arginine-methyl ester (L-NAME) would alter blood flow and oxygen balance in the ischemic cerebrocortex of isoflurane-anesthetized Long-Evans rats.

METHODS

Fifteen minutes after middle cerebral artery occlusion, L-NAME (1.5 mg/min per kilogram) was infused intravenously to the L-NAME group (n = 14), and normal saline was given to the control group (n = 14) for 45 minutes. In each group, regional cerebral blood flow was determined with [14C]iodoantipyrine, and arterial and venous oxygen saturations were determined by microspectrophotometry.

RESULTS

In both groups regional cerebral blood flow of the ischemic cortex was significantly lower than that of the contralateral cortex ([mean +/- SD] 55 +/- 13 versus 110 +/- 29 mL/min per 100 g in the control group and 35 +/- 13 versus 90 +/- 24 mL/min per 100 g in the L-NAME group). Compared with the blood flow in the ischemic cortex of the control group, L-NAME significantly reduced ischemic blood flow by 36%. Venous oxygen saturation was significantly increased in the ischemic cortex (41 +/- 1% versus 44 +/- 3%) but decreased in the contralateral cortex (65 +/- 3% versus 61 +/- 4%) by L-NAME. Calculated ischemic cortical oxygen consumption in the L-NAME group was 39% lower than that in the corresponding control group, whereas the difference was only 11% in the contralateral sides between groups. In both groups, the ratio of oxygen supply to consumption was lower in the ischemic than in the nonischemic regions. In the ischemic cortex, this ratio was significantly lower in the control group than in the L-NAME group (1.7 +/- 0.1 versus 1.9 +/- 0.1). In contrast, the ratio tended to be decreased by L-NAME in nonischemic regions.

CONCLUSIONS

These observations suggest that despite a decrease in cerebral blood flow, inhibition of nitric oxide synthesis mildly improves the oxygen supply and consumption balance in the ischemic cortex.

Targeted disruption of the neuronal nitric oxide synthase gene

By homologous recombination, we have generated mice that lack the neuronal nitric oxide synthase (NOS) gene. Neuronal NOS expression and NADPH-diaphorase (NDP) staining are absent in the mutant mice. Very low level residual catalytic activity suggests that other enzymes in the brain may generate nitric oxide. The neurons normally expressing NOS appear intact, and the mutant NOS mice are viable, fertile, and without evident histopathological abnormalities in the central nervous system. The most evident effect of disrupting the neuronal NOS gene is the development of grossly enlarged stomachs, with hypertrophy of the pyloric sphincter and the circular muscle layer. This phenotype resembles the human disorder infantile pyloric stenosis, in which gastric outlet obstruction is associated with the lack of NDP neurons in the pylorus.