Role of nitric oxide synthase inhibition in leukocyte-endothelium interaction in the rat pial microvasculature

Spatiotemporal dynamics of red blood cells in capillaries in layer I of the cerebral cortex and changes in arterial diameter during cortical spreading depression and response to hypercapnia in anesthetized mice

OBJECTIVE

Control of red blood cell velocity in capillaries is essential to meet local neuronal metabolic requirements, although changes of capillary diameter are limited. To further understand the microcirculatory response during cortical spreading depression, we analyzed the spatiotemporal changes of red blood cell velocity in intraparenchymal capillaries.

METHODS

In urethane-anesthetized Tie2-green fluorescent protein transgenic mice, the velocity of fluorescence-labeled red blood cells flowing in capillaries in layer I of the cerebral cortex was automatically measured with our Matlab domain software (KEIO-IS2) in sequential images obtained with a high-speed camera laser-scanning confocal fluorescence microscope system.

RESULTS

Cortical spreading depression repeatedly increased the red blood cell velocity prior to arterial constriction/dilation. During the first cortical spreading depression, red blood cell velocity significantly decreased, and sluggishly moving or retrograde-moving red blood cells were observed, concomitantly with marked arterial constriction. The velocity subsequently returned to around the basal level, while oligemia after cortical spreading depression with slight vasoconstriction remained. After several passages of cortical spreading depression, hypercapnia-induced increase of red blood cell velocity, regional cerebral blood flow and arterial diameter were all significantly reduced, and the correlations among them became extremely weak.

CONCLUSIONS

Taken together with our previous findings, these simultaneous measurements of red blood cell velocity in multiple capillaries, arterial diameter and regional cerebral blood flow support the idea that red blood cell flow might be altered independently, at least in part, from arterial regulation, that neuro-capillary coupling plays a role in rapidly meeting local neural demand.

Post-ischemic vascular adhesion protein-1 inhibition provides neuroprotection in a rat temporary middle cerebral artery occlusion model

We examined the neuroprotective efficacy associated with post-ischemic vascular adhesion protein-1 (VAP-1) blockade in rats subjected to transient (1 h) middle cerebral artery occlusion (MCAo). We compared saline-treated control rats to rats treated with a highly selective VAP-1 inhibitor, LJP-1586 [Z-3-fluoro-2-(4-methoxybenzyl) allylamine hydrochloride]. Initial intraperitoneal LJP-1586 (or saline control) treatments were delayed until 6 h or 12 h reperfusion. At 72-h reperfusion, LJP-1586-treated rats displayed 51% and 33% smaller infarct volumes, relative to their controls, in the 6- and 12-h treatment groups, respectively. However, only in the 6-h treatment group was the infarct volume reduction significant (p < 0.05). On the other hand, we observed significantly improved neurologic functions in both 6- and 12-h treatment groups, versus their matched controls (p < 0.05). Also, the effect of 6-h LJP-1586 treatment on post-ischemic leukocyte trafficking in pial venules overlying the ischemic cortex was evaluated using intravital microscopy. These experiments revealed that: 1) LJP-1586 did not affect intravascular leukocyte (largely neutrophil) adhesion, at least out to 12-h reperfusion; and 2) the onset of neutrophil extravasation, which occurred between 6-8-h reperfusion in control rats, was prevented by LJP-1586-treatment. In conclusion, in rats subjected to transient MCAo, selective VAP-1 pharmacologic blockade provided neuroprotection, with a prolonged therapeutic window of 6-12-h reperfusion.

Control of brain capillary blood flow

While it has been widely confirmed that cerebral blood flow is closely coupled with brain metabolism, it remains a matter of controversy whether capillary flow is directly controlled to meet the energy demands of the parenchyma. Since the capillary is known to lack smooth muscle cells, it has generally been considered that capillary flow is not regulated in situ. However, we now have increasing data supporting the physiological control of capillary flow. The observation of heterogeneity in the microcirculation in vivo has suggested that intravascular factors may be involved in the flow control, including non-Newtonian rheology, red blood cell flow, leukocyte adhesion, release of vasoactive mediators, and expression of glycoproteins on the endothelial cells. Astrocytes, a key mediator of the neurovascular unit, and intrinsic innervation may also regulate capillary flow. In addition, recent findings on pericyte contractility have attracted the attention of many researchers. Finally, based on these findings, we present a new model of flow control, the proximal integration model, in which localized neural activity is detected at nearby capillaries and the vasodilation signal is transmitted proximally along the vessel. Signals are then integrated at the precapillary arterioles and other arterioles further upstream and regulate the capillary flow.

Oscillating neuro-capillary coupling during cortical spreading depression as observed by tracking of FITC-labeled RBCs in single capillaries

Coupling between capillary red blood cell (RBC) movements and neuronal dysfunction during cortical spreading depression (CSD) was examined in rats by employing a high-speed camera laser-scanning confocal fluorescence microscope system in conjunction with our Matlab domain software (KEIO-IS2). Following microinjection of K(+) onto the surface of the brain, changes in electroencephalogram (EEG), DC potential and tissue optical density were all compatible with the occurrence of a transient spreading neuronal depression. RBC flow in single capillaries was not stationary. Unpredictable redistribution of RBCs at branches of capillaries was commonly observed, even though no change in diameter was apparent at the reported site of the capillary sphincter and no change of arteriolar-venule pressure difference was detected. There appeared to be a slow morphological change of astroglial endfeet. When local neurons were stunned transiently by K(+) injection, the velocity and oscillation frequency of RBCs flowing in nearby capillaries started to decrease. The flow in such capillaries was rectified, losing oscillatory components. Sluggish floating movements of RBCs in pertinent capillaries were visualized, with occasional full stops. When CSD subsided, RBC movements recovered to the original state. We postulate that neuronal depolarization blocks oscillatory signaling to local capillaries via low-shear plasma viscosity increases in the capillary channels, and a complex interaction between the RBC surface and the buffy coat on the capillary wall surface increases the capillary flow resistance. Then, when CSD subsides and oscillatory neuronal function is recovered, the normal physiological conditions are restored.

Estrogen replacement therapy in diabetic ovariectomized female rats potentiates postischemic leukocyte adhesion in cerebral venules via a RAGE-related process

In this study, we tested the hypothesis that the documented transformation of 17beta-estradiol (E2) from a counterinflammatory hormone in nondiabetic (ND) rats to a proinflammatory agent in rats with diabetes mellitus (DM) is due to an enhanced contribution from the receptor for advanced glycation end products (RAGE). Rhodamine 6G-labeled leukocytes were observed through a closed cranial window in rats. In vivo pial venular leukocyte adherence and infiltration were measured over 10 h reperfusion after transient forebrain ischemia in DM (streptozotocin) versus ND intact, ovariectomized (OVX), and E2-replaced (for 7-10 days) OVX (OVE) females. The role of RAGE was examined in two ways: 1) RAGE knockdown via topical application of RAGE antisense versus missense oligodeoxynucleotide or 2) intracerebroventricular injection of the RAGE decoy inhibitor, soluble RAGE. Among diabetic rats, the lowest levels of cortical RAGE mRNA and immunoreactivity of the RAGE ligand, AGE, were seen in OVX females, with significantly higher levels exhibited in intact and OVE females. However, results from the analysis of cortical RAGE protein only partially tracked those findings. When comparing ND to DM rats, cortical AGE immunoreactivity was significantly lower in OVE and intact females but similar in OVX rats. In DM rats, the level of postischemic leukocyte adhesion and infiltration (highest to lowest) was OVE>intact>>untreated OVX. In NDs, adhesion was highest in the untreated OVX group. Leukocyte extravasation was observed at >6 h postischemia but only in diabetic OVE and intact females and in ND OVX (untreated) rats. Pretreatment with RAGE antisense-oligodeoxynucleotide or soluble RAGE attenuated postischemic leukocyte adhesion and prevented infiltration but only in the diabetic OVE and intact groups. These results indicate that the exacerbation of postischemic leukocyte adhesion by chronic E2 replacement therapy in diabetic OVX females involves a RAGE-related mechanism. Targeting RAGE may restore the neuroprotective effect of E2 replacement therapy in diabetic females.

The central hypotensive effect induced by alpha 2-adrenergic receptor stimulation is dependent on endothelial nitric oxide synthase

OBJECTIVE

The aim of the present study was to determine whether the central antihypertensive effect of drugs that act via central alpha 2-adrenergic receptors is mediated by the nitric oxide-ergic system.

METHODS

The hypotensive effects of dexmedetomidine, a 'pure' alpha2-adrenergic agonist, were compared in endothelial nitric oxide synthase knockout and wild-type control mice.

RESULTS

When injected intravenously (5 mug/kg) in wild-type mice, dexmedetomidine elicited a depressor response (60 +/- 4 to 34 +/- 1 mmHg, P < 0.05), but had no hypotensive effect in endothelial nitric oxide synthase (eNOS) knockout mice (84 +/- 7 to 84 +/- 7 mmHg, P > 0.05). In the presence of N-omega-nitro-L-arginine, a nonselective nitric oxide synthase (NOS) blocker that does not cross the blood-brain barrier, the hypotensive effect of dexmedetomidine was not abolished (Delta MAP = 21 +/- 2 mmHg vs. Delta MAP = 26 +/- 3 mmHg, P > 0.05).

CONCLUSIONS

It is concluded that the central cardiovascular effects of alpha 2-adrenergic agonists, such as dexmedetomidine, require an intact expression of eNOS within the brain. This study raises the interesting question of whether central eNOS itself might be considered as a target for new cardiovascular drugs regardless of any activation of alpha 2-adrenergic receptors.

Changes in nitric oxide synthase expression in young and adult rats after spinal cord injury

OBJECTIVE

To examine the clinical meaning of the changes in nitric oxide synthase (NOS) expression and activity after spinal cord injury (SCI) according to the age of the experiment animal.

MATERIAL AND METHOD

Ten 5- and 16-week-old Sprague-Dawley rats were laminectomized at T10 and SCI induced at this level using a New York impactor. Outcome measures to assess SCI utilized the Basso-Beatti-Bresnahan scale to quantitate hind limb motor dysfunction as a functional outcome measure. NOS isoforms (nNOS, neuronal NOS; iNOS, inducible NOS; and eNOS, endothelial NOS) were also immunolocalized in sections of control and spinal cord injury in the two sample groups using specific monoclonal antibodies. Student's t-test evaluated the difference between the young and adult rats, and P<0.05 was considered as significant value.

RESULT

As the expression of nNOS on the spinal gray matter of the adult rat decreased, eNOS activity increased. Different from the adult rat, expression of the nNOS in the young rat was maintained until 1 day after SCI, and compared with the adult rat; eNOS activity was increased in the vessels from the damaged gray matter area after 7 days of SCI. iNOS expression was maintained until the 7th day of SCI on the adult rat, but iNOS expression after 7 days of SCI on young rat decreased. The young rat showed relatively less motor disability on the hind limb when compared with the adult rat, and had a rapid recovery.

CONCLUSION

Neural protective eNOS activity increased after SCI in the young rat, and neural destructive iNOS expression was more remarkable in the adult rat.

Effects of the chemokine CCL2 on blood-brain barrier permeability during ischemia-reperfusion injury

The chemokine CCL2 is considered as one of the main effectors driving postischemic infiltration of monocytes into the brain parenchyma. New experimental data, however, suggest that CCL2 could also participate in blood-brain barrier (BBB) 'opening' during the transmigration of monocytes. The current study examines the role of CCL2 in regulating BBB permeability after ischemia in vitro. To address this issue, an in vitro BBB model (coculture of astrocytes and brain endothelial cells) was subjected to 5 h of oxygen glucose deprivation, followed by reoxgenation (in vitro ischemia/reperfusion (I/R)) for 0 to 48 h. During reperfusion, there was a biphasic enhancement of barrier permeability, with a 200-fold increase in barrier permeability to FITC-albumin at 6 h and a further period of disruption around 24 h. The latter coincided with increased secretion of CCL2 by both astrocytes and brain endothelial cells and increased levels of the CCL2 receptor, CCR2. Applying antisense oligonucleotide or neutralizing antibody to block CCL2 significantly decreased I/R-induced enhancement of BBB permeability (approximately twofold) and redistribution of tight-junction (TJ) proteins (occludin, zonula occluden-1, 2, claudin-5). Similarly, absence of CCR2 from endothelial cells caused stabilization of TJ complexes and decreased the permeability of brain endothelial barrier during in vitro I/R. These data suggest CCL2/CCR2 has an important role in regulating brain endothelial permeability and might be a potential novel therapeutic target for stroke.

Cerebral Endothelial Nitric Oxide Synthase Expression is Reduced After Very Low Flow Bypass

BACKGROUND

In previous studies we have shown that delayed capillary reperfusion after low flow bypass predicts neurologic injury. In this acute study, we hypothesized that low flow reduces endothelial nitric oxide synthase (eNOS) expression, which may lead to more profound inflammatory response and delayed capillary perfusion.

METHODS

Twelve piglets (13.2 +/- 0.7 kg) had a cranial window placed over the parietal cerebral cortex for direct examination of the microcirculation using intravital fluorescence microscopy. Animals were cooled to 15 degrees C or 34 degrees C on cardiopulmonary bypass (pH stat, hematocrit 30%, pump flow 100 mL/kg/minute) followed by 2 hours of low flow (50 mL/kg/minute) or very low flow (10 mL/kg/minute). Rhodamine staining was used to observe adherent and rolling leukocytes in postcapillary venules. The eNOS protein expression was determined by Western immunoblotting.

RESULTS

High temperature and low flow rate correlated with significantly reduced eNOS expression (p < 0.01). Univariate comparisons based on Student t tests indicated that eNOS protein levels were lower at 34 degrees C than at 15 degrees C (0.7 +/- 0.6 vs 1.7 +/- 0.5, p < 0.01) and at 10 mL/kg per minute compared with 50 mL/kg per minute (0.8 +/- 0.7 vs 1.6 +/- 0.5, p = 0.03). Moreover, two-way analysis of variance revealed that temperature (F = 21.6, p < 0.001) and flow rate (F = 13.8, p = 0.005) were independent multivariate predictors of eNOS expression. During low flow bypass, eNOS was inversely correlated with numbers of adherent (p = 0.002) and rolling (p = 0.006) leukocytes, following an exponential decay curve closely.

CONCLUSIONS

eNOS expression is reduced after very low flow bypass, particularly at a higher bypass temperature. This is associated with delayed capillary reperfusion. Reduced eNOS is also associated with increased white cell activation which may lead to greater neurologic injury.

Estrogen replacement treatment in diabetic ovariectomized female rats potentiates postischemic leukocyte adhesion in cerebral venules

BACKGROUND AND PURPOSE

Chronic 17beta-estradiol (E2) replacement therapy in ovariectomized (OVX) female rats reduces leukocyte adhesion and brain damage after transient forebrain ischemia. Recently, we found that E2 treatment in diabetic OVX females was associated with enhanced postischemic neuropathology. We tested the hypothesis that in chronically hyperglycemic diabetic OVX females, chronic E2 replacement potentiates post-transient forebrain ischemia leukocyte adhesion.

METHODS

Pial venules were observed through closed cranial windows. Adherence of rhodamine 6G-tagged leukocytes was monitored before and 10 hours after transient forebrain ischemia (20 minutes right common carotid artery occlusion plus hemorrhagic hypotension) in intact, untreated OVX and E2-treated OVX females rendered diabetic via streptozotocin. Leukocyte adhesion was quantitated as the percentage venular area occupied by adherent leukocytes.

RESULTS

At 2 hours after transient forebrain ischemia, a similar low level of leukocyte adhesion was seen in the 3 groups (<3% of the venular area). Starting at approximately 4 hours after ischemia, leukocyte adhesion in the E2-treated OVX females rose to significantly higher levels compared with the other groups. Relative to the 2-hour value, the level of adhesion at 10 hours was 12.5-fold, 4-fold, and 5-fold greater in the E2-treated OVX, OVX, and intact groups, respectively. Leukocyte extravasation (beginning after 6 hours of reperfusion) was observed in a majority (64%) of the E2-treated animals, with limited or no extravasation seen in the intact or OVX groups.

CONCLUSIONS

These results suggest that factors associated with diabetes and chronic hyperglycemia convert E2 from a counterinflammatory to a proinflammatory substance in an ischemic setting.

Cerebrovascular inflammation after brief episodic hypoxia: modulation by neuronal and endothelial nitric oxide synthase

Obstructive sleep apnea, apnea of prematurity, and sudden infant death syndrome are associated with a high risk of morbidity and mortality secondary to the neuronal and cerebrovascular consequences of the associated intermittent hypoxia. We hypothesized that episodic hypoxia (EH) promotes inflammation in the cerebral microcirculation and that nitric oxide (NO) produced by the endothelial and neuronal isoforms of NO synthase (eNOS and nNOS, respectively) modulates this response. Anesthetized and ventilated Swiss-Webster ND4 mice, wild-type mice, and NO synthase knockout mice were subjected to a 1-h period of EH (twelve 30-s periods of hypoxia every 5 min). Four, 24, or 48 h later, mice were reanesthetized for imaging of leukocyte dynamics in the cortical venular microcirculation by epifluorescence videomicroscopy through closed cranial windows. In Swiss-Webster ND4 mice, leukocyte adherence increased 2.1-fold at 4 h, 3.4-fold at 24 h, and 1.8-fold at 48 h relative to time-matched, normoxic controls; there was no evidence of delayed hippocampal CA1 pyramidal cell death. A similar response was noted in wild-type mice. However, in eNOS knockouts, leukocyte-endothelial cell adherence was elevated to 4.4-fold over baseline 24 h after EH, and a significant fraction of these animals showed evidence of delayed CA1 cell death. Conversely, in nNOS knockouts, no increase in adherence was noted at 24 h and CA1 viability remained unaffected. We conclude that NO derived from nNOS promotes an inflammatory response in the cerebrovascular microcirculation after short-term EH and that NO produced by eNOS blunts the extent of this response and exerts neuroprotective effects.

Role of nitric oxide in the regulation of spinal arteriolar tone

OBJECTIVE

The important role played by nitric oxide (NO) in the maintenance of cerebral arterial tone, both in the resting state and after injury, has been demonstrated, but the role of NO in spinal arteries is unknown. The aim of this study was to verify, by topical application of a NO synthase inhibitor in vivo, the hypothesis that NO plays an important role in the maintenance of spinal arteriolar tone.

METHODS

Closed spinal windows were prepared at the C6 level, for observation of arteriolar reactivity. Male Wistar rats were divided into four groups in the resting state and three groups in the compressed state (which was produced by increasing the intrathecal window pressure). The control group underwent superfusion of artificial cerebrospinal fluid into the spinal window. The nitro-L-arginine-methyl ester (L-NAME) and nitro-D-arginine-methyl ester groups underwent superfusion of the NO synthase inhibitor L-NAME and its inactive enantiomer nitro-d-arginine-methyl ester, respectively. The L-NAME/S-nitroso-acetylpenicillamine (SNAP) group underwent mixed superfusion of L-NAME and the direct NO donor SNAP, for investigation of the effects of an exogenous NO donor.

RESULTS

In the resting state, the arterioles constricted significantly in the L-NAME group, compared with values before L-NAME superfusion and those for the other groups. In the L-NAME/SNAP group, the arterioles dilated significantly after SNAP superfusion, compared with values before superfusion and those for the other groups. In the compressed state, the arterioles dilated after compression in all three groups but the dilation was significantly attenuated in the L-NAME group, compared with values for the control and nitro-D-arginine-methyl ester groups.

CONCLUSION

It is suggested that NO plays an important role in the maintenance of spinal arteriolar tone in the resting and compressed states of the spinal cord.

Combined endothelial nitric oxide synthase upregulation and caveolin-1 downregulation decrease leukocyte adhesion in pial venules of ovariectomized female rats

BACKGROUND AND PURPOSE

We recently found that chronic estrogen depletion enhances leukocyte adhesion in pial venules in the female rat, while estrogen repletion decreases it. Estrogen-associated repression of inflammation may be due to upregulation of the endothelial isoform of nitric oxide synthase (eNOS) and concomitant downregulation of the endogenous inhibitor of eNOS, caveolin-1 (CAV-1). In this study we examined the effects of estrogen-independent eNOS upregulation (via simvastatin) and/or CAV-1 downregulation (antisense) on pial venular leukocyte adhesion in ovariectomized (OVX) rats.

METHODS

Intact and OVX rats were prepared with closed cranial windows. Adherent rhodamine 6G-labeled leukocytes were viewed by intravital microscopy. To demonstrate the importance of pial venular eNOS in the resistance to leukocyte adhesion, intact female rats were treated with a nonselective (N(G)-nitro-L-arginine) or a neuronal NOS-selective (7-nitroindazole) inhibitor. In OVX females, leukocyte adhesion was compared in the following groups: (1) untreated; (2) treated with simvastatin; (3) treated with simvastatin plus CAV-1 antisense; (4) treated with simvastatin plus CAV-1 missense; (5) treated with CAV-1 antisense; and (6) treated with CAV-1 missense.

RESULTS

In intact females, pial venular leukocyte adhesion was increased when total NOS activity, but not neuronal NOS activity alone, was blocked. In OVX rats, basal leukocyte adhesion, measured as the percentage of venular area occupied by adherent leukocytes, was attenuated (by approximately equal 60%) only in the presence of combined simvastatin plus CAV-1 antisense treatment.

CONCLUSIONS

Present findings demonstrate that eNOS-derived NO plays an important role in limiting cerebral venular leukocyte adhesion in female rats. These data also suggest that simvastatin-induced upregulation of eNOS expression in OVX rats will not restore eNOS function, as measured by decreased leukocyte adhesion, unless CAV-1 levels are reduced as well.

Lack of endothelial nitric oxide synthase aggravates murine pneumococcal meningitis

Nitric oxide (NO) plays a central role in the pathogenesis of bacterial meningitis. However, the role of NO produced by endothelial NO synthase (eNOS) in meningitis is still unclear. We investigated the influence of eNOS depletion on the inflammatory host response, intracranial complications, and outcome in experimental pneumococcal meningitis. Leukocyte accumulation in the cerebrospinal fluid was more pronounced in infected eNOS-deficient mice than in infected wild type mice. This effect could be attributed to an increased expression of P-selectin, macrophage inflammatory protein-2, keratinocyte-derived cytokine, and interleukin (IL)-1beta in the brain of infected eNOS-deficient mice. However, no differences in the cerebral expression of intercellular adhesion molecule-1, tumor necrosis factor-alpha, and IL-6 as well as of neuronal NOS and inducible NOS could be detected between infected wild type and mutant mice. In addition to enhanced leukocyte infiltration into the CSF, meningitis-associated intracranial complications including blood-brain barrier disruption and the rise in intracranial pressure were significantly augmented in infected eNOS-deficient mice. The aggravation of intracranial complications was paralleled by a worsening of the disease, as evidenced by a more pronounced hypothermia, an enhanced weight reduction, and an increased death rate. The current data indicate that eNOS deficiency is detrimental in bacterial meningitis. This effect seems to be related to an increased expression of (certain) cytokines/chemokines and adhesion molecules; thus leading to increased meningeal inflammation and, subsequently, to aggravated intracranial complications.

Effects of estrogen on leukocyte adhesion after transient forebrain ischemia

BACKGROUND AND PURPOSE

Recent findings indicate that estrogen (ie, 17beta-estradiol [E(2)]) provides neuroprotection in models of transient global and focal ischemia. Enhanced postischemic leukocyte adhesion and infiltration have been linked to neuropathology in the brain as well as other tissues. We recently showed that estrogen reduces leukocyte adhesion in the cerebral circulation of female rats during resting conditions.

METHODS

We compared leukocyte adhesion in pial venules in vivo in intact, ovariectomized (OVX), and E(2)-treated OVX female rats subjected to transient forebrain ischemia (30-minute right common carotid artery occlusion and hemorrhagic hypotension) and reperfusion. Adherent rhodamine-6G-labeled leukocytes were viewed through a closed cranial window with the use of intravital microscopy. Leukocyte adhesion was measured before ischemia and at different times after reperfusion.

RESULTS

Before ischemia, leukocyte adhesion (measured as a percentage of venular area occupied by adherent leukocytes) was 2 to 3 times greater in OVX versus intact or E(2)-treated OVX rats (7.0%, 3.4%, and 2.2%, respectively). This difference disappeared at 120 minutes of reperfusion, when comparable levels of enhanced leukocyte adhesion were observed in all groups. In OVX rats, leukocyte adhesion remained elevated after 4 and 6 hours of reperfusion (11.6% and 12.9%, respectively), while the other 2 groups showed significantly lower levels (5.0% and 5.8% for intact rats and 7.0% and 7.2% for E(2)-treated OVX rats).

CONCLUSIONS

Present results demonstrate that estrogen modulates leukocyte adhesion in the cerebral circulation after transient forebrain ischemia. This effect suggests that decreased leukocyte adhesion may be an important mechanism in estrogen-mediated neuroprotection.

Leukocyte adherence contributes to disruption of the blood-brain barrier during activation of mast cells

The goal of this study was to examine the role of leukocytes in disruption of the blood-brain barrier during activation of mast cells using compound 48/80. We examined the pial microcirculation in rats using intravital fluorescence microscopy. Permeability of the blood-brain barrier (clearance of fluorescent-labeled dextran; molecular weight 10000 daltons; FITC-dextran-10 K) was determined while suffusing with vehicle or compound 48/80 (10 or 25 microg/ml). During superfusion with vehicle (saline), clearance of FITC-dextran-10 K from pial vessels was modest and remained relatively constant during the experimental period (0.52+/-0.05 ml/sx10(-6) at 80 min). In addition, diameter of pial arterioles remained constant (32+/-5 microm) while suffusing with vehicle. In contrast, topical application of compound 48/80 produced marked disruption of the blood-brain barrier to FITC-dextran-10 K. For example, suffusion with compound 48/80 (25 microg/ml) increased clearance of FITC-dextran-10 K about 4-fold to 2.26+/-0.25 ml/sx10(-6) at 80 min. In addition, superfusion with compound 48/80 (25 microg/ml) constricted pial arterioles by 26+/-9% at 80 min. To determine a potential role for leukocyte adhesion to endothelium in disruption of the blood-brain barrier during suffusion with compound 48/80, we examined permeability during suffusion with compound 48/80 (25 microg/ml) in the presence of WT.3 (2 mg/kg i.v.), a monoclonal antibody directed against the functional epitope of the leukocyte adhesive glycoprotein (CD18; LFA-1beta). We found that infusion of WT.3 markedly attenuated disruption of the blood-brain barrier to FITC-dextran-10 K in response to compound 48/80. The clearance of FITC-dextran-10 K during superfusion with compound 48/80 in the presence of WT.3 was 1.29+/-0.14 ml/sx10(-6) at 80 min (P<0.05). Thus, the findings of the present study suggest that application of compound 48/80, to degranulate mast cells, activates the adhesion of leukocytes to cerebral venular endothelium which contributes to disruption of the blood-brain barrier.

Modulation by nitric oxide of cerebral neutrophil accumulation after transient focal ischemia in rats

A beneficial role of nitric oxide (NO) after cerebral ischemia has been previously attributed to its vascular effects. Recent data indicate a regulatory role for NO in initial leukocyte-endothelial interactions in the cerebral microcirculation under basal and ischemic conditions. In this study, the authors tested the hypothesis that endogenous NO production during and/or after transient focal cerebral ischemia can also be neuroprotective by limiting the process of neutrophil infiltration and its deleterious consequences. Male Sprague-Dawley rats were subjected to 2 hours occlusion of the left middle cerebral artery and the left common carotid artery. The effect of NG-nitro-L-arginine methyl ester (L-NAME) (10 mg/kg, intraperitoneally), an NO synthase inhibitor, was examined at 48 hours after ischemia on both infarct size and myeloperoxidase activity, an index of neutrophil infiltration. L-NAME given 5 minutes after the onset of ischemia increased the cortical infarct volume by 34% and increased cortical myeloperoxidase activity by 60%, whereas administration of L-NAME at 1, 7, and 22 hours of reperfusion had no effect. Such exacerbations of infarction and myeloperoxidase activity produced when L-NAME was given 5 minutes after the onset of ischemia were not observed in rats rendered neutropenic by vinblastine. These results suggest that after transient focal ischemia, early NO production exerts a neuroprotective effect by modulating neutrophil infiltration.

Cerebral Endothelial Cells Release TNF-α After Stimulation with Cell Walls of Streptococcus pneumoniae and Regulate Inducible Nitric Oxide Synthase and ICAM-1 Expression Via Autocrine Loops

TNF-α, inducible NO synthase (iNOS), and ICAM-1 are considered to be key proteins in the inflammatory response of most tissues. We tested the hypothesis that cell walls of Streptococcus pneumoniae (PCW), the most common cause of adult bacterial meningitis, induce TNF-α, iNOS, and ICAM-1 expression in rat primary brain microvascular endothelial cell cultures. We detected TNF-α mRNA by RT-PCR already 1 h after stimulation with PCW, while TNF-α protein peaked at 4 h (9.4 ± 3.6 vs 0.1 ± 0.1 pg/μg protein). PCW induced iNOS mRNA 2 h after stimulation, followed by an increase of the NO degradation product nitrite (18.1 ± 4 vs 5.8 ± 1.8 at 12 h; 18.1 ± 4 vs 5.8 ± 1.8 pmol/μg protein at 72 h). The addition of TNF-α Ab significantly reduced nitrite production to 62.2 ± 14.4% compared with PCW-stimulated brain microvascular endothelial cells (100%). PCW induced the expression of ICAM-1 (measured by FACS), which was completely blocked by TNF-α Ab (142 ± 18.6 vs 97.5 ± 12.4%; 100% unstimulated brain microvascular endothelial cells). Cerebral endothelial cells express TNF-α mRNA as well as iNOS mRNA and release the bioactive proteins in response to PCW. PCW-induced NO production is mediated in part by an autocrine pathway involving TNF-α, whereas ICAM-1 expression is completely mediated by this autocrine loop. By these mechanisms, cerebral endothelial cells may regulate critical steps in inflammatory blood-brain-barrier disruption of bacterial meningitis.

Estrogen reduces leukocyte adhesion in the cerebral circulation of female rats

In this study the authors addressed the hypothesis that estrogen (i.e., 17beta-estradiol) acts to repress leukocyte adhesion. The experiments involved comparing leukocyte adhesion in cerebral venules in vivo, in intact ovariectomized and 17beta-estradiol-treated (100 microg/kg/day for 1 week) ovariectomized female rats using topical applications of the adhesion-promoting drug, phorbol 12-myristate 13-acetate (PMA). Adherent Rhodamine-6G-labeled leukocytes were viewed through a closed cranial window using intravital microscopy/videometry. Leukocyte dynamics were recorded at baseline and after each dose of PMA. The PMA was suffused (1.0 mL/min) at increasing concentrations (0.01, 0.1, and 1.0 micromol/L, 15 minutes at each level). A videotape record of each experiment was made for subsequent analysis of leukocyte adhesion. The data showed that the percentage venular area occupied by adherent leukocytes at baseline was significantly greater in the ovariectomized compared to the intact and 17beta-estradiol-treated groups (12.2%, 3.4%, and 4.2% respectively). That relationship was maintained during PMA treatments to the extent that the percentage venular area occupied by adherent leukocytes increased to 26.4% in the untreated ovariectomized group compared to 14.4% and 11.3% in the intact and 17beta-estradiol-treated groups, respectively. In conclusion, the authors found chronic estrogen depletion enhances leukocyte adhesion in the rat cerebral circulation. Estrogen repletion in such animals is accompanied by a significant reduction in leukocyte adhesion. These findings could, at least in part, account for the ischemic brain damage seen in ovariectomized versus intact females, and the restored neuroprotection observed upon 17beta-estradiol treatment reported in earlier studies.

Modulation of basal and postischemic leukocyte-endothelial adherence by nitric oxide

BACKGROUND AND PURPOSE

Recent studies indicate that leukocytes are important contributors to secondary vascular and parenchymal injury after cerebral ischemia. The present study was undertaken to define nitric oxide (NO)-based mechanisms that regulate leukocyte-endothelial interactions in the cerebral vasculature, how these mechanisms are affected by cerebral ischemia, and whether NO-based therapies can affect postischemic leukocyte dynamics.

METHODS

Leukocyte adherence to pial venules of anesthetized newborn piglets was quantified by in situ fluorescence videomicroscopy through closed cranial windows during basal conditions and during reperfusion after 9 minutes of asphyxia. Nitric oxide synthase (NOS) was inhibited by local window superfusion of L-nitroarginine; superfusion of sodium nitroprusside was used to donate NO.

RESULTS

Local inhibition of NOS under resting conditions increased leukocyte-endothelial adherence 2.2-fold and 3.9-fold over baseline values after 1 hour and 2 hours, respectively; this response was completely blocked by cosuperfusion with L-arginine. Cosuperfusion of superoxide dismutase reversed L-nitroarginine-induced leukocyte adherence by 89% and 63% at these respective time points. The extent of acute leukocyte adherence elicited by NOS inhibition was similar in magnitude to that observed during the initial 2 hours of reperfusion after asphyxia. Leukocyte adherence was not additionally increased in asphyxic animals treated with L-nitroarginine. Sodium nitroprusside robustly inhibited asphyxia-induced leukocyte adherence back to control levels.

CONCLUSIONS

NO exerts a tonic antiadherent effect in the cerebral microcirculation by inactivation of adherence-promoting superoxide radical formation. Cerebral ischemia is associated with an inhibition of NOS or lower levels of NO, which results in leukocyte-endothelial adherence that can be prevented by NO donors. The latter may be useful therapeutically to prevent the purported vascular and parenchymal dysfunction and injury caused by activated leukocytes in ischemic brain.

In vivo fibre optic confocal imaging of microvasculature and nerves in the rat vas deferens and colon

A fluorescence confocal microscopy technique was employed to obtain subsurface images of nerve and microvascular structure in the vas deferens and colon of the living rat. The use of dual labelling with vital dyes and 2-channel confocal acquisition allowed differentiation of microscopic structure at both low and higher magnification. Characteristic staining patterns of nerves and blood vessels were repeatedly obtained in each tissue, suggesting the potential of this technique for studying morphological changes associated with surgical procedures and/or models of neuronal or vascular pathology.

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.

Early white blood cell dynamics after traumatic brain injury: effects on the cerebral microcirculation

Increasing clinical and experimental evidence suggests that traumatic brain injury (TBI) elicits an acute inflammatory response. In the present study we investigated whether white blood cells (WBC) are activated in the cerebral microcirculation early after TBI and whether WBC accumulation affects the posttraumatic cerebrovascular response. Twenty-four anesthetized rabbits had chronic cranial windows implanted 3 weeks before experimentation. Animals were divided into four experimental groups and were studied for 7 hours (groups I, IIa, and III) or 2 hours (group IIb). Intravital fluorescence videomicroscopy was used to visualize WBC (rhodamine 6G, intravenously), pial vessel diameters, and blood-brain barrier (BBB) integrity (Na+-fluorescein) at 6 hours (groups I, IIa, and III) or 1 hour (group IIb) after TBI. Group I (n = 5) consisted of sham-operated animals. Groups IIa (n = 7) and IIb (n = 5) received fluid-percussion injury at 1 hour. Group III (n = 7) received fluid-percussion injury and 1 mg/kg anti-adhesion monoclonal antibody (MoAb) "IB4" 5 minutes before injury. Venular WBC sticking, intracranial pressure (ICP), and arterial vessel diameters increased significantly for 6 hours after trauma. IB4 reduced WBC margination and prevented vasodilation. Intracranial pressure was not reduced by treatment with IB4. Blood-brain barrier damage occurred at 1 hour but not at 6 hours after TBI and was independent of WBC activation. This first report using intravital videomicroscopy to study the inflammatory response after TBI reveals upregulated interaction between WBC and cerebral endothelium that can be manipulated pharmacologically. White blood cell activation is associated with pial arteriolar vasodilation. White blood cells do not induce BBB breakdown less than 6 hours after TBI and do not contribute to posttraumatic ICP elevation. The role of WBC more than 6 hours after TBI should be investigated further.