Absence of N-methyl-D-aspartate receptors on ovine cerebral microvessels

Amyloid-β disrupts unitary calcium entry through endothelial NMDA receptors in mouse cerebral arteries

Transient increases in intracellular Ca2+ activate endothelium-dependent vasodilatory pathways. This process is impaired in cerebral amyloid angiopathy, where amyloid-β(1-40) accumulates around blood vessels. In neurons, amyloid-β impairs the Ca2+-permeable N-methyl-D-aspartate receptor (NMDAR), a mediator of endothelium-dependent dilation in arteries. We hypothesized that amyloid-β(1-40) reduces NMDAR-elicited Ca2+ signals in mouse cerebral artery endothelial cells, blunting dilation. Cerebral arteries isolated from 4-5 months-old, male and female cdh5:Gcamp8 mice were used for imaging of unitary Ca2+ influx through NMDAR (NMDAR sparklets) and intracellular Ca2+ transients. The NMDAR agonist NMDA (10 µmol/L) increased frequency of NMDAR sparklets and intracellular Ca2+ transients in endothelial cells; these effects were prevented by NMDAR antagonists D-AP5 and MK-801. Next, we tested if amyloid-β(1-40) impairs NMDAR-elicited Ca2+ transients. Cerebral arteries incubated with amyloid-β(1-40) (5 µmol/L) exhibited reduced NMDAR sparklets and intracellular Ca2+ transients. Lastly, we observed that NMDA-induced dilation of pial arteries is reduced by acute intraluminal amyloid-β(1-40), as well as in a mouse model of Alzheimer's disease, the 5x-FAD, linked to downregulation of Grin1 mRNA compared to wild-type littermates. These data suggest that endothelial NMDAR mediate dilation via Ca2+-dependent pathways, a process disrupted by amyloid-β(1-40) and impaired in 5x-FAD mice.

Multiple Actions of Phencyclidine and (+)MK-801 on Isolated Bovine Cerebral Arteries

This study examines the direct effects of 3 noncompetitive N-methyl-D-aspartate receptor antagonists, phencyclidine (PCP), (+)MK-801, and (-)MK-801, on bovine middle cerebral arteries (BMCA). Rings of BMCA were mounted in isolated tissue chambers equipped with isometric tension transducers to obtain pharmacologic dose-response curves. In the absence of endogenous vasoconstrictors, the 3 N-methyl-D-aspartate antagonists each produced direct constriction of BMCA. The thromboxane A2 receptor antagonist SQ-29,548, the TxA2 synthase inhibitor furegrelate, the calcium antagonist nimodipine, and calcium-deficient media all inhibited maximal phencyclidine or (+)MK-801-induced constriction. Direct constriction by PCP or (+)MK-801 was independent of the presence of endothelium. When BMCA were preconstricted with potassium-depolarizing solution, PCP, (+)MK-801, and (-)MK-801 each produced only concentration-dependent relaxation. When BMCA were preconstricted with the stable TxA2 analog U-46,619 and exposed to increasing concentrations of PCP, (+)MK-801, or (-)MK-801, tension increased. Thromboxane A2 may contract BMCA by acting as a potassium channel blocker; iberiotoxin and tetraethylammonium both constrict BMCA. In Ca-deficient media containing either potassium or U-46,619, phencyclidine and (+)MK-801 each produced competitive inhibition of subsequent Ca-induced constriction. In additional experiments, arterial strips were mounted in isolated tissue chambers to directly measure calcium uptake, using Calcium as a radioactive tracer. Both phencyclidine and (+)MK-801 blocked potassium-stimulated or U-46,619-stimulated Ca uptake into arterial strips. These results suggest that phencyclidine and (+)MK-801 have 2 separate actions on BMCA. They may constrict arterial rings by releasing TxA2 from cerebrovascular smooth muscle, and relax arterial rings by acting as calcium antagonists.

Effects of interleukin-6 on the expression of tight junction proteins in isolated cerebral microvessels from yearling and adult sheep

OBJECTIVES

The blood-brain barrier is a selective diffusion barrier between brain parenchyma and the intravascular compartment. Tight junctions are integral components of the blood-brain barrier. Pro-inflammatory cytokines are important in the pathogenesis of brain injury and could modify the protein constituents of tight junctions. We hypothesized that interleukin-6 (IL-6) downregulates key protein constituents of endothelial tight junctions (e.g. occludin and claudin-5).

METHODS

We examined the effects of IL-6 on tight junction protein expression using an in vitro blood-brain barrier model. We isolated microvessels from yearling and adult ovine cerebral cortex and placed them into culture with IL-6 concentrations of 0 (control, phosphate-buffered saline), 1, 10, and 100 ng/ml for 24 h. Cerebral microvessels were harvested, Western immunoblot performed for occludin and claudin-5, densitometry performed, and results expressed as a ratio to control values.

RESULTS

Western immunoblot analysis showed that treatment with 100 ng/ml of IL-6, but not the lower concentrations, reduced (p < 0.05) occludin expression in microvessels from yearling and adult sheep and claudin-5 in microvessels from adult sheep. However, treatment with 10 ng/ml of IL-6 increased claudin-5 in microvessels from yearling sheep. The percent of lactate dehydrogenase released from the microvessels into the surrounding media was not increased by IL-6 treatment, suggesting that the reductions in tight junction proteins did not result from cell death. Treatment of adult cerebral cortical microvessels with IL-6 preincubated with anti-IL-6 monoclonal antibodies partially attenuated the reduction in claudin-5.

CONCLUSION

We conclude that IL-6 modulates tight junction protein expression in cerebral cortical microvessels from yearling and adult sheep.

Cerebromicrovascular endothelial cells are resistant to L-glutamate

Cerebral microvascular endothelial cells (CMVECs) have recently been implicated as targets of excitotoxic injury by l-glutamate (l-glut) or N-methyl-d-aspartate (NMDA) in vitro. However, high levels of l-glut do not compromise the function of the blood-brain barrier in vivo. We sought to determine whether primary cultures of rat and piglet CMVECs or cerebral microvascular pericytes (CMVPCs) are indeed sensitive to l-glut or NMDA. Viability was unaffected by 8-h exposure to 1-10 mM l-glut or NMDA in CMVECs or CMVPCs isolated from both species. Furthermore, neither 1 mM l-glut nor NMDA augmented cell death induced by 12-h oxygen-glucose deprivation in rat CMVECs or by 8-h medium withdrawal in CMVPCs. Additionally, transendothelial electrical resistance of rat CMVEC-astrocyte cocultures or piglet CMVEC cultures were not compromised by up to 24-h exposure to 1 mM l-glut or NMDA. The Ca(2+) ionophore calcimycin (5 microM), but not l-glut (1 mM), increased intracellular Ca(2+) levels in rat CMVECs and CMVPCs assessed with fluo-4 AM fluorescence and confocal microscopy. CMVEC-dependent pial arteriolar vasodilation to hypercapnia and bradykinin was unaffected by intracarotid infusion of l-glut in anesthetized piglets by closed cranial window/intravital microscopy. We conclude that cerebral microvascular cells are insensitive and resistant to glutamatergic stimuli in accordance with their in vivo role as regulators of potentially neurotoxic amino acids across the blood-brain barrier.

Effect of memantine on CBF and CMRO2 in patients with early Parkinson's disease

OBJECTIVES

Parkinson's disease (PD) may be associated with increased energy metabolism in overactive regions of the basal ganglia. Therefore, we hypothesized that treatment with the N-methyl-d-aspartate receptor (NMDAR) antagonist memantine would decrease regional cerebral blood flow (rCBF) and oxygen metabolism in the basal ganglia of patients with early-stage PD.

METHODS

Quantitative positron emission tomography (PET) recordings were obtained with 15O]water and 15O]oxygen in 10 patients, scanned first in a baseline condition, and again 6 weeks after treatment with a daily dose of 20 mg memantine. Dynamic PET data were analyzed using volume of interest and voxel-based approaches.

RESULTS

The treatment evoked rCBF decreases in basal ganglia, and in several frontal cortical areas. The regional cerebral metabolic rate of oxygen (rCMRO2) did not decrease in any of the a priori defined regions, and consequently the oxygen extraction fraction was increased in these regions. Two peaks of significantly decreased rCMRO2 were detected near the frontal poles in both hemispheres, using a posteriori voxel-based analysis.

CONCLUSIONS

Although we did not find the predicted decrease in basal ganglia oxygen consumption, our data suggest that treatment with memantine actively modulates neuronal activity and/or hemodynamic response in basal ganglia of PD patients. This finding may be relevant to the putative neuroprotective properties of NMDAR antagonists.

Vasoconstrictive neurovascular coupling during focal ischemic depolarizations

Ischemic depolarizing events, such as repetitive spontaneous periinfarct spreading depolarizations (PIDs), expand the infarct size after experimental middle cerebral artery (MCA) occlusion. This worsening may result from increased metabolic demand, exacerbating the mismatch between cerebral blood flow (CBF) and metabolism. Here, we present data showing that anoxic depolarization (AD) and PIDs caused vasoconstriction and abruptly reduced CBF in the ischemic cortex in a distal MCA occlusion model in mice. This reduction in CBF during AD increased the area of cortex with 20% or less residual CBF by 140%. With each subsequent PID, this area expanded by an additional 19%. Drugs that are known to inhibit cortical spreading depression (CSD), such as N-methyl-D-aspartate receptor antagonists MK-801 and 7-chlorokynurenic acid, and sigma-1 receptor agonists dextromethorphan and carbetapentane, did not reduce the frequency of PIDs, but did diminish the severity of episodic hypoperfusions, and prevented the expansion of severely hypoperfused cortex, thus improving CBF during 90 mins of acute focal ischemia. In contrast, AMPA receptor antagonist NBQX, which does not inhibit CSD, did not impact the deterioration in CBF. When measured 24 h after distal MCA occlusion, infarct size was reduced by MK-801, but not by NBQX. Our results suggest that AD and PIDs expand the CBF deficit, and by so doing negatively impact lesion development in ischemic mouse brain. Mitigating the vasoconstrictive neurovascular coupling during intense ischemic depolarizations may provide a novel hemodynamic mechanism of neuroprotection by inhibitors of CSD.

Contribution of adenosine A2A and A2B receptors and heme oxygenase to AMPA-induced dilation of pial arterioles in rats

Nitric oxide (NO) has been implicated in mediation of cerebral vasodilation during neuronal activation and, specifically, in pharmacological activation of N-methyl-d-aspartate (NMDA) and kainate receptors. Possible mediators of cerebral vasodilation to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) have not been well studied in mature brain, although heme oxygenase (HO) activity has been implicated in newborn pigs. In anesthetized rats, 5 min of topical superfusion of 30 and 100 microM AMPA on the cortical surface through a closed cranial window resulted in increases in pial arteriolar diameter. The dilatory response to AMPA was not inhibited by superfusion of an NO synthase inhibitor, a cyclooxygenase-2 inhibitor, or a cytochrome P-450 epoxygenase inhibitor, all of which have been shown to inhibit the cortical blood flow response to sensory activation. However, the 48 +/- 13% dilation to 100 microM AMPA was attenuated 56-71% by superfusion of the adenosine A(2A) receptor antagonist ZM-241385, the A(2B) receptor antagonist alloxazine, and the HO inhibitor chromium mesoporphyrin. Combination of the latter three inhibitors did not attenuate the dilator response more than the individual inhibitors, whereas an AMPA receptor antagonist fully blocked the vasodilation to AMPA. These results indicate that cortical pial arteriolar dilation to AMPA does not require activation of NO synthase, cyclooxygenase-2, or cytochrome P-450 epoxygenase but does depend on activation of adenosine A(2A) and A(2B) receptors. In addition, CO derived from HO appears to play a role in the vascular response to AMPA receptor activation in mature brain by a mechanism that is not additive with that of adenosine receptor activation.

Carbon monoxide and hydrogen sulfide: gaseous messengers in cerebrovascular circulation

This review focuses on two gaseous cellular messenger molecules, CO and H2S, that are involved in cerebrovascular flow regulation. CO is a dilatory mediator in active hyperemia, autoregulation, hypoxic dilation, and counteracting vasoconstriction. It is produced from heme by a constitutively expressed enzyme [heme oxygenase (HO)-2] expressed highly in the brain and by an inducible enzyme (HO-1). CO production is regulated by controlling substrate availability, HO-2 catalytic activity, and HO-1 expression. CO dilates arterioles by binding to heme that is bound to large-conductance Ca2+-activated K+ channels. This binding elevates channel Ca2+ sensitivity, that increases coupling of Ca2+ sparks to large-conductance Ca2+-activated K+ channel openings and, thereby, hyperpolarizes the vascular smooth muscle. In addition to dilating blood vessels, CO can either inhibit or accentuate vascular cell proliferation and apoptosis, depending on conditions. H2S may also function as a cerebrovascular dilator. It is produced in vascular smooth muscle cells by hydrolysis of l-cysteine catalyzed by cystathione gamma-lyase (CSE). H2S dilates arterioles at physiologically relevant concentrations via activation of ATP-sensitive K+ channels. In addition to dilating blood vessels, H2S promotes apoptosis of vascular smooth muscle cells and inhibits proliferation-associated vascular remodeling. Thus both CO and H2S modulate the function and the structure of circulatory system. Both the HO-CO and CSE-H2S systems have potential to interact with NO and prostanoids in the cerebral circulation. Much of the physiology and biochemistry of HO-CO and CSE-H2S in the cerebral circulation remains open for exploration.

Effects of metabotropic glutamate receptor stimulation on blood-brain barrier permeability during focal cerebral ischemia

This investigation was performed to evaluate whether ACPD [(1S, 3R)-1-aminocyclopentane-1, 3-dicarboxylic acid], a metabotropic glutamate receptor agonist, would enhance the degree of increase in blood-brain barrier (BBB) permeability caused by focal cerebral ischemia. In this study, male Wistar rats were placed in control (n = 7) and ACPD (n = 7) groups under isoflurane anesthesia. Twenty minutes after middle cerebral artery (MCA) occlusion, patches of 10(-5) M ACPD or normal saline were placed on the ischemic cortex (IC) for a period of 40 min. Patches were changed every 10 min. One hour after MCA occlusion, BBB permeability was determined by measuring the transfer coefficient (Ki) of [alpha-14C] aminoisobutyric acid. There were no statistical differences in systemic blood pressures and heart rates between these groups. Blood gases were within normal limits. In the control group, the Ki of ischemic cortex (IC) was 2.1 times that of the contralateral cortex (CC) (3.7+/-0.9 vs. 1.8+/-0.3 microl/g/min). In the ACPD group, the Ki of the IC was 3.3 times that of the CC (5.0+/-0.7 vs. 1.5+/-0.4 microl/g/min). The increase in Ki of the ACPD group in the ischemic cortex was significantly greater than that in the control group. There was no significant difference in the Ki of the CC between these groups. Our data suggest that activation metabotropic glutamate receptors in the cortex can further augment the increase in BBB permeability caused by focal ischemia.

Down-regulation of AMPA glutamate receptors reduces cerebrocortical metabolic response to stimulation

We tested the hypothesis that chronic stimulation of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) glutamate receptors with an agonist causes down-regulation of the receptor protein and a decrement in basal and/or stimulated cerebral O2 consumption. Male Wistar rats were intradurally infused with 10 microM AMPA by an osmotic pump at a rate of 1 microl/h for 6 days. As a result, the specific binding of (S)-[3H]-5-fluorowillardiine to AMPA receptors in the cerebral cortex decreased 46% from 2.7 +/- 0.3 to 1.5 +/- 0.6 (density units). Under isoflurane anesthesia and after topical stimulation to the right cerebral cortex with 10(-3) M AMPA, cerebral blood flow (14C-iodoantipyrine method) and O2 consumption (cryomicrospectrophotometrically determined) were determined in control and down-regulated rats. Down-regulation of AMPA receptors did not alter basal O2 consumption. In control, after agonist stimulation, the O2 consumption in the ipsilateral cortex increased by 34%, (4.7 +/- 0.5 ml O2 x min(-1) x 100 g(-1) compared to 3.5 +/- 0.4 in the contralateral cortex). In the down-regulated rats, the O2 consumption did not significantly increase (4.0 +/- 1.5 ml O2 x min(-1) x 100 g(-1) compared to 3.3 +/- 1.7 in the contralateral cortex) after AMPA. In conclusion, following chronic simulation, AMPA receptors underwent down-regulation, but such down-regulation did not alter basal cerebrocortical blood flow or O2 consumption. AMPA down-regulation reduced the agonist stimulated increase in cortical O2 consumption.

Carbon monoxide mediates vasodilator effects of glutamate in isolated pressurized cerebral arterioles of newborn pigs

The excitatory neurotransmitter glutamate causes dilation of newborn pig cerebral arterioles in vivo that is blocked by inhibition of carbon monoxide (CO) production. CO, a potent dilator in cerebral circulation in vivo, is produced endogenously in cerebral microvessels via heme oxygenase (HO). In isolated pressurized cerebral arterioles (approximately 200 microm) from newborn pigs, we investigated the involvement of CO and the endothelium in response to glutamate. A CO-releasing molecule, dimanganese decacarbonyl (10(-8)-10(-6) M), dilated cerebral arterioles. Glutamate (10(-6)-10(-4) M) and 1-aminocyclopentane-cis-1,3-dicarboxylic acid (cis-ACPD; 10(-6)-10(-5) M), a N-methyl-D-aspartate (NMDA) receptor agonist, caused cerebral vascular dilation. Dilation of cerebral arterioles to glutamate and cis-ACPD was abolished by chromium mesoporphyrin (CrMP; 10(-6) M), a HO inhibitor. In contrast, CrMP did not alter dilation to isoproterenol, a -adrenergic receptor agonist. Endothelium-denuded cerebral arterioles did not dilate to glutamate or bradykinin (endothelium-dependent dilator), whereas responses to isoproterenol were preserved. These data indicate that cerebral arterioles from newborn pigs may directly respond to glutamate and the NMDA receptor agonists by endothelium-dependent dilation that involves stimulation of CO production via the HO pathway in the endothelium.

Ionotropic glutamate receptors in cerebral microvascular endothelium are functionally linked to heme oxygenase

Vasodilator effects of glutamate in the cerebral circulation are, in part, mediated by carbon monoxide (CO), which is formed from heme via the heme oxygenase (HO) pathway. The hypothesis addressed was that glutamate receptors (GluRs) in cerebral microvascular endothelium are functionally linked to HO. Using a radioligand binding and immunoblotting, GluRs were characterized in cerebral microvascular endothelial cells (CMVEC) from newborn pigs. High-affinity (80 nmol/L) reversible binding of [3H]glutamate ([ 3H]Glu) was detected in CMVEC membranes. The -methyl-d-aspartate (NMDA) receptor ligands-NMDA, quinolinic acid, (+/-)1-aminocyclopentane- -1,3-dicarboxylic acid ( ACPD), AP5, 4C3HPG, and CPP-and the (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptor ligands-AMPA, kainic acid, quisqualic acid, DNQX, and CNQX-displaced 20% to 30% of bound [3H]Glu in CMVEC membranes. Metabotropic GluRs antagonists (4CPG, PHCC, and CPPG) did not displace bound [3H]Glu. l-Aspartate, an agonist of GluRs and glutamate transporters, displaced 80% or more of bound [3H]Glu. Ionotropic (NR1 and GluR1) and metabotropic (mGluR1alpha) GluRs were detected in CMVEC by immunoblotting. Glutamate, aspartate, ACPD, AMPA, (RS)-2-amino-(3-hydroxy-5- -butylisoxazol-4-yl)propanoic acid (ATPA), and kainate (10(-5) mol/L) increased HO-directed CO formation by isolated cerebral microvessels and by cultured CMVEC. These data in newborn pigs suggest that CMVEC express ionotropic GluRs that are functionally linked to HO. GluR-mediated increases in CO formation by vascular endothelium may result in increase in cerebral blood flow.

Ketamine and fMRI BOLD signal: distinguishing between effects mediated by change in blood flow versus change in cognitive state

No human fMRI studies have examined ketamine effects on the BOLD signal change associated with cognitive task performance. We wished to distinguish between effects on 1) cerebral blood flow, with resultant change in BOLD signal; and 2) cognition and neural mechanisms underlying BOLD signal change associated with task performance. Eight right-handed men (mean age 28.75 years) received ketamine or saline i.v. in a randomized, double-blind manner (bolus 0.23 mg/kg; 0.5 mg/kg over 45 min to a maximum 1 hr). Subjects viewed 10 alternating 30-sec blocks of faces with neutral expressions and a fixation cross and discriminated gender of faces. Gradient echo echoplanar images were acquired on a GE Signa 1.5 T Neurovascular system. One hundred T2-weighted images depicting BOLD contrast were acquired over 5 min (for each task) at each of 14 near-axial noncontiguous 7-mm thick planes. Ketamine significantly increased dissociative phenomena and negative symptoms, but did not affect performance of the gender discrimination task. Significant BOLD signal change was demonstrated predominantly in occipitotemporal cortex with both ketamine and placebo. Only two clusters in middle occipital gyrus (BA 18) and precentral gyrus (BA 4) showed significantly decreased BOLD signal change during ketamine compared to placebo. BOLD signal change was not significantly greater in any region during ketamine. Our findings demonstrate subtle rather than major differences between the effects of ketamine and placebo upon the BOLD signal change during perception of face-non face contrast. We suggest that they represent task-dependent effects of the drug/placebo, rather than task-independent effects of the drug per se, and indicate that the effects of ketamine on cerebral blood flow are predominantly focal and task-dependent, rather than global and task-independent.

Effects of hypothermia on neuronal-vascular function after cerebral ischemia in piglets

We determined whether cerebral arteriolar dilation to N-methyl-d-aspartate (NMDA), a response dependent on stimulation of cortical neurons and inhibited by anoxic stress, would be preserved by hypothermia during and following ischemia. Pial arteriolar diameters in anesthetized piglets were determined via intravital microscopy. Arteriolar responses to NMDA (10, 50, and 100 micromol/l) were measured before and 1 h after 10 min of global ischemia. Piglets were exposed to either total body or selective brain cooling (33-34 degrees C). Arteriolar dilation to lower doses or to 100 micromol/l NMDA was not affected by hypothermia alone (51 +/- 3 vs. 46 +/- 7%, normothermia vs. hypothermia; n = 7) in nonischemic animals. However, arteriolar responses to 100 micromol/l NMDA were clearly attenuated after ischemia despite body cooling during ischemia (53 +/- 3 vs. 32 +/- 6%; n = 8), hypothermia during ischemia and early reperfusion (49 +/- 10 vs. 20 +/- 3%; n = 8), or selective brain cooling (48 +/- 5 vs. 20 +/- 5%; n = 10). In contrast, pretreatment with indomethacin resulted in complete preservation of NMDA-induced vasodilation after ischemia. Thus, hypothermia fails to protect against neuronal dysfunction during ischemia.

Main subunits of ionotropic glutamate receptors are expressed in isolated rat brain microvessels

Excitatory amino acids are known to modulate blood-brain barrier (BBB) permeability, however, the information on glutamate receptors in cerebral capillaries is inconsistent. In the present study, freshly isolated microvessels obtained from saline-perfused rat brains were used. Gene expression of the main N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) receptor subunits NMDAR1 and GLUR1, respectively, were investigated by reverse transcription-polymerase chain reaction (RT-PCR). The results confirmed the presence of both NMDAR1 and GLUR1 mRNAs in microvessels of seven brain regions studied. Moreover, specific binding of [3H]glutamate to capillary membranes and its displacement by AMPA, NMDA and metabotropic, but not kainate receptor agonists were observed. These results suggest that rat brain capillaries and/or albuminally adhering astrocyte processes possess functional glutamate receptors. Thus, the effects of glutamate agonists and antagonists in modulation of BBB function might be mediated directly by cerebral microvessels.

Cerebral vascular endothelial heme oxygenase: expression, localization, and activation by glutamate

Endogenous carbon monoxide (CO) contributes to vasodilator responses of cerebral microvessels in newborn pigs. We investigated the expression, intracellular localization, and activity of heme oxygenase (HO), the key enzyme in CO production, in quiescent cerebral microvascular endothelial cells (CMVEC) from newborn pigs. HO-1 and HO-2 isoforms were detected by RT-PCR, immunoblotting, and immunofluorescence. HO-1 and HO-2 are membrane-bound proteins that have a strong preference for the nuclear envelope and perinuclear area of the cytoplasm. Betamethasone (10(-6) to 10(-4) M for 48 h) was associated with upregulation of HO-2 protein by approximately 50% and inhibition of Cox-2 but did not alter HO-1 or endothelial nitric oxide synthase expression in CMVEC. In vivo betamethasone treatment of newborn pigs (0.2 and 5.0 mg/kg im for 48 h) upregulated HO-2 in cerebral microvessels by 30-60%. HO activity as (14)CO production from [(14)C]glycine-labeled endogenous heme was inhibited by chromium mesoporphyrin (10(-6) to 10(-4) M). L-Glutamate (0.3-1.0 mM) stimulated HO activity 1.5-fold. High-affinity specific binding sites for L-[(3)H]glutamate suggestive of the glutamate receptors were detected in CMVEC. Altogether, these data suggest that, in cerebral circulation of newborn pigs, endothelium-derived CO may contribute to basal vascular tone and to responses that involve glutamate receptor activation.

The effects of topical and intravenous ketamine on cerebral arterioles in dogs receiving pentobarbital or isoflurane anesthesia

To evaluate the effects of ketamine on cerebral arterioles, we used a closed cranial window technique in mechanically ventilated, anesthetized dogs. Fourteen dogs were assigned to one of the following two basal-anesthesia groups: pentobarbital 2 mg. kg(-1). h(-1) or isoflurane 0.5 MAC (n = 7 each). We administered three different concentrations of ketamine (10(-7), 10(-5), and 10(-3) M) under the window and measured arteriolar diameters. For comparison, in another 14 dogs we examined the effect of systemic (IV) ketamine (1 mg/kg and 5 mg/kg) using the same two basal anesthetics. We measured diameters before and after ketamine administration, and we evaluated the effect of ketamine on CO(2) reactivity of the cerebral arterioles. Neither topical nor systemic ketamine dilated pial arterioles in either basal-anesthesia group. CO(2) reactivity of pial arterioles was reduced under systemic ketamine in both basal-anesthesia groups. The results indicate that although ketamine does not dilate pial arteriolar diameters when topically or IV administered, IV ketamine does attenuate hypercapnic vasodilation in dogs under basal pentobarbital or isoflurane anesthesia. These results provide some insight that ketamine is suitable for supplementary neurosurgical anesthesia.

P-450 epoxygenase and NO synthase inhibitors reduce cerebral blood flow response to N-methyl-D-aspartate

Epoxyeicosatrienoic acids are cerebral vasodilators produced in astrocytes by cytochrome P-450 epoxygenase activity. The P-450 inhibitor miconazole attenuates the increase in cerebral blood flow (CBF) elicited by glutamate. We evaluated whether epoxygenase activity is involved in the CBF response to activation of the N-methyl-D-aspartate (NMDA) receptor subtype by using two structurally distinct inhibitors, miconazole and N-methylsulfonyl-6-(2-propargyloxyphenyl) hexanamide (MS-PPOH), a selective epoxygenase substrate inhibitor. Drugs were delivered locally through microdialysis probes in striata of anesthetized rats. Local CBF was measured by hydrogen clearance and compared with CBF in contralateral striatum receiving vehicle. Microdialysis perfusion of NMDA doubled CBF and increased nitric oxide (NO) production estimated by recovery of labeled citrulline in the dialysate during labeled arginine infusion. Perfusion of miconazole or MS-PPOH blocked the increase in CBF without decreasing citrulline recovery. Perfusion of N(omega)-nitro-L-arginine decreased baseline CBF and inhibited the CBF response to NMDA. Perfusion of MS-PPOH did not inhibit the CBF response to sodium nitroprusside. We conclude that both the P-450 epoxygenase and NO synthase pathways are involved in the local CBF response to NMDA receptor activation, and that the signaling pathway may be more complex than simply NO diffusion from neurons to vascular smooth muscle.

Neurovascular relationships in hippocampal slices: physiological and anatomical studies of mechanisms underlying flow-metabolism coupling in intraparenchymal microvessels

Experiments were carried out to investigate the functional and anatomical relationships between neuronal elements and cerebral microvessels in 300-350-microm thick coronal hippocampal slices maintained at 33-35 degrees C, obtained from 150-200 g male Wistar rats. Cerebral arterioles (9-22 microm in diameter) were visualized in situ and pre-constricted by 22.0+/-6.6% by the addition of the thromboxane A2 agonist U46619 (75 nM), to the bathing medium. The glutamate agonist N-methyl-D-aspartate (0.01-1 mM) produced a dose-related increase in luminal diameter of pre-constricted vessels. In the presence of 4 microM haemoglobin to scavenge nitric oxide from the extravascular environment of the slice, the increase in diameter evoked by 0.1 mM N-methyl-D-aspartate was significantly reduced from 17.5+/-4.6% to 4.8+/-1.7% indicating that N-methyl-D-aspartate-induced vasodilatation of cerebral microvessels is mediated via a mechanism which involves neuronally-derived nitric oxide. In a parallel anatomical study, beta-nicotinamide adenine dinucleotide phosphate-dependent diaphorase staining was used to reveal the enzyme nitric oxide synthase in vascular endothelium and neurons in slices. A small subpopulation (< 11 cells per slice) of darkly-stained multipolar neurons, 21-32 microm in diameter was observed to give rise to a dense network of fine diaphorase-reactive nerve fibres that ramified throughout the whole of the hippocampus and appeared to come into close apposition with arterioles. Morphometric analysis of the relationship between cerebral microvessels, beta-nicotinamide adenine dinucleotide phosphate, reduced form-dependent diaphorase-reactive neuronal elements and individual pyramidal layer neurons, identified by filling with biocytin, revealed that for a given point on a pyramidal layer neuron, the proximity of the nearest diaphorase-reactive nerve fibre was less than 10 microm, whilst the distance to the nearest arteriole (the smallest functional unit for controlling blood flow) was in excess of 70 microm. Such a distance would probably preclude diffusion of vasoactive metabolites in effective concentrations from the area of increased neuronal activity. We therefore propose that the diaphorase-reactive nerve network constitutes the functional link. It is possible that during periods of increased neuronal activity, spillover of glutamate from synapses may activate the diaphorase-reactive network. Release of nitric oxide from the network in the vicinity of local cerebral arterioles may then produce relaxation of the vascular smooth muscle, enabling increased blood flow into the capillary network supplying the region of increased metabolic activity. This study has shown that the process whereby increases in neuronal activity elicit a local change in cerebral blood flow remains functionally intact in hippocampal slice preparations. Nitric oxide of neuronal origin appears to be involved in mediating the coupling between neurons and cerebral arterioles. Stereological analysis of the relationship between neuronal and vascular elements within hippocampal slices suggested that a small subpopulation of nitric oxide synthase-containing neurons which give rise to a diffuse network of fine nitric oxide synthase-containing nerve fibres that lie in close apposition to cerebral arterioles may provide the anatomical substrate for coupling of blood flow to metabolism.

Cerebrovascular reactivity remains intact after cortical depolarization in newborn piglets

Cerebrovascular reactivity is severely affected by ischemia, and changes in vascular responses have been reported after cortical spreading depression and head trauma as well. Cortical depolarization (CD) occurs during ischemia, cortical spreading depression, and head trauma, but its effects on cerebrovascular reactivity are unclear. We tested the hypothesis that CD induced by KCl diminishes the vascular responsiveness to various vasodilatory stimuli in piglets. Responses of pial arterioles were determined by changes in vascular diameter by use of a closed cranial window and intravital microscopy. Baseline arteriolar diameters were 105 +/- 3 microm (mean +/- SEM, n = 27). CD was elicited by topical administration of 1 mol/L KCl for 3 min. Vascular responses were measured before and 1 h after CD. KCl elicited CD and constricted arterioles by 54 +/- 4% (n = 27). N-methyl-D-aspartate induced dose-dependent vasodilation that was unaffected by CD; the percent changes were 9 +/- 1 versus 8 +/- 1 (before and after CD) at 10(-5) mol/L, 19 +/- 2 versus 18 +/- 3 at 5 x 10(-5) mol/L, and 29 +/- 2 versus 26 +/- 3 at 10(-4) mol/L (n = 9). Hypercapnic vasodilation was not diminished by CD; the percent changes were 15 +/- 2 versus 16 +/- 4 at 5%, and 27 +/- 5 versus 27 +/- 6 at 10% inspired CO2 (n = 8). Aprikalim and forskolin caused dilation that was also resistant to prior CD; the percent change values were 21 +/- 4 versus 18 +/- 3 and 16 +/- 2 versus 16 +/- 4 at 10(-6) mol/L, 36 +/- 5 versus 34 +/- 5 and 34 +/- 7 versus 37 +/- 7 at 10(-5) mol/L (n = 8), respectively. Finally, calcitonin gene-related peptide-induced vasodilation was unaffected by CD; percent changes were 15 +/- 3 versus 16 +/- 2 at 10(-7) mol/L and 26 +/- 4 versus 22 +/- 3 at 10(-6) mol/L (n = 8). The intact vascular responses after CD suggest that this component is not responsible for decreased cerebrovascular reactivity after ischemia, head trauma, or cortical spreading depression.

Lack of evidence for direct involvement of NMDA receptors or polyamines in blood-brain barrier injury after cerebral ischemia in rats

It is hypothesized that after various types of brain injury, blood-brain barrier (BBB) opening and vasogenic edema result from excessive neuronal release of glutamate and stimulation of capillary N-methyl-d-aspartate (NMDA) receptors linked to polyamine (putrescine) synthesis in endothelial cells. We produced cerebral ischemia in rats and measured BBB opening 6 h later as the increase in regional transfer constants (Ki) for blood to brain diffusion of [3H]sucrose. Such BBB opening was not mitigated by drugs which block NMDA receptors (MK801 or AR-R 15896AR) or polyamine synthesis (difluoromethylornithine). These results question generality of the capillary NMDA receptor/polyamine hypothesis.

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

OBJECTIVES

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

DESIGN

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

SETTING

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

PATIENTS

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

MEASUREMENTS AND MAIN RESULTS

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

CONCLUSIONS

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

Effect of N-methyl-D-aspartate receptor blockade on the control of cerebral O2 supply/consumption balance during hypoxia in newborn pigs

Using dizocilpine (MK-801), we tested the hypothesis that N-methyl-D-aspartate (NMDA) receptors are important controllers of cerebral O2 supply/consumption balance in newborn piglets both during normoxia and hypoxia. Twenty-five 2 to 7-day-old piglets were anesthetized and divided into four groups: (1) Normoxia (n = 6), (2) Normoxia + MK-801 (n = 6), (3) Hypoxia (n = 6), and (4) Hypoxia + MK-801 (n = 7). Regional cerebral blood flow (rCBF) in ml/min/100 g was measured using 14C-iodoantipyrine, and we determined arterial and venous O2 saturations by microspectrophotometry, calculating cerebral O2 consumption (VO2) in ml O2/min/100 g in the cortex, hypothalamus and pons. MK-801 did not significantly affect regional VO2 or rCBF in normoxic piglets. Hypoxia resulted in an increase in local rCBF compared to controls: from 41 +/- 6 to 103 +/- 18 in the cortex; 34 +/- 7 to 101 +/- 20 in the hypothalamus; and 45 +/- 10 to 95 +/- 11 in the pons. Pretreatment with MK-801 abolished this hypoxic flow effect in the cortex (51 +/- 2) and hypothalamus (49 +/- 5), but not in the pons (91 +/- 17). Similar results were observed for VO2 with control values of 1.9 +/- 0.3, 1.6 +/- 0.2 and 2.1 +/- 0.3 for the cortex, hypothalamus and pons respectively. Hypoxia resulted in an increase in the VO2 to 3.9 +/- 0.4 (cortex), 3.8 +/- 0.6 (hypothalamus) and 3.9 +/- 0.8 (pons). Pretreatment with MK-801 prior to hypoxia abolished these effects in the cortex (2.1 +/- 0.2) and hypothalamus (2.1 +/- 0.2), but not in the pons (2.9 +/- 0.2). These findings suggest that NMDA receptors may play a role in the control of cerebral metabolism during hypoxia in this immature porcine model.

Evidence that functional glutamate receptors are not expressed on rat or human cerebromicrovascular endothelial cells

Excitatory amino acids can modify the tone of cerebral vessels and permeability of the blood-brain barrier (BBB) by acting directly on endothelial cells of cerebral vessels or indirectly by activating receptors expressed on other brain cells. In this study we examined whether rat or human cerebromicrovascular endothelial cells (CEC) express ionotropic and metabotropic glutamate receptors. Glutamate and the glutamate receptor agonists N-methyl-d-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA), and kainate failed to increase [Ca2+]i in either rat or human microvascular and capillary CEC but elicited robust responses in primary rat cortical neurons, as measured by fura-2 fluorescence. The absence of NMDA and AMPA receptors in rat and human CEC was further confirmed by the lack of immunocytochemical staining of cells by antibodies specific for the AMPA receptor subunits GluR1, GluR2/3, and GluR4 and the NMDA receptor subunits NR1, NR2A, and NR2B. We failed to detect mRNA expression of the AMPA receptor subunits GluR1 to GluR4 or the NMDA receptor subunits NR1(1XX); NR1(0XX), and NR2A to NR2C in both freshly isolated rat and human microvessels and cultured CEC using reverse transcriptase polymerase chain reaction (RT-PCR). Cultured rat CEC expressed mRNA for KA1 or KA2 and GluR5 subunits. Primary rat cortical neurons were found to express GluR1 to GluR3 and NR1, NR2A, and NR2B by both immunocytochemistry and RT-PCR and KA1, KA2, GluR5, GluR6, and GluR7 by RT-PCR. Moreover, the metabotropic glutamate receptor agonist 1-amino-cyclopentyl-1S, 3R-dicorboxylate (1S,3R-trans-ACPD), while eliciting both inositol trisphosphate and [Ca2+]i increases and inhibiting forskolin-stimulated cyclic AMP in cortical neurons, was unable to induce either of these responses in rat or human CEC. These results strongly suggest that both rat and human CEC do not express functional glutamate receptors. Therefore, excitatory amino acid-induced changes in the cerebral microvascular tone and BBB permeability must be affected indirectly, most likely by mediators released from the adjacent glutamate-responsive cells.

Potassium channel activators protect the N-methyl-D-aspartate-induced cerebral vascular dilation after combined hypoxia and ischemia in piglets

BACKGROUND AND PURPOSE

Cerebral arteriolar dilation to N-methyl-D-aspartate (NMDA) is a neuronally mediated multistep process that is sensitive to cerebral hypoxia and ischemia (H/I). We tested the hypothesis that topical pretreatment with the selective potassium channel agonists NS1619 and aprikalim preserves the vascular response to NMDA after consecutive H/I.

METHODS

Pial arteriolar diameters were measured in anesthetized piglets with the use of a closed cranial window and intravital microscopy. Arteriolar responses to NMDA (10(-5), 5 x 10(-5), and 10(-4) mol/L) were recorded before and 1 hour after 10 minutes of hypoxia (8.5% O2 in N2) plus 10 minutes of ischemia (H/I). Ischemia was induced by increasing intracranial pressure. Subgroups were topically pretreated with 10(-5) mol/L NS1619, 10(-6) mol/L aprikalim, 10(-6) mol/L calcitonin gene-related peptide (CGRP), or 10(-5) mol/L papaverine. We also examined the effects of H/I on vascular responses to kainate (10(-4) mol/L) to assess specificity of neuronal injury.

RESULTS

Arteriolar responses to NMDA were significantly attenuated after H/I. Baseline compared with post-H/I arteriolar diameters were 9+/-4% versus 3+/-2% at 10(-5) mol/L, 22+/-4% versus 4+/-2% at 5 x 10(-5) mol/L, and 33+/-4% versus 7+/-2% at 10(-4) mol/L (mean+/-SE; all P<.05, n=7). Pretreatment with NS1619 and aprikalim preserved the arteriolar responses to NMDA after H/I. For NS1619 (n=6), values were as follows: 9+/-2% versus 6+/-4% at 10(-5) mol/L, 19+/-6% versus 21+/-5% at 5 x 10(-5) mol/L, and 35+/-3% versus 31+/-5% at 10(-4) mol/L. For aprikalim (n=7), values were as follows: 6+/-2% versus 8+/-2% at 10(-5) mol/L, 22+/-6% versus 15+/-3% at 5 x 10(-5) mol/L, and 41+/-5% versus 32+/-6% at 10(-4) mol/L. In contrast, piglets pretreated with CGRP (n=6) or papaverine (n=5) showed no preservation of the vascular response to NMDA after H/I, although these compounds dilated the arterioles to an extent similar to that with NS1619/aprikalim. Kainate-induced arteriolar dilation (n=6) was largely preserved after H/I compared with preischemic responses.

CONCLUSIONS

(1) Vascular responses of cerebral arterioles to NMDA after H/I are preserved by pretreatment with NS1619 or aprikalim, indicating a neuroprotective effect. (2) CGRP and papaverine do not preserve the vascular response to NMDA despite causing vasodilation similar to that with NS1619 or aprikalim. This suggests that activation of potassium channels on neurons accounts for the protective effect of potassium channel agonists. (3) Preserved arteriolar dilation to kainate suggests largely intact functioning of neuronal nitric oxide synthase after H/I.

Inhibitory effects of hypoxia and adenosine on N-methyl-D-aspartate-induced pial arteriolar dilation in piglets

Our previous studies have indicated that oxygen radicals, produced during reoxygenation following short-term arterial hypoxia, lead to sustained suppression of cerebral arteriolar responses to N-methyl-D-aspartate (NMDA). However, whether arteriolar dilator responses to NMDA are reduced during arterial hypoxia has never been examined. In this study, we determined whether hypoxia or hypoxia-related metabolites such as adenosine or nitric oxide (NO) will reduce NMDA-induced arteriolar dilation. We have also determined the location of NMDA receptor- and brain nitric oxide synthase (bNOS)-positive neurons in the cerebral cortex. In anesthetized piglets, pial arteriolar diameters were determined using intravital microscopy. Baseline arteriolar diameters were approximately 100 microns. Topical application of NMDA at concentrations of 10(-5), 5 x 10(-5) and 10(-4) M resulted in dose-dependent vasodilation (9 +/- 2, 18 +/- 2 and 29 +/- 2% above baseline, respectively, n = 21). Administration of theophylline (20 mg/kg, i.v.) had no effect on NMDA-dependent vasodilation, but it did block dilation to hypoxia (inhalation of 8.5% O2). In theophylline-treated animals, NMDA responses were completely abolished during hypoxia (28 +/- 2 vs. 2 +/- 1%, respectively to 10(-4) M, n = 7) while sodium nitroprusside (SNP, 10(-4) M) still dilated pial arterioles normally. NMDA-induced vasodilation was not modified after application and removal of adenosine (10(-4) M; n = 5) or SNP (10(-5) M; n = 4), or when SNP (10(-7) M) was coapplied with NMDA (n = 6). Conversely, coapplication of adenosine (10(-6) M) attenuated NMDA responses (31 +/- 5 vs. 20 +/- 3%, n = 7). We also found that NMDA receptor- and bNOS-containing neurons were located predominantly in layers II/III of the cortex. Proximity of these neurons to the cortical surface is consistent with diffusion of NO to pial arterioles as the mechanism of dilation to NMDA. We conclude that NMDA-induced cerebral arteriolar dilation is inhibited by hypoxia alone and by exogenous adenosine, but not by NO.

Effects of ifenprodil, a polyamine site NMDA receptor antagonist, on reperfusion injury after transient focal cerebral ischemia

Polyamines and N-methyl-D-aspartate (NMDA) receptors are both thought to play an important role in secondary neuronal injury after cerebral ischemia. Ifenprodil, known as a noncompetitive inhibitor of polyamine sites at the NMDA receptor, was studied after transient focal cerebral ischemia occurred. Spontaneously hypertensive male rats, each weighing between 250 and 350 g, underwent 3 hours of tandem middle cerebral artery (MCA) and common carotid artery occlusion followed by reperfusion for a period of 3 hours or 21 hours. Intravenous ifenprodil (10 microg/kg/minute) or saline infusion was started immediately after the onset of MCA occlusion and continued throughout the ischemic period. Physiological parameters including blood pressure, blood gas levels, blood glucose, hemoglobin, and rectal and temporal muscle temperatures were monitored. Six rats from each group were evaluated at 6 hours postocclusion for brain water content, an indicator of brain edema, and Evans blue dye extravasation for blood-brain barrier breakdown. Infarct volume was also measured in six rats from each group at 6 and 24 hours postocclusion. Ifenprodil treatment significantly reduced brain edema (82.5 +/- 0.4% vs. 83.5 +/- 0.4%, p < 0.05) and infarct volume (132 +/- 14 mm3 vs. 168 +/- 25 mm3, p < 0.05) compared with saline treatment, with no alterations in temporal muscle (brain) or rectal (body) temperature (35.9 +/- 0.4 degrees C vs. 36.2 +/- 0.2 degrees C; 37.7 +/- 0.4 degrees C vs. 37.6 +/- 0.6 degrees C; not significant). These results demonstrate that ifenprodil has neuroprotective properties after ischemia/reperfusion injury in the absence of hypothermia. This indicates that antagonists selective for the polyamine site of the NMDA receptors may be a viable treatment option and helps to explain some of the pathophysiological mechanisms involved in secondary injury after transient focal cerebral ischemia has occurred.

Effects of excitatory amino acids on cerebral oxygen consumption and blood flow in rat

This investigation tested the importance of excitatory amino acids' effects on regional cerebral O2 consumption and the concomitant changes in cerebral blood flow (rCBF) in isoflurane anesthetized rats. In the glutamate or N-methyl-D-aspartate (NMDA) groups, 10(-2) M glutamate or NMDA was topically applied to the right cortex and the left cortex was used as a control. One mg/kg dizocilpine maleate (MK-801), a non-competitive NMDA receptor antagonist, was administered (iv) to the MK-801 group and saline was given to the control group. Cortical rCBF was determined using 14C-iodoantipyrine and regional O2 extraction was measured microspectrophotometrically. Cerebral O2 consumption increased 77% after glutamate (contralateral cortex: 9.0 +/- 1.1 ml O2/min/100 g, glutamate treated cortex: 15.9 +/- 3.9), while a 46% increase was observed with the same concentration of NMDA (contralateral cortex: 9.8 +/- 2.0, NMDA treated cortex: 14.3 +/- 5.5). After MK-801, the O2 consumption decreased to 37% of the control value (control cortex: 7.0 +/- 1.3, MK-801 treated cortex: 2.6 +/- 3.9). MK-801 significantly decreased cerebral O2 extraction from 7.1 +/- 1.3 ml O2/100 ml (control cortex) to 5.3 +/- 0.6 (MK-801 treated cortex). However, there was no significant difference in cerebral O2 extraction between treated and contralateral cortex in either the glutamate or NMDA groups. The increase in O2 consumption caused by glutamate or NMDA was coupled with increased rCBF. Glutamate increased rCBF from 95 +/- 5 ml/min/100 g (contralateral cortex) to 165 +/- 31 (treated cortex), while NMDA increased rCBF from 114 +/- 12 (contralateral cortex) to 178 +/- 60 (treated cortex). MK-801 decreased O2 consumption with a lesser decrease of rCBF. The rCBF was 48 +/- 9 in the MK-801 treated cortex and 99 +/- 22 in the control cortex. Some substances produced by the activation of NMDA receptors may be related to the coupling of cerebral metabolism and blood flow, since after blockade of NMDA receptors with MK-801, this relationship is uncoupled. These findings suggest that glutamatergic processes have a major effect on cerebral O2 consumption and that this is at least partly due to NMDA receptors.

Regulation of cerebral microvessels by glutamatergic mechanisms

This study examined the role of glutamate receptor activation in the regulation of microvascular tone in the hippocampus and neocortex of the rat. Microvascular and neuronal responses were simultaneously recorded in brain slices using videomicroscopic analysis in conjunction with electrophysiological recording. Glutamate and other glutamate receptor agonists, including NMDA, kainic acid, and ACPD elicited dose-dependent dilation in preconstricted hippocampal microvessels. The lower concentrations of NMDA elicited dilation with an increase in neuronal excitability while dilatory responses to other agonists were associated with substantial depolarization. NMDA-mediated dilation was inhibited completely with a sodium channel blocker (TTX), an NOS inhibitor (L-NNA), or a specific inhibitor of neuronal NOS (7-NI). Inhibition of the GABA(A) or the A2 adenosine receptor did not attenuate the NMDA-induced dilation. The role of spontaneous glutamate receptor activation by endogenous glutamate in the regulation of resting dilatory tone was also examined. Blocking AMPA or metabotropic glutamate receptors did not induce significant responses in resting hippocampal vessels. However, the NMDA receptor antagonist, APV, elicited a dose-dependent constriction. In surface vessels of the neocortex, NMDA elicited a comparable dose-dependent dilation, and APV elicited a significantly smaller dose-dependent constriction. A 60 min period of hypoxia elicited a significant dilation of preconstricted hippocampal microvessels. APV did not significantly influence this dilatory response indicating that hypoxia-induced dilation is not mediated by NMDA receptor activation. Taken together, these results indicate that glutamate contributes to the dilatory tone of cerebral microvessels under physiologic conditions and that this effect is mediated by NMDA receptors. Glutamatergic vasodilation is dependent on neuronal discharge activity and the neuronal production of NO. The tonic influence is more pronounced in hippocampal microvessels than in neocortical vessels suggesting that the contribution of NMDA receptor activation to resting dilatory tone is dependent on the location of vessels within the brain.

Effect of up-regulation of NMDA receptors on cerebral O2 consumption and blood flow in rat

We tested the hypothesis that cerebrocortical blood flow and O2 consumption would be proportional to an up-regulated number of functional N-methyl-D-aspartate (NMDA) receptors. Previous work had shown a relationship between cerebral metabolism and NMDA receptor activity. We increased the specific binding to NMDA receptors in the cerebral cortex, from 2.2 +/- 0.9 to 4.5 +/- 0.8 (density units) in male Long-Evans rats by daily giving two intraperiotoneal injections (30 mg/kg) of CGS-19755, an NMDA receptor inhibitor, for 7 consecutive days (discontinued for 20 h before experiment). Twelve up-regulated (CGS treated) and 12 control rats were used in this study. Under isoflurane anesthesia and after topical stimulation of the right cerebral cortex with 10(-2) M NMDA, the blood flow (14C-iodoantipyrine method) increased from 98 +/- 11 ml/min/100 g in the unstimulated cortex of the control rats to 161 +/- 37 ml/min/100 g in the stimulated cortex. The unstimulated value for blood flow (95 +/- 7 ml/min/100 g) did not change in the upregulated group but it doubled (194 +/- 69 ml/min/100 g) in the stimulated, upregulated cortex. Similarly, O2 consumption (cryomicrospectrophotometrically determined) in normal rats increased 46%, from 9.3 +/- 1 ml/min/100 g to 13.6 +/- 4 after NMDA stimulation. While in the upregulated animals, O2 consumption increased 103% from 7.9 +/- 0.6 to 16 +/- 6.5 after NMDA stimulation. In conclusion, NMDA receptor upregulation does not alter basal cerebrocortical blood flow or O2 consumption but in the NMDA-stimulated cortex, the blood flow and O2 consumption increase is dependent on the number of NMDA receptors present.

NMDA- and endothelin-1-induced increases in blood-brain barrier permeability quantitated with Lucifer yellow

At 48 h following intrastriatal injection of N-methyl-D-aspartate (NMDA; 100 nmol/microliter) or endothelin-1 (ET-1; 143 pmol/microliter), significant increases in brain penetration of the highly polar, fluorescent tracer Lucifer yellow were observed. The competitive NMDA receptor antagonist selfotel (CGS-19755; 30 nmol/microliter, i.c.) significantly reduced the NMDA-induced increases in blood-brain barrier permeability, but not those induced by ET-1. These results suggest that NMDA receptors can mediate increases in blood-brain barrier permeability but do not primarily mediate increases in blood-brain barrier permeability caused by ET-1. This is the first study to our knowledge investigating the relationship between excitotoxicity and disruption of the blood-brain barrier, a major pathophysiological event in stroke and traumatic brain injury.

The effects of N-methyl-D-aspartate agonists and antagonists on isolated bovine cerebral arteries

This pharmacologic study examines the direct cerebrovascular effects of N-methyl-D-aspartate (NMDA) receptor agonists and antagonists to determine whether large cerebral arteries have NMDA receptors. Bovine middle cerebral arteries were cut into rings to measure isometric tension development in vitro. Two competitive agonists, L-glutamate and NMDA, each had negligible effects on ring tension in the absence of exogenous vasoconstrictors. L-glutamate (in high concentrations) produced direct relaxation of potassium (K+)-constricted arteries, but the relaxation was not selective for L-glutamate, D-glutamate, or mannitol. Relaxation with L-glutamate was abolished when it was isosmotically substituted in the K(+)-rich medium. NMDA (in the absence or presence of glycine) and two competitive antagonists, 2-amino-5-phosphopentanoic acid (AP5) and (+/-)-3-(s-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), each had little effect on the tone of arteries preconstricted with potassium or the stable thromboxane A2 analog U-46,619. Three noncompetitive antagonists (S(+)-ketamine, dizocilpine, and dextrorphan) and their steroeisomers (R(-)-ketamine, (-)MK-801, and levorphanol) each produced dose-dependent relaxation of K(+)- or U-46,619-constricted arteries; relaxation was not selective for the (+) or (-) stereoisomers. These results suggest that large cerebral arteries lack NMDA receptors mediating constriction or relaxation. All noncompetitive antagonists dilated cerebral arteries, but by mechanisms that were not stereospecific.

Post-ischemic administration of HU-211, a novel non-competitive NMDA antagonist, protects against blood-brain barrier disruption in photochemical cortical infarction in rats: a quantitative study

We examined the effect of HU-211, a synthetic non-psychotropic cannabinoid with non-competitive N-methyl-D-aspartate (NMDA) antagonist properties, on blood-brain barrier (BBB) integrity after photochemically induced cortical infarction. Evans blue dye was used as a BBB permeability indicator after unilateral thrombotic cortical infarction was produced photochemically by 560 nm light irradiation of the cortex in male Wistar rats receiving rose bengal intravenously. HU-211 was injected in a dose of 4 mg/kg i.v. 30 min after stroke. Fluorometric measurement of Evans blue was performed 24 h later in six brain regions. Treatment with HU-211 significantly decreased extravasation of dye into the area of infarct (406 +/- 19 vs. 539 +/- 33 micrograms/g, mean +/- S.E.M.) as well as other sites of the affected hemisphere (866 +/- 68 vs. 1096 +/- 68 micrograms/g) compared to the vehicle group. These data indicate that HU-211 is an effective drug in protecting against the effects of focal ischemia-induced BBB disruption in the rat and suggest that the drug may be an effective treatment against the ischemic cell death and BBB disruption that can occur clinically following a stroke or cardiac arrest.

A dual role for nitric oxide in NMDA-mediated toxicity in vivo

Nitric oxide has been implicated in N-methyl-D-aspartate (NMDA)-mediated damage in vitro; however, its role in excitotoxic damage in vivo is not clear. In the present study we evaluated the histopathological and hemodynamic consequences of intrastriatal injections of various doses of NMDA and determined the effects of nitric oxide synthase inhibition on these changes. NMDA was injected into the striatum at doses of 50, 150, and 300 nmol with or without N omega-nitro-L-arginine methyl ester (L-NAME; 100 micrograms, locally). Three days following injections histopathological assessment was performed by morphometric analysis of the lesion area in multiple sections taken from the anterior to the posterior borders of the lesion. In animals injected with 150 and 300 nmol of NMDA (+/- L-NAME), local CBF (lCBF) was determined 30 min following injections using 14C-iodoantipyrine autoradiography. All NMDA-treated animals showed a well-demarcated lesion extending beyond the injection site. The volume of the lesion correlated significantly with the NMDA dose injected. The effects of L-NAME on lesion size were dependent on the dose of the NMDA. The lesion induced by 50 nmol of NMDA was not affected by L-NAME. With a dose of 150 nmol of NMDA, L-NAME induced a 43% increase in lesion volume. In contrast, a 38% decrease in lesion size was observed in animals treated with 300 nmol of NMDA combined with L-NAME. At a dose of 150 nmol, NMDA induced a significant elevation in lCBF, which was restricted to regions close to the injection site including the center areas of the anterior and middle striatum. The increase in lCBF observed with 150 nmol of NMDA was significantly attenuated in the NMDA + L-NAME-treated group. The lCBF changes induced by 300 nmol of NMDA were not significantly different from those in the 150-nmol group; however, the extent of the regions involved was larger. The increases in lCBF were observed in all striatal regions including the central and peripheral areas. L-NAME did not have a significant effect on the lCBF changes induced by NMDA at a dose of 300 nmol. These data suggest that in vivo the involvement of nitric oxide in NMDA toxicity depends on the NMDA dose and on the participation of hemodynamic mechanisms secondary to NMDA exposure.

Glutamate-induced cerebral vasodilation is mediated by nitric oxide through N-methyl-D-aspartate receptors

BACKGROUND AND PURPOSE

It was found that glutamate, a major neurotransmitter, is vasoactive in the cerebral circulation. However, the mechanism is unclear. This study was designed to investigate the role of nitric oxide (NO) and N-methyl-D-aspartate (NMDA) receptors in cerebral arteriolar dilation to glutamate.

METHODS

Newborn, chloralose-anesthetized pigs were equipped with a closed cranial window. The diameter of pial arterioles was measured by means of intravital microscopy, and NO synthase (NOS) activity in brain cortex was determined by the conversion assay of [14C]arginine to [14C]citrulline.

RESULTS

Topical application of glutamate at 10(-7), 10(-6), and 10(-5) mol/L (n = 5) increased the mean diameter by 12 +/- 3%, 13 +/- 2%, and 18 +/- 3% (+/- SEM), respectively (baseline, 91 +/- 10 microns; P < .05). Similarly, NMDA application at the above doses (n = 5) dilated arterioles by 10 +/- 2%, 16 +/- 3%, and 18 +/- 6%, respectively (baseline, 97 +/- 4 microns; P < .05). Topical application of 10(-4) mol/L NG-nitro-L-arginine (L-NNA), which inhibited NOS activity by 93%, blocked the arteriolar dilation to glutamate or NMDA. Furthermore, administration of MK-801, a potent inhibitor of NMDA receptors, blocked glutamate-induced vasodilation completely in both topical application (10(-5) mol/L; n = 6) and intravenous administration (5 to 10 mg/kg; n = 5). In addition, neither L-NNA nor MK-801 attenuated the vasodilation to hypercapnia (PCO2 = 40 to 68 mm Hg).

CONCLUSIONS

Glutamate-induced cerebral arteriolar dilation is mediated by NO through NMDA receptors, and NO does not play a major role in the cerebral arteriolar dilation to hypercapnia (PCO2 = 40 to 68 mm Hg) in newborn pigs.

Nitric oxide and adenosine mediate vasodilation during functional activation in cerebellar cortex

Activation of the cerebellar parallel fibers (PF) releases glutamate and leads to depolarization of Purkinje cells and interneurons. These cells, in turn, release GABA. We have studied the role of glutamate, GABA, nitric oxide (NO) and adenosine in the increases in cerebellar cortex blood flow (BFcrb) elicited by PF stimulation. In anesthetized rats (halothane 1%) the cerebellar vermis was exposed and the site was superfused with Ringer (37 degrees C, pH 7.4). The PF were stimulated electrically (50-100 microA; 30 Hz) and the increases in BFcrb were recorded using a laser-Doppler flowmeter. Field potentials were recorded using glass microelectrodes. During Ringer superfusion, PF stimulation increased BFcrb by 58 +/- 5% (P < 0.001; analysis of variance; n = 6). Superfusion with the broad spectrum glutamate receptor antagonist kynurenic acid (Kyn; 5 mM) abolished the negative component of the field potential (n = 4), a finding reflecting lack of depolarization of Purkinje cells and interneurons, and blocked the increase in BFcrb (P > 0.05 from Ringer; n = 6). In contrast, Kyn did not influence the increase in BFcrb evoked by hypercapnia (pCO2 55.4 +/- 1.1 mmHg) or by superfusion with the NO donor SIN-1 (0.1, 1 mM; P > 0.05; n = 6). Superfusion with the adenosine receptor antagonist 8-sulphophenyltheophylline (8-SPT; 100 microM) reduced the elevation in BFcrb by 45 +/- 4% (P < 0.05; n = 6) and co-application of 8-SPT and of the NO synthase inhibitor nitro-L-arginine (L-NA; 1 mM) attenuated the vasodilation further (-82 +/- 4% from Ringer; P < 0.01 from 8-SPT alone).(ABSTRACT TRUNCATED AT 250 WORDS)

Response of cerebral endothelial cells to hypoxia: modification by fructose-1,6-bisphosphate but not glutamate receptor antagonists

Damage to the cerebral endothelium from ischemia could exacerbate brain injury by altering vascular integrity, but little is known concerning the response of cerebral endothelial cells to hypoxia. To address this issue, cerebral capillary endothelial cells were isolated from 14-day-old rats, grown to confluence, and placed in hypoxic chambers for up to 62 h. Cells were undamaged by 24 hours of hypoxia as assessed by lactate dehydrogenase release and ethidium bromide staining, but 48 h resulted in marked damage. Hypoxia was probably exacerbated by hypoglycemia because glucose levels fell to < 1 mM by 24 h, at which point ATP levels began to fall in hypoxic cultures (3.25 +/- 1.48 nmol/mg protein; mean +/- S.D.) relative to normoxic cultures (9.52 +/- 1.41 nmol/mg protein). Cells treated with 4 mM fructose-1,6-bisphosphate (FBP) had significantly less damage at 48 h of hypoxia than controls. FBP had little effect on rate of glucose depletion from the media, but ATP depletion due to hypoxia was significantly less. Thus, the protective effect of FBP may be mediated by the ability of treated cells to maintain higher ATP levels. Unlike FBP, glutamate receptor antagonists including MK-801, NBQX, DNQX, and kynurenic acid were ineffective in ameliorating hypoxia-induced endothelial cell injury.

Differential effects of competitive (CGS19755) and non-competitive (MK 801) NMDA receptor antagonists upon local cerebral blood flow and local cerebral glucose utilisation in the rat

The effects of the selective non-competitive NMDA receptor antagonist dizocilpine (MK801) and the competitive NMDA receptor antagonist CGS19755 upon local blood flow (lCBF) and local glucose utilisation (lCGU) were examined in 81 neuroanatomically discrete regions of the conscious rat brain using the [14C]iodoantipyrine and [14C]2-deoxyglucose quantitative autoradiographic techniques, respectively. Animals received dizocilpine (0.3 mg/kg), CGS19755 (30 mg/kg) or saline vehicle (2 ml/kg) 10 min prior to the initiation of lCGU studies while blood flow determinations were performed in parallel groups of animals 20 min after drug administration. Dizocilpine significantly increased lCGU in 33 of the 81 regions measured (most notably in cortical and subcortical limbic structures and in the basal ganglia) while reducing glucose use in seven brain areas (frontoparietal and somatosensory cortex, and in areas subserving auditory function). In contrast, CGS19755 significantly reduced lCGU use in 39 of the 81 areas examined while increases were observed in only three areas (anterior piriform cortex, substantia nigra pars reticulata, and posterior thalamic nucleus). Following Dizocilpine administration, there was evidence of widespread (64 of the 81 areas studied) increases in lCBF, while blood flow was reduced in the inferior colliculus. Significant increases in lCBF were also noted in 26 brain areas of CGS19755-treated rats while in one area (flocculus) blood flow was reduced. In saline-treated rats there was a close correlation between lCBF and lCGU. Dizocilpine administration was associated with an increase in the overall lCBF:lCGU ratio from 1.56 ml/mumol (in saline-treated rats) to 2.34 ml/mumol. In some brain areas (CA1 subfield of the dorsal hippocampus, somatosensory cortex and nucleus accumbens) there was evidence of focal disturbances in flow-metabolism relationship. While a similar increase in the overall lCBF-lCGU use ratio was evident in CGS19755 treated animals, there was no evidence of focal uncoupling of the flow metabolism relationship in any of the 81 brain areas examined. These data show that whilst both competitive and non-competitive NMDA receptor antagonists increased cerebral tissue perfusion beyond that required to meet underlying metabolic demand, focal disturbances in the flow metabolism relationship were observed only in dizocilpine-treated rats.

Reduction of vasogenic edema and infarction by MK-801 in rats after temporary focal cerebral ischemia

Blood-brain barrier permeability alteration, vasogenic brain edema, and infarction, which are more extensive after 3 hours of temporary middle cerebral artery occlusion (MCAO) and 3 hours of reperfusion than after 6 hours of permanent MCAO, develop in rats after prolonged focal cerebral ischemia. Protective effects of excitatory amino acid receptor antagonists have been previously demonstrated after temporary global ischemia and permanent focal ischemia in rats. The purpose of this study was to evaluate the effectiveness of MK-801, a noncompetitive N-methyl-D-aspartate receptor antagonist, in temporary middle cerebral artery occlusion in rats maintained at physiological levels of brain temperature. Rats were anesthetized with chloral hydrate (350 mg/kg, intraperitoneally). The MCAO of rats was occluded by cannulation with a nylon suture for 3 hours, followed by 3 hours of reperfusion accomplished by withdrawing the suture. MK-801 (1 mg/kg, intravenously) or saline (S) was injected immediately before the onset of MCAO. Water content (MK-801, n = 6; S, n = 6), Evans blue dye extravasation (MK-801, n = 6; S, n = 6), infarct volume (MK-801, n = 10; S, n = 10), histology (MK-801, n = 6; S, n = 6), and neurological deficit (MK-801, n = 15; S, n = 18) were measured at the end of 3 hours of reperfusion. Brain temperature was monitored during the experiment. The infarction area (measured by 2, 3, 5-triphenyltetrazolium chloride staining) was reduced (P < 0.001) in the MK-801-treated rats, as was the infarct volume and the severity of neuronal damage (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

The blood-brain barrier in vitro: the second decade

Ever since the discovery of Paul Ehrlich (1885 Das Sauerstoff-bedürfnis des Organismus: Hirschwald, Berlin) about the restricted material exchange, existing between the blood and the brain, the ultimate goal of subsequent studies has been mainly directed towards the elucidation of relative importance of different cellular compartments in the peculiar penetration barrier consisting the structural basis of the blood-brain barrier (BBB). It is now generally agreed that, in most vertebrates, the endothelial cells of the central nervous system (CNS) are responsible for the unique penetration barrier, which restricts the free passage of nutrients, hormones, immunologically relevant molecules and drugs to the brain. After an era of studying with endogenous or exogenous tracers the unique permeability properties of cerebral endothelial cells in vivo, the next generation, i.e. the in vitro blood-brain barrier model system was introduced in 1973. Recent advances in our knowledge of the BBB have in part been made by studying the properties and function of cerebral endothelial cells (CEC) with this in vitro approach. This review summarizes the results obtained on isolated brain microvessels in the second decade of its advent.

Nitric oxide mediates vasodilatation in response to activation of N-methyl-D-aspartate receptors in brain

Neurons release nitric oxide (NO) in response to activation of receptors for the excitatory amino acid N-methyl-D-aspartate (NMDA). We examined the hypothesis that activation of receptors for NMDA produces dilatation of the cerebral microcirculation that is mediated by NO. Diameters of cerebral arterioles were measured using a closed cranial window in anesthetized rabbits. Under control conditions, topical NMDA produced concentration-related dilatation of pial arterioles. Dilatation in response to NMDA was inhibited selectively by MK-801 (an NMDA receptor antagonist) and tetrodotoxin, suggesting that responses to NMDA were receptor mediated and dependent on neuronal activation. Increases in arteriolar diameter in response to NMDA were not affected by L-arginine but were inhibited by NG-nitro-L-arginine, suggesting that the vasodilatation was mediated by NO. Dilatation of cerebral arterioles in response to NMDA was not inhibited by indomethacin, suggesting that cyclooxygenase products do not mediate the response. Using isolated cerebral arteries, we also examined whether NMDA elicited direct cerebral vascular effects. In intact arteries studied in vitro, NMDA had no effect on vascular tone, suggesting that cerebral arteries lack receptors for NMDA. These findings suggest that NO generated in response to activation of receptors for NMDA in vivo is neuronally derived and not due to a direct vascular effect. Thus, NO may mediate increases in local blood flow during increases in neuronal activity in response to excitatory amino acids.

Interaction between free radicals and excitatory amino acids in the blood-brain barrier disruption after iron injury in the rat

Excitatory amino acids and oxygen free radicals have been reported to cooperate in the genesis of brain injury in vivo and in vitro. In this study, we tested the capacity of a noncompetitive N-methyl-D-aspartate receptor antagonist, MK-801, and a 21-aminosteroid, U-74006F, tirilazad mesylate, to block the opening of the blood-brain barrier after subarachnoid injection of FeCl2, which is believed to cause a primarily "pure" free radical insult. Subarachnoid injection of FeCl2 resulted in a significant 10-fold increase in Evans blue extravasation while sham injection or NaCl injection had no effect. Pretreatment with either MK-801 or U-74006F significantly reduced the FeCl2-induced increase in capillary permeability by 43 and 63%, respectively (p < 0.05). Combined treatment with MK-801 and U-74006F resulted in a 65% reduction in vascular leakage that was not significantly greater than pretreatment with either drug alone. These results show that both excitatory amino acids and free radicals can damage the cerebral microvasculature and that an excitatory amino acid antagonist can partially protect the blood-brain barrier after free radical-induced injury.

Acute experimental neuronal injury in the newborn lamb: US characterization and demonstration of hemodynamic effects

BACKGROUND AND PURPOSE

Microinjection into the brain with N-methyl-D-aspartate (NMDA), a synthetic analogue of glutamate, has been used as a chemical model of perinatal hypoxic-ischemic injury. Little is known about the sonographic characteristics and hemodynamic consequences of these cytotoxic lesions. An understanding of these features may be useful in the early sonographic identification of stroke in newborns.

METHODS

Twenty newborn lambs were anesthetized, paralyzed, and mechanically ventilated. Between 0.5 and 5 mu mole NMDA in 0.2 ml phosphate buffered saline, n = 18), or buffered saline only (n = 2) was injected into the right putamen under sonographic guidance. Serial grey-scale and color Doppler images of the brain, Doppler spectra of the middle cerebral and thalamostriate arteries, cerebral blood flow (CBF) determinations using radiolabeled microspheres (n = 9), and cerebral oxygen extraction (n = 4) were obtained before, and at 15.60, and 120 min after NMDA injection. Pathologic examination was obtained in 11 animals.

RESULTS

Homogeneous, well defined, moderately echogenic lesions surrounded by marked focal hyperemia on color Doppler were identified in every animal injected with 5 mu mole NMDA within minutes of injection. Lesions were characterized by focal areas of chromatolysis and cytoplasmic shrinkage, with scattered petechial hemorrhage. No lesions or hyperemia were observed in the animals injected with normal saline. Mean supratentorial CBF increased from 64 +/- 9 ml/min/100 g (control) to 152 +/- 30, 115 +/- 19, and 102 +/- 8 ml/min/100 g at 15, 60, and 120 min after injection respectively. The most marked increases occurred in right midbrain (467% of control), diencephalon (388%), and temporal lobe (282%), but were also observed in homotopic regions of the left hemisphere, and in pons, medulla, and cerebellum. Mean blood flow velocity in the middle cerebral artery, and thalamoperforator artery correlated well with changes in hemispheric and midbrain. CBF respectively. (r = 0.57-0.74, p = 0.0001, and r = 0.65-067, p = 0.0001 respectively).

CONCLUSIONS

Focal brain lesions may by identified by sonography within minutes after experimentally induced neuronal injury. Alterations in echotexture are primarily due to intracellular cytoplasmic changes and microscopic hemorrhage. Local intracerebral injection of NMDA in newborn lambs increases both local and global CBF.