Effect of acute electrode placement on regional CBF in the gerbil: a comparison of blood flow measured by hydrogen clearance, [3H]nicotine, and [14C]iodoantipyrine techniques

Mild hypothermia therapy attenuates early BBB leakage in acute ischemic stroke

Reperfusion therapy inevitably leads to brain-blood barrier (BBB) disruption and promotes damage despite its benefits for acute ischaemic stroke (AIS). An effective brain cytoprotective treatment is still needed as an adjunct to reperfusion therapy. Here, we explore the potential benefits of therapeutic hypothermia (HT) in attenuating early BBB leakage and improving neurological outcomes. Mild HT was induced during the early and peri-recanalization stages in a mouse model of transient middle cerebral artery occlusion and reperfusion (tMCAO/R). The results showed that mild HT attenuated early BBB leakage in AIS, decreased the infarction volume, and improved functional outcomes. RNA sequencing data of the microvessels indicated that HT decreased the transcription of the actin polymerization-related pathway. We further discovered that HT attenuated the ROCK1/MLC pathway, leading to a decrease in the polymerization of G-actin to F-actin. Arachidonic acid (AA), a known structural ROCK agonist, partially counteracted the protective effects of HT in the tMCAO/R model. Our study highlights the importance of early vascular protection during reperfusion and provides a new strategy for attenuating early BBB leakage by HT treatment for ischaemic stroke.

Neurovascular dynamics of repeated cortical spreading depolarizations after acute brain injury

Cortical spreading depolarizations (CSDs) are increasingly suspected to play an exacerbating role in a range of acute brain injuries, including stroke, possibly through their interactions with cortical blood flow. We use simultaneous wide-field imaging of neural activity and hemodynamics in Thy1-GCaMP6f mice to explore the neurovascular dynamics of CSDs during and following Rose Bengal-mediated photothrombosis. CSDs are observed in all mice as slow-moving waves of GCaMP fluorescence extending far beyond the photothrombotic area. Initial CSDs are accompanied by profound vasoconstriction and leave residual oligemia and ischemia in their wake. Later, CSDs evoke variable responses, from constriction to biphasic to vasodilation. However, CSD-evoked vasoconstriction is found to be more likely during rapid, high-amplitude CSDs in regions with stronger oligemia and ischemia, which, in turn, worsens after each repeated CSD. This feedback loop may explain the variable but potentially devastating effects of CSDs in the context of acute brain injury.

Zhao et al. use wide-field optical mapping of neuronal and hemodynamic activity in mice, capturing CSDs immediately following photothrombosis. Initial CSDs are accompanied by strong vasoconstriction, leaving persistent oligemia and ischemia. Region-dependent neurovascular responses to subsequent CSDs demonstrate a potential vicious cycle of CSD-dependent damage in acute brain injury.

Neurometabolic and electrophysiological changes during cortical spreading depolarization: multimodal approach based on a lactate-glucose dual microbiosensor arrays

Spreading depolarization (SD) is a slow propagating wave of strong depolarization of neural cells, implicated in several neuropathological conditions. The breakdown of brain homeostasis promotes significant hemodynamic and metabolic alterations, which impacts on neuronal function. In this work we aimed to develop an innovative multimodal approach, encompassing metabolic, electric and hemodynamic measurements, tailored but not limited to study SD. This was based on a novel dual-biosensor based on microelectrode arrays designed to simultaneously monitor lactate and glucose fluctuations and ongoing neuronal activity with high spatial and temporal resolution. In vitro evaluation of dual lactate-glucose microbiosensor revealed an extended linear range, high sensitivity and selectivity, fast response time and low oxygen-, temperature- and pH- dependencies. In anesthetized rats, we measured with the same array a significant drop in glucose concentration matched to a rise in lactate and concurrently with pronounced changes in the spectral profile of LFP-related currents during episodes of mechanically-evoked SD. This occurred along with the stereotypical hemodynamic response of the SD wave. Overall, this multimodal approach successfully demonstrates the capability to monitor metabolic alterations and ongoing electrical activity, thus contributing to a better understanding of the metabolic changes occurring in the brain following SD.

Regional temperature and quantitative cerebral blood flow responses to cortical spreading depolarization in the rat

Regional temperature and quantitative regional cerebral blood flow responses to cortical spreading depolarization in the rat were continuously monitored in the same tissue using a microfabricated thermal diffusion sensor that recalibrates and measures in 5-s cycles. The regional cerebral blood flow response had four phases, including early hyperemia (peak: 226% of baseline; duration: 113.1 ± 14.4 s) and late oligemia (minimum: 57%, duration: 28.4 ± 3.7 min). Temperature rose with the start of the regional cerebral blood flow response to a peak increase of 0.28 ± 0.06℃ and returned to baseline near the start of oligemia. This technology may be useful for multimodal monitoring in both the laboratory and clinic.

Spreading Depression, Spreading Depolarizations, and the Cerebral Vasculature

Spreading depression (SD) is a transient wave of near-complete neuronal and glial depolarization associated with massive transmembrane ionic and water shifts. It is evolutionarily conserved in the central nervous systems of a wide variety of species from locust to human. The depolarization spreads slowly at a rate of only millimeters per minute by way of grey matter contiguity, irrespective of functional or vascular divisions, and lasts up to a minute in otherwise normal tissue. As such, SD is a radically different breed of electrophysiological activity compared with everyday neural activity, such as action potentials and synaptic transmission. Seventy years after its discovery by Leão, the mechanisms of SD and its profound metabolic and hemodynamic effects are still debated. What we did learn of consequence, however, is that SD plays a central role in the pathophysiology of a number of diseases including migraine, ischemic stroke, intracranial hemorrhage, and traumatic brain injury. An intriguing overlap among them is that they are all neurovascular disorders. Therefore, the interplay between neurons and vascular elements is critical for our understanding of the impact of this homeostatic breakdown in patients. The challenges of translating experimental data into human pathophysiology notwithstanding, this review provides a detailed account of bidirectional interactions between brain parenchyma and the cerebral vasculature during SD and puts this in the context of neurovascular diseases.

Preconditioning cortical lesions reduce the incidence of peri-infarct depolarizations during focal ischemia in the Spontaneously Hypertensive Rat: interaction with prior anesthesia and the impact of hyperglycemia

The relationship between peri-infarct depolarizations (PIDs) and infarction was investigated in a model of preconditioning by cortical freeze lesions (cryogenic lesions, CL) in the Spontaneously Hypertensive Rat. Small (< 5 mm(3)) lesions produced 24 hours before permanent focal ischemia were protective, without impacting baseline cerebral blood flow (CBF) and metabolism. Prior CL reduced infarct volume, associated with improved penumbral CBF as previously showed for ischemic preconditioning. The brief initial procedure avoided sham effects on infarct volume after subsequent occlusion under brief anesthesia. However, under prolonged isoflurane anesthesia for perfusion monitoring both sham and CL rats showed reduced PID incidence relative to naive animals. This anesthesia effect could be eliminated by using α-chloralose during perfusion imaging. As an additional methodological concern, blood glucose was frequently elevated at the time of the second surgery, reflecting buprenorphine-induced pica and other undefined mechanisms. Even modest hyperglycemia (>10 mmol/L) reduced PID incidence. In normoglycemic animals CL preconditioning reduced PID number by 50%, demonstrating associated effects on PID incidence, penumbral perfusion, and infarct progression. Hyperglycemia suppressed PIDs without affecting the relationship between CBF and infarction. This suggests that the primary effect of preconditioning is to improve penumbral perfusion, which in turn impacts PID incidence and infarct size.

Impaired autoregulation of cerebral blood flow in an experimental model of traumatic brain injury

In order to study the pathophysiology and the intracranial hemodynamics of traumatic brain injury, we have developed a modified closed-head injury model of impact-acceleration that expresses several features of severe head injury in humans, including acute and long-lasting intracranial hypertension, diffuse axonal injury, neuronal necrosis, bleeding, and edema. In view of the clinical relevance of impaired autoregulation of cerebral blood flow after traumatic brain injury, and aiming at further characterization of the model, we investigated the autoregulation efficiency 24 h after experimental closed-head injury. Cortical blood flow was continuously monitored with a laser-Doppler flowmeter, and the mean arterial blood pressure was progressively decreased by controlled hemorrhage. Relative laser-Doppler flow was plotted against the corresponding mean arterial blood pressure, and a two-line segmented model was applied to determine the break point and slopes of the autoregulation curves. The slope of the curve at the right hand of the break point was significantly increased in the closed head injury group (0.751 +/- 0.966%/mm Hg versus -0.104 +/- 0.425%/mm Hg,p = 0.028). The break point tended towards higher values in the closed head injury group (62.2 +/- 20.8 mm Hg versus 46.9 +/- 12.7 mm Hg; mean +/- SD, p = 0.198). It is concluded that cerebral autoregulation in this modified closed head injury model is impaired 24 h after traumatic brain injury. This finding, in addition to other characteristic features of severe head injury established earlier in this model, significantly contributes to its clinical relevance.

Extravasation of radiographic contrast is an independent predictor of death in primary intracerebral hemorrhage

BACKGROUND AND PURPOSE

Hematomas that enlarge following presentation with primary intracerebral hemorrhage (ICH) are associated with increased mortality, but the mechanisms of hematoma enlargement are poorly understood. We interpreted the presence of contrast extravasation into the hematoma after CT angiography (CTA) as evidence of ongoing hemorrhage and sought to identify the clinical significance of contrast extravasation as well as factors associated with the risk of extravasation.

METHODS

We reviewed the clinical records and radiographic studies of all patients with intracranial hemorrhage undergoing CTA from 1994 to 1997. Only patients with primary ICH were included in this study. Univariate and multivariate logistic regression analyses were performed to determine the associations between clinical and radiological variables and the risk of hospital death or contrast extravasation.

RESULTS

Data were available for 113 patients. Contrast extravasation was seen in 46% of patients at the time of CTA, and the presence of contrast extravasation was associated with increased fatality: 63.5% versus 16.4% in patients without extravasation (P=0.011). There was a trend toward a shorter time (median+/-SD) from symptom onset to CTA in patients with extravasation (4.6+/-19 hours) than in patients with no evidence of extravasation (6.6+/-28 hours; P=0.065). Multivariate analysis revealed that hematoma size (P=0.022), Glasgow Coma Scale (GCS) score (P=0.016), extravasation of contrast (P=0.006), infratentorial ICH (P=0.014), and lack of surgery (P<0.001) were independently associated with hospital death. Variables independently associated with contrast extravasation were hematoma size (P=0.024), MABP >120 mm Hg (P=0.012), and GCS score of </=8 (P<0.005).

CONCLUSIONS

Contrast extravasation into the hematoma after ICH is associated with increased fatality. The risk of contrast extravasation is increased with extreme hypertension, depressed consciousness, and large hemorrhages. If contrast extravasation represents ongoing hemorrhage, the findings in this study may have implications for therapy of ICH, particularly with regard to blood pressure management.

Cerebral blood flow during hemodilution and hypoxia in rats : role of ATP-sensitive potassium channels

BACKGROUND AND PURPOSE

Hypoxia and hemodilution both reduce arterial oxygen content (CaO(2)) and increase cerebral blood flow (CBF), but the mechanisms by which hemodilution increases CBF are largely unknown. ATP-sensitive potassium (K(ATP)) channels are activated by intravascular hypoxia, and contribute to hypoxia-mediated cerebrovasodilatation. Although CaO(2) can be reduced to equal levels by hypoxia or hemodilution, intravascular PO(2) is reduced only during hypoxia. We therefore tested the hypothesis that K(ATP) channels would be unlikely to contribute to cerebrovasodilatation during hemodilution.

METHODS

Glibenclamide (19.8 microg) or vehicle was injected into the cisterna magna of barbiturate-anesthetized rats. The dose of glibenclamide was chosen to yield an estimated CSF concentration of 10(-4) M. Thirty minutes later, some animals underwent either progressive isovolumic hemodilution or hypoxia (over 30 minutes) to achieve a CaO(2) of approximately 7.5 mL O(2)/dL. Other animals did not undergo hypoxia or hemodilution and served as controls. Six groups of animals were studied: control/vehicle (n=4), control/glibenclamide (n=4), hemodilution/vehicle (n=10), hemodilution/glibenclamide (n=10), hypoxia/vehicle (n=10), and hypoxia/glibenclamide (n=10). CBF was then measured with (3)H-nicotine in the forebrain, cerebellum, and brain stem.

RESULTS

In control/vehicle rats, CBF ranged from 72 mL. 100 g(-1). min(-1) in forebrain to 88 mL. 100 g(-1) x min(-1) in the brain stem. Glibenclamide treatment of control animals did not influence CBF in any brain area. Hemodilution increased CBF in all brain areas, with flows ranging from 128 mL. 100 g(-1) x min(-1) in forebrain to 169 mL. 100 g(-1) x min(-1) in the brain stem. Glibenclamide treatment of hemodiluted animals did not affect CBF in any brain area. Hypoxia resulted in a greater CBF than did hemodilution, ranging from 172 mL. 100 g(-1) x min(-1) in forebrain to 259 mL. 100 g(-1) x min(-1) in the brain stem. Glibenclamide treatment of hypoxic animals significantly reduced CBF in all brain areas (P<0.05).

CONCLUSIONS

Both hypoxia and hemodilution increased CBF. Glibenclamide treatment significantly attenuated the CBF increase during hypoxia but not after hemodilution. This finding supports our hypothesis that K(ATP) channels do not contribute to increasing CBF during hemodilution. Because intravascular PO(2) is normal during hemodilution, this finding supports the hypothesis that intravascular PO(2) is an important regulator of cerebral vascular tone and exerts its effect in part by activation of K(ATP) channels in the cerebral circulation.

Reperfusion in a gerbil model of forebrain ischemia using serial magnetic resonance FAIR perfusion imaging

BACKGROUND AND PURPOSE

Existing methods for the quantitative measurement of the changing cerebral blood flow (CBF) during reperfusion suffer from poor spatial or temporal resolution. The aim of this study was to implement a recently developed MRI technique for quantitative perfusion imaging in a gerbil model of reperfusion. Flow-sensitive alternating inversion recovery (FAIR) is a noninvasive procedure that uses blood water as an endogenous tracer.

METHODS

Bilateral forebrain ischemia of 4 minutes' duration was induced in gerbils (n=8). A modified version of FAIR with improved time efficiency was used to provide CBF maps with a time resolution of 2.8 minutes after recirculation had been initiated. Quantitative diffusion imaging was also performed at intervals during the reperfusion period.

RESULTS

On initiating recirculation after the transient period of ischemia, the FAIR measurements demonstrated either a symmetrical, bilateral pattern of flow impairment (n=4) or an immediate side-to-side difference that became apparent with respect to the cerebral hemispheres in the imaged slice (n=4). The flow in each hemisphere displayed a pattern of recovery close to the preocclusion level or, alternatively, returned to a lower level before displaying a delayed hypoperfusion and a subsequent slow recovery. The diffusion measurements during this latter response suggested the development of cell swelling during the reperfusion phase in the striatum.

CONCLUSIONS

The CBF during the reperfusion period was monitored with a high time resolution, noninvasive method. This study demonstrates the utility of MRI techniques in following blood flow changes and their pathophysiological consequences.

Implementation of quantitative FAIR perfusion imaging with a short repetition time in time-course studies

Flow-sensitive alternating inversion recovery (FAIR) is a pulsed arterial spin labeling magnetic resonance imaging method for perfusion quantification. In its standard implementation for quantification with full longitudinal relaxation between acquisitions, its use in time-course investigations of rapidly changing flow values is limited. The time efficiency can be improved by decreasing the repetition time but quantification becomes problematic. This situation is further complicated if a whole-body radiofrequency transmit coil is not used since fresh blood spins will flow in from outside the coil. To alleviate these problems, the use of global pre-saturation is proposed. The resulting expression for the flow signal depends on the relationship between the imaging parameters and the coil inflow time and can be significantly simplified under certain combinations of these parameters. With this implementation of FAIR, quantitative flow maps of gerbil brains were obtained with a 3 minute time resolution in a study of the effects of reperfusion. The pre-occlusion flow measurements were in good agreement with values obtained by the standard FAIR implementation and by other techniques, but the low values following occlusion were underestimated due to the increased transit times.

Effect of alpha-adrenergic blockade on the cerebrovascular response to increased intracranial pressure after hemorrhage

OBJECT

In this study the authors tested the hypothesis that hemorrhagic hypotension and high intracranial pressure induce an increase in cerebrovascular resistance that is caused by sympathetic compensatory mechanisms and can be modified by alpha-adrenergic blockade.

METHODS

Continuous measurements of cerebral blood flow were obtained using laser Doppler microprobes placed in the cerebral cortex in anesthetized pigs during induced hemorrhagic hypotension and high cerebrospinal fluid pressure. Eight pigs received 2 mg/kg phentolamine in 10 ml saline, and 13 pigs served as control animals. During high intracranial pressure occurring after blood loss, cerebral perfusion pressure (CPP) (p < 0.01) and cerebral blood flow (p < 0.01) decreased in both groups. Cerebrovascular resistance increased (p < 0.05) in the control group and decreased (p < 0.005) in the phentolamine-treated group. The cerebrovascular resistance was significantly lower in the phentolamine-treated group (p < 0.05) than in the control group. Cerebrovascular resistance increased at lower CPPs in the control group (linear correlation, r = 0.39, p < 0.01) and decreased with decreasing CPP in the phentolamine-treated group (linear correlation, r = 0.76, p < 0.001).

CONCLUSIONS

This study shows that the deleterious effects on cerebral hemodynamics induced by blood loss in combination with high intracranial pressure are inhibited by alpha-adrenergic blockade. This suggests that these responses are caused by alpha-adrenergically mediated cerebral vasoconstriction.

Effect of reduced cerebral perfusion pressure on cerebral blood flow following inhibition of nitric oxide synthesis

OBJECT

The authors tested the hypothesis in a porcine model that inhibition of nitric oxide synthesis during reduced cerebral perfusion pressure (CPP) affected the relative cerebral blood flow (CBF) and the cerebrovascular resistance.

METHODS

The CPP was reduced by inducing high cerebrospinal fluid pressure and hemorrhagic hypotension. With continuous blood and intracranial pressure monitoring, relative CPP was estimated using the laser Doppler flowmetry technique in nine pigs that received 40 mg/kg nitro-L-arginine methyl ester (L-NAME) and in nine control animals. The L-NAME caused a decrease in relative CBF (p < 0.01) and increases in cerebrovascular resistance (p < 0.01), blood pressure (p < 0.05), and CPP (p < 0.001). During high intracranial pressure there were no significant differences between the treated animals and the controls. After hemorrhage, there was no significant difference between the groups initially, but 30 minutes later the cerebrovascular resistance was decreased in the control group and increased in the L-NAME group relative to baseline (p < 0.05). Combined hemorrhage and high intracranial pressure increased the difference between the two groups with regard to cerebrovascular resistance (p < 0.05).

CONCLUSIONS

These results suggest that nitric oxide synthesis inhibition affects the autoregulatory response of the cerebral circulation after cardiovascular compensation has taken place. Nitric oxide synthesis inhibition enhanced the undesirable effects of high intracranial pressure during hypovolemia.

How do the currently used prophylactic agents work in migraine?

Since migraine attacks are often frequent they require management with agents that reduce their number. Such agents, although often effective, are mechanistically ill-understood. They have been suggested to work through four main mechanisms, 5HT2 antagonism, modulation of plasma protein extravasation, modulation of central aminergic control mechanisms and membrane stabilizing effects through actions at voltage-sensitive channels. The evidence for these mechanisms, except plasma protein extravasation (see Cutrer, this supplement) is examined in the light of current thoughts concerning the pathophysiology of migraine.

Cortical spreading depression protects against subsequent focal cerebral ischemia in rats

The possibility that cortical spreading depression (CSD) may have neuroprotective action during subsequent focal cerebral ischemia was examined in rats. Three days before the imposition of focal cerebral ischemia CSDs were elicited by applying potassium chloride (KC1) for 2 h through a microdialysis probe implanted in the occipital cortex. Control animals were handled identically except that saline was infused instead of KC1. Focal ischemia was produced by the intraluminal suture method and cortical and subcortical infarct volumes were measured 7 days later. Neocortical infarct volume was reduced from 124.8 +/- 49.5 mm(3) in the controls to 62.9 +/- 59.5 mm(3) in the animals preconditioned with CSD (p = 0.012). There was no difference between the two groups in the subcortical infarct volume or in CBF, measured by the hydrogen clearance method, during or immediately after the ischemic interval. Our data indicate that preconditioning CSD applied 3 days before middle cerebral artery occlusion may increase the brain's resistance to focal ischemic damage and may be used as a model to explore the neuroprotective molecular responses of neuronal and glial cells.

Optimization of epoxyeicosatrienoic acid syntheses to test their effects on cerebral blood flow in vivo

Epoxyeicosatrienoic acids (EETs), normally present in brain and blood, appear to be released from atherosclerotic vessels in large amounts. Once intravascular, EETs can constrict renal arteries in vivo and dilate cerebral and coronary arteries in vitro. Whether EETs in blood will alter cerebral blood flow (CBF) in vivo is unknown. In the present study, the chemical synthesis of four EET regioisomers was optimized, and their identity and structural integrity established by chromatographic and mass spectral methods. The chemically labile EETs were converted to a sodium salt, complexed with albumin, and infused into anesthetized rats via the common carotid. The objective was to test whether sustained, high levels of intravascular EETs alter CBF. The CBF (cortical H2 clearance) was measured before and 30 min after the continuous infusion of 14,15- (n = 5), 11,12- (n = 5), 8,9- (n = 7) and 5,6-EET (unesterified or as the methyl ester, n = 5 for each). Neither the CBF nor the systemic blood pressure was affected by EETs. Because the infusions elevated the plasma concentrations of EETs about 700-fold above normal levels (1.0 nM), it is unlikely that EETs released from atherosclerotic vessels will alter CBF.

Digital imaging of umbelliferone clearance: a method for repeated measurements of cerebral cortical blood flow with high temporal and spatial resolution

We have developed a procedure for digital imaging of the exposed cerebral cortex during elution of a fluorescent dye. This avoids disturbing the cortex and has provided a method for the repeated estimation of regional CBF (rCBF) with a high topographical resolution. Under varying conditions of MABP and arterial blood gases, grey-level images of the exposed cortex irradiated with ultraviolet light (340 or 370 nm) were digitised (8 bits) at 15-s intervals after the injection of 1-2 ml of saturated umbelliferone solution into the lingual or external carotid artery of anaesthetised cats and rabbits. Specifically designed software allowed (a) regions of interest (ROIs) in the exposed cortex to be defined that were automatically applied to the sequence of images in a selected clearance and (b) solution of the initial slope equation for rCBF from the decay in grey-level fluorescence by exponential regression. Separate software that solved the equation at the level of a single pixel allowed a pseudocolour map of cortical rCBF to be generated. The factors affecting the resolution of this technique have been identified and quantified. Thus consistent and reproducible results were obtained provided that the fluorescence enhancement exceeded 20 grey levels and the r2 coefficient for regression was 90% or above. Mean rCBF values of 99.5 [95% confidence interval (CI), 89.4-110] ml 100 g-1 min-1 were obtained for rabbits (N = 12; mean MABP = 75.2; mean PaCO2 = 32.9; PaO2 = 111.8; pH 7.38) and 65.1 (95% CI, 55.1-75.1) ml 100 g-1 min-1 for cats (N = 8; mean MABP = 92.8; PaCO2 = 31.5; PaO2 = 114.6, pH 7.40).(ABSTRACT TRUNCATED AT 250 WORDS)

Calcitonin gene-related peptide reduces brain injury in a rat model of focal cerebral ischemia

BACKGROUND AND PURPOSE

Calcitonin gene-related peptide is an endogenous vasodilating neuropeptide with a dense concentration in the trigeminocerebrovascular system. It is hypothesized that depletion of this peptide contributes to delayed cerebral ischemia after subarachnoid hemorrhage and that an exogenous supply of calcitonin gene-related peptide will augment ischemic cerebral blood flow and reduce neuronal injury.

METHODS

In this study we have investigated the effect of an intravenous infusion of calcitonin gene-related peptide (100 ng/kg per minute), started 1 hour before and continued throughout 4 hours of focal cerebral ischemia, on cerebral blood flow and the volume of brain injury in a rat model of middle cerebral artery occlusion.

RESULTS

Calcitonin gene-related peptide produces a significant improvement in ischemic cerebral blood flow (32 +/- 2 compared with 13 +/- 2 mL/100 g per minute in the controls; t = 6.92, P < .0001) with a concomitant reduction in the volume of ischemic brain injury (102 +/- 22 compared with 234 +/- 19 mm3; t = 4.47, P < .001).

CONCLUSIONS

These findings lend support for the potential use of this peptide in the prophylactic treatment of delayed cerebral ischemia after aneurysmal subarachnoid hemorrhage.

The hemispheric cerebrovascular response to hemodilution is attenuated by a focal cryogenic brain injury

Experimental brain injury attenuates the normal increase in cerebral blood flow (CBF) that accompanies a fall in PaO2, and this may contribute to the well-known detrimental effects of hypoxia following closed head injury. Anemia is also known to adversely affect posttraumatic survival, and it is reasonable to hypothesize that this too may be related to an altered cerebrovascular response. Therefore, to examine this possibility, pentobarbital-anesthetized rabbits were subjected to a left posterior parietal cryogenic cortical injury, followed 90 min later by isovolemic hemodilution with hetastarch. Unlesioned control animals underwent an identical degree of hemodilution. CBF was measured using radioactive microspheres. In control animals, hemodilution was accompanied by a marked increase in CBF in all brain regions. For example, in the left posterior cortex, CBF increased from 30 +/- 14 mL/100 g/min (baseline Hct = 42 +/- 2%, mean +/- SD) to 100 +/- 20 mL/100 g/min at Hct = 12 +/- 1%. By contrast, there was a markedly attenuated response throughout the left (ipsilateral) hemisphere of injured animals, even in cortical regions distant from the injury. For example, in the left posterior cortex, CBF changed from a baseline of 32 +/- 21 mL/100 g/min (baseline) to 40 +/- 14 mL/100 g/min at the lowest Hct. CBF responses to hemodilution were unaltered in the contralateral hemisphere and in the hindbrain. These data indicate that a localized brain lesion can produce widespread ipsilateral alterations in the CBF response to hemodilution, with resultant compromise in cerebral O2 delivery. These data support the argument that the CBF increase produced by hemodilution is an active vasodilatory process rather than a passive response to changing blood viscosity.

Effect of phorbol myristate acetate on cerebral blood flow in normal and neutrophil-depleted rats

BACKGROUND AND PURPOSE

Recent evidence suggests a possible role for leukocytes in ischemic brain injury. This study examined the effect of activation of endogenous circulating leukocytes on cerebral blood flow in normal and neutrophil-depleted rats.

METHODS

Leukocytes were activated by rapid injection of either 50 micrograms/kg phorbol 12-myristate 13-acetate, a protein kinase C activator, or an equimolar amount of the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine, into the right carotid artery. Control rats received an equal volume of dimethyl sulfoxide in saline vehicle. H2-clearance cerebral blood flow was measured in each of the three groups and in vinblastine-treated, neutrophil-depleted rats after carotid artery injection of phorbol.

RESULTS

Phorbol 12-myristate 13-acetate dramatically decreased circulating leukocyte and platelet counts from 5 to 120 minutes after infusion and decreased regional cerebral blood flow in the ipsilateral parietal cortex from a baseline of 119 +/- 14 mL.min-1.100 g-1 (mean +/- SEM) to 49 +/- 5 mL.min-1.100 g-1 at 30 minutes (P < .05). Decreased flow persisted for the 2-hour study. Neither N-formyl-methionyl-leucyl-phenylalanine or vehicle had an effect on cerebral blood flow. In the neutrophil-depleted rats the initial decrease in cerebral blood flow at 30 and 60 minutes after infusion of phorbol was observed, but cerebral blood flow was restored to 70% to 80% of its baseline value (P > .05 versus baseline) by 90 to 120 minutes.

CONCLUSIONS

The early phorbol 12-myristate 13-acetate-induced decrease in cerebral blood flow may be due to the effects of protein kinase C activation on vascular smooth muscle or on platelet aggregation, whereas the persistent decrease in cerebral blood flow appears to be mediated in part by neutrophil activation.

Rapid changes in extracellular glucose levels and blood flow in the striatum of the freely moving rat

The dynamics of regional cerebral blood flow and brain extracellular glucose were studied in the freely moving rat. These two variables were measured in the striatum during and following both mild tail pinch and restraint stress. Blood flow was monitored using a refinement of the hydrogen clearance technique that allowed repeated measurements at 5-min intervals. A slow stream of hydrogen was directed at the rat's snout for 10-20 s through lightweight tubing attached to the animal's head and detected at a chronically implanted platinum electrode. Extracellular glucose was monitored with microdialysis in a separate group of animals using an on-line, enzyme-based assay that provided 2.5-min time resolution. Mean striatal blood flow 24 h following implantation was 89.9 +/- 2.5 ml.(100 g)-1.min-1. A 5-min tail pinch caused flow to increase immediately to 169.5 +/- 20 ml.(100 g)-1.min-1. In contrast, there was no change in blood flow during restraint stress, although there was a small increase following the end of the stress. Significant increases in blood flow were also observed in the striatum during periods of eating and grooming. Extracellular glucose levels increased following both forms of stress, to a maximum of 170 +/- 22% of baseline with restraint compared to 110 +/- 2% with tail pinch. In both cases, the increase occurred after the stress had ended and persisted while blood flow returned to basal levels.

Application of carbon-11 labelled nicotine in the measurement of human cerebral blood flow and other physiological parameters

Using positron emission tomography (PET), we measured the regional cerebral blood flow (rCBF) in five normal human subjects after intravenous injection of carbon-11 labeled (R)nicotine. The rCBF of the same subjects was measured by PET using the C15O2 inhalation steady-state method. The distribution of 11C activity in the brain after injection of 11C-(R)nicotine was almost equivalent to the CBF image obtained with the C15O2 inhalation stead-state method. The kinetics of 11C-(R)nicotine in the brain was analysed using a two-compartment model consisting of vascular and brain tissue compartments. The rCBF values obtained with 11C-(R)nicotine were higher than with C15O2 gas. The relatively long fixed distribution of 11C-(R)nicotine with a short uptake period allows stimulation studies by measurement of CBF to be performed with better photon flux and a longer imaging time than are possible with H215O.

Spreading depression induces prolonged reduction of cortical blood flow reactivity in the rat

The purpose of the present study was to examine the dynamic aspects of the cerebrovascular events occurring during and up to 2 h following cortical spreading depression (CSD) in the rat, using the mass spectrometry technique which enables continuous measurements of the cortical tissue PO2 and PCO2 and repeated blood flow measurements (CoBF) by helium clearance. We mostly sought to determine whether cortical perforation by a stimulation electrode induced long-lasting perturbation of the cortical vasoreactivity to hypercapnia and basal forebrain electrical stimulation. Cortical perforation in the animal under alpha-chloralose anesthesia, chronically implanted with mass spectrometry probes, was associated with biphasic changes in tissue gases. PO2 first briefly decreased (-7.8%) and then strongly increased (+79%) while PCO2 changed in the opposite direction (+7%, -13%) in the ipsilateral frontal cortex. Qualitatively similar changes occurred in the ipsilateral parietal cortex. The CoBF measurements showed a marked vasodilation (131 and 108% in the frontal and parietal cortex, respectively) in parallel with the PO2 increase, followed by a prolonged (60 min), moderate hypoperfusion (maximum -17% at 20 min after CSD). There was a pronounced reduction of vascular reactivity to both hypercapnia (20.3% of the control response) and substantia innominata stimulation (1/6 of the response obtained 80 min later) at 10 min after CSD. Both reactivities progressively recovered in approximately 2 h. Since the issue of CSD in human has become a popular hypothesis for migraine, the reduced cerebrovascular reactivity could constitute the basis of a test for the involvement of CSD in migraine.

The oligemic phase of cortical spreading depression is not blocked by tirilazad mesylate (U-74006F)

Cortical spreading depression is characterised by a wave of depolarization that moves across the cortex leaving in its wake a state of hyperpolarization. Characteristic changes in cerebral blood flow are also seen and these consist of a wave of hyperemia followed by an oligemia, the latter lasting some hours in some experimental animals including the cat. In this study cerebral blood flow was measured using laser Doppler flowmetry in the anesthetised ventilated cat. Spreading depression was initiated with a pin-prick injury prior to or following administration of U74006F (tirilizad mesylate; 3 mg/kg, ivi) or an identical volume of vehicle. Laser Doppler probes were placed bilaterally and in each case studied both the hyperemic and oligemic phases of spreading depression were preserved after administration of either U74006F or its vehicle. These data suggest that free radical mechanisms have no significant role in mediating the blood flow changes of spreading depression and are consistent with data in the literature of a quantitative using single-point measurements that again U74006F does not affect spreading depression.

Subcortical cerebral blood flow and metabolic changes elicited by cortical spreading depression in rat

Changes in cerebral cortical perfusion (CBFLDF), local cerebral blood flow (lCBF) and local cerebral glucose utilization (lCGU) elicited by unilateral cortical spreading depression (SD) were monitored and measured in separate groups of rats anesthetized with alpha-chloralose. CBFLDF was recorded with laser Doppler flowmetry, while lCBF and lCGU were measured by the quantitative autoradiographic [14C]iodoantipyrine and [14C]-2-deoxyglucose methods, respectively. SD elicited a wave of hyperemia after a latency of 2 to 3 min followed by an oligemic phase. Ninety minutes following the onset of SD cortical (frontal, parietal and occipital) lCBF and lCGU were essentially the same as on the contralateral side and in sham-treated rats. However, alteration in the lCBF and lCGU in upper and lower brainstem persisted. The present results demonstrate, for the first time, that long-lasting cerebrovascular and metabolic alterations take place within the subcortical regions following SD. These regions provide an attractive site to integrate observations in man concerning spreading depression and the aura of migraine with the other features of the syndrome.

The role of electrode size on the incidence of spreading depression and on cortical cerebral blood flow as measured by H2 clearance

Cerebral blood flow was measured by the H2 clearance method 30 and 60 min after the implantation of 300, 250, 125, or 50 microns diameter platinum-iridium electrodes 2 mm deep into the right parietal cortex of normothermic, normocarbic halothane-anesthetized rats. Another group of animals had 50 microns electrodes inserted 1 mm. In all animals, the presence or absence of a wave of spreading depression (SD) was noted at the time of implantation, with recordings made with glass micropipettes. H2 flow values were compared with those measured in gray matter from the same anatomical region (but from different rats), using [3H]nicotine. The incidence of SD ranged from 60% following insertion of 300 microns electrodes to 0% with 50 microns electrodes. H2 clearance flows also varied with electrode size, from 77 +/- 21 ml 100 g-1 min-1 (mean +/- standard deviation) with 300 microns electrodes to 110 +/- 31 and 111 +/- 16 ml 100 g-1 min-1 with 125 and 50 microns electrodes, respectively (insertion depth of 2 mm). A CBF value of 155 +/- 60 ml 100 g-1 min-1 was obtained with 50 micron electrodes inserted only 1 mm. Cortical gray matter blood flow measured with [3H]nicotine was 154 +/- 35 ml 100 g-1 min-1. When the role of SD in subsequent flow measurements was examined, there was a gradual increase in CBF between 30 and 60 min after electrode insertion in those animals with SD, while no such change was seen in rats without SD.(ABSTRACT TRUNCATED AT 250 WORDS)

Construction, calibration and evaluation of pO2 electrodes for chronical implantation in the rabbit brain cortex

Aiming at continuous polarographic measurement of the mean pO2 in the rabbit brain cortex before, during and after photochemically induced infarction, we designed and constructed monopolar platinum oxygen electrodes of the open type for chronical implantation. The measuring tip (length 1 mm, diameter 0.1 mm) is covered with a homogenous membrane of cellulose acetate. The electrode currents are measured by a four-channel amplifier of proper design; the device permits accurate and stable polarisation, identical for each channel. Moreover, a calibration device has been constructed. It consists of a Buchner funnel filled with Ringer solution and mounted in a temperature-controlled bath. In order to create a specific partial pressure of oxygen in the calibration chamber, predetermined gasmixtures are bubbled through the solution using computer controlled mass flow regulators. The calibration device thus permits the determination of primary and secondary electrode parameters, i.e. linearity, oxygen sensitivity and residual current, and polarisation dependency, temperature dependency, sensitivity to CO2, electrode stability, dynamic behaviour and oxygen consumption. Three groups, each of them containing ten electrodes, have been tested with regard to electrode parameters: the first group contains bare electrodes, the second and the third group contain membrane covered electrodes, with a membrane thickness of 10 and 20 microns respectively. In order to evaluate acute and long-term effects of implantation on the brain cortical tissue and on the sensors' measuring qualities, electrodes have been implanted for different time periods (51 days, 30 days, 9 days, 5 min). pO2 was recorded regularly and polarograms have been registered. The effects on cortical tissue have been studied with the aid of light microscopy.

Local cerebral blood flow response to locally infused 2-chloroadenosine during hypotension in piglets

Brain interstitial adenosine increases during hypotension in piglets. If adenosine is to participate in the regulation of neonatal cerebral blood flow (CBF) during hypotension, it must retain its vasodilatory action under that condition. To examine this issue, we studied the effects of locally infused 2-chloroadenosine (2-CADO), a stable adenosine analog, on local CBF in the piglet frontal cortex during normotension and graded hemorrhagic hypotension. We used the modified brain microdialysis/hydrogen clearance technique to simultaneously infuse 2-CADO into the frontal cortex and measure local CBF from the same area. When 2-CADO from 10(-8) M to 10(-3) M was infused under control conditions (n = 7), CBF increased 61% at 10(-5) M, 167% at 10(-4) M, and 210% at 10(-3) M. In hypotension experiments, local infusion of 10(-5) M 2-CADO (n = 8) caused significant increases in CBF (P less than 0.05) under control conditions (MABP = 65 mmHg) and at hypotensive blood pressures of 55 mmHg and 44 mmHg, respectively. At a blood pressure of 33 mmHg, however, infusion of the analog failed to increase CBF. Local infusion of 10(-3) M 2-CADO also produced a similar change in CBF during graded hypotension. These results indicate that 2-CADO dilates intracerebral vessels during normotension, and mild and moderate hypotension, and support the hypothesis that endogenous adenosine mediates autoregulatory adjustments of CBF during hypotension in newborn piglets.

Somatosensory evoked potentials in intracranial hypertension: analysis of the effects of hypoxia

The loss of somatosensory evoked potentials (SSEP's) was investigated in a feline model of intracranial hypertension. Threshold values of cerebral perfusion pressure (CPP) and cerebral blood flow (CBF) required for maintenance of SSEP's are defined using a mathematical model. The model describes loss of amplitude of SSEP's using the form of a dose-response curve. Amplitude of the SSEP's declined to 50% of control values at a CBF of 15 ml/100 gm/min and a CPP of 20 mm Hg in the normoxic animal; in the presence of mild hypoxia (8 to 9 kPa), a significant increase in these values to 18 ml/100 gm/min and 32 mm Hg, respectively, occurred. No reliable changes in latency or central conduction time were demonstrated. It is concluded that given adequate oxygenation, evoked electrical activity is lost at too low a level of CPP for this parameter to be useful in clinical monitoring. However, even mild hypoxia, when combined with intracranial hypertension, produces a major risk to neuronal integrity.

Effects of tirilazad mesylate on postischemic brain lipid peroxidation and recovery of extracellular calcium in gerbils

We describe the effects of the 21-aminosteroid tirilazad mesylate (U-74006F) on postischemic lipid peroxidation (depletion of brain vitamin E) and cortical extracellular calcium recovery in gerbils subjected to 3 hours of unilateral carotid artery occlusion. Male gerbils were treated with either 0.2 ml vehicle (0.05N HCl) or 10 mg/kg i.p. U-74006F 10 minutes before the induction of ischemia and again immediately after the initiation of reperfusion. In the first series of experiments, the brain concentration of vitamin E, which was unaffected by ischemia without reperfusion, was decreased after 2 hours of reperfusion by an average of 60% in vehicle-treated animals compared with sham-operated animals; in the U-74006F-treated gerbils, the 2-hour postischemic vitamin E loss was only 27% (p less than 0.002 different from vehicle-treated animals). In the second series, unilateral carotid artery occlusion produced a decrease in the cortical extracellular calcium concentration from 1.05 mM before ischemia to 0.11 mM by the end of the ischemic episode in both vehicle- and U-74006F-treated gerbils. After 2 hours of reperfusion, the calcium concentration had recovered to only 0.22 mM in the vehicle-treated animals compared with 0.56 mM in the U-74006F-treated group (p less than 0.01). Cortical blood flow, mean arterial blood pressure, and blood gases did not differ significantly between the two treatment groups. Administration of only the immediate postreperfusion dose (i.e., no pretreatment) also significantly improved the recovery of cortical extracellular calcium. The results indicate that U-74006F inhibits postischemic lipid peroxidation as assessed by the preservation of brain vitamin E and that, secondary to this membrane-protective effect, the processes responsible for the reversal of ischemia-triggered intracellular calcium accumulation are preserved.

Blood-brain transfer of L-phenylalanine declines after peripheral but not central nervous administration of vasopressin

To determine whether a previously reported effect of vasopressin on blood-brain transfer of leucine extends to other large neutral amino acids, we measured the regional blood-brain transfer of L-phenylalanine with the integral technique. Intravenous co-injection of L-phenylalanine and arginine vasopressin (30 nmol to 10 pmol) resulted in a decrease of the permeability-surface area (PaS) product of phenylalanine of between 11 and 39%. In addition, the peptide elicited a decrease of the cerebral blood flow of between 11 and 56% combined with a drastic decrease of the cardiac output (32-64%) and an elevation of the blood pressure to approximately 150% of control values. However, we found no changes of the cardiac output, the blood pressure, or the PaS product of phenylalanine after microdialysis (30 min, 5 microliters min-1) of arginine vasopressin (15 mumol L-1) into the dorsal hippocampus, but cerebral blood flow was decreased. The results support the hypothesis that arginine vasopressin receptors at the blood-brain barrier are involved in the regulation of large neutral amino acid transfer from blood to brain and indicate that these receptors are located at the luminal membrane of the endothelial cells.

Diurnal variation of cerebral blood flow in rat hippocampus

We measured local cerebral blood flow over 24 hours in 10 unanesthetized, freely moving rats to determine whether blood flow in the hippocampus fluctuated as a function of time of day. We measured hydrogen clearance at 1-hour intervals using a polyurethane-coated platinum electrode with a 1-mm bare tip implanted in the dorsal hippocampus. Individual rats displayed a wide range of local cerebral blood flow values (from 30 to 100 ml/min/100 g tissue) in a day. In seven of the 10 rats, the overall mean hippocampal blood flow for the dark cycle (7 PM-5 AM) was significantly (p less than 0.001, 0.01, or 0.05) greater than that for the light cycle (6 AM-6 PM), showing an average increase of 20%. Further, the maximum mean hippocampal blood flow at 11 PM in all 10 rats was 42% greater than the minimum at noon. Our study demonstrates for the first time that local cerebral blood flow in the hippocampus shows diurnal variation.

Functional impairment due to white matter ischemia after middle cerebral artery occlusion in cats

We recorded regional cerebral blood flow, somatosensory evoked potentials, and auditory evoked potentials in the thalamic relay nuclei (ventral posterior lateral nucleus and medial geniculate body) and in the somatosensory and auditory cortices during and after 1 hour of transient left middle cerebral artery occlusion in nine cats. Regional cerebral blood flow was also measured in the thalamocortical tracts of five of these cats. Additionally, the integrity of thalamocortical connections was tested by retrograde labeling of the thalamic nuclei with horseradish peroxidase in eight cats (three of which experienced no ischemia). Regional cerebral blood flow was severely reduced during middle cerebral artery occlusion in the left primary auditory cortex (8.5 ml/100 g/min) and in white matter pathways (6.4-7.6 ml/100 g/min). In contrast, regional cerebral blood flow did not change significantly in the somatosensory cortex or in either thalamic nucleus. Evoked potentials were abolished in both cortices but remained unchanged in the thalamic nuclei. Cortical somatosensory evoked potentials disappeared 5-8 minutes later than auditory evoked potentials. Recirculation after 1 hour of ischemia resulted in rapid and almost complete recovery (94%) of somatosensory evoked potentials and little recovery (18.4%) of auditory evoked potentials. We conclude that in the auditory pathway both cortical and fiber tract ischemia are (perhaps synergistically) responsible for dysfunction, while in the somatosensory cortex evoked potentials are abolished due to white matter ischemia. The delayed disappearance and better recovery of somatosensory than of auditory evoked potentials indicate that ischemic tolerance is higher in fiber tracts than in cortex.

Local cerebral blood flow following the intrastriatal administration of vasoactive intestinal peptide or peptide histidine isoleucine in the rat

The effect of the intrastriatal microinjection of either vasoactive intestinal peptide (VIP) or a related peptide-peptide histidine isoleucine (PHI)-on local cerebral blood flow in the striatum was examined using iodo[14C]antipyrine quantitative autoradiography. In 37 rats, an injection needle was inserted into a chronically implanted guide cannula and 1 microliter of vehicle, VIP or PHI was injected into the striatum. Blood flow in sham controls was reduced by 15% in proximity to the injection site, when compared with blood flow in the contralateral uninjected control side (P less than 0.01). Similarly, following PHI administration (20 pmol), blood flow in the striatum was reduced by 14% when compared to that contralaterally (P less than 0.02). In contrast, following VIP administration (20 pmol), blood flow in proximity to the injection site was increased compared to flow in the contralateral striatum in 4/8 animals with the mean flow being elevated by 10% (n.s.) compared to blood flow contralaterally. VIP and PHI had similar effects on local cerebral glucose utilization in the caudate nucleus, their response being equivalent to that of sham animals. These experiments suggest that VIP and PHI have a differential influence on the microvasculature of the caudate nucleus, with VIP but not PHI mediating cerebrovascular dilation.

Assessment of damage from implantation of microdialysis probes in the rat hippocampus with silver degeneration staining

We used a sensitive silver degeneration staining method to study the effects of insertion of microdialysis probes in rat dorsal hippocampus and neocortex. Nine animals were sacrificed 24 h, 3 days or 7 days after implantation of dialysis tubing. Although mild neuronal cell death and small petechial hemorrhages were seen in close proximity to the implantation site, the striking finding was the presence of degenerating axons both adjacent to the implantation site and in remote sites such as the corpus callosum and contralateral hippocampus. The observed changes could alter brain function near or remote from the implantation site and should be considered in analysis of dialysis experiments.

Laser-Doppler flowmetry. A review of its application for measuring cerebral and spinal cord blood flow

Laser-Doppler flowmetry is a new technique for noninvasive and continuous measurement of local microcirculatory cerebral and spinal-cord blood flow. The flow estimate by this technique is based on the assessment of the Doppler shift of low-power laser light, which is scattered by moving red blood cells. Laser-Doppler flowmetry has been validated for various organs, including the central nervous system. These studies revealed a linear relationship between relative changes of the Doppler signal and blood flow over a wide range of pharmacological as well as pathological flow alterations, including cerebral ischemia. The usefulness of laser-Doppler flowmetry in experimental as well as clinical applications has received growing attention. The superiority of the technique lies in its high spatial and temporal resolution. Disadvantages are the difficulty of obtaining absolute flow values and the sensitivity to artifacts. The versatility and on-line capacity of laser-Doppler flowmetry might allow new insights into the pathophysiology of alterations of the cerebral and spinal-cord microcirculation.