Measurement of regional cerebral blood flow with antipyrine-14C in awake cats

Beyond monoamines: II. Novel applications for PET imaging in psychiatric disorders

Early applications of positron emission tomography (PET) in psychiatry sought to identify derangements of cerebral blood flow and metabolism. The need for more specific neurochemical imaging probes was soon evident, and these probes initially targeted the sites of action of neuroleptic (dopamine D₂ receptors) and psychoactive (serotonin receptors) drugs. For nearly 30 years, the centrality of monoamine dysfunction in psychiatric disorders drove the development of an armamentarium of monoaminergic PET radiopharmaceuticals and imaging methodologies. However, continued investments in monoamine-enhancing drug development realized only modest gains in efficacy and tolerability. As patent protection for many widely prescribed and profitable psychiatric drugs lapsed, drug development pipelines shifted away from monoamines in search of novel targets with the promises of improved efficacy, or abandoned altogether. Over this period, PET radiopharmaceutical development activities closely parallelled drug development priorities, resulting in the development of new PET imaging agents for non-monoamine targets. In part two of this review, we survey clinical research studies using the novel targets and radiotracers described in part one across major psychiatric application areas such as substance use disorders, anxiety disorders, eating disorders, personality disorders, mood disorders, and schizophrenia. Important limitations of the studies described are discussed, as well as key methodologic issues, challenges to the field, and the status of clinical trials seeking to exploit these targets for novel therapeutics.

Quantitative blood flow measurement in rat brain with multiphase arterial spin labelling magnetic resonance imaging

Cerebral blood flow is an important parameter in many diseases and functional studies that can be accurately measured in humans using arterial spin labelling (ASL) MRI. However, although rat models are frequently used for preclinical studies of both human disease and brain function, rat CBF measurements show poor consistency between studies. This lack of reproducibility is due, partly, to the smaller size and differing head geometry of rats compared to humans, as well as the differing analysis methodologies employed and higher field strengths used for preclinical MRI. To address these issues, we have implemented, optimised and validated a multiphase pseudo-continuous ASL technique, which overcomes many of the limitations of rat CBF measurement. Three rat strains (Wistar, Sprague Dawley and Berlin Druckrey IX) were used, and CBF values validated against gold-standard autoradiography measurements. Label positioning was found to be optimal at 45°, while post-label delay was optimised to 0.55 s. Whole brain CBF measures were 109 ± 22, 111 ± 18 and 100 ± 15 mL/100 g/min by multiphase pCASL, and 108 ± 12, 116 ± 14 and 122 ± 16 mL/100 g/min by autoradiography in Wistar, SD and BDIX cohorts, respectively. Tumour model analysis shows that the developed methods also apply in disease states. Thus, optimised multiphase pCASL provides robust, reproducible and non-invasive measurement of CBF in rats.

APOE and Alzheimer's Disease: Neuroimaging of Metabolic and Cerebrovascular Dysfunction

Apolipoprotein E4 (ApoE4) is the strongest genetic risk factor for late onset Alzheimer’s Disease (AD), and is associated with impairments in cerebral metabolism and cerebrovascular function. A substantial body of literature now points to E4 as a driver of multiple impairments seen in AD, including blunted brain insulin signaling, mismanagement of brain cholesterol and fatty acids, reductions in blood brain barrier (BBB) integrity, and decreased cerebral glucose uptake. Various neuroimaging techniques, in particular positron emission topography (PET) and magnetic resonance imaging (MRI), have been instrumental in characterizing these metabolic and vascular deficits associated with this important AD risk factor. In the current mini-review article, we summarize the known effects of APOE on cerebral metabolism and cerebrovascular function, with a special emphasis on recent findings via neuroimaging approaches.

History of International Society for Cerebral Blood Flow and Metabolism

Interest in the brain's circulation dates back more than a century and has been steadily growing. Quantitative methods for measurements of cerebral blood flow (CBF) and energy metabolism became available in the middle of the 20th century and gave a new boost to the research. Scientific meetings dealing with CBF and metabolism were arranged, and the fast growing research led to a demand for a specialized journal. In this scientific environment, the International Society for Cerebral Blood Flow and Metabolism (ISCBFM) and its official Journal of Cerebral Metabolism were established in 1981 and has since then been a major success. The development of new brain imaging methods has had a major impact. Regulation of CBF and ischemia has been the main topics at the meetings. A new field of brain mapping research emerged and has now its own society and meetings. Brain emission tomography research has grown within the society and is now an integrated part. The ISCBFM is a sound society, and support of young scientists is among its goals. Several awards have been established. Other activities including summer schools, courses, satellite meetings, and Gordon conferences have contributed to the success of the society and strengthened the research.

Visualization of the ischemic core on native human brain slices by potassium staining method

The potassium staining method is based on the formation of potassium cobaltnitrite crystals after the treatment by sodium cobaltnitrite of brain tissue. The degree of staining correlates with the distinct potassium content of infracted and non-infarcted brain areas. The aim of the present study was to prove that potassium staining technique is a reliable method for localization of ischemic core on native whole hemisphere cryosections of stroke patients. Furthermore, potassium stained sections have been compared with appropriate postmortem MRI images of respective brains. Brains of stroke patients were removed within 24h after death and postmortem MRI scanning was performed. Horizontal cryosections of frozen brains were taken and potassium staining was performed. Using the stained whole hemisphere sections as "map" tissue sampling has been made in order to determine water and potassium content. Potassium content of infarcted samples was significantly decreased in comparison with intact regions (0.7346+/-0.2142 mg/L and 1.928+/-0.447 mg/L, respectively, p<0.01) (mean values+/-SD). Water content of affected areas (expressed in percents) has been found to be above non-infarcted regions (81.657%+/-4.07 and 72.96%+/-6.37, respectively, p<0.01). According to our results the potassium staining method of human whole hemisphere brain sections reliably differentiates focal ischemic areas from intact brain regions. In conclusion, the postmortem examination of ischemic brain could be started with making the potassium map of infarcted whole hemisphere cryosections providing guidance for targeted tissue sampling and base of comparison for further examinations.

Pittfalls of MRI measurement of white matter perfusion based on arterial spin labeling

Although arterial spin labeling (ASL) MRI has been successfully applied to measure gray matter (GM) perfusion in vivo, accurate detection of white matter (WM) perfusion has proven difficult. Reported literature values are not consistent with each other or with perfusion measured with other modalities. In this work, the cause of these inconsistencies is investigated. The results suggest that WM perfusion values are substantially affected by the limited image resolution and by signal losses caused by the long transit times in WM, which significantly affect the label. From gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) bolus-tracking experiments (N=6), it is estimated that the transit time can be several seconds long in deep WM. Furthermore, simulations show that even at a spatial resolution of 7 microl voxel size, contamination by the GM signals can exceed 40% of the actual WM signal. From 10-min long flow-sensitive alternating inversion recovery ASL (FAIR-ASL) measurements at 3T in normal subjects (N=7), using highly sensitive detectors, it is shown that single-voxel (7 mul) deep WM perfusion values have an signal-to-noise ratio (SNR) less than 1. The poor sensitivity and heterogeneous transit time limit the applicability of ASL for measurement of perfusion in WM.

Receptor binding techniques

This overview first discusses issues relating to the selection of radioligand for receptor binding assays, including the isotopic label and considerations pertaining to the pharmacological and chemical profile of the ligand. This is followed by a section on characterization of ligand-binding assays, starting with tissue preparation methods, followed by detection of specific binding, determination of incubation and washing conditions and a discussion of saturation and competition assay formats. Quantification of the assay results can be accomplished by autoradiography or film densitometry. Finally, methods and considerations for analysis of the resulting data are presented.

Cerebral blood flow and metabolism

To date, regional measurements of cerebral blood flow in humans have not been especially helpful in predicting the outcome of acute brain injury. The recent appearance within the medical environment of biologically significant pharmaceuticals labelled with the positron-emitting isotopes 15O, 13N and 11C has made possible the quantitative regional measurement of cerebral metabolism in humans. This advance, coupled with the development of radically new detection systems, will allow the safe measurement of truly regional blood flow and metabolism in humans in the near future. Such information is basic to an understanding of the pathophysiology of acute injury to the brain and the rational prediction of outcome in the individual case.

Establishing a photothrombotic 'ring' stroke model in adult mice with late spontaneous reperfusion: quantitative measurements of cerebral blood flow and cerebral protein synthesis

This study sought to establish the photothrombotic 'ring' stroke model with late spontaneous reperfusion in adult mice. By applying a 3.0-mm diameter laser ring-beam (514 nm, 0.21 mm thick, 0.65 W/cm2) onto the exposed skull for 60 secs with concurrent erythrosin B (4.25 mg/kg) intravenous infusion for 15 secs, the centrally located cortical region within the ring locus was progressively encroached by an annular ring-shaped perfusion deficit. In this region, local cerebral blood flow (lCBF) measured by laser-Doppler flowmetry declined promptly after irradiation to 43% of the baseline value at 30 mins poststroke. Using double tracer autoradiography, quantitative lCBF measured with [14C]iodoantipyrine was 46 to 17 to 58 ml/100 g/mins at 4 h to 48 h to 7 days postischemia in this area. Cerebral protein synthesis (CPS), as detected by [3H]leucine incorporation into protein, transiently decreased to 57% to 38% to 112% at 4 h to 48 h to 7 days postischemia in the center region. Morphologically, some neurons in the center region appeared swollen at 4 h. At 48 h, the majority of neurons were severely swollen with eosinophilia and pyknosis, whereas at 7 days poststroke' the tissue morphology became partly restored. The center within the mouse photothrombotic ring lesion thus exhibits reversible alterations of local CBF, CPS and tissue morphology that are reminiscent of the cortical penumbra in other models of focal cerebral ischemia.

Resolution, sensitivity and precision with autoradiography and small animal positron emission tomography: implications for functional brain imaging in animal research

Quantitative autoradiographic methods for in vivo measurement of regional rates of cerebral blood flow, glucose metabolism, and protein synthesis contribute significantly to our understanding of phsysiological and biochemical responses of the brain to changes in the environment. A disadvantage of these autoradiographic methods is that experimental animals can be studied only once. With the advent of small animal positron emission tomography (PET) and with increases in the sensitivity and spatial resolution of scanners it is now possible to use adaptations of these methods in experimental animals with PET. These developments allow repeated studies of the same animal, including studies of the same animal under different conditions, and longitudinal studies. In this review we summarize the tradeoffs between the use of autoradiography and small animal PET for functional brain imaging studies in animal research.

Investigations of the cholinergic deficit hypothesis in the hippocampus of the aged rat brain with physostigmine and scopolamine

Using histochemically demonstrated acetylcholinesterase activity and (14)C-2-deoxyglucose uptake as the respective indices, a study was set up to determine whether cerebral (hippocampal) metabolism was stimulated by a cholinergic agonist and/or inhibited by a cholinergic antagonist. For this 36 12-month-old (adult) and 48 27-month-old (aged) Fischer 344 rats were given intraperitoneal injections of physostigmine 0.05, 0.1 or 0.2 mg/kg or scopolamine 0.01, 0.03 or 0.1 mg/kg for 5 days. In the aged rats there was a slight increase in acetylcholinesterase activity after physostigmine but no convincing evidence of enhanced (14)C-2-deoxyglucose uptake. In neither age group was glucose uptake significantly reduced by scopolamine; it was in fact increased, as was - slightly but significantly - acetylcholinesterase activity. Findings for acetylcholinesterase activity and (14)C-2-deoxyglucose uptake in aged Fischer 344 rats thus do not provide firm corroboration of physostigmine-induced stimulation of mental performance found in behavioural studies, while scopolamine did not adversely affect the hippocampal variables studied. It is concluded that cholinergic agents such as physostigmine and scopolamine have only a marginal effect on the functional and metabolic deficits associated with cerebral aging.

Quantitation and localization of blood-to-brain influx by magnetic resonance imaging and quantitative autoradiography in a model of transient focal ischemia

The ability of gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) enhanced MRI to localize and quantitate blood-brain barrier (BBB) opening was evaluated against quantitative autoradiographic (QAR) imaging of (14)C-alpha-aminoisobutyric acid (AIB) distribution. The blood-to-brain transfer constant (K(i)) for Gd-DTPA was determined by MRI in rats after 3 h of focal cerebral ischemia plus 2.5 h of reperfusion (n = 9), and that of AIB was determined by QAR shortly thereafter. Tissue regions of interest (ROIs) for Gd-DTPA leakage were identified by ISODATA segmentation of pre- and post-Gd-DTPA Look-Locker (L-L) T(1) maps. Patlak plots were constructed using time course of blood and tissue T(1) changes induced by Gd for estimating K(i). Among the nine rats, 14 sizable regions of AIB uptake were found; 13 were also identified by ISODATA segmentation. Although the 13 MRI-ROIs spatially approximated those of AIB uptake, the segmentation sometimes missed small areas of lesser AIB uptake that did not extend through more than 60% of the 2.0-mm-thick slice. Mean K(i)'s of AIB were highly correlated with those of Gd-DTPA across the 13 regions; the group means (+/-SD) were similar for the two tracers (7.1 +/- 3.3 x 10(-3) and 6.8 +/- 3.5 x 10(-3) ml.g(-1) . min(-1), respectively). In most instances, Gd-DTPA MRI accurately localized areas of BBB opening.

Dynamic imaging of perfusion and oxygenation by functional magnetic resonance imaging

Cerebral blood flow can be measured with magnetic resonance imaging (MRI) by arterial spin labeling techniques, where magnetic labeling of flowing spins in arterial blood water functions as the endogenous tracer upon mixing with the unlabeled stationary spins of tissue water. The consequence is that the apparent longitudinal relaxation time (T1) of tissue water is attenuated. A modified functional MRI scheme for dynamic CBF measurement is proposed that depends on extraction of T1 weighting from the blood oxygenation level-dependent (BOLD) image contrast, because the functional MRI signal also has an intrinsic T1 weighting that can be altered by variations of the excitation flip angle. In the alpha-chloralose-anesthetized rat model at 7T, the authors show that the stimulation-induced BOLD signal change measured with two different flip angles can be combined to obtain a T1-weighted MRI signal, reflecting the magnitude of the CBF change, which can be deconvolved to obtain dynamic changes in CBF. The deconvolution of the T1-weighted MRI signal, which is a necessary step for accurate reflection of the dynamic changes in CBF, was made possible by a transfer function obtained from parallel laser-Doppler flowmetry experiments. For all stimulus durations (ranging from 4 to 32 seconds), the peak CBF response measured by MRI after the deconvolution was reached at 4.5 +/- 1.0 seconds, which is in good agreement with (present and prior) laser-Doppler measurements. Because the low flip angle data can also provide dynamic changes of the conventional BOLD image contrast, this method can be used for simultaneous imaging of CBF and BOLD dynamics.

Adventures in the pathophysiology of brain ischemia: penumbra, gene expression, neuroprotection: the 2002 Thomas Willis Lecture

BACKGROUND

The pathophysiology of cerebral ischemia is well studied in small-animal models, which offer reproducibility and control of confounding variables-factors essential to hypothesis-testing. This presentation first highlights insights into the ischemic penumbra enabled by a multimodal experimental approach; second, discusses gene expression in ischemia; and third, confronts the challenges of neuroprotectant therapy.

SUMMARY OF REVIEW

The ischemic penumbra: Transient (2-hour) middle cerebral artery suture-occlusion in anesthetized rats gives rise to highly consistent neurological and histopathological sequelae. Autoradiographic local cerebral blood flow (LCBF) studies at 2 hours of occlusion define the penumbra as a region of intermediate CBF depression (20% to 40% of control) surrounding the densely ischemic core (5% to 20% of control) and constituting one half of the entire lesion. Local glucose metabolic rate in the acute penumbra is not reduced despite the critical CBF reduction, so that the penumbral metabolism/blood flow ratio is markedly elevated. In contrast, following 1 hour of recirculation, glucose metabolism throughout the previously ischemic hemisphere has become markedly depressed, and the metabolism/flow ratio has pseudonormalized. By correlating these data with histopathology using multimodal image analysis, the probability of infarction is shown to be highly determined by the degree of antecedent CBF reduction. These animal data agree strikingly with published results in patients with acute stroke studied by positron emission tomography. This remarkable correspondence belies the assertion that data from lower species may not be relevant to human stroke. Gene expression: Perfusion gradients also determine differential patterns of gene expression in ischemia. This can be demonstrated by correlating in situ hybridization autoradiographs for gene expression with autoradiographic LCBF data and histological infarct maps derived from replicate series. In other studies, DNA microarray technology is used to screen for thousands of expressed genes. In the 2-hour middle cerebral artery occlusion model with 3-hour recirculation, we have identified 28 known ischemia-hypoxia response genes that are upregulated and 6 that are downregulated, together with 35 upregulated and 41 downregulated genes newly connected with ischemia. These findings underscore the enormous complexity of ischemic biology and suggest possible novel mechanisms for future exploration. NEUROPROTECTION: A desirable neuroprotectant would, in theory, antagonize multiple injury mechanisms. We have explored 2 such therapies of particular promise. Mild brain hypothermia (32 degrees C target temperature, for 5 hours) is highly neuroprotective even when initiated at the onset of recirculation. Another highly protective agent is human albumin, administered in doses of 1.25 to 2.5 g/kg--a therapy that reduces infarct volume in this ischemia model by 60% to 65%, markedly diminishes brain swelling, and has a therapeutic window extending to 4 hours.

CONCLUSION

The careful study of rodent ischemia models can yield valuable, clinically relevant insights into the pathophysiology of ischemic stroke.

Cortical spreading depression causes a long-lasting decrease in cerebral blood flow and induces tolerance to permanent focal ischemia in rat brain

Cortical spreading depression (CSD) has previously been shown to induce tolerance to a subsequent episode of transient cerebral ischemia. The objective of the present study was to determine whether CSD also induces tolerance to permanent focal ischemia and, if so, whether tolerance may be mediated by alterations in cerebral blood flow (CBF). Sprague-Dawley rats were preconditioned by applying potassium chloride to one hemisphere for 2 hours, evoking 19 +/- 5 episodes of CSD (mean +/- SD, n = 19). Three days later, the middle cerebral artery (MCA) was permanently occluded using an intraluminal suture. In a subset of animals, laser Doppler blood flow (LDF) was monitored over the parietal cortex before and during the first 2 hours of MCA occlusion. Preconditioning with CSD reduced the hemispheric volume of infarction from 248 +/- 115 mm3 (n = 18) in sham-conditioned animals to 161 +/- 81 mm3 (n = 19, P< 0.02). Similarly, CSD reduced the neocortical volume of infarction from 126 +/- 82 mm3 to 60 +/- 61 mm3 (P < 0.01). Moreover, preconditioning with CSD significantly improved LDF during MCA occlusion from 21% +/- 7% (n = 9) of preischemic baseline in sham-conditioned animals to 29% +/- 9% (n = 7, P< 0.02). Preconditioning with CSD therefore preserved relative levels of CBF during focal ischemia and reduced the extent of infarction resulting from permanent MCA occlusion. To determine whether CSD may have altered preischemic baseline CBF, [14 C]iodoantipyrine was used in additional animals to measure CBF 3 days after CSD conditioning or sham conditioning. CSD, but not sham conditioning, significantly reduced baseline CBF in the ipsilateral neocortex to values 67% to 75% of those in the contralateral cortex. Therefore, CSD causes a long-lasting decrease in baseline CBF that is most likely related to a reduction in metabolic rate. A reduction in the rate of metabolism may contribute to the induction of tolerance to ischemia after preconditioning with CSD.

The concordance of early antipyrine and thiopental distribution kinetics

Studies of factors affecting the initial disposition of drugs with a rapid onset of effect following i.v. administration have used antipyrine as a surrogate for lipophilic drugs because it lacks cardiovascular effects. The present study tested the assumption that antipyrine is a useful surrogate for the flow-dependent tissue distribution of the lipophilic drug thiopental by comparing the recirculatory pharmacokinetic models of antipyrine and thiopental disposition after concomitant administration to five dogs anesthetized with 1.5% halothane. The pharmacokinetics of indocyanine green, a marker of the intravascular behavior of antipyrine and thiopental, and antipyrine in these dogs was nearly identical to that described previously in dogs anesthetized with 1.5% halothane but not given thiopental. The total volume of distribution of the highly lipophilic drug thiopental was more than 60% larger than that of antipyrine, 53 versus 33 liters, respectively. Nonetheless, the initial distribution kinetics of the two drugs, including the pulmonary tissue volume and the volume of the nondistributive pathway as well as the clearance to it, were nearly identical. As a result, the fraction of cardiac output involved in distribution of the two drugs to peripheral tissues was similarly identical, although the distribution of cardiac output between clearance to the rapidly equilibrating tissues and clearance to the slowly equilibrating tissues differed slightly. This study validates the assumption that antipyrine is a useful surrogate for lipophilic drugs in pharmacokinetic studies in which physiologic stability is desirable to meet the assumption of system stationarity.

Brain tissue sodium is a ticking clock telling time after arterial occlusion in rat focal cerebral ischemia

BACKGROUND AND PURPOSE

Many patients with acute stroke are excluded from receiving thrombolysis agents within the necessary time limit (3 or 6 hours from stroke onset) because they or their family members are unable provide the time of stroke onset. Brain tissue sodium concentration ([Na(+)]) increases gradually and incessantly during the initial hours of experimental focal cerebral ischemia but only in severely damaged brain regions. We propose that this steady increase in [Na(+)] can be used to estimate the time after arterial occlusion in the rat middle cerebral artery occlusion model of ischemic stroke.

METHODS

Sixteen anesthetized Sprague-Dawley rats underwent permanent middle cerebral artery occlusion combined with bilateral common artery occlusion. After 100 to 450 minutes, diffusion-weighted MRI was used to generate apparent diffusion coefficient (ADC) maps, cerebral blood flow (CBF) was determined with (14)C-iodoantipyrine (in a subset of 7 animals), and the brain was frozen. Autoradiographic CBF sections and punch samples for Na(+) analysis were obtained from the brain at the same level of the MR image. Severely at risk regions were identified with an ADC of <520 microm(2)/s and, in the subset, with both ADC of <520 microm(2)/s and CBF of <40 mL. 100 g(-1). min(-1).

RESULTS

Both CBF and the ADC dropped quickly and remained stable in the initial hours after ischemic onset. Linear regression revealed strong linearity between [Na(+)] and time after onset, with a slope of 0.95 or 1.00 (mEq/kg DW)/min, with both ADC and ADC-plus-CBF criteria, respectively. The 95% CIs at 180 and 360 minutes were between 41 and 52 minutes.

CONCLUSIONS

The time after ischemic onset can be estimated with this 2-step process. First, ADC and CBF are used to identify severely endangered regions. Second, the [Na(+)] in these regions is used to estimate time after onset. The favorable 95% CIs at the time limits for thrombolytic therapy and the availability of measurements of ADC, CBF, and [Na(+)] in humans through the use of MRI suggest that this time-estimation scheme could be used to assess the appropriateness of thrombolysis for patients who do not know when the stroke occurred.

Quantitative measurement of local cerebral blood flow in the anesthetized mouse using intraperitoneal [14C]iodoantipyrine injection and final arterial heart blood sampling

Autoradiographic measurement of local cerebral blood flow (CBF) with [14C]iodoantipyrine (IAP) is limited in mice by the difficulty in cannulating vessels and the blood loss for repeated blood sampling. The authors modified and validated the method to measure local CBF with [14C]IAP in mice by combining intraperitoneal tracer application with a single blood sampling from the heart at the end of the experiment. Experiments were carried out in male SV129 mice under halothane anesthesia. After intraperitoneal administration of 15 microCi [14C]IAP, arterial blood samples were collected repeatedly and anesthetized animals were immersed in liquid nitrogen. In addition, frozen blood from the heart was sampled to obtain the final blood [14C]radioactivity. Correlation analysis between the sampling time and [14C]radioactivity of the arterial blood revealed a highly significant linear relationship (P < 0.001, r = 0.978) and a lag time of the [14C]tracer in arterial blood of 3.3 +/- 0.6 seconds. [14C]radioactivity of the final arterial blood sample (444 +/- 264 nCi/mL) was almost equal to that of the heart blood (454 +/- 242 nCi/mL), and the absolute difference in each animal was 3.3 +/- 4.2% (mean +/- SD). The convolution integrals for the CBF calculation were determined either by integrating the radioactivity of individual arterial blood samples or by assuming a linear rise from [14C]tracer lag time after intraperitoneal [14C]IAP injection to the value measured in the blood sample from the frozen heart. Regional flow values calculated by the two methods differed by less than 11% (not significant). This method allows the quantitative measurement of local CBF in anesthetized mice without any vessel catheterization and will make mutant mice a more powerful tool to elucidate the molecular mechanisms of brain injuries by combining flow studies with molecular-biological methods.

Postmortem diffusion of autoradiographic blood flow tracers

The heterogeneity of blood flow in the brain under normo- and pathophysiological conditions, as well as during functional activation, has stimulated an interest in the use of autoradiography as a technique for the measurement of local cerebral blood flow. [14C]iodoantipyrine is the most prevalent tracer for the autoradiographic measurement of local cerebral blood flow since it is inert, nonvolatile, and is readily diffusible through the blood-brain barrier. The ability to diffuse freely in cerebral tissue, however, can lead to significant errors if the time duration between when the animal is sacrificed and when the tissue is frozen becomes appreciable, leading to significant postmortem diffusion of the tracer. Using an in vitro technique, the bulk diffusion coefficient for [14C]iodoantipyrine was measured in brain tissue (2.1 x 10(-6) cm(2)/s). Cerebral blood flow was measured with [14C]iodoantipyrine in anesthetized rats. At the end of the radiotracer infusion, the brain was freeze-captured using a device consisting of two rapidly spinning stainless steel blades that were pneumatically driven through the head, freezing the tissue several hundred milliseconds following sacrifice. Autoradiograms from these brains exhibit considerable heterogeneity in blood flow. Computer simulations of the effect of tracer diffusion on these autoradiograms show significant degradation of the images highlighting the importance of very rapid postmortem freezing.

Mice overexpressing extracellular superoxide dismutase have increased resistance to focal cerebral ischemia

Transgenic mice, which had been transfected with the human extracellular superoxide dismutase gene, causing an approximate five-fold increase in brain parenchymal extracellular superoxide dismutase activity, were used to investigate the role of extracellular superoxide dismutase in ischemic brain injury. Transgenic (n = 21) and wild-type (n = 19) mice underwent 90 min of intraluminal middle cerebral artery occlusion and 24 h of reperfusion. Severity of resultant hemiparesis and cerebral infarct size were measured. Wild-type mice had larger infarcts (cortex: wild type =37+/-14 mm3, transgenic = 27+/-13 mm3, P=0.03; subcortex: wild type = 33+/-14 mm3, transgenic = 23+/-10 mm3, P = 0.02). Neurological scores, however, were similar (P = 0.29). Other mice underwent autoradiographic determination of intra-ischemic cerebral blood flow. The volume of tissue at risk of infarction (defined as volume of tissue where blood flow was <25 ml/100g/min) was similar between groups (cortex: wild type = 51+/-15 mm3, transgenic = 47+/-9 mm3, P=0.65; subcortex: wild type = 39+/-16 mm3, transgenic= 37+/-17 mm3, P=0.81). These results indicate that antioxidant scavenging of free radicals by extracellular superoxide dismutase plays an important role in the histological response to a focal ischemic brain insult.

Effects of NMDA receptor glycine recognition site antagonism on cerebral metabolic rate for glucose and cerebral blood flow in the conscious rat

Glycine is a requisite cofactor for glutamatergic activation of the N-methyl-D-aspartate (NMDA) receptor. Antagonism of glutamate at the NMDA receptor has been shown to cause substantial changes in regional cerebral metabolic rate for glucose utilization (CMRglu) and blood flow (CBF). This study examined CMRglu and CBF changes caused by antagonism of glycine at the NMDA receptor recognition site. Rats were anesthetized with halothane and vascular access was obtained. The animals were then awakened. One hour later, either vehicle (control) or ACEA 1021 (5 mg/kg followed by 3.5 mg x kg(-1) x h(-1) or 10 mg/kg followed by 7 mg x kg(-1) x h(-1)) was infused intravenously. CMRglu and CBF were then determined. Autoradiographic analysis of 25 regions revealed effects of ACEA 1021 on CMRglu in the frontal, sensory, parietal and auditory cortices and the anteroventral and subthalamic nuclei. These changes deviated less than 15% from control. Effects on CBF were also small. The CMRglu and CBF effects of ACEA 1021 are substantially less than those previously observed for either competitive or non-competitive glutamate NMDA antagonists. We conclude that inhibition of the NMDA glycine recognition site has little or no effect on CMRglu or CBF at the doses examined. This is consistent with the absence of psychotomimetic effects observed for this class of drugs.

Perfusion to colorectal cancer liver metastases is not uniform and depends on tumor location and feeding vessel

Future effective therapies for hepatic metastases may depend on a better understanding of perfusion to these tumors. The purpose of this project was to define blood flow to colorectal cancer liver metastases using quantitative autoradiography (QAR). Liver tumors were established in F1 hybrids of WF x BN rats by intrasplenic injection of a DMH-induced rat colon adenocarcinoma. Rats underwent laparotomy 4-5 weeks later and [14C]iodoantipyrine (a radiotracer) was infused via the hepatic artery (HA) or portal vein (PV). Livers were harvested, frozen in liquid nitrogen, and sectioned at 20 microns through all tumors. QAR compared optical density of cross sections of tumors to surrounding normal liver tissue. Tumor:liver perfusion ratios (T/L PR) and tumor center:tumor periphery perfusion ratios (C/P PR) were calculated. All groups were analyzed with regard to tumor location and size. Seventy-seven tumors in 6 rats in the HA infusion group were analyzed; 74 tumors in 8 rats in the PV group were analyzed. Statistical analysis was by repeated measures analysis of variance. Mean HA T/L PR = 0.97 +/- 0.13, mean PV T/L PR = 0.25 +/- 0.11. Mean HA T/L PR for deep tumors was 1.38 +/- 0.17 and for superficial tumors was 0.57 +/- 0.15 (P < 0.01). Mean HA T/L PR for small tumors was 1.09 +/- 0.12 and for large tumors was 0.86 +/- 0.21 (P = 0.27). Mean PV T/L PR for deep tumors was 0.27 +/- 0.14 and for superficial tumors was 0.24 +/- 0.15 (P = 0.71). Mean PV T/L PR for small tumors was 0.31 +/- 0.15 and for large tumors was 0.20 +/- 0.14 (P = 0.54). Mean HA C/P PR = 1.15 +/- 0.15, mean PV C/P PR = 0.81 +/- 0.14 (P = 0.06). Mean HA C/P PR for small tumors was 1.37 +/- 0.16 and for large tumors was 0.92 +/- 0.17 (P = 0.01). Mean PV C/P PR for small tumors was 0.78 +/- 0.18 and for large tumors was 0.72 +/- 0.13 (P = 0.71). HA perfusion of tumors is significantly higher than PV perfusion compared to surrounding normal liver tissue. HA perfusion varies significantly depending on tumor location. There was a trend toward HA perfusion to the tumor center being slightly greater than to the periphery whereas the reverse was seen for PV perfusion. Tumor size did not affect overall perfusion but it did affect regional HA tumor perfusion.

Mechanism of dexamethasone suppression of brain tumor-associated vascular permeability in rats. Involvement of the glucocorticoid receptor and vascular permeability factor

Brain tumor-associated cerebral edema arises because tumor capillaries lack normal blood-brain barrier function; vascular permeability factor (VPF, also known as vascular endothelial growth factor, VEGF) is a likely mediator of this phenomenon. Clinically, dexamethasone reduces brain tumor-associated vascular permeability through poorly understood mechanisms. Our goals were to determine if suppression of permeability by dexamethasone might involve inhibition of VPF action or expression, and if dexamethasone effects in this setting are mediated by the glucocorticoid receptor (GR). In two rat models of permeability (peripheral vascular permeability induced by intradermal injection of 9L glioma cell-conditioned medium or purified VPF, and intracerebral vascular permeability induced by implanted 9L glioma), dexamethasone suppressed permeability in a dose-dependent manner. Since 80% of the permeability-inducing activity in 9L-conditioned medium was removed by anti-VPF antibodies, we examined dexamethasone effects of VPF expression in 9L cells. Dexamethasone inhibited FCS- and PDGF-dependent induction of VPF expression. At all levels (intradermal, intracranial, and cell culture), dexamethasone effects were reversed by the GR antagonist mifepristone (RU486). Dexamethasone may decrease brain tumor-associated vascular permeability by two GR-dependent mechanisms: reduction of the response of the vasculature to tumor-derived permeability factors (including VPF), and reduction of VPF expression by tumor cells.

Distribution of bismuth in the brain after intraperitoneal dosing of bismuth subnitrate in mice: implications for routes of entry of xenobiotic metals into the brain

Bismuth (Bi) can produce neurotoxic effects in both humans and animals under certain dosing conditions, but little else is known about the effects of Bi in the brain. In the present study we determined the distribution of Bi in the brains of adult female Swiss-Webster mice 4, 7, 14, 21 and 28 days after a single 2500 mg/kg i.p. injection of Bi subnitrate (BSN), which establishes a depot of absorbable Bi and produces morphological signs of neurotoxicity. Sections of brains were processed by autometallographic (AMG) procedures that produced silver grains at the site of Bi localization (AMGBi). Ventricular dilation was observed in all BSN-dosed mice. Among treated mice there were marked interanimal differences in the absolute amount of AMGBi, but consistent regional and cellular patterns of AMGBi were observed. AMGBi was observed in many cell types in brain regions adjacent to fenestrated blood vessels of the circumventricular organs (CVOs) and olfactory epithelium. Prominent intrasomal AMGBi was observed in nuclei containing large cell bodies, including cranial motor neurons innervating somatic muscle, lateral vestibular and red nucleus and pontine/medullary reticular nuclei. In the hypothalamus, the supraoptic and paraventricular nuclei demonstrated the densest AMGBi. In the cerebellum, Purkinje and granule cell layers with the densest AMGBi were in folia adjacent to the fourth ventricle. In the hippocampus, AMGBi was densest in the fasciola cinerum, polymorph cells of the dentate gyrus, and pyramidal cell layer of the CA3 regions. Neuropil of subcortical auditory nuclei (cochlear nucleus, trapezoid body, lateral lemniscus and nucleus of lateral lemniscus, medial geniculate nucleus and inferior colliculus) had a high density of AMGBi. Among nonneuronal cells, ependyma and meninges lining the ventricular and subarachnoid spaces were labeled extensively. Glial labeling was prominent adjacent to CVOs, in subependymal regions, and in fiber tracts. Presumptive perivascular cells lining large blood vessels had extremely dense AMGBi as early as 4 days after dosing. Smaller blood vessels had moderate AMGBi. However, in regions (e.g. cerebral cortex, striatum) known to have low brain Bi levels after i.p. dosing, vascular deposits accounted for most of the AMGBi. Several animals had foci of AMGBi which suggested that vascular or perivascular aberrations may have contributed to the unusually dense accumulations. The results of the present studies indicate that Bi accumulates predictably in certain regions and cell types. The pattern of regions and cells with the highest AMGBi accumulations is very similar to pattern reported for other xenobiotic metals (i.e. mercury, silver, gold), and supports the hypothesis that these metals may share some mechanisms for entry, distribution and storage in the brain.

Reduced local cerebral blood flow in periventricular white matter in experimental neonatal hydrocephalus-restoration with CSF shunting

The extent to which the reduction in CBF occurring in hydrocephalus is a primary or secondary event in the pathogenesis of the brain injury that ensues has not been clearly established. This is particularly true in neonatal hydrocephalus, where the disorder is most common, and where timing of the treatment of the developing nervous system is so important. We investigated the changes in local CBF (lCBF) in an animal model of severe progressive neonatal hydrocephalus before and after CSF shunting. Hydrocephalus was induced in 27 1-week-old kittens by percutaneous injection of 0.05 ml of 25% kaolin into the cisterna magna. Fourteen littermates acted as controls. The lCBF was measured by 14C-iodoantipyrine quantitative autoradiography after 1 week in 15 animals (8 hydrocephalic, 7 controls) and after 3 weeks in 26 animals (19 hydrocephalic, 7 controls) following induction of hydrocephalus. Twelve of the 3-week hydrocephalic group received a ventriculoperitoneal shunt 10 days following kaolin injection. At 1 week following induction of hydrocephalus, lCBF was globally reduced in cortical gray matter and white matter as well as deep subcortical structures. The maximum reduction was in the parietal white matter, to 37% of control levels. At 3 weeks a significant reduction in lCBF persisted only in the white matter (parietal, occipital, and corpus callosum; average, 42% of control levels), whereas cortical gray and deep subcortical structures had returned to normal levels spontaneously.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoradiographic assessment of blood flow heterogeneity in the hamster heart

OBJECTIVE

Provide regional flow measurement in the hearts of small mammals using a new, higher-resolution technique based on the deposition of a molecular marker.

METHODS

We determined the instantaneous extraction and retention of the "molecular microsphere" radiolabeled desmethylimipramine in retrogradely perfused hamster hearts. In a separate series of experiments, autoradiography was used to measure regional myocardial deposition densities in hamster hearts of about 0.5 g with spatial area resolution of 16 x 16 microns.

RESULTS

Radiolabeled desmethylimipramine is almost 100% extracted during a single transcapillary passage and is retained in the tissue for many minutes. Autoradiographic images demonstrated a spatial flow heterogeneity with standard deviations of 31 +/- 4% of the mean flow (N = 5) in 16 x 16 x 20-micronm3 voxels. This is equivalent to the projections made using fractal relationships from cruder observations obtained with microspheres in the hearts of baboons, sheep, and rabbits.

CONCLUSIONS

Autoradiography using a molecular deposition marker provides quantitative information on myocardial flow heterogeneities with resolution at the size of cardiac myocytes. Because the regions resolved are smaller than the volume of regions supplied by single arterioles, the results must slightly exaggerate the true heterogeneity of regional flows.

Experimental delayed postischemic spinal cord hypoperfusion after aortic cross-clamping

BACKGROUND

As in the brain, recent evidence has suggested a defect in the microcirculation during the reperfusion period after spinal cord ischemia. This investigation was undertaken in order to delineate blood flow dynamics in the postischemic spinal cord of the rat.

METHODS

Male Sprague-Dawley rats underwent cross-clamping of the aorta and subclavian arteries (XC) for 11 minutes. Spinal cord blood flow (SCBF) was measured by autoradiography in the gray and white matter of cervical (Ce), thoracic (Th) and lumbar (Lu) regions during XC, 1 h, 6 h and 24 h (XC n = 8, 1 h n = 9, 6 h n = 9, and 24 h n = 11, groups) after XC. Control groups underwent surgical manipulations and SCBF measurement but no XC (Sham 1, n = 8), or clamping of the subclavian arteries only (Sham 2, n = 8).

RESULTS

In Ce cord, there was no difference between SCBF of 1 h, 6 h, 24 h and Sham 1. In Th cord, SCBF was reduced during XC (P < 0.003 vs. Sham 2), 1 h, 6 h (P < 0.04 and P < 0.01 vs. Sham 1). In Lu cord, SCBF was not detectable in XC, and depressed in 1 h (P < 0.003) and 6 h (P < 0.003). There was no difference between 24 h and Sham 1 in Ce, Th, and Lu cords.

CONCLUSIONS

The study demonstrated a period of delayed postischemic hypoperfusion in the white and gray matter of Th and Lu cord segments lasting 6 h after XC. The phenomenon may play an important role in the ultimate fate of neural elements with borderline viability after ischemic injury.

Three-dimensional image analysis of brain glucose metabolism-blood flow uncoupling and its electrophysiological correlates in the acute ischemic penumbra following middle cerebral artery occlusion

The relationship between local cerebral glucose utilization (LCMRglc) and local CBF (LCBF) is known to be disturbed in regions surrounding an acute focal ischemic lesion--areas that undergo repeated transient depolarizations. In this study, we evaluated the relationship between LCMRglc and LCBF in the acute focal ischemic penumbra to quantify metabolism-flow uncoupling, and we related these findings to local electrophysiological measurements. A novel strategy utilizing three-dimensional (3D) autoradiographic image averaging yielded group 3D reconstructions of LCBF, LCMRglc, and the CMR/CBF ratio. The distal right middle cerebral artery of Sprague-Dawley rats was occluded by laser-driven photothrombosis following administration of the photosensitizing dye rose bengal; this was coupled with permanent ipsilateral and 1-h contralateral common carotid artery occlusions. LCBF (n = 7) and LCMRglc (n = 7) were measured autoradiographically at 1.25 and 1.5-2 h postocclusion, respectively, in matched animal groups. Within the ischemic penumbra (defined as having LCBF of 20-40% of control or 0.23-0.47 ml g-1 min-1), LCMRglc showed a heterogeneous pattern with values ranging from near normal to markedly increased. The resulting CMRglc/CBF ratio in this zone was 234 +/- 100 mumol/100 ml (mean +/- SD), representing a severe degree of metabolism-flow dissociation when compared with the CMRglc/CBF ratio of 51.0 +/- 28.7 mumol/100 ml of the contralateral (normal) hemisphere. Metabolism-flow uncoupling was confined to the ipsilateral cortex and was most prominent at the anterior and posterior coronal poles of the ischemic lesion. In the frontoparietal penumbra, where marked uncoupling was observed, sustained deflections of the DC potential were recorded, which increased significantly in duration over the initial 65 min postocclusion. Both the heterogeneous pattern of LCMRglc and the widespread distribution of increased CMRglc/CBF ratio in the ischemic penumbra are thought to reflect the metabolic consequences of periinfarct depolarizations. Analysis of averaged 3D autoradiographic data sets provides a powerful means for assessing metabolism-flow uncoupling surrounding an ischemic focus.

Uncoupling of LCBF and LCGU in two different models of hydrocephalus: a review

We have used two different experimental models to examine the relationship between local cerebral blood flow and metabolism in hydrocephalus. In our first experiments local cerebral blood flow (LCBF) and local cerebral glucose utilization (LCGU) were measured by quantitative autoradiographic methods in adult rats rendered hydrocephalic, though asymptomatic, by the injection of kaolin intracisternally at 3 weeks of age and in control animals. There were no significant differences in LCGU or LCBF in any of the 29 areas of grey matter examined, including layer IV of the cerebral cortex. Scanning across the cerebral cortex revealed an appreciable fall in LCGU and LCBF towards the inside and the outside of the mantle in control animals. Hydrocephalus had no significant effect on this "transmantle" pattern of reduction in cortical metabolism towards the periphery, but in contrast, significantly enhanced the reduction in cortical blood flow in 7 out of the 10 cortical regions examined. Hence, in this model of asymptomatic hydrocephalus there is relative uncoupling of LCBF and LCGU in the inner and outer layers of the cerebral mantle. In a study performed in congenitally hydrocephalic H-Tx rats at 10, 20 and 28 days we found that uptake of deoxyglucose was impaired in hydrocephalic rats compared with their non-hydrocephalic siblings. Small changes were seen at 10 and 21 days, but statistically significant changes were seen only at 28 days. A small reduction in LCBF was observed in all regions at 10 days, with statistically significant differences between control and hydrocephalic rats in auditory and parietal cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative measurement of regional blood flow with gadolinium diethylenetriaminepentaacetate bolus track NMR imaging in cerebral infarcts in rats: validation with the iodo[14C]antipyrine technique

NMR bolus track measurements were correlated with autoradiographically determined regional cerebral blood flow (rCBF). The NMR method is based on bolus infusion of the contrast agent gadolinium diethylenetriaminepentaacetate and high-speed T*2-sensitive NMR imaging. The first pass of the contrast agent through the image plane causes a transient decrease of the signal intensity. This time course of the signal intensity is transformed into relative concentrations of the contrast agent in each pixel. The mean transit time and relative blood flow and volume are calculated from such indicator dilution curves. We investigated whether this NMR technique correctly expresses the relative rCBF. The relative blood flow data, calculated from NMR bolus track experiments, and the absolute values of iodo[14C]antipyrine autoradiography were compared. A linear relationship was observed, indicating the proportionality of the transient NMR signal change with CBF. Excellent interindividual reproducibility of calibration constants is observed (r = 0.963). For a given NMR protocol, bolus track measurements calibrated with autoradiography after the experiment allow determination of absolute values for rCBF and regional blood volume.

Degeneration of axons in the corticospinal tract secondary to spinal cord ischemia in rats

Occlusion of the thoracic aorta and both subclavian arteries (XC) in the rat model produces spastic paraplegia. In order to characterize the lesion of white matter, 14 male Sprague-Dawley rats underwent XC for 10.5 to 12 min, were observed for 32 days and assessed with a lesion score. A sham group of eight underwent surgical manipulations without XC. The spinal cords were studied by optical microscopy and electron microscopy. An additional group of normal animals (n = 8) underwent spinal cord blood flow measurement with the autoradiographic technique. Optical microscopy showed normal histology in sham operated rats and rats with aortic cross-clamp and lesion score = 2-4 (n = 5), rare changes in the white matter of rats with lesion score = 8 (n = 2), and demyelination of the anterior and lateral tracts of the white matter and motor neuron loss in the gray matter of rats with lesion score = 13-15 (n = 7) and spastic paraplegia. In this last group, electron microscopy disclosed severe axonal degeneration of corticospinal tracts. In the same region spinal cord blood flow was higher than the remaining white matter. This study confirms that spastic paraplegia observed in the rat model after XC is due to degeneration of the pyramidal tracts, perhaps more susceptible to injury due to the high spinal cord blood flow.

Levodopa-induced local cerebral blood flow changes in Parkinson's disease and related disorders

Local cerebral blood flow (CBF) in the steady state and after intravenous administration of levodopa (1 mg/kg) was measured by xenon-enhanced computed tomography in patients with Parkinson's disease (PD, n = 16), progressive supranuclear palsy (PSP, n = 6), olivopontocerebellar atrophy (OPCA, n = 5), and arteriosclerotic parkinsonism (AP, n = 7). Three patterns of local CBF changes following levodopa were observed: (1) diffuse CBF increases, especially in striatum and thalamus, as found in patients with PD; (2) no significant changes in CBF, as in patients with OPCA and AP; and (3) CBF reductions in basal ganglia and thalamus, as seen in patients with PSP. The CBF increases after levodopa in PD may be secondary to metabolic activation of the nigrostriatal dopaminergic system. The poor CBF responses in patients with OPCA, AP, and PSP appeared to reflect degeneration of the dopaminergic neurons and dopamine receptors to various degrees. The CBF increases, especially in striatum and thalamus, tended to be greater (not significant) among responders to oral levodopa therapy. Levodopa-induced CBF measurements may be useful for the differential diagnosis of parkinsonian syndromes of various etiologies, but are not necessarily sufficient for predicting outcomes of long-term levodopa therapy.

Analysis of the attention deficit in schizophrenia: a study of patients and their relatives in Ireland

County Roscommon, a rural area in the western part of Ireland, was the site of a family study of schizophrenia. As part of this study, we have assessed several elements of attention, identified by principal components analysis in previous investigations, in a group of subjects with schizophrenia, first-degree relatives of subjects with schizophrenia and age- and education-matched controls. The schizophrenic subjects performed significantly more poorly than the controls; the performance of the relatives fell somewhere between the other two groups. Those relatives with a DSM-III-R diagnosis (most frequently, alcohol abuse or an affective disorder) tended to perform more poorly on some of the attention elements than relatives without a diagnosis; in contrast, control subjects with diagnoses were not distinguishable from other controls. The attention elements appeared to differ in their capacity to differentiate the groups and each seemed to have a distinctive profile. The effects of alcohol abuse were also considered. The results obtained with this cohort may provide clues concerning the pathophysiological basis of schizophrenia and the heterogeneity of its expression.

Why do all drugs work in animals but none in stroke patients? 1. Drugs promoting cerebral blood flow

Medical treatments which presumably alter cerebral blood flow (CBF) have been quite unimpressive in their effect on stroke outcome. In considering experimental and clinical data from the use of haemodilution and of the antiplatelet agent prostacyclin in focal cerebral ischaemia, and the current work with fibrinolytic agents in acute stroke, several lessons are apparent. Often agents hypothesized to affect CBF receive an underserved reputation based on sparse experimental evidence. Significant even unsuspected differences between species limit application to the clinical setting. Limitations of CBF measurements in experimental models and in humans raise questions about apparent responses to those agents. The failure to confirm a relationship between CBF enhancement and reduction in infarct development experimentally has plagued these approaches. The need for early application of agents which may modulate CBF during cerebral ischaemia is critical. Attention to these general issues and careful application of appropriate models are necessary so that a potentially useful therapeutic intervention is not overlooked.

Cerebral blood flow and glucose metabolism in the squirrel monkey during the late phase of cerebral vasospasm

A double-isotope autoradiography technique was used to evaluate cerebral blood flow (CBF) and cerebral glucose metabolism (CMRglu) during the late phase of vasospasm in a squirrel monkey subarachnoid haemorrhage (SAH) model. Cisternal blood injections induced both global and focal changes in CBF and CMRglu six days following SAH, the timepoint of maximal late spasm in this model. There was a global decrease in CBF of about 30% accompanied by an increase in deoxyglucose uptake of about 50%. Four of seven animals also had foci with flow decreased to 40% of control and deoxyglucose uptake increased to 300% of control. There was an altered but still present interdependence between flow and metabolism post SAH.