Regulation of oxygen supply in the cerebral circulation

Cerebral Microcirculation, Perivascular Unit, and Glymphatic System: Role of Aquaporin-4 as the Gatekeeper for Water Homeostasis

In the past, water homeostasis of the brain was understood as a certain quantitative equilibrium of water content between intravascular, interstitial, and intracellular spaces governed mostly by hydrostatic effects i.e., strictly by physical laws. The recent achievements in molecular bioscience have led to substantial changes in this regard. Some new concepts elaborate the idea that all compartments involved in cerebral fluid homeostasis create a functional continuum with an active and precise regulation of fluid exchange between them rather than only serving as separate fluid receptacles with mere passive diffusion mechanisms, based on hydrostatic pressure. According to these concepts, aquaporin-4 (AQP4) plays the central role in cerebral fluid homeostasis, acting as a water channel protein. The AQP4 not only enables water permeability through the blood-brain barrier but also regulates water exchange between perivascular spaces and the rest of the glymphatic system, described as pan-cerebral fluid pathway interlacing macroscopic cerebrospinal fluid (CSF) spaces with the interstitial fluid of brain tissue. With regards to this, AQP4 makes water shift strongly dependent on active processes including changes in cerebral microcirculation and autoregulation of brain vessels capacity. In this paper, the role of the AQP4 as the gatekeeper, regulating the water exchange between intracellular space, glymphatic system (including the so-called neurovascular units), and intravascular compartment is reviewed. In addition, the new concepts of brain edema as a misbalance in water homeostasis are critically appraised based on the newly described role of AQP4 for fluid permeation. Finally, the relevance of these hypotheses for clinical conditions (including brain trauma and stroke) and for both new and old therapy concepts are analyzed.

Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation

Brain function critically depends on a close matching between metabolic demands, appropriate delivery of oxygen and nutrients, and removal of cellular waste. This matching requires continuous regulation of cerebral blood flow (CBF), which can be categorized into four broad topics: 1) autoregulation, which describes the response of the cerebrovasculature to changes in perfusion pressure; 2) vascular reactivity to vasoactive stimuli [including carbon dioxide (CO2)]; 3) neurovascular coupling (NVC), i.e., the CBF response to local changes in neural activity (often standardized cognitive stimuli in humans); and 4) endothelium-dependent responses. This review focuses primarily on autoregulation and its clinical implications. To place autoregulation in a more precise context, and to better understand integrated approaches in the cerebral circulation, we also briefly address reactivity to CO2 and NVC. In addition to our focus on effects of perfusion pressure (or blood pressure), we describe the impact of select stimuli on regulation of CBF (i.e., arterial blood gases, cerebral metabolism, neural mechanisms, and specific vascular cells), the interrelationships between these stimuli, and implications for regulation of CBF at the level of large arteries and the microcirculation. We review clinical implications of autoregulation in aging, hypertension, stroke, mild cognitive impairment, anesthesia, and dementias. Finally, we discuss autoregulation in the context of common daily physiological challenges, including changes in posture (e.g., orthostatic hypotension, syncope) and physical activity.

The effects of capillary transit time heterogeneity on the BOLD signal

Neurovascular coupling mechanisms give rise to vasodilation and functional hyperemia upon neural activation, thereby altering blood oxygenation. This blood oxygenation level dependent (BOLD) contrast allows studies of activation patterns in the working human brain by functional MRI (fMRI). The BOLD-weighted fMRI signal shows characteristic transients in relation to functional activation, such as the so-called initial dip, overshoot, and post-stimulus undershoot. These transients are modulated by other physiological stimuli and in disease, but the underlying physiological mechanisms remain incompletely understood. Capillary transit time heterogeneity (CTH) has been shown to affect oxygen extraction, and hence blood oxygenation. Here, we examine how recently reported redistributions of capillary blood flow during functional activation would be expected to affect BOLD signal transients. We developed a three-compartment (hemoglobin, plasma, and tissue) model to predict the BOLD signal, incorporating the effects of dynamic changes in CTH. Our model predicts that the BOLD signal represents the superposition of a positive component resulting from increases in cerebral blood flow (CBF), and a negative component, resulting from elevated tissue metabolism and homogenization of capillary flows (reduced CTH). The model reproduces salient features of BOLD signal dynamics under conditions such as hypercapnia, hyperoxia, and caffeine intake, where both brain physiology and BOLD characteristics are altered. Neuroglial signaling and metabolism could affect CBF and capillary flow patterns differently. Further studies of neurovascular and neuro-capillary coupling mechanisms may help us relate BOLD signals to the firing of certain neuronal populations based on their respective BOLD "fingerprints."

No oxygen delivery limitation in hepatic encephalopathy

Hepatic encephalopathy is a condition of reduced brain functioning in which both blood flow and brain energy metabolism declined. It is not known whether blood flow or metabolism is the primary limiting factor of brain function in this condition. We used calculations of mitochondrial oxygen tension to choose between cause and effect in three groups of volunteers, including healthy control subjects (HC), patients with cirrhosis of the liver without hepatic encephalopathy (CL), and patients with cirrhosis with acute hepatic encephalopathy. Compared to HC subjects, blood flow and energy metabolism had declined in all gray matter regions of the brain in patients with HE but not significantly in patients with CL. Analysis of flow-metabolism coupling indicated that blood flow declined in HE as a consequence of reduced brain energy metabolism implied by the calculation of increased mitochondrial oxygen tensions that patients with HE were unable to utilize. We ascribe the inability to use the delivered oxygen of patients with HE to a specific inhibition associated with oxidative metabolism in mitochondria.

RBC velocities in single capillaries of mouse and rat brains are the same, despite 10-fold difference in body size

Employing high-speed camera laser-scanning confocal microscopy with RBC-tracking software, we previously showed that RBC velocities in intraparenchymal capillaries of rat cerebral cortex are distributed over a wide range. In the present work, we measured RBC velocities in mice, whose body weights are less than one-tenth of that of rats. In an isoflurane-anesthetized mouse, a cranial window was opened in the left temporo-parietal region. Intravenously administered FITC-labeled RBCs were automatically recognized and tracked frame-by-frame at 500fps, and the velocities of all RBCs recognized were calculated with our Matlab-domain software, KEIO-IS2. Among 15241 RBCs detected in the ROI in 21 mice, 1655 were identified as flowing in capillaries. The velocities of these RBCs ranged from 0.15 to 8.6mm/s, with a mean of 2.03+/-1.42mm/s. A frequency distribution plot showed that RBC velocities were clustered at around 1.0mm/s, tailing up to 8.6mm/s, and 59% of the RBCs in capillaries showed velocities within the range of 0.5 to 2.0mm/s. Unexpectedly, these characteristics of RBC velocities in mice were very similar to those of rats, despite differences in RBC diameter (6.0 vs. 6.5microm), body size (25 vs. 327g), heart rate (461 vs. 319bpm) and arterial blood pressure (86 vs. 84mmHg). We speculate that physical factors relating to oxygen exchange may constrain general RBC velocity in capillaries to a certain range for optimum oxygen exchange, regardless of species.

Cerebral haemodynamics in patients with glutaryl-coenzyme A dehydrogenase deficiency

In glutaric aciduria type 1, glutaryl-coenzyme A and its derivatives are produced from intracerebral lysine and entrapped at high concentrations within the brain, where they interfere with energy metabolism. Biochemical toxicity is thought to trigger stroke-like striatal degeneration in susceptible children under 2 years of age. Here, we explore vascular derangements that might also contribute to brain damage. We studied injured and non-injured Amish glutaric aciduria type 1 patients using magnetic resonance imaging (n = 26), transcranial Doppler ultrasound (n = 35) and perfusion computed tomography (n = 6). All glutaric aciduria type 1 patients had wide middle cerebral, internal carotid and basilar arteries. In non-injured patients, middle cerebral artery velocities were 18-26% below control values throughout late infancy and early childhood, whereas brain-injured children had an early velocity peak (18 months) and low values thereafter. Perfusion scans from six patients showed that tissue blood flow did not undergo a normal developmental surge. We observed four different perfusion patterns. (i) Three children (two non-injured) had low cerebral blood flow, prolonged mean transit time, elevated cerebral blood volume and high mean transit time/cerebral blood flow and cerebral blood volume/cerebral blood flow ratios. This pattern optimizes substrate extraction at any given flow rate but indicates low perfusion pressure and limited autoregulatory reserve. (ii) Ten hours after the onset of striatal necrosis in an 8-month-old infant, mean transit time and cerebral blood volume were low relative to cerebral blood flow, which varied markedly from region to region. This pattern indicates disturbed autoregulation, regional perfusion pressure gradients, or redistribution of flow from functional capillaries to non-exchanging vessels. (iii) In an infant with atrophic putaminal lesions, striatal flow was normal but mean transit time and cerebral blood volume were low, consistent with perfusion in excess of metabolic demand. (iv) Finally, a brain-injured adult with glutaric aciduria type 1 had regional perfusion values within the normal range, but the putamina, which normally have the highest regional perfusion, had cerebral blood flow values 24% below cortical grey matter. Although metabolic toxicity appears central to the pathophysiology of striatal necrosis, cerebrovascular changes probably also contribute to the process. These changes may be the primary cause of expanded cerebrospinal fluid volume in newborns, intracranial and retinal haemorrhages in infants and interstitial white matter oedema in children and adults. This pilot study suggests important new areas for clinical investigation.

Pial arteriolar vasomotion changes during cortical activation in rats

The oscillatory pattern of pial arterioles, i.e. vasomotion, has been described since early 1980s, but the impact of neural activation on such oscillations has never been formally examined. Sciatic nerve stimulation, a well characterized model for studying neurovascular coupling (NVC), leads to a neural activity-related increase of pial arteriolar diameter in the contralateral hindlimb somatosensory cortex. Exploiting such an experimental model, the aim of the present study was to explore vasomotion and its changes during NVC with a novel analytical approach. Indeed, to characterize oscillations, we evaluated the total spectral power in the range 0.02-2.00 Hz and subdivided this frequency interval into seven 50% overlapping frequency bands. Results indicated that only arterioles overlying the stimulated hindlimb cortex showed a significant increase of total power, unlike arterioles overlaying the whisker barrel cortex, used as control for the vascular response specificity. The total power increase was sustained mainly by marked increments in the low frequency range, with two peaks at 0.03 and 0.08 Hz, and by a wide increase in the high frequency range (0.60-2.00 Hz) in the averaged spectrum. These activity-related spectral changes suggest: (i) that it is possible to assess the vascular responses by using total power; (ii) the existence of at least three distinct mechanisms involved in the control of NVC, two with a feedback frequency loop in the low frequency range and another one in the high range; (iii) a potential involvement of vasomotion in NVC. Moreover, these findings highlight the oscillatory nature of the mechanisms controlling NVC.

Multimodal imaging of striatal degeneration in Amish patients with glutaryl-CoA dehydrogenase deficiency

Despite early diagnosis, one-third of Amish infants with glutaryl-CoA dehydrogenase deficiency (GA1) develop striatal lesions that leave them permanently disabled. To better understand mechanisms of striatal degeneration, we retrospectively studied imaging results from 25 Amish GA1 patients homozygous for 1296C>T mutations in GCDH. Asymptomatic infants had reduced glucose tracer uptake and increased blood volume throughout gray matter, which may signify a predisposition to brain injury. Nine children (36%) developed striatal lesions: three had sudden motor regression during infancy whereas six had insidious motor delay associated with striatal lesions of undetermined onset. Acute striatal necrosis consisted of three stages: (1) an acute stage, within 24 h of motor regression, characterized by cytotoxic oedema within the basal ganglia, cerebral oligemia, and rapid transit of blood throughout gray matter; (2) a sub-acute stage, 4-5 days after the onset of clinical signs, characterized by reduced striatal perfusion and glucose uptake, and supervening vasogenic oedema; and (3) a chronic stage of striatal atrophy. Apparent diffusion coefficient maps revealed that at least two of the six patients with insidious motor delay suffered striatal injuries before or shortly after birth, followed by latent periods of several months before disability was apparent. Thus, acute and insidious presentations may occur by similar mechanisms, and differ only with regard to the timing of injury. Intravenous fluid and dextrose therapy for illnesses during the first 2 years of life was the only intervention that was clearly neuroprotective in this cohort (odds ratio for brain injury = 0.04, 95% confidence interval = 0.01-0.34; P < 0.001).

Functional Imaging with Intraoperative Ultrasound: Detection of Somatosensory Cortex in Dogs with Color-duplex Sonography

OBJECTIVE

To evaluate the capability of intraoperative color-duplex sonography to detect eloquent flow-activated areas and their anatomic relationship in dogs.

METHODS

After craniotomy, the sensory cortex of eight dogs was identified by recording the highest amplitude detected with a grid electrode evoked with somatosensory evoked potential stimulation of the nervus ischiadicus. A 7.5-MHz linear array transducer was placed on the dura, and eight images were taken in color-coded capture mode during baseline and somatosensory evoked potential stimulation of the ipsilateral (nonevoked) and contralateral (evoked) sensory cortex. The differences in flow velocity intensities were statistically compared (Wilcoxon test) in three arbitrary velocity ranges and across all colored pixels in a region of interest between baseline and stimulation in both hemispheres.

RESULTS

Comparing both hemispheres during stimulation, the evoked sensory cortex demonstrated an increase of 10% in the number of counted colored pixels during stimulation, whereas the number of counted colored pixels in the ipsilateral sensory cortex decreased by 2% (P < 0.05), indicating an overall increase in measured flow during stimulation. Comparing differences during nonstimulation and stimulation in single hemispheres, the lowest of the three velocity ranges (approximately 10-20 mm/s) demonstrated a statistically significant (P = 0.01) increase during stimulation, whereas no change was observed during stimulation in the ipsilateral hemisphere. This increase has been confirmed by regional cerebral blood flow measurement with colored microspheres.

CONCLUSION

This study indicates, for the first time, the capability of intraoperative ultrasound to detect functionally important areas during evoked stimulation.

Dynamic observation of oxygenation-induced contraction of and transient fiber-network formation-disassembly in cultured human brain microvascular endothelial cells

Oxygenation-induced contraction of nonconfluent cultured human brain microvascular endothelial cells (HBECs, n = 30) was examined by video-enhanced contrast-differential interferential contrast microscopy. After administering a continuous gentle blow of pure oxygen gas to the surface of the medium just above the flattened HBEC, the plasma membrane exhibited tensioning and wrinkling, resulting in a strong contraction of the cell body by 14 +/- 7% (P < 0.001). When the cell stopped contracting, transient formation of a fiber network starting from certain spots (possibly adhesion plaques, though these were not visible in the majority of cases) and expanding to the whole cell was observed. The occurrence of fiber network formation was statistically significant (26 of 30 separate cells, P < 0.05). After cessation of oxygen delivery, the observed network of fibers broke up rapidly (in a period of 3.3 +/- 1.2 seconds) into small particles of <0.5 microm in diameter, which subsequently fused into the cellular structure. The HBEC completely recovered the control appearance. The sequential process was completed within 30 seconds and was reproduced in individual cells each time that oxygen gas was supplied. The authors conclude that the HBEC strongly contracts in response to a transient oxygenation stimulus, followed by rapid formation/disassembly of a network structure.

Relation between cerebral blood flow and metabolism explained by a model of oxygen exchange

The cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRo(2)) are major determinants of the contrast in functional magnetic resonance imaging and optical imaging. However, the coupling between CBF and CMRo(2) during cerebral activation remains controversial. Whereas most of the previous models tend to show a nonlinear coupling, experimental studies have led to conflicting conclusions. A physiologic model was developed of oxygen transport through the blood-brain barrier (BBB) for dynamic and stationary states. Common model simplifications are proposed and their implications for the CBF/CMRo(2) relation are studied. The tissue oxygen pool, the BBB permeability, and the hemoglobin dissociation curve are physiologic parameters directly involved in the CBF/CMRo(2) relation. We have been shown that the hypothesis of a negligible tissue oxygen pool, which was admitted by most of the previous models, implies a tight coupling between CBF and CMRo(2). By relaxing this hypothesis, a real uncoupling was allowed that gives a more coherent view of the CBF/CMRo(2) relation, in better agreement with the hypercapnia data and with the variability reported in experimental works for the relative changes of those two variables. This also allows a temporal mismatch between CBF and CMRo(2), which influences the temporal shape of oxygenation at the capillary end.

Absolute Frequency-Domain Pulse Oximetry of the Brain: Methodology and Measurements

A new method to non-invasively measure the absolute tissue oxygen saturation (SO2) and arterial oxygen saturation (fdSaO2) by frequency-domain spectroscopy is described. This method is based on the quantitative measurement of the tissue absorption spectrum, which is used to determine global SO2. From the amplitude of absorption changes caused by arterial pulsation oscillations, in the range of 633-841 nm, the fdSaO2 can be calculated. During deoxygenation (air/N2 mixture) experiments, we measured the fdSaO2 and SO2 on the forehead of three healthy volunteers and compared them to the arterial oxygen saturation measured by conventional pulse oximetry (poSaO2) on the finger. fdSaO2 and poSaO2 agree very well (mean difference: -1.2 +/- 2.6%). Changes in SO2 were systematically smaller than in fdSaO2 or poSaO2 probably due to autoregulation. The measurements with 4 and 8 wavelengths had comparable quality.

Spatial integration of vascular changes with neural activity in mouse cortex

The authors evaluated representations of discretely activated, neighboring brain regions using real-time optical intrinsic signals by transcranial imaging with 540-nm and 610-nm broadband illumination of the mouse barrel cortex. Iron filings were glued to two neighboring whiskers (C2 + D2) that were stimulated magnetically, singly and together. Real-time images were collected, averaged, and analyzed statistically. Postmortem filling of arteries with fluorescent beads was shown in relation to histochemical staining of barrels to accurately relate surface changes to functional cortical columns. Significant optical intrinsic signal changes are related to overlapping distributions of arterioles that feed the two separate areas. Activation of adjacent and interacting cortical columns leads not only to increased magnitude of vascular responses in those columns, but also to wider spatial extent of absorption changes occurring principally in areas of cortex fed by vessels upstream of the active cortex. The localization of changing hemoglobin absorption around upstream blood vessels and their vascular domains suggests that propagated vasodilation of upstream parent vessels is greater when vasodilatory signals from separate areas of active cortex converge on common arterioles that feed them.

Precise measurement of cerebral blood flow in newborn piglets from the bolus passage of indocyanine green

Indocyanine green (ICG) is a near-infrared dye that has the potential to be used as a tracer for the minimally invasive measurement of cerebral blood flow (CBF). In order to examine the technique, the arterial and cerebral concentrations of ICG were measured in newborn piglets during the bolus passage of ICG at normocapnia and two levels of mild hypercapnia. The results were analysed by applying the Fick principle in both integral and differential forms using a linear regression technique to improve the precision of calculated values of CBF. It was found that the integral method, which has been used previously, is particularly sensitive to errors in the time registration between the arterial and tissue signals whereas the differential method is less so. In addition, the differential method allows the venous outflow to be calculated which gives further information on the state of the capillary bed. CBF was 39.7 +/- 4.6 ml 100 g(-1) min(-1) at an arterial carbon dioxide tension (PaCO2) of 33.0+/-2.2 mmHg and increased to 53.7+/-9.1 and 75.4+/-15.2 ml 100 g(-1) min(-1) at a PaCO2 of 42.1 +/- 2.6 and 54.2 +/- 3.1 mmHg respectively (mean +/- SD, n = 7). There was no significant change in cerebral metabolic rate for oxygen, validating the value of blood flow to an arbitrary scaling factor. When the inspired CO2 fraction was returned to zero, calculated CBF returned to baseline with a variation of 7% of the mean, indicating that this technique is highly precise.

Oxygen dependency and precision of cytochrome oxidase signal from full spectral NIRS of the piglet brain

Oxidation changes of the copper A (Cu(A)) center of cytochrome oxidase in the brain were measured during brief anoxic swings at both normocapnia and hypercapnia (arterial PCO(2) approximately 55 mmHg). Hypercapnia increased total hemoglobin from 37.5 +/- 9.1 to 50.8 +/- 12.9 micromol/l (means +/- SD; n = 7), increased mean cerebral saturation (Smc(O(2))) from 65 +/- 4 to 77 +/- 3%, and oxidized Cu(A) by 0.43 +/- 0.23 micromol/l. During the onset of anoxia, there were no significant changes in the Cu(A) oxidation state until Smc(O(2)) had fallen to 43 +/- 5 and 21 +/- 6% at normocapnia and hypercapnia, respectively, and the maximum reduction during anoxia was not significantly different at hypercapnia (1.49 +/- 0.40 micromol/l) compared with normocapnia (1.53 +/- 0.44 micromol/l). Residuals of the least squares fitting algorithm used to convert near-infrared spectra to concentrations are presented and shown to be small compared with the component of attenuation attributed to the Cu(A) signal. From these observations, we conclude that there is minimal interference between the hemoglobin and Cu(A) signals in this model, the Cu(A) oxidation state is independent of cerebral oxygenation at normoxia, and the oxidation after hypercapnia is not the result of increased cerebral oxygenation.