Three-dimensional reconstruction of the rat brain cortical microcirculation in vivo

Brain Cortical Activation during Imagining of the Wrist Movement Using Functional Near-Infrared Spectroscopy (fNIRS)

Background:

fNIRS is a useful tool designed to record the changes in the density of blood’s oxygenated hemoglobin (oxyHb) and deoxygenated hemoglobin (deoxyHb) molecules during brain activity. This method has made it possible to evaluate the hemodynamic changes of the brain during neuronal activity in a completely non-aggressive manner.

Objective:

The present study has been designed to investigate and evaluate the brain cortex activities during imagining of the execution of wrist motor tasks by comparing fMRI and fNIRS imaging methods.

Material and Methods:

This novel observational Optical Imaging study aims to investigate the brain motor cortex activity during imagining of the right wrist motor tasks in vertical and horizontal directions. To perform the study, ten healthy young right-handed volunteers were asked to think about right-hand movements in different directions according to the designed movement patterns. The required data were collected in two wavelengths, including 845 and 763 nanometers using a 48 channeled fNIRS machine.

Results:

Analysis of the obtained data showed the brain activity patterns during imagining of the execution of a movement are formed in various points of the motor cortex in terms of location. Moreover, depending on the direction of the movement, activity plans have distinguishable patterns. The results showed contralateral M1 was mainly activated during imagining of the motor cortex (p<0.05).

Conclusion:

The results of our study showed that in brain imaging, it is possible to distinguish between patterns of activities during wrist motion in different directions using the recorded signals obtained through near-infrared Spectroscopy. The findings of this study can be useful in further studies related to movement control and BCI.

A review of flux considerations for in vivo neurochemical measurements

The mass transport or flux of neurochemicals in the brain and how this flux affects chemical measurements and their interpretation is reviewed. For all endogenous neurochemicals found in the brain, the flux of each of these neurochemicals exists between sources that produce them and the sites that consume them all within μm distances. Principles of convective-diffusion are reviewed with a significant emphasis on the tortuous paths and discrete point sources and sinks. The fundamentals of the primary methods of detection, microelectrodes and microdialysis sampling of brain neurochemicals are included in the review. Special attention is paid to the change in the natural flux of the neurochemicals caused by implantation and consumption at microelectrodes and uptake by microdialysis. The detection of oxygen, nitric oxide, glucose, lactate, and glutamate, and catecholamines by both methods are examined and where possible the two techniques (electrochemical vs. microdialysis) are compared. Non-invasive imaging methods: magnetic resonance, isotopic fluorine MRI, electron paramagnetic resonance, and positron emission tomography are also used for different measurements of the above-mentioned solutes and these are briefly reviewed. Although more sophisticated, the imaging techniques are unable to track neurochemical flux on short time scales, and lack spatial resolution. Where possible, determinations of flux using imaging are compared to the more classical techniques of microdialysis and microelectrodes.

A chronic scheme of cranial window preparation to study pial vascular reactivity in murine cerebral malaria

OBJECTIVE

The acute implantation of a cranial window for studying cerebroarteriolar reactivity in living animals involves a highly surgically invasive craniotomy procedure at the time of experimentation, which limits its application in severely ill animals such as in the experimental murine model of cerebral malaria (ECM). To overcome this problem, a chronic window implantation scheme was designed and implemented.

METHODS

A partial craniotomy is first performed by creating a skull bone flap in the healthy mice, which are then left to recover for one to two weeks, followed by infection to induce ECM. Uninfected animals are utilized as control. When cranial superfusion is needed, the bone flap is retracted and window implantation completed by assembling a perfusion chamber for compound delivery to the exposed brain surface. The presurgical step is intended to minimize surgical trauma on the day of experimentation.

RESULTS

Chronic preparations in uninfected mice exhibited remarkably improved stability over acute ones by significantly reducing periarteriolar tissue damage and enhancing cerebroarteriolar dilator responses. The chronic scheme was successfully implemented in ECM mice, which unveiled novel preliminary insights into impaired cerebroarteriolar reactivity and eNOS dysfunction.

CONCLUSION

The chronic scheme presents an innovative approach for advancing our mechanistic understanding on cerebrovascular dysfunction in ECM.

Neurovascular coupling: in vivo optical techniques for functional brain imaging

Optical imaging techniques reflect different biochemical processes in the brain, which is closely related with neural activity. Scientists and clinicians employ a variety of optical imaging technologies to visualize and study the relationship between neurons, glial cells and blood vessels. In this paper, we present an overview of the current optical approaches used for the in vivo imaging of neurovascular coupling events in small animal models. These techniques include 2-photon microscopy, laser speckle contrast imaging (LSCI), voltage-sensitive dye imaging (VSDi), functional photoacoustic microscopy (fPAM), functional near-infrared spectroscopy imaging (fNIRS) and multimodal imaging techniques. The basic principles of each technique are described in detail, followed by examples of current applications from cutting-edge studies of cerebral neurovascular coupling functions and metabolic. Moreover, we provide a glimpse of the possible ways in which these techniques might be translated to human studies for clinical investigations of pathophysiology and disease. In vivo optical imaging techniques continue to expand and evolve, allowing us to discover fundamental basis of neurovascular coupling roles in cerebral physiology and pathophysiology.

Confocal microscopy for astrocyte in vivo imaging: Recycle and reuse in microscopy

In vivo imaging is one of the ultimate and fundamental approaches for the study of the brain. Two-photon laser scanning microscopy (2PLSM) constitutes the state-of-the-art technique in current neuroscience to address questions regarding brain cell structure, development and function, blood flow regulation and metabolism. This technique evolved from laser scanning confocal microscopy (LSCM), which impacted the field with a major improvement in image resolution of live tissues in the 1980s compared to widefield microscopy. While nowadays some of the unparalleled features of 2PLSM make it the tool of choice for brain studies in vivo, such as the possibility to image deep within a tissue, LSCM can still be useful in this matter. Here we discuss the validity and limitations of LSCM and provide a guide to perform high-resolution in vivo imaging of the brain of live rodents with minimal mechanical disruption employing LSCM. We describe the surgical procedure and experimental setup that allowed us to record intracellular calcium variations in astrocytes evoked by sensory stimulation, and to monitor intact neuronal dendritic spines and astrocytic processes as well as blood vessel dynamics. Therefore, in spite of certain limitations that need to be carefully considered, LSCM constitutes a useful, convenient, and affordable tool for brain studies in vivo.

The intravascular susceptibility effect and the underlying physiology of fMRI

In this article, I will first give a brief account of my work at MGH on characterizing the intravascular susceptibility effect. Then I will describe studies into the underlying physiology of BOLD-fMRI which has become of interest to my group in the following decade. I will touch issues such as signal source of BOLD fMRI, capillary recruitment, the elusive initial dip and others.

Optical detection of brain function: simultaneous imaging of cerebral vascular response, tissue metabolism, and cellular activity in vivo

It is known that a remaining challenge for functional brain imaging is to distinguish the coupling and decoupling effects among neuronal activity, cerebral metabolism, and vascular hemodynamics, which highlights the need for new tools to enable simultaneous measures of these three properties in vivo. Here, we review current neuroimaging techniques and their prospects and potential limitations for tackling this challenge. We then report a novel dual-wavelength laser speckle imaging (DW-LSI) tool developed in our labs that enables simultaneous imaging of cerebral blood flow (CBF), cerebral blood volume, and tissue hemoglobin oxygenation, which allows us to monitor neurovascular and tissue metabolic activities at high spatiotemporal resolutions over a relatively large field of view. Moreover, we report digital frequency ramping Doppler optical coherence tomography (DFR-OCT) that allows for quantitative 3D imaging of the CBF network in vivo. In parallel, we review calcium imaging techniques to track neuronal activity, including intracellular calcium approach using Rhod2 fluorescence technique that we develop to detect neuronal activity in vivo. We report a new multimodality imaging platform that combines DW-LSI, DFR-OCT, and calcium fluorescence imaging for simultaneous detection of cortical hemodynamics, cerebral metabolism, and neuronal activities of the animal brain in vivo, as well as its integration with microprobes for imaging neuronal function in deep brain regions in vivo. Promising results of in vivo animal brain functional studies suggest the potential of this multimodality approach for future awake animal and behavioral studies.

Long-term, repeated measurements of mouse cortical microflow at the same region of interest with high spatial resolution

A method for long-term, repeated, semi-quantitative measurements of cerebral microflow at the same region of interest (ROI) with high spatial resolution was developed and applied to mice subjected to focal arterial occlusion. A closed cranial window was chronically implanted over the left parieto-occipital cortex. The anesthetized mouse was placed several times, e.g., weekly, under a dynamic confocal microscope, and Rhodamine B-isothiocyanate-dextran was each time intravenously injected as a bolus, while microflow images were video recorded. Left and right tail veins were sequentially catheterized in a mouse three times at maximum over a 1.5 months' observation period. Smearing of the input function resulting from the use of intravenous injection was shown to be sufficiently small. The distal middle cerebral artery (MCA) was thermocoagulated through the cranial window in six mice, and five sham-operated mice were studied in parallel. Dye injection and video recording were conducted four times in this series, i.e., before and at 10 min, 7 and 30 days after sham operation or MCA occlusion. Pixelar microflow values (1/MTT) in a matrix of approximately 50×50 pixels were displayed on a two-dimensional (2-D) map, and the frequency distribution of the flow values was also calculated. No significant changes in microflow values over time were detected in sham-operated mice, while the time course of flow changes in the ischemic penumbral area in operated mice was similar to those reported in the literature. This method provides a powerful tool to investigate long-term changes in mouse cortical microflow under physiological and pathological conditions.

Imaging separation of neuronal from vascular effects of cocaine on rat cortical brain in vivo

MRI techniques to study brain function assume coupling between neuronal activity, metabolism and flow. However, recent evidence of physiological uncoupling between neuronal and cerebrovascular events highlights the need for methods to simultaneously measure these three properties. We report a multimodality optical approach that integrates dual-wavelength laser speckle imaging (measures changes in blood flow, blood volume and hemoglobin oxygenation), digital-frequency-ramping optical coherence tomography (images quantitative 3D vascular network) and Rhod(2) fluorescence (images intracellular calcium for measure of neuronal activity) at high spatiotemporal resolutions (30 μm, 10 Hz) and over a large field of view (3×5 mm(2)). We apply it to assess cocaine's effects in rat cortical brain and show an immediate decrease (3.5±0.9 min, phase 1) in the oxygen content of hemoglobin and the cerebral blood flow followed by an overshoot (7.1±0.2 min, phase 2) lasting over 20 min whereas Ca(2+) increased immediately (peaked at t=4.1±0.4 min) and remained elevated. This enabled us to identify a delay (2.9±0.5 min) between peak neuronal and vascular responses in phase 2. The ability of this multimodality optical approach for simultaneous imaging at high spatiotemporal resolutions permits us to distinguish the vascular versus cellular changes of the brain, thus complimenting other neuroimaging modalities for brain functional studies (e. g., PET, fMRI).

Long-term in vivo investigation of mouse cerebral microcirculation by fluorescence confocal microscopy in the area of focal ischemia

This study was designed to assess that mouse pial and cortical microcirculation can be monitored in the long term directly in the area of focal ischemia, using in vivo fluorescence microscopy. A closed cranial window was placed over the left parieto-occipital cortex of C57BL/6J mice. Local microcirculation was recorded in real time through the window using laser-scanning confocal fluorescence microscopy after intravenous injection of fluorescent erythrocytes and dextran. The basal velocity of erythrocytes through intraparenchymal capillaries was 0.53+/-0.30 mm/sec (n=121 capillaries in 10 mice). Two branches of the middle cerebral artery were topically cauterized through the window. Blood flow evaluated by laser-Doppler flowmetry in two distinct areas indicated the occurrence of an ischemic core (15.2%+/-5.9% of baseline for at least 2 h) and a penumbral zone. Magnetic resonance imaging and histology were used to characterize the ischemic area at 24 h after occlusion. The infarct volume was 7.3+/-3.2 mm(3) (n=6). Microcirculation was repeatedly videorecorded using fluorescence confocal microscopy over the next month. After the decrease following arterial occlusion, capillary erythrocyte velocity was significantly higher than baseline 1 week later, and attained 0.74+/-0.51 mm/sec (n=76 capillaries in six mice, P<0.005) after 1 month, while venous and capillary network remodeling was assessed, with a marked decrease in tortuosity. Immunohistochemistry revealed a zone of necrotic tissue into the infarct epicenter, with activated astrocytes at its border. Such long-term investigations in ischemic cortex brings new insight into the microcirculatory changes induced by focal ischemia and show the feasibility of long-term fluorescence studies in the mouse cortex.

Morphological changes in capillaries in the ischemic brain in Wistar rats

The microvasculature in the brain plays a vital role in the maintenance of brain perfusion, and fulfills the dynamic requirements of normal brain functions. It is well known that collateral circulation can be induced by ischemia in cerebral infarctions, but it is not known whether cerebral ischemia affects microvasculatures in the ischemic region. In the present study, we examined quantitatively serial changes in capillaries following bilateral common carotid artery ligation in Wistar rats. After the animals were perfused with tetramethylrhodamine isothiocyanate-labeled gelatin 3 h (n = 9), 1 day (n = 9), 7 days (n = 9) and 28 days (n = 9) after the ligation, capillary diameters in the brain sections were measured with a confocal laser-scanning microscope. Capillary diameters of the cerebellum did not differ among all groups, while those in the ischemic regions decreased significantly 3h after the ligation (p<0.01), thereafter gradually returned toward the baseline level, and became significantly larger (168% of the control) 28 days after the ligation (p< 0.01). The density of capillaries in the frontal and parietal cortices increased approximately to 1.3-fold of those of the control level 28 days after the ligation. Transmission electron microscopy showed that the mean ratio of the inner diameter to the outer diameter of capillaries in the frontal cortex became significantly greater 28 days after the ligation (p<0.05). Our data indicate that capillaries dilate in the ischemic brain region in the chronic phase of cerebral ischemia. It is also suggested that neovascularization occurs in the ischemic brain region.

Capillary flow and diameter changes during reperfusion after global cerebral ischemia studied by intravital video microscopy

The reaction of cerebral capillaries to ischemia is unclear. Based on Hossmann's observation of postischemic "delayed hypoperfusion," we hypothesized that capillary flow is decreased during reperfusion because of increased precapillary flow resistance. To test this hypothesis, we measured cerebral capillary erythrocyte velocity and diameter changes by intravital microscopy in gerbils. A cranial window was prepared over the frontoparietal cortex in 26 gerbils anesthetized with halothane. The animals underwent either a sham operation or fifteen minutes of bilateral carotid artery occlusion causing global cerebral ischemia. Capillary flow velocities were measured by frame-to-frame tracking of fluorescein isothiocyanate labeled erythrocytes in 1800 capillaries after 1-hour reperfusion. Capillary flow velocities were decreased compared to control (0.25 +/- 0.27 mm/s vs. 0.76 +/- 0.45 mm/s; P<0.001). Precapillary arteriole diameters in reperfused animals were reduced to 76.3 +/- 6.9% compared to baseline (P<0.05). Capillary diameters in reperfused animals (2.87 +/- 0.97 microm) were reduced (P<0.001) compared to control (4.08 +/- 1.19 microm). Similar reductions of precapillary (24%) and capillary vessel diameters (30%) and absolute capillary flow heterogeneity indicate that delayed (capillary) hypoperfusion occurs as a consequence of increased precapillary arteriole tone during reperfusion.

Microvascular structure after embolic focal cerebral ischemia in the rat

OBJECTIVES

We analyze morphological alterations of cerebral neovascularization after stroke using a new 3D imaging software program.

METHODS

Male Wistar rats underwent unilateral embolic middle cerebral artery occlusion (MCAo) by a single fibrin rich clot. Subjects were sacrificed from 1 to 28 days post infarct. Vessel perimeters were measured on coronal sections stained with endothelial cell-specific antibody to von Willebrand's factor. Vessel segment lengths, diameters and number of vessels were analyzed on cerebral microvessels perfused with FITC-dextran 14 days after ischemia using LSCM and a 3-D vessel quantification program.

RESULTS

The mean number of microvessels with enlarged perimeters significantly increased in the ipsilateral cortex at day 7 when compared to the contralateral cortex (29.7+/-14.7 vs. 3.7+/-2.5, P<0.05). Subsequently, differences in the number of microvessels with enlarged perimeters decreased on days 14 and 28. Fourteen days post-MCA occlusion, microvessel segment length (15.0 vs. 26.0 microm, P<0.05) and diameter (3.14 vs. 3.75 microm, P<0.05) significantly decreased in the ipsilateral hemisphere when compared to the contralateral hemisphere, respectively. Furthermore, the mean total number of these smaller microvessels increased in the ipsilateral hemisphere (57.33+/-14.5 vs. 32.22+/-11.7, P<0.05).

CONCLUSIONS

Focal cerebral ischemia induces morphological changes (early dilated microvessels followed by decreased microvessel segment length and diameter) that are consistent with newly generated microvessels.

Spatial/temporal correlation of BOLD and optical intrinsic signals in humans

Comparing the BOLD signal with electrophysiological maps and other perfusion-dependent signals, such as the optical intrinsic signal (OIS), within subjects should provide insight into the etiology of the BOLD signal. Tongue activations were compared in five human subjects using BOLD fMRI, 610-nm OIS, and the electrocortical stimulation map (ESM). Robust fMRI activations centered on the lateral inferior aspect of the central sulcus and extended into pre- and post-central gyri, adjacent to ESM tongue loci. OIS and fMRI maps colocalized, although optical responses were spatially larger (P <.001 across multiple thresholds) and contained more gyral components. The timecourses of the fMRI and OIS signals were similar, appearing within 2.5 s and peaking 6-8 s after task onset. Although many processes contribute to increased 610-nm reflectance, optical spectroscopy and fluorescent dye imaging suggest that a significant part of this signal is due to a concomitant decrease in deoxyhemoglobin and increase in oxyhemoglobin concentrations. The spatial/temporal correlation of BOLD and the positive 610-nm response within subjects suggests that the two signals may share similar etiologies. The OIS/fMRI inconsistencies may be due to cell swelling and light-scattering contributions to OIS and fMRI sensitivity. This study also demonstrates that fMRI maps do not precisely colocalize with ESM, rather they emphasize changes in adjacent venous/sulcal structures.

Albumin therapy of transient focal cerebral ischemia: in vivo analysis of dynamic microvascular responses

BACKGROUND AND PURPOSE

To study whether intravascular or hemodynamic factors contribute to the marked neuroprotective effect of albumin therapy in focal cerebral ischemia, 2 complementary methods were applied: laser-scanning confocal microscopy (LSCM) and laser-Doppler perfusion imaging (LDPI).

METHODS

In the LSCM study, Sprague-Dawley rats were anesthetized with halothane/nitrous oxide, and a cranial window was placed over the dorsolateral frontoparietal cortex. Rats received 2-hour middle cerebral artery occlusion (MCAO) by an intraluminal suture and were treated with human albumin (1.25 g/kg; n=4) or saline (n=3) after 30 minutes of recirculation. Video images of cortical vessels were continually acquired and were digitized offline to measure diameters and fluorescent erythrocyte velocities. In the LDPI study, cortical perfusion was measured in anesthetized Sprague-Dawley rats that received 2-hour MCAO and were treated with albumin (2.5 g/kg; n=6) or saline (n=5) at 30 minutes after recirculation.

RESULTS

In the LSCM study, MCAO was associated with arteriolar dilation and slowing of capillary and venular erythrocyte perfusion. During the first 15 to 30 minutes of postischemic recirculation, prominent foci of vascular stagnation developed within cortical venules, associated with thrombuslike foci and adherent corpuscular structures consistent in size with neutrophils. Saline administration failed to affect these phenomena, while albumin therapy was followed by significant increases in arteriolar diameter ( approximately 12%; P=0.007) and by a prompt improvement of venular and capillary erythrocyte perfusion and a partial disappearance of adherent thrombotic material. Albumin therapy increased erythrocyte flow velocity in both capillaries (288+/-73% versus 76+/-18% in the saline group; P=0.023) and venules (2.7-fold [P=0.001] versus 1.0-fold in the saline group [P=NS]). In the LDPI study, cortical perfusion declined during MCAO and rose initially with recirculation (to approximately 135% of baseline) in both groups. Mean cortical perfusion improved slightly (approximately 14%; P=NS) in albumin-treated animals.

CONCLUSIONS

These results reveal a beneficial effect of albumin therapy in reversing stagnation, thrombosis, and corpuscular adherence within cortical venules in the reperfusion phase after focal ischemia and support its utility in the treatment of acute ischemic stroke.

Three-dimensional imaging of hormone-secreting cells and their microvessel environment in estrogen-induced prolactinoma of the rat pituitary gland by confocal laser scanning microscopy

This study focused on the three-dimensional imaging of hormone-secreting cells and their microvascular environment in estrogen-induced prolactinoma of the rat pituitary gland. Adult female Wistar-Imamichi rats were injected with estradiol dipropionate and killed 7 weeks later. Some rats given estrogen for 7 weeks also were injected with bromocriptine before killing. To obtain a detailed three-dimensional image of microvessels, dialyzed fluorescein isothiocyanate (FITC)-conjugated gelatin was injected into the left ventricle of the rat heart. After the perfusion, the pituitary glands were resected and subjected to immunohistochemistry (IHC). To evaluate the effects of estrogen and bromocriptine, IHC was performed with antibodies against prolactin (PRL), adrenocorticotropic hormone (ACTH), and growth hormone (GH). With the combination, microvessels and cells containing PRL, ACTH, and GH could be clearly identified by confocal laser scanning microscopy (CLSM). The PRL cells increased in number and became hypertrophic after prolonged exposure to estrogen. With bromocriptine administration after estrogen treatment, however, PRL cells decreased in number and became atrophic. The current study revealed that estrogen and bromocriptine had significant effects on PRL secretion and the microvascular environment. Therefore, this technique (FITC injection and IHC) with CLSM is suitable for the three-dimensional imaging of hormone-secreting mechanisms under various conditions.

A new approach to three-dimensional reconstructed imaging of hormone-secreting cells and their microvessel environments in rat pituitary glands by confocal laser scanning microscopy

There has been considerable interest in the relationship between hormone- secreting endocrine cells and their microvessels in human pituitary gland. However, microcirculatory networks have rarely been studied in three dimensions (3D). This study was designed to visualize and to reveal the relationship between hormone-secreting endocrine cells and their microvessel environment in 3D, using rat pituitary glands under various (hyper/hypo) experimental conditions by confocal laser scanning microscopy (CLSM). Female adult Wistar rats were used after bilateral adrenalectomy or ACTH administration for 2 weeks. Clear 3D reconstructed images of ACTH cells, the microvessel network and counterstained nuclei were obtained at a maximal focus depth of 1 mm by CLSM without any background noise. In the hyperfunctional state, slender cytoplasmic processes of hypertrophic stellate ACTH cells frequently extended to the microvessels. In the hypofunctional state, ACTH cells appeared atrophic and round with scanty cytoplasm, and cytoplasmic adhesions to microvessel network patterns were inconspicuous. Therefore, 3D reconstructed imaging by CLSM is a useful technique with which to investigate the microvessel environment of hormone-secreting cells and has the potential to reveal dynamic hormone-secreting pathways.

Dynamic in vivo measurement of erythrocyte velocity and flow in capillaries and of microvessel diameter in the rat brain by confocal laser microscopy

A new method for studying brain microcirculation is described. Both fluorescently labeled erythrocytes and plasma were visualized on-line through a closed cranial window in anesthetized rats, using laser-scanning two-dimension confocal microscopy. Video images of capillaries, arterioles, and venules were digitized off-line to measure microvessel diameter and labeled erythrocyte flow and velocity in parenchymal capillaries up to 200 microm beneath the brain surface. The method was used to analyze the rapid adaptation of microcirculation to a brief decrease in perfusion pressure. Twenty-second periods of forebrain ischemia were induced using the tour-vessel occlusion model in eight rats. EEG, arterial blood pressure, and body temperature were continuously controlled. In all conditions, labeled erythrocyte flow and velocity were both very heterogeneous in capillaries. During ischemia, capillary perfusion was close to 0, but a low blood flow persisted in arterioles and venules, while EEG was flattening. The arteriole and venule diameter did not significantly change. At the unclamping of carotid arteries, there was an instantaneous increase (by about 150%) of arteriole diameter. Capillary erythrocyte flow and velocity increased within 5 seconds, up to, respectively, 346 +/- 229% and 233 +/- 156% of their basal value. No capillary recruitment of erythrocytes was detected. All variables returned to their basal levels within less than 100 seconds after declamping. The data are discussed in terms of a possible involvement of shear stress in the reperfusion period.

Blood flow structuring and its alterations in capillaries of the cerebral cortex

Various manifestations of blood flow structuring were investigated in rabbit cerebral cortex capillaries, which possess the most narrow lumina of all parts of the body. Blood flow structuring in the capillaries was characterized by the presence of a stable and comparatively large parietal plasma layer, which changed insignificantly under control and ischemic conditions, but disappeared when blood stasis developed inside the capillaries. The axial core of the blood flow in the capillaries, which occupied almost two-thirds of the intracapillary volume under normal conditions, consisted of significantly deformed (stretched along the microvessels' axes) and nonaggregated erythrocytes. During ischemia the shape of the erythrocytes did not change appreciably; only the blood plasma intervals between them increased significantly, demonstrating reduction of the local hematocrit. During primary blood stasis caused by enhanced intravascular erythrocyte aggregation, typical blood flow structuring became significantly disturbed: red cells filled the whole, or almost the whole, capillary lumina and did not leave visible space for plasma inside the microvessel lumina. We concluded that normal blood flow structuring is a deciding factor in the blood rheological properties of microvessels. Its disturbance, caused by fast accumulation of erythrocytes in the capillary lumina, results in blood rheological disorders and in a slow down to a full stop of the blood flow, despite a preserved arteriolovenular pressure difference.

Understanding functional neuroimaging methods based on neurovascular coupling

Functional neuroimaging techniques are usually grouped according to the employed apparatus into functional magnetic resonance imaging techniques (fMRI), nuclear medicine approaches such as single photon emission tomography (SPET) or positron emission tomography (PET), and optical approaches (measurement of intrinsic signals, near infrared spectroscopy (NIRS)). However, the physiological parameters that are measured with these methods do not necessarily follow this technical classification. On the one hand, using different imaging modalities the same physiological parameters are measured and on the other hand, using the same imaging devices completely different physiological parameters can be assessed. The present article covers those functional neuroimaging methods which measure the vascular response to functional brain activation (PET, SPET, fMRI and NIRS). First, starting with the traditional grouping of these methods, it is outlined how the specific methods assess vascular changes associated with brain activation in order to localize brain function. Based on the understanding of the underlying physiological events, subsequently, a new classification of functional neuroimaging methods is proposed.

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

We investigated the role of nitric oxide (NO) in leukocyte-endothelium interaction, blood-brain barrier (BBB) function and oxygen free-radical production in the rat pial microcirculation. In a closed cranial window preparation (dura removed) over the parietal cortex of pentobarbital-anesthetized Wistar rats, NO synthase (NOS) was inhibited by systemic and/or topical application of N omega-nitro-L-arginine (L-NNA) under physiological conditions and during leukotriene B4 (LTB4) activation. Circulating leukocytes were labeled by intravenous injection of rhodamine 6G. We used a confocal laser scanning microscope (CLSM) and studied leukocyte rolling and sticking in pial veins and arteries before and after NOS inhibition. At the end of the experiments, sodium-fluorescein was injected intravenously to test BBB integrity. Brain cortex oxygen free-radical production was investigated in the cranial window preparation using lucigenin-enhanced chemiluminescence (CL). L-NNA application did not lead to significant changes in leukocyte-endothelium interaction, BBB function, and oxygen free-radical production under physiological conditions [leukocyte-endothelium interaction: control (n = 5), L-NNA systemically (n = 5), L-NNA topically (n = 5): at baseline rollers/100 microns: 0.76 +/- 0.55, 0.64 +/- 0.94, 0.44 +/- 0.55 and stickers/100 microns: 0.90 +/- 0.28, 0.76 +/- 0.24, 0.84 +/- 0.42; at 60 min rollers/100 microns: 1.49 +/- 0.66, 1.21 +/- 0.99, 0.67 +/- 0.66 and stickers/100 microns: 1.04 +/- 0.20, 1.19 +/- 0.23, 1.21 +/- 0.54; oxygen free-radical production (n = 4): CL count before L-NNA application 35 +/- 17 cps, after 1 h of topical superfusion of L-NNA 38 +/- 14 cps; p < 0.05]. In contrast to the results achieved under physiological conditions, a significant further increase of rolling leukocytes and BBB permeability occurred due to NOS inhibition under LTB4-activated conditions [76 +/- 47% significant (p < or = 0.01, n = 7) further increase of rollers/100 microns due to 60 min L-NNA application following the activation period of 120 min LTB4 superfusion]. Our results support a modulatory role for NO in leukocyte-endothelium interaction and BBB permeability in the pial microcirculation when this interaction is increased.

Global cerebral ischemia in the rat: online monitoring of oxygen free radical production using chemiluminescence in vivo

Using online in vivo chemiluminescence (CL), we studied for the first time continuously the production of reactive oxygen species (ROS) after global cerebral ischemia and the relationship of ROS production to CBF. In anesthetized rats equipped with a closed cranial window, the CL enhancer, lucigenin (1 mM), was superfused onto the brain topically. CL was measured through the cranial window with a cooled photomultiplier, and CBF was measured simultaneously with laser-Doppler flowmetry. Reperfusion after 10 min (n = 8) of global cerebral ischemia led to a CL peak to 188 +/- 77% (baseline = 100%) within 10 +/- 4 min. After 2 h of reperfusion, CL had returned to 102 +/- 28%. Reperfusion after 20 min (n = 8) of ischemia increased CL to 225 +/- 48% within 12 +/- 3 min. After 2 h, CL was still increased (150 +/- 44%, p < 0.05 compared with 10 min of ischemia). CL after 10 min of ischemia was neither affected by brain topical free CuZn-superoxide dismutase (SOD) (100 U/ml, n = 3) nor by i.v. administration of free CuZn-SOD (104 U/kg, followed by 104 U/kg/h, n = 3). The CBF hyperfusion peak on reperfusion preceded the CL peak in all experiments by several minutes. In additional in vitro experiments we investigated the source of CL: Intracellular loading of lucigenin was demonstrated in cultured CNS cells, and a very similar pattern of CL as in the in vivo preparation after ischemia developed in rat brain slices after 15 min of hypoxia, which was unaffected by free CuZn-SOD (100 U/ml) but strongly attenuated by liposome-entrapped CuZn-SOD. We conclude that lucigenin-enhanced CL is a promising tool to study ROS production continuously from the in vivo brain of experimental animals and brain slices, and that the CL signal most likely derives from the intracellular production of superoxide. The production of ROS is preceded by reperfusion, is burst-like, and is dependent on the duration of the ischemic interval.

Functional increases in cerebral blood volume over somatosensory cortex

We have examined the relationship between cerebral blood volume (CBV) and electrophysiology over primary somatosensory cortex (S-I) in the rat. We did this by comparing the spatial characteristics and time course of activity-related changes in plasma fluorescence, intrinsic optical reflectance signals, and single unit electrophysiology in S-I to identical stimuli. S-Is of urethane-anesthetized male Sprague-Dawley rats were exposed, and fluorescent Texas Red dextran dye (MW 70,000) was administered intravenously. Subsequently, foredigit electroshock or vibrissal deflection was associated with fluorescence increases over contralateral forelimb or posteromedial barrel subfield cortex. Fluorescence was delayed and prolonged, indicating that CBV increases at 1-1.5 s and peaks 2-2.5 s after the onset of stimulation in both regions. When stimulus intensity was adjusted to produce barely detectable fluorescence foci (10% above back-ground), significant electrophysiologic spiking was seen. At these parameters, fluorescence change overlay areas of increased cortical layer III cell firing on single unit recordings. However, surface boundaries of the smallest observable fluorescence foci at their peak spatial extents consistently overspilled electrophysiologic center receptive fields. Corresponding intrinsic optical reflectance decreases were seen at 610 and 850 nm, exhibiting similar timing and colocalizing closely with fluorescence increase at both wavelengths after identical stimuli. These signals similarly overspilled electrophysiologic activity. Thus, we observed delayed increases in vascular fluorescence (related to CBV) over activated cortex. The smallest detectable fluorescence changes overspilled the center receptive field boundaries and were associated with appreciable electrophysiologic firing. In addition, the striking spatial and temporal similarity between intrinsic optical reflectance and fluorescence activity suggests that changes in intrinsic cortical reflectance are strongly related to changes in CBV.

Patterns of capillary plasma perfusion in brains in conscious rats during normocapnia and hypercapnia

The present study aimed to investigate the distribution pattern of plasma flow velocities in brain capillaries. We tested the hypothesis that plasma flow velocities are heterogeneous in the brain capillaries of normocapnic conscious rats and become more homogeneous during increased cerebral blood flow induced by hypercapnia. We developed a method that makes it possible to detect the distribution pattern of plasma flow velocities from the intravascular dye concentrations measured in different capillaries. Evans blue was injected intravenously as a bolus, and 3 to 4 seconds later the rats were decapitated. During this period, a steep increase in arterial dye concentration was verified by frequent arterial blood sampling. Under such conditions, divergent plasma flow velocities in different capillaries yield unequal intravascular dye concentrations. Dye concentrations were measured in several hundred capillaries of brain cryosections using quantitative fluorescence microscopy based on calibration curves obtained from anesthetized rats. The results show a high degree of variation in the intravascular dye concentration during normocapnia. During increasing stages of hypercapnia, the variation was gradually reduced. The coefficient of variation (SD/mean-100) of intracapillary dye concentration decreased from 76% at normocapnia to 22% at extreme hypercapnia (PCO2 of 87 mm Hg), thus showing an inverse correlation with arterial PCO2 (r = .97). The heterogeneity of intravascular dye concentrations observed in the present experiments indicates heterogeneous velocities of plasma perfusion in different brain capillaries during normocapnia and a more homogeneous distribution pattern during hypercapnic hyperemia.

Capillary perfusion of the rat brain cortex. An in vivo confocal microscopy study

Confocal laser-scanning microscopy was used to visualize subsurface cerebral microvessels labeled with intravascular fluorescein in a closed cranial window model of the anesthetized rat. In noninvasive optical sections up to 250 microns beneath the brain surface, plasma perfusion and blood cell perfusion of individual capillaries were studied. Under resting conditions, in all cerebral capillaries the presence of plasma flow as demonstrated by the appearance of an intravenously injected fluorescent tracer within 20 seconds after injection. Plasma flow was verified even in capillaries that contained stationary erythrocytes or leukocytes; 91.1% of the capillaries contained flowing blood cells, 5.2% contained stationary blood cells, and no blood cells were seen in 3.6%. Mean blood cell velocity was 498.3 +/- 443.9 microns/s, and the mean blood cell supply rate was 35.75 +/- 28.01 cells per second. When capillaries were continuously observed for 1 minute, "on" and "off" periods of blood cell flow were noted. During hypercapnia (increase of PCO2 from 33.25 to 50.26 mm Hg), mean blood cell flux increased from 38.6 +/- 17.2 to 55.5 +/- 12.2 per second (P < .005, paired t test of mean values in six animals), and blood cell velocity increased from 519.5 +/- 254.8 to 828.5 +/- 460.8 microns/s (P = .074, paired t test of mean values in six animals). Homogeneity of blood cell flux increased as indicated by the coefficient of variation decreasing from 44.6% to 22.0%, and the portion of poorly perfused capillaries (blood cell flux, < 40 per second) decreased from 59.2% to 22.4%.(ABSTRACT TRUNCATED AT 250 WORDS)

Cortical hypoperfusion after global forebrain ischemia in rats is not caused by microvascular leukocyte plugging

BACKGROUND AND PURPOSE

We tested the hypothesis that cerebral hypoperfusion after experimental global cerebral ischemia is caused by plugging of the microcirculation with activated leukocytes using in vivo microscopic observation of the behavior of leukocytes in the cortical microcirculation during the transition from postischemic hyperperfusion to hypoperfusion.

METHODS

Anesthetized and ventilated rats (n = 24) were equipped with a closed cranial window. Physiological variables and cortical regional cerebral blood flow (laser-Doppler flowmetry) were measured continuously. Leukocytes were labeled intravitally with rhodamine 6G and visualized in the microcirculation of the brain surface and outer layers of the cortex with confocal laser scanning microscopy from preischemia to 4 hours after reperfusion that followed 10 minutes of global cerebral ischemia (rCBF < 10% of control).

RESULTS

In controls (n = 8), there were no signs of leukocyte activation over the 4-hour observation period. In ischemic rats (n = 16), during the transition from hyperperfusion to hypoperfusion there was no change in the behavior of leukocytes. Most notably, no capillary pluggers were seen. In the postischemic period only a slight increase of the number of leukocytes rolling along or sticking to the venular endothelium was seen, and very few capillaries were plugged by leukocytes. Extravasation of leukocytes into the brain tissue was observed in 8 rats beginning 2 hours after ischemia with a variable degree between animals.

CONCLUSIONS

Because there was only mild activation of leukocyte-endothelium interaction within the first hours of reperfusion after 10 minutes of global forebrain ischemia, because no leukocytes plugged superficial cortical capillaries during the transition from hyperperfusion to hypoperfusion, and because the regional cerebral blood flow transition was very rapid, we speculate that leukocyte plugging is not responsible for the early cortical hypoperfusion seen after brief global ischemia in rats.

Imaging of leukocytes within the rat brain cortex in vivo

Confocal laser scanning microscopy was used in a rat closed cranial window preparation in order to study rhodamin 6G-labeled leukocytes within the brain cortex in vivo. Leukocytes were visualized up to 150 microns beneath the rat brain surface in noninvasive optical sections. In pial venules, leukocytes were seen flowing with the blood stream, rolling along or sticking to the endothelium, and migrating through the vessel wall. Within cerebral capillaries, leukocyte flux, velocities, and leukocyte plugging were measured. After additional intravenous administration of fluorescein, the plasma, leukocytes, and erythrocytes were visualized simultaneously. Based on stacks of optical sections of fluorescein-labeled capillaries, the individual capillaries were localized within the three-dimensional microvascular network. The usefulness of this technique was illustrated in a feasibility study in which leukocyte sticking to the vascular walls of venules, leukocyte extravasation, and intracapillary leukocyte plugging were monitored in a model of global cerebral ischemia.