Successive depth variations in microvascular distribution of rat somatosensory cortex

Improved laminar specificity and sensitivity by combining SE and GE BOLD signals

The most widely used gradient-echo (GE) blood oxygenation level-dependent (BOLD) contrast has high sensitivity, but low specificity due to draining vein contributions, while spin-echo (SE) BOLD approach at ultra-high magnetic fields is highly specific to neural active sites but has lower sensitivity. To obtain high specificity and sensitivity, we propose to utilize a vessel-size-sensitive filter to the GE-BOLD signal, which suppresses macrovascular contributions and to combine selectively retained microvascular GE-BOLD signals with the SE-BOLD signals. To investigate our proposed idea, fMRI with 0.8 mm isotropic resolution was performed on the primary motor and sensory cortices in humans at 7 T by implementing spin- and gradient-echo (SAGE) echo planar imaging (EPI) acquisition. Microvascular-passed sigmoidal filters were designed based upon the vessel-size-sensitive ΔR₂*/ΔR₂ value for retaining GE-BOLD signals originating from venous vessels with ≤ 45 μm and ≤ 65 μm diameter. Unlike GE-BOLD fMRI, the laminar profile of SAGE-BOLD fMRI with the vessel-size-sensitive filter peaked at ∼ 1.0 mm from the surface of the primary motor and sensory cortices, demonstrating an improvement of laminar specificity over GE-BOLD fMRI. Also, the functional sensitivity of SAGE BOLD at middle layers (0.75-1.5 mm) was improved by ∼ 80% to ∼100% when compared with SE BOLD. In summary, we showed that combined GE- and SE-BOLD fMRI with the vessel-size-sensitive filter indeed yielded improved laminar specificity and sensitivity and is therefore an excellent tool for high spatial resolution ultra-high filed (UHF)-fMRI studies for resolving mesoscopic functional units.

Three-dimensional microvascular network reconstruction from in vivo images with adaptation of the regional inhomogeneity in the signal-to-noise ratio

OBJECTIVE

Quantification of angiographic images with two-photon laser scanning fluorescence microscopy (2PLSM) relies on proper segmentation of the vascular images. However, the images contain inhomogeneities in the signal-to-noise ratio (SNR) arising from regional effects of light scattering and absorption. The present study developed a semiautomated quantification method for volume images of 2PLSM angiography by adjusting the binarization threshold according to local SNR along the vessel centerlines.

METHODS

A phantom model made with fluorescent microbeads was used to incorporate a region-dependent binarization threshold.

RESULTS

The recommended SNR for imaging was found to be 4.2-10.6 that provide the true size of imaged objects if the binarization threshold was fixed at 50% of SNR. However, angiographic images in the mouse cortex showed variable SNR up to 45 over the depths. To minimize the errors caused by variable SNR and a spatial extent of the imaged objects in an axial direction, the microvascular networks were three-dimensionally reconstructed based on the cross-sectional diameters measured along the vessel centerline from the XY-plane images with adapted binarization threshold. The arterial volume was relatively constant over depths of 0-500 µm, and the capillary volume (1.7% relative to the scanned volume) showed the larger volumes than the artery (0.8%) and vein (0.6%).

CONCLUSIONS

The present methods allow consistent segmentation of microvasculature by adapting the local inhomogeneity in the SNR, which will be useful for quantitative comparison of the microvascular networks, such as under disease conditions where SNR in the 2PLSM images varies over space and time.

Early fMRI responses to somatosensory and optogenetic stimulation reflect neural information flow

fMRI has revolutionized how neuroscientists investigate human brain functions and networks. To further advance understanding of brain functions, identifying the direction of information flow, such as thalamocortical versus corticothalamic projections, is critical. Because the early hemodynamic response at microvessels near active neurons can be detected by ultrahigh field fMRI, we propose using the onset times of fMRI responses to discern the information flow. This approach was confirmed by observing the ultrahigh spatiotemporal resolution BOLD fMRI responses to bottom-up somatosensory stimulation and top-down optogenetic stimulation of the primary motor cortex in anesthetized mice. Because ultrahigh field MRI is increasingly available, ultrahigh spatiotemporal fMRI will significantly facilitate the investigation of functional circuits in humans.

Blood oxygenation level–dependent (BOLD) functional magnetic resonance imaging (fMRI) has been widely used to localize brain functions. To further advance understanding of brain functions, it is critical to understand the direction of information flow, such as thalamocortical versus corticothalamic projections. For this work, we performed ultrahigh spatiotemporal resolution fMRI at 15.2 T of the mouse somatosensory network during forepaw somatosensory stimulation and optogenetic stimulation of the primary motor cortex (M1). Somatosensory stimulation induced the earliest BOLD response in the ventral posterolateral nucleus (VPL), followed by the primary somatosensory cortex (S1) and then M1 and posterior thalamic nucleus. Optogenetic stimulation of excitatory neurons in M1 induced the earliest BOLD response in M1, followed by S1 and then VPL. Within S1, the middle cortical layers responded to somatosensory stimulation earlier than the upper or lower layers, whereas the upper cortical layers responded earlier than the other two layers to optogenetic stimulation in M1. The order of early BOLD responses was consistent with the canonical understanding of somatosensory network connections and cannot be explained by regional variabilities in the hemodynamic response functions measured using hypercapnic stimulation. Our data demonstrate that early BOLD responses reflect the information flow in the mouse somatosensory network, suggesting that high-field fMRI can be used for systems-level network analyses.

UV-Induced Neuronal Degeneration in the Rat Cerebral Cortex

Irradiation with ultraviolet (UV) light on the cortical surface can induce a focal brain lesion (UV lesion) in rodents. In the present study, we investigated the process of establishing a UV lesion. Rats underwent UV irradiation (365-nm wavelength, 2.0 mWh) over the dura, and time-dependent changes in the cortical tissue were analyzed histologically. We found that the majority of neurons in the lesion started to degenerate within 24 h and the rest disappeared within 5 days after irradiation. UV-induced neuronal degeneration progressed in a layer-dependent manner. Moreover, UV-induced terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) positivity and heme oxygenase-1 (HO-1) immunoreactivity were also detected. These findings suggest that UV irradiation in the brain can induce gradual neural degeneration and oxidative stress. Importantly, UV vulnerability may vary among cortical layers. UV-induced cell death may be due to apoptosis; however, there remains a possibility that UV-irradiated cells were degenerated via processes other than apoptosis. The UV lesion technique will not only assist in investigating brain function at a targeted site but may also serve as a pathophysiological model of focal brain injury and/or neurodegenerative disorders.

Dynamic Contrast Optical Coherence Tomography reveals laminar microvascular hemodynamics in the mouse neocortex in vivo

Studies of flow-metabolism coupling often presume that microvessel architecture is a surrogate for blood flow. To test this assumption, we introduce an in vivo Dynamic Contrast Optical Coherence Tomography (DyC-OCT) method to quantify layer-resolved microvascular blood flow and volume across the full depth of the mouse neocortex, where the angioarchitecture has been previously described. First, we cross-validate average DyC-OCT cortical flow against conventional Doppler OCT flow. Next, with laminar DyC-OCT, we discover that layer 4 consistently exhibits the highest microvascular blood flow, approximately two-fold higher than the outer cortical layers. While flow differences between layers are well-explained by microvascular volume and density, flow differences between subjects are better explained by transit time. Finally, from layer-resolved tracer enhancement, we also infer that microvascular hematocrit increases in deep cortical layers, consistent with predictions of plasma skimming. Altogether, our results show that while the cortical blood supply derives mainly from the pial surface, laminar hemodynamics ensure that the energetic needs of individual cortical layers are met. The laminar trends reported here provide data that links predictions based on the cortical angioarchitecture to cerebrovascular physiology in vivo.

Seizure epicenter depth and translaminar field potential synchrony underlie complex variations in tissue oxygenation during ictal initiation

Whether functional hyperemia during epileptic activity is adequate to meet the heightened metabolic demand of such events is controversial. Whereas some studies have demonstrated hyperoxia during ictal onsets, other work has reported transient hypoxic episodes that are spatially dependent on local surface microvasculature. Crucially, how laminar differences in ictal evolution can affect subsequent cerebrovascular responses has not been thus far investigated, and is likely significant in view of possible laminar-dependent neurovascular mechanisms and angioarchitecture. We addressed this open question using a novel multi-modal methodology enabling concurrent measurement of cortical tissue oxygenation, blood flow and hemoglobin concentration, alongside laminar recordings of neural activity, in a urethane anesthetized rat model of recurrent seizures induced by 4-aminopyridine. We reveal there to be a close relationship between seizure epicenter depth, translaminar local field potential (LFP) synchrony and tissue oxygenation during the early stages of recurrent seizures, whereby deep layer seizures are associated with decreased cross laminar synchrony and prolonged periods of hypoxia, and middle layer seizures are accompanied by increased cross-laminar synchrony and hyperoxia. Through comparison with functional activation by somatosensory stimulation and graded hypercapnia, we show that these seizure-related cerebrovascular responses occur in the presence of conserved neural-hemodynamic and blood flow-volume coupling. Our data provide new insights into the laminar dependency of seizure-related neurovascular responses, which may reconcile inconsistent observations of seizure-related hypoxia in the literature, and highlight a potential layer-dependent vulnerability that may contribute to the harmful effects of clinical recurrent seizures. The relevance of our findings to perfusion-related functional neuroimaging techniques in epilepsy are also discussed.

Laminar fMRI: What can the time domain tell us?

The rapid developments in functional MRI (fMRI) acquisition methods and hardware technologies in recent years, particularly at high field (≥7 T), have enabled unparalleled visualization of functional detail at a laminar or columnar level, bringing fMRI close to the intrinsic resolution of brain function. These advances highlight the potential of high resolution fMRI to be a valuable tool to study the fundamental processing performed in cortical micro-circuits, and their interactions such as feedforward and feedback processes. Notably, because fMRI measures neuronal activity via hemodynamics, the ultimate resolution it affords depends on the spatial specificity of hemodynamics to neuronal activity at a detailed spatial scale, and by the evolution of this specificity over time. Several laminar (≤1 mm spatial resolution) fMRI studies have examined spatial characteristics of the measured hemodynamic signals across cortical depth, in light of understanding or improving the spatial specificity of laminar fMRI. Few studies have examined temporal features of the hemodynamic response across cortical depth. Temporal features of the hemodynamic response offer an additional means to improve the specificity of fMRI, and could help target neuronal processes and neurovascular coupling relationships across laminae, for example by differences in the onset times of the response across cortical depth. In this review, we discuss factors that affect the timing of neuronal and hemodynamic responses across laminae, touching on the neuronal laminar organization, and focusing on the laminar vascular organization. We provide an overview of hemodynamics across the cortical vascular tree based on optical imaging studies, and review temporal aspects of hemodynamics that have been examined across cortical depth in high spatiotemporal resolution fMRI studies. Last, we discuss the limits and potential of high spatiotemporal resolution fMRI to study laminar neurovascular coupling and neuronal processes.

Laminar microvascular transit time distribution in the mouse somatosensory cortex revealed by Dynamic Contrast Optical Coherence Tomography

The transit time distribution of blood through the cerebral microvasculature both constrains oxygen delivery and governs the kinetics of neuroimaging signals such as blood-oxygen-level-dependent functional Magnetic Resonance Imaging (BOLD fMRI). However, in spite of its importance, capillary transit time distribution has been challenging to quantify comprehensively and efficiently at the microscopic level. Here, we introduce a method, called Dynamic Contrast Optical Coherence Tomography (DyC-OCT), based on dynamic cross-sectional OCT imaging of an intravascular tracer as it passes through the field-of-view. Quantitative transit time metrics are derived from temporal analysis of the dynamic scattering signal, closely related to tracer concentration. Since DyC-OCT does not require calibration of the optical focus, quantitative accuracy is achieved even deep in highly scattering brain tissue where the focal spot degrades. After direct validation of DyC-OCT against dilution curves measured using a fluorescent plasma label in surface pial vessels, we used DyC-OCT to investigate the transit time distribution in microvasculature across the entire depth of the mouse somatosensory cortex. Laminar trends were identified, with earlier transit times and less heterogeneity in the middle cortical layers. The early transit times in the middle cortical layers may explain, at least in part, the early BOLD fMRI onset times observed in these layers. The layer-dependencies in heterogeneity may help explain how a single vascular supply manages to deliver oxygen to individual cortical layers with diverse metabolic needs.

Cerebral autoregulation in the microvasculature measured with near-infrared spectroscopy

Cerebral autoregulation (CA) is the mechanism that allows the brain to maintain a stable blood flow despite changes in blood pressure. Dynamic CA can be quantified based on continuous measurements of systemic mean arterial pressure (MAP) and global cerebral blood flow. Here, we show that dynamic CA can be quantified also from local measurements that are sensitive to the microvasculature. We used near-infrared spectroscopy (NIRS) to measure temporal changes in oxy- and deoxy-hemoglobin concentrations in the prefrontal cortex of 11 human subjects. A novel hemodynamic model translates those changes into changes of cerebral blood volume and blood flow. The interplay between them is described by transfer function analysis, specifically by a high-pass filter whose cutoff frequency describes the autoregulation efficiency. We have used pneumatic thigh cuffs to induce MAP perturbation by a fast release during rest and during hyperventilation, which is known to enhance autoregulation. Based on our model, we found that the autoregulation cutoff frequency increased during hyperventilation in comparison to normal breathing in 10 out of 11 subjects, indicating a greater autoregulation efficiency. We have shown that autoregulation can reliably be measured noninvasively in the microvasculature, opening up the possibility of localized CA monitoring with NIRS.

Ultra-high spatial resolution basal and evoked cerebral blood flow MRI of the rat brain

Cerebral blood flow (CBF) is tightly coupled to metabolism and neural activity under normal physiological conditions, and is often perturbed in disease states. The goals of this study were to implement a high-resolution (up to 50×38μm(2)) CBF MRI protocol of the rat brain, create a digital CBF atlas, report CBF values for 30+ brain structures based on the atlas, and explore applications of high-resolution CBF fMRI of forepaw stimulation. Excellent blood-flow contrasts were observed among different cortical and subcortical structures. CBF MRI showed column-like alternating bright and dark bands in the neocortices, reflecting the layout of descending arterioles and ascending venules, respectively. CBF MRI also showed lamina-like alternating bright and dark layers across the cortical thicknesses, consistent with the underlying vascular density. CBF profiles across the cortical thickness showed two peaks in layers IV and VI and a shallow trough in layer V. Whole-brain CBF was about 0.89ml/g/min, with the highest CBF values found amongst the neocortical structures (1ml/g/min, range: 0.89-1.16ml/g/min) and the lowest CBF values in the corpus callosum (0.32ml/g/min), yielding a gray:white matter CBF ratio of 3.1. CBF fMRI responses peaked across layers IV-V, whereas the BOLD fMRI responses showed a peak in the superficial layers II-III. High-resolution basal CBF MRI, evoked CBF fMRI, and CBF brain atlas can be used to study neurological disorders (such as ischemic stroke).

Large arteriolar component of oxygen delivery implies a safe margin of oxygen supply to cerebral tissue

What is the organization of cerebral microvascular oxygenation and morphology that allows adequate tissue oxygenation at different activity levels? We address this question in the mouse cerebral cortex using microscopic imaging of intravascular O2 partial pressure and blood flow combined with numerical modeling. Here we show that parenchymal arterioles are responsible for 50% of the extracted O₂ at baseline activity and the majority of the remaining O₂ exchange takes place within the first few capillary branches. Most capillaries release little O₂ at baseline acting as an O₂ reserve that is recruited during increased neuronal activity or decreased blood flow. Our results challenge the common perception that capillaries are the major site of O₂ delivery to cerebral tissue. The understanding of oxygenation distribution along arterio-capillary paths may have profound implications for the interpretation of BOLD fMRI signal and for evaluating microvascular O₂ delivery capacity to support cerebral tissue in disease.

Optical Coherence Tomography angiography reveals laminar microvascular hemodynamics in the rat somatosensory cortex during activation

The BOLD (blood-oxygen-level dependent) fMRI (functional Magnetic Resonance Imaging) signal is shaped, in part, by changes in red blood cell (RBC) content and flow across vascular compartments over time. These complex dynamics have been challenging to characterize directly due to a lack of appropriate imaging modalities. In this study, making use of infrared light scattering from RBCs, depth-resolved Optical Coherence Tomography (OCT) angiography was applied to image laminar functional hyperemia in the rat somatosensory cortex. After defining and validating depth-specific metrics for changes in RBC content and speed, laminar hemodynamic responses in microvasculature up to cortical depths of >1mm were measured during a forepaw stimulus. The results provide a comprehensive picture of when and where changes in RBC content and speed occur during and immediately following cortical activation. In summary, the earliest and largest microvascular RBC content changes occurred in the middle cortical layers, while post-stimulus undershoots were most prominent superficially. These laminar variations in positive and negative responses paralleled known distributions of excitatory and inhibitory synapses, suggesting neuronal underpinnings. Additionally, the RBC speed response consistently returned to baseline more promptly than RBC content after the stimulus across cortical layers, supporting a "flow-volume mismatch" of hemodynamic origin.

Practical steps for applying a new dynamic model to near-infrared spectroscopy measurements of hemodynamic oscillations and transient changes: implications for cerebrovascular and functional brain studies

RATIONALE AND OBJECTIVES

Perturbations in cerebral blood volume (CBV), blood flow (CBF), and metabolic rate of oxygen (CMRO2) lead to associated changes in tissue concentrations of oxy- and deoxy-hemoglobin (ΔO and ΔD), which can be measured by near-infrared spectroscopy (NIRS). A novel hemodynamic model has been introduced to relate physiological perturbations and measured quantities. We seek to use this model to determine functional traces of cbv(t) and cbf(t) - cmro2(t) from time-varying NIRS data, and cerebrovascular physiological parameters from oscillatory NIRS data (lowercase letters denote the relative changes in CBV, CBF, and CMRO2 with respect to baseline). Such a practical implementation of a quantitative hemodynamic model is an important step toward the clinical translation of NIRS.

MATERIALS AND METHODS

In the time domain, we have simulated O(t) and D(t) traces induced by cerebral activation. In the frequency domain, we have performed a new analysis of frequency-resolved measurements of cerebral hemodynamic oscillations during a paced breathing paradigm.

RESULTS

We have demonstrated that cbv(t) and cbf(t) - cmro2(t) can be reliably obtained from O(t) and D(t) using the model, and that the functional NIRS signals are delayed with respect to cbf(t) - cmro2(t) as a result of the blood transit time in the microvasculature. In the frequency domain, we have identified physiological parameters (e.g., blood transit time, cutoff frequency of autoregulation) that can be measured by frequency-resolved measurements of hemodynamic oscillations.

CONCLUSIONS

The ability to perform noninvasive measurements of cerebrovascular parameters has far-reaching clinical implications. Functional brain studies rely on measurements of CBV, CBF, and CMRO2, whereas the diagnosis and assessment of neurovascular disorders, traumatic brain injury, and stroke would benefit from measurements of local cerebral hemodynamics and autoregulation.

3D BrainCV: simultaneous visualization and analysis of cells and capillaries in a whole mouse brain with one-micron voxel resolution

Systematic cellular and vascular configurations are essential for understanding fundamental brain anatomy and metabolism. We demonstrated a 3D brainwide cellular and vascular (called 3D BrainCV) visualization and quantitative protocol for a whole mouse brain. We developed a modified Nissl staining method that quickly labeled the cells and blood vessels simultaneously in an entire mouse brain. Terabytes 3D datasets of the whole mouse brains, with unprecedented details of both individual cells and blood vessels, including capillaries, were simultaneously imaged at 1-μm voxel resolution using micro-optical sectioning tomography (MOST). For quantitative analysis, we proposed an automatic image-processing pipeline to perform brainwide vectorization and analysis of cells and blood vessels. Six representative brain regions from the cortex to the deep, including FrA, M1, PMBSF, V1, striatum, and amygdala, and six parameters, including cell number density, vascular length density, fractional vascular volume, distance from the cells to the nearest microvessel, microvascular length density, and fractional microvascular volume, had been quantitatively analyzed. The results showed that the proximity of cells to blood vessels was linearly correlated with vascular length density, rather than the cell number density. The 3D BrainCV made overall snapshots of the detailed picture of the whole brain architecture, which could be beneficial for the state comparison of the developing and diseased brain.

Dynamic model for the tissue concentration and oxygen saturation of hemoglobin in relation to blood volume, flow velocity, and oxygen consumption: Implications for functional neuroimaging and coherent hemodynamics spectroscopy (CHS)

This article presents a dynamic model that quantifies the temporal evolution of the concentration and oxygen saturation of hemoglobin in tissue, as determined by time-varying hemodynamic and metabolic parameters: blood volume, flow velocity, and oxygen consumption. This multi-compartment model determines separate contributions from arterioles, capillaries, and venules that comprise the tissue microvasculature, and treats them as a complete network, without making assumptions on the details of the architecture and morphology of the microvascular bed. A key parameter in the model is the effective blood transit time through the capillaries and its associated probability of oxygen release from hemoglobin to tissue, as described by a rate constant for oxygen diffusion. The solution of the model in the time domain predicts the signals measured by hemodynamic-based neuroimaging techniques such as functional near-infrared spectroscopy (fNIRS) and functional magnetic resonance imaging (fMRI) in response to brain activation. In the frequency domain, the model yields an analytical solution based on a phasor representation that provides a framework for quantitative spectroscopy of coherent hemodynamic oscillations. I term this novel technique coherent hemodynamics spectroscopy (CHS), and this article describes how it can be used for the assessment of cerebral autoregulation and the study of hemodynamic oscillations resulting from a variety of periodic physiological challenges, brain activation protocols, or physical maneuvers.

Critical state of energy metabolism in brain slices: the principal role of oxygen delivery and energy substrates in shaping neuronal activity

The interactive vasculo-neuro-glial system controlling energy supply in the brain is absent in vitro where energy provision is determined by experimental conditions. Despite the fact that neuronal activity is extremely energy demanding, little has been reported on the state of energy metabolism in submerged brain slices. Without this information, the arbitrarily chosen oxygenation and metabolic provisions make questionable the efficient oxidative metabolism in slices. We show that in mouse hippocampal slices (postnatal day 19–44), evoked neuronal discharges, spontaneous network activity (initiated by 4-aminopyridine), and synaptic stimulation-induced NAD(P)H autofluorescence depend strongly on the oxygen availability. Only the rate of perfusion as high as ~15 ml/min (95% O₂) provided appropriate oxygenation of a slice. Lower oxygenation resulted in the decrease of both local field potentials and spontaneous network activity as well as in significant modulation of short-term synaptic plasticity. The reduced oxygen supply considerably inhibited the oxidation phase of NAD(P)H signaling indicating that the changes in neuronal activity were paralleled by the decrease in aerobic energy metabolism. Interestingly, the dependence of neuronal activity on oxygen tension was clearly shifted toward considerably larger pO₂ values in slices when compared to in vivo conditions. With sufficient pO₂ provided by a high perfusion rate, partial substitution of glucose in ACSF for β-hydroxybutyrate, pyruvate, or lactate enhanced both oxidative metabolism and synaptic function. This suggests that the high pO₂ in brain slices is compulsory for maintaining oxidative metabolism, and glucose alone is not sufficient in fulfilling energy requirements during neuronal activity. Altogether, our results demonstrate that energy metabolism determines the functional state of neuronal network, highlighting the need for the adequate metabolic support to be insured in the in vitro experiments.

Recent progress in high-resolution functional MRI

PURPOSE OF REVIEW

For functional MRI (fMRI), as for any imaging technique, the higher the spatial resolution, the more the details it can reveal. This review will discuss the factors restricting the spatial resolution of fMRI, describe high-resolution fMRI (HR-fMRI) applications in neuroscience and outline a few research areas for future HR-fMRI studies.

RECENT FINDINGS

HR-fMRI has been successfully used to map fine cortical architectures and reveal cortical laminar structures and subcortical structures. HR-fMRI has also played important roles in resolving controversies regarding modular representations in the ventral visual pathway and interpretations of multivariate pattern analysis results.

SUMMARY

Real-time HR-fMRI as well as high-resolution anatomical MRI may emerge as indispensable tools for surgical planning, diagnosis of neurological diseases and targeting of deep brain stimulation.

Microvascular organization of the cat inferior colliculus

Brain neural activity depends critically on the blood supply to a given structure. The blood supply can differ within and between divisions, which may have functional significance. We analyzed the microvascular organization of the cat inferior colliculus (IC) to determine if the capillary distribution is homogenous throughout. The IC consists of the central nucleus (CN), the dorsal cortex (DC), and the lateral cortex (LC), each with different roles in auditory behavior and perception. Plastic-embedded tissue was studied from adult cats in 1-μm thick semi-thin sections stained with toluidine blue; tissue was sampled from the IC in a caudal-rostral series of sections. The architectonic subdivisions were drawn independently based on Golgi impregnations. We used the nearest neighbor distance (NND) method to quantify capillary density between subdivisions. Overall, the distribution of capillary density was non-homogenous across the IC. We found significant capillary NND differences between the CN and LC (Mann-Whitney test; p ≤ 0.05), CN and DC (Mann-Whitney test; p ≤ 0.05), and LC and DC (Mann-Whitney test; p ≤ 0.05). The CN had the lowest NND values among all three divisions, indicating the highest capillary density. NND values changed gradually as analysis moved from the center of the IC towards the periphery. The significantly higher microvascular density in the CN may imply that the lemniscal auditory pathway has higher levels of blood flow and metabolic activity than non-lemniscal areas of the IC. The non-homogenous microvascular organization of the IC supports parcellation schemes that delineate three major subdivisions and confirms that the borders between the three regions are not sharp.

Cerebral oxygen delivery and consumption during evoked neural activity

Increases in neural activity evoke increases in the delivery and consumption of oxygen. Beyond observations of cerebral tissue and blood oxygen, the role and properties of cerebral oxygen delivery and consumption during changes in brain function are not well understood. This work overviews the current knowledge of functional oxygen delivery and consumption and introduces recent and preliminary findings to explore the mechanisms by which oxygen is delivered to tissue as well as the temporal dynamics of oxygen metabolism. Vascular oxygen tension measurements have shown that a relatively large amount of oxygen exits pial arterioles prior to capillaries. Additionally, increases in cerebral blood flow (CBF) induced by evoked neural activation are accompanied by arterial vasodilation and also by increases in arteriolar oxygenation. This increase contributes not only to the down-stream delivery of oxygen to tissue, but also to delivery of additional oxygen to extra-vascular spaces surrounding the arterioles. On the other hand, the changes in tissue oxygen tension due to functional increases in oxygen consumption have been investigated using a method to suppress the evoked CBF response. The functional decreases in tissue oxygen tension induced by increases in oxygen consumption are slow to evoked changes in CBF under control conditions. Preliminary findings obtained using flavoprotein autofluorescence imaging suggest cellular oxidative metabolism changes at a faster rate than the average changes in tissue oxygen. These issues are important in the determination of the dynamic changes in tissue oxygen metabolism from hemoglobin-based imaging techniques such as blood oxygenation-level dependent functional magnetic resonance imaging (fMRI).

Optical monitoring of oxygen tension in cortical microvessels with confocal microscopy

Evaluating cerebral oxygenation is of critical importance for the understanding of brain function and several neuropathologies. Although several techniques exist for measuring cerebral oxygenation in vivo, the most widely accepted techniques offer limited spatial resolution. We have developed a confocal imaging system for minimally invasive measurement of oxygen tension (pO(2)) in cerebral microvessels with high spatial and temporal resolution. The system relies on the phosphorescence quenching method using exogenous porphyrin-based dendritic oxygen probes. Here we present high-resolution phosphorescence images of cortical microvasculature and temporal pO(2) profiles from multiple locations in response to varied fraction of inspired oxygen and functional activation.

Oxygen transport in brain tissue

Oxygen is essential to maintaining normal brain function. A large body of evidence suggests that the partial pressure of oxygen (pO(2)) in brain tissue is physiologically maintained within a narrow range in accordance with region-specific brain activity. Since the transportation of oxygen in the brain tissue is mainly driven by a diffusion process caused by a concentration gradient of oxygen from blood to cells, the spatial organization of the vascular system, in which the oxygen content is higher than in tissue, is a key factor for maintaining effective transportation. In addition, a local mechanism that controls energy demand and blood flow supply plays a critical role in moment-to-moment adjustment of tissue pO(2) in response to dynamically varying brain activity. In this review, we discuss the spatiotemporal structures of brain tissue oxygen transport in relation to local brain activity based on recent reports of tissue pO(2) measurements with polarographic oxygen microsensors in combination with simultaneous recordings of neural activity and local cerebral blood flow in anesthetized animal models. Although a physiological mechanism of oxygen level sensing and control of oxygen transport remains largely unknown, theoretical models of oxygen transport are a powerful tool for better understanding the short-term and long-term effects of local changes in oxygen demand and supply. Finally, emerging new techniques for three-dimensional imaging of the spatiotemporal dynamics of pO(2) map may enable us to provide a whole picture of how the physiological system controls the balance between demand and supply of oxygen during both normal and pathological brain activity.

Measurement of cerebral blood volume in mouse brain regions using micro-computed tomography

Micro-computed tomography (micro-CT) is an X-ray imaging technique that can produce detailed 3D images of cerebral vasculature. This paper describes the development of a novel method for using micro-CT to measure cerebral blood volume (CBV) in the mouse brain. As an application of the methodology, we test the hypotheses that differences in CBV exist over anatomical brain regions and that high energy demanding primary sensory regions of the cortex have locally elevated CBV, which may reflect a vascular specialization. CBV was measured as the percentage of tissue space occupied by a radio-opaque silicon rubber that fills the vasculature. To ensure accuracy of the CBV measurements, several innovative refinements were made to standard micro-CT specimen preparation and analysis procedures. Key features of the described method are vascular perfusion under controlled pressure, registration of the micro-CT images to an MRI anatomical brain atlas and re-scaling of micro-CT intensities to CBV units with selectable exclusion of major vessels. Histological validation of the vascular perfusion showed that the average percentage of vessels filled was 93+/-3%. Comparison of thirteen brain regions in nine mice revealed significant differences in CBV between regions (p<0.0001) while cortical maps showed that primary visual and auditory areas have higher CBV than primary somatosensory areas.

Developmental and neurochemical features of cholinergic neurons in the murine cerebral cortex

Background

The existence and role of intrinsic cholinergic cells in the cerebral cortex is controversial, because of their variable localization and morphology in different mammalian species. We have applied choline acetyltransferase (ChAT) immunocytochemistry to study the distribution of cholinergic neurons in the murine cerebral cortex, in the adult and during postnatal development. For more precise neurochemical identification of these neurons, the possible colocalization of ChAT with different markers of cortical neuronal populations has been analyzed by confocal microscopy. This method was also used to verify the relationship between cholinergic cells and cortical microvessels.

Results

ChAT positive cells appeared at the end of the first postnatal week. Their density dramatically increased at the beginning of the second postnatal week, during which it remained higher than in perinatal and adult stages. In the adult neocortex, cholinergic neurons were particularly expressed in the somatosensory area, although their density was also significant in visual and auditory areas. ChAT positive cells tended to be scarce in other regions. They were mainly localized in the supragranular layers and displayed a fusiform/bipolar morphology.

The colocalization of ChAT with pyramidal neuron markers was negligible. On the other hand, more than half of the cholinergic neurons contained calretinin, but none of them expressed parvalbumin or calbindin. However, only a fraction of the ChAT positive cells during development and very few in adulthood turned out to be GABAergic, as judged from expression of GABA and its biosynthetic enzymes GAD67/65. Consistently, ChAT showed no localization with interneurons expressing green fluorescent protein under control of the GAD67 promoter in the adult neocortex. Finally, the cortical cholinergic cells often showed close association with the microvessel walls, as identified with the gliovascular marker aquaporin 4, supporting previous hypotheses on the role of cholinergic cells in modulating the cortical microcirculation.

Conclusion

Our results show that the development of the intracortical cholinergic system accompanies the cortical rearrangements during the second postnatal week, a crucial stage for the establishment of cortical cytoarchitecture and for synaptogenesis. Although intrinsic ChAT positive cells usually expressed calretinin, they displayed a variable GABAergic phenotype depending on marker and on cortical developmental stage.

Dynamic magnetic resonance imaging of cerebral blood flow using arterial spin labeling

Modern functional neuroimaging techniques, including positron emission tomography, optical imaging of intrinsic signals, and magnetic resonance imaging (MRI) rely on a tight coupling between neural activity and cerebral blood flow (CBF) to visualize brain activity using CBF as a surrogate marker. Because the spatial and temporal resolution of neuroimaging modalities is ultimately determined by the spatial and temporal specificity of the underlying hemodynamic signals, characterization of the spatial and temporal profiles of the hemodynamic response to focal brain stimulation is of paramount importance for the correct interpretation and quantification of functional data. The ability to properly measure and quantify CBF with MRI is a major determinant of progress in our understanding of brain function. We review the dynamic arterial spin labeling (DASL) method to measure CBF and the CBF functional response with high temporal resolution.

Apparent diffusion time of oxygen from blood to tissue in rat cerebral cortex: implication for tissue oxygen dynamics during brain functions

To investigate the dynamics of tissue oxygen demand and supply during brain functions, we simultaneously recorded Po(2) and local cerebral blood flow (LCBF) with an oxygen microelectrode and laser Doppler flowmetry, respectively, in rat somatosensory cortex. Electrical hindlimb stimuli were applied for 1, 2, and 5 s to vary the duration of evoked cerebral metabolic rate of oxygen (CMR(O(2))). The electrical stimulation induced a robust increase in Po(2) (4-9 Torr at peak) after an increase in LCBF (14-26% at peak). A consistent lag of approximately 1.2 s (0.6-2.3 s for individual animals) in the Po(2) relative to LCBF was found, irrespective of stimulus length. It is argued that the lag in Po(2) was predominantly caused by the time required for oxygen to diffuse through tissue. During brain functions, the supply of fresh oxygen further lagged because of the latency of LCBF onset ( approximately 0.4 s). The results indicate that the tissue oxygen supports excess demand until the arrival of fresh oxygen. However, a large drop in Po(2) was not observed, indicating that the evoked neural activity demands little extra oxygen or that the time course of excess demand is as slow as the increase in supply. Thus the dynamics of Po(2) during brain functions predominantly depend on the time course of LCBF. Possible factors influencing the lag between demand and supply are discussed, including vascular spacing, reactivity of the vessels, and diffusivity of oxygen.

Depth-resolved optical imaging and microscopy of vascular compartment dynamics during somatosensory stimulation

The cortical hemodynamic response to somatosensory stimulus is investigated at the level of individual vascular compartments using both depth-resolved optical imaging and in-vivo two-photon microscopy. We utilize a new imaging and spatiotemporal analysis approach that exploits the different characteristic dynamics of responding arteries, arterioles, capillaries and veins to isolate their three-dimensional spatial extent within the cortex. This spatial delineation is validated using vascular casts. Temporal delineation is supported by in-vivo two-photon microscopy of the temporal dynamics and vascular mechanisms of the arteriolar and venous responses. Using these techniques we have been able to characterize the roles of the different vascular compartments in generating and controlling the hemodynamic response to somatosensory stimulus. We find that changes in arteriolar total hemoglobin concentration agree well with arteriolar dilation dynamics, which in turn correspond closely with changes in venous blood flow. For 4-s stimuli, we see only small changes in venous hemoglobin concentration, and do not detect measurable dilation or ballooning in the veins. Instead, we see significant evidence of capillary hyperemia. We compare our findings to historical observations of the composite hemodynamic response from other modalities including functional magnetic resonance imaging. Implications of our results are discussed with respect to mathematical models of cortical hemodynamics, and to current theories on the mechanisms underlying neurovascular coupling. We also conclude that our spatiotemporal analysis approach is capable of isolating and localizing signals from the capillary bed local to neuronal activation, and holds promise for improving the specificity of other hemodynamic imaging modalities.

Activity-induced tissue oxygenation changes in rat cerebellar cortex: interplay of postsynaptic activation and blood flow

Functional neuroimaging relies on the robust coupling between neuronal activity, metabolism and cerebral blood flow (CBF), but the physiological basis of the neuroimaging signals is still poorly understood. We examined the mechanisms of activity-dependent changes in tissue oxygenation in relation to variations in CBF responses and postsynaptic activity in rat cerebellar cortex. To increase synaptic activity we stimulated the monosynaptic, glutamatergic climbing fibres that excite Purkinje cells via AMPA receptors. We used local field potentials to indicate synaptic activity, and recorded tissue oxygen partial pressure (P(tiss,O2)) by polarographic microelectrodes, and CBF using laser-Doppler flowmetry. The disappearance rate of oxygen in the tissue increased linearly with synaptic activity. This indicated that, without a threshold, oxygen consumption increased as a linear function of synaptic activity. The reduction in P(tiss,O2) preceded the rise in CBF. The time integral (area) of the negative P(tiss,O2) response increased non-linearly showing saturation at high levels of synaptic activity, concomitant with a steep rise in CBF. This was accompanied by a positive change in P(tiss,O2). Neuronal nitric oxide synthase inhibition enhanced the initial negative P(tiss,O2) response ('dip'), while attenuating the evoked CBF increase and positive P(tiss,O2) response equally. This indicates that increases in CBF counteract activity-induced reductions in P(tiss,O2), and suggests the presence of a tissue oxygen reserve. The changes in P(tiss,O2) and CBF were strongly attenuated by AMPA receptor blockade. Our findings suggest an inverse relationship between negative P(tiss,O2) and CBF responses, and provide direct in vivo evidence for a tight coupling between activity in postsynaptic AMPA receptors and cerebellar oxygen consumption.