Laminar analysis of cerebral blood flow in cortex of rats by laser-Doppler flowmetry: a pilot study

Neurovascular coupling and oxygenation are decreased in hippocampus compared to neocortex because of microvascular differences

The hippocampus is essential for spatial and episodic memory but is damaged early in Alzheimer's disease and is very sensitive to hypoxia. Understanding how it regulates its oxygen supply is therefore key for designing interventions to preserve its function. However, studies of neurovascular function in the hippocampus in vivo have been limited by its relative inaccessibility. Here we compared hippocampal and visual cortical neurovascular function in awake mice, using two photon imaging of individual neurons and vessels and measures of regional blood flow and haemoglobin oxygenation. We show that blood flow, blood oxygenation and neurovascular coupling were decreased in the hippocampus compared to neocortex, because of differences in both the vascular network and pericyte and endothelial cell function. Modelling oxygen diffusion indicates that these features of the hippocampal vasculature may restrict oxygen availability and could explain its sensitivity to damage during neurological conditions, including Alzheimer's disease, where the brain's energy supply is decreased.

Assessment of Cerebral Vasodilatory Capacity as Part of Catheter-Based Cerebral Angiography

BACKGROUND AND PURPOSE

Cerebral vasodilatory capacity assessment for risk stratification in patients with extracranial arterial stenosis or occlusion may be useful. We describe a new method that assesses cerebral vasodilatory capacity as part of catheter-based cerebral angiography.

METHODS

We prospectively assessed regional cerebral blood volume (rCBV) in the arterial distribution of interest using a controlled contrast injection in the common carotid or the subclavian arteries. rCBV maps were created using a predefined algorithm based on contrast distribution in the venous phase (voxel size: .466 mm³ ). rCBV maps were acquired again after selective administration of intra-arterial nicardipine (2.0 mg) distal to the stenosis. Two independent observers graded the change in rCBV in 10 predefined anatomical regions within the tributaries of the artery of interest (0 = reduction, 1 = no change, 2 = increase) and total rCBV change scores were summated.

RESULTS

Twenty-five patients with internal carotid artery stenosis (n = 18; 0-90% in severity) or extracranial vertebral artery stenosis (n = 7; 0-100% in severity) were assessed. There was an increase in rCBV in a tributary of the artery of interest in 18 of 25 after intra-arterial nicardipine (mean score: 11.98; range 0-19.5). There was no change or decrease in rCBV in 7 of 25 patients. The mean rCBV change score was similar in patients with an assessment of internal carotid artery or vertebral artery distributions (12.2 ± 5.3; 11.4 ± 2.5; P = .68).

CONCLUSION

Selective vasodilatory response to intra-arterial nicardipine in the affected arterial distribution during catheter-based cerebral angiography may provide new data for risk stratification.

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.

Dissociation between CSD-Evoked Metabolic Perturbations and Meningeal Afferent Activation and Sensitization: Implications for Mechanisms of Migraine Headache Onset

The onset of the headache phase during attacks of migraine with aura, which occur in ∼30% of migraineurs, is believed to involve cortical spreading depression (CSD) and the ensuing activation and sensitization of primary afferent neurons that innervate the intracranial meninges, and their related large vessels. The mechanism by which CSD enhances the activity and mechanosensitivity of meningeal afferents remains poorly understood, but may involve cortical metabolic perturbations. We used extracellular single-unit recording of meningeal afferent activity and monitored changes in cortical blood flow and tissue partial pressure of oxygen (tpO₂) in anesthetized male rats to test whether the prolonged cortical hypoperfusion and reduction in tissue oxygenation that occur in the wake of CSD contribute to meningeal nociception. Suppression of CSD-evoked cortical hypoperfusion with the cyclooxygenase inhibitor naproxen blocked the reduction in cortical tpO₂, but had no effect on the activation of meningeal afferents. Naproxen, however, distinctly prevented CSD-induced afferent mechanical sensitization. Counteracting the CSD-evoked persistent hypoperfusion and reduced tpO₂ by preemptively increasing cortical blood flow using the ATP-sensitive potassium [K(ATP)] channel opener levcromakalim did not inhibit the sensitization of meningeal afferents, but prevented their activation. Our data show that the cortical hypoperfusion and reduction in tpO₂ that occur in the wake of CSD can be dissociated from the activation and mechanical sensitization of meningeal afferent responses, suggesting that the metabolic changes do not contribute directly to these neuronal nociceptive responses.SIGNIFICANCE STATEMENT Cortical spreading depression (CSD)-evoked activation and mechanical sensitization of meningeal afferents is thought to mediate the headache phase in migraine with aura. We report that blocking the CSD-evoked cortical hypoperfusion and reduced tissue partial pressure of oxygen by cyclooxygenase inhibition is associated with the inhibition of the afferent sensitization, but not their activation. Normalization of these CSD-evoked metabolic perturbations by activating K(ATP) channels is, however, associated with the inhibition of afferent activation but not sensitization. These results question the contribution of cortical metabolic perturbations to the triggering mechanism underlying meningeal nociception and the ensuing headache in migraine with aura, further point to distinct mechanisms underlying the activation and sensitization of meningeal afferents in migraine, and highlight the need to target both processes for an effective migraine therapy.

The CGRP receptor antagonist BIBN4096 inhibits prolonged meningeal afferent activation evoked by brief local K+ stimulation but not cortical spreading depression-induced afferent sensitization

Calcitonin gene-related peptide mediates K⁺-evoked delayed and prolonged activation of cranial meningeal afferents but does not contribute to their enhanced responsiveness following cortical spreading depression.

Introduction:

Cortical spreading depression (CSD) is believed to promote migraine headache by enhancing the activity and mechanosensitivity of trigeminal intracranial meningeal afferents. One putative mechanism underlying this afferent response involves an acute excitation of meningeal afferents by cortical efflux of K⁺ and the ensuing antidromic release of proinflammatory sensory neuropeptides, such as calcitonin gene-related peptide (CGRP).

Objectives:

We sought to investigate whether (1) a brief meningeal K⁺ stimulus leads to CGRP-dependent enhancement of meningeal afferent responses and (2) CSD-induced meningeal afferent activation and sensitization involve CGRP receptor signaling.

Methods:

Extracellular single-unit recording were used to record the activity of meningeal afferents in anesthetized male rats. Stimulations included a brief meningeal application of K⁺ or induction of CSD in the frontal cortex using pinprick. Cortical spreading depression was documented by recording changes in cerebral blood flow using laser Doppler flowmetery. Calcitonin gene-related peptide receptor activity was inhibited with BIBN4096 (333 μM, i.v.).

Results:

Meningeal K⁺ stimulation acutely activated 86% of the afferents tested and also promoted in ∼65% of the afferents a 3-fold increase in ongoing activity, which was delayed by 23.3 ± 4.1 minutes and lasted for 22.2 ± 5.6 minutes. K⁺ stimulation did not promote mechanical sensitization. Pretreatment with BIBN4096 suppressed the K⁺-induced delayed afferent activation, reduced CSD-evoked cortical hyperemia, but had no effect on the enhanced activation or mechanical sensitization of meningeal afferents following CSD.

Conclusion:

While CGRP-mediated activation of meningeal afferents evoked by cortical efflux of K⁺ could promote headache, acute activation of CGRP receptors may not play a key role in mediating CSD-evoked headache.

'Spreading depression of Leão' and its emerging relevance to acute brain injury in humans

A new research field in translational neuroscience has opened as a result of the recognition since 2002 that "spreading depression of Leão" can be detected in many patients with acute brain injury, whether vascular and spontaneous, or traumatic in origin, as well as in those many individuals experiencing the visual (or sensorimotor) aura of migraine. In this review, we trace from their first description in rabbits through to their detection and study in migraine and the injured human brain, and from our personal perspectives, the evolution of understanding of the importance of spread of mass depolarisations in cerebral grey matter. Detection of spontaneous depolarisations occurring and spreading in the periphery or penumbra of experimental focal cortical ischemic lesions and of their adverse effects on the cerebral cortical microcirculation and on the tissue glucose and oxygen pools has led to clearer concepts of how ischaemic and traumatic lesions evolve in the injured human brain, and of how to seek to improve clinical management and outcome. Recognition of the likely fundamental significance of spreading depolarisations for this wide range of serious acute encephalopathies in humans provides a powerful case for a fresh examination of neuroprotection strategies.

Differential contribution of COX-1 and COX-2 derived prostanoids to cortical spreading depression-Evoked cerebral oligemia

Cortical spreading depression (CSD) is considered a significant phenomenon for human neurological conditions and one of its key signatures is the development of persistent cortical oligemia. The factors underlying this reduction in cerebral blood flow (CBF) remain incompletely understood but may involve locally elaborated vasoconstricting eicosanoids. We employed laser Doppler flowmetry in urethane-anesthetized rats, together with a local pharmacological blockade approach, to test the relative contribution of cyclooxygenase (COX)-derived prostanoids to the oligemic response following CSD. Administration of the non-selective COX inhibitor naproxen completely inhibited the oligemic response. Selective inhibition of COX-1 with SC-560 preferentially reduced the early reduction in CBF while selective COX-2 inhibition with NS-398 affected only the later response. Blocking the action of thromboxane A2 (TXA2), using the selective thromboxane synthase inhibitor ozagrel, reduced only the initial CBF decrease, while inhibition of prostaglandin F2alpha action, using the selective FP receptor antagonist AL-8810, blocked the later phase of the oligemia. Our results suggest that the long-lasting oligemia following CSD consists of at least two distinct temporal phases, mediated by preferential actions of COX-1- and COX-2-derived prostanoids: an initial phase mediated by COX-1 that involves TXA2 followed by a later phase, mediated by COX-2 and PGF2alpha.

Mapping cerebral pulse pressure and arterial compliance over the adult lifespan with optical imaging

Cerebrovascular health is important for maintaining a high level of cognitive performance, not only in old age, but also throughout the lifespan. Recently, it was first demonstrated that diffuse optical imaging measures of pulse amplitude and arterial compliance can provide estimates of cerebral arterial health throughout the cortex, and were associated with age, estimated cardiorespiratory fitness (eCRF), neuroanatomy and cognitive function in older adults (aged 55-87). The current study replicates and extends the original findings using a broader age range (a new adult sample aged 18-75), longer recording periods (360 s), and a more extensive optical montage (1536 channels). These methodological improvements represent a 5-fold increase in recording time and a 4-fold increase in coverage compared to the initial study. Results show that reliability for both pulse amplitude and compliance measures across recording blocks was very high (r(45) = .99 and .75, respectively). Pulse amplitude and pulse pressure were shown to correlate with age across the broader age range. We also found correlations between arterial health and both cortical and subcortical gray matter volumes. Additionally, we replicated the correlations between arterial compliance and age, eCRF, global brain atrophy, and cognitive flexibility. New regional analyses revealed that higher performance on the operation span (OSPAN) working memory task was associated with greater localized arterial compliance in frontoparietal cortex, but not with global arterial compliance. Further, greater arterial compliance in frontoparietal regions was associated with younger age and higher eCRF. These associations were not present in the visual cortex. The current study not only replicates the initial one in a sample including a much wider age range, but also provides new evidence showing that frontoparietal regions may be especially vulnerable to vascular degeneration during brain aging, with potential functional consequences in cognition.

Dynamic diameter response of intraparenchymal penetrating arteries during cortical spreading depression and elimination of vasoreactivity to hypercapnia in anesthetized mice

Cortical spreading depression (CSD) induces marked hyperemia with a transient decrease of regional cerebral blood flow (rCBF), followed by sustained oligemia. To further understand the microcirculatory mechanisms associated with CSD, we examined the temporal changes of diameter of intraparenchymal penetrating arteries during CSD. In urethane-anesthetized mice, the diameter of single penetrating arteries at three depths was measured using two-photon microscopy during passage of repeated CSD, with continuous recordings of direct current potential and rCBF. The first CSD elicited marked constriction superimposed on the upstrokes of profound dilation throughout each depth of the penetrating artery, and the vasoreaction temporally corresponded to the change of rCBF. Second or later CSD elicited marked dilation with little or no constriction phase throughout each depth, and the vasodilation also temporally corresponded to the increase of rCBF. Furthermore, the peak dilation showed good negative correlations with basal diameter and increase of rCBF. Vasodilation induced by 5% CO2 inhalation was significantly suppressed after CSD passage at any depth as well as hyperperfusion. These results may indicate that CSD-induced rCBF changes mainly reflect the diametric changes of the intraparenchymal arteries, despite the elimination of responsiveness to hypercapnia.

Glutamate-system defects behind psychiatric manifestations in a familial hemiplegic migraine type 2 disease-mutation mouse model

Migraine is a complex brain disorder, and understanding the complexity of this prevalent disease could improve quality of life for millions of people. Familial Hemiplegic Migraine type 2 (FHM2) is a subtype of migraine with aura and co-morbidities like epilepsy/seizures, cognitive impairments and psychiatric manifestations, such as obsessive-compulsive disorder (OCD). FHM2 disease-mutations locate to the ATP1A2 gene encoding the astrocyte-located α₂-isoform of the sodium-potassium pump (α₂Na⁺/K⁺-ATPase). We show that knock-in mice heterozygous for the FHM2-associated G301R-mutation (α₂^+/G301R) phenocopy several FHM2-relevant disease traits e.g., by mimicking mood depression and OCD. In vitro studies showed impaired glutamate uptake in hippocampal mixed astrocyte-neuron cultures from α₂^G301R/G301R E17 embryonic mice, and moreover, induction of cortical spreading depression (CSD) resulted in reduced recovery in α₂^+/G301R male mice. Moreover, NMDA-type glutamate receptor antagonists or progestin-only treatment reverted specific α₂^+/G301R behavioral phenotypes. Our findings demonstrate that studies of an in vivo relevant FHM2 disease knock-in mouse model provide a link between the female sex hormone cycle and the glutamate system and a link to co-morbid psychiatric manifestations of FHM2.

Spreading Depression, Spreading Depolarizations, and the Cerebral Vasculature

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

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.

GABAA Receptor-Mediated Bidirectional Control of Synaptic Activity, Intracellular Ca2+, Cerebral Blood Flow, and Oxygen Consumption in Mouse Somatosensory Cortex In Vivo

Neural activity regulates local increases in cerebral blood flow (ΔCBF) and the cortical metabolic rate of oxygen (ΔCMRO2) that constitutes the basis of BOLD functional neuroimaging signals. Glutamate signaling plays a key role in brain vascular and metabolic control; however, the modulatory effect of GABA is incompletely understood. Here we performed in vivo studies in mice to investigate how THIP (which tonically activates extrasynaptic GABAARs) and Zolpidem (a positive allosteric modulator of synaptic GABAARs) impact stimulation-induced ΔCBF, ΔCMRO2, local field potentials (LFPs), and fluorescent cytosolic Ca(2+) transients in neurons and astrocytes. Low concentrations of THIP increased ΔCBF and ΔCMRO2 at low stimulation frequencies. These responses were coupled to increased synaptic activity as indicated by LFP responses, and to Ca(2+) activities in neurons and astrocytes. Intermediate and high concentrations of THIP suppressed ΔCBF and ΔCMRO2 at high stimulation frequencies. Zolpidem had similar but less-pronounced effects, with similar dependence on drug concentration and stimulation frequency. Our present findings suggest that slight increases in both synaptic and extrasynaptic GABAAR activity might selectively gate and amplify transient low-frequency somatosensory inputs, filter out high-frequency inputs, and enhance vascular and metabolic responses that are likely to be reflected in BOLD functional neuroimaging signals.

Device for simultaneous positron emission tomography, laser speckle imaging and RGB reflectometry: validation and application to cortical spreading depression and brain ischemia in rats

Brain function critically relies on the supply with energy substrates (oxygen and glucose) via blood flow. Alterations in energy demand as during neuronal activation induce dynamic changes in substrate fluxes and blood flow. To study the complex system that regulates cerebral metabolism requires the combination of methods for the simultaneous assessment of multiple parameters. We developed a multimodal imaging device to combine positron emission tomography (PET) with laser speckle imaging (LSI) and RGB reflectometry (RGBR). Depending on the radiotracer, PET provides 3-dimensional quantitative information of specific molecular processes, while LSI and RGBR measure cerebral blood flow (CBF) and hemoglobin oxygenation at high temporal and spatial resolution. We first tested the functional capability of each modality within our system and showed that interference between the modalities is negligible. We then cross-calibrated the system by simultaneously measuring absolute CBF using (15)O-H2O PET (CBF(PET)) and the inverse correlation time (ICT), the LSI surrogate for CBF. ICT and CBF(PET) correlated in multiple measurements in individuals as well as across different animals (R(2)=0.87, n=44 measurements) indicating that ICT can be used for absolute quantitative assessment of CBF. To demonstrate the potential of the combined system, we applied it to cortical spreading depression (CSD), a wave of transient cellular depolarization that served here as a model system for neurovascular and neurometabolic coupling. We analyzed time courses of hemoglobin oxygenation and CBF alterations coupled to CSD, and simultaneously measured regional uptake of (18)F-2-fluoro-2-deoxy-D-glucose ((18)F-FDG) used as a radiotracer for regional glucose metabolism, in response to a single CSD and to a cluster of CSD waves. With this unique combination, we characterized the changes in cerebral metabolic rate of oxygen (CMRO2) in real-time and showed a correlation between (18)F-FDG uptake and the number of CSD waves that passed the local tissue. Finally, we examined CSD spontaneously occurring during focal ischemia also referred to as peri-infarct depolarization (PID). In the vicinity of the ischemic territory, we observed PIDs that were characterized by reduced CMRO2 and increased oxygen extraction fraction (OEF), indicating a limitation of oxygen supply. Simultaneously measured PET showed an increased (18)F-FDG uptake in these regions. Our combined system proved to be a novel tool for the simultaneous study of dynamic spatiotemporal alterations of cortical blood flow, oxygen metabolism and glucose consumption under normal and pathologic conditions.

Increased 20-HETE synthesis explains reduced cerebral blood flow but not impaired neurovascular coupling after cortical spreading depression in rat cerebral cortex

Cortical spreading depression (CSD) is associated with release of arachidonic acid, impaired neurovascular coupling, and reduced cerebral blood flow (CBF), caused by cortical vasoconstriction. We tested the hypothesis that the released arachidonic acid is metabolized by the cytochrome P450 enzyme to produce the vasoconstrictor 20-hydroxyeicosatetraenoic acid (20-HETE), and that this mechanism explains cortical vasoconstriction and vascular dysfunction after CSD. CSD was induced in the frontal cortex of rats and the cortical electrical activity and local field potentials recorded by glass microelectrodes, CBF by laser Doppler flowmetry, and tissue oxygen tension (tpO(2)) using polarographic microelectrodes. 20-HETE synthesis was measured in parallel experiments in cortical brain slices exposed to CSD. We used the specific inhibitor HET0016 (N-hydroxy-N'-(4-n-butyl-2-methylphenyl)formamidine) to block 20-HETE synthesis. CSD increased 20-HETE synthesis in brain slices for 120 min, and the time course of the increase in 20-HETE paralleled the reduction in CBF after CSD in vivo. HET0016 blocked the CSD-induced increase in 20-HETE synthesis and ameliorated the persistent reduction in CBF, but not the impaired neurovascular coupling after CSD. These findings suggest that CSD-induced increments in 20-HETE cause the reduction in CBF after CSD and that the attenuation of stimulation-induced CBF responses after CSD has a different mechanism. We suggest that blockade of 20-HETE synthesis may be clinically relevant to ameliorate reduced CBF in patients with migraine and acute brain cortex injuries.

Assessment of retinal and choroidal blood flow changes using laser Doppler flowmetry in rats

PURPOSE

A new noninvasive laser Doppler flowmetry (LDF) probe (one emitting fiber surrounded by a ring of eight collecting fibers, 1-mm interaxis distance) was tested for its sensitivity to assess the retinal/choroidal blood flow variations in response to hypercapnia, hyperoxia, diverse vasoactive agents and following retinal arteries photocoagulation in the rat.

MATERIALS AND METHODS

After pupil dilation, a LDF probe was placed in contact to the cornea of anesthetized rats in the optic axis. Hypercapnia and hyperoxia were induced by inhalation of CO(2) (8% in medical air) and O(2) (100%) while pharmacological agents were injected intravitreously. The relative contribution of the choroidal circulation to the LDF signal was estimated after retinal artery occlusion by photocoagulation.

RESULTS

Blood flow was significantly increased by hypercapnia (18%), adenosine (14%) and sodium nitroprusside (16%) as compared to baseline values while it was decreased by hyperoxia (-8%) and endothelin-1 (-11%). Photocoagulation of retinal arteries significantly decreased blood flow level (-45%).

CONCLUSIONS

Although choroidal circulation most likely contributes to the LDF signal in this setting, the results demonstrate that LDF represents a suitable in vivo noninvasive technique to monitor online relative reactivity of retinal perfusion to metabolic or pharmacological challenge. This technique could be used for repeatedly assessing blood flow reactivity in rodent models of ocular diseases.

Activity-dependent increases in local oxygen consumption correlate with postsynaptic currents in the mouse cerebellum in vivo

Evoked neural activity correlates strongly with rises in cerebral metabolic rate of oxygen (CMRO(2)) and cerebral blood flow (CBF). Activity-dependent rises in CMRO(2) fluctuate with ATP turnover due to ion pumping. In vitro studies suggest that increases in cytosolic Ca(2+) stimulate oxidative metabolism via mitochondrial signaling, but whether this also occurs in the intact brain is unknown. Here we applied a pharmacological approach to dissect the effects of ionic currents and cytosolic Ca(2+) rises of neuronal origin on activity-dependent rises in CMRO(2). We used two-photon microscopy and current source density analysis to study real-time Ca(2+) dynamics and transmembrane ionic currents in relation to CMRO(2) in the mouse cerebellar cortex in vivo. We report a direct correlation between CMRO(2) and summed (i.e., the sum of excitatory, negative currents during the whole stimulation period) field EPSCs (∑fEPSCs) in Purkinje cells (PCs) in response to stimulation of the climbing fiber (CF) pathway. Blocking stimulus-evoked rises in cytosolic Ca(2+) in PCs with the P/Q-type channel blocker ω-agatoxin-IVA (ω-AGA), or the GABA(A) receptor agonist muscimol, did not lead to a time-locked reduction in CMRO(2), and excitatory synaptic or action potential currents. During stimulation, neither ω-AGA or (μ-oxo)-bis-(trans-formatotetramine-ruthenium) (Ru360), a mitochondrial Ca(2+) uniporter inhibitor, affected the ratio of CMRO(2) to fEPSCs or evoked local field potentials. However, baseline CBF and CMRO(2) decreased gradually with Ru360. Our data suggest that in vivo activity-dependent rises in CMRO(2) are correlated with synaptic currents and postsynaptic spiking in PCs. Our study did not reveal a unique role of neuronal cytosolic Ca(2+) signals in controlling CMRO(2) increases during CF stimulation.

Neuronal inhibition and excitation, and the dichotomic control of brain hemodynamic and oxygen responses

Brain's electrical activity correlates strongly to changes in cerebral blood flow (CBF) and the cerebral metabolic rate of oxygen (CMRO(2)). Subthreshold synaptic processes correlate better than the spike rates of principal neurons to CBF, CMRO(2) and positive BOLD signals. Stimulation-induced rises in CMRO(2) are controlled by the ATP turnover, which depends on the energy used to fuel the Na,K-ATPase to reestablish ionic gradients, while stimulation-induced CBF responses to a large extent are controlled by mechanisms that depend on Ca(2+) rises in neurons and astrocytes. This dichotomy of metabolic and vascular control explains the gap between the stimulation-induced rises in CMRO(2) and CBF, and in turn the BOLD signal. Activity-dependent rises in CBF and CMRO(2) vary within and between brain regions due to differences in ATP turnover and Ca(2+)-dependent mechanisms. Nerve cells produce and release vasodilators that evoke positive BOLD signals, while the mechanisms that control negative BOLD signals by activity-dependent vasoconstriction are less well understood. Activation of both excitatory and inhibitory neurons produces rises in CBF and positive BOLD signals, while negative BOLD signals under most conditions correlate to excitation of inhibitory interneurons, but there are important exceptions to that rule as described in this paper. Thus, variations in the balance between synaptic excitation and inhibition contribute dynamically to the control of metabolic and hemodynamic responses, and in turn the amplitude and polarity of the BOLD signal. Therefore, it is not possible based on a negative or positive BOLD signal alone to decide whether the underlying activity goes on in principal or inhibitory neurons.

Columnar specificity of microvascular oxygenation and blood flow response in primary visual cortex: evaluation by local field potential and spiking activity

The relation of cortical microcirculation, oxygen metabolism, and underlying neuronal network activity remains poorly understood. Anatomical distribution of cortical microvasculature and its relationship to cortical functional domains suggests that functional organizations may be revealed by mapping cerebral blood flow responses. However, there is little direct experimental evidence and a lack of electrophysiological evaluation. In this study, we mapped ocular-dominance columns in primary visual cortex (V1) of anesthetized macaques with capillary flow-based laser speckle contrast imaging and deoxyhemoglobin-based intrinsic optical imaging. In parallel, the local field potentials (LFPs) and spikes were recorded from a linear array of eight microelectrodes, carefully positioned into left and right eye columns in V1. We found differential activation maps of blood flow, after masking large superficial draining vessels, exhibited a column-like pattern similar as the oximetric maps. Both the activated spikes and γ-band LFP demonstrated corresponding eye preference, consistent with the imaging maps. Our results present direct support in favor of previous proposals that the regulation of microcirculation can be as fine as the submillimeter scale, suggesting that cortical vasculature is functionally organized at the columnar level in a manner appropriate for supplying energy demands of functionally specific neuronal populations.

Investigation of Linear Coupling Between Single-Event Blood Flow Responses and Interictal Discharges in a Model of Experimental Epilepsy

A successful outcome of epilepsy neurosurgery relies on an accurate delineation of the epileptogenic region to be resected. Functional magnetic resonance imaging (fMRI) would allow doing this noninvasively at high spatial resolution. However, a clear, quantitative description of the relationship between hemodynamic changes and the underlying epileptiform neuronal activity is still missing, thereby preventing the systematic use of fMRI for routine epilepsy surgery planning. To this aim, we used a local epilepsy model to record simultaneously cerebral blood flow (CBF) with laser Doppler (LD) and local field potentials (LFP) in rat frontal cortex. CBF responses to individual interictal-like spikes were large and robust. Their amplitude correlated linearly with spike amplitude. Moreover, the CBF response added linearly in time over a large range of spiking rates. CBF responses could thus be predicted by a linear model of the kind currently used for the interpretation of fMRI data, but including also the spikes’ amplitudes as additional information. Predicted and measured CBF responses matched accurately. For high spiking frequencies (above ∼0.2 Hz), the responses saturated but could eventually recover, indicating the presence of multiple neurovascular coupling mechanisms, which might act at different spatiotemporal scales. Spatially, CBF responses peaked at the center of epileptic activity and displayed a spatial specificity at least as good as the millimeter. These results suggest that simultaneous electroencephalographic and blood flow-based fMRI recordings should be suitable for the noninvasive precise localization of hyperexcitable regions in epileptic patients candidate for neurosurgery.

Persistent increase in oxygen consumption and impaired neurovascular coupling after spreading depression in rat neocortex

Cortical spreading depression (CSD) is associated with a dramatic failure of brain ion homeostasis and increased energy metabolism. There is strong clinical and experimental evidence to suggest that CSD is the mechanism of migraine, and involved in progressive neuronal injury in stroke and head trauma. Here we tested the hypothesis that single episodes of CSD induced acute hypoxia, and prolonged impairment of neurovascular and neurometabolic coupling. Cortical spreading depression was induced in rat frontal cortex, whereas cortical electrical activity and local field potentials (LFPs) were recorded by glass microelectrodes, cerebral blood flow (CBF) by laser-Doppler flowmetry, and tissue oxygen tension (tpO(2)) with polarographic microelectrodes. Cortical spreading depression increased cerebral metabolic rate of oxygen (CMRO(2)) by 71%+/-6.7% and CBF by 238%+/-48.1% for 1 to 2 mins. For the following 2 h, basal tpO(2) and CBF were reduced whereas basal CMRO(2) was persistently elevated by 8.1%+/-2.9%. In addition, within first hour after CSD we found impaired neurovascular coupling (LFP versus CBF), whereas neurometabolic coupling (LFP versus CMRO(2)) remained unaffected. Impaired neurovascular coupling was explained by both reduced vascular reactivity and suppressed function of cortical inhibitory interneurons. The protracted effects of CSD on basal CMRO(2) and neurovascular coupling may contribute to cellular dysfunction in patients with migraine and acutely injured cerebral cortex.

Persisting depletion of brain glucose following cortical spreading depression, despite apparent hyperaemia: evidence for risk of an adverse effect of Leão's spreading depression

Rapid sampling microdialysis (rsMD) directed towards the cerebral cortex has allowed identification of a combined time-series signature for glucose and lactate that characterizes peri-infarct depolarization in experimental focal ischaemia, but no comparable data exist for 'classical' cortical spreading depression (CSD) associated with hyperaemia in the normally perfused brain. Here, we examined the rsMD responses of dialysate glucose and lactate to five hyperaemic spreading depressions induced with intracortical microinjections, typically of 1 mol/L KCl, in open-skull preparations in five cats under chloralose anaesthesia. Depolarization was verified with microelectrodes, and laser speckle flowmetry was used to examine propagation of the events and perfusion responses near the MD probe. Ten minutes after depolarization, dialysate glucose fell and lactate rose by 28% and 58% respectively. There was no recovery of dialysate glucose 30 mins after depolarization. Mean baseline indicative cerebral blood flow was 25.5+/-4.1 mL/100 g/min and mean maximum hyperaemic increase was by 29.6+/-6 mL/100 g/min; hyperaemia remained present 30 mins after CSD. As CSD events are repetitive, frequent, and often clustered temporally in human acute brain injury, these results indicate a high risk of depletion of extracellular glucose in association with depolarization events of a pattern previously thought to be largely benign.

Mechanisms involved in the cerebrovascular dilator effects of cortical spreading depression

Cortical spreading depression (CSD) leads to dramatic changes in cerebral hemodynamics. However, mechanisms involved in promoting and counteracting cerebral vasodilator responses are unclear. Here we review the development and current status of this important field of research especially with respect to the role of perivascular nerves and nitric oxide (NO). It appears that neurotransmitters released from the sensory and the parasympathetic nerves associated with cerebral arteries, and NO released from perivascular nerves and/or parenchyma, promote cerebral hyperemia during CSD. However, the relative contributions of each of these factors vary according to species studied. Related to CSD, axonal and reflex responses involving trigeminal afferents on the pial surface lead to increased blood flow and inflammation of the overlying dura mater. Counteracting the cerebral vascular dilation is the production and release of constrictor prostaglandins, at least in some species, and other possibly yet unknown agents from the vascular wall. The cerebral blood flow response in healthy human cortex has not been determined, and thus it is unclear whether the cerebral oligemia associated with migraines represents the normal physiological response to a CSD-like event or represents a pathological response. In addition to promoting cerebral hyperemia, NO produced during CSD appears to initiate signaling events which lead to protection of the brain against subsequent ischemic insults. In summary, the cerebrovascular response to CSD involves multiple dilator and constrictor factors produced and released by diverse cells within the neurovascular unit, with the contribution of each of these factors varying according to the species examined.

Gamma-aminobutyric acid modulates local brain oxygen consumption and blood flow in rat cerebellar cortex

In the awake brain, the global metabolic rate of oxygen consumption is largely constant, while variations exist between regions dependent on the ongoing activity. This suggests that control mechanisms related to activity, that is, neuronal signaling, may redistribute metabolism in favor of active networks. This study examined the influence of gamma-aminobutyric acid (GABA) tone on local increases in cerebellar metabolic rate of oxygen (CeMR(O(2))) evoked by stimulation of the excitatory, glutamatergic climbing fiber-Purkinje cell synapse in rat cerebellum. In this network, the postsynaptic depolarization produced by synaptic excitation is preserved despite variations in GABAergic tone. Climbing fiber stimulation induced frequency-dependent increases in synaptic activity and CeMR(O(2)) under control conditions. Topical application of the GABA(A) receptor agonist muscimol blocked the increase in CeMR(O(2)) evoked by synaptic excitation concomitant with attenuation of cerebellar blood flow (CeBF) responses. The effect was reversed by the GABA(A) receptor antagonist bicuculline, which also reversed the effect of muscimol on synaptic activity and CeBF. Climbing fiber stimulation during bicuculline application alone produced a delayed undershoot in CeBF concomitant with a prolonged rise in CeMR(O(2)). The findings are consistent with the hypothesis that activity-dependent rises in CeBF and CeMR(O(2)) are controlled by a common feed-forward pathway and provide evidence for modification of cerebral blood flow and CMR(O(2)) by GABA.

High-resolution in vivo imaging of the neurovascular unit during spreading depression

Spreading depression (SD) is a propagating wave of neuronal depolarization and ionic shifts, seen in stroke and migraine. In vitro, SD is associated with astrocytic [Ca2+] waves, but it is unclear what role they play and whether they influence cerebral blood flow, which is altered in SD. Here we show that SD in vivo is associated with [Ca2+] waves in astrocytes and neurons and with constriction of intracortical arterioles severe enough to result in arrest of capillary perfusion. The vasoconstriction is correlated with fast astrocytic [Ca2+] waves and is inhibited when they are reduced. [Ca2+] waves appear in neurons before astrocytes, and inhibition of astrocytic [Ca2+] waves does not depress SD propagation. This suggests that astrocytes do not drive SD propagation but are responsible for the hemodynamic failure seen deep in the cortex. Similar waves occur in anoxic depolarizations (AD), supporting the notion that SD and AD are related processes.

Nonlinear neurovascular coupling in rat sensory cortex by activation of transcallosal fibers

Functional neuroimaging and normal brain function rely on the robust coupling between neural activity and cerebral blood flow (CBF), that is neurovascular coupling. We examined neurovascular coupling in rat sensory cortex in response to direct stimulation of transcallosal pathways, which allows examination of brain regions inaccessible to peripheral stimulation techniques. Using laser-Doppler flowmetry to record CBF and electrophysiologic recordings of local field potentials (LFPs), we show an exponential relation between CBF responses and summed LFP amplitudes. Hemodynamic responses were dependent on glutamate receptor activation. CNQX, an AMPA receptor blocker, strongly attenuated evoked CBF responses and LFP amplitudes at all stimulation frequencies. In comparison, N-methyl D-aspartate (NMDA) receptor blockade by MK801 attenuated CBF responses at high (>7 Hz) but not low (<7 Hz) stimulation frequencies, without affecting evoked LFP amplitudes. This shows the limitation of using LFP amplitudes as indicators of synaptic activity. 7-Nitroindazole, a neuronal nitric oxide synthase inhibitor, and indomethacin, a nonspecific cyclooxygenase inhibitor, attenuated the hemodynamic responses by 50%+/-1% and 48%+/-1%, respectively, without affecting LFP amplitudes. The data suggest that preserved activity of both AMPA and NMDA receptors is necessary for the full CBF response evoked by stimulation of rodent interhemispheric connections. AMPA receptor activation gives rise to a measurable LFP, but NMDA receptor activation does not. The lack of a measurable LFP from neural processes that contribute importantly to CBF may explain some of the difficulties in transforming extracellular current or voltage measurements to a hemodynamic response.

Simultaneous laser Doppler flowmetry and arterial spin labeling MRI for measurement of functional perfusion changes in the cortex

This study compares laser Doppler flowmetry (LDF) and arterial spin labeling (ASL) for the measurement of functional changes in cerebral blood flow (CBF). The two methods were applied concurrently in a paradigm of electrical whisker stimulation in the anaesthetised rat. Multi-channel LDF was used, with each channel corresponding to different fiber separation (and thus measurement depth). Continuous ASL was applied using separate imaging and labeling coils at 3 T. Careful experimental set up ensured that both techniques recorded from spatially concordant regions of the barrel cortex, where functional responses were maximal. Strong correlations were demonstrated between CBF changes measured by each LDF channel and ASL in terms of maximum response magnitude and response time-course within a 6-s-long temporal resolution imposed by ASL. Quantitatively, the measurements of the most superficial LDF channels agreed strongly with those of ASL, whereas the deeper LDF channels underestimated consistently the ASL measurement. It was thus confirmed that LDF quantifies CBF changes consistently at a superficial level, and for this case the two methods provided concordant measures of functional CBF changes, despite their essentially different physical principles and spatiotemporal characteristics.

Near-infrared spectroscopy measurement of the pulsatile component of cerebral blood flow and volume from arterial oscillations

We describe a near-infrared spectroscopy (NIRS) method to noninvasively measure relative changes in the pulsate components of cerebral blood flow (pCBF) and volume (pCBV) from the shape of heartbeat oscillations. We present a model that is used and data to show the feasibility of the method. We use a continuous-wave NIRS system to measure the arterial oscillations originating in the brains of piglets. Changes in the animals' CBF are induced by adding CO(2) to the breathing gas. To study the influence of scalp on our measurements, comparative, invasive measurements are performed on one side of the head simultaneously with noninvasive measurements on the other side. We also did comparative measurements of CBF using a laser Doppler system to validate the results of our method. The results indicate that for sufficient source-detector separation, the signal contribution of the scalp is minimal and the measurements are representative of the cerebral hemodynamics. Moreover, good correlation between the results of the laser Doppler system and the NIRS system indicate that the presented method is capable of measuring relative changes in CBF. Preliminary results show the potential of this NIRS method to measure pCBF and pCBV relative changes in neonatal pigs.

Concurrent fMRI and optical measures for the investigation of the hemodynamic response function

Functional magnetic resonance imaging (fMRI) signal variations are based on a combination of changes in cerebral blood flow (CBF) and volume (CBV), and blood oxygenation. We investigated the relationship between these hemodynamic parameters in the rodent barrel cortex by performing fMRI concurrently with laser Doppler flowmetry (LDF) or optical imaging spectroscopy (OIS), following whisker stimulation and hypercapnic challenge. A difference between the positions of the maximum blood oxygenation level-dependent (BOLD) and CBV changes was observed in coronal fMRI maps, with the BOLD region being more superficial. A 6.5% baseline blood volume fraction in this superficial region dropped to 4% in deeper cortical layers (corresponding to total hemoglobin baseline volumes Hbt0 = 110 microM and 67 microM, respectively), as inferred from maps of deltaR2*. Baseline volume profiles were used to parameterize the Monte Carlo simulations (MCS) to interpret the 2D OIS. From this it was found that the optical blood volume measurements (i.e., changes in total hemoglobin) equated with CBV-MRI measurements when the MRI data were taken from superficial cortical layers. Optical measures of activation showed a good spatial overlap with fMRI measurements taken in the same plane (covering the right hemisphere surface). Changes in CBV and CBF followed the scaling relationship CBV = CBF(alpha), with mean alpha = 0.38 +/- 0.06.

Spatial extent of oxygen metabolism and hemodynamic changes during functional activation of the rat somatosensory cortex

The spatial extent of the changes in oxy-hemoglobin (HbO), deoxy-hemoglobin (HbR), total hemoglobin concentration (HbT), cerebral blood flow (CBF), and the cerebral metabolic rate of oxygen (CMRO(2)) in response to forepaw and whisker stimulation were compared in the rat somatosensory cortex using a combination of multi-wavelength reflectance imaging and laser speckle contrast imaging of cerebral blood flow. The spatial extents of the response of each hemodynamic parameter and CMRO(2) were found to be comparable at the time of peak response, and at early times following stimulation onset, the spatial extent of the change in HbR was smaller than that of HbO, HbT, CBF, and CMRO(2). In addition, a slight spatial dependence was found in the power law coefficient relating changes in CBF and HbT. Although the CMRO(2) response is a metabolic measure and thus expected to have a more localized response than the hemodynamic parameters, the results presented here suggest that this may not be the case in general, possibly due to the increased sensitivity of optical imaging techniques to superficial cortical layers where the lateral extent of the metabolic and neuronal activation is larger compared to that in layer IV. In addition, we found that the measured spatial extent of the CMRO(2) changes was insensitive to assumptions made in the calculation of the CMRO(2) changes such as baseline hemoglobin concentrations, vascular weighting constants, and wavelength dependence of tissue scattering. Multi-parameter full field imaging of the functional response provides a more complete picture of the hemodynamic response to functional activation including the spatial and temporal estimation of CMRO(2) changes.

Cerebral pressure autoregulation and vasoreactivity in the newborn rat

Perinatal brain injury has been associated with impaired cerebral blood flow (CBF) pressure autoregulation. The brain of 3- to 5-d-old rat pups is immature and similar to that of a preterm infant, and therefore we tested cerebral vasoreactivity in that animal. CBF pressure autoregulation was tested in 20 Wistar pups during normocapnia and hypercapnia, respectively. Hypotension was induced by hemorrhage and cerebral perfusion was monitored with laser Doppler flowmetry and near-infrared spectroscopy. Systolic blood pressure was measured noninvasively from the tail. During normocapnia, the autoregulatory plateau was narrow. Resting systolic blood pressure (SBP) was 39.2 mm Hg and CBF remained constant until SBP decreased below 36.0 mm Hg (SE 0.8). Below the lower limit, CBF declined by a mean of 2.7% per mm Hg [95% confidence interval (CI), 2.4-3.0%], and hemoglobin difference (HbD) and total hemoglobin (HbT) changed proportionally to CBF. After inhalation of carbon dioxide, CBF increased significantly by a mean of 17.7% (95% CI, 13.7-22.8%). The CBF-CO2 reactivity was estimated to 13.4% per kPa (95% CI, 2-24.8%), p=0.026. Over the range of SBP (6-54 mm Hg), a linear relationship between CBF and SBP was found during hypercapnia, indicating abolished pressure autoregulation. A linear correlation between CBF and HbD was found (r=0.80). CBF pressure autoregulation and reactivity to CO2 operate in the newborn rat. This model may be useful for future investigations concerning perinatal pathophysiology in the immature brain.

Laminar optical tomography: demonstration of millimeter-scale depth-resolved imaging in turbid media

Laminar optical tomography (LOT) is a new technique that combines the advantages of diffuse optical tomography image reconstruction and a microscopy-based setup to allow noncontact imaging with 100-200-microm resolution effective over depths of 0-2.5 mm. LOT is being developed primarily for multispectral imaging of rat cortex, for which resolving functional dynamics in various layers of the brain's cortex (to depths of 1500 microm) is of increasing interest to neurophysiologists. System design and image reconstruction techniques are described, along with simulation and phantom results that demonstrate the characteristics and limitations of system accuracy and resolution.

Spatiotemporal quantification of cerebral blood flow during functional activation in rat somatosensory cortex using laser-speckle flowmetry

Laser-speckle flowmetry was used to characterize activation flow coupling after electrical somatosensory stimulation of forepaw and hindpaw in the rat. Quantification of functional activation was made with high transverse spatial (microm) and temporal (msec) resolution. Different activation levels and duration of stimulation were quantitatively investigated, and were in good agreement with previous laser-Doppler measurements. Interestingly, the magnitude but not the overall shape of the response was found to scale with stimulus amplitude and the distance from the activation centroid. The results provide new insights about the spatial characteristics of cerebral blood flow response to functional activation, and the method should lead to improved understanding of the coupling of neuronal activity and hemodynamics under normal and pathologic conditions.

Contribution of somatosensory cortex to evoked cerebellar blood flow responses

Projections from the trigeminocerebellar pathway and the somatosensory cortex coincide spatially in the granule cell layer of Crus I/II of the cerebellar hemisphere. A biphasic field potential was seen: one peak at 10 ms (trigeminal input) and another at 20 ms (somatosensory input). Linear correlation analysis revealed only a weak coupling between somatosensory input and cerebellar blood flow responses to infraorbital nerve stimulation. In separate experiments, cortical spreading depression attenuated the field potential peak at 20 ms while blood flow responses remained unaltered. Thus, trigeminocerebellar activity explained the evoked blood flow responses. Our data provide further evidence that activity-dependent blood flow responses are context-sensitive and that interaction between excitatory neuronal circuits targeting the same cells may occlude vascular responses.

A newborn rat model for the study of cerebral hemodynamics by near-infrared spectroscopy and laser-Doppler flowmetry in the immature brain

An animal model for the study of cerebrovascular physiology in the immature brain was developed. Twelve 3- to 5-day-old rat pups were maintained on spontaneous breathing under light anesthesia for either 1 or 2 h. Transcutaneous carbon dioxide tension and arterial oxygen saturation were monitored. Continuous infusion of doxapram limited respiratory acidosis. Cerebral blood flow (CBF) and volume (CBV) could be monitored by near-infrared spectroscopy (NIRS) and laser-Doppler flowmetry (LDF) in spite of some movement artifacts. CBV and CBF were 6.0 +/- 0.3 SE ml/100 g and 36.3 +/- 3.1 SE ml/100 g/min, respectively, and remained stable during the study. Cerebrovascular responses, as monitored by LDF and NIRS, to hypoxic and hypercapnic gas mixtures were consistent.

Moment analysis of microflow histogram in focal ischemic lesion to evaluate microvascular derangement after small pial arterial occlusion in rats

The authors' high-spatial-resolution optical method was used to examine microvascular derangement in a focal cerebral cortex lesion in 12 Sprague-Dawley rats anesthetized with alpha-chloralose-urethane. A pial artery (approximately 40- to 50 microm diameter) was occluded by laser-beam cauterization (n = 6). Diluted carbon black suspension was injected into the internal carotid artery, and images in a 2-mm x 2-mm region of interest during tissue dye-dilution were recorded. Sequential frames were analyzed with Matlab software to evaluate blood distribution and mean transit time, affording a two-dimensional microflow map and histogram with first, second, third, and fourth moments. In the early phase of ischemia, blood distribution and average flow decreased (both P < 0.01), and the second moment (microflow heterogeneity) and third moment (skew to the left owing to increase in low-flow components) increased (P < 0.05 and P < 0.01, respectively). At approximately 2 hours, blood distribution decreased further in 3 cases, apparently because capillary stasis prevented carbon black filling. However, average microflow unexpectedly increased in 4 of 5 rats, presumably due to exclusion of unperfused (low flow at the earlier stage) channels from the calculation. The authors conclude that flow in ischemic tissue is quite heterogeneous and that an averaged flow value tends to smear important information about ischemic microvascular derangement.

Temporal profile of cortical perfusion and microcirculation after controlled cortical impact injury in rats

Impaired cerebral perfusion contributes to evolving posttraumatic tissue damage. Spontaneous reversibility of reduced perfusion within the first days after injury could make a persisting impact on secondary tissue damage less likely and needs to be considered for possible therapeutic approaches. The present study was designed to characterize the temporal profile and impact of trauma severity on cortical perfusion and microcirculation during the first 48 h after controlled cortical impact injury (CCI). In 10 rats, pericontusional cortical perfusion and microcirculation using laser Doppler flowmetry (LDF) and orthogonal polarization spectral (OPS) imaging were assessed before, and at 4, 24, and 48 h after CCI. Influence of trauma severity was studied by varying the penetration depth of the impactor rod (0.5 vs. 1 mm), thereby inducing a less and a more severe contusion. Mean arterial blood pressure (MABP), arterial blood gases, and blood glucose were monitored. With unchanged MABP and paCO2, cortical perfusion and microcirculation were significantly impaired during the first 48 h following CCI. Hypoperfusion observed at 4 h related to vasoconstriction and microcirculatory stasis preceded a long-lasting phase of hyperperfusion at 24 and 48 h reflected by vasodilation and increased flow velocity in arterioles and venules. Hyperperfusion was mostly pronounced in rats with a less severe contusion. Following CCI, trauma severity markedly influences changes in pericontusional cortical perfusion and microcirculation. Overall, pericontusional cortical hypoperfusion observed within the early phase preceded a long lasting phase of hyperperfusion up to 48 h after CCI.

Endothelin-1 potently induces Leão's cortical spreading depression in vivo in the rat: a model for an endothelial trigger of migrainous aura?

According to the 'neuronal' theory, cortical spreading depression (CSD) is the pathophysiological correlate of migrainous aura. However, the 'vascular' theory has implicated altered vascular function in the induction of aura symptoms. The possibility of a vascular origin of aura symptoms is supported, e.g. by the clinical observation that cerebral angiography frequently provokes migrainous aura. This suggests that endothelial irritation may somehow initiate one of the pathways resulting in migrainous aura. Up to now, an endothelium-derived factor has never been shown to trigger CSD. Here, for the first time, we demonstrate and characterize the ability of the vasoconstrictor and astroglial/neuronal modulator endothelin-1 to trigger Leão's 'spreading depression of activity' in vivo in rats. At a concentration range between 10 nM and 1 microM, endothelin-1 induced changes characteristic of CSD with regard to the rate of propagation, steady (direct current) potential and extracellular K(+)-concentration. A spreading hyperaemia followed by oligaemia was observed similar to those in K(+)-induced CSD. Endothelin-1 did not provoke changes characteristic of a terminal depolarization. The mechanism by which endothelin-1 generated CSD involved the N-methyl-D-asparate receptor. Cerebral blood flow decreased slightly, but significantly, before endothelin-1 generated CSD. A vasodilator (NO*-donor) shifted the threshold for CSD induction to higher concentrations of endothelin-1. Endothelin-1, in contrast to K(+), did not induce CSD in rat brain slices suggesting indirectly that endothelin-1 may require intact perfusion to exert its effects. In conclusion, endothelin-1 was found in the experiment to be the most potent inducer of CSD currently known. We propose endothelin-1 as a possible candidate for the yet enigmatic link between endothelial irritation and migrainous aura.

Effects of anterior inferior cerebellar artery occlusion on cochlear blood flow--a comparison between laser-Doppler and microsphere methods

The effects of anterior inferior cerebellar artery (AICA) occlusion on blood flow were investigated in rats using the laser-Doppler and microsphere methods. A specially designed microclamp was held in a micromanipulator and positioned to occlude the left AICA. After the middle ear mucosa had been removed, a 1.0-mm laser-Doppler probe was placed on the basal turn of the left cochlea. Non-radioactive microspheres were injected intracardially during the AICA occlusion and the numbers of microspheres in various parts of the cochlea were counted, including in the bone surrounding the cochlea. Upon AICA clamping, the blood flow measured by laser-Doppler flowmetry decreased to 46.9+/-11.9% of the baseline value, and the number of microspheres trapped in the cochlea was 17.2+/-13.3% compared with the contralateral side in 15 animals. The number of microspheres in the bone surrounding the cochlea in the AICA-clamped side was 81+/-15% of that of the contralateral side. In animals in which there were few if any microspheres in the cochlea, laser-Doppler output was 30-40% of the baseline value. From these findings, we infer that during complete interruption of cochlear blood flow in rats, residual laser-Doppler output was essentially attributable to blood flow in the bone surrounding the cochlea.

Relationship of spikes, synaptic activity, and local changes of cerebral blood flow

The coupling of electrical activity in the brain to changes in cerebral blood flow (CBF) is of interest because hemodynamic changes are used to track brain function. Recent studies, especially those investigating the cerebellar cortex, have shown that the spike rate in the principal target cell of a brain region (i.e. the efferent cell) does not affect vascular response amplitude. Subthreshold integrative synaptic processes trigger changes in the local microcirculation and local glucose consumption. The spatial specificity of the vascular response on the brain surface is limited because of the functional anatomy of the pial vessels. Within the cortex there is a characteristic laminar flow distribution, the largest changes of which are observed at the depth of maximal synaptic activity (i.e. layer IV) for an afferent input system. Under most conditions, increases in CBF are explained by activity in postsynaptic neurons, but presynaptic elements can contribute. Neurotransmitters do not mediate increases in CBF that are triggered by the concerted action of several second messenger molecules. It is important to distinguish between effective synaptic inhibition and deactivation that increase and decrease CBF and glucose consumption, respectively. In summary, hemodynamic changes evoked by neuronal activity depend on the afferent input function (i.e. all aspects of presynaptic and postsynaptic processing), but are totally independent of the efferent function (i.e., the spike rate of the same region). Thus, it is not possible to conclude whether the output level of activity of a region is increased based on brain maps that use blood-flow changes as markers.

Coupling and uncoupling of activity-dependent increases of neuronal activity and blood flow in rat somatosensory cortex

1. Electrical stimulation of the infraorbital nerve was used to examine the coupling between neuronal activity and cerebral blood flow (CBF) in rat somatosensory cortex by laser Doppler flowmetry and extracellular recordings of field potentials. 2. The relationship between field potential (FP) and CBF amplitudes was examined as a function of the stimulus intensity (0--2.0 mA) at fixed frequency (3 Hz). FP amplitudes up to 2.0-2.5 mV were unaccompanied by increases of CBF. Above this threshold, CBF and FP amplitudes increased proportionally. 3. At fixed stimulus intensity of 1.5 mA, CBF increases were highly correlated to FP amplitudes at low frequencies of stimulation (< 2 Hz), but uncoupling was observed at stimulation frequencies of 2--5 Hz. The evoked responses were independent of stimulus duration (8--32 s). 4. The first rise in CBF occurred within the first 0.2 s after onset of stimulation in the upper 0--250 microm of the cortex. Latencies were longer (1.0--1.2 s) in lower cortical layers in which CBF and FP amplitudes were larger. 5. Local AMPA receptor blockade attenuated CBF and FP amplitudes proportionally. 6. This study showed that activity-dependent increases in neuronal activity and CBF were linearly coupled under defined conditions, but neuronal activity was well developed before CBF started to increase. Consequently, the absence of a rise in CBF does not exclude the presence of significant neuronal activity. The CBF increase in upper cortical layers preceded the rise in lower layers suggesting that vessels close to or at the brain surface are the first to react to neuronal activity. The activity-dependent rise in CBF was explained by postsynaptic activity in glutamatergic neurons.

Scanning laser-Doppler flowmetry of rat cerebral circulation during cortical spreading depression

Scanning laser-Doppler flowmetry (SLDF) generates two-dimensional images of blood flow. This study compared SLDF to conventional laser-Doppler flowmetry (LDF) in the cerebral circulation. Test stimuli were episodes of cortical spreading depression (CSD) elicited in brains of halothane anaesthetised rats (n = 9). The LDF instrument used two wavelengths of laser light to record relative changes of cerebral blood flow (CBF) up to an approximate depth of 250 microm (543 nm) and 500 microm (780 nm). Under resting conditions, SLDF images showed a heterogeneous pattern of flow in pial vessels with high flow rates in arterioles, and lower rates in venules and small vessels (<30 microm). Arterioles constituted about 6%, venules 12% and small vessels 2% of the image area, while approximately 80% were background with a laser-Doppler signal corresponding to zero calibration. During CSD, the relative increase of area was largest for small vessels and less for venules and arterioles. Similar changes were observed for blood flow in the three vessel structures. For both wavelengths of LDF, flow changes correlated with SLDF (r approximately 0.7). In conclusion, SLDF provides images of flow in pial vessels and capillaries at, or just beneath the cortical surface. SLDF and LDF are complementary, but cannot substitute for one another as they measure flow in different layers of the cortex.

A sealed cranial window system for simultaneous recording of blood flow, and electrical and optical signals in the rat barrel cortex

We have developed a new sealed cranial window technique which allows the manipulation of simultaneously and independently multiple sensor probes, such as a glass microelectrode and a laser-Doppler probe. possible. Furthermore, normal intracranial pressure (4 mmHg) can be maintained throughout the craniectomy and the experiment. Using this technique, we have measured the neuronal activity and local cerebral blood flow together with the intrinsic optical properties in the rat barrel cortex during mechanical stimulation of the whiskers. The onset of the field response recorded by an extracellular electrode in the principal barrel columns occurred about 8 ms from the beginning of stimulation. These responses were well correlated with the whisker displacements (3 Hz, 2 s). The local cerebral blood flow, measured by laser-Doppler flowmetry, started to increase about 0.5 s after the first field response, peaked at about 1.7 s, and then gradually waned. A similar time-course of changes in the local blood volume was observed by simultaneous intrinsic optical imaging at the hemoglobin-isosbestic wavelength (570 nm). These results suggest that our technique would be useful for assessing the mechanism underlying neurovascular coupling under physiological conditions in vivo.

Can the green laser doppler measure skin-nutritive perfusion in patients with peripheral vascular disease?

The recently developed green laser (GL; wavelength 543 nm) is thought to measure perfusion derived from a more superficial skin layer than does the standard near-infrared laser (RL; wavelength 780 nm). These lasers were used to investigate the disturbances in the different layers of skin perfusion in ischaemic legs before and after treatment and compared with capillary microscopy. Eighteen patients (20 legs) with different stages of leg ischaemia scheduled for a vascular intervention (11 males, 7 females; median age 73, range: 52-81 years; Fontaine stages II-IV) were investigated by means of capillary microscopy, visualising the nail fold capillary perfusion, and a laser Doppler, equipped with a special dual probe conducting both GL and RL. The probe was attached to the pulp and the dorsum of the big toe to assess skin perfusion at rest and during reactive hyperaemia, while sitting and while supine. Resting and hyperaemic perfusion using GL was low and significantly lower (p < 0.01) than with RL in both areas and positions. Laser Doppler perfusion was higher in the pulp than on the dorsum with both wavelengths (p < 0.05). The hyperaemia response was highest using GL and differed among the three techniques. Postural reduction of capillary and RL flow was reduced, but not with GL. After treatment, skin capillary perfusion improved more clearly than did the laser Doppler perfusion with either wavelength, while postural vasoconstriction improved only when measured with the capillary microscope. The differences found between RL and GL Doppler perfusion, but also between GL and capillary microscopy measurements suggest that the GL does measure the more superficial, but not exclusively the nutritive skin perfusion. Clinically, the use of the green laser in its present form in patients with leg ischaemia offers no advantage over the red laser.

Temporal coupling between neuronal activity and blood flow in rat cerebellar cortex as indicated by field potential analysis

1. Laser-Doppler flowmetry and extracellular recordings of field potentials were used to examine the temporal coupling between neuronal activity and increases in cerebellar blood flow (CeBF). 2. Climbing fibre-evoked increases in CeBF were dependent on stimulus duration, indicating that increases in CeBF reflected a time integral in neuronal activity. The simplest way to represent neuronal activity over time was to obtain a running summation of evoked field potential amplitudes (runSigmaFP). RunSigmaFP was calculated for each stimulus protocol and compared with the time course of the CeBF responses to demonstrate coupling between nerve cell activity and CeBF. 3. In the climbing fibre system, the amplitude and time course of CeBF were in agreement with the calculated postsynaptic runSigmaFP (2-20 Hz for 60 s). This suggested coupling between CeBF and neuronal activity in this excitatory, monosynaptic, afferent-input system under these conditions. There was no correlation between runSigmaFP and CeBF during prolonged stimulation. 4. Parallel fibre-evoked increases in CeBF correlated with runSigmaFP of pre- and postsynaptic potentials (2-15 Hz for 60 s). At higher stimulation frequencies and during longer-lasting stimulation the time course and amplitudes of CeBF responses correlated with runSigmaFP of presynaptic, but not postsynaptic potentials. This suggested a more complex relationship in this mixed inhibitory-excitatory, disynaptic, afferent-input system. 5. This study has demonstrated temporal coupling between neuronal activity and CeBF in the monosynaptic, excitatory climbing-fibre system. In the mixed mono- and disynaptic parallel fibre system, temporal coupling was most clearly observed at low stimulation frequencies. We propose that appropriate modelling of electrophysiological data is needed to document functional coupling of neuronal activity and blood flow.

Modification of activity-dependent increases in cerebellar blood flow by extracellular potassium in anaesthetized rats

1. The hypothesis that potassium ions mediate activity-dependent increases of cerebral blood flow was examined in rat cerebellar cortex using ion-selective microelectrodes and laser-Doppler flowmetry. Increases of cerebellar blood flow (CeBF) and extracellular potassium concentration ([K+]o) were evoked by stimulation of parallel fibres and climbing fibres, and by microinjection of KCl into the cortex. 2. For parallel fibre stimulation, there was a maximal increase in [K+]o to 6.3 +/- 0.5 mM and in CeBF of 122 +/- 11 %. Climbing fibre stimulation gave a maximal increase in [K+]o to 4.4 +/- 0.2 mM and in CeBF of 157 +/- 20 %. This indicates different maxima for [K+]o and CeBF, dependent on the afferent system activated. 3. [K+]o and CeBF responses evoked by parallel or climbing fibre stimulation increased rapidly at the onset of stimulation, but exhibited different time courses during the remainder of the stimulation period and during return to baseline. 4. Microinjections of KCl into the cortex increased [K+]o to levels comparable to those evoked by parallel fibre stimulation. The corresponding CeBF increases were the same as, or smaller than, for parallel fibre stimulation, and much smaller than for climbing fibre stimulation. This suggests that mediators other than [K+]o are important for activity-dependent cerebral blood flow increases. 5. The present study showed that increased [K+]o is involved in CeBF regulation in the parallel fibre system, but is of limited importance for CeBF regulation in the climbing fibre system. The hypothesis that K+ is a major mediator of activity-dependent blood flow increases is probably not generally applicable to all brain regions and all types of neuronal stimulation.

Red and green laser Doppler compared with capillary microscopy to assess skin microcirculation in the feet of healthy subjects

Skin microvasculature consists of nutritive capillaries and subpapillary arteriolar and venular plexus connected by arteriolovenular anastomoses. Capillary perfusion is of paramount importance for skin viability. Recently a new combined laser Doppler instrument has become available, featuring a combination of near-infrared (RL; 780 nm) and green (GL; 543 nm) laser light sources. Theoretically, the red laser will penetrate deeper, whereas the green laser will read fairly superficially. This may enable differentiation between the more superficial, i.e., capillary, and the deeper skin layers. To test this hypothesis, the combined laser Doppler technique was compared with nail fold capillary microscopy in the feet of 10 healthy subjects. Seven males and 3 females with a median age of 26 (range 20-42) years and without arterial pathology were investigated. The laser Doppler (Periflux 4001, Perimed) was equipped with a special dual probe conducting both GL and RL. The probe was attached to the pulp of the big toe (with many AV-shunts) and to the nail fold, at the site where capillary microscopy was performed too. Laser Doppler and capillary perfusion was assessed at rest and during postocclusive reactive hyperemia. These measurements were performed both in the sitting and the supine positions to test the postural vasoconstriction response. Median resting and hyperemic skin perfusion with GL were lower (P < 0.01) than with the RL in both areas and positions, except for the resting value in the sitting position on the dorsum of the toe. Plantar perfusion was found significantly higher than dorsal perfusion only with the RL in the supine position (P < 0.01). GL and RL on the plantar, but not the dorsal, side showed a significantly decreased perfusion upon dependency (P < 0.05), both at rest and during hyperemia. In contrast, resting and peak capillary velocity did show a decrease on dependency (P < 0.05). Although the green laser measures a lower perfusion than does the red laser, which is likely to be derived from more superficial skin layers, it does not show a reactivity similar to that measured with capillary microscopy. Thus, it is questionable whether the green laser exclusively measures capillary perfusion.