Real time laser-Doppler perfusion imaging of cortical spreading depression in rat neocortex

The Transient Receptor Potential Ankyrin Type 1 Plays a Critical Role in Cortical Spreading Depression

The transient receptor potential ankyrin type-1 (TRPA1) channels have been proposed as a potential target for migraine therapy. Yet the role of cortical TRPA1 channels in migraine mechanism has not been fully understood. Cortical spreading depression (CSD) is known as an underlying cause of migraine aura. The aim of this study is to investigate if cortical TRPA1 activity is required for CSD genesis and propagation. A mouse brain slice CSD model with intrinsic optical imaging was applied for TRPA1 signaling pharmacology. The results showed that the TRPA1 agonist, umbellulone, facilitated the propagation of submaximal CSD. Correspondingly, an anti-TRPA1 antibody and two selective TRPA1 antagonists, A967079 and HC-030031, prolonged the CSD latency and reduced magnitude, indicating a reduced cortical susceptibility to CSD under TRPA1 deactivation. Furthermore, the TRPA1 agonist, allyl-isothiocyanate (AITC), reversed the suppression of CSD by HC-030031, but not by A967079. Interestingly, the inhibitory action of A967079 on CSD was reversed by exogenous calcitonin-gene-related peptide (CGRP). Consistent to TRPA1 deactivation, the prolonged CSD latency was observed by an anti-CGRP antibody in the mouse brain slice, which was reversed by exogenous CGRP. We conclude that cortical TRPA1 is critical in regulating cortical susceptibility to CSD, which involves CGRP. The data strongly suggest that deactivation of TRPA1 channels and blockade of CGRP would have therapeutic benefits in preventing migraine with aura.

Effect of serotonin depletion on cortical spreading depression evoked cerebrovascular changes

Abstract Background: The cortical spreading depression (CSD) is a phenomenon associated with several pathological conditions including migraine. It can induce alterations in both neural and vascular compartments. Serotonin (5-HT) depletion is known as a condition involved in migraine pathophysiology. The hyper-excitability of the cortical neurons to the CSD activation in the low 5-HT state has been previously reported. However, the cerebrovascular responses to CSD activation in this condition have never been studied yet. Objectives: Determine the effect of 5-HT depletion on the cerebrovascular responses to CSD activation. Methods: Wistar rats (weighing 250-300 grams) were divided into three groups: control, CSD, and low 5-HT with CSD group (five rats per group). To induce the low 5-HT state, the para-chlorophenylalanine was injected intraperitoneally into the rats three days before the experiment. CSD was induced by the application of solid KCl (3 mg) on the parietal cortex. NaCl instead of KCl was applied to the control group. Cerebral cortical blood flow was monitored using Laser Doppler flowmetry. The ultrastructure of cerebral microvessels was examined using electron microscopy to determine the cerebral microcirculatory responses to CSD. Results: Depletion of serotonin induced a significant increase in the peak amplitude of CSD-evoked cerebral hyperaemia. This condition also enhanced the development of CSD-induced endothelial pinocytosis and microvillus formation in cerebrocortical microvessels. Conclusion: 5-HT was an important neurotransmitter involved in the control of cerebrovascular responses to CSD activation. The hypersensitivity of the cerebrovascular responses observed in the 5-HT depleted state may explain the relationship between headache and 5-HT depletion.

Laser application in neurosurgery

BACKGROUND

Technological innovations based on light amplification created by stimulated emission of radiation (LASER) have been used extensively in the field of neurosurgery.

METHODS

We reviewed the medical literature to identify current laser-based technological applications for surgical, diagnostic, and therapeutic uses in neurosurgery.

RESULTS

Surgical applications of laser technology reported in the literature include percutaneous laser ablation of brain tissue, the use of surgical lasers in open and endoscopic cranial surgeries, laser-assisted microanastomosis, and photodynamic therapy for brain tumors. Laser systems are also used for intervertebral disk degeneration treatment, therapeutic applications of laser energy for transcranial laser therapy and nerve regeneration, and novel diagnostic laser-based technologies (e.g., laser scanning endomicroscopy and Raman spectroscopy) that are used for interrogation of pathological tissue.

CONCLUSION

Despite controversy over the use of lasers for treatment, the surgical application of lasers for minimally invasive procedures shows promising results and merits further investigation. Laser-based microscopy imaging devices have been developed and miniaturized to be used intraoperatively for rapid pathological diagnosis. The multitude of ways that lasers are used in neurosurgery and in related neuroclinical situations is a testament to the technological advancements and practicality of laser science.

Demonstration of impaired neurovascular coupling responses in TG2576 mouse model of Alzheimer's disease using functional laser speckle contrast imaging

Increasing evidence from epidemiological, clinical, and experimental studies indicates that cerebromicrovascular dysfunction and microcirculatory damage play critical roles in the pathogenesis of many types of dementia in the elderly, including both vascular cognitive impairment (VCI) and Alzheimer's disease. Vascular contributions to cognitive impairment and dementia (VCID) include impairment of neurovascular coupling responses/functional hyperemia ("neurovascular uncoupling"). Due to the growing interest in understanding and pharmacologically targeting pathophysiological mechanisms of VCID, there is an increasing need for sensitive, easy-to-establish methods to assess neurovascular coupling responses. Laser speckle contrast imaging (LSCI) is a technique that allows rapid and minimally invasive visualization of changes in regional cerebromicrovascular blood perfusion. This type of imaging technique combines high resolution and speed to provide great spatiotemporal accuracy to measure moment-to-moment changes in cerebral blood flow induced by neuronal activation. Here, we provide detailed protocols for the successful measurement in neurovascular coupling responses in anesthetized mice equipped with a thinned-skull cranial window using LSCI. This method can be used to evaluate the effects of anti-aging or anti-AD treatments on cerebromicrovascular health.

'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.

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.

Identification of the extent of cortical spreading depression propagation by Npas4 mRNA expression

Cortical spreading depression (CSD) is a phenomenon associated with a propagating large shift in direct current (DC) potential followed by suppression of electrophysiological activity. For temporal analysis of CSD propagation, electrophysiological recording is the most reliable tool. However, it is difficult to completely identify the spatial area of the brain influenced by CSD, because recording sites are technically limited. Histological post hoc identification of activated neurons by labeling the induction of an immediate early gene (IEG) could determine areas of CSD propagation. We found that cortical application of potassium chloride induced expression of Npas4 IEG mRNA in the ipsilateral dorsal cortex. Interestingly, induction of Npas4 was never observed in the ipsilateral hippocampus and there was a clear boundary to the area of Npas4 expression. To determine whether the boundary of the area of Npas4 mRNA expression was the limit of CSD propagation, we recorded local field potentials from multiple sites that crossed the boundary of Npas4 expression. We found that the area of Npas4 mRNA expression coincided with the area of DC-potential shift propagation. We propose that induction of Npas4 identifies the area influenced by CSD propagation.

Alpha 1-antitrypsin therapy mitigated ischemic stroke damage in rats

Our objective is to develop a new therapy for the treatment of stroke. Currently, the only effective therapy for acute ischemic stroke is the thrombolytic agent recombinant tissue plasminogen activator. α1-Antitrypsin (AAT), a serine proteinase inhibitor with potent anti-inflammatory, anti-apoptotic, antimicrobial, and cytoprotective activities, could be beneficial in stroke. The goal of this study is to test whether AAT can improve ischemic stroke outcome in an established rat model. Middle cerebral artery occlusion was induced in male rats via intracranial (i.c.) microinjection of endothelin-1. Five to 10 minutes after stroke induction, rats received either i.c. or intravenous delivery of human AAT. Cylinder and vibrissae tests were used to evaluate sensorimotor function before and 72 hours after middle cerebral artery occlusion. Infarct volumes were examined via either 2,3,5-triphenyltetrazolium chloride assay or magnetic resonance imaging 72 hours after middle cerebral artery occlusion. Despite equivalent initial strokes, at 72 hours, the infarct volumes of the human AAT treatment groups (local and systemic injection) were statistically significantly reduced by 83% and 63% (P < .0001 and P < .05, respectively) compared with control rats. Human AAT significantly limited sensory motor system deficits. Human AAT could be a potential novel therapeutic drug for the protection against neurodegeneration after ischemic stroke, but more studies are needed to investigate the protective mechanisms and efficacy in other animal models.

Perfusion imaging of spinal cord contusion: injury-induced blockade and partial reversal by β2-agonist treatment in rats

OBJECT

Traumatic injury to the spinal cord results in considerable delayed tissue loss. The authors investigated the extent to which ischemia occurs following contusion-induced spinal cord injury and whether ischemia exacerbates tissue damage that leads to the loss of locomotor function. They also determined if ischemia is reversed with β2-adrenoceptor agonist treatment, which has been established to be neuroprotective following contusion injury.

METHODS

The extent and role of circulation loss in spinal cord injury was determined in an established experimental model of contusion injury. The spinal cord dura mater of Wistar rats was exposed by performing a laminectomy at T-8 to T-11. Laser Doppler perfusion imaging was then used to measure microcirculation in the exposed spinal cord. After imaging, a moderately severe contusion injury was produced using a weight-drop device unto the exposed dura at T-10. Perfusion imaging was again performed, scans were quantitated, and integrated intensities were compared.

RESULTS

Postinjury imaging revealed an 18%-27% reduction in perfusion in regions rostral and caudal to the injury site, and a 68% reduction was observed at the contusion epicenter. These perfusion losses persisted for at least 48 hours. At 24 hours after injury, some rats were intraperitoneally injected with 2 mg/kg of the β2-adrenoceptor agonist clenbuterol, which has been shown to promote the partial recovery of locomotor function and spare spinal cord tissue when administered within 2 days after contusion injury. Clenbuterol injection caused a gradual increase in perfusion, which was detectable at 30 minutes postinjection and continued over time, resulting in an 127% overall increase in perfusion at the epicenter 24 hours after treatment.

CONCLUSIONS

These results suggest that the occurrence of chronic perfusion loss after contusion contributes to delayed damage and tissue loss. In contrast, β2-adrenoceptor agonist treatment may exert neuroprotection by restoring perfusion, thereby preventing ischemic neurodegeneration. The ability of laser Doppler imaging to measure the loss of perfusion and its restoration upon treatment suggests that it may have clinical utility in the assessment and treatment of spinal cord injury.

Cortical spreading depression and calcitonin gene-related peptide: a brief review of current progress

Although detailed disease mechanisms of migraine remain poorly understood, migraine is known to have a complex pathophysiology with both vascular and neuronal mechanisms. The neuronal mechanisms of migraine may be attributed to cortical spreading depression (CSD); consequently, CSD has been widely studied for understanding the pathophysiology of migraine. Well validated CSD models have been developed for evaluating anti-migraine drugs. Neuropeptides, mainly, calcitonin gene-related peptide (CGRP), have been proposed as an emerging class of effective drugs against migraine headache. The central role of this neuropeptide has led to research into CSD for understanding disease mechanisms of migraine. This review briefly summarizes our current understanding of CSD and CGRP involvement in CSD. Although CSD can also worsen strokes, this brief paper has excluded the possible connection between the neuropeptide and CSD associated with them. Instead it has focused solely on CGRP in CSD associated with migraine.

Subarachnoid hemorrhage, spreading depolarizations and impaired neurovascular coupling

Aneurysmal subarachnoid hemorrhage (SAH) has devastating consequences on brain function including profound effects on communication between neurons and the vasculature leading to cerebral ischemia. Physiologically, neurovascular coupling represents a focal increase in cerebral blood flow to meet increased metabolic demand of neurons within active regions of the brain. Neurovascular coupling is an ongoing process involving coordinated activity of the neurovascular unit-neurons, astrocytes, and parenchymal arterioles. Neuronal activity can also influence cerebral blood flow on a larger scale. Spreading depolarizations (SD) are self-propagating waves of neuronal depolarization and are observed during migraine, traumatic brain injury, and stroke. Typically, SD is associated with increased cerebral blood flow. Emerging evidence indicates that SAH causes inversion of neurovascular communication on both the local and global level. In contrast to other events causing SD, SAH-induced SD decreases rather than increases cerebral blood flow. Further, at the level of the neurovascular unit, SAH causes an inversion of neurovascular coupling from vasodilation to vasoconstriction. Global ischemia can also adversely affect the neurovascular response. Here, we summarize current knowledge regarding the impact of SAH and global ischemia on neurovascular communication. A mechanistic understanding of these events should provide novel strategies to treat these neurovascular disorders.

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

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

Visualizing odor representation in the brain: a review of imaging techniques for the mapping of sensory activity in the olfactory glomeruli

The brain transforms clues from the external world, the sensory stimuli, into activities in neuroglial networks. These circuits are activated in specialized sensory cortices where specific functional modules are responsible for the spatiotemporal coding of the stimulus. A major challenge in the neuroscience field has been to image the spatial distribution and follow the temporal dynamics of the activation of such large populations in vivo. Functional imaging techniques developed in the last 30 years have enabled researchers to solve this critical issue, and are reviewed here. These techniques utilize sources of contrast of radioisotopic, magnetic and optical origins and exploit two major families of signals to image sensory activity: the first class uses sources linked to cellular energy metabolism and hemodynamics, while the second involves exogenous indicators of neuronal activity. The whole panel of imaging techniques has fostered the functional exploration of the olfactory bulb which is one of the most studied sensory structures. We summarize the major results obtained using these techniques that describe the spatial and temporal activity patterns in the olfactory glomeruli, the first relay of olfactory information processing in the main olfactory bulb. We conclude this review by describing promising technical developments in optical imaging and future directions in the study of olfactory spatiotemporal coding.

Evidence for a vascular factor in migraine

OBJECTIVE

It has been suggested that migraine is caused by neural dysfunction without involvement of vasodilatation. Because dismissal of vascular mechanisms seemed premature, we examined diameter of extra- and intracranial vessels in migraine without aura patients.

METHODS

A novel high-resolution direct magnetic resonance angiography imaging technique was used to measure arterial circumference of the extracranial middle meningeal artery (MMA) and the intracranial middle cerebral artery (MCA). Data were obtained at baseline, during migraine attack, and after treatment with the migraine abortive drug sumatriptan (a 5-hydroxytryptamine agonist).

RESULTS

We found dilatation of both MMA and MCA during migraine attack (p = 0.001). Sumatriptan administration caused amelioration of headache (p < 0.001) and contraction of MMA (p < 0.001), but MCA remained unchanged (p = 0.16). Exploratory analysis revealed that in migraine attacks with half-sided headache, there was only dilatation on the headache side of MMA of 12.49% (95% confidence interval [CI], 4.16-20.83%) and of MCA of 12.88% (95% CI, 3.49-22.27%) and no dilatation on the non headache side of MMA (95% CI, -4.27 to 11.53%) and MCA (95% CI, -6.7 to 14.28%). In double-sided headache we found bilateral vasodilatation of both MMA and MCA (p < 0.001).

INTERPRETATION

These data show that migraine without aura is associated with dilatation of extra- and intracerebral arteries and that the headache location is associated with the location of the vasodilatation. Furthermore, contraction of extracerebral and not intracerebral arteries is associated with amelioration of headache. Collectively, these data suggest that vasodilatation and perivascular release of vasoactive substances is an integral mechanism of migraine pathophysiology.

Quantitative cerebral blood flow with optical coherence tomography

Absolute measurements of cerebral blood flow (CBF) are an important endpoint in studies of cerebral pathophysiology. Currently no accepted method exists for in vivo longitudinal monitoring of CBF with high resolution in rats and mice. Using three-dimensional Doppler Optical Coherence Tomography and cranial window preparations, we present methods and algorithms for regional CBF measurements in the rat cortex. Towards this end, we develop and validate a quantitative statistical model to describe the effect of static tissue on velocity sensitivity. This model is used to design scanning protocols and algorithms for sensitive 3D flow measurements and angiography of the cortex. We also introduce a method of absolute flow calculation that does not require explicit knowledge of vessel angles. We show that OCT estimates of absolute CBF values in rats agree with prior measures by autoradiography, suggesting that Doppler OCT can perform absolute flow measurements in animal models.

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.

Direct, live imaging of cortical spreading depression and anoxic depolarisation using a fluorescent, voltage-sensitive dye

Perilesion depolarisations, whether transient anoxic depolarisation (AD) or spreading depression (SD), occur in stroke models and in patients with acute brain ischaemia, but their contribution to lesion progression remains unclear. As these phenomena correspond to waves of cellular depolarisation, we have developed a technique for their live imaging with a fluorescent voltage-sensitive (VS) dye (RH-1838). Method development and validation were performed in two different preparations: chicken retina, to avoid any vascular interference; and cranial window exposing the cortical surface of anaesthetised rats. Spreading depression was produced by high-K medium, and AD by complete terminal ischaemia in rats. After dye loading, the preparation was illuminated at its excitation wavelength and fluorescence changes were recorded sequentially with a charge-coupled device camera. No light was recorded when the VS dye was omitted, ruling out the contribution of any endogenous fluorophore. With both preparations, the changes in VS dye fluorescence with SD were analogous to those of the DC (direct current) potential recorded with glass electrodes. Although some blood quenching of the emitted light was identified, the VS dye signatures of SD had a good signal-to-noise ratio and were reproducible. The changes in VS dye fluorescence associated with AD were more complex because of additional interferents, especially transient brain swelling with subsequent shrinkage. However, the kinetics of the AD-associated changes in VS dye fluorescence was also analogous to that of the DC potential. In conclusion, this method provides the imaging equivalent of electrical extracellular DC potential recording, with the SD and AD negative shifts translating directly to fluorescence increase.

Dose effect of sevoflurane and isoflurane anesthetics on cortical blood flow during controlled hypotension in the pig

BACKGROUND

The ability of the brain to preserve adequate cerebral blood flow (CBF) during alterations in systemic perfusion pressure is of fundamental importance. At increasing concentrations, isoflurane and sevoflurane have been known to alter CBF, which may be disadvantageous for patients with increased intracranial pressure. The aim was to examine the effects of isoflurane and sevoflurane at increasing minimum alveolar concentrations (MAC) on CBF, during controlled hypotension.

METHODS

We studied eight pigs during variations in perfusion pressure induced by caval block (100, 60, 50, and 40 mmHg) under normocapnia. CBF was measured locally in a defined area (4 x 5 measurement points covering 1 cm(2)) of the motor cortex using laser Doppler perfusion imaging. Physiological variables, assessed by analysis of arterial O(2) and CO(2), hemoglobin and hematocrit, were controlled. CBF was measured during propofol (10 mg x kg(-1)x h(-1)) and fentanyl (0.002 mg x kg(-1)x h(-1)) anesthesia, and then during anesthesia with either isoflurane or sevoflurane (given in random order) at increasing MAC (0.3-1.2). After a washout period, the measurements were repeated with the other gas.

RESULTS

CBF was significantly higher in the cortex during normotensive (control) settings, MAP approximately 100 mmHg, compared with during hypotension (MAP 40-60 mmHg). Neither different anesthetic nor MAC or local measurement sites were found to influence CBF at any perfusion pressure.

CONCLUSION

In this experimental model, the effect of hypotension on CBF was not altered by the anesthetics used [isoflurane, sevoflurane (MAC 0.3-1.2) or propofol (10 mg x kg(-1)x h(-1))]. In this aspect (cortical tissue perspective), these volatile agents appear as suitable as propofol for neurosurgical anesthesia for patients at risk.

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.

Hemodynamic responses in cortex investigated with optical imaging methods. Implications for functional brain mapping

During the last 20 years, optical imaging methods - either alone or in combination with other recording techniques - has proven a fruitful approach to explore both the physiological and the functional aspects of activity-evoked hemodynamic responses in cortex. One of the main advantages of optical imaging consists in its high spatio-temporal resolution (in the order of few microns and milliseconds), allowing not only to unambiguously distinguish between activity patterns relating to the underlying functional architecture and those originating from the activation of medium/large blood vessels, but also to investigate the various activity-evoked hemodynamic processes at very fine detail. Here, we briefly review the principal findings obtained by optical imaging about the spatio-temporal properties of the various hemodynamic responses in cortex, i.e., changes in blood-oxygenation, blood-volume, and, to some extent, blood-flow. We also discuss the implications of those findings for non-invasive high-resolution functional brain imaging, in particular for fMRI. Finally, we underscore the importance of novel approaches for high-resolution blood-flow imaging, in the context of the need to gather information at fine spatial detail about the blood-flow response, necessary to constrain the multiple free parameters of hemodynamic response models.

Functional imaging with Laser Speckle Contrast Analysis: Vascular compartment analysis and correlation with Laser Doppler Flowmetry and somatosensory evoked potentials

Laser Speckle Contrast Analysis (LASCA), a novel, high-resolution blood flow imaging method, was performed on rat somatosensory cortex during functional activation. In the same animals, cerebral blood flow (CBF) was measured with Laser Doppler Flowmetry. To obtain a quantitative estimate of the underlying neuronal activity, somatosensory evoked potentials were recorded simultaneously with an epidural EEG. Our results show that: 1. CBF changes measured by LASCA or LDF are nonlinearly dependent on the magnitude of electrical neural activity revealed by somatosensory evoked potentials. 2. The magnitude of relative CBF changes measured by LASCA and LDF shows a strong correlation. 3. LASCA imaging localizes the highest relative changes of CBF in microcirculatory areas, with a smaller contribution by larger vessels. This study demonstrates that LASCA is a reliable method that provides 2D-imaging of CBF changes that are comparable to LDF measurements. It further suggests that functional neuroimaging methods based on CBF enhance areas of microcirculation and thus might prove more accurate in localizing neural activity than oxygenation related methods like BOLD-fMRI.

Spindle-shaped constriction and propagated dilation of arterioles during cortical spreading depression

A dual illumination technique was used to examine the behavior of parenchymal arterioles of the cortex during cortical spreading depression in six cats. K+ produced a wave-ring spread of optical density changes concentrically from the injection site. At locations where the wave was passing, arterioles first constricted at one or two spots (55+/-43% of control), forming a spindle-shape in the early phase of cortical spreading depression, and then markedly dilated (155+/-57% of control) within 1 min. The dilation started at the constricted spots, propagated bidirectionally and finally resulted in full-length dilation of the arteriole. Although it varied in magnitude and time-course, this arteriolar behavior was observed in all six cats. Despite these changes, no associated downstream tissue microvascular flow changes were discernable.

Spatial extent of CBF response during whisker stimulation using trial averaged laser Doppler imaging

The spatial pattern of activation in response to multiple whisker stimulation was studied using high-resolution laser Doppler (LD) imaging in urethane-anesthetized rats. LD flux change representing cerebral blood flow (CBF) responses were analyzed from a single trial or after averaging a number of similar trials. CBF change in a single trial was observed predominantly over pixels having low baseline flux values (microvessels), which included diffuse circular patterns of activation 400-800 microm in diameter similar to the histological dimensions of individual barrels established in the layer IV of the rat somatosensory cortex. The overall activation pattern varied considerably between each trial (only about 9-10% overlap); however, the diffuse circular pattern of activation was reproducible in every single trial within and across all rats. With trial averaging, no significant increase was observed in the outer boundary of activation, but the number of activated pixels increased within the diffuse circular patterns of activation. Emergence of further active pixels primarily within the diffuse circular regions of activity with trial averaging indicates distinct CBF responses in the septal and barrel regions, with a lesser LD signal to noise ratio in the barrel core.

Diffuse optical correlation tomography of cerebral blood flow during cortical spreading depression in rat brain

Diffuse optical correlation methods were adapted for three-dimensional (3D) tomography of cerebral blood flow (CBF) in small animal models. The image reconstruction was optimized using a noise model for diffuse correlation tomography which enabled better data selection and regularization. The tomographic approach was demonstrated with simulated data and during in-vivo cortical spreading depression (CSD) in rat brain. Three-dimensional images of CBF were obtained through intact skull in tissues(~4mm) deep below the cortex.

Bootstrap resampling method to estimate confidence intervals of activation-induced CBF changes using laser Doppler imaging

Laser Doppler imaging (LDI) signal and noise characteristics can vary significantly depending upon the underlying vascular caliber. Further, noise characteristics are not constant over time (non-stationary) and can vary during resting and activated conditions in a typical experiment. Since only a limited number of images can be acquired in a single run, concatenation of data from similar experimental trials becomes necessary which can induce further variation in temporal noise due to instrumental response. In conventional statistical analysis methods such as cross-correlation, a fixed significance threshold is generally used (for the entire image) to detect activation assuming constant noise over time and a normal distribution. As a consequence, statistical significance can become strong or weak due to temporal differences in baseline LD noise, which can possibly deviate from a normal distribution. The main emphasis of this study was the application of bootstrap resampling in conjunction with cross-correlation to estimate the confidence intervals on a pixel-by-pixel basis to avoid distributional specifications on the additive measurement error leading to reliable whisker activation-induced CBF changes. At a 95% confidence level, bootstrap resampling followed by confidence intervals for the correlation coefficient distribution increased the number of active pixels by almost 45% when compared to conventional cross-correlation. These pixels were mostly confined to areas with intermediate and large baseline LD flux with considerable deviation from normality. It is suggested that confidence intervals of the bootstrap estimates can lead to unbiased detection of CBF change in the cerebral cortex, particularly in regions with large temporal variation in noise and low CNR.

Temporal clustering analysis of cerebral blood flow activation maps measured by laser speckle contrast imaging

Temporal and spatial orchestration of neurovascular coupling in brain neuronal activity is crucial for comprehending the mechanism of functional cerebral metabolism and pathophysiology. Laser speckle contrast imaging (LSCI) through a thinned skull over the somatosensory cortex is utilized to map the spatiotemporal characteristics of local cerebral blood flow (CBF) in anesthetized rats during sciatic nerve stimulation. The time course of signals from all spatial loci among the massive dataset is hard to analyze, especially for the thousands of images, each of which composes millions of pixels. We introduce a temporal clustering analysis (TCA) method, which is proven as an efficient method to analyze functional magnetic resonance imaging (fMRI) data in the temporal domain. The timing and location of CBF activation shows that contralateral hindlimb sensory cortical microflow is activated to increase promptly in less than 1 s after the onset of 2-s electrical stimulation and is evolved in different discrete regions. This pattern is similar but slightly elaborated from the results obtained from laser Doppler flowmetry (LDF) and fMRI. We present this combination to investigate interacting brain regions, which might lead to a better understanding of the nature of brain parcellation and effective connectivity.

Cortical spreading depression produces long-term disruption of activity-related changes in cerebral blood volume and neurovascular coupling

Cortical spreading depression (CSD) is a pronounced depolarization of neurons and glia that spreads slowly across the cortex followed by a period of depressed electrophysiological activity. The vascular changes associated with CSD are a large transient increase in blood flow followed by a prolonged decrease lasting greater than 1 h. Currently, the profile of functional vascular activity during this hypovolemic period has not been well characterized. Perfusion-based imaging techniques such as functional magnetic resonance imaging (fMRI) assume a tight coupling between changes in neuronal and vascular activity. Under normal conditions, these variables are well correlated. Characterizing the effect of CSD on this relationship is an important step to understand the impact acute pathophysiological events may have on neurovascular coupling. We examine the effect of CSD on functional changes in cerebral blood volume (CBV) evoked by cortical electrophysiological activity for 1 h following CSD induction. CBV signal amplitude, duration, and time to peak show little recovery at 60 min post-induction. Analysis of spontaneous vasomotor activity suggests a decrease in vascular reactivity may play a significant role in the disruption of normal functional CBV responses. Electrophysiological activity is also attenuated but to a lesser degree. CBV and evoked potentials are not well correlated following CSD, suggesting a breakdown of the neurovascular coupling relationship.

Optical imaging of the spatiotemporal dynamics of cerebral blood flow and oxidative metabolism in the rat barrel cortex

Oxidative metabolism and cerebral blood flow (CBF) are two of the most important measures in neuroimaging. However, results from concurrent imaging of the two with high spatial and temporal resolution have never been published. We used flavoprotein autofluorescence (AF) and laser speckle imaging (LSI) in the anaesthetized rat to map oxidative metabolism and CBF in response to single vibrissa stimulation. Autofluorescence responses reflecting oxidative metabolism demonstrated a fast increase with a delay of 0.1 s. The sign-reversed speckle contrast reflecting CBF started to rise with a delay of 0.6 s and reached its maximum 1.4 s after the stimulation offset. The fractional signal changes were 2.0% in AF and 9.7% in LSI. Pixelwise modelling revealed that CBF maps spread over an area up to 2.5-times larger than metabolic maps. The results provide evidence that the increase in cerebral oxidative metabolism in response to sensory stimulation is considerably faster and more localized than the CBF response. This suggests that future developments in functional imaging concentrating on the metabolic response promise an increased spatial resolution.

Instant myocardial blood flow monitor: its calibration and assessment of flow capacity of the intracoronary shunt tube

BACKGROUND

We developed a new instant regional myocardial blood flow (RMBF) monitor utilizing the thermal diffusion method in which the RMBF value is presented as the value inversely proportional to the thermocouple voltage output (1/V). The purposes of this study were (1) to validate the accuracy of RMBF measurement by the instant RMBF monitor in comparison with the colored microsphere method for calibration; (2) to investigate influences of it on the RMBF; and (3) to assess changes in RMBF caused by the shunt tube insertion.

METHODS

Twenty pigs were used for this study: 4 for comparison between the instant RMBF meter and the colored microsphere method, 4 for validation of reproducibility, and 6 for measurement of RMBF during shunt tube.

RESULTS

The relation between RMBF values obtained by the colored microsphere method and 1/V values by instant RMBF monitor was colored microsphere = 140,992 (1/V)--231 in epicardial layer (R2 = 0.819) and colored microsphere = 111,381 (1/V)--165 in endocardial layer (R2 = 0.693). The correlation coefficient and R2 values between RMBF values measured by both methods were 0.985 and 0.839 in epicardial layer, and 0.963 and 0.679 in endocardial layer, respectively. The RMBF at each layer did not change after the attachment of the monitor. Fifteen minutes after shunt tube insertion, RMBF measured by the colored microsphere method decreased to 31.1% (p = 0.0001) and 33.7% (p = 0.0001) in epicardium and endocardium, respectively, and no difference was observed from the value measured by the instant RMBF monitor.

CONCLUSIONS

This instant RMBF monitor can provide instantaneous and continuous information of RMBF without requiring tissue examination.

Laser speckle flowmetry for the study of cerebrovascular physiology in normal and ischemic mouse cortex

Laser speckle flowmetry (LSF) is useful to assess noninvasively two-dimensional cerebral blood flow (CBF) with high temporal and spatial resolution. The authors show that LSF can image the spatiotemporal dynamics of CBF changes in mice through an intact skull. When measured by LSF, peak CBF increases during whisker stimulation closely correlated with simultaneous laser-Doppler flowmetry (LDF) measurements, and were greater within the branches of the middle cerebral artery supplying barrel cortex than within barrel cortex capillary bed itself. When LSF was used to study the response to inhaled CO2 (5%), the flow increase was similar to the response reported using LDF. For the upper and lower limits of autoregulation, mean arterial pressure values were 110 and 40 mm Hg, respectively. They also show a linear relationship between absolute resting CBF, as determined by [C]iodoamphetamine technique, and 1/tau(c) values obtained using LSF, and used 1/tau(c) values to compare resting CBF between different animals. Finally, the authors studied CBF changes after distal middle cerebral artery ligation, and developed a model to investigate the spatial distribution and hemodynamics of moderate to severely ischemic cortex. In summary, LSF has distinct advantages over LDF for CBF monitoring because of high spatial resolution.

Negative functional response to sensory stimulation and its origins

Functional imaging studies typically give prominence to positive responses. However, negative changes that accompany activation-induced positive responses are not yet clearly understood. The authors investigated the origin of sustained negative CBF responses that accompanied positive CBF changes. Measurements were made in the rat somatosensory cortex in response to whisker stimulation using laser-Doppler imaging. Flux images indicative of CBF were obtained at rest and during whisker stimulation with a spatial resolution of 200 microm. Large and intermediate blood vessels in the cortical surface exhibiting high flux values were clearly resolved. This greatly reduced the contamination of the tissue pixel volume with macroscopic blood vessels. Regions that responded positively to whisker stimulation were from areas with intermediate to low baseline flux and distinctly away from high flux areas. Stimulation-induced change in signal intensity was the largest in pixels with low baseline flux, presumably from tissue and microvessels. Simultaneously, a sustained decrease in signal intensity was observed in regions with high baseline flux values. The temporal coherence, macrovascular origin, lesser trial-to trial variability, and complete absence of the negative CBF response in the microvascular regions suggest that it may be purely hemodynamic in nature.

Real-time cortical cerebral blood flow follow-up in conscious, freely moving rats by laser Doppler flowmetry

This article describes a laser Doppler flowmetry (LDF) system that enables repeated measurements and thereby long-term followup of cortical cerebral blood flow (CBF) in awake and freely moving rats. The system consists of a specially designed flow probe adapter, a flow probe connector, and a LDF flow probe, which may thereby rotate through its own axis. During the experiment, the flow adapter is permanently mounted onto the rat's skull bone. A thin layer of skull bone is left intact at the site for cortical CBF measurements. The probe connector and the flow probe may be repeatedly detached and remounted to the adapter, which allows for cortical cerebral blood flow recording from exactly the same anatomical location. The laser Doppler flowmetry system enables stable cortical CBF recordings in the conscious rat while it moves freely in a bowl cage.

Automated registration of laser Doppler perfusion images by an adaptive correlation approach: application to focal cerebral ischemia in the rat

Hemodynamic changes are extremely important in analyzing responses from a brain subjected to a stimulus or treatment. The Laser Doppler technique has emerged as an important tool in neuroscience research. This non-invasive method scans a low-power laser beam in a raster pattern over a tissue surface to generate the time course of images in unit of relative flux changes. Laser Doppler imager (LDI) records cerebral perfusion not only in the temporal but also in the spatial domain. The traditional analysis of LD images has been focused on the region-of-interest (ROI) approach, in which the analytical accuracy in an experiment that necessitates a relative repositioning between the LDI and the scanned tissue area will be weakened due to the operator's subjective decision in data collecting. This report describes a robust image registration method designed to obviate this problem, which is based on the adaptive correlation approach. The assumption in mapping corresponding pixels in two images is to correlate the regions in which these pixels are centered. Based on this assumption, correlation coefficients are calculated between two regions by a method in which one region is moved around over the other in all possible combinations. To avoid ambiguity in distinguishing maximum correlation coefficients, an adaptive algorithm is adopted. Correspondences are then used to estimate the transformation by linear regression. We used a pair of phantom LD images to test this algorithm. A reliability test was also performed on each of the 15 sequential LD images derived from an actual experiment by imposing rotation and translation. The result shows that the calculated transformation parameters (rotation: theta =7.7+/-0.5 degrees; translation: Delta x =2.8+/-0.3, Deltaŷ=4.7+/-0.4) are very close to the prior-set parameters (rotation: theta=8 degrees; translation: Delta x=3, Delta y=5). This result indicates that this approach is a valuable adjunct to LD perfusion monitoring. An original sequence of LD images that recorded cerebral perfusion through a cranial window before, during and after middle cerebral artery occlusion (MCAo) is presented, together with the registered image sequence. Cerebral perfusion data acquired in a pixel-based manner from different anatomic locations of the registered LD image sequence are also presented over the whole time-course of the experiment.

Cortical blood flow autoregulation revisited using laser Doppler perfusion imaging

Methods of laser Doppler perfusion monitoring (LDPM) and imaging (LDPI) have been validated and found useful for measurements of brain blood flow in several studies. The present work was undertaken to examine the cortical blood flow autoregulatory phenomenon as it has lately been questioned and claimed to be method-dependent and related to sample volume. Spatial variations in cerebral cortical blood flow (CBF(cortex)) in the pressure range 20-140 mmHg (static cerebral autoregulation; caval block/angiotensin infusion) were studied in six mechanically ventilated (hypocapnic, normocapnic and hypercapnic) pigs anaesthetized with propofol and fentanyl. Although the cortical blood flow values sampled were highly heterogeneously distributed, they were strongly pressure-dependent as well as CO2-dependent (P < 0.001). A cumulative cerebral blood flow (CBF)-pressure (MAP) plot comprising all values obtained indicated a pressure range between 70 and 120 mmHg where CBF remained almost constant. However, at the local level in the cortex (mm2) the same type of 'classic' autoregulatory flow : pressure graphs (FPG) were found in only a few of the cases of the cortical areas examined (n = 96). Alterations in blood P(a)CO2 saturation did not affect the pressure : flow relationship at low perfusion pressures, whereas at normal or above normal values, and as anticipated, hypercapnia considerably increased CBF (P < 0.001). 'Classic' autoregulatory FPGs were found only when all values sampled were clustered together, whereas, as a new finding, data are presented indicating that autoregulatory capacity is lacking at the local level at some cortical surface areas.

Multiwavelength optical intrinsic signal imaging of cortical spreading depression

Cortical spreading depression (CSD) is an important disease model for migraine and cerebral ischemia. In this study, we exploit the high temporal and spatial resolution of optical imaging to characterize perfusion-dependent and -independent changes in response to CSD and to investigate the etiology of reflectance changes during CSD. In this experiment, we characterized the optical response to CSD at wavelengths that emphasize perfusion-related changes (610 and 550 nm), and we compared these results with 850 nm and blood volume data. Blood volume changes during CSD were recorded using an intravascular fluorescent dye, Texas Red dextran. We observed triphasic optical signals at 850 and 550 nm characterized by spreading waves of increased, decreased, then increased reflectance (Fig. 1) which expanded at a rate of approximately 3-5 mm/min. The signal at 610 nm had a similar initial phase, but the phase 2 response was slightly more complex, with a parenchymal decrease in reflectance but a vascular increase in reflectance. Reflectance values decreased in phase three. Blood volume signals were delayed relative to the optical intrinsic signals and corresponded temporally to phases 2 and 3. This is the first study to characterize optical imaging of intrinsic signal responses to CSD, in vivo, at multiple wavelengths. The data presented here suggest that changes in light scattering precede perfusion responses, the blood volume increase (phase 2) is accompanied by a reduction in deoxyhemoglobin, and the blood volume decrease (phase 3) is accompanied by an increase in deoxyhemoglobin. Previous studies have suggested the oligemia of spreading depression was a result of decreased metabolic demand. This study suggests that during the oligemic period there is a greater reduction in oxygen delivery than in demand.

On the conservation of fast calcium wave speeds

Calcium waves were first seen about 25 years ago as the giant, 10 micro m/s wave or tsunami which crosses the cytoplasm of an activating medaka fish egg [J Cell Biol 76 (1978) 448]. By 1991, reports of such waves with approximately 10 micro m/s velocities through diverse, activating eggs and with approximately 30 micro m/s velocities through diverse, fully active systems had been compiled to form a class of what are now called fast calcium waves [Proc Natl Acad Sci USA 88 (1991) 9883; Bioessays 21 (1999) 657]. This compilation is now updated to include organisms from algae and sponges up to blowflies, squid and men and organizational levels from mammalian brains and hearts as well as chick embryos down to muscle, nerve, epithelial, blood and cancer cells and even cell-free extracts. Plots of these data confirm the narrow, 2-3-fold ranges of fast wave speeds through activating eggs and 3-4-fold ones through fully active systems at a given temperature. This also indicate Q(10)'s of 2.7-fold per 10 degrees C for both activating eggs and for fully activated cells.Speeds through some ultraflat preparations which are a few-fold above the conserved range are attributed to stretch propagated calcium entry (SPCE) rather than calcium-induced calcium release (CICR).

Laser Doppler flowmetry mapping of cerebrocortical microflow: characteristics and limitations

The aim of this study was to quantitatively analyze the amount of methodological noise and the spatial and temporal variability of laser Doppler flowmetry (LDF) signals mapping cerebrocortical microflow. In an experimental setup with latex beads, the methodological LDF-signal variability was determined (coefficient of variation or CV(method)). The biological variability of the LDF signals was measured in animal experiments using 10 anesthetized rabbits. One stationary reference probe was used to assess temporal heterogeneity (CV(temp)) and a micromanipulator-driven scanning probe was used to assess spatial heterogeneity (CV(spat)) in a cortical area of 3.5 x 4.5 mm with 252 measurement points. CO(2) tests were used to modulate cerebrovascular resistance. CV(method) was found to be 4.94 +/- 1.7. The CV(temp) for the LDF-velocity signal was assessed to be 13.93 +/- 5.9 during normocapnia. Scanning of the brain surface with the scanning probe revealed a CV(spat) for LDF velocity of 65.0 +/- 16.2 during normocapnia. CO(2) modulation (hypocapnia --> normocapnia --> hypercapnia) of the cerebral resistance did not show a significant change in temporal heterogeneity (10.84 +/- 3.1 --> 13.93 +/- 5.9 --> 14.82 +/- 3.9), whereas spatial heterogeneity decreased significantly (81.31 +/- 12.0 --> 65.0 +/- 16.2 --> 54.04 +/- 21.8). Although the spatial and temporal variability of LDF signals evoked by cerebrocortical microflow is in the same range as with other methods and in other organs, LDF cerebrocortical mapping is restricted by the large temporal and spatial heterogeneity of the cerebrocortical vasculature. The definitions of sample volume, scanning step width, probe to brain surface distance, and average time per scanning point are critical concerning reliable LDF cerebrocortical mapping techniques.

Laser Doppler, speckle and related techniques for blood perfusion mapping and imaging

Laser Doppler velocimetry uses the frequency shift produced by the Doppler effect to measure velocity. It can be used to monitor blood flow or other tissue movement in the body. Laser speckle is a random interference effect that gives a grainy appearance to objects illuminated by laser light. If the object consists of individual moving scatterers (such as blood cells), the speckle pattern fluctuates. These fluctuations provide information about the velocity distribution of the scatterers. It can be shown that the speckle and Doppler approaches are different ways of looking at the same phenomenon. Both these techniques measure at a single point. If a map of the velocity distribution is required, some form of scanning must be introduced. This has been done for both time-varying speckle and laser Doppler. However, with the speckle technique it is also possible to devise a full-field technique that gives an instantaneous map of velocities in real time. This review article presents the theory and practice of these techniques using a tutorial approach and compares the relative merits of the scanning and full-field approaches to velocity map imaging. The article concludes with a review of reported applications of these techniques to blood perfusion mapping and imaging.

Neuroprotective effect of treatment with human albumin in permanent focal cerebral ischemia: histopathology and cortical perfusion studies

In recent experimental studies, we demonstrated a highly beneficial neuroprotective effect of moderate- to high-dose human albumin treatment of transient focal cerebral ischemia, but we did not define the effect of albumin therapy in permanent focal cerebral ischemia. In this study, anesthetized Sprague-Dawley rats were subjected to permanent middle cerebral artery occlusion by retrograde insertion of an intraluminal nylon suture coated with poly-L-lysine. Albumin was administered i.v. at 2 h after onset of middle cerebral artery occlusion, in doses of either 1.25 (n=8) or 2.5 g/kg (n=6). In a separate group of animals, albumin (2.5 g/kg) was given 1 h after middle cerebral artery occlusion (n=6). Vehicle-treated rats (n=6) received 0.9% saline in equivalent volumes. Neurological status was evaluated during and 24 h after middle cerebral artery occlusion. One day after middle cerebral artery occlusion, infarct volumes and brain edema were determined. In a separate group of animals, cortical perfusion was assessed by Laser-Doppler perfusion imaging. Albumin (1.25 g/kg; n=3) or vehicle (sodium chloride 0.9%; n=3) was administered at 2 h after onset of middle cerebral artery occlusion. Higher-dose albumin therapy (2.5 g/kg) significantly improved the neurological score compared to vehicle rats at 24 h, when administered at either 1 or 2 h after middle cerebral artery occlusion. Total infarct volume was reduced by albumin (2.5 g/kg given at 2 h) by 32% compared with vehicle-treated rats. Both albumin doses (1.25 and 2.5 g/kg) significantly reduced cortical and striatal infarct areas at several coronal levels when administered at 2 h after middle cerebral artery occlusion. Brain swelling was not affected by albumin treatment. Cortical perfusion declined during middle cerebral artery occlusion in both groups. Treatment with albumin led to 48% increases in cortical perfusion (P<0.002), but saline caused no change. These results support a beneficial effect of albumin therapy in permanent focal cerebral ischemia.

Dynamic imaging of cerebral blood flow using laser speckle

A method for dynamic, high-resolution cerebral blood flow (CBF) imaging is presented in this article. By illuminating the cortex with laser light and imaging the resulting speckle pattern, relative CBF images with tens of microns spatial and millisecond temporal resolution are obtained. The regional CBF changes measured with the speckle technique are validated through direct comparison with conventional laser-Doppler measurements. Using this method, dynamic images of the relative CBF changes during focal cerebral ischemia and cortical spreading depression were obtained along with electrophysiologic recordings. Upon middle cerebral artery (MCA) occlusion, the speckle technique yielded high-resolution images of the residual CBF gradient encompassing the ischemic core, penumbra, oligemic, and normally perfused tissues over a 6 x 4 mm cortical area. Successive speckle images demonstrated a further decrease in residual CBF indicating an expansion of the ischemic zone with finely delineated borders. Dynamic CBF images during cortical spreading depression revealed a 2 to 3 mm area of increased CBF (160% to 250%) that propagated with a velocity of 2 to 3 mm/min. This technique is easy to implement and can be used to monitor the spatial and temporal evolution of CBF changes with high resolution in studies of cerebral pathophysiology.

Laser doppler imaging of activation-flow coupling in the rat somatosensory cortex

Activation-flow coupling (AFC) provides a physiological basis for mapping cerebral activation using cerebral blood flow (CBF) as a surrogate marker for neuronal function. Laser Doppler offers a minimally invasive approach for measuring changes in cerebral blood flow but the spatial resolution of this technique is limited by the number of individual probes that can be used. Recently, laser Doppler imaging (LDI) scanners, which use computer-driven optics to scan and measure LD changes in two dimensions, have successfully measured flow changes in the exposed cortex of animals. Here we demonstrate the use of an LDI device through a thinned skull to determine the spatiotemporal characteristics of AFC in alpha-chloralose anesthetized rats in response to electrical forepaw stimulation. The spatial and temporal characteristics of the AFC response measured by LDI are in agreement with prior results obtained using a single LD probe. These results suggest a promising role for LDI in the characterization of the spatiotemporal characteristics of AFC in animal models and possibly for intraoperative monitoring in the human brain.

Functional recruitment of red blood cells to rat brain microcirculation accompanying increased neuronal activity in cerebellar cortex

Scanning laser-Doppler flowmetry (SLDF) combines laser-Doppler flowmetry and laser scanning to provide images of cerebral blood flow (CBF) with high spatial and temporal resolution. We investigated the contribution of single vascular elements to the local increase of CBF accompanying increased neuronal activity in halothane-anesthetized rats. CBF was examined in the cerebellar cortex under control conditions and in response to electrical stimulation of parallel and climbing fibers. At rest, arterioles contributed 9%, venules 11-13% and small vessels (< 20 microm) 8-14%, while the background constituted 64-72% of the total SLDF signal. During activation the background signal decreased to 55-60% while the signal from arterioles increased to 11-12%, from venules to 14-15% and from small vessels to 14-19%. The signal increase in small vessels that did not give any laser-Doppler signal at rest was due to functional recruitment of red blood cells to the capillary bed. We conclude that functional recruitment may be an integral part of the hemodynamic response accompanying neuronal activity.

Changes in the complex permittivity during spreading depression in rat cortex

With recent developments in current density imaging (CDI), it is feasible to utilize this new technique in brain imaging applications. Since CDI's ability to measure changes in current density depends on a concomitant activity-dependent change in the conductivity of the brain tissue, we have examined the changes in complex conductivity during spreading depression (SD) in rodent neocortex using a coaxial probe. SD was chosen because it is often referred to as an animal model of cerebral ischemia and migraine with aura. The conductivity measurements revealed a change with short latency (30-60 s) followed by a change with a longer latency (200-300 s). This change in conductivity with short latency has not been reported before, and we conjecture that it may be the priming or triggering mechanism prior to the main SD episode. A 20% change in conductivity during SD is sufficiently large to be measured by CDI. Therefore, the ability to measure changes in the conductivity, as opposed to metabolic changes, makes CDI a viable approach to the study of ischemia and migraine with aura.

Intercellular calcium waves in glia

Glial cells are capable of communicating increases in [Ca2+]i from a single cell to many surrounding cells. These intercellular Ca2+ waves have been observed in glia in multiple different preparations, including dissociated brain cell cultures, glial cell lines, organotypic brain slice cultures, and intact retinal preparations. They may occur spontaneously, or in response to a variety of stimuli. Ca2+ waves occurring under different conditions in different preparations may have distinctive patterns of initiation and propagation, and distinctive pharmacological characteristics consistent with the involvement of different intracellular and intercellular signaling pathways. This paper presents original data supporting a combination of gap junction and extracellular messenger-mediated signaling in mechanically induced glial Ca2+ waves. Additional new observations provide evidence that a rapidly propagated signal may precede the glial Ca2+ wave and may mediate rapid glial-neuronal communication. This original data is discussed in the context of a review of the literature and current concepts regarding the potential mechanisms, physiological and pathological roles of this dynamic pattern of glial intercellular signaling.

On the conservation of calcium wave speeds

Most long distance calcium signals are believed to take the form of actively propagated calcium waves. In 1991, when this proposal was first advanced, all such waves were thought to belong to one class, for which fertilization waves were the prototype. Moreover, the speeds of such waves were found to be conserved at about 10 microns/s for primary fertilization waves and 30 microns/s for waves through fully active systems at 20 degrees C. In 1993, preliminary evidence for a second class of such waves was published and the prototype for these were ones which drive cell cleavage. These move at only about 1 micron/s at 20 degrees C and were, therefore, called slow calcium waves as opposed to the fast ones first considered. Here we compile compelling evidence that slow waves comprise a second distinct class of actively propagated calcium waves. This is based on 30 papers which yield evidence of slow calcium waves in organisms ranging from Dictyostelium to mammals and phenomena ranging from the surface contraction waves seen long ago in axolotl eggs to embryonic cleavage and mitotic waves and to ones recently seen to accompany primary neural induction in axolotls. Ultraslow and ultrafast calcium waves are also considered.

Induced hypertension improves regional blood flow and protects against infarction during focal ischemia: time course of changes in blood flow measured by laser Doppler imaging

OBJECTIVE

To characterize changes in regional blood flow (rCBF) during and after a period of arterial occlusion and determine the effect on rCBF and on the extent of infarction when the mean arterial blood pressure is increased during the period of occlusion.

METHODS

rCBF in the middle cerebral artery (MCA) territory of rabbits was monitored using laser Doppler perfusion imaging before, during, and after a 1- or 2-hour period of MCA occlusion, and the size of the infarction was assessed by 2,3,5-triphenyltetrazolamine chloride staining after 2 hours of reperfusion. Test animals, the mean arterial blood pressure of which was increased by 65 mm Hg with intravenous phenylephrine during the ischemia, were compared with control animals that remained normotensive. The laser Doppler perfusion imager (Lisca Developments Co., Linköping, Sweden) scanned a 3-cm2 area of cortex with a resolution of 4 mm2 every 15 minutes.

RESULTS

MCA occlusion reduced rCBF to 71 +/- 2% of the control level (n = 24, P < 0.001). Hypertension (HTN) restored rCBF to 84 +/- 3% of the control level (n = 12, P < 0.01), but the HTN-induced improvement diminished with time, so that after 1 hour, there was no longer a significant difference between hypertensive and normotensive animals. HTN during the MCA occlusion caused a 97% reduction in infarct size (P < 0.05) in the animals subjected to 1 hour of occlusion but caused only a 45% reduction (P approximately 0.1) in the animals subjected to 2 hours of occlusion.

CONCLUSION

This study supports the use of HTN to minimize ischemic injury from short intervals of major intracranial vessel occlusion but fails to demonstrate protection when HTN is maintained during occlusions of more than 1 hour.