Isocortical Hyperemia and Allocortical Inflammation and Atrophy Following Generalized Convulsive Seizures of Thalamic Origin in the Rat

1. Generalized convulsive seizures can be elicited by a single unilateral microinjection of the cholinergic muscarinic agonist, carbachol, into the specific sites of the thalamus including ventral posterolateral and the reticular thalamic nuclei. The implication of the thalamic specific and reticular neurons is reviewed and discussed.2. On the basis of the c-fos regional expression and well-known efferent and afferent pathways linking these regions, a neuronal network relating the limbic, thalamo–striatal–cortical, and central autonomic systems, was constructed.3. The pattern of Fos immunoreactivity associated with long-lasting isocortical vasodilatation elicited by generalized convulsive seizures in anesthetized rat following cholinergic stimulation of the thalamus can be attributed to both the electrocortical activity and the long-lasting increase in cortical blood flow. We propose that the sustained cerebral cortical blood flow response during convulsive epileptic seizures may implicate intracerebral vasodilatory and vasoconstrictory neural mechanisms. Double-labeled NADPH-d and Fos-positive neurons implicated in maintaining the sustained isocortical vasodilatory response were found in the anterior lateral hypothalamic area. Inhibition of these neurons prevented the increase in cortical blood flow despite an increased metabolic demand manifested by the ictal electrocortical activity.4. Medial temporal lobe atrophy, including hippocampus, amygdala, and parahippocampal gyrus (piriform and entorhinal cortices) are the most common pathology in man. However the origin of medial lobe atrophy remain uncertain. Our results provide evidence that the allocortical microvascular inflammation may be in origin of the neurovascular degenerative processes leading to atrophy.     

Cerebral Hemodynamic Changes of Mild Traumatic Brain Injury at the Acute Stage

Mild traumatic brain injury (mTBI) is a significant public health care burden in the United States. However, we lack a detailed understanding of the pathophysiology following mTBI and its relation to symptoms and recovery. With advanced magnetic resonance imaging (MRI), we can investigate brain perfusion and oxygenation in regions known to be implicated in symptoms, including cortical gray matter and subcortical structures. In this study, we assessed 14 mTBI patients and 18 controls with susceptibility weighted imaging and mapping (SWIM) for blood oxygenation quantification. In addition to SWIM, 7 patients and 12 controls had cerebral perfusion measured with arterial spin labeling (ASL). We found increases in regional cerebral blood flow (CBF) in the left striatum, and in frontal and occipital lobes in patients as compared to controls (p = 0.01, 0.03, 0.03 respectively). We also found decreases in venous susceptibility, indicating increases in venous oxygenation, in the left thalamostriate vein and right basal vein of Rosenthal (p = 0.04 in both). mTBI patients had significantly lower delayed recall scores on the standardized assessment of concussion, but neither susceptibility nor CBF measures were found to correlate with symptoms as assessed by neuropsychological testing. The increased CBF combined with increased venous oxygenation suggests an increase in cerebral blood flow that exceeds the oxygen demand of the tissue, in contrast to the regional hypoxia seen in more severe TBI. This may represent a neuroprotective response following mTBI, which warrants further investigation.     

Time-dependent spatial specificity of high-resolution fMRI: insights into mesoscopic neurovascular coupling

High-resolution functional magnetic resonance imaging (fMRI) is becoming increasingly popular because of the growing availability of ultra-high magnetic fields which are capable of improving sensitivity and spatial resolution. However, it is debatable whether increased spatial resolutions for haemodynamic-based techniques, like fMRI, can accurately detect the true location of neuronal activity. We have addressed this issue in functional columns and layers of animals with haemoglobin-based optical imaging and different fMRI contrasts, such as blood oxygenation level-dependent, cerebral blood flow and cerebral blood volume fMRI. In this review, we describe empirical evidence primarily from our own studies on how well these fMRI signals are spatially specific to the neuronally active site and discuss insights into neurovascular coupling at the mesoscale.This article is part of the theme issue ‘Key relationships between non-invasive functional neuroimaging and the underlying neuronal activity’.     

Multispectral optoacoustic tomography of muscle perfusion and oxygenation under arterial and venous occlusion: A human pilot study

Perfusion and oxygenation are critical parameters of muscle metabolism in health and disease. They have been both the target of many studies, in particular using near‐infrared spectroscopy (NIRS). However, difficulties with quantifying NIRS signals have limited a wide dissemination of the method to the clinics. Our aim was to investigate whether clinical multispectral optoacoustic tomography (MSOT) could enable the label‐free imaging of muscle perfusion and oxygenation under clinically relevant challenges: the arterial and venous occlusion. We employed a hybrid clinical MSOT/ultrasound system equipped with a hand‐held scanning probe to visualize hemodynamic and oxygenation changes in skeletal muscle under arterial and venous occlusions. Four (N = 4) healthy volunteers were scanned over the forearm for both 3‐minute occlusion challenges. MSOT‐recorded pathophysiologically expected results during tests of disturbed blood flow with high resolution and without the need for contrast agents. During arterial occlusion, MSOT‐extracted Hb‐values showed an increase, while HbO₂‐ and total blood volume (TBV)‐values remained roughly steady, followed by a discrete increase during the hyperemic period after cuff deflation. During venous occlusion, results showed a clear increase in intramuscular HbO₂, Hb and TBV within the segmented muscle area. MSOT was found to be capable of label‐free non‐invasive imaging of muscle hemodynamics and oxygenation under arterial and venous occlusion. We introduce herein MSOT as a novel modality for the assessment of vascular disorders characterized by disturbed blood flow, such as acute limb ischemia and venous thrombosis.     

Interpretation of near-infrared spectroscopy signals: a study with a newly developed perfused rat brain model.

Using a newly developed perfused rat brain model, we examined direct effects of each change in cerebral blood flow (CBF) and oxygen metabolic rate on cerebral hemoglobin oxygenation to interpret near-infrared spectroscopy signals. Changes in CBF and total hemoglobin (tHb) were in parallel, although tHb showed no change when changes in CBF were small (< or =10%). Increasing CBF caused an increase in oxygenated hemoglobin (HbO(2)) and a decrease in deoxygenated hemoglobin (deoxy-Hb). Decreasing CBF was accompanied by a decrease in HbO(2), whereas changes in direction of deoxy-Hb were various. Cerebral blood congestion caused increases in HbO(2), deoxy-Hb, and tHb. Administration of pentylenetetrazole without increasing the flow rate caused increases in HbO(2) and tHb with a decrease in deoxy-Hb. There were no significant differences in venous oxygen saturation before vs. during seizure. These results suggest that, in activation studies with near-infrared spectroscopy, HbO(2) is the most sensitive indicator of changes in CBF, and the direction of changes in deoxy-Hb is determined by the degree of changes in venous blood oxygenation and volume.     

Models of focal cerebral ischemia in the nonhuman primate

Ischemic stroke is a uniquely human disease syndrome. Models of focal cerebral ischemia developed in nonhuman primates provide clinically relevant platforms for investigating pathophysiological alterations associated with ischemic brain injury, microvascular responses, treatment responses, and clinically relevant outcomes that may be appropriate for ischemic stroke patients. A considerable number of advantages attend the use of nonhuman primate models in cerebral vascular research. Appropriate development of such models requires neurosurgical expertise to produce single or multiple vascular occlusions. A number of experimentally and clinically accessible outcomes can be measured, including neurological deficits, neuron injury, evidence of non-neuronal cell injury, infarction volume, real-time imaging of injury development, vascular responses, regional cerebral blood flow, microvascular events, the relation between neuron and vascular events, and behavioral outcomes. Nonhuman primate models of focal cerebral ischemia provide excellent opportunities for understanding the vascular and cellular pathophysiology of cerebral ischemic injury, which resembles human ischemic stroke, and the appropriate study of pharmacological interventions in a human relevant setting.     

Cerebral blood flow and metabolism during exercise: implications for fatigue.

During exercise: the Kety-Schmidt-determined cerebral blood flow (CBF) does not change because the jugular vein is collapsed in the upright position. In contrast, when CBF is evaluated by (133)Xe clearance, by flow in the internal carotid artery, or by flow velocity in basal cerebral arteries, a approximately 25% increase is detected with a parallel increase in metabolism. During activation, an increase in cerebral O(2) supply is required because there is no capillary recruitment within the brain and increased metabolism becomes dependent on an enhanced gradient for oxygen diffusion. During maximal whole body exercise, however, cerebral oxygenation decreases because of eventual arterial desaturation and marked hyperventilation-related hypocapnia of consequence for CBF. Reduced cerebral oxygenation affects recruitment of motor units, and supplemental O(2) enhances cerebral oxygenation and work capacity without effects on muscle oxygenation. Also, the work of breathing and the increasing temperature of the brain during exercise are of importance for the development of so-called central fatigue. During prolonged exercise, the perceived exertion is related to accumulation of ammonia in the brain, and data support the theory that glycogen depletion in astrocytes limits the ability of the brain to accelerate its metabolism during activation. The release of interleukin-6 from the brain when exercise is prolonged may represent a signaling pathway in matching the metabolic response of the brain. Preliminary data suggest a coupling between the circulatory and metabolic perturbations in the brain during strenuous exercise and the ability of the brain to access slow-twitch muscle fiber populations.     

Patient-Specific Detection of Cerebral Blood Flow Alterations as Assessed by Arterial Spin Labeling in Drug-Resistant Epileptic Patients

Electrophysiological and hemodynamic data can be integrated to accurately and precisely identify the generators of abnormal electrical activity in drug-resistant focal epilepsy. Arterial Spin Labeling (ASL), a magnetic resonance imaging (MRI) technique for quantitative noninvasive measurement of cerebral blood flow (CBF), can provide a direct measure of variations in cerebral perfusion associated with the epileptic focus. In this study, we aimed to confirm the ASL diagnostic value in the identification of the epileptogenic zone, as compared to electrical source imaging (ESI) results, and to apply a template-based approach to depict statistically significant CBF alterations. Standard video-electroencephalography (EEG), high-density EEG, and ASL were performed to identify clinical seizure semiology and noninvasively localize the epileptic focus in 12 drug-resistant focal epilepsy patients. The same ASL protocol was applied to a control group of 17 healthy volunteers from which a normal perfusion template was constructed using a mixed-effect approach. CBF maps of each patient were then statistically compared to the reference template to identify perfusion alterations. Significant hypo- and hyperperfused areas were identified in all cases, showing good agreement between ASL and ESI results. Interictal hypoperfusion was observed at the site of the seizure in 10/12 patients and early postictal hyperperfusion in 2/12. The epileptic focus was correctly identified within the surgical resection margins in the 5 patients who underwent lobectomy, all of which had good postsurgical outcomes. The combined use of ESI and ASL can aid in the noninvasive evaluation of drug-resistant epileptic patients.     

Asymmetric regional cerebral blood flow in sedated baboons measured by positron emission tomography (PET)

The analysis of structural brain asymmetry has been a focal point in anthropological theories of human brain evolution and the development of lateralized behaviors. While physiological brain asymmetries have been documented for humans and animals presenting with pathological conditions or under certain activation tasks, published studies on baseline asymmetries in healthy individuals have produced conflicting results. We tested for the presence of cerebral blood flow asymmetries in 7 healthy, sedated baboons using positron emission tomography, a method of in vivo autoradiography. Five of the 7 baboons exhibited hemispheric asymmetries in which left-sided flow was significantly greater than right-sided flow. Furthermore, the degree of asymmetry in 8 of 24 brain regions was found to be significantly correlated with age; older individuals exhibited a higher degree of asymmetry than younger individuals. Cerebral blood flow itself was uncorrelated with age, and differences between males and females were not significant.     

Dynamic Imaging of Coherent Sources Reveals Different Network Connectivity Underlying the Generation and Perpetuation of Epileptic Seizures

The concept of focal epilepsies includes a seizure origin in brain regions with hyper synchronous activity (epileptogenic zone and seizure onset zone) and a complex epileptic network of different brain areas involved in the generation, propagation, and modulation of seizures. The purpose of this work was to study functional and effective connectivity between regions involved in networks of epileptic seizures. The beginning and middle part of focal seizures from ictal surface EEG data were analyzed using dynamic imaging of coherent sources (DICS), an inverse solution in the frequency domain which describes neuronal networks and coherences of oscillatory brain activities. The information flow (effective connectivity) between coherent sources was investigated using the renormalized partial directed coherence (RPDC) method. In 8/11 patients, the first and second source of epileptic activity as found by DICS were concordant with the operative resection site; these patients became seizure free after epilepsy surgery. In the remaining 3 patients, the results of DICS / RPDC calculations and the resection site were discordant; these patients had a poorer post-operative outcome. The first sources as found by DICS were located predominantly in cortical structures; subsequent sources included some subcortical structures: thalamus, Nucl. Subthalamicus and cerebellum. DICS seems to be a powerful tool to define the seizure onset zone and the epileptic networks involved. Seizure generation seems to be related to the propagation of epileptic activity from the primary source in the seizure onset zone, and maintenance of seizures is attributed to the perpetuation of epileptic activity between nodes in the epileptic network. Despite of these promising results, this proof of principle study needs further confirmation prior to the use of the described methods in the clinical praxis.     

How do we select perceptions and actions? Human brain imaging studies.

The selective nature of human perception and action implies a modulatory interaction between sensorimotor processes and attentional processes. This paper explores the use of functional imaging in humans to explore the mechanisms of perceptual selection and the fate of irrelevant stimuli that are not selected. Experiments with positron emission tomography show that two qualitatively different patterns of modulation of cerebral blood flow can be observed in experiments where non-spatial visual attention and auditory attention are manipulated. These patterns of modulation of cerebral blood flow modulation can be described as gain control and bias signal mechanisms. In visual and auditory cortex, the dominant change in cerebral blood flow associated with attention to either modality is related to a bias signal. The relation of these patterns of modulation to attentional effects that have been observed in single neurons is discussed. The existence of mechanisms for selective perception raises the more general question of whether irrelevant ignored stimuli are nevertheless perceived. Lavie''s theory of attention proposes that the degree to which ignored stimuli are processed varies depending on the perceptual load of the current task. Evidence from behavioural and functional magnetic resonance imaging studies of ignored visual motion processing is presented in support of this proposal.     

Effect of penicillin G-induced epileptic seizures on hemorheological parameters in rats

Normally, cerebral blood flow (CBF) is quantitatively coupled to cerebral metabolic rate like other tissues and maintained basically by altering vascular geometry and appropriate perfusion pressure. However, the rheological properties of the blood are important factors for effective tissue perfusion. Although a lot of studies have reported that hemorheological parameters are affected by a wide range of pathophysiological conditions, to our knowledge no research related to the effects of epileptic seizures on hemorheological parameters has been carried out. Thus, the aim of this study was to explore possible changes in rheological parameters including red blood cell (RBC) deformability, rigidity and aggregation, whole blood and plasma viscosity during epileptic seizures induced by penicillin G in rats. Eighteen female albino rats were divided into three groups that included sham operated controls (Group S), epileptic group (Group E), intraperitoneal penicillin group (Group IPP). Epilepsy was induced by intracortical injections of penicillin G. Hemorheological studies had been carried out 3 h after the induction of epilepsy. Among the studied hemorheological parameters, only RBC deformability was found to be different in the E group compared to S group. Epileptic seizures led to an increase in RBC deformability in the E group. In conclusion, these results suggest that in addition to an increase in CBF, RBC deformability may also improve to better match brain metabolic demands during seizures.     

In vivo optical mapping of epileptic foci and surround inhibition in ferret cerebral cortex

The population of neurons participating in an epileptiform event varies from moment to moment. Most techniques currently used to localize epileptiform events in vivo have spatial and/or temporal sampling limitations. Here we show in an animal model that optical imaging based on intrinsic signals is an excellent method for in vivo mapping of clinically relevant epileptiform events, such as interictal spikes, ictal onsets, ictal spread and secondary homotopic foci. In addition, a decrease in the optical signal correlates spatially with a decrease in neuronal activity recorded from cortex surrounding an epileptic focus. Optical mapping of epilepsy might be a useful adjunct in the surgical treatment of neocortical epilepsy, which critically depends on the precise localization of intrinsically epileptogenic neurons.     

La TEP et la TEMP pour l’étude des épilepsies

Positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging are very useful for the management of patients with medically refractory partial epilepsy. Presurgical evaluation of patients with medically refractory partial epilepsy often included PET imaging using FDG. The use of SPECT in these patients adds some more information and gives the clinicians the possibility of having ictal imaging. Furthermore, PET and SPECT imaging are performed to better understand the pathophysiology of epilepsy.     

Utilité de la TEP au 18FDG dans l’épilepsie. Méthodes et indications

Positron emission tomography (PET) using ¹⁸fluorodeoxyglucose (¹⁸FDG) is currently used in presurgical work-up for drug-resistant partial epilepsies in children as in adults, in addition to MRI. Recent cameras with less than 5 mm spatial resolution allow to obtain thin slices (about 2 mm thickness) in 3D planes. ¹⁸FDG is intravenously injected at the mean dose of 3 MBq/kg of body weight in interictal and resting state, in a quiet, dimly lit environment and careful monitoring for head movements and ictal events. In children, sedation may be necessary. Image acquisition starts 30 min after injection and ended 15 to 20 min later. Semiquantitative analysis is visually assessed in clinical practice using colour scales. PET sensibility is improved by superimposition of metabolic imaging on MRI. Statistical analysis with SPM may be useful but comparison with health subjects database is required. In medial temporal lobe epilepsy associated with hippocampal sclerosis, hypometabolism ipsilateral to the epileptogenic focus is found in 70 to 90% of the cases and is predictive of surgical outcome. In other types of temporal and extratemporal epilepsy with negative MRI, focal hypometabolism can be detected, allowing identification of minor gyral abnormalities corresponding to focal cortical dysplasias. In such MRI negative cases, PET findings may improve surgical outcome.     

Neural network analysis of the pattern of functional connectivity between cerebral areas in schizophrenia

 Recent evidence suggests that the core abnormality of cerebral function in schizophrenia is a disruption of functional connectivity between diverse cerebral sites. Functional connectivity is defined as the correlation between neuronal activity at remote sites. It can be measured using functional imaging techniques such as positron emission tomography (PET). This paper reports an analysis using a neural network to discriminate between the patterns of functional connectivity in schizophrenic patients and healthy subjects. The data was derived from a PET study of regional cerebral blood flow during word generation in 6 healthy subjects and 16 schizophrenic patients with established illness, in whom the clinical diagnosis could be made with confidence. After training on data from two healthy subjects and seven schizophrenic patients, the neural network successfully assigned all members of a test set of four healthy subjects and nine schizophrenic patients to the correct diagnostic category. While this result should be interpreted with caution on account of the small sample size, it indicates that neural network analysis is potentially of value in the diagnosis of schizophrenia. Received: 2 August 1999 / Accepted in revised form: 30 June 2000     

Multimodality molecular imaging of disease progression in living subjects

The enormous advances in our understanding of the progression of diseases at the molecular level have been supplemented by the new field of ‘molecular imaging’, which provides for in vivo visualization of molecular events at the cellular level in living organisms. Molecular imaging is a noninvasive assessment of gene and protein function, protein–protein interaction and/or signal transduction pathways in animal models of human disease and in patients to provide insights into molecular pathogenesis. Five major imaging techniques are currently available to assess the structural and functional alterations in vivo in small animals. These are (i) optical bioluminescence and fluorescence imaging techniques, (ii) radionuclide-based positron emission tomography (PET) and single photon emitted computed tomography (SPECT), (iii) X-ray-based computed tomography (CT), (iv) magnetic resonance imaging (MRI) and (v) ultrasound imaging (US). Functional molecular imaging requires an imaging probe that is specific for a given molecular event. In preclinical imaging, involving small animal models, the imaging probe could be an element of a direct (‘direct imaging’) or an indirect (‘indirect imaging’) event. Reporter genes are essential for indirect imaging and provide a general integrated platform for many different applications. Applications of multimodality imaging using combinations of bioluminescent, fluorescent and PET reporter genes in unified fusion vectors developed by us for recording events from single live cells to whole animals with high sensitivity and accurate quantification are discussed. Such approaches have immense potential to track progression of metastasis, immune cell trafficking, stem cell therapy, transgenic animals and even molecular interactions in living subjects.     

Positron emission tomography: a clinical and biological research tool

Medical and biological imaging has undergone a revolution in the past decade. Positron emission tomography (PET) has been developed to visualize biochemical and physiological phenomena in living humans and animals. For instance, blood flow, blood volume, glucose metabolism, amino acid metabolism, can be quantitatively estimated by means of PET with various radioactive tracers. This functional and molecular imaging technique has progressed rapidly from being a research technique in laboratories to a routine clinical imaging modality. The most widely used radiotracer in routine is 18F-fluorodeoxyglucose (18FDG), which is an analogue of glucose. Since glucose metabolism is increased many fold in malignant tumors, PET has a major role in the field of clinical oncology and recently in cardiology and neurology. PET is also a valuable tool to study cerebral or cardiac binding sites and to image the expression of reporter genes in small animals. In this review, we summarize the most recent developments in PET imaging with particular reference to the radiotracers available and their application.     

Physiological studies of cortical spreading depression

Cortical spreading depression (CSD) produces propagating waves of transient neuronal hyperexcitability followed by depression. CSD is initiated by K+ release following neuronal firing or electrical, mechanical or chemical stimuli. A triphasic (30-50 s) cortical potential transient accompanies localized transmembrane redistributions of K+, glutamate, Ca2+, Na+, Cl- and H+. Accumulated K+ in the restricted interstitial space can cause both further neuronal depolarisation and inward movement of K+ into astrocytes that buffers this increased extracellular K+ concentration ([K+])o. However, astrocyte interconnections may then propagate the CSD wave by K+ liberation through an opening of remote K+ channels by volume, Ca2+ or N-methyl-D-aspartate receptor activation. Changes in cerebral blood volume and in apparent water diffusion co-efficient (ADC) accompanying CSD were first studied using magnetic resonance imaging (MRI) in whole lissencephalic brains. Diffusion-weighted echoplanar imaging in gyrencephalic brains went on to demonstrate CSD features that paralleled classical migraine aura. The ADC activity persisted minutes/hours post KCl stimulus. Pixelwise analyses distinguished single primary events and multiple, spatially restricted, slower propagating, secondary events whose detailed features varied with the nature of the originating stimulus. These ADC changes varied reciprocally with T2*-weighted (i.e. referring to spin-spin relaxation times) waveforms reflecting local blood flow. There followed prolonged decreases in cerebral blood flow culminating in late cerebrovascular changes blocked by the antimigraine agent sumatriptan. CSD phenomena have possible translational significance for human migraine aura and other cerebral pathologies such as the peri-infarct depolarisation events that follow ischaemia and brain injury.     

A model of brain circulation and metabolism: NIRS signal changes during physiological challenges

We construct a model of brain circulation and energy metabolism. The model is designed to explain experimental data and predict the response of the circulation and metabolism to a variety of stimuli, in particular, changes in arterial blood pressure, CO(2) levels, O(2) levels, and functional activation. Significant model outputs are predictions about blood flow, metabolic rate, and quantities measurable noninvasively using near-infrared spectroscopy (NIRS), including cerebral blood volume and oxygenation and the redox state of the Cu(A) centre in cytochrome c oxidase. These quantities are now frequently measured in clinical settings; however the relationship between the measurements and the underlying physiological events is in general complex. We anticipate that the model will play an important role in helping to understand the NIRS signals, in particular, the cytochrome signal, which has been hard to interpret. A range of model simulations are presented, and model outputs are compared to published data obtained from both in vivo and in vitro settings. The comparisons are encouraging, showing that the model is able to reproduce observed behaviour in response to various stimuli.