Positron Emission Tomography in Basic Epilepsy Research: A View of the Epileptic Brain

Preclinical PET imaging in epileptogenesis: towards identification of biomarkers and therapeutic targets

BACKGROUND

Epilepsy is a neurological disorder that affects a significant portion of the global population. However, its complexity and the lack of biomarkers hinder the study of its etiology, resulting in a lack of effective treatments to slow down or halt disease development, also called epileptogenesis.

MAIN BODY

Animal models have proven to be a crucial tool for studying epileptogenesis, many exhibiting cellular, molecular, and functional alterations that resemble those found in human patients. This review examines preclinical studies that have utilized positron emission tomography, a non-invasive neuroimaging technique that has demonstrated correlation with the pathological features and behavioral comorbidities of the disease and a high predictive value for the severity of epileptogenesis.

CONCLUSION

Positron emission tomography imaging has fostered the knowledge of the mechanisms driving epileptogenesis. This translational technique might be crucial for identifying biomarkers of epilepsy, identifying novel treatment targets and selecting and monitoring patients for potential future therapies.

Innovations in the Neurosurgical Management of Epilepsy

Technical limitations and clinical challenges have historically limited the diagnostic tools and treatment methods available for surgical approaches to the management of epilepsy. By contrast, recent technological innovations in several areas hold significant promise in improving outcomes and decreasing morbidity. We review innovations in the neurosurgical management of epilepsy in several areas, including wireless recording and stimulation systems (particularly responsive neurostimulation [NeuroPace]), conformal electrodes for high-resolution electrocorticography, robot-assisted stereotactic surgery, optogenetics and optical imaging methods, novel positron emission tomography ligands, and new applications of focused ultrasonography. Investigation into genetic causes of and susceptibilities to epilepsy has introduced a new era of precision medicine, enabling the understanding of cell signaling mechanisms underlying epileptic activity as well as patient-specific molecularly targeted treatment options. We discuss the emerging path to individualized treatment plans, predicted outcomes, and improved selection of effective interventions, on the basis of these developments.

Divergent metabolic substrate utilization in brain during epileptogenesis precedes chronic hypometabolism

Alterations in metabolism during epileptogenesis may be a therapy target. Recently, an increase in amino acid transport into the brain was proposed to play a role in epileptogenesis. We aimed to characterize alterations of substrate utilization during epileptogenesis and in chronic epilepsy. The lithium-pilocarpine post status epilepticus (SE) rat model was used. We performed longitudinal O-(2-[(18)F]fluoroethyl)-l-tyrosine (18F-FET) and 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) and calculated 18F-FET volume of distribution (Vt) and 18F-FDG uptake. Correlation analyses were performed with translocator protein-PET defined neuroinflammation from previously acquired data. We found reduced 18F-FET Vt at 48 h after SE (amygdala: -30.2%, p = 0.014), whereas 18F-FDG showed increased glucose uptake 4 and 24 h after SE (hippocampus: + 43.6% and +42.5%, respectively; p < 0.001) returning to baseline levels thereafter. In chronic epileptic animals, we found a reduction in 18F-FET and 18F-FDG in the hippocampus. No correlation was found for 18F-FET or 18F-FDG to microglial activation at seven days post SE. Whereas metabolic alterations do not reflect higher metabolism associated to activated microglia, they might be partially driven by chronic neuronal loss. However, both metabolisms diverge during early epileptogenesis, pointing to amino acid turnover as a possible biomarker and/or therapeutic target for epileptogenesis.

Structural and Functional Alterations at Pre-Epileptic Stage Are Closely Associated with Epileptogenesis in Pilocarpine-induced Epilepsy Model

Pilocarpine-induced rat epilepsy model is an established animal model that mimics medial temporal lobe epilepsy in humans. The purpose of this study was to investigate neuroimaging abnormalities in various stages of epileptogenesis and to correlate them with seizure severity in pilocarpine-induced rat epilepsy model. Fifty male Sprague-Dawley rats were subject to continuous video and electroencephalographic monitoring after inducing status epilepticus (SE) and seizure severity was estimated by frequency and total durations of class 3 to 5 spontaneous recurrent seizures (SRS) by modified Racine's classification. The 7.0 Tesla magnetic resonance imaging (MRI) with high resolution flurodeoxyglucose positron emission tomography (FDG-PET) was performed at 3 hours, 1, 3, 7 days and 4 weeks after the initial insult. The initial SRS was observed 9.7±1.3 days after the pilocarpine injection. MRI revealed an abnormal T2 signal change with swelling in both hippocampi and amygdala in acute (day 1 after injection) and latent phases (days 3 and 7), in association with PET hypometabolism in these areas. Interestingly, the mean frequency of class 3 to 5 SRS was positively correlated with abnormal T2 signals in hippocampal area at 3 days. SRS duration became longer with more decreased glucose metabolism in both hippocampi and amygdala at 7 days after pilocarpine injection. This study indicates that development and severity of SRS at chronic phase could be closely related with structural and functional changes in hippocampus during the latent period, a pre-epileptic stage.

Preserved Hippocampal Glucose Metabolism on 18F-FDG PET after Transplantation of Human Umbilical Cord Blood-derived Mesenchymal Stem Cells in Chronic Epileptic Rats

Human umbilical cord blood-derived mesenchymal stem cells (hUCB-MSCs) may be a promising modality for treating medial temporal lobe epilepsy. (18)F-fluorodeoxyglucose positron emission tomography (FDG-PET) is a noninvasive method for monitoring in vivo glucose metabolism. We evaluated the efficacy of hUCB-MSCs transplantation in chronic epileptic rats using FDG-PET. Rats with recurrent seizures were randomly assigned into three groups: the stem cell treatment (SCT) group received hUCB-MSCs transplantation into the right hippocampus, the sham control (ShC) group received same procedure with saline, and the positive control (PC) group consisted of treatment-negative epileptic rats. Normal rats received hUCB-MSCs transplantation acted as the negative control (NC). FDG-PET was performed at pre-treatment baseline and 1- and 8-week posttreatment. Hippocampal volume was evaluated and histological examination was done. In the SCT group, bilateral hippocampi at 8-week after transplantation showed significantly higher glucose metabolism (0.990 ± 0.032) than the ShC (0.873 ± 0.087; P < 0.001) and PC groups (0.858 ± 0.093; P < 0.001). Histological examination resulted that the transplanted hUCB-MSCs survived in the ipsilateral hippocampus and migrated to the contralateral hippocampus but did not differentiate. In spite of successful engraftment, seizure frequency among the groups was not significantly different. Transplanted hUCB-MSCs can engraft and migrate, thereby partially restoring bilateral hippocampal glucose metabolism. The results suggest encouraging effect of hUCB-MSCs on restoring epileptic networks.

Workshop on Neurobiology of Epilepsy appraisal: new systemic imaging technologies to study the brain in experimental models of epilepsy

Modern functional neuroimaging provides opportunities to visualize activity of the entire brain, making it an indispensable diagnostic tool for epilepsy. Various forms of noninvasive functional neuroimaging are now also being performed as research tools in animal models of epilepsy and provide opportunities for parallel animal/human investigations into fundamental mechanisms of epilepsy and identification of epilepsy biomarkers. Recent animal studies of epilepsy using positron emission tomography, tractography, and functional magnetic resonance imaging were reviewed. Epilepsy is an abnormal emergent property of disturbances in neuronal networks which, even for epilepsies characterized by focal seizures, involve widely distributed systems, often in both hemispheres. Functional neuroimaging in animal models now provides opportunities to examine neuronal disturbances in the whole brain that underlie generalized and focal seizure generation as well as various types of epileptogenesis. Tremendous advances in understanding the contribution of specific properties of widely distributed neuronal networks to both normal and abnormal human behavior have been provided by current functional neuroimaging methodologies. Successful application of functional neuroimaging of the whole brain in the animal laboratory now permits investigations during epileptogenesis and correlation with deep brain electroencephalography (EEG) activity. With the continuing development of these techniques and analytical methods, the potential for future translational research on epilepsy is enormous. A PowerPoint slide summarizing this article is available for download in the Supporting Information section here.

PET imaging of brain inflammation during early epileptogenesis in a rat model of temporal lobe epilepsy

BACKGROUND

Recently, inflammatory cascades have been suggested as a target for epilepsy therapy. Positron emission tomography (PET) imaging offers the unique possibility to evaluate brain inflammation longitudinally in a non-invasive translational manner. This study investigated brain inflammation during early epileptogenesis in the post-kainic acid-induced status epilepticus (KASE) model with post-mortem histology and in vivo with [18F]-PBR111 PET.

METHODS

Status epilepticus (SE) was induced (N = 13) by low-dose injections of KA, while controls (N = 9) received saline. Translocator protein (TSPO) expression and microglia activation were assessed with [125I]-CLINDE autoradiography and OX-42 immunohistochemistry, respectively, 7 days post-SE. In a subgroup of rats, [18F]-PBR111 PET imaging with metabolite-corrected input function was performed before post-mortem evaluation. [18F]-PBR111 volume of distribution (Vt) in volume of interests (VOIs) was quantified by means of kinetic modelling and a VOI/metabolite-corrected plasma activity ratio.

RESULTS

Animals with substantial SE showed huge overexpression of TSPO in vitro in relevant brain regions such as the hippocampus and amygdala (P < 0.001), while animals with mild symptoms displayed a smaller increase in TSPO in amygdala only (P < 0.001). TSPO expression was associated with OX-42 signal but without obvious cell loss. Similar in vivo [18F]-PBR111 increases in Vt and the simplified ratio were found in key regions such as the hippocampus (P < 0.05) and amygdala (P < 0.01).

CONCLUSION

Both post-mortem and in vivo methods substantiate that the brain regions important in seizure generation display significant brain inflammation during epileptogenesis in the KASE model. This work enables future longitudinal investigation of the role of brain inflammation during epileptogenesis and evaluation of anti-inflammatory treatments.

Finding a better drug for epilepsy: antiinflammatory targets

This monograph summarizes one of the sessions of the XI Workshop on Neurobiology of Epilepsy (WONOEP), and provides a critical review of the current state of the field. Speakers and discussants focused on several broad topics: (1) the coexistence of inflammatory processes encompassing several distinct signal-transduction pathways with the epileptogenic process; (2) evidence for the contribution of specific inflammatory molecules and processes to the onset and progression of epilepsy, as well as to epilepsy-related morbidities including depression; (3) the complexity and intricate cross-talk of the pathways involved in inflammation, and the discrete, often opposite roles of a given mediator in neurons versus other cell types. These complexities highlight the challenges confronting the field as it aims to define inflammatory molecules as promising targets for epilepsy prevention and treatment.

Applications of positron emission tomography in animal models of neurological and neuropsychiatric disorders

Positron emission tomography (PET) provides dynamic images of the biodistribution of radioactive tracers in the brain. Through application of the principles of compartmental analysis, tracer uptake can be quantified in terms of specific physiological processes such as cerebral blood flow, cerebral metabolic rate, and the availability of receptors in brain. Whereas early PET studies in animal models of brain diseases were hampered by the limited spatial resolution of PET instruments, dedicated small-animal instruments now provide molecular images of rodent brain with resolution approaching 1mm, the theoretic limit of the method. Major applications of PET for brain research have consisted of studies of animal models of neurological disorders, notably Parkinson's disease (PD), Alzheimer's disease (AD), and Huntington's disease (HD), stroke, epilepsy and traumatic brain injury; these studies have particularly benefited from selective neurochemical lesion models (PD), and also transgenic rodent models (AD, HD). Due to their complex and uncertain pathophysiologies, corresponding models of neuropsychiatric disorders have proven more difficult to establish. Historically, there has been an emphasis on PET studies of dopamine transmission, as assessed with a range of tracers targeting dopamine synthesis, plasma membrane transporters, and receptor binding sites. However, notable recent breakthroughs in molecular imaging include the development of greatly improved tracers for subtypes of serotonin, cannabinoid, and metabotropic glutamate receptors, as well as noradrenaline transporters, amyloid-β and neuroinflammatory changes. This article reviews the considerable recent progress in preclinical PET and discusses applications relevant to a number of neurological and neuropsychiatric disorders in humans.

Neuroimaging in Animal Seizure Models with (18)FDG-PET

Small animal neuroimaging has become increasingly available to researchers, expanding the breadth of questions studied with these methods. Applying these noninvasive techniques to the open questions underlying epileptogenesis is no exception. A major advantage of small animal neuroimaging is its translational appeal. Studies can be well controlled and manipulated, examining the living brain in the animal before, during, and after the disease onset or disease treatment. The results can also be compared to data collected on human patients. Over the past decade, we and others have explored metabolic patterns in animal models of epilepsy to gain insight into the circuitry underlying development of the disease. In this paper, we provide technical details on how metabolic imaging that uses 2-deoxy-2[(18)F]fluoro-D-glucose ((18)FDG) and positron emission tomography (PET) is performed and explain the strengths and limitations of these studies. We will also highlight recent advances toward understanding epileptogenesis through small animal imaging.

Presurgical epilepsy localization with interictal cerebral dysfunction

Localization of interictal cerebral dysfunction with 2-[(18)F]fluoro-2-D-deoxyglucose (FDG) positron emission tomography (PET) and neuropsychological examination usefully supplements electroencephalography (EEG) and brain magnetic resonance imaging (MRI) in planning epilepsy surgery. In MRI-negative mesial temporal lobe epilepsy, correlation of temporal lobe hypometabolism with extracranial ictal EEG can support resection without prior intracranial EEG monitoring. In refractory localization-related epilepsies, hypometabolic sites may supplement other data in hypothesizing likely ictal onset zones in order to intracranial electrodes for ictal recording. Prognostication of postoperative seizure freedom with FDG PET appears to have greater positive than negative predictive value. Neuropsychological evaluation is critical to evaluating the potential benefit of epilepsy surgery. Cortical deficits measured with neuropsychometry are limited in lateralizing and localizing value for determination of ictal onset sites, however. Left temporal resection risks iatrogenic verbal memory deficits and dysnomia, and neuropsychological findings are useful in predicting those at greatest risk. Prognostication of cognitive risks with resection at other sites is less satisfactory.

Microglial ablation and lipopolysaccharide preconditioning affects pilocarpine-induced seizures in mice

Activated microglia have been associated with neurodegeneration in patients and in animal models of Temporal Lobe Epilepsy (TLE), however their precise functions as neurotoxic or neuroprotective is a topic of significant investigation. To explore this, we examined the effects of pilocarpine-induced seizures in transgenic mice where microglia/macrophages were conditionally ablated. We found that unilateral ablation of microglia from the dorsal hippocampus did not alter acute seizure sensitivity. However, when this procedure was coupled with lipopolysaccharide (LPS) preconditioning (1 mg/kg given 24 h prior to acute seizure), we observed a significant pro-convulsant phenomenon. This effect was associated with lower metabolic activation in the ipsilateral hippocampus during acute seizures, and could be attributed to activity in the mossy fiber pathway. These findings reveal that preconditioning with LPS 24 h prior to seizure induction may have a protective effect which is abolished by unilateral hippocampal microglia/macrophage ablation.

The current status of neuroimaging for epilepsy

PURPOSE OF REVIEW

Neuroimaging research continues apace and is being applied to further understanding of the epilepsies, and improve clinical management.

RECENT FINDINGS

Structural imaging has become more sensitive with developments of MRI hardware, acquisition and postprocessing methods. Tractography is being used to define critical pathways prior to surgery. Functional MRI for language lateralization is now a clinical tool. PET studies with specific ligands reveal neurochemical changes associated with specific epilepsy syndromes.

SUMMARY

MRI at 3T with FLAIR and multiple channel coils identifies and clarifies relevant abnormalities in 20% of patients with previously unremarkable scans. Voxel-based analysis of diffusion scans may identify abnormalities in group comparisons. Identification of relevant abnormalities using voxel-based methods in individual patients requires a careful balance of sensitivity and specificity, and has a 10-30% yield. The PROPELLER sequence improves the detail of hippocampal anatomy and correlation with histological slices shows the pathological basis of MRI signal changes. Tractography has shown the connections of the language cortex and visualizes specific tracts. Electroencephalograms with simultaneous functional MRI and perfusion have shown that perfusion changes are a major determinant of changes in blood-oxygen-level-dependent signal. Functional MRI of language and memory are becoming used as a predictor of deficits as a result of temporal lobe resection.Increased uptake of the PET tracer 11C-alpha-methyl tryptophan shows promise for localizing epileptogenic malformations of cortical development. Abnormalities of 5HT-1A receptor ligands have been reported in temporal lobe epilepsy, with controversial association with depression. Dopamine uptake abnormalities have been noted in autosomal dominant nocturnal frontal lobe epilepsy.

Cognitive disorders associated with epilepsy: diagnosis and treatment

INTRODUCTION

Cognitive disorders are common in patients with epilepsy. Their aetiology is multifactorial, being affected by the type and location of the epileptogenic lesion, epileptic syndrome, type of seizures, age of onset, frequency and severity. Timely diagnosis and treatment can help to reduce their impact on the patient's quality of life.

RESULTS

The most significant cognitive deficits are associated with focal epilepsy, although some, usually mild, neuropsychological disorders can be found in idiopathic generalized epilepsy. The use of antiepileptic drugs (AEDs) can cause additional neuropsychological disorders that are of particular concern in learning-age children and elderly patients with cognitive disorders before the start of treatment. Recent studies have raised the concern that the use of some AEDs during pregnancy may cause cognitive disorders in the child exposed to them in utero. Cognitive disorders can also present as a complication of surgery for refractory epilepsy. Some risk factors for significant memory loss after surgery for temporal lobe epilepsy have been described. They include intervention in the dominant hemisphere, good preoperative function and poor functional reserve in the contralateral hippocampus.

CONCLUSIONS

The heterogeneity of different types of epilepsy makes case-control studies difficult; however, thanks to the growing interest in the neuropsychological deficits associated with epilepsy, we now know some factors that could lead to the appearance of these disorders and their prognosis. Special care must be taken to detect cognitive side effects associated with AEDs, which seem to be more common with classic than with new AEDs, and in those patients receiving polytherapy. Neuropsychological assessment should be routinely performed before epilepsy surgery to predict possible postsurgical cognitive deficits.

Anterior thalamic nucleus stimulation modulates regional cerebral metabolism: an FDG-MicroPET study in rats

The mechanism underlying the antiepileptic function of deep brain stimulation (DBS) of the anterior thalamic nucleus (ATN) remains unknown, presumably related to functional lesioning of target. We measured the regional normalized cerebral metabolic rate of glucose (nCMRglc) with (18)F-fluorodeoxyglucose (FDG)-MicroPET in animals receiving either ATN stimulation or lesioning. Bilateral ATN stimulation reversibly increased glucose uptake in the target region, the thalamus and hippocampus, and decreased glucose uptake in the cingulate cortex and frontal cortex. However, bilateral ATN lesioning decreased glucose uptake only in the target region. Animals with bilateral ATN lesions showed no metabolic changes after ATN stimulation. Thus, bilateral DBS of the ATN reversibly induces metabolic activation of the target area and modulates energy metabolism in remote brain regions via efferent or afferent fibers in non-epileptic rats. DBS of the ATN may work by a different mechanism than ATN lesioning.