In vivo imaging of dopamine receptors in a model of temporal lobe epilepsy

Rodent Models of Audiogenic Epilepsy: Genetic Aspects, Advantages, Current Problems and Perspectives

Animal models of epilepsy are of great importance in epileptology. They are used to study the mechanisms of epileptogenesis, and search for new genes and regulatory pathways involved in the development of epilepsy as well as screening new antiepileptic drugs. Today, many methods of modeling epilepsy in animals are used, including electroconvulsive, pharmacological in intact animals, and genetic, with the predisposition for spontaneous or refractory epileptic seizures. Due to the simplicity of manipulation and universality, genetic models of audiogenic epilepsy in rodents stand out among this diversity. We tried to combine data on the genetics of audiogenic epilepsy in rodents, the relevance of various models of audiogenic epilepsy to certain epileptic syndromes in humans, and the advantages of using of rodent strains predisposed to audiogenic epilepsy in current epileptology.

Potential Pleiotropic Genes and Shared Biological Pathways in Epilepsy and Depression Based on GWAS Summary Statistics

Current epidemiological and experimental studies have indicated the overlapping genetic foundation of epilepsy and depression. However, the detailed pleiotropic genetic etiology and neurobiological pathways have not been well understood, and there are many variants with underestimated effect on the comorbidity of the two diseases. Utilizing genome-wide association study (GWAS) summary statistics of epilepsy (15,212 cases and 29,677 controls) and depression (170,756 cases and 329,443 controls) from large consortia, we assessed the integrated gene-based association with both diseases by Multimarker Analysis of Genomic Annotation (MAGMA) and Fisher's meta-analysis. On the one hand, shared genes with significantly altered transcripts in Gene Expression Omnibus (GEO) data sets were considered as possible pleiotropic genes. On the other hand, the pathway enrichment analysis was conducted based on the gene lists with nominal significance in the gene-based association test of each disease. We identified a total of two pleiotropic genes (CD3G and SLCO3A1) with gene expression analysis validated and interpreted twenty-five common biological process supported with literature mining. This study indicates the potentially shared genes associated with both epilepsy and depression based on gene expression, meta-data analysis, and pathway enrichment strategy along with traditional GWAS and provides insights into the possible intersecting pathways that were not previously reported.

The functions of repressor element 1-silencing transcription factor in models of epileptogenesis and post-ischemia

Epilepsy is a debilitating neurological disorder characterised by recurrent seizures for which 30% of patients are refractory to current treatments. The genetic and molecular aetiologies behind epilepsy are under investigation with the goal of developing new epilepsy medications. The transcriptional repressor REST (Repressor Element 1-Silencing Transcription factor) is a focus of interest as it is consistently upregulated in epilepsy patients and following brain insult in animal models of epilepsy and ischemia. This review analyses data from different epilepsy models and discusses the contribution of REST to epileptogenesis. We propose that in healthy brains REST acts in a protective manner to homeostatically downregulate increases in excitability, to protect against seizure through downregulation of BDNF (Brain-Derived Neurotrophic Factor) and its receptor, TrkB (Tropomyosin receptor kinase B). However, in epilepsy patients and post-seizure, REST may increase to a larger degree, which allows downregulation of the glutamate receptor subunit GluR2. This leads to AMPA glutamate receptors lacking GluR2 subunits, which have increased permeability to Ca²⁺, causing excitotoxicity, cell death and seizure. This concept highlights therapeutic potential of REST modulation through gene therapy in epilepsy patients.

ERK1/2 kinases and dopamine D2 receptors participate in the anticonvulsant effects of a new derivative of benzoylpyridine oxime and valproic acid

Inhibition of the activity of extracellular signal-regulated kinases (ERK1/2) induced by the activation of the dopamine D2 receptor signalling cascade may be a promising pharmacological target. The aim of this work was to study the involvement of ERK1/2 and dopamine D2 receptor in the mechanism of the anticonvulsant action of valproic acid (VA) and a new benzoylpyridine oxime derivative (GIZH-298), which showed antiepileptic activity in different models of epilepsy. We showed that subchronic exposure to maximal electroshock seizures (MES) for 5 days reduced the density of dopamine D2 receptors in the striatum of mice. GIZH-298 counteracted the decrease in the number of dopamine D2 receptors associated with MES and increased the number of ligand binding sites of dopamine D2 receptors in mice without MES. The affinity of dopamine D2 receptors to the ligand was not changed by GIZH-298. MES caused an increase in ERK1/2 and synapsin I phosphorylation in the striatum while GIZH-298, similar to VA, reduced the levels of both phospho-ERK1/2 and phosphosynapsin I after MES, which correlated with the decrease in the intensity of seizure in mice. In addition, GIZH-298 suppressed ERK1/2 phosphorylation in SH-SY5Y human neuroblastoma cells at therapeutic concentrations, while VA inhibited ERK1/2 phosphorylation in vivo but not in vitro. The data obtained expand the understanding of the mechanisms of action of VA and GIZH-298, which involve regulating the activity of ERK1/2 kinases, probably by modulating dopamine D2 receptors in limbic structures, as well as (in the case of GIZH-298) directly inhibiting of the ERK1/2 cascade.

Revisiting the role of neurotransmitters in epilepsy: An updated review

Epilepsy is a neurologicaldisorder characterized by persistent predisposition to recurrent seizurescaused by abnormal neuronal activity in the brain. Epileptic seizures maydevelop due to a relative imbalance of excitatory and inhibitory neurotransmitters. Expressional alterations of receptors and ion channelsactivated by neurotransmitters can lead to epilepsy pathogenesis.

AIMS

In this updated comprehensive review, we discuss the emerging implication of mutations in neurotransmitter-mediated receptors and ion channels. We aim to provide critical findings of the current literature about the role of neurotransmitters in epilepsy.

MATERIALS AND METHODS

A comprehensive literature review was conducted to identify and critically evaluate studies analyzing the possible relationship between epilepsy and neurotransmitters. The PubMed database was searched for related research articles.

KEY FINDINGS

Glutamate and gamma-aminobutyric acid (GABA) are the main neurotransmitters playing a critical role in the pathophysiology of this balance, and irreversible neuronal damage may occur as a result of abnormal changes in these molecules. Acetylcholine (ACh), the main stimulant of the autonomic nervous system, mediates signal transmission through cholinergic and nicotinic receptors. Accumulating evidence indicates that dysfunction of nicotinic ACh receptors, which are widely expressed in hippocampal and cortical neurons, may be significantly implicated in the pathogenesis of epilepsy. The dopamine-norepinephrine-epinephrine cycle activates hormonal and neuronal pathways; serotonin, norepinephrine, histamine, and melatonin can act as both hormones and neurotransmitters. Recent reports have demonstrated that nitric oxide mediates cognitive and memory-related functions via stimulating neuronal transmission.

SIGNIFICANCE

The elucidation of the role of the main mediators and receptors in epilepsy is crucial for developing new diagnostic and therapeutic approaches.

Neuroimaging Biomarkers of Experimental Epileptogenesis and Refractory Epilepsy

This article provides an overview of neuroimaging biomarkers in experimental epileptogenesis and refractory epilepsy. Neuroimaging represents a gold standard and clinically translatable technique to identify neuropathological changes in epileptogenesis and longitudinally monitor its progression after a precipitating injury. Neuroimaging studies, along with molecular studies from animal models, have greatly improved our understanding of the neuropathology of epilepsy, such as the hallmark hippocampus sclerosis. Animal models are effective for differentiating the different stages of epileptogenesis. Neuroimaging in experimental epilepsy provides unique information about anatomic, functional, and metabolic alterations linked to epileptogenesis. Recently, several in vivo biomarkers for epileptogenesis have been investigated for characterizing neuronal loss, inflammation, blood-brain barrier alterations, changes in neurotransmitter density, neurovascular coupling, cerebral blood flow and volume, network connectivity, and metabolic activity in the brain. Magnetic resonance imaging (MRI) is a sensitive method for detecting structural and functional changes in the brain, especially to identify region-specific neuronal damage patterns in epilepsy. Positron emission tomography (PET) and single-photon emission computerized tomography are helpful to elucidate key functional alterations, especially in areas of brain metabolism and molecular patterns, and can help monitor pathology of epileptic disorders. Multimodal procedures such as PET-MRI integrated systems are desired for refractory epilepsy. Validated biomarkers are warranted for early identification of people at risk for epilepsy and monitoring of the progression of medical interventions.

Antiepileptic effect of uridine may be caused by regulating dopamine release and receptor expression in corpus striatum

Uridine is a potential endogenous neuromodulator studied for several decades for its antiepileptic effect, but the results were controversial. One remarkable feature of uridine is its regulatory action on the dopaminergic pathways. In this study, the changes in uridine and dopamine (DA) release were examined in the mouse corpus striatum after pilocarpine (PC) intraperitoneal injection. Then, the effect of uridine pre-treatment on DA release and expression of dopamine receptor (DR) was determined. The results revealed an increased uridine release initially, followed by a downward trend after an injection of 400-mg/kg PC. However, the DA release continuous increased significantly. The expression of dopamine receptor-1 (D1R) increased in a dose-dependent manner while that of dopamine receptor-2 (D2R) decreased significantly. Prophylactic administration of uridine significantly relieved the high-frequency and high-amplitude expression induced by PC as well as dose-dependently reversed the PC-induced changes in DA and DRs levels. These findings suggested that uridine produced an antiepileptic effect, which might have been mediated in part by interfering with the dopaminergic system.

The role of the basal ganglia in the control of seizure

Epilepsy is a network disorder and each type of seizure involves distinct cortical and subcortical network, differently implicated in the control and propagation of the ictal activity. The role of the basal ganglia has been revealed in several cases of focal and generalized seizures. Here, we review the data that show the implication of the basal ganglia in absence, temporal lobe, and neocortical seizures in animal models (rodent, cat, and non-human primate) and in human. Based on these results and the advancement of deep brain stimulation for Parkinson's disease, basal ganglia neuromodulation has been tested with some success that can be equally seen as promising or disappointing. The effect of deep brain stimulation can be considered promising with a 76% in seizure reduction in temporal lobe epilepsy patients, but also disappointing, since only few patients have become seizure free and the antiepileptic effects have been highly variable among patients. This variability could probably be explained by the heterogeneity among the patients included in these clinical studies. To illustrate the importance of specific network identification, electrophysiological activity of the putamen and caudate nucleus has been recorded during penicillin-induced pre-frontal and motor seizures in one monkey. While an increase of the firing rate was found in putamen and caudate nucleus during pre-frontal seizures, only the activity of the putamen cells was increased during motor seizures. These preliminary results demonstrate the implication of the basal ganglia in two types of neocortical seizures and the necessity of studying the network to identify the important nodes implicated in the propagation and control of each type of seizure.

Advances of Molecular Imaging in Epilepsy

Positron emission tomography (PET) is a neuroimaging method that offers insights into the molecular functioning of a human brain. It has been widely used to study metabolic and neurotransmitter abnormalities in people with epilepsy. This article reviews the development of several PET radioligands and their application in studying the molecular mechanisms of epilepsy. Over the last decade, tracers binding to serotonin and γ-aminobutyric acid (GABA) receptors have been used to delineate the location of the epileptic focus. PET studies have examined the role of opioids, cannabinoids, acetylcholine, and dopamine in modulating neuronal hyperexcitability and seizure termination. In vivo analyses of drug transporters, e.g., P-glycoprotein, have increased our understanding of pharmacoresistance that could inform new therapeutic strategies. Finally, PET experiments targeting neuroinflammation and glutamate receptors might guide the development of novel biomarkers of epileptogenesis.

Hippocampal asymmetry: differences in the left and right hippocampus proteome in the rat model of temporal lobe epilepsy

The hippocampus is a complex brain structure and undergoes severe sclerosis and gliosis in temporal lobe epilepsy (TLE) as the most common type of epilepsy. The key features of the TLE may be reported in chronic animal models of epilepsy, such as pilocarpine model. Therefore, the current study was conducted in a rat pilocarpine model of acquired epilepsy. Two-dimensional gel electrophoresis based proteomic technique was used to compare the proteome map of the left and right hippocampus in both control and epileptic rats. Generally, 95 differentially expressed spots out of 1300 spots were identified in the hippocampus proteome using MALDI-TOF-TOF/MS. Within identified proteins, some showed asymmetric expression related to the mechanisms underlying TLE imposed by pilocarpine. Assessment of lateralization at the molecular level demonstrated that expression of proteins involved in dopamine synthesis was significantly more in the right hippocampus than the left one. In the epileptic model, reduction in dopamine pathway proteins was accompanied by an increase in the expression of proteins involved in polyamine synthesis, referring to a new regulating mechanism. Our results revealed changes in the laterality of protein expression due to pilocarpine-induced status epilepticus that could present some new proteins as potential candidates for antiepileptic drug design.

BIOLOGICAL SIGNIFICANCE

In the current study, two-dimensional gel electrophoresis (2-DE) based proteomic technique was used to profile changes in the left and right hippocampus proteome after pilocarpine induced status epilepticus. Spots of proteome maps for two hemispheres were excised and identified with MALDI-TOF-TOF/MS. Analysis of proteome map of the left and right hippocampus revealed a lateralization at the molecular level, in which the expression of proteins involved in dopamine synthesis and release were significantly more in right hippocampi than the left ones in the normal rats. Also, the expression of proteins involved in polyamine synthesis significantly increased in epileptic hippocampus (considerably higher in right hippocampi), whilst the proteins which included in dopamine pathways were decreased. Our results revealed changes in the laterality of protein expression due to pilocarpine-induced status epilepticus that could present some new proteins as potential candidates for antiepileptic drug design.

Reduced D2/D3 Receptor Binding of Extrastriatal and Striatal Regions in Temporal Lobe Epilepsy

Objective

Dopamine is an endogenous neuromodulator in cortical circuits and the basal ganglia. In animal models of temporal lobe epilepsy (TLE), seizure threshold is modulated to some extent by dopamine, with D1-receptors having a pro- and D2-receptors an anticonvulsant effect. We aimed to extend our previously reported results on decreased D2/D3 receptor binding in the lateral epileptogenic temporal lobe and to correlate them with demographic and seizure variables to gain a more comprehensive understanding of the underlying involvement of the dopaminergic system in the epileptogenesis of TLE.

Methods

To quantify D2/D3 receptor binding, we studied 21 patients with TLE and hippocampal sclerosis (13 left- and eight right-sided) and 18 controls using PET with the high-affinity dopamine D2/D3-receptor ligand ¹⁸F-Fallypride to image striatal and extrastriatal binding. TLE was defined by interictal and ictal video-EEG, MRI and ¹⁸F-Fluorodeoxyglucose PET. Voxel-based statistical and regions-of-interest analyses were performed.

Results

¹⁸F-Fallypride binding potential was significantly reduced in the affected temporal lobe and bilateral putamen. A positive correlation between age at onset of epilepsy and [¹⁸F]FP BP_nd (binding potential non-displaceable) in temporal regions on the epileptogenic side was found, as well as a negative correlation between epilepsy duration and [¹⁸F]FP BP_nd in the temporal pole on the epileptogenic side and a positive correlation between the estimated number of lifetime GTCS and [¹⁸F]FP BP_nd in the hippocampus on the epileptogenic side.

Significance

The areas of reduced D2/D3 receptor availability correspond to “the irritative zone” surrounding the epileptogenic area. Moreover, reduced D2/D3 receptor availability was detectable in the basal ganglia, which are suspected to be involved in a control circuit for epileptic seizures. The correlational analysis additionally suggests that increased epilepsy duration leads to increasing impairment of the dopaminergic system.

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.

Effects of glutathione S-transferase M1 and T1 deletions on epilepsy risk among a Tunisian population

Glutathione-S-transferases enzymes are involved in the detoxification of several endogenous and exogenous substances. In this present study, we evaluated the effects of two glutathione-S-transferase polymorphisms, (GSTM1 and GSTT1) on epilepsy risk susceptibility in a Tunisian population. These polymorphisms were analyzed in 229 healthy subjects and 98 patients with epilepsy, using a polymerase chain reaction (PCR). Odds ratio (ORs) was used for analyzing results. The study results demonstrated that individuals with the GSTM1 null genotype were at an increased risk of developing epilepsy [OR=3.80, 95% confidence interval (CI) (2.15-4.78)], whereas no significant effects were observed between individuals with GSTT1 null genotype and epilepsy risk [OR=1.15, 95% CI (0.62-2.12)]. These genotyping finding revealed that the absence of GSTM1 activity could be contributor factor for the development of epilepsy disease.

An overview of PET neuroimaging

Over the past 35 years or so, PET brain imaging has allowed powerful and unique insights into brain function under normal conditions and in disease states. Initially, as PET instrumentation continued to develop, studies were focused on brain perfusion and glucose metabolism. This permitted refinement of brain imaging for important, non-oncologic clinical indications. The ability of PET to not only provide spatial localization of metabolic changes but also to accurately and consistently quantify their distribution proved valuable for applications in the clinical setting. Specifically, glucose metabolism brain imaging using (F-18) fluorodeoxyglucose continues to be invaluable for evaluating patients with intractable seizures for identifying seizure foci and operative planning. Cerebral glucose metabolism also contributes to diagnosis of neurodegenerative diseases that cause dementia. Alzheimer disease, dementia with Lewy bodies, and the several variants of frontotemporal lobar degeneration have differing typical patterns of hypometabolism. In Alzheimer disease, hypometabolism has furthermore been associated with poorer cognitive performance and ensuing cognitive and functional decline. As the field of radiochemistry evolved, novel radioligands including radiolabeled flumazenil, dopamine transporter ligands, nicotine receptor ligands, and others have allowed for further understanding of molecular changes in the brain associated with various diseases. Recently, PET brain imaging reached another milestone with the approval of (F-18) florbetapir imaging by the United States Federal Drug Administration for detection of amyloid plaque accumulation in brain, the major histopathologic hallmark of Alzheimer disease, and efforts have been made to define the clinical role of this imaging agent in the setting of the currently limited treatment options. Hopefully, this represents the first of many new radiopharmaceuticals that would allow improved diagnostic and prognostic information in these and other clinical applications, including Parkinson disease and traumatic brain injury.

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.

The role of dopamine signaling in epileptogenesis

Clinical and experimental studies implicate most neuromodulatory systems in epileptogenesis. The dopaminergic system has a seizure-modulating effect that crucially depends on the different subtypes of dopamine (DA) receptors involved and the brain regions in which they are activated. Specifically, DA plays a major role in the control of seizures arising in the limbic system. Studies performed in a wide variety of animal models contributed to illustrate the opposite actions of D1-like and D2-like receptor signaling in limbic epileptogenesis. Indeed, signaling from D1-like receptors is generally pro-epileptogenic, whereas D2-like receptor signaling exerts an anti-epileptogenic effect. However, this view might appear quite simplistic as the complex neuromodulatory action of DA in the control of epileptogenesis likely requires a physiological balance in the activation of circuits modulated by these two major DA receptor subtypes, which determines the response to seizure-promoting stimuli. Here we will review recent evidences on the identification of molecules activated by DA transduction pathways in the generation and spread of seizures in the limbic system. We will discuss the intracellular signaling pathways triggered by activation of different DA receptors in relation to their role in limbic epileptogenesis, which lead to the activation of neuronal death/survival cascades. A deep understanding of the signaling pathways involved in epileptogenesis is crucial for the identification of novel targets for the treatment of epilepsy.

Neuroimaging biomarkers for epilepsy: advances and relevance to glial cells

Glial cells play an important role in normal brain function and emerging evidence would suggest that their dysfunction may be responsible for some epileptic disease states. Neuroimaging of glial cells is desirable, but there are no clear methods to assess neither their function nor localization. Magnetic resonance imaging (MRI) is now part of a standardized epilepsy imaging protocol to assess patients. Structural volumetric and T2-weighted imaging changes can assist in making a positive diagnosis in a majority of patients. The alterations reported in structural and T2 imaging is predominantly thought to reflect early neuronal loss followed by glial hypertrophy. MR spectroscopy for myo-inositol is a being pursued to identify glial alterations along with neuronal markers. Diffusion weighted imaging (DWI) is ideal for acute epileptiform events, but is not sensitive to either glial cells or neuronal long-term changes found in epilepsy. However, DWI variants such as diffusion tensor imaging or q-space imaging may shed additional light on aberrant glial function in the future. The sensitivity and specificity of PET radioligands, including those targeting glial cells (translocator protein) hold promise in being able to image glial cells. As the role of glial function/dysfunction in epilepsy becomes more apparent neuroimaging methods will evolve to assist the clinician and researcher in visualizing their location and function.

Altered expression of vesicular monoamine transporter 2 in epileptic patients and experimental rats

In the central nervous system (CNS), vesicular monoamine transporter 2 (VMAT2) transports cytoplasmic monoamines such as dopamine into synaptic vesicles for storage and subsequent exocytotic release. Recent studies have provided direct evidence for VMAT2-regulated monoamine neurotransmitter involvement in the neurophysiological activities of neurological disease. This study investigated the expression pattern of VMAT2 in patients with temporal lobe epilepsy (TLE) and in a rat model of epilepsy. We assessed the expression of VMAT2 in the temporal neocortex in 24 TLE patients using western blotting and quantitative real time PCR (qRT-PCR) analyses. These results showed that VMAT2 expression dynamically decreased in TLE patients when compared with the control subjects (n = 12). And that VMAT2 protein transiently increased in acute stages (1 day and 3 days) after epileptic seizures in pilocarpine-treated rats; however, it clearly decreased after spontaneous recurrent seizures (7 days, 21 days, and 60 days after seizures). In addition, double immunofluorescence and immunohistochemical labeling studies performed in patient and experimental animal tissue revealed that VMAT2 protein was mainly expressed in the cytoplasm and in the axons of neurons but not glial cells in the hippocampus and temporal lobe cortex. These data suggested that the abnormal expression of VMAT2 mRNA and protein in epileptic brain tissue may contribute to vulnerability toward epilepsy-related psychiatric disorders and cognitive impairment.

Changes on D2-like receptor induced Gi protein activation and hippocampal dopamine release in kindled rats

The present study aimed to characterize dopamine release in the hippocampus and D2-like receptor-induced Gi protein activation in several brain areas of fully kindled rats. During the interictal period, kindled rats showed lower extracellular levels of dopamine when compared with those obtained in the control group under basal conditions, a situation that was not modified when an afterdischarge was evoked. Hippocampal perfusion of sulpiride, a D2 receptor antagonist, enhanced dopamine release, which was more evident when an afterdischarge was induced in kindled rats. In addition, sulpiride perfusion was associated with longer seizure duration. Functional autoradiography experiments revealed increased [(35)S]GTPγS incorporation as a consequence of D2-like receptor activation in different brain areas of fully kindled animals, including the ventral hippocampus. The present study reveals that hippocampal kindling is associated with alterations in dopamine release and D2-like receptor-induced neurotransmission.

The use and importance of liposomes in positron emission tomography

Among different imaging modalities, Positron Emission Tomography (PET) gained importance in routine hospital practice depending on ability to diagnose diseases in early stages and tracing of therapy by obtaining metabolic information. The combination of PET with Computed Tomography (CT) forms hybrid imaging modality that gives chance to obtain better images having higher resolution by fusing both functional and anatomical images in the same imaging modality at the same time. Therefore, better contrast agents are essentially needed. The advance in research about developing drug delivery systems as specific nanosized targeted systems gained an additional importance for obtaining better diagnosis and therapy of different diseases. Liposomes appear to be more attractive drug delivery systems in delivering either drugs or imaging ligands to target tissue or organ of diseases with higher accumulation by producing in nano-scale, long circulating by stealth effect and specific targeting by modifying with specific ligands or markers. The combination of positron emitting radionuclides with liposomes are commonly in research level nowadays and there is no commercially available liposome formulation for PET imaging. However by conjugating positron emitter radionuclide with liposomes can form promising diagnostic agents for improved diagnosis and following up treatments by increasing image signal/contrast in the target tissue in lower concentrations by specific targeting as the most important advantage of liposomes. More accurate and earlier diagnosis of several diseases can be obtained even in molecular level with the use of stable and effectively radiolabeled molecular target specific nano sized liposomes with longer half-lived positron emitting radionuclides.

Do the basal ganglia inhibit seizure activity in temporal lobe epilepsy?

There is substantial evidence in the literature that the basal ganglia (BG), namely the striatum and pallidum, are involved in temporal lobe epilepsy (TLE). The BG are probably not involved in elaborating clinical seizures, as they do not produce specific epileptiform activity and there is no evident change in the electrical activity in the BG immediately after seizure onset. The data we obtained by direct ictal recording in the BG [1,2], as well as a large body of experimental and clinical evidence reported by other groups, suggest an inhibitory role of the BG during temporal lobe seizures. The BG may have a remote influence on cortical oscillatory processes related to control of epileptic seizures via their feedback pathways to the cortex.

Classification and lateralization of temporal lobe epilepsies with and without hippocampal atrophy based on whole-brain automatic MRI segmentation

Brain images contain information suitable for automatically sorting subjects into categories such as healthy controls and patients. We sought to identify morphometric criteria for distinguishing controls (n = 28) from patients with unilateral temporal lobe epilepsy (TLE), 60 with and 20 without hippocampal atrophy (TLE-HA and TLE-N, respectively), and for determining the presumed side of seizure onset. The framework employs multi-atlas segmentation to estimate the volumes of 83 brain structures. A kernel-based separability criterion was then used to identify structures whose volumes discriminate between the groups. Next, we applied support vector machines (SVM) to the selected set for classification on the basis of volumes. We also computed pairwise similarities between all subjects and used spectral analysis to convert these into per-subject features. SVM was again applied to these feature data. After training on a subgroup, all TLE-HA patients were correctly distinguished from controls, achieving an accuracy of 96 ± 2% in both classification schemes. For TLE-N patients, the accuracy was 86 ± 2% based on structural volumes and 91 ± 3% using spectral analysis. Structures discriminating between patients and controls were mainly localized ipsilaterally to the presumed seizure focus. For the TLE-HA group, they were mainly in the temporal lobe; for the TLE-N group they included orbitofrontal regions, as well as the ipsilateral substantia nigra. Correct lateralization of the presumed seizure onset zone was achieved using hippocampi and parahippocampal gyri in all TLE-HA patients using either classification scheme; in the TLE-N patients, lateralization was accurate based on structural volumes in 86 ± 4%, and in 94 ± 4% with the spectral analysis approach. Unilateral TLE has imaging features that can be identified automatically, even when they are invisible to human experts. Such morphometric image features may serve as classification and lateralization criteria. The technique also detects unsuspected distinguishing features like the substantia nigra, warranting further study.

Changes in glucose metabolism and metabolites during the epileptogenic process in the lithium-pilocarpine model of epilepsy

PURPOSE

  The metabolic and biochemical changes that occur during epileptogenesis remain to be determined. (18) F-Fluorodeoxyglucose positron emission tomography (FDG-PET) and proton magnetic resonance spectroscopy ((1) H MRS) are noninvasive techniques that provide indirect information on ongoing pathologic changes. We, therefore, utilized these methods to assess changes in glucose metabolism and metabolites in the rat lithium-pilocarpine model of epilepsy as markers of epileptogenesis from baseline to chronic spontaneous recurrent seizures (SRS).

METHODS

  PET and MRS were performed at baseline, and during the acute, subacute, silent, and chronic periods after lithium-pilocarpine induced status epilepticus (SE). Sequential changes in glucose metabolism on (18) F-FDG PET using SPM2 and the ratios of percent injected dose per gram (%ID)/g of regions of interest (ROIs) in the bilateral amygdala, hippocampus, basal ganglia with the thalamus, cortex, and hypothalamus normalized to the pons were determined. Voxels of interest (VOIs) on (1) H MRS were obtained at the right hippocampus and the basal ganglia. NAA/Cr levels and Cho/Cr at various time points were compared to baseline values.

KEY FINDINGS

  Of 81 male Sprague-Dawley rats, 30 progressed to SRS. (18) F-FDG PET showed widespread global hypometabolism during the acute period, returning to baseline level during the subacute period. Glucose metabolism, however, declined in part of the hippocampus during the silent period, with the hypometabolic area progressively expanding to the entire limbic area during the chronic period. (1) H MRS showed that the NAA/Cr levels in the hippocampus and basal ganglia were reduced during the acute period and were not restored subsequently from the subacute to the chronic period without any significant change in the Cho/Cr ratio throughout the entire experiment.

SIGNIFICANCE

  Serial metabolic and biochemical changes in the lithium-pilocarpine model of epilepsy indirectly represent the process of human epileptogenesis. Following initial irreversible neural damage by SE, global glucose metabolism transiently recovered during the subacute period without neuronal recovery. Progressive glucose hypometabolism in the limbic area during the silent and chronic periods may reflect the important role of the hippocampus in the formation of ongoing epileptic network during epileptogenesis.

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.

Voxel-based statistical analysis of fractional anisotropy and mean diffusivity in patients with unilateral temporal lobe epilepsy of unknown cause

PURPOSE

To determine regional alterations of fractional anisotropy (FA) and mean diffusivity (MD) in patients with magnetic resonance imaging (MRI)-negative temporal lobe epilepsy with unknown cause (TLEu) using diffusion tensor imaging (DTI) and voxel-based statistics (VBS).

METHODS

Ten patients with left TLEu and no abnormality on conventional MRI and 81 age-matched neurological healthy controls were studied. VBS analyses were used to compare FA and MD differences between patients and controls. All results were reported using stringent statistical thresholds corrected for multiple comparisons.

RESULTS

Patients with TLEu had widespread and bilateral reduction of white matter FA, encompassing the temporal lobes, entire corpus callosum, thalamus, and other regions relative to controls. Increased MD was more spatially limited in patients, but was also observed in the thalamus. FA of the putamen was significantly increased bilaterally in patients relative to controls, which correlated with increasing macroscopic atrophy of the putamen.

DISCUSSION

Water diffusion abnormalities are widespread and bilaterally distributed in patients with unilateral TLEu, which are beyond the resolution of conventional MRI. FA alterations are more widespread relative to MD alterations. This is the first study to show evidence of interrelated microscopic (ie, FA increase) and macroscopic (ie, atrophy) alterations of the putamen in patients with TLEu.

Increased neurokinin-1 receptor availability in temporal lobe epilepsy: a positron emission tomography study using [(11)C]GR205171

PURPOSE

Activation of the neurokinin-1 (NK1) receptor by neuropeptide substance P (SP) induces and maintains epileptic activity in various experimental models of epilepsy. The primary objective of this study was to investigate whether neurobiological changes linked to NK1-SP receptor system are associated with hyperexcitability in patients with temporal lobe epilepsy (TLE). A secondary objective was to investigate the relationship between seizure frequency and NK1 receptor availability.

METHODS

A positron emission tomography study was conducted with the selective NK1 receptor antagonist [(11)C]GR205171 in nine patients with TLE and 18 healthy control participants. Parametric PET images were generated using the Patlak graphical method, with cerebellum as reference region. Data analyses including group comparisons were performed using statistical parametric mapping.

RESULTS

Patients with TLE showed increased NK1 receptor availability in both hemispheres with the most pronounced increase in anterior cingulate gyrus ipsilateral to seizure onset. A positive correlation between NK1 receptor availability and seizure frequency was observed in the medial temporal lobe and in the lentiform nucleus ipsilateral to the seizure onset.

CONCLUSION

Our results suggest that there is an intrinsic network using the NK1-SP receptor system for synaptic transmission and epileptiform activity in TLE.

Dopamine D2/D3 receptor availability and venturesomeness

The construct of impulsivity is considered as a major trait of personality. There is growing evidence that the mesolimbic dopamine system plays an important role in the modulation of impulsivity and venturesomeness, the two key components within the impulsivity-construct. The aim of the present study was to explore an association between trait impulsivity measured with self-assessment and the dopaminergic neurotransmission as measured by positron emission tomography (PET) in a cohort of healthy male subjects. In vivo D2/D3 receptor availability was determined with [(18)F]fallypride PET in 18 non-smoking healthy subjects. The character trait impulsivity was measured using the Impulsiveness-Venturesomeness-Empathy questionnaire (I7). Image processing and statistical analysis was performed on a voxel-by-voxel basis using statistical parametric mapping (SPM) software. The I7 subscale venturesomeness correlated positively with the D2/D3 receptor availability within the left temporal cortex and the thalamus. Measures on the I7 subscale impulsiveness and empathy did not correlate with the D2/D3 receptor availability in any brain region investigated. Our results suggest the involvement of extrastriatal dopaminergic neurotransmission in venturesomeness, a component of impulsivity.

Genotype specific age related changes in a transgenic rat model of Huntington's disease

We aimed to characterize the transgenic Huntington rat model with in vivo imaging and identify sensitive and reliable biomarkers associated with early and progressive disease status. In order to do so, we performed a multimodality (DTI and PET) longitudinal imaging study, during which the same TgHD and wildtype (Wt) rats were repetitively scanned. Surprisingly, the relative ventricle volume was smaller but increased faster in TgHD compared to Wt animals. DTI (mean, axial, radial diffusivity) revealed subtle genotype-specific aging effects in the striatum and its surrounding white matter, already in the presymptomatic stage. Using ¹⁸F-FDG and ¹⁸F-Fallypride PET imaging, we were not able to demonstrate genotype-specific aging effects within the striatum. The outcome of this longitudinal study was somewhat surprising as it demonstrated a significant differential aging pattern in TgHD versus Wt animals. Although it seems that the TgHD rat model does not have a sufficient expression of disease yet at the age of 12 months, further validation of this model is highly beneficial since there is still an incomplete understanding of the early disease mechanisms of Huntington's disease.

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.

Alteration of dopamine D2/D3 receptor binding in patients with juvenile myoclonic epilepsy

PURPOSE

To quantify extrastriatal and striatal D2/D3 receptor binding in patients with juvenile myoclonic epilepsy (JME) using the high-affinity dopamine D2/D3 receptor positron emission tomography (PET) ligand (18) F-Fallypride ([(18) F]FP).

METHODS

Twelve patients with JME and 21 age-matched control subjects were studied. Dynamic images (180 min) were acquired after injection of [(18) F]FP. Patients had been seizure-free of all seizure types for at least 10 days before scanning. Parametric images of binding potential (BP) were created using the simplified reference tissue model. The images were stereotactically normalized using a ligand-specific template. We performed a voxel-based analysis with statistical parametric mapping (SPM2). Region of interest (ROI) analysis was done comparing the BP of the thalamus, caudate nucleus, anterior (ventral) and posterior (dorsal) putamen, ventral striatum, and temporal lobe.

RESULTS

Compared to controls, patients with JME showed a significant decrease in [(18) F]FP BP (SPM analysis corr. p < 0.001 at cluster level) restricted to the bilateral posterior putamen. There was no significant alteration of [(18) F]FP binding in other brains regions. ROI analysis revealed a significant (p < 0.05) decrease of [(18) F]FP BP in the left (mean -14.8%) and right (mean -16.9%) posterior putamen, but not in the anterior putamen, caudate, ventral striatum, thalamus, or temporal lobe.

DISCUSSION

Patients with JME showed a reduction in D2/3 receptor binding restricted to the bilateral posterior putamen, suggesting a specific alteration of the dopaminergic system. Whether these changes can be regarded as merely functional or whether they relate to the pathophysiology of juvenile myoclonic epilepsy still remains unclear.

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.