A GABAA receptor mutation causing generalized epilepsy reduces benzodiazepine receptor binding

Established and emerging GABAA receptor pharmacotherapy for epilepsy

Drugs that modulate the GABAA receptor are widely used in clinical practice for both the long-term management of epilepsy and emergency seizure control. In addition to older medications that have well-defined roles for the treatment of epilepsy, recent discoveries into the structure and function of the GABAA receptor have led to the development of newer compounds designed to maximise therapeutic benefit whilst minimising adverse effects, and whose position within the epilepsy pharmacologic armamentarium is still emerging. Drugs that modulate the GABAA receptor will remain a cornerstone of epilepsy management for the foreseeable future and, in this article, we provide an overview of the mechanisms and clinical efficacy of both established and emerging pharmacotherapies.

GABAA receptors in epilepsy: Elucidating phenotypic divergence through functional analysis of genetic variants

Normal brain function requires a tightly regulated balance between excitatory and inhibitory neurotransmissions. γ-Aminobutyric acid type A (GABAA ) receptors represent the major class of inhibitory ion channels in the mammalian brain. Dysregulation of these receptors and/or their associated pathways is strongly implicated in the pathophysiology of epilepsy. To date, hundreds of different GABAA receptor subunit variants have been associated with epilepsy, making them a prominent cause of genetically linked epilepsy. While identifying these genetic variants is crucial for accurate diagnosis and effective genetic counselling, it does not necessarily lead to improved personalised treatment options. This is because the identification of a variant does not reveal how the function of GABAA receptors is affected. Genetic variants in GABAA receptor subunits can cause complex changes to receptor properties resulting in various degrees of gain-of-function, loss-of-function or a combination of both. Understanding how variants affect the function of GABAA receptors therefore represents an important first step in the ongoing development of precision therapies. Furthermore, it is important to ensure that functional data are produced using methodologies that allow genetic variants to be classified using clinical guidelines such as those developed by the American College of Medical Genetics and Genomics. This article will review the current knowledge in the field and provide recommendations for future functional analysis of genetic GABAA receptor variants.

Region-specific associations between gamma-aminobutyric acid A receptor binding and cortical thickness in high-functioning autistic adults

The neurobiology of autism has been shown to involve alterations in cortical morphology and gamma-aminobutyric acid A (GABA_A ) receptor density. We hypothesized that GABA_A receptor binding potential (GABA_A R BP_ND ) would correlate with cortical thickness, but their correlations would differ between autistic adults and typically developing (TD) controls. We studied 50 adults (23 autism, 27 TD, mean age of 27 years) using magnetic resonance imaging to measure cortical thickness, and [¹⁸ F]flumazenil positron emission tomography imaging to measure GABA_A R BP_ND . We determined the correlations between cortical thickness and GABA_A R BP_ND by cortical lobe, region-of-interest, and diagnosis of autism spectrum disorder (ASD). We also explored potential sex differences in the relationship between cortical thickness and autism characteristics, as measured by autism spectrum quotient (AQ) scores. Comparing autism and TD groups, no significant differences were found in cortical thickness or GABA_A R BP_ND . In both autism and TD groups, a negative relationship between cortical thickness and GABA_A R BP_ND was observed in the frontal and occipital cortices, but no relationship was found in the temporal or limbic cortices. A positive correlation was seen in the parietal cortex that was only significant for the autism group. Interestingly, in an exploratory analysis, we found sex differences in the relationships between cortical thickness and GABA_A R BP_ND , and cortical thickness and AQ scores in the left postcentral gyrus. LAY SUMMARY: The thickness of the brain cortex and the density of the receptors associated with inhibitory neurotransmitter GABA have been hypothesized to underlie the neurobiology of autism. In this study, we found that these biomarkers correlate positively in the parietal cortex, but negatively in the frontal and occipital cortical regions of the brain. Furthermore, we collected preliminary evidence that the correlations between cortical thickness and GABA receptor density are sexdependent in a brain region where sensory inputs are registered.

Hyperexcitability and impaired intracortical inhibition in patients with fragile-X syndrome

Fragile-X syndrome (FXS) is characterized by neurological and psychiatric problems symptomatic of cortical hyperexcitability. Recent animal studies identified deficient γ-aminobutyricacid (GABA) inhibition as a key mechanism for hyperexcitability in FXS, but the GABA system remains largely unexplored in humans with the disorder. The primary objective of this study was to assess GABA-mediated inhibition and its relationship with hyperexcitability in patients with FXS. Transcranial magnetic stimulation (TMS) was used to assess cortical and corticospinal inhibitory and excitatory mechanisms in 18 patients with a molecular diagnosis of FXS and 18 healthy controls. GABA-mediated inhibition was measured with short-interval intracortical inhibition (GABA_A), long-interval intracortical inhibition (GABA_B), and the corticospinal silent period (GABA_A+B). Net intracortical facilitation involving glutamate was assessed with intracortical facilitation, and corticospinal excitability was measured with the resting motor threshold. Results showed that FXS patients had significantly reduced short-interval intracortical inhibition, increased long-interval intracortical inhibition, and increased intracortical facilitation compared to healthy controls. In the FXS group, reduced short-interval intracortical inhibition was associated with heightened intracortical facilitation. Taken together, these results suggest that reduced GABA_A inhibition is a plausible mechanism underlying cortical hyperexcitability in patients with FXS. These findings closely match those observed in animal models, supporting the translational validity of these markers for clinical research.

Human GABRG2 generalized epilepsy: Increased somatosensory and striatothalamic connectivity

Objective

To map functional MRI (fMRI) connectivity within and between the somatosensory cortex, putamen, and ventral thalamus in individuals from a family with a GABAergic deficit segregating with febrile seizures and genetic generalized epilepsy.

Methods

We studied 5 adults from a family with early-onset absence epilepsy and/or febrile seizures and a GABA_A receptor subunit gamma2 pathogenic variant (GABRG2[R43Q]) vs 5 age-matched controls. We infer differences between participants with the GABRG2 pathogenic variant and controls in resting-state fMRI connectivity within and between the somatosensory cortex, putamen, and ventral thalamus.

Results

We observed increased fMRI connectivity within the somatosensory cortex and between the putamen and ventral thalamus in all individuals with the GABRG2 pathogenic variant compared with controls. Post hoc analysis showed less pronounced changes in fMRI connectivity within and between the primary visual cortex and precuneus.

Conclusions

Although our sample size was small, this preliminary study suggests that individuals with a GABRG2 pathogenic variant, raising risk of febrile seizures and generalized epilepsy, display underlying increased functional connectivity both within the somatosensory cortex and in striatothalamic networks. This human network model aligns with rodent research and should be further validated in larger cohorts, including other individuals with generalized epilepsy with and without known GABA pathogenic variants.

Double-edged GABAergic synaptic transmission in seizures: The importance of chloride plasticity

GABAergic synaptic inhibition, which is a critical regulator of neuronal excitability, is closely involved in epilepsy. Interestingly, fast GABAergic transmission mediated by Cl- permeable GABAA receptors can bi-directionally exert both seizure-suppressing and seizure-promoting actions. Accumulating evidence suggests that chloride plasticity, the driving force of GABAA receptor-mediated synaptic transmission, contributes to the double-edged role of GABAergic synapses in seizures. Large amounts of Cl- influx can overwhelm Cl- extrusion during seizures not only in healthy tissue in a short-term "activity-dependent" manner, but also in chronic epilepsy in a long-term, irreversible "pathology-dependent" manner related to the dysfunction of two chloride transporters: the chloride importer NKCC1 and the chloride exporter KCC2. In this review, we address the importance of chloride plasticity for the "activity-dependent" and "pathology-dependent" mechanisms underlying epileptic events and provide possible directions for further research, which may be clinically important for the design of GABAergic synapse-targeted precise therapeutic interventions for epilepsy.

Reduced local input to fast-spiking interneurons in the somatosensory cortex in the GABAA γ2 R43Q mouse model of absence epilepsy

Objective

Absence seizures in childhood absence epilepsy are initiated in the thalamocortical (TC) system. We investigated if these seizures result from altered development of the TC system before the appearance of seizures in mice containing a point mutation in γ‐aminobutyric acid A (GABA_A) receptor γ2 subunits linked to childhood absence epilepsy (R43Q). Findings from conditional mutant mice indicate that expression of normal γ2 subunits during preseizure ages protect from later seizures. This indicates that altered development in the presence of the R43Q mutation is a key contributor to the R43Q phenotype. We sought to identify the cellular processes affected by the R43Q mutation during these preseizure ages.

Methods

We examined landmarks of synaptic development at the end of the critical period for somatosensory TC plasticity using electrophysiologic recordings in TC brain slices from wild‐type mice and R43Q mice.

Results

We found that the level of TC connectivity to layer 4 (L4) principal cells and the properties of TC synapses were unaltered in R43Q mice. Furthermore, we show that, although TC feedforward inhibition and the total level of GABAergic inhibition were normal, there was a reduction in the local connectivity to cortical interneurons. This reduction leads to altered inhibition during bursts of cortical activity.

Significance

This altered inhibition demonstrates that alterations in cortical circuitry precede the onset of seizures by more than a week.

Mutant GABA(A) receptor subunits in genetic (idiopathic) epilepsy

The γ-aminobutyric acid receptor type A (GABAA receptor) is a ligand-gated chloride channel that mediates major inhibitory functions in the central nervous system. GABAA receptors function mainly as pentamers containing α, β, and either γ or δ subunits. A number of antiepileptic drugs have agonistic effects on GABAA receptors. Hence, dysfunctions of GABAA receptors have been postulated to play important roles in the etiology of epilepsy. In fact, mutations or genetic variations of the genes encoding the α1, α6, β2, β3, γ2, or δ subunits (GABRA1, GABRA6, GABRB2, GABRB3, GABRG2, and GABRD, respectively) have been associated with human epilepsy, both with and without febrile seizures. Epilepsy resulting from mutations is commonly one of following, genetic (idiopathic) generalized epilepsy (e.g., juvenile myoclonic epilepsy), childhood absence epilepsy, genetic epilepsy with febrile seizures, or Dravet syndrome. Recently, mutations of GABRA1, GABRB2, and GABRB3 were associated with infantile spasms and Lennox-Gastaut syndrome. These mutations compromise hyperpolarization through GABAA receptors, which is believed to cause seizures. Interestingly, most of the insufficiencies are not caused by receptor gating abnormalities, but by complex mechanisms, including endoplasmic reticulum (ER)-associated degradation, nonsense-mediated mRNA decay, intracellular trafficking defects, and ER stress. Thus, GABAA receptor subunit mutations are now thought to participate in the pathomechanisms of epilepsy, and an improved understanding of these mutations should facilitate our understanding of epilepsy and the development of new therapies.

Multimodal neuroimaging: potential biomarkers for response to antiepileptic drugs?

Neuroimaging techniques in epilepsy are used widely for definition of the epileptogenic lesion and surgical decision. However, its applications extend to the knowledge of epileptic mechanisms and include the identification of prognostic features that can help our decisions on the appropriate type of treatment on an individual basis. Structural neuroimaging may be able to identify patients more likely to respond to antiepileptic drug (AED) treatment and also patients who are better candidates for earlier surgical treatment. In the past decades, quantitative analyses have also improved our knowledge about epileptogenic lesions and networks as well as the following prognoses: seizure control, cognitive outcome, and comorbidities. New advanced neuroimaging techniques such as functional magnetic resonance imaging (MRI) and the development biotracers that could be associated with inflammation and specific genetic patterns will add further knowledge to the development of epilepsy treatments.

The impact of positron emission tomography imaging on the clinical management of patients with epilepsy

Clinical positron emission tomography (PET) imaging of human epilepsy has a 30-year history, but it is still searching for its exact role among rapidly advancing neuroimaging techniques. The vast majority of epilepsy PET studies used this technique to improve detection of epileptic foci for surgical resection. Here, we review the main trends emerging from three decades of PET research in epilepsy, with a particular emphasis on how PET imaging has impacted on the clinical management of patients with intractable epilepsy. While reviewing the latest studies, we also present an argument for a changing role of PET and molecular imaging in the future, with an increasing focus on epileptogenesis and newly discovered molecular mechanisms of epilepsy. These new applications will be facilitated by technological advances, such as the use of integrated PET/MRI systems and utilization of novel radiotracers, which may also enhance phenotype-genotype correlations and assist rational, individualized treatment strategies.

GABAB-ergic motor cortex dysfunction in SSADH deficiency

OBJECTIVE

Succinic semialdehyde dehydrogenase (SSADH) deficiency is a rare autosomal recessive disorder of GABA degradation leading to elevations in brain GABA and γ-hydroxybutyric acid (GHB). The effect of chronically elevated GABA and GHB on cortical excitability is unknown. We hypothesized that use-dependent downregulation of GABA receptor expression would promote cortical disinhibition rather than inhibition, predominantly via presynaptic GABAergic mechanisms.

METHODS

We quantified the magnitude of excitation and inhibition in primary motor cortex (M1) in patients with SSADH deficiency, their parents (obligate heterozygotes), age-matched healthy young controls, and healthy adults using single and paired pulse transcranial magnetic stimulation (TMS).

RESULTS

Long interval intracortical inhibition was significantly reduced and the cortical silent period was significantly shortened in patients with SSADH deficiency compared to heterozygous parents and control groups.

CONCLUSIONS

Since long interval intracortical inhibition and cortical silent period are thought to reflect GABA(B) receptor-mediated inhibitory circuits, our results point to a particularly GABA(B)-ergic motor cortex dysfunction in patients with SSADH deficiency. This human phenotype is consistent with the proposed mechanism of use-dependent downregulation of postsynaptic GABA(B) receptors in SSADH deficiency animal models. Additionally, the results suggest autoinhibition of GABAergic neurons. This first demonstration of altered GABA(B)-ergic function in patients with SSADH deficiency may help to explain clinical features of the disease, and suggest pathophysiologic mechanisms in other neurotransmitter-related disorders.

Temperature elevation increases GABA(A) -mediated cortical inhibition in a mouse model of genetic epilepsy

A missense mutation (R43Q) in the γ2 subunit of the γ-aminobutyric acid (GABA)(A) receptor is associated with generalized (genetic) epilepsy with febrile seizures plus (GEFS+). Heterozygous GABA(A) γ2(R43Q) mice displayed a lower temperature threshold for thermal seizures as compared to wild-type littermates. Temperature-dependent internalization of GABA(A) γ2(R43Q)-containing receptors has been proposed as a mechanism underlying febrile seizure genesis in patients with this mutation. We tested this idea using the GABA(A) γ2(R43Q) knockin mouse model and analyzed GABAergic miniature postsynaptic inhibitory currents (mIPSCs) in acute brain slices after exposure to varying temperatures. Incubation of slices at an elevated temperature increased mIPSC amplitude in neurons from heterozygous mice, with no change seen in wild-type controls. [³H]Flumazenil binding measured in whole-brain homogenates from mutant and control mice following elevation of body temperature showed no temperature-dependent differences in γ2-containing receptor density. Therefore, in vivo mouse data do not support earlier in vitro observations that proposed temperature-dependent internalization of γ2 R43Q containing GABA(A) receptors as the cellular mechanism underlying febrile seizure genesis in patients with the GABA(A) γ2(R43Q) mutation.

Neuronal network pharmacodynamics of GABAergic modulation in the human cortex determined using pharmaco-magnetoencephalography

Neuronal network oscillations are a unifying phenomenon in neuroscience research, with comparable measurements across scales and species. Cortical oscillations are of central importance in the characterization of neuronal network function in health and disease and are influential in effective drug development. Whilst animal in vitro and in vivo electrophysiology is able to characterize pharmacologically induced modulations in neuronal activity, present human counterparts have spatial and temporal limitations. Consequently, the potential applications for a human equivalent are extensive. Here, we demonstrate a novel implementation of contemporary neuroimaging methods called pharmaco‐magnetoencephalography. This approach determines the spatial profile of neuronal network oscillatory power change across the cortex following drug administration and reconstructs the time course of these modulations at focal regions of interest. As a proof of concept, we characterize the nonspecific GABAergic modulator diazepam, which has a broad range of therapeutic applications. We demonstrate that diazepam variously modulates θ (4–7 Hz), α (7–14 Hz), β (15–25 Hz), and γ (30–80 Hz) frequency oscillations in specific regions of the cortex, with a pharmacodynamic profile consistent with that of drug uptake. We examine the relevance of these results with regard to the spatial and temporal observations from other modalities and the various therapeutic consequences of diazepam and discuss the potential applications of such an approach in terms of drug development and translational neuroscience. Hum Brain Mapp, 2010. © 2009 Wiley‐Liss, Inc.

Mutations affecting GABAergic signaling in seizures and epilepsy

The causes of epilepsies and epileptic seizures are multifactorial. Genetic predisposition may contribute in certain types of epilepsies and seizures, whether idiopathic or symptomatic of genetic origin. Although these are not very common, they have offered a unique opportunity to investigate the molecular mechanisms underlying epileptogenesis and ictogenesis. Among the implicated gene mutations, a number of GABAA receptor subunit mutations have been recently identified that contribute to several idiopathic epilepsies, febrile seizures, and rarely to certain types of symptomatic epilepsies, like the severe myoclonic epilepsy of infancy. Deletion of GABAA receptor genes has also been linked to Angelman syndrome. Furthermore, mutations of proteins controlling chloride homeostasis, which indirectly defines the functional consequences of GABAA signaling, have been identified. These include the chloride channel 2 (CLCN2) and the potassium chloride cotransporter KCC3. The pathogenic role of CLCN2 mutations has not been clearly demonstrated and may represent either susceptibility genes or, in certain cases, innocuous polymorphisms. KCC3 mutations have been associated with hereditary motor and sensory polyneuropathy with corpus callosum agenesis (Andermann syndrome) that often manifests with epileptic seizures. This review summarizes the recent progress in the genetic linkages of epilepsies and seizures to the above genes and discusses potential pathogenic mechanisms that contribute to the age, sex, and conditional expression of these seizures in carriers of these mutations.

Mutations in GABAA receptor subunits associated with genetic epilepsies

Mutations in inhibitory GABAA receptor subunit genes (GABRA1, GABRB3, GABRG2 and GABRD) have been associated with genetic epilepsy syndromes including childhood absence epilepsy (CAE), juvenile myoclonic epilepsy (JME), pure febrile seizures (FS), generalized epilepsy with febrile seizures plus (GEFS+), and Dravet syndrome (DS)/severe myoclonic epilepsy in infancy (SMEI). These mutations are found in both translated and untranslated gene regions and have been shown to affect the GABAA receptors by altering receptor function and/or by impairing receptor biogenesis by multiple mechanisms including reducing subunit mRNA transcription or stability, impairing subunit folding, stability, or oligomerization and by inhibiting receptor trafficking.

Decreased GABA-A binding on FMZ-PET in succinic semialdehyde dehydrogenase deficiency

OBJECTIVE

Succinic semialdehyde dehydrogenase (SSADH) deficiency is an autosomal recessive disorder of GABA metabolism characterized by elevated levels of GABA and gamma-hydroxybutyric acid. Clinical findings include intellectual impairment, hypotonia, hyporeflexia, hallucinations, autistic behaviors, and seizures. Autoradiographic labeling and slice electrophysiology studies in the murine model demonstrate use-dependent downregulation of GABA(A) receptors. We studied GABA(A) receptor activity in human SSADH deficiency utilizing [(11)C]-flumazenil (FMZ)-PET.

METHODS

FMZ binding was measured in 7 patients, 10 unaffected parents, and 8 healthy controls. Data analysis was performed using a reference region compartmental model, with time-activity curve from pons as the input function. Relative parametric binding potential (BP(ND)) was derived, with MRI-based pixel by pixel partial volume correction, in regions of interest drawn on coregistered MRI.

RESULTS

In amygdala, hippocampus, cerebellar vermis, frontal, parietal, and occipital cortex, patients with SSADH deficiency had significant reductions in FMZ BP(ND) compared to parents and controls. Mean cortical values were 6.96 +/- 0.79 (controls), 6.89 +/- 0.71 (parents), and 4.88 +/- 0.77 (patients) (F ratio 16.1; p < 0.001). There were no differences between controls and parents in any cortical region.

CONCLUSIONS

Succinic semialdehyde dehydrogenase (SSADH) deficient patients show widespread reduction in BZPR binding on [(11)C]-flumazenil-PET. Our results suggest that high endogenous brain GABA levels in SSADH deficiency downregulate GABA(A)-BZPR binding site availability. This finding suggests a potential mechanism for neurologic dysfunction in a serious neurodevelopmental disorder, and suggests that PET may be useful to translate studies in animal models to human disease.

[Current clinical-neurophysiological findings in absence epilepsies]

According to the international classification of epileptic seizures (1981) generalized seizures present ictal signs indicating seizure onset in both hemispheres whereas focal seizures present ictal signs pointing to one hemisphere at seizure onset. Recent neurophysiological studies using MEG, EEG, and fMRI suggest that in case of idiopathic generalized absence epilepsies, activity predominates in specific networks, which include frontoparietal cortical as well as subcortical areas of both hemispheres. In these network regions, epileptic activity can occur circumscribed and focal as spike or spike-wave activity but also widely distributed, bilateral, and homologous. As a consequence to the classification of "generalized" epileptic seizures and syndromes, it might be possible to distinguish a subgroup: regional bilateral homologous epilepsies. These must be distinguished from a pure culture of focal frontal lobe epilepsies, which can also be accompanied by the lead symptom of absences.

Network characteristics of idiopathic generalized epilepsies in combined MEG/EEG

Seven patients with idiopathic generalized epilepsies were investigated using MEG and EEG. Spike-wave series were seen in all patients, single spikes in six. For both, source analysis showed most often involvement of frontal, perinsular and subcortical/thalamic areas. In all patients, a unilateral frontal accentuation of activity could be observed. Patients with juvenile myoclonic and myoclonic absence epilepsy presented with localizations mainly in the central and premotor regions versus prefrontal accentuation in the other absence patients. MEG/EEG source localization provides important information concerning regional network involvement in idiopathic generalized epilepsies. As a consequence for a neurophysiological concept of "generalized" epileptic seizures and syndromes, a third subgroup with regional activity in bilateral homologous regions can be differentiated next to pure focal and multifocal generalized epilepsies.

A fusion PET-MRI system with a high-resolution research tomograph-PET and ultra-high field 7.0 T-MRI for the molecular-genetic imaging of the brain

We have developed a positron emission tomography (PET) and magnetic resonance imaging (MRI) fusion system for the molecular-genetic imaging (MGI) of the in vivo human brain using two high-end imaging devices: the HRRT-PET, a high-resolution research tomograph dedicated to brain imaging on the molecular level, and the 7.0 T-MRI, an ultra-high field version used for morphological imaging. HRRT-PET delivers high-resolution molecular imaging with a resolution down to 2.5 mm full width at half maximum (FWHM), which allows us to observe the brain's molecular changes using the specific reporter genes and probes. On the other front, the 7.0 T-MRI, with submillimeter resolution images of the cortical areas down to 250 mum, allows us to visualize the fine details of the brainstem areas as well as the many cortical and subcortical areas. The new PET-MRI fusion imaging system will provide many answers to the questions on neurological diseases as well as cognitive neurosciences. Some examples of the answers are the quantitative visualization of neuronal functions by clear molecular and genetic bases, as well as diagnoses of many neurological diseases such as Parkinson's and Alzheimer's. The salient point of molecular-genetic imaging and diagnosis is the fact that they precede the morphological manifestations, and hence, the early and specific diagnosis of certain diseases, such as cancers.

GABA(A) receptors in normal development and seizures: friends or foes?

GABA(A) receptors have an age-adapted function in the brain. During early development, they mediate excitatory effects resulting in activation of calcium sensitive signaling processes that are important for the differentiation of the brain. In more mature stages of development and in adults, GABA(A) receptors transmit inhibitory signals. The maturation of GABA(A) signaling follows sex-specific patterns, which appear to also be important for the sexual differentiation of the brain. The inhibitory effects of GABA(A) receptor activation have been widely exploited in the treatment of conditions where neuronal silencing is necessary. For instance, drugs that target GABA(A) receptors are the mainstay of treatment of seizures. Recent evidence suggests however that the physiology and function of GABA(A) receptors changes in the brain of a subject that has epilepsy or status epilepticus.This review will summarize the physiology of and the developmental factors regulating the signaling and function of GABA(A) receptors; how these may change in the brain that has experienced prior seizures; what are the implications for the age and sex specific treatment of seizures and status epilepticus. Finally, the implications of these changes for the treatment of certain forms of medically refractory epilepsies and status epilepticus will be discussed.

Reduced GABAA benzodiazepine receptor binding in veterans with post-traumatic stress disorder

Gamma-aminobutyric acid (GABA(A)) receptors are thought to play an important role in modulating the central nervous system in response to stress. Animal data have shown alterations in the GABA(A) receptor complex by uncontrollable stressors. SPECT imaging with benzodiazepine ligands showed lower distribution volumes of the benzodiazepine-GABA(A) receptor in the prefrontal cortex of patients with post-traumatic stress disorder (PTSD) in one, but not in another study. The objective of the present study was to assess differences in the benzodiazepine-GABA(A) receptor complex in veterans with and without PTSD using [(11)C]flumazenil and positron emission tomography (PET). Nine drug naive male Dutch veterans with deployment related PTSD and seven male Dutch veterans without PTSD were recruited, and matched for age, region and year of deployment. Each subject received a [(11)C]flumazenil PET scan and a structural magnetic resonance imaging scan. Dynamic 3D PET scans with a total duration of 60 min were acquired, and binding in template based and manually defined regions of interest (ROI) was quantified using validated plasma input and reference tissue models. In addition, parametric binding potential images were compared on a voxel-by-voxel basis using statistical parametric mapping (SPM2). ROI analyses using both template based and manual ROIs showed significantly reduced [(11)C]flumazenil binding in PTSD subjects throughout the cortex, hippocampus and thalamus. SPM analysis confirmed these results. The observed global reduction of [(11)C]flumazenil binding in patients with PTSD provides circumstantial evidence for the role of the benzodiazepine-GABA(A) receptor in the pathophysiology of PTSD and is consistent with previous animal research and clinical psychopharmacological studies.

Reduced cortical inhibition in a mouse model of familial childhood absence epilepsy

Mutations in the GABA(A) receptor gamma2 subunit are associated with childhood absence epilepsy and febrile seizures. To understand better the molecular basis of absence epilepsy in man, we developed a mouse model harboring a gamma2 subunit point mutation (R43Q) found in a large Australian family. Mice heterozygous for the mutation demonstrated behavioral arrest associated with 6-to 7-Hz spike-and-wave discharges, which are blocked by ethosuximide, a first-line treatment for absence epilepsy in man. Seizures in the mouse showed an abrupt onset at around age 20 days corresponding to the childhood nature of this disease. Reduced cell surface expression of gamma2(R43Q) was seen in heterozygous mice in the absence of any change in alpha1 subunit surface expression, ruling out a dominant-negative effect. GABA(A)-mediated synaptic currents recorded from cortical pyramidal neurons revealed a small but significant reduction that was not seen in the reticular or ventrobasal thalamic nuclei. We hypothesize that a subtle reduction in cortical inhibition underlies childhood absence epilepsy seen in humans harboring the R43Q mutation.

Estimating neurotransmitter kinetics with ntPET: a simulation study of temporal precision and effects of biased data

We recently introduced neurotransmitter PET (ntPET), an analysis technique that estimates the kinetics of stimulus-induced neurotransmitter (NT) release. Here, we evaluate two formulations of ntPET. The arterial (ART) approach measures the tracer input function (TIF) directly. The reference (REF) approach derives the TIF from reference region data. Arterial sampling is considered the gold standard in PET modeling but reference region approaches are preferred for reduced cost and complexity. If simulated PET data with unbiased TIFs were analyzed using ART or REF, temporal precision was better than 3 min provided NT concentration peaked less than 30 min into the scanning session. The consequences of biased TIFs or stimulus-induced changes in tracer delivery were also evaluated. ART TIFs were biased by the presence of uncorrected radiometabolites in the plasma whereas REF TIFs were biased by specific binding in the reference region. Simulated changes in tracer delivery emulated ethanol-induced blood flow alterations observed previously with PET. ART performance deteriorated significantly if metabolites amounted to 50% of plasma radioactivity by 60 min. The accuracy and precision of REF were preserved even if the reference region contained 40% of the receptor density of the target region. Both methods were insensitive to blood flow alterations (proportional changes in K(1) and k(2)). Our results suggest that PET data contain information--heretofore not extracted--about the timing of NT release. The REF formulation of ntPET proved to be robust to many plausible model violations and under most circumstances is an appropriate alternative to ART.

Intracortical hyperexcitability in humans with a GABAA receptor mutation

A missense mutation of the gamma2 subunit of the gamma-aminobutyric acid A (GABA(A)) receptor has been linked to an inherited human generalized epilepsy. As synaptic inhibition in the human brain is largely mediated by the GABA(A) receptor, we tested the hypothesis that the GABRG2(R43Q) mutation alters cortical excitability. Fourteen subjects affected by the GABRG2(R43Q) mutation (5 males, mean age: 44 +/- 15 years) and 24 controls (11 males, mean age: 38 +/- 11 years) were studied with transcranial magnetic stimulation (TMS). To assess the specificity of the effect of the mutation, 4 additional family members unaffected by the GABRG2(R43Q) mutation (2 males, mean age: 41 +/- 16 years) were included. Subjects affected by the GABRG2(R43Q) mutation demonstrated reduced net short-interval intracortical inhibition and increased intracortical facilitation assessed with paired-pulse stimulation. Subjects with the mutation had similar motor thresholds to controls both at rest and with weak voluntary activation. No significant differences were noted between groups in the cortical silent period. Our findings provide in vivo evidence for increased intracortical excitability in subjects affected by the GABRG2(R43Q) mutation. These findings are also likely to represent an important clue to the mechanisms linking this gene defect and the epilepsy phenotype.

The use of SPECT and PET in routine clinical practice in epilepsy

PURPOSE OF REVIEW

The aim of this article is to give a subjective review of the usefulness of single photon emission computed tomography (SPECT) and positron emission tomography (PET) imaging in clinical practice in epilepsy for 2007.

RECENT FINDINGS

Both ictal perfusion SPECT and interictal fluorodeoxyglucose PET can provide new information in the presurgical evaluation of intractable partial epilepsy. These functional imaging modalities reflect dynamic seizure-related changes in cerebral cellular functions. Although asymmetry of fluorodeoxyglucose PET metabolism has been useful to localize the epileptic temporal lobe, which tends to be more hypometabolic than the contralateral one, both frontal lobes are more hypometabolic than the epileptic temporal lobe, and may represent a region of 'surround inhibition'. Due to its low temporal resolution, ictal perfusion SPECT hyperperfusion patterns often contain both the ictal onset zone and propagation pathways. These patterns often have a multilobulated 'hourglass' appearance. The largest and most intense hyperperfusion cluster often represents ictal propagation, and does not always need to be resected in order to render a patient seizure free.

SUMMARY

Optimized interictal FDG-PET and ictal perfusion SPECT as part of a multimodality imaging platform will be important tools to better understand the neurobiology of epilepsy and to better define the epileptogenic, ictal onset, functional deficit and surround inhibition zones in refractory partial epilepsy.

Cellular and network mechanisms of genetically-determined absence seizures

The absence epilepsies are characterized by recurrent episodes of loss of consciousness associated with generalized spike-and-wave discharges, with an abrupt onset and offset, in the thalamocortical system. In the absence of detailed neurophysiological studies in humans, many of the concepts regarding the pathophysiological basis of absence seizures are based on studies in animal models. Each of these models has its particular strengths and limitations, and the validity of findings from these models for the human condition cannot be assumed. Consequently, studies in different models have produced some conflicting findings and conclusions. A long-standing concept, based primarily from studies in vivo in cats and in vitro brain slices, is that these paroxysmal electrical events develop suddenly from sleep-related spindle oscillations. More specifically, it is proposed that the initial mechanisms that underlie absence-related spike-and-wave discharges are located in the thalamus, involving especially the thalamic reticular nucleus. By contrast, more recent studies in well-established, genetic models of absence epilepsy in rats demonstrate that spike-and-wave discharges originate in a cortical focus and develop from a wake-related natural corticothalamic sensorimotor rhythm. In this review we integrate recent findings showing that, in both the thalamus and the neocortex, genetically-determined, absence-related spike-and-wave discharges are the manifestation of hypersynchronized, cellular, rhythmic excitations and inhibitions that result from a combination of complex, intrinsic, synaptic mechanisms. Arguments are put forward supporting the hypothesis that layer VI corticothalamic neurons act as 'drivers' in the generation of spike-and-wave discharges in the somatosensory thalamocortical system that result in corticothalamic resonances particularly initially involving the thalamic reticular nucleus. However an important unresolved question is: what are the cellular and network mechanisms responsible for the switch from physiological, wake-related, natural oscillations into pathological spike-and-wave discharges? We speculate on possible answers to this, building particularly on recent findings from genetic models in rats.