GABA(B) receptor 1 polymorphism (G1465A) is associated with temporal lobe epilepsy

Modulation of GABAB receptors in the insula bidirectionally affects associative memory of epilectic rats in both spatial and non-spatial operant tasks

Background

Stimulation of gamma-aminobutyric acid (GABA) activity through GABA receptor agonists is the basic mechanism of many anticonvulsant drugs. Nevertheless, many of these GABergic drugs have adverse cognitive effects. We previously found that GABAB receptors (GABA_BRs) in the insula regulate operant associative memory in healthy rats. The present study aimed at investigating the effects of GABA_BR modulation in the insula on operant associative memory in epileptic rats, along with the underlying mechanisms.

Methods

The lithium-pilocarpine model of temporal lobe epilepsy (TLE) was established in male Sprague-Dawley rats. A 22-gauge stainless-steel guide cannula was surgically implanted into the granular insula cortex of the epileptic rats. Baclofen (125 ng/μl, 1 μl), CGP35348 (12.5 μg/μl, 1 μl), or saline (1 μl) was slowly infused through the guide cannula. The Intellicage automated behavioral testing system was used to evaluate operant associative memory of the epileptic rats, including non-spatial operant tasks (basic nosepoke learning and skilled nosepoke learning) and spatial operant tasks (chamber position learning). The expression of the GABA_BR subunits GB1 and GB2 in the insula was examined by immunofluorescence and Western blotting.

Results

The Intellicage tests demonstrated that baclofen significantly impaired basic nosepoke learning, skilled nosepoke learning and chamber position learning of the epileptic rats, while CGP35348 boosted these functions. Immunofluorescence staining revealed that GB1 and GB2 were expressed in the insula of the epileptic rats, and Western blotting analysis showed that baclofen enhanced while CGP35348 inhibited the expression of these subunits.

Conclusion

GABA_BRs in the insula bidirectionally regulate both spatial and non-spatial operant associative memory of epileptic rats. Effects of GABA_BRs on cognition should be taken into account when evaluating new possible treatments for people with epilepsy.

Rare antibody-mediated and seronegative autoimmune encephalitis: An update

Paralleling advances with respect to more common antibody-mediated encephalitides, such as anti-N-methyl-D-aspartate receptor (NMDAR) and anti-leucine-rich glioma-inactivated 1 (LGI1) Ab-mediated encephalitis, the discovery and characterisation of novel antibody-mediated encephalitides accelerated over the past decade, adding further depth etiologically to the spectrum of antibody-mediated encephalitis. Herein, we review the major mechanistic, clinical features and management considerations with respect to anti-γ-aminobutyric acid B (GABA_B)-, anti-α-amino-3-hydroxy-5-methyl-4-isoxazolepropinoic receptor- (AMPAR), anti-GABA_A-, anti-dipeptidyl-peptidase-like protein-6 (DPPX) Ab-mediated encephalitides, delineate rarer subtypes and summarise findings to date regarding seronegative autoimmune encephalitis.

Pharmacogenetics of Drug-Resistant Epilepsy (Review of Literature)

Pharmacogenomic studies in epilepsy are justified by the high prevalence rate of this disease and the high cost of its treatment, frequent drug resistance, different response to the drug, the possibility of using reliable methods to assess the control of seizures and side effects of antiepileptic drugs. Candidate genes encode proteins involved in pharmacokinetic processes (drug transporters, metabolizing enzymes), pharmacodynamic processes (receptors, ion channels, enzymes, regulatory proteins, secondary messengers) and drug hypersensitivity (immune factors). This article provides an overview of the literature on the influence of genetic factors on treatment in epilepsy.

COR758, a negative allosteric modulator of GABAB receptors

Allosteric modulators of G protein coupled receptors (GPCRs), including GABA_BRs (GABA_BRs), are promising therapeutic candidates. While several positive allosteric modulators (PAM) of GABA_BRs have been characterized, only recently the first negative allosteric modulator (NAM) has been described. In the present study, we report the characterization of COR758, which acts as GABA_BR NAM in rat cortical membranes and CHO cells stably expressing GABA_BRs (CHO-GABA_B). COR758 failed to displace the antagonist [³H]CGP54626 from the orthosteric binding site of GABA_BRs showing that it acts through an allosteric binding site. Docking studies revealed a possible new allosteric binding site for COR758 in the intrahelical pocket of the GABA_B1 monomer. COR758 inhibited basal and GABA_BR-stimulated O-(3-[³⁵Sthio)-triphosphate ([³⁵S]GTPγS) binding in brain membranes and blocked the enhancement of GABA_BR-stimulated [³⁵S]GTPγS binding by the PAM GS39783. Bioluminescent resonance energy transfer (BRET) measurements in CHO-GABA_B cells showed that COR758 inhibited G protein activation by GABA and altered GABA_BR subunit rearrangements. Additionally, the compound altered GABA_BR-mediated signaling such as baclofen-induced inhibition of cAMP production in transfected HEK293 cells, agonist-induced Ca²⁺ mobilization as well as baclofen and the ago-PAM CGP7930 induced phosphorylation of extracellular signal-regulated kinases (ERK1/2) in CHO-GABA_B cells. COR758 also prevented baclofen-induced outward currents recorded from rat dopamine neurons, substantiating its property as a NAM for GABA_BRs. Altogether, these data indicate that COR758 inhibits G protein signaling by GABA_BRs, likely by interacting with an allosteric binding-site. Therefore, COR758 might serve as a scaffold to develop additional NAMs for therapeutic intervention.

The Role of Phospholipase C in GABAergic Inhibition and Its Relevance to Epilepsy

Epilepsy is characterized by recurrent seizures due to abnormal hyperexcitation of neurons. Recent studies have suggested that the imbalance of excitation and inhibition (E/I) in the central nervous system is closely implicated in the etiology of epilepsy. In the brain, GABA is a major inhibitory neurotransmitter and plays a pivotal role in maintaining E/I balance. As such, altered GABAergic inhibition can lead to severe E/I imbalance, consequently resulting in excessive and hypersynchronous neuronal activity as in epilepsy. Phospholipase C (PLC) is a key enzyme in the intracellular signaling pathway and regulates various neuronal functions including neuronal development, synaptic transmission, and plasticity in the brain. Accumulating evidence suggests that neuronal PLC is critically involved in multiple aspects of GABAergic functions. Therefore, a better understanding of mechanisms by which neuronal PLC regulates GABAergic inhibition is necessary for revealing an unrecognized linkage between PLC and epilepsy and developing more effective treatments for epilepsy. Here we review the function of PLC in GABAergic inhibition in the brain and discuss a pathophysiological relationship between PLC and epilepsy.

Possible interplay between the theories of pharmacoresistant epilepsy

Epilepsy is one of the oldest known neurological disorders and is characterized by recurrent seizure activity. It has a high incidence rate, affecting a broad demographic in both developed and developing countries. Comorbid conditions are frequent in patients with epilepsy and have detrimental effects on their quality of life. Current management options for epilepsy include the use of anti-epileptic drugs, surgery, or a ketogenic diet. However, more than 30% of patients diagnosed with epilepsy exhibit drug resistance to anti-epileptic drugs. Further, surgery and ketogenic diets do little to alleviate the symptoms of patients with pharmacoresistant epilepsy. Thus, there is an urgent need to understand the underlying mechanisms of pharmacoresistant epilepsy to design newer and more effective anti-epileptic drugs. Several theories of pharmacoresistant epilepsy have been suggested over the years, the most common being the gene variant hypothesis, network hypothesis, multidrug transporter hypothesis, and target hypothesis. In our review, we discuss the main theories of pharmacoresistant epilepsy and highlight a possible interconnection between their mechanisms that could lead to the development of novel therapies for pharmacoresistant epilepsy.

Genetic Landscape of Common Epilepsies: Advancing towards Precision in Treatment

Epilepsy, a neurological disease characterized by recurrent seizures, is highly heterogeneous in nature. Based on the prevalence, epilepsy is classified into two types: common and rare epilepsies. Common epilepsies affecting nearly 95% people with epilepsy, comprise generalized epilepsy which encompass idiopathic generalized epilepsy like childhood absence epilepsy, juvenile myoclonic epilepsy, juvenile absence epilepsy and epilepsy with generalized tonic-clonic seizure on awakening and focal epilepsy like temporal lobe epilepsy and cryptogenic focal epilepsy. In 70% of the epilepsy cases, genetic factors are responsible either as single genetic variant in rare epilepsies or multiple genetic variants acting along with different environmental factors as in common epilepsies. Genetic testing and precision treatment have been developed for a few rare epilepsies and is lacking for common epilepsies due to their complex nature of inheritance. Precision medicine for common epilepsies require a panoramic approach that incorporates polygenic background and other non-genetic factors like microbiome, diet, age at disease onset, optimal time for treatment and other lifestyle factors which influence seizure threshold. This review aims to comprehensively present a state-of-art review of all the genes and their genetic variants that are associated with all common epilepsy subtypes. It also encompasses the basis of these genes in the epileptogenesis. Here, we discussed the current status of the common epilepsy genetics and address the clinical application so far on evidence-based markers in prognosis, diagnosis, and treatment management. In addition, we assessed the diagnostic predictability of a few genetic markers used for disease risk prediction in individuals. A combination of deeper endo-phenotyping including pharmaco-response data, electro-clinical imaging, and other clinical measurements along with genetics may be used to diagnose common epilepsies and this marks a step ahead in precision medicine in common epilepsies management.

Relationship between GABRB2 gene polymorphisms and schizophrenia susceptibility: a case-control study and in silico analyses

PURPOSE

Converging evidence has recently established the significance of γ-aminobutyric acid neurotransmitter (GABA) system in the development of schizophrenia (SCZ). We aimed to determine the association of two markers of the GABA_A receptor β₂ subunit gene (GABRB2), rs12187676 G/C and rs1816072 T/C, with the risk of SCZ in Iranian population.

MATERIALS AND METHODS

In this case-control study, 190 patients with SCZ and 200 healthy controls were recruited from December 2018 to February 2020. Genotyping was done using the Tetra-ARMS-PCR technique. In silico analyses were performed to determine the potential effects of the variants.

RESULTS

The C allele and genotypes of codominant CC vs.TT and CT vs.TT, dominant TT vs. TC + CC, recessive TT + TC vs. CC of rs1816072 polymorphism, as well as codominant CC vs. GG and recessive GG + GC vs. CC genetic models of rs12187676 polymorphism were significantly associated with SCZ susceptibility. Compared to the TC/GC model, we have found that the TC/CC combination significantly increased the risk of SCZ by 4.32 fold while the TT/GG combination conferred a protective role against SCZ. Haplotypes analysis indicated that GABRB2 polymorphisms are in weak linkage disequilibrium with each other (LD = 0.1). However, bioinformatics analyses predicted that these polymorphisms do not have significant effects on the secondary structure and the splicing of GABRB2-mRNA.

CONCLUSIONS

We found that intronic GABRB2 polymorphisms were associated with SCZ risk in a sample of the Iranian population. These findings provided proof of concept for the involvement of the GABAergic neurotransmission system in SCZ development. These observations should be validated across other ethnicities and clinical subtypes.

Late drug-resistance in mild MTLE: Can it be influenced by preexisting white matter alterations?

OBJECTIVE

To identify early structural alterations preceding the development of drug-resistance in mild mesial temporal lobe epilepsy (mMTLE), a drug-responsive syndrome ideal for investigating epilepsy pathophysiology and potential prognostic markers of long-term clinical outcome, using magnetic resonance imaging (MRI) at baseline and after 12-year follow-up.

METHODS

Since 2002, a total of 55 participants with a baseline diagnosis of mMTLE underwent three-dimensional (3D) T1 1.5T MRI. Based on long-term outcome (follow-up 12 ± 3 years), we identified 39 patients with stable mMTLE (smMTLE) and 16 patients who had developed drug-resistance overtime (refractory MTLE [rMTLE]). At follow-up, 21 smMTLE and 13 rMTLE patients underwent 3T-MRI including diffusion-weighted scans. Structural images were processed using longitudinal voxel-based morphometry and standard Freesurfer analysis. Statistical analyses were carried out accounting for age, age at onset, gender, hippocampal volume, and hippocampal sclerosis (Hs).

RESULTS

Patients presented similar demographic, clinical, and Hs features. White matter volume of the arcuate fasciculi, corticospinal tracts, left retrosplenial cingulum, and left inferior longitudinal fasciculus was reduced only in rMTLE patients before the development of drug-resistance. At follow-up, rMTLE showed decreased fractional anisotropy in the corpus callosum, superior longitudinal fasciculi, and major bundles of the right hemisphere.

SIGNIFICANCE

White matter temporal and extratemporal abnormalities are preexisting in patients with mild MTLE who will develop drug-resistance, independently from the presence of Hs. Thus, these changes might be due to an inherited genetic alteration rather than a subordinate worsening after repeated seizures, multiple antiepileptic drugs, or initial precipitating factors.

Genetic variations associated with pharmacoresistant epilepsy (Review)

Epilepsy is a common, serious neurological disorder worldwide. Although this disease can be successfully treated in most cases, not all patients respond favorably to medical treatments, which can lead to pharmacoresistant epilepsy. Drug-resistant epilepsy can be caused by a number of mechanisms that may involve environmental and genetic factors, as well as disease- and drug-related factors. In recent years, numerous studies have demonstrated that genetic variation is involved in the drug resistance of epilepsy, especially genetic variations found in drug resistance-related genes, including the voltage-dependent sodium and potassium channels genes, and the metabolizer of endogenous and xenobiotic substances genes. The present review aimed to highlight the genetic variants that are involved in the regulation of drug resistance in epilepsy; a comprehensive understanding of the role of genetic variation in drug resistance will help us develop improved strategies to regulate drug resistance efficiently and determine the pathophysiological processes that underlie this common human neurological disease.

Network and Pathway-Based Analysis of Single-Nucleotide Polymorphism of miRNA in Temporal Lobe Epilepsy

Temporal lobe epilepsy (TLE) is a complex disease with its pathogenetic mechanism still unclear. Single-nucleotide polymorphisms (SNPs) of miRNA (miRSNPs) are SNPs located on miRNA genes or target sites of miRNAs, which have been proved to be associated with neuropsychic disease development by interfering with miRNA-mediated regulatory function. In this study, we integrated TLE-related risk genes and risk pathways multi-dimensionally based on public data resources. Furthermore, we systematically screened candidate functional miRSNPs for TLE and constructed a TLE-associated pathway-based miRSNP switching network, which included 92 miRNAs that target 12 TLE risk pathways. Moreover, we dissected thoroughly the correlation between 5 risk genes of 4 risk pathways and TLE development. Additionally, the biological function of several candidate miRSNPs were validated by luciferase reporter assay. In silico approach facilitates to select potential "miRSNP-miRNA-risk gene-pathway" axis for experimental validation, which provided new insights into the mechanism of miRSNPs as potential genetic risk factors of TLE.

Validation of in vivo MRS measures of metabolite concentrations in the human brain

PURPOSE

In vivo magnetic resonance spectroscopy (MRS) is the only technique capable of non-invasively assessing metabolite concentrations in the brain. The lack of alternative methods makes validation of MRS measures challenging. The aim of this study is to assess the validity of MRS measures of human brain metabolite concentrations by comparing multiple MRS measures acquired using different MRS acquisition sequences.

METHODS

Single-voxel SPECIAL and MEGA-PRESS MR spectra were acquired from both the dorsolateral prefrontal cortex and primary motor cortices in 15 healthy subjects. The SPECIAL spectrum, as well as both the edit-off and difference spectra of MEGA-PRESS were each analyzed in LCModel to obtain estimates of the absolute concentrations of total choline (TCh; glycerophosphocholine + phosphocholine), total creatine (TCr; creatine + phosphocreatine), N-acetylaspartate (NAA), N-acetylaspartylglutamate (NAAG), NAA + NAAG, glutamate (Glu), glutamine (Gln), Glu + Gln, scyllo-inositol (Scyllo), myo-inositol (Ins), glutathione (GSH), γ-aminobutyric acid (GABA), lactate (Lac) and aspartate (Asp). Then, having obtained up to three independent measures of each metabolite per brain region per subject, correlations between the different measures were assessed.

RESULTS

The degree of correlation between measures varied greatly across both the metabolites and sequences tested. As expected, metabolites with the most prominent spectral peaks (TCh, TCr, NAA + NAAG, Ins and Glu) had the most well-correlated measures between methods, while metabolites with less prominent spectral peaks (Lac, Gln, GABA, Asp, and NAAG) tended to have poorly-correlated measures between methods. Some metabolites with relatively less prominent spectral peaks (GSH, Scyllo) had fairly well-correlated measures between some methods. Combining metabolites improved the agreement between methods for measures of NAA + NAAG, but not for Glu + Gln.

CONCLUSIONS

Given that the ground truth for in vivo MRS measures is never known, the method proposed here provides a promising means to assess the validity of in vivo MRS measures, which has not yet been explored widely.

Isolated seizures are a common early feature of paraneoplastic anti-GABAB receptor encephalitis

OBJECTIVE

To report the clinical features and long-term outcome of 22 newly diagnosed paraneoplastic patients with GABA_B receptor antibodies (GABA_BR-Abs).

METHODS

Retrospective clinical study of CSF-confirmed cases of GABA_BR-Abs encephalitis.

RESULTS

We identified 22 patients (4 female) with GABA_BR-Abs, with a median age of 64 years (range 55-85). All were paraneoplastic: 20 small-cell lung cancer, one malignant thymoma, and one uncharacterized lung mass. The most frequent first symptom was the isolated recurrent seizures without cognitive inter-ictal impairment in 17 patients (77%). In the other, three presented the first behavioral disorders and two presented de novo status epilepticus (SE). After a median delay of 10 days (range 1-30), the recurrent seizures' phase was followed by an encephalitic phase characterized by confusion in 100% of cases and SE in 81% (n = 17), with 53% (n = 9) non-convulsive SE. Dysautonomic episodes were frequent (36%, n = 8, bradycardia and central apnea) and killed three patients. CSF study was abnormal in 95% of the cases (n = 21). At the encephalitic phase, MRI showed a temporal FLAIR hypersignal in 73% (n = 16) of the cases. First-line immunotherapy was initiated after a median delay of 26 days (range 6-65) from disease onset, and a partial response was observed in 10 out of 20 patients (50%). There was no complete response. Two years after onset, a massive anterograde amnesia affected all still alive patients. Nine patients died from cancer progression (median survival: 1.2 years).

CONCLUSION

Paraneoplastic GABA_BR-Abs encephalitis is characterized by a stereotype presentation with an epilepsy phase before an encephalitic phase with dysautonomia. The functional prognosis is poor.

A Caenorhabditis elegans assay of seizure-like activity optimised for identifying antiepileptic drugs and their mechanisms of action

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Worms with mutant GABA_A receptors exhibit convulsions upon exposure to pentylenetetrazol.

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Convulsions are prevented by the approved anti-epileptic drug, ethosuximide.

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C. elegans model is a higher throughput, ethical alternative to rodent seizure models.

Background

Epilepsy affects around 1% of people, but existing antiepileptic drugs (AEDs) only offer symptomatic relief and are ineffective in approximately 30% of patients. Hence, new AEDs are sorely needed. However, a major bottleneck is the low-throughput nature of early-stage AED screens in conventional rodent models. This process could potentially be expedited by using simpler invertebrate systems, such as the nematode Caenorhabditis elegans.

New method

Head-bobbing convulsions were previously reported to be inducible by pentylenetetrazol (PTZ) in C. elegans with loss-of-function mutations in unc-49, which encodes a GABA_A receptor. Given that epilepsy-linked mutations in human GABA_A receptors are well documented, this could represent a clinically-relevant system for early-stage AED screens. However, the original agar plate-based assay is unsuited to large-scale screening and has not been validated for identifying AEDs. Therefore, we established an alternative streamlined, higher-throughput approach whereby mutants were treated with PTZ and AEDs via liquid-based incubation.

Results

Convulsions induced within minutes of PTZ exposure in unc-49 mutants were strongly inhibited by the established AED ethosuximide. This protective activity was independent of ethosuximide’s suggested target, the T-type calcium channel, as a null mutation in the worm cca-1 ortholog did not affect ethosuximide’s anticonvulsant action.

Comparison with existing method

Our streamlined assay is AED-validated, feasible for higher throughput compound screens, and can facilitate insights into AED mechanisms of action.

Conclusions

Based on an epilepsy-associated genetic background, this C. elegans unc-49 model of seizure-like activity presents an ethical, higher throughput alternative to conventional rodent seizure models for initial AED screens.

Diversity of structure and function of GABAB receptors: a complexity of GABAB-mediated signaling

γ-aminobutyric acid type B (GABA_B) receptors are broadly expressed in the nervous system and play an important role in neuronal excitability. GABA_B receptors are G protein-coupled receptors that mediate slow and prolonged inhibitory action, via activation of Gαi/o-type proteins. GABA_B receptors mediate their inhibitory action through activating inwardly rectifying K⁺ channels, inactivating voltage-gated Ca²⁺ channels, and inhibiting adenylate cyclase. Functional GABA_B receptors are obligate heterodimers formed by the co-assembly of R1 and R2 subunits. It is well established that GABA_B receptors interact not only with G proteins and effectors but also with various proteins. This review summarizes the structure, subunit isoforms, and function of GABA_B receptors, and discusses the complexity of GABA_B receptors, including how receptors are localized in specific subcellular compartments, the mechanism regulating cell surface expression and mobility of the receptors, and the diversity of receptor signaling through receptor crosstalk and interacting proteins.

Advances in the development of biomarkers for epilepsy

Over 50 million people worldwide have epilepsy. In nearly 30% of these cases, epilepsy remains unsatisfactorily controlled despite the availability of over 20 antiepileptic drugs. Moreover, no treatments exist to prevent the development of epilepsy in those at risk, despite an increasing understanding of the underlying molecular and cellular pathways. One of the major factors that have impeded rapid progress in these areas is the complex and multifactorial nature of epilepsy, and its heterogeneity. Therefore, the vision of developing targeted treatments for epilepsy relies upon the development of biomarkers that allow individually tailored treatment. Biomarkers for epilepsy typically fall into two broad categories: diagnostic biomarkers, which provide information on the clinical status of, and potentially the sensitivity to, specific treatments, and prognostic biomarkers, which allow prediction of future clinical features, such as the speed of progression, severity of epilepsy, development of comorbidities, or prediction of remission or cure. Prognostic biomarkers are of particular importance because they could be used to identify which patients will develop epilepsy and which might benefit from preventive treatments. Biomarker research faces several challenges; however, biomarkers could substantially improve the management of people with epilepsy and could lead to prevention in the right person at the right time, rather than just symptomatic treatment.

Epilepsy and intellectual disability linked protein Shrm4 interaction with GABABRs shapes inhibitory neurotransmission

Shrm4, a protein expressed only in polarized tissues, is encoded by the KIAA1202 gene, whose mutations have been linked to epilepsy and intellectual disability. However, a physiological role for Shrm4 in the brain is yet to be established. Here, we report that Shrm4 is localized to synapses where it regulates dendritic spine morphology and interacts with the C terminus of GABA_B receptors (GABA_BRs) to control their cell surface expression and intracellular trafficking via a dynein-dependent mechanism. Knockdown of Shrm4 in rat severely impairs GABA_BR activity causing increased anxiety-like behaviour and susceptibility to seizures. Moreover, Shrm4 influences hippocampal excitability by modulating tonic inhibition in dentate gyrus granule cells, in a process involving crosstalk between GABA_BRs and extrasynaptic δ-subunit-containing GABA_ARs. Our data highlights a role for Shrm4 in synaptogenesis and in maintaining GABA_BR-mediated inhibition, perturbation of which may be responsible for the involvement of Shrm4 in cognitive disorders and epilepsy.

Mutations in the gene encoding Shrm4 are associated with epilepsy and intellectual disability. The authors show that Shrm4 interacts with GABA_B receptors and regulates tonic inhibition in the hippocampus, and knockdown of Shrm4 in rats leads to anxiety-like behaviour and seizures.

Long-term outcome of mild mesial temporal lobe epilepsy: A prospective longitudinal cohort study

OBJECTIVE

To identify clinical and imaging features at presentation that might predict long-term outcome in patients with mild mesial temporal lobe epilepsy (mMTLE), which is defined by at least 24 seizure-free months with or without antiepileptic medication.

METHODS

In the setting of a prospective, population-based, longitudinal cohort study, we followed up 101 patients, all with mMTLE at enrolment. By protocol, patients underwent clinical evaluation every 3-12 months. Independent t test, Mann-Whitney test, or χ(2) test was used for comparing 2 groups. The incidence rate of refractory MTLE (rMTLE) was expressed as number of cases every 100 person-years.

RESULTS

After a mean follow-up of 12.2 ± 3.7 years, 16 patients dropped out and 85/101 (mean age 46.5 ± 13.3 years) were available for the present analysis. Of these, 64/85 (75%) patients remained seizure-free and 21/85 (25%) became refractory (rMTLE), the latter corresponding to 2.0 cases per 100 persons per year. Patients with rMTLE showed a longer duration of epilepsy (p < 0.001), earlier age at epilepsy onset (p = 0.006), more frequent febrile convulsions (p = 0.02), and hippocampal sclerosis (HS) at MRI (p = 0.004) as compared to those with mMTLE.

CONCLUSIONS

mMTLE is a syndrome representing the mildest form of the wide spectrum of MTLE. Earlier age at onset, history of febrile convulsions, longer duration of epilepsy, and the presence of HS on MRI predict a worse outcome.

Gatekeepers Controlling GPCR Export and Function

Regulated export of G protein-coupled receptors (GPCRs) from intracellular stores involves chaperones and escort proteins, which promote their progression to the cell surface, and gatekeepers, which retain them in intracellular compartments. Functional γ-aminobutyric acid (GABA)B receptors, the paradigm of this phenomenon, comprise GB1 and GB2 subunits forming a heterodimer. GB1 is retained in the endoplasmic reticulum (ER) in the absence of GB2. A specific ER-resident gatekeeper, prenylated Rab acceptor family 2 (PRAF2), is involved in GB1 retention and prevents its progression into the biosynthetic pathway. GB1 can be released from PRAF2 only on competitive interaction with GB2. PRAF2 is ubiquitous and belongs to a subgroup of the mammalian Ypt-interacting protein (Yip) family. Several other GPCRs are likely to be regulated by Yip proteins, which might be involved in the pathophysiology of human diseases that are associated with impaired receptor targeting to the cell surface.

Developmental and degenerative modulation of GABAergic transmission in the mouse hippocampus

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter involved in synaptic plasticity. GABAergic transmission is also implicated in developmental and degenerative processes in the brain. The goal of the present study was to understand the developmental and degenerative regulation of GABAergic transmission in the mouse hippocampus by examining changes in GABA receptor subunit mRNA levels and GABA-related protein expression during postnatal development of the hippocampus and trimethyltin (TMT)-induced neurodegeneration in the juvenile (postnatal day [PD] 24) and adult hippocampus (PD 56). During postnatal development, the mRNA levels of GABA A receptor (GABAAR) subunits, including α1, α4, β1, β2, and δ; GABA B receptor (GABABR) subunit 2; and the expression of GABA-related proteins, including glutamic acid decarboxylase, vesicular GABA transporter (VGAT), and potassium chloride cotransporter 2 increased gradually in the mouse hippocampus. The results of seizure scoring and histopathological findings in the hippocampus revealed a more pronounced response to the same administered TMT dose in juvenile mice, compared with that in adult mice. The mRNA levels of most GABA receptor subunits in the juvenile hippocampus, excluding GABAAR subunit β3, were dynamically altered after TMT treatment. The mRNA levels of GABAAR subunits γ2 and δ decreased significantly in the adult hippocampus following TMT treatment, whereas the level of GABABR subunit 1 mRNA increased significantly. Among the GABA-related proteins, only VGAT decreased significantly in the juvenile and adult mouse hippocampus after TMT treatment. In conclusion, regulation of GABAergic signaling in the mouse hippocampus may be related to maturation of the central nervous system and the degree of neurodegeneration during postnatal development and TMT-induced neurodegeneration in the experimental animals.

Potential Similarities in Temporal Lobe Epilepsy and Alzheimer’s Disease: From Clinic to Pathology

Alzheimer’s disease (AD) is clinically characterized by insidious onset of memory and cognitive impairments, which are also presented in patients with temporal lobe epilepsy (TLE). Many studies have shown that seizures occur in some patients with AD, and AD is a risk factor for epilepsy, mainly complex partial and secondary generalized seizure. Here, we focus on the relationship between TLE and AD in clinical and pathological aspects, as they are having similar comorbidities and mechanisms. In this study, we first reviewed the clinical observations that showed concomitant AD and TLE. Then, we picked up common genetic and pathological changes in both the diseases from neurobiological researches. Although both the diseases have delicate differences in many aspects, their common characteristics intrigue more detailed research to be done by newer technology.

Anticonvulsant effects of structurally diverse GABA(B) positive allosteric modulators in the DBA/2J audiogenic seizure test: Comparison to baclofen and utility as a pharmacodynamic screening model

The GABA(B) receptor has been indicated as a promising target for multiple CNS-related disorders. Baclofen, a prototypical orthosteric agonist, is used clinically for the treatment of spastic movement disorders, but is associated with unwanted side-effects, such as sedation and motor impairment. Positive allosteric modulators (PAM), which bind to a topographically-distinct site apart from the orthosteric binding pocket, may provide an improved side-effect profile while maintaining baclofen-like efficacy. GABA, the major inhibitory neurotransmitter in the CNS, plays an important role in the etiology and treatment of seizure disorders. Baclofen is known to produce anticonvulsant effects in the DBA/2J mouse audiogenic seizure test (AGS), suggesting it may be a suitable assay for assessing pharmacodynamic effects. Little is known about the effects of GABA(B) PAMs, however. The studies presented here sought to investigate the AGS test as a pharmacodynamic (PD) screening model for GABA(B) PAMs by comparing the profile of structurally diverse PAMs to baclofen. GS39783, rac-BHFF, CMPPE, A-1295120 (N-(3-(4-(4-chloro-3-fluorobenzyl)-6-methoxy-3,5-dioxo-4,5-dihydro-1,2,4-triazin-2(3H)-yl)phenyl)acetamide), and A-1474713 (N-(3-(4-(4-chlorobenzyl)-3,5-dioxo-4,5-dihydro-1,2,4-triazin-2(3H)-yl)phenyl)acetamide) all produced robust, dose-dependent anticonvulsant effects; a similar profile was observed with baclofen. Pre-treatment with the GABA(B) antagonist SCH50911 completely blocked the anticonvulsant effects of baclofen and CMPPE in the AGS test, indicating such effects are likely mediated by the GABA(B) receptor. In addition to the standard anticonvulsant endpoint of the AGS test, video tracking software was employed to assess potential drug-induced motor side-effects during the acclimation period of the test. This analysis was sensitive to detecting drug-induced changes in total distance traveled, which was used to establish a therapeutic index (TI = hypoactivity/anticonvulsant effects). Calculated TIs for A-1295120, CMPPE, rac-BHFF, GS39783, and A-1474713 were 5.31x, 5.00x, 4.74x, 3.41x, and 1.83x, respectively, whereas baclofen was <1. The results presented here suggest the DBA/2J mouse AGS test is a potentially useful screening model for detecting PD effects of GABA(B) PAMs and can provide an initial read-out on target-related motor side-effects. Furthermore, an improved TI was observed for PAMs compared to baclofen, indicating the PAM approach may be a viable therapeutic alternative to baclofen.

Exome sequencing identifies SUCO mutations in mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (mTLE) is the main type and most common medically intractable form of epilepsy. Severity of disease-based stratified samples may help identify new disease-associated mutant genes. We analyzed mRNA expression profiles from patient hippocampal tissue. Three of the seven patients had severe mTLE with generalized-onset convulsions and consciousness loss that occurred over many years. We found that compared with other groups, patients with severe mTLE were classified into a distinct group. Whole-exome sequencing and Sanger sequencing validation in all seven patients identified three novel SUN domain-containing ossification factor (SUCO) mutations in severely affected patients. Furthermore, SUCO knock down significantly reduced dendritic length in vitro. Our results indicate that mTLE defects may affect neuronal development, and suggest that neurons have abnormal development due to lack of SUCO, which may be a generalized-onset epilepsy-related gene.

Regulating hippocampal hyperexcitability through GABAB Receptors

Disturbances of GABAergic inhibition are a major cause of epileptic seizures. GABA exerts its actions via ionotropic GABAA receptors and metabotropic G protein‐coupled GABAB receptors. Malfunction of GABAA inhibition has long been recognized in seizure genesis but the role of GABAB receptors in controlling seizure activity is still not well understood. Here, we examined the anticonvulsive, or inhibitory effects, of GABAB receptors in a mouse model of hippocampal kindling as well as mouse hippocampal slices through the use of GS 39783, a positive allosteric GABAB receptor modulator, and CGP 55845, a selective GABAB receptor antagonist. When administered via intraperitoneal injections in kindled mice, GS 39783 (5 mg/kg) did not attenuate hippocampal EEG discharges, but did reduce aberrant hippocampal spikes, whereas CGP 55845 (10 mg/kg) prolonged hippocampal discharges and increased spike incidences. When examined in hippocampal slices, neither GS 39783 at 5 μmol/L nor the GABAB receptor agonist baclofen at 0.1 μmol/L alone significantly altered repetitive excitatory field potentials, but GS 39783 and baclofen together reversibly abolished these field potentials. In contrast, CGP 55845 at 1 μmol/L facilitated induction and incidence of these field potentials. In addition, CGP 55845 attenuated the paired pulse depression of CA3 population spikes and increased the frequency of EPSCs in individual CA3 pyramidal neurons. Collectively, these data suggest that GABABB receptors regulate hippocampal hyperexcitability by inhibiting CA3 glutamatergic synapses. We postulate that positive allosteric modulation of GABAB receptors may be effective in reducing seizure‐related hyperexcitability.

GABAB positive modulator GS 39783 attenuated, whereas GABAB antagonist CGP55845 facilitated hippocampal EEG spikes in kindled mice and excitatory field potentials in hippocampal slices. We postulate that GABAB receptors may inhibit CA3 glutamate synapses and hence regulate hippocampal hyperexcitability.

Molecular and cellular mechanisms underlying anti-neuronal antibody mediated disorders of the central nervous system

Over the last decade multiple autoantigens located on the plasma membrane of neurons have been identified. Neuronal surface antigens include molecules directly involved in neurotransmission and excitability. Binding of the antibody to the antigen may directly alter the target protein's function, resulting in neurological disorders. The often striking reversibility of symptoms following early aggressive immunotherapy supports a pathogenic role for autoantibodies to neuronal surface antigens. In order to better understand and treat these neurologic disorders it is important to gain insight in the underlying mechanisms of antibody pathogenicity. In this review we discuss the clinical, circumstantial, in vitro and in vivo evidence for neuronal surface antibody pathogenicity and the possible underlying cellular and molecular mechanisms. This review shows that antibodies to neuronal surface antigens are often directed at conformational epitopes located in the extracellular domain of the antigen. The conformation of the epitope can be affected by specific posttranslational modifications. This may explain the distinct clinical phenotypes that are seen in patients with antibodies to antigens that are expressed throughout the brain. Furthermore, it is likely that there is a heterogeneous antibody population, consisting of different IgG subtypes and directed at multiple epitopes located in an immunogenic region. Binding of these antibodies may result in different pathophysiological mechanisms occurring in the same patient, together contributing to the clinical syndrome. Unraveling the predominant mechanism in each distinct antigen could provide clues for therapeutic interventions.

The search for circulating epilepsy biomarkers

Few would experience greater benefit from the development of biomarkers than those who suffer from epilepsy. Both the timing of individual seizures and the overall course of the disease are highly unpredictable, and the associated morbidity is considerable. Thus, there is an urgent need to develop biomarkers that can predict the progression of epilepsy and treatment response. Doing so may also shed light on the mechanisms of epileptogenesis and pharmacoresistance, which remain elusive despite decades of study. However, recent advances suggest the possible identification of circulating epilepsy biomarkers – accessible in blood, cerebrospinal fluid or urine. In this review, we focus on advances in several areas: neuroimmunology and inflammation; neurological viral infection; exemplary pediatric syndromes; and the genetics of pharmacoresistance, as relevant to epilepsy. These are fertile areas of study with great potential to yield accessible epilepsy biomarkers.

Emerging neurotrophic role of GABAB receptors in neuronal circuit development

The proper development of highly organized structures in the central nervous system is a complex process during which key events – neurogenesis, migration, growth, differentiation, and synaptogenesis – have to take place in an appropriate manner to create functional neuronal networks. It is now well established that GABA, the main inhibitory neurotransmitter in the adult mammalian brain, plays more than a classical inhibitory role and can function as an important developmental signal early in life. GABA binds to chloride-permeable ionotropic GABA_A receptors and to G-protein-coupled GABA_B receptors (GABA_B-Rs). Although most of the trophic actions of GABA have been attributed to the activation of GABA_A receptors, recent advances show that GABA_B-Rs also regulate fundamental steps of network development. This review summarizes some of the recent progress about the neurotrophic role of GABA_B-Rs to neuronal development.

Therapeutic potential of GABA(B) receptor ligands in drug addiction, anxiety, depression and other CNS disorders

Glutamate and γ-aminobutyric acid (GABA) are the major excitatory and inhibitory neurotransmitter systems, respectively in the central nervous system (CNS). Dysregulation, in any of these or both, has been implicated in various CNS disorders. GABA acts via ionotropic (GABA(A) and GABA(C) receptor) and metabotropic (GABA(B)) receptor. Dysregulation of GABAergic signaling and alteration in GABA(B) receptor expression has been implicated in various CNS disorders. Clinically, baclofen-a GABA(B) receptor agonist is available for the treatment of spasticity, dystonia etc., associated with various neurological disorders. Moreover, GABAB receptor ligands has also been suggested to be beneficial in various neuropsychiatric and neurodegenerative disorders. The present review is aimed to discuss the role of GABA(B) receptors and the possible outcomes of GABA(B) receptor modulation in CNS disorders.

Genetics of temporal lobe epilepsy

The most common partial epilepsy, temporal lobe epilepsy (TLE) consists of a heterogeneous group of seizure disorders originating in the temporal lobe. TLE had been thought to develop as a result of acquired structural problems in the temporal lobe. During the past two decades, there has been growing evidence of the important influence of genetic factors, and familial and non-lesional TLE have been increasingly described. Here, we focus on the genetics of TLE and review related genes which have been studied recently. Although its molecular mechanisms are still poorly understood, TLE genetics is a fertile field, awaiting more research.

A functional genetic variation of the 5-HTR2A receptor affects age at onset in patients with temporal lobe epilepsy

The 1354C>T polymorphism of the 5-hydroxytryptamine receptor 2A gene (5-HTR2A) was implicated in human memory performance. We investigated the relationship between this polymorphism and cognitive function in patients with temporal lobe epilepsy (TLE). We also evaluated if this polymorphism could influence the phenotype. There were 138 patients with TLE: 25% (34/138) of them found to be cognitively impaired, while the remaining 104 of 138 (75%) were found to be cognitively preserved after a comprehensive neuropsychological evaluation. dHPLC followed by DNA sequencing was used to detect the genetic variation. The distribution of 1354C>T did not differ between these two TLE groups, both in the comparison of genotype distribution (P= 0.177) and allele frequencies (P = 0.065). Nonetheless, patients with the T allele had a significantly earlier age at onset of the disease (P= 0.006). This effect was even stronger in patients with impaired memory (P= 0.00015). A second independent sample of 86 individuals with TLE satisfactorily confirmed the relationship between T allele and age at epilepsy onset. The results of this study have demonstrated that the T variant of 5-HTR2A may influence an earlier age of onset of TLE, especially in those with impaired memory. Nonetheless, this polymorphism has no major impact on memory functions in such TLE patients.

Loss of dendritic inhibition in the hippocampus after repeated early-life hyperthermic seizures in rats

Seizures are relatively common in children and are a risk factor for subsequent temporal lobe epilepsy. To investigate whether early-life seizures themselves are detrimental to the proper function of the adult brain, we studied whether dendritic excitation and inhibition in the hippocampus of adult rats were altered after hyperthermia-induced seizures in immature rats. In particular, we hypothesized that apical dendritic inhibition in hippocampal CA1 pyramidal cells would be disrupted following hyperthermia-induced seizures in early life. Seizure rats were given three hyperthermia-induced seizures per day for three days from postnatal day (PND) 13 to 15; control rats were handled similarly but not heated. At PND 65-75, paired-pulse inhibition in area CA1 was evaluated under urethane anesthesia, using CA3 and medial perforant path (MPP) stimulation to excite the proximal and distal apical-dendrites, respectively, and the evoked field potentials were analyzed by current source density. There was no difference in the CA1 response to single-pulse stimulation of CA3 or MPP. In control rats, a high-intensity CA3 stimulus inhibited a subsequent MPP-evoked CA1 distal dendritic excitatory sink, and the inhibition at 150-200 ms was blocked by a GABA(B) receptor antagonist. Seizure as compared to control rats showed a decrease in a CA3-evoked inhibition of the CA1 distal dendritic excitation, 30-400 ms after the CA3 stimulus. In addition, seizure as compared to control rats showed a reduced early (20-80 ms) inhibition of a CA1 mid-apical dendritic sink following paired-pulse CA3 stimulation. In conclusion, long-term alterations in dendritic inhibition in CA1 were found following early-life seizures.

Natural history of temporal lobe epilepsy: antecedents and progression

Temporal lobe epilepsy represents the largest group of patients with treatment resistant/medically intractable epilepsy undergoing epilepsy surgery. The underpinnings of common forms of TLE in many instances begin in early life with the occurrence of an initial precipitating event. The first epileptic seizure often occurs after a variable latency period following this event. The precise natural history and progression following the first seizure to the development of TLE, its subsequent resolution through spontaneous remission or the development of treatment resistant epilepsy remain poorly understood. Our present understanding of the role played by these initial events, the subsequent latency to development of temporal lobe epilepsy, and the emergence of treatment resistance remains incomplete. A critical analysis of published data suggest that TLE is a heterogeneous condition, where the age of onset, presence or absence of a lesion on neuroimaging, the initial precipitating event, association with febrile seizures, febrile status epilepticus, and neurotropic viral infections influence the natural history and outcome. The pathways and processes through which these variables coalesce into a framework will provide the basis for an understanding of the natural history of TLE. The questions raised need to be addressed in future prospective and longitudinal observational studies.

Genetics of temporal lobe epilepsy: a review

Temporal lobe epilepsy (TLE) is usually regarded as a polygenic and complex disorder. To understand its genetic component, numerous linkage analyses of familial forms and association studies of cases versus controls have been conducted since the middle of the nineties. The present paper lists genetic findings for TLE from the initial segregation analysis to the most recent results published in May 2011. To date, no genes have been clearly related to TLE despite many efforts to do so. However, it is vital to continue replication studies and collaborative attempts to find significant results and thus determine which gene variant combination plays a definitive role in the aetiology of TLE.

GABBR1 gene polymorphism(G1465A)isassociated with temporal lobe epilepsy

PURPOSE

γ-Aminobutyric acid B receptor 1(GABBR1) gene G1465A polymorphism has been considered as a potential risk factor for the development of temporal lobe epilepsy (TLE). However, the results were inconsistent. In this study, we performed a meta-analysis to assess the association between GABBR1 G1465A polymorphism and the risk of TLE.

METHODS

Biomedical literature databases including PubMed, ISI web of science and Embase were searched. The studies evaluating the association between GABBR1 G1465A polymorphism and TLE were included. Pooled odds ratio (OR) and 95%CI confidence interval (CI) were calculated using fixed- or random-effects model.

KEY FINDINGS

Seven studies (1011 cases and 2184 controls) met the inclusion criteria and were included in the meta-analysis. The overall result showed that the association between GABBR1 G1465A polymorphism was statistically significant (OR=5.381, 95%CI: 1.726, 16.776, P=0.004). Subgroup analysis showed that the effect estimate was higher in the studies with high quality score (OR=14.220, 95%CI: 6.933, 29.169, P=0.000) than that in the studies with low quality score (OR=1.158, 95%CI: 0.325, 4.123, P=0.821).

SIGNIFICANCE

The present meta-analysis suggests that GABBR1 G1465A polymorphism is associated with the risk of TLE. The role of GABBR1 G1465A polymorphism in the development of TLE merits further investigation.

ASIC1a polymorphism is associated with temporal lobe epilepsy

Recent in vitro and in vivo data show that acid-sensing ion channel 1a (ASIC1a) activation enhances neuronal excitability in the hippocampus and neocortex, indicating that ASIC1a might play a role in the generation and maintenance of epileptic seizures. The aim of this study was to investigate association of the ASIC1a gene with temporal lobe epilepsy (TLE) for the first time. Six tag single-nucleotide polymorphisms (SNPs) of the ASIC1a gene were selected and genotyped using polymerase chain reaction-restriction fragment length polymorphism in 560 TLE patients and 401 healthy controls. There was a significant allelic and genotypic association between rs844347:A>C and TLE compared with controls. The rs844347-A allele frequency was 88.1% in the patients and 83.0% in control subjects (OR=1.516, 95% CI 1.142-2.013, p=0.004). Furthermore, the haplotype analysis revealed a significant association with TLE. The results of this study demonstrate for the first time an association between an ASC1a variant allele and TLE in a Han Chinese population.

A polymorphism in CALHM1 is associated with temporal lobe epilepsy

A recent study suggests that the P86L polymorphism (rs2986017) in the calcium homeostasis modulator 1 (CALHM1) gene interferes with calcium homeostasis and increases amyloid β (Aβ) levels. Moreover, in vitro and in vivo data show that both calcium homeostasis and high levels of Aβ play an important role in the induction and maintenance of epileptic seizures in hippocampus, indicating CALHM1 might play a potential role in pathophysiological pathways involved in temporal lobe epilepsy (TLE). The aim of this study was to investigate the genetic contribution of CALHM1 to TLE. Five single-nucleotide polymorphisms (SNPs) of CALHM1 were selected and genotyped using polymerase chain reaction restriction fragment length polymorphism in 560 patients with TLE and 401 healthy controls. We found a positive association between rs11191692 and TLE, but a negative result between rs2986017 and TLE. The rs11191692-A allele frequency was found in 32.4% of the patients and in 26.2% of control subjects (OR=1.35, 95% CI=1.10-1.65, uncorrected P=0.003, corrected P=0.015). Furthermore, the positive association between rs11191692 and TLE independent of apolipoprotein E ε4 was supported by five SNPs haplotype analysis. The results of this study provide the first evidence that the SNP rs11191692 in CALHM1 confers highly increased susceptibility to TLE.

Neocortical thinning in "benign" mesial temporal lobe epilepsy

PURPOSE

In refractory mesial temporal lobe epilepsy (MTLE) extrahippocampal and neocortical abnormalities have been described in patients with or without mesial temporal sclerosis (MTS). Recently we observed gray matter reductions in regions outside the hippocampus in benign MTLE with or without MTS. Cortical thickness has been proposed as a viable methodologic alternative for assessment of neuropathologic changes in extratemporal regions. Herein, we aimed to use this technique to describe cortical abnormalities in patients with benign TLE.

METHODS

Whole-brain cortical thickness analysis (FreeSurfer) was performed in 32 unrelated patients with benign TLE [16 patients with signs of MTS on magnetic resonance imaging (MRI), pMTLE; 16 without, nMTLE] and 44 healthy controls.

KEY FINDINGS

In the pMTLE group, the most significant thinning was found in the sensorimotor cortex bilaterally but was more extensive in the left hemisphere (false discovery rate, p < 0.05). Other areas were localized in the occipital cortex, left supramarginal gyrus, left superior parietal gyrus, left paracentral sulcus, left inferior/middle/superior frontal gyrus, left inferior frontal sulcus, right cingulate cortex, right superior frontal gyrus, right inferior parietal gyrus, right fusiform gyrus, and cuneus/precuneus. In the nMTLE, a similar neurodegenerative pattern was detected, although not surviving correction for multiple comparisons. Direct comparison between pMTLE and nMTLE did not reveal significant changes.

SIGNIFICANCE

Patients with either benign pMTLE or nMTLE showed comparable cortical thinning, mainly confined to the sensorimotor cortex. This finding that is not appreciated on routine MRI supports the hypothesis that similar to refractory MTLE, even in benign MTLE, pathology in neocortical regions maybe implicated in the pathophysiology of this syndrome.

Benign mesial temporal lobe epilepsy

Benign mesial temporal lobe epilepsy (bMTLE), which is defined as at least 24 months of seizure freedom with or without antiepileptic medication, has probably been under-recognized because of a literature bias toward refractory epilepsy cases. Seizure onset in bMTLE tends to be in adolescence or adulthood, and patients frequently have a family history of febrile seizures and epilepsy. Long-term seizure freedom is observed with or without antiepileptic medication. On brain MRI, nearly 40% of patients with long-standing bMTLE show evidence of hippocampal sclerosis, a feature usually associated with refractory temporal lobe epilepsy. Prospective studies are needed to determine the features that allow prediction of a benign course, and to clarify the significance of hippocampal MRI changes.

Differential effects of GABAB receptor subtypes, {gamma}-hydroxybutyric Acid, and Baclofen on EEG activity and sleep regulation

The role of GABA(B) receptors in sleep is still poorly understood. GHB (γ-hydroxybutyric acid) targets these receptors and is the only drug approved to treat the sleep disorder narcolepsy. GABA(B) receptors are obligate dimers comprised of the GABA(B2) subunit and either one of the two GABA(B1) subunit isoforms, GABA(B1a) and GABA(B1b). To better understand the role of GABA(B) receptors in sleep regulation, we performed electroencephalogram (EEG) recordings in mice devoid of functional GABA(B) receptors (1(-/-) and 2(-/-)) or lacking one of the subunit 1 isoforms (1a(-/-) and 1b(-/-)). The distribution of sleep over the day was profoundly altered in 1(-/-) and 2(-/-) mice, suggesting a role for GABA(B) receptors in the circadian organization of sleep. Several other sleep and EEG phenotypes pointed to a more prominent role for GABA(B1a) compared with the GABA(B1b) isoform. Moreover, we found that GABA(B1a) protects against the spontaneous seizure activity observed in 1(-/-) and 2(-/-) mice. We also evaluated the effects of the GHB-prodrug GBL (γ-butyrolactone) and of baclofen (BAC), a high-affinity GABA(B) receptor agonist. Both drugs induced a state distinct from physiological sleep that was not observed in 1(-/-) and 2(-/-) mice. Subsequent sleep was not affected by GBL whereas BAC was followed by a delayed hypersomnia even in 1(-/-) and 2(-/-) mice. The differential effects of GBL and BAC might be attributed to differences in GABA(B)-receptor affinity. These results also indicate that all GBL effects are mediated through GABA(B) receptors, although these receptors do not seem to be involved in mediating the BAC-induced hypersomnia.

Antibodies to the GABA(B) receptor in limbic encephalitis with seizures: case series and characterisation of the antigen

BACKGROUND

Some encephalitides or seizure disorders once thought idiopathic now seem to be immune mediated. We aimed to describe the clinical features of one such disorder and to identify the autoantigen involved.

METHODS

15 patients who were suspected to have paraneoplastic or immune-mediated limbic encephalitis were clinically assessed. Confocal microscopy, immunoprecipitation, and mass spectrometry were used to characterise the autoantigen. An assay of HEK293 cells transfected with rodent GABA(B1) or GABA(B2) receptor subunits was used as a serological test. 91 patients with encephalitis suspected to be paraneoplastic or immune mediated and 13 individuals with syndromes associated with antibodies to glutamic acid decarboxylase 65 were used as controls.

FINDINGS

All patients presented with early or prominent seizures; other symptoms, MRI, and electroencephalography findings were consistent with predominant limbic dysfunction. All patients had antibodies (mainly IgG1) against a neuronal cell-surface antigen; in three patients antibodies were detected only in CSF. Immunoprecipitation and mass spectrometry showed that the antibodies recognise the B1 subunit of the GABA(B) receptor, an inhibitory receptor that has been associated with seizures and memory dysfunction when disrupted. Confocal microscopy showed colocalisation of the antibody with GABA(B) receptors. Seven of 15 patients had tumours, five of which were small-cell lung cancer, and seven patients had non-neuronal autoantibodies. Although nine of ten patients who received immunotherapy and cancer treatment (when a tumour was found) showed neurological improvement, none of the four patients who were not similarly treated improved (p=0.005). Low levels of GABA(B1) receptor antibodies were identified in two of 104 controls (p<0.0001).

INTERPRETATION

GABA(B) receptor autoimmune encephalitis is a potentially treatable disorder characterised by seizures and, in some patients, associated with small-cell lung cancer and with other autoantibodies.

FUNDING

National Institutes of Health.