Arjune Sen

Characteristics of surround inhibition in cat area 17

Abstractu2002The effects of stimuli falling outside the ’classical receptive field’ and their influence on the orientation selectivity of cells in the cat primary visual cortex are still matters of debate. Here we examine the variety of effects of such peripheral stimuli on responses to stimuli limited to the receptive field. We first determined the extent of the classical receptive field by increasing the diameter of a circular patch of drifting grating until the response saturated or reached a maximum, and by decreasing the diameter of a circular mask in the middle of an extended grating, centred on the receptive field, until the cell just began to respond. These two estimates always agreed closely. We then presented an optimum grating of medium-to-high contrast filling the classical receptive field while stimulating the surround with a drifting grating that had the same parameters as the central stimulus but was varied in orientation. For all but five neurons (of 37 tested), surround stimulation produced clear suppression over some range of orientations, while none showed explicit facilitation under these conditions. For 11 cells (34% of those showing suppression), the magnitude of suppression did not vary consistently with the orientation of the surround stimulus. In the majority of cells, suppression was weakest for a surround grating oriented orthogonal to the cell’s optimum. Nine of these cells (28%) exhibited maximum inhibition at the optimum orientation for the receptive field itself, but for 12 cells (38%) there was apparent ’release’ from inhibition for surround gratings at or near the cell’s optimum orientation and direction, leaving inhibition either maximal at angles flanking the optimum (9 cells) or broadly distributed over the rest of the orientation range (3 cells). This implies the existence of a subliminal facilitatory mechanism, tightly tuned at or near the cell’s optimum orientation, extending outside the classical receptive field. For just two cells of 13 tested the preferred orientation for a central grating was clearly shifted towards the orientation of a surrounding grating tilted away from the cell’s optimum. The contrast gain for central stimulation at the optimal orientation was measured with and without a surround pattern. For nine of 25 cells tested, surround stimulation at the cell’s optimum orientation facilitated the response to a central grating of low contrast (≤0.1) but inhibited that to a higher-contrast central stimulus: the contrast-response gain is reduced but the threshold contrast is actually decreased by surround stimulation. Hence the receptive field is effectively larger for low-contrast than for high-contrast stimuli. Inhibition from the periphery is usually greatest at or around the cell’s optimum, while suppression within the receptive field has been shown to be largely non-selective for orientation. Inhibition by orientations flanking the optimum could serve to sharpen orientation selectivity in the presence of contextual stimuli and to enhance orientational contrast; and it may play a part in orientation contrast illusions.

Nova2 Interacts with a Cis-Acting Polymorphism to Influence the Proportions of Drug-Responsive Splice Variants of SCN1A

An intronic polymorphism in the SCN1A gene, which encodes a neuronal sodium-channel alpha subunit, has been previously associated with the dosing of two commonly used antiepileptic drugs that elicit their pharmacologic action primarily at this ion-channel subunit. This study sought to characterize the functional effects of this polymorphism on alternative splicing of SCN1A and to explore the potential for modulating the drug response in the pharmacologically unfavorable genotype by identification of a splice modifier acting on SCN1A. The effects of the genotype at the SCN1A IVS5N+5 G-->A polymorphism on SCN1A splice-variant proportions and the consequences of increased expression of splice modifiers were investigated both in human temporal neocortex tissue and in a cellular minigene expression system. Quantitative real-time polymerase chain reaction was used to quantify the amounts of SCN1A transcripts forms. We show that the polymorphism has a dramatic effect on the proportions of neonate and adult alternative transcripts of SCN1A in adult brain tissue and that the effect of the polymorphism also appears to be modified by Nova2 expression levels. A minigene expression system confirms both the effect of the polymorphism on transcript proportions and the role of Nova2 in the regulation of splicing, with higher Nova2 expression increasing the proportion of the neonate form. A larger Nova2-mediated effect was detected in the AA genotype that is associated with increased dose requirements. The effects of Nova2 on modulation of the alternative splicing of 17 other neuronally expressed genes were investigated, and no effect was observed. These findings emphasize the emerging role of genetic polymorphisms in modulation of drug effect and illustrate both alternative splicing as a potential therapeutic target and the importance of considering the activity of compounds at alternative splice forms of drug targets in screening programs.

Alternative ion channel splicing in mesial temporal lobe epilepsy and Alzheimer's disease

BackgroundAlternative gene transcript splicing permits a single gene to produce multiple proteins with varied functions. Bioinformatic investigations have identified numerous splice variants, but whether these transcripts are translated to functional proteins and the physiological significance of these alternative proteins are largely unknown. Through direct identification of splice variants associated with disease states, we can begin to address these questions and to elucidate their roles in disease predisposition and pathophysiology. This work specifically sought to identify disease-associated alternative splicing patterns in ion channel genes by comprehensively screening affected brain tissue collected from patients with mesial temporal lobe epilepsy and Alzheimers disease. New technology permitting the screening of alternative splice variants in microarray format was employed. Real time quantitative PCR was used to verify observed splice variant patterns.ResultsThis work shows for the first time that two common neurological conditions are associated with extensive changes in gene splicing, with 25% and 12% of the genes considered having significant changes in splicing patterns associated with mesial temporal lobe epilepsy and Alzheimers disease, respectively. Furthermore, these changes were found to exhibit unique and consistent patterns within the disease groups.ConclusionThis work has identified a set of disease-associated, alternatively spliced gene products that represent high priorities for detailed functional investigations into how these changes impact the pathophysiology of mesial temporal lobe epilepsy and Alzheimers disease.

Pathognomonic seizures in limbic encephalitis associated with anti-LGI1 antibodies

A 62-year-old retired supermarket worker developed a rash on her trunk after eating sausages on Good Friday, 2012. On the way to hospital, her right arm jerked involuntarily, striking her husband in the face. The rash resolved quickly with antihistamine, but over the next 13 days she had increasingly frequent stereotypical, involuntary movements of her left arm and face, lasting less than 1 s. During an event, the left side of her face would contort, her left arm would fl ex at the elbow, wrist, and metacarpophalangeal joints with fi nger extension (video), and she sometimes made brief guttural noises. She had no loss of awareness. By the time she presented to our hospital she was having these seizures up to eight times per hour. Her medical history was unremarkable and she took no regular medication. Systemic and neurological examinations, including cognition, were normal. Her serum sodium was normal. MR brain imaging showed small vessel disease, but no changes in limbic structures. Video-EEG captured multiple events associated with movement and muscle artifact, but no abnormality. The inter-ictal EEG was normal. Neuropsychological assessment showed some mild executive dysfunction, which is compatible with cerebral small vessel disease. The events were characteristic of faciobrachial dystonic seizures, and we made a clinical diagnosis of limbic encephalitis associated with antibodies to LGI1 (leucinerich glioma inactivated-1). We started treatment with intravenous methylprednisolone 1 g daily for 5 days, followed by oral tapered prednisolone for 6 weeks. There was an immediate reduction in frequency of faciobrachial dystonic seizures, and these had completely stopped by day three of treatment (appendix). 3 weeks after presentation, the result of testing for serum LGI1 antibodies returned strongly positive (319 pmol/m3; reference range <85 pmol/m3), confi rming the clinical diagnosis. The patient has remained well in the community for 18 months, with no further seizures and no cognitive sequelae. Faciobrachial dystonic seizures are a newly described seizure type that seems to be pathognomonic for limbic encephalitis associated with antibodies to the LGI1 component of the voltage-gated potassium channel complex in the brain (LGI1-antibody associated limbic encephalitis). As well as faciobrachial dystonic seizures, patients with LGI1-antibody associated limbic encephalitis may have behavioural abnormalities and cognitive and memory decline. There is frequently hyponatraemia, and MR brain imaging may show hyperintensity within the medial temporal lobe structures. All manifestations of LGI1-antibodyassociated limbic encephalitis usually respond rapidly and completely to immunotherapy, but poorly to antiepileptic drugs. Delays in diagnosis and starting treatment often mean that patients are not restored to their baseline, and may have ongoing functional limitation from their memory defi cits. Faciobrachial dystonic seizures may be the earliest manifestation of limbic encephalitis associated with LGI1-antibody, so recognition of this unique seizure type off ers an opportunity for early treatment, preventing the development of other manifestations of limbic encephalitis. Our patient was treated within 14 days of symptom onset, which is shorter than the usual interval to start of treatment. She did not develop any features of LGI1antibody associated limbic encephalitis beyond very frequent dystonic seizures, and her initial investigations were normal. We avoided the need for CSF analysis as we made the diagnosis clinically. The patient was not given anti-epileptic medications and has not needed them subsequently. In an era of increasing reliance on investigations, identifi cation of a pathognomonic clinical sign is rare. Prompt recognition of faciobrachial dystonic seizures in primary or secondary care enables treatment to be initiated while awaiting laboratory confi rmation. Faciobrachial dystonic seizures exemplify the continuing need for bedside clinical acumen.

Comparing neurostimulation technologies in refractory focal-onset epilepsy

For patients with pharmacoresistant focal epilepsy in whom surgical resection of the epileptogenic focus fails or was not feasible in the first place, there were few therapeutic options. Increasingly, neurostimulation provides an alternative treatment strategy for these patients. Vagal nerve stimulation (VNS) is well established. Deep brain stimulation (DBS) and cortical responsive stimulation (CRS) are newer neurostimulation therapies with recently published long-term efficacy and safety data. In this literature review, we introduce these therapies to a non-specialist audience. Furthermore, we compare and contrast long-term (5-year) outcomes of newer neurostimulation techniques with the more established VNS. A search to identify all studies reporting long-term efficacy (>5u2005years) of VNS, CRS and DBS in patients with refractory focal/partial epilepsy was conducted using PubMed and Cochrane databases. The outcomes compared were responder rate, percentage seizure frequency reduction, seizure freedom, adverse events, neuropsychological outcome and quality of life. We identified 1 study for DBS, 1 study for CRS and 4 studies for VNS. All neurostimulation technologies showed long-term efficacy, with progressively better seizure control over time. Sustained improvement in quality of life measures was demonstrated in all modalities. Intracranial neurostimulation had a greater side effect profile compared with extracranial stimulation, though all forms of stimulation are safe. Methodological differences between the studies mean that direct comparisons are not straightforward. We have synthesised the findings of this review into a pragmatic decision tree, to guide the further management of the individual patient with pharmacoresistant focal-onset epilepsy.

An investigation of the expression of G1-phase cell cycle proteins in focal cortical dysplasia type IIB

Balloon cells (BCs) are the pathologic hallmark of focal cortical dysplasia type IIB, a common cause of pharmacoresistent epilepsy. Expression of markers of cell immaturity and of the proliferation marker minichromosome maintenance protein 2 (mcm2) have been previously shown in BCs, suggesting that these cells might represent a pool of less-differentiated cells licensed for replication. An alternative explanation is that these cells are the remnants of early cortical plate cells that have failed to differentiate or to be eliminated during development and are arrested in the cell cycle, a hypothesis that this study aims to explore. Using immunohistochemical methods and semiquantitative analysis in 19 cases of focal cortical dysplasia (ages 1-81 years), we studied the expression of cell cycle proteins important either in regulating progression through the G1 phase or inducing cell arrest and promoting premature senescence. Only a small fraction of BCs expressed geminin, suggesting that few BCs enter the S phase or complete the cell cycle. Variable expression of nonphosphorylated retinoblastoma protein (Rb), cdk4, and p53 was noted in BCs. Cyclin E, D1, cdk2, phosphorylated Rb (795 and 807/811), and checkpoint 2 expression levels were low in BCs. These findings suggest early rather than late G1 arrest. Cell senescence could be induced by an undefined cerebral insult during development or alternatively represent a physiologic replicative senescence. These findings also suggest that dysregulation of cell cycle pathways may occur in focal cortical dysplasia, which opens further areas for exploration as potential new treatment avenues.

YouTube as a potential learning tool to help distinguish tonic-clonic seizures from nonepileptic attacks.

Medical students are increasingly turning to the website YouTube as a learning resource. This study set out to determine whether the videos on YouTube accurately depict the type of seizures that a medical student may search for. Two consultant epileptologists independently assessed the top YouTube videos returned following searches for eight terms relating to different categories of seizures. The videos were rated for their technical quality, concordance of diagnosis with an epileptologist-assigned diagnosis, and efficacy as a learning tool for medical education. Of the 200 videos assessed, 106 (63%) met the inclusion criteria for further analysis. Technical quality was generally good and only interfered with the diagnostic process in 8.5% of the videos. Of the included videos, 40.6-46.2% were judged to depict the purported diagnosis with moderate agreement between raters (75% agreement, κ=0.50). Of the videos returned after searching tonic-clonic seizure, 28.6-35.7% were judged to show nonepileptic seizures with almost perfect interrater agreement (92.9% agreement, κ=0.84). Of the videos returned following the search pseudoseizure, 77.8-88.9% of videos were judged to show nonepileptic seizures with substantial agreement (88.9% agreement, κ=0.61). Across all search terms, 19.8-33% of videos were judged as potentially useful as a learning resource, with fair agreement between raters (75.5% agreement, κ=0.38). These findings suggest that the majority of videos on YouTube claiming to show specific seizure subtypes are inaccurate, and YouTube should not be recommended as a learning tool for students. However, a small group of videos provides excellent demonstrations of tonic-clonic and nonepileptic seizures, which could be used by an expert teacher to demonstrate the difference between epileptic and nonepileptic seizures.

Increased immunoreactivity of cdk5 activators in hippocampal sclerosis

Cyclin-dependent kinase 5 is important in several in-vitro neurodegeneration paradigms. Whether cyclin-dependent kinase 5 contributes to cell death in human neurodegenerative diseases remains uncertain, particularly because post-mortem delay and other extrinsic factors might influence cyclin-dependent kinase 5 activity. Here we demonstrate increased immunoreactivity for the activators of cyclin-dependent kinase 5 in post-mortem human hippocampi affected by the neurodegenerative condition hippocampal sclerosis, but not in histologically normal hippocampi. Moreover, in post-mortem brain tissue from patients with unilateral hippocampal sclerosis, increased immunoreactivity for cyclin-dependent kinase 5 activators was detected in the hippocampus with sclerosis, but not in the contralateral hippocampus, suggesting that extrinsic factors are unlikely to account for the differential staining observed. Our findings suggest that deregulation of cyclin-dependent kinase 5 might contribute to the pathogenesis of hippocampal sclerosis.

The potential role of cyclin-dependent kinase 5 in focal cortical dysplasia.

Focal cortical dysplasia (FCD) is the most common malformation of cortical development found in epilepsy surgical series. Characterised by cortical mislamination, dysplastic neurons and, in a subgroup of cases, balloon cells, FCD is potently epileptogenic. Despite decades of study, the underlying aetiology of FCD remains uncertain and research has been hampered by the lack of a good animal model in which to simulate the condition. In this article we review some of the potential molecular mechanisms that might underpin human FCD. In particular we examine the potential role of cyclin-dependent kinase 5 and its principal activator p35 in FCD and estimate the contribution that deregulation of cyclin-dependent kinase 5 might make to the pathogenesis of this condition.

Current practice and recommendations in UK epilepsy monitoring units. Report of a national survey and workshop

PURPOSEnInpatient video-EEG monitoring (VEM) is an important investigation in patients with seizures or blackouts, and in the pre-surgical workup of patients with epilepsy. There has been an expansion in the number of Epilepsy Monitoring Units (EMU) in the UK offering VEM with a necessary increase in attention on quality and safety. Previous surveys have shown variation across centres on issues including consent and patient monitoring.nnnMETHODnIn an effort to bring together healthcare professionals in the UK managing patients on EMU, we conducted an online survey of current VEM practice and held a one-day workshop convened under the auspices of the British Chapter of the ILAE. The survey and workshop aimed to cover all aspects of VEM, including pre-admission, consent procedures, patient safety, drug reduction and reinstatement, seizure management, staffing levels, ictal testing and good data recording practice.nnnRESULTSnThis paper reports on the findings of the survey, the workshop presentations and workshop discussions. 32 centres took part in the survey and there were representatives from 22 centres at the workshop. There was variation in protocols, procedures and consent processes between units, and levels of observation of monitored patients. Nevertheless, the workshop discussion found broad areas of agreement on points.nnnCONCLUSIONnA survey and workshop of UK epilepsy monitoring units found that some variability in practice is inevitable due to different local arrangements and patient groups under investigation. However, there were areas of clear consensus particularly in relation to consent and patient safety that can be applied to most units and form a basis for setting minimum standards.