Medicine (Austin & Northern Health) - Research Publications

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Subject: Seizures ×

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    Enhanced in vitro CA1 network activity in a sodium channel β1(C121W) subunit model of genetic epilepsy
    Hatch, RJ ; Reid, CA ; Petrou, S (WILEY-BLACKWELL, 2014-04)
    OBJECTIVE: A NaV β1(C121W) mouse model of human genetic epilepsy has enhanced neuronal excitability and temperature sensitivity attributed to a decreased threshold for action potential firing in the axon initial segment. To investigate the network consequences of this neuronal dysfunction and to establish a genetic disease state model we developed an in vitro assay to investigate CA1 network properties and antiepileptic drug sensitivity. METHODS: CA1 network oscillations were induced by tetanic stimulation and average number of spikes, interspike interval (ISI), duration, and latency were measured in slices from control and NaV β1(C121W) heterozygous mice in the presence and absence of retigabine or carbamazepine. Retigabine was also tested in a thermogenic seizure model. RESULTS: Oscillations were reliably induced by tetanic stimulation and were maintained after severing connections between CA3 and CA1, suggesting a local recurrent circuit. Blocking α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), γ-aminobutyric acid receptor A (GABAA ), Ih , and T-type Ca(2+) channels/receptors reduced the number of spikes. Slices from NaV β1(C121W) heterozygous mice displayed several hallmarks of increased network excitability including increases in duration of the oscillation, the number and frequency of spikes and a decrease in their onset latency. The effect of genotype on network excitability was temperature sensitive, as it was seen only at elevated temperatures. Carbamazepine and retigabine were more effective in reducing network excitability in slices from NaV β1(C121W) heterozygous mice. Retigabine appeared to be more effective in suppressing time to thermogenic seizures in NaV β1(C121W) heterozygous mice compared to wild-type (WT) controls. SIGNIFICANCE: Hippocampal networks of the NaV β1(C121W) heterozygous mouse model of genetic epilepsy show enhanced excitability consistent with earlier single neuron studies bridging important scales of brain complexity relevant to seizure genesis. Altered pharmacosensitivity further suggests that genetic epilepsy models may be useful in the development of novel antiepileptic drugs that target disease state pathology. A PowerPoint slide summarizing this article is available for download in the Supporting Information section here.
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    Network-specific mechanisms may explain the paradoxical effects of carbamazepine and phenytoin
    Thomas, EA ; Petrou, S (WILEY, 2013-07)
    PURPOSE: A common notion of the mechanism by which the antiepileptic drugs (AEDs) carbamazepine and phenytoin act is that they block sodium channels by binding preferentially to the inactivated state, thereby allowing normal neuronal firing while blocking ictal activity. However, these drugs have unpredictable efficacy and, in some cases, may exacerbate seizures. Previous studies have suggested that reducing sodium channel availability in the dentate gyrus (DG) paradoxically increases excitability. We used a biophysically detailed computer model of the DG to test the hypothesis that AEDs increase excitability by disproportionately reducing negative feedback mechanisms. METHODS: We built a Markov model of sodium channel gating that reproduces responses to voltage clamp experiments in the presence of carbamazepine and phenytoin. We incorporated this validated Markov model into a biophysically realistic computer model of DG neurons and networks. Simulated drug concentrations were similar to those measured in cerebral spinal fluid in medicated patients. Single neuron models were stimulated with current injections, and networks were stimulated with perforant path synaptic input. In the network model, environmental effects were studied by introducing mossy fiber sprouting. KEY FINDINGS: As expected, drugs reduced sodium channel availability, which in turn reduced action potential amplitude. This had only a small effect on action potential (AP) firing rate during brief (100 msec) current injections. Paradoxically, long current injections (2,500 msec) increased AP firing rates. This was caused by reduced calcium entry and consequently reduced activation of calcium activated potassium channels. It is important to note that the main determinant of drug effect was resting membrane potential (RMP) and not action potential firing rate. Binding of phenytoin and carbamazepine is slow and, thus drug effects are largely determined by the long term state of the RMP. This paradoxical AP firing increase was dependent on the unusually large calcium-activated potassium conductances expressed by DG granule cells. This predicts that drug efficacy in a given network will depend on the precise makeup of conductances in the network. RMP is expected to vary with the level of activity in the network. We simulated the effects of drugs on single shot stimulus responses in networks with mossy fiber sprouting and varied the RMP in all neurons as a model for network activity. For an RMP of -50 mV, representing an active network, drugs had no effect, or in some cases, increased excitability. Drugs had an increasingly larger inhibitory effect on network responses as RMP decreased. An important prediction is that drugs will be unable to block ictal activity invading an active network. SIGNIFICANCE: Our key findings are that drug effects depend on both intrinsic properties of the network and its behavioral state. This may explain the paradoxical and unpredictable effects of some AEDs on seizure control in some patients.
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    SCN1A testing for epilepsy: Application in clinical practice
    Hirose, S ; Scheffer, IE ; Marini, C ; De Jonghe, P ; Andermann, E ; Goldman, AM ; Kauffman, M ; Tan, NCK ; Lowenstein, DH ; Sisodiya, SM ; Ottman, R ; Berkovic, SF (WILEY-BLACKWELL, 2013-05)
    This report is a practical reference guide for genetic testing of SCN1A, the gene encoding the α1 subunit of neuronal voltage-gated sodium channels (protein name: Nav 1.1). Mutations in this gene are frequently found in Dravet syndrome (DS), and are sometimes found in genetic epilepsy with febrile seizures plus (GEFS+), migrating partial seizures of infancy (MPSI), other infantile epileptic encephalopathies, and rarely in infantile spasms. Recommendations for testing: (1) Testing is particularly useful for people with suspected DS and sometimes in other early onset infantile epileptic encephalopathies such as MPSI because genetic confirmation of the clinical diagnosis may allow optimization of antiepileptic therapy with the potential to improve seizure control and developmental outcome. In addition, a molecular diagnosis may prevent the need for unnecessary investigations, as well as inform genetic counseling. (2) SCN1A testing should be considered in people with possible DS where the typical initial presentation is of a developmentally normal infant presenting with recurrent, febrile or afebrile prolonged, hemiclonic seizures or generalized status epilepticus. After age 2, the clinical diagnosis of DS becomes more obvious, with the classical evolution of other seizure types and developmental slowing. (3) In contrast to DS, the clinical utility of SCN1A testing for GEFS+ remains questionable. (4) The test is not recommended for children with phenotypes that are not clearly associated with SCN1A mutations such as those characterized by abnormal development or neurologic deficits apparent at birth or structural abnormalities of the brain. Interpreting test results: (1) Mutational testing of SCN1A involves both conventional DNA sequencing of the coding regions and analyses to detect genomic rearrangements within the relevant chromosomal region: 2q24. Interpretation of the test results must always be done in the context of the electroclinical syndrome and often requires the assistance of a medical geneticist, since many genomic variations are possible and it is essential to differentiate benign polymorphisms from pathogenic mutations. (2) Missense variants may have no apparent effect on the phenotype (benign polymorphisms) or may represent mutations underlying DS, MPSI, GEFS+, and related syndromes and can provide a challenge in interpretation. (3) Conventional methods do not detect variations in introns or promoter or regulatory regions; therefore, a negative test does not exclude a pathogenic role of SCN1A in a specific phenotype. (4) It is important to note that a negative test does not rule out the clinical diagnosis of DS or other conditions because genes other than SCN1A may be involved. Obtaining written informed consent and genetic counseling should be considered prior to molecular testing, depending on the clinical situation and local regulations.