Anatomy and Neuroscience - Theses
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ItemGenetic, metabolic and pharmacological modulation of seizure susceptibility in mouse models of genetic epilepsy( 2013)Epilepsy is a common neurological disorder that is poorly understood. A large proportion of epilepsies have a strong familial component. The GABAA γ2 (R43Q) mutation was discovered in an Australian family with genetic epilepsy with febrile seizures + (GEFS+) that predominantly have febrile seizures (FS) and childhood absence epilepsy (CAE). A mouse model based on the mutation recapitulates these seizure types and is sensitive to first-line antiepileptic drugs. The model therefore provides an opportunity to study aspects of the genesis of epilepsy with relevance to the human condition. The work performed in this thesis describes the use of this syndrome specific mouse model to investigate aspects of seizure genesis and modulation. Three research questions are addressed; the genetic mechanisms underlying seizure genesis, metabolic and dietary modulation of seizure activity and pharmacological sensitivity to new anti-epileptic drugs in the GABAA γ2 (R43Q) mouse. Clinical heterogeneity in genetic epilepsy is common and is typically characterized by multiple seizure types and incomplete penetrance for a given protein mutation. However, the molecular and genetic basis of clinical heterogeneity is not well understood. Here, two models, GABAA γ2 (R43Q) knock-in and GABAA γ2 knock-out were used to determine the fundamental molecular mechanisms of the GABAA γ2 (R43Q) mutation underlying individual seizure phenotype. Spike-wave discharges (SWD) recorded on electroencephalogram from the GABAA γ2 (R43Q) mouse are associated with behavioural arrest and model absence epilepsy. A reduced latency to first heat-induced tonic-clonic seizure is consistent with a FS phenotype. Both the knock-in and knock-out models expressed SWDs while only the knock-in had a reduced latency to thermogenic seizures. This comparison demonstrates that two fundamental molecular mechanisms independently cause the two major seizure types in the mouse model. Haploinsufficiency could account for the SWD phenotype while a dominant impact of the mutation must be required for the FS phenotype. Subsequent investigation using mice of varying genetic background showed that the SWD phenotype required additional genetic susceptibility. In contrast, FS phenotype occurred independently of background genetics consistent with its higher penetrance compared to absence epilepsy in the GABAA γ2 (R43Q) family. Environmental modulation of neuronal excitability has been long known to alter seizure susceptibility. Altered metabolism using dietary intervention, such as the ketogenic diet, is a well recognized epilepsy therapy. The ketogenic diet conveys its anticonvulsant effects presumably through the stabilization of blood glucose and/or providing an alternative energy substrate. Here, the impact of a number of metabolic manipulations was investigated in the GABAA γ2 (R43Q) mouse model. Overnight fasting lowered blood glucose levels and increased SWD occurrence suggesting it as a potential seizure precipitant. Low-GI and triheptanoin diets on the other hand reduced SWD activities suggesting that both stabilization of blood glucose levels and provision of additional energy substrates may independently offer anticonvulsant effects. Importantly, these diets have less tolerability issues making them a potential alternative to the poorly tolerated ketogenic diet. In-vivo drug testing is a critical step for drug discovery. Oxcarbazepine (OXC) is a second-generation drug that is typically used to control partial seizures. Like its older generation carbamazepine, OXC is contraindicated in patients with generalized epilepsy. OXC is metabolized to monohydroxy derivatives (MHD) in two enantiomeric-forms, S-(+)-licarbazepine and R-(+)-licarbazepine. The effects of individual metabolites have not been adequately characterized. In this study, OXC increased the frequency of SWDs in the GABAA γ2 (R43Q) model, consistent with clinical observation. Similarly, both MHDs also caused seizure aggravation. However, OXC and MHDs were ineffective at altering the sensitivity of mice to thermogenic seizures. The findings indicate that like OXC, its derivatives may be contraindicated in certain forms of generalized epilepsy.