Abstract
Childhood absence epilepsy (CAE) is a generalised, non-convulsive subtype of epilepsy, accounting for 10-17% of all childhood epilepsies. Absence seizures are characterised by a brief loss of consciousness accompanied by bilateral hypersynchronous brain oscillations known as short wave discharges (SWDs) that are 3-4 Hz. These seizures can initiate spontaneously hundreds of times per day, making them incredibly disruptive to daily life. A variety of comorbidities such as anxiety, depression, cognitive deficits, learning difficulties and behavioural disorders are associated with CAE. These children are at an increased risk of being injured if these seizures occur during a potentially dangerous activity. Therefore, anti-epileptic drug (AED) therapy is recommended to treat absence seizures. However, these AEDs often either fail to suppress seizures, cause intolerable side effects or in some cases intensify seizures. In order to develop more targeted and personalised therapies that cater to individual needs, a more thorough understanding of epileptogenic pathways is necessary.
Animal models such as the stargazer mouse model of absence epilepsy, have been useful in studying CAE. In both humans and stargazer mice, SWDs are thought to arise in the somatosensory cortex (SSCtx) from disturbances within the corticothalamocortical (CTC) network, where normal spindle-like oscillatory activity is morphed into pathological SWDs as a result of alterations within this network. However, the precise mechanisms involved are not yet fully understood. The initial aim of this thesis was to use immunofluorescence confocal microscopy to investigate whether cortical somatostatin positive (SOM+) interneurons were affected by the loss of stargazin and showed a decrease in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR), as was previously described in parvalbumin positive (PV+) interneurons. As SOM+ interneurons are the second largest group of cortical inhibitory interneurons, providing feedback inhibition onto cortical neurons in the CTC network, alterations to SOM+ firing in the cortex could also contribute to the disruptions in the CTC network that lead to seizure generation. Transmission electron microscopy was also used to investigate excitatory synapses within the cortex, to see whether changes in AMPAR expression was reflected as a change in the post synaptic density thickness/length.
As SOM+ interneurons were the focus of this thesis and γ-aminobutyric acid (GABA) is co-expressed within these neurons, GABAergic signalling within the stargazers was investigated as well. As part of a team of researchers, high performance liquid chromatography was conducted to investigate total GABA levels within the SSCtx. While double immunolabelling was conducted to investigate relative GABA levels within cortical GABAergic presynaptic terminals and SOM+ interneurons as well. Key results from this study indicates that an increase in total GABA in the SSCtx alongside a decrease in intracellular GABA within presynaptic GABAergic terminals, suggests an increase in extracellular GABA levels. This could potentially lead to an increase in tonic inhibition as was observed previously in the thalamocortical neurons of the CTC network.
Overall, the findings from this project demonstrates alterations to GABAergic signaling within the SSCtx, reflected as alterations to the relative levels of GABA in this region. As some AED therapies that act on GABAergic signalling within the brain have been shown to aggravate seizures, understanding the pathological alterations in GABAergic signalling could contribute to developing novel AED therapies that work to suppress seizures rather than aggravate them.