Abstract
Childhood absence epilepsy (CAE) is the most common early-onset epilepsy accounting for about 10-17% of all paediatric epilepsies. It is a genetic generalized, non-convulsive form of epilepsy characterized by a sudden and brief loss of consciousness, which can occur up to hundreds of times a day in affected individuals. The frequent episodes of abrupt unconsciousness are hugely disruptive, and are associated with adverse effects on learning, behaviour and social adjustment, as well as physical wellbeing in some patients. Thus, anti-seizure medication is recommended to combat the frequent episodes of ‘absences’. However, currently available antiepileptic drugs fail to control seizures or induce intolerable side effects in about a third of patients, and even exacerbate seizures in some cases. This variability in patient response to drug treatment as well as disease outcomes suggests that complex and multifactorial mechanisms may underlie seizure generation in patients from different genetic backgrounds. Hence, it is important to decipher potential mechanisms involved in generation of absence seizures in order to identify novel therapeutic targets with higher efficacy for patient-specific treatment.
The electroclinical hallmark of absence seizures, 2.5-4 Hz spike wave discharges (SWDs), arise from disruptions within the corticothalamocortical (CTC) circuit, which switch the normal CTC oscillatory firing pattern into pathological SWDs. However, the precise cellular and molecular mechanisms in genetically different patients are still under investigation. Notably, in all these patients, the various gene mutations appear to be capable of causing perturbations in the CTC network that alter neuronal excitability and the synchronous interactions within the cortical and thalamic nodes. There appear to be multiple mechanisms through which this aberration can occur; with altered glutamatergic excitation, mainly due to altered receptor function, implicated as one facilitator of epileptogenesis in many experimental models. Recent studies in some models have specifically implicated alterations in the expression and function of AMPA receptors (AMPARs), which mediate most of the fast excitatory glutamatergic synaptic transmission in the brain, in SWD generation.
Since SWDs are thought to originate in the somatosensory cortex, it is necessary to establish the aberrations that promote this pathological switch into SWDs within the cortical network. The well-established stargazer mouse model of absence epilepsy presents with a genetic deficit in stargazin, which is involved in the modulation and synaptic trafficking of AMPARs. In the cortex, stargazin is predominantly expressed in parvalbumin-containing (PV+) inhibitory interneurons, and concomitantly, recent evidence alludes to dysregulation of the cortical inhibitory microcircuit in SWDs in stargazers. However, it is still unclear if this aberration is a direct consequence of loss of excitatory activation of these local inhibitory interneurons and/or if a loss of inhibitory neurons also contributes. Hence, this study was aimed at examining how the stargazin mutation impacts AMPAR subunits expression in epileptic somatosensory cortex and whether PV+ inhibitory neurons are affected. This involved the examination of subunit-specific expression of AMPARs and cortical PV+ interneuron density using western blotting and fluorescence confocal immunohistochemistry. Synaptic changes in AMPAR expression were evaluated in biochemically isolated subcellular fractions and also at excitatory input synapses of PV+ neurons using double post-embedding immunogold cytochemistry. Finally, an examination of the developmental expression profile of AMPARs was conducted using western blotting, to identify changes that precede seizure onset in stargazer mice and potentially contribute to the epileptic phenotype. Results indicate that whereas PV+ inhibitory interneuron number was unchanged between stargazers and control non-epileptic littermates, AMPAR GluA1-4 expression was reduced in the stargazer cortex, particularly at excitatory input synapses onto the PV+ interneurons. Furthermore, this reduction in AMPAR expression occurred prior to seizure onset, and thus potentially involved in SWD generation in stargazers.
Overall, the findings from this thesis suggest a potential impairment of cortical feed-forward inhibition due to reduced AMPAR excitatory activation of inhibitory PV+ neurons in the stargazer cortex. This deficit could ultimately alter cortical excitation-inhibition balance, and result in SWDs in stargazers. Importantly, this suggests that an underlying cause of SWD in some patients may involve disruptions in AMPAR activation in specific neurons, and thus may represent a potential therapeutic target for a subpopulation of human patients. This has clinical implications, as for instance; any drugs that further reduce AMPAR activation in these human cases could potentially exacerbate seizures in these patients.