In vitro characterisation and modulation of evolving epileptiform activity

Abstract

Understanding the role of different neuronal populations in the evolution of epileptic activity remains a major goal for epilepsy research. Physiological neuronal networks may become hyperexcitable if they tip over some apparent threshold level of excitation, or below some threshold level of inhibition, although this process, termed ictogenesis is not understood. This hyperexcitable state of the network underlies the pathological condition of epilepsy. Clinical evidence suggests strongly that different regions in the brain have different epileptic-activity patterns and seizure susceptibility. The reasons for this differential susceptibility, however, are also not known. In this thesis, two widely used in vitro models of epilepsy were used – zero-magnesium, and 4-aminopyridine (4AP) models – to characterise the evolution of epileptiform activity in naïve cortical networks in different regions of brain slices taken from wild-type mice. Various metrics were then used to develop assays for measuring (1) the action of disease-modifying drugs and (2) the effects of genetic mutations on seizure susceptibility. Lastly, the firing properties of neocortical parvalbumin-positive (PV+) interneurons in 4AP were characterised. Different cortical areas showed notable differences in seizure susceptibility and activity patterns in the two models. In zero-magnesium, development of epileptiform activity in hippocampal regions facilitated transformation of early-stage epileptiform activity to late-stage in the neocortex. Furthermore, activity in the hippocampus entrained neocortical events, and this phenomenon was mediated, at least in part, by non-synaptic mechanisms, providing strong evidence for propagation through non-synaptic pathways. The effects of diazepam and baclofen were also examined. They showed distinct effects on different cortical areas. Pharmacological suppression of glial functions induced spontaneous activity patterns, and also affected the development of epileptiform activity in the neocortex. Lastly, 4AP was found to alter the firing capability of PV+ interneurons in an input intensity-dependent manner, and induced spontaneous membrane potential oscillations.