Optogenetic study of cell excitability over long timescales

The research carried out in this thesis builds on the pioneering discoveries by Larry Abbott, Shimon Marom, Eve Marder, and others, who's research provided key insights on the dynamics of neuronal excitability over long timescales. By combining Optogenetics with substrate-integrated microelectrode arrays, I developed a novel approach to observe and characterise single-neuron activity, in response to repetitive light stimulation and upon pharmacological isolation of neurons from synaptic interactions. This offers a detailed and systematic observation window of neuronal excitability over very long periods of time, which I inferred by probing the input-output properties of individual cells. A distinctive feature of our approach is the methodological advantage offered by Optogenetics, as we can restrict my investigation to genetically-identified cell types e.g., putative glutamatergic cells. My method also allows for stable recordings of a large number of simultaneous neurons, undergoing repeated wide-field photo-activation and retaining long experimental stability of the spike waveform at very low and comparatively high stimulation rates. I also develop an algorithm to resolve and separate individual neuronal units at each microelectrode. Through the analysis of my experimental results, I characterise rich dynamical processes underlying neuronal excitability, reflected in power-law relations across various stimulation frequencies and various degrees of cross-correlations along multiple timescales. As an important consequence, besides further clarifying the physiological processes of biological excitability, my results also provide the community with insights and quantitative experimental data, very much needed for designing novel mathematical models of cells and circuits, capable to capture neuronal dynamics to a full extent.