Learning and memory are the basis of our behaviour and mental well-being. Understanding the mechanisms of structural and cellular plasticity in defined neuronal circuits in vivo will be crucial to elucidate principles of circuit-specific memory formation and their relation to changes in neuronal ensemble dynamics. Structural plasticity studies were so far technically limited to dorsal cortex, excluding deeper brain areas. Furthermore, these studies mainly focused on the input site (dendritic spines), whilst the plasticity of the axon initial segment, a neuron’s site of output generation, was so far not studied in vivo. The axon initial segment is the site of action potential generation. Length and location of the axon initial segment were found to be plastic in cultured neurons or ex vivo, which strongly affected a neurons spike output. However, it remains unknown if axon initial segment plasticity regulates neuronal activity upon learning in vivo. The objective of this research programme was to combine the expression of live markers of the axon initial segments with novel deep brain imaging techniques via gradient index (GRIN) lenses to investigate how axon initial segment location and length are regulated upon associative learning in brain circuits beyond cortex in vivo. Two-photon time-lapse imaging of the axon initial segment of neurons was combined with associative threat conditioning to track learning-driven axon initial segment location dynamics. It was furthermore important to understand how the structure of the axon initial segment relates to neuronal activity in vivo. This will provide fundamental insights into the cellular mechanisms underlying sensory processing upon learning and relate network level plasticity with the cellular level.
