Dynamic of axonal activity during fast network oscillations and plasticity

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In hippocampal pyramidal cells, as in other neurons, initiation of action potentials (APs) occurs in the axon close to the soma. APs can also be generated at regions remote from the soma, at ectopic sites of the axon, as an emergent intrinsic property of the neuronal network. We will examine the possibility that such ectopically generated APs (EAPs) may be initiated at presynaptic axon terminals of principal cells of the hippocampus during normal physiological network activity such as gamma frequency oscillations. Parvalbumin containing perisomatic targeting axo-axonic cells (AACs) form synapses at strategic sites of pyramidal cells, namely at the axon initial segment (AIS). We will investigate the possibility that this cell type could routinely confine the propagation of EAPs to the soma during the gamma oscillations and therefore may have critical impact on the excitability of principal cells. Paired recordings will show whether activation of presynaptic AACs results in suppression of EAPs invasion to the parent soma of monosynaptically coupled principal cells. We will further examine whether suppression of AP back-propagation is mediated through a membrane potential hyperpolarization/ depolarization or a shunting inhibition. We hypothesise that the modulation/suppression of spike backpropagation along the pyramidal cell axon by AACs allows axons to fire independently from their somata and to transmit their message to the postsynaptic targets, whilst at the same time to spare the complex dendritic integration during relevant physiological network activity.