Principles of calcium and membrane voltage imaging

Orateur : Marco CANEPARI (Laboratoire Interdisciplinaire de Physique, Université de Grenoble - Alpes)

In our laboratory, we recently developed an optical technique to image Ca2+ currents as they physiologically occur in neuronal dendrites (Jaafari et al. 2014 ; 2015). In this seminar, I will present the basics of this novel method and the results we obtained by applying the technique to investigate Ca2+ currents in CA1 hippocampal pyramidal neurons (CA1HPNs) and in cerebellar Purkinje neurons (PNs). At CA1HPNs, action potentials generated in the axon hillock back-propagate into the dendrites activating different types of voltage-gated Ca2+ channels (VGCCs). The consequent Ca2+ signal represents a precise code for the dendritic sites where the cell receives synaptic inputs and plays itself a role in the propagation of electrical signals within the dendrites. We found that all types of high-voltage activated (HVA) VGCCs (L-type, N-type and P/Q type) contribute to the kinetics of action potentials, in particular by activating K+ channels (Jaafari and Canepari 2016). The longer lasting action potential prolongs the duration of activation of low-voltage activated Ca2+ channels (T-type). The result is a compensation of the loss of the Ca2+ signal component mediated by LVA VGCCs with the larger component mediated by LVA VGCCs. Thus, functional coupling of VGCCs underlies the high-fidelity of fast dendritic Ca2+ signals during individual action potentials which occur in the majority of the dendritic field. In PNs, the large dendritic depolarisation associated with climbing fibre (CF) excitatory synaptic potentials (EPSPs) activates P/Q-type and T-type Ca2+ voltage-gated calcium channels. We extended our optical approach to measure Ca2+ currents in PNs where the Ca2+ signal is distorted by the presence of the fast Ca2+-binding protein Calbindin-28k (Ait Ouares et al. 2016). We found that the Ca2+ currents associated with CF-EPSPs are spatially inhomogeneous and strongly dependent on the initial membrane potential. If the CF-EPSP occurs when the cell is not firing, the Ca2+ current has a small fast component mediated by P/Q-type channels and associated with a dendritic spike, followed by slower component mainly mediated by T-type channels. If the CF-EPSP occurs when the cell is firing, the Ca2+ current component mediated by P/Q-type channels becomes dominant. These results represent the first direct measurement of the physiological Ca2+ current associated with CF-EPSPs opening the gate to the final understanding of the Ca2+-mediated signalling associated with this important synaptic input.

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