<br/>Water is at the heart of biology and comprises ~75 % of the brain. Within the brain, neurons communicate via electrochemical signals that are confined to the cellular membrane. These spatiotemporal changes in membrane potentials are traditionally measured by electrical recordings or by using optical probes. Since both types of probes are inevitably invasive and damaging to the cells, label-free imaging approaches are sought for. For many years second harmonic (SH) imaging has been a promise for delivering direct label-free neuronal membrane potential information. However, to date this promise has not been delivered, owing to the intrinsic low sensitivity of the method. Here, we present an improvement in label-free second harmonic neuroimaging sensitivity by ~3 orders of magnitude using a wide-field medium repetition rate illumination. Comparing patch-clamp and second harmonic imaging we demonstrate the theoretically predicted linear correlation between whole neuron membrane potential changes and the square root of the second harmonic intensity. Using SH imaging of free standing lipid bilayer membranes, we assign the induced changes to the SH intensity to changes in the orientation of membrane interfacial water, which is used to image spatiotemporal changes in the membrane potential and K<sup>+</sup> ion flux. We observe a non-uniform spatial distribution and temporal activity of ion channels in the mouse brain neurons.<br/>

Water perspective on neuronal activity using wide field second harmonic imaging

2019 Fall National Meeting

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