Simultaneous fluorescence imaging of tilted focal planes at two depths in thick neural tissue: Implementation with remote focus in a widefield electrophysiological microscope

Wide-field imaging conventionally results in a single image plane oriented perpendicular to the optical axis. However, in brain slice or in vivo recording, neuronal or circuit morphologies lie in arbitrarily tilted planes. Consequently the spatiotemporal advantages of wide-field non-scanned imaging are lost because of the time required for stepwise focal readjustments to view an entire neuron or network. We describe an application of remote focus that views simultaneously two planes separated by up to 100 µm, each with variable tilt from the conventional image plane. This permits fluorescence detection of ion fluxes or membrane potential across neuronal compartments and their correlation with electrical activity. Further, two fluorophores can be viewed simultaneously in each plane. We show (i) neuronal images tilted to optimise simultaneous aquisition of somatic, dendritic and axonal compartments; (ii) networks viewed simultaneously at 2 depths separated by up to 100 µm, (iii) widefield imaging at 30 Hz of Gcamp5 fluorescence during spontaneous spiking in motoneuron layers of zebrafish spinal cord separated by 30-40 microns.