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Functional Ultrasound (fUS) imaging is a novel neuroimaging modality, which probes the activity of deep vascular networks in a wide range of experimental contexts including mobile, head-fixed and sleeping animals. Its key features include large field of view, good spatial (150 microns) and temporal (200 ms) resolutions, lightweight apparatus and compatibility with electrophysiological recordings. In rodent studies, fUS imaging is becoming a tool of choice as it provides an insight into brain-wide functional networks in an unbiased fashion.
Here we reveal brain-wide spatiotemporal hemodynamics of vascular networks in two contexts: first during a basic locomotion task and second during spontaneous REM sleep episodes. During locomotion, we characterized the precise timing of hemodynamic responses in cortical, hippocampal and thalamic structures and showed that they follow a sequential activation pattern starting in the dorsal thalamus, spreading to the dentate gyrus and reaching the CA1 and CA3 region, at the second timescale. The analysis of individual runs revealed that vascular responses were highly plastic within a single 30mn recording, strongly potentiating in hippocampal regions but depressing in cortical ones, independent of mere behavioral parameters (speed, acceleration) or theta activity.
During REM sleep, we demonstrate the close association between massive hyperemic events and fast gamma. We show that vascular activity divides into tonic and phasic regimes. The phasic component of this vascular activity contained vascular surges (VS), i.e. large-amplitude spatially-extended hyperemic waves. The amplitude of these VS outmatched waking levels and were robustly preceded by sustained theta (6-10 Hz) and fast gamma oscillations (80-110 Hz), the power of fast gamma being strongly correlated to the amplitude and duration of each subsequent VS. Our findings question the evolutionary benefit of such high energy-demanding vascular patterns and opens the way for the combined local controlled manipulation of brain rhythms and global imaging of sleep hemodynamics.