Spatiotemporal features of neurovascular (un)coupling with stimulus-induced activity and hypercapnia challenge in...

Carbon dioxide (CO₂) is traditionally considered as metabolic waste, yet its regulation is critical for brain function. It is well accepted that hypercapnia initiates vasodilation, but its effect on neuronal activity is less clear. Distinguishing how stimulus- and CO₂-induced vasodilatory responses are (dis)associated with neuronal activity has profound clinical and experimental relevance. We used an optical method in mice to simultaneously image fluorescent calcium (Ca²⁺) transients from neurons and reflectometric hemodynamic signals during brief sensory stimuli (i.e., hindpaw, odor) and CO₂ exposure (i.e., 5%). Stimuli-induced neuronal and hemodynamic responses swiftly increased within locally activated regions exhibiting robust neurovascular coupling. However, hypercapnia produced slower global vasodilation which was temporally uncoupled to neuronal deactivation. With trends consistent across cerebral cortex and olfactory bulb as well as data from GCaMP6f/jRGECO1a mice (i.e., green/red Ca²⁺ fluorescence), these results unequivocally reveal that stimuli and CO₂ generate comparable vasodilatory responses but contrasting neuronal responses. In summary, observations of stimuli-induced regional neurovascular coupling and CO₂-induced global neurovascular uncoupling call for careful appraisal when using CO₂ in gas mixtures to affect vascular tone and/or neuronal excitability, because CO₂ is both a potent vasomodulator and a neuromodulator.