The influence of basal CO2 on neurofluid dynamics measured using resting-state BOLD fMRI

Abstract

An understanding of neurofluid dynamics has been gaining importance, in part given the link between neurofluid dynamics and glymphatic flow. Recently, CSF pulsations have been attributed to widespread changes in cerebral blood volume (CBV) driven by sleep-state slow-wave electrocortical activity (Fultz et al., 2019), by localized neuronal activity (Williams et al., 2023), by respiration-related autonomic tone (Picchioni et al., 2022) and by vigilance (Z. Yang et al., 2024). It was further suggested that the slow-wave induced CSF pulsations are in fact driven by autonomic (ANS) regulation (Picchioni et al., 2022), and that CSF dynamics are ultimately modulated by ANS mechanisms instead of by sleep per se. To further understand the role of this ANS regulation of vascular tone independently of sleep, and given the established influence of carbon dioxide (CO2) on both ANS tone and vascular tone, we hypothesized that a modulation of basal CO2, producing altered global vascular tone and respiration, may highlight the role of ANS regulation in driving CSF flow, and more broadly, neurofluid flow. In this work, we report on observations of neurofluid dynamics at awake normocapnia as well as mild hyper- and hypocapnia steady states. We use the resting-state BOLD fMRI time courses in neurofluid regions (i.e. blood vessels, CSF compartments) as a surrogate of neurofluid dynamics. We found that 1) the manner biomechanical does not drive the variations in neurfluid dynamics across capnias; 2) besides respiration, cardiac pulsation also independently drives neurofluid flow as an indication of the ANS pathway of control; 3) changed CO2 alters neurofluid dynamics primarily through frequency rather than amplitude of heart-rate and respiratory-volume variability. These findings suggest that hyper- and hypocapnia both represent a disruption of homeostasis that engages ANS regulation, as reflected by the deviations in CRF and RRF from normocapnia. Our work demonstrates in awake humans previously reported ANS regulation observed during sleep. As basal CO2 can modulate this ANS regulation, it represents a new avenue for modulating neurofluid dynamics independently of sleep, attention or neuronal activation. More broadly, individuals with different basal capnic states may manifest differences in CSF dynamics, giving rise to a novel paradigm for modulating neurofluid flow in awake humans.