Dependence of BOLD signal fluctuation on arterial blood CO₂ and O₂: Implication for resting-state functional connectivity

Blood oxygenation level dependent (BOLD) functional MRI signal is known to be modulated by the CO₂ level. Typically only end-tidal CO₂, rather than the arterial partial pressure of CO₂ (paCO₂), was measured while the arterial partial pressure of O₂ (paO₂) level was not controlled due to free breathing, making their contribution not separable. Especially, the influences of paO₂ and paCO₂ on resting-state functional connectivity are not well studied. In this study, we investigated the relationship between paCO₂ and resting as well as stimulus-evoked BOLD signals under hyperoxic and hypercapnic manipulation with tight control of arterial paO₂. Rats under isoflurane anesthesia were subjected to six inspired gas conditions: 47% O₂ in air (Normal), adding 1%, 2% or 5% CO₂, carbogen (95% O₂/5% CO₂), and 100% O₂. Somatosensory BOLD activation was significantly increased under 100% O₂, while reduced with increased paCO₂ levels. However, while resting BOLD connectivity pattern expanded and bilateral correlation increased under 100% O₂, the correlation coefficient between the left and right somatosensory cortex was generally not dependent on paCO₂ or paO₂. Interestingly, the correlation in 0.04-0.07Hz range significantly increased with CO₂ levels. Intracortical electrophysiological recordings showed a similar trend as the BOLD but the neurovascular coupling varied. The results suggest that paO₂ and paCO₂ together rather than paCO₂ alone alter the BOLD signal. The response is not purely vascular in nature but has strong neuronal origins. This should be taken into consideration when designing calibrated BOLD experiment and interpreting functional connectivity data especially in aging, under drug, or neurological disorders.