Abstract
Evoked neural activity correlates strongly with rises in cerebral metabolic rate of oxygen (CMRO(2)) and cerebral blood flow (CBF). Activity-dependent rises in CMRO(2) fluctuate with ATP turnover due to ion pumping. In vitro studies suggest that increases in cytosolic Ca(2+) stimulate oxidative metabolism via mitochondrial signaling, but whether this also occurs in the intact brain is unknown. Here we applied a pharmacological approach to dissect the effects of ionic currents and cytosolic Ca(2+) rises of neuronal origin on activity-dependent rises in CMRO(2). We used two-photon microscopy and current source density analysis to study real-time Ca(2+) dynamics and transmembrane ionic currents in relation to CMRO(2) in the mouse cerebellar cortex in vivo. We report a direct correlation between CMRO(2) and summed (i.e., the sum of excitatory, negative currents during the whole stimulation period) field EPSCs (∑fEPSCs) in Purkinje cells (PCs) in response to stimulation of the climbing fiber (CF) pathway. Blocking stimulus-evoked rises in cytosolic Ca(2+) in PCs with the P/Q-type channel blocker ω-agatoxin-IVA (ω-AGA), or the GABA(A) receptor agonist muscimol, did not lead to a time-locked reduction in CMRO(2), and excitatory synaptic or action potential currents. During stimulation, neither ω-AGA or (μ-oxo)-bis-(trans-formatotetramine-ruthenium) (Ru360), a mitochondrial Ca(2+) uniporter inhibitor, affected the ratio of CMRO(2) to fEPSCs or evoked local field potentials. However, baseline CBF and CMRO(2) decreased gradually with Ru360. Our data suggest that in vivo activity-dependent rises in CMRO(2) are correlated with synaptic currents and postsynaptic spiking in PCs. Our study did not reveal a unique role of neuronal cytosolic Ca(2+) signals in controlling CMRO(2) increases during CF stimulation.