Changes in Blood Flow, Oxygenation, and Volume Following Extended Stimulation of Rodent Barrel Cortex

Abstract

Simultaneous optical imaging spectroscopy and laser–Doppler flowmetry were used in rodent barrel cortex to examine the hemodynamic response to extended electrical stimulation (20 s, 5 Hz) of the whisker pad. Stimulation results in a fast early increase in deoxyhemoglobin concentration (Hbr) followed by a later decrease to a “plateau” phase ∼4 s after stimulation onset. There was no corresponding decrease in oxyhemoglobin (HbO2), which simply increased after stimulation, reaching a plateau at ∼5 s. The time series of flow and volume had similar onset times and did not differ significantly throughout the presentation of the stimuli. Following stimulation cessation all aspects of the hemodynamic response returned to baseline with a long decay constant (>20 s), CBV doing so at a slower rate than CBF. The time courses of CBF, CBV, Hbr, and HbO2 were very similar to that produced by a brief stimulus up to peak. The relationship between the flow and the volume changes is well approximated by the expression CBV = CBFφ. We find φ to be slightly lower under stimulation (0.26 ± 0.0152) than during hypercapnia (0.32 ± 0.0172). Saturation and flow data were used to estimate changes in CMRO2 for a range of baseline oxygen extraction fractions (OEF). In the case of hypercapnia CMRO2 was biphasic, increasing after onset and sharply decreasing below baseline following cessation. If it is assumed that there is no “net” increase in CMRO2 (i.e., ∑ΔCMRO2 = 0) following the onset and offset of hypercapnia, then the corresponding estimate of baseline OEF is 0.45. Evidence for increased oxygen consumption was obtained for all stimulation intensities assuming a baseline OEF of 0.45.