Dual responses of tissue partial pressure of oxygen after functional stimulation in rat somatosensory cortex

To compare the spatial heterogeneity of brain tissue partial pressure of oxygen (pO₂) among local brain regions, we focused on functional and anatomical variations in rat somatosensory cortex. Tissue pO₂ was measured by using an oxygen microelectrode with high spatio-temporal resolution, and investigated in three somatosensory areas including hindlimb (HL), forelimb (FL), and trunk region (Tr). Their anatomical structures were determined with histological techniques (Nissl stain). In addition to the measurement of baseline tissue pO₂, we examined temporal shifts in tissue pO₂ distribution elicited by functional stimulation using the brushing stimulation to the hindlimb, forelimb, and trunk regions of the body. We observed that average tissue pO₂ in the Tr (14±10 Torr) was significantly lower than those in the HL (25±13 Torr) and FL (24±13 Torr). Such regional differences in tissue pO₂ were closely related to the cytoarchitectonic variations among these three areas. In addition, the functional stimulation enlarged the regional differences in the pO₂ depending on each somatosensory area; the pO₂ in the HL increased by 3.6±2.9% after the stimulation to hindlimb, whereas that in the Tr decreased by -2.9±2.5% after the stimulation to trunk region. Such dual responses of tissue pO₂ (i.e. increase or decrease) after the functional stimulation to the corresponding body regions may provide a criterion to clinically predict regions susceptible to tissue hypoxia, because considerable decrease in tissue pO₂ occurred in the Tr showing the lowest baseline pO₂.