Functional MRI of somatosensory activation in rat: Effect of hypercapnic tip-regulation on perfusion- and BOLD-imaging

Functional activation of somatosensory cortex was studied in alpha-chloralose anesthetized rats by functional magnetic resonance imaging (fMRI), using both perfusion-weighted and T-2*-weighted (blood oxygenation level dependent, BOLD) imaging, The sensitivity of functional activation was altered by ventilating animals for 3 minutes with 6% CO2. Before hypercapnic conditioning, electrical stimulation of the left forepaw at a frequency of 3 Hz led tt, an increase of signal intensity (relative to the unstimulated baseline condition) in the right somatosensory cortex by 6 +/- 2% (means +/- SD) in T-2*-weighted images and by 45% +/- 48% in perfusion-weighted images, After hypercapnic conditioning the signal intensity increase in perfusion-weighted images doubled to 91% +/- 62% (P = 0.034), whereas that of T-2*-weighted images only marginally increased to 7 +/- 4% (not significant), This different behavior in both imaging modalities is interpreted as evidence for an increased flow response in combination with a higher oxygen extraction, Thus, the fMRI data reflect hypercapnia-induced resetting of the functional-metabolic coupling of the tissue during activation.