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

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Abstract
Functional activation of somatosensory cortex was studied in α‐chloralose anesthetized rats by functional magnetic resonance imaging (fMRI), using both perfusion–weighted and T2*–weighted (blood oxygenation level dependent, BOLD) imaging. The sensitivity of functional activation was altered by ventilating animals for 3 minutes with 6% CO2. Before hyper–capnic conditioning, electrical stimulation of the left forepaw at a frequency of 3 Hz led to an increase of signal intensity (relative to the unstimulated baseline condition) in the right somatosensory cortex by 6 ± 2% (means ± SD) in T2*–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 T2*–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 hypercapniainduced resetting of the functional–metabolic coupling of the tissue during activation.

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