FUNCTIONAL MRI OF SOMATOSENSORY ACTIVATION IN RAT - EFFECT OF HYPERCAPNIC UP-REGULATION ON PERFUSION-IMAGING AND BOLD-IMAGING

Functional activation of somatosensory cortex was studied in alpha-chl oralose anesthetized rats by functional magnetic resonance imaging (fM RI), using both perfusion-weighted and T-2-weighted (blood oxygenatio n level dependent, BOLD) imaging, The sensitivity of functional activa tion was altered by ventilating animals for 3 minutes with 6% CO2. Bef ore hypercapnic conditioning, electrical stimulation of the left forep aw at a frequency of 3 Hz led tt, an increase of signal intensity (rel ative to the unstimulated baseline condition) in the right somatosenso ry 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 ma rginally increased to 7 +/- 4% (not significant), This different behav ior in both imaging modalities is interpreted as evidence for an incre ased flow response in combination with a higher oxygen extraction, Thu s, the fMRI data reflect hypercapnia-induced resetting of the function al-metabolic coupling of the tissue during activation.