Imaging oxygen consumption in forepaw somatosensory stimulation in rats under isoflurane anesthesia

The cerebral metabolic rate of oxygen (CMRO₂) was dynamically evaluated on a pixel-by-pixel basis in isoflurane-anesthetized and spontaneously breathing rats following graded electrical somatosensory forepaw stimulations (4, 6, and 8mA). In contrast to α-chloralose, which is the most widely used anesthetic in forepaw-stimulation fMRI studies of rats under mechanical ventilation, isoflurane (1.1-1.2%) provided a stable anesthesia level over a prolonged period, without the need to adjust the ventilation volume/rate or sample blood gases. Combined cerebral blood flow signals (CBF) and blood oxygenation level-dependent (BOLD) fMRI signals were simultaneously measured with the use of a multislice continuous arterial spin labeling (CASL) technique (two-coil setup). CMRO₂ was calculated using the biophysical BOLD model of Ogawa et al. (Proc Natl Acad Sci USA 1992;89:5951-5955). The stimulus-evoked BOLD percent changes at 4, 6, and 8mA were, respectively, 0.5% ± 0.2%, 1.4% ± 0.3%, and 2.0% ± 0.3% (mean ± SD, N = 6). The CBF percent changes were 23% ± 6%, 58% ± 9%, and 87% ± 14%. The CMRO₂ percent changes were 14% ± 4%, 24% ± 6%, and 43% ± 11%. BOLD, CBF, and CMRO₂ activations were localized to the forepaw somatosensory cortices without evidence of plateau for oxygen consumption, indicative of partial coupling of CBF and CMRO ₂. This study describes a useful forepaw-stimulation model for fMRI, and demonstrate that CMRO₂ changes can be dynamically imaged on a pixel-by-pixel basis in a single setting with high spatiotemporal resolution.