Larminar-dependent CBF responses induced by visual and hypercapnic stimulation

Background and aims: Metabolic changes accompanying neural activity modulate hemodynamic responses including CBF and CBV. Recently, relative CBF and CBV responses induced by neural activity are the highest in middle cortical layer IV where the highest capillary density and highest metabolic responses are observed, suggesting that hemodynamic responses are highly correlated to metabolic responses induced by neural activity (1,2). However, it is still possible that laminar-dependent hemodynamic responses may not relate to metabolic responses, but merely reflect intrinsic laminar-scale regulation. To evaluate this issue, we measured laminar-dependent CBF responses induced by global, non-metabolic hypercapnic stimulation in addition to visual stimulation in isoflurane-anesthetized cats. Methods: Four studies in three female adolescent cats were performed on a 9.4T MRI (Varian) system. The cat was orally intubated and mechanically ventilated under 0.8 - 1.2% isoflurane anesthesia in a 2:1 air:O2 mixture. Pancuronium bromide (∼0.2 mg/Kg/h) was delivered through the cephalic vein. The end-tidal CO2 level was maintained in the range of 3.4 ? 3.6% by adjusting the respiration rate and volume, and the rectal temperature was controlled at 38.5 +/- 0.5 C with a feedback regulated hot water circulator. The animal's head was fixed with an in-house head frame. Binocular full-field visual stimuli were presented with square-wave high-contrast moving gratings (2 cycles/s) with 0.15 cycles/degree of spatial frequency during 30-s stimulation. For hypercapnic stimulation, a gas mixture with ~6% CO2 was inhaled during 30-s without changing other inhalation levels. CBF fMRI images were measured by the slice-selective (SS) and nonslice-selective (NS) inversion recovery images of a flow alternating inversion recovery (FAIR) method (TE = 20 ms, TR = 3 s including TI = 1.5 s) using a gradient echo echo-planar imaging pulse sequence on a single slice with matrix size = 64 * 64, field of view = 2 * 2 cm, and slice thickness = 0.2 cm. The acquisition order of NS and SS was alternatively changed. Cortical depth profile analysis for layer specificity was performed in area 18 within the visual cortex. Results and discussion: CBF values in baseline conditions (without stimulation) were consistent in both visual and CO2 stimulation (R = 0.90 +/- 0.05). Large baseline CBF signal was observed in upper cortical layers, possibly due to the signal contribution from large arterial vessels. Functional CBF maps were successfully obtained during both visual and CO2 stimulation. Consistent with previous observations during neural activation, the largest relative changes in CBF occur at the middle of the cortex (~1 mm depth from the surface of the cortex) during visual stimulation. Interestingly, the similar observation was also found during hypercanic stimulation, which differs from our expectation of homogeneous CBF responses across layers. Our data indicate that larger CBF changes in middle cortical layers due to visual stimulation may not be caused by metabolic demand, but purely intrinsic layer-dependent vascular regulation.