Layer-dependent BOLD and CBV-weighted fMRI responses in the rat olfactory bulb

Abstract

The olfactory bulb is a laminarized brain structure involved in odor sensation that has important implications to basic neuroscience research, like mechanisms for neurovascular coupling and early disease diagnosis. To investigate laminar-dependent responses to odor exposure, blood oxygenation level-dependent (BOLD) and cerebral blood volume weighted (CBVw) fMRI with iron oxide nanoparticle contrast agent were obtained with 110 × 110 × 500 μm3 resolution in urethane-anesthetized rats at 9.4 T. The baseline total CBV is the largest at the olfactory bulb surface and midline, and decreases in the deeper layers, while a band of increasedmicrovasculature density is observed at the glomerular, external plexiform andmitral cell layers. With odor exposure, CBVw fMRI is more sensitive and reproducible than BOLD. BOLD fMRI had the greatest activation on the bulb surface, midline, olfactory nerve and glomerular layers, while CBVw activation peaked in glomerular and external plexiform layers, but was still significant in mitral cell layer. Negative BOLD responses were observed in the bulb midline and near large blood vessels. CBVw laminar profiles are similar to the layer-dependent metabolic changes to the same odor exposure reported by previous glucosemetabolismstudies. Unique activation patterns for two different odor conditionswere also differentiatedwith CBVwfMRI. Our study suggests that CBVwactivation better represents the spatial location of metabolic activity in the olfactory bulb than BOLD. © 2014 Elsevier Inc. All rights reserved.