title: Layer-Specific fMRI Responses to Excitatory and Inhibitory Neuronal Activities in the Olfactory Bulb.
abstract: High-resolution functional magnetic resonance imaging (fMRI) detects localized neuronal activity via the hemodynamic response, but it is unclear whether it accurately identifies neuronal activity specific to individual layers. To address this issue, we preferentially evoked neuronal activity in superficial, middle, and deep layers of the rat olfactory bulb: the glomerular layer by odor (5% amyl acetate), the external plexiform layer by electrical stimulation of the lateral olfactory tract (LOT), and the granule cell layer by electrical stimulation of the anterior commissure (AC), respectively. Electrophysiology, laser-Doppler flowmetry of cerebral blood flow (CBF), and blood oxygenation level-dependent (BOLD) and cerebral blood volume-weighted (CBV) fMRI at 9.4 T were performed independently. We found that excitation of inhibitory granule cells by stimulating LOT and AC decreased the spontaneous multi-unit activities of excitatory mitral cells and subsequently increased CBF, CBV, and BOLD signals. Odor stimulation also increased the hemodynamic responses. Furthermore, the greatest CBV fMRI responses were discretely separated into the same layers as the evoked neuronal activities for all three stimuli, whereas BOLD was poorly localized with some exception to the poststimulus undershoot. In addition, the temporal dynamics of the fMRI responses varied depending on the stimulation pathway, even within the same layer. These results indicate that the vasculature is regulated within individual layers and CBV fMRI has a higher fidelity to the evoked neuronal activity compared with BOLD. Our findings are significant for understanding the neuronal origin and spatial specificity of hemodynamic responses, especially for the interpretation of laminar-resolution fMRI.
  
  
  CONCLUSIONS
  Functional magnetic resonance imaging (fMRI) is a noninvasive, in vivo technique widely used to map function of the entire brain, including deep structures, in animals and humans. However, it measures neuronal activity indirectly by way of the vascular response. It is currently unclear how finely the hemodynamic response is regulated within single cortical layers and whether increased inhibitory neuronal activities affect fMRI signal changes. Both laminar specificity and the neural origins of fMRI are important to interpret functional maps properly, which we investigated by activating discrete rat olfactory bulb circuits.
authors.affiliation: Departments of Radiology and.
authors.forename: Alexander John
authors.initials: AJ
authors.lastname: Poplawsky
authors.affiliation: Departments of Radiology and.
authors.forename: Mitsuhiro
authors.initials: M
authors.lastname: Fukuda
authors.affiliation: Ophthalmology, University of Pittsburgh, Pittsburgh, Pennsylvania 15203.
authors.forename: Matthew
authors.initials: M
authors.lastname: Murphy
authors.affiliation: Departments of Radiology and Center for Neuroscience Imaging Research, Institute for Basic Science, Suwon, 440-330, Korea, and Departments of Biomedical Engineering and Biological Sciences, Sungkyunkwan University, Suwon, 440-330, Korea seonggikim@skku.edu.
authors.forename: Seong-Gi
authors.initials: SG
authors.lastname: Kim
mesh_terms: D000818:Animals; D001931:Brain Mapping; D004558:Electric Stimulation; D007091:Image Processing, Computer-Assisted; D017078:Laser-Doppler Flowmetry; D008279:Magnetic Resonance Imaging; D008297:Male; D009415:Nerve Net / Q000098:blood supply*; D009433:Neural Inhibition / Q000502:physiology*; D009474:Neurons / Q000502:physiology; D009812:Odorants; D009830:Olfactory Bulb / Q000098:blood supply*; D009833:Olfactory Pathways / Q000098:blood supply*; D010100:Oxygen / Q000097:blood; D051381:Rats; D017207:Rats, Sprague-Dawley; D013997:Time Factors
chemical_list: D010100:Oxygen
keywords: BOLD fMRI; GABA; cerebral blood flow; cerebral blood volume fMRI; hemodynamic response