Mapping at glomerular resolution: fMRI of rat olfactory bulb

Abstract

The rat olfactory bulb contains ϳ2000 functional units called glomeruli which are used to recognize specific characteristics of odorants. Activity localization of individual glomerulae (ϳ0.002 L) has important consequences for understanding mechanisms in olfactory information encoding. High-resolution functional MRI (fMRI) data from the rat olfactory bulb are presented using the blood oxygenation level dependent (BOLD) method at 7 T. Either individual or clusters of fMRI voxels suggestive of activity in the olfactory nerve and glomerular layers were reproducibly detected with repeated 2-min exposures of iso-amyl acetate at spatial resolution of 0.001-0.003 L. The importance of glomerular clustering for olfaction and the implications of BOLD mapping with even higher spatial resolution (i.e., Ӷ0.001 L voxels) are discussed. High-resolution in vivo mapping of the rat olfactory bulb with fMRI at high magnetic field promises to provide novel data for understanding olfaction.

Two-photon imaging of capillary blood flow in olfactory bulb glomeruli

“…Such a venular regulation of blood flow could explain the odor responses observed by using BOLD fMRI. Shulman and colleagues (18)(19)(20) reported that odors evoke BOLD signals spatially confined to the GL with an extension to the ONL. In the absence of capillaries, the BOLD signal detected in the nerve layer must relate to an increase in deoxyhemoglobin concentration in the veins downstream of activated glomeruli and not to a local increase in metabolism triggered by the activation of olfactory nerve axon bundles.…”

“…As a result, odor stimulation reproducibly activates small glomeruli ensembles. These patterns of glomerular activation have been revealed by several methods including (i) BOLD fMRI (18)(19)(20), (ii) optical imaging of calcium (21), voltage (22), and intrinsic signals (23)(24)(25)(26)(27)(28), (iii) electrophysiological recordings (29)(30)(31), and (iv) measurements of 2-deoxyglucose consumption (32,33). Here, we use two-photon imaging to determine with a high spatial resolution the relationship between RBC flow in individual capillaries and neuronal activation.…”

Discrimination between the Enantiomers of Carvone and of Terpinen-4-ol Odorants in Normal Rats and Those with Lesions of the Olfactory Bulbs

“…Other methods, including the use of voltage-sensitive dyes Spors et al, 2006), unit recording from multiple-electrode arrays (Lehmkuhle et al, 2003), functional magnetic resonance imaging (Kida et al, 2002;Pautler and Koretsky, 2002), or probes that selectively label presynaptic terminals (Bozza et al, 2004), may provide greater insight to the neural events that must code for odor detection and discrimination during the brief odor samples made by the behaving animal. .…”

What Does Local Functional Hyperemia Tell about Local Neuronal Activation?

“…This correlation is surprisingly strong and independent of odor stimulation intensity, suggesting that presynaptic Ca 2ϩ signals could be considered as solid markers of the amount of glutamate released onto the glomerular dendrites that generate LFP negativities. Because odors also trigger blood flow increases in glomeruli, a response termed glomerular functional hyperemia (Kida et al, 2002;Chaigneau et al, 2003Chaigneau et al, , 2007Petzold et al, 2008;Shepherd and Charpak, 2008), we examined whether this functional hyperemia is correlated to presynaptic Ca 2ϩ signals in ORN terminals under three conditions of neuronal activation: in the absence or presence of peripheral adaptation and below the ORN activation threshold.…”