Central Processing of Behaviorally Relevant Odors in the Brain of Awake Rats, as Revealed by Functional Manganese-enhanced MRI

Abstract

An overall view of olfactory structures activated by biologically relevant odors in conscious rats has not been available up to now. Manganese (Mn)-enhanced MRI (MEMRI) could be an appropriate method to provide such a view because it is based on the detection of Mn, a persistent intra-cellular functional contrast agent. Whereas MEMRI has been used for anatomical labeling of olfactory pathways, functional imaging analyses have not yet been performed beyond the olfactory bulb. Here, we have used MEMRI for the functional imaging of rat central olfactory structures and for comparing activation maps obtained with odors conveying different biological messages. The odors used were those of male fox feces, that triggers an innate fear reaction, and of chocolate-flavored cereals, that is linked to a food reward. They were used to stimulate conscious rats previously treated by intranasal instillation of Mn at a dose optimized to ensure reproducible contrast in MR images of whole brain at 4.7 T, while sparing olfactory perception. MEMRI activation maps showed Mn enhancement throughout the primary olfactory cortex. Mn enhancement elicited by fox odor differed from that elicited by deodorized air (used as a control), whereas Mn enhancement elicited by chocolate odor did not. By providing an overall image of brain structures activated in conscious rats by odorous stimulation, these results demonstrate the interest of MEMRI for functional studies of olfaction in small laboratory animals. Our results also suggest that the activation of deep brain olfactory structures depends on the biological significance of the odorous message.