Cerebral odour treatment revealed by functional NMR imaging in awake rats

A comprehensive functional image of olfactory structures activated by odours has not hitherto been produced in animal models. The main difficulty comes from the image acquisition, which requires the subject to remain immobile. The animal must be anaesthetised or accustomed to restraint. Anaesthesia depresses brain function, while restraint generates stress that contaminates the images.

NMR imaging (MRI) enhanced by manganese (MEMRI) is a way of overcoming these difficulties. Manganese is an MRI contrast agent. It is also an analogue of calcium, sensed by the activated neurons and transported by them to the synapse where it is released. It is a marker of both anatomical and functional neural networks. The cellular mechanisms that excrete calcium excrete manganese very slowly, which ensures a prolonged MRI signal.

To study the sense of smell, manganese neurotoxicity has essentially restricted the use of MEMRI to tracing only anatomical neural pathways. Our research teams are the first to have used MEMRI for functional purposes in the deeper areas of the rat brain and in conditions close to natural. After administration of manganese, olfactory stimulation was performed on the conscious animal, the residual magnetism of manganese in neurons then allowing the acquisition of images reflecting brain activation.

By this method, we have shown that different biologically important smells activate the primary olfactory cortex. MEMRI appears to be an appropriate technique for functional imaging of sensations in small laboratory animals.

We plan now to improve manganese administration conditions and to use the new high field magnet at the INRA site at Theix. This will allow a more detailed quantitative analysis of the activation of the olfactory system and the study of its functional plasticity.

Partners

This work was conducted in partnership with the Neurobiology of Olfaction unit of INRA at Jouy-en-Josas (Phase department) as part of a thesis in co-supervision with these two units and co-financed by the ANR AROMALIM project and the INRA research departments Cepia, Phase and AlimH.