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Mounir Bendahmane"Fasting-induced changes in the olfactory system"

Abstract :

Olfactory cues are important for many vital functions as reproduction, social interactions, food detection and feeding behavior.
However, the nutritional impact on the olfactory system has not been precisely described. During my thesis, we studied the effects of fasting on the spatiotemporal activities in the main olfactory bulb (OB), a structure supporting the first step of odor coding in the brain.

Odorant signals undergo a complex processing that starts in the Main Olfactory Epithelium where the olfactory sensitive neurons (OSN) are located. OSN project into the OB where odorant features (among them identity and concentration) evoke spatiotemporal patterns of activities located respectively at the glomerular and mitral/granule cells levels. The spatial coding at the glomerular level is classically studied by optical imaging of odor-evoked activities on the surface of the MOB in response to different odors at different concentrations. The temporal coding is characterized by specific variations in the oscillatory activity of the Local Field Potential (LFP) in deeper layers.

We tested odor responses in the OB in 17 hours-fasted rats compared to ad libitum-fed rats. We used two olfactory stimuli at different concentrations: the first one was a food odor and the second was a new and neutral odor. Using these odorants at low concentrations to probe threshold modifications, we analyzed both types of OB coding (spatial and temporal) at the levels where they occur (glomerular layer and mitral/granule cell layers respectively) in anesthetized rats. Using intrinsic optical signals imaging and local field potential recording, we observed that fasting deeply impacts the overall odor threshold detection in the OB for a food and a novel odor.

Then, we sought to identify putative molecular candidates that could trigger this olfactory plasticity in fasted conditions. We have checked the variation of receptor expression for leptin and insulin, two anorexigen hormones, in the OB but did not find any significant changes between the two groups. In addition, i.p. injection of leptin did not reverse the oscillatory profile induced in fasted rats. We concluded that leptin does not act solely in signalling the nutritional state that induces the OB plasticity and there are probably other mecanisms involved in this kind of plasticity.

We are still looking for specific modifications of OB circuits during fasting and we present interesting preliminary data about astrocytic plasticity.

Giovanni Marsicano du Neurocentre Magendie