Interaction between olfaction and food intake: first evidence for a role of astroglial plasticity in the olfactory bulb
Résumé
Olfaction participates in the elaboration of the hedonic value of foods and play an important role in food choice and the regulation of food intake. Reciprocally, the detection of food odors is influenced by the metabolic state (hunger/satiety), thereby contributing to the initiation and termination of eating behavior. Fasting increases olfactory performances , notably by increasing neuronal activation in the olfactory bulb (OB) (Prud’homme et al, 2009), the first integration site of olfactory signals towards the central nervous system. Within the OB glomeruli, the glutamatergic synapses between olfactory sensory neurons (OSN) and mitral cells are regulated by peri-glomerular neurons and astrocytes. Astrocytes are now fully recognized as synaptic partners (Halassa & Haydon 2010) and are thought to generate some kind of “metaplasticity” in the central nervous (Min et al, 2012) as well as in the OB glomerular region (Roux et al, 2011). The neuron-astrocyte cross-talk in the glomeruli albeit poorly known, may be a central player in the metabolic regulation of olfactory response. To test the hypothesis that OB glomeruli astrocytes are involved in the metabolic sensing of the olfactory system, we have compared the expansion of the astroglial processes within the glomeruli in fed or fasted rats. We have characterized the fasting status of 5-week-old Wistar rats after 17h, 24h or 48h of food deprivation by measuring their food olfactory seeking behavior, body weight loss, glycemia and leptinemia. The astroglial deployment within the OB glomeruli was evaluated by immunohistochemical quantification of the GFAP area. We have also quantified the expression (by western-blotting) of the Y1 receptor of NPY, a major orexigenic peptide involved in food intake regulation. Our results show that 24h- and 48h-fastinged rats exhibiting strong food seeking behavior, moderate (-6.4%) to high (-18%) weight loss, low glycemia, and strongly decreased leptinemia (-82% after 48h of fasting), also have a lower GFAP deployment within the dorsal glomeruli (-30%, p<0.01) as compared to fed rats. Fasting also increases the expression of the NPY-Y1R, which is mainly located on the astrocytes, in the OB. These results show that the deployment of astroglial processes within the OB glomeruli varies according to the metabolic status of the rats. This morphological plasticity may be influenced by peptides involved in food intake regulation, such as NPY, and may participate to the adaptation of olfactory sensitivity to food intake.