María Abellán Álvaro ¹ Anna Teruel-Sanchis ¹ Maria F. Madeira ² Enrique Lanuza ¹ Mónica Santos ² Carmen Agustín-Pavón ^1*

• ¹ University of Valencia, Valencia, Valencian Community, Spain
• ² Centre for Neuroscience and Cell Biology, University of Coimbra, Coimbra, Coimbra, Portugal

The olfactory system is a niche of continuous structural plasticity, holding postnatal proliferative neurogenesis in the olfactory bulbs and a population of immature neurons in the piriform cortex. These neurons in the piriform cortex are generated during embryonic development, retain the expression of immaturity markers such as doublecortin, and slowly mature and integrate into the olfactory circuit as the animal ages. To study how early life experiences affect this population of cortical immature neurons, we submitted mice of the C57/Bl6J strain to a protocol of maternal separation for 3 hours per day from postnatal day 3 to postnatal day 21. Control mice were continuously with their mothers. After weaning, mice were undisturbed until 6 weeks of age, when they were weighted and tested in the elevated plus-maze, a standard test for anxiety-like behaviour, to check for phenotypical effects. Mice were then perfused, and their brains processed for the immunofluorescent detection of doublecortin and the endogenous proliferation marker Ki67. We found that maternal separation induced a significant increase in the body weight of males, but not females. Further, maternally separated mice displayed increased exploratory-like behaviour (i.e. head dipping, velocity and total distance travelled in the elevated plus maze), but no significant differences in anxiety-like behaviour or corticosterone levels after behavioural testing. Finally, we observed a significant increase in the number of complex doublecortin neurons in the piriform cortex, but not in the olfactory bulbs, of mice submitted to maternal separation. Interestingly, most doublecortin neurons in the piriform cortex, but not the olfactory bulb, express the epigenetic reader MeCP2. In summary, mild early life stress results, during adolescence, in a male-specific increase in body weight, alteration of the exploratory behaviours, and an increase in doublecortin neurons in the piriform cortex, suggesting an alteration in their maturation process.