Addressing the role of the olfactory dopaminergic population in processing ethologically relevant odors by using a multidisciplinary approach

Olfaction is one of the most conserved senses in animal evolution, and many species rely heavily on it for behavior, including mate choice and reproduction. In mice, the main olfactory bulb (MOB) is the first and only relay structure for odors before reaching the olfactory cortex and hosts the most numerous dopaminergic (DA) population of the forebrain. These inhibitory interneurons are typically identified by tyrosine hydroxylase (TH) expression and, notably, they co-release GABA and dopamine as neurotransmitters. At least two main subpopulations can be recognized (i.e., small and large cells) based on their soma size, in vitro and in vivo electrophysiological properties, and generation time. Although increasing evidence places the role of these cells in the early stages of sensory processing, their exact contribution in odor processing remains unknown. Moreover, while dopamine action in the brain has been associated with decoding the valence of a sensory experience, whether MOB DA cells play an important role in the processing of ethologically relevant odors with innate valence is still unclear. To verify this hypothesis, in the first part of this PhD thesis I contributed to characterize the activation patterns in response to sexual odors contained in the urine of the opposite sex in the MOB of adult C57BL/6J and wild-caught house female mice and to investigate the recruitment of OB DA cells in the same regions. I also assessed the contribution of OB DA neurons to olfactory responses related to odor novelty and familiarity. Using 3D mapping of the immediate early gene (IEG) c-Fos and light-sheet fluorescence microscopy (LSFM), the results show that, in adult female mice, brief and acute exposure to opposite-sex odorants increases neuronal activation mainly in the posterior region of the OB. In this region, I detected a selective recruitment of olfactory DA cells and a higher fraction of large DA cells compared to the small subtype. Interestingly, unfamiliar cues elicited a stronger DA response than familiar ones in adult female mice raised with their father, highlighting a sensitivity of OB DA circuits to the novelty of social olfactory stimuli. Notably, this difference in the level of DA cell recruitment was abolished in females reared without their father, suggesting that, in the absence of odor imprinting, paternal signals may be perceived as novel odors. Moreover, to further analyze the in vivo physiological response of DA interneurons to opposite-sex odors, I validated a virally delivered, time-controlled Cre-lox technique that allows to express either morphological or functional reporters in OB DA cells selectively. This method could target OB DA subpopulations enriched in either small or large cells whether it was performed in adult mice or in embryos and newborns, respectively. Finally, by expressing GCaMP indicators in OB DA cells, I performed functional in vivo 2-photon calcium imaging in adult C57BL/6J female mice to assess the contribution of DA subpopulations of the posterior OB in the processing of opposite-sex odors. Therefore, the main aims of this PhD thesis were to dissect the early coding process of salient olfactory information at brain level and assess a possible differential role for the two DA subpopulations in processing ethologically relevant odors, specifically those associated to sexual behaviors, to eventually unravel the role of dopamine in the mouse MOB regarding the reproductive behavior.