Ruolo dei neuroni dopaminergici nel processamento di stimoli olfattivi salienti

Selective mating is a potent driver of evolution and optimizing the chances to obtain vigorous offspring is a priority especially for female individuals, that have fewer gametes than their male counterparts. Urine contains signalling peptides and proteins that act as chemosignals in mice, like major histocompatibility complex (MHC) and major urinary protein complex (MUP) and previous studies demonstrated that female mice preference for partners can be linked to the perception of a larger dissimilarity of their father's and their partner's urinary chemosignals through smell. This ability to smell kinship is imprinted in the early stages of life and it is heavily affected in mice whose olfactory system is impaired. Classically, the main olfactory system (MOS) was believed to integrate non-social olfactory signals, while accessory olfactory system was thought to be responsible for processing the ethologically salient cues. Nowadays, the perception of ethologically relevant odorants in macrosmatic mammals has been attributed to both systems. The dopaminergic (DA) cells of the MOB constitute the most common source of juxtaglomerular connection in the adult mouse brain, with a proposed role in the sharpening of olfactory information and odour discrimination. They are located in the glomerular layer of the MOB, where they inhibit synaptic transmission between olfactory sensory neurons and mitral/tufted cells by co-releasing dopamine and GABA.They are an intriguing population under different points of view. First, they express tyrosine hydroxylase (TH) - the rate-limiting enzyme for dopamine synthesis – in an activity-dependent way. Moreover, they can be divided in at least two different cell subpopulations based on their time of origin, morphology and biophysical properties. Accordingly, the majority of DA cells are small in size and they are constantly regenerated lifelong, while we can distinguish a minor subpopulation represented by cells displaying a bigger soma and an overall greater excitability. Importantly, the exact role of these cells has not been ruled out yet, their involvement in processing olfactory information mediating mate choice is still unknown. In this study we investigated how each of the two OB dopaminergic subpopulations processes ethologically relevant odors to understand how salient cues, specifically sexual odors contained in urine of the opposite sex, are represented in the brain. With whole-brain labelling of c-fos and light-sheet microscopy, we showed that short and acute exposure to unfamiliar opposite sex odorants increases neuronal activation in the most dorsal-posterior area of the female olfactory bulb, and that TH cells of the glomerular layer are more activated by unfamiliar scents respect to the controls, in a likely cell-specific way. Moreover, we started to dissect how the two different subpopulations respond to salient cues, with the larger cells showing a higher basal activation than the smaller ones in the same region. To further analyze the in vivo physiological response of DA cells to ethologically relevant stimuli, we validated a Cre-lox based method to specifically label dopaminergic cells in the mouse MOB for 2-photon in vivo calcium imaging. This way, we aim to further dissect the coding of salient olfactory information in the brain and unravel the role of glomerular dopamine in the context of reproduction.