LONG-LASTING MOLECULAR ALTERATIONS FOLLOWING EARLY LIFE STRESS IN RATS: ROLE OF GUT MICROBIOTA AND INFLAMMATION

It is well known that stress, especially when experienced early in life, affects the brain developmental trajectories leading to an enhanced vulnerability for several mental disorders later in life. The adverse outcomes depend on the timing of the stressful experiences, and in this contest, early stages, such as pregnancy and adolescence, represent key sensitive periods for brain development. Several studies in animal models have been carried out to understand the mechanisms underlying the negative outcomes associated with early adverse exposures. The presence of a chronic low-grade inflammatory status has been suggested to play a central role, however, a better knowledge of the inflammatory processes, as well as of the neurodevelopmental temporal window where such alterations become manifested, is still needed. Moreover, among the biological systems involved, the gut microbiota has recently been proposed to act at the interplay between the stress response system, brain functionality and inflammatory processes, through the gut-brain-immune axis communication. To better elucidate the involvement of inflammation and the gut microbiota in the long-lasting functional effects of stress exposure, we have used the prenatal stress (PNS) paradigm, a well-established preclinical model of stress during pregnancy that causes impairments in self-care, motivation and sociability and we have investigated possible long-lasting alterations in the inflammatory mechanisms. Specifically, we found that adult animals exposed to PNS show a significant cortical activation status of the p38 MAPK, NF-kB, Neuroinflammation and Dendritic cell maturation signalling pathways. We also found that PNS causes the most pronounced alterations in inflammatory mechanisms already at adolescence, highlighting the biological vulnerability of this temporal window. As we reported adolescence as the temporal window where biological vulnerability begins to manifest, we wanted to test whether pharmacological interventions during adolescence could modulate the impact of stress early in life. Therefore, we treated adolescent animals exposed to PNS with lurasidone, and we analysed their whole genome transcriptome. Our analyses indicated that none of the pathways related to pro-inflammatory processes were present in PNS animals treated with lurasidone, whereas we reported the implication of several processes related to inositol metabolism among the top involved mechanisms. As we also aimed at investigating the role of the gut microbiota, we tested changes induced by PNS exposure in the gut microbiota composition during different temporal windows both in male and female offspring. We reported that the gut microbiota significantly evolves during ageing both in female and male rats and that restraint stress during the last week of gestation produced several short- and long-lasting changes in the gut microbiota composition with a significant modulation of bacterial taxa such as Parabacteroides, Lachnoclostridium, Eubacterium ventriosum, Eisenbergiella, Turicibacter, Lactobacillus, and Muribaculum. As we observed that stress exposure during the prenatal period affects behaviour and it is associated with alterations in inflammatory mechanisms and in gut microbiota composition, we then wanted to evaluate the consequence of stress during a different temporal window (e.g., adolescence) on the same biological outcomes. Therefore, we investigated the effects of stress exposure during adolescence, by applying a social isolation paradigm, on central inflammation and gut microbiota. 16S sequencing analysis revealed that microbial changes were influenced by age in both isolated and control rats, regardless of sex, whereas social isolation impacted the microbial composition in a sex-dependent manner. Specifically, we found that social isolation induced short-term gut microbiota alterations in females but not in males. We also identified several stress-related bacterial taxa such as Odoribacter, Akkermansia, Parabacteroides, Ruminiclostridum 5. At the central level, we found a specific inflammatory pattern in dorsal and ventral hippocampus that significantly correlated with gut microbiota composition. Overall, our findings support the notion that the negative behavioural outcomes related to early life stress exposure are associated with specific molecular signatures in inflammatory pathways and in the gut microbiota community structure and composition. The alterations become fully manifested in adolescence, pointing out this temporal window as the most sensitive and vulnerable period where interventions could be already effective. Indeed, we observed that a pharmacological treatment with lurasidone during this period moderates PNS-induced alterations in inflammatory processes, representing a possible intervention tool to restore or moderate the detrimental long-lasting effects caused by stress early in life exposure.