Gut microbiota interventions, including probiotic and prebiotic use can alter behavior in adult animals and healthy volunteers. However, little is known about their effects in younger individuals. To investigate this, male Sprague Dawley rats (post-natal day 21, PND21) received Lactobacillus casei 54-2-33 (10(4) cfu/ml), inulin as prebiotic (16 mg/ml), or both together (synbiotic) via drinking water for 14 days. Control rats received water alone. Open field (OF) and elevated plus maze (EPM) behaviors were evaluated at PND34 and 35, respectively. 30 min after EPM, brains and trunk blood were collected to evaluate hippocampal 5-HT1A (mRNA and protein) and plasma corticosterone (CORT). Lactobacillus, inulin and synbiotic-treated rats had fewer entries to the OF's center and spent more time in its periphery than controls. Synbiotic-fed rats explored the EPM's open arms longer than probiotic and inulin-fed rats. Synbiotic, but not Lactobacillus nor inulin-fed rats had lower levels of EPM-evoked CORT than controls. Basal CORT levels, evaluated in a naive cohort, were higher in Lactobacillus-and inulin-fed rats than controls. In naive synbiotic-fed rats, 5-HT1A mRNA levels were higher in dentate gyrus and cornus ammonis 1 layer (CA1), than in all other naive groups, while hippocampal 5-HT1A protein levels were lower in bacteria fed rats than controls. 5-HT1A mRNA changes suggest complex effects of gut microbes on hippocampal gene expression machinery, probably involving endogenous/exogenous bacteria and prebiotics interactions. Importantly, age might also influence their behavioral outcomes. Together, these data suggest that interventions in young rat microbiota evoke early behavioral changes upon stress, apparently in a hypothalamus-pituitary-adre nal axis independent fashion. (C) 2017 IBRO. Published by Elsevier Ltd. All rights reserved.