There is growing evidence that the administration of psychobiotics may have protective and ameliorative effects on behavioral disorders. The present study was conducted to provide a new insight into the effects of psychobiotics on psychological behaviors in three major brain regions that play important roles in the regulation of behaviors. According to our findings, the administration of L.plantarum and synbiotic could alleviate depression and anxiety-like behavior and enhance learning, whereas inulin intake could not significantly improve behavioral responses. However, serum and brain oxidative stress markers were significantly declined by synbiotic consumption. Also, intake of L.plantarum resulted in the decreased oxidative stress markers in the hippocampus and amygdala. In addition, hippocampal serotonin and BDNF concentrations were significantly elevated following symbiotic and L.plantarum intake. Furthermore, there are strong correlations between serum and brain parameters with behavioral responses (Figure. 5).
Administration of synbiotic resulted in ameliorated oxidative status in the serum, hippocampus, and amygdala. Furthermore, L.plantarum intake could significantly improve oxidative stress markers in the amygdala and hippocampus (Figure. 1). Oxidative stress can be derived from a variety of sources that observed even in normal conditions [31]. As mentioned earlier, oxidative stress can damage the CNS function and behavioral process [16, 17]. Recently, oxidative stress indices, known as risk factors for some diseases, appear to be prospective biomarkers for early prediction of healthy people [32]. In our previous works, we demonstrated that consumption of synbiotic, L.plantarum, and inulin in diabetic rats could improve antioxidant levels in the serum, hypothalamus, and amygdala; Antioxidant enzymes could be a potential target for the prevention of memory deterioration. Liu et al. [33] found that SOD and catalase protect cognitive functions from damages. D'souza et al. [19] showed that probiotic supplements are effective antioxidants and may be beneficial for combating the adverse effects of ROS via reducing inflammation and increasing antioxidant enzymes such as SOD and GPx. Huang et al. [34] demonstrated that L.plantarum K68 (109 CFU/ mL) intake could increase the activity of SOD, catalase, and GPx, resulting in improved hyperglycemia, IR, and hyperlipidemia in rats with insulin resistance. In contrary, Davari et al. [12] indicated that daily consumption of a mixture of probiotics (L.acidophilus, B.lactis, and L.fermentum) for 56 days could not significantly ameliorate oxidative stress markers in healthy rats. In their study, the number of probiotics (CFU/mL) for intervention was not mentioned and each rat was kept in a separate cage (one animal per cage). While in our study evry four rats were housed in a cage. Moreover, our results demonstrated that there is a positive correlation between serum and brain regions oxidative stress markers with anxiety and cognition performances (Figure.5). According to the evidence, it can be concluded that the species and strain of probiotics could beneficial effects on oxidative stress status.
In this work, we demonstrated that a significant increase in hippocampal serotonin and BDNF concentration was observed after symbiotic and L.plantarum intake. As well as, administration of symbiotic could increase PFC BDNF and amygdala serotonin concentrations (Figure. 2). BDNF and serotonin are extensively distributed in the CNS and affect various brain functions such as survival, maintenance, growth, differentiation of neurons, and finally, physiological behaviors [20, 21]. It has been shown that the levels of these parameters decrease in anxiety and depression-like behaviors and other disorders that lead to cognitive impairment [22]. In a study, it was shown that any change in the levels of neurotrophins in the hippocampus and amygdala of mice is strongly correlated with anxiety [35]. Likewise, our previous work showed a strong correlation between the increase of MDA and the reduction of the amygdala BDNF and serotonin levels in the diabetic rats [9]. Salim et al. [36] demonstrated that oxidative stress leads to a decrease in the amygdala and hippocampal BDNF concentration. Moreover, Shankaran et al. [37] indicated that induction of oxidative stress leads to depletion of brain serotonin level in the striatum and hippocampus which can be improved by vitamin E and vitamin C intake. Probably one of the mechanisms to improve the antioxidant capacity in the brain is via increased serotonin concentration; however, its clear mechanism is unknown [38]. Hence, boosting the antioxidant system could protect against brain damage as well as prevent behavioral disorders. Psychobiotics could prevent nerve oxidative damage via increasing antioxidative enzyme levels. Consistent with our finding, in a study conducted by Toldy et al. [39], nettle consumption as an antioxidant could not increase BDNF and nerve growth factor (NGF), despite reducing oxidative stress in both cerebellum and frontal lobes. Therefore, other mechanisms may also be involved with changes in the levels of nerve parameters such as BDNF.
In another work, Burokas et al. [10] reported that administration of fructo-oligosaccharides (FOS) and galacto-oligosaccharides (GOS) combination (FOS+GOS) in chronic stress could increase expression of gamma-aminobutyric acid B1 (GABAB1), GABAB2, BDNF and in the hippocampus of mice. In addition, after FOS+GOS intake, elevation of serotonin level in the PFC was observed. In their study, FOS+GOS consumption through regulated HPA activity (plasma and hypothalamic corticosterone), improved dysbiosis, and increased short-chain fatty acids (SCFAs) concentrations could ameliorate behavioral responses. In addition, the combination of the two prebiotics was more effective than their separate use. In our study, inulin was not probably able to significantly alter the gut microbial composition. On the other hand, it seems that the prebiotic intake may be more helpful in pathological conditions [8, 9].
Our results on behavior responses indicated that the synbiotic and L.plantarum consumption had significant effects on learning, memory, depression, and anxiety in the healthy rats (Figure. 3, 4). Also, a strong correlation was found between the oxidative stress markers with serotonin and BDNF concentration in different regions of the brain with behavioral tests (Figure. 5). The beneficial effects of psychobiotics on behavioral disorders have been reported in several diseases, as well [9, 40, 41]. But studies about healthy individuals or animals are confined. Allen et al. [14] demonstrated that administration of B.longum 1714 (109 CFU/mL) resulted in modulation of stress and amelioration of memory performance in addition to cortisol reduction in healthy volunteers. In another study Jeong et al. [42] indicated that, compared to the control group, elderly rats treated with L.plantarum C29 (2×109 CFU/mL) for 8 weeks had a significant increase in the expression of hippocampal BDNF and cAMP response element binding protein (CREB) genes as well as improvement in cognitive behavior via inhibiting NF-κB signaling pathway. Our findings also indicated that there is a positive correlation between memory and learning with PFC and hippocampal BDNF (Figure.5). In addition, in this study, the improvement of anxiety like-behavior was consistent with increasing levels of serotonin in the amygdala and hippocampus (Figure.5). In another research by Takada et al. [43], it was demonstrated that administration of L.casei could regulate stress in both healthy subjects and rats. They showed that L.casei intake declined levels of corticotropin-releasing factor (CRF) and cortisol in the hypothalamus of rats which probably affected the vagus nerve signaling to the brain and decreased activity of the HPA axis. Furthermore, Bravo et al. [44] indicated that L.rhamnosus (109 CFU/mL) consumption resulted in reduced anxiety (EPM) and depression (FST) as well as decreased levels of mRNA expression of GABAB1b in the hippocampus, amygdala, and locus coeruleus, compared with healthy control mice.
According to the evidence as well as our previous works [9, 14, 42, 44], several mechanisms have been proposed in relation to the effects of psychobiotics on behavioral responses through intestinal microbial changes. First, as a result of improved microbial composition, lipopolysaccharide (LPS) production decrease and immune and inflammatory responses are reduced, subsequently leading to alleviation of ROS production [43, 45]. Secondly, the regulation of HPA hyperactivity in most studies has a direct correlation with the anxiety and depression. The most important factor involved in this pathway is cortisol, directly linked to increased neuropsychological disorders [43, 46]. Third, the effect of psychobiotics on the gut- vagus nerve- brain axis which results in improved CNS neurotrophins (like BDNF) and neurotransmitters (like serotonin) [9, 47]. Although valuable findings were obtained in the present study, there were some limitations. We could not assess changes of the microbial population which could be very helpful. Our results could be more comprehensive if the microbial composition of the rats were also examined. Finally, supplementation with other probiotics and prebiotics in different doses is proposed, as varied results might be produced.