Oral treatment with Lactobacillus reuteri attenuates depressive-like behaviors and serotonin metabolism alterations induced by chronic social defeat stress

Abstract

Background

Alterations in bidirectional gut–brain interactions are believed to be involved in the pathogenesis of neuropsychiatric diseases. Considering the putative connections among gut microbiota, neural function, and behavior, this study investigated the potential of microbe-induced gut–to–brain signaling to modulate the impact of stress on depressive-like behaviors and serotonin metabolism.

Methods

Depression-susceptible mice induced by chronic social defeat stress received oral treatment of either Lactobacillus reuteri 3 (L. reuteri 3) or vehicle for 28 days, and alterations in behavior and serotonin metabolism were assessed. 16S rRNA sequencing and gas chromatograph were employed to analyze the gut microbiota community and short-chain fatty acids (SCFAs).

Results

Treatment with L. reuteri 3 ameliorated depressive-like behaviors, suppressed the increase in the relative abundances of Clostridiales and Adlercreutzia, improved the decrease in abundances of Lactobacillus, Allobaculum, and Sutterella induced by stress, and significantly increased the proportion of Bifidobacterium. L. reuteri 3 reduced the acetate and total SCFAs levels in the depression group. Blood and colon 5-HT were decreased in depressive-like mice but were significantly ameliorated after L. reuteri 3 treatment. Moreover, L. reuteri 3 administration increased the expression of enzymes involved in serotonin biosynthesis but suppressed that of the enzymes involved in tryptophan metabolism along the kynurenine pathway in colon and prefrontal cortex.

Conclusions

Despite the complexity of the gut microbiota, exposure to a single microbial strain L. reuteri 3 can protect against depressive-like behaviors induced by chronic social defeat stress. The anti-depressive effects of L. reuteri 3 were associated with improved gut microbiota and serotonin metabolism.

Introduction

Depression is a devastating disorder, afflicting up to 300 million global people of all ages and representing one of the leading causes of disability worldwide (Global Burden of Disease Study, 2013 Collaborators; The United Nations World Health Organization stated in a February 2017 Fact Sheet on Depression). Although effective treatments are available, approximately one third of all patients with depression fail to respond to conventional antidepressant therapies (Rush et al., 2006), further contributing to the global burden of the disease. The last decade has witnessed a growing interest in the potential contribution of microbiota–gut–brain axis signaling to psychiatric disorders (Burokas et al., 2017; Cryan et al., 2019; Fung et al., 2017; Kelly et al., 2016; Scriven et al., 2018), although precise biological mechanisms remain unclear. Mice lacking a normal gut flora display reduced basal and stress-induced behavioral impairments associated with anxiety and depression. These phenotypes are restored following intestinal colonization (Diaz Heijtz et al., 2011). Furthermore, patients with major depressive disorder (MDD) harbor a microbiota distinct from that of healthy controls. Fecal transfer of the MDD microbiota to gut flora mice resulted in depression-like behaviors, including increased immobility time during the forced swim and reduced center distance traveled in the open-field test (Zheng et al., 2016). Similarly, the rats display increased immobility during the forced swim test and elevated pro-inflammatory cytokines in the maternal separation model of depression. The cytokine levels are restored to normal following treatment with the probiotic Bifidobacterium infantis (Desbonnet et al., 2010).

The pathways that mediate microbiota–gut–brain interactions in stress-associated depression involve the modulation of neuroimmunity (Bharwani et al., 2017), host metabolism (Zheng et al., 2016), neurotransmitter systems (Zhu et al., 2010), and vagal nerve (Bravo et al., 2011). Given the pivotal importance of neurotransmission to mood regulation, the impact of gut microbiota on the monoamine serotonin and on the excitatory amino acid glutamate was considered (Zhu et al., 2010). Serotonin (5-hydroxytryptamine, 5-HT) synthesis is reduced in patients with MDD. The increased 5-HT content in synaptic cleft is one of the most common targets in depression treatment (Marije aan het Rot et al., 2009). Liang et al. (2015) indicated that an antidepressant effect of L. helveticus NS8 in rats subjected to chronic restraint stress was due to restored hippocampal 5-HT and norepinephrine levels. More than 90% of 5-HT content in the body is synthesized in the gut by enterochromaffin cells (ECs), mucosal mast cells, and myenteric neurons (Gershon and Tack, 2007) and then distributed to various body sites. However, molecular mechanisms underlying the control for gut 5-HT metabolism remain unclear. Recent study demonstrated that the indigenous spore-forming bacteria (Sp) plays a critical role in regulating host 5-HT by direct metabolic (a-tocopherol, tyramine, butyrate and para aminobenzoic acid) signaling to promote 5-HT biosynthesis from colonic ECs (Yano et al., 2015). In addition, the human gut microbiota-induced 5-HT production in the colon of Germ-free (GF) mice is induced by microbiota-derived short-chain fatty acids (SCFAs), which stimulate EC cells to increase the transcription of TPH1 (tryptophan hydroxylase 1) mRNA (Reigstad et al., 2014).

This study aimed to investigate the role of microbe-induced gut–to–brain signal on depressive-like behaviors and serotonin metabolism alterations induced by chronic exposure to a psychosocial stressor. An animal model of susceptible mice to chronic social defeat stress (Aubry et al., 2019; Golden et al., 2011) was used to test whether the oral administration of a bacterium with immunomodulatory properties could modulate psychosocial stress-induced depressive-like behaviors, gut microbiota alteration, and serotonin metabolism. Lactobacillus reuteri 3 (L. reuteri 3) was isolated from the Peyer's patches, a well-characterized gut-associated lymphoid tissue and the entry site for luminal antigens (Kiyono et al., 2008). L. reuteri 3 was selected as a test organism because it showed anti-inflammatory in vitro when isolated from normal chow-diet mice and lacking-in-obesity mice (Sun et al., 2016). Furthermore, treatment with this strain decreases the pro-inflammatory cytokine levels and increases the ambulatory activity compared with the obese mice induced by high-fat diet (Qiao et al., 2015). The psychosocial stress-induced disruptions in the microbiota were examined, and whether the administration of a single bacterial strain can facilitate recovery of the dysbiosis community was investigated.