While an imbalance of the gut microbial community (induced by stress or diet) can lead to inflammatory processes and activation of the HPA axis, experimental work points to potential beneficial roles of probiotics, including lactobacilli and bifidobacteria in this process (Table 1).

Table 1 Selection of probiotics with reported preclinical effects on the neuroendocrine system Full size table

A recent study showed that the probiotic Bifidobacterium pseudocatenulatum CECT 7765, administered from postnatal day 2 (P2) until P21, modified maternal separation (MS) stress-induced neuroendocrine alterations [72]. When tested at P21, B. pseudocatenulatum did not attenuate MS stress-induced increases in corticosterone levels, whereas stress-induced increases in catecholamines in the hypothalamus and small intestine were attenuated by the probiotic. However, at P41 B. pseudocatenulatum completely abolished stress-induced increases of corticosterone in stool and catecholamines in the hypothalamus, as well as anxiety-like behavior [72]. The potential involvement of immunomodulatory effects of B. pseudocatenulatum is suggested by a marked decrease of basal and stress-induced interferon (IFN)-γ levels in the small intestine, as well as of stress-induced interleukin (IL)-18 levels in the serum of probiotic-treated mice. In parallel to these immunomodulatory effects the probiotic prevented MS-induced increases of operational taxonomic units belonging mainly to the Bacteroidetes phylum [76].

Further studies by the same laboratory demonstrated that B. pseudocatenulatum CECT 7765 is able to attenuate various neuroendocrine alterations in response to administration of a HFD to adult male mice [73]. Again, the probiotic normalized HFD-induced fecal corticosterone levels (basal and in response to an acute stressor) and, in addition, reversed the tendency of HFD to decrease hippocampal GR levels. Moreover, B. pseudocatenulatum exerted anti-inflammatory effects and blunted HFD-induced behavioral and metabolic disturbances (Table 1) [73]. Finally, the probiotic affected DA, NE, epinephrine, and 5-hydroxytryptamine (5-HT) levels in the small intestine and brain. While the precise pathways of these probiotic effects have not been delineated, the data clearly show that B. pseudocatenulatum is able to ameliorate the effects of HFD-induced obesity, likely through anti-inflammatory actions and modulation of the neuroendocrine system.

While the studies referred to above point to the ability of certain strains of Bifidobacterium to affect the neuroendocrine system, other groups using different experimental protocols and different strains of Bifidobacterium did not see changes in corticosterone, although other parameters were affected. Thus, Bifidobacterium infantis 35624, given to maternally separated rats for 6 weeks from P50 to the day of sacrifice (P95), did not change baseline corticosterone levels, whereas stress-induced levels were not assessed [74]. However, B. infantis normalized MS-induced depression-like behavior, displayed certain anti-inflammatory effects, and affected central NE and 5-hydroxyindole acetic acid levels, the main metabolite of 5-HT [74]. Another study, conducted by the same group compared the effects of Bifidobacterium longum 1714 and Bifidobacterium breve 1205 in adult male BALB/c mice [75]. Again, while both probiotics displayed varying anxiolytic and antidepressant-like effects, they did not affect baseline or stress-induced corticosterone levels [75]. From these latter studies it could therefore be concluded that certain strains of Bifidobacterium exert particular behavioral effects that are not dependent on the neuroendocrine system. In contrast to these findings, the combination of Bifidobacterium animalis subsp. lactis BB-12 and Propionibacterium jensenii 702 (a probiotic isolated from dairy) administered to Wistar rats 10 days before conception until weaning increased corticosterone levels in female offspring [76].

Various groups have also investigated the central and neuroendocrine effects of different strains of Lactobacillus. In particular, the finding that chronic treatment of nonstressed adult male BALB/c mice with the probiotic Lactobacillus rhamnosus (JB-1) for 28 days is able to decrease corticosterone levels, reduce depressive-like behavior in the forced swim test, and decrease anxiety-like behavior on the elevated plus maze (EPM), attracted a lot of attention within and outside the scientific community [77]. However, these effects of L. rhamnosus (JB-1) appear to be strain- and species-dependent, given that the probiotic fails to affect the baseline social and anxiety-like behavior in male C57BL/6 mice, whereas it did attenuate stress-induced behavioral disturbances [78]. Likewise, an 8-week intake of L. rhamnosus (JB-1) by 29 healthy male subjects did not alter salivary cortisol, mood, anxiety, sleep quality, subjective stress, visuospatial memory performance, attention switching, rapid visual information processing, emotion recognition, and associated electroencephalography parameters [79].

As the behavioral effects in response to L. rhamnosus in BALB/c mice are not present in vagotimized mice, the vagus nerve is suggested to be a critical pathway of communication between probiotic signaling and the CNS [77]. Likewise, vagotomy prevented some changes in central GABA receptor expression induced by L. rhamnosus, whereas corticosterone levels in vagotomized mice were not reported [77].

In addition to these emotional-affective and neurochemical changes, lactobacilli have been repeatedly reported to exert beneficial effects in various stress protocols (Table 1). A 2-week treatment of female Wistar rats with Lactobacillus farciminis followed by partial restraint stress is able to block stress-induced increases in ACTH and corticosterone, as well as CRF in the PVN [80]. In addition, the probiotic was able to inhibit stress-induced hyperpermeability of the intestinal barrier, as well as increased LPS levels in the portal blood. Consistent with these effects, L. farciminis prevented stress-induced increases of IL-1β, IL-6, and tumor necrosis factor (TNF)-α mRNA expression in the hypothalamus [80]. In a subsequent study the same group reported that a 2-week pretreatment with a probiotic formulation containing Lactobacillus helveticus R0052 and B. longum R0175 blunts increased levels of corticosterone, NE, and epinephrine of adult male mice in response to water avoidance stress (WAS) [81]. These effects were paralleled by prevention of gut barrier impairment in response to stress, reduced neuronal activity (c-Fos expression) in stress-sensitive brain areas, including the PVN and amygdala, and an increase in hippocampal neurogenesis in control and stressed animals. In addition, the probiotic formulation affected hypothalamic genes involved in synaptic plasticity in stressed mice, increasing the expression of brain-derived neurotrophic factor (BDNF), while decreasing the expression of microglial activation markers [81]. Similarly, the probiotic combination of L. rhamnosus R0011 and L. helveticus R0052, administered to maternally separated rat pups, attenuated the increased corticosterone levels, as well as the increased permeability of the colonic barrier [82]. Furthermore, administration of Lactobacillus fermentum CECT5716 to newborn rats has been recently reported to reduce intestinal permeability in vivo under basal conditions and in response to MS or WAS [83]. The rise of corticosterone levels in response to MS or WAS was attenuated by the probiotic. Moreover, the probiotic increased IFN-γ [a marker of T helper (Th)1 response] secretion of activated splenocytes, whereas IL-4 (a marker of Th2 response) secretion was inhibited. Finally, L. fermentum increased locomotion and exploratory behavior [83].

As it has been suggested that a combination of several probiotic species may exert additive effects, a recent study investigated the interaction of a HFD (starting at 4 weeks of age) with a multispecies probiotic formulation (starting at 9 weeks of age) containing 8 bacterial strains (Bifidobacterium bifidum W23, Bifidobacterium lactis W52, Lactobacillus acidophilus W37, L. brevis W63, Lactobacillus casei W56, Lactobacillus salivarius W24, Lactococcus lactis W19, Lactococcus lactis W58) in male Sprague–Dawley rats [84]. While the probiotic formulation did not affect HFD-induced increase of plasma LPS levels and metabolic changes, the formulation exerted various diet-independent effects. Thus, probiotic treatment reduced depression-like behavior and hippocampal mRNA expression of CRF receptor (CRFR)1 and CRFR2, indicating a potentially reduced activity of the HPA axis. In keeping with other reports, the probiotic formulation modulated the cytokine production by stimulated blood mononuclear cells towards T lymphocyte-derived cytokines (IFN-γ, IL-2, IL-4), whereas macrophage-associated cytokines (TNF-α, IL-6) were reduced. Furthermore, the probiotic formulation increased the levels of indole-3-propionic acid, a microbial tryptophan metabolite that has been demonstrated to be neuroprotective and reduce CNS inflammation [85].

The findings of predominantly beneficial effects of probiotics in rodents are matched by some positive effects of different probiotic strains in humans. For instance, in a randomized, double-blind, placebo-controlled study, Lactobacillus plantarum 299v was administered to 41 students with an upcoming exam and associated with decreased corticosterone levels after 10 days [86]. Similarly, a probiotic formulation consisting of L. helveticus R0052 and B. longum R0175 (which had been effective in mice), decreased median urinary free cortisol levels and psychological distress, when given to 26 healthy volunteers for 30 days in a double-blind, controlled, randomized, parallel study [87]. Similarly, B. longum 1714 given to 22 healthy male volunteers in a placebo-controlled study attenuated increases in cortisol levels and subjective anxiety in response to an acute stressor [88]. These effects were paralleled by improvements in hippocampus-dependent visuospatial memory performance and changes in brain activity, as assessed by electroencephalography [88].

In addition to probiotics, prebiotics—non-digestible fiber compounds that stimulate the growth of beneficial bacteria—have also been reported to exert positive effects on the microbiota–gut–brain axis. A recent study compared the effects of a 3-week administration of the prebiotics fructo-oligosaccharides (FOS), galacto-oligosaccharides (GOS), and their combination in adult male mice [89]. Prebiotic administration reduced stress-induced plasma corticosterone levels, with GOS and the combination of FOS and GOS having the strongest effects and attenuated hippocampal mRNA expression of CRFR1. In line with this finding, GOS and FOS + GOS administration reduced anxiety and depression-like behavior. In particular, the combination of FOS + GOS increased the expression of BDNF and the GABA B receptor in the hippocampus and 5-HT levels in the prefrontal cortex. Prebiotics also induced changes in gut microbiota composition, with surprising decreases of the relative abundance of lactobacilli and bifidobacteria. In contrast, the cecal SCFAs acetate and propionate were increased, whereas those of isobutyrate were decreased by the prebiotics. Remarkably, the changes in SCFA levels correlated with the behavioral alterations. In addition to these baseline effects, a 3-week administration of FOS + GOS was demonstrated to blunt the effects of chronic social stress in increasing corticosterone levels and spleen cytokine production in response to stimulation with concanavalin A and in exacerbating stress-induced behavioral disturbances [89]. In line with these beneficial effects in mice, a clinical study reported that GOS, but not FOS, are able to decrease the salivary cortisol awakening response in healthy volunteers [90].

It should not go unnoticed, however, that pre- and probiotics may also have adverse effects on HPA axis and behavior. Thus, a recent study administering L. casei 54-2-33, the prebiotic inulin, or a mixture of the probiotic and prebiotic (synbiotic) to male Sprague–Dawley rats starting at P21 for 14 days, found increased baseline corticosterone levels in response to the probiotic or prebiotic alone, whereas the synbiotic attenuated stress-induced increases of corticosterone [91]. Surprisingly, all treatments had anxiogenic effects in the open-field test (OFT), whereas the synbiotic exerted anxiolytic effects in the EPM test [91].