The microbiome-gut-brain axis during early life regulates the hippocampal serotonergic system in a sex-dependent manner

Abstract

Bacterial colonisation of the intestine has a major role in the post-natal development and maturation of the immune and endocrine systems. These processes are key factors underpinning central nervous system (CNS) signalling. Regulation of the microbiome-gut-brain axis is essential for maintaining homeostasis, including that of the CNS. However, there is a paucity of data pertaining to the influence of microbiome on the serotonergic system. Germ-free (GF) animals represent an effective preclinical tool to investigate such phenomena. Here we show that male GF animals have a significant elevation in the hippocampal concentration of 5-hydroxytryptamine and 5-hydroxyindoleacetic acid, its main metabolite, compared with conventionally colonised control animals. Moreover, this alteration is sex specific in contrast with the immunological and neuroendocrine effects which are evident in both sexes. Concentrations of tryptophan, the precursor of serotonin, are increased in the plasma of male GF animals, suggesting a humoral route through which the microbiota can influence CNS serotonergic neurotransmission. Interestingly, colonisation of the GF animals post weaning is insufficient to reverse the CNS neurochemical consequences in adulthood of an absent microbiota in early life despite the peripheral availability of tryptophan being restored to baseline values. In addition, reduced anxiety in GF animals is also normalised following restoration of the intestinal microbiota. These results demonstrate that CNS neurotransmission can be profoundly disturbed by the absence of a normal gut microbiota and that this aberrant neurochemical, but not behavioural, profile is resistant to restoration of a normal gut flora in later life.

Gutsy Moves: The Amygdala as a Critical Node in Microbiota to Brain Signaling

“…[88] Interestingly, there seems to be a divergence between behavioral and hormonal measures of the stress response in GF mice, which exhibit heightened corticosterone levels following exposure to a stressor despite their low-anxiety behavioral profile. [8,10] However, in agreement with the neuroendocrine profile, heightened anxietylike behavior has been observed in GF rats. [89] GF animals also exhibit increased depressive-like behavior.…”

“…It receives sensory inputs from the thalamus and cortical areas as well as extensive inputs from other regions in the limbic system, including the prefrontal cortex and hippocampus, [15] both of which are markedly altered in germ-free (GF) animals. [10,12,16] In fact, the connectivity between these three major regions, not just the function of the individual structures, is fundamental to appropriate emotional responses. [17] Although tracer studies have shown afferents reaching all amygdala nuclei, sensory inputs often reach the amygdala through the LA while inputs from other brain regions target primarily the LA and BLA.…”

“…[72][73][74][75] To this end, GF animals exhibit striking alterations in circulating tryptophan levels and serotonergic neurotransmission, at least in the hippocampus. [10,76] Similarly, alterations in the microbiota in a murine model of autism are associated with deficient tryptophan metabolism and gastrointestinal dysfunction. [77] Prebiotic and probiotic treatments have been shown to alter central levels of serotonin or serotonin receptors in the hippocampus and prefrontal cortex, both regions that are highly connected to the amygdala.…”

“…[90,96] Finally, different probiotic strains have been shown to reduce symptoms of anxiety and depression across several studies in clinical and healthy populations. [97,98] While there have also [5,9,10,88,90] Anxiolytic effects, sometimes sex-specific, in mice (light-dark box, elevated-plus maze) [72,113] Anxiolytic effects in mice (open field, defensive marble burying, elevated-plus maze, light-dark box) [91,93] Strain-specific anxiolytic effects in humans (e.g., Beck Anxiety Index, Hospital Anxiety & Depression Scale) and rodents (defensive marble burying, elevated-plus maze, open field) [11,62,133,134] Transplant from humans with depression or comorbid IBS and anxiety increases anxietylike behavior in mice (open field, step-down test, light-dark box) [96,125] Heightened anxiety in rats (open field) [89] Depression Increased depressive-like behavior in mice (forced swim) [90] Increased depressive-like behavior in rats (forced swim) [94] Antidepressant effects in mice and rats (forced swim, tail suspension, learned helplessness after inescapable shock) [91,92] Strain-specific antidepressant effects in humans (e.g. Beck Depression Inventory, Hospital Anxiety & Depression Scale) and rodents (tail suspension test, forced swim, sucrose preference) [11,97,98,133,134] Transplant from depressed human donors induces depressive-like behavior in mice (sucrose preference, forced swim [varied results], tail suspension test) [90,96] Learned fear Impaired fear recall in adult mice [101] Acute administration enhances fear extinction in rodents and exposure therapy in humans, [103,104] reduces fear recall in huma...…”

“…[7] Thus far, the use of microbial manipulations to test for brain region-specific effects has largely focused on the hippocampus and the prefrontal cortex. [8][9][10][11][12] However, we are now beginning to understand that the same microbial manipulations may also have implications for the amygdala.…”