Su1992 Regulation of Serotonergic Neurotransmission and Behaviour by the Brain-Gut-Microbiome Axis

Background: The developing gut microbiome is particularly susceptible to neonatal manipulation and such interventions may have consequences that persist into adulthood. Post-natal development and maturation of the immune and endocrine systems is dependant on bacterial colonisation of the intestine in early life. These processes are key factors underpinning central nervous system (CNS) signalling. Dysregulation of the brain-gut-enteric microbiota axis is implicated in functional disorders including irritable bowel syndrome (IBS) and previous studies have demonstrated CNS serotonergic disturbances following disruption of the intestinal microflora. It is unclear at present whether such alterations may be reversed by microbiome directed therapies later in life and germ-free animals represent an ideal preclinical tool to investigate such phenomena. Aim: To compare germ-free animals with conventional and recolonised germ-free animals for concentrations of serotonin (5-HT) and its main metabolite, 5-HIAA, in the hippocampus. The availability of plasma tryptophan, the amino acid precursor of serotonin, was also assessed along with anxiety-like behaviours. Methods: Germ-free (GF), conventionally colonised (CC) and recolonised GF Swiss Webster mice were compared. High performance liquid chromatography (HPLC) was used to assess the concentrations of 5-HT and 5-HIAA in the hippocampus and tryptophan concentrations in plasma. Anxiety-like behaviours were assessed in the light-dark box. Results: There was a significant elevation in the hippocampal concentration of 5-HT (461.5 & plusmn; 24 vs 534.3 & plusmn; 11.30 ng/g tissue, p < 0.05) and 5-HIAA (289.7 & plusmn; 14.07 vs 380.7 & plusmn; 35.49 ng/g, p < 0.05) in GF animals compared to their CC counterparts. Moreover, plasma tryptophan concentrations were elevated in GF animals (15539 & plusmn; 1454 vs 21080 & plusmn; 2000 ng/ml, p < 0.05) who also displayed less anxiety-like behaviours than their CC counterparts as determined by an increased number of transitions in the light-dark box (33.60 & plusmn; 5.66 vs 56.56 & plusmn;3.920, p < 0.01). Recolonisation of the GF animals proved insufficient to reverse the hippocampal serotonergic alterations found but did restore both plasma tryptophan concentrations and anxiety-like behaviours to baseline values. Conclusion: The results demonstrate that CNS neurotransmission can be profoundly disrupted by the absence of a normal gut microbiota. Moreover we have demonstrated that the neurochemical, but not the behavioural, consequences of growing up germ-free are resistant to interventions later in life aimed at restoring a normal gut flora. These findings should inform future studies seeking to exploit the therapeutic potential of microbiome manipulation.