Emerging evidence indicates that disruption of the gut microbial community (dysbiosis) impairs mental health. Germ-free mice and antibiotic-induced gut dysbiosis are two approaches to establish causality in gut microbiota-brain relationships. However, both models have limitations, as germ-free mice display alterations in blood-brain barrier and brain ultrastructure and antibiotics may act directly on the brain. We hypothesized that the concerns related to antibiotic-induced gut dysbiosis can only adequately be addressed if the effect of intragastric treatment of adult mice with multiple antibiotics on (i) gut microbial community, (ii) metabolite profile in the colon, (iii) circulating metabolites, (iv) expression of neuronal signaling molecules in distinct brain areas and (v) cognitive behavior is systematically investigated. Of the antibiotics used (ampicillin, bacitracin, meropenem, neomycin, vancomycin), ampicillin had some oral bioavailability but did not enter the brain. 16S rDNA sequencing confirmed antibiotic-induced microbial community disruption, and metabolomics revealed that gut dysbiosis was associated with depletion of bacteria-derived metabolites in the colon and alterations of lipid species and converted microbe-derived molecules in the plasma. Importantly, novel object recognition, but not spatial, memory was impaired in antibiotic-treated mice. This cognitive deficit was associated with brain region-specific changes in the expression of cognition-relevant signaling molecules, notably brain-derived neurotrophic factor, N-methyl-d-aspartate receptor subunit 2B, serotonin transporter and neuropeptide Y system. We conclude that circulating metabolites and the cerebral neuropeptide Y system play an important role in the cognitive impairment and dysregulation of cerebral signaling molecules due to antibiotic-induced gut dysbiosis.

The gut-brain axis refers to the bidirectional communication between the gut and the brain. Four information carriers (vagal and spinal afferent neurons, immune mediators such as cytokines, gut hormones and gut microbiota-derived signalling molecules) transmit information from the gut to the brain, while autonomic neurons and neuroendocrine factors carry outputs from the brain to the gut. The members of the neuropeptide Y (NPY) family of biologically active peptides, NPY, peptide YY (PYY) and pancreatic polypeptide (PP), are expressed by cell systems at distinct levels of the gut-brain axis. PYY and PP are exclusively expressed by endocrine cells of the digestive system, whereas NPY is found at all levels of the gut-brain and brain-gut axis. The major systems expressing NPY comprise enteric neurons, primary afferent neurons, several neuronal pathways throughout the brain and sympathetic neurons. In the digestive tract, NPY and PYY inhibit gastrointestinal motility and electrolyte secretion and in this way modify the input to the brain. PYY is also influenced by the intestinal microbiota, and NPY exerts, via stimulation of Y1 receptors, a proinflammatory action. Furthermore, the NPY system protects against distinct behavioural disturbances caused by peripheral immune challenge, ameliorating the acute sickness response and preventing long-term depression. At the level of the afferent system, NPY inhibits nociceptive input from the periphery to the spinal cord and brainstem. In the brain, NPY and its receptors (Y1, Y2, Y4, Y5) play important roles in regulating food intake, energy homeostasis, anxiety, mood and stress resilience. In addition, PP and PYY signal to the brain to attenuate food intake, anxiety and depression-related behaviour. These findings underscore the important role of the NPY-Y receptor system at several levels of the gut-brain axis in which NPY, PYY and PP operate both as neural and endocrine messengers.

Objectives: The biological mechanisms linking diet-related obesity and depression remain unclear. Therefore, we examined the impact of high-fat diet (HFD) on murine behaviour, intestinal microbiome, brain metabolome, neuropeptide Y (NPY) expression, and dipeptidyl peptidase-4 (DPP-4) activity. Methods: Male C57Bl/6J mice were fed an HFD (60 kJ% from fat) or control diet (12 kJ% from fat) for 8 weeks, followed by behavioural phenotyping. Caecal microbiome was analysed by 16S rDNA sequencing, brain metabolome by ¹H nuclear magnetic resonance, NPY expression by PCR and immunoassay, and dipeptidyl peptidase-4 (DPP-4) activity by enzymatic assay. The effect of a 4-week treatment with imipramine (7 mg/kg/day) and the DPP-4 inhibitor sitagliptin (50 mg/kg/day) on HFD-induced behavioural changes was also tested. Results: HFD led to a depression-like phenotype as revealed by reduced sociability and sucrose preference. In the caecum, HFD diminished the relative abundance of Bacteroidetes and increased the relative abundance of Firmicutes and Cyanobacteria. In the brain, HFD modified the metabolome of prefrontal cortex and striatum, changing the relative concentrations of molecules involved in energy metabolism (e.g. lactate) and neuronal signalling (e.g. γ-aminobutyric acid). The expression of NPY in hypothalamus and hippocampus was decreased by HFD, whereas plasma NPY and DPP-4-like activity were increased. The HFD-induced anhedonia remained unaltered by imipramine and sitagliptin. Discussion: The depression-like behaviour induced by prolonged HFD in mice is associated with distinct alterations of intestinal microbiome, brain metabolome, NPY system, and DPP-4-like activity. Importantly, the HFD-evoked behavioural disturbance remains unaltered by DPP-4 inhibition and antidepressant treatment with imipramine.