Search

Key points: The vagus nerve has been implicated in mediating behavioural effects of the gut microbiota on the central nervous system. This study examined whether the secretory products of commensal gut bacteria can modulate the excitability of vagal afferent neurons with cell bodies in nodose ganglia.Cysteine proteases from commensal bacteria increased the excitability of vagal afferent neurons via activation of protease‐activated receptor 2 and modulation of the voltage dependence of Na+ conductance activation.Lipopolysaccharide, a component of the cell wall of gram‐negative bacteria, increased the excitability of nodose ganglia neurons via TLR4‐dependent activation of nuclear factor kappa B.Our study identified potential mechanisms by which gut microbiota influences the activity of vagal afferent pathways, which may in turn impact on autonomic reflexes and behaviour. Behavioural studies have implicated vagal afferent neurons as an important component of the microbiota‐gut‐brain axis. However, the mechanisms underlying the ability of the gut microbiota to affect vagal afferent pathways are unclear. We examined the effect of supernatant from a community of 33 commensal gastrointestinal bacterial derived from a healthy human donor (microbial ecosystem therapeutics; MET‐1) on the excitability of mouse vagal afferent neurons. Perforated patch clamp electrophysiology was used to measure the excitability of dissociated nodose ganglion (NG) neurons. NG neuronal excitability was assayed by measuring the amount of current required to elicit an action potential, the rheobase. MET‐1 supernatant increased the excitability of NG neurons by hyperpolarizing the voltage dependence of activation of Na+ conductance. The increase in excitability elicited by MET‐1 supernatant was blocked by the cysteine protease inhibitor E‐64 (30 nm). The protease activated receptor‐2 (PAR2) antagonist (GB 83, 10 μm) also blocked the effect of MET‐1 supernatant on NG neurons. Supernatant from Lactobacillus paracasei 6MRS, a component of MET‐1, recapitulated the effect of MET‐1 supernatant on NG neurons. Lastly, we compared the effects of MET‐1 supernatant and lipopolysaccharide (LPS) from Escherichia coli 05:B5 on NG neuron excitability. LPS increased the excitability of NG neurons in a toll‐like receptor 4 (TLR4)‐dependent and PAR2‐independent manner, whereas the excitatory effects of MET‐1 supernatant were independent of TLR4 activation. Together, our findings suggest that cysteine proteases from commensal bacteria increase the excitability of vagal afferent neurons by the activation of PAR2. Key points: The vagus nerve has been implicated in mediating behavioural effects of the gut microbiota on the central nervous system. This study examined whether the secretory products of commensal gut bacteria can modulate the excitability of vagal afferent neurons with cell bodies in nodose ganglia.Cysteine proteases from commensal bacteria increased the excitability of vagal afferent neurons via activation of protease‐activated receptor 2 and modulation of the voltage dependence of Na+ conductance activation.Lipopolysaccharide, a component of the cell wall of gram‐negative bacteria, increased the excitability of nodose ganglia neurons via TLR4‐dependent activation of nuclear factor kappa B.Our study identified potential mechanisms by which gut microbiota influences the activity of vagal afferent pathways, which may in turn impact on autonomic reflexes and behaviour. [ABSTRACT FROM AUTHOR]

Increased bile acids in the colon can evoke increased epithelial secretion resulting in diarrhea, but little is known about whether colonic bile acids contribute to abdominal pain. This study aimed to investigate the mechanisms underlying activation of colonic extrinsic afferent nerves and their neuronal cell bodies by a major secondary bile acid, deoxycholic acid (DCA). All experiments were performed on male C57BL/6 mice. Afferent sensitivity was evaluated using in vitro extracellular recordings from mesenteric nerves in the proximal colon (innervated by vagal and spinal afferents) and distal colon (spinal afferents only). Neuronal excitability of cultured dorsal root ganglion (DRG) and nodose ganglion (NG) neurons was examined with perforated patch clamp. Colonic 5-HT release was assessed using ELISA, and 5-HT immunoreactive enterochromaffin (EC) cells were quantified. Intraluminal DCA increased afferent nerve firing rate concentration dependently in both proximal and distal colon. This DCA-elicited increase was significantly inhibited by a 5-HT3 antagonist in the proximal colon but not in the distal colon, which may be in part due to lower 5-HT immunoreactive EC cell density and lower 5-HT levels in the distal colon following DCA stimulation. DCA increased the excitability of DRG neurons, whereas it decreased the excitability of NG neurons. DCA potentiated mechanosensitivity of high-threshold spinal afferents independent of 5-HT release. Together, this study suggests that DCA can excite colonic afferents via direct and indirect mechanisms but the predominant mechanism may differ between vagal and spinal afferents. Furthermore, DCA increased mechanosensitivity of high-threshold spinal afferents and may be a mechanism of visceral hypersensitivity.NEW & NOTEWORTHY Deoxycholic acid (DCA) directly excites spinal afferents and, to a lesser extent, indirectly via mucosal 5-HT release. DCA potentiates mechanosensitivity of high-threshold spinal afferents independent of 5-HT release. DCA increases vagal afferent firing in proximal colon via 5-HT release but directly inhibits the excitability of their cell bodies. [ABSTRACT FROM AUTHOR]