Introduction: Dysbiosis of the microbiome is associated with functional bowel disorders such as IBS and dyspepsia and CNS diseases such as depression, anxiety, parkinsons and autism, but how the microbiota in the external environment of the gut signal to the brain is unclear. This is important in both health and disease and is likely to occur with a healthy intact epithelial layer. Thus, a mechanism must exist for the microbiota to communicate with the CNS. In this study, we have investigated one bacterial product, indole, a breakdown product of tryptophan, which interacts with enteroendocrine L-cells to stimulate the secretion of GLP-1. This study aimed to investigate L-cells act as a signalling cell, transmitting microbial signals to the internal milieu and on to the CNS. Methods: A novel dissection technique facilitated the recording of extracellular nerve activity in mesenteric and celiac plexi and vagus nerves following mucosal stimulation of the distal colon adult male Sprague Dawley rats. Nerve activity was recorded using a bipolar electrode and the signal amplified, recorded and analysed using Chart7. Additional calcium imaging recordings were carried out in colonic submucosal plexus preparations loaded with the ratiometric calcium indicator, Fura 2-AM. Immunofluorescence images of GLP-1 receptor expression in colonic submucosal neurons were recorded using a confocal microscope. Results: When the mucosa was exposed to indole, increased GLP-1R immunostaining was evident in the submucosal ganglia. Although direct application of indole had no effect on intracellular calcium in submucosal neuronal, mucosal application of indole potentiated GLP-1-evoked calcium responses (P<0.05, n=3). Excitingly, exposure of the distal colonic mucosa to indole stimulated vagal nerve activity (n=3, P<0.05), a response that was attenuated by a GLP-1 receptor antagonist. Conclusion: Our findings provide the first tangible evidence of a signalling mechanism from the luminal bacteria in the external environment of the distal colon to the CNS. L-cells appear to be key in transmitting the microbial signal across the gut barrier to the intrinsic and extrinsic nerves including the vagus nerve.