Background: Irritable Bowel Syndrome (IBS) is a chronic condition characterised by bouts of cramping, abdominal pain, constipation and/or diarrhoea and it afflicts up to 10-20% of the population. Symptom flares are exacerbated by factors such as stress or food intake and support is now growing for a role for immune activation in symptom manifestation. Indeed, the pro-inflammatory cytokine, interleukin (IL)-6, is elevated in IBS plasma and can activate enteric neurons leading to changes in gastrointestinal secretion. However, other plasma constituents such as the satiety hormone, leptin are also altered in IBS plasma. This may reflect a protective role of leptin in IBS or it may be important in regulating the function of IL-6. Indeed, leptin can also activate enteric neurons and alter gut function. With similar signalling pathways, these studies aim to investigate potential crosstalk between leptin and IL-6 in the pathophysiology of IBS.Methods: Whole mount preparations of submucosal and myenteric neurons were prepared from adult male Sprague Dawley rats and were loaded with the ratiometric calcium indicator, Fura 2AM (7μM). Real-time calcium imaging experiments were conducted using a standard epifluorescence imager. Neurons were also fixed, permeabilized and incubated with antibodies for immunofluorescence staining.Results: Leptin induced a significant increase in intracellular calcium in submucosal (0.146±0.02, n=34) neurons but there was no effect on myenteric neurons. Leptin attenuated IBS plasma-evoked activation in both myenteric neurons and submucosal neurons (P<0.001). Recombinant IL-6 caused a significant increase in calcium which was reduced in the presence of leptin (P<0.01, n=38) in myenteric neurons, but leptin did not significantly alter the response in submucosal neurons. In submucosal neurons, leptin-evoked responses were unaffected by the MAPK inhibitor, PD98059 but potentiated the response to IL-6 and leptin (P<0.001). In contrast, the STAT3 inhibitor, WP1006 attenuated the response to leptin (P<0.001) but also increased the response to IL-6 and leptin (P<0.05). Immunofluorescent expression of both leptin and leptin receptors co-localised with IL-6 receptors in both submucosal (64%) and myenteric (39%) neurons.Conclusions: These data provide tangible evidence of interaction between leptin and IL-6 in colonic enteric neurons, with leptin having a moderating effect on the stimulatory actions of IL-6 in myenteric but not in submucosal neurons. These data suggest a potentially protective effect of leptin in functional bowel disorders.