Visceral pain is a prominent cause of morbidity in gastrointestinal (GI) diseases, like inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS), but many current analgesics produce GI side effects. To develop much needed, novel treatments, further understanding of the pro-inflammatory mediators and mechanisms underpinning visceral nociception in disease states is required. In GI disease, mast cell number is elevated in the gut and activation leads to synthesis and release of many pro-inflammatory mediators, which modulate afferent function and induce hypersensitivity, and contribute to persistent pain in clinical remission. Using Ca2+ imaging of cultured DRG neurons, I first examined the ability of several mast cell mediators to activate sensory neurons. I observed that mast-cell derived cytokines (TNFα & IL-4), lipids (PGD2 & LTC4) and growth factors (NGF & SCF) all increased [Ca2+]i in distinct populations of DRG neurons, the majority of which also responded to the TRPV1 agonist capsaicin, thus indicating that these neurons were nociceptors.
TNFα levels correlate with pain in IBD and IBS, and TNFα is known to sensitise colonic sensory neurons. Somatic, TNFα-triggered thermal and mechanical hypersensitivity is mediated by TRPV1 signalling and p38 MAPK activity respectively, downstream of TNFR1 receptor activation. I therefore sought to investigate whether TNFR1-evoked p38 MAPK activity is also involved in TNFα sensitisation of colonic afferent responses to capsaicin and noxious distension of the bowel. In Ca2+ imaging of DRG neurons, TNFα sensitised neuronal capsaicin responses in a TNFR1-dependent manner, an effect attenuated by p38 MAPK inhibition. In ex vivo colonic afferent fibre recordings, pre-treatment with TNFα enhanced responses to both noxious ramp distention of the bowel and bath perfusion with capsaicin. TNFα-induced sensitisation was reversed by p38 MAPK inhibition and absent in TNFR1 knockout mice, thus demonstrating a role for TNFR1, p38 MAPK and TRPV1 in TNFα-induced sensitisation of colonic afferents.
IL-13 is also released from mast cells during inflammation and, whilst inhibition of IL-13 signalling pathways has proven effective in treating other inflammatory conditions, little is known about its role in GI disease. I therefore aimed to determine whether IL-13 could sensitise colonic afferent responses to mechanical and chemical stimuli and whether effects were reversed by inhibition of downstream p38 MAPK and JAK signalling pathways. I observed an IL-13-dependent increase in colonic afferent activity to noxious distension, but not capsaicin, which was reversed by p38 MAPK or JAK inhibition.
Overall, I have demonstrated that TNFα and IL-13-induced mechanosensitivity in colonic afferents is reversed by inhibition of p38 MAPK and/or JAK signalling pathways. This work highlights the potential utility of targeting TNFα and IL-13 signalling for treating visceral pain in GI disease.
