Visceral hyposensitivity induced by congenitally elevated gut permeability turns to hypersensitivity after stress in CA-MLCK mice

Background: Elevated gut permeability is linked to visceral hypersensitivity in patients with IBS. Using transgenic mice that express constitutively-active myosin light chain kinase (CA-MLCK) specifically within the intestinal epithelium in this way having gut hyperpermeability, we aimed to evaluate the pattern of visceral sensitivity and to determine putative mechanisms under basal and stress conditions.

Materials and Methods: Adult CA-MLCK transgenic mice or WT littermates were used for experiments. To reveal gut permeability we used in vivo gavage method with fluorescein (400 Da). To measure visceral sensitivity, electrodes were inserted into external oblique muscle of the abdomen. Mice underwent 1-h water avoidance stress (WAS) sessions for 4 consecutive days. Colorectal distension (CRD) was performed and abdominal muscle cramps measured by EMG as an index of visceral pain. In separate experiments mice were treated with naloxone in basal condition, in other series WAS stressed animals received doxantrazole before each stress session.

Results: CA-MLCK mice showed elevated gut permeability. In basal condition, CA-MLCK mice exhibited a decreased visceral sensitivity to CRD. This hyposensitivity was abolished by naloxone. QPCR analysis revealed significantly decreased level of neuronal NO synthase (nNOS) in the colon. After WAS, CA-MLCK mice had a significantly increased visceral sensitivity to CRD, relative to WT. Both strains had similarly elevated nNOS levels upon WAS. Doxantrazole pretreatment abolished WAS-induced visceral hypersensitivity in WT animals, while CA-MLCK mice sensitivity was not restored at their unstressed level, but lowered at the level of WT mice.

Conclusions: In CA-MLCK mice, congenitally elevated gut permeability was linked to basal visceral hyposensitivity coupled with a hyperactivation of the descending endogenous analgesic system and decreased colonic nNOS levels. Conversely, they showed the same visceromotor profile than WT upon stress, evidencing a greater difference in the response to painful visceral stimuli. This was correlated with nNOS expression level and involved mast cells. These data indicate that elevated gut permeability just followed by stress can trigger visceral hypersensitivity. Based on our results CA-MLCK mice represent a valuable model to study IBS pathophysiology.