The aim of this study was to determine if changes in the commensal bacterial content and activity of resident inflammatory cells in the intestine of healthy mice alter visceral sensitivity. It was not our purpose to define the role of specific flora in modulating sensory function as this would involve performing comparative studies on germ free and colonised mice in a gnotobiotic facility.
Our results show that mice maintained under normal, but non‐sterile, conditions may exhibit a time dependent increase in visceral sensitivity which is accompanied by an increase in the activity of inflammatory cells in the intestine. These findings suggest that a small increment in inflammatory activity that is well below that seen in our models of intestinal inflammation17,26
and which is not accompanied by signs of ill health is sufficient to induce hyperalgesia. This is supported by demonstration that the time dependent increase in visceral sensitivity and myeloperoxidase activity could be reversed by dexamethasone. Dexamethasone administration has been reported to prevent hypersensitivity in dental nerves and to reduce sensory neuropeptides.27,28
Increased degradation of tachykinins released from sensory nerves by glucocorticoids may contribute to this effect.29
However, we found that instillation of dexamethasone directly into the colon of naïve mice without hypersensitivity failed to alter responses to CRD. We conclude that the antinociceptive effect of dexamethasone observed in mice with spontaneous visceral hyperalgesia was due to a primary suppressive effect on inflammatory cell activity. Taken together, these results suggest that the degree of immune activation in the gut modulates visceral perception, and is in keeping with other recent findings that the integrity of the mucosal immune system is necessary for normal visceral perception.30
As a time dependent shift in visceral sensitivity and increases in MPO activity were not observed in mice housed under similar conditions but handled using sterile techniques, we speculate that enhanced MPO and sensory state reflected low grade bacterial contamination. Support for the concept that shifts in resident intestinal bacteria may alter visceral sensitivity also comes from our demonstration that antibiotic administration to healthy mice produced visceral hyperalgesia. The antibiotics caused a decrease in Bacteroides
and enterococci and eliminated lactobacilli from the gut. Commensal bacteria are involved in maintenance of normal mucosal immune responses and inflammatory activity in the gut.11,12
We believe that antibiotic treatment altered the bacterial content of the gut, eliminating bacteria that possess counterinflammatory properties and thus favouring a proinflammatory milieu and a hyperalgesic state.
There are other explanations for the antibiotic induced changes in this study that merit discussion. Neomycin has been reported to produce increased infiltrates of macrophages in the lamina propria and cause histological changes in small bowel mucosa.31
Although we cannot rule out direct induction of low grade inflammation, a direct toxic effect is unlikely as at the dose used in this study this antibiotic combination has previously been determined to be safe and no toxicity has been reported in rodents or humans.20,21
Moreover, while direct intrarectal administration of histamine significantly increased CRD responses, no increases in CRD responses were observed after intrarectal perfusion with the antibiotic combination. We also reason that any direct toxic effects would have also been present in antibiotic treated mice receiving L paracasei
in SCM. These results suggest that an immediate toxic effect of the antibiotic combination leading to visceral hypersensitivity is unlikely. We have not tested the effect of repeated intrarectal administration of the antibiotic combination. It is possible that such a protocol would result in intestinal microbiota changes and altered visceral sensitivity.
Administration of L paracasei
in SCM prevented the antibiotic induced increase in visceral sensitivity and inflammatory activity but did not restore normal counts of lactobacillus in the gut. This is in accordance with our recent study showing that supplementation with live L paracasei
improved gut dysfunction without restoring counts of lactobacilli in the intestine.23
Moreover, in that study we showed the ability of the SCM medium to attenuate inflammation induced changes in muscle contractility following primary infection by the nematode Trichinella spiralis
Taken together with our previous results, and because L paracasei
was undetectable on day 10 after antibiotic therapy, we suggest that the beneficial effect after antibiotic therapy may be mediated by a soluble product of L paracasei
present in the SCM in which the bacteria were resuspended. Alternatively, bacterial fragments released as a consequence of antibiotic therapy may be involved, as probiotic DNA has been shown to attenuate inflammation in models of experimental colitis.32
In the present study, we showed that the antibiotic induced change in visceral sensitivity was accompanied by increased SP immunoreactivity, which was localised primarily in the myenteric and submucous plexus. It is likely that the change in sensory neurotransmitter content was secondary to the increase in inflammatory activity, as previous studies have shown increases in enteric SP following exposure of the myenteric plexus to interleukin 1β33
and in experimental colitis.34
However, we cannot exclude the possibility that bacteria directly influence neurotransmitter content. Previous studies have suggested that pathogenic microbes can upregulate SP in infected tissue.35,36
Direct communication between commensals and the enteric nervous system was suggested in the study of Hooper et al
in which expression of genes encoding enteric neural transmission differed in germ free and colonised mice.37
In addition, Kamm et al
showed changes in the localisation patterns of neuronal markers in myenteric neurones of the pig jejunum of Saccharomyces boulardi
While an oligopeptidase produced by L paracasei
has been shown to hydrolyse the Pro‐Gln, Gln‐Phe, and Phe‐Gly bonds of SP,39
we consider this an unlikely explanation for its prevention of antibiotic induced increases in SP and visceral sensitivity as this action would not account for the decrease in MPO activity.
There are several implications of our results. Firstly, the hygiene status of mice maintained in conventional facilities may influence responsiveness to colorectal distension, and this may be important in disease models, which are not associated with overt inflammation. Secondly, the notion that subtle changes in inflammatory activity in the gut alter visceral perception may have bearing on the finding that IBS symptoms are less prevalent in those patients receiving oral corticosteroids for other indications,40
and on the association between antibiotic usage and expression of IBS.5,6,41
Thirdly, these findings provide a rationale for the use of selected probiotics in the management of IBS.