We examined the effects of oral treatment of a probiotic bacteria, Lactobacillus reuteri, on the cardio‐autonomic response to colorectal distension in Sprague Dawley rats. The findings of this study were as follows:
- Noxious colorectal distension caused a decrease in heart rate in anaesthetised healthy control rats.
- Oral administration of both live and non‐viable (heat killed and gamma irradiated) Lactobacillus reuteri attenuated the cardiac and colonic afferent fibre responses to colorectal distension. This inhibitory effect on pain was also reproduced with conditioned medium.
- There were no effects of these treatments on somatic pain.
It is widely accepted that the intestinal microflora play an important role in the health of the host and also possesses immunomodulatory capacity.33,34,35
A disturbance in the balance of normal intestinal microflora, or the host response to this, has been shown to play a critical role in the pathogenesis of IBD.33,36
Non‐pathogenic bacteria may modify immune responses of the intestinal mucosa through interaction and signaling at mucosal surfaces. Probiotic bacteria offer a means of modifying the enteric microflora, and their therapeutic effects may include competitive interaction with commensal and pathogenic flora, production of antimicrobial metabolites, or influence the immune response by modulating mucosal and systemic immunity,33,34
and improving nutritional and microbial balance in the intestinal tract. However, the exact mechanisms of probiotic action are not well understood.
The most interesting finding in this study is the elimination of any cardio‐autonomic response to the highest colorectal distension pressures in normal rats after treatment with live or dead probiotic bacteria and even after treatment with conditioned medium. The existence of brain‐gut relations suggests that autonomic pathways through sensory afferent nerves are involved in informing the brain of the physiological and pathological events that occur in visceral organs. Several models of visceral perception have been demonstrated23,24,30
in animals based on the study of visceromotor or cardiovascular responses to gastrointestinal and colonic distension. Cardiovascular responses to colorectal distension reflect the intensity of sensory perception or the integrated autonomic function. In IBS patients, some investigators suggest that visceral hypersensitivity is involved in the aetiology of the disorder, while others emphasise the importance of disturbed autonomic function.38
Our results suggest that a particular probiotic, live or dead, or even products found in culture medium, can reduce the normal visceral sensitivity to colorectal distension.
There is little evidence from published research that similar effects of probiotic organisms have been seen before on the autonomic nervous system. Kamm et al
have recently shown in pigs that a probiotic yeast, Saccharomyces boulardii
seemed to decrease the number of calbindin positive myenteric neurons.39
This was the only marker of many tested which showed any change following probiotic treatment: these included calcitonin gene related peptide, nitric oxide synthase, vasoactive intestinal polypeptide, and substance P. L farciminis
treatment inhibited a delayed hypersensitivity model of colitis in rats, probably through NO release.40
Monocontamination of germ free rats with L acidophilus
or Bifidobacterium bifidum
has been shown to reduce the migrating myoelectric complex period.41
In the latter study neuropeptide Y was decreased in the blood following conventionalisation with full intestinal microflora, suggesting reduced inhibitory control of intestinal propulsion.
In our study, administration of conditioned media and non‐viable bacteria (both heat killed and gamma irradiated) inhibited the autonomic response to colorectal distension. Previous studies indicated that heat killed Lactobacillus acidophilus
retained adhesion capacity to the epithelial receptor sites.42
Furthermore, bacterial peptidoglycan, an essential component of Gram positive bacterial cell walls, is known to activate toll‐like receptors (TLR2). Indeed, the composition of lipoteichoic acid in the cell wall of a probiotic bacterium, Lactobacillus plantarum
, has been recently shown to modulate both pro‐ and anti‐inflammatory immune responses.43
Bacterial lipopolosaccharide also interacts with TLR2 and TLR4.44,45
Many commensal organisms can influence innate immune mechanisms through toll receptors. On the other hand, non‐viable gamma irradiated bacteria, but not heat killed bacteria, had an attenuating effect on experimental colitis in mice, and this immunoregulation was mediated by TLR9‐probiotic DNA motif interaction.15,46
However, because heat killed organisms were equally effective as gamma irradiated in our studies, we infer that there are differences between the mechanisms of action of probiotics on inhibition of mucosal inflammation through toll receptors and the elimination of autonomic responses to distension.
We found that oral administration of Lactobacilli
had an inhibitory effect on constitutive discharge in colonic afferent fibres in the dorsal root ganglia. Furthermore, the measurements of single fibre discharge frequency in response to colorectal distension paralleled the cardioautonomic effects observed on colorectal distension. This clearly showed that the effect was localised to the intestine or its neuronal connections to the spinal cord. We previously demonstrated that colonic distension produced a volume dependent bradycardia mediated through sympathetic afferent and cholinergic vagal efferent pathways in Sprague‐Dawley rats.47
Under anaesthesia, neither the thalamus nor the cerebral cortex are necessary for the evocation of cardioautonomic response to colorectal distension. The brainstem and hypothalamus integrate autonomic responses. We could find no effect of probiotic treatment on somatic pain in either of the standard tests employed. Taking these results on somatic pain into consideration, our results obtained with the DRG single unit discharge suggest that the observed effects were peripheral and not central (brain).
An interesting recent study has shown that another strain of Lactobacilli
) attenuated smooth muscle hypercontractility post Trichinella
This effect was heat labile and a component in the conditioned medium also had the same effect. The effect of gamma irradiated organisms was not tested. The authors concluded that the effect was likely mediated by the immune response to infection and direct effects on smooth muscle. Again Verdu et al
very recently showed that administration of L paracasei
or the spent culture medium prevented the antibiotic induced increase in visceromotor response and inflammatory activity, and effectively decreased immunostaining for substance P.49
In their model the direct effect of probiotic treatment on modulation of visceral perception by the autonomic nervous system in healthy animals was not tested. These results clearly show that another Lactobacillus
strain can modulate inflammation associated visceral hypersensitivity responses in a murine model. In view of our findings in healthy conventionally housed rats, an additional important mechanism to explain their observations might be through a more direct effect of the probiotic on a component of the nervous system. Alternatively, the involvement of visceral pain perception resulting from inflammation may have a different functional mechanism.
In conclusion, this is the first study to examine the effect of probiotic bacteria on autonomic function and visceral perception in vivo in normal animals. Oral administration of live or dead Lactobacillus reuteri and even conditioned medium showed marked inhibitory effects on the cardio‐autonomic response to colorectal distension in Sprague‐Dawley rats. These data provide supportive evidence for further explanation of the effect of probiotics on visceral pain and provide a novel mechanism of the effect of probiotics in the treatment of patients with functional bowel disorders such as IBS.