The probiotic Bifidobacterium infantis 35624 displays visceral antinociceptive effects in the rat D. P MCKERNAN*, P. FITZGERALD*, T. G. DINAN*,  & J. F. CRYAN*, à,§ *Laboratory of Neurogastroenterology, Alimentary Pharmabiotic Centre, University College Cork, Cork, Ireland  Department of Psychiatry, University College Cork, Cork, Ireland àDepartment of Pharmacology & Therapeutics, University College Cork, Cork, Ireland §School of Pharmacy, University College Cork, Cork, Ireland Abstract Background Irritable bowel syndrome (IBS) is charac- terized by recurrent abdominal pain and altering bowel habit with a high percentage of patients displaying comorbid anxiety. Growing clinical and preclinical evidence suggests that probiotic agents may restore the altered brain–gut communication in IBS. In this study, we evaluated the efficacy of repeated treatment with three different probiotics in reducing visceral pain in visceral normosensitive (Sprague–Dawley [SD]) and visceral hypersensitive (Wistar–Kyoto [WKY]) rat strains. Methods Following 14 days oral gavage of Lactobacillus salivarius UCC118, Bifidobacterium infantis 35624, or Bifidobacterium breve UCC2003 both SD and WKY rats were exposed to a novel stress, the open field arena and their behavior was recorded. Subsequently, the effects of probiotics on visceral nociceptive responses were analyzed by recording pain behaviors during colorectal distension (CRD). Key Results It was found that there was a difference in the open field behavior between strains but none of the probiotic treatment altered behavior within each strain. Interestingly, the probiotic B. infantis 35624 but not others tested significantly reduced CRD-induced visceral pain behaviors in both rat strains. It significantly increased the threshold pressure of the first pain behavior and also reduced the total number pain behaviors during CRD. Conclusions & Inferences These data confirm that probiotics such as B. infantis 35624 are effective in reducing visceral pain and may be effective in treating certain symptoms of IBS. Keywords brain–gut axis, IBS, probiotic, stress, visceral hypersensitivity. INTRODUCTION Irritable bowel syndrome (IBS) is a functional disorder of the gastrointestinal tract characterized by recurrent abdominal pain or discomfort at least 3 days month )1 for the past 3 months, associated with improvement of symptoms with defecation, onset associated with a change in the frequency and form/appearance of stool. 1 Irritable bowel syndrome is one of the most common disorders seen by gastroenterologists and it is esti- mated to have a prevalence of 10–15% worldwide. 2 Chronic stress and anxiety, immune activation, gut microbiota, and visceral hypersensitivity are thought to be major contributors to some of the symptoms of IBS. 3 Chronic stress is known to activate the hypotha- lamic–pituitary–adrenal (HPA) axis leading to the release of corticotropin releasing hormone, adrenocor- ticotropic hormone, and cortisol. 4 Moreover, IBS patients were reported to show dysregulation of the HPA axis. 5 The microbiota is known to play numerous roles in the maintenance of gut homeostasis from metabolic activities, vitamin production, immune stimulation, satiety signaling, and pain signaling. 6–8 Disruption of these organisms may lead to alterations in gut homeo- stasis and thus result in disease. 9 Of note, it has been reported that IBS patients display changes in the number and species of the gut microbiota compared with healthy controls and that these alterations were dependent on the subgroup classification of the IBS patients. 10 It is plausible that such disruptions in microbiota may have implications for pain signaling and result in the lowering of the visceral pain threshold observed in IBS patients. Commensal bacteria have already been shown to be important in developing inflammatory hypernocicep- tion. 11 In addition, a Lactobacillus probiotic species has Address for Correspondence John F. Cryan, Laboratory of Neurogastroenterology, Alimentary Pharmabiotic Centre, University College Cork, Cork, Ireland. Tel.: +353 21 4901676; fax: +353 21 4901656; e-mail: j.cryan@ucc.ie Received: 20 January 2010 Accepted for publication: 14 April 2010 Neurogastroenterol Motil (2010) 22, 1029–e268 doi: 10.1111/j.1365-2982.2010.01520.x Ó 2010 Blackwell Publishing Ltd 1029