Cytokines in the colon, central nervous system and serum of irritable bowel syndrome rats

Irritable bowel syndrome (IBS) is defined as a functional gastrointestinal disease without organic abnormalities and is characterized by chronic abdominal pain or discomfort [1]. Its incidence is about 10–20% in European and American [2], at least 6.5–10.1% in Asian and it has been continuing to grow in recent years [3]. Although IBS is one of the most frequent diagnosis in gastrointestinal diseases, the underlying mechanism of its chronic symptoms remains unknown [4]. Some factors like infection, food, and psychiatric factors have been stated as the possible etiology connected with the development of IBS [5]. It is clear that some IBS patients show remarkable inflammation and/or immunological disturbance in the colon [6]. Further studies revealed that transforming growth factor (TGF)-β1 could suppress the intestinal inflammation by modulating the growth and differentiation of different immunocytes and non-immunocytes [7]. And interleukin- (IL-)1β was reported to have the effect of regulating the initial stages of inflammation [8]. The study has shown that TGF-β1 and IL-1β are connected with inflammation, so we suspect they might play a role in the inflammation progression of IBS. Corticotropin-releasing factor (CRF) is the key regulatory factor in stress reaction. Some experiments indicated that the colon would be stimulated when injecting CRF in the cerebral ventricle or peripheral vein of rats, presented as the accelerated transmission of the colon and hasty excrementitious venting [9], which might be connected with IBS. Therefore, this study was to detect the expression of IL-1β and TGF-β1 in the colonic tissue and serum of IBS rats by the liquid chip and RT-PCR technology to find the relationship between them.

IBS is a chronic, intermittent disease characterized by abdominal pain or discomfort without specific pathophysiologic factors [11]. According to the Rome III criteria, the global prevalence of IBS ranges from 1.1 to 29.2 percent and shows a female predominance [12]. More than one mechanism contributes to the development of IBS [13]. It’s known clearly that dysregulation of the autonomic nervous system, maladjustment of the hypothalamic–pituitary–adrenal axis (HPA), increased pain sensitivity, imbalanced gut microbial population, and altered enteric physiology participate in the pathophysiology of IBS [14]. In recent years, extensive attention has also been paid to the roles of inflammation and immunity in IBS. The intestinal inflammatory response to infection has been stated as a kind of remarkable defense response [15]. A large number of T cells in different lymphoid compartments were reported in IBS patients intestine [16]. And the activated T lymphocytes can produce many cytokines such as IL-1β, IL-10, and IL-17, which were closely related to the inflammatory process [15]. Moreover, some studies revealed that the level of IL-1β was much higher in IBS-D patients [17, 18]. This may have something to do with the inflammatory response caused by them [19]. And it is consistent with our results, in our present research, the IL-1β level in the colonic mucous membrane and blood of the IBS group was significantly increased, compared to those of the control group. As we have known, IL-1β is an important pro-inflammatory factor which can cause an inflammatory reaction and destruction on the mucosal barrier, or result in constriction and inflammatory pain for its direct action on mucosa and colonic smooth muscle [17]. However, some other researches reported that the expression of IL-1β showed no difference between the IBS group and the control group. We speculate that it may be related to the regulation of different T cells and the interaction between various cytokines. This network structure is worthy of further study to verify. At present, many studies confirmed that peripheral and local cytokines may affect GI function. Actually, not only cytokines and inflammatory reactions, but also psychological factors play pivotal roles in the development of IBS [20]. Our findings have also suggested that psychological status effects on the levels of cytokines, inducing or aggravating the digestive symptoms of IBS rats. Therefore, our further research is supposed to explore whether there is a cross-factor path to regulate cytokines such as IL-1β in IBS patients.

TGF-β1 is a member of the growth factor supper family. Stadnicki et al [21] reported that the high level of TGF-β1 was detected in the muscle of pro-inflammation type IBS, which could last for at least one month and effectively improve the excitability of smooth muscle. It was thought to contribute to the inflammation in IBS [22]. On the other hand, Gonsalkorale’s research [23] showed no significant difference in the TGF-β1 level between IBS rats and the normals, which was the same as our results. This may have to do with the fact that the production of cytokines is under genetic control. The genetic susceptibility of specific cytokines with gene polymorphism may change the sensitivity to certain diseases or affect their clinical expression [23]. What’s more, it should be noticed that many points need to be considered in the explanation of research findings. TGF-β1 is just one of the cytokines relevant to the regulation of immune and inflammatory, and the possible involvement of other cytokines in the process cannot be ruled out. Accordingly, the specific role of TGF-β1 in IBS still needs further study.

CRF is a kind of neuroendocrine peptide, which distributes in the stress-related important area of the central nervous system [24]. As a key regulatory factor in the stress reaction, CRF can produce a series of biological effects, such as removing the system to consent stress excitation, accommodating endocrine, autonomic nerve, immunifaction, and regulating behavioral response [25]. Combining with its receptor, CRF could keep gastrointestinal motility in a stringent state, which accelerates colonic transmission and causes watery diarrhea [26, 27]. During colonic dynamo-actuation, the nucleus of the hypothalamus and locus caeruleus were major locus to stimulate colonic movement. When microinjection of CRF was done to the nucleus of the hypothalamus and locus caeruleus of rats, colonic movement time decurtated, and the fecal output effectiveness could increase 20 times [28]. Correspondingly, when the injection of CRF was done to the centrum of the rats, visceral pain could aggravate; whereas, peripheral injection of CRF to mankind could degrade the pain threshold of the colon and increase bowel movement. Besides, some animal experiments found that injection of CRF to the center could cause turbulence of animals, such as gripping, scratching, and so on. As the dose increases, it could lead to spastic reactions, such as the increase of defecation and water sample stool [29]. This suggested that connections existed between psycho-behavioral changes caused by CRF with bowel movement. The latest clinical researches also pointed out that 57% of IBS patients had manifested visceral hypersensitivity. Based on the above discussion, this study aimed to investigate the distribution and expression of CRF in the spinal cord and brain of the visceral hypersensitivity rats and to reveal the key factor in the process of visceral hypersensitivity conduction.

The results showed that the level of CRF was quite high in the spinal cord, hypothalamus, and the third ventricle of the cerebrum. While low and sporadic CRF positive reactant distributed in neuronic endochylema. This stated that these three places all had neuroendocrine functions for physiological functions. CRF positive reactants in the model group were close, yellow, and deeper, with a positive index number increased obviously. It was clear that the expression of CRF increased in the models, and the visceral hypersensitivity was directly proportional to it. According to previous studies, the release of CRF was connected with emotional depression, and the special smell could induce depression. Importantly, CRF was expressed obviously in the hypothalamus, which suggested that central nervous system changes also occurred in visceral hypersensitivity. CRF and locus caeruleus–arterenol system might participate in the formation of visceral hypersensitivity besides the HPA axis. So we believe that CRF released in the hypothalamus could affect intestinal movement. The intestinal sensory receptor in visceral hypersensitivity could induce nervous excitation of CRF in the hypothalamus and other correlated areas, resulting in intestinal movement as well as externalization of corporal, which could explain the relationship between IBS and CRF. The mechanism might be that the release of CRF increased under a stimulative state, in turn, the released CRF could increase the releasing frequency of locus caeruleus, then induce neurofibrae projections to descend to myelonic sympathetic neuron and cause overexpression of CRF in the spinal cord. Therefore, locus caeruleus was the transition of power of bowel movement to cause visceral hypersensitivity. Through this experiment, we thought that CRF could be considered as an important factor to study the visceral hypersensitivity conduction system. Combining with many studies performed on the mechanism of visceral hypersensitivity, we confirmed that CRF could be an original target to treat IBS. At the same time, it would play an important role in the evaluation of the curative effect as well.

This study suggested that increased CRF and IL-1β are involved in the mechanism of IBS. And CRF may be an original target in the pathogenesis of IBS. But further research need to be conducted to detect the interacting and interadjusting mechanism of IL-1β, TGF-β1, and other cytokines in IBS.