Intestinal motility and the irritable bowel

Increase in neurokinin-1 receptor-mediated colonic motor response in a rat model of irritable bowel syndrome

AIM: Irritable bowel syndrome (IBS) is a functional bowel disorder. Its major symptom is bowel dysmotility, yet the mechanism of the symptom is poorly understood. Since the neurokinin-1 receptor (NK₁R)-mediated signaling in the gut is important in the control of normal bowel motor function, we aimed to investigate whether the NK₁R-mediated bowel motor function was altered in IBS, using a rat IBS model that was previously reported to show colonic dysmotility in response to restraint stress.

METHODS: IBS symptoms were produced in male Sprague-Dawley rats by inducing colitis with acetic acid. Rats were left to recover from colitis for 6 d, and used for experiments 7 d post-induction of colitis. Motor activities of distal colon were recorded in vitro.

RESULTS: The contractile sensitivity of isolated colon to a NK₁R agonist [Sar⁹,Met(O₂)¹¹]-substance P (1-30 nmol/L) was higher in IBS rats than that in normal rats. After the enteric neurotransmission was blocked by tetrodotoxin (TTX, 1 μmol/L), the contractile sensitivity to the NK₁R agonist was increased in normal colon but not in IBS rat colon. The NK₁R agonist-induced contraction was not different between the two groups when the agonist was challenged to the TTX-treated colon or the isolated colonic myocytes. A nitric oxide synthase inhibitor Nω-nitro-L-arginine methyl ester (L-NAME, 100 μmol/L) augmented the NK₁R agonist-induced contraction only in normal rat colon.

CONCLUSION: These results suggest that the NK₁R-meidated colonic motor response is increased in IBS rats, due to the decrease in the nitrergic inhibitory neural component.

Re-adaptation of the colonic mucosa following protracted stress in rats

BACKGROUND

We previously found that DNA synthesis of the colonic mucosa rises under protracted physical stress. At 8 stress weeks DNA values regained those of untreated animals, and was regarded as DNA adaptation to stress.

AIMS

To assess whether the 8 weeks' DNA adaptation to stress could be maintained following stress-free intervals, ending with a stress stimulus.

METHODS

Sixty rats were transported to the stress laboratory. Thirty were water plunged and 30 sham handled once a day, 5 days a week, for 8 weeks. After 8 stress weeks, groups of five animals were allowed to rest for 1, 2, 3 and 4 weeks, ending with single water plunging or sham handling. Five additional rats were untreated. All 65 animals received an i.p. injection of 3H-thymidine before sacrifice. The ratio radioactive DNA/total DNA reflected the DNA synthesis of the colonic mucosa.

RESULTS

Following DNA adaptation at 8 stress weeks, DNA replication in the right colon of the stressed rats significantly increased at 9, 10 and 11 weeks (P < 0.05), but at 12 weeks it had decreased to 8-week values.

CONCLUSIONS

DNA replication in the right colon of the stressed rats did not prevail: it increased considerably in the following weeks, but at 12 weeks (only 4 weeks later), re-adaptation to DNA synthesis had occurred. The review of the literature indicates that this is the first report in which, following adaptation to protracted stress, a re-adaptation phase to DNA replication is demonstrated in the colonic mucosa of experimental animals.

In vitro excitatory actions of corticotropin-releasing factor on rat colonic motility

Corticotropin-releasing factor (CRF) has been shown to affect gastrointestinal functions, however, a direct effect of CRF on the intestine has not been demonstrated. To determine the direct effect of CRF and its antagonist alpha-helical-CRF9-41 (alpha-h-CRF) on the enteric nervous system, we studied the action of these substances on electrical and mechanical parameters of peristaltic activity on isolated distal colon of the rat. The effects of CRF were evaluated in vitro on rat isolated colonic segments in which intraluminal pressure, longitudinal displacement, ejected fluid volume and extracellular electrical activity were simultaneously recorded during colonic peristaltic reflex. The addition of CRF (10(-10) - 10(-8) M) to the bath fluid provoked a concentration-dependent increase of both mechanical and electrical peristaltic activity. The CRF-receptor antagonist alpha-h-CRF dose-dependently (10(-10) - 10(-7) M) induced a decrease of the colonic mechanical and electrical activity and prevented (10(-8) - 10(-6) M) CRF (10(-8) M) maximal effects. These results indicate: (a) CRF can exert its effects on colon functions by a direct action, (b) a specific CRF-receptor is present in the rat colon. Indeed, CRF effects are antagonized by the specific CRF antagonist alpha-h-CRF, (c) the fact the alpha-h-CRF displays an activity on its own reveals that colonic functions are controlled by an endogenous CRF tonic activity.

A model to evaluate acute and chronic stress in the colonic mucosa of rats

The effect of acute and chronic stress on the colonic mucosa of the rat was investigated at various time intervals, ranging from one day to eight weeks. The amount of DNA synthesized by the mucosa was used as a marker of time-related events. A total of 75 Sprague-Dawley rats were investigated. Acute stress was considered for rats briefly plunged (psychic stimulus) or swimming for two hours (physical stimulus). Chronic stress was determined when rats were briefly plunged or swam for two hours for periods ranging from one to eight weeks. "Sham-transported" rats were used as controls. DNA in the descending colon decreased significantly in rats swimming for two weeks, but increased (even for plunged rats) at four and eight weeks. In the ascending colon, a substantial increase in DNA content was found in rats plunged or swimming for eight weeks. The descending colon appears to be quantitatively more affected by various stressors than the ascending colon. It is apparent that, in control rats, the mucosa of the descending colon differs from the ascending colon. Fluctuations in the colonic DNA synthesis throughout the experiment suggest that this phenomenon may be connected to compensatory mechanisms toward cell adaptation to stress conditions. The model may prove of value in studies of the therapeutic abrogation of the fluctuations of the DNA replication of the colonic mucosa during the acute and chronic phases of a given stress.