Oxytocin ameliorates oxidative colonic inflammation by a neutrophil-dependent mechanism

Abstract

Oxytocin (OT), a nonapeptide produced in the paraventricular and the supraoptical nuclei in the hypothalamus has a wide range of effects in the body. However, the role of OT on the gastrointestinal (GI) tract has to be settled. OT may participate in the regulation of motility, secretion, blood flow, cell turnover and release of neurotransmitters and/or peptides in the GI tract, possesses antisecretory and antiulcer effects, facilitates wound healing and is involved in the modulation of immune and inflammatory processes. The present work was conducted to assess the possible therapeutic effects of OT against the acetic acid-induced colonic injury in the rat. Methods: Colitis was induced by intracolonic administration of acetic acid (5%) in Sprague–Dawley rats (200–250 g). Either saline or OT (0.5 mg/kg) was injected subcutaneously, immediately after the induction of colitis and repeated two times a day for 4 days. On the 4th day, rats were decapitated and distal 8 cm of the colon were removed for the macroscopic and microscopic damage scoring, determination of tissue wet weight index (WI), malondialdehyde (MDA) levels, an end product of lipid peroxidation; glutathione (GSH) levels, a key antioxidant; and myeloperoxidase (MPO) activity, as an indirect index of neutrophil infiltration. Colonic collagen content, as a fibrosis marker was also determined. Lactate deydrogenase (LDH) and tumor necrosis factor-alpha (TNF-α) levels were assayed in serum samples. In the acetic acid-induced colitis, macroscopic and microscopic damage scores, WI, MDA and MPO levels were significantly increased, while GSH levels were decreased when compared to control group (p < 0.05–<0.001). Treatment with OT abolished the colitis-induced elevations in damage scores, WI, MDA and MPO levels and restored the GSH levels (p < 0.05–0.001). Similarly, acetic acid increased the collagen content of colonic tissues and OT-treatment reduced this value to the level of the control group. Serum LDH and TNF-α levels were also elevated in the acetic acid-induced colitis group as compared to control group, while this increase was significantly decreased by OT treatment. The results suggest that OT, which improves the antioxidative state of the colonic tissue and ameliorates oxidative colonic injury via a neutrophil-dependent mechanism, requires further investigation as a potential therapeutic agent in colonic inflammation.

Introduction

Ulcerative colitis (UC) is an idiopathic inflammatory bowel disease (IBD) with diffuse, recurrent inflammation of the colon and rectum, which is predominantly characterized by cycles of acute inflammation, ulceration and bleeding of the colonic mucosa [46]. Among various animal models of intestinal inflammation, acetic acid-induced colitis is one of the widely used models [28], [37], [48], which uses intrarectal administration of dilute solutions of acetic acid to produce a diffuse colitis in rats, and the resulting colonic inflammation is characterized by increased leukocyte infiltration, edema and ulceration. Although the inflammation produced in this model of colitis is not identical to human ulcerative colitis, it does have some important histological and biochemical similarities to human UC [28]. The etiology of UC is poorly understood, but inflammatory mediators such as cytokines, eicosanoids, and excessive production of reactive oxygen species (ROS) by the inflamed mucosa have been proposed to contribute significantly to the development of tissue injury [5]. Activated neutrophils and macrophages are major components of active lesions in ulcerative colitis [23], [51]. Large number of neutrophils and macrophages pass out of the circulation and enter the inflamed mucosa and submucosa of the large intestine during acute inflammation, leading to over production of oxygen free radicals [25], [50], [55], which appear to be associated with the increased lipid peroxidation observed in the mucosa of ulcerative colitis in experimental animals and humans [1], [64], [65].

Oxytocin (OT), synthesized in the supraoptic and paraventricular nuclei of the hypothalamus, is a neurohypophysial nonapeptide hormone, which performs its diverse actions through the OT receptor, a G-protein coupled receptor [6]. Among the biological functions of OT are the stimulation of the uterine contractions at parturition and myoepithelial contractions in the mammary gland during suckling, vasoconstrictory or vasodilatory effects on different vascular beds [3], [16], [56] and the enhancement of atrial natriuretic peptide (ANP) release with ensuing increased excretion of sodium [24], [53]. OT and OT-receptor mRNAs are expressed throughout the GI tract [35]. Systemic administration of OT in humans has been shown to increase gastric emptying [21] and colonic motility [39]. However, OT was shown to relax the antral smooth muscle cells of the guinea pigs [13] and decrease the gastric motility in dogs and rats [34]. OT possesses an antiulcer effect in certain experimentally induced gastric and duodenal ulcers, which was attributed principally to its modest antisecretory activity [4]. Oxytocin increases the survival of ischaemic skin flaps in rats via the activation of growth factors or antiinflammatory mechanisms [40]. Recently, OT was shown to have an antiinflammatory effect in carrageenan-induced neutrophil accumulation and hyperalgesia in the hindpaw. It was proposed that oxytocin, released in high amounts during parturition and breastfeeding, serves to protect females against inflammation [41].

There is a large body of evidence indicating that stress has a prominent role in the pathophysiology and/or clinical presentation of gastrointestinal (GI) conditions [11], [30] and that chronic stress increases the severity of intestinal inflammation [20] through the stimulation of the hypothalamic–pituitary–adrenal (HPA) axis [32]. On the other hand, hypothalamic hormone OT is well known to exert potent physiological antistress effects [58], [59]. On the basis of this background, the presence of insufficient information on the OT-receptor function in the gastrointestinal tract has inspired us to study the potential protective effects of oxytocin in a rat model of colitis.