Studies of the mechanisms involved in the production of stress and stress-atropine ulcers in rats

Animal models of gastrointestinal inflammation and cancer

Inflammation and cancer are the two major disorders in the gastrointestinal tract. They are causally related in their pathogenesis. It is important to study animal models' causal relationship and, in particular, to discover new therapeutic agents for such diseases. There are several criteria for these models in order to make them useful in better understanding the etiology and treatment of the said diseases in humans. In this regard, animal models should be similar as possible to human diseases and also be easy to produce and reproducible and also economic to allow a continuous replication in different laboratories. In this review, we summarize the various animal models for inflammatory and cancerous disorders in the upper and lower gastrointestinal tract. Experimental approaches are as simple as by giving a single oral dose of alcohol or other noxious agents or by injections of multiple dosages of ulcer inducing agents or by parenteral administration or in drinking water of carcinogens or by modifying the genetic makeups of animals to produce relatively long-term pathological changes in particular organs. With these methods they could induce consistent inflammatory responses or tumorigenesis in the gastrointestinal mucosa. These animal models are widely used in laboratories in understanding the pathogenesis as well as the mechanisms of action for therapeutic agents in the treatment of gastrointestinal inflammation and cancer.

Tungstic acid reduction of cold-resistant stress-induced ulceration in rats

Sprague-Dawley rats were restrained at 4 degrees C for 2 h (stress). Tungstic acid in a single dose of 0.01, 0.1, 1, 10, 100 or 300 mg/kg (dissolved in distilled water) was administered intragastrically to animals 30 min prior to stress. Stress induced significant gastric mucosal damage, whereas tungstic acid pretreatment dose-dependently reduced lesion formation. Doses of tungstic acid of 1 mg/kg and higher significantly (P < 0.05-0.001) decreased ulcers. The mucosal mast cell counts in rats pretreated with tungstic acid were significantly higher than those of control rats. In motility experiments using oral administration of amberlite pellets, pretreatment with tungstic acid dose-dependently reduced the gastric emptying rate during a 1 h period of stress. Gastric mucosal xanthine oxidase and superoxide dismutase (SOD) activities, after pretreatment with a single dose of tungstic acid, were not altered in stressed animals. It is suggested that tungstic acid effectively antagonizes stress-induced gastric ulcers, possibly by decreasing motility and mass cell degranulation. Xanthine oxidase and SOD activities and mucous content were not changed in the gastric mucosa by the present method of tungstic acid administration.

How to study the pharmacology of medicinal plants in underdeveloped countries

This paper presents a reflection based on 15 years' experience of studies on the pharmacology of medicinal plants in an underdeveloped country, Brazil. In these countries the investment in research is small and frequently interrupted. There is no new-medicines development program. Brazilian pharmaceutical companies have been short-sighted and have not developed new drugs. Although the diversity of the Brazilian flora is a remarkable opportunity for the development of new medicine products, natural product research is limited to a small group. These difficulties are common to all underdeveloped countries. Strategies for the pharmacological study of medicinal plants are proposed, the main difficulties are identified and a discussion of possible ways to overcome them is presented.

Vagal hyperactivity in stress induced gastric ulceration in rats

Indirect evidence suggests that stress ulceration is provoked by vagal hyperactivity. However, direct evidence of hypervagal activity during stress conditions is lacking. Experiments were designed to directly measure vagal activity under different stress conditions in rats. Starvation stress for 48 h did not change the mean amplitude of action potentials, but their frequency was significantly decreased. Restraint stress at 22 degrees C increased vagal activity, both amplitude and frequency, in the first 60 min; these responses were markedly enhanced by cold (4 degrees C) and persisted for at least 2 h. Starvation for 48 h did not induce any gastric mucosal lesions. Restraint alone produced petechiae in the gastric mucosa, but cold restraint induced severe haemorrhagic ulcers. It is concluded that cold restraint stress provokes a prolonged vagal hyperactivity, which is one of the causative factors for gastric ulceration.

Involvement of vagal pathway in the anti-secretory effect of a novel xanthine derivative

The inhibitory action of a novel xanthine derivative, 3-ethyl-1-(6-hydroxy-6-methylheptyl)-7-propylxanthine (A90 6119) on gastric acid secretion was studied in rats. In conscious pylorus-ligated rats, A90 6119 (3 mg/kg intraduodenally, i.d.), inhibited gastric acid output stimulated by carbachol and by 2-deoxy-D-glucose by 49% and 100% respectively. Basal acid secretion was inhibited by 61% by 10 mg/kg, i.d. A90 6119. In urethane anesthetized stomach-lumen-perfused rats, A 90 6119 at 1 and 3 mg/kg, i.d. significantly reduced the acid secretion stimulated by 2-deoxy-D-glucose, by 83% and 100%, respectively. The stable thyrotropin-releasing hormone (TRH) analog, RX 77368, injected intracisternally (i.c.) at a 30 ng dose, induced concomitant increases in acid secretion and gastric mucosal blood flow. A90 6119 (10 micrograms/rat, i.c.) inhibited by 93% and 132% the increase in acid secretion and gastric mucosal blood flow induced by i.c. injection of TRH analog, respectively. These data suggest that the anti-secretory effect of A90 6119 involves inhibition of both central and peripheral vagal cholinergic pathways.

Nitric oxide inhibition intensifies cold-restraint induced gastric ulcers in rats

Treatment 20 min beforehand with an inhibitor of nitric oxide (NO) synthesis NW-nitro-1-arginine methyl ester (L-NAME) (12.5, 25, 50 or 100 mg/kg, s.c.), dose-dependently intensified gastric glandular mucosal ulceration produced by cold-restraint stress. Hexamethonium (20 mg/kg) or atropine (1 mg/kg) pretreatment s.c. 20 min before stress strongly antagonised stress-evoked ulceration, as well as the ulcer-potentiating effects of L-NAME when either cholinoceptor antagonist was given concurrently with the NO inhibitor. Stress-induced mast cell degranulation was not worsened by L-NAME pretreatment. The findings suggest that NO could confer partial protection against stress-induced gastric ulcer formation; its activity is triggered off by the ulcerogenic mechanism of stress.

Effects of nicotine on activity and stress-induced gastric ulcers in rats

Nicotine is known to influence locomotor activity. The alkaloid also intensifies gastric ulcer formation in stressed rats. The effects of nicotine on locomotor activity in relation to gastric lesions induced by restraint at 4 degrees C for 2 h (stress) were, therefore, studied. Ten-day treatment with nicotine 25 or 50 micrograms/ml drinking water potentiated stress-evoked ulceration and mast cell degranulation. These same doses of nicotine increased vertical motor activity; only the higher dose of the alkaloid enhanced horizontal movements. Phenobarbitone (12.5, 25, or 50 mg/kg, SC) dose dependently reduced vertical activity, as well as stress-induced gastric ulceration and mucosal mast cell degranulation. The drug also lessened the potentiating effects of nicotine on motor activity and stress-evoked gastric lesion formation. It is concluded that the ability of chronic nicotine treatment to intensify stress-induced gastric ulceration most likely owes part of its action to a mechanism evoking increased activity, which possibly reflects an influence on the CNS, as well as to enhancement of mast cell degranulation in the stomach glandular mucosa.

The influence of chronic nicotine treatment on stress-induced gastric ulceration and emptying rate in rats

Ten-day treatment with nicotine (5, 25 or 50 micrograms/ml drinking water) dose-dependently intensified gastric ulceration induced by cold-restraint, and emptying rate. Stomach contractions produced by graded doses of bethanechol i.v. were elevated further by nicotine treatment. It is suggested that chronic nicotine administration produces hypersensitivity of the gastric muscarinic receptors; stomach hypermotility contributes to the ulcer-worsening action of the alkaloid.

Luminal acid in stress ulceration and the antiulcer action of verapamil in rat stomachs

The role of luminal acid and the influence of the antisecretory action of verapamil in stress ulcer prevention in rat stomachs have been studied. Intraperitoneally injected verapamil, 4 mg kg-1, inhibited gastric acid secretion and ulcer formation, however, a 2 mg kg-1 dose, which did not significantly influence acid output, also had an antiulcer effect. Intraperitoneal injection of bethanechol, 1.2 or 3.6 mg kg-1, increased gastric acid output, but did not influence stress-induced ulcer formation. Oral administration of HCl, 25 or 50 mu equiv, aggravated stress ulceration in a dose-dependent manner; this lesion-worsening effect was prevented by pretreatment with verapamil or bethanechol. The gastric luminal acid content in 2 h pylorus-ligated rats was similar in the groups given either bethanechol or HCl. These findings indicate that the antisecretory action of verapamil may not account for its antiulcer effect. It is suggested that endogenous and exogenous luminal acid may have different influences on stress ulcer formation.

Restraint stress in biomedical research: a review

The use of restraint or immobilization for investigations of animal physiology, pathology and pharmacology has an extensive history. The major use of this technique has been as a "stressor" for the induction of stress response syndromes in animals. Many such syndromes have been characterized from the behavioral level to the neurochemical concomitants of stress. As a consequence of this particular use of the restraint procedure, much information concerning drug effects on stress response syndromes has been obtained. Indeed, many researchers in the area of gastrointestinal drugs routinely screen their new compounds in a restraint model of gastric stress ulcer. The purpose of this review is to present for researchers, a summary of the methods for, the parameters of, and known drug effects on, restraint-induced pathology. In our experience, this technique has proven to be a very useful one for the examination of both central and peripheral mechanisms of stress-related disorders, as well as for studying drug effects upon these disorders.

Autonomic drug effects and gastric secretion in a new experimental model of stress ulcers in rats

A psychological procedure which does not involve the application of physical stimulation was used to produce gastric ulcers experimentally. Ulceration was induced in rats by exposing the animals to the aggressive attacks of rats treated with 6-hydroxy-dopamine (6-OHDA). Gastric secretion and the effects of autonomic drugs on ulcer formation were investigated. Atropine methylbromide did not significantly inhibit the occurrence of erosions. Phentolamine or hexamethonium bromide significantly inhibited the production of erosions, and combined administration of an anticholinergic agents and alpha-blocking agent led to a complete inhibition, with no notable behavioral change. In case of pylorus ligation, gastric secretion during exposure to attack of 6-OHDA-treated rats was significantly less than that in the controls. We suggest that the sympathetic nervous system plays an important role in the production of gastric erosions, as induced by the methods reported in this study.

Cholinergic-mediated gastric mast cell degranulation with subsequent histamine H1-and H2-receptor activation in stress ulceration in rats

The effects of atropine, mepyramine, metiamide or NaHCO3 on gastric ulceration, gastric secretion and gastric mast cell degranulation were studied in stressed pylorus-occluded rats. The influence of dexamethasone pretreatment on stress ulcers in animals without pylorus occlusion (intact rats) was also examined. Stress produced a high glandular lesion incidence and ulcer index, and markedly lowered gastric secretion and glandular wall mast cell counts. Injected 0.5 h before stress, atropine, mepyramine or metiamide strongly antagonised ulceration. Atropine or metiamide, but not mepyramine, reduced gastric secretion. Only atropine prevented stress-induced mast cell changes. NaHCO3, given intragastrically before stress, did not prevent ulceration or mast cell degranulation despite complete neutralisation of gastric acid. Dexamethasone-induced gastric mucosal mast cell depletion could reduce stress ulceration. The findings show that stress degranulates stomach mast cells via a cholinergic pathway; released histamine from this source is largely responsbile for gastric ulceration through H1- and H2-receptor effects. Histamine H2-receptor-mediated gastric acid may play only a small contributory role in stress ulcers in rats. The antiulcer mechanisms of histamine H1- and H2-receptor blockade are discussed.

Histamine H1- and H2-receptor-mediated gastric microcirculatory effects in the aetiology of stress ulceration in the rat stomach

Stress produced severe mucosal ulcers, increased mucosal microcirculation and lowered mast cell counts in the glandular wall of rat stomachs. Mepyramine i.m. or metiamide i.p. effectively prevented both ulceration and microcirculatory changes but not stress-reduced mast cell counts.

A correlative study of the antiulcer effects of zinc sulphate in stressed rats

The effects on zinc sulphate pretreatment of rats on stress-induced gastric ulcers and on changes in mast cell counts were studied and correlated with changes in gastric mucosal microcirculation. The effects on zinc sulphate on blood pressure responses and on growth were also examined. Stress (2 h restraint at 4 degrees C) produced marked glandular mucosal ulceration, lowered the stomach wall mast cell counts and increased the microcirculatory blood volume in the superficial glandular mucosa. Zinc sulphate (22, 44 or 88 mg/kg; injected i.p. 48 h before stress) reversed all these changes in a dose-related manner. Blood pressure responses to i.v. acetylcholine, adrenaline or histamine were unaffected and growth of the rats as observed for 7 days after injection was not impaired. On the basis of these findings the mechanism of the antiulcer action of zinc sulphate is the following: inhibition of the stress-induced release of vasoactive agents from gastric mast cells and thus prevention of the subsequent microciculatory changes known to produce mucosal ulceration. Interference with vascular responses through direct blockade or toxicity is unlikely.

A new model of stress ulcer in the rat with pylorus ligation and its pathogenesis

Water-immersion stress for 7, 14, or 20 hr consistently induced linear or punctate stress ulcers (mucosal erosions) in the corpus of the stomach in intact rats. When the pylorus of the stomach had been ligated prior to stressing, the stress ulcers changed their morphological feature (mainly punctate and in one place elongated) and location (both in corpus and antrum). Histologically, the stress ulcer developed in the proximal antrum of pylorus ligated rats and penetrated into the muscularis mucosa. Sodium bicarbonate, chlorpromazine, hexamethonium, atropine, metiamide, and bilateral vagotomy markedly inhibited the stress ulcers which developed in the pylorus-ligated rats. Phentolamine and propranolol hardly affected the development of stress ulcers. Amylopectine evoked a new type of stress ulcer in the corpus when it was given to the pylorus-ligated rats.

Effects of zinc chloride on gastric secretion and ulcer formation in pylorus-occluded rats

The effects of 10-day pretreatment with i.p. injections of zinc chloride, 16 mg/kg, on gastric secretion and on gastric ulceration induced by stress or by acid accumulation were examined in pylorus-occluded rats. Zinc chloride pretreatment significantly reduced the volume of gastric secretion and the total acid output as well as the incidence of gastric ulcers induced either by stress or by acid accumulation. The findings support the idea zinc compounds may be useful in the treatment of gastric ulcers.

Intragastric NaHCO3 perfusion and vagal-induced ulcer formation in the rat stomach

The effects of electrical vagal stimulation on gastric acid output and ulcer formation were studied in rats intragastrically perfused with saline or NaHCO3 solutions. Vagal stimulation produced a 100% incidence of glandular lesions and a significant increase in total acid output in saline-perfused stomachs. Antacid perfusion failed to prevent ulcer formation despite complete neutralization of the increased acid output. It is considered that vagal-induced gastric glandular lesions are not acid dependent.

Acute gastric ulcer formation in response to electrical vagal stimulation in rats

Intermittent electrical stimulation of the left cervical vagus increased intragastric pressure and induced a 100% incidence of haemorrhagic ulcers in the glandular mucosa of rat stomachs. Atropine pretreatment of sub-diaphragmatic vagotomy prevented these effects. The findings substantiate the idea that stress-induced glandular ulcers result from vagal-mediated increased gastric contractions.

The effects of metiamide on gastric secretion and stress ulceration in rats

The effects of metiamide, a histamine H2 blocker, on gastric secretion and ulcer formation in stressed pylorus-occluded rats were investigated. Metiamide, like atropine, significantly reduced the volume of gastric secretion and total acid output in unrestrained pylorus-occluded rats. Both drugs produced greater decreases in the volumes of gastric secretion in stressed rats than in their corresponding unrestrained groups. Stress itself reduced both parameters. Metiamide, like atropine, significantly reduced the incidence of gastric stress ulcers. When given together these two drugs did not provide greater protection. The results obtained with metiamide indicate that histamine plays a role in basal gastric secretion and in the pathogenesis of stress ulcers. As no correlation between gastric acid secretion and ulcer formation was demonstrated in this study, it is suggested that H2 receptors may also be involved in gastric motility. However, the possibility that metiamide could exert its ulcer-protecting effects through other mechanisms cannot yet be excluded.

Effects of stress and of autonomic blockers on gastric mucosal microcirculation in rats

Changes in gastric mucosal microcirculation in rats were studied by using the method of intra-aortic injection of India ink, followed by microdissection of the mucosa. Acute stress, induced by restraint and exposure to cold for 2 hr, caused marked and significant vasodilatation in the gastric mucosa. This vasodilatation was prevented by pretreatment with atropine or chlorpromazine, but not by alpha- or theta-adrenoceptor blocking agents. Phentolamine caused significant vasoconstriction in the gastric mucosa of non-stressed rats, but when animals were stressed phentolamine induced a greater vasodilatation than was obtained with stress alone. These observations provide added support for the hypothesis that stress induces vagal overactivity, probably of central origin. The resulting strong contractions of the gastric wall, and compression of the intramural vessels, are probably responsible for degeneration of the mucosal cells leading to the formation of stress-induced ulcers in the rat.