Role of thyrotropin-releasing hormone in stress ulcer formation in the rat

Crucial role of c-Jun NH2-terminal kinase 1 (JNK1) in cold-restraint stress-induced gastric lesions in mice

c-Jun NH2-terminal kinase 1 /JNK1, is activated in response to a broad array of cellular stresses. We investigated the role of JNK1 in the pathophysiology of cold-restraint stress-induced gastric lesions in mice. B6/129, wild type (WT) mice, or mutant mice deficient in Jnk1 (Jnk1-/- mice) were exposed to cold-restraint stress for different time periods. Gastric lesions were identified and quantitated by morphometric analysis. JNK1 activity in mucosal homogenates was quantitated by immunoprecipitation and in-vitro kinase assays. JNK1 expression and Akt activation were assessed by Western blots with anti-JNK1 and anti-phospho Akt antibodies, respectively. Gastric mucosal homogenates from Jnk1-/- mice exhibited no significant expression of JNK1 and no detectable level of JNK1 activation. Exposure of WT mice to cold-restraint stress led to the development of significant gastric lesions and to a greater than three-fold induction in JNK1 activity, while no lesions were detected in the gastric mucosa of Jnk1-/- mice. Since cold-restraint stress-induced gastric lesions involve the activation of cholinergic pathways, we tested the effect of atropine on both the development of gastric lesions and JNK1 activation. Pretreatment of WT mice with atropine completely inhibited both cold-restraint stress-induced lesions and JNK1 activation. Cold-restraint stress induced protein kinase B/Akt to a similar level in the gastric mucosa of both WT and Jnk1-/- mice indicating the integrity of other signaling pathways. JNK1 plays a key role in the development of cold-restraint stress-induced gastric lesions in mice through the activation of cholinergic, atropine sensitive pathways.

Preconditioning stress prevents cold restraint stress-induced gastric lesions in rats: roles of COX-1, COX-2, and PLA2

We investigated the protective effect of mild stress on gastric lesions induced by cold-restraint stress, especially concerning prostaglandins (PGs)/cyclo-oxygenase (COX) isozymes. Rats were exposed to severe stress (cold-restraint stress at 10 degrees C for 6 hr) or mild stress (cold-restraint stress at 10 degrees C for 30 min and kept at room temperature for 60 min) followed by severe stress. Severe stress induced gastric lesions, with a concomitant decrease in body temperature (BT). The ulcerogenic response was inhibited by atropine but worsened by indomethacin and SC-560 but not rofecoxib, although none of these agents had any effect on the change in BT. Mild stress suppressed the gastric ulceration and the decrease in BT induced by severe stress, and these effects were reversed by both COX-1 and COX-2 inhibitors. The expression of COX-2 in the stomach was up-regulated from 4 hr after severe stress and this response was slightly expedited by mild stress. COX-2 was also expressed in the hypothalamus under normal and stressed conditions. Quinacrine (phospholipase A(2) inhibitor) attenuated the protective effect of mild stress on the ulceration and decrease in BT caused by severe stress. TA-0910 (TRH analogue) at a low dose also prevented the gastric ulceration and the decrease in BT induced by severe stress. These results suggest that mild stress protects against cold-restraint stress-induced gastric ulceration, and the effect is peripherally and centrally mediated by PGs derived from both COX-1 and COX-2 through the activation of phospholipase A(2). TRH may also be involved in the protective effect of mild stress, probably through regulation of the thermogenic system.

Role of brainstem TRH/TRH-R1 receptors in the vagal gastric cholinergic response to various stimuli including sham-feeding

Pavlov's pioneering work established that sham-feeding induced by sight or smell of food or feeding in dogs with permanent esophagostomy stimulates gastric acid secretion through vagal pathways. Brain circuitries and transmitters involved in the central vagal regulation of gastric function have recently been unraveled. Neurons in the dorsal vagal complex including the dorsal motor nucleus of the vagus (DMN) express thyrotropin-releasing hormone (TRH) receptor and are innervated by TRH fibers originating from TRH synthesizing neurons in the raphe pallidus, raphe obscurus and the parapyramidal regions. TRH injected into the DMN or cisterna magna increases the firing of DMN neurons and gastric vagal efferent discharge, activates cholinergic neurons in gastric submucosal and myenteric plexuses, and induces a vagal-dependent, atropine-sensitive stimulation of gastric secretory (acid, pepsin) and motor functions. TRH antibody or TRH-R1 receptor oligodeoxynucleotide antisense pretreatment in the cisterna magna or DMN abolished vagal-dependent gastric secretory and motor responses to sham-feeding, 2-deoxy-D-glucose, cold exposure and chemical activation of cell bodies in medullary raphe nuclei. TRH excitatory action in the DMN is potentiated by co-released prepro-TRH-(160-169) flanking peptide, Ps4 and 5-HT, and inhibited by a number of peptides involved in the stress/immune response and inhibition of food-intake. These neuroanatomical, electrophysiological and neuropharmacological data are consistent with a physiological role of brainstem TRH in the central vagal stimulation of gastric myenteric cholinergic neurons in response to several vagal dependent stimuli including sham-feeding.

Intracerebroventricular calcitonin prevents stress-induced gastric dysfunction

BACKGROUND

Restraint stress produces gastric hypercontractility and acidity leading to stress ulceration. Intracerebroventricular (ICV) salmon calcitonin (sCT) decreases restraint injury and acidity, but its effects on restraint-induced hypercontractility are unknown.

METHODS

Using stereotactic guidance, ICV catheters were placed into the lateral ventricle of adult male rats and calibrated gastric strain gauge transducers were implanted 5 days prior to restraint stress. sCT rats (n = 8) were pretreated with 5 microg of calcitonin ICV (10 microl volume), while controls (n = 10) received 10 microl of ICV saline prior to restraint for 2 h at 20 degrees C followed by 2 h at 4 degrees C. Gastric motility data were collected with AT-CODAS and analyzed with ADVANCED CODAS. Gastric volume, pH, and lesions were recorded following the stress.

RESULTS

ICV calcitonin prevented gastric mucosal injury in all animals (0% vs 100%, P <.01) and elevated pH slightly (2.5 +/-.3 vs 1.6 +/-.1, P <.05). Stress caused the force of contractions to increase from 0.35 +/-.1 to 1.38 +/-.4 g in controls (P <.01), while treated animal's force fell from.42 +/-.1 to 0.2 +/-.05 g (P <.01 vs control). Stress did not affect contractions/min (3.4 +.6 vs 3.5 +.3), but sCT increased frequency (2.5 +.4 to 5.0 +.2, P <.01). Stress prolonged contraction duration (11.5 + 1 to 16.5 + 1.7 s, P <.01), but stress's effect was prevented by sCT (11.0 +.5 to 11.2 +.3, P <.01 vs control).

CONCLUSIONS

Pretreatment with 5 microg central sCT prevents the increased amplitude and duration of gastric contractions produced by restraint stress for 2 h, in association with gastroprotection.

Importance of medullary thyrotropin-releasing hormone in brain-gut circuits regulating gastric integrity: preclinical studies

Cold restraint stress (CRS) induced vagal-dependent corpus mucosal erosions with hemorrhage in rats. Aggressive factors such as gastric acid, pepsin, and high-amplitude contractions have been reported to be responsible for inducing the formation of gastric lesions. Thyrotropin-releasing hormone (TRH) microinjected into the cisterna magna or the dorsal motor nucleus (DMN) of the vagus induced similar vagally mediated gastric injuries as CRS, and both were prevented by immunoneutralization with TRH antibody injected centrally. These findings indicated that TRH action in the DMN may contribute to CRS-induced gastric mucosal lesions. However, either exogenous or endogenous TRH at a subthreshold dose, which did not increase gastric acid secretion, alleviated gastric injury induced by intragastric administration of a strong irritant in rats. Previous studies have shown that the vagus participates in adaptive cytoprotection. Vagotomy or intracisternal injection of TRH-antibody completely abolished the protective effect of a mild irritant pretreatment in rats. The number of c-Fos protein-positive cells in the DMN increased in the process of adaptive cytoprotection. These results suggest that vagal afferent nerves activated by a topical gastric irritant influence DMN activity by releasing endogenous TRH, leading to protection against injury induced by a subsequent strong irritant. The dual vagally mediated action of TRH in the medulla to regulate the gastric mucosa's response to injury reflects the balance between the aggressive (acid, pepsin, motility) and protective (prostaglandin, calcitonin gene-related peptide, nitric oxide) factors recruited by the level of vagal activation. These data indicate a crucial role of medullary TRH and gastric vagal efferent and afferent circuits in the modulation of gastric integrity.

Effect of thyroid hormones on stress ulcer formation

BACKGROUND

Stress ulcers are gastric mucosal lesions that may cause life-threatening upper gastrointestinal bleeding. Although it is known that hyperthyroid status prevents stress ulcer formation, the effect of thyroid hormones given just as the stress is beginning has not been studied. The aim of this study was to assess the effect of thyroid hormone supplementation on gastric stress ulcers at the beginning of the restraint stress.

METHODS

Thyroid hormones were administered to rats 2 days before or at the beginning of the restraint stress. The linear length of the gastric mucosal lesions, mucosal pH and thyroid hormone levels were measured and histopathological examinations were carried out.

RESULTS

It was found that both triiodothyronin and thyroxin reduce the length and depth of the stress ulcers (P < 0.001).

CONCLUSION

Although the mechanisms by which the thyroid hormones act on stress ulcers are uncertain, current experimental studies suggest that thyroid hormones reduce the formation of stress ulcers in rats when given before or at the beginning of the stress.

Effect of psychogenic stress on gastrointestinal function

This review summarizes the studies published over the last twenty years on the effects of psychogenic stress on gastrointestinal function, using animal models. The effects of stress on gastric ulceration have received wide attention and the central and local mechanisms of mucosal damage have been, for the most part, clearly delineated. In comparison, relatively few studies have focused on the impact of stress on intestinal and colonic physiology, even though its influence on intestinal motility, mucosal permeability and inflammation has been established. More work is necessary in this field, especially considering the importance of irritable bowel syndrome in modern society.

Roles of endogenous prostaglandins and nitric oxide in gastroduodenal ulcerogenic responses induced in rats by hypothermic stress

We examined the roles of endogenous prostaglandins (PGs) and nitric oxide (NO) in the gastroduodenal ulcerogenic responses to hypothermic stress (28 approximately 30 degrees C) in anesthetized rats. Lowering body temperature provoked damage in the gastroduodenal mucosa, with an increase of gastric acid secretion and motility. These responses were completely abolished by bilateral vagotomy or atropine, while 16,16-dimethyl PGE2 decreased the mucosal ulcerogenic response with no effect on acid secretion. The non-selective COX inhibitors, indomethacin or aspirin, worsened these lesions with enhancement of gastric motility and no effect on acid secretion, while the selective COX-2 inhibitor NS-398 did not affect any of these responses. On the other hand, the non-selective NOS inhibitor L-NAME but not aminoguanidine (a relatively selective inhibitor of iNOS), significantly potentiated the acid secretory and mucosal ulcerogenic responses in the stomach but reduced the duodenal damage in response to hypothermia, the effects being antagonized by co-administration of L-arginine. Hypothermia itself decreased duodenal HCO3- secretion under both basal and mucosal acidification-stimulated conditions. Both indomethacin and aspirin further decreased the HCO3- response to the mucosal acidification, while L-NAME significantly increased the HCO3- secretion even under hypothermic conditions, similar to 16,16-dimethyl PGE2. These results suggest that 1) hypothermic stress caused an increase of acid secretion and motility as well as a decrease of duodenal HCO3-secretion, resulting in damage in both the stomach and duodenum, 2) the COX-1 but not COX-2 inhibition worsened these lesions by enhancing gastric motility and further decreasing duodenal HCO3- response, 3) the cNOS but not iNOS inhibition worsened gastric lesions by increasing acid secretion but decreased duodenal damage by increasing HCO3- secretion. Thus, it is assumed that the gastroduodenal ulcerogenic and functional responses to hypothermic stress are modified by cNOS/NO as well as COX-1/PGs.

Cold restraint-induced gastric lesions in individual- and group-stressed rats

The aim of the present study was to 1) determine the intensity of cold restraint-induced gastric lesions and core body temperature in single- and group-stressed rats, and establish a correlation between them; and 2) determine the influence of visual contact among animals during cold restraint on development of gastric stress ulcer. Therefore, adult male Wistar rats were put into individual or group restraint boxes (composed of two, three, six or nine single boxes) with or without possibility of visual contact and then exposed 2 hr to the cold (4 degrees C). Core body temperature was measured just before and after cold restraint using a digital rectal thermometer. The results showed that: 1) single stressed animals expressed significantly higher ulcer index than those stressed in group of three, six and nine rats; 2) there was no significant difference in degree of hypothermia among rats exposed to various group paradigms; and 3) there was no significant difference in ulcer index among animals stressed in conditions with or without visual contact. An absence of significant difference in ulcer index between single and paired stressed rats implies that three is the lowest number of animals per group at which an influence of group size on behavioral and adaptive mechanisms in rats exposed to cold restraint becomes manifest.

Gastric ECL-cell hyperplasia produces enhanced basal and stimulated gastric acid output but not gastric erosion formation in the rat

1. The purpose of this study was to examine the change in gastric acid output and gastric erosion formation produced by inducing gastric enterochromaffin-like (ECL) cell hyperplasia in female rats. 2. Rats were treated with vehicle or ranitidine (1,200 mumol/kg/day x 4 wks) administered via SC Alzet minipumps. Experiments were performed 24 hours after removing the minipump, when the inhibitory effect of ranitidine on gastric acid secretion had been lost. 3. Basal gastric acid secretion was 7-fold higher in chronic ranitidine animals than in sham control. 4. Both total and net gastric acid secretions stimulated by carbachol/pentagastrin infusion or histamine injection were significantly higher in the chronic ranitidine animals than in controls. 5. By contrast, intracisternal injection of the chemical vagal stimulant RX77368 (100 ng) resulted in no net increase in acid output of recovered ranitidine-pretreated group. 6. No significant changes in gastric erosions produced experimentally by cold exposure plus restraint or indomethacin pretreatment were noted in recovered chronic ranitidine animals compared to sham controls. 7. These findings suggest that achlorhydria-induced ECL cell hyperplasia augments both basal and stimulated gastric acid secretory function. The histamine results implicate an enhanced parietal cell mass, upregulation of H2 receptors, and/or second-messenger events at the parietal cell as the mechanism for the enhanced gastric secretory response.

Use of animal models in peptic ulcer disease

Considerable progress has been made in the understanding of the formation of gastric erosions by the use of animals. The role of gastric acid secretion in their pathogenesis has been clarified. Gastric erosions are associated with the presence of acid in the stomach and slow gastric contractions. With several different experimental procedures, the animal's body temperature falls; preventing the fall averts erosions. A fall in body temperature or exposure to cold are associated with the secretion of thyrotropin-releasing hormone (TRH), and both increased and decreased concentration of corticotropin-releasing factor (CRH) in discrete regions of rat brains. Thyrotropin-releasing hormone when injected into specific sites in the brain produces gastric erosions and increases acid secretion and slow contractions, whereas CRH has the opposite effects. One of the major sites of interaction of the two peptides is in the dorsal motor complex of the vagus nerve. Thyrotropin-releasing hormone increases serotonin (5-HT) secretion into the stomach. Serotonin counter-regulates acid secretion and slow contractions. Many other peptides injected into discrete brain sites stimulate or inhibit gastric acid secretion.

Caudal raphe-dorsal vagal complex peptidergic projections: role in gastric vagal control

Subpopulations of raphe pallidus (Rpa) and raphe obscurus (Rob) neurons containing TRH, serotonin (5-HT), and substance P contribute projections to the dorsal vagal complex (DVC). Activation of Rpa and Rob neurons induces a vagal cholinergic-dependent stimulation of gastric secretory and motor function and modulates resistance of the gastric mucosa to gastric injury in rats and cats. The caudal raphe nuclei-DVC pathways containing TRH/5-HT are involved in mediating cold-induced vagal stimulation of gastric function and erosion formation. These results suggest that Rpa/Rob-DVC projections containing TRH/5-HT may be an important pathways in the medullary regulation of vagal activity to the viscera.

Cold exposure elevates thyrotropin-releasing hormone gene expression in medullary raphe nuclei: relationship with vagally mediated gastric erosions

The stimulation of thyrotropin release by cold is associated with an increase in thyrotropin-releasing hormone gene expression in the paraventricular nucleus of the hypothalamus. Cold exposure also stimulates autonomic outflow to viscera. There is evidence that caudal raphe nuclei are involved in autonomic regulation through thyrotropin-releasing hormone projections to the dorsal vagal complex and spinal cord. To determine whether cold modulates thyrotropin-releasing hormone gene expression in the caudal raphe nuclei, the effect of cold exposure on thyrotropin-releasing hormone messenger RNA levels in the rat lower brainstem was examined by quantitative Northern blot analysis and thyrotropin-releasing hormone messenger RNA was localized by in situ hybridization. The gastric responses to cold exposure were also assessed in sham or vagotomized rats with pylorus ligation. Thyrotropin-releasing hormone messenger RNA signal was detected in the RNA extracted from the medulla and hypothalamus but not from the amygdala, periaqueductal gray or cerebellum. Cold exposure (4 degrees C) for 1 or 3 h increased thyrotropin-releasing hormone messenger RNA levels in the medulla by 77 +/- 37 and 142 +/- 39% respectively. In situ hybridization histochemistry showed that the increase in silver grain density occurred exclusively in the raphe pallidus and raphe obscurus. Exposure to cold stress for 2 h stimulated gastric acid secretion and resulted in gastric lesion formation in sham but not vagotomized rats. There are established thyrotropin-releasing hormone projections from the raphe pallidus and obscurus to the dorsal vagal complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of Fos immunoreactivity in the rat brain after cold-restraint induced gastric lesions and fecal excretion

Cold-restraint alters gastrointestinal function through vagal pathways. Immunohistochemical detection of the nuclear phosphoprotein Fos (Fos-IR) was used to map brain neuronal pathways activated by cold exposure for 3 h in fasted rats maintained individually in semi-cylindrical restraining cages. Gastric lesions and fecal pellet output were also monitored. In rats exposed to cold (4 degrees C) restraint for 3 h, numerous Fos-positive nuclei were observed in the dorsal motor nucleus of the vagus, raphe pallidus, locus coeruleus, and paraventricular nucleus of the hypothalamus, and, to a lesser extent, in the raphe obscurus, parapyramidal region, and medullary noradrenergic region, bed nucleus of the stria terminalis and septum. Fecal pellet output was increased by 8 fold and gastric lesions covered 19.5 +/- 1.1% of the corpus mucosa. Rats restrained at room temperature under otherwise same conditions had little or no Fos-positive cells in these brain nuclei, no gastric erosion and a low pellet output (1.3 +/- 0.5 nb/3 h). These data, in addition to previous functional studies, provide anatomic support for the involvement of neurons in the caudal raphe nuclei, dorsal motor nucleus of the vagus and paraventricular nucleus of the hypothalamus in the autonomic and endocrine responses to cold-restraint.

Restraint stress in biomedical research: an update

Since the publication of our initial review of restraint stress in 1986, much work has continued with this technique, either as a tool for the investigation of other pharmacological, physiological, or pathologic phenomena or with restraint stress itself serving as the object of the study. As we noted in 1986, the major use of restraint has been for the induction of stress responses in animals and, more specifically, for the investigation of drug effects, particularly as they affect typical stress-related pathology--gastrointestinal, neuroendocrine, and immunological agents have been extensively studied. In compiling this update on restraint stress and its effects, we noted an increasing emphasis on central nervous system mechanisms in peripheral disease, especially gastrointestinal disease. In particular, many CNS-active agents have been tested for their effects on gastric and duodenal lesion formation and gastric secretion, including antidepressants, antipsychotics, anxiolytics, noradrenergic, serotonergic, dopaminergic, and peptidergic compounds. Some of these agents are especially active in the gastrointestinal tract even when administered centrally, further solidifying the concept of a brain-gut axis. The present update includes studies of: methods and procedures, pre-restraint manipulations, post-restraint/healing effects, and drug effects. In addition, a current bibliography of reports that have employed restraint is included.

Raphe pallidus stimulation increases gastric contractility via TRH projections to the dorsal vagal complex in rats

The role of thyrotropin releasing hormone (TRH) in the dorsal vagal complex (DVC) in mediating the enhanced gastric contractility induced by glutamate (100 pmol) microinjected into the raphe pallidus (Rpa) was investigated in urethane-anesthetized rats acutely implanted with miniature strain gauge force transducers on the corpus of the stomach. Glutamate-induced stimulation of gastric contractility was dose-dependently inhibited by bilateral microinjection into the DVC of TRH antibody (0.17, 0.85 or 1.7 micrograms/100 nl/site) but not by vehicle. TRH antibody microinjected into the dorsal medullary reticular field had no effect. These data indicate that activation of Rpa neurons by glutamate increases gastric motor function through TRH release in the DVC.

Heterogeneous distribution of gastric mucosal blood flow with restraint stress in the rat

Cold water immersion restraint (CWIR) is associated with gastric hypercontractility and gastric corpus erosions in the rat. Because the gastric blood flow response to CWIR has not been well defined, we performed the following study. Rats were implanted with force transducers, subjected to CWIR for 2 hr, and then blood flow was determined by the iodo[14C]antipyrine autoradiographic (IAP) technique. When compared to control animals, the CWIR-treated animals displayed foci of gastric corpus hyperemia with a marked and significant increase in blood flow in all layers of the gastric corpus. There was approximately a 100% increase in the mucosa and a 50% increase in the muscularis externa. The hyperemia was not uniform, but rather alternated every 2.1 +/- 0.2 mm with regions of low blood flow. Blood flow in the antrum and duodenum was unaffected by CWIR. We conclude that CWIR is associated with alternating regions of high and low blood flow only in the gastric corpus. Reduction of corpus mucosal blood flow might be due to the powerful gastric contractions associated with CWIR.

CNS effects on gastric functions: from clinical observations to peptidergic brain-gut interactions

Clinical observations as early as the last century pointed to the stomach's link to the brain. Animal studies in this century have given us detailed information about the neuroanatomical and neurophysiological basis of brain-gut interactions. Psychological stress models and stereotaxic brain procedures have been important tools in gaining this information. During the last 10 years, there has been much focus on the effects of neuropeptides on gastric functions. Several CNS-peptides have indeed been shown to influence multiple gastric functions such as: acid secretion, bicarbonate secretion, mucus secretion, motility, blood flow and prostaglandin synthesis. Accordingly, direct CNS-application of these peptides also influences the development of gastric erosions during experimental stress procedures.

Thyrotropin-releasing hormone analogue and serotonin interact within the dorsal vagal complex to augment gastric acid secretion

The effects of serotonin (5HT) and a thyrotropin-releasing hormone (TRH) analogue, RX77368, on vagal control of gastric acid secretion were studied. Microinjection of RX77368 (0.66 pmol in 10 nl), but not 5HT (8 pmol in 10 nl), into the dorsal vagal complex (DVC) evoked a significant increase in acid output. When the same doses of RX77368 and 5HT were co-injected, the amount of acid secreted was significantly greater than that due to RX77368 alone. Thus, 5HT and the TRH analogue interact within the DVC to enhance vagal stimulation of acid secretion. The study suggests a possible functional significance of raphe TRH/serotonergic tracts projecting to the DVC.

The effects of stress on gastric ulceration, T3, T4, reverse T3 and cortisol in neonatal foals

Sixteen neonatal foals stressed by disease underwent endoscopic examination of their stomachs and blood was assayed for triiodothyronine (T3), reverse T3 (rT3), thyroxine (T4) and cortisol, to determine the effects of severe physiological stress and the occurrence of gastric ulcers. compared with eight age-matched controls, six foals had abnormal cortisol, seven had abnormal T3 and 12 had abnormal T4. Eleven of 13 foals had rT3 outside the 95 per cent confidence interval for clinically normal foals of comparable ages. Gastric lesions were seen more frequently in stressed foals, and gastric glandular mucosal lesions were noted in 40 per cent of the stressed foals. Previous studies report low (3 per cent) occurrence of gastric mucosal lesions. The frequency of squamous mucosal lesions was not different from that reported previously, indicating that stress has little effect on the development of lesions at this site.

Role of thyrotropin-releasing hormone in acid secretory response induced by lowering of body temperature in the rat

Acid secretory and mucosal ulcerogenic responses to hypothermia (36-24 degrees C) were examined in anesthetized rats, and the role of thyrotropin-releasing hormone (TRH) in these responses was investigated. Lowering of body temperature (less than 32 degrees C) induced acid hypersecretion and damage in the gastric mucosa. These responses reached a maximum at a body temperature of 28 degrees C and were completely abolished by bilateral cervical vagotomy and significantly inhibited by intracerebroventricular (i.c.v.) administration of TRH antiserum (10 microliters/rat). TRH (10 micrograms/rat) given i.c.v. to the normothermia rat, caused an increase of acid secretion with a pattern similar to those observed during hypothermia. The blood levels of thyroid-stimulating hormone rose significantly during exposure of cold, and this response preceded the onset of acid hypersecretion and lesion formation. Thus, lowering of body temperature induces vagal-dependent gastric acid secretion, probably mediated by TRH released in response to cold exposure, and may be an important element in the etiology of stress ulceration.

Vagally mediated acid hypersecretion and lesion formation in anesthetized rat under hypothermic conditions

The pathophysiological changes associated with hypothermia were investigated in the rat stomach under anesthetized conditions. The animal was placed in a styrene foam box and the core body temperature was kept between 24 and 36 degrees C using a heat lamp and refrigerant pack. Lowering of body temperature (less than 30 degrees C) produced acid hypersecretion and induced hemorrhagic lesions in the gastric mucosa; these responses reached the maximum at 28 degrees C, and a significant relationship was found between acid output and lesion score. Hypothermia (28 degrees C) also caused a marked increase of gastric contractile activity and mucosal blood flow (MBF), but the ratio of acid output to MBF became greater when compared to that obtained under normothermic conditions. These changes induced by hypothermia (28 degrees C) were completely blocked by vagotomy and were significantly inhibited by atropine, hexamethonium, clonidine, or TRH antiserum. However, lowering body temperature did not significantly affect acid secretory, motility, and ulcerogenic responses induced by carbachol in the vagotomized rat, excluding local mechanisms (suppression of the inhibitory nerves) in the hypothermia-induced changes. We conclude that hypothermia alone stimulates vagally dependent acid secretion and motility, resulting in damage in the gastric mucosa. These changes may be centrally mediated by TRH, which is released in association with the thermogenic response to hypothermia.

Administration of thyrotropin-releasing hormone into the central nucleus of the amygdala induces gastric lesions in rats

Studies utilizing microinjections of thyrotropin-releasing hormone (TRH) were performed to determine potentially sensitive limbic brain sites to the ulcerogenic effect of TRH. Administration of TRH into the central nucleus of the amygdala (CEA) produced a high (80%) incidence of gastric lesions and also significantly stimulated acid secretion. Microinjections of TRH into other brain sites including superior colliculus, medial septum, substantia nigra and the hippocampus (CA1 area) were ineffective. Intra-CEA TRH-induced gastric lesions and acid secretion were prevented by vagotomy. These results indicate that the amygdala is of importance for mediating the ulcerogenic effect of brain TRH through peripheral vagal pathways.

Imipramine prevents gastric lesions induced by centrally administered thyrotropin-releasing hormone (TRH) in rats

Increasing evidence indicates that thyrotropin-releasing hormone (TRH), and endogenous brain-gut peptide may play a role in experimental ulcerogenesis. Potential interactions between TRH and imipramine (a typical tricyclic antidepressant (TCA] on the development of TRH-induced gastric lesions have not been investigated. Imipramine (0.05, 0.5 and 5 mg/kg, i.p.) dose-dependently inhibited gastric lesion formation induced by intracisternal (i.c.) administration of TRH (1 micrograms). In addition, imipramine (5 mg/kg, i.p.) significantly decreased gastric acid secretion in response to i.c. TRH (1 microgram) in rats with pyloric ligation. These findings suggest the TCAs may be effective drug agents against centrally initiated gastric ulcerations. The mechanism of this response probably involves blockade of cholinergic (muscarinic) and H2 histamine receptors.

Neurobiology of brain-gut interactions. Implications for ulcer disease

Clinical and laboratory evidence indicates that the brain exerts major control on the gastrointestinal tract. Specific brain loci and circuits that send efferent viscerotropic projections to the gut have been described. A variety of aminergic and peptidergic neurotransmitters have been shown to occur along these cerebrogastrointestinal pathways and to influence motor and secretory functions of the gut. Some of the newly identified peptides have been shown to influence the development of gastroduodenal ulcers. Findings with thyrotropin-releasing hormone (TRH) indicate that this endogenous tripeptide induces a full spectrum of gut effects, prominent among which is production of gastric ulcers. By contrast, other peptides including beta-endorphin, neurotensin, and bombesin induce gut effects opposite to those of TRH, namely, inhibition of gastric acid and motility and prevention of experimental ulcers. These laboratory findings suggest that ulcer disease may represent a brain-driven event, which may be the result of a neurochemical imbalance within the brain. Further neurobiological research will generate additional data on brain-gut interactions and will probably disclose new information to explain certain functional and organic disorders of the gut.

Ultrastructural localization of thyrotropin-releasing hormone immunoreactivity in the dorsal vagal complex in rat

Thyrotropin-releasing hormone-like immunoreactivity (TRH-LI) was localized at the ultrastructural level in the dorsal vagal complex (DVC: dorsal motor nucleus of the vagus (DMV) and the nucleus of the solitary tract (NST] in rat. TRH-LI was concentrated in large granular vesicles in axons, presynaptic terminals, and non-synaptic axon varicosities. TRH-LI presynaptic terminals established both asymmetric and symmetric synaptic contacts with dendrites. These observations are consistent with recently described direct inhibitory and facilitatory effects of TRH on the electrical activity of neurons in the DVC.