Prevention by bombesin of cold-restraint stress induced hemorrhagic lesions in rats

Brainstem neuropeptides and vagal protection of the gastric mucosal against injury: role of prostaglandins, nitric oxide and calcitonin-gene related peptide in capsaicin afferents

Earlier experimental studies indicated that the integrity of vagal pathway was required to confer gastric protection against damaging agents. Several peptides located in the brainstem initially identified to influence vagal outflow to the stomach, as assessed by electrophysiological approach or by vagal dependent alterations of gastric secretory and motor function, were investigated for their influence in the vagal regulation of the resistance of the gastric mucosa to injury. Thyrotropin releasing hormone (TRH), or its stable TRH analog, RX-77368, injected at low doses into the cisterna magna or the dorsal motor nucleus (DMN) was the first peptide reported to protect the gastric mucosa against ethanol injury through stimulation of vagal cholinergic pathways, inducing the release of gastric prostaglandins/nitric oxide (NO) and the recruitment of efferent function of capsaicin sensitive afferent fibers containing calcitonin-gene related peptide (CGRP). Activation of endogenous TRH-TRH1 receptor signaling located in the brainstem plays a role in adaptive gastric protection against damaging agents. Since then, an expanding number of peptides, namely peptide YY, CGRP, adrenomedullin, amylin, glugacon-like peptide, opioid peptides acting on µ, δ1 or δ2 receptors, nocicpetin, nocistatin, ghrelin, leptin and TLQP-21, a peptide derived from VGF prohormone, have been reported to act in the brainstem to afford gastric protection against ethanol injury largely through similar peripheral effectors mechanisms than TRH. Therefore gastric prostaglandins and CGRP/NO pathways represent a common final mechanism through which brain peptides confer vagally mediated gastroprotection against injury. A better understanding of brain circuitries through which these peptides are released will provide new strategies to recruit integrated and multifaceted gastroprotective mechanisms.

Bombesin and the brain-gut axis

Bombesin is the first peptide shown to act in the brain to influence gastric function and the most potent peptide to inhibit acid secretion when injected into the cerebrospinal fluid (CSF) in rats and dogs. Bombesin responsive sites include specific hypothalamic nuclei (paraventricular nucleus, preoptic area and anterior hypothalamus), the dorsal vagal complex as well as spinal sites at T9-T10. The antisecretory effect of central bombesin encompasses a variety of endocrine/paracrine (gastrin, histamine) or neuronal stimulants. Bombesin into the CSF induces an integrated gastric response (increase in bicarbonate, and mucus, inhibition of acid, pepsin, vagally mediated contractions) enhancing the resistance of the mucosa to injury through autonomic pathways. The physiological significance of central action of bombesin on gastric function is still to be unraveled.

Pentagastrin cytoprotection in ethanol-induced gastric mucosal lesions in rats

To establish pentagastrin cytoprotection, the effectiveness of various doses of pentagastrin on ethanol induced gastric mucosal lesions was investigated in Wistar rats. Significant protection was obtained only after parenteral pretreatment with the exception of the lowest dose (1 microgram/kg b.w.). Pentagastrin cytoprotection is not mediated either by a dopamine, muscarinic or gastrin/CCK receptor or by prostaglandin synthesis. However, the protective effect of pentagastrin is abolished by prior vagotomy, although this procedure alone or sham operation is ineffective to influencing control-ethanol lesions. In secretory studies pentagastrin increased both the volume of gastric juice and total acid output. Unlike cytoprotection, these were reversed by vagotomy, but also with atropine and problumide, whereas domperidone and indomethacin were ineffective.

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.

Inhibition of gastric acid secretion and ulcers by calcitonin [correction of calciton] gene-related peptide

A central action of CGRP to inhibit gastric acid secretion, demonstrated in rats and dogs, is mediated at least in rats through modulation of parasympathetic outflow to the stomach. The centrally mediated protective effects of CGRP against ethanol-induced lesions is unique to this peptide and not shared by other centrally acting inhibitors of gastric function. It may be related to the increase in gastric mucosal blood flow induced by central CGRP. The presence of CGRP-like immunoreactivity and receptors in medullary nuclei receiving visceral information and influencing vagal outflow suggests a possible role of the peptide in the central regulation of gastric function. Peripheral injection of CGRP is well established to inhibit acid secretion in rats, dogs, rabbits, and humans. Its antisecretory effect is unlikely to be related to a direct action on the parietal cells. It involves specific and marked release of gastric somatostatin through an interaction with CGRP receptors characterized on D cells and coupled with cAMP. In addition, CGRP induces a decrease in acetylcholine transmission in the enteric nervous system, which may contribute to the inhibition of acid. The rich innervation of the stomach with CGRP-like immunoreactivity, which forms the major component of gastric sensory fibers, along with peptide release by sensory stimulation and potent actions on gastric secretions suggests a role of the peptide in the regulation of gastric function.

The neurobiology of stress ulcers

We have reviewed the neurobiology of stress ulcers from animal models to potential pharmacotherapeutic mechanisms. The evidence strongly supports the hypothesis that certain stress-related gastric lesions are 'brain-driven' events which may be more effectively managed through central manipulations than by altering local, gastric factors. Recent advances in the use of anxiolytic and antidepressant drugs in the management of stress-related gastric mucosal injury further supports the contention that a brain-gut axis, which may have nervous, peptidergic and classic monoaminergic components, modulates the intricate and complicated pattern of communication between the brain and the stomach. Delineation of the precise pathways which make up this communication as well as their manipulation by various pharmacological agents will be the focus of future research endeavour.

Gastric cytoprotection by intracisternal interleukin-1 beta in the rat

The effects of intracisternal (ic) injection of recombinant interleukin-1 beta (IL-1) on absolute ethanol-induced gastric necrotic lesions were studied in conscious rats. IL-1 given ic inhibited ethanol-induced gastric lesions. The cytoprotective effect was dose dependent (ED175 ng/rat), long lasting with a maximal action when given 1-3 h prior to ethanol, blocked by ic injection of a IL-1 receptor antagonist protein (IRAP), and by intraperitoneal injection of indomethacin. IL-1, injected ic, was detected in the peripheral blood. However, IL-1 serum levels were lower after IL-1 injection ic than after ip at a dose giving equal gastric protection. These data show that ic IL-1 induces long lasting gastric protection mediated by interaction with IL-1 receptors and prostaglandin pathways at central and/or peripheral sites that remain to be localized.

Central and peripheral actions of calcitonin gene-related peptide on gastric secretory and motor function

CGRP exerts a potent central action to inhibit gastric acid secretion in rats and dogs and gastric emptying, contractility and ulcer formation in rats. The site of action to inhibit acid secretion has been localized in the dorsal vagal complex. The inhibition of acid secretion is related primarily to the decrease in vagal efferent activity whereas the inhibition of gastric motor functions involves increases in sympathetic outflow. The central action of CGRP to prevent ethanol-induced lesions is unique to this peptide and not shared by other centrally acting inhibitors of gastric function. It may be related to the increase in gastric mucosal blood induced by central CGRP. The presence of CGRP-like immunoreactivity and receptors in medullary nuclei receiving visceral information and influencing vagal outflow suggests a possible role of the peptide in the vagal regulation of gastric secretion. Peripheral injection of CGRP also inhibits acid secretion when administered peripherally in rats, dogs, rabbits and humans. Its antisecretory effect is unlikely to be related to a direct action on the parietal cells. It involves specific and marked release of gastric somatostatin through an interaction with CGRP receptors characterized on D cells and coupled with cAMP. In addition, CGRP induces a decrease in acetylcholine transmission in the enteric nervous system which may contribute to the inhibition of acid. Peripheral CGRP inhibits gastric emptying and motility by a direct action on smooth muscles through receptors linked with cAMP. The release of CGRP from spinal afferents innervating the stomach in response to stimulation of capsaicin-sensitive fibers suggests a role of the peptide in the regulation of gastric function.

Changes in gastric secretion after intracerebroventricular infusion of bombesin in dogs

Synthetic bombesin (BBS) was infused intracerebroventricularly in 14 mongrel dogs, to study the effects of the peptide on gastric secretion and on gastrin and neurotensin levels. The infusion was performed with a specific apparatus, and gastric fluid was collected with a Pavlov pouch. BBS was given in two series of experiments: as a bolus intracerebroventricular injection of 308.6 pmol/kg and as a continuous intracerebroventricular infusion at a rate of 617.3 pmol/kg/h for 30 min. The bolus injection caused a very significant decrease of gastric fluid volume, a significant decrease of HCl output, and a significant increase of its pH, while serum immunoreactive gastrin increased significantly. The continuous infusion of BBS caused similar changes in gastric secretion. The plasma neurotensin levels did not change. In conclusion, the intracerebroventricular administration of BBS increases the serum gastrin levels, decreases the volume and HCl content of gastric fluid, and increases its pH.

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.

Central nervous system action of TRH to influence gastrointestinal function and ulceration

There is clear evidence in rats that TRH acts in the brain to stimulate gastric acid, pepsin, and serotonin secretion, mucosal blood flow, contractility, emptying, and ulceration through activation of parasympathetic outflow to the stomach (TABLE 3). A number of TRH analogues, including some devoid of TSH-releasing activity, mimic the effects of TRH. The most sensitive TRH sites of action to elicit gastric acid secretion and motility are located in the dorsal vagal complex and include the dorsal vagal, nucleus tractus solitarius, and nucleus ambiguus. The gastrointestinal tract is one of the most responsive visceral systems to the central effects of TRH, because doses in the range of 1-10 pmol in the dorsal vagal complex stimulate gastric function, whereas stimulation of cardiovascular and respiratory function on microinjection of the brainstem nuclei requires higher doses. Although fewer investigations have been carried out in other species, evidence from the available data clearly indicates that TRH acts in the brain to increase gastric secretion and motility in the rabbit, sheep, and cat. Lack of stimulation of gastric acid secretion after third ventricle injection in the dog may be related to species difference or to rapid degradation of the peptide before it reaches its site of action. TRH acts centrally to stimulate gastric function and also intestinal secretion, motility, and transit as reported mostly in rabbits (TABLE 3). TRH produces enteropooling and release of serotonin in portal blood, increases duodenal and intestinal contractility and colonic transit, and elicits diarrhea. All these effects were shown to be vagally mediated. Stimulation of intestinal motility and transit by central injection of TRH has been observed in rats and sheep. The biological activity of centrally injected TRH is well correlated with the presence of TRH immunoreactivity and receptors in the dorsal vagal complex containing afferent and efferent connections to the stomach. Moreover, endogenous release of brain TRH in rats mimics the stimulatory effect of centrally injected TRH on gastric function. Although the lack of a specific TRH antagonist has hampered assessment of the physiological role of TRH, converging neuropharmacological, neuroanatomical, and physiological findings support the concept that TRH in the dorsal vagal complex may play a physiological role in the vagal regulation of gastrointestinal function.(ABSTRACT TRUNCATED AT 400 WORDS)

Central nervous system action of calcitonin to alter experimental gastric ulcers in rats

The central nervous system action of calcitonin to influence various experimental models of gastric ulcers and gastric function was studied in rats fasted for 24 h. Intracisternal injection of salmon calcitonin (5 micrograms) completely suppressed gastric ulcerations induced by exposure to cold restraint stress, intracisternal injection of a stable thyrotropin-releasing hormone analogue, or peroral administration of aspirin. By contrast, intracisternal calcitonin enhanced gastric lesions elicited by peroral administration of 40% ethanol or 0.6 N HCl. Calcitonin action was dose-dependent (0.01-1 microgram) and central nervous system mediated inasmuch as intravenous calcitonin, given at a dose 50-fold higher than that effective intracisternally, did not significantly modify gastric mucosal injuries elicited by aspirin or ethanol. Intracisternal injection of calcitonin at 0.01 microgram inhibited gastric acid output by 90% in pylorus-ligated rats and suppressed gastric emptying of a liquid meal by 63%-94% in doses ranging from 0.01 to 5 micrograms. Prostaglandin generation in the gastric mucosa was not modified by intracisternal injection of calcitonin. These results demonstrate that intracisternal calcitonin acts within the brain to potently prevent ulcer formation elicited by stress, thyrotropin-releasing hormone analogue, or aspirin, but is not cytoprotective against necrotizing agents. Calcitonin action is not related to modification of gastric prostaglandin generation but it may involve the inhibition of gastric secretory and motor function.

Central nervous system action of TRH to stimulate gastric function and ulceration

Intracisternal or intracerebroventricular injection of TRH (0.1-10 micrograms) in rats stimulated the secretion of gastric acid and pepsin secretion, increased gastric mucosal blood flow and gastric contractility and emptying, induced gastric hemorrhagic lesions and aggravated experimental ulcers elicited by aspirin, serotonin or indomethacin. TRH action was dose-dependent, rapid in onset and central nervous system-mediated by activation of the parasympathetic outflow to the stomach and cholinergic receptors. The stable TRH analog, RX 77368, was more potent and longer lasting than TRH. TRH and its stable analog appear as new chemical probes to produce centrally-mediated vagal-dependent stimulation of gastric function and experimental ulcers. The physiologic role of endogenous TRH in the central regulation of gastric function and ulceration remains to be established.

Effect of hypophysectomy, adrenalectomy, pituitary hormone secretion and gastric acid secretion on neurotensin induced gastric protection against stress gastric lesions

In previous studies we have established that intracisternal (i.c.) but not peripheral (intravenous) administration of neurotensin (NT), a brain and gastrointestinal tridecapeptide, totally prevents the development of gastric lesions produced by cold-restraint stress (CRS) with food-deprived rats. In this investigation, removal of the pituitary and adrenal gland, anterior pituitary hormone secretion and gastric acid secretion were evaluated independently as potential intermediates for NT's protective effect. NT (30 micrograms) produced a significant reduction of gastric lesions incidence and severity in intact and sham-operated controls. Adrenalectomy, but not hypophysectomy totally blocked the protective effect of i.c. NT. In addition, replacement therapy with s.c. prednisone (1 mg/kg) for 5 days following adrenalectomy did not restore the protective activity of central (i.c.) NT in adrenalectomized rats. A significant reduction of serum levels of TSH, PRL and GH following i.c. NT (30 micrograms) was observed after 2h of CRS. The gastrosecretory studies revealed that i.c. NT (30 micrograms) did not affect gastric acid secretion in pylorus ligated rats. However, blockade of peripheral (gut) cholinergic (muscarinic) receptors with i.p. atropine methylbromide (1 mg/kg) significantly raised gastric pH and reduced gastric acid concentration and output. In conclusion, the results of this study indicate that the acute protective effect of brain NT appears to be mediated, at least in part, by the sympathoadrenomedullary axis, and not by the pituitary gland or substances derived from the pituitary or by inhibition of gastric acid secretion.

Thyrotropin-releasing hormone (TRH) acts centrally to stimulate gastric contractility in rats

Changes in gastric contractility induced by intracisternal (ic) injection of thyrotropin-releasing hormone (TRH) or a stable TRH analog, RX77368 [p-Glu-His-(3,3'-dimethyl)-Pro NH2] were investigated in 24 h fasted-conscious rats. Gastric contractility was monitored using chronically implanted extraluminal force transducers sutured to the corpus. Response elicited by a standard meal was used as a physiologic standard. Intracisternal injection of TRH (1 microgram) or RX77368 (100 ng), unlike saline, stimulated high amplitude gastric contractions. The stimulation of gastric contractions induced by ic RX77368 was dose dependent (3-100 ng), rapid in onset, long lasting and not mimicked by the intravenous route of administration. Atropine (0.1 mg/kg) partially antagonized and vagotomy totally blocked the RX77368 (100 ng, ic)-induced stimulation of gastric contractility. These results demonstrated that TRH or RX77368 acts within the brain to elicit potent contractions of the stomach; TRH action appears vagally mediated probably through cholinergic mechanism.

The peptidergic brain-gut axis: influence on gastric ulcer formation

The existence of a relationship between the brain and the formation of gastric ulcers has been suspected since the last century. The advancement of stereotaxic procedures and the use of electrical lesion or stimulation have allowed localization within the limbic system, hypothalamus and brain stem, of discrete nuclei that influence the formation of gastric ulceration in experimental animals. Recently, further progress in the understanding of how the brain may influence gastric pathogenesis has been made by the demonstration that specific peptides act in the central nervous system to induce or prevent the formation of gastric ulcers and to markedly alter gastric secretory and motor function. Peptides established to have a centrally mediated protective effect are bombesin, calcitonin, corticotropin-releasing factor, neurotensin and opioid peptides. Growing evidence suggests a possible role for endogenous thyroptropin-releasing hormone in mediating cold-restraint stress induced gastric lesions. Circadian variations of the content and release of these peptides have been demonstrated in specific brain structures. To what extent such rhythms of peptide secretion are potentially linked to the circadian changes in the susceptibility to ulcer formation is worth investigating.

Chronobiologic factors in experimental stress ulcer

The available literature on chronobiologic factors in experimental stress ulcer is extremely small and thematically limited. It focuses almost exclusively on circadian rhythms and, within that, on rhythms related to light-dark cycles, activity and body temperature. Among these, only differences in ulcer induction related to circadian activity patterns have been adequately demonstrated. Other circadian patterns and other temporal phase relationships might be profitably explored, including those related to postnatal development. It is also likely that the important relationships between biorhythms and stress ulcer are not limited to ulcer induction. Future studies should address chronobiologic factors in predisposition, severity of illness, the probability of recovery and response to various therapeutic interventions.

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.

Neuroendocrine mechanisms of stress ulceration: focus on thyrotropin-releasing hormone (TRH)

It is generally accepted that stress ulceration, a multifactorial or pluricausal gastrointestinal disorder, may be the result of mechanistic interrelationships between mucosal, vascular, hormonal and neurogenic factors. The relative importance of each of these independent mechanisms remains unclear. This minireview represents an attempt to interpret many recent studies on certain neurogenic mechanisms and to integrate these observations into the existing body of knowledge. A variety of in vitro techniques and animal models to manipulate actual structures, organ systems, and certain well-defined hormonal influences have been utilized. The peripheral studies have followed, for the most part, the established observation that the stomach is under reciprocal control by sympathetic inhibitory and parasympathetic excitatory autonomic fibers. As a result, several autonomic adrenergic neurotransmitter substances have been found to promote mucosal resistance. Some of these include dopamine, epinephrine, and norepinephrine. Others in contrast, appear to promote vulnerability of the mucosa, and of these, the most well-studied include acetylcholine and histamine.

Differential effects of intracisternal neurotensin and bombesin on stress- and ethanol-induced gastric ulcers

This study evaluated the effect of intracisternal (I.C.) administration of the brain and gastrointestinal peptides neurotensin (NT) and bombesin (BOM) on the acute development of gastric ulcers induced by cold-restraint stress (CRS) and ethanol in rats. The results of this study can be summarized as follows: In confirmation of previous observations I.C. NT (30 micrograms) and BOM (1 microgram) significantly reduced gastric ulcer incidence and severity induced by 3h of CRS. The results of the ethanol preparation indicate that although I.C. BOM (1 microgram) significantly (P less than 0.05) increased intraluminal gastric pH and mucus, it did not prevent gastric ulcer formation. NT (30 micrograms), by contrast, was totally inactive in this ethanol model. These findings support a role for brain NT and BOM in protection against psycho-behavioral, but not chemical forms of ulcer-producing stress.

Role of brain neurotransmitters on neurotensin-induced gastric cytoprotection

We have reported previously that intracisternal (IC) administration of neurotensin (NT) prevents stress-induced gastric ulcers in rats. This effect of NT appears to be mediated by the central nervous system because peripheral (IV) NT is totally ineffective. The present study sought to clarify the central mechanism of the cytoprotective effect of NT by utilizing pharmacological treatments which alter the function of brain neurotransmitter systems. Pretreatment with intracerebroventricular (ICV) administration of agonists and antagonists of acetylcholine (ACh), gamma-aminobutyric acid (GABA), and serotonin (5-HT) receptors or with an anti-opiate (naloxone) agent did not significantly alter NT-induced cytoprotection. However, pretreatment with ICV haloperidol, a dopamine (DA) receptor antagonist, totally blocked NT's cytoprotective effect. In addition, pretreatment with methylphenidate, a DA receptor agonist, produced cytoprotection similar to IC NT. These data indicate that NT-induced cytoprotection is not mediated by 5-HT, GABA, ACh (muscarinic) receptors, or endogenous opiate systems, but suggest interactions between brain DA systems and NT.

The effects of centrally administered neuropeptides on the development of gastric lesions in the rat

Centrally administered neuropeptides were investigated for their effects on the development of gastric lesions in rats. Thyrotropin releasing hormone (TRH), vasoactive intestinal peptide (VIP) and gonadotropin releasing hormone (LHRH) produced gastric lesions acutely, with TRH demonstrating the most pronounced effect in terms of incidence and severity. Ten-fold higher doses of the same peptides administered intravenously produced none or very few gastric lesions. Moreover, pretreatment with atropine partially inhibited their production. Corticotropin releasing factor (CRF) exhibited only mild ulcerogenic effects, and the gastric lesions induced with this peptide developed more slowly than with TRH, VIP and LHRH. Although ulcerogenic in their own right, none of these four neuropeptides significantly potentiated the potent ulcerogenic effects of cold-restraint stress. Since other neuropeptides, including somatostatin, human pancreatic growth hormone releasing factor (hpGRF), substance P, bombesin, and neurotensin, had no demonstrable effects on gastric mucosa, we can conclude that the lesions were not a general effect of intracisternal administration of neuropeptides. The results suggest that within the central nervous system, there are several neuropeptides that play a significant role in the development of gastric lesions via, at least in part, vagal-dependent mechanisms.

Prolactin inhibits the development of stress-induced ulcers in the rat

Hyperprolactinaemia, as induced by pituitary homografts under the kidney capsule, was accompanied by an inhibition of development of gastric ulcers following the application of cold-plus-restraint stress in male rats. This effect was mimicked by intracisternal administration of a low dose of the hormone. Peripheral injection of the dopamine receptor antagonist, domperidone, also inhibited the development of stress-induced ulcers. However, no effect was found after peripheral injection of another dopamine receptor antagonist, haloperidol. This latter drug appeared to antagonize the cytoprotective effect of prolactin (PRL) on stress-induced ulcers. Furthermore, peripheral injection of the prostaglandin synthesis inhibitor, indomethacin, increased the incidence of gastric ulcers in hyperprolactinaemic rats subjected to cold -plus-restraint stress. These data suggest that the cytoprotective effect of PRL on development of gastric ulcers in stressed animals may involve both central (i.e. dopamine transmission) and peripheral (i.e. prostaglandin synthesis) mechanisms.

Gastric erosions induced by intracisternal thyrotropin-releasing hormone (TRH) in rats

Intracisternal injection of TRH (1 microgram) under light ether anesthesia induced within 4 hr gastric lesions in 24-hr fasted rats maintained unrestrained at room temperature. Saline, ovine corticotropin-releasing factor (oCRF, 10 micrograms), or human pancreatic growth hormone-releasing factor [hpGRF(1-40), 10 micrograms] tested under the same conditions did not modify the integrity of the gastric mucosa. TRH injected intravenously (100 micrograms/kg) proved to be ineffective. The production of gastric erosions elicited by intracisternal TRH (0.1-1 microgram) or by a stabilized TRH analog, RX 77368 [pGlu-His-(3,3'-dimethyl)-ProNH2, (0.01-0.1 microgram)] was dose-dependent. RX 77368 shows an enhanced potency over TRH. TRH action on gastric mucosa was reversed by atropine, omeprazole and cimetidine. These results demonstrate that TRH, unlike the other hypothalamic releasing factors CRF or GRF, is able to act within the brain to cause the formation of gastric erosions probably through mechanisms involving changes in gastric acid secretion. Intracisternal injection of TRH or its potent analog RX 77368 appears also as a new, simple method to produce centrally mediated experimental gastric erosions in 24 hr-fasted rats.

Enhancement by intracerebroventricular thyrotropin-releasing hormone of indomethacin-induced gastric lesions in the rat

Effects of the intracerebroventricular thyrotropin-releasing hormone (TRH) on gastric mucosa were studied in rats. TRH (3 and 10 micrograms rat-1 i.c.v.) produced slight gastric lesions and also aggravated indomethacin-, aspirin- or 5-hydroxytryptamine (5-HT)-induced gastric lesions, while restraint and cold stress-induced lesions were not influenced by TRH. Bethanechol used at a dose sufficient to produce acid secretion did not influence the gastric mucosa in intact or indomethacin-treated rats. Enhancement of indomethacin-induced gastric lesions by TRH was not inhibited to any significant degree by atropine 0.1 mg kg-1 s.c., which prevented TRH-induced gastric acid secretion, but tended to be inhibited by phentolamine, 2.5 mg kg-1 i.p. It is concluded that the enhancement by TRH of indomethacin-induced gastric lesions is due to a combination of the central and peripheral actions of the ulcerogenic agents.

The effect of centrally administered neuropeptides on the development of stress-induced gastric ulcers in rats

Neurotensin (NT) and bombesin, which are heterogeneously distributed in both brain and gastrointestinal tissue of several mammalian species, inhibit the formation of stress-induced gastric ulcers in rats. Many other endogeneous neuropeptides have also been reported to be present in brain and gastrointestinal tissue. The present study was conducted to evaluate the effect of some of these peptides on the development of cold-restraint stress (CRS)-induced gastric ulcers in rats. In addition, the effect of thyrotropin-releasing hormone (TRH), which antagonizes many of the CNS effects of NT, was investigated to determine whether this tripeptide antagonizes the cytoprotective effect of NT in this CRS model. All peptides were initially administered intracisternally (ic) in doses equimolar to 30 micrograms NT. As previously reported, NT (30 micrograms, ic) completely prevented the development of gastric ulcers in rats exposed to three hours of CRS. Bombesin, beta-endorphin, substance P, and somatostatin also exhibited cytoprotective activity. Several other peptides studied in the CRS model exerted no significant effects on the development of gastric ulcers; these included cholecystokinin octapeptide, gastrin, leu-enkephalin, met-enkephalin, and bradykinin. Two peptides, vasoactive intestinal polypeptide and TRH, significantly increased the severity of gastric ulcerations. The cytoprotective effect of NT was dose dependent. In contrast, lower doses of beta-endorphin, substance P, and somatostatin were cytoprotective whereas higher doses were not. Finally, concomitant ic injections of TRH antagonized the cytoprotective effects of NT and bombesin, but not that of beta-endorphin. The present results suggest that certain brain peptides may participate in modulating the gastric mucosal barrier, thereby increasing or decreasing its vulnerability to stress-induced lesions.

The effect of intracisternally administered bombesin on cysteamine-induced duodenal ulcers in rats

Cysteamine (420 mg/kg s.c.) increased gastric acid secretion in rats 3-6 h post-injection; duodenal pH fell from 6.0 +/- 0.2 (S.E.M.) at 0 time to 2.2 +/- 0.3 at 12 h. A high (60-80%) incidence of duodenal ulcerations was observed 18 h after cysteamine injection. Intracisternal (i.c.) administration of bombesin (1 microgram) significantly inhibited both cysteamine-induced increases in gastric acid secretion and the development of duodenal ulcers. This effect of bombesin was dose-dependent and appears to be relatively specific since i.c. neurotensin (1 or 30 micrograms) was not cytoprotective. Peripheral cholinergic (muscarinic) blockade with atropine methylbromide (10 mg/kg i.p.) was as effective as i.c. bombesin (1 microgram) in inhibiting cysteamine-induced gastric hypersecretion and duodenal ulcers.

Endogenous opiates and stress ulceration

Parenteral administration of the opiate antagonist, naltrexone, had a cytoprotective effect against stress-induced ulceration. This effect appears to be due to blockade of peripheral rather than central endogenous opiates and is not related to the central inhibitory effect of opiates on gastric acid secretion. Opiates have complex effects on gastric mucosal blood flow which may explain their role in stress ulceration.

Is somatostatin or a somatostatin-like peptide involved in central nervous system control of gastric secretion?

Intracisternal injection of octapeptide analogs of somatostatin (SS), Cys-Phe-Phe-D-Trp-Lys-Thr-Phe-Cys (des-AA1,2,4,5,12,13-[D-Trp8]SS (ODT8-SS)) and Cys-Phe-Phe-D-Trp-Lys-Thr-Phe-D-Cys, increased the volume and the acid output of gastric secretion in rats. ODT8-SS given intravenously did not affect basal gastric secretion. The gastrosecretory effect of ODT8-SS, administered intracisternally is dose-dependent (0.01-1 micrograms), long acting, reversible, specific, and abolished by vagotomy, or systemic injection of atropine or SS. SS (5-10 micrograms) or [D-Trp8]SS (1 microgram) had no effect on gastric secretion when given intracisternally. These results demonstrate that some octapeptide SS analogs, unlike SS or other SS analogs, have the capability to act in the brain to induce a vagal dependent stimulation of gastric secretion.

Brain regulation of gastric acid secretion in rats by neurogastrointestinal peptides

Brain alteration of catecholaminergic, serotoninergic, dopaminergic, gabaergic and cholinergic pathways by intracisternal injection of agonists, antagonists or specific neurotoxic drugs did not significantly affect gastric acid secretion in 2 hr pylorus-ligated rats. In contrast, several neuropeptides were found to be very potent in influencing gastric secretion when administered intracisternally but not when given intravenously. Bombesin-like peptides, opioid peptides and arginine vasopressin acted within the brain to inhibit gastric acid secretion through yet unknown neurohumoral pathways, whereas TRH and some somatostatin analogs elicited brain stimulation of gastric acid output through vagal-dependent mechanisms. These observations have led to the concept that some specific neuropeptides may be important chemical messengers involved in physiologic brain processes regulating gastric acid secretion, and appear as new chemical probes to further investigate the brain-gut relationship.

Thyrotropin-releasing hormone--CNS action to stimulate gastric acid secretion

Much physiological and pharmacological evidence has accumulated to suggest that the autonomic nervous system has an important role in the peripheral modulation of gastric secretion, although the neurochemical mediators in the brain which initiate or modulate autonomic input are poorly understood. Recently, the demonstration that some oligopeptides present in mammalian brain act in the central nervous system (CNS) to influence profoundly glucoregulation, thermoregulation, blood pressure, sympathetic outflow, muscular activity of gut and stress-induced gastric haemorrhagic lesions have led us to examine a possible role for some of these endogenous brain oligopeptides as chemical messengers involved in the CNS modulation of gastric secretion. We report here that thyrotropin-releasing hormone (TRH) acts within the CNS to elicit a vagus-dependent stimulation of gastric acid secretion.

Brain regulation of gastric secretion: influence of neuropeptides

Several neuropeptides injected intracisternally were assessed for their effects on gastric secretion in rats. Bombesin (1 microgram) completely suppressed gastric acid secretion, produced the volume of gastric secretion, and partially blocked insulin- or 2-deoxy-D-glucose-induced stimulation of gastric acid output. The inhibitory effect of this peptide is dose-dependent, long-acting, reversible, and specific. Bombesin response appears to be central nervous system-mediated; its expression is not dependent on the vagus nerve or the adrenal glands, and does not rely on a decrease in gastrin secretion. Among seven other peptides tested, only beta-endorphin and a potent gonadotropin releasing-factor (gonadoliberin) agonist significantly reduced gastric acid secretion, with an activity ca. 100 times less than that of bombesin. The presence of bombesin-like material in rat brain and the high potency of bombesin to inhibit gastric secretion suggest that this peptide may be of physiologic significance as a chemical messenger involved in brain modulation of gastric secretion.

Changes in body temperature after administration of acetylcholine, histamine, morphine, prostaglandins and related agents

This survey, the second in a series, presents extensive tabulations of literature, primarily since 1965, on thermoregulatory effects of cholinergic agonists and antagonists, histamine and H1- and H2-receptor antagonists, narcotic analgesics and antagonists in both non-tolerant and tolerant subjects and of prostaglandins and related agents. The information listed includes the species used, route of administration and dose of drug, the environmental temperature at which the experiments were performed, the number of tests, the direction and magnitude of body temperature change and remarks on the presence of special conditions, such as age or lesions, or on the influence of other drugs, such as antagonists, on the response to the primary drug.

Bombesin-induced poikilothermy in rats

The effect of ambient temperature (Ta) on the rectal temperature (Tre) response to intraventricular injection of bombesin has been evaluated in conscious adult male rats. At Ta = 4 degrees C, bombesin (50 ng-1 microgram) caused a marked hypothermia which was dose-dependent both in terms of the magnitude and of the duration of the response. The bombesin-induced hypothermia was reduced at Ta = 24 degrees C, whereas at Ta = 31 or 33 degrees C, the peptide (1 microgram) failed to affect Tre.AtTa = 36 degrees C, bombesin 1-10 micrograms induced an elevation in Tre. The hyperthermia observed at high Ta could be reversed to hypothermia by transferring rats to cold. The analogs [d-Trp8]bombesin or [d-Leu13]bombesin, tested under the same conditions, failed to produce significant changes in Tre. These findings demonstrate that bombesin appears to act in the brain as a poikilothermic agent by disrupting thermoregulation at temperatures below or above thermoneutrality.

Changes in body temperature after administration of amino acids, peptides, dopamine, neuroleptics and related agents

Drugs may alter body temperature by acting on any component of the thermoregulatory system. These components include heat production, heat conservation and heat loss effectors and their efferent pathways, thermosensors and their afferent pathways and neurons within the central nervous system that coordinate thermoregulatory effector activities. A thermostat is often thought to be involved although thermoregulation can be explained by models that do not incorporate a thermostat. An action on a particular component can be assessed by determining the effect of a drug on body temperature over a range of environmental temperatures and by observation and measurement of associated changes in effector activities. A scheme for such assessment is presented along with examples of its use. The study of drug-induced changes in body temperature has expanded greatly within the past decade. The primary purpose of this review is to provide a readily available source of information on interactions between certain drugs and the thermoregulatory system. Extensive tables are presented of body temperature changes after administration of amino acids, peptides, dopamine and related agents, phenothiazine neuroleptics and also phenothiazines that lack neuroleptic activity, butyrophenones, diphenylbutylpiperidines such as pimozide and miscellaneous neuroleptics. The information tabulated includes the species used, route of administration and dose of drugs, the environmental temperature at which the experiments were performed, the number of tests, the direction and magnitude of body temperature change and remarks on the presence of special conditions, such as age or lesions, or on the influence of other drugs, such as antagonists, on the response to the primary drug. Most of the cited literature was published since 1965.