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

Role of the autonomic nervous system in the cytoprotective effect of neurotensin against gastric stress ulcers in rats

Pharmacologic agents were used to study the role of the autonomic nervous system in the cytoprotection produced by intracisternal neurotensin against cold plus restraint stress-induced gastric ulcers in rats. Drugs which stimulated alpha- or beta-adrenergic receptors or blocked muscarinic cholinergic receptors reduced the incidence of ulcers to a similar degree as intracisternal neurotensin; alpha-adrenergic or beta-adrenergic blockade as well as cholinergic stimulation prevented neurotensin's beneficial effect. However, pretreatment with indomethacin blocked only the cytoprotective effect of neurotensin or beta-adrenergic stimulation, but not that of muscarinic cholinergic blockade. In addition, pretreatment with reserpine or guanethidine also was effective in preventing cytoprotection by intracisternal neurotensin. These data indicate that the mechanism for cytoprotection by centrally administered neurotensin is mediated at least in part through activation of the sympathetic nervous system. This activation by neurotensin appears to produce cytoprotection by stimulation of gastric mucosal prostaglandin synthesis.

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.

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.

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.

Thyrotropin-releasing hormone: medullary site of action to induce gastric ulcers and stimulate acid secretion

This study evaluated the hypothesis that the dorsal motor nucleus (DMN) of the brainstem may mediate the ulcerogenic and acid-stimulatory effects of thyrotropin-releasing hormone (TRH) in rats. To accomplish this, intra-DMN microinjections of TRH (50 and 500 ng) were performed and their effects on acid secretion and gastric ulcer formation evaluated in the pylorus-ligation model. The high (500 ng), but not the low dose of TRH (50 ng) produced gastric glandular lesions in 64% of the rats with a mean severity index (no. of ulcers/rat) of 6.4 +/- 0.98 and significantly increased gastric acid output. The ulcerogenic and gastric secretory response to intra-DMN TRH was site-specific. We conclude that presynaptic TRH fibers may modulate vagal activity at the level of the DMN and propose that descending TRH pathways may play a role in experimental ulcerogenesis through acid hypersecretion.

Brain vasoactive intestinal peptide: a potent stimulant of gastric acid secretion

Vasoactive intestinal peptide (VIP) belongs to a rapidly expanding family of cerebrogastrointestinal oligopeptides. This report describes a potent dose-, time- and vagus-dependent stimulation of gastric acid secretion by brain VIP. Physiologic evidence favoring a role of VIP on acid secretion was provided by the finding that immunoneutralization of endogenous brain VIP produced a significant decrease of acid secretion. These and other data contained herein suggest that the mechanism by which central VIP stimulates acid secretion appears to involve peripheral cholinergic pathways.

Potent CNS action of calcitonin to inhibit cysteamine-induced duodenal ulcers in rat

Intracisternal injection of calcitonin (0.01-5 micrograms) dose dependently prevented the development of duodenal ulcers induced by cysteamine in female rats. By contrast, intravenous infusion of the peptide at a dose 50 times higher than an effective intracisternal dose, had no effect. Intracisternal injection of calcitonin increased by three fold the generation of 6-keto-PGF1 alpha, the stable hydrolysis product of PGI2, in the duodenal mucosa. These studies demonstrated that calcitonin acts within the brain to potently suppress duodenal ulcers induced by cysteamine. The mechanisms of the antiulcer effect may involve changes in prostaglandin generation along with alterations of gastrointestinal secretion and motility associated the central injection of calcitonin. Growing evidence suggests that salmon calcitonin may act as a neuromodulator or neurotransmitter in the central nervous system. Specific binding sites have been demonstrated for calcitonin in the hypothalamus, brain stem and dorsal horn of the spinal cord using homogenate and membrane preparations or in vitro autoradiography methods. The peptide injected into the cerebrospinal fluid (CSF) produces a wide spectrum of biological effects including analgesia, hyperthermia, changes in pituitary hormone release, decrease in food and water intake, locomotor activity, and blood pressure. Numerous studies also demonstrated that calcitonin acts within the brain to markedly influence gastrointestinal secretory and motor function in rats and dogs and gastric ulceration in rats. In particular, intracisternal injection of salmon calcitonin was found very potent to selectively inhibit gastric ulcers elicited by stress, aspirin and central thyrotropin-releasing factor but not by necrotizing agents. In the present study, we further investigated the antiulcer effect of salmon calcitonin using the well established cysteamine experimental model to induce duodenal ulcers in rats. Part of this work has been reported in abstract form.

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.

Development and significance of cysteamine and propionitrile models of duodenal ulcer

Cysteamine is widely used in rodents to induce duodenal ulcer. Herein, the pathogenesis of duodenal ulceration in its earliest stages was reviewed using findings from cysteamine- and propionitrile-induced duodenal ulcer in rodent models, especially taking into account changes in the secretion of gastric acid, duodenal and pancreatic bicarbonate as well as gastroduodenal motility. The effect of cysteamine-HCl in inducing ulcers in rats is circadian rhythm-dependent. The effect is greatest from just before the end of diurnal rest to just after the start of nocturnal activity. The chronobiologic effect may be in part due to the circadian rhythm-dependent increased gastric acid production from cysteamine. Titratable acidity was found to be twice as great in the gastric juice of rodents when cysteamine was given by injection at 2000 (just after the start of nocturnal activity) in comparison to when given at 0800 or 1200 (at the beginning or middle span of daily rest). Further studies have shown that adrenalectomy of rats 7 days before cysteamine administration obliterated the observed circadian susceptibility to ulcer formation. Duodenal ulceration, at least in the cysteamine model, appears to be under chronobiologic neuroendocrine control or influence, seemingly mediated by the adrenal glands.

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.

Gastric mucosal protection against ulcerogenic factors in the rat mediated by capsaicin-sensitive afferent neurons

Treatment of newborn rats with capsaicin (0.16 mmol/kg) is known to cause a permanent degeneration of certain, primarily unmyelinated, afferent neurons. In this study, experimentally induced gastric ulceration was investigated in adult rats treated with capsaicin as neonates. It was found that in capsaicin-treated animals the formation of gastric mucosal lesions in response to indomethacin, ethanol, or cysteamine was significantly enhanced as compared with vehicle-treated controls. The duodenal ulceration caused by cysteamine was not altered after capsaicin treatment. In further experiments the possible pathways involved in the effect of capsaicin treatment on gastric mucosal protection were explored. It was found that the capacity of gastric tissue to release prostaglandins E2 and I2 was unchanged after capsaicin treatment. Atropine, hexamethonium, cimetidine, or terbutaline all reduced gastric ulceration in response to indomethacin in both solvent- and capsaicin-treated rats but did not counteract the enhancement of the ulcerogenic effect of indomethacin in capsaicin-treated rats. Guanethidine enhanced the ulcerogenic effect of indomethacin in solvent-treated but not in capsaicin-treated animals. Ethanol-induced formation of gastric lesions remained unaltered by guanethidine in both solvent- and capsaicin-treated rats. These results indicate that capsaicin-sensitive afferent neurons are involved in gastric mucosal protection against ulcerogenic factors. The data further suggest that this type of gastric defense is primarily due to a local mechanism initiated by sensory nerve endings in the gastric mucosa.

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.

Neural pathways involved in intracisternal bombesin-induced inhibition of gastric secretion in rats

Possible humoral and neural components mediating intracisternal bombesin (500 ng)-induced inhibition of gastric secretion were studied in rats. Intracisternal bombesin induced a marked rise in gastric pH from 2.0 +/- 0.5 up to 6.5 +/- 0.6 whereas gastric secretion in parabiotic saline-treated partners was not modified. Bombesin inhibited pentagastrin-stimulated gastric acid output by 88 +/- 5% in intact rats and 71 +/- 7% in vagotomized animals. Spinal cord transection at the level of the 5th or 7th cervical but not the 8th or 13th thoracic vertebrae significantly reversed the rise in pH and the decrease in acid concentration induced by bombesin and abolished the hyperglycemic effect of the peptide. These results suggest that bombesin inhibitory action on gastric secretion is not mediated through humoral factors but through neural pathways in part related to the sympathetic nervous system along the spinal cord, whereas the parasympathetic outflow from the vagus or the sacral spinal nerves does not seem to play an important role.

Central nervous system action of bombesin to inhibit gastric acid secretion

Bombesin or gastrin releasing peptide injected into the lateral, third, or fourth ventricle, or into the cisterna magna, inhibited gastric acid secretion induced by a wide variety of gastric acid stimulants in several animal models. Studies of bombesin microinfusion into specific hypothalamic nuclei of intact rats, or injection into the cisterna magna of midbrain transected rats, indicated that the peptide can trigger inhibition of gastric acid secretion from both forebrain and hindbrain structures. The neural pathways mediating bombesin action required the integrity of the cervical spinal cord; the vagus did not play an important role. Spantide, a substance P and bombesin receptor antagonist, was not useful in studying the physiological role of bombesin. This was due both to its inability to reverse the central action of bombesin on gastric secretion, and to its in vivo toxicity.

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.

Cysteamine and propionitrile inhibit the rise of duodenal mucosal alkaline secretion in response to luminal acid in rats

Effects of subulcerogenic doses of cysteamine (100 mg/kg s.c.) and propionitrile (5 mg/kg) on alkaline secretion by duodenal surface epithelium and pH at the surface of this mucosa were assessed in duodenum of anesthetized rats. Alkaline secretion was titrated in situ, using segments of duodenum just distal to the Brunner's glands area and devoid of pancreatic HCO3-. Surface pH was measured by advancing pH-sensitive microelectrodes from the luminal solution to the epithelial cell surface. Proximal duodenum from bullfrogs was used to study effects of cysteamine on alkaline secretion in vitro. Cysteamine caused an increase in alkaline secretion in the rat during the first hour after administration, but rates after 5 and 20 h were the same as in controls and cysteamine (1 mg/ml) had no effect on secretion in vitro. Neither in vitro nor in vivo did cysteamine affect the rise in alkaline secretion in response to exogenous prostaglandin E2 (and dibutyryl-cyclic adenosine monophosphate). Luminal acid is a potent stimulant of duodenal mucosal alkaline secretion. By delayed (5 h) actions, both cysteamine and propionitrile inhibited the rise in alkaline secretion in response to a 5-min exposure to luminal acid with pH 2.00 in the rat. Cysteamine also depressed the ability of this mucosa to maintain a high rate of alkaline secretion during sustained exposure at pH 2.00 but had no such effect at pH 5.00. The former resulted in acidification of the pH gradient at the mucosal surface. Cysteamine is thus probably without effect on the HCO3- secretory process itself but impairs the ability of the duodenal mucosa to respond to acid. Inhibition of mechanisms mediating this response may contribute to the duodenal ulcerogenic actions of cysteamine and propionitrile.

Prevention of stress-induced gastric ulcers by dopamine agonists in the rat

Dopamine (DA) and DA agonists have been shown to exert a protective role against the formation of duodenal ulcers. The effect of stimulation of DA receptors on the development of stress-induced gastric ulcers is currently unknown. Accordingly, we evaluated the effect of several DA agonists on the development of gastric ulcers induced by 3 h of cold + restraint stress (CRS) in rats. Apomorphine, d-amphetamine, methylphenidate, and threo-dl-p-hydroxymethylphenidate (an hydroxylated analog of methylphenidate), significantly reduced both the incidence and severity of CRS-induced gastric ulcers. The gastric cytoprotection afforded by these agents was dose-related, and completely antagonized by pretreatment with the peripherally acting DA antagonist domperidone. Because domperidone blocks peripheral, but not central, DA receptors, and since the entry of threo-dl-p-hydroxymethylphenidate across the blood-brain barrier into the brain is restricted to a great extent, we conclude that stimulation of peripheral DA receptors is primarily involved in the gastric cytoprotection induced by dopamimetics.

Contractile effects of cysteamine on the guinea-pig ileum

Cysteamine (beta-mercaptoethylamine HCl) (1.0-40.0 mM) induced a concentration-dependent increase in tonic and phasic contractions of segments of guinea-pig ileum in vitro. Myenteric plexus-longitudinal muscle (MPLM) preparations also responded with an increase in tonic contractions but phasic contractions were either greatly reduced or absent, indicating that these were a response of the circular muscle. Atropine (5 microM) inhibited the cysteamine-induced contractions, whereas hexamethonium and guanethidine had no effect, suggesting that cysteamine was acting at least partly via a cholinergic mechanism involving muscarinic receptors. Tetrodotoxin increased the phasic contractions of ileal segments, but had no effect on the tonic component. Treatment of MPLM preparations with morphine (1 microM) resulted in a small reduction in responsiveness to cysteamine, and blocked electrically-induced contractions by at least 90%. Since morphine acts by inhibiting acetylcholine release via hyperpolarization of intrinsic neurones, a small but significant part of the cysteamine-induced contractions probably resulted from stimulation of acetylcholine release from intrinsic neurones. Following a response to high cysteamine concentrations (greater than 15 mM) tissues were refractory to subsequent cysteamine administration. Cross-desensitization between cysteamine and acetylcholine also occurred, as short-term (1-3 min) incubation of MPLM preparations with high concentrations of either compound (1-10 microM acetylcholine or 20 mM cysteamine) resulted in a reduced responsiveness to both. A reduced sensitivity to acetylcholine or cysteamine was obtained following long-term (45 min) incubation with acetylcholine (1 microM). Removal of Na+ from the incubation medium negated this effect. In contrast, the refractoriness to acetylcholine or cysteamine following long-term (45 min) incubation with cysteamine (20 mM) was accentuated in low Na+ medium. It is suggested that cysteamine induces a contraction of both the circular and longitudinal muscle of the guinea-pig ileum by stimulating the release of acetylcholine from intrinsic neurones, by an action at the level of the smooth muscle muscarinic receptor, and possibly by a non-cholinergic mechanism. However, the mechanisms by which acetylcholine and cysteamine induce tissue refractoriness probably differ.