CGRP antagonists enhance gastric acid secretion in 2-h pylorus-ligated rats

Virtual drug repurposing study for the CGRPR identifies pentagastrin and leuprorelin as putative candidates

Calcitonin gene-related peptide receptor (CGRPR) is a heterodimer consisting of CLR and RAMP1 proteins. Activation of the CGRPR with the endogenous peptide CGRP is known to play a crucial role in migraine pathophysiology. CGRP occupies two regions in the CGRPR upon binding, namely ectodomain and transmembrane sites (sites 1 and 2, respectively). The disruption of the CGRPR heterodimer interface is one of the main strategies to prevent CGRPR activation and its resulting effects. So far, FDA approved monoclonal antibodies and small molecule gepant inhibitors are considered for the treatment of acute or chronic migraine symptoms. However, most of these gepants have severe side effects. Thus, in this study, a virtual drug repurposing approach is applied to CGRPR to find alternative or better molecules that would have a potential to inhibit or block the CLR - RAMP1 interface compared to known gepant molecules. A small molecule library of FDA-approved molecules was screened in these two different binding sites, further simulations were performed and analyzed. The objectives of this study are (i) to repurpose an FDA-approved drug having more potent features for CGRPR inhibition compared to gepants, and (ii) to examine whether the transmembrane binding site (site 2) accepts small molecules or small peptide analogues for binding. As a result of this extensive in silico analysis, two molecules were identified, namely pentagastrin and leuprorelin. It is shown that FDA approved compound rimegepant and the identified pentagastrin molecules form and maintain the interactions through CLR W72 and RAMP1 W74, which are the residues revealed to have an important role in CGRPR antagonism at binding site 1. At binding site 2, the interactions needed to be formed for CGRP binding are not captured by rimegepant nor leuprorelin, yet leuprorelin forms more interactions throughout the simulations, meaning that small molecules are also capable of binding to site 2. Moreover, it is found that the crucial interactions for receptor signaling and heterodimerization occurred between CLR and RAMP1 interface are disrupted more with the ligands bound to ectodomain site, rather than the transmembrane domain. These findings of pentagastrin and leuprorelin molecules are recommended to be considered in further de novo drug development and/or experimental studies related to CGRPR signaling blockade and antagonism.

The Effect of Cochinchina momordica Seed Extract on Gastric Acid Secretion and Morphologic Change in Aged Rat Stomach

Background/Aims

Cochinchina momordica seed extract (SK-MS10) has a gastric protective effect. We aimed to assess the effect of SK-MS10 on gastric acid secretion with morphologic changes in the aged rat.

Methods

Acid secretions were evaluated in the male F344 rats of four different ages (6-, 31-, 74-week, and 2-year). The 31-week-old rats were divided to three groups and continuously administered chow containing vehicle, SK-MS10 and lansoprazole, respectively. At the age of 74 weeks and 2 years, basal and stimulated acid was measured and the expression of mRNA and protein of H⁺-K⁺-ATPase were determined. The area of connective tissue of lamina propria was measured.

Results

Basal and stimulated gastric acid significantly decreased and connective tissue of lamina propria increased with age. The expression of mRNA and protein of H⁺-K⁺-ATPase significantly decreased with age. However, 74-week-old rats in the SK-MS10 group had higher stimulated gastric acid secretion than those in the vehicle and lansoprazole groups. In 2-year-old rats of SK-MS10 group, there was no increase of connective tissue.

Conclusions

As SK-MS10 kept the capacity of acid secretion as well as connective tissue area to comparable to young rats, it might valuable to perform further research regarding mechanism of SK-MS10 as an antiaging agent in the stomach.

H. pylori acutely inhibits gastric secretion by activating CGRP sensory neurons coupled to stimulation of somatostatin and inhibition of histamine secretion

Acute Helicobacter pylori infection produces hypochlorhydria. The decrease in acid facilitates survival of the bacterium and its colonization of the stomach. The present study was designed to identify the pathways in oxyntic mucosa by which acute H. pylori infection inhibits acid secretion. In rat fundic sheets in an Ussing chamber, perfusion of the luminal surface with H. pylori in spent broth (10(3)-10(8) cfu/ml) or spent broth alone (1:10(5) to 1:10(0) final dilution) caused a concentration-dependent increase in somatostatin (SST; maximal: 200 ± 20 and 194 ± 9% above basal; P < 0.001) and decrease in histamine secretion (maximal: 45 ± 5 and 48 ± 2% below basal; P < 0.001); the latter was abolished by SST antibody, implying that changes in histamine secretion reflected changes in SST secretion. Both responses were abolished by the axonal blocker tetrodotoxin (TTX), the sensory neurotoxin capsaicin, or the CGRP antagonist CGRP8-37, implying that the reciprocal changes in SST and histamine secretion were due to release of CGRP from sensory neurons. In isolated rabbit oxyntic glands, H. pylori inhibited basal and histamine-stimulated acid secretion in a concentration-dependent manner; the responses were not affected by TTX or SST antibody, implying that H. pylori can directly inhibit parietal cell function. In conclusion, acute administration of H. pylori is capable of inhibiting acid secretion directly as well as indirectly by activating intramural CGRP sensory neurons coupled to stimulation of SST and inhibition of histamine secretion. Activation of neural pathways provides one explanation as to how initial patchy colonization of the superficial gastric mucosa by H. pylori can acutely inhibit acid secretion.

Capsaicin and gastric ulcers

In recent years, infection of the stomach with the organism Helicobacter Pylori has been found to be the main cause of gastric ulcers, one of the common ailments afflicting humans. Excessive acid secretion in the stomach, reduction in gastric mucosal blood flow, constant intake of non-steroid anti-inflammatory drugs (NSAIDS), ethanol, smoking, stress etc. are also considered responsible for ulcer formation. The prevalent notion among sections of population in this country and perhaps in others is that "red pepper" popularly known as "Chilli," a common spice consumed in excessive amounts leads to "gastric ulcers" in view of its irritant and likely acid secreting nature. Persons with ulcers are advised either to limit or avoid its use. However, investigations carried out in recent years have revealed that chilli or its active principle "capsaicin" is not the cause for ulcer formation but a "benefactor." Capsaicin does not stimulate but inhibits acid secretion, stimulates alkali, mucus secretions and particularly gastric mucosal blood flow which help in prevention and healing of ulcers. Capsaicin acts by stimulating afferent neurons in the stomach and signals for protection against injury causing agents. Epidemiologic surveys in Singapore have shown that gastric ulcers are three times more common in the "Chinese" than among Malaysians and Indians who are in the habit of consuming more chillis. Ulcers are common among people who are in the habit of taking NSAIDS and are infected with the organism "Helicobacter Pylori," responsible for excessive acid secretion and erosion of the mucosal layer. Eradication of the bacteria by antibiotic treatment and avoiding the NSAIDS eliminates ulcers and restores normal acid secretion.

Protective effect of proteinase-activated receptor 2 activation on motility impairment and tissue damage induced by intestinal ischemia/reperfusion in rodents

We hypothesized that proteinase-activated receptor-2 (PAR(2)) modulates intestinal injuries induced by ischemia/reperfusion. Ischemia (1 hour) plus reperfusion (6 hours) significantly delayed gastrointestinal transit (GIT) compared with sham operation. Intraduodenal injection of PAR(2)-activating peptide SLIGRL-NH(2) significantly accelerated transit in ischemia/reperfusion but not in sham-operated rats. GIT was significantly delayed in ischemia/reperfusion and sham-operated PAR(2)(-/-) mice compared with PAR(2)(+/+). SLIGRL-NH(2) significantly accelerated transit in ischemia/reperfusion in PAR(2)(+/+) but not in PAR(2)(-/-) mice. Prevention of mast cell degranulation with cromolyn, ablation of visceral afferents with capsaicin, and antagonism of calcitonin gene-related peptide (CGRP) and neurokinin-1 receptors with CGRP(8-37) and RP67580, respectively, abolished the SLIGRL-NH(2)-induced stimulatory effect on transit in ischemia/reperfusion. Tissue damage was significantly reduced by SLIGRL-NH(2); this effect was not observed in cromolyn-, capsaicin-, or RP67580-treated rats but was detected following CGRP(8-37). Intestinal PAR(2) mRNA levels were not affected by SLIGRL-NH(2) in ischemia/reperfusion. We propose that PAR(2) modulates GIT and tissue damage in intestinal ischemia/reperfusion by a mechanism dependent on mast cells and visceral afferents. PAR(2) effect on transit might be mediated by CGRP and substance P, whereas the effect on tissue damage appears to involve substance P but not CGRP. PAR(2) might be a signaling system in the neuroimmune communication in intestinal ischemia/reperfusion.

Nitric oxide synthase activity in rat gastric mucosa contributes to mucin synthesis elicited by calcitonin gene-related peptide

The majority of research for the calcitonin gene-related peptide (CGRP) in the stomach has been devoted to the submucosal blood flow, and only slight attention has been paid to its involvement in the gastric epithelial function. In this study, we examined the age-related change in the CGRP-containing nerves and its effects on the mucus metabolism. We compared the immunoreactivity for CGRP in the gastric mucosa of 7-week-old rats (young) to that of 52-week-old animals (middle-aged). The effects of CGRP on the mucin biosynthesis were compared using the stomachs from both young and middle-aged rats. The nitric oxide synthase (NOS) activity was measured in the surface and deep mucosa of the gastric corpus. The density of the CGRP nerve fibers was reduced in both the lamina propria and submucosa of the middle-aged rats compared to the young rats. CGRP stimulated the mucin biosynthesis in the cultured corpus mucosa from the 7-week-old rats, but not from the 52-week-old rats. The total NOS activity of the surface layer in the corpus mucosa was markedly reduced in the middle-aged rats compared to the young rats. These findings demonstrate the age-dependent reduction in the CGRP-induced mucin biosynthesis, as well as in the density of the CGRP fibers in the rat stomach. The decreased NOS activity in the surface layer of the oxyntic mucosa in the aged rats may also be a principal cause for the lack of regulation of the mucin biosynthesis by CGRP.

Lafutidine, a newly developed antiulcer drug, elevates postprandial intragastric pH and increases plasma calcitonin gene-related peptide and somatostatin concentrations in humans: comparisons with famotidine

Lafutidine, a newly developed histamine H(2)-receptor antagonist, inhibits daytime (i.e., postprandial) as well as nighttime gastric acid secretion in clinical studies. It also has gastroprotective activity that particularly affects mucosal blood flow in rats. This study focused on the efficacy of lafutidine on plasma concentrations of gastrointestinal peptides in humans. Six healthy male volunteers aged 23-32 years without Helicobacter pylori infection were orally administered either 10 mg lafutidine, 20 mg famotidine, or water only (control) 30 min after a standard meal (650 kcal). Plasma concentrations of lafutidine and famotidine were highest from 90 to 150 min after administration. Intragastric pH was elevated after both lafutidine and famotidine compared with the control. Plasma concentrations of calcitonin gene-related peptide (CGRP) and somatostatin were significantly increased after lafutidine at 60 and 90 min. We concluded that lafutidine increases plasma concentrations of CGRP and somatostatin in humans, which may result in inhibition of postprandial acid secretion and gastroprotective activity.

Role of somatostatin receptors on gastric acid secretion in wild-type and somatostatin receptor type 2 knockout mice

Somatostatin, probably acting through somatostatin type 2 receptors (SSTR2), is the main inhibitor of gastric acid secretion. We characterized gastric acid secretion in SSTR2 knockout mice, and used preferential somatostatin receptor agonists to assess the relative role of SSTR1, 2, 3, 4, and 5 on gastric acid secretion. Basal gastric acid secretion and the secretory response to a meal were similar in conscious wild-type and knockout mice. However, under urethane anesthesia, which releases endogenous somatostatin, SSTR2 knockout mice had a basal secretion 11-15-fold higher than wild-type animals (micromol/10 min:1.40+/-0.09 vs. 0.10+/-0.01, p<0.05). Gastrin immunoneutralization or H(2) receptors blockade (cimetidine), but not cholinergic blockade (atropine), reduced the high basal secretion in SSTR2 knockout mice. In SSTR2 knockout mice, gastrin and histamine stimulated acid secretion with similar efficacy, while in wild-type mice histamine was more effective than gastrin. SSTR2 knockout mice showed also a hypersecretory response to pylorus ligation compared with wild-type animals. In wild-type mice, somatostatin-14, SMS 201-995, and the SSTR2-preferential agonist, DC 32-87, inhibited gastrin-stimulated acid secretion with an order of potency SMS 201-995>DC 32-87>somatostatin-14. Preferential agonists for the SSTR1, 3, 4, and 5 were devoid of any effect. None of the compounds tested affected the high basal secretion observed under urethane anesthesia in SSTR2 knockout mice. These results show that gastric antisecretory effects of peripheral somatostatin are mediated solely through SSTR2. In the absence of functional SSTR2 other somatostatin receptors do not compensate for the lack somatostatin-SSTR2-mediated inhibition. Basal acid secretion and the response to a meal are normal in conscious SSTR2 knockout mice, suggesting the presence of somatostatin-independent mechanisms that compensate for the lack of somatostatin-SSTR2-mediated inhibitory responses.

Peripheral PACAP inhibits gastric acid secretion through somatostatin release in mice

1. Studies in rats suggest that PACAP modulates gastric acid secretion through the release of both histamine and somatostatin. 2. We characterized the effects of exogenous PACAP on gastric acid secretion in urethane-anesthetized mice implanted with a gastric cannula and in conscious 2-h pylorus ligated mice, and determined the involvement of somatostatin and somatostatin receptor type 2 (SSTR2) by using somatostatin immunoneutralization, the SSTR2 antagonist, PRL-2903, and SSTR2 knockout mice. 3. Urethane-anesthetized wild-type mice had low basal acid secretion (0.10+/-0.01 micromol (10 min)(-1)) compared with SSTR2 knockout mice (0.93+/-0.07 micromol (10 min)(-1)). Somatostatin antibody and PRL-2903 increased basal secretion in wild-type mice but not in SSTR2 knockout animals. 4. In wild-type urethane-anesthetized mice, PACAP-38 (3-270 microg kg(-1) h(-1)) did not affect the low basal acid secretion, but inhibited the acid response to pentagastrin, histamine, and bethanechol. 5. In wild-type urethane-anesthetized mice pretreated with somatostatin antibody or PRL-2903 and in SSTR2 knockout mice, peripheral infusion of PACAP-38 or somatostatin-14 did not inhibit the increased basal gastric acid secretion. 6. In conscious wild-type mice, but not in SSTR2 knockout mice, PACAP-38 inhibited gastric acid secretion induced by 2-h pylorus ligation. The antisecretory effect of PACAP-38 was prevented by immunoneutralization of somatostatin. 7. These results indicate that, in mice, peripheral PACAP inhibits gastric acid secretion through the release of somatostatin and the activation of SSTR2 receptors. There is no evidence for stimulatory effects of PACAP on acid secretion in mice.

Amylin, released from the gastric fundus, stimulates somatostatin and thus inhibits histamine and acid secretion in mice

BACKGROUND & AIMS

Amylin, a peptide that displays 50% homology with calcitonin gene-related peptide (CGRP), is colocalized with somatostatin in endocrine cells of the gastric fundus. The present study was designed to determine the mechanism of action of amylin on gastric exocrine and endocrine secretion.

METHODS

Acid secretion was measured in the isolated mouse stomach by titration. Somatostatin and histamine secretion were measured in rat fundic segments by radioimmunoassay.

RESULTS

In isolated mouse stomach, amylin caused a concentration-dependent decrease in acid secretion. In rat fundic segments, amylin and CGRP each caused a concentration-dependent increase in somatostatin and a decrease in histamine secretion. Changes in histamine secretion induced by amylin reflected changes in somatostatin secretion and could be abolished by addition of somatostatin antibody. Both the somatostatin and the histamine responses to amylin were abolished by the selective amylin antagonist AC187 but were unaffected by the CGRP antagonist CGRP8-37. In contrast, the responses to CGRP were abolished by CGRP8-37 but were unaffected by AC187. AC187 alone decreased somatostatin and increased histamine in fundic segments and increased acid secretion in isolated stomach, indicating that endogenous amylin participates in the regulation of gastric endocrine (somatostatin and histamine) and exocrine (acid) secretion.

CONCLUSIONS

In gastric fundus, release of amylin from somatostatin cells interacts with distinct amylin receptors to enhance somatostatin secretion via an autocrine pathway that leads to inhibition of histamine and acid secretion.

The role of sensory neurons in the antiulcer effect of centrally injected amylin in rat

Central administration of amylin (2.2 microg/rat, i.c.v.) reduces (from a minimum of 67% to 83%) indomethacin (Indo, 20 mg Kg(-1), orally) induced ulcers in rats. The anti-ulcer effect of the peptide is not removed by the administration of prokinetic drugs like domperidone or neostigmine but it is reduced by 35% in rats treated with capsaicin or with the CGRP antagonist, CGRP(8-37). These data indicate that amylin gastroprotection involves capsaicin-sensitive nerve fiber leading to CGRP-dependent gastric vasodilatory effect. Additional mechanisms could involve noradrenergic alpha(2) receptors as the peptide gastroprotective activity is reduced from 67% to 20% by the alpha(2) antagonist yohimbine.

CGRP modulates mucin synthesis in surface mucus cells of rat gastric oxyntic mucosa

We examined the effects of the calcitonin gene-related peptide (CGRP), including the possible participation of nitric oxide (NO), on mucin biosynthesis in the surface epithelium and remaining deep mucosa as well as the entire mucosa and compared the distribution of CGRP and NO synthase (NOS) using a combination of double immunofluorescence labeling and multiple dye filter. Pieces of tissue obtained from the corpus and antrum were incubated in a medium containing [(3)H]glucosamine and CGRP, with or without the NOS inhibitor. CGRP dose-dependently enhanced [(3)H]glucosamine incorporation into the corpus mucin but had no effect on antral mucin biosynthesis. The CGRP receptor antagonist, CGRP-(8-37), prevented the increase in (3)H-labeled corpus mucin. This stimulation of corpus mucin synthesis disappeared after removal of the surface mucus cell layer. CGRP activated the mucin biosynthesis in the surface mucus cells. In the full-thickness corpus mucosa, CGRP-induced activation was completely blocked by the NOS inhibitor. CGRP-immunoreactive fibers were intertwined within the surface mucus cell layer with type I NOS immunoreactivity. These results show that CGRP-stimulated mucin biosynthesis mediated by NO is limited to surface mucus cells of the rat gastric oxyntic mucosa.

Adrenomedullin promotes epithelial restitution of rat and human gastric mucosa in vitro

We have investigated the effect of adrenomedullin (AM) on restitution of mucosal integrity following damage in rat and human gastric mucosa, measuring the potential difference (PD) on a mucosal strip mounted on an Ussing chamber. Mucosal damage was induced by 0.5, 1.0, and 2.0 M NaCl solution, and it caused an immediate and significant decrease in PD. In the rat AM group, PD recovered significantly more than in control group at 120 min after exposure to 0.5 M (p < 0.01) and 1.0 M (p < 0.05) NaCl solution. In the human AM group, PD completely recovered at 120 min after exposure to 0.5 M (p < 0.05) NaCl solution. In rat mucosa damaged by 0.5 M NaCl solution, the effect was inhibited by human (h)-CGRP(8-37) and there was no significant difference between the h-CGRP(8-37) group and control group. On immunohistochemical examination of rat gastric mucosa, AM was detected within the chief cell. AM probably promotes epithelial restitution primarily through the CGRP receptor, but it does not ameliorate more severe damage of gastric mucosa in vitro.

High basal gastric acid secretion in somatostatin receptor subtype 2 knockout mice

BACKGROUND & AIMS

Somatostatin receptor subtype 2 (sst2) agonists inhibit gastric secretion. The role of sst2 in the regulation of acid secretion was assessed using sst2 knockout mice and urethane to induce somatostatin release.

METHODS

Acid secretion was monitored every 10 minutes by gastric perfusion and backtitration of perfusates in fasted, urethane-anesthetized C57/129 sst2 (-/-) mice and wild-type (+/+) mice. The ileal vein was cannulated for drug injection. Intragastric pH and serum gastrin were monitored 1 hour after anesthesia without perfusion.

RESULTS

Gastric pH values were lower in sst2 (-/-) mice (3.8 +/- 0.3) than in wild-type mice (7.1 +/- 0.1, P < 0.05), and there was no difference in gastrin levels. Basal acid output per 2 hours was 10-fold higher in sst2 knockout mice compared with wild-type mice. The gastrin antibody abolished the high basal acid secretion in sst2 (-/-) mice and had no effect in wild-type mice. The somatostatin antibody increased basal secretion by 4-fold in wild-type and had no effect in knockout mice. Somatostatin 14 or the sst2 agonist DC 32-87 inhibited pentagastrin-stimulated acid secretion in wild-type mice, but did not alter basal secretion in knockout mice.

CONCLUSIONS

These results indicate that sst2 is the main subtype whereby endogenous somatostatin suppresses gastric acid secretion through inhibition of gastrin action.

Neural emergency system in the stomach

The maintenance of gastric mucosal integrity depends on the rapid alarm of protective mechanisms in the face of pending injury. Afferent neurons of extrinsic origin constitute an emergency system that is called into operation when the gastric mucosa is endangered by acid and other noxious chemicals. The function of these chemoceptive afferents can be manipulated selectively and explored with the excitotoxin capsaicin. Most of the homeostatic actions of capsaicin-sensitive afferents are brought about by peptides released from their peripheral endings in the gastric wall. When stimulated, chemoceptive afferents enhance gastric blood flow and activate hyperemia-dependent and hyperemia-independent mechanisms of protection and repair. In the rodent stomach, these local regulatory roles of sensory neurons are mediated by calcitonin gene-related peptide acting via calcitonin gene-related peptide 1 receptors and neurokinin A acting via neurokinin 2 receptors, with both peptides using nitric oxide as their common messenger. In addition, capsaicin-sensitive neurons form the afferent arc of autonomic reflexes that control secretory and motor functions of the stomach. The pathophysiological potential of the neural emergency system is best portrayed by the gastric hyperemic response to acid backdiffusion, which is signaled by afferent nerve fibers. This mechanism limits damage to the surface of the mucosa and creates favorable conditions for rapid restitution and healing of the wounded mucosa.

Adrenomedullin, amylin, calcitonin gene-related peptide and their fragments are potent inhibitors of gastric acid secretion in rats

Adrenomedullin, amylin and calcitonin gene-related peptides (CGRP) share close sequence homology and have overlapping spectra of biological activities, particularly with respect to cardiovascular and gastrointestinal functions. Comparisons of the effects of these three peptides on gastric acid release have been made by i.v. infusions in conscious rats equipped with gastric fistulae. All peptides were extremely potent inhibitors of basal, pentagastrin- and 2-deoxy-D-glucose-stimulated gastric acid secretion with IC50 values in the subnanomolar to nanomolar range. These effects were not inhibited by C-terminal extra-cyclic fragments of the peptides which often act as competitive receptor antagonists in other biological systems. At high concentrations C-terminal fragments of human adrenomedullin and rat alpha-calcitonin gene-related peptide displayed some receptor agonist activity. Furthermore, the N-terminally situated disulfide-bridged ring fragments, human adrenomedullin-(15-22), rat amylin-(1-8) and rat alpha-calcitonin gene-related peptide-(1-8), retained significant gastric acid inhibitory potencies thus suggesting involvement of receptor(s) with significantly differing ligand binding profiles than those characterized previously.