Cerebroventricular calcitonin gene-related peptide inhibits rat duodenal bicarbonate secretion by release of norepinephrine and vasopressin

Diabetes mellitus influences the degree of colocalization of calcitonin gene-related peptide with insulin and somatostatin in the rat pancreas

OBJECTIVES

Although calcitonin-gene related peptide (CGRP) (C162H262N50O50S2) has been shown to be present in pancreatic islet cells, no data have been reported on the pattern of its distribution in the islets of Langerhans of diabetic rats.

METHODS

The present study used immunohistochemical and immunofluorescence techniques to examine the pattern of distribution of CGRP-like immunoreactive (CGRP-LIR) cells and nerves in the pancreata of rats with streptozotocin-induced diabetes. The effect of CGRP on insulin secretion from rat pancreatic tissue fragments was also investigated using a radioimmunoassay technique.

RESULTS

Numerous CGRP-LIR cells were observed in both the peripheral and central regions of the islets of Langerhans of normal pancreata where they colocalized with large and small subsets of insulin-LIR and somatostatin-LIR cells, respectively. By contrast, the islets of diabetic rat pancreata contained significantly (P < 0.0001) fewer CGRP-LIR cells compared with normal rats. In diabetic rat pancreata, CGRP was colocalized with larger and smaller subsets of somatostatin-LIR and insulin-LIR cells, respectively. CGRP-LIR nerve fibers were discerned in the perivascular and periacinar regions of the pancreata of both normal and diabetic rats. CGRP (10 M) induced a significant (P < 0.02) increase in insulin secretion from the pancreas of normal rat.

CONCLUSION

CGRP is colocalized with insulin and somatostatin in the pancreata of normal and diabetic rats and may play an important role in the humoral and neural regulation of the endocrine pancreas.

Inhibitory effect of central calcitonin-gene related peptide (CGRP) on pancreatic secretion in conscious rats

The inhibitory effect of central administration of calcitonin-gene related peptide (CGRP) on pancreatic secretion stimulated by bile-pancreatic juice diversion was determined in conscious rats. Rats were prepared with separate cannulae for draining bile and pancreatic juice and with a duodenal cannula and an extrajugular vein cannula. In addition, another cannula was stereotactically implanted into the left lateral cerebral ventricle. Rats were placed in restraint cages and experiments were conducted 4 d after the operation without anesthesia. An injection of CGRP (1 nmol/10 microl) into the left lateral cerebral ventricle (i.c.v.) inhibited pancreatic secretion as well as cholecystokinin (CCK) release induced by bile-pancreatic juice diversion. Intravenous infusion of alpha- and beta-adrenergic receptor antagonist, phentolamine and propranolol did not reverse the inhibition of pancreatic secretion. Intravenous infusion of CGRP did not affect pancreatic secretion or plasma CCK concentrations. The inhibitory action of central CGRP (i. c.v.) on pancreatic secretion and CCK release stimulated by bile-pancreatic juice diversion is partially mediated by an alpha-adrenergic mechanism, although its precise mechanism has not been elucidated.

Digestive motor effects and vascular actions of CGRP in dog are expressed by different receptor subtypes

Calcitonin gene-related peptide (CGRP) is a 37 AA peptide localized in blood vessels and nerves of the GI tract. Activation of CGRP receptors (subtypes 1 or 2) usually induces vasodilation and/or muscle relaxation, but its effects in dog and on gastroduodenal motility are still unclear. This study looked for the effect of CGRP and the antagonist CGRP8-37, specific for CGRP type 1 receptor, 1) on GI motility (interdigestive and postprandial), and 2) on hemodynamy, in conscious dogs. During the interdigestive period, the infusion of CGRP1-37 (200 pmol/kg/h) or CGRP8-37 (2000 pmol/kg/h) did not modify the duration of the migrating motor complex nor the release nor the motor action of plasma motilin. The gastric emptying of a solid meal (15 g meat/kg) was reduced by the administration of CGRP1-37 (AUC: 2196 +/- 288.6 versus 3618 +/- 288.4 with saline or T12: 78 +/- 7.3 versus 50 +/- 4.3 min; P < 0.01) and this effect was reversed by the antagonist CGRP8-37. CGRP1-37 significantly (P < 0. 01) diminished arterial pressures (118 +/- 1.6/64 +/- 1.4 vs. 125 +/- 1.4/75 +/- 1.2 mmHg with saline) and accelerated the basal cardiac rhythm (110 +/- 1.4 versus 83 +/- 1.6 beats/min). However, CGRP8-37 failed to block the cardiovascular effects of CGRP1-37. In dog, CGRP could influence digestive motility by slowing the gastric emptying of a meal through an action on CGRP-1 receptors. Hemodynamic effects of CGRP were not blocked by CGRP8-37 and seem therefore mediated by CGRP-2 receptor subtype.

Central CGRP inhibits pancreatic enzyme secretion by modulation of vagal parasympathetic outflow

Calcitonin gene-related peptide (CGRP) is a potent inhibitor of pancreatic enzyme secretion in vivo. Recent studies have shown that CGRP exerts its inhibitory action at a central vagal site. The present study investigates the mechanism responsible for the central action of CGRP. Rats were fitted with lateral cerebroventricular cannulas, using stereotaxic instruments, 4 days before pancreatic secretion studies. In anesthetized rats, administration of 2-deoxy-D-glucose (2-DG) (75 mg/kg iv) or CCK-8 (40 pmol. kg-1. h-1) produced a 100 and 75% increase in protein secretion, respectively, which was completely blocked by atropine. Intracerebroventricular (ICV) administration of CGRP (0.03-0.6 nmol/h) resulted in a dose-related inhibition of pancreatic protein secretion evoked by 2-DG or CCK-8. CGRP administered by the ICV route was 10-40 times more potent than CGRP given by the intravenous route. In contrast, ICV administration of CGRP had no significant effect on pancreatic protein secretion evoked by electrical vagal stimulation or bethanechol, which directly activates the pancreatic muscarinic receptor. Chemical sympathectomy induced by pretreatment with guanethedine (20 mg/kg ip, 2 days) or alpha-adrenergic receptor blockade with phentolamine did not alter the inhibitory effects of CGRP. We recently demonstrated that CCK stimulated the enteropancreatic neural pathways to mediate pancreatic secretion in rats with a chronic vagotomy. ICV-administered CGRP did not affect CCK-stimulated pancreatic secretion in rats with a chronic vagotomy. In conclusion, CGRP in the central nervous system inhibits pancreatic enzyme secretion stimulated by 2-DG and CCK-8, which act through vagal pathways. The inhibitory action of CGRP is not mediated by the sympathetic nervous system but appears to depend on intact vagus nerves.

Calcitropic peptides: neural perspectives

In mammals and higher vertebrates, calcitropic peptides are produced by peripheral endocrine glands: the parathyroid gland (PTH), thyroid or ultimobranchial gland (calcitonin) and the anterior pituitary gland (growth hormone and prolactin). These hormones are, however, also found in the neural tissues of lower vertebrates and invertebrates that lack these endocrine organs, suggesting that neural tissue may be an ancestral site of calcitropic peptide synthesis. Indeed, the demonstration of CNS receptors for these calcitropic peptides and their induction of neurological actions suggest that these hormones arose as neuropeptides. Neural and neuroendocrine roles of some of these calcitropic hormones (calcitonin and parathyroid hormone) and related peptides (calcitonin gene related peptide, stanniocalcin and parathyroid hormone related peptide) are thus the focus of this review.

Review article: gastroduodenal bicarbonate secretion

The gastroduodenal epithelium is covered by an adherent mucus layer into which bicarbonate is secreted by surface epithelial cells. This mucus-bicarbonate barrier is an important first line of defence against damage by gastric acid and pepsin, and has been demonstrated in all species including human. Similar to gastric acid secretion, regulation of gastric and duodenal bicarbonate secretion can be divided into three phases: cephalic, gastric and duodenal. In humans, sham-feeding increases bicarbonate secretion in both the stomach and duodenum which is mediated by cholinergic vagal fibres in the stomach, but seems to be noncholinergic in the duodenum. Gastric distention and luminal acidification increases gastric bicarbonate production. Whereas there are no data relating to the gastric phase of human duodenal bicarbonate secretion, in animals, food and acid in the stomach independently stimulate duodenal bicarbonate output. To date, the duodenal phase of human gastric bicarbonate secretion has not been studied, but data from animals reveal that duodenal acidification augments bicarbonate secretion in the stomach. In all species tested, direct acidification of the duodenum is a potent stimulant of local bicarbonate production. In humans, the pH threshold for bicarbonate secretion is pH 3.0. Mediation of gastroduodenal bicarbonate secretion is provided by a variety of agonists and antagonists, tested mainly in animals, but some have been evaluated in humans. Prostaglandins of the E class and VIP are major factors that control bicarbonate secretion. Bicarbonate secretion, and the mucus-bicarbonate layer in general, is adversely effected by ulcerogenic factors such as aspirin, NSAIDs, bile salts, and cigarette smoking. Furthermore, duodenal ulcer patients have an impairment in bicarbonate production within the duodenal bulb, at rest and in response to stimulation. These findings indicate that the mucus-bicarbonate barrier is an important first line of defence in the pathogenesis of peptic ulcer disease.

On the point of action for sympatho-adrenergic inhibition of duodenal alkaline secretion in the rat

Experiments were performed on chloralose anaesthetized rats. A duodenal segment was decentralized by means of bilateral vagotomy and splanchnicotomy. The mucosal alkaline secretion by the segment was measured using an in situ pH-stat titration. The duodenal alkaline secretion was raised by about 100% utilizing two different secretagogues; vasoactive intestinal polypeptide (VIP) and prostaglandin E2 (PGE2). Direct splanchnic nerve stimulation (10 Hz, 5 min, bilaterally), or exogenous administration of the alpha-2-adrenoceptor agonist clonidine (15 micrograms h-1 kg-1), inhibited the secretion stimulated by PGE2, but not by VIP. The results suggest that the sympathetic nerves exert their inhibitory effect mainly on enteric secretomotor neurones.

Mechanism of action of calcitonin gene-related peptide in inhibiting pancreatic enzyme secretion in rats

BACKGROUND

Recently, calcitonin gene-related peptide (CGRP) receptors have been identified in the central nervous system. Therefore whether CGRP inhibits pancreatic enzyme secretion at a central site was investigated.

METHODS

In vivo studies were performed on rats to examine the effect of CGRP on pancreatic enzyme secretion evoked by stimulants that act on different sites: (1) 2-Deoxy-D-glucose (2DG), a central vagal stimulant; (2) cholecystokinin, which acts via vagal afferent pathways under physiologic conditions; (3) electric vagal nerve stimulation, which stimulates vagal release of acetylcholine in the pancreas; and (4) bethanechol, which directly activates pancreatic muscarinic receptors.

RESULTS

CGRP produced a dose related inhibition of pancreatic secretion evoked by 2DG. Complete inhibition was observed at a dose of 25 micrograms.kg-1 x h-1. Similarly, CGRP at a dose of 50 micrograms.kg-1 x h-1 completely inhibited pancreatic protein secretion in response to a physiological concentration of cholecystokinin octapeptide (CCK-8). In contrast, pancreatic protein secretion evoked by bethanechol or electrical stimulation of the vagal trunk were unaffected by CGRP. It was also shown that perivagal capsaicin treatment impaired pancreatic responses to CCK-8 but not to 2DG ruling out an effect of CGRP on vagal afferent pathway.

CONCLUSIONS

Our data indicates that CGRP inhibits pancreatic enzyme secretion evoked by 2DG or CCK-8 via vagal pathways. CGRP exerts its inhibitory action at a central vagal site.

Regulation of canine gallbladder motility by brain peptides

BACKGROUND

Brain peptides alter most gastrointestinal functions, but their effects on gallbladder motility have not been examined in detail.

METHODS

Studies were conducted in awake, male beagle dogs.

RESULTS

Of 30 brain peptides evaluated, thyrotropin-releasing hormone (TRH) and calcitonin gene-related peptide (CGRP) inhibited CCK- and meal-induced gallbladder contraction. These responses were abolished by ganglionic blockade. Truncal vagotomy prevented the central inhibitory action of TRH but not that of CGRP, whereas noradrenergic blockade prevented the central inhibitory action of CGRP but not that of TRH. Muscarinic blockade did not prevent the relaxing effect of cerebral TRH but pretreatment with the vasoactive intestinal peptide (VIP) antagonist, (4Cl-D-Phe6-Leu17)VIP, significantly attenuated gallbladder relaxation induced by cerebral TRH: the combination of both VIP and muscarinic antagonists abolished TRH-induced gallbladder relaxation. alpha-Adrenergic receptor blockade but neither beta-adrenergic blockade nor adrenalectomy abolished gallbladder relaxation induced by cerebral CGRP. Intravenous infusion of VIP and norepinephrine inhibited CCK-induced gallbladder contraction and these responses were abolished dose dependently by intravenous infusion of (4Cl-D-Phe6-Leu17)VIP and phentolamine, respectively.

CONCLUSIONS

Cerebral TRH inhibits canine gallbladder contraction by stimulation of vagal outflow and subsequent release of VIP acting via its specific receptor whereas cerebral CGRP inhibits gallbladder contraction by stimulation of sympathetic, noradrenergic outflow via alpha-adrenergic receptors.

Stimulation of mucosal alkaline secretion in rat duodenum by dopamine and dopaminergic compounds

BACKGROUND

The catechol-O-methyl-transferase (COMT) inhibitor nitecapone, which prevents mucosal degradation of dopamine, and some dopamine receptor agonists ameliorate gastroduodenal mucosal damage. Therefore, their effects on mucosal bicarbonate secretion were studied.

METHODS

Duodenum just distal to the Brunner's glands area was cannulated in situ in anesthetized rats. Bicarbonate secretion into the luminal perfusate and transmucosal electrical potential difference (PD) were recorded.

RESULTS

Intravenous dopamine (50 micrograms.kg-1 x h-1) increased bicarbonate secretion twofold, and a higher rate of infusion (250 micrograms.kg-1 x h-1) resulted in a further increase. Neither dose affected the PD. The dopamine D1 agonist SKF-38393 (10-50 micrograms/kg) and the COMT inhibitor nitecapone (50-500 micrograms/kg) caused dose-dependent increases in secretion, similar to that observed with dopamine. Domperidone, a peripherally acting dopamine antagonist, inhibited the stimulatory effects of SKF-38393 and nitecapone. Hexamethonium or the alpha-adrenoceptor antagonist phentolamine, in contrast, did not affect the response to nitecapone. Intracerebroventricular administration of nitecapone was without effect.

CONCLUSIONS

A probable electroneutral component of duodenal mucosal bicarbonate secretion is stimulated via peripheral dopamine D1 receptors. This may contribute to the previously observed ulceroprotective actions of dopaminergic compounds.

Effects of intracerebroventricular administration of calcitonin gene-related peptide on passive avoidance behaviour in rats

The effects of different doses of calcitonin gene-related peptide (CGRP) on passive avoidance behaviour were studied in male rats following its intracerebroventricular (i.c.v.) administration. Treatment with doses of 200 ng, 300 ng, 500 ng or 1 microgram was performed prior to learning, immediately and 6 h after the learning session and 30 min before the 24 h retention test. CGRP enhanced the avoidance latency in a dose-dependent manner at each time studied, with the exception of 6 h after the learning session. It is concluded that CGRP might be able to lengthen the passive avoidance latency by facilitating learning and memory formation.

Effects of diazepam and Ro 15-1788 on duodenal bicarbonate secretion in the rat

Bicarbonate secretion by duodenal mucosa just distal to the Brunner's glands area and devoid of pancreatic secretions was titrated in situ in anesthetized rats. Intravenous injection of diazepam (0.1 and 0.5 mg/kg) significantly increased the secretion; this stimulation was abolished by proximal bilateral vagotomy. Ro 15-1788, a benzodiazepine antagonist that also has well-known intrinsic activity, caused similar stimulation of the secretion when administered IV (0.01 and 0.1 mg/kg). Intracerebroventricular infusion of Ro 15-1788 (10 micrograms/h) resulted in a greater increase in secretion; again, this stimulation was prevented by vagotomy. Adrenoceptor blockade by phentolamine increased basal alkaline secretion but did not affect the stimulation by diazepam. The tricyclic antidepressant trimipramine (2.5 mg/kg IV) did not affect the duodenal bicarbonate secretion. For comparison, effects of diazepam and Ro 15-1788 (10(-6)-10(-4) mol/L) were also tested in isolated bullfrog duodenal mucosa. Neither drug effected the alkaline secretion in vitro. The combined results strongly suggest that benzodiazepines, as previously shown for certain brain peptides, influence the central nervous control of duodenal mucosal alkaline secretion and that their stimulation of secretion is vagally mediated. This action benzodiazepines might be used in modulating mucosal protection against acid.

Central nervous system binding sites for calcitonin and calcitonin gene-related peptide

Alternative splicing of the primary RNA transcript of the calcitonin gene leads to the generation of two distinct peptides, calcitonin (CT) and calcitonin gene-related peptide (CGRP). These peptides share only limited sequence homology and generally subserve different biological functions through their own distinct binding sites, which differ in specificity and distribution. Additionally, a binding site with high-affinity binding for both peptides that has a restricted pattern of distribution has been identified. The present article reviews the biochemical and morphological characteristics of centra CT and CGRP binding sites.

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.

Stimulation of duodenal bicarbonate secretion in conscious rats by cerebral somatostatin-28. Role of neurohumoral pathways

The central nervous system effects of somatostatin-28 on proximal duodenal bicarbonate secretion were studied in freely moving rats. Cerebroventricular administration of somatostatin-28 (0.2-2.0 nmol) significantly stimulated duodenal bicarbonate secretion in a dose-dependent fashion. Somatostatin-28 was approximately twice as effective as somatostatin-14. Intravenous administration of somatostatin-28 or somatostatin-14 did not significantly alter the bicarbonate response. Ganglionic blockade with chlorisondamine and truncal vagotomy abolished the stimulatory effect of somatostatin-28 while bretylium, naloxone, indomethacin, and adrenalectomy did not. Furthermore, atropine methylnitrate significantly attenuated and the vasoactive intestinal peptide antagonist 4Cl-D-Phe6, Leu17-vasoactive intestinal peptide abolished the bicarbonate response produced by cerebroventricular somatostatin-28. In contrast, hypophysectomy and pretreatment with the vasopressin V1-receptor antagonist [1-deaminopenicillamine, 2-(0-methyl)Tyr, 8-Arg]-vasopressin significantly enhanced the bicarbonate response produced by cerebroventricular somatostatin-28. These findings indicate that somatostatin-28 acts within the central nervous system to stimulate duodenal bicarbonate secretion in freely moving rats via vagal efferents by release of vasoactive intestinal peptide and, in part, by a muscarinic pathway and not by catecholamine, opiate, or prostaglandin release.