Central nervous system actions of calcitonin gene-related peptide on gastric acid secretion in the rat

Quantification of FAM20A in human milk and identification of calcium metabolism proteins

BACKGROUND

FAM20A, a recently discovered protein, is thought to have a fundamental role in inhibiting ectopic calcification. Several studies have demonstrated that variants of FAM20A are causative for the rare autosomal recessive disorder, enamel-renal syndrome (ERS). ERS is characterized by defective mineralization of dental enamel and nephrocalcinosis suggesting that FAM20A is an extracellular matrix protein, dysfunction of which causes calcification of the secretory epithelial tissues. FAM20A is a low-abundant protein that is difficult to detect in biofluids such as blood, saliva, and urine. Thus, we speculated the abundance of FAM20A to be high in human milk, since the secretory epithelium of lactating mammary tissue is involved in the secretion of highly concentrated calcium. Therefore, the primary aim of this research is to describe the processes/methodology taken to quantify FAM20A in human milk and identify other proteins involved in calcium metabolism.

METHOD

This study used mass spectrometry-driven quantitative proteomics: (1) to quantify FAM20A in human milk of three women and (2) to identify proteins associated with calcium regulation by bioinformatic analyses on whole and milk fat globule membrane fractions.

RESULTS

Shotgun MS/MS driven proteomics identified FAM20A in whole milk, and subsequent analysis using targeted proteomics also successfully quantified FAM20A in all samples. Combination of sample preparation, fractionation, and LC-MS/MS proteomics analysis generated 136 proteins previously undiscovered in human milk; 21 of these appear to be associated with calcium metabolism.

CONCLUSION

Using mass spectrometry-driven proteomics, we successfully quantified FAM20A from transitional to mature milk and obtained a list of proteins involved in calcium metabolism. Furthermore, we show the value of using a combination of both shotgun and targeted driven proteomics for the identification of this low abundant protein in human milk.

Spotlight on Anti-CGRP Monoclonal Antibodies in Migraine: The Clinical Evidence to Date

Migraine, a common neurovascular brain disorder, represents a severe and widespread health problem; along with medication‐induced (medication‐overuse) headache, it is the third‐leading cause of disability worldwide. Currently, its therapeutic management remains unsatisfactory for several reasons; up to 40% of migraineurs are eligible for prophylactic treatment, but there are issues of efficacy, safety, and adherence. In recent years the evidence on the role of calcitonin gene‐related peptide (CGRP) in migraine pathophysiology has been consolidated, so new and promising treatments for migraine pain and its possible prevention have been developed. The following review reports the results of the clinical trials conducted so far with each of the new monoclonal antibodies targeting CGRP or its receptor, with particular reference to safety, tolerance, and efficacy in migraine prevention. Moreover, the pharmacological characterization and further developments of each monoclonal antibody are reported, based on current knowledge.

Stimulation of calcitonin gene-related peptide release through targeting capsaicin receptor: a potential strategy for gastric mucosal protection

Calcitonin gene-related peptide (CGRP) is a predominant neurotransmitter from capsaicin-sensitive sensory nerves, which are widely distributed in the gastrointestinal system. These sensory nerves are reported to be involved in the protection of gastric mucosa against damage by various stimuli, and CGRP is a potential mediator in this process. In addition to increase in gastric mucosal blood flow, the beneficial effects of CGRP on gastric mucosa include inhibition of gastric acid secretion, prevention of cellular apoptosis and oxidative injury. The synthesis and release of CGRP is regulated by the capsaicin receptor which is known as transient receptor potential vanilloid subfamily member 1 (TRPV1) and the agonists of TRPV1 have the potential for gastric mucosal protection. So far, multiple TRPV1 agonists, including capsaicin, capsiate, anandamide and rutaecarpine are reported to exert beneficial effects on gastric mucosal injury induced by various stimuli. Therefore, the TRPV1/CGRP pathway represents a novel target for therapeutic intervention in human gastric mucosal injury.

CGRP stimulates gill carbonic anhydrase activity in molluscs via a common CT/CGRP receptor

The physiological significance of calcitonin gene-related peptide (CGRP) during biomineralization was investigated by assessing the effect of human CGRP on the carbonic anhydrase activity in gill membranes of the pearl oyster, Pinctada margaritifera. Salmon CT and human CGRP were able to induce a 150% increase of the basal activity. No additive effect was observed suggesting that both activities are mediated by the same receptor. The CGRP-stimulated effect was specific as demonstrated by the inhibition produced by the CGRP antagonist, hCGRP8-37. So, CGRP by its specific action on gill carbonic anhydrase controls the calcification process, an ancient role both in invertebrates and non-mammalian vertebrates.

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.

Role of neuropeptide receptor systems in vanilloid VR1 receptor-mediated gastric acid secretion in rat brain

Previously, we reported that the injection of capsaicin into the lateral cerebroventricle (i.c.v.) stimulated gastric acid secretion via vanilloid VR1 receptors and the vagal cholinergic pathways in anesthetized rats. In the present study, we investigated the involvement of receptor systems for neurokinin A, calcitonin gene-related peptide (CGRP) and glutamate in the vanilloid VR1 receptor-mediated response. The i.c.v. injection of neurokinin A (30 nmol) stimulated gastric acid secretion in the presence of cis-2-(diphenylmethyl)-N-[(2-iodophenyl)methyl]-1-azabicyclo[2.2.2]octan-3-amine oxalate (L-703606, a tachykinin NK1 receptor antagonist, 30 nmol) and the effect was inhibited by cyclo[Gln-Trp-Phe-Gly-Leu-Met] (L-659877, a tachykinin NK2 receptor antagonist, 30 nmol); the values were 145.9 +/- 32.3 and 21.1 +/- 16.6 microEq HCl per 120 min, respectively. The value in the control group was 14.3 +/- 3.8 microEq HCl. The tachykinin NK2 receptor-mediated secretion was inhibited by i.c.v. injections of antagonists of the CGRP1 receptor (human CGRP fragment 8-37, 15 nmol) and non-N-methyl-D-aspartate (non-NMDA)-type glutamate receptor (6-cyano-7-nitroquinoxaline-2,3-dione, 10.9 nmol); the values were 30.8+/-29.8 and 5.7+/-16.9 microEq HCl, respectively. Gastric acid secretion induced by the i.c.v. injection of 30 nmol capsaicin (178.4 +/- 34.0 microEq HCl) was inhibited by antagonists of tachykinin NK2 (23.7 +/- 6.2) and CGRP1 (21.2 +/- 8.5), but not tachykinin NK1 (181.4 +/- 37.0), receptors. The gastric acid secretion induced by capsaicin was decreased by the i.c.v. pre-injection of low doses of neurokinin A or CGRP, which alone had no effect on the secretion. These findings suggest the involvement of tachykinin NK2, CGRP and non-NMDA receptor systems in the vanilloid VR1 receptor-mediated regulation of gastric acid secretion in the rat brain regions close to the lateral cerebroventricle.

Effects on digestive secretions

Rat amylin subcutaneously injected into rats dose-dependently inhibits pentagastrin-stimulated gastric acid secretion and protects the stomach from ethanol-induced gastritis. The ED50s for these actions (0.050 and 0.036 microg, respectively) are the lowest for any dose-dependent effect of amylin thus far described, and their similar potencies are consistent with a mechanistic (causal) association. At higher amylin doses, inhibition of gastric acid secretion was almost complete (93.4%). Gastric injury (measured by a subjective analog scale) was inhibited by up to 67%. The observation that effective doses of amylin result in plasma concentrations of 7-10 pM (i.e., within the reported range; Pieber et al., 1994) supports the interpretation that inhibition of gastric acid secretion and maintenance of gastric mucosal integrity are physiological actions of endogenous amylin. The pharmacology of these responses fits with one mediated via amylin-like receptors. Rat amylin inhibited CCK-stimulated secretion of pancreatic enzymes,amylase, and lipase by up to approximately 60% without having significant effect in the absence of CCK. ED50s for the effect were in the 0.1-0.2 microg range, calculated to produce plasma amylin excursions within the physiological range. Effects of informative ligands are consistent with the concept of amylin receptor mediation. Amylin was effective in ameliorating the severity of pancreatitis in a rodent model. The amylin analog pramlintide inhibited gallbladder emptying in mice as measured by total weight of acutely excised gallbladders. Amylin inhibition of gastric acid secretion, pancreatic enzyme secretion, and bile secretion likely represents part of an orchestrated control of nutrient appearance. Modulation of digestive function fits with a general role of amylin in regulating nutrient uptake. Rate of ingestion, rate of release from the stomach, and rate of digestion of various food groups appear to be under coordinate control.

Neuromedin U acts in the central nervous system to inhibit gastric acid secretion via CRH system

Neuromedin U (NMU) is a hypothalamic peptide involved in energy homeostasis and stress responses. NMU, when administered intracerebroventricularly, decreases food intake and body weight while increasing body temperature and heat production. In addition, NMU, acting via the corticotropin-releasing hormone (CRH) system, induces gross locomotor activity and stress responses. We studied the effect of intracerebroventricularly administered NMU (0.5-4 nmol) in the regulation of gastric functions in conscious rats. Intracerebroventricular administration of NMU significantly decreased gastric acid output to 30-60% and gastric emptying to 35-70% in a dose-dependent manner. Vagotomy did not abolish the inhibitory effect of NMU on pentagastrin-induced gastric acid secretion. Pretreatment with indomethacin (10 mg/kg), an inhibitor of prostaglandin synthesis, also did not affect NMU-induced acid inhibition. Pretreatment with anti-CRH IgG (1 microg/rat), however, completely blocked NMU-induced acid inhibition (P < 0.01). Administration of yohimbine (4 mg/kg), an alpha(2)-adrenergic receptor antagonist, also abolished NMU-induced acid inhibition (P < 0.01). These findings suggest that NMU is critical in the central regulation of gastric acid secretion via CRH.

Cocaine-amphetamine-regulated transcript (CART) acts in the central nervous system to inhibit gastric acid secretion via brain corticotropin-releasing factor system

Recent study has indicated that cocaine-amphetamine-regulated transcript (CART) is an anorectic chemical in the brain. In the present study, we examined the hypothesis that CART may act in the central nervous system to alter gastric function. Food consumption, gastric acid secretion, and gastric emptying were measured after injection of CART into the cerebrospinal fluid in 24-h fasted Sprague Dawley rats. Central injection of CART inhibited food intake, gastric acid secretion, and gastric emptying. In contrast, ip injection of CART failed to inhibit gastric acid secretion and gastric emptying, suggesting that CART acts in the brain to suppress gastric acid secretion and gastric emptying. In the vagotomized animals, centrally administered CART did inhibit pentagastrin-stimulated gastric acid secretion. The CART-induced acid inhibition was also observed in rats treated with indomethacin, a cyclooxygenase inhibitor. In contrast, pretreatment with central administration of a CRF receptor antagonist, alpha-helical CRF9-41, completely blocked the central CART-induced inhibition of gastric acid secretion. All these results suggest that CART acts in the brain to inhibit gastric function via brain CRF system. The vagal pathway and the prostaglandin system are not involved in the acid inhibition.

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.

Three-dimensional spatial relationship of neuropeptides and receptors in the rat dorsal vagal complex

Retrograde tracing, multi-label fluorescence immunohistochemistry, confocal microscopy and three-dimensional (3-D) reconstruction techniques were combined to examine the spatial relationship of immunoreactive nerve terminals containing either calcitonin gene-related polypeptide (CGRP) or substance P (SP) to identified gastric efferent neurons in the dorsal motor nucleus of the vagus (DMV) in the brainstem of the rat. The availability of an antibody to the receptor for SP (NK-1r) permitted observation of the association between peptide and receptor. Although both SP-IR and CGRP-IR nerve fibres came in close spatial proximity to identified gastric efferent neurons, few discrete contacts between these fibres and the neuronal membrane were observed. In addition, NK-1r-IR was localized to the somatic and dendritic membranes of a subpopulation of neurons within the DMV, with the majority of receptor labelling not in close spatial proximity to SP-IR nerve fibres. The methodology described in this study permitted the simultaneous observation of the spatial relationship between neuropeptide and an identified neuron (and the corresponding receptor in the case of SP) in 3-D, which is something that cannot be achieved using conventional microscopic techniques

Neuroanatomical localization, pharmacological characterization and functions of CGRP, related peptides and their receptors

Calcitonin generelated peptide (CGRP) is a neuropeptide discovered by a molecular approach over 10 years ago. More recently, islet amyloid polypeptide or amylin, and adrenomedullin were isolated from human insulinoma and pheochromocytoma respectively, and revealed between 25 and 50% sequence homology with CGRP. This review discusses findings on the anatomical distributions of CGRP mRNA, CGRP-like immunoreactivity and receptors in the central nervous system, as well as the potential physiological roles for CGRP. The anatomical distribution and biological activities of amylin and adrenomedullin are also presented. Based upon the differential biological activity of various CGRP analogs, the CGRP receptors have been classified in two major classes, namely the CGRP1 and CGRP2 subtypes. A third subtype has also been proposed (e.g. in the nucleus accumbens) as it does not share the pharmacological properties of the other two classes. The anatomical distribution and the pharmacological characteristics of amylin binding sites in the rat brain are different from those reported for CGRP but share several similarities with the salmon calcitonin receptors. The receptors identified thus far for CGRP and related peptides belong to the G protein-coupled receptor superfamily. Indeed, modulation of adenylate cyclase activity following receptor activation has been reported for CGRP, amylin and adrenomedullin. Furthermore, the binding affinity of CGRP and related peptides is modulated by nucleotides such as GTP. The cloning of various calcitonin and most recently of CGRP1 and adrenomedullin receptors was reported and revealed structural similarities but also significant differences to other members of the G protein-coupled receptors. They may thus form a new subfamily. The cloning of the amylin receptor(s) as well as of the other putative CGRP receptor subtype(s) are still awaited. Finally, a broad variety of biological activities has been described for CGRP-like peptides. These include vasodilation, nociception, glucose uptake and the stimulation of glycolysis in skeletal muscles. These effects may thus suggest their potential role and therapeutic applications in migraine, subarachnoid haemorrhage, diabetes and pain-related mechanisms, among other disorders.

Receptor subtypes mediating renal actions of calcitonin gene-related peptide

We previously reported that the renal arterial infusions of non-hypotensive doses of calcitonin gene-related peptide (CGRP) caused renal vasodilatation and increases in glomerular filtration rate at a low dose, but renal vasoconstriction, natriuresis and kaliuresis at a high dose. In the present study, we examined the effects of the specific CGRP1 receptor antagonist (CGRP-(8-37) (1 and 10 nmol/kg) and the putative CGRP receptor antagonist, [Tyr(0)]CGRP-(28-37)(3 and 30 nmol/kg), on the renal vascular and tubular effects of CGRP in inactin-anaesthetized Sprague-Dawley rats. Renal arterial infusion of single doses of CGRP (0.3-300 pmol/kg per min) did not significantly alter mean arterial pressure or heart rate. However, during the continuous renal arterial infusion of either CGRP-(8-37) or [Tyr(0)CGRP-(28-37) incompletely inhibited the vasoconstriction but did not inhibit diuresis, natriuresis and kaliuresis elicited by a high but non-hypotensive dose of CGRP. On the basis that CGRP-(8-37) is a competitive CGRP1 receptor antagonist, our results suggest: (1) the renal vascular effect of CGRP is completely mediated via the activation of CGRP1 receptors, (2) the renal tubular effects of CGRP are not mediated via CGRP1 receptors, and (3) [Tyr(0)]CGRP-(28-37) is a CGRP1 receptor antagonist with potency and efficacy less than those of CGRP-(8-37).

Effects of CGRP on active avoidance behavior in rats

The effects of calcitonin gene-related peptide (CGRP) on the extinction of active avoidance behaviour was examined in rats. Three doses (250 ng, 500 ng and 1 microgram) of the peptide were administered into the lateral brain ventricle (i.c.v.). CGRP delayed the extinction of an active avoidance response in a dose-dependent manner. To reveal any role of the transmitter systems in the action of exogenously administered CGRP, the animals were pretreated with different receptor blockers. CGRP induced a delay in the extinction of an active avoidance response, which could be prevented by haloperidol, propranolol, methysergide and naloxone. Phenoxybenamine, atropine and bicuculline were ineffective. The data suggest that dopaminergic, beta adrenergic, serotonergic and opiate transmission are involved in the CGRP-induced behavioral alterations.

CGRP prevents electroconvulsive shock-induced amnesia in rats

The effects of calcitonin gene-related peptide (CGRP) on electroconvulsive shock (ECS)-induced amnesia and the possible involvement of neurotransmitters in this action were studied in rats. ECS-induced amnesia was elicited in a passive avoidance paradigm. The possible roles of different transmitters involved in mediating CGRP action were followed by pretreating the animals with different receptor blockers in doses which themselves could not influence the paradigm. CGRP facilitated learning in the passive avoidance paradigm and prevented ECS-induced amnesia in a dose-dependent manner. Pretreatment with atropine, naloxone, phenoxybenzamine, or propranolol blocked the antiamnesic action of CGRP. Other receptor blockers, such as bicuculline, methysergide, and haloperidol, were ineffective. The results support our previous findings that CGRP facilitates passive avoidance learning and prevents ECS-induced amnesia. In the antiamnesic action of CGRP, cholinergic, opiate, and alpha- and beta-adrenergic mediators are involved.

Modulation of active avoidance behavior of rats by ICV administration of CGRP antiserum

The effects of ICV administration of CGRP antiserum on active avoidance behavior were studied in rats. CGRP antiserum dilutions of 1:5, 1:10, 1:20, and 1:40 facilitated the extinction of active avoidance responses in a platform-jumping situation in the 3- and 6-h tests. In the 24-h test, only the CGRP antiserum dilution of 1:5 was effective. The present findings suggest that the endogenous CGRP of the brain may be a physiological modulator in extinction processes of active avoidance behavior.

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.

Isolation, purification, and characterization of calcitonin gene-related peptide receptor

Intact calcitonin gene-related peptide (CGRP) receptors were solubilized from porcine neural membranes using sodium cholate: potassium buffer. The solubilized receptors were purified sequentially by hydrophobic interaction and ion-exchange chromatography followed by specific affinity chromatography. Using these procedures, we have isolated 2 nmol of highly purified active CGRP receptor to a homogeneity (5 x 10(8)-fold purification). The isolated receptors retained their specificity and the capacity to bind to 125I-CGRP, and showed no cross-reactivity with a number of other peptides, except with amylin having 46% amino acid sequence homology to h-CGRP. The solubilized receptors were adsorbed by WGA-agarose and concanavalin-A, suggesting a glycoprotein nature. SDS-PAGE, size-exclusion HPLC, and autoradiography confirmed that CGRP receptor is a monomeric membrane protein with M(r) 66 kDa.

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 calcitonin gene-related peptide on extrapyramidal motor system

The effects of central administration of calcitonin gene-related peptide (CGRP, 1 or 100 ng/rat) on behavioral and biochemical parameters related to the extrapyramidal motor system were investigated in male rats. The peptide-induced catalepsy occurred only at the dose of 100 ng/rat and hypomotility at both doses used. Calcitonin gene-related peptide increased haloperidol-induced catalepsy and decreased apomorphine-induced hypermotility at the doses of 1 and 100 ng/rat. Although these behaviors are related to dopamine, no significant change of striatal DA or DOPAC concentration were observed after central administration of the peptide. Other neurotransmitters may be directly or indirectly involved in these behavioral effects of CGRP.

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.

Calcitonin gene-related peptide: multiple actions, multiple receptors

Calcitonin gene-related peptide (CGRP) shows diversity both in its effects and its receptors. It is likely to have roles as a neurotransmitter, neuromodulator, local hormone and trophic factor. Its effects include rapid changes in neuronal activity, relaxation of many types of smooth muscle, actions on metabolism and changes in gene expression. Receptor heterogeneity has been revealed from experiments comparing agonist potency ratios and antagonist affinities. The evidence from these approaches is reviewed in this article and a speculative receptor classification scheme is proposed. Some of the likely future directions for CGRP research are discussed.

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.

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

Proximal duodenal bicarbonate secretion is an important factor in humans and animals protecting the mucosa against acid-peptic damage. This study examined the mechanisms responsible for the central nervous system regulation of duodenal bicarbonate secretion by calcitonin gene-related peptide (CGRP) in unrestrained rats. Cerebroventricular administration of rat CGRP significantly inhibited basal duodenal bicarbonate secretion as well as the stimulatory effects of vasoactive intestinal peptide, neurotensin, a luminal PGE1 analogue, misoprostol, and hydrochloric acid. The inhibitory effects of cerebroventricular CGRP were abolished by ganglionic blockade with chlorisondamine, significantly attenuated by noradrenergic blockade with bretylium, and enhanced by vagotomy. Inhibition of duodenal bicarbonate secretion induced by CGRP coincided with significant increases in plasma norepinephrine (NE) and vasopressin concentrations. The alpha adrenergic receptor antagonist, phentolamine, and the vasopressin V1 receptor antagonist, (1-deaminopenicillamine, 2-[O-methyl]Tyr, 8-Arg)-vasopressin, given intravenously reversed the central inhibitory effect of CGRP by approximately 50% each. Pretreatment of the animals with both phentolamine and the vasopressin antagonist completely abolished the central inhibitory effect of CGRP. Peripheral vasopressin and NE significantly decreased duodenal bicarbonate secretion, and their inhibitory effects were additive and prevented by phentolamine and the vasopressin antagonist, respectively. We conclude that cerebroventricular CGRP inhibits rat duodenal bicarbonate secretion by activation of sympathetic efferents and subsequent release of NE and vasopressin that act on alpha adrenergic and vasopressin receptors, respectively.

The calcitonin gene peptides: biology and clinical relevance

The calcitonin/CGRP multigene complex encodes a family of peptides: calcitonin, its C-terminal flanking peptide, katacalcin, and a third novel peptide, calcitonin gene-related peptide (CGRP). The 32-amino acid peptide calcitonin inhibits the osteoclast, thereby conserving skeletal mass during periods of potential calcium lack, such as pregnancy, growth, and lactation. This hormonal role is emphasized by observations that lower circulating calcitonin levels are associated with bone loss and that calcitonin replacement prevents further bone loss. Structurally, CGRP resembles calcitonin and has been implicated in neuromodulation and in the physiological regulation of blood flow. Here we review the molecular genetics, structure, and function of the calcitonin-gene peptides as analyzed in the laboratory and focus on more recent clinical studies relating to disorders and therapeutics.

Neuroendocrine regulation of canine gastric secretion, emptying and blood flow

Abstract Several peptides have been implicated as central nervous system transmitters regulating various peripheral organ systems. This study examined the central nervous system effects of rat corticotrophin-releasing factor (CRF), salmon calcitonin (CT), beta-endorphin (beta-End), neurotensin (NT), rat calcitonin gene-related peptide (CGRP) and bombesin (BOM) on gastric acid secretion, gastric emptying and left gastric artery flow in conscious dogs. All of these peptides, injected into the third cerebral ventricle, significantly inhibited gastric acid secretion but not plasma gastrin concentrations stimulated by a liquid protein meal. Ganglionic blockade with chlorisondamine abolished the gastric inhibitory action of CRF, CT, beta-End and NT but not of CGRP and BOM. Truncal, subdiaphragmatic vagotomy prevented the gastric inhibitory actions of beta-End and NT only, while bilateral adrenalectomy did not affect gastric acid inhibition induced by any of the six peptides. Cerebroventricular administration of CRF, NT and BOM significantly delayed gastric emptying of the protein meal while beta-End, CT and CGRP were not effective. Only BOM significantly increased left gastric artery flow. These results indicate that various neuropeptides alter gastric functions in a differentiated fashion and via distinct pathways. CRF and CT inhibit meal-stimulated gastric acid secretion by activation of sympathetic efferents. beta-End and NT inhibit meal-stimulated acid secretion by inhibition of vagal efferents while the pathways that mediate CT- and CGRP-induced gastric acid inhibition in the dog are unknown. Gastrin, the adrenal glands and changes in gastric emptying or blood flow do not play a role in mediating gastric acid inhibition produced by Cerebroventricular administration of CRF, CT, beta-End, NT, CGRP and BOM.

Intrathecal injection of bombesin inhibits gastric acid secretion in the rat

Bombesin (100-500 ng) injected intrathecally (T9-10) inhibited gastric acid secretion stimulated by pentagastrin and the GABAB agonist baclofen in urethane-anesthetized rats and basal gastric acid secretion in conscious, pylorus-ligated rats. Peptide action was dose-related, occurred within 30 min, and lasted for greater than 1 h. Bombesin-induced inhibition of pentagastrin-stimulated gastric acid secretion was not altered by cervical cord transection. Intravenous infusion of the monoclonal bombesin antibody 2A11 abolished intravenous bombesin (10 micrograms/kg.h)-induced 33% inhibition of gastric response to pentagastrin but did not alter intrathecal bombesin (200 ng)-induced 38% inhibition of gastric response to pentagastrin. The inhibitory effect of bombesin (200 ng) on pentagastrin-stimulated gastric secretion was reversed by bilateral adrenalectomy or removal of celiac and mesenteric ganglia. Intrathecal injections of rat calcitonin gene-related peptide, neuromedin B, neuromedin U, and the stable substance P analogue (pGlu5, MePhe8, MeGly9)-substance P(5-11) did not alter pentagastrin-stimulated gastric acid secretion. These results demonstrate that bombesin injected into the subarachnoid space of the spinal cord inhibits vagally stimulated and pentagastrin-stimulated gastric secretion in rats. Bombesin action is peptide specific, exerted at a spinal site, and expressed through the sympathetic nervous system.

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)

Intracerebroventricular administration of calcitonin enhances glucose-stimulated release of insulin

Intracerebroventricular (i.c.v.) administration of salmon calcitonin (500 ng) augmented glucose-stimulated release of insulin in rats. Vagotomy increased this enhancement effect of i.c.v. calcitonin significantly, whereas peripheral atropine treatment did not change it. Adrenal catecholamines did not participate in the centrally mediated insulinotropic effect of calcitonin since acute adrenalectomy did not modify the enhancement effect of i.c.v. calcitonin. Destruction of the sympathetic ganglia by neonatal treatment with 6-hydroxydopamine abolished the enhancement effect of i.c.v. calcitonin, which suggests that the sympathetic nervous system participates in the central action of calcitonin to enhance glucose-stimulated release of insulin.

Stress-induced gastrointestinal secretory and motor responses in rats are mediated by endogenous corticotropin-releasing factor

Corticotropin-releasing factor (CRF) has been implicated as a central nervous system mediator of stress. This study examined the effects of CRF and stress on gastric secretory and gastrointestinal motor functions in rats. Partial body restraint as a stress-producing stimulus significantly decreased gastric acid secretion, gastric emptying, and small bowel transit but markedly increased large bowel transit. Corticotropin-releasing factor given cerebroventricularly mimicked the gastrointestinal secretory and motor responses induced by partial body restraint. Cerebroventricular administration of a specific CRF receptor antagonist, alpha-helical CRF-(9-41), but not of the CRF fragment CRF-(1-20), prevented the gastrointestinal secretory and motor responses elicited either by partial body restraint or by exogenous administration of CRF in a dose-dependent fashion. These results suggest that the gastrointestinal secretory and motor responses in rats produced by stress (partial body restraint) are mediated by the endogenous release of CRF. They also indicate that CRF exerts its central nervous system actions on the gastrointestinal tract by a receptor-mediated event.

Central nervous system action of calcitonin gene-related peptide to inhibit gastric emptying in the conscious rat

The central nervous system action of rat alpha-calcitonin gene-related peptide (alpha-CGRP) on gastric emptying of a liquid, noncaloric, methylcellulose solution was assessed in 24-hr fasted, conscious rats using phenol red method as a marker. Intracisternal injection of alpha-CGRP (0.75-250 pmol) dose-dependently inhibited gastric emptying by 27-94% as measured 20 min after oral administration of the solution. The ED50 was 6.2 pmol. alpha-CGRP injected intravenously at 250 pmol delayed gastric emptying by 71% whereas a lower dose (75 pmol) was inactive. Intracisternal alpha-CGRP-induced inhibition of gastric emptying was completely abolished by bilateral adrenalectomy and partially suppressed by subdiaphragmatic vagotomy or coeliac/superior mesenteric ganglionectomy. Adrenalectomy or vagotomy in saline-treated animals did not significantly modify the rate of gastric emptying whereas coeliac/superior mesenteric ganglionectomy caused a significant 29% inhibition as compared to the nonoperated group. These results demonstrate that alpha-CGRP is a potent centrally acting inhibitor of gastric emptying of a nonnutrient liquid. The inhibitory effect of intracisternal injection of CGRP appears to be mediated by the adrenal gland and in part by the sympathetic and parasympathetic nervous system.

Central nervous system effects of corticotropin-releasing factor on gastrointestinal transit in the rat

Corticotropin-releasing factor (CRF) administered into the lateral cerebral ventricle significantly inhibited gastric emptying and small bowel transit, but most markedly increased large bowel transit in a dose-related fashion in freely moving rats. Inhibition of gastric emptying induced by central administration of CRF was completely abolished by pretreatment of the animals with either the ganglionic blocking agent chlorisondamine or the opioid antagonist naloxone, or by noradrenergic blockade with bretylium, but not by truncal vagotomy. Either chlorisondamine, naloxone, or vagotomy--but not bretylium--reversed the inhibitory effect of central CRF on small bowel transit. Chlorisondamine or vagotomy, but neither bretylium nor naloxone, abolished the stimulatory effect of central CRF on large bowel transit. Neither hypophysectomy nor adrenalectomy altered the gastrointestinal motor responses induced by central administration of CRF. Intraperitoneal administration of CRF also significantly inhibited gastric emptying and stimulated large bowel transit but did not alter small bowel transit. These peripheral effects of CRF were not prevented by blockade of autonomic efferents with bretylium or chlorisondamine. It is concluded that (a) CRF acts within the central nervous system to delay gastric emptying, to inhibit small bowel transit, and to increase large bowel transit in freely moving rats and (b) CRF exerts these biological actions by modulation of the autonomic nervous system and, in part, by opioid pathways.

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.

Distribution and characterization of calcitonin gene-related peptide immunoreactivity in the digestive system of normal and capsaicin-treated rats

The distribution and characterization of calcitonin gene-related peptide immunoreactivity in the digestive system of normal, capsaicin-treated, and littermate control rats were studied by radioimmunoassay, chromatography, and immunohistochemistry. The highest concentrations of calcitonin gene-related peptide immunoreactivity were found in the stomach (45 +/- 2.8 pmol/g wet wt, nonsecretory region; 38.7 +/- 4.4 pmol/g wet wt, secretory region) and rectum (30.9 +/- 1.6 pmol/g wet wt). Significant amounts of peptide were also found in the other regions of the gut and in the pancreas. Neonatal treatment with capsaicin, which causes a permanent degeneration of most of the small-diameter sensory neurons, reduced calcitonin gene-related peptide content by greater than 95% in the esophagus and stomach, by 60% in the pancreas, and by less than 50% in the intestine, when compared with littermate controls. Separation of extracts from the gut, pancreas, and brain by chromatography gave major peaks corresponding to the predicted rat calcitonin gene-related peptide and small unidentified peaks, which presumably arise from metabolism of the peptide. Immunohistochemical studies demonstrated that in the esophagus and stomach, calcitonin gene-related peptide immunoreactivity is restricted to nerve fibers, whereas in the intestine it is localized in both nerve fibers and enteric ganglion cells. In capsaicin-treated rats there was a virtually complete elimination of calcitonin gene-related peptide immunoreactive fibers innervating the esophagus and stomach, whereas in the small and large intestine there was a dramatic reduction and often a complete elimination of those associated with blood vessels and a slighter reduction of the nonvascular immunoreactive fibers. The results of this study indicate that calcitonin gene-related peptide immunoreactive nerve fibers innervating the rat digestive system originate from both intrinsic (enteric) and extrinsic (presumably sensory) sources and that both the intrinsic and extrinsic components appear to contain a substance that corresponds to the predicted calcitonin gene-related peptide.

Central nervous system actions of beta-endorphin on gastric acid secretion

We assessed the central nervous system (CNS) actions of beta-endorphin on gastric acid secretion in awake dogs. Synthetic beta-endorphin (0.2-2.0 nmol X kg-1), but not Leu- or Met-enkephalin, microinjected into the third cerebral ventricle, significantly (P less than 0.01) decreased gastric acid secretion stimulated by pentagastrin. beta-Endorphin given intracerebroventricularly inhibited gastric acid secretion following 2-deoxy-D-glucose (P less than 0.01), but not after stimulation with histamine. Intravenous administration of beta-endorphin did not inhibit gastric acid secretion. beta-Endorphin decreased gastric acid secretion but not the concomitant release of gastrin stimulated by a 200-ml liquid meal containing 8% peptone. Pretreatment of the animals with the opioid antagonist, naloxone, prevented the gastric inhibitory effect of beta-endorphin. Furthermore, either ganglionic blockade with chlorisondamine or truncal vagotomy completely abolished the gastric inhibitory action of beta-endorphin. These findings indicate that beta-endorphin, but neither Leu- nor Met-enkephalin, acts within the CNS to inhibit gastric acid secretion in awake dogs. beta-Endorphin-induced inhibition of gastric acid secretion is mediated by an opiate-dependent pathway and by the autonomic (parasympathetic) nervous system.

Intracerebroventricular administration of human calcitonin and human calcitonin gene-related peptide inhibits meal-stimulated gastric acid secretion in the dog

This study was designed to assess the central nervous system actions of human calcitonin (hCalc) and human calcitonin gene-related peptide (hCGRP) on meal-stimulated gastric acid secretion in awake beagle dogs. hCalc (0.1-1.0 nmol/kg) and hCGRP (0.01-1.0 nmol/kg) injected into the third cerebral ventricle significantly inhibited gastric acid secretion stimulated by an 8% peptone meal. hCGRP was ten times more potent than hCalc in inhibiting gastric secretion. Neither hCalc nor hCGRP significantly altered plasma gastrin concentrations compared to control values. Truncal vagotomy did not prevent the gastric inhibitory actions of hCalc and hCGRP. Ganglionic blockade with chlorisondamine completely abolished the gastric inhibitory action of hCalc but had no effect on gastric acid inhibition induced by hCGRP. The results of this study indicate that intracerebroventricular administration of hCalc and hCGRP inhibits meal-stimulated gastric acid secretion in awake dogs. Inhibition of gastric acid secretion by hCalc and hCGRP in the dog is not mediated by inhibition of gastrin release or by the vagus nerves. Human Calc but not human CGRP appears to inhibit meal-stimulated gastric acid secretion in the dog by activation of the autonomic (sympathetic) nervous system.

Basal and stimulated release of calcitonin gene-related peptide (CGRP) in patients with medullary thyroid carcinoma

Calcitonin gene-related peptide (CGRP) is a recently discovered peptide whose existence was first predicted following sequence analysis of the rat calcitonin gene. In the present study, plasma levels of CGRP were measured in patients with medullary thyroid carcinoma, both basally and following calcium or pentagastrin stimulation. Using a sensitive radioimmunoassay for CGRP, 19 of 21 patients with medullary thyroid carcinoma had elevated plasma levels of immunoreactive CGRP (median value 114 pmol/l) while 23 normal subjects had plasma CGRP levels below the detection limit of the assay (less than 10 pmol/l). Calcium or pentagastrin infusion in medullary thyroid carcinoma patients were potent stimuli to CGRP release, increasing plasma levels two- to five-fold. Chromatographic characterization using high pressure liquid chromatography of the CGRP immunoreactivity in plasma from two patients with medullary carcinoma of the thyroid demonstrated the presence of two molecular forms, neither of which co-eluted with synthetic human CGRP. The role of CGRP in the pathophysiology of medullary thyroid carcinoma is not clear but it may contribute to some of the clinical features associated with the disease. Furthermore, measurement of CGRP in conjunction with calcitonin could help in determining the prognosis of these patients.