Calcitonin gene-related peptide-alpha receptor binding sites in the gastrointestinal tract

Characterization of erenumab and rimegepant on calcitonin gene-related peptide induced responses in Xenopus Laevis oocytes expressing the calcitonin gene-related peptide receptor and the amylin-1 receptor

BACKGROUND

The clinical use of calcitonin gene-related peptide receptor (CGRP-R) antagonists and monoclonal antibodies against CGRP and CGRP-R has offered new treatment possibilities for migraine patients. CGRP activates both the CGRP-R and structurally related amylin 1 receptor (AMY1-R). The relative effect of erenumab and the small-molecule CGRP-R antagonist, rimegepant, towards the CGRP-R and AMY-R needs to be further characterized.

METHODS

The effect of CGRP and two CGRP-R antagonists were examined in Xenopus laevis oocytes expressing human CGRP-R, human AMY1-R and their subunits.

RESULTS

CGRP administered to receptor expressing oocytes induced a concentration-dependent increase in current with the order of potency CGRP-R> > AMY1-R > calcitonin receptor (CTR). There was no effect on single components of the CGRP-R; calcitonin receptor-like receptor and receptor activity-modifying protein 1. Amylin was only effective on AMY1-R and CTR. Inhibition potencies (pIC50 values) for erenumab on CGRP induced currents were 10.86 and 9.35 for CGRP-R and AMY1-R, respectively. Rimegepant inhibited CGRP induced currents with pIC50 values of 11.30 and 9.91 for CGRP-R and AMY1-R, respectively.

CONCLUSION

Our results demonstrate that erenumab and rimegepant are potent antagonists of CGRP-R and AMY1-R with 32- and 25-times preference for the CGRP-R over the AMY1-R, respectively. It is discussed if this difference in affinity between the two receptors is the likely reason why constipation is a common and serious adverse effect during CGRP-R antagonism but less so with CGRP binding antibodies.

Constipation Caused by Anti-calcitonin Gene-Related Peptide Migraine Therapeutics Explained by Antagonism of Calcitonin Gene-Related Peptide's Motor-Stimulating and Prosecretory Function in the Intestine

The development of small-molecule calcitonin gene-related peptide (CGRP) receptor antagonists (gepants) and of monoclonal antibodies targeting the CGRP system has been a major advance in the management of migraine. In the randomized controlled trials before regulatory approval, the safety of these anti-CGRP migraine therapeutics was considered favorable and to stay within the expected profile. Post-approval real-world surveys reveal, however, constipation to be a major adverse event which may affect more than 50% of patients treated with erenumab (an antibody targeting the CGRP receptor), fremanezumab or galcanezumab (antibodies targeting CGRP). In this review article we address the question whether constipation caused by inhibition of CGRP signaling can be mechanistically deduced from the known pharmacological actions and pathophysiological implications of CGRP in the digestive tract. CGRP in the gut is expressed by two distinct neuronal populations: extrinsic primary afferent nerve fibers and distinct neurons of the intrinsic enteric nervous system. In particular, CGRP is a major messenger of enteric sensory neurons which in response to mucosal stimulation activate both ascending excitatory and descending inhibitory neuronal pathways that enable propulsive (peristaltic) motor activity to take place. In addition, CGRP is able to stimulate ion and water secretion into the intestinal lumen. The motor-stimulating and prosecretory actions of CGRP combine in accelerating intestinal transit, an activity profile that has been confirmed by the ability of CGRP to induce diarrhea in mice, dogs and humans. We therefore conclude that the constipation elicited by antibodies targeting CGRP or its receptor results from interference with the physiological function of CGRP in the small and large intestine in which it contributes to the maintenance of peristaltic motor activity, ion and water secretion and intestinal transit.

Immunity and pain in the eye: focus on the ocular surface

Most ocular diseases are associated with pain. While pain has been generally considered a mere (deleterious) additional symptom, it is now emerging that it is a key modulator of innate/adaptive immunity. Because the cornea receives the highest nerve density of the entire body, it is an ideal site to demonstrate interactions between pain and the immune response. Indeed, most neuropeptides involved in pain generation are also potent regulators of innate and adaptive leukocyte physiology. On the other hand, most inflammatory cells can modulate the generation of ocular pain through release of specific mediators (cytokines, chemokines, growth factors, and lipid mediators). This review will discuss the reciprocal role(s) of ocular surface (and specifically: corneal) pain on the immune response of the eye. Finally, we will discuss the clinical implications of such reciprocal interactions in the context of highly prevalent corneal diseases.

The two-faced effects of nerves and neuropeptides in corneal diseases

Corneal nerves are instrumental to maintain cornea integrity through regulation of key physiological functions such as tear secretion, blink reflex, and neuropeptide turnover. Corneal nerve injury/stimulation can follow many insults including mechanical/chemical trauma, infections and surgeries. Nerve disruption initiates a process named neurogenic inflammation which leads to edema, pain, and recruitment and activation of leukocytes. Interestingly, leukocyte influx in the cornea can further damage nerves by releasing inflammatory mediators-including neuropeptides. The clinical outcome of neuroinflammation can be beneficial or detrimental to corneal integrity. On one side, it ensures prompt wound healing and prevents infections. On the other, prolonged and/or deranged neuroinflammation can permanently disrupt corneal integrity and impair vision. The cornea is an ideal site to study peripheral neuroinflammation and neurogenic inflammation since it receives the highest density of sensory nerves of the entire body. We will review the corneal nerve anatomy and neurochemistry, discuss the beneficial and detrimental effects of neurogenic inflammation in corneal wound healing, inflammatory processes, and pain. We will also examine the emerging remote impact of corneal nerve disruption on the trigeminal ganglion and the brain, highlighting the key role of neuropeptide Substance P. Finally, we will discuss the clinical relevance of such neuroinflammatory network in the context of severe and highly prevalent ocular diseases, including potential treatments.

Capsaicin receptor as target of calcitonin gene-related peptide in the gut

Calcitonin gene-related peptide (CGRP), a 37 aminoacid-residue peptide, is a marker of afferent fibers in the upper gastrointestinal tract, being almost completely depleted following treatment with the selective neurotoxin capsaicin that targets these fibers via transient receptor potential vanilloid type-1 (TRPV-1). It is widely distributed in the peripheral nervous system of mammals where it is present as alpha isoform, while intrinsic neurons of the enteric nervous systems express predominantly CGRP-beta. Many gastrointestinal functions involve CGRP-containing afferent fibers of the enteric nervous system such as defense against irritants, intestinal nociception, modulation of gastrointestinal motility and secretion, and healing of gastric ulcers. The main effects on stomach homeostasis rely on local vasodilator actions during increased acid-back diffusion. In humans, release of CGRP through the activation of TRPV-1 has been shown to protect from gastric damage induced by several stimuli and to be involved in gastritis. In both dyspepsia and irritable bowel syndrome the repeated stimulation of TRPV-1 induced an improvement in epigastric pain of these patients. The TRPV-1/CGRP pathway might be a novel target for therapeutics in gastric mucosal injury and visceral sensitivity.

Localization of calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1) in human gastrointestinal tract

Calcitonin gene-related peptide (CGRP) exerts its diverse effects on vasodilation, nociception, secretion, and motor function through a heterodimeric receptor comprising of calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1). Despite the importance of CLR·RAMP1 in human disease, little is known about its distribution in the human gastrointestinal (GI) tract, where it participates in inflammation and pain. In this study, we determined that CLR and RAMP1 mRNAs are expressed in normal human stomach, ileum and colon by RT-PCR. We next characterized antibodies that we generated to rat CLR and RAMP1 in transfected HEK cells. Having characterized these antibodies in vitro, we then localized CLR-, RAMP1-, CGRP- and intermedin-immunoreactivity (IMD-IR) in various human GI segments. In the stomach, nerve bundles in the myenteric plexus and nerve fibers throughout the circular and longitudinal muscle had prominent CLR-IR. In the proximal colon and ileum, CLR was found in nerve varicosities of the myenteric plexus and surrounding submucosal neurons. Interestingly, CGRP expressing fibers did not co-localize, but were in close proximity to CLR. However, CLR and RAMP1, the two subunits of a functional CGRP receptor were clearly localized in myenteric plexus, where they may form functional cell-surface receptors. IMD, another member of calcitonin peptide family was also found in close proximity to CLR, and like CGRP, did not co-localize with either CLR or RAMP1 receptors. Thus, CGRP and IMD appear to be released locally, where they can mediate their effect on their receptors regulating diverse functions such as inflammation, pain and motility.

Peripheral adrenomedullin inhibits gastric emptying through CGRP8-37 -sensitive receptors and prostaglandins pathways in rats

The effects of intravenous (iv) adrenomedullin (AM) on gastric emptying were investigated in conscious rats. AM induced a maximal 50% inhibition of gastric emptying at a dose of 1.2 nmol/kg. AM was about two-fold less potent than alpha-calcitonin gene-related peptide (alpha-CGRP), which induced a similar 50% maximal inhibition of gastric emptying at 0.6 nmol/kg. Delayed gastric emptying induced by i.v. AM and alpha-CGRP was prevented by peripheral injection of the selective CGRP1 antagonist, CGRP8-37, and by pretreatment with indomethacin, while not altered by blockade of the sympathetic nervous system with propranolol. These data indicate that peripheral AM inhibits gastric emptying through the interaction with CGRP8-37 -sensitive receptors, likely CGRP1 receptors, and the recruitment of prostaglandin-dependent mechanisms.

Co-localization of TRPV1-expressing nerve fibers with calcitonin-gene-related peptide and substance P in fundus of rat stomach

The localization of vanilloid receptor TRPV1 was studied in rat gastric fundus by an immunohistochemical technique. Numerous TRPV1-immunoreactive nerve fibers were found around arterioles in the submucosal layer. A large number of the nerve fibers were also seen in the smooth muscle layer and in the myenteric nerve plexus, but the cell bodies could not be found. TRPV1 nerve fibers within the circular muscle layers were running parallel to the muscle fibers. Virtually all TRPV1 axons were immunoreactive for calcitonin-gene-related peptide (CGRP), with particularly extensive double labeling seen in axons of the submucosa around blood vessels. TRPV1 nerve fibers containing substance P were found running in longitudinal muscle and circular muscle. The TRPV1 axons seem to be predominantly extrinsic and contain CGRP and substance P in gastric fundus. TRPV1 neurons are thought to be sensory afferent neurons that operate to maintain gastric motility and blood flow.

Localization of calcitonin receptor-like receptor and receptor activity modifying protein 1 in enteric neurons, dorsal root ganglia, and the spinal cord of the rat

Calcitonin receptor-like receptor (CLR) and receptor activity modifying protein 1 (RAMP1) comprise a receptor for calcitonin gene related peptide (CGRP) and intermedin. Although CGRP is widely expressed in the nervous system, less is known about the localization of CLR and RAMP1. To localize these proteins, we raised antibodies to CLR and RAMP1. Antibodies specifically interacted with CLR and RAMP1 in HEK cells coexpressing rat CLR and RAMP1, determined by Western blotting and immunofluorescence. Fluorescent CGRP specifically bound to the surface of these cells and CGRP, CLR, and RAMP1 internalized into the same endosomes. CLR was prominently localized in nerve fibers of the myenteric and submucosal plexuses, muscularis externa and lamina propria of the gastrointestinal tract, and in the dorsal horn of the spinal cord of rats. CLR was detected at low levels in the soma of enteric, dorsal root ganglia (DRG), and spinal neurons. RAMP1 was also localized to enteric and DRG neurons and the dorsal horn. CLR and RAMP1 were detected in perivascular nerves and arterial smooth muscle. Nerve fibers containing CGRP and intermedin were closely associated with CLR fibers in the gastrointestinal tract and dorsal horn, and CGRP and CLR colocalized in DRG neurons. Thus, CLR and RAMP1 may mediate the effects of CGRP and intermedin in the nervous system. However, mRNA encoding RAMP2 and RAMP3 was also detected in the gastrointestinal tract, DRG, and dorsal horn, suggesting that CLR may associate with other RAMPs in these tissues to form a receptor for additional peptides such as adrenomedullin.

Extrinsic and intrinsic sources of calcitonin gene-related peptide immunoreactivity in the lamb ileum: a morphometric and neurochemical investigation

To investigate extrinsic origins of calcitonin gene-related peptide immunoreactive (CGRP-IR) nerve fibres in the sheep ileum, the retrograde fluorescent tracer Fast Blue (FB) was injected into the ileum wall. Sections of thoraco-lumbar dorsal root ganglia (DRG) and distal (nodose) vagal ganglia showing FB-labelled neurons were processed for CGRP immunohistochemistry. The distribution of CGRP-IR in fibres and nerve cell bodies in the ileum was also studied. CGRP-IR enteric neurons were morphometrically analysed in myenteric (MP) and submucosal plexuses (SMP) of lambs (2-4 months). Sensory neurons retrogradely labelled with FB were scattered in T5-L4 DRG but most were located at the upper lumbar levels (L1-L3); only a minor component of the extrinsic afferent innervation of the ileum was derived from nodose ganglia. In the DRG, 57% of retrogradely labelled neurons were also CGRP-IR. In cryostat sections, a dense network of CGRP-IR fibres was observed in the lamina propria beneath the epithelium, around the lacteals and lymphatic follicles (Peyer's platches), and along and around enteric blood vessels. Rare CGRP-IR fibres were also present in both muscle layers. Dense pericellular baskets of CGRP-IR fibres were observed around CGRP-negative somata. The only CGRP-IR nerve cells were well-defined Dogiel type II neurons localised in the MP and in the external and internal components of the SMP. CGRP-IR neurons in the myenteric ganglia were significantly larger than those in the submucosal ganglia (mean profile areas: about 1,400 mum(2) for myenteric neurons, 750 mum(2) for submucosal neurons). About 6% of myenteric neurons and 25% of submucosal neurons were CGRP-IR Dogiel type II neurons. The percentages of CGRP-IR neurons that were also tachykinin-IR were about 9% (MP) and 42% (SMP), whereas no CGRP-IR neurons exhibited immunoreactivity for vasoactive intestinal peptide, nitric oxide synthase or tyrosine hydroxylase in either plexus. Thus, CGRP immunoreactivity occurs in the enteric nervous system of the sheep ileum (as in human small intestine and MP of pig ileum) in only one morphologically defined type of neuron, Dogiel type II cells. These are probably intrinsic primary afferent neurons.

Differential and cell-specific expression of calcitonin receptor-like receptor and receptor activity modifying proteins in the human uterus

The calcitonin receptor-like receptor (CRLR) can function as a receptor for either calcitonin gene-related peptide (CGRP) or adrenomedullin (AM), depending upon co-expression with members of a novel family of receptor activity-modifying proteins (RAMP). RAMP1 presents the CRLR at the cell surface as a CGRP/AM receptor. RAMP2- and RAMP3-transported CRLR receptors act as AM-specific receptors. However, it is still unknown if this signalling system operates in vivo. Of particular interest is the uterus, where both peptides and their binding sites are known to be present and where both mitogenic and vasodilatory responses to AM and CGRP have been demonstrated. In this study, we examined whether CRLR and RAMP are co-expressed in the same populations of cells in human uterine tissue. Analysis by in-situ hybridization and immunocytochemistry revealed a heterogeneous and cell type-specific distribution of components of this AM/CGRP signalling system. Adrenomedullin mRNA was expressed and evenly distributed across all cell types. CRLR mRNA was predominantly found in blood vessels. RAMP1 expression was specific to myometrial myocytes and vascular smooth muscle cells in uterine arteries. RAMP2 and RAMP3 mRNA were not detectable by in-situ hybridization. The pattern of differential and cell-specific expression of CRLR and RAMP suggests the involvement of CRLR/RAMP1 in the processes of vasodilation, smooth muscle relaxation and angiogenesis in response to AM and CGRP in the human uterus, but also indicates that other receptors may be implicated.

The effect of calcitonin gene-related peptide on pancreatic blood flow and secretion in conscious dogs

The effects of human alpha-calcitonin gene-related peptide (alpha-CGRP) and beta-CGRP on pancreatic arterial (PA), superior mesenteric (SMA) and left gastric arterial (LGA) blood flows were studied by ultrasound transit-time blood flow meters in five conscious dogs. Intravenous injections of alpha-CGRP and beta-CGRP (5-200 pmol/kg) induced a dose-related increase in PA flow and a dose-related decrease in its resistance. At lower doses, alpha-CGRP was more potent than beta-CGRP, but their maximal responses were similar. The blood flow responses to alpha-CGRP (200 pmol/kg) were 153% of the basal flow in LGA, 313% in PA, and 534% in SMA, while those to VIP (100 pmol/kg) were 467% in LGA, 953% in PA and 163% in SMA. Somatostatin reduced blood flow in all arteries. alpha-CGRP, but not beta-CGRP, at higher doses induced gastric contractions and pancreatic protein-rich secretion, which were blocked by atropine. These results suggest that CGRP in perivascular nerves in the pancreas may regulate pancreatic blood flow in dogs but its physiological function remains to be studied.

Calcitonin gene-related peptide regulates amino acid absorption across rat jejunum

The calcitonin gene related peptide (CGRP) is widely distributed in the enteric nervous system and gut afferents. Its role in normal digestion and absorption is not characterised. This study is conducted to elucidate whether CGRP regulates amino acid absorption in the small intestine. In in vivo experiments using the single-pass perfusion technique, intravenous infusion of CGRP (250-750 pmol/kg-min) reduced alanine absorption by 35-40%. The effects were completely blocked by the antagonist hCGRP (8-37). Moreover, intravenous infusion of CGRP antagonist blocked the inhibitory effect of intraluminal capsaicin perfusion on alanine absorption. Similarly, intracerebral injection of CGRP decreased alanine absorption, an effect which was reduced by vagotomy. In vitro experiments using isolated jejunal strips showed that CGRP reduced alanine absorption in a dose-dependent manner. At 6 pM, CGRP decreased alanine absorption by 33%. Similarly, CGRP reduced the absorption of proline and taurine by 20 and 11.5%, respectively. Kinetic studies revealed that CGRP reduces alanine influx into intestinal epithelial cells by inhibiting the affinity of the carriers. It is demonstrated that CGRP is involved in the regulation of jejunal amino acid absorption through intrinsic (enteric) and extrinsic (central) neural mechanisms.

Specific calcitonin gene-related peptide binding sites present throughout rat intestine

Calcitonin gene-related peptide is found extensively in the innervation of the intestine and has potent pharmacological effects on secretion, blood flow, and motility. Although essential for assessing the physiological significance of CGRP, detailed information concerning the distribution of its receptor(s) within the intestine is lacking. By using autoradiographic methods, we identified specific binding sites for 125I-tyr0-CGRP-alpha in all regions of the rat small and large intestine. Particularly dense saturatable binding is observed within the lamina propria. There is moderate saturatible binding in the myenteric plexuses. These findings clearly support the notion that CGRP has a neuroeffector role in intestinal functions.

Vagally dependent protective action of calcitonin gene-related peptide on colitis

This work evaluates the mechanism of action of calcitonin gene-related peptide (CGRP) on colitis. Firstly, Wistar rats were intracolonically instilled with trinitrobenzenesulfonic acid (TNBS) and i.v. treated by either alphaCGRP, or hCGRP(8-37), or by vehicle. The inflammatory level was evaluated 8 h and 4 days after TNBS. Secondly, intracerebroventricular alphaCGRP was assessed on the 4-day group with colitis. Finally, i.v. alphaCGRP was administered in vagotomized animals, and tested on the 4-day group with colitis. Colitis was aggravated by hCGRP(8-37), and decreased by peripheral but not central alphaCGRP. AlphaCGRP was inactive on inflammatory parameters in vagotomized colitic rats. This suggests that endogenous peripheral CGRP has an anti-inflammatory role in TNBS-induced colitis, depending upon the integrity of the vagus.

CGRP upregulation in dorsal root ganglia and ileal mucosa during Clostridium difficile toxin A-induced enteritis

We have previously reported that pretreatment of rats with capsaicin (an agent that ablates sensory neurons) or CP-96345 (a substance P receptor antagonist) dramatically inhibits fluid secretion and intestinal inflammation in ileal loops exposed to Clostridium difficile toxin A. The aim of this study was to determine whether calcitonin gene-related peptide (CGRP), a neuropeptide also found in sensory afferent neurons, participates in the enterotoxic effects of toxin A. Administration of toxin A was also found to increase CGRP content in dorsal root ganglia and ileal mucosa 60 min after toxin exposure. Furthermore, immunohistochemical studies demonstrated increased neuronal staining for CGRP 2 h after toxin A treatment. Pretreatment of rats with CGRP-(8-37), a specific CGRP antagonist, before instillation of toxin A into ileal loops significantly inhibited toxin-mediated fluid secretion (by 48%), mannitol permeability (by 83%), and histological damage. We conclude that CGRP, like substance P, contributes to the secretory and inflammatory effects of toxin A via increased production of this peptide from intestinal nerves, including primary sensory afferent neurons.

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.

Distribution of several gut neuropeptides and their effects on motor activity in muscularis mucosae of guinea-pig proximal colon

The distribution of peptide-containing nerve fibers and the effect of their neuropeptides on motor activity were studied in the muscularis mucosae of the guinea-pig proximal colon. In the immunohistochemical study, it was shown that the tachykinin (TK)-containing nerve fibers densely innervated the muscularis mucosae. In the superfusion study, three kinds of TKs, i.e., neurokinin A (NK-A), neurokinin B (NK-B) or substance P (SP), enhanced the spontaneous activity on the strips of muscularis mucosae with a tetrodotoxin (TTX)-insensitive manner. Their potency was in the rank order of NK-A > SP. This suggests that the muscle has a predominant NK2 receptor. Calcitonin gene-related peptide (CGRP)-immunoreactive fibers were commonly observed in the muscle. CGRP induced a potent inhibition on spontaneous activity and a concentration-dependent inhibition on the NK-A-elicited excitation in the presence of TTX, indicating its direct effect on the receptor in the muscle. On the other had, gastrin releasing peptide (GRP), galanin, neuropeptide Y or somatostatin were more or less immunopositive in nerve fibers, but they had no effect on the motility of the muscle except that GRP sometimes showed a faint increase in spontaneous activity. Neither methionine-enkephalin nor gastrin-17/cholecystokinin was immunoreactive and had any effect on the muscle. These neuropeptides other than TKs and CGRP do not seem to be neuromediators of motor activity of muscularis mucosae. The results suggest the possibility that TK-, especially NK-A- and CGRP-containing neurons, participate in the regulation of motor activity of the muscularis mucosae in the guinea-pig proximal colon.

Neurochemical coding in the myenteric plexus of the upper gastrointestinal tract of hibernating hamsters

As part of our investigation of the plasticity of autonomic nerves in physiological and pathological conditions, we have examined the effect of hibernation on the neurochemical content of myenteric nerves and nerve cell bodies of the upper gastrointestinal tract of the non-seasonal hibernator, the golden hamster. Age matched hamsters kept at room temperature and those kept at 5 degrees C but which failed to hibernate, were used as controls. Possible changes in nerve fibres and nerve cell bodies containing the general neuronal marker, protein gene product 9.5, the peptides, vasoactive intestinal polypeptide, substance P (SP) and calcitonin gene-related peptide (CGRP), the catecholamine synthesizing enzyme tyrosine hydroxylase and the enzyme responsible for synthesizing nitric oxide, nitric oxide synthase, were examined in the oesophagus, proventriculus and proximal and distal stomach of the golden hamsters using immunohistochemical techniques. The results of the present study revealed a significant increase in the number of nerve cell bodies and density of nerve fibres containing SP-immunoreactivity and increased number of CGRP-immunoreactive cell bodies but not the other markers examined in the proximal stomach and proventriculus. In contrast, there was no change in the distribution of any of the neuroactive substances examined in the myenteric plexus of the oesophagus and distal stomach. It is suggested that the change in the environment of the hibernating hamsters perturbs the normal digestive physiology in the proximal stomach and proventriculus that is reflected by the selective changes in SP- and CGRP-containing enteric nerves; these changes may be part of protective reflex mechanisms to the environmental changes resulting from hibernation, where upgrading of nerve cell bodies expressing CGRP and SP has occurred.

Islet amyloid polypeptide in the gut and pancreas: localization, ontogeny and gut motility effects

The occurrence of islet amyloid polypeptide (IAPP) in the gut and pancreas of several species and during ontogeny of the rat, was studied using immunocytochemistry. Effects of IAPP on rat ileal smooth muscle were assessed in vitro. Islets of most, but not all, species examined, displayed IAPP in insulin cells and, in some species, also in somatostatin- and peptide YY (PYY)-containing cells. In the gut, expression of IAPP varied among species; when present, IAPP was most abundant in the proximal part and co-localized with somatostatin, PYY, gastrin/cholecystokinin, enteroglucagon or serotonin. IAPP was first demonstrated at embryonic day 12 and 16 in islet and gastrointestinal endocrine cells, respectively. IAPP relaxed gut muscle and reduced electrical field stimulation-evoked contractions, presumably by inhibiting acetylcholine release. Thus, IAPP expression in islets is consistent with an important role for IAPP in fuel metabolism; the gastrointestinal expression and motor effects of IAPP suggest that IAPP may modulate gastrointestinal function.

The calcitonin gene-related peptide activates both cAMP and NO pathways to induce relaxation of circular smooth muscle cells of guinea-pig ileum

The direct effects and the intracellular pathways of rCGRP were investigated on smooth muscle cells (SMC) isolated by enzymatic digestion from the circular and longitudinal layers of guinea-pig ileum. In circular SMC, rCGRP inhibited CCK8-induced contraction in a concentration-dependent manner (Cmax = 100 microM and EC50 = 0.7 +/- 0.4 nM). Preincubation of SMC with 1 microM Rp-cAMPs, a cAMP antagonist, abolished the relaxing effect of rCGRP; moreover, preincubation of SMC with 100 microM L-NAME, an inhibitor of NOS, inhibited the relaxing effect of rCGRP, hCGRP(8-37), a selective antagonist of rCGRP receptors, inhibited the rCGRP-induced relaxation in a concentration dependent manner whereas the vasoactive intestinal polypeptide (VIP) antagonist had no significant effect. In longitudinal SMC, rCGRP-induced relaxation was abolished by Rp-cAMPs, whereas L-NAME had no effect. In conclusion, rCGRP triggers different intracellular pathways to induce relaxation of circular or longitudinal intestinal SMC; cAMP is involved in cells from both layers while nitric oxide (NO) is involved only in relaxation of circular SMC.

Calcitonin gene-related peptide is a potent intestinal, but not gastric, vasodilator in conscious dogs

The effects of human alpha-calcitonin gene-related peptide (alpha-CGRP), beta-CGRP, and vasoactive intestinal polypeptide (VIP) on left gastric (LGA) and superior mesenteric arterial (SMA) blood flow, heart rate and systemic arterial blood pressure were investigated in 6 conscious beagle dogs. Both intravenous injections of alpha-CGRP and beta-CGRP (5-200 pmol/kg) and infusion of alpha-CGRP (25-100 pmol/kg per h) induced a dose-related increase in SMA flow and a dose-related decrease in its resistance. At lower doses, alpha-CGRP was more potent than beta-CGRP, but their maximal responses were the same. alpha-CGRP and beta-CGRP had little effect on LGA flow. However, alpha-CGRP at 200 pmol/kg, but not beta-CGRP, stimulated gastroduodenal contractions that were associated with a phasic increase of LGA flow. Atropine inhibited gastric, but not duodenal, motor and circulatory responses to alpha-CGRP. Tachycardia and hypotension induced by beta-CGRP were significantly less than those by alpha-CGRP. VIP, on the other hand, increased mainly LGA flow. These results suggest that blood vessels of the small intestine of dogs are more sensitive to CGRP than those of the stomach, while the sensitivity to VIP is reversed.

Protection by gastrin in the rat stomach involves afferent neurons, calcitonin gene-related peptide, and nitric oxide

BACKGROUND & AIMS

Certain gut peptides exert gastroprotective effects. However, the underlying mechanism is not fully understood. This study examines the contribution of afferent neurons, calcitonin gene-related peptide, and nitric oxide to the protection conferred by gastrin 17 in the rat stomach.

METHODS

Gastroprotection by gastrin 17 against ethanol-induced gross and histological damage was studied after capsaicin-induced defunctionalization of afferent neurons, pretreatment with the calcitonin gene-related peptide receptor antagonist human calcitonin gene-related peptide8-37, anti-calcitonin gene-related peptide antibodies, and the NO synthase inhibitor NG-nitro-L-arginine.

RESULTS

Gastrin 17 (1-25 pmol/kg) dose-dependently prevented mucosal damage caused by ethanol. Protection was inhibited by functional ablation of afferent neurons or pretreatment with human calcitonin gene-related peptide8-37 (50% inhibitory dose, 86 pmol.kg-1.min-1), anticalcitonin gene-related peptide antibodies, or NG-nitro-L-arginine (50% inhibitory dose, 1 mg/kg). L-Arginine but not D-arginine reversed the effect of NG-nitro-L-arginine. Effects on gross damage were paralleled by histology. Protective doses of gastrin 17 increased gastric mucosal blood flow and, in addition, elevated plasma gastrin concentrations to the same extent as intragastric peptone perfusion.

CONCLUSIONS

Gastrin 17 has potent gastroprotective activity that involves afferent neurons, calcitonin gene-related peptide, and NO.

Diverse interactions of calcitonin gene related peptide and nitric oxide in the gastric and cutaneous microcirculation

Calcitonin gene related peptide (CGRP) is the major mediator of afferent nerve mediated vasodilatation in the gastric mucosa and skin of the rat. Since receptors for CGRP occur on both the vascular endothelium and smooth muscle, it is conceivable that the vascular actions of CGRP involve multiple mechanisms. The vasodilator effect of rat CGRP-alpha in the rat gastric mucosa is indeed inhibited by blockage of nitric oxide (NO) synthesis, as is the gastric mucosal hyperemia in response to gastric acid challenge, which is mediated by CGRP release from afferent nerve fibres. In contrast, the vasodilator response to rat CGRP-alpha in the rat hind paw and the CGRP-mediated vasodilatation evoked by antidromic stimulation of afferent nerve fibres do not depend on the formation of NO. These data indicate that NO plays regionally different roles in the local vasodilator action of CGRP. NO is a secondary vasorelaxant messenger of CGRP in the gastric, but not in the cutaneous, microcirculation. However, this L-arginine-derived autacoid may have a role in the irritant-induced CGRP release from afferent vasodilator fibres in the skin.

Effects of calcitonin gene-related peptide on cyclic AMP production and relaxation of longitudinal muscle of guinea pig ileum

The effects of rat calcitonin gene-related peptide (rCGRP) on the content of cyclic nucleotides in the cells of plexus-free longitudinal muscle (LM) of guinea pig ileum relative to smooth muscle relaxation were investigated. In the absence of isobutyl-methyl-xanthine (IBMX), a nonspecific inhibitor of phosphodiesterases, rCGRP significantly increased cyclic 3'5'-adenosine monophosphate (cAMP) content that correlated temporally to the relaxation of the muscle. Cyclic 3'5'-guanosine monophosphate (cGMP) content was not affected by this peptide. IBMX potentiated both the increase in cAMP content and the muscle relaxation induced by rCGRP. rCGRP increased cAMP content in a concentration-dependent manner, with an ED50 value of 7.5 x 10(-9) M. Both the rCGRP-stimulated increase in cAMP content and the rCGRP-induced muscle relaxation were blocked by hCGRP(8-37), a selective antagonist of CGRP receptors. The pA2 value of hCGRP(8-37) for rCGRP-induced inhibition of tension was calculated to be 6.48. These results suggest that CGRP relaxes the LM through its specific receptors and involves the generation of cAMP, but not the generation of cGMP.

Morphological relationships of choleragenoid horseradish peroxidase-labeled spinal primary afferents with myenteric ganglia and mucosal associated lymphoid tissue in the cat esophagogastric junction

The goal of the present study was to gain insight into the environmental factors influencing the activity of primary spinal afferent fibers in the different layers of the esophagogastric junction of the cat and, thus, to analyze the relationships of these afferents with various cellular components. Spinal primary afferent fibers were selectively labeled by anterogradely transported choleragenoid horseradish peroxidase conjugate (B-HRP). B-HRP was injected into the thoracic dorsal root ganglion at the T8-T13 levels. 6-Hydroxydopamine-induced sympathectomy was performed prior to B-HRP injection in order to prevent otherwise unavoidable labeling of sympathetic fibers in the gut wall. Numerous labeled fibers ran between, around, and within the myenteric ganglia. Others crossed the muscle layers directly and entered the mucosa, where some ran near granulocytes and around or through solitary lymphoid follicles. Labeled fibers were observed in the squamous esophageal epithelium but not in the fundic glandular epithelium. The fibers in the myenteric area are probably connected to the muscular tension receptors that have been detected by electrophysiologic techniques. This assumption is based on the observation that only a few fibers appear to terminate in muscle layers and on the fact that the myenteric area is very narrow and subject to powerful forces. Fibers in the myenteric ganglia could be involved in local efferent functions. Fibers in the mucosa could act as nociceptors and might be involved in local immunological responses.

Characterization of binding sites and effects of calcitonin gene-related peptide (CGRP) and CGRP 8-37 on regional blood flow in rabbit salivary glands

Calcitonin gene-related peptide (CGRP) is a neuropeptide present in nerve fibres of salivary glands in several species, including man. One of the major targets of these nerve fibres are blood vessels of the glands. The presence and distribution of specific binding sites for CGRP in the rabbit major salivary glands was here investigated autoradiographically. In order to determine the physiological role of CGRP, regional blood flow was measured after intravenous (i.v.) or intra-arterial (i.a.) administration of CGRP or the antagonist CGRP 8-37, using a microsphere technique. Specific binding sites for CGRP were found in the parotid, submandibular and sublingual glands, distributed mainly in the muscular and endothelial layers of the blood vessel walls. CGRP injected i.a. (10 pmol/kg) caused a significant increase in regional blood flow in all major salivary glands. However, i.v. infusion of CGRP (120 pmol/kg) decreased regional blood flow in the parotid and sublingual glands, due to a general decrease in peripheral resistance and redistribution of peripheral blood flow. CGRP 8-37 given i.a. together with CGRP significantly inhibited the blood-flow increase by CGRP alone. It is concluded that most of the CGRP receptors in the rabbit salivary glands are localized in vascular elements. The physiological data show that CGRP acts as a vasodilator in the major salivary glands of the rabbit in vivo, and that the effect of CGRP is inhibited by the CGRP antagonist CGRP 8-37.

Effect of CGRP antagonist, alpha-CGRP 8-37, on acid secretion in the dog

The recently synthesized calcitonin gene-related peptide (CGRP) antagonist, human alpha-CGRP 8-37, was used to study its effects on gastric acid secretion. Four dogs with gastric fistula were used to measure the antagonist's physiologic effects in the stomach. All dogs received a bactopeptone dextrose meal (intragastric titration to pH 5.5) with either continuous CGRP 8-37 (1000 pmol/kg/hr) or saline (control). Additionally, intravenous bombesin (75-600 ng/kg/hr) and bethanechol (12.5-100 micrograms/kg/hr) was tested in the presence of the antagonist. Plasma gastrin levels also were measured via radioimmunoassay (RIA) in control and CGRP 8-37-stimulated animals. Gastric acid secretion increased by 100% with infusion of 1000 pmol/kg/hr CGRP 8-37 when compared to the control. Acid output increased 98% with both intravenous antagonist and 600 ng/kg/hr bombesin when compared to bombesin alone. However, no augmentation of acid secretion by CGRP 8-37 was shown with 25 micrograms/kg/hr bethanechol. RIA of plasma gastrin demonstrated no effect with the antagonist when given alone and did not increase bombesin-stimulated gastrin release. We conclude that CGRP 8-37 blocks native CGRP inhibitory effects on gastric acid secretion. Our findings of potentiation of acid secretion by bombesin as well as no change in gastrin levels in the presence of the antagonist is likely due to a blockage in a noncholinergic neuron to the somatostatin cell. Furthermore, CGRP 8-37 did not increase bethanechol-stimulated acid secretion, most likely due to bethanechol's (acetylcholine) nearly ubiquitous positive effects on acid secretion.

Role of spinal afferents and calcitonin gene-related peptide in the postoperative gastric ileus in anesthetized rats

OBJECTIVE

The object of this study was to investigate the mechanisms of postoperative gastric ileus in an experimental model of abdominal surgery in anesthetized rats.

SUMMARY BACKGROUND DATA

Sensory neurons partly mediate postoperative gastric ileus. Among other neuropeptides, sensory neurons contain calcitonin gene-related peptide (CGRP) and release CGRP in response to noxious stimulation. Because CGRP inhibits gastric motility, it was hypothesized that abdominal surgery stimulates sensory neurons, which then releases CGRP, thereby inhibiting gastric motility.

METHODS

Postoperative ileus was induced by abdominal surgery. Gastric corpus motility was measured by an intragastric catheter. CGRP action was blocked by CGRP immunoneutralization or by a CGRP receptor antagonist. Spinal sensory neurons were ablated by application of a sensory neurotoxin (capsaicin) to the celiac and superior mesenteric ganglia.

RESULTS

Abdominal surgery decreased gastric corpus motility in the first 5 minutes after abdominal surgery by 59 +/- 5% and by 24 +/- 4% during the 1st postoperative hour. Capsaicin pretreatment of the celiac and superior mesenteric ganglia, CGRP immunoneutralization, or CGRP receptor antagonism reversed the postoperative decrease in gastric corpus motility during the 1st postoperative hour by 50%, 100%, and 59%, respectively.

CONCLUSIONS

These data indicate that spinal sensory neurons and CGRP partly mediate postoperative gastric ileus. CGRP may be released from spinal sensory neuron terminals in the celiac and superior mesenteric ganglia as part of an extraspinal intestinogastric inhibitory reflex activated by abdominal surgery.

CGRP antagonists and capsaicin on celiac ganglia partly prevent postoperative gastric ileus

The role of capsaicin-sensitive pathways and CGRP in postoperative gastric ileus was investigated. Abdominal surgery was performed under enflurane anesthesia, and 5 min later, the 20-min rate of gastric emptying was measured by the phenol red method in conscious rats. Surgery inhibited gastric emptying by 76-83% compared with rats receiving anesthesia alone. Capsaicin on the celiac/mesenteric ganglia (10-21 days before) reduced gastric ileus by 33 +/- 8%, whereas perivagal capsaicin had no effect. The IV CGRP-induced inhibition of gastric emptying was completely reversed by the CGRP antagonist, CGRP(8-37) (30 micrograms, IV); CGRP(8-37) (15, 30, or 60 micrograms) or CGRP monoclonal antibody #4901 (2 mg protein) decreased the inhibition of gastric emptying by 11 +/- 7%, 51 +/- 13%, 47 +/- 3%, and 45 +/- 17%, respectively. These results indicate that CGRP and splanchnic capsaicin-sensitive afferents are involved in mediating part of the gastric ileus observed immediately after abdominal surgery.

Neural and non-neural origin of calcitonin gene-related peptide (CGRP) in the gastric mucosa

We have used immunohistochemistry and in situ hybridization histochemistry to visualize CGRP and the mRNA encoding the CGRP precursor in the stomach. CGRP is present in nerve fibers in the mucosa. CGRP mRNA and CGRP itself are also found in non-neural cells in the lamina propria. These cells are likely to be macrophages or B-lymphocytes.

Calcitonin gene-related peptide neurons innervating the canine digestive system

The pattern of nerve cells and fibers containing calcitonin gene-related peptide immunoreactivity (CGRP-IR) was investigated in the canine digestive tract by means of immunohistochemistry. CGRP-IR nerve fibers innervate all the layers of the gut, including the vasculature, with different densities depending on the region. CGRP-IR processes are sparse in the esophagus and stomach, where they are mostly confined to the enteric plexuses and vasculature. CGRP-IR fibers are quite abundant in the small and large intestine, where they form dense arborizations in the mucosa, and are numerous in the muscularis mucosae, deep muscular plexus and circular muscle. The myenteric and submucous plexuses of the intestine contain dense networks of CGRP-IR fibers and numerous CGRP-IR ganglion cells. On the other hand, in the enteric ganglia of the esophagus and stomach, in the intrapancreatic ganglia and in the ganglionated plexus of the gallbladder, CGRP-IR is restricted to non-varicose processes. A moderate density of CGRP-IR fibers supplies the endocrine and exocrine pancreas, and the fibromuscular layer and lamina propria of the gallbladder. The density of CGRP innervation in different regions can be summarized as follows: intestine >> pancreas and gallbladder > or = antrum > cardia > gastric corpus and distal esophagus. CGRP- and tachykinin (TK)-IRs are colocalized in a substantial population of fibers, particularly those distributed to the mucosa, muscularis mucosae and vasculature, whereas there was no evidence of colocalization in intrinsic ganglion cells. The present results suggest that (1) the CGRP innervation of the dog digestive system includes an intrinsic and an extrinsic component, and (2) CGRP- and TK-IRs are co-expressed in extrinsic nerve fibers. These findings extend previous observations in rats and guinea pigs and provide insights into the sites of action of CGRP in the digestive system of the dog, which has served as a model for CGRP functional studies.

Receptor binding sites for cholecystokinin, galanin, somatostatin, substance P and vasoactive intestinal polypeptide in sympathetic ganglia

Sympathetic ganglia are innervated by neuropeptide-containing fibers originating from pre- and postganglionic sympathetic neurons, dorsal root ganglion neurons, and in some cases, myenteric neurons. In the present report receptor autoradiography was used to determine whether sympathetic ganglia express receptor binding sites for several of these neuropeptides including bombesin, calcitonin gene-related peptide-alpha, cholecystokinin, galanin, neurokinin A, somatostatin, substance P, and vasoactive intestinal polypeptide. The sympathetic ganglia examined included the rat and rabbit superior cervical ganglia and the rabbit superior mesenteric ganglion. High levels of receptor binding sites for cholecystokinin, galanin, somatostatin, substance P, and vasoactive intestinal polypeptide were observed in all sympathetic ganglia examined, although only discrete neuronal populations within each ganglion appeared to express receptor binding sites for any particular neuropeptide. These data suggest that discrete populations of postganglionic sympathetic neurons may be regulated by neuropeptides released from pre- and postganglionic sympathetic neurons, dorsal root ganglion neurons, and myenteric neurons.

Direct inhibitory effect of calcitonin gene-related peptide and atrial natriuretic peptide on gastric smooth muscle cells via different mechanisms

Smooth muscle cells isolated from the gastric muscle layers of the guinea pig were used to determine whether calcitonin gene-related peptide (CGRP) and atrial natriuretic peptide (ANP) can inhibit the contractile response produced by 10(-6) M carbachol by exerting a direct action on muscle cells. In addition, the inhibitory effect of 2', 5'-dideoxyadenosine, an inhibitor of adenylate cyclase, on the CGRP-induced or ANP-induced relaxation of gastric smooth muscle cells were examined. CGRP and ANP inhibited the contractile response produced by carbachol in a dose-dependent manner, and the values of IC50 were 3 nM and 2 nM, respectively. 2',5'-dideoxyadenosine significantly inhibited the relaxation produced by CGRP. On the other hand, 2',5'-dideoxyadenosine did not have any significant effect on the relaxation produced by ANP. These results demonstrate the difference between intracellular mechanism responsible for gastric smooth muscle relaxation by CGRP and the mechanism responsible for muscle relaxation by ANP, and strongly suggest that the action of CGRP is mediated by adenosine 3',5'-cyclic monophosphate.

Calcitonin gene-related peptide-containing neurons supplying the rat digestive system: differential distribution and expression pattern

In the enteric nervous system, calcitonin gene-related peptide (CGRP) immunoreactivity is localized to a substantial number of capsaicin-sensitive afferent fibers and to intrinsic neurons and processes. CGRP immunoreactivity detected by immunohistochemistry represents the expression of two distinct genes, the calcitonin/alpha-CGRP and the beta-CGRP genes, which have different tissue distributions. In the present study, we used (1) in situ hybridization histochemistry and ribonucleic acid (RNA) blot hybridization with RNA probes complementary to the divergent sequences of alpha- and beta-CGRP messenger RNAs (mRNAs) to differentiate which CGRP gene was expressed in enteric and afferent neurons; and (2) axonal transport approaches in combination with CGRP immunohistochemistry to define the location of CGRP-containing afferent neurons supplying the digestive system. In situ hybridization histochemistry with [35S]-labeled RNA probes indicated that in the gastrointestinal tract beta-CGRP mRNA, but not alpha-CGRP mRNA, was expressed in enteric neurons confined to the myenteric and submucous plexuses of the small and large intestine. In dorsal root and vagal sensory ganglia, mRNAs for alpha-CGRP and beta-CGRP were both present in a vast population of neurons, with an overlapping pattern, even though the alpha-CGRP signal appeared more intense. RNA blot hybridization analysis showed a single band of hybridization at 1.2 Kb with the beta-CGRP RNA probe in RNA extracts from muscle layer-myenteric plexus and submucosal layer preparations of the ileum, and from dorsal root ganglia; it also showed a single band at 1.3 Kb with the alpha-CGRP RNA probe in extracts from dorsal root ganglia, but not from the intestine. These findings further support the differential expression of alpha- and beta-CGRP mRNAs. Retrograde transport of fast blue or fluorogold coupled with CGRP immunohistochemistry demonstrated that the vast majority of CGRP-containing afferent neurons supplying the stomach, proximal duodenum, and pancreas were located in dorsal root ganglia at the middle and lower thoracic and at the upper lumbar levels, and represented a major component of the afferent innervation of these viscera (up to 89%). Approximately 50% of CGRP-immunoreactive afferent neurons also expressed tachykinin (TK) immunoreactivity, as shown by triple labeling. Only a minor component of the afferent innervation of the stomach, duodenum, and pancreas derived from vagal CGRP-containing neurons (less than 8%). A large portion of these neurons (an average of 62%) also contained TK immunoreactivity.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuropeptide control of rat gastric mucosal blood flow. Increase by calcitonin gene-related peptide and vasoactive intestinal polypeptide, but not substance P and neurokinin A

Submucosal blood vessels of the mammalian stomach are densely innervated by neurons containing calcitonin gene-related peptide (CGRP), substance P, neurokinin A, and vasoactive intestinal polypeptide (VIP). Because all these peptides are vasodilators in certain vascular beds, we tested the hypothesis that rat alpha-CGRP, rat VIP, substance P, and neurokinin A are candidate mediators of noncholinergic vasodilator neurons in the gastric mucosa and submucosa. The experiments were performed on urethane-anesthetized Sprague-Dawley rats. Gastric mucosal blood flow (GMBF) was measured by the hydrogen gas clearance technique, and the peptides were infused close arterially to the stomach via a catheter inserted retrogradely in the splenic artery. Basal GMBF was in the range of 35-50 ml/min/100 g. Infusion of rat alpha-CGRP (15 and 75 pmol/min) significantly increased GMBF in a dose-dependent manner, whereas mean arterial blood pressure was significantly lowered only by the higher dose of CGRP. Substance P (125 and 625 pmol/min) and neurokinin A (50 and 250 pmol/min) failed to alter GMBF, although the higher dose of each peptide led to a significant decrease in mean arterial blood pressure. Infusion of rat VIP (25 pmol/min) failed to affect GMBF and mean arterial blood pressure, whereas a fivefold higher dose of VIP (125 pmol/min) led to a significant rise of GMBF and to significant hypotension. These findings indicate that substance P and neurokinin A are unlikely to be of physiological significance for the regulation of GMBF. CGRP and VIP, however, can be considered as candidate mediators of submucosal nerve endings involved in the neural control of GMBF.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and developmental evolution of a high-affinity binding site for calcitonin gene-related peptide on chick skeletal muscle membrane

Calcitonin gene-related peptide is a putative neurotransmitter of central and peripheral nervous systems which coexists with acetylcholine in motor nerve terminals and exerts multiple effects on skeletal muscle, suggesting a trophic role for this neuropeptide. Using radiolabeled calcitonin gene-related peptide as a probe in a specific binding assay, we have characterized calcitonin gene-related peptide binding sites on chick skeletal muscle membranes. Binding is time-dependent, saturable and reversible. Scatchard analyses revealed two classes of sites: high-affinity sites with a KD value of 62 pM, and low-affinity sites with a KD value of 3.3 nM. The maximal number of sites is, respectively, 22 and 155 fmol/mg protein for high- and low-affinity binding sites. Specific binding was not affected by the presence, in excess, of other neuropeptides such as salmon calcitonin or somatostatin or vasoactive intestinal polypeptide. Affinity of the binding site for calcitonin gene-related peptide was decreased in the presence of 5'-guanylyl-imidodiphosphate, suggesting a physiological coupling of calcitonin gene-related peptide receptor to a GTP binding protein. In a developmental study of chick muscle, we found the highest activity of calcitonin gene-related peptide binding sites in 11-14 day embryos, following a pattern of evolution similar to that of acetylcholine receptors (constant ratio of 12 acetylcholine receptors per calcitonin gene-related peptide binding site). However, both receptors appear differentially regulated: while the number of acetylcholine receptors increases 5-16-fold after denervation, calcitonin gene-related peptide binding sites slightly diminish in number. These results are discussed in terms of the physiological significance of calcitonin gene-related peptide binding sites on chick skeletal muscle membrane.