Calcitonin gene-related peptide neurons innervating the canine digestive system

Mapping the Organization and Morphology of Calcitonin Gene-Related Peptide (CGRP)-IR Axons in the Whole Mouse Stomach

Nociceptive afferent axons innervate the stomach and send signals to the brain and spinal cord. Peripheral nociceptive afferents can be detected with a variety of markers [e.g., substance P (SP) and calcitonin gene-related peptide (CGRP)]. We recently examined the topographical organization and morphology of SP-immunoreactive (SP-IR) axons in the whole mouse stomach muscular layer. However, the distribution and morphological structure of CGRP-IR axons remain unclear. We used immunohistochemistry labeling and applied a combination of imaging techniques, including confocal and Zeiss Imager M2 microscopy, Neurolucida 360 tracing, and integration of axon tracing data into a 3D stomach scaffold to characterize CGRP-IR axons and terminals in the whole mouse stomach muscular layers. We found that: 1) CGRP-IR axons formed extensive terminal networks in both ventral and dorsal stomachs. 2) CGRP-IR axons densely innervated the blood vessels. 3) CGRP-IR axons ran in parallel with the longitudinal and circular muscles. Some axons ran at angles through the muscular layers. 4) They also formed varicose terminal contacts with individual myenteric ganglion neurons. 5) CGRP-IR occurred in DiI-labeled gastric-projecting neurons in the dorsal root and vagal nodose ganglia, indicating CGRP-IR axons were visceral afferent axons. 6) CGRP-IR axons did not colocalize with tyrosine hydroxylase (TH) or vesicular acetylcholine transporter (VAChT) axons in the stomach, indicating CGRP-IR axons were not visceral efferent axons. 7) CGRP-IR axons were traced and integrated into a 3D stomach scaffold. For the first time, we provided a topographical distribution map of CGRP-IR axon innervation of the whole stomach muscular layers at the cellular/axonal/varicosity scale.

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.

Dawn of a New RAMPage

Receptor activity-modifying proteins (RAMPs) interact with G-protein-coupled receptors (GPCRs) to modify their functions, imparting significant implications upon their physiological and therapeutic potentials. Resurging interest in identifying RAMP-GPCR interactions has recently been fueled by coevolution studies and orthogonal technological screening platforms. These new studies reveal previously unrecognized RAMP-interacting GPCRs, many of which expand beyond Class B GPCRs. The consequences of these interactions on GPCR function and physiology lays the foundation for new molecular therapeutic targets, as evidenced by the recent success of erenumab. Here, we highlight recent papers that uncovered novel RAMP-GPCR interactions, human RAMP-GPCR disease-causing mutations, and RAMP-related human pathologies, paving the way for a new era of RAMP-targeted drug development.

The neuropeptide receptor subunit RAMP1 constrains the innate immune response during acute pancreatitis in mice

OBJECTIVES

The importance of the Calcitonin-gene-related-peptide-pathway (CGRP) as neuronal modulator of innate immune responses in mice has been previously demonstrated. The CGRP-receptor is composed of two subunits: the receptor-activity-modifying-protein-1 (RAMP1) and the calcitonin-receptor-like-receptor (CLR). CGRP can influence immune cells and their capacity of producing inflammatory cytokines. Using a RAMP1 knockout-mouse (RAMP1^-/-) we examined the role of the CGRP-receptor in the acute-phase of cerulein-induced pancreatitis.

METHODS

Hourly cerulein-injections for a period of 8 h in RAMP1^-/- and wild-type mice were performed. To compare severity and extent of inflammation in RAMP1^-/- and wild-type mice, histological analyses were done and cytokine levels were assessed using qRT-PCR 8 h, 24 h, 2 days, and 7 days post-cerulein-treatment. Furthermore, serum activities of LDH and lipase were determined.

RESULTS

After 8 h RAMP1^-/- mice showed a higher pancreas-to-body-weight-ratio, increased tissue edema and immune cell infiltration with higher amount of F4/80-positive cells as compared to wild-type mice. Overall infiltration of immune cells at 24 h was increased in RAMP1^-/- mice and composed predominantly of MPO-positive neutrophils. In addition, after 24 h RAMP1^-/- mice presented a higher pancreas-to-body-weight-ratio, higher expression of Ccl3, Il6, and Il1b and increased number of cleaved caspase 3 positive cells. Serum lipase correlated with the extent of tissue damage in RAMP1^-/- compared to wild-type mice 24 h post-cerulein treatment.

CONCLUSION

Mice lacking RAMP1 showed increased inflammation, tissue edema, and pancreas injury particularly in the early phase of acute pancreatitis. This study highlights the essential role of CGRP for dampening the innate immune response in acute pancreatitis.

Alterations in enteric calcitonin gene-related peptide in patients with colonic diverticular disease: CGRP in diverticular disease

Diverticular disease (DD) is one of the most prevalent diseases of the large bowel. Lately, imbalance of neuro-muscular transmission has been recognized as a major etiological factor for DD. Neuronal calcitonin gene-related peptide (CGRP) is a potent gastrointestinal smooth muscle relaxant shown to have a widespread effect within the alimentary tract. Nevertheless, CGRPergic innervation of the enteric ganglia has never been considered in the context of motility impairment observed in DD patients. Changes in CGRP and calcitonin receptor-like receptor (CRLR) abundance within enteric ganglia were investigated in sigmoid samples from symptomatic and asymptomatic DD patients using quantitative fluorescence microscopy. CGRP effect on gastrointestinal smooth muscle was investigated using organ bath technique. We found CGRP levels within the enteric ganglia to be declined by up to 52% in symptomatic DD patients. Conversely, CRLR within the enteric ganglia was upregulated by 41% in symptomatic DD. Longitudinal smooth muscle displayed an elevated (+10.5%) relaxant effect to the exogenous application of CGRP in colonic strips from symptomatic DD patients. Samples from asymptomatic DD patients consistently showed intermediate values across different experiments. In conclusion, the present study demonstrates that CGRPergic signaling is subject to alteration in DD. Our results suggest that a hypersensitization mechanism to gradually decreasing levels of CGRP-IR nerve fibers takes place during DD progression. Alterations to CGRPergic signaling in DD disease may have implications for physiological abnormalities associated with colonic DD.

The protective effects of rutaecarpine on acute pancreatitis

Acute pancreatitis (AP) is the acute inflammation of the pancreas. The morbidity of AP has increased in recent years. Certain patients eventually develop severe AP (SAP), which rapidly progresses to multiple organ dysfunction; the incidence of this occurring in patients with AP is 20-30%. To date, no specific drugs or methods exist to treat this disease. Rutaecarpine relaxes vascular smooth muscle by stimulating calcitonin gene-related peptide (CGRP) release via activation of vanilloid receptor subtype 1 (VR1). It has been demonstrated that rutaecarpine induces a therapeutic effect on SAP. The present study was conducted to characterize the molecular mechanisms underlying the protective effects of rutaecarpine against AP using a rat model of AP. Gross pathological changes of the pancreas, as well as the pancreatic tissue histopathological score, were assessed following treatment with rutaecarpine, capsazepine or a combination of the two. Serum amylase activity was detected using an automatic biochemistry analyzer. Changes in the serum concentrations of interleukin (IL)-6, tumor necrosis factor (TNF-α), IL-10 and CGRP were assessed by ELISA and radioimmunoassay. The results demonstrated that pre-treatment with rutaecarpine markedly decreased pancreatic inflammation and necrosis, reduced the volume of ascites, and significantly increased the plasma concentration of CGRP and the serum concentration of IL-10, an anti-inflammatory cytokine. However, serum concentrations of the inflammatory cytokines IL-6 and TNF-α were decreased. The effect of rutaecarpine treatment markedly improved with increases in the drug dose. Capsazepine, as a competitive vanilloid receptor antagonist, abolished these protective effects of rutaecarpine against AP. Therefore, the results of the present study indicate that rutaecarpine protects against AP in rats by upregulating endogenous CGRP release via activation VR1 of, to improving the microcirculation of the pancreatic tissue and regulate the expression of inflammatory factors.

Capsaicin reduces tissue damage in experimental acute pancreatitis

OBJECTIVES

Calcitonin gene-related peptide (CGRP) is released from perivascular pancreatic nerves. It effects vasomotion and cytokine liberation in inflammatory processes, including acute pancreatitis (AP). Calcitonin gene-related peptide liberation is stimulated by capsaicin, a substance of red hot chili peppers. Aim of the study was to investigate the influence of exogenous capsaicin on experimental AP.

METHODS

Acute pancreatitis was induced in rats by glycodeoxycholic acid and cerulein. Animals were divided into 4 groups: (1) severe AP, (2) severe AP+capsaicin, (3) control without AP, and (4) control+capsaicin. After 24 hours, survival, histology, and CGRP were evaluated (n=6/group). In additional animals, erythrocyte flow and leukocyte activation were evaluated by intravital microscopy 6 hours after AP induction (n=6/group).

RESULTS

In the control groups, all animals survived without histological alterations. Mortality in severe AP was 67%. Capsaicin reduced mortality to 16% (P<0.05). Acute pancreatitis animals developed pancreatic inflammation and necrosis, which was significantly less after capsaicin application. Intravital microscopy in severe AP showed reduced erythrocyte velocity and increased leukocyte adhesion, which was nearly normalized by capsaicin (P<0.01). Calcitonin gene-related peptide increased in both capsaicin groups, indicating endogenous CGRP liberation (P<0.01).

CONCLUSION

Capsaicin releases endogenous CGRP with improved pancreatic microcirculation and reduced inflammation in experimental AP. This underlines neuropeptide activity in the pathogenesis of AP.

HCl-activated neural and epithelial vanilloid receptors (TRPV1) in cat esophageal mucosa

To test whether transient receptor potential channel vanilloid subfamily member-1 (TRPV1) mediates acid-induced inflammation in the esophagus, a tubular segment of esophageal mucosa was tied at both ends, forming a sac. The sac was filled with 0.01 N HCl (or Krebs buffer for control) and kept in oxygenated Krebs buffer at 37 degrees C. The medium around the sac (supernatant) was collected after 3 h. Supernatant of the HCl-filled sac abolished contraction of esophageal circular muscle strips in response to electric field stimulation. Contraction was similarly abolished by supernatant of mucosal sac filled with the TRPV1 agonist capsaicin (10(-6) M). These effects were reversed by the selective TRPV1 antagonist 5'-iodoresiniferatoxin (IRTX) and by the platelet-activating factor (PAF) receptor antagonist CV9388. Substance P and CGRP levels in mucosa and in supernatant increased in response to HCl, and these increases were abolished by IRTX and by tetrodotoxin (TTX) but not affected by CV9388, indicating that substance P and CGRP are neurally released and PAF independent. In contrast, the increase in PAF was blocked by IRTX but not by TTX. Presence of TRPV1 receptor was confirmed by RT-PCR and by Western blot analysis in whole mucosa and in esophageal epithelial cells enzymatically isolated and sorted by flow cytometry or immunoprecipitated with cytokeratin antibodies. In epithelial cells PAF increased in response to HCl, and the increase was abolished by IRTX. We conclude that HCl-induced activation of TRPV1 receptors in esophageal mucosa causes release of substance P and CGRP from neurons and release of PAF from epithelial cells.

Distribution and neurochemical identification of pancreatic afferents in the mouse

Dysfunction of primary afferents innervating the pancreas has been shown to contribute to the development of painful symptoms during acute and chronic pancreatitis. To investigate the distribution and neurochemical phenotype of pancreatic afferents, Alexa Fluor-conjugated cholera toxin B (CTB) was injected into the pancreatic head (CTB-488) and tail (CTB-555) of adult male mice to label neurons retrogradely in both the dorsal root ganglia (DRG) and nodose ganglia (NG). The NG and DRG (T5-T13) were processed for fluorescent immunohistochemistry and visualized by using confocal microscopy. Spinal pancreatic afferents were observed from T5 to T13, with the greatest contribution coming from T9-T12. The pancreatic afferents were equally distributed between right and left spinal ganglia; however, the innervation from the left NG was significantly greater than from the right. For both spinal and vagal afferents there was significantly greater innervation of the pancreatic head relative to the tail. The total number of retrogradely labeled afferents in the nodose was very similar to the total number of DRG afferents. The neurochemical phenotype of DRG neurons was dominated by transient receptor potential vanilloid 1 (TRPV1)-positive neurons (75%), GDNF family receptor alpha-3 (GFRalpha3)-positive neurons (67%), and calcitonin gene-related peptide (CGRP)-positive neurons(65%) neurons. In the NG, TRPV1-, GFRalpha3-, and CGRP-positive neurons constituted only 35%, 1%, and 15% of labeled afferents, respectively. The disparity in peptide and receptor expression between pancreatic afferents in the NG and DRG suggests that even though they contribute a similar number of primary afferents to the pancreas, these two populations may differ in regard to their nociceptive properties and growth factor dependency.

Neuronal Control of Skin Function: The Skin as a Neuroimmunoendocrine Organ

This review focuses on the role of the peripheral nervous system in cutaneous biology and disease. During the last few years, a modern concept of an interactive network between cutaneous nerves, the neuroendocrine axis, and the immune system has been established. We learned that neurocutaneous interactions influence a variety of physiological and pathophysiological functions, including cell growth, immunity, inflammation, pruritus, and wound healing. This interaction is mediated by primary afferent as well as autonomic nerves, which release neuromediators and activate specific receptors on many target cells in the skin. A dense network of sensory nerves releases neuropeptides, thereby modulating inflammation, cell growth, and the immune responses in the skin. Neurotrophic factors, in addition to regulating nerve growth, participate in many properties of skin function. The skin expresses a variety of neurohormone receptors coupled to heterotrimeric G proteins that are tightly involved in skin homeostasis and inflammation. This neurohormone-receptor interaction is modulated by endopeptidases, which are able to terminate neuropeptide-induced inflammatory or immune responses. Neuronal proteinase-activated receptors or transient receptor potential ion channels are recently described receptors that may have been important in regulating neurogenic inflammation, pain, and pruritus. Together, a close multidirectional interaction between neuromediators, high-affinity receptors, and regulatory proteases is critically involved to maintain tissue integrity and regulate inflammatory responses in the skin. A deeper understanding of cutaneous neuroimmunoendocrinology may help to develop new strategies for the treatment of several skin diseases.

Signalling the state of the digestive tract

The gastrointestinal tract has a rich sensory innervation. Extrinsic afferents in vagal, splanchnic and pelvic nerves project to the CNS where gut reflex function is coordinated and integrated with behavioural responses (e.g. regulation of food intake) and mediate sensations. The afferent information conveyed by vagal and spinal mechanosensitive afferents can be very different. Vagal afferents have low thresholds of activation and reach maximal responses within physiological levels of distension. In contrast, spinal afferents, although many have corresponding thresholds for activation, are able to respond beyond the physiological range and encode both physiological and noxious levels of stimulation. However, mechanosensitivity is not fixed but can be influenced by a wide range of chemical mediators released as a consequence of ischemia, injury and inflammation. Indeed, previously mechanical insensitive afferents can develop mechanosensitivity during inflammation and a variety of chemical mediators are implicated in this sensitisation process. Chemosensitivity is also a property of vagal mucosal afferents that detect the chemical milieu for chemicals absorbed across the epithelium or released from enteroendocrine cells that are strategically positioned to "taste" luminal contents. Thus, there exists a complex interplay between immunomodulators, neurotransmitters and neuroendocrine factors that underlie gastrointestinal sensing mechanisms and enable orchestration of appropriate host responses.

Innervation of the extrahepatic biliary tract

The extrahepatic biliary tract is innervated by dense networks of extrinsic and intrinsic nerves that regulates smooth muscle tone and epithelial cell function of extrahepatic biliary tree. Although these ganglia are derived from the same set of precursor neural crest cells that colonize the gut, they exhibit structural, neurochemical, and physiological characteristics that are distinct from the neurons of the enteric nervous system. Gallbladder neurons are relatively inexcitable, and their output is driven by vagal inputs and modulated by hormones, peptides released from sensory fibers, and inflammatory mediators. Gallbladder neurons are cholinergic and they can express a number of other neural active compounds, including substance P, galanin, nitric oxide, and vasoactive intestinal peptide. Sphincter of Oddi (SO) ganglia, which are connected to ganglia of the duodenum, appear to be comprised of distinct populations of excitatory and inhibitory neurons, based on their expression of choline acetyltransferase and substance P or nitric oxide synthase, respectively. While SO neurons likely receive vagal input and their activity is modulated by release of neuropeptides from sensory fibers, a significant source of excitatory synaptic input to these cells arise from the duodenum. This duodenum-SO circuit is likely to play an important role in the coordination of SO tone with gallbladder motility in the process of gallbladder emptying. Now that we have gained a relatively thorough understanding of the innervation of the biliary tree under healthy conditions, the way is paved for future studies of altered neural function in biliary disease.

Effects of the calcitonin gene-related peptide (CGRP) receptor antagonist BIBN4096BS on alpha-CGRP-induced regional haemodynamic changes in anaesthetised rats

Several studies suggest that a calcitonin gene-related peptide (CGRP) receptor antagonist may have antimigraine properties, most probably via the inhibition of CGRP-induced cranial vasodilatation. We recently showed that the novel selective CGRP receptor antagonist, BIBN4096BS (1-piperidinecarboxamide, -N-[2-[[5-amino-1-[[4-(4-pyridinyl)-1-piperazinyl] carbonyl] pentyl]amino]-1-[(3,5-dibromo-4-hydroxyphenyl) methyl]-2-oxoethyl]-4-(1,4-dihydro-2-oxo-3(2H)-quinazolinyl)-, [[R-(R,(R*,S*)]), attenuated the CGRP-induced porcine carotid vasodilatation in a model predictive of antimigraine activity. In order to evaluate the potential safety of BIBN4096BS in migraine therapy, this study was designed to investigate the effects of intravenous BIBN4096BS on alpha-CGRP-induced systemic and regional haemodynamic changes in anaesthetised rats, using radioactive microspheres. In vehicle-pretreated animals, consecutive intravenous infusions of alpha-CGRP (0.25, 0.5 and 1 microg kg(-1) min.(-1)) dose-dependently decreased mean arterial blood pressure with an accompanying increase in heart rate and systemic vascular conductance whereas cardiac output remained unchanged. Alpha-CGRP also increased the vascular conductance to the heart, brain, gastrointestinal tract, adrenals, skeletal muscles and skin, whilst that to the kidneys, spleen, mesentery/pancreas and liver remained unaltered. The above systemic and regional haemodynamic responses to alpha-CGRP were clearly attenuated in BIBN4096BS (3 mg kg(-1) intravenously)-pretreated animals. These results indicate that exogenously administered alpha-CGRP dilates regional vascular beds via CGRP receptors on the basis of the antagonism produced by BIBN4096BS. Moreover, the fact that BIBN4096BS did not alter baseline haemodynamics suggests that endogenously produced CGRP does not play an important role in regulating the systemic and regional haemodynamics under resting conditions.

Effects of Ninjin-to on Levels of Calcitonin Gene-Related Peptide and Substance P in Human Plasma

The herbal medicine Ninjin-to has been used for the treatment of gastroenteritis, esogastritis, gastric atony, gastrectasis, vomiting, and anorexia. One of the mechanisms of the empirical effects is assumed to be due to local changes in neuropeptide levels. Sensory afferent neurons in the gastrointestinal mucosa regulate neuropeptides [calcitonin gene-related peptide (CGRP), substance P, etc.], which play various physiologic roles. To determine whether the pharmacologic effects of Ninjin-to on the gastrointestine are due to changes in gastrointestinal mucosa regulatory peptide levels, we examined the effects of Ninjin-to on the levels of CGRP-like immunoreactive substances (IS) and substance P-IS in plasma taken from five healthy subjects. A single oral administration of 6.0 g of Ninjin-to caused significant increases in plasma CGRP-IS at 40 min and 60 min, and in substance P-IS levels at 90 min, compared with a placebo group. These results may indicate that the pharmacologic actions of Ninjin-to are closely related to changes in CGRP-IS and substance P-IS levels.

Alterations of neurokinin receptors and interstitial cells of Cajal during and after jejunal inflammation induced by Nippostrongylus brasiliensis in the rat

Substance P (SP) and its receptors NK1 and NK2 are widely expressed in the intestinal wall by neurones, interstitial cells of Cajal (ICC) and smooth muscle cells. Changes in SP and/or its NK receptors have been documented during experimental inflammation in animals or inflammatory bowel diseases in humans, but the data concern the acute phase of the inflammatory process. We determined immunohistochemically whether NK receptors and SP were altered in the muscle coat during jejunal inflammation induced by the nematode Nippostrongylus brasiliensis and whether these alterations persisted when inflammation had spontaneously resolved 30 days postinfection. An ultrastructural analysis was also conducted on ICC, nerves and muscle. At day 14, when inflammation peaked, there was a reduction in NK1 receptors in myenteric neurones and in SP-immunoreactive nerve endings. There were also ultrastructural anomalies in synaptic vesicles and NK2 receptor loss in the circular muscle layer. The SP decrease persisted at day 30, whereas neurones and circular muscle cells re-expressed NK1 and NK2 receptors, respectively. The ICC at the deep muscular plexus, located near to the inflammatory site, underwent alterations leading to their complete loss at day 30. These morphological changes are probably associated with impairment in tachykinergic control of jejunal functions leading to the alterations of motility and sensitivity to distension already described in these animals.

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.

Localisation and neural control of the release of calcitonin gene-related peptide (CGRP) from the isolated perfused porcine ileum

By immunohistochemistry, CGRP-like immunoreactive (CGRP-LI) nerve fibres were found in the lamina propria along small vessels and in the lamina muscularis mucosae in the porcine ileum. Immunoreactive nerve cell bodies were found in the submucous and myenteric plexus. Upon HPLC-analysis of ileal extracts, CGRP-LI corresponded entirely to porcine CGRP plus smaller amounts of oxidised CGRP. Using isolated vascularly perfused segments of the ileum, we studied the release of CGRP-LI in response to electrical stimulation of the mixed extrinsic periarterial nerves and to infusion of different neuroblockers. In addition, the effect of infusion of capsaicin was studied. The basal output of CGRP-LI was 2.9+/-0.7 pmol/5 min (mean+/-S.D.). Electrical nerve stimulation (8 Hz) significantly increased the release of CGRP-LI to 167+/-16% (mean+/-S.E.M.) of the basal output (n=13). This response was unaffected by the addition of atropine (10(-6) M). Nerve stimulation during infusion of phentolamine (10(-5) M) with and without additional infusion of atropine resulted in a significant further increase in the release of CGRP-LI to 261+/-134% (n=5) and 240+/-80% (n=9), respectively. This response was abolished by infusion of hexamethonium (3x10(-5) M). Infusion of capsaicin (10(-5) M) caused a significant increase in the release of CGRP-LI to 485+/-82% of basal output (n=5). Our results suggest a dual origin of CGRP innervation of the porcine ileum (intrinsic and extrinsic). The intrinsic CGRP neurons receive excitatory input by parasympathetic, possibly vagal, preganglionic fibres, via release of acetylcholine acting on nicotinic receptors. The stimulatory effect of capsaicin suggests that CGRP is also released from extrinsic sensory neurons.

Co-distribution of NK2 tachykinin receptors and substance P in nerve endings of guinea-pig ileum

The distribution of NK2 tachykinin receptors-immunoreactivity (NK2r-IR) in the guinea-pig ileum and the co-distribution of NK2r-IR with substance P-immunoreactivity (SP)-IR were investigated. NK2r-IR was detected in varicose fibers of myenteric and submucous ganglia and nerve strands, in the longitudinal and circular muscle layers and at the deep muscular plexus (DMP). Except for the submucous plexus, some of the NK2r-IR varicose fibers were co-distributed with SP-IR ones and quantitative analysis showed significant regional differences in the percentages of these fibers. These results demonstrate that presynaptic NK2 receptors are located at varicose fibers likely originating from motor neurons projecting to muscle layers and DMP, and from interneurons. Furthermore, the NK2r/SP-IR co-distribution suggests that some of these receptors are autoreceptors on SP nerve endings.

Primary structure, distribution, and effects on motility of CGRP in the intestine of the cod Gadus morhua

Calcitonin gene-related peptide (CGRP) was isolated from an extract of the intestine of the cod Gadus morhua. The primary structure of this 37-amino acid peptide was established as follows: ACNTA TCVTH RLADF LSRSG GIGNS NFVPT NVGSK AF-NH2. The peptide shows close structural similarities to other nonmammalian (3-4 amino acid substitutions) and mammalian (5-8 amino acid substitutions) CGRPs, and it contains the two residues Asp14 and Phe15 that seem to be characteristic for CGRP in nonmammalian vertebrates. Cod CGRP (10(-9)-10(-7) M) inhibited the motility of spontaneously active ring preparations from the cod intestine and was significantly (P < 0.05) more potent than rat alpha-CGRP. Neither prostaglandins nor nitric oxide is involved in the inhibitory response produced by cod CGRP, and the lack of effect of tetrodotoxin suggests an action of CGRP on receptors on the intestinal smooth muscle cells. The competitive CGRP antagonist human alpha-CGRP-(8-37) significantly (P < 0.05) reduced the response to cod CGRP. Immunohistochemistry demonstrated CGRP-immunoreactive neurons intrinsic to the intestine, and a dense innervation with immunoreactive nerve fibers was observed in the myenteric plexus and the circular muscle layer. Myotomy studies show that CGRP-containing nerves project orally and anally in the myenteric plexus, whereas nerve fibers in the circular muscle layer project mainly anally, indicating a role for CGRP in descending inhibitory pathways of the cod intestine.

NK1 receptor expression in the interstitial cells of Cajal and neurons and tachykinins distribution in rat ileum during development

The origin and function of the interstitial cells of Cajal (ICCs) that are located at the level of the deep muscular plexus (DMP) have not been completely identified. It has been recently reported that these cells express neurokinin-1 (NK1) receptors to which substance P (SP) shows the highest affinity. Studies during pre- and postnatal life have demonstrated that ICCs are identifiable in the rat ileum soon after birth and already show adult features at 7 days of postnatal life. Several neurotransmitters have been identified at the DMP which appear at specific times during development. We have studied the expression of NK1 receptors by ICCs and enteric neurons and the timing of the appearance of SP in the DMP, myenteric plexus (MP) and submucous plexus (SMP) of rat ileum during development. Rats, aged from 18 days of fetal life to adulthood, were used. NK1 receptors and SP were identified by using NK1 polyclonal antibodies and tachykinin (SP/TK) polyclonal antibodies, respectively. NK1-immunoreactivity (IR) was detected in the ICCs immediately after birth and reached maximal intensity at 7 days. From birth, SP/TK-IR fibers originated from short excitatory neurons at the MP and reached the DMP at 1 week of postnatal life. NK1- and SP/TK-IR appeared in the MP neurons in the fetus and in the SMP neurons at weaning. The present study demonstrates that by the first days of postnatal life, the NK1-IR might be used as a marker of the ICCs at the DMP and suggests that these cells may participate in the actions exerted by tachykinins on muscle cells.

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.

Tachykinins in the gut. Part II. Roles in neural excitation, secretion and inflammation

The preprotachykinin-A gene-derived peptides substance (substance P; SP) and neurokinin (NK) A are expressed in intrinsic enteric neurons, which supply all layers of the gut, and extrinsic primary afferent nerve fibers, which innervate primarily the arterial vascular system. The actions of tachykinins on the digestive effector systems are mediated by three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Within the enteric nervous system, SP and NKA are likely to mediate, or comediate, slow synaptic transmission and to modulate neuronal excitability via stimulation of NK3 and NK1 receptors. In the intestinal mucosa, tachykinins cause net secretion of fluid and electrolytes, and it appears as if SP and NKA play a messenger role in intramural secretory reflex pathways. Secretory processes in the salivary glands and pancreas are likewise influenced by tachykinins. The gastrointestinal arterial system may be dilated or constricted by tachykinins, whereas constriction and an increase in the vascular permeability are the only effects seen in the venous system. Various gastrointestinal disorders are associated with distinct changes in the tachykinin system, and there is increasing evidence that tachykinins participate in the hypersecretory, vascular and immunological disturbances associated with infection and inflammatory bowel disease. In a therapeutic perspective, it would seem conceivable that tachykinin antagonists could be exploited as antidiarrheal, antiinflammatory and antinociceptive drugs.

Tachykinins in the gut. Part I. Expression, release and motor function

The preprotachykinin-A gene-derived peptides substance P and neurokinin (NK) A are expressed in distinct neural pathways of the mammalian gut. When released from intrinsic enteric or extrinsic primary afferent neurons, tachykinins have the potential to influence both nerve and muscle by way of interaction with three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Most prominent among the effects of tachykinins is their excitatory action on gastrointestinal motor activity, which is seen in virtually all regions and layers of the mammalian gut. This action depends not only on a direct activation of the muscle through NK1 and/or NK2 receptors, but also on stimulation of excitatory enteric motor pathways through NK3 and/or NK1 receptors. In addition, tachykinins can inhibit motor activity by stimulating either inhibitory neuronal pathways or interrupting excitatory relays. A synopsis of the available data indicates that endogenous substance P and NKA interact with other enteric transmitters in the physiological control of gastrointestinal motor activity. Derangement of the regulatory roles of tachykinins may be a factor in the gastrointestinal dysmotility associated with infection, inflammation, stress and pain. In a therapeutic perspective, it would seem conceivable, therefore, that tachykinin agonists and antagonists are adjuncts to the treatment of motor disorders that involve pathological disturbances of the gastrointestinal tachykinin system.

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.

Calcitonin gene-related peptide innervation of the rat hepatobiliary system

The digestive system is densely innervated by calcitonin gene-related peptide (CGRP)-immunoreactive neurons. The present study investigated a) the distribution and origin of CGRP-immunoreactive fibers in the rat hepatobiliary tract, and b) their relation with substance P/tachykinin (SP/TK) immunoreactivity using immunohistochemical and radioimmunoassay techniques. CGRP-containing fibers form dense networks in the fibromuscular layer of the biliary tree and surrounding the portal vein. Thin, varicose fibers are present at the base of the mucosa of the ducts. In the liver, labeled fibers are restricted to the portal areas and the stromal compartment. Neonatal treatment with capsaicin, a neurotoxin for primary afferent neurons, or celiac/superior mesenteric ganglionectomy depletes CGRP-containing fibers in the biliary tract, and reduces those associated with the portal vein. In contrast, subdiaphragmatic vagotomy does not appreciably modify the density of these fibers. Radioimmunoassay studies show a reduction of CGRP-immunoreactive contents in the biliary tract and portal vein by 84% and 65%, respectively, following capsaicin treatment, and by 80% and 66%, respectively, following ganglionectomy. By contrast, CGRP concentrations in vagotomized animals are comparable to those of controls. Most CGRP-positive fibers appear to contain SP/TK immunoreactivity, as indicated by double-label studies. These results demonstrate that the rat hepatobiliary tract is prominently innervated by CGRP- and CGRP/SP/TK-immunoreactive fibers, which are likely to originate from spinal afferent neurons. The abundance of these fibers and their association with a variety of targets are in line with the involvement of these peptidergic visceral afferents in regulating hepatobiliary activities, including hemodynamic functions of the hepatic vasculature.

Peptide immunoreactivities in the ganglionated plexuses and nerve fibers innervating the human gallbladder

The mammalian gallbladder is innervated by a well-developed intrinsic neural network. However, little is known about the neurochemistry and organization of the innervation of this organ in humans. The aim of this study was to analyze the distribution of immunoreactivity (IR) for the neuropeptides, vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), tachykinins (TK) and calcitonin gene-related peptide (CGRP) in the human gallbladder by means of immunohistochemistry. Neuropeptide-IRs are found in neurons and processes of the two ganglionated plexuses, i.e., the innermost plexus located in the lamina propria at the base of the mucosal folds, and the outermost plexus situated within the fibro-muscular layer. In these two plexuses, VIP-, NPY- and TK-IRs are present in ganglion cells and varicose fibers, whereas CGRP-IR is confined to nerve processes. VIP-IR is present in most, if not all, neurons. NPY- and TK-IRs are also found in many neurons. The densities of the peptide-IR nerves in the mucosa are NPY and VIP > TK >> CGRP, and in the fibro-muscular layer are NPY > VIP and TK > CGRP. The vasculature is richly innervated by NPY-IR nerves, which are mostly perivascular. CGRP-, VIP- and TK-IR processes are found only occasionally around blood vessels and in a paravascular position. Double-label studies demonstrated that a large number of VIP-containing neurons expresses NPY- or TK-IR. On the other hand, all neurons positive for either NPY- or TK-IR are immunostained for VIP. In agreement with these findings, most of the NPY-IR fibers in the lamina propria and fibro-muscular layer contain VIP-IR, and numerous TK-IR fibers are positive for VIP. However, the perivascular NPY-IR processes do not contain VIP-IR, suggesting an extrinsic origin. In addition, a population of TK-IR processes contains CGRP-IR and presumably originates from extrinsic sources, since CGRP/TK-IR intrinsic neurons could not be detected in the gallbladder. Peptide-IRs have a similar distribution in the neck, body and fundus of the gallbladder. No peptide-containing endocrine/paracrine cells are observed in the epithelium. The presence of peptide-IRs in the ganglionated plexuses and the abundance of peptidergic innervation suggest that peptides exert their effects on gallbladder function by acting directly on tissue targets and influencing intrinsic ganglion cells. Furthermore, the co-localization of more than one peptide in the same neuron raises the possibility that peptides are co-released upon stimulation and might interact at the same target.