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

An Intraperitoneal Treatment with Calcitonin Gene-Related Peptide (CGRP) Regulates Appetite, Energy Intake/Expenditure, and Metabolism

Calcitonin gene-related peptide (CGRP) is a 37-amino acid neuropeptide expressed both centrally and peripherally. CGRP has been shown to be involved in arteriolar dilation, cardiovascular regulation, pain transmission, migraine, and gastrointestinal physiology. Our current research is aimed at analyzing CGRP's impact on appetite/satiety, body metabolism, and energy homeostasis. Our study investigated the effects of a single-dose intraperitoneal (IP) treatment with CGRP on food and water consumption, energy expenditure, physical activity, respirometry, and a panel of plasma metabolic hormones in C57Bl/6 wild-type (WT) mice. After a CGRP IP injection at a dose of 2 nmol (10 μM CGRP in 200 μl of saline), a significant reduction in food intake and metabolic parameters as RQ, VCO₂, and VO₂ was observed. CGRP-injected mice had also significantly lower total energy expenditure (TEE) with no changes in activity levels compared to vehicle-injected controls. CGRP treatment in mice induced significantly lower plasma levels of glucagon and leptin but higher levels of amylin. Our data show that a single dose of CGRP peptide significantly decreased food consumption and altered calorimetric parameters and plasma metabolic hormone levels, thus confirming that CGRP plays a pivotal role in the regulation of appetite and metabolism. Future studies are necessary to analyze CGRP's long-term impact on body metabolism and its potential effects on appetite, obesity, and metabolic disorders.

The DH31/CGRP enteroendocrine peptide triggers intestinal contractions favoring the elimination of opportunistic bacteria

The digestive tract is the first organ affected by the ingestion of foodborne bacteria. While commensal bacteria become resident, opportunistic or virulent bacteria are eliminated from the gut by the local innate immune system. Here we characterize a new mechanism of defense, independent of the immune system, in Drosophila melanogaster. We observed strong contractions of longitudinal visceral muscle fibers for the first 2 hours following bacterial ingestion. We showed that these visceral muscle contractions are induced by immune reactive oxygen species (ROS) that accumulate in the lumen and depend on the ROS-sensing TRPA1 receptor. We then demonstrate that both ROS and TRPA1 are required in a subset of anterior enteroendocrine cells for the release of the DH31 neuropeptide which activates its receptor in the neighboring visceral muscles. The resulting contractions of the visceral muscles favors quick expulsion of the bacteria, limiting their presence in the gut. Our results unveil a precocious mechanism of defense against ingested opportunistic bacteria, whether they are Gram-positive like Bacillus thuringiensis or Gram-negative like Erwinia carotovora carotovora. Finally, we found that the human homolog of DH31, CGRP, has a conserved function in Drosophila.

The intestine is the first barrier to fight non-commensal bacteria ingested along with the food. The innate immune system is the main mean mounted by the gut lining in response to ill-causing bacteria to avoid detrimental impact. Intestinal cells produce chlorine bleach and antimicrobial peptides that destroy exogenous bacteria. Here, we identified and characterized a new mechanism of gut defense that occurs rapidly after ingestion of exogenous bacteria. We found that the enteroendocrine cells perceive the presence of chlorine bleach in the lumen thanks to a sensor. This sensor promotes a calcium flux within enteroendocrine cells that allows the release of a hormone. This hormone acts locally on the visceral muscle surrounding the intestine by provoking its strong contractions (or spasms). We show that these strong but brief visceral contractions are helping to the quick expulsion of the ingested bacteria thus limiting their potential detrimental impact on the intestine. Markedly, the bleach-sensor is well known to be involved in pain. Therefore we have deciphered in this study a biological mechanism that has so far been described only empirically by medicine, potentially explaining intestinal pain and visceral spasms upon food poisoning.

Neurochemical characterization of myenteric neurons in the juvenile gilthead sea bream (Sparus aurata) intestine

We evaluated the chemical coding of the myenteric plexus in the proximal and distal intestine of gilthead sea bream (Sparus aurata), which represents one of the most farmed fish in the Mediterranean area. The presence of nitric oxide (NO), acetylcholine (ACh), serotonin (5-HT), calcitonin-gene-related peptide (CGRP), substance P (SP) and vasoactive intestinal peptide (VIP) containing neurons, was investigated in intestinal whole mount preparations of the longitudinal muscle with attached the myenteric plexus (LMMP) by means of immunohistochemical fluorescence staining. The main excitatory and inhibitory neurochemicals identified in intestinal smooth muscle were ACh, SP, 5HT, and NO, VIP, CGRP. Some neurons displayed morphological features of ascending and descending interneurons and of putative sensory neurons. The expression of these pathways in the two intestinal regions is largely superimposable, although some differences emerged, which may be relevant to the morphological properties of each region. The most important variances are the higher neuronal density and soma size in the proximal intestine, which may depend on the volume of the target tissue. Since in the fish gut the submucosal plexus is less developed, myenteric neurons substantially innervate also the submucosal and epithelial layers, which display a major thickness and surface in the proximal intestine. In addition, myenteric neurons containing ACh and SP, which mainly represent excitatory motor neurons and interneurons innervating the smooth muscle were more numerous in the distal intestine, possibly to sustain motility in the thicker smooth muscle coat. Overall, this study expands our knowledge of the intrinsic innervation that regulates intestinal secretion, absorption and motility in gilthead sea bream and provides useful background information for rational design of functional feeds aimed at improving fish gut health.

Biochemical, Anatomical, and Pharmacological Characterization of Calcitonin-Type Neuropeptides in Starfish: Discovery of an Ancient Role as Muscle Relaxants

Calcitonin (CT) is a peptide hormone released by the thyroid gland that regulates blood Ca²⁺ levels in mammals. The CT gene is alternatively spliced, with one transcript encoding CT and another transcript encoding the CT-like neuropeptide calcitonin-gene related peptide (α-CGRP), which is a powerful vasodilator. Other CT-related peptides in vertebrates include adrenomedullin, amylin, and intermedin, which also act as smooth muscle relaxants. The evolutionary origin of CT-type peptides has been traced to the bilaterian common ancestor of protostomes and deuterostomes and a CT-like peptide (DH31) has been identified as a diuretic hormone in some insect species. However, little is known about the physiological roles of CT-type peptides in other invertebrates. Here we characterized a CT-type neuropeptide in a deuterostomian invertebrate—the starfish Asterias rubens (Phylum Echinodermata). A CT-type precursor cDNA (ArCTP) was sequenced and the predicted structure of the peptide (ArCT) derived from ArCTP was confirmed using mass spectrometry. The distribution of ArCTP mRNA and the ArCT peptide was investigated using in situ hybridization and immunohistochemistry, respectively, revealing stained cells/processes in the nervous system, digestive system, and muscular organs, including the apical muscle and tube feet. Investigation of the effects of synthetic ArCT on in vitro preparations of the apical muscle and tube feet revealed that it acts as a relaxant, causing dose-dependent reversal of acetylcholine-induced contraction. Furthermore, a muscle relaxant present in whole-animal extracts of another starfish species, Patiria pectinifera, was identified as an ortholog of ArCT and named PpCT. Consistent with the expression pattern of ArCTP in A. rubens, RT-qPCR revealed that in P. pectinifera the PpCT precursor transcript is more abundant in the radial nerve cords than in other tissues/organs analyzed. In conclusion, our findings indicate that the physiological action of CT-related peptides as muscle relaxants in vertebrates may reflect an evolutionarily ancient role of CT-type neuropeptides that can be traced back to the common ancestor of deuterostomes.

Endocrine cells producing peptide hormones in the intestine of Nile tilapia: distribution and effects of feeding and fasting on the cell density

Endocrine cells (ECs) act as a luminal surveillance system responding to either the presence or absence of food in the gut through the secretion of peptide hormones. The aim of this study was to analyze the effects of feeding and fasting on the EC peptide-specific distribution along the intestine of Nile tilapia. We assessed the density of ECs producing gastrin (GAS), cholecystokinin-8 (CCK-8), neuropeptide Y (NPY), and calcitonin gene-related peptide (CGRP) in nine segments of the intestine using immunohistochemistry. Our results show that ECs immunoreactive to CCK-8, GAS, NPY, and CGRP can be found along all the intestinal segments sampled, from the midgut to hindgut, although differences in their distribution along the gut were observed. Regarding nutrient status, we found that the anterior segments of the midgut seem to be the main site responding to luminal changes in Nile tilapia. The NPY+ and CGRP+ EC densities increased in the fasted group, while the amount of CCK-8+ ECs were higher in the fed group. No effects of fasting or feeding were found in the GAS+ EC densities. Changes in ECs density were found only at the anterior segments of the intestine which may be due to the correlation between vagus nerve anatomy, EC location, and peptide turnover. Lastly, ECs may need to be considered an active cell subpopulation that may adapt and respond to different nutrient status as stimuli. Due to the complexity of the enteroendocrine system and its importance in fish nutrition, much remains to be elucidated and it deserves closer attention.

The presence and role of interstitial cells of Cajal in the proximal intestine of shorthorn sculpin (Myoxocephalus scorpius)

Rhythmic contractions of the mammalian gastrointestinal tract can occur in the absence of neuronal or hormonal stimulation owing to the generation of spontaneous electrical activity by interstitial cells of Cajal (ICC) that are electrically coupled to smooth muscle cells. The myogenically driven component of gastrointestinal motility patterns in fish probably also involves ICC; however, little is known of their presence, distribution and function in any fish species. In the present study, we combined immunohistochemistry and in vivo recordings of intestinal motility to investigate the involvement of ICC in the motility of the proximal intestine in adult shorthorn sculpin (Myoxocephalus scorpius). Antibodies against anoctamin 1 (Ano1, a Ca²⁺-activated Cl^- channel), revealed a dense network of multipolar, repeatedly branching cells in the myenteric region of the proximal intestine, similar in many regards to the mammalian ICC-MY network. The addition of benzbromarone, a potent blocker of Ano1, altered the motility patterns seen in vivo after neural blockade with TTX. The results indicate that ICC are integral for the generation and propagation of the majority of rhythmic contractile patterns in fish, although their frequency and amplitude can be modulated via neural activity.

Expression of neuropeptides and anoctamin 1 in the embryonic and adult zebrafish intestine, revealing neuronal subpopulations and ICC-like cells

This immunohistochemical study in zebrafish aims to extend the neurochemical characterization of enteric neuronal subpopulations and to validate a marker for identification of interstitial cells of Cajal (ICC). The expression of neuropeptides and anoctamin 1 (Ano1), a selective ICC marker in mammals, was analyzed in both embryonic and adult intestine. Neuropeptides were present from 3 days postfertilization (dpf). At 3 dpf, galanin-positive nerve fibers were found in the proximal intestine, while calcitonin gene-related peptide (CGRP)- and substance P-expressing fibers appeared in the distal intestine. At 5 dpf, immunoreactive fibers were present along the entire intestinal length, indicating a well-developed peptidergic innervation at the onset of feeding. In the adult intestine, vasoactive intestinal peptide (VIP), pituitary adenylate cyclase-activating peptide (PACAP), galanin, CGRP and substance P were detected in nerve fibers. Colchicine pretreatment enhanced only VIP and PACAP immunoreactivity. VIP and PACAP were coexpressed in enteric neurons. Colocalization stainings revealed three neuronal subpopulations expressing VIP and PACAP: a nitrergic noncholinergic subpopulation, a serotonergic subpopulation and a subpopulation expressing no other markers. Ano1-immunostaining revealed a 3-dimensional network in the adult intestine containing multipolar cells at the myenteric plexus and bipolar cells interspersed between circular smooth muscle cells. Ano1 immunoreactivity first appeared at 3 dpf, indicative of the onset of proliferation of ICC-like cells. It is shown that the Ano1 antiserum is a selective marker of ICC-like cells in the zebrafish intestine. Finally, it is hypothesized that ICC-like cells mediate the spontaneous regular activity of the embryonic intestine.

Brain neuropeptides in central ventilatory and cardiovascular regulation in trout

Many neuropeptides and their G-protein coupled receptors (GPCRs) are present within the brain area involved in ventilatory and cardiovascular regulation but only a few mammalian studies have focused on the integrative physiological actions of neuropeptides on these vital cardio-respiratory regulations. Because both the central neuroanatomical substrates that govern motor ventilatory and cardiovascular output and the primary sequence of regulatory peptides and their receptors have been mostly conserved through evolution, we have developed a trout model to study the central action of native neuropeptides on cardio-ventilatory regulation. In the present review, we summarize the most recent results obtained using this non-mammalian model with a focus on PACAP, VIP, tachykinins, CRF, urotensin-1, CGRP, angiotensin-related peptides, urotensin-II, NPY, and PYY. We propose hypotheses regarding the physiological relevance of the results obtained.

Autonomic control of gut motility: a comparative view

Gut motility is regulated to optimize food transport and processing. The autonomic innervation of the gut generally includes extrinsic cranial and spinal autonomic nerves. It also comprises the nerves contained entirely within the gut wall, i.e. the enteric nervous system. The extrinsic and enteric nervous control follows a similar pattern throughout the vertebrate groups. However, differences are common and may occur between groups and families as well as between closely related species. In this review, we give an overview of the distribution and effects of common neurotransmitters in the vertebrate gut. While the focus is on birds, reptiles, amphibians and fish, mammalian data are included to form the background for comparisons. While some transmitters, like acetylcholine and nitric oxide, show similar distribution patterns and effects in most species investigated, the role of others is more varying. The significance for these differences is not yet fully understood, emphasizing the need for continued comparative studies of autonomic control.

Effect of calcitonin gene-related peptide (CGRP), adrenomedullin and adrenomedullin-2/intermedin on food intake in goldfish (Carassius auratus)

The purpose of the present study was to elucidate the possible role of calcitonin gene-related peptide (CGRP), adrenomedullin (AM) and adrenomedullin-2/intermedin (IMD) on food intake regulation in goldfish (Carassius auratus). We examined the effects of intracerebroventricular (ICV) administration of these related hormones on food intake. Food-deprived goldfish were subjected to ICV injections of CGRP, AM and IMD and their food intake were quantified. CGRP at 10ng/g body weight (bw) significantly decreased food intake as compared to saline-treated fish. IMD at 10 and 50ng/g bw both significantly decreased food intake as compared to saline group. No significant differences were observed after AM administration. Our results suggest, for the first time in fish, a role for both CGRP and IMD in the central regulation of feeding in fish.

Occurrence of a CGRP-like molecule in siki (Centroscymnus coelolepsis) hydrolysate of industrial origin

Fish protein hydrolysates (FPH) may have potential as bioactive components in functional foods as nutraceuticals. This study focused on the identification of calcitonin gene-related peptide (CGRP) molecules in FPH. CGRP is a neuropeptide belonging to the calcitonin/CGRP family and is known as potent arterial and venous vasodilator in humans. Hydrolysates of industrial origin were prepared from siki (Centroscymnus coelolepsis) heads and were analyzed for the presence of CGRP-like molecules using specific radioimmunoassays and radioreceptorassays. The biological activity of the CGRP-related molecules was assessed by their ability to stimulate the adenylate cyclase activity in rat liver membranes. They were finally purified using gel exclusion chromatography and high-performance liquid chromatography (HPLC). These molecules presented a molecular weight around 1500-2500 Da and were obtained with a purification factor of 79. The incorporation of FPH with CGRP-like molecules in functional foods could lead to the development of new useful products for health and nutrition markets.