Distribution of peptide- and catecholamine-containing neurons in the gastro-intestinal tract of rat and guinea-pig: immunohistochemical studies with antisera to substance P, vasoactive intestinal polypeptide, enkephalins, somatostatin, gastrin/cholecystokinin, neurotensin and dopamine beta-hydroxylase

Rotavirus Downregulates Tyrosine Hydroxylase in the Noradrenergic Sympathetic Nervous System in Ileum, Early in Infection and Simultaneously with Increased Intestinal Transit and Altered Brain Activities

While rotavirus diarrhea has been considered to occur only due to intrinsic intestinal effects within the enteric nervous system, we provide evidence for central nervous system control underlying the clinical symptomology. Our data visualize infection by large-scale three-dimensional (3D) volumetric tissue imaging of a mouse model and demonstrate that rotavirus infection disrupts the homeostasis of the autonomous system by downregulating tyrosine hydroxylase in the noradrenergic sympathetic nervous system in ileum, concomitant with increased intestinal transit. Interestingly, the nervous response was found to occur before the onset of clinical symptoms. In adult infected animals, we found increased pS6 immunoreactivity in the area postrema of the brain stem and decreased phosphorylated STAT5-immunoreactive neurons in the bed nucleus of the stria terminalis, which has been associated with autonomic control, including stress response. Our observations contribute to knowledge of how rotavirus infection induces gut-nerve-brain interaction early in the disease.

Combinatorial Transcriptional Profiling of Mouse and Human Enteric Neurons Identifies Shared and Disparate Subtypes In Situ

BACKGROUND & AIMS

The enteric nervous system (ENS) coordinates essential intestinal functions through the concerted action of diverse enteric neurons (ENs). However, integrated molecular knowledge of EN subtypes is lacking. To compare human and mouse ENs, we transcriptionally profiled healthy ENS from adult humans and mice. We aimed to identify transcripts marking discrete neuron subtypes and visualize conserved EN subtypes for humans and mice in multiple bowel regions.

METHODS

Human myenteric ganglia and adjacent smooth muscle were isolated by laser-capture microdissection for RNA sequencing. Ganglia-specific transcriptional profiles were identified by computationally subtracting muscle gene signatures. Nuclei from mouse myenteric neurons were isolated and subjected to single-nucleus RNA sequencing, totaling more than 4 billion reads and 25,208 neurons. Neuronal subtypes were defined using mouse single-nucleus RNA sequencing data. Comparative informatics between human and mouse data sets identified shared EN subtype markers, which were visualized in situ using hybridization chain reaction.

RESULTS

Several EN subtypes in the duodenum, ileum, and colon are conserved between humans and mice based on orthologous gene expression. However, some EN subtype-specific genes from mice are expressed in completely distinct morphologically defined subtypes in humans. In mice, we identified several neuronal subtypes that stably express gene modules across all intestinal segments, with graded, regional expression of 1 or more marker genes.

CONCLUSIONS

Our combined transcriptional profiling of human myenteric ganglia and mouse EN provides a rich foundation for developing novel intestinal therapeutics. There is congruency among some EN subtypes, but we note multiple species differences that should be carefully considered when relating findings from mouse ENS research to human gastrointestinal studies.

CCK-1 and CCK-2 receptor agonism do not stimulate GLP-1 and neurotensin secretion in the isolated perfused rat small intestine or GLP-1 and PYY secretion in the rat colon

Gastrin and cholecystokinin (CCK) are hormones released from endocrine cells in the antral stomach (gastrin), the duodenum, and the jejunum (CCK). Recent reports, based on secretion experiments in an enteroendocrine cell line (NCI-H716) and gastrin receptor expression in proglucagon-expressing cells from the rat colon, suggested that gastrin could be a regulator of glucagon-like peptide-1 (GLP-1) secretion. To investigate these findings, we studied the acute effects of CCK-8 (a CCK1/CCK2 (gastrin) receptor agonist) and gastrin-17 (a CCK2(gastrin) receptor agonist) in robust ex vivo models: the isolated perfused rat small intestine and the isolated perfused rat colon. Small intestines from Wistar rats (n = 6), were perfused intraarterially over 80 min. During the perfusion, CCK (1 nmol/L) and gastrin (1 nmol/L) were infused over 10-min periods separated by washout/baseline periods. Colons from Wistar rats (n = 6) were perfused intraarterially over 100 min. During the perfusion, CCK (1 nmol/L), vasoactive intestinal peptide (VIP) (10 nmol/L), and glucose-dependent insulinotropic polypeptide (GIP) (1 nmol/L) were infused over 10-min periods separated by washout/baseline periods. In the perfused rat small intestines neither CCK nor gastrin stimulated the release of GLP-1 or neurotensin. In the perfused rat colon, neither CCK or VIP stimulated GLP-1 or peptide YY (PYY) release, but GIP stimulated both GLP-1 and PYY release. In both sets of experiments, bombesin, a gastrin-releasing peptide analog, served as a positive control. Our findings do not support the suggestion that gastrin or CCK participate in the acute regulation of intestinal GLP-1 secretion, but that GIP may play a role in the regulation of hormone secretion from the colon.

Gastric Peptides-Gastrin and Somatostatin

Gastric acid secretion (i) facilitates digestion of protein as well as absorption of micronutrients and certain medications, (ii) kills ingested microorganisms, including Helicobacter pylori, and (iii) prevents bacterial overgrowth and enteric infection. The principal regulators of acid secretion are the gastric peptides gastrin and somatostatin. Gastrin, the major hormonal stimulant for acid secretion, is synthesized in pyloric mucosal G cells as a 101-amino acid precursor (preprogastrin) that is processed to yield biologically active amidated gastrin-17 and gastrin-34. The C-terminal active site of gastrin (Trp-Met-Asp-Phe-NH₂ ) binds to gastrin/CCK₂ receptors on parietal and, more importantly, histamine-containing enterochromaffin-like (ECL) cells, located in oxyntic mucosa, to induce acid secretion. Histamine diffuses to the neighboring parietal cells where it binds to histamine H₂ -receptors coupled to hydrochloric acid secretion. Gastrin is also a trophic hormone that maintains the integrity of gastric mucosa, induces proliferation of parietal and ECL cells, and is thought to play a role in carcinogenesis. Somatostatin, present in D cells of the gastric pyloric and oxyntic mucosa, is the main inhibitor of acid secretion, particularly during the interdigestive period. Somatostatin exerts a tonic paracrine restraint on gastrin secretion from G cells, histamine secretion from ECL cells, and acid secretion from parietal cells. Removal of this restraint, for example by activation of cholinergic neurons during ingestion of food, initiates and maximizes acid secretion. Knowledge regarding the structure and function of gastrin, somatostatin, and their respective receptors is providing novel avenues to better diagnose and manage acid-peptic disorders and certain cancers. Published 2020. Compr Physiol 10:197-228, 2020.

Trim33 (Tif1γ) is not required for skeletal muscle development or regeneration but suppresses cholecystokinin expression

The expression of Trim33 (Tif1γ) increases in skeletal muscles during regeneration and decreases upon maturation. Although Trim33 is required for the normal development of other tissues, its role in skeletal muscle is unknown. The current study aimed to define the role of Trim33 in muscle development and regeneration. We generated mice with muscle-specific conditional knockout of Trim33 by combining floxed Trim33 and Cre recombinase under the Pax7 promoter. Muscle regeneration was induced by injuring mouse muscles with cardiotoxin. We studied the consequences of Trim33 knockdown on viability, body weight, skeletal muscle histology, muscle regeneration, and gene expression. We also studied the effect of Trim33 silencing in satellite cells and the C2C12 mouse muscle cell line. Although Trim33 knockdown mice weighed less than control mice, their skeletal muscles were histologically unremarkable and regenerated normally following injury. Unexpectedly, RNAseq analysis revealed dramatically increased expression of cholecystokinin (CCK) in regenerating muscle from Trim33 knockout mice, satellite cells from Trim33 knockout mice, and C2C12 cells treated with Trim33 siRNA. Trim33 knockdown had no demonstrable effect on muscle differentiation or regeneration. However, Trim33 knockdown induced CCK expression in muscle, suggesting that suppression of CCK expression requires Trim33.

Three-dimensional analysis of fibroblast-like cells in the lamina propria of the rat ileum using serial block-face scanning electron microscopy

Serial block-face scanning electron microscopy (SBF-SEM) is useful for three-dimensional observation of tissues or cells at high-resolution. In this study, SBF-SEM was used to three-dimensionally analyze the characteristics of fibroblast-like cells (FBLCs) in the rat ileal lamina propria (LP). The results revealed that FBLCs in LP could be divided into four types, tentatively named FBLC type I-IV, based on the external cellular appearance, abundance or shape of each organelle, detailed distribution in the LP and relationship with surrounding cells. FBLC-I and -II localized around the intestinal crypt (InC), FBLC-III localized from the lateral portion of InC to the apical portion of the intestinal villus (InV), and FBLC-IV localized in the InV. FBLC-I, -II and -III, but not FBLC-IV, localized beneath the epithelium. FBLC-II possessed thin lamellar-shaped endoplasmic reticula, whereas FBLC-I possessed expanded endoplasmic reticula that occasionally showed a spherical shape. FBLC-III and -IV possessed a cytoplasmic region with high-electron density and no organelles immediately beneath the cellular membrane; this region was found at the epithelial sides in FBLC-III and scattered in FBLC-IV. FBLC-IV were in constant close proximity to villous myocytes throughout the InV and also in contact with FBLC-III especially in the apical portion of the InV. FBLC-I, -II and -IV, but not -III, were constantly found to be in contact with various immunocompetent cells in the LP. Three-dimensional analysis using SBF-SEM indicates that four types of FBLC localized in the rat ileal LP.

Evodiamine inhibits gastrointestinal motility via CCK and CCK1 receptor in water-avoidence stress rat model

AIM

Evodiamine (EVO) has been reported to play an important role in regulating gastrointestinal motility, but the evidence is insufficient, and the mechanism remains unknown. The aim of this study is to investigate the possible role of EVO in stress-induced colonic hypermotility and the potential mechanisms via both in vivo and in vitro investigations.

METHODS

Male Sprague-Dawley rats were exposed to water avoidance stress (WAS) for 1 h or sham WAS daily for 10 consecutive days to construct the rat model. The colonic contractile activity was studied in an organ bath system. The serum CCK-8 level was detected using an enzyme immunoassay kit, and gastrointestinal transit was detected by intragastric administration of India ink.

RESULTS

WAS induced gastrointestinal hypermotility in male rats. EVO significantly inhibited the contractile activity of colonic muscle strips; this effect was not blocked by TTX and the CCK₁ receptor antagonist devazepide. Chronic WAS induced a slight but nonsignificant increase in the serum CCK-8 level, while EVO elevated the serum CCK-8 level in the WAS rats in a dose-dependent manner. Exogenous CCK-8 significantly inhibited the contractile activity of the colonic muscle strips; this effect was not blocked by TTX but was completely blocked by devazepide. Both EVO and CCK-8 inhibited gastrointestinal transit, and the effect of EVO could be partially blocked by devazepide.

SIGNIFICANCE

EVO can reverse stress-induced gastrointestinal hypermotility. This effect may partially occur as a result of promoting the release of CCK and then activating the CCK₁ receptor instead of directly activating the CCK₁ receptor.

The influence of experimental inflammation and axotomy on leucine enkephalin (leuENK) distribution in intramural nervous structures of the porcine descending colon

Background

The enteric nervous system (ENS), located in the intestinal wall and characterized by considerable independence from the central nervous system, consists of millions of cells. Enteric neurons control the majority of functions of the gastrointestinal tract using a wide range of substances, which are neuromediators and/or neuromodulators. One of them is leucine–enkephalin (leuENK), which belongs to the endogenous opioid family. It is known that opioids in the gastrointestinal tract have various functions, including visceral pain conduction, intestinal motility and secretion and immune processes, but many aspects of distribution and function of leuENK in the ENS, especially during pathological states, remain unknown.

Results

During this experiment, the distribution of leuENK – like immunoreactive (leuENK-LI) nervous structures using the immunofluorescence technique were studied in the porcine colon in physiological conditions, during chemically-induced inflammation and after axotomy. The study included the circular muscle layer, myenteric (MP), outer submucous (OSP) and inner submucous plexus (ISP) and the mucosal layer. In control animals, the number of leuENK-LI neurons amounted to 4.86 ± 0.17%, 2.86 ± 0.28% and 1.07 ± 0.08% in the MP, OSP and ISP, respectively. Generally, both pathological stimuli caused an increase in the number of detected leuENK-LI cells, but the intensity of the observed changes depended on the factor studied and part of the ENS. The percentage of leuENK-LI perikarya amounted to 11.48 ± 0.96%, 8.71 ± 0.13% and 9.40 ± 0.76% during colitis, and 6.90 ± 0.52% 8.46 ± 12% and 4.48 ± 0.44% after axotomy in MP, OSP and ISP, respectively. Both processes also resulted in an increase in the number of leuENK-LI nerves in the circular muscle layer, whereas changes were less visible in the mucosa during inflammation and axotomy did not change the number of leuENK-LI mucosal fibers.

Conclusions

LeuENK in the ENS takes part in intestinal regulatory processes not only in physiological conditions, but also under pathological factors. The observed changes are probably connected with the participation of leuENK in sensory and motor innervation and the neuroprotective effects of this substance. Differences in the number of leuENK-LI neurons during inflammation and after axotomy may suggest that the exact functions of leuENK probably depend on the type of pathological factor acting on the intestine.

Neurochemical Markers in the Mammalian Brain: Structure, Roles in Synaptic Communication, and Pharmacological Relevance

BACKGROUND

Knowledge of molecular marker (typically protein or mRNA) expression in neural systems can provide insight to the chemical blueprint of signal processing and transmission, assist in tracking developmental or pathological progressions, and yield key information regarding potential medicinal targets. These markers are particularly relevant in the mammalian brain in the light of its unsurpassed cellular diversity. Accordingly, molecular expression profiling is rapidly becoming a major approach to classify neuron types. Despite a profusion of research, however, the biological functions of molecular markers commonly used to distinguish neuron types remain incompletely understood. Furthermore, most molecular markers of mammalian neuron types are also present in other organs, therefore complicating considerations of their potential pharmacological interactions.

OBJECTIVE

Here, we survey 15 prominent neurochemical markers from five categories, namely membrane transporters, calcium-binding proteins, neuropeptides, receptors, and extracellular matrix proteins, explaining their relation and relevance to synaptic communication.

METHOD

For each marker, we summarize fundamental structural features, cellular functionality, distributions within and outside the brain, as well as known drug effectors and mechanisms of action.

CONCLUSION

This essential primer thus links together the cellular complexity of the brain, the chemical properties of key molecular players in neurotransmission, and possible biomedical opportunities.

Cholecystokinin-From Local Gut Hormone to Ubiquitous Messenger

Cholecystokinin (CCK) was discovered in 1928 in jejunal extracts as a gallbladder contraction factor. It was later shown to be member of a peptide family, which are all ligands for the CCK₁ and CCK₂ receptors. CCK peptides are known to be synthetized in small intestinal endocrine I-cells and cerebral neurons. But in addition, CCK is expressed in several endocrine glands (pituitary cells, thyroid C-cells, pancreatic islets, the adrenals, and the testes); in peripheral nerves; in cortical and medullary kidney cells; in cardial myocytes; and in cells of the immune system. CCK peptides stimulate pancreatic enzyme secretion and growth, gallbladder contraction, and gut motility, satiety and inhibit acid secretion from the stomach. Moreover, they are major neurotransmitters in the brain and the periphery. CCK peptides also stimulate calcitonin, insulin, and glucagon secretion, and they may act as natriuretic peptides in the kidneys. CCK peptides are derived from proCCK with a C-terminal bioactive YMGWMDFamide sequence, in which the Y-residue is partly O-sulfated. The plasma forms are CCK-58, -33, -22, and -8, whereas the small CCK-8 and -5 are potent neurotransmitters. Over the last decades, CCK expression has also been encountered in tumors (neuroendocrine tumors, cerebral astrocytomas, gliomas, acoustic neuromas, and specific pediatric tumors). Recently, a metastastic islet cell tumor was found to cause a specific CCKoma syndrome, suggesting that circulating CCK may be a useful tumor marker.

Biological redundancy of endogenous GPCR ligands in the gut and the potential for endogenous functional selectivity

This review focuses on the existence and function of multiple endogenous agonists of the somatostatin and opioid receptors with an emphasis on their expression in the gastrointestinal tract. These agonists generally arise from the proteolytic cleavage of prepropeptides during peptide maturation or from degradation of peptides by extracellular or intracellular endopeptidases. In other examples, endogenous peptide agonists for the same G protein-coupled receptors can be products of distinct genes but contain high sequence homology. This apparent biological redundancy has recently been challenged by the realization that different ligands may engender distinct receptor conformations linked to different intracellular signaling profiles and, as such the existence of distinct ligands may underlie mechanisms to finely tune physiological responses. We propose that further characterization of signaling pathways activated by these endogenous ligands will provide invaluable insight into the mechanisms governing biased agonism. Moreover, these ligands may prove useful in the design of novel therapeutic tools to target distinct signaling pathways, thereby favoring desirable effects and limiting detrimental on-target effects. Finally we will discuss the limitations of this area of research and we will highlight the difficulties that need to be addressed when examining endogenous bias in tissues and in animals.

Spatiotemporal expression pattern of DsRedT3/CCK gene construct during postnatal development of myenteric plexus in transgenic mice

Cholecystokinin (CCK) is an early marker of both neuronal and endocrine cell lineages in the developing gastrointestinal tract. To determine the quantitative properties and the spatial distribution of the CCK-expressing myenteric neurones in early postnatal life, a transgenic mouse strain with a CCK promoter-driven red fluorescent protein (DsRedT3/CCK) was established. The cell-specific expression of DsRedT3/CCK was validated by in situ hybridization with a CCK antisense riboprobe and by in situ hybridization coupled with immunohistochemistry involving a monoclonal antibody to CCK. A gradual increase in the DsRedT3/CCK-expressing enteric neurones with clear regional differences was documented from birth until the suckling to weaning transition, in parallel with the period of rapid intestinal growth and functional maturation. To evaluate the proportion of myenteric neurones in which DsRedT3/CCK transgene expression was colocalized with the enteric neuronal marker peripherin, immunofluorescence techniques were applied. All DsRedT3/CCK neurones were peripherin-immunoreactive and the proportion of DsRedT3/CCK-expressing myenteric neurones in the duodenum was the highest after the third week of life, when the number of peripherin-immunoreactive myenteric neurones in this region had decreased. Nearly all of the DsRedT3/CCK-expressing neurones also expressed 5-hydroxytryptophan (5-HT). Thus, by utilizing a new transgenic mouse strain, we have demonstrated a small number of CCK-expressing myenteric neurones with a developmentally regulated spatiotemporal distribution. The coexistence of CCK and 5-HT in the majority of these neurones suggests their possible regulatory role in feeding at the suckling to weaning transition.

Biosynthesis of tachykinins (substance P, neurokinin A and neuropeptide K) in neurons of the guinea pig myenteric plexus

Incubation of the myenteric plexus-containing longitudinal muscle layer of guinea pig small intestine with [(35)S]methionine for up to 9 h resulted in a progressive increase in the incorporation of radioactivity into newly synthesized substance P, neurokinin A and neuropeptide K. The radiolabelled peptides were isolated from tissue extracts by immunoprecipitation using regionally-specific antisera and purified by reverse-phase high performance liquid chromatography. Incorporation of radioactivity into the tachykinins was abolished by cycloheximide and was not observed when [(35)S]cysteine was substituted for [(35)S]methionine. The method may be used to study the regulation of biosynthesis and posttranslational modification of protachykinins in the gut.

Age-related changes in substance P-immunoreactive nerve structures of the rat recto-anal region

The recto-anal region is innervated by extrinsic and intrinsic nerves and a number of neuropeptides including substance P (SP) have been suggested to participate in the regulation of intestinal movements. We examined the age-related changes in the distribution of SP-immunoreactive nerve structures in the distal part of the rat large intestine. Using immunohistochemistry, the presence of SP was studied in fresh tissues from Wistar rats at different ages taken at three sampling sites, the distal rectum, anal canal and internal anal sphincter. In the 15-day old rats the myenteric plexus of the distal rectum and anal canal was well outlined by numerous SP-immunoreactive varicose nerve fibres encircling immunonegative perikarya. In the circular muscle layer, nerve fibres and small nerve bundles ran parallel to the muscle cells, while in the longitudinal muscle layer, only occasional nerve fibres were seen. At the level of the internal anal sphincter, no myenteric ganglia were present. Here, thin varicose fibers ran parallel to the smooth muscle cells. In the 3-month old rats, a larger number of intensely staining SP-immunoreactive nerve fibres were found and in the circular muscle layer, thicker nerve strands were observed. In the 26-month old rats, the density and staining intensity of SP-immunopositive nerve fibres in the myenteric plexus was lower than in the 3-month-old rats. Similar changes in the SP-immunostained fibres in the internal anal sphincter were observed. Degenerative alterations in SP-containing fibres during aging appear to play a role in ano-rectal motility and sphincter control.

Substance P in nerve tissue in the gut

Substance P is present in intrinsic nerves in the intestine and in some extrinsic sensory nerves. It can be released from these nerves to act on the muscle layers and on other enteric neurons. Immunohistochemical studies show that the enteric substance P neurons are short and branch profusely to supply all intestinal layers. Pharmacological studies indicate that enteric substance P neurons receive inputs from enteric cholinergic interneurons. This suggests that these neurons are part of multineuronal nerve pathways in the intestine. The substance P neurons are not essential for the enteric reflexes which subserve peristalsis but they appear to modulate these reflexes.

Myenteric plexus of the hind-gut: developmental abnormalities in humans and experimental studies

Intraluminal pressure studies on patients with congenital aganglionosis showed that the aganglionic rectum contracted in an uncoordinated manner and failed to relax. Histochemical assessment of the innervation helped to explain the variable severity of the symptoms in this condition. It is concluded that (1) absence of ganglia prevents normal coordinated peristalsis and creates an obstructive element; (2) absence of reflex relaxation adds to the obstruction; (3) the degree of uncoordinated motor activity in distal aganglionic bowel probably relates to the number of cholinesterase-positive nerves in the circular muscle and adds another variable obstructive element; and (4) deficient innervation of distal ganglionic bowel probably creates a poor propulsive force and so accentuates more distal obstructive factors. Neurohistochemical and functional studies in the anorectum of cats reveal a somewhat different innervation pattern from that in humans but show that sphincteric tone is mainly due to alpha-adrenergic neural activity. Reflex relaxation of the internal anal sphincter is a complex function in which inhibitory responses override motor responses, and it involves an important non-adrenergic non-cholinergic component. The role of cholinergic nerves in the sphincter remains uncertain. Neurohistochemical assessment of full thickness biopsy specimens of rectal muscle from patients with disabling constipation shows that developmental neuronal dysplasias of the hind-gut may be divided into three main categories: (1) aganglionosis (Hirschsprung's disease), (2) hypoganglionosis and (3) hyperganglionosis, and that the different neuronal elements may be affected to differing degrees in individuals within each group. Resection of the aganglionic bowel is required in congenital aganglionosis but the combined diagnostic-therapeutic procedure of anorectal myotomy has been found beneficial in patients with hypoganglionosis.

Distribution of substance P in brain and periphery and its possible role as a co-transmitter

Substance P is widely distributed in the nervous system. In brain and spinal cord it may act as a transmitter, for example at the central branches of primary sensory neurons. It may also be released from the sensory nerve endings and is thought to be involved in antidromic vasodilatation and in synaptic transmission in autonomic ganglia. In some central neurons substance P is stored together with 5-hydroxytryptamine and thyrotropin-releasing hormone. These neurons project to the ventral horn of the spinal cord, amongst other places. In another system substance P coexists with a cholecystokinin-like peptide. These neurons are localized in the periaqueductal central grey matter and also project to the spinal cord. Finally, injection of a substance P antagonist into the ventral mesencephalon causes marked morphological changes in neurons that contain dopamine, substance P and gamma-aminobutyric acid (GABA).

A functional comparison of recombinant and native somatostatin sst2 receptor variants in epithelia

BACKGROUND AND PURPOSE

Somatostatin (SRIF-14) exerts broad spectrum antisecretory effects by activating the somatostatin 2 (sst(2)) receptor. The rat (r) sst(2) receptor exists in 'long' (sst(2a)) and 'short' (sst(2b)) forms that differ in their C termini, while a single human (h) sst(2a) exists. This study compares the characteristics of recombinant rsst(2a), rsst(2b) and hsst(2a) activation in human epithelia, and with native sst(2) responses in rat colon.

EXPERIMENTAL APPROACH

Epithelial layers of each clone or rat colon were placed in Ussing chambers and short-circuit current (I (SC)) measured in response to SRIF-14 and chosen analogues. The relative potencies and ability to cause desensitization to SRIF-14 were assessed, and the affinities of the sst(2) antagonist, D-Tyr(8) CYN154806 for hsst(2a), rsst(2a) and native rat colon sst(2) receptors were established.

KEY RESULTS

Basolateral SRIF-14 responses were transient in hsst(2a) and rsst(2a) epithelia, but prolonged in rsst(2b)-expressing cells. Activation of rsst(2a) resulted in significant desensitization to SRIF-14 and receptor phosphorylation, whereas the rsst(2b) receptor did neither. Sst(2)-preferred agonists (BIM23190C and BIM23027) reduced I (sc) with similar potency and both caused complete desensitization to SRIF-14. CYN154806 antagonized hsst(2a) and rsst(2a) receptors with pK (B) values of 7.9 and 7.8, respectively. In rat colon mucosa, CYN154806 blocked SRIF-14 responses with a pA (2) value of 8.2, and BIM23190C responses with a pK (B) of 8.4.

CONCLUSIONS AND IMPLICATIONS

SRIF-14 caused rapid rsst(2a) receptor phosphorylation and desensitization of epithelial antisecretory responses, neither of which occurred with the rsst(2b) receptor. These mechanisms are most likely to be a prerequisite for sensitivity to sst(2)-analogues with radiotherapeutic potential.

High and complementary expression patterns of alcohol and aldehyde dehydrogenases in the gastrointestinal tract: implications for Parkinson's disease

Parkinson's disease (PD) is a heterogeneous movement disorder characterized by progressive degeneration of dopamine neurons in substantia nigra. We have previously presented genetic evidence for the possible involvement of alcohol and aldehyde dehydrogenases (ADH; ALDH) by identifying genetic variants in ADH1C and ADH4 that associate with PD. The absence of the corresponding mRNA species in the brain led us to the hypothesis that one cause of PD could be defects in the defense systems against toxic aldehydes in the gastrointestinal tract. We investigated cellular expression of Adh1, Adh3, Adh4 and Aldh1 mRNA along the rodent GI tract. Using oligonucleotide in situ hybridization probes, we were able to resolve the specific distribution patterns of closely related members of the ADH family. In both mice and rats, Adh4 is transcribed in the epithelium of tongue, esophagus and stomach, whereas Adh1 was active from stomach to rectum in mice, and in duodenum, colon and rectum in rats. Adh1 and Adh4 mRNAs were present in the mouse gastric mucosa in nonoverlapping patterns, with Adh1 in the gastric glands and Adh4 in the gastric pits. Aldh1 was found in epithelial cells from tongue to jejunum in rats and from esophagus to colon in mice. Adh3 hybridization revealed low mRNA levels in all tissues investigated. The distribution and known physiological functions of the investigated ADHs and Aldh1 are compatible with a role in a defense system, protecting against alcohols, aldehydes and formaldehydes as well as being involved in retinoid metabolism.

The vagus regulates histamine mobilization from rat stomach ECL cells by controlling their sensitivity to gastrin

The ECL cells in the oxyntic mucosa secrete histamine in response to gastrin, stimulating parietal cells to produce acid. Do they also operate under nervous control? The present study examines histamine mobilization from rat stomach ECL cells in situ in response to acute vagal excitation and to food or gastrin following vagal or sympathetic denervation. Applying the technique of microdialysis, we monitored the release of histamine by radioimmunoassay. Microdialysis probes were placed in the submucosa on either side of the stomach, 3 days before experiments. The rats were awake during microdialysis except when subjected to electrical vagal stimulation. One-sided electrical vagal stimulation raised serum gastrin and mobilized gastric histamine. However, gastrin receptor blockade prevented the histamine mobilization, indicating that circulating gastrin accounts for the response. Vagal excitation by hypoglycaemia (insulin) or pylorus ligation did not mobilize either gastrin or histamine. The histamine response to food was almost abolished by gastrin receptor blockade, and it was halved on the denervated side after unilateral subdiaphragmatic vagotomy. While the histamine response to a near-maximally effective dose of gastrin was unaffected by vagotomy, the response to low gastrin doses was reduced significantly. Abdominal ganglionic sympathectomy failed to affect the histamine response to either food or gastrin. In conclusion, gastrin is responsible for most of the food-evoked mobilization of ECL-cell histamine. The histamine response to electrical vagal stimulation reflects the effect of circulating gastrin rather than a direct action of the vagus on the ECL cells. Vagal denervation was accompanied by an impaired histamine response to food intake, probably reflecting the right-ward shift of the serum gastrin concentration-histamine response curve. The results suggest that the vagus controls the sensitivity of the ECL cells to gastrin.

Clinical endocrinology and metabolism. Cholecystokinin

Cholecystokinin (CCK) is a peptide hormone discovered in the small intestine. Together with secretin and gastrin, CCK constitutes the classical gut hormone triad. In addition to gallbladder contraction, CCK also regulates pancreatic enzyme secretion and growth, intestinal motility, satiety signalling and the inhibition of gastric acid secretion. CCK is, however, also a transmitter in central and intestinal neurons. Notably, CCK is the most abundant neuropeptide in the human brain. Owing to difficulties in developing accurate assays, knowledge about CCK secretion in disease is limited. Available data indicate, however, that proCCK is expressed in certain neuroendocrine tumours and sarcomas, whereas the secretion of CCK is impaired in celiac disease and bulimia nervosa. Stimulation with exogenous CCK has proved useful in diagnostic tests of gallbladder and pancreatic diseases, as well as medullary thyroid carcinomas.

Effects of dopamine on ion transport across the rat distal colon

Dopamine (5.10(-6)-5.10(-4) M) induced a concentration-dependent decrease in short-circuit current (I(sc)) across the rat distal colon. This response was preceded by a transient and inconsistent increase in I(sc). The alpha-adrenoceptor blocker phentolamine and the inhibitors of dopamine-2-like (D(2)-like) receptors L-741,626 and L-745,870 inhibited the dopamine response, suggesting a contribution of adrenergic and dopaminergic receptors. The decrease in I(sc) evoked by dopamine was inhibited by bumetanide, an inhibitor of the basolateral Na(+)-K(+)-2 Cl(-) cotransporter responsible for the uptake of K(+), and by quinine, a blocker of apical K(+) channels, indicating that stimulation of K(+) secretion contributes to the measured change in I(sc). In patch-clamp experiments dopamine hyperpolarized the membrane and increased cellular K(+) current. This response was not concomitant with a change in the intracellular [Ca(2+)] as demonstrated in parallel fura-2 experiments. These results demonstrate that dopamine, like other catecholamines, stimulates colonic K(+) secretion.

Cellular localization and distribution of neurokinin-1 receptors in the rat stomach

In the stomach, the majority of substance P's effects are mediated by the activation of neurokinin-1 (NK1) receptors. The gastric cellular distribution of these receptors in Wistar and Sprague-Dawley rats was determined using immunocytochemistry. The localization of the NK1 receptors with respect to von Willebrand's factor, protein gene product 9.5, substance P, vasoactive intestinal peptide, and calcitonin gene-related peptide was also determined. Results show that NK1 receptor immunoreactivity was dependent on the duration of fixation. In corpus and antrum tissues that were fixed in 4% paraformaldehyde for 30 min, the presence of NK1 receptor immunoreactivity was demonstrated on nerve fibers throughout the stomach, on the surface and in the cytoplasm of myenteric cell bodies, on circular smooth muscle cells, and on vascular endothelial cells. This was observed in tissues from both rodent strains. Overnight fixation in the same fixative, however, demonstrated the presence of NK1 receptor immunoreactivity only on nerve fibers and cell bodies of the myenteric plexus, and on circular smooth muscle cells. In 30-min fixed tissues, the localization of NK1R immunoreactivity on vascular endothelial cells and nerve fibers was confirmed by co-localization with von Willebrand's factor and protein gene product 9.5 immunoreactivity, respectively. In both rodent strains, NK1 receptor immunoreactivity was co-localized with substance P immunoreactivity on nerve fibers of the longitudinal and circular muscle. In the Wistar rat, NK1 receptor immunoreactivity was co-localized with vasoactive intestinal peptide immunoreactivity or calcitonin gene-related peptide immunoreactivity throughout the stomach. However, in the Sprague-Dawley rat, NK1 receptor immunoreactivity was only co-localized with calcitonin gene-related peptide immunoreactivity in a minority of fibers of the circular muscle. The overall results of this study show that the antigenic epitopes of the NK1 receptor are sensitive to overfixation. When tissues were not overfixed, NK1 receptor immunoreactivity was distributed more extensively throughout the rat stomach than has been described previously. The results of this study provide the anatomical basis for many of the actions of substance P in the rat stomach.

Involvement of a purinergic pathway in the sympathetic regulation of motility in rat ileum

We investigated extrinsic neuronal regulation of intestinal motility. The mesenteric nerve stimulation (MNS; duration 0.5 ms, 10 Hz for 30 s) evoked relaxation in the longitudinal muscle direction of the isolated rat ileum. The MNS-induced relaxation was abolished by guanethidine (2 microM) or propranolol (10 microM), but was not affected by prazosin (10 microM), rauwolscine (10 microM), hexamethonium (100 microM) or capsaicin (1 microM). Exposure to a high concentration (100 microM) of ATP (ATP-desensitization) or ADP (ADP-desensitization) reduced the MNS-induced relaxation to 44.7% or 32.5% of the control (P<0.01), respectively. P2 purinoceptor antagonists [suramin (100 microM) and reactive blue-2 (RB-2, 50 microM)] or small conductance Ca(2+)-activated K(+) channel blocker, apamin (0.5 microM), significantly decreased the relaxation to 54.4% and 25.6% or 19.4% of the control (P<0.01), respectively, whereas selective P2Y(1) purinoceptor antagonist MRS2179 (10 microM) failed to affect the relaxation. Furthermore, exogenous ATP (1 microM) or ADP (1 microM) elicited relaxation in the rat ileum, which was almost abolished by reactive blue-2 (50 microM, 9.1% of control remained, P<0.05). In contrast, relaxation induced by noradrenalin (10 microM) was not antagonized by ATP-desensitization, apamin (0.5 microM) or reactive blue-2 (50 microM). From the present results, we conclude that noradrenergic sympathetic nerves might regulate intestinal motility mediated through a purinergic inhibitory neuronal pathway in the rat small intestine.

Effects of peripheral CCK receptor blockade on feeding responses to duodenal nutrient infusions in rats

Type A cholecystokinin receptor (CCKAR) antagonists differing in blood-brain barrier permeability were used to test the hypothesis that duodenal delivery of protein, carbohydrate, and fat produces satiety in part by an essential CCK action at CCKARs located peripheral to the blood-brain barrier. Fasted rats with open gastric fistulas received devazepide (1 mg/kg iv) or A-70104 (700 nmol. kg(-1). h(-1) iv) and either a 30-min intravenous infusion of CCK-8 (10 nmol. kg(-1). h(-1)) or duodenal infusion of peptone, maltose, or Intralipid beginning 10 min before 30-min access to 15% sucrose. Devazepide penetrates the blood-brain barrier; A-70104, the dicyclohexylammonium salt of Nalpha-3-quinolinoyl-d-Glu-N,N-dipentylamide, does not. CCK-8 inhibited sham feeding by approximately 50%, and both A-70104 and devazepide abolished this response. Duodenal infusion of each of the macronutrients dose dependently inhibited sham feeding. A-70104 and devazepide attenuated inhibitory responses to each macronutrient. Thus endogenous CCK appears to act in part at CCKARs peripheral to the blood-brain barrier to inhibit food intake.